Acidic Strengths of Brønsted and Lewis Acid Sites in Solid Acids Scaled by 31P NMR Chemical Shifts of Adsorbed Trimethylphosphine

Abstract

The validity of using 31P NMR of adsorbed trimethylphosphine (TMP) as a probe molecule for discerning the types (Brønsted vs Lewis) and strengths of acid sites in solid acid catalysts have been studied by density functional theory (DFT) calculations. Brønsted acid sites with varied acidic strengths covering from weak, strong, to superacid, mimicked by 8T zeolite cluster models having different Si−H bond lengths and hence proton affinities, were examined together with Lewis acid systems having different metallic centers, e.g., BClnF3−n (n = 0−3), AlClnF3−n (n = 0−3), and TiClnF4−n (n = 0−4) and their mixed halides. The theoretical 31P chemical shifts predicted for the hydrogen-bonded TMP···H complex and the TMPH+ adducts were −61 ± 1 and −3 ± 1 ppm, respectively, in good agreement with the experimental data. For the TMP−Lewis acid complex, a linear correlation between the calculated 31P chemical shifts and corresponding binding energies was observed for the B-, Al-, and Ti-containing Lewis acids, respectively, indicating the feasibility of using the 31P chemical shift of adsorbed TMP as a scale for Lewis acidic strength.