Milan Kocirik

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.

Room temperature negative differential resistance in molecular nanowires

Electric-field assisted assembly has been used to place rod-shaped metal nanowires containing 4-{[2-nitro-4-(phenylethynyl)phenyl]ethynyl}benzenethiol molecules onto lithographically defined metal pads. In these systems the current–voltage characteristics exhibited negative differential resistance at room temperature with on–off peak-to-valley ratios of 1.80 to 2.21.

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.

Diffusion of n-butane, isobutane and ethane in a MFI-zeolite membrane investigated by gas permeation and ZLC measurements

The diffusion of n-butane, isobutane and ethane in a composite-membrane alumina-MFI zeolite has been investigated using gas permeation and ZLC (zero-length column) techniques. The diffusion of isobutane in the sample is faster than that of n-butane although isobutane molecule has a larger kinetic diameter, and its activation energy is comparable to the results obtained from the gas permeation and QENS measurements reported in the literature. For ethane, the diffusivity is much higher in comparison to n-butane and isobutane. When isobutane is present at a high concentration, the diffusion of ethane is remarkably hindered due to a relatively strong adsorption of isobutane in the membrane micropores. By comparing the diffusivity data obtained from the permeation and ZLC measurements, it was possible to evaluate the thickness of the zeolite membrane effective for gas permeation.

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.

Quasiisothermal degradation kinetics of tetrapropylammonium cations in silicalite-1 matrices

The present work deals with the kinetics of template removal investigated via thermogravimetric measurements both in the flow of non-oxidative and oxidative agent. Apart from linear temperature programs, quasiisothermal conditions were used which consisted in keeping the sample at the temperature of measurement Tmax. A simple kinetic model was formulated which involves the effect of mobilisation reaction, rate of mass transfer from crystals to intercrystalline space, intercrystalline diffusion and the rate of mass transfer from the layer of crystals into the main stream of template removing agent. The model qualitatively appreciates all the principal features of template removal process including the effect of the percolation threshold.

REMOVAL OF DIQUATERNARY AMMONIUM CATIONS FROM AS-SYNTHESIZED SSZ-16 ZEOLITE

Zeolites are stable microporous aluminosilicates with numerous applications in chemical technology such as separation of species and catalytic transformations. Our study is focused on a weakly explored zeolite SSZ-16 with pore constrictions defined by 8-membered oxygen rings. Key results are the preparation of Et6-diquat-5 dication used as a structure directing agent (SDA) and finding the optimum synthesis conditions with respect to zeolite phase purity. Stability of SDA was examined in conditions similar to those of autoclave synthesis (concentration, pH, temperature). Moreover, the content and location of SDA species in zeolite phase and conditions of SDA decomposition were investigated.