Adsorption Interface-Induced H...F Charge Transfer in Ultramicroporous Metal–Organic Frameworks for Perfluorinated Gas Separation

Self-adaptive Coordination Evolution Mediated Pore-Space-Partition in Metal-Organic Frameworks for Boosting SF6/N2 Separation

The controllable and precise structural regulation of metal-organic frameworks (MOFs) based on isoreticular chemistry is an effective strategy for creating functional material platforms, such as efficient porous adsorbents. Herein, for the first time, mediated by an unprecedented self-adaptive coordination evolution (SACE) on pseudo-D2h-symmetric [M4(μ3-O)2(COO)6] (M=Mn/Fe) clusters, two pore space partitioned MOFs (CTGU-47-Mn/Fe, CTGU=China Three Gorges University) have been successfully constructed. Owing to the more confined adsorption space and dense binding sites produced by pore space partitioning (PSP), the CTGU-47-Mn/Fe exhibit significantly enhanced performance in the capture or recovery SF6 (greenhouse/electronic specialty gas) from SF6/N2 mixture compared to their non-partitioned homologous structures (CTGU-46-Mn/Fe) with adsorption selectivity increased from 37/72 to 634/157 (v/v, 10/90, 100 kPa). The theoretical calculations also elucidated that the implementation of PSP within CTGU-47-Mn/Fe leads to dramatically strengthened binding affinity for SF6 over N2 through extra multiple F⋅⋅⋅H interactions. This study represents a valuable advance in crystal engineering field: the SACE of polynuclear metal clusters is expected to be useful in the structural regulation of MOFs and the fabrication of advanced porous adsorbents.

Molecular Mechanism Behind the Capture of Fluorinated Gases by Metal-Organic Frameworks

Fluorinated gases (F-gases) play a vital role in the chemical industry and in the fields of air conditioning, refrigeration, health care, and organic synthesis. However, the direct emission of waste gases containing F-gases into the atmosphere contributes to greenhouse effects and generates toxic substances. Developing porous materials for the energy-efficient capture, separation, and recovery of F-gases is highly desired. Recently, as a highly designable porous adsorbents, metal-organic frameworks (MOFs) exhibit excellent selective sorption performance toward F-gases, especially for the recognition and separation of different F-gases with highly similar properties, showing their great potential in F-gases control and recovery. In this review, we discuss the capture and separation of F-gases and their azeotropic, near-azeotropic, and isomeric mixtures in various application scenarios by MOFs, specifically classify and analyze molecular interaction between F-gases and MOFs, and interpret the mechanisms underlying their high performance regarding both adsorption capacity and selectivity, providing a repertoire for future materials design. Challenges faced in the transformation research roadmap of MOFs adsorbent separation technologies toward F-gases are also discussed, and areas for future research endeavors are highlighted.

Metal-Organic Framework-Based NF3 Nanotrap for the Separation of NF3 and CF4

As an electronic specialty gas, nitrogen trifluoride (NF3) has been widely used in the semiconductor, photovoltaic, and display industries. However, NF3 in industrial settings is typically mixed with CF4, and the distillation method commonly used for their separation consumes significant energy. Herein, we propose a novel NF3 nanoadsorber featuring relatively proximal unsaturated metal sites. The potential field overlap between the two unsaturated metal centers creates an NF3-affinitive environment. The adsorption mechanism of NF3 molecules in the metal-organic frameworks (MOFs) was investigated through density functional theory (DFT) optimization. The results from ideal adsorbed solution theory (IAST) calculations and breakthrough experiments reveal that the MOF-based NF3 nanotrap possesses the highest adsorption capacity and excellent NF3/CF4 selectivity among the physically adsorptive materials reported to date. This work presents a novel solution for the separation and purification of NF3, contributing to the field of gas separation technology.

Application and Challenge of Metal/Covalent Organic Frameworks in Ammonia Sorption and Separation

As both a critical chemical feedstock and an environmental pollutant, the production and utilization of ammonia (NH3) are accompanied by the progress of social civilization. In recent years, research on metal/covalent organic framework materials as NH3 adsorbents has attracted increasing attention due to their high porosity, versatile architecture and tunable functionality. This review was organized to highlight the recent advancement of MOF/COF materials for NH3 sorption, which successively presented the key properties of solid adsorbents and summarized the strategies along with their mechanisms for enhancing NH3 adsorption. In addition, perspectives and outlook regarding the future development of MOF/COF-based NH3 adsorbents were outlined to meet the requirements of practical applications under various condition.

