Gas-phase hydrogenation of hexanoic acid over P–Mo–V heteropoly compounds in comparison with the constituent Mo and V oxides

Abstract Bulk Keggin heteropoly acids (HPAs) H 3+ n [PMo 12− n V n O 40 ] ( n  = 0–2) and their Cs + salts catalyse the gas-phase hydrogenation of hexanoic acid at 350 °C and 1 bar H 2 pressure, yielding hexanal together with 6-undecanone and C 4 –C 7 hydrocarbons as the main products. Catalyst acidity (controlled by Cs substitution) has crucial effect on reaction selectivity. Partially substituted Cs salts exhibit high aldehyde selectivity (up to 76% at 70% conversion). Further increasing Cs substitution in HPAs, increases the selectivity to 6-undecanone up to 71–76% at the expense of hexanal and hydrocarbons. Initially crystalline, the post-reaction catalysts become amorphous, with their surface area significantly reduced. This shows that the as-made heteropoly compounds are catalyst precursors rather than the true catalysts. The constituent V 2 O 5 and MoO 3 oxides make more active catalysts for hexanoic acid hydrogenation than the P–Mo–V heteropoly compounds. Under reaction conditions, the bulk MoO 3 reduces to MoO 2 , which performs steadily to give 75% hexanal selectivity at 95% conversion. The bulk V 2 O 5 , inactive itself, slowly, over 26 h, reduces to the active V 2 O 3 oxide, which shows excellent hydrogenation performance: >97% hexanal selectivity at 80–83% conversion over 72 h on stream.