Metal organic framework (MOF) based mixed matrix membranes for Co2 separation: Microporous metal imidazolate framework (MMIF) and strontium-based mofs

Polymeric gas separation membrane systems have been widely considered as an alternative to conventional methods such as adsorption and distillation due to their ease of fabrication and low cost. One of the most challenging applications is carbon dioxide separation, i.e. natural gas purification and carbon capture from post-combustion gases. Reduction of carbon dioxide as the chief culprit of global warming from atmosphere has played a crucial rule in polymeric membrane development research in the last 10 years. An important class of polymers which finds extensive use in gas separation membrane development is polyimides. This is mainly due to their comparatively better transport properties and superior chemical and thermal stability. The trade-off limitation of polymeric membranes prohibits the widespread use of membranes for commercially important applications. This problem motivated researchers to push the borders and come up with the idea of mixed matrix membranes (MMMs). Mixed matrix membranes are a combination of two main phases known as continuous phase or polymeric phase and dispersed phase or fillers. Due to superior performance of fillers, MMMs may overcome the performance limitations whereas the polymeric phase provides processability to produce large surface area modules. Zeolites, as the most common fillers, attracted great interest in MMM fabrication. Zeolite based MMMs have been studied for the last two decades and much effort has been spent to ensure good adhesion between organic and inorganic phases. In the last decade, a new class of crystalline nanoporous materials known as metal organic frameworks (MOFs) have been employed in mixed matrix membranes because of their high gas adsorption capacity over other conventional nanoporous materials such as zeolites. MOFs are organic-inorganic hybrid materials which consist of an organic bridging ligand and metal ions with tunable chemistry. These materials, with high potential in gas sorption, are mostly chemically stable and robust enough to be utilized as the fillers in polymer matrices. The organic linkers in MOFs have affinity to polymer chains so the control of MOF/polymer interface is easier than zeolite/polymer interface. The ultimate goal of this thesis is to develop MOF/polyimide mixed matrix membranes for CO2/CH4 (natural gas purification) and CO2/N2 (carbon capture from post-combustion gases) separation applications. For this purpose, first, a theoretical analysis is carried out to determine promising polyimide and MOF combinations for improved MMM performance. Polyimide separation properties are predicted using a group contribution method. MOF permeabilities are obtained from the molecular simulation studies reported in the literature. Simulated permeabilities of some new MOFs together with predicted permeabilities of polyimides were introduced into the Maxwell equation in order to predict MMM performance and identify new combinations which are able to overcome the trade-off limitation for CO2/CH4 and CO2/N2 separations. This study revealed two MOF structures as outstanding candidates for MOF-based MMMs. One of these MOFs is a strontium-cobalt based MOF: [SrCo(C3H2O4)2(H2O)5].2H2O, named as FOHQUO in the Cambridge Structural Database (CSD). Another MOF which became prominent for CO2/CH4 separation is microporous metal–imidazolate framework (MMIF) with the general formula of (C12H8CuN4)n. No experimental data is available in the literature to confirm the predicted performances of these MOFs, in particular no MMM studies with these MOFs are reported. The experimental part of this thesis involves synthesis/characterization of two different strontium-based MOFs and microporous metal imidazolate framework (MMIF), and investigation of their potential for use in CO2 separation and mixed matrix membrane formation. Strontium-based MOFs on the basis of malonate as ligand were synthesized with rapid room temperature method using two different co-metal ions, cobalt and copper, which are designated as FOHQUO and VAMQOP in the CSD, respectively. Nanoparticles of MMIF were synthesized using sonication method. All synthesized MOFs are characterized by X-ray diffraction (XRD) analysis. XRD patterns showed that the structures are in good agreement with those of patterns obtained by molecular simulations which confirm the accuracy of the syntheses. Thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and intelligent gravimetric analyzer (IGA) were used to characterize thermal stability, morphology, and gas sorption capacity of aforementioned MOFs, respectively. The relatively larger particle size (above 20 microns) of synthesized FOHQUO and VAMQOP crystals posed obstacles in the way of membrane preparation. In addition to this, X-ray diffraction (XRD) analysis proved that FOHQUO framework integrity collapsed above 80 °C. TGA analysis of the same samples reflected dehydration at 80°C. The crystalline lattice of as-synthesized MOFs is typically filled with guest molecules of solvents used in the synthesis. Solvent removal can pose obstacles in crystallinity of MOFs and collapse the framework integrity and convert to amorphous materials. These challenges prevented the use of strontium based MOFs in fabrication of MMMs. The dynamic gas sorption analysis of VAMQOP samples activated at 110°C, 150 °C, and 210 °C showed that the structure of VAMQOP at 210 °C converted to a new crystalline porous material after an amorphous period around 150-200 °C. Interestingly an extraordinary CH4 sorption capacity was observed for VAMQOP activated at 110 °C and 240 °C. The synthesized MMIF nanoparticles (40-1570 nm) were found to be thermally and structurally stable up to 150 °C. The nanoparticles are dispersed into Matrimid, a commercial polyimide, to fabricate MMMs. Matrimid was chosen as the continuous phase because of its commercial availability and thermal stability to test the performance of the dispersed fillers. In addition, there are several studies reported in the literature which uses Matrimid as the continuous phase and other MOFs as the dispersed phase, therefore it provides a good basis for comparison before a better polyimide match is tested. Membranes were characterized by SEM analysis for their morphology, TGA analysis for their thermal behavior, differential scanning calorimetry (DSC) for glass transition temperature (Tg) measurement, and single and mixed gas permeation measurements at 35 C and 4 bar feed pressure to reveal their separation performance. MMIF/Matrimid membranes with 10 and 20 wt.% MMIF loading were prepared by casting-evaporation technique and annealed up to 150 °C and 120 °C for 48 hours, respectively. Homogenous dispersion of MMIF particles in MMMs were observed from SEM analysis. Both 10% and 20% MMIF containing Matrimid membranes exhibited enhanced gas permeabilities for all gases (CO2, CH4, N2) tested. However, ideal selectivity of membranes were not improved. On the other hand, mixed gas permeability measurements showed significant improvement in CO2/CH4 selectivity due to competitive adsorption.