Photocatalysis is a potentially promising approach to harvest aromatic compounds from lignin. However, the development of an active and selective solid photocatalyst is still challenging for lignin ...transformation under ambient conditions. We herein report a mild photocatalytic oxidative strategy for C–C bond cleavage of lignin β-O-4 and β-1 linkages using a mesoporous graphitic carbon nitride catalyst. Identifications by solid-state NMR techniques and density functional theory (DFT) calculations indicate that π–π stacking interactions are most likely present between the flexible carbon nitride surface and lignin model molecule. Besides, low charge recombination efficiency and high specific surface area (206.5 m2 g–1) of the catalyst also contribute to its high catalytic activity. Mechanistic investigations reveal that photogenerated holes, as the main active species, trigger the oxidation and C–C bond cleavage of lignin models. This study sheds light on the interaction between complex lignin structures and the catalyst surface and provides a new strategy of photocatalytic cleavage of lignin models with heterogeneous photocatalysts.
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.
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•Mechanism of formations H2 and CH4 by hydrolysis and alcoholysis of lignin model compounds were investigated using DFT method.•Hydrolysis energy barrier is lower than that of alcoholysis reaction, so the formation of H2 is easier than that of CH4.•High reaction temperature can promote the formation of H2 and CH4 in lignin degradation.
•The state-of-the-art strategies for the lignin conversion to alkanes over different multifunctional catalysts are summarized.•The depolymerization and hydrodeoxygenation process of lignin and ...lignin-derived compounds are detailed discussed.•The roles of different active metals for the catalysts and conversion of lignin to alkanes are described.•The major challenges and the future research trend in this field have been prospected.
Lignin exists widely in nature and is a potential substitute for traditional fossil fuels. The technology of producing alkanes from lignin and its derivatives has been extensively studied in recent decades. This review summarizes state-of-the-art strategies for the conversion of lignin to alkanes over different multifunctional catalysts. The hydrodeoxygenation (HDO) process of lignin and lignin-derived compounds at different metal sites and the possible depolymerization pathways are detailed discussed. The roles of different active metals and their advantages for the catalysts and conversion of lignin to alkanes are also described. Finally, the major challenges and the future research trend in this field have been prospected.
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•A continuously catalytic system was developed for lignin oxidative depolymerization in a near-neutral aqueous solution.•Heterogeneous Mo-based catalysts combined with H2O2 increased ...the selectivity and yield of main platform products.•The acid properties of solid catalysts influenced vanillin and lactic acid yields.•A hypothetical mechanism was proposed to elucidate the formation pathways of vanillin and lactic acid based on the results of lignin model compounds.
A heterogeneous molybdenum (Mo)-based oxide catalyst system in combination with hydrogen peroxide as an oxidant was developed for oxidative depolymerization of lignin to yield high-value platform chemicals in a nearly neutral aqueous solution. The impact of various reaction conditions, including the ratios of H2O2 and/or MoO3/Al2O3 catalyst to lignin, reaction temperature and duration, and lignin concentration, were investigated in a batch system or a continuous flow fixed-bed system. The structural changes between dealkaline lignin and the recovered lignin (O-lignin and M−lignin) were analyzed using UV–vis, FT-IR, and GPC techniques. The combination of MoO3 and H2O2 resulted in a superior peroxo complex, promoting the formation of vanillin and lactic acid. The medium and weak acid feature of MoO3/Al2O3 catalyst induced the highest vanillin yield (5.6%) and the selectivity (86.9%) of main platform products. However, the highest yield of lactic acid (3.4%) was observed on MoO3/SiO2 with more weak acid sites. Hypothetical formation mechanisms for vanillin and lactic acid were proposed based on the depolymerization of lignin model compounds.
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•A nearly total conversion and > 98% selectivity of C-C bond cleavage is achieved.•Sufficient superoxide radicals are necessary for highly selective C-C bond cleavage under visible ...light.•Efficient photocatalytic C-C bonds breaking towards lignin β-O-4 model compounds.•Ru single atoms outperform the nanoparticles for photocatalytic C-C bond cleavage.•Cβ-H extraction by photogenerated hole is the critical step of the transformation.
The selective cleavage of the CC bonds in lignin is highly pursued to obtain aromatic monomers from renewable resources but remains challenging due to their high dissociation energies and steric hindrance between bulky aromatic groups. Herein, a mesoporous carbon nitride-supported ruthenium single-atom photocatalyst (Ru1/MCN) is reported for efficient oxidative Cα-Cβ bonds scissoring towards lignin β-O-4 model compounds with high activity and selectivity under visible–light irradiation and room temperature. With this strategy, a nearly total conversion of > 99% and > 98% selectivity of CC bond cleavage can be achieved upon 2–phenoxy–1–phenylethanol. In-depth investigations verify that the Cβ-H extraction by photogenerated holes is the critical step of the transformation, which directly generates the Cβ radical intermediates and thus promotes the selective Cα-Cβ bonds breaking by coupling with superoxide radicals induced by photogenerated electrons. This work explores the potential of single-atom catalysts for photocatalytic production of aromatics from renewable biomass feedstocks.
