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This is to certify that the
dissertation entitled

REGIO AND CHEMO SELECTIVE HYDROGENATION BY
PALLADIUM NANOPARTICLES EMBEDDED IN
POLYELECTROLYTE FILMS

presented by

Somnath Bhattacharjee

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Ph.D. degree in Chemistry

 

 

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REGIO AND CHEMO SELECTIVE HYDROGENATION BY PALLADIUM
NANOPARTICLES EMBEDDED IN POLYELECTROLYTE FILMS

By

Somnath Bhattachaijee

A DISSERTATION

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

DOCTOR OF PHILOSOPHY

Chemistry

2008

ABSTRACT
REGIO AND CHEMO SELECTIVE HYDROGENATION BY PALLADIUM
NANOPARTICLES EMBEDDED IN THE POLYELECTROLYTE FILMS
By

Somnath Bhattacharjee

Transition-metal nanoparticles (NPs) are attractive catalytic materials because
they possess a high surface area to volume ratio and electronic properties that are a
function of particle size. Without a suitable support, however, aggregation of NPs tends
to reduce their surface area and alter their electronic properties. This research examines
the catalytic properties of NPs embedded in multilayer polyelectrolyte ﬁlms. The
polyelectrolyte matrix restricts NP aggregation, and layer-by-layer assembly on alumina
supports yields a heterogeneous catalyst that can be easily recycled. Encapsulated NPs
exhibit remarkable catalytic selectivities that are a strong function of NP diameter.

Pd NPs embedded in multilayer polyelectrolyte ﬁlms can be easily prepared
through layer-by—layer adsorption of poly(acrylic acid) (PAA) and poly(ethylenimine)-
Pd2+ (PEI-Pd(II)) complexes followed by chemical reduction of Pd(II) with NaBH4.
Transmission electron microscopy conﬁrms the formation of Pd particles with diameters
of 1-3 nm. Remarkably, [PAA/PEI-Pd(0)]3PAA ﬁlms catalyze the hydrogenation of
monosubstituted alkenes with turnover frequencies (TOFs) that are as much as IOO-fold
higher than TOFs for hydrogenation of multiply substituted double bonds. These
selectivities are higher than those of Wilkinson’s catalyst. Regioselective hydrogenation
of monosubstituted C=C bonds occurs when substrate molecules contain both mono and

multiply substituted C=C bonds. The combination of the encapsulating polyelectrolyte

ﬁlm and small N'Ps likely hinders access of multiply substituted double bonds to catalytic
sites.

NP-containing ﬁlms can also be prepared through deposition of PAA-Pd(II)/PEI
ﬁlms and subsequent reduction of Pd(II). The use of different ratios of PAA to Pd(II) in
deposition solutions gives a series of ﬁlms with Pd NPs whose average diameters range
from 2.2 to 3.4 nm, and the catalytic selectivities of these NPs vary dramatically with
their size. TOFs for the hydrogenation of monosubstituted unsaturated alcohols increase
with decreasing average NP size, whereas multisubstituted unsaturated alcohols show the
opposite trend. Hence, the ratio of TOFs for the hydrogenation of allyl alcohol and crotyl
alcohol is 39 with average particle diameters of 2.2 nm and only 1.3 with average particle
diameters of 3.4 nm. Ratios of TOFs for hydrogenation of allyl alcohol and B-methallyl
alcohol are as high as 240. Increasing selectivity with decreasing average particle size
occurs with both ﬁlms deposited on alumina powder and NPs stabilized by
polyelectrolytes in solution. Presumably, the highest selectivities occur on the smallest
NPs because hydrogenations of monosubstituted and multisubstituted double bonds take
place on different types of atoms

[PAA-Pd(0)/PEI]3 ﬁlms also catalyze chemoselective hydrogenation of aromatic
nitro compounds that contain other readily reducible functional groups. The resulting
amines may serve as intermediates in the synthesis of a wide range of important organic
compounds. In the reduction of 4—nitrobenzaldehyde to 4-aminobenzaldehyde, [PAA-
Pd(O)/PEI]3 ﬁlms are more selective than either Pd/C or Pd/alumina. Chemoselective
hydrogenation of aromatic nitroamides, nitroesters, nitroaldehydes, and nitronitriles is

also possible with these catalysts.

TO MY FAMILY

iv

ACKNOWLEDGEMENT

I wish to express my gratitude with humility to some special people who
contributed enormously to my success as a graduate student. First I want to thank my
thesis advisor Dr. Merlin Bruening for his encouragement, enthusiastic words, guidance
and invaluable suggestions during this fascinating study. I especially thank him very
much for accepting me into his group during a tough time as a graduate student and
allowing me to independently explore my ideas. I primarily owe my achievement in this
dissertation to him. I wish I could be able to grasp everything that he tried to teach me.

I am extremely grateful to Dr. Babak Borhan for not only accepting me to be part
of his group but also his generous guidance and invaluable suggestions during my stay in
his group as well as the time after I leﬁ his group. Moreover, I convey my thanks to him
for serving on my PhD committee. I express my special gratitude to him for providing
me an opportunity to work with Dr. Jennifer Schomaker to learn several great tricks
regarding organic synthesis.

I am very thankful to Dr. Schomaker for her suggestions, guidance and
enthusiastic words. I express my gratitude to her especially for sharing her experience
regarding chemistry and life during my stay in Dr. Borhans’s group.

I thank Dr. Gregory Baker and Dr. Kevin Walker for being on my PhD
committee. I am grateful to Dr. Baker in particular for his motivating words and
invaluable suggestions whenever we talked.

I am thankful to all past and present colleagues whom I learn a lot from about

chemistry and life. I would like to thank David Dotzauer, Sebastian Grajales, Dr.

Srividhya Kidambi, Maneesha Adusumill, Lu Ouyang, Fei Xu, Weihan Wang, Yujing
Tan, Guanqing Lin, Dr. Jamie Dunn, Dr. Lei Sun, Dr. Malaisamy Ramamoorthy, Dr.
Jinhua Dai, Parul Jain, Randy Benedict as well as Dr. Chrysoula Vasileiou, Dr. Courtney
Olmsted, Dr. Tao Zheng, Dr. Marina Tanasova, Dr. Montserrat Rabago Smith, Dr. Jun
Yan, Zhihua Shang, Stewart Hart, Daniel Whitehead, Xiaoyong Li, Abra, Kate. Dr.
Rachael Crist, Dr. Ben Travis, Dr. Qifei Yang. I especially would like to thank Dr.
Srividhya Kidambi in particular for generously sharing her experience and knowledge of
her research project with me.

I would like to convey my special thanks to David Dotzauer for being an
awesome colleague as well as a great ﬁiend and provide me all possible help and sharing
his word of wisdom during my stay as a Dr. Bruening’s group member. I would also like
to thank Sebastian Grajales very much for his help and wonderful discussion on several
issues including chemistry.

Last, but not least, I owe the most gratitude to my family members for their
constant support, patience and encouragement. I express my special gratitude with
humility to bapi, ma, baramama, puspita, didi, chordibhai, goutamda, subhanduda,
didibhai, baba and ma (in-laws), mesomasai (Dr. Shyama Prasad Bhattacharyya) and
many others for always being there for me.

Finally, I gratefully acknowledge the generous ﬁnancial support from Department
of Chemistry, Michigan State University, US Department of Energy, American Chemical

Society Petroleum Research Funding.

vi

TABLE OF CONTENTS

LIST OF TABLES ................................................................................. X
LIST OF FIGURES .............................................................................. XII
LIST OF SCHEMES ............................................................................. XVI
LIST OF ACRONYMS ....................................................................... XVIII
CHAPTER 1: BACK GROUND INFORMATION ........................................... 1
1.1. Introduction .................................................................................... 1
. . Early Development of Catalytically Active Transition-Metal NPs. . . . . . . . . . . . . . ......4
1.3. Recent Developments in the Preparation of Stable, Catalytic Metal NPs. . . . . .......6
1.3.1. Immobilization of Metal NPs on Solid Supports ................................ 7

1.3.2. Solution-based Metal NP Stabilizers ............................................ 10

1.4. Layer-by-Layer Assembly of Oppositely Charged Polyelectrolytes.. .................. 15
1.5. Motivation and Research Goal ............................................................ 18
1.6. Outline of the Thesis ....................................................................... 20
1.7. References ................................................................................... 23

CHAPTER 2: SELECTIVE HYDROGENATION OF MONOSUBSTITUTED
ALKENES BY Pd NANOPARTICLES EMBEDDED IN POLYELECTROLYTE

FILMS ................................................................................................ 40
2.1. Introduction .................................................................................. 40
2.2. Experimental ................................................................................. 42
2.2.1. Materials ............................................................................ 42
2.2.2. Preparation of Pd NPs Encapsulated in Polyelectrolyte Multilayer
Films ......................................................................................... 42
2.2.3. TEM of Pd NPs .................................................................... 43
2.2.4. Determination of the Amount of Pd in Catalyst and Deposition
Solutions .................................................................................... 43
2.2.5. Hydrogenation Reaction .......................................................... 44
2.2.6. Recycling Experiment ............................................................ 45
2.2.7. Hydrogenation Reactions by Wilkinson’s Catalyst ........................... 45
2.2.8. Isolation of Products .............................................................. 46

vii

2.3.

2.4.
2.5.
2.6.

2.2.9. Transport Studies .................................................................. 47

Results and Discussions .................................................................... 48
2.3.1. NP Size .............................................................................. 48
2.3.2. Intermolecular Selectivity ........................................................... 50
2.3.3. Intramolecular Selectivity ........................................................ 54
Conclusion ................................................................................... 58
Appendix ..................................................................................... 59
References .................................................................................... 66

CHAPTER 3: SELECTIVITY AS A FUNCTION OF NANOPARTICLE SIZE IN
THE CATALYTIC HYDROGENATION OF UNSATURATED ALCOHOLS......69

3.1.
3.2.

3.3.

3.4.
3.5.

3.6.

Introduction .................................................................................. 69
Experimental Section ....................................................................... 7 1
3.2.1. Materials ............................................................................ 71

3.2.2. Preparation of Pd NPs Encapsulated in Polyelectrolytes .................... 71

3.2.2. 1. [PAA-Pd(0)/PEI]3 on Alumina ......................................... 71
3.2.2.2. [Pd(O)/PEI] on Alumina .................................................. 72
3.2.2.3. PAA-Pd(0)/PEI in Solution ........................................................... 72
3.2.3. Characterization of Pd NPs ....................................................... 73
3.2.4. Hydrogenation Reactions ............................................................ 76
Result and Discussion ...................................................................... 77
3.3.1. Catalyst Synthesis and Variation of Particles Size ............................ 77
3.3.2. Selective Hydrogenation with Films on Alumina Particles .................. 90
3.3.3. Hydrogenation with NP/Polyelectrolyte Complexes in Solution. . . . . .......98
3.3.4. Solvent Effects on Hydrogenation ................................................ 99

. 3.3.5. Hydrogenation of Other Unsaturated Alcohols .............................. 101

Possible Explanation of Selectivity ..................................................... 102
Conclusion .................................................................................. 105
References .................................................................................. 107

CHAPTER 4: CHEMOSELECTIVE NITROARENE HYDROGENATION
CATALYZED BY PALLADIUM NPs EMBEDDED IN POLYELECTROLYTE

MULTILAYERS .................................................................................. 110
4.1. Introduction ................................................................................. 110
4.2. Experimental ................................................................................ 114
4.2.1. Materials ........................................................................... 114
4.2.2. Preparation of Pd NPs on Alumina ............................................ 115
4.2.2.1. [PAA-Pd(0)/PEI]3 ....................................................... 115
4.2.2.2. Reduced PdC142‘ on alumina ........................................... 115
4.2.3. Characterization of Pd NPs ...................................................... 116

viii

4.2.4. Determination of the Pd Content of Catalysts Used in

Hydrogenation .............................................................................. l 16
4.2.5. General Procedure for the Hydrogenation of Nitroaromatic
Compounds ................................................................................. 116
4.3. Result&Discussion ....................................................................... 119
4.3.1. Catalyst Preparation and Characterization ...................................... 119
4.3.2. Hydrogenation of nitrobenzene as a function of solvent and nitrobenzene
concentration. .............................................................................. 121
4.3.3. Hydrogenation of nitrobenzaldehyde .......................................... 123
4.3.4. Hydrogenation of Other Nitroarenes ............................................ 125
4.4. Conclusion .................................................................................. 127
4.5. Appendix ................................................................................... 128
4.6. References ................................................................................... 140
CHAPTER 5: CONCLUSIONS AND FUTURE WORK ................................ 144
5.1. Selective Catalysis ......................................................................... 144
5.2. Selective Hydrogenation of Unsaturated Alcohols as a Function of NP
Diameter .................................................................................... 145
5.3. Chemoselective Hydrogenation of Aromatic N‘itro Compounds by Pd NPs
encapsulated in polyelectrolyte ﬁlms ................................................... 145
5.4. Future Work ................................................................................ 146
5.4.1. Effect of NP Size on the Chemoselective Hydrogenation of 4-
nitrobenzaldehyde ......................................................................... 146
5.4.2. The Contribution of Size-Dependent Electronic Properties to Selective
Hydrogenation .............................................................................. 146
5.4.3. Synergistic Effects Using Bimetallic NPs ..................................... 147
5.5. Summary Statement ....................................................................... 148
5.6. References .................................................................................. 149

ix

LIST OF TABLES

Table 2.1. TOFs for the hydrogenation of a series of unsaturated alcohols by different
Pd catalysts ........................................................................................... 51

Table 2.2. TOFs for the hydrogenation of a series of unsaturated alcohols by
Wilkinson’s catalyst ................................................................................. 54

Table 2.3. TOFs (moles hydrogenated/ mole Pd/ h) for selective hydrogenation of
monosubstituted double bonds in the presence of di and tri substituted double bonds in
the same molecules. The reaction was catalyzed by [PAA/PEI—Pd(0)]3PAA on alumina
in a 3:1 mixture of water and ethanol ............................................................. 56

Table 2. A-l Time-dependent percentage of crotyl alcohol hydrogenation catalyzed by
[PAA/PEI-Pd(0)]3PAA in water .................................................................. 60

Table 3.1. Amounts and Pd Content of Catalysts Used in Hydrogenation
Reactions ............................................................................................. 75

Table 3.2. Calculation of the relative surface area of large (ranging from ~50-300
nm) particles and NPs (< 5 nm) in [PAA-Pd(0)/PEI]3 ﬁlms and reduced [PdCl42'/PEI]3
ﬁlms on carbon-coated TEM grids. The initial concentration of Pd(II) was 15 mM in the
solution containing PAA-PdCl42' or PdCl42'. Large particles were not prevalent when
using lower concentrations of Pd(II) .............................................................. 84

Table 3.3. TOFs and TOF ratios for unsaturated-alcohol hydrogenation catalyzed by
[PAA-Pd(O)/PEI]3 ﬁlms that contain Pd NPs with different diameters. The catalytic ﬁlms
were deposited on micron-sized alumina. Values in parentheses are the sum of TOFs for
hydrogenation and isomerization .................................................................. 92

Table 3.4. TOFs and TOF ratios for unsaturated-alcohol hydrogenation catalyzed by
[PAA-Pd(0)/PEI]3/(PAA/PEI)3 ﬁlms that contain Pd NPs with different diameters. The
ﬁlms were deposited on micron-sized alumina. Values in parentheses are the sum of
TOFs for hydrogenation and isomerization ...................................................... 93

Table 3 5. TOFs and TOF ratios for unsaturated-alcohol hydrogenation catalyzed by
reduced [PdCl42'/PEI]3 ﬁlms that contain Pd NPs with different diameters. The ﬁlms
were deposited on micron-sized alumina. Values in parentheses are the sum of TOFs for
hydrogenation and isomerization .................................................................. 97

' Table 3.6. TOFs and TOF ratios for unsaturated-alcohol hydrogenation catalyzed by
a solution of PAA—Pd(O)/PEI that contains Pd NPs with different diameters. Values in
parentheses are the sum of TOFs for hydrogenation and isomerization ..................... 99

Table 3.7. TOFs and TOF ratios for unsaturated-alcohol hydrogenation catalyzed by
[PAA-Pd(O)/PEI]3 ﬁlms in different solvents. The ﬁlms contain Pd NPs with different

diameters and are deposited on micron-sized alumina. Values in parentheses are the sum
of TOFs for hydrogenation and isomerization ................................................ 100

Table 3.8. TOFs and TOF ratios for unsaturated-alcohol hydrogenation catalyzed by
[PAA-Pd(0)/PEI]3 ﬁlms that contain Pd NPs with different diameters. The catalytic ﬁlms
were deposited on micron-sized alumina. Values in parentheses are the sum of TOFs for
hydrogenation and isomerization ................................................................ 102

Table.3.9. Formula used to calculate m, the number of shells for NPs having
cuboctahedron geometry, and the number of Pd atoms at the various positions available .
on NP surfaces ...................................................................... ' ............... 104

Table 4.1. TOFs for nitrobenzene hydrogenation catalyzed by alumina-supported
[PAA-Pd(II)/PEI]3 ﬁlms in different solvents ................................................. 122

Table 4.2 TOFs (moles hydrogenated/moles Pd/h) for nitrobenzene hydrogenation
by different catalysts in EtOH ..................................................................... 122

Table 4.3. Product distributions from 4—nitrobenzaldehyde hydrogenation (>99%
conversion) using several catalysts and two solvent systems ................................ 124

Table 4.4. Catalytic hydrogenation of various aromatic nitro compound by [PAA-
Pd(O)/PEI]3 ﬁlms and reduced [PdCluz'] on alumina .......................................... 126

xi

LIST OF FIGURES

Figure 1.1. Number of publications found in a search for the key word “nanoparticles”
using SciFinder Scholar (Chemical Abstracts search tool) ...................................... 1

Figure 1.2. Number of publications found for the key phrase “Nanoparticles for
catalysis” in a SciFinder Scholar (American Chemical Society Search tool) search ........ 6

Figure 1.3. Encapsulation of metal nanoparticles in a dendrimer matrix ................ 14

Figure 1.4. Formation of bilayer on the substrate through layer-by-layer deposition of

polyelectrolytes ...................................................................................... 16
Figure 1.5. Intermolecularly selective hydrogenation of allyl alcohols by [PAA/PEI-

Pd(0)]3PAA on alumina ............................................................................ 19
Figure 2.1. Home-built diffusion dialysis set-up for transport experiments ............ 47

Figure 2.2. TEM image of [PAA/PEI/Pd(0)]3PAA on carbon-coated cupper grid. . . ..48

Figure 2.3. TEM image of a [PAA/PEI-Pd(0)]3PAA ﬁlm on a carbon-coated copper
grid. This image was taken at a lower magniﬁcation than Figure 2.4 in to better show the
distribution of NPs .................................................................................. 49

Figure 2.4. Histogram of Pd NP diameters in several TEM images. The average
particle size is 2.2 nm ................................................................................ 49

Figure 2.5. Time evolution of the receiving phase concentrations of allylic alcohols
during diffusion dialysis of 1 (squares), 2 (triangles), 3 (diamonds), and 4 (circles)
through porous alumina membranes coated with a [PAA/PEI]7 ﬁlm. The source phase
initially contained 20 mM of the allylic alcohols, and the receiving phase was initially
deionized water ...................................................................................... 58

Figure 2. A-l Example gas chromatograms of the reaction mixture from
hydrogenation of crotyl alcohol (3) after 10 min (black line) and 40 min (red line) of
reaction time. The reaction was catalyzed by [PAA/PEI-Pd(0)]3PAA in water.
Integration was performed using instrument software .......................................... 61

Figure 2. A-2. Plot of time-dependent percentage of crotyl alcohol hydrogenation
catalyzed by [PAA/PEI-Pd(0)]3PAA in water ................................................... 61

Figure 2. A—3 . 1H NMR spectrum of isolated crude 2-heptene-4-ol (compound 6) in
CD3OD. Impurity peaks are indicated with a * ................................................ 62

xii

Figure 2. A-4 '3 C NMR spectrum of isolated crude 2-heptene-4-ol (compound 6) in
CD3OD. Impurity peaks are indicated with a * ................................................ 63

Figure 2. A-S. IH NMR spectrum of puriﬁed 3,7-dimethyl-6-octen—3-ol (compound
8) in CD3OD. Impurity peaks are indicated with a * .......................................... 64

Figure 2. A-6. '3 C NMR spectrum of puriﬁed 3,7-dimethyl-6-octen-3-ol (compound
8, chapter 2) in CD301). Impurity peaks are indicated with a * .............................. 65

Figure 3.1. (a) TEM image of a [PAA-Pd(0)/PEI]3 ﬁlm on a carbon-coated copper
grid. 1 mM Pd(II) was present in the PAA-KdeCl4 solution used for ﬁlm deposition. A
few of the NPs are circled. (b) Histogram of Pd NP diameters in several TEM images.
The average particle size is 2.2:t0.6 nm. The area-average particle size is 2.5i0.7 nm...78

Figure 3.2. (a) TEM image of a [PAA-Pd(O)/PEI]3 ﬁlm on a carbon-coated copper
grid. 4 mM Pd(II) was present in the PAA-KdeCl4 solution used for ﬁlm deposition. A
few NPs are circled. (b) Histogram of Pd NP diameters in several TEM images. The
average particle size is 3.0i0.8 nm. The area-average particle size is 3.1d:0.5 nm. . .......79

Figure 3.3. (a) TEM image of a [PAA-Pd(O)/PEI]3 ﬁlm on a carbon-coated copper
grid. 8 mM Pd(II) was present in the PAA-KdeCl4 solution used for ﬁlm deposition.

