Carbon cycle in advanced coal chemical engineering

Seawater electrolysis for fuels and chemicals production: fundamentals, achievements, and perspectives

Seawater electrolysis for the production of fuels and chemicals involved in onshore and offshore plants powered by renewable energies offers a promising avenue and unique advantages for energy and environmental sustainability. Nevertheless, seawater electrolysis presents long-term challenges and issues, such as complex composition, potential side reactions, deposition of and poisoning by microorganisms and metal ions, as well as corrosion, thus hindering the rapid development of seawater electrolysis technology. This review focuses on the production of value-added fuels (hydrogen and beyond) and fine chemicals through seawater electrolysis, as a promising step towards sustainable energy development and carbon neutrality. The principle of seawater electrolysis and related challenges are first introduced, and the redox reaction mechanisms of fuels and chemicals are summarized. Strategies for operating anodes and cathodes including the development and application of chloride- and impurity-resistant electrocatalysts/membranes are reviewed. We comprehensively summarize the production of fuels and chemicals (hydrogen, carbon monoxide, sulfur, ammonia, etc.) at the cathode and anode via seawater electrolysis, and propose other potential strategies for co-producing fine chemicals, even sophisticated and electronic chemicals. Seawater electrolysis can drive the oxidation and upgrading of industrial pollutants or natural organics into value-added chemicals or degrade them into harmless substances, which would be meaningful for environmental protection. Finally, the perspective and prospects are outlined to address the challenges and expand the application of seawater electrolysis.

Development of Multifunctional Catalysts for the Direct Hydrogenation of Carbon Dioxide to Higher Alcohols

The direct hydrogenation of greenhouse gas CO2 to higher alcohols (C2+OH) provides a new route for the production of high-value chemicals. Due to the difficulty of C-C coupling, the formation of higher alcohols is more difficult compared to that of other compounds. In this review, we summarize recent advances in the development of multifunctional catalysts, including noble metal catalysts, Co-based catalysts, Cu-based catalysts, Fe-based catalysts, and tandem catalysts for the direct hydrogenation of CO2 to higher alcohols. Possible reaction mechanisms are discussed based on the structure-activity relationship of the catalysts. The reaction-coupling strategy holds great potential to regulate the reaction network. The effects of the reaction conditions on CO2 hydrogenation are also analyzed. Finally, we discuss the challenges and potential opportunities for the further development of direct CO2 hydrogenation to higher alcohols.

Defective Nb2C MXene Cocatalyst on TiO2 Microsphere for Enhanced Photocatalytic CO2 Conversion to Methane

Sustainable and scalable solar-energy-driven CO2 conversion into fuels requires earth-abundant and stable photocatalysts. In this work, a defective Nb2C MXene as a cocatalyst and TiO2 microspheres as photo-absorbers, constructed via a coulombic force-driven self-assembly, is synthesized. Such photocatalyst, at an optimized loading of defective Nb2C MXene (5% def-Nb2C/TiO2), exhibits a CH4 production rate of 7.23 µmol g-1 h-1, which is 3.8 times higher than that of TiO2. The Schottky junction at the interface improves charge transfer from TiO2 to defective Nb2C MXene and the electron-rich feature (nearly free electron states) enables multielectron reaction of CO2, which apparently leads to high activity and selectivity to CH4 (sel. 99.5%) production. Moreover, DFT calculation demonstrates that the Fermi level (EF) of defective Nb2C MXene (-0.3 V vs NHE) is more positive than that of Nb2C MXene (-1.0 V vs NHE), implying a strong capacity to accept photogenerated electrons and enhance carrier lifetime. This work gives a direction to modify the earth-abundant MXene family as cocatalysts to build high-performance photocatalysts for energy production.

