HCl Treatment of Mixed-Phase MoVTeNbOx Catalysts for Enhanced Performance in Selective Oxidation of Propane

Abstract

Hydrothermally synthesized mixed-phase MoVTeNbO_x catalysts are active for catalyzing the selective oxidation of propane to acrylic acid but suffer from the presence of the amorphous phase and low specific surface areas. Herein we report that HCl treatment preferentially dissolves the amorphous phase in hydrothermally synthesized mixed-phase MoVTeNbO_x catalysts and increases the catalytic performance. An optimal HCl treatment significantly increases the C₃H₈ conversion from 38.9% to 58.2% without changing the acrylic acid selectivity in the selective oxidation of propane to acrylic acid at 380 °C. The original and HCl treated catalysts exhibit similar apparent activation energies, while HCl treatment increases the specific surface area, surface acid sites, surface V⁵⁺ density, and C₃H₈ and C₃H₆ irreversible adsorption amounts but decreases the C₃H₈ and C₃H₆ irreversible adsorption heats. The C₃H₈ conversion rate is proportional to the surface V⁵⁺ density and C₃H₈ irreversible adsorption amount, and the TOF is measured as 3.31 ± 0.08 × 10^–5 s^–1 at 340 °C. Thus, HCl treatment enhances the catalytic performance of mixed-phase MoVTeNbO_x catalysts mainly by increasing the active site density rather than by increasing the active site activity. Our results provide an effective approach to prepare highly active mixed-phase MoVTeNbO_x catalysts for the selective oxidation of propane to acrylic acid.