ISOMERIZATION/HYDROGEN TRANSFER RELATIVE RATE

Whereas step 1 is stoichiometric, steps 2 and 3 form a catalytic cycle involving the continuous generation of carbenium ions via hydride transfer from a new hydrocarbon molecule (step 3) and isomerization of the corresponding carbenium ion (step 2). This catalytic cycle is controlled by two kinetic and two thermodynamic parameters that can help orient the isomer distribution, depending on the reaction conditions. Step 2 is kinetically controlled by the relative rates of hydrogen shifts, alkyl shifts, and protonated cyclopropane formation, and it is thermodynamically controlled by the relative stabilities of the secondary and tertiary ions. (This area is thoroughly studied see Chapter 3.) Step 3, however, is kinetically controlled by the hydride transfer from excess of the starting hydrocarbon and by the relative thermodynamic stability of the various hydrocarbon isomers. 

The main competing reaction to isomerization of the n-carbenium ion is its hydrogen transfer to the n-paraffin which should then be relatively unreactive. By maximizing the relative rates of isomerization to hydrogen transfer (k,/kH), the yield of branched products, especially branched olefins, should also be maximized. 

Whatever the explanation, there is no doubt that different zeolites even at the same relative aluminum and silicon contents have dramatically different relative rates of isomerization and hydrogen transfer (43). Using the reaction of cyclohexene to form either cyclohexane, cyclopropane, or cyclopropene the relative rates of isomerization for three different zeolite structures were measured for samples prepared at the same Si/Al ratio 12. The results showed significant differences of nearly an order of magnitude in the relative rates of isomerization to hydrogen transfer. The structural details that provide these differences remain elusive. 

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