CO₂ facilitated transport membranes for hydrogen purification and flue gas carbon capture

Membrane technology is a promising alternative to aqueous amine scrubbing process for CO₂ separation and capture, because of its superior energy efficiency and cost effectiveness. CO₂ facilitated transport membrane, based on the reversible reaction between amine carriers and CO₂, is a good candidate membrane to achieve both high permeability and high selectivity. In this research, facilitated transport membranes with controlled compositions were synthesized and studied for H₂ purification for fuel cell applications and CO₂/N₂ separation for flue gas carbon capture.

In Chapter 2, the water vapor transport mechanism of CO₂ facilitated transport membranes is elucidated. In Chapter 3, various approaches are discussed for improving membrane performance at a high temperature (120°C) with a low water vapor partial pressure (~0.54 atm) on the feed side. The best CO₂/H₂ separation performance (CO₂ permeance of 146 GPU and CO₂/H₂ selectivity of 107) was obtained on the membrane with the incorporation of a fluoride-containing polyelectrolyte and a quaternaryammonium hydroxide into an amine-containing membrane, which was about 80% higher than the performance of crosslinked poly(vinyl alcohol) based membranes containing amine carriers. Chapter 4 mainly discusses the CO₂/N₂ separation performance of different types of amine-containing membranes for flue gas carbon capture. In Chapter 5, steric hindrance effect is demonstrated on polyvinylamine-based membranes. Poly(N-methyl-N-vinylamine) membranes showed almost 4 times of the performances of unhindered polyvinylamine membranes at 102°C with 80% water vapor content on the feed side. Chapter 6 addresses the oxidative stability of two types of facilitated transport membranes, including the amine-containing membranes and hydroxide- and fluoride-containing membranes.

In conclusions, the water vapor transport property and oxidative stability studied in this work were rarely reported in the literature, yet important to industrial applications of CO₂ facilitated transport membranes. Different membrane compositions were developed to improve membrane performance at high temperature and low water partial pressure. The membranes containing quaternaryammonium hydroxide and amine carriers at appropriate amounts demonstrated an outstanding membrane performance, which indicated the excellent synergy of amine-catalyzed CO₂-hydroxide reaction in CO₂ facilitated transport. In addition, sterically hindered polyvinylamines were synthesized and studied, indicating that poly(N-methyl-N-vinylamine) could be the next-generation fixed-site carrier for CO₂ facilitated transport.