Synthesis and characterization of PILCs with single and mixed oxide pillars prepared from two different bentonites. A comparative study

Abstract

Different PILCs with single oxide pillars of Fe, Cr and Zr and mixed oxide pillars of these metals and Al have been prepared from two bentonites: a commercial bentonite from Fisher Scientific Company (F bentonite) and a natural bentonite from Peru (P bentonite). Several Al/metal, OH/metal and metal/clay ratios were used in order to investigate the effect on the chemical and physical properties, specifically the thermal stability of the synthesized pillared clays. The structure of the pillared materials has been studied by XRD, UV Diffuse Reflectance Spectroscopy (UV DRS) and detailed analysis of N 2 adsorption isotherms. The thermal stability, acidity and reducing behaviour of these products were determined by using, respectively, thermogravimetric measurements (TGA), ammonia-TPD and TPR. Likewise, the methane adsorption capacity for different samples was evaluated. The resulting materials exhibited basal spacings in the range of 7–25 Å, with high surface areas (213–331 m 2 g −1) and a high contribution of micropore area (between 50 and 80%) over the total surface area. Pillared clays prepared from F bentonite generally showed larger basal spacings and surface areas than those from P bentonite. Except for PILCs with Cr in the pillar, whose structure collapsed below 400°C, most of the PILCs materials with single oxide pillars were found to be thermally stable up to 500°C. A relatively strong interaction between metal and aluminium in the pillars was observed. The surface acidity, methane adsorption and thermal stability were increased by incorporating aluminium into the single oxide pillars of Fe, Cr and Zr. Likewise, it was observed that the pillar structure in PILCs with mixed oxide pillars was affected by the Al/metal ratio. Thus, the Keggin structure in samples with a higher Al/metal ratio seems to predominate, whereas the pillar structure in materials with a lower Al/metal ratio is similar to that of PILCs with single oxide pillars containing no aluminium.