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3 1293 020611

This is to certify that the

dissertation entitled

Biomass Conversion: Sugar Hydrogenolysis, Glycerol
Dehydroxylation and HF Saccharification

presented by

Keyi Wang

has been accepted towards fulfillment
of the requirements for

Ph . D . degree in Chemical Engineering

 

 

Major professor
DateW7

MSU it an Affirmativtt Action/Equal Opportunity Institution 0-12771

 

 

IJBRAHY

Michigan State
University

 

 

 

PLACE IN RETURN BOX to remove this checkout from your record.
TO AVOID FINE return on or before date due.
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11/00 c/CIFICIDIuDqus-pu

BIOMASS CONVERSION: SUGAR HYDROGENOLYSIS, GLYCEROL
DEHYDROXYLATION AND HF SACCHARIFICATION

By

Keyi Wang

A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Chemical Engineering

1999

BlOI

In the l
hydrogenolysi:
sugar hydroger
glycol; the glyc
the HF sacchan

The tuner
reaction mechai
mechanism stud;

maul-Sh the me“

ABSTRACT

BIOMASS CONVERSION: SUGAR HYDROGENOLYSIS, GLYCEROL
DEHYDROXYLATION AND HF SACCHARIFICATION

By

Keyi Wang

In the current research, three biomass conversion processes, namely sugar
hydrogenolysis, glycerol dehydroxylation and HF saccharification, are investigated. The
sugar hydrogenolysis process converts sugars into glycerol, propylene glycol and ethylene
glycol; the glycerol dehydroxylation process converts glycerol to 1,3-propanediol; while
the HF saccharification process converts lignocellulosic biomass into sugars.

The current research on sugar hydrogenolysis has focused on understanding of the
reaction mechanism and of the factors controlling the reaction selectivity. In the
mechanism study, a series of 1,3-diol model compounds have been hydrogenolyzed, to
establish the mechanism of C-C and CO bond cleavage in sugar hydrogenolysis. The
experimental work continued our theoretical conception that the CC bond breaks via the
retro-aldol reaction of a B-hydroxyl carbonyl precusor, and it also provided for the first
time direct evidence that the C-0 cleavage is through dehydration of the same B-hydroxyl
carbonyl precursor just mentioned. Establishment of the above mechanism makes it
possible for one to adopt a more rational approach than the current practice in control of

the selectivity of sugar hydrogenolysis.

lire s
in sugar hi
study. has
concentratl
selectivity.

In the
Proposed rt
8T0Up. and
Concept is
tosylates at:
In this Worl

mp are l
mihemum c2

The in:
adsorption p]
with a bencl
”actor. Two
blSed 0n GXp‘
anilygis Of ti.
Mum-Hg in l

l“Sight into th
C

The selectivity study in the current work has focused on control of the C-0 cleavage
in sugar hydrogenolysis. Specifically, an experimental study, as well as a simulation
study, has been carried out in this work to investigate the effect of temperature, base
concentration, hydrogen pressure, catalyst amount and catalyst type on the C—C vs 00
selectivity. These studies, both experimental and theoretical, yielded many useful results.

In the research on conversion of glycerol to 1,3-propanediol, a new concept is
proposed to selectively transform the second hydroxyl group of glycerol into a tosyloxy
group, and then to remove it by hydrogenolysis. The technical feasibility of this new
concept is verified experimentally in this work. Before this work, hydrogenolysis of

tosylates are generally effected with base metal hydrides, such as LiAlH4 and LiHBEt3.

In this work, for the first time, tosylates with a hydroxy group adjacent to the tosyloxy
group are hydrogenolyzed with molecular hydrogen in the presence of a nickel or
ruthenium catalyst. The mechanism for this hydrogenolysis reaction is also established.
The work on vapor-phase HF saccharification has focused on modeling of the HF
adsorption process in a packed-bed reactor. However, experiments were also conducted
with a bench-scale reactor to study the behavior of HF adsorption in the packed-bed
reactor. TWO models of HF adsorption have been developed. One is developed purely
based on experimental observations, and the other one is developed based on a detailed
analysis of the HF flow, the intrinsic HF adsorption and the heat transfer processes
occurring in the reactor during HF adsorption. These models provide a great deal of

insight into the HF adsorption process in the packed-bed reactor.

First 0
guidance am
he gave me d
Dr. Kris A. r
Dale for sen-
Scot J. De Ari.
Plant Biotec.
Bloprocessmg
Engineering a

SUppOrL

ACKNOWLEDGMENTS

First of all, I wish to thank my academic advisor Dr. Martin C. Hawley for his
guidance and support in this investigation. I am especially grateful for the encouragement
he gave me during the final stage of my dissertation preparation. I would also like to thank
Dr. Kris A. Berglund, Dr. Dannis J. Miller Dr. Rawle I. Howlingsworth and Dr. Bruce E.
Dale for serving on my graduate committee, and to thank Mr. Todd D. Fumey and Mr.
Scot J. DeAthos for their assistance in completing this investigation. Tire Consortium for
Plant Biotechnology Research, the Michigan State University Crop and Food
Bioprocessing Center, the Amoco Foundation and the Department of Chemical
Engineering at Michigan State University are recognized for their generous financial

support.

iv

LIST OF
LIST OF

LIST OF.‘

CHAP'

PART I. c.

CHIP]

2.1

f")

h)

TABLE OF CONTENTS

LIST OF TABLES ................................................... viii
LIST OF FIGURES ................................................... ix
LIST OF SYMBOLS AND ABBREVIATIONS ............................ xi

CHAPTER 1. GENERAL INTRODUCTION ............................ 1

PART I. CATALITIC SUGAR HYDROGENOLYSIS

CHAPTER 2. INTRODUCTION: SUGAR HYDROGENOLYSIS .......... 10
2.1 Literature Review ........................................... 10
2.1.1 Studies on Process Development .......................... 11

2.1.2 Studies on Reaction Mechanism ........................... 14

2.1.3 Studies on Reaction Selectivity ............................ 17

2.1.4 Studies on Reaction Kinetics ............................. 18

2.2 Objective and Scope of Current Research ........................ 19
CHAPTER 3. EXPERIMENTAL SYSTEM AND METHODS ............. 21
3.1 Experimental System and Procedure ............................ 21

3.2 Reaction Regents and Product Analysis ......................... 25
CHAPTER 4. MECHANISM OF SUGAR HYDROGEN OLYSIS .......... 29
4.1 Theoretical Considerations .................................... 29
4.2 Experimental Verification .................................... 31

V

CHAP

5.1

5.3

PART II. S

CRAFT

6.1

6.3 t

CHAPTER 5. SELECTIVITY OF SUGAR HYDROGENOLYSIS .......... 41

5.1 Selectivity Study Using 2,4-PD Model Compound: Rationale ....... 41
5.2 Experimental Study ........................................ 44
5.3 Simulation Study .......................................... 50
5.3.1 Model Development .................................... 50
5.3.2 Kinetic Parameter Determination ......................... 55
5.3.3 Simulation Results and Discussion ......................... 65

PART H. SELECTIVE DEHYDROXYLATION OF GLYCEROL

CHAPTER 6. INTRODUCTION: GLYCEROL DEHYDROXYLATION . . . . 74

6.1 On-going Research on Glycerol Dehydroxylation .................. 74
6.2 A New Conversion Concept and Supportive Work ................. 77
6.3 Objective and Scope of Current research ......................... 83
CHAPTER 7. EXPERIMENTAL PROCEDURES ....................... 85
7.1 Acetalization .............................................. 85
7.2 Tosylation ................................................. 89
7.3 Hydrolysis ................................................ 89
7.4 Hydrogenolysis ............................................ 91
CHAPTER 8. RESULTS AND DISCUSSION .......................... 92
8.1 Acetalization .............................................. 92
8.2 Tosylation ................................................. 94
8.3 Detosyloxylation ........................................... 95
8.4 Design Concept of Industrial Conversion Process .................. 100

PART III.

CHAP

CHAR
10.
10.1

10.2

CRAP]
11.]
11.2

11.3

APPESDLX

APPEXE

APPEXE
APPEND

LIST OF REF

PART III. VAPOR-PHASE HF SACCHARIFICATION

CHAPTER 9. MODELING THE HF ADSORPTION PROCESS

IN A PACKED-BED REACTOR ........................ 112

CHAPTER 10. SUMMARY AND CONCLUSIONS ..................... 116
10.1 Sugar Hydrogenolysis ...................................... 116
10.2 Glycerol Dehydroxylation ................................... 117
10.3 HF Saccharification ........................................ 118
CHAPTER 11. RECOMMENDATIONS ON FUTURE WORK ............. 120
11.1 Future Work on Sugar Hydrogenolysis ......................... 120
11.2 Future Work on Glycerol Dehydroxylation ...................... 121
11.3 Future Work on HF Saccharification ........................... 124

APPENDIX

APPENDIX A. COMPUTER PROGRAMS FOR SIMULATION STUDY ..... 125

APPENDIX B. RETRO-ALDOL AND DEHYDRATION RATE CONSTANT
DERIVATION PROCEDURE ........................... 131
APPENDIX C. EXPERIMENTAL DATA OF 2,4»PD HYDROGENOLYSIS . . . 134

LIST OF REFERENCES .............................................. 165

vii

Table 3-1.
Table 32
Table 4—1,
Table 4-2~
Table 5-1_
Table 5-2_
Table 5.3_
Table 5-4_
Table 5-5.
Table 5‘6.
Table 5-7_
Table 5-3.
Table 7‘ 1.
Table D-1.

Table D‘Z,

Table 3-1.

Table 3-2.

Table 4-1.

Table 4-2.

Table 5-1.

Table 5-2.

Table 5-3.

Table 5-4.

Table 5-5.

Table 5-6.

Table 5-7.

Table 5-8.

Table 7-1.

Table D— 1.

Table D-2.

LIST OF TABLES

HPLC Methods for 1,3-Diol Hydrogenolysis Products ............... 26
GC Methods for 1,3-Diol Hydrogenolysis Products ................. 27
Structures of 1,3-diols Used in Mechanism Study .................. 33
Results of 1,3—Diol Hydrogenolysis ............................. 34
Effect of Temperature on Selectivity ............................. 46
Effect of Base Concentration on Selectivity ....................... 46
Effect of Hydrogen Pressure on Selectivity ....................... 47
Effect of Catalyst Amount on Selectivity ......................... 47
Effect of Catalyst Type on Selectivity ........................... 49
Equilibrium Constants for Relevant Dehydrogenation Reactions ...... 56
Summary of Product Concentration and Reaction Fluxes ............. 60
Summary of Determined Reaction Rate Constants .................. 64
GC Retention Tunes for Acetalization products .................... 88
Rate Constants Directly from Jenson and Hassan (197 6) .......... 158
Determined Rate Constants ................................. 159

viii

Figure 3-1.
Figure 3-2.
Figure H.
Figure 5-1,

Fi{lure 5-2.

F1Sure 5-3.

Figure 54.
Flgm‘e 5‘5

I"Tgure 5-6.

Figure 5-8.
Figure 5‘9

figure 5~10.

Figure 3-1.
Figure 3-2.
Figure 4-1.
Figure 5-1.

Figure 5-2.

Figure 5-3.

Figure 5-4.
Figure 5-5.

Figure 5-6.

Figure 5-7.

Figure 5-8.
Figure 5-9.

Figure 5-10.

LIST OF FIGURES

Illustration of Hydrogenolysis Reactor .......................... 22
Schematic Illustration of Experimental System ................... 23
Mechanism of Sugar and Sugar Alcohol Hydrogenolysis ............ 30
Reaction Pathway of 2,4-Pentanediol Hydrogenolysis .............. 42

Alternate Hydrogenation Pathway of (LB-Unsaturated Ketone
in Sugar Hydrogenolysis ..................................... 43

Product v.s Reaction Time Profiles. Experimental Conditions: 220 °C,
5.0 MPa H2 pressure, 0.1 g Ni/kieselguhr catalyst, 0.4 ml 1 N NaOH

and 40 ml water solvent. Symbols: 0 2,4-PD; I EtOH; O acetone;

A i-PrOH; CI 2-pentanone; A 2-pentanol. ......................... 45
Selectivity v.s. Reaction Time Profile Derived from Figure 5-3 ....... 45
Detailed Reaction Scheme of 2,4.PD Hydrogenolysis .............. 51

Simulated Concentration Profiles of Major Products.
Simulation Conditions: 220 °C, [H]=100 mM, [M]0=28 mM,

[OH]O=10 mM, [A]o=120 mM. ................................ 66

Simulated Selectivity Profile. S is calculated as ratio of

the sum of [C] and [D] to the sum of [G], [I] and [J]. .............. 66
Simulated Concentration Profiles of Minor products ............... 67
Simulated Base Concentration profile .......................... 67

Simulated Effect of Base Concentration on Selectivity.
Simulation Conditions: 220 °C, [H]=100 mM, [M]0=28 mM,
[A]0=120 mM, varying [OH]0. ................................ 68

ix

Figure 5-2

Figure 5-1

Figure 5-1

Figure 6-1
Figure 6-2.
Figure 7-1.
Figure 8-1.
Figure 8-2.
Figure 8-3,
Fl{lure 8-4.
Figure 8-5,
Ifigure 86

figure 87.

Figure 5-11.

Figure 5-12.

Figure 5-13.

Figure 6-1.
Figure 6-2.
Figure 7-1.
Figure 8-1.
Figure 8-2.
Figure 8-3.
Figure 8—4.
Figure 8-5.
Figure 8-6.

Figure 8-7.

Simulated Effect of Catalyst Concentration on Selectivity.
Simulation Conditions: 220 °C, [H]=100 mM, [OH]O=10 mM,

[A]o=120 mM, varying [M]o. ................................. 68

Simulated Effect of Hydrogen Concentration on Selectivity.
Simulation Conditions: 220 °C, [M]O=28 mM, [OH]0=10 mM,

[A]o=120 mM, varying [H]. ................................... 69

Simulated Effect of k6 on Selectivity. Simulation Conditions:
220 °C, [H]=180 mM, [M]o=28 mM, [OH]o=10 mM,

[A]o=120 mM, but varying k6. ................................ 69
A simplified Reaction Pathway of Glycerol Fermentation ........... 76
Illustration of the New Concept of Glycerol Dehydroxylation ....... 78

Experimental Set-up for Acetalization of Glycerol with Benzaldehyde .86

Reaction Pathway of TCH Hydrogenolysis ...................... 97
Reaction Pathway of TPD Hydrogenolysis ...................... 99
Glycerol Dehydroxylation Processes - Acetalization .............. 101
Glycerol Dehydroxylation Processes - Tosylation (Design #1) ...... 102
Glycerol Dehydroxylation Processes - Tosylation (Design #2) ...... 103
Glycerol Dehydroxylation Processes - Detosyloxylation ........... 104
Material Flow in Glycerol Dehydroxylation ..................... 109

Mathemat

1:1. It].

~ .

3" ’V...

RP!

LIST OF SYMBOLS AND ABBREVIATIONS

Mathematical Symbols

k], k_,, ...... , k8, kg

i» “ck

a a

[A10

reaction rate constants defined in Figure 5-5
forward reaction rate constants of Reaction No. i in Figure 5-5
backward reaction rate constants of Reaction No. i in Figure 5-5

reaction equilibrium constant

equilibrium constant of Reaction No. i in Figure 5-5

equilibrium constant of dehydrogenation reaction

equilibrium constant of dehydrogenation corresponding to
Reaction No. i

reversibility parameter of Reaction No. 2, defined by Eq.(37)
reversibility parameter of Reaction No. 5, defined by Eq.(38)
C-C v.s. C—O selectivity

reaction time

reaction flux rate of Reaction No.1 through No.8, respectively

free energy change of dehydrogenation reaction

free energy change defined in Page 57.

concentration of 2,4-pentandiol

initial concentration of 2,4-pentanediol

[B]
[C]
[D]
[E]

[G]
[H]

U]
[M]
[MlO
[MH]
[on]
{Dino

Chemical

[B]
[C]
[D]
[E]

[G]

[J]
[M]
[M].
[MH]
[0H]
tomo

concentration of 4-hydroxy-2-pentanone
concentration of acetone
concentration of isopropanol
concentration of acetaldehyde
concentration of ethanol

concentration of 3-penten-2-one
concentration of dissolved hydrogen
concentration of 2-pentanone
concentration of 2-pentanol
concentration of metal catalyst

initial concentration of metal catalyst
concentration of metal hydride
concentration of base or hydroxide ion

initial concentration of base or hydroxide ion

Chemical Abbreviations

l ,2-PD
1 ,3-PD
2,4-PD
HPA
HPD
HMPD
PTD
TPD

1,2-propanediol

1 ,3-propanediol

2,4-pentanediol

3-hydropropionaldehyde
5-hydroxy-2-phenyl- 1,3-dioxane
4-hydroxymethyl-2-phenyl-1,3-dioxolane
2-phenyl-5-tosy1- 1,3-dioxane
2-tosyloxy-1,3-propanediol

xii

CAT

PART I

CATALYTIC H YDROGENOLYSIS OF S UGARS

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“ESTES. III

CHAPTER 1
GENERAL INTRODUCTION

Currently, a large portion of the chemical process industry depends on petroleum and
natural gas for feedstocks. Considering the finite and nonrenewable nature of these fossil
resources, this dependence is a severe handicap. As the reserves of petroleum and natural
gas diminish, the chemical process industry will possibly face feedstock problems in the
future. Consequently, we will face a shortage of synthetic chemicals, which are largely
responsible for our current living standard. In light of these possibilities, development of
the biomass conversion processes that produce useful chemicals from the renewable
biomass resource is of great significance.

The significance of biomass conversion is also comprehensible from the perspective
of enhancing biomass utilization. Biomass (lignocellulose) is an abundant material
occurring mainly in the form of forestry residues, agriculture residues and municipal solid
wastes. It is estimated that each year the US. alone generates about 1 billion dry tons of

these residues and wastes (Barrier and Bulls, I992). The annual production of biomass in

the world is estimated to be as high as 1011 to 1012 dry tons (Grohman et aL, 1993). At the
current time, the greatest use of biomass is to burn it as a low-valued fuel, and a large
portion of biomass is simply discarded as waste. Biomass conversion processes can
convert biomass into a variety of value-added useful chemicals, and thus upgrade the use

of biomass.

 

In the
hydrogenoly
sugar hydro ;
glycol; the g
HF sacchar.
propylene gl
While sugar i

In spite
10 sugar h)
hi'drogenoly
figDOCeIIUTOS
as Sugar Crop
is “OI neceg.

which also a
which Consis
hydrogenglys
condUCted is

proYTde d in U‘

2

In the current research, three biomass conversion processes, namely sugar
hydrogenolysis, glycerol dehydroxylation and HF saccharification, are investigated. The
sugar hydrogenolysis process converts sugars into glycerol, propylene glycol and ethylene
glycol; the glycerol dehydroxylation process converts glycerol to 1,3-propanediol; and the
HF saccharification process converts lignocellulosic biomass into sugars. Glycerol,
propylene glycol, ethylene glycol and 1,3-propanediol are value-added useful chemicals;
while sugar is an important intermediate in the conversion of biomass to useful chemicals.

In spite of the fact that the HF saccharification process naturally provides feedstock
to sugar hydrogenolysis, it needs to be noted here that the feedstock of sugar
hydrogenolysis is not necessarily from HF saccharification. It may be from other
lignocellulose saccharification processes. It may also be from other sugar sources, such
as sugar crops and starchy materials. Similarly, the feedstock of glycerol dehydroxylation
is not necessarily from sugar hydrogenolysis. It may come from sugar fermentation,
which also converts sugars to glycerol, or it may come from processing of fatty materials,
which consists of a different type of biomass. In this research, studies on the sugar
hydrogenolysis, glycerol dehydroxylation and HF saccharification processes have been
conducted as independent sub-projects. A brief discussion on each of these projects is

provided in the following.

W The purpose of sugar hydrogenolysis is to convert sugars to

glycerol, propylene glycol and ethylene glycol. These chemicals have extensive uses.
Glycerol is used in alkyd resin, ester gums, polyethers, pharmaceuticals, perfumeries, food
additives and humectants. Ethylene glycol is used in antifreeze, hydraulic fluids, paints,

deicers and alkyd and polyester resins. Propylene glycol has similar applications to

 

 

4 3.. .._——_4

ethylene gly
cosmetic sot
glycerol. pro
At the
as an intern
chlorohydrin
produce glyc
epichlorohyd;
gli'cidol as int
as a b)’-ptodu<
Pmpanedjol a4!
Chemicals and
able [0 prOdUCl
Pmpylene g1.“
enl'imnmentauy
methOds.
A5 menu-0n

fennentauOn, in .

3

ethylene glycol, and is additionally used as tobacco humectants, food additives and
cosmetic softening agents, owing to its non-toxic nature. Because of their extensive uses,
glycerol, propylene glycol and ethylene glycol are currently produced in large volumes.

At the current time, ethylene glycol is produced from ethylene with ethylene oxide
as an intermediate. Propylene glycol is produced from propylene with propylene
chlorohydrin and propylene oxide as intermediates. Several commercial routes exist to
produce glycerol from propylene. The major one has ally chloride, dichlorhydrin and
epichlorohydrin as intermediates. The more recent one has acrolein, glycidaldehyde and
glycidol as intermediates. A small portion of glycerol is also produced from fatty material
as a by-product of soap production. Glycerol, propylene glycol, ethylene glycol and 1,3-
propanediol are all petroleum-based chemicals. In view of the importance of these
chemicals and the inevitability of petroleum depletion in the future, it is significant to be
able to produce them from renewable biomass resources. Production of glycerol and
propylene glycol by sugar hydrogenolysis also has potential to eliminate the
environmentally unfriendly chlorohydrin intermediates from the current production
methods.

As mentioned previously, the conversion of sugars to glycerol can also be done by
fermentation, in lieu of hydrogenolysis. Nevertheless, the hydrogenolysis process has
some unique advantages. Theoretically, the hydrogenolysis reaction preserves all the
carbon atoms in the original sugar molecules, and thus can potentially provide a high
glycerol yield. The theoretical glycerol yield of fermentation, in contrast, is limited to
75% in aerobic fermentation and to 58% in anaerobic fermentation (Wise, 1983). This

limitation results from the carbon loss as carbon dioxide in fermentation. The second

advantage
example.
Consideri
lignocellu
pentose. I
The disad‘
not yet 5:
glycol. etl
ensisionec
process do
The .
rEaction n
tnechamSn
Plausible ,
Cleavage i]
1.3116ng

Perfoxme d

4

advantage of the hydrogenolysis process is that it can also use pentose, xylose for
example, as feedstock. In contrast, the feedstock of fermentation is limited to hexose.
Considering that hemicellulose, which is one of the three major components of
lignocellulose and accounts for 20 to 30% of its total weight, is composed mainly of
pentose, this feature of sugar hydrogenolysis is of great importance to biomass utilization.
The disadvantage of the hydrogenolysis process is that, at the current stage, this process is
not yet selective. The current hydrogenolysis processes produce glycerol, propylene
glycol, ethylene glycol, as well as other polyhydric alcohols, in a mixture. However, it is
envisioned that the selectivity of sugar hydrogenolysis can be improved through proper
process development.

The current research on sugar hydrogenolysis has focused on understanding of the
reaction mechanism and of the factors controlling the reaction selectivity. Reaction
mechanism and selectivity control are two major issues of sugar hydrogenolysis. Several
plausible mechanisms have been proposed by previous researchers to explain the C-C
cleavage in sugar hydrogenolysis. In the current work, these mechanisms are critically
reviewed on the theoretical grounds and against the hydrogenolysis experiments
performed in this research using 1,3-diol model compounds. This leads us to identify the
retro-aldol reaction of a B—hydroxyl carbonyl precursor as the CC cleavage mechanism of
sugar hydrogenolysis. Other mechanisms are excluded based on theoretical
considerations and experimental results. The hydrogenolysis experiments performed in
this work have also provided for the first time direct evidence that the C-0 cleavage is
through dehydration of the same B-hydroxyl carbonyl precursor just mentioned.

Establishment of the above reaction mechanisms makes it possible for one to adopt a

5

rational approach in control of the selectivity of the sugar hydrogenolysis process.

The selectivity study in the current work has focused on control of the C-0 cleavage
in sugar hydrogenolysis. It is envisioned that, if the C-0 cleavage could be somehow
controlled, the reaction products of sugar hydrogenolysis would be significantly
simplified, and the reaction would be directed to produce the highest-valued product,
glycerol. The selectivity study has also been carried out using a 1,3-diol model compound
(2,4-pentanediol). The study on the competition between CC and C-0 cleavage in sugar
hydrogenolysis is greatly facilitated by use of this model compound. Thus, an
experimental study has been carried out in this work to investigate the effect of
temperature, base concentration, hydrogen pressure, catalyst amount and catalyst type on
the C-C v.s C-O selectivity, and a simulation study has been conducted to rationalize the
experimental results and to look into ways to improve the selectivity. It is found that the
temperature has little effect on the selectivity; increasing base concentration can increase
the selectivity to certain extent; increase in the amount of catalysts used has an adverse
effect on the selectivity; and the selectivity varies widely with the type of catalysts used.
The effect of hydrogen pressure on the selectivity is the most interesting. At low pressure,
the selectivity increases with the pressure; at medium pressure, the selectivity is not
sensitive to the change in the pressure; while at high pressure, the selectivity decreases as
the pressure is increased. Of the seven different types of catalysts that has been studied in
our experiments, the best one is barium-promoted copper chromite, which has furnished a
C-C vs. C-O selectivity of 2.5 in hydrogenolysis of 2,4—PD. Our simulation study also
identifies a catalyst with strong capability to hydrogenate ketones and aldehydes but only

moderate ability to hydrogenate (LB-unsaturated ketones as the one with high potential to

6

deliver improved selectivity. These results, both experimental and theoretical, point out

the possible directions that further selectivity study may follow.

