(157d) The Chemical Properties Allowing the Tetrabutylphosphonium Chloride-Water Mixture to Efficiently Dissolve Cellulose in High Water Concentrations Via Molecular Dynamics

Our society is gradually moving away from fossil-based fuels, as we develop and transition to renewable fuel alternatives. Many of these renewable energy sources such as wind, solar, and tidal power can easily distribute power to stationary locations, via our existing electric grid. However, we also need to provide power to mobile sources, such as cars, tractor trailers, cargo ships, and other heavy equipment. In the near future, renewable biofuels can supply the energy dense and portable power to our vehicles, heavy equipment, and the shipping industry, much like gasoline and diesel do today.

These biofuels can be derived from waste biomass, such as corn husks at the end of a growing season, waste tree limbs from logging operations, or sawdust from a sawmill. The cellulose, contained in the biomass, needs to be broken down before the enzymes or other catalysts can turn it into fuels, such as ethanol. Many processes currently require high temperatures to sufficiently break down the cellulose, so it can efficiently be catalyzed in the downstream processes. Since a large part of this energy expenditure is due to the elevated temperature, it would be economically ideal to operate at near room temperature. It’s also critical that these solvents work in the presence of water, since cellulose naturally contains water, and an additional pre-drying step may thwart our efforts in developing an economical and sustainable process.

There are a variety of ionic liquids (ILs) capable of dissolving cellulose. However, many ILs fail to dissolve cellulose when even small amounts of water are added to them at moderate temperatures.^1-5 The IL and co-solvent mixtures of tetrabutylphosphonium hydroxide (TBPH) and water are capable of dissolving at least 15 wt% of cellulose at room temperature, within a working concentration range of 30 to 50 wt % water.⁶ Therefore, studying the less reactive derivatives of the TBP class of ILs, such as tetrabutylphosphonium chloride (TBPCl), could lead to an optimized IL and co-solvent mixture for dissolving cellulose.

In this work, we analyze the TBPCl-water mixtures physical and chemical interactions that take place during the cellulose dissolution process. We investigate the hydrogen bonding strengths and disruption capabilities between the TBPCl-water mixture and the cellulose bundle. The chloride anion and TBP cation show similar binding and cleaving properties as previous studies.⁷ Water plays a supporting role by assisting in the cellulose separation process and preventing cellulose reformation, providing the TBP molecule with added time to attack and cleave the weaken cellulose region. The pairwise interaction energies of the system are also calculated, which portray favorable interactions between the cellulose and the IL-water mixture. Understanding the physical and chemical properties that allow the TBPCl-water solution to work efficiently in water, is critical to developing more economical IL-water mixtures in the future.

¹Fang, Z., Smith, R. L., Jr., Qi, Z., Eds. Production of Biofuels and Chemicals with Ionic Liquids; Springer: Heidelberg, 2014.

²Wang, H.; Gurau, G.; Rogers, R. ”Ionic Liquid Processing of Cellulose”. Chem. Soc. Rev. 2012, 41, 1519–1537.

³Fang, Z., Smith, R. L., Jr., Qi, Z., Eds. Production of Biofuels and Chemicals with Ionic Liquids; Springer: Heidelberg, 2014.

⁴Mazza, M.; Catana, D.-A.; Vaca-Garcia, C.; Cecutti, C. Influence of Water on the Dissolution of Cellulose in Selected Ionic Liquids. Cellulose 2009, 16, 207–215.

⁵Swatloski, R. P.; Spear, S. J.; Holbrey, J. D.; Rogers, R. D. Dissolution of Cellulose with Ionic Liquids. J. Am. Chem. Soc. 2002, 91, 4974–4975.

⁶Mitsuru Abe, Yukinobu Fukaya and Hiroyuki Ohno, 2011, Fast and Facile Dissolution of Cellulose with Tetrabutylphosphonium Hydroxide Containing 40 wt% Water

⁷Brooks D. Rabideau, Animesh Agarwal, and Ahmed E. Ismail, 2013, Observed Mechanism for the Breakup of Small Bundles of Cellulose 1α and 1β Ionic Liquids from Molecular Dynamic Simulations