Abstract

Many recent investigations have confirmed the presence of surface barriers in a variety of zeolites. These surface barriers can have a significant effect on the overall rates of mass transfer and have the potential to significantly reduce the effectiveness of zeolites in industrial applications. In this study, the strength of the surface barrier was manipulated in a well-defined way and correlated with the rate of isobutane uptake.

Silicalite-1 crystals were synthesized and were surface treated with trimethyl-, triethyl- and tripropylchlorosilanes. It was hypothesized that with the increasing size of the alkyl group the pore windows would be increasingly blocked, thereby effectively decreasing the pore size and increasing the surface barriers. The rates of adsorption and desorption of isobutane in untreated and treated samples were monitored by infrared microscopy.

For differential pressure steps, adsorption and desorption responses were mirror images of one another. For large pressure steps, the rate of adsorption was much higher than that of desorption, indicating a significant variation of intracrystalline diffusivity with loading. The adsorption/desorption curves were analyzed considering (1) the presence of only intracrystalline diffusion and (2) surface barrier followed by intracrystalline diffusion.

The measured rates of adsorption/desorption were significantly lower for the surface treated samples when compared with those for the untreated samples. Moreover, the strength of the surface resistance increased with the size of the alkyl group in chlorosilanes used for surface treatment.