W The purpose of glycerol dehydroxylation is to convert
glycerol to 1,3-propanediol. 1,3-propanediol is a very high-valued specialty chemical,
with a current marketing price of $12 per pound. Because of its high price, the use of 1,3-
propanediol is limited to specialty polyester fibers, films and coatings. 1,3-propanediol
does not impart the stiffness of ethylene glycol, yet avoids the floppiness of 1,4-butanediol
and 1,6-hexanediol. Polyesters based on 1,3-propanediol have unique features that are
difficult to obtain from other glycols. For instance, polytrimethylene terephthalate (P'IT),
a new polyester based on 1,3-propanediol has the elastic recovery of nylon and the
chemical resistance of polyester (Stinson, 1995). It is positioned by its manufacturers as a
rival to tradition nylon in carpet area (Chapman, 1997).

At the current time, the production scale of 1,3-propanediol is small, but is quickly
expanding. Dedussa, a German-based company, has just installed 2.2 million lb per year
capacity at its plant in Antwerp, Belgium (Stinson, 1995). Shell has also plan to install a
160-million-pound unit at Geiser, Louisiana by early 1999 (Chapman, 1997). The need
for 1,3-propanediol is great in production of specialty polymers. Dedussa, Shell and
Dupont all have plan to startup or expand their PTI‘ plants (Chapman, 1996). If a low cost
process can be found, it is expected that 1,3-propanediol could quickly become a
commodity chemical.

Cunently, 1,3~propanediol is made by hydration of acrolein to B-
hydroxypropionaldehyde, which yields 1,3-propanediol upon hydrogenation. Acrolein is

a very dangerous chemicals. It must be synthesized on site. The yield from the acrolein

7

process is very low (Gunzel et al., 1991). The problem is with the first step of the process.
Acrolein has a large tendency to polymerize through self-condensation, and hydration
must compete with this reaction. 1,3-propanediol may also be made via carbonylation of
ethylene oxide to B—hydroxypropionaldehyde. The yield of 1,3—propanediol in this process
is also poor, however. Lack of conversion efficiency in the current commercial processes
is the main reason for the current high price and small production scale of 1,3-
propanedioL

Effort to convert glycerol to 1,3-propanediol has been reported. These processes will
be reviewed later in Part 2 of this dissertation. Briefly, these processes are still under
investigation and are not yet commercializable.

In the current research, a new concept to convert glycerol to 1,3-propancdiol is
proposed. This new concept selectively transforms the second hydroxyl group of glycerol
into a tosyloxy group with the help of a group protection technique, and then removes it by
hydrogenolysis. The technical feasibility of this new concept is verified experimentally in
this work. Before this work, hydrogenolysis of tosylates are generally effected with base

metal hydrides, such as LiAlH4 and LiHBEt3. In this work, for the first time, tosylates

with a hydroxy group adjacent to the tosyloxy group are hydrogenolyzed with molecular

hydrogen in the presence of a nickel or ruthenium catalyst.

W As previously mentioned, in addition to the HF process, other
saccharification concepts, including dilute mineral acid, concentrated mineral acid and
enzymatic processes, are available to convert lignocellulosic biomass into sugars. The
research on HF saccharification in the current project is motivated by the potential

advantages it has over other processes. As summarized by Hawley et al. (1986) and

 

Rotter
to 95"
recon

theli

CXpe‘

are.

80m

Hall
bell

the

in

ma:

8

Rorrer (1989), the HF saccharification processes is characterized by high sugar yield (85
to 95% of the theoretical yield), easy catalyst recovery (up to 99.6% of the HF may be
recovered), and little pretreatment requirement (only drying and chipping). In addition,
the HF process operates under ambient reaction time, needs short reaction time and can
produce a lignin residue suitable for further conversion. The HF vapor used in the process
is recycled internally. The HF residue left with the reacted substrates is dissolved in water
and neutralized in post-hydrolysis. Both measures minimize the HF emission from the
HF saccharification process.

The current work on vapor-phase HF saccharification is a continuation of the
research initiated by Rorrer et al (1988), Moring (1989), Rorrer (1989) and Reath (1989)
at Michigan State University. It has focused on modeling of the HF adsorption process in
a packed-bed reactor. However, experiments have also been performed with a bench-scale
reactor to study the behavior of HF adsorption in the packed-bed reactor. Similar
experiments has been done previously by Reath (1989), but the experiments in this work
are conducted in a broader range of operating conditions, which leads to the finding of
some behavior of HF adsorption that has not been observed by Reath.

TWO models of HF adsorption have been developed. One is phenomenological
nature, and is developed based on experimental observations of the HF adsorption
behavior in a bench-scale, packed-bed reactor. The second model is more advanced than
the first one. It is developed based on a detailed analysis of the HF flow, the intrinsic HF
adsorption and the heat transfer processes occurring in the reactor during HF adsorption.
This model provides a great deal of insight into the HF adsorption process in a packed-bed

reactor, and it can be used to predict the HF loading distribution, as well as the overall HF

 

 

 

 

loading. as
modeling Wt

adsorption p:

Results
Pan l preset
dehydroxylal

Pan lc
Chapter 3 de
01 the mechar

Pan llc
concept as v
documents th
”inpatient
adiscllssionc

Based 0:
Published in
me“ Papers
To avoid m
Therefore, or
briefly descn'
sacchaliftcatjc

inchapmr 10$

9

loading, as functions of the adsorption time and operating conditions. The above
modeling work provides a theoretical foundation to design and operation of the HF

adsorption process in a packed-bed reactor.

Results from the current research are presented in this dissertation in three parts.
Part I presents the work on sugar hydrogenolysis; Part 11 presents the work on glycerol
dehydroxylation; Part III presents the work on HF saccharification.

Part I contains four chapters: Chapter 2 is an introduction to sugar hydrogenolysis;
Chapter 3 describes the experimental system and methods; Chapter 4 presents the results
of the mechanism study; and Chapter 5 presents the results of the selectivity study.

Part 11 contains three chapters: Chapter 6 presents the new glycerol dehydroxylation
concept, as well as a review of the current glycerol conversion processes; Chapter 7
documents the experimental procedures used to carry out the conversion of glycerol to
1,3-propanediol; and Chapter 8 presents the experimental results. Chapter 8 also includes
a discussion on the future work that may be done on glycerol dehydroxylation.

Based on the research on HF saccharification, two papers have been written and been
published in Chemical Engineering Science (Wang et al., 1994; Wang et al., 1995a).
These papers document basically all the work that has been done on HF saccharification.
To avoid rewriting, only abstracts of these papers are included in this dissertation.
Therefore, only one chapter, Chapter 9, is allocated to Part III in this dissertation, to
briefly describe the rationale, scope and accomplishments of the research on HF
saccharification. Following Part III, a general summary of the current research is provided

in Chapter 10 and recommendations on future work are made in Chapter 11.

2.1

CHAPTER 2
INTRODUCTION: SUGAR HYDROGEN OLYSIS

2.1 Literature Review

Under high temperature and high hydrogen atmosphere, sugars can be catalytically
hydrocracked into lower polyhydric alcohols in the presence of transition metal catalysts.
This process is called hydrogenolysis, because the bond cleavage reactions are

accompanied with hydrogen addition as shown below:

R3C —CR’3 + H2 -—.-> R3CH + HCR’3

R3C —OH + H2 ——.> R3CH '1' H20

Sugars are available from a variety of biomass raw materials. Hydrogenolysis provides a
potential process to convert sugars to synthetically important polyols, including glycerol,
propylene glycol and ethylene glycol.

In the literature, sugar hydrogenolysis is discussed indistinguishably from sugar
alcohol hydrogenolysis, because of the close relationship between these two reactions.
Sugar hydrogenolysis and sugar alcohol hydrogenolysis take place under the same
conditions, yield the same products and have identical bond cleavage mechanisms and
bond cleavage precursors (sugars), as will be shown later in Chapter 4. As a matter of
fact, in sugar hydrogenolysis, a significant portion of the starting sugar will be

hydrogenated to sugar alcohol before being finally hydrogenolyzed into lower polyhydric

10

alcohols I
hydrogenolys
understood a
context indie

The sug
(Connor and
sasstarted n
Laboratory (1
been increas
511331 hydrug
mitten in bi(
hl‘drogenoly
mLSPIOCcss,
Studies 011 l:

Sawfly ar

11.1 studie

In the
Purpose of .
upenmenm’

nickel 0n he

ll

alcohols. In some occasion, sugar alcohols have been used to study the sugar
hydrogenolysis process. In the following, references to sugar hydrogenolysis should be
understood as the hydrogenolysis of both sugars and sugar alcohols, except where the
context indicates otherwise.

The sugar hydrogenolysis reactions have been performed since as early as the 1930’s
(Connor and Adkins, 1932). However, the research for the purpose of biomass conversion
was started much later. The first one was reported in 1950’s, at the US. Forestry Products
Laboratory (Clark, 1958). Since then, the body of literature on sugar hydrogenolysis has
been increasing steadily. Like many other biomass conversion projects, the research on
sugar hydrogenolysis underwent a “boom” period in the 1980’s, as a result 'of the general
interest in biomass conversion stimulated by the oil crisis in 1970’s. The research on sugar
hydrogenolysis has been conducted in many countries, and has covered various aspects of
this process. In the following, the available literature is reviewed according to four topics:
studies on process development, studies on reaction mechanism, studies on reaction

selectivity and studies on reaction kinetics.

2.1.1 Studies on Process Development

In the 1950’s, Clark (1958) started the first research on sugar hydrogenolysis for
purpose of biomass conversion at the US. Forestry Products Laboratory. In his
experiments, sorbitol was reacted under the hydrogenolysis conditions in the presence of a
nickel on kieselguhr catalyst. Calcium hydroxide was added to promote the reaction. The
reaction was carried out in aqueous phase in a batch reactor. The temperature was at 215

to 240 'C, and the hydrogen pressures was at 2000 to 5600 psi. The identified products

 

and

(Boel

ofcat
and.B

Pedon

12

included glycerol, propylene glycol, ethylene glycol, erythritol and xylitol. The product
composition varied with the reaction time. The concentration of xylitol. erythritol,
glycerol increased during the initial period of an experimental run, but after reaching a
maximum, they started declining. The maximum yield of glycerol obtained in Clark’s
experiments was 40%.

At about the same time as Clark’s work, according to Boelhouwer er al., Geerards
and co-workers investigated the hydrogenolysis of sucrose with HI as a catalyst
(Boelhouwer er al., 1960). The products they obtained had very low oxygen contents.

In their own work, Boelhouwer er al. (1960) hydrogenolyzed sucrose with a variety
of catalysts, including nickel on kieselguhr, nickel-thorium oxide on kieselguhr, and MgO
and BeO promoted copper chromite. Calcium oxide was also added. The reaction was
performed in a rotating autoclave with methanol as the reaction medium. The temperature
range was between 195 and 250 °C, and the hydrogen pressure range was between 150
and 200 atrn. The products were fractionated by distillation. In one experiment, the
glycerol fraction was reported to account for 61% of the product. Since each fraction
covered a broad boiling range, exact products were not determined. However, it was
believed that glycerol, propylene glycol and ethylene glycol were included in the products.

Van Ling and co-workers (van Ling et al., 1967; van Ling and Vlugter, 1969)
hydrogenolyzed sucrose, glucose, fructose and inan with CuO-CeOz-SiOZ. The reaction
was done in a 250 mL autoclave, at a temperature of 200-250 °C, a hydrogen pressure of
100-300 atm, with addition of caladium hydroxide. In the hydrogenolysis of sucrose,
ethylene glycol, propylene glycol, glycerol, teritols, pentinols, hexitols and dehydrated

hexitols were obtained. In hydrogenolysis of other sugars, the products were similar.

 

Van Lll
process [0 pi
firStWaS a I“!
m The teat“
Because 015
yield tbelow I
[an]: reactors
operating V'Oll.
reanon syster
(CuOCeOI-Si

A great
hydrogenolyzet
hydrogenolyzet
platinum on car
carbonaceousp
using a sulfur-:
64.6% in one e:

mtheniurn and r

13

Van Ling er al. (1970) further developed a continuous pilot-scale hydrogenolysis
process to produce glycerol from sucrose. Two reactor configurations were studied. The
first was a tubular reactor, composed of a 2 m long stainless steel tube with a diameter of 2
cm. The reactor was filled with 1/8—inch Raschig rings to promote the gas-liquid contact.
Because of sucrose decomposition, this reaction system furnished only a poor glycerol
yield (below 20%). The second reaction system was composed of two continuous stirred-
tank reactors (CSTR) in series. Each reactor had a total volume Of 500 ml and an
operating volume of 250 ml. Sucrose was fed in 75% methanol-water solution. With this
reaction system, a glycerol yield up to 30% was obtained. The catalyst used in this study
(CuO-CeOz-SiOz) was the same as in their previous research.

A great deal of work was accomplished in the 1980’s. Chao et al. (1982)
hydrogenolyzed sorbitol with a supported nickel catalyst in aqueous phase. Arena (1983)
hydrogenolyzed sorbitol, glucose and sucrose in the presence of nickel on kieselguhr,
platinum on carbon, or metals (including palladium, rhenium and copper) deposited on a
carbonaceous pyropolymer structure. Duback and Knapp (1984) hydrogenolyzed sorbitol
using a sulfur-modified ruthenium catalyst, and achieved a propylene glycol yield of
64.6% in one experiment Ariono er al. (1986) hydrogenolyzed glucose with palladium,
ruthenium and rhodium supported on carbon. Tanikella (1983a; 1983b) hydrogenolyzed
sorbitol in both aqueous and non-aqueous solutions to produce ethylene glycol. In the
work by the Motassier group (Sohounloue et al., 1983; Montassier et al., 1988, 1989 and
1991), various sugar alcohols, including sorbitol, xylitol, erythritol and even glycerol,

were subject to the hydrogenolysis conditions. The catalysts used included RulSiOz,

Ranney Cu and other metal based catalysts.

In all t
hgtgrpgelldtlu
(Andrews an.
and We}
pyrrolidinone
arm The It
heterogenous
lithlnrctose
glycerol relat‘
hydrogenation
to the previous
favored than in

Studies or
al.1975;Piltlt
llWage, lon
pmdthlS. The

“lililibn'um.

 

14

In all the work reviewed above, the hydrogenolysis has been carried out with
heterogeneous catalysts, but a process was also reported using homogenous catalysts
(Andrews and Klaeren, 1989). Andrews and Klaeren hydrogenolyzed fructose, glucose

and manna-heptulose with H2Ru(PPh3). The reaction was done in N-methyl-Z-

pyrrolidinone solvent, at a temperatures of 50 to 100 “C and a hydrogen pressures of 20
arm. The reaction conditions were significantly less severe than those used with
heterogenous catalysts. In most of the experiments, KOH was added as a co-catalyst.
With fructose as the starting compound, this process provided very good selectivity toward
glycerol relative to other degradation products. However, because of the rapid simple
hydrogenation of fructose (to sorbitol and mannitol), the overall glycerol yield was similar
to the previous processes. With glucose as the starting compound, glycerol was much less
favored than in the case of fructose.

Studies on separation of the hydrogenolysis products were also reported (Sokolvo er
al., 1975; Pikkov et al., 1973; Solkovo et al., 1977). Sokolvo et al., (1975) developed a
two-stage, low pressure distillation process to separated glycerol from other reaction
products. They also determined the activity coefficients for the glycerol-water vapor

equilibrium.

2.1.2 Studios on Reaction Mechanism

In sugar hydrogenolysis, both CC and 00 are subject to cleavage. These reactions
are promoted by transition metals and base co—catalysts. Cleavage of C-0 bonds, as

Montassier er al. (1988 and 1989) proposed, is through dehydration of a B—hydroxyl

carbonyl:

on or
RCHCHCH
On

The structure

molecule. and

The direct pro

carbonyl. whiej
just discussed.
hydrogenation s
The origin
“Plain the C{

reaction:

9H on
RCHCHCHR';
!
0H

Wrens and K15
that the Primary
Accord“ g to this
Omit of this
Timmy hl'drc

Lam bOWeye

15

carbonyl:
OH OH O OH O OH
I I -H2 II 1 +120 ll +H2 |
RCHCIZHCHR’ ———> RCCIZHCHR’ ——-> RCCII= CHR’ —-> RCHCHCHZR’
OH OH OH OH

The structure of this B-hydroxyl carbonyl is already contained in an open-chain sugar
molecule, and it may be generated from a sugar alcohol molecule by dehydrogenation.
The direct product from dehydration of the B-hydroxyl carbonyl is an Ot,B-unsaturated
carbonyl, which yields a polyhydric alcohol upon hydrogenation. In the reaction scheme
just discussed, the dehydration step is catalyzed by bases, and the dehydrogenation and
hydrogenation steps are catalyzed by transition metal catalysts.

The original mechanism proposed by the Montassier group (Sohounloue, 1983) to

explain the CC cleavage in sugar and sugar alcohol hydrogenolysis is the retro-aldol

reaction:
(I’ll 9“ I”) (RH O o H OH
, 'H II II 1’ 2 l
RCHCIZHCHR _.l Rcclzncrm' _. Rccnzom HCR’ -— RCHCH20H +HOCH2R’
OH OH Retro-aldol

Andrews and Klaeren (1989) suggested the same mechanism, based on their observation
that the primary C-C cleavage site is B to the carbonyl group in sugar hydrogenolysis.
According to this mechanism, the QC cleavage precursor is again a B-hydroxyl carbonyl.
Cleavage of this B—hydroxyl carbonyl leads to an aldehyde and a ketone, which are
subsequently hydrogenated to alcohols.

Later, however, Montassier er al. (1988) felt that it was difficult to explain the

absence of“

and other 51
anc‘her mecl

hydrogenoll'f

on or
cugcncn

1

OH

This mechan:
hydrogenolysi:
allows format
formaldehyde ;
xylitol and sort

the mechanism:

0H

RCHCHCHCT
' l

onOnor
Thh mechanism
The reactio
hsuga; hldngc.‘

NIPPON am" of [hf

16

absence of methanol and the presence of C02 in the hydrogenolysis products of glycerol

and other sugar alcohols with the retro-aldol mechanism. Therefore, they proposed

another mechanism, namely the retro-Claisen reaction, for the C-C cleavage in glycerol

hydrogenolysis:
OH OH o O ”
l -2H2 ll III-151V.“ Retro—Claisen HCOH
CHZC'IHCHZ —> HCCIEHCH —> HO-C\r\ +
J H/ CH
OH H20 OH OHCH=CHOH

OH

This mechanism allows formation of formic acid which decomposes under the

hydrogenolysis conditions to form C02. The mechanism based on the retro-aldol reaction

allows formation of formaldehyde rather than formic acid. Upon hydrogenation,
formaldehyde yields methanol. TO explain the CC cleavage in the hydrogenolysis of
xylitol and sorbitol, Montassier et al. (1988) also proposed the retro-Michael reaction as

the mechanism:

?“ 1’” 2n it it it it
' 2
RCHCIZHCIIHCHCHR’ —-- RCC|HC|HCIHCR’ ———- RC(|I = (IZH + OHCHZCR’
OHOH OH OHOH OH Retro-Michael OH OH

This mechanism requires a S-dicarbonyl as the bond cleavage precursor.
The reaction mechanisms just reviewed are all consistent with the products obtained
in sugar hydrogenolysis. Other than that, no direct experimental evidence is available to

support any of the bond cleavage mechanisms proposed.

2.1.3 Studi

Studies
Sohounloue
noose, glu
selectivity g
representing
Ell'COU in the
the percentag
Percentage. It
Pmducts inch;
that the CS ‘
Variables. such
dipendgm Ont
incheased. It yy
matimplhmo

Sbhounlo
sorbitoh. He

inchted. He;

17
2.1.3 Studios on Reaction Selectivity

Studies on the reaction selectivity have been reported by van Ling er al. (1969) and
Sohounloue er al. (1983). Ling er al. studied the effect—hf the starting sugars, including
sucrose, glucose, fructose and inulin, on the reaction selectivity. He defined three
selectivity parameters for sugar hydrogenolysis: 1) CS, or cleavage selectivity,
representing the percentage of the C3-C3 cleavage products (glycerol and propylene
glycol) in the total cleavage products; 2) HS, or hydrogenation selectivity, representing
the percentage of glycerol in the total C3-C3 cleavage products; and 3) CR, or cleavage
percentage, representing the percentage of the bond cleavage products in all reaction
products including hexitols from the starting sugar via simple hydrogenation. He found
that the CS varies with the starting sugar, but was independent of the other reaction
variables, such as catalyst type and base concentration. The H.S, as he reported, is also
dependent on the starting sugar, and in addition it decreases as the reaction temperature is
increased. It was also found that the CF increases with decreasing hydrogen pressure, and
that it is promoted by small addition of Ca(OH)2.

Sohounloue et al. (1983) studied the reaction selectivity in the hydrogenolysis of
sorbitols. He observed that the CC cleavage became random as the temperature
increased. He also Observed that the CC v.s. C-O selectivity, as defined by the ratio of the
glycerol to propylene glycol, decreased with temperature in a basic reaction medium but
increased in a neutral medium. Both of the studies by Ling er al. and by Sohounloue et al.
were experimental in nature, and the mechanisms controlling the reaction selectivity were

not well reflected.

1.1.4 Studit

The tir:
(1958?). who
hydrogen pre
in a pressure
activation ene
determined to

Chang 6.
5113th differ:
Rho nickel a
in the concentr
mil11150.8 orde.

am011111 01 cam
exl‘el‘lrrlentg by
m sufficienu)

Chang ct al. (_

Wilding 01 5U.

h,
Merton-5'5 ‘

TWWMU

18
2.1.4 Studies on Reaction Kinetics

The first empirical kinetic study on sugar hydrogenolysis was reported by Clark
(1958), who found that the hydrogenolysis of sorbitol is first-order in the substrate. The
hydrogen pressure does have effect on the reaction rate, but this effect has been considered
in a pressures dependent activation energy. For a hydrogen pressure of 5600 psi, the
activation energy was determined to be 44 kcal; for a hydrogen pressure of 2900 psi, it was
determined to be 33 kcal.

Chang et al. (1985) studied the kinetics of sorbitol hydrogenolysis also, but used a
slightly different catalyst. Instead of nickel on kieselguhr as in Clark’ work, they used
Raney nickel as the catalyst. They found that the hydrogenolysis of sorbitol is first order
in the concentration of sorbitol, as Clark did. In addition, they found that this reaction is
minus 0.8 order with respect to the hydrogen pressure and second order with respect to the
amount of catalyst. The activation energy was determined to be about 22 kcal/mol. The
experiments by Chang et al. were conducted in an one-liter autoclave. The agitation rate
was sufficiently high to eliminate the effect of external mass transfer. In the work of
Chang et al. (1985), the kinetics of glycerol hydrogenolysis was also studied. The
reaction was also found to be first-order in the substrate.

The Work by Tronconi et al. (1992) was the only one reportedly on mathematical
modeling of sugar hydrogenolysis. This work provided a kinetic model for sorbitol
hydrogenolysis which was different from the previous work. The authors claimed that this
model had been developed based on their analysis of the whole reaction network of
hydrogenolysis (which is not shown in the paper). The developed model was

subsequently adopted in a simple pseudo-homogeneous non-isothermal plug flow reactor

model. Corr
and pilot-scz
kinetic erpre

A kinet

(1973). Hon

2.2 Object

A major
control. As di
generally 110n-
ih‘tol are proc
levels. It is cl:
h«‘d”«‘t‘=’irlolysis

Another r
KI”white oil.
imMince for
Review, three
Michaey have b
fishl'drition mgc
hhhr’cauy n0“
heCCcymgt3

vilified

II] View of [m

19

model. Comparison of this reactor model against their experimental data from their micro
and pilot-scale reactors allowed them to determine the model parameters present in the
kinetic expression.

A kinetic study on sugar hydrogenolysis was also reportedly done by Stefoglo er al.

(1973). However, the detail of this study is not clear.

2.2 Objective and Scope of Current Research

A major issue in development of the sugar hydrogenolysis process is selectivity
control. As discussed in the Literature Review, current sugar hydrogenolysis processes are
generally non-selective. In these processes, glycerol, propylene glycol and ethylene
glycol are produced in a mixture, along with other polyhydric alcohols occurring at lower
levels. It is clear that a more selective process needs to be developed, in order for sugar
hydrogenolysis to be commercially viable.

