Ionic Liquids for Conversion of Biomass to Sugars or HMF Without Additional Catalysts

Researchers at Colorado State University have shown that under relatively mild conditions (≤140 °C, 1 atm) and in the absence of added acid catalysts typically employed in biomass conversion, cellulose dissolved in certain ionic liquids (ILs) can convert into water-soluble reducing sugars in high total reducing sugar yield (up to 97%), or directly into the biomass
platform chemical 5-hydroxymethyl furfural (HMF) in high conversion (up to 89%) when CrCl₂ is added.

The results, they say, are broadly relevant to reactions involving the use of IL-H₂O mixtures (as solvents, reactants, or catalysts), including, but not limited to, organic catalysis, electrochemistry, and biomass
processing or conversion. A paper describing the work was published 10 March in the ACS journal Energy & Fuels.

The majority (60-90 wt %) of plant biomass is the biopolymer carbohydrates stored in the form of cellulose and hemicelluloses. As cellulosic material is the most abundant renewable biomass resource on earth, it can potentially meet our future energy needs if it can be efficiently converted into sugar molecules (such as glucose) with higher energy densities than the parent biomass. The biomass-derived sugars can be converted into fuels and value-added
chemicals by liquid-phase catalytic processing.

Alternatively, lignocellulosic materials can be directly converted into the biomass platform chemical 5-hydroxymethyl furfural (HMF), a versatile intermediate for top-value added chemicals and fuels (e.g., 2,4-dimethylfuran, a biofuel
with a 40% higher energy density than ethanol).

—Zhang et al.

Converting cellulosic biomass in ionic liquids is an alternative to enzymatic and chemical hydrolysis under typically heterogeneous conditions, or the hydrolysis in hot-compressed water under hydrothermal (high temperature and pressure) conditions.

However, ionic liquid-water mixtures are currently only viewed as a solvent currently recognized only as solvent (IL for solubilizing cellulose) and reactant (H₂O for hydrolysis). As a result, Zhang et al. note, the current cellulosic conversion process still employs additional mineral or organic acids (as catalyst). The process demonstrated by the Colorado State team avoids the use of the additional acid catalysts.

Their study, a combination of experimental methods and ab initio calculations, demonstrated that the significantly increased K_w [dissociation constant of water] by ILs in the IL-water mixture is responsible for the catalysis seen in the current efficient biomass conversion system without added acid catalysts. The finding that the water in ILs under mild
conditions can exhibit high K_w values (up to 3 orders of magnitude higher than the pure water under ambient conditions) is significant because such high Kw values are typically achievable by the water under harsh high-temperature or subcritical water conditions, they note.

…this work has demonstrated experimentally
the significant [H⁺] in the aqueous solution of water-stable ILs
[R(D)MIM]Cl, which has been attributed to the significantly
increased K_w of the water by ILs in the IL-water mixture through the theoretical study using combined quantum mechanical and continuum solvation methods. This intrinsic
property of the IL-water mixture has been utilized, in
absence of any additional acid catalysts, for the near quantitative
conversion of cellulosic biomass into water-soluble
reducing sugars.

Not only glucose, but also other reducing
sugars can be subsequently converted to HMF (with aid of the
catalyst CrCl₂). The TRS and HMF yields are highly sensitive
to the reaction temperature and time as well as the amount of
water added.

—Zhang et al.

Resources

• Article
  Yuetao Zhang, Hongbo Du, Xianghong Qian and Eugene Y.-X. Chen (2010) Ionic Liquid-Water Mixtures: Enhanced Kw for Efficient Cellulosic Biomass Conversion. Energy Fuels, Article ASAP doi: 10.1021/ef1000198