Formation of a Stable Surface Oxametallacycle that Produces Ethylene Oxide

Abstract

Temperature programmed desorption, high-resolution electron energy loss spectroscopy (HREELS), and density functional theory (DFT) were used to investigate the adsorption and reaction of ethylene oxide (EO) on the Ag(111) surface. When EO is dosed onto Ag(111) at 140 K it adsorbs molecularly, desorbing without reaction at approximately 200 K. On the other hand, when EO is dosed at 250 K, the ring-opening of EO is activated, and a stable surface intermediate is formed. This intermediate reacts at 300 K to re-form EO plus a few other products. HREELS and DFT studies suggest that this stable intermediate is a surface oxametallacycle. Moreover, the activation energies observed for the reaction of the oxametallacycle to form EO are in an excellent agreement with the values reported for the steady-state ethylene epoxidation process. This work represents the first demonstration of surface oxametallacycle ring-closure to form EO. Comparison of the spectroscopic results obtained from silver single crystals and supported catalysts strongly suggests that oxametallacycles are important intermediates in silver-catalyzed ethylene epoxidation.

Combined in situ XPS and PTRMS study of ethylene epoxidation over silver

“…This means that this oxygen species, which has been studied many, many times in the past, has no direct concern to the ethylene epoxidation. Its possible role, proposed also in our previous papers [7,[10][11]48], could be a creation of Ag δ+ sites for the adsorption of ethylene [50] that then can react with electrophilic oxygen to ethylene oxide in the reaction:…”

“…The main idea of the model is that epoxidation and combustion go through common intermediate (oxametallacycle) as suggested by Campbell et al [1,2] and Barteau [11,12]. Oxametallacycle, which is produced in reaction of ethylene and electrophilic oxygen, can branch to ethylene oxide and to acetaldehyde.…”

“…Two aspects are mainly under discussion: on one hand the nature of the epoxidizing oxygen [1][2][3][4], and on the other hand the nature of the key intermediate that leads to the formation of ethylene oxide. Both oxametallacycle [11,12] and π-bonded ethylene [3,8,13] are discussed in the literature, but on the basis of the currently available data it is difficult to determine which species (or both) is indeed the intermediate in the epoxidation route. Recent microkinetic modeling investigations [14,15] allowed the elucidation of some aspects of the reaction mechanism.…”

Preparation and particle size effects of Ag/α-Al2O3 catalysts for ethylene epoxidation

“…Computational results supported an oxometallocycle (OMC) mechanism, in which a key step is the formation of an OMC surface intermediate: an ethylene molecule adsorbed on an ensemble of surface oxygen atoms and a metal site [21,22]. This OMC can then form ethylene oxide (EO) or acetaldehyde, of which the latter is subsequently fully oxidized over silver [11].…”

Ag–Cu alloy surfaces in an oxidizing environment: A first-principles study

“…In industrial applications, the catalyst used for this important chemical reaction is silver, and the reaction is carried out at atmospheric pressure and temperatures T =500-600 K. [16] Both the selective (formation of ethylene oxide) and unselective (total oxidation to CO 2 , which occurs via the formation of acetaldehyde) paths have been found to originate from a common intermediate, an oxametallacycle, in which the molecule is bound to both a chemisorbed oxygen atom and a surface atom. [17,18] This intermediate can react to form either ethylene oxide or acetaldehyde with similar activation barriers. On the basis of both first-principles calculations [19] and experiments [20] it has been suggested that if an Ag-Cu alloy, rather than pure Ag, is used as a catalyst, the selectivity toward ethylene oxide will be improved.…”