Another major issue of sugar hydrogenolysis concerns the reaction mechanism.
Knowledge of how the CC and C-0 bonds are broken in sugar hydrogenolysis is of great
importance for control of the reaction selectivity. So far, as discussed in the Literature
Review, three competing mechanisms, namely retro-aldol, retro-Claisen and retro-
Michael, have been proposed to explain the CC cleavage in sugar hydrogenolysis. The
dehydration mechanism of C-0 cleavage is not disputed, but direct experimental evidence
is basically non-existent. Before a useful strategy for selectivity control can be developed,
the C-C cleavage mechanism must be clarified and the C-0 cleavage mechanism must be
verified.

In view of the above situation, the objectives of the current research are determined

20

as: 1) to carry out a mechanism study to discriminate and verify the existing bond cleavage
mechanisms of sugar hydrogenolysis; and 2) to carry out a selectivity study to investigate
the factors controlling the selectivity of sugar hydrogenolysis. In this capacity, a series of
1,3-diol model compounds are hydrogenolyzed, to establish the mechanism of CC and C-
0 bond cleavage in sugar hydrogenolysis. The experimental work confirmes our
theoretical conception that the QC bond breaks via the retro-aldol reaction of a [3-
hydroxyl carbonyl precusor, and it also provides for the first time direct evidence that the
C-0 cleavage is through dehydration of the same B-hydroxyl carbonyl precursor just
mentioned.

In the selectivity study, the research focuses on control of the C-0 cleavage in sugar
hydrogenolysis. It is envisioned that the reaction products of sugar hydrogenolysis would
be considerably simplified, and the reaction would be directed to produce the highest-
valued glycerol, if the C-0 cleavage can be controlled somehow. Thus in this work, the
competition between the C-C and C-0 cleavage is studied again using a 1,3-diol model
compound (2,4-pentanediol). Experiments, as well as computer simulation, are conducted
to investigate the effects of temperature, base concentration, hydrogen pressure, catalyst
amount and catalyst type on the selectivity, as well as possible ways to improve this

selectivity to C-C bond cleavage.

CHAPTER 3
EXPERIMENTAL SYSTEM AND METHODS

3.1 Experimental System and Procedure

In this work, 1,3-diols with various structural features were hydrogenolyzed as
model compounds to study the reaction mechanism and selectivity of sugar
hydrogenolysis. These hydrogenolysis experiments were performed in a specially
designed, stainless steel reactor of 50 mL capacity. The detailed design of this reactor is
given in Figure 3-1.

This hydrogenolysis reactor is basically a pressure vessel (note that hydrogenolysis
is effected at high temperature and high hydrogen pressure), and is equipped with a
magnetic stirring bar. Because of the need to regularly take samples during the reaction, a
sampling port is provided with the reactor. This sampling system is composed of a
sampling valve, a sampling tubing and an inlet filter emerged in the reaction medium.
This filter has a 0.45 u rating. It can prevent solid catalyst particles from entering the
sampling system and generate clean samples suitable for direct HPLC and GC injection.
This filter also reduces the pressure exerted on the sampling valve, which levitates the
requirement on the sampling valve. The sampling tubing is very thin, with an inner
diameter of about 0.1 mm. The dead volumes of the sampling valve and the inlet filter are
0.2 and 0.4 uL, respectively. The whole sampling system has a total hold volume of only

several microliters. This guarantees that the samples taken from this system always

21

 

 

 

 

 

 

///%

/ /
/ w /<
Z \[]\

 

 

 

Figure 3-1. Illustration of Hydrogenolysis Reactor

23

 

 

 

 

 

 

 

 

 

 

 

 

To vacuum
H2
—’
Sampling valve
Temperature controller
_J¢ .— ..... -
I O O
Stirring gas (N2) ! 5' ' ' '—
—-* i I
3 I
l
; I
I I Heating coil
Oil bath\ __ . _|__ /
x - O — . '-
@ § @//Thermal couple
0 a
@ o 32 @
o ' @
. @ ° . Magnetic stirrer
/

 

 

 

 

 

 

 

 

 

 

Figure 3-2. Schematic Illustration of Experimental System

24

represent the current condition in the reactor.

The reactor is heated in a silicone oil bath. The heat is provided by an electric
heating coil emerged in the oil bath, which is constantly stirred by bubbling a nitrogen
flow into it. The temperature of the oil bath is controlled by a temperature controller,
which controls the oil temperature within :l: 3 °C of the set point. The reaction medium is
stirred by the magnetic stirring bar inside the reactor. To provide the magnetic force to the
stirring bar, the whole oil bath is placed on a magnetic stirrer.

The hydrogen pressure is provided to the reactor by a hydrogen cylinder with high
pressure regulator for pressure control. To prevent over-pressurization, a pressure relief

valve is installed in the H2 supply line as a safety device. A vacuum line is also connected

to the reactor for the purpose of initial reactor purging. A schematic illustration of the
whole experimental system is provided as Figure 3-2.

To carry out the hydrogenolysis reaction, the starting diol, the catalyst, a certain
amount of sodium hydroxide (if needed) and a proper amount of distilled water are placed
in the reactor. The reactor is purged by alternately connecting it to hydrogen and vacuum.
After purging, the reactor is pressurized with hydrogen, and heating and stirring (both
inside and outside of the reactor) are started. Thereupon, the reaction starts. During the
reaction course, reaction samples are taken regularly from the reactor and are analyzed for
composition of reaction products. After the the desired conversion has occurred the

reaction is stopped by shutting off the heater and hydrogen supply.

25

3.2 Reaction Regents and Product Analysis

All the 1,3-diols and catalysts used in this work are purchased from Aldrich and are
used as received. The hydrogen (99.9% pure) is obtained from Purity Cylinder Gases and
AGA Gas Products.

The hydrogenolysis products from the mechanism study are analyzed by GC and
HPLC. The GC column used in this work is a non-polar DB624 column, supplied by
J&W Scientific Company. With this column, samples can be injected in the aqueous
phase. The HPLC column used in this work is a 25 x 0.46 cm C18 column obtained from
Whatrnan (Cat.# 4621-1502). The detectors used with GC and HPLC are a FID detector
and a differential refractometer, respectively. In both cases, samples taken from the
reactor are injected directly.

In the mechanism study, seven different 1,3-diols are hydrogenolyzed, giving seven
different sets of products. GC and HPLC methods are developed for each set of these
products, and are documented here in Table 3-1 and Table 3-2, respectively. However, the
hydrogenolysis products from the selectivity study are analyzed by GC only. Since the
selectivity study demands more accurate measurement of the product concentrations,
calibration curves are prepared for all the products from 2,4-pentanediol hydrogenolysis
(2,4.pentanediol is the model compound used in selectivity study).

In the HPLC chromatograms of actual reaction products, a skewed peak has often
been seen at a retention time well earlier than those for the expected reaction products.
This peak is identified as the impurities introduced into the reaction medium with Raney
Cu and Ni catalysts. An experiment without any 1,3-diol substrate was performed.

Samples taken from this experiment are found to give the same skewed peak. Since no

26

Table 3-1. HPLC Methods for 1,3-Diol Hydrogenolysis Products .

 

Conditions

Retention Time

 

Product Set #1: 2,4-dimethy1-2,4-pentanediol

 

solvent: 25% acetonitrile/water
flow rate: 1.5 mllmin (isocratic)

temperature: 40 ° C

 

1. 2,4-dimethyl-2.4-pentanediol: 2.92 min

 

Product Set #2: 2-methyl-2,4-pentanediol

 

solvent: 42% methanol/water
flow rate: 0.8 mllmin (isocratic)

temperature: 40 °C

 

1. acetone: 3.98 min

2. isopropanol: 4.25 min

3. 2-methyl-2,4-pcntanediol: 4.91 min
4. 4-methyl-2-pentanol: 11.85 min

 

Product Set #3: 2,2,4-trimethyl-1,3-pentanediol

 

solvent: 25% acetonitrile/water
flow rate: 2.0 mllmin (isocratic)
temperature: 40 °C

 

l. methanol: 1.30 min

2. isobutanol: 2.31 min

3. 2.2,4-trimethyl-1,3-pentanedi01: 3.62 min
4. 2,4-dimethyl-3-pentanol: 9.98 min

 

Product Set #4: 2,2-dimethy1-1,3-propanediol

 

solvent: 10% acetonitrile/water
flow rate: 1.5 mllmin (isocratic)

temperature: 60 ° C

 

1. methanol: 1.80 min
2. 2,2-dimethyl-l,3-propanediol: 2.70 min
3. isobutanol: 5.15 min

 

Product Set #5: 2,4-pentanediol

 

solvent: 42% methanol/water
flow rate: 0.8 mllmin (isocratic)

temperature: 40 ° C

 

1. ethanol: 3.62 min

2. 2,4-pentanediol: 3.84 min
3. isopropanol: 4.22 min
4. 2-pentanone: 6.54 min

5. 2-pentanol: 8.21 min

 

Product Set #6: 1,3-butanediol

 

solvent: 3% acetonitrilclwater
flow rate: 1.2 mllmin (isocratic)

temperature: 60 ° C

 

1. methanol: 2.58 min

2. ethanol: 3.15 min

3. 1.3-butanediol: 3.23 min
4. acetone: 4.50 min

5. isopropanol: 5.00 min
6. 2-butanone: 9.48 min

7. 2-butanol: 10.98 min

8. n-butanol: 13.85min

 

Product Set #7: 1,3-propanediol

 

solvent: 0.3% acetonitrilelwater
flow rate: 1.5 mllmin (isocratic)

temperature: 60 ° C

 

1. methanol: 2.04 min

2. 1.3-propanediol: 2.32 min
3. ethanol: 2.90 min

4. n-propanol: 5.51 min

 

27
Table 3-2. GC Methods for 1,3-Diol Hydrogenolysis Products.

 

Conditions Retention Time

 

Product Set #1: 2,4-dimethyl-2,4-pentanediol

 

initial temperature/time: 150 °C/ 1 min
temperature ramp: 40 ° C/min 1. 2,4-dimethyl-2,4-pentanediol: 2.10 min
final temperature/time 260 °C/3 min

 

 

Product Set #2: 2-methy1—2,4-pentanediol

 

initial temperature/time: 80 °C/l min 1- isopropanol: 1-14 min

2. acetone: 1.20 min

. 3. 4-methyl-2-pentanol: 2.50 min
final temperature/time 260 010 min 4. 2-methyl-2,4-pentanediol: 3.76 min

temperature ramp: 40 o C/min

 

 

Product Set #3: 2,2,4-trimethyl-1,3-pentanediol

 

initial temperature/time: 150 °C/l min 1- 1119111311013 091 min
. o . 2. isobutanol: 1.11 min
temperature ramp. 30 C/rrun 3. 2,4-dimethy1-3-pentanol: 1.57 min
final temperature/time 260 010 min 4. 2,2,4-trimethy1-1,3-pentanediol: 3.35 min

 

 

Product Set #4: 2,2-dimethyl-1,3-propanediol

 

initial temperature/time: 150 °C/l min 1. methanol: 0.92 min

temperature ramp: 30 °C/min 2. isobutanol: 1.11 min
3. 2,2-dimethyl-1,3-propanediol: 2.16 min

 

final temperature/time 260 °C/10 min

 

Product Set #5: 2,4-pentanediol

 

. ethanol: 1.43 min

. isopropanol: 1.69 min

. 2-pentanone: 3.09 min

. 2-pentanol: 3.19 min

. 2.4-pentanediol: 4.68 min

initial temperature/time: 4o °C/1 min
temperature ramp: 40 °C/min
final temperature/time 260 °C/3 min

Ur«bU->NH

 

 

Product Set #6: 1,3-butanediol

 

methanol: --- min
ethanol: 1.49 min
1,3-butanedi01: --- min
acetone: -- min
isopropanol: 1.69 min
2-butanone: --- min
2-butanol: --- min
n-butanol: -- min

initial temperature/time: 40 °C/1 min
temperature ramp: 40 ° C/min
final temperature/time 260 oC/3 min

PSQEAPP'U‘N‘

 

 

Product Set #7: 1,3-propanediol

 

methanol: 0.90 min
ethanol: 0.96 min
n-propanol: 1.09 min
1.3-propanediol: 2.32 min

initial temperature/time: 120 °Cll min
temperature ramp: 30 ° C/min
final temperature/time 260 °C/10 min

99’3"!"

 

 

28

1,3-diol is added to the reactor in this experiment, this skewed peak can only result from

the catalyst added.

4.] The:

As It
Clilsen ;
hldfllgtn
tiller tel
amid
require a
mullet i
“Klimt
carbon)
mom
dehj'dtz
fal'Orab
fisultec
d‘Omina
re5101017

lbe deb“

CHAPTER 4
MECHANISM OF SUGAR HYDROGENOLYSIS

4.1 Theoretical Considerations

As reviewed in the previous chapter, three competing mechanisms (retro-aldol, retro-
Claisen and retro-Michael) have been proposed for the C-C cleavage in sugar
hydrogenolysis. However, two theoretical considerations prevent us from believing that
either retro-Claisen or retro-Michael is a dominating C-C cleavage mechanism over the
retro-aldol in hydrogenolysis. First, both the retro-Claisen and retro-Michael mechanisms
require a dicarbonyl precursor. The formation of a dicarbonyl presumedly occurs through
further dehydrogenation of a mono-carbonyl. As dehydrogenation is thermodynamically
unfavorable, the dicarbonyl formed is unlikely to be significant relative to the mono-
carbonyl, the precursor of the retro-aldol reaction. Second, the dehydrogenation of the
mono-carbonyl is in competition with dehydration, the C-0 cleavage reaction. Since the
dehydration is both thermodynamically and kinetically (in presence of bases) much more
favorable than the dehydrogenation, the hydrogenolysis products would have exclusively
resulted from CD cleavage, had either the retro-Claisen or the retro-Michael been the
dominating mechanism of CC cleavage in hydrogenolysis. In contrast, the retro-aldol
reaction shares the same precusor as the dehydration, and has the ability to compete with

the dehydration. Based on these reasons, we tend to believe that the C-C cleavage occurs

29

though ill

lite.

absence 0
)ltrtissle
not have 1
reaction .
by *ogen
lormlc at
“Miller

catalysts

30

through the retro-aldol reaction.

The need for a formic acid intermediate to explain the formation of C02 and the

absence of methanol in the hydrogenolysis of glycerol was the direct motivation for
Montassier et aL to propose the retro-Claisen mechanism. However, this formic acid does
not have to be from the retro-Claisen reaction. It may be produced from the Cannizzaro
reaction of formaldehyde, which is expected by the retro-aldol mechanism in the
hydrogenolysis of glycerol. In the Cannizzaro reaction, formaldehyde is oxidized to
formic acid while other carbonyl compounds are reduced to alcohols. This reaction is
catalyzed by bases, but has been reported to be greatly enhanced by transition metal co-

catalysts (Cook and Mailis, 1981). The Cannizzaro reaction enhanced by the transition

OH
II II +H2 |
RCCH20H 'I“ HCR’ —-"’ RCHCHon 'I' HOCHzR’
III IV V VI
retro-aldol T
OH OH O OH
I I ' H2 II I ,
RCHCIIHCHR’ —> RCCEHCHR
OH 0H 11
I
dehydration I - H20
0 OH
II “*2 I
RCC|3=CHR’ —'* RCHCEHCHZR'
OH OH
VII VIII

Figure 4-1. Mechanism of Sugar and Sugar Alcohol Hydrogenolysis

 

math
in the hyd
absence nl

lne
dehylntl.
mechanisr
processe i
Inl-lgnre
the bond

alcohol I

4.2 E)

Iound s
Vellfica
dchldr.
that del
”natal;

iICOIlQl
41%;
the P111115
”one Sllg

HD‘VEVer,

31

metal is likely to be the side reaction competing with the hydrogenation of formaldehyde
in the hydrogenolysis of glycerol and other sugar alcohols, which is responsible for the
absence of methanol and the presence of C02 in the hydrogenolysis product.

The retro-aldol mechanism needs a B—hydroxyl carbonyl precursor. Considering that
dehydration is a well-known reaction of B-hydroxyl carbonyls. The dehydration
mechanism of C-0 cleavage is very sound, at least in theory. Thus, the bond-cleaving
processe in the hydrogenolysis of sugars and sugar alcohols can be pictured as Figure 4-1.
In Figure 4—1, sugars are represented by H and sugar alcohols by 1. Based on Figure 4-1,
the bond cleavage mechanisms in sugar hydrogenolysis ought to be the same as in sugar

alcohol hydrogenolysis.

4.2 Experimental Verification

The reaction mechanism described in Figure 4—1 can explain all the reaction products
found so far in sugar hydrogenolysis. Nevertheless, it is still subject to experimental
verification. The currently available data provide no clue that the CO bond is broken by
dehydration of a B—hydroxyl carbonyl in sugar hydrogenolysis. Experimental evidence
that dehydrogenation is a necessary step in the hydrogenolysis of sugar alcohols is also
unavailable, and no sugar intermediate has been identified so far in any of the sugar
alcohol hydrogenolysis experiments (Clark, 1958; Sohounloue et al. 1983; Motassier et
al., 1988 and 1989; Chang et al., 1985). Andrew and Klaeren’s (1989) observation that
the primary C-C cleavage site is B to the carbonyl group in sugar hydrogenolysis may be a
strong suggestion that the C-C cleavage has occurred through the retro-aldol reaction.

However, bond breakage at this site is also expected by the retro-Claisen mechanism.

 

accomnh'
dehydrng
sugar alt
pursued
rnechml
cnmpnu
the Start
I.3~dlnl
underg:
Is a re
In the s

have IC

32

Clearly, more evidence is desired to validate the mechanism described in Figure 4-1.

In the current research, verification of the reaction mechanism in Figure 4-1 is
accomplished using 1,3-diol model compound. According to Figure 4-1, a 1,3-diol or its
dehydrogenation product is the basic structural unit giving the reactivity to sugar and
sugar alcohol molecules. Therefore, a mechanism study of sugar hydrogenolysis can be
pursued with 1,3-diol model compounds. Using 1,3-diol model compounds in the
mechanism study has several advantages over direct use of the sugar or sugar alcohol
compounds. First, the bond cleavage pattern is indicated by the reaction products, since
the starting molecule undergoes only one bond-cleaving reaction in the hydrogenolysis of
1,3-diols. In the hydrogenolysis of sugars and sugar alcohols, the starting molecule can
undergo a chain of bond-cleaving reactions before further hydrogenolysis is impossible.
As a result, the bond cleavage pattern is difficult to recognize based on the final products
in the sugar and sugar alcohol hydrogenolysis. Second, sugar and sugar alcohol molecules
have too many functional groups, which makes it possible to interpret the experimental
results in a variety of ways. In contrast, 1,3-diols possess only the functional groups
necessary for the hydrogenolysis to occur, which minimizes the ways to interpret the
results. The retro-Michael mechanism is excluded from the hydrogenolysis of 1,3-diols,
for instance, because 1,3-diols are incapable of forming the 5—dicarbonyl as required by
the mechanism. Last and the most important, 1,3-diols with special structural features
can be used, to verify the role played by individual reaction steps in the hydrogenolysis.

The 1,3—diols model compounds used in this work include 2,4-dimethyl-2,4—
pentanediol (l), 2-methy1-2,4-pentanediol (2), 2,2,4-trimethyl-l,3-pentanediol (3), 2,2-

dimethyl-l,3-propanediol (4), 2,4»pentanediol (5), 1,3-butanediol (6) and 1,3-propanediol

 

III- lhe st

 

2.411131”:

Z-ne'lzyl-I

‘l‘lr

Jet-mm
ziillllrt'l
lipcnti
Bhutan

I.3~pt0p:
\

hj'dltlgg
lr‘ins'rtli
Club's
(101,. .
*3. to.
FIgure
miss n
Where
imm-
10min

AC4

Milk»

33

(7). The structures of these compounds are provided in Table 4-1.

Table 4-1. Structures of 1,3-diols Used in Mechanism Study

 

N 81116 R1 R2 R3 R4 R5 R6 1 ,3-(1101

 

2,4-dimethyl-2,4-pentanediol (1) CH3 CH3 H H CH3 CH3

2-methyl-2,4-pentanediol (2) CH3 CH3 H H H CH3

2,2,4-trimethyl-1,3-pentanediol (3) H H CH3 CH3 H CH(CH3)2 ‘EH I“ ?"
“rec‘- ere-(R6

2.2-dimethyl-1,3-propanediol (4) H H CH3 CH3 H H R2 R, I,

2.4-pentanediol (5) CH3 H H H H CH3

1,3-butanediol (6) H H H H H CH3

1,3-propanediol (7) H H H H H H

 

All the hydrogenolysis experiments in this work are performed at 210 °C and 5 MPa
hydrogen pressure in the aqueous phase. NaOH is added to promote the reaction. The
transition catalysts used in this work are Raney Cu and Raney Ni, two of the typical
catalysts used in sugar and sugar alcohol hydrogenolysis (Montassier er al., 1988; Chang
et al., 1985; Sohounloue et al., 1983). The experimental results are summarized in Table
4-2, together with a documentation of the expected products from each 1,3-diol based on
Figure 4-1. The yields presented in Table 4-2 are based on the reacted 1,3-diols. The
mass is not balanced perfectly, but is within the experimental error. In all the experiments
where methanol is an expected product, it is found only in a trace amount. The reason is
attributed to the transition-metal-catalyzed Cannizzaro reaction, which intercepts
formaldehyde, the hydrogenation precusor to methanol, as discussed earlier.

According to Figure 4—1, the hydrogenolysis of a 1,3-diol is initiated by

dehydrogenating one of its hydroxyl groups. This postulation is strongly supported by the

 

a. Hawk-OCUMe-bhfih .Cmcrflin he emu-Edi:

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35

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ll
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36

results from hydrogenolysis of 1. Because of the absence of a hydrogen atom on both the
or and y carbons, 1 is incapable of undergoing dehydrogenation, and thus is expected to be
inactive under the hydrogenolysis conditions. As indicated in Table 4-2, 1 is indeed found
to be inactive. The hydrogenolysis of 1 has been carried out for six hours with Raney Cu
as a catalyst (Run 1) and for four hours with Raney Ni as a catalyst (Run 8). No reaction
products have been identified at the end of either reaction. For comparison, other 1,3-diols
used in this work are all found to be reactive under the same conditions, because they can
undergo the dehydrogenation reaction. That dehydrogenation is a necessary step for
hydrogenolysis is additionally supported by the results from hydrogenolysis of 2. As 2
can not be dehydrogenated at the or carbon, the C-C and C-0 bonds of 2 are not expected
to be broken between the B and y carbons and at the 7 carbon, respectively, during
hydrogenolysis, although they may be broken between the or and B carbons and at the or
carbon, respectively. The experimental results turned out again exactly as expected (Runs
2 and 9).

The results from hydrogenolysis of 2 (Runs 2 and 9) also provide good evidence
against the retro—Claisen as the dominating C-C cleavage mechanism. The retro-Claisen
reaction requires a dicarbonyl precursor. Since 2 is incapable of forming such a
dicarbonyl, the retro-Claisen mechanism predicts that the CC bonds of 2 are unbreakable
in hydrogenolysis. However, the experimental results show otherwise: the hydrogenolysis
rate of 2 is not impaired by its incapability of undergoing the retro-Claisen reaction, as
shown by the conversion data in Table 4-2.

In contrast to the retrooCIaisen mechanism, the retro-aldol mechanism predicts that

the CC bond of 2 may be broken between the or and B carbons to form two isopropanol

molecule
predicts
i2. Isnl
dnnutit
Idenut‘u
.Icetnn:
Rises ‘
lnthel

hydtug

cnnsu

again

i CIiI
they
Our t

1in

37

molecules, as 2 can be dehydrogenated at the 7 carbon. The retro-aldol mechanism also
predicts that the isopropanol is formed through an acetone intermediate. As seen in Table
4-2, isopropanol is not only found at the end of the reaction (Runs 2 and 9), but is a
dominating product of 2-methyl-2,4-pentanediol hydrogenolysis. Furthermore, acetone is
identified in the reaction when Raney Cu is used as the hydrogenolysis catalyst (Run 2).
Acetone is not detected in the reaction when Raney Ni is used as the catalyst, because
Raney Ni is a more efficient hydrogenation catalyst than Raney Cu. The acetone formed
in the hydrogenolysis of 2 may be quickly hydrogenated to isopropanol. The results from
hydrogenolysis of 2 are very supportive of the retro-aldo mechanism.

The results from hydrogenolysis of 3, 4, 5, 6 and 7 are also supportive of the retro-
aldol mechanism. In all these experiments, the C-C bond cleavage patterns are found to be
consistent with the retro-aldol mechanism. In hydrogenolysis of 6 (Run 6), acetone is
again identified as an intermediate as predicted by the retro-aldol mechanism.

