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dissertation entitled

Aqueous-Phase Catalytic Hydrogenation of Organic Acids and
Their Mixtures

presented by

Yuqing Chen

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Aqueous-Phase Catalytic Hydrogenation of
Organic Acids and Their Mixtures

By

Yuqing Chen

A DISSERTATION
Submitted to
Michigan State University

in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY
Department of Chemical Engineering and Materials Science

2007

ABSTRACT

Aqueous-Phase Catalytic Hydrogenation of Organic Acids and Their Mixtures
By
Yuqing Chen

Biomass-based organic acids are attractive feedstocks for chemicals production
because they are available in large quantities and can undergo a variety of reactions to
useful products. Hydrogenation of organic acids yields value-added alcohol products that
are important building blocks for pharmaceuticals, foods, agricultural chemicals, and
polymers. Carrying out hydrogenation in water over heterogeneous metal catalysts
eliminates waste generation associated with traditional hydride reagents, facilitates viable
reaction rates at mild conditions, allows easy catalyst separation and reuse, and avoids
organic solvents. Further, the mild aqueous environment allows transfer of chirality
present in bio-derived organic acids to their alcohol products, giving high-value, optically
pure materials in a single step. Development of low-cost, high-efficiency hydrogenation
routes could open economically viable pathways from renewable resource-derived
materials as alternatives to today’s petroleum-based chemicals.

We investigated the aqueous-phase hydrogenation of lactic acid and propionic
acid and their mixtures, as well as mixtures of the acids with their alcohol products
propylene glycol and l-propanol, over Ru/C and Ru sponge catalysts in a three-phase
stirred batch reactor. The goals of the work are to examine the acids' relative
hydrogenation reactivities, the substrates' and products' relative affinities for metal

surface sites, and how one substrate's conversion is influenced by other acid or alcohol

species in solution. By relating adsorption affinity and reactivity to substrate structure
and feedstock composition, we aim to provide a rational basis for design of aqueous-
phase heterogeneous hydrogenation catalysts and processes.

For the study of acid hydrogenation over Ru/C catalyst, kinetic data were
collected for reactions at 343-423 K, 3.4-10.3 MPa hydrogen pressure and 0.05-5 M acid
feed concentrations. Mass-transfer analysis showed that acid conversion rates were not
limited by mass-transport resistances over the reaction conditions studied. A two-site
Langmuir-Hinshelwood (L-H) kinetic model with a single set of rate and adsorption
constants fits the conversion kinetics of both individual and mixed acid hydrogenations
over a wide range of condition. Competitive adsorption of acids and their alcohol
products strongly affects hydrogenation rates.

Hydrogenation reactions over Ru sponge catalyst were conducted at 403 K, 3.4-
7.9 MPa hydrogen pressure and 0.1-1 M acid feed concentrations. The same two-site L-H
model with a new set of kinetic constants was used to characterize acid hydrogenations
over Ru sponge. Hydrogenation reactivity/selectivity and competitive adsorption of the
reacting species on Ru sponge are significantly different from that on Ru/C.

The activated carbon support material facilitates selective adsorption of acids or
alcohols into the carbon pore structure, which results in higher local concentrations in the
vicinity of the catalyst and thus influences the observed hydrogenation rates. Adsorption
of reactant and product species into the activated carbon support was studied for the
prediction of local pore concentrations. A global model that incorporates local pore
concentrations into the hydrogenation kinetics over Ru sponge was used to predict

hydrogenation rates of organic acids over Ru/C catalyst in water.

ACKNOWLEDGMENT

I would like to thank Dr. Dennis Miller for his guidance and support as my
research advisor through my Ph.D. studies. I would also like to thank Dr. James Jackson,
Dr. Lawrence Drzal, Dr. Milton Smith and Dr. Ramani Narayan for serving on my
graduate committee. Thanks to Miller-Jackson group members Lars Peereboom, Navin
Asthana, Yaoyan Xi, Ketan Pimparkar and Xi Hong for discussion and help during this

project.

iv

TABLE OF CONTENTS

List of Tables ....................................................................................... viii
List of Figures .......................... ' ............................................................... x
List of Schemes .................................................................................... xiv
List of Symbols and Abbreviations ............................................................... xv
Chapter 1. Background ........................................................................... l
l .1 . Introduction .................................................................................... 1
1.2. Research objectives ........................................................................... 1
1.3. Organic acid hydrogenation ................................................................. 2
1.3.1. Literature review ........................................................................ 4
1.3.2. Previous work on organic acid hydrogenation in our research group. . . . . . . . ....5
1.4. Competitive adsorption ...................................................................... 6
1.4.1. Traditional adsorption theory .......................................................... 6
1.4.2. Methods for aqueous-phase adsorption studies ..................................... 7
1.4.3. Relevant results from literature and the previous work in our research
group ............................................................................................ 10
1.5. Kinetic model for catalytic reaction ...................................................... 11
1.5.1. Langmuir-Hinshelwood (L-H) kinetic model ...................................... 11
1.5.2. Relevant results from literature and the previous work in our research
group ............................................................................................ 13
1.6. References .................................................................................... 14
Chapter 2. Experimental Methods ............................................................ 16
2.1. Hydrogenation of organic acids over carbon-supported and unsupported ruthenium
catalysts ............................................................................................ 16
2.1.1. Materials ................................................................................ 16
2.1.2. Reactor system ......................................................................... 17
2.1.3. Operating procedure ................................................................... 18
2.1.4. Analytical ............................................................................... 19
2.2. Catalyst characterization ................................................................... 20
2.2.1. Physisorption ........................................................................... 21
2.2.2. Chemisorption .......................................................................... 21
2.2.3. Pore volume and pore size measurement ........................................... 22
2.3. Liquid phase adsorption studies ........................................................... 23
2.3.1. Adsorption onto activated carbon support .......................................... 23
2.3.2. Adsorption onto Ru/C catalyst ....................................................... 23
2.3.3. Adsorption onto Ru sponge .......................................................... 24
2.4. References .................................................................................... 26

Chapter 3. Aqueous-Phase Hydrogenation of Organic Acids and Their Mixtures

over Carbon-Supported Ruthenium Catalyst ................................................ 27
3.1. Introduction .................................................................................. 27
3.2. Hydrogenation of single lactic acid and propionic acid over Ru/C catalyst. . . . . ....30

3.2.1. Effect of temperature .................................................................. 30
3.2.2. Effect of hydrogen pressure .......................................................... 31
3.2.3. Effect of acid feed concentration .................................................... 33
3.2.4. Effect of catalyst loading .............................................................. 35
3.2.5. Degradation of the alcohol products of hydrogenation ........................... 35
3.2.6. Determination of reaction rate, reaction order and rate constant ................ 37
3.2.7. Activation energy calculation ........................................................ 42
3.3. Mixed acid/alcohol hydrogenation over Ru/C catalyst - Effect of competitive
adsorption .......................................................................................... 44
3.4. Uncertainty and error ....................................................................... 47
3.5. References .................................................................................... 50

Chapter 4. Aqueous-Phase Hydrogenation of Organic Acids and Their Mixtures

over Ruthenium Sponge .......................................................................... 51
4.1. Introduction .................................................................................. 51
4.2. Effect of microporous carbon support on hydrogenation reactivity and
selectivity .......................................................................................... 53
4.3. Hydrogenation of single lactic acid and propionic acid over Ru sponge. . . . . . . . ....56

4.3.1. Effect of catalyst loading ............................................................. 56
4.3.2. Effect of acid feed concentration ..................................................... 57
4.3.3. Effect of hydrogen pressure .......................................................... 59
4.3.4. Effect of catalyst reuse ................................................................ 59
4.4. Mixed acid/alcohol hydrogenation over Ru sponge - Effect of competitive
adsorption .......................................................................................... 64
4.5. References .................................................................................... 68

Chapter 5. Adsorption Study on Activated Carbon Support ............................ 69
5.1 . Introduction .................................................................................. 69
5.2. Carbon characterization .................................................................... 69
5.3. Isothermal adsorption ....................................................................... 69
5.4. Elevated temperature adsorption .......................................................... 71
5.5. Mixed-component adsorption ............................................................. 72
5.6. Octanol-water partition coefficient estimation .......................................... 73
5.7. References .................................................................................... 75

Chapter 6. Mass Transfer Analysis ............................................................ 76
6.1 . Introduction .................................................................................. 76
6.2. Catalyst suspension ......................................................................... 77
6.3. Gas-liquid mass transfer ................................................................... 78
6.4. Liquid-solid mass transfer .................................................................. 81
6.5. Comparison of reaction rates with mass transfer rates ................................. 83

vi

6.6. Intraparticle mass transfer .................................................................. 84

6.7. Results of mass transfer calculations ..................................................... 86
6.8. References .................................................................................... 88
Chapter 7. Kinetic Modeling ..................................................................... 89
7.1. Hydrogenation of organic acids over carbon-supported ruthenium catalyst” .. ..89
7.1.1. Introduction ............................................................................. 89
7.1.2. Hydrogenation of single acids over Ru/C catalyst ................................. 90
7.1..3 Hydrogenation of acid mixtures over Ru/C catalyst .............................. 98
7.1.4. Error analysis ......................................................................... 102
7.1.5. Summary .............................................................................. 104
7.2. Hydrogenation of organic acids over ruthenium sponge catalyst ................... 105
7.2.1. Introduction ........................................................................... 105

7.2.2. L-H model parameters for acid hydrogenation over Ru sponge catalyst. . ....107
7.2.3. Comparison of the model parameters for acid hydrogenation over Ru/C

and Ru sponge catalysts ..................................................................... 113

7.3. Global model for the hydrogenation of organic acids over Ru/C catalyst —
incorporating local pore concentrations into kinetic modeling ........................... 115
7.4. References .................................................................................. 121
Chapter 8. Conclusions and Future Work ................................................. 122

vii

LIST OF TABLES

Table 1.1. Comparison of physisorption and Chemisorption ................................... 6
Table 3.1. Summary of reaction conditions for the hydrogenation of lactic acid and

propionic acid over 5 wt % Ru/C catalyst ....................................................... 29
Table 3.2. Effect of temperature on the hydrogenation selectivity ........................... 31
Table 3.3. The solubility of hydrogen in water ................................................. 33
Table 3.4. Effect of hydrogen pressure on the hydrogenation selectivity ................... 33
Table 3.5. Degradation of 1,2-propanediol and l-propanol under the hydrogenation

conditions ............................................................................................. 36
Table 3.6. Degradation rates of l-propanol at different initial concentrations ............. 36

Table 3.7. Results of activation energy calculation for acid hydrogenation over 5 wt %
Ru/ carbon catalyst ................................................................................... 43

Table 4.1. Summary of reaction conditions for the hydrogenation of lactic acid and
propionic acid over Ru sponge .................................................................... 52

Table 4.2. Comparison of the acid hydrogenation rates over carbon-supported and un-

supported ruthenium catalysts ..................................................................... 54
Table 4.3. Comparison of the l-propanol selectivity over carbon-supported and un-
supported ruthenium catalysts ..................................................................... 56
Table 4.4. Characterization of the ruthenium sponge .......................................... 61

Table 4.5. Initial rates of acid hydrogenation over Ru sponges with different age. . . . . ...63

Table 4.6. Catalytic activity of Ru sponges with different age (the basis for rate data
normalization) ....................................................................................... 63

Table 5.1. Langmuir isotherm coefficients for LA, PA, PG and l-PrOH adsorption on the

# 3310 activated carbon ............................................................................ 71
Table 5.2. The logKow values of LA, PA, PG, and l-PrOH ................................. 74
Table 6.1. Results of mass transfer analysis ..................................................... 87
Table 7.1. Kinetic constants for single acid hydrogenation over Ru/C at 403 K ........... 94

viii

Table 7.2. Sensitivity analysis of the kinetic constants for single acid hydrogenation over

Ru/C at 403 K ....................................................................................... 97
Table 7.3. Average error between experimental and predicted rates over Ru/C at

403K ................................................................................................. 103
Table 7 .4. Kinetic constants for acid hydrogenation over Ru sponge at 403 K ........... 108

Table 7.5. Average error between experimental and predicted rates over Ru sponge at
403K ................................................................................................. 112

Table 7.6. Comparison of the kinetic constants for acid hydrogenation over Ru/C and Ru
sponge at 403 K .................................................................................... 114

Table 7.7. Kinetic constants for acid hydrogenation over Ru/C at 403 K (global
model) ............................................................................................... 117

Table 7.8. Average error between experimental and predicted rates over Ru/C at 403K
(global model) ...................................................................................... 120

ix

LIST OF FIGURES

Figure 1.1. ATR cell used for in situ adsorption measurements ............................... 9
Figure 2.1. Parr series 5000 multiple reactor system .......................................... 17
Figure 2.2. The reactor set-up for charging and sampling .................................... 19
Figure 2.3. R1 chromatograph of the typical liquid reaction species ........................ 20
Figure 2.4. Recirculating microreactor system... . .. ........................................... 25
Figure 3.1. Effect of temperature on lactic acid hydrogenation rate over Ru/C. . . . . . . . ....30
Figure 3.2. Effect of temperature on propionic acid hydrogenation rate over Ru/C ...... 31

Figure 3.3. Effect of hydrogen pressure on lactic acid hydrogenation rate over Ru/C. . ..32

Figure 3.4. Effect of hydrogen pressure on propionic acid hydrogenation rate over

Ru/C ................................................................................................... 32
Figure 3.5. Effect of initial acid concentration on lactic acid hydrogenation rate over
Ru/C ................................................................................................... 34
Figure 3.6. Effect of initial acid concentration on propionic acid hydrogenation rate over
Ru/C ................................................................................................... 34
Figure 3.7. Effect of catalyst loading on lactic acid hydrogenation rate over Ru/C ....... 35
Figure 3.8. The plot of concentrations vs. reaction time for the hydrogenation of lactic
acid over Ru/C at 423 K ............................................................................ 38
Figure 3.9. The plot of reaction rates vs. reaction time for the hydrogenation of lactic
acid over Ru/C at 423 K ............................................................................ 39
Figure 3.10. Determination of the reaction order and rate constant for the hydrogenation
of lactic acid over Ru/C at 423 K .................................................................. 39
Figure 3.1]. Determination of the rate constant for a first order reaction .................. 40
Figure 3.12. Concentration dependence of the initial hydrogenation rate .................. 41
Figure 3.13. The plot of concentrations vs. reaction time for the hydrogenation of 2M
lactic acid over Ru/C at 403 K ..................................................................... 41
Figure 3.14. Arrhenius plot for activation energy of hydrogenation ........................ 43

Figure 3.15. Effect of propionic acid on lactic acid hydrogenation rate over Ru/C ....... 45
Figure 3.16. Effect of lactic acid on propionic acid hydrogenation rate over Ru/C ....... 46

Figure 3.17. Effect of 1,2-pr0panediol and l-propanol on lactic acid hydrogenation rate
over Ru/C ............................................................................................. 46

Figure 3.18. Effect of 1,2-propanediol and 1-propanol on propionic acid hydrogenation
rate over Ru/C ....................................................................................... 47

Figure 3.19. Replicate experiments for lactic acid hydrogenation over Ru/C -
Experiments 23 and 50 .............................................................................. 48

Figure 3.20. Replicate experiments for lactic acid hydrogenation over Ru/C -
Experiments 15 and 38 .............................................................................. 48

Figure 3.21. Replicate experiments for lactic acid hydrogenation over Ru/ C -
Experiments 26 and 116 ............................................................................ 49

Figure 3.22. Replicate experiments for propionic acid hydrogenation over Ru/C -
Experiments 22 and 42 .............................................................................. 49

Figure 4.1. Temperature dependent adsorption of lactic acid and propionic acid on
activated carbon 3310 .............................................................................. 55

Figure 4.2. Effect of catalyst loading on lactic acid hydrogenation rate over Ru
sponge ................................................................................................ 57

Figure 4.3. Effect of initial acid concentration on propionic acid hydrogenation rate over
Ru sponge ............................................................................................. 58

Figure 4.4. Concentration dependence of the propionic acid hydrogenation rate over Ru
sponge ................................................................................................ 58

Figure 4.5. Effect of hydrogen pressure on acid hydrogenation rate over Ru sponge. . ...59

Figure 4.6. Change in the catalytic activity of Ru sponge for lactic acid
hydrogenation ....................................................................................... 61

Figure 4.7. Change in the catalytic activity of Ru sponge for propionic acid
hydrogenation ....................................................................................... 62

Figure 4.8. Effect of propionic acid on lactic acid hydrogenation rate over Ru
sponge ................................................................................................ 65

Figure 4.9. Effect of lactic acid on propionic acid hydrogenation rate over Ru
sponge ................................................................................................ 66

xi

Figure 4.10. Effect of 1,2-propanediol and l-propanol on lactic acid hydrogenation rate

over Ru sponge ...................................................................................... 66
Figure 4.11. Effect of 1,2-prcpanediol and l-propanol on propionic acid hydrogenation

over Ru sponge rate ................................................................................. 67
Figure 6.1. Mass transfer in a three-phase catalytic reaction ................................. 77

Figure 7 .1. Experimental and predicted rates for individual lactic acid hydrogenation
over Ru/C ............................................................................................. 95

Figure 7.2. Experimental and predicted rates for individual propionic acid hydrogenation
over Ru/C ............................................................................................. 95

Figure 7 .3. Experimental and predicted conversion rates for individual lactic acid
hydrogenation over Ru/C ........................................................................... 96

Figure 7.4. Experimental and predicted conversion rates for individual propionic acid
hydrogenation over Ru/C ........................................................................... 96

Figure 7.5. Experimental and predicted lactic acid rates in the hydrogenation of acid
mixtures over Ru/C ................................................................................ 100

Figure 7.6. Experimental and predicted propionic acid rates in the hydrogenation of acid
mixtures over Ru/C ................................................................................ 100

Figure 7.7 . Experimental and predicted lactic acid conversion rates in the hydrogenation
of acid mixtures over Ru/C ....................................................................... 101

Figure 7.8. Experimental and predicted propionic acid conversion rates in the
hydrogenation of acid mixtures over Ru/C ..................................................... 101

Figure 7 .9. Experimental and predicted rates for lactic acid hydrogenation over Ru
sponge (all data) .................................................................................... 109

Figure 7.10. Experimental and predicted rates for propionic acid hydrogenation over Ru
sponge (all data) .................................................................................... 109

Figure 7.1]. Experimental and predicted rates for individual lactic acid hydrogenation
over Ru sponge .................................................................................... 1 10

Figure 7.12. Experimental and predicted rates for individual propionic acid
hydrogenation over Ru sponge .................................................................. 110

Figure 7.13. Experimental and predicted lactic acid rates in the hydrogenation of acid
mixtures over (Ru sponge ......................................................................... 111

Figure 7.14. Experimental and predicted propionic acid rates in the hydrogenation of
acid mixtures over Ru sponge ................................................................... 111

xii

Figure 7.15. Experimental and predicted rates for individual lactic acid hydrogenation
over Ru/C (global model; use pore concentrations) .......................................... 118

Figure 7.16. Experimental and predicted rates for individual propionic acid
hydrogenation over Ru/C (global model; use pore concentrations) ........................ 118

Figure 7.17. Experimental and predicted lactic acid rates in the hydrogenation of acid
mixtures over Ru/C (global model; use pore concentrations) ............................... 119

Figure 7.18. Experimental and predicted propionic acid rates in the hydrogenation of
acid mixtures over Ru/C (global model; use pore concentrations) .......................... 119

xiii

LIST OF SCHEMES

Scheme 3.1. Catalytic hydrogenation of lactic acid and propionic acid ..................... 28

xiv

LIST OF SYMBOLS AND ABBREVIATIONS

English:

a = effective interfacial area per unit volume reaction fluid [1 / m]

ap = external area of catalyst particles per unit volume reaction fluid [1 / m]
CA = concentration of species A at time t

CA0 = initial concentration of species A

CA...“ = bulk liquid concentration of the acid [kmol / m3]

CAMS = catalyst sm'face concentration of the acid [kmol / m3]

CH; = gas-phase concentration of hydrogen [kmol / m3]

CH1], = bulk liquid concentration of hydrogen = aqueous-phase hydrogen solubility at the
hydrogen partial pressure of the experiment [kmol / m3]

CHMw = catalyst surface concentration of hydrogen [kmol / m3]

C, = liquid phase concentration at the catalyst surface [mol / L]

Cu = total catalyst site concentration for acid and alcohol adsorption [kmol / kg catalyst]
Cg = total catalyst site concentration for hydrogen adsorption [kmol / kg catalyst]

D = diffusion rate [m / 32]

D. = £20,. = effective diffusivity [in2 / 8]

DJ- = diffusivity of reactant j in the liquid phase estimated using the Wilke-Chang
equation

(I, = impeller diameter [m]

dp = particle diameter [m]

dR = reactor diameter [m]

E A“ = activation energy [J / mol]

ee = enantiomeric excess

XV

g = gravitation constant [9.8066 m / 82]

HA (7) = Henry’s Law constant for A in a specific solvent [atm / (mole A/mole solution)]
K Acid = adsorption constant for each acid [m3 / krnol]

K Alcohol = adsorption constant for each alcohol product [m3 / krnol]

K H2 = adsorption constant for hydrogen [1 / MPa]

Kow = octanol-water partition coefficient

k = reaction rate constant

k, = pre-exponential factor

ka = rate constant of adsorption process

kAa-d = composite rate constant for each acid [In3 / kg catalyst / MPa / sec]

k4 = rate constant of desorption process

k5,, = surface reaction rate constant for each acid [(kg catalyst)2 / l~rmol2 / sec]
kmAcidap = L-S mass transfer coefficient of the acid [1 / s]

kLgflzap = L-S mass transfer coefficient of hydrogen [1 / s]

L = characteristic length of the catalyst [m] = dp / 6 for spherical particles
LA = lactic acid

L-H kinetic model = Langmuir-Hinshelwcod kinetic model

M3 = molecular weight of solvent [g / mol]

N = number of adsorbed molecules [kmol / kg catalyst]

Nm = minimum stirring speed for complete catalyst suspension [rotations / sec]
P = bulk gas pressure [atm]

PA = partial pressure of species A in the gas phase [atm]

PA = propionic acid

PG = propylene glycol; 1,2-prcpanediol

l-PrOH = l-propanol

xvi

R = ideal gas constant [J / mol / K]

4,, = conversion rate of species A [kmol species A / m3 / s]
4,4,,“ = observed reaction rate of the acid [kmol / m3 / s]
-er = observed reaction rate of hydrogen [kmol / m3 / s]

S = total number of available sites [kmol / kg catalyst]

Sr = surface tension [N / m]

Sh = Sherwood number [dimensionless]

T = absolute temperature [K]

ug = superficial gas velocity [cm / s]

V,- = molar volume of j as liquid at its normal boiling point [cm3 / g mol]
VL = volume of the solution [cm3]

xA = mole fraction of species A in the liquid phase

y = mole fraction of species A in the gas phase

Greek:

8 = support porosity

77¢? = observable modulus

0 = fraction of surface coverage

pi, = liquid viscosity [kg / m's]

pp = particle density [kg / m3]

pL = liquid density [kg / m3]

db; = association parameter for solvent = 2.6 for water

w = catalyst loading [grams of catalyst / 100 grams of solution]

xvii

Chapter 1. Background

1.1. Introduction

Organic acids are attractive biorenewable intermediates because they can be
produced in large quantities via fermentation of carbohydrate monomers and can undergo
a variety of reactions to usefirl products.l Hydrogenation of organic acids yields alcohol
products that are important building blocks for pharmaceuticals, foods, agricultural
chemicals, and polymers.2 Carrying out hydrogenation in water, the “native” solvent for
fermentation-derived substrates, in the presence of heterogeneous supported metal
catalysts eliminates waste generation associated with traditional hydride reagents (e. g.
LiAll-I4), facilitates viable reaction rates at mild conditions, allows easy catalyst
separation and reuse, and avoids organic solvents. Further, the mild aqueous environment
allows transfer of chirality present in bio-derived organic acids to their alcohol products,
giving high-value, optically pure materials in a single step.3 The development of low-
cost, high-efficiency hydrogenation routes will open economically viable pathways from
renewable resource-derived materials as alternatives to today’s petroleum-based

chemicals.

