Zeolites single-component diffusion

For multi-component systems it seems intuitive that single-component diffusion and adsorption data would enable one to predict which component would be selectively passed through a membrane. This is only the case where molecular sieving is observed for all other separations where the molecules interact with one another and with the zeolite framework their behavior is determined by these interactions. Differences in membrane properties such as quahty, microstructure, composition and modification can also play a large role in the observed separation characteristics. In many cases, these properties can be manipulated in order to tailor a membrane for a specific apphcation or separation. 

In order to apply this technique for quantitative characterization of counter-transport phenomena in zeolites it had to be checked first for the more simple case of single-component diffusion. As an example, benzene diffusion in H-ZSM-5 was chosen, because results for this case had been already reported by several authors. Their results, obtained from sorption kinetics as well as from NMR experiments, were in very good agreement and, thus, provided a reliable basis for comparison [13,14]. 

In this section, single-component diffusion in zeolites [20,87-92], with the help of the case study of the diffusion of p-xylene and o-xylene in H-ZSM-11 and H-SSZ-24 zeolites, is discussed [90], SSZ-24 is a 12-MR zeolite that was first obtained as the silicon counterpart of AlP04-5, and, later, was obtained as a borosilicate (B-SSZ-24), which could be exchanged with A1 to yield the H-SSZ-24 zeolite [111], This zeolite exhibits the AFI framework, which embodies a one-dimensional channel network without cavities, consisting of parallel 12-MR channels with a free-channel diameter, ow = 7.0A [112], Additionally, ZSM-11 encloses an intersecting two-directional 10-MR channel system, where the two-dimensional channel system is characterized by the free-channel diameter, ow = 5.8 A [112],... 

For the systems described in this chapter, the values for om and ow are om = 5.8 and om = 7.0 A for / -xylene and o-xylene, respectively [11,12] and ow = 7 A for the SSZ-24 channel windows and ow = 5.8 A for the ZSM-11 zeolite [112], Therefore, p-xylene and o-xylene relatively, freely move in H-SSZ-24 during single-component diffusion, inasmuch as r p x = 0.83 and T 0, = 1.00. In addition, for H-ZSM-11, the single-component diffusion of o-xylene is hindered by steric factors inasmuch as rp= 1.21, but the single-component diffusion for / -xylene is relatively free since rp, = 1. These facts are reflected on the reported single-component diffusion coefficients (see Table 5.3). Besides, the results reported in Table 5.3 reasonably agree with data previously reported in the literature for the diffusion of xylenes in zeolites with 10- and 12-MR channels [88,116-120],... 

Figure 13 displays the self-diffusivities of n-hexane and 2-methylpentane in silicalite-1 and H-ZSM-5 as a function of the ratio of the hydrocarbons. The self-diffusivities of both hexanes linearly decrease with increasing gas-phase fraction of the branched hexane in the gas phase for the non-acidic and acidic zeolite. In H-ZSM-5, the mobility of alkanes is approximately two times slower than in silicalite-1. Obviously, the presence of acid sites strongly affects the molecular transport due to stronger interactions with the n-hexane molecules. A similar effect of Bronsted sites on the single component diffusion of aromatics was observed in MFI zeolites with different concentration of acid sites [63-65]. The frequency response (FR) technique provided similar results... 

The current work indicates the strong effect of acid sites on the interaction and diffusivity of hydrocarbons. To further study this effect, we determined the single-component diffusion coefficients and specifically the activation energy for diffusion. Activated diffusion is described by the Arrhenius-type Eq. 8. The pre-exponential factor Djnf is related to the jump frequency between adsorption sites in the zeolite lattice, while the exponential expresses the chance that the molecules are able to overcome the free energy barrier - act between these sites. The loadings of n-hexane and 2-methylpentane in H-ZSM-5 and silicalite-1 have been measured at temperatures between 373 and 533 K at intervals of 20 K. The hydrocarbon pressure was taken identical... 

For zeolites, the following relationship can be used to directly estimate the concentration dependence of the effectiveness factor for a first-order reaction (involving single-component diffusion) in a flat plate (Ruthven, 1972) ... 

For single-component gas permeation through a microporous membrane, the flux (J) can be described by Eq. (10.1), where p is the density of the membrane, ris the thermodynamic correction factor which describes the equilibrium relationship between the concentration in the membrane and partial pressure of the permeating gas (adsorption isotherm), q is the concentration of the permeating species in zeolite and x is the position in the permeating direction in the membrane. Dc is the diffusivity corrected for the interaction between the transporting species and the membrane and is described by Eq. (10.2), where Ed is the diffusion activation energy, R is the ideal gas constant and T is the absolute temperature. 