(b) Histogram of Pd NP diameters in several TEM images. The average particle size is
3.2:t0.6 nm. The area-average particle size is 3.5:t0.7 nm .................................... 80

Figure 3.4. (a) TEM image of a [PAA-Pd(0)/PEI]3 ﬁlm on a carbon-coated copper
grid. 15 mM Pd(II) was present in the PAA-KdeCl4 solution used for ﬁlm deposition.
(b) Histogram of Pd NP diameters in several TEM images. The average particle size is
3.4i1.3 nm. The area-average particle size is 3.8i1.1 nm .................................... 81

Figure 3.5. Low-magniﬁcation TEM images of large particles and NPs in [PAA-
Pd(0)/PEI]3 ﬁlms on carbon-coated copper grids. The ﬁlms were prepared with 15 mM
Pd(II) in the PAA deposition solution. (a) Image showing large particles with diameters
of 160d:100 nm, as determined from several TEM images. Such large particles appeared
in ~30% of similar images. (b) Image of NPs. Larger spots are overlapping multiple
particles. The dark spot in the center of low-magniﬁcation images is due to a defect in
the lens where it was burned by the electron beam ............................................. 83

Figure 3.6. (a) TEM image of a dried PAA-Pd(0)/PEI solution on a carbon-coated
copper grid. 1 mM Pd(II) was initially present in the PAA solution. (b) Histogram of Pd
NP diameters in several TEM images. The average particle size is 2.6i0.8 nm. The area-
average particle size is 3.1i0.8 nrn ............................................................... 86

Figure 3.7 . (a) TEM image of a [PAA-Pd(O)/PEI] solution on a carbon-coated copper
grid. 4 mM Pd(II) was initially present in the PAA-PdCluz' solution. (b) Histogram of Pd

xiii

NP diameters in several TEM images. The average particle size is 3.2:t0.7 nm. The area-
average particle size is 3.4:t0.7 nm ............................................................... 87

Figure 3.8. (a) TEM image of a dried PAA-Pd(0)/PEI solution on a carbon-coated
copper grid. 8 mM Pd(II) was initially present in the PAA solution. (b) Histogram of Pd
NP diameters in several TEM images. The average particle size is 3.5:t0.7 nm. The area-
average particle size is 3.7i0.7 nm ............................................................... 88

Figure 3.9. (a) TEM image of a [PAA-Pd(O)/PEI] solution on a carbon-coated copper
grid. 15 mM Pd(II) wasinitially present in the PAA-PdCl42‘ solution. (b) Histogram of
Pd NP diameters in several TEM images. The average particle size is 5.1:t1.3 nm. The
area-average particle size is 5.8:l:1.4 nm ......................................................... 89

Figure.3.10. (a) TEM image of a dried PAA-Pd(O)/PEI solution on a carbon-coated
copper grid. 15 mM Pd(II) was initially present in the PAA solution. This image was
taken at a relatively low magniﬁcation to show the distribution of NPs and the absence of
large particles on the TEM grid. The dark spot in the center of low-magniﬁcation images
is due to a defect in the lens where it was burned by the electron beam ..................... 90

Figure 3.11. (a) TEM image of a [Pd(0)/PEI]3 ﬁlm on a carbon-coated copper grid. The
concentration of PdCl42' used in depositing the precursor [PdCl42’/PEI]3 ﬁlm was 1 mM.
(b) Histogram of Pd NP diameters in several TEM images. The average particle size is
3.0:t0.9 nm. The area-average particle size is 3.5i1.0 nm .................................... 94

Figure 3.12. (a) TEM image of a [Pd(0)/PEI]3 ﬁlm on a carbon-coated copper grid. The
concentration of PdCl42' used in depositing the precursor [PdCl42'/PEI]3 ﬁlms was 15

mM. (b) Histogram of Pd NP diameters in several TEM images. The average particle size
is 3.4i0.8 nm. The area-average particle size is 3.8i0.9 nm ................................. 95

Figure 3.13. (a) Low-magniﬁcation TEM image of a reduced [PdCl42'/PEI]3 ﬁlm on a
carbon-coated copper grid. 15 mM PdCl42’ was initially present in the deposition solution.
The average size of these large particles in several TEM images is 170i110 nm. Large
particles are visible in about 60% of these low-magniﬁcation images, but we estimate that
the surface area of the large particles constitutes ~33% of the surface area in the ﬁlm
(Table 3.2) ............................................................................................ 96

Figure 3.14. Plot of the calculated percentage of surface, defect and face atoms with
respect to the total atoms for each particle size ................................................ 105

Figure 4.1. (a) TEM image of a [PAA-Pd(O)/PEI]3 ﬁlm on a carbon-coated copper

grid. 15 mM Pd(II) was present in the PAA-KdeCl4 solution used for ﬁlm
deposition ............................................................................................. 120

Figure 4.1. (b) Histogram of Pd NP diameters in several TEM image based on their
area..... ............................................................................................................................ 121

xiv

Figure 4. A-l. 1H NMR spectrum of puriﬁed 2-Aminotoluene (compound 7) in
CDC13 ............................................................................................... 128

Figure 4. A-2. '3 C NMR spectrum of puriﬁed 2-Aminotoluene (compound 7) in
CDC13 ............................................................................................................................... 129

Figure 4. A-3. 1H NMR spectrum of puriﬁed Methyl-4-aminobenzoate (compound
4) in CDC13 ......................................................................................... 130

Figure 4. A-4. '3 C NMR spectrum of puriﬁed Methyl-4-aminobenzoate (compound
4) in CDC13 ......................................................................................... 131

Figure 4. A-5. 1H NMR spectrum of puriﬁed 4-Aminobenzamide (compound 6) in
CD3OD .............................................................................................. 132

Figure 4. A-6. '3 C NMR spectrum of puriﬁed 4-Aminobenzamide (compound 6) in
CD30D .............................................................................................. 133

Figure 4. A-7. 1H NMR spectrum of puriﬁed 4-Aminobenzaldehyde and residual
AcOEt (compound 2) in CDC13 .................................................................. 134

Figure 4. A-8. ’3 C NMR spectrum of puriﬁed 4-Aminobenzaldehyde and residual
AcOEt (compound 2) in CDC13 .................................................................. 135

Figure 4. A-9. IH NMR spectrum of puriﬁed 4-Aminobenzonitrile (compound 3) in
CDC13 ................................................................................................ 136

Figure 4. A-10. '3 C NMR spectrum of puriﬁed 4-Aminobenzonitrile (compound 3)
in CDC13 ............................................................................................. 137

Figure 4 A-ll. 1H NMR spectrum of puriﬁed 4-Aminoacetophenone (compound 5)
in CDCl3 ............................................................................................. 138

Figure 4. A-12. '3 C NMR spectrum of puriﬁed 4-Aminoacetophenone (compound
5) in CDC]; ......................................................................................... 139

XV

LIST OF SCHEMES

Scheme 1.1 Heck reaction of styrene and iodobenzene catalyzed by Pd/C and Pd NPs
on single-wall carbon nanotubes ..................................................................... 8

Scheme 1.2. Oxidation of cinnamyl alcohol by Pd/C and Pd NPs on single-wall carbon
nanotubes .............................................................................................. 8

Scheme 1.3. Hydrogenation of cinnamaldehyde by Pd/C and Pd NPs on single-wall
carbon nanotubes ..................................................................................... 8

Scheme 1.4. Pauson-Khand reactions catalyzed by Co and Pd NPs supported on silica
for a one-pot syntheses of bicyclic enones ......................................................... 9

Scheme 1.5. 3-component cascade process by palladium NPs generated in situ to form
isoindolinone ......................................................................................... 1 l

Scheme1.6. Regioselective silylation of sugars through Pd NP-catalyzed silane
alcoholysis ............................................................................................ 1 1

Scheme 1.7. Selective hydrogenation of Dehydrolinalool to linalool by PS-b-P4VP-Pd
on alumina ............................................................................................. 13

Scheme 1.8. Regio selective hydrogenation of Linalool to dihydrolinalool by

[PAA/PEI-Pd(0)]3PAA ﬁlms on alumina ........................................................ 20
Scheme 1.9. Selectivity as a function of NP size in the catalytic hydrogenation of
unsaturated alcohols ................................................................................. 21
Scheme 2.]. [PAA/PEI-Pd(0)]3PAA ﬁlm formation on Alumina .......................... 41
Scheme 3.]. Cuboctahedra Pd NP with different atoms .................................... 103
Scheme 4.1. Chemoselective hydrogenation of aromatic nitro compounds by a
montrnorillonitesilylpalladium (II) complex ................................................... 111
Scheme 4.2. Preparation of a montmorillonitesilylpalladium (II) complex .............. 112

Scheme 4.3. Silicon hydride reduction of nitro benzaldehyde catalyzed by Pd NPs
generated in-situ ............................................................................................................... 1 13

Scheme 4.4. Chemoselective hydrogenation of p-nitrobenzonitrile catalyzed by Pd NPs
on a carbon nanoﬁber .............................................................................. 113

xvi

Scheme 4.5. Hydrogenation of 4-nitrobenzaldehyde to several possible products. . 124

xvii

LIST OF ACRONYMS

LBL: Layer-by-layer

PEI: Poly(ethylenimine)

PAA: Poly(acrylic acid)

TEM: Transmission Electron Microscopy
AES: Atomic Emission Spectroscopy.
NP: Nanoparticles

DMF: Dimethylformamide

MSI: Metal-Support Interaction.
TOF: Turnover Frequencies

PMHS: Polymethylhydrosiloxane
EtOAc: Ethyl Acetate

EtOH: Ethanol

THF: Tetrahydrofuran

xviii

CHAPTER 1

BACK GROUND INFORMATION

1.]. Introduction

During the last 15 years, the formation, characterization, and application of
nanoparticles (N Ps) have become one of the most prominent areas of chemical research.
The yearly number of publications in this ﬁeld has grown exponentially as shown in
Figure 1.1. Much of the interest in NPs stems ﬁ'om the fact that they often manifest

properties that are different ﬁom the properties of the bulk material of the same

 

30000

20000 -'

10000 ‘

# Hits for Nanoparticles

 

 

1990
1995
2000
2005

Publication Year

Figure 1.1. Number of publications found in a search for the key word “nanoparticles”
using SciFinder Scholar (Chemical Abstracts search tool).
“Images in this dissertation are presented in color”

composition,"2 and classical laws of physics are sometimes not applicable to small NPs.
By deﬁnition, NPs have diameters that range from 1 to 100 nm, so they contain between
about 10 and 106 atoms or molecules per particle.3’4 Because of their unique properties,

NPs are being intensively studied for various applications in sensingf"l3

24-29 30-49

optoelectronicsy"18 tissue engineering,”"23 biotechnology, and catalysis.

Because of their high surface area to volume ratio, perhaps the most natural
application area for NPs is catalysis. Berzelius introduced the term catalysis in 1836 to
classify certain types of chemical reactions whose progress is greatly affected by
additional substances even though they are neither reactants nor products.50 Since then, a
molecular understanding of catalytic processes has been emerging, and catalytic reactions
constitute about 90% of chemical processes.“ Solid-state materials, including transition
metals, are frequently the materials of choice for catalysis because they can be readily
separated from a reaction mixture and frequently reused. One disadvantage of
heterogeneous catalysts, however, is that typically only surface atoms serve as active sites
for catalytic processes. The percentage of atoms that reside at the surface of a material
increases with decreasing particle size, so small metal particles are highly reactive
catalysts for many reactions. However, optimal use of metal atoms for catalysis requires
extremely small particles. Even in a 20 nm-diameter particle, only 10% of the atoms
reside at the particle surface. In contrast, in a 3 nm-diameter particle, 50% of the atoms
are on the surface. The large surface areas of nanostructured metals allow efﬁcient use of
metal atoms in various catalytic reactions.

During the twentieth century, chemists made tremendous achievements in

heterogeneous catalysis, which is responsible for the vast majority of chemical

- - 9
production.52 5

Nevertheless, heterogeneous catalysts sometimes show relatively low
selectivity, and it is often difﬁcult to elucidate catalytic mechanisms. On the other hand,
homogeneous catalysts, which have been primarily developed since the early l970$,60'63
can sometimes be tailored to selectively catalyze speciﬁc reactions. Still, homogeneous
catalysts are not often used on a large scale because they cannot be easily recovered.

Transition-metal NPSM’65

might bridge the gap between homogeneous and heterogeneous
catalysis because they can be either suspended in solution as homogeneous catalysts or
ﬁxed onto a support. Although by deﬁnition the diameters of NPs vary between one nm
and 100 nm, the most active NPs in catalysis have diameters of only a few 11mm“58 NPs
are also relevant to homogeneous catalysis because there is a full continuum between
small metal clusters and larger NPs. Moreover, NP catalysts are frequently selective,
efﬁcient, and recyclable, and thus they meet the modern requirements for green catalysts.
Already, numerous applications of transition-metal NP catalysts are emerging.”97

As discussed above, the high surface area of NPs is attractive for catalysis.
However, the high fraction of active surface atoms also gives rise to a high surface
energy that oﬁen causes metal NPs to aggregate. This aggregation yields larger, stable
particles with less surface area and less catalytic activity and provides a serious challenge
in catalysis where control over NP size is often vital for improving the reactivity and
selectivity of a catalyst. Fortunately, several methods have been developed to stabilize
metal NPs and prevent aggregation. Perhaps the most common of these is to cap the

particles with ligands that reduce surface energy or provide repulsive interactions

between particles.98"°l However, the capping ligands may isolate the particle surface and

decrease catalytic activity. Other methods of NP stabilization that may be appropriate for
catalysis include immobilization on solid supports and in polymer ﬁlms.'02"03

This dissertation focuses on the layer-by-layer (LBL) assembly of polyanions and
polycations to incorporate stable Pd NPs in a polyelectrolyte matrix. These NPs show
remarkable regio and chemoselective hydrogenation, and selectivities vary dramatically
with NP diameter. In this introduction I attempt to place my work in context by
providing background on previous studies of NP catalysis and layer-by-layer deposition.
Section 1.2 discusses the historical background of transition-metal NPs and their
preliminary development for catalytic applications. Section 1.3 describes more recent
work including stabilization of NPs using ligands, surfactants, dendrimers/polymers, and
solid supports. Finally, the last section of this chapter (section 1.4) introduces the general

procedure of layer-by-layer assembly, which can be implied to prepare stable and

catalytically active homogeneous and heterogeneous metal NPs for catalytic applications.

1.2. Early Development of Catalytically Active Transition-Metal N Ps

In the mid-nineteenth century, Michael Faraday was the ﬁrst to record the
synthesis of NPs.82"°4 Faraday noted the formation of deep red solutions upon the
reduction of aqueous NaAuCl4 by phosphorous in C82. Faraday believed that the
particles formed via his method had diameters smaller than wavelength of visible light.
Unfortunately, at the time there were no tools for determining the actual size of the gold
particles. In the 19805 J. M. Thomas reproduced the formation of gold particles
according to F araday’s method and generated NPs with diameters ranging from 3-30

nm.105 However, these gold NPs were not used for catalytic applications.

The use of transition-metal NPs as catalysts started in the second half of the twentieth
century. Initial studies included a small but important group of reactions including
hydrogenation, hydrosilylation and hydration of unsaturated organic substrates. In the
19405, Nord reported pioneering catalytic applications of NPs in the reduction of
nitrobenze.'06"08 In 1970, Parravano used Au NP catalysts for hydrogen-atom transfer
between benzene and cyclohexane, as well as oxygen-atom transfer between CO and
C02.'09 However, the real breakthrough in NP catalysis came in the 19805 when Haruta
observed that oxide-supported Au NPs catalyze CO oxidation by 02 at low temperatures
and that the small size of gold is crucial for the tranforrnation.”0'”3 In the 19705 and
19805, NP catalysts were also used in the ﬁelds of redox catalysis, photocatalysis
including photo-water splitting and photo-hydrogenation of C02 to methane, and
hydrogenation and oxidation of unsaturated compoundsm'122 In the 19905, Reetz
reported seminal studies of Pd NPs as catalysts for Heck coupling reactions between
butyl acrylate and iodobenzene or aryl bromides and styrenem'I30 Moreover, Figure 1.2
illustrates that in the ﬁrst few years of the twenty-ﬁrst century, there has been enormous
growth in the number of publications concerning the application of metal NPs for
catalysis. The fundamental goals of recent research on NP catalysts include preparation
of stable NPs; improvement of catalyst activities and selectivities; and understanding the
catalytic mechanisms which cause high activity and selectivity. The next section

discusses studies in these speciﬁc areas.

 

 

 

 

# Hits for Nanoparticles
for Catalysrs
N
O
G
O

1991
1996
2001
2006

Publication Year

Figure 1.2. Number of publications found for the key phrase “Nanoparticles for
catalysis” in a SciFinder Scholar (American Chemical Society Search tool) search.
1.3. Recent Developments in the Preparation of Stable, Catalytic Metal NPs

As mentioned in section 1.1, the preparation of catalytically active, stable metal
NPs is challenging due to their tendency to aggregate. To prevent aggregation, metal
NPs are immobilized on various solid supports such as carbon and metal oxides, or
stabilized by surfactants, ligands, or polymers. Immobilization on a support physically
prevents NP aggregation, whereas ligands, polymers, and surfactants typically surround
particles to lower surface energy or provide repulsive forces between particles. NPs
stabilized by surfactants, ligands, and small polymers can serve as homogeneous
catalysts, whereas NPs on solid supports act as heterogeneous catalysts that can be
readily recovered. The following subsections focus on the different techniques for

preparing stable metal NPs and the catalytic applications of these particles.

1.3.1. Immobilization of Metal N Ps on Solid Supports
Carbon Supports

Charcoal is a common solid support for catalysts, and Pd on activated C (Pd/C) is
an archetypical catalyst. Activated carbon is produced by treatment of charcoal with acid
and elution to remove ash and other contaminants, and to prepare a highly porous
material. Puriﬁed activated carbon often provides optimal interactions with the precious
metal catalysts and ensures dispersibility in the reaction media. A recent report suggests
that Pd NPs on single wall carbon nanotubes are more active than commercially available
Pd on activated carbon for the Heck reaction of styrene and iodobenzene (Scheme 1.1).
However, Pd on activated C is the more active catalyst in the case of oxidation of
cinnamyl alcohol to cinnamaldehyde by molecular oxygen and the hydrogenation of

cinnamaldehyde to 3-phenylpropionaldehyde (Schemes 1.2 and 1.3).13'

The average
diameter of Pd NPs on single-wall carbon nanotubes is 5 nm, whereas the average
diameter of Pd on activated carbon (1 wt %) is 14.5 nm. A number of studies show the
high activity of metal NPs on carbon in a wide range of reactionsm"35 Another report
shows that catalytic activities of cobalt NPs on graphite, mesoporous carbon, and
charcoal are almost same for the Pauson-Khand reaction. This suggests that in this case,

the catalytic activity depends on the NPs and not on the nature of support '36

| \

Tributylamine, DMF, 140°C 0 / O
+ s
O Pd Catalyst

Single-Wall Carbon Nanotube 60 min 100% conversion

 

PdlC 60 min 65% conversion

Scheme 1.1. Heck reaction of styrene and iodobenzene catalyzed by Pd/C and Pd NPs on
single-wall carbon nanotubes.

O
./\/\OH 02, EtOH, 80°C \ H
Pd Catalyst
Single-Wall Carbon Nanotube 10 h 2% yield
PdIC 10 h 11% yield

 

Scheme 1.2. Oxidation of cinnamyl alcohol by Pd/C and Pd NPs on single-wall carbon
nanotubes. '

O
O
\ H H2, o-xylene, 80 C = O \ OH
Pd Catalyst
Single-Wall Carbon Nanotube 2 h 10% yield
PdIC 2 h 100% yield

 

Scheme 1.3. Hydrogenation of cinnamaldehyde by Pd/C and Pd NPs on single-wall
carbon nanotubes.

Metal Oxide Supports

A large number of recent publications focus on catalysis with NPs supported on
oxides of Si 137-152 A169.150.153.155 Ti I56-158 Zr 159-168 Ca 76 Mg 69,155,169-173 and Zn 174 These
supports are attractive because they are stable at high temperatures and easy to remove
from the reaction medium. The performances of these supported catalysts were

examined for various reactions such as hydrogenation of unsaturated substrates,175

l24-I27,129,l30,l76-l79 93,126,]37,140,16I,l77.180,181 165,166,182 - 183
Heck, Stllle,

Suzuki, Sonogashira,

Negishi,184 and Kumada185 ’186 coupling reactions, oxidation of CO using molecular

 

oxygen,77’82‘187"194 Mannich reactions,74 and Pauson—Khand reactions (Scheme 1.4) .141,195
Co NPs,
Pd NPs CH3
MeozcL on Sio2 MeOﬁ)“:
MeOZC WOAC MeOZC

Scheme 1.4. Pauson—Khand reactions catalyzed by Co and Pd NPs supported on silica for
a one-pot syntheses of bicyclicenones.

Catalyst particles are anchored to oxide supports through a metal-support
interaction (MSI), which prevents NP aggregation and can potentially inﬂuence the
activity of the catalyst.”””’197 Studies indicate that the strength of the MSI depends on
both the support and the metal. For example, the extent of aggregation of supported
silver NPs in a vacuum decreases in the order Si02>A1203>C, while for Pt nanoparticles,

the extent of interaction decreases in the order A1203> Si02>C.'98

In particular, alumina powders have surface areas that range from 0.1 to 400 m2/ g,
pore volumes between 0.1 and 1.5 cm3/g, and average pore sizes between 2 nm and 170

199

pm The large surface area seem to help keep prevent NP aggregation and increase NP

200,201

loadin . Thus, aluminum oxide powder is erha s the most attractive su ort
g P P PP

material for NP catalysis. Additionally, alumina membranes can be used as a support to

catalytic NPs.202’203

1.3.2. Solution-based Metal NP Stabilizers
Surfactants

Surfactants are common stabilizers of metal-NP suspensions. These molecules
interact with small NPs to reduce their surface energy. A recent report indicates that
suspensions of Pd NPs stabilized by N,N-dimethy1-N-cetyl-N-(2-hydroxyethyl)
ammonium chloride successfully catalyze hydrogenation and dehalogenation of various
halogenoarenes.204 Cetyltrimethylammonium bromide also stabilizes Cu NPs on several
metal oxides, and these supported Cu NPs were employed for the selective
dehydrogenation of methanol to produce formaldehyde.205 Metal oxide supports help to
recover the catalyst at the end of the reaction.
Ligands

The use of ligands as NP stabilizers is especially interesting because variation of
the ligand composition allows some optimization of the NP reactivity for use in speciﬁc
catalytic reactions. Treatment of molecular Pd complexes with CO in DMF or toluene at

room temperature produces Pd NPs that catalyze nucleophilic

10

substitution/carbonylation/amination to afford isoindolinones at room temperature as

shown in the scheme 1.5.206 Pd NPs capped with special ligands such as cyclodextrins

 

o
l 1 mol % Pd(OAC)2. co (1 atm)/RNH2
cc -
3’ K2003, DMF
\ I /
CE/NHR

Scheme 1.5. 3-component cascade process by palladium NPs generated in situ to form
isoindolinone.

are active for the catalysis of the hydrogenation of unsaturated substrates and of the

Suzuki, Heck, and Stille coupling reactions.94'96

Dodecanethiolate-capped Pd NPs
catalyze the Suzuki-Miyaura reaction of aryl halides, including, with phenylboronic acid

at ambient temperature.93 Another simple method of stabilization involves addition of

 

OH OSiMDBT
HO O Pd-nanoparticles _ HO O
HO TBDMSi-H, DMA TBDMSiO
HO OCH3 25°C, -H2 H0 OCHs

TBDMSiH= t-BuMeZSiH
DMA = Dimethylacetamide

Scheme1.6. Regioselective silylation of sugars through Pd NP-catalyzed silane
alcoholysis.

11

silanes (R3SiH), such as tert—butyldimethylsilane, to PdC12 in N, N-dimethylacetamide.
The black NP solution formed in this way catalyzes silane alcoholysis of sugars207
(Scheme 1.6), and selective cross—coupling of the silane with phenyl and vinyl thioethers

gives the corresponding thiosilanes and silthianes.208

Polymers

Polymers are particularly attractive stabilizers because they weakly bind metal
nanoparticles via heteroatoms in the interior of the polymer and build a protective “shell”
of polymer chains. In addition to stabilization, the steric bulk of the polymer framework
may restrict access to metal NPs to impart selectivity. The polymer can be a soluble
linear or branched macromolecule that “wraps” around the metal NP in solution to
prevent metal aggregation and precipitation.

