Direct alkylation of benzene with branched alkanes using solid acids: Unexpected product selectivity based on the tertiary carbon position

Abstract

The direct alkylation of benzene with alkanes offers significant advantages, such as the use of simple alkanes readily available from petroleum and natural gas, and no formation of byproducts containing halogens. In this study, the direct alkylation of benzene with branched alkanes was investigated using solid acid catalysts. The solid acid-catalyzed reaction of benzene with 2-methylhexane was significantly accelerated by the addition of hydrotalcite-supported Pt nanoparticles (Pt/HT). The highest alkylation product yield was approximately 20 % in the presence of H-beta and Pt/HT. The main products from the reaction of benzene and 2-methylhexane were heptylbenzenes (Ph-C7). In contrast, when 3-methylhexane was used as the branched alkane, the main products were pentylbenzenes (Ph-C5). The reaction with 3-methylhexane proceeded well with only the solid acid catalyst, whereas the acceleration effect of Pt/HT was limited. Analysis of the reaction intermediate and reaction pathway revealed that the acid-catalyzed cracking of 3-methylhexane to 2-pentene occurred, followed by benzene alkylation to afford Ph-C5. Arrhenius analysis of the alkylation of benzene with 2-methylhexane, 3-methylhexane, and n-heptane indicated that the activation energy of the reaction using 3-methylhexane was approximately 15–18 kJ mol−1 lower than those of 2-methylhexane and n-heptane, demonstrating the different alkylation pathways.