WO2004056944A1

WO2004056944A1 - Process for the production of propylene and ethylene mixtures and the catalytic systems used therein

Info

Publication number: WO2004056944A1
Application number: PCT/EP2003/013345
Authority: WO
Inventors: Maria Angela Mantegazza; Pierluigi Crocetta; Massimo Romagnoli; Marco Ferrari; Paolo Pollesel
Original assignee: Polimeri Europa S.P.A.; Enitecnologie S.P.A.
Priority date: 2002-12-20
Filing date: 2003-11-25
Publication date: 2004-07-08
Also published as: AU2003292134A8; ITMI20022706A1; AU2003292134A1

Abstract

Process for the production of mixtures containing propylene and ethylene having an ethylene/propylene molar ratio <1, starting from C4-C8 hydrocarbon mixtures, prevalently olefinic, which comprises putting said hydrocarbon mixtures in contact with a catalytic system, under cracking conditions, at a temperature ranging from 400°C and 600°C, at a weight hourly space velocity (WHSV) ranging from 0.1 h-1 to 20 h-1 and at a pressure ranging from 0.1 to 30 atm, characterized in that the catalytic system contains a first zeolite with a Constraint Index (C.I.) of between 1.5 and 2.5, in a quantity ranging from 20 to 80% w/w, and a second zeolite with a Constraint Index (C.I.) of between 6.9 and 11.0, in a quantity ranging from 20 to 80% w/w, the molar ratio Si02/Al203 of both said zeolites being higher than 50. Catalytic systems of ZSM-12/ZSM-5 and ZSM-12/ZSM-23 mixtures are described in particular.

Description

PROCESS FOR THE PRODUCTION OF PROPYLENE AND ETHYLENE MIXTURES AND THE CATALYTIC SYSTEMS USED THEREIN

The present invention relates to a process for the production of mixtures rich in propylene and ethylene from prevalently olefinic hydrocarbon streams and the relevant catalytic systems used.

The hydrocarbon streams used are essentially C-C₈, preferably C-C6, fractions coming from the steam cracking of naphtha, with an olefin content preferably of at least 60% w/w, more preferably of at least 70% w/w.

Propylene is one of the most important chemical products as far as demand and production volume are concerned, and is mainly used in the field of polymer production. The steam cracking process is the main source of propylene, wherein hydrocarbon charges with a high paraffin content are thermally treated in the presence of vapour. Propylene and ethylene are the main products of steam cracking, and leave the process in a ratio of around 0.5. As the market demand for propylene has grown at a higher rate than that of ethylene in the last few years, and as the propyl- ene/ethylene ratio from steam cracking cannot be significantly changed, the necessity of increasing the production of propylene in some other way has arisen. During 2000, a considerable deficit of propylene was registered in Western Europe, making it necessary to import it in quantities of over 400,000 tons. The production of relevant quantities of propylene through processes which suitably treat steam cracking by-products, can allow a variation in the overall propylene/ethylene ratio, thus satisfying market demands. The possibility of having flexible processes which allow a certain elasticity to be obtained in the propylene/ethylene ratio, would therefore lead to great economic advantages. An interesting option consists in a selective cracking catalytic process which can convert C₄-C₅ fractions into propylene. The fractions can come specifically from steam cracking, but it is also possible to extend the process to other similar streams coming, for example, from FCC (Fluid Catalytic Cracking) . The charges can also come from the above-mentioned fractions after extraction and/or enrichment of olefins .

The use of acid solid catalysts - among which amorphous silico-aluminas, in particular zeolites - in the cracking reaction of hydrocarbons, is known in literature (see, for example, J. Scherzer, Cata. Rev. - Sci . Eng., 31 ( 3 ) , 215-354 , 1989 ) .

The most important application of said materials in cracking reactions, from an industrial point of view, is that called FCC (Fluid Catalytic Cracking) , whose purpose is to produce lighter hydrocarbon cuts starting from heavy charges such as vacuum gas oil, in particular within the boiling range of gasolines. The catalysts currently used in this process are Y-type zeolites (IUPAC abbreviation: FAU) with various additions . A different type of cracking, for the charges used and types of products to be obtained, is called "selective cracking" . The purpose of selective cracking is to produce light olefins, such as ethylene and propylene, starting from C-C₈ hydrocarbon fractions, and therefore already in themselves light cuts, if compared to a vacuum gas oil. The advantage of this process consists in transforming low cost hydrocarbon fractions, not easily marketable, into olefins having a higher added value.

