Harnessing the Hybridization of a Metal-Organic Framework and Superbase-Derived Ionic Liquid for High-Performance Direct Air Capture of CO2

Direct air capture (DAC) of CO₂ has emerged as the most promising “negative carbon emission” technologies. Despite being state-of-the-art, sorbents deploying alkali hydroxides/amine solutions or amine-modified materials still suffer from unsolved high energy consumption and stability issues. In this work, composite sorbents are crafted by hybridizing a robust metal-organic framework (Ni-MOF) with superbase-derived ionic liquid (SIL), possessing well maintained crystallinity and chemical structures. The low-pressure (0.4 mbar) volumetric CO₂ capture assessment and a fixed-bed breakthrough examination with 400 ppm CO₂ gas flow reveal high-performance DAC of CO₂ (CO₂ uptake capacity of up to 0.58 mmol g⁻¹ at 298 K) and exceptional cycling stability. Operando spectroscopy analysis reveals the rapid (400 ppm) CO₂ capture kinetics and energy-efficient/fast CO₂ releasing behaviors. The theoretical calculation and small-angle X-ray scattering demonstrate that the confinement effect of the MOF cavity enhances the interaction strength of reactive sites in SIL with CO₂, indicating great efficacy of the hybridization. The achievements in this study showcase the exceptional capabilities of SIL-derived sorbents in carbon capture from ambient air in terms of rapid carbon capture kinetics, facile CO₂ releasing, and good cycling performance.     

CO2 capture at high temperature using calcium-based sorbents

Calcium-based sorbents synthesized from CaO, CaCO₃, and Ca(OH)₂ precursors were demonstrated as high-temperature CO₂ capture materials. The effect on CO₂ capture capability of calcium-based sorbents receiving different activations was also investigated. After proper activation, the best carbon capturing material is CaO that captured 75% of available CO₂ in nine cyclic tests and captured 61% even after 40 cyclic experiments. The correlation of the structural difference in the three activated sorbents and CO₂ conversion has been discussed. The sintering effect is presumably a major cause for activity decline of calcium-based sorbents… after cyclic carbonation/decarbonation runs.     

Porous Organic Polymers for Post‐Combustion Carbon Capture

One of the most pressing environmental concerns of our age is the escalating level of atmospheric CO₂. Intensive efforts have been made to investigate advanced porous materials, especially porous organic polymers (POPs), as one type of the most promising candidates for carbon capture due to their extremely high porosity, structural diversity, and physicochemical stability. This review provides a critical and in‐depth analysis of recent POP research as it pertains to carbon capture. The definitions and terminologies commonly used to evaluate the performance of POPs for carbon capture, including CO₂ capacity, enthalpy, selectivity, and regeneration strategies, are summarized. A detailed correlation study between the structural and chemical features of POPs and their adsorption capacities is discussed, mainly focusing on the physical interactions and chemical reactions. Finally, a concise outlook for utilizing POPs for carbon capture is discussed, noting areas in which further work is needed to develop the next‐generation POPs for practical applications.     

Multishelled CaO Microspheres Stabilized by Atomic Layer Deposition of Al2O3 for Enhanced CO2 Capture Performance

CO₂ capture and storage is a promising concept to reduce anthropogenic CO₂ emissions. The most established technology for capturing CO₂ relies on amine scrubbing that is, however, associated with high costs. Technoeconomic studies show that using CaO as a high‐temperature CO₂ sorbent can significantly reduce the costs of CO₂ capture. A serious disadvantage of CaO derived from earth‐abundant precursors, e.g., limestone, is the rapid, sintering‐induced decay of its cyclic CO₂ uptake. Here, a template‐assisted hydrothermal approach to develop CaO‐based sorbents exhibiting a very high and cyclically stable CO₂ uptake is exploited. The morphological characteristics of these sorbents, i.e., a porous shell comprised of CaO nanoparticles coated by a thin layer of Al₂O₃ (<3 nm) containing a central void, ensure (i) minimal diffusion limitations, (ii) space to accompany the substantial volumetric changes during CO₂ capture and release, and (iii) a minimal quantity of Al₂O₃ for structural stabilization, thus maximizing the fraction of CO₂‐capture‐active CaO.     

