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, HLaNb 2 O 7 · x H 2 O (HLaNb), has been successfully modified with various organophosphonic acids [phenylphosphonic acid (PhPO(OH) 2, PPA) and n -alkylphosphonic acids ( n -C n H 2 n +1 PO(OH) 2 with n = 4−18, APAs)] to produce graft-type organic derivatives using an n -decoxy derivative of HLaNb (C 10 O-HLaNb) as an intermediate. The interlayer distances of the products are changed from that of the intermediate, 2.73 nm, to 2.31 (PPA/C 10 O-HLaNb) and 2.31−5.26 (APAs/C 10 O-HLaNb) nm. IR and solid-state 13 C CP/MAS NMR spectra of the products reveal that n -decoxy groups are removed and phenyl (PPA/C 10 O-HLaNb) or n -alkyl groups (APA/C 10 O-HLaNb) are introduced. Elemental analysis reveals that the amounts of PPA- and APA-moieties are 0.88−0.99 per [LaNb 2 O 7 ], corresponding approximately to the amount of the n -decoxy groups in C 10 O-HLaNb. The environment of interlayer species in PPA/C 10 O-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/C 10 O-HLaNb and n -butylamine (−NH 2 /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 C 10 O-HLaNb, this reaction is assumed to proceed via the formation of an (HO)NbO 5 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/C 10 O-HLaNb, and a structural model of APAs/C 10 O-HLaNb with a n -alkyl chain tilt angle of 57° is proposed.