EAGER: LbL Polymer Thin Films for Reaction-Assisted Acid Gas Removal

Texas A&M Engineering Experiment Station, College Station TX

Investigators

Abstract

1202447-Wilhite Single composite membrane for reaction-assisted separations. Hydrogen, which can be produced via catalytic reforming of virtually any hydrocarbon resource, has emerged as a promising global energy currency. However, no elegant solution for mitigating the undesired by-products of hydrogen production (e.g., CO, CO2) currently exists- thus, presenting a grand challenge. We propose to simultaneously purify H2, destroy CO and isolate CO2 using a single composite membrane comprised of a layer-by-layer (LbL) assembled polymeric thin film for selective removal of CO2 and an inorganic catalytic membrane for converting CO to CO2 via water-gas-shift reaction. The resulting composite catalytic-permselective membranes represent a unique and transformative approach to hydrogen purification by integrating layer-by-layer assembly techniques for constructing permselective polymer films with washcoating methods to construct catalytic films to achieve reaction-assisted gas separations in a single composite membrane. Proton-exchange membrane fuel cells (PEMFCs), which use hydrogen as a fuel, are a leading candidate for next-generation power systems, owing to their durability, portability and/or scalability. However, by-products from hydrogen production such as CO can poison the PEMFC and dramatically limit lifetime and performance; CO2, another by-product, dilutes the hydrogen stream and must be removed prior to endpoint usage. Current strategies for reducing CO-levels involve coupling of the equilibrium-limited water-gas-shift catalysts (WGS) with palladium-based hydrogen-permselective membranes. Because of palladium's cost and low hydrogen permeability, replacing palladium with alternative materials is viewed as a grand challenge in realizing cost-effective high-purity hydrogen. In the proposed work the LbL membranes may compete with or even surpass palladium. Recent work in reverseselective polymeric membranes indicates that select polymers (i.e., poly(ethylene oxide) (PEO) and poly(allylamine) (PAH)) are very cost-effective at separating CO2 from H2. The proposed LbL thin films containing PAH, coupled with WGS catalyst to destroy CO, may potentially produce high-purity, high-pressure hydrogen from hydrogen reformate streams containing undesired by-products at low cost. This exploratory grant will explore the use of LbL membranes for permselective gas separation, their compatibility with reforming chemistries and with catalytic thin-film deposition techniques, with the ultimate goal of demonstrating a prototype composite catalytic-permselective membrane capable of permselective CO2 removal from reformate mixtures at typical (100 - 180C) reaction temperatures. The proposed research is high-risk, as all three central hypotheses are untested to-date. Gas transfer in LbL assemblies is relatively unexplored, partly because there is little crossover in the fields of LbL assembly and gas separations. The compatibility of LbL deposition techniques with catalytic washcoating methods has not been explored in the literature to-date. Lastly, the durability and performance of LbL thin films have not been investigated under reaction environments or at elevated temperatures. Scientific results regarding each of these hypotheses are of substantial intellectual value to the separations community. Validation of the proposed coupling of catalytic and LbL polymeric films in a composite catalytic-permselective membrane will enable the rigorous development of an innovative approach to realizing low-cost, highly selective gas separation membranes. For the specific case of a water-gas-shift catalytic layer enhancing the permselectivity of a CO2-selective LbL film, recent theoretical predictions by Wilhite indicate that H2-CO permselectivities in excess of 250:1 (comparable to Pd films) may be achieved at roughly 1/100th the cost. By itself, this achievement could transform the field of hydrogen purification membranes. Planned outreach activities include mentoring of undergraduate researchers through the Department of Chemical Engineering's REU program and Engineering Scholars program. The PIs will also host an international research internship through the International Scholars Program at TAMU. Summer research opportunities will also be available to K-12 teachers through the College of Engineering's RET program.