Degradation of solvolysis lignin using Lewis acid catalysts

Elucidating the Reaction Pathways of Veratrylglycero-β-Guaiacyl Ether Degradation over Metal-Free Solid Acid Catalyst with Hydrogen

Efficient cleavage of β-O-4 bonds in lignin to high-yield aromatic compounds for the potential production of fuels and chemicals is vital for the economics of the modern biorefinery industry. This work is distinct in that a detailed mechanistic analysis of the reaction pathways of veratrylglycero-β-guaiacyl ether (VGE) catalyzed by transition-metal-free solid acid zeolite in aqueous conditions at high hydrogen pressure has been performed. VGE degradation produced high monomers yields (≈87 %), including guaiacol (48.2 %), 1-(3,4-dimethoxyphenyl)ethanol (10.3 %), 1-(3,4-dimethoxyphenyl)-2-propanol (6.1 %), 3,4-dimethoxyphenylpropanol (4.7 %), 3,4-dimethoxycinnamyl alcohol (4.1 %), and 1,2-dimethoxy-4-propylbenzene (2 %). The products were identified and confirmed by the in situ solid-state magic angle spinning (MAS) ¹³ C NMR spectroscopy in real-time conditions and the two-dimensional gas chromatography (GC×GC). A variety of products reveal the crucial role of hydrogen, water, and acid sites for heterolytic cleavage of the β-O-4 bond in VGE. Decarbonylation, hydrogenolysis, hydrogenation, and dehydration reaction pathways are proposed and further validated using first-principles calculations.

Catalytic conversion of enzymatic hydrolysis lignin into cycloalkanes over a gamma-alumina supported nickel molybdenum alloy catalyst

The efficient depolymerization and hydrodeoxygenation of enzymatic hydrolysis lignin are achieved in cyclohexane solvents over a gamma-alumina supported nickel molybdenum alloy catalyst in a single step. Under initial 3 MPa hydrogen at 320 °C, the highest overall cycloalkane yield of 104.4 mg/g enzymatic hydrolysis lignin with 44.4 wt% selectivity of ethyl-cyclohexane was obtained. The reaction atmosphere and temperature have significant effects on enzymatic hydrolysis lignin conversion, product type and distribution. The conversion of enzymatic hydrolysis lignin was also investigated over different nickel and molybdenum-based catalysts, and the gamma-alumina supported nickel molybdenum alloy catalyst exhibited the highest activity among those catalysts. To reveal the reaction pathways of alkylphenol hydrodeoxygenation, 4-ethylphenol was tested as a model compound. Complete conversion of 4-ethylphenol into cycloalkanes was achieved. A two-step mechanism of 4-ethylphenol dihydroxylation - hydrogenation is proposed, in which the benzene ring saturation is deemed as the rate-determining step.

A review on high catalytic efficiency of solid acid catalysts for lignin valorization

It is imminent to develop renewable resources to replace fossil-derived energies as fossil resources are on the brink of exhaustion. Lignin is one of the major components of lignocellulosic biomass, which is a natural amorphous three-dimensional polymer with abundant C-O bonds and aromatic structure. Hence, valorization of lignin into high value-added liquid fuels and chemicals is regarded as a promising strategy to mitigate fossil resource shortages. Solid acid catalysts are extensively studied due to environmentally friendly in terms of the ease of separation, recovery and reduced amount of wastes. Hence, this review focuses on summarizing the recent progress of catalytic valorization of lignin over different kinds of solid acid catalysts including zeolites, heteropolyacids, metal oxides, amorphous SiO₂-Al₂O₃, metal phosphates, and Lewis acid. Based on reviewing of current progress of lignin conversion, the challenges and future prospects are emphasized.

Selective catalytic degradation of a lignin model compound into phenol over transition metal sulfates

Transition metal salts were employed as the catalysts to improve the selective degradation of the α-O-4 lignin model compound (benzyl phenyl ether (BPE)) in the solvothermal system. The results concluded that most of the transition metal salts could enhance BPE degradation. Among which, NiSO₄·6H₂O exhibited the highest performance on BPE degradation (90.8%) for 5 h and phenol selectivity (53%) for 4 h at 200 °C. In addition, the GC-MS analysis indicated that the intermediates during BPE degradation included a series of aromatic compounds, such as phenol, benzyl methyl ether and benzyl alcohol. Furthermore, the mechanisms for BPE degradation and phenol selectivity in the NiSO₄·6H₂O system involved the synergetic effects between the acid catalysis and coordination catalysis, which caused the effective and selective cleavage of the C–O bonds.

An efficient method for degradation of benzyl phenyl ether using NiSO₄·6H₂O as catalyst.

