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 ) DME oxidation turnover rates (per Mo-atom) were approximately three times greater than on unmodified Al2O3 samples containing predominately polymolybdate domains ( 7 Mo nm ). 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 ), 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.