Eliminating lattice defects in metal-organic framework molecular-sieving membranes.

Metal-organic framework (MOF) membranes are energy-efficient candidates for molecular separations, but it remains a considerable challenge to eliminate defects at the atomic scale. The enlargement of pores due to defects reduces the molecular-sieving performance in separations and hampers the wider application of MOF membranes, especially for liquid separations, owing to insufficient stability. Here we report the elimination of lattice defects in MOF membranes based on a high-probability theoretical coordination strategy that creates sufficient chemical potential to overcome the steric hindrance that occurs when completely connecting ligands to metal clusters. Lattice defect elimination is observed by real-space high-resolution transmission electron microscopy and studied with a mathematical model and density functional theory calculations. This leads to a family of high-connectivity MOF membranes that possess ångström-sized lattice apertures that realize high and stable separation performance for gases, water desalination and an organic solvent azeotrope. Our strategy could enable a platform for the regulation of nanoconfined molecular transport in MOF pores.

Zeolites and metal-organic frameworks for gas separation: the possibility of translating adsorbents into membranes.

Zeolites and metal-organic frameworks (MOFs) represent an attractive class of crystalline porous materials that possesses regular pore structures. The inherent porosity of these materials has led to an increasing focus on gas separation applications, encompassing adsorption and membrane separation techniques. Here, a brief overview of the critical properties and fabrication approaches for zeolites and MOFs as adsorbents and membranes is given. The separation mechanisms, based on pore sizes and the chemical properties of nanochannels, are explored in depth, considering the distinct characteristics of adsorption and membrane separation. Recommendations for judicious selection and design of zeolites and MOFs for gas separation purposes are emphasized. By examining the similarities and differences between the roles of nanoporous materials as adsorbents and membranes, the feasibility of zeolites and MOFs from adsorption separation to membrane separation is discussed. With the rapid development of zeolites and MOFs towards adsorption and membrane separation, challenges and perspectives of this cutting-edge area are also addressed.

PDMS with Tunable Side Group Mobility and Its Highly Permeable Membrane for Removal of Aromatic Compounds.

Polydimethylsiloxane (PDMS), as the benchmark of organophilic membrane materials, still faces challenges for removal of aromatic compounds due to the undesirable transport channels. In this work, we propose to reconstruct the PDMS conformation with tunable side group mobility by introducing phenyl as rigid molecular spacer to relieve steric hindrance of large-sized aromatic molecules; meanwhile, polymer segments are loosely stacked to provide additional degrees of freedom as increasing the permeant size. Moreover, the reconstructed PDMS is engineered into the composite membrane with prevention of condensation of aromatic compounds in the substrate pores. The resulting thin-film composite membrane achieved one order of magnitude higher flux (11.8 kg m-2 h-1 ) with an equivalent separation factor (12.3) compared with the state-of-the-art membranes for aromatic removal. The permeant-customized membrane molecular and microstructure designing strategy opens a new avenue to develop membranes for specific separation targets.

Aliquots of MIL-140 and Graphene in Smart PNIPAM Mixed Hydrogels: A Nanoenvironment for a More Eco-Friendly Treatment of NaCl and Humic Acid Mixtures by Membrane Distillation.

The problem of water scarcity is already serious and risks becoming dramatic in terms of human health as well as environmental safety. Recovery of freshwater by means of eco-friendly technologies is an urgent matter. Membrane distillation (MD) is an accredited green operation for water purification, but a viable and sustainable solution to the problem needs to be concerned with every step of the process, including managed amounts of materials, membrane fabrication procedures, and cleaning practices. Once it is established that MD technology is sustainable, a good strategy would also be concerned with the choice of managing low amounts of functional materials for membrane manufacturing. These materials are to be rearranged in interfaces so as to generate nanoenvironments wherein local events, conceived to be crucial for the success and sustainability of the separation, can take place without endangering the ecosystem. In this work, discrete and random supramolecular complexes based on smart poly(N-isopropyl acrylamide) (PNIPAM) mixed hydrogels with aliquots of ZrO(O2C-C10H6-CO2) (MIL-140) and graphene have been produced on a polyvinylidene fluoride (PVDF) sublayer and have been proven to enhance the performance of PVDF membranes for MD operations. Two-dimensional materials have been adhered to the membrane surface through combined wet solvent (WS) and layer-by-layer (LbL) spray deposition without requiring further subnanometer-scale size adjustment. The creation of a dual responsive nanoenvironment has enabled the cooperative events needed for water purification. According to the MD's rules, a permanent hydrophobic state of the hydrogels together with a great ability of 2D materials to assist water vapor diffusion through the membranes has been targeted. The chance to switch the density of charge at the membrane-aqueous solution interface has further allowed for the choice of greener and more efficient self-cleaning procedures with a full recovery of the permeation properties of the engineered membranes. The experimental evidence of this work confirms the suitability of the proposed approach to obtain distinct effects on a future production of reusable water from hypersaline streams under somewhat soft working conditions and in full respect to environmental sustainability.

