Methylation of benzene with methane induced by strong adsorption of benzene on Co ion at -position in zeolite with moderate Al-Al distance

First principles calculations are applied to find a pathway for methylation of benzene with methane into toluene and dihydrogen catalyzed by MFI type zeolite-supported Co species. On Co2+ at the alpha-position of MFI framework which is exposed to relatively large space, benzene is strongly adsorbed, while the adsorption on Co at the beta-position is weak due to steric hindrance. Energy required for dissociation (activation) of methane was high, but the large energy evolved by adsorption of benzene on Co cancels the activation energy for the methane dissociation to suppress the energies of this step and following formation of toluene precursor. The activation energy of methane dissociation is sensitively changed by Al-Al distance also. Short distance results in difficulty of cleavage of Co-O bond, because Co is fully and strongly coordinated to basic oxygen atoms next to Al atoms, i.e., O in Al-O-Si. On the contrary, Co is partly coordinated to O in Si-O-Si with moderate Al-Al distance, and the cleavage of coordination bond is easy. Too long distance causes difficulty for hopping H+ to reach base site apart from the reaction field. At an appropriate Al-Al distance, the activation energies of the methane dissociation and toluene precursor formation are low. The latter is the rate-determining of benzene methylation with methane, and its energy, i.e., the apparent activation energy is in agreement with the experimental measurements. Experimentally, high activity at the alpha-position, strong dependence of activity on the framework Al content and the high activity at a moderate framework Al content were observed, supporting the theoretical conclusions. The driving force of benzene methylation with methane, a reaction between a pair of inactive reactants (methane and benzene), is thus clarified to be the strong adsorption of benzene. The origin of catalytic activity is therefore proposed to be strong electron withdrawing nature (Lewis acidity) of Co species held by the ion exchange site of zeolite.