Chemically Modified Surfaces in Catalysis and Electrocatalysis

14 I n t e r c a l a t i o n of M o l e c u l a r C a t a l y s t s i n L a y e r e d Silicates T. J. PINNAVAIA

Downloaded by CORNELL UNIV on July 21, 2016 | http://pubs.acs.org Publication Date: July 2, 1982 | doi: 10.1021/bk-1982-0192.ch014

Michigan State University, Department of Chemistry, East Lansing, MI 48824

A variety of cationic catalysts species can be intercalated in swelling layered silicate clay minerals such as hectorite. Under appropriate conditions of interlayer swelling by a polar solvent, the immobilized catalyst is accessible for reaction with reagents from solution. Intercalated rhodium phosphine complexes have been found to be active for the hydrogenation of olefins, alkynes, dienes, and prochiralα-enamidesand for the hydroformylation of olefins. Since the reactions occur in solvated interlayers of more-or-less uniform thickness, spacial factors and polarization effects can lead to significant enhancement in substrate selectivity or product distribution. The ability to control interlayer swelling offers the possibility of inducing size selectivity which may not be realized for the metal complex catalysts in homogeneous solution. Large enzyme molecules can also be intercalated in layered silicates. With glucose oxidase as the intercalant, loadings up to 50 wt% can be achieved. The versatility of layered silicates as matrices for the immobilization of molecular catalysts is emphasized. The immobilization of metal complex catalysts on polymers and inorganic oxides has received considerable attention as a means of combining the best advantages of homogeneous and hetereogeneous catalysis (1-6). The swelling layer l a t t i c e s i l i c a t e s known as smectite clay minerals have added an important new dimension to metal complex immobilization. These compounds have mica-type structures i n which two-dimensional s i l i c a t e sheets are separated by monolayers of a l k a l i metal or alkaline earth cations (7). The structure of a typical smectite, hectorite, i s illustrated i n Figure 1. 0097-6156/82/0192-0241 $6.00/0 © 1982 American Chemical Society Miller; Chemically Modified Surfaces in Catalysis and Electrocatalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


242

CHEMICALLY MODIFIED SURFACES

Figure 1. The hectorite structure, an idealized unit cell formula being Na . 6[Lio.ee; Mgs.sJfSis.oofOgofOH^. Key: O, oxygen; • , OH and occasionally fluoride. The tetrahedral sites are occupied mainly by silicon; magnesium and lithium occupy the octahedral sites. 0

Miller; Chemically Modified Surfaces in Catalysis and Electrocatalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

6

14.

PiNNAVAiA

243

Molecular Catalysts in Layered Silicates

Unlike the micas, the interlayer regions occupied by the a l k a l i metal and alkaline earth ions i n smectites can be swelled by the adsorption of water and other polar molecules, and the interlayer ions can be replaced by ion exchange with almost any desired cation* Large differences i n charge density on the s i l i cate sheets contribute s i g n i f i c a n t l y to the differences i n swelling and cation exchange properties of smectites and micas. The charge per unit c e l l for a t y p i c a l smectite i s ^0.7 vs. 2.0 for mica when the unit c e l l i s taken to contain 20 oxide ions and 4 OH groups. The r e l a t i v e l y low charge density on the smectite sheets and the large internal sujface area (y 750 m /g) results i n substantial separation 10 A) between charge centers. Thus a s i g n i f i c a n t fraction of the exchange cations i n the smectite can be replaced by a variety of large complex cations, such as those containing triphenylphosphine ligands. However, i n t e r c a l a t i o n i s not limited to metal complexes. Even large enzyme molecules 70 À i n diameter can be intercalated i n smectites. Since layered s i l i c a t e intercalation compounds have ordered structures, they offer certain advantages over amorphorus metal oxides as s o l i d supports* By varying the p o l a r i t y of the swelling solvent, one can vary the thickness of the interlayer regions i n which the c a t a l y t i c reaction i s taking place. The a b i l i t y to control interlayer swelling offers the p o s s i b i l i t y of inducing size or shape s e l e c t i v i t y which may not be realized for the metal complex catalysts i n homogeneous solution* Cationic rhodiumphosphine complexes have been especially useful i n demonstrating the v e r s a t i l i t y of layered s i l i c a t e intercalation catalysts for the hydrogénation of o l e f i n s , alkynes, dienes, and prochiral ot-eneamides for the hydroformylation of terminal o l e f i n s .


