EP0515270A1 - Hydrocracking of Fischer-Tropsch paraffines with catalysts based on zeolithe-H-Y - Google Patents

Abstract

Process for hydrocracking of feedstocks originating from the Fischer-Tropsch process, in which a) hydrogen is reacted with the feedstock in contact with a catalyst 1 in a first reaction zone, the said catalyst 1 comprising at least one alumina-based matrix and at least one component for hydrodehydrogenation, b) the effluent originating from the first reaction zone is brought into contact with a catalyst 2 in a second reaction zone, the said catalyst 2 comprising - from 20 to 97 % by weight of at least one matrix - from 3 to 80 % by weight of at least one zeolite Y in hydrogen form, the said zeolite being characterised by an SiO2/Al2O3 molar ratio higher than 4.5, a sodium content lower than 1 % by weight, determined on zeolite calcined at 1100 DEG C, an elementary crystal lattice constant a0 lower than 24.70 x 10<-10> m, and a specific surface, determined by the BET method, greater than 400 m<2>.g<-1>, - at least one component for hydrodehydrogenation. <

Description

The present invention relates to a process for converting paraffins from the Fischer-Tropsch process. In particular, it uses bifunctional zeolitic catalysts for the hydrocracking of paraffins originating from the Fischer-Tropsch process, making it possible to obtain highly recoverable products such as kerosene, diesel oil and especially base oils.

The present invention relates to a process for converting paraffins from the Fischer-Tropsch process using a bifunctional catalyst containing a faujasite-type zeolite which can be specially modified, dispersed in a matrix generally based on alumina, silica, silica-alumina, alumina-boron oxide, magnesia, silica-magnesia, zirconia, titanium oxide or based on a combination of at least two of the preceding oxides, or based on a clay, or d 'a combination of the preceding oxides with clay. The role of this matrix is in particular to help shape the zeolite, in other words to produce it in the form of agglomerates, beads, extrudates, pellets, etc., which can be placed in an industrial reactor. The proportion of matrix in the catalyst is between 20 and 97% by weight and preferably between 50 and 97% by weight.

In the Fischer-Tropsch process, the synthesis gas (CO + H₂) is catalytically transformed into oxygenated products and hydrocarbons, essentially linear, in gaseous, liquid or solid form. These products are free of heteroatomic impurities such as for example sulfur, nitrogen or metals. However, these products cannot be used as such, in particular because of their cold resistance properties which are not very compatible with the usual uses of petroleum fractions. For example, the pour point of a linear hydrocarbon containing 20 carbon atoms per molecule (boiling point equal to approximately 344 ° C, that is to say included in the diesel cut) is +37 ° C approximately when the customs specifications require a pour point below -7 ° C for commercial gas oils. These hydrocarbons from the Fischer-Tropsch process must then be transformed into more recoverable products such as kerosene and diesel after having undergone catalytic hydrocracking reactions.

The catalysts used in hydrocracking are all of the bifunctional type combining an acid function with a hydrogenating function. The acid function is provided by supports of large surfaces (generally, 150 to 800 m².g⁻¹) having a surface acidity, such as halogenated aluminas (chlorinated or fluorinated in particular), combinations of oxides of boron and aluminum, amorphous silica-aluminas and zeolites. The hydrogenating function is provided either by one or more metals from group VIII of the periodic table of elements, such as iron, cobalt, nickel, ruthenium, rhodium, palladium osmium, iridium and platinum, or by a combination of at least one metal of group VI of the periodic table such as chromium, molybdenum and tungsten and of at least one metal of group VIII.

The balance between the two acid and hydrogenating functions is the fundamental parameter which governs the activity and the selectivity of the catalyst. A weak acid function and a strong hydrogenating function give catalysts which are not very active and selective towards isomerization whereas a strong acid function and a weak hydrogenating function give very active and selective catalysts towards cracking. It is therefore possible, by judiciously choosing each of the functions, to adjust the activity / selectivity pair of the catalyst.

Among the acid supports, there are, in order of increasing acidity, aluminas, halogenated aluminas, amorphous silica-aluminas and zeolites.

