GB2117367A - Activation of zeolites - Google Patents

Abstract

A method of restoring or improving the activity of zeolite catalysts, whether or not loaded with gallium, for use in a dehydrocyclodimerisation reaction comprises heating the catalyst initially in a reducing atmosphere and finally in an oxidising atmosphere. The reduction may also be preceded by an oxidation.

Description

SPECIFICATION Activation of aluminosilicate zeolites The present invention relates to a method of improving the catalytic activity of an aluminosilicate zeolite, especially the activity of a freshly made zeolite catalyst loaded with a gallium compound or gallium ions.

Methods of preparing aluminosilicates with a high silica to alumina ratio and in which the cations have been exchanged for gallium ions are claimed and described in our British Patent Specification Serial No. 1561 590 and in our published copending European Patent Application No. 0002900. These catalysts find use in hydrocarbon conversion reactions, in particular in the dehydrocyclodimerisation of the lower paraffinic hydrocarbons into aromatics.

Prior art publications refer to various methods of restoring the activity of catalysts deactivated during use.

It has now been found that the activity of unused, freshly made catalysts can be improved by a series of pretreatments.

Accordingly the present invention is a process for improving the catalytic activity of an aluminosilicate zeolite having a silica to alumina ratio of at least 5:1, said process comprising heating the zeolite initially in a reducing atmosphere and subsequently in an atmosphere containing oxygen.

According to a further embodiment, the present invention is a process for improving the catalytic activity of a catalyst composition comprising an aluminosilicate zeolite loaded with a gallium compound or gallium ions which have either been impregnated upon or exchanged with cations in the zeolite, said aluminosilicate zeolite having a silica to alumina ratio of at least 5:1 which process comprises heating the catalyst composition at an elevated temperature initially in a reducing atmosphere and subsequently in an atmosphere containing oxygen.

The aiuminosilicate zeolite having a silica to alumina ratio of at least 5:1 is suitably a zeolite which has a general formula: MO.W203.yYO2zH20 wherein M is a cation which its a proton or a positively charged ion selected from a metal ion and an organic ion of valence n, W is either aluminium or mixtures thereof with gallium, Y is silicon, y is an integer greater than 5 and z is from 0 to 40. The metal ion is preferably an alkali metal ion or an alkaline earth metal ion, preferably sodium or potassium ions.Aiuminosilicate zeolites of this type are preferably produced by a process claimed and described in our published copending European Patent Application No. 0002900 in which the zeolite is crystallised from an aqueous solution comprising a mixture of a source of silica, a source of alumina, a source of alkali metal and an organic nitrogen-containing base.

The organic nitrogen-containing base is preferably an alkanolamine. A particularly suitable method of synthesizing such zeolites using ammonia as the nitrogen-containing base is described in our published European Patent Application no.0030811.

The zeolite may be loaded with a gallium compound or with gallium ions. This may be achieved either by simple impregnation is by preparing a solution, suitably an aqueous solution, of a gallium compound such as for example gallium nitrate and adding the conventional aluminosilicate zeolite to this aqueous solution with thorough stirring to form a paste. The paste is subsequently dried at an elevated temperature in vacuum. Alternatively, the crystalline zeolite, after water washing and calcination, may be refluxed with a solution of a gallium compound.

Whichever method of catalyst preparation is used, the amount of gallium present in the catalyst compositions may vary for instance between 0.05 and 10 per cent by weight of the total aluminosilicate in the catalyst composition. The gallium-loaded zeolite thus obtained may be combined with a porous matrix, e.g. silica or alumina or other inorganic compositions to improve the mechanical strength of the catalyst.

The gallium-loaded aluminosilicate thus produced is the catalyst used in hydrocarbon conversion reactions, especially for the dehydrocyclodimerisation of lower hydrocarbon feedstock. More specifically, the dehydrocyclodimerisation reaction is a catalytic process for the conversion of C3 and C4 gases into a mixture of aromatic compounds which can be used as high octane blending components for gasoline or as feedstock for petrochemicals.

For example, n-butane reacts in the presence of this catalyst at a temperature between 400 and 6000C to form a mixture of aromatic hydrocarbons, including benzene, toluene and xylene and generates hydrogen as a by-product. Similarly, propane can be used as a feed to give a similar range of products. Such dehydrocyclodimerisation reactions using catalysts containing gallium are claimed and described in our British Patent Nos. 1 507778 and 1 561 590.

