Current pulping technologies only valorize the cellulosic fiber giving total yields from biomass below 50 %. Catalytic fractionation enables valorization of both cellulose, lignin, and, optionally, also the hemicellulose. The process consists of two operations occurring in one pot: (1) solvolysis to separate lignin and hemicellulose from cellulose, and (2) transition metal catalyzed reactions to depolymerize lignin and to stabilized monophenolic products. In this article, new insights into the roles of the solvolysis step as well as the operation of the transition metal catalyst are given. By separating the solvolysis and transition metal catalyzed hydrogen transfer reactions in space and time by applying a flow-through set-up, we have been able to study the solvolysis and transition metal catalyzed reactions separately. Interestingly, the solvolysis generates a high amount of monophenolic compounds by pealing off the end groups from the lignin polymer and the main role of the transition metal catalyst is to stabilize these monomers by transfer hydrogenation/hydrogenolysis reactions. The experimental data from the transition metal catalyzed transfer hydrogenation/hydrogenolysis reactions was supported by molecular dynamics simulations using ReaXFF.

A reductive lignocellulose fractionation in a flow-through system in which pulping and transfer hydrogenolysis steps were separated in time and space has been developed. Without the hydrogenolysis step or addition of trapping agents to the pulping, it is possible to obtain partially depolymerized lignin (21 wt% monophenolic compounds) that is prone to further processing. By applying a transfer hydrogenolysis step 37 wt% yield of lignin derived monophenolic compounds was obtained. Pulp generated in the process was enzymatically hydrolyzed to glucose in 87 wt% yield without prior purification.

A mild Pd/C-catalyzed hydrogenolysis of the C-O bond of model Compounds representing the dibenzodioxocin motif in lignin using polymethylhydrosiloxane (PMHS) and water as hydrogen sources was developed. The efficiency of the reaction is highly dependent on both water Concentration and the addition of a base. The results from mechanistic studies showed that the benzylic C-O bond is cleaved faster than the terminal C-O bond, which only cleaves upon the presence of the neighboring phenol. We propose a hydrogen bond formation between an oxygen atom of an ether group and a proton of a neighboring phenol under the employed mild reaction Conditions, which facilitates cleavage of the C-O bond.

This review discusses the challenges within the research area of modern biomass fractionation and valorization. The current pulping industry focuses on pulp production and the resulting cellulose fiber. Hemicellulose and lignin are handled as low value streams for process heat and the regeneration of process chemicals. The paper and pulp industry have therefore developed analytical techniques to evaluate the cellulose fiber, while the other fractions are given a low priority. In a strive to also use the hemicellulose and lignin fractions of lignocellulosic biomass, moving towards a biorefining concept, there are severe shortcomings with the current pulping techniques and also in the analysis of the biomass. Lately, new fractionation techniques have emerged which valorize a larger extent of the lignocellulosic biomass. This progress has disclosed the shortcomings in the analysis of mainly the hemicellulose and lignin structure and properties. To move the research field forward, analytical tools for both the raw material, targeting all the wood components, and the generated fractions, as well as standardized methods for evaluating and reporting yields are desired. At the end of this review, a discourse on how such standardizations can be implemented is given.

Density functional theory (DFT) calculations, combined with a constrained minima hopping algorithm (global minimum search while preserving the molecular identity), have been performed to investigate important reaction intermediates for the heterogeneously catalyzed beta-O-4' bond cleavage in lignin derivatives. More specifically, we have studied the adsorption properties of a keto tautomer (1-methoxypropan-2-one) and its enol form on a catalytically active Pd(111) surface. In agreement with experiments, we find that for the gas phase molecules the keto tautomer is the most stable. Interestingly, the enol tautomer has a higher affinity to the Pd catalyst than the keto form, and becomes the most stable molecular form when adsorbed on the catalyst surface. The global minimum complex found on the metal surface corresponds to an enolate structure formed when the enol tautomer chemisorbs onto the surface and donates its pi-electrons from the C=C region to two adjacent palladium atoms. The actual formation of a chemical bond to the surface in the case of the enol molecule could be the key to understanding why the enol derivative is needed for an efficient beta-O-4' bond cleavage.

A novel lignin-based synthon, diglycidylether of iso-eugenol (DGE-isoEu) is used as a prepolymer for the preparation of thermosetting resins. DGE-isoEu is synthesized in a two-step procedure with a satisfactory yield from bio-based iso-eugenol (isoEu, 2-methoxy-4-(1-propenyl)phenol) catalytically fragmented from lignin in an organosolv process. DGE-isoEu was fully characterized by NMR, MS and FTIR. Curing of the DGE-isoEu monomer has then been investigated in the presence of several carboxylic acid derivatives hardeners. The thermal and mechanical properties of each material were recorded showing, in particular, a high T-g and instantaneous modulus values in the range of 78-120 degrees C and 4.6-5.5 GPa, respectively. The lignin derived new materials give very attractive thermo-mechanical properties comparable to that of common BPA-containing epoxy resins.

The pulping industry could become a biorefinery if the lignin and hemicellulose components of the lignocellulose are valorized. Conversion of lignin into well-defined aromatic chemicals is still a major challenge. Lignin depolymerization reactions often occur in parallel with irreversible condensation reactions of the formed fragments. Here, we describe a strategy that markedly suppresses the undesired condensation pathways and allows to selectively transform lignin into a few aromatic compounds. Notably, applying this strategy to woody biomass at organosolv pulping conditions, the hemicellulose, cellulose, and lignin were separated and in parallel the lignin was transformed into aromatic monomers. In addition, we were able to utilize a part of the lignocellulose as an internal source of hydrogen for the reductive lignin transformations. We hope that the presented methodology will inspire researchers in the field of lignin valorization as well as pulp producers to develop more efficient biomass fractionation processes in the future.

Current processes for the fractionation of lignocellulosic biomass focus on the production of high-quality cellulosic fibers for paper, board, and viscose production. The other fractions that constitute a major part of lignocellulose are treated as waste or used for energy production. The transformation of lignocellulose beyond paper pulp to a commodity (e.g., fine chemicals, polymer precursors, and fuels) is the only feasible alternative to current refining of fossil fuels as a carbon feedstock. Inspired by this challenge, scientists and engineers have developed a plethora of methods for the valorization of biomass. However, most studies have focused on using one single purified component from lignocellulose that is not currently generated by the existing biomass fractionation processes. A lot of effort has been made to develop efficient methods for lignin depolymerization. The step to take this fundamental research to industrial applications is still a major challenge. This review covers an alternative approach, in which the lignin valorization is performed in concert with the pulping process. This enables the fractionation of all components of the lignocellulosic biomass into valorizable streams. Lignocellulose fractions obtained this way (e.g., lignin oil and glucose) can be utilized in a number of existing procedures. The review covers historic, current, and future perspectives, with respect to catalytic lignocellulose fractionation processes.