Date of Award

Spring 1-1-2017

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Richard D. Noble

Second Advisor

Douglas L. Gin

Third Advisor

Hans Funke

Fourth Advisor

Wei Zhang

Fifth Advisor

Will Medlin

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™, 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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