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.