This dissertation addresses the rational design of intrinsically microporous solutionprocessable polyimides and ladder polymers for highly permeable and highly selective gas transport in cornerstone applications of membrane-based gas separation – that is, air enrichment, hydrogen recovery and natural gas sweetening. By virtue of rigid and contorted chains that pack inefficiently in the solid state, polymers of intrinsic microporosity (PIMs) have the potential to unite the solution-processability, mechanical flexibility and organic tunability of commercially relevant polymers with the microporosity characteristics of porous crystalline materials. The performance enhancements of PIMs over conventional low-free-volume polymers have been primarily permeability-driven, compromising the selectivity essential to commercial viability. An approach to unite high permeability with high selectivity for performance transcending the state-of-the-art in air and hydrogen separations was demonstrated via a fused-ring integration of a three-dimensional, shape persistent triptycene moiety optimally substituted with short, branched isopropyl chains at the 9,10-bridgeheads into a highly inflexible backbone. The resulting polymers exhibited selectivities (i.e., O2/N2, H2/N2, H2/CH4) similar to or higher than commercial materials matched with permeabilities up to three hundred times higher. However, the intra-chain rigidity central to such conventional PIM-design principles was not a singular solution to suppression of CO2-induced plasticization in CO2/CH4 mixedgas separations. Plasticization diminishes the sieving capacity of the membrane, resulting in costly hydrocarbon losses that have significantly limited the commercialization of new polymers. Unexpectedly, the most permeable and selective PIMs designed for air and hydrogen separations strongly plasticized in 50:50 CO2/CH4 mixtures, enduring up to three-fold increases in mixed-gas CH4 permeability by 30 bar and strong drops in selectivity. Instead, a paradigm shift emphasizing inter-chain rigidity was demonstrated for the PIM-type polyimides via introduction of a flexible diamine functionalized with hydroxyl groups. Intra-chain flexibility may permit short-range coplanarization of backbone segments which facilitates inter-chain interactions likely comprising charge transfer complexes over the N-phenyl imide bond and hydrogen bonding. Relative to commercial cellulose acetate membranes at a representative 10 bar CO2 partial pressure, the resulting polyimide exhibited two-fold increases in mixed-gas CO2/CH4 selectivity (~50) concurrent with nearly 10-fold higher CO2 gas permeability. Similar design principles were drawn for ladder polymers.

Control at the molecular level over porous solid-state materials is of prime importance for fine-tuning the local structures, as well as the resultant properties. Traditional porous solid-state materials such as zeolite and activated carbon are the benchmarks in the current market with vital applications in sorption and heterogeneous catalysis. However, the adjustments of pore size and geometry of those materials, which are essential for the broader aspect of modern prominent applications, remain challenging. Reticular chemistry has emerged as a dominant tool toward the ‘designed syntheses’ of porous crystalline frameworks (e.g. metal-organic frameworks (MOFs)) with a specific pore system. This dissertation illustrates the power of reticular chemistry and its use in the directional assembly of highly coordinated MOF materials, as well as their potential applications such as gas storage, natural gas upgrading, and light hydrocarbon separation. Highly connected minimal edge-transitive derived and related nets, obtained via the deconstruction of nodes of the edge-transitive nets, are suitable blueprints and can potentially be deployed in the future ‘designed syntheses’ of MOFs. The further employment of the conceptual net-coded building units (e.g. highly connected MBBs and edge-transitive SBLs) in the practical reticular synthesis results in the rational design and construction of functional MOF platforms like shp-, alb-, kce-, kex- and eea- MOFs. In addition, the isoreticular synthesis of Al-cea-MOF-2 with functionalized pendant acid moieties inside pore channels in comparison to the parent Al-cea-MOF-1 led to enhanced light hydrocarbons separation performance. Moreover, controlling the molecular defects in Zr-fum-fcu-MOFs resulted in the development of an ultramicroporous adsorbent with an engineered aperture size for the highly efficient separation of butane/iso-butane.

