Zeolites: Superior Mass Transfer Properties of Technical Zeolite Bodies with Hierarchical Porosity (Adv. Funct. Mater. 2/2014) (original) (raw)
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Advanced Functional Materials, 2012
Engineering levels of porosity in hierarchical zeolites is a vibrant area of research with remarkable application potential. To gain practical relevance, the superior properties observed for the as-synthesized powders have to be preserved when they are shaped into suitable technical geometries. Herein, mechanically stable millimeter-sized bodies are prepared by granulation of mesoporous ZSM-5 zeolite powders using an attapulgite clay binder. Alkaline treatment of conventional zeolite granules is demonstrated to be unsuitable for this purpose. Multiple techniques are applied to characterize mesoporous zeolite granules with respect to their conventional zeolite counterparts, thus establishing the impact of binder inclusion and granulation on their respective properties. The intrinsic structure and acidity of the zeolite are retained post-structuring. Gas adsorption and mercury porosimetry confi rm the presence of interconnected micro-, meso-, and macropores. A wide range of imaging techniques permits visualization of the particle properties, phase distribution, and consequent origins of the distinct levels of porosity within the zeolite granules. The superior adsorption properties of the hierarchical ZSM-5 zeolite granules are demonstrated using cyclohexane, toluene, and isopropyl alcohol as probe molecules.
Modeling of Adsorption Properties of Zeolites: Correlation with the Structure
The Journal of Physical Chemistry B, 1997
The adsorption of N 2 and CO in Na X-zeolites has been studied for different framework structures and extraframework cation distributions. To this aim, the cation-molecule system modeling one site has been embedded in a set of external point charges which simulate the zeolite environment of the site and has been treated quantum chemically, using a method based on density functional theory. This procedure has been applied to the 64 cationic sites accessible for adsorption in a crystal unit cell of an ideal X-zeolite with a Si/Al ratio equal to 1. These calculations have shown that only a few cations are favorable for initial adsorption and that those cations are always of type III(III′). Their efficiency depends both on the framework geometry and on their location in the supercages. The analysis of the quantum chemical results in terms of a classical description involving electrostatic and induction interaction energies with the framework has led to the conclusion that the direction of the electric field vector created by the zeolite in the supercages is an important factor determining the zeolite adsorption properties.
Sorption Uptake of the Molecular Mobility of n-Paraffins in ZSM-5 Type Zeolite
Adsorption Science & Technology
Sorption uptake of propane and n-hexane on ZSM-5 type zeolite monocrystals has been investigated at 313 K and 323 K, respectively, by means of a constant-volume/variable-pressure method. In the case of propane no reliable information on the molecular mobility could be found. The intracrystalline diffusivity of n-hexane in the Na,H-ZSM-5 zeolite sample is 10−8 to 10−9 cm2s−1.
Adsorption and diffusion in zeolites: the pitfall of isotypic crystal structures
Molecular Simulation, 2011
The influence of isotypic crystal structures on adsorption and diffusion of methane in all-silica LTA, SAS and ITE zeolites is studied. Results obtained with the experimental structures are compared with structure predictions and approximations that are commonly employed. The results indicate that diffusion coefficients are much more affected than Henry coefficients. In fact, orders of magnitude deviations in the diffusivity can be observed and a systematic parameter study finally gives rise to the correlation between structure sensitivity ...
The improvement of molecular transport properties of hierarchical H-ZSM-5 obtained by desilication was evidenced by studying the desorption of o-xylene and isooctane by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). This technique enabled monitoring simultaneously bands associated with the molecular probes and the zeolite, using powdered sample masses as low as 1 mg. Two H-ZSM-5 samples with markedly different crystal sizes were investigated. The first sample was commercial and consisted of small crystallites (ca. 250 nm). The second sample were laboratory-made large crystals with coffin-like shape (ca. 17 Â 4 Â 4 lm 3 ). The hierarchical derivatives of the small and large zeolite crystals displayed 250 and 120 m 2 g À1 of mesopore surface area, respectively, in contrast to the 62 and 5 m 2 g À1 of the parent counterparts. The data based on o-xylene desorption were partly disguised by site-desorption limitations. Desorption experiments using isooctane evidenced a 4-fold reduction in the characteristic diffusion path length on both mesoporous small and large zeolites with respect to their purely microporous analogues. These results confirm the substantial potential for improvement of commercial nanocrystalline zeolites in diffusion-limited reactions upon the introduction of intra-crystalline mesoporosity by post-synthesis modification.