GB2096481A - Catalyst for the production of aromatic hydrocarbons and process for its preparation - Google Patents

Abstract

A monofunctional catalyst for the dehydrocyclization of paraffins, characterised in that it contains: 0.1 to 1.5% of platinum 0.1 to 1.5% of rhenium incorporated in the form of carbonyl a small proportion of sulphur introduced by a sulphur compound which is reducible or decomposable by hydrogen, such that the ratio x of the number of sulphur atoms to the number of platinum and rhenium atoms deposited on the catalyst is between 0.05 and 0.6, the support being a zeolitic crystalline aluminosilicate compensated to more than 90% by alkaline cations, having a pore diameter larger than 6.5 ANGSTROM .

Description

SPECIFICATION Catalyst for the production of aromatic hydrocarbons and process for its preparation This invention relates to a new catalyst for the dehydrocyclization of C.6 to C.10 paraffins, and its method of preparation. This catalyst has a particularly good performance for the production of light aromatics from petroleum fractions obtained by direct distillation.

The conventional methods of carrying out aromatization reactions are based on the use of catalysts comprising a noble metal on a support, more particularly catalysts containing from 0.2 to 0.8% by weight of platinum on an 0.5 - 2% by weight chlorinated alumina support. It is generally accepted that these catalysts operate in accordance with a bifunctional mechanism combining the hydrogenating-dehydrogenating activity of the metal and the isomerizing-cyclizing activity of the acid support. A very significant improvement in these catalysts has consisted in adding a second metal to the catalyst, thus giving it increased stability and the possibility of operating at lower pressure under conditions in which the aromatization reactions are favoured. The US patent 3415737 (H. E. KLUKSDAHL, Chevron Res.Co) shows, in particular, the advantage of using catalysts containing from 0.1 to 3% of Pt and from 0.1 to 3% of rhenium.

It also claims the presence of 0.05 to 2% of sulphur on the catalyst, this sulphur being intended to limit the hydrogenolyzing activity due to the rhenium, such activity resulting in rapid de-activation of the dehydrocyclization catalyst.

Another type of aromatization catalyst is described in US patents 3775502 and 3819507 (M. OISHI, Sun Res. and Dev. Co). It comprises platinum deposited on zeolites X or lithium, sodium of potassium exchanged zeolites. This catalyst may also contain from 0.1 to 1.2% by weight of rhenium. Although this type of catalyst is new, its aromatizing activity is comparable to that of the previous catalysts. The presence of rhenium in this new type of catalyst does not appearto provide any advantage, since the results obtained with the platinum-rhenium zeolite X are inferior to those of Pt on zeolite X.

Another type of catalyst on a molecular sieve is described in US patent 4 104320 (J. Nury, J. R. Bernard, ERAP). It contains from 0.1 to 1.5% by weight of platinum deposited on alkaline zeolite L, preferably potassium, rubidium or cesium exchanged. When charged with platinum, zeolite L results in a greater aromatics selectively and yield than all the other catalysts described above. Although the operating mechanism of these catalysts has not been completely explained, it may be thought that this catalyst is monofunctional and that it is therefore due solely to the platinum, the properties of this platinum being modified by the zeolite L support.The zeolite L usable for these catalysts is compensated by alkaline cations and hence the platinum which is in the metallic state under the catalysis conditions is greatly influenced by these alkaline cations since the activity of the catalyst increases passing from lithium to potassium and to cesium.

Although they have remarkable aromatizing activity and selectivity qualities, the catalyst described in the cited patent have low stability which is less than that of the bifunctional monometallic catalysts based on platinum on chlorinted alumina, and hence they can be used under high hydrogen pressures if suitable cycle times are required.

To try to stabilize these catalysts it is possible to use the techniques described in the previous patents, i.e.

the incorporation of rhenium in the catalyst by deposition of perrhenic acid in aqueous solution. This incorporation, whether carried out in co-impregnation with the platinum, or successive impregnation, does not result in an improvement of the catalytic stability and in addition the activity of these bimetallic catalysts is very much inferior to that of the zeolite L based monometallic catalysts. This is in keeping with the observations of the authors of the US patent 3819 507, who find no beneficial effect of rhenium in the case of platinum charged zeolites X and Y.

