Sorption Rate Measurements

Condition 1 Direct determination of diffusivities in ZSM-5 via sorption rate measurements showed Dp/DQ > 103 and and DQ Dm. 

Diffusion of hydrocarbons and other simple molecules in A, X and Y zeolites has been studied by a range of experimental methods including direct sorption rate measurements, chromatography and NMR. The advantages and limitations of these techniques are considered and results of recent experimental studies are reviewed with emphasis on the detailed microdynamic information obtainable by NMR. 

In 200-mL deionized water 0.1 g of humic acid particles were suspended at 298 K, stirred at 430 rpm, 0.02 mmole metal ions added, and the sorption rate measured as described above. At the end of the first experiment, the humic acid particles, (which were now partially saturated with the metal ion) were allowed to settle, the supernatant solution was withdrawn, the particles in the reaction vessel were washed with deionized water, resuspended again in 200 mL water, 0.02 mmole metal ions added as before, and the sorption rate was determined. This procedure was repeated several times for each metal ion. More details of this technique have been given previously (Bunzl et al., 1976b). 

NMR PFG measurements determine the tracer or self-diffusivity (D ) under equilibrium conditions with no concentration gradient. n any sorption rate measurement it is the transport diffusivity under the influence of a concentration gradient which is measured. In general these two quantities are not the same but the relationship between them can be established from irreversible thermodynamics. (17,18) In the low concentration limit the thermodynamic correction factor vanishes and the transport and self diffusivities should approach the same limit. Since ZLC measurements are made at low concentrations within the Henry s Law region the diffusivity values should be directly comparable with the NMR self-dif fusivities. ... 

In recent years there has been a tendency to accept NMR diffusivity data as correct and to assume that where lower diffusivity values are obtained from sorption rate measurements the latter must be in error. The present results suggest that this perspective may not always be correct. Until the origins of the discrepancy are resolved, one must be cautious of accepting NMR self diffusivity data without independent confirmation. 

The generalized Maxwell-Stefan equation provides a rational basis for the analysis of sorption rate measurements and membrane permeation in multi-component systems. For a binary Langmuir system ... 

The consistency between sorption rate measurements and PFG NMR measurements in large crystals of 5A zeolite was noted many years ago. In more recent studies a similar pattern of consistency between sorption rate, ZLC, and PFG NMR data has been observed for Xe, CO2, and C3H8 in 5A zeoHte (Fig. 13). 

The striking discrepancy between the results of sorption rate measurements with large (100 xm, 250 xm) crystals, and the PFG NMR data for the xylenes in NaX was pointed out in 1989 [72]. The behavior of benzene in NaX is similar [55]. Diffusion of benzene is about an order of magnitude faster than the xylenes, and direct derivation of diffusivities from uptake rate measurements is therefore possible only over a very limited range of conditions. This system... 

As an alternative to conventional sorption rate measurements it is also possible to derive diffusional time constants from the dynamic response of a packed column to a change in sorbate concentration. In a chromatographic system the broadening of the response peak results from the combined effects of axial dispersion and mass transfer resistance. By making measurements over a range of gas velocities it is possible to separate the dispersion and mass transfer effects and so to determine the effective overall mass transfer coefficient or the diffusional time constant. Further details are given in Section 8.5. 

Sorption Rates in Batch Systems. Direct measurement of the uptake rate by gravimetric, volumetric, or pie2ometric methods is widely used as a means of measuring intraparticle diffusivities. Diffusive transport within a particle may be represented by the Fickian diffusion equation, which, in spherical coordinates, takes the form... 

The zeolites and catalysts used in this study were prepared as described previously (1,16,18,20). The ortho-xylene sorption rate data, obtained on a computer-controlled Du Pont 951 TGA, were measured at 120°C and P(o-xylene) 3.8 torr. The isomerization and disproportionation data were obtained using a... 

An intriguing aspect of these measurements is that the values of D determined from NMR and from sorption kinetics differ by several orders of magnitude. For example, for methane on (Ca,Na)-A the value of the diffusion coefficient determined by NMR is 2 x 10 5 cm2 sec-, and the value determined for sorption rates only 5 x 10"10 cm2 sec-1. The values from NMR are always larger and are similar to those measured in bulk liquids. The discrepancy, which is, of course, far greater than the uncertainty of either method, remained unexplained for several years, until careful studies (267,295,296) showed that the actual sorption rates are not determined by intracrystalline diffusion, but by diffusion outside the zeolite particles, by surface barriers, and/or by the rate of dissipation of the heat of sorption. NMR-derived results are therefore vindicated. Large diffusion coefficients (of the order of 10-6 cm2 sec-1) can be reliably measured by sorption kinetics... 

