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

• Main Author: Vasudevan, Vallabh
• Other Authors: Samir Hemant Mushrif
• Format: Theses and Dissertations
• Language: English
• Published: 2015
• Subjects: DRNTU::Science::Chemistry::Physical chemistry::Catalysis; DRNTU::Science::Chemistry::Physical chemistry::Reactions and kinetics
• Online Access: https://hdl.handle.net/10356/65833
• Institution: Nanyang Technological University

Biomass is a carbon neutral renewable alternative to fossil fuels, and can help in reducing the demand for fossil fuels in the near future. This study is relevant to the liquid phase catalytic conversion of biomass to fuels, fuel additives and chemicals. Products obtained during the catalytic conversion can be used as platform chemicals for further upgradation to bulk chemicals, fuels and fuel additives. One such platform chemical is 5-hydroxymethylfurfural (HMF), which can be produced by the acid catalyzed dehydration of glucose, which is the largest monomeric component of lignocellulosic biomass. Recent research on the conversion of glucose to HMF has shown that the conversion and yield of the reaction are affected by the choice of solvents used. It has been noted that the solvents not only play the role of a benign solvation medium but also are involved in the conversion process and significantly affect the conversion and yield of the reaction. However, the exact role of solvents, as the reaction media, is not yet well understood. It would be extremely difficult to decouple physical solvation and chemical participation effects using experimental methods, so as to study them individually. To overcome this difficulty, computational modeling is used. The present work focuses on identifying key physical interactions between biomass molecules and solvents using molecular modeling. We perform molecular dynamics simulations, using OPLS/AA force field to investigate the physical solvation of glucose in four common solvents used in Biomass conversion studies, namely, water, Dimethylsulfoxide (DMSO), N,N-Dimethylformamide (DMF) and Tetrahydrofuran (THF). The parameters for the solvents were selected by careful consideration and were benchmarked with existing data from the literature to ensure accuracy. Force-field parameters for DMF, as reported in the literature with a negative partial charge on the amide nitrogen, resulted in poor prediction of the bulk properties like density and dielectric constant and showed reduced miscibility with water in a DMF-water mixed system. To address these issues, we proposed a new set of parameters with a small positive partial charge on the amide nitrogen. The new parameters were benchmarked with both, preexisting experimental data and using ab initio molecular dynamics, to ensure better accuracy of the DMF molecule. 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. From our simulation studies and analysis it was observed that i) 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; ii) though fewer water molecules are directly coordinated with glucose in the presence of co-solvents, they are bound strongly to it; iii) DMSO, THF and DMF tend to localize more around the hydrogen atom of the hydroxyl groups of selected carbon atoms of glucose; iv) the 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; v) 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 co-solvents might be correlated to the experimentally observed reduction in the rate of formation of polymerization/condensation products and humins.