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This work demonstrates an additive and hydrogen‐free 2‐step lignin‐first fractionation in flow‐through. First, solvolytic delignification renders lignin liquors with its native chemical structure largely intact; and second, ß‐zeolite catalytic depolymerization of these liquors leads to similar monomer yields as the corresponding 1‐step fractionation process. Higher delignification temperatures lead to slightly lower ß−O−4 content in the solvated lignin, but does not affect significantly the monomer yield, so a higher temperature was overall preferred as it promotes faster delignification. Deposition of Pd on ß‐zeolite resulted in a bifunctional hydrogenation/dehydration catalyst, tested during the catalytic depolymerization of solvated lignin with and without hydrogen addition. Pd/ß‐zeolite displays synergistic effects (compared to the Pd/γ‐Al2O3 and ß‐zeolite tested individually and as a mixed bed), resulting in higher monomer yield. This is likely caused by increased acidity and the proximity between the metallic and acid active sites. Furthermore, different ß‐zeolite with varying SAR and textural properties were studied to shed light onto the effect of acidity and porosity in the stabilization of lignin monomers. While some of the catalysts showed stable performance, characterization of the spent catalyst reveals Al leaching (causing acidity loss and changes in textural properties), and some degree of coking and Pd sintering. [ABSTRACT FROM AUTHOR]

Utilization of lignin is among the most pressing problems for biorefineries that convert lignocellulosic biomass to fuels and chemicals. Recently "lignin-first" biomass fractionation has received increasing attention. In most biorefining concepts, carbohydrate portions of the biomass are separated, and their monomeric sugar components released, while the relatively chemically stable lignin rich byproduct remains underutilized. Conversely, in lignin-first processes, a one-pot fractionation and depolymerization is performed, leading to an oil rich in phenolic compounds and a cellulosic pulp. Usually, the pulp is considered as a fermentation feedstock to produce ethanol. Herein, the results of a study where various cellulosic pulps are tested for their potential to produce valuable products via pyrolysis processes, assessed via analytical pyrolysis (py-GC), are presented. Samples of herbaceous (switchgrass) and woody biomass (oak) were subjected to both an acid-catalyzed and a supported-metal-catalyzed reductive lignin-first depolymerization, and the pulps were compared. Fast pyrolysis of the pulps produced levoglucosan in yields of up to about 35 wt %. When normalized for the amount of biomass entering the entire process, performing the lignin-first reductive depolymerization resulted in 4.0–4.6 times the yield of levoglucosan than pyrolysis of raw biomass. Pulps derived from switchgrass were better feedstocks for levoglucosan production compared with pulps from oak, and pulps produced from metal-on-carbon catalyzed depolymerization produced more levoglucosan than those from acid-catalyzed depolymerization. Catalytic pyrolysis over HZSM-5 produced aromatic hydrocarbons from the pulps. In this case, the yields were similar from both feedstocks and catalyst types, suggesting that there is no advantage to lignin fractionation prior to zeolite-catalyzed catalytic pyrolysis for hydrocarbons. [ABSTRACT FROM AUTHOR]

Reductive Catalytic Fractionation (RCF) of lignocellulosic materials produces lignin oil rich in monomer products and high‐quality cellulosic pulps. RCF lignin oil also contains lignin oligomers/polymers and hemicellulose‐derived carbohydrates. The variety of components makes lignin oil a complex matrix for analytical methods. As a result, the signals are often convoluted and overlapped, making detecting and quantifying key intermediates challenging. Therefore, to investigate the mechanisms underlining lignin stabilization and elucidate the structural features of carbohydrates occurring in the RCF lignin oil, fractionation methods reducing the RCF lignin oil complexity are required. This report examines the solvent fractionation of RCF lignin oil as a facile method for producing lignin oil fractions for advanced characterization. Solvent fractionation uses small volumes of environmentally benign solvents (methanol, acetone, and ethyl acetate) to produce multigram lignin fractions comprising products in different molecular weight ranges. This feature allows the determination of structural heterogeneity across the entire molecular weight distribution of the RCF lignin oil by high‐resolution HSQC NMR spectroscopy. This study provides detailed insight into the role of the hydrogenation catalyst (Raney Ni) in stabilizing lignin fragments and defining the structural features of hemicellulose‐derived carbohydrates in lignin oil obtained by the H‐transfer RCF process. [ABSTRACT FROM AUTHOR]

