Enhanced CO2/CH4 separation performance of mixed-matrix membranes through dispersion of sorption-selective MOF nanocrystals

Mixed matrix membrane of poly(4-methyl-1-pentyne) and ZIF-8 for enhanced CO2 separation over H2 and CH4

Carbon dioxide (CO2) generally exists as the main impurity in natural gas, whose main component is methane. The presence of CO2 reduces the energy content of natural gas and also causes the corrosion of pipelines. To prevent such problems, natural gas must contain a small concentration of CO2 (less than 2% by weight). Membrane technology is an attractive separation method that has been widely studied due to its advantages such as high efficiency, low operating costs, and low energy requirements. However, in the last decade, Mixed Matrix Membranes (MMMs) have attracted the attention of many researchers due to their suitable capabilities in separating polar from non-polar gases. In this research, a new MMMs was obtained by adding imidazole zeolite nanoparticle (ZIF-8) to the poly methyl pentene (PMP) polymer matrix. The polymer part of this membrane can provide high permeability and suitable mechanical and thermal stability. In addition, ZIF-8 particles enhance CO2 separation by offering high CO2 adsorption capacity and molecular sieving, improving selectivity. The gas permeability test was performed on pure and mixed matrix membranes at 30 ℃ and pressures of 2, 6 and 10 bar. In addition, the fabricated membranes were evaluated by FESEM, FTIR-ATR, BET, DMA and TGA tests. The results indicated that in the MMMs containing 30 wt% of nanoparticles in the polymer, the permeability of CO2 gas improved by more than 180% and reached about 278.95 barrer, compared to the pure polymer membrane at a pressure of 10 bar. Moreover, the selectivity of CO₂/CH₄ and CO₂/H₂ increased by 142% and 155%, respectively, primarily due to the preferential sorption of CO₂ over H₂ and CH₄ facilitated by ZIF-8 particles.

Plasmonic Nanocrystal-MOF Nanocomposites as Highly Active Photocatalysts and Highly Sensitive Sensors for CO2 Reduction over a Wide Range of Solar Wavelengths

Plasmonic nanocrystals have the potential to be widely used in green energy-related applications, due to their excellent optical properties and high reactivity over a wide range of solar wavelengths. Another benefit of using plasmonic nanocrystals for optical applications is that these nanocrystals strongly enhance Raman scattering and are therefore widely used in sensors. Recently, nanocomposites of porous materials deposited on plasmonic nanocrystals are demonstrated to enhance chemical reactivity by concentrating reactants on the surface of plasmonic nanocrystals. Here, three different plasmonic nanocrystals producing plasmonic responses within 400-900 nm are used as templates, and MOF-801 (Zr-based MOF) is produced on these nanocrystals as photocatalysts for the CO2 reduction reaction. Using nanocomposites as CO2 reduction reaction photocatalysts, the CO2 conversion rate can reach >50% within 30 min. The CO2 reduction reactivity of nanocomposites can be improved by the composition and morphology of plasmonic nanocrystals (increased by 40-50%), due to stronger synergistic effects and higher surface area to volume ratio. This report demonstrates that by controlling the plasmonic responses of nanocrystals, it is possible to realize photocatalysts that can be used for CO2 reduction reactions over a wide range of solar wavelengths.

Ionic porous materials: from synthetic strategies to applications in gas separation and catalysis

Ionic porous materials possess a unique combination of tunable pore sizes and task-specific interactions between guest molecules and the charged frameworks, which endow them with versatility across diverse domains in chemistry and materials science. Significant advancements in their applications for gas separation and catalysis have been achieved in recent years due to the incorporation of ionic functionalities and ultra-microporous structures that enable molecular-scale recognition of guest molecules. This review summarizes recent advancements in the synthetic strategies of ionic porous materials, establishing design guidelines for the incorporation of ionic moieties into the backbone to fine-tune pore sizes and chemistry. It highlights the synergistic interplay of task-specific interactions with custom-designed pore structures in key applications, including adsorption separation, membrane separation, and gas conversion. Additionally, it examines structure-property relationships, offering deeper insights into enhancing performance. The report also addresses the current challenges in the practical application of these materials. Finally, the review provides future perspectives on ionic porous materials from both scientific and industrial viewpoints. Overall, this review aims to provide insights into pore structure and chemistry, supporting the precise placement of ionic functionalities.

