Dehydrogenation of n-butane over vanadia catalysts supported on θ-alumina

Dehydrogenation of n-butane was studied over three θ-alumina supported catalysts with different loadings of VOx. The catalysts were characterised by UV–visible spectroscopy in the as-prepared state and the reduced state. The catalyst with a 1% V loading exhibited a 4+ oxidation state after reduction, whereas the catalysts with 3.5 and 8% V loadings were reduced to a 3+ oxidation state. Oxygen chemisorption gave oxidation state values of 3.8 for the 1% catalyst and 2.6 for the 3.5% catalyst. Both the rate and turnover frequency data confirmed that the sample with the highest proportion of polyvanadate species (3.5V) was the most effective dehydrogenation catalyst. Analysis of the deactivation indicated that butane isomerisation was slow, and deactivation of the isomerisation site was faster than that of the dehydrogenation site. The primary product of butane dehydrogenation was 1-butene, which then isomerised to trans-2-butene. More than three-quarters of the deactivation was due to strongly bound reaction intermediates, which can be removed from the surface at room temperature by treatment in 2% O2/Ar. The amount of carbon found on each catalyst after removal of the reaction intermediates was alumina <1V<3.5V<8V<1V<3.5V<8V. Therefore, the VOx species plays a role in generation of the non-C4 carbonaceous deposits, suggesting either that spillover occurs from the VOx to the support or that some VOx sites catalyse the production of coke but do not catalyse dehydrogenation.