Study of intracrystalline diffusion in zeolites 7. Numerical modeling of nonisothermal diffusion in crystals

The kinetics of nonisothermal adsorption from a limited volume with consideration of the dependence of the differential heat of adsorption and the intracrystalline diffusion coefficient on the degree of filling were mathematically modeled. A model of the adsorption kinetics which explains the appearance of extremes on the experimental kinetic adsorption curves in adsorption of water by CaA zeolite was proposed. It was shown that a decrease in the differential heat of adsorption with an increase in the degree of filling is a necessary condition for the appearance of extremes on the kinetic adsorption curves.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1228–1233, June, 1990.     

Intracrystalline diffusion in zeolites 9. Determination of intracrystalline diffusion coefficients from exothermic calorimetric curves

A method has been proposed for the determination of the intracrystalline diffusion coefficients for polar substances in zeolites from the exothermic calorimetric curves. If the differential heat of adsorption falls as the degree of filling increases, diffusion is an exothermic process and, therefore, the release of heat in a system with a steep adsorption isotherm (in the case of a measured addition) continues even though the adsorption process may have finished. The exothermic calorimetric curve for NH₃ adsorption by the zeolite NU has been taken as an illustration.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 301–305, February, 1991.     

Study of intracrystalline diffusion in zeolites 6. Comparison of the results obtained by the method of densitometry of light and the adsorption method

The use of the method of densitometry of light passing through a layer of zeolite crystals permits determining the limiting stage of diffusion during adsorption by zeolites. Diffusion in transport pores and external heat exchange play the basic role in adsorption of benzene by NaX zeolite, while diffusion in crystals is the determining mechanism of transport in adsorption of water by NaA and NaX zeolites. The diffusion coefficients of water in NaX zeolite have an order of magnitude of 10^–17 m²/sec and increase with an increase in the degree of filling. An explanation for the anomalous behavior of the kinetic adsorption curve for brief times of adsorption of water by zeolite is proposed.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1224–1228, June, 1990.     

Study of intracrystalline diffusion in zeolites. 8. Effect of the parameters of the adsorption systems on the kinetics of adsorption in numerical modeling of nonisothermal diffusion in crystals

The factors which affect the magnitude of the extremes on the kinetic adsorption curve were determined by the method of numerical modeling. A decrease in the adsorbate reserve in adsorption from a restricted volume, a decrease in the change in the temperature of the adsorbent, and attenuation of the decrease in the intracrystalline diffusion coefficient with an increase in the degree of filling decrease the intensity of the extremes on the kinetic adsorption curves. In contrast to the decrease in the differential heat of adsorption with the degree of filling, these factors are not necessary conditions for the appearance of extremes on the kinetic adsorption curves. The extremes can be eliminated by altering any factor, but extremes appear again with a corresponding change in the other factors.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1703–1707, August, 1990.     

Intracrystalline diffusion in zeolites

Conclusions A technique for measuring diffusion coefficients for rapid sorption processes has been proposed and applied for studying intracrystalline mass transfer in zeolites.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1217–1219, June, 1981.     

Polyelectrolyte-surfactant complex as a template for the synthesis of zeolites with intracrystalline mesopores

Mesoporous zeolite silicalite-1 and Al-ZSM-5 with intracrystalline mesopores were synthesized with polyelectrolyte-surfactant complex as the template. Complex colloids were first formed by self-assembly of the anionic polymer poly(acrylic acid) (PAA) and the cationic surfactant cetyltrimethylammonium bromide (CTAB) in basic solution. During the synthesis procedure, upon the addition of the silica source, microporous template (tetrapropylammonium hydroxide), and NaCl, these PAA/CTA complex colloids underwent dissociation and gave rise to the formation of hollow silica spheres with mesoporous shells templated by CTAB micelles and PAA domains as the core. Under hydrothermal treatment, the hollow silica spheres gradually merged together to form larger particles with the PAA domains embedded as the space occupant, which acted as a template for intracrystalline mesopores during the crystallization of the zeolite framework. Amphiphilic organosilane was used to enhance the connection between the PAA domain and the silica phase during the synthesis. After calcination, single crystal-like zeolite particles with intracrystalline mesopores of about 5-20 nm were obtained, as characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and N(2) adsorption measurements. With the addition of an aluminum source in the synthesis, mesoporous zeolite Al-ZSM-5 with intracrystalline mesopores was also synthesized, and enhanced catalytic property was observed with mesoporous Al-ZSM-5 in acetalization of cyclohexanone with methanol.     

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.     

