Separation technologies with polymeric membranes are widely studied and have a wide range of applications. The membrane's heart is a dense selective layer whose permeability should strongly depend on the permeating species' properties. In turn, permeability depends on the diffusivity and solubility of the permeating species in the selective layer, which are considered intrinsic properties of the polymer forming the selective layer. When developing new membrane materials, the ultimate objective is to exceed the famous "upper bound" limit by achieving simultaneously higher selectivity and higher permeability. This objective is impossible without a reliable and accurate characterization method to determine the selective layer's intrinsic transport properties. The time-lag method is the most common membrane characterization technique, initially developed for polymeric membranes. However, as the membrane technology and material science advance, the selective layer structure becomes more complex and not limited to organic polymers. As a result, the time-lag method needs to be reviewed and adapted to these more complicated cases, which was the main objective of this thesis. Numerical simulation of dynamic gas permeation experiments is a powerful tool to examine different aspects of the time-lag method. Therefore, we have established a comprehensive variable-mesh finite-difference scheme, which was used throughout the thesis. It allowed us to investigate the effect of different random and resolution errors and an extrapolation error on the resulting time lag of an ideal membrane. We then considered more complex systems, particularly those of glassy polymers and mixed matrix membranes, to investigate the effect of different transport mechanisms on the results of dynamic and steady-state gas permeation experiments. In parallel, we also focused on developing a novel gas permeation system that would monitor dynamic gas permeation experiments based on pressure decay at the feed side. All the existing constant-volume gas permeation systems rely on monitoring pressure to rise at the membrane's permeate side. Although this work is still ongoing, we have made considerable progress. Among the numerous contributions made through this thesis, there are three of particular significance. We have developed an analytical model to predict mixed matrix membranes' relative permeability with the uniformly dispersed non-permeable fillers of different shapes. The model requires three structural parameters arising from the filler's shape and size, and it is superior to all existing analytical models, including the famous Maxwell model. We have also demonstrated that the diffusivity of mixed matrix membranes determined by the time-lag method depends on the number of layers of dispersed particles. In the limiting case of a single layer of uniformly impermeable fillers, it is possible for the diffusivity determined by the time-lag method to be greater than that of the host polymer, which might appear as counterintuitive in the absence of defects at the polymer-particle interface. In the case of glassy polymers, it is possible to observe an upward deviation from the steady-state flux, resulting from a non-instantaneous equilibrium between permeating species in Henry's and Langmuir adsorption sites.

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The vast majority of commercial gas separation membrane systems are polymeric because of processing feasibility and cost. However, polymeric membranes designed for gas separations have been known to have a trade-off between permeability and selectivity as shown in Robeson's upper bound curves. The search for membrane materials that transcend Robeson's upper bound has been the critical issue in research focused on membranes for gas separation in the past decade. To that end, many researchers have explored the idea of mixed matrix membranes (MMMs). These membranes combine a polymer matrix with inorganic molecular sieves such as zeolites. The ideal filler material in MMMs should have excellent properties as a gas adsorbent or a molecular sieve, good dispersion properties in the polymer matrix of submicron thickness, and should form high quality interfaces with the polymer matrix. In order to increase gas permeance and selectivity of polymeric membranes by fabricating MMMs, we have fabricated mixed matrix membranes using carbon nanotubes (CNTs) and nano-sized mesoporous silica. Mixed matrix membranes containing randomly oriented CNTs showed that addition of nanotubes to a polymer matrix could improve its selectivity properties as well as permeability by increasing diffusivity. Overall increases in permeance and diffusivity for all tested gases suggested that carbon nanotubes can provide high diffusivity tunnels in the CNT within the polymer matrix. This result agreed well with molecular simulation estimations. In order to prepare ordered CNTs membranes, we have developed a simple, fast, commercially attractive, and scalable orientation method. The oriented CNT membrane sample showed higher permeability by one order of magnitude than the value predicted by a Knudsen model. This CNT membrane showed higher selectivities for CO₂ over other gas molecules because of preferential interaction of CO₂ with the amine functionalized nanotubes, demonstrating practical applications in gas separations. Recently, mesoporous molecular sieves have been used in MMMs to enhance permeability or selectivity. However, due to their micrometer scale in particle size, the composite membrane was extremely brittle and tended to crack at higher silica loading. In this study, we have developed fabrication techniques to prepare MMMs containing mesoporous MCM-41 nanoparticles on the order of ~50 nm in size. This smaller nanoparticle lead to higher polymer/particle interfacial area and provides opportunity to synthesize higher loading of molecular sieves in polymer matrix up to ~80 vol%. At 80 vol% of nano-sized MCM-41 silica loading, the permeability of the membrane increased dramatically by 300 %. Despite these increases in permeability, the separation factor of the MMMs changed only slightly. Therefore, these nanoscale molecular sieves are more suitable for commercialization of MMMs with very thin selective layers than are micro-sized zeolites or molecular sieves. / Ph. D.

