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J . Am. Chem. SOC.1991, 113, 1636-1639
Carbon Molecular Sieves as Catalysts and Catalyst Supports Gerald C. Grunewald and Russell S. Drago* Contribution from the Department of Chemistry, Unhersity of Florida. Gainesville, Florida 3261 1. Received May 29, 1990
Abstract: Ultrahigh surface area (>2500 m 2 / g ) carbon molecular sieves (CMS's) a r e shown to be very effective catalysts and catalyst supports for the oxidative dehydrogenation and dehydration of a variety of substrates. Studies of methanol, ethanol, 1 - and 2-propanol, and propanal provide mechanistic insight concerning the reactivity of these materials. T h e activities a r e superior t o many inorganic oxide based systems. As catalysts, CMS systems a r e shown to have t h e ability t o function via hydride or hydrogen atom abstraction mechanisms, depending on the nature of the substrate. As catalyst supports, a synergism is demonstrated between the CMS support and metal dopants, which enables the system t o have greater activity than that of either of the constituents alone. T h e highly reactive surface, the ability to disperse and stabilize metal clusters, and the extraordinary adsorption capabilities of the CMS materials a r e the key contributors to their high activity. One of the catalysts studied (a 15% MoO,/CMS system) has pronounced activity toward methanol oxidation: 70% of the substrate is converted to methyl formate in a single pass with over 95% selectivity. A further significant finding involves the metal-catalyzed conversion of the carbonaceous support itself t o small molecular weight products via reactions with methanol fragments. This finding has fundamental implications in heterogeneous catalyzed CO reductions and in the conversion of methanol to gasoline in zeolites.
Introduction The introduction of synthetic zeolites in the early 1960s sparked a g r e a t deal of research in m a t e r i a l s science and catalysis.' In t h e ensuing years, hundreds of patents, books, and papers have described the application of zeolites t o a plethora of molecular sieve separation processes and shape-selective catalysis systems. Currently a new class of m a t e r i a l s has been discovered t h a t has the potential to equal or surpass the novel properties and reactivity of aluminosilicate compounds. Carbon molecular sieves ( C M S ' S ) ~ with extraordinarily high surface areas (>2500 m 2 / g ) and relatively uniform pore sizes have recently been ~ y n t h e s i z e d . ~Initial investigations have shown these materials to be very effective in gas-separation ~ t u d i e s , ~in- ~pressure-swing absorption experim e n t ~and , ~ in a variety of catalytic systems.*-" Despite these encouraging preliminary results, there have been surprisingly few f u n d a m e n t a l studies of t h e reactivity and properties of t h e
Table I . Reactivity of C, Oxygenated Substrates over CMS
CMS-based m a t e r i a l s as catalyst supports.12-17 We have recently reported the reactivity of CMS catalysts in the oxidative dehydrogenation of ethylbenzene t o styrene.I8 We are particularly interested in obtaining information t o characterize t h e CMS catalyst as both a reactive species and a s u p p o r t with possible synergistic interactions in catalytic processes. In this light,
our major focus is on factors t h a t affect how the catalyst adsorbs and transforms various substrates, how t h e chemical structure of t h e substrates affects t h e reactivity shown by the support, and how various promoters interact with t h e CMS t o give rise to novel synergism. For this purpose, we have chosen t h e oxidation of alcohols and aldehydes as s u b s t r a t e s for s t u d y because their mechanisms of oxidation are reasonably well understood. In t h e process, a low-temperature oxidation of methanol t o m e t h y l formate was found. M e t h y l formate is becoming increasingly i m p o r t a n t a s a s t a r t i n g material for t h e preparation of a c e t i c dimethylformamide,22 and ethylene
