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University Microfilms International 300 N. Zeeb Road Ann Arbor, Ml 48106
8403525
Hatton, Kimi Susan
PALLADIUM/POLY(ETHYLENIMINE)/SILICA CATALYSTS
The Ohio State University Ph.D. 1983
University Microfilms International300 N. Zeeb Road, Ann Arbor, Ml 48106
PALLADIUM/POLY(ETHYLENIMINE)/SILICA CATALYSTS
DISSERTATION
Presented in Partial Fulfillment of the Requirements for the
Degree Doctor of Philiosophy in the Graduate School
of the Ohio State University
By
Kimi S. Hatton, 13.S.
*****
The Ohio State University
1983
Reading Committee: Approved by
Dr. Garfield P. Royer
Dr. George A. Barber
Dr. Edward J. Behrman Adviser ^ Dr. David H. Ives The Department of Biochemistry In Memory of
My Mother
(May 3, 1936-March 27, 1982)
and
To My
Father
ii ACKNOWLEDGMENTS
I would like to thank Dr. Garfield P. Royer for his
encouragement, guidance, and patience these past four years.
Thanks is extended to the Department of Biochemistry of the Ohio
State University and Dow Chemical Company for financial support.
I would like to give credit to Dr. George Andrykovitch for
introducing me to science and remaining a good friend after my departure from George Mason University.
I have enjoyed the many discussions with Dr. David J. Hart of the Ohio State Chemistry Department. He is truly a scholar and a delight to have had as a professor.
The critical review of this document by my reading committee,
Drs. Behrman, Ives, and Barber, is greatly acknowledged. Their attention has helped make this a better document.
The encouragement of Dr. Timothy Lee and Dr. Wen-Shiung Chow and their help in the laboratory is deeply appreciated.
My closest friends, Mary Smith, Martha Garifo, Susan Kobs, and David Grahame have helped to lift my spirits when times were bad. To them I am eternally grateful.
And finally to my father and mother and brother David who without their support this work would not have been possible.
iii VITA
January 5, 1957 Born, Parapanga Phillipines
1975-1979 B.S., George Mason University Fairfax, Virginia
1979-1983 Graduate Research and Teaching Associate The Ohio State University Col unibus, Ohio
PUBLICATIONS
"Palladium/Poly(ethylenimine)/Silica Catalysts" (in preparation).
"Palladium/Poly(ethylenimine)/Silica Catalysts " presented at the A.C.S National Meeting, Washington, D.C., September, 1983.
FIELDS OF STUDY
Major Field: Biochemistry
Studies in Fjizymology. Professor Garfield P. Royer
Studies in Organic Chemistry. Professor John S. Swenton
Studies in Organic Chemistry. Professor Harold Shechter
Studies in Organic Chemistry. Professor David J. Hart
Studies in Advanced Organic Synthesis. Professor David J. Hart TABLE OF CONTENTS
Page DEDICATION ...... ii
ACKNOWLEDGMENTS ...... iii
VITA ...... iv
LIST OF TABLES ...... x
LIST OF FIGURES ...... xii
LIST OF ABBREVIATIONS...... xiii
INTRODUCTION ...... 1
1. Enzymes and Synzymes ...... 1
2. Heterogeneous Catalysis ...... 4
3. Survey of Some Catalyst Supports ...... 5
4. Catalytic Transfer Hydrogenation ...... 8
5. The Pd/PEl/Silica Bead Catalyst...... 9
6. Purpose of Research and Specific Aims ...... 10
7. Research Plan and Strategy...... 11
EXPERIMENTAL ...... 16
8. Materials ...... 16
9. Analytical Procedures ...... 16
9.1 The ninhydrin assay ...... 16
9.2 The Cbz-alanine assay ...... 17
9.3 The Cbz-glycine-OtBu a s s a y ...... 18
v page 9.4 The reduction of nitrobenzene ...... 18
10. The Binding of PEI to Silica Beads ...... 19
11. Preparations of PEl/Silicas ...... 20
11.1 Preparation of reagents for PEl/silica bead synthesis ...... 20
11.2 Preparation of PGP/silica beads ...... 20
11.3 Preparation of PG/silica beads .... 21
11.4 Preparation of PEl/Xama/ silica beads ...... 22
11.5 Preparation of PEl/silica gel ...... 23
11.6 Preparation of PEl/Xama/silica gel ...... 23
12. Leaching of PEl/silica beads ...... 23
12.1 A general leaching scheme ...... 24
12.2 Determination of the silica- organic remaining after leaching...... 24
13. Stability of PEl/Silica Preparations ...... 24
13.1 Stability under flew conditions ...... 24
13.2 Stability under wrist-action shaking conditions ...... 25
14. Preparation of Hydrophobic PEl/Silicas ...... 26
14.1 Preparation of octanoyl PEl/ silica beads ...... 26
14.2 Preparation of octyl PEI/ silica beads ...... 26
14.3 Preparation of lauryl PEl/ silica beads ...... 27
14.4 Preparation of hexanoyl PEl/silica gel...... 27
vi page 14.5 Preparation of hexyl PEl/silica gel ...... 28
14.6 Preparation of benzyl PEl/silica gel ...... 28
15. Synthesis of Cbz-glycine- O-tert-butyl ester ...... 29
16. Preparation of Palladium Catalysts ...... 30
16.1 Adsorption of Pd^+ ...... 30
16.2 Sodium borohydride reduction in various solvents ...... 30 2+ 16.3 A general method for the reduction of Pd in water ...... 32
16.4 Preparation of palladium catalysts with different levels of Pd loading (0.5, 1.0, 1.5, and 2.0% Pd) ...... 33
16.5 Preparation of Pd/PEI/silica gel ...... 34
16.6 Preparation of hydrophobic Pd/ PEl/silica gel and bead catalysts...... 34
17. Reproducibility of the wrist-action shaking assay ...... 35
18. Stability Studies of Catalyst Activity...... 35
18.1 Reuse of the Pd/PEl/silica bead and silica gel catalysts in formic acid ...... 35
18.2 Reuse of the Pd/PEl/silica gel catalyst in hydrogen gas ...... 35
18.3 Storage stability of catalyst activity ...... 36
19. Study of Catalyst Poisons ...... 36
20. Preparation of Pd/PEl/silica Samples for ESCA studies ...... 36
vii page RESULTS AND DISCUSSION ...... 33
21. Evaluation of the Preparation of PEl/silica beads ...... 38
22. Evaluation of the Preparation of Pd/PEl/silica beads...... 51
22.1 Leaching of PEl/silica matrixes...... 51
22.2 Pd^+ Absorption...... 51
22.3 The reduction step of catalyst preparation and the reproducibility of catalyst synthesis ...... 62
23. Evaluation of Pd/PEl/silica beads as catalysts...... 71
23.1 Comparison of Pd on carbon, Pd(OH) , and Pd/PEl/silica bead catalyst...... 71
23.2 Comparison of Pd/PEl/silica bead catalysts with different levels of Pd loading (0.5, 1.0, 1.5, and 2.0% Pd) ...... 75
23.3 Reuse of the Pd/PEl/silica bead catalyst in formic acid ...... 75
23.4 Study of catalyst poisons ...... 86
23.5 Storage stability of the Pd/PEl/ silica bead catalyst under atmospheric pressure...... 86
24. Evaluation of Hydrophobic Pd/PEl/silica beads...... 88
25. Evaluation of the Pd/PEl/Silica gel catalyst...... 96
25.1 Reuse of Pd/PEl/silica gel catalyst in hydrogen gas and formic acid ...... 96
26. Evaluation of the Hydrophobic Derivatives of Pd/PEl/Silica Gel ...... 105
27. Physical properties and Palladium content of Pd/PEl/Silicas ...... 109 viii page 28. Palladium Surface Composition and Palladium Surface Analysis ...... 113
29. Conclusions and Prospectives ...... 117
BIBLIOGRAPHY ...... 120
ix LIST OF TABLES
Table page 1. Some applications of enzymes in organic synthesis...... 2
2. Stability of PEl/silica preparations under wrist-action shaking conditions...... 52
3. Reproduciblity of the wrist-action shakingassay ...... 67
4. Reproducibility of catalyst preparation (no temperature or solubility control) ...... 69 2+ 5. Sodium borohydride reduction of Pd /PEI/ silica beads in various solvents (below 5 C) ...... 70
6. Reproducibility of catalyst preparation (2 C in water in water) ...... 72
7. Study of catalyst poisons ...... 87
8. The primary amine content of hydrophobic PEl/silica beads ...... 81
9. The primary amine content of hydrophobic PEl/silica gel ...... 108
10. Physical properties of Pd/PEl/silica catalysts ...... 112
11. Elemental Pd analysis ...... 114
12. Surface Composition...... 114
13. Pd Surface analysis...... 115
x LIST OF FIGURES
Figure page
1. The preparation the of Pd/PEl/silica bead catalyst ...... 13
2. The binding of PEI to silica beads ...... 40
3. Elution of PEl/silica samples ...... 43
4. Stability of PGP resin and catalyst in HCOOHsEtOH:water (2:9:10) ...... 45
5. Stability of the reduced PEl/silica preparations HCOOH:EtOH:water (2:9:10) ....47
6. New scheme for the synthesis of PEl/silica beads ...... 50
7. The leaching of PGP/silica beads ...... 54
8. Schematic diagram showing metal chelation and reduction ...... 56
9. Absorbance spectrum of the PdCl^-sodium acetate solution (1:9 dilution) ...... 59
10. Beer's Law plot for the PdCl -sodium acetate solution ...... 61
11. Schematic representation of the leaching of silica ...... 64
12. Uptake of Pd^+ by PG/silica and leached silica beads ...... 66
13. Comparison of 1 % Pd on carbon and 1 % Pd/PEl/silica beads in the removal of the Cbz group from Cbz-glycine-O-tBu in formic ...... 74 Figure page 14. Comparison of Pd/PEl/silica catalysts with different levels of Pd loading (0.5, 1.0, 1.5, and 2.0% Pd)...... 77
15. Reuse of the Pd/PGP/silica catalyst in formic acid ...... 80
16. Reuse of the Pd/PG/silica catalyst in formic acid ...... 83
17. Reuse of the Pd/PEl/Xama catalyst in formic acid ...... 85
18. Schematic diagram showing the preparation of hydrophobic Pd/PEl/silica catalysts ....90
19. Comparison of hydrophobic Pd/PEI silica bead catalysts and unmodified Pd/PEl/silica bead catalysts ...... 94
20. Comparison of octanoyl PEl/silica beads and Pd PEl/silica beads with 0.5% Pd loading ...... 98
21. The absorbance spectrum of aniline ...... 100
22. Change in absorbance spectrum as nitrobenzene becomes reduced to aniline .. 102
23. Reuse of the Pd/PEl/silica gel catalyst in (g) ...... 104
24. Reuse of the Pd/PEl/silica gel catalyst in formic acid ...... 107
25. Comparison of the hydrophobic Pd/PEl/silica gel catalysts ...... Ill
xii LIST OF ABBREVIATIONS
EtOH ethanol
MeOH methanol
PEI poly(ethylenimine)
DMF dimethyl formamide
HMPA hexamethylphosphoramide
°C degrees centigrade
Rb roundbottom g grams ml milliliters
NaAc sodium acetate
1 liters
HCOOH formic acid
(g) gas
Pd palladium
Rh rhodium
Ni nickel
THF tetrahydrofuran
Cbz benzyloxycarbonyl gly glycine
-O-tBu tertiary butyl ester
xiii Hi microliters min minutes hr hours INTRODUCTION
1. Enzymes and Synzymes
In recent years, enzymes have been exploited apart from
living cells, the biological source from which they arise.
Enzymes have been widely used in a variety of industrial applications in the synthesis of biological compounds. The
literature is replete with examples of the use of enzymes, both soluble and immobilized, in organic synthesis. Table 1
(1) shows a few of these examples. Enzymes have two unique catalytic properties: greater rate enhancement and specificity.
These two properties have guided chemists' attempts to design synthetic enzyme analogs or "synzymes", the synthetic mimics of biological catalysts ( for a review see reference 2). In this dissertation is described an attempt to make synthetic hydrogenation catalysts with enzyme-like properties using palladium as the metal component.
There are problems with using enzymes in synthesis, however.
Enzymes are derived from biological sources and are expensive.
Biological reactions are carried out in aqueous solution with a defined dependence on the pH of the medium. An enzymatic reaction would preclude the use of an organic solvent since enzymes are denatured in hydrophobic media. These drawbacks have 2
Table 1: Some applications of enzymes in synthesis
Compound or process Enzyme or source Reference (1)
6-aminopenacillanic acid penicillin amidase (4) cebhalosporin acetyl esterase (5) cortisol 11- /S -hydroxylase (6) prednisolone 4 '*■'^-dehydrogenase (6) deblocking in peptide Carboxypeptidase Y (7) synthesis acyl-DL-amino acids L-amino acid acvlase (8) amino acids microorganisms (9) 3
initiated the search for synthetic catalysts with enzyme-like
properties, catalysts with the specificity properties of enzymes that can be used in organic solvents. These catalysts can be
soluble or attached to solid supports.
Asymmetric hydrogenation could be viewed as a chemist's attempt to reproduce the specificity of enzymatic reactions.
