Influence of hydrothermal ageing time on the performance of in situ prepared VMCM-41 catalysts in the selective oxidation of methane to formaldehyde

VMCM-41 catalysts, prepared at different hydrothermal ageing times, were applied for the selective oxidation of methane to formaldehyde. An increase in hydrothermal ageing time leads to higher V content as well as higher content of oligomeric VOx species in the fresh catalysts. The VMCM-41 catalysts undergo structural changes on stream at 650 °C, positively affecting the catalytic performance. A formaldehyde selectivity up to 38% at methane conversion of about 4% was achieved over a catalyst synthesized with hydrothermal ageing time of 3 h, which corresponds to a high formaldehyde production rate of rCH2O = 829 h−1. In contrast, V/MCM-41 prepared via a surface sensitive grafting method showed only a rCH2O of about 323 h−1 under the same conditions. Factors for the superior behavior of VMCM-41 are i) the generation of monomeric VOx on stream which promote formaldehyde formation as well as ii) lower acidities of the restructured catalysts reducing further formaldehyde decomposition. [Display omitted] •VMCM-41 samples were prepared hydrothermally at different hydrothermal ageing times.•Rate of formaldehyde production is around 2.5 times higher than that over grafted VOx.•Monomeric VOx species and lower activity of VMCM-41 lead to the superior performance.