Abstract
The determination of the true molar mass distribution (MMD) of lignin is highly important to understand the physicochemical characteristics for lignin-based value-added applications. It is imperative to develop a universal method to quantify accurate MMD of lignin using size exclusion chromatography (SEC), as the conventional method with polymer standards provides irregular MMD results. This work aims to evaluate the MMD of five lignin samples (Protobind 1000, Organosolv, Indulin, Pine Kraft and Eucalyptus Kraft) in THF. Different derivatization methods (acetylation, fluorobenzylation and fluorobenzoylation) were performed. FTIR and 19F NMR analyses were used to follow derivatization. The MMDs of derivatized and underivatized lignins were determined by the conventional method and compared with the universal calibration method developed using intrinsic viscosity. The 19F NMR spectra provided the information to quantify the degree of substitution of lignin hydroxyl groups, to calculate the true molar mass of the derivatives of lignin monomers. The obtained MMDs values for all the derivatized lignin by universal calibration were found to be three to five times higher than that of the conventional calibration. The polydispersity values obtained with the acetylation method were higher than the fluoro-derivatives. The results demonstrated that fluoro-derivatization is an appropriate method to apply to higher molar mass technical lignins and lacks solubility and aggregation issues.
Highlights
Lignin is the second most abundant biopolymer produced as a by-product during the wood pulping process and is mostly utilized to produce energy through burning [1,2]
Four industrial lignin samples and one laboratory-cooked lignin were used in this study: (i) Soda lignin from wheat straw (Protobind 1000) (PB) was purchased from Green Value Enterprises LLC, Frederick, MD, USA; (ii) Kraft lignin from pine (KR) was provided by the Centre Technique du Papier (CTP), Grenoble, France; (iii) Organosolv lignin (ORG) from wheat straw was purchased from CIMV Co., Labège, France; (iv) Kraft Indulin AT lignin (IND) was purchased from DKSH Ltd., Zurich, Switzerland [37]; and (v) Eucalyptus Kraft lignin (EU-KR) was prepared in the laboratory and the detailed procedure can be found in [38]
The author stated that chromatographic separation in size exclusion chromatography (SEC) columns is based on the molar hydrodynamic volume of the polymer pellet rather than on its molar mass, which is the principle of universal calibration in SEC
Summary
Lignin is the second most abundant biopolymer produced as a by-product during the wood pulping process and is mostly utilized to produce energy through burning [1,2]. The physicochemical properties and molecular weight distribution of lignin are highly dependent on the source of the wood species, pulping and isolation processes [4]. The obtained molar mass is rather inaccurate and deviates from the absolute molar mass of lignin because the cross-linked three-dimensional structural conformation of lignin greatly differs from that of the narrow polystyrene (PS) standards [8,9]. Another drawback of methods based only on elution time is the phenomenon of flow dispersion through columns, valves, and tubing that cause peak enlargement, which is known as band broadening [10–12]. Mathematical corrections are theoretically possible to account for this, with variable accuracy [13]
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