FR3059325A1 - USE OF ZEOLITE IZM-2 CATALYST AND EUO ZEOLITE CATALYST FOR ISOMERIZING C8 AROMATIC CUTTING - Google Patents

Abstract

An isomerization process of an aromatic section comprising at least one aromatic compound with eight carbon atoms per molecule, said process involving the successive use of two isomerization catalysts based on zeolite IZM-2 (A ) then based on EUO zeolite (B).

Description

® FRENCH REPUBLIC

NATIONAL INSTITUTE OF INDUSTRIAL PROPERTY © Publication number: 3,059,325 (to be used only for reproduction orders)

©) National registration number: 16 61673

COURBEVOIE © Int Cl ⁸ : C 07 C 15/08 (2017.01), C 07 C 5/27, B 01 J 29/74

A1 PATENT APPLICATION

©) Date of filing: 30.11.16. © Applicant (s): IFP ENERGIES NOUVELLES Etablis- (© Priority: public education - FR. @ Inventor (s): BOUCHY CHRISTOPHE. ©) Date of public availability of the request: 01.06.18 Bulletin 18/22. ©) List of documents cited in the report preliminary research: Refer to end of present booklet (© References to other national documents ® Holder (s): IFP ENERGIES NOUVELLES Etablisse- related: public. ©) Extension request (s): © Agent (s): IFP ENERGIES NOUVELLES.

USE OF A CATALYST BASED ON ZEOLITHE IZM-2 AND A CATALYST BASED ON ZEOLITHE EUO FOR THE ISOMERIZATION OF AROMATIC C8 CUTS.

©) A process for the isomerization of an aromatic cut comprising at least one aromatic compound containing eight carbon atoms per molecule is described, said process using the successive use of two isomerization catalysts based on zeolite IZM-2 (A) then based on EUO zeolite (B).

FR 3 059 325 - A1

The present invention relates to a process for the isomerization of an aromatic cut comprising at least one aromatic compound with eight carbon atoms per molecule, said process implementing the use of two isomerization catalysts: one based on IZM zeolite -2 (A) and another based on EUO zeolite (B).

State of the art

The sources of aromatics with eight carbon atoms mainly come from the reforming process (reformate) and steam cracking (pyrolysis essences). The distribution of aromatics with eight carbon atoms within these cuts is variable: generally from 10 to 30% of ethylbenzene with the three isomers of xylenes as a complement: para-xylene, meta-xylene and ortho-xylene . Typically the distribution within this complement of xylenes is 50% in meta-xylene, 25% in ortho-xylene and 25% in para-xylene. Within this complement of xylenes, para-xylene is a particularly desirable isomer. Indeed the latter, via dimethyl terephthalate and terephthalic acid, allows the production of polyester fibers used for clothing and resins and films of polyethylene terephthalate (PET). It is thus desirable to maximize the production of para-xylene to the detriment of other aromatics with eight carbon atoms. This is achieved by implementing catalytic isomerization processes. After extraction of the paraxylene, the residual cut, rich in meta-xylene, ortho-xylene and ethylbenzene is sent to a catalytic isomerization unit which gives a mixture of aromatics with eight carbon atoms in which the proportion of xylenes is close to thermodynamic equilibrium and the quantity of ethylbenzene reduced thanks to the conversion of ethylbenzene. This mixture is again sent to a para-xylene extraction unit and the residual cut sent to the isomerization unit. This creates a "C ₈ aromatic loop" which maximizes the production of para-xylene (E. Guillon, P. Leflaive, Techniques de l'Ingénieur, J5920, V3). An isomerization unit can be used to isomerize the xylenes to para-xylene and convert ethylbenzene to benzene by the dealkylation reaction of ethylbenzene. In this case we speak of "dealkylating" isomerization of the cut. The residual cut can also be sent to an isomerization catalytic unit, to isomerize the xylenes to para-xylene and convert ethylbenzene to xylenes by the isomerization reaction of ethylbenzene. This is called “isomerizing” isomerization of the section. These industrial processes generally use heterogeneous catalysts implemented in a fixed bed and operating in the vapor phase under hydrogen pressure. These two types of process are distinguished by the operating conditions and by the formulation of the catalysts used (by their nature and / or their content in hydro-dehydrogenating function and / or in acid). The present invention is in the field of "isomerizing" isomerization.

