Carbon dioxide capture in gallate-based metal-organic frameworks

Eco-friendly one-pot hydrothermal synthesis of cyclodextrin metal-organic frameworks for enhanced CO2 capture

Polysaccharide-based metal-organic frameworks have attracted widespread attention due to their combination of the biocompatibility and flexibility of polysaccharides. Cyclodextrin are interesting bio-ligands in the construction of polysaccharide-based MOFs. Conventional methods for preparing cyclodextrin metal-organic frameworks (CD-MOFs) are often time-consuming and inefficient. In this study, cost-effective and environmentally friendly α- and β-CD-MOFs were successfully synthesized using a hydrothermal method, with optimized incubation time and solvent ratios. The materials were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and N₂ adsorption/desorption measurements. The CO₂ adsorption mechanism was also examined using Fourier transform infrared spectroscopy (FTIR). The results demonstrated excellent thermal and cycling stability of the materials. The CO₂ uptake capacities of α- and β-CD MOF-K were 10.8 and 11.2 cm3/g, respectively. Additionally, the CD-MOFs showed strong selectivity for CO₂ over N₂. Given the straightforward operational procedures, safety characteristics, and mild reaction conditions of CD-MOFs, it is reasonable to conclude that they are promising candidates for use as CO₂ adsorption materials.

The First Kr-Selective Carbon Molecular Sieve for Inverse Adsorption of Krypton Over Xenon at Ambient Temperature

The efficient adsorption-based separation of krypton (Kr) and xenon (Xe) is of paramount importance but is challenged by their similar physicochemical properties. While carbon adsorbents are theoretically promising for Kr/Xe sieving, practical success has remained elusive. Here, a series of ultramicroporous carbon molecular sieves synthesized from sucrose-derived hydrochar is reported. The study employs careful characterization and controlled thermal pyrolysis to tailor ultramicropore formation and elucidate the evolution of the carbon framework. The leading material, C-Suc-750, has an ideal pore size of ≈4.0 Å. In particular, C-Suc-750 has achieved a remarkable Kr/Xe uptake ratio of 39.3 at ambient conditions, setting a new benchmark for selective Kr adsorption and molecular sieving of Kr/Xe. Breakthrough experiments further confirm the superior molecular sieving performance of C-Suc-750, highlighting its potential for Kr recovery in nuclear waste treatment. Moreover, molecular dynamics (MD) simulations demonstrate the critical role of narrow slit-pore of the carbon molecular sieve in molecular sieving separation of Kr/Xe, providing insights into the mechanism driving this selectivity.

Planar Group Functionalization of Quasi-Discrete Pores in Metal-Organic Frameworks for Enhanced Isomeric Separation in Simulated Moving Bed Processes

The efficient separation of 4-methyl-1-pentene (4MP1) from its structural isomers is crucial for industrial applications but remains challenging due to the similar physicochemical properties of these compounds. This study introduces a novel strategy using metal-organic frameworks (MOFs), specifically an engineered variant of ZIF-108, which demonstrates remarkable improvements in the thermodynamic and kinetic properties for 4MP1 separation. By substituting the methyl groups in ZIF-8 with planar nitro groups, we achieved a strategic resizing of the pore windows and cavity dimensions in ZIF-108. This adjustment not only enhanced the molecular affinity and selectivity toward 4MP1 but also facilitated a diffusion rate that is 164 times faster than that observed in ZIF-8. These properties significantly elevated the performance of ZIF-108 in simulated moving bed (SMB) processes, achieving up to 96.5% recovery of high-purity 4MP1, outperforming traditional adsorbents. Comprehensive characterization, including density functional theory (DFT) calculations and molecular dynamics (MD) simulations, provided insights into the interactions and the stability of the adsorption process. The findings suggest that the strategic modification of the pore architecture in MOFs holds significant potential for optimizing the separation processes of industrially relevant mixtures.

