Adsorption enthalpy differential

The site-selectively derived thermodynamic parameters obtained by adaptation of Equation 1.17 (Table 1.8) clearly revealed that the heat of adsorptions are exothermic on both enantioselective and nonenantioselective sites, and the difference in the adsorption enthalpies on enantioselective and nonenantioselective sites is about 10 and 15 kJ mol for/ - and 5-enantiomers, respectively. The differential enthalpy change upon adsorption of R- and 5-enantiomers at the enantioselective site AAEIg... 

The differential enthalpy of adsorption may be obtained indirectly by the isosteric method (cf. Section 2.6.1). In the past, it was often referred to as the isosteric heat and denoted by -qa. This term is now discouraged, and should be replaced by the isosteric enthalpy of adsorption. The differential energy and differential enthalpy of adsorption are related by the expression ... 

Differential heat of adsorption or adsorption enthalpy Qiiff APiads J mof Defined from integral heat by Qditf ( Qjnt/ a)r,A... 

Reviews by Gorte and coworkers [35, 36] deal with the adsorption complexes formed by strong and weak bases with acid sites in zeolites. They examine the adsorption enthalpies of a series of strongly basic molecules such as alkylamines, pyridines and imines. These workers also performed studies of the adsorption properties of weak bases, including water, alcohols, thiols, olefins, aldehydes, ketones and nitriles. They report a poor correlation between the differential heats of adsorption on H-MFl zeolites and the enthalpies of protonation in aqueous solutions, but a much better correlation with gas-phase proton affinities [37]. 

Instead of TPD, microcalorimetry of adsorption shows the heat evolved during the adsorption of probe molecules, usually ammonia, on acid sites [91-94]. This measurement can determine the distribution of adsorption enthalpies but cannot differentiate between adsorption on Lewis and Br0nsted acid sites. 

Fbr type II isotherms with multilayer adsorption, the differential enthalpy for the second and higher layers approaches the entWpy of condensation of the liquid. Does the differential enthalpy for type I isotherms approadi some limit at saturation ... 

With the increase in the alkyl chain, the slope of the plots, i.e. the change in the differential adsorption enthalpy (—AH) increases. Fig. 11 also shows that branching of an alkyl chain with a given number of carbon atoms leads to a weakening in the retention and to a decrease in — AH. 

Figure 11. Adsorption of DTAB onto precipitated silica at 298 K and different ionic strengths (a) isotherms of adsorption (b) differential molar enthalpies of displacement (c) electrophoretic mobilities of silica particles.

One, called isosteric (because, for gas adsorption, it requires comparing two states with same amount adsorbed, i.e., same volume adsorbed), is the calculation of the differential adsorption enthalpy by using a set of two (or, better, three) adsorption isotherms at different temperatures. In dilute solution, the calculation of the isosteric enthalpy from adsorption isotherms at different temperatures is done by applying the following equation ... 

The differential adsorption enthalpy given in kJ/kg adsorbed substance, is exchanged when the adsorbent load is increased from A to A -I- dA . The integral adsorption enthalpy Ah, given in kJ/kg adsorbent, is the evolution of heat if 1 kg adsorbent is loaded with adsorbate, starting from 0 to the final load A, ... 

The differential adsorption enthalpy depends on the adsorption system, the adsorption conditions, pressure and temperature, and the adsorbate concentration X. Its value decreases with increasing X. 

Figure 4-13 gives the differential molar adsorption enthalpy for the adsorption of different hydrocarbons on active carbon. 

On principle calorimetric data of adsorbed phases can be calculated from adsorption equilibria data, i. e. adsorption isotherms, if these are available for different temperatures. From the Clausius-Clapeyron equation applied to the phase equilibrium in the ideal sorptive gas (f) - adsorbate (a) system, one can derive an equation for the isosteric differential adsorption enthalpy or isosteric differential heat of adsorption [2.2, p. 43], [2.26, p. 38]... 

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. 

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. 

Thus, the molar adsorption heat measured by differential calorimetry at constant temperature corresponds to the differential molar adsorption enthalpy. This value is equivalent to the isosteric enthalpy of adsorption [11]. 

In the case of multi-components adsorption, the partial molar differential adsorption enthalpies and entropies of each component i present in the gas mixture cannot be directly measured by experiments. However, it is possible to estimate them by mean of the tangent method based on the well-known Gibbs-Duhem 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. 

Fig. 7.21 Differential molar adsorption enthalpies of single p-xylene and m-xylene as a function of filling in NaY and BaY zeolites at 423 K...

Fig. 7. 31 Adsorption enthalpies of ESH, HEP and TOE on NaX zeolite determined by differential calorimetry at 298 K as a function of the loading rate...

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