2

Alkene and Alkyne Hydrogénations* Dirhodium acetate complex cations intercalated i n hectorite have been shown to react with triphenylphosphine from methanol solution to form intercalated Rh(PPh3> + species which are catalyst precursors for the hydrogénation of olefins (8): n

PPh (MeOH) > 0

*V

Q A

'>tx

2 R h ( P P h

3>n

a

(1

>

wherein x = 1,2, n 2 or 3, and the horizontal l i n e s represent the s i l i c a t e sheets* Table I compares the results for the hydrogénation of 1-hexene i n methanol with the intercalated and homogeneous catalyst systems. Under the reaction conditions employed, the hydrogen uptake rate i s lower for the intercalated catalyst than for the homogeneous catalyst. However, the intercalated catalyst greatly reduces the extent of 1-hexene to 2-hexene isomerization, r e l a t i v e to homogeneous solution* The a b i l i t y of the intercalated catalyst to i n h i b i t substrate isomerization has been attributed to the existence of a surface equilibrium between a monohydride



244 and dihydride complex: RhH (PPh )^ 2

3

>

abbreviated P-P . Table VI compares the results for the hydroformylation of 1-hexene i n acetone with three different catalyst precursor systems containing P-P* as a ligand (20). For each of the intercalated catalysts, a l l of the a c t i v i t y occurred i n the s o l i d phase; no c a t a l y s t i c a c t i v i t y was observed for the clear f i l t r a t e s . Thus, +

pp

CH

Pn


250


Table V Assymmetric Hydrogénation of Prochiral Olefins with Rh(NBD)(4-Me-(R)-Prophos)


I Substrate

o

' Interc. Catalyst

. C00H ^C—C^ ^NHCOCH^

Optical Y i e l d (%)-

Homo. Catalyst* ~

89.6

92.6

— Reactions were carried out at 25°, 1 atm pressure, i n 95% ethanol.

v

The chemical yields were > 98% i n each case.


14.

PiNNAVAiA

251


Table VI Hydroformylation of 1-Hexene i n Acetone^ Product Distribution (%) Rh Precursor

+

P-P /Rh

n-Heptanal

2-Me-Hexanol

2-Hexene

A. Homogeneous Catalyst [RhCl(COD)]

2

[Rh(CO) Cl] 2


[Rh(COD)]

2

+

4

55

22

23

3

54

26

20

2

60

30

10

B. Intercalated Catalyst [RhCl(COD)]

2

[Rh(CO) Cl] 2

+

2

[Rh(COD)] ~

4

63

23

8

3 2

71

23 23

6

70

0

- 100°C, 600 p s i C0/H (1/1). - This system gave 7% of an unident i f i e d reaction product. 2

the layered s i l i c a t e s not only provide a convenient means of immobilizing the hydroformylation catalyst, but they also provide some chemical advantages over the homogeneous catalysts. The yields of the synthetically more valuable normal chain aldehyde are c o n s i s t e n t l y higher for the intercalated catalysts. Also, the extent of the 1-hexene isomerization to 2-hexene i s lower f o r the intercalated catalyst than for the homogeneous catalyst. Apparently, the r e s t r i c t e d interlayers of the intercalated catal y s t favors the formation of the s t e r i c a l l y less demanding aa l k y l intermediate. Similar s t e r i c factors may also be affecting the isomerization pathway. Enzyme Intercalation. As noted e a r l i e r the intercalation of molecular catalysts i n layered s i l i c a t e s i s not limited to metal complexes. Large enzyme molecules can also be intercalated at pH values below their i s o e l e c t r i c points. Because the large internal surface area of smectite, very large enzyme loadings can be anticipated. For hexagonal close packing of glucose oxidase (M.W. - 160,000), for example, the anticipated loading i s 2.3 g enzyme per g of s i l i c a t e . In practice, loadings up to 1 g enzyme/g s i l i c a t e can be achieved (21). In comparison, t y p i c a l loadings for enzymes immobilized by conventional methods on metal oxides are i n the range 0 . 1 - 5 wt%.


