The conversion of light paraffins to olefins and the secondary reactions of the unsaturated compounds were investigated on H-ZSM5 and H-Y zeolites between 733 and 823 K. Steady stateand transient response-isotope tracing studies revealed that two mechanisms of dehydrogenation are operative. The main pathway is represented by monomolecular, protolytic dehydrogenation. This reaction contributes most to steady state olefin production. Additionally, at the initial stages of the reaction, extra framework aluminum moieties are speculated to participate in high dehydrogenation activity. This pathway is blocked at later stages of the reaction by product (hydrogen) inhibition. The intrinsic rates of protolytic dehydrogenation and olefin desorption range in the same order of magnitude. At high protolytic dehydrogenation rates, olefin desorption represents the rate determining step. Depending on the process conditions, olefins undergo secondary cracking, oligomerization, or isomerization. The latter proceeds via intramolecular rearrangement, possibly via a cyclopropylcarbenium ion at high temperatures and low conversions. At reaction conditions where bimolecular cracking prevails, isomerization is concluded to occur via secondary cracking of di-or oligomers.

Steady-state isotope tracer studies and isotope transient response experiments of n-butane conversion on H-ZSM-5 (Si/Al = 35) were carried out between 673 and 823 K. Among the three main reactions, the rate of H/D-exchange is at least one order of magnitude higher compared to the rates of cracking or dehydrogenation. Its apparent energy of activation is lower than that of the latter two processes. The rates of H/D-exchange are higher for larger molecules than for smaller ones and faster with olefins than with paraffins. Proton exchange proceeds stepwise, i.e., only one hydrogen (deuterium) of the substrate is exchanged with one deuterium (hydrogen) in a single catalytic turnover. A kinetic isotope effect was found for protolytic cracking, but not for dehydrogenation. Protonation of the feed (deprotonation of the zeolite) is concluded to be involved in the rate determining step of cracking.

The conversion of light linear and branched alkanes on two faujasite samples containing different concentrations of free Brcnsted acid sites and extraframework alumina (EFAL) was studied between 733 K and 813 K. Protolytic cracking and bimolecular hydride transfer proceeded solely on BrCnsted acid sites. For cracking of n-alkanes, the variation of the concentration of extraframework aluminum did not affect the catalytic activity per accessible BrCnsted acid site. The activity to dehydrogenation is enhanced in the presence of EFAL and, unlike protolytic cracking, it decreased with time on stream. At high conversions relatively high concentrations of olefins change the selectivity and decrease the turnover frequencies. Compared to n-alkanes, the catalytic activity to convert iso-alkanes is enhanced in the presence of extralattice alumina.

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The conversion of n-hexadecane over fluid catalytic cracking (FCC) catalysts was studied at 788 K and compared with the conversion of n-hexane over these FCC catalysts and the conversion of a vacuum gas oil from the micro activity test (MAT). The product distribution could be fully explained by reaction pathways identical with those in n-hexane conversion (i.e., protolytic cracking, dehydrogenation and hydrogen transfer). The rates via all three reaction pathways decreased at long time on stream. As with n-hexane, two reaction pathways to dehydrogenate n-hexadecane were identified; one that is affiliated with the protolytic activation over strong Brønsted acid sites, and the other that is catalyzed by Lewis acid sites and rapidly deactivates with time on stream. With time on stream, the rate of aromatic formation in parallel with coking markedly decreased within the first 2500 s. n-Hexadecane conversion varied in parallel with the MAT activity. The octane numbers observed in the MAT test were directly proportional to the iso/n-paraffin ratio and the paraffin/olefin ratio of n-hexadecane cracking. The rate of conversion to isoalkanes (characteristic of hydride transfer) is directly correlated to the amount of coke formed in the MAT experiments.

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