Development Trends in Porous Adsorbents for Carbon Capture

Ca-Based Layered Double Hydroxides for Environmentally Sustainable Carbon Capture

The process of carbon dioxide capture typically requires a large amount of energy for the separation of carbon dioxide from other gases, which has been a major barrier to the widespread deployment of carbon capture technologies. Innovation of carbon dioxide adsorbents is herein vital for the attainment of a sustainable carbon capture process. In this study, we investigated the electrified synthesis and rejuvenation of calcium-based layered double hydroxides (Ca-based LDHs) as solid adsorbents for CO2. We discovered that the particle morphology and phase purity of the LDHs, along with the presence of secondary phases, can be controlled by tuning the current density during electrodeposition on a porous carbon substrate. The change in phase composition during carbonation and calcination was investigated to unveil the effect of different intercalated anions on the surface basicity and thermal stability of Ca-based LDHs. By decoupling the adsorption of water and CO2, we showed that the adsorbed water largely promoted CO2 adsorption, most likely through a sequential dissolution and reaction pathway. A carbon capture capacity of 4.3 ± 0.5 mmol/g was measured at 30 °C and relative humidity of 40% using 10 vol % CO2 in nitrogen as the feed stream. After CO2 capture occurred, the thermal regeneration step was carried out by directly passing an electric current through the conductive carbon substrate, known as the Joule-heating effect. CO2 was found to start desorbing from the Ca-based LDHs at a temperature as low as 220 °C as opposed to the temperature above 700 °C required for calcium carbonate that forms as part of the Ca-looping capture process. Finally, we evaluated the cumulative energy demand and environmental impact of the LDH-based capture process using a life cycle assessment. We identified the most environmentally concerning step in the process and concluded that the postcombustion CO2 capture using LDH could be advantageous compared with existing technologies.

Infrared spectroscopic monitoring of solid-state processes

We put a spotlight on IR spectroscopic investigations in materials science by providing a critical insight into the state of the art, covering both fundamental aspects, examples of its utilisation, and current challenges and perspectives focusing on the solid state.

Design of Graphene/Ionic Liquid Composites for Carbon Capture

Pore size is a crucial factor impacting gas separation in porous separation materials, but how to control the pore size to optimize the separation performance remains a challenge. Here, we propose a design of graphene/ionic liquid composites with tunable slit pore sizes, where cations and anions of ionic liquids are intercalated between graphene layers. By varying the sizes of the ions, we show from first-principles density functional theory calculations that the accessible pore size can be tuned from 3.4 to 6.0 Å. Grand canonical Monte Carlo simulations of gas sorption find that the composite materials possess high CO₂ uptake at room temperature and 1 bar (up to ∼8.5 mmol/g). Further simulations of the sorption of gas mixtures reveal that high CO₂/N₂ and CO₂/CH₄ adsorption selectivities can be obtained when the accessible pore size is <5 Å. This work suggests a new strategy to achieve tunable pore sizes via the graphene/IL composites for highly selective CO₂/N₂ and CO₂/CH₄ adsorption.

Selective capture of carbon dioxide from hydrocarbons using a metal-organic framework

Efficient and sustainable methods for carbon dioxide capture are highly sought after. Mature technologies involve chemical reactions that absorb CO2, but they have many drawbacks. Energy-efficient alternatives may be realised by porous physisorbents with void spaces that are complementary in size and electrostatic potential to molecular CO2. Here, we present a robust, recyclable and inexpensive adsorbent termed MUF-16. This metal-organic framework captures CO2 with a high affinity in its one-dimensional channels, as determined by adsorption isotherms, X-ray crystallography and density-functional theory calculations. Its low affinity for other competing gases delivers high selectivity for the adsorption of CO2 over methane, acetylene, ethylene, ethane, propylene and propane. For equimolar mixtures of CO2/CH4 and CO2/C2H2, the selectivity is 6690 and 510, respectively. Breakthrough gas separations under dynamic conditions benefit from short time lags in the elution of the weakly-adsorbed component to deliver high-purity hydrocarbon products, including pure methane and acetylene.

