Doctoral Dissertations

Date of Award

12-2017

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemistry

Major Professor

Alexei P. Sokolov

Committee Members

Jimmy W. Mays, Charles S. Feigerle, Jaan Mannik

Abstract

Membrane separation is highlighted as one of the most promising approaches to mitigate the excessive CO2 [carbon dioxide] emission, due to its significant reduction of energy cost compared with many conventional separation techniques. Unfortunately, the separation performance of current membranes does not meet the practical CO2/N2 [nitrogen] separation requirements. And due to the huge volume of industrial flue gas, membranes with exceptionally high permeability are needed for practical reasons.

Currently, the separation mechanism of most polymeric membranes is based on size-sieving. However, this method is not sufficient for CO2/N2 separations due to the similar kinetic diameters of CO2 (3.30 Å [angstrom]) and N2 (3.64 Å). Thus, developing a new method based on a non-size sieving mechanism could offer a solution to the improvement of CO2/N2 separation efficiency.

In this dissertation, (bicycloheptenyl) ethyl terminated polydimethylsiloxane (PDMSPNB) membranes were firstly studied. The developed polymer membranes show higher permeability and better selectivity than those of conventional PDMS [polydimethylsiloxane] membrane. The achieved performance (CO2 [CO2 permeability] 6800 Barrer and α[CO2/N2] [CO2/N2 selectivity] 14) is very promising for practical applications. The key to achieving this high performance is the use of an in-situ cross-linking method of the difunctional PDMS macromonomers, which provides lightly crosslinked membranes. By combining positron annihilation lifetime spectroscopy, broadband dielectric spectroscopy and gas sorption measurements, we have elucidated the key parameters for achieving their excellent performance.

In the following work, two CO2-philic groups, amidoxime (AO) and polyethylene oxide (PEO), were successfully incorporated into the rubbery PDMS and glassy poly(1- trimethylsilyl-1-propyne) (PTMSP) systems. By the careful tuning of functional group composition, the membranes showed controlled CO2/N2 solubility selectivity. The combination of CO2-philic groups and highly permeable polymer matrix showed one of the highest CO2/N2 separation performance among all polymeric membranes. The overall gas separation performance (PCO2 6800 Barrer and α[CO2/N2] 19 for AO-PDMSPNB; PCO2 3400 Barrer and α[CO2/N2] 19 for PEO-PDMSPNB; PCO2 6000 Barrer and α[CO2/N2] 17 for AO-PTMSP) of the highest performing membranes have exceeded/achieved the Robeson upper bound line. These studies provide a roadmap to enhancing gas separation performance in rubbery and glassy polymers by tuning gas solubility selectivity.

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