Lignin Depolymerization: A Comparison of Methods to Analyze Monomers and Oligomers

Abstract

Catalytic liquefaction of lignin is an attractive process to produce fuels and chemicals, but it forms a wide range of liquid products from monomers to oligomers. Oligomers represent an important fraction of the products and their analysis is complex. Therefore, rapid characterization methods are needed to screen liquefaction conditions based on the distribution in monomers and oligomers. For this purpose, UV spectroscopy is proposed as a fast and simple method to assess the composition of lignin‐derived liquids. UV absorption and fluorescence were studied on various model compounds and liquefaction products. Liquefaction of Soda lignin was conducted in an autoclave, in ethanol and with Pt/C catalyst (H2, 250 °C, 110 bar). Liquids were sampled at isothermal conditions every 30 min for 4 h. UV fluorescence spectroscopy is related to GC–MS, gel‐permeation chromatography (GPC), MALDI‐TOF MS, and NMR characterizations. A depolymerization index is proposed from UV spectroscopy to rapidly assess the relative distribution of monomers and oligomers.

Depolymerization of Technical Lignins in Supercritical Ethanol: Effects of Lignin Structure and Catalyst

“…The UV fluorescence spectra are presented in supporting material (figure S12). They depict 3 main peaks as previously unraveled 46 : at around 375nm (lignin), 350nm (oligomers), 305nm (monomers). They show a progressive decrease of the lignin peak accompanied with an increase in oligomers and monomers peaks upon the conversion time.…”

“…The liquid products were analyzed by UV fluorescence, GPC and GC/MS. We have justified in our previous article 46 the interest of UV fluorescence as a fast method to assess the distribution in monomers and oligomers produced by lignin depolymerization. The analytical methods were developed under different conditions (Soda lignin, Pt/C catalyst) 46 than the ones presented in this work.…”

“…It is of high importance to analyze this continuum of molecules for understanding the mechanisms of lignin depolymerization. We have previously demonstrated that UV fluorescence is an interesting technique to assess fastly this continuum without samples' pre-treatment 46 . The emission peak of UV fluorescence is related to the number of undisrupted conjugated bonds in molecules and, therefore, to the molecular weight of the conjugated molecules.…”

“…Possible condensation reactions of lignin fragments were reviewed recently. 61 We have proposed a depolymerization index (DI) based on the deconvolution of UV fluorescence peaks 46 . It relates the signal of the depolymerization products (monomers and oligomers) to the global signal (products + lignin).…”

“…65 Some ethanol-derived products were detected only at early reaction times and at small yields (1-Butanol; 1,1-diethoxy-Ethane; Butanoic acid, ethyl ester, etc.) 46 . The conversion of ethanol under similar conditions has been studied in more details by other authors 50,66 .…”