The most commonly encountered polymer stabilizers are commercially available

poly(N-vinyl-Z-pyrrolidone) (PVP),209'2'2 polyurea,213 polyacrylonitrile,”4 poly(acrylic

215 5

acid),214 polysiloxane, oligosaccharides,l4 poly(4-vinylpyridine),216 poly(N,N-

l9

dialkylcarbodiimide),217 polyethylene glycol,218 chitosan,2 and a hyperbranched

aromatic polyamide.220

Metal NPs protected in these polymers are used as soluble
catalysts, and they can also be immobilized in ionic liquids for various kinds of reactions.
Hirai’s pioneering work in the 19705 and 19805 is especially impressive. He generated
Rh NPs using aqueous methanol or NaOH in methanol as a reducing agent for
RhC13-3H20 to produce polyvinyl alcohol (PVA)-stabilized Rh NPs.22"222 Toshima

pursued research on metal-NP catalysts in the early 19905 with polymer-stabilized

bimetallic NPs generated by direct co-reduction of salts of two different metals (Pt and

12

Au for example), by sequential reduction (the initial NPs serve as seeds for the surface
condensation of the second metal), or by galvanometric reduction of a metal salt by NPs
of another more easily reduced metal.2°9’223’224 In the late 19905, Andres et al.
synthesized PVP-stabilized Pt, Pd and Rh NPs by ethanolic reduction of the
corresponding metal halide. These NPs were immobilized in an ionic liquid, 1-n-butyl-3-
methylimidazolium hexaﬂuorophosphate ([BMI][PF6]), and found to be very efﬁcient
catalysts for benzene hydrogenation at 40 °C.2'0 The polymer-stabilized metal NPs could
also be recycled without loss of activity. Additionally, Pd and bimetallic (PdAu, PdPt)
NPs were stabilized in polystyrene-block-poly-4-vinylpyridine (PS-b-P4VP) micelles and
studied in the selective hydrogenation of dehydrolinalool as shown in the scheme
1.7.225’226 Cyclohexene hydrogenation was catalyzed with Pd NPs stabilized by highly

branched amphiphilic polyglycerol.227

 

\ - - -
\ \ Catalyst. PS b-P4VP Pd/AIzOgA' / \
OH H2 (1 atm), Toluene, 90°C 0”
selectivi 99.8"/ b GC .
Dehydrolinalool ty ° y llnalool

Scheme 1.7. Selective hydrogenation of Dehydrolinalool to linalool by PS-b-P4VP-Pd on
alumina.

Dendrimers
Dendrimers are a unique class of polymeric materials that often ﬁnd application

as templates to stabilize metal NPs. The formation of NPs stabilized by dendrimers was

228.229 230,231 232-234

reported in 1998 by the research groups of Crooks, Tomalia and Esumi.

Crooks pioneered the use of dendrimer-encapsulated metal NPs for catalysis. In his

13

approach, metal cations such as Pd2+, Pt“, or Au3+ that are complexed with the nitrogen

atoms of interior tertiary amines are reduced to NPs upon treatment with NaBH4 as

 

Metal
Cation NaBH4
— ﬂ
0
Dendrimer—metal cation Dendrimer-metal NP

Dendrimer

. = Metal NP

Figure 1.3. Encapsulation of metal nanoparticles in a dendrimer matrix.235

3228335440 The procedure used by Crooks and Tomalia

illustrated in ﬁgure 1
encapsulated metal NPs inside the dendrimers, whereas Esumi’s process incorporated
NPs at the dendrimer periphery. Dendrimer-encapsulated metal NPs are attractive for
catalysis for several reasons. First, the dendrimer templates are fairly uniform structures,
and they yield well-deﬁned NP5.229'24"242 Second, NPs are stabilized by encapsulation
within the dendrimer through weak interactions with amines so they retain their reactivity
but do not agglomeratem'244 Third, the dendrimer branches can be used as selective
gates to limit the access of large substrates to the encapsulated nanoparticlesumu
Finally, the terminal groups on the dendrimer periphery can be tailored to control the

solubility of the hybrid nanocomposite?“249

1.4. Layer-by-Layer Assembly of Oppositely Charged Polyelectrolytes

The previous sections described a number of ways to stabilize NPs, including
stabilization with polymers. Because this research speciﬁcally employs NPs in
multilayer polyelectrolyte ﬁlms, this section discusses the layer-by-layer assembly of
such ﬁlms. In 1966, Iler reported the formation of layers of charged particles by
adsorption from solutions.250 In the early 1990’525"252 Decher and coworkers
rediscovered surface modiﬁcation through layer-by-layer deposition, and greatly
expanded the technique by using sequential assembly of oppositely charged polymers.
This pioneering work spawned thousands of subsequent studies because of the simplicity
and versatility of the approach. The stepwise growth of multilayer polyelectrolyte ﬁlms
(Figure 1.4) is an entropy driven process due to the release of salt counter ions that were

253.254

originally neutralizing the polyelectrolytes in solution. The stability of each

polyanion/polycation “bilayer” is presumably maintained through the formation of

5 (Other interactions

multiple ion pairs between oppositely charged polyelectrolytes.25
such as hydrogen bonding can also be employed for layer-by-layer deposition,256 but this
work focuses solely on electrostatic interactions.) The procedure for layer-by-layer
deposition of polyelectrolyte ﬁlms is very simple and begins with immersion of a charged
substrate in a solution containing an oppositely charged polyelectrolyte. The polymer
adsorbs on the surface, presumable due to a gain in entropy through the release of many
counter ions, as mentioned above. The presence of the single-layer coating on the surface
reverses the sign of the substrate charge. After rinsing to remove loosely attached

polyelectrolyte, immersion of the substrate into a solution containing a polyelectrolyte of

opposite charge gives a bilayer consisting of two oppositely charged polyelectrolytes, as

15

depicted schematically in the Figure 1.4.257 Additional bilayers are added similarly

(Figure 1.4).

 

 

 

    

Polyanion
solution

Polycation
solution

”l

i) Polyanion
ii) Rinse

L >

i) Polycation
ii) Rinse
C

 

 

 

 

 

 

 

 

 

_/

Substrate 1-bilayer
Formation

Figure 1.4. Formation of bilayer on the substrate through layer-by-layer deposition of
polyelectrolytes.257

Multilayer polyelectrolyte ﬁlms should consist of stratiﬁed structures because
they are formed via sequential adsorption of polycations and polyanions. However,
layers are highly intertwined in most cases due to the loops and tails in polyions and
interactions between polyelectrolytes in several different layersm'259 These tails and
loops are partially responsible for the charge overcompensation at the ﬁlm surface. 260

Additionally, the entanglement of neighboring layers also aids charge compensation

16

within the multilayer ﬁlms. Several studies did not detect any counter ions in the bulk of
polyelectrolyte ﬁlms, which suggested the complete compensation of opposite charge in

the ﬁlms.260’261 However, more recent studies show that charge compensation depends

on the compositions of the ﬁlm as well as deposition and post-deposition treatments.262

The layer-by-layer deposition method has several attractive features. 1. It is an

easy and convenient method that usually involves a simple dip and rinse procedure. 2.

Film thickness can be controlled at the nm scale by varying the number of layers 257363

and the ionic strength or the pH of the deposition solutions.263'266 The pH controls the
charge density of weak polyelectrolytes by changing the degree of protonation or
deprotonation, and polyelectrolytes with high charge densities typically give thin ﬁlms
with polymer chains are highly extended. A high concentration of salt in the deposition
solution usually increases the ﬁlm thickness because it shields neighboring charges on the
polyelectrolytes and allows the formation of loops and tails. 3. Films can form on any
surface that will adsorb polyelectrolytes. This requirement is easily met because most

common solid substrates are charged. Adsorption is also relatively independent of

267

substrate geometry and can occurs on materials ranging from small particles to test

268 3

tubes to spongy membranes20 .

4. Many different polyelectrolytes are available for
deposition. This technique initially started primarily with polyamines and polysulfonates,

but subsequent reports deomonstrated layer-by-layer deposition of electro-active

269.270

polymers, quantum dots,271'273 DNA,274‘276 charged viruses,277’278 inorganic

269,279,280 281-284 285,286

sheets, proteins, and enzymes. 5. Film formation is relatively

environmentally friendly in that it typically uses water as the solvent. Due to the

17

advantages of layer-by-layer assembly, these ﬁlms. are being examined for applications in

areas such as sensors, electronics, tissue engineering, and biotechnology.

1.5. Motivation and Research Goal

Section 1.3 of this chapter indicates that a plethora of methods were developed to
obtain stable metal NPs for use in catalysis. However, systematic studies of selective
catalysis with a series of well-controlled, highly reactive NPs are lacking. There are two
important challenges to prepare homogeneous/heterogeneous NPs for use as efﬁcient
catalysts. Firstly, to obtain maximum catalytic efﬁciency, the NPs must be stabilized to
eliminate aggregation without blocking most of the active sites on the NP surfaces.
Secondly, the stabilizer should afford control over NP size and reactivity to manipulate
catalytic activity. We envisioned that the incorporation of metal NPs in thin
polyelectrolyte multilayers prepared by layer-by-layer deposition could address these
challenges quite effectively. To form NPs in these ﬁlms, we ﬁrst adsorb polyelectrolyte
complexes of transition-metal ions and subsequently reduce these ions to form metal
NPs. Variation of the concentration of metal ions in the ﬁlm affords control over the NP
size, and the polyelectrolyte ﬁlm may also serve as a ﬁlter to enhance selectivity.
Previous work in our group demonstrated the preparation of Pd NPs incorporated in
poly(acrylic acid)(PAA)/polyethylenimine (PEI) ﬁlms on alumina ([PAA/PEI-Pd(0)]3)
and their catalytic activity (Figure 1.5). Those studies demonstrated moderate catalytic
selectivity with allylic alcohols that had different substituents at the position a to the

double bond as shown in the ﬁgure 1.5. However, that research did not investigate the

18

effect of NP size on selectivity or investigate a very wide range of hydrogenation

reactions.

   

K 1

+ H2 + H2
Figure 1.5. Interrnolecularly selective hydrogenation of allyl alcohols by [PAA/PEI-
Pd(0)]3PAA on alumina.

This research examines selective, NP-catalyzed hydrogenation of a much broader
series of compounds including allylic alcohols with various substituents on the double
bond, molecules with more than one double bond, and nitroaromatic compounds. The
NP catalysts show selectivities >100 for hydrogenation of mono over disubstituted
double bonds, and these selectivities vary dramatically with NP size. These studies are
predicated on careful formation and characterization of ﬁlms with controlled NP size.
Both intramolecular and intermolecular selectivities can be achieved. Additionally, the
NP ﬁlms are chemoselective in the hydrogenation of nitroaromatic compounds. These

studies certainly demonstrate the unique properties of NP catalysts in selective reactions.

1.6. Outline of the Thesis

In this introduction, I have attempted to provide background on NP stabilization
and catalysis with NPs to show how my work ﬁts in the broader context of this ﬁeld.
Chapter 2 expands on our previous work with Pd NPs polyelectrolyte ﬁlms and shows
unprecedented selectivities in hydrogenation of mono and multisubstituted double bonds.
Initially, the chapter shows that Pd NPs embedded in multilayer polyelectrolyte ﬁlms can
be easily prepared through layer-by-layer adsorption of poly(acrylic acid) (PAA) and
poly(ethylenimine)—Pd(II) (PEI-Pd(II)) complexes followed by reduction of Pd(II) with

NaBI-Ia. Transmission electron microscopy conﬁrms the formation of Pd particles with

 

Scheme 1.8. Regio selective hydrogenation of Linalool to dihydrolinalool by [PAA/PEI-
Pd(0)]3PAA ﬁlms on alumina

diameters of 1-3 nm. Remarkably, [PAA/PEI-Pd(0)]3PAA ﬁlms catalyze the
hydrogenation of allyl alcohols (monosubstituted unsaturated alcohols) with turnover
frequencies (TOFs) that are as much as lOO-fold higher than turnover frequencies for
hydrogenation of 3-methyl-2-buten-1-ol (multisubstituted unsaturated alcohols).
Intramolecular selectivity for the hydrogenation of monosubstituted unsaturated alcohols

also occurs when substrate molecules contain both mono and multiply substituted double

20

bonds, and one example is illustrated in scheme 1.8. The combination of the
encapsulating polyelectrolyte ﬁlm and small NPs apparently results in hindered access of
multiply substituted double bonds to catalytic sites.

Chapter 3 investigates selectivity in hydrogenation as a function of NP size, and
shows that in the hydrogenation of mono and multisubstituted double bonds, selectivity
increases dramatically as NP diameter decreases from 3.9 to 2.2 nm. The use of different
ratios of poly(acrylic acid) to Pd(II) in deposition solutions for PAA-Pd(0)/PEI ﬁlms
gives a series of catalysts with Pd NPs whose average diameters range from 2.2 to 3.4

nm. Turnover frequencies (TOFs) for the hydrogenation of monosubstituted unsaturated

1 k 240

relative_ relative-

krelative=3 k

 

Scheme 1.9. Selectivity as a function of NP size in the catalytic hydrogenation of
unsaturated alcohols.

alcohols increase with decreasing average NP size, whereas multisubstituted unsaturated
alcohols show the opposite trend. Hence, the ratios of TOFs for hydrogenation of allyl
alcohol and B-methallyl alcohol are as high as 240 with the smallest NPs as illustrated in

Scheme 1.9. Presumably, high selectivities occur on the smallest NPs because

21

hydrogenations of monosubstituted and multisubstituted double bonds take place on

different types of atoms (e. g., terraces versus edges) on the Pd NPs.

Chapter 4 expands the scope of [PAA-Pd(O)/PEI]3 catalysts and demonstrates
chemoselective hydrogenation of aromatic nitro compounds. Chemoselective
hydrogenation of nitroarenes is often challenging due to the formation of unwanted side
products. The [PAA-Pd(0)/PEI]3 catalysts allow hydrogenation of various nitroarenes to
the corresponding amines with functional group tolerance in high yield (>90 %) and short
reaction times (15 to 60 mins). Higher chemoselectivity is observed when the
hydrogenation reaction is performed in a mixture of water and Ethanol than in pure
Ethanol.

In chapter 5, I provide concluding remarks along with future prospects for these

catalysts.

22

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39

CHAPTER 2
SELECTIVE HYDROGENATION OF MONOSUBSTITUTED ALKENES BY Pd
NANOPARTICLES EMBEDDED IN POLYELECTROLYTE FILMS
2.1. Introduction
As shown in chapter 1, metal NPs are attractive catalytic materials because of
their high surface area to volume ratio and their size-dependent, tunable electronic

6

properties." However, aggregation of metal NPs reduces surface area and restricts

control over particle size. Thus, practical catalysts that rely on these materials generally

ll-15 l6-l8 19-38

contain a support, such as carbon,7"° metal oxides, zeolites, or polymers, onto
which the NPs are deposited to prevent particle aggregation and facilitate catalyst
recovery. In particular, polymeric supports are attractive because they provide unique
possibilities for modifying the environment around catalytic sites and controlling access
to these sites. Incorporation of NPs in multilayer polyelectrolyte ﬁlm523’24 or
dendrimersfs‘26 for example, allows size-based selectivity in the hydrogenation of
oleﬁns. This chapter demonstrates that catalytic NPs in multilayer polyelectrolyte ﬁlms
also show remarkable intramolecular selectivity in the hydrogenation of alkenes based on
the degree of substitution of the double bond. Selectivity in the hydrogenation of
monosubstituted double bonds over di or trisubstituted double bonds is. equal to or greater
than that of Wilkinson’s catalyst, tris(triphenylphosphine)rhodium(I) chloride, which is
the prototype homogenous catalyst for such reactions.39 Additionally, alumina-supported
NP/polyelectrolyte catalysts can be easily separated from the reaction mixture and
recycled.

Several reports show that the layer-by-layer deposition of polyelectrolyte ﬁlms

. . . . . . . . 4.4 ‘4
provrdes a convenient method for syntheSls or immoblllzatlon of nanopartrcles.23’2 ’ 0 4

40

In the present case, we prepare selective catalysts using layer-by-layer adsorption of
poly(acrylic acid) (PAA) and poly(ethylenimine)—Pd(11) (PEI-Pd(II)) complexes on
alumina particles and subsequent reduction of Pd(II) by NaBI-Ia to form NPs as illustrated
schematically in scheme 2.1.23'24 The resulting embedded NPs contain catalytic sites that

show unusually high selectivities, partly because the polymer restricts access to catalytic

   

sites.
. ._ J Repeat step 1-4
3 11:11:; 3) PE] ,‘ 36mg followed J a
Alumina ’
[PAA-Pd(ID/PEIbPAA

3) NaBH4

4) Rinse

-—————.

 

[PAA-Pd(oyPE113PAA
Scheme 2.1. [PAA/PEl-Pd(0)]3PAA ﬁlm formation on Alumina.

Our previous research reported that the size-based selectivity of these catalysts in
hydrogenation of allyl alcohols containing different alkyl substituents in the position a to
the double bond?”4 Smaller molecules exhibited higher hydrogenation rates,
presumably because of more rapid transport to catalytic sites. The work reported in this
chapter shows that [PAA/PEI-Pd(0)]3PAA ﬁlms on alumina also exhibit high selectivities
based on the substitution on the double bond itself. Moreover, intramolecular selectivity
demonstrates that speciﬁcity cannot be explained solely by differences in the rate of

diffusion through the polyelectrolyte ﬁlm to catalytic sites.

41

2.2. Experimental
2.2.1. Materials

Poly(ethylenimine) (PEI, MW = 25 000 Da), poly(acrylic acid) (PAA, Mw = 90
000 Da), palladium (5 wt % on alumina powder), a-alumina (100 mesh, typical particle

size 75-100 um), allyl alcohol (99%), crotyl alcohol (97%, mixture of isomers), 2-
methyl-2-propen-l-ol (98%), 3-methy-2-buten-1-ol (99%), propionaldehyde, 3-methyl-1-
butanol (98.5+%), isovaleraldehyde (97%), 2-heptene-4-ol, 1,5-heptadien—4-ol, 3,7-
dimethyl-l,6-octadien-3-ol (97%), potassium tetrachloropalladate(II) (99.99%), and
sodium borohydride (98%) were purchased ﬁ'om Aldrich and used as received. l-Butanol
(J .T. Baker), n-propyl alcohol (Columbus Chemical), iso-butyl alcohol (Spectrum),
ethanol (100%, Pharmco), and calcium hydride were also used as received. Benzene was
purchased from EMD chemicals and distilled over calcium hydride. Aqueous solutions
were prepared with deionized water (Milli-Q, 18.2 M!) cm), and hydrogen (99.9%) was

obtained from AGA gases.

2.2.2. Preparation of Pd NPs Encapsulated in Polyelectrolyte Multilayer Films
Preparation of [PAA/PEI-Pd(0)]3PAA ﬁlms on alumina began with vigorous
stirring of 15 g of til-alumina in 100 mL of 20 mM PAA (pH adjusted to 4.0 with 0.1 M
NaOH, molarity of PAA is given with respect to the repeating unit) for 10 min.
Subsequently, the alumina was allowed to settle, the supernatant was decanted, and the
alumina was washed with three 100-mL aliquots of deionized water (stirring in water for
5 min each time) to remove excess PAA. The polycation layer was deposited by
vigorous stirring of PAA-coated alumina in a 100-mL solution containing the PEI-Pd(II)

complex (1 mg/mL PEI, 2 mM KdeCla, pH adjusted to 9.0 with 0.1 M HCl), followed

42

by washing with three lOO-mL aliquots of deionized water. This procedure was repeated
until 3.5 bilayers were deposited on alumina. Reduction of Pd(II) was performed by
stirring the [PAA/PEI-Pd(II)]3PAA ﬁlms on alumina in 100 mL of freshly prepared 0.1
mM NaBH4 for 30 min, followed by washing with three 100-mL aliquots of deionized
water. The water was decanted, and the residual water was removed under reduced

pressure to afford a catalytic powder.

2.2.3. TEM of Pd NPs

For TEM imaging, ﬁlms were deposited on carbon-coated copper grids that were
treated in a UV/ozone cleaner for 1 min. [PAA/PEI-Pd(0)]3PAA ﬁlms were deposited as
described above, except the grid was simply immersed for 5 min in solutions without
stirring, and rinsing between each step consisted of a 1-min washing with deionized
water ﬁ'om a squirt bottle. Imaging was performed on a JEOL lOOCX microscope using
an accelerating voltage of 100 kV. Particle sizes were determined manually on digital

images.

2.2.4. Determination of the Amount of Pd in Catalyst and Deposition Solutions
Calculation of turnover frequencies (TOFs), i.e., the moles of substrate that are
hydrogenated per mole of Pd per hour, requires knowing the amount of Pd in the catalyst.
To determine Pd content, Pd in the catalyst was dissolved using aqua regia, and atomic
emission spectroscopy (Varian Spectra AA-200) was employed to analyze these solutions

as described previously.23‘24

43

2.2.5. Hydrogenation Reactions

Catalytic hydrogenation was performed in a 100-mL, three-neck, round-bottomed
ﬂask. During the reaction, hydrogen at a gauge pressure of 50 kPa was bubbled through
a frit at the bottom of a solution that was vigorously stirred. Prior to the reaction,
suspended catalyst (250 mg of [PAA/PEI-Pd(0)]3PAA on alumina or 10 mg of
commercial 5% Pd on alumina) in 25 mL of H20 was bubbled with H2 for 30 min before
adding 25 mL of a 50.0 mM solution of substrate in water or in a 1:1 (v:v) mixture of
ethanol and water. Much less mass of the commercial catalyst was used for
hydrogenation reactions because this material has a 36-fold higher Pd loading than
[PAA/PEI-Pd(0)]3PAA ﬁlms on alumina. (The amount of Pd in 10 mg commercial
catalyst is about 1.42 times higher than that in 250 mg of [PAA/PEI-Pd(0)]3PAA on
alumina.) Gas chromatography (Shimadzu GC-17A equipped with an RTx-
BAClcolumn) was employed to monitor reactions using response factors that were
determined with a standard mixture containing 10 mM reactant, product, and isomer (if
applicable), except for the case of 2-methyl-2-propen-1-ol, for which we did not have
pure isomer. In this case only, the isomer response factor was assumed to be the same as
the product, as these compounds have the same number of C and O atoms. IH NMR
spectra obtained on 300 MHz or 500 MHz spectrometers and '3 C NMR spectra obtained
on 75 MHz or 125 MHz spectrometers were plotted with chemical shift reported relative
to the residue peaks of methanol (5 3.30 for 1H NMR and 49.0 for 13C NMR). For
molecules containing multiple double bonds, GC-MS (Hewlett Packard G1800B GCD
system with a non-polar HP-SMS column) with appropriate response factors was used to
determine TOF. TOFs were calculated from the slope of the linear portions of plots of

percent hydrogenation versus time (conversion less than 50 %) with the intercept of the

44

line forced through zero. Each experiment was repeated twice and the i values represent
the difference between the average and the data points. Representative calculation of

response factor and TOF are shown in appendix.

