The (oxidative) dehydroisomerization of n-butane to isobutene - Effect of butadiene on catalyst deactivation

The oxidativc dehydroisomerization of n-butane to isobutene was studied over physical mixtures of a supported vanadia-niobia mixed oxide with the zeolites ZSM5, FER and MCM36. The zeolites converted a fraction of 10 to 30% of the butènes formed on the vanadia-niobia catalyst to isobutene. The rate of isobutene formation decreased very rapidly, however, due to the presence of butadiene in the reaction mixture. The deactivation was aggravated by the fact that zeolites themselves have some oxidative dehydrogenation activity and transformed butène to butadiene in the presence of oxygen. Only ZSM5 showed a fairly stable activity and selectivity due to its two-dimensional pore-structure, which is least sensitive to deactivation. In dehydroisomerization experiments over Pt-ZSM5 in the presence of H2, the butène isomerization activity was perfectly stable. This was attributed to (i) the presence of H2 and Pt and (ii) the higher ratio of butène to butadiene in the reaction mixture compared to oxidative dehydroisomerization.