Lignin in lignocellulosic biomass is the only renewable source for aromatic compounds, and effective valorization of lignin remains a significant challenge in biomass conversion processes. We have ...performed density functional theory calculations and experiments to investigate the cleavage mechanism of the C–O ether bond in the lignin model compound 2-phenoxy-1-phenylethanol with a β-O-4 linkage over a Pd(111) catalyst surface model. We propose the favorable reaction pathway to proceed as follows: the dilignol reactant gets dehydrogenated first on the α-carbon and then on the −OH group to generate its corresponding ketone 2-phenoxy-1-phenylethanone; the ketone continues to get dehydrogenated on the β-carbon by first a equilibrated keto–enol tautomerization to its enol form and then −OH dehydrogenation; the C–O ether bond cleavage happens afterward, leading to one-aromatic-ring surface intermediates followed by hydrogenation to yield acetophenone and phenol.
As a natural aromatic polymer, lignin has great potential but limited industrial application due to its complex chemical structure. Among strategies for lignin conversion, biodegradation has ...attracted promising interest recently in term of efficiency, selectivity and mild condition. In order to overcome the issues of poor stability and non-reusability of enzyme in the biodegradation of lignin, this work explored a protocol of immobilized laccase on magnetic nanoparticles (MNPs) with rough surfaces for enhanced lignin model compounds degradation. Scanning electron microscope with energy dispersive spectrometer (SEM-EDS), flourier transformation infrared spectroscopy (FTIR) and thermal gravimetric analysis (TGA) were utilized to characterize the immobilization of laccase. The results showed a maximum activity recovery of 64.7% towards laccase when it was incubated with MNPs and glutaraldehyde (GA) with concentrations of 6 mg·ml−1 and 7.5 mg·ml−1 for 5 h, respectively. The immobilized laccase showed improved thermal stability and pH tolerance compared with free laccase, and remained more than 80% of its initial activity after 20 days of storage at 4 °C. In addition, about 40% residual activity of the laccase remained after 8 times cycles. Gas chromatography–mass spectrometry (GC–MS) was utilized to characterize the products of lignin model compound degradation and activation, and the efficiency of immobilized laccase was calculated to be 1–5 times that of free laccase. It was proposed that the synergistic effect between MNPs and laccase displays an important role in the enhancement of stability and activity in lignin model compound biodegradation.
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•Catalytic effects of K and Ca on lignin pyrolysis were explored.•Cβ-O homolytic reaction dominates the initial depolymerization of dimers.•Potassium and calcium additives are beneficial to the ...formation of phenol.•The Cβ-O homolytic reaction is further enhanced by both potassium and calcium.•Ca2+ is more conducive to the concerted decomposition of side chains than K+.
In this work, to investigate the catalytic effects of potassium and calcium additives on lignin pyrolysis, two β-O-4 type model compounds for lignin containing different oxygen-containing functional groups (a carbonyl group or a hydroxyl group) on Cα were prepared, and their catalytic fast pyrolysis experiments were carried out on Py-GC/MS. Furthermore, the possible catalytic mechanism of potassium and calcium additives is further investigated using density functional theory (DFT) calculations. The experimental results show that the addition of potassium acetate and calcium acetate is beneficial to the formation of phenol, while the promotion effect of potassium addition is stronger than that of calcium addition. Moreover, considering the contributions of temperature and activation entropy ΔS‡, the activation Gibbs free energy ΔG‡ is required as the basis to explain the pyrolysis mechanism of lignin. It is determined that the Cβ-O homolytic reaction performs the most favorable in the initial depolymerization of both dimers. By comparing the ΔG‡, it is confirmed that the addition of K+ and Ca2+ are beneficial to the Cβ-O homolytic reaction and further promote the conversion of the intermediate to phenol. Furthermore, the different effects of K+ and Ca2+ additives are compared on the side chains cleavage through homolytic reaction and concerted decomposition. The calculation results show that K+ is more conducive to the simple homolytic reaction, while Ca2+ is more conducive to the concerted decomposition.
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•Base catalyzed depolymerization of lignin was studied in a continuous reactor system at low temperature (≤180 ℃).•Low concentration (<5 %) of alkaline ionic liquid N-allylpyridinium ...hydroxide (APyOH) as a co-solvent in ethylene glycol solution decreased feedstock viscosity and reduced the cost.•APyOH as a catalyst also presented a synergistic effect with alkali metal salt (NaCl) and exhibited high aromatics yield.•A hypothetical mechanism was proposed to explain the depolymerization pathway of lignin.
A novel alkaline ionic liquids N-allylpyridinium hydroxide (APyOH) was successfully designed and developed as a catalyst and co-solvent with ethylene glycol in the continuous depolymerization of lignin. Various reaction conditions, like lignin concentration, water content, and reaction temperature, were investigated under atmospheric pressure. Ethylene glycol was used as main solvent and repolymerization inhibitor of the aldehydes produced. The combination of APyOH and alkali metal salts exhibited a good synergistic effect for lignin depolymerization. Then, the depolymerization of three different lignins, including dealkaline lignin (DA-lignin), alkaline lignin (A-lignin), and sodium lignosulfonate (S-lignin), were also studied. S-lignin exhibited the highest yields of total liquid products (21.3 %) and aromatics (10.7 %). The change in lignin structure after depolymerization was observed by means of FT-IR and DTG-DTA techniques. Lastly, a hypothetical pathway for lignin depolymerization was proposed based on the catalytic results of lignin model compounds (LMCs) with different linkages.
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