ISOTHERM OF BENZENE ADSORPTION AND DIFFERENTIAL HEAT OF ADSORPTION ON AgZSM-5 ZEOLITE

Abstract

Benzene, toluene, and para-xylene (kinetic diameters ~0.58 nm) are easily adsorbed onto ZSM-5 zeolites. The molecules of aromatic hydrocarbons repeatedly attach to the OH groups, migrate along the external surface of the zeolite before entering the micropores or desorbing. Molecules that can enter the micropores, such as benzene, toluene, and xylene, are adsorbed more rapidly onto the SiOHA1 groups compared to the SiOH groups. For o-xylene molecules, which find it difficult to enter the pores, the adsorption rate on the remaining SiOH groups significantly increases. As the adsorption volume reaches saturation, the cation-free part of the zeolite, i.e., the silicalite portion, continues the adsorption process, resulting in two maxima and two minima. At an adsorption value of 1.53 mmol/g, the differential heat decreases to 54 kJ/mol. If we assume the density of benzene at experimental temperature to be the same as that in normal liquid form and calculate the volume occupied by the benzene molecules at saturation, we find that the AgZSM-5 zeolite occupies about ~0.16 cm³/g of the sorption volume, which constitutes approximately 94.7%. Thus, based on the experimental data, the following conclusion can be drawn: In the adsorption of benzene on AgZSM-5 zeolite, the benzene molecules localize around the active centers, forming a sandwich-like structure around the silver sites