Mechanistic insights into Lewis and Brønsted acid catalyzed conversion of sugars to platform furan derivatives in aqueous media
Date
2013
Authors
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Publisher
University of Delaware
Abstract
Biomass, the only renewable source of organic carbon, can play a key role in mitigating the rising concerns over greenhouse gas emissions and diminishing petroleum reserves. A promising approach in this regard entails biomass conversion to furanic compounds that have potential to substitute petroleum precursors in producing chemicals, polymers, and fuels. This thesis is focused on gaining thermodynamic, catalytic, and mechanistic insights into the production of versatile platform furan derivatives 5-(hydroxymethyl)furfural (HMF) and furfural from biomass derived simple sugars, glucose and xylose, respectively, in aqueous media. ☐ The role of solvent in the thermochemistry of sugar dehydration reactions has been investigated. The reaction free energies were calculated using ab initio (G4) calculations combined with the COSMO-SAC solvation model. It is shown that aldose (glucose, xylose)-to-ketose (fructose, xylulose) isomerization is an equilibrium-limited reaction, whereas the ketose dehydration to the respective furan derivative is practically an irreversible reaction. Further, the choice of solvent can affect product distribution in biomass conversion; as an example, a small fraction of water in organic media suppresses anhydroglucose formation from glucose and can improve HMF selectivity. ☐ In kinetic experiments, we found that the Brønsted acid catalyzed sugar dehydration strongly depends on its molecular structure. The ketose dehydration to the respective furan derivative is much faster than the aldose dehydration. High yields of furfural (~75%) and HMF (~60%) from xylose and glucose, respectively, were obtained by combining the aldose-ketose isomerization with the ketose dehydration in a cascade of reactions in aqueous media at much lower temperature (413 K) than reported earlier. This is achieved using a Lewis acid (CrCl<sub>3</sub> or Sn-beta) as the isomerization catalyst with a Brønsted acid (HCl or Amberlyst-15) as the dehydration catalyst in a single reactor. ☐ In order to gain mechanistic insights, the role of various metal ions in the aldose-ketose isomerization using a metal salt such as CrCl<sub>3</sub> and AlCl<sub>3</sub>, has been investigated. Speciation of the metal salts in aqueous media was modeled, and used in conjunction with kinetics experiments; we revealed for the first time that the partially hydrolyzed ions, [Cr(H<sub>2</sub>O)<sub>5</sub>OH]<super>2+</super> or [Al(H<sub>2</sub>O)<sub>5</sub>OH]<super>2+</super>, respectively, are the most active species for the aldose-ketose isomerization. Additionally, complex interactions between the Lewis and Brønsted acid catalysts have been explained in the sugar conversion: Brønsted acid decelerates the aldose-ketose isomerization by suppressing the equilibrium concentration of the partially hydrolyzed ions, whereas the Lewis acid promotes side reactions during fructose dehydration and HMF rehydration reactions. Thus, the catalyst loading and reactions conditions in sugar conversion need to be optimized. ☐ A strong interaction between the Cr cation and the glucose molecule in the first coordination sphere of the metal ion is indicated using extended X-ray absorption fine structure spectroscopy analysis and Car-Parrinello molecular dynamics simulation. More interestingly, mechanistic similarities are revealed among the homogeneous (CrCl<sub>3</sub>, AlCl<sub>3</sub>) and heterogeneous (Sn-beta) catalysts for the aldose-ketose isomerization using isotopic-labeling experiments. An intra-hydride transfer (C2 to C1) is the dominant reaction channel for the isomerization and a Lewis acid-Brønsted base bifunctional site is the most active site for all three catalysts. A reaction mechanism has been proposed for the Lewis acid-catalyzed isomerization and the energetics of the reaction pathway was calculated using ab initio for the Sn-beta catalyzed isomerization. Mechanistic insights are qualitatively consistent with the experimental data. ☐ The reaction mechanism of the Brønsted acid catalyzed fructose dehydration to HMF was investigated using isotopic-labeling experiments with nuclear magnetic resonance spectroscopy (NMR). The hydrogen transfer from C1 is the rate limiting step in the dehydration mechanism in both water and dimethyl sulfoxide (DMSO). Additionally, resonances corresponding to alkene and carbonyl groups were observed that compare well with an intermediate identified in the fructose dehydration reaction in DMSO. These observations suggest that the fructose dehydration in water and DMSO follows a similar reaction mechanism.