Role of the Surface Modification on the Transport of Hexane Isomers in ZSM-5

The role of the surface modification on the transport properties of nanosized HZSM-5 particles was investigated by the analysis of the sorption kinetics of hexane isomers. The rate of diffusion is enhanced by this modification, if the initial adsorption is the limiting step, i.e., for n-hexane, 2-methylpentane, and 3-methylpentane. If the intracrystalline transport is the rate-determining step, i.e., for 2,2-dimethylbutane, the surface modification leads to a significantly decreased sorption rate. This reduction in the transport rate is caused by a lower pre-exponential factor; i.e., it is entropically induced. Due to the surface modification, a certain fraction of the pores is completely blocked for the sterically demanding molecules reducing the probability of entering the pores. The results show that surface modification of microporous materials allows selectively enhancing or decreasing the transport rate of defined systems.