Diffusion of Mixtures of Light Alkanes and Benzene in Nano-Sized H-ZSM5

The joint transport of linear C4-C6 alkanes and benzene has been studied over nanosized H-ZSM5 crystals by fast-time-resolved infrared spectroscopy. The internal and external concentration profiles during the coadsorption of alkanes and aromatics were directly monitored for a zeolite in which transport is controlled by the pore entrance step. The detailed kinetic analysis revealed a decrease in the adsorption and pore entrance rates of light hydrocarbon molecules upon addition of benzene by about 10-30%. This was caused by both a reduction in the rate constants and in the coverage at the external surface. On the other hand the apparent diffusion coefficient of benzene decreased up to 80% in comparison to the pure component. This decrease is attributed to a competitive adsorption at sites near the pore mouth. The extent increased with decreasing chain length of the cosorbed alkane, which corresponds to a simultaneous decrease in occupation of pore mouth sites by light hydrocarbons.