Development of 'Lignin-First' Approaches for the Valorization of Lignocellulosic Biomass.

Tamás I Korányi,Bálint Fridrich,Katalin Barta,Antonio Pineda

doi:10.3390/molecules25122815

Abstract

Currently, valorization of lignocellulosic biomass almost exclusively focuses on the production of pulp, paper, and bioethanol from its holocellulose constituent, while the remaining lignin part that comprises the highest carbon content, is burned and treated as waste. Lignin has a complex structure built up from propylphenolic subunits; therefore, its valorization to value-added products (aromatics, phenolics, biogasoline, etc.) is highly desirable. However, during the pulping processes, the original structure of native lignin changes to technical lignin. Due to this extensive structural modification, involving the cleavage of the β-O-4 moieties and the formation of recalcitrant C-C bonds, its catalytic depolymerization requires harsh reaction conditions. In order to apply mild conditions and to gain fewer and uniform products, a new strategy has emerged in the past few years, named ‘lignin-first’ or ‘reductive catalytic fractionation’ (RCF). This signifies lignin disassembly prior to carbohydrate valorization. The aim of the present work is to follow historically, year-by-year, the development of ‘lignin-first’ approach. A compact summary of reached achievements, future perspectives and remaining challenges is also given at the end of the review.

Highlights

Plant cells’ composite material is lignocellulose, which mainly consists of cellulose, hemicellulose, and lignin and in total accounts for ca. 90% of dry matter of land-based biomass
Lignocellulose can be divided into three main categories—i.e., softwood, hardwood, and grass
With environmental concerns and increasing need to shift away from the dependence on fossil resources, interest in biomass as a sustainable feedstock has been resurgent, and lignocellulose has been identified as an important feedstock because it does not compete with the food supply

Summary

Introduction

Plant cells’ composite material is lignocellulose, which mainly consists of cellulose, hemicellulose, and lignin and in total accounts for ca. 90% of dry matter of land-based biomass. 4-O-5 in Figure 1) are cleaved and new stable C-C linkages are formed, resulting in more condensed structure of native lignin is altered during conventional lignocellulose fractionation methods: ether and unreactive technical lignins [4]. As a conversion of all lignocellulose components to lignin monomers and C2–C6 alcohols and/or alkanes is possible by one-step reductive catalytic processing (route c in Figure 2) [3,13]. Reactive fragments or tointermediates that originate during pulping form with fractionation, or parallel cellulose processing Under these conditions, immediately undergo stabilization that (typically by during hydrogenation see Figure 3)undergo to form stabilization more stable reactive fragments or intermediates originate pulping immediately molecules.byPossible lignin-derived monomers obtained far from routes c and d combined

From 1940 to 2014

Summary

Findings

Future Perspectives and Challenges