Efficient depolymerization of lignin in supercritical ethanol by a combination of metal and base catalysts

Impact of Temperature an Order of Magnitude Larger Than Electrical Potential in Lignin Electrolysis with Nickel

Lignin, a major component of plant biomass, is a promising sustainable alternative carbon-based feedstock to petroleum as a source of valuable aromatic compounds such as vanillin. However, lignin upgrading reactions are poorly understood due to its complex and variable molecular structure. This work focuses on electrocatalytic lignin upgrading, which is efficient and sustainable at moderate temperatures and pressures and does not require stoichiometric reagents. We used a meta-analysis of published lignin conversion and product yield data to define the operating range, to select the catalyst, and then performed electrocatalytic experiments. We quantified the impact of temperature and electrical potential on the formation rate of valuable products (vanillic acid, acetovanillone, guaiacol, vanillin, and syringaldehyde). We found that increasing temperature increases their formation rate by an order of magnitude more than increasing electrical potential. For example, increasing temperature from 21 to 180 °C increases the vanillin formation rate by +16.5 mg⋅L-1 ⋅h-1 ±1.7 mg⋅L-1 ⋅h-1 , while increasing electrical potential from 0 to 2 V increases the vanillin formation rate by -0.6 mg⋅L-1 ⋅h-1 ±1.4 mg⋅L-1 ⋅h-1 .

Solvothermal liquefaction of orange peels into biocrude: An experimental investigation of biocrude yield and energy compositional dependency on process variables

The efficient valorization of biomass for energy-derived biocrudes is essential for effective waste management. However, the production of biocrudes with high energy and reduced oxygen contents during the liquefaction process requires further insight. Therefore, the impact of reaction temperature, residence time, and ethanol: acetone on the energy compositions and bioproduct's yield enhancement were investigated. The biocrudes obtained were characterized using elemental analysis, GC-MS, FTIR, GPC and TGA to understand the effects of process parameters on the biocrudes' compositions. An improved HHV (38.18 MJ/kg) and lower O/C ratio (0.11) were obtained at 430 °C, 35 min and 50% ethanol with a significant improvement in the enhancement factor, deoxygenation, and percentage hydrogenation of 2.63, 36.88%, and 77.87%, respectively. The presence of ketones, hydrocarbons, phenolics and aromatics of 23.74, 4.28, 37.20 and 17.81% respectively indicate the potential of the obtained biocrude as renewable energy sources upon further upgrading.

Catalytic hydrothermal liquefaction of alkali lignin for monophenols production over homologous biochar-supported copper catalysts in water

Constructing an advanced catalytic system for the purposeful liquefaction of lignin into chemicals has presented a significant prospect for sustainable development. In this work, the catalytic process of mesoporous homologous biochar (HBC) derived from alkali lignin supported copper catalysts (Cu/HBC) was reported for catalytic liquefaction of alkali lignin to monophenols. The characterization results revealed HBC promoted the formation of metal-support strong interaction and the generation of oxygen vacancies, enhancing the acid sites of Cu/HBC. Under the optimal conditions (0.2 g alkali lignin, 280 °C, 0.05 g Cu/HBC, 6 h, 18 mL water), the monophenol yield reached 75.01 ± 0.76 mg/g, and the bio-oil yield was 57.98 ± 1.76%. The copious mesopores, high surface area, and rich acidic sites were responsible for the high activity of Cu/HBC, which significantly outperformed the controlled catalysts, such as HBC, commercial activated carbon (AC), and reported Ni/AC, Ni/MCM-41, etc. In four consecutive runs, the catalytic performance of Cu/HBC was only reduced by 3.65% per cycle. Interestingly, catechol was selectively produced with Cu/HBC, which provided an effective strategy for the conversion of G/S-type lignin to catechyl phenolics (C-type). These findings indicate that the Cu/HBC will be a promising substitution of noble metal-supported catalysts for conversion biomass into high value-added phenolics.

