Microwave-assisted catalytic depolymerization of lignin from birch sawdust to produce phenolic monomers utilizing a hydrogen-free strategy

Catalytic depolymerization of Camellia oleifera shell lignin to phenolic monomers: Insights into the effects of solvent, catalyst and atmosphere

Camellia oleifera shell (COS) is a renewable biomass resource abundant in lignin with significant potential for producing phenolic monomers. However, the dearth of research has led to considerable resource wastage and environmental pollution. Herein, reductive catalytic fractionation (RCF) of COS was performed using noble metal catalysts in different solvents. An 11.1 wt% yield of phenolic monomers was achieved with 91% selectivity toward propylene-substituted monomers in H2O/EtOH (3:7, v/v) cosolvent under N2 atmosphere. Notably, the highest phenolic monomer yield of 17.0 wt% was obtained with impressive selectivity (86.9%) toward propanol-substituted monomers in the presence of H2. The GPC analysis and 2D HSQC NMR spectra indicated the effective depolymerization of lignin oligomers with catalysts. Phenolic monomers with ethyl, propyl, or propanol side chain could be produced from lignin-derived oligomers through hydrogenolysis, hydrogenation, and decarboxylation reactions. Overall, this study has paved the way for the valorization of COS lignin through the RCF strategy.

Production of Oximes Directly from Sustainable Lignocellulose-Derived Aldehydes and Ammonia over HTS-1 Catalyst

Oxime chemicals are the building blocks of many anticancer drugs and widely used in industry and laboratory. A simple but robust hierarchically porous zeolite (HTS-1) catalyst was prepared by hydrothermal methods and used for the preparation of vanillin oxime from vanillin in NH3  ⋅ H2 O/DIO (v/v 1/10) system. The results of the catalyst characterization showed that the larger pore size and more framework Ti were conducive to promote the transformation of the substrates. The conversion of vanillin and the yield of vanillin oxime were both higher than 99 % under optimized reaction conditions. It was found that the reaction proceeded by oxidation of NH3 to hydroxylamine (NH2 OH), and oximation of hydroxylamine with vanillin to obtain vanillin oxime, where the rate-controlling step was the hydroxylamine formation, and the apparent activation energy was 26.22 kJ/mol. The corresponding oximation products could also be obtained by extending this method to other compounds derived from lignin. Furthermore, the catalytic system was used directly to the conversion of birch biomass to obtain oxime products such as vanillin oxime, syringaldehyde oxime, and furfural oxime etc. This work might give insights into the sustainable production of N-containing high-value products from lignocellulose.

Advances in Heterogeneous Catalysts for Lignin Hydrogenolysis

Lignin is the main component of lignocellulose and the largest source of aromatic substances on the earth. Biofuel and bio-chemicals derived from lignin can reduce the use of petroleum products. Current advances in lignin catalysis conversion have facilitated many of progress, but understanding the principles of catalyst design is critical to moving the field forward. In this review, the factors affecting the catalysts (including the type of active metal, metal particle size, acidity, pore size, the nature of the oxide supports, and the synergistic effect of the metals) are systematically reviewed based on the three most commonly used supports (carbon, oxides, and zeolites) in lignin hydrogenolysis. The catalytic performance (selectivity and yield of products) is evaluated, and the emerging catalytic mechanisms are introduced to better understand the catalyst design guidelines. Finally, based on the progress of existing studies, future directions for catalyst design in the field of lignin depolymerization are proposed.

Unraveling the Lignin Structural Variation in Different Bamboo Species

The structure of cellulolytic enzyme lignin (CEL) prepared from three bamboo species (Neosinocalamus affinis, Bambusa lapidea, and Dendrocalamus brandisii) has been characterized by different analytical methods. The chemical composition analysis revealed a higher lignin content, up to 32.6% of B. lapidea as compared to that of N. affinis (20.7%) and D. brandisii (23.8%). The results indicated that bamboo lignin was a p-hydroxyphenyl-guaiacyl-syringyl (H-G-S) lignin associated with p-coumarates and ferulates. Advanced NMR analyses displayed that the isolated CELs were extensively acylated at the γ-carbon of the lignin side chain (with either acetate and/or p-coumarate groups). Moreover, a predominance of S over G lignin moieties was found in CELs of N. affinis and B. lapidea, with the lowest S/G ratio observed in D. brandisii lignin. Catalytic hydrogenolysis of lignin demonstrated that 4-propyl-substituted syringol/guaiacol and propanol guaiacol/syringol derived from β-O-4' moieties, and methyl coumarate/ferulate derived from hydroxycinnamic units were identified as the six major monomeric products. We anticipate that the insights of this work could shed light on the sufficient understanding of lignin, which could open a new avenue to facilitate the efficient utilization of bamboo.

Upgrading bio-oil model compound over bifunctional Ru/HZSM-5 catalysts in biphasic system: Complete hydrodeoxygenation of vanillin

A complete hydrodeoxygenation(HDO) of vanillin to yield cycloalkanes was performed using bifunctional Ru loaded HZSM-5 catalysts with different metal loadings (0.1, 0.5, 1, 3, and 5 wt%) and Si/Al₂ ratios (Si/Al₂ = 23,300) in n-octane/water biphasic system. Both the reaction pathway and product distribution were influenced by the metal/acid balance of the catalysts. Higher metal/acid ratio promoted C_aryl-C cleavage reaction, resulting in the increased yield of cyclohexane. Synergetic effect of metal and acid sites was observed in the bifunctional catalyst, attaining as high as 40-fold increase of metal efficiency in the ring hydrogenation reaction, compared to lone metal site catalyst. The effect of solvent composition was evaluated, revealing that the presence of water promoted the overall HDO reaction. By balancing metal/acid and introducing appropriate solvent system, efficient catalytic system that minimized carbon loss and improved metal efficiency for vanillin HDO was obtained.

