and Fuels Changzhi Li,† Xiaochen Zhao,† Aiqin Wang,† George W. Huber,†,‡ and Tao Zhang*,† †State Key Laborotary of Catalysis, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China ‡Department of Chemical and Biological Engineering, University of WisconsinMadison, Madison, Wisconsin 53706, United States

Catalytic Transformation of Lignin for the Production of Chemicals and Fuels

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.

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

Lignin is an abundant biopolymer with a high carbon content and high aromaticity. Despite its potential as a raw material for the fuel and chemical industries, lignin remains the most poorly utilised of the lignocellulosic biopolymers. Effective valorisation of lignin requires careful fine-tuning of multiple "upstream" (i.e., lignin bioengineering, lignin isolation and "early-stage catalytic conversion of lignin") and "downstream" (i.e., lignin depolymerisation and upgrading) process stages, demanding input and understanding from a broad array of scientific disciplines. This review provides a "beginning-to-end" analysis of the recent advances reported in lignin valorisation. Particular emphasis is placed on the improved understanding of lignin's biosynthesis and structure, differences in structure and chemical bonding between native and technical lignins, emerging catalytic valorisation strategies, and the relationships between lignin structure and catalyst performance.

Paving the Way for Lignin Valorisation: Recent Advances in Bioengineering, Biorefining and Catalysis.

Increasing energy demand, especially in the transportation sector, and soaring CO2 emissions necessitate the exploitation of renewable sources of energy. Despite the large variety of new energy carriers, liquid hydrocarbon still appears to be the most attractive and feasible form of transportation fuel taking into account the energy density, stability and existing infrastructure. Biomass is an abundant, renewable source of energy; however, utilizing it in a cost-effective way is still a substantial challenge. Lignocellulose is composed of three major biopolymers, namely cellulose, hemicellulose and lignin. Fast pyrolysis of biomass is recognized as an efficient and feasible process to selectively convert lignocellulose into a liquid fuel—bio-oil. However bio-oil from fast pyrolysis contains a large amount of oxygen, distributed in hundreds of oxygenates. These oxygenates are the cause of many negative properties, such as low heating value, high corrosiveness, high viscosity, and instability; they also greatly limit the application of bio-oil particularly as transportation fuel. Hydrocarbons derived from biomass are most attractive because of their high energy density and compatibility with the existing infrastructure. Thus, converting lignocellulose into transportation fuels via catalytic fast pyrolysis has attracted much attention. Many studies related to catalytic fast pyrolysis of biomass have been published. The main challenge of this process is the development of active and stable catalysts that can deal with a large variety of decomposition intermediates from lignocellulose. This review starts with the current understanding of the chemistry in fast pyrolysis of lignocellulose and focuses on the development of catalysts in catalytic fast pyrolysis. Recent progress in the experimental studies on catalytic fast pyrolysis of biomass is also summarized with the emphasis on bio-oil yields and quality.

Catalytic fast pyrolysis of lignocellulosic biomass

The utilization of fossil fuels has enabled an unprecedented era of prosperity and advancement of well-being for human society. However, the associated increase in anthropogenic carbon dioxide (CO2) emissions can negatively affect global temperatures and ocean acidity. Moreover, fossil fuels are a limited resource and their depletion will ultimately force one to seek alternative carbon sources to maintain a sustainable economy. Converting CO2 into value-added chemicals and fuels, using renewable energy, is one of the promising approaches in this regard. Major advances in energy-efficient CO2 conversion can potentially alleviate CO2 emissions, reduce the dependence on nonrenewable resources, and minimize the environmental impacts from the portions of fossil fuels displaced. Methanol (CH3OH) is an important chemical feedstock and can be used as a fuel for internal combustion engines and fuel cells, as well as a platform molecule for the production of chemicals and fuels. As one of the promising approaches, thermocatalytic CO2 hydrogenation to CH3OH via heterogeneous catalysis has attracted great attention in the past decades. Major progress has been made in the development of various catalysts including metals, metal oxides, and intermetallic compounds. In addition, efforts are also put forth to define catalyst structures in nanoscale by taking advantage of nanostructured materials, which enables the tuning of the catalyst composition and modulation of surface structures and potentially endows more promising catalytic performance in comparison to the bulk materials prepared by traditional methods. Despite these achievements, significant challenges still exist in developing robust catalysts with good catalytic performance and long-term stability. In this review, we will provide a comprehensive overview of the recent advances in this area, especially focusing on structure-activity relationship, as well as the importance of combining catalytic measurements, in situ characterization, and theoretical studies in understanding reaction mechanisms and identifying key descriptors for designing improved catalysts.

Recent Advances in Carbon Dioxide Hydrogenation to Methanol via Heterogeneous Catalysis.

