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

Abstract

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 α-YB₅O₉ generate medium-strength basic sites. Experimentally, NH₃- and CO₂-TPD experiments prove that the La³⁺ doping of α-Y_1-xLa_xB₅O₉ (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 α-YB₅O₉ to ∼90% of acetone for α-Y_0.76La_0.24B₅O₉, which means the La³⁺ doping gradually turns the solid from a Lewis acid to a Lewis base. Besides, α-Y_0.76RE_0.24B₅O₉ (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.