Hydrogenolysis of 2, 6 and 7 has been canied out by Conner and Adkins (1932) with
a catalyst prepared by precipitation of Ni carbonates. However, in hydrogenolysis of 2,
they identified only the end products, isopropanol (11) and 4-methyl-2-pentanol (19), as in
our experiment with Raney Ni as the catalyst In hydrogenolysis of 6 and 7, they did not
report the C-C cleavage at all. The successful identification of CC bond cleavage in
hydrogenolysis of 6 and 7 and of the retro—aldo intermediates in hydrogenolysis of 2 and 6
in this work strengthens the theory that the C-C cleavage is through the retro-aldol
reaction.

Also according to Figure 4-1, the dehydration reaction is responsible for the C-0

cleavage in hydrogenolysis. This postulation is strongly supported by the results from

hidt
illit‘
suc't
ncc'

0c

the

he

Minds

Its

 

38

hydrogenolysis of 3 and 4. Based on the dehydration mechanism, a hydrogen atom
attached to the B carbon is necessary for the hydrogenolysis of C-0 bonds to occur. Since
such a hydrogen does not exist in 3 and 4, cleavage of the C-0 bonds is not expected to
occur in the hydrogenolysis of these compounds. Indeed, as indicated in Table 4-2, no C-
O cleavage is found in hydrogenolysis of either 3 or 4.

The dehydration mechanism is also supported by the C-0 cleavage pattern found in
the hydrogenolysis of 2 (Runs 2 and 9). The dehydration mechanism expects the C-0
bond to be broken only at the or carbon in hydrogenolysis of 2, as 2 can be dehydrogenated
only at the 7 carbon. The results from hydrogenolysis of 2 shows that this expectation is
well founded. The additional products 2-pentanone (l8) and 2-butanone (16) identified in
the hydrogenolysis of 5 (Runs 5 and 12) and 6 (Run 6), respectively, provide additional
evidence to the dehydration mechanism. These compounds are not the direct products of
dehydration, or VII in Figure 4-1, but intermediates from VII to VIII. The hydrogenation
of VI] to VIII is believed to take place in two sequential steps, as shown in the following
with hydrogenolysis of S as an example:

0 O H
II + H2

II II
CH3CH=CHCCH3 LL CH3CH2CH2CCH3 —. CH3CH2CH2CHCH3

VII = 3-pentene-2-one 2-pentanone VIII = 2-pentanol

The accumulation of 2-pentanone in hydrogenolysis of 5 and of 2-butanone in
hydrogenolysis of 6 suggests that, with Raney Cu and Ni as catalysts, the hydrogenation of
C=C double bonds is faster than the hydrogenation of C=O double bonds.

The mechanism pictured in Figure 4-1 is supported additionally by the selectivity

39

data collected from our experiments. In hydrogenolysis of a 1,3-diol, typically four bond
cleavage selectivities can be defined, namely; Ca'Cp vs. Ca—O, Cfl-CY vs. CY-O, Ca—Cp vs.
CB-Cy. and Ca-O vs. CTO. These selectivities are calculated for each experiment and are
documented in Table 4-2.

According to Figure 4-1, the retro-aldol reaction responsible for the CC cleavage
and the dehydration reaction responsible for the C-0 cleavage share the same precursor.
Therefore, the C-C v.s. C-O selectivities should be affected by relative rate of the retro-
aldol reaction to the dehydration reaction. This expectation is justified by the CC v.s. C-
O selectivity data in Table 4-2. In hydrogenolysis of straight-chain 1,3-diols (5, 6 and 7),
the reaction is found to favor the C-0 cleavage products, because the dehydration is faster
than the retro-aldo reaction in this case. In hydrogenolysis of a branched-chain 1,3-diol
(2), the reaction is found to greatly favor the C-C cleavage products, because the retro-
aldol reaction is faster than the dehydration in this case (Neilson and Houlihan, 1968).

When the retro-aldol reaction occurs, the aldol reaction is also possible, which often
leads to scrambling of the retro-aldol fragments. Thus, in the hydrogenolysis of 1,3-diols,
one might expect 1,3-diols other than the starting ones to be formed as the result of the
scrambling aldol reaction. However, no such by-products have been observed in any
experiments carried out in this work. The reason is attributed to the fast hydrogenation of
aldehydes intermediates under the hydrogenolysis conditions and the unfavorable
condensation between ketone intermediates, which minimize the formation of any new
1,3-diols.

As discussed above, the hydrogenolysis mechanism described in Figure 4-1 is

strongly supported by the results from hydrogenolysis of 1,3-diol model compounds. To

Slim

and

the

rut

res;

.J

sun
arc

“It‘tul

 

4o

summarize the experimental results, the hydrogenolysis of 2,4-dimethyl-2,4-pentanediol
and 2-methyl-2,4-pentanediol demonstrates that the dehydrogenation is a necessary step in
the hydrogenolysis of sugar alcohols. The hydrogenolysis of 1,3-dimethyl-1,3—
propanediol and 2,2,4-trimethyl-1,3-pentanediol demonstrates that the dehydration is
responsible for the C-0 cleavage in hydrogenolysis. The hydrogenolysis of 2-methy1-2,4-
pentanediol also provides good evidence against the retro-Claisen as the dominating C-C
cleavage mechanism. The likelihood of the retro-Michael reaction being a dominating C-
C cleavage mechanism is all but eliminated by the capability of all the 1,3-diols (except
2,4-dimethyl-2,4-pentanediol) to undergo hydrogenolysis. On the other hand, these
experimental results are very supportive to the retro-aldol mechanism. In all the
experiments but hydrogenolysis of 2,4-dimethy1-2,4-pentanediol, where no reaction is
supposed to occur, the C-C cleavage is found to follow the patterns predicted by the retro-
aldol mechanism. In addition, some ketone intermediates expected by this mechanism are
identified in the hydrogenolysis of 2-methyl-2,4-pentanediol, 2,4-pentanediol and 1,3-
butanediol. In no reaction, has a product other than the expected been found, and in no
reaction, has an expected product not been found.

The experiments have been performed with two different catalysts (raney Cu and
Raney Ni), the reaction patterns are not affected by the change of catalysts. These results
strongly support the mechanism described in Figure 4-1. Now that the mechanism of C-C
and C-0 cleavage is known, it becomes possible for one to adopt a rational approach in

control of the selectivity of sugar hydrogenolysis.

5.1

3. try:
('0 cl
It can
IIIliter

I“ Intel

 

CHAPTER 5
SELECTIVITY OF SUGAR HYDROGEN OLYSIS

5.1 Selectivity Study Using 2,4-PD Model Compound: Rationale

Because of the multiple bond-cleavage reactions involved in sugar hydrogenolysis,
three types of selectivities can be defined for this process, namely the CC vs. C—C, C-O
vs. 00 and CC vs. 00 selectivities. In the current work, the CC v.s. C-O selectivity of
sugar hydrogenolysis is studied using 2,4-pentanediol (2,4-PD) model compounds. The
purpose is to find a way to minimize C-O cleavage in sugar hydrogenolysis. As the C-0
cleavage is to a great extent responsible for the complication of sugar hydrogenolysis
products, minimizing C-O cleavage is expected to make the reaction more selective.
Reduction of C-0 cleavage in sugar hydrogenolysis is also expected to increase the yield
of glycerol. As mentioned in Chapter 3, glycerol is the highest-valued major product of
sugar hydrogenolysis, and it preserves all the oxygen atoms in the starting sugar
molecules.
I Using 2,4-PD model compound in the selectivity study has a major advantage, that
is, hydrogenolysis of 2,4—PD gives only one pair of CC cleavage products and one pair of
C-0 cleavage products, and these products do not undergo further bond cleavage reactions
to complicate the process. Figure 5-1 shows the reaction pathway of 2,4-PD
hydrogenolysis, which is derived based on the hydrogenolysis mechanism established in

our mechanism study. From this figure, it is seen that aldehyde, ethanol, acetone and

41

42

OH OH

I
CH3CHCH25HCH3
2.4-pentanediol

In.

I II I)” n o II
' 2
CH3CCH3 + CH3CHO ‘— CH3CCH2CHCH3 —-> CH3CH=CHCCH3
3061006 acetaldehyde 4-hydroxy-2-pentanone 3-penten—2-pentanone
¢ + H2 \I' H2 ¢ + H2
3H 0
CH CH CH II
CH3 HCH3 3 2
isopropanol ethanol CH3C§§§E§§EH3
(In (H;
CH3CH2CH2CHCH3
2-pentanol

Figure 5-1. Reaction Pathway of 2,4-Pentanediol Hydrogenolysis

isopropanol result only from the C-C cleavage, and that 3-penten-2-one, 2-pentanone and
2-pentanol result only from the C-0 cleavage. Therefore, the C-C cleavage in 2,4-PD
hydrogenolysis can be quantified by the sum of acetone and isopanol, and the C-0
cleavage by the sum of 3-penten-2-one, 2-pentanone and 2-pentanol, which makes the
selectivity study easy to carry out In contrast, in hydrogenolysis of real sugars, it is
difficult to tie any reaction products to a specific bond-cleavage reaction, because of the
general existence of more than one pathway to the same product.

In spite of the convenience of using 2,4-pentanediol model in selectivity study, a

difference between 2,4—pentanediol and sugars needs to be noted. Compared to 2,4-PD,

43

sugar and sugar alcohol are characterized by a third hydroxy group at the B position,
which will slightly alter the hydrogenation pathway of the ocB-unstaurated ketone from
the C-0 cleavage, or dehydration, reaction. In the presence of a hydroxy group at the B
position, the or,B-unstaurated ketone can tautomerize to give a 1,2-diketone, as shown in
Figure 5-2. Therefore, hydrogenation of the or,B-unstaurated ketone may occur via two

pathways in the hydrogenolysis of real sugars.

 

O o
dehydration ll tautomerization II
——" RC? = CHR = RCCCHZR’
OH
I hydrogenation I hydrogenation

Figure 5-2. Alternate Hydrogenation Pathway of or,B-Unstaurated Ketone
in Sugar Hydrogenolysis

At 25 °C, the equilibrium ratio of the or,B.unstaurated ketone to the 1,2-diketone is
about 1:99 (Alexander, 1954). At 220 °C, a typical temperature used in hydrogenolysis,
this ratio still remains at about 1:20. However, because the hydrogenation of an or,B-
unstaurated ketone is generally much faster than the hydrogenation of an 1,2-diketone,
direct hydrogenation of the or,B-unstaurated ketone intermediate is still expected to be a
major pathway, even in sugar hydrogenolysis.

The 00 cleavage in sugar hydrogenolysis is expected to be less favorable than in
2,4-PD hydrogenolysis. The tautomerization reaction following the dehydration converts
a better hydrogenation precursor to a worse one, and thus may reduce the hydrogenation

rate of the dehydration product in sugar hydrogenolysis. Based on this reason, a catalyst

Illljdj

 

44

which gives a good C-C v.s. C-O selectivity in 2,4-PD hydrogenolysis is expected to also

deliver a good selectivity in sugar hydrogenolysis.

5.2 Experimental Study

In the current work, 2,4-PD has been hydrogenolyzed under various reaction
conditions to investigate the effects of temperature, hydrogen pressure, base concentration,
catalyst amount and catalyst type on the C-C v.s C-O selectivity. Figure 5-3 presents the

result from a typical experiment run, carried out at 220 °C, 5 MPa H2 pressure and with Ni

on kieselguhr as the catalyst. In addtion to the end products, ethanol, isopropanol and 2-
pentanol, two intermediate products, acetone and 2-pentanone, are normally observed at a
significant level in 2,4-PD hydrogenolysis. Other intermediate products, including 4-
hydroxy-Z-pentanone, acetaldehyde and 3-penten-2-one, are sometimes detected during
the initial period of reaction, but generally exist only at low levels. These products are not
quantified and therefore are not shown in Figure 3.

Figure 5-4 shows how the CC v.s C-O selectivity changes during the
hydrogenolysis. This selectivity is defined as the ratio of the mole of C-C bonds to the
mole of C-0 bonds broken in the reaction. Since the amount of 3-penten-2-one is

negligible in the reaction product, the selectivity is calculated as:

_ [acetone] + [isopropanol]
_ [2—pentanone] + [2-pentanol]

 

S

As seen from Figure 5-4, the selectivity is typically stabilized at a constant level after the
initial period of the reaction. It is this stabilized selectivity that is compared in the

following for different reaction conditions.

45

 

 

 

 

 

120 . -
groo— -
é
a 80: _
E
8 cos .
O
C
O
0
:1 j x
U -
9
(1 20» «
, /" i g . :
0 so 10 150 200 250

Time (min)

Figure 5-3 Product v.s Reaction Time Profiles. Experimental Conditions:
220 °C, 5.0 MPa H2 pressure, 0.1 g Ni/kieselguhr catalyst, 0.4 ml 1 N NaOH

and 40 ml water solvent. Symbols: 0 2,4-PD; I EtOH; O acetone; Ai-
PrOH; D 2-pentanone; A 2-pentanol.

 

 

 

 

 

3 . . fl
2.5" 4
2h 4
2‘
E .
“as K
CD :
1t... . , , . ..
0.5" .
0 1 L 1 4
0 50 100 150 200 250

Time (min)

Figure 5-4 Selectivity v.s. Reaction Time Profile Derived from Figure 5-3

Table 5-1 Effect of Temperature on Selectivity

46

 

 

 

 

 

 

Experiment Series # 1 “ Experiment Series # 2 b
Temperature (°C) Selectivity Temperature (C) Selectivity
180 1.38 — —
200 1.42 200 1.06
220 1.42 220 1.02
240 1.45 240 1.10

 

 

 

 

0. Reaction Conditions: 5 MPa H2 pressure. 0.1 g Ni/kieselguhr. 0.4 ml 1 N NaOH. 0.5 starting 2,4-
PD and 40 ml water. b. Reaction Conditions: 5.0 MPa H2 pressure, 0.25 g Raney Cu, 0.2 m1 1 N

NaOH. 0.5 g starting 2,4-PD and 40 ml water.

Table 5-2 Effect of Base Concentration on Selectivity ‘

 

 

 

 

 

Experiment Series # l b Experiment Series # 2 C
NaOH Added (ul) Selectivity NaOHl Added (til) Selectivity
0 0.46 0 0.26
400 1.42 200 1.02
1000 1.56 500 1.73

 

 

 

 

a. In all cases, base is added as 1 N aqueous N aOH solution. b. Reaction Conditions: 200 °C, 5.0
MPa H2 pressure. 0.1 g Ni/kieselguhr, 0.5 starting 2,4-PD and 40 ml water. c. Reaction Conditions:

220 °C, 5.0 MPa H2 pressure, 0.25 g Raney Cu, 0.5 g starting 2,4-PD and 40 ml water.

Table 5-3 Effect of Hydrogen Pressure on Selectivity

 

Experiment Series # 1 0

Experiment Series # 2 b

 

 

 

 

Pressure (MPa) Selectivity Pressure (MPa) Selectivity
3.0 1.50 3.0 0.80
5.0 1.42 5.0 1.02
7.0 1.49 7 .0 l. 15

 

 

 

 

0. Reaction Conditions: 200 °C, 0.1 g Ni/kieselguhr, 0.4 ml 1 N NaOH. 0.5 starting 2,4-PD and 40
ml water. b. Reaction Conditions: 220 °C, 0.25 g Raney Cu. 0.2 ml 1 N NaOH. 0.5 g starting 2,4-

PD and 40 ml water.

Table 5-4 Effect of Catalyst Amount on Selectivity

 

Experiment Series # l 0

Experiment Series # 2 b

 

 

 

 

Catalyst (g) Selectivity Catalyst (g) Selectivity
0.1 1.42 0.1 1.22
0.2 1.49 0.25 1.02
0.5 0.90 0.5 0.75

 

 

 

 

0. Reaction Conditions: 220 °C, 5.0 MPa H2 pressure, Ni/kieselguhr, 0.4 ml 1 N NaOH. 0.5 start-
ing 2,4-PD and 40 ml water. 17. Reaction Conditions: 220 °C, 5.0 MPa H2 pressure. Raney Cu,
0.2 m1 1 N NaOH. 0.5 g starting 2,4-PD and 40 ml water.

the
rim
hydro
tnndir
(Ibsen)

CGIISlSle

£ij

Writs.

8let 0,, I)

48

Efleet of Temperature To investigate the effect of temperature, 2,4-PD is
hydrogenolyzed under two sets of base conditions with varying temperature. The
selectivity, as Table 5-1 shows, is found to barer change with the temperature, although

increasing temperature greatly enhances the reaction rate.

Eflect of Base Concentration The base concentration, as Table 5-2 shows, has a
profound effect on the selectivity, as well as on the reaction rate. Increase in the amount of
base added to the reaction solution increases the reaction rate, and favors the cleavage of
CC bonds. This effect on the selectivity is most easily seen when the data from reactions
with base addition are compared with those from reactions without base addition. One
thing that needs to be pointed out is that the base concentration, as indicated by the PH of
the reaction solution, is generally found to be lower than the initial concentration after the

initial period of reaction.

Eflect of Hydrogen Pressure The effect of hydrogen pressure seems to vary with
the catalyst used. With Raney Cu as the catalyst, an increase in the hydrogen pressure
increases the selectivity; while with Ni on kieselghur as the catalyst, the increase in the
hydrogen pressure causes little change in the selectivity within the experimental
conditions explored in this work In the case of Raney Cu, hydrogen pressure is also
observed to have an inhibiting effect on the rate of 2,4-PD hydrogenolysis. This effect is

consistent with the observation made by Chang et al. (1985) in hydrogenolysis of sobitol.

Effect of Catalyst Amount The effect of catalyst amount again varies with the
catalysts. However, the general trend is that high catalyst concentration has an adverse

effect on the selectivity. With Raney Cu as the catalyst, the selectivity steadily decreases

 

49

with the catalyst amount. With Ni on kieselghur as the catalyst, the effect of catalyst

amount only becomes significant at high level.

Efiect of Catalyst Type A good catalyst is considered to be the key to achieve the
control over the C-0 cleavage in sugar hydrogenolysis. In this current work, seven
different types of catalysts have been investigated. The results are presented in Table 5-5.
As can be seen from this table, the selectivity varies widely with the type of catalyst used.
The best selectivity is furnished by barium-promoted copper chromite, and is as high as
2.5, which is yet to be optimized against other reaction conditions. In contrast to the
barium-promoted copper chromite, the plain copper chromite catalyst only provides a
selectivity of 1.21 in our experiments. It is interesting to see the difference made by the
presence of barium oxide. The effect of the amount of barium oxide used in preparation of

the catalyst may be another factor deserving study in the context of selectivity

 

 

 

 

 

 

 

 

improvement.
Table 5-5 Effect of Catalyst Type on Selectivity "
Catalyst Type Selectivity
nickel on silica-alumina 1.12
nickel kieselguhr 1.18
copper chromite 1.21
ruthenium on carbon 1.36
Raney nickel 1.46
Raney copper 1.73
barium-promoted copper chromite 2.50

 

 

a. All reaction are run at 210 °C, 3.5 MPa H2 pressure, with 0.05 g metal catalyst. 0.2 ml
1 N NaOH. 0.5 g starting 2,4-PD and 40 ml water.

tea
frnrr

5—5 at

Iheret
IIIISQIW‘
II'Ittde
ICS‘tlIl of l

 

 

50
5.3 Simulation Study

In order to rationalize the experimental results just presented, a simulation study has
been carried out on the hydrogenolysis of 2,4wPD. In this study, the effect of base
concentration, hydrogen pressure, catalyst amount and catalyst type are simulated and
compared against the experimental results. The effect of temperature is not studied in this
initial simulation effort, because first it is more difficult to do and second this effect is not
as profound as other factors. Therefore, all the simulated reactions are assumed to be

isothermal and take place at an arbitrarily chosen temperature of 220 °C.

5.3.1 Model Development

At this time, the reaction pathway of 2,4-PD hydrogenolysis is clear to us. However,
the pathway alone is not sufficient to allow us to develop the process model. It does not

provide enough reaction details on how the H2 and the metal and base catalysts are

involved in the reaction, which are necessary for process model development. For this
reason, a more detailed reaction scheme of 2,4-pentanediol hydrogenolysis is developed
from the reaction pathway in Figure 5-1. This detailed reaction scheme is shown in Figure
5-5 and is used as the basis to develop the process model.

In the absence of transitional metal catalysts, molecular hydrogen is not reactive.
Therefore, it must be activated by the metal catalyst before it can be used to reduce any

unsaturated compounds. The result of the reaction of H2 with the metal catalyst is a metal
hydride species, which has the ability to hydrogenate C=C and C=O double bonds. As a
result of this hydrogenation reaction, the metal catalyst must be regenerated, so that it can

be re-used in the next cycle of catalysis. The above mechanism has been used by many

IESI
Its'd;

PDh

IIIE me
“thin-
I IUllh:
Winn

womb:

51

 

 

2.4-pentanediol
(A)
M.0H'
*1
MH H 0 ‘
r 2 I k- I
OH‘ OH’
acetone + acetaldehyde 11,—— 4-hydroxy-2-pentanone :p: 3-penten-2-one
(C) (E) k? (B) k (G)
MH. H20 MH. H20 MB. H20
k4 k6
: b3 ”'0”. ‘ k1 M. OH'
isopropanol ethanol 2-pentanone
(D) (F) (1)
MH, H20
ks
H2+M+OH' —-:f> MH+H20 :k-7
It'ir

2-pentanol
(D

Figure 5-5. Detailed Reaction Scheme of 2,4-PD Hydrogenolysis

researchers to explain the hydrogenation process of C=C and C=O bonds with molecular
hydrogen, and is adopted in the current work to help develop the reaction scheme of 2,4-
PD hydrogenolysis.

The hydrogenation of carbonyl compounds by the metal hydride yields alcohol and
the metal catalyst. Since this reaction is reversible, alcohols must be also able to react
with the metal catalyst to get dehydrogenated and to release a metal hydride. Therefore, it
is further pictured in Figure 5-5 that the hydrogenolysis of 2,4-PD is initiated by the
reaction of this starting compound with the metal catalyst, and that the metal hydride

works as a hydrogen carrier to transport hydrogen from 2,4-PD and molecular H2 to those

nil
hurl

equil

fitted
“Inc on
IIIII IIIIIS
littered

III he I

52

unsaturated intermediate products.

For the convenience of later mathematical model development, the reactions shown
in Figure 5-5 are all numbered (the reaction number corresponds to the subscript of the
rate constants shown in Figure 5-5), and the products are also designated by single letters.
From Figure 5-5, it may be noticed that the base catalyst (hydroxide ion in this case) is not
only involved in the retro-aldol and dehydration reactions as a real catalyst, but also
participates directly in the dehydrogenation of 2,4-PD and activation of H2. Participation
of bases in H2 activation and alcohol dehydrogenation has been well documented in
literature. As the dehydrogenation step is most likely to be the rate limiting step in 2,4-PD
hydrogenolysis, the above postulation is in consistence with the fact that 2,4-PD
hydrogenolysis rate is enhanced by the addtion of bases.

It may also be noticed from Figure 5-5 that all the reaction step in 2,4-PD
hydrogenolysis, except the hydrogenation of 3-penten-2-one, are assumed to be reversible.
Retro-aldolization and dehydration are well—known reversible reactions. Hydrogenation
of ketones and aldehydes and dehydrogenation of alcohols are also known to be reversible,
but hydrogenation of alkene is generally considered to be irreversible, because the reaction
equilibrium lies far to the product end.

To derive the mathematical model from Figure 5-5, we assume that the reaction is
carried out in a batch reactor, and thus the product concentrations are functions of reaction
time only. We also assume that the reaction is carried out under isothermal conditions,

and thus the reaction rate constants do not change with time. The concentration of H2 in

the reaction solution may also be assumed to be time-independent and be in equilibrium

with the gaseous phase, as the reaction is assumed to take place under a constant H2

In t.
COIICE
flirts
Ctlllten

andl0t

53

pressure and the solution is under constant violent stirring during the reaction. A mass

balance on each of the species present in Figure 5-5 except H2 leads to:

d
d—I[A] = ‘XI (1)
d
$13] = Xr'xz‘xs (2)
d
d
EID] = X3 (4)
d
EMS] = Xz-X4 (5)
d
Eur] = X4 (6)
d
are] = X5_X6 ‘7’
d
EII] = X6‘X7 (8)
d
Em = X7 ‘9)

d d d
25”“ =EIOH] =_-d—t[MI-l] =—X1+X3+X4+X6+X7—X8 (10)

In the above equations, [A], [B], [C], [D], [E], [F], [G], [I] and [J] are the molar
concentration of 2,4—PD, 4-hydroxy-2-pentanone, acetone, isopropanol, acetaldehyde,
ethanol, 3-penten-2-one, 2-pentanone and 2-pentanol, respectively. [M] is the molar
concentration of the metal catalyst, [MH] is the molar concentration of the metal hydride,

and [OH] is the molar concentration of hydroxide ion in the reaction solution. X 1 to X8

54

are the reaction fluxes of Reactions No.1 to No.8 in Figure 5-5, respectively. Assuming all

these reactions are first order in the substrates and catalysts, we further have

X, = 1:1 [A] [M] [0H] —k_1 [B] [MH] (11)
X2 = 1:213] [01!] —k_2[C] [El [0H] (12)
X3 = k3IC] [MH] —k_3[Dl [M] [0H] (13)
X. = ME] [MH] -k4[F] [M] [OH] (14)
X5 = kslB] [0H] —k_5[G] [0H] (15)
X(5 = 1:616] [MH] (16)
X7 = k7rlt [MH] -—k., in [M] [OH] (17)
X8 = kslfll [Ml [0H] —k_8IMHl (18)

In the last equation, [H] is the molar concentration of H2 dissolved in the reaction solution.

