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.

Over the past decade, membrane based gas separations have gained traction in industry as an attractive alternative to traditional thermally based separations due to their potential to offer lower operational and capital expenditures, greater ease of operation and lower environmental impact. As membrane research evolves, new state-of-the-art membrane materials as well as processes utilizing membranes will likely be developed. Therefore, their incorporation into existing thermally based units as a debottlenecking step or as a stand-alone separation unit is expected to become increasingly more common. Specifically for natural gas, utilization of smaller, more remote natural gas wells will require the use of less equipment intensive and more flexible separation technologies, which precludes the use of traditional, more capital and equipment intensive thermally based units. The use of membranes is, however, not without challenges. Perhaps the most important hurdle to overcome in membrane development for natural gas purification is the ability to maintain high efficiency in the presence of harsh feed components such as CO₂ and H₂S, both of which can swell and plasticize polymer membranes. Additionally, as this project demonstrates, achievement of similarly high selectivity for both CO₂ and H₂S is challenged by the different governing factors that control their transport through polymeric membranes. However, as others have suggested and shown, as well as what is demonstrated in this project, when CO₂ is the primary contaminant of interest, maintaining high CO₂/CH₄ efficiency appears to be more important in relation to product loss in the downstream. This work focuses on a class of fluorinated, glassy polyamide-imides which show high plasticization resistance without the need for covalent crosslinking. Membranes formed from various polyamide-imide materials show high mixed gas selectivities with adequate productivities when subjected to feed conditions that more closely resemble those that may be encountered in a real natural gas well. The results of this project highlight the polyamide-imide family as a promising platform for future membrane material development for materials aimed at aggressive natural gas purifications due to their ability to maintain high selectivities under aggressive feed conditions without the need for extensive stabilization methods.

The main theme of this dissertation is to engineer nanoporous materials and nanostructured surfaces for applications in gas separation membranes. Tunable methods have been developed to create inorganic hydroxide nanostructures on zeolite surfaces, and used to control the inorganic/polymer interfacial morphology in zeolite/polymer composite membranes. The study of the structure-property relationships in this material system showed that appropriate tuning of the surface modification methods leads to quite promising structural and permeation properties of the membranes made with the modified zeolites. First, a facile solvothermal deposition process was developed to prepare roughened inorganic nanostructures on zeolite pure silica MFI crystal surfaces. The functionalized zeolite crystals resulted in high-quality ̒mixed matrix̕ membranes, wherein the zeolite crystals were well-adhered to the polymeric matrix. Substantially enhanced gas separation characteristics were observed in mixed matrix membranes containing solvothermally modified MFI crystals. Gas permeation measurements on membranes containing nonporous uncalcined MFI revealed that the performance enhancements were due to significantly enhanced MFI-polymer adhesion and distribution of the MFI crystals. Solvothermal deposition of inorganic nanostructures was successfully applied to aluminosilicate LTA surfaces. Solvothermal treatment of LTA was tuned to deposit smaller/finer Mg(OH)₂ nanostructures, resulting in a more highly roughened zeolite surface. Characterization of particles and mixed matrix membranes revealed that the solvothermally surface-treated LTA particles were promising for application in mixed matrix membranes. Zeolite LTA materials with highly roughened surfaces were also successfully prepared by a new method: the ion-exchange-induced growth of Mg(OH)₂ nanostructures using the zeolite as the source of the Mg²⁺ ions. The size/shape of the inorganic nanostructures was tuned by adjusting several parameters such as the pH of the reagent solution and the amount of magnesium in the substrates and systematic modification of reaction conditions allowed generation of a good candidate for application in mixed matrix membranes. Zeolite/polymer adhesion properties in mixed matrix membranes were improved after the surface treatment compared to the untreated bare LTA. Surface modified zeolite 5A/6FDA-DAM mixed matrix membranes showed significant enhancement in CO₂ permeability with slight increases in CO₂/CH₄ selectivity as compared to the pure polymer membrane. The CO₂/CH₄ selectivity of the membrane containing surface treated zeolite 5A was much higher than that of membrane with untreated zeolite 5A. In addition, the use of metal organic framework (MOF) materials has been explored in mixed matrix membrane applications. ZIF-90 crystals with submicron and 2-μm sizes were successfully synthesized by a nonsolvent induced crystallization technique. Structural investigation revealed that the ZIF-90 particles synthesized by this method had high crystallinity, microporosity and thermal stability. The ZIF-90 particles showed good adhesion with polymers in mixed matrix membranes without any compatibilization. A significant increase in CO₂ permeability was observed without sacrificing CO₂/CH₄ selectivity when Ultem® and Matrimd® were used as the polymer matrix. In contrast, mixed matrix membranes with the highly permeable polymer 6FDA-DAM showed substantial enhancement in both permeability and selectivity, as the transport properties of the two phases were more closely matched.