Review on the preparation of fuels and chemicals based on lignin

Lignin is by far the most abundant natural renewable aromatic polymer in nature, and its reserves are second only to cellulose. In addition to the rich carbon content, the structure of lignin contains functional groups such as benzene rings, methoxyl groups, and phenolic hydroxyl groups. Lignin degradation has become one of the high value, high quality and high efficiency methods to convert lignin, which is of great significance to alleviating the current energy shortage and environmental crisis. This article introduces the hydrolysis methods of lignin in acidic, alkaline, ionic liquids and supercritical fluids, reviews the heating rate, the source of lignin species and the effects of heating rate on the pyrolysis of lignin, and briefly describes the metal catalysis, oxidation methods such as electrochemical degradation and photocatalytic oxidation, and degradation reduction methods using hydrogen and hydrogen supply reagents. The lignin degradation methods for the preparation of fuels and chemicals are systematically summarized. The advantages and disadvantages of different methods, the selectivity under different conditions and the degradation efficiency of different catalytic combination systems are compared. In this paper, a new approach to improve the degradation efficiency is envisioned in order to contribute to the efficient utilization and high value conversion of lignin.

Machine learning based analysis of reaction phenomena in catalytic lignin depolymerization

Heterogeneously catalyzed lignin solvolysis opens the possibility of transforming low value biomass into high value, useful aromatic chemicals, however, its reaction behavior is poorly understood due to the many possible interactions between reaction parameters. In this study, a novel predictive model for bio-oil yield, char yield and reaction time is developed using Random Forest (RF) regression method using data available from the literature to study the impact of surface properties of the catalyst and the weight averaged molecular weight of the lignin (M_w) used in the reaction. The models achieved a coefficient of determination (R2) score of 0.9062, 0.9428 and 0.8327, respectively, and feature importance for each case was explained and tied to studies that provide a mechanistic explanation for the performance of the model. Surface properties and lignin M_w showed no importance to the prediction of bio-oil yield and average pore diameter contributed 3% of feature importance to reaction time.

Catalytic hydrothermal liquefaction of alkali lignin over activated bio-char supported bimetallic catalyst

Carbon-based support catalysts are beneficial on account of low material cost, prominent surface area, and stability at high temperature. In this study, biochar derived activated carbon (AC) supported metal catalysts were tested for hydrothermal liquefaction (HTL) of alkali lignin. Catalytic HTL of alkali lignin was carried out at various temperatures (260 to 300 °C) with varying catalysts quantity (5 to 20 wt%), and solvents (water, ethanol, methanol) for 15 min reaction time. As the reaction temperature increased from 260 to 300 °C, conversion increased from 76.2 to 85.5 wt%. Bimetallic catalyst Ni-Co/AC with ethanol solvent system at 280 °C gave highest bio-oil yield (72.0 wt%). Lignin catalytic depolymerization produces monomer phenolic compounds due to efficient breaking of the lignin macromolecule. Thus, the presence of catalyst and solvent increased the cleavage of β-O-4 bonds resulting in increased selectivity towards vanillin (32.3-36.2%).

Mild depolymerization of the sinocalamus oldhami alkali lignin to phenolic monomer with base and activated carbon supported nickel-tungsten carbide catalyst composite system

In this study, the sinocalamus oldhami alkali lignin was depolymerized into phenolic products in a combined system by using the composite alkali and Ni-W₂C/activated carbon (AC) as catalysts. FT-IR, GPC, TG, 2D-HSQC and GC-MS were used to analyze the composition, structure and distribution of degradation products, and the synergistic effect of metal and alkali catalysts on the depolymerization of lignin was also studied. The results showed that Ni-W₂C/AC and composite alkali could effectively improve the catalytic degradation efficiency of lignin under mild conditions, 94.4% of lignin was converted and 17.18% of phenolic monomers were obtained under 260 °C for 5 h. In this composite system, the synergism of the basic sites, the metal active sites and the Lewis acid sites could promote the cleavage of C-O bonds in the lignin molecule and lower the char formation during the base-catalyzed solvolysis. Phenolic monomers were mainly composed of phenol, 2-methyl-phenol and p-cresol etc.