1.2. Research objectives

Hydrogenation of organic carboxylic acids to alcohols has been extensively
investigated. Most of the previous research has focused on catalyst modifications, and
control of process variables as a means to improve the yield and selectivity to the desired

alcohols. However, some potentially important mechanistic details are far from

understood, such as competitive adsorption of species on the catalyst surface, and the
relationship between substrate-metal interactions and reactivity. Ignoring these
mechanistic details leads to poor assumptions when determining the correct model
describing the reaction system.

To improve our understanding of organic acid hydrogenation to alcohol products,
we investigate the hydrogenation of lactic acid (2-hydroxypropanoic acid, herein LA) and
propionic acid (propanoic acid, herein PA) alone, together, and in mixtures with their
alcohol products propylene glycol (1,2-propanediol, herein PG) and l-propanol (herein 1-
PrOH), over ruthenium metal catalysts in a three-phase stirred batch reactor. Such
competition studies illuminate the acids‘ relative hydrogenation reactivities, the
substrates' and products' relative affinities for metal surface sites, and how one substrate's
conversion is influenced (usually inhibited) by other acid or alcohol species in solution.
By relating adsorption affinity and reactivity to substrate structure and feedstock
composition, we aim to provide a rational basis for design of aqueous-phase

heterogeneous hydrogenation catalysts and processes.

1.3. Organic acid hydrogenation

Organic acids compose a major class of renewable-resource feedstock chemicals.
Some organic acids are traditionally produced via fermentation of biomass-derived
glucose, including citric acid, gluconic acid, itaconic acid, lactic acid, and acetic acid.
Fermentation technologies to several other organic acids are currently under
development, including routes to succinic acid, propionic acid, butyric acid, fumaric acid,

and poly-(3-hydroxybutyric) acid.1 In addition, amino acids are formed predominantly by

fermentation or enzymatic conversion. Because of their multiple functional groups and
high reactivity, organic acids undergo a variety of reactions to valuable products,
including esterification, condensation, dehydration, oxidation, and hydrogenation.

Hydrogenation is the addition of gaseous hydrogen to saturate a double bond (e. g.
C = O or C = C). Generally, hydrogenation of organic carboxylic acids to alcohols is not
easy, and vigorous conditions are needed for successful reduction on a synthetic scale.
Because molecular hydrogen is not reactive chemically at low temperature and it’s
solubility in aqueous phase is very low at low pressure, almost all hydrogenation
reactions need catalysts, elevated temperature and high pressure.

The most commonly used hydrogenation catalysts are metals, metal oxides and

some salts in solid state, such as Raney Ni, Nl/A1203, Pd/carbon, Ru/carbon, Ru/TiOz,

Ru/A1203, Rh203 and CuCrOx. To increase the surface area, and therefore the number of
active sites, solid catalysts are often finely dispersed on the surface of a supporting
material. Hydrogenation selectivity is directly influenced by the choice of catalyst and the
quantity of catalyst used. In spite of the hundreds of techniques available for studying
catalyst surfaces, it is much more difficult to study heterogeneous reactions than it is to
study reactions where all species are in solution. Also, many heterogeneous catalysts
produce a wide range of products and have no single mechanism of action. Because of
this, the exact mechanisms for many seemingly simple reactions catalyzed by metals are

still poorly understood.

 

"U

 

1.3.1. Literature review

Hydrogenation of organic acids and esters over metal catalysts has been the
subject of numerous studies over the last several decades. In 1934, Bowden and Adkins4
studied the hydrogenation of optically active esters and ketones to optically active
alcohols over copper-chromium oxide and nickel catalysts under 15—20 MPa H2 pressure
at 250°C. The paper of Adkins and Pavlic5 published in 1947 was about hydrogenation of
arrrino acid esters over Raney nickel in ethanol. By using high loadings of catalyst and
lower temperatures (ZS-75°C), they could obtain the corresponding amino alcohols at
increased yields and purities. In the 1950’s, Camahan and Ford6 used ruthenium based
catalysts (RuO2 and Ru/C) to directly hydrogenate organic acids such as acetic acid,
oxalic, adipic, succinic, and hydroxyacetic acid at low temperatures ( 94-192°C) and high
H2 pressures (over 50 MPa) with yields ranging from 47 % to 88 %. In Broadbent’s
paper,7 they described the hydrogenation of organic acids catalyzed by rhenium powder
prepared in situ at 137-286°C and 15-33 MPa H2 pressures, including formic, acetic,
propionic, butyric, capric, lauric, stearic, lactic, maleic, succinic, and glutaric acid.
Miroslav’s patent8 showed the hydrogenation of fluorine-containing alkyl, cycloalkyl,
and benzene carboxylic acids using solid rhodium or iridium as the catalyst. The patent
granted to Antons9 reported their study about hydrogenation of lactic and maleic acid
over ruthenium catalysts. At temperatures of 60-70°C and 10 MPa H2 pressure, they
produced optically active alcohols with enantiomeric excess over 90 %. Dumesiclo
investigated the vapor phase lactic acid hydrogenation over silica-supported copper. They
produced propylene glycol with 88 % yield and high selectivity at 200°C and a hydrogen

pressure of 0.7 MPa. Recently, Mac et a1” published their work on hydrogenation of

carboxylic acids catalyzed by magnesia-supported poly-y-aminopropylsiloxane-Ru
complex (MgO-NHz-Ru). This complex catalyst was able to catalyze the hydrogenation
of acetic acid, propionic acid, lactic acid and isobutyric acid to the corresponding

alcohols with 100 % yields in the presence of water at 240°C and 5 MPa H2.

1.3.2. Previous work on organic acid hydrogenation in our research group

Worldwide production of lactic acid (LA) has increased steadily since the early
19905, and is expected to rise still further. Propylene glycol (PG) is a valuable
commodity chemical that finds many uses in foods and consumer products. Lactic acid
hydrogenation to PG in aqueous solution was investigated by Zhang, Z. et al.2
Ruthenium on activated carbon was identified as an active and selective catalyst for this
reaction. Under optimal conditions (150°C and 10-14 MPa H2), over 90 % PG selectivity
with 95 % LA conversion was achieved in both a batch autoclave reactor and continuous
trickle bed reactor. At temperatures lower than 100°C, optically active S-PG is formed
from fermentation-derived L-LA with about 100 % ee. These initial studies demonstrated
higher sustained PG yields at milder reaction conditions than prior studies that used either
lactate esters or aqueous-phase free LA as the starting material.

Amino alcohols are important building blocks in agricultural, pharmaceutical, and
peptide chemistry, and are also utilized as chiral auxiliaries. Hydrogenation of L-alanine
(2-aminopropanoic acid) to the corresponding amino alcohol was studied by Jere, F. T. et
a1.3 At 100°C and 6.9 MPa of hydrogen, aqueous L-alanine undergoes facile
hydrogenation to L-alaninol over a 5 % Ru/C catalyst. In the presence of sufficient

phosphoric acid, protonated L-alaninol was formed with selectivity exceeding 95% and

enantiomeric excess (ee) over 99 %. The retention of stereochernistry at the amine-
bearing sp3 C site was the most unexpected finding in this work. Understanding the
configuration of alanine adsorbed on the catalyst, and thus the mode by which
stereochernistry is retained represents a major technical challenge associated with this

condensed-phase catalytic hydrogenation.

1.4. Competitive adsorption

1.4.1. Traditional adsorption theory

Adsorption is the binding of molecules or particles to a surface. It is usually an
exothermic process. Molecules can interact with a surface via a physical process
(physisorption) or a chemical process (Chemisorption). A comparison of physisorption
and Chemisorption is shown in Table 1.1.

Table 1.1. Comparison of physisorption and chemisorption12

 

 

 

 

 

 

 

 

Comparison Physisorption Chemisorption
Attractive ~ Van der Waals; strong chemical bonds formation;
forces electrostatic interactions involve sharing or transfer of electrons
Adsorption less than 80 kJ / mol 200 kJ / mol or more
energy
A dso tion takes place on all highly selective; occurs only between
1.? surfaces provided that T chemically active surface and certain
conditions
and P are favorable adsorbate
Adsorption . .
layer multr-layer smgle-layer
Reversibility reversible irreversible

 

 

 

In an adsorption process, molecules in the free gaseous state are in a dynamic
equilibrium with adsorbed molecules. The relationship between the quantity of molecules

adsorbed and the pressure of gas at a fixed temperature is called the adsorption isotherm.

The famous Langmuir isotherm” developed in 1916 was based on three assumptions: (1)
monolayer adsorption; (2) all active surface sites are equivalent and only one molecule at
a time can occupy a site; (3) there are no attractive or repulsive interactions between
adsorbed molecules, even at high fractional coverage. Consider the reaction:

A (g) + M (active surface site) (—) A-M

 

At equilibrium:
9 _ E _ KP
s 1+KP “'1’
k
K = —“
kd (1.2)
Where:

6 = fraction of surface coverage

N = number of adsorbed molecules [kmol / kg catalyst]
S = total number of available sites [kmol / kg catalyst]
P = bulk gas pressure [atm]

ka = rate constant of adsorption process

k4 = rate constant of desorption process

1.4.2. Methods for aqueous-phase adsorption studies

To understand the mechanism of a catalytic reaction, it is advantageous to
characterize the adsorbed species in detail — to have information on their structure,
energetics (MM) and dynamics. Over the past few decades, many advanced techniques

have been developed for studying the metal catalyst surface and the absorbed molecules.

For our special aqueous-phase catalytic hydrogenation systems, the most commonly used
methods are solution analysis, ATR-IR technique, and isotopic labeling.

Solution analysis is a simple and straightforward method to determine the
quantity of species adsorbed on metal surfaces by measming the difference between the
initial and final concentrations in solution before and after adsorption. The major
disadvantage of this method is its low sensitivity due to the relatively large quantity of
solution per gram of catalyst present. The quantity adsorbed must often be determined by
small differences in solution concentration.

Vibrational spectroscopy, most prominently infrared spectroscopy (IR), plays an
important role in heterogeneous catalysis research. A vibrational spectrum contains
detailed information about the nature of adsorbed molecules, interaction mode between
the surface and the adsorbate, and intermolecular interactions within the adsorbate layer.
The use of infrared spectroscopy to investigate aqueous systems has been previously
hindered by the strong IR absorptions of water. Attenuated total reflection infrared
spectroscopy (ATR-IR) holds promise as an effective means for probing the solid-liquid
interfaces in aqueous solution. It overcomes the water adsorption because the evanescent
wave selectively probes the region near the interface, so that the contribution from the
liquid can be kept reasonably small.l4 Figure 1.1 depicts an ATR cell used for in situ
adsorption measurements. In contrast to transmission IR spectroscopy where the IR beam
passes directly through the sample, in the ATR mode the IR radiation is coupled into the
internal reflection element (IRE), an IR transparent crystal of high refractive index in
contact with the sample. The IR radiation propagates through the IRE at an angle of

incidence larger than the critical angle, such that total reflection occurs at the IRE-sample

interface. An evanescent electromagnetic field is generated that penetrates into the
sample and is attenuated by the sample, thus producing an IR spectrum. Since the earliest
work in the late 19803 and early 19908,15’16 several research groups have implemented
ATR for catalytic studies.“18 These researchers have presented methods for depositing
thin layers of catalyst particles on ATR windows as well as detailed descriptions of
background subtraction techniques. ATR-IR technique provides a starting point to gain

structural and semi-quantitative adsorption data for our special reaction system.

   

 

thin film evanescent field
E WI
2 J l.
4 3’\
‘L/TNI

 

 

 

 

Figure 1.1. ATR cell used for in situ adsorption measurements.

Isotopic labeling (I-I-D exchange) provides non-destructive measure of substrate-
catalyst interactions. Examining H-D exchange rate of a specific organic acid helps to
determine the reactivity of hydrogen atoms on different carbons in the acid. H-D
exchange between D20 and hydrogen yields important information about water-catalyst
interaction and thus the role of water in aqueous phase hydrogenation. The extent of H-D

exchange can be measured by lH-NMR or indirectly (inorganic molecules) via 13C-NMR.

1.4.3. Relevant results from literature and the previous work in our research group

Competitive adsorption for surface sites is created when the adsorption system
involves more than one adsorbate. A few studies have probed catalyst adsorption of
organic acids, particularly acetic and formic acid, relative to their configurations on the
metal surface.19 In examining the catalytic production of ethylene glycol (EG) and
propylene glycol (PG) from glycerol with 5 wt% Ru/C catalyst, Lahr and Shanks20
addressed competitive adsorption involving those three polyols, but few other analyses of
substrate-catalyst interactions in heterogeneous aqueous phase catalysis have been
reported. Lahr and Shanks examined the degradation of EG and PG and found that PG
degraded at a slightly faster rate than EG. However, introducing the mixture of EG and
PG into the reactor resulted in significantly higher fiactional turnover for EG than that for
PG. This result indicated that PG was less competitive for active sites than EG. The
difference in adsorption behavior for EG and PG was attributed to their different
structures; these polyols adsorb presumably to ruthenium through oxygen, so the
presence of a hydrophobic nonoxygenated end in PG may cause it to be partially repelled
by the polar catalytic surface, thus decreasing its binding energy and allowing EG to
adsorb more readily on the surface sites. In the kinetic study of glycerol conversion, they
found that the reaction rate of glycerol decreased in proportion to the amount of EG
added while the addition of PG had a rrrinirnal effect on this reaction rate. This behavior
indicated that glycerol and EG competed relatively equally for active sites on the
ruthenium metal surface.

Some results firom the previous work in our research group provided an important

starting point in addressing the issue of competitive adsorption of species on the catalyst

10

from aqueous solution. D. Kovacs21 studied C-H activation in PG over Ru by using
chirality and isotopic exchange. She found that without lactic acid present, H-D exchange
at C2 of optically pure S-PG in solution was accompanied by rapid racemization, but, if
an equal molar quantity of L-lactic acid was added to the solution along with the S-PG,
the observed rate of H-D exchange was significantly decreased and the extent of PG
racernization was minimized. This behavior clearly showed that compared to lactic acid
PG was weakly adsorbed onto Ru. More importantly, the reduced racemization of PG
upon adding lactic acid indicates that lactic acid may exhibit a “templating” behavior that
retains or generates chirality. Works by Zhang2 and J ere3 showed that the presence of 3
wt % (0.33M) alanine reduced lactic acid (1.3M feed concentration) conversion fi'om 60
% to 25 %, and this effect was completely reversible. This result indicated that alanine
was able to exclude lactic acid from the catalyst surface by blocking adsorption sites.
Moreover, the fact that alanine conversion was only about 50 % at these conditions
indicated that adsorption ability was not necessarily related to reactivity; otherwise nearly

complete conversion of alanine would be expected.

1.5. Kinetic model for catalytic reactions

1.5.1. Langmuir-Hinshelwood (L-H) kinetic model

A catalytic reaction proceeds through a particular sequence of individual steps
called the reaction mechanism. This series of steps serves as the basis for development of
the mathematical description of the reaction kinetics, called the rate expression. The first

goal of any kinetic study is to get an accurate predictive relationship between the rate of

11

reaction and the experimental parameters controlling it, which are primarily the
temperature and concentrations (or partial pressures) of the reactants. If conversions are
high enough, then consideration of the influence of the products may also be required,
both in regard to competitive adsorption and to the reverse reactions if conversions are
too close to the equilibrium conversion.22 It is important that one works with kinetic data
free from any significant artifacts, particularly heat and mass transfer limitations, in order
that the postulated rate expression describes the intrinsic reaction kinetics.

In 19203 Langmuir” studied the kinetics of surface reactions and introduced what
is now known as the “Langmuir isotherm” to relate the coverage, or surface
concentration, of a reactant or product species with its gas-phase or liquid-phase
concentration, which can be measured. This approach was further developed by
Hinshelwood2425 into the “Langmuir-Hinshelwood mechanism” of heterogeneous
reactions, and the resulting rate expressions are referred to as “Langmuir-Hinshelwood
models”. The Langmuir-Hinshelwood (L—H) mechanism has found use in correlating and
interpreting rate data for many reactions taking place on solid surfaces. This mechanism
consists of the following types of steps: (1) adsorption of the reactants on the active sites;
(2) dissociation of molecules at the surface; (3) reactions between adsorbed molecules on
the surface; (4) desorption of the products from the active sites. Two simplifying
assumptions for the model are that: (1) one step is the rate-determining step, and (2) one
surface intermediate is dominant and thus all the other intermediates are present in
relatively insignificant amounts. When Langmuir-Hinshelwood kinetics are postulated, it
is reasonable to expect that the rate constant will show Arrhenius temperature

dependence, While the adsorption equilibrium constants will decrease with temperature.26

12

1.5.2. Relevant results from literature and the previous work in our research group
We have previously demonstrated aqueous-phase hydrogenation of lactic acid and
L-alanine over 5 wt% Ru/C catalyst, and have reported reaction kinetics for each
system.”28 For the hydrogenation of lactic acid to propylene glycol, Zhang27 proposed a
one-site Langmuir-Hinshelwood (L-H) kinetic model in which lactic acid and molecular
hydrogen adsorb on the same surface catalytic site. A two-site L-H model was first used
in our group by J ere28 for stereoretentive amino acid hydrogenation, where protonated
alanine and undissociated phosphoric acid compete for one set of surface sites and
hydrogen dissociatively adsorbs on a second set of sites. The molecular modeling results
of Neurock29 for the hydrogenation of acetic acid over palladium support this two-site
approach, as they show that the adsorbed acid is bound by carboxylate ~2 A above
catalyst surface palladium atoms while hydrogen nestles in the 3-fold coordinated
interstices between surface metal atoms. Dissociative hydrogen adsorption on metal

surfaces is widely accepted in the literature.30

13

1.6. References
(l) Srirarn Varadarajan, S.; Miller, D. J ., Catalytic upgrading of fermentation-derived
organic acids. Biotechnol. Prog. 1999, 15, 845.

(2) Zhang, Z.; Jackson, J. E.; Miller, D. J., Aqueous-phase hydrogenation of lactic
acid to propylene glycol. Appl. Catal. A. 2001, 219, 89.

(3) Jere, F. T.; Miller, D. J .; Jackson, J. E., Stereoretentive C-H Bond Activation in
the Aqueous Phase Catalytic Hydrogenation of Amino Acids to Arrrino Alcohols.
Org. Lett. 2003, 5, 527.

(4) Bowden, 13.; Adkins, H., J. Am. Chem. Soc, 1934, 56, 689.
(5) Adkins, H.; Pavlic, A.A., J. Am. Chem. Soc, 1947, 69, 3039.
(6) Camahan, 1.13. et al., J. Am. Chem. Soc., 1955, 77, 3766.

(7) Broadbent, H. S.; Campbell, G. C.; Bartley, W. J .; Johnson, J. H. J. Am. Chem.
Soc. 1959, 24, 1847.

(8) Novotny; Miroslav. US. Patent 2,607,807, 1981.
(9) Antons, S.; Tilling, A. S.; Wolters, E. US. Patent 6,355,848, 2002.

(10) 'Cortright, R. D.; Sanchez-Castillo, M.; Dumesic J. A. Applied Catal B:
Env. 2002, 39, 353.

(11) Bao-Wei Mao; Zhong-Zhan Cai, Mei-Yu Huang and Ying-Yan Jiang.
Polym. Adv. T echnol. 2003, I4, 278.

(12) Paul A. Webb, Introduction to Chemical Adsorption Analytical
Techniques and their Applications to Catalysis. MIC Technical Publications,
January 2003.

(13) Michael J. Pilling and Paul W. Seakins, Reaction Kinetics, page 179,
1995, New York.

(14) Davide Ferri, Thomas Bulrgi, and Alfons Baiker. J. Phys. Chem. B, 2001,
105, 3187.

(15) Zippel, E.; Breiter, M. W.; Kellner, R. J. Chem. Soc. 1991, 87(4), 637.

(16) Blesa, M. A.; Weisz, A. D.; Morando, P. J.; Salfity, J. A.; Magaz, G. E.;
Regazzoni, A. E. Coordin. Chem. Rev. 2000, 196, 31.

(17) McQuillan, J. Adv. Mater. 2001, 13, 1034.

14

(18) Baiker, A. Chimia 2001, 55, 796.

(19) Pallassana, V.; Neurock, M. Reaction paths in the hydrogenolysis of acetic
acid to ethanol over Pd (III), Re (0001), and PdRe alloys. J. Catal. 2002, 209 (2),
289.

(20) Daniel G. Lahr and Brent H. Shanks. Kinetic analysis of the
hydrogenolysis of lower polyhydric alcohols: glycerol to glycols. Ind. Eng. Chem.
Res. 2003, 42, 5467.

(21) Kovacs, D., D. J. Miller, and J. E. Jackson, “On the mechanism of
catalytic hydrogenation of lactic acid to propylene glycol”, Paper #340, 221St ACS
National Meeting, San Diego, CA, April, 2001.

(22) _ M. Albert Vannice, Kinetics of Catalytic Reactions, page 106, 2005, New
York.

(23) I. Langmuir, Trans. Faraday Soc. 1921, I 7, 621 .

(24) C. N. Hinshelwood, “Kinetics of Chemical Change in Gaseous Systems”,
Clarendon, Oxford, 1926.

(25) C. N. Hinshelwood, “The Kinetics of Chemical Change”, 4th Ed., Oxford
Press, Oxford, 1940.

(26) Lee, Hong H., Heterogeneous Reactor Design, page 63, 1985, Butterworth
Publishers.

(27) Zhang, Z.; Jackson, J. E.; Miller, D. J. Kinetics of aqueous phase

hydrogenation of lactic acid to propylene glycol. Ind. Eng. Chem. Res. 2002, 41,
691.

(28) Jere, E; Jackson, J. E.; Miller, D. J. Kinetics of aqueous phase
hydrogenation of L-alanine to L-alaninol. Ind. Eng. Chem. Res. 2004, 43, 3297.