The incentive for use of renewable resources to replace fossil sources is motivating extensive research on new and alternative fuels derived from biomass. Bio-oils derived from cellulosic biomass offer the prospect of becoming a major feedstock for production of fuels and chemicals, and lignin is a plentiful, underutilized component of cellulosic biomass. Lignin conversion requires depolymerization and removal of oxygen. Likely processes for lignin conversion involve depolymerization (e.g., by pyrolysis) and catalytic upgrading of the resultant bio-oils. A major goal of the upgrading is catalytic hydrodeoxygenation (HDO), which involves reactions with hydrogen that produce hydrocarbons and water. The aim of this review is to present a critical introduction to HDO chemistry focused on compounds derived from lignin, including a summary of HDO reactions and those that accompany them, with a comparison of catalysts addressing their activities, selectivities, and stabilities. The reactions are evaluated in terms of reaction pathways of compounds representative of lignin-derived bio-oils, including anisole, guaiacol, and phenol. The review includes recommendations for further research and an attempt to place HDO in a context of options for renewable fuels and chemicals, but it does not provide an economic assessment.

Upgrading of lignin-derived bio-oils by catalytic hydrodeoxygenation

Landfill leachate, is a contaminated liquid consisted of different toxic and hazardous pollutants which could penetrate the ground and reach the groundwater, leading to the soil and water resources pollution. Hence, it is regarded as a substantial risk to the environment. Accordingly, in this work, a sustainable and efficacious sulfate radical-based oxidation approach using persulfate and heterogeneous magnetic CuFe2O4 nanocatalyst was suggested to treat municipal stabilized landfill leachate. In this study, CuFe2O4 catalyst was synthesized through the straightforward sonochemical co-precipitation method, and it was characterized by FTIR, EDX, VSM, XRD, and TEM analysis techniques. The dominant mechanism of the degradation process was discussed in detail, and the influence of various operating conditions such as pH, persulfate dosage and catalyst loading was investigated on the treatment performance of the process based on COD, ammonia-nitrogen (NH3-N) and color removal efficiencies. Under optimized condition (pH = 2, persulfate amount = 5 g/L and catalyst dosage = 1.5 g/L), the treatment process showed acceptable efficiency removals (COD, NH3-N and color removal efficiencies of 57%, 71%, and 63%, respectively). Moreover, performance of the current system was compared with performances of distinct persulfate-based and CuFe2O4-based treatment systems. The reusability of the catalyst was also evaluated over the sequential treatment runs, and the catalyst showed high reactivity over four repeated cycles. To prove the capability of the treatment process in pollutants degradation ATR-FTIR analysis of the leachate samples before and after the oxidation process was conducted, and the results showed that aromatic compounds were converted to aliphatic components after the oxidation.

Heterogeneous degradation of stabilized landfill leachate using persulfate activation by CuFe2O4 nanocatalyst: an experimental investigation

Novel heterogeneous degradation of mature landfill leachate using persulfate and magnetic CuFe2O4/RGO nanocatalyst

In the present work, carbon dioxide hydrogenation to methanol via reverse water gas shift reaction (CAMERE) in an industrial scale was modeled and optimized. In this process, syngas is first produced by CO2 hydrogenation through reverse water gas shift (RWGS) reaction over Ni/Al12O19 catalyst and next the syngas is conveyed to a reactor as the feedstock to produce methanol. The inner tubes of methanol synthesis reactor were coated by a water perm-selective membrane for removal of H2O, as the cause of catalyst poisoning. A precise two-dimensional model solved by finite-difference procedure was employed to evaluate both RWGS and methanol synthesis membrane reactors performance. Also, the operating conditions of the RWGS reactor were optimized by differential evolution (DE) technique to gain a maximum methanol production rate. Moreover, the results of the methanol production reactor from the CAMERE process were compared with the conventional route (CR) in which methanol is produced from coal and natural gas. In the case of methanol synthesis membrane reactor, CAMERE process was superior to CR due to achieving 20.8% increase in methanol production rate. The results of this process modeling provide a good initial insight for green methanol production form indirect CO2 conversion.

Dornaz Karimipourfard

Papers

Upgrading of lignin-derived bio-oils by catalytic hydrodeoxygenation

Heterogeneous degradation of stabilized landfill leachate using persulfate activation by CuFe2O4 nanocatalyst: an experimental investigation

Novel heterogeneous degradation of mature landfill leachate using persulfate and magnetic CuFe2O4/RGO nanocatalyst

Green methanol synthesis process from carbon dioxide via reverse water gas shift reaction in a membrane reactor

A novel axial-flow spherical packed-bed membrane reactor for dimethyl ether synthesis: Simulation and optimization