GB680610A - Improvements in and relating to the production of silica-alumina cracking catalyst - Google Patents

Abstract

A silica-alumina cracking catalyst is prepared by producing under acid conditions a concentrated silica sol containing at least 85 grams SiO2 per litre and having a final pH between 2.8 and 4.1, aging the sol for a time which is at least 0.5 but less than 1 times the time of set of the sol, preventing the sol setting to a gel by diluting it with water at a rate so as to maintain the viscosity constant, increasing the pH to 7 by the addition of an alkali while preventing setting of the sol, adding a concentrated solution of an aluminium salt (such as a 1 molar solution of the sulphate) to provide between 18 per cent and 30 per cent Al2O3 in the finished catalyst, hydrolysing the aluminium salt by the addition of a basic precipitant and washing and drying the product. The above procedure is applicable at 20 DEG C. and deviations from this temperature may be adjusted by heating or refrigeration. The initial silica sol preferably contains 90 to 125 grams of silica per litre and may be prepared by adding sodium silicate to an acid while mixing or simultaneously adding the silicate solution and an acid to an acid medium, the final pH being adjusted to between 2.8 and 4.1 by the addition of an alkali such as sodium silicate. The dilution of the sol with water is delayed until the sol has almost gelled, and may continue for 30 minutes to 2.5 hours, after which the pH is reduced to 6.0 to 7.0 when a further aging period may ensue. After the addition of the aluminium salt, and the hydrolysis, the sodium salts are removed by water washing optionally followed by washing with dilute hydrochloric, hydrofluoric or acetic acids. Alternatively aluminium chloride, sulphate or nitrate or ammonium chloride, sulphate or nitrate or acetate solutions may be employed. Drying may be effected by passing the material in a thin layer through an oven at 80 DEG -130 DEG C. in a stream of air or alternatively the slurry may be passed through a colloid mill and spray dried to form microspheres which may be further calcined.