Figure 10. Calculated 1-hexene adsorption capacities on all ion-exchanged zeolite X samples.
NaX. Such phenomenon is due to decrease in the number of adsorption sites.59 In contrast, ion-exchange of Na+ with Co2+, Ni2+, Cu2+, Zn2+ and Ag+ increases the 1-hexene – zeolite X interaction energies to 54.10 kJ/mol for Co-NaX, 56.20 kJ/mol for Ni-NaX, 55.51 kJ/mol for Cu-NaX, 53.09 kJ/mol for Zn-NaX, and 56.66 kJ/mol for Ag-NaX. Such enhancement is beneficial and will lead to more effective uptake of olefin from olefin/paraffin mixtures. Indeed, the enhanced guest – host interaction energies are due to π-complexation, which enables weak and reversible bonds between transition metal ions and olefin molecules, resulting in olefin/paraffin separation with better selectivity.60,61 According to the calculated interaction energies, compared with Co-NaX, Cu-NaX and Zn-NaX, the higher adsorption capacity seen on Ni-NaX may be enabled by the interplays between more favorable cation-substituted structure and larger interaction energy.