Figure 11. Calculated 1-hexene – zeolite interaction energies
of ion-exchanged samples with different degrees of ion-exchange.
The adsorption properties of zeolite X samples with different
ion-exchange degrees were calculated to further evaluate the effects of
ion type and degree of ion-exchange (see Figures 10 and 11). In summary,
introduction of Mg2+ and Ca2+ into
zeolite X leads to increased adsorption capacity due to pore volume
increase yet results in decrease in interaction energy. Indeed, such
effect tends to be more significant as the degree of
Mg2+ and Ca2+ exchange increases. In
contrast, for Co-NaX, Ni-NaX, Cu-NaX and Zn-NaX, as ion-exchange
proceeds, the magnitudes of both 1-hexene adsorption capacity and
interaction energy increase. This is mainly caused by the π-complexation
among 1-hexene molecules and transition metal ions. Moreover, the
adsorption capacity of Ag-exchanged zeolite X decreases mainly due to
the increase of molecular weight, while the magnitude of interaction
energy increases owing to π-complexation. Generally, the impacts of
ion-exchange degree on both adsorption capacity and interaction energy
tend to be less significant as the ion-exchange reaches above 60%.
According to the calculated adsorption capacity, fully ion-exchanged MgX
presents the highest adsorption capacity of 195.0 mg/g. Interestingly,
at a low ion-exchange degree of 12.8%, Co-NaX shows a higher uptake
(178.5 mg/g) as compared to other partially cation-replaced samples at
the same exchange level. Additionally, full replacement of
Na+ with Ag+ in zeolite X leads to a
decreased adsorption capacity of 115.1 mg/g. We also found that the
calculated 1-hexene – zeolite X interaction energy at the same
ion-exchanged degree follow the transition metal electronegativity order
of Ag > Ni ≈ Co ≈ Cu > Zn > Mg
> Ca. This is because that in zeolite X framework
transition metal ion with a greater electronegativity has stronger
bonding with 1-hexene.61 We also noticed that metals
with very high electronegativity values lead to poor (incomplete)
regeneration.
In summary, according to our experimental and simulation results,
Mg-NaX, Ca-NaX, Co-NaX, Ni-NaX, Cu-NaX and Zn-NaX exhibit higher
adsorption capacities, and exchange of Co2+,
Ni2+, Cu2+, Zn2+and Ag+ into zeolite X structure increases the
interaction energy between 1-hexene and zeolite X. Ion-exchange enhances
the adsorption performance of binderless zeolite X as a promising
olefin/paraffin separation sorbent. On the other hand, our GCMC
simulation provides experimental guidance to elucidate the relationships
among ion type, exchange degree and adsorption performance. This study
has significant implications for the controllable preparation of
function-strengthened porous materials for adsorption and may benefit
chemical engineers by providing fundamental knowledge governing the
performance of zeolite-based olefin/paraffin separation materials.