Influence of doping with copper on the texture of pillared montmorillonite catalysts

Abstract

Modification of the textural properties of Al-, Zr-, and Ti-pillared montmorillonites upon doping with copper has been investigated by nitrogen adsorption/desorption at 77 K. The doping methods included addition of copper during the post-pillaring treatment of uncalcined or calcined solids, and co-pillaring, i.e., the use of the pillaring agent containing copper ions. All investigated pillared samples are predominantly microporous, the dominant micropore radii of Al- and Zr-pillared samples (<3.50 Å) being much lower than in the case of the Ti-pillared clay (5.25 Å). The influence of the copper additive on the texture of pillared matrices depends on the nature of the pillars and on the method of doping, and affects both the micropore and the mesopore system. The results are interpreted in terms of the physicochemical properties of the samples as determined by the chemical, XRD and ESR analyses. The strong influence of post-pillaring doping on the texture of the Al-pillared clay, resulting in a decrease of microporosity, is assigned to the enhanced formation of clustered copper ions. An increase of the adsorption potential of the co-pillared samples is attributed mainly to a better structural order induced by the competitive exchange. It is suggested that the sensitivity of the mesopore system of the Ti-pillared clay to the repeated post-pillaring doping is due to crystallisation effects in the extra-framework titania.