Enzyme-catalyzed reactions utilize or produce one enantiomer of a
D,L pair. Asymmetric hydrogenation is an attempt to produce a single enantiomer (either 0 or L) from a prochiral molecule using a chiral catalyst. In this manner, the chiral catalyst specifies the configuration of the product. Asymmetric hydrogenation using rhodium complexed to a ligand named DiPAMP (shown below) has become the basis for a commercial process for the synthesis of
L-DOPA (3,4-dihydroxyphenylalanine), a drug for the treatment of the nervous disorder, Parkinson's disease (3). In this reaction, the desired enantiomer was produced in 95% enantiomeric excess.
C6H5
:p mu c H o 1 O An
DiPAMP
OAn = 0 Anisyl residue 2. Heterogeneous Catalysis
Heterogeneous catalysis is a field of chemistry dealing with insolubilized or immobilized catalysts. Usually some
catalytic moiety is attached to a solid support such as graphitic carbon, silica, or polystyrene to name a few. An immobilized enzyme is indeed a heterogeneous catalyst. The use of an immobilized system is practical. Products of a particular reaction can be removed from the heterogeneous catalyst simply by filtration. In so doing, the catalyst is also easily recovered since it is the solid fraction from a suspension of catalyst and reaction mixture. Recovery of the catalyst allows for potential reuse of the catalyst. A heterogeneous system may also alleviate contamination of the reaction products with catalyst since separation of the two in a homogeneous system might be difficult (10). Another advantage of an immobilized system is that potentially reactive groups are isolated from one another, essentially fixed on the support.
In solution, these groups could react with each other.
Attachment of a metal to particular ligands afixed to a solid may permit the iso.lation of metal atoms, and prevent the formation of metal-metal bonds.
Heterogeneous catalysts are studied for their unique catalytic properties as well. For a particular catalyst, a heterogeneous matrix might alter some of the catalyst's properties. A catalyst may become more selective (increasing the ring size of a particular olefin results in a decrease 5 in the rate of double bond reduction) (11), or the immobilized catalyst may show altered activity relative to the homogeneous counterpart (10). Undoubtedly, these observations are the direct effect of the catalyst support.
3. Survey of Some Catalyst Supports
Many different types of heterogeneous catalysts exist.
Rather than discuss in detail the types of reactions catalyzed by heterogeneous catalysts, I will focus on the type of support used and the metal component. Some interesting supoorts include nylon fibers, modified polystyrenes, and silicas containing covalently attached ligands.
Nylon is a polymer of repeating amides. MacDonald and
Winterbottom (12) have made some palladium catalysts from nylon 66 fibers. The catalysts were active in the hydrogenation of some unsaturated compounds. Other workers have made both palladium
(13,14) and platinum (13) catalysts anchored to nylon 66 which will partially hydrogenate benzene. A Russian group (15) has made a homogeneous counterpart to the insoluble nylon catalyst, namely a Rh and a Pd complex of nylon 548. The Rh catalyst was able to hydrogenate the benzene ring at room temperature and atmospheric pressure. The Pd-polyamide catalyst will hydrogenate olefins and aromatic nitro groups.
Probably the most studied organic polymer in heterogeneous catalysis is the polystyrene-divinyl benzene copolymer. The ability to functionalize this resin allows for the attachment of 6
various functional groups which are good ligands for metals. The
polystyrene resin is commercially available in bead form. To
functionalize the resin, the resin is first chloromethylated or
simply brominated by standard procedures. Then, a ligand or
ligands may be attached by nucleophilic displacement of the halide. Both bifunctional and heterobifunctional reagents have been incorporated. Also, some workers have taken modified styrene monomer and copolymerized with it another molecule containing a
ligand. In this fashion, the overall properties of the polymer could be changed; polymerization with a hydrophilic molecule would result in a polymer which would show increased ability to
swell in polar solvents.
Anthranilic acid (shown belcw) has been anchored to polystyrene-divinyl benzene copolymer beads (16-19). The utility of anthranilic acid as a chelating ligand for metals was demonstrated by the synthesis of Rh, Pd, and Ni anthranilic acid anchored polystyrene beads. The catalysts promote the reduction of many different types of functional groups.
Polymer bound anthranilic acid 7
Phosphorus-bearing ligands are typically found in combination
with polystyrene based supports for heterogeneous catalysts. A
phosphide salt is reacted with a brominated polymer to give a
phosphorus containing ligand attached to a styrene support. In
one reacted with brominated
styrene-divinyl benzene copolymer followed by ligand exchange with
RhH(CO) (PPh^)^ to give a rhodium hydrogenation catalyst (19).
In another case, lithium diphenyl phosphide, Ph^P Li+,
reacted with brominated polystyrene-divinyl benzene (10). To the
polymer was then attached either palladium or rhodium. Also,
chloromethylated polystyrene has been used for the attachment of
the diphenyl phosphide group (11).
Another interesting variation of the polystyrene based
supports is that reported by Stille (21). The reaction of
(-)-l,4-ditosylthreitol with 4-vinylbenzaldehyde in the presence of p~toluenesulfonic acid gave a new styrene monomer,
2-p-styryl-4,5-bis(tosyloxymethyl)-l,3-dioxolane. This new styrene monomer was polymerized with hydroxyethyl methacrylate to give a new polystyrene-based polymer that would swell in polar solvents. Sodium diphenyl phosphide was used to displace the tosyl groups. A rhodium catalyst was then prepared and used .in the asymmetric synthesis of optically active amino acids.
Amine-containing ligands have been covalently attached to poystyrene-divinyl benzene. Drago and Gaul (22) have attached bis-(3-aminopropyl) amine to chloromethylated polystyrene beads.
The bis-(3-aminopropyl) amine was attached to the beads by 8
reaction of the chloromethyl group with bis-(2-cyanoethyl) amine.
The cyano groups were reduced with borane-THF complex (BH^THF)
to the primary amines. Different metals were attached to the
polymer for use in catalytic oxidation reactions.
Heterogeneous supports have been made from silica gels with
covalently attached amines or phosphines. Sharf and coworkers
(23) have attached amine ligands by reaction of silanol groups on
silica gel with (C_H_0)Si(CH) NH„. Palladium z d o z n z catalysts prepared from these resins were active in the reduction
of double bonds and imino (unsaturated) groups. Trost and Keinan have prepared phosphinylated silica gel (24). To the silica gel
was added palladium, by ligand exchange with tetrakis
(triphenylphoshine) palladium. The catalyst was used in allylic
substitution reactions.
Although slighty less exotic than the examples cited above,
the standard hydrogenation catalysts Pd or Rh on carbon or alumina
can be thought of as heterogeneous catalysts. These are simply metals deposited on insoluble supports. For some good general
information on these more common supports and hydrogenation reactions in general, see the treatises by Rylander (25,26,27),
Freifelder (28), or Augustine (29).
4. Catalytic Transfer Hydrogenation
Catalytic transfer hydrogenation is a process in which hydrogen is donated not by hydrogen gas but by some other hydrogen donor. Transfer hydrogenation can be divided into three major 9 categories (30,31): 1) hydrogen migrations taking place within one molecule? 2) hydrogen disproportionation, transfer between identical donor and acceptor units? and 3) transfer hydrogenation-dehydrogenation, occurring between unlike donor and acceptor units. For most of the studies in this dissertation, formic acid was used as the hydrogen donor. Formic acid would be included in 3) above.
Formic acid has been used in combination with ruthenium, osmium, rhodium, platinum, and palladium complexes to reduce olefins and acetylenes (32). Also, formic acid has been used for the deprotection of peptides bearing benzyl type protecting groups
(33). In this case, 85% formic acid was used as the hydrogen donor with Pd black at room temperature. The butyloxycarbonyl group was removed under these conditions. FI Amin et a l . have reported the use of formic acid (4.4%) to remove benzyl type protecting groups using a column of palladium black (34).
5. The Palladium/PFl/Silica Bead Catalyst
In Dr. Royer's laboratory, a novel hydrogenation catalyst was discovered (35,36). This catalyst is made by the deposition of palladium on a poly(ethylenimine)/silica bead. The PFI/alumina matrix (completely leached) was first described by Meyers and
Royer as an insoluble support for the application of synthetic enzyme analogs (37,38). The Pd/PEl/silica bead catalyst was successfully used in the synthesis of peptides bearing the tert-butyl protecting group for carboxyl protection. 10
Formic acid was used as the hydrogen donor for the removal of benzyl type protecting groups from the N-terminus of the peptide.
By using formic acid at low concentrations, 4.4-14 %, the t-butyl
group was not removed in the synthetic sequence.
6. Purpose of Research and Specific Aims
The purpose of this research was to optimize the conditions
for synthesis of the Pd/PEl/silica bead catalyst. Once the
synthesis of the Pd/PEI/silica beads had been optimized, the
longevity of the catalyst was investigated under conditions
typically found during peptide synthesis, using formic acid as the hydrogen donor. An additional purpose of this reseach was to prepare novel Pd catalysts by changing the environment of the metal on the support in a way that the catalyst might resemble an enzyme.
The specific aims of this work were:
1) to optimize the preparation of the PEI/silica beads;
2) to find the simplest method for reduction of the
Pd^+/PEI/silica beads;
3) to synthesize the catalyst reproducibly;
4) to demonstrate the necessity of the amine matrix in the 2 + PEl/silica composite for the uptake of Pd ;
5) to study the reutilization of the Pd/PEl/silica bead catalyst in formic acid, the conditions used for peptide synthesis;
6) to elucidate the role of catyalyst poisons in catalyst inactivation; 11
7) to introduce a Pd/PEl/silica gel catalyst and to compare the
reuse properties of the Pd/PEl/silica gel catalyst to the
Pd/PEI/silica head catalyst in formic acid;
8) to delineate the reuse properties of the Pd/PEl/silica gel
catalyst in hydrogen gas;
9) to prepare hydrophobic PEl/silica beads and hydrophobic
PEl/silica gel supports by attaching octyl, octanoyl, hexyl, hexanoyl, lauryl, or benzyl groups to the PEI/silicas;
10) to prepare Pd catalysts from the hydrophobic PEl/silicas and
to evaluate the hydrophobic Pd/PEl/silicas as hydrogenation
catalysts using a non-polar substrate, Cbz-glycine-OtBu or
Cbz-phenytalanine-OtBu, and compare their activity to the
unmodified Pd/PEl/silica catalysts;
11) and finally, to compare and contrast the physical properties,
surface composition, and palladium surface chemistry of the
Pd/PEl/silica bead and Pd/PEl/silica gel catalyst.
7. Research Plan and Strategy
Although the utility of the Pd/PEl/silica bead catalyst was demonstrated in the synthesis of peptides, we realized that no rigorous study of the catalyst itself existed. Figure 1 shews the synthesis of the catalyst. The synthesis of the catalyst involved several steps, and we wanted to investigate whether or not all of the steps were necessary. To begin, we wanted to look at the initial PEI deposition step. In previous syntheses of the catalyst, PEI was deposited in excess and then washed exhaustively Figure 1: The preparation of the Pd/PEl/silica bead catalyst
12 13
Inorganic Core
I PEI, (-CH j CH j NH,),
1 Glutaraldehyde
Y PEI/NaBH
y Leach with Acid or B ase
Hollow *Ghost"
oy Pd(OAc)2/ NaBH4
Figure 1 Pd-PEI Catalyst 14
(5 times). We thought that these excessive washing steps could be
eliminated if a small initial coating of PEI was deposited.
Another aspect of the catalyst synthesis was the solvent used to
effect crosslinking of the PEI bound to silica. When crosslinking
takes place, the beads turn a characteristic light orange-yellow
color. In water, the solvent previously used as the crosslinking
medium, the supernatant of the suspension of PEl/silica in
glutaraldehyde-water will also turn color. This observation
suggested using a solvent which did not dissolve PEI, such as an
non-hydroxylie organic solvent. The necessity of reducing the
resin with sodium borohydride prior to metal chelation and
reduction was another aspect we wanted to investigate. It was
conjectured that all crosslinking steps took place by Michael
addition; the crosslinks then would not require reduction for
stabilization. And finally, any new synthesis of the beads must be tested for the preparation's stability in solvents that the catalyst would encounter.
Other aspects of the catalyst's preparation required further 2+ study. In particular, the question of whether or not the Pd was being complexed by the silica itself or the amine components 2+ of PEI had not been addressed. Also, a simple assay for Pd depletion of the supernatant was needed to ensure equivalent palladium uptake between different PEl/silica samples.
Reproducibility of the catalyst's preparation was another uncertainty. One crucial question was the selection of a solvent 2 + for the reduction of the Pd /PEl/silica beads. 15 During the course of studies of Pd/PEl/silica beads, a new
Pd/PEl/silica gel catalyst was discovered. A additional objective of this work was to compare the properties of the Pd/PEl/silica bead catalyst to the Pd/PEl/silica gel catalyst. The silica beads are 40 mesh and highly porous with an average pore diameter 0 2 of 265 A and a surface area of 51 m /g. On the other hand, the silica gel is essentially a solid with interstitial spaces with
° 2 an average diameter of 14 A and a surface area of 500 m"/g. We wanted to investigate the potential for reuse of these catalysts, and we wanted to compare their viability under conditions which would imitate peptide synthesis using formic acid as the hydrogen donor. Since the silica gel catalyst seemed physically more amenable to shaking conditions (the gel is a powder rather than a bead), the reuse of the silica gel catalyst using H2 (g) in the reduction of nitrobenzene was investigated.
