HYDROGEN SEPARATION AND CARBON CAPTURE BY CARBON MOLECULAR SIEVE MEMBRANES DERIVED FROM INTERFACIALLY POLYMERIZED POLYARAMIDS

Abstract

Due to its high energy density and zero-emission combustion, hydrogen (H2) has emerged as a clean fuel for energy generation and transportation. Also, H2 is an important chemical used in petrochemical refining, metal production, and fertilizer manufacture. In the United States, more than 10 million metric tons of H2 is produced each year by steam methane reforming, which gives 100 million metric tons of carbon dioxide (CO2) by-product. Downstream H2/CO2 separation is therefore needed to produce high-purity H2 product while simultaneously capturing the CO2 by-product. State-of-the-art separation technologies such as pressure-swing adsorption (PSA) and amine absorption are energy intensive with large footprints. Membrane-based H2/CO2 separation provides an energy-efficient alternative with smaller footprints. Commercial implementation of membrane-based H2/CO2 separation requires scalable membranes with high H2/CO2 selectivity to produce high-purity H2 product.The overarching goal of this PhD dissertation is to understand the formation and pore structure-transport property relationships in novel carbon molecular sieve (CMS) membranes derived from interfacially polymerized aromatic polyamides (polyaramids) for H2/CO2 separation. Polyaramid precursor hollow fiber membranes were fabricated by solution spinning of an uncrosslinked polyaramid precursor synthesized by stirred interfacial polymerization, which gave polyaramid-derived CMS membranes following pyrolysis. The formation, pore structure, and transport properties of the novel polyaramid-derived CMS membranes were systematically investigated. The polyaramid-derived CMS membrane pyrolyzed at 925 °C showed unprecedented H2/CO2 separation performance under single-gas permeation. Further increasing the pyrolysis temperature to 1050 °C dramatically enhanced the mixed-gas H2/CO2 separation factor to more than one order of magnitude higher than the most selective CMS membrane reported in literature. Modeling further demonstrates the attractiveness of the polyaramid-derived CMS membrane for enrichment of highly-pure H2 from the reaction product of steam methane reforming. Finally, the effect of precursor amide moiety on CMS membrane pore structure and transport properties was studied by comparing the polyaramid-derived CMS membrane with CMS membranes derived from a polyimide precursor and a polyamide-imide copolymer precursor under identical pyrolysis conditions. The results show that introducing precursor amide moiety is a powerful tool to tailor the H2/CO2 transport properties of CMS membranes via controlling the precursor hydrogen bonding and CMS membrane pore structure.