2.2.6. Recycling Experiment

Catalytic hydrogenation was performed in 100% water as described above. Most
of the solvent was decanted after the reaction, the catalyst was transferred to another
ﬂask, and the residual water was removed in vacuum for more than 4 hours prior to
determination of the remaining catalyst mass. About 5 % of the catalyst was lost during
the recycling process. The hydrogenation reaction was repeated with this catalyst prior

to another recycling step and a third hydrogenation reaction.

2.2.7. Hydrogenation Reactions by Wilkinson’s Catalyst

Catalytic hydrogenation was performed in a lOO-mL oven dried three-neck,
round-bottomed ﬂask containing a condenser and a bubbler. N2 was bubbled through the
round-bottom ﬂask for 10 min before the reaction. During the reaction, hydrogen at a
gauge pressure of 50 kPa was bubbled through a frit at the bottom of a solution that was
vigorously stirred with magnetic stir bar. Prior to the reaction, 23 mg (0.025 mole) of
RhCl(PPh3)3, Wilkinson’s catalyst, was stirred in 18 mL of benzene with bubbling of H2
for 20 min before adding 2.5 mmole substrate in 2 mL of benzene using a glass syringe.
Sample was collected using a glass syringe at various time intervals that depended on the
rate of the reaction. Gas chromatography was employed to monitor the reaction using

response factors as described above.

45

2.2.8. Isolation of Products

After 97% hydrogenation of compounds 5 (0.140 g, 50 mM solution in 25 mL 1:1
water and ethanol, later mixed with 25 mL water) and 7 (0.193 g, 50 mM solution in 25
mL 1:] water and ethanol, later mixed with 25 mL water), the catalyst was removed from
the solution by ﬁltration through cotton, and about lg of NaCl was added to the solution.
Products (6 and 8) were extracted with 25 mL of anhydrous diethyl ether. The organic
layer was collected and dried over anhydrous sodium sulfate, and solvent was removed
under reduced pressure to afford crude compounds 6 and 8. Pure compound 8 was
afforded in 81% yield (0.158 g) after column chromatography with 9:1 hexane and ethyl
acetate. In the case of hydrogenation of compound 5, 0.120 g of crude product (84%
yield) was isolated after extraction. GC/MS analysis showed that the ratio of 6 and 9 in
crude 6 is 8:1. 1H NMR indicated the presence of small amounts of impurities in the

crude product as shown in the spectra, which are given in the appendix.

Compound 6: lH NMR (500 MHz, CD30D): 8 0.90 (t, 3H), 1.27-1.51 (m, 4H), 1.67 (dd,
3H), 3.95 (m, 1H), 5.41 (m, 1H), 5.61 (m, 1H), 13C NMR (125 MHz, CD3OD): 5 14.3,
17.8, 19.7, 40.5, 73.4, 127.1 and 135.4. Spectral data were consistent with commercially

available material.

Compound 8: 1H NMR (300 MHz, CD30D): 8 0.88 (t, 3H), 1.11 (s, 3H), 1.37-1.51 (m,
4H), 1.61 (s, 3H), 1.66 (s, 3H) 2.00 (m, 2H), 5.11 (m, 1H), 13C NMR (75 MHz, CD3OD):

5 8.5, 17.6, 23.6, 25.9, 26.2, 34.9, 42.2, 73.4, 125.8 and 132.0

46

2.2.9. Transport Studies
Diffusion dialysis experiments were performed using a home-built dialysis

apparatus (shown in Figure 2.1) in which a porous alumina membrane (Whatman

Anodisc, 0.02 um-diameter surface

Stirrers

Alumina
— Membrane

   

 

 

 

 

 

1;

Receiving Source Phase O-ring

Phase 0
Membrane

 

 

 

 

Figure 2.1. Home-built diffusion dialysis set-up for transport experiments.
(Figure courtesy of Dr. Jinhua Dai).

pores) coated with a [PAA/PEI]7 ﬁlm was sandwiched between two glass cells. (More
layers were used in these experiments than in preparing catalysts to ensure complete
coverage of the pores in the underlying alumina support.) The source cell was ﬁlled with
90 mL of a 20 mM aqueous solution of the desired unsaturated alcohol (allyl alcohol,
crotyl alcohol, 2-methy1-2-propen—1-ol, or 3-methyl-2-buten-1-ol), while the receiving
cell initially contained 90 mL of deionized water. Both the source and the receiving sides

were stirred vigorously to minimize concentration polarization at the membrane

47

interface, and samples of the receiving phase were taken every 20 min and analyzed by

gas chromatography using methanol as an internal standard.

2.3. Results and Discussions
2.3.1. NP Size

Figure 2.2 & 2.3 shows the TEM image of a [PAA/PEI-Pd(0)]3PAA ﬁlm on a
carbon-coated grid. Consistent with prior results,23 the distribution of particle diameters
ranged from 1-3 mm as shown in Figure 2.4. The estimated average particles size is 2.2
nm. The relatively impermeable PAA/PEI ﬁlms likely inhibit ion and NP diffusion to

give small particles.

 

Figure 2.2. TEM image of [PAA/PEUPd(0)]3PAA on carbon-coated cupper grid.

48

 

Figure 2.3. TEM image of a [PAA/PEI-Pd(0)]3PAA ﬁlm on a carbon-coated copper grid.
This image was taken at a lower magniﬁcation than Figure 2.2 in to better show the

distribution of NPs.

30.

% of particles

 

 

1.1 1.3 1.5 1.7 1.9 2.1 2.3 2.5 2.7 2.9 3.1 3.3

Diameter (nm)

Figure 2.4. Histogram of Pd NP diameters in several TEM images of [PAA/PEI-
Pd(0)]3PAA ﬁlm deposited on a TEM grid. The average particle size is 2.2 nm.

49

2.3.2. Intermolecular Selectivity

Using similar ﬁlms on alumina particles, we initially compared the rates of
hydrogenation of unsaturated alcohols with different substituents on the double bond and
found very high selectivities (Table 2.1). With a [PAA/PEI-Pd(0)]3PAA-on-alumina
catalyst in water, allyl alcohol (1) could be 82% hydrogenated in 15 min (formation of
about 18% propionaldehyde isomer prohibits complete hydrogenation) with an initial
turnover frequency (TOF) of 3500 moles hydrogenated/mole catalyst/h.45 In contrast, 2-
methyl-2-propen-1-ol (2), crotyl alcohol (3), and 3-methyl-2-buten-l-ol (4) exhibited
TOFs that ranged from 14 to 100-fold lower (Table 2.1, column 4). By comparison, a
commercially available 5% Pd on alumina catalyst facilitated hydrogenation of 1, 2, 3
and 4 with TOFs that varied by at most a factor of 2 when water was the solvent (Table

2.1, column 2).

50

Table 2.1. TOFs for the hydrogenation of a series of unsaturated alcohols by different Pd

 

 

catalysts.
TOF (moles hydrogenated/ mol Pd/ h)a
5%Pd on 5%Pd on [PAA/PEI- [PAA/PEI-
Substrates Alumiréa Alumina Pd(O)]3P€xA Pd(O)]3PAA
(H2O) (H2O/EtOH)c (H2O) (H2O/EtOH)c
1 =¥ 1680i30 1160a120 3505s175 2700i100
0H (2770120) (1700i170) (53001500) (5590i50)
2 =< 835d:65 415245 120i5 75a5
OH (2275a25) (1270a130) (365:1:35) (230i20)
3 /=\_OH 1705215 1060i50 250115 175.215
4 >=\_
OH 1220a190 ssoaso 35a5 25a5

 

[a] Values in parentheses represent TOFs where the isomer is included as part of the
product, although it is not hydrogenated. Each experiment was repeated twice, and the i
values represent the difference between the average and the data points. [b] 25 mM
substrate was hydrogenated in water. [c] 25 mM substrate was hydrogenated in a 3:1
(v:v) mixture of water and ethanol.

Interestingly, although the TOP for hydrogenation of allyl alcohol using the
[PAA/PEI-Pd(0)]3PAA on alumina catalyst is higher than that of the commercially
available 5% Pd on alumina, selectivity is still much higher for the Pd NPs in the
polyelectrolyte multilayer. However, the commercial catalyst has a 36-fold higher wt %
of Pd than [PAA/PEI-Pd-(0)]3PAA on alumina, as determined by atomic emission
spectroscopy. To keep the amount of Pd in the different reactions about the same, 250
mg of [PAA/PEI-Pd(0)]3PAA on alumina was added to the reaction mixtures while only

10 mg of 5 wt % Pd on alumina was employed. Overall, the molar ratio of substrate to

51

Pd atoms was 400 for hydrogenation using [PAA/ PEI-Pd(0)]3PAA on alumina and 280
for hydrogenation using the commercial 5% Pd on alumina.

Hydrogenation was also performed in 3:1 mixtures of water and ethanol because
of the relatively low solubility of the substituted unsaturated alcohols in water. For the
[PAA/PEI—Pd(0)]3PAA on alumina catalyst, the TOF for allyl alcohol decreased by 23%
in the less polar mixture, possibly because the reduced swelling of the polyelectrolyte
ﬁlms decreases the rate of substrate diffusion to the catalytic sites. However, TOFs also
decreased for the commercial catalyst, suggesting that other factors inﬂuence the
hydrogenation rate. Nevertheless, the observed selectivities are similar in the two solvent
systems (compare columns 4 and 5 of Table 2.1), reaching 2 orders of magnitude (1/4)
for the [PAA/PEI-Pd(0)]3PAA on alumina catalyst.

As a control experiment, hydrogenation of allyl alcohol and its isomer
propionaldehyde was attempted in the absence of hydrogen gas using [PAA/PEI-
Pd(0)]3PAA as a catalyst. No reaction occurred, which rules out the possibility of other
reductants, such as trace amounts of residual NaBH4, driving the reaction.

The propionaldehyde and 2-methylpropionaldehyde isomers of 1 and 2,
respectively, are not hydrogenated under the reaction conditions, so their formation
results in a decreased rate of hydrogenation. Isomer formation was particularly extensive
for 2 (ratio of the isomer to product was 1.5-2 with 50% of the reactant remaining) and
also occurred for allyl alcohol (1) when using either the commercial 5% Pd on alumina or
the [PAA/PEI-Pd(0)]3PAA catalyst. Hence the TOFs of these molecules are somewhat
lower than what might be expected, especially in the case of 2 (row 2, Table 2.1). The
sum of TOFs for isomerization and hydrogenation are given in parentheses in Table 2.1

for cases where signiﬁcant isomer formation occurred.

52

To examine the recylability of the [PAA/PEI-Pd(0)]3PAA on alumina, this
catalyst was recovered after the hydrogenation of allyl alcohol in water and dried prior to
use in a subsequent hydrogenation. Remarkably, the TOF was 3200i100 moles
hydrogenated/mol Pd/h for three consecutive hydrogenations of allyl alcohol. Thus,
unlike homogeneous catalysts such as tris-(triphenylphosphine)rhodium(I) chloride,
[PAA/PEI-Pd(0)]3PAA on alumina can be readily recovered and reused.

The selectivity of the embedded Pd NPs compares very well with that of
Wilkinson’s catalyst, which previously showed a rate of hydrogenation for l-hexene
(monosubstituted unsaturated alcohols) in benzene that is 1.1 and 49 times higher than
that for 2-methylpent-1-ene (a disubstituted unsaturated alcohol) and l-
methylcyclohexene methylcyclohexene (a trisubstituted unsaturated alcohol),46
respectively. To better compare the catalytic selectivity of [PAA/PEI-Pd(0)]3PAA ﬁlms
with that of Wilkinson’s catalyst, we determined the TOFs for the hydrogenation of
compounds 1, 2, and 4 with Wilkinson’s catalyst (Table 2.2). (In this case, the reactions
had to be performed in benzene rather than water because Wilkinson’s catalyst is not
soluble in aqueous solutions.) The selectivities of Wilkinson’s catalyst for 1/2 and 1/4
were 2.6 and 54, respectively, which is in reasonable agreement with the data mentioned
above for l-hexene, 2-methylpent-1-ene, and l-methylcyclohexene. (The GC peak
corresponding to l-butanol, the hydrogenated product of 3, overlapped with that of
benzene, so hydrogenation of this compound with Wilkinson’s catalyst was not
examined.) Wilkinson’s catalyst shows only minor selectivity for mono versus
disubstituted unsaturated alcohols, whereas the embedded NPs show selectivities ranging
from 14 to 36 for allyl alcohol over 2 and 3. In the case of the trisubstituted unsaturated

alcohol 4, the selectivity (1/4) of the embedded NPs is 2-fold higher than that of the

53

Wilkinson’s catalyst. However, unlike the [PAA/PEI-Pd(0)]3PAA ﬁlms, Wilkinson’s
catalyst showed minimal formation of the isomer of 1 and none of the isomer of 2. In
contrast, Wilkinson’s catalyst showed formation of the isomer of 4 (the amounts of

isomer and hydrogenated product were the same although rates of formation of both were

low), while the [PAA/PEI-Pd(0)]3PAA ﬁlm did not show any readily detectable isomer.

Table 2.2. TOFs for the hydrogenation of a series of unsaturated alcohols by Wilkinson’s

 

 

catalyst.
SUbStTateS TOF (moles hydrogenated/ mol of catalyst/ h)al
1. _ 270i10
—\—0H (290i10)
2. : 105i25
OH
4. 5
>=\_ (10)
OH

 

[a] Values .in parentheses represent the sum of TOFs for the hydrogenation and
isomerlzatlon.
2.3.3. Intramolecular Selectivity

We further examined the catalytic behavior of [PAA/PEI-Pd(0)]3PAA ﬁlms on
alumina for intramolecularly selective hydrogenation of molecules containing a
combination of mono and di or tli substituted double bond (Table 2.3). In this case, 1,5-
heptadien-4-ol (5) and 3,7-dimethyl-1,6-octadien-3-ol (7) were selectively hydrogenated
into 2-hepten-4-ol (6) and 3,7-dimethyl-6-octen—3-ol (8), respectively, when
hydrogenation was catalyzed by [PAA/PEI-Pd(0)]3PAA. In a control experiment, the

TOF for the hydrogenation of 2-hepten—4-ol (6) to 4-heptanol (9) was 3% of the TOF for

54

reduction of 5 to 6, indicating much slower conversion for the disubstituted double bond.
This is consistent with the fact that at 97% conversion of 5, the isolated product was an
8:1 mixture of 6 and 9 (84% overall yield). (Reactions are ﬁrst order in substrate 50
product ratios cannot be calculated as simple ratios of TOFs.) No product with both
hydrogenated double bonds was observed by GC-MS and NMR for compound 7 (81%
yield of 8 after puriﬁcation) because trisubstituted double bond are hydrogenated very
slowly (Table 2.1).

The TOF for the hydrogenation of 5 to 6 is 2.6 times that for hydrogenation of 7
to 8, probably because the more bulky groups 0. to the monosubstituted carbon-carbon
double bond in 7 restrict access to catalytic sites and diminish the rate of the reaction. A5
a consequence, compounds 5 and 7 required 15 and 135 min, respectively, to achieve
about 97% conversion to hydrogenated products. Control experiments with the 5% Pd on
alumina also showed some intramolecular selectivity in the hydrogenation of 5 and 7, but
much less than that for the [PAA/PEI-Pd(0)]3PAA catalyst. At about 99% hydrogenation
of 5 by the commercial catalyst, the product distribution was 34% 6 and 65% 9. The
TOF for the hydrogenation of compound 6 (550i130) was also lO-fold greater with the
commercial catalyst than with [PAA/PEI-Pd(0)]3PAA. In the case of hydrogenation of 7
using the commercial catalyst, after 240 min, 95% of the mixture was the doubly
hydrogenated product, but 25 min after starting, the mixture contained 94% 8 after only
25 min of reaction. In this case, the commercial catalyst may also be a viable choice for

somewhat selective hydrogenation, but one would need to carefully monitor the reaction.

55

Table 2.3. TOFs (moles hydrogenated/ mole Pd/ h) for selective hydrogenation of
monosubstituted double bonds in the presence of di and tri substituted double bonds in
the same molecules. The reaction was catalyzed by [PAA/PEI—Pd(0)]3PAA on alumina
in a 3:1 mixture of water and ethanol.

 

Substrates Major Products TOFsa

 

H H

5 /\j\/\ 6 Ai/\ 1835i”

/ / /
7 W 3 W 700i140

HO HO

H /\j:/\
6 Ai/\ 9
/ SSiS

 

[a] The reaction was catalyzed by [PAA/PEI-Pd(0)]3PAA on alumina in a 3:1 mixture of
water and ethanol. TOF is expressed as moles/hydrogenated/mol Pd/h.

Our prior studies of diffusion of allyl alcohol derivatives through porous alumina
membranes (0.02-um surface pore size) coated with [PAA/PEI]7 ﬁlms suggested that
different rates of diffusion to catalytic sites imparted selectivity.23’24 Figure 2.5 presents
analogous dialysis studies with compounds 1-4 and also shows selective transport
through [PAA/PEI]7 ﬁlms on alumina. The ratios of transport rates (slopes in Figure 2.5)
for 1/2, 1/3, and 1/4 were 2.2, 2.0 and 6.3, respectively. However, relative rates of
hydrogenation in water are 25, 12 and 82 for 1/2, 1/3, and 1/4, respectively, suggesting
that differences in transport rates are insufﬁcient to account for the hill hydrogenation
selectivity among these compounds. Moreover, selective hydrogenation within
molecules 5 and 7 shows that these compounds can diffuse through the ﬁlm to reach
catalytic sites and yet only one double bond is predominantly hydrogenated. Diffusion

through the ﬁlm cannot be limiting hydrogenation of the second double bond. On the

56

other hand, compounds 1-4 are all hydrogenated at relatively similar rates (1/2=2.1,
1/3=0.95, and 1/4=1.2) with the commercial catalyst (5% Pd on Alumina) suggesting that
selectivity cannot have a completely electronic basis (Table 2.1, column 2). We think
that steric constraints for binding of multiply substituted double bonds to embedded NPs
must be signiﬁcantly greater than similar constraints in binding to the 5% Pd on alumina.
I investigate the selectivity in the hydrogenation of unsaturated alcohols as a function of

the NP size in chapter 3.

57

0.08 -
0.07 -

'8

0.05 i
0.04 -
0.03 -

0.02 - /
/
0.01 . /

0 T“ 1 l I I
0 20 40 60 80
Time (min)

 

Receiving-phase concentration
(mM)

 

Figure 2.5. Time evolution of the receiving phase concentrations of allylic alcohols
during diffusion dialysis of 1 (squares), 2 (triangles), 3 (diamonds), and 4 (circles)
through porous alumina membranes coated with a [PAA/PEI]7 ﬁlm. The source phase
initially contained 20 mM of the allylic alcohols, and the receiving phase was initially
deionized water.

2.4. Conclusion

Deposition of Pd (II) in polyelectrolyte ﬁlms followed by reduction yields
catalytic, NP-containing ﬁlms that can be used as catalysts in inter and intramolecularly
selective hydrogenation of unsaturated alcohols. Remarkably, the selectivity of these
heterogeneous catalysts exceeds that of Wilkinson’s homogenous catalyst.
Intramolecular selectivity and studies of diffusion through PAA/PEI ﬁlms suggest that
selectivity stems from steric constraints for binding of multiply substituted double bonds

to embedded NPs.

58

2.5. Appendix

Calculation of TOFs From GC Data With Response Factors

Determination of Response Factors

 

 

 

sP cp*k,
sR = cR
SI C|*k2
8.. z C—R

Where Sp = Peak area corresponding to product
S1 = Peak area corresponding to isomer

SR = Peak area corresponding to Reactant

Cp = Concentration of product

CR = Concentration of reactant

C1 = Concentration of isomer

k1 = Response factor for product to reactant

k2 = Response factor for isomer to reactant

Response factors were calculated from the signals due to a solution containing 10 mM
product, 10 mM reactant, and 10 mM isomer, and three chromatograms with three

standard solutions were averaged to determine the value.

59

Calculation of % Hydrogenation

1 mL samples were removed every few minutes (time intervals depended on the rate)
from the hydrogenation mixture and placed in autosampler vials for GC analysis. Percent

hydrogenation was determined as shown below.

 

 

 

 

C 11:
% hydrogenation = P x 100% = SP k2 x 100%
CP+CR+C' SP*k2 + SRikkfl'kz + S1*k1
When the rates of hydrogenation and isomerization were added
% reaction = % of hydrogenation + % of isomerization:
C + C S *k + S*k
%reaction= P ' x100% = P 2 ' ‘ x100%
CP+CR+C| Sp*k2 + SR*k1*k2 + S|*k1

Table A-1 shows some sample data.

Table 2. A-l Time-dependent percentage of crotyl alcohol hydrogenation catalyzed by
[PAA/PEI-Pd(0)]3PAA in water.

 

 

Time (min) % of hydrogenation
0.0 0
10.0 7.93
20.0 24.3
30.0 33.89
40.0 45.3

 

6O

Crotyl Alcohol
(Realetle‘nt)
Butanol A

(Desired
Product)

 

 

r r

5.5 5.6 5.7 5.3 1
Minutes

Figure 2. A-l Example gas chromatograms of the reaction mixture from hydrogenation
of crotyl alcohol (3) after 10 min (black line) and 40 min (red line) of reaction time. The
reaction was catalyzed by [PAA/PEI-Pd(0)]3PAA in water. Integration was performed

using instrument software.

 

Time Vs % of Hydrogenation

  
  

 

 

 

50 —

1: y =1.1313x

° 40 —

‘8

0

31 30 ~

2. 20 ..

Q-

0 10 _

.\°

0 ' l l T j
0 10 20 3O 40 50
Time (min)

 

 

 

Figure 2. A-2. Plot of time-dependent percentage of crotyl alcohol hydrogenation
catalyzed by [PAA/PEI-Pd(0)]3PAA in water.

61

r T—T
pen

 

 

U1 !
+ Q
C) 1
9° l
31' i

 

7 1
6 j r— n
5 :1
HO 4
3
2/
*.___1
1 *,____1 —n
JMJ

 

 

 

 

Figure 2. A-3 . 1H NMR spectrum of isolated crude 2-heptene-4-ol (compound 6) in
CD30D. Impurity peaks are indicated with a *.

62

IIIIII
pen

 

 

 

40

 

IIIIIIIIIIIIIIII‘IIII

IIIIIII
60

171

l
80

100

 

120

 

III]IIITIFIIIIITTTTFIIIIFIll

1
1‘0

lllrnﬁﬁjﬁlr
160

1111

180

 

rTI—lﬁrfTT I:

Figure 2. A-4 13C NMR spectrum of isolated crude 2-heptene-4-ol (compound 6) in
CD3OD. Impurity peaks are indicated with a *.

