Diffusion of ethane in silicalite-1 by frequency response, sorption uptake and nuclear magnetic resonance techniques

Abstract

The diffusion of ethane in ZSM-5 has been studied using four different experimental methods: a frequency-response technique, a single-step frequency-response method (an improved sorption uptake–desorption technique), an n.m.r. pulsed field gradient spin–echo technique and, finally, an n.m.r. tracer desorption technique. Large and uniform silicalite-1 crystals were prepared and used in all four techniques. Different concentrations of tetrapropylammonium ions in the synthesis batch alter systematically both the crystal habit and the relative amounts of internal silanol groups in the silicalite-1 framework, indicating different concentrations of defect Si—O—Si bonds. For the genesis of the defective siloxane bonds, a model has been presented. Comparing the translational mobility of ethane on a selected sample, the diffusion coefficients measured by the two n.m.r. methods were found to be ca. 150 times larger than the corresponding coefficients derived from the frequency response and sorption uptake–desorption methods. This experimental finding is explained by crystal bed depth influences on the molecular uptake in the latter sorption experiments.