Patent EP-A.147873 describes a catalyst comprising an element of group VIII on a support during a process carrying out the Fischer-Tropsch synthesis reaction then hydrocracking. Patent application EP.B.356560 describes the preparation of a very specific Y zeolite which can be used in a catalyst of the Fischer-Tropsch reaction or in a hydrocracking catalyst.

The catalyst of the present invention contains a Y zeolite with a faujasite structure (Zeolite Molecular Sieves Structure, chemistry and use, DW Breck, J. Willey and Sons, 1973). Among the Y zeolites that can be used, one will preferably use a stabilized Y zeolite, commonly called ultrastable or USY, either in the form partially exchanged with rare earth cations of atomic number. 57 to 71 inclusive so that its rare earth content expressed in% by weight of rare earth oxides is less than 10% by weight, preferably less than 6% by weight, or in hydrogen form.

The significant research carried out by the applicant on numerous zeolites led him to discover that, surprisingly, the use of a catalyst comprising a Y zeolite makes it possible to convert the charges resulting from the Fischer-Tropsch process in order to obtain more valuable products.

The zeolite used in the catalyst of the present invention is preferably an acidic zeolite HY characterized by different specifications: a SiO₂ / Al₂O₃ molar ratio greater than 4.5 and preferably between 8 and 70; a sodium content of less than 1% by weight and preferably less than 0.5% by weight, determined on the zeolite calcined at 1100 ° C; a crystalline parameter a _o of the elementary mesh less than 24.70 x 10⁻¹⁰ meter and preferably between 24.24 x 10⁻¹⁰ meter and 24.55 x 10⁻¹⁰ meter; a specific surface area determined by the BET method greater than 400m² / g and preferably greater than 550m² / g.

• le rapport molaire SiO₂/Al₂O₃ est mesuré par fluorescence X. Quand les quantités d'aluminium deviennent faibles, par exemple inférieures à 2% en poids, il est opportun d'utiliser une méthode de dosage par spectrométrie d'adsorption atomique pour plus de précision.
• le paramètre de maille est calculé à partir du diagramme de diffraction aux rayons X, selon la méthode décrite dans la fiche ASTM D3.942-80.
• la surface spécifique est déterminée par mesure de l'isotherme d'adsorption diazote à la température de l'azote liquide et calculée selon la méthode classique B.E.T.. Les échantillons sont prétraités, avant la mesure, à 500°C sous balayage d'azote sec.

The different characteristics are measured by the methods specified below:

• the SiO₂ / Al₂O₃ molar ratio is measured by X-ray fluorescence. When the quantities of aluminum become low, for example less than 2% by weight, it is advisable to use an assay method by atomic adsorption spectrometry for more precision.
• the mesh parameter is calculated from the X-ray diffraction diagram, according to the method described in sheet ASTM D3.942-80.
• the specific surface is determined by measuring the nitrogen adsorption isotherm at the temperature of liquid nitrogen and calculated according to the conventional BET method. The samples are pretreated, before measurement, at 500 ° C under dry nitrogen sweep.

This zeolite is known from the prior art (French patent 2,561,946). The NaY zeolite generally used as raw material has more than 5% by weight of sodium and has a SiO₂ / Al₂O₃ molar ratio of between 4 and 6. It is not used as such, but it must be subjected to a series of treatments stabilization aimed at increasing its acidity and thermal resistance.

It can be stabilized by different methods.

The stabilizations of zeolite Y are most commonly carried out, either by the introduction of rare earth cations or group IIA metal cations, or by hydrothermal treatments. All these treatments are described in French patent FR 2 561 946.

However, there are other stabilization methods known from the prior art. These include the extraction of aluminum with chelating agents, such as ethylene tetraacetic diammine or acetylacetone. It is also possible to make partial substitutions of aluminum atoms of the crystal lattice with exogenous silicon atoms. This is the principle of the treatments carried out at high temperature with silicon tetrachloride, described in the following reference: H.R. BEYER et al., Catalysis by zéolites, Ed. B. Imelik et al., Elsevier, Amsterdam -1980- p. 203. This is also the principle of treatments carried out in the liquid phase with fluorosilicic acid, or salts of this acid, method described in the following patents: US-A-3,594,331, US-A-3,933,983 and EP-B-002211.

