Abstract
Mixed matrix composite membranes (MMCMs) hold great potential to realize efficient CO2 removal from natural gas. However, the reduction of separation performance arising from the interfacial defects, significant plasticization and aging effect in the thin films severely limit their application. Herein, we fabricated a series of polyimide MMCMs with MOF-protruding structure wherein amino-functionalized ZIF-8 nanocrystals nearly penetrate the thin selective layer. Through engineering the interfacial interactions, e.g., covalent or hydrogen bondings, we successfully fabricated defect-free MMCMs with the thickness ranging from 140 to 280 nm. The stronger interfacial interactions eliminate the interfacial defects and restrict the mobility of polymer chains under high pressure. Accordingly, the MMCM displays a high CO2permeance of 778 GPU and a CO2/CH4selectivity of 34 with significantly improved resistance to plasticization and aging. Considering the superior performance, we anticipate our work could provide guidelines on designing advanced MMMs to tackle critical separations.
Keywords: Natural gas purifications, mixed matrix membranes, thin-film composite membranes, interfacial interactions.
Introduction
Efficient CO2 removal from natural gas has been regarded as one of the most important processes in modern chemical industries because the presence of CO2 can reduce the calorific value and make the natural gas streams acidic and corrosive.1-3 Among CO2 separation processes, membrane technologies hold great potential to siginificantly increase the energy efficiencies and reduce the environmental impacts compared with traditional separation technologies such as adsorption and absorption.3-8 To date, polymeric membranes is dominating the membrane separation market because of the ease of processing polymers into thin selective membranes.2,9-11 Nevertheless, the separation performance of polymeric membranes suffers from the trade-off effect, as depicted by the Robeson plot for typical gas pairs including CO2/CH4.12-14 In this context, it is essential to develop membranes that can outperform conventional polymeric membranes.
Metal-organic framework (MOF) based mixed matrix membrane (MMM), which combines the advantages of molecular-sieving capability of microporous fillers and solution processibility of polymers, has been widely researched as an effective approach to surpass the performance upper-bound of polymeric membranes.14-21Recently, MMMs with MOFs penetrating matrix have attracted extensive attention because the fillers are expected to dominate the membrane separation performance which approaches to that from pure MOF crystals.22-25For example, Huang and co-workers fabricated highly H2permeable MMMs with ZIF-7 microplates penetrating the polymer matrix and yielded a 14-fold higher H2 permeance and 9-fold higher H2/CO2 selectivity, respectively.22 Our group has proposed a MMM with ZIF-8 microcrystals nearly penetrate the polyimide matrix and form a “Direct-through Channel” structure which achieves nearly 70% of C3H6/C3H8separation performance from pure ZIF-8 crystals according to the calculation results of the Maxwell model.24 Overall, MMMs with MOF-penetrated structure are still in its infancy stage and usually a self-standing membrane with large thickness is fabricated in most studies. For practical application, mixed matrix composite membranes (MMCMs) with thin selective layers are much more attractive attributing to the fact that the thin selective layer is expected to minimize the trans-membrane resistance and thus maximize the gas permeance.26-28 Till now, fabricating ultrathin selective films with nano-sized MOF penetrated the selective layer has been rarely reported. Wang’s group developed MMMs with penetrated oligomer-grafted MIL-101 nanoparticles serving as gas transport channels in rubbery poly-vinylamine (PVAm) matrix, with a CO2permeance of 823 GPU and a CO2/N2selectivity of 242 under humidified conditions.23
Glassy polymeric membranes have excellent mechanical/chemical stabilities which benefit the operation under extreme conditions such as high pressure.10,11,29-32 However, the fabrication of high-performance MMCMs using glassy polymer as matrix remains elusive. In theory, there are two critical issues remaining unresolved. Firstly, non-selective defects or pinholes can be easily generated at the interfaces when the membrane thickness reduces to sub-micrometer or nanometer scale due to the weak interfacial interactions between the fillers and rigid chains of glassy polymers which are detrimental to the separation performance.26,27 Secondly, the plasticization and aging effect would become more serious for thin-selective layer, owing to the enhanced chain mobility of polymers, leading to drastic decrease of the overall separation performance.29,33,34
To overcome the above-mentioned issues, we develop a series of MMCMs, containing MOF-protruding structure wherein the MOF crystals nearly penetrate the thin selective layer. The thickness of the selective layer can be reduced to 140-280 nm through engineering the interfacial hydrogen or covalent bonds, as illustrated in Fig. 1. Attributing to the affinity of amine groups to CO2 and low permeation resistance of the selective layer with protruding MOFs, the MMCM yields a CO2 permeance of 778 GPU which is three-fold higher than that of polyimide membrane with a CO2/CH4 selectivity of 34, and demonstrated improved resistance to plasticization (up to 40 bar) and aging performance (for 28 days). Considering the superior CO2 separation performance as well as the good resistance to plasiticization and aging, we anticipate that the MMCMs hold great potential for real applications in natural gas purification processes.