Insights into the solvation of glucose in water, dimethyl sulfoxide (DMSO), tetrahydrofuran (THF) and N,N-dimethylformamide (DMF) and its possible implications on the conversion of glucose to platform chemicals

Abstract

Cellulosic biomass derived molecules such as glucose can be converted into specific platform chemicals like 5-hydroxymethylfurfural (HMF), levulinic acid and gamma valerolactone (GVL). The solvation medium plays an important role in the selective conversion of glucose to these platform chemicals and it is shown that the addition of co-solvents increases the selectivity towards desired products and minimizes the formation of undesired condensation/polymerization products and humins. Hence, it becomes imperative to understand the implications of the solvation of glucose by co-solvents on glucose conversion reactions. In the present paper, we implement OPLS-AA force-field based molecular dynamics simulations to investigate the solvation of glucose in water, in the presence of dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF) and tetrahydrofuran (THF). The local arrangement of solvents around the glucose molecule is analyzed using 2-dimensional radial pair distribution functions and 3-dimensional volumetric maps. Additionally, lifetimes and activation free energies of hydrogen bonds between solvents and glucose and the tendency of glucose molecules to agglomerate were studied. It was observed that all the aforementioned co-solvents compete with water to be in the first solvation shell of glucose and significant amount of water is pushed to the second coordination shell.Though fewer water molecules are directly coordinated with glucose in the presence of co-solvents, they are bound strongly to it. Additionally, DMSO, THF and DMF tend to localize more around the hydrogen atom of the hydroxyl groups of selected carbon atoms of glucose. This preferential arrangement of co-solvents and water around glucose may play a role in facilitating the reaction pathway for the formation of HMF and levulinic acid and may reduce the likelihood of glucose' degradation to unwanted dehydration/rehydration products. Increasing the proportion of co-solvents also increases the hydrogen bond lifetimes between water and glucose and reduces the mobility of glucose molecules within the solvent. The reduced mobility of glucose molecules in the presence of cosolvents might be correlated to the experimentally observed reduction in the rate of formation of polymerization/condensation products and humins. † Electronic supplementary information (ESI) available: Centre of mass RDFs for glucose-co-solvents, co-solvent-water and water-water pairs are provided. See

Organic Solvent Effects in Biomass Conversion Reactions

“…Biomass liquefaction reactions typically involve the liquid‐phase deconstruction of biomass to small organic oxygenates at temperatures between 200 and 380 °C . In these processes, organic solvents can be used to improve and accelerate lignin dissolution and to affect cellulose crystallinity, thus decreasing the amount of solid residues formed .…”

“…Vasudevan et al. found that, with the presence of an organic solvent in water, the mobility of glucose molecules was reduced, forming longer‐lived hydrogen bonds with decreasing water content (Table ) . This was attributed to the increased hydrogen‐bonding strength of glucose with water and with organic solvents in the presence of increasing amount of organic solvents.…”

The challenge of converting biomass polysaccharides into levulinic acid through heterogeneous catalytic processes

“…Like homogeneous systems, solvent selection is also of particular importance to determine the kind of final product. Some authors even claim that the solvent choice has a more important role in the selectivity of reactions than the working temperature and / or the chosen catalyst because the solvation medium is a key parameter that determines final yields, selectivity, activation energy, and reaction kinetics . A large number of solvents can be used in these catalytic reactions: Polar aprotic solvents, which are molecules with electric dipole or a multipole moment, which do not have a hydrogen atom bound to an oxygen or a nitrogen, but are generally double‐bounded to a carbon atom, like DMSO, dimethylformamide (DMF), dimethylacetamide (DMA), tetrahydrofuran (THF), ethyl acetate, dioxane, gamma‐valerolactone (GVL), and acetonitrile (ACN) …”

“…Dimethylsulfoxide is widely used to produce HMF because it improves the selectivity of the reaction . Since water protonation is non‐selective, water forms hydrogen bonds with oxygen and hydrogen atoms of the OH in polysaccharides, whilst other organic solvents (such as DMSO, THF, or DMF) do it only with the hydrogen atoms, increasing final HMF yields (see Table ) . Several authors even report that, when using fructose / sulfated zirconia system, final HMF yields of 65.6% and 29.9 were obtained in acetone‐DMSO and water, respectively .…”

“…On the other hand, in DMSO/water mixtures rich in water (water molar fraction >0.5), the reaction medium has properties similar to those of water, and LA formed . In DMSO / water mixtures, DMSO compete with water to be the first in the solvation and protonation of sugars generating less competition for sugar protons than water . This means that introduction of this organic solvent into the aqueous phase generates greater stability of the hydrogen bonds between polysaccharides and water, increasing their strength and lifetime .…”