Figure 9. (a) Experimental and simulated 1-hexene adsorption capacity and (b) calculated 1-hexene – zeolite X interaction energies of all fully ion-exchanged zeolite X.
Gas phase adsorption of 1-hexene was carried out to evaluate the adsorption capacity of ion-exchanged zeolite X samples (see Figure 9a for the experiment results). Compared with the as-synthesized zeolite NaX (178.2 mg/g), six ion-exchanged samples have larger adsorption capacities. Specifically, they are 193.9 mg/g for Mg-NaX, 181.7 mg/g for Ca-NaX, 194.5 mg/g for Co-NaX, 196.2 mg/g for Ni-NaX, 185.2 mg/g for Cu-NaX and 182.0 mg/g for Zn-NaX. The enhanced adsorption capacities of Mg-NaX and Ca-NaX samples are ascribed to the decrease in molecular weight as well as the increased pore volume (0.313 cm3·g-1 for NaX, 0.326 cm3·g-1 for Mg-NaX, and 0.317 cm3·g-1 for Ca-NaX). Unlike Mg-NaX and Ca-NaX samples, the enhanced adsorption capacities of Co, Ni, Cu and Zn-exchanged samples are ascribed to the increase of adsorption centers (π-complexation) and interaction energy.30 For Ag-NaX, a 23.4 % increase in molecular weight leads to significant decrease of 1-hexene adsorption capacity. Furthermore, we used GCMC simulation to calculate the adsorption of 1-hexene on these ion-exchanged zeolite X samples. The unit cell structures are presented in Figure S2. Overall, the simulation results are consistent with the experimental data for all ion-exchanged samples (see Figure 9a).
To further evaluate the effect of ion-exchange on adsorption performance, the van der Waals interaction energy between 1-hexene and zeolite was calculated by the Locate Task in the Sorption module of the Accelrys Package (see Figure 9b). The adsorption process was strictly restricted to feature one 1-hexene molecule confined in each zeolite X cage to exclude the intermolecular force among adsorbates. According to the GCMC simulation results, introduction of Mg2+ and Ca2+ into zeolite NaX frameworks leads to decreased interaction energies from 52.22 kJ/mol for NaX to 51.42 kJ/mol for Mg-NaX and 50.97 kJ/mol for Ca-