Several zeolite materials active in selective cracking reactions, are described in literature. For example EP-A- 109,059 and EP-A-109,060 describe the use of the zeolite ZSM-5 (IUPAC abbreviation: MFI) for selective cracking reactions . These documents show that the best catalytic performances, referring to the yield to ethylene and propyl- ene, are obtained when the Si0₂/Al₂0₃ ratio of the zeolite is high. More specifically, EP-A-109,060 discloses, for zeolites of the MFI (ZSM-5) type, Si0₂/Al₂0₃ ratios higher or equal to 350 (mol/mol) .

WO 99/57226 describes a method for the conversion of hydrocarbon charges, with boiling points within the naphtha range, to propylene, in the presence of medium-pore zeolites having an Si0₂/Al₂0₃ ratio higher than 200 (mol/mol) . In particular, ZSM-22, ZSM-23, ZSM-35, ZSM-48 and ZSM-57, are claimed. The above-mentioned document provides two ex- perimental examples: in the first, three medium-pore zeolites are compared ZSM-48 (SiO₂/Al₂O₃>150) , ZSM-22 (SiO₂/Al₂O₃>150) and ZSM-5 (Si0₂/Al₂0₃=55) , showing that the selectivity to propylene of the two first catalysts is higher with respect to ZSM-5. In the second example, two ZSM-22 zeolites having a different Si0₂/Al₂0₃ ratio (>150 and 120) are compared. The one with the higher Si0₂/Al₂0₃ ratio proves to have the higher selectivity to propylene.

WO 99/29805 describes a process for the production of propylene starting from C and higher olefin streams, in the presence of the MFI (ZSM-5) zeolite having an Si0₂/Al0₃ molar ratio of at least 180.

WO 01/90034 describes a process for producing propylene starting from hydrocarbon mixtures, prevalently olefins, in the presence of a large-pore zeolite with an Si0₂/Al₂0₃ molar ratio lower than 200, in particular ZSM-12. Finally, US-4975401 claims a mixture of ZSM-5 and ZSM-

12 zeolites in which both can vary from 10 to 90% w/w, which can be used for other types of reactions (cracking of alkyl benzenes, to produce mainly benzene, toluene and xy- lenes) .

Technical experts in the field still feel the necessity, however, of using materials suitable for giving ever- increasing conversions, and at the same time having a high stability of the catalytic activity with time. A very im- portant problem, in fact, but with scarce consideration in literature, specifically consists of the poor stability of the catalytic material over a period of time.

A process has now been found, which uses catalytic systems, containing two different types of zeolites, capa- ble of enhancing the yield to propylene and ethylene and obtaining an ethylene/propylene molar ratio < 1, with the further advantage of maintaining the 'catalytic performances almost constant with time.

The process, object of the present invention, for the production of mixtures containing propylene and ethylene with an ethylene/propylene molar ratio < 1, starting from C-Cs, preferably C₄-C₆, hydrocarbon mixtures, prevalently with an olefin component, comprises putting said hydrocarbon mixtures in contact, under cracking conditions, with a catalytic system, at a temperature ranging from 400°C to 600°C, preferably from 500°C to 600°C, at a weight hourly space velocity (WHSV) from 0.1 if¹ to 20 if¹, preferably from 1 h^-1 to 6 h^_1, at a pressure ranging from 0.1 atm to 30 atm, preferably from 1 atm to 3 atm, and is character- ized in that the catalytic system contains a first zeolite with a Constraint Index (C.I) between 1.5 and 2.5, in a quantity ranging from 20 and 80% w/w, preferably from 25 to 75%, more preferably from 25 to 55%, and a second zeolite with a Constraint Index (C.I) between 6.9 and 11.0, in a quantity ranging from 20 to 80% w/w, preferably from 25 to 75%, more preferably from 45 to 75% w/w, the Si0₂/Al₂0₃ molar ratio of both said zeolites being higher than 50, preferably higher than 100.

The Constraint Index (C.I., US Patent 4,016,218) rep- resents the experimental ratio between the cracking rate of n-hexane and 3-methyl pentane, in an equimolecular mixture of the two components .

The first zeolite, having a C.I. between 1.5 and 2.5, is preferably ZSM-12, whereas the second zeolite with a C.I. between 6.9 and 11.0, is preferably selected from ZSM- 5 and ZSM-23.

