Lignin Model Compounds Research Articles

Ionic liquid-trimetallic electrocatalytic system for C-O bond cleavage in lignin model compounds and lignin under ambient conditions

Ionic liquid-trimetallic electrocatalytic system for C-O bond cleavage in lignin model compounds and lignin under ambient conditions

Synergy of electronic and steric effects of Br-Ni catalysts for selective hydrogenolysis of diphenyl ether to phenol

Selective hydrogenolysis of 4-O-5 bond in lignin under relatively mild conditions is an important strategy for the production of valuable aromatic products, e.g. phenols, from renewable carbon resources. However, the easy saturation of benzene rings under reductive conditions over metal catalyst reduces the selectivity to aromatics. In this work, we investigated the effect of bromination of supported Ni nanoparticles on the phenol selectivity during the hydrogenolysis of diphenyl ether (DPE), a commonly used lignin model compound. Compared with the unmodified catalysts, Br-Ni/Al2O3 catalyst derived from Ni-Al layered double oxides exhibits enhanced phenol selectivity (37% vs. 15%) under similar DPE conversions (60%). Various characterizations including transmission electron microscopy (TEM), in-situ X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure (EXAFS), and CO-Fourier transform infrared spectroscopy (FTIR) indicate that Br preferentially located at the terrace site of Ni nanoparticles, deactivating the continuous Ni sites for benzene ring hydrogenation. In addition, the electron-withdrawing effect of Br creates positively charged Ni sites at the corners, facilitating the hydrogenolysis of C–O aryl ether bonds. During the hydrogenation of real lignin, the selective poisoning and electronic effects introduced by Br synergistically increased the yield of phenols from 12.20% on the initial Ni/Al2O3 to 30.47% over the Br-Ni/Al2O3 catalyst. This work provides an advanced strategy for the catalytic valorization of lignin by halogen modified metal-based catalysts.

The mechanism of self-assembly of lignin in deep eutectic solvent based on sulfamic acid and urea through molecular dynamics simulation

Due to the diversity of industrial lignin sources and the complexity of its structure, its application as a high-value material is limited. Lignin nanoparticles (LNPs) have emerged as a hotspot for research due to their advantages of high specific surface area and high dispersion and the solvent transfer method is commonly used for the preparation of LNPs. In this paper, LNPs were prepared by solvent transfer method using DES based on sulfamic acid and urea (S/U DES) as solvent and water as anti-solvent. To explore the internal mechanism of the self-assembly of nanoparticles, a theoretical model of the solvent system and model lignin compound was constructed with the assistance of quantum chemistry and molecular dynamics theories. Through classical molecular dynamics (MD) simulations, the interaction energy, radius of gyration (ROG), solvent accessible surface area (SASS), radial and spatial distribution function (RDFs/SDFs), hydrogen bonding, and the morphology changes were analyzed to reveal the internal mechanism of self-assembly of model lignin compounds in S/U DES. This study is useful in revealing the mechanism of interaction between lignin and DES, as well as providing a benchmark for the green and efficient preparation of lignin nanoparticles.

Catalytic Selective Oxidation of β-O-4 Bond in Phenethoxybenzene as a Lignin Model Using (TBA)5[PMo10V2O40] Nanocatalyst: Optimization of Operational Conditions.

The catalytic oxidation of phenethoxybenzene as a lignin model compound with a β-O-4 bond was conducted using the Keggin-type polyoxometalate nanocatalyst (TBA)5[PMo10V2O40]. The optimization of the process's operational conditions was carried out using response surface methodology. The statistically significant variables in the process were determined using a fractional factorial design. Based on this selection, a central circumscribed composite experimental design was used to maximize the phenethoxybenzene conversion, varying temperature, reaction time, and catalyst load. The optimal conditions that maximized the phenethoxybenzene conversion were 137 °C, 3.5 h, and 200 mg of catalyst. In addition, under the optimized conditions, the Kraft lignin catalytic depolymerization was carried out to validate the effectiveness of the process. The depolymerization degree was assessed by gel permeation chromatography from which a significant decrease in the molar mass distribution Mw from 7.34 kDa to 1.97 kDa and a reduction in the polydispersity index PDI from 6 to 3 were observed. Furthermore, the successful cleavage of the β-O-4 bond in the Kraft lignin was verified by gas chromatography-mass spectrometry analysis of the reaction products. These results offer a sustainable alternative to efficiently converting lignin into valuable products.

