Support and promoter effects in the selective oxidation of ethane to acetic acid catalyzed by Mo-V-Nb oxides

Catalysts based on Mo-V-Nb oxides were examined in bulk and supported forms for the oxidation of ethane to ethene and acetic acid. Bulk Mo0.61V0.31Nb0.08Ox powders showed rates and selectivities similar to those in previous reports. Precipitation in the presence of colloidal TiO2 led to a 10-fold increase in ethene and acetic acid rates (per active oxide) without significant changes in selectivity relative to unsupported samples. Precipitation in the presence of colloidal ZrO2 and Al2O3 suspensions, however, introduced unselective combustion sites without improving ethane oxidation rates. Mo5O14 structures, containing low-valent metal cation centers were detected in bulk Mo0.61V0.31Nb0.08Ox and TiO2-supported samples by Raman and UV-visible spectra and consistent with X-ray diffraction patterns, but not in Al2O3- or ZrO2-containing catalysts. The introduction of trace amounts of Pd (0.0025-0.01 wt.%), as a physical mixture of separate 0.3 wt.% Pd/SiO2, led to the near complete depletion of ethene intermediates and to a significant increase in acetic acid synthesis rate. Small PdOx catalyze ethene oxidation to acetaldehyde, but require the rapid scavenging of these molecules by Mo-V-Nb oxides to prevent acetaldehyde combustion and loss of selectivity. Dispersed VOx domains on TiO2 were able to catalyze all steps required for ethane oxidation to acetic acid. CO, selectivities, however, were much higher than on bulk and TiO2-supported Mo0.61V0.31Nb0.08Ox catalysts. Dispersed MoOx domains were essentially inactive at these reaction conditions but their concurrent presence with VOx increased acetic acid selectivity by titrating unselective sites and stabilizing more reducible VOx species. (c) 2007 Elsevier B.V. All rights reserved.