Lignin Model Compounds Research Articles

An experimental and density functional theory investigation of the influence of HZSM-5 on pyrolysis oils produced from vanillin, a model lignin compound

Lignin, as a renewable energy source, has attracted much attention due to its high valorization potential. Due to the complex composition of lignin, vanillin, an important intermediate of lignin, was chosen as a model compound in this study. The study examined the influence of the HZSM-5 molecular sieve on the quality of pyrolysis oils during the catalytic pyrolysis of vanillin at 600 °C using a horizontal tube furnace and Py-GC/MS analysis. The results revealed that the HZSM-5 catalyst significantly increased the yields of toluene, guaiacol, and 4-hydroxyisophthalaldehyde by 2.49 %, 19.26 %, and 1.8 %, respectively, whereas reducing the contents of phenol and 3,4-dimethoxyphenol by 6.01 % and 3.79 %, respectively. These findings indicate that the HZSM-5 molecular sieve effectively improves the quality of pyrolysis oils during the catalytic pyrolysis of vanillin. The evolution of the main functional groups in the pyrolysis of vanillin was analyzed using an in situ diffuse reflectance infrared Fourier transform spectroscopy. The addition of HZSM-5 promoted the cleavage of oxygen-containing functional groups and the generation of aromatic hydrocarbons during the pyrolysis of vanillin. Furthermore, the pyrolysis reaction pathways of vanillin were simulated under the B3LYP/6-311 g (d, p) basis set using the density functional theory. The acidic sites of HZSM-5 interacted with vanillin via the hydrogen bonds that affected the energy barriers of the pyrolysis reaction pathways of vanillin. HZSM-5 inhibited the generation of phenol and 3,4-dimethoxyphenol by increasing the energy barriers by 32.95 and 31.03 kJ/mol, respectively; however, HZSM-5 promoted toluene production by decreasing the energy barrier by 37.38 kJ/mol. The effects of HZSM-5 on the products in the simulation results were consistent with the experimental results.

Kraft Lignin Decomposition by Forest Soil Bacterium Pseudomonas kribbensis CHA-19.

Identification of the biochemical metabolic pathway for lignin decomposition and the responsible degradative enzymes is needed for the effective biotechnological valorization of lignin to renewable chemical products. In this study, we investigated the decomposition of kraft lignin by the soil bacterium Pseudomonas kribbensis CHA-19, a strain that can utilize kraft lignin and its main degradation metabolite, vanillic acid, as growth substrates. Gel permeation chromatography revealed that CHA-19 decomposed polymeric lignin and degraded dehydrodivanillin (a representative lignin model compound); however, the degradative enzyme(s) and mechanism were not identified. Quantitative polymerase chain reaction with mRNAs from CHA-19 cells induced in the presence of lignin showed that the putative genes coding for two laccase-like multicopper oxidases (LMCOs) and three dye-decolorizing peroxidases (DyPs) were upregulated by 2.0- to 7.9-fold compared with glucose-induced cells, which indicates possible cooperation with multiple enzymes for lignin decomposition. Computational homology analysis of the protein sequences of LMCOs and DyPs also predicted their roles in lignin decomposition. Based on the above data, CHA-19 appears to initiate oxidative lignin decomposition using multifunctional LMCOs and DyPs, producing smaller metabolites such as vanillic acid, which is further degraded via ortho- and meta-ring cleavage pathways. This study not only helps to better understand the role of bacteria in lignin decomposition and thus in terrestrial ecosystems, but also expands the biocatalytic toolbox with new bacterial cells and their degradative enzymes for lignin valorization.

Effective breaking of β-o-4 bonds in lignin model compounds by g-C3N4/ZnIn2S4 photocatalyst under visible light

In this study, cauliflower-like g-C3N4/ZnIn2S4 composite photocatalysts were prepared via a hydrothermal method by loading ZnIn2S4 on g-C3N4 nanosheet layers. The crystal structures of the prepared samples were characterized by X-ray diffractometer. TEM and XPS characterization indicated the successful catalyst compounding, while PL and UV–Vis-DRs characterization illustrated the broadening of the visible light absorption interval of g-C3N4/ZnIn2S4 and the low photogenerated electron-hole complexation rate. The lignin model compounds were degraded under visible light, and the GC-MS analysis of the composition of the products showed that CN/ZIS-15 degraded, with 94 % conversion of 2-phenoxy-1-phenylethanol(PP-ol) in 3 h, which was 2.5 times higher than that of pure g-C3N4, and the main products being acetophenone (AP) and phenol (Phol). The reactive radical trapping experiments were carried out to investigate the main reactive species in the reaction and to explore the mechanism for the breaking of β-O-4 bond.

