Impact of VAPO molecular sieve configuration on catalytic oxidation of ethyl lactate to ethyl pyruvate and its reaction mechanism

The presence of the template agent within the pore or cage during the synthesis process of the molecular sieve plays a crucial role in the formation of its porous structure. Using triethylamine and diisopropylamine as structure-guiding agents, VAPO molecular sieve with AFI and AEL topologies were synthesized by hydrothermal method. The physical and chemical properties of hierarchical VAPO-5 and VAPO-11 molecular sieves were characterized by XRD, SEM, N2-adsorption desorption, UV–Vis, XPS, NH3-TPD and Py-IR, etc. It was verified that V isomorphically displaced the P-site of the molecular sieve in pentavalent form, and more acid sites were obtained on the surface of VAPO-5 molecular sieve. This is closely related to the performance of the catalytic oxidation of ethyl lactate to ethyl pyruvate. The activity evaluation showed that the VAPO-5 molecular sieve outperformed the VAPO-11 molecular sieve in terms of catalytic performance. The theoretical study on the reaction mechanism showed that the rate-determining step energy barriers of the catalytic oxidation of EL to EP were 118.7 kJ/mol and 150.18 kJ/mol, respectively. They were consistent with the apparent activation energies of 98.92 kJ/mol and 119.24 kJ/mol obtained by kinetic experiments. The conjecture of the reaction mechanism was confirmed in which the VAPO molecular sieve adsorbs H2O2 molecules and produces surface reactive oxygen species, thus promoting the catalytic oxidation of EL to EP.