Positron emission profiling : a study of hydrocarbon diffusivity in MFI zeolites

Zeolites are of prime importance to the petrochemical industry as catalysts for hydrocarbon conversion. In their molecule-sized micropores, hydrocarbon diffusion plays a pivotal role in the final catalytic performance. Here, we present the results of Positron Emission Profiling experiments with labeled hydrocarbons in zeolites with the MFI morphology. Single-component self-diffusion coefficients of hexanes in silicalite-1 and its acidic counterpart H-ZSM-5 are determined. For the first time, self-diffusion co efficients of n-pentane and n-hexane in mixtures are studied. This shows that Positron Emission Profiling is a powerful technique for in situ investigations of the adsorptive and diffusive properties of hydrocarbons in zeolites. The diffusion of hydrocarbons in medium-pore zeolites is determined by a complex interplay of factors such as the loading, the temperature, the preference for certain pore locations, the interactions with other hydrocarbon molecules of the same type or of other types and the presence of acid protons. In the diffusion of mixtures, pore blockage by one of the components might occur, thus strongly decreasing the diffusivity of the faster diffusing hydrocarbon.