Oxidative Cleavage of α-O-4, β-O-4, and 4-O-5 Linkages in Lignin Model Compounds Over P, N Co-Doped Carbon Catalyst: A Metal-Free Approach.

Developing efficient metal-free catalysts for lignin valorization is essential but challenging. In this study, a cost-effective strategy is employed to synthesize a P, N co-doped carbon catalyst through hydrothermal and carbonization processes. This catalyst effectively cleaved α-O-4, β-O-4, and 4-O-5 lignin linkages, as demonstrated with model compounds. Various catalysts were prepared at different carbonization temperatures and thoroughly characterized using techniques such as XRD, RAMAN, FTIR, XPS, NH3-TPD, and HRTEM. Attributed to higher acidity, the P5NC-500 catalyst exhibited the best catalytic activity, employing H2O2 as the oxidant in water. Additionally, this metal-free technique efficiently converted simulated lignin bio-oil, containing all three linkages, into valuable monomers. Density Functional Theory calculations provided insight into the reaction mechanism, suggesting substrate and oxidant activation by P-O-H sites in the P5NC-500, and by N-C-O-H in the CN catalyst. Moreover, the catalyst's recyclability and water utilization enhance its environmental compatibility, offering a highly sustainable approach to lignin valorization with potential applications in various industries.

Efficient and selective cleavage of lignin C–C bonds under visible light facilitated by the Zn(II)-doped mesoporous graphitic carbon nitride photocatalyst

Despite the promise of photocatalysis for extracting aromatic compounds from renewable lignin feedstocks, the selective cleavage of Cα–Cβ bonds in lignin under mild conditions poses a persistent challenge, primarily due to their elevated dissociation energies and steric hindrance. Herein, an efficient photocatalytic strategy for selectively breaking the Cα–Cβ bonds of lignin with visible-light irradiation and room temperature is proposed, facilitated by the Zn(II)-doped porous graphitic carbon nitride (Zn/CN) photocatalyst. Reduced energy band gap and photoluminescence, along with intensified visible-light absorption and enhanced photocurrent of Zn/CN, contribute to its efficacy in photocatalytic activity. Consequently, the lignin model compound (2-phenoxy-1-phenylethanol) achieves a conversion rate of 99 %, demonstrating a notable selectivity of 97 % in specifically breaking the Cα–Cβ bonds. Mechanistic investigations identify that photogenerated holes play a pivotal role during the photocatalytic conversion. Additionally, the activity and selectivity of Zn/CN photocatalyst are further confirmed by the successful photocatalytic conversion of pine kraft lignin, yielding high-value chemicals such as benzoic acid and vanillin. This research proposes an economical, efficient, and facile method for utilizing renewable lignin feedstocks under photocatalysis, thus advancing the production of high-value aromatic compounds.

Enhanced reductive catalytic fractionation of lignocellulose using a water-resistant RuNiZnOx/Nb2O5 catalyst with synergistic hydrogen spillover and acidic properties

Eco-friendly lignin depolymerization into monomeric phenols in an aqueous system is essential for the sustainable and green valorization of lignocellulosic biomass. However, a major challenge in this process is the development of efficient and stable catalysts for aqueous reductive catalytic fractionation (RCF), as water can cause catalyst deactivation and reduce monomer yields. In this study, a water-resistant RuNiZnOx/Nb2O5 catalyst is developed that demonstrates excellent catalytic activity and stability in aqueous systems, achieving a near-theoretical monomeric phenol yield of 40.1 wt% under reaction conditions of 240℃ and 3 MPa H2 for 4 h. The excellent performance of this catalyst is attributed to the synergistic hydrogen spillover effect between Ni and Ru and the strong acidic properties of ZnO, as revealed by in-situ XPS analysis and NH3-TPD, respectively. This hydrogen spillover effect significantly alters the product composition, promoting the generation of POH-S/G from lignin model compounds. Additionally, screening of the catalyst support materials revealed that Nb2O5 not only enhances Ru dispersion but also improves its stability, resulting in a minor decrease in lignin monomer yield observed after three cycles.

Tracking Thermo-Oxidation Reaction Products and Pathways of Modified Lignin Structures from Reactive Molecular Dynamics Simulations.

