GB983395A - A method of producing molecular sieve bodies - Google Patents

Abstract

A crystalline zeolitic molecular sieve is produced by shaping acturated kaolin-type clay into a preformed body which is reacted as part of an aqueous reactant system until crystals of zeolitic molecular sieve are produced in the body. The aqueous reactant system contains in the aggregate, water and oxide of aluminium, silicon and alkali metal in the proportions required to form crystals of the desired zeolitic molecular sieve. Reference is made to the production of synthetic zeolites X, Y, L, D, T, R, A. The kaolin-type clay is actuated by thermal treatment at a temperature between 550-850 DEG C., before or after shaping. Bodies may be shaped from mixtures of raw kaolin and at least 10% reactive kaolin and subsequently fired for at least one hour to make them reactive. The shaping and forming step may be accomplished by moulding, extruding, tumbling, drum-rolling, casting, slip-casting, disc-forming, belt-forming, prilling, tableting and briquetting. Certain materials may be combined with the kaolin-type substance prior to shaping. For example, in the production of silica rich zeolites, silica in the form of colloidal silica solution, silica gel, silicic acid and alkali metal silicates may be included in the mixture to be shaped. It is easier to incorporate the silica when small amounts of organic gelling or thickening agents are used in the mixture to be shaped. Resins of the water-soluble ethylene oxide type are especially useful, when these are added, a heating step at 200-700 DEG C. is conducted after shaping to wholly or partly carbonize the resin in the shaped body. Alumina over and above that supplied by the reactive kaolin can be added by incorporating aluminium trihydrate, activated alumina, gamma alumina or alkali metal aluminate in the kaolin material prior to shaping. Alkali metal hydroxide in amounts of at least 0.1 mol. of alkali metal oxide per mol. of silica may also be incorporated in the kaolin material prior to shaping. Incorporation of 1.30% of combustible diluents in the raw kaolin-type materials improves the purity of the zeolitic body and forms macroporis. Diluents which can be used are sawdust, powdered carbon, lamp black, powdered coke, methyl or ethyl cellulose or other organic resins, wood flour and starch. After the forming or shaping step the bodies are dried so that the moisture content is less than 30 weight per cent. Digestion and crystallization of the shaped body may now be accomplished by treating the reaction system in a single step or in two separate steps. The first step takes place at a temperature of from room temperature to 55 DEG C. for 3-80 hours. In the second step, which need not of necessity employ a reaction mixture of the same composition as that used in the first step, the reaction temperature is preferably maintained between 75-175 DEG C. for 1-40 hours until crystals of the desired product form in situ. The aqueous reactant system, made up of the ambient liquid and the shaped body have on an aggregate an initial overall composition defined in terms of oxide-mol. ratios as follows: M2O/SiO2 = a SiO2/Al2O3 = b H2O/M2O = c. The ambient liquor contains proportions of silica and alumina, added in the form of materials specified above, and alkali metal oxide, in amounts needed to achieve the overall initial reactant composition, e.g. for sodium zeolite if M = Na a = 0.5-1.5 b = 1.0-2.5 c = 10-100. In the preparation of silica rich zeolite shapes having an SiO2/Al2O3 ratio greater than 3, e.g. sodium zeolite 7, the product purity can be improved by first contacting reactive kaolin powder with 0.1-5 mols. per mol. of alumina of a suitable aqueous reactant, e.g. a water-soluble alkali metal salt, in a predigestive step. In another aspect of the invention, high purity zeolite bodies are produced from a mixture of finely-divided kaolin-type material, either raw or reactive, and fine crystals of a pure zeolite. In determining the quantity of reactant required to provide the proper values of a, b and c (as defined above), the composition of the added zeolite is not included in the determination. The crystalline zeolite does not need to be the same as the final product, unless alkali metal hydroxides are used to form the shaped objects. The amount of zeolite incorporated in the to be shaped is from 30-80 weight per cent, unless an alkali metal hydroxide is incorporated when the preferred percentage is 5-80 weight per cent. On firing to produce the reactive kaolin, the firing temperature must be adjusted to the thermal stability limit of the particular zeolite. After crystallization, the zeolite bodies are filtered and washed until the effluent wash-water has a pH of between 9-12. Thereafter, the bodies are dried at a temperature between 25-150 DEG C. The zeolite is characterized by X-ray diffraction or adsorption measurements. Specifications 777,232, 777,233, 841,812, 868,846, 909,264, 909,266, 912,936 and 972,831 are referred to.