Covalent Organic Frameworks: The Rising-Star Platforms for the Design of CO2 Separation Membranes

Membrane-based carbon dioxide (CO₂) capture and separation technologies have aroused great interest in industry and academia due to their great potential to combat current global warming, reduce energy consumption in chemical separation of raw materials, and achieve carbon neutrality. The emerging covalent organic frameworks (COFs) composed of organic linkers via reversible covalent bonds are a class of porous crystalline polymers with regular and extended structures. The inherent structure and customizable organic linkers give COFs high and permanent porosity, short transport channel, tunable functionality, and excellent stability, thereby enabling them rising-star alternatives for developing advanced CO₂ separation membranes. Therefore, the promising research areas ranging from development of COF membranes to their separation applications have emerged. Herein, this review first introduces the main advantages of COFs as the state-of-the-art membranes in CO₂ separation, including tunable pore size, modifiable surfaces property, adjustable surface charge, excellent stability. Then, the preparation approaches of COF-based membranes are systematically summarized, including in situ growth, layer-by-layer stacking, blending, and interface engineering. Subsequently, the key advances of COF-based membranes in separating various CO₂ mixed gases, such as CO₂/CH₄, CO₂/H₂, CO₂/N₂, and CO₂/He, are comprehensively discussed. Finally, the current issues and further research expectations in this field are proposed.     

Liquid-in-Aerogel Porous Composite Allows Efficient CO2 Capture and CO2/N2 Separation

The pursuit of efficient CO₂ capture materials remains an unmet challenge. Especially, meeting both high sorption capacity and fast uptake kinetics is an ongoing effort in the development of CO₂ sorbents. Here, a strategy to exploit liquid-in-aerogel porous composites (LIAPCs) that allow for highly effective CO₂ capture and selective CO₂/N₂ separation, is reported. Interestingly, the functional liquid tetraethylenepentamine (TEPA) is partially filled into the air pockets of SiO₂ aerogel with left permanent porosity. Notably, the confined liquid thickness is 10.9–19.5 nm, which can be vividly probed by the atomic force microscope and rationalized by tailoring the liquid composition and amount. LIAPCs achieve high affinity between the functional liquid and solid porous counterpart, good structure integrity, and robust thermal stability. LIAPCs exhibit superb CO₂ uptake capacity (5.44 mmol g⁻¹, 75 °C, and 15 vol% CO₂), fast sorption kinetics, and high amine efficiency. Furthermore, LIAPCs ensure long-term adsorption–desorption cycle stability and offer exceptional CO₂/N₂ selectivity both in dry and humid conditions, with a separation factor up to 1182.68 at a humidity of 1%. This approach offers the prospect of efficient CO₂ capture and gas separation, shedding light on new possibilities to make the next-generation sorption materials for CO₂ utilization.     

Porous liquids as solvents for the economical separation of carbon dioxide from methane

Current CO₂ separation technologies by liquid solvents suffer from either high regeneration costs or low selectivity/capacity of the solvent. Dispersing ca. 12.5 wt% CO₂-selective zeolite rho solid into the commercial CO₂ capture solvent Genosorb® converts it into a porous liquid with at least 2.5 times greater CO₂ capacity and CO₂/CH₄ selectivity compared to Genosorb® itself. This is predicted to result in more economical separation processes, particularly for biogas upgrading.     

2D Transition Metal Carbides (MXenes) for Carbon Capture

Global warming caused by burning of fossil fuels is indisputably one of mankind's greatest challenges in the 21st century. To reduce the ever‐increasing CO₂ emissions released into the atmosphere, dry solid adsorbents with large surface‐to‐volume ratio such as carbonaceous materials, zeolites, and metal–organic frameworks have emerged as promising material candidates for capturing CO₂. However, challenges remain because of limited CO₂/N₂ selectivity and long‐term stability. The effective adsorption of CO₂ gas (≈12 mol kg^?1) on individual sheets of 2D transition metal carbides (referred to as MXenes) is reported here. It is shown that exposure to N₂ gas results in no adsorption, consistent with first‐principles calculations. The adsorption efficiency combined with the CO₂/N₂ selectivity, together with a chemical and thermal stability, identifies the archetype Ti₃C₂ MXene as a new material for carbon capture (CC) applications.     

Low-temperature CO2 capture technologies – Applications and potential

CO₂ capture by chemical or physical sorption and membrane separation have been the dominant fields of research within post- and pre-combustion CO₂ capture from power cycles and industrial processes. Except for oxy-combustion capture applications, limited attention has been given to low-temperature capture from flue gas and synthesis gas by phase separation. This paper gives an overview of common CO₂ capture conditions for a broad range of different power cycles and industrial processes. For a selected range of capture conditions, potential applications for low-temperature CO₂ capture have been evaluated with respect to energy consumption and CO₂ capture ratio. For all applications of low-temperature capture, specific power consumption and obtainable CO₂ capture ratio are sensitive to flue-gas or synthesis-gas feed CO₂ concentration. However, for certain applications such as synthesis gas from coal gasification, low-temperature capture shows promising potential and highly competitive energy figures compared to baseline technology.     