Functionalized membranes provide an elegant platform for selective separations and sorptions. In this dissertation, application of functionalized membranes for surfactant and metal sorption studies are discussed. Sorption behavior of surfactants is also studied using quartz crystal microbalance (QCM) and other techniques. Adsorption of the ethoxylated surfactants on polymeric materials (cotton and polyester) and model gold surface was quantified from a non-aqueous siloxane based solvent (D5) and water. The role of ethylene oxide group and the effect of nature of polymeric materials on adsorption behavior was quantified and established. In the case of gold-water interface, the adsorption data was fitted to calculate adsorption/desorption rate constants. The study is important towards applications involving use of the surfactants in cleaning operations. PAA functionalized membranes were prepared and used for separation of the surfactants from the siloxane solvent. Finally the pH sensitivity of the PAA-surfactant complex was verified by successful regeneration of the membrane on permeation of slightly alkaline water. The preparation and application of thiol and sulfonic acid functionalized silica mixed matrix membranes for aqueous phase metal ion sorption is also studied. The functionalized particles were used as the dispersed phase in the polysulfone or cellulose acetate polymer matrix. The effects of the silica properties such as particle size, specific surface area, and porous/nonporous morphology on the metal ion sorption capacity were studied. Silver and ferrous ions were studied for metal sorption capacities. The ferrous ions were further reduced to prepare membrane immobilized iron nanoparticles which are attractive for catalytic applications. One dimensional unsteady state model with overall volumetric mass transfer coefficient was developed to model the metal ion sorption using mixed matrix membrane. The study demonstrates successful application of the functionalized mixed matrix membranes for aqueous phase metal capture with high capacity at low transmembrane pressures. The technique can be easily extended to other applications by altering the functionalized groups on the silica particles. The study is important towards water treatment applications and preparation of membrane immobilized metal nanoparticles for catalytic applications.

Developing new membrane morphologies and modifying the existing membrane materials are required to obtain membranes with improved gas separation performances. The incorporation of zeolites and low molecular-weight additives (LMWA) into polymers are investigated as alternatives to modify the permselective properties of polymer membranes. In this study, these two alternatives were applied together to improve the separation performance of a polymeric membrane. The polycarbonate (PC) chain characteristics was altered by incorporating p-nitroaniline (pNA) as a LMWA and the PC membrane morphology was modified by introducing zeolite 4A particles as fillers. For this purpose, pure PC and PC/pNA dense homogenous membranes, and PC/zeolite 4A and PC/pNA/zeolite 4A mixed matrix membranes (MMM) were prepared by solvent-evaporation method using dichloromethane as the solvent. The pNA and zeolite 4A concentrations in the casting solutions were changed between 1-5% (w/w) and 5-30% (w/w), respectively. Membranes were characterized by SEM, DSC, and single gas permeability measurements of N2, H2, O2, CH4 and CO2. They were also tested for their binary gas separation performances with CO2/CH4, CO2/N2 and H2/CH4 mixtures at different feed gas compositions. DSC analysis of the membranes showed that, incorporation of zeolite 4A particles into PC/pNA increased the glass transition temperatures, Tg, but incorporation of them to pure PC had no effect on the Tg, suggesting that pNA was a necessary agent for interaction between zeolite 4A and PC matrix. The ideal selectivities increased in the order of pure PC, PC/zeolite 4A MMMs and PC/pNA/zeolite 4A MMMs despite a loss in the permeabilities with respect to pure PC. A significant improvement was achieved in selectivities when the PC/pNA/zeolite 4A MMMs were prepared with pNA concentrations of 1 % and 2 % (w/w) and with a zeolite loading of 20 % (w/w). The H2/CH4 and CO2/CH4 selectivities of PC/pNA (1%)/zeolite 4A (20%) membrane were 121.3 and 51.8, respectively, which were three times higher than those of pure PC membrane. Binary gas separation performance of the membranes showed that separation selectivities of pure PC and PC/pNA homogenous membranes were nearly the same as the ideal selectivities regardless of the feed gas composition. On the other hand, for PC/zeolite 4A and PC/pNA/zeolite 4A MMMs, the separation selectivities were always lower than the respective ideal selectivities for all binary gas mixtures, and demonstrated a strong feed composition dependency indicating the importance of gas-membrane matrix interactions in MMMs. For CO2/CH4 binary gas mixture, when the CO2 concentration in the feed increased to 50 %, the selectivities decreased from 31.9 to 23.2 and 48.5 to 22.2 for PC/zeolite 4A (20%) and PC/pNA (2%)/zeolite 4A (20%) MMMs, respectively. In conclusion, high performance PC based MMMs were prepared by blending PC with small amounts of pNA and introducing zeolite 4A particles. The prepared membranes showed promising results to separate industrially important gas mixtures depending on the feed gas compositions.

