Conversion of n-hexane and n-dodecane over H-ZSM-5, H-Y and Al-MCM-41 at supercritical conditions

Catalytic reactions of supercritical n-hexane and n-dodecane were studied over H-ZSM-5, H-Y and Al-MCM-41 at pressures between 4 and 6 MPa and temperatures between 558 and 748 K, using a combination of experimental and theoretical approaches. The primary reaction for n-hexane conversion over H-ZSM-5 was cracking, whereas the primary reaction over H-Y and Al-MCM-41 was isomerization. In contrast, cracking was the primary reaction of n-dodecane over all of the aluminosilicates. Turnover frequencies (TOFs) for n-hexane and n-dodecane conversion over H-ZSM-5 were much greater than for H-Y and Al-MCM-41 under all tested reaction conditions. The invariance of TOFs for n-hexane conversion over H-ZSM-5 and H-Y over the pressure range studied indicated that the reaction was nearly zero-order in hydrocarbon at the supercritical conditions used here. Configurational-bias Monte Carlo simulations indicate that the loadings of reactant molecules reached only about half of their saturation values at the experimental conditions, but plateaus in the adsorption isotherms of n-hexane in H-ZSM-5 and of n-dodecane in both H-ZSM-5 and H-Y zeolites account for the nearly zero-order kinetics. At supercritical conditions, the apparent activation energy of n-dodecane cracking over H-ZSM-5, H-Y and Al-MCM-41 as well as that for n-hexane cracking over H-ZSM-5 was associated with the rate constant for C[sbnd]C bond cleavage (156–200 kJ mol⁻¹). Density-functional theory calculations show that for n-hexane conversion over H-ZSM-5, β-scission from the bound alkoxide possesses the highest barriers, ranging from 167–177 kJ mol⁻¹ depending on the mode of cracking. The intrinsic activation barriers for isomerization and hydride transfer reactions were calculated to be significantly lower, at 94 and 96–124 kJ mol⁻¹, respectively, although still higher than the experimentally observed value of 74 ± 6 kJ mol⁻¹ for n-hexane conversion over H-Y. Rate expressions derived for monomolecular cracking, bimolecular cracking, and isomerization reactions suggest that the differences between the apparent barriers determined experimentally and the intrinsic barriers calculated theoretically are likely due to the competing hydrocarbon adsorption or quasi-equilibrated exothermic reaction steps. The increased selectivity to cracking over isomerization for n-hexane in H-ZSM-5 was attributed to the difficulty of product removal from the zeolite and, for n-dodecane, to the greatly reduced β-scission barriers and the much stronger product adsorption.