Mladen Eić

A new experimental technique for measurement of intracrystalline diffusivity

A new experimental technique has been developed for measuring the intracrystalline diffusivities of strongly adsorbed species by following chromatographically the desorption curve for a small sample of zeolite, saturated at a low concentration level. Results obtained by this method for o -xylene in 50-μm and 100-μm NaX crystals are self-consistent and in good agreement with previously determined gravimetric values.

Diffusion of C8 aromatic hydrocarbons in silicalite

Diffusion of p -xylene, ethylbenzene, and o -xylene in large (105 × 50 × 40 μm) crystals of silicalite has been studied by the zero-length column (ZLC) method and by gravimetric uptake rate measurements. The form of the uptake curves suggests that diffusion occurs primarily through the straight channels of the silicalite pore structure so both ZLC and uptake rate curves were interpreted on the basis of diffusion in a parallel-sided slab. The experimental data obtained by both techniques are consistent. The diffusivity of p -xylene is almost the same as that of benzene and about an order of magnitude larger than that of o -xylene, but there is very little difference in activation energy (~ 30 kJ/mol). The results, therefore, do not support the commonly accepted view that the difference in diffusivity between the ortho and para isomers results from the difference in their critical molecular diameters

Diffusion of linear paraffins and cyclohexane in NaX and 5A zeolite crystals

Diffusion of a series of linear paraffins and cyclohexane in 13X and 5A zeolite crystals has been studied by the recently developed zero length column (ZLC) method. The results obtained for butane in commercial 5A crystals and in larger laboratory-synthesized 5A crystals agree well with our earlier gravimetric uptake-rate data. The large difference in diffusivity between the different zeolite preparations and the previously observed agreement with n.m.r. self-diffusivity data (for the laboratory-synthesized crystals) are both confirmed. The ZLC data for NaX, however, yield diffusivities that are about two orders of magnitude smaller than the values obtained by the n.m.r. pulsed-field gradient method, although the trends of both diffusivity and activation energy with carbon number are qualitatively similar.

Diffusion of benzene in NaX and natural faujasite

Diffusion of benzene in large crystals of both natural faujasite and NaX zeolite has been measured by conventional gravimetric methods and by the new “zero length column” chromatographic method. Results of both types of measurement are consistent, and the derived time constants show the expected square dependence on crystal radius, confirming intracrystalline diffusion control. As with the xylene isomers, there is no significant difference in diffusivity between natural faujasite and synthetic NaX. However, the corrected diffusivities are much smaller than the self-diffusivities derived from n.m.r. measurements.

Diffusion of n-paraffins in offretite-erionite type zeolites

Abstract Intracrystalline diffusivities are reported for a series of n -paraffins (C 6 to C 20 ) in four different offretite-erionite intergrowths with constraint indices (Cl) varying from 2.1 to 115 (a low value of Cl corresponds to nearly pure offretite, whereas a high value corresponds to erionite). The intracrystalline diffusivities for each sample show a monotonic decrease with carbon number, except for one of the intermediate samples (HZSM-34), which has the highest proportion of extraframework Al. For the sorbates studied, the diffusivities decrease with the Cl. Activation energies show different trends with carbon number, depending on the erionite/offretite character of the sample. The experimental evidence does not support the existence of a window effect, as suggested by Gorring (Gorring, R.L. J. Catal. 1973, 31 , 13) on the basis of measurements with a zeolite of similar structure.

Intracrystalline Diffusion of Linear Paraffins and Benzene in Silicalite Studied by the ZLC Method

The zero length column (ZLC) method has been applied to study diffusion of benzene and several linear paraffins in large crystals of silicalite. The ZLC diffusivity data are broadly consistent with recently reported membrane, piezometric and frequency response values but the values (for propane) are much smaller than the NMR self diffusivities. The trends of diffusivity with carbon number for 5A, 13X and silicalite are qualitatively similar, with the diffusivities for silicalite lying between the corresponding values for 5A and 13X. For carbon numbers greater than 5, activation energies for silicalite are essentially the same as for NaX.

Breakthrough study of the adsorption and separation of sulfur dioxide from wet gas using hydrophobic zeolites

The adsorptive and kinetic behaviour patterns of SO2 and water vapour on mordenites and pentasil zeolites were investigated using the breakthrough curve method. For all the zeolites studied, the breakthrough experimental data show a decrease in the equilibrium capacities for both SO2 and H2O with increasing SiO2/Al2O3 ratio. At the lower ratios SO2 adsorption is believed to be influenced by the basicity of the zeolite. The presence of water in the gas reduces its SO2 capacity to varying degrees, depending on the SiO2/Al2O3 ratio. In contrast, the presence of CO2 (in the wet SO2-containing gas) has very little effect. The hydrophobic indices, which were used to interpret the selectivity of SO2 adsorption, showed different trends with SiO2/Al2O3 ratios. The Langmuir-Freundlich and extended Langmuir-Freundlich equilibrium models were used to predict equilibrium properties for the single-component and binary systems, respectively. The linear driving force-based non-isothermal model was used to fit experimental breakthrough curves for the single-component systems. Overall mass transfer resistances derived from the model were compared with the values obtained for SO2 and water vapour adsorption in pelleted samples using a simplified biporous adsorbent model. Breakthrough curves for the binary systems were calculated using kinetic and equilibrium data of the single-component systems.

Measurement of Diffusion in Microporous Solids by Macroscopic Methods

In this chapter the main macroscopic experimental methods for measuring diffusion in microporous solids are reviewed and the advantages and disadvantages of the various techniques are discussed. For several systems experimental measurements have been made by more than one technique, and in Part 3 the results of such comparative studies are reviewed. While in some cases the results show satisfactory consistency, there are also many systems for which the apparent intracrystalline diffusivities derived from macroscopic measurements are substantially smaller than the values from microscopic measurements such as PFG NMR. Recent measurements of the transient intracrystalline concentration profiles show that surface resistance and intracrystalline barriers are both more common and more important than has been generally recognized. This may explain such discrepancies. Part 4 comprises a brief review of recent experimental studies of diffusion in mesoporous silica structures, which, in contrast to zeolites, have a well-defined bimodal distribution of pore size.

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.

Adsorption of SO2 from Wet Mixtures on Hydrophobic Zeolites

Breakthrough curve measurements of SO2 and water vapor were carried out on a number of selected mordenite and pentasil zeolites from their binary and ternary mixtures with CO2 at 50 and 100°C. SO2 capacities of these samples were found to be significantly reduced by the presence of water. Competitive adsorption led to unusually high overshoot peaks of SO2 breakthrough curves. On the other hand, SO2 was found to displace water on the samples with very high silica to alumina ratio. A linear driving force, isothermal model was used to predict the breakthrough curves. Langmuir and extended Langmuir equilibrium models were used to describe the equilibrium properties of water and SO2, respectively. The overall mass transfer resistance obtained from the model was compared to the values calculated from a simplified biporous adsorbent model to shed some light on the adsorption kinetics.