Catalytic Strategies Towards Lignin-Derived Chemicals

Abstract

Lignin valorization represents a crucial, yet underexploited component in current lignocellulosic biorefineries. An alluring opportunity is the selective depolymerization of lignin towards chemicals. Although challenged by lignin's recalcitrant nature, several successful (catalytic) strategies have emerged. This review provides an overview of different approaches to cope with detrimental lignin structural alterations at an early stage of the biorefinery process, thus enabling effective routes towards lignin-derived chemicals. A first general strategy is to isolate lignin with a better preserved native-like structure and therefore an increased amenability towards depolymerization in a subsequent step. Both mild process conditions as well as active stabilization methods will be discussed. An alternative is the simultaneous depolymerization-stabilization of native lignin towards stable lignin monomers. This approach requires a fast and efficient stabilization of reactive lignin intermediates in order to minimize lignin repolymerization and maximize the envisioned production of chemicals. Finally, the obtained lignin-derived compounds can serve as a platform towards a broad range of bio-based products. Their implementation will improve the sustainability of the chemical industry, but equally important will generate opportunities towards product innovations based on unique biobased chemical structures.

Aldehydes-Aided Lignin-First Deconstruction Strategy for Facilitating Lignin Monomers and Fermentable Glucose Production from Poplar Wood

“…In general, the direct catalytic degradation of native lignin in lignocellulose resulted in a high yield of lignin monomers [20,35]. However, the subsequent conversion of the substrate after lignin-first biomass fractionation has only been reported occasionally.…”

Catalytic Transfer Hydrogenolysis Reactions for Lignin Valorization to Fuels and Chemicals

“…In fast pyrolysis, lignin is heated up to 400-700 • C under high heating/cooling rates in the absence of oxygen, with or without catalyst [34,50]. The main products of no-catalytic, thermal fast pyrolysis of lignin are bio-oil (containing substituted alkoxyphenols and few aromatics), char and gases (mainly CO, CO 2 , CH 4 ).…”

“…In contrary to the oxidative depolymerization, reductive depolymerization is taking place in the presence of reducing agents and redox catalysts. Sels and co-workers have reported a categorization of reductive depolymerization process based on hydrogen source and reaction temperature [50,58]. When H 2 gas is used as the reducing agent, the process is called hydroprocessing and when hydrogen donor solvents are used, the process is usually called liquid phase reforming.…”

“…Similar results were obtained for Alcell lignin by the use of supported noble metals at 400 • C, for 4 h reaction time and initial H 2 pressure of 100 bar, with Ru/TiO 2 , exhibiting the highest catalytic activity providing bio-oil yield 78.3 wt.%, and 9.1 wt.% alkylphenolics, 2.5 wt.% aromatics, and 3.5 wt.% catechols [63]. The bifunctional hydroprocessing with metals supported on acidic materials has been also identified as a separate case, leading to alkane production via additional hydrolysis and dehydration reactions due to the acidic nature of the support, at temperatures below 320 • C [50].…”

“…Lignin first strategy refers to the reductive catalytic fractionation of the whole biomass feedstock and not to the conversion of isolated lignins. Actually, this process is a combination of lignocellulosic biomass fractionation and simultaneous conversion of lignin to monomers [50]. The composition and the structure of the primary biomass influences significantly the obtained products.…”