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-