Destructive Adsorption of Nitrogen Trifluoride (NF3) Using M-MOF-74 with Open Metal Sites

Using solid adsorbents for the destructive sorption of nitrogen trifluoride (NF3) presents a potential solution to its dual challenges as a potent greenhouse gas and hazardous compound in microelectronics. In this study, a series of MOFs (M-MOF-74, M = Mg, Co, Ni, Zn) with open metal sites (OMSs) are utilized for NF3 adsorption. By employing single-component adsorption isotherms and the ideal adsorbed solution theory (IAST) selectivity calculations, the adsorption performance of various adsorbents is evaluated. The results indicate that Mg, Co, and Ni-MOF-74 exhibit high adsorption capacities for NF3, while Zn-MOF-74 shows a lower adsorption capacity, likely due to the weaker Lewis acidity of Zn2+. Experimental findings from PXRD and gas adsorption studies indicate structural pore alteration in the MOF-74 series following NF3 gas adsorption. Theoretical computational analyses reveal that the MOF-74 series has a higher adsorption affinity for NF3 compared to N2. This research provides insights into the use of efficient MOF sorbents for the destructive adsorption of NF3.

Pore Environmental Modification by Amino Groups in Robust Microporous MOFs for SF6 Capturing and SF6/N2 Separation

Capturing and separating the greenhouse gas SF6 from nitrogen N2 have significant greenhouse mitigation potential and economic benefits. We used a pore engineering strategy to manipulate the pore environment of the metal-organic framework (MOF) by incorporating organic functional groups (-NH2). This resulted in an enhanced adsorption of SF6 and separation of the SF6/N2 mixture in the MOF. The introduction of amino (-NH2) groups into YTU-29 resulted in a reduction of the Brunauer-Emmett-Teller surface but an increase in interactions with SF6 within the confined pores. Water-stable YTU-29-NH2 showed a significantly higher SF6 uptake (95.5 cm3/g) than YTU-29 (77.4 cm3/g). The results of the breakthrough experiments show that YTU-29-NH2 has a significantly improved separation performance for SF6/N2 mixtures, with a high SF6 capture of 0.88 mmol/g compared to 0.56 mmol/g by YTU-29. This improvement is due to the suitable pore confinement and accessible -NH2 groups on pore surfaces. Considering its excellent regeneration ability and cycling performance, ultrastable YTU-29-NH2 demonstrates great potential for SF6 capturing and SF6/N2 separation.

Construction of Hierarchical Porous UiO-66-Br2@PS/DVB-Packed Columns by High Internal Phase Emulsion Strategy for Enhanced Separation of CF4/N2 and SF6/N2

Recovery and separation of anthropogenic emissions of electronic specialty gases (F-gases, such as CF4 and SF6) from the semiconductor sector are of critical importance. In this work, the hierarchical porous UiO-66-Br2@PS/DVB-packed column was constructed by a high internal phase emulsions strategy. UiO-66-Br2@PS/DVB exhibits a superior selectivity of CF4/N2 (2.67) and SF6/N2 (3.34) predicted by the IAST due to the diffusion limitation in the micropore and the gas-framework affinity. Especially, UiO-66-Br2@PS/DVB showed significant CF4 and SF6 retention and enabled the successful separation of CF4/N2 and SF6/N2 with a resolution of 2.37 and 8.89, respectively, when used as a packed column in gas chromatography. Compared with the Porapak Q column, the HETP of the UiO-66-Br2@PS/DVB-packed column decreased and showed good reproducibility. This research not only offers a convenient method for fabricating a hierarchical porous MOF-packed column but also showcases the prospective utilization of MOFs for the separation of the F-gas/N2 mixture.

Selective adsorption of fluorinated super greenhouse gases within a metal-organic framework with dynamic corrugated ultramicropores

Perfluorocompound (PFC) gases play vital roles in microelectronics processing. Requirements for ultra-high purities traditionally necessitate use of virgin sources and thereby hinder the capture, purification, and reuse of these costly gases. Most importantly, gaseous PFCs are incredibly potent greenhouse gases with atmospheric lifetimes on the order of 103-104 years, and thus any environmental emissions have an outsized and prolonged impact on our climate. The development of sorbents that can capture PFC gases from industrial waste streams has lagged substantially behind the progress made over the last decade in capturing CO2 from both point emission sources and directly from air. Herein, we show that the metal-organic framework Zn(fba) (fba2- = 4,4'-(hexafluoroisopropylidene)bis-benzoate) displays an equilibrium selectivity for CF4 adsorption over N2 that surpasses those of all water-stable sorbents that have been reported for this separation. Selective adsorption of both CHF3 and CH4 over N2 is also evident, demonstrating a general preference for tetrahedral C1 gases. This selectivity is enabled by adsorption within narrow corrugated channels lined with ligand-based aryl rings, a site within this material that has not previously been realized as being accessible to guests. Analyses of adsorption kinetics and X-ray diffraction data are used to characterize sorption and diffusion of small adsorbates within these channels and strongly implicate rotation of the linker aryl rings as a gate that modulates transport of the C1 gases through a crystallite. Multi-component breakthrough measurements demonstrate that Zn(fba) is able to resolve mixtures of CF4 and N2 under flow-through conditions. Taken together, this work illuminates the dynamic structure of Zn(fba), and also points toward general design principles that can enable large CF4 selectivities in sorbents with more favorable kinetic profiles.

