Thermal Effects and Patterns of Chloromethanes Adsorption on Zeolite 13X

Abstract

The aim of the work is to study the adsorption of chloromethanes on zeolite 13X. A sample of zeolite 13X was used in the work. The choice of substances is due to the lack of data on the heats of adsorption of chloromethanes on this zeolite in the scientific literature. The elemental composition of the zeolite sample was determined by X-ray fluorescence method. It was found that the zeolite underwent partial ion exchange of sodium for calcium, which should cause a change in the size of a part of the pores of the zeolite. The specific surface area is 630 ± 63 m²/g, total pore volume of the zeolite is 0.27 ± 0.03 cm³/g, and micropore volume is 0.26 ± 0.03 cm³/g were determined by the low-temperature nitrogen adsorption method. The molecular diameters of di-, tri-, and tetrachloromethane were estimated using the Sutherland equation. The obtained values were compared with the literature data, and adsorption of these compounds on zeolite 13X is predicted. The prediction is experimentally confirmed for di- and trichloromethane, but not for carbon tetrachloride. Carbon tetrachloride retention time is almost identical to the column dead time. This result suggests that carbon tetrachloride molecules do not enter the zeolite pores and are adsorbed predominantly on the external surface. Possible reasons for the absence of carbon tetrachloride adsorption are suggested: semi-empirical nature of the Sutherland formula, semi-empirical method for determining the zeolite window size, failure to take into account the dependence of the zeolite window size on temperature, and ion exchange in it. The integral heats of adsorption of dichloromethane and trichloromethane, 1 μl sample, were calculated: 49.8 ± 0.9 and 45.7 ± 1.5 kJ/mol, respectively. The heat of adsorption reflects the energy of interaction between the adsorbate molecules and the active centers of the zeolite. The dependence of the heat of dichloromethane adsorption on the volume of the sorbate (from 1 to 3 μl) was studied – a linear decrease with increasing volume, which is explained by the participation in the process of adsorption centers of different activity. Adsorption on more active centers occurs first and with the release of more heat than adsorption on less active centers. The obtained data on the thermal effects of adsorption and the structure of the zeolite surface are important for understanding the mechanisms of interaction between the adsorbent and the adsorbed substances. This information is also necessary for predicting the efficiency and selectivity of the adsorption process.