Density Functional Theory Investigations on the Structure and Dissolution Mechanisms for Cellobiose and Xylan in an Ionic Liquid: Gas Phase and Cluster Calculations

Abstract

Density functional theory (DFT) calculations have been carried out for cellobiose and xylan chosen as models for cellulose and hemicellulose, respectively, in gas phase, implicit and explicit solvent (water, methanol, and the ionic liquid, 1,3-dimethylimidazolium acetate) media using plane wave and atom centered basis set approaches in order to find out lowest energy conformers and configurations. Geometry, vibrational properties, and (1)H and (13)C NMR chemical shift values have been discussed under all three conditions. Calculations predict that inter- and intramolecular hydrogen bonding play an important role in the dissolution processes. In the gas phase and in implicit solvent, the anti-anti conformer of cellobiose and the anti-syn conformer of xylan are the most stable due to the formation of a large number of intramolecular hydrogen bonds. However, in the cluster calculations containing ion pairs of the ionic liquid (IL) surrounding the cellulosic units, the anti-syn conformer of cellobiose is more stable as intramolecular hydrogen bonds are substituted by intermolecular ones formed with the ions of the IL. The complexes of cellobiose (or of xylan) with the ions of the ionic liquid are stable with large negative binding energies ranging between -21 and -55 kcal mol(-1). The predicted (1)H NMR values of the lowest energy cellobiose conformers are in good agreement with the experimental value. Xylan binds stronger with the IL than cellobiose does by 20 kcal mol(-1). Furthermore, the two pentose rings in xylan are rotated by 60° to each other in contrast to their coplanarity in cellobiose, which can explain the higher solubility and the amorphous nature of hemicellulose in ionic liquids. The fewer number of hydroxyl groups in xylan (relative to cellobiose) does not affect the number of cations present in its first solvation shell while the number of anions is reduced.

Lignosulfonate-based heterogeneous sulfonic acid catalyst for hydrolyzing glycosidic bonds of polysaccharides

“…This assumption was consistent with previous reports on disaccharide and oligosaccharide hydrolysis [22,32,34]. Fig.…”

“…Binding functions of COOH and OH groups on cellobiose is supported by theoretical calculation (see supporting information) [32]. When these two groups act as binding sites to attract cellobiose to SO 3 H sites, two hydrogen bonds (1.94 and 1.832Å) with cellobiose may be formed with the binding energy of LF catalyst is −20.1 kcal mol −1 , which is lower than 12.3 kcal mol −1 of Amberlyst-15 with cellobiose.…”

“…The apparent activation energy of cellobiose hydrolysis is 98.5 kJ mol −1 . The value is smaller than 110-138 kJ mol −1 for cellobiose hydrolysis catalyzed by mineral acids, organic acid homogeneous catalysts [3,32]. We note that respecting the complexity of catalysis invoked in the catalytic system, more in-depth investigations are required in material synthesis and catalytic reactions.…”

Dissolution of cellulose in ionic liquid: A review

“…Agreeing to a molecular simulation experiment conducted by Gino et al [14], they verified that both anions and cations were involved in the dissolution process. This finding also supported by density functional theory (DFT) analysis, which was directed by Payal et al [16]. Some imidazolium-based ionic liquids, such halide (F-, CI-, bromine-and I-), phosphonate ([R1O (R2O) PO2], [R1O (R2) PO2] − and [R1R2PO2] −), carboxylate and thiocyanate were reported to dissolve cellulose at rather high concentrations.…”

Unveiling the initial pyrolytic mechanisms of cellulose by DFT study

“…Cellobiose is the cycle unit of cellulose which contains all the chemical characters appear in cellulose. Thus, in this work, cellobiose is selected and is investigated to revel the primary chemical reactions at the initial stage of cellulose pyrolysis [31,32].…”