Lignin Depolymerization and Conversion: A Review of Thermochemical Methods

Abstract

The structure of lignin suggests that it can be a valuable source of chemicals, particularly phenolics. However, lignin depolymerization with selective bond cleavage is the major challenge for converting it into value-added chemicals. Pyrolysis (thermolysis), gasification, hydrogenolysis, chemical oxidation, and hydrolysis under supercritical conditions are the major thermochemical methods studied with regard to lignin depolymerization. Pyrolytic oil and syngases are the primary products obtained from pyrolysis and gasification. A significant amount of char is also produced during pyrolysis. Thermal treatment in a hydrogen environment seems very promising for converting lignin to liquid fuel and chemicals like phenols, while oxidation can produce phenolic aldehydes. Reaction severity, solvents, and catalysts are the factors of prime importance that control yield and composition of the product. IntroductionThe depleting stocks of fossil fuels and the growing concern over the excessive emission of greenhouse gases have forced researchers to investigate renewable, abundant and comparably cleaner alternatives to liquid fuels and chemicals produced from petroleum [1][2][3]. Biomass appears to be a promising alternative and a renewable source for fuels and chemicals. Significant achievements have already been made in the production of ethanol fuel from biomass, primarily starch and sugarrich components. However, research on second-generation bioethanol is focused on a more abundant [4,5] and often relatively cheap [6][7][8] biomass feedstock known as lignocellulosic biomass. Lignocellulosic biomass consists of three basic components: cellulose, hemicellulose, and lignin. Cellulose is a linear polymer of glucose consisting of parts with crystalline structure and parts with amorphous structure [9], while hemicellulose is an amorphous, heterogeneously branched polymer of pentoses and hexoses, mainly xylose, arabinose, mannose, galactose, and glucose. Lignin is an amorphous and highly branched polymer of phenylpropane units, which can account for up to 40 % of the dry biomass weight [10]. The concept of a biorefinery that integrates processes and technologies for biomass conversion demands efficient utilization of all three components [11]. Most of the biorefinery schemes, however, are focused on utilizing easily convertible fractions while lignin remains relatively underutilized to its potential [11]. The lignocellulosics-to-ethanol process makes use of the cellulose and hemicelluloses, leaving lignin as waste. In addition, pulp and paper refineries also generate huge amounts of lignin. Presently, lignin is being utilized as a low-grade boiler fuel to provide heat and power to the process [3]. However, the chemical structure of lignin suggests that it may be a good source of valuable chemicals if it could be broken into smaller molecular units [7].Several studies have been done to convert lignin to more value-added products. These include attempts to convert lignin to liquid fuel additives and commercially important chem...

Integrating a Biorefinery into an Operating Kraft Mill

“…Hydrothermal liquefaction can be used to hydrolyze lignocellulose to C5 and C6 sugars and lignin sequentially (Pandey and Kim 2011;Colakyan 2012). Papermill sludge and black liquor can be readily converted to biooil for fuel or chemical platforms by hydrothermal treatment (Xu and Lancaster 2008;Knez et al 2015;Huet et al 2016).…”

Catalysing Sustainable Fuel and Chemical Synthesis

“…High temperature calcination induced Mg surface segregation, resulting in MgO nanocrystals dispersed over CaO/(OH) 2 particles, while the attendant loss of CO 2 increases both the surface area and basicity. The resulting calcined dolomite proved an effective catalyst for the transesterification of C 4 , C 8 and TAGs with methanol and longer chain C [16][17][18] Fig . 9 Superior catalytic performance of a hierarchical macroporous-microporous Mg-Al hydrotalcite solid base catalyst for TAG transesterification to biodiesel versus a conventional microporous analogue.…”

“…Thermal pyrolysis offers one avenue by which to obtain transportation fuels, and wherein catalysis will undoubtedly play a significant role in both pyrolysis of raw biomass and subsequent upgrading of bio-oils via deoxygenation and carbon chain growth. Catalytic depolymerisation of lignin may also unlock opportunities for the production of phenolics and related aromatic compounds for fine chemical and pharmaceutical applications [16].…”

Low-Power Microwave Radiation-assisted Depolymerization of Ethanol Organosolv Lignin in Ethanol/Formic Acid Mixtures

“…Several studies have investigated the mechanism of lignin degradation, including fast pyrolysis, hydrogenolysis, oxidation, lignin depolymerization in supercritical solvents, ionic liquid pretreatment, and catalysts in lignin conversion (Pandey and Kim 2011). Under rapid pyrolysis conditions, lignin is converted directly into a liquid product, which is referred to as "bio-crude" or "bio-oil."…”