Electrochemically Modulated Separation of Olefin-Paraffin Gas Mixtures in Membrane Electrode Assemblies

Abstract

Olefin-Paraffin separations are ubiquitous chemical processes used in purifying monomers for the manufacture of a large variety of polymers and chemical intermediates. Some of the highly demanded olefin-made products include polyethylene, ethylene oxide, vinyl acetate, polypropylene, acrylonitrile, propylene oxide, and acrylic acid. Olefin-paraffin mixtures are relatively difficult to separate because of the similarity in physical properties of each component. Currently, olefins are typically purified via cryogenic distillations, a highly energy-intensive process that liquifies the hydrocarbon gas mixture and re-vaporizes them repeatedly, requiring approximately 3.6 MJ per kg of olefin purified. Because of the large energy-intensive of the process, olefin separations are responsible for approximately 0.3% of the global energy consumption, to produce 200 and 130 million metric tons per year of ethylene and propylene respectively (2020).Alternative non-thermal olefin-paraffin separation methods have the potential to increase the efficiency of this separation, and thus increase its sustainability. In this presentation we will discuss an alternative separation method based on an electrochemically modulated swing absorption system. In this system, Nickel(II) maleonitriledithiolate, an electrochemically active organometallic complex, is dispersed in an ionogel and integrated in a membrane electrode assembly (MEA). The oxidation state of Nickel(II) maleonitriledithiolate can be electrochemically modulated based on potential. At its oxidized state [i.e., Ni (IV)], the complex has a strong affinity towards the olefin’s pi-bond, while at its reduced state [i.e., Ni (II)] the complex loses this affinity. Our results show that when exposed to propylene-propane gas mixtures, propylene was selectively captured by the organometallic species during oxidative phase, and then released or decoupled during reductive phase. Its paraffin counterpart, propane, behaved as spectator molecules throughout the whole process, demonstrating consistent olefin separation through this selective capture-and-release mechanism. I will discuss how varying the composition of the MEA exposed potential mass transport limitations that play an important role in the separation efficiency and throughput. Experiments conducted under varying potentials exhibited operational robustness of the MEA in gas separations if electric potentials are carefully tuned between 1-3V in the oxidative cycle, and at approximately -2V in the reductive cycle. This proof-of-concept demonstration represents an important step towards achieving high-performing electrochemically-modulated olefin-paraffin separations that can integrate renewable electricity sources directly and help decarbonize the chemical manufacturing industry. Figure 1