Li–CO2 and Na–CO2 Batteries: Toward Greener and Sustainable Electrical Energy Storage

Metal–CO₂ batteries, especially Li–CO₂ and Na–CO₂ batteries, offer a novel and attractive strategy for CO₂ capture as well as energy conversion and storage with high specific energy densities. However, some scientific issues and challenges existing restrict their practical applications. Here, recent progress of crucial reaction mechanisms on cathodes in Li–CO₂ and Na–CO₂ batteries are summarized. The detailed reaction pathways can be modified by operation conditions, electrolyte compositions, and catalysts. Besides, specific discussions from aspects of catalyst design, stability of electrolytes, and anode protection are presented. Perspectives of several innovative directions are also put forward. This review provides an intensive understanding of Li–CO₂ and Na–CO₂ batteries and gives a useful guideline for the practical development of metal–CO₂ batteries and even metal–air batteries.     

Covalent Organic Frameworks for CO2 Capture

As an emerging class of porous crystalline materials, covalent organic frameworks (COFs) are excellent candidates for various applications. In particular, they can serve as ideal platforms for capturing CO₂ to mitigate the dilemma caused by the greenhouse effect. Recent research achievements using COFs for CO₂ capture are highlighted. A background overview is provided, consisting of a brief statement on the current CO₂ issue, a summary of representative materials utilized for CO₂ capture, and an introduction to COFs. Research progresses on: i) experimental CO₂ capture using different COFs synthesized based on different covalent bond formations, and ii) computational simulation results of such porous materials on CO₂ capture are summarized. Based on these experimental and theoretical studies, careful analyses and discussions in terms of the COF stability, low‐ and high‐pressure CO₂ uptake, CO₂ selectivity, breakthrough performance, and CO₂ capture conditions are provided. Finally, a perspective and conclusion section of COFs for CO₂ capture is presented. Recent advancements in the field are highlighted and the strategies and principals involved are discussed.     

Facile Fabrication of Monodispersed Carbon Sphere: A Pathway Toward Energy-Efficient Direct Air Capture (DAC) Using Amino Acids

Direct removal of carbon dioxide (CO₂) from the atmosphere, known as direct air capture (DAC) is attracting worldwide attention as a negative emission technology to control atmospheric CO₂ concentrations. However, the energy-intensive nature of CO₂ absorption-desorption processes has restricted deployment of DAC operations. Catalytic solvent regeneration is an effective solution to tackle this issue by accelerating CO₂ desorption at lower regeneration temperatures. This work reports a one-step synthesis methodology to prepare monodispersed carbon nanospheres (MCSs) using trisodium citrate as a structure-directing agent with acidic sites. The assembly of citrate groups on the surface of MCSs enables consistent spherical growth morphology, reduces agglomeration and enhances water dispersibility. The functionalization-assisted synthesis produces uniform, hydrophilic nanospheres of 100–600 nm range. This work also demonstrates that the prepared MCSs can be further functionalized with strong Brønsted acid sites, providing high proton donation ability. Furthermore, the materials can be effectively used in a wide range of amino acid solutions to substantially accelerate CO₂ desorption (25.6% for potassium glycinate and 41.1% for potassium lysinate) in the DAC process. Considering the facile synthesis of acidic MCSs and their superior catalytic efficiency, these findings are expected to pave a new path for energy-efficient DAC.     

Optimal Bidding and Operation of a Power Plant with Solvent-Based Carbon Capture under a CO2 Allowance Market: A Solution with a Reinforcement Learning-Based Sarsa Temporal-Difference Algorithm

