Conference Dates

March 8-13, 2009

Abstract

Numerous studies have shown that the properties of metal catalysts can in principle be fine-tuned by controlling the composition of the metal surface with high precision. The ability to design catalysts capable of high selectivity towards the conversion of a single functional group in a multifunctional molecule is a major objective for heterogeneous catalysis research. This need for high selectivity toward a single functional group is of growing importance in efforts to improve biorefining operations, where biomass-derived multifunctional carbohydrates are key "building block" intermediates that must be converted to a vast range of commodity chemical products such as fuels, pharmaceuticals, food products, and more. This work focuses on results from high resolution electron energy loss spectroscopy (HREELS) and temperature programmed desorption (TPD) experiments combined with selective use of density functional theory (DFT) calculations on single-crystal surfaces under ultrahigh vacuum conditions to study structure-property relations for a series of C4 cyclic oxygenates on catalytic metal surfaces. The objective of this work is to identify methods to tailor surfaces that are able to selectively catalyze conversions of one functional group in the multifunctional molecule.

Two types of cyclic probe molecules have been studied in particular: 3-membered epoxide rings (in which ring-strain is high and the character of the oxygenate function is therefore more reactive) and 5-membered furanone rings (in which the ring is relatively stable). Both the epoxides and furanones contain an unsaturated C=C bond; for many biorefining applications it is desirable to selectively hydrogenate the olefin while keeping the oxygenate functionality intact. In this contribution, we explore the role of surface structure and composition in dictating the reaction pathways for multifunctional C4 cyclic oxygenates on key transition metal and bimetallic surfaces.

Results for the epoxide probe molecule studies indicate differing modes of interaction with different metal surfaces. On a platinum or palladium surface, the epoxide ring opens irreversibly while the C=C functional group has a strong interaction with the surface. However, on a silver surface, the epoxide ring also opens, but can be made to close reversibly. An effective catalyst design strategy, then, is to combine silver on a predominantly platinum or palladium surface in order to create a bimetallic catalyst with high selectivity toward reaction of the olefin while keeping the epoxide ring intact. Recent studies of the chemistry of furanone species indicate that the olefin group interacts strongly with a platinum or palladium metal surface, and therefore is very likely to also determine how the molecule reacts. In this presentation, relationships between catalyst design strategies for epoxides versus furanones will be discussed, as will the likely biorefining reactions that such strategies can impact.

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