Superacidity in Sulfated Metal-Organic Framework-808

Superacids, defined as acids with a Hammett acidity function H-0 <=-12, are useful materials, but a need exists for new, designable solid state systems. Here, we report superacidity in a sulfated metal-organic framework (MOP) obtained by treating the microcrystalline form of MOF-808 [MOF-808-P: Zr6O5(OH)(3)-(BTC)(2)(HCOO)(5)(H2O)(2), BTC = 1,3,5-benzenetricarboxylate] with aqueous sulfuric acid to generate its sulfated analogue, MOF-808-2.5SO(4) [Zr6O5(OH)(3)(BTC)(2)-(SO4)(2.5)(H2O)(2.5)]. This material has a Hammett acidity function H-0 <= -14.5 and is thus identified as a superacid, providing the first evidence for superacidity in MOFs. The superacidity is attributed to the presence of zirconium-bound sulfate groups structurally characterized using single-crystal X-ray diffraction analysis.