Hydrotreating and hydrothermal treatment of alkaline lignin as technological valorization options for future biorefinery concepts: a review

Abstract

Lignin has the potential to be a sustainable resource for producing biobased chemicals (e.g. phenols, aromatic hydrocarbons, vanillin) for multiple applications. However, given its heterogeneous and rigid structure, its efficient conversion to value‐added products remains one of the most important limiting factors for the successful viability of the biobased economy. Hydrotreating and hydrothermal treatment (including liquefaction, gasification and wet oxidation) are promising technologies that can convert lignin into biobased products. This review article provides a literature overview of how key process parameters of hydrotreating and hydrothermal treatment (operating conditions, catalysts, solvents, type of starting lignin) may influence the conversion of alkaline lignin into valuable chemical products. It was observed that low selectivity to products (and subsequent required separation and purification) and char formation are the main hurdles for effective conversion of alkaline lignin. However, experimental work in alternative catalytic systems, solvents and hydrogen sources has shown that promising opportunities exist to overcome these drawbacks. Certain catalysts (e.g. Ru/Al2O3) have been found to improve selectivity and the use of alcohol solvents (especially methanol or ethanol) as a hydrogen source has been found to improve product yields and reduce char formation at lower working temperatures and pressures. © 2016 Society of Chemical Industry

Base-Catalyzed Depolymerization of Lignin: History, Challenges and Perspectives

“…The latter consumes the produced hydrogen equivalents [13]. Besides, hydrotreating and hydrothermal treatment of alkaline lignin in methanol over Ru/Al 2 O 3 have been found to improve product yield and selectivity and reduce char formation at lower working temperatures and pressures [53].…”

Oxidized Lignin Depolymerization using Formate Ionic Liquid as Catalyst and Solvent

“…Many papers have reported catalytic methods to oxidize β‐ O ‐4 linkages in lignin, making lignin more reactive for further depolymerization …”

“…The utilization of lignin has been studied for decades and many different approaches to lignin valorization have been explored. Selective depolymerization of lignin to produce well‐defined chemicals remains very challenging owing to the complex structure of lignin . Numerous studies focused on lignin depolymerization, have reported lignin catalytic cracking, hydrolysis, reductive depolymerization, and oxidative depolymerization .…”

Successive ethanol–water fractionation of enzymatic hydrolysis lignin to concentrate its antimicrobial activity

“…1 As a major natural resource, plants have been emphasized as alternatives due to their high content of antimicrobial agents, such as alkaloids and diverse polyphenols (i.e. 6 With the emergence of biomass refining, there is an increasing interest in exploring valuable applications for this underused resource 7 and antimicrobial development is likely to be a feasible solution for this issue. 1,2 Lignin is a dominant aromatic polymer in terrestrial plants, which consists of three phenylpropane units [namely, guaiacyl (G), syringyl (S), and p-hydroxyphenyl (H) units].…”

“…4,5 Despite its abundance and critical role for plant resistance to microbes, lignin is typically underused because it is traditionally viewed as a waste material or a low value byproduct from pulping and bio-energy industries. 6 With the emergence of biomass refining, there is an increasing interest in exploring valuable applications for this underused resource 7 and antimicrobial development is likely to be a feasible solution for this issue.…”