Hydrogenation of CO2 into Value-added Chemicals Using Solid-Supported Catalysts

Reducing CO2 emissions is an urgent global priority. In this context, several mitigation strategies, including CO2 tax and stringent legislation, have been adopted to halt the deterioration of the natural environment. Also, carbon recycling procedures undoubtedly help reduce net emissions into the atmosphere, enhancing sustainability. Utilizing Earth's abundant CO2 to produce high-potential green chemicals and light fuels opens new avenues for the chemical industry. In this context, many attempts have been devoted to converting CO2 as a feedstock into various value-added chemicals, such as CH4 , lower methanol, light olefins, gasoline, and higher hydrocarbons, for numerous applications involving various catalytic reactions. Although several CO2 -conversion methods have been used, including electrochemical, photochemical, and biological approaches, the hydrogenation method allows the reaction to be tuned to produce the targeted compound without significantly altering infrastructure. This review discusses the numerous hydrogenation routes and their challenges, such as catalyst design, operation, and the combined art of structure-activity relationships for the various product formations.

Brominated metal phthalocyanine-based covalent organic framework for enhanced selective photocatalytic reduction of CO2

Covalent organic frameworks (COFs) have great potential for photocatalytic CO2 reduction, owing to their adjustable structures, porous characteristics, and highly ordered nature. However, poor light absorption, fast recombination of photogenerated electron-hole pairs, and suboptimal coordination conditions have contributed to the hindered efficiency and selectivity observed in photocatalytic CO2 reduction processes. In this work, the integration of bromine (Br) atoms into COFs was achieved through the synthesis process involving nickel (II) tetraaminophthalocyanine (NiTAPc) and 3,6-dibromopyromellitic dianhydride (BPMDA) using a solvothermal approach. The coupling of a porous framework structure alongside the incorporation of Br atoms yields a significant enhancement in photoelectric properties compared to bromine-free COFs. Meanwhile, X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations revealed that the introduction of Br atoms can facilitate the adjustment of the electron configuration around the phthalocyanine unit and diminish the required energy for the photocatalytic reaction. When subjected to visible light irradiation, the NiTAPc-BPMDA COF showcased a CO yield of 148.25 μmol g-1 over a 5-hour period, accompanied by an impressive selectivity of 98%. This work highlights the collaborative influence of phthalocyanines and Br atoms within COF-based photocatalysts, offering an alternative approach for designing and constructing high-performance photocatalysts with elevated yield and selectivity. The synergistic role of phthalocyanines and Br atoms within the COF-based photocatalysts provides an alternative strategy for photocatalysts with high yield and selectivity in the future.

Valorizing spent mushroom substrate into syngas by the thermo-chemical process

This study investigated the conversion of agricultural biomass waste (specifically, spent mushroom substrate) into syngas via pyrolysis. Carbon dioxide was used to provide a green/sustainable feature in the pyrolysis process. All the experimental data highlight the mechanistic role of carbon dioxide (CO2) in the process, demonstrated by the enhanced carbon monoxide (CO) yield from pyrolysis under CO2. Carbon dioxide was indeed reactive at ≥ 500 ˚C. Carbon dioxide was reduced and subsequently oxidized volatiles stemming from the thermolysis of spent mushroom substrate via the gas-phase reaction, thereby resulting in the enhanced formation of CO. Carbon dioxide radically diverted the carbon distribution patterns of the pyrogenic products, as more carbon in the oil was allocated to syngas by the gas-phase reaction of volatiles and CO2. To enhance the mechanistic role of CO2, a Ni-based catalyst was added to the pyrolysis process, which greatly accelerated the gas-phase reaction of volatiles and CO2.

Synergistically Coupled Ni/CeOx@C Electrocatalysts for the Hydrogen Evolution Reaction: Remarkable Performance to Pt/C at High Current Density