Interaction-selective molecular sieving adsorbent for direct separation of ethylene from senary C2-C4 olefin/paraffin mixture

Olefin/paraffin separations are among the most energy-intensive processes in the petrochemical industry, with ethylene being the most widely consumed chemical feedstock. Adsorptive separation utilizing molecular sieving adsorbents can optimize energy efficiency, whereas the size-exclusive mechanism alone cannot achieve multiple olefin/paraffin sieving in a single adsorbent. Herein, an unprecedented sieving adsorbent, BFFOUR-Cu-dpds (BFFOUR = BF4-, dpds = 4,4'-bipyridinedisulfide), is reported for simultaneous sieving of C2-C4 olefins from their corresponding paraffins. The interlayer spaces can be selectively opened through stronger guest-host interactions induced by unsaturated C = C bonds in olefins, as opposed to saturated paraffins. In equimolar six-component breakthrough experiments (C2H4/C2H6/C3H6/C3H8/n-C4H8/n-C4H10), BFFOUR-Cu-dpds can simultaneously divide olefins from paraffins in the first column, while high-purity ethylene ( > 99.99%) can be directly obtained through the subsequent column using granular porous carbons. Moreover, gas-loaded single-crystal analysis, in-situ infrared spectroscopy measurements, and computational simulations demonstrate the accommodation patterns, interaction bonds, and energy pathways for olefin/paraffin separations.

Quasi-Discrete Pore Engineering via Ligand Racemization in Metal-Organic Frameworks for Thermodynamic-Kinetic Synergistic Separation of Propylene and Propane

The adsorptive separation of propylene and propane offers an energy-efficient alternative to the conventional cryogenic distillation technology. However, developing porous adsorbents with both high equilibrium and kinetic selectivity remains extremely challenging due to the similar size and physical properties of these gases. Herein, this work reports a ligand racemization strategy to construct quasi-discrete pores in MOFs for a synergistically enhanced thermodynamic and kinetic separation performance. The use of enantiopure l-malic acid versus racemic dl-malic acid as ligands afforded isoreticular Ni-based MOFs with contrasting one-dimensional channels (l-mal-MOF) and quasi-discrete cavities connected by small windows (dl-mal-MOF). The periodic pore constrictions in dl-mal-MOF significantly increased the differentiation in diffusion rates and binding energies between propylene and propane. dl-mal-MOF exhibited an exceptional propylene uptake of 1.82 mmol/g at 0.05 bar and 298 K along with an ultrahigh equilibrium-kinetic combined selectivity of 62.6. DFT calculations and MD simulations provided insights into the synergistic mechanism of preferential propylene adsorption and diffusion. Breakthrough column experiments demonstrated the excellent separation and high-purity recovery of propylene over propane on dl-mal-MOF. The robust stability and facile regeneration highlight its potential for propylene purification applications.

Exquisitely Constructing a Robust MOF with Dual Pore Sizes for Efficient CO2 Capture

Developing metal-organic framework (MOF) adsorbents with excellent performance and robust stability is of critical importance to reduce CO2 emissions yet challenging. Herein, a robust ultra-microporous MOF, Cu(bpfb)(bdc), with mixed ligands of N, N'-(1,4-phenylene)diisonicotinamide (bpfb), and 1,4-dicarboxybenzene (bdc) was delicately constructed. Structurally, this material possesses double-interpenetrated frameworks formed by two staggered, independent frameworks, resulting in two types of narrow ultra-micropores of 3.4 × 5.0 and 4.2 × 12.8 Å2, respectively. The above structural properties make its highly selective separation at 273~298 K with a CO2 capacity of 71.0~86.2 mg/g. Its adsorption heat over CO2 and IAST selectivity were calculated to be 27 kJ/mol and 52.2, respectively. Remarkably, cyclic breakthrough experiments corroborate its impressive performance in CO2/N2 separation in not only dry but also 75% RH humid conditions. Molecular simulation reveals that C-H···OCO2 in the pores plays a pivotal role in the high selectivity of CO2 adsorption. These results point out the huge potential application of this material for CO2/N2 separation.

Machine learning implemented exploration of the adsorption mechanism of carbon dioxide onto porous carbons

