Peter Kölsch

Zeolite membranes – state of their development and perspective

Abstract An ideal zeolite membrane combines the general advantages of inorganic membranes (temperature stability, solvent resistance) with a perfect shape selectivity. Due to their “molecular sieve” function, zeolite membranes can principally discriminate the components of gaseous or liquid mixtures dependent on their molecular size. This molecular sieving principle requires a pinhole- and crack-free zeolite membrane. Remarkable separation effects can also be achieved by the interplay of mixture adsorption and mixture diffusion. This review focuses on composite membranes which consist of a zeolite top layer on a mesoporous ceramic or metal support. Special attention is given to supported MFI membranes.

MFI membranes of different Si/Al ratios for pervaporation and steam permeation

Abstract For pervaporation and steam permeation, in one- and two-step crystallizations, respectively, two kinds of MFI membranes of different SiO 2 /Al 2 O 3 ratio on a mesoporous alumina ceramic support have been prepared. In the one-step crystallization, using TPA-OH as template, a supported silicalite membrane was obtained by in situ crystallization. In the two-step crystallization, by first attaching seed crystals to the ceramic surface with electrostatic forces and then crystallizing a continuous MFI layer in a template-free synthesis, a ZSM-5 membrane was prepared. The membranes have been characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) in combination with electron dispersive X-ray (EDX) line scan as well as by single gas and binary mixture permeation. In pervaporation and steam permeation measurements of water/iso-propanol and water/methanol mixtures, the silicalite membrane showed a hydrophobic and the ZSM-5 membrane a hydrophilic separation behavior. However, only the silicalite membrane obtained by one-step crystallization allows shape selective permeation such as the separation of n/iso-butane and methanol/MTBE mixtures. In the case of the polycrystalline ZSM-5 membrane with columnar microstructure, a superposition of mass transport through the crystal boundaries and the crystals takes place.

Molecular Sieve Membranes for Industrial Application: Problems, Progress, Solutions

Molecular sieve membranes are characterized by their high thermal and chemical resistance and by their monodisperse micropore system. Mixtures can be separated by their molecular size as well as by adsorptive interactions and differences in the diffusion coefficients. In this paper, techniques for a reproducible preparation of large area tubular zeolite membranes together with ways for their regeneration and repair are shown. The separation figures demonstrate the efficiency of the molecular sieve membranes developed. Potential application fields are discussed.

Preparation of MFI membranes of enlarged area with high reproducibility

Abstract In the last 10 years extensive work was done in the preparation and characterization of molecular sieve membranes, especially of the types MFI, LTA and FAU. There is a remarkable progress in the synthesis of MFI membranes from the pure silica crystallization system. These silicalite-1 membranes show both separation properties by molecular size exclusion and by molecular interaction because of the hydrophobic interaction of the silica surface with permeating molecules. Silicalite-1 membranes exhibit promising permeation properties for industrially interesting mixtures as n-/iso-alkanes or the xylene isomers. However, so far these membranes are available only with areas of 1–10 cm2, and the reproducibility of the membrane preparation is unsatisfactory. The yield of high-quality membranes is in the range of 10–30% of the preparations. In this work the membrane area was enlarged by the factor of 30, and the support shape was changed from a flat disc to a tube geometry. This was achieved by the optimization of the synthesis gel composition and the synthesis conditions. Furthermore, by changing the prepared membrane type from a mono- to a polylayer and by varying the calcination conditions, the density and size of leaks could be reduced. As a result of all these measures the yield of high-quality membranes with a minimum permselectivity of H2 to SF6 of 43 (which is five times the Knudsen factor) as a quality criterium was increased to 70% of all preparations. Special attention was given to the economy of the synthesis of MFI membranes which implies to save material, energy and time.

