JOURNAL OF MATERIALS SCIENCE 29 (1994) 4927 4932 Characterization of the microporosity of pillared clays by nitrogen adsorption- application of the Horvath-Kawazoe approach A. GIL, A. D[AZ, M. MONTES Grupo de Ingenieria Quimica, Departamento de Quimica Aplicada, Facultad de Quimica de San Sebastian, Universidad del Pals Vasco, Apdo. 1072, 20080 San Sebastian, Spain D. R. ACOSTA Instituto de Fisica, Universidad Nacional Aut6noma de M6xico, A.P. 20-364, 01000 Mexico, D.F., Mexico The microporosity of aluminium pillared montmorillonite has been investigated by nitrogen adsorption, X-ray diffraction and electron microscopy. Control of the preparation conditions allowed the synthesis of solids with two types of micropores. Comparison of XRD measurements and Horvath-Kawazoe analysis of very low,pressure nitrogen adsorption have shown a new promising way to increase the micropore characterization. Disagreement between data from different techniques could be related to the different experimental conditions during measurements and to uncertain values of the interaction parameter in the Horvath-Kawazoe analysis. 1. Introduction Important efforts have been devoted in recent years to the synthesis and characterization of pillared clays [1, 2]. This new kind of nano-composite is prepared by intercalating metal complex cations between the silicate layers of clays. Pillared oxides, formed after calcination, are capable of preventing the collapse of the interlayer spaces, generating therefore a micro- porous structure and a high specific surface area. Pillared clays offer promise as new shape selective catalysts for reactions involving large molecules and as molecular adsorbents [3, 6]. Their usefulness is related to the micropore structure, which is not easy to characterize. X-ray diffraction is normally used to evaluate the interlayer spacing [7], but the pore size of pillared clays is determinated by both the interlayer distance and the lateral distance between pillars. More interesting information has been obtained using adsorption of probe molecules of known kinetic size [4, 8-14]. Recently, Horvath and Kawazoe have pro- posed a new method to determine the micropore volume distribution based on the adsorption isotherm data obtained at very low relative pressure (e.g. from P/Po = 10-7-10-1 for nitrogen adsorption at 77K) [15]. The Horvath-Kawazoe equation used in this analysis includes an interaction parameter dependent on the nature of both the adsorbent and the adsorbate. Reliable values for this parameter have been proposed only for active carbon [15] and zeolites [16]. As a first approach, based on the chemical similarity between zeolites and clays, the interaction parameter value derived for zeolites may be used to study pillared clays. Nevertheless, the validity of this approach 0022-2461 9 1994 Chapman & Hall should be checked by comparison with different characterization techniques. In this work we studied three pillared clays and the starting material. Preparation conditions were chosen in order to obtain samples with different micro- porosity. Conventional nitrogen adsorption measure- ments, X-ray diffraction and electron microscopy have been used to characterize samples and these results are compared with the micropore size distribution obtained using the Horvath-Kawazoe equation to analyse low-pressure nitrogen adsorption data. 2. Experimental procedure The starting material used in this work was montmor- illonite (Kunipia F), kindly supplied by Kunimine Co. This commercial product is available in the sodium- exchanged form with a size less than 2 ~tm, Sodium montmorillonite was dispersed in water and aged for at least 2 months. It was then washed by dialysis until the conductivity of the surrounding water was less than 1.5 ~tS. The solid content of the dialysed clay dispersion was 7-10 gl-1. The montmorillonite trea- ted in this way and dried at 393 K for 16 h is referred to as (S-I). Aluminium polycations were prepared by slow ti- tration of 0.2M A1C13 solution with 0.2M NaOH under vigorous stirring [17, 18]. The sample referred to as (S-2), was prepared with an OH/AI mole ratio equal to 1 and aged at room temperature for two days (pH = 3.8). That referred to as (S-3) was prepared with an OH/A1 mole ratio equal to 2 and aged at 363 K for 4h (pH = 3.9) [19]. 4927