Sankar Nair

A High-Performance Gas-Separation Membrane Containing Submicrometer-Sized Metal–Organic Framework Crystals†

Metal–organic frameworks (MOFs) are an emerging class of nanoporous materials comprising metal centers connected by various organic linkers to create one-, two-, and threedimensional porous structures with tunable pore volumes, surface areas, and chemical properties. Several thousand MOF materials have been synthesized and their numbers continue to grow rapidly. MOFs are predicted to be highly attractive for application in gas-separation membranes and also have a range of other potential applications, for example in selective gas adsorption, hydrogen storage, catalysis, and sensing. Recently, thin continuous MOFmembranes for gas separation have been reported by several authors using MOFs such as MOF-5, HKUST-1 (Cu3(BTC)2), [8] Cu(hfipbb)(H2hfipbb)0.5, [9] and ZIF-8. However, the gas-permeation properties (permeability and selectivity) have so far not been found to be technologically attractive. This may have several reasons, such as membrane defects and related processing issues, use of MOFs with low selectivity, and unfavorable orientation of crystals in the membrane. An alternative route to high-performance MOF membranes is to incorporate them into polymers to obtain nanocomposite (mixed-matrix) membranes. The incorporation of nanoporous molecular sieves such as zeolites into polymeric membranes has attracted much attention, since one can in principle combine the size/shape selectivity of nanoporous materials with the processibility and mechanical stability of polymers. However, zeolite/polymer composite membranes often have defective morphologies characterized by void spaces between the zeolite particles and the polymeric matrix, leading to poor gas-separation performance since the gas molecules bypass the zeolite particles. Recent approaches to address the issue of interface compatibilization are emerging. On the other hand, the use of MOFs in mixed-matrix membranes provides several potential advantages over zeolites. The control of MOF/polymer interface morphology is easier than that of the zeolite/polymer interface, since the organic linkers in MOFs have better affinity with polymer chains than the inorganic zeolites do, and the surface properties of MOFs can be easily tuned by functionalization with various organic molecules if necessary. In general, MOFs also have higher pore volumes and lower density than zeolites, and hence their effect on the membrane properties can be greater for a given mass loading. Recently, several MOFmixed-matrix membranes such as Cu-BPY-HFS (Cu-4,40-bipyridine hexafluorosilicate) in Matrimid, HKUST-1 in poly(sulfone), MOF-5 in Matrimid, and Cu-TPA (terephthalic acid) in poly(vinyl acetate) have been reported. Although a high degree of MOF/polymer adhesion (as characterized by scanning electron microscopy) was found, the gas-separation performance of these membranes was not high. In addition to the control of interface morphology, the selection of appropriate MOF/polymer pairs is indispensable for high-performance mixed-matrix membranes, a fact emphasized in theoretical predictions. ZIF-90 (zeolitic imidazolate framework-90) is an attractive MOF for application in CO2-selective mixed-matrix membranes. ZIF-90 has a sodalite cagelike structure with 0.35 nm pore windows, through which size exclusion of CH4 from CO2/CH4 mixtures is possible. [20] Furthermore, the imidazole linker in ZIF-90 contains a carbonyl group, which has a favorable chemical noncovalent interaction with CO2. [21] Submicrometer-sized crystals of a related MOF material, ZIF-8, have recently been reported. So far, ZIF-90 crystals have been synthesized by the conventional solvothermal method. However, their size (ca. 100 mm) is too large for use in thin mixed-matrix membranes (which require submicrometer-sized crystals). Herein, we describe the synthesis of submicrometer-sized ZIF-90 crystals by a novel method, namely nonsolvent-induced crystallization. The ZIF-90 crystals were thoroughly characterized, and we compare them with solvothermally synthesized ZIF-90. Mixed-matrix membranes were then fabricated using three poly(imide)s as polymer matrices, and their CO2/CH4 separation properties were investigated. In particular, we demonstrate the first MOF-containing gas-separation membranes with technologically attractive properties. The morphology of our ZIF-90 crystals is shown in Figure 1. In general, the synthesis of smaller crystals requires reaction conditions that favor nucleation over crystal growth. Particle-size control proved difficult in conventional solvothermal synthesis. We crystallized small ZIF-90 particles at room temperature by the rapid addition of a nonsolvent to the reagent solution (see the Supporting Information), leading to supersaturation of the solution. The nucleation rate can be [*] Dr. T.-H. Bae, J. S. Lee, Dr. W. Qiu, Prof. Dr. W. J. Koros, Prof. Dr. C. W. Jones, Prof. Dr. S. Nair School of Chemical & Biomolecular Engineering Georgia Institute of Technology 311 Ferst Drive NW, Atlanta, GA 30332-0100 (USA) Fax: (+1)404-894-4200 E-mail: christopher.jones@chbe.gatech.edu sankar.nair@chbe.gatech.edu

