Immobilization of molecular H3PW12O40 heteropolyacid catalyst in alumina-grafted silica-gel and mesostructured SBA-15 silica matrices

The effects of grafting the alumina species at the surface of silica support on the H3PW12O40 (HPW) immobilization capacity and performance of loaded HPW in acid-catalyzed reactions were studied with regular silica-gel and ordered mesostructured silica SBA-15. The states of alumina and HPW, in terms of the acidity/basicity, material texture, and structure of adsorbed HPW species, were characterized by XRD, XPS, NH3-TPD, UV-vis-, and FTIR-spectroscopy, as well as 1H and 31P MAS NMR, NH3- and CO2-TPD, and N2 adsorption. It was demonstrated that grafting of silica with small alumina clusters at partial to full surface coverage produces isolated basic sites of the same strength as at the surface of pure alumina, but with a surface concentration that is an order of magnitude lower. These sites anchor the molecular species of HPW, retaining their polyanion structure and acidity-catalytic activity patterns in several acid-catalyzed reactions. The reason for the low activity of HPW moieties immobilized at the surface of pure alumina is a polydentate adsorption of polyanions yielding a high extent of HPW acidity neutralization. The use of mesostructured SBA-15 as a support yielded catalysts with 30-70% higher activity compared with that based on regular silica gel. This is a result of higher surface area and surface concentration of silanols in SBA-15, which makes it possible to immobilize more HPW as molecular species.