Michael Tsapatsis

Hierarchical nanofabrication of microporous crystals with ordered mesoporosity

Shaped zeolite nanocrystals and larger zeolite particles with three-dimensionally ordered mesoporous (3DOm) features hold exciting technological implications for manufacturing thin, oriented molecular sieve films and realizing new selective, molecularly accessible and robust catalysts. A recognized means for controlled synthesis of such nanoparticulate and imprinted materials revolves around templating approaches, yet identification of an appropriately versatile template has remained elusive. Because of their highly interconnected pore space, ordered mesoporous carbon replicas serve as conceptually attractive materials for carrying out confined synthesis of zeolite crystals. Here, we demonstrate how a wide range of crystal morphologies can be realized through such confined growth within 3DOm carbon, synthesized by replication of colloidal crystals composed of size-tunable (about 10-40 nm) silica nanoparticles. Confined crystal growth within these templates leads to size-tunable, uniformly shaped silicalite-1 nanocrystals as well as 3DOm-imprinted single-crystal zeolite particles. In addition, novel crystal morphologies, consisting of faceted crystal outgrowths from primary crystalline particles have been discovered, providing new insight into constricted crystal growth mechanisms underlying confined synthesis.

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.

Synthesis of self-pillared zeolite nanosheets by repetitive branching

Go with the Flow Effective absorption or filtration can be achieved by having a material with multiple levels of porosity, so that the main flow can occur in the larger channels, while smaller passageways can be used to sequester a secondary material. It can be difficult to make these materials because the pores need to be different sizes, but still fully connected to each other. Zhang et al. (p. 1684) show that a hierarchical zeolite can be made through a simple process using a single structure-directing agent that causes repetitive branching. This leads to a material with improved transport and catalytic properties. Single-step synthesis of pillared zeolite nanosheets is achieved with a common structure-directing agent. Hierarchical zeolites are a class of microporous catalysts and adsorbents that also contain mesopores, which allow for fast transport of bulky molecules and thereby enable improved performance in petrochemical and biomass processing. We used repetitive branching during one-step hydrothermal crystal growth to synthesize a new hierarchical zeolite made of orthogonally connected microporous nanosheets. The nanosheets are 2 nanometers thick and contain a network of 0.5-nanometer micropores. The house-of-cards arrangement of the nanosheets creates a permanent network of 2- to 7-nanometer mesopores, which, along with the high external surface area and reduced micropore diffusion length, account for higher reaction rates for bulky molecules relative to those of other mesoporous and conventional MFI zeolites.

Dispersible exfoliated zeolite nanosheets and their application as a selective membrane

Thin zeolite films prepared through a polymer exfoliation method were used as selective membranes. Thin zeolite films are attractive for a wide range of applications, including molecular sieve membranes, catalytic membrane reactors, permeation barriers, and low-dielectric-constant materials. Synthesis of thin zeolite films using high-aspect-ratio zeolite nanosheets is desirable because of the packing and processing advantages of the nanosheets over isotropic zeolite nanoparticles. Attempts to obtain a dispersed suspension of zeolite nanosheets via exfoliation of their lamellar precursors have been hampered because of their structure deterioration and morphological damage (fragmentation, curling, and aggregation). We demonstrated the synthesis and structure determination of highly crystalline nanosheets of zeolite frameworks MWW and MFI. The purity and morphological integrity of these nanosheets allow them to pack well on porous supports, facilitating the fabrication of molecular sieve membranes.

Layer Structure Preservation during Swelling, Pillaring, and Exfoliation of a Zeolite Precursor

MCM-22(P), the precursor to zeolite MCM-22, consists of stacks of layers that can be swollen and exfoliated to produce catalytically active materials. However, the current swelling procedures result in significant degradation of crystal morphology along with partial loss of crystallinity and dissolution of the crystalline phase. Fabrication of polymer nanocomposites and coatings with MCM-22 for separation, barrier, and other applications requires a swelling method that does not alter drastically the crystal morphology and layer structure and preserves the high aspect ratio of the layers. Here, we demonstrate such a method by swelling MCM-22(P) at room temperature. The low-temperature process does not disrupt the framework connectivity present in the parent MCM-22(P) material. By extensive washing with water, the swollen material, MCM-22(PS-RT), evolves to a new ordered layered structure. Interestingly, the swelling procedure is reversible and the swollen material can be restored back to MCM-22(P) by acidification of the sample. The swollen material can also be pillared to produce an MCM-36 analogue. It can also be exfoliated, and layers can be incorporated in a polymer matrix to make nanocomposites.

Grain boundary defect elimination in a zeolite membrane by rapid thermal processing.

