Catalytic performance of mesoporous VO x/SBA-15 catalysts for the partial oxidation of methane to formaldehyde

Abstract

The ordered hexagonal mesoporous pure silica SBA-15 has been used as a support for preparing highly dispersed V-containing catalysts. VO x /SBA-15 samples with different V loading (1.26–5.54 wt.%) have been prepared by impregnation with an aqueous solution of NH 4VO 3, characterized by N 2 adsorption, UV-Vis spectroscopy, and H 2-TPR and then evaluated for the partial oxidation of methane to formaldehyde with oxygen. For vanadium coverages below the theoretical monolayer capacity (corresponding to ca. 4 wt.% V) the SBA-15 surface was predominantly covered by monomeric and low oligomeric vanadium oxide species, while at higher vanadium coverages agglomerated vanadium entities and even some small amounts of microcrystalline V 2O 5 were also formed. The conversion of methane (at constant temperature and GHSV) passed a maximum at a vanadium loading of 3.85 wt.%, while the maximum of activity typically occurs at lower V contents (1–2 wt.%) in amorphous vanadia-silica catalysts. This feature can be related with the much higher surface area of the mesoporous support, which allows the monolayer capacity to be reached at higher loading than in amorphous silica. The influence of the main reaction parameters (temperature, GHSV, CH 4:O 2 ratio) on the productivity of formaldehyde was studied. A maximum space–time yield (STY) of formaldehyde of about 2.4 kg kg −1 h −1 was obtained in this work for the most active VO x /SBA-15 catalyst (3.85 wt.% V) at 618 °C, GHSV=417,000 l (N) kg −1 h −1, and CH 4:O 2 molar ratio of 8:1. This value of STY HCHO is significantly higher than the maximum STY (ca. 1.3 kg kg −1 h −1) reported for VO x /SiO 2, and comparable to that recently reported for VO x /MCM-41 (ca. 2.2 kg kg −1 h −1), thus confirming the promising use of high surface area mesoporous supports for the POM reaction on vanadia based catalysts.