Mechanistic Investigations of Liquid-phase Direct Alkylation of Benzene with n-Heptane Using Proton-exchanged Montmorillonite Catalysts

The direct alkylation of benzene with n-heptane was investigated using noble-metal-free montmorillonite as a solid acid catalyst. It was found that the catalytic activity of proton-exchanged montmorillonite increased after pretreatments such as heating and ultrasonic irradiation. Aluminum-exchanged montmorillonite was also found to be a good catalyst. Moreover, during the reaction using proton-exchanged montmorillonite, the product selectivity depended on the interlayer distance of the catalyst. Detailed time-course analysis of the selectivity of the alkylation product revealed that, in the case of the catalysts with smaller interlayer distances, the bimolecular reaction occurred preferentially at the surface, resulting in higher selectivity with respect to the target C7 alkylated products (Ph-C7). On the other hand, the monomolecular cracking of heptane in the interlayer spaces resulted in a stable t-butyl cation, yielding t-butylbenzene. Monomolecular cracking also occurred when H-ZSM-5 was used as the catalyst, and the main product was isopropylbenzene.