GB2082157A - Gallium ion exchanged zeolites - Google Patents

Abstract

Active gallium-ion exchanged zeolite catalysts are made by washing conventional crystalline zeolites of high silica:alumina ratio with water or acid followed by the deionised water, calcining the water-washed product and directly refluxing the calcined zeolite with a solution of a gallium compound. The gallium ion exchanged zeolites are suitable catalysts for hydro-carbon conversion reactions.

Description

SPECIFICATION Improved method of preparing gallium exchanged zeolites The present invention relates to a method of preparing active ion-exchanged zeolite catalysts.

Catalyst compositions based on crystalline aluminosilicates, i.e. zeolites, having a high silica to alumina ratio, particularly crystalline aluminosilicates prepared using organic nitrogen cations, are known. Catalyst compositions of this type comprising a zeolite which has been exchanged with a metal such as gallium are described, for example, in our European Patent Application Nos. 78300773.5 and 78300774.3, and Belgian Patent No. 862051. In our copending British Application No.7930679, it is also disclosed that the activity of such catalyst compositions may be improved by subjecting the zeolites to a series of pre-treatments prior to exchange with gallium.

The presence of alkali metal ions in a zeolite usually reduces catalytic activity. Therefore treatments, such as exchange with ammonium ions followed by calcination, are required to remove alkali metal ions which are present in zeolites crystallised from an alkaline medium. However, zeolites produced using alkanol amines, particularly diethanolamine, contain low concentrations of alkali metal ions.

It has now been found that such improved activity of zeolites produced using alkanolamines may be achieved by a relatively simple process in which the initially crystallised, zeolite is washed with dilute acid and/or water, calcined and then directly exchanged with gallium ions.

Accordingly the present invention is an improved method of preparing a catalyst composition comprising a zeolite in which some or all of the cations have been exchanged for gallium ions, the zeolite having a high silica to alumina ratio and being prepared by crystallsation from an aqueous solution comprising a mixture of a source of silica, a source of alumina, a source of alkali metal and an alkanolamine, washing the crystallised zeolite with deionised water or with dilute acid followed by deionised water and calcining the water washed product at an elevated temperature, the improvement comprising refluxing the calcined product directly with a solution of a gallium compound to produce a gallium ion exchanged zeolite.

The zeolites having a high silica to alumina ratio are well known in the art. These are generally prepared by reacting in aqueous solution a mixture of a source of silica, a source of alumina, a source of alkali metal and an organic nitrogen-containing base in appropriate proportions. The zeolite is allowed to crystallise from the solution by maintaining the solution at an elevated temperature under autogenous pressure.

Suitable sources of silica include, for example, sodium silicate, silica hydrosol, silica gel, silica sol, and silicic acid. The preferred source of silica is an aqueous colloidal dispersion of silica particles. A suitably commercially available source of silica is LUDOX Colloidal Silica manufactured by Du Pont (LUDOX is a Registered Trade Mark).

Suitable sources of alumina include, for example, sodium aluminate, aluminium sulphate and alumina.

The preferred source of alumina is sodium aluminate prepared by dissolving alumina particles in excess sodium hydroxide solution.

Suitable sources of alkali metal include alkali metal hydroxides and alkali metal oxides. Preferably, the alkali metal is sodium.

It will be appreciated that each source of silica, alumina and alkali metal can be supplied by one or more initial reactants and then mixed together in any order. For example, a sodium silicate is a source of both sodium and silica.

The alkanolamine may be a mono- or di-alkanolamine such as mono-ethanolamine, di-ethanolamine, mono-propanolamine or di-propanolamine, or a tri-alkanolamine as described in European Patent Applications Nos. 78300773.5 and 78300774.3. The use of di-ethanolamine is particularly preferred.

The reaction conditions which affect the formation of the alumino-silicate may be, for example, a temperature in the range from 80 to 210"C, preferably from 160 to 190"C, and a pressure in the range from 70 to 400 psig, preferably from 100 to 250 psig. The mixture may be held under these conditions for a time not less than 4 hours, preferably from 3 to 15 days.

The source of silica, alumina and alkali metal, water and the alkanolamine may be mixed in quite wide proportions. Thus the ratio of the silica source to the alumina source may be in the range 10:1 to 150:1, preferably rom 20:1 to 100:1 based on the equivalent moles of silica and alumina in the respective sources.

The alkali metal source may be present in an amount from 0.01 to 10, preferably from 0.04 to 3 moles of alkali metal per mole equivalent of alumina. The alkanolamine e.g diethanolamine, may suitably be present in an amount from 0.02 to 50, preferably from 0.1 to 25 moles per mole of alumina. The amount of water is not critical to the performance of the invention.

The reaction is suitably carried out in a closed vessel capable of withstanding the elevated pressures generally employed during the process. Furthermore the reaction mixture may be agitated during the formation of the aluminosilicate.

The crystalline zeolite is suitably separated from the mother liquor by decantation. The crystals are then washed thoroughly with de-ionised water or with dilute acid followed by water and the water-washed product is dried, for example, by heating in air at temperatures up to 120"C. The dried zeolite is then calcined at an elevated temperature suitably between 500 and 600 C for at least 12 hours. The calcined zeolite is then subjected to a gallium exchange step. This step may be carried out by refluxing the calcined zeolite with a solution of gallium compound, e.g. gallium nitrate. The gallium exchanged zeolite mav be thereafter washed with de-ionised water.The water-washed gallium-exchanged zeolite is thereafter dried as previously described by heating in air at temperatures up to 1200C.

