Alkaline wet oxidation of lignin over Cu-Mn mixed oxide catalysts for production of vanillin

Production of vanillin via oxidation depolymerization of lignin over Fe- and Mn-modified TS-1 zeolites

Converting lignin into specific aromatic chemicals for utilization through depolymerization of lignin is an effective way to achieve high-value applications. There are many depolymerization methods that can do this, but there are problems such as harsh reaction conditions, low depolymerization efficiency and uncontrollable target products that need to be solved. This study reports a novel system for the oxidative depolymerization of alkali lignin using Fe- and Mn- modified TS-1 as a catalyst to assist in the highly selective production of vanillin. We also proposed a possible reaction pathway for the oxidative depolymerization of alkali lignin to produce vanillin catalyzed by Fe-Mn/TS-1 catalyst. The catalytic effects of TS-1, Fe/TS-1, and Fe-Mn/TS-1 catalysts on the oxidative depolymerization of lignin to produce phenolic monomers and vanillin were investigated. The results show that the modified catalysts can effectively improve the efficiency of linkage bond breaking in lignin, especially the β-O-4 bond, in which the inter-band transitions of Fe and Mn play an important role. The synergistic effect of the bimetallic-loaded catalyst (Fe-Mn/TS-1) could catalyze the oxidative depolymerization of lignin more efficiently than the monometallic-loaded catalyst (Fe/TS-1). This lignin oxidative depolymerization system produced 40.59 wt% bio-oil including 12.24 wt% phenolic monomers and 16.17 wt% re-lignin after the addition of Fe-Mn/TS-1 catalyst, owning the highest phenolic monomer yield. Surprisingly, this lignin oxidative depolymerization system exhibited high yield for vanillin (8.36 wt%) production. These results demonstrated that the Fe-Mn/TS-1 catalytic system has potential to produce vanillin from lignin under mild conditions.

From Waste to Value: Recent Insights into Producing Vanillin from Lignin

Vanillin, one of the most widely used and appreciated flavoring agents worldwide, is the main constituent of vanilla bean extract, obtained from the seed pods of various members belonging to the Orchidaceae family. Due to the great demand in the food confectionery industry, as well as in the perfume industry, medicine, and more, the majority of vanillin used today is produced synthetically, and only less than one percent of the world's vanilla flavoring market comes directly from the traditional natural sources. The increasing global demand for vanillin requires alternative and overall sustainable new production methods, and the recovery from biobased polymers, like lignin, is an environmentally friendly alternative to chemical synthesis. The present review provides firstly an overview of the different types of vanillin, followed by a description of the main differences between natural and synthetic vanillin, their preparation, the market of interest, and the authentication issues and the related analytical techniques. Then, the review explores the real potentialities of lignin for vanillin production, presenting firstly the well-assessed classical methods and moving towards the most recent promising approaches through chemical, biotechnological and photocatalytic methodologies, together with the challenges and the principal issues associated with each technique.

Catalytic hydrothermal liquefaction of alkali lignin for monophenols production over homologous biochar-supported copper catalysts in water

Constructing an advanced catalytic system for the purposeful liquefaction of lignin into chemicals has presented a significant prospect for sustainable development. In this work, the catalytic process of mesoporous homologous biochar (HBC) derived from alkali lignin supported copper catalysts (Cu/HBC) was reported for catalytic liquefaction of alkali lignin to monophenols. The characterization results revealed HBC promoted the formation of metal-support strong interaction and the generation of oxygen vacancies, enhancing the acid sites of Cu/HBC. Under the optimal conditions (0.2 g alkali lignin, 280 °C, 0.05 g Cu/HBC, 6 h, 18 mL water), the monophenol yield reached 75.01 ± 0.76 mg/g, and the bio-oil yield was 57.98 ± 1.76%. The copious mesopores, high surface area, and rich acidic sites were responsible for the high activity of Cu/HBC, which significantly outperformed the controlled catalysts, such as HBC, commercial activated carbon (AC), and reported Ni/AC, Ni/MCM-41, etc. In four consecutive runs, the catalytic performance of Cu/HBC was only reduced by 3.65% per cycle. Interestingly, catechol was selectively produced with Cu/HBC, which provided an effective strategy for the conversion of G/S-type lignin to catechyl phenolics (C-type). These findings indicate that the Cu/HBC will be a promising substitution of noble metal-supported catalysts for conversion biomass into high value-added phenolics.

