Unfolding of Lignin Structure Using Size-Exclusion Fractionation

Abstract

The heterogeneous and recalcitrant structure of lignin hinders its practical application. Here, we describe how new approaches to lignin characterization can reveal structural details that could ultimately lead to its more efficient utilization. A suite of methods, which enabled mass balance closure, the evaluation of structural features, and an accurate molecular weight (MW) determination, were employed and revealed unexpected structural features of the five alkali lignin fractions obtained with preparative size-exclusion chromatography (SEC). A thermal carbon analysis (TCA) provided quantitative temperature profiles based on sequential carbon evolution, including the final oxidation of char. The TCA results, supported with thermal desorption/pyrolysis gas chromatography–mass spectrometry (TD-Py-GC-MS) and 31P NMR spectroscopy, revealed the unfolding of the lignin structure as a result of the SEC fractionation, due to the disruption of the interactions between the high- and low-MW components. The “unraveled” lignin revealed poorly accessible hydroxyl groups and showed an altered thermal behavior. The fractionated lignin produced significantly less char upon pyrolysis, 2 vs. 47%. It also featured a higher occurrence of low-MW thermal evolution products, particularly guaiacol carbonyls, and more than double the number of OH groups accessible for phosphitylation. These observations indicate pronounced alterations in the lignin intermolecular association following size-exclusion fractionation, which may be used for more efficient lignin processing in biorefineries.

Advancing Molecular Weight Determination of Lignin by Multi-Angle Light Scattering

“…Lignin, due to its complex structure combined with a relatively low MW, cannot be automatically assumed to occur in solutions as a near-spherical, random coil, unlike many other polymers [ 38 ]. Thus, the shape of lignin will not abide by the confinements of well-defined scatterers presented by Podzimek [ 39 ].…”

“…Coupling a SEC column to a mass spectrometer with appropriate ionization techniques has been a recent trend in the analysis of biomass-derived products [ 63 , 64 , 65 ]. Nonetheless, the bias towards lower MW fragments should be taken into account since ionization and vaporization of larger MW species is hindered [ 38 ]. The benefit of MS detection is that characteristic ions of the analyte can be identified while providing a mass distribution [ 66 ].…”

“…Mass spectrometry with appropriate ionization techniques (usually atmospheric pressure ionization) have reported MW values and structural information [ 37 ]. However, MS yields MW values that are inherently biased toward lower values, due to the hindered transfer of high MW species to the gas phase prior to, during, or after ionization [ 38 ].…”

A Complementary Multitechnique Approach to Assess the Bias in Molecular Weight Determination of Lignin by Derivatization-Free Gel Permeation Chromatography

“…Moreover, they found that derivatization reactions may skew the results, distorting the determined MW toward higher values . Recently, several studies reported methods employing silica-based stationary phases with more polar eluents such as DMSO and DMF using salt additives. , Nevertheless, THF-based GPC with PS-calibration is still a commonly employed method. − …”

“… 18 , 19 Nevertheless, THF-based GPC with PS-calibration is still a commonly employed method. 20 − 23 …”