Superacidity
in Sulfated Metal–Organic Framework-808
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Abstract
Superacids,
defined as acids with a Hammett acidity function <i>H</i><sub>0</sub> ≤ −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:
Zr<sub>6</sub>O<sub>5</sub>(OH)<sub>3</sub>(BTC)<sub>2</sub>(HCOO)<sub>5</sub>(H<sub>2</sub>O)<sub>2</sub>, BTC = 1,3,5-benzenetricarboxylate]
with aqueous sulfuric acid to generate its sulfated analogue, MOF-808-2.5SO<sub>4</sub> [Zr<sub>6</sub>O<sub>5</sub>(OH)<sub>3</sub>(BTC)<sub>2</sub>(SO<sub>4</sub>)<sub>2.5</sub>(H<sub>2</sub>O)<sub>2.5</sub>]. This material has a Hammett acidity function <i>H</i><sub>0</sub> ≤ −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
