Low-Temperature Reverse Water-Gas Shift Process and Transformation of Renewable Carbon Resources to Value-Added Chemicals

Catalytic Conversion of Lignin into Valuable Chemicals: Full Utilization of Aromatic Nuclei and Side Chains

ConspectusIn recent years, significant efforts have been directed toward achieving efficient and mild lignocellulosic biomass conversion into valuable chemicals and fuels, aiming to address energy and environmental concerns and realize the goal of carbon neutrality. Lignin is one of the three primary building blocks of lignocellulose and the only aromatic renewable feedstock. However, the complex and diverse nature of lignin feedstocks, characterized by their three-dimensional, highly branched polymeric structure and intricate C-O/C-C chemical bonds, results in substantial challenges. To tackle these challenges, we carried out extensive research on selectively activating and transforming chemical bonds in lignin for chemical synthesis. In this Account, we discuss our recent progress in catalytic lignin conversion.Our work is focused on two main objectives: (i) achieving precise and selective transformation of C-O/C-C bonds in lignin (and its model compounds) and (ii) fully utilizing the aromatic nuclei and side chains present in lignin to produce valuable chemicals. Lignin consists of interconnected phenylpropanoid subunits linked by interlaced C-C/C-O bonds. To unlock the full potential of lignin, we propose the concept of "the full utilization of lignin", which encompasses both the aromatic nuclei and the side chains (e.g., methoxyl and polyhydroxypropyl groups).For the conversion of aromatic nuclei, selective activation of C-O and/or C-C bonds is crucial in synthesizing targeted aromatic products. We begin with model compounds (such as anisole, phenol, guaiacol, etc.) and then transition to protolignin feedstocks. Various reaction routes are developed, including self-supported hydrogenolysis, direct Caryl-Csp3 cleavage, coupled Caryl-Csp3 cleavage and Caryl-O hydrogenolysis, and tandem selective hydrogenation and hydrolysis processes. These tailored pathways enable high-yield and sustainable production of a wide range of aromatic (and derived) products, including arenes (benzene, toluene, alkylbenzenes), phenols, ketones, and acids.In terms of side chain utilization, we have developed innovative strategies such as selective methyl transfer, coupling depolymerization-methyl shift, selective acetyl utilization, and new activation methods such as amine-assisted prefunctionalization. These strategies enable the direct synthesis of methyl-/alkyl-derived products, such as acetic acid, 4-ethyltoluene, dimethylethylamine, and amides. Additionally, aromatic residues can be transformed into chemicals or functionalized ingredients that can serve as catalysts or functional biopolymer materials. These findings highlight promising opportunities for harnessing both the aromatic nuclei and side chains of lignin in a creative manner, thereby improving the overall atom economy of lignin upgrading.Through innovative catalyst engineering and reaction route strategies, our work achieves the sustainable and efficient production of various valuable chemicals from lignin. By integrating side chains and aromatic rings, we have successfully synthesized methyl-/alkyl-derived and aromatic-derived products with high yields. The full utilization of lignin not only minimizes waste but also opens up new possibilities for generating chemical products from lignin. These novel approaches unlock the untapped potential of lignin, expand the boundaries of lignin upgrading, and enhance the efficiency and economic viability of lignin biorefining.

MIL-100(Fe)-derived catalysts for CO2 conversion via low- and high-temperature reverse water-gas shift reaction

Fe-derived catalysts were synthesized by the pyrolysis of MIL-100 (Fe) metal-organic framework (MOF) and evaluated in the reverse water-gas shift (RWGS) reaction. The addition of Rh as a dopant by in-situ incorporation during the synthesis and wet impregnation was also considered. Our characterization data showed that the main active phase was a mixture of α-Fe, Fe₃C, and Fe₃O₄ in all the catalysts evaluated. Additionally, small Rh loading leads to a decrease in the particle size in the active phase. Despite all three catalysts showing commendable CO selectivity levels, the C@Fe* catalyst showed the most promising performance at a temperature below 500 °C, attributed to the in-situ incorporation of Rh during the synthesis. Overall, this work showcases a strategy for designing novel Fe MOF-derived catalysts for RWGS reaction, opening new research opportunities for CO₂ utilization schemes.

Shvo-Catalyzed Hydrogenation of CO2 in the Presence or Absence of Ionic Liquids for Tandem Reactions

Ionic liquids (ILs) have been widely used in transition metal-catalyzed processes, but the precise behavior of ILs and catalysts in these reactions is unknown. Herein, the role of ILs and the interaction pattern between Shvo's catalyst and ILs have been revealed with characterization by ¹H NMR and crystallography based on the catalytic hydrogenation of CO₂. ILs promote the dissociation of Shvo's catalyst and enhance the rate of production of CO. The CO that is produced is subsequently used in the tandem hydroformylation-reduction of alkenes to produce valuable alcohols. In the absence of ILs, formamides can be obtained by N-formylation of most primary or secondary amines.

