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

Abstract

The selective oxidation of dimethyl ether (CH 3OCH 3) to formaldehyde (HCHO) was carried out on MoO x species with a wide range of MoO x surface density and structure supported on MgO, Al 2O 3, ZrO 2, and SnO 2. Raman and X-ray absorption spectroscopies were used to probe the structure of these MoO x domains, as they evolved from monomeric species into two-dimensional polymolybdate domains and MoO 3 clusters with increasing MoO x surface density. Primary HCHO synthesis rates (per Mo atom) initially increased with increasing MoO x surface density (1.5–7 Mo/nm 2) on all supports, indicating that MoO x domain surfaces become more active as two-dimensional monolayers form via oligomerization of monomer species. The incipient formation of MoO 3 clusters at higher surface densities led to inaccessible MoO x 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 SnO 2, lowest on Al 2O 3, 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 MoO x 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 MoO x domains by H 2 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 MoO x domain size; they were highest on Al 2O 3 and lowest on SnO 2 supports. The Lewis acidity of the support cations appears to influence HCHO binding energy. HCHO reactions leading to CO x 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 CO x and methyl formate. When reported on a CH 3OH-free basis, because of the pathways available for CH 3OH oxidation to HCHO and for CH 3OCH 3CH 3OH interconversion, primary HCHO selectivities reached values greater than 95% on Al 2O 3-supported polymolybdate monolayers.