The catalytic activity of the aluminosilicate zeolite is improved by heating the zeolite, whether or not impregnated or exchanged with other cations, initially to an elevated temperature in a reducing atmosphere as described below. The treatment in a reducing atmosphere, hereafter referred to as "reduction treatment," is suitably carried out in an atmosphere of hydrogen or mixtures thereof with methane and/or ethane. The gases necessary for providing the reducing atmosphere may be derived from the by-products of the dehydrocyclodimerisation reaction. The reduction treatment is suitably carried out at a temperature between 350" and 7500C, preferably between 500 and 7000C, most preferably between 600 and 7000C. The duration of the reduction treatment may be up to 24 hours but is usually not more than 10 hours.Thereafter the product resulting from the reduction treatment is heated at an elevated temperature in an atmosphere containing oxygen eg air or oxygen-containing gases. This latter treatment in an atmosphere containing oxygen is hereafter referred to as "oxidation treatment". The oxidation treatment is suitably carried out at a temperature between 3500 and 6500 C, preferably between 500 and 6000 C. The catalytic activity of the zeolite may be improved by subjecting the catalyst to a single reduction treatment followed by a single oxidation treatment.

In a further embodiment of the present invention the catalytic activity of the aluminosilicate zeolite is improved in a three stage process. In the first stage the zeolite is subjected to an initial oxidation treatment similar to that described above; thereafter the second stage comprises a reduction treatment as described above and the product resulting from the second stage is subjected to a final oxidation treatment. There may be a series of first stage and second stage treatments but a single final treatment in an oxygen-containing atmosphere is usually sufficient. Moreover, this three stage procedure is particularly suitable for improving the catalytic activity of an unused or freshly prepared aluminosilicate zeolite. A zeolite subjected to a sequential three-stage treatment as above shows improved activity especially in respect of hydrocarbon conversion reactions.In the three stage process each stage is preferably of a duration lasting between 3 and 6 hours. The successive reduction/oxidation treatment whether done in two stages or in three stages can be interspersed with a purge with an inert gas such as nitrogen after each stage. The inert gas purge treatment may also be used before the commencement of and/or after the conclusion of any of the treatments.

The present invention is further illustrated with reference to the following Examples and Comparative Tests.

The catalysts used in the Examples were prepared in a similar manner to the procedure given below. Only the scale of the synthesis varied between the catalysts described.

Zeolite Synthesis In the synthesis of the zeolite the following reactants were used: Sodium hydroxide 10.0 g Sodium aluminate 28.0 g Diethanolamine 262.0 g Ludox AS 40 (Registered Trade Mark) 714.0 g (40% w/w colloidal silica) Deionised water 850.0 g Sodium hydroxide and sodium aluminate were dissolved in deionised water (350 g) by warming and stirring for 10 minutes. The solution was then filtered and placed in a 3-iitre flask. Diethanolamine was melted and added to this solution and the whole stirred for 10 minutes maintaining the temperature at 400C. The colloidal silica was then diluted with the remainder of the deionised 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 400C 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 separated from the mother liquor by decantation.

Pretreatment of zeolite The crystalline zeolite was then washed thoroughly first with deionised water and then with a 10% nitric acid solution. Thereafter the acid treated zeolite was washed thoroughly with deionised water to remove any traces of acid. This was then dried in a vacuum oven at 1 000C for 1 6 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 refluxed in 1.6 litres of 10% w/w nitric acid for 2.5 hours and then water washed and dried in a vacuum oven as before.

The acid-washed zeolite was then subjected to ammonia exchange by refluxing in 1.5 litres of 0.67 molar ammonium nitrate solution for 4 hours. It was then water-washed and dried as previously to give the ammonia exchanged zeolite.

The ammonia exchanged zeolite was recalcined by raising the temperature to 5000C and maintained at that temperature for 1 6 hours to give H-zeolite.

(c) Gallium-exchange The H-zeolite from the recalcination step was placed in 1.65 litres of a 0.065 molar solution of gallium nitrate and refluxed for 4 hours. The gallium exchanged material was then water-washed and dried in a vacuum oven as before.