Direct Conversion of Syngas to Higher Alcohols via Tandem Integration of Fischer-Tropsch Synthesis and Reductive Hydroformylation

The selective conversion of syngas to higher alcohols is an attractive albeit elusive route in the quest for effective production of chemicals from alternative carbon resources. We report the tandem integration of solid cobalt Fischer–Tropsch and molecular hydroformylation catalysts in a one‐pot slurry‐phase process. Unprecedented selectivities (>50 wt %) to C₂₊ alcohols are achieved at CO conversion levels >70 %, alongside negligible CO₂ side‐production. The efficient overall transformation is enabled by catalyst engineering, bridging gaps in operation temperature and intrinsic selectivity which have classically precluded integration of these reactions in a single conversion step. Swift capture of 1‐olefin Fischer–Tropsch primary products by the molecular hydroformylation catalyst, presumably within the pores of the solid catalyst is key for high alcohol selectivity. The results underscore that controlled cooperation between solid aggregate and soluble molecular metal catalysts, which pertain to traditionally dichotomic realms of heterogeneous and homogeneous catalysis, is a promising blueprint toward selective conversion processes.

Catalytic self-transfer hydrogenolysis of lignin with endogenous hydrogen: road to the carbon-neutral future

Due to the depletion of fossil sources, it is imperative to develop a sustainable and carbon-neutral biorefinery for supporting the fuel and chemical supply in modern society. Lignin, the only renewable aromatic source, is still an underutilized component in lignocellulose. Very recently, it has been found that hydrogenolysis is a promising technology for lignin valorization. However, high-pressure H₂ is necessary during lignin hydrogenolysis, resulting in safety problems. Furthermore, H₂ is mainly produced from steam reforming of fossil sources in industry, which makes the conversion of renewable lignin unsustainable and costly. Plentiful aliphatic hydroxyl and methoxy groups exist in native lignin and offer a renewable alternative to H₂, and can be hydrogen sources for the depolymerization and upgradation of lignin via the intramolecular catalytic transfer hydrogenation. The hydrogen source in situ generated from lignin is a type of green hydrogen, decreasing the carbon footprint. The purpose of this review is to provide a summary and perspective of lignin valorization via self-transfer hydrogenolysis, mainly focusing on a comprehensive understanding of the mechanism of catalytic self-transfer hydrogenolysis at the molecular level and developing highly effective catalytic systems. Moreover, some opportunities and challenges within this attractive field are given to discuss future research directions.

K-Promoted Ni-Based Catalysts for Gas-Phase CO2 Conversion: Catalysts Design and Process Modelling Validation

The exponential growth of greenhouse gas emissions and their associated climate change problems have motivated the development of strategies to reduce CO₂ levels via CO₂ capture and conversion. Reverse water gas shift (RWGS) reaction has been targeted as a promising pathway to convert CO₂ into syngas which is the primary reactive in several reactions to obtain high-value chemicals. Among the different catalysts reported for RWGS, the nickel-based catalyst has been proposed as an alternative to the expensive noble metal catalyst. However, Ni-based catalysts tend to be less active in RWGS reaction conditions due to preference to CO₂ methanation reaction and to the sintering and coke formation. Due to this, the aim of this work is to study the effect of the potassium (K) in Ni/CeO₂ catalyst seeking the optimal catalyst for low-temperature RWGS reaction. We synthesised Ni-based catalyst with different amounts of K:Ni ratio (0.5:10, 1:10, and 2:10) and fully characterised using different physicochemical techniques where was observed the modification on the surface characteristics as a function of the amount of K. Furthermore, it was observed an improvement in the CO selectivity at a lower temperature as a result of the K-Ni-support interactions but also a decrease on the CO₂ conversion. The 1K catalyst presented the best compromise between CO₂ conversion, suppression of CO₂ methanation and enhancing CO selectivity. Finally, the experimental results were contrasted with the trends obtained from the thermodynamics process modelling observing that the result follows in good agreement with the modelling trends giving evidence of the promising behaviour of the designed catalysts in CO₂ high-scale units.

Ru-Catalyzed Reverse Water Gas Shift Reaction with Near-Unity Selectivity and Superior Stability

Cascade catalysis of reverse water gas shift (RWGS) and well-established CO hydrogenation holds promise for the conversion of greenhouse gas CO₂ and renewable H₂ into liquid hydrocarbons and methanol under mild conditions. However, it remains a big challenge to develop low-temperature RWGS catalysts with high activity, selectivity, and stability. Here, we report the design of an efficient RWGS catalyst by encapsulating ruthenium clusters with the size of 1 nm inside hollow silica shells. The spatially confined structure prevents the sintering of Ru clusters while the permeable silica layer allows the diffusion of gaseous reactants and products. This catalyst with reduced particle sizes not only inherits the excellent activity of Ru in CO₂ hydrogenation reactions but also exhibits nearly 100% CO selectivity and superior stability at 200-500 °C. The ability to selectively produce CO from CO₂ at relatively low temperatures paves the way for the production of value-added fuels from CO₂ and renewable H₂.

