31 research outputs found

    Monomolecular conversion of light alkanes over H-ZSM-5

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    The monomolecular conversion of light n-alkanes (propane to n-hexane) over H-ZSM-5 was investigated between 723 and 823 K. The rates and energies of activation of the individual reactions were determined and a kinetic model for the conversion is presented. The results suggest that carbonium ions are intermediates for all primary reactions. Depending upon the nature of the carbonium ion in the transition state, three parallel primary reactions were identified, leading to hydrogen exchange, dehydrogenation, and cracking. With increasing size of the n-alkane, the rate of reaction increases due to the increase in the adsorption constant of the hydrocarbon. The true energies of activation are independent of the hydrocarbon chain length and the position of the carbon-carbon bond to be cleaved

    Dehydrogenation of light alkanes over zeolites

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    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 state- and 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

    Dehydrogenation of light alkanes over zeolites

    Get PDF
    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 state- and 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
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