Study of catalysts comprising heteropoly acid H3PW12O40 supported on MCM-41 molecular sieve and amorphous silica

With the aim of obtaining pure Keggin-type heteropoly acid (HPA) species on siliceous surfaces, the preparation of HPA catalysts, comprising of 10 to 50 wt.% H3PW12O40 (PW) supported on an amorphous silica or a mesoporous all-silica molecular sieve MCM-41, was studied. The state of HPA on the siliceous surface was characterized by P-31 NMR, XRD and TEM. Impregnating MCM-41 or amorphous SiO2 with an aqueous solution of PW gave catalysts with, in general, two HPA species: one with intact Keggin structure (A) and the other with a different structure (B), supposedly, H6P2W18O62 (P2W18) and/or H6P2W21O71 (P2W21). The relative amount of species A and B depends on HPA loading, with A dominating. At higher loadings, 30-50 wt.%, A is practically the only one present on the surface; at lower loadings, both species exist, the amount of B increasing as the HPA loading decreases. In contrast, catalysts prepared by impregnation with a methanol solution of HPA contained exclusively Keggin-type A over the whole range of PW loading. In the PW/MCM-41 catalysts, as shown by TEM, the PW species are mainly located inside the MCM-41 pores. The B species was about 8 times as active as A in the liquid-phase trans-de-t-butylation of 2,6-di-t-butyl-4-methylphenol.