Defect-Engineered Metal-Organic Framework/Polyimide Mixed Matrix Membrane for CO2 Separation

Defect-engineered metal-organic frameworks (MOFs) with outstanding structural and chemical features have become excellent candidates for specific separation applications. The introduction of structural defects in MOFs as an efficient approach to manipulate their functionality provides excellent opportunities for the preparation of MOF-based mixed matrix membranes (MMMs). However, the use of this strategy to adjust the properties and develop the separation performance of gas separation membranes is still in its early stages. Here, a novel defect-engineered MOF (quasi ZrFum or Q-ZrFum) was synthesized via a controlled thermal deligandation process and incorporated into a CO2-philic 6FDA-durene polyimide (PI) matrix to form Q-ZrFum loaded MMMs. Defect-engineered MOFs and fabricated MMMs were investigated regarding their characteristic properties and separation performance. The incorporation of defects into the MOF structure increases the pore size and provides unsaturated active metal sites that positively affect CO2 molecule transport. The interfacial compatibility between the Q-ZrFum particles and the PI matrix increases via the deligandation process, which improves the mechanical strength of Q-ZrFum loaded membranes. MMM containing 5 wt.% of defect-engineered Q-ZrFum exhibits excellent CO2 permeability of 1308 Barrer, which increased by 99 % compared to the pure PI membrane (656 Barrer) at a feed pressure of 2 bar. CO2/CH4 and CO2/N2 selectivity reached 44 and 26.6 which increased by about 70 and 16 %, respectively. This study emphasizes that defect-engineered MOFs can be promising candidates for use as fillers in the preparation of MMMs for the future development of membrane-based gas separation applications.

Naphthalene Tetrazole-Based Nickel Metal-Organic Framework as a Filler of Polycarbonate Membranes to Improve CO2 and H2 Separation

A tetrazole-naphthalene linker was used to prepare a nickel MOF (metal-organic framework) (NiNDTz) with interesting properties: a specific surface area SBET of 320 m2g-1 (SLangmuir 436 m2g-1), high thermal stability (Tdonset = 300 °C), and CO2 uptake of 1.85 mmolg-1, attributed to the tetrazole groups to be used as fillers in gas separation membranes. The role of these groups was crucial in the mechanical properties of mixed membranes prepared using polycarbonate as a polymer matrix, providing a very homogeneous filler distribution and also in the gas separation properties since a simultaneous increase in permeability and selectivity was achieved, especially in the hybrid membrane containing 20% filler (PC@NiNDTz-20%). This membrane exhibited an excellent balance between permeability (P) and selectivity (α) with an increase in the permeability of CO2 and H2, 177 and 185%, respectively, and improvements in the selectivity of these gases against greenhouse gases such as methane and ethylene (between 15 and 28% improvement). These results make this membrane competitive to deal with separations in which these gases are involved, and are of special interest for the H2/CH4 separation since it clearly improves the performance of pure PC and no better PC-based membranes have been reported in the literature for this separation.

Fabrication, Facilitating Gas Permeability, and Molecular Simulations of Porous Hypercrosslinked Polymers Embedding 6FDA-Based Polyimide Mixed-Matrix Membranes

Novel polymers applied in economic membrane technologies are a perennial hot topic in the fields of natural gas purification and O₂ enrichment. Herein, novel hypercrosslinked polymers (HCPs) incorporating 6FDA-based polyimide (PI) MMMs were prepared via a casting method for enhancing transport of different gases (CO₂, CH₄, O₂, and N₂). Intact HCPs/PI MMMs could be obtained due to good compatibility between the HCPs and PI. Pure gas permeation experiments showed that compared with pure PI film, the addition of HCPs effectively promotes gas transport, increases gas permeability, and maintains ideal selectivity. The permeabilities of HCPs/PI MMMs toward CO₂ and O₂ were as high as 105.85 Barrer and 24.03 Barrer, respectively, and the ideal selectivities of CO₂/CH₄ and O₂/N₂ were 15.67 and 3.00, respectively. Molecular simulations further verified that adding HCPs was beneficial to gas transport. Thus, HCPs have potential utility in fabrication of MMMs for facilitating gas transport in the fields of natural gas purification and O₂ enrichment.

