Structural Features of Lanthanum Aluminum Mixed Oxides and Stability of Their Catalytic Properties in Oxidative Coupling of Methane

Abstract

The catalytic properties in the oxidative coupling of methane (OCM) and the structural features of a series of lanthanum aluminum mixed oxides with a constant La : Al atomic ratio of 1 : 1 were studied. Samples were prepared from precursors containing lanthanum and aluminum nitrates and an organic component: filter paper or starch. After drying and burning of the organic component, the samples were subjected to additional heat treatment and treatment with water fluid (WF) with a density of 0.2 g/cm³ at 415°C. After calcination at 900°C, all the samples contained a phase of lanthanum aluminate LaAlO₃ with a cubic perovskite structure. Varying the type of organic component and the sequence of procedures and treatment conditions gave samples with different morphologies and catalytic properties. The minimum activity, selectivity for OCM products, and stability over time was demonstrated by the sample obtained under conditions conducive to the formation of the most ordered structure, namely, the sample sequentially calcined at 900°C, treated in a WF medium, and recalcined at 900°C. The most efficient and stable sample turned out to be the one whose crystal structure was formed mainly in the WF medium. It was noted that there is no correlation between the morphology of the particles and the specific surface area of the samples, on the one hand, and their catalytic properties, on the other. It was assumed that the catalytic properties are determined by the type and number of point defects in the crystal structure—primarily cation vacancies and, as a consequence, hole sites of the [O^–] type in the anion sublattice.