Cyclohexene synthesis by catalytic cracking of cyclohexylarenes.

Abstract

The cracking behavior of cyclohexylarenes over various catalysts was investigated to develop an effective method for cyclohexene synthesis. The reaction was carried out in the vapor phase under atmospheric pressure using a conventional fixed bed flow apparatus. Cyclohexylbenzene (CHB) was cracked quantitatively over silica-alumina catalysts, but the main products were methylcyclopentenes and only a small amount of cyclohexene was obtained. By loading molybdena on the silica-alumina catalysts, the selectivity of cyclohexene increased but the conversion of CHB decreased, and significant amounts of methylcyclopentenes and some biphenyl, dehydrogenated product of CHB, were found. On the other hand, 2, 4-dimethyl-1-cyclohexylbenzene (2, 4-DMCHB) and 2, 4, 6-trimethyl-1-cyclohexylbenzene (2, 4, 6-TMCHB) compared with CHB were more easily cracked over the MoO3-SiO2-Al2O3 catalysts, but methylcyclopentenes still were considerably produced. The selective cracking of 2, 4, 6-TMCHB to cyclohexene occurred on NaOH-Al2O3 catalysts. Conversion and selectivity under the most suitable reaction conditions were 91% and 95%, respectively. These results suggest that the selective interaction between aryl group and acid sites on the catalyst is important for selective cracking to cyclohexene, and it will occur when the acidity of the catalyst is weak and the basicity of the aryl group is strong. Further, the kinetics of the cracking reaction of 2, 4, 6-TMCHB over NaOH-Al2O3 was studied, and some mechanistic schemes of the reaction were proposed.