Doctor of Philosophy / Department of Chemical Engineering / Mary E. Rezac / Polyimide membranes have been used extensively in gas separation applications because of their attractive gas transport properties and the ease of processing these materials. Other applications of membranes, such as membrane reactors, which could compete with more traditional packed and slurry bed reactors across a wider range of environments, could benefit from improvements in the thermal and chemical stability of polymeric membranes. This work focuses on blending polyimide and polybenzimidazole polymers to improve the thermal and chemical stability of polyimide membranes while retaining the desirable characteristics of the polyimide. Blended dense films and asymmetric membranes were fabricated and characterized. Dense film properties are useful for studying intrinsic properties of the polymer blends. Transport properties of dense films were characterized from room temperature to 200°C. Properties including miscibility, density, chain packing and thermal stability were investigated. A process for fabricating flat sheet blended integrally skinned asymmetric membranes by phase inversion was developed. The transport properties of membranes were characterized from room temperature to 300°C. A critical characteristic of gas separation membranes is selectivity. Post-treatments including thermal annealing and vapor and liquid surface treatments were investigated to improve the selectivity of blended membranes. Vapor and liquid surface treatments with common, benign solvents including an alkane, an aldehyde and an alcohol resulted in improvements in selectivity.

Metal-Organic Frameworks (MOFs) are nanoporous framework materials with tunable pore size and functionality, and hence attractive for gas separation membrane applications. Zeolitic Imidazolate Frameworks (ZIFs), a subclass of MOFs, are known for their high thermal and chemical stability. ZIF-8 has demonstrated potential to kinetically separate propane/propene in powder and membrane form. ZIF-8 membranes propane-propene separation performance is superior in comparison to polymer, mixed matrix and carbon membranes. The overarching theme of my research is to address challenges that hinder fabrication of MOF membranes on a commercial scale and in a reproducible and scalable manner. 1. Current approaches, are specific to a given ZIF, a general synthesis route is not available. Use of multiple steps for surface modification or seeding causes reproducibility and scalability issues. 2. Conventional fabrication techniques are batch processes, thereby limiting their commercialization. Here we demonstrate two new approaches that can potentially address these challenges. First, we report one step in situ synthesis of ZIF-8 membranes on more commonly used porous α-alumina supports. By incorporating sodium formate in the in situ growth solution, well intergrown ZIF-8 membranes were synthesized on unmodified supports. The mechanism by which sodium formate promotes heterogeneous nucleation was investigated. Sodium formate reacts with zinc source to form zinc oxide layer, which in turn promotes heterogeneous nucleation. Sodium formate promotes heterogeneous nucleation in other ZIF systems as well, leading to ZIF-7, Zn(Im)2 (ZIF-61 analogue), ZIF-90, and SIM-1 films. Thus one step in situ growth using sodium formate provides a simplified, reproducible and potentially general route for ZIF film fabrication. One step in situ route, although advantageous; is still conventional in nature and batch process with long synthesis time. This limits commercialization, due to scalability and manufacturing cost issues. Taking advantage of coordination chemistry of MOFs and using temperature as driving force, continuous well-intergrown membranes of HKUST-1 and ZIF-8 in relatively short time (15 min) using Rapid Thermal Deposition (RTD). With minimum precursor consumption and simplified synthesis protocol, RTD provides potential for a continuous, scalable, reproducible and commercializable route for MOF membrane fabrication. RTD-prepared MOF membranes show improved separation performances, indicating improved microstructure.