One-pot transformation of lignocellulosic biomass into crude bio-oil with metal chlorides via hydrothermal and supercritical ethanol processing

Grape seeds were deconstructed in both hydrothermal and supercritical ethanol media with a combination of two metal chlorides (TiCl₄:MgCl₂) to produce bio-oils. The use of metal chloride additives in supercritical ethanol achieved the highest bio-oil yield of 49.2 wt% (300 °C, 30 min). Both the hydrothermal and supercritical ethanol deconstruction with the additives (TiCl₄:MgCl₂ = 4 mmol:4mmol) produced the bio-oils with a higher heating value (HHV) of 35 MJ/Kg. Gas chromatography-mass spectrometry (GC-MS) analysis of the bio-oils showed that the major products in bio-oils from the hydrothermal deconstruction were acids while the majority products in bio-oils form the supercritical ethanol deconstruction were esters. Nuclear magnetic resonance (NMR) data of the bio-oils suggested that both hydrothermal and supercritical ethanol deconstruction with metal chlorides significantly reduced the non-condensed OH and oxygenated lignin sub-units in bio-oils; while only supercritical ethanol deconstruction with metal chlorides reduced the aliphatic OH and O-alkylated structures in bio-oils.

Catalytic Strategies Towards Lignin-Derived Chemicals

Lignin valorization represents a crucial, yet underexploited component in current lignocellulosic biorefineries. An alluring opportunity is the selective depolymerization of lignin towards chemicals. Although challenged by lignin's recalcitrant nature, several successful (catalytic) strategies have emerged. This review provides an overview of different approaches to cope with detrimental lignin structural alterations at an early stage of the biorefinery process, thus enabling effective routes towards lignin-derived chemicals. A first general strategy is to isolate lignin with a better preserved native-like structure and therefore an increased amenability towards depolymerization in a subsequent step. Both mild process conditions as well as active stabilization methods will be discussed. An alternative is the simultaneous depolymerization-stabilization of native lignin towards stable lignin monomers. This approach requires a fast and efficient stabilization of reactive lignin intermediates in order to minimize lignin repolymerization and maximize the envisioned production of chemicals. Finally, the obtained lignin-derived compounds can serve as a platform towards a broad range of bio-based products. Their implementation will improve the sustainability of the chemical industry, but equally important will generate opportunities towards product innovations based on unique biobased chemical structures.

Hydrodeoxygenation of Bio-Derived Phenol to Cyclohexane Fuel Catalyzed by Bifunctional Mesoporous Organic-Inorganic Hybrids

In this work, mesoporous silica materials SBA-15 functionalized with propyl/ phenyl-sulfonic acid group were synthesized and loaded with Pt to form bifunctional catalysts. SAXRD, WAXRD, N₂ adsorption-desorption, TEM techniques were used to characterize the above bifunctional catalysts. These bifunctional catalysts were applied to the reaction of hydrodeoxygenation (HDO) of bio-derived phenol (PhOH) to produce cyclohexane fuel and showed very good catalytic performances. There were strong synergies between the metal sites and the acid sites on the bifunctional catalysts. This reaction of phenol HDO provides a model system for the catalytic upgrading of biomass-derived fuel.

Chemicals from lignin: an interplay of lignocellulose fractionation, depolymerisation, and upgrading

In pursuit of more sustainable and competitive biorefineries, the effective valorisation of lignin is key. An alluring opportunity is the exploitation of lignin as a resource for chemicals. Three technological biorefinery aspects will determine the realisation of a successful lignin-to-chemicals valorisation chain, namely (i) lignocellulose fractionation, (ii) lignin depolymerisation, and (iii) upgrading towards targeted chemicals. This review provides a summary and perspective of the extensive research that has been devoted to each of these three interconnected biorefinery aspects, ranging from industrially well-established techniques to the latest cutting edge innovations. To navigate the reader through the overwhelming collection of literature on each topic, distinct strategies/topics were delineated and summarised in comprehensive overview figures. Upon closer inspection, conceptual principles arise that rationalise the success of certain methodologies, and more importantly, can guide future research to further expand the portfolio of promising technologies. When targeting chemicals, a key objective during the fractionation and depolymerisation stage is to minimise lignin condensation (i.e. formation of resistive carbon-carbon linkages). During fractionation, this can be achieved by either (i) preserving the (native) lignin structure or (ii) by tolerating depolymerisation of the lignin polymer but preventing condensation through chemical quenching or physical removal of reactive intermediates. The latter strategy is also commonly applied in the lignin depolymerisation stage, while an alternative approach is to augment the relative rate of depolymerisation vs. condensation by enhancing the reactivity of the lignin structure towards depolymerisation. Finally, because depolymerised lignins often consist of a complex mixture of various compounds, upgrading of the raw product mixture through convergent transformations embodies a promising approach to decrease the complexity. This particular upgrading approach is termed funneling, and includes both chemocatalytic and biological strategies.

