Low-temperature o-xylene oxidation over KOH pre-modified OMS-2 catalyst: Dual effect of K on oxygen activation and carboxylate intermediate desorption

Abstract

A defective KR-OMS-2 catalyst was prepared by refluxing an OMS-2 precursor in KOH solution and investigated for the deep oxidation of o-xylene. The activity test results showed that the T100, T50 and T20 over KR-OMS-2 were 160, 146 and 132 °C, respectively, which were 20, 16 and 20 °C lower than that over unmodified OMS-2 (500 ppm o-xylene/air, W/F = 0.6 g s mL−1). The characterization results demonstrated the generation of Mn vacancies (VMn). K+ anchored to nearby VMn for charge balance and formed bonds with lattice oxygen, which deformed the OMS-2 structure and generated additional surface OH groups. As a result, surface active oxygen species increased, and the low-temperature reducibility of the catalyst and the replenishment capacity of molecular oxygen were improved. Moreover, the surface acid-base properties were modified to improve the desorption of carboxylate intermediates. In situ DRIFTS revealed that the synergistic effect of surface oxygen species and surface OH groups first induced the oxidation of aromatic rings to produce maleate. Then, gaseous oxygen formed highly-active oxygen species, oxidizing maleate to formate, and finally CO2 and H2O. This reaction pathway was more efficient. This study provides a useful method for BTX catalytic oxidation at low temperatures.