Lignin is a highly abundant yet recalcitrant polymer finding only limited use. In this study, we subjected alkali lignin to size exclusion chromatography verified by extensive calibration using both polar and non-polar standards, to obtain six lignin fractions distinguished by molecular weight (MW). Then we performed a comprehensive characterization of these fractions utilizing a suite of methods including high performance SEC, direct infusion high resolution mass spectrometry, gas chromatography-mass spectrometry (GC-MS), thermal desorption/pyrolysis-GC-MS, phosphorus-31 NMR spectroscopy and thermal carbon analysis. Unlike commonly used fractionation methods, i.e., precipitation, solvent extraction and membrane filtration, preparative SEC with a properly selected stationary phase enabled the fraction collection with a desired narrow MW range. Alkali lignin was shown to contain impurities of a lower MW, apparently of carbohydrate nature, which eluted prior to high MW lignin species in SEC; this common feature may skew the determined MW values if the detector other than DAD is used. NMR data suggest that the obtained SEC fractions feature a different functional groups composition. A preliminary TEM evaluation of the particle size distribution in the solution showed that the high-MW fractions form larger size nanoparticles in water than original lignin. By contrast, lower MW fractions either featured high zeta-potential, forming unstable nanoparticles or none, with zero diameter.

Alkali lignin fractionation by size exclusion chromatography: Analysis of fractions

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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