Energetic heterogeneity of the surface of a micro/mesoporous zeolite adsorbent

Abstract

A complex approach based on treatment of calorimetric data using the Guggenheim–Andersen–de Boer and modified Jovanoviae equations was developed to evaluate the energetic and structural heterogeneity of microporous and mixed micro/mesoporous zeolite adsorbents. The calculated dependence of the hexane adsorption energy distribution function for the Y-US-Ex micro/mesoporous zeolite on the adsorbate pressure is in good accord with the calorimetric curves for the differential heats of adsorption, which indicates the suitability of this theoretical approach. Energetic heterogeneity is characteristic for the surface of solids. The structural heterogeneity arises, for example, due to energetic inequivalence of the different crystal faces, existence of various types of dislocations on the surface of microcrystals, the existence of surface micropores, the breakdown of order in the deposition of adjacent layers in the structure, and domain distribution of isomorphic impurities. Chemical heterogeneity is related both to the different chemical composition of the adsorbent, which determines the type of active adsorption sites and their coordination number. Such heterogeneity is characteristic for silicas, aluminas, and carbon adsorbents. The silica surface features isolated and vicinal silanediol groups. Aluminas are characterized by tetrahedral and octahedral coordination of the surface aluminum atoms and, thus, different activity of the attached hydroxyl groups. Carbon adsorbents are characterized by carboxyl, phenolic, hydroxyl, and other active sites. The reasons for energetic heterogeneity have been analyzed in detail by various workers [1-5]. Various physical methods are used to study the energetic heterogeneity. Atomic force microscopy [6] and high resolution electron microscopy [7] are among the most common methods for oxides and silicates. However, analysis of the corresponding publications indicates that physical methods are capable of distinguishing heterogeneity of nonporous adsorbents with a step on the order of 1.0-1.5 nm. Thus, adsorption and chromatographic methods retain their importance for porous materials. Analysis of studies in this field [3, 8] shows that major attention has been given to mesoporous materials using inert gases and saturated hydrocarbons as the adsorbates. The literature lacks studies on the energetic heterogeneity of microporous systems using kinetic theory and statistical mechanics. In the present work, we examine a method for analyzing micropore heterogeneity in zeolite employing adsorption isotherms of hexane and benzene relative to a micro/mesoporous zeolite adsorbent. These results are compared with the calorimetric date for the heats of adsorption of hexane and benzene vapors on this adsorbent.