A. Zikánová

Investigation of sorption and transport of sorbate molecules in crystals of MFI structure type by iodine indicator technique

Abstract Iodine indicator technique (IIT) involving light microscopy has been introduced in our laboratory to investigate sorption and mass transport phenomena in zeolites and peculiarities of crystal morphology via coloring of zeolite crystals. The coloring of silicalite-1 90°-intergrowths was performed using pure iodine vapors and binary solutions of iodine in organic solvents (benzene, cyclohexane, toluene, ethylbenzene, p -xylene, and decahydronaphthalene). Coloring from the liquid phase was carried out either under co- or counter-diffusion conditions. The rate of coloring from the vapor phase, and under co-diffusion conditions from the liquid phase, was found to be limited by external mass transport and the coloring was uniform. Under counter-diffusion conditions, the rate of coloring was to about 2 to 3 orders of magnitude lower than in the latter case. Except for decahydronaphthalene, the coloring patterns were non-uniform, and they visualized at least in the beginning stage of the coloring process the interfaces of crystal sections. Also, the cracks were visualized being decorated with iodine. There is a marked difference between the development of coloring patterns of benzene and cyclohexane on one side and their linear alkyl derivatives (toluene, ethylbenzene and p -ethyltoluene) on the other side. In the former case, the iodine enters the bulk of crystal sections via section interfaces. In the latter case, the bulk of the sections appears to be inaccessible for iodine molecules. Other information on channel system accessibility provides IIT for decahydronaphthalene, where the molecular sieve effect of 10-membrane oxygen rings with respect to decahydronaphthalene is indicated.

A comparative study of template removal from silicalite-1 crystals in pyrolytic and oxidizing regimes

Abstract The removal of tetrapropylammonium cations (TPA + ) from silicalite-1 crystals with a morphology of 90°-intergrowth and different crystal sizes was investigated. The effects of the nature of the gas (air or nitrogen) and of the hydrodynamics were examined using a stream flowing in parallel or across the crystal layer. The only process variable was the plateau temperature ( T max ). Crystal domain boundaries became permeable to template degradation products when T max exceeded 300 °C. The template removal was monitored by (i) light microscopy, (ii) degree of removal of the total organics α , of nitrogen species α N and of carbon species α C and (iii) accessibility of the channel system for N 2 molecules. The sorption isotherms for N 2 exhibited two steps. The upper step started to be perceptible for a degree of organic removal higher than 55%. Its height increased with α , and its position moved to lower pressure values. The plots of the gas accessibility versus the degree of removal were modelled. For template removal in air, all plots were linear. In a non-oxidizing atmosphere, the accessibility lagged behind α N . Light microscopy showed that template degradation started along domain boundaries. Sorption kinetics of iodine into silicalite-1 crystals treated in a non-oxidizing atmosphere was considerably slower than that observed into crystals calcined in air. The tendency to crack formation increased with crystal size. The template removal efficiency was higher when using cross-flow than parallel flow calcination.

Synthesis and characterization of the all-silica 8-ring Clathrasil DD3R comparison of adsorption properties with the hydrophilic zeolite A

The adsorption properties of the clathrasil Deca-dodecasil 3R (DD3R) have been studied. This porous tectosilicate becomes microporous when the organic template 1-adamantanamine is removed by calcination, which gives this clathrasil zeolitical properties. By means of adsorption of water, DD3R has been proven to be very hydrophobic. These adsorption data, together with the isosteric heats of adsorption of carbon dioxide at different loadings, reveal the absence of strong adsorption sites for polar or polarizable molecules, in contrast to the adsorbent zeolite A.

Diffusion and Immobilization Mechanisms in Zeolites Studied by ZLC Chromatography

Zero Length Column chromatography was used to study mass transfer in zeolites involving coupled diffusion and immobilization mechanisms. A modeling based on Volterra integral equation technique was utilized to simulate sorption and desorption kinetic curves and compare results of the simulations with experimentally obtained curves. This approach was applied to analyze sorption kinetics in the model system: toluene/silicalite-1 (75°C–178°C). The system generally shows a non-Fickian behavior and can be described by diffusion coupled with immobilization.

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.

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.

14-P-38-Topochemical changes in large MFI-type crystals upon thermal treatment in oxidizing and non-oxidizing atmosphere

Publisher Summary This chapter discusses topochemical changes in large MFl-type crystals upon thermal treatment in oxidizing and nonoxidizing atmosphere. Tetrapropylammonium hydroxide (TPAOH) removal from large as syntesized silicalite-1 crystals with internal morphology of 90°-intergrowths has been investigated in regimes with gas flow in parallel to and through the crystal layer both in the absence and presence of oxygen. The void space accessibility of crystals is estimated from the sorption isotherms of nitrogen (N2), which is exhibited as a rule two step. The topochemical changes in crystals after a partial template removal are evaluated using light microscopy, electron spectroscopy for chemical analysis (ESCA) measurements, and elemental analysis of organic residues.

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.