63

 

 

 

 

 

 

 

l i
,,r
1 I ,1 c:
L £29“. ﬂ
4 *4; :
‘\\ a;
9 __\_, a; w
10 ‘ j g.
\ a," o
6 \ Cited" {‘4
8 9 , i
i 1n
‘0 \ 7 e;

 

I
O
010’

1 CD300

 

 

 

Figure 2. A-5. 1H NMR spectrum of puriﬁed 3,7-dimethyl-6-octen-3-ol (compound 8) in
CD3OD. Impurity peaks are indicated with a *

64

 

 

 

 

 

 

 

 

 

 

Figure 2. A-6. 13C NMR spectrum of puriﬁed 3,7-dimethyl-6-octen-3-ol (compound 8,
chapter 2) in CD3OD. Impurity peaks are indicated with a *

65

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(2) Chen, S.; Ingram, R. S.; Hostetler, M. J .; Pietron, J. J.; Murray, R. W.; Schaaff, T.
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(4) Rao, C. N. R.; Kulkami, G. U.; Thomas, P. J.; Edwards, P. P. Chemistry-A
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(5) Fendler J. H. Nanoparticles and Nanostructuredﬁlms: Preparation,
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66

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(26) Oh, S.-K.; Niu, Y.; Crooks, R. M. Langmuir 2005, 21 , 10209.

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67

(36) Hou, Z.; Theyssen, N.; Brinkmann, A.; Leitner, W. Angewandte Chemie
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(45) The TOF reported for hydrogenation of allyl alcohol by [PAA/PEI-Pd(0)]3PAA
on alumina is 3.9-fold higher than what we reported previously. The higher value is due
in large part to a 2.5-fold higher concentration of allyl alcohol.

(46) Takaya, H.; Noyori, R.; In Comprehensive Organic Synthesis: Selectivity,

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68

CHAPTER 3

SELECTIVITY AS A FUNCTION OF NANOPARTICLE SIZE IN THE
CATALYTIC HYDROGENATION OF UNSATURATED ALCOHOLS

3.1. Introduction
Control over particle size is essential in creating highly active NP catalysts for
alkene hydrogenation,"5 alcohol oxidationf’7 Suzuki8 and Heck9 coupling, Norrish type

0

II reactions,l reduction of aromatic nitro compounds,ll CO oxidation,”’13 and

dl4,15 I.l6

electrooxidation of formic aci and methane NP size is also a critical parameter
for achieving partial hydrogenation in the conversion of alkynes to alkenes,”18 01,13
unsaturated aldehydes to unsaturated alcohols,'9 and conjugated alkenes to
monoalkenes.2°’2' Moreover, NP catalysts show high intermolecular selectivities in the
hydrogenation of alkenes22 and allylic alcohols,”27 but selectivity has not been studied as
a function of particle size. Examination of selectivities is particularly attractive for
understanding the effect of particle size on reactivity because rate enhancements due

solely to increases in surface area should cancel when calculating the ratios of reaction

rates for structurally related compounds.

This chapter shows that both the rates and selectivities of hydrogenation reactions
vary dramatically with the diameter of catalytic NPs. Remarkably, in the hydrogenation
of mono and disubstituted double bonds, NPs with average diameters of 2.2 nm exhibit
selectivities as high as 39 for allyl alcohol over crotyl alcohol, whereas NPs with average
diameters of 3.4 nm show corresponding selectivities less than 1.3. Such variations in
selectivity occur with polymer-encapsulated NPs immobilized on alumina as well as

polymer-capped NPs in solution.

69

The ﬁrst step in examining the effect of NP size on catalytic activity is
development of a method to synthesize NP catalysts with several different average
particle sizes and narrow size distributions. Among the methods for controlling NP size,
a number of studies demonstrate that layer-by-layer deposition of polyelectrolyte ﬁlms
containing metal ions and subsequent reduction of these metal ions provide a
straightforward technique for creating encapsulated NP5.23'25’28'32 The amount of metal
ion present during ﬁlm deposition dictates the particle size, and layer-by-layer deposition
on micron—sized alumina provides heterogeneous catalysts. Polyelectrolyte-NP catalysts
can also form in solution, which allows comparison of particle-size effects in supported

ﬁlms and solution-based catalysts.

Palladium NPs in polyelectrolyte ﬁlms are particularly attractive for investigating
relationships between particle size and reactivity/selectivity because they show high
selectivity in the hydrogenation of unsaturated alcohols.23'24 In the previous chapter we
described the preparation of Pd NPs encapsulated in multilayer polyelectrolyte ﬁlms
through layer-by-layer adsorption of poly(acrylic acid) (PAA) and poly(ethylenimine)-
Pd(II) (PEI-Pd(II)) complexes and reduction of Pd(II) with NaBH4. Transmission
electron microscopy conﬁrmed the formation of Pd NPs, and 'the selectivities of
[PAA/PEI-Pd(0)]3PAA ﬁlms in the hydrogenation of monosubstituted over
multisubstituted double bonds were as high as 100.23 However, we employed only one
average particle size. This work examines the hydrogenation of a series of unsaturated
alcohols using NP/polyelectrolyte catalysts with several average NP sizes and shows that

selectivity is an extremely sensitive function of particle diameter.

70

3.2. Experimental Section

3.2.1. Materials
Poly(ethylenimine) (PEI, MW = 25 000 Da), poly(acrylic acid) (PAA, Mw = 90

000 Da), ct-alumina (100 mesh, typical particle size 75-100 um), allyl alcohol (99.0%),
crotyl alcohol (97.0%, mixture of isomers), 2-methyl-2-propen—l-ol (98.0%), 3-methyl-2-
buten-l-ol (99.0%), propionaldehyde, 3-methyl-1-butanol (98.5+%), isovaleraldehyde
(97.0%), isobutyraldehyde (98.0%), 3-methyl-1-penten-3-ol (99.0%), 3-methyl-3-
pentanol (99.0%), cis-3-hepten-l-ol (2 95.0%) potassium tetrachloropalladate(11)
(99.99%), and sodium borohydride (98%) were purchased from Aldrich. l-Heptanol and
3-buten-l-ol were obtained from TCI, and butyraldehyde (2 99.0%) was purchased from
Fluka. l-Butanol (J.T. Baker), n-propyl alcohol (Columbus Chemical), iso-butyl alcohol
(Spectrum) and ethanol (100%, Pharmco) were used as received. Aqueous solutions
were prepared with deionized water (18.2 MQ cm, Milli-Q puriﬁcation system), and

hydrogen (99.9%) was obtained from AGA gases.

3.2.2. Preparation of Pd NPs Encapsulated in Polyelectrolytes

3.2.2.1. [PAA-Pd(0)/PEI]3 on Alumina

Preparation of PAA/PEI-Pd(0) ﬁlms on alumina was described previously,”24
and formation of PAA-Pd(0)/PEI ﬁlms occurred similarly. Alumina was alternately
suspended for 10 min in a solution that contained 20 mM PAA and 1, 4, 8, or 15 mM
K2PdCl4 and for 10 min in a solution that contained PEI (1 mg/mL). (The molarity of

PAA is given with respect to the repeating unit.) Prior to their use in multilayer

assembly, the PAA solution was adjusted to pH 4.0 with 0.1 M NaOH, and the PEI

71

solution was adjusted to 9.0 with 0.1 M HCl. After the deposition of each polyelectrolyte
layer, the alumina was allowed to settle, the supernatant was decanted, and the solid was
suspended similarly (vigorous stirring) in three lOO-mL aliquots of deionized water for 5
min to remove excess polyelectrolyte. Overall, three PAA-Pd(II)/PEI bilayers were
deposited. Reduction to Pd(0) was performed by stirring the [PAA-Pd(II)/PEI]3 ﬁlms on
alumina in 100 mL of freshly prepared 0.1 mM NaBHa for 30 min, followed by washing
with three 100-mL aliquots of deionized water for 5 min each. After decanting the ﬁnal
water rinse, the residual water was removed under reduced pressure overnight to afford a

catalytic powder.

3.2.2.2. [Pd(0)/PEI] on Alumina

[Pd(0)/PEI]3 ﬁlms on alumina were prepared as described previously.25 In brief,
15 g of el-alumina powder was stirred for 30 min in 100 mL of a solution that contained
K2PdC14 (1 or 15 mM) and 0.1 M KCl. The alumina was washed as described above
prior to suspension in PEI (2 mg/mL PEI, pH adjusted to 9.0 with 0.1 M HCl) for 10 min
and additional washing. After deposition of three PdC142'/PEI bilayers, the reduction of

Pd(II) to Pd (0) was performed as described above.

3.2.2.3. PAA-Pd(0)/PEI in Solution
PAA-Pd(II) was prepared by mixing 20 mM PAA and K2PdCl4 (l, 4, 8, or 15

mM) and adjusting the pH of the solution to 4 with 0.1 M NaOH. Five mL of this
solution and 5 mL of PEI solution (1 mg/mL PEI, pH adjusted to 9.0 with 0.1 M HCl)
were mixed together in a 20 mL glass vial at which point the solution became cloudy.

Freshly prepared 0.1 M NaBH4 (5 mL) was added to the cloudy PAA-Pd(II)/PEI, which

72

was then stirred vigorously for 3 h in a loosely capped via]. The resulting clear, brown
solution was then immediately used for hydrogenation.
Crooks et al. previously reported that dendrimer-encapsulated Pd NPs, a

3 We also

homogeneous catalytic system, are unstable in the presence of oxygen.3
observed that the brown color of the PAA-Pd(0)/PEI solution faded to pale yellow over
24 h if the solution was not capped tightly and reﬁigerated. This was especially apparent

when the concentration of Pd(II) in the deposition solution was 1 mM.

3.2.3. Characterization of Pd NPs

For TEM imaging, ﬁlms were deposited on carbon-coated copper grids that were
pretreated in a UV/ozone cleaner for 1 min. [PAA-Pd(O)]/PEI]3 and reduced [PdClaz'
/PEI]3 ﬁlms were formed as described above, except the grid was simply immersed for 5
min in deposition solutions without stirring, and rinsing between each step consisted of a
1-min washing with deionized water.

To characterize the PAA-Pd(0)/PEI solutions prepared with different initial
concentrations of Pd(II) (1, 4, 8, 15 mM), we ﬁrst diluted the reduced solutions with
sufﬁcient deionized water to obtain 0.33 mM Pd. We then spotted a drop of these
solutions on separate carbon-coated copper grids that had been pretreated with UV/ozone
for 1 min, and allowed the solutions to dry at room temperature overnight. Imaging was
performed on a JEOL-2200FS microscope using an accelerating voltage of 200 kV.
Diameters of at least 250 particles for each sample were measured manually on digital
images, and uncertainties in average particle diameters represent standard deviations.
The amounts of palladium in different catalysts were determined by atomic emission

spectroscopy (AES). Standard solutions (0.1 to 0.5 mM) were prepared by dissolving

73

K2PdC14 in 0.1 M HNO3, and sample solutions were prepared by stirring the desired
amount of alumina-supported catalyst (60 to 500 mg, depending on the Pd content) in 2
mL of aqua regia for 15 min. The aqua regia solutions were diluted to 12 mL with water
and centrifuged (the (ll-alumina support does not dissolve in aqua regia), and the
supernatant was analyzed using its emission at 340.5 nm. Table 3.1 lists the amounts of
Pd in each of the different catalysts. For the PAA-Pd(0)/PEI solutions, the amount of

Pd(0) was calculated by assuming total conversion of Pd(Il) to Pd(O).

74

Table 3.]. Amounts and Pd Content of Catalysts Used in Hydrogenation Reactions.

 

 

Catalyst Amount of Moles of Pd in
Catalyst the Catalyst
[PAA-Pd(0)/PEI]3 011 A1203 500 mg 1.91
(Pd(II) = 1 mM), Catalyst A
[PAA-Pd(0)/PEI]3 on A1203 250 mg 4.15
(Pd(II) = 4 mM), Catalyst B
[PAA-Pd(0)/PEI]3 on A1203 125 mg 7.66
(Pd(II)= 8 mM), Catalyst C
[PAA-Pd(0)/ PEI]3 on A1203 125 mg 12.8
(Pd(II) = 15 mM), Catalyst D
[PAA-Pd(O)/PEI]3(PAA/PEI)3 on A1203 500 mg 1,13
(Pd(II) = 1 mM), Catalyst E
[PAA-Pd(O)/PEI]3 (PAA/PEI)3on A1203 125 mg 112
(Pd(II) = 15 mM), Catalyst F
Reduced [PdCl42'/PEI]3 on A1203 250 mg 3,71
(Pd(II) = 1 mM), Catalyst G
Reduced [PdCl42'/PEI]3 on A1203 100 mg 110
(Pd(II)= 15 mM), Catalyst H
PAA-Pd(0)/PEI solution 500 mL 1.67
(Pd(II)= 1 mM), Catalyst I
PAA-Pd(0)/PEI solution 5_00 mL 6.65
(Pd(II) = 4 mM), Catalyst J
PAA-Pd(0)/PEI solution 300 mL 8.00
(Pd(II)= 8 mM), Catalyst K
PAA-Pd(O)/PEI solution 300 mL 1500

(Pd(II) = 15 mM), Catalyst L

 

75

3.2.4. Hydrogenation Reactions

Catalytic hydrogenation was performed in a 100-mL, three—neck, round-bottomed
ﬂask. Suspended alumina-supported catalyst in 25 mL of H20 (or 25 mL of diluted
catalyst solution in the case of NPs stabilized by polyelectrolytes in solution) was
bubbled with H2 for 30 min before adding 25 mL of a 50 mM solution of substrate in
water or in a 1:1 (v:v) mixture of ethanol or methanol or THF and water. During the
reaction, hydrogen at a gauge pressure of 50 kPa was bubbled through a frit at the bottom
of a solution that was vigorously stirred. Different amounts of catalyst (Table 3.1) were
used for hydrogenation reactions because each type of material had a different wt% of
Pd. 0.5-mL aliquots were removed from the reaction vessel at speciﬁc times, ﬁltered
with cotton to remove the catalyst, and diluted by addition of l-mL deionized water prior
to analysis by gas chromatography (Shimadzu GC-17A equipped with an RTx-
BAClcolumn). In the GC analysis of hydrogenation of 3, product and reactant peaks
showed a small overlap, and one limit of integration was placed at the minimum between
the peaks. Response factors were determined to calculate the turnover frequency (TOF)
as described previously.23 TOFs were calculated from the slope of the initial portion
(total conversion of reactant to isomer and hydrogenated product <50%) of plots of
percent hydrogenation versus time. In ﬁtting the initial data, the intercept of the line was
forced through zero. Each experiment was repeated twice and the i values represent the

difference between the average and the data points.

76

3.3. Result and Discussion
3.3.1. Catalyst Synthesis and Variation of Particles Size

The initial step in studying the effect of NP size on catalytic selectivity is to
prepare a series of catalysts with different particle sizes. Previously, we made [PAA/PEI-
Pd(II)]3PAA ﬁlms using 2 mM Pd(II) in the PEI-Pd(II) deposition solution. Subsequent
reduction of the Pd(II) gave [PAA/PEI-Pd(0)]3PAA coatings containing Pd NPs with

diameters of 1-3 nm.23

We tried to enhance NP size by increasing the concentration of
Pd(II) in the PEI deposition solution, but at a Pd(II) concentration of 12 mM, the PEI
solution became cloudy, perhaps because the Pd(II) serves as a cross- linking agent for
PEI chains. To overcome this challenge, we add Pd(II) to the PAA, rather than PEI,
deposition solution. The 20 mM PAA deposition solution (PAA concentration is given
with respect to the repeating unit) remains clear at Pd(II) concentration as high as 15 mM.
Prior studies demonstrated deposition of metal ions in PAA/protonated poly(allylamine)
ﬁlms and subsequent formation of nanoparticles,3"32 but in the present case we add the
metal ions during ﬁlm formation.

Most importantly, reduction of [PAA-Pd(II)/PEI]3 using NaBH4 yields [PAA-
Pd(0)/PEI]3 ﬁlms, and the sizes of the resulting Pd NPs increase with increasing

concentrations of Pd(II) in PAA deposition solutions. Figures 3.1, 3.2, 3.3, 3.4, and 3.5

present TEM images of ﬁlms on carbon-coated grids and histograms of particle sizes.

77

 

N N
O 0|
1 1

  
   
    
 

 

1n
2

.2

E 15-

8

g 10-

n.

0

.\° 5'

o
1.2 1.8 2.4 3

Nanoparticle Diameter (nm)

Figure 3.1. (a) TEM image of a [PAA-Pd(0)/PEI]3 ﬁlm on a carbon-coated copper grid. 1
mM Pd(II) was present in the PAA-K2PdCl4 solution used for ﬁlm deposition. A few of
the NPs are circled. (b) Histogram of Pd NP diameters in several TEM images. The
average particle size is 2.2iO.6 nm. The area-average particle size is 2.5:l:0.7 nm.

78

 

30-
251
20.
15.
1o-
5.
o

 

% of Nanoparticles

 

1.8 2.1 2.4 2.7 3.0 3.3 3.6 3.9 4.2

Nanoparticle Diameter (nm)

Figure 3.2. (a) TEM image of a [PAA—Pd(0)/PEI]3 ﬁlm on a carbon-coated copper grid. 4
mM Pd(II) was present in the PAA-K2PdCl4 solution used for ﬁlm deposition. A few
NPs are circled. (b) Histogram of Pd NP diameters in several TEM images. The average
particle size is 3.03:0.8 nm. The area-average particle size is 3.1:k0.5 nm.

79

% of Nanoparticles

 

2.1 2.7 3.3 3.9 4.5

Nanoparticle Diameter (nm)

Figure 3.3. (a) TEM image of a [PAA-Pd(O)/PEI]3 ﬁlm on a carbon-coated copper grid.
8 mM Pd(II) was present in the PAA-K2PdC14 solution used for ﬁlm deposition. (b)
Histogram of Pd NP diameters in several TEM images. The average particle size is
3.2i0.6 mn. The area-average particle size is 3.5i0.7 nm.

80

 

M
(II
I

O'

    
  
 
 

.3 A
0 0|
1 1

% of Nanoparticles
UI

 

O

2.4 3 3.6 4.2 4.8 5.4 6

Nanoparticle Diameter (nm)

.5
.
on

Figure 3.4. (a) TEM image of a [PAA-Pd(0)/PEI]3 ﬁlm on a carbon-coated copper grid.
15 mM Pd(II) was present in the PAA-K2PdCl4 solution used for ﬁlm deposition. (b)
Histogram of Pd NP diameters in several TEM images. The average particle size is
3.4:l:1.3 nm. The area-average particle size is 3.8:l:1.1 nm.

81

With Pd(II) concentrations of 1, 4, 8, or 15 mM in the PAA deposition solutions,
the average particle diameters in [PAA-Pd(0)/PEI]3 ﬁlms on TEM grids are 2.2iO.6,
2.9i0.5, 3.2i0.6, and 3.4:t0.9 nm, respectively. Area-average particle sizes might be
more representative of catalytic activities because larger particles make up a
disproportionate share of the total catalyst surface area. We calculate the area-average

diameters, DA, using equation 1, where D, represents the diameters of individual particles.
ZD i D i2
i

1'

DA (1)

Values of DA are 2.5:t0.7, 3.1i0.5, 3.53:0.7, and 3.8i1.1 nm for ﬁlms prepared using 1, 4,
8, and 15 mM Pd(II), respectively, in the 20 mM PAA deposition solutions.
Occasionally, TEM images of [PAA-Pd(0)/PEI]3 ﬁlms prepared with 15 mM Pd(II) in the
PAA deposition solutions exhibit areas containing large particles with average diameters
of 160i100 nm (Figures 3.5). We did not include these larger particles in the calculation
of average particle diameters because they constitute only about 18 % of the total particle
surface area (Table 3.2), and we are not sure whether they are representative of all
samples. However, the absolute values of TOFs for catalysis with alumina-supported
[PAA-Pd(0)/PEI]3 ﬁlms that are prepared using 15 mM Pd(II) should be viewed with

caution.

82

 

Figure 3.5. Low-magniﬁcation TEM images of large particles and NPs in [PAA-
Pd(0)/PEI]3 ﬁlms on carbon-coated copper grids. The ﬁhns were prepared with 15 mM
Pd(II) in the PAA deposition solution. (a) Image showing large particles with diameters
of 160i100 nm, as determined from several TEM images. Such large particles appeared
in ~30% of similar images. (b) Image of NPs. Larger spots are overlapping multiple
particles. The dark spot in the center of low-magniﬁcation images is due to a defect in
the lens where it was burned by the electron beam.

83

Table 3.2. Calculation of the relative surface area of large (ranging from ~50-300 nm)
particles and NPs (< 5 nm) in [PAA-Pd(O)/PEI]3 ﬁlms and reduced [PdC142'/PEI]3 ﬁlms
on carbon-coated TEM grids. The initial concentration of Pd(II) was 15 mM in the
solution containing PAA-PdClaz‘ or PdC142'. Large particles were not prevalent when
using lower concentrations of Pd(II).

 

% occupy by big particles

and NPs
Speciﬁc Descriptions

 

[PAA- [Pdle'
Pd(0)/PEI]3 rpm],

 

Average percentage of image area covered by large particles

~ 0 ~ 0
(when present) 11 /° 1 1 /o

 

Percentage of low-magniﬁcation images containing large

particles ~30% ~6O%

 

Percentage of image area covered by large particles in all
low-magniﬁcation images (including images with and ~3 % ~7 %
without large particles)

 

Average percentage of image area covered by NPs

 

~ 0 ~ 0
(determined from high magniﬁcation images) 14 /o 14 A:
Fraction of total Pd surface area due to small NPs
(determined from the image areas covered by large and small 0.82 0.67

NPs)a

 

 

 

 

 

[a] Because both the surface area of a particle and the image area it covers are
proportional to the square of the particle diameter, the fraction of the total Pd surface area
due to NPs is equivalent to the image area covered by NPs divided by the sum of the
image areas covered by NPs (< 5 nm) and larger particles (ranging from ~ 50-300 nm).

Although we use ﬁlms deposited on alumina particles for catalysis, we perform
the TEM analyses with ﬁlms on carbon-coated Cu grids because it is very difﬁcult to
visualize NPs on the rough alumina microparticles. The NPs likely have similar
diameters on the TEM grids and on the alumina, but we cannot verify the diameters of
particles on alumina. To address this problem, we also prepare polyelectrolyte/NP

composites in water. These materials serve as catalysts and are amenable to TEM

84

imaging, which allows a direct correlation between catalytic activity and particle size. A
recent study showed that in many cases, NPs in water do not aggregate in the presence of
polyelectrolytes.34

The preparation of the suspended polyelectrolyte/NP composites includes the
addition of an aqueous solution of PEI (1 mg/ml, pH adjusted to 9) to a solution of PAA
(20 mM, pH adjusted to 4) that contains various concentrations of K2PdC14 (1, 4, 8, or 15
mM). The yellow-colored PAA-Pd(II) solution becomes cloudy after addition of PEI and
then turns to a clear brown color after addition of 0.1 M aqueous NaBHa and vigorous
stirring. The pH of the PAA-Pd(II)/PEI mixture is about 5, but the addition of 0.1 M
NaBHa gives a clear solution with a pH of ~9. Raising the pH of PAA-Pd(II)/PEI
solutions by addition of 0.1 M NaOH also yields a clear solution, presumably because
less protonation of PEI results in weaker PAA/PEI complexes. However, both
polyelectrolytes are important for stabilizing the NPs. Addition of 0.1 M NaBHa to
solutions containing either PAA and 8 mM Pd(lI) or PEI and 8 mM Pd(II) produces a
black precipitate.