On the other hand, we have surprisingly found that the incorporation of rhenium in the form of rhenium carbonyl Re2 (cm) ,0 in the zeolite, preceded orfollowed by the deposition of platinum by known methods (impregnation, ion exchange of platinum complexes or salts) give good catalytic stability and activity after reduction by hydrogen.

UK patent No. 2 004 764 describes a catalyst prepared by pyrolysis of rhenium carbonyl on a porous support also containing platinum. This patent shows that the catalysts thus prepared are more active than catalysts prepared by means of perrhenic acid, since they give a higher octane number for the same reaction temperature by reforming of the oils. However, these catalysts produce more hydro-cracking than the catalysts prepared from perrhenic acid and their advantages are therefore not really clear.

On the other hand, the use of rhenium carbonyl in the Pt-Re zeolite catalysts gives an active catalyst while the use of perrhenic acid results in a catalyst of little activity.

However, with these catalysts, the paraffin dehydrocyclization selectivity is poor because hydrogenolysis occurs in the proportions recommended for the bifunctional catalysts with a chlorinated alumina support, the resulting catalyst is completely poisoned, and is practically inactive for dehydrocyclization.

We have unexpectedly found that the incorporation of sulphur in the catalyst in much lower proportions than those described previously results in good selectivity values while giving excellent catalytic stability.

For example, the sulphur content currently recommended for ready-to-use reforming catalysts is of the order of 0.2% by weight, deposited by impregnation by a sulphur compound or presulphurization before catalysis, which is equivalent to a molar ratio x of approximately 2 for 0.3% Pt and 0.3% Re contents. Thex denotes the ratio of the number of sulphur atoms deposited on the catalyst to the number of Pt + Re atoms contained in the catalyst.

The platinum-rhenium-zeolite catalysts are not active with such a value. The molar ratio x must be reduced to between 0.05 and 0.6 to obtain an active and stable catalyst as regards dehydrocyclization. With the Pt and Re contents indicated above this is equivalent to a range of from 0.005% to 0.06% by weight of sulphur to catalyst, which is clearly below the 0.2% by weight mentioned hereinbefore.

The invention consists in a new paraffin dehydrocyclization catalyst containing from 0.1 to 1.5% of platinum, 0.1 to 1.5% of rhenium, sulphur in a form which is reducible or decomposable by hydrogen in such a manner that the molar ratio x is between 0.05 and 0.06, the support being a zeolitic crystalline alumino-sillicate of a pore size greater than 6.5 A, compensated to more than 90% by alkaline cations.

According to another aspect, the invention relates to a new process for the preparation of a catalyst, the steps of which are as follows, the sequence in which these steps are carried out being arbitrary: A non-acidic zeolitic crystalline aluminosilicate of a pore size greater than 6.5 A is charged with rhenium, the operation being carried out by means of rhenium carbonyl Re2(CO)1O, either by sublimation on the support or by impregnation with an organic solution of Re2(CO)10.

Platinum is deposited by the prior art methods, i.e., impregnation is carried out by means of a solution of a compound of platinum such as hexachloroplatinic acid, platinum tetramine chloride, dinitrodiaminoplatinum or ion exchange by a platinum cationic complex.

Sulphur is incorporated by impregnation with a sulphur compound solution which is reducible or decomposable under the catalysis conditions (oxyanion, sulphide, mercaptan, etc), it being possible for this operation to be carried out together with the previous one. It is also possible to pre-treat the catalyst in the dehydrocyclization reactor by a sulphur compound (H2S, dimethyl disulphide, CS2) before or after the reduction by hydrogen.

After these steps have been carried out, the catalyst is dried, possibly calcined, and then reduced by hydrogen and possibly suphurized in accordance with the previous recommendations.

