Consecutive determination of softwood kraft lignin structure and molar mass from NMR measurements

Abstract

The general molecular properties and in particular, the molar mass of lignin are of central importance for industrial applications, as these data govern important thermal and mechanical characteristics. The focus of the present paper is pulsed field gradient-nuclear magnetic resonance (PFG-NMR), which is suitable for determination of lignins’ weight-average molar mass, based on diffusion constants. The method is calibrated by lignin fractions characterized by matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF-MS). It could be demonstrated on a set of softwood kraft lignins that the PFG-NMR approach gives results in very good agreement with those obtained using conventional size exclusion chromatography (SEC).

Fractionation and DOSY NMR as Analytical Tools: From Model Polymers to a Technical Lignin

“…23 Surprisingly, some recent results of the analysis of kraft lignin fractions suggest the opposite. 10,22 This could be possibly explained by the difference in fractionation methods or a different source of kraft lignin with a different degree of condensation and branching.…”

Identification of a diagnostic structural motif reveals a new reaction intermediate and condensation pathway in kraft lignin formation

“…For example acetylation with aminolysis, 42 permanganate oxidation, 43 mannich reaction, 44 aqueous titration, 45 FTIR, 45 and oximation with UV-spectroscopy. 46 More recently NMR methods have come to the fore in kraft lignin analysis, with 1D 13 C, 47 – 51 31 P NMR, 51 – 53 DOSY 54 , 55 and 2D HSQC/HMQC, 32 , 56 – 62 methods being used to study the kraft lignin structure and functional group abundance. These analyses have shown that the kraft lignin structure is indeed very different to in planta lignin, with significantly reduced levels of native lignin linkages and increased levels of phenolic groups, consistent with results from other analytical methods and modes of reactivity proposed based on model compound studies.…”

Using Fractionation and Diffusion Ordered Spectroscopy to Study Lignin Molecular Weight.

“…One option is to use MALDI-TOF MS however, it is challenging to both ionize lignin and ensure uniform ionization across its polydisperse MWD. [15] Recently, a multiangle laser light scattering (MALLS) method was developed that circumvented issues arising from lignin's florescence when more conventional analyses of this type were performed. [16] The most popular method used to analyse lignin MW is gel permeation chromatography (GPC).…”

“…[21] This technique initially measures the diffusivity of molecules in solution and this parameter can then be related to the MW of both small molecules [22,23] and polymers including lignin. [15,[24][25][26] In our previous report, diffusion coefficients of lignin fractions of variable MW were linearly correlated with MW data generated via GPC using the Mark-Houwink-Sakurada (MHS) relationship. [21] These fractions were generated by a selective dissolution solvent fractionation that used acetylation to enhance the solubility of Kraft lignin in acetone.…”