Comments on “Enhanced Thermal Stability of Al-Pillared Smectites Modified with Ce And La” By J.L. Valverde, P. Cañizares, M.R. Sun Kou, and C.B. Molina

Abstract

Valverde et al. (2000) recently reported on the preparation and characterization of Al-pillared smectites modified with Ce and La. Pillaring of clays with A1 polyoxocations (Keggin type Al~3 ) and a large variety of other complexing cations has been reported many times since the seventies. McCauley (1988) described in a patent the preparation of thermally stable pillared clays with large basal spacings of -28 A, followed by publications by Sterte (1991aSterte ( , 1991b and Booij and coworkers (Booij et al., 1996a(Booij et al., , 1996b. These authors used hydrothermally treated or refluxed solutions containing the A1 polyoxocation and the rare earth elements in the form of their chloride salts. The presence of cerium or lanthanum seems to promote polymerization of the A1 polyoxocation. Until now however, nobody has been able to assess the structure of the newly formed pillaring molecules. Although the interlayer spacing is about twice that of the normal A1-pillared clay, chemical analyses indicate smaller amounts of Ce and La than expected from polymerization of the A1 polymer alone. Several other studies on the combination of A1 pillars with Ce or La resulted in the formation of pillared clays with basal spacings characteristic of Al-pillared clays. However, catalytic activity is enhanced owing to Ce or La (e.g., Gonz~ilez et al., 1992;Mendioroz et aL, 1993;Trillo et al., 1993).Unfortunately, Valverde and coworkers (Valverde et aL, 2000) missed most of the important publications on the thermally stable large-pore pillared clays based on AI pillars modified by Ce or La. The basal spacings reported by Valverde et al. (2000) are in a similar range to those reported by others for Al-pillared smectites (Kloprogge, 1998 and references therein), but -6--7 A smaller than those reported for the A1 and rare-earth element (A1/REE) large-pore pillared smectites (McCauley, 1988; Sterte, 1991a Sterte, , 1991bBooij et al., 1996aBooij et al., , 1996b. This indicates that rare earth elements do not form an integral part of the pillars within the interlayer. Although the surface areas are in the order of what is expected for a pillared clay, the micropore volume is significantly lower as, for example, observed for Al-pillared montmorillonite SWy-1 (Kloprogge et al., 1994). However, their micropore volume is much higher than that of large-pore A1/REE pillared clays in which no micropores are present at all because the average pore diameter is --60-70 A (Booij et aL, 1996b). The relatively low micropore volume can probably be explained by the incorporation of a relatively large amount of Ce or La in the interlayer between the pillars during the synthesis of the pillared clays where initial REE/A1 ratios of --<1.8 were maintained. Although the thermal stability of these pillared clays is enhanced, the access to the catalytically active sites in the pillared clays may be severely hindered. However, Valverde et al. (2000) did not report any catalytic testing on their materials.Valverde et al. (2000) also reported on the infrared s...