(29) Pallassana, V.; Neurock, M. Reaction paths in the hydrogenolysis of acetic
acid to ethanol over Pd (III), Re (0001), and PdRe alloys. J. Catal. 2002, 209 (2),
289.

(30) Singh, U.K.; Vannice, M.A. Kinetics of liquid phase hydrogenation

reactions over supported metal catalysts — A review. Appl. Catal. A: Gen. 2001,
213(1), 1.

15

Chapter 2. Experimental Methods

2.1. Hydrogenation of organic acids over carbon-supported and unsupported
ruthenium catalysts

Hydrogenation of lactic acid, propionic acid, acid mixtures, and combinations of
acids with their alcohol products in aqueous solution were performed in a laboratory-
scale stirred batch reactor. Liquid phase products were analyzed using high performance
liquid chromatography (HPLC) and gas phase samples were analyzed on a Varian 3300

GC instrument.

2.1.1. Materials

The main reactants for hydrogenation were lactic acid, propionic acid, 1,2-
propanediol (propylene glycol), l-propanol and hydrogen. Reagent-grade D,L-lactic acid
(85% solution in water) and l-propanol (99 %) were purchased from Aldrich Chemical
Co. Propionic acid (>99%) was obtained from J .T. Baker, and propylene glycol (99.5%)
was ordered fiom Jade Scientific. Feed materials were prepared using HPLC grade water
(J .T. Baker). Ultrahigh-purity hydrogen gas (99.999%, AGA) was used without further
purification in all experiments.

The catalysts used in the experiments were (1) a 5 wt % ruthenium on activated
carbon powder (Ru/C) or (2) a nonporous ruthenium sponge. The 5 wt % Ru/C was
obtained from PMC, Inc. with a mean particle diameter of 150 pm, a N2 BET surface area
of 715.6 mz/g, a porosity of 0.6, and a dry particle density of 800 kg/m3. The ruthenium

metal dispersion of the as-received catalyst, measured by volumetric hydrogen

l6

chemisorption in a Micromeritics ASAP 2010 instrument, was 8.8 % corresponding to
the metallic surface area of 1.6 m2/g catalyst. The as-received catalyst is a 50.9 wt%
slurry in water; The Ru sponges (-100 mesh, 99.9 %) were purchased from Aldrich
Chemical Co. with a metallic surface area of 0.2-0.4 m2/g catalyst for the old Ru sponge
(with unknown batch No.) and 0.055-0.22 mzlg catalyst for the new Ru sponge (batch
No. 09814PB). Catalyst loadings and reaction rates in the thesis are reported on a dry

catalyst basis.

2.1.2. Reactor system

Reactions were carried out in a Parr series 5000 multiple reactor system (Figure
2.1) equipped with a magnetically coupled stirring unit. The system contains six 75 ml
reactors constructed of 316 stainless steel and rated to a maximum pressure of 20 MPa at
573 K. Each reactor is equipped with an individual heating mantle and ports for gas and

liquid sampling.

 

Figure 2.1. Parr series 5000 multiple reactor system.

2.1.3. Operating procedure

In a typical reaction, 0.5 g of Ru/C catalyst (dry basis) or 5 g of Ru sponge was
pre-reduced in a dry reactor by purging with nitrogen and heating to 423 -473 K, charging
with hydrogen to 3.4 MPa, and holding at these conditions for 12 hours. Afler cooling to
room temperature, the reactor was vented and 50 ml of organic acid feed solution (0.05-5
M) was forced into the reactor from a sample cylinder under hydrogen pressure. The
stirring speed was set to 1000 rpm, a value determined to be high enough to avoid
external mass transport limitations in the reactor. It took 10-15 min for the reaction
temperature to reach the desired value (343-423 K). When the reaction temperature
stabilized at the desired value, the reactor was pressurized with hydrogen gas (3.4-10.3
MPa) to initiate the reaction and that time was recorded as “time zero”. Due to adsorption
and pre-reaction of the starting materials, non-zero conversions were usually observed at
“time zero”. Liquid samples (1-2 ml) were typically taken at one hour intervals
throughout the reaction via a sampling loop system. For each sample, the loop was
purged with compressed air and then flushed with reaction mixture several times to
ensure that a representative composition was obtained; the ~1 ml liquid sample taken was
then filtered through a syringe microfilter prior to analysis. At the end of the reaction
time, which ranged from 5 to 80 hours, the reactor was cooled and the pressure was
reduced. The reactor set-up for charging and sampling is shown in Figure 2.2.

The Ru sponge was recovered by separating fi'om the final reaction solution by
filtration or centrifirgation and then washed thoroughly with HPLC water. The wet
catalyst was put in an oven and calcined at 350 °C for 12 hours. After pre-reduction the

metal catalyst was ready to use for the next experiment.

18

   

(b)

Figure 2.2. The reactor set-up for (a) charging and (b) sampling.

2.1.4. Analytical

Liquid-phase samples were analyzed on a Spectra Tech P1000 HPLC (Thermo
Separation Products, Inc.) with R1 detection and UV detection at 250 nm. The separation
was carried out on a Biorad Aminex HPX-87H column at 323 K. The mobile phase was
0.005 M H2SO4 in HPLC grade water with a flow rate of 0.45 ml/min. A RI
chromatogram of the typical liquid reaction species is shown in Figure 2.3. Gas-phase
samples were analyzed on a 1/8” x 6 ft Spherocarb packed column in a Varian 3300 gas
chromatograph with thermal conductivity detector and helium as carrier gas. For both
chromatographic analyses, multipoint calibration curves were developed to obtain peak

response factors. Data from the HPLC and GC were used to calculate acid conversion,

19

product yields and selectivities, and overall reaction material balances (the variability of

         

 

 

 

HPLC data is ~5%).
-30 a
A
-§ a c: A 7'.“
s = a a E
V) ‘1' "' 0°
N. '(3 3 )9. .75.
'40 7 2 hi ”‘5 C" tr;
3 78 “a ‘9' V '8
o te i8 E 9 g
> .0 9. to a a.
E *3 a o e o
, o H
.3 8‘ a a 9'
'50 ‘ H 9 I 0|
9 cu —-
or 8
-—t Q4
-60 I I fi
10 20 3O 40

Retention Time (min)
Figure 2.3. R1 Chromatograph of the typical liquid reaction species.
HPLC operation conditions: Aminex HPX-87H column temperature = 323 K,

mobile phase = 0.005 M H2SO4 in HPLC grade water,
flow rate of the mobile phase = 0.45 ml / min.

2.2. Catalyst characterization

Catalysts used in the experiments were characterized using a number of standard
methods to determine the total (BET) surface area, average pore size and pore size
distribution, micropore and total pore volume, metal surface area, as well as the metal
dispersion on the catalysts. All of the gases used in these analysis were of ultra high

purity (99.999%, AGA/BOC).

20

~ h ' “i!

 

2.2.1. Physisorption

The BET surface area was measured by nitrogen physisorption at 77 K using the
Accelerated Surface Area and Porosirnetry System (Micromeritics ASAP 2010), and
analyzed using the method proposed by Brunauer, Emmett and Teller. 1 Catalyst sample
was loaded to a pre-weighed quartz sample tube and heated to 423 K to remove water and
other potential contaminants. The sample was then reweighed to determine the actual
amount of catalyst present in the sample tube. Next, the evacuated sample tube was
placed in a Dewar flask of liquid nitrogen and further evacuated until equilibrium was
reached. At this point, analysis commenced with dosing of the analysis gas and
subsequent measurement of the adsorbed gas. The amount of nitrogen adsorbed at a given
composition and pressure was used to determine the BET surface area down to the micro

pore level.

2.2.2. Chemisorption

The metal surface area and metal dispersion on the catalysts were calculated using
Chemisorption methods. Adsorption of hydrogen or carbon monoxide on to activated
metal sites provides information about the fraction of active metal that is available for
reaction. Chemisorption measurements were conducted using the Micromeritics ASAP
2010 instrument. Samples were first prepared by degassing the material in a pre-weighed
glass sample tube to remove water and other potential contaminants. The sample was
then reweighed to determine the actual amount of catalyst present. Once complete, the
sample tube was placed in the analysis port of the ASAP 2010 system. The sample tube

was then placed under vacuum and heated to remove adsorbed materials on the catalyst.

21

Next, the catalyst was reduced under hydrogen to activate the metal sites. Then, the
sample tube was cooled and evacuated again. At this point, analysis commenced with
dosing of the analysis gas and subsequent measurement of the adsorbed gas. The metal
surface area and metal dispersion of the catalyst were calculated using the corresponding

Micromeritics software.

2.2.3. Pore volume and pore size measurement

A simple model for a porous catalyst is to assume it to be composed of a
collection of cylindrical pores of radius r. A volume of liquid can be forced under
pressure to fill the void space and thus determine pore volumes and the pore size
distribution of larger pores with radii larger than 10 nm. This liquid is invariably mercury
that does not wet the pore wall surface due to its high surface tension (mercury
porosimetry method 2). A nitrogen desorption technique can be used to determine the
distribution of pores with diameters smaller than 20 nm by using the Kelvin equation3 to
relate pore radius to the vapor pressure. Combining these two techniques can give the
overall pore size distribution. For the ruthenium catalysts used in our reaction systems,
the micropore volume was determined using the t-plot method in nitrogen physisorption,
and total pore volume was characterized as volume adsorbed at the maximum relative

pressure (P/Po) of 0.99.

22

2.3. Liquid phase adsorption studies
The quantity of species adsorbed onto a surface is determined by measuring the
difference between the initial and final species concentrations in solution, which were

measured prior to and following exposure to the surface, respectively.

2.3.1. Adsorption onto activated carbon support

Isothermal adsorption experiments at room temperature were performed using 8.5
ml glass vials with Teflon-lined plastic lids. The vials were cleaned and air-dried, and
then the activated carbon powder (# 3310) was weighed and added to the vials based on
the solution concentration to be examined in order to maintain a detectable change in
solution concentration before and after adsorption. The vials were capped and put on a
rotator overnight to ensure thorough mixing and equilibration. Upon removal from the
rotator, the vials were either centrifuged or left standing to allow the suspended carbon in
the solution to settle. Liquid samples were then taken and analyzed using HPLC.

Temperature dependent adsorption experiments were conducted using the 75 m1
batch reactor shown in Figure 2.1. The operating procedure was about the same as that

described in section 2.3.2 except that the carbon support doesn’t need to be pre-reduced.

2.3.2. Adsorption onto Ru/C catalyst

Adsorption onto the 5 wt % Ru/C catalyst was canied out in the 75 ml batch
reactor shown in Figure 2.1. A specified amount of catalyst was pre-reduced at 473 K and
3.4 MPa hydrogen pressure for 5-12 hours in order to activate the metal surface. Then the

reactor was cooled and charged with 50 ml aqueous solution of the substrates

23

investigated. The reactor was purged with hydrogen to remove air present in the solution,
and then sealed. To begin the temperature variation adsorption experiments, the reactor
was first stirred at room temperature overnight to equilibrate and the first liquid sample
was taken. The reactor temperature was then set to 30°C and increased by increments of
20°C up to 150°C. Liquid samples were taken at each temperature setting after several
hours of equilibration. An in-line filter was connected to the dip tube within the reactor in
order to separate the solid catalyst from the liquid solution during sampling. The samples
were analyzed by HPLC and the solution concentrations were used to calculate the
amount of substrates adsorbed on catalyst at each temperature. The sampling loss and the

change of solution volume were taken into account during the calculation.

2.3.3. Adsorption onto Ru sponge

The quantity of substrate adsorbed onto ruthenium sponge is extremely small due
to the low surface area of the catalyst (0.1-0.2 mZ/g Ru). The high cost of Ru metal led us
to focus on decreasing the solution volume in order to maximize the metal to solution
volume ratio and thus increase the sensitivity of detection. Lars Peereboom4 from Miller
group designed a recirculating rnicroreactor system (Figure 2.4) consisting of a pump,
packed bed reactor, thermal stabilization components, inline sampling system, and a
HPLC. The system allows us to have a large amount of catalyst (1-10 g) with a minimal
amount of liquid volume (10-20 ml). The inline sampling and HPLC system allows
removal of a minimal amount of liquid sample (10 pl) and allows accurate measurement
of concentration changes down to 10'5 M. The detailed operating procedure of this system

can be found in the Ph.D. dissertation of Lars Peereboom.

24

reactor..K

oil bath

J

l'"""

-------_-‘.---_

   

 

 

 

 

 

  

 

gas equilibration
space

recirculating j

pump

 

 

 

 

 

 

 

Figure 2.4. Recirculating microreactor system.

25

2.4. References

(l) Brunauer, S.; Emmett, P.H.; Teller, E., J. Am. Chem. Soc, 1938, 60 (2), 309.

(2) Gregg, S.J.; Sing, K.S.W., Adsorption, Surface Area and Porosity, 2nd Edition,
1982, 173-190, New York.

(3) M. Albert Vannice, Kinetics of Catalytic Reactions, page 18, 2005, New York.

(4) L. Peereboom, J. E. Jackson, and D. J. Miller, “The Effects of Temperature and
Hydrogen on Glycerol Adsorption on Ruthenium Metal”, AIChE Annual
Meeting, San Francisco, CA, November, 2006.

26

Chapter 3. Aqueous-Phase Hydrogenation of Organic Acids
and Their Mixtures over Carbon-Supported Ruthenium

Catalyst

3.1. Introduction

Our research aims to develop an improved mechanistic understanding of the
conversion fi'om organic acids to the corresponding alcohols. We seek to investigate
substrate-metal interactions in aqueous solution, and then to relate those adsorption
properties to the efficacy for hydrogenation. We approach this goal by conducting
adsorption and reduction studies of individual acids, acid mixtures, and combinations of
acids with their alcohol products. These studies provide valuable information on the
relative hydrogenation reactivity of different acids, the relative binding strength of
different substrates on the metal catalyst surface, and how the presence of one species at
the surface influences the reactivity of another.

The aqueous-phase hydrogenation of lactic acid (LA) and propionic acid (PA)
alone, together, and in mixtures with their alcohol products propylene glycol (PG) and l-
propanol (l-PrOH), over a 5 wt % Ru/C catalyst was performed in a three-phase stirred
batch reactor. Experiments were performed at several acid feed concentrations from 0.05
to 5 M, at temperatures from 343 to 423 K, and at hydrogen pressures from 3.4 to 10.3
MPa. To maintain reasonable reaction rates in the kinetic regime of reaction, a hydrogen
pressure of 6.9 MP3 (1000 psi) and a temperature of 403 K were used for most

experiments. Catalyst loading was 0.5 g (dry basis) in 50 ml aqueous solution for all

27

experiments. The reactions are depicted in Scheme 3.1 and a summary of the reaction

conditions investigated is reported in Table 3.1.

Scheme 3.1. Catalytic hydrogenation of lactic acid and propionic acid

 

 

 

0 CH4
C2H6
OH + 2 H2 5 ' Y\OH + H20 + C3Hs
OH OH C2H50H
Lactic Acid (LA) 1,2-Propanediol (PG) C3H70H
5 % Ru/C
* $1 "- .. 0 side roducts
403 K (selectrvrty. 80 99 A») ( P )
6.9 MPa H2
0 CH4
\/U\ + 2 H2 J [ V\OH + H2O + CZH 6
OH
l-Propanol (l-PrOH) C3Hs
Propionic Acid (PA) _
( selectivity: 30 - 60 % ) ( srde products )

28

Table 3.1. Summary of reaction conditions for the hydrogenation of lactic acid and
propionic acid over 5 wt % Ru/C catalyst

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

T P Concentrations of the starting Conversion of Selectivity to

(K) (M?!) t (hour) materials (M) acids (%) alcohols (%)
LA PA PG l-PrOH LA PA PG l-PrOH

343 6.9 24 0.5 24 90
363 6.9 8 0.5 30 85
383 6.9 8 0.5 64 87
383 6.9 7 0.5 60 89
403 6.9 8 0.1 100 85
403 6.9 7 0.5 97 86
403 6.9 6 0.5 88 97
403 6.9 8 2 77 90
403 6.9 5 2 50 78
403 6.9 8 5 50 66
403 3.4 30 0.5 99 78
403 3.4 30 2 96 77
403 10.3 12 0.5 99 100
403 10.3 12 2 93 86
423 6.9 4 0.5 83 85
383 6.9 8 0.5 20 56
393 6.9 28 0.5 69 54
403 6.9 8 0.1 80 44
403 6.9 8 0.5 41 54
403 6.9 6 0.5 26 65
403 6.9 6 2 13 48
403 6.9 8 5 12 33
403 8.3 l l 0.5 61 55
403 8.3 l l 2 26 50
403 3.4 31 0.5 69 33
403 3.4 31 2 45 43
413 6.9 12 0.5 77 40
423 6.9 12 0.5 91 39
383 6.9 8 0.5 0.5 53 l l 80 57
403 6.9 8 0.1 0.1 100 60 100 75
403 6.9 8 0.5 0.5 90 28 84 60
403 6.9 8 2 2 59 18 73 29
403 6.9 8 0.05 0.5 100 41 68 42
403 6.9 8 0.5 2 59 9 82 87
403 6.9 8 0.5 5 55 l l 68 29
403 6.9 8 2 0.5 62 17 84 60
403 6.9 8 5 0.5 52 21 59 35
403 10.3 12 2 2 83 24 81 44
403 3.4 30 0.5 0.5 100 73 100 44
403 3.4 31 2 2 83 26 75 44
403 6.9 3 0.5 0.5 58 79
403 6.9 7 0.5 0.1 93 85
403 6.9 7 0.5 0.5 90 87
403 6.9 10 0.5 solvent 68 20
403 6.9 8 0.5 0.5 29 0
403 6.9 8 0.5 0.36 29 0
403 6.9 7 0.5 0.5 35 56
403 6.9 8 0.5 0.5 0.5 89 28 74 56
403 6.9 8 0.5 0.5 0.5 0.5 81 23 80 0

'r P i LA PA PG l-PrOH LA PA PG l-PrOH
(K) (M‘Il’za) t (hour) Concentrations of the starting Conversion of Selectivity to

materials (M) acids (%) alcohols (%)

 

29

3.2. Hydrogenation of single lactic acid and propionic acid over RulC catalyst

The conversion rates of LA and PA are influenced dramatically by temperature
and hydrogen pressure, with LA reacting nearly four times faster than PA over the range
of conditions examined. Selectivity to PG fi'om LA ranged from 80 to 99%, while
selectivity to l-PrOH from PA was less than 60% under most conditions. Methane,

ethane, and propane were detected as the chief side products for these reactions.

3.2.1. Effect of temperature

The effects of hydrogenation temperature on acid conversion rates are shown in
Figures 3.1 and 3.2. The rate of conversion for both LA and PA increase dramatically
with temperature. Lower temperatures, as shown in Table 3.2, favored selectivity to the

alcohol products.

 

100%

80% r

60%3

40%“

Lactic Acid Conversion

 

 

 

 

 

 

Re action Time (hour)

Figure 3.1. Effect of temperature on lactic acid hydrogenation rate over Ru/C.
Conditions: LA concentration = 0.5 M; Pm = 6.9 MPa; 0.5 g (5 wt % Ru/C) /50 ml

aqueous solution; 1000 rpm. (0) 343 K; (A) 363 K; (I) 383 K; (0)403 K; (A) 423 K.

30

l

Propionic Acid Conversion

Figure 3.2. Effect of temperature on propionic acid hydrogenation rate over Ru/C.
Conditions: PA concentration = 0.5 M; Pm = 6.9 MPa; 0.5 g (5 wt % Ru/C) /50 ml

aqueous solution; 1000 rpm. (9) 383 K; (A) 393 K; (I) 403 K; (0)413 K; (A) 423 K.

 

00%

80% 7

60%“

40%~

20% ‘

 

 

 

 

0%:

4

6 8
Re action Time (hour)

10 12

Table 3.2. Effect of temperature on the hydrogenation selectivity

 

 

 

 

 

 

 

 

 

 

 

 

 

Lactic acid hydrogenation Pro bionic acid h droggration
T LA conv. PG sele. T PA conv. l-PrOH sele.

(K) (%) (%) (K) I%) (%)

343 24 90 383 20 56

363 30 85 393 69 54

383 64 87 403 41 54

403 97 86 41 3 77 40

423 83 85 423 91 39

 

Conditions: Acid feed concentration = 0.5 M, Pm = 6.9 MPa, 0.5 g (5 wt % Ru/C)/50 ml

solution, 1000 rpm.

3.2.2. Effect of hydrogen pressure

Due to the low solubility of hydrogen (Table 3.3)1’2 in aqueous solution, elevated
hydrogen pressure should be used to achieve reasonable reaction rates. Conversion rates
of LA and PA at three different hydrogen pressures (3.4, 6.9, and 10.3 MPa for LA; 3.4,

6.9, and 8.3 MPa for PA) and two different acid concentrations (0.5 and 2 M) are

31

 

reported in Figures 3.3 and 3.4. Higher hydrogen pressures favored the conversion of LA

and PA, and the selectivity (Table 3.4) to alcohol products.

 

100%

80%-

60% 7

40%-

Lactic Acid Conversion

 

 

 

 

0 2 4 6 8 10 12
Reaction Time (hour)

Figure 3.3. Effect of hydrogen pressure on lactic acid hydrogenation rate over Ru/C.
Conditions: T = 403 K; 0.5 g (5 wt % Ru/C) /50 ml aqueous solution; 1000 rpm. (A)

0.5M LA, 3.4 MPa; (in) 0.5M LA, 6.9 MPa; (o) 0.5M LA, 10.3 MPa; (a) 2M LA, 3.4
MPa; (:1) 2M LA, 6.9 MPa; (0) 2M LA, 10.3 MPa.

 

 

Propionic Acid Conversion

 

 

 

0 2 4 6 8 10 12
Re action Time (hour)

Figure 3.4. Effect of hydrogen pressure on propionic acid hydrogenation rate over Ru/C.
Conditions: T = 403 K; 0.5 g (5 wt % Ru/C) /50 ml aqueous solution; 1000 rpm. (A)

0.5M PA, 3.4 MPa; (u) 0.5M PA, 6.9 MPa; (0) 0.5M PA, 8.3 MPa; (a) 2M PA, 3.4
MPa; (:1) 2M PA, 6.9 MPa; (0) 2M PA, 8.3 MPa.

32

[ml Hz
(STD)/ml

Table 3.3. The solubility of hydrogen in water"2

 

Table 3.4. Effect of hydrogen pressure on the hydrogenation selectivity

 

 

 

 

 

 

 

 

 

 

 

Lactic acid hydro renation Propionic acid hydrogenation

P32 LA conv. PG sele. P112 PA conv. l-PrOH sele.
(MP8) (%) (%) (MPa) (%) (%)

3.4 99 78 3.4 69 33

6.9 97 86 6.9 41 54

10.3 99 100 8.3 61 55

 

Conditions: Initial concentration of the acid = 0.5 M, T = 403 K, 0.5 g (5 wt % Ru/C)/50

ml solution, 1000 rpm.

