Abstract
Technical lignins are increasingly available at industrial scale, offering opportunities for valorization, such as by (partial) depolymerization. Any downstream lignin application requires careful tailoring of structural properties, such as molecular weight or functional group density, properties that are difficult to control or predict given the structure variability and recalcitrance of technical lignins. Online insight into changes in molecular weight (M w), to gauge the extent of lignin depolymerization and repolymerization, would be highly desired to improve such control, but cannot be readily provided by the standard ex‐situ techniques, such as size exclusion chromatography (SEC). Herein, operando attenuated total reflectance infrared (ATR‐IR) spectroscopy combined with chemometrics provided temporal changes in M w during lignin depolymerization with high resolution. More specifically, ex‐situ SEC‐derived M w and polydispersity data of kraft lignin subjected to aqueous phase reforming conditions could be well correlated with ATR‐IR spectra of the reaction mixture as a function of time. The developed method showed excellent regression results and relative error, comparable to the standard SEC method. The method developed has the potential to be translated to other lignin depolymerization processes.
Highlights
Lignin, an aromatic biopolymer that gives plants their structural integrity, holds considerable potential for the production of renewable chemicals and materials.[1,2,3,4,5,6,7,8,9,10] Several biorefining operations, including the pulp and paper industry and 2ndgeneration bio-ethanol plants generate streams of so-called technical lignins
The depolymerization of kraft lignin was monitored over time under alkaline (3.5 wt% NaOH, pH = 13.58) Aqueous phase reforming (APR) conditions (225 °C, 30 bar He) using a commercially available Pt/Al2O3 catalyst in an autoclave setup equipped with a bottommounted attenuated total reflectance infrared (ATR-IR) probe (Figure S1)
19 samples were taken over the course of the reaction for ex-situ size exclusion chromatography (SEC) determination of the molecular weight (Mw) and PD values, data needed as input to build the chemometrics model
Summary
An aromatic biopolymer that gives plants their structural integrity, holds considerable potential for the production of renewable chemicals and materials.[1,2,3,4,5,6,7,8,9,10] Several biorefining operations, including the pulp and paper industry and 2ndgeneration bio-ethanol plants generate streams of so-called technical lignins. Partial least squares (PLS) regression combined with FT-IR spectroscopy has already been used for quantification of some structural units and functional groups in lignin,[28,29] and the characterization of whole biomass.[30,31,32,33,34,35,36] Recently, it was reported that Mw values and inter-unit linkage abundances could be obtained for a wide range of technical lignins directly from their attenuated total reflectance (ATR)-IR spectra by chemometrics.[37] The FT-IR spectra were acquired from the solid lignin powder under ambient conditions using a standard ATR-IR accessory This demonstrates that information on Mw is in principle tractable from FT-IR spectra and poses the question if this method for off-line solid sample analysis could be extended to on-line monitoring of a lignin depolymerization reaction, for example, as part of a prospective biorefinery operation. A multivariate regression model was developed with the potential to replace off-line SEC measurements with online operando spectroscopy measurements to monitor changes in the Mw value of kraft lignin
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