(Invited) Nitroxyl-Derivatives As Cocatalyst for Lignin Decomposition by Dye-Sensitized Photoelectrochemical Cell

For stable and chemoselective decomposition of lignin model compound by DSPEC, metal based sensitizer was replaced by organic dye, 5-[4-(diphenylamino)phenyl]thiophene-2-cyanoacrylic acid (L1). For effective regeneration of dyes and oxidation of lignin model compounds(LMCs), bicyclic nitroxyl derivates, such as 2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) and 9-azabicyclo[3,3,1]nonan-3-one-9-oxyl (Keto-ABNO) were used. Keto-ABNO exhibited higher photo-electrocatalytic activity than TEMPO for the oxidation and decomposition of LMC. The superior reactivity with high selectivity of L1 sensitizer with Keto-ABNO mediator is attributed to the effective Caryl-Cα bond cleavage with 100% conversion yield for 24hrs at room temperature while it was 86% with TEMPO. The D–π–A type organic dye with a bicyclic nitroxyl radical mediator provides a possible strategic pathway for heterogeneous photocatalytic C–C bond cleavage at ambient condition.

Photoion Mass-Selected Threshold Photoelectron Spectroscopy to Detect Reactive Intermediates in Catalysis: From Instrumentation and Examples to Peculiarities and a Database.

Photoion mass-selected threshold photoelectron spectroscopy (ms-TPES) is a synchrotron-based, universal, sensitive, and multiplexed detection tool applied in the areas of catalysis, combustion, and gas-phase reactions. Isomer-selective vibrational fingerprints in the ms-TPES of stable and reactive intermediates allow for unequivocal assignment of spectral carriers. Case studies are presented on heterogeneous catalysis, revealing the role of ketenes in the methanol-to-olefins process, the catalytic pyrolysis mechanism of lignin model compounds, and the radical chemistry upon C-H activation in oxyhalogenation. These studies demonstrate the potential of ms-TPES as an analytical technique for elucidating complex reaction mechanisms. We examine the robustness of ms-TPES assignments and address sampling effects, especially the temperature dependence of ms-TPES due to rovibrational broadening. Data acquisition approaches and the Stark shift from the extraction field are also considered to arrive at general recommendations. Finally, the PhotoElectron PhotoIon Spectral Compendium (https://pepisco.psi.ch), a spectral database hosted at Paul Scherrer Institute to support assignment, is introduced.

Catalytic Conversion of Lignin Model Compound and Kraft Lignin over Reduced Ru/C Catalyst

AbstractCatalytic conversion of lignin into valuable liquid products has been a promising approach. Herein, we reported the catalytic conversion of the lignin model compound and Kraft lignin over reduced Ru/C catalyst with different industrial organic solvents using a high‐pressure/ high‐temperature reactor in a nitrogen atmosphere. The results showed reduced Ru/C catalyst is stable for two runs and active for the conversion of the lignin model compound (2‐phenoxy‐1‐phenyl ethanol) with a breaking of the C−O ether bond at high temperature. In the case of Kraft lignin, 2‐butanol and ethanol were potential solvents for the lignin conversion. The Ru/C catalyst enhanced the conversion of lignin, and the highest Kraft lignin conversion (~68 %) was obtained by using ethanol solvent at 220 °C for 1 h reaction. The product distribution with most small phenolic compounds demonstrated the potential of Kraft lignin conversion over the Ru/C catalyst.