Numerical Simulation of the Effect of Injected CO2 Temperature and Pressure on CO2-Enhanced Coalbed Methane

The injection of CO2 to displace CH4 in coal seams is an effective method to exploit coalbed methane (CBM), for which the CO2 injection temperature and pressure are important influential factors. We performed simulations, using COMSOL Multiphysics to determine the effect of CO2 injection temperature and pressure on CO2-enhanced coalbed methane (CO2-ECBM) recovery, according to adsorption/desorption, seepage, and diffusion of binary gas (CO2 and CH4) in the coal seam, and deriver a thermal–hydraulic–mechanical coupling equation of CO2-ECBM. The simulation results show that, as CO2 injection pressure in CO2-ECBM increases, the molar concentration and displacement time of CH4 in the coal seam significantly decrease. With increasing injection temperature, the binary gas adsorption capacity in the coal seam decreases, and CO2 reserves and CH4 production decrease. High temperatures are therefore not conducive for CH4 production.

Construction of a hierarchical-structured MgO-carbon nanocomposite from a metal–organic complex for efficient CO2 capture and organic pollutant removal

A hierarchical-structured porous MgO/C nanocomposite derived from a metal–organic complex performs as a remarkable adsorbent for CO₂ adsorption and organic pollutant removal.

Understanding Hysteresis in Carbon Dioxide Sorption in Porous Metal-Organic Frameworks

Two new isostructural microporous coordination frameworks [Mn₃(Hpdc)₂(pdc)₂] (1) and [Mg₃(Hpdc)₂(pdc)₂] (2) (pdc^2- = pyridine-2,4-dicarboxylate) showing primitive cubic (pcu) topology have been prepared and characterized. The pore aperture of the channels is too narrow for the efficient adsorption of N₂; however, both compounds demonstrate substantially higher uptake of CO₂ (119.9 mL·g^-1 for 1 and 102.5 mL·g^-1 for 2 at 195 K, 1 bar). Despite of their structural similarities, 2 shows a typical reversible type I isotherm for adsorption/desorption of CO₂, while 1 features a two-step adsorption process with a very broad hysteresis between the adsorption and desorption curves. This behavior can be explained by a combination of density functional theory calculations, sorption, and X-ray diffraction analysis and gives insights into the further development of new sorbents showing adsorption/desorption hysteresis.

Ratiometric Monitoring of Thorium Contamination in Natural Water Using a Dual-Emission Luminescent Europium Organic Framework

Traditional analytical methods of thorium contamination suffer from several disadvantages such as time-consuming procedures and an equipment-intensive nature, leading to substantial challenges in rapid and on-site monitoring of thorium concentrations in complex natural water systems. We report here the first case of a luminescent metal organic framework based probe (ThP-1) for highly sensitive and selective self-calibrated sensing of Th⁴⁺ contamination in natural fresh water media with a notably facilitated detection procedure. The detection limit of ThP-1 was determined to be 24.2 μg/L, much lower than the thorium contamination standard of 246 μg/L in drinking water defined by the World Health Organization. Importantly, the detection procedure based on the rarely reported self-calibration manner is greatly beneficial in improving the detection accuracy. The self-calibrated luminescence evolution process shows a great anti-interference ability capable of detecting thorium contamination in a wide concentration range from 24.2 μg/L to 300 mg/L, which can not be achieved directly by the traditional methods. The Th⁴⁺-selective luminescence response originates from the selective uptake and efficient enrichment of Th⁴⁺ by the host framework of ThP-1 through inner-sphere coordination, which is further confirmed by batch experiments, X-ray absorption spectroscopic study, and DFT calculations.

Developmental trends in CO2 methanation using various catalysts

Co₂ methanation-two edged sword to counter global warming and energy crisis.

Charge, adsorption, water stability and bandgap tuning of an anionic Cd(ii) porphyrinic metal–organic framework

By changing the occupancies of the metal ions and counterions, the tuning of the framework charge, band-edge position and bandgap of a novel Cd(ii) porphyrinic MOF 1 was achieved.

Structure stability of HKUST-1 towards water and ethanol and their effect on its CO2 capture properties

Water and ethanol stabilities of the crystal structure of the Cu-based metal-organic framework (MOF) HKUST-1 have been investigated. Vapour (water and ethanol) sorption isotherms and cyclability were measured by a dynamic strategy. The ethanol sorption capacity of HKUST-1 at 303 K remained unchanged contrasting water sorption (which decreased along with the sorption experiment time). Considering the binding energy of each sorbate with the open Cu(ii) sites, obtained by the use of diffusion coefficients, we showed the superior crystal stability of the HKUST-1 framework towards ethanol. Finally, a small quantity of ethanol (pre-adsorbed) slightly enhanced CO₂ capture without crystal structure degradation.