3.2.3. Effect of acid feed concentration

Conversion rates of LA and PA at four different feed concentrations (0.1, 0.5, 2
and 5 M) with otherwise identical reaction conditions are reported in Figures 3.5 and 3.6.

Higher acid feed concentration results in lower conversion after the same reaction time

33

 

although the absolute reaction rate increases with the concentration. The dependence of
rate on concentration is minimized when the acid concentration is over 2 M, indicating

that the catalyst surface is nearly saturated with acids at this concentration.

 

100%

80% 7

60%7

40%7

20% 7

Lactic Acid Conversion

 

 

 

 

m/O I I I I I I I
0 l 2 3 4 5 6 7 8
Re action Time (hour)

Figure 3.5. Effect of initial acid concentration on lactic acid hydrogenation rate over
Ru/C.
Conditions: T = 403 K; PH2 = 6.9 MPa; 0.5 g (5 wt % Ru/C) /50 ml aqueous solution;

1000 rpm. (9) 0.1M LA; (A) 0.5M LA; (o) 2M LA; (I) 5M LA.

 

100%

80% 7

60% 7

40% —

 

20% 7

Propionic Acid Conversion

 

 

 

(P/o I I I I I fir
0 l 2 3 4 5 6 7 8
Re action Time (hour)

Figure 3.6. Effect of initial acid concentration on propionic acid hydrogenation rate over
_ Ru/C.
Conditions: T = 403 K; PH2 = 6.9 MPa; 0.5 g (5 wt % Ru/C) /50 ml aqueous solution;

1000 rpm. (6) 0.1M PA; (a) 0.5M PA; (0) 2M PA; (a) 5M PA.

34

3.2.4. Effect of catalyst loading

Initial hydrogenation rates of LA at three different catalyst loadings (0.25, 0.5,
and 1 g Ru/C) with otherwise identical reaction conditions are represented in Figure 3.7.
The observation that the acid conversion rate increases in proportion to the catalyst
loading indicates that mass transport resistances across phase boundaries are negligible at
these experimental conditions. This would not be the case if phase boundary mass

transfer resistances limited reaction rate (Chapter 6).

 

 

 

0.15
o
*-' y=0.1201x
& A 2 ..o'.
3 g R =0.9957
o: 3 0.10 .. ’
E - v
u 1-I

o a ’

.3 E 0.05
3 °"
u-I

0.00

 

0.0 0.2 0.4 0.6 0.8 1.0 1.2
Catalyst Loading (g)

Figure 3.7 . Effect of catalyst loading on lactic acid hydrogenation rate over Ru/C.
Conditions: The catalyst is 5 wt % Ru/C; 50 ml 0.2 M LA aqueous solution; T = 403 K;
PH2 = 6.9 MPa; 1000 rpm.

3.2.5. Degradation of the alcohol products of hydrogenation

The alcohol products (PG and l-PrOH) of hydrogenation react further under the
hydrogenation conditions. Degradation of the alcohol products of hydrogenation was
investigated under typical reaction conditions. As shown in Table 3.5, for 0.5 M solutions

of the alcohols, 11% of the PG and 25% of the I-PrOH were degraded after 8.5 hours of

35

reaction at 403 K and 6.9 MPa. Examination of l-PrOH degradation over a concentration
range of 0.5 to 2 M showed zero-order behavior with a constant rate of 0.012
mol/liter/hour (Table 3.6). Degradation of PG was also studied by Shanks et (11,3 and was
found to be zero order reaction for the concentration range of 0.3 to 1.5 M. Degradation
products included methane, ethane, and propane from both l-PrOH and PG, along with
significant quantities of ethanol and l-PrOH fi'om PG. The relatively rapid degradation of
l-PrOH, coupled with the slow hydrogenation rate of PA, together contribute to the low
selectivity to l-PrOH from PA relative to that of PG from LA. In new studies probing
adsorption of acids and alcohols on the catalyst Ru metal and carbon support components
(Chapter 5), we have found that 1-PrOH adsorbs more strongly than PG into the carbon,
creating locally high pore concentrations of l-PrOH near catalytic Ru sites that contribute

to its elevated degradation rate.

Table 3.5. Degradation of 1,2-propanediol and l-propanol under the hydrogenation

 

 

 

 

 

 

 

 

 

conditions
Starting Alcohol conv. Yield of liquid phase Yield of gas phase
materials (carbon %) products (carbon %) products (carbon %)
0 ethanol and l-propanol methane, ethane, and
0'5M PG 11 A) (2.7 %) propane (2.4 %)
_ o methane, ethane, and
0.5M l PrOH 25 A) None propane (4.9 %)
Conditions: T = 403 K, PH2 = 6.9 MPa, t = 8.5 hour, 0.5 g (5 wt % Ru/C)/50 ml solution,
1000 rpm.

Table 3.6. Degradation rates of l-propanol at different initial concentrations

 

Initial concentration of l-PrOH (M) 0.5 2 5
[Degradation rate of 1-PrOH(mol/L/hour) 0.0123 0.0120 0.0111
Conditions: T = 403 K, Pm = 6.9 MPa, 0.5 g (5 wt % Ru/C)/50 ml solution,
1000 rpm.

 

 

 

 

 

 

 

36

3.2.6. Determination of reaction rate, reaction order and rate constant

Acid hydrogenation rates were calculated via differential analysis: acid
concentration vs. time data were fit to an nth-order polynomial (n = 2-4), which was then
differentiated and evaluated at various times over the course of reaction. Initial rates were
calculated by extrapolating the rate curve to time zero without forcing it through the
origin. By incorporating the acid concentration and the quantity and properties of the
catalyst, reaction rates on a catalyst mass basis (kmol acid/kg catalyst/s) or on a reactor
fluid volume basis (kmol acid/m3 solution/s) were determined. Initial rate data were used
to estimate the activation energy and the dependence of rates on acid feed concentrations.
This initial rate evaluation avoids possible complications from side reactions and catalyst
deactivation.

For a batch reactor, the plot of ln(-rA) vs. ln(C,4) yields the reaction order (n) from

the slope and the rate constant (k) from the intercept based on equations 3.1 and 3.2:

 

dC ,,
_rA =- th :kCA (3.1)
ln(— rA)= n1n(CA)+lnk (3.2)

For a 1St order reaction (n = l), the rate constant can be determined by the slope of

ln(CAo/CA) vs. t:

 

-r.r =kC... (3.3)
l—xA CA '

The method of data analysis was illustrated by the following example

(experimental conditions: 50 ml 0.5 M LA, 423 K, 6.9 MPa hydrogen pressure, and 0.5 g

37

Ru/C catalyst): the data of concentration vs. time were fit to a 4th order polynomial
(Figure 3.8); the equation of Cu, = fit) from the polynomial was then differentiated to
yield the reaction rates at various times over the course of reaction (Figure 3.9); the plot
of ln(-ru) vs. ln(Cu) (Figure 3.10) indicated that the reaction was nearly 1St order (n =
1.01) at this experimental conditions, and the rate constant was calculated from the
intercept (Ink = -0.78, k = 0.46); plotting ln(CLAo/Cu) vs. t (Figure 3.11) gave a straight
line through the origin, which confirmed that the reaction was 1St order, and the rate

constant was determined from the slope ( k = 0.45).

 

    

 

 

 

0.5
0.4 ‘ y= 0.0013x4 - 0.0131x3 + 0.0606x2 - 0.205x + 0.4322
A R2 = 0.9984
g 0.3
E
3 0.2
U
0.1
0.0 r r r r . r r
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
t(hr)
Figure 3.8. The plot of concentrations vs. reaction time for the hydrogenation of lactic
acid over Ru/C at 423 K.

Conditions: LA feed concentration = 0.5 M, T = 423K, Pm = 6.9 MPa, 0.5 g (5 wt %
Ru/C)/50 ml solution, 1000 rpm.

38

 

0.20 1’ y = 0.0105x2 — 0.0801x + 0.1954
R2 = 0.986

 
   

(-n.A)
(mol/L/ho ur)

O

8

 

 

 

0.05 7
e
0.“) I I I I I I
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
t0“)
Figure 3.9. The plot of reaction rates vs. reaction time for the hydrogenation of lactic
acid over Ru/C at 423 K.

Conditions: LA feed concentration = 0.5 M, T = 423K, PH2 = 6.9 MPa, 0.5 g (5 wt %
Ru/C)/50 ml solution, 1000 rpm.

 

    

 

 

 

A -2 q
E
'3 ‘ y= 1.0121x- 0.7852
° R2 = 0.989
-4 I r T r r 1 r r
-2.6 -2.4 -2.2 -2.0 -l.8 -l.6 -l.4 -l.2 -l.0 -0.8

In(CLA)

Figure 3.10. Determination of the reaction order and rate constant for the hydrogenation
of lactic acid over Ru/C at 423 K.
Conditions: LA feed concentration = 0.5 M, T = 423K, Pm = 6.9 MPa, 0.5 g (5 wt %
Ru/C)/50 ml solution, 1000 rpm.

39

 

 

 

 

2.0
y=0.4475x

A "5 ° R2=0.9982
3
Q
g 1.0-
E

0.5 7

0.0 1 I I I I I I

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

fair)

Figure 3.11. Determination of the rate constant for a first order reaction.
Conditions: LA feed concentration = 0.5 M, T = 423K, Pm = 6.9 MPa, 0.5 g (5 wt %
Ru/C)/50 ml solution, 1000 rpm.

Initial LA and PA hydrogenation rates at four different feed concentrations
(approximately 0.1, 0.5, 2 and 5 M) with otherwise identical reaction conditions are
reported in Figure 3.12. The non-linear dependence of initial rate on initial concentration
indicates that at the higher concentrations (2 2 M) the hydrogenation reactions are not
simply first order with respect to LA or PA, and suggests that a Langmuir-Hinshelwood
type of rate expression would better describe the reaction kinetics. Figure 3.13 shows the
data of concentration vs. time for the hydrogenation of 2 M LA. The linear dependence of
LA concentration on reaction time indicates that the hydrogenation reaction is nearly zero

order at this concentration with a constant rate of 0.2 mol/L/hour.

40

 

p
w

.°
N

_o
s—s

 

Initial Rate (mol/L/hour)

 

 

 

0 l 2 3 4 5
Initial Concentration (mol/L)

Figure 3.12. Concentration dependence of the initial hydrogenation rate.
Conditions: T = 403 K; PH2 = 6.9 MPa; 0.5 g (5 wt % Ru/C)/50 ml aqueous solution;
1000 rpm. (A) LA; (1:) PA.

 

  

 

 

 

2.0
1.5 -
g.
g 1.0 -
:‘i
o
0.5 _ y = -0.l968x + 2.0337 >
R2 = 0.9981
0.0 r r r r i r
0 l 2 3 4 5 6 7 8

to")

Figure 3.13. The plot of concentrations vs. reaction time for the hydrogenation of 2M
lactic acid over Ru/C at 403 K.
Conditions: LA feed concentration = 2 M, T = 403K, PH2 = 6.9 MPa, 0.5 g (5 wt %
Ru/C)/50 ml solution, 1000 rpm.

41

3.2.7. Activation energy calculation
The activation energy (E A“) is related to the reaction rate constant (k) by

Arrhenius’ law":

‘ _ E C!
k = koeXP(——R;, ' J (3.5)
Transforming the equation:
E c, 1

where:
k = reaction rate constant [hour '1]
k0 = pre-exponential factor [hour '1]
E Ac, = activation energy [J /mol]
R = ideal gas constant [J / mol / K]

T = Temperature [K]

The activation energy for acid hydrogenation was estimated by plotting the
natural log of the initial rate versus reciprocal absolute temperature. The data points were
fitted with a linear trendline, and the activation energy was then calculated from the slope
of the trendline (-EAC,,/R). Plots are shown in Figure 3.14 for PA at 0.5 M and for LA at
0.5 M and 2.0 M. The initial activation energies for LA (average) and PA hydrogenation
are 51 kJ/mol and 68 kJ/mol, respectively. The data used for the calculation of activation

energies were shown in Table 3.7.

42

 

 

ln(-nnm.l ) [kmol/kg catalyst/s]

 

 

 

 

~16 r r r
0.002 0.0024 0.0026 0.0028 0.0030

l/T [K']

Figure 3.14. Arrhenius plot for activation energy of hydrogenation.
Conditions: 0.5 g (5 wt% Ru/C)/50 ml solution; Pm = 6.9 MPa. (A) 0.5 M LA; (I) 2 M
LA; (:1) 0.5 M PA.

Table 3.7. Results of activation energy calculation for acid hydrogenation

 

 

 

 

 

 

 

 

 

 

 

 

 

 

over 5 wt % Ru/C catalyst
Initial cone. T (K) Initial rate x 10° Equation of the EA...
of the acid (kmol/kg catalyst/s) Arrhenius plot (kJ/mol)

343 0.16
363 0.50 _

0.5M LA 383 0.95 y “ -:525:.3r;;935.4749 54
403 3.32 '
423 5.80
383 2.31 __

2M LA 403 5.47 y " “172933;; '199 48

423 9.65 '
383 0.25
393 0.44 _

0.5M PA 403 0.70 y " -t;129:.(9)x949-461.2173 68
413 1.32 '
423 1.81

 

 

 

 

 

 

 

Conditions: PH2 = 6.9 MPa, 0.5 g (5 wt % Ru/C)/50 ml solution, 1000 rpm.

43

3.3. Mixed acid/alcohol hydrogenation over Ru/C catalyst - Effect of competitive
adsorption

The simultaneous hydrogenation of two acids and combinations of acids with
product alcohols provides insight into the interactions of the reaction species with the
catalyst surface that cannot be ascertained from single acid hydrogenation. These studies
provide valuable information on the relative hydrogenation reactivity of acids and
alcohols, the relative binding strength of different substrates on the metal catalyst surface,
and how the presence of one acid or alcohol species in solution influences hydrogenation
rate of another species.5

Hydrogenation of acid mixtures and combinations of acids with product alcohols
was performed at several concentration ratios. The results of these experiments are
represented in Figures 3.15-3.18 (lines represent polynomial fits for rate calculations). As
seen in Figures 3.15 and 3.16, adding a second acid significantly decreases the
conversion rate of a given acid, and this inhibitory effect increases with the concentration
of the added acid. This observation indicates that LA and PA compete for active sites on
the catalyst surface, and, consistent with the above concentration study (Figure 3.12),
suggests that surface hydrogenation of the adsorbed acid is the rate-limiting step in the
reaction. The inhibitory effect becomes nearly constant at added acid concentrations over
2 M, suggesting that the catalyst is saturated with acids at this concentration.

The influence of the alcohol products on acid reaction rates is shown in Figures
3.17 and 3.18. The presence of PG in the starting solution has little effect on the

hydrogenation rates of either acid, indicating that PG adsorption on the catalyst surface is

weak. Adding l-PrOH to the initial reaction solution decreases PA conversion rate

(Figure 3.18), while the effect of l-PrOH on LA is less significant (Figure 3.17). All of
these observations suggest that competitive adsorption of the reaction species on the
catalyst surface affects acid hydrogenation rates, and that any kinetic model describing
the reaction system must include these inhibitory effects in order to properly describe the

reaction.

 

100%

80% 7

60%7

40%7

20% 7

Lactic Acid Conversion

 

 

 

 

0%
2 3 4 5 6 7 8
Re action Time (hour)

Figure 3.15. Effect of propionic acid on lactic acid hydrogenation rate over Ru/C.
Conditions: T = 403 K; Pm = 6.9 MPa; 0.5 g (5 wt % Ru/C)/50 ml aqueous solution;

1000 rpm. (0) 0.5 M LA; (I) 0.5 M LA and 0.5 M PA; (A) 0.5 M LA and 2 M PA; (0)
0.5MLAand5MPA.

45

 

50%

40% 7

 

30%7

20%7

10%

 

Propionic Acid Conversion

 

 

00A) I I I I r I I l
0 1 2 3 4 5 6 7 8
Re action Time (hour)

Figure 3.16. Effect of lactic acid on propionic acid hydrogenation rate over Ru/C.
Conditions: T = 403 K; Pm = 6.9 MPa; 0.5 g (5 wt % Ru/C)/50 ml aqueous solution;

1000 rpm. (0) 0.5 M PA; (I) 0.5 M PA and 0.5 M LA; (A) 0.5 M PA and 2 M LA; (0)
0.5 M PA and 5 M LA.

 

100%

80% 7

60%7

40% 7

20% 7

 

Lactic Acid Conversion

 

 

 

0 l 2 3 4 5 6 7
Re action Time (hour)
Figure 3.17. Effect of 1,2-propanedicl and l-propanol on lactic acid hydrogenation rate

over Ru/C.
Conditions: T = 403 K; Pm = 6. 9 MPa; 0. 5 g (5 wt % Ru/C)/50 ml aqueous solution;

lOOOrpm. (0)0.;5MLA (I)0.5.,MLAand01MPG (A)0..5MLAand05MPG; (O)
OHSMLAandOSMl -.PrOH

46

 

50%

 

 

 

Propionic Acid Conversion

 

mA) I I I I I I I
0 l 2 3 4 5 6 7 8

Re action Time (hour)

Figure 3.18. Effect of 1,2-propanediol and l-propanol on propionic acid hydrogenation
rate over Ru/C.
Conditions: T = 403 K; Pm = 6.9 MPa; 0.5 g (5 wt % Ru/C)/50 ml aqueous solution;
1000 rpm. (0) 0.5 M PA; (I) 0.5 M PA and 0.5 M PG; (A) 0.5 M PA and 0.36 M 1-

PrOH; (o) 0.5 M PA and 0.5 M l-PrOH.

3.4. Uncertainty and error

The experimental uncertainties were estimated by performing replicate reactions
over the course of the experiments and checking the consistency of the data collected.
Shown in Figures 3.19-3.22 are conversion data fiom replicate runs for the hydrogenation
of LA and PA over Ru/C at different reaction conditions. The errors of these conversion

data are within 10 %. The variability of concentration data from the HPLC is ~ 5 %.

47

Lactic Acid Conversion

100%

80% 7
60% 7
40% 7
20% 7

0% "

 

 

 

 

 

3 4 5 6

Re action Time (hour)

Figure 3.19. Replicate experiments for lactic acid hydrogenation over Ru/C -
Experiments 23 and 50.

Conditions: 0.5M LA; T = 383 K; Pm = 6.9 MPa; 0.5 g (5 wt % Ru/C)/50 ml aqueous
solution. (I) Experiment 50; ([3) Experiment 23.

Lactic Acid Conversion

100%

80% 7
60% 7
40% 7
20% 7

0% "

 

 

 

 

3 4 5 6 7

Re action Time (hour)

Figure 3.20. Replicate experiments for lactic acid hydrogenation over Ru/ C -
Experiments 15 and 38.
Conditions: 0.5M LA; T = 403 K; Pm = 6.9 MPa; 0.5 g (5 wt % Ru/C)/50 ml aqueous

solution. (0) Experiment 38; (0) Experiment 15.

48

 

100%

80% 7

60%7

40%7

20% 7

Lactic Acid Conversion

 

 

 

 

0% ’ . . . . . . .
0 1 2 3 4 5 6 7 8
Re action Time (hour)
Figure 3.21. Replicate experiments for lactic acid hydrogenation over Ru/C -

Experiments 26 and 116.
Conditions: 2M LA; T = 403 K; P32 = 6.9 MPa; 0.5 g (5 wt % Ru/C)/50 ml aqueous

solution. (A) Experiment 26; (A) Experiment 116.

 

100%

80% 7

60%7

40% 7

20% 7

 

 

 

Propionic Acid Conversion

 

0% 1 r r r r r r
0 l 2 3 4 5 6 7 8
Re action Time (hour)
Figure 3.22. Replicate experiments for propionic acid hydrogenation over Ru/C -
Experiments 22 and 42.

Conditions: 0.5M PA; T = 403 K; PH2 = 6.9 MPa; 0.5 g (5 wt % Ru/C)/50 ml aqueous
solution. (0) Experiment 42; (0) Experiment 22.

49

3.5. References

(1) Hydrogen and Deuterium; Young, C. L., Ed.; Solubility Data Series; IUPAC,
Pergarnon Press; New York, 1981; Vol. 5/6.

(2) Stephan, H., The solubility of inorganic and organic compounds, The Macmillan
company, New York, 1963. '

(3) Daniel G. Lahr and Brent H. Shanks. Kinetic analysis of the hydrogenolysis of
lower polyhydric alcohols: glycerol to glycols. Ind. Eng. Chem. Res. 2003, 42,
5467.

(4) Levenspiel, 0., Chemical Reaction Engineering, 31rd Edition, page 27, Wiley, New
York, 1999.

(5) Yuqing Chen, Dennis J. Miller and James E. Jackson. Kinetics of Aqueous-Phase

Hydrogenation of Organic Acids and Their Mixtures over Carbon Supported
Ruthenium Catalyst. Ind. Eng. Chem. Res. 2007, 46, 3334-3340.

50

Chapter 4. Aqueous-Phase Hydrogenation of Organic Acids

and Their Mixtures over Ruthenium Sponge

4.1. Introduction

The aqueous-phase hydrogenation of LA and PA alone, together, and in mixtures
with their alcohol products PG and l-PrOH, over bulk ruthenium metal catalyst (un-
supported Ru sponge) was performed in a three-phase stirred batch reactor. This group of
experiments was conducted to help understand the effects of the activated carbon support
on hydrogenation reactivity and selectivity, by comparing the results of acid adsorption
and hydrogenation over carbon supported (5 wt % Ru/ C) and un-supported (Ru sponge)
ruthenium catalysts. Since the role of the activated carbon support is often to facilitate
selective adsorption of organic species out of aqueous solution, it may be that selective
adsorption of acids or alcohols into the carbon pore structure results in higher local
concentrations in the vicinity of the catalyst and thus affects the observed rates of organic
acid hydrogenation.

Hydrogenation experiments were performed at temperature of 403 K, at several
acid feed concentrations fiom 0.1 to 1 M, and at hydrogen pressures from 3.4 to 7.9 MPa.
Catalyst loading was 5 g Ru sponge in 50 ml aqueous solution for most experiments. A

summary of the reaction conditions investigated is reported in Table 4.1.