Solid-solvent processing of ultrathin, highly loaded mixed-matrix membrane for gas separation.

Mixed-matrix membranes (MMMs) that combine processable polymer with more permeable and selective filler have potential for molecular separation, but it remains difficult to control their interfacial compatibility and achieve ultrathin selective layers during processing, particularly at high filler loading. We present a solid-solvent processing strategy to fabricate an ultrathin MMM (thickness less than 100 nanometers) with filler loading up to 80 volume %. We used polymer as a solid solvent to dissolve metal salts to form an ultrathin precursor layer, which immobilizes the metal salt and regulates its conversion to a metal-organic framework (MOF) and provides adhesion to the MOF in the matrix. The resultant membrane exhibits fast gas-sieving properties, with hydrogen permeance and/or hydrogen-carbon dioxide selectivity one to two orders of magnitude higher than that of state-of-the-art membranes.

Pebax-Based Membrane Filled with Two-Dimensional Mxene Nanosheets for Efficient CO2 Capture.

Mixed matrix membranes (MMMs) made from inorganic fillers and polymers is a kind of promising candidate for gas separation. In this work, two-dimensional MXene nanosheets were synthesized and incorporated into a polyether-polyamide block copolymer (Pebax) matrix to fabricate MMM for CO2 capture. The physicochemical properties of MXene nanosheets and MXene/Pebax membranes were studied systematically. The introduction of MXene nanosheets provided additional molecular transport channels and meanwhile enhanced the CO2 adsorption capacity, thereby enhancing both the CO2 peremance and CO2 /N2 selectivity of Pebax membrane. The optimized MXene/Pebax membrane with a MXene loading of 0.15 wt % displayed a high separation performance with a CO2 permeance of 21.6 GPU and a CO2 /N2 selectivity of 72.5, showing potential application in CO2 capture.

Ultrathin Membranes with a Polymer/Nanofiber Interpenetrated Structure for High-Efficiency Liquid Separations.

Ultrathin-film composite membranes comprising an ultrathin polymeric active layer have been extensively explored in gas separation applications benefiting from their extraordinary permeation flux for high-throughput separation. However, the practical realization of an ultrathin active layer in liquid separations is still impeded by the trade-off effect between the membrane thickness (permeation flux) and structural stability (separation factor). Herein, we report a general multiple and alternate spin-coating strategy, collaborating with the interface-decoration layer of copper hydroxide nanofibers (CHNs), to obtain ultrathin and robust polymer-based membranes for high-performance liquid separations. The structural stability arises from the poly(dimethylsiloxane) (PDMS)/CHN interpenetrated structure, which confers the synergistic effect between PDMS and CHNs to concurrently resist PDMS swelling and avoid CHNs from collapsing, while the ultrathin thickness is enabled by the sub-10 nm pore size of the CHN layer, the rapid cross-linking reaction during spin-coating, and the small thickness of the CHN layer. As a result, the as-prepared membrane possesses an exceptional butanol/water separation performance with a flux of 6.18 kg/(m2 h) and a separation factor of 31, far exceeding the state-of-the-art polymer membranes. The strategy delineated in this work provides a straightforward method for the design of ultrathin and structurally stable polymer membranes, holding great potential for the practical application of high-efficiency separations.