Switching of selectivity from benzaldehyde to benzoic acid using MIL-100(V) as a heterogeneous catalyst in aerobic oxidation of benzyl alcohol†

A vanadium-centered metal organic framework [MIL-100(V)] was synthesized as a heterogeneous catalyst allowing the selectivity to be switched from almost quantitative formation of benzaldehyde (Bz-CHO) to quantitative formation of benzoic acid (Bz-COOH) by changing only the temperature in the aerobic oxidation of benzyl alcohol (Bz-OH). The aerobic oxidation of Bz-OH was performed using molecular oxygen or air in the temperature range of 60–120 °C. A Bz-CHO formation yield of 98.1% was obtained with quantitative Bz-OH conversion at 80 °C. When the oxidation temperature was set to 100 °C, a Bz-COOH formation yield of 100% was achieved with quantitative Bz-OH conversion. The suitability of a serial reaction mechanism including Bz-CHO formation from Bz-OH and Bz-COOH formation from Bz-CHO as the first and second stage reactions, respectively was investigated for the aerobic oxidation process. The apparent first-order rate constants determined for first and second stage reactions demonstrated that the first-stage reaction was faster with respect to the second one. The proposed kinetic model allowed the calculation of apparent activation energies for Bz-CHO formation from Bz-OH and Bz-COOH formation from Bz-CHO as 77.3 and 149.2 kJ mol⁻¹, respectively. The presence of hydroxyl (·OH) and superoxide anion (O₂˙⁻) radicals in the aerobic oxidation was demonstrated by radical scavenging runs. A mechanism was proposed based on the crystalline structure of MIL-100(V) and the radical types identified by the scavenging runs. This study opens a new path for tuning of selectivity towards Bz-CHO or Bz-COOH, for the first time, using a transition metal based catalyst synthesized by a one-pot hydrothermal reaction.