The final objective of this work was to prepare novel hydrogenation catalysts by attaching hydrophobic groups to the support. Both derivatives, PEl/silica beads and silica gel, were investigated. It was postulated that hydrophobic substitution of the matrix might augment interaction of the apolar substrate with the support surface. In this fashion, we attempted to mimic an enzyme and make a synthetic enzyme analog. EXPERIMENTAL
S. Materials
Palladium chloride, octanoyl chloride, benzyl bromide, octyl
chloride, hexanal, p-nitrophenyl caproate, glutaraldehyde (25% in water), and hexamethylphosphoramide were from Aldrich, Xama-2 and
Corcat (PEI, MW 40,000 - 60,000, 33% PEI in water) were obtained
from Cordova Chemical Company. Mallinckrodt was the provider of methylene chloride, 30% hydrogen peroxide, ethyl acetate and
sodium acetate. THF, NaOH, DMF, methanol, formic acid, and Norit
A were supplied by MCE reagents. Toluene was from Chemical
Samples company. Bio-Rad provided 2-mercaptoethanol. Cbz-glycine was purchased from either Sigma or Aldrich. N-ethylmorpholinium acetate and sodium borohvdride were supplied by Sigma. Celite was a product of Sargent-Welch. Ethanol was from Aaper Alcohol and Chemical Company. Matheson, Coleman and Bell provided diglyme. Silica gel (grade 923) was from Davison Chemical.
Silica beads were silica carrier, code number 691952 (30/45 o mesh, 275 A average pore diameter), from Corning Glass Works.
9. Analytical Procedures
9.1 The ninhydrin assay (39,40)
To a 12 ml screw cap tube is added the sample , one ml of water, and one ml of ninhydrin reagent. The 16 i 17 tube is tightly capped, mixed by vortexing for three seconds, and then placed in a boiling water bath for 20 minutes. To the tube is then added 8 ml of 50% ethanol in water. After mixing, the sample absorbance is read at 570 nm against a blank consisting of reagents (without sample) carried through the same procedure.
The extinction coefficient was determined under the experimental conditions employed.
When ninhydrin analysis was performed on silica for the determination of PEI content, 2 ml of water and 2 ml of ninhydrin reagent were added to approximately 10 mg of solid in a 12 ml screw cap tube. Boiling of the tubes was continued for 90 minutes with intermittent agitation. The sample was diluted to
100 ml with 50% ethanol in water. The solid was allowed to settle, and the absorbance of the solution was read at 570 nm.
For PEI as the primary amine component in the ninhydrin reaction, an extinction coefficient of 17,500 cm ^ was assumed.
9.2 The Cbz-alanine assay
An assay using Cbz-alanine as substrate was used for scanning purposes. Cbz-alanine, (0.1 g), was placed in 3.0 ml of
HCOOHsMeOHiH^O (1:3:5) in a 25 ml Erlenmeyer flask. The flask was placed on the wrist-action shaker. The reaction was started by adding catalyst. The flask was stoppered with a perforated cork stopper, and the speed of shaking was brought to maximum. The shaking motion of the flask can be described as a
20° deflection from the horizontal with approximately 239 18
excursions per minute. The distance travelled perpendicular to
the horizontal was 2.5 cm. A 4 til sample was removed at timed
intervals, and the extent of reaction was assessed by ninhydrin
assay.
9.3 The Cbz-glycine-O-tBu assay
The tert-butyl ester of Cbz-glycine (Cbz-glycine-O-tBu) was
used for the determination of catalyst stability during reuse
since it is a better peptide model than Cbz-alanine. A stock
solution of Cbz-glycine-O-tBu in ethanol was made and stored at 4 0
C. The formic acid solution (1:5 HCOOHsH^O) was prepared daily.
The assay mixture consisted of 3.5 ml of solution
(HCOOH:EtOH:U^O, 2:9:10) and 0.120 g of Cbz-glycine-O-tBu. The
assay was performed on a wrist-action shaker. The reaction was
considered to have started when assay mixture was delivered to the
flask; the flask was stoppered with a perforated cork, and the
speed of shaking was brought to 10 maximum. 4 u \ samples were taken at timed intervals and the extent of reaction determined by
ninhydrin analysis.
9.4 The reduction of nitrobenzene
To a 500 ml three-neck roundbottom flask equipped with an overhead stirrer was added 10.3 ml of nitrobenzene (0.1 mole),
89.7 ml of methanol, and an appropriate amount of Pd catalyst (for example, 2.128 g of 1% Pd on PEl/glutaraldehyde/silica gel).
Hydrogenation was performed at atmospheric pressure. The flow rate of hydrogen gas was fixed at a constant rate with a Gilmont flowmeter (0.5 l/minute, calibrated by air). The rate of over head stirring was just fast enough so that settling of the catalyst did not occur. Also, the stirrer was positioned such that it did not touch the bottom of the flask. At timed intervals an aliquot (10 /il) was removed and diluted with 5 ml of MeOH and
05 nl of water. rrrhe samples were analyzed spectrophotometrically
(220-320 nm). Nitrobenzene has an absorption maximum at 268 nm while aniline has an absorption maximum at 230 nm. The absorp tion change can be calculated at 230 or 260 nm.
10. The Binding of PEI to Silica Beads
A series of stock PEI solutions were prepared. A weighed sample (approximately 0.25 g) of Corcat (Corcat is 33% w/ w PEI in water) was placed in a beaker (100 ml) and dissolved by the addition of methanol with vigorous swirling. The solution was transferred quantitatively to a 259 ml volumetric flask and diluted to 250 ml. Prom this solution, appropriate dilutions were made to give different concentrations of PEI in methanol.
The binding of PEI was assessed in the following manner. A weighed sample of silica (approximately 0.1 q each) was placed into a 12 ml screw cap tube and overlayed with a large volume of methanol (about 7 ml). The tubes were placed in a heated desiccator equipped with a vacuum (via aspirator) for removal of trapped air. After entrapped air was removed, the methanol was removed with a dispo pipet fitted with nylon net. Five ml of the 20 desired PEI solution were then added, and the tube was tightly capped. The PEl/silica suspensions were rotated end over end for three hours. At the end of three hours, the supernatant was removed (by dispo pipet). Five ml of methanol was added, and the mixture was agitated by vortex for five seconds. The wash was removed, and the beads were dried in a heated desiccator and then ground to a fine powder prior to the determination of primary amine content.
11. Preparations of PEl/Silicas
11.1 Preparation of reagents for PEl/silica bead synthesis
A 1% PEI in methanol solution was prepared by placing 18.0 g of Corcat in a beaker and diluting to 600 ml with methanol. A
0.5% glutaraldehyde in THF solution was prepared by placing 2.0 ml of 25% aqueous glutaraldehyde (Aldrich) in a graduated cylinder and diluting to 100 ml with THF. THF was refluxed and distilled from lithium aluminum hydride before use.
11.2 Synthesis of PGP/silica beads
PEl/Si02 beads were synthesized using a rotary evaporator,
Buchi-Brinkmann rotavapor-R. Silica beads (50 g) were placed in a 1000 ml roundbottom flask, and 200 ml of 1% PEI in methanol was added. Trapped air was removed by lowering the pressure while the flask was rotated at maximum speed. After removing trapped air at room temperature, heat was applied (35 °C) to evaporate the methanol. The resulting PEI beads were dried in a heated 21
desiccator and then passed through a sieve to provide a more
homogeneous preparation.
To 50 g of PEI-SiC^ (in a 250 or 500 ml roundbottom flask)
is added 125 ml of 0.5% glutaraldehyde in THF. Trapped air is
removed for five minutes while the flask is rotated at maximum
(maximum speed for a 250 ml roundbottom flask is roughly 260 to
280 revolutions per minute). The vacuum is then turned off and
rotation is continued for 25 additional minutes. The THF is re
moved by suction through a gas dispersion tube or by decanting.
Finally, 100 ml of 1% PEI was added and the flask was rotated
at maximum speed for 30 minutes. The beads were washed twice with
200 ml of methanol and then dried in a heated desiccator
overnight.
11.3 Preparation of PG/silica beads
Twenty grams of PEl/silica beads (4% w/w) were placed in a
250 ml roundbottom flask, and 50 ml of 0.5% glutaraldehyde in THF
was added. The flask was placed on the rotary evaporator and a
vacuum was applied for removal of trapped air for five minutes.
The flask was rotated at 1/4 maximum speed (60-70revolutions/min
at a 45 angle) for an additional 25 minutes. The glutaraldehyde
solution was removed by suction with a gas dispersion tube and was
replaced with 100 ml of a 1% solution of NaBH^ in water. The
flask was rotated at 1/4 maximum speed for one hour. The boro- hydride solution was decanted, and 100 ml of 15% formic acid (v/v) was added. After rotation for one hour at 1/4 maximum speed, 22
the formic acid solution was decanted, and 200 ml of 5 N NaOH was
added. After leaching the preparation for one hour at 1/4 maximum
speed, the beads were washed to neutrality with water. All oper
ations were performed at room temperature (23-24 C).
11.4 Preparation of PEl/Xana/silica beads
Corcat (2.44 g) was placed in a 250 ml roundbottom flash. To
the flask was added 50 ml of methanol. After thorough mixing of
the PEI-methanol solution, 0.66 ml of 6% Xama-2 in methanol was
added. Silica beads (20 g) were then added, and the flask was
placed on the rotary evaporator and rotated at maximum speed while
trapped air was removed by lowering the pressure for ten minutes.
The vacuum was turned off and the flask was immersed in a boiling water bath with rotation of the flask still at maximum speed.
When all of the methanol was removed, the flask was placed in a heated desiccator for at least one hour. The preparation was
passed through a sieve.
The above preparation (15 g) was added to a 250 ml round bottom flask. Formic acid in water (15%, 150 ml) was added, and the flask was rotated for one hour at room temperature. At the end of that time, the supernatant was decanted and replaced with
150 ml of 5 N NaOH. Rotation was continued for one hour. The preparation was washed exhaustively with water and dried. 23
11.5 Preparation of PEl/silica gel
Ten grams of silica gel were coated with 5% PEI (0.5 g PEI/10 g silica gel) by rotary evaporation. The preparation was then washed briefly with methanol (to remove extraneous PEI) and dried in a vacuum desiccator. 25 ml of 1% (w/v) glutaraldehyde in water was added, and the suspension was rotated at room temperature for
30 minutes. The glutaraldehyde solution was replaced with 25 ml of 1% PEI in methanol and rotated for an additional 30 minutes.
The PEI solution was decanted, and 25 ml of methanol and 300 mg of NaEH. was added, ^fter rotation for 30 minutes the prepar- ation was washed with water and methanol, and dried in a vacuum desiccator.
11.6 Preparation of PEl/Xama silica gel
PEI (10 g, 33% aqueous) was mixed with 0.3 g of Xama-2 in 10 ml of methanol and refluxed for ten minutes. The volume was then adjusted to 50 ml with methanol. Five ml of the PEI-Xama solution was mixed with 10 g of silica gel suspended in 100 ml of MeOH.
The mixture was stirred with an overhead stirrer and heated on a boiling water bath for 15 minutes. The methanol was then evapor ated and the PEI-silica was repeatedly suspended in water until the decanted wash water gave a negative ninhydrin test. The support was then dried in a vacuum desiccator. 24
12. Leaching of PEl/silica Beads
12.1 A general leaching scheme
To a 250 ml roundbottom flask was added 10 g of PEI-SiO^
cross-linked with glutaraldehyde and 100 ml of 5 N NaOH. The
flask was placed on the rotary evaporator and rotated at maximum
speed without application of a vacuum. Catalysts were generally
prepared with a supnort leached in this manner for one hour.
12.2 Determination of silica-organic remaining after leaching
PEI-silica preparations were exposed to 5 N NaOH for various time intervals. PGP/silica (10.0 g) was weighed into a 250 ml roundbottom flask. The flask was rotated on the rotary evaporator
for 1, 2, or 4 hours after the addition of 100 ml of 5 N NaOH. At the end of the time interval, the supernatant was removed by suction through a gas dispersion tube. The preparation was washed with water to neutrality and then was washed twice with methanol.
The preparation was dried in a heated desiccator and weighed. The weight after leaching was compared to the weight before leaching for the different leach times used.
13. Stability of PEl/Silica Bead Preparations
13.1 Stability under flow conditions
To determine the stability of various PEI-silica samples, 1.0 g of silica-organic material was applied to a Pyrex column equipped with a Teflon frit and a small amount of glass wool.
Five ml of eluant was added and a vacuum was applied (to the top 25
of the column with-the stopcock closed) for removal of trapped
air. Elution of the column with methanol or N-ethylmorpholinium
acetate (pH 7, 1 mM) was carried out on several different PEI-
silica preparations.
Stability studies were carried out directly on the column
eluate. Eight 25 ml fractions were collected and quantitatively
transferred to 12 ml screw cap tubes by successive removal of an
aliquot of the sample and evaporation of the solvent by placing
the tube and sample in a hot water bath and blowing a stream of
dry air over the sample. Ninhydrin analysis was performed on the
residue. The amount of ninhydrin positive material in the 25 ml
fractions was compared to the amount of ninhydrin positive
material found on the beads before elution with solvent.