Incredibly active electrocatalysts comprising earth-abundant materials that operate as effectively as noble metal catalysts are essential for the sustainable generation of hydrogen through water splitting. However, the vast majority of active catalysts are produced via complicated synthetic processes, making scale-up considerably tricky. In this work, a facile strategy is developed to synthesize superhydrophilic Ni/CeO_x nanoparticles (NPs) integrated into porous carbon (Ni/CeO_x@C) by a simple two-step synthesis strategy as efficient hydrogen evolution reaction (HER) electrocatalysts in 1.0 M KOH. Benefiting from the electron transport induced by the heterogeneous interface between Ni and CeO_x NPs and the superhydrophilic structure of the catalyst, the resultant Ni₂Ce₁@C/500 catalysts exhibit a low overpotential of 26 and 184 mV at a current density of 10 and 300 mA cm^-2, respectively, for HER with a small Tafel slope of 62.03 mV dec^-1 and robust durability over 300 h, and its overpotential at a high current density is much better than the benchmark commercial Pt/C. Results revealed that the electronic rearrangement between Ni and CeO_x integrated into porous carbon could effectively regulate the local conductivity and charge density. In addition, the oxygen vacancies and Ni/CeO_x heterointerface promote water adsorption and hydrogen intermediate dissociation into H₂ molecules, which ultimately accelerate the HER reaction kinetics. Notably, the electrochemical results demonstrate that structural optimization by regulating synthesis temperature and metal concentration could improve the surface features contributing to high electrical conductivity and increase the number of electrochemically active sites on the Ni/CeO_x@C heterointerface, high crystal purity, and better electrical conductivity, resulting in its exceptional electrocatalytic performance toward the HER. These results indicated that the Ni/CeO_x@C electrocatalyst has the potential for practical water-splitting applications because of its controlled production strategy and outstanding Pt-like HER performance.

Pinch Analysis for Heat Integration of Pulverized Coke Chemical Looping Gasification Coupled with Coke-Oven Gas to Methanol and Ammonia

Methanol and ammonia are important chemical materials in the chemical industry. During the production of methanol and ammonia, a large amount of waste heat is released. The waste heat can be used to save energy and reduce CO2 emissions. In this study, pinch analysis is used to design the heat exchanger network (HEN) of pulverized coke (PC) chemical looping gasification coupled with coke-oven gas (COG) to methanol and ammonia (PCCLHG-CGTMA). The heat integration process is accomplished in two ways, as mentioned below. (1) The HENs in each of the three heat exchange units are designed individually; (2) the HENs of the three heat exchange units are treated as a whole and designed simultaneously. Compared to the HEN designed individually, when the HENs are designed as a whole, a total of 112.12 MW of hot and cold utilities are saved. In the HENs designed as a whole, the reduction in operating cost is sufficient to offset the increase in capital cost; the total annual cost (TAC) is reduced by 10.9%. These results reveal that the HENs designed as a whole have more scope for energy saving, which can be a reference for new HEN design and modification to realize more heat recovery and lower investment.

Recent progress on covalent organic framework materials as CO2 reduction electrocatalysts

CO₂ emission caused by fuel combustion and human activity has caused severe climate change and other subsequent pollutions around the world. Carbon neutralization via various novel technologies to alleviate the CO₂ level in the atmosphere has thus become one of the major topics in modern research field. These advanced technologies cover CO₂ capture, storage and conversion, etc., and electrocatalytic CO₂ reduction reaction (CO₂RR) by heterogeneous catalysts is among the most promising methods since it could utilize renewable energy and generate valuable fuels and chemicals. Covalent organic frameworks (COFs) represent crystalline organic polymers with highly rigid, conjugated structures and tunable porosity, which exhibit significant potential as heterogeneous electrocatalysts for CO₂RR. This review briefly introduces related pioneering works in COF-based materials for electrocatalytic CO₂RR in recent years and provides a basis for future design and synthesis of highly active and selective COF-based electrocatalysts in this direction.

