Adsorption of pentane isomers on metal-organic frameworks Cu-BTC and Fe-BTC

Adsorption isotherms of n-pentane, isopentane and cyclopentane on Cu-BTC and Fe-BTC metal-organic frameworks, measured in a temperature range from 273 to 333 K, have been examined. For correlations, adsorption of nitrogen at 77 K and argon at 87 K has been studied as well. The analysis of adsorption isotherms was based on the calculation of differential distributions of the amount adsorbed with respect to the adsorption potential. It appears that the filling of Cu-BTC micropores with nitrogen occurs in two steps due to specific interaction of nitrogen molecules with open metal sites. In contrast to nitrogen, the filling of Cu-BTC micropores with argon takes place in one step. The micropores of Fe-BTC are filled with nitrogen and argon gradually in two featureless stages indicating the unordered porous structure. Adsorption isotherms of n-pentane, isopentane and cyclopentane display especially at 273 and 293K a rectangular course with a long vertical part. This shape of C5 hydrocarbon isotherms on Cu-BTC is consistent with the shape of argon isotherms since the interactions with saturated hydrocarbons are nonspecific and thus the pores of Cu-BTC are filled in one step. The isotherms of C5 hydrocarbons on Fe-BTC show one saturation step being less sharp than in the case of Cu-BTC. Calculated isosteric adsorption heats suggest that the energy of lateral interactions between adsorbed molecules dominates the gas-solid energy heterogeneity. This domination is especially pronounced for cyclopentane adsorbed on Cu-BTC. In the case of Fe-BTC, almost constant isosteric adsorption heats show that the variations of the gas-solid interaction energy are compensated by the energy of lateral interactions. (C) 2014 Elsevier B.V. All rights reserved.