Influence of Postsynthetic Surface Modification on Shape Selective Transport of Aromatic Molecules in HZSM-5

Fast, time-resolved infrared spectroscopy was utilized to determine the transport and sorption properties of benzene, toluene, and p-xylene oil a series of zeolites with increasing degree of surface modification. Postsynthetic modification of H-ZSM5 with a mesoporous silica overlayer generates novel hierarchical materials. which significantly increase the sorption rates for benzene and decrease it gradually for alkyl-substituted aromatic molecules with increasing radius of gyration of the aromatic molecule. The ratio between the sorption rates of benzene and p-xylene increased from,in initial value of 4.3 to about 27 after modification. The reason for the enforced differentiation is the combination of the intrinsic size exclusion properties of the zeolite micropores and the variation of the sticking probability of the molecules on the modified surface, which enhanced the mass transfer into the porous overlayer. The size of the overlayer pores and the radius of gyration of the sorbate were identified as the parameters determining the sorption rates. Our results highlight that hierarchical composites generated by deposition of highly porous silica overlayers on microporous materials allow tailoring the separation properties of porous materials and allow introducing a new concept to kinetically separate molecules of identical minimum (kinetic) diameters.