Mechanistic study of cellobiose conversion to 5-hydroxymethylfurfural catalyzed by a Brønsted acid with counteranions in an aqueous solution.

The fundamental understanding of the cooperativity of a Brønsted 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 ß-cellobiose conversion to 5-hydroxymethylfurfural (HMF) catalyzed by a Brønsted 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 ß-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.

Molecular mechanism comparison of decarbonylation with deoxygenation and hydrogenation of 5-hydroxymethylfurfural catalyzed by palladium acetate.

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.

Theoretical study on molecular mechanism of aerobic oxidation of 5-hydroxymethylfurfural to 2,5-diformyfuran catalyzed by VO2 + with counterpart anion in N,N-dimethylacetamide solution.

Vanadium-containing catalysts exhibit good catalytic activity toward the aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformyfuran (DFF). The aerobic oxidation mechanism of HMF to DFF catalyzed by VO2 + with counterpart anion in N,N-dimethylacetamide (DMA) solution have been theoretically investigated. In DMA solution, the stable VO2 +-containing complex is the four-coordinated [V(O)2(DMA)2]+ species. For the gross reaction of 2HMF + O2 → 2DFF + 2H2O, there are three main reaction stages, i.e., the oxidation of the first HMF to DFF with the reduction of [V(O)2(DMA)2]+ to [V(OH)2(DMA)]+, the aerobic oxidation of [V(OH)2(DMA)]+ to the peroxide [V(O)3(DMA)]+, and the oxidation of the second HMF to DFF with the reduction of [V(O)3(DMA)]+ to [V(O)2(DMA)2]+. The rate-determining reaction step is associated with the C-H bond cleavage of -CH2 group of the first HMF molecule. The peroxide [V(O)3(DMA)]+ species exhibits better oxidative activity than the initial [V(O)2(DMA)2]+ species, which originates from its narrower HOMO-LUMO gap. The counteranion Cl- exerts promotive effect on the aerobic oxidation of HMF to DFF catalyzed by [V(O)2(DMA)2]+ species.