Characterization of Surface Acidity of Carbonated Materials by IR-Sensitive Molecular Probes: Advantages of Using tert-Butyl Cyanide

International audienceBulk carbonates or carbonated surfaces are increasingly relevant materials applied to many base-catalyzed reactions. Various basic molecular probes were considered for the assessment of the surface acidity of a highly carbonated catalyst, namely a high surface area MgO prepared by sol gel and activated at 240 degrees C. Using such a low activation temperature results in the presence of large amounts of residual superficial carbonates that hinders analysis with common IR acidity probes. Compared with pyridine, acetonitrile, and its deuterated counterpart CD(3)CN, tert-butyl cyanide (noted TBC, also known as pivalonitrile) appeared as the most appropriate probe to assess surface acidity at room temperature on this material. TBC did not lead to any measurable dissociative adsorption and easily enabled the differentiation between Lewis and Bronsted (i.e., surface hydroxyls) acid sites on carbonated solids on which the characteristic pyridine adsorption region (1700-1200 cm(-1)) is completely obscured by the signal of carbonates. The interpretation of the TBC-based spectra was also much more straightforward than those based on acetonitrile and deuterated acetonitrile, which both led to significant probe dissociation on the surface of the sample. TBC therefore appears as a suitable acidity probe for carbonated materials, opening the way to facile characterization of carbonate-rich base catalysts activated at low temperatures