Catalytic mechanism for the isomerization of glucose into fructose over an aluminium-MCM-41 framework

Abstract

Al-Containing MCM-41 catalysts exhibit good catalytic activity toward glucose-to-fructose isomerization. However, the catalytic contribution of typical functional groups over Al active sites is a little unclear. Here, on the pore surface of Al-doped MCM-41, there are two different active sites, i.e., –[SiO]₂[SiO(H)]Al[OH] ([Al-1]) with one hydroxyl and –[SiO][SiO(H)]Al[OH]₂ ([Al-2]) with two hydroxyls. Over these two active sites, the catalytic mechanisms for the isomerization of glucose to fructose have been theoretically studied, combining molecular mechanics and hybrid quantum mechanics calculations. The glucose-to-fructose isomerization usually involves three successive reaction stages, i.e., ring-opening, tautomerization, and ring-closure. Among them, aldose–ketose tautomerization is the rate-determining step, which is related to the intramolecular C2 → C1 H-shift. The catalytic activity of the Al-containing active sites originates from three factors, i.e., the Lewis acidity of the Al site, a proton donor in the form of an Al–O(H)–Si group and a proton acceptor in the form of an Al–OH group. The [Al-2] active site with two hydroxyls displays higher catalytic activity than the [Al-1] active site with one hydroxyl, which mainly depends on the Lewis acidity of the aluminium atom in the active site.