To determine the sizes of NPs suspended in these solutions, we deposit a drop of
solution on a carbon-coated copper grid and then image the NPs with TEM. These TEM
images along with histograms of particle sizes (Figures 3.6, 3.7, 3.8, 3.9) indicate the
formation of NPs whose size is a function of the concentration of Pd(II) in solution prior
to reduction. In this case, the average particle diameters that result from the use of 1, 4,
8, and 15 mM Pd(II) to form NPs are 2.6i0.8, 32:20.7, 3.5i0.7 and 5.1i1.3 nm,
respectively. Corresponding area-average diameters are 3.1:t0.8, 3.4i0.7, 3.7:l:0.7 and

5.8i1.4 nm. We do not observe any large (>9 nm) particles in PAA-Pd(0)/PEI solutions

85

prepared with Pd(II) concentrations of 15 mM in the PAA deposition solutions (Figure

3.10).

 

 

% of Nanoparticles

 

1.2 1.8 2.4 3 3.6 4.2 4.8
Nanoparticle Diameter (nm)

Figure 3.6. (a) TEM image of a dried PAA-Pd(0)/PEI solution on a carbon-coated copper
grid. 1 mM Pd(II) was initially present in the PAA solution. (b) Histogram of Pd NP
diameters in several TEM images. The average particle size is 2.6108 nm. The area-
average particle size is 3.1i0.8 nm.

86

N
0|
;1

% of Nanoparticles

 

 

..n .1. N
0 0| 0 (II D
1 r l 1

A
.
on

2.4 3 3.6 4.2 4.8

Nanoparticle Diameter (nm)

Figure 3.7. (a) TEM image of a [PAA-Pd(0)/PEI] solution on a carbon-coated copper
grid. 4 mM Pd(II) was initially present in the PAA-PdCl42' solution. (b) Histogram of Pd
NP diameters in several TEM images. The average particle size is 3.2iO.7 nm. The area-
average particle size is 3.4i0.7 um.

87

N
O

a
0|

0|

 

% of Nanoparticles
3

O

1.8 2.4 3 3.6 4.2 4.8

Nanoparticle Diameter (nm)

Figure 3.8. (a) TEM image of a dried PAA-Pd(0)/PEI solution on a carbon-coated copper
grid. 8 mM Pd(II) was initially present in the PAA solution. (b) Histogram of Pd NP
diameters in several TEM images. The average particle size is 3.5:l:0.7 nm. The area-
average particle size is 3.7:l:0.7 um.

88

14 -
12 -
10 -
3 .
5 .
4 .
2 .
o .
2.4 3 3.6 4.2 4.8 5.4 6 6.6 7.2 7.8

Nanoparticle Diameter (nm)

%of Nanoparticles

 

 

Figure 3.9. (a) TEM image of a [PAA-Pd(0)/PEI] solution on a carbon-coated copper
grid. 15 mM Pd(II) was initially present in the PAA-PdClaz' solution. (b) Histogram of
Pd NP diameters in several TEM images. The average particle size is 5.1:l:1.3 nm. The
area-average particle size is 5.8:l:1.4 nm.

89

 

Figure.3.10. (a) TEM image of a dried PAA-Pd(0)/PEI solution on a carbon-coated
copper grid. 15 mM Pd(II) was initially present in the PAA solution. This image was
taken at a relatively low magniﬁcation to show the distribution of NPs and the absence of
large particles on the TEM grid. The dark spot in the center of low-magniﬁcation images
is due to a defect in the lens where it was burned by the electron beam.

3.3.2. Selective Hydrogenation with Films on Alumina Particles

To investigate catalytic selectivity as a ﬁmction of NP diameter, we ﬁrst
investigated hydrogenation of allyl alcohol (1, a monosubstituted unsaturated alcohol), 2-
methyl-Z-propen-l-ol (2, a disubstituted unsaturated alcohol), and crotyl alcohol (3, a
disubstituted unsaturated alcohol). TOFs (moles hydrogenated/mol Pd/h) at low substrate
conversion provide a quantitative comparison of the reactivities and selectivities of the
different catalysts. Unforttmately, interpretation of the TOFs is complicated by
isomerization of the unsaturated alcohols to the corresponding aldehydes, which are not

readily hydrogenated. Thus, Table 3.3 contains TOFs for initial hydrogenation along

90

with values in parentheses that reﬂect the sum of the initial hydrogenation and initial
isomerization TOFs. Isomerization is most extensive for 2, presumably because its
isomer is a trisubstituted unsaturated alcohol that is more stable than 2 and readily
tautomerizes to the corresponding aldehyde.

Table 3.3 shows that regardless of NP size, both the TOF for hydrogenation and
the combined hydrogenation/isomerization TOF are higher for 1 than for 2 and 3, which
contain disubstituted double bonds. In addition, selectivity varies dramatically with the
particle size. The general trend in Table 3.3 is that the rate of hydrogenation of 1
(column 3) decreases with increasing NP size, while the corresponding TOFs for 2
(column 4) and 3 (column 5) increase with increasing NP size, even though the surface
area to volume ratio for the catalyst is smaller for larger particles. Most remarkably, this
trend yields selectivities (ratios of TOFs for different compounds, columns 6 and 7 of
Table 3.3) that are as much as 90—fold larger with the smallest NPs than with the largest.
These trends also hold when considering the sum of hydrogenation and isomerization.
The selectivities for 1/2 and 1/3 with the smaller NPs are especially notable because
Wilkinson’s catalyst, the prototypical homogeneous catalyst for selective hydrogenation,

shows essentially no selectivity in the hydrogenation of mono- and disubstituted double

bonds.23’35

91

Table 3.3. TOFs and TOF ratios for unsaturated-alcohol hydrogenation catalyzed by
[PAA-Pd(O)/PEI]3 ﬁlms that contain Pd NPs with different diameters. The catalytic ﬁlms
were deposited on micron-sized alumina. Values in parentheses are the sums of TOFs for
hydrogenation and isomerization.

 

Average TOFs (moles hydrogenated/ mol Pd/ h)a Ratio 0f TOPS

 

nano-
particle
diameter _
Catalyst (nm) _\_OH OH /=\_OH 1/2 1/3
1 2 3
CatalystA b 2 2 2900i570 12d:1 753:15 240 39
[pd(11)]=1 mM ' (6200i2400) (85i5) (85i15) (73) (73)
Catalyst B b 2.9 1700i830 160i10 450i80 11 3.8
[Pd(11)]=4 mM (4100a 800) (730i20) (6202.430) (5.6) (6.6)
CatalystC b 3 2 1100.600 165:1:5 610i60 6.7 1.8
[pd(11)]=3 mM ' (2400:}:800) (895i15) (760i80) (2.7) (3.2)
Catalyst D b 3 4 760i40 290i70 5801100 2.6 1.3
[pd(11)]=15 mM ' (2050i80) (1400i100) (790:1:160) (1.5) (2.6)

 

[a] 50 mL of 25 mM substrate was hydrogenated in water. TOFs are initial values
determined at conversions less than 50%. [b] The initial concentration of KdeCl4 in the
PAA deposition solutions.

Several studies show that polyelectrolyte multilayers provide a signiﬁcant barrier
to the diffusion of small compounds, and even molecules as small as glycerol and

36 Thus, one

methanol show much slower transport than water in polyelectrolyte ﬁlms.
possible explanation for the high selectivities of NPs in polyelectrolyte ﬁlms is that the
polyelectrolytes decrease the rate at which bulkier molecules reach catalytic sites. To
examine whether simple difﬁrsion through polyelectrolyte ﬁlms enhances selectivity, we
coated ﬁlms containing the smallest and largest NPs with additional layers of

polyelectrolytes. Pd was reduced prior to the deposition of additional layers. As

comparison of Table 3.3 and Table 3.4 shows, addition of three PAA/PEI bilayers on the

92

 

Table 3.4. TOFs and TOF ratios for unsaturated-alcohol hydrogenation catalyzed by
[PAA—Pd(O)/PEI]3/(PAA/PEI)3 ﬁlms that contain Pd NPs with different diameters. The
ﬁlms were deposited on micron-sized alumina. Values in parentheses are the sums of
TOFs for hydrogenation and isomerization.

 

Average TOFs (moles hydrogenated/ mol Pd/ h)a Ratio 0f TOFS

 

nano-
Catalyst particle _ —
diameter —\—OH 0H /—\—0H
(m) 1 2 3 1/2 1/3
Catalyst E 2.2 1500i250 10+.1 35:5 150 43
[Pd(11)]=1 me (3000i1000) (40i5) (404.5) (75) (75)
Catalyst F 3.4 1200i90 85i5 40015 14 3.0
[pd(11)]=15 me (2600i600) (3702215) (540i35) (7.0) (4.8)

 

[a] 25 mM substrate was hydrogenated in water. TOFs are initial values determined at
conversions less than 50%. [b] The initial concentration of KdeCl4 in the PAA
deposition solutions.

top of a ﬁlm containing 2.2 nm-diameter particles does not have a large effect on the
relative TOFs of 1 and 2 or 1 and 3, although the magnitudes of TOF values decrease by
a factor of ~2. Addition of three PAA/PEI bilayers to ﬁlms containing NPs with a
diameter of 3.4 nm increases selectivity as much as 6-fold, but the selectivity of these
catalysts is still ll-(1/2) and lS-fold (1/3) lower than that with the 2.2-um NPs (Table
3.4). These data suggest that the most important factor behind selectivity is NP size.

To further demonstrate that NP size is the primary factor behind selectivity, we
studied catalysis with reduced [PdCl42’/PEI]3 coatings on alumina. (The nomenclature
PdCl42' reﬂects deposition conditions and not the state of Pd in the ﬁlms). These NP-
containing ﬁlms should be more permeable than [PAA-Pd(O)/PEI] coatings because they
contain only one polyelectrolyte.25 After reduction of the Pd(II) in these ﬁlms, the

average particle size is 3.43:0.8 nm when using 15 mM PdCl42' to prepare the ﬁlms and

93

 

% of Nanoparticles

 

1.5 2.1 2.7 3.3 3.9 4.5

Nanoparticle Diameter (nm)

Figure 3.11. (a) TEM image of a [Pd(0)/PEI]3 ﬁlm on a carbon-coated copper grid. The
concentration of PdCl42' used in depositing the precursor [PdC142'lPEI]3 ﬁlm was 1 mM.
(b) Histogram of Pd NP diameters in several TEM images. The average particle size is
3.0:t0.9 nm. The area-average particle size is 3.5i1.0 nm.

94

 

25-
20- b
15-

10-

   

5..

 

% of Nanoparticles

0
1 .8 2.4 3 3.6 4.2 4.8 5.4
Nanoparticle Diameter (nm)
Figure 3.12. (a) TEM image of a [Pd(0)/PEI]3 ﬁlm on a carbon-coated copper grid. The
concentration of PdCl42' used in depositing the precursor [PdCl42‘/PEI]3 ﬁlms was 15

mM. (b) Histogram of Pd NP diameters in several TEM images. The average particle size
is 3.4i0.8 nm. The area-average particle size is 3.8i0.9 nm.

95

 

 

Figure 3.13. (a) Low-magniﬁcation TEM image of a reduced [PdCl42’lPEI]3 ﬁlm on a
carbon-coated copper grid. 15 mM PdCl42' was initially present in the deposition solution.
The average size of these large particles in several TEM images is 170i110 nm. Large
particles are visible in about 60% of these low-magniﬁcation images, but we estimate that
the surface area of the large particles constitutes ~33% of the surface area in the ﬁlm
(Table 3.2).

3.0:t0.9 run when using a 1 mM PdCl42' solution (See Figures 3.11 and 3.12).
Corresponding area-average diameters are 3.8i0.9 and 3.5:hl.0 nm respectively. In this
case, NP sizes are not very different because PdCl42‘ does not bind to a polymer in
solution before adsorbing to the ﬁlm, but rather adsorbs directly to the polyelectrolyte
ﬁlm. Because the amount of adsorbed PdCl42' in Pdle/PEI ﬁlms is somewhat self-
limiting as the ﬁlm becomes negatively charged, PdChz’ concentration should not have a

dramatic effect on the amount of Pd in the ﬁlm. However, in some areas of reduced

96

[PdCl42'/PEI]3 ﬁlms prepared with 15 mM Pd(II), TEM images show the presence of
large particles (average particle size 170i110 nm) as well as NPs (Figure 3.13). We
estimate that the large particles represent ~33 % of the total Pd surface area. We did not
include these larger particles in the calculation of average particle diameters because we
are not sure whether they are representative of all samples. The absolute values of TOFs
for catalysis with reduced [PdCl42'/PEI]3 ﬁlms that are prepared using 15 mM Pd(II)
should, thus, be viewed with caution.

Table 3.5 summarizes the TOFs for the hydrogenation of 1, 2 and 3 using the two
different reduced [PdCl42'/PEI] ﬁlms on alumina. The smaller NPs in catalyst G give
selectivities that are an order of magnitude higher than hydrogenation using catalyst H,

even though the average NP diameters in the two catalysts differ by only 0.40 nm.

Table 3.5. TOFs and TOF ratios for unsaturated-alcohol hydrogenation catalyzed by
reduced [PdCl42'/PEI]3 ﬁlms that contain Pd NPs with different diameters. The ﬁlms
were deposited on micron-sized alumina. Values in parentheses are the sums of TOFs for
hydrogenation and isomerization.

 

TOFs (moles hydrogenated! mol Pd/ h)a Ratio 0f TOPS

 

Avarage
nano- _ _
Catalyst particle —\—OH {OH /—\—OH
diameter 1 2 3 1/ 2 1/3
(11m)
Catalyst G 3.0 2230i70 25d:5 220i40 89 10
[pd(11)]=1 me (4600i38 (150:1:40) (250i40) (30) (19)
0)
Catalyst H 3.4 880:1:110 380i110 580i80 2.3 1.5
[pd(11)]=15 me (2300i10 (1700d:350) (7501100) (1.4) (3.1)
0)

 

[a] 25 mM substrate was hydrogenated in water. TOFs are initial values determined at
conversions less than 50%. [b] The initial concentration of KdeCl4 in the deposition
solutions. 9

97

3.3.3. Hydrogenation with NP/Polyelectrolyte Complexes in Solution

Because we cannot effectively image NP catalysts on alumina, we also examined
hydrogenation with suspended PAA-Pd(0)/PEI catalysts prepared by addition of a PEI (1
mg/mL) solution to 20 mM PAA solutions containing different concentrations of Pd(II).
Reduction of the Pd(II) with NaBH.; yields suspended PAA-Pd(0)/PEI catalysts with a
variety of Pd NP sizes. One concern when using these NPs is that excess NaBH4, which
is not removed from the NP solution, might promote side reactions. As a control
experiment, we attempted reduction of allyl alcohol in the absence of hydrogen gas using
the PAA-Pd(0)/PEI solution containing excess Nth. No reaction occurs, which rules
out the possibility of NaBH4 participating in side reactions.

Similar to catalysis by NP ﬁlms on alumina, with the PAA-Pd(0)/PEI solution, the
TOF for catalytic hydrogenation of 1 decreases with increasing particle size, whereas the
TOFs for 2 and 3 either increase or remain approximately constant as particle size
increases (Table 3.6). Thus, overall selectivity shows an order of magnitude decrease on
going from an average NP diameter of 2.6 nm to 5.1 nm. Comparison of Tables 3.3, 3.4,
3.5 and 3.6 shows that selectivities are frequently higher for these suspended NPs than
they are for [PAA-Pd(0)/PEI]3 and reduced [PdCl42'/PEI]3 ﬁlms on alumina. The
suspended NPs in solution may present a more uniform set of catalytic sites than the
ﬁlms on alumina powder, which might favor high selectivity. In general, supported
catalysts are complex materials due to the presence of many types of potential active

sites.

98

Table 3.6. TOFs and TOF ratios for unsaturated-alcohol hydrogenation catalyzed by
solutions of PAA-Pd(0)/PEI that contain Pd NPs with different diameters. Values in
parentheses are the sum of TOFs for hydrogenation and isomerization.

 

TOFs (moles hydrogenated/ mol Pd/ h)a Ratio 0f TOFS

 

Average
nano- — _
Catalys‘ particle —1¥°” 0” /_3\—°” 1/2 1/3
diameter
(nm)
Catalyst 1 37001860 12 30110 310 120
[Pd(II)]=l me 2.6 (580011900) (7515) (35115) (73) (170)
Catalyst J 16001190 12 4515 130 35
[Pd(II)]=4 me 3.2 (29001600) (5015) (50110) (59) (59)
Catalyst K 11001560 1214 3515 95 33
[Pd(II)]=8 me 3.5 (24001950) (6015) (4515) (40) (54)
Catalyst L 8901210 30110 80115 30 10
[Pd(II)]=15 me 5-1 (17001350) (100115) (100115) (16) (18)

 

[a] 25 mM substrate was hydrogenated in water. TOFs are initial values determined at
conversions less than 50%. [b] The initial concentration of KdeCl4 in the PAA
deposition solutions.

3.3.4. Solvent Effects on Hydrogenation

Previous studies of catalytic hydrogenation showed that variations in solvent can
lead to changes in reaction rates, differences in the extent of double-bond isomerization,
or changes in selectivity.23’24’27’37'38 Presumably, molecules of solvents such as MeOH,
EtOH and THF adsorb to the catalyst to alter reactivities. Additionally, swelling of
polyelectrolyte ﬁlms in water may increase reaction rates.39 We examined hydrogenation
with catalysts A and D using solvents consisting of 3 parts water and 1 part of either
MeOH or EtOH or THF. These two catalysts represent the widest range of particle
diameters among the NPs deposited on alumina. Table 3.7 summarizes the TOFs for
hydrogenation of l, 2, and 3 in different mixed solvents. For catalyst D (average particle

diameter of 3.4 nm), the rate of hydrogenation for all of the compounds decreases in the

99

Table 3.7. TOFs and TOF ratios for unsaturated-alcohol hydrogenation catalyzed by
[PAA-Pd(0)/PEI]3 ﬁlms in different solvents. The ﬁlms contain Pd NPs with different
diameters and are deposited on micron-sized alumina. Values in parentheses are the sum
of TOFs for hydrogenation and isomerization.

 

TOFs (moles hydrogenated/ mol Pd/ h)a Ratio 0f TOPS

 

 

=\_ =< ﬂ
Catalysts Solvent 1 OH 2 OH 3 OH “2 1/3
Mlxtures
Water & 14001100 4 1515 350 93
MeOH (25001250) (30110) (2015) (83) (125)
(3: 1)
b
Catalyst A Water & EtOH 16001400 611 4015 267 40
[Pd(II)]=l mMc (3:1) (29001650) (4515) (4515) (64) (64)
Water & THF 6001200 - 25115 - 24
(3:1) (1050150) (-) (-)
Water & 12001350 28015 7001100 4.3 1.7
MeOH (390011150) (14001200) (1050120 (2.8) (3.7)
(3: 1) 0)
Catalyst Db Water & EtOH 8101320 220110 4001100 3.7 2.0
[P d(II)]=15 (3:1) (290011 100) (15001220) (6601160) (1.9) (4.4)
mM°
Water & THF 300160 - 250120 - 1.2
(3: 1) (130013 90) (-) (-)

 

[a] Several TOFs could not be estimated because GC signals of the substrates, products,
or isomers overlapped with the very large peaks corresponding to solvents. [b] Average
nanoparticle sizes in catalysts A and D are 2.2 and 3.4 nm respectively. [c] The initial
concentration of KdeCl4 in the PAA deposition solutions.

order water > water/methanol, water/ethanol > water/THF, which may reﬂect the degree
of swelling of the polyelectrolyte. Higher swelling in more polar solvents should lead to
higher reaction rates. With catalyst A, TOFs are higher in water (data in Table 3.3) than
the mixed solvent systems (Table 3.7), but there is no clear trend in the different solvents.

Table 6 shows that in most cases, selectivities are higher in 3:1 water/methanol than any

100

of the other solvents. The small methanol molecules might more effectively compete for

binding sites with 2 and 3 than with 1.

3.3.5. Hydrogenation of Other Unsaturated Alcohols

To examine the generality of these results, we studied the hydrogenation of
several additional molecules using catalysts A and D, which have approximate NP
diameters of 2.2 and 3.4 nm, respectively. As Table 3.8 shows, the rate of hydrogenation
of compounds 4-7 decreases in the order monosubstituted double bonds>disubstituted
double bond> trisubstituted double bonds. (These reactions occur in 3:1 water/ethanol to
ensure substrate solubility.) Compounds 4 and 1 both contain a monosubstituted double
bond, but in 3:1 waterzethanol, hydrogenation of 4 by catalyst A occurs at a slower rate
than hydrogenation of 1 (compare Tables 3.7 and 3.8). Nevertheless, the TOF for the
hydrogenation of 4 is still 11 times higher than that for 6, which contains a disubstituted
double bond. Interestingly, using the smaller NPs, the TOF for hydrogenation of 5 is 11-
fold lower than that for hydrogenation of 1 in 3:1 waterzethanol. This suggests that when
using the smaller NPs, rates are affected by the addition of a bulky substituent at to the
double bond, which is consistent with previous studies.24 However, even with 5,
hydrogenation using catalyst A is much faster than with the di and trisubstituted
compounds 2, 3, 6 and 7. In contrast, catalyst D, which has much larger NPs, shows little
difference in the TOFs among the molecules containing monosubstituted double bonds
(1, 4, and 5). Furthermore, selectivities for hydrogenation of 4 and 5 vs di and
trisubstituted double bonds (6 and 7, respectively) are much smaller when using the

larger NPs in catalyst D than when using catalyst A.

101

Table 3.8. TOFs and TOP ratios for unsaturated-alcohol hydrogenation catalyzed by
[PAA-Pd(0)/PEI]3 ﬁlms that contain Pd NPs with different diameters. The catalytic ﬁlms
were deposited on micron-sized alumina. Values in parentheses are the sum of TOFs for
hydrogenation and isomerization.

 

TOFs (moles hydrogenated/ mol Pd/ h)a’b

OH _ OH
M / /
Catalysts / OH /\|/\ /'_—/_\__\OH v
4 5 6 7

 

Catalyst Ac 290:th 150130 2713 812
[Pd(II)]-l de (“03:10) H H (')
Catalyst 11" 895145 760110 23312 130110
d (2175165) () (..) (15015)

[Pd(II)]=15 mM

 

[a] 25 mM substrate was hydrogenated in 3:1 waterzethanol. TOFs are initial values
determined at conversions less than 50%. [b] In some cases, the TOP was not estimated
because the amount of isomer detected was negligible. [c] Average nanoparticle sizes for
catalysts A and D are 2.2 and 3.4 nm respectively. [d] The initial concentration of
KdeCl4 in the PAA deposition solutions.