Thermo-oxidation of biomass is an important process that occurs through a variety of reaction pathways depending on the chemical nature of the molecules and reaction conditions. These processes can be modeled using reactive molecular dynamics to study chemical reactions and the evolution of converted molecules over time. The advantage of this approach is that many molecules can be modeled, but it is challenging to use the large amount of data obtained from such a simulation to determine reaction products and pathways. In this study, we developed a tracking approach to identify the reaction pathways of the dominant reaction products from reactive molecular dynamics simulations. We demonstrated the approach for thermo-oxidation reactions of modified model lignin compounds. For two modified lignin structures, we tracked the evolving chemical species to find the most common reaction products. Subsequently, we monitored specific bonds to determine the individual steps in the reaction process. This combined approach of reactive molecular dynamics and tracking enabled us to identify the most likely thermo-oxidation pathways. The methodology can be used to investigate the thermo-oxidative pathways of a wider range of chemical compounds.

Mechanism of formation H2 and CH4 by hydrolysis and alcoholysis of lignin: A density functional theory study

Hydrolysis and alcoholysis are considered as viable routes for efficient degradation and recycling high-quality gas fuel of lignin. Mechanism of formation H2 and CH4 by hydrolysis and alcoholysis of various phenylic lignin model compounds were studied using density functional theory method at M06–2X/6–31++G(d,p) level. Formation H2 by eight hydrolysis reaction pathways and formation CH4 by eight alcoholysis reaction pathways were designed. The standard kinetic and thermodynamic parameters of hydrolysis and alcoholysis reaction pathways were calculated. The results show that energy barriers for hydrolysis of phenyl (p-hydroxyphenyl, guaiacyl, and syringyl) formaldehyde paths (about 270.0 kJ/mol) are higher than those for hydrolysis of phenyl (p-hydroxyphenyl, guaiacyl, and syringyl) acetaldehyde paths (about 250.0 kJ/mol). Thus, H2 is easily formed during the decomposition of phenyl (p-hydroxyphenyl, guaiacyl, and syringyl) acetaldehyde. The energy barriers for alcoholysis of phenyl (p-hydroxyphenyl, guaiacyl, and syringyl) formaldehyde paths (about 370.0 kJ/mol) are lower than those for alcoholysis of phenyl (p-hydroxyphenyl, guaiacyl, and syringyl) acetaldehyde paths (about 355.0 kJ/mol). Thus, CH4 is easily formed during the decomposition of phenyl (p-hydroxyphenyl, guaiacyl, and syringyl) acetaldehyde. In addition, temperature is the key factor in the lignin degradation process, the calculation results indicate the reaction rate increases with the increase of temperature, and the reaction in water molecular environment is better than that in methanol medium.

Self-hydrogen transfer hydrogenolysis of β-O-4 bonds in lignin model compounds over NiCu/Al2O3 catalyst

Hydrogenolysis of β-O-4 linkages in lignin to produce aromatic compounds is attractive but challenging. Currently, external hydrogen source like hydrogen gas or hydrogen-donor solvent is necessary during hydrogenolysis, leading to high cost and low selectivity of products. Herein, a NiCu/Al2O3 catalyst was prepared for self-hydrogen transfer hydrogenolysis (SHTH) of Cβ-O bond using inherent CαH-OH structure as H-source. A β-O-4 model compound, 2-phenoxy-1-phenethanol, was completely transformed into acetophenone and phenol at 220 ℃ under 1 MPa N2 for 2h. The SHTH reaction proceeded through a dehydrogenation reaction followed by a hydrogenolysis reaction. Hydrogen radicals released from CαH-OH group generated a “hydrogen pool” on the catalyst surface, which inserted into the formed β-O-4 ketone intermediate to cleave the Cβ-O bond. The catalyst presented outstanding reusability and universality during the SHTH reaction. Moreover, the methoxy groups at aryl positions impeded the SHTH process, and the steric effect was related to the amount and position of methoxy groups. This study realizes the utilization of structural hydrogen from lignin model compounds for its Cβ-O bond cleavage, providing an economical and sustainable method toward lignin valorization.

Combining experiment and theory to study the mechanism of lignin supercritical water gasification

Supercritical water gasification (SCWG) presents notable advantages in biomass energy utilization. The exploration of the reaction mechanism of lignin in supercritical water holds significance in advancing and refining the biomass SCWG process. In this work, syringol was selected as a lignin model compound to study the SCWG mechanism of lignin through a combination of molecular simulation and experiments. The results show that the ether bond is rapidly cleaved in the reaction initial stages. While the phenolics dearomatization has a slower rate and temperature exerts a considerable influence on the dearomatization process. Four degradation pathways are proposed based on the molecular simulations. In the context of the dearomatization reaction, DFT calculations are performed to determine the energy barriers associated with each of the four pathways. Notably, the energy barrier for the most favorable pathway is found to be 291.96 kJ/mol. A lumped kinetic model is constructed based on the reaction pathway, and the reliability of the model is verified by both experiments and DFT calculations. This paper offers valuable assistance in optimizing reaction pathways.

