Synthesis, Structure, and Reactions of Stable Oxametallacycles from Styrene Oxide on Ag(111)

Abstract

Styrene oxide undergoes an activated ring opening on Ag(111) at temperatures above 200 K. The product of this reaction is a stable oxametallacycle intermediate. The structure of this species has been obtained by density functional theory calculations and the computed vibrational spectrum is consistent with the experimental spectrum obtained using high-resolution electron energy loss spectroscopy. The oxametallacycle formed by ring-opening styrene oxide is structurally analogous to that previously observed for ring opening of epoxybutene on Ag(110) and represents the largest member of this adsorbate structure class yet isolated. In both cases, the epoxide ring opens at the carbon bearing the pendant unsaturated group, and the pendant group (phenyl in styrene oxide) is oriented nearly parallel to the surface plane. The oxametallacycle formed from styrene oxide reacts at 485 K to regenerate styrene oxide plus small amounts of phenylacetaldehyde. This peak temperature is similar to that previously reported for generation of styrene oxide from adsorbed styrene and oxygen atoms on Ag(111), suggesting that the epoxidation proceeds via the oxametallacycle intermediate isolated in the present work.

Investigation of Ethylene Oxide on Clean and Oxygen-Covered Ag(110) Surfaces

“…This approach permits consistent comparison of the barriers for reactions of different oxametallacycle species on different surfaces. The peak shapes for reaction of epoxybutenederived and styrene oxide-derived oxametallacycles at higher temperatures on Ag(111) and (110) surfaces are consistent with first order processes [5,8,9,24], but those for EO-derived oxametallacycles are less consistent owing to the procedures for isolating these peaks from the molecular state, as noted above. Nevertheless, the barriers determined are consistent with DFT calculations and previous results on the (111) surface.…”

“…However, without this step, it was difficult to achieve the desired separation of the 280 K peak from the molecular states in the vacuum system used for these experiments. Wu [3,5,9,24] generated strongly-bound oxametallacycle intermediates by ringopening epoxides on Ag surfaces and reported that activation of this process required adsorption at surface temperatures above 200 K; the oxametallacycles underwent ring-closure during TPD to reform the parent epoxides at temperatures well above those reported for desorption of the molecular epoxide states. The similar surface chemistry of the intermediate derived from ethylene oxide on Ag(110) to that of the oxametallacycle formed from ethylene oxide on Ag(111) suggests that the species formed on the Ag(110) surface is also an oxametallacycle.…”

“…Ethylene oxide is the largest volume compound produced by the chemical process industry via selective oxidation, with annual production valued at close to $9 billion USD ([1.4 9 10 10 kg) [2]. Despite over 70 years of industrial practice, the mechanism of the reaction remained obscure until recently [3][4][5][6][7][8][9], and the majority of significant advances in Ag-catalyst development occurred primarily through empirical approaches [10]. Silver, however, remains the cornerstone of commercial catalysts for ethylene oxide production due to its unique ability to selectively epoxidize ethylene with high selectivity.…”

“…Surface science and computational studies [3][4][5][6][7][8][9] have circumvented these limitations and have identified the central intermediate in epoxidations of both ethylene and butadiene by examination of the reverse reactions. This intermediate is a surface oxametallacycle that can be formed in UHV experiments by opening the epoxide ring.…”

“…The aldehyde has been proposed as the gateway species to combustion products [21]. Stable, spectroscopically verified oxametallacycles have been synthesized from 2-iodo-ethanol on Ag(110) [22] and Ag(111) [6,23], ethylene oxide on Ag(111) [3], epoxybutene on Ag(110) [5] and Ag(111) [8], and styrene oxide on Ag(110) [24] and Ag(111) [9].…”