Optimizing Molecular Sieve Production Microporous membranes composed of aluminosilicate minerals are known as zeolites and are often called molecular sieves because of their ability to filter or separate small molecules. The separation performance is partly governed by the selectivity for one species over another, and this can be compromised by defects, which allow for easy diffusion pathways. To create the porosity, structure-directing agents are used, which need to be removed during a long thermal treatment that can generate defects. Choi et al. (p. 590) show that for the silicalite-1 system, a rapid thermal treatment significantly reduces the defect density, with corresponding improvement in the filtration of very similar species, such as xylene isomers. A reduction in the formation of defects in silicalite-1 zeolite membranes improves their isomer separation capabilities. Microporous molecular sieve catalysts and adsorbents discriminate molecules on the basis of size and shape. Interest in molecular sieve films stems from their potential for energy-efficient membrane separations. However, grain boundary defects, formed in response to stresses induced by heat treatment, compromise their selectivity by creating nonselective transport pathways for permeating molecules. We show that rapid thermal processing can improve the separation performance of thick columnar films of a certain zeolite (silicalite-1) by eliminating grain boundary defects, possibly by strengthening grain bonding at the grain boundaries. This methodology enables the preparation of silicalite-1 membranes with high separation performance for aromatic and linear versus branched hydrocarbon isomers and holds promise for realizing high-throughput and scalable production of these zeolite membranes with improved energy efficiency.

Hydrothermal synthesis of zeolites with three-dimensionally ordered mesoporous-imprinted structure.

Zeolites are microporous materials with pores and channels of molecular dimensions that find numerous applications in catalysis, separations, ion exchange, etc. However, whereas uniformity of micropore size is a most desirable and enabling characteristic for many of their uses, in certain cases, for example in reactions involving bulky molecules, it is a limitation. For this reason, synthesis of hierarchical zeolites with micro- and mesoporosity is of considerable interest as a way to control molecular traffic for improved catalytic and separation performance. Herein, we report a general synthesis route for the confined synthesis of zeolites within three-dimensionally ordered mesoporous carbon templates by conventional hydrothermal synthesis. Various zeolites, including BEA, LTA, FAU, and LTL, with three-dimensionally ordered mesoporous-imprinted structure have been synthesized by this approach. It is expected that these hierarchical zeolite materials will provide building blocks for thin-film and other syntheses and may provide a basis for quantitatively studying the mass-transfer limitation on the catalytic performance of zeolite catalysts.

Structure and aging characteristics of H2-permselective SiO2-Vycor membranes

Hydrogen-permselective membranes were prepared by atmospheric pressure chemical vapor deposition (APCVD) of SiO_2 layers in porous Vycor tubes. The deposition was carried out by passing the reactants SiCl_4 and H_2O through the bore of the support tubes at temperatures ranging from 600 to 800°C. The deposit layers were examined by TEM, SEM, and EPMA. When the deposit was confined inside the pores of the Vycor substrate, the membranes were mechanically stable but when it extended substantially outside of the porous matrix the stresses induced by thermal cycling led to crack formation and propagation. Electron microscopy revealed that the SiO_2 deposit density is maximum in a region ≈0.5 μm thick adjacent to the bore surface and gradually declines to zero within a depth of ≈10 μm from the surface. The thin region of maximum deposit density is responsible for permselectivity, for it essentially blocks the permeation of nitrogen and larger molecules while allowing substantial permeation of hydrogen. This region contains ≈10% by volume trapped voids and as a result has relatively high permeability as suggested by the percolation theory. Annealing at high temperatures causes densification of the deposited material as evidenced by increased activation energy for H_2 permeation and correspondingly reduced permeance. The presence of H_2O vapor accelerates the densification process. The densified membranes had a H_2 permeance as high as 0.1 cm^3(STP)/min-atm-cm^2 at 500°C and a H_2/N_2 permeance ratio above 500.

Preparation and characterization of a glassy fluorinated polyimide zeolite-mixed matrix membrane

Mixed matrix membranes of 6FDA-6FpDA-DABA, a glassy polyimide, and modified zeolites (ZSM-2) were successfully fabricated using the procedure outlined in this paper. The membranes were cast from solution and then exposed to different gases for the purpose of determining and comparing the diffusivity coefficients, the solubility coefficients, and the permeation rates of He, O2, N2, CH4 and CO2 of the pure polyimide and the composite membrane. FTIR spectra were collected from the pure polyimide, the polyimide and untethered zeolite solutions, and the mixed matrix membrane (MMM) solution. Comparison of the spectra revealed the presence of hydrogen bonding in the MMM solution not present in the other samples. FESEM images and TEM images did not reveal the presence of voids between the polymer and the zeolite. These images also revealed that when given ample time for the solvent to evaporate, the zeolites sediment to one side of the membrane. This develops a polymer-rich phase and a zeolite-rich phase, and many of the ZSM-2 zeolites appear to adopt an orientation with their largest face orthogonal to the direction of the gas flow.

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.