From Lignin to Valuable Aromatic Chemicals: Lignin Depolymerization and Monomer Separation via Centrifugal Partition Chromatography

Abstract

Lignin has long been recognized as a potential feedstock for aromatic molecules; however, most lignin depolymer­ization methods create a complex mixture of products. The present study describes an alkaline aerobic oxidation method that converts lignin extracted from poplar into a collection of oxygenated aromatics, including valuable commercial compounds such as vanillin and p-hydroxybenzoic acid. Centrifugal partition chroma­tography (CPC) is shown to be an effective method to isolate the individual compounds from the complex product mixture. The liquid–liquid extraction method proceeds in two stages. The crude depolymerization mixture is first subjected to ascending-mode extraction with the Arizona solvent system L (pentane/ethyl acetate/methanol/water 2:3:2:3), enabling isolation of vanillin, syringic acid, and oligomers. The remaining components, syringaldehyde, vanillic acid, and p-hydroxybenzoic acid (pHBA), were resolved by using ascending-mode extraction with solvent mixture comprising dichloromethane/methanol/water (10:6:4) separation. These results showcase CPC as an effective technology that could provide scalable access to valuable chemicals from lignin and other biomass-derived feedstocks.

Monomers and Macromolecular Materials from Renewable Resources: State of the Art and Perspectives

“…These processes are known as pulping with a “lignin-first” approach, a term that indicates that the valorization of lignin is prioritized over the valorization of carbohydrates [ 68 , 69 ]. An alkaline aerobic oxidation method that converts lignin into a collection of oxygenated aromatics, including vanillin and p-hydroxybenzoic acid, was recently published, and centrifugal partition chromatography was proven effective to isolate the individual compounds from the complex product mixture [ 70 ]. In all cases, a careful choice of the biomass fractionation process, ideally avoiding extensive side reactions of lignin, allows the production of promising aromatic building blocks for polymer synthesis.…”

Simple and Practical Conversion of Benzoic Acids to Phenols at Room Temperature

“…We decided to employ p-hydroxybenzoic acid as the starting material because it can be readily extracted from biomass, which is beneficial in the context of green chemistry. 19 To facilitate the purification, the acid was converted to its methyl ester. Following esterification/bromination/Suzuki coupling, the first arene was introduced to an ortho position of the −OH group (69).…”

“…On the other hand, employing benzoic acids as the aryl precursors to prepare phenols is advantageous because (1) unlike direct C-H oxidation of arenes, benzoic acids' oxidation requires no directing groups and can avoid the regioselectivity issue since the reactive sites of benzoic acids have been fixed without any bias; (2) unlike aryl (pseudo)halides and aryl metal species, benzoic acids are stable, not moisture-and airsensitive, and readily available with a wide range of structural diversity and can also be extracted from the biomass platform. 19 Thus, a simple and efficient approach to convert benzoic acids to phenols would be highly desirable.…”

“…Current strategies to synthesize phenols from various aryl precursors include the following: (1) direct oxidation of the C–H bond to the C–O bond; − (2) conversion of the C–X bond to the C–O bond in a redox-neutral manner assisted by transition metals; − and (3) oxidation of the C–M bond to the C–O bond , (Figure left). On the other hand, employing benzoic acids as the aryl precursors to prepare phenols is advantageous because (1) unlike direct C-H oxidation of arenes, benzoic acids’ oxidation requires no directing groups and can avoid the regioselectivity issue since the reactive sites of benzoic acids have been fixed without any bias; (2) unlike aryl (pseudo)­halides and aryl metal species, benzoic acids are stable, not moisture- and air-sensitive, and readily available with a wide range of structural diversity and can also be extracted from the biomass platform . Thus, a simple and efficient approach to convert benzoic acids to phenols would be highly desirable.…”

Striding the threshold of photocatalytic lignin-first biorefining via a bottom-up approach: from model compounds to realistic lignin

“…10 From the perspective of sustainable technologies and carbon neutrality, the efficient utilization of renewable lignin and its derivatives that builds the industrial platform of aromatic chemicals and drop-in-fuels would alleviate the intense dependence on fossil energy and minimize greenhouse gas emission. 11,12 At present, inspired by the largest renewable aromatic biopolymer, the directional depolymerization of lignin can yield valuable phenyl intermediates including phenol, benzene, toluene, and xylene. 13 Apart from that, it is of utmost importance to reform the current methods of lignin upgradation (e.g., pyrolysis, as well as chemical and biological catalysis), which generally require harsh reaction conditions (e.g., high temperature and pressure, acid/base, or extra hydrogen) and expensive catalysts that inevitably hinder their large-scale application.…”