Could preoxidation always promote the subsequent hydroconversion of lignin? Two counterexamples catalyzed by Cu/CuMgAlOx in supercritical ethanol

In this study, two counterexamples of lignin preoxidation-hydroconversion were reported. First, two lignin feedstocks were preoxidized with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in acetonitrile with various dosages (15%, 30%, and 60%). Then, these preoxidized lignins (HELOs and MWLOs) were hydroconverted in supercritical ethanol catalyzed by Cu/CuMgAlO_x. Total yields from HELOs were all higher than those from HEL, indicating the good promotion of DDQ preoxidation on the subsequent hydroconversion of HELOs, especially with the DDQ dosage of 15%. Differently, the promotion effect of DDQ preoxidation on the hydroconversion of MWLOs depended on the DDQ dosage as well as the reaction time. Through the comparison of two counterexamples, this work bursted the myth that preoxidation can always promote the subsequent hydroconversion of lignin, revealed the influence of lignin property, preoxidation degree, and reaction conditions on the subsequent hydroconversion of preoxidized lignin, and presented the new insight into the preoxidation-hydroconversion strategy for lignin.

Chemicals from Lignin: A Review of Catalytic Conversion Involving Hydrogen

Lignin is the most abundant biopolymer with aromatic building blocks and its valorization to sustainable chemicals and fuels has extremely great potential to reduce the excessive dependence on fossil resources, although such conversions remain challenging. The purpose of this Review is to present an insight into the catalytic conversion of lignin involving hydrogen, including reductive depolymerization and the hydrodeoxygenation of lignin-derived monomers to arenes, cycloalkanes and phenols, with a main focus on the catalyst systems and reaction mechanisms. The roles of hydrogenation sites (Ru, Pt, Pd, Rh) and acid sites (Nb, Ti, Mo), as well as their interaction in selective hydrodeoxygenation reactions are emphasized. Furthermore, some inspirational strategies for the production of other value-added chemicals are mentioned. Finally, some personal perspectives are provided to highlight the opportunities within this attractive field.

Effective Depolymerization of Sodium Lignosulfonate over SO42−/TiO2 Catalyst

In this paper, liquefaction of sodium lignosulfonate (SL) over SO42−/TiO2 catalyst in methanol/glycerol was investigated. Effects of temperature, time, the ratio of methanol to glycerol and catalyst dosage were also studied. It was indicated that optimal reaction condition (the temperature of 160 °C, the time of 1 h, solvent ratio (methanol/glycerol) of 2:1, catalyst dosage of 5 wt % (based on lignin input)) was obtained after sets of experiments. The maximum yields of liquefaction (89.8%) and bio-oil (86.8%) were gained under the optimal reaction conditions. Bio-oil was analyzed by elemental analysis, FT-IR and gas chromatogram and mass spectrometry (GC/MS). It was shown that the functional groups of bio-oil were enriched and calorific value of bio-oil was increased. Finally, it can be seen from GC/MS analysis that the type of products included alcohols, ethers, phenols, ketones, esters and acids. Phenolic compounds mainly consisted of G (guaiacyl)-type phenols.