At the beginning of the reaction, we may assume that all the products, but the

substrate (2,4-PD) and the metal and base catalysts, have a zero concentration. Therefore,
[A] = [Ar,.rMi = [moron] =t0H1,
and
[B]=[Cl=[D]=[E]=IFI=[G]=III=[JI=[MH]=0
for t = 0.
The above equations and initial conditions constitute a closed problem. If all the

relevant reaction rate constants are obtained, the above equations can be solved to give

product concentrations as functions of reaction time.

55
5.3.2 Determination of Kinetic Parameters

As mentioned previously, the simulation study is to be carried out at a constant
temperature to simulate the effects of reaction conditions other than temperature on the
selectivity. Therefore, we need only to determine the reaction rate constants for one
temperature at which we choose to run the simulation. This temperature is chosen here to
be 220 °C. This way, we could use the experimental data presented in Figure 5-3 to help
determine these rate constants. The data in Figure 5-3 are obtained at 220 °C.

Among the eight reactions shown in Figure 5-5, reactions N o. 2 (retro-aldo) and No.

5 (dehydration) are, though at low temperature, well-studied. From the kinetic data

published by Jensen and Hassan (1976), it is derived that k2=752 M‘lmin‘l, k_2=4480 M‘
2min'1, k5=14100 M'lmin'1 and k,5=6590 M'lmin'l at 220 °c (see Appendix D). The
rate constants (eleven in total) for the rest of the reactions shown in Figure 5-5 vary with
the metal catalyst involved, and are not available from literature. Therefore, they are
determined below, indirectly, from our 2,4-PD hydrogenolysis data

The eleven rate constants to be determined are not all independent They are bound
together by thermodynamic relations. Of these relations, an obvious one is that, for any
reversible reaction, the ratio of the forward reaction rate constant to the backward reaction

rate constant is equal to the reaction equilibrium constant. Thus,
kr‘
I; = K i (19)

for i=1, 3, 4, 7 and 8. In the above equation, K,- is the equilibrium constant of reaction No.

i. The reaction is assumed to take place under dilute conditions.

The relations which are not so obvious are that the above equilibrium constants are

56

also interdependent. These relations become apparent when the following two chemical

equations are examined:

AG”, K
RICHOI-IRZ + M + OH“ 2;": RICORZ + MH + H20

AG°,r_r
RICHOHRZ - H20) 2‘2 Rlcorz2

The first equation in the above is a general representation of the dehydrogenation/
hydrogenation reactions shown in Figure 5-5. This first equation is also the result of the

second equation above added to Reaction No.8 in Figure 5-5. Therefore, we have
A0” = A9" + A080 (20)

where A60 and A9” are the free energy changes of the first and second reactions above,
respectively. From Eq.(20), we further have
K = I_(K8 (21)
where K and K are the equilibrium constants for the first and second reactions,
respectively.
Unlike AG and K, ag" and K are well defined and are not dependent on the catalyst

involved. Values of KS relevant to 2,4-PD hydrogenolysis have been estimated in this
study from the free energy data documented in Guthrie (1977) for ketones and aldehydes
and in Young (1981) for hydrogen dissolved in aqueous phase, and are listed in Table 5-6.

Due to lack of free energy data at high temperature, these equilibrium constants have been

calculated using free energies at 25 °C. Also because of lack of data, A9” '3 for Reactions

No.1 and No.7 are assumed to be the same as Reaction No.3. The rationale is that these

reactions are all about dehydrogenation of a secondary alcohol to a ketone.

57

Table 5-6 Equilibrium Constants of Relevant Dehydrogenation Reactions

 

 

No. Alcohol Ketone/Aldehyde ag" (kcal) K
1 2,491) 4-hydroxy-2-pentanol 12.54 2.81x10‘6
3 isopropanol acetone 12.54 2.81x10‘6
4 ethanol acetaldehyde 16.59 4.52x10"8
7 2-pentanol 2-pentanone 12.54 2.81x10'6

 

Substituting Eq.(21) into Eqs.(19) for each i and rearranging gives

k1 = Eth’ht (22)
k—3 = §3K8k3 (23)
ka=fiflh on
k—7 = §7K8k7 (25)
18 = K814, (26)

Elimination of K8 from the above equations leaves four relations relating the rate

constants. Therefore, of the eleven rate constants to be determined, only seven are
independent. The remaining four have to be determined from the seven independent rate
constants chosen, in order to assure thermodynamical consistency.

Besides the thermodynamic consistency, it is also important for the determined rate
constants to fall within a reasonable range, so that the simulated processes are as close as
possible to those actually occurring in our experiments. To achieve this, the best approach

would be to accommodate as much experimental information as possible into the rate

58

constant determination process. Therefore, data from the experiments presented
previously in Figure 5-3 are used in the following to help determine the rate constants.
That experiment was carried out at 220 “C.

Figure 5-3 shows how the concentration of 2,4-PD (A), acetone (C), isopropanol (D),
ethanol (F), 2-pentanone (I) and 2-pentanol (J) changes with reaction time in the above
experiment Concentration of 4-hydroxy-2-pentanone (B), acetaldehyde (E) and 3-
penten-2-one (G) are low (estimated to be less than 1 mM) and are not determined in the
experiment. The concentration of metal catalyst (M), metal hydride (MH) and hydroxide
ion (OH) are also undetermined, but at beginning of the reaction they are calculated as 28,
0 and 10 mM, respectively, from the amounts of regents used in the experiment. The
initial concentration of 2,4-PD (A) is 120 mM. The H2 concentration in the reaction
solution is found to be about 0.1 M in equilibrium with 5 MPa hydrogen pressure at 220
°C (Young, 1981).

From Figure 5-3, it is seen that, after 1:100 min, [C] and [I] are quite steady.
Considering also that [B], [E], [G] and [OH] are low, we may assume that the reaction is at
a pseudo-steady state after the initial period. Thus from Eqs.(2), (3), (5), (7), (8) and (10),

it is derived that

x2 = x3 = x4 (27)
X5 = X6 = X7 (28)
and
X1 = X3 = X2+X5 (29)

From Figure 5-3, x, and x7 can be determined to be 8.85 x 10-5 M min-1 and 6.07 x 10-5 M

59

min'l, respectively, at t=l75 min. All the other reaction fluxes at 1: 175 min are calculated

with Eqs.(27) to (29) from X 3 and X7, and are listed in Table 5-7, together with the product

concentrations at t=l75 min and t=0 min. Concentrations of ethanol (F) and 2,4»PD (A)
in Table 5-7 have been re-calculated to make the mass perfectly balanced. These data are
used below in conjunction with Eqs.(l 1) through (18) to determine the reaction rate
constants.

From Eq.(l l), we have

x1 = k, [A] [Mi [0H1 —k_1[B] [MH]

k 0
= a. tMHiIgj-IW [A] - [Bi]
0
= k_1[MHl(If1Kg%fl]-IA1 — 131)
Let
0H
[(0 = Kalb—{13%;}; (30)

and substitute it into the above equation. We get
X1 = k_1[MH] (15,19, [A] — [31) (31)

Similarly, from Eqs.(13), (14), (17) and (18) we may get

x, = k31MH1 ([Cl—I_<3KOID]) (32)
X4 = k4 [MH] ([5] ‘E4K0 I”) (33)
X7 = k7IMHI ([1] -I_{7KOIJ]) (34)

and

X8 = k_8[MH] (K0 [H] — 1) (35)

Table 5-7 Summary of Product Concentrations and Reaction Fluxes

 

 

t = 0 min t = 175 min Unit
[A] 120 28.0 mM
[B] 0 less than 1 mM mM
[C] 0 5.89 mM
[13} 0 48.0 mM
[E] 0 less than 1 mM mM
[F] 0 53.9 mM
[G] 0 less than 1 mM mM
[1] 0 6.26 mM
[J] 0 31.7 mM
[H] 100 100 mM
[OH] 10 unknown mM
[M] 28 unknown mM
[MH] 0 unknown mM
X1 _ 1.49 x 10“ M min“l
X2 _ 8.85 x 10" M min'1
X3 _ 8.85 x 10" M min'l
X4 _ 8.85 x 10'5 M Ma1
X5 _ 6.07 x 10'5 M min'1
X6 _ 6.07 x 10" M min‘l
X7 _ 6.07 x 10'5 M min'1
X3 _ 1.49 x 10" M Ma1

 

61
Combining Eqs.(3l) through (35) with Eqs.(12), (15) and (16), there are now eight

equations available in total for eleven unknowns. These unknowns include six rate

constants (k_ 1’ lg, lg, k6, 1:7 and k8), four concentrations ( [B], [E], [G] and [OH] ) and a
new variable K0. [M] and [MH] are related to [01-1] through the following equation:
[MH] = [M],- [M] = rom,-10H1 (36)
which is obtained by integration of Eq.(10). As the available equations are not enough for
unknowns, three of them have to be assumed before others can be solved. In the
following, this liberty is taken to make sure that the determined [B], [E], [G] and [OH]
conform with our experimental observations and that the relative magnitudes of the

determined rate constants are consistent with our knowledge of hydrogenation reactions.

If the dehydration (Reaction No.5) were irreversible during hydrogenolysis, the

maximum selectivity of 2,4-PD hydrogenolysis would be k2/k5 = 0.053 , or the CO

bond cleavage would be tremendously favored over the C-C bond cleavage. The
experiment shows otherwise. Therefore, the dehydration reaction must be reversible in
reality in 2,4-PD hydrogenolysis. The retro-aldol reaction (Reaction No.2) may also be
reversible in 2,4-PD hydrogenolysis, depending on the reaction conditions. In order to
describe the degree that the above reactions are reversed, a reversibility parameter may be
defined for each of these reactions, as the ratio of the forward reaction rate to the reverse

reaction rate, or

R _ k_2[Cl [51 [OH] _ k_2[Cl [£1
2" 1:218] [0H1 ‘ 1:213]

 

(37)

and

62

_ k-5IGI [0H] _ k_5 [G]
5 ’ kslB] [0H] ‘ k5 [B] (38)

 

As later we will see, the reaction selectivity is closely related to these two parameters.

With the newly defined reversibility parameters, we may rewrite Eqs.(2) and (5) as

X2

1,181 [0H] (1 -112) (39)

and

X5 1:513] [0H1 (1-R5) (40)
Dividing Eq.(39) by Eq.(40) and solving for R 5 yields:

kZXS
R5 = 1 — @(r—Rz) = O.97+0.03R2 (41)

By definition, 0< R2 < 1. Hence, 0.97 < R5 < l, which implies that the dehydration

reaction is highly reversible in the 2,4-PD hydrogenolysis process.

Also derivable from the above equations are:

 

 

B X2
I l ' k2(1—R2) [0H] (42)
szs
[E] = k_,(1-R,)1C110H1 ‘43)
and
ex x
[G] - 5 2 5 (44)

 

_ k2k_5(1—R2) [0H] _k__5[0H]
Thus, [B], [E] and [G] can be determined if [OH] and R2 are assumed. The pH of the

reaction solution has been checked during the experiment to be around 11. The retro-aldol

reaction is speculated to be reversible but to a much less degree than the dehydration

63
reaction. Let [OH] = 1 mM and R2 = 0.2. It is obtained that [B] = 0.147 mM,
[E] = 0.838 mMand [G] = 0.305 mM, allless than 1 mM. From Eqs.(36) and (41), it

is also obtained that [M] = 19 mM, [MH] = 9 mMand R5 = 0.976.

Now that all the product concentrations and reaction fluxes are known, all the
concerned reaction rate constants are ready to be determined, providing that an

appropriate value is assigned to K0 . The equations needed are derived from Eqs.(3l) to

(35) and Eq.(16), and are given as follows:

 

 

 

 

 

XI
k = 4
-‘ [MH] (15X, [A] - [81) ( 5)
X3
k = 46
3 [MH1([C]-I_(3KOID1) ( )
X4
= 47
"4 [MH]([E]—I_(4KOIFT) ‘ ’
X6
"6 = [MH] [Gr (48)
X7
k = 49
7 [MH]([ll 45,19,111) ( )
and
X
k 8 (50)

-8 "" [MH] (X01111 -1)

Let K0 = 5000. k,,, k3, k4, k6, k7 and k_8 are calculated as 67.1, 1.89, 11.9, 22.1, 1.16 and

3.22x10'5 M" min", respectively. These rate constants satisfy that k_1 > k7 and that

64

k6 > k4 > k3 > k7 , and thus are consistent with our knowledge of hydrogenation reactions.

With the above just determined rate constants, k1, k,3, k,.,, k; and k3 can be further

calculated from Eqs.(22) through (26). Table 5-8 below summarizes all the rate constants

obtained from the above procedure.

Table 5-8 Summary of Determined Reaction Rate Constants

 

In 16 k. k. In k6 k7 kt
(M'Zmin‘l) (Mi-him") (M‘zmin'j) (M’zmin'l) (M‘zrnr'n'l) (M‘zmin'l) (M‘Zmin'l) (M'zmin'l)

 

435 752 1.89 11.9 14100 22.1 1.16 76.2

 

k_1 k_2 k,3 k4 k,5 k,6 k_7 k,8
(M‘Zmin‘I) (M‘Zmin‘l) (M‘zmin'l) (M’min‘l) (M‘znu'n‘l) (M‘zmin'l) (M‘zmin‘l) ("tin")

 

67.4 4480 12.2 1.24 6590 o 7.48 3.32x10'5

 

Although some arbitrary measure has been used in the above rate constant

determination process to assign values to R2 and K0, the determined rate constants remain

sensible in the sense that:
a) they are thermodynamically consistent;
b) they are within a reasonable range of absolute and relative magnitude; and
c) most importantly, the simulation results obtained with these rate constants are
comparable to those obtained experimentally, as will be seen in next section, which is
the ultimate test for the determined rate constants.

These reaction rate constants may not represent Ni on kieselguhr or any other catalysts

65

that have actually been used in our experiments, but they may represent a hypothetical
catalyst which is not very different from them. If this is the case, the effects of reaction
conditions on the selectivity should be similarly reflected on the actual and the
hypothetical catalysts. Thus, the simulation study based on these rate constants would be
useful at least qualitatively in interpretation of the experimental results, which is our goal

in this study.

5.3.3 Simulation Results and Discussion

Figures 5-6 to 5-9 present the results of a simulated run of 2,4»PD hydrogenolysis.
The assumed reaction conditions for this simulated run are the same as those for the
experimental run shown in Figures 5-3 and 5-4, except that the experimental run has
undergone a temperature rise from ambient temperature to 220 °C during the initial period
of reaction, while the simulated run is computed under isothermal condition. Because of
this difference in temperature history, the concentration and selectivity profiles in Figures
5-6 and 5-7 do not match those in Figures 5-3 and 5-4 exactly. Nevertheless, they still bear
the major features of the later profiles. For example, the selectivity profile in Figure 5-7
also becomes time-independent after the initial period of reaction. The concentration of
acetone (C) and 2-pentanone (I) in the simulated run also undergo an up-and-down
behavior during the reaction, as in the experimental run. In addition to the information on
major hydrogenolysis products, the simulation study also reveals how the concentration of
minor products and the concentration of the base catalyst (hydroxide ion) change during
the reaction (Figures 5-8 and 5-9). All the minor products shown in Figure 5-8 seem at a

higher level than that observed in the experimental run, which, however, may result from

66

 

 

 

 

3 : : :
55.100 --------- .i -------------------- : ---------- : --------- —
§ 5 ‘ E
A 80 ------------------------ ‘7 -------- : --------- —t
9 ‘ : :'
g eo---------3- ............................. 3 ......... -
I . t , D
I I / ’ IK
4o----—-----.+ ------------------ 12"": ----- ; --------- -
I , /I l
/ l r
I .. / ...... 1- , J
20I'"-—-‘:-:.-:€_V'5’-’:’-;‘-;“= ........ :___ _ — _"‘_-_._*____
Slur : ' C
0 ,4” ’ l . '. l A
O 50 100 150 200 250

Time (min)

Figure 5-6 Simulated Concentration Profiles of Major Products. Simulation
conditions: 220 °C, [H]=100 mM, [M]o=28 mM, [OH]O=10 mM, [A]o=120 mM.

(as)

 

____________________________________

N

([C]+[D])/II)GI+[|I+IJII

S:
in

 

 

 

 

 

0 50 100 150 200 250
Time (min)

Figure 5-7 Simulated Selectivity Profile. S is calculated as ratio of the sum of
[C] and [D] to the sum of [G], [I] and [J].

[B]. [E]. [G] (mM)

[0H] (mM)

 

 

 
   

 

 

   

 

10 I I T I
: r :
I I |
I I I
I ‘ I
8------——----: ----------- : ----------- «
I t I I
r t t t
t t r
i J '
6»-——--—------T -------------------------------------- -—
.o—“fl‘x
r ,,,,, x.
l‘"\—-"" ..... I \‘\
4.4 ____________________________ ‘34.; ———————————————————— _
I \5.‘
\-
' \'\\‘. I
I \\\E '
2IC".I..'1'¥"”""""‘~"’\"."_('37""\<~Z"'T”"‘
‘ , \ ~ \‘xJ
W i \ ~ :‘X
0 I r B 1 L '
O 50 100 150 200

Time (min)

Figure 5-8 Simulated Concentration Profiles of Minor Products

10

 

 

_________________________________________

 

 

 

O 50 1 00 1 50 200 250

Time (min)

Figure 5-9 Simulated Base Concentration Profile

68

 

 

Selectivity, S

 

 

 

l

O 5 10 15 20 25 30 35 40
Base Concentration, [OHlo (mM)

 

Figure 5-10 Simulated Effect of Base Concentration on Selectivity.
Simulation Conditions: 220 °C, [H]=100 mM, [M]o=28 mM, [A]o=120 mM,

varying [OH]0.

 

Selectivity, S
o: '00

—L
1:.

 

1.2

 

 

 

1 1 1 1 1
10 1 5 20 25 30 35 4O 45 50
Catalyst Concentration, [M10 (mM)

Figure 5-11 Simulated Effect of Catalyst Concentration on Selectivity
Simulation Conditiom: 220 °C, [H]=100 mM, [OH]0=10 mM, [A]o=l20 mM,

varying [M]..-

Selectivity, S
is a: in

.NA

 

 

 

 

 

O 50
Hydrogen Concentration, [H] (mM)

100

150

200

Figure 5-12 Simulated Effect of Hydrogen Concentration on Selectivity
Simulation Conditions: 220 °C, [M]o=28 mM, [OH]0=10 mM, [A]o=120 mM,

varying

[H].

 

UT

Selectivity, S
h

(A)

 

 

L

 

 

100

150

Rate constant. k6 (1/M/min)

200

Figure 5-13 Simulated Effect of k6 on Selectivity. Simulation Conditions: 220
°C, [H]=180 mM, [M]o=28 mM, [OH]0=10 mM, [A]o=120 mM, but varying k6.

70

the temperature history difference that exists between the experimental and simulated
runs.

As in the experimental study, the effects of reaction conditions on the selectivity are
also investigated in the simulation study. Figure 5-10 shows how the selectivity is affected
by the change in base concentration. As observed in our experiments, the simulation study
also indicates that the selectivity increases with the base concentration. This effect is most
profound at low base concentration, but diminishes as the base concentration gets high.
Because of the limited range of base concentration explored, the independence of the
selectivity on base concentration at high base concentration is not revealed by the
experiment

Figure 5-11 shows how the selectivity is affected by the change in the amount of
catalyst used in the reaction. Again as in our experiment, the selectivity is found to
decrease with the catalyst amount. This effect remains over the whole range of catalyst
concentration that has been explored. Lower catalyst concentration makes the reaction
more selective, but it also makes the reaction slower.

As for the effect of hydrogen pressure on the selectivity, one set of experimental data
(with Raney Cu as catalyst) show that it increases with the hydrogen pressure, while the
other set of data (with Ni on Kieselguhr as catalyst) show that it is basically independent
of the hydrogen pressure. The simulation study reveals a more complete picture and
provides a good explanation to the above apparently different behaviors of the selectivity.
It shows that, for a given catalyst, the selectivity increases with the hydrogen pressure at
low pressure, is indifierent to the change in hydrogen pressure within the medium pressure

range, and decreases with the hydrogen pressure at high pressure (Figure 5-12). These

71

low, medium and high pressures are relative, and are catalyst-dependent. For Raney Cu, 7
MPa must be still in its low pressure range, while for Ni on kieselguhr, 3 to 7 MPa must
happen to be its medium range. This explains why with different catalysts the selectivity
behaves differently within the same hydrogen pressure range.

The efiects of reaction conditions on the selectivity may be understood at a deeper
level with the help of some mathematical equations relating the selectivity to base
concentration, metal hydride concentration and the reversibility parameter for the retro-
aldol reaction. These equations are derived as follows.

To start, we have

)12 k2(1—R2) [0H] [B] k2(1-R2)

S = Y. = k,(1-R,)10H1 [B] = err-R.) ‘5”

 

Assuming [G] is at a pseudo-steady state, we have

 

£161 = "sIBI [0H] "£5101 [0H1 -k6[Gl [MH] so (52)
which leads to
[G] *5 [OH]
1731 = k_5 [0H] + 16 [MH] (53)
Substituting the above equation into Eq.(38) and rearranging yields
R [1 k6 [MH] I1 (54)
5 ’ 1 14510111
Further substituting Eq.(54) into Eq.(51), we obtain
".2 k-swm 55
S=k5(l-R2) l-l-W ( )

01'

72
k2 k_5 [cm]0
s = E‘l’R2)[l+k_6(W—HT’1)] (56)

Replace the rate constants in Eq.(56) with numbers. We get

 

s = 0.053 (1 —R2) [1 +298( [5;];- 1)] (57)

which provides additionally some quantitative ideas.

Eqs.(56) and (57) clearly show that the selectivity will increase as [OH]0 increases, if
other variables are kept unchanged. However, increase in [OI-I]o will inevitably lead to
increase in [MH]. Increase in [MH], as well as increase in R2 , leads the selectivity to
decrease. The level of R2 depends on levels of [OH] and [MH], as well as other factors.
The simulation study reveals that, at low base concentration, R2 decreases as the [OH]0 is
increased. The combination of the effects of [0H]oand R2 overrides the effect of [MH].

As a result, the selectivity shows an overall increase as the base concentration is increased.

However, at high base concentration, R2 increases with [OH]0. The effect of [OI-I]o is
offset by the combination of the effects of [MH] and R2. The overall result is a selectivity
independent of base concentration.

Increase in the amount of catalysts used simply increases the metal hydride level in
the solution. Change in R2 is minor. Therefore, the selectivity decreases with the catalyst
amount. Increase in hydrogen pressure decreases both [MI-I] and R2. At low hydrogen
pressure, R2 is very sensitive to the change in hydrogen pressure. As the pressure
increases, it becomes less and less sensitive, which explains why the selectivity responds
to the change in hydrogen pressure diflerenfly in different pressure range.

From Eq.(56), it is also seen that the selectivity should increase as the rate constant k6

73

decreases. This deduction is confirmed by the simulation results as shown in Frgure 5-13.
Since different catalysts feature different k6 values, the dependence of the selectivity on k6
explains why it varies widely with the type of catalysts used. To improve the reaction
selectivity, catalysts with small k6 value should be used. Figure 5-13 suggests that the
effect of k6 on the selectivity is very profound. Therefore, the selectivity may be well
improved by use of a good catalyst. A good catalyst implies that this catalyst possesses
only a moderate capability to hydrogenate the cab-unsaturated ketone while still keeping a
reasonable capability to hydrogenate saturated ketones and aldehydes.

Lowering the reaction temperature may also lead to a reduced k6. However, it will
also change other rate constants, including k2, k5 and k5 in Eq.(56). As a result, the
selectivity may not be improved by reduction of the reaction temperature. As a matter of
fact, our experiments indicate that the selectivity is not sensitive to the change in

temperature, at least within the temperature range we have explored.

PART II

SELECTIVE DEH YDROX YLAT I 0N 0F GLYCEROL

CHAPTER 6
INTRODUCTION: GLYCEROL DEHYDROXYLATION

6.1 On-going Research on Glycerol Dehydroxylation

The purpose of glycerol dehydroxylation is to convert glycerol to 1,3-propanediol
(1,3-PD) by removing the second hydroxyl group of glycerol. Research on this process
has been motivated mainly by two considerations. First, the current commercial processes
to produce 1,3-PD from acrolein and ethylene oxide are not efficient, as has been
discussed in Chapter 1. Second, glycerol is a biomass-derivable chemical. It can not only
be potentially produced from sugars through fermentation and hydrogenolysis, but also be
obtained from processing of animal fats and vegetable oils. Considering the promise of
1,3-PD in polyester fibers, films and coatings, it is significant being able to derive this
chemical from biomass sources.