The C-C₈, preferably C₄-C₆, hydrocarbon mixtures, coming, from example, from steam cracking and from catalytic cracking, have an olefin content preferably of at least 60% w/w, more preferably of at least 70% w/w. Typical examples of hydrocarbons which form the above hydrocarbon mixtures are: 1-butene, trans-2-butene, cis-2- butene, n-butane, isobutane, propane, pentane, isopentane, 1-pentene, 2-pentene, n-hexane, 1-hexene, 2-hexene. The process of the present invention can be carried out using any reactor solution, for example a fixed bed reactor, a moving bed, a "riser" or a fluid bed reactor, preferably a fixed bed reactor.

The zeolite can be used as such or mixed with inert products, in granular or pellet form.

A further object of the present patent application relates to the catalytic system containing two zeolites and possibly a binder, the first zeolite being a ZSM-12, in an amount ranging from 20 to 80% w/w preferably between 25 and 55%, and the second zeolite being a ZSM-23 in an amount ranging from 20 to 80% w/w preferably between 45 and 75%, the Si0₂/Al₂0₃ molar ratio of both said zeolites being higher than 50, preferably higher than 100.

The binder is an inorganic oxide, preferably selected from alumina, silica, zirconia and magnesia. The binder percentage is between 10 and 90%, preferably between 20 and 50%.

The catalytic system can be obtained either by mixing the two calcined zeolites, as powder or as tablets and "meshed" or as extruded products, or by putting the gels of the two syntheses of ZSM-12 and ZSM-23 in contact with each other.

When the binder is present, the preparation of the catalytic system also includes the extrusion phase. The binder (for example silica) is mixed with the zeolite; this mixture is extruded in the desired form, pellets, for example. The extruded product is then calcined in air, at a temperature between 300 and 800°C for a period of time between 1 and 48 h. A further object of the present invention relates to the catalytic system consisting of two zeolites, the first zeolite being a ZSM-12 in an amount ranging from 25 to 55% w/w, preferably between 35 and 51%, the second zeolite being a ZSM-5 in an amount ranging from 45 to 75% w/w, pref- erably between 49 and 65%, the Si0₂/Al₂θ₃ molar ratio of both said zeolites being higher than 50, preferably higher than 100.

The catalytic system can be obtained either by mixing the two calcined zeolites, as powder or as tablets and "meshed" or as extruded products, or by putting the gels of the two syntheses of ZSM-12 and ZSM-5 in contact with each other .

The catalytic system can optionally consist of a binder preferably selected from alumina, silica, zirconia and magnesia, in addition to said zeolites ZSM-12 and ZSM-5 in the above-mentioned amounts .

Should the binder also be present, the preparation of the catalytic system also includes the extrusion phase. The binder (silica, for example) is mixed with the zeolite; this mixture is extruded in the desired form, for example pellets. The extruded product is then calcined in air, at a temperature ranging from 300 to 800°C, for a period of time ranging from 1 to 48 h.

The following examples are provided for a better un- derstanding of the present invention. EXAMPLE 1 (Synthesis of ZSM-12)

ZSM-12 was obtained by crystallizing, at 160°C for 72 h, a reaction mixture having the following composition in molar ratios: Si0₂/Al₂0₃ = 100, TEA-OH/ Si0₂ = 0.15, H₂0/Si0₂ = 7.3 and Na/Si0₂ = 0.02. 151.2 gr of an aqueous solution of tetraethyl ammonium hydroxide (TEA-OH) at 35% by weight (Sachem) are heated to 60°C; 4.38 g of sodium aluminate (Carlo Erba: 56% by weight of alumina) are slowly added. 360 g of LUDOX HS 40% colloidal silica (Du Pont) are charged into an autoclave, regulating the stirring at 119 m/min and the aqueous solution of NaAl0/TEAOH previously prepared is then added. The autoclave is thermostat- regulated at 55°C and the mixture is left under stirring for 30 minutes; the crystallization phase is initiated by lowering the stirring rate (73 m/min) and increasing the temperature (160°C for 72h) . A homogenous and latescent slurry is obtained, from which the zeolite is isolated in powder form. The crystallized reaction mixture is filtered, the precipitate is then re-suspended in demineralized water and filtered again. The washing in water is repeated two or three times, the mixture is then dried at 100°C and calcined at 550°C for 5 h. The zeolite in sodium form obtained after this treatment, is treated with a solution of ammonium acetate at 60°C to obtain ion exchange. The acid form is obtained after a second calcination at 550°C. EXAMPLE 2 (Synthesis of ZSM-5)