51

Table 4.1. Summary of reaction conditions for the hydrogenation of lactic acid and
propionic acid over Ru sponge

 

Catalyst Concentrations of the

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

T Pm Catalyst. Loading Starting Materials (M) t (116;. Cm“
(K) (MPa) (g/50 ml LA PA PG 1- (hour) (%) (%)
. 2.. [ solution) - PrOH H 7
403 6.9 °ld R“ 2.7 0.1 5 75
sponge
403 6.9 °ld R“ 2.7 0.1 5 l7
sponge
403 6.9 °1° R“ 2.7 0.5 52.5 57
sponge
403 6.9 °1° R“ 2.7 2 52 37
sponge
403 6.9 °ld R“ 2.7 0.1 0.1 5 74 8.7
sponge
403 6.9 Ru-l-O 2.5 0.1 51 35
403 6.9 Ru-I-O 5 0.1 51 73
403 6.9 Ru-I-O 10 0.1 51 99
403 3.4 Ru-II-O 5 0.1 5 47
403 6.9 Ru-I-5 5 0.1 5 69
403 6.9 Ru-I-4 5 0.1 0.5 5 71
403 6.9 Ru-l-4 5 0.1 0.1 5 68
403 6.9 Ru-I-4 5 0.1 0.5 5 78
403 7.9 Ru-II-l 5 0.1 5 59
403 3.4 Ru-l-6 5 0.1 6 4.7
403 6.9 Ru-l-5 5 0.1 46.25 82
403 6.9 Ru-l-3 5 0.1 0.5 77 95
403 6.9 Ru-I-3 5 0.1 0.1 53.5 92
403 6.9 Ru—I-3 5 0.1 0.5 77 95
Ru-II-l
403 7.9 5 0.1 5 18
and 2
403 3.4 Ru-II-O 5 0.1 0.1 6 34 2.9
403 6.9 Ru-I-S 5 0.1 0.1 5 66 8.2
403 6.9 Ru-l-2 5 0.5 0.1 52 84 13
403 6.9 Ru-I-2 5 1 0.1 52 71 3.5
403 6.9 Ru-l-l 5 0.1 0.5 74.5 85 22
403 6.9 Ru-I-l 5 0.1 1 74.5 88 20
403 7.9 ”1‘6 5 0.1 0.1 5 47 4.2
and 7
Catalyst 1—
T Pm Catalyst‘ Loading LA PA PG PrOH t CIdnAv C52;
(K) (MPa) (ng0 ml Concentrations of the (hour) (%) ' (%) °
solution) Starting Materials (M)

 

aCatalyst: “Ru” means Ru sponge; “I, II” are the catalyst batch numbers; “0,1, ...... 7” are
the numbers of times the catalyst is reused.
For example: “Ru-I-6” means the Ru sponge with batch number “I” has been reused for
six times”.

52

4.2. Effect of microporous carbon support on hydrogenation reactivity and
selectivity

The aqueous-phase hydrogenation of LA, PA and their mixtures were performed
in a Parr series 5000 batch reactor at 403 K and 6.9 MPa hydrogen pressure. Both 5 wt %
ruthenium supported on 3310 activated carbon and nonporous ruthenium bulk sponge
were used as catalysts to determine the effect of microporosity on reactivity. The metallic
sru‘face area of each catalyst, measured by volumetric hydrogen chernisorption in a
Micromeritics ASAP 2010 instrument, is 1.6 mz/g for the Ru/C, 0.2 ~ 0.4 mz/g for the old
Ru sponge (with unknown batch No.), and 0.055 ~ 0.22 m2/g for the new Ru sponge
(batch No. 09814PB). For direct comparison of results fi'om these catalysts, all ruthenium
catalysts were pre-reduced at the same conditions (473 K and 3.4 MPa hydrogen pressure
for 12 hours). The acid conversion rates were calculated on a metallic surface area basis
(mol acid / m2 metallic surface area / s).

The initial hydrogenation rates of LA, PA, and their mixtures are summarized in
Table 4.2. The rates per unit metal area are, within experimental uncertainty (:1: 40%) of
measuring the active surface area, the same for the old and new ruthenium sponges. Both
acids react faster on the carbon-supported ruthenium catalyst than on the ruthenium
sponges. The LA initial rate on Ru/C is 1.4 times the initial rate on Ru sponge, and the
PA initial rate on Ru/C is three times that on Ru sponge. Hydrogenation of an equimolar
LA / PA mixture gives an initial rate of LA ten times that of PA over the un-supported
ruthenium sponge catalyst, while on the carbon-supported ruthenium catalyst the initial
rate of LA is only five times that of PA. The difference in hydrogenation reactivity of

acids over carbon-supported and un-supported ruthenium catalysts indicates that the

53

activated carbon support may play a role in influencing relative rates of organic acid
hydrogenation. Since the role of activated carbon is often to selectively adsorb organic
species out of aqueous solution, the selective adsorption of acids into the carbon pore
structure may result in higher local concentrations inside the carbon micropores and thus

increase the observed hydrogenation rates.

Table 4.2. Comparison of the acid hydrogenation rates over carbon-supported and
un-supported ruthenium catalysts

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Initial rate x 107 .
Metal surface Starting (mol/m2 metal surface Initial.
Catalysts 2 area materials are a/s) rate ratio
(m lg catalyst) LA PA (LA/PA)
0.1MLA ' 7.1 ' ' " '
0.1M PA 2.1 . ,
0 . .,
0.5g(5thRu/C) 1.6 (0.1MLA)and 73 g. 14‘, p 51
(9.1M PA) ' ' . '
2.7 g old Ru sponge 0.1M LA 5.0 (i 2.0)
(unknown batch 0.3 0.1M PA 0.53 (i 0.21)
No.; reused for (0.2~0.4) (0.1M LA) and
many times) ( .1M PA) 4.2 (i 1.7) 0.39 (:t 0.16) 11
5 newRus on e (.1MLA 4.7 (:l: 1.9)
‘5 (batch N: g 0.16 0.1M PA 0.59 (s 0.24) . .
. _‘ (0.055~0.22) (0.1M LA)and ;~1 , ~ . .
09814PB, Ru I 5) [0.1M PA) 4.0 (:1: 1.6) 0.43 (:i: 0.17) 1. 9'2-
Conditions: T = 403 K, PH2 = 6.9 MPa, 50 ml reaction so;:ution, 1000 rpm.

Adsorption experiments with LA and PA were performed on the 3310 activated
carbon support by L. Peereboom1 from the Miller group. Representative results of these
adsorption studies are shown in Figure 4.1, as the quantity of each acid adsorbed as a
function of temperature at 0.25 M solution concentration. The results indicate that PA is
more strongly adsorbed into the carbon pore structure than is LA. Based on a micrcopore
volume of 0.17 ems/g for 3310 carbon, the calculated local concentration of PA at 100-

150 °C, which is the typical range of reaction temperatures for hydrogenation, is 7-8

54

times that of the bulk concentration of 0.25 M. In contrast, the pore concentration of LA
is only 2.5 — 3 times the bulk concentration at those temperatures. These observations are
consistent with the results of reactivity studies discussed previously: The ratio of initial
rates (LA/PA) on the carbon-supported ruthenium is much lower than that on the
nonporous Ru sponge because of the local enhancement of PA concentration inside the
carbon micropores, resulting in its enhanced reaction rate. The rates measured on the
nonporous Ru sponge represent real reaction kinetics because local and bulk
concentrations are the same. We thus conclude that the actual ratio of reactivity of LA to
PA is about ten (see Table 4.2), but that value is disguised over Ru/C catalyst because of

pore concentration enhancement.

 

 

 

 

 

2.0
fl
.3;
ac
Ben
«231.0-
3.8
as
5'3 0.5—
9 ...................... ° 00 0°
0° ““““““““ 6 '00 OOOOO
0.0 I r I I fir I I I

0 20 40 60 80 100 120 140 160 180
Temperature (°C)

Figure 4.1. Temperature dependent adsorption of lactic acid and propionic acid on
activated carbon 3310.
Conditions: mixture of (0.25 M LA) and (0.25 M PA); 50 m1 aqueous solution; 3 g
carbon 3310; (0) LA; (I) PA.

55

Selectivity to the alcohol products of hydrogenation was observed over carbon
supported and un-supported ruthenium catalysts. The selectivity to PG in LA
hydrogenation is similar on Ru/C and Ru sponge catalysts (80 - 90 % selectivity). The
selectivity to l-PrOH in PA hydrogenation, as shown in Table 4.3, is obviously higher
when using Ru sponge as the catalyst. In adsorption studies of acids and alcohols on the
carbon support (Chapter 5), we have found that l-PrOH adsorbs more strongly than PG
into the carbon rrricropore, creating locally high pore concentrations of l-PrOH near
catalytic Ru sites that contribute to its elevated degradation rate and lower selectivity on

Ru/C.

Table 4.3. Comparison of the l-propanol selectivity over carbon-supported and un-
supported ruthenium catalysts

 

 

 

 

 

 

 

 

 

 

 

 

 

2.7g Ru spong; 0.5g wt% Ru/C
. . t PA l-PrOH t PA l-PrOH
Starting materials (hour) conv. sele. (hour) conv. sele.
‘ (%) CA) (%) (%)
0.1 M PA 5 17 99 8 80 44
0.5 M PA 52.5 57 73 8 41 54
2 M PA 52 37 63 6 13 48
(0.1M LA) and (0.1M PA) 5 13 96 8 60 75

 

Conditions: T = 403 K, Pm = 6.9 MPa, 50 m1 reaction solution, 1000 rpm.

 

4.3. Hydrogenation of single lactic acid and pr0pionic acid over Ru sponge

4.3.1. Effect of catalyst loading
Initial hydrogenation rates of 0.1 M LA at three different catalyst loadings (2.5, 5,
and 10 g new Ru sponge) with otherwise identical reaction conditions (403 K and 6.9

MPa hydrogen pressure) are represented in Figure 4.2. The acid conversion rate increased

56

linearly with the catalyst loading, indicating that the reaction was controlled by the

availability of catalytic surface sites. Selectivity to PG was not affected by catalyst

loading at the temperature and pressure investigated.

0.020

Lactic Acid Initial Rate
[mol / L / hour]

0.000

 

0.015 7

0.010 7

0.005 7

 

 

 

y = 0.0016x
R2 = 0.9844 A
t» .
.7
2 4 6 10 12
Catalyst Loading (a)

Figure 4.2. Effect of catalyst loading on lactic acid hydrogenation rate over Ru sponge.
Conditions: The catalyst is new Ru sponge with a metallic surface area of 0.1 mz/g; 50 ml
0.1 M LA aqueous solution; T = 403 K; PH2 = 6.9 MPa; 1000 rpm.

4.3.2. Effect of acid feed concentration

PA conversion rates at three different feed concentrations (approximately 0.1, 0.5,

and 2 M) but otherwise identical reaction conditions are reported in Figure 4.3. Initial PA

hydrogenation rates at these concentrations are represented in Figure 4.4. The non-linear

dependence of initial rate on initial concentration in PA is similar to the observation on

Ru/C catalyst (Figure 3.12), which indicates that the hydrogenation over Ru sponge is not

simply first order in PA, and suggests that the reaction kinetics on Ru sponge would be

better described using a Langmuir-Hinshelwood type of rate expression.

57

 

 

60%
'6‘ "3""
'2' 50% . “-7..-’
o O..-
E 97"
o 40% 7 ..o’
u ..“I __,_9
.6 '0' If
'3 30% - ,../“’
< o. '. ’9.”
5:3 20% ~ ,4/
.2 x’
D:
E 10% -

 

 

 

W/o l I I l l I l I I
O 5 10 15 20 25 30 35 40 45 50 55
Re action Time (hour)

Figure 4.3. Effect of initial acid concentration on propionic acid hydrogenation rate over
Ru sponge.
Conditions: The catalyst is 2.7 g old Ru sponge with a metal surface area of 0.2-0.4 mz/g;
T = 403 K; Pm = 6.9 MPa; 1000 rpm. (A) 0.1M PA; (I) 0.5M PA; (0) 2M PA.

 

(mol/L/hour)
§

9
§

 

Propionic Acid Initial Rate

 

 

T I

0.5 1.0 1.5 2.0
Propionic Acid Initial Concentration (mol/L)

.0
.C
c:

Figure 4.4. Concentration dependence of the propionic acid hydrogenation rate over Ru

sponge.
Conditions: The catalyst is 2.7g old Ru sponge with a metal surface area of 0.2-0.4 m2/g;
T = 403 K; Pm = 6.9 MPa; 1000 rpm.

58

4.3.3. Effect of hydrogen pressure

Conversion rates of LA and PA at three different hydrogen pressures (3.4, 6.9,
and 7.9 MPa) are reported in Figure 4.5. Higher hydrogen pressures favored the
conversion of LA and PA, and hydrogen pressure has little effect on the selectivity to

product alcohols on Ru sponge.

 

60% ,-
50% 4 ’,r
40% ~
30% .

20% 7

Acid Conversion

 

 

 

 

 

 

Re action Time (hour)

Figure 4.5. Effect of hydrogen pressure on acid hydrogenation rate over Ru sponge.
Conditions: Starting materials = 0.1M LA or 0.1M PA; T = 403 K; 5 g Ru sponge/50 ml
aqueous solution; 1000 rpm.

lFor LA: (0) 3.4 Mpa, Ru-II-O; (A) 6.9 Mpa, Ru—II-l; (I) 7.9 Mpa, Ru-II-l.
For PA: (0) 3.4 Mpa, Ru-I-6; (A) 6.9 Mpa, Ru-I-S; (D) 7.9 Mpa, Ru-II-l and Ru-II-2.

4.3.4. Effect of catalyst reuse

The expensive ruthenium sponge catalyst for hydrogenation reactions can be
reused for many times without obvious decrease in catalytic activity. To recover the
catalyst, the Ru Sponge was first separated from the final reaction solution by filtration or

centrifugation, washed thoroughly with HPLC water, and then calcined at 350-400 °C for

59

12 hours. The recovered catalyst was pre-reduced in situ at 200 °C and 500 psi hydrogen
pressure for 12 hours before the next use. Although the activity of a Ru sponge catalyst
did not decrease following its repeated use in hydrogenation, an increase in catalytic
activity was observed in the used Ru sponge when compared with the new Ru sponge.
Shown in Table 4.4 is the characterization of several Ru sponges with different number
of times of reuse. The BET surface area and metallic surface area in Table 4.4 were
measured by nitrogen physisorption and hydrogen Chemisorption, respectively, in a
Micromeritics ASAP 2010 instrument. The BET surface area does not change after
repeated use of the catalyst. The metallic surface area of new and used Ru sponge,
however, are quite different. To test the catalytic activity of new and used Ru sponge,
hydrogenation of LA and PA were conducted with several different Ru sponges at
otherwise identical reaction conditions. The conversion curves of LA and PA for these
experiments are reported in Figures 4.6 and 4.7. As can be seen, the acid hydrogenation
reactivity is higher on the used Ru sponge, and the catalytic activity tends to stabilize
after the catalyst has been reused for more than three times. The degree of activity
increase is dependent on the reaction conditions, catalyst recovery conditions, and the
number of times the catalyst is reused. The possible cause for the increase in catalytic
activity is a physical change of the active site of the catalyst itself caused by reaction
circumstances such as reaction temperature and pressure, or the accumulation of a foreign
metal (XPS analysis showed that the used Ru sponge was contaminated by Chromium

that presumably from the stainless steel reactor vessel).

60

Table 4.4. Characterization of the Ruthenium Sponge

 

 

 

 

 

 

 

 

 

 

 

 

 

Ru Sponge BET Sigrface Metal Dispersion Metallic Surface
Area (w Ru) (%) Area (m IERu)
Ru-I-O
(Batch No. I; New 0.31, 0.33 0.028, 0.029 0.10, 0.11
Sponge)
Ru—I-6
(Batch No. I; Reused 0.43 0.015 0.054
for six times)
Ru-I-7
(Batch No. I; Reused 0.39 0.016 0.060
for seven times)
Ru-II-O
(Batch No. II; New 0.42 0.043, 0.029 0.12, 0.11
sponge)
Ru-II-l
(Batch No. II; 0.34 0.060, 0.048 0.22, 0.17
Reused for once)
80%
60% ~ 1,- 7"

Lactic Acid Conversion
” §
§ .\

 

 

 

 

0%

Re action Time (hour)

Figure 4.6. Change in the catalytic activity of Ru sponge for lactic acid hydrogenation.
Conditions: 0.1 M LA; T = 403 K; Pm = 6,9 MPa; 5 g Ru sponge/50 ml aqueous

solution; 1000 rpm. (0) Ru-I-O; (A) Ru-I-S; (I) Ru-I-7; (A) Ru-II-O; (D) Ru-II-l.

61

 

HXB6

80967

60967

40967

20967

Propionic Acid Conversion

 

 

 

 

00/0 I I l l l l l I
0 5 10 15 20 25 30 35 40 45 50 55
Re action Time (hour)

Figure 4.7. Change in the catalytic activity of Ru sponge for pr0pionic acid
hydrogenation.
Conditions: 0.1 M PA; T = 403 K; Pm = 6.9 MPa; 5 g Ru sponge/50 ml aqueous

solution; 1000 rpm. (6) Ru-I-2; (A) Ru-I-5.

Since the activity of ruthenium sponge changes with the catalyst age, it is
necessary to normalize the rate data collected, before using them for kinetic model
development, based on the stabilized catalytic activity (the number of times for catalyst
reuse 2 3). The method of normalization is described as follows: the catalysts Ru-I-3, Ru-
I-4, Ru-I-S, Ru-I-6, and Ru-I-7 have the same activity that is directly proportional to the
total concentration (C,) of active sites; the activity of Ru-I-O, Ru-I-l, Ru-I—2, Ru-II-O, Ru-
II-l, or Ru-II-2 was calculated according to the initial rate data available in Table 4.5, and
was represented as a certain fraction of C, (shown in Table 4.6); for example, the
observed initial rate of LA hydrogenation was 0.0088 mol/I/hour over the catalyst Ru-I-O

(its activity was represented as 0.4C,), then the rate used for kinetic modeling should be

62

0.022 mol/I/hour (= 0.0088/0.4) after being normalized to the stabilized catalytic activity

(represented as C,).

Table 4.5. Initial rates of acid hydrogenation over Ru sponges with different age

 

 

 

 

 

 

 

 

 

 

 

 

 

_ Ru Spong 7 LA Initiaerate (mol/L/hour) 7 PA Initial Rate (mol/L/hour)

Ru-I-O 0.0088

Ru-I-l

Ru-I-2 0.0018
Ru-I-3

Ru-I-4

Ru-I-S 0.024 0.0034
Ru-I-6

Ru-I-7 0.020

Ru-lI-O 0.009

Ru-II-l 0.012

Ru-II-Z

 

 

 

 

Conditions: 0.1 M LA or PA; T = 403 K; Pm = 6.9 MPa; 5 g Ru sponge/50 ml aqueous
solution; 1000 rpm.

Table 4.6. Catalytic activity of Ru sponges with different age (the basis for rate data

 

 

 

 

 

 

 

 

 

 

 

 

normalization)
Total Concentration of Total Concentration of
Ru Sponge Active Sites Active Sites

(for LA hydrogenation) (for PA hydrgenation) ‘
Ru-I-0 0.40C, 0.40C,
Ru-I-l 0.51 C, 0.51 C,
Ru-I-2 0.56C, 0.56C,
Ru-I-3 C, C,
Ru-I-4 C, C:
Ru-I-S C, C,
Ru-I-6 C, C,
RU'I'7 C1 Ct
Ru-II-O 0.43C, 0.43 C,
Ru-II-l 0.55C, 0.55C,
Ru—II-2 0.61 C, 0.61 C,

 

 

 

 

 

63

4.4. Mixed acid/alcohol hydrogenation over Ru sponge - Effect of competitive
adsorption

The simultaneous hydrogenation of two acids and combinations of acids with
product alcohols were performed over Ru sponge catalyst at several concentration ratios.
These studies provide insight into the interactions of the reaction species with the Ru
metal, and how these species compete for active sites on the metal surface. By comparing
the competition behavior of reaction species on Ru sponge with that on Ru/C, we expect
to understand the role of the activated carbon support in acid adsorption and
hydrogenation.

The results of these experiments are shown in Figures 4.8-4.11 (lines represent
polynomial or exponential fits for rate calculations). As seen in Figures 4.8 and 4.9, LA
conversion rates are not obviously affected by adding PA (consider the fact that Ru-l-S
has higher catalytic activity than Ru-I-l); while adding LA significantly decreases the
conversion rate of PA, and this inhibitory effect increases with the concentration of the
added LA. This observation indicate that PA is less competitive for active sites than LA
on the Ru metal surface. The influence of the alcohol products on acid reaction rates is
shown in Figures 4.10 and 4.11. The presence of PG and l-PrOI-l in the starting solution
has little effect on the hydrogenation rates of either acid, indicating that the adsorption of
PG and l-PrOH on the Ru metal surface is much weaker compared to the acids.

The results of competitive adsorption and hydrogenation over Ru sponge shown
here are quite different from the previous results over Ru/C (Figures 3.15-3.18): LA and
PA compete relatively equally for surface reaction sites on Ru/C; PG is weakly adsorbed

on the Ru/C surface and it has little effect on the hydrogenation rates of either acid;

64

adding l-PrOH to the initial reaction solution decreases PA conversion rate on Ru/C,
while the effect of l-PrOH on LA is less significant. We attribute these differences in
observed relative reactivity of hydrogenation to the different adsorption affinity of
reaction species on activated carbon support and on ruthenium metal surface: LA adsorbs
more strongly than PA onto the ruthenium metal sites, while it adsorbs to a much lesser
extent onto the support material compared to PA - consistent with the fact that PA
competes relatively equally with LA over Ru/C while it has little effect on decreasing the
LA conversion rates over Ru sponge; PG is weakly adsorbed onto both the metal active
sites and the carbon micropores; the decreased PA conversion rates on Ru/C upon adding

1-PrOH is evidence of strong adsorption of l-PrOH onto the activated carbon support.

 

 

10096

80967

6096 7

4096 7

2096 7

 

 

Lactic Acid Conversion

 

 

00/0 I r r r l r r I 1 1 g T r 1
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75
Re action Time (hour)

Figure 4.8. Effect of propionic acid on lactic acid hydrogenation rate over Ru sponge.
Conditions: T = 403 K; Pm = 6.9 MPa; 5 g Ru sponge/50 ml aqueous solution; 1000 rpm.
(0) 0.1 M LA; Ru-I-S; (I) 0.1 M LA and 0.1 M PA; Ru-I-S; (A) 0.1 M LA and 0.5 M PA;

Ru-I-l; (O) 0.1 M LA and 1 M PA; Ru-I-l.

65

100%

 

80% 7

60%7

40%7

N
§
1

 

Propionic Acid Conversion

 

 

 

§

0510152025303540455055
Reaction Time (hour)

Figure 4.9. Effect of lactic acid on propionic acid hydrogenation rate over Ru sponge.
Conditions: T = 403 K; Pm = 6.9 MPa; 5 g Ru spongeJSO ml aqueous solution; 1000 rpm.
(0) 0.1 M PA; Ru-I-S; (I) 0.1 M PA and 0.1 M LA; Ru-I-S; (A) 0.1 M PA and 0.5 M LA;

Ru-I-2; (o) 0.1 M PA and 1 M LA; Ru-I-2.

1 00%

 

80% 7

60%7

 

Lactic Acid Conversion

 

 

 

Re action Time (hour)

Figure 4.10. Effect of 1,2-propanediol and l-propanol on lactic acid hydrogenation rate
over Ru sponge.
Conditions: T = 403 K; Pm = 6.9 MPa; 5 g Ru sponge/50 ml aqueous solution; 1000 rpm.
(0) 0.1 M LA; Ru-l-S; (I) 0.1 M LA and 0.1 M 1-PrOH;Ru-I-4; (A) 0.1 M LA and 0.5 M

l-PrOH; Ru-I-4; (O) 0.1 M LA and 0.5 M PG; Ru-I-4.