The catalysts according to the invention have exceptional aromatizing properties and excellent stability.

Their support is zeolitic crystalline aluminosilicate or a molecular sieve. For dehydrocyclization it is essential that the molecular sieve acting as the support should have low acidity, or zero acidity or a low basicity. For this reason the zeolite should have cation exchange sites compensated to more than 90% by alkaline cations, all other cations introducing some acidity, either because they are polyvalent and thus create acidic sites, or because they are reducible or decomposable under the catalysis conditions, the reduction or decomposition then corresponding to the formation of protons on the zeolite. It is evident that the alkaline zeolite pores have an aperture at least equal to the dimensions of the benzene and of the zeolites that can be used we may cite the faujasites X and Y, zeolite L, zeolite omega and zeolite ZSM 4.These zeplites may be used in their ex synthesis form, apart from the last two, which contain alkylammonium cations which must be replaced by alkaline cations by the methods known to those versed in the art, such as thermal decomposition followed by neutralization by an alkaline base. Their synthesis cations can also be exchanged by other alkaline cations, and the zeolites in question can therefore contain lithium, sodium, potassium, rubidium and/or cesium.

Amongst these zeolites, the preferred support is zeolite L which gives exceptionai yields of aromatics from aliphatic fractions. This zeolite is synthesized in its potassium form and it can be economically used in that form, but it may also contain sodium and particularly rubidium or cesium.

The zeolite support must be made up into the required form for industrial use and this can be done either before or after the description of the platinum, rhenium and sulphur, and use will be made of the methods known to those versed in the art, such as mixing with aluminia or clay binders followed by extrusion or putting into the form of balls by the dragee-making or oil drop technique. Another technique that can be used is to make balls or extrusions from clay, such as meta-kaolin and convert it to zeolite by the appropriate techniques. The zeolite can also be used in the form of pellets, tablets, or in powder form if it is made into a fluidized bed.

The platinum can be deposited by the methods described in the prior art. The aqueous method will generally be used, more particularly impregnation and ion exchange. Impregnation can be carried out with any water-soluble platinum compound, e.g. hexachloro-platinic acid. Although this compound is satisfactory, it imparts some acidity to the catalyst, and it is preferably to use Keller's complex or platinum tetramine chloride or alternatively dinitrodiaminoplatinum. Since the latter is not very water-soluble, this impregnation can be carried out with the application of heat. When a cationic complex of platinum is used it is also possible to carry out ion exchange, in which case the support is immersed in the solution containing the platinum, is then withdrawn after some time, and washed and dried. Since the quantities of platinum to be deposited represent only a very small fraction of the zeolite cations and the zeolite has a strong affinity for platinum, all of this metal then remains fixed on the support. The Keller complex and various other cationic complexes of platinum, and dinitrodiaminoplatinum, can thus be deposited, although the latter is not an ionized compound. It is also possible to carry out this ion exchange in the presence of an excess of a salt of the cation of the zeolite, e.g. potassium chloride in the case of zeolite KL, so as to distribute the platinum more homogeneously in the zeolite crystal.

The percentage of platinum to be introduced may vary from 0.1 to 1.5% by weight. In the case of the bifunctional catalysts based on platinum on chlorinated alumina, the percentage of platinum does not affect the dehydrocylizing activity beyond 0.3%, because it is the acid group which becomes activity-limiting. The catalysts according to the invention, however, have only the metallic function, the dehydrocyclizing activity of which is very high. This activity also increases monotonously with the quantity of metal present in the catalyst. In practic, the activity is substantial as from 0.1% Pt. It increases again beyond 1.5% Pt, but with these contents the price of the catalyst becomes too high.

The rhenium can be deposited only by using rhenium carbonyl Re2(CO)1,. The reason for this is that the perrhenic acid aqueous impregnation method results in catalysts with little activity and stability. The method according to the invention therefore comprises mixing the solid catalyst which may or may not contain platinum with the carbonyl powder of rhenium, then heating the mixture to temperatures between 50"C and 200"C. This step may be carried out in air, nitrogen or hydrogen, but is preferably carried out in vacuo ranging from 0.01 to 100 Torr to facilitate sublimation.