Defluorination study of spent carbon cathode by microwave high-temperature roasting

Spent carbon cathode (SCC) produced in the process of aluminum electrolysis is a typical toxic and hazardous solid waste. Therefore, the harmless treatment of SCC is extremely important for the green development of aluminum electrolysis industry. In this paper, the microwave-assisted high-temperature roasting technology was developed to remove fluorides in SCC for recycling of this cathode. The melting point, dielectric parameter, crystalline structure, surface chemical property, elemental composition, morphological structure, carbon graphitization and surface area were characterized using thermogravimetric analysis and differential scanning calorimetry, high-temperature composite conductivity analyzer, X-ray diffraction, X-ray fluorescence, X-ray photoelectron spectroscopy, scanning electronic microscopy, transmission electron microscopy, Raman spectroscopy and isothermal N₂ adsorption-desorption method. The content of fluorides in raw and treated SCC was measured by ion activity meter. The results showed that the phase of sodium fluoride and cryolite would transform from solid to liquid when the temperature was higher than 1098.5 °C, and the SCC exhibited good performance on wave absorption with the action depth of 1 cm. The SCC was mainly composed of 57.94 wt% C, 14.23 wt% NaF, 1.80 wt%, CaF₂, 15.06 wt% Na₃AlF₆, and 10.97 wt% Other. After treatment under microwave, the graphite carbon exhibited pitting structure and the fluorides could be effectively removed. In addition, the average layer spacing of graphite was increased from 0.34 to 0.36 nm. The defluorination of SCC could be enhanced with the increase of roasting temperature, which would attain 95.4% at 1500 °C. Compared with the traditional roasting method, the process under microwave showed more defects, which would provide a new guidance for the treatment and recycling of spent SCC.

Rapid and efficient microwave-assisted guanidine hydrochloride deep eutectic solvent pretreatment for biological conversion of castor stalk

Microwave-assisted guanidine hydrochloride deep eutectic solvents (DESs) was developed for rapid and efficient pretreatment of castor stalk. The DES synthesized with guanidine hydrochloride and lactic acid showed a better delignification (92.02%) and enzymatic saccharification yield (96.3%) than choline chloride and lactic acid DES resulted. In addition, high-purity (up to 98%) lignin was recovered from the pretreatment liquor. The good recyclability of the guanidine hydrochloride-based DES was also proven with up to 90% cellulose hydrolysis with third-time recycled DES without post purification. The proposed microwave-assisted guanidine hydrochloride/lactic acid DES showed its great potentials as a highly effective and recyclable pretreatment solvent for future biorefinery strategies.

Lignin-Derived Syringol and Acetosyringone from Palm Bunch Using Heterogeneous Oxidative Depolymerization over Mixed Metal Oxide Catalysts under Microwave Heating

Biomass valorization to building block chemicals in food and pharmaceutical industries has tremendously gained attention. To produce monophenolic compounds from palm empty fruit bunch (EFB), EFB was subjected to alkaline hydrothermal extraction using NaOH or K2CO3 as a promotor. Subsequently, EFB-derived lignin was subjected to an oxidative depolymerization using Cu(II) and Fe(III) mixed metal oxides catalyst supported on γ-Al2O3 or SiO2 as the catalyst in the presence of hydrogen peroxide. The highest percentage of total phenolic compounds of 63.87 wt% was obtained from microwave-induced oxidative degradation of K2CO3 extracted lignin catalyzed by Cu-Fe/SiO2 catalyst. Main products from the aforementioned condition included 27.29 wt% of 2,4-di-tert-butylphenol, 19.21 wt% of syringol, 9.36 wt% of acetosyringone, 3.69 wt% of acetovanillone, 2.16 wt% of syringaldehyde, and 2.16 wt% of vanillin. Although the total phenolic compound from Cu-Fe/Al2O3 catalyst was lower (49.52 wt%) compared with that from Cu-Fe/SiO2 catalyst (63.87 wt%), Cu-Fe/Al2O3 catalyst provided the greater selectivity of main two value-added products, syringol and acetosyrigone, at 54.64% and 23.65%, respectively (78.29% total selectivity of two products) from the NaOH extracted lignin. The findings suggested a promising method for syringol and acetosyringone production from the oxidative heterogeneous lignin depolymerization under low power intensity microwave heating within a short reaction time of 30 min.

Advances in Versatile Nanoscale Catalyst for the Reductive Catalytic Fractionation of Lignin

In the past five years, biomass-derived biofuels and biochemicals were widely studied both in academia and industry as promising alternatives to petroleum. In this Review, the latest progress of the synthesis and fabrication of porous nanocatalysts that are used in catalytic transformations involving hydrogenolysis of lignin is reviewed in terms of their textural properties, catalytic activities, and stabilities. A particular emphasis is made with regard to the catalyst design for the hydrogenolysis of lignin and/or lignin model compounds. Furthermore, the effects of different supports on the lignin hydrogenolysis/hydrogenation are discussed in detail. Finally, the challenges and future opportunities of lignin hydrogenolysis over nanomaterial-supported catalysts are also presented.