Isosteric heats of sorption

As a consequence of the assumed temperature dependence of the molecular volume of the sorbate, the isosteric heats of sorption, calculated... 

The isosteric heats of sorption reported in Table VI are also in acceptable agreement with values based on sorption measurements made near ambient temperature by other investigators, as the comparisons in Table VIII show. 

Table VIII. Reported Isosteric Heats of Sorption (kcal mol...

The curves of Figure 2 may be thought of as "titration" curves for the sites involved, those in zeolite NaA equalling the number of 8-rings. That sites of similar nature are involved is also shown by the observed constancy of isosteric heats of sorption with coverage. It is then clear that the initial, steep fall of the CaA zeolite curves corresponds to sites of greater number in the A structure. These curves have the appearance of being the sum of two lines of different slope. 

A variety of studies have shown that as temperature increases, sorption may increase, decrease, or remain the same, with isosteric heats of sorption being very low (Table 21.7). These studies have used the batch slurry technique, so the impact of temperature on water-triazine interactions may mask surface-triazine interactions. In contrast, at 10% soil moisture isosteric heats of atrazine sorption ranged from -10 to -12kcal/mol determined with the SF technique (Koskinen and Rochette, 1996). Sorption coefficients in field-moist soils were much greater than are typically obtained with the batch slurry system, while heats of sorption were much more negative, indicating greater sorption at low moisture contents. 

FIGURE 2 Variation of isosteric heat of sorption -AH0 with coverage c showing the difference in trends between polar and nonpolar sorbates. (Reprinted from Ruthven, D. M. (1976). Sep. Purif. Methods 5 (2), 184, copyright Marcel Dekker, Inc., New York.)... 

There are an increasing number of reports of useful, specific chemical and physical properties in MOPs. Recent examples include high gas separation efficiencies in PIM membranes [41, 78], catalytically-active phthalocyanine and porphyrin network polymers [52], and HCPs with elevated isosteric heats of sorption for CH [39], Many more possibilities for exploration exist - the following list suggests just a few such opportunities ... 

Sorptional capacity varies with tanperature, and the thermal effect associated with this phenomenon is isosteric heat of sorption, which can be numerically calculated using the Clausius-Clapeyron equation... 

Moreover, the isotherms determine the proper storage environment and the packaging conditions, especially for foods. Through the isotherms, the isosteric heat of sorption can be determined and, hence an accurate prediction can be made of the energy requirements for the drying of a solid. The utility of the isotherm is extended to the determination of the moisture sorption mechanism as well as to the degree of bound water. 

ABSTRACT. An isosteric sorption system has been used to study the sorption of methane, ethane, ethene, propane, N2 and CO2 and some of their binary mixtures in silicalite-1. Isotherms of some of these sorbates have been determined at equilibrium pressures up to 20 atmospheres. Isosteric heats of sorption have been obtained from the slopes of the isosteres. Separation factors calculated from the Henry s Law constants determined from the initial slopes of the single conq>onent isotherms are found to be in good agreement with experim tal separation factors. The Langmuir-Freundlich equation has be used to lit the single component data and the Ideal Adsorbed Solution theory has be used to predict a binary sorption isotherm from the respective single component data. Comparison of the sorption behaviour of the hydrocarbons in silicalite-1 and NaY zeolites has been made. 

The isosteric heats of sorption, Qst, obtained from the slopes of the isosteres are plotted in Figures 4 as a function of coverage for ethane, ethene and propane sorbed in silicalite-1. It will be shown later that the slope of the isosteres obtained from the isosteric apparatus used in these studies increases when die corrections referred to above are allowed for. However, for these hydrocarbon/silicalite-1 systems the corrections are too small to affect the slopes significantly so the isosteric heats shown in Figure 4 can be accepted as accurate. 

The initial isosteric heats of sorption of these hydrocarbons in silicalite-1 are listed in Table 1 which includes the corresponding heats of sorption in NaY for comparison and, also, the heats of sorption of CH4 for completeness but these methane sorption studies will be described, more fiilly, later. 

TABLE 1. Initial isosteric heats of sorption of hydrocarbons in NaY and Silicalite-1... 

Isosteric heats of sorption of various hydrocarbons in Silicalite-1. ( )C2H4(0)C2H6 ( )C2H8. 

Isosteric heats of sorption of C02 in silicalite-1 (0) from original set of isosteres ( ) from corrected set of isosteres... 

The isotherm of n-butane and 2-butyne sorbed in Silicalite-1 are shown in Figure 11. The amount of 2-butyne sorbed exceeds that of n-butane by -20% at a given pressure and temperature which may be due to bofii the smaller dynamic dimensions of the 2-butyne molecules and the specific interaction of the triple bond wifii the silanol groups which are present in the Silicalite-1 channels and intersections. [7,17] This latter interaction gives an additional lOkJ/mol to the isosteric heat of sorption of 2-butyne over that of n-butane at low loadings. Above loadings of 2 molecules perunit cell (m/uc) die isosteric heats of both sorbates are very similar increasing from 52 to 57 kJmol" because of sorbate-sorbate interactions. 

The assumptions of identical sites with no interaction between adsorbed molecules imply that the heat of adsorption is independent of coverage. It follows by differentiation of Eq. (2.27) that the isosteric heat of sorption (-A//,) is the same as the limiting heat of sorption (-A/Zg) ... 

FIGURE 4.5. Variation of isosteric heat of sorption with coverage, calculated from Eq. (3.101), for light paraffins in 5A zeolite. Points are experimental data of Schirmcr el for butanc-5A. (From ref. 10.)... 

Sorption affinity of fluid components with molecular sieves is governed by the type of their mutual interactions, i.e., fluid-solid interactions, on the microphysical level. For the sorption systems given, besides van der Waals-type atom-atom dispersion interactions of all types of molecules involved, with the solid, zeolitic ion-dipole interactions for H2O, H2S and COS as well as zeolitic ion-quadrupole interaction for CO2 have to be taken into account. A measure of the overall strength of interaction energy is the isosteric heat of sorption, qisosteric-This quantity differs from the differential heat of sorption or sorption enthalpy, - AH, usually measured calorimetrically, by the mechanical work term, RT qisosteric AH -t RT. 

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