A discrete model of diffusion in zeolites

The diffusion of adsorbed molecules in a zeolite crystal is regarded as a random walk in a semi-infinite three-dimensional lattice. It is assumed that the probability of the movement of a molecule from a cavity in which it is situated into a definite adjacent cavity in a period of time t greater than the mean lifetime of the molecule in the given cavity is exponentially dependent on the height of the potential barrier separating the cavities. The evolution of the system, with various additional limitations, is described by an infinite matrix which can be used to find the probability P(n, m, N) characterizing the diffusion of adsorbed molecules in the crystal.     

A Monte-Carlo simulation of diffusion and reaction in zeolites

Investigations of two-dimensional models to simulate diffusion and reaction in zeolites with a Monte-Carlo method are presented herein. New results of the simulation of single component diffusion and binary diffusion and a possible method to rescale the obtained diffusion coefficients in real units is presented. The estimation of an accurate activation energy was found to be the most important factor, the other parameters were estimated by simple assumptions.Complex reactions such as a consecutive reaction and xylene isomerisation were simulated with a new model. With this model the selective behaviour of the two reactions was investigated. A Type-III selectivity according to Wheeler was found for the consecutive reaction. For xylene isomerisation, reaction paths were simulated and compared with experimental reaction paths. The composition of the thermodynamic equilibrium was obtained for high transition probabilities and the relative rate constants from the literature could be confirmed. According to the results of the simulations one can conclude that a ratio of the diffusion coefficients of o-, m-, and p-xylene=1:1:1000 is too large to explain the experimentally observed distribution of the xylene isomers and a difference of only one order of magnitude was found.     

Tracing pore-space heterogeneities in X-type zeolites by diffusion studies

Pore-space homogeneity of zeolite NaX was probed by pulsed field gradient (PFG) NMR diffusion studies with n-butane as a guest molecule. At a loading of 0.75 molecules per supercage, a wide spectrum of diffusivities was observed. Guest molecules in the (well-shaped) zeolite crystallites were thus found to experience pore spaces of quite different properties. After loading enhancement to 3 molecules per supercage, however, molecular propagation ideally followed the laws of normal diffusion in homogeneous media. At sufficiently high guest concentrations, sample heterogeneity was thus found to be of no perceptible influence on the guest mobilities anymore.     

乙硫醇在MFI和MOR沸石中扩散行为的分子动力学模拟

采用分子动力学方法模拟计算了乙硫醇分子在MFI和MOR两种拓扑结构沸石中的动力学性质,比较了乙硫醇不同负载量和沸石晶格原子刚性和柔性两种状态下乙硫醇分子在沸石孔道中扩散系数的大小。模拟结果表明,对于这两种沸石,负载量增加,扩散系数减小。对于MFI沸石,在刚性和柔性沸石结构的情况下,扩散系数分别为3×10－10m2/s和4×10－9m2/s;对于MOR沸石,在刚性和柔性结构的情况下,扩散系数分别为2×10－7m2/s和3×10－8m2/s。对于MFI沸石,柔性结构下的扩散系数比刚性结构下的扩散系数要大,而对于MOR却呈现出相反的行为。对比这两种沸石,MOR中的扩散系数要比MFI沸石中大,这是因为乙硫醇分子在MOR12元环中扩散更易进行所致。相互作用能的计算结果表明,MFI中交叉型和直型孔道活性位相近,MOR孔道中12元环与4元环孔道相差较大。     

Pore mouth versus intracrystalline adsorption of isoalkanes on ZSM-22 and ZSM-23 zeolites under vapour and liquid phase conditions

Methylbranched C5-C9 alkanes do not adsorb in the intracrystalline void space of ZSM-23, neither from the vapour nor the liquid phase, but are adsorbed in ZSM-22, if only from the liquid phase, and this despite the small difference in pore size.     

The role of mesopores in intracrystalline transport in USY zeolite: PFG NMR diffusion study on various length scales

PFG NMR has been applied to study intracrystalline diffusion in USY zeolite as well as in the parent ammonium-ion exchanged zeolite Y used to produce the USY by zeolite steaming. The diffusion studies have been performed for a broad range of molecular displacements and with two different types of probe molecules (n-octane and 1,3,5-triisopropylbenzene) having critical molecular diameters smaller and larger than the openings of the zeolite micropores. Our experimental data unambiguously show that, in contrast to what is usually assumed in the literature, the intracrystalline mesopores do not significantly affect intracrystalline diffusion in USY. This result indicates that the intracrystalline mesopores of USY zeolite do not form a connected network, which would allow diffusion through crystals only via mesopores.