Benchmarking Nitrous Oxide Adsorption and Activation in Metal-Organic Frameworks Bearing Coordinatively Unsaturated Metal Centers

Abstract

Anthropogenic emissions of N₂O, the third most abundant greenhouse gas after CO₂ and CH₄, are contributing to global climate change. Although metal-organic frameworks (MOFs) have been widely studied as adsorbents for CO₂ and CH₄, less effort has focused on the use of MOFs to remove N₂O from emission streams or from air. Further, N₂O activation would enable its use as an inexpensive oxidant for fine chemical synthesis. Herein, we identify features that contribute to strong binding and high uptake of N₂O at coorinatively unsaturated metal sites in the M₂Cl₂(btdd) (M= Mn, Co, Ni, Cu; btdd^2– = bis(1,2,3-triazolo[4,5-b],[4′,5′-i])dibenzo[1,4]dioxin) and M₂(dobdc) (M = Mg, Mn, Fe, Co, Ni, Cu, Zn; dobdc⁴⁻ = 2,5-dioxido-1,4-benzenedicarboxylate) series of MOFs. Combined experimental and computational studies suggest that N₂O adsorption at open-metal-sites is primarily based on electrostatic interactions, rather than π-backbonding, causing MOFs with more Lewis acidic metal centers to be superior N₂2O adsorbents. As a result, Mg₂(dobdc) demonstrates strong binding and record-setting N₂O uptake (8.75 mmol/g at 1 bar and 298 K). Using density functional theory (DFT) to characterize reactive intermediates and transition states, we demonstrate that N₂O activation to form a M(IV)-oxo species and N₂ is thermodynamically favorable in Mn₂(dobdc) and Fe₂(dobdc) but appears to be kinetically limited in Mn₂(dobdc). Our work lays a foundation for understanding N2O adsorption and activation in MOFs, paving the way for the design of promising next-generation materials for N₂O capture and utilization.