Interlayer surface modification of the protonated ion-exchangeable layered perovskite HLaNb₂O₇·xH₂O with organophosphonic acids

The interlayer surface of a protonated form of the Dion-Jacobson-type ion-exchangeable layered perovskite, HLaNb₂O₇· xH₂O (HLaNb), has been successfully modified with various organophosphonic acids [phenylphosphonic acid (PhPO(OH)₂, PPA) and n -alkylphosphonic acids (n-C_nH_2n+1-PO(OH)₂ with n = 4-18, APAs)] to produce graft-type organic derivatives using an n-decoxy derivative of HLaNb (C₁₀O-HLaNb) as an intermediate. The interlayer distances of the products are changed from that of the intermediate, 2.73 nm, to 2.31 (PPA/C₁₀O-HLaNb) and 2.31-5.26 (APAs/C₁₀O-HLaNb) nm. IR and solid-state ¹³C CP/MAS NMR spectra of the products reveal that n-decoxy groups are removed and phenyl (PPA/C₁₀O-HLaNb) or n-alkyl groups (APA/C₁₀O-HLaNb) are introduced. Elemental analysis reveals that the amounts of PPA- and APA-moieties are 0.88-0.99 per [LaNb ₂O₇], corresponding approximately to the amount of the n-decoxy groups in C₁₀O-HLaNb. The environment of interlayer species in PPA/C₁₀O-HLaNb is assumed to be monodentate PhPO(OH)(ONb) based on the IR results (the P-O stretching and P-OH stretching bands at ∼1030 and ∼950 cm^-1) and the reaction between PPA/C₁₀O-HLaNb and n-butylamine (-NH₂/POH = 1.0). Scanning electron micrographs of the products reveal that the morphology is clearly preserved during the reactions with PPA or APAs, indicating that they are graft-type rather than dissolution-recrystallization-type reactions. Because water is required for the reaction between PPA and C₁₀O-HLaNb, this reaction is assumed to proceed via the formation of an (HO)NbO₅ site and its subsequent reaction with PPA.Alinear relationship is clearly observed between the number of carbon atoms in the n-alkyl chains and the interlayer distances of APAs/C ₁₀O-HLaNb, and a structural model of APAs/C₁₀O-HLaNb with a n-alkyl chain tilt angle of 57° is proposed.