The rhenium can thus be distributed homogeneously in the zeolite crystal and after the reduction it will be dispersed more satisfactorily and can thus interact with the platinum.

Another technique is to impregnate the zeolite support with a solution of Re2(CO)10 in an appropriate solvent, e.g. acetone.

The quantity of rhenium present in the catalyst is between 0.1 and 1.5%. This quantity must in fact be adjusted to the platinum content of the catalyst, for if there is too much rhenium in relation to the platinum, the rhenium is then responsible for a hydrogenolysis reaction which particularly produces methane and ethane to the detriment of the production of aromatics and hydrogen. This hydrogenolysis can be inhibited by sulphur, but only partially, and it is in fact preferable to maintain the percentage of rhenium at values which are not too high in relation to those of the platinum.

However, the rhenium content shou Id not be too low compared with that of the platinum if the catalytic stabilization effect is to be achieved.

In fact, the proportion of rhenium in relation to the platinum must be adjusted according to the working conditions of the catalyst, for a reduction in pressure increases aromatization, and reduces hydrogenolysis and stability. Rhenium is responsible for most of the hydrogenolysis and the good stability of catalysts. At high utilization pressure (e.g. 15 to 35 bars), the proportion of rhenium must be less than that of the platinum, for the pressure of itself sufficiently stabilizes the catalyst, and it is preferable to limit hydrogenolysis. At low utilization pressure, it may be advantageous to use a catalyst which contains more rhenium than platinum, for the catalyst must be stabilized by a high quantity of rhenium which the no longer has a high hydrogenolyzing activity.

Rhenium has marked hydrogenolyzing properties. If a Pt-Re catalyst on zeolite is produced, it will be seen that this catalyst is very hydrogenolyzing and the production of aromatics from aliphatics is low. Unlike the phenomena described in the US patent 3415737 for the alumina support, this hydrogenolysis decreases little in the course of time and is accompanied by deactivation of the aromatizing function. In these conditions it is not possible to achieve good selectivity of the monometallic catalyst by a relatively insevere start to the cycle, which allows the hydrogenolyzing function of the catalyst to be de-activated. It is then essential to poison the hydrogenolyzing function selectively by means of sulphur. This operation is conventionally carried out on bifunctional catalysts and the prior art gives sulphur contents of the order of 0.05 to 2% to weight.

In fact, since the sulphur selectively poisons the metallic function of the catalyst, the sulphur content must of course be related to the metal content and we shall express this content by the value: S x = Pt+Re Inatoms Sulphurization of the dehydrocyclization catalysts in the prior art is carried out with x = 2 approximately. In the case of our invention such a value is prohibitive, for the catalysts are then completely poisoned and the recommendable value of xis between 0.1 and 0.6. With such values, the prior art catalysts are in fact too hydrogenolyzing. For example, in the case of a bifunctional catalyst containing 0.3% of Re and 0.3% of Pt on chlorinated alumina, if x = 0.6, the percentage by weight of sulphur is 0.05%. In these conditions a catalyst of this kind is not selective in respect of aromatization.

The sulphur can be impregnated by the aqueous method in a separate step or together with the platinum.

For this purpose, oxyanions of sulphur are used, e.g. the sulphate, thiosulphate, sulphite and other ions.

During reduction of the catalyst, this sulphur is reduced at least partially in the for of H2S and selectively poisons the catalyst. Sulphide ions can also be used.

Another method comprises injecting a sulphurized compound on the catalyst in the presence of hydrogen after reduction of the catalyst. Hydrogen sulphide, mercaptans, and disulphides such as dimethyl disulphide can thus be used. All these methods are equivalent and give very good catalysts.

