Transport phenomena through intercrystalline and intracrystalline pathways of silicalite zeolite membranes

Zeolite membranes are, generally, polycrystalline structures. Penetrant molecules pass through zeolite crystals (intracrystalline pathways) and intercrystalline pathways. The transport phenomena of ethanol through intracrystalline and intercrystalline pathways of silicalite membranes were separately examined. An intercrystalline-intracrystalline model (i-i model) is proposed for quantitative discussion of fluxes through intercrystalline and intracrystalline pathways. The concentration in the interface of the intracrystalline region is of the Langmuir type, and the concentration in the intercrystalline region is proportional to the activity on the outside of the membrane. This i-i model was investigated using silicalite membranes and silicalite membranes modified by chemical vapor deposition (CVD). CVD modification was carried out using a tetramethylorthosilicate (TMOS)/O₃ counter-diffusion CVD technique. The i-i model can quantitatively explain the permeate properties of ethanol transport both through the silicalite membrane and the CVD modified membrane. According to this model, ethanol is permselective from ethanol/water mixtures through intercrystalline pathways.