( I ) Breck, D. W. Zeolite Molecu1ar Sieoes; Wiley: New York, 1974. (2) The name carbon molecular sieve is broadly applied to a wide range of carbon-based adsorbants, which vary greatly in their molecular sieving ability. As one reviewer pointed out, these materials are perhaps better described as ultramicroporous carbons to distinguish them from crystalline inorganic molecular sieves such as zeolites. (3) (a) Konnert, J.; D'Antoniw, P. Carbon 1983, 21, 193. (b) Marsh, H.; Crawford, D.; O'Grady, T.M.; Wennerberg, A. N. Carbon 1982, 15, 419. (4) Wennerberg, A. N.; O'Grady, T. M. U S . Pat. 4,082,694, 1978. ( 5 ) Ruthven, D. M.; Raghavan, N. S.;Hassan, M. M. Chem. Enn. Sei. 1986, 41, 1325. (6) Koresh, J. E.: Sofer. A. Sep. Sci. Techno/. 1983, 18, 723. (7) Schumacher. H.: Juentaen. H.; Preuss, E.; Zuendorf, D.; Hodek, W.; Romky, I . US. Pat. 4,261,831, 1981. (8) Hassan, M.M.: Raghavan. N . S.;Ruthven, D. M. Chem. Eng. Sei. 1987, 4 2 , 2037. (9) Dessau. R. M. U.S. Pat. 4,413,154, 1983. (IO) Connolly, J. F.; Wennerberg, A. N.; Waters, R. F. US. Patent 4,478,954, 1984. (II ) Foley, H. C. Novel Carbon Molecular Siece Catalysts for Wax Suppression in the Fischer-Tropsch Reaction; DOE Contract No. DEAC22-84 PC 70031, Final Report, 1987. See also: Foley, H. C., Flank, W. H., Whyte, T. E., Eds.; ACS Symposium Series 368: American Chemical Society: Washington, DC, 1988; pp 335-360. (12) Kashiwase, M. Jap. Pat. JP 61/191510, 1986. (13) Ida. S.;Matsuoka, M.; Matsufuji, Y . Jap. Pat. JP 61/68312, 1986. (14) Wennerberg, A. N . Eur. Pat. Appl. EP 120, 604, 1983. (15) Connor, H. Ger. Pat. DE 3006105, 1980. (16) Song, G.; Jiang, Z. Sepu, 1987. 5 , 58. ( I 7) Surinova. S. 1.; Kostomarova. M. A,; Petukhov, S.S.Zh. Prikl. Khim. (Leningrod) 1987. 60, 640. (18) Grunewald. G. C.; Drago, R. S. J . Mol. Catal. 1990, 58. 227.
0002-7863/91/ 15 13- I636$02.50/0
Catalyst" substrate
products
selectivityb (%)
1 -propanol
propene propanol propene acetone acetaldehyde ethanol ethylene
63 13 70 20 69 10 8
2-propanol propanal
conversionC(%) 40 46 48
acetone
350 "C) with a H z O feed (Le., in the absence of methanol as a substrate), no hydrocarbons are formed. The degradation product is simply COz. Thus, the C M S fragments actually form new bonds with the surface methanol moieties. Similar processes may be involved with carbonaceous deposits formed on the surface of heterogeneous catalysts in the conversion of methanol to gasoline with ZSM-5 and in heterogeneous C O reduction systems for Fischer-Tropsch processes.
Conclusions Ultrahigh surface area C M S materials have been shown to possess novel reactivity capable of hydride and hydrogen atom abstractions in the oxidative dehydrogenation and dehydration of a variety of substrates. These materials have also been shown to be excellent supports for metal dopants. The dopant/support catalytic system is a synergistic one, which exhibits an activity greater than the sum of those of the constituents. The highly reactive nature of these materials, together with their excellent dispersion and adsorption properties, is shown to make them quite attractive supports for catalytic applications. Novel reactivity of the support with an intermediate generated from methanol oxidation has implications for both Fischer-Tropsch and methanol to gasoline reaction mechanisms. Acknowledgment. We acknowledge support of this research by the National Science Foundation and the US. Army CRDEC. Professor H. Foley also provided helpful comments on the paper. Registry No. C, 7440-44-0; EtOH, 64-17-5; PrOH, 71-23-8; i-PrOH, 67-63-0; H 3 C C H 2 C H 0 , 123-38-6; M e O H , 67-56-1; ( M e 0 ) $ H 2 , 10987-5; MOO,, I3 13-27-5; ( N H 4 ) 2 M 0 0 7 .I3 1685-38-6; Na2MoOp,763195-0; H C 0 2 M e . 107-31-3.