On conclusion of these element deposition steps the catalyst is dried. It can then be calcined in air between 100"C and 600"C but calcination is not essential. It is then placed in a reactor and reduced by hydrogen at a temperature between 300 and 550"C.

The catalyst is used for aromatics production processes from petroleum fractions either for the production of fuels or for the production of petrochemical bases (benzene, toluene, xylene).

The operating conditions depend on the type of charge and products. In fact, the operating pressure may be between atmospheric pressure and 30 bars, low pressures promoting and aromatics yields, and accelerating coking of the catalyst. A pressure between 5 and 25 bars will preferably be used.

The temperature is between 420 and 600"C, for high temperatures promote good aromatics yields. The temperature factor has a considerable influence on the yields, for if the pressure is, for example, 14 bars with ann hexane charge, the bifunctional catalysts of the prior art give a 22% aromatics optimum for temperatures of about 525"C. Beyond this hydro-cracking predominates and the aromatic yields falls.

Against this, with the catalysts according to the invention, 30% aromatics are obtained in these conditions. If the temperature is increased to 550"C, up to 50% aromatics can be obtained from n hexane at 14 bars.

The charge should be injected on the catalyst with hydrogen in order to ensure its stability. 1 - 10 mols of hydrogen will preferably be injected per hydrocarbon. The injected charge volume per apparent catalyst volume and per hour will be from 0.2 to 5 h-1.

The catalysts according to the invention are usable for reforming and aromatization processes and the charges that can be used are the charges for those processes. They must be desulphurized and contain 1 ppm of S or less, for the catalyst is sensitive to poisoning. For the process to be economically advantageous, they should contain aliphatic or alicyclic hydrocarbons. The ideal charges are desulphurized petroleum distillation oils, the initial distillation point of which is from 50 to 1200C and the final point from 70 to 200"C.

A 50 to 80"C fraction contains essentially hydrocarbons containing 6 atoms of carbon and produces essentially benzene. A 60 - 1 OO"C fraction produces a mixture of benzene and toluene. Finally, an 80 - 180"C fraction produces a reformate having a good octane number in an excellent yield, but because of the properties of the catalysts this reformate contains substantially more benzene and toluene than the conventional reformates and its final point is only slightly increased.

The present invention will be more readily understood from the following examples.

In these examples, the oil used is from the 60-80"C distillation fraction containing 91 % C 6 hydrocarbons, of which 1% benzene and 1.5% cyclohexane. This charge was desulphurized and the catalysts were reduced with hydrogen at 500"C, and then tested in a reactor without hydrogen recycling, under the following conditions: Pressure 15 bars, hydrogen: hydrocarbon molar ratio: 5, weight of charge injected by weight of catalyst and by hour 2.5 h-'. The temperature was kept constant at 525"C during the time and the yield development was followed in order to evaluate the stability.Since the charge contained 3.5% benzene and C 6 naphthenes it may be considered that the dehydrocyclizing activity is zero when the aromatics yield is 4% or less.

Example 1 A monometallic catalyst containing 0.6% Pt deposited by Keller complex exchange on zeolite L in its potassium form (KL) was prepared by impregnation with a solution of Pt (NH3)4C12 and KCI, washing, drying and calcining in air at 4800C.

The following results were obtained after reduction with hydrogen: Operating C1- C2 yield (% by weight) Aromatics yield time (h) (% by weight) 5 13.1 37.7 29 7.0 22.9 77 2.5 6.9 This example shows the excellent initial de-hydrocyclization activity but the low stability of monometallic catalysts on zeolite.

Example 2 A commercial catalyst containing 0.3% platinum, 0.3% rhenium, 1.3% chlorine on alumina was placed in a reactor. After reduction by hydrogen, the catalyst was sulphurized by dimethly disul phide in the presence of hydrogen at 370"C, so as to inject 0.2% by weight of sulphur on the catalyst, i.e. x = 2. The reaction was then carried out under the aforementioned conditions.