13.2 Stability under wrist-action shaking conditions
In order to assess the physical stability of several PEI-
silica preparations, a sample of the PEI-silica preparation was
subjected to wrist-action shaking conditions. A 0.2 g sample
was placed in a 25 ml Erlenmeyer flask, and 3.5 ml of EtOH:
HCOOHiH^O (9:2:10) was added. The flask was stoppered and
allowed to shake on the wrist-action shaker for one hour. At the
end of that time, a 1.0 ml sample was removed and placed in a 12
ml screw cap tube. The sample was evaporated to dryness and nin hydrin analysis was performed on the dried residue. 26
14. Preparation of Hydrophobic PEl/Silicas
14.1 Preparation of octanoyl PEl/silica beads
Three g of PEl/glutaraldehyde/silica beads were placed in a
100 ml roundbottom flask, and 25 ml of 20% (v/v) triethylamine in
DMF was added. A vacuum was applied for 30 minutes for the
removal of trapped air. The base solution was removed by de-
cantation, and 0.277 g of octanoyl chloride in 10 ml of DMF was
added. An additional 1 ml of 20% TEA in DMF was added. The
suspension was rotated at one-fourth maximal speed on the rotary
evaporator for 43 hours at room temperature. The modified sup port was treated with 20% TEA in DMF, DMF, methanol, water, and methanol, and dried in a heated vacuum desiccator (via as pirator) .
14.2 Preparation of octyl PEl/silica beads
Two g of PEI silica beads were placed in a 100 ml roundbottom flask and 25 ml of 20% TEA in hexamethylphosphoramide was added.
A vacuum was applied for the removal of trapped air as the flask was slcwly rotated on the rotary evaporator. The wash was removed by suction with a gas dispersion tube, and an additional 25 ml of
20% TEA in HMPA was added as a wash. After removing the second wash, 10 ml of HMPA was added. To this suspension was added 1.23 g of octyl bromide and 1.29 g of TEA. The mixture was rotated at 50 °C at 1/4 maximal speed on the rotary evaporator for 72 hours. The HMPA solution was removed by suction with a gas dis-
•persion tube, and the modified beads were washed with additional 27
20% TEA in HMPA, HMPA, and methanol and dried in a heated vacuum desiccator (via aspirator).
14.3 Preparation of lauryl PEl/silica beads
Two g of PEl/silica beads were added to a 100 ml roundbottom flask. To the flask was added 50 ml of 20% (v/v) TEA in DMF, and a vacuum was applied for the removal of trapped air. The preparation was washed again with an additional 50 ml of 20% TEA in DMF. Next, 3 g of lauryl bromide, 15 ml of DMF, and 0.35 ml of
TEA were added. The suspension was rotated at lew speed for 72 hours at 50 °C. The preparation was washed with 20% TEA in DMF,
DMF, and methanol and dried in a heated vacuum desiccator (via aspirator).
14.4 Preparation of hexanoyl PEl/silica gel
PEI-Xama silica gel (5 g) was washed with 2% TEA in DMF (50 ml) to obtain the free base form. DMF was added in sufficient quantity to just cover the support. To this suspension, p-nitrophenyl caproate (4 eg, 0.9 g) and TEA (1 eg, 0.13 ml) were added. The reaction mixture was continuously rotated with a ro tary evaporator at 50 °C for 24 hours. At the end of this time, the support was filtered, washed with methanol and dried in a heated vacuum desiccator (via water aspirator). 28
14.5 Preparation of hexyl PEl/silica gel
a) Aqueous buffer/methanol mixture. PEl/Xama/silica gel was mixed with hexanal (50 ^1) in a mixture of 0.1 M borate buffer (25 ml, pH 8.5) and methanol at 4 °C. Sodium borohydride (70 mg) was added in six equal portions over a period of one hour with occasional agitation. The support was then washed with water, methanol, and dried in a vacuum desiccator. The decrease in the primary amine content was 14%.
b) In methanol. PEl/Xama/silica gel (2 g) was washed with 2% triethylamine in DMF (50%) and suspended in 10 ml of methanol. To this suspension was added hexanal (340 #il) in 2 ml of ethanol.
The suspension was agitated occasionally. After 30 minutes,
NaBH^ (25 mg) was added in five equal portions over a period of one hour. The support was then washed with water and methanol, and then dried in a vacuum desiccator. The decrease in the pri mary amine content was 32.5%.
14.6 Preparation of benzyl PEl/silica gel
PEl/Xama/silica gel (2 g) was suspended in 10 ml of DMF containing 100 //I of TEA. To this suspension was added 100 //I of benzyl bromide, and the mixture was rotated at 90 °C for one hour.
The support was washed with methanol and dried in a vacuum desiccator. The decrease in the primary amine content was 28%. 29
15. Synthesis of Cbz-glycine-O-tBu (41)
Cbz-gly(OH) (25.0 g) was placed in a pressure bottle and 50
ml of THF was added. After the substrate was dissolved by mag
netic stirring, 170 ml of dichloromethane was added. The THF-
CH2C^2 solution was cooled in a dry ice-acetone bath, and 1.5 ml of concentrated was ac^ ec^ • Cold isobutylene (130 ml) was added, and the bottle was stoppered with a rubber stopper
covered with Teflon tape. The stopper was secured with a metal
plate bolted to a collar fitted to the bottle with flanking metal
screws. The reaction mixture was allowed to stir magnetically for
4 to 6 days at room temperature.
Before the pressure bottle was opened for work-up of the
reaction mixture, the vessel was cooled in a dry ice-acetone bath.
After opening the pressure bottle, 50 ml of 1 M Na^CO^ was
added and the reaction vessel was allowed to stand at room temperature for one hour with no stirring. The organic phase was transferred to a roundbottom flask and the solvents were removed by rotary evaporation. The residual viscous liquid was par titioned between ethyl acetate (250 ml) and 1 M sodium carbonate
(100 ml). The organic layer was washed to neutrality and dried over magnesium sulfate. The ethyl acetate was removed in vacuo.
The resulting oil was diluted with methanol and cooled to 0° C to precipitate isobutylene polymers, and then Celite was added.
After filtration through Norit A, methanol was removed in vacuo to produce the oil in 30-60% yield. The purity of the product was at least 95% as judged by TLC. 30
16. Preparation of Palladium Catalysts 2 + 16.1 Adsorption of Pd
0.2 g of P^Cl^ was dissolved in 2 ml of concentrated HOI by
heating. The solution was then diluted to 48 ml with 15% sodium
acetate trihydrate. This solution contains 10.9 mg of palladium/
4 ml of solution. When catalysts were being prepared, 1 g of
PEl/silica beads was placed in an Erlenmeyer flask and 4 ml of the
palladium chloride-sodium acetate solution was added. The flask was swirled immediately to ensure efficient mixing. Usually a
catalyst was prepared from beads that had been exposed to the
solution for 0.5 to 1.0 hour.
A 1:9 dilution of the PdCl^-NaAc 3 ^ 0 revealed an ab
sorption maximum at 375 nm. In order to generate the data shown
in Figure 15, the following experiment was performed. Four ml of the PdCl2~NaAcsolution were placed in two 25 ml Erlenmeyer flasks and the two flasks were placed on a wrist- action shaker. To one flask was added 1 g of PEl/silica beads, and to the other flask was added 1 g of 1 hour leached silica beads (no PEI) as the control. At timed intervals, 0.1 ml of the supernatant was removed and diluted with 0.8 ml of 15% NaAc •
3^0. The absorbance was read at 375 nm.
16.2 Sodium borohydride reduction in various solvents
Several different solvents were used in the reduction step of catalyst synthesis. The concentrations of NaBH4 in the solvents were as follows: 0.05 g/ml DMF, 0.025 g/ml diglyme, 0.02 g/ml 31
ethanol, and 0.25 g/ml water. These solutions were made by main
taining the temperature of the solvents below 5 °C in an ice-water
bath and adding the specified amount of NaBH^.
Several 1.0 g samples of support were placed in 25 ml 2 + Erlenmeyer flasks. &fter the Pd was adsorbed from the
PdCl^-NaAc solution, the supernatant was removed by suction 2+ through a dispo pipet fitted with polyester net. The Pd
PEl/silica beads were washed two times (10 ml each time) with the
solvent chosen for the reduction, and then 1.0 ml of the solvent was added to the beads. The flask was cooled to below 5 °C. The
solutions of NaBH in the chosen solvent were maintained at the 4 same temperature. 2 + Reduction of the Pd to Pd metal was carried out by pipetting an appropriate amount (a total of 0.05 g/l of support) of the cold borohydride solution into the flask followed by a vigorous swirling motion. Reduction was allowed to continue for
15 minutes with agitation of the flask about every 5 minutes. For a 5% reduced catalyst, the supernatant was removed; the catalyst was then washed three times with 15 ml of water and two times with
15 ml of methanol and dried in a heated vacuum desiccator (via aspirator). For a 10% or 15% reduced catalyst, the supernatant was removed and the flask was washed three times with water (15 ml) and then two times with the solvent being studied (15 ml each time). One ml of the solvent was then added, and the reduction step was carried out one more time (for a 10% reduced catalyst) or two more times (for a 15% reduced catalyst) as described above. 32
Washing of the preparation (3 X 15 ml of water and 2 X 15 ml of
methanol) was carried out before drying in a heated vacuum
desiccator. For a 100% reduced catalyst, the sodium borohydride
solution was added all at once. The 100% preparations were
allowed to stand at room temperature for one hour before washing
and drying.
2+ 16.3 A general method for the reduction of Pd /PEl/silica beads
in water using sodium borohydride
Since the reduction with sodium borohydride invariably
evolves hydrogen gas, open flames or heat of any sort should not be within close range of the reaction mixture. The procedure should be performed in a hood. The following procedure is a gen- 2 + eral procedure employed after Pd adsorption; this description 2+ is for treatment of 1 g of catalyst. After the Pd was adsorbed to a PEl/silica bead preparation, the liquid phase was removed from the solid beads by suction through a dispo pipet fitted with polyester net. The beads were washed with water two times (15 ml each). One ml of water was then added and the sus pension was cooled in an ice-water bath to between 0 and 2 °C. A solution of the sodium borohydride in water is made by dissolving
NaBH4 in water that is orecooled to 0 to 2 °C. For the reduc tion of 1 g of support, 0.150 g of sodium borohydride was dissolved in 3 ml of cold water. One ml of this solution was 2+ added to the Pd /silica beads in water, and the beads were swirled rapidly for about one minute. The beads were then allowed to sit in an ice-water bath for one-half hour with inter mittent swirling. At the end of 30 minutes, the supernatant was removed with a dispo pipet fitted with polyester net. The blackened beads were then washed twice with 10-15 ml of water.
One ml of water was added and the flask was cooled in an ice- water bath. Reduction was again performed by adding 1.0 ml of the borohydride solution followed by a vigorous swirling motion. The beads were allowed to sit in an ice-water bath for 30 minutes with intermittent agitation. The beads were washed four times with water and two times with methanol (10-15 ml each time) prior to drying in a heated vacuum desiccator (via water aspirator).
Reduction in this fashion requires 10 mg of sodium borohydride oer gram of beads.
16.4 Preparation of palladium catalysts with different levels of palladium loading (0.5, 1.0, 1.5, and 2.0% Pd)
To change the amount of palladium on the silica beads, an appropriate change in the amount of PdCl^ used in making the
PdC^-NaAc solution was made. For a 0.5% Pd catalyst, 0.1 g of
PdC^ was heated in 1 ml of concentrated HC1 to dissolve the solid. The PdCl^-HCl was diluted to 48.0 ml with 15% NaAc •
3^0. For a 1.5% Pd catalyst, 0.3 g of palladium chloride was heated in 3 ml of concentrated HC1. Dilution to 48 ml with 15%
NaAcOH^O provided 0.15 g Pd/4.0 ml of solution. A 2.0% catalyst was made by using a proportionate amount of anc^ 2+ concentrated HC1. After adsorption of the Pd (within 10 34
minutes), the supernatant was removed and the beads were reduced
in water as previously described.
16.5 Preparation of Pd/PEl/glutaraldehyde/silica gel
A palladium stock solution is made by mixing 1 ml of 1 N HC1
with 35 mg of palladium chloride and heating to dissolve the mix
ture. Ten ml of 15% N a A c ' S ^ O and 4.4 ml of palladium stock
solution were added to 2 g of PHl/glutaraldehvde/silica gel.
Uptake of palladium was complete within ten minutes. The prepar
ation was then reduced with 300 mg of sodium borohydride in 10 ml
of MeOH. After washing with water, 10% HCOOH, water, and MeOH,
the gel was dried in a vacuum desiccator.
16.6 Synthesis of hydrophobic PEl/silica bead and gel catalysts
Hydrophobic PEI/silica catalysts were synthesized by the same general procedure as underivatized PEl/silica catalysts. The only
change in the protocol was the amount of time the hydrophobic 2+ derivatives were exposed to solutions of Pd . The underiva- tized PEl/silica beads and gel can decolorize solutions of Pd2+ within 5 to 10 minutes while the hydrophobic derivatives required
24 to 48 hours. Since the hydrophobic PEl/silicas tend to float in aqueous solution, the suspension was rotated to ensure maximum 2 + exposure of the PEl/silica to the Pd solution. After the 2 + supernatant of the Pd solution-silica suspension was colorless, the preparation was reduced with sodium borohydride as previously described. 35
17. Reproducibility of the Wrist-Act ion Shaking Assay
The reproducibility of the wrist-action shaking assay was assessed by repeating the standard assay five times in exactly the same manner. Catalyst (0.2 g, from the same batch) was placed in a 25 ml Erlenmeyer flask. The reaction was started when assay mixture was added to the flask (3.5 ml, 0.12 g Cbz-gly-O-tBu, 1.5 ml EtOH, 0.33 ml HCOOH and 1.67 ml water).