Review on the oxidative catalysis methods of converting lignin into vanillin

Vanillin plays an important role not only in food and flavouring, but also as a platform compound for the synthesis of other valuable products, mainly derived from the oxidative decarboxylation of petroleum-based guaiacol production. In order to alleviate the problem of collapsing oil resources, the preparation of vanillin from lignin has become a good option from the perspective of environmental sustainability, but it is still not optimistic in terms of vanillin production. Currently, catalytic oxidative depolymerization of lignin for the preparation of vanillin is the main development trend. This paper mainly reviews four ways of preparing vanillin from lignin base: alkaline (catalytic) oxidation, electrochemical (catalytic) oxidation, Fenton (catalytic) oxidation and photo (catalytic) oxidative degradation of lignin. In this work, the working principles, influencing factors, vanillin yields obtained, respective advantages and disadvantages and the development trends of the four methods are systematically summarized, and finally, several methods for the separation and purification of lignin-based vanillin are briefly reviewed.

Bio-separation of value-added products from Kraft lignin: A promising two-stage lignin biorefinery via microbial electrochemical technology

The most ubiquitous aromatic biopolymer in nature, lignin offers a promising foundation for the development of bio-based chemicals with wide-ranging industrial uses attributable to its aromatic structure. Lignin must first be depolymerized into smaller oligomeric and monomeric units at the initial stage of lignin bioconversion, followed by separation to recover valuable products. This study demonstrates an integrative biorefinery idea based on in-situ depolymerization of the lignin via microbial electro-Fenton reaction in a microbial peroxide-producing cell and recovery of the identified products i.e., phenolic or aromatic monomers by one step high throughput chromatography. The yield percentage of acetovanillone, ethylvanillin, and ferulic acid recovered from the depolymerized lignin using the integrative biorefinery strategy were 2.1 %, 9.1 %, and 9.04 %, respectively. These products have diverse industrial usage and can be employed as platform chemicals. The development of a novel system for efficient simultaneous lignin depolymerization and subsequent quality separation are demonstrated in this study.

Catalytic ozonation of toluene over amorphous Cu-Mn bimetallic oxide: Influencing factors, degradation mechanism and pathways

Herein, amorphous catalysts were employed to investigate the catalytic ozonation system, revealing the degradation mechanism and influencing factors (O₃ concentration, temperature, and humidity) for toluene catalytic ozonation. Cu_0.2MnO_x exhibited the highest toluene oxidized and excellent stability (∼85% at 60 h) based on the suitable value of O_ads/O_lat and potent synergy between Cu with Mn. To explore the effect of factors, the change of fresh and post-reaction samples was compared as revealed in the relevant characterization results (SEM, XRD, BET, XPS, TGA), DRIFTS and GC-MS identified the intermediates and byproducts. The results show that appropriate temperature (100 °C) and O₃ concentration (2100 ppm) can effectively enhance the number of reactive oxygen species. Although H₂O can increase the production of ·OH to promote degradation, it is easier to quench the active sites on the surface of amorphous catalysts. During the reaction, the main role of Cu in Cu-Mn bimetallic oxides is adsorption of toluene and O₃, formation of benzoic acid, and oxidation of short-chain products. As for the adjacent Mn, it works on the cleavage of O-O in O₃ and the ring-opening of benzene. Then, the mainly catalytic ozonation pathway of toluene was proposed and followed the order: toluene, benzoic acid, benzene, maleic anhydride, short-chain carbon species, CO₂, and H₂O.

On the Oxidative Valorization of Lignin to High-Value Chemicals: A Critical Review of Opportunities and Challenges

The efficient valorization of lignin is crucial if we are to replace current petroleum-based feedstock and establish more sustainable and competitive lignocellulosic biorefineries. Pulp and paper mills and second-generation biorefineries produce large quantities of low-value technical lignin as a by-product, which is often combusted on-site for energy recovery. This Review focuses on the conversion of technical lignins by oxidative depolymerization employing heterogeneous catalysts. It scrutinizes the current literature describing the use of various heterogeneous catalysts in the oxidative depolymerization of lignin and includes a comparison of the methods, catalyst loadings, reaction media, and types of catalyst applied, as well as the reaction products and yields. Furthermore, current techniques for the determination of product yields and product recovery are discussed. Finally, challenges and suggestions for future approaches are outlined.