ZSM-5 was obtained by crystallizing, at 180°C for 4 h, a reaction mixture with the following composition in molar ratios: Si0₂/Al₂0₃ = 700, TPA-OH/Si0₂ = 0.25 and H₂0/Si0₂ = 40. 0.32 g of aluminum isopropoxide are dissolved in 70.9 g of an aqueous solution of tetrapropyl ammonium hydroxide at 40% by weight (Sachem) heated to 50°C; a yellowish solution is obtained, to which 113 g of demineralized water are added. After regulating the stirring rate to 42 m/min, 114.5 g of tetraethyl orthosilicate (TEOS) are added. 240 g of demineralized water are added and the mixture is left to age at room temperature for 16 h. The stirring rate is reduced to 30 m/min and the crystallization is effected by thermostat-regulation at 180°C for 4 h. A latescent slurry is obtained, from which the ZSM-5 can be isolate in powder form. The crystallized reaction mixture is centrifuged, the precipitate is suspended in demineralized water and centrifuged again. The washing in water is repeated two or three times, the mixture is then dried at 100°C and calcined at 550°C for 5 h. EXAMPLE 3

The reaction mixtures discharged from the synthesis reactor of ZSM-12 (EXAMPLE 1) or ZSM-5 (EXAMPLE 2) after crystallization, can be preserved as such, until they are mixed. In order to mix the slurries and obtain an end mixture with the zeolites in the desired ratio, the percentage of solid present in each slurry is measured by collecting a small weighed portion thereof. The quantities used for obtaining a mixture of ZSM-12/ZSM-5 with a weight ratio of 50:50, are indicated below.

138.3 g of ZSM-12 slurry (14.2 wt% of solid fraction) are added to 297.3 g of ZSM-5 slurry (6.6 wt% of solid fraction) . The mixture obtained is left under stirring at room temperature for 1 h and is then centrifuged, suspend- ing the solid in demineralized water and repeating the washing twice. The solid obtained is dried at 100°C and calcined at 550°C. The end product is suspended in a solution of ammonium acetate at 60°C to obtain the ion exchange. After two exchange treatments, the product is dried and calcined at 550°C. EXAMPLE 4

ZSM-5 and ZSM-12 zeolites are mixed as powders, in a quantity equal to 50% by weight of each; the mixture is then pelletized and meshed (20-40 mesh) . The ZSM-5 was syn- thesized according to Example 2, whereas the ZSM-12 was synthesized according to Example 1. 4.33 g of the mixture of ZSM-5 and ZSM-12 zeolites are mixed with an equal volume of Al₂0₃ (corundum) and charged into the central, isotherm zone of an AISI 321 steel reactor, with a fixed bed having a diameter of 10 mm, a length of 50 cm and a thermocouple inserted and fixed at the centre of the catalytic bed.

The feeding consisted of a gaseous mixture having the following composition: n-butane = 5.0%; isobutane = 0.2%; 1-butene = 25.0%; 2-butene = 15.0%; isobutene = 54.8%.

The percentage of olefins in the feeding is 94.8%. The catalytic test is carried out at T = 525°C; WHSV = 1.9 h Table 1 indicates the yields to products after 24 and 48 h of reaction time (t.o.s). The data of Table 1 indicate the high yields to ethylene and propylene, increasing with time, whereas the aromatic (BTEX and higher products) are formed in a lower quantity and decreasing with time. The catalyst corresponding to the invention is therefore highly selective, under the process conditions adopted, with respect to the light olefins, in particular propylene. EXAMPLE 5

ZSM-5 and ZSM-12 zeolites are mixed as powders, in a quantity equal to 50% by weight of each; the mixture is then pelletized and meshed (20-40 mesh) . The ZSM-5 was synthesized according to Example 2, whereas the ZSM-12 was synthesized according to Example 1. 4.33 g of the mixture of ZSM-5 and ZSM-12 zeolites are mixed with an equal volume of Al₂0₃ (corundum) and charged into the central, isotherm zone of an AISI 321 steel reactor.

The feeding consisted of a C₄ gaseous mixture (47.9% by weight: n-butane = 5.0%; isobutane = 0.2%; 1-butene = 25.0%; 2-butene = 15.0%; isobutene = 54.8%), and a C₅ liquid mixture (52.1% by weight: n-pentane = 40.0%; 1-pentene = 22.0%; 2-methyl-2-butene = 24.0%; cyclopentene = 14.0%).

The percentage of olefins in the feeding is therefore 76.7%. The two mixtures are contemporaneously fed from two different pumps, with an equal flow-rate.

The catalytic test is carried out at T = 525°C; WHSV = 1.9 hA

Table 2 indicates the yields to products after 24 and 48 h of reaction time (t.o.s).