ABSTRACT EFFECT OF PREPARATION AND OPERATION PARAMETERS ON PERFORMANCE OF POLYETHERSULFONE BASED MIXED MATRIX GAS SEPARATION MEMBRANES Karatay, Elif M.Sc., Department of Chemical Engineering Supervisor : Prof. Dr. Levent Yilmaz Co-supervisor : Assoc. Prof. Dr. Halil Kalip&ccedil / ilar August 2009, 126 pages Membrane processes have been considered as promising alternatives to other competing technologies in gas separation industry. Developing new membrane morphologies are required to improve the gas permeation properties of the membranes. Mixed matrix membranes composing of polymer matrices and distributed inorganic/organic particles are among the promising, developing membrane materials. In this study, the effect of low molecular weight additive (LMWA) type and concentration on the gas separation performance of neat polyethersulfone (PES) membranes and zeolite SAPO-34 containing PES based mixed matrix membranes was investigated. Membranes were prepared by solvent evaporation method and annealed above the glass transition temperature (Tg) of PES in order to remove the residual solvent and erase the thermal history. They were characterized by single gas permeability measurements of H2, CO2, and CH4 as well as scanning electron microscopy (SEM), thermal gravimetric analysis (TGA), and differential scanning calorimetry (DSC). Various LMWAs were added to the neat PES membrane at a concentration of 4 wt %. Regardless of the type, all of the LMWAs had an anti-plasticization effect on PES gas permeation properties. 2-Hydroxy 5-Methyl Aniline, HMA, was selected among the other LMWAs for parametric study on the concentration effect of this additive. The incorporation of SAPO-34 to PES membranes increased the permeabilities of all gases with a slight loss in selectivities. However, the addition of HMA to PES/SAPO-34 membranes increased the ideal selectivities well above the ideal selectivities of PES/HMA membranes, while keeping the permeabilities of all the gases above the permeabilities of both pure PES and PES/HMA membranes.

This research investigates the effect of feed composition on the separation performance of pure polyethersulfone (PES) and different types of PES based mixed matrix membranes (MMMs) in order to develop high performing membranes for CO2/CH4 separation. MMMs were prepared by solvent evaporation method using PES as the polymer matrix with SAPO-34 particles as fillers, and 2-hydroxy 5-methyl aniline (HMA) as the low molecular weight additive. Four types of membranes were used throughout the study, namely pure PES membrane, PES/HMA (4, 10%w/w) membrane, PES/SAPO-34 (20%w/w) MMM, PES/SAPO-34 (20%w/w)/HMA (4, 10%w/w) MMM. The effect of CO2 composition on the performance of the membranes was investigated in detail with a wide feed composition range changing between 0 and 100%. In addition to separating CO2/CH4 binary gas mixtures, the separation performances of these membranes were determined by measuring single gas permeabilities at 35&ordm / C, with a feed pressure of 3 bar. Moreover, the membranes were characterized by scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and thermal gravimetric analyzer (TGA). The separation selectivities of all types of membranes generally observed to be independent of feed composition. The composition independency of these membranes eliminates the need of investigating at which feed gas composition the prepared membranes are best performing for practical applications. PES/SAPO-34/HMA MMMs with HMA loading of 10% and SAPO-34 loading of 20% demonstrated the highest separation selectivity of about 40, and the ideal selectivity of 44, among the used membranes.

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.

Pervaporation with a "reverse-selective" (hydrophobic) membrane is a promising technology for the energy-efficient separation of alcohols from dilute alcohol-water streams, such as those formed in the production of biofuels. Pervaporation depends on the selectivity and throughput of the membrane, which in turn is highly dependent on the membrane material. A nanocomposite approach to membrane design is desirable in order to combine the advantages and eliminate the individual limitations of previously-reported polymeric and zeolitic membranes. In this work, a hollow-fiber membrane composed of a thin layer of polymer/zeolite nanocomposite material on a porous polymeric hollow fiber support is developed. The hollow fiber geometry offers considerable advantages in membrane surface area per unit volume, allowing for easier scaling and higher throughput than flat-film membranes. Poly(dimethyl siloxane) (PDMS) and pure-silica MFI zeolite (silicalite-1) were investigated for these membranes. Iso-octane was used to dilute the dope solution to provide thinner coatings. Previously-spun non-selective Torlon hollow fibers were used as the support layer for the nanocomposite coatings. To determine an acceptable method for coating fibers with uniform, defect-free coatings, flat-film membranes (0 to 60 wt% MFI on a solvent-free basis) and hollow-fiber membranes (0 and 20 wt% MFI) were fabricated using different procedures. Pervaporation experiments were run for all membranes at 65C with a 5 wt% ethanol feed. The effects of membrane thickness, fiber pretreatment, coating method, zeolite loading, and zeolite surface treatment on membrane pervaporation performance were investigated.