It is possible after all these stabilizing treatments to carry out exchanges with cations of metals of group IIA, with cations of rare earths, or cations of chromium and zinc, or with any other element useful for improving the catalyst.

The HY or NH₄Y zeolite thus obtained or any HY or NH₄Y zeolite having these characteristics can be introduced at this stage, into the matrix described above in the form of an alumina gel. The catalyst thus obtained comprises, by weight, from 20 to 97% of matrix, 3 to 80% of zeolite and at least one hydro-dehydrogenation component. One of the preferred methods in the present invention, for the introduction of the zeolite into the matrix, consists in co-kneading the zeolite and the gel and then passing the dough thus obtained through a die to form extrudates of diameter between 0.4 and 4 millimeters.

The hydro-dehydrogenation component of the catalyst of the present invention is for example at least one compound, for example an oxide, of a metal from group VIII of the periodic table of the elements, (in particular nickel, palladium or platinum ), or a combination of at least one compound of a group VI metal (especially molybdenum or tungsten) and at least one compound of a group VIII metal (especially cobalt or nickel) of the periodic classification of the elements.

The concentrations of the metal compounds, expressed by weight of metal relative to the finished catalyst, are the following: between 0.01 and 5% by weight of group VIII metals, and preferably between 0.03 and 3% by weight, in the case where it is they are only noble metals of the palladium or platinum type; between 0.01 and 15% by weight of group VIII metals, and preferably between 0.05 and 10% by weight, in the case of non-noble metals of group VIII of the nickel type for example; when at least one metal or group VIII metal compound and at least one group VI metal compound are used, between 5 and 40% by weight of a combination of at least one compound is used (oxide in particular) of a group VI metal (molybdenum or tungsten in particular) and at least one metal or compound of group VIII metal (cobalt or nickel in particular) and preferably between 12 and 30% by weight, with a weight ratio (expressed as metal oxides) metals of group VIII to metals of group VI between 0.05 and 0.8 and preferably between 0.13 and 0.5. This catalyst may advantageously contain phosphorus; in fact, it is known in the prior art that this compound brings two advantages to hydrotreatment catalysts: ease of preparation during in particular the impregnation of nickel and molybdenum solutions, and better hydrogenation activity. The phosphorus content, expressed as a concentration of phosphorus oxide P₂O₅, will be less than 15% by weight and preferably less than 10% by weight. The hydrogenating function as defined above can be introduced into the catalyst at various levels of the preparation and in various ways as described in French patent FR 2 561 946.

The NH₄Y or HY zeolite catalysts as described above undergo, if necessary, a final calcination step in order to finally obtain a Y zeolite catalyst in hydrogen form. The catalysts thus finally obtained are used for the hydrocracking of paraffin feeds from the Fischer-Tropsch process under the following conditions: hydrogen is reacted with the feed in contact with a catalyst 1 contained in the reactor R1 (or first reaction zone R1) whose role is to eliminate unsaturated and oxygenated hydrocarbon molecules produced during Fischer-Tropsch synthesis. The effluent from reactor R1 is brought into contact with a second catalyst 2 contained in reactor R2 (or second reaction zone R2) whose role is to ensure the hydrocracking reaction. The effluent from reactor 2 is divided into various conventional petroleum fractions such as gas, light petrol, heavy petrol, kerosene, diesel and "residue"; the fraction called "residue" represents the heaviest fraction obtained during fractionation. The choice of temperatures during the effluent fractionation step from reactor 2 can vary very greatly depending on the specific needs of the refiner. The adjustment of the reaction temperature makes it possible to obtain variable yields from each cut.

Various modifications can be made. It is possible to recycle at reactor 1 or preferably at reactor 2 at least one of these fractions; if a single reactor comprising the two catalysts is used, that is to say if a single reactor comprises the two reaction zones, it is possible to recycle at the inlet of the reactor. Finally, it is possible to use only the reactor 2 if the contents of unsaturated products in the feed do not lead to too great deactivation of the catalytic system. The fraction called "residue" can also undergo dewaxing operations in order to recover base oil.