(d) Incorporation of binder: Catalyst 1 The gallium exchanged zeolite was mixed with Ludox AS40 (Registered Trade Mark containing 40% w/w SiO2) and the resulting slurry dried in a vacuum as described previously. The ratio of zeolite:binder was 75:25 by weight. The dried product was then broken and sieved to between 12 and 30 mesh BSS.

(e) Incorporation of Binder: Catalyst 2 and 3 The gallium exchanged zeolite was mixed with alumina and water and extruded into 1.6 mm extrudates. This was dried and then calcined in dry flowing air at elevated temperature. The ratio of zeolite:binder was 70:30 by weight.

EXAMPLE 1 A fixed bed reactor was loaded with catalyst 1(5.7 g) and placed in a tubular furnace. The activation was then carried out for four hours by heating the catalyst to the required temperature in a flow of the appropriate gas. Thus, the following sequence of treatments was performed.

a) catalyst heated at 5500C in dry air (flow rate 1 ml/sec) for 4 hours b) catalyst purged with dry nitrogen whilst temperature raised to 6500C (1 hour) c) catalyst heated at 6500C in dry hydrogen (flow rate 1 ml/sec) for 4 hours d) catalyst purged with dry nitrogen whilst temperatures dropped to 5500C (1 hour) e) catalyst heated at 5500C in dry air (flow rate 1 ml/sec) for 4 hours f) catalyst purged with dry nitrogen for 0.5 hour The catalyst was then tested for activity in the dehydrocyclodimerisation (DHCD) reaction. A feed of n-butane was preheated to 550 C and passed over the catalyst at 3.4 WHSV and atmospheric pressure. The reactor furnace was maintained at 5500C during the run. Reaction products were collected between 0.5 and 1.5 hours on stream. These were analysed and the results obtained shown in Table 1.After 6 hours the butane feed was stopped and the catalyst allowed to cool under a flow of nitrogen.

Comparative Test A: In a comparative test not according to the invention Example 1 was repeated except that only the treatments (a) and (f) were performed on the catalyst before the activity was tested for the DHCD reaction. These results are also shown in Table 1.

EXAMPLE2 A similar procedure to that described for Example 1 was carried out except that catalyst 2 was used. The activation sequence and temperatures are shown in Table 2 together with the results.

Comparative Tests B, C and D A series of Comparative Tests were carried out using catalyst 2. The activation sequence and temperatures are shown in Table 2 together with the results.

Examples 3 and 4 A similar procedure to that described for Example 1 was carried out except that catalyst 3 was used. The activation sequence and temperature are shown in Table 3 together with the results.

Comparative Tests E and F Two Comparative Tests were carried out using catalyst 3. The activation sequence and temperatures are shown in Table 3 together with the results.

EXAMPLE 5 The catalyst test was carried out on a small scale pilot plant using catalyst 3 activated according to Example 1 (200 mix). Butane was passed over the catalyst at 2 LHSV and at a pressure of 6 bar absolute. The average catalyst bed temperature was maintained at 5350C. Reaction products were analysed at intervals throughout the run and the results are shown in Table 4.

Comparative Test G The procedure of Example 5 was repeated using catalyst 3 except that the activation sequence was the same as Comparative Test A. The results of catalyst activity are shown in Table 4.

Explanation of Terms used in Tables Weight of components higher than C4 in product Yield C4 = x 100 Weight butane fed Yield of components higher than C4 Selectivity = ----------- x 100 Conversion (C3 + C4) Weight butane fed - Weight of C3 + C4 components in product Conversion (C3 + C4) = x 100 Weight butane fed Weight carbon on catalyst after 6 hours on stream % carbon on fresh catalyst = x 100 Weight fresh catalyst Explanation of the Results Table 1: The improvement obtained by carrying out the air/hydrogen/air activation is clearly seen from Example 1. Not only is the yield and selectivity increased, but the rate of deactivation is reduced as evidenced by the lower carbon formation.

Table 2: Catalyst performance is improved by carrying out an air/hydrogen activation. A considerably greater improvement is, however, achieved by carrying out the air/hydrogen/air activation. No improvement resulted from an air/nitrogen treatment.

Tables 3 and 4: Again the large improvement on carrying out the air/hydrogen/air activation is clearly seen. The results show the lower rate of deactivation obtained with the activated catalyst.

