Coadsorption enthalpy

If the van t Hoff and isosteric methods are simple ways for estimating the adsorption enthalpy of single component from isothermal adsorption data, they have the disadvantage to not take into account the temperature dependence on the enthalpy and entropy and to be not enough accurate. Moreover they are not adapted to the adsorption of gas mixtures. The best mean to determine the adsorption and coadsorption enthalpy is to measure them by using a differential calorimetry technique coupled with others techniques allowing the measure of adsorbed amount and composition as for example the manometry and the chromatography. 

However, if the adsorbed phase can be considered as an ideal solution, in which the molecular interactions are the same as for the adsorption of single components, it is possible to calculate the molar differential coadsorption enthalpy by mean of the relation ... 

Thus we can obtain the dependence of the adsorbed amount on the differential coadsorption Gibbs energy in a similar way as for the differential coadsorption enthalpy measured by calorimetry. Practically, ArG g can be calculated by numerical derivation of the plot AG = f(n ). In the case of the adsorption of a single component, the equations for the calculation of the Gibbs adsorption energy are exactly the same, we have just to take i equal to one. 

The differential molar coadsorption entropy can be easily calculated knowing the differential molar coadsorption enthalpy measured by calorimetry and the differential molar coadsorption Gibbs energy calculated as explained above by mean of the relation ... 

It may be noticed that in the case of an ideal adsorbed solution the partial molar differential values ArZ j is constant and equal to the partial molar value of single component ArZ j. If Z represents the enthalpy, we retrieve from the Eq. 7.97 the Eq. 7.60, which allows the prediction of the coadsorption enthalpy from the adsorption enthalpies of single components. 

Figure 7.26 shows the coadsorption enthalpies as a function of the tilling. For NaY, the calorimetric curve measured with the equimolar mixture is very similar to... 

Fig. 7.26 Coadsorption enthalpies of p-xylene and m-xylene on NaY and BaY zeolites at 423 K as a function of the total filling [Doted lines adsorption enthalpies of single components]...

Fig. 7.34 Coadsorption enthalpies of ESH + HEP and ESH+TOL as a function of the total filling of NaX zeolite at 298 K and for a sulphur content in the initial gas mixture of 25 %. Red points experimental green solid line calculated from adsorption enthalpies of single components with Eq.7.60]...

Abstract This chapter is devoted to the study of coadsorption of gases in nanoporous solids by using the differential calorimetry. In the first part, the thermodynamic principles of adsorption of gases are recalled. Some of them have already presented in chapter one. However a special attention has been paid here to the determination of the adsorption enthalpies and entropies and we focused on the selective adsorption of binary mixtures. Then the specific experimental technique based on the combination of differential calorimetry with manometry and gas phase chromatography or mass spectrometry is shown in details. In the last part, the thermodynamic concepts on coadsorption are illustrated with experimental results taken from studies on gas separation by selective adsorption in mlcroporous solids. 

Let us consider now the coadsorption of two gases or more, the definition of the calorimetric heat is exactly the same as for the adsorption of a single component. In this case, it corresponds obviously to the differential molar enthalpy of coadsorption. It is not possible to measure directly by calorimetry the differential enthalpy of adsorption of each component present in the mixture. Thus, for the coadsorption of two components A and B, the molar calorimetric coadsorption heat is equal to the molar differential enthalpy of coadsorption ... 

Let us consider the adsorption of a binary gas mixture A+B and Z a function of state, which denotes either the enthalpy or the entropy. The differential molar coadsorption value ArZ is equal to ... 

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