A separação de gases é um processo que está presente na grande maioria das indústrias e a utilização de membranas vem ganhando cada vez mais importância e destaque. Neste trabalho foram fabricadas membranas de carbono suportadas em um tubo cerâmico de alumina TCB99 (99 % de alumina) a partir da pirólise (atmosfera inerte de N2) do polímero poli(éter imida) (PEI). A técnica de recobrimento por imersão foi utilizada para formar uma camada de solução polimérica sobre o suporte. Os filmes poliméricos e de carbono e as membranas de carbono suportadas foram caracterizados em relação à estrutura utilizando diferentes técnicas morfológicas, químicas e de estrutura. Além disso, foram realizados testes de sorção nos filmes de carbono e permeação de gases nas membranas de carbono. Também foram realizadas simulações de dinâmica molecular para obter um melhor entendimento das etapas de degradação do polímero, da conformação das cadeias poliméricas e formação da estrutura de carbono durante o processo de pirólise. As membranas de carbono suportadas apresentaram uma camada seletiva bem definida e com pouca ou nenhuma intrusão nos poros do suporte. Os resultados das análises estruturais mostraram que as membranas de carbono são constituídas em sua maior parte por carbono amorfo, podendo conter carbono grafite. Foi observado pelos resultados de FTIR, CNS e DRX que o polímero precursor, poli(éter imida), não foi totalmente pirolisado na temperatura máxima empregada, contendo possivelmente grupamentos de amida e anéis benzênicos. Através de simulação de dinâmica molecular foi possível obter um melhor entendimento sobre o processo de formação e composição da membrana de carbono, confirmando os resultados obtidos experimentalmente em relação à morfologia e estrutura destas, isto é, a formação de uma estrutura amorfa contendo nanodomínios de grafite. A sorção dos gases pelo material da membrana de carbono (MC) seguiu um padrão prescrito pela literatura, onde o gás CO2 apresentou maior sorção, seguido pelos gases CH4, N2 e He. Além disso, verificou-se que para as membranas produzidas em maior temperatura obtiveram um aumento na sorção dos gases He e N2, o qual atribui-se a formação de uma estrutura mais porosa. Os testes de desempenho de permeação de gases (He, CO2, O2, N2, CH4, C3H6 e C3H8) para as membranas de carbono nas diferentes concentrações de polímero (10, 15 e 20 % (m/m)) indicaram a formação de uma estrutura porosa, onde os mecanismos de permeação predominantes foram difusão de Knudsen e difusão superficial, indicando a presença de poros maiores e/ou defeitos na estrutura da membrana, possivelmente causado pela grande área de suporte utilizada. Ainda foi observado que um aumento na concentração de polímero, acarretou em um aumento da permeância para todos os gases, o que pode estar relacionado com o aumento de volume livre no polímero precursor quando submetido à pirólise. Os resultados obtidos demonstram que as membranas de carbono desenvolvidas neste trabalho apresentam potencial para serem aplicadas em processos de separação de gases. / The gas separation is a process that is present in the vast majority of industries and the membranes use is gaining more and more importance and prominence. In this work carbon membranes supported on a TCB99 alumina ceramic tube (99 % alumina) were manufactured from the pyrolysis (N2 inert atmosphere) of the poly(imid ether) (PEI) polymer. The immersion coating technique was used to form a layer of polymer solution on the support. The polymer and carbon films and the supported carbon membranes were characterized in relation to the structure using techniques of microscopy and spectroscopy, among others. In addition, sorption tests were performed on carbon films and gas permeation on carbon membranes. Molecular dynamics simulations were also performed to obtain a better understanding of polymer degradation steps, polymer chain conformation and carbon structure formation during the pyrolysis process. The supported carbon membranes presented a well defined selective layer with little or no intrusion into the pores of the support. The results of the structural analyzes showed that the carbon membranes are composed mostly of amorphous carbon and may contain graphite carbon. It was observed by the FTIR, CNS and XRD results that the precursor polymer, poly(imide ether), was not completely pyrolyzed at the maximum temperature employed, possibly containing amide groups and benzene rings. By means of molecular dynamics simulation it was possible to obtain a better understanding of the formation and composition of the carbon membrane, confirming the results obtained experimentally in relation to the morphology and structure of these, this is, the formation of an amorphous structure containing graphite nanodomains. The gases sorption by the MC material followed a standard prescribed in the literature, where the CO2 gas presented higher sorption, followed by CH4, N2 and He gases. In addition, it was observed that for the gases He and N2, the membranes produced at higher temperature obtained an increase in the sorption, which is attributed to the formation of a more porous structure. The gas permeation tests (He, CO2, O2, N2, CH4, C3H6 and C3H8) for the carbon membranes at the different polymer concentrations (10, 15 and 20 % (m/m)) indicated the formation of a porous structure, where the predominant permeation mechanisms were Knudsen diffusion and surface diffusion, indicating the presence of larger pores and/or defects in the membrane structure, possibly caused by the large support area used. It was further noted that an increase in polymer concentration resulted in increased permeability for all gases, which may be related to the increase in free volume in the precursor polymer when subjected to pyrolysis. The results obtained demonstrate that the carbon membranes developed in this work have the potential to be applied in gas separation/adjustment processes.