MULTIPLE EQUILIBRIA ANALYSES OF GAS-POROUS SOLID ISOTHERMS

This article confirms the physical significance of a new method for de termining adsorption energy distributions in porous materials. The pre mise of the model is that an adsorption isotherm consists of several e quilibrium processes corresponding to adsorption into a distinct pore size regime. A distribution type equilibrium constant, K, involving th e gas and adsorbate in a pore of capacity n describes each process. In order to define the n's and K's for all the processes involved, isoth erms are collected at several temperatures to minimize the ratio of un knowns to knowns. In this article the model is extended to a series of adsorptives and it is shown that the resulting K's, Delta H's, and n' s are not meaningless empirical fit parameters but have the meaning su ggested by the model. The Delta G's vary linearly with Delta H for eac h pore size regime and both correlate linearly with the square root of the van der Waals a parameter, a(1/2). In addition to providing stron g support for the physical significance of the parameters, these corre lations enable prediction of the K values for adsorption of a new adso rptive by a characterized adsorbent given the a parameter of the adsor ptive. The correlations show that the strongest binding corresponds to the adsorptive selecting pores from the distribution available that m atch its molecular dimensions. The n's for the different adsorptives p rovide insight into the pore distribution in the solid and about the p ores utilized in the adsorption of different adsorptives. Prediction o f the K's from a and estimating n's from molecular diameters is sugges ted as a way to attain the long-range goal of predicting the total iso therm for a new adsorptive from molecular properties. The practical ap plication of this information for use in separations is illustrated. T he concept of effective pressure, P-eff, is introduced for catalysis t o allow comparison of the concentrating effect of different microporou s solids.