In this paper, a reinforcement learning (RL)-based Sarsa temporal-difference (TD) algorithm is applied to search for a unified bidding and operation strategy for a coal-fired power plant with monoethanolamine (MEA)-based post-combustion carbon capture under different carbon dioxide (CO₂) allowance market conditions. The objective of the decision maker for the power plant is to maximize the discounted cumulative profit during the power plant lifetime. Two constraints are considered for the objective formulation. Firstly, the tradeoff between the energy-intensive carbon capture and the electricity generation should be made under presumed fixed fuel consumption. Secondly, the CO₂ allowances purchased from the CO₂ allowance market should be approximately equal to the quantity of CO₂ emission from power generation. Three case studies are demonstrated thereafter. In the first case, we show the convergence of the Sarsa TD algorithm and find a deterministic optimal bidding and operation strategy. In the second case, compared with the independently designed operation and bidding strategies discussed in most of the relevant literature, the Sarsa TD-based unified bidding and operation strategy with time-varying flexible market-oriented CO₂ capture levels is demonstrated to help the power plant decision maker gain a higher discounted cumulative profit. In the third case, a competitor operating another power plant identical to the preceding plant is considered under the same CO₂ allowance market. The competitor also has carbon capture facilities but applies a different strategy to earn profits. The discounted cumulative profits of the two power plants are then compared, thus exhibiting the competitiveness of the power plant that is using the unified bidding and operation strategy explored by the Sarsa TD algorithm.     

Metal–CO2 Batteries on the Road: CO2 from Contamination Gas to Energy Source

Rechargeable nonaqueous metal–air batteries attract much attention for their high theoretical energy density, especially in the last decade. However, most reported metal–air batteries are actually operated in a pure O₂ atmosphere, while CO₂ and moisture in ambient air can significantly impact the electrochemical performance of metal–O₂ batteries. In the study of CO₂ contamination on metal–O₂ batteries, it has been gradually found that CO₂ can be utilized as the reactant gas alone; namely, metal–CO₂ batteries can work. On the other hand, investigations on CO₂ fixation are in focus due to the potential threat of CO₂ on global climate change, especially for its steadily increasing concentration in the atmosphere. The exploitation of CO₂ in energy storage systems represents an alternative approach towards clean recycling and utilization of CO₂. Here, the aim is to provide a timely summary of recent achievements in metal–CO₂ batteries, and inspire new ideas for new energy storage systems. Moreover, critical issues associated with reaction mechanisms and potential directions for future studies are discussed.     

Valuing Metal–Organic Frameworks for Postcombustion Carbon Capture: A Benchmark Study for Evaluating Physical Adsorbents

The development of practical solutions for the energy‐efficient capture of carbon dioxide is of prime importance and continues to attract intensive research interest. Conceivably, the implementation of adsorption‐based processes using different cycling modes, e.g., pressure‐swing adsorption or temperature‐swing adsorption, offers great prospects to address this challenge. Practically, the successful deployment of practical adsorption‐based technologies depends on the development of made‐to‐order adsorbents expressing mutually two compulsory requisites: i) high selectivity/affinity for CO₂ and ii) excellent chemical stability in the presence of impurities. This study presents a new comprehensive experimental protocol apposite for assessing the prospects of a given physical adsorbent for carbon capture under flue gas stream conditions. The protocol permits: i) the baseline performance of commercial adsorbents such as zeolite 13X, activated carbon versus liquid amine scrubbing to be ascertained, and ii) a standardized evaluation of the best reported metal–organic framework (MOF) materials for carbon dioxide capture from flue gas to be undertaken. This extensive study corroborates the exceptional CO₂ capture performance of the recently isolated second‐generation fluorinated MOF material, NbOFFIVE ‐1‐Ni, concomitant with an impressive chemical stability and a low energy for regeneration. Essentially, the NbOFFIVE ‐1‐Ni adsorbent presents the best compromise by satisfying all the required metrics for efficient CO₂ scrubbing.     

Cement and carbon emissions

Because of its low cost, its ease of use and relative robustness to misuse, its versatility, and its local availability, concrete is by far the most widely used building material in the world today. Intrinsically, concrete has a very low energy and carbon footprint compared to most other materials. However, the volume of Portland cement required for concrete construction makes the cement industry a large emitter of CO₂. The International Energy Agency recently proposed a global CO₂ reduction plan. This plan has three main elements: long term CO₂ targets, a sectorial approach based on the lowest cost to society, and technology roadmaps that demonstrate the means to achieve the CO₂ reductions. For the cement industry, this plan calls for a reduction in CO₂ emissions from 2 Gt in 2007 to 1.55 Gt in 2050, while over the same period cement production is projected to increase by about 50 %. The authors of the cement industry roadmap point out that the extrapolation of existing technologies (fuel efficiency, alternative fuels and biomass, and clinker substitution) will only take us half the way towards these goals. According to the roadmap, the industry will have to rely on costly and unproven carbon capture and storage technologies for the other half of the required reduction. This will result in significant additional costs for society. Most of the CO₂ footprint of cement is due to the decarbonation of limestone during the clinkering process. Designing new clinkers that require less limestone is one means to significantly reduce the CO₂ footprint of cement and concrete. A new class of clinkers described in this paper can reduce CO₂ emissions by 20 to 30 % when compared to the manufacture of traditional PC Clinker.     