The prerequisites of the evaluation of data characteristic of intracrystalline processes in the case of zeolite sorbents are discussed, along with the conditions under which diffusion can be compared to self-diffusion. Selected results of investigations carried out in the author s laboratory are given in order to demonstrate the consistency of sorption kinetic data with intracrystalline mobility data of single components on molecular sieves (HS). Various types of surface barrier which may influence the uptake rate are also described. 

Conversely, the correct approach to formulate the diffusion of a single component in a zeolite membrane is to use the MaxweU-Stefan (M-S) framework for diffusion in a nonideal binary fluid mixture made up of species 1 and 2 where 1 and 2 stands for the gas and the zeohtic material, respectively. In the M-S theory it is recognized that to effect relative motions between the species 1 and 2 in a fluid mixture, a force must be exerted on each species. This driving force is the chemical potential gradient, determined at constant temperature and pressure conditions [68]. The M-S diffiisivity depends on coverage and fugacity, and, therefore, is referred to as the corrected diffiisivity because the coefficient is corrected by a thermodynamic correction factor, which can be determined from the sorption isotherm. 

The equations and plots presented in the foregoing sections largely pertain to the diffusion of a single component followed by reaction. There are several other situations of industrial importance on which considerable information is available. They include biomolecular reactions in which the diffusion-reaction problem must be extended to two molecular species, reactions in the liquid phase, reactions in zeolites, reactions in immobilized catalysts, and extension to complex reactions (see Aris, 1975 Doraiswamy, 2001). Several factors influence the effectiveness factor, such as pore shape and constriction, particle size distribution, micro-macro pore structure, flow regime (bulk or Knudsen), transverse diffusion, gross external surface area of catalyst (as distinct from the total pore area), and volume change upon reaction. Table 11.8 lists the major effects of all these situations and factors. 

Figure 20 shows the diffusivities of isopropanol, acetone and propene under the conditions of single-component adsorption on zeolite Na-X [170]. All three compounds are involved in a well-established test reaction to discriminate between acid and basic zeolites [171,172] Isopropanol is dehydrated to propene on acid catalysts, while it is dehydrogenated to acetone on basic catalysts. Figure 20 shows that the diffusivity of propene, i.e. of the product of the acid-catalyzed reaction is more than one order of magnitude larger than the diffusivities of the reactant (isopropanol) and of the product (acetone) of the base-catalyzed reaction. Hence, if the acid- and base-catalyzed reactions both were to occur in parallel, the difference in... 

The FTIR technique has proven to be a powerful method for investigating adsorption, desorption, and diffusion of single components or binary mixtures in microporous solids such as zeolites. In the latter case of mixtures, the phenomena of codiffusion and counter-diffusion became accessible to measurement, which was not possible with methods of investigation based on changes of weight, volume, or pressure. Even with the powerful and most important NMR techniques (see Chap. 3 of the present volume), the study of multicomponent (e.g., H2-D2) self-diffusion rather than co- and counterdiffusion experiments is possible (see Sect. 1 and [6]). The only prerequisite for the IR method is that the IR spectra, which are contributed by the components of the mixture, can be sufficiently decomposed. This, however, was easily achieved for all systems studied so far, owing to appropriate computer programs nowadays available. Certainly, the computational methods... 

Abstract Zeolites are of prime importance to the petrochemical industry as catalysts for hydrocarbon conversion. In their molecule-sized micropores, hydrocarbon diffusion plays a pivotal role in the flnal catalytic performance. Here, we present the results of Positron Emission Profiling experiments with labeled hydrocarbons in zeolites with the MFI morphology. Single-component self-diffnsion coefficients of hexanes in silicalite-1 and its acidic connterpart H-ZSM-5 are determined. For the first time, self-diffnsion co-... 

The last two chapters have addressed the adsorption kinetics in homogeneous particle as well as zeolitic (bimodal diffusion) particle. The diffusion process is described by a Fickian type equation or a Maxwell-Stefan type equation. Analysis presented in those chapters have good utility in helping us to understand adsorption kinetics. To better understand the kinetics of a practical solid, we need to address the role of surface heterogeneity in mass transfer. The effect of heterogeneity in equilibria has been discussed in Chapter 6, and in this chapter we will briefly discuss its role in the mass transfer. More details can be found in a review by Do (1997). This is started with a development of constitutive flux equation in the presence of the distribution of energy of interaction, and then we apply it firstly to single component systems and next to multicomponent systems. 

Choudhary, V.R. Nayak, V.S., and Choudhary, T.V., Single-component sorption/diffusion of cyclic compounds from their bulk liquid phase in H-ZSM-5 zeolite. Ind Eng. Chem. Res.. 36(5), 1812-1818 (1997). 

As we have seen previously, the separation mechanism in pervaporation is explained by an adsorption-diffusion process. In this way, the selective adsorption of the components in the zeolite will be responsible for the selectivity in the separation. Adsorption is an exothermic nonactivated process. In general, the isotherm of adsorption on zeohtes follows a single site Langmuir-type isotherm [74]. 

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