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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.

The rate-determining reaction step is associated with the C–H bond cleavage of –CH2 group of the first HMF molecule oxidized by [V(O)2(DMA)2]+ species, while counteranion Cl− exhibits catalytically promotive effect.

V-containing catalysts exhibit good catalytic performance toward the selective oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF). Here, we report our study on the catalytic mechanism of –(SiO)3V(O) ([V-0]) and –(SiO)2V(O)(OH) ([V-1]) on a V-doped Al-MCM-41 pore model (V/Al-MCM-41) for the aerobic oxidation of HMF to DFF. For the two active sites, there are three types of oxygen-containing functional groups, which are hydroxyl-oxygen ([V–OH]), lattice-oxygen ([V–O–Si]), and terminal-oxygen ([VO]). We show that the catalytic cycle involves two HMF molecules, and there are mainly two activation steps, i.e., both O–H and C–H bond cleavages of HMF, and the rate-determining step is associated with the C–H bond cleavage of the first HMF molecule. We illustrate the efficiency of the catalytic contribution as [V–OH] > [V–O–Si] > [VO], and the [V-1] active site with a hydroxyl group displays higher catalytic activity than the [V-0] active site without a hydroxyl group. The present study not only brings an in-depth understanding of the activation of both O–H and C–H bonds which has been proposed based on experimental results for biomass molecules, but also makes one step forward toward the mechanism-guided design and synthesis of efficient, environmentally-friendly, and low temperature recyclable heterogeneous catalysts.

Mo-Containing catalysts exhibit good catalytic performance toward the selective oxidation of platform chemicals from biomass. However, for the aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF), the development of an environmentally-friendly and promising catalytic system remains a significant challenge to date. In the present study, on the pore surface of MCM-41, two kinds of active sites, which are, –(SiO)2Mo(O)2 ([Mo-4]) and –(SiO)3Mo(OH)(O) ([Mo-5]), have been theoretically designed, over which the catalytic mechanisms for the aerobic oxidation of HMF to DFF have been theoretically examined, combining the hybrid quantum mechanics and molecular mechanics calculations. Over both [Mo-4] and [Mo-5] active sites, the oxidative reaction includes two main reaction steps, the O–H and C–H bond cleavages of HMF, in which the crucial reaction step is associated with the C–H bond cleavage of the CH2 group of the first HMF. Over the –(O)4–MoO active site on the Keggin heteropolyacid ([Mo-HPA]), [Mo-4] and [Mo-5] active sites, the MoO group is responsible for the O–H bond activation in the HMF moiety, and the Mo–OH group is in charge of the C–H bond activation in the HMF moiety, both of which take a synergic catalytic effect. The catalytic activity decreases as [Mo-5] > [Mo-4] > [Mo-HPA]. Based on the activation strain analysis, the lowering activation energy stems mainly from the lowering activation strain of the active site, which correlates positively with the corresponding HOMO–LUMO gap. These findings provide deep insight for the rational design of a recyclable catalyst to activate both C–H and O–H bonds at low temperatures, especially for some typical platform molecules from biomass.