Chemically modified ceramic membranes

Abstract Commercial ultrafiltration ceramic filters have been modified by chemical treatment. The γ-Al 2 O 3 top layer of the asymmetric ceramic filter was modified (1) by in situ hydrolysis of tetraethylorthosilicate, giving a hydrophilic nanoporous SiO x top layer rich in Si–OH, (2) by silylation of the γ-Al 2 O 3 layer, providing an organophilic functionalization, and (3) by reaction of the γ-Al 2 O 3 layer with alkyl/aryl phosphonic acids, resulting in an organophobic behaviour. By these chemical treatments both a pore narrowing and a hydrophilic/hydrophobic functionalization can be achieved. Permeation results of single gases and pervaporation data of liquid mixtures on the novel membranes are presented.

Membrane supported catalytic dehydrogenation of iso-butane using an MFI zeolite membrane reactor

Abstract A H2-selective MFI zeolite membrane has been crystallized as layer on a porous ceramic tube. At 510 ° C , this membrane can separate H2 from iso-butane/iso-butene with mixture separation factors of 70 and H2 permeances of ca. 1 m 3 ( STP )/ m 2 hbar. In membrane supported iso-butane dehydrogenation the conversion of iso-butane was increased by almost a factor of 2. However, a detailed analysis of the experiment showed that 2/3 of the conversion increase is based on the dilution of the feed by the sweep gas and 1/3 is due to the H2 removal.

New one-dimensional membrane: aligned AlPO4-5 molecular sieve crystals in a nickel foil

Oriented AlPO4-5 crystals can be embedded into a metallic grid and fixed by galvanic nickel deposition to form a membrane that is thermally stable up to 650 K.. Such an “AlPO4-5-in-nickel-membrane” was tested in the separation of binary mixtures of n-heptane and various aromatic compounds of different molecular geometries.

Zeolite-in-metal membranes: preparation and testing

By galvanic metal deposition, silicalite crystals have been embedded in silver or nickel foils; the resulting composites are self-supporting high-temperature membranes (thermally stable up to 650 K) and have been tested by permeation/pervaporation studies of a binary n-heptane–toluene mixture; a separation factor αtol/n-C7ca. 4 ± 1.5 is found over a wide temperature range.

Ceramic Membranes for Water Separation from Organic Solvents

Novel hydrophilic SiO x modified alumina membranes with high separation factors and flux rates have been prepared for the separation of water from organic solvents. For the preparation of the membranes, SiO x networks are deposited inside the γ-Al 2 O 3 layer of a commercial ultrafiltration membrane by hydrolysis of tetraethylorthosilicate in autoclaves at 250°C. The transport resistances of the individual membrane layers for the permeation flux are described by a model. The membranes are stable towards solvents to temperatures of at least 150°C. Pervaporation studies show that water can be separated from solvents such as acetonitrile, tetrahydrofurane, 2-propanol, ethyl alcohol, dimethylsulfoxide, N,N-dimethylformamide, and phenol. The separation performance of the membranes allows their use in technical separation processes, especially for the removal of water.

Development of a H2-selective SiO2-membrane for the catalytic dehydrogenation of propane

Abstract Commercial alumina ceramic tubes of asymmetric cross section were used as supports for the deposition of SiO 2 layers by the sol–gel technique. The SiO 2 gel layer has been built up on the γ-alumina ultrafiltration (UF)-layer of the support by dip-coating with a SiO 2 -polymer sol. The sol was prepared by a two-step acidic-base hydrolysis of tetraethylorthosilicate (TEOS) in alcoholic solution with a relative low content of water (H 2 O:TEOS=4.3:1). The following step of hydrolysis and condensation was varied by different amounts of NH 4 OH. These sols of lowly branched SiO 2 polymers with 60–70% Q 3 units were used for dip-coating. After drying (2 h at ambient temperature) the membranes have been calcined at 600°C. The resulting membranes show hydrogen fluxes ≥20 m 3 (STP)/m 2 h bar and permselectivities hydrogen:propane between 30 and 75 in a temperature region of 450–550°C. The membranes were tested in membrane supported propane dehydrogenation. The propene yield in the membrane reactor can be higher than in a conventional fixed-bed reactor if the hydrogen is removed by a H 2 -selective membrane. After several regeneration cycles, stable and acceptable membrane properties are found.