Catalytic Transfer Hydrogenolysis Reactions for Lignin Valorization to Fuels and Chemicals

Abstract

Lignocellulosic biomass is an abundant renewable source of chemicals and fuels. Lignin, one of biomass main structural components being widely available as by-product in the pulp and paper industry and in the process of second generation bioethanol, can provide phenolic and aromatic compounds that can be utilized for the manufacture of a wide variety of polymers, fuels, and other high added value products. The effective depolymerisation of lignin into its primary building blocks remains a challenge with regard to conversion degree and monomers selectivity and stability. This review article focuses on the state of the art in the liquid phase reductive depolymerisation of lignin under relatively mild conditions via catalytic hydrogenolysis/hydrogenation reactions, discussing the effect of lignin type/origin, hydrogen donor solvents, and related transfer hydrogenation or reforming pathways, catalysts, and reaction conditions.

Unlocking Structure–Reactivity Relationships for Catalytic Hydrogenolysis of Lignin into Phenolic Monomers

“…Performing the reaction under an ambient pressure of N 2 (1 atm) led to a significant reduction of both lignin oily products (57 wt %) and phenolic monomers (10.7 wt %) but a higher M w (1571 g mol −1 ; Tables S4 and S5, Figure S3 in the Supporting Information). This lower yield of aromatic phenols could be ascribed to the catalytic transfer hydrogenolysis of lignin in the absence of additional H 2 , and MeOH acted as a hydrogen‐donor source, in good agreement with previous reports under the Pd/C catalytic system . The above results indicate that H 2 assists the Pd/C catalyst in favor of increasing the selective C−O cleavage rates, thus accelerating the fragmentation–hydrogenolysis process .…”

https://researchopenworld.com/synthesis-of-nanolignin-following-ozonation-of-lignocellulosic-biomass/#

“…The lignocellulosic biomass has to be treated before any widespread utilization of its components. As a decrease in the lignin content in plants results in an increase in biodegradability, lignin removal from this biomass is a crucial pretreatment step [4,5]. A large number of chemophysical pretreatment approaches has been investigated on a wide variety of feedstock [6].…”

Monometallic Cerium Layered Double Hydroxide Supported Pd-Ni Nanoparticles as High Performance Catalysts for Lignin Hydrogenolysis

“…In addition to metal nanoparticles, a suitable support, which further promotes their performance through synergistic interactions, is crucial for the materials' catalytic performance. Traditionally, metal oxides have been considered in the context of the reductive lignin depolymerization, as they provide key characteristics for efficient and effective cleavage of ether bonds [4,21]. Firstly, most metal oxides contain both Lewis acid sites and Brønsted or Lewis basic sites that can attract hydrides (H:) or protons (H + ) from alcohols, respectively, lowering the activation energy of reaction steps involving a hydrogen transfer.…”

“…Secondly, metal oxides seem to stabilize the reactive intermediates, which are formed during reductive lignin depolymerization, e.g., through their alkylation with solvent molecules (e.g., ethanol or methanol). As a result, unwanted repolymerization can be avoided [21]. Alumina (Al 2 O 3 ) and silica (SiO 2 ), which show many of the aforementioned beneficial properties, have been examined extensively for the reductive depolymerization of lignin [4].…”

“…Moreover, specifically beneficial for the hydrogenolysis of lignin, ceria is known to promote the water gas shift and steam reforming reactions, which are closely related to the ability of the support to generate hydrogen in situ. Additionally, ceria has the ability to store and release hydrogen [21,22]. AuPd/CeO 2 catalysts have already been tested in reductive depolymerization of lignin and model compounds and yielded promising results [16].…”