DEHYDROGENATION OF LIGHT ALKANES OVER ZEOLITES

The conversion of light paraffins to olefins and the secondary reactio ns of the unsaturated compounds were investigated on H-ZSM5 and H-Y ze olites between 733 and 823 K. Steady state- and transient response-iso tope tracing studies revealed that two mechanisms of dehydrogenation a re operative. The main pathway is represented by monomolecular, protol ytic dehydrogenation. This reaction contributes most to steady state o lefin production. Additionally, at the initial stages of the reaction, extra framework aluminum moieties are speculated to participate in hi gh dehydrogenation activity. This pathway is blocked at later stages o f the reaction by product (hydrogen) inhibition, The intrinsic rates o f protolytic dehydrogenation and olefin desorption range in the same o rder of magnitude. At high protolytic dehydrogenation rates, olefin de sorption represents the rate determining step. Depending on the proces s conditions, olefins undergo secondary cracking, oligomerization, or isomerization. The latter proceeds via intramolecular rearrangement, p ossibly via a cyclopropylcarbenium ion at high temperatures and low co nversions. At reaction conditions where bimolecular cracking prevails, isomerization is concluded to occur via secondary cracking of di-or o ligomers. (C) 1997 Academic Press.