In situ dual doping for constructing efficient CO2-to-methanol electrocatalysts

Methanol is a highly desirable product of CO₂ electroreduction due to its wide array of industrial applications. However, the development of CO₂-to-methanol electrocatalysts with high performance is still challenging. Here we report an operationally simple in situ dual doping strategy to construct efficient CO₂-to-methanol electrocatalysts. In particular, when using Ag,S-Cu₂O/Cu as electrocatalyst, the methanol Faradaic efficiency (FE) could reach 67.4% with a current density as high as 122.7 mA cm^-2 in an H-type cell using 1-butyl-3-methylimidazolium tetrafluoroborate/H₂O as the electrolyte, while the current density was below 50 mA cm^-2 when the FE was greater than 50% over the reported catalysts. Experimental and theoretical studies suggest that the anion S can effectively adjust the electronic structure and morphology of the catalysts in favor of the methanol pathway, whereas the cation Ag suppresses the hydrogen evolution reaction. Their synergistic interactions with host material enhance the selectivity and current density for methanol formation. This work opens a way for designing efficient catalysts for CO₂ electroreduction to methanol.

Boosted CO2 photoreduction performance on Ru-Ti3CN MXene-TiO2 photocatalyst synthesized by non-HF Lewis acidic etching method

Photocatalytic CO₂ reduction to produce value-added products is considered a promising solution to solve the global energy crisis and the greenhouse effect. In this study, Ti₃CN MXene was synthesized using a Lewis acidic etching method without the usage of toxic hydrofluoric acid (HF). Ti₃CN MXene was then used as a support for the in situ hydrothermal growth of TiO₂ and Ru nanoparticles. In the presence of 0.5 wt% Ru, Ru-Ti₃CN-TiO₂ shows CO and CH₄ production rates of 99.58 and 8.97 μmol/g, respectively, in 5 h under Xenon lamp irradiation, more than 20.5 and 9.3 times that of commercial P25. The enhancement in photocatalytic activity was attributed to the synergy between the in-situ growth of TiO₂ on Ti₃CN MXene and Ru nanoparticles. It was proven experimentally that Ti₃CN MXene can provide abundant pathways for electron transfer. The separation and transfer of the photo-induced charge were further increased with the help of Ru and Ti₃CN MXene, leaving more electrons to participate in the subsequent CO₂ reduction reaction. We believe that this work will encourage more attention to designing environment-friendly MXene-based photocatalysts for CO₂ photoreduction using the non-HF method.

Wool-Based Carbon Fiber/MoS2 Composite Prepared by Low-Temperature Catalytic Hydrothermal Method and Its Application in the Field of Gas Sensors

Under the background of the Paris Agreement on reducing greenhouse gases, waste wools were converted into wool carbon fiber (WCF) and WCF–MoS₂ composites by low-temperature catalytic hydrothermal carbonization. Their structures and gas-sensing performances were studied for the first time. Due to the existence of heterojunctions, the responses of the WCF–MoS₂ composite to the five analytes were 3–400 times those of MoS₂ and 2–11 times those of WCF. Interestingly, because of the N, P, and S elements contained in wools, the WCF prepared by the hydrothermal method was realized the doping of N, P, and S, which caused the sensing curves of WCF to have different shapes for different analytes. This characteristic was also well demonstrated by the WCF–MoS₂ composite, which inspired us to realize the discriminative detection only by a single WCF–MoS₂ sensor and image recognition technology. What’s more, the WCF–MoS₂ composite also showed a high sensitivity, a high selectivity, and a rapid response to NH₃. The response time and the recovery time to 3 ppm NH₃ were about 16 and 5 s, respectively. The detection of limit of WCF–MoS₂ for NH₃ was 19.1 ppb. This work provides a new idea for the development of sensors and the resource utilization of wool waste.

Improving the steric hindrance effect of linear sulfonated acetone–formaldehyde dispersant and its performance in coal–water slurry

PSAF gains a significant steric hindrance effect from the introduction of phenol groups into its molecular structure. It exhibits stand-up adsorption rather than lie-down adsorption on SAF, resulting in a stronger steric hindrance effect and improved rheological properties.

