Depolymerization of organosolv lignin to aromatic compounds over Cu-doped porous metal oxides

Abstract

Isolated, solvent-extracted lignin from candlenut (Aleurites moluccana) biomass was subjected to catalytic depolymerization in methanol with an added pressure of H 2 , using a porous metal oxide catalyst (PMO) derived from a Cu-doped hydrotalcite-like precursor. The Cu-PMO was effective in converting low-molecular weight lignin into simple mixtures of aromatic products in high yield, without char formation. Gel permeation chromatography was used to track changes in molecular weight as a result of the catalytic treatments and product mixtures were characterized by 1 H and 13 C NMR spectroscopy. In the temperature range 140-220 °C, unusual C9 catechols were obtained with high selectivity. Lignin conversion of >90% and recovery of methanol-soluble products in yields of was >70% was seen at 180 °C with optimized catalyst and biomass loadings. At 140 °C, 4-(3-hydroxypropyl)-catechol was the major product and could be isolated in high purity. † Electronic supplementary information (ESI) available: GPC traces and details of the extraction and purification of lignin from candlenut nutshells; spectroscopic details of low-and high-molecular weight lignins; representative powder XRD data for synthesized hydrotalcite-like catalyst precursor; GPC and NMR spectroscopic data for representative product mixtures after catalytic treatment in the supercritical regime. See

Heterogeneously catalyzed lignin depolymerization

“…Cu-doped porous catalysts in supercritical methanol reportedly suppress chars, with hydrogen obtained by in situ solvent reforming [41]. A similar approach was adopted by Anastas et al [42], employing porous metal oxides containing Cu 2? , Mg 2?…”

“…, achieving almost complete lignin conversion to catechols under mild conditions of 140-220°C. The role of Cu as a char inhibitor was attributed to enhanced hydrogenolysis, dehydration, and hydrogenation steps after cleavage of b-O-4 linkers, eliminating product aromaticity [41,42].…”

Mapping reactivities of aromatic models with a lignin disassembly catalyst. Steps toward controlling product selectivity

“…Analogous results were found when no catalyst was added. Cu-doped PMO catalysts were subsequently shown to be effective in the H 2 hydrogenation of candlenut lignin to aromatic products 27 and for the reduction of 5-hydroxymethylfurfural to tetrahydrofuran derivatives. 28,29 A related study recently demonstrated disassembly of organosolv lignin using a copper-doped magnesium-aluminum mixed metal oxide catalyst in supercritical ethanol.…”

Developing energy efficient lignin biomass processing – towards understanding mediator behaviour in ionic liquids

“…Therefore, depolymerisation of lignin into aromatic products would greatly benefit both the fine chemicals industry and the further development of sustainable chemical processes. [4][5][6] Over the years, several methods have been explored and documented for the depolymerisation of lignin, such as pyrolysis, 4,7 chemical oxidation including electrochemical oxidation, 6,8,9 hydrolysis 10, 11 and enzymatic reactions. [12][13][14] Among these methods, enzymatic reactions and electrochemical oxidation are preferable due to their environmentally friendly characteristics and lower energy demands.…”