Lattice Strain Engineering on Metal-Organic Frameworks by Ligand Doping to Boost the Electrocatalytic Biomass Valorization.

As an efficient and environmental-friendly strategy, electrocatalytic oxidation can realize biomass lignin valorization by cleaving its aryl ether bonds to produce value-added chemicals. However, the complex and polymerized structure of lignin presents challenges in terms of reactant adsorption on the catalyst surface, which hinders further refinement. Herein, NiCo-based metal-organic frameworks (MOFs) are employed as the electrocatalyst to enhance the adsorption of reactant molecules through π-π interaction. More importantly, lattice strain is introduced into the MOFs via curved ligand doping, which enables tuning of the d-band center of metal active sites to align with the reaction intermediates, leading to stronger adsorption and higher electrocatalytic activity toward bond cleavage within lignin model compounds and native lignin. When 2'-phenoxyacetophenone is utilized as the model compound, high yields of phenol (76.3%) and acetophenone (21.7%) are achieved, and the conversion rate of the reactants reaches 97%. Following pre-oxidation of extracted poplar lignin, >10 kinds of phenolic compounds are received using the as-designed MOFs electrocatalyst, providing ≈12.48% of the monomer, including guaiacol, vanillin, eugenol, etc., and p-hydroxybenzoic acid dominates all the products. This work presents a promising and deliberately designed electrocatalyst for realizing lignin valorization, making significant strides for the sustainability of this biomass resource.

Bonding of Lignin and Coniferyl Alcohol by a Redox Shuttle of Low-Molecular-Weight Lignols in Enzymatic Oxidative Dehydrogenative Polymerization.

Lignin is an aromatic polymer that constitutes plant cell walls. The polymerization of lignin proceeds by radical coupling, and this process requires radicalization of the phenolic end of lignin by enzymes. However, due to the steric hindrance between enzymes, lignin, and polysaccharides, the direct oxidation of the phenolic end of lignin by the enzyme would be difficult, and the details of the growth of lignin are still unknown. In this study, enzymatic dehydrogenative polymerization experiments were conducted using coniferyl alcohol (CA) and the deuterium-labeled lignin model compound (D-LM) under a noncontact condition in which horseradish peroxidase cannot directly oxidize D-LM due to separation by a dialysis membrane. Analysis of deuterium-labeled degraded compounds obtained by a combination of methylation and thioacidolysis revealed the formation of the bond between the phenolic end of D-LM and CA, suggesting that membrane-permeable, low-molecular-weight lignols functioned as a redox shuttle mediator.

Revisiting the mechanism of β-O-4 bond cleavage during acidolysis of lignin part 10: reactions of C6–C2-type non-phenolic syringyl model compounds and comparison of the reactions with those of the guaiacyl analogues

A C6–C2-type non-phenolic syringyl lignin model compound (I, 2-(2-methoxyphenoxy)-1-(3,4,5-trimethoxyphenyl)ethanol) as well as its derivatives was acidolyzed in aqueous 82vol% 1,4-dioxane containing 0.2 mol/L HBr, HCl, or H2SO4 at 85ºC. The results were compared with those on the guaiacyl analogues (V as well as its derivatives) obtained in our previous reports. The acidolysis of compound I was slower than that of compound V, which is in accordance with our previous result on the formation rates of the benzyl cation intermediates (BC) from a syringyl compound and its guaiacyl analogue. The enol ether-type compound was primarily produced in the acidolysis of compound I, similar to that of compound V. The acidolysis using HBr or HCl was faster than that using H2SO4, indicating the participation of Br¯ or Cl¯, respectively, in the acidolysis. It was suggested that Br¯ (as well as Cl¯) adds to the cation center of the BC to afford the adduct in the acidolysis of compound I, similar to that of compound V, and hence, this adduct formation is a major bypass of the common route. The bypass activity in the acidolysis of compound I was about half in that of compound V.

