Inverse Gas Chromatography Study of Modified Smectite Surfaces

-Inverse gas chromatography at infinite dilution, employing alkanes and alkenes as probes, has been used to characterize the surface properties of a series of smectites of varying chemical composition. The results of this study show that the acidic centers and the interlayer distances have a great influence on the specific interaction of the smectite surface with ~r-bonds of alkenes. High values of the specific interaction parameter, ~, are caused by the existence of strong acidic centers that are connected with interlayer cations as well as with the chemical structure of the mineral sheets. On the other hand, alkanes, whose interaction with the smectites is predominantly dispersive, are unaffected by changes in the clays' composition and/or structure. Key Words--Smectite, Intercalation, Inverse gas chromatography, Alkane and alkene sorption, Surface acidity, Surface energy. I N T R O D U C T I O N The intercalation of smectites with hydroxy-metal polycations is a method of mineral modification developed in the late 1970's (Brindley and Sempels, 1977; Lahav et al., 1978; Vaughan et al., 1979), The method involves the introduction of large metal polycations into the interlayer spaces of clay minerals using ion exchange from aqueous solutions. The polycations act as "pillars" supporting the layers and additionally, after calcination, make the structure rigid. After heating treatment the hydroxycation "pillars" are converted into oxide "pillars" (Occelli and Tindwa, 1983). The intercalated smectite is characterized by large specific surface area, developed porous structure, and high surface acidity (Lahav et al., 1978; Vaughan et al., 1979; Occelli and Tindwa, 1983; Brindley and Yamanaka, 1979). Because intercalated smectite also has high thermal resistance, it has application in a variety of chemical processes as a catalyst or catalyst carrier. In addition, it shows promise as a molecular sieve and sorbent (Occelli and Tindwa, 1983; Pinnavaia, 1983; Diddams et aL, 1984). The sorbent properties of smectites depend on the manner of modification of the initial mineral, and on the heat treatment of the resulting product. Many studies in the literature have been devoted to the study of the surface properties of clay minerals (Brindley and Kao, 1980; Yamanaka and Brindley, 1979; Yamanaka et al. 1984). Such properties are of importance when Permanent address: Institute of Energochemistry of Coal and Physicochemistry of Sorbents, University of Mining and Metallurgy, 30-059 Krak6w, Poland. 3 Author to whom correspondence should be addressed. Copyright 9 1992, The Clay Minerals Society these materials are used as catalysts (Diddams et ak, 1984; Kikutchi et al., 1985), adsorbents, or as reinforcing fillers. In our laboratory we have applied chromatographic methods to determine the surface properties of wide classes of materials. It is the objective of this study to present the results of applying the inverse gas chromatographic method at infinite dilution, and to demonstrate that this method provides a convenient way to evaluate surface properties of clay minerals. We show that by using alkane in conjunction with alkene molecular probes, we can determine such surface properties as the acidity, the dispersive component of the surface free energy, and the surface area of a mineral. The advantages of this method are speed, reliability of measurements, and direct thermodynamic interpretation of the experimentally derived parameters. Inverse gas chromatography at infinite dilution Gas-solid chromatography is called inverse gas chromatography (Gilbert, 1984) when applied to the investigation of solid surface properties. This method is based on the study of interactions of gas molecules of known properties with solid surface. Small amounts o f molecular probes are injected into the stream of the carrier gas flowing through the chromatographic column filled with the solid to be investigated. The net retention time tN is measured as a difference between retention times of a given probe and a non-adsorbing gas used as a reference. The net retention volume, VN, which is the fundamental quantity utilized in this method, is calculated from the formula