The results were as follows: Operating C,-C2 yield (% by weight) Aromatics yield time (h) (% by weight 5 24.6 20.1 29 22.1 22.3 77 19.2 22.8 The bifunctional commercial catalyst was very stable but the aromatics yield was moderately high.

Example 3 Another sample of the commercial catalyst described in the previous experiment was sulphurized to 0.06% by weight, i.e. x = 0.6, all the other operations being the same.

The results were as follows: Operating C,-C2 yield (% by weight) Aromatics yield time (h) (% by weight) 5 72 10 29 65 15 77 61 14 This example clearly shows that the prior art catalysts cannot tolerate low sulphurization with x = 0.6, for then the C1C2 hydrogenolysis becomes very considerable, the aromatizing function is extremely reduced and never reaches the level of example 2.

Example 4 A catalyst containing 0.6% Pt and 0.3% Re on zeolite KL was prepared from the uncalcined catalyst of example 1. This catalyst was impregnated with a perrhenic acid solution, then dried and calcined in air at 480"C.

The results were as follows: Operating C7- C2 yield {% by weight) Aroma tics yield time (h) (% by weight) 24 17 11 72 9 3.5 These results show that unlike the alumina support catalysts the introduction of rhenium in the form of perrhenic acid is unfavourable to the catalyst from every aspect: Its activity is lower than that of the monometallic, while aromatization and hydrogenolysis remain low and unstable. At 72 hours the dehydrocyclizing activity becomes zero.

Example 5 A catalyst containing 0.6% Pt - 0.6% Re on zeolite KL was prepared as follows: The support was mixed with the required quantity of Re2(CO)O, then heated in a vacuum of 1 Torr at 1 100C for 3 hours. After cooling, 60 g of catalyst were impregnated with 66 ml of 0.1 N KCI solution containing the required quantity of Pt(NH3)4C12.

The catalyst was then washed, dried and calcined for 3 hours at 480"C.

Operating C7-C2 yield (% by weight) Aromatics yield time (h) (% by weight) 5 76.7 6 28 70.4 5.6 52 64.3 5 Although not selective as regards aromatization, this catalyst becomes active, since it is highly hydrogenolyzing. As compared with the previous example, this shows that the use of rhenium carbonyl instead of perrhenic acid introduces strong activity. However, most of this activity lies in the hydrogenolysis, which is not a desirable reaction.

Example 6 The catalyst of the previous example was impregnated with an aqueous solution containing sodium sulphate so as to introduce 0.1 mol of sulphur per Pt + Re (x = 0.1). It was tested after drying: Operating C,-C2 yield (% by weight) Aromatics yield time (h) (% by weight) 5 56.7 10.7 29 36.6 14.6 77 22.5 13.5 The hydrogenolysis decreases distinctly while the aromatization is greater than that of the previous example and it remains stable.

Example 7 The catalyst of example 5 was impregnated with an aqueous solution containing sodium sulphate so that x = 1.

Operating C,-C2 yield (% by weight) Aromatics yield time (hi (% by weight) 5 12.6 5.3 24 8.5 4.3 52 6.1 4.6 It will be seen that one atom of sulphur introduced per atom of metal poisons the catalyst excessively since hydrogenolysis decreases remarkably in comparison with the previous example, and the same applies to aromatization. An excess of sulphur poisons the two functions, aromatization and hydrogenolysis, and the best value of xis between 0 and 1.

Example 8 A catalyst identical to that in example 6 was prepared but containing 0.6% Pt and 0.3% Re intead of 0.6% Pt and 0.6% Re. xwas again 0.1.

Operating C,-C2 yield (% by weight) Aromatics yield time (h) {% by weight) 5 56.2 14.9 29 34.8 21.1 77 18.4 20.5 A smaller quantity of rhenium slightly reduces the hydrogenolysis but greatly increases the aromatics yield.

