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.