A titanosilicate molecular sieve with adjustable pores for size-selective adsorption of molecules

Zeolites and related crystalline microporous oxides—tetrahedrally coordinated atoms covalently linked into a porous framework—are of interest for applications ranging from catalysis to adsorption and ion-exchange. In some of these materials (such as zeolite rho) adsorbates, ion-exchange, and dehydration and cation relocation can induce strong framework deformations. Similar framework flexibility has to date not been seen in mixed octahedral/tetrahedral microporous framework materials, a newer and rapidly expanding class of molecular sieves. Here we show that the framework of the titanium silicate ETS-4, the first member of this class of materials, can be systematically contracted through dehydration at elevated temperatures to ‘tune’ the effective size of the pores giving access to the interior of the crystal. We show that this so-called ‘molecular gate’ effect can be used to tailor the adsorption properties of the materials to give size-selective adsorbents suitable for commercially important separations of gas mixtures of molecules with similar size in the 4.0 to 3.0 Å range, such as that of N2/CH4, Ar/O2 and N2/O2.

Interfacial microfluidic processing of metal-organic framework hollow fiber membranes

High-surface-area gas separation membranes Membranes for gas separation require a combination of high surface area and selective transport pathways. Brown et al. present a potentially scalable route for making high-quality gas separation membranes in a high-surface-area configuration. Using two different solvents flowing in opposite directions, a metal-organic framework material was selectively deposited within hollow polymer fibers. The membranes showed high-performance separation capabilities when tested with mixtures of hydrocarbon gases. Science, this issue p. 72 Gas separation membranes are assembled from metal organic frameworks at the interfaces of porous polymer hollow fibers. Molecular sieving metal-organic framework (MOF) membranes have great potential for energy-efficient chemical separations, but a major hurdle is the lack of a scalable and inexpensive membrane fabrication mechanism. We describe a route for processing MOF membranes in polymeric hollow fibers, combining a two-solvent interfacial approach for positional control over membrane formation (at inner and outer surfaces, or in the bulk, of the fibers), a microfluidic approach to replenishment or recycling of reactants, and an in situ module for membrane fabrication and permeation. We fabricated continuous molecular sieving ZIF-8 membranes in single and multiple poly(amide-imide) hollow fibers, with H2/C3H8 and C3H6/C3H8 separation factors as high as 370 and 12, respectively. We also demonstrate positional control of the ZIF-8 films and characterize the contributions of membrane defects and lumen bypass.

Efficient Calculation of Diffusion Limitations in Metal Organic Framework Materials: A Tool for Identifying Materials for Kinetic Separations

The very large number of distinct structures that are known for metal-organic frameworks (MOFs) and related materials presents both an opportunity and a challenge for identifying materials with useful properties for targeted applications. We show that efficient computational models can be used to evaluate large numbers of MOFs for kinetic separations of light gases based on finding materials with large differences between the diffusion coefficients of adsorbed gas species. We introduce a geometric approach that rapidly identifies the key features of a pore structure that control molecular diffusion and couple this with efficient molecular modeling calculations that predict the Henrys constant and diffusion activation energy for a range of spherical adsorbates. We demonstrate our approach for >500 MOFs and >160 silica zeolites. Our results indicate that many large pore MOFs will be of limited interest for separations based on kinetic effects, but we identify a significant number of materials that are predicted to have extraordinary properties for separation of gases such as CO(2), CH(4), and H(2).

Continuous Polycrystalline Zeolitic Imidazolate Framework‐90 Membranes on Polymeric Hollow Fibers

Headed for a membrane: Continuous, polycrystalline ZIF-90 membranes (picture, left) can be grown at 65 °C from methanolic precursor solutions on nanocrystal-seeded surfaces of poly(amide-imide) macroporous hollow fibers (right). The ZIF-90 membranes exhibit good separation properties for linear over cyclic hydrocarbons, as well as gas permeation selectivities higher than Knudsen values.

Finding MOFs for Highly Selective CO2/N2 Adsorption Using Materials Screening Based on Efficient Assignment of Atomic Point Charges

Electrostatic interactions are a critical factor in the adsorption of quadrupolar species such as CO(2) and N(2) in metal-organic frameworks (MOFs) and other nanoporous materials. We show how a version of the semiempirical charge equilibration method suitable for periodic materials can be used to efficiently assign charges and allow molecular simulations for a large number of MOFs. This approach is illustrated by simulating CO(2) and N(2) adsorption in ~500 MOFs; this is the largest set of structures for which this information has been reported to date. For materials predicted by our calculations to have promising adsorption selectivities, we performed more detailed calculations in which accurate quantum chemistry methods were used to assign atomic point charges, and molecular simulations were used to assess molecular diffusivities and binary adsorption isotherms. Our results identify two MOFs, experimentally known to be stable upon solvent removal, that are predicted to show no diffusion limitations for adsorbed molecules and extremely high CO(2)/N(2) adsorption selectivities for CO(2) adsorption from dry air and from gas mixtures typical of dry flue gas.

