Quantification and Classification of Carbonyls in Industrial Humins and Lignins by ¹⁹F NMR

Abstract

Lignin and humins are both (by-)products of biorefining processes aimed at the valorization of lignocellulosic biomass. In order to improve the efficiency of such biorefineries and to develop new valorization pathways for these polymeric materials, detailed insight into their complex chemical structure and functional group distribution is required. Here, we report on the quantification and classification of the ketone and aldehyde carbonyl functional groups contained in these two polymers by 19 F NMR. The known method of carbonyl derivatization with 4-(trifluoromethyl)phenylhydrazine to the corresponding hydrazone has been improved and simplified, allowing derivatization directly in an NMR tube, requiring no additional workup before quantification by 19 F NMR. Furthermore, the scope of the method was assessed and expanded, with model compound studies, which included monomeric and dimeric compounds as well as synthetic polymers, showing that the carbonyl functions can indeed be reproducibly quantified. Using this model compound library, the carbonyl functional groups in two technical lignins (Indulin Kraft and Alcell) and, for the first time, an industrial humin could be quantified and classified. The industrial humin was found to contain 6.6 wt % of carbonyl functions, with aliphatic and conjugated carbonyls being detected. The relatively high abundance of such functional groups, which are amenable to further chemical modification, provides opportunities for the use of these humin byproducts in various applications, e.g., as materials after derivatization.

Catalytic pyrolysis of recalcitrant, insoluble humin byproducts from C6 sugar biorefineries

“…The industrial humins were further purified using solvent extractions to remove the residual monomers (MMF, levulinic acid). Details of the extraction procedure and characterisation of the purified industrial humins (PIH) are given in reference [31].…”

Structural Characterization of Lignin and Its Degradation Products with Spectroscopic Methods

“…Constant et al [78] carried out the quantification and classification of carbonyls in industrial humins and lignins by 19 F NMR. The carbonyl groups were transformed to corresponding hydrazone with 4-(trifluoromethyl)phenylhydrazine before quantification by 19 F NMR.…”

Selective Catalytic Oxidation of Humins to Low‐Chain Carboxylic Acids with Tailor‐Made Polyoxometalate Catalysts

“…Only little is hitherto known about the structure of humins. Up to date, only a high furan content linked via aliphatic groups is reported ,. Therefore, the usage as feedstock for reactions that cut a certain bond is not yet possible.…”