Roles of Re and Cs Promoters and Organochlorine Moderators in the Synthesis of Ethylene Oxide on Ag‐based Catalysts

Abstract

Abstract C 2 H 4 −O 2 reactions form ethylene oxide (EO) on Ag nanoparticles dispersed on α‐Al 2 O 3 and promoted with alkali and other elements, with Re and Cs most frequently used in practice. Traces of alkyl chlorides (e. g., C 2 H 5 Cl) and alkanes in much larger amounts are added to reactants to balance the rate of deposition and removal (and the coverage) of Cl adatoms (Cl*) on Ag surfaces. Such Cl adlayers retain Ag surface ensembles that form O 2 ‐derived intermediates that favor EO synthesis, but typically decrease O 2 activation rates. A series of Ag/α‐Al 2 O 3 catalysts (with and without Re or Cs) are used here to examine the role of promoters and Cl moderators through an analysis of the effects of reaction conditions and C 2 H 5 Cl levels using convection‐reaction constructs and a mechanistic formalism that considers O 2 (as chemisorbed O 2 *) as the reactant in EO synthesis through one electrophilic O‐atom, with the second O‐atom (O*) acting as a unselective nucleophile that must be scavenged by a sacrificial reductant (C 2 H 4 or EO); such channels are precluded by any intervening O 2 dissociation events (that form two O*). O 2 consumption rates in C 2 H 4 −O 2 reactions decrease 10‐fold upon exposing Ag/α‐Al 2 O 3 to C 2 H 5 Cl, evincing persistent Cl* species that block active sites and require >10 4 C 2 H 4 oxidation turnovers to be removed. Activation barriers do not change but relative rates of epoxidation versus combustion increase in the presence of this refractory Cl* adlayer, which reduces the number and shrinks the size of available site ensembles without altering their intrinsic reactivity for O 2 activation to form O 2 * but attenuating rates of its subsequent dissociation. Refractory O* adlayers also form on Ag surfaces, as shown by the accumulation of persistent O* species upon exposing reduced Ag particles to N 2 O. Inter‐atom repulsive forces weaken O* binding and destabilize N 2 O decomposition transition states to eventually allow stable N 2 and O 2 evolution for catalytic N 2 O decomposition, which occurs at landing ensembles formed at interstices of refractory adlayers. Cl* forms denser adlayers than O*, as evidenced by a 10‐fold decrease not only to C 2 H 4 −O 2 rates but also to N 2 O stoichiometric and catalytic rates and O* uptakes. The occasional evolution of O 2 , which forms larger landing ensembles that more readily form 2O* from O 2 , occurs with higher frequency from O* than mixed O*/Cl* adlayers, and this effect of Cl* to inhibit (O−O)* dissociation is demonstrated in CO probe reaction studies, which show that highly reactive bound dioxygen species are retained to a greater extent when adlayers comprise both Cl* and O*. Re and Cs do not influence the nature of such adlayers but increase and decrease, respectively, the number of acid sites and selectivity losses via EO combustion. Their respective domains may also block a modest fraction of Ag surfaces without consequences on intrinsic O 2 activation rates or EO selectivities. The effects of Re beyond those reported here may emerge upon synthetic protocols that alter Re location and Ag−Re intimacy to enable channels which utilize both O‐atoms in epoxidation.

Topics & Concepts