Milan Kočiřík

Prediction and Evaluation of Time-Dependent Effective Self-diffusivity of Water and Other Effective Transport Properties Associated with Reconstructed Porous Solids

We reconstructed pore structures of three porous solids that differ from each other in morphology and topology of pore space. To achieve this, we used a stochastic method based on simulated annealing and X-ray computed microtomography. Simulated annealing was constrained by the following microstructural descriptors sampled along the principal and diagonal directions: the two-point probability function for the void phase and the lineal-path functions for both void and solid phases. The stochastic method also assumed the isotropic pore structures in accordance with a recent paper (Čapek et al. in Transp Porous Media 88(1): 87–106 (2011)). With the exception of the solid with the widest pores, we made tomographic volume images in high and low resolution, which enabled us to study the effect of resolution on microstructural descriptors and effective transport properties. A comparison of the two-point probability function and the lineal-path function sampled in the principal directions revealed that the pore structures derived from the tomographic volume images were slightly anisotropic, in opposition to the assumption of the stochastic method. Besides the anisotropy, other microstructural descriptors including the pore-size function and the total fraction of percolating cells indicated that the morphological and topological characteristics of the pore structures depended on the reconstruction method and its parameters. Particularly, the pore structures reproduced using the stochastic method contained wider pores than those obtained using X-ray tomography. Deviations between the pore structures derived from low- and high-resolution tomographic volume images were also observed and imputed to partial volume artefacts. Then, viscous flow of incompressible liquid, ordinary diffusion, Knudsen flow and self-diffusion of water in the reconstructed pore spaces were simulated. As counterparts, experimental data were measured by means of permeation and Wicke–Kallenbach cells and pulsed field gradient NMR. Deviations between the simulated quantities on the one hand and experimental data on the other hand were generally acceptable, which corroborated the pore-space models. As expected, the predictions based on the tomographic models of pore space were more successful than those derived from the stochastic models. The stationary effective transport properties, i.e. the effective permeability, the effective pore size and the geometric factor, were sensitive to a bias in long-range pore connectivity. Furthermore, the time-dependent effective diffusivity was found to be especially sensitive to relatively small morphological deviations between the real and reconstructed pore structures. It is concluded that the combined predictions of the effective permeability, the effective pore size, the geometric factor and time-dependent effective self-diffusivity of water are needed for the reliable evaluation of pore-space reconstruction.

Heterogeneous membranes based on zeolites for separation of small molecules

Abstract Heterogeneous zeolite-based membranes with polymeric binder were tested for the separation of hydrogen and methane. The separation process was characterized by parameters such as permeability, selectivity, and diffusion flux. The prepared membranes are superior to homogeneous polymeric membranes in permeability and diffusion flux. For H 2 /CH 4 separation selectivity equal to 10 was attained.

Incorporation of zeolites in polyimide matrices

3-Aminopropyltriethoxysilane terminated polyimide precursors (polyamic acids) and Silicalite-1 were used to prepare zeolite-filled polyimide films. The accessibility of the pores of the zeolite built in both non-treated and treated polyimide matrices was studied using sorption of iodine from the vapour phase. Transport properties of the filled films were investigated using small probe molecules of hydrogen and methane.

Formation of a frozen-like adsorbed layer of ethyltoluenes in ZSM-5 crystals and their release upon heating

Abstract The long-term sorption kinetics of p -ethyltoluene on HZSM-5 zeolite was investigated using a McBain balance. The kinetic data suggested that the transport is coupled with immobilization. In situ isomerization of p -ethyltoluene as well as temperature-programmed desorption (TPD) of the samples containing a preloaded mixture of ethyltoluenes were followed by Fourier transform infrared spectroscopy (FTIR). TPD results indicate that no significant release of immobilized m -ethyltoluene was observed at temperatures below 543xa0K. TPD patterns of p -ethyltoluene were found to depend strongly on the temperature of the zeolite preloading. The removal and/or isomerization of p -ethyltoluene molecules trapped by the m -ethyltoluene species immobilized on the strong acid sites can occur only when the meta -isomer leaves the zeolite channels at temperatures above 543xa0K.