Example 9 A catalyst containing 0.6% Pt - 0.3% Re on zeolite KL was prepared by dry impregnation of Pt (NH3)2(NO2)2 dissolved in an 0.1 N solution of KCI at boiling point. After washing and drying, the catalyst was calcined at 400"C, and then rhenium carbonyl was introduced by the method described previously.The catalyst was finally impregnated with a solution of Na2SO4 so that x = 0.1 Operating C-C2 yield (% by weight) Atomatics yield time (h) {% by weight) 5 48 20 29 28 23.2 77 18 22 Example 10 The catalyst of the preceding example was not treated with Na2SO4, but after reduction by hydrogen was treated in the reactor with an equivalent quantity of sulphur (x = 0.1) in the form dimethyl disulphide, at 370"C.

Operating C,-C2 yield (% by weight) Aromatics yield time (h) (% by weight) 25 40.1 25.1 73 32.0 21.0 Sulphurization by dimethyl disulphide is substantially equivalent to that obtained with Na2SO4.

Examples 71 to 15 A series of catalysts with varying platinum, rhenium and sulphur contents were prepared as follows: The zeolite KL support was impregnated with Pt (NH3)2(NO2)2 dissolved in an 0.1 N KCI solution at boiling point - washing - drying.

Rhenium carbonyl was deposited by sublimation on the zeolite at 2 Torr and a temperature of 110"C.

Impregnation with a solution of Na2SO4 and drying.

The following results were obtained after 77 hours of operation: Examples %Pt % Re x %C1-C2 % Aromatics 11 1 0.67 0.22 18.9 28.2 12 1 0.67 0.47 16.5 27.0 13 0.6 0.55 0.6 8.3 10.3 14 1 0.32 0.35 6.2 10.9 16 0.6 0.55 0.35 19.0 22.6 Example 16 An identical catalyst to that in Example 11 was prepared by using perrhenic acid impregnation instead of rhenium carbonyl sublimation. The results were as as follows: Operating C7- C2 yield (% by weight) Aromatics yield time (h) (% by weight) 5 8.0 16.1 53 4.1 7.9 This example again shows that the use of perrhenic acid is harmful to the good catalytic properties of the Pt-Re-S-KL system.

Example 17 A catalyst containing 0.87% Pt - 1% Re on zeolite KLwith x = 0.1 was prepared as described in example 11 except that the Re2(CO)lo sublimation step was replaced by impregnation of a solution of Re2(CO)O in acetone.

The following results were obtained: Operating C7- C2 yield (O/o by weight) Aromatics yield time (h) (% by weight) 3 46.5 16.8 24 30.5 17.2 72 25.1 16.5 Although this catalyst contains a considerable amount of rhenium and little sulphur it has an interesting activity and excellent stability, showing that the catalysts of the invention can be prepared both by impregnation of rhenium carbonyl in acetone and by direct sublimation.

Example 18 A catalyst identical to example 11 was prepared with a faujasite NaX support instead of zeolite L.

Operating C7- C2 yield (% by weight) Aromatics yield time (h) (% by weight) 5 42 23 24 36 24.2 77 28.6 23.1 Although this catalyst is inferior to the corresponding zeolite L based catalyst, it has advantageous aromatization properties and is stable.

Example 19 An identical preparation to that of example 11 was made so as to give a catalyst containing 0.87% Pt, 1% Re and x = 0.1. After reduction by hydrogen at 500"C, this catalyst was tested under the following conditions: pph 2.5 h-1, H2/HC 5,500 C instead of 525"C and 8 bars instead of 15 bars.

The following results were obtained: Operating C,-C2 yield (% by weight) Aromatics yield time (h) (% by weight) 24 13.0 38.2 73 9.5 35.0 Although this catalyst contains more rhenium than platinum, it gives good yields and aromatics selectivity while maintaining good stability, because it was used at a lower pressure than in the preceding examples.

The marked hydrogenolysis produced by the rhenium is inhibited by the low pressure, while the aromatics yield is promoted and the high rhenium content contributes towards an improvement of the stability of the catalyst.

