CRITICAL INFLUENCE OF THE AMORPHOUS SILICA-TO-CRISTOBALITE PHASE TRANSITION ON THE PERFORMANCE OF MN/NA_2WO_4/SIO_2 CATALYSTS FOR THE OXIDATIVE COUPLING OF METHANE

摘要

Currently, natural gas plays only a relatively small role as a feedstock for thechemical industry, although it has long been recognised that oxidative coupling of methane (OCM) to C_2 hydrocarbons, especially ethylene, is an attractive possible route for CH_4, usage. To this end, a key technological goal is the identification of suitable catalysts capable of high ethylene selectivity at useful levels of methane conversion. Since the pioneering work of Keller and Bhasin, the subject has received a great deal of attention, although there is little consensus about the identity of the active surface species nor about the reaction mechanism. Keller and Bhasin tested 26 different catalyst formulations at temperatures between 773 K and 1273 K; these consisted of metal oxides supported on alumina. Subsequently, a large number of oxide-based systems has been investigated, and effective OCM catalysts may be broadly classified into irreducible metal oxides, rare earth oxides and reducible metal oxides. Additional complexities are introduced by the use of promoters, supports and composites, and there is continuing controversy about the extent to which homogeneous gas phase chemistry contributes to the overall conversion of methane into ethane, ethylene carbon dioxide and water. Although the very wide range of conditions used for catalyst testing by different authors makes quantitative comparisons difficult, the indications are that the most promising materials show performances on the edge of commercial usefulness. A mechanism involving gas phase coupling of surface-generated methyl radicals to yield ethane, followed by oxidative dehydrogenation of the latter to yield ethylene, seems to be generally accepted for all catalysts. There is also broad consensus that deep oxidation to yield CO_2 is a predominantly heterogeneous process. Useful reviews are to be found in references. Recently, Fang et al identified 1.9 wt% Mn/ 5% Na_2WO_4/SiO_2 as a promising OCM catalyst. They proposed tetrahedral WO_4 surface species with one W=O and three W-O-Si surface bonds as the OCM active site, with manganese oxide enhancing the exchange between gaseous and lattice oxygen. Subsequently, this model was elaborated to include a redox mechanism involving lattice oxygen ions and the W~(6+)/W~(5+) couple. However, although it is true that the formation of trigrafted (SiO)_3W=O units is plausible on, say, the (111) face of α-cristobalite, the results do not permit digrafted or monografted species to be ruled out. The same catalyst was studied by Lunsford et al [1], although rather little agreement emerged as to the roles of the various chemical components in the full trimetallic working system. Moreover, the possible role of the alkali component appears to have been entirely overlooked by both groups. Here, we demonstrate that the presence of Na is critically important for the genesis of an active and selective OCM catalyst. This is achieved by progressively "taking apart" the trimetallic system. In particular, it is shown that the alkali appears to play a dual role as both structural and chemical promoter and that the presence of Mn is not crucial for obtaining high ethylene yields.