Investigation of the Performance of Heterogeneous MOF-Silver Nanocube Nanocomposites as CO2 Reduction Photocatalysts by In Situ Raman Spectroscopy

Here, we fabricated two different heterogeneous nanocomposites, core-shell MOF-AgNC and corner MOF-AgNC, as photocatalysts for CO₂ conversion by generating metal-organic frameworks (MOFs) on silver nanocube templates. These MOF-AgNC nanocomposites showed good CO₂ adsorption features and high CO₂ reduction reactivity. The performances of these MOF-AgNC nanocomposites in CO₂ adsorption and CO₂ reduction reactions can be characterized by in situ Raman spectrum measurement. The corner MOF-AgNC nanocomposite exhibited a faster CO₂ adsorption rate than the core-shell MOF-AgNC nanocomposite, which was due to the higher surface area/volume ratio of the MOF in corner MOF-AgNC. The CO₂ reaction reactivity and mechanisms (products of the reaction) of CO₂ reduction also depended on the morphologies of MOF-AgNC nanocomposites, which were caused by different reaction environments at the interface between the MOF and AgNCs. The CO₂ reduction reactivity of MOF-AgNC nanocomposites also exhibited high sensitivity to the irradiation intensity and wavelength, which was caused by the variation of the number of hot electrons and their positions in AgNCs with the irradiation intensity and irradiation wavelength, respectively. This method for the synthesis of heterogeneous nanocomposites should make it possible to design photocatalysts for various reactions by carefully designing the morphology and composition of nanocomposites.

Fluorinated metal-organic frameworks for gas separation

Fluorinated metal-organic frameworks (F-MOFs) as fast-growing porous materials have revolutionized the field of gas separation due to their tunable pore apertures, appealing chemical features, and excellent stability. A deep understanding of their structure-performance relationships is critical for the synthesis and development of new F-MOFs. This critical review has focused on several strategies for the precise design and synthesis of new F-MOFs with structures tuned for specific gas separation purposes. First, the basic principles and concepts of F-MOFs as well as their structure, synthesis and modification and their structure to property relationships are studied. Then, applications of F-MOFs in adsorption and membrane gas separation are discussed. A detailed account of the design and capabilities of F-MOFs for the adsorption of various gases and the governing principles is provided. In addition, the exceptional characteristics of highly stable F-MOFs with engineered pore size and tuned structures are put into perspective to fabricate selective membranes for gas separation. Systematic analysis of the position of F-MOFs in gas separation revealed that F-MOFs are benchmark materials in most of the challenging gas separations. The outlook and future directions of the science and engineering of F-MOFs and their challenges are highlighted to tackle the issues of overcoming the trade-off between capacity/permeability and selectivity for a serious move towards industrialization.

Rational design of mixed-matrix metal-organic framework membranes for molecular separations

Conventional separation technologies to separate valuable commodities are energy intensive, consuming 15% of the worldwide energy. Mixed-matrix membranes, combining processable polymers and selective adsorbents, offer the potential to deploy adsorbent distinct separation properties into processable matrix. We report the rational design and construction of a highly efficient, mixed-matrix metal-organic framework membrane based on three interlocked criteria: (i) a fluorinated metal-organic framework, AlFFIVE-1-Ni, as a molecular sieve adsorbent that selectively enhances hydrogen sulfide and carbon dioxide diffusion while excluding methane; (ii) tailoring crystal morphology into nanosheets with maximally exposed (001) facets; and (iii) in-plane alignment of (001) nanosheets in polymer matrix and attainment of [001]-oriented membrane. The membrane demonstrated exceptionally high hydrogen sulfide and carbon dioxide separation from natural gas under practical working conditions. This approach offers great potential to translate other key adsorbents into processable matrix.