The glycerol dehydroxylation processes reported so far are mostly fermentation
processes (Gunzel at al., 1991; Boenigk et al., 1993; Zeng er al., 1993; Biebl, 1995;
Barbirato, 1996; Cameron, 1998; Reimann, 1998). Nevertheless, Che (1987) has invented
an one-step catalytic process to convert glycerol to 1,3- and 1.2-PD’s. This process uses a
synthesis gas co-feed, and is typically carried out under a temperature of 100-200 'C and a
pressure of loo-10,000 psi, in the presence of tungsten and a Group VIII component. Like
the current 1,3-PD production processes, however, this process furnishes only a low yield

of 1,3-PD. The main products of this process are propanol and propylene glycol. 1,3-PD

74

75

accounts for only a small portion of the reaction product (generally less than 20%).

The fermentation processes are based on the observation that, under appropriate
conditions, certain bacteria, such as Colosm'um, K. pneumoniae and Citrvbacterfi'eundii,
can convert glycerol to 1,3-PD. This conversion is generally accompanied with formation
of by-product ethanol, acetic acid, butyric acid, lactic acid, hydrogen and carbon dioxide,
but by careful control of the fermentation conditions the yield of 1,3-PD can be
maximized. Research on glycerol fermentation has been started since the 1960’s. At the
current time, this process is able to furnish a 1,3-PD yield as high as 0.66 mole per mole
glycerol or 55% on weight basis (Zeng et al., 1993), which, unfortunately, is about the
highest yield that one can obtain from the glycerol fermentation process.

Having a low theoretical yield is one of the major disadvantages of the fermentation
process. The reason for this low theoretical yield can be understood by examination of the
reaction pathway of glycerol fermentation. Figure 6-1 shows a simplified reaction
pathway of glycerol fermentation, which is derived from a more complex one in Zeng er
al. (1993). From Figure 6-1, it is seen that the conversion of glycerol to 1,3-PD is
completed through dehydration of glycerol followed by hydrogenation of the resulting 3-
hydropropionaldehyde (HPA). Hydrogenation of HPA consumes NADHZ, which is
regenerated in the oxidation of glycerol to pyruvate, the precursor to by—product ethanol,
acetate, lactate, butyrate, H2 and C02 (the latter species are not shown in Figure 6-1). In
other words, the conversion of glycerol to 1,3-PD is sustained by the formation of by-
products, which inevitably reduces the theoretical yield of 1,3-PD. The theoretical yield
that a fermentation process can provide varies with the by-product it produces. In the ideal

case, only acetate would be produced as the by-product. As conversion of pyruvate to

76

 

Glycerol

 

 

 

1120

3-hydroxypropionaldehyde

NADH2

NAD

 

1,3-propanediol

 

 

 

2NAD

2116113112

Pyruvate

l

 

Acetic acid

 

 

 

2mm2

ZNAD

 

Ethanol

 

 

 

11.4113112

NAD

 

 

Lactic acid

 

Figure 6-1. A Simplified Reaction Pathway of Glycerol Fermentation

acetate does not consume NADHZ, the theoretical yield of 1,3-PD would be 67% in this

case. When other by-products are produced, the theoretical yield is expected to be lower

than 67%.

Another major disadvantage of the fermentation process is that this process is
substrate-inhibited. The bacteria used in the fermentation are generally not able to stand a
glycerol concentration above 17%. As a result, both the product concentration and the

productivity are low. In a continuous process, Zeng et al. (1993) have obtained a 1,3-

propanediol concentration of 30 g l‘1 with a productivity of 8.1 g 1'1 h'l, which are the best

results reported so far in literature.

 

77

6.2 The New Conversion Concept and Supportive Work

In the current research, we propose a new glycerol dehydroxylation concept to
selectively remove the second hydroxy group of glycerol and thus to convert it to 1,3-PD.
This new conversion concept has potential to provide a high 1,3-PD yield and does not
have the disadvantages of the current glycerol dehydroxylation processes.

The new glycerol dehydroxylation concept is presented in Figure 6-2. It consists of
three steps, namely acetalization, tosylation and detosyloxylation. The idea is to
selectively transform the second hydroxyl group of glycerol into a tosyloxy group
(tosylation) and then remove it by catalytic hydrogenolysis (detosyloxylation). Compared
to the hydroxyl group, the tosyloxy group is a better leaving group and is easier to replace
with a hydride ion. In the following, a detailed discussion is made on each of the

conversion steps.

Wu, In order to selectively tosylate the second hydroxyl group of
glycerol, the first and third hydroxyl groups of glycerol need to be protected. According
to the proposed conversion concept, this is completed by acetalizing glycerol with
benzaldehyde. Acetalization is an equilibrium reaction, but can be driven to completion
by removing the water formed in the reaction. The difficulty with this step is that, besides
the desired 1,3-product (S-hydroxyl-Z-phenyl-l,3-dioxane, or HPD), the undesired 1,2-
product (4—hydroxylmethy-2-phenyl—1,3-dioxolane, or HMPD) may also be formed.
Nevertheless, these products can be separated, and the separated 1.2-product can be
returned to the acetalization reactor, where either it itself can be converted to the 1,3-

product or it can help shift the reaction toward the 1,3-product.

78

Step 1: Acetalization

OH + PhCHO H OH
’ + O
OH H
h h

 

glycerol
S-hydroxy-Z-phenyl- 4-hydroxymethy-2-phenyl-
1,3-dioxane (HPD) 1,3-dioxolane (HMPD)
Step 2: Tosylation
H S
TsCl
»
h h
HPD 2-phenyl-5-tosyloxy-1,3-dioxane (PTD)

Step 3: Detosyloxylation and Group Protection Removal

S
+ H20
01‘ng ———-> OIH !H
- PhCHO - TsOH
h 2-tosyloxy- 1 3-pl'0panediol
1.3-propanediol (TPD)
PTD
or
S
+ H2 + H20
———> —-——>
' T30“ - PhCHO OH H
h h 1.3-propanediol
pTD 2-phenyl-l.3-droxane

Figure 6-2. Illustration of the New Concept of Glycerol Dehydroxylation

79
Besides benzaldehyde, many other carbonyl compounds could also be used as the

hydroxyl group protection regent. The condensation between glycerol and various
aldehydes and ketones has been extensively reviewed by Showler and Barley (1967).
Generally, the reaction between glycerol and aldehydes gives more 1,3-product than the
reaction between glycerol and ketones (Trosr; 1991). For our purpose, it is obvious that
aldehydes are preferred to ketones.

Besides the product yield, the ease of separating the 1,3- from the 1,2-product is
another important factor that needs to be considered in selection of the group protection
regent. In terms of the later aspect, benzaldehyde is superior to aliphatic aldehydes. The
products from aliphatic aldehydes are generally high-boiling-point oils (Piasecki and
Burczyk, 1980; Hilbbert and Carter, 1928), and the differences between the boiling points
of the 1,2 and 1,3 products are usually small. For example, the condensation between
glycerol and acetaldehyde gives four products: the cis- and trans- 5-hydroxyl-2-methyl-
1,3-dioxanes (1,3-product) and the cis- and trans- 4-hydroxylmethyl-2-methyl-1,3-
dioxolanes (1,2-product). As documented in Beilstein, the boiling points of these products
are 73, 100, 86, and 94 °C, respectively, under 18 torr.

The 1,2- and 1,3- products from benzaldehyde have considerably different
solubilities in benzene-ligroin mixture. The 1,3-product can be readily crystallized from
the benzene-ligroin solution (Hill et al., 1928; Baggett er al., 1960).

The reaction between glycerol and benzaldehyde has been carried out previously, in
several ways. The early methods (Irvine et al., 1915; Baggett er al., 1960) allowed direct

contact of glycerol with benzaldehyde. In some cases, acids, such as H2804, were added

as catalysts. The late methods (Baggett et al., 1965; Szeja, 1983) are featured by use of a

80

benzene solvent. The advantage of doing the reaction in benzene is that the water formed
in the reaction can be removed azeotropically, and thus the reaction can be driven to
completion. This advantage can be further enhanced by using a Dean-Stark u'ap (Casey, et
al., 1990), as Szeja (1983) has done. Acid catalysts were generally used in the late
methods.

The condensation between glycerol and benzaldehyde gives four products: the cis
and trans- HPD’s and the cis- and trans- HMPD’s. As mentioned above, separation of the
1,3-product HPD from the 1,2-product HMPD has been achieved by Hill et al. (1928) and
Baggett et aL (1960), through crystallization from benzene-ligroin solution. Although the
trans—HPD (b.p. 63.5 'C) can also be potentially crystallized, this procedure yields mainly
cis-HPD (b.p. 83.5 °C) . Cis- HPD is the more stable diastereomer of HPD (Showler and
Darley, 1967).

The equilibrium between the 1,2- and 1,3- products under the acetalization
conditions has been studied. Hill et al. (1928) showed that the 1,2- product is favored over
the 1,3-product by a ratio of 5.7: l in the absence of solvent. As stated by the authors, this
result was more qualitative than quantitative, because only the fractionated part of the
products was assumed as the 1,3-product in their study. The data from Baggett er al.
(1965) also favor the 1,2-product, but only by a ratio of 1.9:1. The data documented in
Iochims and Kobayshi (1974) show the contrast to the previous work: the 1,3-product is

favored over the 1,2-product (by a ratio of 1.5:1). Both the data of Baggett er al. and of

Jochims and Kobayshi were obtained by integration of the characteristic H1 NMR signals
of the products, but the former was done in dioxane and the latter was done in chloroform.

The solvents used may have effect on the position of the reaction equilibrium. Another

81

example that favors the 1,3-product is given by Juaristi and Antiunez (1992). They
obtained 11 47:30:23 mixture of HMPD, cis- HPD and trans- HPD from the reaction
between glycerol and benzaldehyde. Upon continued exposure to acid, this ratio changed
to 22:34:44. Therefore, they concluded that the reaction thermodynamics favors the 1,3-
products. No solvent was mentioned in their work.

Clearly, more research is needed to accurately determine the equilibrium ratio of the
1,2- and 1,3- products. However, the previous work should have cleared any doubts about
the feasibility to quantitatively produce the 1,3-product (mainly cis-HPD) from
condensation of glycerol with benzaldehyde, and about the ready interconversion between

the 1,2- and 1,3- products under acidic conditions.

W In this step, the unprotected hydroxy group of glycerol is
transformed into a tosyloxy group, so that it can be removed in the subsequent
detosyloxylation reaction. This group transformation is completed by treating HPD, the
1,3—product from the first-step reaction, with tosyl chloride. Tosylation of alcohols is a
well-known reaction and is generally carried out under basic conditions. Bases are needed
to neutralize the hydrogen chloride formed in this reaction. If not neutralized, the chlorine
ion can potentially attack the newly formed tosyl group (Tipson, 1953).

A general procedure exists for tosylation of alcohols in pyridine (Fieser and Fieser,
1967). Pyridine functions both as a solvent and as a base in this reaction. The hydrogen
chloride produced in tosylation may form a complex with pyridine, which crystallizes
from pyridine at 0 °C. Based on this procedure, Juaristi and Antiunez (1992) have
tosylated both the cis- and trans- HPD’s in 98% yields. In another example, Hesse] et al.

(1954), by using a similar procedure, was able to obtain 2-phenyl—5-tosyl-1,3-dioxane

82

(PTD) in 94% yield in the tosylation of HPD. Besides the organic base, inorganic bases,
such as sodium hydroxide and sodium carbonate, can also be used in alcohol tosylation,
which has been reported in several incidents to tosylate other alcohols (Tipson, 1953;

Flowers, 197 1).

W In order to finally convert

the selectively tosylated glycerol to 1,3-PD, the tosyloxy group, as well as the group
protection, needs to be removed from PTD. As shown in Figure 6-2, there are two
potential pathways to go from PTD to 1,3-PD. Either the hydroxy group protection can be
removed, followed by removal of the tosyloxy group; or alternately the tosyloxy group
can be removed, followed by removal of the hydroxy group protection. In either approach,
removal of the group protection can be done with a hydrolysis reaction, and removal of the
tosyloxy group is to be done with a hydrogenolysis reaction. The hydrolysis reaction
regenerates benzaldehyde, which can be reused in the acetalization reaction.

If the hydrolysis reaction is canied out first, 2-phenyl-l,3-dioxane results as an
intermediate. Hydrolysis of this compound is expect to be easy, because no additional
functional groups other than the ether linkages may be affected. If the hydrolysis reaction
is carried out first, however, caution needs to be exercised to prevent the potential attack of
solvents on the tosyloxy group of PTD and/or TPD. Although according to Flowers
(1971) tosylates are exceedingly stable under the acid hydrogenolysis conditions, and
many successful examples of tosylate hydrolysis have been documented by Tipson (1953),
our experiments show that the tosyloxy group is subject to the attack of protic solvents at
temperatures above 60 °C.

Hydrolysis of PTD yields TPD. Although this reaction has never been reportedly

83
done, TPD has been synthesized by Chong and Sokoll (1993) through a different route,

which makes both the H1 and C13 NMR data available for characterization of this
compound.

Removal of the tosyloxy group is expected to be the most difficult step of the
proposed glycerol conversion concept. According to Figure 6-2, detosyloxylation is to be
done through a hydrogenolysis reaction with molecular hydrogen as the reducing regent
In the literature, hydrogenolysis of tosylates is generally effected with a lithium hydride,
such as lithium aluminum hydride (LiAlH4) or lithium triethylborohydride (LiHBEt3).
This reaction has never been reportedly done using molecular hydrogen as the reducing

regent. Although LiAlH4 and LiHBEt3 are effective as detosyloxylation regents,

however, they are too expensive to use on industrial scale. Therefore, it is up to us to
develop a catalytic tosylate hydrogenolysis process with molecular hydrogen as the
hydrogen source.

In summary, the new glycerol dehydroxylation concept presented in Figure 6-2 has
provided us with an alternate and promising route to convert glycerol to 1,3-propanediol.
The first and second step reactions of the new concept have been successfully performed
by previous researchers, although for different purposes. The third step reaction,
detosyloxylation with molecular hydrogen as reducing regent, is the key to the success of

this new conversion concept, which has never been reportly done in literature.

6.3 Objective and Scope of Current Research

The objective of the current research is to explore the technical feasibility of the new

glycerol dehydroxylation concept presented in Figure 6-2. As discussed above, removal

34

of the tosyloxy group from the selectively tosylated glycerol is expected to be the most
difficult step of the new conversion concept. Therefore, our primary task in this work is to
develop a catalytic hydrogenolysis process to remove the tosyloxy group from ether PTD
or TPD. In this capacity, the two potential pathways shown in Figure 6-2 to carry out the
conversion from PTD to 1,3-PD has been tested and compared.

The reactions involved in the acetalization and tosylation steps have been performed
by previous investigators for different purposes. The results from their work are
promising. In this work, these reactions are re-performed to verify the published
procedures and to provide the PTD needed in studying the detosyloxylation and protection

removal reactions.

CHAPTER 7
EXPERIMENTAL PROCEDURES

7.1 Acetalization

The acetalization between glycerol and benzaldehyde is an equilibrium reaction. To
drive this reaction to completion, the water formed in this reaction needs to be constantly
removed from the reaction flask. In this experiment, water removal is done by performing
the reaction in benzene solvent and removing the water as an azeotrope with benzene.

Figure 7-1 shows the experimental set-up used in this experiment. This set-up
includes a round-bottomed reaction flask, a condenser and a Dean-Stark trap. The
function of the Dean-Stark trap is to trap the water formed in the reaction. The reaction
takes place in the flask. The water formed in the reaction is boiled off from the reaction
flask as an azeotrope with benzene (boiling point: 65 'C). The vaporized water and
benzene flow condense in the condenser and flow down into the Dean-Stark trap. As
water and benzene are immiscible and have different densities, they separate in the Dean-
Stark trap with water forming the bottom layer. The Dean-Stark trap is pre-filled with
benzene. The condensed benzene is returned to the flask by overflow.

In a typical experiment, 100 g glycerol, 120 g benzaldehyde (c.a. 6% excess) and 300
mL benzene, together with l g p-toluene sulfonic acid catalyst are placed in a 500 mL
round-bottom flask, equipped with a magnetic stirring bar. The reaction is initiated by

bringing the reaction solution to a boiling state. Thereupon, vaporization and

85

86

 

 

 

 

 

 

Condenser _
\K
W.../
Dean-Stark Thermometer

Reaction flask

/ Sand bath

  

 

 

 

 

 

 

 

 

0 © Magnetic stirrer
/ with hot plate

 

 

 

Figure 7-1. Experimental Set-up for Acetalization of Glycerol with Benzaldehyde

87

condensation commences in the reaction flask and the condenser, respectively, and the
water is seen to accumulate at the bottom of the Dean-Stark trap. The Dean-ka trap is
graduated. Therefore, progress of the reaction can be monitored by the volume of the
water accumulated in the Dean—Stark trap. When the water in the Dean-Stark trap reaches
a steady level, the reaction is completed. Completion of the reaction is also indicated by
the temperature change of the reaction solution. Before completion, the temperature stays
constant at about 80 °C. When the reaction is close to completion, the temperature starts
to climb, until a new level (about 160 °C) is reached. After the reaction is completed, the
valve with the Dean-Start trap is turned open, and the benzene is boiled off, leaving only
the product in the reaction flask.

The reaction product is a mixture of cis- and trans- 5-hydroxy-2-phenyl-1,3-dioxane
(HPD, the 1,3-product) and cis- and tran- 4-hydroxy-methyl-2-phenyl-1,3—dioxolane
(HMPD, the 1,2-product). A small amount of benzaldehyde may also exist in the product,
because the excess benzaldehyde used in the reaction. To isolated HPD from this mixture,
it is crystallized from a benzene-ligroin solution. The procedure is taken from Hill et al.
(1928) and is documented in the following:

1) Dissolve the reaction product in a benzene-ligroin mixture. Typically, for an amount of
60 g of reaction mixture, about 90 mL of benzene and 150 mL of ligroin are needed.

2) Leave the above solution in a freezer for about one hour. At this time, crystals may not
form from the solution.

3) Slowly add benzene while shaking, until the oil layer disappear. Crystals may or may
not form at this time. Place the solution in the freezer, for about another hour. Crystals

should form at this point.

88

4) Filter the solution with a Buckner filter under vacuum, and wash the crystals with a
50% benzene-ligroin solvent. Continue washing until the yellow color disappears. The
yellow color indicates the presence of benzaldehyde in the crystals.

At the end of this procedure, a white, wax-like crystal product yields, which is cis-HDP.
The trans-HDP has a better solubility in the benzene-ligroin solvent and is more difficult
to crystallize than the Cir-HPD. The above procedure can be repeated to further purify the
HDP product obtained.

Analysis of the raw reaction product is performed with GC. Characterization of the

purified HPD is done with 1H and 13C nmr. The GC column used is a DB-S column from
J&W. The carrier gas is hydrogen (at a flow rate of 1 mllmin). The oven temperature is
programmed so that it stays at 150 °C for one minutes, then increases at a rate of 40 'C/
min and then stays at 320 °C for 3 minutes. The retention times for the relevant

compounds are documented in Table 7-1.

Table 7-1. GC Retention Times for Acetalization Products

 

 

Compounds Retention time (min)
benzene 1.30
benzaldehyde 1.42
cis-HPD 3.36
cis- and trans- HMPD 3.48
trans-HPD 3.83

 

 

89
7.2 Tosylation

Tosylation of 5-hydroxy-2-phenyl-1,3-dioxane (HPD) is carried out in pyridine. To
start the experiment, HPD and TsCl are placed in an Erlenmeyer flask. Then to the flask
is added cold dry pyridine. The flask is shaken until both HPD and TsCl dissolve in
pyridine. Then the flask is placed in a refrigerator (c.a. 5 °C) to allow the reaction to
continue for about 12 hours. Typically, 5-10% excess TsCl is used in this reaction, and for
each gram of HPD, about 20 mL pyridine is needed.

Progress of this reaction can be monitored by formation of needle crystals. These
crystals are pyridine-hydrochloride complex. When no more crystals are forming, the
reaction is complete. At this time, the flask is removed from the refrigerator. The content
of the flask is poured with stirring into a beaker containing about 6 g ice for each milli-
liter pyridine. The tosylate product (2-phenyl-5-tosyl-1,3-dioxane, or PTD) precipitates
immediately.

Stirring is continued until all the ice melts. Then the solution is filtered with a
Buckner funnel under vacuum, and the crystals is washed with water for three times.
Finally, the crystals are dried in a desiccator under vacuum. After this procedure, PTD is

obtained as white dry wax-like crystals. Characterization of PTD is accomplished using

1H and 13C nmr.

7.3 Hydrolysis

Hydrolysis of PTD has been performed in both water and methanol. To carried out

the reaction in water, PI'D from the previous reaction is placed in a glass vial equipped

90

with a magnetic stirring bar. Then to the glass vial is added a small amount of p-toluene
sulfonic acid (T sOH) and a large amount of water. PTD is not soluble in water.
Therefore, it floats on the water at beginning of the reaction.

The glass vial is heated in a water or sand bath. The reaction solution is constantly
stirred. After 4 to 6 hours' heating and stirring, the PTD floating on the water disappears,
and yellow oil droplets are seen at the bottom of the glass vial. These yellow dmplets are
the benzaldehyde formed in the reaction. Upon cooling and sitting, benzaldehyde
separates from the aqueous phase. The benzaldehyde phase is removed from the glass
vial. The aqueous phase is neutralized with NaOH, and is transferred to the
hydrogenolysis reactor for further reaction. Possibly because of the two hydroxyl groups
present in 2-tosyl-1,3-propanediol (TPD), most of the TPD produced in the hydrolysis
reaction stays in the aqueous phase.

To carry out the hydrolysis reaction in methanol, PTD is again placed in a glass vial.
Then to this glass vial, the methanol solution of TsOH is added. The glass vial is simply
placed in an oven at 40 °C. After 2 hours, the reaction is completed. The benzaldehyde
formed in the reaction does not separate from the methanol phase. To isolate PTD, the
reaction product is first extracted with hexane to remove benzaldehyde, and then it is dried
at a temperature below 40 °C to remove the methanol solvent. Thus obtained PTD is re-
dissolved in dioxane and then transfer to the hydrogenolysis reactor.

Comparing the above two procedures, the later one is more successful than the
former. The hydrolysis reaction in water is usually canied out at a temperature above 80
°C. At this temperature, tosylates are subject to the attack of protic solvents, as is

discovered in this work. The hydrolysis reaction in methanol is carried out at a

91

temperature of 40 °C. At this temperature, the solvent attack on the tosyloxy group is

insignificant.

7.4 Hydrogenolysis

In this work, hydrogenolysis of tosylates is carried out under hydrogen pressure with
either Ni on kieselguhr or Ru on carbon as the catalyst. The same experimental system
and the same experimental procedure as described in Chapter 3 are used, but the reactor
used in this experiment has a smaller capacity (10 mL). The 1,3-PD produced in TPD

hydrogenolysis is quantified using GC.

CHAPTER 8
RESULTS AND DISCUSSION

8.] Acetalization

Acetalization with benzaldehyde protects the first and third hydroxyl groups of
glycerol from being tosylated and removed in the subsequent reactions. This reaction has
two important features: a) it is an equilibrium reaction, but can be driven to completion;
and b) it is non-selective: besides the desired 1,3-product HPD, the undesired 1,2-product
HMPD is also produced, which has to be separated from HDP and returned to the
acetalization reactor. Because of the later feature, the percentage of HDP in the
acetalization products and the ease to separate HPD from HMPD are our primary concern
in this research.

Three acetalization experiments have been performed. TWo are done in benzene, and
one is done in a 1:1 benzene-ligroin mixture. In the first experiment, 50 g glycerol and 60
g benzaldehyde (c.a. % excess) are condensed in 300 ml benzene. The reaction takes
about 3 hours to complete. GC analysis of the raw reaction product shows three product
peaks. Of these three peaks, two are determined to result from HPD (cis— and tran- HPD),
and one from HMPD. The two diastereomers of HMPD can not be separated with the GC
column we were using.

To separate HPD from the product mixture, it is crystallized from a benzene-ligroin

mixture with the procedure described in Chapter 7. The crystallized product is

92

93

characterized with 1H and 13C nmr. The obtained spectra are found to be consistent with
the structure of HPD and with those documented for this compound in the literature.
Measurement of the melting point of this crystallized product (75-80 'C) further identifies
it as the cis diastereomer of HDP. The previously reported melting points are 37 'C for
HMDP, 65 ’C for trans-HDP and 83 'C for cis-HDP. The crystallized cis-HPD is not
100% pure. It contains a small amount of trans-HPD, but very little HMPD. HMPD is
believed to not crystallize with cis-I-IPD, and therefore can be removed in the process of
filtration and washing. Trans-HPD may crystallize with cis-HPD, although to a much less
extent. Both cis-HPD and trans-HPD are desired in the acetalization reaction.