Quantifying Large Effects of Framework Flexibility on Diffusion in MOFs: CH4 and CO2 in ZIF‐8

Breathe in, breathe out: efficient methods are introduced for assessing the role of framework flexibility on molecular diffusion in metal-organic frameworks (MOFs) that does not require defining a classical forcefield for the MOF. These methods combine ab initio MD of the MOF with classical MD simulation of the diffusing molecules. The effects of flexibility are shown to be large for CH(4), but not for CO(2), in ZIF-8.

Separation of close-boiling hydrocarbon mixtures by MFI and FAU membranes made by secondary growth

Abstract We summarize and discuss recent results on the separation of close-boiling hydrocarbon mixtures by means of zeolite membranes. We focus on the separation of xylene isomers using silicalite (MFI) membranes, as well as several other hydrocarbon mixtures using faujasite membranes. In the case of the silicalite membranes, the selectivity is found to depend on the membrane microstructure. Permeation of xylene isomers through the silicalite membranes occurs through both zeolitic and non-zeolitic (intercrystalline) nanopores. This hypothesis is supported by vapor-phase permeation results on silicalite membranes synthesized with different microstructures, and by confocal microscopy experiments. In addition, a simple method for repairing calcination-induced membrane defects is presented, and its application is found to be essential in obtaining high (20–300) p -xylene/ o -xylene separation factors. The faujasite membranes are found to have high selectivities (40–150) in the separation of binary mixtures containing one aromatic component, and modest selectivities (4–9) for the separation of unsaturated from saturated low-molecular-weight hydrocarbons.

Single-walled aluminosilicate nanotube/poly(vinyl alcohol) nanocomposite membranes.

The fabrication, detailed characterization, and molecular transport properties of nanocomposite membranes containing high fractions (up to 40 vol %) of individually-dispersed aluminosilicate single-walled nanotubes (SWNTs) in poly(vinyl alcohol) (PVA), are reported. The microstructure, SWNT dispersion, SWNT dimensions, and intertubular distances within the composite membranes are characterized by scanning and transmission electron microscopy (SEM and TEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), XRD rocking curve analysis, small-angle X-ray scattering (SAXS), and solid-state NMR. PVA/SWNT nanocomposite membranes prepared from SWNT gels allow uniform dispersion of individual SWNTs in the PVA matrix with a random distribution of orientations. SAXS analysis reveals the length (∼500 nm) and outer diameter (~2.2 nm) of the dispersed SWNTs. Electron microscopy indicates good adhesion between the SWNTs and the PVA matrix without the occurrence of defects such as voids and pinholes. The transport properties of the PVA/SWNT membranes are investigated experimentally by ethanol/water mixture pervaporation measurements, computationally by grand canonical Monte Carlo and molecular dynamics, and by a macroscopic transport model for anisotropic permeation through nanotube-polymer composite membranes. The nanocomposite membranes substantially enhance the water throughput with increasing SWNT volume fraction, which leads to a moderate reduction of the water/ethanol selectivity. The model is parameterized purely from molecular simulation data with no fitted parameters, and shows reasonably good agreement with the experimental water permeability data.

Dehydration, Dehydroxylation, and Rehydroxylation of Single-Walled Aluminosilicate Nanotubes

Single-walled metal oxide (aluminosilicate) nanotubes are excellent candidates for addressing the long-standing issue of functionalizing nanotube interiors, due to their high surface reactivity and controllable dimensions. However, functionalization of the nanotube interior is impeded by its high surface silanol density (9.1 -OH/nm(2)) and resulting hydrophilicity. Controlled dehydration of the nanotubes is critical for the success of functionalization efforts. We employ a range of solid-state characterization tools to elucidate dehydration and dehydroxylation phenomena in the nanotubes as a function of heat treatment up to 450 degrees C. Vibrational spectroscopy (Fourier transform infrared, FT-IR), thermogravimetric analysis-mass spectrometry (TGA-MS), nitrogen physisorption, solid-state NMR, and X-ray diffraction (XRD) reveal that a completely dehydrated condition is achieved at 250 degrees C under vacuum and that the maximum pore volume is achieved at 300 degrees C under vacuum due to partial dehydroxylation of the dehydrated nanotube. Beyond 300 degrees C, further dehydroxylation partially disorders the nanotube wall structure. However, a unique rehydroxylation mechanism can partially reverse these structural changes upon re-exposure to water vapor. Finally, detailed XRD simulations and experiments allow further insight into the nanotube packing, the dimensions, and the dependence of nanotube XRD patterns on the water content.