• le but principal recherché est la conversion hydrocraquante de la charge c'est-à-dire la transformation de la charge en produits plus légers. Cette conversion hydrocraquante est souvent comprise entre 20 et 100% en poids et de préférence entre 25 et 98% en poids.
• la pression partielle d'hydrogène est comprise entre 5 et 200 bars et de préférence entre 30 et 150 bars.
• les conditions opératoires sont, au niveau de la zone R2, une Vitesse Volumique Horaire (VVH) comprise entre 0.2 et 10 M³ de charge/m³ de catalyseur/heure et de préférence entre 0.3 et 2, une température de réaction comprise entre 150°C et 450°C et de préférence entre 250°C et 420°C. Les conditions opératoires appliquées au niveau de la zone R1 peuvent être très variables suivant la charge et ont pour but de diminuer les concentrations en composés insaturés et/ou hétéroatomiques à des valeurs convenables. Dans ces conditions opératoires, la durée de cycle du système catalytique est d'au moins un an et de préférence égale à 2 ans et la désactivation du catalyseur, c'est-à-dire l'augmentation de température que doit subir le système catalytique pour que la conversion soit constante, est inférieure à 5°C/mois et de préférence inférieure à 2.5°C/mois.
• les distillats moyens et les huiles de base obtenus grâce au procédé de l'invention présentent de très bonnes caractéristiques, du fait de leur caractère très paraffinique. Par exemple, il est possible d'obtenir une coupe kérosène d'intervalle de distillation compris entre 150°C et 250°C ayant un point de fumée supérieur à 50 millimètres, une coupe gas-oil d'intervalle de distillation compris entre 250°c et 380°C d'indice de cétane égal ou supérieur à 65; le VI de l'huile obtenue, après déparaffinage au solvant MEK/toluène de la coupe 380⁺, est égal ou supérieur à 135 et le point d'écoulement obtenu est inférieur ou égal à -12°C. Le rendement en huile par rapport au résidu dépend de la conversion globale de la charge. Dans le cas du catalyseur zéolithique, ce rendement est compris entre 5 et 70% en poids environ et de préférence entre 10 et 60% en poids.

The use of such a process has several characteristics:

• the main goal sought is the hydrocracking conversion of the charge, that is to say the transformation of the charge into lighter products. This hydrocracking conversion is often between 20 and 100% by weight and preferably between 25 and 98% by weight.
• the partial pressure of hydrogen is between 5 and 200 bars and preferably between 30 and 150 bars.
• the operating conditions are, at the level of the zone R2, an Hourly Volume Speed (VVH) of between 0.2 and 10 M³ of charge / m³ of catalyst / hour and preferably between 0.3 and 2, a reaction temperature of between 150 ° C. and 450 ° C and preferably between 250 ° C and 420 ° C. The operating conditions applied in the R1 zone can be very variable depending on the charge and are intended to reduce the concentrations of unsaturated and / or heteroatomic compounds to suitable values. Under these operating conditions, the cycle time of the catalytic system is at least one year and preferably equal to 2 years and the deactivation of the catalyst, that is to say the increase in temperature which the catalytic system must undergo so that the conversion is constant, is less than 5 ° C / month and preferably less than 2.5 ° C / month.
• middle distillates and base oils obtained by the process of the invention have very good characteristics, due to their very paraffinic nature. For example, it is possible to obtain a kerosene cut with a distillation interval between 150 ° C. and 250 ° C. having a smoke point greater than 50 millimeters, a diesel cut with a distillation interval between 250 ° C. and 380 ° C. with a cetane number equal to or greater than 65; the VI of the oil obtained, after dewaxing with MEK / toluene solvent from section 380⁺, is equal to or greater than 135 and the pour point obtained is less than or equal to -12 ° C. The oil yield relative to the residue depends on the overall conversion of the charge. In the case of the zeolitic catalyst, this yield is between 5 and 70% by weight approximately and preferably between 10 and 60% by weight.

