From Lewis Acid to Lewis Base by La³⁺-to-Y³⁺ Substitution in -YB₅O₉: Local Structure Modification Induced Lewis Basicity

Different from the common perspective of average structure, we propose that the locally elongated metal-oxygen bonds induced by La3+-to-Y3+ substitution to a Lewis acid alpha-YB5O9 generate medium-strength basic sites. Experimentally, NH3- and CO2-TPD experiments prove that the La3+ doping of alpha-Y1-xLaxB5O9 (0 <= x <= 0.24) results in the emergence of new medium-strength basic sites and the increasing La3+ concentration modifies the number, not the strength, of the acidic and basic sites. The catalytic IPA conversion exhibits a reversal of the product selectivity, i.e., from 93% of propylene for alpha-YB5O9 to similar to 90% of acetone for alpha-Y0.76La0.24B5O9, which means the La3+ doping gradually turns the solid from a Lewis acid to a Lewis base. Besides, alpha-Y0.76RE0.24B5O9 (RE = Ce, Eu, Gd, Tm) compounds were prepared to consolidate the above conjecture, where the acetone selectivity exhibits a linear dependence on the ionic radius (or electronegativity). This work suggests that the substitution-induced local structure change deserves more attention.