Abstract: In this work, carbon molecular sieve (CMS) membranes were prepared based on a polymer of intrinsic microporosity, named PIM-6FDA-OH. The goal of this work was to examine the effect of the fabrication parameters of the CMS membranes on the gas separation performance of the final CMS membranes produced. Furthermore, the performance changes are reported for membranes physically aged over 7, 30, 60, and 90 days. The membranes prepared consisted of thin-film (about 3 !m thick) CMS selective layers supported by a stainless-steel tube. The experiments were split into four projects. The first project aimed to determine the effect the layer thickness had on the final performance of the produced CMS membranes. Five pairs of membranes were prepared using different coating solution concentrations, and different number of layers. The concentrations used were 5 (1 layer), 7.5 (1 layer), and 9 wt% (1, 2, and 3 layers) polymer in THF. The membranes had the same soak time of 15 minutes and pyrolysis temperature of 650 °C. The results showed that the increase in number of layers did not provide any benefits and was unnecessary. Moreover, the decrease in concentration produced membranes with higher permeances but with a greater loss in selectivity. Therefore, the 9 wt% concentration solution with one layer was chosen for the remaining experiments. The second project examined the effect of the pyrolysis temperature on the performance of the final membranes produced. All membranes were made with the 9 wt% solution and the soak time was held constant at 15 minutes. The soak temperatures tested were: 700, 750, 850, and 950. °C. The membranes pyrolyzed at temperatures above 650 °C were severely defective. This suggests that either the precursor polymer could not form defect-free thin membranes using high soak temperatures, or another potential reason is related to interfacial defect formation between the CMS layer and the porous stainless-steel support. Further experiments are required to fully understand the soak temperature effect on the formation of thin CMS films on porous supports. The third project examined the effect of the soak time (i.e. time the membranes are held isothermally at the pyrolysis temperature) on the final performance of the membranes. The same 9 wt% solution was used, and the pyrolysis temperature was 650 °C. The pyrolysis soak times were 15 minutes, 1 hour, 3 hours, and 10 hours, respectively. The results showed that as the soak time increased the membranes became denser and provided higher selectivities and lower permeances. Furthermore, the membranes with longer soak times became more size-sieving earlier during physical aging than the membranes made with shorter soak times. Physical aging was accelerated with an increase in soak time, i.e., membranes made by soaking over 10 hours reached stable permeance over time starting at day 7. The fourth project aimed to investigate the preparation process, as well as to test the performance of the membranes under different environments. Two types of polyimide precursor membranes were made, one set with the pristine polyimide and the other one with a PDMS top coating. The results showed that the membranes with PDMS had similar selectivities but far slower permeances than the CMS membranes, the membranes made without PDMS coating had much lower selectivities and permeances. CMS membranes soaked for 15 minutes and 3 hours, respectively, were tested to check the permeances of all the five gases (H2, O2, N2, CH4, and CO2) under pressure cycles from 2 to 8 bar. The membranes passed the tests and their permeances were not affected by exposing them to high pressures and back, except for the membranes soaked for 3 hours when tested with CO2.

No description available.

The objective of this study was to examine the effects of annealing polyimide-organosilicate hybrid membranes on gas transport. In addition, the effects of carbon dioxide pressure on the gas transport of unannealed polyimide-organosilicate hybrid membranes were evaluated. The membranes in both studies consisted of sol-gel derived organosilicate domains covalently bonded to a 6FDA-6FpDA-DABA polyimide using partially hydrolyzed tetramethoxysilane (TMOS), methyltrimethoxysilane (MTMOS) or phenyltrimethoxysilane (PTMOS). The first study subjected the hybrid membranes to a 400°C annealing process to enhance gas separation performance by altering the organosilicate structures. The hybrid membranes were evaluated before and after annealing using pure gases (He, O₂, N₂, CH₄, CO₂) at 35°C and a feed pressure of 4 atm. The permeability for most of the membranes increased 200-500% after the annealing process while the permselectivity dropped anywhere from 0 to 50%. The exceptions were the 6FDA-6FpDA-DABA-25 22.5 wt% TMOS and MTMOS hybrid membranes, both of which exhibited increases in the CO₂ permeability and CO₂-CH₄ permselectivity. The increase in permeation was attributed to increases in the free volume and enhanced segmental mobility of the chain ends resulting from the removal of sol-gel condensation and polymer degradation byproducts. For the second study, the transport properties of four membranes, 6FDA-6FpDA polyimide, 6FDA-6FpDA-DABA polyimide, MTMOS and PTMOS-based hybrid materials, were characterized as a function of feed pressure to evaluate how the hybrid materials reacted to CO₂ plasticization. Steady-state gas permeation experiments were performed at 35°C using pure CO₂ and CH₄ gases at feed pressures ranging from 4 to 30 atm. All four materials exhibited dual mode sorption up to feed pressures of 17 atm, at which point the effects of CO₂ plasticization were observed. / Master of Science