Effects of the novel catalyst Ni-S2O8 2--K2O/TiO2 on efficient lignin depolymerization

To improve the utilization of lignin, much effort has been devoted to lignin depolymerization with the aim to decrease waste and enhance profitability. Here, a dual property (acid and base) catalyst, namely S₂O₈²⁻–K₂O/TiO₂, was carefully researched. Upon loading S₂O₈²⁻ and K₂O onto TiO₂, acid and base sites emerged, and S₂O₈²⁻ and K₂O mutually enhanced the acid and base strengths of the catalyst enormously; this indeed facilitated lignin depolymerization. Under appropriate conditions, the yields of liquid product, petroleum ether soluble (PE-soluble) product and total monomer products were 83.76%, 50.4% and 28.96%, respectively. The constituents of the PE-soluble fraction, which are mainly monomers and dimers, can be used as liquid fuels or additives. In addition, after the catalyst was modified by Ni, better results were obtained. Surprisingly, it was found that the Ni enhanced not only the hydrogenation capacity but also the acidity. The highest high heating value (HHV) of the liquid product (33.6 MJ kg⁻¹) was obtained, and the yield of PE-soluble product increased from 50.4 to 56.4%. The product can be utilized as a fuel additive or be converted to bio-fuel. This catalysis system has significant potential in the conversion of lignin to bio-fuel.

To improve the utilization of lignin, much effort has been devoted to lignin depolymerization with the aim to decrease waste and enhance profitability.

Polysaccharides of mushroom Pleurotus spp.: New extraction techniques, biological activities and development of new technologies

The biodiversity of mushrooms Pleurotus spp. is impressive due to its complexity and diversity related to the composition of chemical structures such as polysaccharides, glycoproteins and secondary metabolites such as alkaloids, flavonoids and betalains. Recent studies of polysaccharides and their structural elucidation have helped to direct research and development of technologies related to pharmacological action, production of bioactive foods and application of new, more sophisticated extraction tools. The diversity of bioactivities related to these biopolymers, their mechanisms and routes of action are constant focus of researches. The elucidation of bioactivities has helped to formulate new vaccines and targeted drugs. In this context, in terms of polysaccharides and the diversity of mushrooms Pleurotus spp., this review seeks to revisit the genus, making an updated approach on the recent discoveries of polysaccharides, new extraction techniques and bioactivities, emphasising on their mechanisms and routes in order to update the reader on the recent technologies related to these polymers.

Effect of HCOOK/Ethanol on Fe/HUSY, Ni/HUSY, and Ni-Fe/HUSY Catalysts on Lignin Depolymerization to Benzyl Alcohols and Bioaromatics

We have investigated the production of benzyl alcohols and bioaromatics via the reductive lignin depolymerization process over Fe/H-style ultrastable Y (HUSY), Ni/HUSY, and Ni-Fe/HUSY catalysts using HCOOK/ETOH in air. Synergy effect between HCOOK and the catalysts improved the depolymerization process, resulting in a higher bio-oil recovery. HCOOK does not act solely as an in situ hydrogen source; it also interacts with lignin to enable its initial depolymerization via a base-catalyzed mechanism to low-molecular-weight fragments, and in tandem with the catalyst, the hydrogenolysis rate of the depolymerized lignin monomers was enhanced. Fe/HUSY displayed an excellent activity for the catalytic reductive step in contrast to Ni/HUSY and Ni-Fe/HUSY by facilitating methoxy group removal via hydrogenolysis, thereby contributing to the yield and stabilization of the low-molecular-weight aromatics [diethyl ether (DEE)-soluble products]. Fe/HUSY gave the highest DEE product yield of >99 wt % and a total benzyl alcohol yield of 16 wt % with a total selectivity of 47 wt % (60 wt % for aromatic alcohols). Fe/HUSY was reused for the lignin depolymerization reaction without much loss of its initial activity, giving 13 wt % yield of benzyl alcohols with a selectivity of 58 wt % (77 wt % for aromatic alcohols).

