Design and Evaluation of New Mixed-Matrix Membranes for CO2/CH4 Separations

Singh, Zoban Veer

Graduate Thesis Or Dissertation

Design and Evaluation of New Mixed-Matrix Membranes for CO2/CH4 Separations Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/zc77sq231

Abstract

Many polymeric materials are mechanically robust and thus can be used to form membranes for CO₂/CH₄ separations. Most of these polymers, however, do not have sufficiently high CO₂ permeabilities and CO₂/CH₄ selectivities. Some microporous solids have a high CO₂/CH₄ adsorption selectivity, and CO₂ diffuses fast in their pores, but the synthesis scale-up of thin membranes formed from these materials is difficult. Small particles of such microporous solids can be embedded in a polymeric matrix to form mechanically stable mixed-matrix membranes (MMMs). In this work, three new types of defect-free mixed-matrix membranes with both high CO₂ permeabilities and CO₂/CH₄ selectivities were synthesized by combining three organic and inorganic microporous solids with different polymers.

Room-temperature ionic liquid (RTIL)-based materials formed the matrix for the zeolites SAPO-34 and SSZ-13 that have been shown to be selective for the separation of CO₂ from CH₄. The structures of the polymerized RTIL (poly(RTIL)) materials were modified to achieve CO₂/CH₄ separation that surpassed the Robeson upper bound. The interface between the zeolite particles and the surrounding poly(RTIL)-matrix that often causes non-selective transport or pore blocking in other MMMs was optimized by adding free RTILs to obtain mechanically stable membranes with high solid loadings and good separation performance.

Novel pillar[5]arene supra-molecular organic framework (P5-SOF) particles were used to form defect-free MMMs by solvent-casting with Matrimid-5218^TM, a commercially available uncharged polymer. These all-organic MMMs had very high CO₂/CH₄ selectivities that surpassed the Robeson upper bound but permeances were low. The CO₂ permeability of these membranes was improved by adding different low-vapor-pressure liquids during membrane formation to optimize the interface between the polymer matrix and the P5-SOF particles. MMMs containing n-dodecane as the interfacing agent displayed the largest enhancements in CO₂ permeability while also maintaining high CO₂/CH₄ selectivity.

In this thesis work, we gained a new fundamental understanding of the microstructure of MMMs to optimize their transport properties for CO₂/CH₄ gas separation. The chemical compatibility between the different components of the membrane used in this study, polymer-solid interfacing, and particle loading all strongly affected the separation performance. Defect-free MMMs that are mechanically stable with separation properties that exceed those of other types of MMM’s were synthesized and these membranes show potential for scale-up and commercialization.

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