ADSORPTIVE SEPARATIONS BASED ON THE DIFFERENCES IN SOLUTE SORBENT HYDROGEN-BONDING STRENGTHS

Selectivity in adsorptive separations can be enhanced by limiting solute-sorbent interactions to a single or a few specific mechanisms. This work examines the potential of exploiting solute-sorbent hydrogen bonding as a selective adsorption mechanism, for solute adsorption from a nonpolar solvent onto a polycarboxylic ester sorbent. The hydrogen bond is believed to be formed between a proton-donating group on the solute and the carbonyl group on the sorbent. Studies were conducted for three classes of solutes, all of which can hydrogen bond, to determine whether differences in the strengths of adsorption can be exploited for separations. The enthalpies for adsorption from the nonpolar solvent onto the polycarboxylic ester sorbent were determined from calorimetry to be -5.1, -6.4, and -8.2 kcal/mol for the adsorption of N-methylaniline, alcohols, and phenols, respectively. In single-solute-adsorption studies with these solutes, we also observed a strong correlation between the adsorption affinity and the adsorption enthalpy. In studies on the adsorption from mixtures of two solutes, we observed that the solute with the higher adsorption enthalpy was preferentially adsorbed and that the temperature dependency of the separation factor could be related to the difference in the adsorption enthalpies of the two solutes. A simple thermodynamic framework, using data from single-solute studies, was capable of successfully predicting separation factors and the temperature dependence of separation factors.