Effect of ZIFs loading
Pure gas permeation experiments were conducted at a pressure of 1.5 bar and a temperature of 40°C, for the evaluation of pure PDMS, and MMMs. Figure 3 demonstrates the permeation of BD in pure PDMS and MMMs with ZIF-8 and Ni-ZIF-8 loadings. It can be observed that the permeation improved by the increase in ZIFs loading in both types of MMMs, while the Ni-ZIF-8 based MMMs showed a significant rise in permeation with respect to the ZIF-8 based MMMs. The highest permeation results were observed at 20% ZIFs loading, which were 400 GPU for Ni-ZIF-8 based MMM and 325 GPU for ZIF-8, and showed improvements of 98%, and 61% as compared with pure PDMS membrane, respectively. The reason for Ni-ZIF-8/PDMS MMMs has more permeation than pure ZIF-8/PDMS MMMs can be described in three different parameters. The first parameter was the large surface area, high micro, and total pore volumes of Ni-ZIF-8 relative to the pure ZIF-8. The second parameter was the tunability of the effective aperture size of the ZIF-8 by the inclusion of the Ni atoms in its cluster. Pure ZIF-8 has experimentally demonstrated an effective aperture size of 4.0-4.2 Å (0.6-0.8Å higher than its aperture size (3.4Å) showed by XRD analysis)48, which is smaller than the kinetic diameter of the BD molecule (4.3 Å). In addition, the simulation findings have shown that the organic linker in the ZIF-8 is able to fluctuate the opening size up to 1Å, which means that the pure ZIF-8 will expand its pores up to 4.4Å49. Based on these facts, we assumed the same organic linker fluctuation in Ni-ZIF-8 that enhanced its aperture size by 0.8-1.0 Å, and it showed an effective aperture size of 4.4-4.6 Å, which is larger than the size of BD molecule. These evidences proved that the Ni-ZIF-8 pore/aperture has more compatibility with the size of the BD molecule than the ZIF-8 pore/aperture, making it more convenient for the BD molecules penetration. The third parameter was the presence of more metal sites (Zn and Ni) in Ni-ZIF-8 cluster relative to the ZIF-8 (Zn), which provided more affinity to butadiene molecules.
BD molecule is unsaturated due to two C = C double bonds in its structure. This unsaturation gives it the capacity to interact with the metal sites that are normally electron-rich. The BD molecule attracted towards the two-metal sites of Ni-ZIF-8, which have different adsorption and binding behavior based on the π-complexation mechanism for the BD molecules. Owing to this discrepancy, it gave the asymmetric adsorption/desorption of BD from Ni-ZIF-8, which enhanced the permeation from Ni-ZIF-8/PDMS MMMs. BD adsorption in the ZIFs based on metal-to-BD molecule interplay polarization that can form a reversible complex with transition metals. Based on previous work50, it was postulated that the BD interacted with the Ni and Zn metals atomic orbitals, forming a complex. During this complex, BD and metals act as an electron acceptor and electron donor, respectively. By overlapping the metal’s external s-orbitals with the BD’s bonding π-molecular orbital, a σ -bond is formed in the complex. Additionally, a π-bond formed in the complex due to the transfer of electrons from the BD’s vacant antibonding π∗-molecular orbital and filled d-atomic metal orbital. In desorption, the same reverse effect happened, releasing the BD molecule from metal sites. In Ni-ZIF-8, the presence of two metals created the synergistic effect and reinforced BD affinity.
On the other hand, Figure 3 shows the ideal selectivity of pure PDMS membrane and MMMs for BD/N2 separation with respect to both types of ZIFs loadings. It can be found that the selectivity improved by increasing the ZIFs loading in the PDMS matrix up to 15%, and thereafter the selectivity decreased in both types of MMMs. The contradiction between permeance and selectivity at 20% ZIF loading is the so-called “trade-off effect” of polymeric membranes. Compared to the pure PDMS membrane, the Ni-ZIF-8 displayed an overall improvement in selectivity about 81%, while the ZIF-8 showed 32% at the same (15%) loadings. Thus, ZIFs with a load of not more than 15% broke the trade-off law between permeability and selectivity. The reduction of selectivity at higher loading may be due to internal interface defects in the agglomeration of the ZIFs within the PDMS matrix. Ni-ZIF-8/MMMs has more selectivity and permeation than the ZIF-8/MMMs, which can be explained by the high affinity of Ni-ZIF-8 with BD, and low interaction with the nitrogen molecules. It was also observed that the nitrogen permeance was slightly higher in ZIF-8 based MMMs than the Ni-ZIF-8/MMMs, which also affected the selectivity of ZIF-8/MMMs.
Based on the aforementioned results of permselectivity, 15% of Ni-ZIF-8 MMM were selected for further evaluation under different pressure and temperature conditions. Improved permeation properties, particularly 81% improvement in BD/N2 ideal selectivity, strongly confirmed that the synthesized Ni-ZIF-8/MMMs up to 15 wt% loadings are defect-free. For the 99% hydrocarbons recovery from nitrogen mixture, rubbery membranes with 100 GPU permeability and 10 selectivity can be used with the proper design of the membrane process29. Therefore, it is believed to use 15% Ni-ZIF-8 MMMs made in this study, having 323 GPU permeance and 19.5 selectivity for efficient separation of BD from the N2 mixture.