Synthesis and characterization of hydroxy-CrAl pillared clays

Abstract

This paper describes the preparation and characterization of new PILC materials with mixed pillars of Cr 3+ and Al 3+ intercalated between the layers of montmorillonite. Various Cr/Al ratios were studied to obtain the most suitable physicochemical products. The resulting materials showed basal spacing between 2.49 nm and 1.90 nm for air-dried samples and surface areas between 232 and 283 m 2/g. The pillar structure, thermal stability, surface acidity, and reducing behavior of the pillared clay products were determined using 27Al n.m.r., X-ray diffraction (XRD), d.t.a. diffuse reflectance spectroscopy (d.r.s.), e.p.r., i.r., NH 3-t.p.d., t.p.r. The results suggest that Cr/Al ratios greatly affect the pillar structure, surface area, acidity, and thermal stability of hydroxy-CrAl pillared clay (CrAl-PILC). The pillar in CrAl-PILC with Cr/Al ratio <1 has a Keggin structure similar to that of hydroxy-Al pillared clay (Al-PILC), but the pillar structures of CrAl-PILC with Cr/Al ratio ⩾1 are large polyoxochromium complexes similar to those of hydroxy-Cr pillared clay. The surface acidity and thermal stability are increased by incorporating aluminum into a chromia pillar. There is relatively strong interaction between chromium and alumina in the pillars, which is enhanced with decreasing Cr/Al ratios. Reduction of the chromium(III) was made difficult because of the interaction of chromium with alumina. Catalytic behavior responded to Cr/Al ratio, and incorporating Cr(III) into an alumina pillar increased the kinetic stability and selectivity of the catalyst in the disproportionation of 1,2,4-trimethylbenzene compared with a PILC catalyst containing no chromium.