(462a) Ionicity and Hydrogen–Bonding As Critical Factors for Protic Ionic Liquids (PILs) Dissolution of Polysaccharides and Lignin

Authors: 
Achinivu, E., North Carolina State University
Effectively partitioning lignocellulosic biomass into its various fractions―cellulose, hemicellulose and lignin―is essential for the implementation of a biofuel/biorefinery-based economy. In particular, an efficient, low-cost technique for the selective removal and recovery of lignin, the component that largely renders biomass intractable, is necessary to facilitate easier access to the polysaccharides and the production of valuable side-product streams based on lignin. Current separation techniques for lignin removal suffer significant drawbacks such as a high energy intensity and a harmful environmental impact, and need to be optimized to minimize waste generation and resource (lignin) under-utilization.

A highly effective method has been developed for the facile extraction of lignin from lignocellulosic biomass using potentially inexpensive protic ionic liquids (PILs). With systematic variations in the cation of acetate-based PILs, several PILs were evaluated to reveal the important structural features that pertain to lignin extraction. Solubility tests (with commercially available biomass components), in conjunction with the physical and chemical properties of the PILs, and force field energy minimizations (MM2), were correlated with the PILs’ lignin extraction efficiency. These results indicate that the PIL’s ability to partially dissolve xylan (i.e., hemicellulose) and disperse the cellulose in a stable suspension results in greater fiber disruption/penetration, and consequently, enhances the effectiveness of the lignin extraction. This observation was also directly linked to two significant PIL properties: ionicity and hydrogen bonding interactions.

PILs from cyclic amines (e.g pyrrolidinium acetate – [Pyrr][Ac]) and PILs from alkanolamines (e.g. ethanolammonium acetate – [Eth][Ac]) are two noteworthy PIL groups that were identified with a superior performance for this process (i.e lignin extraction). Further analyses revealed that these PILs are associated with ions that are strongly interacting, which is linked to key structural features of the PIL, the ease of accessibility of the PIL ions, and the ability of the PIL ions to form hydrogen bonds with ideal bond lengths. In particular, the PIL, [Eth][Ac], is able to selectively extract up to 85% of the lignin found in cornstover (a biomass resource). After the lignin-extraction step, the PILs are easily recovered using distillation leaving the separated lignin and cellulose-rich residues available for further processing.

Careful selection of the ions present in PILs is essential for their utilization in many applications especially in the biomass/bioenergy industry. The strategies employed in this research project can be easily applied as an approach for designing task-specific PILs for targeted applications (such as biomass processing), which will enable improved process efficiencies and product yield/purity.