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.