Superacidity in Sulfated Metal–Organic Framework-808

Superacids, defined as acids with a Hammett acidity function H0 ≤ −12, are useful materials, but a need exists for new, designable solid state systems. Here, we report superacidity in a sulfated metal–organic framework (MOF) 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-808-2.5SO4 [Zr6O5­(OH)3­(BTC)2­(SO4)2.5(H2O)2.5]. This material has a Hammett acidity function H0 ≤ −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.