From the first experiment, about 10 g cis-HPD is obtained after purification. The
yield is calculated as 25%. Before purification, the yield of cis-HPD is estimated to be
about 42%, based on the sizes of GC peaks. The ratio of cis-HPD to trans-HPD to HMPD
is 42:33:25. In the second experiment, 100 g glycerol is condensed with 120 g
benzaldehyde in 300 m1 benzene. The (cis-) HPD yield from this experiment is similar to
that of the first experiment.

From these results, it is seen that the yield of HPD from the acetalization reaction is
in high percentage. The procedure to separate HPD from the product mixture is fairly
simple and easy to carry out. It is also seen, however, that the undesired HMDP and
unrecovered HDP in combination still account for about 75% of the reaction product.
They need to be recycled back to the acetalization reactor, if an industrial process is to be
designed.

After crystallization, the undesired HMDP and unrecovered HDP are left in the

benzene-ligroin mixture. If the acetalization is carried out in the same solvent, recycle of

94

this residue will be straightforward. It is motivated by this consideration that the third
experiment is run in a 1:1 benzene-ligroin mixture to explore the feasibility to carry out
the acetalization in such a solvent. This experiment turned out to be a success and

furnished an HPD yield of 23%.

8.2 Tosylation

The purpose of tosylation is to transform the unprotected hydroxy group of HPD into
a good leaving group, so that it can be removed in the subsequent reactions. In the current
research, this group transformation is accomplished by treating HPD with tosyl chloride
(T sCl) in pyridine.

TWo experiments have been performed. In one experiment, 2 g HPD is treated with
2.4 g TsCl (c.a. 12% excess), which gives 3.1 g PTD. The yield is calculated as 83%. In
the other experiment, 5 g HPD is treated with 6 g TsCl (c.a. 12% excess), which furnishes
8.1 g PTD. The yield is 87%. In the literature, a yield as high as 98% has been reported in
tosylation of HPD (Juaristi and Antiunez, 1992). The lower yield in our experiments may
result from the unpurified TsCl and the undried pyridine used in this experiment, which
are believed to affect the yield. However, no additional effort has been made in the current
research to maximize the product yield. We are convinced that higher yield is achievable

if more care is exercised in performing the reaction.
The obtained PTD is characterized with 1H and 13c nmr. These spectra are

consistent with the structure of PTD and with the data documented in the literature

(J uaristi and Antiunez, 1992).

95
8.3 Detosyloxylation

Detosyloxylation, or removal of the tosyloxy group, is the focus of this work. This
reaction has never been done with molecular hydrogen as the reducing regent. In this
work, we attempt to develop such a hydrogenolysis process to remove the tosyloxy group
from either TPD or PTD, and to create a pathway from PTD to 1,3-PD.

As discussed previously, the conversion of PTD to 1,3-PD can potentially be
approached in two different ways. One approach is to first remove the hydroxy group
protection and then remove the tosyloxy group. The other is to first remove the tosyloxy
group and then remove the hydroxy group protection. Both approaches have been
explored in this work.

The second approach proves to be unsuccessful. PTD is found to be inactive under
hydrogenolysis conditions. The experiment has been carried out in dioxane with Ni on
kieselguhr as the catalyst. The reaction is run under 3 MPa H2 pressure. The temperature
has been kept at 120 ’C for two hours, at 130 “C for two hours, at 140 °C for 10 hours and
then at 150 “C for another 10 hours. No product is found at the end of the experiment

The initial attempt to convert PTD to 1,3-PD via the first approach did furnish 1,3-
PD. However, the yield was low (less than 15%). To start this experiment, PTD is
hydrolyzed at 80 ‘C in 0.01 N aqueous p-toluene sulfonic acid solution. This reaction
normally takes about 4 to 5 hours. After the reaction, the benzaldehyde released from the
reaction, which separates itself from the aqueous phase, is removed. The aqueous phase,
which contains most of the TPD resulting from the hydrolysis, is neutralized with NaOH
and transferred to the hydrogenolysis reactor to remove the tosyloxy group. Three

catalysts, including Ni on kieselguhr, Raney Ni and Ru on carbon, have been used in the

96
hydrogenolysis, and none of them has provided a 1,3-PD yield higher than 27%. A more

detailed account of the about results may be found in DeAthos (1995), who has assisted
this author in performing the above experiments.

In spite of the low 1,3-PD yield delivered by this experiment, it shows that TPD, in
contrast to PTD, is reactive under the hydrogenolysis condition. Comparing the structures
of PTD and TPD, the tosyloxy group of PTD is isolated, while the tosyloxy group of TPD
is next to a hydroxy group. Possession of this adjacent hydroxy group must have
somehow facilitated the removal of the tosyloxy group in hydrogenolysis of TPD.

To confirm the above speculation, experiments are carried out to hydrogenolyze 6-
tosyloxy-hexanol (TH) and 2-tosyloxy-cyclohexanol (TCH). Like PTD, TH has an
isolated tosyloxy group; while like TPD, the tosyloxy group of TCH is next to a hydroxy
group. As expected, TB is found to be inactive under the hydrogenolysis conditions.
TCH, on the hand, is hydrogenolyzed at a temperature above 140 °C. Below 140 °C, the
reaction is slow. Both the hydrogenolysis of TH and of TCH have been conducted in
dioxane. Hydrogenolysis of TCH has been conducted with both Ni on kieselguhr and Ru
on carbon. Hydrogenolysis of TH, however, has been conducted only with Ni on
kieselguhr. The results of these experiments confirm our speculation that the presence of a
hydroxy group next to the tosyloxy group facilitates its removal in hydrogenolysis.

Hydrogenolysis of TCH yields two products. One is cyclohexanol and the other is
cyclohexanone, as found in our GC-MS analysis. With Ni on kieselguhr as the catalyst,
the ratio of cyclohexanol to cyclohexenone in the product is about 1:1; with Ru on carbon
as the catalyst, the ratio is about 3:1. No other products are found. Based on the observed

reaction products, the hydrogenolysis of TCH may occur as shown in Figure 8-1, with

97

OH OH O OH
on slow fast slow
—> —> ——>
- TsOH + H2

Figure 8-1. Reaction Pathway of TCH Hydrogenolysis

cyclohexanone as the precursor to cyclohexanol. The function of transition metal catalysts
in this reaction is not limited to catalyzing the hydrogenation of cyclohexanone to
cyclohexanol in the last step. They must be involved in the initial detosyloxylation step as
well, because, in the absence of transition metal catalysts, TCH is found to be inactive at
140 °C. .

The assistance of the adjacent hydroxy group to detosyloxylation can be understood
on the assumption that the initial detosyloxylation step in the above reaction scheme is
thermodynamically unfavorable. Presence of the adjacent hydroxy group stabilize the
initial detosyloxylation product through a subsequent tautomerization reaction. In the
absence of such a hydroxy group, as in the cases of PTD and TH, the subsequent
tautomerization is impossible and thus the initial detosyloxylation product is not
stabilized. This explains the inactivity of PTD and TH under hydrogenolysis conditions.

To find out the reason for the low 1,3-PD yield in our initial attempt to convert PTD
to 1,3-PD, further experiments are done to hydrogenolyze TCH in 90% aqueous methanol.
To our surprise, this reaction yields neither cyclohexanol nor cyclohexanone. The reaction
does provide two products, but these products are from the reactions of TCH with water

and with methanol, respectively. Further experiments show that the reactions of TCH with

98

water and methanol can both be effected simply by heating TCH in these solvents over 60
“C. At 80 'C, these reactions becomes fairly fast. It has been suggested in the literature
that tosylate is stable under hydrolysis conditions (Flower, 1971). This statement appears
not true to TCH, and is probably not true to PTD and TPD either. In hydrolysis of PTD, it
has been observed that the reaction solution becomes substantially more acidic at the end
of the reaction than at the beginning of the reaction. Increase in the acidity of the reaction
solution is a strong sign that the attack of water on the tosylate has occurred. The reaction
of water (and methanol) with tosylates releases TsOH, which increases the acidity of the
reaction solution.

Based on the above study on TCH hydrogenolysis, a new procedure is developed to
convert PTD to 1,3-PD. This procedure starts with a hydrolysis reaction in methanol.
PTD has a much better solubility in methanol than in water. Therefore, the reaction can be
carried out at a temperature as low as 40 ’C, while still giving a reasonable reaction rate
(normally complete in two hours). No acidity increase of the reaction solution is observed
at the end of the reaction, which implies that the solvent attack on the tosyloxy group is
insignificant at 40 °C.

Hydrolysis of PTD yields TPD and benzaldehyde (in the form of acetal with
methanol). To separate benzaldehyde from TPD, the raw product is extracted with
hexane. Then the reaction solution is dried at the room temperature to remove methanol.
The dried product is mostly TPD. To hydrogenolyze TPD, it is dissolved in dioxane, and
then heated up to 140 'C in the presence of Ni on kieselguhr. Dioxane is an aprotic
solvent, and it does not attack tosylate.

The first experiment performed with the new procedure starts with 213 mg PTD. At

99

the end of hydrogenolysis, 27 mg 1,3-PD is obtained in the product. The yield is
calculated as 56%. In addition to 1,3-PD, hydrogenolysis of TPD also gives n-propanol
(24%). This is understandable, because hydrogenolysis of TPD has 3-
hydropropionaldehyde (HPA) as an intermediate, which may also undergo a dehydration

pathway to give n-propanol, as shown in Figure 8-2.

L - TsOH

-H O
HOCHzCHzCHO —2—> CH2=CHCHO

¢+H2 ¢+H2

1,3-PD n-propanol

 

 

 

 

 

 

 

 

Figure 8-2. Reaction Pathway of TPD Hydrogenolysis

From Figure 8-2, the selectivity of TPD hydrogenolysis is controlled by the relative
hYCII‘Ogenation and dehydration rates of HPA. Increasing the amount of the metal catalyst
increases the hydrogenation rate. Therefore, the selectivity to 1,3-PD should increase as
more catalyst is used. This proves to be true. When the mole ratio of Ni catalyst to
subStl’ate is increased from 1:1 in the frrst experiment to 2:1 in the second experiment we
Carried out, it is observed that the mole ratio of 1,3-PD to n-propanol in the product
inc“? ases from 2.3:1 in the first experiment to 4:1 in the second experiment In the second
exPetirnenr, 34 mg 1,3-PD has been obtained from 210 mg 1711). The 1,3-PD yield is

72%. The n—propanol yield is calculated as 18%. No further experiments are conducted to

100

maximize the 1,3-PD yield. Nevertheless, the above experimental results show that the
new procedure is promising in carrying out the conversion of PTD to 1,3-PD.

In addition to the 1,3-PD yield, the catalytic nature of TPD hydrogenolysis consists
of another major concern of the current research. The stoichiometrical reaction between
transition metal and tosylate has been previously reported, in spite of the fact that in the
reported reaction it is the O-S bond, instead of the CO bond as in TPD hydrogenolysis, '
that is broken. Therefore, in the current work, the Ni on kieselguhr catalyst used in the
second TPD hydrogenolysis experiment is collected after the reaction, and is reused in
hydrogenolysis of TCH. It is found that the catalyst is still active and a turnover number

of about 50 has been achieved in 24 hours, at 160 “C and 3 MPa H2 pressure. At this

point, there should be no doubt that the hydrogenolysis of TPD in the presence of Ni on

kieselguhr is a catalytic reaction in its nature.

8.4 Design Concept of Industrial Conversion Process

As the goal of this research, the technical feasibility of the new glycerol
dehydroxylation concept presented in Figure 6-2 is verified on the laboratory scale by the
experiments just discussed. On the industrial scale, processes as shown in Figures 8-3 to
8-6 may be designed to produce 1,3-propanediol from glycerol.

The acetalization process shown in Figure 8-3 features a continuous acetalization
reactor, a continuous forced-circulation HPD crystallizer and a continuous vacuum filter.
The reactor is equipped with a water trap of the same principle as the Dean-Stark trap used
in our bench-top experiments. The purpose is to remove the water formed in the reaction.

The design of this process has incorporated the following observations made in our

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acetalization experiments:

a) acetalization of glycerol with benzaldehyde occurs readily in the presence of acid

catalyst, and can be driven to completion by using a Dean-Stark trap to remove the

water formed in the reaction.

b) the desired 1,2-product HPD can be readily purified from the product mixture by

crystallizing it from benzene-ligroin mix. The unreacted glycerol and benzaldehyde

and the undesired 1,3-product HMPD will stay in the solution.

c) acetalization of glycerol with benzaldehyde can be carried out in benzene-ligroin

mix, as well as benzene only, thus allowing direct crystallization of HPD from the

reaction solution and direct recycling of the unreacted glycerol and benzaldehyde and

the undesired HMPD back to the reactor, if benzene-ligroin is used as the solvent.

According to the design, the HPD rich product will be directly fed into the
crystallizer from the reactor. The desired HPD is crystallized from the solution and then
separated from it by the filter. Owing to the nature of the selected crystallizer, some
solvent will be vaporized in the crystallizer and then condensed in the condenser equipped
with the crystallizer. This solvent will be collected and used to wash the HPD cake
formed on the filter surface. The filter cake discharged from the filter may need further
drying. The filtrate, composed of the solvent and the undesired HMPD and the unreacted
glycerol and benzaldehyde dissolved in it, is lean in HPD and is recycled back to the
reactor. The benzaldehyde used in acetalization will also be recycled, but from Step 3, the
detosyloxylation process.
Because of the recycling of HMPD, the process shown in Figure 8-3 produces no by-

product. Under acetalization condition, HMPD and HPD are readily inter-convertible. As

106

a result, the HMPD formed in the reaction can only accumulate to a certain level in the
reactor, in equilibrium with the HPD present. Once that level is reached, no HMPD will
be further produced in the reactor. HPD will be the only product of the reaction of
glycerol and benzaldehyde, as it is continuously removed from the reactor and process.
Therefore, with the process shown in Figure 8-3, one mole HPD will be expected from
each mole glycerol fed into the reactor, or the HPD yield will be 100% (or close to 100%
if possible loss in filter is considered), under stable conditions. High HPD yield is a major
advantage of the continuous process shown in Figure 8-3.
The tosylation process shown in Figure 8-4 is designed strictly based on our lab

tosylation procedure described in Chapter 7. It involves three major steps:

a) allowing HPD to reactor with TsCl in pyridine at low temperature, so that the

hydrochloride formed in the reaction can precipitate from the reaction solution as

pyridine-hydrochloride complex.

b) mixing the reaction product with large quantity of ice following Step a). The

purpose is to further lower the temperature and to force the tosylation product PTD to

crystallize from the product solution. and

c) separating PTD crystals by filtration
High PTD yield has been obtained with this procedure in lab experiments. As an
industrial process, however, this procedure has a major disadvantage: mixing reaction
product with ice in Step b) creates a difficulty to the recycling of pyridine.

At industrial scale, the pyridine solvent has to be recycled, to minimize the process

cost and waste discharge. After PTD removal, the pyridine is in a mixture with water in

Figure 8-4. To reuse pyridine in tosylation, it has to be first separated from water and then

107

thoroughly dried. Tosyl chloride is attacked by very small quantity of water present in
pyridine.

To overcome the above difficulty, an alternate process as shown in Figure 8—5 is
considered. This design eliminates ice from the process and attempts to crystallize PTD
directly from the reaction medium, thus allowing direct recycling of the medium, together
with the unreacted starting material in it. The reaction medium for this process may be
pure pyridine or a mixture of pyridine with another solvent, ligroin for example, which
helps lower the solubility of PTD in the reaction medium. Pyridine, as a base, is needed in
the reaction medium to neutralize the hydrochloride released from the reaction. To
remove the pyridine-hydrochloride complex precipitated from the reaction medium, the
tosylation reactor in Figure 8-5 is incorporated with a circulation loop, which contains a
continuous in-line filter. The reaction product sent to the crystallizer is designed to be free
of hydrochloride. The crystallizer in Figure 8-5 is of evaporative type, so that the solvent
can be partially removed to give a PTD saturated solution in the crystallizer. The solvent
removed from the solution may be used to wash the PTD crystal in filtration step.

At the time being, the solubility data of PTD in pyridine or other relevant solvents are
not available yet. It is also left for speculation wether the tosylation reaction can be
carried out in a mix of pyridine with another solvent as smoothly as in pyridine alone. It is
clear that further experiments are needed to verify the feasibility of this process. However,
if successful, this process has great advantages over the one shown in Figure 8-4, in terms
of both processing cost and waste discharge. Like the acetalization process in Figure 8-3,
this process can potentially furnish a product yield of 100%, under ideal condition.

The detosyloxylation process as shown in Figure 8-6 is designed based on the

108

discovery made in this research: the detosyloxylation of PTD can be accomplished by a
hydrolysis (or methnolysis more accurately) reaction carried out in methanol at moderate
temperature, followed by a catalytic hydrogenolysis reaction carried out in dioxane. It
features a hydrolysis reactor, an extraction column to remove benzaldehyde from the
hydrolysis product solution, a distillation column to separate the extracted benzaldehyde
from the hexane extractant, a thin-film evaporator/dryer to remove the methanol solvent
from the hydrolysis product TPD, and a hydrogenolysis reactor to finally convert TPD to
1,3-PD. Separation of 1,3-PD from the solvent dioxane and other hydrogenolysis
products, including TsOH and propanol, is yet to be added to this process.

According to the design, both the methanol and the hexane solvents used in this
process are recycled. The benzaldehyde reclaimed in this process is also recycled, but
back to the Step 1 acetalization process.

Figure 8-7, obtained from combining and simplifying the processes in Figures 8-3,
8-5 and 8-6, illustrates the material flow in glycerol dehydroxylation. It is seen from this
diagram that the acetalization process, if designed as in Figure 8-3, produces no other
product but HPD, at 100% yield. Similarly, the tosylation process, if designed as in Figure
8-5, gives only PTD, at 100% yield. Therefore, the overall yield of the glycerol
dehydroxylation process is solely determined by the yield of the detosyloxylation process.
In our experiment, a 1,3-PD yield as high as 72% has been achieved in PTD
detosyloxylation, under un-optimized conditions. This yield can be taken as the overall
yield of the new glycerol dehydroxylation process. For comparison, the theoretical yield
of glycerol fermentation is 67 %.

From Figure 8-7, it is also seen that production of 1,3-PD is at the expense of tosyl

109

Glycerol, 1 mole

 

 

 

 

 

 

 

 

 

 

 

 

 

V
$ ACETALIZATION .———> H20
Yield=100%
HPD, 1 mole
V
TsOH, 1 mole—+ TOSYLATTON —> HCl
Yield=100%
PTD, 1 mole
,_ _ _ _ _ _ _ ._ _
l
HZO/MeOH———h HYDROLYSIS

 

 

 

 

 

I
|
1r TPD Yield = 72%

 

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I
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L__..___ _____ J

1,3-PD, 0.72 mole
Propanol
V TsOH

 

Figure 8-7. Material flow in glycerol dehydroxylation

110

chloride, gaseous hydrogen, as well as glycerol. Benzaldehyde is not consumed in the
process because of recycling. Assuming the overall yield of the process is 72%,
production of each pound of 1,3-PD will consume 1.68 lb glycerol, 3.61 lb tosyl chloride
and 4.12 mole hydrogen, which cost about $1.68, $5.60 and $0.12, respectively. The total
raw material cost will be $7.40 per pound of 1,3-PD, of which 76% results from tosyl
chloride. The above costs are calculated based on a unit price of $1.00 per pound for
glycerol, $1.55 per pound for tosyl chloride, and $0.03 per mole for hydrogen gas. These
prices are from the Chemical Market Report, Aceto Corp and AGA Gas, respectively.

It is seen from the above that the majority of the raw material cost of 1,3-PD
production results from tosyl chloride. The price of this chemical is fairly high on the
market, as a result of the small demand and thus small production of this chemical. As a
solution to reduce the raw material cost, on—site synthesis of tosyl chloride may be
considered. On-site synthesis of tosyl chloride may require a large scale glycerol
conversion process to justify it The glycerol conversion processes shown in Figures 8-
3, 8-5 and 8-6 are mostly continuous, and are therefore suitable for large scale production.

Another way to improve the economics of the glycerol dehydroxylation process is to
recover and sell the r-toluene sulfuric acid (TsOH) produced in this process. As can be
seen from Figure 8-7, the glycerol dehydroxylation process produces two by-products:
propanol and TsOH. Propanol competes the raw materials with 1,3-PD and is to be
minimized from the process; while TsOH is produced at the same time as TsCl is
consumed in the process, on the mole to mole basis. Spectrum Chemical MFG has quoted
a price of $5.87 per pound on TsOH. Selling of TsOH could basicly compensate the cost

of tosyl chloride. Then the net raw material cost for production of 1,3-propanediol from

11]

glycerol would be only $1.80 per pound of 1,3-propanediol, which is extremely favorable.

PART III

VAPOR-PHASE HF SACCHARIF ICAT ION

CHAPTER 9
MODELING THE HF ADSORTION PROCESS IN A
PACKED-BED REACTOR

HF vapor is a potent and selective regent for saccharification of lignocellulose. The
vapor-phase HF saccharification consists of two sequential operations: HF adsorption and
solvolysis (or simply HF adsorption as it is more commonly called) followed by HF
desorption. The current research focuses on the first operation, that is, HF adsorption.

Research on vapor-phase HF saccharification can be traced back to the early
experiments by German scientists (Fredenhagen and Cadenbach, 1933; Luers, 1938).
More recently, this process has been studied by Bentsen (1982), Ostrovski er al. (1984 and
1985), and Franz er al. (1982 and 1987) on pilot-scales. These research was aimed to
demonstrate the technical and economic promise of vapor-phase HF saccharification.
Because of its empirical and proprietary nature, however, these research provided very
limited data for process design and development.

The research at Michigan State University (MSU) was started in the late 1980’s,
aiming to provide the necessary fundamental chemical engineering data for process
development and design. Specifically, Mohring and Hawley (1989) studied the intrinsic
adsorption of HF on wood chips. Rorrer er al. (1988) and Rorrer (1989) studied the
intrinsic reaction of HF with lignocellulose. Reath (1989) observed the behavior of HF
adsorption on wood chips in a bench-scale, packed-bed reactor. These studies have

yielded a great deal of useful results, but are not yet complete; as is especially true to the

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research on HF adsorption in a packed-bed reactor. Questions of primary importance to
design and Operation of the HF adsorption process in a packed-bed reactor but not yet
explored before the current work include: a) under given operating conditions, how long
does the adsorption need to be carried out to achieve a certain overall HF loading level?
and b) how is the adsorbed HF distributed through the reactor?

The current work is a continuation of the research initiated by Hawley, Mohring,
Rorrer and Reath, and has focused on modeling the HF adsorption process on wood chips
in a packed-bed reactor. A predictive model relating HF loading to the adsorption time
and conditions is considered to be the best solution to the two questions raised above.

Several breakthroughs have been made in this work. Experimentally, the HF
adsorption behavior in the bench-scale, packed-bed reactor has been studied in a broader
temperature and HF supply rate ranges than in the previous work, which leads to the
finding that the rate of HF adsorption is controlled by two factors: the intrinsic adsorption
rate and the HF supply rate. These two rate-limiting mechanisms take control at different
phases of the adsorption process and leads to difierent behavior of HF adsorption. Based
on this finding, a conceptual model has been developed, which explains all the observed
behavior of HF adsorption in the bench-scale, packed-bed reactor. Based on this
conceptual model, a mathematical model has also been developed, which can be used to
calculate the overall HF loading as a function of the adsorption time. However, the
significance of this model is undermined, by its inability to provide information on the HF
loading distribution in the reactor.

Further work has led to a more advanced model of the HF adsorption process in the

packed-bed reactor. This model is developed based on a detailed analysis of the HF flow,

114

the intrinsic HF adsorption and the heat transfer processes occurring in the reactor during
HF adsorption. This advanced model provides a great deal of insight into the HF
adsorption process in the packed-bed reactor, and can be used to predict the HF loading
distribution, as well as the overall HF loading, as a function of the adsorption time and
operating conditions. This model has been validated by the experimental data from the
bench-scale, packed-bed reactor under study.

Based on this work, two papers have been written and been published in Chemical
Engineering Science (Wang et al., 1994; Wang et al., 1995a). These papers basically
document all the work that has been done in the current research on HF adsorption. To
avoid rewriting, only abstracts of these papers are included in this dissertation.

The first paper is entitled “Adsorption model of HF on Wood Chips in a Bench-
Scale, Packed-Bed Reactor”, representing the experimental work and the work on the
more primitive lumped HF adsorption model. the abstract of this paper is quoted below:

Abstract: The adsorption process of HF on wood chips in a packed-bed reactor was
studied. 7W0 adsorption rate limiting mechanisms exist for this process, namely HF
supply control and intrinsic adsorption control. In the initial period of this process, the
adsorption rate is determined by the HF supply rate; afler the initial period, the
adsorption rate is determined by the intrinsic adsorption rate. Consequently, the HF
loading profile for this adsorption process includes two distinct parts: an initial linear
part and following non-linear part, corresponding to the two difi’erent rate limiting
mechanism. This conceptual model of HF adsorption in the packed-bed reactor was
supported by experimental observations from a bench-scale. packed-bed reactor: Based
on this conceptual model, a mathematical model was developed to predict the HF loading
for a given adsorption time under given operating conditions. To demonstrate its
application, the mathematical model was used to analyze the efect of the heat dissipation
rate of the reactor on the adsorption process.