Oxidative catalytic valorization of industrial lignin into phenolics: Effect of reaction parameters and metal oxides

Oxidative depolymerization of an industrial lignin was performed to study the effect of various metal oxides in oxygen and air atmosphere. CeO₂ exhibited excellent catalytic property, and promoted the production of bio-oil yield up to a maximum of 49 wt% in 10 bar O₂, whereas 31 wt% bio-oil was noticed in atmospheric air. GC-MS analysis of bio-oil showed that high selectivity towards acetosyringone was observed in the presence of air (70.5 area%) as compared to oxygen (48.1 area%). Herein, we have also applied transitional metals (Co, Mn and Cu) doped CeO₂ catalysts. Compared to Cu and Mn, Co metal showed efficient activity that promoted the breaking of labile β-O-4 linkages via the conversion of C_α-OH in to carbonyl group in atmospheric air resulting in the formation of acetosyringone up to 78 area%. Moreover, it exhibited excellent catalytic activity up to four successive cycles. Catalyst has been characterized by XRD, BET, TEM, FT-IR and Raman spectroscopy.

Interfacial engineering coupling with tailored oxygen vacancies in Co2Mn2O4 spinel hollow nanofiber for catalytic phenol removal

Herein, one-dimensional Co₂Mn₂O₄ (CMO) hollow nanofibers with a general spinel structure were constructed by electrospinning and tunning thermal-driven procedures. The resultant catalyst was endowed with appreciable active interfacial engineering effect, which revealed improved peroxymonosulfate (PMS) activation efficiency in catalytic phenol degradation with nearly 12.9 folds increment in reaction rate constant compared to the hydrothermally synthesized counterpart. Besides, tailored oxygen-vacancy sites including chemical environment and contents in the bimetallic spinel were rationally validated compared to the monometal spinel counterparts. The improved catalytic phenol degradation by reactive-oxidative-species (ROS) from PMS was well correlated with the more active Co(II) and Mn(II) species, reactive active oxygen-vacancy and the interfacial engineering effect in the CMO catalyst. These correlations were comprehensively demonstrated by various characterization techniques, catalytic results, and Density-Functional-Theoretical (DFT) calculations of the adsorption and activation of PMS. Besides, the results revealed that the specific content of cobalt species in the structural unit of the Co₂Mn₂O₄ spinel resulting from the optimized thermal treatment could further improve the catalytic activity by the intermetallic synergy along with the beneficial electron transfer cycles. This work provides a practical understanding of the improvement of interfacial systems in catalysis efficiency and environmental remediation.

Degradation mechanism of a lignin model compound during alkaline aerobic oxidation: formation of the vanillin precursor from the β-O-4 middle unit of softwood lignin

We have proposed plausible reaction pathways involved in the chemical conversion of softwood lignin to vanillin through alkaline aerobic oxidation.

Self-regeneration of supported transition metals by a high entropy-driven principle

The sintering of Supported Transition Metal Catalysts (STMCs) is a core issue during high temperature catalysis. Perovskite oxides as host matrix for STMCs are proven to be sintering-resistance, leading to a family of self-regenerative materials. However, none other design principles for self-regenerative catalysts were put forward since 2002, which cannot satisfy diverse catalytic processes. Herein, inspired by the principle of high entropy-stabilized structure, a concept whether entropy driving force could promote the self-regeneration process is proposed. To verify it, a high entropy cubic Zr0.5(NiFeCuMnCo)0.5Ox is constructed as a host model, and interestingly in situ reversible exsolution-dissolution of supported metallic species are observed in multi redox cycles. Notably, in situ exsolved transition metals from high entropy Zr0.5(NiFeCuMnCo)0.5Ox support, whose entropic contribution (TΔSconfig = T⋆12.7 J mol-1 K-1) is predominant in ∆G, affording ultrahigh thermal stability in long-term CO2 hydrogenation (400 °C, >500 h). Current theory may inspire more STWCs with excellent sintering-resistance performance.

Recent Advances in Renewable Polymer Production from Lignin-Derived Aldehydes

Lignin directly derived from lignocellulosic biomass has been named a promising source of platform chemicals for the production of bio-based polymers. This review discusses potentially relevant routes to produce renewable aromatic aldehydes (e.g., syringaldehyde and vanillin) from lignin feedstocks (pre-isolated lignin or lignocellulose) that are used to synthesize a range of bio-based polymers. To do this, the processes to make aromatic aldehydes from lignin with their highest available yields are first presented. After that, the routes from such aldehydes to different polymers are explored. Challenges and perspectives of the production the lignin-derived renewable chemicals and polymers are also highlighted.