The separation and purification of chemicals remains a significant challenge. On a global scale, processes used in chemical separations account for 10-15% of the world’s total energy consumption. In the United States alone, improvements in separation technology in the petroleum, chemical, and paper manufacturing sectors could save up to 100 million metric tons of CO2 emissions per year and $4 billion dollars in annual energy costs. Despite the obvious need to develop alternative processes and materials aimed at improving energy efficiency and lowering the overall carbon footprint for industrial separation processes, the development of alternative, commercially viable routes have shown limited success. In a new study published by an international team of scientists in the journal Angewandte Chemie International Edition and highlighted on the cover page, the researchers show that taking a new approach to the development of chemical adsorbents may pave the way to novel materials that can compete with traditional industrial distillation processes. The study, completed in collaboration with beamline scientists at the U.S. Department of Energy’s Advanced Photon Source (APS) at Argonne National Laboratory, describes the structure and reactivity of two new copper(I) complexes that exhibit a unique combination of low heat adsorption, high selectivity, good uptake capacity, and rapid kinetics for the separation of gaseous alkenes (i.e., ethene and propene) from alkanes. These findings have exciting implications for the development of new adsorbent materials that can significantly improve the energy efficiency and carbon footprint of industrial-scale alkene purification processes.
Ethene and propene are vitally important to the global economy. Both are among the top five most produced chemicals in the world―combined annual production of around 230 million tons―with primary applications focused on the production of polymer and chemical products. However, 75% of alkene production costs come from the capital and energy-intensive cryogenic distillation process. Alternative separation techniques such as membrane, adsorption, molecular sieving, or hybrid processes all face road blocks for industrial-scale implementation. Specifically, traditional approaches to improve adsorbing materials (e.g., increased capacity, increased selectivity) for alkene purification require the tradeoff of desirable properties. For example, increasing the capacity of adsorbents by increasing surface area results in a decrease in selectivity.
To overcome these challenges, the researchers developed two new olefin-responsive copper(I) complexes: {[4-Br-3,5-(CF3)2Pz]Cu}3 ([Cu-Br]3) and {[3,5-(CF3)2Pz]Cu}3 ([Cu-H]3). The unique aspect of these materials, as opposed to traditional porous adsorbent materials, is that they undergo a reversible structural rearrangement upon exposure to ethene and propene, resulting in dimeric copper(I)-alkene analogues: [Cu-Br•(alkene)]2 and [Cu-H•(alkene)]2. The alkene is released from the complex upon removal of the alkene source, and the complexes convert back to their original trimeric structure. [Cu-H]3 is a highly attractive candidate for commercial applications because the complex undergoes ethene adsorption above 1 bar at near ambient temperature and rapid desorption when exposed to the atmosphere. These attributes would allow for operation conditions near atmospheric pressure and avoid complicated adsorption process designs.
Two of the most remarkable pieces of data demonstrating the unique reversibility of this material were collected at the high-energy x-ray diffraction beamline 17-BM operated by the X-ray Science Division, Structural Science Group (SRS) at the APS. The team of scientists from The University of Texas at Arlington, the University of Canterbury (New Zealand), and Massey University (New Zealand), together with colleagues from the SRS, utilized x-ray powder diffraction under high-pressure ethene flow to confirm stoichiometric ethene coordination by [Cu-H]3 and the formation and breaking of several bonds in the solid-state to form [Cu-H•(C2H4)]2 (Fig. 1). Based on crystal structures of the final product, the team were able to confirm that greater than 95% of the starting material [Cu-H]3 converted into the dimeric-alkene derivative within 2 minutes at 10 bar. Furthermore, complete removal of ethene under helium flow accompanied by recovery of the original trimeric starting material occurred within 1 hour with most of the transformation occurring within the first 15 minutes.
These promising results illustrate the potential to use this approach to develop new adsorbent materials capable of separating high-value chemical targets like alkenes from alkanes. Specifically, [Cu-H]3 is an attractive target for alkene-alkane separation due to its ability to rapidly adsorb ethene at near atmospheric conditions. Furthermore, [Cu-H]3 is air-stable and can be prepared using commercially available raw materials which will lower the barrier for commercial manufacturing. These new complexes represent the “tip of the iceberg” when it comes to the potential for this new approach to the development of adsorbent materials.
― Alicia Surrao
See: Devaborniny Parasar1, Ahmed H. Elashkar2, Andrey A. Yakovenko3, Naleen B. Jayaratna1, Brian L. Edwards1, Shane G. Telfer4, H. V. Rasika Dias1* and Matthew G. Cowan2**, “Overcoming Fundamental Limitations in Adsorbent Design: Alkene Adsorption by Non-porous Copper(I) Complexes,” Angew. Chem. Int. Ed. 59, 21001 (2020). DOI: 10.1002/anie.202010405
Author affiliations: 1The University of Texas at Arlington, 2University of Canterbury, 3Argonne National Laboratory, 4Massey University
Correspondence: *dias@uta.edu, **matthew.cowan@canterbury.ac.nz
M.G.C. thanks the Royal Society of New Zealand for awarding a Rutherford Fellowship to support this research (RFT-UOC1601-PD) and the Marsden Fund (MFP-19-UOC-072), and acknowledges the support of the MacDiarmid Institute. H.V.R.D. acknowledges the Robert A. Welch Foundation (Y-1289) for the funding of this research. A.H.E. thanks the New Zealand Ministry of Foreign Affairs and Trade for provision of a New Zealand Aid Scholarship. This research used the resources of the Advanced Photon Source, a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by the Argonne National Laboratory under contract no. DE-AC02-06CH11357.
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