3.4. Possible Explanation of Selectivity

The above data demonstrate that selectivity in hydrogenation varies greatly with
particle size, and related studies of NP reactivity imply that such trends arise from
geometrical effects.40 Doyle, et al. suggest that the sensitivity of catalytic hydrogenation
rates to NP size occurs because the reaction takes place on speciﬁc types of atoms that

are more (or less) prevalent on small particles.2

Large particles mainly contain terraces
with atoms of high coordination numbers, whereas small particles contain comparatively
high numbers of surface atoms with low coordination numbers (edge and corner atoms).
A recent study based on STM images shows that Pd NPs with diameters <4 nm are

highly defective, whereas NPs with diameters >4 nm start to develop large and well

deﬁned facets?"41 Additionally, in some cases, if a TOF is calculated with respect to a

102

Face Atoms

Defect Atoms

 

Scheme 3.1. Cuboctahedra Pd NP with different atoms.

deﬁned type of atom (edge atoms, for example), the dependence of TOF on particle size
disappears, and the reaction is deemed structure-sensitive but size-independents'lu'
Using the Van Hardeveld and Hartog statistics, we calculated the number of different
types of surface atoms on cuboctahedra Pd NPs (Scheme 3.1) containing the same
number of atoms as average NPs in catalysts I, J, K and L (NPs suspended in
polyelectrolyte solutions) (Table 3.9).42 Figure 3.14 presents the calculated percentage of
surface, defect (edge and comer), and face atoms with respect to the total atoms for each
average particle size and shows that the percentage of face atoms changes minimally
(from 26% to 20%) as the average particle size increases from 2.6 to 5.1 nm. However,
the percentage of defect atoms decreases from 18% to 5% as the average particles size
increases from 2.6 to 5.1 nm. In general, the TOFs for hydrogenation of monosubstituted

compounds decrease with increasing NP diameter, while the TOFs for di- and

trisubstituted compounds increase slightly or are unaffected as NP size increases. One

103

Table 3.9. Formula used to calculate m, the number of shells for NPs having
cuboctahedron eometry, and the number of Pd atoms at the various positions available
on NP surfaces. 2

 

Total no. of atoms (l/3)(2m—1) (5m2-5m+3)
No. of surface atoms 10m2-20m+12
Vertex atoms (5-coordinate) 12
Edge atoms (7-coordinate) 24(m-2)
Sq. face (8-coordinate) 6(m-2)2
Triangular face (9-coordinate) 4(m-3)(m—2)

 

The total number of atoms (N) was calculated by the following equation1

N = 471(r)3/(3Vg), where Vg = the volume of one Pd atom =15 A3

explanation for this observation is that in the hydrogenation of monosubstituted double
bonds, defect atoms are the primary active centers for catalysis, whereas hydrogenation
of multisubstituted double bonds occurs on terraces. Hence, TOFs for the
monosubstituted compounds increase as NP size decreases because of a larger fraction of
defect atoms in the small NPs. Several recent studies show that interactions of alkenes
with catalysts change with alkene size, and hydrogenation of higher alkenes such as
trans-2-pentene takes place on face atoms.2’5'43 Rates of hydrogenation on terrace sites
should be much less dependent on particle size because the fraction of total atoms that are

on terraces is relatively independent of the NP diameter. Thus, it is possible that

104

preferential hydrogenation of monosubstituted double bonds on defect sites leads to high

selectivities with small NPs.

50.
45-
40*
35-
30-
251‘ ‘ A +%ofeurfaceatome
20'. A I %ofdefectatoms
15- A %offaceatome
10‘ '

5-1 l

0 t t .

2.5 3.5 4.5 5.5
Average Nanoparticle Diameter (nm)

 

 

 

 

% of Different Atoms

 

 

Figure 3.14. Plot of the calculated percentage of surface, defect and face atoms with
respect to the total atoms for each particle size.

3.5. Conclusion .

Selectivity in the hydrogenation of monosubstituted over disubstituted double
bonds increases dramatically as the diameter of catalytic NPs decreases, and this size-
dependent selectivity occurs with both suspended NPs and NPs deposited on micron-
sized alumina. High selectivities stem from the fact that TOFs for the hydrogenation of
monosubstituted double bonds decrease as particle size increases, whereas the TOFs for
the hydrogenation of multisubstituted double bonds show the opposite trend. When using
the smallest NPs (average diameters 2.2 run), these trends result in selectivities >100 for

the hydrogenation of monosubstituted over disubstituted double bonds. Selectivities

105

might occur because monosubstituted double bonds are hydrogenated on defect sites,

while hydrogenation of multisubstituted double bonds takes place on terraces.

106

3.6. References

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(18) Semagina, N.; Renken, A.; Kiwi-Minsker, L. Journal of Physical Chemistry C
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(19) Claus, P.; Brueckner, A.; Mohr, C.; Hofmeister, H. Journal of the American
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109

CHAPTER 4
CHEMOSELECTIVE NITROARENE HYDROGENATION CATALYZED BY
PALLADIUM NPs EMBEDDED IN POLYELECTROLYTE MULTILAYERS
4.1. Introduction
Functionalized aniline derivatives are important synthetic intermediates in the
production of dyes, herbicides, pesticides, and pharmaceuticals, and aromatic nitro
compounds routinely serve as precursors to their corresponding anilines."7 The
transformation of nitrobenzene to aniline proceeds through intermediate nitroso and
hydroxylamine compounds,8 but slow conversion of hydroxylamine to aniline sometimes

9-12

results in the accumulation of hydroxylamine, as well as the formation of

hydrazines,'3"4 azoarenes'5 or azoxyarenes.”21 Reduction of other functional groups in
nitroaromatic compounds can also lead to undesired side products.”23
Transformation of nitro compounds to aromatic amines is generally accomplished

22,24-26

through catalytic reduction 112337-33

or hydrogenation, although electrochemical34 or
enzymatic reactions have also been used.”36 Stoichiometric reagent-based processes are
selective towards the reduction of the nitro functional group, but such processes are
frequently expensive and not environmentally friendly. The catalytic hydrogenation of
nitroarenes using molecular hydrogen is both a clean and convenient method, but the
chemoselective hydrogenation of a nitro group in the presence of other reducible groups
is sometimes challenging. Catalytic hydrogenation by H2 is typically less chemoselective

than stoichiometric methods.2 Thus, a number of recent studies aimed at developing new

catalysts and protocols for chemoselective hydrogenation of nitroarenes with H2.

110

Several methods for chemoselective hydrogenation of nitroarenes catalyzed by 4

supported Pd catalysts were reported.27’37‘39

Holy, et a1. use polystyrene-supported
anthranilic acid to anchor a Pd(II) complex for the hydrogenation of aromatic nitro
compounds at temperatures ranging from 90-100°C and pressures ranging from 400-1000
psi.38 The reaction times vary from 1 h to 15 h, but hydrogenation of nitrobenzene for 48
h affords only 60% aniline at room temperature and 60 psi pressure. Nitro compounds
with different reducible functional groups were not tested in this study. F réchet, et al..
described a polybenzimidazole-supported Pd(O) catalyst for hydrogenation of
nitrobenzene at room temperature and under 45 psi H2 pressure.39 However, this catalyst
was also not used for hydrogenation of various aromatic nitro compounds containing
other reducible functional group. Choudary, et al. showed the use of a
montmorillonitesilylpalladium (II) complex for chemoselective hydrogenation of nitro
aromatic compounds at room temperature in EtOH, which is illustrated in Scheme 4.1.

However, the preparation of the montrnorillonitesilylpalladium (II) complex requires a

long procedure as shown in scheme 4.2.37

MW

N02 I I . NH2
O-Sl—(CH2)"H2N’PdC'z

W

H2, EtOH, 5-10 mins

 

 

 

‘
V

R= -CHO, -COCH3

Scheme 4.1. Chemoselective hydrogenation of aromatic nitro compounds by a
montrnorillonitesilylpalladium (II) complex.37

111

 

M

NaCl W, (EtO)3Si(CH2)3NH£

 

 

 

 

 

 

Montmorillonite ———> 1 .
0.1 N HCI Toluene,reflux 48h 0’?"’(CH2)3NH2
H-Montmorillonite
m 1. Pd(PhCHZCNhCIZ
I 1 Benzene, rt
Q’~?'i"((3Hz)3'\'HzF’dC'2 f 2. Soxhlet extraction
8h

 

Scheme 4.2. Preparation of a montmorillonitesi1ylpalladium (1.1) complex.

As mentioned in previous chapters, metal NPs are frequently better catalytic
materials than bulk metals because of their high surface area to volume ratio and their
size-dependent electronic properties.‘“"12 The use of metal NPs for the chemoselective
reduction of nitroarene derivatives recently attracted tremendous interest.22'24’28’30’3l’43’44
Recent work by Sema, et al. published in Science shows that titania-supported gold NPs
provide selectivity for nitro group hydrogenation in the presence of C=C, C=O, and CEN
functional groups. However, these reactions occur under high temperature (1 10-140°C)
and pressure (9-25 bar) with relatively long reaction times (0.03 to 9.5 h).3| Ranu, et al.
used Cu NPs and ammonium formate in ethylene glycol for reduction of aromatic nitro
compounds with high chemoselectivity. However, these reactions take place under high
temperature (120°C) and very long reaction times (8-12 h).24

Pd NPs, in particular, generated in situ22 or supported on nanoﬁbers23 are used for
reduction of nitro compounds. Maleczka, et al. prepared Pd NPs in situ using

polymethylhydrosiloxane (PMHS), palladium (II) acetate, and aqueous potassium

ﬂuoride. Pd NPs generated in situ chemoselectively reduce aromatic nitro compounds in

112

a high yield with short reaction times (30 min). However, in some cases either reduction
of aromatic nitro compounds is not highly chemoselective (see scheme 4.3) or the
reduction stops at an intermediate stage. Motoyama, et al. prepared Pd and Pt NPs
supported on carbon nanoﬁber catalyst and used them for chemoselctive hydrogenation
of several aromatic nitro compounds containing other reducible functional groups.
However, the reactions took place under high pressure (10 atrn) and required relatively
long times (4 h) to complete. Additionally, in this case, hydrogenation of aromatic nitro
compounds is not always highly chemoselective as shown in scheme 4.4. Therefore, an

alternative efﬁcient yet simple chemoselective procedure is highly desirable.

 

4 equiv PMHS NO
NC)2 2 equiv KF aq NH2 2
5 mol% Pd(OAc)2,
THF, rt, 30 min 1 +
CHO CHO CH20H
73% 27%

Scheme 4.3. Silicon hydride reduction of nitro benzaldehyde catalyzed by Pd NPs
generated in-situ.22

 

N02 H2 (3-10 atm)
PtIPd supported on
carbon nanofiber (catalyst):
AcOEt, rt-5 50°C
CN CH2NH2

64% 32%

Scheme 4. 4. Chemosezl3ective hydrogenation of p-nitrobenzonitrile catalyzed by Pd NPs
on a carbon nanoﬁber.23

113

This chapter describes a very simple, room-temperature protocol for highly
chemoselective hydrogenation of aromatic nitro compounds containing other readily
reducible functional groups. The procedure uses the catalytic [PAA-Pd(0)/PEI]3 ﬁlms
described in previous chapters and a new catalyst prepared by reducing PdC142'
impregnated in alumina. These catalysts show nearly complete selectivity for
hydrogenation of nitro groups in the presence of aldehyde, nitrile, amide, ketone, and
ester groups.

4.2. Experimental
4.2.1. Materials

Poly(ethylenimine) (PEI, Mw = 25 000 Da), poly(acrylic acid) (PAA, Mw = 90
000 Da), a-alumina (100 mesh, typical particle size 75-100 um), potassium
tetrachloropalladate(ll) (99.99%), sodium borohydride (98%) and all nitro compounds
were purchased from Aldrich and used as received. Ethanol (100%) was obtained from
Pharmco. Aqueous solutions were prepared with deionized water (18.2 M!) cm, Milli-Q
puriﬁcation system), and hydrogen (99.9%) was obtained from AGA gases. Flash
chromatography was performed with silica gel 60, particle size 0.040-0.063 mm (230—
400 mesh), which was purchased from EM science. 1H NMR and '3C NMR spectra were
recorded on a Varian spectrometer (300 MHz for 1H and 75 MHz for 13C, respectively),
with chemical shiﬁs reported relative to the residue peaks of solvent chloroform (6 7.24
for ]H and 77.0 for 13C). Synthetic yields refer to chromatographically and
spectroscopically pure compounds unless otherwise stated. GC-MS (Hewlett-Packard
G18OOB GCD system with a nonpolar HP-5 MS column) was used to analyze

hydrogenation reactions. All reactions were carried out in clean glassware with magnetic

114

stirring and were monitored by thin-layer chromatography with precoated silica gel

plates, unless otherwise noted.

4.2.2. Preparation of Pd N Ps on Alumina
4.2.2.1. [PAA-Pd(0)/PEI]3

[PAA-Pd(0)/PEI]3 ﬁlms on alumina were prepared following the procedure in
Chapter 3. To form an initial PAA layer, alumina was vigorously stirred for 10 min in a
pH 4 solution containing 20 mM PAA (molarity of PAA is given with respect to the
repeating unit) and 15 mM K2PdC14. After rinsing, the alumina coated with PAA was
stirred in a pH 9 solution containing PEI (1 mg/mL) to form a PAA-Pd2+/PEI ﬁlm. After
the deposition of each layer, the coated alumina was allowed to settle, the supernatant
was decanted, and the solid was washed through vigorous stirring in three 100-mL
aliquots of deionized water (3 x 5 min) to remove excess polyelectrolyte. In total, three
PAA-Pd(II)/PE1 bilayers were deposited on the alumina powder. Pd(0) nanoparticles
were formed by stirring the [PAA-Pd(II)/PEI]3 on alumina in 100 mL of ﬁeshly prepared
0.1 mM NaBH4 for 30 min, and the reduced ﬁlms were washed with three lOO-mL
aliquots of deionized water (3 x 5 min). After decanting the ﬁnal water rinse, the residual
water was removed under reduced pressure overnight to afford a catalytic powder, which

was stored in a clean dry vial until use.

4.2.2.2. Reduced Pdle' on alumina
Fifteen mg of a-alumina was stirred in a 100-mL solution containing 0.5 mM

PdC142' and 0.1 M KCl for 30 min. The powder was then washed with three lOO-mL

115

aliquots of deionized water (3 x 5 min), and the Pd(II) was reduced to Pd(O) as described

above to afford a catalytic powder.

4.2.3. Characterization of Pd NPs

To obtain TEM images, [PAA-Pd(0)]/PEI]3 ﬁlms were deposited on carbon-
coated copper grids that were pretreated in a UV/ozone cleaner for 1 min. The deposition
procedure on the TEM grid is similar to that described above, except that the grid was
simply immersed for 5 min in solutions without stirring, and rinsing between each step
consisted of a l-min washing with deionized water. Imaging was performed on a JEOL

2200F S microscope using an accelerating voltage of 200 kV.

4.2.4. Determination of the Pd Content of Catalysts Used in Hydrogenation

The amounts of palladium in different catalysts were determined by atomic
emission spectrophotometry (AES). Standard solutions (0.1 to 0.5 mM) were prepared
by dissolving K2PdCl4 in 0.1 M HNO3, and sample solutions were prepared by stirring 60
mg of catalyst in 2 mL of aqua regia for 15 min. The sample solutions were diluted to 12
mL with water and centrifuged (the a—alumina support does not dissolve in aqua regia) to

collect supernatant to analyze using its emission at 340.5 nm.

4.2.5. General Procedure for the Hydrogenation of N itroaromatic Compounds
To determine catalyst turnover frequencies (TOFs), catalytic hydrogenation was
performed in a 100-mL, three-neck, round-bottomed ﬂask. 30 mg of catalyst was

suspended in a 25-mL aliquot 'of deionized water and bubbled with H2 for 30 min.

116

Subsequently, a solution of nitrobenzene (2.5, 7.5, 15, 22.5, or 30 mM) in 25 mL of a 1:1
(v:v) mixture of ethanol or methanol or THF and water was prepared and added to the
reaction vessels. During the reaction, hydrogen at a gauge pressure of 50 kPa was
bubbled through a frit at the bottom of the solution, which was vigorously stirred.
Sample aliquots (0.5 mL) were removed from the reaction vessel at speciﬁc times and
diluted by addition of 1 mL of THF prior to analysis by GC-MS with appropriate
response factors. The response factors were determined to calculate the TOF as
described in chapter 2.45 TOFs were calculated from the slope of the initial portion (total
conversion of reactant to isomer and hydrogenated product <50%) of plots of percent
hydrogenation versus time. In ﬁtting the initial data, the intercept of the line was forced
to be zero.

In reactions used for product isolation, 125 mg of catalyst (more catalyst was used
to speed up the reaction) was suspended in 25 mL of EtOH or a mixture of 20 mL water
and 5 mL of Ethanol and bubbled with H2 for 30 min. A 30 mM solution of substrate in
25 mL EtOH was added to the reaction vessels and the mixture was stirred until the
hydrogenation was complete as judged by TLC. The reaction mixture was ﬁltered
through cotton with a top layer of Celite to remove the catalyst, and the solvent was
evaporated under reduced pressure, dissolved in a minimum volume of solvent, and

subjected to ﬂash chromatography using hexanes/EtOAc.
4-Aminobenzaldehyde (2): Hydrogenation of 4-nitrobenza1dehyde (0.75 mmol, 0.113 g)

in 3:2 EtOH and water for 15 min afforded >99% conversion. After collection of the

crude product and ﬂash chromatography (hexanes/EtOAc: 5:1), nitroarenes were

117

collected as a yellow liquid in the presence of solvent. Upon complete removal of the
solvent, a yellow solid formed, which was insoluble in all solvents tested (presumably
because of polymerization). Therefore, NMR in CDC13 was performed with a sample that
contained residual EtOAc. '11 NMR (300 MHz, CDC13): 8 9.64 (s, 1H), 7.60 (d, r = 6.59
Hz, 2H), 6.61 (d, J = 6.59 Hz, 2H), 4.50 (bs, 2H); 13C NMR (75 MHz, CDC13): 5 190.3,
152.8, 132.2, 126.9, 113.8.22

4-Aminobenzonitrile (3): 4-nitrobenzonitrile (0.75 mmol, 0.111 g) was subjected to the
general procedure for reducing nitroarenes with stirring for 90 min followed by ﬂash
chromatography (hexanes/EtOAc: 5:1) to afford an inseparable mixture (0.084 g) of 4-
aminobenzonitrile (95%) and 4-hydroxyaminobenzonitrile (5%) as a bright yellow solid.
For 4-aminobenzonitrile: ‘H NMR (500 MHz, CDC13): 8 7.36 (d, J = 8.79 Hz, 2H), 6.61
(d, J = 8.79 Hz, 2H), 4.23 (bs, 2H); 13C NMR (125 MHz, CDC13)I 8 150.6, 133.6, 120.2,
114.3, 99.6.22

Methyl 4-aminobenzoate (4): Methyl 4-nitrobenzoate (0.75 mmol, 0.136 g) was
subjected to the general procedure for reducing nitroarenes with stirring for 15 min
followed by ﬂash chromatography (hexanes/EtOAc: 5:1) to afford 0.11 g (97% yield) of
methyl 4-aminobenzoate (14) as a yellow solid. 1H NMR (300 MHz, CDCl3): 8 7.83 (d, J
= 8.14 Hz, 2H), 6.61 (d, J = 8.70 Hz, 2H), 4.06 (bs, 2H), 3.79 (s, 3H); 13C NMR (75
MHz, CDC13): 8 167.1, 150.8, 131.6, 119.7, 113.7, 51.6.22

4-Aminoacetophenone (5): 4-nitroacetophenone (0.75 mmol, 0.124 g) was subjected to
the general procedure for reducing nitroarenes with stirring for 15 min followed by ﬂash
chromatography (hexanes/EtOAc: 5:1) to afford 4-aminoacetophenone (5) 0.094 g (96%

yield) as a yellow solid. 1H NMR (300 MHz, CDC13): 6 7.78 (d, J = 6.15 Hz, 2H), 6.62

118

(d, J = 6.08 Hz, 2H), 4.06 (bs, 2H), 2.48 (s, 3H); 13C NMR (75 MHz, CDC13): 8 196.5,
151.3, 130.7, 127.6, 113.6, 26.0.22

4-Aminobenzamide (6): 4-nitrobenzamide (0.75 mmol, 0.125 g) was subjected to the
general procedure for reducing nitroarenes with stirring for 15 min followed by ﬂash
chromatography (EtOAc/MeOH, 100/0 then 9:1) to afford 0.096 g (94% yield) of 3-
aminobenzamide (6) as a yellow solid. 1H NMR (300 MHz, CD30D): 8 7.63 (d, J = 7.58
Hz, 2H), 6.65 (d, J = 7.49 Hz, 2H), 4.87 (s, 4H); 13C NMR (75 MHz, CD3OD): 8 172.8,
153.5, 130.5, 122.2, 114.6.22

2-Aminotoluene (7): 2-nitrotoluene (0.75 mmol, 0.088 mL) was subjected to the general
procedure for reducing nitroarenes with 60 min of stirring followed by ﬂash
chromatography (hexanes/EtOAc: 5:1) to afford 0.77 g (95% yield) of 2-aminotoluene as
a yellow oil. 1H NMR (300 MHz, CDCl3)! 6 7.04 (m, 2H), 6.74-6.66 (m, 2H), 3.55 (bs,
2H), 2.17 (s, 3H); 13C NMR (75 MHz, CDC13)I 8 144.5, 130.4, 126.9, 122.3, 118.6,

114.9, 17.3.22

4.3. Result & Discussion
4.3.]. Catalyst Preparation and Characterization

We prepared [PAA-Pd(0)/PEI]3 ﬁlms on micron-size alumina particles using 15
mM Pd(II) in the PAA-Pd(II) deposition solution and reduction of Pd(II) to Pd(O) by
NaBI-I4. Pd NPs embedded in these polyelectrolyte multilayers are stable and can be
stored for months without losing their catalytic activity. Moreover, as shown in chapter
2, the catalytic activity of this material is not altered by recycling.45 The average

diameters of small NPs in these [PAA-Pd(0)/PEI]3 ﬁlms on TEM grids are 3.4109 nm

119

(see chapter 3). Additionally, some big particles with average diameters of 1601100 nrn
appear in some images, but we roughly estimate that these particles constitute only 18%
of the total particle surface area (see chapter 3). Figure 4.1 shows the TEM image of
small particles in a [PAA-Pd(0)/PEI]3 ﬁlm on a TEM grid and as well as a histogram of

the diameters of the small NPs.

 

Figure 4.1. (a) TEM image of a [PAA-Pd(0)/PEI]3 ﬁlm on a carbon-coated copper grid.
15 mM Pd(II) was present in the PAA-K2PdCl4 solution used for ﬁlm deposition.

120

'0
0|
1

  
  
 
 

 

tn

.2 20- b
.2

E
Q15-
2
«110-
2

h

0 5.
,,\°

0

.3
on

2.4 3 3.6 4.2 4.8 5.4 6

Nanoparticle Diameter (nm)

Figure 4.1. (b) Histogram of Pd NP diameters in several TEM image based on their area.
4.3.2. Hydrogenation of nitrobenzene as a function of solvent and nitrobenzene
concentration

To maximize TOFs, we initially evaluated the catalytic activity of [PAA-
Pd(0)/PEI]3 ﬁlms on alumina for hydrogenation of nitrobenzene in several solvent
systems. As Table 1 shows, hydrogenation in THF and a mixture of water and THF gives
TOFs that are signiﬁcantly lower than hydrogenation in pure EtOH or in a mixture of
EtOH and water. Previous reports also showed that solvents affect the rate of the
hydrogenation of aromatic nitro compounds,46 and EtOH is commonly used to perform
these transformations.27’30’37'39 We further examined hydrogenation using different initial
nitrobenzene concentrations. The data in Table 4.2 show that TOFs do not vary
signiﬁcantly with nitrobenzene concentrations ranging from 2.5 mM to 30 mM, which
suggests that even at 2.5 mM nitrobenzene, the catalyst is essentially saturated with

substrate. The calculated TOF for the hydrogenation of 15 mM nitrobenzene by Pd NPs

121

in [PAA-Pd(O)/PEI]3 ﬁlms is about the same as the TOF with commercially available 5%
Pd on alumina (Table 4.2), and ﬁlms prepared by impregnation of the alumina with

[PdC142'] and subsequent reduction of Pd(II) with NaBI-I4 also show similar TOFs.