In the case of "isomerizing" isomerization, the catalyst is of the bifunctional type and has both an acid function (generally provided by at least one zeolite) and a hydro-dehydrogenating function provided by a noble metal (generally platinum) . It has indeed been shown that the isomerization of ethylbenzene into xylenes involves a bifunctional type mechanism. Ethylbenzene is first hydrogenated to ethylcyclohexenes on the metal sites, these cycloolefin intermediates are then isomerized to dimethylcyclohexenes on the acid sites of Bronsted. Finally the dimethylcyclohexenes are dehydrogenated into xylenes on the metal sites. The use of a strong hydro-dehydrogenating function such as platinum also induces the production of naphthenic cycles by hydrogenation of the corresponding aromatic cycles.

In addition to the desired isomerization reactions, it is desirable to limit side reactions of the type:

- dealkylation of ethylbenzene to benzene and ethylene;

- disproportionation of ethylbenzene to diethylbenzene and benzene, or xylenes to toluene and aromatics with 9 carbon atoms;

- transfer of alkyls between ethylbenzene and xylenes;

- opening of naphthenic cycles and cracking.

All of these reactions generate the production of less recoverable molecules, which are not recycled in the "C ₈ aromatic loop" and are considered as net losses for the process. All molecules other than cyclic molecules with eight carbon atoms are thus considered to be net losses.

Isomerization reactions as well as side reactions are mainly catalyzed by the acid function. The properties of the zeolite (number and strength of Bronsted acid sites, topology of the microporous network, etc.), acting as an acid function, thus have a direct impact on the properties of the bifunctional catalyst, and in particular its selectivity.

The catalysis of the isomerization of an aromatic C ₈ cut into xylenes has thus been the subject of numerous patents relating to different zeolites. Among the zeolites used in isomerization of an aromatic C ₈ cut, there is ZSM-5, used alone or in mixture with other zeolites, such as for example mordenite. These catalysts are described in particular in US patents 4,467,129 B and US 4,482,773 B. Other catalysts mainly based on mordenite have been described for example in patent FR 2,477,903 B. A catalyst has also been proposed. based on a EUO structural type zeolite (EP 923 987) or based on a MTW structural type zeolite (WO 2005 065380 A, WO 2010 000652 A, US 2014 0296601 A) or also based on a zeolite UZM-8 in US Pat. No. 7,091,390 B. More recently, patents claim the use of catalysts based on zeolite IZM-2 (patent FR 2,934,793 B). Other patents claim methods for isomerizing C ₈ aromatic cuts using sequenced isomerization steps. Patent EP 1 620 374 B1 thus describes a first step of isomerization of the section on a first isomerization catalyst comprising a zeolite chosen from the zeolites ZSM, MCM, PSH-3 and SSZ-25, under operating conditions. isomerizing the xylenes to produce an effluent with an increased para-xylene concentration; at least part of said effluent undergoes a second isomerization step on a second isomerization catalyst comprising a zeolite chosen from the zeolites ZSM, SAPO-11 and NU-87, under operating conditions for isomerization of ethylbenzene in order to d 'Isomerize at least partially ethylbenzene to para-xylene and thus obtain an effluent having a para-xylene concentration in xylenes greater than its concentration at thermodynamic equilibrium under the isomerization conditions of ethylbenzene. US patent 2007 0004947 A1 describes a sequential isomerization in two stages which can be distinguished by the operating conditions used, the isomerization catalyst of the first stage not containing substantially platinum group metal, unlike the second catalyst. The concentration of para-xylene within the xylenes in the final effluent is greater than its concentration at thermodynamic equilibrium under the operating conditions of the second isomerization step.

These examples illustrate the continuous research carried out to develop ever more efficient processes for the isomerization of aromatic C ₈ cuts in order to maximize the production of para-xylene and limit the production of losses. These methods can use an isomerization catalyst or combinations of isomerization catalysts.

During its work, the Applicant has thus discovered that, surprisingly, the successive use of an isomerization catalyst using an IZM-2 (A) zeolite and then an isomerization catalyst using a zeolite of structural type EUO (B), makes it possible to obtain performances in terms of couple activity / net losses higher than those obtained when said catalysts based on IZM-2 (A) and based on zeolite of structural type EUO (B) are used individually and separately or as a mixture or in a different order of the process according to the invention.