Molecular properties of metal difluorides and their interactions with CO2 and H2O molecules: a DFT investigation

A computational study of metal difluorides (MF₂; M = Ca to Zn) and their interactions with carbon dioxide and water molecules was performed. The structural parameter values obtained and the results of AIM analysis and energy decomposition analysis indicated that the Ca-F bond is weaker and less ionic than the bonds in the transition metal difluorides. A deformation density plot revealed the stablizing influence of the Jahn-Teller effect in nonlinear MF₂ molecules (e.g., where M= Sc, Ti, Cr). An anaysis of the metal K-edge peaks of the difluorides showed that shifts in the edge energy were due to the combined effects of the ionicity, effective nuclear charge, and the spin state of the metal. The interactions of CO₂ with ScF₂ (Scc3 geometry) and TiF₂ (Tic2 geometry) caused CO₂ to shift from its usual linear geometry to a bent geometry (η²(C=O) binding mode), while it retained its linear geometry (η¹(O) binding mode) when it interacted with the other metal difluorides. Energy decomposition analysis showed that, among the various geometries considered, the Scc3 and Tic2 geometries possessed the highest interaction energies and orbital interaction energies. Heavier transition metal difluorides showed stronger affinities for H₂O, whereas the lighter transition metal (Sc and Ti) difluorides preferred CO₂. Overall, the results of this study suggest that fluorides of lighter transition metals with partially filled d orbitals (e.g., Sc and Ti) could be used for CO₂ capture under moist conditions. Graphical abstract Interaction of metal difluorides with carbon dioxide and water.

Hierarchical Nanocomposite by the Integration of Reduced Graphene Oxide and Amorphous Carbon with Ultrafine MgO Nanocrystallites for Enhanced CO2 Capture

Exploring efficient and low-cost solid sorbents is essential for carbon capture and storage. Herein, a novel class of high-performance CO₂ adsorbent (rGO@MgO/C) is engineered based on the controllable integration of reduced graphene oxide (rGO), amorphous carbon, and MgO nanocrystallites. The optimized rGO@MgO/C nanocomposite exhibits remarkable CO₂ capture capacity (up to 31.5 wt % at 27 °C, 1 bar CO₂, and 22.5 wt % under the simulated flue gas), fast sorption rate, and strong process durability. The enhanced capture capability of CO₂ is the best among all of the MgO-based sorbents reported so far. The high performance of rGO@MgO/C nanocomposite can be ascribed to the hierarchical architecture and special physicochemical features, including the sheet-on-sheet sandwich-like structure, ultrathin nanosheets with abundant nanopores, large surface area, and highly dispersed ultrafine MgO nanocrystallites (ca. 3 nm in size), together with the rGO sheets and in situ generated amorphous carbon that serve as a dual carbon support and protectant system with which to prevent MgO nanocrystallites from agglomeration. In addition, the CO₂-uptake capacity at intermediate temperature (e.g., 350 °C) can be further improved threefold through alkali metal salt promotion treatment. This work provides a facile and effective strategy with which to engineer advanced graphene-based functional nanocomposites with rationally designed compositions and architectures for potential applications in the field of gas storage and separation.

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.

Synthetic Architecture of MgO/C Nanocomposite from Hierarchical-Structured Coordination Polymer toward Enhanced CO2 Capture

Highly efficient, durable, and earth-abundant solid sorbents are of paramount importance for practical carbon capture, storage, and utilization. Here, we report a novel and facile two-step strategy to synthesize a group of hierarchically structured porous MgO/C nanocomposites using flowerlike Mg-containing coordination polymer as a precursor. The new nanocomposites exhibit superb CO₂ capture performance with sorption capacity up to 30.9 wt % (at 27 °C, 1 bar CO₂), fast sorption kinetics, and long cycling life. Importantly, the achieved capacity is >14 times higher than that of commercial MgO, and favorably exceeds the highest value recorded to date for MgO-based sorbents under similar operating conditions. On the basis of the morphological and textural property analysis, together with CO₂ sorption mechanism study using CO₂-TPD and DRIFT techniques, the outstanding performance in CO₂ uptake originates from unique features of this type of sorbent materials, which include hierarchical architecture, porous building blocks of nanosheets, high specific surface area (ca. 300 m²/g), evenly dispersed MgO nanocrystallites (ca. 3 nm) providing abundant active sites, and the in situ generated carbon matrix that acts as a stabilizer to prevent the growth and agglomeration of MgO crystallites. The nanocomposite system developed in this work shows good potential for future low-cost CO₂ abatement and utilization.

