Sulfation of metal–organic frameworks: Opportunities for acid catalysis and proton conductivity

A new post-functionalization method for metal–organic frameworks (MOFs) has been developed to introduce acidity for catalysis. Upon treatment with a mixture of triflic anhydride and sulfuric acid, chemically stable MOF structures MIL-101(Cr) and MIL-53(Al) can be sulfated, resulting in a Brønsted sulfoxy acid group attached to up to 50% of the aromatic terephthalate linkers of the structure. The sulfated samples have been extensively characterized by solid-state NMR, XANES, and FTIR spectroscopy. The functionalized acidic frameworks show catalytic activity similar to that of acidic polymers like Nafion® display in the esterification of n-butanol with acetic acid (TOF ∼ 1 min−1 @ 343 K). Water adsorbs strongly up to 4 molecules per sulfoxy acid group, and an additional 2 molecules are taken up at lower temperatures in the 1-D pore channels of S-MIL-53(Al). The high water content and Brønsted acidity provide the structure S-MIL-53(Al) a high proton conductivity up to moderate temperatures.

A new post-functionalization method to incorporate sulfoxy acid moieties in different MOFs has been developed. The sulfated frameworks show outstanding catalytic activity in esterification and a high proton conductivity.Figure optionsDownload high-quality image (43 K)Download as PowerPoint slideHighlights
► A new post-functionalization method for MOFs has been developed.
► The sulfated frameworks show outstanding catalytic activity in esterification.
► The sulfated MIL-53(Al) shows a high proton conductivity.