Summary of the invention

The present invention relates to a process for the isomerization of an aromatic cut comprising at least one aromatic compound containing eight carbon atoms per molecule, said process using two isomerization catalysts and comprising the following steps:

(a) bringing said aromatic cut into contact with an isomerization catalyst comprising at least one IZM-2 zeolite at a temperature between 300 ° C. and 500 ° C., a partial hydrogen pressure ranging between 0.3 and 1 , 5 MPa, a total pressure between 0.45 and 1.9 MPa and a spatial feed speed, expressed in kilograms of aromatic fraction introduced per kilogram of catalyst and per hour, from 0.25 to 60 h ¹ to produce an isomerized hydrocarbon effluent, (b) contacting at least part of the isomerized hydrocarbon effluent from step (a) with a second isomerization catalyst comprising at least one zeolite of structural type EUO at a temperature between 300 ° C and 500 ° C, a partial hydrogen pressure between 0.3 and 1.5 MPa, a total pressure between 0.45 and 1.9 MPa and a space velocity of supply, expressed in kilogram of aromatic cut introduced per kilogram of catalyst e t per hour, from 0.25 to 60 h ¹ to produce an isomerized effluent.

The sequence of an isomerization catalyst using an IZM-2 zeolite (A) and then an isomerization catalyst using a zeolite of structural type EUO (B) according to the invention makes it possible to obtain a higher activity towards the production of para-xylene than that obtained with the catalyst based on IZM-2 (A) used alone, without increasing the production of net losses for the same yield of para-xylene. The sequence of an isomerization catalyst using an IZM-2 zeolite (A) and then an isomerization catalyst using a zeolite of structural type EUO (B) according to the invention makes it possible to reduce the production net losses for the same para-xylene yield compared to the EUO structural type zeolite catalyst (B) used alone. Finally, the sequence of an isomerization catalyst using an IZM-2 zeolite (A) and then an isomerization catalyst using a zeolite of structural type EUO (B) according to the invention is presented surprisingly a reduction in the production of net iso-yield losses in para-xylene compared to a mixture of catalyst A and catalyst B, or to a sequence of a different order, namely the sequence of an isomerization catalyst using an EUO structural type zeolite (B) and then an isomerization catalyst using an IZM-2 zeolite (A).

Detailed description of the invention

Load processed

According to the invention, the feedstock treated in the process according to the present invention is a hydrocarbon fraction comprising an aromatic compound with eight carbon atoms per molecule.

Preferably, said filler is chosen from the cuts resulting from catalytic reforming and / or steam cracking. Preferably, the feedstock treated in the present invention may contain ethylbenzene and / or a mixture of xylenes poor in para-xylene or free of para-xylene, for example after selective extraction of para3059325 xylene or by selective adsorption on molecular sieves. Said filler can also contain non-aromatic hydrocarbons such as naphthenes or paraffins.

Step a)

According to the invention, said method comprises a step a) of bringing said aromatic cut into contact with an isomerization catalyst comprising at least one IZM-2 zeolite (A) at a temperature between 300 ° C and 500 ° C , a partial pressure of hydrogen of between 0.3 and 1.5 MPa, a total pressure of between 0.45 and 1.9 MPa and a space supply speed, expressed in kilograms of aromatic fraction introduced per kilogram of catalyst and per hour, from 0.25 to 60 h ¹ to produce an isomerized hydrocarbon effluent.

Preferably, said step a) is carried out at a temperature between 350 ° C and 450 ° C, a partial pressure of hydrogen generated between 0.4 and 1 MPa, a total pressure between 0.6 and 1, 2 MPa and a spatial feed speed, expressed in kilograms of aromatic fraction introduced per kilogram of catalyst and per hour, from 1 to 30 h ¹ .

Description of the isomerization catalyst A based on zeolite IZM-2

According to the invention, the isomerization catalyst A used in step a) of the process according to the invention comprises at least one IZM-2 zeolite. Preferably, said catalyst A also comprises at least one matrix and at least one metal from group VIII of the periodic table.

IZM-2 Zeolite

The IZM-2 zeolite presents an X-ray diffraction diagram including at least the lines listed in Table 1. IZM-2 has a crystal structure. Advantageously, the diffraction diagram is obtained by radiocrystallographic analysis by means of a diffractometer using the conventional method of powders with the radiation K _a1 of copper (λ = 1.5406 Å). From the position of the diffraction peaks represented by the angle 2Θ, we calculate, by the Bragg relation, the reticular equidistances d _hk i characteristic of the sample. The measurement error A (d _hW ) on d _hk i is calculated using the Bragg relation as a function of the absolute error Δ (2θ) assigned to the measurement of 2Θ. An absolute error Δ (2θ) equal to ± 0.02 ° is commonly accepted. The relative intensity l _re i assigned to each value of d _hk is measured according to the height of the corresponding diffraction peak. The X-ray diffraction diagram of IZM-2 according to the invention comprises at least the lines with the values of d _hk i given in table 1. In the column of d _hk i, the mean values of the inter-reticular distances are indicated. in Angstroms (Â). Each of these values must be assigned the measurement error A (d _hk i) between ± 0.6Â and ± 0.01 Â.