Mesoporous SBA‐15 Supported CdS through Organosilane Ligand for Photo Driven Cleavage of Aryl Ether in Lignin and its Model Compound

AbstractMesoporous silica (SBA‐15) was synthesized hydrothermally followed by calcination and then functionalization with 3‐(triethoxysilyl)‐1‐propanethiol through surface grafting. The thiol moiety of the grafted ligand was utilized for the uniform anchoring of cadmium into the channels of the material. The synthesised material was subjected for its photo catalytic applications after characterization through standard characterization protocol. The synthesised mesoporous silica (i.e. SAB‐15), ligand modified material (i.e.SBA‐15‐S and Cd loaded ( i.e. SAB‐15‐CdS) materials were characterized through standard characterization protocol like powder XRD, FT‐IR, UV‐DRS, MAS NMR, N2 sorption, FE‐SEM, HR‐TEM and TG‐DTA studies. After successful characterization, the screening of the catalyst was carried out on a model reaction involving photo catalytic cleavage of α‐O‐4 aryl ether linkage present in benzyl phenyl ether, which is an important lignin model compound. The catalyst exhibited good efficiency along with the selectivity for desired monoaromatic platform chemicals. The synthesized catalyst was also carried out for the photo catalytic depolymerization of alkali lignin at the optimized reaction conditions which further showed the promising results for getting value added chemicals from lignin. The reaction products were characterized by using Gas chromatography‐mass spectrometry (GC‐MS) techniques.

Effect of Dynamic and Preferential Decoration of Pt Catalyst Surfaces by WOx on Hydrodeoxygenation Reactions.

Catalysts containing Pt nanoparticles and reducible transition-metal oxides (WOx, NbOx, TiOx) exhibit remarkable selectivity to aromatic products in hydrodeoxygenation (HDO) reactions for biomass valorization, contrasting the undesired aromatic hydrogenation typically observed for metal catalysts. However, the active site(s) responsible for the high selectivity remains elusive. Here, theoretical and experimental analyses are combined to explain the observed HDO reactivity by interrogating the organization of reduced WOx domains on Pt surfaces at sub-monolayer coverage. The SurfGraph algorithm is used to develop model structures that capture the configurational space (∼1000 configurations) for density functional theory (DFT) calculations of a W3O7 trimer on stepped Pt surfaces. Machine-learning models trained on the DFT calculations identify the preferential occupation of well-coordinated Pt sites (≥8 Pt coordination number) by WOx and structural features governing WOx-Pt stability. WOx/Pt/SiO2 catalysts are synthesized with varying W loadings to test the theoretical predictions and relate them to HDO reactivity. Spectroscopy- and microscopy-based catalyst characterizations identify the dynamic and preferential decoration of well-coordinated sites on Pt nanoparticles by reduced WOx species, consistent with theoretical predictions. The catalytic consequences of this preferential decoration on the HDO of a lignin model compound, dihydroeugenol, are clarified. The effect of WOx decoration on Pt nanoparticles for HDO involves WOx inhibition of aromatic ring hydrogenation by preferentially blocking well-coordinated Pt sites. The identification of preferential decoration on specific sites of late-transition-metal surfaces by reducible metal oxides provides a new perspective for understanding and controlling metal-support interactions in heterogeneous catalysis.

Silica Biomineralization with Lignin Involves Si-O-C Bonds That Stabilize Radicals.

Plants undergo substantial biomineralization of silicon, which is deposited primarily in cell walls as amorphous silica. The mineral formation could be moderated by the structure and chemistry of lignin, a polyphenol polymer that is a major constituent of the secondary cell wall. However, the reactions between lignin and silica have not yet been well elucidated. Here, we investigate silica deposition onto a lignin model compound. Polyphenyl propanoid was synthesized from coniferyl alcohol by oxidative coupling with peroxidase in the presence of acidic tetramethyl orthosilicate, a silicic acid precursor. Raman, Fourier transform infrared, and X-ray photoelectron spectroscopies detected changes in lignin formation in the presence of silicic acid. Bonds between the Si-O/Si-OH residues and phenoxyl radicals and lignin functional groups formed during the first 3 h of the reaction, while silica continued to form over 3 days. Thermal gravimetric analysis indicated that lignin yields increased in the presence of silicic acid, possibly via the stabilization of phenolic radicals. This, in turn, resulted in shorter stretches of the lignin polymer. Silica deposition initiated within a lignin matrix via the formation of covalent Si-O-C bonds. The silica nucleants grew into 2-5 nm particles, as observed via scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy. Additional silica precipitated into an extended gel. Collectively, our results demonstrate a reciprocal relation by which lignin polymerization catalyzes the formation of silica, and at the same time silicic acid enhances lignin polymerization and yield.