Earlier studies of intracrystalline diffusion in zeolites were carried out almost exclusively by direct measurement of sorption rates but the limitations imposed by the intrusion of heat transfer and extra-crystalline mass transfer resistances were not always fully recognized. As a result the reported diffu-sivities showed many obvious inconsistencies such as differences in diffusivity between adsorption and desorption measurements(l-3), diffusivities which vary with fractional uptake (4) and large discrepancies between the values measured in different laboratories for apparently similar systems. More recently other experimental techniques have been applied, including chromatography and NMR methods. The latter have proved especially useful and have allowed the microdynamic behaviour of a number of important systems to be elucidated in considerable detail. In this paper the advantages and limitations of some of the common experimental techniques are considered and the results of studies of diffusion in A, X and Y zeolites, which have been the subject of several detailed investigations, are briefly reviewed. 

NMR Sorption Comparison Diffusion of the large tri-ethylamine molecule is sufficiently slow that reliable dif-fusivities can be determined from uptake rate measurements in 50]jm crystals(57) at least over a limited range of conditions. A comparative study, carried out with the same zeolite samples showed good agreement between the sorption and PFG NMR measurements, both as to the magnitude of the diffusivity ( 10 cm2.s- at 445K) and the trends with concentration and temperature(41). 

By combining thermodynamically-based monomer partitioning relationships for saturation [170] and partial swelling [172] with mass balance equations, Noel et al. [174] proposed a model for saturation and a model for partial swelling that could predict the mole fraction of a specific monomer i in the polymer particles. They showed that the batch emulsion copolymerization behavior predicted by the models presented in this article agreed adequately with experimental results for MA-VAc and MA-Inden (Ind) systems. Karlsson et al. [176] studied the monomer swelling kinetics at 80 °C in Interval III of the seeded emulsion polymerization of isoprene with carboxylated PSt latex particles as the seeds. The authors measured the variation of the isoprene sorption rate into the seed polymer particles with the volume fraction of polymer in the latex particles, and discussed the sorption process of isoprene into the seed polymer particles in Interval III in detail from a thermodynamic point of view. 

Skujins and McLaren (1967) co-lyophilized urease and [ CJurea. The rate of reaction, determined by the level of C02, was measured as a function of water content. Onset of enzyme reaction occurred at 0.6 relative humidity. The samples contained a 25 1 weight ratio of urea to urease. Sorption isotherms measured separately for enzyme and urea showed that below 0.75 relative humidity the urea adsorbed no water, and thus that the enzyme changes reflected adsorption of water by the urease. From the sorption isotherm for urease, 0.6 relative humidity corresponds to 0.15 h. 

This effect was confirmed by calculation of the loading of the free phenol molecule and the complex phenol-sulfolane (expressed as the number of loaded molecules in a crystal elementary unit of TS-1), using the software Sorption (Cerius 2), which turned out to be 13.6 and 0.8, respectively. Alternatively, the protective effect exerted by sulfolane can be evaluated by measuring the reaction rate, expressed as the turnover frequency (TOF moles of reacted substrate/moles of Ti per hour) for the oxidation of benzene and phenol, carried out separately in acetone and sulfolane as co-solvents. In the case of acetone, the phenol oxidation (TOF = 190) was ten times faster than that measured for benzene (TOF = 19) conversely, operating in sulfolane the rate measured for phenol (TOF = 51) was only 1.6 time higher than that measured for benzene (TOF = 31), according to the higher value of the observed selectivity. 

Figure 3.11. Time dependence predicted for Mn sorption on -MnO , using the rate constants determined, compared with the sorption rate curve measured for initial Mn"+ concentrations of (a) 25 piM and ( ) 40 piM. (From Fendorf et al., 1993.)...

In the dry particles, oxygen uptake occurs on a measurable time scale (minutes). Here the span of the experiment is the particle radius. Such experiments hold the promise of providing sorption rates into the stabilizer phase and into the interphase. We are investigating whether it is in fact possible to obtain information in sufficient detail to measure the magnitude of the interphase in the dry particles and to obtain the solubilities and diffusion coefficients of various sorbents within the various individual phases of the material. 

A new experimental technique (ZLC) has been developed and applied to study the diffusion of a range of hydrocarbons (xylene, benzene, cyclohexane and linear paraffins) in unaggregated crystals of zeolites A and X. The validity of the method was confirmed by varying the crystal size and the nature and flowrate of the purge gas. The method has advantages of speed and simplicity but the major advantage is that the intrusion of extraneous heat and mass transfer resistances is much less significant than in conventional uptake rate measurements. As a result, the new method can be applied to systems in which diffusion is too rapid to follow in a conventional sorption experiment. 

---