Zeolitic imidazolate frameworks (ZIFs) are a subclass of nanoporous metal−organic frameworks (MOFs) that exhibit zeolite-like structural topologies and have interesting molecular recognition properties, such as molecular sieving and gate-opening effects associated with their pore apertures. The synthesis and characterization of hybrid ZIFs with mixed linkers in the framework are described in this work, producing materials with properties distinctly different from the parent frameworks (ZIF-8, ZIF-90, and ZIF-7). NMR spectroscopy is used to assess the relative amounts of the different linkers included in the frameworks, whereas nitrogen physisorption shows the evolution of the effective pore size distribution in materials resulting from the framework hybridization. X-ray diffraction shows these hybrid materials to be crystalline. In the case of ZIF-8-90 hybrids, the cubic space group of the parent frameworks is continuously maintained, whereas in the case of the ZIF-7-8 hybrids there is a transition from a cubic to a rhombohedral space group. Nitrogen physisorption data reveal that the hybrid materials exhibit substantial changes in gate-opening phenomena, either occurring at continuously tunable partial pressures of nitrogen (ZIF-8-90 hybrids) or loss of gate-opening effects to yield more rigid frameworks (ZIF-7-8 hybrids). With this synthetic approach, significant alterations in MOF properties may be realized to suit a desired separation or catalytic process.

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.

The fabrication of advanced molecular separation membranes is attracting great interest because of their potential to significantly increase the energy efficiency, and reduce the cost, of renewable/clean fuel production, bio-based chemical production, greenhouse gas capture, and water purification. Currently available polymeric membranes have the advantage of low cost and high processibility. They can be engineered into different morphologies, such as asymmetric hollow fiber membranes (ca. 100-300 mm in diameter) comprised of mechanically strong macroporous fibers with a thin, dense polymeric layer that performs molecular separation. [1] These fibers are used to fabricate modules with large membrane areas in excess of 1000 m 2 m À3 of module volume. [1,2] However, polymeric membranes are limited by an intrinsic trade-off between permeability and selectivity, [3] and higher-performance materials are desired to decisively improve the economics of emerging separation technologies. Inorganic (zeolitic) molecular sieving membranes with very high throughput and high selectivity can be fabricated by hydrothermal processing on flat and tubular ceramic supports, but currently have limited applications. This is due to the high cost of the support materials and difficulties in the scale-up and reliability of hydrothermal growth and high-temperature calcination to remove the organic structure-directing agents. [4] Metal-organic frameworks (MOFs) are a large emerging class of crystalline nanoporous materials composed of metal centers linked by organic ligands. The synthesis and applications of MOF membranes have generated a great deal of interest. [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] MOFs possess attractive properties, such as finetuning of the pore structure by the judicious choice of metallic and organic components, and elimination of the high-temperature calcination step that is usually necessary for inorganic molecular sieving membranes. Zeolitic imidazolate frameworks (ZIFs), a subclass of MOFs, have emerged as candidates for use in the synthesis of membranes for gas and hydrocarbon separation, owing to their attractive crystallographic pore sizes (0.3-0.5 nm), and good thermal and chemical stability. [22] Taking a processing route similar to that of zeolitic membranes, several reports have described the growth and characterization of continuous polycrystalline membranes of materials such as ZIF-7, ZIF-8, ZIF-69, and ZIF-90 on ceramic (a-Al 2 O 3 , TiO 2 ) disks and tubular supports. [8][9][10][11][12][13][14][15][16][17][18][19] Further attention has been paid to the modification of ceramic support surfaces with polymeric coatings or organosilane reagents, to enhance the adhesion and growth of MOF and ZIF membranes. [7,8,11,[13][14][15][16][17][18][19] Table S1 (see the Supporting Information) reviews currently available data [8][9][10][11][12][13][14][15][16][17][18][19][20] on the gas permeation characteristics of ZIF membranes in a temperature range of 25-225 8C and a pressure range of ...

A multiscale experimental study of the structural, compositional, and morphological characteristics of aluminosilicate (LTA) and pure-silica (MFI) zeolite materials surface-modified with MgO x H y nanostructures is presented. These characteristics are correlated with the suitability of such materials in the fabrication of LTA/ Matrimid mixed-matrix membranes (MMMs) for CO 2 /CH 4 separations. The four functionalization methods studied in this work produce surface nanostructures that may appear superficially similar under SEM observation but in fact differ considerably in shape, size, surface coverage, surface area/ roughness, degree of attachment to the zeolite surface, and degree of zeolite pore blocking. The evaluation of these characteristics by a combination of TEM, HRTEM, N 2 physisorption, multiscale compositional analysis (XPS, EDX, and ICP-AES elemental analysis), and diffraction (ED and XRD) allows improved understanding of the origin of disparate gas permeation properties observed in MMMs made with four types of surface-modified zeolite LTA materials, as well as a rational selection of the method expected to result in the best enhancement of the desired properties (in the present case, CO 2 /CH 4 selectivity increase without sacrificing permeability). A method based on ion exchange of the LTA with Mg 2+ , followed by base-induced precipitation and growth of MgO x H y nanostructures, deemed "ion exchange functionalization" here, offers modified particles with the best overall characteristics resulting in the most effective MMMs. LTA/ Matrimid MMMs containing ion exchange functionalized particles had a considerably higher CO 2 /CH 4 selectivity (∼40) than could be obtained with the other functionalization techniques (∼30), while maintaining a CO 2 permeability of ∼10 barrers. A parallel study on pure silica MFI surface nanostructures is also presented to compare and contrast with the zeolite LTA case.

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