Acid-type catalytic properties of heteropolyacid H3PW12O40 supported on various porous silica-based materials

In order to obtain highly dispersed heteropolyacid species, H3PW12O40 (HPA) was supported on various silica-based supports. Three lamellar silicates were used: a natural montmorillonite with high alkaline metal content, an 'activated' montmorillonite strongly delaminated with low alkaline metal content, a kenyaite which is a lamellar sodium silicate as well as a mesoporous hexagonal silica (HMS). The state of the supported HPA was studied by X-ray diffraction, molecular spectroscopies, HRTEM and EDX-STEM and particularly by their catalytic behavior in the methanol conversion, which is considered as a bulk-type reaction, and in the n-butane isomerization considered as a surface-type reaction. The presence of a high content of charge compensating alkaline cations in the lamellar structures was shown to decompose the Keggin structure, leading to solids with poor acidic properties. The influence of the HPA content, ranging between 10 and 50 wt%, was studied on the 'activated' montmorillonite. An exchange reaction between the charge compensating cations of the clay and the protons of the HPA was shown to occur for low HPA content. The Keggin structure was preserved, but the dispersion was not as high as on the mesoporous silica, and the presence of residual charge compensating cations decreased the catalytic activities. The HPA/HMS sample was highly active for the two reactions and we suggest that for this sample the catalytic activity was due to isolated HPA molecules entrapped within the pores. (C) 1998 Elsevier Science B.V. All rights reserved.