Enhancement of the Catalytic Activity of a Dawson-type Heteropoly Acid Induced by the Loading on a Silica Support

摘要

The Friedel–Crafts alkylation, benzylation of anisole with benzyl alcohol, was carried out over heteropoly acids (HPAs) supported on SiO2. A Dawson-type HPA, in which Nb atoms were partially substituted with W atoms, exhibited the highest activity among various kinds of HPAs. The catalyst was reusable in the reaction after the filtration and it kept high activity even after the washing with hot water when the catalyst was calcined at 743 K. In contrast, the unsupported catalyst was almost inactive after the thermal treatment. The change in the activity was in good agreement with the degree in the acid amount measured by NH3-TPD. Unlike the Dawson-type heteropoly acid, the activity of the Keggin-H3PW12O40/SiO2 remarkably dropped through the repeated use for the reaction. Based on the structural analysis of Dawson-type HPAs, the partially decomposed HPA on the support of SiO2 was ascribed to the active species.