The preparation of thermally stable washcoat aluminas from low cost gibbsite

Study of the formation and decomposition of aluminium sulphates has shown that low cost gibbsite, can readily be converted to thermally stable washcoat alumina through the intermediate formation of the sulphate. Optimal reaction between hydroxidic aluminium starting materials and sulphuric acid occurred in the presence of water. A protective sulphate layer was formed on the surface of gibbsite on reaction with concentrated sulphuric acid which limited conversion. Higher conversion could be achieved by reaction with diluted acid. Conversion of the resultant aluminium sulphate to alumina was essentially complete on calcination at about 1000°C for 4 hours. No differences in the product were observed on changing the starting material to pseudoboehmite. The alumina produced from both starting materials was thermally stable with a surface area of ca 120-130m²g^-1. The surface area dropped to ca 12-13 m²g^-1 on heating to 1200°C for 4 hours. Addition of 2% La₂O₃ or 4% BaO was found to stabilise the alumina, and samples were produced with surface areas of 70-100m²g^-1 after heating at 1200°C for 4 hours. It was found to be necessary to grind the material to obtain a solution with solids loading suitable for washcoat preparation. Up to 67wt% suspension could be achieved at some cost in thermal stability.