2.4 Gas separation performance
The transport properties of 30/70 (v/v)
CO2/CH4 mixture of the MMCMs were
evaluated. The separation performance of MMCMs has a strong dependence
on the MOF loading as clearly shown in Fig. 5a and b. The
CO2 permeance of 6FDA-DAM membranes with a 300-nm
thickness is 242 GPU, with a CO2/CH4selectivity of 26. When the MOF loading increases to 20 wt%, the
CO2 permeance of MMCM-A increases to 642 GPU, with the
CO2/CH4 selectivity slightly increasing
to 33 (Fig. 5a). Fig. 5b shows the separation performance of MMCM-B with
different NH2-ZIF-8 loading (0 to 40 wt%). With increasing the
NH2-ZIF-8 loading from 0-20 wt%, the MMCM-B exhibits a drastically
enhanced CO2 permeance (from 304 to 778 GPU) and an
increased CO2/CH4 selectivity (from 22
to 34). This is possibly due to the protruding amino-functionalized MOF
crystals with high specific surface area fortified the
CO2 adsorption and solution processes and facilitate the
low-resistance diffusion of gas molecules. Moreover, the selectivity of
all the MMCMs declines drastically with the NH2-ZIF-8 loading increasing
to over 30 wt%, owing to the crystal aggregation and the generation of
interfacial voids, wherein non-selective diffusion reduces the
selectivity.
Plasticization effect of polymer-based membranes is also a challenging
problem in industrial applications.24,33,34,43,44 In
general, under high pressure, polar gas molecules with high critical
temperatures such as CO2,
C2H4 and
C3H6 can swell and promote the mobility
of the polymer chains, resulting in a significant drop of the
selectivity and a slight increase of the permeance. To evaluate the
plasticization-resistant properties, we utilized the plasticization
point which refers to the pressure wherein the CO2permeance upturns.44 In this regard, we tested the
separation performance of MMCMs under equimolar mixed gas with
CO2 partial pressure increasing to 20 bar. For 6FDA-DAM
membrane, the plasticization point appears at CO2partial pressure of 15 bar, wherein the CO2 permeance
reaches its minimum value of 66 GPU (Fig. 5c). With regard to the MMCMs,
both the CO2 permeance of MMCM-A and MMCM-B decreases
monotonically and we did not observe a plasticization point. It is
noteworthy that when the
CO2 partial pressure increasing to 20 bar, the
CO2/CH4 selectivity of polyimide, MMCM-A
and MMCM-B decreases by 71.2%, 54.9% and 28.5% (Fig. 5d),
respectively. Overall, the above results show that MMCMs yield a
significant plasticization resistant performance. Moreover, because the
interfacial covalent bonds in MMCM-B are much more robust than the
hydrogen bonds in MMCM-A membranes, wherein the stronger interfacial
bonds potentially restrict the swelling of polyimide chains at high feed
gas pressure, MMCM-B exhibits a superior pressure stability.
Aging effect in polymeric membranes is another challenging problem for
practical applications,29,43,44 which is typically
prominent in thin-film glassy polymers.18,33,45-47 We
tested the long-term stabilities of the MMCMs for nearly one month. As
shown in Fig. 5e-f, the pure polyimide membranes exhibit a typical aging
performance, and the CO2 permeance decreases 64.5%
after 28 days, from 242 to 86 GPU. In contrast, the MMCMs exhibited an
improved aging-resistance performance. For MMCM-A,
the CO2 permeance
reduces from 652 to 417 GPU, with 36% of declination after 28 days.
With regard to MMCM-B, the CO2 permeance decreases from
778 to 583 GPU, retaining 75% of the initial separation performance.
The above results indicate that the MMCM-B with robust covalent-bonding
exhibit a better long-term stability for nearly one month than the
MMCM-A with hydrogen-bonding and pure polyimide membranes.
As shown in Fig. 6, compared with the state-of-the-art composite
membranes, the MMCMs exhibit an outstanding
CO2/CH4 separation performance. The
yellow dashed lines in Fig 6 depict the target performance zone of
CO2/CH4 separation membranes,
indicating that a membrane should yield a CO2 permeance
higher than 100 GPU and a CO2/CH4selectivity higher than 30 to achieve economical demands for natural gas
purification.48 The separation performances of both
MMCM-A and MMCM-B surpass all the polyimide-based composite membranes
and the MMCMs reported in the literature and are comparable to the
facilitated transport membranes.