Claims (10)

1. A monofunctional bimetallic catalyst for the dehydrocyclization of paraffins, characterised in that it contains the following on a support consisting of a zeolitic crystalline aluminosilicate compensated to more than 90% by alkaline cations, and having a pore diameter greater than 6.5 : 0.1 to 1.5% of platinum introduced by impregnation 0.1 to 1.5% of rhenium incorporated in the form of carbonyl sublimed or dissolved in acetone, the rhenium/platinum proportion ratio varying according to the pressure used a small proportion of sulphur introduced by a sulpharized compound which is reducible or decomposable by hydrogen, such that the ratio x of the number of atoms of sulphur to the number of atoms of platinum is between 0.05 and 0.6.

2. A dehydrocyclization catalyst according to claim 1, characterised in that for high-pressure uses from 15 bars to 35 bars the proportion of rhenium is less than that of platinum.

3. A dehydrocyclization catalyst according to claim 1, characterised in that for low-pressure use the rhenium content is greater than the platinum content in orderto maintain its excellent stability properties.

4. A catalyst according to claim 1, characterised in that the support is selected from the group comprising the faujasities X and Y, zeolite L, zeolite omega and zeolite Z S M 4.

5. A catalyst according to claims 1 and 4, characterised in that the support is preferably an exchanged zeolite L containing one of the alkali metals selected from the group comprising sodium, potassium, rubidium and cesium.

6. A catalyst according to claims 1,4 and 5, characterised in that the support is made into the form of balls, tablets or pellets before of after the deposition of platinum, rhenium and sulphur, by extrusion, drageefication, drop coagulation, or any other known method.

7. A process for the production of the catalyst according to claim 1, characterised by the following steps taken in any order: Charging of the support with rhenium is effected by means of rhenium carbonyl Re2(CO)O, by sublimation of the latter in contact with the support in an atmosphere comprising air, nitrogen or hydrogen, and preferably a reduced pressure of from 0.1 to 100 Torr, Deposition of platinum is effected by known impregnation methods using a solution of a platinum compound selected from the group comprising hexachloroplatinic acid, platinum tetraminechloride, dinitrodiaminoplatinum, or by ion exchange with a cationic complex of platinum, possibly in the presence of an excess of a salt of the cation of the zeolite, Incorporation of the sulphur is effected by impregnation of the support by means of a solution of sulphurized compounds selected from the group comprising oxyanions: sulphates, thiosulphates, sulphites, The support having received the rhenium, platinum and sulphur charges is then dried, possibly calcined and necessarily reduced by hydrogen between 300"C and 550"C before use.

8. A process for the production of the catalyst according to claims 1 and 7, characterised in that incorporation of the sulphur is effected by pretreatment of the support charged with rhenium and platinum in the dehydrocyclization reactor by a sulphur compound selected from the group comprising hydrogen sulphide, dimethyldisulphide, carbon disulphide, before of after reduction by hydrogen.

9. A process for the production of the catalyst according to claim 1, characterised by the following steps taken in any sequence: Charging of the support by rhenium is effected by impregnation with an organic solution of rhenium carbonyl in acetone, Deposition of the platinum is effected by known methods of impregnation using a solution of a compound of platinum selected from the group comprising hexachloroplatinic acid, platinum tetramine chloride, dinitrodiaminoplatinum, or by ion exchange with a cationic complex of platinum, possibly in the presence of an excess of a salt of the cation of the zeolite, Incorporation of the sulphur is effected by impregnation of the support by means of a solution of a sulphurized compound selected from the group comprising oxyanions: sulphates, thiosulphates, sulphites, The support having received the rhenium, platinum and sulphur charges is then dried, possibly calcined, and necessarily reduced by hydrogen btween 300"C and 550"C before use.

10. A process for the production of the catalyst according to claims 1 and 9, characterised in that incorporation of the sulphur is effected by pretreating the support charged with rhenium and platinum in the dehydrocyclization reactor by a sulpharized compound selected from the group comprising hydrogen sulphide, dimethyl disulphide, carbon disulphide, before or after reduction by hydrogen.