The catalyst 1 of the first stage consists of an alumina-based matrix, preferably not containing a zeolite, and of at least one metal having a hydrodehydrogenating function. Said matrix can also contain silica-alumina, boron oxide, magnesia, zirconia, titanium oxide, clay or a combination of these oxides. The hydro-dehydrogenating function is provided by at least one metal or group VIII metal compound such as nickel and cobalt in particular. It is possible to use a combination of at least one metal or compound of group VI metal (in particular molybdenum or tungsten) and of at least one metal or compound of group VIII metal (in particular cobalt or nickel). The total concentration of metals of groups VI and VIII, expressed in metal oxides, is between 5 and 40% by weight and preferably between 7 and 30% by weight and the weight ratio expressed in metal oxide metal (or metals) of the group VI on metal (or metals) of group VIII is between 1.25 and 20 and preferably between 2 and 10. In addition, this catalyst may contain phosphorus. The phosphorus content, expressed as a concentration of phosphorus oxide P₂O₅, will be less than 15% by weight and preferably less than 10% by weight.

The catalyst contained in the R2 repeater is the catalyst described in the main part of the text. It especially comprises at least one HY zeolite characterized by a SiO₂ / Al₂O₃ molar ratio greater than 4.5 and preferably between 8 and 70; a sodium content of less than 1% by weight and preferably less than 0.5% by weight determined on the zeolite calcined at 1100 ° C; a crystalline parameter a _o of the elementary mesh less than 24.70 x 10⁻¹⁰ meter and preferably between 24.24 x 10⁻¹⁰ meter and 24.55 x 10⁻¹⁰ meter; a specific surface area determined by the BET method greater than 400m².g⁻¹ and preferably greater than 550m².g⁻¹.

The examples presented below illustrate the characteristics of the present invention without however limiting its scope.

Example 1 Preparation of catalyst A (not in accordance with the invention)

The alumina gel used is supplied by the company Condéa under the reference SB3. After mixing, the dough obtained is extruded through a die with a diameter of 1.4mm. The extrudates are calcined and then impregnated to dryness with a solution of mixture of ammonium heptamolybdate, nickel nitrate and orthophosphoric acid, and finally calcined in air at 550 ° C. The weight contents, expressed in active oxides, are as follows (relative to the catalyst):

Example 2 Preparation of catalyst B (according to the invention)

rapport SiO₂/Al₂O₃ molaire

6.6

paramètre cristallin

24.55 x 10⁻¹⁰ mètre

surface spécifique

690 m²/g

est malaxée avec de l'alumine de type SB3 fournie par la société Condéa. La pâte malaxée est ensuite extrudée à travers une filière de diamètre 1.4 min. Les extrudés sont ensuite calcinés puis imprégnés à sec par une solution d'un mélange d'heptamolybdate d'ammonium, de nitrate de nickel et d'acide orthophosphorique, et enfin calcinés sous air à 550°C. Les teneurs pondérales, exprimées en oxydes actifs, sont les suivantes (par rapport au catalyseur):

A HY zeolite of formula H AlO₂ (SiO₂) _{3.3 is used,} supplied by the company Contéka under the reference CBV500. This zeolite whose characteristics are:

molar SiO₂ / Al₂O₃ ratio

6.6

crystal setting

24.55 x 10⁻¹⁰ meter

specific surface

690 m² / g

is kneaded with SB3 type alumina supplied by the company Condéa. The kneaded dough is then extruded through a die with a diameter of 1.4 min. The extrudates are then calcined then impregnated to dryness with a solution of a mixture of ammonium heptamolybdate, nickel nitrate and orthophosphoric acid, and finally calcined in air at 550 ° C. The weight contents, expressed in active oxides, are as follows (relative to the catalyst):

Example 3 Preparation of catalyst C (according to the invention)

