Mixed Matrix Dual Layer Hollow Fiber Membranes For Natural Gas Separation
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Mixed matrix membranes offer an attractive route to the development of high performance and efficiency membranes required for demanding gas separations. Such membranes combine the advantageous processing characteristics of polymers with the excellent separation productivity and efficiency of molecular sieving materials. This research explores the development of mixed matrix membranes, namely in the form of asymmetric hollow fiber membranes using zeolites as the molecular sieving phase and commercially available high performance polymers as the continuous matrix. Lack of adhesion between the typically hydrophobic polymer and the hydrophilic native zeolite surface is a major hurdle impeding the development of mixed matrix membranes. Silane coupling agents have been used successfully to graft polymer chains to the surface of the zeolite to increase compatibility with the bulk polymer in dense films. However, transitioning from a dense film to an asymmetric structure typically involves significant processing changes, the most important among them being the use of phase separation to form the asymmetric porous structure. During the phase separation, it is believed that hydrophilic sieves can act as nucleating agents for the hydrophilic polymer lean phase. Such nucleation tendencies are believed to lead to the formation of gaps between the polymer and sieve resulting in poor mixed matrix performance. This research focuses on defining procedures and parameters to form successful mixed matrix hollow fiber membranes. The first part of this dissertation describes dope mixing procedures and unsuccessful results obtained using a silane coupling agent to enhance polymer-zeolite adhesion. The next section follows the development of a highly successful surface modification technique, discovered by the author, employing the use of a Grignard reagent. As a test case, two zeolites of different silicon-to-aluminum ratios are successfully modified and used to develop mixed matrix membranes with greatly increased gas separation efficiencies. The broad applicability of the surface treatment is also demonstrated by the successful incorporation of the modified zeolites in a second polymer matrix. The final section of the work describes the novel occurrence of large defects (macrovoids) caused by the presence of large zeolite particles proposing a particle size effect in the formation of such defects.