Ethanol/1,4-dioxane/formic acid as synergistic solvents for the conversion of lignin into high-value added phenolic monomers

In this study, a mixture solvent of ethanol/1,4-dioxane/formic acid (FA) is firstly reported to efficaciously depolymerize industrial lignin to produce high-value added phenolic monomers, in which 1,4-dioxane acts as lignin solvent, ethanol acts as solvent, reactant and in situ hydrogen donor, and FA acts as acid catalyst and in situ hydrogen donor. The effects of solvent composition and reaction conditions on the lignin conversion and product yields were explored, resulting in a low residue yield of 6.57% and a high phenolic monomers yield of 22.4% at 300 °C for 2 h when Kraft lignin was depolymerized in the mixture solvent of ethanol/1,4-dioxane/FA (10:10:2, v/v). Moreover, possible reaction mechanism on lignin depolymerization in the mixture solvent was illustrated, suggesting a favorable synergistic effect among the three components of the mixture solvent. In addition, the satisfactory applicability of the mixture solvent was approved through the feedstock adaptability and recyclability experiments.

Catalytic Transfer Hydrogenolysis Reactions for Lignin Valorization to Fuels and Chemicals

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.

From lignin to valuable products-strategies, challenges, and prospects

The exploration of effective approaches for the valorization of lignin to valuable products attracts broad interests of a growing scientific community. By fully unlocking the potential of the world's most abundant resource of bio-aromatics, it could improve the profitability and carbon efficiency of the entire biorefinery process, thus accelerate the replacement of fossil resources with bioresources in our society. The successful realization of this goal depends on the development of technologies to overcome the following challenges, including: 1) efficient biomass pretreatment and lignin separation technologies that overcomes its diverse structure and complex chemistry challenges to obtain high purity lignin; 2) advanced chemical analysis for precise quantitative characterization of the lignin in chemical transformation processes; 3) novel approaches for conversion of biomass-derived lignin to valuable products. This review summarizes the latest cutting-edge innovations of lignin chemical valorization with the focus on the aforementioned three key aspects.

Ethanol: A Promising Green Solvent for the Deconstruction of Lignocellulose

Growing energy demand, environmental impact, energy security issues, and rural economic development have encouraged the development of sustainable renewable fuels. Nonfood lignocellulosic biomass is a suitable source for sustainable energy because the biomass feedstocks are low cost, abundant, and carbon neutral. Recent thermochemical conversion studies are frequently directed at converting biomass into high-quality liquid fuel precursors or chemicals in a single step. Supercritical ethanol has been selected as a promising solvent medium to deconstruct lignocellulosic biomass because ethanol has extraordinary solubility towards lignocellulosic biomass and can be resourced from cellulosic ethanol facilities. This review provides a critical insight into both catalytic and noncatalytic strategies of lignocellulose deconstruction. In this context, the supercritical ethanol deconstruction pathways are thoroughly reviewed; GC-MS, 1D and 2D NMR spectroscopy, and elemental analysis strategies towards liquid biomass deconstruction products are also critically presented. This review aims to provide readers a broad and accurate roadmap of novel biomass to biofuel conversion techniques.

A Review of Microwave Assisted Liquefaction of Ligninin Hydrogen Donor Solvents: Effect of Solvents and Catalysts

Lignin, a renewable source of aromatic chemicals in nature, has attracted increasing attention due to its structure and application prospect. Catalytic solvolysis has developed as a promising method for the production of value-added products from lignin. The liquefaction process is closely associated with heating methods, catalysts and solvents. Microwave assisted lignin liquefaction in hydrogen donor solvent with the presence of catalysts has been confirmed to be effective to promote the production of liquid fuels or fine chemicals. A great number of researchers should be greatly appreciated on account of their contributions on the progress of microwave technology in lignin liquefaction. In this study, microwave assisted liquefaction of lignin in a hydrogen donor solvent is extensively overviewed, concerning the effect of different solvents and catalysts. This review concludes that microwave assisted liquefaction is a promising technology for the valorization of lignin, which could reduce the reaction time, decrease the reaction temperature, and finally fulfill the utilization of lignin in a relatively mild condition. In the future, heterogeneous catalysts with high catalytic activity and stability need to be prepared to achieve the need for large-scale production of high-quality fuels and value-added chemicals from lignin.