NaX Zeolite Crystals Research Articles

Measurement of intracrystalline diffusion in NaX zeolite by capillary column gas chromatography

Chromatographic HETP (height equivalent to a theoretical plate) measurements are reported for a series of hydrocarbon sorbates ( n-butane, n-hexane, cyclohexane, n-octane, benzene and decalin) in a capillary column (150 μm diameter) packed with 100-μm crystals of NaX zeolite. Such a column is superior to a conventional-packed column in terms of both heat transfer and external mass transfer characteristics. Dimensionless plots of reduced HETP versus reciprocal Peclet numbers show clearly that, for the more strongly adsorbed species, the HETP can be quantitatively accounted for by external mass transfer resistance and axial dispersion so that only lower limits for intracrystalline diffusivities can be obtained. The HETP data for butane, cyclohexane and decalin show clearly the intrusion of internal (micropore) diffusional resistance so, for these species, reasonably reliable intracrystalline diffusivities can be derived. The diffusivities so obtained (for butane and cyclohexane) are smaller than previously reported nuclear magnetic resonance (NMR) self-diffusivities but larger than the zero-length column (ZLC) values.

Adsorption of aromatic hydrocarbons in NaX zeolite. 1. Equilibrium

Equilibrium isotherms and isobar have been measured for a series of aromatic hydrocarbons (benzene, toluene, xylene, mesitylene, tetramethylbenzene, naphthalene, hexamethylbenzene, dimethylnaphthalene, and anthracene) on well-defined crystals of NaX zeolite. Henry constants, energies of adsorption, and complete isotherms are reported. The isotherms for the higher molecular weight species approach the ideal Langmuir form with a saturation capacity close to 1 molecule/cage. Heats of sorption increase linearly with carbon number, and there appears to be little difference in either the isotherms or the heats of sorption for different aromatic sorbates of the same carbon number

Adsorption of aromatic hydrocarbons in NaX zeolite. 2. Kinetics

Kinetic data are reported for the sorption of a range of aromatic sorbates in unaggregated crystals of NaX zeolite. Diffusivities (at 600 K) range from order 10 -6 -10 -7 cm 2 /s for benzene and p-xylene to 10 -14 -10 -15 cm 2 /s for hexamethylbenzene and anthracene. The diffusivities decrease with increasing sorbate size, but the relationship is complex and does not follow the sequence of either molecular weight or critical molecular diameter. Rather the correlation appears to be with the moment of inertia of the sorbate molecule, suggesting that restriction of the rotational freedom of the diffusing molecule is probably a major factor. For several of the sorbates the validity of the diffusion model was confirmed by replicate measurements with different crystal size fractions

Selective two-component self-diffusion measurement of adsorbed molecules by pulsed field gradient Fourier transform NMR

The two-component self-diffusion of C 2 H 4 and C 2 H 6 adsorbed in NaX zeolite crystals has been measured by 1 H pulsed field gradient (PFG) Fourier transform NMR. The results so obtained are compared with single-component self-diffusion measurements and with values for the intracrystalline mean residence times, determined by means of NMR tracer desorption experiments

Evaluation of experimental parameters influencing the growth of large crystals of NaX zeolite

AbstractFollowing the Charnell's procedure, the influence of materials, methods of preparation and changes in composition of the gel on the process of NaX zeolite crystallization have been investigated in order to find conditions providing both the growth of crystals as large as possible and also good reproducibility of the result. The experimental conditions used are detailed described.

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.