The second paper is entitled “Modeling the HF Adsorption Process on Wood Chips in a
Packed-Bed Reactor”, representing the work on the more advanced detailed HF adsorption

model. The abstract of this paper is also quoted below:

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Abstract: In this work, a detailed model, along with the numerical procedure to
solve this model, was developed for the HF adsorption process on wood chips in a packed-
bed reactor: This model is aimed to predict the HF loading distribution, as well as the
overall HF loading, with respect to the adsorption time. However; the temperature field in
the HF reactor may also be obtained using this model, as a function of time. To validate
the developed model, the computed overall HF loading profiles were compared with the
experimental data acquired previously from a bench-scale, packed-bed reactor filled with
big-tooth aspen wood chips, and a good agreement was obtained. By taking advantage of
the newly developed adsorption model, the HF loading distribution and temperature field
in the bench-scale, packed-bed reactor was examined It was found that a uniform HF
loading distribution and temperature field is always obtained at the end of the adsorption,
provided that the adsorption is carried out sufi'iciently long. This finding is significant,
since a uniform HF loading distribution is always desired in operation of the HF
adsorption process.

The above modeling work carried out in the current research provides a theoretical

foundation to design and control of the HF adsorption process in the packed-bed reactor.

CHAPTER 10
SUMMARY AND CONCLUSIONS

In the current research. three biomass conversion processes have been investigated,
namely sugar hydrogenolysis, glycerol dehydroxylation and HF saccharification, all
aiming to produce value-added, useful chemicals from the biomass resources. Results

from these investigations are summarized as follows.

10.1 Sugar Hydrogenolysis

The research on the sugar hydrogenolysis process has focused on understanding of
the reaction mechanism and improvement of the reaction selectivity. In the mechanism
study, a series of 1,3-diol model compounds have been hydrogenolyzed, to establish the
mechanism of C-C and CO bond cleavage in sugar hydrogenolysis. The experimental
work confirmed our theoretical conception that the C-C bond breaks via the retro-aldol
reaction of a B-hydroxyl carbonyl precusor and the CO bond breaks via the dehydration
of the same carbonyl precusor. Establishment of the above mechanism makes it possible
for one to adopt a more rational approach than the current practice in control of the
selectivity of sugar hydrogenolysis.

The selectivity study in the current research is also canied out with a 1,3-diol model
compound, namely 2,4-pentanediol(2,4-PD). Focus of this selectivity study has been

placed on understanding of the factors controlling the CC v.s C-O selectivity. In this

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117

capacity, the effects of temperature, base concentration, hydrogen pressure, catalyst
amount and catalyst type on the selectivity are experimentally investigated. Additionally,
a mathematical model is also developed and the 2,4-PD hydrogenolysis process is
simulated with the developed model, to rationalize the experimental results and to look
into ways to improve the selectivity. It is found out that the temperature has little effect on
the selectivity; increasing the base concentration can increase the selectivity to certain
extent; increase in the amount of catalysts used has an adverse effect on the selectivity;
and the selectivity varies widely with the type of catalysts used. The effect of hydrogen
pressure on the selectivity is the most interesting. At low pressure, the selectivity
increases with the pressure; at medium pressure, the selectivity is not sensitive to the
change in the pressure; while at high pressure, the selectivity decreases as the pressure is
increased. 0f the seven different types of catalysts that has been studied in our
experiments, the best one is barium—promoted copper chromite, which has furnished a C-
C vs. 00 selectivity of 2.5 in hydrogenolysis of 2,4—PD. Our simulation study
additionally points out that, to improved the selectivity, a catalyst with a reasonable
capability to hydrogenate ketones and aldehydes but only moderate ability to hydrogenate

cab-unsaturated ketones should be sought.

10.2 Glycerol Dehydroxylation

The purpose of glycerol dehydroxylation is to convert glycerol to 1,3-propanediol, a
very-high-valued chemical. In this research, a new concept has been presented to
selectively transform the second hydroxyl group of glycerol into a tosyloxy group with the

help of a group protection technique, and then to remove it by hydrogenolysis. To verify

118

the technical feasibility of this new concept, experiments are carried out in this work to
investigate each of the steps involved in the new conversion concept. The results are
promising: an overall yield of 72% or higher is found to be achievable with this new

conversion concept.

Another major accomplishment of this research is that, for the first time, tosylates are
hydrogenolyzed catalytically with molecular hydrogen. The standard method to
hydrogenolyze tosylates at the current time is to use base metal hydrides, such as lithium

aluminum hydride (LiAlH4) and lithium triethylborohydride (Lil-IBEt3). It is discovered

in the current research that the tosyloxy group of a tosylate, when assisted by a hydroxy
group next to it, can be removed with molecular hydrogen, in the presence of a nickel or
ruthenium catalyst. This discovery is vital to the new concept of glycerol conversion. It
makes it possible to detosyloxylate TPD economically with gaseous hydrogen.
Otherwise, hydrogenolysis of TPD has to be canied out with lithium aluminum hydride or

lithium triethylborohydride. Both are too expensive to use at industrial scale.

10.3 HF Saccharification

The work on HF saccharification has focused on modeling of the HF adsorption
process in a packed-bed reactor. No models have been developed. The first one is
phenomenological in nature, and is developed based on experimental observations of the
HF adsorption behavior in a bench-scale, packed-bed reactor. The second model is more
advanced than the first one. It is developed based on a detailed analysis of the HF flow,

the intrinsic HF adsorption and the heat transfer processes occurring in the reactor during

119

HF adsorption. This model provides a great deal of insight into the HF adsorption process
in a packed-bed reactor, and it can be used to predict the HF loading distribution, as well
as the overall HF loading, as functions of the adsorption time and operating conditions.
The above modeling work accomplished in this research provides a theoretical foundation

to design and operation of the HF adsorption process in a packed-bed reactor.

CHAPTER 11
RECOMMENDATIONS ON FUTURE WORK

11.1 Future Work On Sugar Hydrogenolysis

Future work on sugar hydrogenolysis should continue focusing on control of the C-
O bond cleavage. The selectivity study may continue being carried out with the 2,4-
pentanediol model (2,4—PD) compound (ideally 2,3,4-pentanetriol though, because of its
closer analogy to sugars). Analysis of both the products and data are simple with this
model compound, and the study may also be assisted by computer simulation, since a
process model is now available for this purpose and has proven to be useful.

In our simulation study, the process model has predicted some phenomena that have
not been observed in the experiment, possibly because the limited range of reaction
conditions within which our experiments have been conducted. Included in these
predictions are that the selectivity will become independent of the base concentration
when the concentration gets too high, and that the selectivity will decrease with hydrogen
pressure when the pressure gets too high. In further work, experiments may be performed
in a larger base concentration and hydrogen pressure ranges to verify the above
predictions.

Changing the catalyst is the most efiective way to change the selectivity. The
simulation study suggests that catalysts with strong ability to hydrogenate saturated

ketones and aldehydes but only moderate ability to hydrogenate or,0unsaturated carbonyls

120

121

have potential to deliver high selectivity. Some homogeneous rhodium catalysts with the
above properties have been reported by Mesuoni er al. (1978). These catalysts may be
tested in 2,4-PD hydrogenolysis experiments, to see if they can provide improved
selectivity.

The most promising catalyst found so far in our experiments is barium-promoted
copper chromite. It provides a much better selectivity than other catalysts tested,
including the plain copper chromite catalyst. These results clearly indicate that the
selectivity of copper chromite is enhanced by the presence of barium. Therefore, future
work may also be canied out to study the effect of addition of barium, as well as other
elements, in preparation of the catalyst on the selectivity of 2,4-PD hydrogenolysis.

Finally and most importantly, the results from studying the model compound 2,4-
propanediol (2,4-PD) should be applied to sugars in the future, so as to improve the
selectivity of sugar hydrogenolysis. The optimal conditions for sugar hydrogenolysis may
be different from those for 2,4-PD hydrogenolysis. However, the effects of operating
conditions on the former process should parallel to those on the later. This is yet to be

verified in the future research.

11.2 Future Work on Glycerol Dehydroxylation

Future work on glycerol dehydroxylation has two immediate goals: a) to modify the
procedure of tosylation, so as to facilitate the recycling of solvent; and b) to optimize the
detosyloxylation condition, so as to increase the yield of 1,3-PD.

The standard lab procedure used in this research furnishes high PTD yield.

However, introduction of ice following the reaction makes it difficult to recycle the

 

122

solvent, as the solvent has to be separated from water and thoroughly dried before it can be
retumed back to the reactor. Solvent recycling is extremely important at industrial scale.
A better process would allow both the reaction and crystallization to be carried out in the
same solvent, so that the solvent can be directly recycled as in Figure 8-5. Therefore,
future work should check the feasibility to directly crystallize PTD from pyridine. If
direct crystallization of PTD from pyridine proves to be difficult, a co-solvent may be
found, to help the precipitation of PTD from pyridine. If a co-solvent is needed to
facilitate the crystallization, it ought to be verified that the tosylation can proceed in a
mixture Of this co-solvent with pyridine as smoothly as in pyridine alone. In both
experiments, it may become necessary to proceed the crystallization with a pre-
evaporation step to remove a portion of the solvent from the PTD solution.

As discussed in Chapter 8, the overall yield of the glycerol dehydroxylation process
is determined by the yield of the detosyloxylation step. Although a 1,3-PD yield as high
as 72% has been obtained in the current work, it is not optimized against the reaction
conditions. Therefore, future work is needed to study the effect of reaction conditions,
including temperature, medium acidity, and catalyst, on the product yield.

The 1,3-PD yield of the detosyloxylation step is mainly affected by the formation of
by-product propanol in TPD hydrogenolysis. Propanol competes with 1,3-PD for starting
TPD. The ratio of 1,3-PD to propanol produced in this reaction is determined by the rate
of 3—hydropropionaldehyde hydrogenation relative to the 3-hydropropionaldehyde
dehydration (Figure 8-2). Any factors affecting this rate has potential to be optimized to
improve the yield of 1,3-PD.

As a general rule, hydrogenation has a higher activation energy than dehydration.

 

123

improve the yield of 1,3—PD.

As a general rule, hydrogenation has a higher activation energy than dehydration.
Therefore, increase in temperature should increase the rate of hydrogenation relative to
dehydration, and thus increase the yield of 1,3-PD in TPD hydrogenolysis. In the current
work, TPD hydrogenolysis is canied out at a relatively low temperature (140 °C), due to
the concern that the 1,3—PD formed in this reaction might be further hydrogenolyzed. In
retrospection, this concern may not be well founded, as our experiments on 1,3-diol
hydrogenolysis has shown that hydrogenolysis of 1,3-PD is slow even at 210 °C. Hence,
the effect of temperature on TPD hydrogenolysis should be investigated in future research,
so that the reaction can be carried out at an optimal temperature to maximize the yield of
1,3-PD.

In addtion to 1,3-PD and n-propanol, TPD hydrogenolysis also generates p-toluene
sulfonic acid (TsOH). In the current work, the TsOH generated during TPD
hydrogenolysis is not neutralized. Since dehydration of HPA is catalyzed by acids,
neutralization of the reaction medium during the reaction may be helpful to control the
formation of by-product n-propanol. This measure to improve 1,3-PD yield should also be
investigated in the future research.

Future research should also investigate the effect of catalyst on TPD hydrogenolysis.
A good catalyst enhances the hydrogenation of 3-hydropropionaldehyde, and thus may
improve the yield of 1,3-PD.

In addition to the 1,3-PD yield, the possibility of catalyst poisoning by the sulfur
containing compounds in TPD hydrogenolysis also needs to be addressed. Although it has

been found in our experiments that the Ni catalyst used in TPD hydrogenolysis is still

 

124

was stopped only after the catalyst completed a turnover number of about 50, to
demonstrate the catalytic nature of the reaction. Further work should furnish, and improve
if necessary, the overall turnover number that a catalyst can achieve in its lifetime.
Theoretically, mesyl chloride may work just as well as tosyl chloride in the new
concept of glycerol conversion. Future work may look into the feasibility of using this
alternative compound. Because of its smaller molecular weight, mesyl chloride is about
40% cheaper than tosyl chloride per mole, although the prices of these two compounds are

similar on per pound basis. This prices difference provides a material cost advantage.

11.3 Future Work on HF Saccharification

Focus of the future work on HF saccharification should shift from the HF adsorption
process to the HF desorption process. Due to the low price of sugars and the relatively
high price of hydrogen fluoride, recovery and reuse of this regent is crucial to the
economical viability of the HF saccharification process.

Although not reported here, some work has been done on HF desorption in the
current research. It was found that, under some conditions, the desorption process was
extremely slow, and it took more than 30 hours for the process to complete. Under these
conditions, the HF residue level was normally observed to be high and it ranged from 5 to
10% of the HF loaded onto the wood chip substrate. However, some desorption runs were
also completed in a time less than one hour, with an HF residue level less than 0.5%.
Further work is needed to investigate the reason causing the above difference. It may
possibly be linked to the temperature history experienced by the substrate in the HF

adsorption process.

APPENDIXES

APPENDIX A

COMPUTER PROGRAMS F OR SIMULATION STUDY

APPENDIX A
COMPUTER PROGRAMS FOR SIMULATION STUDY

This appendix documents the computer programs used to solve the process model of
2,4-PD hydrogenolysis. These programs are wrinen in Matlab language, and must be run
in Matlab environment. The function of each program in solving the problem are

explained in the comments included in the program.

Main Program:

% Program SIMUM (Script M file)

% Version 4.0, June 1, 1997

% This program is used to simulate the hydrogenolysis reaction of 2,4-pentanediol.
% It solves the differential equation system as defined in function M file CPRIMEM,
% and displays and saves the results.

% t =-- time (not used); c --= concentrations; y = rates of concentration change
% Units: time = min; concentration = mol/l; flux and rate = monin

% Table of reactant and products:

% 1. A, 2,4»pentanediol

% 2. B, 4-hydroxyl-2-pentanone
% 3. C, acetone

% 4. D, 2—propanol

% 5.13, acetaldehyde

% 6. F, ethanol

% 7. G, 3-pentene-2-one

% 8.1, 2-pentanone

% 9. J, 2-pentanol

% 10. M, metal catalyst

% 11. MH, metal hydride

% 12. OH, hydroxide ion

% Additional species: H, molecular hydrogen; H20, water.

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126
%%% Main Body of Program

% Load Parameter (Including initial conditions)
clear; clear global %clean work space
para %sub-program containing rate constants and initial conditions

% Initial Concentrations:
c0 = [A, B, C, D, E, F, G, I, J, M, MH, OH]’; % A, B provided by Program PARAM

% Reaction time

ti = 0;

tf = 210; % tf-ti == duration of reaction
% Solve the Differential Equation System

[t, c] = ode45(‘cprime’, ti, tf, c0, 1e-6, l);

% Save the Solution in file: rsl2\sol and “Reaction Conditions” in rsl\condi, if n=2.
l=1ength(t); sol = [301; [c(1:10:l,:),t(1:10:l,:)]];
dir = [‘rsl’ int23tr(n)];
eval([‘ !mkdir ‘ dir])
eval([‘!copy para.m ‘ dir ‘/condi’])
eval([‘save ‘ dir ‘Isol sol -ascii’])

% Display Results

% Data preparation
A=sol(:,l); B=sol(:,2); C=sol(:,3); D=sol(:,4); B=sol(:,5); F=sol(:,6);
G=sol(:,7); I=sol(:,8); I=sol(:,9); M=sol(:,10); MH=sol(:,11); OH=sol(:,12);
t=sol(:,l3); l = 1ength(t);

% Main products
figure(l)
plot(t,A*1000, t,C*100 t,D*1000,’-.’, t,l*1000,’:’, t,J*1000)
axis([0 250 0 120])
title(‘Major Products’)
xlabel(‘T1me (min)’)
ylabel(‘[Al. [C]. [D]. [I]. [1] (mM)’)
grid

% Minor products
figure(2)
plot(t,B*1000, LE*1000,’--’, LG*1000,’-.’
axis([0 250 0 5]);
title(‘Minor Products’)
xlabel(‘T1rne (min)’)
ylabel(‘[B]. [E]. [(3] (mM)’)
grid

127

% Selectivity
figure(3)
S = (C+D) J (G+I+J);
P10t(t. S. ‘-’)
axis([0 250 0 5])
title(‘Selectivity’)
xlabel(‘Trme (min)’)
ylabeK‘ s = ( [C]+[Dl )/ ( [G]+[I]+[Jl )’)
grid

%Base concentration
figure(4)
plot(t, OH*1000)
title(‘Base Concentration’)
xlabel(‘Trme (min)’)
ylabel(‘[OH] (mM)’)
grid
eval([ ‘print -deps ‘ dir ‘/base.eps’]) % save the figure

%%% End End End End End
Sub-Program #1:

% Program PARAM (Scrip M file)
% Version 4.0, June 1, 1997

% This program is used to simulate the hydrogenolysis reaction of 2,4-pentanediol.

% It defines the rate constants and initial conditions

% t == time (not used); c = concentrations; y --= rates of concentration change

% Units: time = min; concentration = mol/l; flux and rate = monin
% Table of reactant and products:

% 1. A, 2,4-pentanediol

% 2. B, 4-hydroxyl-2-pentanone
% 3. C, acetone

% 4. D, 2-propanol

% 5. E, acetaldehyde

% 6. F, ethanol

% 7. G, 3-pentene-2-one

% 8.1, 2-pentanone

% 9. J, 2-pentanol

% 10. M, metal catalyst

% 11. MH, metal hydride

% 12. OH, hydroxide ion

% Additional species: H, molecular hydrogen; H20, water.

128

% Kinetic Parameters

klf: 435; klb = 67.4;
k2f = 752; k2b = 4480;
k3f= 1.89; k3b = 12.2;
k4f = 11.9; k4b = 1.24;
k5f: 14100; 161) = 6590;
k6f = 22.1;

k7f: 1.16; k7b = 7.48;
k8f = 76.2; k8b = 3.32e-5;

% Reaction Conditions:

% Water Concentration
H20 = 46.5; % M (not used)

% Hydrogen pressure (base: 5 MPa)
H = 0.1; % M, expressed as the concentration of molecular hydrogen
% dissolved in the reaction solution.

% Initial Base Concentration (0.4 m1 l N NaOH)
0H0 = 0.010; % M

% Initial catalyst concentration (base: 0.1 g Ni/K)
M0 = 0.028; % M

% Initial 2,4-Pentanediol Concentration (base 0.5 g diol in 40 ml water)
A0 = 0.12; % M

% Pass data to CPRIMEM
global klf klb k2f k2b k3f k3b k4f k4b
global k5f k5b k6f k7f k7b k8f k8b
global H20 H 0H0 M0

% Conditions to start “STARTM”
A=A0; B=0; C=0; D=0; E=0; F=0; G=0; I=0; J=0; OH=OH0; M=M0; MH=0;

% Define the number of the directory to save the results
n = l;
%%% End End End End End

129
Sub-Program:
function y = cprime(t, c)

% Program CPRIMEM (function M file)

% Version 4.0, June 1, 1997

% This program defines the problem of 2.4-pentanediol hydrogenolysis
% It is to be used with program SIMU.

% t =2 time (not used); c == concentrations; y = rates of concentration change
% X’s == reaction fluxes
% Units: time = min; concentration = mol/l; flux and rate = monin

% Table of reactant and products:

% 1. A, 2,4-pentanediol

% 2. B, 4-hydroxyl-2-pentanone
% 3. C, acetone

% 4. D, 2-propanol

% 5. E, acetaldehyde

% 6. F, ethanol

% 7. G, 3-pentene-2-one

% 8. I, 2-pentanone

% 9. J, 2-pentanol

% 10. M, metal catalyst

% 11. MH, metal hydride

% 12. OH, hydroxide ion

% Additional species: H, molecular hydrogen; H20, water.

%Get Data from PARA.M

global klf klb k2f k2b k3f k3b k4f k4b\
global k5f k5b k6f k7f k7b k8f k8b
global H H20

% Flux Expressions
X1 = klf*c(l)*c(10)*c(12) - k1b*c(2)*c(ll);
X2 = k2f*c(2)*c(12) - k2b*c(3)*c(5)*C(12);
X3 = k3f*c(3)*c(11) - k3b*c(4)*c(10)*c(12);
X4 = k4f*c(5)*c(1 1) - k4b*c(5)*c(10)*c(12);
X5 = k5f*c(2)*c(12) - k5b*c(7)*c(12);
X6 = k6f*c(7)*c(l 1);
X7 = k7f*c(8)*c(1 l) - k7b*c(9)*c(10)*c(12);
X8 = k8f*H*c(10)*c(12) - k8b*c(l 1);

130

% Differential Equations

y(1) = -Xl; % A
y(2)=X1-X2-X5; %B
y(3) = X2 - X3; % C
y(4) = X3; % D
y(5) = X2 - X4; % E
y(6) = X4; % F
y(7)=X5-X6; %G
y(8) = X6 - X7; % I

y(9) = X7; % J

y(10)=-Xl +X3+X4+X6+X7-X8; % M
you = -y(10); %MH
y(12>= y(10); %OH

% End End End End End

APPENDIX B

RE T RO-ALDOL AND DEHYDRA T ION RA TE

CONSTANT DERI VA T I ON PROCEDURE

 

APPENDIX B
RETRO-ALDOL AND DEHYDRATION RATE CONSTANT
DERIVATION PROCEDURE

This appendix documents the procedure used to derive the rate constants for the
retro-aldol and dehydration reactions of 4-hydroxy-2-pentanone, or the Reactions No.2
and No.5 in Figure 5-5. The basis of this derivation is the kinetic data obtained by Jenson
and Hassan (1976) on the hydration of 3-penten-2-one.

In Jenson and Carre’s experiments, 3-penten-2-one was allowed to undergo the

following reactions at 40 and 50 °C in 0.1 N NaOH aqueous solution:

0 OH 0
II km II km
CH3CH=CHCCH3 kt: CH3CHCH2CCH3 —> CH3CHO + CH2COCH3
dehy
3-penten-2-one 4-hydroxyl-2-pentanone acetaldehyde and acetone

The hydration rate constant khyd was determined at 40 and 50 °C, and was presented along
with an observed rate constant hobs. This observed rate constant is related to khyd, kdehy

and kmm as follows:

kh ydk retro
kdehyd + kretro

 

k (D-l)

abs =

Thus, if the reaction equilibrium constant for the hydration/dehydration reaction pair, or

the ratio of khyd to kdeh), is obtained, the dehydration rate constant kdehy may be calculated

131

 

Tr) '.. .
‘r

 

132

from khyd, and the retro-aldol rate constant kmm can then be solved from Eq.(D-l), which

is exactly what has been in this work. The relevant data and results are all given in Table
D-l. The equilibrium constants in Table D-l are from Jenson and Carre (1974), and are

measured in 2.57 M HClO4 solution.

Table D-l Rate Constants Directly from Jenson and Hassan (1976) a

 

 

4b 4b d 4d
40 °C .50 9.5 3.14 3.03 0.168
50 °C 1.1 21 2.67 7.87 0.435

 

a. unit for all rate constants: s"; b. from J enson and Hassan (1976); c. from Jenson and Carre (1974);
d calculated in this work.

The rate constants listed in Table D-l are the products of those in Figure 5-3 for the
same reactions with the concentration of hydroxide ion in the solution. As the hydroxide
ion concentration at which the above rate constants are obtained are known (0.1 M), the
rate constants k5, k_5 and k2 in Figure 5-3 can be determined from the rate constants in
Table D2, and then from these rate constants at low temperatures, the rate constants for
220 °C can be extrapolated with the help of the Arrhenius equation. To obtain the rate
constant k,2, the equilibrium constant for the retro-aldol and aldol reaction pair is needed.
In the current work, it is estimated from the free energy data at 25 “C (again data at other
temperature are unavailable) in' Cuthrie (1977) to be 0.168. With this equilibrium

constant, k_2 is calculated, and is documented in Table D-2, together with other determined

rate constants.

 

133

Table D-2 Determined Rate Constants

 

k5 (M‘lmin'l) k_5 (M'lmin'l) k2 (M'lmin'l) k_2 (M'zmin'l)

 

40 °C 0.182 0.570 0.0101
50 °C 0.472 1.260 0.0261
220 °C 14100 6590 752 4480

 

rd

 

 

APPENDIX C
EXPERIMENTAL DATA OF 2,4-PD

H YDR OGEN OL YSI S

APPENDIX C
EXPERIMENTAL DATA OF 2,4-PD HYDROGENOLYSIS

This appendix documents the experimental data of 2,4-pentanediol (2,4—PD)
hydrogenolysis, collected in the selectivity of sugar hydrogenolysis. In this study, 2,4-PD

is used as a model compound to facilitate the rersearch.

134

 

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