Sorption isosteres

Sorption thermodynamic functions of nitrous oxide, N2O, are described for zeolites NaLSX and CaLSX in shapes cA clay-bound beads. They were determined by the Sorption Isosteric Method (SIM) over complete ranges of sorption-phase concentration and compared with those for carbon dioxide, C(>2. nepo earlier for the same NaLSX sorbent... 

The Sorption Isosteric Method (SIM) was used to determine thermoifynamic data of N2O sorption by zeolites NaLSX and CaLSX. This technique and its utilization for an investigation of sorption-thermodynamic properties of CO2 on identical NaLSX beads were described in detail in refs. [4-8]. 

Sorption Isosteres ofN20 on NaLSX and CaLSX Zeolites... 

Sorption isosteres measured for the N20-NaLSX and N20-CaLSX systems over complete of sorption-phase concentrations, n, are presented in Figs. I and 2, respectively. Slopes... 

Figure 1. Sorption isosteres fbrN20 on NaLSX zeolite (clay-bound beads).

Fig. 18 Sorption isosteres of the sorbate ethylbenzene from simultaneous sorption of the binary mixture of ethylbenzene and benzene pressure as a function of the reciprocal temperature at constant loading (in mmolg" ) (1) 0.10 (2) 0.15 (3) 0.20 (4) 0.25 (5) 0.30...

Since the function X = X (T,py) expresses sorption equi-librium, sorption isotherms X =X (p,)t, sorption isobars X =X (Dp, and sorption isosteres p, = p (Dx applied to drying. Since sorption isotherms are most frequently applied, the method of their determination is dealt with here. Sorption isotherms are determined from point to point ... 

This paper deals with the principles, advantages and limitations of measurement of sorption equilibria under isosteric conditions. It further assesses the sorption-isosteric method (SIM) as an effective tool for providing complete sets of sorption-thermodynamic functions, viz., enthalpy, standard entropy and standard Gibbs free energy of sorption, for nanoporous solids, i.e., micro- and mesoporous ones, as functions of sorption-phase concentration, n, over its entire range, and to approach such data for mixtures. The usefulness of SIM is exemplified by sorption systems that comprise atmospheric gases on zeolites and carbon dioxide, CO2, on carbonaceous sorbents, as well as several of their mixtures. 

The basic idea of direct measurement of sorption isosteres for microporous sorption systems was first expressed by Serpinsky in 1967 [6] and published by Bering et al. in 1969 [7]. Fundamental thermodynanric features related to sorption isosteres and their direct measurement were discussed frequently by Bering, Serpinsky, Fomkin et al, e.g., in [8-11]. The first direct measurement of single-component sorption isosteres was carried out by the Schirmer school for n-paraffin compounds on FAU- and LTA-type zeolites, reported in 1969 and published in 1971 [12]. Extended basic research performed by that school, specifically for hydrocarbon-zeolite systems, utilized SIM in close connection with other techniques, e.g., calorimetry, and theoretical methods such as Monte Carlo and statistical thermodynamics [13-17]. 

The basic principle of SIM follows from a fundamental phenomenological experience that stems from basic research executed in the area of physical sorption over many decades, viz., sorption isosteres may presumptively be considered as straight lines at constant sorption-phase composition, n = const., in Clausius-Clapeyron plots, In p vs. I / T.ln accordance with [41-43], this finding allows to calculate the differential molar sorption heat, Q, as difference between the molar enthalpy of the gas phase, Hg, and the partial molar enthalpy of the sorbed substance, // ... 

Hi) An apparent sorption isostere may deviate from linearity due to in(de)creasing desorbed amount with T in(de)crease to an extent that is specific for a considered sorption system, over a given T range. A correction of such a de(ad)sorbed amount can be applied to a single-component sorption isostere based on considerations below. An analogous correction is practically impossible for the mixture-sorption case, because of exact knowledge needed for mixture-sorption isotherms, which is very difficult to obtain. 

In case of multi-component mixtures, total isosteres can be measured at constant sorption-phase composition by changing, in successive steps, the total amount of gas mixture sorbed at constant mole fractions of sorption phase [31]. On the other hand, a point-bypoint measurement of partial pressures of a multi-coinKtnent mixture sorbed leads to partial mixture-sorption isosteres that can be evaluated further by solution thermodynamics [57]. 

Sorption isosteres were measured for CO2, N2O, N2 and O2 on NaLSX pellets (13 wt.-% binder Si/Al mole ratio of pellets 1.28), coded as FAU-I (c/. Table 2 in [30]), over wide ranges of p, T and n as seen from the sorption isosteric plots in Figures 3-6, respectively. 

Figure 3. Sorption isosteres of COj on NaLSX beads, FAU-I (the notation refers to the isostere ...

Figure 4. Sorption isosteres for N2O on NaLSX beads at phase concentrations indicated.

Sorption isosteres and sorption thermodynamic data for CO2 on specific FAU zeolite modifications, NaLSX and NaX, i.e., FAU-I and FAU-n (data obtained for a DAY zeolite, viz., dealuminated sub-type of the FAU-framework species. Figure 10 shows sorption isosteres measured for a DAY san tle with a framework elemental Si/Al ratio of c. 56, supplied by Germany. 

Figure 12. Sorption isosteres for CO2 on CarboTech D 47/2 activated carbon.

Sorption isosteres were investigated for CO2 on a series of carbonaceous sorbents, specifically on materials D 47/2, D 55/2 and DGK that were kindly provided by CarboTech, Germany. These materials differ in their degree of activation (as manufacture step) and, thus, in their sorption capacity for CO2, especially in the micro-mesoporous range. As an example, sorption isosteres for CO2 on the D 47/2 sorbent are shown in Figure 12. 

The sorption isosteres cover entire sorption-phase concentration ranges, from c. 0.06 mol/kg to c. 12.4 mol/kg. The sorption isosteres determined appear to be Unear within the experimental conditions, indicating that no sorption-phase transition occurs in the system. 

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... 

Figure 24. Sorption isosteres of binary N2 - O2 mixtures on Li,RE-LSX zeolite at sorption-phase composition of 80 % N2 and 20 % O2.

A modem version of the sorption-isosteric method has been shown to be a very useful tool for sorption-thermodynamic studies. Concenhation dependences of thermodynamic functions over entire sorption-phase concentration ranges can be determined. During an isosteric measurement, fluid-component transfer between co-existing phases is kept to aminimum to ensure that isosteric conditions are maintained, and to accelerate equilibration between phases. Isostere linearity is assumed to occur, and its validity is discussed. 

Billow M. and Shen D., Direct Measurement of Sorption Isosteres for Gases on Microporous Solids, in Proc. Topical Conf. Sep. Sci. Technol., Part II, AIChE 1997 Annual Meeting, ed. by W.S.W. Ho and R.G Luo (Los Angeles, November 16-21, 1997) pp. 1150-1155. 

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