A liquid phase chromatographic study of sorption and diffusion of glucose and fructose in NaX and KX zeolite crystals

Adsorption and diffusion of glucose and fructose in NaX and KX zeolite crystals has been studied by liquid chromatography, using a column packed with 50-μm (unaggregated) crystals. This eliminates macropore resistance and maximizes intracrystalline diffusion resistance, thus facilitating the reliable determination of intracrystalline diffusivity. However, even under these conditions, reliable diffusivities cannot be determined when the equilibrium constant is small ( −9 cm 2 s −1 at room temperature. Keywordsffusion; fructose; glucose; liquid chromatography; zeolite NaX; zeolite KX

Self-diffusion behaviour of n-heptane/benzene mixtures in the intercrystalline space of packings of NaX zeolite crystals as observed by the NMR pulsed field gradient technique

Self-diffusion of n-heptane, as well as of n-heptane/benzene mixtures, in the intercrystalline space of packings of NaX zeolite has been investigated by means of the NMR pulsed field gradient technique. Both the temperature and the concentration dependence of the long-range self-diffusion of single components and the concentration dependence of mixtures in zeolite beds can be described by means of simple theoretical relations. By determination of the long-range self-diffusion coefficients, it is possible to calculate the sorptive separation factors of binary hydrocarbon mixtures with an adequate accuracy.

Experimental evidence of the influence of sorption-heat release processes on the sorption kinetics of benzene in NaX zeolite crystals

Under constant-volume–variable-pressure conditions the sorption uptake of benzene by NaX zeolite crystals has been investigated with respect to the influence of external thermal conditions on the uptake rate. The information obtained permits quantitative conclusions concerning the influence of the sorption heat generated on the uptake rate, i.e. the decrease in apparent diffusivities calculated under the erroneous presumption of the isothermicity of the sorption process considered. A corresponding tentative mechanism which takes into account the generation of thermal interface barriers is proposed. Furthermore, there are experimental conditions for which the uptake of benzene by NaX zeolite crystals should be considered as an isothermic process. Within that parameter region agreement was obtained between the sorption diffusion data (corrected by the Darken equation) and the n.m.r. self-diffusivities, as reported earlier.

Self-diffusion behaviour of n-heptane/benzene mixtures in the intercrystalline space of packings of NaX zeolite crystals as observed by the NMR pulsed field gradient technique

Self-diffusion behaviour of n-heptane/benzene mixtures in the intercrystalline space of packings of NaX zeolite crystals as observed by the NMR pulsed field gradient technique

Thermal desorption spectroscopy from high-specific-area solids—Hydrocarbon adsorption and diffusion in NaX zeolite crystals

This work illustrates a novel application of the thermal desorption spectroscopy technique to investigate adsorption, desorption, and surface diffusion phenomena on small zeolite crystals. Ultrahigh-vacuum procedures employed with zeolite samples of very small mass permit measurements to be made without the presence of intergranular diffusion effects. Studies of 1-butene and C 2H 4 adsorption have been carried out. A diffusion energy of ~2.7 kcal/mole has been measured for 1-butene on zeolite-X; for C 2H 4 the measured diffusion energy is ~1.9 kcal/mole. The desorption energy for C 2H 4 on zeolite-X is kinetically measured to be 8.9 kcal/mole in excellent agreement with the isosteric heat of adsorption (8.9 kcal/mole). The characteristic behavior of adsorbate thermal desorption states upon annealing may be employed to discriminate between species adsorbed on the outer surface of the zeolite crystals, and species which are present within the pores of the zeolite.

Investigations on the crystallization of larger NaA and NaX zeolite crystals in the presence of triethanolamine (I). The influence of Na2O and SiO2 content

AbstractSodium silicate‐aluminate aqueous systems with various contents of Na2O and SiO2 containing triethanolamine were crystallized. Optimum Na2O and SiO2 contents have been observed to give NaA and NaX zeolite crystals with dimensions up to 80 μm. For NaA crystals the values are: ∼ 3.5 mass% Na2O, ∼ 1.65 mass% SiO2; for NaX crystals ∼ 2.5 mass% Na2O, ∼ 1.5 mass% SiO2. – The effect of other additives on crystal size is proved. – The results are discussed in terms of nucleation and growth, reaction rate, and supersaturation. Supposition are made to gain larger crystals by maintaining the excess of solute over a longer period of time.