High throughput screening of pure silica zeolites for CF4 capture from electronics industry gas

The capture and separation of CF4 from CF4/N2 mixture gas is a crucial issue in the electronics industry, as CF4 is a commonly used etching gas and the ratio of CF4 to N2 directly affects process efficiency. Utilizing high-throughput computational screening techniques and grand canonical Monte Carlo (GCMC) simulations, we comprehensively screened and assessed 247 types of pure silicon zeolite materials to determine their adsorption and separation performance for CF4/N2 mixtures. Based on screening, the relationships between the structural parameters and adsorption and separation properties were meticulously investigated. Four indicators including adsorption selectivity, working capacity, adsorbent performance score (APS), and regenerability (R%) were used to evaluate the performance of adsorbents. Based on the evaluation, we selected the top three best-performing zeolite structures for vacuum swing adsorption (LEV, AWW and ESV) and pressure swing adsorption (AVL, ZON, and ERI) processes respectively. Also, we studied the preferable adsorption sites of CF4 and N2 in the selected zeolite structures through centroid density distributions at the molecule level. We expect the study may provide some valuable guidance for subsequent experimental investigations on adsorption and separation of CF4/N2.

Non-CO2 greenhouse gas separation using advanced porous materials

Global warming has become a growing concern over decades, prompting numerous research endeavours to reduce the carbon dioxide (CO2) emission, the major greenhouse gas (GHG). However, the contribution of other non-CO2 GHGs including methane (CH4), nitrous oxide (N2O), fluorocarbons, perfluorinated gases, etc. should not be overlooked, due to their high global warming potential and environmental hazards. In order to reduce the emission of non-CO2 GHGs, advanced separation technologies with high efficiency and low energy consumption such as adsorptive separation or membrane separation are highly desirable. Advanced porous materials (APMs) including metal-organic frameworks (MOFs), covalent organic frameworks (COFs), hydrogen-bonded organic frameworks (HOFs), porous organic polymers (POPs), etc. have been developed to boost the adsorptive and membrane separation, due to their tunable pore structure and surface functionality. This review summarizes the progress of APM adsorbents and membranes for non-CO2 GHG separation. The material design and fabrication strategies, along with the molecular-level separation mechanisms are discussed. Besides, the state-of-the-art separation performance and challenges of various APM materials towards each type of non-CO2 GHG are analyzed, offering insightful guidance for future research. Moreover, practical industrial challenges and opportunities from the aspect of engineering are also discussed, to facilitate the industrial implementation of APMs for non-CO2 GHG separation.

Recent progresses, challenges and proposals on SF6 emission reduction approaches

The increasing utilization and emission of sulfur hexafluoride (SF6) pose severe threats to the climate and the environment, owing to its potent greenhouse gas properties. In this paper, we comprehensively review the recent progresses of SF6 emission reduction approaches. Currently, the use and emission of SF6 are still on the rise, and mainly concentrated in the power industry. Restrictive use and emission reduction policies are fundamental step in guiding SF6 emission, but they are poor promoted in developing economies. More specific policies and regulations are needed in conjunction with timely and accurate assessments of SF6 atmospheric properties and emissions. SF6 recovery is the direct emission reduction approach, but defects in recovery methods and equipment limit its applications. The development of SF6 purification technologies and optimizations in recovery devices and processes are needed for its treatment of different regions and SF6 volumes. SF6 degradation is the final step of waste gas treatment, and its development needs to better balance the degradation rate and product selectivity, as well as to improve their multi-scenario responsiveness. SF6 substitution is a necessity for future large-scale SF6 emission reduction. Improvements in SF6-free applications and its long-term stability are critical via new gas design, gas mixture optimization and equipment updates. Finally, all the emission reduction approaches are closely related, and promoting their synergistic development and complementarity is the ultimate way to realize SF6 lifecycle management.