On the active site in H₃PW₁₂O₄₀/SiO₂ catalysts for fine chemical synthesis

The surface environment and structural evolution of silica supported phosphotungstic acid (H₃PW₁₂O₄₀) catalysts have been investigated as a function of acid loading. H₃PW₁₂O₄₀ clusters are deposited intact upon the silica surface, adopting a Stranksi-Krastanov growth mode forming a two-dimensional adlayer which saturates at 45wt% acid. Intimate contact with the silica support perturbs the local chemical environment of three tungstate centres, which become inequivalent with those in the remaining cluster, suggesting an adsorption mode involving three terminal W==O groups. Above the monolayer, H₃PW₁₂O₄₀ clusters form three-dimensional crystallites with physico-chemical properties indistinguishable from those in the bulk heteropoly acid. These H₃PW₁₂O₄₀/SiO₂ materials are efficient for the solventless isomerisation of α-pinene under mild reaction conditions. Activity scales directly with the number of accessible perturbed tungstate sites at the silica interface; these are the active species.