IS. Stability Studies of Catalyst Activity
15.1 Reuse of Pd/PEl/silica beads and silica gel catalysts in formic acid
A 0.2 g sample of catalyst was placed in a 25 ml Erlenmeyer flask and was assayed for activity using Cbz-gly-O-tBu in HCOOH:
EtOH:H^O (2:0:10). After about 70 minutes, the assay mixture was removed by dispo pi pet fitted with polyester net. The catalyst was washed four times with 15 ml of methanol and dried in vacuo. The Cbz-gly-O-tBu assay was performed on this 0.2 g sam ple ten times and the was determined as a function of use.
13.2 Reuse of the Pd/PEl/silica gel catalyst in hydrogen gas
The stability test (reuse) of 1% Pd/PEl/silica gel was performed by reusinq 2.12R g of catalyst. At the end of the reaction, the catalyst was filtered, washed with methanol, dried and recycled. 36
18.3 Storage stability of catalvst activity
A batch of catalyst was placed in a beaker and stored on the
bench top. Samples were taken on various days, and the activity
of the catalyst was assessed using the Cbz-gly-O-tBu assay.
19. Study of Catalyst Poisons
The Cbz-gly-O-tBu assay was used in the determination of
catalyst poisons. Immediately after the reaction mixture was
added to the flask containing catalyst, the catalyst poison was
added to the flask via Hamilton syringe or pipet. When CO^ was being tested as a catalyst inhibitor, a CO^ generating apparatus was made. It consisted of a squirt bottle to which a long small bore tyqon tubing was attached. The other end of the tubing was placed below the level of the reaction mixture in the assay flask.
Solid carbon dioxide (about 1 g) was placed in the squirt bottle and the top was capped. Gaseous CO^ was bubbled through the reaction mixture throughout the assay. Toluene was used as a catalyst poison in two different ways. First, it was added to the reaction mixture directly. Secondly, a sample of catalyst was soaked in an excess of toluene and dried. The presoaked catalyst was then used in the assay.
20. Preparation of Pd/PEl/Silica Samples for ESCA. Studies
A sample of Pd/PEl/silica (PG resin) and a sample of the same catalyst treated with formic acid were submitted for analysis of the palladium content, physical properties, surface composition, 37 and palladium surface chemistry. In addition, a sample of the
Pd/PEl/silica gel was submitted for the same analyses. All sam ples were prepared as described in the text. The samples were dried in a heated vacuum desiccator (via water aspirator) for 1-2 days prior to submitting the samples for analysis. No special precautions were taken to keen air out of the sample vials. The samples were analyzed at Amoco Research Center, Naperville,
Illinois. RESULTS AND DISCUSSION
21. Evaluation of the preparation of PEl/siica beads
Our approach to the study of Pd/PEl/silica beads was to first
elucidate a synthesis of the polymer ghosts that consisted of few
manipulations and that was reproducible. After attempting various
methods to deposit the first PEI coating, we finally arrived at a
possible route to ghost synthesis by studying the binding
characteristics of PEI to silica beads. We reasoned that
deposition of a single layer of PEI on the silica beads followed by crossliriking would be sufficient to form a stabilized PEI bead.
The binding of PEI to silica beads is shown in Figure 2.
With unwashed beads, layers of PEI appeared to build up (data not
shown); while with washed beads, the amount of PEI bound to the
silica approached 0.04 g PEl/g of silica.
Following the deposition of PEI, a crosslinking step using
glutaraldehyde in THF was included to stabilize the PEI layer.
PEI is insoluble in THF; this step would effectively precipitate the PEI onto the silica. In this fashion, no PEI would elute into the solvent as is observed with preparations using glutaraldehyde in water as the crosslinking medium. Next, the beads were exposed to a PEI solution once again to react with free aldehydes that may have remained on the beads and to increase the amine capacity of this preparation. 38 Figure 2: The binding of PEI to silica beads 1 g of silica beads was equilibrated with methanolic soltions of PEI. The beads were then washed twice with methanol and dried. The beads were then ground to a fine powder, and the primary amine content was determined. Amine content determined twiceO— O (all error bars fall within the circles); determined once 0 - 0
39 iQ (D ►J C CM g of PEI Bound/g Si02 x 10 8.0 0 - .0 7 6.0 .0 4 0.0 2.0 5.0 3.0 1.0 0 20 0 60 . 1.1. 40 60 0 24.0 4 2 .0 0 2 16.0 14.0 10.012.0 8.0 6.0 .0 4 2.0 .0 0 / o PI n eH 0 3 10 MeOH in x PEI of l g/m .0 5 3 O 41
The above procedure was used for several years without the
appearance of any adverse aspects. The preparation is stable
under flow conditions in methanol and in N-ethyl morpholinium
acetate buffer pH 7, IrrM (Figure 3). However, when the beads are
exposed to EtOH:HCOOH: H^O (9:2:10), the solvent system used to assay the catalyst, a significant amount of primary amine was
found in the eluted fractions (Figure 4). It is also interesting that from a Pd catayst prepared from the preparation described above ninhydrin positive material will also elute.
It was suspected that much of this acid labile material was due to reverse condensation in acid of imines or enamines formed between PEI and glutaraldehyde. In order to illustrate that imines or enamines were indeed contributing to the acid labile fraction of primary amine material, the preparation was exposed to sodium borohydride to reduce these unsaturated linkages. Figure
5 shows that reduction of the PEI matrix with NaBH. stabilizes 4 the PEl/silica/PEI matrix to acid conditions.
Recent work on the crosslinking of proteins and simple amine models by glutaraldehyde has led to the identification of pyridinium containing compounds as the products of amine and glutaraldehyde condensation (42). In light of this finding, it would not be unreasonable to find precursors of pyridinium compounds in these PEI/glutaraldehyde preparatons. These compounds may be contributing to the material that elutes in acid. Figure 3: Elution of PEl/silica samples Several 1 g samples of PEl/silica were applied to a column and eluted with the indicated solvent. Eight 25 ml fractions of the column eluate were collected. The primary amine content of each fraction was compared to the total primary amine content of the material in the column before elution, and the percentage of total primary amine content lost in each fraction was determined. PEI coated on silica (not crosslinked) elued with methanol ; N-ethylnorpholinium acetate (pH 7, lmM) elution of PGP resin ? MeOH elution of PGP resin ? methanol elution of 1.5 hr leached PGP resin and NaBH reduced resin - - - (all values fall below this level).
42 Percentage of TotalPEI Lost in Fraction 15.0 IN) OJ Ul (J) >J oo cd o — o5 £ o• o• •o •o •o • o O• •O •O #O •O O •o n — i— i-- 1— i--- r Figure 4: Stability of PGP resin and catalvst in HCOOH:EtOH:water (2:9:10). Samples of PEl/silica beads (1 g) were applied to a column and eluted with the indicated solvent. Eight 25 ml fractions of the column eluate were collected. The primary amine content of each fraction was compared to the total primary amine content of the material in the column before elution, and the percentage of total primary amine content lost in each fraction was determined. PGP resin 0 ~ O * catalyst D-D-
44 4^ U1 __ U1 O H - CD CD - ro - cm Fraction number
5 5 Percentage of PEI Lost in Fraction * O O I 03 i Figure 5: Stability of reduced PEl/silica preparations in HCOOH: EtOH-.water (2:9:10). Note expanded scale relative to Figure 4. The elution of the samples was performed as described in Figure 4 and 5. A PGP resin refers to the preparation of the beads in which the beads were coated with 4 g PEl/g silica beads, crosslinked with glutaraldehyde in THF, recoated with PEI, and leached with base for 1 hr. A PG resin refers to the preparation of the beads in which the beads were coated with 4 g PEl/g silica beads, crosslinked with glutaraldehyde in THF, reduced with sodium borohydride, washed with acid, and leached with base for 1 hr. A sample of PGP resin from Figure 4 ,reduced D -D ; new synthesis of the beads, PG resin A - A -
46
— CJ1 O o • o * _ *
Percentage of Total - PEI Lost in Fraction - 00 no no - 0 1 3 §• §• * . H o .- CT> CJ 01- a o
Figure 5 48
One aim of this project was to synthesize the beads in as few
as steps possible. If a reduction step were to be included in the
synthesis, an additional acid wash step would be needed to remove borate anion which would bind strongly to the cationic matrix. To
circumvent the need for many additional steps, a practical
preparartion seemed the following. First, PEI would be coated
on silica and dried. Second, the beads would be crosslinked in
glutaraldehyde THF. The beads would then be reduced with
NaBHj, washed with acid, and leached with base. Figure 6 shows the current scheme for PEl/silica synthesis.
The major difference between this synthesis and the last is that this synthesis does not involve a second PEI coating step.
Also, a reduction step and and an acid wash are included. Both
syntheses of the beads have been used in studies in this dissertation, and both syntheses will produce active catalysts.
This preparation of the beads was then subjected to elution in
HCOOH:EtOH:water (2:9:10) (Figure 5). The results show that this synthesis results in improved acid stability. For the sake of simplicity, PGP refers to the synthesis which includes an additional PEI coating step and no reduction of the resin; and PG refers to the synthesis of the beads which includes a reduction step and an acid wash.
We wanted an additional measure of the stability of
PEI/glutaraldehyde silica bead preparations under the conditions employed for catalyst use. PG resin, PGP resin, and PEI on silica Figure 6: New scheme for preparation of PEl/silica beads
4 9 Inorganic Core
PEI,(-CH2CH2NHx)n
Glutaraldehyde,THF
X
X
1. NaBHU 2. H + 4 3. Leach with NaOH
OH
OH PEI/Silica 51
(not crosslinked) were exposed to wrist-action shaking conditions
in both methanol and HCOOH: EtOH: H^O as shown in Table 2. The
results show that PG resin is by far the most stable preparation.
22. Evaluation of the preparation of Pd/PEl/silica beads
22.1 Leaching of PEl/silica matrices
Previous work in this laboratory has involved PEl/inorganic
preparations in which all of the core had been removed (38) or preparations in which the core had been partially removed (35).
A simple measure of the amount of silica removed by base leaching can be quantitated by measuring the weight of PEl/silica remaining after exposure of the beads to 5 N NaOH (Figure 7). In all of the studies preseted in this dissertation, PEl/silica beads were exposed to base solution for 1 hr. The data shown in Figure 7 show that 1 hr leached PEl/silica has approximately 20 % (wt/wt) of the silica core removed.
22.2 Pd^+ Adsorbtion 2 + Figure 8 shows the adsorption of the Pd and the synthesis of the Pd/PEl/silica beads. Once a plausible synthesis of the beads had been constructed, we concentrated on the deposition of 2+ Pd onto the bead matrix. We wanted a simple measure of 2+ Pd uptake by the beads since it would be necessary in 24- future work to ensure equivalent Pd loading when different
PEl/silica bead preparations were being compared. An example of 24- the importance of depositing equal amounts of Pd 52
Table 2: Stability of PEl/silica preparations under wrist-action shaking conditions (a)
PEl/silica sample Solvent Percentage of Amine Lost
PEl/silica HCOOH: EtOH :H 0 100 (b) (not crosslinked) (2:9:10)
PEl/silica EtOH 100 (b) (not crosslinked)
PGP/silica HCOOH:EtOH:H O 6.0 (2:9:10) 2
PGP/silica EtOH 0.034
PG/silica HCOOH: EtOH: H O 0.72 (2:9:10) 2
PG/silica EtOH
(a) In this experiment, 0.2 g samples of PEl/silica beads were placed in Erlenmeyer flasks, and 3.5 ml of solvent was added. The flask was shaken at 10 maximum on the wrist-action shaker as described in the text. At the end of one hour, the primary amine content of the supernatant was determined and compared to the amount of primary amine material on the beads before assay. (b) The amount detected was too large to be determined by ninhydrin under the experimental conditions employed. 53
Figure 7: The leaching of PGP/silica beads Samples of PGP/silica beads (10 g) were placed in 250 ml Rb flasks. The samples were leached with NaOH (5N, 100 ml) for various times. The weight of PEl/silica remaining was determined. A 1 hr leached resin has approximately 20% of the silica core removed ((10-8)/l0) ( X 100). iue 7 Figure Weight of PEI/Silica Remaining (g ) 10.0 .0 9 6.0 .0 7 11.0 .0 4 8.0 o a 2.0 .0 3 5.0 1.0 or o Leaching of Hours 54 Figure 8: Schematic diagram showing metal chelation and reduction to form catalyst
55 56
PEI/Silica Bead
Pd 2 +
N aB H 4
Pd/PEI/Silica Bead C atalyst Figure 8 57
would be inherent in the comparison of, for example, the Pd/PEl/
silica catalyst with hydrophobic Pd/PEl/silica bead preparation. 2 + The purpose of studying the Pd deposition was two-fold,
however. Not only was it necessary to have a simple method for 2+ looking at Pd uptake, but also it was necessary to have a 2+ simple assay system to look at whether or not Pd adsorption
was accomplished by the amine component of PEl/silica beads or by
the silica beads themselves. After all, the PEI is only 4 % by
weight while the silica is approximately 100 % by weight. It is
known that hydroxyl groups are a component of silicas. Also,
hydroxyl groups have been used as ligands for other insolubilized
transition metal cations (43). A simple assay system would allow 2 + the determination of whether Pd uptake was through interaction 2+ of Pd with the amines on the beads or the surface 2+ hydroxyl groups by comparing adsorbtion of Pd by leached
silica (no PEI) and PEl/silica beads.
An absorption spectrum of a 1:9 dilution of the
PdCl2_sodium acetate stock solution revealed an absorbance
maximum at 375 nm (Figure 9) which obeyed Beer's Law (Figure 10).
