Vanadium-Containing MCM-41 for Partial Oxidation of Lower Alkanes

Abstract

Vanadium-containing mesoporous molecular sieves synthesized by both template-ion exchange (TIE) and direct hydrothermal (DHT) methods have been studied for partial oxidation of lower alkanes. UV–vis and in situ laser Raman spectroscopic studies suggest that the former synthetic method can provide tetrahedrally coordinated vanadium species mainly dispersed on the wall surface of MCM-41, while the latter method leads to vanadium sites mainly incorporated into the framework of MCM-41. H2-TPR measurements show that the vanadium species in the TIE samples can be reduced at lower temperatures than those in the DHT samples. NH3-TPD investigations suggest that weak acid sites mainly exist over MCM-41 along with a small amount of medium ones. The introduction of vanadium by the TIE method increased the amount of weak acid sites, while both weak and medium acid sites of MCM-41 are decreased with introducing vanadium up to a certain content by the DHT method. In the oxidations of ethane and propane, the alkane conversions increase remarkably with increasing vanadium content, and moderate selectivities to ethylene and propylene are obtained over the TIE catalysts. The same catalysts, however, are not selective for the oxidative dehydrogenation of isobutane. On the other hand, propylene and isobutene are obtained with high selectivity over the DHT catalysts with vanadium content exceeding 1 wt% in the oxidations of propane and isobutane, respectively. Acrolein and methacrolein can also be formed respectively with considerable selectivity over the DHT catalysts with lower vanadium content. It is likely that the medium acid sites that remained in these samples play roles in the formation of oxygenates through the adsorption of alkenes or allylic intermediates.