Co-pyrolysis induced strong metal-support interaction in N-doped carbon supported Ni catalyst for the hydrogenolysis of lignin

Designing highly active and stable catalysts for the selective cleavage of the C-O bond of lignin is important to convert it into high-valued aromatic compounds. Herein, a facile synthetic strategy was developed using melamine as a soft template to prepare a highly active catalyst consisting of highly dispersed Ni nanoparticles on defective N-doped carbon support. The results indicated that the decomposition of melamine during the pyrolysis process induced the nitrogen atoms into aromatic carbon. The introduction of N-doped carbon support acted as anchor points for the Ni nanoparticles, strengthening the metal-support interaction. Approximately 100 conversion of the representative lignin model compounds, including 2-phenylethyl phenyl ether, were converted into their aromatic monomers under mild conditions. The as-prepared catalyst was stable and reusable after five cycles with no apparent signs of deactivation. Moreover, the theoretical calculation revealed that N-doped carbon supported Ni-based catalyst possessed a lower activation energy of C-O bond than only carbon supported Ni-based catalyst for the C-O bond cleavage in hydrogenolysis of lignin and its model compounds.

Photocatalytic lignin C-C bonds cleavage in lignin models via Fe-doped graphene carbon nitride in micellar aqueous medium

Photocatalytic depolymerization of lignin to prepare value-added chemicals by breaking lignin Cα-Cβ bonds (LCCD) is sustainable because of its green and mild reaction conditions. However, photocatalytic cleavage of LCCD still faces great challenges because of the complexity and stubbornness of lignin structure. Here, Fe-doped graphene carbon nitride (g-C3N4) photocatalysts were successfully prepared. The doping of Fe ions reduces the band gap of g-C3N4 and accelerates the migration ability of photogenerated carriers. 8FeCN can efficiently break LCCD in β-1 lignin model compounds and ethanol organosolv lignin, producing aromatic aldehydes, alcohol and acids as the main products. Hydroxyl radicals are generated through Fe-induced Fenton reactions, which increase the reaction pathway for breaking LCCD. The mechanism studies showed that photocatalytic cleavage of LCCD mainly followed the improved Cβ radical and single electron transfer mechanism. In addition, micellar aqueous media further accelerates the cleavage of LCCD, as water participates in the photocatalytic reaction and provided the O and H atoms needed for the formation of photocatalytic products. This work offers a fresh concept for the high-value utilization of lignin by oxidizing cleavage of LCCD.

A copper decorated porphyrin-based porous organic polymer: Evaluation in the oxidation of lignin model compounds

A novel porous organic polymer containing porphyrin units was synthesized and decorated with copper to produce the metallated POP, Cu-TriPOP. The native POP, Nat-TriPOP and metal-decorated Cu-TriPOP exhibited BET surface areas of 595 m2/g and 373 m2/g, respectively. Excellent thermal stability up to 300 °C was observed by thermogravimetric analysis. Transmission electron microscopy (TEM) showed a sea urchin/bush-like morphology. The metal-decorated POP, Cu-TriPOP, was evaluated in the oxidation of veratryl alcohol and other related substrates. Excellent catalytic performance was achieved with conversions of around 80%. The addition of ascorbic acid, a well-known radical scavenger, significantly reduced the activity, suggesting that a radical mechanism is most likely operative. In addition, excellent catalyst recovery and recyclability were achieved for up to five cycles with minimal decrease in activity.

Polyoxometalates-modified and facets-engineered TiO2 nanocomposites for selective photocatalytic cleavage of C–C bond of lignin models

Selective cleavage of C-C bond of lignin derivatives and models via oxidation is vital to realize value-added utilization of lignocellulosic biomass. Herein, based on the strategy of facet engineering and pseudo-heterojunction construction, a series of P2W17V/{001}-TiO2 heterogenous photocatalysts were constructed by loading monovanadium-substituted Dawson-type polyoxometalates P2W17V onto TiO2 with exposed {001} facets. The morphologies, structures, photoelectrical properties, and photocatalytic performances of the nanocomposites were researched. P2W17V/{001}-TiO2 could effectively photocatalyze the transformation of 2-phenoxy-1-phenylethanol, the lignin model compound with characteristic β-O-4 structure, to benzaldehyde and phenol. Photogenerated electrons and O2•- were disclosed critical to lignin model conversion that could induce C-C bond cleavage, with a selectivity above 85%, through a Cβ-centered radical. Neither pure O2 nor precious metal was required in the photocatalysis. P2W17V acted as electron pools. The engineered facets of TiO2 and the pseudo-heterojunction between P2W17V and TiO2 suppressed electron-hole recombination, facilitated charge transfer, and promoted photocatalytic performance. This work paved a new avenue to convert lignin and lignin derivatives to high-value chemicals by selectively breaking C-C bond under mild conditions.