66

 

100%

80%7

60%7

40%-

 

 

Propionic Acid Conversion

 

 

0 10 20 30 40 50 60 70 80
Re action Time (hour)

Figure 4.11. Effect of 1,2-propanediol and l-pr0panol on propionic acid hydrogenation
rate over Ru sponge.
Conditions: T = 403 K; Pm = 6.9 MPa; 5 g Ru sponge/50 ml aqueous solution; 1000 rpm.
(0) 0.1 M PA; Ru-I-5; (I) 0.1 M PA and 0.1 M l-PrOH; Ru-I-3; (A) 0.1 M PA and 0.5 M

1-PrOH; Ru-I-3; (O) 0.1 M PA and 0.5 M PG; Ru-I-3.

The above observations on adsorption affinity and hydrogenation reactivity over
carbon supported and un-supported ruthenium catalysts demonstrate the importance of
catalyst support material on the observed rate of acid hydrogenation. In order to properly
characterize kinetics of acid conversion over lRu/C catalyst, particularly when mixtures of
materials are present, it is necessary to consider the adsorption behavior of reaction

species onto the activated carbon micropores in kinetic modeling and reactor design.

67

4.5. References

(1) L. Peereboom, Y. Chen, J. E. Jackson, and D. J. Miller, “Polyols and Organic
Acids Adsorption onto Activated Carbon, and Its Role in Aqueous-Phase
Catalytic Hydrogenation Rates”, AIChE Annual Meeting, San Francisco, CA,
November, 2006.

68

Chapter 5. Adsorption Study on Activated Carbon

5.1. Introduction

Selective adsorption of reactant and product species onto the porous catalyst
support materials can have a major effect on hydrogenation rates by causing difference in
concentration between solution and pore. The adsorption of acids (LA and PA) and
product alcohols (PG and l-PrOH) on the 3310 activated carbon support was studied by
Lars Peereboom"2 in the Miller group. The objective of his work is to develop adsorption
models to characterize local concentrations in the catalyst pores and eventually
incorporate local pore concentrations into kinetic modeling and reactor design of organic
acid hydrogenation. In this Chapter, we briefly describe the adsorption work done by
Lars Peereboom as a basis for the later development of a global model for organic acid

hydrogenations over Ru/C catalyst (Chapter 7: Section 7.3).

5.2. Carbon characterization

The # 3310 activated carbon powder is the support material for ruthenium catalyst
we used in organic acid hydrogenation. The BET surface area, micropore volume and
total pore volume of this carbon support were characterized in a Micrometrics ASAP

2010 instrument using nitrogen physisorption method described in Section 2.2.

5.3. Isothermal adsorption
Adsorption experiments were conducted at room temperature (25 °C) using the

method described in Section 2.3.1 for LA, PA, PG, and l-PrOH with solution

69

concentration ranging from 0.01 M to 2 M. The quantity of species adsorbed onto the
carbon support was determined by the difference in initial and final species concentration
in solution, which was measured by HPLC prior to and following exposure to the
activated carbon, respectively (Eq. 5.1). The local pore concentrations were then

calculated based on the measured micropore volume of 0.17 cm3/ g carbon.

C = (CA0 — CA )* Kolution
AS m (51)

 

carbon
where:

C, = observed solution concentration of species A in equilibrium with the carbon
suvport [M]

CA0 = initial solution concentration of species A prior to adsorption [M]

CA5 = concentration of species A in the carbon pore structure [mol/kg carbon]

mmbon = mass of carbon in solution [kg]

The adsorption data from single component experiments were modeled using the
Langmuir isotherm (Eq. 5.2) to obtain adsorption constants at room temperature. A plot
of experimental data as (CA/CA5) vs. C, gives a slope of l/CTA and an intercept of
I/KACTA. Comparing the experimental quantities adsorbed with those predicted by the
model indicated that Langmuir isotherm gives the best fit of individual LA and PA
adsorption at concentrations below 0.5 M. Shown in Table 5.1 are the Langmuir isotherm
coefficients for LA, PA, PG and l-PrOH adsorption on the # 3310 activated carbon.

= KACACTA
AS 1+KACA

 

(5.2)

70

where:

Cm = maximum concentration of species A in the carbon pore structure [mol/kg
carbon]

K, = equilibrium constant for- adsorption of species A in Langmuir isotherm
model [I/M]

Table 5.1. Langmuir isotherm coefficients for LA, PA, PG and l-PrOH adsorption
on the # 3310 activated carbon

 

 

 

 

 

Maximum Concentration of the Equilibrium Constant
Substrates Substrate in the Carbon Pore Structure for Adsorption
Cu (mol/kg carbon) KA (lfM)
LA 2.71 2.38E-02
PA 1.61 1.87E-02
PG 1 .43 1 .80E-02
l-PrOH 2.62 1 .80E-02

 

 

 

 

 

5.4. Elevated temperature adsorption

Quantities of reaction species adsorbed onto the 3310 activated carbon were
measured at concentrations from 0.1 M to 0.5 M and temperatures from 303 to 423 K,
using the method described in Section 2.3.2. At each temperature, the equilibrium
constant (K4) for the Langmuir isotherm was calculated using Cm determined fi'om
isothermal adsorption experiments at 25 °C. The equilibrium constant varies with
temperature based on the Van’t Hoff Equation (Eq. 5.3). For each substrate, the plot of
ln(K,.,) vs. l/T was fitted with a straight line via least-squares linear regression to give the
heat of adsorption (AH) from the slope (-AH/R) and the preexponential constant (K0)

fi'om the intercept (ln(K0)).

71

l
m(KA)=——*§:+ln(Ko) (5.3)

where:

AH = heat of adsorption [kJ/mol]; this is a constant since Langmuir isotherm
assumes all adsorption sites have equal binding energies.

K0 = Van’t Hoff constant [l/M]

K A = equilibrium constant for adsorption of species A in Langmuir isotherm
model [l/M]

R = ideal gas constant [8.31 J/mol/K]

T = absolute temperature [K]

5.5. Mixed-component adsorption

Isothermal (298 K) and temperature-dependent (303-423 K) adsorption
experiments were conducted for acid mixtures and combinations of acids with their
alcohol products at several concentration ratios. The extended Langmuir model (Eqs. 5.4a
and 5.4b) was applied to characterize mixed-substrate adsorption on the carbon support.
The denominator of the extended Langmuir isotherm accounts for competitive adsorption
of the two substrates into the carbon micropores. All parameters required in the extended

model were taken from the single component modeling results.

 

 

: KACACTA

”‘5 1+KACA +KBCB (5°43)
= KBCBCTB

BS 1+KACA+KBCB (5'41”

72

where subscripts A and B denotes different component in the mixture.

5.6. Octanol-water partition coefficient estimation

The octanol-water partition coefficient (Kow) is defined as the ratio of a
chemical's equilibrium concentration in the octanol phase to its concentration in the
aqueous phase of a two-phase octanol/water system (Equation 5.5). Measured values of
Kow for organic chemicals have been found as low as 10'3 and as high as 107. In terms of
IogKow, this range is from -3 to 7. Values of Kow represent the tendency of the chemical
to partition itself between an organic phase and an aqueous phase. Chemicals with low
Kow values may be considered relatively hydrophilic; they tend to have high water
solubilities and small adsorption coefficients into organic matter. Conversely, chemicals
with high Kow values are very hydrophobic; they preferentially partition into organic
matter rather than water. The LogKow (KowWin) program estimates the IogKow values
of organic chemicals using an atom/fragment contribution method3 developed at SRC.
The logKow values of LA, PA, PG, and l-PrOH are reported in Table 5.2. The relative
magnitude of these logKow values indicates that PA and l-PrOH are strongly adsorbed
into the activated carbon micropores and functionalized carbon surfaces, while the

adsorption of LA and PG is very weak on the carbon support.

Concentration in octanol phase

 

Kow = (5.5)

Concentration in aqueous phase

73

Table 5.2. The logKow values of LA, PA, PG, and l-PrOH

 

 

 

 

 

 

Chemicals IogKow values
LA -0.72
PA 0.33
PG -0.92
l-PrOH 0.25

 

 

 

74

5.7. References

(1) Lars Peereboom, Benjamin Koenigsknecht, Margaret Hunter, J arnes E. Jackson,
and Dennis J. Miller. Aqueous-Phase adsorption of propylene glycol onto
activated carbon. Carbon 2007, 45, 579-586.

(2) L. Peereboom, Y. Chen, J. E. Jackson, and D. J. Miller, “Polyols and Organic
Acids Adsorption onto Activated Carbon, and Its Role in Aqueous-Phase
Catalytic Hydrogenation Rates”, AIChE Annual Meeting, San Francisco, CA,
November, 2006.

(3) W. M. Meylan and P. H. Howard. Atom/fiagment contribution method for
estimating octanol-water partition coefficients. J. Pharm. Sci. 1995, 84, 83-92.

75

Chapter 6. Mass Transfer Analysis

6.1. Introduction

Aqueous phase catalytic hydrogenation is a three-phase reaction involving
hydrogen gas, aqueous solution of the organic acid and its alcohol product, and the solid
catalyst. For reaction to occur, the following steps must take place: (1) hydrogen
transport from the gas phase to the liquid phase across the gas-liquid interface; (2)
hydrogen and acid transport from the bulk liquid phase to the catalyst across the liquid-
solid interface; (3) hydrogen and the acid diffusion through the porous catalyst support to
the metal surface sites; (4) adsorption and conversion of the acid to its alcohol product
via a sequence of surface chemical reaction steps; (5) product desorption from the
catalyst surface; (6) diffusion of products through the porous catalyst support to the liquid
phase. For three-phase reactions, the rate of mass transfer between phases can
significantly affect overall reaction rate. Collection of experimental rate data suitable for
kinetic model development requires that the reactor contents are well mixed and that the
reaction rates are not limited by mass transport resistances. Correlations in the literature
have been developed to estimate the minimum stirring rate necessary for complete
catalyst suspension and calculate the mass-transfer coefficients across the phase

boundaries. Figure 5.1 illustrates the concentration profile of a three-phase reaction.

76

 
   

 

 

 

 

 

 

A

A' 5 5

:I 5 AL 1
3 : : .
g : .As!
= s 5% g! l
g : g in: i
° E 1: PE i
U 8 - 5 '5 ‘2‘: § i
“l is: m .2' ‘9: .a'
a mi :3 1'] :5: n":
a :5 E i 3., g i
U U; A an A! m I
A I I I
RadialPosition R 0

Figure 6.1. Mass transfer in a three-phase catalytic reaction

6.2. Catalyst suspension

For maximum utilization of the catalyst, it is necessary to keep the entire solid
catalyst suspended and evenly distributed in the reaction solution. The Zwietering
correlation1 is used to estimate the minimum stirring rate necessary for complete catalyst
suspension.

1.33
0.45 013

2 — dPMPLOJgOASLOP —pL) W

min _ 0.55 0.85 (6.1)
1. d1

 

where:

NM," = minimum stirring speed for complete catalyst suspension [rotations / sec]

77

d), = reactor diameter [m]

d, = impeller diameter [m]

(1,» = particle diameter [m]

p, = liquid viscosity [kg / m-s]

g = gravitation constant [9.8066 In / 82]

,0,» = particle density [kg / m3]

)0], = liquid density [kg / m3]

w = catalyst loading [grams of catalyst / 100 grams of solution]
For this reaction system, with 0.5 g of 5 wt % Ru/carbon in 50 ml aqueous acid solution,
the minimum stirring speed required for complete catalyst suspension is about 500 rpm.

A stirring rate of 1000 rpm was therefore used in all experiments.

6.3. Gas-liquid mass transfer
The mass transfer rate across the gas-liquid (G-L) phase boundary is expressed by

the following equation:

a:

- rH, : kGL,H2 “(CH2 " CH,,L) (6.2)
where:
-er = observed reaction rate of hydrogen [kmol / m3 / s]
kGLflza = G-L mass transfer coefficient of H2 [1 / s]
a = effective interfacial area per unit volume reaction fluid [1 / m]
C H2. = gas-phase concentration of hydrogen [kmol / m3]
C H2, 1. = bulk liquid concentration of hydrogen = aqueous-phase hydrogen

solubility at the hydrogen partial pressure of the experiment [kmol / m3]

78

The mass transfer coefficient (karma) across the gas-liquid (G-L) phase boundary
for hydrogen was estimated using the correlations of Bern et a1.2 and of Yagi and
Yoshida3.

In 1976, Bern et al proposed the following correlation (Equation 6.3) for kGLa in
an agitated slurry reactor from studies of the hydrogenation of oil in a gas-liquid mass
transfer controlled regime and this correlation satisfied their data for both 30 and 500 liter

reaction vessels.
kGLa = 1.099 x 1072 N"16d,1'979ug°'32VL—0'521 (6.3)

where:
kGLa = G-L mass transfer coefficient [1 / s]
a = gas-liquid interfacial area per unit volume of solution [1 / cm]
d1 = impeller diameter [cm]
N = stirring speed [s’l]
ug = superficial gas velocity [cm / s]
VL = volume of the solution [cm3]
The correlation (Equation 6.4) from Yagi and Yoshida in 1975 was used to

satisfactorily predict C02 adsorption in the glycerol-water system at 30°C.

 

 

Sh=kGLad,2
D
1.5 0.19 0.5 0.6 0.32
=0.06.dfiNp. [w] [,1] (mg) N6 .4
3 #1. g IOLD ST “g (l)

79

where:

Sh = Sherwood number [dimensionless]
kGLa = G-L mass transfer coefficient [1 / s]
d, = impeller diameter [m]

D = diffusion rate [m / s2]

N= stirring speed [5"]

p, = liquid density [kg / m3]

y, = liquid viscosity [kg / m - s]

g = gravitational constant [9.8066 m / 32]
u3 = superficial gas velocity [cm / 3]

ST: surface tension [N / m]

The gas phase concentration of hydrogen (CHZ') was calculated using the ideal gas

law. The bulk liquid concentration of hydrogen (CHu) can be measured4 by a volumetric

technique or determined by Henry’s Law5 :

where:

PA=YAP=XAHA(D (65)

PA = partial pressure of species A in the gas phase [atm]

y, = mole fraction of species A in the gas phase

P = total pressure in the gas phase [atm]

x, = mole fi'action of species A in the liquid phase

H, (T) = Henry’s Law constant for A in a specific solvent [atm / (mole A / mole

solution)]

80

6.4. Liquid-solid mass transfer

The mass transfer rates across the liquid-solid (L-S) phase boundary are expressed

by the following equations:
_ r Acid = kLS,Acida p (CAcid,L — CAcid,S ) (6.6)
_ r H2 = Ls,H2 a p (CH, ,L — C1125 ) (6.7)
where:

4,4,.“ = observed reaction rate of the acid [kmol / m3 / s]

-er = observed reaction rate of hydrogen [kmol / m3 / s]

kmAcwap = L-S mass transfer coefficient of the acid [1 / s]

kLsflzap = L-S mass transfer coefficient of hydrogen [1 / 5]

ap = external area of catalyst particles per unit volume reaction fluid [1 / m]
CAM, = bulk liquid concentration of the acid [kmol / m3]

CAMS = catalyst surface concentration of the acid [kmol / m3]

CH2), = bulk liquid concentration of hydrogen [kmol / m3]

CH2; = catalyst surface concentration of hydrogen [kmol / m3]

81

 

The liquid-solid mass transfer coefficient, km, for hydrogen and liquid phase

reactants were calculated using the Boon-Long correlation6 proposed in 1978 for agitated

 

 

vessels.
kwdp
D
0233 0.173 —0.011 0.019 0.461
:004 27’2‘1ppLdTN Pzzgdpa _“”VL g1 [ ”L J
”L ”L2 dp3 dP pLD
(6.8)
where:

kg = liquid-solid mass transfer coefficient [m / s]

dp = catalyst particle diameter [m]

D = diffusion rate of species [m2/ 3]

dr = reactor diameter [m]

N = stirring speed [5"]

,uL = liquid viscosity [kg / m - s]

pL = liquid density [kg / m3]

g = gravitational constant [9.8 m / s2]

w ’ = catalyst loading [grams of catalyst / 100 grams of solution]

VL = liquid volume [m3]

82

The mass transfer area for the liquid-solid interface is calculated using properties

of the catalyst and conditions of the experiment:

_ 6W. 10L

dppp

P

where:
ap = ratio of catalyst surface area to volume of reaction fluid [1 / m]
w ’ = catalyst loading [grams of catalyst / 100 grams of solution]
pL = liquid density [kg / m3]
(1,, = catalyst particle diameter [m]

pp = catalyst particle density [kg / m3]

6.5. Comparison of reaction rates with mass transfer rates

(6.9)

For three-phase reactions, the rate of mass transfer between phases can

significantly affect the overall reaction rate. The driving force for mass transfer is the

difference in species concentration between phases, which can be measured directly or

calculated. The maximum mass transfer rate is calculated as the product of the mass

transfer coefficient and the initial species concentration in bulk solution. Hydrogen

concentrations were calculated fi'om solubility data7.
Maximum G-L mass transfer rate for hydrogen = (1601.112) a (Cmt)
Maximum L-S mass transfer rate for hydrogen = (10.5112) ap (CHM)

Maximum L-S mass transfer rate for acids 7—7 (haunt) ap (CA,,,,_L)

83

(6.10)
(6.11)

(6.12)

The results of mass transfer calculations (section 6.7) showed that the maximum
rates of G-L and L-S mass transfer for each reactant are orders of magnitude larger than
the observed reaction rates, indicating that mass transport resistances across phase

boundaries were negligible at the experimental conditions studied.

6.6. Intraparticle mass transfer
The effect of mass transport inside the catalyst pores can be characterized by the
Weisz-Prater criterion8 via the observable modulus:

2 —rL2

77¢ =CD

S e

 

(6.13)

where:

77¢: = observable modulus

-r = initial species reaction rate [mol / L - s]

L = characteristic length of the catalyst [m] = (1,, / 6 for spherical particles

dp = catalyst particle diameter [m]

C, = liquid phase concentration at the catalyst surface [mol / L]

D. = 21),- : effective diffusivity [m2 / s]

e = support porosity = 0.6 for the 5 wt % Ru/C catalyst

D,- = diffusivity of reactant j in the liquid phase estimated using the Wilke-Chang
equation9:

.. (¢ M )“T
D,=7.4x10 8 ”W586

J

(6.14)

84

 

where:
D,- = diffusivity of reactant j [em2 / s]
T = absolute temperature [K]
,u = viscosity of solution [cp]
V,- = molar volume of j as liquid at its normal boiling point [cm3 / g mol]
¢B = association parameter for solvent = 2.6 for water

M3 = molecular weight of solvent [g / mol]

The observable modulus is the ratio of the observed reaction rate to the effective
diffusion rate inside the catalyst particle. If 77402 is less than 1, then the rate of mass
transport is greater than the reaction rate and the reaction is not limited by intraparticle
diffusion. As shown in Table 6.1, the observed modulus for each reactant is less than 0.1
(section 6.7), therefore mass-transport resistance inside the catalyst pores can be
neglected.

The limiting reactant inside the catalyst particles was estimated by calculating the

ratio of the observable modulus of hydrogen to that of the acid:

77¢H2 2 = l CAcidDe,Acid
”(o/W2 b CH, De,H, (6.15)

 

where b is the stoichiometric ratio of hydrogen to acid (= 2.0). For all reactions we
investigated, the ratio of observable moduli is much greater than 1, indicating that

hydrogen is the limiting reactant.

85

6.7. Results of mass transfer calculations

Mass transfer coefficients and the maximum mass transfer rates were calculated
for the experiment with the highest hydrogenation rate at which mass transfer is most
likely to influence the reaction (50 ml 2 M lactic acid, 403 K, 6.9 MPa hydrogen
pressure, and 0.5 g 5 wt % Ru/C catalyst). The results of mass transfer calculations are
shown in Table 6.1: although G-L mass transfer of hydrogen is slower than L-S mass
transfer of either acid or hydrogen, the maximum G-L and L-S mass transfer rates for
both hydrogen and the acid are at least an order of magnitude larger than the observed
maximum reaction rates, indicating that mass transport resistances across phase
boundaries are negligible at the experimental conditions studied. This conclusion is
supported by experiments represented in Figure 3.7 in which the acid conversion rate
increases in proportion to the catalyst loading. This would not be the case if phase
boundary mass transfer resistances limited reaction rate. Diffusion inside the catalyst
pores is not limited because the observed modulus for each reactant is much less than
one. The results of mass transfer analysis showed that surface reaction of the adsorbed
species is the rate-limiting step. The experimental data collected represent intrinsic

reaction kinetics and are suitable for kinetic model development.

86

Table 6.1. Results of Mass Transfer Analysis“

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Description Value
G-L mass transfer coefficient of H2 km [NS] 7.1X10'I
L-S mass transfer coefficient of H2 k1,; H2 [m/s] 3.5><10'5
L-S mass transfer coefficient of LA kw,“ [m/s] 3.8><10'5

L-S mass transfer area per volume fluid a, [l/m] 28000
Observed modulus for H2 ”01,122 4.2><10’2
Observed modulus for LA Wu] 4.9><10'4
Maximum G-L mass transfer rate of H2 (kaflz) a (CH5) [mol/cm3/s] 3.8x 10‘
Maximum L-S mass transfer rate of H2 (123,”) a, (Cm) [mol/cm3/s] 5.4x 10'5
Maximum L-S mass transfer rate of LA (kmAcu) ap (Cm-u) [mol/cm3/s] 2.1><10'3
Observed maximum reaction rate of H2 (4112),“, [mol/cm3/s] 1.1><10'7

Observed maximum reaction rate of LA (-rL,,),,,a,, [mol/cm3/s] 5.5><10'

 

aConditions: 50 ml 2 M lactic acid solution; 403 K; 6.9 MPa H2; 0.5 g (5 wt %

Ru/carbon)/50 m1 aqueous solution; 1000 rpm.

bMass transfer rates and reaction rates are reported on a unit fluid volume basis.

87

 

6.8. References

(1) Zwietering, T. N. Suspending of solids in liquid by agitators. Chem. Eng. Sci.
1958, 8, 244.

(2) Bern, L.; Lidefelt, J .; Schoon,,N. Mass transfer and scaleup in fat hydrogenation.
J. Am. Oil Chem. Soc. 1976, 53, 463.

(3) Yagi, H.; Yoshida, F. Gas absorption by Newtonian and non-Newtonian fluids in
sparged agitated vessels. Ind. Eng. Chem. Process Des. Dev. 1975, I4 (4), 488.

(4) Zhang, 2.; Jackson, J. E.; Miller, D. J. Kinetics of aqueous phase hydrogenation
of lactic acid to propylene glycol. Ind. Eng. Chem. Res. 2002, 41, 691.

(5) F elder, R.M.; Rousseau, R.W., Elementary Principles of Chemical Processes, 2ml
Ed., page 248, Wiley, New York, 1986.

