Crosslinked MOF-polymer to enhance gas separation of mixed matrix membranes

Interfacial Property Modulation of PIM-1 through Polydopamine-Derived Submicrospheres for Enhanced CO2/N2 Separation Performance

Polydopamine-modified additives have been thus far widely used in the mixed matrix membranes (MMMs) for gas separation. However, very few reports focus on the polydopamine alone and investigate its contribution to the gas separation performance. Herein, the polydopamine-derived submicrospheres (PDASS) were paired with polymers of intrinsic microporosity (PIM-1) to fabricate high-performance gas separation membranes, through which the effects of PDASS on gas permeability and CO₂/N₂ separation performance were systematically investigated. The addition of PDASS provides a 1.6-fold enhancement in CO₂/N₂ selectivity together with acceptable gas permeability as compared to the original polymeric membrane. Such enhanced separation behavior is supposed to stem from the densified membrane microstructure induced by the strong intermolecular interactions between PIM-1 and PDASS (i.e., charge transfer, π-π stacking, and hydrogen bonding). Importantly, the physical aging behavior, as judged by gas permeability, is retarded for PIM/PDASS membranes after 4 months of testing.

Defect-Free MOF-Based Mixed-Matrix Membranes Obtained by Corona Cross-Linking

Functionalized UiO-66 metal-organic frameworks (MOF) particles were covalently grafted with hydride-terminated poly(dimethylsiloxane) (PDMS) via postsynthetic modification. These PDMS-coated MOF particles (termed here "corona-MOF") were used in the preparation of mixed-matrix membranes (MMMs). Defect-free MMMs with weight loadings of 50% were achieved with corona-MOF particles, attributed to the improved dispersibility of the corona-MOF particles and covalent linkages between the corona-MOF particles and the polymer matrix. The PDMS MMMs showed distinct separation features in single gas permeation tests, displaying much higher CO₂ gas permeation with no decrease in selectivity when compared to MMMs prepared with unmodified UiO-66 particles. Single gas separation tests with CO₂, N₂, and propane were performed to probe the separation mechanism of the corona-MOF MMMs, demonstrating that these MMMs avoid nonideal "sieve-in-a-cage" and "plugged sieves" scenarios. Additionally, due to covalent bond formation between both the MOF and the polymer matrix in corona-MOF MMMs, particle aggregation is negligible during film curing, allowing for the formation of flexible, self-standing MMMs of <1 μm in thickness. Low quantities of polymer covalently attached to the MOF surface (<5 wt %) are sufficient to fabricate thin, defect-free, high MOF-loading MMMs.

Constructing Connected Paths between UiO-66 and PIM-1 to Improve Membrane CO2 Separation with Crystal-Like Gas Selectivity

Most metal-organic-framework- (MOF-) based hybrid membranes face the challenge of low gas permeability in CO₂ separation. This study presents a new strategy of interweaving UiO-66 and PIM-1 to build freeways in UiO-66-CN@sPIM-1 membranes for fast CO₂ transport. In this strategy, sPIM-1 is rigidified via thermal treatment to make polymer voids permanent, and concurrently polymer chains are mutually linked onto UiO-66-CN crystals to minimize interfacial defects. The pore chemistry of UiO-66-CN is kept intact in hybrid membranes, allowing full utilization of MOF pores and selective adsorption for CO₂ . Separation results show that UiO-66-CN@sPIM-1 membranes possess exceptionally high CO₂ permeability (15433.4-22665 Barrer), approaching to that of UiO-66-NH₂ crystal (65-75% of crystal-derived permeability). Additionally, the CO₂ /N₂ permeation selectivity for a representative membrane (23.9-28.6) moves toward that of single crystal (24.6-29.6). The unique structure and superior CO₂ /N₂ separation performance make UiO-66-CN@sPIM-1 membranes promising in practical CO₂ separations.