These results provided a simple spectrophotometric assay system to 2 + quantitate the amount of Pd adsorbed by the beads. 24- In order to visualize the uptake of Pd by the beads, the beads were exposed to the P d C ^ - s o d i u m acetate solution; and
aliquots of the supernatant were taken at timed intervals. The 2 + rate of uptake of Pd by the PEl/silica beads was compared to
the rate of uptake by leached silica beads (no PEI). Leached Figure 9: Pfosorbance spectrum of the PdCl^-sodium acetate solution (1:9 dilution)
E375=297 cm‘1M~1, 2.61 mM PdCl2 in 15% Na^c 3 H20
58 Figure 9 Figure A b sorb an ce 0.00 0-20 0.60 0.80 0-40 375 aeegh (nanometers) Wavelength 0 0 6 59 Figure 10: Beer's Law plot for the PdCl^-sodium acetate solution
60 Figure 10 Figure Absorbance at 3 7 5 nm 0.3 0.5 0.7 0.8 0.2 0.4 0.6 .0 5 f dI ml ouin x solution l /m PdCI2 of g
10.0
20.0
30.0
.0 0 4 I0"5 61 62
silica was used as the control because leaching silica can be
envisioned as a process which increases the concentration of hydroxyl groups on the silica surface (Figure 11).
24- Figure 12 shows that uptake of the Pd by the PEl/silica beads is rapid and complete within 5-20 minutes. The uptake of 2+ Pd by the control beads is insignificant. Thus, uptake of 2+ Pd is through interaction with the amino groups of the composite.
22.3 The reduction step of catalyst preparation and the reproducibility of catalyst preparation
Previous to the studies in this dissertation, no study of the reproducibility of the reduction step in catalyst synthesis had been performed. Probably the most frequently written observation about catalyst syntheses in general is that different batches of catalyst vary in activity. A method was needed that could reproduce the activity of the catalyst each time the catalyst was synthesized.
First, the reproducibility of the wrist-action shaking assay was studied. The assay was performed five times with 0.2 g of catalyst that was derived from the same batch. Table 3 shows that the assay can be reproduced. Next, we wanted to know if the catalyst itself could be reproducibly synthesized.
In the reduction step of catalyst synthesis, usually the 2+ Pd /PEl/silica beads were placed in EtOH; and solid sodium Figure 11: Schematic representation of the leaching of silica
63 64
OH OH OH OH OH I I I I I Si—0 — Si —0 Si—0- Si — 0 Si — I I I I 0 0 0 0 0 1 I I I I — Si — 0 — Si—O'— Si— 0- Si—0- Si —0 I I I 0 0 0
1) OH- 2) H20
OH OH OH I I I 0 — Si —OH HO- Si— OH HO— Si —0 ^ ^ I I 0 OH 0 OH 0 I I I I -- Si —0 Si— 0 — Si —0 —Si—0 — Si — 0 '■v-'v-' I I I I 0 0 0 0
Figure 11 2 + Figure 12: Uptake of Pd by PG/silica and leached silica beads. One g of PG/silica and 1 g of leached silica (no PEI) were added to 25 ml Erlenmeyer flasks. The PdCl -NaAc solution was added and the flask was shaken at maximum speed on the wrist-action shaker. At timed intervals, 0.1 ml of the supernatant was removed and diluted with 0.8 ml of 15% NaAc • 3 H O. The absorbance of the samples was read at 375 nm. Leached silica (no PEI) , PG/silica Q — Q .
65 < 375nm 0.2 .3 0 0.5 0.6 .4 0 0 . 7 , 10 me (minutes) e im T
15
20
25
30 66 67
★ Table 3: Reproducibility of the wrist-action shaking assay
Entry k _[minutes
1 0.12
2 0.12
3 0.10
4 0.12
5 0.11
A 0.2 g sample of catalyst was placed in a 25 ml Erlenmeyer flask. Assay mixture was added (3.5 ml of HC00H:Et0H:water (2:0:10) containing 0.120 g Cbz-gly-O-tBu), and the flask was shaken as described in the text. Samples of the reaction mixture (4 ftl) were taken at timed intervals. The amount of product at 5, 10, 15, 30, 45, and 60 minutes was assesed by ninhydrin analysis. This assay was performed five times on five fresh samples of catalyst from the same batch. The rate constant, k, was determined. The assays were performed at 23 °C. 68
borohydride was added to the suspension in batches. Two points
deserve mention here. First, sodium borohydride is not extremely
soluble in ethanol, complicating the reproducibility of efficient 2 *t, , contact between the Pd /PEl/silica beads while mixing the two.
Second, the reduction process is highly exothermic providing the
potential for uncontrolled temperature rise in the reduction
process. Indeed, an attempt to synthesize the catalyst
reproducibly by the above method showed large differences from
batch to batch (Table 4).
To reproduce the synthesis of catalyst more reliably, a
scheme for the reduction step should then circumvent the two
problems discussed above. Four solvents were chosen for their
solubility properties with sodium borohydride. These solvents were DMF, ethanol, diglyme and water. Also the reduction step o was carried out near 0 C to control the temperature,and to
retard the decomposition of borohydride. Table 5 shows the 2+ reduction of the Pd /PEl/silica beads in various solvents.
The data shown in Table 5 show that all the solvents could be used in catalyst synthesis. Water was chosen for its solubility properties as well as its availability. Problems that might arise from hydrolysis of the borohydride anion in water are
lessened by maintaining the temperature near 0° C where hydro lysis would be minimal compared to that at room temperature. One other piece of information is evident in Table 4. The data show that two additions of reductant (0.05 g/g of beads) are sufficient to produce an optimally active catalyst. A third addition of 69
Table 4: Reproducibility of catalyst preparation (no temperature
it or solubility control)
Entry g NaBH^/g beads fci/2 ^™-nutes)
1 0.05 11
2 0.05 25
3 0.05 60
4 0.10 8
5 0.10 13
6 0.10 16
7 0.10 23
8 0.20 11
9 0.20 12
10 0.20 17
11 0.20 38
12 0.20 51
* 2+ The catalysts were prepared by placing the Pd /PEl/silica beads in ethanol and adding the solid NaBH in the amounts specified in the table. The activity of the catalyst was determined by using 0.100 q of catalyst and the Cbz-alanine assay as described in the text (0.100 g Cbz-alanine in HC00H:MeOH water (1:3:5)). The assays were performed at 23 °C. 70
24- Table 5: Sodium borohydride reduction of Pd /PEl/silica beads * in various solvents (below 5 °C )
Entry Solvent g NaBH^/g of beads tl/2
1 DMF 0.05 33.0
2 DMF 0.10 15.0
3 DMF 0.15 16.0
4 DMF 1.0 19.0
5 Diqlyme 0.05 33.0
6 Diglyme 0.10 27.0
7 Diglyme 0.15 10.0
8 Diglyme 1.0 18.0
9 EtOH 0.05 15.0
10 EtOH 0.10 14.0
11 EtOH 0.15 13.0
12 EtOH 1.0 15.0
13 H20 0.05 25.0
14 H20 0.10 15.0
15 H20 0.15 15.0
16 H20 1.0 16.0
* The activity of the catalysts was measured using 0.100 g of catalyst and the Cbz-alanine assay as described in the text (0.100 g of Cbz-alanine in HCOOH:MeOH:H20 (1:3:5), 3 ml total volume of assay mixture). The assays were performed at 23 °C. 71
reductant shows little effect on ultimate catalyst activity.
When the catalyst was synthesized several times using borohydride
in water as the reducing medium a vast improvement in the
reproducibility of catalyst synthesis was seen (Table 6). After
this experiment had been performed, a related patent describing
the reduction of transition metals in with NaBH^
was found (44). By virtue of its simplicity, reduction of the 2+ Pd /PEl/silica beads with sodium borohydride in water is
recommended as the method of choice for the reduction step of
catalyst synthesis.
23. Evaluation of the Pd/PEl/silica beads as catalysts
23.1 Comparison of Pd on C, Pd(0H)9 , and Pd/PEl/silica bead
catalysts
When the catalyst is compared to Pd on carbon in the hydrogenolytic removal of Cbz from Cbz-gly-O-tBu using formic acid as hydrogen donor, the catalyst's performance is by far superior to Pd on carbon (Figure 13). In this experiment, equivalent amounts of Pd were used (1 mg of Pd for each catalyst).
Pd(0H)2» which was also used as a catalyst in this reaction, showed no activity (data not shown). Previous studies using Pd black at this concentration of Pd have also shown very little activity(35,36) . Thus, the Pd/PEl/silica bead-formic acid system appears to be superior to the Pd on C, PdfOH)^/ and Pd black. 72
o * Table 6: Reproducibility of catalyst preparation (2 C)
Entry (minutes) tl/2 1 5.1
2 5.0
3 5.5
4 3.1
5 4.0
* The catalyst was prepared five separate times using NaBH in water at low temperature. The activity of the resulting catalysts was assessed by using 0.200 g of catalyst in the Cbz-gly-O-tBu assay as described in the text. The assays were performed at 23 °c. Figure 13: Comparison of 1% Pd on carbon and 1% Pd/PEl/silica beads in the removal of the Cbz group from Cbz-glycine-OtBu in formic acid The catalysts (0.100 g each) were placed in 25 ml Erlenmeyer flasks and 3.5 ml of assay mixture (0.12 g Cbz-gly-O-tBu in HCOOd:EtOH:water (2:9:10)) was added. Samples of the reaction mixture (4 fi 1) were taken at timed intervals and the amount of product present was determined by ninhydrin. The assays were performed at 23 °C. Pd/PEl/silica beads Q— Q ' p,3/C Q — Q .
73 iue 13 Figure Percentage Completion of Reaction 100 40 20 60 90 80 - 10
20 i (minutes) e Tim
30
40
50
60
70
80 74 75
23.2 Comparison of Pd catalysts with different levels of Pd loading
( 0.5, 1.0, 1.5 and 2.0% Pd)
Figure 14 shows how the rate constant of the reaction increases when the Pd content of the Pd/PEl/silica bead increases.
At 2% palladium loading, the catalyst appears to have the highest specific rate of Pd). While these catalysts were being handled, no pyrophoric behavior was observed.
23.3 Reuse of the Pd/PEl/silica bead catalyst in formic acid
Not only was attention paid to the catalytic activity of the catalyst, but also to the reuse of the catalyst under conditions that would mimic the catalyst's reuse during peptide synthesis.
Coleman and Royer have shown the applicability of the Pd/PEl/ silica bead catalyst-formic acid system using the t-butyl group as temporary side chain protection (35). Thus far, no quantitative study of the reuse of the catalyst in formic acid has been made.
Several studies of the reuse of the Pd/PEl/silica bead catalyst using formic acid as the hydroqen donor were made. In these experiments, a simple peptide model (Cbz-gly-0-t!3u) was used as the substrate. It was thought that other simple substrates such as Cbz-alanine did not contain the usual functional groups encountered during peptide synthesis. Cbz-alanine has a free carboxyl group and might bind to the catalyst surface. Usually, the carboxyl function is protected during synthesis. Also, the
Cbz-alanine in HCOOHtMeOHsF^O (1:3:5) system shewed signs of product insolubility near the end of the reaction (either alanine Figure 14: Comparison of Pd/PEl/silica catalysts with different levels of palladium loadinq (0.5, 1.0, 1.5, and 2.0% Pd) Samples of catalyst (0.100 g) were placed in 25 ml Erlenmeyer flasks, and 3.5 ml of assay mixture (0.12 g of Cbz-gly-O-tBu in HCOOH:EtOH:water (2:9:10) was added). The flask was agitated by wrist-action shaking as described in the text. Samples (4 H1) of the reaction mixture were taken at timed intervals and the amount of product was determined by nirihydrin. The assays were performed at 23 °C. The rate constant is shown as k , = obs k ' QcatalystJ= k ’ ^percentage of Pd by weight for 100 mg of catalystj .
76 iue 14 Figure kobs (s'1) X 10 2.0 . - 0.5 2.5 1.0 - ecnae f d y Weight by Pd of Percentage 0.5 1.0 1.5 2.0 77 78
or toluene) as was evidenced by a cloudy appearance of the
solution and the appearance of an oily layer that appeared on the surface of the reaction mixture. Cbz-gly-O-tBu is soluble in
formic acid:ethanol:water (2:9:10) and the end products of the
reaction are also soluble.
In the typical reuse experiment, a weighed sample of catalyst was placed in a 25 ml Erlenmeyer flask and repeatedly assayed for activity using Cbz-qly-O-tBu as the substrate. At the end of the reaction, the reaction solvent was removed and the catalyst was washed exhaustively with methanol or ethanol and dried. The weight loss of the catalyst between successive assays was monitored and never found to exceed 4% by weight for a total of 10 runs (the rate constant was not corrected for weight loss of the catalyst). In this fashion, the same batch of catalyst was reused; the t ^ ^ was comP are^ f°r ten successive runs.
