Abstract
Lignin is the second most abundant component, next to cellulose, in lignocellulosic biomass. Large amounts of this polymer are produced annually in the pulp and paper industries as a coproduct from the cooking process—most of it burned as fuel for energy. Strategies regarding lignin valorization have attracted significant attention over the recent decades due to lignin’s aromatic structure. Oxidative depolymerization allows converting lignin into added-value compounds, as phenolic monomers and/or dicarboxylic acids, which could be an excellent alternative to aromatic petrochemicals. However, the major challenge is to enhance the reactivity and selectivity of the lignin structure towards depolymerization and prevent condensation reactions. This review includes a comprehensive overview of the main contributions of lignin valorization through oxidative depolymerization to produce added-value compounds (vanillin and syringaldehyde) that have been developed over the recent decades in the LSRE group. An evaluation of the valuable products obtained from oxidation in an alkaline medium with oxygen of lignins and liquors from different sources and delignification processes is also provided. A review of C4 dicarboxylic acids obtained from lignin oxidation is also included, emphasizing catalytic conversion by O2 or H2O2 oxidation.
Highlights
Lignocellulosic biomass, including hardwood, softwood, and herbaceous crops, is an abundant renewable resource mainly composed of cellulose, hemicellulose, and lignin [1,2]
The main lignin valorization strategies are focused on the depolymerization of lignin into valuable compounds that could be used as platform molecules for industry
Vanillin (V, 4-hydroxy-3-methoxybenzaldehyde) is the most commonly produced aroma compound worldwide, with an annual production of 20,000 tons, 15% of which comes from lignin, and only 40 to 50 tons per year are produced from natural vanilla extract [6]
Summary
Lignocellulosic biomass, including hardwood, softwood, and herbaceous crops, is an abundant renewable resource mainly composed of cellulose, hemicellulose, and lignin [1,2]. Most biorefineries are currently focused on the valorization of cellulose and hemicellulose, a so-called sugar-based platform In this context, lignin is usually considered as a low-value residual product and has significant potential as a renewable resource to produce bio-based materials, fuels, and valuable chemicals [4]. In addition to the phenolic product conversion, dicarboxylic acids have been obtained from the aromatic ring cleavage of lignin or its fragments using more potent oxidants and/or more severe reaction conditions [11] Dicarboxylic acids such as muconic acid, maleic acid, succinic acid, and malonic acid are valuable platform chemicals and intermediates used in the polymer, pharmaceutical, and food industries [12,13,14]. It includes recent advances on lignin depolymerization towards C4 dicarboxylic acids, mainly by catalytic oxidation
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