Non-catalytic oxidative depolymerization of lignin in perfluorodecalin to produce phenolic monomers

We demonstrate for the first time non-catalytic, oxidative cracking with molecular oxygen (O2) to depolymerize native lignin into oxygenated phenolic monomers. Maximum monomer yield of 10.5 wt% was achieved at 250 °C after only 10 min of reaction and included vanillin, syringaldehyde, vanillic acid, and syringic acid. High rates of oxidation are attributed to the use of perfluorodecalin as solvent. Perfluorodecalin is a perfluorocarbon (PFC), characterized by their chemical stability and exceptionally high solubility for O2. Monomer yields were typically five-fold higher in perfluorodecalin compared to solvents more commonly employed in lignin conversion, such as methanol, butanol, acetonitrile, and ethyl acetate. Phenolic monomer production in perfluorodecalin favors high temperatures and short reaction times to prevent further oxidation of the produced monomers. Lignin oil obtained under oxidative conditions in perfluorodecalin showed lower molecular weight and smaller polydispersity compared to other solvents. Increasing the reaction time further decreased the molecular weight, while increasing reaction time in an inert atmosphere increased the molecular weight of the lignin oil. High concentrations of O2 in perfluorodecalin not only increased lignin depolymerization but suppressed undesirable condensation reactions. Depolymerization is likely initiated by thermally induced homolytic cleavage of ether linkages in lignin to form phenoxy and carbon-based radicals. These radicals bind with O2 as a radical scavenger and further react to form phenolic monomeric products rather than repolymerizing to large oligomers. The PFC process was scaled from 5 mL to 250 mL without any loss of yield. Because most organic compounds are not soluble in perfluorodecalin, recycling is easily achieved via liquid–liquid separation