Also in this case, the catalyst corresponding to the invention proves to be highly selective with respect to ethylene and propylene (especially propylene) . The overall yield to ethylene + propylene increases with time, whereas the yield to aromatics (BTEX and higher products) decreases .

EXAMPLE 6 (Comparative) 4.33 g of ZSM-5 zeolite synthesized according to Example 2, are mixed with an equal volume of Al₂0₃ (corundum) and charged into the central, isotherm zone of an AISI 321 steel reactor. The zeolite is pelletized and meshed (20-40 mesh) . The feeding consisted of a gaseous mixture having the following composition: n-butane = 5.0%; isobutane = 0.2%; 1-butene = 25.0%; 2-butene = 15.0%; isobutene = 54.8%.

The percentage of olefins is therefore 94.8%. The catalytic test is carried out at T = 525°C; WHSV = 1.9 h Table 3 indicates the yields to products after 24 and 48 h of reaction time (t.o.s) .

A comparison of the data of Tables 3 and 1 (the same feeding but a different catalyst) shows the higher yield to ethylene + propylene (in particular propylene) of the catalyst according to the invention, with respect to the use of ZSM-5 alone. Furthermore, the aromatic substances, whose formation is not only undesired but is also a source of coke and consequently more rapid deactivation of the cata- lyst, have lower yields, in the presence of the catalyst prepared according to the invention. EXAMPLE 7 (Comparative)

4.33 g of ZSM-5 zeolite synthesized according to Example 2, are mixed with an equal volume of Al₂0₃ (corundum) and charged into the central, isotherm zone of an AISI 321 steel reactor. The zeolite is pelletized and meshed (20-40 mesh) .

The feeding consisted of a C gaseous mixture (47.9% by weight: n-butane = 5.0%; 1-butene = 25.0%; 2-butene = 15.0%; isobutene = 55.0%), and a C₅ liquid mixture (52.1% by weight: n-pentane = 40.0%; 1-pentene = 22.0%; 2-methyl-

2-butene = 24.0%; cyclopentene = 14.0%).

In this case, the percentage of olefins is equal to 76.7%. The two mixtures are contemporaneously fed from two different pumps, with an equal flow-rate.

The catalytic test is carried out at T = 525°C; WHSV = 1.9 h

Table 4 indicates the yields to products after 24 and 48 h of reaction time (t.o.s) . A comparison of the data of Tables 4 and 2 (the same feeding, but different catalysts) demonstrates the necessity of mixing the two zeolites ZSM-5 and ZSM-12, rather than using ZSM-5 alone. EXAMPLE 8 (Comparative) 4.16 g of ZSM-12 zeolite synthesized according to Ex- ample 1, are mixed with an equal volume of A1₂0₃ (corundum) and charged into the central, isotherm zone of an AISI 321 steel reactor. The zeolite is pelletized and meshed (20-40 mesh) . The feeding consisted of a gaseous mixture having the following composition: n-butane = 5.0%; isobutane = 0.2%; 1-butene = 25.0%; 2-butene = 15.0%; isobutene = 54.8%.

The percentage of olefins is 94.8%. The catalytic test is carried out at T = 525°C; WHSV = 2.0 h

Table 5 indicates the yields to products after 24 and 48 h of reaction time (t.o.s).

By using ZSM-12 alone (rather than a mixture of ZSM-5 and ZSM-12, indicated in the invention and Table 1), a lower yield both to ethylene and propylene, is obtained, which also decreases with time. The percentage of Cβ-Cg oligomers, undesired products, on the other hand, is extremely high. EXAMPLE 9 (Comparative)

4.34 g of ZSM-12 zeolite synthesized according to Example 1, are mixed with an equal volume of Al₂0 (corundum) and charged into the central, isotherm zone of an AISI 321 steel reactor. The zeolite is pelletized and meshed (20-40 mesh) . The feeding consisted of a C gaseous mixture (47.8% by weight: n-butane = 5.0%; 1-butene = 25.0%; 2-butene =

15.0%; isobutene = 55.0%), and a C₅ liquid mixture (52.2% by weight: n-pentane = 38.8%; 1-pentene = 22.0%; 2-methyl- 2-butene = 24.8%; cyclopentene = 14.4%).

The percentage of olefins is therefore equal to 77.3% by weight. The two mixtures are contemporaneously fed from two different pumps, with an equal flow-rate.

