Overcoming Fundamental Limitations in Adsorbent Design: Alkene Adsorption by Non-porous Copper(I) Complexes

Abstract

Purifying alkenes from alkanes requires cryogenic distillation. This consumes energy equivalent to countries of ca. 5 million people. Replacing distillation with adsorption processes would significantly increase energy efficiency. Trade-offs between kinetics, selectivity, capacity, and heat of adsorption have prevented production of an optimal adsorbent. We report adsorbents that overcome these trade-offs. [Cu-Br]₃ and [Cu-H]₃ are air-stable trinuclear complexes that undergo reversible solid-state inter-molecular rearrangements to produce dinuclear [Cu-Br⋅(alkene)]₂ and [Cu-H⋅(alkene)]₂ . The reversible solid-state rearrangement, confirmed in situ using powder X-ray diffraction, allows adsorbent design trade-offs to be overcome, coupling low heat of adsorption (-10 to -17 kJ mol^-1_alkene ), high alkene:alkane selectivity (47; 29), and uptake capacity (>2.5 mol_alkene mol^-1_Cu3 ). Most remarkably, [Cu-H]₃ displays fast uptake and regenerates capacity within 10 minutes.

Keywords: X-ray diffraction; adsorption; alkenes; copper; olefin separation.