Steric stabilization of colloidal UiO-66 nanocrystals with oleylammonium octadecylphosphonate

We report the synthesis and characterization of octahedral UiO-66 nanocrystals (d = 17-25 nm) terminated with amine, oleate, and octadecylphosphonate ligands. Acetate capped UiO-66 nanocrystals were dispersed in toluene using oleic acid and oleylamine. Ligand exchange with octadecylphosphonic acid produces ammonium octadecylphosphonate terminated nanocrystals with coverages of 2.6-3.2 chains per nm² that stabilize colloidal dispersions in nonpolar solvents. Liquid phase ¹H and ³¹P nuclear magnetic resonance (NMR) spectra of the linkers and surface ligands display line shapes that are broadened by slow tumbling of the nanocrystals. Octadecylphosphonate functionalized MOFs have up to ∼30% carbon dioxide absorption capacities compared to bulk UiO-66 after correcting for the ligand mass. These results illustrate the intriguing perspective that MOF nanocrystals can be characterized and manipulated like a macromolecular complex and represent an important milestone in the nascent field of MOF surface science.