Numerical study of hydrogen production by the sorption-enhanced steam methane reforming process with online CO<Subscript>2</Subscript> capture as operated in fluidized bed reactors

A three-dimensional (3D) Eulerian two-fluid model with an in-house code was developed to simulate the gas-particle two-phase flow in the fluidized bed reactors. The CO₂ capture with Ca-based sorbents in the steam methane reforming (SMR) process was studied with such model combined with the reaction kinetics. The sorption-enhanced steam methane reforming (SE-SMR) process, i.e., the integration of the process of SMR and the adsorption of CO₂, was carried out in a bubbling fluidized bed reactor. The very high production of hydrogen in SE-SMR was obtained compared with the standard SMR process. The hydrogen molar fraction in gas phase was near the equilibrium. The breakthrough of the sorbent and the variation of the composition in the breakthrough period were studied. The effects of inlet gas superficial velocity and steam-to-carbon ratio (mass ratio of steam to methane in the inlet gas phase) on the reactions were studied. The simulated results are in agreement with the experimental results presented by Johnsen et al. (2006a, Chem Eng Sci 61:1195–1202).     

Optimizing Sieving Effect for CO2 Capture from Humid Air Using an Adaptive Ultramicroporous Framework

Excessive CO₂ in the air can not only lead to serious climate problems but also cause serious damage to humans in confined spaces. Here, a novel metal–organic framework (FJI-H38) with adaptive ultramicropores and multiple active sites is prepared. It can sieve CO₂ from air with the very high adsorption capacity/selectivity but the lowest adsorption enthalpy among the reported physical adsorbents. Such excellent adsorption performances can be retained even at high humidity. Mechanistic studies show that the polar ultramicropore is very suitable for molecular sieving of CO₂ from N₂, and the distinguishable adsorption sites for H₂O and CO₂ enable them to be co-adsorbed. Notably, the adsorbed-CO₂-driven pore shrinkage can further promote CO₂ capture while the adsorbed-H₂O-induced phase transitions in turn inhibit H₂O adsorption. Moreover, FJI-H38 has excellent stability and recyclability and can be synthesized on a large scale, making it a practical trace CO₂ adsorbent. This will provide a new strategy for developing practical adsorbents for CO₂ capture from the air.     

Rapid Fabrication of High-Permeability Mixed Matrix Membranes at Mild Condition for CO2 Capture

Mixed matrix membranes (MMMs), conjugating the advantages of flexible processing-ability of polymers and high-speed mass transfer of porous fillers, are recognized as the next-generation high-performance CO₂ capture membranes for solving the current global climate challenge. However, controlling the crystallization of porous metal-organic frameworks (MOFs) and thus the close stacking of MOF nanocrystals in the confined polymer matrix is still undoable, which thus cannot fully utilize the superior transport attribute of MOF channels. In this study, the “confined swelling coupled solvent-controlled crystallization” strategy is employed for well-tailoring the in-situ crystallization of MOF nanocrystals, realizing rapid (<5 min) construction of defect-free freeway channels for CO₂ transportation in MMMs due to the close stacking of MOF nanocrystals. Consequently, the fabricated MMMs exhibit approximately fourfold enhancement in CO₂ permeability, i.e., 2490 Barrer with a CO₂/N₂ selectivity of 37, distinctive antiplasticization merit, as well as long-term running stability, which is at top-tier level, enabling the large-scale manufacture of high-performance MMMs for gas separation.     

Metal‐Organic Frameworks for CO2 Chemical Transformations

Carbon dioxide (CO₂), as the primary greenhouse gas in the atmosphere, triggers a series of environmental and energy related problems in the world. Therefore, there is an urgent need to develop multiple methods to capture and convert CO₂ into useful chemical products, which can significantly improve the environment and promote sustainable development. Over the past several decades, metal‐organic frameworks (MOFs) have shown outstanding heterogeneous catalytic activity due in part to their high internal surface area and chemical functionalities. These properties and the ability to synthesize MOF platforms allow experiments to test structure‐function relationships for transforming CO₂ into useful chemicals. Herein, recent developments are highlighted for MOFs participating as catalysts for the chemical fixation and photochemical reduction of CO₂. Finally, opportunities and challenges facing MOF catalysts are discussed in this ongoing research area.