Polymer-metal organic frameworks (MOFs) mixed matrix membranes for gas separation applications

Abstract

The plasticization behavior of pure polymers is well studied in literature. However, there are only few studies on the plasticization behavior of mixed matrix membranes. In Chapter 2 of this thesis, pure and mixed gas plasticization behavior of MMMs prepared from mesoporous Fe(BTC) nanoparticles and the polymer Matrimid® is investigated. All experiments were carried with solution casted dense membranes. Mesoporous Fe(BTC) MOF particles showed reasonably good compatibility with the polymer. Incorporation of Fe(BTC) in Matrimid® resulted in membranes with increased permeability and selectivity. At low pressures of 5 bar the MMMs showed an increase of 60 % in CO2 permeability and a corresponding increase of 29 % in ideal selectivity over pure Matrimid® membranes. It was observed that the presence of Fe(BTC) particles increases the plasticization pressure of Matrimid® based MMMs. Furthermore, this pressure increases more with increasing MOF loading. This delay in plasticization is attributed to the reduced mobility of the polymer chains in the vicinity of the Fe(BTC) particles. Also, at higher Fe(BTC) loadings, the membranes showed more or less constant selectivity over the whole pressure range investigated. Chapter 3 subsequently presented the preparation and plasticization behavior of MMMs based on three distinctively different MOFs (MIL-53(Al) (breathing MOF), ZIF-8 (flexible MOF) and Cu3(BTC)2 (rigid MOF)) dispersed in Matrimid®. The ideal and mixed gas performance of the prepared MMMs was determined and the effect of MOF structure on the plasticization behavior of MMMs was investigated. Among the three MOF-MMMs, membranes based on Cu3(BTC)2 showed highest selectivity while ZIF-8 based membranes showed highest permeability. The respective increase in performance of the MMMs is very much dependent on the MOF crystal structure and its interactions with CO2 molecules. Chapter 4 described the preparation of Matrimid® polyimide (PI)/polysulfone (PSF)-blend membranes containing ZIF-8 particles for high pressure gas separation. An optimized PI/PSF blend ratio (3:1) was used and performance and stability of PI/PSF mixed matrix membranes filled with different concentrations of ZIF-8 were investigated. PI and PSF were miscible and provided good compatibility with the ZIF-8 particles, even at high loadings. The PI/PSF-ZIF-8 MMMs showed significant enhancement in CO2 permeability with increased ZIF-8 loading, which was attributed to a moderate increase in sorption capacity and faster diffusion through the ZIF-8 particles. In pure gas measurements, pure PI/PSF blend (3:1) membranes showed a plasticization pressure of ~18 bar while the ZIF-8 MMMs showed a higher plasticization pressures of ~25 bar. Mixed gas measurements of PI/PSF-ZIF-8 MMMs showed suppression of plasticization as confirmed by a constant mixed gas CH4 permeability and a nearly constant selectivity with pressure but the effect was stronger at high ZIF-8 loadings. Gas separation results of the prepared PI/PSF-ZIF-8 MMMs show an increased commercial viability of Matrimid® based membranes and broadened their applicability, especially for high-pressure CO2 gas separations. In Chapter 5, a novel route for the preparation of mixed matrix membranes via a particle fusion approach was introduced. Surface modification of the polymer with 1-(3-aminopropyl)-imidazole led to an excellent ZIF-8-Matrimid® interfacial compatibility. It was possible to successfully prepare MMMs with MOF loadings as high as 30 wt.% without any non-selective defects. Upon increasing the ZIF-8 loading, MMMs showed significantly better performance in the separation of CO2/CH4 mixtures as compared to the native polymer. The CO2 permeability increased up to 200 % combined with a 65 % increase in CO2/CH4 selectivity, compared to the native Matrimid®. Chapter 6 finally discussed the conclusions and directions for future research based on the findings presented in this thesis.