Novel layered & three-dimensional oxides of perovskite & related structures: synthesis & investigation of ion-exchange & intercalation

Abstract

In this chapter, we have described the synthesis of a new series of protonated layered perovskites of the formula, HCa???La?Nb???Ti?O??, from the corresponding potassium compounds by ion-exchange. The protonated compounds are isostructural with the parent HCa?Nb?O??, which has been reported in the literature to be a strong Brønsted acid. Investigation of the Brønsted acidity of HCa???La?Nb???Ti?O?? series for 0 < x < 2.0 by intercalation of a variety of organic bases has shown that the acidity varies in an interesting manner. The x = 1 member (HCaLaNb?TiO??) is nearly as acidic as HCa?Nb?O??, while the x = 2 member (HLa?NbTi?O??) is a weak Brønsted acid just being able to intercalate piperidine (pKa = 11.3). The protons attached to NbO? octahedra are expected to be more acidic than the protons attached to TiO? octahedra. Accordingly, the variation of the Brønsted acidity across the series is probably due to an ordering of niobium/titanium atoms in the triple octahedral perovskite slabs, [Ca???La?Nb???Ti?O??], which results in protons being attached to NbO? octahedra in the x = 1 member and to TiO? octahedra in the x = 2 member. The present work implies that substitution of tungsten/molybdenum for niobium in a similar series would produce phases of much higher Brønsted acidity.