Date of Award

Spring 1-1-2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical & Biochemical Engineering

First Advisor

Douglas L. Gin

Second Advisor

Richard D. Noble

Third Advisor

Stephanie J. Bryant

Abstract

Supported ionic liquid membranes (SILMs) possess very attractive CO2 permeabilities and CO2/light gas permeability selectivities. However, the liquid RTIL in SILMs is physically displaced at elevated trans-membrane pressures (e.g., > 1 atm). The RTIL component can, however, be "stabilized" by forming a solid, polymerized RTIL (poly(RTIL)) membrane. To compensate for a reduction in CO2 permeability, "free" liquid RTIL can be incorporated into the poly(RTIL) to form a composite poly(RTIL)-RTIL material. Optimizing the performance of poly(RTIL)-RTIL membranes can be achieved by individually tailoring the liquid (i.e, RTIL) and solid (i.e., poly(RTIL)) components to maximize CO2 permeability and CO2/light gas permeability selectivity. A thermodynamics-based, "rational" design guide for the synthesis of new, highly selective RTIL materials was presented and verified experimentally with CO2 solubility and CO2/light gas selectivity measurements. Appending the RTIL imidazolium cation with groups that possess large molar attraction constants (i.e., nitrile or propargyl) was found to increase the RTIL solubility parameter, reduce CO2 solubility, and increase CO2/light gas solubility selectivity relative to alkyl-functionalized RTIL analogues. The synthesis and CO2 separation characterization of several new RTIL-based polymeric membrane materials were also investigated. It was also found that composite structures formed by blending these polymers with liquid RTILs affords enhanced CO2 flux and CO2 light gas selectivity. For example, the CO2 permeability and CO2/N2 permeability selectivity of a disiloxane-functionalized poly(RTIL)-RTIL (20 mol% liquid) composite were 190 barrers and 19, respectively. To maximize both CO2 flux and CO2/light gas selectivity, new cross-linked poly(RTIL)-RTIL gel membranes were developed, and the CO2 separation performances of these membranes were studied. This membrane configuration effectively "stabilizes" the liquid RTIL component while maintaining a good degree of membrane mechanical stability. These materials demonstrated excellent CO2/light gas separation performance. The CO2 permeability of these membranes were found to range from 130 to 520 barrers with no change in CO2/N2 or CO2/CH4 selectivity (ca. 34 and 20, respectively). The CO2/H2 selectivity improved with RTIL content to a maximum of 12 at 75 wt. % liquid loading. As a new class of RTIL-based membrane materials, these next generation of RTIL-based membranes, cross-linked poly(RTIL)-RTIL gels were found to be very promising and potentially viable candidates for industrial CO2 membrane separations.

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