Adsorption of CO₂ on porous carbons has been identified as one of the promising methods for carbon capture, which is essential for meeting the sustainable developmental goal (SDG) with respect to climate action, i.e., SDG 13. This research implemented six supervised machine learning (ML) models (gradient boosting decision tree (GBDT), extreme gradient boosting (XGB), light boost gradient machine (LBGM), random forest (RF), categorical boosting (Catboost), and adaptive boosting (Adaboost)) to understand and predict the CO₂ adsorption mechanism and adsorption uptake, respectively. The results recommended that the GBDT outperformed the remaining five ML models for CO₂ adsorption. However, XGB, LBGM, RF, and Catboost also represented the prediction in the acceptable range. The GBDT model indicated the accurate prediction of CO₂ uptake onto the porous carbons considering adsorbent properties and adsorption conditions as model input parameters. Next, two-factor partial dependence plots revealed a lucid explanation of how the combinations of two input features affect the model prediction. Furthermore, SHapley Additive exPlainations (SHAP), a novel explication approach based on ML models, were employed to understand and elucidate the CO₂ adsorption and model prediction. The SHAP explanations, implemented on the GBDT model, revealed the rigorous relationships among the input features and output variables based on the GBDT prediction. Additionally, SHAP provided clear-cut feature importance analysis and individual feature impact on the prediction. SHAP also explained two instances of CO₂ adsorption. Along with the data-driven insightful explanation of CO₂ adsorption onto porous carbons, this study also provides a promising method to predict the clear-cut performance of porous carbons for CO₂ adsorption without performing any experiments and open new avenues for researchers to implement this study in the field of adsorption because a lot of data is being generated.

Strategic Fast Induction Heating to Combat Hysteresis Barriers in a Flexible MOF for Rapid CO2 Desorption in Biogas Upgrading

A major challenge in Cyclic Swing Separation using flexible adsorbents that have high equilibrium CO₂  adsorption capacity is their very low-pressure hysteresis that hinders efficient desorption. Mg-Gallate MOF is such a flexible adsorbent that only begins to release CO₂ at its pore closing pressure at 0.08 bar and 30 °C, showing very slow and inefficient desorption in pressure or temperature swing. Therefore, a novel strategy is presented that combines state of art technique Magnetic Induction Heating with a vacuum swing for fast and efficient CO₂ desorption from flexible adsorbents at a moderately elevated temperature (70 °C).

Customizing Metal-Organic Frameworks by Lego-Brick Strategy for One-Step Purification of Ethylene from a Quaternary Gas Mixture

One-step purification of ethylene (C₂ H₄ ) from a quaternary gas mixture of C₂ H₆ /C₂ H₄ /C₂ H₂ /CO₂ by adsorption is a promising separation process, yet developing adsorbents that synergistically capture various gas impurities remains challenging. Herein, a Lego-brick strategy is proposed to customize pore chemistry in a unified framework material. The ethane-selective MOF platform is further modified with customized binding sites to specifically adsorb acetylene and carbon dioxide, thus one-step purification of C₂ H₄ with high productivity of polymer-grade product (134 mol kg^-1 ) is achieved on the assembly of porous coordination polymer-2,5-furandicarboxylic acid (PCP-FDCA) and PCP-5-aminoisophthalic acid (IPA-NH₂ ). Computational studies verify that the low-polarity surface of this MOFs-based platform provides a delicate environment for C₂ H₆ recognition, and the specific binding sites (FDCA and IPA-NH₂ ) exhibit favorable trapping of C₂ H₂ and CO₂ via CH^δ+ ···O^δ- and C^δ+ ···N^δ- electrostatic interactions, respectively. The proposed Lego-brick strategy to customize binding sites within the MOFs structure provides new ideas for the design of adsorbents for compounded separation tasks.

Ideal Adsorbed Solution Theory (IAST) of Carbon Dioxide and Methane Adsorption Using Magnesium Gallate Metal-Organic Framework (Mg-gallate)

Ideal Adsorbed Solution Theory (IAST) is a predictive model that does not require any mixture data. In gas purification and separation processes, IAST is used to predict multicomponent adsorption equilibrium and selectivity based solely on experimental single-component adsorption isotherms. In this work, the mixed gas adsorption isotherms were predicted using IAST calculations with the Python package (pyIAST). The experimental CO2 and CH4 single-component adsorption isotherms of Mg-gallate were first fitted to isotherm models in which the experimental data best fit the Langmuir model. The presence of CH4 in the gas mixture contributed to a lower predicted amount of adsorbed CO2 due to the competitive adsorption among the different components. Nevertheless, CO2 adsorption was more favorable and resulted in a higher predicted adsorbed amount than CH4. Mg-gallate showed a stronger affinity for CO2 molecules and hence contributed to a higher CO2 adsorption capacity even with the coexistence of a CO2/CH4 mixture. Very high IAST selectivity values for CO2/CH4 were obtained which increased as the gas phase mole fraction of CO2 approached unity. Therefore, IAST calculations suggest that Mg-gallate can act as a potential adsorbent for the separation of CO2/CH4 mixed gas.