The catalytic test is carried out at T = 525°C; WHSV = 2.0 hA

Table 6 indicates the yields to products after 24 and 48 h of reaction time (t.o.s).

By using ZSM-12 alone (rather than a mixture of ZSM-5 and ZSM-12, indicated in the invention and Table 2), a lower yield both to ethylene and propylene, is obtained, which also decreases with time. The percentage of C₆-C9 oligomers, undesired products, on the other hand, is extremely high.

Table 1 - Yield (%)to products at 24 and 48 h (Example 4]

Table 2 - Yield (%)to products at 24 and 48 h (Example 5)

- Ii Table 3 - Yield (%)to products at 24 and 48 h (Example 6 ]

Table 4 - Yield (%)to products at 24 and 48 h (Example 7)

Table 5 - Yield (%)to products at 24 and 48 h (Example 8)

Table 6 - Yield (%)to products at 24 and 48 h (Example 9)

Claims

1. A process for the production of mixtures containing propylene and ethylene having an ethylene/propylene molar ratio <1, starting from C-C₈ hydrocarbon mix- tures, prevalently olefinic, which comprises putting said hydrocarbon mixtures in contact with a catalytic system, under cracking conditions, at a temperature ranging from 400°C and 600°C, at a weight hourly space velocity (WHSV) ranging from 0.1 h^"1 to 20 h^"1 and at a pressure ranging from 0.1 to 30 atm, characterized in that the catalytic system contains a first zeolite with a Constraint Index (C.I.) of between 1.5 and 2.5, in a quantity ranging from 20 to 80% w/w, and a second zeolite with a Constraint Index (C.I.) of between 6.9 and 11.0, in a quantity ranging from 20 to 80% w/w, the molar ratio Si0₂/Al₂0₃ of both said zeolites being higher than 50.

2. The process according to claim 1, wherein the cracking is carried out at a temperature of between 500°C and 600°C, at a weight hourly space velocity (WHSV) ranging from 1 h^"1 to 6 h^_1 and at a pressure ranging from 1 to 3 atm.

3. The process according to claim 1, wherein the cracking is carried out in a fixed bed reactor.

4. The process according to claim 1, wherein the first zeolite having a C.I. of between 1.5 and 2.5 is a ZSM- 12.

5. The process according to claim 1, wherein the second zeolite having a C.I. of between 6.9 and 11.0, is se- lected from ZSM-5 and ZSM-23.

6. The process according to claim 1, wherein the content of the first zeolite is between 25 and 75% weight and the content of the second zeolite is between 25 and 75% weight.

7. The process according to claim 1, wherein the content of the first zeolite is between 25 and 55% weight and the content of the second zeolite is between 45 and 75% weight.

8. The process according to claim 1, wherein the molar ratio Si0/Al₂0₃ of both zeolites is higher than 100.

9. A catalytic system containing two zeolites, the first zeolite being a ZSM-12 in an amount of between 25 and 75% w/w and the second zeolite being a ZSM-23 in an amount of between 25 and 75% w/w, the molar ratio Si0₂/Al₂0₃ of both said zeolites being higher than 50.

10. The catalytic system according to claim 9 wherein a binder selected from alumina, silica, zirconia and magnesia is also present.

11. The catalytic system according to claim 9 or 10 wherein the content of ZSM-12 in the catalytic system is between 30 and 60% weight and the content of ZSM-23 is between 40 and 70% weight.

12. The catalytic system according to claim 9 or 10 wherein the molar ratio Si0₂/Al₂0₃ of both ZSM-12 and ZSM-23 is higher than 100.

13. A catalytic system consisting of two zeolites, the first zeolite being a ZSM-12 in an amount of between 25 and 75% weight and the second zeolite being a ZSM-5 in an amount of between 25 and 75% weight, the molar ratio Si0₂/Al₂0₃ of both ZSM-12 and ZSM-5 being higher than 50.

14. A catalytic system consisting of two zeolites and a binder, the first zeolite being a ZSM-12 in an amount of between 25 and 55% weight and the second zeolite being a ZSM-5 in an amount of between 45 and 75% weight, the molar ratio Si0₂/Al₂0₃ of both said zeolites being higher than 50, the binder being selected from alumina, silica, zirconia and magnesia.

15. The catalytic system according to claim 13 or 14 wherein the content of ZSM-12 in the catalytic system is between 35 and 51% weight, and the content of ZSM-5 is between 49 and 65% weight.

16. The catalytic system according to claim 13 or 14 wherein the molar ratio Si0₂/Al₂03 both of ZSM-12 and ZSM-5 is higher than 100.