Study of catalysts comprising heteropoly acid H 3PW 12O 40 supported on MCM-41 molecular sieve and amorphous silica

Abstract

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.% H 3PW 12O 40 (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 31P NMR, XRD and TEM. Impregnating MCM-41 or amorphous SiO 2 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, H 6P 2W 18O 62 (P 2W 18) and/or H 6P 2W 21O 71 (P 2W 21). 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.