Adsorption isotherms pulse methods

Using this methodology via measurement of adsorption isotherms, Guiochon and coworkers investigated site-selectively the thermodynamics of TFAE [51] and 3CPP [54] on a tBuCQD-CSP under NP conditions using the pulse method [51], the inverse method with the equilibrium-dispersive model [51, 54], and frontal analysis [54]. 

Chromatographic methods are widely used for the study of both physisorption and chemisorption. In its simplest form the technique consists of passing a pulse of the adsorbate through a column of the adsorbent and measuring the retention time and registering the elution curve. Measurement of the variation in the retention time as a function of temperature permits the evaluation of the enthalpy of adsorption, and analysis of the shape of the elution curve provides information about the adsorption isotherm. 

Figure 21-6. Experimental setup of ECP (a), MDM (b), and ADM (c) method for the determination of adsorption isotherms. The concentration-time relation of the dispersed taU in the ECP approach (a) is completely defined by the course of the adsorption isotherm, as can be visuahzed by the injection of increasing samples amounts. Solvent injections at defined concentrations will result in pulses in the MDM approach (b) which are linked to the adsorption isotherms. Although very precise during application of the ADM method, the data points of the adsorption isotherms (c) have to be measnred individually.

Harlicl PJ.E. and Tezel, F.H., A Novel Solution Method for Interpreting Binary Adsorption Isotherms from Concentration Pulse Chromatography Data , Adsorption, 6 (2000), pp.293-309... 

The primary use of isotherm data measurements carried out on single-component elution profiles or breakthrough curves is the determination of the single-component adsorption isotherms. This could also be done directly, by conventional static methods. However, these methods are slow and less accurate than chromatographic methods, which, for these reasons, have become very popular. Five direct chromatographic methods are available for this purpose frontal analysis (FA) [132,133], frontal analysis by characteristic point (FACP) [134], elution by characteristic point (ECP) [134,135], pulse methods e.g., elution on a plateau or step and pulse method) [136], and the retention time method (RTM) [137]. 

Jandera et al. [179] showed excellent agreement between the adsorption isotherm data obtained by the pulse method for benzophenone on a Cig-bonded silica using two columns of different diameters, 3.2 and 0.32 mm, made with exactly the same packing material. They showed that the pulse method might be far easier to use for accurate measurements than the FA method. 

The tracer pulse method was also used by Bliimel et al. [112] to determine the binary isotherms of the enantiomers of l-phenoxy-2-propanol on Chiralcel OD, by Lindholm et al. [113] to determine the binary isotherms of methyl-mandelate on Chiral AGP, and by Mihlbachler et al. [1] to determine those of the enantiomers of Troger s base on Chiralpak AD. In this last case, an imusual isotherm was obtained, illustrated in Figure 4.28. The adsorption of the more retained (+) enantiomer is not competitive the amoimt adsorbed by the chiral stationary phase at equilibrium with a constant concentration of the (+) enantiomer is independent of the concentration of the (-) enantiomer. On the other hand, the adsorption of the less retained enantiomer is cooperative the amoimt of this (-) enantiomer adsorbed by the CSP at equilibrium with a constant concentration of this enantiomer increases with increasing concentration of the (+) enantiomer. The isotherm data are best accounted for by an isotherm model derived assuming multilayer adsorption. 

Here p is the bed porosity. The dead time % is the time reqrrired for the passage of a sample pulse through the empty volume of the connecting tube from the injection point to the detector plus the void space in the packed column This method can be applied to determine binary adsorption isotherms (Harlick et al. 2003, 2004). 

The chromatography analysis presented so far for a number of practical adsorption models illustrates its usefulness in determining the adsorption equilibria constant in the form of Henry constant and the various kinetics parameters. This technique usefulness is not limited to the very low concentration range where we extract the Henry constant, it can also be applied to any concentration and if applied appropriately we can obtain the slope of the adsorption isotherm at any concentration. The appropriate method is the perturbation chromatography and its operation is as follows. First the column is equilibrated with a concentration, say C, until all void space within the column and particle have a solute concentration of C and the adsorbed phase has a concentration of f(C ) where f is the functional form for the adsorption isotherm. After the column has been equilibrated with a flow of concentration C, we inject into the column a pulse of adsorbate having a concentration of C + AC where AC C. With this small perturbation in concentration, the responses of the concentration in the column and in the particle will take the following asymptotic form ... 

Also in large pulses, overlapping of the mixture for a considerable length of a traveling distance in a column may introduce interaction effect of both components. This effect may be quantitatively checked by using numerical methods based on a bi-component adsorption isotherm. 

