(2fd) Synthesis and Computational Investigation of Novel Antioxidants Prepared By Oxidative Depolymerization of Lignin and Aldol Condensation of Aromatic Aldehydes

Owing to the shortage of fossil resources and the increasing severity of climate change, conversion of biomass to fuels and chemicals has attracted great interest in recent years. Lignin is one of the main components of lignocellulosic biomass, and utilization of lignin to produce phenolic compounds is an interesting and important direction for biomass conversion. The global yield of industrial lignin produced by the pulping and paper industries exceeds 50 million tons each year, but most of them are just burned or discharged as waste. Therefore, the use of lignin to produce value-added products will significantly improve the economic benefits of biomass utilization. At present, valorization of lignin is mainly achieved through depolymerization techniques, including pyrolysis, reductive depolymerization, and oxidative depolymerization. However, because of its complex structure and biological recalcitrance, lignin depolymerization is still facing certain challenges, though considerable progress has been made. The primary goals of lignin depolymerization are preparation of bio-oil fuel, bulk and fine chemicals including short-chain fatty acids and phenolic compounds. One of the most classic lignin-derived products is vanillin, which has been used as a flavoring and fragrance ingredient in food or cosmetic industries, a ripening agent to increase the amount of sucrose in sugar cane, a base compound for preparation of sunscreen, or as a precursor for synthesis of bio-based polymers. Moreover, aromatic aldehydes such as p-hydroxybenzaldehyde and syringaldehyde also can be produced by oxidative depolymerization of lignin, both of which show great promise in application as chemical or pharmaceutical intermediates. Currently, there are various methods to produce aromatic aldehydes by oxidative depolymerization of lignin either in the aqueous phase or solvent medium. The most representative method of lignin depolymerization to produce aromatic aldehydes is the oxidation with nitrobenzene, which could partially oxidize the linkages without destroying the aromatic rings, thereby obtaining a higher yield of aromatic aldehydes. However, this method has not been commercialized due to the use of toxic organic chemicals. Alkaline aerobic oxidation of lignin has been employed for production of aromatic aldehydes for a relatively long history and it has been commercially used to produce vanillin from lignosulfonate. Antioxidants are a category of naturally existing or synthetic compounds that could prevent oxidation. They are not only widely used in biological systems including food and medicine fields, but also in many industrial products that are susceptible to oxidative degradation. Antioxidants are essential to protect oxidation-prone materials in industrial applications. Commercial organic polymers such as polypropylene (PP) and polyethylene (PE) require antioxidant additives to scavenge free radicals to inhibit degradation and prolong the lifetimes of the polymer. Moreover, antioxidants are also widely used in lubricants, fuels, synthetic diesel and biodiesel. Owing to the unsaturated carbon chain in biodiesel and formation of hydroperoxides, the oxidative degradation of biodiesel will damage the car engine. Antioxidant additives must be added to biofuels in order to prevent related issues. Antioxidant additives in biodiesel can efficiently reduce nitrogen oxide emissions by inhibiting or slowing the formation of free radicals during fuel storage and combustion. Additionally, these additives can help lower particle emissions through enhancing combustion efficiency. Therefore, additives are considered necessary to overcome the technical issues with the usage of biodiesel, such as low oxidation stability and exhaust emissions. Phenolic antioxidants are particularly considered because of their ability to directly capture peroxyl radicals formed during oxidative degradation, thereby breaking the autoxidative chain reaction. There have been numerous reports on the synthetic phenolic antioxidants and their use, such as tert-butyl hydroxyquinone (TBHQ), butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA) and propyl gallate (PG). However, these antioxidants are relatively expensive and have some drawbacks, such as the high volatility and poor thermal stability of BHT, and the poor solubility of PG in oils and fats. Therefore, a more sustainable option is to obtain antioxidants from natural sources, especially biomass and its derivative platform chemicals. Lignin has a long history as an antioxidant with certain antioxidation properties because of its phenolic structure. However, the solubility of lignin is poor due to its relatively large molecular weight. Several modification treatments have been applied to lignin, including the modification of macromolecular functional groups, and degradation into small molecules to increase the content of phenolic hydroxy groups. In addition, to increase the solubility of antioxidants, it is better to decrease the molecular weight of lignin, but the antioxidation ability of lignin depolymerized products still needs to be improved. Among the lignin-derived phenolic compounds, ferulic acid (FA) is one of the most efficient antioxidants, because its phenolic nucleus and unsaturated side chain can readily form a resonance stabilized phenoxy radical that accounts for its potent antioxidation ability. In this work, inspired by the chemical structure and good antioxidation ability of FA, we aimed to synthesize novel lignin-based antioxidants with similar structure to FA, and interpret the antioxidation mechanism by studying the kinetics of aldol condensation between lignin-derived aromatic aldehydes and methyl ethyl ketone (MEK), as well as density functional theoretical calculation to reveal the structure-activity relationship. In this study, a new technology has been developed to produce antioxidants by oxidative depolymerization of lignin to form aromatic aldehydes followed by aldol condensation to improve the antioxidation ability. Oxidative depolymerization of alkali lignin was achieved by CuSO4-catalyzed oxygen oxidation. The antioxidation ability of the lignin depolymerized products was greatly improved by aldol condensation with MEK. The prepared lignin-based antioxidants could well improve the oxidation stability of biodiesel. This process may provide new technical routes for conversion of lignin to high added-value products. New antioxidants have been successfully synthesized by aldol condensation of lignin derived aromatic aldehydes, namely p-hydroxybenzaldehyde, vanillin, and syringaldehyde, resulting in successful synthesis of 1-(4-hydroxyphenyl)pent-1-en-3-one (HPPEO), 1-(4-hydroxy-3-methoxyphenyl)pent-1-en-3-one (HMPPEO) and 1-(4-hydroxy-3,5-dimethoxyphenyl)pent-1-en-3-one (HDMPPEO ), respectively. The aldol condensation products showed significantly improved antioxidation ability, and syringaldehyde-derived product (HDMPPEO) displayed the best activity. Especially, HDMPPEO showed even better antioxidation ability than ferulic acid, which might be related to the electron-donating conjugation effect achieved by aldol condensation reaction and functional groups in the structure. Kinetic modeling illustrated that p-hydroxybenzaldehyde had the highest reaction rate to form the main condensation product, followed by vanillin and then syringaldehyde, which was probably affected by the presence of methoxy groups. The formed main product was an intermediate, which also could be consumed by its self-condensation to form heavier products. Characterization information including FTIR, NMR and GC-MS spectra were provided, which have never been reported before. The antioxidation mechanisms of the synthesized antioxidants were interpreted by density functional theoretical calculation, which corroborates that electron-donating group such as methoxyl group and conjugated side chain could effectively improve the antioxidation ability. A hydrogen atom transfer (HAT) mechanism tends to occur in nonpolar solvents, whereas a sequential proton-loss electron transfer (SPLET) mechanism is favored in polar solvents. This finding thus may provide good inspiration for design and synthesis of efficient lignin-based antioxidants in future work. Additionally, this work thus can provide novel technology for preparing oil-soluble antioxidants that could be used in fossil fuels or biofuels, and even in food and cosmetics. Therefore, we believe that this work has made novel progress in this field, and may well enlighten new technology development for conversion of lignin to high value-added products.

Research Interests ：The lignin fuel cell, the electrochemical conversion of lignin to produce high value-added products and the value-added utilization of lignin depolymerization products.