The Effect of the Ln Type on the Structure and Catalytic Properties of Ln Chromites

Abstract

The influence of the lanthanide cation type on the structure and catalytic properties of both individual and high-entropy (HE) Ln chromites (Ln = La – Yb) was studied. Ln chromites crystallized in a distorted perovskite structure with the orthorhombic symmetry (sp. gr. Pbnm). The distortion of the perovskite structure increases with a decrease in \({Ln}^{3+}\) cation radius. In HE Ln chromites \({Ln}^{3+}\) cations retain certain independence in behavior despite occupying the same crystallographic site in the structure. The use of Ln chromites significantly reduces the conversion onset temperature and increases the percentage of its conversion. It was found that the ionic radius and electron structure of the \({Ln}^{3+}\) cation determine the acidity of the active sites and the differential heat of adsorption at zero surface coverage, which are the main factors determining the adsorption capacity and pathways of the propane conversion process. The use of light Ln chromites with the maximum concentration of Lewis acid sites promotes the propane dehydrogenation reaction with the formation of propylene. A decrease in the number of Lewis acid sites and an increase in the number of Brønsted acid sites as the 4f shell of the \({Ln}^{3+}\) cations is filled promotes the propane cracking reaction with the formation of a mixture of methane and ethylene. A characteristic feature of the catalytic behavior of HE Ln chromites is the formation of butadiene in the conversion products, which is attributed to the increased acidity of the active sites and the differential heat of adsorption at zero surface coverage.