[Display omitted] • Constructing N-doped carbon supported Co catalyst for lignin-first strategy. • Catalyst morphology, metal dispersion and N content were regulated by melamine dosage. • Pyridinic N enhanced lignin adsorption and decreased C–O bond disassociation energy. • Atom-cluster Co was loaded by the presence of pyridinic N in the carrier. • 40.62 wt% of monophenolics from lignin and 84.02 wt% of cellulose were obtained. The lignin-first strategy is a crucial approach for highly efficient depolymerization of lignin to produce aromatic monomers as the condensation of lignin occurred during the separating process which increased the difficulty in depolymerization. In this work, a cobalt atom clusters supported on nitrogen-doped carbon catalysts (Co/NC-15) was prepared by adjusting the dosage of melamine to control the morphology, structure, and pyridinic nitrogen content of the catalysts. DFT calculations based on lignin model showed that the doping of nitrogen increased the adsorption energy of the lignin on the supports and decreased the dissociation energy of C-O bonds. Under lignin first strategy, Co/NC-15 exhibited good ability to break C-O bonds at 220 °C of mild condition, in which 84.32 wt% of lignin in lignocellulosic biomass was converted into 40.62 wt% of aromatic monomers, while 84.02 wt% of cellulose was preserved. The atom clusters Co/NC-15 exhibited good catalytic activity to lignin depolymerization due to efficient cleavage of C-O bonds, and hence showed the promising prospect in potential industrial application. [ABSTRACT FROM AUTHOR]

Depolymerization of lignin can yield phenolic monomeric platform molecules, but depolymerization and repolymerization are proposed as concurrent processes, which greatly limit the yield of monomeric products. Molecular-level characterization of heavy intermediates during depolymerization process is crucial for developing efficient methods to depolymerize raw lignin into monomers. In this study, a reactor-integrated negative-ion electrospray ionization high-resolution mass spectrometry was used to real-time analyze the lignin depolymerization process under operando conditions. Elemental composition, structural information as well as time-resolved evolution of a series of dimers and oligomers were obtained, which is hard to achieve with traditional GC-MS methods. The real-time evolution profiles of monomers, dimers, and oligomers reveals a possible repolymerization process during lignin depolymerization. These results provide insight into the complex lignin depolymerization process.

Obtaining chemical building blocks from biomass is attractive for meeting sustainability targets. Herein, an effective approach was developed to convert the lignin part of woody biomass into phenol, which is a valuable base chemical. Monomeric alkylmethoxyphenols were obtained from pinewood, rich in guaiacol‐type lignin, through Pt/C‐catalyzed reductive depolymerization. In a second step, an optimized MoP/SiO2 catalyst was used to selectively remove methoxy groups in these lignin monomers to generate 4‐alkylphenols, which were then dealkylated by zeolite‐catalyzed transalkylation to a benzene stream. The overall yield of phenol based on the initial lignin content in pinewood was 9.6 mol %. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The utilization of lignocellulosic biomass promotes the concept of sustainability and contribute to carbon neutrality. In contrast to traditional biomass pretreatment, lignin-first biomass pretreatment considers both the integrity of lignin and sugar production. There has been increasing interest in applying the concept of lignin-first biomass pretreatment to traditional pretreatment processes. Progress has been made in the field in the past three years or so. It is timely to offer a systematic analysis of the current state of lignin-first biomass pretreatment, and identify existing challenges and future trends in the field. Herein, we first present a brief introduction to biomass recalcitrance and pretreatment, followed by a discussion of lignin repolymerization/condensation during pretreatment and different strategies to stabilize lignin. Next, we focus on biomass pretreatments with active protection of the β-O-4 bonds in lignin. Ionic liquid (IL) pretreatment, deep eutectic solvent (DES) pretreatment, acidified organosolv pretreatment, and organic acid pretreatment are the main pretreatment technologies that have been studied with the active protection of the β-O-4 linkages. We emphasize the importance of retention of high β-O-4 content in valorizing lignin by depolymerization approach. Finally, we provide our perspectives on the challenges and opportunities in this field, in support of efforts to advance lignin-first biomass pretreatment. [Display omitted] • Lignin-first biomass pretreatment aims to maximize both sugar yield and β-O-4 bonds preservation. • Active protection of the β-O-4 bonds has been extended to several typical pretreatment processes. • Ionic liquids, deep eutectic solvents, organic solvents, and organic acids were discussed. • Mild pretreatment conditions and chemical stabilization of lignin were analyzed. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel pre-treatment strategy for LCB for 2G ethanol biorefinery. • Complete and effective utilization of all the components of lignocellulosic biomass. • Depolymerization of native lignin into phenolic monomers. • Reaction chemistry behind lignin valorisation. • Role of RCF biorefinery to achieve SDGs. Presently, the use of lignocellulosic biomass is mainly focused on creating pulp/paper, energy, sugars and bioethanol from the holocellulose component, leaving behind lignin to be discarded or burned as waste despite of its highest aromatic carbon and energy content (22–29 KJ/g). During the pulping process, lignin undergoes significant structural changes to yield technical lignin. For a circular bioeconomy, there is an urgent need to enhance the use of native lignin for generating more valuable products. Over the last few years, a new method called 'lignin-first', or 'reductive catalytic fractionation' (RCF), has been devised to achieve selective phenolic monomers under mild reaction conditions. This involves deconstructing lignin before capitalizing on carbohydrates. The objective of this study is to record the recent developments of the 'lignin-first' process. This review also underlines the contribution of RCF biorefinery towards achieving sustainable development goals (SDGs) and concludes with an overview of challenges and upcoming opportunities. [ABSTRACT FROM AUTHOR]