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 La 3+ -to-Y 3+ substitution to a Lewis acid α-YB 5 O 9 generate medium-strength basic sites. Experimentally, NH 3 -and CO 2 -TPD experiments prove that the La 3+ doping of α-Y 1−x La x B 5 O 9 (0 ≤ x ≤ 0.24) results in the emergence of new medium-strength basic sites and the increasing La 3+ 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-YB 5 O 9 to ∼90% of acetone for α-Y 0.76 La 0.24 B 5 O 9 , which means the La 3+ doping gradually turns the solid from a Lewis acid to a Lewis base. Besides, α-Y 0.76 RE 0.24 B 5 O 9 (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.