Roth preparations of Pd/PEl/silica beads were compared; that 2+ is, both PGP resin (no reduction of the resin prior to Pd chelation) and PG resin (resin which had been reduced with sodium borohydride prior to metal chelation and reduction). Figure 15 shows the reuse of the Pd/PGP/silica beads in formic acid
(which was performed twice). A reduction in activity was observed with use (an increase in t ^ ^ ) - T^at this loss in activity was permanent was shown by the inability of the catalyst to recover activity when the preparation was re-reduced with sodium borohydride after the tenth use. During the tenth or Figure 15: Reuse of the Pd/PGP/silica bead catalyst in formic acid The catalyst (0.200 g) was placed in a 25 ml Erlenmeyer flask, and assay mixture (3.5 ml of HCOOH:EtOH:water (2:9:10) containing 0.12 g of Cbz-glycine-OtBu) was added. The flask was agitated by wrist-action shaking (as described in the text). Samples of the reaction mixture (4 Ail) were withdrawn at timed intervals and analyzed for product by ninhydrin. The assay was performed at 23° C. ZVt the end of the assay, the assay mixture was removed; and the catalyst was washed four times with MeOH and dried. To this once used catalyst was added a second 3.5 ml of assay mixture. The rate of the reaction was again determined. This procedure was repeated until the catalyst had been used ten times.
79 o 00 — PO o o o GO cn oi CD CD CD 3 n> o C = c cr 2 W ) (minutes) for Cbz Removal in IICOOH
Figure 15 81
eleventh use, a dark-colored material was observed to elute
from the resin.
Figure 16 shows the reuse of the Pd/PG resin in formic acid.
In this case, a more constant '*'s maintaine(3 through successive uses, but the activity still decreases with use.
During the tenth assay, a dark-colored material was observed to elute from the resin, and a large increase in the twas observed. The pre-reduced resin (PG) thus serves to maintain a more constant with use, but does little to stop the ultimate elution of a dark-colored material from the resin.
Some aspects of the catalysis in acid need to be pointed out, however. The major application of the Pd/PEI/silica bead catalyst/HCOOH system is in the synthesis of peptides where the catalyst would be reused in the deprotection of Cbz-protected peptides of various lengths. At the end of the tenth run in the reuse experiment, the catalyst had processed 240 moles of sub strate for every mole of palladium, or (for this substrate) 600 q of substrate for every 1 g of nalladium. For a synthesis involving peptides, these numbers could easily approach 1000 g of peptide for every 1 g of palladium.
One additional Pd/PEl/silica bead catalyst was thought, perhaps, to be of utility. This catalyst was derived from a PEI/ silica resin in which the crosslinking reagent was Xama-2, a multifunctional aziridine. Initial studies showed that the PEI/
Xama/silica bead was stable in methanol, and a Pd catalyst pre pared from this resin was as active as the catalyst prepared Figure 16: Reuse of the Pd/PG/silica bead catalyst in formic acid Catalyst (0.2 g) was reused ten times. The description for the typical reuse experiment can be found in the legend for Figure 15.
82 iue 16 Figure
CVJ for Cbz Removal in HCOOH (minutes) 40 60 80 20 I 10 9 8 7 6 5 4 3 2 ubr f Uses of Number 33 Figure 17; Reuse of the Pd/PEl/Xama/silica bead catalyst in formic acid Catalyst (0.2 g) was reused ten times. The description of the typical reuse experiment can be found in the legend of Figure 15.
84 Figure 17 Figure for Cbz Removal in HCOOH (minutes) 100 140 120 40 20 60 80 I 10 9 8 7 6 5 4 3 2 ubr f Uses of Number 85 86
from the glutaraldehyde crosslinked resin. The reuse of this
catalyst in formic acid showed that the catalyst was much less
resilient to acid conditions than Pd/PG catalyst (Figure 17).
23.4 Study of catalyst poisons
During the course of the studies of catalyst reuse in formic
acid, it became apparent that the decrease in activity might be
attributed to poisoning of the catalyst with one of the end
products of the reaction. This study was initiated by adding
toluene or carbon dioxide to the reaction mixture and observing
the effect on the rate of the reaction. In one experiment, the
catalyst was soaked in toluene prior to the determination of ac
tivity. Since substances other than end products may also act as
inhibitors, other compounds were included in this study. Table 7
shows that no end products were catalyst poisons. On the other hand, 2-mercaptoethanol and hydrogen peroxide were drastic in hibitors of the catalyst. It is unlikely that the decrease in the rate of the reaction with use is caused by end product inhibition; perhaps some reactive intermediate generated during the reaction may cause catalyst inactivation.
23.5 Stability of Pd/PEl/silica beads at atmospheric pressure
The Pd/PEl/silica beads lost activity during storage (data not shown). A.fter 42 days of storage, the activity of the catalyst was the same as the initial activity. However, after 146 days of storage, the catalyst was completely inactive. The 87
Table 7: Study of catalyst poisons
Catalyst poison moles poison/ Activity mole Pd
H202 (a) 167 none * 2-mercaptoethanol (a ) 5 none
t-butyl hydroperoxide (a) 1.6 activity
5 activity
carbon dioxide (b) activity toluene (a) activity
toluene (presoak) (c) activity
(a) These catalyst poisons were added at time 0 in the Cbz-gly-O-tBu assay as described in the text.
(b) Carbon dioxide was bubbled through the assay mixture throughout the Cbz-qlv-O-tBu assay.
(c) The catayst was soaked in toluene and dried prior to being assayed for activity 88
inactive catalyst was reduced with sodium borohydride (0.05 g/g of
catalyst) and all of the activity was recovered. Thus, catalysts
which are inactivated by storage can be reactivated by an addi
tional reduction.
24. Evaluation of Hydrophobic PEl/Silica Reads
We wanted to investigate catalysts with selectivity as well
as efficiency. Hydrophobic substitution of the PEl/silica matrix
was performed in order to augment interaction of the substrate
with the catalyst. The proposed scheme for synthesis of hydro-
phobic catalysts is shown in Figure 18. First, a PEl/silica bead
would be modified with a hydrophobic group such as an octyl or 2 + octanoyl moiety. Pd would then be adsorbed and reduced. A
hydrophobically modified catalyst could then be compared to an
unmodified catalyst.
Table 8 shows the modified supports and their primary amine
content. The decrease in primary amine content was used as a
relative measure of support modification. Primary, secondary, and
tertiary amines are present in the preparation and available for
reaction with the modification reagent. Since ninhydrin is used
to quantitate only those primary amines that have reacted, each
modified preparation in Table 8 should be thought of at least 15% modified as in the case of octyl PEl/silica. In other words, more hydrophobic groups could be present in the preparation than indi
cated by the decrease in primary amine content. However, the Figure 18: Schematic diagram showing the preDaration of hydrophobic Pd/PEl/silica bead catalysts
89 90
PEI/Silica Bead
I CH3(CH2)6-X o . X= -CH2CI or-CCI O
Pd2+/, Y
Na BH- Y
Hydrophobic Pd/PEI/Silica Catalyst
Figure 18 Table 8: The primary amine content of hydrophobic PEl/silicas
PEI /Silica Group Primary Amine % Modification of Sample Incorporated Content (meq/g) Primary Amine
PEI /Silica 0 .2 2 - 0 .3 0 Bead 0 Octanoyl PEI/Silica CH3-fCH2^ C - 0.145 46 Bead
Octyl PEI/Silica CH3-tC H 2-kCH — 0.187 15 Bead
Lauryl P E I/S ilica CH3 - f CHz-t0CH- 0.194 35 Bead
vO 92
secondary and tertiary amines would be expected to react more
slowly than the primary amines•
Catalysts were made from the modified PEl/silica beads. The
only notable exception in the syntheses was the amount of time the
PEl/silica beads were exposed to the palladium chloride-sodium
acetate solution. When unmodified PEl/silica beads are exposed to 2+ a Pd solution, decolorization of the solution is rapid and
complete within five minutes. When a hydrophobic PEl/silica
preparation is exposed to the palladium solution, decolorization
takes place in 24 to 43 hours. Each time a catalyst was made from
a hydrophobic PEl/silica bead, a catalyst was made from an
unmodified PEl/silica bead. Both preparations were exposed to the 2+ Pd solution for 24 to 43 hours. In this fashion, the effect
of prolonged exposure to the PdCl^-WaAc solution was the same
for the unmodified support and the modified support.
When the hydrophobic Pd/PEl/silica catalysts are compared to
the unmodified Pd/PEl/silica catalysts in the removal of the Cbz
group from Cbz-gly-O-tBu in formic acid, the unmodified catalysts
in all cases were superior to the hvdrophobically modified deriv
atives (Figure 19). Both Pd/octyl PEl/silica beads and
Pd/octanoyl PEl/silica beads were less active than Pd/PEl/silica beads. Although the data are not shown, a Pd/lauryl PEl/silica bead catalyst was also not as active as the control. Hydrophobic substitution of the bead matrix appears to decrease the rate of the reaction. Figure 19: Comparison of hydrophobic Pd/PEl/silica bead catalysts and unmodified Pd/PEl/silica bead catalysts The activity of the catayst was measured using the removal of the Cbz-group from Cbz-glycine-O-tEu in HCOOH:EtOH:water as described in the text. The assays were performed at 23 °C. Pd/PEl/silica beads Pd octanoyl PEl/silica beads Q — O ' and Pd octyl PEl/silica beads / V —.
93 iue 19 Figure
Absorbance at 570nm 0.8 0.7 0.9 0.5 0.6 0.2 0.3 0.4 0.1 1.0 510 15 510 ie (minutes) Time 0 5 0 75 60 45 30 94 95
Several explanations may account tor this decrease in rate.
First, the palladium may not be dispersed in the same manner in
the unmodified catalysts as in the modified catalysts. In partial
support of this speculation is the time required to deposit all 2+ the Pd onto the silica bead. Presumably, the most accessible
amino groups are taken up by reaction with the modification rea- 2+ gent. The longer time needed for the uptake of the Pd by the 2+ modified support may reflect the necessity of the Pd to react
with less accessible amine functional groups within the bead
matrix. The palladium, once reduced, may be in an altered form or
perhaps a different surface structure that is less catalytically
active.
It has been hypothesized that the reaction of non-polar sub
stances with polar surfaces may cause the deposition of non-polar
patches of material along the support surface (45). This phenom
enon is brought about because the reaction of one hydrophobic
group in a particular area may facilitate the binding and there
fore the reaction of a second and perhaps a third non-polar group
near a non-polar area. If this is the case, then the palladium
deposition would have to occur over a much smaller surface area.
The catalyst may have two alternating areas on the surface— a hydrophobic area and an area in which the palladium is concen
trated. This arrangement of palladium and hydrophobic groups would not be expected to resemble the unmodified catalyst.
It was thought that the ratio of palladium to hydrophobic groups may be too high. If the ratio of palladium to hydrophobic 96
groups was excessive, then the presence of the hydrophobic groups
would be insignificant and reaction would take place at an al
tered palladium surface. In this manner, 1/2 palladium concen
tration may be too high and the specific activity of the palladium
may be low. When the palladium concentration was lowered to 0.5%,
still no increase in the rate of reaction was noted. In fact, the
rate was the same in both cases (Figure 20).
25. Evaluation of Pd/PEl/Silica Gel Catalyst
25.1 Reuse of the Pd/PEl/silica gel catalyst in hydrogen gas and
formic acid
During the work on Pd/PEl/silica beads, a new catalyst (Pd/
PEl/silica gel) was discovered by Dr. W.-S. Chow in our labora tory. The main difference between the silica bead catalyst and the silica gel catalyst is that the silica gel catalyst is essentially a powder. Deposition of the PEI and the actual syn thesis of the catalyst are all very similar to the bead prepara tions. One advantage of the gel preparations was that the gel could be used under more vigorous shaking conditions. Unlike the bead preparations, the gel preparations can be used in a Parr shaker or with overhead stirring provided the stirrer does not touch the bottom of the flask. We found that the Pd/PEl/silica gel catalyst could reduce nitrobenzene to aniline. We chose TJV spectroscopy to follow this reaction. Figure 21 shows the absorption spectrum of the product aniline. Figure 22 illus trates how the spectrum changes as the starting material becomes Figure 20: Comparison of Pd/PEl/silica beads and Pd octanoyl/PEl/silica beads with 0.5% Pd loading These data show the removal of the Cbz group from Cbz-glycine-OtBu in HCOOHsEtOH:water as described in the text (0.100 g of catalyst was used in each case). The assays were perfomed at 23 °C.
97 Figure 20 Figure Absorbance at 570 nm 0.6 0.2 0.5 0.9 0.3 0.4 0.8 0.7 0.1 1.0 1015 5 ie (minutes) Time 0 5 0 75 60 45 30 98 Figure 21: The absorbance spectrum of aniline (0.1 mM aniline in 5% MeOH in water, cell path length 1 cm)
99 Figure 21 Figure
Absorbance 0.2 0.6 0.0 0.4 0.8 0 20 4 20 8 300 280 260 240 220 200 aeegh (nm) Wavelength 100 Figure 22: Change in absorbance spectrum as nitrobenzene becomes reduced to aniline The starting concentration of nitrobenzene was 0.1 mM.
101 102
0.9
0.8
0.7
0.6
0.5
j d 0.4 <
0.3
0.2
0.0 220 240 260 280 300 320 Wavelength (nm)
Figure 22 Figure 23: Reuse of the Pd/PEl/silica gel catalyst in H (g) Catalyst (2.128 g f 1% Pd/PEl/silica gel). To a 500 ml Rb flask equipped with an overhead stirrer was added 10.3 ml of nitrobenzene (0.1 mole), 89.7 ml of MeOH, and 2.128 g of 1% Pd/PEl/silica gel. The mixture was agitated by overhead stirring at a speed sufficient to maintain the catalyst from settling. The stirrer was positioned such that it did not touch the bottom of the flask. Hydrogenation was performed at atmospheric pressure. The flow rate of hydrogen gas was fixed at a constant rate with a Gilmont flowmeter (0.5 l/min, calibrated by air). At timed intervals, an aliquot (10 /yl) of the supernatant was removed and diluted with 5 ml of MeOH and 95 ml of water. The samples were analyzed spectrophotometrically (220-320 nm). At the end of the reaction, the catalyst was washed with methanol and dried. The same batch of catalyst was reused ten times. For each run, the initial starting temperature was 23 °C; however, a rise in temperature was observed after the reaction had started (the reacton was exothermic).