Analysis of the effect of temperature and reaction time on yields, compositions and oil quality in catalytic and non-catalytic lignin solvolysis in a formic acid/water media using experimental design

The catalytic solvolysis of Norway spruce (Picea abies L.) lignin in a formic acid/water media was explored at different temperatures and reaction times (283-397°C and 21-700min, respectively). Non-catalyzed experiments were compared with the effect of three different type of bifunctional catalysts (Pd/Al₂O₃, Rh/Al₂O₃ and Ru/Al₂O₃) and a solid Lewis acid (γ-Al₂O₃). We demonstrated that surface response methodology (RSM) and principal component analysis (PCA) were an adequate tool to: (i) evaluate the effect of the catalysts, temperature and reaction time in the oil yield, oil quality (H/C and O/C ratios, and M_w) and composition of the oil, (ii) establish the differences and/or similarities between the three bifunctional catalyst and (iii) to determine the role of the noble metal and the alumina support in the reaction system. In addition, the most active catalysts, Ru/Al₂O₃, and the optimum reaction conditions were determined (i.e. 340°C and 6h).

A ternary composite oxides S2O82−/ZrO2–TiO2–SiO2 as an efficient solid super acid catalyst for depolymerization of lignin

The solid, super, acid catalyst S₂O₈²⁻/ZrO₂–TiO₂–SiO₂, which has both a strong Brønsted acid and Lewis acid, was prepared and applied in lignin depolymerization.

Towards lignin valorisation: comparing homogeneous catalysts for the aerobic oxidation and depolymerisation of organosolv lignin

Lignin is a unique potential source of aromatic chemicals derived from renewable resources.

Kinetic study of wood pyrolysis in presence of metal halides

The purpose of this work was to study the kinetics of wood pyrolysis in the presence of inorganic salts, representatives of classes of alkali and alkaline earth metal halides (NaCl, KCl, KBr, CaCl2, BaCl2·2H2O) and Lewis acids (AlCl3·6H2O, FeCl3·6H2O, CuCl2, CuBr2, ZnCl2·1.5H2O, NiCl2·6H2O, SnCl2·2H2O) using TG-DSC. The activity of these catalysts was estimated by the temperature of the beginning of pyrolysis, charcoal yield and kinetic parameters, such as energy of activation and reaction order. Using the Lewis acids as catalysts for pyrolysis leads to a decrease in the temperature of the process beginning and the activation energy. In the presence of other catalysts activation energy does not significantly change. The increase of a seeming reaction order in the presence of Lewis acids possibly is a consequence of complication of the thermodestruction mechanism, with the appearance of new parallel competing stages.

Recent Development in Chemical Depolymerization of Lignin: A Review

This article reviewed recent development of chemical depolymerization of lignins. There were five types of treatment discussed, including base-catalyzed, acid-catalyzed, metallic catalyzed, ionic liquids-assisted, and supercritical fluids-assisted lignin depolymerizations. The methods employed in this research were described, and the important results were marked. Generally, base-catalyzed and acid-catalyzed methods were straightforward, but the selectivity was low. The severe reaction conditions (high pressure, high temperature, and extreme pH) resulted in requirement of specially designed reactors, which led to high costs of facility and handling. Ionic liquids, and supercritical fluids-assisted lignin depolymerizations had high selectivity, but the high costs of ionic liquids recycling and supercritical fluid facility limited their applications on commercial scale biomass treatment. Metallic catalyzed depolymerization had great advantages because of its high selectivity to certain monomeric compounds and much milder reaction condition than base-catalyzed or acid-catalyzed depolymerizations. It would be a great contribution to lignin conversion if appropriate catalysts were synthesized.

Conversion of lignin to aromatic-based chemicals (L-chems) and biofuels (L-fuels)

Conversion of lignin into chemicals and biofuels was performed using the commercial Kraft lignin, Indulin AT. Lignin was depolymerised in an aqueous alkaline solution using a continuous flow reactor generating four fractions. First is the gas fraction (mainly CO(2)), the second includes methanol, acetic acid and formic acid, thus defined as small organic compounds and third one (up to 19.1 wt.% of lignin) is mostly composed of aromatic monomers. The fourth fraction (45-70 wt.%) contains oligomers (polyaromatic molecules) and modified lignin. Pyrocatechol was the most abundant product at high severities (315°C) with selectivity up to 25.8%. (31)P NMR showed the loss of almost all aliphatic OH groups and apparition of catechol groups during depolymerisation.