It can be seen by comparing Tables 2 and 3 that catalysts made by similar routes can have slightly different activities. Thus catalyst 3 gave a lower yield and selectivity as compared to catalyst 2 when only given an air treatment at 5500C. A further advantage of the air/hydrogen/air activation is that the initially poorer catalyst 3 was improved even mote than catalyst 2.

TABLE 1 Catalyst 1

Example/ Comparative % carbon on Test Activation Yield C4 L Selectivity fresh catalyst A Air5500C 47 64 1.9 Air 5500C/ 1 H26500C/ 54 67 1.2 Air 5500C TABLE 2 Catalyst 2

Example/ Comparative % carbon on Test Activation Yield C4 Selectivity fresh catalyst B Air5500C 42 61 2.6 Air5500C/ 44 63 C H26250C Air 550 C/ 40 59 N26250C Air 55Oc/ 2 H26250C/ 47 64 2.0 Air5500C TABLE 3 Catalyst 3

Example/ Comparative % carbon on Test Activation Yield C4 Selectivity fresh catalyst E Air 5500C 39 56 F Air 5500C/ 43 60 3.8 H2 550 C Air 550 C/ 3 H26250C/ 51 63 3.1 Air 550 C Air 55O0C/ 4 H27000C/ 52 66 2.8 Air5500C TABLE 4 Catalyst 3

Example/ Comparative Hours on Test Activation stream Yield C4 3 49 Air55O0C/ 12 46 5 H26500C/ 24 43 Air 550" 36.5 38 48 32 3 43 Air 550 C 8 41 G for4hours 16 32 27.5 14 EXAMPLE 6 The procedure of Example 1 was repeated but this time only steps (c) to (f) of that Example were used for activation. The results are shown below.

TABLE 5

% carbon on Example Activation Yield C4 Selectivity fresh catalyst 6 H2 650"C/ 53 67 Not determined Air55O0C

Claims (9)

1. A process for improving the catalytic activity of an aluminosilicate zeolite having a silica to alumina ratio of at least 5:1, said process comprising heating the zeolite initially in a reducing atmosphere and subsequently in an atmosphere containing oxygen.

2. A process for improving the catalytic activity of a catalyst composition comprising an aluminosilicate zeolite loaded with a gallium compound or gallium ions which have either been impregnated upon or exchanged with cations in the zeolite, said aluminosilicate zeolite having a silica to alumina ratio of at least 5:1 which process comprises heating the catalyst composition at an elevated temperature initially in a reducing atmosphere and subsequently in an atmosphere containing oxygen.

3. A process according to claim 1 or 2 wherein the aluminosilicate zeolite having a silica to alumina ratio of at least 5:1 is a zeolite which has a general formula: MO.W2O3.yYO2zH29 wherein M is a cation which is a proton or a positively charged ion selected from a metal ion and an organic ion of valence n, W is either aluminium or mixtures thereof with gallium, Y is silicon, y is an integer greater than 5 and z is from 0 to 40.

4. A process according to claim 2 or 3 wherein the amount of gallium present in the catalyst compositions varies between 0.05 and 10 percent by weight of the total aluminosilicate in the catalyst composition.

5. A process according to any one of the preceding claims 2 to 4 wherein the gallium loaded zeolite is combined with a porous matrix.

6. A process according to any one of the preceding claims wherein the treatment in a reducing atmosphere, hereafter referred to as "reduction treatment", is carried out in an atmosphere of hydrogen or mixtures thereof with methane and/or ethane.

7. A process according to claim 6 wherein the reduction treatment is carried out at a temperature between 3500 and 7500C.

8. A process according to any one of the preceding claims wherein the product resulting from the reduction treatment is heated at an elevated temperature in an atmosphere containing air or oxygen containing gases hereafter referred to as "oxidation treatment".

9. A process according to claim 8 wherein the oxidation treatment is carried out at a temperature between 3500 and 6500C.

1 0. A three-stage process for improving the catalytic activity of the aluminosilicate zeolite whether or not loaded with gallium in which the zeolite is subjected in the first stage to an oxidation treatment, followed in a second stage by a reduction treatment and the resulting product from the second stage is subjected in a third stage to a final oxidation treatment.