Isolation and Recovery of Lignin from Lignocellulosic Biomass Using Recyclable Protic Ionic Liquids (PILs) for a Cost-Effective Biomass Processing Technique.

摘要

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 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 being energy intensive and environmentally harmful, and need to be optimized to minimize waste generation and resource (lignin) underutilization.;A highly effective method has been developed for the simple extraction of lignin from lignocellulosic biomass using potentially inexpensive protic ionic liquids (PILs). Solubility tests with commercially available biomass components, in conjunction with the physical and chemical properties of the PILs, were correlated with the lignin extraction efficiency of the PILs in actual biomass (cornstover). These results indicate that increasing the xylan (i.e., hemicellulose) solubility in the PILs and dispersing the cellulose results in greater fiber disruption/penetration, which significantly enhances the effectiveness of the lignin extraction. Systematic variations in the cation supported by Raman spectroscopy and force field calculations confirms that PILs from cyclic amines favor xylan solubility as these PILs are more ionic and PILs from alkanolamines are able to participate in hydrogen bonding with the polysaccharides and disperse the cellulose. In particular, the PIL ethanolammonium acetate, i.e., [Eth][Ac], is able to extract up to 85% of the lignin found in cornstover.;The effect of hydrogen bonding in the PIL is further analyzed in order to design a PIL with increased lignin extraction efficiency. Analysis of lactate-PILs indicates that hydrogen bonds play an important role in the physiochemical properties of the PILs, but the trend in lignin extraction for the PILs still follow the trend in ionicity expected. Therefore, ionic interactions are the dominant factor that contributes to lignin removal--with hydrogen bonding slightly improving the ionic effect. The developed method for lignin extraction was also effectively applied to remove lignin from switchgrass, but woody materials such as pinewood and beech wood were not favorable for this lignin removal process.;After the lignin-extraction step, the PILs are easily recovered using distillation leaving the separated lignin and cellulose-rich residues available for further processing. Complete PIL recovery was hindered by the formation of unwanted side products (amides) in the [Pyrr][Ac] PIL. The [Eth][Ac] PIL, however, showed no formation of amides, but have a PIL lower recovery rate (90%) due to the high temperature requirement for distillation induced by the increased hydrogen bonding in this PIL. Careful selection of the PIL ions is necessary to develop a PIL with balanced properties for the recovery of pure PILs at a high yield.;The lignin extract and the cellulose-rich pulp were physically, chemically, and thermally characterized. Lignin characterization shows that the PILs fragment the lignin molecules during the extraction/dissolution process. The more ionic [Pyrr][Ac] PIL fragments the lignin to smaller sized particles and results in a more homogenous distribution of lignin particle sizes. Cellulose-rich residues are shown to have increased fiber size (swelling) and the formation of pores on the fiber surface is observed. The crystallinity (phase and amount), however, is largely unchanged. Using PILs, a simple yet effective method has been developed for the removal of lignin from biomass, which should greatly aid in the implementation of an economically viable integrated multi-product biorefinery.