Structure and support effects on the selective oxidation of dimethyl ether to formaldehyde catalyzed by MoOx domains

The selective oxidation of dimethyl ether (CH3OCH3) to formaldehyde (HCHO) was carried out on MoOx species with a wide range of MoOx surface density and structure supported on MgO, Al2O3, ZrO2, and SnO2. Raman and X-ray absorption spectroscopies were used to probe the structure of these MoOx domains, as they evolved from monomeric species into two-dimensional polymolybdate domains and MoO3 clusters with increasing MoOx surface density. Primary HCHO synthesis rates (per Mo atom) initially increased with increasing MoOx surface density (1.5–7 Mo/nm2) on all supports, indicating that MoOx domain surfaces become more active as two-dimensional monolayers form via oligomerization of monomer species. The incipient formation of MoO3 clusters at higher surface densities led to inaccessible MoOx species and to lower HCHO synthesis rates (per Mo). Areal rates reached constant values as polymolybdate monolayers formed. These areal rates depend on the identity of the support; they were highest on SnO2, lowest on Al2O3, and undetectable on MgO, indicating that the surface properties of polymolybdate structures are strongly influenced by their atomic attachment to a specific support. The catalytic behavior of MoOx domains reflects their ability to delocalize electron density during the formation of transition states required for rate-determining CH bond activation steps within redox cycles involved in HCHO synthesis from dimethyl ether. These conclusions are consistent with the observed parallel increase in the rates of HCHO synthesis and of incipient stoichiometric reduction of MoOx domains by H2 as the domain size increases and as the supports become less insulating and more reducible, and as the energy required for ligand-to-metal electronic transitions in the UV–visible spectrum decreases. HCHO selectivities increased with increasing MoOx domain size; they were highest on Al2O3 and lowest on SnO2 supports. The Lewis acidity of the support cations appears to influence HCHO binding energy. HCHO reactions leading to COx and methyl formate via primary and secondary pathways are favored on weaker Lewis acids (with stronger conjugate bases). The MoO–support linkages prevalent at low surface densities also favor primary and secondary pathways to COx and methyl formate. When reported on a CH3OH-free basis, because of the pathways available for CH3OH oxidation to HCHO and for CH3OCH3CH3OH interconversion, primary HCHO selectivities reached values greater than 95% on Al2O3-supported polymolybdate monolayers.