The NaY zeolite is subjected to two exchanges in ammonium chloride solutions so that the sodium level is 2.6% by weight. The product is then introduced into a cold oven and calcined in air up to 400 ° C. At this temperature, a flow of water corresponding to a partial pressure of 50.7 kPa is introduced into the calcination atmosphere. The temperature is then brought to 565 ° C for two hours. The product is then subjected to an exchange with an ammonium chloride solution followed by a very gentle acid treatment under the following conditions: volume of hydrochloric acid 0.4 N on weight of solid of 10, duration of 3 hours. The sodium level then drops to 0.6% by weight, the SiO₂ / Al₂O₃ ratio is 7.2. This product is then subjected to a brutal calcination in a static atmosphere at 780 ° C. for 3 hours, then again taken up in acid solution with hydrochloric acid of normality 2 and a ratio volume of solution to weight of zeolite of 10. The crystalline parameter is 24.28 x 10⁻¹⁰ meter, the specific surface of 825m² / g, the water recovery capacity is 11.7, the sodium ion recovery capacity is 1.0 expressed by weight of sodium per 100 grams of dealuminated zeolite .
The zeolite thus obtained is kneaded with alumina of the SB3 type supplied by the company Condéa. The kneaded dough is then extruded through a die with a diameter of 1.4 mm. The extrudates are then calcined then impregnated to dryness with a solution of a mixture of ammonium heptamolybdate, nickel nitrate and orthophosphoric acid, and finally calcined in air at 550 ° C. The weight contents, expressed in active oxides, are as follows (relative to the catalyst):

Example 4 Preparation of catalyst D (not in accordance with the invention)

using a silica-alumina prepared in the laboratory containing 25% by weight of SiO₂ and 75% by weight of Al₂O₃. 3% by weight of pure nitric acid is added to 67% relative to the dry weight of silica-alumina powder in order to obtain the peptization of the powder. After mixing, the dough obtained is extruded through a die with a diameter of 1.4mm. The extrudates are calcined then impregnated to dryness with a solution of a salt of platinum chloride tetramine Pt (NH₃) ₄Cl₂ and finally calcined in air at 550 ° C. The platinum content of the final catalyst is 0.6% by weight.

Example 5 Evaluation of catalysts A, B, C and D in a hydrocracking test without recycling of the "residue" fraction

point initial

114°C

point 10%

285°C

point 50%

473°C

point 90%

534°C

point final

602°C

point d'écoulement

+67°C

densité (20/4)

0.825

The catalysts, the preparations of which are described in the preceding examples, are used under the conditions of hydrocracking on a charge of paraffins resulting from the Fischer-Tropsch synthesis, the main characteristics of which are as follows:

initial point

114 ° C

point 10%

285 ° C

point 50%

473 ° C

point 90%

534 ° C

period

602 ° C

pour point

+ 67 ° C

density (20/4)

0.825

The catalytic test unit comprises a single reactor in a fixed bed, with upward flow of charge ("up-flow"), into which 80 ml of catalyst is introduced. Catalysts A, B and C are sulfurized by an n-hexane / DMDS + aniline mixture up to 320 ° C. Catalyst D undergoes a reduction under hydrogen in situ in the reactor. The total pressure is 5 MPa, the hydrogen flow rate is 1000 liters of gaseous hydrogen per liter of charge injected, the hourly volume speed is 0.5.

The catalytic performances are expressed by the temperature which makes it possible to reach a net conversion level of 50% and by the gross selectivity. These catalytic performances are measured on the catalyst after a stabilization period, generally at least 48 hours, has been observed.

The net CN conversion is taken equal to:

The gross selectivity SB is taken equal to:

In the case of Example 3, a yield of 32% by weight of oil relative to the residue is obtained by dewaxing, said oil having an IV of 152.

The use of such a zeolite makes it possible to decrease the net conversion temperature CN substantially, since a gain of approximately 100 ° C. is observed between the catalyst without zeolite (catalyst of Example 1) and the catalysts in container (catalysts of Examples 2 and 3). Likewise, a gain of approximately 78 ° C. is observed between the silica-alumina-based catalyst (catalyst of Example 4) and the catalysts containing it (catalysts of Examples 2 and 3).

Compared to a zeolite which has not undergone extensive dealumination like that of Example 2, the use of a dealuminated zeolite such as that used in Example 3 makes it possible to improve the selectivity clearly.

