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.