103 iue 23 Figure (minutes) 40 60 80 20 I 23456789 I10 23456789 ubr f Uses of Number 104 105
reduced to aniline by the action of the Pd/PEl/silica gel catalyst in hydrogen gas.
Since this catalyst seemed to hold up under more vigorous conditions than the bead catalyst, we explored the reuse of this catalyst in hydrogen gas and in acid conditions. Figure 23 shows the reuse of Pd/PEl/silica gel in the reduction of nitrobenzene using hydrogen gas as the hydrogen donor. Remarkably, reuse of the same batch of Pd/PEl/silica gel catalyst ten times results in no loss of activity.
When the Pd/PEl/silica gel catalyst is reused in the hydrogenolytic removal of the Cbz-group from Cbz-gly-O-tBu using formic acid as the hydrogen donor, the catalyst drastically loses activity after one or two uses (Figure 24). The reuse of the
Pd/PEl/silica gel catalyst was performed twice as shown in A and B
(Figure 24). \fter an initial use or two, the rate of the reaction greatly increases. In one case, the catalyst was re reduced with sodium borohydride, and a large portion of the activity was restored. The grave difference between the behavior of the Pd/PEl/silica bead catalyst and the Pd/PEl/silica gel catalyst in acid could be due to the increased sensitivity of the
Pd/PEI/silica gel catalyst to the acid conditions.
26. Evaluation of Hydrophobic Derivatives of Pd/PEl/Silica Gel
Hydrophobic substitution of PEl/silica gel was performed in the same general manner as the PEl/silica beads. Table 9 shows the primary amine content of the modified silica gel. In these Figure 24: Reuse of the Pd/PEl/silica gel catalyst in formic acid Catalyst (0.2 g) was placed in an 25 ml Erlenmeyer flask. Assay mixture (3.5 mi of HCOOH:EtOHrwater (2:9:10) containing 0.12 g of Cbz-glycine-0-t3u) was added. Samples (4 ^1) were taken at timed intervals and analyzed for product by ninhydrin analysis. At the end of the reaction, the assay mixture was removed and the catalyst was washed with MeOH and dried. In A, the assay was performed seven times on the same 0.2 g batch. In B, the assay was performed four times on another 0.2 g batch. The four-times used catayst was reduced with sodium borohydride (0.01 g sodium borohydride/0.2 g of catalyst) in water at 2 °C as indicated by the event marker in B. This four-times used, regenerated catalyst was then assayed for activity two more times as decribed above. All catalyst assays in formic acid were performed at 23 °c.
106 107
B 80 NaBH 60
X 20
O I 2 3 4 5 6
A
I 2 3 4 5 6 7 Number of Uses
Figure 24 Table 9: The primary amine content of hydrophobic PEl/silica qels
PET / Silica Group Primary Amine % Modification of Sample Incorporated Content (meq/g) Primary Amine
PEI/Silica 0.16 Gel 0 Hexanoyl II 0.045 72 PEI/Silica CHa-fCH^C- Gel 0.108 32.5
Hexyl PEI/Silica CH3-f-CH2-^CH2— 0.13 19 Gel
Benzyl PEI/Silica 0.114 28 Gel CHh~ 108 109 studies, a PEI crosslinked with Xama/silica gel was used instead of the glutaraldehyde crosslinked resin. Figure 25 shows that the unmodified derivatives of the Pd/PEl/silica gel were more active than the hydrophobically modified derivatives.
27. Physical Properties and Palladium Content of Pd/PEI/Silica
Catalysts
Table 10 shows the physical properties of the Pd/PEl/silica catalysts. The Pd/PEl/silica bead catalyst has a surface area 2 around 51 m /g. The Pd/PEl/silica gel catalyst has a surface 2 area around 500 m /g. This is the total surface area of the preparation and not just the surface area of CTie palladium. The surface area of the preparation nay be seen as the total potential surface area that the palladium has available to cover in the deposition step.
An. interesting finding was that the silica beads contain only about 2.5% water (or other volatiles by weight).
This value was obtained after the preparation was dried in a heated vacuum dessicator (via water aspirator) for 1-2 days, placed in a bottle that was not sealed with parafilm, and sat on the shelf three months before analysis. It had been thought that these silica preparations were hygroscopic and they appear not to be. The highest percentage of water was found in the
Pd/PEl/silica gel catalyst which was 6.8%. Figure 25: Comparison of the hydrophobic Pd/PEl/silica gel catalysts The amount of catalyst used in each experiment was 0.100 g. The assay mixture consisted of EtOH (1.5 ml), formic acid (0.33 ml), and water (0.5 ml). The substrate used was Cbz-phenylalanine-OtBu (0.1 g). ^t timed intervals, aliquots of the supernatant were withdrawn? and the amount of product was determined by ninhydrin assay. The assays were performed at 23 °C. 1% Pd/PEl/Xama/silica gel catalyst A — A ? 1% Pd/PEl/glutaraldehyde/silica gel catalystQ— □ ? 1% Pd/PEl/silica gel catalyst A — A ? 1% Pd/hexyl/PEl/silica gel catalyst ; 1% Pd/benzyl/PEI/silica gel catalyst ■ — 3
110 111 — % Completion of Reaction _ ID o CD o~ O- (J1 (J1 o-
Time (minutes) -d K- g g oooooooooo to to roai-kmo-jcoioo — 112
Table 10: Physical properties of Pd/PEl/silica catalysts (46)
Sample Weight loss Surfaop area Cumulative Average on drying BET m /g pore volume pore 0 at 250°C cc/g radius (A) for 4 hrs (%)
Pd/PG/ 2.4 47 0.70 265 silica bead
Pd/PEI/ 6.8 499 0.30 14 silica gel
Pd/PG/ 2.2 51 0.65 206 silica bead (HCOOH) 113
Another physical characteristic that appears to be quite
different between the two preparations is the average pore radius.
The silica beads preparation has extremely large pores, around o 265 A as the pore diameter. The silica gel preparations, on the o other hand, have much smaller pores (14 A).
Table 11 shows the elemental palladium analysis of Pd/PEl/
silica samples. The Pd/PEl/silica beads contain 1.09% palladium
by weight, in agreement with the theoretical value (and shown by
the colorimetric assay). The Pd/PEl/silica gel contains 1.05%
Pd, very close to the value for the Pd/PEl/silica beads.
28. Palladium Surface Composition and Palladium Surface
Analysis
The surface composition of the two silica catalysts are
remarkably similar (Table 12). The main difference in the
silica bead and gel preparations is that the silica bead preparation seems to have more carbon on the surface than the silica gel preparation. Another notable difference between the two is the absence of nitrogen in the silica bead preparation.
This may be due to experimental error since the same Pd/PEl/ silica bead preparation (entry three) shows than nitrogen is present. The only difference between Sample 1 and Sample 3 is that the last sample had been treated with formic acid and dried.
The palladium surface chemistry is shown in Table 13. As can be seen in the table, the Pd/PEl/silica preparations are a mixture of palladium forms. One interesting result was the finding of a 114
Table 11: Elemental Pd analysis (46)
Sample Pd (wt/wt %)
Pd/PG/silica 1.09 bead
Pd/PEl/silica 1.05 gel
Pd/PEl/silica 1.04 bead (HCOOH)
Table 12: Surface composition of Pd/PEl/silica catalysts (46)
Sample Si O Pd C N Na
Pd/PG/silica 21.4 45.3 1.6 31.6 ND 0.2 bead
Pd/PEl/silica 19.8 49.4 2.0 22.7 6.0 0.1 gel
Pd/PG/silica 19.4 41.7 1.7 29.7 7.5 trace bead (HCOOH) Table 13: Palladium surface analysis (46)
Sample Pd 3d 5/2 % assignment
Pd/PG/silica 335.4 47 Pd metal bead +4 337.9 53 Pd
Pd/PEl/silica 334.4 70 Pd 6 - qel +2 337.1 30 Pd
Pd/PEl/silica 335.4 65 Pd metal bead (HCOOH) 338. 2 35 Pd+4 116
6~ Pd on the Pd/PEI/silica gel. The analyst who performed these
studies remarked that this alluded to a negatively charged
palladium {46). The Pd/PEl/silica bead preparation seems to be a 4+ mixture of both Pd metal and Pd . An interesting difference
between the silica bead preparation that had seen formic acid and
the silica bead preparation that had not seen formic acid is that 4+ exposure to formic acid seems to shift some of the Pd to Pd 4+ metal. As much as 18% of the Pd is converted to palladium
metal upon exposure to formic acid. Whether this phenomenon is
the culprit of the loss of activity with repeated use in acid
remains to be determined.
The analysis of the palladium surfaces and palladium
chemistry are all very interesting, but as yet the significance of these findings can not be tied to their dynamic catalytic activity. It remains to be seen whether or not the palladium of the Pd/PEl/silica gel can be responsible for the differences in activity between the Pd/PEl/silica gel in acid conditions and the
Pd/PEl/silica beads in acid conditions. Also it is not possible to determine whether catalysis is brought about by palladium metal, an ion of palladium, or a mixture of the two. Further experimentation in the palladium chemistry of these catalysts and how the palladium chemistry changes with use may provide insight into just what is happening to the catalyst with repeated uses in acid. It may be that the +2 or +4 forms of the palladium may not be essential to catalysis and are just formed by exposure to air
(46). Further experimentation in this area might also provide a 117
clue as to the exact active catalytic species in these preparations and perhaps ways to make a better catalyst.
29. Conclusions and Prospectives
The preparation of PEl/silica beads was optimized. It was found that deposition of a minimum coating of PEI followed by crosslinking with glutaraldehyde in THF, reduction with sodium borohydride, acid washing, and base leaching produced a bead of improved stability in acid conditions. A preparation of
PEl/silica beads which involved a minimum coating of PEI followed by crosslinking with glutaraldehyde in THF, recoating with PEI, and base leaching showed the elution of a significant amount of primary amine material in acid conditions. An analysis of the leaching of PEl/silica beads showed that a 1 hour leached support has approximately 20% of the silica core removed by weight.
Experiments presented in this dissertation showed the absolute requirement for the PEI matrix to obtain binding of the 2+ Pd to the PEl/silica beads. The silanols of silica did not 2 + act as ligands. The uptake of Pd by the PEl/silica beads is through interaction with the amine component of the PEl/silica beads.
The palladium catalyst produced from the PEl/silica beads is active in the removal of the Cbz group from Cbz-amino acids. The catalyst can be reproducibly prepared by reduction with NaBH^ in 2+, # water at low temperature. The method of placing the Pd /PEI/ silica beads in ethanol and adding the sodium borohydride in 118
solid form gave large differences in the initial t*ie
catalysts prepared. The catalyst can be reused in formic acid, but the activity decreases with use.
The Pd/PEl/silica gel catalyst is active in the hydrogenation of nitrobenzene to aniline. The catalyst can be reused 10 times with no loss in activity. When the Pd/PEl/silica gel catalyst is reused in formic acid, a large decrease in activity is observed after one or two uses. This activity may be recovered by treatment of the catalyst with sodium borohydride. The differences in catalytic behavior in acid conditions observed between the Pd/PEl/silica gel and the Pd/PEl/silica beads are not explicable. The answer may lie in the differences between the two types of silica or the differences in the state of the Pd on the surfaces of the catalyst.
Studies of the oxidation state of the Pd on the silica catalysts showed the major palladium form on the Pd/PEl/silica gel to be a Pd with some Pd . The Pd/PEI/silica beads contained 4+ Pd metal and Pd . The role of these forms of Pd in the catalytic properties of the two catalysts was not evaluated.
Although our experiments with hydrophobic modifications of the PEl/silicas did not lead to a definable synthetic enzyme analog, the experiments which lead up to their syntheses have yielded information of practical benefit in the preparation and usage properties of the unmodified catalysts. The hydrophobic
PEl/silicas do show altered catalytic properties from the un modified PEl/silicas; however, these altered catalytic properties 119 are not explainable in terms of selective complexation and productive turnover of the non-polar substrate with the hydrophobic groups on the catalyst surface. It may be that the substrate is bound to a hydrophobic region and is not free to react with a region of palladium. In all cases studied, the rate of the reaction for the hydrophobic PEl/silicas was less than the rate of the reaction for the unmodified PEl/silicas.
Two alternative lines of research could develop from these studies. One investigation would focus on the use of PEl/silicas containing hydrophobic groups as ion-exchange resins. Experiments have shown that the hydrophobic PEl/Xama/silica gel shows altered ion-exchange properties (47) relative to the unmodified PEl/silica gel (48). The other area of investigation would focus on a purely chemical investigation of the mechanism of catalysis for the
Pd/PEl/silica catalysts. In particular the exact role of the different palladium forms and their relevance to the catalytic behavior of these catalysts would be intriguing to address. These insights might illuminate the Pd/PEl/silica catalysts’ superiority to Pd/C, Pd(0H)2 and Pd black in reactions employing formic acid as the hydrogen donor. BIBLIOGRAPHY
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