Multifunctionality in an Ion-Exchanged Porous Metal–Organic Framework

Abstract

Porous robust materials are typically the primary selection of several industrial processes. Many of these compounds are, however, not robust enough to be used as multifunctional materials. This is typically the case of Metal-Organic Frameworks (MOFs) which rarely combine several different excellent functionalities into the same material. In this report we describe the simple acid-base postsynthetic modification of isotypical porous rare-earth-phosphonate MOFs into a truly multifunctional system, maintaining the original porosity features: [Ln(H₃pptd)]·<i>xSolvent</i> [where Ln³⁺ = Y³⁺ (<b>1</b>) and (Y_0.95Eu_0.05)³⁺ (<b>1_Eu</b>)] are converted into [K₃Ln(pptd)]·<i>zSolvent</i> [where Ln³⁺ = Y³⁺ (<b>1K</b>) and (Y_0.95Eu_0.05)³⁺ (<b>1K_Eu</b>)] by immersing the powder of <b>1</b> and <b>1_Eu</b> into an ethanolic solution of KOH for 48 h. The K⁺-exchanged Eu³⁺-based material exhibits a considerable boost in CO₂ adsorption, capable of being reused for several consecutive cycles. It can further separate C₂H₂ from CO₂ from a complex ternary gas mixture composed of CH₄, CO₂, and C₂H₂. This high adsorption selectivity is, additionally, observed for other gaseous mixtures, such as C₃H₆ and C₃H₈, with all these results being supported by detailed theoretical calculations. The incorporation of K⁺ ions notably increases the electrical conductivity by 4 orders of magnitude in high relative humidity conditions. The conductivity is assumed to be predominantly protonic in nature, rendering this material as one of the best conducting MOFs reported to date.