Unexpected linker-dependent Brønsted acidity in the (Zr)UiO-66 metal organic framework and application to biomass valorization

Abstract

The catalytic activity of UiO-66 type Zr metal–organic frameworks can be tuned by the linker employed, e.g. nitro- or aminoterephthalic acid. Here we study composition–activity relations for the UiO-66 catalyzed dehydration of fructose into hydroxymethylfurfural, a carbon-neutral feedstock for chemicals and fuels. Variation of the substituents on the NO₂/NH₂ functionalized solids has a clear influence on the number of missing linkers around the Zr oxo clusters, on the hydrophobicity/hydrophilicity of the eventual metal–organic framework and on the Brønsted acidity. The thermally modulated UiO-66-NH₂ is able to yield 25 molecules of HMF per defective Zr site after 1 h at 100 °C, maintaining its activity in 4 consecutive runs.