Inorganic Pillar Center-Facilitated Counterdiffusion Synthesis for Highly H2 Perm-Selective KAUST-7 Membranes

Fluorinated metal-organic framework materials (NbOFFIVE-1-Ni, also referred to as KAUST-7) have attracted widespread attention because of their high chemical stability and thermal stability, outstanding tolerance with water and H₂S, and high CO₂-adsorption selectivity over H₂ and CH₄. KAUST-7 was expected to be a new membrane material candidate for H₂/CO₂ separation because of the hindered permeation of CO₂ resulting from the interaction between CO₂ and (NbOF₅)^2- of the KAUST-7 framework. A highly H₂ perm-selective KAUST-7 membrane was first achieved using a novel strategy of inorganic pillar center-facilitated counterdiffusion (IPCFCD) proposed by us. The IPCFCD method not only effectively avoided the corrosion of hydrofluoric acid to α-Al₂O₃ tubes in the process of preparing KAUST-7 membranes, but also better reduced grain boundary defects because of the faster nucleation rate and resultant high crystallinity. The KAUST-7 membrane exhibited a high H₂/CO₂ separation factor (SF) of 27.30 for the 1:1 H₂/CO₂ binary gas mixture with a high H₂ permeance of 5.30 × 10^-7 mol m^-2 s^-1 Pa^-1 under ambient conditions and a slight decrease of the H₂/CO₂ SF with increasing operation temperature and presence of steam. This study highlighted the importance of pre-synthesizing inorganic pillar centers (NiNbOF₅ intermediate) and the innovation of a membrane formation process for synthesizing polycrystalline KAUST-7 membranes. Most important of all, our study provided a novel approach to overcome the challenge in fabricating metal-organic framework membranes containing corrosive reactants for the corresponding supports.

A Highly Efficient and Stable Composite of Polyacrylate and Metal-Organic Framework Prepared by Interface Engineering for Direct Air Capture

We present a kilogram-scale experiment for assessing the prospects of a novel composite material of metal-organic framework (MOF) and polyacrylates (PA), namely NbOFFIVE-1-Ni@PA, for trace CO₂ capture. Through the interfacial enrichment of metal ions and organic ligands as well as heterogeneous crystallization, the sizes of microporous NbOFFIVE-1-Ni crystals are downsized to 200-400 nm and uniformly anchored on the macroporous surface of PA via interfacial coordination, forming a unique dual-framework structure. Specifically, the NbOFFIVE-1-Ni@PA composite with a loading of 45.8 wt % NbOFFIVE-1-Ni yields a superior CO₂ uptake (ca. 1.44 mol·kg^-1) compared to the pristine NbOFFIVE-1-Ni (ca. 1.30 mol·kg^-1) at 400 ppm and 298 K, indicating that the adsorption efficiency of NbOFFIVE-1-Ni has been raised by 2.42 times. Meanwhile, the time cost for realizing a complete adsorption/desorption cycle in a fluidized bed has been shortened to 25 min, and the working capacity (ca. 0.84 mol·kg^-1) declines only by 1.3% after 2000 cycles. The device is capable of harvesting 2.1 kg of CO₂ per kilogram of composite daily from simulated air with 50% relatively humidity (RH). To the best of our knowledge, the excellent adsorption/desorption performances of NbOFFIVE-1-Ni@PA position it as the most advantageous and practically applicable candidate for trace CO₂ capture.

Catalyst-free development of N-doped microporous carbons for selective CO2 separation

Owing to their catalyst-free development, high yield, notable CO₂ uptake performance, and excellent CO₂/CH₄ selectivity, the fabricated N-doped microporous carbons (NMCs) are highly suitable for selective CO₂ separation.