Selective hydrogenolysis of C-O bonds in lignin model compounds and Kraft lignin over highly efficient NixCoyAl catalysts

The selective hydrogenolysis of CO bonds is critical for the effective valorization of lignin. Herein, we synthesized a series of highly efficient NixCoyAl-T catalysts by a coprecipitation-calcination-reduction method and evaluated their catalytic hydrogenolysis performance towards the CO bonds of lignin-derived aryl ether, using isopropanol as the hydrogen donor source. The results revealed that the Ni0.5Co1.5Al-600 catalyst displayed superior activity and its catalytic performances were strongly influenced by the Ni/Co molar ratios and calcination temperatures. The enhanced dispersion of metal particles and the electron transfer from Ni to Co in Ni0.5Co1.5Al-600 catalyst facilitated the cleavage of CO bonds in lignin model compounds. Furthermore, the β-benzyl-H atom in 2-phenoxy-1-phenylethanol (PPE) also contributes to the cleavage of Caliphatic-O bonds. Under optimized reaction conditions (180 ℃, reaction time 2 h), PPE was completely converted over the Ni0.5Co1.5Al-600 catalyst, and the yield of ethylbenzene and cyclohexanol reached 95.91wt% and 85.73wt%, respectively. These findings demonstrate that the NixCoyAl-T catalytic system holds great promise for the production of valuable chemicals from lignin under mild conditions.

Single-step conversion of wood lignin into phenolic amines

•Woody lignin is directly converted into aromatic amines •Lignin is selectively removed while (hemi)cellulose is kept intact •The process does not compromise the further processing property of (hemi)cellulose •The aromatic amines have the potential to be upgraded into useful fine chemicals Although the transformation of lignin monomers and model compounds into nitrogenated chemicals has been actively investigated, the production of organonitrogen chemicals directly from lignin has been scarcely reported. Here, we communicate a single-step catalytic strategy for producing phenolic amines directly from wood lignin via reductive fractionation in an aqueous ammonia-alcohol mixture over Ru catalyst. Depending on the type of alcohol used, primary, secondary, and ternary phenolic amines can all be synthesized and reach a maximum yield of 26.6%. During the process, native lignin was selectively removed from woody biomass, leaving other constituents, especially (hemi)cellulose, largely unaffected. The obtained phenolic amines were further upgraded into products with enhanced functions. This work exemplifies the possibility for the production of N-functional compounds from lignin, expanding the options for the lignin-first strategy in biomass refinery. Although the transformation of lignin monomers and model compounds into nitrogenated chemicals has been actively investigated, the production of organonitrogen chemicals directly from lignin has been scarcely reported. Here, we communicate a single-step catalytic strategy for producing phenolic amines directly from wood lignin via reductive fractionation in an aqueous ammonia-alcohol mixture over Ru catalyst. Depending on the type of alcohol used, primary, secondary, and ternary phenolic amines can all be synthesized and reach a maximum yield of 26.6%. During the process, native lignin was selectively removed from woody biomass, leaving other constituents, especially (hemi)cellulose, largely unaffected. The obtained phenolic amines were further upgraded into products with enhanced functions. This work exemplifies the possibility for the production of N-functional compounds from lignin, expanding the options for the lignin-first strategy in biomass refinery.

Selective Cleavage of Lignin Model Compounds via a Reverse Biosynthesis Mechanism.

Selective depolymerization of lignin remains a significant challenge in biomass conversion. The biosynthesis of lignin involves the polymerization of monolignol building blocks through oxidative radical coupling reactions. A strategy for lignin degradation leverages photoredox deoxygenative radical formation to trigger reverse biosynthesis, which cleaves model compounds of the β-O-4 and β-5-β-O-4 linkages to produce monolignols, precursors to flavoring compounds. This mild method preserves important oxygen functionality and serves as a platform for achieving selective lignin depolymerization.

