Comparison of nuclear magnetic resonance tracer exchange and molecular uptake of benzene on pentasils

Abstract

Conventional NMR spectroscopy is applied to study tracer exchange in adsorbate–adsorbent systems. In the proposed experimental procedure, heat effects are completely excluded and possible transport resistances due to gas-phase diffusion and intercrystalline diffusion are reduced to a minimum. The influence of deviations from the spherical shape of the adsorbent particles as well as possible anisotropy of intracrystalline diffusion on tracer exchange curves is estimated. The obtained relations are likewise applicable to uptake studies. For benzene on different ZSM-5 specimens, the tracer exchange data are compared with the results of uptake measurements. Though carried out independently from each other with different techniques and different specimens, the experiments provide coinciding results. In particular, it appears that the ratio between the self-diffusion coefficient obtained in the tracer experiments and the diffusion coefficient resulting from uptake studies is indeed given by the logarithmic derivative of the adsorbate concentration versus the adsorbate pressure as conventionally assumed on comparing self-diffusion and uptake measurements.