252



The s p e c i f i c a c t i v i t y and longevity of glucose oxidase i n t e r calated i n hectorite i s dependent i n part on the extent of surface coverage. As i l l u s t r a t e d i n Figure 3, the enzyme a c t i v i t y decays by two pathways: a fast pathway which i s loading dependent, and a slow pathway which i s loading independent. The fast decomposition pathway can be almost completely eliminated by incorporating i n the interlayer regions alkylammonium ions which may be acting as hydrogen bonding disruptors. Thus the fast decomposition pathways appears to be due to conformational denaturation of the enzyme through hydrogen bonding with the s i l i c a t e oxygens. The slower decomposition pathway may be due to protein hydrolysis or loss of FAD cofactor.

Figure 3. Specific activity of glucose oxidase intercalated in a smectite layered silicate versus time of aging at 20°C. Loading is 20 g of enzyme/g of silicate. The dashed line shows the specific activity in the presence of tetrabutylammonium ion.

Acknowledgements I wish to acknowledge the contributions of R. Raythatha, J.G.S. Lee, L. Halloran, J . Hoffman, H.M. Chang, F. Farzaneh, W.H. Quayle, and G. Garwood to various aspects of this work. I also wish to thank my colleague Professor M.M. Mort land f o r many useful discussions. P a r t i a l support of t h i s work by the National Science Foundation i s gratefully acknowledged.



14. PINNAVAIA


253

Literature Cited 1. Whitehurst, D.D. Chemtech, 1980, 44. 2. Gates, B.C.; Lieto, J. Chemtech, 1980, 195. 3. Grubbs, R.H. Chemtech, 1977, 512. 4. Hartley, F.R.; Vezey, P.N. Adv. Organomet. Chem., 1977, 15, 189. 5. Yermakov, Yu.I. Catal. Rev.-Sci. Eng., 1976, 13, 77. 6. Bailar, J.C. Jr. Catal. Rev.-Sci. Eng., 1974, 10, 17. 7. Grim, R.E. "Clay Mineralogy", 2nd ed., McGraw-Hill, New York, 1968, pp. 77-92. 8. Pinnavaia, T . J . ; Raythatha, R.; Lee, J.G.S.; Halloran, L.J.; Hoffman, J.F. J Amer. Chem. Soc., 1979, 101, 6891. 9. Schrock, R.R.; Osborn, J.A. J. Amer. Chem. Soc., 1976, 98, 2134. 10. Cady, S.S.; Pinnavaia, T . J . , Inorg. Chem., 1978, 17, 1501. 11. Raythatha, R.; Pinnavaia, T . J . , unpublished results. 12. Quayle, W.H.; Pinnavaia, T.J. Inorg. Chem., 1979, 18, 2840. 13. Schrock, R.R.; Osborn, J.A. J. Amer. Chem. Soc., 1976, 98, 4450. 14. Raythatha, R.; Pinnavaia, T.J. J. Organomet. Chem., in press. 15. Kagan, H.B.; Dang, T.P. J. Amer. Chem. Soc., 1972, 94, 6429. 16. Fryzuk, M.D.; Bosnich, B. J. Amer. Chem. Soc., 1978, 100, 5491. 17. Chang, H.M.; Pinnavaia, T . J . , unpublished results. 18. Mozzei, M.; Marconi, W.; Riocci, M. J. Molec. Catal., 1980, 9, 381. 19. Crabtree, R.H.; Felkin, H. J. Molec. Catal., 1979, 5, 75. 20. Farzaneh F . ; Pinnavaia, T . J . , unpublished results. 21. Garwood, G.; Mortland, M.M.; Pinnavaia, T . J . , unpublished results. RECEIVED November 4, 1981.


Chemically Modified Surfaces in Catalysis and Electrocatalysis

Recommend Documents