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 degrees 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 r(CH2O) = 829 h(-1). In contrast, V/MCM-41 prepared via a surface sensitive grafting method showed only a r(CH2O) 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.