Synthesis of layered perovskite oxides, ACa_2-xLa_xNb_3-xTi_xO₁₀ (A = K, Rb, Cs), and characterization of new solid acids, HCa_2-xLa_xNb_3-xTi_xO₁₀ (0 < x ≤ 2) exhibiting variable bronsted acidity

Abstract

Layered perovskite oxides of the formula ACa_2-xLa_xNb_3-xTi_xO₁₀ (A = K, Rb, Cs and 0 < x ≤ 2) have been prepared. The members adopt the structures of the parent ACa₂Nb₃O₁₀. Interlayer alkali cations in the niobium-titanium oxide series can be ion-exchanged with Li⁺, Na⁺, NG₄⁺, of H⁺ to give new derivatives. Intercalation of the protonated derivatives with organic bases reveals that the Bronsted acidity of the solid solution series, HCa[sub 2-x]La[sub x]Nb[sub 3-x]Ti[sub x]O[sub 10], depends on the titanium content. While the x = 1 member (HCaLaNb₂TiO₁₀) is nearly as acidic as the parent HCa₂Nb₃O₁₀, the x = 2 member (HLA₂NbTi₂O₁₀) is a weak acid hardly intercalating organic bases with pK_a ~ 11.3. The variation of acidity is probably due to an ordering of Nb/Ti atoms in the triple octahedral perovskite slabs, [Ca_2-xLa_xNb_3-xTi_xO₁₀], such that protons are attached to NbO₆ octahedra in the x = 1 member and to TiO₆ octahedra in the x = 2 member.