Table 4.1. TOFs for nitrobenzene hydrogenation catalyzed by alumina-supported [PAA-
Pd(II)/PEI]3 ﬁlms in different solvents.

no2

NH2
30 mg of [PAA-Pd(0)/PEI]3
© on alumina _
H2 (50 kpa). rt.

1 solvent 1 a

 

 

TOFs (moles hydrogenated/ mol Pd/ h)a
(3:1) (3:1) 100% 100%
Water & EtOH Water & THF EtOH THF
1 320 460 1460 450

[a] 50 mL of 7.5 mM substrate was hydrogenated in different solvents. TOFs
are initial values determined at conversions less than 50%.

 

 

 

 

 

 

 

 

Table 4.2. TOFs (moles hydrogenated/moles Pd/h) for nitrobenzene hydrogenation by
different catalysts in EtOH.

 

 

 

 

TOFs (moles hydrogenated/ mol Pd/ h)a
[PAA-Pd(II)/PEI]3 ﬁlm 5% Pd Reduced PdCl42'
on alumina on on
alumina alumina
'tr b 2.5b 7.5 15 22.5 30 b b
[n1 0 enzene] mM me me me me 15 mM 15 mM
TOF 1350 1460 1950 1690 1540 1430 1950

 

 

 

 

 

 

 

 

 

 

[a] TOFs are initial values determined at conversions less than 50%. [b] Substrate’s initial concentration
in the reaction mixture.

122

4.3.3. Hydrogenation of nitrobenzaldehyde

In many synthetic reactions, aromatic nitro compounds must be selectively
hydrogenated in the presence of other functional groups such as aldehydes and nitriles.
Recent report indicate that aminals can be easily accessed from aminobenzaldehyde,‘17 so
we initially investigated whether [PAA-Pd(0)/PEI]3 catalysts can facilitate
chemoselective hydrogenation of 4-nitrobenzaldehyde, 2, to 4-aminobenzaldehyde.

Aminals are present in many natural product48'49

and are also important building blocks in
the preparation of many biological active compounds.”55 The chemoselective reduction
of nitrobenzaldehyde to amino benzaldehyde is challenging due to the presence of the
easily reducible carbonyl group (Scheme 4.5).

Using EtOI-l as a solvent, we carried out the hydrogenation of 2 in the presence of
[PAA-Pd(0)/PEI]3 on alumina, reduced [PdCl42'] on alumina, 5% Pd on alumina, or 10%
Pd on carbon. Table 4.3 shows that in EtOH, [PAA-Pd(0)/PEI]3 ﬁlms and reduced
[PdClaz'] on alumina are the most selective catalysts for the hydrogenation of 2 to 4-
aminobenzaldehyde (2a). The use of [PAA-Pd(0)/PEI]3 ﬁlms and reduced [PdClaz'] on
alumina gives only 2a and 4-aminobenzyl alcohol (2b) as hydrogenation products, and
the ratio of these compounds in the product is about 17:1. The chemoselectivity with the
commercial catalysts in EtOH is much lower than with the [PAA-Pd(0)/PEI]3 ﬁlms, and
the Pd on C shows much more reduction of the aldehyde than 5% Pd on alumina. The
relatively small size of NPs in [PAA-Pd(0)/PEI]3 ﬁlms may account for their high
chemoselectivity in the hydrogenation of 2, but ﬁrrther studies are needed to conﬁrm that

this is the case. In contrast to a previous report (Scheme 4.3), we do not observe the

formation of 4-nitrobenzyl alcohol (2d) with any catalyst.22

123

 

N02 NHZ
Catalyst
H2 (50 kpa), rt solvent
CHO CH20H CH2 OH
2

Scheme 4.5. Hydrogenation of 4-nitrobenzaldehyde to several possible products.

Table 4.3. Product distributions from 4-nitrobenzaldehyde hydrogenation (>99%
conversion) using several catalysts and two solvent systems.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

% of Different Products
Catalyst Solvent 2a 2b 2c 2d
100% EtOH
[PAA-Pd(O)/PEI]3 (15 min) 94 6 - -
A1 '
on umrna (32)
EtOH and water 99.3 0.40 - -
(20 min)
100% EtOH
5% Pd (15 min) 79.2 20.8 - -
. a
on Alumrna (3:2) _ _
EtOH and water 98.7 0.40
(30 min)
100% EtOH
Reduced [PdC142'] (15 min) 95 5 - -
on Alumrna (7: 5)
EtOH and water 100 - - -
(20 min)
10% Pd 100% EtOH
on Carbona (10 min) 39 50 10.5 -
(15 rmn) (3:2)
EtOH and water 94.5 3.5 2 -
(15 min)

 

 

 

 

[a] Substrate concentration was initially 15 mM in the reaction mixture. [b]Substrate concentration was
initially 10 mM in the reaction mixture.

124

We also performed the hydrogenation reaction in a 3:2 (v:v) EtOH and water
mixture. The limited solubility of 2 in water restricts the total water content for this
reaction to about 40%. Water is a byproduct of the hydrogenation of aromatic nitro
compounds. However, interestingly, we observe improved chemoselectivity in the
presence of water for the hydrogenation of 2 to 2a with all of the catalysts. Further
studies needs to be performed to understand the mechanistic reason why chemoselectivity
increases in the presence of water.

4.3.4. Hydrogenation of Other Nitroarenes

To further explore the scope of hydrogenation with [PAA-Pd(0)/PEI]3 ﬁlms and
reduced [PdC142'] on alumina, we examined the hydrogenation of the nitroarenes shown
in Table 4.4. 4-nitrobenzonitrile (3) is an interesting example among them. Maleczka, et
al. reported that the reduction of 3 by a hydride reducing agent PMHS, produced the
corresponding N-hydroxylamine (3b), even when the reaction was performed for 12 h
instead of 30 min22 Cu and Au nanoparticles promoted the catalytic transformation of 3
to 4-aminobenzonitrile (3a) chemoselectively, but these reactions were run at 120°C and
110-140°C, respectively.24’3°’3 l Corma, et al. reported that Pd/C and PVC catalysts are not
chemoselective for the hydrogenation of 4-nitrobenzonitrile at high temperature and
pressure (140°C, 25 bar).31 Motoyama, et al. recently showed that the hydrogenation of 3
catalyzed by nanoﬁber-supported Pd NPs produces 64 % 3a and 32% di(p-aminobenzyl)
amine (see Scheme 4.4).23 In our case, the hydrogenation of 4-nitrobenzonitrile by
[PAA-Pd(0)/PEI]3 ﬁlms in pure EtOH produces 95% of the desired compound, 3a, and
only 5% 3b. The ratio of 3a and 3b in the product did not change when the reaction was

performed for 90 min instead of 15 mi

125

Table 4.4. Catalytic hydrogenation of various aromatic nitro compound by [PAA-
Pd(0)/PEI]3 ﬁlms and reduced [PdClaz'] on alumina.

 

[PAA- Reduced

Pd(O)/PEI]3 [PdClaz‘]
Product

 

Substratesa . b . b
Yleld (%) Yleld (%)

(Time, min) (Time, min)

NC—QNOz NC‘Q'NW 88c 90d

 

 

90 4O
3 31 < ) ( )
H3COZC-_©-N02 H30020‘Q-NH2 97 98
(15) (10)
4 4a

 

92 98
Hecoc—Q—Noe “acOCQNHZ (15) (10)
5 5a
(15) (10)
6 6a

95 98
QNOZ Q'NHZ (60) (120)

7 7a

[a] 15 mM substrate was subjected to hydrogenation conditions in the presence of 125 mg of [PAA-
Pd(0)/PEI]3 on alumina in 50 mL of EtOH at room temperature. [b] >99% consumption (calculated by
GC/MS) of the starting materials takes place. [0] The overall isolated yield is 93%, which consists of 95%
3a and 5% 3b. [(1] The overall isolated yield is 96%, which consists of 94% 3a and 6% 3b.

 

 

 

 

 

 

 

 

A similar ratio of 3a and 3b (17:1) was obtained in the hydrogenation of 3 catalyzed by
reduced PdCl42' on alumina.
With both [PAA-Pd(0)/PEI]3 and reduced PdCl42' on alumina catalysts, the

hydrogenation of nitroarenes containing electron withdrawing substituents such as esters

126

(4), ketones (5) and amides (6) is 100% chemoselective and allow isolation of very high
yields of the desired products 4a, 5a, and 6a after ﬂash chromatography. Long reaction
times are required for hydrogenation of 7 due to the steric hindrance caused by the
methyl group at the ortho position, but >99% conversion can be achieved within a few
hours.

Taken together, these studies demonstrate the high chemoselectivity of [PAA-
Pd(0)/PEI]3 and reduced PdCl42' on alumina catalysts. The high selectivity of these
catalysts relative to commercial Pd/C and 5% Pd on alumina may relate to the size of the
Pd particles in the catalysts. Unfortunately, it is very difﬁcult to image the Pd
nanoparticles on the very rough supports, so we do not know their sizes at this time.
Future studies of selectivity as a ﬁrnction of NP size, similar to those in chapter 3, may

shed light on the mechanism of chemoselectivity.

4.4. Conclusion

In conclusion, NPs embedded in polyelectrolyte multilayer ﬁlms on micron-size
alumina powder provide highly active and chemoselective catalysts for the hydrogenation
of nitroarenes to their corresponding amines. Reduced PdCl42’ on alumina is similarly
active and selective. Reactions in ethanol proceed more rapidly than reactions in THF,
and mixtures of water and ethanol give higher chemoselectivity than that pure ethanol in
the case of hydrogenation of 4-nitrobenza1dehyde. Chemoselective hydrogenation of
aromatic nitroamides, nitroesters, nitroaldehydes, and nitronitriles is also possible with

these catalysts.

127

4.5. Appendix

 

~ 11
r- H
r- N
’— M
NH2
6 r— '
).
‘— II

 

 

 

 

Figure 4. A-l. 1H NMR spectrum of puriﬁed 2—Aminotoluene (compound 7) in CDCl3.

128

 

20

40

NH2

 

100

 

 

120

 

 

140

 

160

II II JITJIJJ
'rrrrrrrrrrrrr IIITTT II]UYJYrIJlT—rlefIIIJIIT‘rrIIYTITIIIIIIIVIT'IWTfTIJIIIIIIITTT I

180

I

 

Figure 4. A-2. '3 C NMR spectrum of puriﬁed 2-Aminotoluene (compound 7) in CDCl3.

129

NH2 1

cozcn3

 

 

 

 

 

JL

 

 

Figure 4. A-3. 1H NMR spectrum of puriﬁed Methyl-4-aminobenzoate (compound 4) in
CDC13.

130

711111I’111]

20

NH2

40

CO2CH3

WWW

 
  

 

[FJIIIIJIIIIIIIIIIWII11111111II

100

FITTrTIITIWIIIIlTIIITI

 

120

140

 

160

 

ITI'I #TTTTllllllljl'rlllllIITTIIT

180

l
r v

 

Figure 4. A-4. 13’C NMR spectrum of puriﬁed Methyl-4-aminobenzoate (compound 4) in
CDC13.

131

raj
pn-

NH2

I I I I

CONH2

 

 

 

 

 

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1
i
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_ .1 ﬂ ___J
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Figure 4. A-S. 1H NMR spectrum of puriﬁed 4-Aminobenzamide (compound 6) in
CD30D.

132

 
    

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NH2

CONH2

100

 

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160

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l‘w ‘ “ ‘ 1 ‘ . ‘ l ‘ y‘ . ‘ l
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180

Jldil I‘lli‘di. [III 1h .5

Figure 4. A-6. '3 C NMR spectrum of puriﬁed 4-Aminobenzamide (compound 6) in
CD30D.

133

 

 

r- N
NH2 i i
+ EtOAc

CHO

 

 

11

. 1111

 

 

 

J I - "_

Figure 4. A-7. 1H NMR spectrum of puriﬁed 4-Aminobenzaldehyde and residual AcOEt
(compound 2) in CDC13.

134

 

NH2

+ EtOAc

CH0 E

 

1
100

 

l
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140

160

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III

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Figure 4. A-8. '3 C NMR spectrum of puriﬁed 4-Aminobenzaldehyde and residual AcOEt
(compound 2) in CDC13.

135

NH2 1

CN

I j

 

 

 

 

)—

Figure 4. A-9. ]H NMR spectrum of puriﬁed 4-Aminobenzonitrile (compound 3) in
CDC13.

136

 

 

 

 

I E
..
_ O
__ N
)—
NH ~
2 a
_ O
_ ‘
__ O
2 90
d r—
b
' o
)—
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—_—_—_4
- Q
___4 ~ H
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t r
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>- H
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Figure 4. A-l0. '3 C NMR spectrum of puriﬁed 4-Aminobenzonitri1e (compound 3) in
CDC13.

137

 

.1. .

NH2 1

 

COCH3

 

JL

 

 

Figure 4.A-11. 1H NMR spectrum of puriﬁed 4-Aminoacetophenone (compound 5) in
CDCl3.

138

NH2

COCH3

 

IT'IIIIIIIIIIITTIIITIITTIIIIIITIIIIII'Ij—r

100

 

120

 

 

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160

180

IIIIITIIIIIWIII‘IrTTIIIIIIFITIthIITrIIIIrITJIII'IHWI]II

 

 

 

Figure 4. A-12. ‘3 C NMR spectrum of puriﬁed 4-Aminoacetophenone (compound 5)
in CDC13.

139

4.6. References

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K.; Searle, P. M.; Ward, G.; Wood, A. S. Organic Process Research & Development
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(19) Ogata, Y.; Mibaye, J. Journal of Organic Chemistry 1962, 27, 2048.

(20) Hutton, J .; Waters, W. A. Journal of the Chemical Society B: Physical Organic
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(51) Mhaske, S. B.; Argade, N. P. Journal of Organic Chemistry 2001, 66, 9038.

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142

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143

CHAPTER 5

CONCLUSIONS AND FUTURE WORK

This dissertation demonstrates that metal ions can be incorporated inside
polyelectrolyte ﬁlms and subsequently reduced by NaBH4 to give polymer-encapsulated
NPs. Most importantly, these NPs are highly selective in the hydrogenation of
unsaturated alcohols and nitroarenes. However, this work has only begun to probe the
potential of NPs to serve as highly selective catalysts. There are likely a number of
reactions in which NP size plays a vital role in effecting catalytic selectivity. Below I
brieﬂy summarize the most important points of my research and suggest a number of

directions in which this research could expand in scope and importance.

5.1. Selective Catalysis

Chapter 2 shows that the rate of hydrogenation of allyl alcohols containing methyl
substituents on the double bond is order(s) of magnitude lower than the rate of
hydrogenation of allyl alcohol. Such selectivity depends on both the number of
substituents on the double bond and the size of these substituents. Polyelectrolyte-
embedded Pd NPs also show intramolecular selectivity for the hydrogenation of
molecules which contain mono and multisubstituted carbon-carbon double bonds.
Additionally, chapter 2 indicates that the presence of the polyelectrolyte matrix restricts
the access of larger substrates to the catalyst. However, size-based selective diffusion of

substrates through the polyelectrolyte matrix only accounts for a small fraction of the

144

total inter and intramolecular selectivity in the hydrogenation of unsaturated alcohols

catalyzed by polyelectrolyte-encapsulated Pd NPs.

5.2. Selective Hydrogenation of Unsaturated Alcohols as a Function of NP Diameter
Chapter 3 demonstrates that selectivity is a very strong function of Pd NP size in

the hydrogenation of unsaturated alcohols. The rate of hydrogenation of allyl alcohol
decreases as the NP size increases, but hydrogenation rates for crotyl alcohol and. 2-
methyl-2-propen-l-ol increase as NP size increases. Chapter 3 further suggests that these
different trends in reactivity for the hydrogenation of unsaturated alcohols are largely due
to interactions with speciﬁc types of surface atoms on the catalytic NPs. Presumably the
hydrogenation of allyl alcohol occurs primarily on defect atoms. Crotyl alcohol and 2-
methyl-2-propen—l-ol, on the other hand, might bind to face atoms during the

hydrogenation reaction.

5.3. Chemoselective Hydrogenation of Aromatic Nitro Compounds by Pd NPs
encapsulated in polyelectrolyte ﬁlms

Chapter 4 extends the catalytic scope of polyelectrolyte-encapsulated Pd NPs to
the chemoselective hydrogenation of aromatic nitro compounds that contain other readily
reducible functional groups. The nanoparticles show higher selectivities than commercial
catalysts such as Pd/C and Pd/alumina. This chapter also demonstrates that the addition

of water improves chemoselectivity, even in case of commercially available Pd catalysts.

145

5.4. Future Work
5.4.1. Effect of NP Size on the Chemoselective Hydrogenation of 4-
nitrobenzaldehyde

In chapter 4, I showed that [PAA-Pd(0)/PEI]3 ﬁlms are more chemoselective
catalysts than commercially available Pd/C or 5% Pd on alumina for the hydrogenation of
4 nitrobenzaldehyde in ethanol. The [PAA-Pd(0)/PEI]3 ﬁlms in this case were prepared
using 15 mM Pd(II) in the PAA deposition solution. Chapter 3 indicates that the average
Pd NP diameter in this catalyst is 3.4 nm. It will be interesting to investigate the
chemoslectivity for the hydrogenation of 4-nitrobenza1dehyde as a function of NP
diameter. This may help to explain why the NP catalysts are more selective than Pd/C or

Pd on alumina.

5.4.2. The Contribution of Size-Dependent Electronic Properties to Selective
Hydrogenation

Chapter 3 suggests that the NP-size-dependent selectivity for hydrogenation of
monosubstituted double bonds is primarily due to geometrical effects. However, several
studies indicate that in some cases, differences in reaction rates as a function of NP size
arise from either electronic or geometrical effects or both."3 As the diameters of NPs
decrease, in addition to changes in the fraction of different atom types (e.g., face versus
edge), electronic properties of the NPs change due to the transition from metal to
insulator to molecule.4'5 This change might contribute to the selective catalytic properties
of NPs because interactions of electron rich and poor substrates with NP surfaces may

vary. Hydrogenation of nitrobenzene containing electron donating (-CHO, -COCH3),

146

withdrawing (-OH, -NH2), and sterically hindered (-CH3) groups would be an ideal set of
reactions for investigating the contribution of changing NP electronic properties to

selective hydrogenation

5.4.3. Synergistic Effects Using Bimetallic NPs

Bimetallic NPs manifest electronic properties that are uncommon in single-metal
NPs, therefore these materials are attractive for developing new catalysts with unique
reactivities and selectivities.6'8 A recent report indicates that the rate of hydrogenation of
allyl alcohol is enhanced in the presence of the bimetallic dendrimer-encapsulated PdPt
NPs compared to only Pt or only Pd NPs supported by the dendrimer.6 Additionally, the
enhanced catalytic activity of dendrimer-supported PdAu catalysts in CO oxidation is
indicative of a synergistic bimetallic interaction.7

Chemoselective hydrogenations of nitro compounds containing other reducible
groups such as oleﬁnic bonds are industrially important reactions. During the reduction
of nitostyrene, transition-metal catalysts usually show poor chemoselectivity due to the
hydrogenation of the oleﬁnic bond to produce unwanted side products. Halogenated
anilines are another industrially important class of compounds due to their presence in

pharmaceuticals.9'1 '

Halogenated aniline is often synthesized from halogenated nitro
benzene, but dehydrohalogenation is a very common, unwanted side reaction.
Sema, et al. showed that titania—supported gold NPs provide selectivity for nitro

group reduction in the presence of oleﬁnic bonds or halogens. However, these reactions

occur under high temperature and pressure with relatively long reaction times as

147

discussed in chapter 4. Pd NPs show very poor chemoselectivity for the hydrogenation
of these nitro compounds, but reactions are very fast at room temperature.

Preparation of bimetallic NPs encapsulated in a polyelectrolyte matrix can be
easily performed by complexing two types of metal ions with polyelectrolytes during the
deposition. Utilization of PdAu bimetallic NPs for chemoselective hydrogenation of
nitro groups in the presence of oleﬁnic bonds or other halogen groups under mild
condition at short reaction times should again expand the scope of reactions available

with NP catalysts.

5.5. Summary Statement

This dissertation showed that NPs encapsulated in a polyelectrolyte matrix can
serve as highly selective catalysts. I envision that there is enormous potential for

signiﬁcant expansion to a wide range of catalytic reactions.

148

5.6. References

(1) Wilson, 0. M.; Knecht, M. R.; Garcia-Martinez, J. C.; Crooks, R. M. Journal of
the American Chemical Society 2006, 128, 4510.

(2) Bratlie, K. M.; Lee, H.; Komvopoulos, K.; Yang, P.; Somorjai, G. A. Nano
Letters 2007, 7, 3097.

(3) Zhou Wei, P.; Lewera, A.; Larsen, R.; Masel Rich, 1.; Bagus Paul, S.;
Wieckowski, A. Journal of Physical Chemistry. B 2006, 110, 13393.

(4) Chen, S.; Murray, R. W.; Feldberg, S. W. Journal of Physical Chemistry B 1998,
102, 9898.

(5) Guo, R.; Murray, R. W. Journal of the American Chemical Society 2005, 127,
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(6) Scott Robert, W. J.; Datye Abhaya, K.; Crooks, R. M. Journal of the American
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(7) Scott Robert, W. J.; Sivadinarayana, C.; Wilson Orla, M.; Yan, Z.; Goodman, D.
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(8) Wilson, 0. M.; Scott, R. W. J.; Garcia-Martinez, J. C.; Crooks, R. M. Journal of
the American Chemical Society 2005, 127, 1015.

(9) Jacobsen, M. F.; Moses, J. E.; Adlington, R. M.; Baldwin, J. E. Organic Letters
2005, 7, 641.

(10) Brickner, S. J.; Hutchinson, D. K.; Barbachyn, M. R.; Manninen, P. R.;
Ulanowicz, D. A.; Garmon, S. A.; Grega, K. C.; Hendges, S. K.; Toops, D. 8.; Ford, C.
W.; Zurenko, G. B. Journal of Medicinal Chemistry 1996, 39, 673.

(11) Dale, D. J.; Dunn, P. J.; Golightly, C.; Hughes, M. L.; Levett, P. C.; Pearce, A.

K.; Searle, P. M.; Ward, G.; Wood, A. S. Organic Process Research & Development
2000, 4, 17.

149

 

 

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