Effect of traditional solvent on thermal decomposition mechanism of lignin: A density functional theory study

In order to understand the effect of traditional solvents on lignin pyrolysis, the decarbonylation and decarboxylation reactions of various phenylic lignin model compounds were theoretically investigated using DFT methods at M06-2×/6–31++G(d,p) level. The calculation results show that activation energy of the decarbonylation and decarboxylation reactions of lignin model compounds can be reduced when H2O/CH3OH existed. There are two types of reaction for the H2O/CH3OH during the pyrolysis. For first type, the synergistic reaction of lignin with H2O/CH3OH as hydrogen transfer carrier. The energy barriers of the main elemental reaction steps during this type of pyrolysis are about 285.0–300.0 kJ/mol (H2O) and 275.0–290.0 kJ/mol (CH3OH) (decarbonylation), 170.0–210.0 kJ/mol and 155.0–200.0 kJ/mol (decarboxylation). For another type, the synergistic reaction of lignin with H2O/CH3OH as hydrogen source. The energy barriers of the main elemental reaction steps during this type of pyrolysis are about 260.0–278.0 kJ/mol and 240.0–260.0 kJ/mol, 303.0–312.0 kJ/mol and 291.0–297.0 kJ/mol. Furthermore, the reaction temperature has the most significant impact on decomposition reaction of lignin in a methanol medium, suggesting that the reaction in the methanol medium is better than that in the water environment.

In situ activated CoAg bimetallic catalyst for selective hydrodeoxygenation of syringol to cyclohexanol

Development of highly effective catalysts for selective hydrodeoxygenation (HDO) of syringol, a model compound of lignin, to cyclohexanol (CYHAOL) is a challenge due to the intricate molecular structure of syringol. Herein, we have synthesized an in–situ activated cobalt–silver bimetallic catalyst, CoAg/SiO2, through a simple incipient–wetness impregnation method. The catalyst without pre–reduction treatment attained a complete conversion with 93.8 mol% CYHAOL selectivity under moderate reaction conditions. Characterizations including HRTEM, XPS, XRD, NH3–TPD, and H2–TPR unanimously revealed the structure–activity relationship of the catalyst. In the reaction system, Ag promoted the in–situ reduction of Co3O4 to form a polycrystalline–structure complex (CoOx) containing oxygen vacancies. Ag also combines with Co to construct strong acid sites. The synergy of CoOx, oxygen vacancies, Ag, and acid sites demonstrates a high–efficiency HDO of syringol to CYHAOL. CoAg/SiO2 exonerates the pre–reduction treatment, simplifying the preparation and maintenance process and preventing severe sintering of the metals.

Front Cover: Ether Bond Cleavage of a Phenylcoumaran β‐5 Lignin Model Compound and Polymeric Lignin Catalysed by a LigE‐type Etherase from Agrobacterium sp. (ChemBioChem 8/2024)

Front Cover: Ether Bond Cleavage of a Phenylcoumaran β‐5 Lignin Model Compound and Polymeric Lignin Catalysed by a LigE‐type Etherase from <i>Agrobacterium</i> sp. (ChemBioChem 8/2024)

Improved nickel nanocatalysts for selective cleavage of lignin model compounds and lignin

The selective hydrogenolysis of lignin offers a promising avenue for converting lignin into valuable chemicals and fuels. However, the development of highly active catalysts for this process remains challenging. In this study, a series of Nix/Al2O3 catalysts were synthesized by calcining and reducing layered double hydroxides (LDHs) precursors. These catalysts were evaluated for their effectiveness in catalyzing the conversion of lignin-derived model compounds (2-phenoxy-1-phenylethanol and diphenyl ether). Remarkably, the optimized Ni3/Al2O3 catalyst demonstrated exceptional performance in cleaving C–O bonds, achieving complete conversion with high selectivity for monomers product under reaction conditions of 200 °C, 1 MPa H2, and 1 h. The catalyst's outstanding performance can be attributed to its relatively large specific surface area, Ni loading exceeding 50 wt%, well-dispersed Ni metal particles, abundant surface oxygen vacancies, and a significant number of acid sites. Additionally, the hydrogenolysis of lignin using the Ni3/Al2O3 catalyst resulted in a substantial production of monophenols (15.0 wt%). This study introduces a novel approach for tailoring highly active Ni nanocatalysts and highlights the capability of Nix/Al2O3 catalysts to efficiently cleave C–O bonds in lignin under mild reaction conditions.