Advances in the utilisation of carbon-neutral technologies for a sustainable tomorrow: A critical review and the path forward

Global industrialisation and overexploitation of fossil fuels significantly impact greenhouse gas emissions, resulting in global warming and other environmental problems. Hence, investigations on capturing, storing, and utilising atmospheric CO2 create novel technologies. Few microorganisms, microalgae, and macroalgae utilise atmospheric CO2 for their growth and reduce atmospheric CO2 levels. Activated carbon and biochar from biomasses also capture CO2. Nanomaterials such as metallic oxides, metal-organic frameworks, and MXenes illustrate outstanding adsorption characteristics, and convert CO2 to carbon-neutral fuels, creating a balance between CO2 production and elimination, thus zeroing the carbon footprint. The need for a paradigm shift from fossil fuels and promising technologies on renewable energies, carbon capture mechanisms, and carbon sequestration techniques that help reduce CO2 emissions for a better tomorrow are reviewed to achieve the world's sustainable development goals. The challenges and possible solutions with future perspectives are also discussed.

Conjugated Microporous Poly(aniline) Enabled Hierarchical Porous Carbons for Hg(II) Adsorption

Hierarchical porous carbons equipped with heteroatoms and diffusion pores have a wide application prospect in adsorption. Herein, we report N-autodoped porous carbons (PTPACs), which were derived from rigid N-rich conjugated microporous poly(aniline)s (CMPAs) and show their all-around applicability in heavy metal adsorption. Their molecular structure could be delicately tuned from 3D organic networks to graphitic carbons through simply adjusting the pyrolysis temperature, affording unique hybrid features of hierarchical micro-meso-macroporosity and amount-tunable nitrogen defects, as validated by the enhanced CO₂ adsorption capacities reaching 5.0 mmol g^-1, a 230% increase compared to the precursor (2.15 mmol g^-1). They therefore show promising a Langmuir adsorption capacity of 434.8 mg g^-1 toward mercury ions, which could be rapidly achieved within a short 20 min. Based on the comprehensive experimental, characterization, and DFT calculation studies, we rationally reveal these impressive adsorptions arise from the hybrid function of chemisorption contributed by populated nitrogen defects and physical adsorption achieved by synergistic functions in the diffusion and storage pores. Outcomes mark the high merits of PTPACs in addressing recent global challenges in environmental engineering.

Surface, Interface and Structure Optimization of Metal-Organic Frameworks: Towards Efficient Resourceful Conversion of Industrial Waste Gases

Industrial waste gas emissions from fossil fuel over-exploitation have aroused great attention in modern society. Recently, metal-organic frameworks (MOFs) have been developed in the capture and catalytic conversion of industrial exhaust gases such as SO₂ , H₂ S, NO_x , CO₂ , CO, etc. Based on these resourceful conversion applications, in this review, we summarize the crucial role of the surface, interface, and structure optimization of MOFs for performance enhancement. The main points include (1) adsorption enhancement of target molecules by surface functional modification, (2) promotion of catalytic reaction kinetics through enhanced coupling in interfaces, and (3) adaptive matching of guest molecules by structural and pore size modulation. We expect that this review will provide valuable references and illumination for the design and development of MOF and related materials with excellent exhaust gas treatment performance.

Molecular Sieving of Propylene from Propane in Metal-Organic Framework-Derived Ultramicroporous Carbon Adsorbents

The development of adsorption-based separation processes alternative to the energy-intensive cryogenic distillation for a mixture of propylene and propane remains essential but challenging in gas industries. Molecular sieving separation of C₃H₆/C₃H₈ on stable carbon adsorbents appeals to be promising, while it is quite challenging to realize due to the random distributions and arrangements of the internal pores in common carbons. Herein, a series of polysaccharide-based CD-MOF-derived ultramicroporous carbon adsorbents with their pore size tuned at a subangstrom level were prepared. Molecular sieving separation of C₃H₆/C₃H₈ was realized on the optimal C-CDMOF-2-700 owing to the delicate structure with an appropriate pore size (5.0 Å). Besides, C-CDMOF-2-700 exhibited a high C₃H₆ uptake of 1.97 mmol g^-1 under ambient conditions. An ultrahigh uptake ratio of C₃H₆/C₃H₈ at 1.0 kPa (403) was also achieved, outperforming all reported adsorbents. Kinetic adsorption tests and breakthrough experiments further demonstrate this well-designed carbon adsorbent to be promising in industrial C₃H₆/C₃H₈ separation.