Interlayer surface modification of the protonated ion-exchangeable layered perovskite HLaNb₂O₇•<i>x</i>H₂O with organophosphonic acids

Abstract

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-CnH2n+1PO(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 [LaNb2O7], 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. A linear relationship is clearly observed between the number of carbon atoms in the n-alkyl chains and the interlayer distances of APAs/C10O-HLaNb, and a structural model of APAs/C10O-HLaNb with a n-alkyl chain tilt angle of 57° is proposed.