(6) Boon-Long, S.; Laguerie, C.; Couderc, J. Mass transfer from suspended solids to
a liquid in agitated vessels. Chem. Eng. Sci. 1978, 33 (7), 813.

(7) Hydrogen and Deuterium; Young, C. L., Ed.; Solubility Data Series; IUPAC,
Pergamon Press; New York, 1981; Vol. 5/6.

(8) Weisz, P.B.; Prater, C. D. Interpretation of measurements in experimental
catalysis. Adv. Catal. 1954, 6, 143.

(9) Wilke, C.; Chang, P. Correlation of diffusion coefficients in dilute solution.
AIChE J. 1955, I, 264.

88

Chapter 7. Kinetic Modeling

7 .1. Hydrogenation of organic acids over carbon-supported ruthenium catalyst

7.1.1. Introduction

The results of mass transfer analysis (Table 6.1) show that the acid conversion
rates are not limited by mass-transport resistances over the ranges of temperature and
acid feed concentrations investigated; therefore, the experimental data collected represent
intrinsic reaction kinetics and are suitable for kinetic model development. To describe the
hydrogenation reactions for LA, PA and their mixtures, a two-site Langmuir-
Hinshelwood (L-H) kinetic model is postulated here in which acids adsorb on one type of
surface catalytic site (8,) and hydrogen dissociatively adsorbs on a second type of site
(S2). This two-site approach was first used in our group by J erel for stereoretentive amino

acid hydrogenation. The following set of elementary reactions is used to model both LA

and PA hydrogenation:
Acid + S, = Acid-S, (fast) (7.1a)
H2 + 282 = 2H-S2 (fast) (7.1b)
2H-S2 + Acid°Si -+ Intermediate-S, + 2S2 (slow) (7.1c)
Intermediate-S, + 2H-S2 = Alcohol-S, + 282 (fast) (7.1d)
Alcohol-S, = Alcohol + S, (fast) (7.1c)

The irreversible surface reaction of the adsorbed acid is assumed to be the rate-

controlling step; all other steps are assumed to be rapid and close to equilibrium. The

89

adsorption of water is neglected. Several variations of the above L-H model were
examined, including those with molecular hydrogen adsorption or same-site adsorption of
the acid and hydrogen. Those models gave negative values of adsorption constants or a
poor fit of data at high acid concentrations. Only the above two-site model gave positive
values for all kinetic constants and reliably predicted the acid hydrogenation kinetics over

a wide range of conditions.2

7.1.2. Hydrogenation of single acids over Ru/C catalyst

Combining the rate expressions for the above elementary reaction steps
(Equations 7.1a-7.1e) and invoking the steady-state assumption for adsorbed
intermediates yields the final rate expressions (Equations 7.12a and 7.12b) for single acid
hydrogenation. The final rate equations are developed as follows:

Equilibrium constants for the adsorption of acid, hydrogen, and alcohol product

are expressed as:

 

 

K __ k Acid __ CAcid-Sl

7.2
-Acid CAcid CV1 ( )

= kHz = (C‘H-Sz)2
2 k—Hz PH2 (Cv2 )2 (7.3)

 

 

KH

_ k Alcohol _ CAIcohol-Sl

K _ _.___ _
Alcohol (74)
k—Alcohol CAlcohol CV1

 

90

where CAMS), CH.s2, CAlcohol-Sla CV), and CV2 represent the concentrations of acid-occupied
sites, hydrogen-occupied sites, alcohol-occupied sites, vacant surface sites 8,, and vacant
surface sites S2, respectively.

The overall reaction rate is determined by the rate-lirniting steps expressed in
Equations (7.1c) and (7.1d). To simplify the model, desorption of the chemisorbed
intermediate is neglected since the rate of consumption is much faster than that of
formation for the intermediate at the present reaction conditions. The concentration of the
adsorbed intermediate can be solved using the steady-state assumption by setting the rate

of accumulation to zero. The surface reaction rate is expressed by Equation (7.5):

_ r Acid = ks CAcid-S, (Czar-52)z (75)

Substituting Equations (7.2) and (7.3) and rearrange the above rate equation:

_ rAcid : ks (KAcidCAcidel XKHZ PH2 (Cv2 )2) (7-6)

2
_ rAcid — ksKAcidKHz Cvl (CvZ ) CAcidPH2 (7.7)
In addition, the relationships for the surface active sites are:

Cu = Cvl + CAcid-Sl + CAIcohoI-S, (73)

C12 = Cv2 + Cms, (7.9)

where C,, and C,2 represent the total concentrations of sites S, and S2, respectively.

91

Substituting Equations (7.2), (7.3) and (7.4) to Equations (7.8) and (7.9), and solve for
Cy] and CV2:

C _ C.
v1 _ q 7.10
(1 + K Acid CAcid + K Alcohol CAIcohoI ) ( )

 

C Ct2

v2 =(1+W) (7.11)

 

Combining Equations (7.10) and (7.11) with Equation (7.7) give the following final rate

expressions for single acid hydrogenation:

— r Acid (kmol / kg catalyst / sec)

kAcid CAcid PH,

(1 + K Acid CAcid + K Alcohol CAlcohol X1 + V K H2 P H2 )2 (7-123)

 

kAcid : ks AKAcid KHZ Ctl (Ct2 )2 (7.12b)

where km, = composite rate constant for each acid [m3/kg catalyst/MPa/sec]
k3,, = surface reaction rate constant for each acid [(kg catalyst)2/kmolz/sec]
K Add = adsorption constant for each acid [m3/kmol]
K Akaho, = adsorption constant for each alcohol product [m3/kmol]

K H2 = adsorption constant for hydrogen [l/MPa]

92

C, 1 = total catalyst site concentration for acid and alcohol adsorption [kmol/kg
catalyst]

C,2 = total catalyst site concentration for hydrogen adsorption [kmol/kg catalyst]

The rate constants and equilibrium constants in Equations (7.12a) and (7.12b) for
single acid hydrogenation were calculated by a least-squares regression analysis of
kinetic data taken from 35 experiments (shown in Table 3.1) at 403 K. The values of
these constants are reported in Table 7.1. The adsorption constant of LA on Ru/C catalyst
is slightly higher than that of PA, indicating that these two acids compete relatively
equally for catalytic surface sites. l-PrOH has the highest affinity for active sites among
these four reaction species, and PG is weakly adsorbed with an adsorption constant of
0.27 m3/kmol. The relative magnitude of the kinetic parameters shown here are consistent
with the results seen in Figures 3.15-3.18. Adding a second acid significantly decreased
the conversion rate of the first acid because both compete relatively equally for surface
reaction sites. The fact that PG has little effect on the reaction rate of both acids is
consistent with its low adsorption constant. The presence of l-PrOH in the starting
solution decreases PA hydrogenation rate — evidence of strong adsorption onto catalyst

sites.

93

Table 7.1. Kinetic Constants for Single Acid Hydrogenation over Ru/C at 403 K"’b

 

 

 

 

 

 

 

 

 

Kinetic Constants Values at 403 K
Composite rate constant of LA k2,, (m3/kg catalyst/MPa/s) 1.0E-05
Composite rate constant of PA km (m3/kg catalyst/MPa/s) 1.7E-06
Adsorption constant of LA KM (my/kmol) 1.7
Adsorption constant of PA 7 K p, (m3/kmol) 1.4
Adsorption constant of PG Kp6(m3/kln01) 0.27
Adsorption constant of l-PrOH Kym” (m3/1tmol) 2.9
Adsorption constant of H2 K”, (MPa‘I) 0.21

 

 

 

 

1‘Conditions: 403 K; 0.5 g (5 wt % Ru/C)/50 ml solution; 1000 rpm.
bUse bulk solution concentrations.

Comparison of the experimental and predicted rates for individual LA and PA
hydrogenation are shown in Figures 7.1 and 7.2. These parity plots illustrate the
agreement between the observed rates and the rates predicted by the model. For both LA
and PA hydrogenation, all of the data points are distributed near the y = x line in the
parity plots, indicating that this two-site L-H model predicts single acid hydrogenation
kinetics over a wide range of concentrations (0.1-5 M) with a high degree of reliability.
Figures 7.3 and 7.4 compare experimental and predicted acid conversion versus time for
experiments conducted at a typical reaction condition (0.5 M acid, 0.5 g Ru/C, 50 ml
solution, 403 K, 6.9 MPa hydrogen). Clearly, the reaction trajectories are properly
reproduced by the model.

For reaction conditions in Figures 7.1 and 7.2, the turnover frequency at a typical
LA reaction rate of 3.3><10‘6 kmol/kg of catalyst/s is ~ 270 mol of LA/mol of Ru surface
atoms/hour; for PA at a rate of 0.7><10*‘3 kmol/kg of catalyst/s the turnover frequency is ~

60/hour.

94

 

00

Individual Lactic Acid Rates at 403 K

   
  
   

0‘ \l
L 1
“Ob

M
1

  

U)
1 1

(kmol/kg of catalyst/s)
N A

Predicted Rate x 106

 

 

 

0 l 2 3 4 5 6 7 8
Experimental Rate x 106 (knnl/kg of catalyst/s)

Figure 7 .1. Experimental and predicted rates for individual lactic acid hydrogenation
over Ru/C.
Conditions: T = 403 K, 0.5 g (5 wt % Ru/C)/50 ml solution, 1000 rpm.

 

2"
00

Individual Propionic Acid Rates at 403 K y = X

2-7 :-
h 0\
1

1.2 7

.O r"
00 o
1

Predicted Rate x 10‘
(kmol/kg of catalyst/s)
_o
O\

 

 

 

 

0.0 I I I I I I I I
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

Experimental Rate x 10‘ (kmol/kg of catalyst/s)

Figure 7 .2. Experimental and predicted rates for individual propionic acid hydrogenation
over Ru/C.
Conditions: T = 403 K, 0.5 g (5 wt % Ru/C)/50 ml solution, 1000 rpm.

95

 

100%

80% 7

60%7

40% 7

20% 7

 

Lactic Acid Conversion

 

 

 

0 1 2 3 4 5 6 7
Re action Time (hour)

Figure 7 .3. Experimental and predicted conversion rates for individual lactic acid
hydrogenation over Ru/C.
Conditions: 0.5 M LA, 0.5 g (5 wt % Ru/C)/50 ml solution, T = 403 K,
Pm = 6.9 MPa, 1000 rpm.
The line represents reaction trajectory predicted by the model.

 

50%
40% . . 0
30% 7 o
20% 7

10% 1

 

Propionic Acid Conversion

 

 

0 1 2 3 4 5 6 7 8
Re action Time (hour)
Figure 7 .4. Experimental and predicted conversion rates for individual propionic acid
hydrogenation over Ru/C.
Conditions: 0.5 M PA, 0.5 g (5 wt % Ru/C)/50 ml solution, T = 403 K,
. PH2 = 6.9 MPa, 1000 rpm.
The line represents reaction trajectory predicted by the model.

96

A sensitivity analysis (Table 7.2) of the kinetic constants, carried out by varying

one constant while holding all others constant, shows that a i20% change in k“, kpA, K“,

K”, and Km results in a substantial increase (SO-700%) in the least squares error of

predicted rate; the model is sensitive to these constants because they appear in the

numerator of the kinetic rate expression. In contrast, the model is relatively insensitive to

the adsorption constant Km and K ”,0” of the alcohol products because of the low value

of Km and because few experiments were conducted at significant concentrations of l—

 

 

    

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

PrOH.
Table 7 .2. Sensitivity Analysis of the Kinetic Constants for Single Acid
Hydrogenation over Ru/C at 403 K
Kinetic Change in the Chan e in the Sum ofDeviation gs uaresg-L 7
Constants Constants Sinae LA . . 7 Sin PA" '
k2,, (m3/kg -20% +195%
catalyst/MPa/s) +20% +705%
kn (m’lkg ~20% +1934 -.
catalyst/MPa/s) +20% +345% 5 .
3 -20% +520%
Kb, (m /kmol) +20% +18%
3 -20% +250%
Km (m Ikrnol) +20% , _ , 144.50% »
3 -20% +2%
KPG (m /kmol) +20% +1%
K1450” -20% ., +429 I F .. .;,
(m3/kmol) +20% +4%
-1 -20% +307% +138%
K”) (MP3 ) +20% +51% +37%

 

 

 

 

 

 

l’I’he sum of deviation squares:

n

H = 2 (Predicted Rate — Experimental Rate)2

i=1

97

7.1.3. Hydrogenation of acid mixtures over Ru/C catalyst

In acid mixtures, competitive adsorption between LA and PA affects their relative
hydrogenation rates. To quantify this effect, the two-site Langmuir-Hinshelwood model
is extended to include adsorption of additional species in the denominator term. For two
acid mixtures, the PG term in the denominator of the L-H rate expression is eliminated
because PG is weakly adsorbed and thus has little effect on rate. The resulting kinetic rate

expressions for mixed acid hydrogenation are given in Equations (7.13) and (7.14).

— rLA (kmol / kg catalyst / sec)

 

 

 

 

kLACLAPHZ
(1+ KLACLA + KPACPA + Kl—PrOHCI—PrOH )(1 + 3’ K192 PH, )2
(7.13a)
2
kLA = ksLAKLAKH2 Ct1(Ct2) (7.13b)
— rPA (kmol / kg catalyst / sec)
___ kPACPAPH2
(1+ KLACLA + KPACPA + Kl—PrOHCI—PrOH )(1 + 3’ KHZPHZ Y
(7.14a)
2
kPA : ksPAKPAKH2 Ctl (C12) (7.14b)
(_rLA) = ksLA KLA £1.31
(— r PA) ksPA K PA CPA (7'15)

98

The terms in the denominator of the rate equations represent the relative
concentrations of vacant (I), acid-occupied (KMCLA and KpACpA), alcohol-occupied (K 1-
”0,202,020, and hydrogen-occupied (KH2PH2) surface sites. For 0.5 M concentrations of
LA and PA at 403 K, the site occupancies are 39.1% vacant, 34.3% LA, and 26.6% PA.
With 0.5 M l-PrOH present, the fractions change to 26.6% vacant, 23.3% LA, 18.1% PA,
and 32.0% l-PrOH. At the typical hydrogen pressure of 6.9 MPa, the fraction of
hydrogen surface sites occupied is 60.1%. These results indicate that at much higher
concentrations (>2 M), the surface is essentially saturated with reactants.

To examine how well the mixed acid L-H model predicts hydrogenation rates in
acid mixtures, all kinetic parameters obtained from single acid reactions (Table 7.1) were
used directly, without adjustment, in Equations (7.13) and (7.14) for prediction of mixed
acid hydrogenation rates. Parity plots of LA and PA hydrogenation rates in acid mixtures
are shown in Figures 7.5 and 7.6. From Figure 7.5, the two-site L-H model works quite
well for LA in the two-acid hydrogenation over the concentration range 0.05-5 M,
although some deviation is seen at the higher reaction rates. In Figure 7.6, most of the PA
rate data follow the trend predicted by the model; however, discrepancy between the
experimental and predicted rates arises at some reaction conditions. Comparisons of
predicted and experimental acid conversions as a function of time for a typical reaction
condition (mixture of 0.5M LA and 0.5M PA, 0.5 g Ru/C, 50 ml solution, 403 K, 7 MPa
hydrogen) are shown in Figures 7.7 and 7.8. The model reliably predicts conversion rates

of both acids for the hydrogenation of acid mixtures under this condition.

99

 

10
9 - Lactic Acid Rates in the Hydrogenation of Acid Mixtures at 403 K

Predicted Rate x 10‘
(kmol/kg of catalyst/s)

 

 

 

 

0 1 2 3 4 5 6 7 8 9 10
Experimental Rate it 106 (kmol/kg of catalyst/s)

Figure 7 .5. Experimental and predicted lactic acid rates in the hydrogenation of acid
mixtures over Ru/C.
Conditions: T = 403 K, 0.5 g (5 wt % Ru/C)/50 ml solution, 1000 rpm.

 

—s
4}

Propionic Acid Rates in the Hydrogenation of y = x
Acid Mixtures at 403 K

S

P
O
1

'39. 0..
o

‘0

p
m

9
a

1
8

.000 .900.
0000...

he.

f F

00 02 04 06 08 10 12 14
Experimental Rate it 106 (kmol/kg of catalyst/s)

Figure 7 .6. Experimental and predicted propionic acid rates in the hydrogenation of acid
mixtures over Ru/C.
Conditions: T = 403 K, 0.5 g (5 wt % Ru/C)/50 ml solution, 1000 rpm.

Predicted Rate x 106
(kmol/kg of catalyst/s)
O
as

P
N

 

 

 

9
o

100 '

 

100%

80% 7

60%7

40%7

20% 7

Lactic Acid Conversion

 

 

 

0% r 1 t . 1 ,
0 l 2 3 4 5 6 7

Reaction Time (hour)

 

Figure 7.7. Experimental and predicted lactic acid conversion rates in the hydrogenation
of acid mixtures over Ru/C.
Conditions: mixture of 0.5M LA and 0.5M PA, 0.5 g (5 wt % Ru/C)/50 ml
solution, T = 403 K, Pm = 6.9 MP3, 1000 rpm.
The line represents reaction trajectory predicted by the model.

 

30%

 

 

 

Propionic Acid Conversion

 

0% r r t t r
0 1 2 3 4 5 6 7
Re action Time (hour)
Figure 7.8. Experimental and predicted propionic acid conversion rates in the
hydrogenation of acid mixtures over Ru/C.
Conditions: mixture of 0.5M LA and 0.5M PA, 0.5 g (5 wt % Ru/C)/50 ml
solution, T = 403 K, Pm = 6.9 MPa, 1000 rpm.
The line represents reaction trajectory predicted by the model.

101

7.1.4. Error analysis

The absolute value of percentage error in rate prediction for each data point was
calculated via the expression in parenthesis in Equation (7.16a) below; the average was
then determined over all individual data points. The average percent error in prediction of
hydrogenation rates of LA and PA, both individually and in mixtures, are shown in Table

7.3.

 

z lPredicted Rate — Experimental Ratel

,, Experimental Rate
Error % = x 100%
n

 

(7.16a)

The average absolute percent error for the single acid hydrogenation is within
accepted range for kinetic modeling. It must be noted that the absolute percent error as
described above weighs small values of reaction rate equally with large values, such that
a small absolute deviation at low reaction rates can give a high percent error. This method
of determining errors is thus conservative. If the error is taken relative to the average rate
over all data points [a method that weighs small rates to a lesser extent, shown in
Equation (7.16b) below], the average errors for LA and PA kinetic models are 9% and

1 1% respectively.

102

Z |Predicted Rate - Experimental Rate|

 

 

Error % = n x 100%
2 (Experimental Rate)

n

 

n

2 lPredicted Rate — Experimental Ratel

= ,. x 1009
2(Experimental Rate) 0 (7.16b)

II

 

Percentage errors in mixed acid hydrogenation are similar for LA but larger for
PA; the trend of PA hydrogenation in some experiments is unexplained at this point and
even is opposite to that predicted by the kinetic model. Many of the experiments with
these difficulties were conducted at relatively high concentrations (>2 M or >20 wt%) of
acid, where the reaction environment may be substantially different from the aqueous

phase media at lower concentrations.

Table 7 .3. Average Error between Experimental and Predicted Rates over Ru/C at

 

 

 

 

 

 

403 K
Use E uation (7.16a) Use E nation (7.16b)
Reaction System Average Average Error Average Average Error
Error of the Initial Error of the Initial
(%) Ratefl%) (%) Rates (%)
Individual LA
hydrogenation 15 14 9'5 1 1
Individual PA
hydrogenation 1 8 17 1 1 13
LA in the hydrogenation
of acid mixtures 1 7 13 1 5 16
PA in the hydrogenation
of acid mixtures 3O 17 22 19

 

 

 

 

 

 

 

103

7.1.5. Summary

Hydrogenation of LA and PA was carried out over a carbon-supported ruthenium
catalyst for the acids alone in solution and for mixed acids in water. A two-site
Langmuir-Hinshelwood kinetic model with surface reaction of the adsorbed acid as the
rate-limiting step was developed to describe the hydrogenation of individual acids, acid
mixtures, and acid mixtures with product alcohols. Competitive adsorption of the reacting
species, reflected in the adsorption constants determined by fitting the L-H model, is
postulated to be responsible for the strong effect of additional species on acid
hydrogenation rates. The same kinetic and adsorption constants were used to characterize
the hydrogenation of single acids and acid mixtures. The fit of the model was excellent
for single acid hydrogenation, and reasonably good for mixed acids over a wide range of
acid concentrations.

Finally, the activated carbon support may play a role in influencing relative rates
of organic acid hydrogenation. Since the role of activated carbon is often to selectively
adsorb organic species from water, the selective adsorption of acids or alcohols into the
carbon micropores may result in higher concentrations in the catalyst vicinity and thus
affects the observed reaction rates. We have characterized the adsorption behavior of the
species presented here (LA, PA, PG, and l-PrOH) on the activated carbon support
(Chapter 5), and it is necessary to incorporate that information into an expanded reaction

model for hydrogenation.

104

 

7.2. Hydrogenation of organic acids over ruthenium sponge catalyst

7.2.1. Introduction

The experimental data from acid hydrogenation over Ru sponge at 403 K were
modeled using the same two-site Langmuir-Hinshelwood mechanism (Equations 7.1a-
7.1e) postulated for acid hydrogenation over Ru/C catalyst: Acids adsorb on one type of
surface catalytic site (S1) and hydrogen dissociatively adsorbs on a second type of site
(S2); The irreversible surface reaction of the adsorbed acid is the rate-limiting step and all
other steps are at equilibrium; The adsorption of water is neglected. The L-H rate
expressions for the hydrogenation of single acid and acid mixtures are given in Equations
7.17-7.l9. The PG and l-PrOH terms in the denominator of the rate equations are
eliminated because both products are weakly adsorbed on the ruthenium surface and thus

have little effect on acid conversion rates (shown in Figures 4.10 and 4.11).