Lignin valorization into bio-based chemicals and fuels

Lignin, the sole component of biomass containing aromatic structures, is a promising feedstock for renewable chemicals. However, lignin is produced chiefly as the by-product of industries focused on recovering carbohydrates, with little attention directed toward preserving lignin for chemical synthesis. For instance, in Kraft pulping, responsible for over 90% of all chemical pulps, the high alkalinity of the process converts lignin into a condensed and more recalcitrant form known as technical lignin, which relegates it to relatively low-value applications such as boiler fuel for heat and power generation. To retain the intrinsic value of virgin lignin, biorefineries will have to employ either mild depolymerization processes, such as ammonia-based fractionation and low-temperature organosolv techniques, or employ processes that continuously stabilize phenolic products as they are released from biomass. This second approac, sometimes referred to as the lignin-first strategy for deconstructing lignocellulose, is receiving increasing attention. In the first part of this study, we demonstrated for the first time non-catalytic, oxidative cracking with molecular oxygen (O2) to depolymerize native lignin into oxygenated phenolic monomers. Maximum monomer yield of 10.5 wt% was achieved at 250°C after only 10 min of reaction and included vanillin, syringaldehyde, vanillic acid, and syringic acid. High rates of oxidation are attributed to the use of perfluorodecalin as solvent. Perfluorodecalin is a perfluorocarbon (PFC), characterized by their chemical stability and exceptionally high solubility for O2. Monomer yields were typically five-fold higher in perfluorodecalin compared to solvents more commonly employed in lignin conversion, such as methanol, butanol, acetonitrile, and ethyl acetate. Phenolic monomer production in perfluorodecalin favors high temperatures and short reaction times to prevent further oxidation of the produced monomers. Lignin oil obtained under oxidative conditions in perfluorodecalin showed lower molecular weight and smaller polydispersity compared to other solvents. Increasing the reaction time further decreased the molecular weight, while increasing reaction time in an inert atmosphere increased the molecular weight of the lignin oil. High concentrations of O2 in perfluorodecalin not only increased lignin depolymerization but suppressed undesirable condensation reactions. Depolymerization is likely initiated by thermally induced homolytic cleavage of ether linkages in lignin to form phenoxy and carbon-based radicals. These radicals bind with O2 as a radical scavenger and further react to form phenolic monomers rather than repolymerizing to large oligomers. The PFC process was scaled from 5 mL to 250 mL without loss in yield. Because most organic compounds are not soluble in perfluorodecalin, recycling is easily achieved via liquid-liquid separation. In the second part of this study, the possibility of phenyl ester formation from lignin as an analog to methyl ester production for use as diesel fuel was studied for the first time. Methyl esters are formed by reacting waste fats and oils with alcohols like methanol or ethanol to produce methyl or ethyl esters via transesterification . Miscible in diesel fuel, the blend is commonly known as bio-diesel. However, the alcohol employed in biodiesel production is derived from fossil fuels, which increases the net carbon emissions of methyl/ethyl esters compared to the lipids from which it is prepared. Biodiesel also has a lower heating value than fossil diesel fuel, reducing its fuel economy (miles per gallon). The net carbon emissions from biodiesel could be decreased if methanol was replaced by a biobased reactant. Here, we proposed replacement of alcohols with phenolic compounds to produce phenyl esters as diesel fuel substitute . These are produced by depolymerizing technical lignin at high temperatures (200°C-250°C) in the presence of acetic acid. Lignin was successfully depolymerized to lower molecular weight, with 8-10 mole equivalent (meq) of ester produced per gram of liquid product after 30 min of reaction. Adding zeolite to the reaction to absorb product water not only increased ester production up to 75 wt% but improved lignin depolymerization during the process. Around 14 (meq) of ester was produced per gram of liquid product after 30min of reaction. Adding THF as co-solvent to acetic acid without zeolite increased ester production by 29 wt% and 22 wt % after 30 min and 60 min, respectively. Increasing reaction time in the absence of carboxylic acids increased the molecular weight of the liquid products while increasing reaction time in the presence of carboxylic acid decreased the molecular weight significantly. Lignin depolymerization and esterification using longer chain carboxylic acids as solvent and esterification agent was more challenging due to decreasing solubility of lignin with increasing molecular weight of the carboxylic acids. FTIR analysis of the liquid product from esterification of lignin in hexanoic acid at 250°C for one hour reaction time showed ester formation, but the conversion of lignin was only about 40 wt%. Adding a co-solvent increased the solubility of lignin, but the molecular weight of the product was still higher than the liquid products obtained from acetic acid reactions. A concept for a lignin-first biorefinery based on oxidative fractionation of lignocellulose was explored for the first time, producing both lignin monomers and processable carbohydrate pulp. A response surface statistical model was used to evaluate the influence of reaction conditions on delignification, monomer yield, and carbohydrate retention in the pulp. Red oak was successfully delignified through alkaline oxidation process, resulting in carbohydrate pulp and lignin oil. Process conditions for optimizing the yield of lignin monomers, carbohydrate retention in the pulp, and delignification were explored. The effects of reaction temperature and time, oxygen partial pressure, the presence of catalyst, and sodium hydroxide concentration were evaluated by a central composite response surface method. Two different operating windows were proposed to get the optimum results for all three responses. Temperature and time were the most significant factors for all the response models. The presence of catalyst was only slightly a significant factor in monomer production when reaction times were short. Under optimum reaction conditions, the lignin oil contained around 40 wt% of phenolic monomers, mainly syringaldehyde and vanillin. The structural features of the lignin oil were further analyzed by GC/MS, GPC, and 2D HSQC NMR techniques. The isolated carbohydrates pulp contained approximately 97 wt% C6 sugars under optimum reaction conditions. C5 sugars always showed lower retention, up to 78 wt%, due to the amorphous structure of hemicellulose, which makes it susceptible to thermal decomposition. The carbohydrate pulp was further analyzed by X-ray diffractometer, which showed the presence of crystalline cellulose in the isolated pulp

Lignin Depolymerization and Esterification with Carboxylic Acids to Produce Phenyl Esters

The possibility of phenyl ester formation from lignin as an alternative to methyl esters for the production of diesel fuel substitute is explored for the first time. The depolymerization of technical lignin at high temperatures (200-250°C) was conducted using carboxylic acids. Successful depolymerization occurred in acetic acid, generating 8-10 mol eq ester per gram of liquid product within 30 minutes. The inclusion of molecular sieve as in situ water sorbent not only increased ester production by 75% but facilitated lignin depolymerization. Additionally, acetic acid played a dual role of esterifying lignin and preventing undesired condensation reactions through the stabilization of depolymerization products. Adding tetrahydrofuran (THF) as a co-solvent with acetic acid boosted ester production by 29% and 22% after 30 and 60 minutes, respectively. 1,4-dioxane as a co-solvent had no significant effect. The substitution of longer-chain carboxylic acids for acetic acid proved to be more challenging due to solubility issues. FTIR analysis of the liquid product confirmed the formation of esters, but the conversion of lignin in hexanoic acid at 250°C after 1 hour of reaction was only approximately 40 wt%. Adding the co-solvent enhanced lignin solubility, but resulting products had higher molecular weights compared to those from acetic acid reactions.This is a manuscript of an article published as Hafezisefat, Parinaz, Long Qi, and Robert C. Brown. "Lignin Depolymerization and Esterification with Carboxylic Acids to Produce Phenyl Esters." ACS Sustainable Chemistry & Engineering (2023). doi: https://doi.org/10.1021/acssuschemeng.3c05221. Posted with Permission. Copyright © 2023 American Chemical Societ