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

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; btdd2– = 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 N2O 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.