GB825414A - Preparation of an alumina hydrate composition - Google Patents

Abstract

Alumina hydrate containing trihydrate and amorphous or monohydrate forms, particularly useful as a catalyst base, is prepared by treating sodium aluminate solution with a water-soluble compound acid reacting therewith to reduce the pH to a value not below 10.5 thereby precipitating 10 to 90% of the alumina as trihydrate and thereafter mixing the slurry with a water-soluble acid reacting compound to reduce the pH to below 10, thereby precipitating monohydrate or amorphous hydrous alumina. The trihydrate precipitation is preferably effected at a pH of 11.5 and after the addition of the acid acting compound the pH is preferably allowed to rise to at least 11.6 as the trihydrate precipitates. Gibbsite or bayerite may be formed. The pH of the monohydrate precipitation is preferably less than 7. The trihydrate and the monohydrate precipitations are preferably effected at 20-50 DEG C. Suitable acid acting compounds for addition to the aluminate are the mineral acids such as sulphuric, hydrochloric and nitric acids, carbon dioxide, sulphur trioxide and hydrogen chloride. When gaseous these may be mixed with inert gases such as air or nitrogen. Aluminium salts such as the sulphate, chloride and nitrate may also be used and so may ammonium bicarbonate and bisulphate and sodium bisulphate. Salts of amines and ammonia such as ammonium acetate carbonate and sulphate and carboxylic acids such as formic, acetic and mono- and dichloro acetic acids may be similarly employed. When using carbon dioxide to produce the trihydrate and another acid-reacting material to provide the monohydrate, the final pH should be carried low enough to release the carbon dioxide from the aqueous solution, e.g. a pH of 5.5. The precipitated hydrate may be washed with or without previous drying and the washed hydrates may be dried at a 110-250 DEG C. to remove free water and calcined or activated at 400-700 DEG C., thereby removing the water of crystallization. When employed as a catalyst the alumina may be used in finely-divided form or in tablets up to one inch long. Chemically reactive components from Groups V-VIII of the Periodic Table, particularly vanadium, molybdenum, tungsten, chromium, cobalt, nickel and platinum or mixtures thereof may be deposited on or mixed with the catalyst base; in particular, from 0.1 to 1.5% by weight of platinum or 1% each of cobalt and molybdenum may be used. According to examples: (1) an aqueous sodium aluminate solution having a Na2O to Al2O3 ratio of 1.41 to 1 was reacted with sulphuric acid at 40 DEG C. to decrease the pH to 11.4. While continuing stirring, the temperature fell to 36 DEG C. and the pH rose to 12.1 at which point further sulphuric acid was added to bring the pH down to 5.3 after which the hydrate was filtered dried at 120 DEG C., washed with ammonium hydroxide solution at a pH of about 9 and finally dried at 120 DEG C.; (2) a similar sodium aluminate solution was treated with aluminium sulphate solution at 40 DEG C. with carbon dioxide to reduce the pH to 11.4. The pH then rose to 11.9 and aluminium sulphate was added to reduce the pH to 5.1. The filter cake contained 66% trihydrate and the remainder was boehmite and amorphous hydrate. Further examples relate to the use in the first addition period of the acid-reacting material of carbon dioxide, aluminium sulphate, ammonium acetate and sulphuric acid and, for the second addition of acid-acting material, of aluminium sulphate. (X) A desulphurization catalyst containing cobalt and molybdenum was prepared by treating the final alumina hydrate of Example I above with cobalt carbonate and molybdenum trioxide as an aqueous slurry which was heated to 71 DEG C. The filter cake after removal of mother liquor was dried at 111 DEG C., mixed with a die lubricant and tableted and then calcined in air at 482 DEG C. (XI) A platinum-alumina catalyst is prepared by slurrying the alumina hydrate prepared by Example II above, and adding chloroplatinic acid solution and thereafter adding a solution of hydrogen sulphide in water. The resulting slurry was dried at 110 DEG C. ground and pressed into tablets. The tablets were burned at 482 DEG C. in an atmosphere containing initially 5 parts of air to 300 parts of nitrogen and finally in pure air.