Zeolites standard entropy

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. 

Barrer (3) makes similar calculations for the entropies of occlusion of substances by zeolites and reaches the conclusion that the adsorbed material is devoid of translational freedom. However, he uses a volume, area or length of unity when considering the partition function for translation of the adsorbed molecules in the cases where they are assumed to be capable of translation in three, two or one dimensions. His entropies are given for the standard state of 6 = 0.5, and the volume, area or length associated with the space available to the adsorbed molecules should be of molecular dimensions, v = 125 X 10-24 cc., a = 25 X 10-16 cm.2 and l = 5 X 10-8 cm. When these values are introduced into his calculations the entropies in column four of Table II of his paper come much closer together, as is shown in Table I. The experimental values for different substances range from zero to —7 cals./deg. mole or entropy units, and so further examination is required in each case to decide... 

The virial isotherm equation, which can represent experimental isotherm contours well, gives Henry s law at low pressures and provides a basis for obtaining the fundamental constants of sorption equilibria. A further step is to employ statistical and quantum mechanical procedures to calculate equilibrium constants and standard energies and entropies for comparison with those measured. In this direction moderate success has already been achieved in other systems, such as the gas hydrates 25, 26) and several gas-zeolite systems 14, 17, 18, 27). In the present work AS6 for krypton has been interpreted in terms of statistical thermodynamic models. 

Figure 26. Concentration dependences of standard sorption entropy for N2, O2 and binary mixtures at sorption-phase composition, 80 % N2 and 20% O2, on Li,RE-LSX zeolite.

The parent 7-oxanorbomane, 1, is commercially available. Its preparation starts with the catalytic hydrogenation of hydroquinone to generate a mixture of trans-and cfs-cyclohexane-l,4-diol [15-18]. cfs-Cyclohexane-l,4-diol can be isomerized into the more stable trans isomer with metallic sodium [17]. Dehydratation of the latter on A4 zeolites, on alumina [19], or over nickel-kieselguhr catalyst [20] provides 1. This reaction is exergonic (ArG° = ArH° — TArS° = -3.1 2.5 kcal/mol) at room temperature as its standard gas phase heat of reaction amounts to ArH° = -1-7.3 2.5 kcal/mol. A variation of entropy of reaction of ca. -1-35 eu is assumed for this fragmentation, what leads to —TArS° = 298(0.035) = — 10.4 kcal/mol. The standard gas phase heat of formation of frans-cyclohexane-1,4-diol (Scheme 1) is estimated from that of cyclohexanol (—69.0 2.0 kcal/mol) and the standard heat of oxidation of cyclohexane into cyclohexanol (—39.5 kcal/ mol) [21]. Chickos and Acree [22] give AfH°(l) = -43.4 0.5 kcal/mol (see also [23-25]). 

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