Oxidation of butane and butadiene to furan over vanadia-silica mixed oxides

Vanadia-silica mixed oxides (aerogels and xerogels), derived from the sol-gel preparation route, were tested in the partial oxidation of n-butane and 1,3-butadiene to furan. The dependence of the furan selectivity on morphological properties and vanadium dispersion in the silica matrix has been studied. The catalysts were characterized by N-2 and Ar physisorption, AAS, XPS and FTIR spectroscopy. Surface acidic centers of the materials were assessed by means of DRIFT-measurements of adsorbed NH3 probe molecules. The structural properties, mainly influenced by the drying procedure of the sol-gel samples, had a major influence on the catalytic performance of the sol-gel materials. Aerogels were the most selective catalyst, converting butane to furan with only 3% selectivity. The maximum furan selectivity in butadiene transformation was about 25%, but considerable coke formation was also observed. Microporosity, found in conventionally dried sol-gel materials seems to be unfavorable for furan selectivity. The presence of Bronsted acidic sites was proved to be necessary for furan formation. Studies involving V2O5, SiO2, and vanadia-silica low-temperature aerogels with 5-20% vanadia contents revealed that the selectivity to furan was diminished by silica due to its intrinsic high activity for total oxidation and cracking. A comparison to literature data indicated that vanadia-silica mixed oxides and vanadia grafted on silica are markedly less selective towards furan than VPO catalysts. (C) 1998 Elsevier Science B.V.