Abstract
Preparation of biphenyl by vapor phase catalytic dehydrogenation of cyclohexylbenzene over chromia-alumina, and the effect on reactivity of the alkyl substituent attached to the phenyl ring in cyclohexylbenzene were studied.
The catalysts used were a commercially available catalyst of the chromia-alumina type (Cr2O3 12-13wt%, MgO 2-3wt%) and the same catalyst alkali-treated. The benzene solutions of cyclohexylbenzenes were used as reactants.
Favorable conditions for dehydrogenation of cyclohexylbenzene to biphenyl were: temperature, 450-550°C and contact time, 30-40g-cat•hr/mol. Under these conditions the conversion of cyclohexylbenzene and selectivity for biphenyl were 70-80% and 94-97%, respectively.
In the catalysis of cyclohexylalkylbenzene, in which an alkyl substituent was attached to the phenyl ring the dehydrogenation and cracking reactions proceeded competitively. m- and/or p-alkylated cyclohexylbenzenes were preferentially dehydrogenated. On the other hand, cracking took place substantially in the reaction of cyclohexylpolymethylbenzenes in which at least one of the ortho positions was substituted by a methyl group. Especially, cyclohexyl-2, 4, 6-trimethylbenzene was selectively cracked.
When the chromia-alumina catalyst was treated with alkali, its cracking activity was repressed almost completely.
Based on these results, catalytic activities of chromia-alumina catalyst and reaction schemes were proposed.