Advanced Porous Materials in Mixed Matrix Membranes

Membrane technology has gained great interest in industrial separation processing over the past few decades owing to its high energy efficiency, small capital investment, environmentally benign characteristics, and the continuous operation process. Among various types of membranes, mixed matrix membranes (MMMs) combining the merits of the polymer matrix and inorganic/organic fillers have been extensively investigated. With the rapid development of chemistry and materials science, recent studies have shifted toward the design and application of advanced porous materials as promising fillers to boost the separation performance of MMMs. Here, first a comprehensive overview is provided on the choices of advanced porous materials recently adopted in MMMs, including metal-organic frameworks, porous organic frameworks, and porous molecular compounds. Novel trends in MMMs induced by these advanced porous fillers are discussed in detail, followed by a summary of applying these MMMs for gas and liquid separations. Finally, a concise conclusion and current challenges toward the industrial implementation of MMMs are outlined, hoping to provide guidance for the design of high-performance membranes to meet the urgent needs of clean energy and environmental sustainability.

ROMP for Metal-Organic Frameworks: An Efficient Technique toward Robust and High-Separation Performance Membranes

Mixed-matrix membranes (MMMs) with excellent mechanical and separation performance are usually challenging to be fabricated due to the significant incompatibility between nanofillers and the polymer matrix. This work provides a facile technique to construct MMMs through covalently attaching metal-organic frameworks (MOFs) within the polymer matrix via ring-opening metathesis polymerization. Norbornene-modified UiO-66-NH₂ was successfully copolymerized into polynorbornene matrix in less than 10 min. Owing to strong covalent interaction among MOFs and polymers, exceptional toughening effects for MMMs through cavitation were observed. For MMMs with 20 wt % MOF loading, 520 times improvement in mechanical toughness was realized in comparison with neat polymers (52 vs 0.1 MJ/m³), far exceeding most of the previous MMMs. Such MMMs exhibited excellent gas separation performance for H₂/CO₂ and H₂/N₂ with high H₂ permeability at 91-230 barrers and H₂/N₂ and H₂/CO₂ selectivity at >1000 and 6-7, respectively, surpassing the 2008 Robeson Upper Bound. As a proof for the scalable preparation of MMMs, a large and thin MMM (dimension: 98 × 165 cm; thickness: 3-5 μm) was also prepared in the factory for gas separation.

Towards High Performance Metal-Organic Framework-Microporous Polymer Mixed Matrix Membranes: Addressing Compatibility and Limiting Aging by Polymer Doping

Membrane separation for gas purification is an energy-efficient and environment-friendly technology. However, the development of high performance membranes is still a great challenge. In principle, mixed matrix membranes (MMMs) have the potential to overcome current materials limitations, but in practice there is no straightforward method to match the properties of fillers and polymers (the main components of MMMs) in such a way that the final membrane performance reflects the high performance of the microporous filler and the processability of the continuous polymer phase. This issue is especially important when high flux polymers are utilized. In this work, we demonstrate that the use of small amounts of a glassy polymer in combination with high performance PIM-1 allow for the preparation of metal-organic framework (MOF)-based MMMs with superior separation properties and low aging rates under humid conditions, meeting the commercial target for post-combustion CO₂ capture.

Fabrication of polyimide and covalent organic frameworks mixed matrix membranes by in situ polymerization for preliminary exploration of CO2/CH4 separation

More and more polyimide (PI) mixed matrix membranes (MMMs) have been reported for gas separation. In this study, a novel PI MMM, named as PI/SNW-1 and composed of PI and Schiff base network (SNW) type covalent organic frameworks (COFs) SNW-1, was used for gas permeation measurements of carbon dioxide (CO2) and methane (CH4). The prepared PI/SNW-1 was investigated by the Fourier transform infrared spectroscopy, the field emission scanning electron microscopy, and the thermal gravimetric analysis. The results indicated that PI/SNW-1 had maintained a high thermal stability and uniform distribution of filler. Compared with the pure PI membrane, MMMs showed an increment of 48.7% in ideal selectivity of CO2/CH4 and an enhancement of 106.4% in CO2 permeability at 5 wt% SNW-1. The enhancement of permeability and selectivity was mainly attributed to the high porosity of SNW-1, the specific sorption affinity for CO2, and the close interface interaction with the PI matrix. It can be seen that PI/SNW-1 has a great potential for actual gas separation.

