Patricia Cheung

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

Abstract The selective oxidation of dimethyl ether (CH 3 OCH 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 2 O 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 2 O 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 2 O 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 3 OH-free basis, because of the pathways available for CH 3 OH oxidation to HCHO and for CH 3 OCH 3 CH 3 OH interconversion, primary HCHO selectivities reached values greater than 95% on Al 2 O 3 -supported polymolybdate monolayers.

Effects of Al2O3 support modifications on MoOx and VOx catalysts for dimethyl ether oxidation to formaldehyde

Dispersed two-dimensional MoOx and VOx oligomers on Al2O3 and SnOx-modified Al2O3 supports were examined for selective dimethyl ether (DME) oxidation to HCHO and their structure and reduction rates in H2 were determined using Raman and X-ray near edge absorption spectroscopies (XANES), respectively. Modifying Al2O3 supports with SnOx or other reducible oxides (ZrOx, CeOx and FeOx) led to MoOx domains with higher rates for catalytic DME oxidation and for reduction in H2, while maintaining the high HCHO selectivity observed on MoOx/Al2O3 catalysts. This appears to reflect the higher reactivity of lattice oxygen atoms as Mo–O–M acquires more reducible M cations. On Al2O3 modified with SnOx species at near monolayer coverages (5.5 Sn nm−2) DME oxidation turnover rates (per Mo-atom) were approximately three times greater than on unmodified Al2O3 samples containing predominately polymolybdate domains (∼7 Mo nm−2). The rates of DME oxidation and of reduction by H2 increased in parallel with increasing Sn surface density. HCHO selectivities decreased slightly with increasing Sn surface density, but they were significantly higher than on MoOx domains supported on bulk crystalline SnO2. The use of more reducible VOx domains instead of MoOx also led to higher DME oxidation rates (per V or Mo atom) without significant changes in HCHO selectivity and to effects of Al2O3 modification by SnOx similar to those observed on MoOx-based catalysts. Al2O3 supports with higher surface area led to catalytic materials with similar rates per V or Mo atom and similar HCHO selectivities for a given surface density (∼7 V or Mo nm−2), because of the prevalence of accessible two-dimensional oligomeric domains of the active oxides on both Al2O3 supports at these surface densities. Higher surface area Al2O3 supports, however, led to proportionately higher rates per catalyst mass, as a result of the larger number of active domains that can be accommodated at higher surface areas. These studies provide a rationale for the design of more efficient catalysts for selective DME oxidation to HCHO and illustrate the significant catalytic productivity improvements available from support modifications in oxidation catalysts.