Phenol tetrahydropyranylation catalyzed by silica-alumina supported heteropolyacids with Keggin structure (original) (raw)

2004, Applied Catalysis A: General

The catalytic behavior of molybdophosphoric or tungstophosphoric acids supported on different silica-aluminas, obtained by sol-gel method, in a hydroxyl protection reaction, phenol tetrahydropyranylation, was studied. Different techniques were used to synthesize the supports. In one of them, commercial alumina was added to a tetraethyl orthosilicate (TEOS) solution and then this was hydrolyzed. In the other preparations, TEOS and aluminum tri-sec-butoxide were hydrolyzed, simultaneously or with TEOS prehydrolysis. The catalysts were prepared by incipient wetness impregnation and, after their leaching with toluene, different thermal treatments were applied. The specific surface area of the solids was measured, and the nature of the Keggin heteropolyacid species present in the catalysts was characterized by diffuse reflectance spectroscopy and X-ray diffraction. In addition, their acidic properties were evaluated by potentiometric titration with n-butylamine. The acetal yield was strongly dependent on the catalyst properties, mainly their acidic characteristics which, in turn, depended on the support synthesis. Moreover, the decrease of the catalyst water content through treatment up to 250 • C led to higher yields, due to the presence of water in the reaction medium favors the hydrolysis of the acetal. hydrofluoric acids, boron trifluoride, have raised the interest in the development of alternative clean processes using solid acid catalysts. Among them, Keggin heteropolyacids (HPA), constituted by a close-packed framework of metal-oxygen octahedra MO 6 (M = Mo or W) surrounding a central PO 4 tetrahedron, exhibit stronger acidity than conventional acids. Advantages to be highlighted of using supported HPA as catalysts are their easiness of recovery and recycling after carrying out liquid phase reactions, and the easier product isolation, when they are compared to that of the homogeneously catalyzed reactions. The preparation of supported catalysts from tungstophosphoric acid (TPA) or molybdophosphoric acid (MPA) on different carriers has been previously studied . The MPA and TPA on silica catalysts were used, obtaining high conversion and selectivity, in cyclization reactions among others [9,10]. Moreover, the 0926-860X/$ -see front matter