E.R. Geus

Synthesis and characterization of zeolite (MFI) membranes on porous ceramic supports

Continuous polycrystalline films of ZSM-5 (MFI-type) crystals have been grown on porous ceramic (clay) supports. The films are thermomechanically stable upon calcination at 400 °C in air to remove template ions (tetrapropylammonium), during which process the porous support seems to have a stabilizing effect. When the hydrothermal conditions induce substantial aluminium leaching from the support, large analcime crystals are grown on the support. Gas permeation experiments with both pure gases (permanent gases, alkanes and difluorodichloromethane) and mixtures thereof have been performed on the MFI composite to investigate the separation potential of this new type of membrane. Expected selectivities as a result of large differences in diffusivity are found to be strongly reduced by differences in sorption of the same order of magnitude, and the reduced mobility of weakly adsorbing (fast moving) molecules caused by the slower moving species. However, from the observed low permeation rates it is expected that, owing to the presence of a porous support with relatively low porosity, the molecular sieving effect is reduced as well.

High-temperature stainless steel supported zeolite (MFI) membranes: Preparation, module construction, and permeation experiments

Abstract Continuous layers of MFI (silicalite-1; Si-rich ZSM-5) have been prepared on porous, sintered stainless steel supports. Similar metal supported MFI membranes of ∼ 50 μm thickness have been grown within stainless steel membrane modules in order to perform (high-temperature) permeation experiments. As-synthesized layers are found to be gas-tight even for small molecules such as neon. The supported MFI layers remain thermomechanically stable upon calcination at 400°C in air to remove template ions (tetrapropylammonium). Gas permeation experiments have been performed using neon, methane, n-butane, and isobutane according to the Wicke—Kallenbach principle with helium as a purge gas. The sequence of the pure gas permeabilities at room temperature and 0.3 bar partial pressure difference is methane >n-butane > neon a isobutane, demonstrating that the permeation is based on both adsorption and diffusion. The deviating permeation behaviour between the butane isomers is attributed to the bulkiness of isobutane, which is also reflected in the substantially lower adsorption capacity as compared with n-butane. In experiments using binary mixtures of strongly (butane isomers) and weakly (methane) adsorbing species, the permeation rate of the former is hardly affected, whereas for the latter a drop in permeability of some two orders of magnitude is observed. At higher temperatures (up to 350°C) with a constant feed composition, the methane permeation rate increases as a result of the decreased adsorption of n-butane. The MFI layer retains its separation potential after several heating and cooling cycles.

Calcination of large MFI-type single crystals, Part 2: Crack formation and thermomechanical properties in view of the preparation of zeolite membranes

Abstract The calcination in air of large, tetrapropylammonium (TPA)-templated silicalite (MFI) crystals has been studied with special attention to the development of cracks. Crystal cracking on activation of TPA-templated MFI crystalline material arranged in a membrane configuration adversely affects the molecular sieve characteristics desired. Techniques used are visible light microscopy, microscope FT i.r. spectroscopy, and thermogravimetry. Differently prepared silicalite crystals have been found to exhibit deviating crystal cracking. Temperature-programmed X-ray powder diffraction has been performed to assess the thermal expansion/contraction behavior of both as-synthesized (TPA-containing) MFI on calcination and activated MFI material when subjected to thermal cycling. The observations from the calcination study assisted in determining a “safe” calcination procedure for supported, polycrystalline silicalite/MFI-type membranes, prepared via in situ growth.

Calcination of large MFI-type single crystals: Part 1. Evidence for the occurrence of consecutive growth forms and possible diffusion barriers arising thereof

Abstract The removal of tetrapropylammonium (TPA) from large, prismatic-shaped single crystals of the MFI-type (silicalite-1) has been studied by light microscopy and microscope FT i.r. spectroscopy. The first stage in the calcination process, the Hofmann elimination of TPA to tripropylamine and propene, proceeds homogeneously below 300°C. The subsequent β-elimination reactions to lower alkylamines takes place in two crystal sections at a somewhat lower calcination temperature as compared to other crystal parts and leads to an “hourglass pattern” within the crystal during calcination. This may be attributed to the hindered diffusion of water from these crystal sections, presumably as a result of interfaces between crystal parts, developed during the stepwise crystal growth process. The microscope FT i.r. technique provides for a direct observation of chemical and/or physical processes within one selected zeolite single crystal and allows for in situ scanning over the crystal for detailed observations.

Single and Multi-Component Transport through Metal-Supported MFI Zeolite Membranes

Continuous zeolite (silicalite; MFI) layers were grown on a porous sintered stainless steel support in an all metal high-temperature membrane reactor. To test the application potentials of these layers, permeation experiments, according the Wicke-Kallenbach method, were performed using noble gases, lower n-alkanes, the butane-isomers, CFC-12 and 2,2,4-trimethylpentane.

Permeation measurements on in situ grown ceramic MFI type films

Continuous MFI type films were grown on macroporous ceramic clay-type supports. Permeation experiments were performed using a Wicke-Kallenbach experimental set-up. Applying helium as an inert carrier gas, the permeation behaviour of strongly (n-butane, isobutane), and weakly adsorbing species (neon, argon, methane) was studied at room temperature for both ’pure’ gases, and binary mixtures. Significant differences in permeation behaviour were found between strongly and weakly adsorbing gases, although steady state permeation rates varied less than one order of magnitude. The lower diffusivity of heavier alkanes is compensated.by the higher sorbate concentration within the zeolite micropores. Accordingly, only low selectivities were found for binary mixtures, where the permeation rate is governed by the slowest moving species. For sorption near or within the Henry region, substantially higher selectivities were obtained. The presence of a macroporous layer on one side of the membrane will lead to reduced permeation rates, especially for strongly adsorbing molecules.

Permeability studies on a silicalite single crystal membrane model

Summary Permeation experiments on large silicalite single crystals (MFI type), embedded in an epoxy resin were performed, using permanent gases and small alkanes. A scaling-up from initially a one zeolite crystal membrane to a multi zeolite crystal membrane is presented. Differences in permeability were small and attributed to a sorption-diffusion mechanism, in qualitative agreement with theory. As permeation may vary within different regimes, maximum selectivities are expected in the Henry region. Some boundary conditions in the preparation of (ceramic) zeolite membranes are formulated.