Sorption of ammonia from gas streams on clinoptilolite impregnated with inorganic acids

The contribution deals with ammonia-removal from waste air by adsorption using low-cost sorbents. The natural zeolite clinoptilolite from the deposit Nižný Hrabovec, Slovakia was tested for ammonia removal from the waste air. Measurements of breakthrough curves for ammonia were performed using fixed beds of clinoptilolite (i) in its natural form and (ii) on clinoptilolite samples pre-treated with acids. The parameters of the experiments were selected to simulate the conditions ruling in animal breeding farms. The pre-treatment of clinoptilolite with acids increased sorption capacity of clinoptilolite for ammonia.

Square root relationship in growth kinetics of silicalite-1 membranes

Conceivable limiting models of growth kinetics of polycrystalline layers involving diffusion of low molecular silicon containing species, Brownian motion of nanoparticles and their sedimentation are analyzed from the point of view of their application to the preparation of zeolite-based membranes. The activation energies derived using these models were evaluated and the effect of support orientation was quantified. A criterion of a relative importance of colloidal particle sedimentation with respect to the Brownian motion was formulated.

Study of the Accessibility of Zeolite Crystals in Polyimide Matrices. A Route to Coatings Exhibiting Selective Permeation

The accessibility of void space in MFI-type zeolite crystals, isolated and embedded in a polyimide (PI) matrix, was studied using optical microscopy coupled with an iodine indicator technique (IIT). IIT was used to estimate the adhesion between the PI matrix and the zeolitic phase, the accessibility of the zeolite void space for gas molecules as well as for characterizing the spatial distribution of embedded crystals in the composite. The channel system of the zeolitic phase in as-synthesized composites with the majority of crystals covered by a layer of PI is occupied by the solvent molecules used in composite synthesis. In untreated composites, the zeolitic phase is inaccessible to iodine sorption. Colouring patterns and colouring kinetics have been used to characterize the efficiency of the treatment for removing the PI covering layer from the crystal surface. The same techniques can be applied to characterize the treatment with regard to the desorption of solvents (N-methyl-2-pyrrolidone, N,N-dimethylformamide, n-heptane) from the silicalite-1 channel system.

Transport-related structure characteristics of FCC catalysts from sorption, porosimetric and PFG NMR measurements

Texture characteristics of fluid catalytic cracking (FCC) catalysts were evaluated from N 2 adsorption and mercury porosimetry measurements. A hierarchy of four levels of voids were found and quantified in the beds of FCC particles. Diffusion coefficients of intraparticle diffusion for n-octane were measured in the catalyst beds by PFG NMR technique. The diffusion coefficients exhibit a linear dependence on parameters ξ that involve the mean value of the pore radii r ¯ in the pertinent porosity region and a function of the corresponding porosity. A strong linear correlation was also found between the parameter ξ for macropores and that for mesopores. The mean sizes of macro- and mesopores thus become key parameters in tuning the FCC catalyst properties.

24-P-10-immobilization and mobilization of surface species during transformation of ethylene over HZSM-5 catalysts

Publisher Summary This chapter discusses the immobilization and mobilization of surface species during transformation of ethylene over HZSM-5 catalysts. Amount of ethylene immobilized in HZSM-5 catalysts is estimated from sorption and reaction dynamics as a function of reactor temperature. Space accessible in loaded crystals is measured using water sorption at 298 K.

Sorption and Transport of p-Ethyltoluene in H-ZSM-5 Zeolites

Long-term adsorption measurements in the system p-ethyltoluene/ H-ZSM-5 zeolites have been performed in the temperature region between 323 K and 393 K. The plots of sorbed amount vs. pressure exhibit irreversible features for temperatures above 333 K. Uptake curves measured by a gravimetric method in small pressure steps show strong deviations from the corresponding solution of the second Fick law; the form of the uptake curves being consistent in a broad time interval with a diffusion-immobilization model. An a posteriori FTIR analysis of zeolite samples loaded with p-ethyltoluene suggests that even at low temperatures the immobilization process may be affected by the isomerization of the sorbing species.