Zeolite Crystal Size Research Articles

Venezuelan natural alumosilicates as a feedstock in the synthesis of zeolite A

Zeolite A has been synthesized from low-cost natural aluminosilicates. The samples were characterized by X-ray powder diffraction, chemical analysis, electron microscopy, FTi.r. spectroscopy, and nitrogen adsorption. Studies of calcination temperature effects and Na 2O SiO 2 and H 2O Na 2O ratios were performed. It was observed that mineral calcination at temperatures in the range of 500–850°C led to zeolite A formation, but when the calcination step was omitted, hydroxysodalite was formed. In samples calcined for 5 h at temperatures higher than 850°C, no zeolite was observed. A maximum in crystallinity was obtained for samples calcined in the range 700–750°C. The zeolite crystal mean size increased as the alkalinity increased. I.r. spectroscopy was used to characterize the products. An area band ratio at 570 and 470 cm −1 showed a linear correlation with % crystallinity determined by XRD.

Shape-Selectivity of Large and Small Crystals of Zeolite ZSM-5

The authors have shown recently that the isomer distribution in the xylene resulting from the methylation of toluene by methanol on zeolite H-ZSM-5 is independent of conversion as long as reactant methanol is not exhausted, and that it depends inter alia on the size of the unmodified zeolite crystals. The interpretation of the observed isomer distribution as resulting from intracrystalline reaction and shape-selective diffusion led to the conclusion that only that para isomer should be generated on sufficiently large crystals of the unmodified zeolite, whereas with extremely small crystals (in the hypothetical limit consisting of only one unit cell) an isomer distribution should be observed which is not influenced by subsequent diffusion and isomerization of the primary product of alkylation. The authors now report briefly the results of experiments which have been conducted to test these conclusions. 3 refs., 2 figs., 3 tabs.

Zeolites in organic reactions: Condensation of formaldehyde with benzene in the presence of HY zeolites

Alkylation of benzene with formaldehyde to give diphenylmethane was carried out at 30–80°C in the presence of a series of HY zeolites at three levels of Na + exchange (HY-21, HY-50 and HY-100), several framework Si-to-Al ratios from 4 to 90 and two zeolite crystal sizes (0.30 and 0.80 μm). Only strong acid sites, i.e., those with p K a < 0, which correspond to centres with 0 and 1 aluminium in the next nearest neighbours, are active in catalysing this reaction. The turnover number of the acid zeolites increases steadily with increasing framework Si-to-Al ratio. Under the conditions studied there is no diffusion control of the reaction in the pores of the zeolite. There is a decay in the catalyst activity during the course of the formaldehyde condensation, which is greater for samples with higher Al content. This catalyst decay is due not only to poisoning by oligomers, but also to the strong adsorption of polar products.

Growth of NaX zeolite monocrystals up to 0.5 mm

This work studies the influence of preliminary ageing of the original solutions, and sedimentation of the crystals during the decrease of gel viscosity at the growth of the crystals, on the NaX zeolite monocrystal size. By optimizing the age of the solutions and slowing down sedimentation using a crystallizer of special construction, NaX zeolite crystal size was increased up to 0.5mm.

Prediction of the zeolite crystal size distribution in batchwise hydrothermal synthesis

Several ways of characterizing the crystal size distribution during batchwise hydrothermal synthesis of molecular sieve zeolites are shown. In particular, we show how the crystal size distribution function can be predicted once the transient birth rate and growth rate functions have been determined, and how average size, standard deviation, and skewness can be used to characterize the size distribution. Simulation results are presented which show the effect of manipulating several key parameters in the synthesis system (e.g. initial reagent concentration and nucleation rate). It is shown how one might change the operating policy of the batch crystallizer to obtain larger particle sizes and/or narrower crystal size distribution.

EFFECT OF CRYSTAL SIZE DISTRIBUTION ON CHROMATOGRAPHIC PEAKS IN MOLECULAR SIEVE COLUMNS

The effect of zeolite crystal size distribution on sorption has been studied for a packed bed subjected to a pulse input. The swelling crystal approach proposed recently has been used lo incorporate the effect of size distribution. The effluent concentration pulse and its moments show significant dependence on size distribution. Use of conventional moment analysis for uniform particles is shown to introduce significant error in the estimates of sorption parameters. A simple technique is suggested to rectify these estimates for size distribution effects.

Sorption kinetics of n-hexane on MgA zeolites of different crystal sizes. Study of the rate-limiting transport mechanism

Starting from the necessity for more substantial evidence with regard to the occurrence of intracrystalline diffusion as the rate-limiting transport mechanism in sorption processes on zeolites, the variation of zeolite crystal size is regarded as a criterion and used in the case of the sorption system n-hexane/MgA zeolite.

Theoretical prediction of sorption curves for molecular sieves

A model is proposed to take into account the non-linearity of equilibrium isotherm, concentration dependence of diffusivity and zeolite crystal size distribution while calculating the sorption curve for a single component adsorption on molecular sieves. The effect of size distribution is modelled by assuming the adsorption to take place on a progressively swelling crystal. Such a model allows the consideration of size distribution to be detached from other aspects while solving the diffusion equation. This reduces the computational efforts considerably.