Renewable chemicals and fuel additives from lignin: an interplay of lignin structure, valorisation, and upgrading

Abstract

Excessive consumption of fossil fuel for the production of energy carriers and commodity chemicals caused depletion of fossil fuel resources, environmental pollution, and global warming. Hence, many researchers investigated the production of renewable energy, and chemical commodities from sustainable energy resources, which are abundant and comparably cleaner alternatives to fossil fuel derivatives. Among these renewable resources, lignocellulosic biomass (vegetal cell component including, cellulose, hemicellulose and lignin) appears to be a promising alternative for the production of commodity chemicals and fuels. Lignocellulosic biomass comprises three primary components: cellulose, hemicellulose, and lignin. Lignin, the second principal constituent of biomass after cellulose, is an amorphous polymer made of aromatic monomers. Lignin has been known as the most abundant renewable source of natural building blocks of aromatics and has high potential to serve as a precursor for the production of functionalised aromatics in the petroleum industry. To achieve this goal, many researchers investigated thedepolymerisation of lignin, for the production of monomeric aromatics.This project aims to use lignin as an abundant natural resource for the production of low molecular weight chemical commodities. To achieve this goal, successful fractionation trailed by chemocatalytic upgrading is required to build an effective lignin depolymerisation sequence. Depolymerisation of lignin carried out in this project using two approaches, namely, reductive depolymerisation and fast pyrolysis reaction.To conduct the reductive depolymerisation reaction titanium nitride (TiN) Nano-catalyst supported with copper nanocatalysts used as the catalyst for hydrogenolysis of lignin. In order to investigate the reactivity of TiN supported copper catalyst (TiN-Cu), hydrogenation of benzaldehyde carried out using a series of TiN-Cu and available commercial catalysts. These tests revealed the high activity of TiN particles loaded with 30 Wt. % copper in hydrogenolysis reaction comparable to industrial noble catalysts. Degradation of model compounds with TiN-Cu catalysts carried out to measure the reactivity of lignin and lignin oxide model compounds in the hydrogenation process, which show higher reactivity for selectively oxidised lignin model compounds. In order to promote real lignin sample reactivity during the hydrogenolysis process, selective oxidation of α hydroxyl group of lignin conducted. Selective aerobic oxidation of lignin source carried with HCl/HNO3/TEMPO ((2,2,6,6-Tetramethylpiperidin-1-yl)oxyl) catalyst, and the obtained lignin oxide used for hydrogenation step with TiN-Cu catalyst. High degradation rate for lignin achieved in this process resulting in the production of oligomers consisting of 2 to 3 interconnected aromatic rings and valuable phenolic monomers. The obtained dimers and monomers from hydrogenolysis of lignin, shown high potential to be used as commodity chemicals, such as phenolic precursor for the petrochemical industry.Fast pyrolysis reaction used as the second approach for depolymerisation of lignin. Solvent-assisted fast pyrolysis of lignin shown high selectivity for depolymerisation of lignin toward dimers and trimers with a low amount of oxygen. This process is capable of lowering the aliphatic hydroxyl contents of lignin, along with increasing the amount of single and double bonded aliphatics. The addition of solvent during fast pyrolysis of lignin lowered the molecular weight distribution of the obtained bio-oil (~49-52% decrease) and prohibited the formation of the high amount of char content. The detailed study for cleavage of complex model compounds using Ethanol assisted fast pyrolysis (EAFP) revealed the active sites during this process are the aliphatic hydroxyl groups and etheric linkages. The EAFP of the deuterated lignin model compounds and solvent concluded that the mechanism for cleavage of lignin in the EAFP process involves the formation of a transition state between solvent and oxygen sites of lignin. This transition state involves the cleavage of the etheric site by in situ transfer of hydrogen from ethanol to this linkage.In the final stage of this project, an approach is designed to upgrade industrial available biooils for production of commodity chemicals such as phenolic precursor, and fuel components like BTX (benzene, toluene, and xylene). In this approach, the obtained bio-oils from fast pyrolysis of the tree, hydrogenated in the presence of an industrial catalyst. This process showed a high capacity for lowering the amount of oxygen and resulted in the production of 22% of phenolic monomers, which has high potential to be used as a chemical precursor in the polymer industry. In the other hand, the approach for upgrading the phenolic monomers to a low oxygenated chemical was successful, in which, the fine phenolic monomers such as catechol, anisole, and aliphatic monomers like propanol obtained by the extended HDO reaction.It is believed that the developed method for depolymerisation of lignin to low molecular weight bio-oil and upgrading of the obtained bio-oil can be an effective environmental friendly bioprocess for the production of commodity chemicals and biofuels from lignin.