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

Abstract

Dispersed two-dimensional MoO_x and VO_x oligomers on Al₂O₃ and SnO_x-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 SnO_x or other reducible oxides (ZrO_x, CeO_x and FeO_x) led to MoO_x domains with higher rates for catalytic DME oxidation and for reduction in H₂, while maintaining the high HCHO selectivity observed on MoO_x/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 SnO_x 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 MoO_x domains supported on bulk crystalline SnO₂. The use of more reducible VO_x domains instead of MoO_x 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 SnO_x similar to those observed on MoO_x-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.