Influence of Terephthalic Acid Substituents on the Catalytic Activity of MIL-101(Cr) in Three Lewis Acid Catalyzed Reactions

Six isostructural MIL-101(Cr)-X (X: H, NO₂, SO₃H, Cl, CH₃, and NH₂) materials have been prepared directly by the reaction of Cr^III salts and the corresponding terephthalic acid or by postsynthetic treatments of preformed MIL-101(Cr) following reported procedures. The materials were characterized by using XRD (crystallinity and coincident diffraction pattern), isothermal N₂ adsorption (specific surface areas range from 2740 m² g⁻¹ for MIL-101(Cr)-H to 1250 m² g⁻¹ for MIL-101(Cr)-Cl), thermogravimetry (thermal stability up to 400 °C), and IR spectroscopy (detection of the corresponding substituents), and the results were all in agreement with the reported data for these materials. The MIL-101(Cr) materials were tested as heterogeneous catalysts for epoxide ring opening by methanol, benzaldehyde acetalization by methanol, and Prins coupling, observing a clear influence of the substituent that in general follows a linear relationship with the Hammett σ_meta constant of the substituent: the catalytic activity increases as the electron-withdrawing ability of the substituents increases. An up to three orders of magnitude enhancement in the presence of the NO₂ substituent was found for some of these reactions. The present study illustrates the versatility that metal–organic frameworks offer as heterogeneous catalysts that allow the design of actives sites with adequate properties tuned for each reaction.