Critical role of support crystal structures on highly selective hydrodeoxygenation of lignin-derived vanillin over Pd/ZrO2 catalysts

Hydrodeoxygenation (HDO) of bio-oils obtained by rapid pyrolysis of biomass into high value-added chemicals generally required harsh conditions. Herein, three kinds of mixed-phase ZrO2 supports (denoted as ZSX, X = 2–4) with different crystal structure proportions were prepared by one-pot hydrothermal synthesis method, and their performance on the properties of Pd/ZrO2 catalysts were investigated for vanillin HDO. Pd/ZS3 exhibited excellent efficiency in HDO of vanillin, a typical model compound of lignin, achieving approximately 100% yield of 2-methoxy-4-methylphenol (MMP) in presence of n-butanol as solvent and polymethyl hydrogen siloxane (PMHS) as hydrogen donor under mild conditions (35 °C, 1 h). It was due to catalytic synergy of exposed metal active sites, oxygen vacancies (Ov) and acid sites to occur electron transfer and enhance interaction between Pd and ZrO2. By tuning the crystal structure of ZrO2 support, the influence of Ov and acid sites of Pd/ZrO2 over vanillin HDO performance was investigated, Pd/ZS2 with abundant Ov and Pd/ZS4 with substantial acid sites promoted hydrogenation and hydrogenolysis, respectively, Pd/ZS3 possessed preferable conversion and selectivity as having the properties of both at once. Additionally, reaction pathway was also proposed in which vanillin was converted to MMP by combining intermediate of 4-hydroxy-methyl-2-methoxyphenol (HMP) with active hydrogen species while removing water. This developed catalysts with different crystalline highlighted an enormous promise for HDO of bio-oils into green fuel under mild conditions.

Effect of the Proton Transfer Pathway on Selective Photoreforming of Lignin Models for Target Products Enabled by Sulfur Vacancy Engineering on Chalcogenide Nanosheets

Photoreforming of lignin has been explored as a fascinating technology to generate clean hydrogen energy and value-added aromatic monomers from biomass. However, its upscaling is impeded by unsatisfactory selectivity due to the lack of mechanistic investigations in the uncontrollable reaction pathways. Herein, we successfully controlled the concentration and position of sulfur vacancies within the ultrathin ZnIn2S4 nanosheets to optimize the photo-driven lignin model reforming process. The competition of proton transfer between the hydrogen evolution and dissociation of the β-O-4 linkage in the model compound of lignin was identified, and the modulation of the proton migration pathway was realized through S vacancy engineering in ZnIn2S4 nanosheets toward target products. As such, excellent selectivity for hydrogen and chemical monomers was achieved with a high concentration of S vacancies in the bulk and on the surface of ZnIn2S4, respectively. This study endows new mechanistic insights into the biomass photoreforming process and elucidates the structure/chemistry-catalysis correlation of ZnIn2S4 photocatalysts, which are beneficial for photocatalyst design and rational solar fuel production.

Experimental evidence that lignin-modifying enzymes are essential for degrading plant cell wall lignin by Pleurotus ostreatus using CRISPR/Cas9.

Lignin-modifying enzymes (LMEs), which include laccases (Lacs), manganese peroxidases (MnPs), versatile peroxidases (VPs), and lignin peroxidases (LiPs), have been considered key factors in lignin degradation by white-rot fungi because they oxidize lignin model compounds and depolymerize synthetic lignin in vitro. However, it remains unclear whether these enzymes are essential/important in the actual degradation of natural lignin in plant cell walls. To address this long-standing issue, we examined the lignin-degrading abilities of multiple mnp/vp/lac mutants of Pleurotus ostreatus. One vp2/vp3/mnp3/mnp6 quadruple-gene mutant was generated from a monokaryotic wild-type strain PC9 using plasmid-based CRISPR/Cas9. Also, two vp2/vp3/mnp2/mnp3/mnp6, two vp2/vp3/mnp3/mnp6/lac2 quintuple-gene mutants, and two vp2/vp3/mnp2/mnp3/mnp6/lac2 sextuple-gene mutants were generated. The lignin-degrading abilities of the sextuple and vp2/vp3/mnp2/mnp3/mnp6 quintuple-gene mutants on the Beech wood sawdust medium reduced drastically, but not so much for those of the vp2/vp3/mnp3/mnp6/lac2 mutants and the quadruple mutant strain. The sextuple-gene mutants also barely degraded lignin in Japanese Cedar wood sawdust and milled rice straw. Thus, this study presented evidence that the LMEs, especially MnPs and VPs, play a crucial role in the degradation of natural lignin by P. ostreatus for the first time.

