Sorption isosteric heats

Sorption-isosteric heats determined by SIM over full concentration ranges can be analyzed to identify, quantify and distinguish between the strengths of sorption sites in nanoporous sorption systems. Figure 17 shows the concentration dependences of isostericsorption heats of N2 and O2 on zeolites CaA (Ca ion... 

The parameter determined from simulations that can be most readily compared to an experimentally observable quantity is the heat of sorption. The mean total internal energy determined by the simulations, (V), can be equated to the isosteric heat of adsorption, Qst, in the limit of low occupancy, as follows ... 

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.)... 

Almog and hrier (1978) made a direct calorimetric measurement of the dependence of the heat of solution of ribonuclease A on water content (Fig. 2). The heat of solution drops strongly in the low hydration range 90% of the heat change is obtained at about half-hydration. The differential heat for transfer of water from the pure liquid to the protein is estimated from the data of Fig. 2 as 8 kcal/mol of water at the lowest hydration studied (the heat of condensation of water should be added for comparison with isosteric heats), and it decreases monotonically with increased hydration. There is no extremum at low hydration, unlike what has been reported based on the temperature dependence of the sorption isotherm. It is not clear whether this difference reflects inaccuracies in the data used in van t Hoff analyses of the sorption isotherms, or a complex hydration path that is not modeled properly in the van t Hoff analyses. 

Relatively speaking, the affinity of the hydrogen-mordenite is greatest for the SO2, then in decreasing order the CO2, N2, and O2 follow. This can be explained by differences in the electronic configurations of these adsorbates no "sieving would be expected in any of these systems since the molecular diameter of these gas molecules is well below the published pore diameters. The sorption of all the species is likely to be physical, since the calculated isosteric heats are all below 5 kcal/mole. 

We have examined whether a simple non-bonded potential can be developed to be (i) transferable from one zeolite to another and (ii) to simulate without parameter adjustment isosteric heats at different temperatures and sorption uptake isotherms. The sorption of methane into Na- and K- zeolite X, and Na-and K-clinoptilolites was considered. Models for Na-X and K-X were constructed based on the averaged crystallographic results. The non-bonded parameters in a Lennard-Jones potential were iteratively adjusted so as to best reproduce the experimental isosteric heats in Na-X and K-X over a small temperature range. Methane-methane interaction parameters were taken from earlier work [89] and a final iteration was made so as to better fit the experimental sorption isotherms in clinoptilolite. This single and simple non-bonded potential parameter set then reproduces to a reasonable degree... 

The selection of adsorbents is critical for determining the overall separation performance of the above-described PSA processes for hydrogen purification. The separation of the impurities from hydrogen by the adsorbents used in these processes is generally based on their thermodynamic selectivities of adsorption over H2. Thus, the multicomponent adsorption equilibrium capacities and selectivities, the multi-component isosteric heats of adsorption, and the multicomponent equilibrium-controlled desorption characteristics of the feed gas impurities under the conditions of operation of the ad(de)sorption steps of the PSA processes are the key properties for the selection of the adsorbents. The adsorbents are generally chosen to have fast kinetics of adsorption. Nonetheless, the impact of improved mass transfer coefficients for adsorption cannot be ignored, especially for rapid PSA (RPSA) cycles. 

Henry constants (b), isosteric heats of adsorption qst and lower limiting temperature for sorption behaviour TiimiHenry for CH4, H2, CO2 and isobutane in microporous silica at P < 125 kPa. After de Lange et al. [59,63]... 

The activation energy for intra-crystalline diffusion for n-butane is 30 kj mol" [72,74,89], For the isosteric heat of adsorption values of =38 kJ mol" are reported by Kapteyn and by Vroon, which value is considerably lower than other values (=50 kJ mol ) reported in literature. For CH4 a good agreement between Ccdculated and measured fluxes is obtained, for n-butane the agreement is reasonable to bad at low pressure and good at higher pressure. A difficult problem is the value of the saturation concentration. In many cases no reliable experimental data are known and theoretical estimates have to be made usually imder the questionable assumption that is independent of temperature. For n-butane the theoretical sorption capacity of MFI/silicalite equals about 2.1x10 mol g" (equivalent to 12 molecules per unit cell). 

The pertinent thermodynamic variable to quantitatively describe the thermal effects in the exo(endo) thermic gas ad(de)sorption process is called the iso-steric heat of adsorption.The isosteric heat of adsorption of component i of an ideal gas mixture (q/) at adsorbate loading of rii and temperature T is given by the following thermodynamic relationship... 

Abstract Infrared spectroscopic methodsfor the measurement of adsorption and adsorption kinetics of some aromatics (benzene, ethylbenzene, p-xylene), pyridine, and paraffins in solid microporous materials such as zeolites (MOR, ZSM-5, silicalite-1) are described as well as the evaluation of the spectroscopically obtained data. The adsorption isotherms are of the Langmuir-Freundlich type. Isosteric heats of adsorption, transport diffusivities, and activation energies of diffusion as deduced from the spectroscopic measurements are compared with literature data as far as available, and they are found to be in reasonable agreement with results provided by independent techniques. Special attention is paid to sorption and sorption kinetics of binary mixtures, especially the problems of co- and counter-diffusion. ... 

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. 

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