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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.

Cellobiose is usually preferred as model compound of cellulose to explore its conversion mechanism, in which the first challenge is to acquire the molecular solvation mechanism. The interactions of β-cellobiose with the hydrated Na+ cation and Cl− anion in aqueous solution have been theoretically investigated, using quantum chemical calculations at LC-ωPBE/6-311++G(d,p), aug-cc-pVTZ level under a polarized continuum model (PCM-SMD), together with molecular dynamics stimulation. In aqueous solution, the most stable form of β-cellobiose is syn-ϕ/syn-φ (CB2), including the exocylcic hydroxyl groups and three intramolecular H-bonds. The favorite solvation structure of NaCl is concerned with [Na(H2O)3]+···Cl− ion pair. The Cl− anion interacts with β-cellobiose, favorably affording $$[\eta_{{\text{H}}6,{\text{H}}2^{\prime}}^2{-}{\text{CB}}2){\text{Cl}}{]^- }$$ and $$[\eta_{{\text{H}}3^{\prime},{\text{H}}4^{\prime}}^2{-}{\text{CB}}2){\text{Cl}}{]^- }$$ with a bridge of OH···Cl···HO interaction. Alternatively, the [Na(H2O)3]+ cation interacts with β-cellobiose, preferably yielding $$[\eta_{{\text{O}}3,{\text{O}}5^{\prime},{\text{O}}6^{\prime}}^3{-}{\text{CB}}2){\text{Na}}{]^+ }$$ . Additionally, the [Na(H2O)3]+···Cl− ion-pair interacts cooperatively with β-cellobiose, preferentially producing $$[\eta_{{\text{O}}3,{\text{O}}5^{\prime},{\text{O}}6^{\prime},{\text{H}}6,{\text{H}}2^{\prime}}^3{-}{\text{CB}}2){\text{Na}}{\left( {{{\text{H}}_2}{\text{O}}} \right)_1}]{\text{Cl}}$$ . For CB2, the reactive sites for the nucleophilic and electrophilic reactions locate at the O5 atom on the reducing end and the H6′ atom on the non-reducing end, respectively. Alternatively, for $$[\eta_{{\text{O}}3,{\text{O}}5^{\prime},{\text{O}}6^{\prime},{\text{H}}6,{\text{H}}2^{\prime}}^3{-}{\text{CB}}2){\text{Na}}{\left( {{{\text{H}}_2}{\text{O}}} \right)_1}]{\text{Cl}}$$ , the O5 and H6′ atoms should no longer be characteristic of nucleophilicity and electrophilicity. Moreover, [Na(H2O)3]+···Cl− ion-pair can break some intramolecular H-bonds of CB2, which may promote the solvation of β-cellobiose in aqueous solution. It also shields the glycosidic bond by steric effect, which make CB2 not readily hydrated at the glycosidic bond. These functions would alter the reaction pathway in the further degradation of β-cellobiose. This study may advance on the novel design for sustainable conversion system of cellulose into high-added value chemicals.

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.

It is still not clear at the molecular level how the acidic silanol and basic alkylamine bifunctional groups over SBA-15 zeolite exert catalytic functions in aldol condensation of 5-hydroxymethylfu...

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.

The fundamental understanding of the cooperativity of a Bronsted acid together with its anion for cellulose conversion in an aqueous solution is limited at present, in which cellobiose has usually been regarded as a bridge that connects monosaccharides and cellulose. The mechanism of β-cellobiose conversion to 5-hydroxymethylfurfural (HMF) catalyzed by a Bronsted acid (H3O+) accompanied by counteranions in an aqueous solution has been studied using quantum chemical calculations at the M06-2X/6-311++G(d,p) level under a polarized continuum model (PCM-SMD). For the formation of the first HMF from cellobiose, there are three reaction pathways, i.e., through cellobiulose and glycosyl–HMF (C/H), through cellobiulose and fructose (C/F/H), and through glucose (C/G/H). For these three reaction pathways, the rate-determining steps are associated with the intramolecular [1,2]-H shift in the aldose–ketose tautomerization. C/H is the thermodynamically predominant pathway, while C/G/H is the kinetically dominant pathway. From cellobiose, the origin of the first HMF results kinetically from a small proportion of both C/H and C/F/H and from a large proportion of C/G/H. For the role of the counteranion in the catalytic activity of H3O+, the halide anions (Cl− and Br−) act as promoters, whereas both NO3− anions and carboxylate-containing anions behave as inhibitors. The roles of these anions in β-cellobiose conversion to HMF can be correlated with their electrostatic potential and atomic number, which may cause a decrease in the relative enthalpy energy and the value of entropy on interacting with the cation moiety. These insights may advance the novel design of sustainable conversion systems for cellulose conversion into HMF.

The selective removal of oxygen from 5-hydroxymethylfurfural (HMF) is challenging for the effective utilization of biomass. The catalytic mechanisms of palladium acetate toward the conversion of HMF to furfuryl alcohol (FFA), 5-methylfurfural (5-MF) and 2,5-dihydroxymethyl furan (DHMF) have been theoretically investigated. The decarbonylation of HMF to FFA includes (i) migratory extrusion, (ii) metal-acetate-co-assisted deprotonation, (iii) decarbonylation, (iv) metal-assisted deprotonation, and (v) migratory extrusion and catalyst regeneration. Both hydrogenation and deoxidation of HMF with HCOOH as the H-source involve (i) migratory extrusion, (ii) oxidative addition, (iii) reductive elimination, (iv) metal-assisted deprotonation, and (v) migratory extrusion and catalyst regeneration. The C–H bond cleavage is the crucial reaction step, in which the metal-acetate-co-assisted deprotonation is kinetically more preferable than the oxidative addition. Both FFA and DHMF are kinetically superior to 5-MF. In terms of selectivity, increasing the temperature is beneficial to decarbonylation and decreasing the temperature is advantageous to hydrogenation. The present finding provides molecular-level insight into the functions of both the metal-center and coordinated-ligand in the Pd(OAc)2 catalyst, which may drive the novel design of catalytic systems toward both decarbonylation and hydrogenation reactions.