Synthesis of C2+ Chemicals from CO2 and H2 via C-C Bond Formation

ConspectusThe severity of global warming necessitates urgent CO₂ mitigation strategies. Notably, CO₂ is a cheap, abundant, and renewable carbon resource, and its chemical transformation has attracted great attention from society. Because CO₂ is in the highest oxidation state of the C atom, the hydrogenation of CO₂ is the basic means of converting it to organic chemicals. With the rapid development of H₂ generation by water splitting using electricity from renewable resources, reactions using CO₂ and H₂ have become increasingly important. In the past few decades, the advances of CO₂ hydrogenation have mostly been focused on the synthesis of C1 products, such as CO, formic acid and its derivatives, methanol, and methane. In many cases, the chemicals with two or more carbons (C₂₊) are more important. However, the synthesis of C₂₊ chemicals from CO₂ and H₂ is much more difficult because it involves controlled hydrogenation and simultaneous C-C bond formation. Obviously, investigations on this topic are of great scientific and practical significance. In recent years, we have been targeting this issue and have successfully synthesized the basic C₂₊ chemicals including carboxylic acids, alcohols, and liquid hydrocarbons, during which we discovered several important new reactions and new reaction pathways. In this Account, we systematically present our work and insights in a broad context with other related reports.1.We discovered a reaction of acetic acid production from methanol, CO₂ and H₂, which is different from the well-known methanol carbonylation. We also discovered a reaction of C₃₊ carboxylic acids syntheses using ethers to react with CO₂ and H₂, which proceeds via olefins as intermediates. Following the new reaction, we realized the synthesis of acetamide by introducing various amines, which may inspire the development of further catalytic schemes for preparing a variety of special chemicals using carbon dioxide as a building block.2.We designed a series of homogeneous catalysts to accelerate the production of C₂₊ alcohols via CO₂ hydrogenation. In the heterogeneously catalyzed CO₂ hydrogenation, we discovered the role of water in enhancing the synthesis of C₂₊ alcohols. We also developed a series of routes for ethanol production using CO₂ and H₂ to react with some substrates, such as methanol, dimethyl ether, aryl methyl ether, lignin, or paraformaldehyde.3.We designed a catalyst that can directly hydrogenate CO₂ to C₅₊ hydrocarbons at 200 °C, not via the traditional CO or methanol intermediates. We also designed a route to couple homogeneous and heterogeneous catalysis, where exceptional results are achieved at 180 °C.

Product-oriented Direct Cleavage of Chemical Linkages in Lignin

Lignin is one of the most important biomacromolecules in the plant biomass and the largest renewable source of aromatic building blocks in nature. Selectively producing value-added chemicals from the catalytic transformation of renewable lignin is of strategic significance and meet sustainability targets owing to the excessive consumption of non-renewable petroleum resource, but remains a long-term challenge owing to the complexity of lignin structure. This Minireview provides a summary and perspective of the extensive research that provides insight into selectively catalytic transformations of lignin and its derived monomers via directed scissor of chemical linkages (C-O and C-C bonds) with product-oriented targets. Furthermore, some challenges and opportunities of lignin catalytic transformation are provided based on existing problems in this field for readers to discuss future research directions.

Research Progress in Lignin-Based Slow/Controlled Release Fertilizer

As a skeleton component of plants, lignin is an organic macromolecule polymer that can be regenerated and naturally degraded. Annually, plant growth produces about 150 billion tons of lignin. In industrial processes such as paper and biomass-refining industry, large amounts of lignin are formed as by-products. Most of technical lignins are directly combusted to obtain heat, which not only is a waste of organic matter but also leads to environmental pollution and other issues. Interestingly, lignin can be used as slow-release carriers and coating materials for fertilizers due to its excellent slow release properties as well as chelating and other functionalities. Preparation of lignin-based slow/controlled release fertilizers can be achieved by sustainable chemical (ammoxidation, Mannich reaction, and other chemical modifications), coating (without or with chemical modification), and chelation modifications. This Review systematically summarizes the methods, mechanisms, and application of the above methods for preparing lignin-based slow/controlled release fertilizers. Although the evaluation standards and methods of lignin-based slow/controlled release fertilizers are not perfect, it is believed that more and more scholars will pay more attention to them to accelerate the development and application of lignin-based slow/controlled release fertilizers, so as to improve their relevant standards. In short, there is an urgent need to improve the preparation process of lignin-based slow/controlled release fertilizers and application as lignin-based slow/controlled release fertilizers to production practice as soon as possible.