Metal-Organic Frameworks as a Platform for CO2 Capture and Chemical Processes: Adsorption, Membrane Separation, Catalytic-Conversion, and Electrochemical Reduction of CO2

The continuous rise in the atmospheric concentration of carbon dioxide gas (CO2) is of significant global concern. Several methodologies and technologies are proposed and applied by the industries to mitigate the emissions of CO2 into the atmosphere. This review article offers a large number of studies that aim to capture, convert, or reduce CO2 by using a superb porous class of materials (metal-organic frameworks, MOFs), aiming to tackle this worldwide issue. MOFs possess several remarkable features ranging from high surface area and porosity to functionality and morphology. As a result of these unique features, MOFs were selected as the main class of porous material in this review article. MOFs act as an ideal candidate for the CO2 capture process. The main approaches for capturing CO2 are pre-combustion capture, post-combustion capture, and oxy-fuel combustion capture. The applications of MOFs in the carbon capture processes were extensively overviewed. In addition, the applications of MOFs in the adsorption, membrane separation, catalytic conversion, and electrochemical reduction processes of CO2 were also studied in order to provide new practical and efficient techniques for CO2 mitigation.

Metal Organic Framework - Based Mixed Matrix Membranes for Carbon Dioxide Separation: Recent Advances and Future Directions

Gas separation and purification using polymeric membranes is a promising technology that constitutes an energy-efficient and eco-friendly process for large scale integration. However, pristine polymeric membranes typically suffer from the trade-off between permeability and selectivity represented by the Robeson's upper bound. Mixed matrix membranes (MMMs) synthesized by the addition of porous nano-fillers into polymer matrices, can enable a simultaneous increase in selectivity and permeability. Among the various porous fillers, metal-organic frameworks (MOFs) are recognized in recent days as a promising filler material for the fabrication of MMMs. In this article, we review representative examples of MMMs prepared by dispersion of MOFs into polymer matrices or by deposition on the surface of polymeric membranes. Addition of MOFs into other continuous phases, such as ionic liquids, are also included. CO2 separation from hydrocarbons, H2, N2, and the like is emphasized. Hybrid fillers based on composites of MOFs with other nanomaterials, e.g., of MOF/GO, MOF/CNTs, and functionalized MOFs, are also presented and discussed. Synergetic effects and the result of interactions between filler/matrix and filler/filler are reviewed, and the impact of filler and matrix types and compositions, filler loading, surface area, porosity, pore sizes, and surface functionalities on tuning permeability are discoursed. Finally, selectivity, thermal, chemical, and mechanical stability of the resulting MMMs are analyzed. The review concludes with a perspective of up-scaling of such systems for CO2 separation, including an overview of the most promising MMM systems.

MOF-Based Membranes for Gas Separations

Metal-organic frameworks (MOFs) represent the largest known class of porous crystalline materials ever synthesized. Their narrow pore windows and nearly unlimited structural and chemical features have made these materials of significant interest for membrane-based gas separations. In this comprehensive review, we discuss opportunities and challenges related to the formation of pure MOF films and mixed-matrix membranes (MMMs). Common and emerging separation applications are identified, and membrane transport theory for MOFs is described and contextualized relative to the governing principles that describe transport in polymers. Additionally, cross-cutting research opportunities using advanced metrologies and computational techniques are reviewed. To quantify membrane performance, we introduce a simple membrane performance score that has been tabulated for all of the literature data compiled in this review. These data are reported on upper bound plots, revealing classes of MOF materials that consistently demonstrate promising separation performance. Recommendations are provided with the intent of identifying the most promising materials and directions for the field in terms of fundamental science and eventual deployment of MOF materials for commercial membrane-based gas separations.

