Manfred Noack

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.

Permeation of single gases and gas mixtures through faujasite-type molecular sieve membranes

Abstract Composite membranes that consist of thin faujasite layers on the surfaces of porous α-Al2O3 support disks were prepared by using a two-step synthesis. First, a seed layer of externally synthesized Na–Y nanocrystals was deposited on the surface of the support. Second, the polycrystalline faujasite layer was grown hydrothermally. Characterization by X-ray diffraction, field emission-scanning electron microscopy and energy dispersive X-ray spectroscopy showed dense intergrown FAU layers with Si/Al ratios of 1.3–1.8 and layer thicknesses in the range of 0.8–6 μm depending on the synthesis conditions. Two different kinds of heating were used in the second synthesis step, either a conventional oven or a microwave oven. The synthesis conditions like crystallization time and temperature as well as the kind of heating influenced the permeation properties of the supported faujasite membranes. The thicker the FAU layers, the lower were the permeances of the single gases which were found to be of the order H2>CH4>N2>O2>CO2>nC4H10>SF6 at 23 °C. The separation factors α of equimolar binary gas mixtures were measured at 23 °C. For the gas mixture N2/CO2 an α of 8.4 was found, for the mixture CH4/CO 2 α was 3.5. From these permeation characteristics one can draw the conclusion that the separation of these small gas molecules by the large and heteropolar FAU (Na–X) pore system with 0.74 nm pore opening diameters is mainly based on differences of adsorption or diffusion rates, rather than on size exclusion like in LTA- or MFI-type zeolite 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.

Change of gas permeation by photoinduced switching of zeolite-azobenzene membranes of type MFI and FAU

Abstract FAU membranes of type Na–X and MFI membranes of type silicalite-1 which contain adsorbed azobenzene (AZB), have photoswitchable permeation properties due to the trans–cis photoisomerization of AZB. The change of gas permeation through these host–guest composite membranes depends on the irradiation wavelength, the quality of the membranes and the amount of adsorbed AZB. The permeances of the gases in the trans-form of the zeolite-encapsulated AZB were higher than those in the cis-form. The separation factors of equimolar mixtures of N2/CO2 and CH4/CO2 fed through the FAU–AZB membrane were higher at trans-switching than at cis-switching. The changes of the permeances of single gases and of the separation factors of equimolar gas mixtures by photoinduced trans–cis switching were found to be reversible over numerous switching cycles. The experimental results are in agreement with predictions made by Monte Carlo simulations for passing of permeant gases through these host–guest systems with the zeolite-hosted AZB in the trans- and in the cis-configuration.

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.

News from AlPO4-5: Microwave synthesis, application as medium to organize molecules for spectroscopy and nonlinear optics, material for one-dimensional membranes

Synthesis of AlPO 4 -5, crystals of high quality results in novel applications of this molecular sieve: (i) Guest molecules become ordered in the pores of the molecular sieve crystal and allow spectroscopy on aligned molecules. (ii) Due to a specific adsorption mechanism, dipolar organic molecules become oriented by entering the pores and give nonlinear optical effects like frequency doubling (SHG). (iii) Aligned AlPO 4 -5 crystals in a nickel film represent a new one-dimensional membrane for molecular sieving.