Green Carbon Science: Efficient Carbon Resource Processing, Utilization, and Recycling Towards Carbon Neutrality

Green carbon science is defined as "Study and optimization of the transformation of carbon containing compounds and the relevant processes involved in the entire carbon cycle from carbon resource processing, carbon energy utilization, and carbon recycling to use carbon resources efficiently and minimize the net CO2 emission." [1] Green carbon science is related closely to carbon neutrality, and the relevant fields have developed quickly in the last decade. In this Minireview, we proposed the concept of carbon energy index, and the recent progresses in petroleum refining, production of liquid fuels, chemicals, and materials using coal, methane, CO2, biomass, and waste plastics are highlighted in combination with green carbon science, and an outlook for these important fields is provided in the final section.

Modulating electron density of vacancy site by single Au atom for effective CO2 photoreduction

The surface electron density significantly affects the photocatalytic efficiency, especially the photocatalytic CO₂ reduction reaction, which involves multi-electron participation in the conversion process. Herein, we propose a conceptually different mechanism for surface electron density modulation based on the model of Au anchored CdS. We firstly manipulate the direction of electron transfer by regulating the vacancy types of CdS. When electrons accumulate on vacancies instead of single Au atoms, the adsorption types of CO₂ change from physical adsorption to chemical adsorption. More importantly, the surface electron density is manipulated by controlling the size of Au nanostructures. When Au nanoclusters downsize to single Au atoms, the strong hybridization of Au 5d and S 2p orbits accelerates the photo-electrons transfer onto the surface, resulting in more electrons available for CO₂ reduction. As a result, the product generation rate of Au_SA/Cd_1−xS manifests a remarkable at least 113-fold enhancement compared with pristine Cd_1−xS.

The electron density of reactive sites significantly affects catalytic performances. Here, authors demonstrate the electron density of different reactive sites can be modulated by regulating the type of vacancy and the size of Au, leading to effective CO₂ photoreduction.

Conceptual Design Development of Coal-to-Methanol Process with Carbon Capture and Utilization

Methanol is a clean fuel and an important feedstock for the petrochemical industry. Conventionally, the coal-to-methanol process generates a substantial amount of CO2 emissions with a low yield of methanol. In this study, we propose the conceptual design development of coal-to-methanol process using captured CO2 from the gasification plant by implying process intensification. The base case and three alternative designs have been developed using the Aspen Plus to analyze the process performance. The four designs have been compared in terms of their energy consumption, economics, methanol production rate, and carbon emissions while maintaining the gasifier operation conditions, sulfur content in the syngas, and stoichiometric number at the methanol synthesis reactor. Among the developed designs, the most feasible design requires an energy requirement and product cost of 21.9 GJ and $142.5 per ton of methanol product. The study also showed that the coal-to-methanol process assisted with external hydrogen from electrolysis plant for achieving higher production rates and low CO2 emissions is currently not economical due to the high H2 cost.

Template-free preparation of anthracite-based nitrogen-doped porous carbons for high-performance supercapacitors and efficient electrocatalysts for the oxygen reduction reaction

The conversion of coal into high-performance electrochemical energy materials, exemplified by electrodes and electrocatalysts for supercapacitors and fuel cells, is currently crucial to the advancement of high value-added, clean and non-fuel utilization of coal resources. In this work, anthracite-based nitrogen-doped porous carbon (ANPC) materials with well-defined pore architectures and adjustable nitrogen concentrations were prepared without any template: ANPC-1 by a one-step activation/doping process and ANPC-2 by a two-step process. The specific capacitance value of the ANPC-1 materials could attain a maximum of 346.0 F g⁻¹ at the current density of 0.5 A g⁻¹ in 6 M KOH. Supercapacitors composed of the ANPC-1 electrodes were able to achieve high energy densities up to 10.3 W h kg⁻¹ and 20.8 W h kg⁻¹, together with good charge/discharge stabilities of 95.4% and 91.3% after 5000 cycles, in KOH and Na₂SO₄ aqueous electrolytes, respectively. The ANPC-2 materials are more associated with the oxygen reduction reaction (ORR): one possessed a comparable ORR electrocatalytic activity to the commercial JM Pt/C (20% Pt) catalyst, and, moreover, its onset potential (0.96 V vs. RHE), half-wave potential (0.85 V vs. RHE), catalyst durability (95.9% activity retained after 40 000 s) and methanol tolerance were all superior to the benchmark electrocatalyst. This study provides a feasible route to rational design of coal-based multifunctional materials towards electrochemical energy storage and conversion.