Solid Sorbents as a Retrofit Technology for CO2 Removal from Natural Gas Under High Pressure and Temperature Conditions

The capture of CO2 under high pressure and temperature is challenging and is required in a number for industrial applications including natural gas processing. In this work, we examine the use of benchmark hybrid ultraporous materials HUMs for their potential use in CO2 adsorption processes under high-pressure conditions, with three varying temperatures (283, 298 and 318 K). NbOFFOVE-1-Ni and SIFSIX-3-Ni were the selected HUMs given their established superior CO2 capacity under low pressure (0-1 bar). Both are microporous with highly ordered crystalline structures as compared to the mesoporous hexagonal silica (Santa Barbara Anhydrous-15 (SBA-15)). SBA-15 was previously tested for both low and high-pressure applications and can serve as a benchmark in this study. Sorbent characterization using XRD, SEM, FTIR and N2 adsorption were conducted to assure the purity and structure of the sorbents. TGA analysis were conducted to establish the thermal stability of the sorbents under various temperatures. High-pressure CO2 adsorption was conducted from 0-35 bar using magnetic suspension balance (Rubotherm). Although the SBA-15 had the highest surface (527 m3/g) are of the three adsorbents, the CO2 adsorption capacity (0.42 mmol/g) was an order of magnitude less than the studies HUMs with SIFSIX-3-Ni having 2.6 mmol/g, NbOFFIVE-1-Ni achieving 2.5 mmol/g at 298 K. Multistage adsorption isotherms were obtained at different pressures. In addition, results indicate that electrostatics in HUMs are most effective at improving isosteric heat of adsorption Qst and CO2 uptake. Higher temperatures had negative effect on adsorption capacity for the HUMs and SBA-15 at pressures between 7-9 bar. In SAB-15 the effect of temperature is reversed in what is known as a cross over phenomena.

Highly Propylene-Selective Mixed-Matrix Membranes by in Situ Metal-Organic Framework Formation Using a Polymer-Modification Strategy

Despite the potential of C₃H₆/C₃H₈ separation, there have been no industrial applications of zeolitic-imidazole framework-8 (ZIF-8) mixed-matrix membranes (MMMs) because of the moderate separation performances and several challenging processing issues. Herein, we present a new paradigm of MMM fabrication, named polymer-modification-enabled in situ metal-organic framework formation (PMMOF), enabling in situ formation of ZIF-8 fillers inside the 4,4-(hexafluoroisopropylidene)diphthalic anhydride 2,4,6-trimethyl-1,3-phenylenediamine polymer. PMMOF consists of four steps including hydrolysis of a polymer, ion-exchange, ligand treatment, and imidization. Each step was thoroughly analyzed and important processing parameters were identified, enabling the structural control of MMMs by PMMOF. The binary C₃H₆/C₃H₈ separation performance of the MMMs showed much higher separation factors than conventionally prepared MMMs at similar filler loadings, satisfying the commercial C₃H₆/C₃H₈ separation performance criteria. PMMOF was successfully applied for other MOFs, demonstrating that the process could be general. Finally, as a proof of concept, asymmetric mixed-matrix hollow fiber membranes (i.d. of 0.45 mm and o.d. of 0.63 mm) with ultrathin selective skin layers were prepared by PMMOF, showing C₃H₆ permeance of 2.17 GPU and C₃H₆/C₃H₈ separation factor of ∼20.

Performance of Mixed Matrix Membranes Containing Porous Two-Dimensional (2D) and Three-Dimensional (3D) Fillers for CO₂ Separation: A Review

Application of conventional polymeric membranes in CO₂ separation processes are limited by the existing trade-off between permeability and selectivity represented by the renowned upper bound. Addition of porous nanofillers in polymeric membranes is a promising approach to transcend the upper bound, owing to their superior separation capabilities. Porous nanofillers entice increased attention over nonporous counterparts due to their inherent CO₂ uptake capacities and secondary transport pathways when added to polymer matrices. Infinite possibilities of tuning the porous architecture of these nanofillers also facilitate simultaneous enhancement of permeability, selectivity and stability features of the membrane conveniently heading in the direction towards industrial realization. This review focuses on presenting a complete synopsis of inherent capacities of several porous nanofillers, like metal organic frameworks (MOFs), Zeolites, and porous organic frameworks (POFs) and the effects on their addition to polymeric membranes. Gas permeation performances of select hybrids with these three-dimensional (3D) fillers and porous nanosheets have been summarized and discussed with respect to each type. Consequently, the benefits and shortcomings of each class of materials have been outlined and future research directions concerning the hybrids with 3D fillers have been suggested.