Creation of Controlled Brønsted Acidity on a Zeotypic Mesoporous Chromium(III) Carboxylate by Grafting Water and Alcohol Molecules

This paper reports a new way of modulating the strength of the Brønsted acid sites of a porous solid. Our approach consists of the grafting of various polar molecules (H2O, CD3OH, CF3CH2OH, and (CF3)2CHOH) on MIL-100(Cr), a mesoporous crystallized chromium carboxylate with accessible unsaturated metal sites. We demonstrate, using infrared spectroscopy, that all these compounds do coordinate on the coordinatively unsaturated Cr3+ sites. This generates controlled Brønsted acid sites whose strength, measured by CO adsorption at 100 K, depends on the nature of the adsorbate. It can vary from H0 = −3 (CD3OH) to −8 ((CF3)2CHOH).