The gallium ion exchanged zeolites produced according to the process of the present invention suitably contain between 0.05 and 5.0% w/w of gallium and exhibit a high degree of activity in hydrocarbon conversion reactions. For example, these gallium-exchanged zeolites may be used as catalysts in the dehydrocyclodimerisation reactions claimed and described in our British Patent Specifications Nos. 1507549 and 1507778 and in our Belgian Patent No.862051. In using these zeolites for such reactions it is preferabley to pelletise or extrude these zeolites together with a base or binder material. Such materials will be known to those skilled in the art. An example of such a binder is silica which may be added to the gallium-exchanged zeolite using a commercially available silica sol such as LUDOX (Registered Trade Mark).The slurry thus formed may be extruded and dried to produce a material sufficiently strong to prevent crushing.

The present invention is further illustrated with reference to the accompanying Examples.

Example 1 Zeolite Synthesis In the synthesis of the zeolite of the zeolite the following reactants were used: Sodium hydroxide 10.0 g Sodium aluminate 28.0 g Di-ethanolamine 262 g Ludox AS 40 714g (40% w/w colloidal silica) (Registered Trade Mark) De-ionised water 850 g Sodium hydroxide and sodium aluminate were dissolved in de-ionised water (350 g) by warming and stirring for 10 minutes. The solution was then filtered and placed in a 3-litre flask. Di-ethanolamine was melted and added to this solution and the whole stirred for 10 minutes maintaining the temperature at 40 to 50"C. The colloidal silica was then diluted with the remainder of the de-ionised water (500 g) and then slowly added to the mixture in the flask, over a period of 1 hour.During this addition the temperature was maintained at 40 to 50"C and the mixture, which gradually thickened, stirred continuously. Stirring was continued for 0.5 hr after the silica had been added. The mixture was charged to a 3-litre rocking autoclave which was agitated for 4 hours while the temperature was raised to 1 750C. The autoclave was then left static at this temperature for 7 days. Thereafter the autoclave was opened and the white crystalline zeolite which had formed was separted from the mother liquor by decantation.

The crystalline zeolite was then washed thoroughly first with de-ionised water and then dried in a vacuum oven at 100 Cfor 16 hours.

The dried zeolite was then calcined in an oven by raising the temperature to 5000C over 4 hours and holding at that temperature for 60 hours.

The calcined zeolite was then subjected to gallium exchange by refluxing in a 0.065 molar solution of gallium nitrate for 4 hours. The gallium exchanged material was then water-washed and dried in a vacuum oven as before.

200 g of gallium exchanged zeolite thus produced were mixed with 213 g of Ludox AS 40 (Registered Trade Mark containing 40% SiO2) and the resulting slurry was dried in a vacuum oven as previously described. The dried product was then broken and sieved to pass 12 to 30 mesh BSS.

Example 2 The gallium exchanged zeolite was prepared as described in Example 1 except that the crystalline zeolite was calcined at 550"C instead of 500"C.

n-Butane was passed over a 15 ml bed of the catalyst at 530"C, atmospheric pressure and a Liquid Hourly Space Velocity of 1. The yield of aromatics was 58% by weight based on the n-butane feed with a 100% conversion of n-butane.

Example 3 A zeolite (SiO2:AI203 motor ratio 35.8:1), prepared as in Example 1 using diethanolamine and sodium hydroxide in addition to sources of silica and alumina and which contained 0.68% wt of sodium, was calcined at 550"C for 64 hours. The calcined material (172 g) was refluxed for 4 hours with a mixture of 86 ml gallium nitrate solution (0.05 g Ga/ml) and 850 ml of water. After separation the gallium containing zeolite was washed with 2 litres of water and dried under vacuum. A sample of the material (150 g) was bound by mixing with a silica sol (50% SiO2) and drying. The resulting cake was broken and sieved to give a catalyst of 12/30 mesh granules.

Butane (85% n-, 15% iso-) was passed over 200 ml of the above catalyst at an LHSV of 2. The average bed temperature was maintained at 535"C and the pressure at 6 bar abs. The reaction products obtained between 1.5 and 2.5 hours on stream contained 105 g of mixed aromatics (44.3% weight yield). After 48 hours on stream the yield of aromatics was 38.3% by weight.

Claims (5)

1. An improved method of preparing a catalyst composition comprising a zeolite in which some or all of the cations have been exchanged for gallium ions, or on which gallium has been deposited, the zeolite having a high silica to alumina ratio and being prepared by crystallisation from an aqueous solution comprising a mixture of a source of silica, a source of alumina, a source of alkali metal and an alkanolamine, washing the crystallised zeolite with deionised water or with dilute acid followed by deionised water and calcining the water washed product at an elevated temperature, characterised in that the calcined product is refluxed directly with a solution of a gallium compound to produce a gallium ion exchanged zeolite.

2. A method according to claim 1 wherein the alkanolamine is diethanolamine.

3. A method according to claim 1 or 2 wherein the gallium compound is gallium nitrate.

4. A method according to any one of the preceding claims wherein the catalyst composition contains between 0.05 and 5.0 per cent by weight of gallium.

5. A method according to any one of the preceding claims wherein the gallium ion exchanged zeolite is pelletised or extruded together with a base or binder material.