KINETICS OF N-BUTYLBENZENE DEHYDROCYCLIZATION OVER CHROMIA-ALUMINA

The kinetics of dehydrogenation, cyclization, and isomerization steps in n-butylbenzene dehydrocyclization over precoked chromia-alumina has been investigated at 683-783 K. Apparent kinetic parameters of a pseu dohomogeneous model were calculated from the initial-rate and integral -rate data measured with n-butylbenzene and 4-phenyl-1-butene in an is othermal fixed-bed reactor. Dehydrocyclization was shown to proceed by dehydrogenation of n-butylbenzene to n-butenylbenzenes, by their cycl ization, and by dehydrogenation of the cyclic products to methylindene s and naphthalene. Direct cyclization of n-butylbenzene was of little importance over the precoked catalyst. According to the initial-rate d ata, dehydrogenation to n-butenylbenzenes followed an apparent order o f 0.5 and the kinetic parameters seemed to be applicable also at highe r conversions. The primary dehydrogenation products were 1-phenyl-1-bu tenes, and a surface model based on hydrogen abstraction from the alph a-carbon has been discussed. The initial cyclization rates measured wi th 4-phenyl-1-butene were appreciably lower than those observed in int egral-rate experiments. The discrepancy was tentatively explained by t he higher reactivity of l-phenyl-l-butenes, which are the main constit uents of the n-butenylbenzene fraction at longer contact times. High c onversion runs with n-butylbenzene also showed that methylindenes and -indans were partly dealkylated and probably polymerized. The apparent orders ranged from 1 to 1.5 for 1.6-cyclization and from 0.5 to 0.8 f or 1,5-cyclization. Skeletal isomerization of n-butylbenzene to sec-bu tyl- and sec-butenylbenzenes was rapidly subsided during coking, and o n the coked catalyst at 783 K the reaction followed an apparent order df -0.5.