In general, the selectivity varies greatly with the conversion. The selectivity is higher the lower the conversion.

Example 6: Evaluation of catalysts A, B, C and D in a hydrocracking test with recycling of the "residue" fraction

The charge and the test conditions are identical to those of Example 5. The use of recycling of the 380 fraction fraction, at the inlet of the reactor, makes it possible to obtain a total conversion of the charge. In this case, the term conversion by pass is used which represents the conversion actually carried out at the level of the catalyst.

The conversion by CP pass is taken equal to: $$\mathit{\text{CP}}\text{=}\frac{{\text{(weight 380⁻}}_{\text{effluents}}\text{)}}{{\text{(weight 380⁻}}_{\text{charge}}{\text{) + (weight 380 ⁺}}_{\text{charge}}\text{)}}\text{* 100}$$

The gross selectivity SB is taken equal to:

As in the case of Example 5, the use of a zeolite makes it possible to reduce the iso-conversion temperature substantially. Compared to a zeolite that has not undergone extensive dealumination like that of Example 2, the use of a dealuminated zeolite such as that used in Example 3 makes it possible to very clearly improve the selectivity .

Claims (10)

1) Process for hydrocracking feedstocks from the Fischer-Tropsch process in which: a) reacting hydrogen with the feedstock in contact with a catalyst 1 in a first reaction zone, said catalyst 1 comprising at least one alumina-based matrix and at least one hydrodehydrogenation component b) the effluent from the first reaction zone is brought into contact with a catalyst 2 in a second reaction zone, said catalyst 2 comprising: - from 20 to 97% by weight of at least one matrix, - from 3 to 80% by weight of at least one zeolite Y in hydrogen form, said zeolite being characterized by a SiO₂ / Al₂O₃ molar ratio greater than 4.5; a sodium content of less than 1% by weight determined on the zeolite calcined at 1100 ° C; a crystalline parameter a _o of the elementary mesh less than 24.70.10⁻¹⁻ m; a specific surface area determined by the BET method greater than 400 m².g⁻¹. - at least one hydro-dehydrogenation component. 2) Method according to claim 1 wherein the Y zeolite is characterized by a SiO₂ / Al₂O₃ molar ratio between 8 and 70; a sodium content of less than 0.5% by weight determined on the zeolite calcined at 1100 ° C; a crystalline parameter a _o of the elementary mesh between 24.24.10⁻¹⁰ and 24.55.10⁻¹⁰ m; a specific surface area determined by the BET method greater than 550 m².g⁻¹. 3) Method according to one of claims 1 and 2 wherein the hydro-dehydrogenation component is the combination of at least one metal or compound of group VIII metal and at least one metal or compound of group metal VI of the periodic table. 4) Method according to claim 3 wherein, in the case of the component of step b), is used between 5 and 40% by weight of metal compounds (relative to the finished catalyst), the weight ratio (expressed in metal oxides ) Group VIII metals on Group VI metals being between 0.05 and 0.8, and, in the case of the component of step a), between 5 and 40% by weight of metal compounds are used (relative to the finished catalyst), the weight ratio (expressed as metal oxides) of Group VIII metals to Group VI metals being between 1.25 and 20. 5) Method according to one of claims 1 and 2 wherein the hydro-dehydrogenation component is at least one metal or metal compound of group VIII of the periodic table of the elements. 6) The method of claim 5 wherein, in the case of the component of step b), the concentration of group VIII metal, expressed by weight relative to the finished catalyst, is between 0.01 and 5% in the case of a noble metal and between 0.01 and 15% by weight in the case of a non-noble metal. 7) Method according to one of claims 1 to 6 wherein the hydro-dehydrogenation component further comprises phosphorus. 8) Method according to claim 7 wherein the phosphorus content, expressed by weight of phosphorus oxide P₂O₅ relative to the finished catalyst, is less than 15%. 9) Method according to one of claims 1 to 8 wherein one performs a recycling of at least one of the fractions of the effluent from the second reaction zone at the entrance to one of the reaction zones. 10) The method of claim 9 wherein the recycling takes place at the entrance to the second reaction zone.