Date of Award

Spring 1-1-2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical & Biochemical Engineering

First Advisor

Richard D. Noble

Second Advisor

Douglas L. Gin

Third Advisor

Wei Zhang

Fourth Advisor

Alan R. Greenberg

Fifth Advisor

Mark P. Stoykovich

Abstract

Room temperature ionic liquids (RTILs) have very attractive CO2 permeabilities and CO2/light gas permeability selectivities as supported membranes. However, RTILs are displaced as supported membranes by a pressure differential, rendering them useless. Polymerized RTILs (poly(RTIL)s) can be blended with unbound RTILs to form composite structures with good CO2 permeabilities and CO2/light gas permeability selectivities. High weight loadings of free RTILs can be achieved with epoxy-amine chemistry to form ion gel membranes that have residual and formed amine groups that interact specifically with CO2 to provide enhanced CO2 transport.

Novel bis(epoxide)-functionalized RTIL monomers were synthesized and reacted with commercially available amine monomers to produce cross-linked, epoxy-amine-based poly(RTIL) resins and ion-gel membranes via step-growth (S-G) polymerization. The amine functionality was controlled by manipulating the S-G monomer stoichiometric ratios. Analysis of the gas permeation data revealed that these materials exhibit a rare case of inverse CO2/CH4 diffusion selectivity (DCO2/DCH4 < 1) for ideal gas permeation testing. This phenomenon was attributed to the interaction of CO2 with residual and formed amine groups in the S-G PIL.

Structural changes to the length and chemical nature (i.e., alkyl vs. ether) between the imidazolium group and epoxide groups were studied to determine their effects on CO2 affinity. The effect of using a primary vs. a secondary amine-containing multifunctional monomer was also investigated. Secondary amine monomers can increase CO2 permeability but also increase the reaction time. By changing either epoxide or amine monomer structure, the CO2 solubility and permeability of the resulting PIL resins and ion-gel membranes can be improved.

The residual and formed amine functional groups in epoxy-amine ion gel membranes allow for the fixed-site-carrier facilitated transport of CO2. As expected for materials operating via the fixed-site facilitated transport mechanism, increased CO2 permeability and CO2/N2 selectivity was observed with decreasing CO2 partial pressure. The hydrophilicity of the free RTIL was determined to play an important role, with more hydrophilic RTILs enhancing the effects of facilitated transport. Several of the membranes reported have CO2/N2 separation performance that exceeds the 2008 Robeson upper bound. Therefore, these represent promising materials and industrially attractive materials for membrane-based CO2 separations.

Share

COinS