(509a) Reactions Of Complex Epoxides On Silver Surfaces

Silver-catalyzed direct oxidation processes, such as ethylene epoxidation, have been extensively researched due both to the uniqueness of the process among epoxidation methods and to the commercial significance of epoxide products. Ethylene oxide is the largest volume product generated via selective oxidation by the chemical process industry with annual production in excess of 8x109 lbs [1]. Union Carbide first commercialized a process that converted ethylene directly to ethylene oxide using oxygen and an a-alumina supported Ag-catalyst in 1937 [2]. Despite almost seventy years of industrial practice, however, the reaction pathways of ethylene epoxidation were not well understood until recently [3-9] and the majority of significant advances in Ag-catalyst development had occurred primarily through empirical methods.

With the exception of the small-scale commercial generation (now discontinued) of epoxybutene from Ag-catalyzed selective epoxidation of 1,3-butadiene by Eastman Chemical, production of ethylene oxide directly from ethylene and molecular oxygen remains unique among industrial epoxidation processes [10]. Implementation of direct oxidation chemistry in the production of more complex epoxides, such as propylene oxide, has yet to be realized because there are no catalysts currently capable of epoxidizing propylene, the simplest homolog of ethylene, directly with acceptably high selectivities. Propylene oxide is a valuable product with a global market of 7 billion lbs/year [11] and is produced via indirect epoxidations.

Surface science and computational studies [3-6,12] identified an oxametallacycle species as the central intermediate in Ag-catalyzed epoxidation of both ethylene and 1,3-butadiene. This species controls selectivity through competitive ring-closure to form the epoxide and isomerization to aldehydes. Oxametallacycles have also been synthesized from 2-iodo-ethanol on Ag(110) [13] and Ag(111) [6,14], epoxybutene on Ag(110) [5] and Ag(111) , and styrene oxide on Ag(111) [12].

Surface science techniques and Density Functional Theory (DFT) are used in this study to probe the reactions of complex epoxides on Ag-surfaces. The initial part of this work explores the role of unsaturated substituents in intermediates derived from epoxides and includes the recent study of styrene oxide on Ag(110) [15], while subsequent discussion explores reactions of the intermediate derived from ring-opening isoprene oxide. on Ag(110). Like propylene, isoprene contains allylic hydrogens, and the facile abstraction of these by adsorbed oxygen species makes direct oxidation difficult.

Styrene oxide undergoes activated ring-opening upon adsorption above 200 K to form stable oxametallacycle on both Ag(111) [12] and (110) [15]; the unsaturated phenyl group interacts with the Ag-surface and stabilizes the oxametallacycle relative to that derived from ethylene oxide. Ring-closure and isomerization of the oxametallacycle forms the epoxide and aldehyde isomers near 505 K on Ag(110). Similar to the styrene oxide-derived oxametallacycle on Ag(111), DFT calculations predict that epoxide ring-opens at the carbon bound to the substituent group and adsorbs with the phenyl rings nearly parallel to the Ag(110) surface.

Isoprene oxide also forms a strongly bound oxametallacycle intermediate on the Ag(110) surface. The oxametallacycle undergoes ring-closure to reform isoprene oxide in two peaks at 320 and 460 K when synthesized by epoxide adsorption at low temperatures; the product distribution in these two states is identical and corresponds to the gas-phase cracking pattern of isoprene oxide. Epoxide doses at higher surface temperatures, ca. 300 K, lead to isomerization of the oxametallacycle and desorption of the aldehyde isomer, 2-methyl-2-butenal, in a single peak at 460 K. This work represents the first demonstration of a surface oxametallacycle species derived from an allylic epoxide. The structure of the isoprene oxide-derived oxametallacycle resembles that formed from ring-opening the non-allylic counterpart, 1-epoxy-3-butene, on Ag(110), according to DFT calculations.

Identification of oxametallacycle intermediates from complex epoxides with similar surface chemistry and structure to the active species in Ag-catalyzed epoxidation of ethylene suggests a common mechanism for olefin epoxidation. Through investigation of a variety of epoxides, we hope to obtain a better understanding of the epoxidation pathways and facilitate the development of catalysts capable of oxidizing higher olefins directly.

References

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