Water-based synthesis of zeolitic imidazolate framework-8 for CO2 capture

ZIF-8 synthesized in aqueous solution has a large surface area and high CO₂ adsorption capacity.

Emerging Multifunctional Metal-Organic Framework Materials

Metal-organic frameworks (MOFs), also known as coordination polymers, represent an interesting type of solid crystalline materials that can be straightforwardly self-assembled through the coordination of metal ions/clusters with organic linkers. Owing to the modular nature and mild conditions of MOF synthesis, the porosities of MOF materials can be systematically tuned by judicious selection of molecular building blocks, and a variety of functional sites/groups can be introduced into metal ions/clusters, organic linkers, or pore spaces through pre-designing or post-synthetic approaches. These unique advantages enable MOFs to be used as a highly versatile and tunable platform for exploring multifunctional MOF materials. Here, the bright potential of MOF materials as emerging multifunctional materials is highlighted in some of the most important applications for gas storage and separation, optical, electric and magnetic materials, chemical sensing, catalysis, and biomedicine.

Selective Adsorption and Selective Transport Diffusion of CO2-CH4 Binary Mixture in Coal Ultramicropores

The adsorption and diffusion of the CO2-CH4 mixture in coal and the underlying mechanisms significantly affect the design and operation of any CO2-enhanced coal-bed methane recovery (CO2-ECBM) project. In this study, bituminous coal was fabricated based on the Wiser molecular model and its ultramicroporous parameters were evaluated; molecular simulations were established through Grand Canonical Monte Carlo (GCMC) and Molecular Dynamic (MD) methods to study the effects of temperature, pressure, and species bulk mole fraction on the adsorption isotherms, adsorption selectivity, three distinct diffusion coefficients, and diffusivity selectivity of the binary mixture in the coal ultramicropores. It turns out that the absolute adsorption amount of each species in the mixture decreases as temperature increases, but increases as its own bulk mole fraction increases. The self-, corrected, and transport diffusion coefficients of pure CO2 and pure CH4 all increase as temperature or/and their own bulk mole fractions increase. Compared to CH4, the adsorption and diffusion of CO2 are preferential in the coal ultramicropores. Adsorption selectivity and diffusivity selectivity were simultaneously employed to reveal that the optimal injection depth for CO2-ECBM is 800-1000 m at 308-323 K temperature and 8.0-10.0 MPa.

Covalent Triazine-Based Frameworks with Ultramicropores and High Nitrogen Contents for Highly Selective CO2 Capture

Porous organic frameworks (POFs) are a class of porous materials composed of organic precursors linked by covalent bonds. The objective of this work is to develop POFs with both ultramicropores and high nitrogen contents for CO2 capture. Specifically, two covalent triazine-based frameworks (CTFs) with ultramicropores (pores of width <7 Å) based on short (fumaronitrile, FUM) and wide monomers (1,4-dicyanonaphthalene, DCN) were synthesized. The obtained CTF-FUM and CTF-DCN possess excellent chemical and thermal stability with ultramicropores of 5.2 and 5.4 Å, respectively. In addition, they exhibit excellent ability to selectively capture CO2 due to ultramicroporous nature. Especially, CTF-FUM-350 has the highest nitrogen content (27.64%) and thus the highest CO2 adsorption capacity (57.2 cc/g at 298 K) and selectivities for CO2 over N2 and CH4 (102.4 and 20.5 at 298 K, respectively) among all CTF-FUM and CTF-DCN. More impressively, as far as we know, the CO2/CH4 selectivity is larger than that of all reported CTFs and ranks in top 10 among all reported POFs. Dynamic breakthrough curves indicate that both CTFs could indeed separate gas mixtures of CO2/N2 and CO2/CH4 completely.

Enhanced selective adsorption of CO2 on nitrogen-doped porous carbon monoliths derived from IRMOF-3

The N-doped porous carbon monoliths prepared by direct carbonization of IRMOF-3, through an in situ activation and self-templating process, were found to exhibit significantly enhanced performance for the selective adsorption of CO₂ compared to pristine IRMOF-3.