Hydrocarbon cracking and isomerisation over acidic catalysts

Hydrocarbon cracking and isomerisation over acidic catalysts 

Cyclohexane dehydrogenation represents another classical example for isotopic studies. Balandin s sextet mechanism predicted direct dehydrogenation of cyclohexane over several metals, assuming a planar reactive chemisorption of the reactant. Cyclohexene is also readily dehydrogenated to benzene. The use of hydrocarbons labelled with established the true reaction pathway. T6tenyi and co-workers[ °di] reacted a mixture of [ 0]-cyclohexane and inactive cyclohexene on different metals and measured the specific radioactivity of the fractions (cyclohexane, cH, cyclohexene, cH= and benzene, Bz) in the product at low conversion values (Table The 

The reaction hexenes — hexadienes was demonstrated without using radiotracers both on oxide and metal catalysts, Nil 1 and Ptj l Mixtures containing [ CJ-hexene contributed to the clarification of the further reaction pathway. These studies showed that neither the hexene cyclohexane nor the hexene — cyclohexene ring closure pathway took place.Table 2 indicates that radioactivity appeared in both the hexatriene and 1,3-cyclohexadiene fractions when their inactive form was admixed to radioactive hexene. The aromatisation of both inactive components was much more rapid than that of hexene, therefore their specific radioactivities showed very low absolute values, however, these were still higher than that of benzene produced mainly from these non-radioactive precursors. The true precursor of ring closure should have been cis-cis-1,3,5-hexatriene. Its ring closure takes place without any catalyst from 513 The stepwise dehydrogenation of open-chain hydrocarbons produces cis- and trans-isomers of alkenes and alkadienes. Any c s-c s-triene 

The above results are valid if the equilibration between the adsorbed and gas phase of corresponding compounds (feed, intermediates and/or products) is more rapid than their further reaction. l In the absence of sufficient hydrogen, this may not be the situation. Davis did not reject the triene intermediates but criticised the stepwise dehydrocychsation as described above, since he doubted that the requirement of gas urface equilibration is fulfilled under every experimental condition. The desorption of surface intermediates with more double bonds is hindered, compared with their further reaction. Davis s argument that the aromatic is as unsaturated as the triene (Ref. 18) is true and so is the second half of the sentence, and readily desorbs as a product , but the two treatments should be separated. Benzene is an end product, thus it will not react further before its hindered desorption takes place. In fact, the degree of dehydrogenation of the surface benzene precursor can be too deep, thus their removal to the gas phase is a hydrogenative process, for example 

The reality of such processes has been confirmed experimentally by at least two other independent methods. Temperature Programmed Reaction — TPRI I and Transient Response Method.  

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