Matrimid-JUC-62 and Matrimid-PCN-250 mixed matrix membranes displaying light-responsive gas separation and beneficial ageing characteristics for CO2/N2 separation

The performance of two generation-3 light-responsive metal-organic framework (MOF), namely JUC-62 and PCN-250, was investigated in a mixed matrix membrane (MMM) form. Both of them were incorporated inside the matrimid as the polymer matrix. Using our custom-designed membrane testing cell, it was observed that the MMMs showed up to 9% difference in CO₂ permeability between its pristine and UV-irradiated condition. This shows that the light-responsive ability of the light-responsive MOFs could still be maintained. Thus, this finding is applicable in designing a smart material. Apart from that, the MMMs also has the potential to be applied for post-combustion carbon capture. At loadings up to 15 wt%, both CO₂ permeability and CO₂/N₂ ideal selectivity could be significantly improved and surpassed the value exhibited by most of the MOF-matrimid MMM. Lastly the long term performance of the MMM was also evaluated and it was observed that both MMM could maintain their performance up to 1 month with only a slight decrease in CO₂ permeability observed for 10 wt% PCN-250-matrimid. This study then opens up the possibility to fabricate a novel anti-aging multifunctional membrane material that is applicable as a smart material and also in post combustion carbon capture applications.

Epoxy Nanocomposites Containing Zeolitic Imidazolate Framework-8

Zeolitic imidazole framework-8 (ZIF-8) is utilized as a functional filler and a curing agent in the preparation of epoxy nanocomposites. The imidazole group on the surface of the ZIF-8 initiates epoxy curing, resulting in covalent bonding between the ZIF-8 crystals and epoxy matrix. A substantial reduction in dielectric constant and increase in tensile modulus were observed. The implication of the present study for utilization of metal-organic framework to improve physical and mechanical properties of polymeric matrixes is discussed.

Chemically Cross-Linked MOF Membrane Generated from Imidazolium-Based Ionic Liquid-Decorated UiO-66 Type NMOF and Its Application toward CO2 Separation and Conversion

Carbon dioxide capture and transformation are of great importance to make cuts in greenhouse gas emissions. Nanometal-organic frameworks (NMOFs) could serve as ideal fillers for polymer membranes owing to their structural diversity and regulable microenvironment of the nanocage. Herein, a bifunctional, robust, and chemically cross-linked NMOF-based membrane was successfully constructed by the postsynthetic polymerization of imidazolium-based ionic liquid (IL)-decorated UiO-66 type nanoparticles (NPs) and the isocyanate-terminated polyurethane oligomer under mild conditions. The IL-modified MOF-polymer membranes exhibit a highly selective adsorption for CO₂ over N₂ and CH₄. In addition, the obtained membrane can also be a highly active heterogeneous catalyst for CO₂ transformation by cycloaddition with epoxide under an ambient pressure.

Metal-Organic Framework UiO-66 Layer: A Highly Oriented Membrane with Good Selectivity and Hydrogen Permeance

The 3D metal-organic framework (MOF) structure UiO-66 [Zr₆O₄(OH)₄(bdc)₆], featuring triangular pores of approximately 6 Å, has been successfully prepared as a thin supported membrane layer with high crystallographic orientation on ceramic α-Al₂O₃ supports. The adhesion of the MOF layer to the ceramic support was investigated in different taxing conditions. Furthermore, by coating this UiO-66 membrane with a thin polyimide (Matrimid) top layer, we prepared a multilayer composite. Said membranes have been evaluated in the separation of hydrogen (H₂) from different binary mixtures at room temperature. H₂ as the smallest molecule (2.9 Å) should pass the UiO-66 membrane preferably since the kinetic diameters of all the other gases under study are larger. The gas mixture separation factors for the neat UiO-66 membrane were indeed found to be H₂/CO₂ = 5.1, H₂/N₂ = 4.7, H₂/CH₄ = 12.9, H₂/C₂H₆ = 22.4, and H₂/C₃H₈ = 28.5. The coating with Matrimid led to a sharp cutoff for gases with kinetic diameters greater than 3.7 Å, resulting in increased separation performance.

CO2 Capture and Separations Using MOFs: Computational and Experimental Studies

This Review focuses on research oriented toward elucidation of the various aspects that determine adsorption of CO₂ in metal-organic frameworks and its separation from gas mixtures found in industrial processes. It includes theoretical, experimental, and combined approaches able to characterize the materials, investigate the adsorption/desorption/reaction properties of the adsorbates inside such environments, screen and design new materials, and analyze additional factors such as material regenerability, stability, effects of impurities, and cost among several factors that influence the effectiveness of the separations. CO₂ adsorption, separations, and membranes are reviewed followed by an analysis of the effects of stability, impurities, and process operation conditions on practical applications.