Measurements of the relationship between e and tracer method [119] and pulse methods [108] led to greatly divergent results. According to [119], adsorption of ethylene on platinum obeys the Langmuir isotherm. In the maximum adsorption region (0.2 -0.4 V), according to [108], the Qorg the values for ethylene and acetylene do not depend on their partial pressures. These differences are evidently due to different conditions for the preliminary treatment of the electrodes and assessment of their true surface areas. 

Volumetric measurements at room temperature showed that by far the greater part of the adsorption of carbon monoxide on Au/TiC>2 occurred on the support it followed the Langmuir equation and most of it was removable by pumping.23,83 About one-third of the titania surface was able to retain it, but there was little uptake on Au/SiC>2. Use of the double-isotherm method with Au/MgO showed that adsorption onto the metal was complete at about 1 atm, but on various samples the coverage never rose above 18%. On model Au/MgO(100) the maximum coverage attained using a pulsed molecular beam at room temperature was < 10%.54... 

The experimental method used in TEOM for diffusion measurements in zeolites is similar to the uptake and chromatographic methods (i.e., a step change or a pulse injection in the feed is made and the response curve is recorded). It is recommended to operate with dilute systems and low zeolite loadings. For an isothermal system when the uptake rate is influenced by intracrystalline diffusion, with only a small concentration gradient in the adsorbed phase (constant diffusivity), solutions of the transient diffusion equation for various geometries have been given (ii). Adsorption and diffusion of o-xylene, / -xylene, and toluene in HZSM-5 were found to be described well by a one-dimensional model for diffusion in a slab geometry, represented by Eq. (7) (72) ... 

The coupling of diazotized /i-aminoacetophenone (DPAAP) with imidazole forms the basis of the differential pulse adsorption stripping voltammetry (DPASV) method for detection of histidine and its metabolites. The absorption of azo-histidine was found to obey Frumkin absorption isotherm <1999TAL319>. 

A possibility to reduce the influence of column efficiency on the results obtained by the ECP method is to detect the position of the peak maximum only, which is called the peak-maximum or retention-time method. Graphs like Fig. 6.23 are then achieved by a series of pulse injections with different sample concentrations. The concentration and position of the maximum is strongly influenced by the adsorption equilibrium due to the compressive nature of either the front or the rear of the peak (Chapter 2.2.3). Thus, the obtained values are less sensitive to kinetic effects than in the case of the ECP method. The isotherm parameters can be evaluated in the same way as described in Section 6.5.7.6, but the same limitations have to be kept in mind. For some isotherm equations, analytical solutions of the ideal model can be used to replace the concentration at the maximum (Golshan-Shirazi and Guiochon, 1989 and Guiochon et al., 1994b). Thus, only retention times must be considered and detector calibration can be omitted in these cases. 

There is a dearth of competitive adsorption data, in a large part because they are difficult to measme, but also because little interest has been devoted to them, as, until recently, there were few problems of importance whose solution depended on their understanding. Besides the static methods, which are extremely long and tedious and require a large amoimt of material, the main methods of measurement of competitive isotherms use column chromatography. Frontal analysis can be extended to competitive binary isotherms [14,73,93-99], as well as pulse techniques [100-104]. The hodograph transform is a powerful method that permits an approach similar to FACP for competitive binary isotherms [105,106]. 

The method consists of monitoring and analyzing the response of an adsorption column to a pulse input or a step change in concentration of an adsorbate. The carrier gas is a mixture of an inert gas and the adsorbate of known composition. The retention time of the pulse is related to slope of the equilibrium curve at the carrier gas composition. The slopes of the equilibrium curve at different points on the curve can be determined by carrying out experiments with different carrier gas compositions. The equilibrium curve can, then, be easily obtained by integration of the slopes of the isotherm curve. For binary sorption equilibria, the experiments are similar except the carrier gas is a mixture of the two adsorbates. 

Recently, some techniques have been presented in the literature making use of volumetric titrations of surface sites in liquids, of different polar and protic characteristics, to determine the amount of acid sites and relevant acid strength effective acidity). Two different methods will be here discussed the first one is based on a pulse liquid chromatographic method (a dynamic method) [8] and the second one on a liquid recirculation chromatographic method (a Masi-static method) [9]. When surface acidity studies are concerned, the measurements may be performed in apolar, aprotic liquid (like cyclohexane), for the determination of the intrinsic acidity, or in several other liquids with polar/protic characteristics, for the determination of the effective acidity. Basic probes of different basicity (following the pK scale) may be used (e.g., 2-phenylethylamine, PEA, aniline, AN, pyridine, PY, etc.). Titration temperatures may be varied from room temperature (r.t.) up to the normal boiling point of the liquid used in order to calculate, from the collected isotherms of adsorption, the isosteric heats of adsorption which can be related to the acid strength of the surface sites [10, 11]. 

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