Zeolites are microporous crystalline aluminosilicates with molecular sieving capabilities (pore size < 1.3 nm).[1±3] The use of zeolite coatings on different supports (metal substrates, porous glass, ceramic honeycombs, mesoporous materials) has recently been proposed for catalysis, adsorption, and separation.[4±8] Open-cell ceramic foams [9±15] (connected pores in the 0.1±4 mm range) are gaining interest as zeolite carriers [11±15] due to their unique properties, for the development of new reactor concepts (monolithic, catalytic wall configurations) for technical catalytic and adsorption±separation processes. [15,16] Here we present, for the first time, a novel zeolite±ceramic composite material, with a bimodal (nano-, macroscale) pore system, obtained by coating the cell walls of microcellular polymer-derived ceramic foams (cell size~8 lm) with a thin, binder-free layer of MFI-type zeolite (Silicalite-1 and ZSM-5), produced by an in-situ conventional supported hydrothermal synthesis. To our knowledge, these materials represent the smallest microreactors/separators fabricated to date.Ceramic foams, with low thermal expansion coefficients and high thermal, mechanical, and chemical stability, are suitable as zeolite supports because they provide enhanced mass and heat transfer and a low pressure drop when compared to zeolite pellets used in fixed-bed reactors. [9,10] The fabrication of ceramic foams from preceramic polymers allows tailoring of the composition and morphology, and therefore of the final properties, of the porous material. [17,18] Very recently, [19,20] we fabricated mechanically stable ceramic microcellular [21,22] foams with a homogeneous interconnected porosity and an average cell size in the range of~1±100 lm, bulk density from 0.2±0.6 g cm ±3, and cell density from~10 7 ±10 12 cells cm ±3 . The starting materials were poly(methyl methacrylate) (PMMA) microbeads, acting as sacrificial template, and a COMMUNICATIONS
The structure of the intermediate species in the clear solution synthesis of the MFI framework (zeolite ZSM 5) has been investigated using the characteristic 13C, 14N, 15N, and 2D spectra of the tetrapropylammonium (TPA) template ions as probes as well as the 29Si spectra of the silicate species present in samples that can be isolated by centrifugation. Comparison with the corresponding spectra of the final products that can be characterized by X-ray diffraction indicates that there is no evidence for the involvement of nanospecies, as has been proposed, and that crystallization is most probably from an amorphous gel. This conclusion is supported by the lack of deuterium rotational echo double-resonance dephasing of the 29Si spectra by deuterated TPA of the earliest intermediate species obtained, while it is clearly observed in the final product. These observations indicate that any TPA ions present in the gel phase are not in intimate contact with the silicon nuclei as they would be if in the local MFI environment. This is supported by the very low amount of TPA found by 14N NMR, which is much less than needed for the proposed nano intermediates and the fact that the TPA present can be removed by simple re-suspension in water and recovery.
Fine-pored, 45 ppi (pores per linear inch) alumina foams are prepared from ceramic slurries with varying contents of additives (deflocculant, binder) and solid loading following a standardized procedure. Rheological key parameters (yield stress, high-shear viscosity) of the respective slurries are determined by approximation of the experimental flow curves with appropriate rheological models. The resulting ceramic foams are characterized by computed tomography (CT) followed by a morphometric analysis of the reconstruction volume data. The main scope of the work involves the development of a procedure to reliably define the binarization threshold during these morphometric calculations, which is based on the analysis of the differential course of the total porosity results from calculations performed at varying binarization threshold values ("differential thresholding"). A very good match of the CT porosity results with experimental data is achieved, despite the unfavorable CT voxel resolution to foam structure fineness relation. The CT evaluation results are finally correlated to the rheological properties of the respective slurries used in foam manufacturing. The dominant slurry composition parameters are the weight fraction of the ceramic powder and the binder concentration. Increasing binder and solid content result in an increased yield stress and viscosity of the respective dispersion and consequently in a decreased porosity and cell size of the finally manufactured cellular ceramic.
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