Selective photocatalytic aerobic oxidative cleavage of lignin C–O bonds over sodium lignosulfonate modified Fe3O4/TiO2

Lignocellulose shows significantly potential in sustainable conversion to high-quality fuel and value-added chemicals with the demands for realizing the rapid cycle of carbon resources and helping to reach carbon neutrality in nature. Selective tailoring of α-O-4, β-O-4, etc. linkages in lignin has always been viewed as “death blow” for its depolymerization. Herein, novel sodium lignosulfonate (SL) modified Fe3O4/TiO2 (SL-Fe3O4/TiO2) spherical particles have been developed and used as catalysts for selectively photocatalytic oxidative cleavage of organosolv lignin. As expected, 80% selective conversion of lignin in C2–C4 esters has been achieved, while C–O bonds in lignin model compounds can be effectively cleaved. Other than normal hydroxyl radical-mediated photocatalytic depolymerization of lignin over TiO2-based materials, in this contribution, mechanism studies indicate that photogenerated holes and superoxide anion radicals are main active species, which trigger the cleavage of α/β-O-4 bond, and the isotope-labeling study confirms the crucial factor of Cβ–H dehydrogenation in cleavage of β-O-4 bonds.

Selective Photocatalytic C–C Bond Cleavage of Lignin Models and Conversion to High-Value Chemical by Polyoxometalate Under a Mild Water-Based Environment

Selective bond cleavage using a photoredox reaction is a powerful technique in the chemical conversion of biomass such as lignin since it enables us to produce value-added aromatic compounds by controlling the activation of certain chemical bonds. Also, a water-based environment would be preferred for the sake of an eco-friendly reaction, additionally having the advantage of utilizing water-originated species such as •OH and H2O2. However, a direct cleavage of the C–C bond is still challenging due to the difficulties in tailoring redox potentials of catalysts relevant to covalent bond decoupling. Herein, we investigate a selective C–C bond cleavage of the β-O-4 lignin model compound using giant Keplerate polyoxometalates as photocatalysts. The redox potentials of photocatalysts are varied by changing metal compositions, and the correlation between redox potential and covalent bond decoupling is elucidated, probing each set of active radicals by HOMO and LUMOs. Especially, Mo132 shows a highly selective C–C bond cleavage with hydroxyl groups in the lignin model and produces phenyl formate─a effective CO surrogate for the synthesis of various organic compounds─as the main product (97.8% of product selectivity) in a water-based condition. Furthermore, Mo132 decomposes other aromatic polymer models, indicating the potential of photocatalytic C–C bond decoupling of polymers under a benign environment.

Mild hydrogenolysis of lignin model compound and organosolv lignin over non-noble bimetallic Ni–Fe/TiN catalyst

Lignin is one of the most promising feedstocks for renewable aromatics production. Conversion of such feedstock into aromatics can be attained through catalytic hydrogenolysis. In this work, NixFey/TiN bimetallic catalysts were evaluated in the hydrogenolysis of both: (i) benzyl phenyl ether (BPE) as a model compound for lignin and (ii) real organosolv lignin feedstock under low temperature (150 °C) and low H2 pressure (12 bar). All bimetallic catalysts exhibited superior performance over single-component materials and were shown to compose of uniformly/highly dispersed and intimately mixed Ni and Fe nanoparticles. Among bimetallic materials, Ni5Fe2/TiN possesses the highest activity in BPE hydrogenolysis, which is comparable to that of a 5% Pd/C commercial catalyst while showing significantly higher aromatic selectivity. Ni5Fe2/TiN catalyst also outperformed Pd/C in hydrogenolysis of organosolv lignin, shown by its higher oil yield, greater content of phenolic monomers, and lower content of dimers. This material exhibited good stability in BPE conversion with no noticeable deactivation over 5 recycling cycles. XANES analysis suggests the electron transfer from Ni to Fe, which explains the superior activity observed with Ni5Fe2/TiN.