MOFs-based Mixed Matrix Membranes for Gas Separation

Abstract

Energy-efficient separation of gases has attracted intensive attentions both in research and in industry.nThe separation of gases by membranes is more effective and energy saving with lower productionnand equipment cost than some traditional gas separation methods, such as adsorption or distillation.nHowever, most of the polymeric membranes suffer from the trade-off between mass transport ratesnand separation efficiency. Polymeric membranes show high gas permeation flux but low selectivity,nand vice versa. To overcome such weakness, mixed matrix membranes (MMMs) can providenpromising potentials in high performance gas separation, by combining high separation properties ofnthe inorganic filler with low cost and flexible of the polymers. For filler selection, metal-organicnframeworks (MOFs) are promising adsorbents for gas storage and separation due to their high surfacenarea and porosity, adjustable pore sizes and controllable surface functionality.This thesis is focused on developing novel MOFs-based MMMs for gas separation with highnpermeability and selectivity, as well as good thermal and chemical stability. The studies includenfabrication and optimization of MOFs-based MMMs, designing MMMs with good filler/polymerninteraction and good interfacial morphology, as well as evaluating the permeation and selectivitynperformance of all the prepared membranes. It aims to establish the guidance for the MOF/polymernpair selection and interface manipulation in the fabrication of MMMs to greatly increase thenmembrane permeability and selectivity.In the first part of the experimental chapters, novel MMM with dispersed MOF into polyimide matrixnwere fabricated and the derived membranes are employed for gas separation. MMMs werensynthesized from 2,2-bis(3,4-anhydrodicarboxyphenyl) hexafluoropropane (6FDA) and 4,4(-diaminodiphenyl ether (oxydianiline, ODA) into which were incorporated Cd-6F MOF filler. Cd-6Fnwas synthesized by using 4,4p-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) as the organicnlinker which is also one of the monomers in the 6FDA-ODA synthesis. A specific interfacialninteraction between MOF crystals and polymer chains was innovatively targeted with this specificnMOF/polymer pair by controlling the in-situ 6FDA-ODA polymerization procedure with thenexistence of MOF particles. The enhanced adhesion between filler particles and polymer phase wasnachieved and the improved interfacial interaction between MOF and polymeric matrix was confirmednby FTIR, NMR and XPS. Moreover, it was found that the MOF/polymer interfacial morphologynstrongly affects the gas permeability and selectivity of the membrane. The MMM prepared by in-situnpolymerization displays excellent interfaces between micron-ized Cd-6F crystals and polymernmatrix as well as increased permeability and selectivity compared to pure 6FDA-ODA polymericmembrane. The interaction between MOF crystals and polymeric matrix can be controlled for theninterfacial voids elimination and optimal membrane transport properties.The second part of the experimental chapters focuses on MMMs with MOF/CNT composite. NovelnMMM filler CNT-MIL was synthesized by in-situ growth of NH2-MIL-101(Al) on the externalnsurface of carbon nanotubes (CNT) and applied to fabricate MMMs for CO2/CH4 separation. Extranamino groups and active sites were introduced to the surface of CNT. Compared to pure polymericnmembrane, MMMs containing synthesized MOF/CNT composite displayed significantly increasednCO2 permeance (up to 150%) and selectivity (up to 37.5%). The separation performance of derivednMMMs clearly transcends the 1991 upper bound and being on the 2008 upper bound for polymericnmembrane performance. MMMs for efficient C3H6/C3H8 separation were fabricated by embeddingnZIF-8/CNT composite into 6FDA-durene polymer matrix. The volume of filler and voids in thenpolymeric matrix as well as the distribution of the fillers and their contact with the polymeric matrixnwere quantitatively evaluated by using the tomographic focused ion beam scanning electronnmicroscopy (FIB-SEM). The dispersion of ZIF-8 in 6FDA-durene polymer was enhanced by growthnof ZIF-8 on the CNT surfaces. Meanwhile, good adhesion between the synthesized MOF/CNT fillersnand polymer matrix was observed, only 0.086% voids volume fraction is determined, even at a highnfiller loading. The improved ZIF-8 dispersion and filler/polymer interface lead to the efficient C3H6separation in MMMs. MMMs containing synthesized MOF/CNT composite displayed significantlynincreased C3H6 permeability (up to 105%) and selectivity (up to 96%) compare to pure 6FDA-durenenmembrane.The third part of the experimental chapters focuses on elimination of interfacial voids bynincorporating ionic liquid (IL) as the MOFs/polymer interfacial binder into MMM. Thin layer of ILnhas been fabricated on CuBTC fillers for MMMs fabrication. With the aid of ionic liquid, theninterfacial voids of the CuBTC-IL MMM was significantly reduced due to the favourable MOF/ILnand IL/polymer adhesion, thus improving CO2 selectivity compare to pure polymer membrane andnMMMs with untreated CuBTC. The performance of 10% CuBTC-IL/6FDA-durene MMM clearlyntranscends the 2008 upper bound for polymer membrane. The strategy of IL decoration method cannprovide an effective way to eliminate interfacial voids and enhance CO2 selectivity and can be appliednin large sized fillers with better gas separation properties.n