Current membrane technology, based on polymeric materials, is subject to a limiting tradeoff between productivity (permeability) and efficiency (selectivity). Other materials with better gas separation performance exist, such as zeolites and carbon molecular sieves, but the physical characteristics of these materials inhibit industrial scale membrane preparation. This research focuses on the application of hybrid membrane technology, which has shown the ability to combine the advantageous properties of these materials, to a system comprised of carbon molecular sieves dispersed in the upper bound polymer 6FDA-6FpDA. Hybrid membranes require effective mass transfer across the interface between the two phases. This work shows the sensitivity of the component materials to processing conditions and the importance of consistency in gas separation membrane production. In particular, milling the sieves to reduce the size and using chemical linkage agents to bond to the polymer have potential to alter the separation performance of the respective materials. Analysis of multiple factors in this work provides important information regarding the source of unexpected properties in the hybrid membranes. Hybrid membrane testing in this work shows a need for active control of particle agglomerates within the dope prior to casting for effective membrane production. Continual sonication during the preparation of the casting dope was able to prevent the excessive agglomerates present in earlier trials. Further reduction of stresses generated during the casting process was also necessary to produce membranes with enhanced selectivity. Annealing the hybrid films above the polymer Tg appears to repair the interfacial morphology and produce effective membranes. The application of this process to enhance the gas separation performance of 6FDA-6FpDA represents the first known report of successful selectivity improvement in an upper bound polymer using the hybrid membrane approach.

Mixed matrix nanocomposite membranes composed of a crosslinkable polyimide matrix and high-silica molecular sieve particles were developed for purifying natural gas. It was shown that ideal mixed matrix effects were not possible without sieve surface modification. A previously developed Grignard procedure was utilized to deposit magnesium hydroxide nanostructures on the sieve surface in order to enhance polymer adhesion. Analyses of Grignard-treated sieves pointed to the formation of non-selective voids within the surface deposited layer. These voids were suspected to lead to lower-than-expected membrane performance. In order to improve membrane transport, a reactive sizing procedure was developed to fill these voids with polyimide-miscible material. In a serendipitous discovery, as-received sieves--when treated with this reactive sizing procedure--resulted in nearly identical membrane performance as reactive-sized, Grignard-treated sieves. This observation lead to the speculation of a non-ideal transport mechanism in mixed matrix membranes.

Traditional gas separation technologies are thermally-driven and can have adverse environmental and economic impacts. Gas separation membrane processes are not thermally-driven and have low capital and operational costs which make them attractive alternatives to traditional technologies. Polymers are easily processed into large, defect-free membrane modules which have made polymeric membranes the industrial standard; however, polymers show separation efficiency-productivity trade-offs and are often not thermally or chemically robust. Molecular sieves, such as zeolites, have gas separation properties that exceed polymeric materials and are more thermally and chemically robust. Unfortunately, formation of large, defect-free molecular sieve membranes is not economically feasible. Mixed matrix membranes (MMMs) combine the ease of processing polymeric materials with the superior transport properties of molecular sieves by dispersing molecular sieve particles in polymer matrices to enhance the performance of the polymers. MMMs with high molecular sieve loadings were made using polyvinyl acetate (PVAc) and various molecular sieves. Successful formation of these MMMs required substantial modifications to low loading MMM formation techniques. The gas separation properties of these MMMs show significant improvements over PVAc properties, especially for high pressure mixed carbon dioxide-methane feeds that are of great industrial relevance.

