Parameter intracrystalline diffusion

Zeolite crystal size can be a critical performance parameter in case of reactions with intracrystalline diffusion limitations. Minimizing diffusion limitations is possible through use of nano-zeolites. However, it should be noted that, due to the high ratio of external to internal surface area nano-zeolites may enhance reactions that are catalyzed in the pore mouths relative to reactions for which the transition states are within the zeolite channels. A 1.0 (xm spherical zeolite crystal has an external surface area of approximately 3 m /g, no more than about 1% of the BET surface area typically measured for zeolites. However, if the crystal diameter were to be reduced to 0.1 (xm, then the external surface area becomes closer to about 10% of the BET surface area [41]. For example, the increased 1,2-DMCP 1,3-DMCP ratio observed with decreased crystallite size over bifunctional SAPO-11 catalyst during methylcyclohexane ring contraction was attributed to the increased role of the external surface in promoting non-shape selective reactions [65]. 

Quantitative Interpretation of Intracrystalline Diffusional Effects. Since a qualitative effect of crystallite size upon selectivity was observed, the next step was to extract some quantitative values for the intracrystalline diffusional parameters. To do this, we must either know the intrinsic or diffusion-free kinetics or be able to make a simplifying assumption so that the diffusional parameters can be extracted from the available data. 

R. W. H. Sargent The COj-No (or rather C02-air) values were obtained indirectly from studies on a fixed-bed separation process for removal of CO2 from air. These studies were made on a bed of pellets 4 ft long and 4 inches in diameter two pellet sizes were used, and a range of air flow rates was covered at pressures from 1 to 25 atm and from +15° to —40°C. All the results were successfully predicted by a model assuming both pellet-pore and intracrystalline diffusion under isothermal conditions the two relevant diffusivities were used as adjustable parameters to fit the experimental break-through curves by least squares for one set of conditions at each temperature, and these values then pre-... 

In Eq. (3). is the flux when intracrystalline transport is rate controlling, Af, is the real flux. 8 AE is the difference between the activation energy for escape from within the crystal to the externally adsorbed layer and the activation energy for diffusion. It is generally a positive quantity [35]. K and K represent the Langmuir parameters for adsorption on the external and internal surfaces, respectively. When internal and external adsorption isotherms are taken to be identical or are within Henry s law range, Eq. (3) becomes... 

It is well known that, under the assumption that the exchange between the product molecules in the interior of the zeolite crystallites and the reactant molecules in the intercrystalline space is limited by intracrystalline diffusion, the effectiveness of reactions catalyzed by the internal surface of zeolite crystallites is a function of a single parameter, the Thiele modulus [3. It is defined by the relation... 

In the decompositions of some particularly stable crystalline materials, the reactant stracture does not undergo recrystallization or disintegration, although there may be modification of lattice parameters following the loss of a small stable molecule, such as HjO or NHj, from the reactant phase. Such molecules diffuse outwards between structural components that are sufficiently stable to survive unmodified. Reaction rates are controlled by Fick s laws, ease of movement being determined by the dimensions of the intracrystalline channels. The participation of diffusion control is often recognized by the appearance of the characteristic A term in the rate equation estabhshed. Theoretical aspects of diffusion control have been discussed by Okhotnikov el uf/. [53-57]. 

The present study reports the measurements of intracrystalline diffusion and adsorption equilibrium for ethanol, propanols and butanols from aqueous solution in silicalite using a modified HPLC technique. The unique feature of the present work is the use of a mathematical model with a nonlinear adsorption isotherm equation to obtain the intracrystalline diffusivity and adsorption isotherm parameters. The adsorption equilibrium data for alcohols from aqueous solution in silicalite measured by the conventional batch method are also reported and compared with the results measured by the HPLC technique. 

Table II Intracrystalline diffusivity and adsorption isotherm parameters of liquid alcohols in silicalite...

Figure 23 shows results from a systematic study dealing with the influence of a hydrothermal pretreatment of granulated zeolite NaCaA on the three main transport parameters accessible by PFG NMR the coefficients of intracrystalline and long-range diffusion, as well as the intracrystalline mean lifetimes [145,... 

Figure 2 summarizes the three main parameters of molecular transport accessible by PEG NMR and illustrates the conditions under which they may be obtained. Sections 3 and 4 provide examples of the message provided by the study of long-range diffusion (Dh ) and intercrystalHne exchange rates (Tjjjtra )- Since the scientific interest in molecular propagation is primarily focussed on intracrystalline diffusion, the main part of this contribution (Sect. 5) will be devoted to the measurement of Dintra. 

Since the heat of adsorption is usually larger than the activation energy of intracrystalline diffusion, the parameter y decreases with an increase in temperature. This means that the micropore diffusion is more important at higher temperatures and the macropore diffusion is more significant at lower temperatures. This is only true for linear isotherm. We will discuss this effect on nonlinear isotherm in the next section. To illustrate the temperature effect on the parameter y for the linear isotherm case, we take the following example ... 

Xe NMR spectra are simulated from the adsorbate concentration profiles. The fit of experimental with calculated spectra (Fig. 3a) using the adjustable parameter, Tintra, allows the determination of the intracrystalline diffusion coefficient of benzene in HZSM-5 zeolite during its adsorption under constant (saturation) pressure. Dint = 7x10 m s" . This value agrees with that obtained with other techniques and reported in literature [4]. The equilibrium time, too, of 8 and 13 h, depends on the bed length, 5 and 15 mm, respectively. The calculation... 

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