Acting as supercapacitor electrodes and ORR electrocatalysts, anthracite-based nitrogen-doped porous carbons are prepared by one/two-step activation/doping processes.

High conversion of CO2 into cyclic carbonates under solvent free and ambient pressure conditions by a Fe-cyanide complex

Methods of converting carbon dioxide into valuable chemicals are of great demand but their development is still challenging.

Promoting helix pitch and trichome length to improve biomass harvesting efficiency and carbon dioxide fixation rate by Spirulina sp. in 660 m2 raceway ponds under purified carbon dioxide from a coal chemical flue gas

The helix pitch and trichome length of Spirulina sp. were promoted to improve the biomass harvesting efficiency and CO₂ fixation rate in 660 m² raceway ponds aerated with food-grade CO₂ purified from a coal chemical flue gas. The CO₂ fixation rate was improved with increased trichome length of the Spirulina sp. in a raceway pond with double paddlewheels, baffles, and CO₂ aerators (DBA raceway pond). The trichome length has increased by 33.3 μm, and CO₂ fixation rate has increased by 42.3% and peaked to 51.3 g/m²/d in a DBA raceway pond. Biomass harvesting efficiency was increased with increased helix pitch. When the day-average greenhouse temperature was 33 °C and day-average sunlight intensity was 72,100 lu×, the helix pitch of Spirulina sp. was increased to 56.2 μm. Hence the biomass harvesting efficiency was maximized to 75.6% and biomass actual yield was increased to 35.9 kg in a DBA raceway pond.

Ruthenium water oxidation catalysts containing the non-planar tetradentate ligand, biisoquinoline dicarboxylic acid (biqaH2)

Two ruthenium complexes containing the tetradentate ligand [1,1'-biisoquinoline]-3,3'-dicarboxylic acid, and 4-picoline or 6-bromoisoquinoline as axial ligands have been prepared. The complexes have been fully characterised and initial studies on their potential to function as molecular water oxidation catalysts have been performed. Both complexes catalyse the oxidation of water in acidic media with Ce^IV as a stoichiometric chemical oxidant, although turnover numbers and turnover frequencies are modest when compared with the closely related Ru-bda and Ru-pda analogues. Barriers for the water nucleophilic attack and intermolecular coupling pathways were obtained from density functional theory calculations and the crucial influence of the ligand framework in determining the most favourable reaction pathway was elucidated from a combined analysis of the theoretical and experimental results.

Alumina-Supported CoFe Alloy Catalysts Derived from Layered-Double-Hydroxide Nanosheets for Efficient Photothermal CO2 Hydrogenation to Hydrocarbons

A series of novel CoFe-based catalysts are successfully fabricated by hydrogen reduction of CoFeAl layered-double-hydroxide (LDH) nanosheets at 300-700 °C. The chemical composition and morphology of the reaction products (denoted herein as CoFe-x) are highly dependent on the reduction temperature (x). CO₂ hydrogenation experiments are conducted on the CoFe-x catalysts under UV-vis excitation. With increasing LDH-nanosheet reduction temperature, the CoFe-x catalysts show a progressive selectivity shift from CO to CH₄ , and eventually to high-value hydrocarbons (C₂₊ ). CoFe-650 shows remarkable selectivity toward hydrocarbons (60% CH₄ , 35% C₂₊ ). X-ray absorption fine structure, high-resolution transmission electron microscopy, Mössbauer spectroscopy, and density functional theory calculations demonstrate that alumina-supported CoFe-alloy nanoparticles are responsible for the high selectivity of CoFe-650 for C₂₊ hydrocarbons, also allowing exploitation of photothermal effects. This study demonstrates a vibrant new catalyst platform for harnessing clean, abundant solar-energy to produce valuable chemicals and fuels from CO₂ .