A new innovative polymer pyrolysis method was proposed for creation of attractive carbon molecular sieve (CMS) membranes. Oxygen exposure at ppm levels during pyrolysis was hypothesized and demonstrated to make slit-like CMS structures more selective and less permeable, which I contrary to ones expectation. Indeed prior to this work, any exposure to oxygen was expected to result in removal of carbon mass and increase in permeability. The results of this study indicated that the separation performance and CMS structure may be optimized for various gas separations by careful tuning of the oxygen level. This finding represents a breakthrough in the field of CMS membranes. Simple replacement of pyrolysis atmospheres from vacuum to inert can enable scale-up. The deviation in CMS membrane performance was significantly reduced once oxygen levels were carefully monitored and controlled. The method was shown to be effective and repeatable not only with dense films but also with asymmetric hollow fiber membranes. As a result, this work led the development of the "inert" pyrolysis method which has overcome the challenges faced with previously studied pyrolysis method to prepare attractive CMS membranes. The effect of oxygen exposure during inert pyrolysis was evaluated by a series of well-controlled experiments using homogeneous CMS dense films. Results indicated that the oxygen "doping" process on selective pores is likely governed by equilibrium limited reaction rather than (i) an external or (ii) internal transport or (iii) kinetically limited reaction. This significant finding was validated with two polyimide precursors: synthesized 6FDA/BPDA-DAM and commercial Matrimid®, which implies a possibility of the "inert" pyrolysis method application extending towards various precursors. The investigation was further extended to prepare CMS fibers. Despite the challenge of two different morphologies between homogeneous films and asymmetric hollow fibers, the "inert" pyrolysis method was successfully adapted and shown that separation performance can be tuned by changing oxygen level in inert pyrolysis atmosphere. Moreover, resulting CMS fibers were shown to be industrially viable. Under the operating condition of ~80 atm high pressure 50/50 CO2/CH4 mixed gas feed, the high separation performance of CMS fibers was shown to be maintained. In addition, elevated permeate pressures of ~20 atm did effect the theoretically predicted separation factor. While high humidity exposures (80%RH) resulted in reduced permeance, high selectivity was sustained in the fibers. Recommendations to overcome such negative effects as well as future investigations to help CMS membranes to be commercialized are provided.

Natural gas is one of the fastest growing primary energy sources in the world today. The increasing world demand for energy requires increased production of high quality natural gas. For the natural gas to be fed into the mainline gas transportation system, it must meet the pipe-line quality standards. Natural gas produced at the wellhead is usually "sub-quality" and contains various impurities such as CO2, H2S, and higher hydrocarbons, which must be removed to meet specifications. Carbon dioxide is usually the most abundant impurity in natural gas feeds and high CO2 partial pressures in the feed can lead to plasticization, which causes loss of some methane product and may ultimately render the membrane ineffective. Moreover, the presence of highly sorbing higher hydrocarbons in the feed can further reduce membrane performance. Covalent crosslinking has been shown to increase plasticization resistance in dense films by suppressing the degree of swelling and segmental chain mobility in the polymer, thereby preserving the selectivity of the membrane. This research focuses on extending the dense film success to asymmetric hollow fibers. In this work, the effect of high pressure CO2 (up to 400 psia CO2 partial pressure) on CO2/CH4 mixed gas separation performance was investigated on defect-free the hollow fiber membrane at different degrees of crosslinking. All the crosslinked fibers were shown to exhibit good resistance to selectivity losses from CO2 induced plasticization, significantly more than the uncrosslinked fibers. Robust resistance of the hollow fiber membranes in the presence of toluene (a highly sorbing contaminant) was also demonstrated as the membranes showed no plasticization. Antiplasticization was found to occur in the presence of toluene feeds with the crosslinkable fibers used in this work.