For single acid hydrogenations:

— r Add (kmol / kg catalyst/ sec)
k Acid CAcid P H2
(1+ KAcidCAcidX1+ V KHZ PH2 )2 (7.17:1)

kAcid : ks AKAcid KHZ Ctl (Ct2 )2 (7.171))

 

105

For the hydrogenation of acid mixtures:

— rLA (kmol / kg catalyst/ sec)

 

_ kLACLAPHz
(1+ KLACLA + K1,,ACM)(1+,lKHZPH2 )2 (“8a) )
l
kLA : ksLAKLAKHZCtl (Ct2 )2 (7.18b) E:

— rPA (kmol / kg catalyst / sec)

 

: kPACPAPH,
(1+ KLACLA + KPACPA)(1+ ,/KH2PH2 )2 (“93)
2
kPA = ksPAKPAKHz Ct1(Ct2) (7.1%)

where km, = composite rate constant for each acid [m3/kg catalyst/MPa/sec]
k,,, = surface reaction rate constant for each acid [(kg catalyst)2/kmolz/sec]
K Add = adsorption constant for each acid [m3/kmol]
K H2 = adsorption constant for hydrogen [l/MPa]
C,, = total catalyst site concentration for acid and alcohol adsorption [kmol/kg
catalyst]

C,2 = total catalyst site concentration for hydrogen adsorption [kmol/kg catalyst]

106

7.2.2. L-H model parameters for acid hydrogenation over Ru sponge catalyst

The hydrogenation of LA and PA (alone and mixed) over Ru sponge was
modeled using Equations (7 .17)-(7.19). The adsorption constants of LA and PA on Ru
sponge were assigned according to the adsorption constants on Ru/C (Kama = 1.7
m3/kmol, KpMu/c = 1.4 m3/kmol) to compensate for pore adsorption effects: as seen in
Figure 4.1, the calculated pore concentration of PA at 100-150 °C is 7-8 times that of the
bulk concentration of 0.25 M; in contrast, the pore concentration of LA is only 2—3 times
the bulk concentration at those temperatures. The calculated values of K1,“, Wong, and

KMR" sponge are 0.7 m3/kmol and 0.18 m3/kmol, respectively, based on Equations (7.20a)

 

 

 

 

and (7.20b).
KIA. Ru/C _ Pore Concentration _ 2 5
KLA, Ru sponge Bulk Concentration ° (7-20a)
KPA,Ru/C _ Pore Concentration _ 7 5
Km, Ru sponge Bulk Concentration ° (7201’)

The rate constants k1,“, sponge and km,“ Sponge in Equations (7.17)-(7.l9) were
calculated by a least-squares regression analysis of the kinetic data taken from 19
experiments at 403 K (shown in Table 4.1; include the reactions of single acids and acid
mixtures). The rate data from these experiments were normalized on the same basis of
catalytic activity using the method discussed in Section 4.3.4. For the hydrogenation of
LA, we observed catalyst deactivation after 24 hours of reaction; therefore, only the
experimental data at t S 24 hours were used for modeling. For the hydrogenation of PA,
only the initial data (t = 0) were applied in modeling for most experiments due to the low

acid conversions (PA conversion 3 10 %).

107

 

 

The values of all kinetic constants for acid hydrogenation over Ru sponge are
reported in Table 7.4. The higher adsorption constant of LA compared to PA indicates
that LA is adsorbed more readily on ruthenium surface sites. Figures 7.9 and 7.10 are
parity plots of acid hydrogenation rates including all experimental data (single acids and
acid mixtures); Figures 7.11 and 7.12 compare the experimental and predicted rates of
individual LA and PA; Shown in Figures 7.13 and 7.14 are the parity plots of LA and PA
rates in mixed acid hydrogenations. The two-site L-H model gives reasonably good fit of
the conversion kinetics of mixed acid hydro genations at 403 K, hydrogen pressures from
3.4 to 7.9 MPa, and total initial concentrations from 0.2 to 1.1 M. All of the rate data
from single acid hydrogenation follow the trend predicted by the model although the
conversion rates are under-predicted. The single acid experiments were all conducted at
very low concentrations (3 0.1 M), which may attribute to the discrepancy observed

between experimental and predicted rates.

Table 7.4. Kinetic Constants for Acid Hydrogenation over Ru Sponge at 403 Kc

 

 

 

 

 

 

 

 

 

 

 

Kinetic Constants Values at 403 K
Composite rate constant of LA k2,, (m3/kg catalyst/MPa/s) 4.2E-07
Composite rate constant of PA [(13); (mj/kg catalyst/MPa/s) 3.6E-08
Adsorption constant of LA K2,, (m3/kmol) 0.7
Adsorption constant of PA KP, (m3/kmol) 0.18
Adsorption constant of PG Km (m3/kmo1) 0
Adsorption constant of 1-PrOH K 149,017 (m3/kmol) 0
Adsorption constant of H2 KHz (MPa'r) 0.22

 

 

cConditions: 403 K; 5 g Ru sponge/50 ml solution; 1000 rpm.

108

 

Predicted (41A) [kmol/kg catalyst/s]

 

4.0E-07

3013-07 7

2013-07 7

 

 

 

LOB-07 7
a"... 8 °
0.01~:+00 . . t
0.0E+OO LOB-07 ZOE-O7 3.0E-07

4.0E—07

Experimntal or“) [knnllkg catalyst/s]

Figure 7 .9. Experimental and predicted rates for lactic acid hydrogenation over Ru
sponge (all data).
Conditions: 403 K; 5 g Ru sponge/50 ml solution; 1000 rpm.

 

4.0E-08 .

3013-08 7

2013-08 7

1.0E-08 7

 

Predicted (an) [kmol/kg catalyst/s]

 

 

 

0.0E+00 v . r T
0.0E+00 LOB-08 2.0E-08 3.0E—08
Experimental (an) [kmol/kg catalyst/s]

4.0E-08

Figure 7 .10. Experimental and predicted rates for propionic acid hydrogenation over Ru
sponge (all data).
Conditions: 403 K; 5 g Ru sponge/50 ml solution; 1000 rpm.

109

 

1 013-07

8013-08 7

6.0E-08 7

4013-08 7

2.0E-08 7

 

Predicted Hut) [kmol/kg catalyst/s]

 

 

 

0.0E+00 r r t r
0.0E+00 2.0E—08 4.0E-08 6.0E-08 8.0E-08 1.0E-07

Experimental (an) [kmol/kg catalyst/s]

Figure 7.1 1. Experimental and predicted rates for individual lactic acid hydrogenation
over Ru sponge.
Conditions: 403 K; 5 g Ru sponge/50 ml solution; 1000 rpm.

 

1.0308
8.0309 -
6.0309 .
4.0309 - - a 2‘ x”

2.0309— A, 4’ X 9

Predicted (an) [kmol/kg catalyst/s]
O

 

 

 

0.0E+OO I I l I
0.0E+00 2.0E—09 4.0E-09 6.0E-09 8.0E-09 1.0E-08

Experimental (4“) [kmol/kg catalyst/s]

Figure 7 .12. Experimental and predicted rates for individual propionic acid
hydrogenation over Ru sponge.
Conditions: 403 K; 5 g Ru sponge/50 ml solution; 1000 rpm.

110

 

4015-07

3013-07 7

2013-07 7

1.0E—07 7

Predicted (ma) [kmol/kg catalyst/s]

 

 

 

 

0.0E+00 1 1 ,
0.0E+00 1.0E-07 2.0E—07 3.0E-07 4.0E—07

Experimental (41A) [knnl/kg catalyst/s]

Figure 7 .13. Experimental and predicted lactic acid rates in the hydrogenation of acid
mixtures over Ru sponge.
Conditions: 403 K; 5 g Ru sponge/50 ml solution; 1000 rpm.

 

4.0308 ,

3.0E-08 7

2.0E—08 7

1.0E-08 7

 

Predicted (an) [kmong catalyst/s]

 

 

 

0.0E+00 . I t t
0.0E+00 1.0E-08 2.0E-08 3.0E-08 4.0E-08

Experimental (4“) [knnl/kg catalyst/s]

Figure 7 .14. Experimental and predicted propionic acid rates in the hydrogenation of
acid mixtures over Ru sponge.
Conditions: 403 K; 5 g Ru sponge/ 50 ml solution; 1000 rpm.

111

The average percent errors in prediction of hydrogenation rates of LA and PA,
both individually and in mixtures, were calculated using Equations (7.16a) and (7.16b),
and the results are shown in Table 7 .5. The average percent errors for acid
hydro genations are within accepted range for kinetic modeling, considering the existence
of uncertainty in measuring metal surface areas and normalizing ruthenium sponge

activities.

Table 7 .5. Average Error between Experimental and Predicted Rates over Ru

 

 

 

 

 

 

 

Sponge at 403 K
Use Equation (7.16a) Use Equation (7.16b)
. Average Average
Reaction System Aggzge Error of the Aggzlge Error of the .
(o/ ) Initial Rates (0/ ) Initial Rates
° (%) " (%)
Individual LA hydrogenation 28 23 30 25
Individual PA hydrogenation 41 34 43 38
LA 1n the hydrogenatron of 34 20 27 25
acrd mrxtures
PA in the hydrogenation of
acid mixtures (all data) 122 127 34 27
PA in the hydrogenation of
acid mixtures (omit PA rate , 34 30 25 21
data from Experiment 94) d

 

 

 

 

 

 

 

‘ e rate data of PA in Experiment 94 (mixture of 1M LA and 0.1M PA; 5g Ru-I-2;
403K; 7MPa H2) were omitted in the error analysis. The PA conversion in this
experiment was less than 4% after 50 hours of reaction. The polynomial fit of PA
concentrations versus time can NOT give accurate prediction of experimental rates. The
PA rate data are far off in the parity plot upon being fit, and thus introduce very large
absolute percent errors (600-700%).

112

7.2.3. Comparison of the model parameters for acid hydrogenation over Ru/C and
Ru sponge catalysts

A comparison of the model parameters for acid hydrogenation over Ru sponge
with that determined from experiments over Ru/C is given in Table 7.6. The surface
reaction rate constant for each acid was calculated on a metallic surface area basis (kmol

acid/m2 metal surface area/s) using Equation (7.21 c):

 

 

 

2
kAcid = ksACt1(Ct2 ) . I(Acid . KHZ . Scatalyst (7.21a)
m3 _[ kmol ]. m3 I: l ] m2 metal

kg catalyst-MPa - s In2 metal~s kmol MPa kg catalyst
(7.21b)

 

 

.(c..)2 1..., km

2 . = 7.21c
m metal 5 K AcidK H2 Scatalyst ( )

ks, C

t

where Scamp“ = metal surface area of the catalyst [m2 metal / kg catalyst].

The surface reaction rate constants for LA and PA are about the same on the
carbon-supported and un-supported ruthenium catalysts, given that we have substantial
uncertainty (i 40%) in measuring the active surface area for the Ru sponge. The
adsorption constant on Ru/C is 2.5 times that on Ru sponge for LA, and 7.5 times for PA.
The relative magnitude of the adsorption constants on Ru/C and on Ru sponge were
determined based on adsorption studies conducted on activated carbon support. The

product l-PrOH is responsible for inhibiting the PA hydrogenation rates on Ru/C due to

113

its strong competitive adsorption into the carbon micropore, while this effect is
minimized on the nonporous Ru sponge. There is no difference in hydrogen adsorption

affinity for the two catalyst cases.

Table 7.6. Comparison of the Kinetic Constants for Acid Hydrogenation over Ru/C

 

 

 

 

 

 

 

 

 

 

and Ru Sponge at 403 K°
Kinetic Constants Ru/C Ru Spon e
Surface reaction rate ksuC,,(C,2)Z 1 7E-08 1.7E-08
constant for LA (kmol/m2 metal surface area/s) ' (:t 6.8E-09)
Surface reaction rate k,p,4Cr1(C,2)T 3 5E-09 5.7E-09
constant for PA (kmol/m2 metal surface area/s) ' (i 2.3E-09)
Composite rate 3
constant of LA k2,, (m /kg catalyst/MPa/s) 1.0E-05 4.2E-07
Composite rate 3
constant of PA kpA (m /kg catalyst/MPa/s) 1.7E-06 3.6E-08
Adsorption constant 3
of LA KL, (m /kmol) 1.7 0.7
Adsorption constant 3
()pr KPA (m /kmol) 1.4 0.18
Adsorption constant 3
of PG KPG (m /kmol) 0.27 0
Ad ti tant
sogglf’lfi‘ggs K,.p,0,,(m3/1ono1) 2.9 0
Adsorption constant KHz (MPa'l) 0.21 0.22
Of H2

 

 

 

 

 

 

°Ihe metal surface areas are 1.6 m2/ g (Ru/C) and 0.16 mz/ g (Ru sponge), respectively.

114

7.3. Global model for the hydrogenation of organic acids over Ru/C catalyst -
incorporating local pore concentrations into kinetic modeling

Adsorption of reactant and product species can have a major effect on local pore
concentrations in activated carbon-supported metal catalysts. This is because the
activated carbon micropores and functionalized carbon surfaces facilitate selective
adsorption of organic species fiom water; at equilibrium this adsorption typically leads to
local reactant and product concentrations in the catalyst vicinity (e.g. in activated carbon
micropores) that are significantly different fiom those in bulk solution phase outside the
carbon support.3 Because chemical kinetics and mass transport are commonly represented
in terms of concentration or mole fraction, a correct description of reaction kinetics must
account for the difference in concentration between solution and pore. To gain a deeper
understanding of aqueous-phase catalysis involving activated carbon-supported catalysts
in general, Lars Peereboom3 in Miller group conducted adsorption study on the carbon
support for the prediction of local pore concentrations. We assume that reaction kinetics
are the same for hydro genations taking place on the surface of Ru sponge and inside the
pore structures of Ru/C, if we compensate for pore effects of the carbon support. By
incorporating local pore concentrations on Ru/C into the reaction kinetics on Ru sponge,
we expect to be able to reliably predict hydrogenation rates of organic acids over Ru/C
catalyst - the global model.

The local concentrations of reaction species LA, PA, PG, and l-PrOH in the 3310
activated carbon micropores were calculated using the (extended) Langmuir adsorption
model discussed in Chapter 5, based on the measured micropore volume of 0.17 cm3/g

carbon. The Langmuir adsorption model works well for bulk solution concentrations

115

below 0.5 M and temperatures from 298 K to 433 K. The kinetic data used in the global
model were taken fiom all hydrogenation experiments performed over Ru/C at 403 K
(shown in Table 3.1: Pm = 3.4-10.3 MPa, and Cacid = 0.05-5 M). For the experiments
with low initial concentrations (5 0.5M) of acids, we applied pore concentrations instead
of solution concentrations in the new modeling process. The pore concentrations are
usually higher than the solution concentrations since organic species tends to fill the
carbon pore. For the high concentration cases (2 2M), we took bulk solution
concentrations for modeling because the catalyst surface is saturated and there is no
significant enhancement in local pore concentrations.

Kinetic parameters of the global model are given in Table 7.7. The surface
reaction rate constants (km, C,1(C,2)2 and kspA C, ,( C,2)2 ) and equilibrium constants (KM,
KpA, Kpg, K1.p,0H, and K32) were taken directly fiom the modeling results of Ru sponge
shown in Table 7.6. The composite rate constants (km and kpA) for pore reactions on
Ru/C were calculated based on surface reaction rate constants on Ru sponge by
accounting for difference in metal surface area for the two catalyst cases (the metal
surface area is 1.6 mZ/g for Ru/C, and 0.16 m2/ g for Ru sponge).

Figures 7.15-7.l8 are parity plots of acid conversion rates for the hydrogenation
of single and mixed acids on Ru/C catalyst. These parity plots illustrate the agreement
between the experimental rates and the rates predicted by the global model. By
incorporating local pore concentrations on Ru/C into the reaction kinetics on Ru sponge,
the global model works reasonably well in prediction of the hydrogenation rates for
single and mixed acids on Ru/C over a wide range of conditions (acid concentrations

fi'om 0.05 to 5M; hydrogen pressures from 3.4 to 10.3 MPa).

116

Table 7.7. Kinetic Constants for Acid Hydrogenation over Ru/C at 403 K (Global

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Model) "3
Kinetic Constants at 403 K
C°mm31tfczlazf$$§m 0f LA k2,, (m3/kg catalyst/MPa/s) 4.2306
Composrtgeczzlrzelngcsl;ant Of PA [(11,] (m3/kg catalyst/MPa/s) 3.6E-07
Surface reaction rate constant for ksuC,,(C,2)I 1 7E 08
LA (kmol/m2 metal surface area/s) ’ -
Surface reaction rate constant for k,” CrI(C,z)z 5 7E-09
PA (kmol/m2 metal surface area/s) '
Adsorption constant of LA K2,, (ms/kmol) 0.7
Adsorption constant of PA KP, (m3/1ttno1) 0.18
Adsorption constant of PG Kpc; (m3/kmol) 0
Adsorption constant of l-PrOH K ”,0” (mi/kmol) 0
Adsorption constant of H2 K”, (MPa‘I) 0.22
1Conditions: 403 K; 0.5 g (5 wt % Ru/C)/50 ml solution; 1000 rpm.
gUse pore concentrations.

117

 

8.0E—06

 

 

 

 

 

3:"
3 6015-06 7
i”
i 4.0E—06 7
a?
g 2.0306 ~
8
II
a.
0.0E+00 r t ,
0.0E+00 ZOE-06 4.0E-06 6.0E-06 8.0E-06
Experimental (an) [knnl/kg catalyst/s]

Figure 7 .15. Experimental and predicted rates for individual lactic acid hydrogenation
over Ru/C (global model; use pore concentrations).
Conditions: T = 403 K, 0.5 g (5 wt % Ru/C)/50 ml solution, 1000 rpm.

 

 

Predicted (an) [kmol/kg catalyst/s]
\O
O
E“
O
\)

 

 

 

0.0E+00 t r 1 r 1
0.0E+00 3.0E—07 6.0E—O7 9.0E-07 1.2E-06 1.5E—06 1.8E—06

Experimental (an) [knnllkg catalyst/s]

Figure 7 .16. Experimental and predicted rates for individual propionic acid
hydrogenation over Ru/C (global model; use pore concentrations).
Conditions: T = 403 K, 0.5 g (5 wt % Ru/C)/50 ml solution, 1000 rpm.

118

 

1.0E—05

  
  
  
  
 

8.0E—06 7

6013-06 7

4013-06 7

2013-06 7

Predicted (din) [kmol/kg catalyst/s]

 

 

 

0.0E+00
0.0E+00 2.0E—06 4.01306 6.0E—06 8.0B-06 1.0E—05

Experimental (41,.) [kmol/kg catalyst/s]

f I T

Figure 7 .17. Experimental and predicted lactic acid rates in the hydrogenation of acid
mixtures over Ru/C (global model; use pore concentrations).
Conditions: T = 403 K, 0.5 g (5 wt % Ru/C)/50 ml solution, 1000 rpm.

 

1.4E-O6
1.2E—06 - y: x
1.0306 -
8.0307 - «I... .
6.0307 -

4.0307— P o . .
0...

2013-07 7

 

 

Predicted (an) [kmol/kg catalyst/s]
o
o
0
g:
3 . 0°
0%
0°.

 

0c0E+OO I I I I I I
0.0E+00 2.0E—07 4.0E-07 6.0E-O7 8.0E—07 1.0E—06 1.2E—06 1.4B-06

Experimental ('1‘pA) [kmol/kg catalyst/s]

Figure 7 .18. Experimental and predicted propionic acid rates in the hydrogenation of
acid mixtures over Ru/C (global model; use pore concentrations).
Conditions: T = 403 K, 0.5 g (5 wt % Ru/C)/50 ml solution, 1000 rpm.

119

 

Table 7.8 shows the results of error analysis for the global model. The average
percent errors in prediction of hydrogenation rates of LA and PA were calculated using
Equations (7.16a) and (7.16b). The relatively large errors seen here are attributed to the
uncertainty involved in measuring metal surface areas and the limited concentration

range (less than 0.5M) applied to the adsorption model.

Table 7.8. Average Error between Experimental and Predicted Rates over Ru/C at

 

 

 

 

 

 

 

 

 

 

403 K (Global Model)
Use E uation (7.16a) Use E nation (7.16b)
Reaction System Average Average Error Average Average Error
Error of the Initial Error of the Initial
(%) Rates (%) (%) Rates (%)
“‘1‘de 13A 57 32 29 24
hydro genatron
Ind‘v‘dual IA 65 59 37 36
hydrogenation
LA in the hydrogenation 30 20 24 21
of acrd mixtures
PA in the hydrogenation
of acid mixtures 62 36 48 40

 

 

120

 

 

7 .4. References

(1) Jere, R; Jackson, J. E.; Miller, D. J. Kinetics of aqueous phase hydrogenation of
L-alanine to L-alaninol. Ind. Eng. Chem. Res. 2004, 43, 3297.

(2) Yuqing Chen, Dennis J. Miller and J arnes E. Jackson. Kinetics of Aqueous-Phase
Hydrogenation of Organic Acids and Their Mixtures over Carbon Supported

Ruthenium Catalyst. Ind. Eng. Chem. Res. 2007, 46, 3334-3340.

(3) Lars Peereboom, Benjamin Koenigsknecht, Margaret Hunter, James E. Jackson,
and Dennis J. Miller. Aqueous-Phase adsorption of propylene glycol onto

activated carbon. Carbon 2007, 45, 579-586.

121

 

Chapter 8. Conclusions and Future Work

Our research aims to develop an improved mechanistic understanding of organic
acid hydrogenation on heterogeneous ruthenium catalysts in aqueous solution. We
approach this goal by investigating the hydrogenation of LA and PA alone, together, and
in mixtures with their alcohol products PG and 1-PrOH, over carbon-supported and un-
supported ruthenium metal catalysts in a three-phase stirred batch reactor. The present
study characterizes (competitive) adsorption and reaction by a generalized two-site L—H
kinetic model that describes the hydrogenation rates of the acids (alone and mixed). The
model includes effects of product adsorption and inhibition, and accounts for the
difference in concentration between solution and carbon micropores. The model reliably
predicted acid hydrogenation kinetics over a wide range of concentrations.

The future work:

(1) Hydrogenation of single acids over Ru sponge was investigated only at low
concentrations (_<_ 0.1 M). For a more robust model, higher concentrations (0.5 M, 1 M,
and 2 M) of initial feedstock may need to be included.

(2) We observed inconsistency in catalytic activity of the Ru sponge catalyst.
Surface techniques, such as x-ray photoelectron spectroscopy (XPS), would provide
insights into the changes to the catalyst due to different reaction conditions, surface
defects, crystal faces, and the effects of foreign metals.

(3) Characterizing the configuration of species adsorbed on the ruthenium metal
catalyst, and relate adsorption affinity and reactivity to substrate structure and feedstock

composition, is still a major challenge of the present work. The continuous microreactor

122

system (Figure 2.4) designed by Lars Peereboom allows us to accurately measure the
quantity of species adsorbed onto metal surfaces. Chemisorption of single acids and acid
mixtures need to be observed at different concentrations (or concentration ratios) and
temperatures for the purpose of characterizing their relative affinities for metal surface
sites. The reversibility of species adsorption can be examined via temperature
programmed desorption (T'PD) technique. Attenuated total reflection infrared
spectroscopy (ATR-IR) is a useful method to characterize substrates adsorption structures
and their linkages with the catalyst surface in aqueous solution.1 Isotopic labeling (H-D
exchange) in combination with l3C-NMR provides non-destructive measure of substrate-
metal interactions, and helps us determine the reactivity of hydrogen atoms on different
carbons of a specific organic acid. H-D exchange between D20 and hydrogen yields
important information about water-catalyst interaction and thus the role of water in

aqueous phase hydrogenation.

123

References

(1) Dalavoy, T. S.; Jackson, J. E.; Swain, G. M.; Miller, D. J.; Li, J .; Lipkowski, J.,
Mild electrocatalytic hydrogenation of lactic acid to lactaldehyde and propylene
glycol. Journal of Catalysis 2007, 246, (1), 15-28.

124