Separation mechanism
For further clarifying the effect of polymer embedding on separation
performance, the solubility and diffusivity of gases through the
membranes were simulated (Figure 5b-d and Table S4). As expected, the
CO2 adsorption capacity of the MOF-NP/PSF, pMOF-MS/PSF,
and pUiO-66-MS/PSF MMMs were much higher than that of the PSF membrane
owing to the specific interaction of
Zr6O4(OH)4 clusters and
-NH2 groups in MOFs to CO2 molecules.
However, because the incorporated MOF fillers simultaneously enhanced
the CO2 and CH4 solubilities, the
CO2/CH4 solubility selectivity reduced
from 5.3 to about 3.2. The CO2 diffusivity of the MMMs
was enhanced after MOF incorporation (Figure 5d), while the
CH4 diffusivity of the MMMs declined. This might be
explained by that the special interaction between the polymer chains and
MOFs narrowed the gas transport channels.42 Therefore,
the CO2/CH4 diffusivity selectivity
increased drastically. In particular, for the MMMs with polymer-embedded
MOF MSs, the CO2/CH4 diffusivity
selectivity of the pMOF-MS/PSF and pUiO-66-MS/PSF MMMs were 9.2 and 8.9,
respectively, which were much higher than that of the MOF-NP/PSF MMM
(5.7) and pristine PSF membrane (3.8). Therefore, it could be deduced
that the great improvement in diffusivity selectivity was the dominate
factor for the higher CO2/CH4selectivity of the pMOF-MS/PSF and pUiO-66-MS/PSF MMMs. For the
MOF-NP/PSF MMMs, the serious filler aggregations and obvious interfacial
defects offered the invalid channels for non-selective gas transports.
For the pMOF-MS/PSF MMMs, the excellent interfacial compatibility,
highly efficient transport channels, and large adsorption capacities
contributed to the greater CO2 selectivity and
permeability.