Studies on the identification of the heteropoly acid generated in the H3PO4–WO3–Nb2O5 catalyst and its thermal transformation process

The precursor of the mixed metal oxide catalyst composed of H3PO4–WO3–Nb2O5, which exhibits excellent activity in Friedel–Crafts alkylations, was identified with 31P NMR. It was revealed that the Keggin-type mixed heteropoly acid, H4PNbW11O40, was spontaneously generated during preparation of the H3PO4–WO3–Nb2O5 catalyst. The partial decomposition of H4PNbW11O40 occurred in the temperature range of 673–823 K to give an amorphous oxide that had a Brønsted acid character. Based on the results of structural and acidic studies, the active species obtained through the calcination of H3PO4–WO3–Nb2O5 was ascribed to the transient state in the course of the thermal decomposition of the H4PNbW11O40 over the WO3–Nb2O5 support. In fact, the catalyst prepared by mixing H4PNbW11O40, niobic oxalate, and ammonium tungstate solutions, followed by calcination at 773 K, exhibited excellent activity in the Friedel–Crafts acylation of anisole with carboxylic acids.