Activity of Brønsted Acid Sites in UiO-66 for Cyclohexanol Dehydration

Brønsted-acid sites are introduced via –OSO₃H groups on the coordinatively unsaturated ZrO₂ nodes of UiO-66 metal organic frameworks (MOFs). Such groups create strong Brønsted acidic sites that are active for cyclohexanol and ethanol dehydration (in liquid organic phase and gas phase, respectively). The intrinsic activity of Brønsted acid sites at nodes increased by increasing the concentration of sulfur, which is attributed to a shift from isolated µ₃ − OSO₃H to two groups [(µ₃ − OSO₃H)₂] interacting via hydrogen bonding. For cyclohexanol dehydration, the relatively low activation enthalpies and negative transition entropies point to an E2 elimination mechanism, similar to dehydration with MFI zeolites in organic solvents. Our results, show that the catalytic activity can be manipulated via the functionalization of zirconia nodes of the MOF framework.