Table 1: mean values of d _Îk i and relative intensities measured on an X-ray diffraction diagram of the calcined IZM-2 crystallized solid.

2 theta (°) 4k, (Â) Irel 2 theta (°) dki (Â) Irel 5.07 17.43 ff 19.01 4.66 ff 7.36 12.01 FF 19.52 4.54 ff 7.67 11.52 FF 21.29 4.17 m 8.78 10.07 F 22.44 3.96 f 10.02 8.82 ff 23.10 3.85 mf 12.13 7.29 ff 23.57 3.77 f 14.76 6.00 ff 24.65 3.61 ff 15.31 5.78 ff 26.78 3.33 f 15.62 5.67 ff 29.33 3.04 ff 16.03 5.52 ff 33.06 2.71 ff 17.60 5.03 ff 36.82 2.44 ff 18.22 4.87 ff 44.54 2.03 ff

where FF = very strong; F = strong; m = medium; mf = medium weak; f = weak; ff = very weak.

The relative intensity l _re i is given in relation to a relative intensity scale where a value of 100 is assigned to the most intense line of the X-ray diffraction diagram: ff <15; 15 <f <30; 30 <mf <50; 50 <m <65; 65 <F <85;FF> 85.

IZM-2 has a chemical composition expressed on an anhydrous basis, in terms of moles of oxides, defined by the following general formula: SiO ₂ : a AI ₂ O ₃ : b M _{2 / n} O, in which M is at least an alkali metal and / or an alkaline earth metal of valence n. In said formula given above, a represents the number of moles of Al ₂ O ₃ and b represents the number of moles of M _{2 / n} O. Advantageously the ratio between the number of moles of silicon and the number of moles of aluminum of the zeolite IZM2 used in step (a) is between 60 and 150, preferably between 60 and 110, very preferably between 60 and 100 and even more preferably between 60 and 95. This ratio is understood as the ratio of the number of moles of silicon divided by the number of moles of aluminum in the lattice of the IZM-2 zeolite.

According to the invention, the number of moles of lattice aluminum per gram of IZM-2 zeolite is determined from the percentage (%) weight (wt) of aluminum in the zeolite and the percentage of tetaco and pentacoordinated aluminum present in the zeolite according to the formula:

n _A i = [(% pdsAI) * (% RMNAI ^IV +% RMNAI ^V )] / [MM (AI) * 10000] with n _A i: moles of lattice aluminum per gram of zeolite, in mole / gram, % pdsAI: percentage by weight of aluminum in the zeolite (dry mass), measured by inductively coupled plasma (ICP) on a SPECTRO ARCOS ICP-OES device from SPECTRO according to the ASTM D7260 method,

MM (AI): molar mass of aluminum, in grams / mole,% RMNAI ^IV : weight percentage of tetracoordinated aluminum in zeolite, measured by nuclear magnetic resonance of ²⁷ AI,% RMNAI ^V : weight percentage of pentacoordined aluminum in the zeolite, measured by nuclear magnetic resonance of ²⁷ AI.

The weight percentage of the tetracoordinated and pentacoordinated aluminum atoms present in the IZM-2 zeolite is determined by nuclear magnetic resonance of the solid of ²⁷ AI. Aluminum NMR is indeed known to be used to identify and quantify the different coordination states of this nucleus (Physico-chemical analysis of industrial catalysts, J. Lynch, Editions Technip (2001) chap. 13, pages 290 and 291). The aluminum NMR spectrum of the IZM-2 zeolite can present three signals, a first being characteristic of the resonance of tetacoordinated aluminum atoms, a second being characteristic of pentacoordinated aluminum atoms and a third being characteristic of the resonance hexacoordinated aluminum atoms. The tetacoordinated aluminum atoms (denoted AI ^IV ) resonate at a chemical shift typically between +50 ppm and +70 ppm, the pentacoordinated aluminum atoms (denoted Al ^v ) resonate at a chemical shift typically between +20 ppm and +40 ppm and the hexacoordinated aluminum atoms (denoted AI ^VI ) resonate with a chemical shift typically between -20 ppm and +10 ppm. The weight percentage of the different aluminum species is quantified from the integration of the signals corresponding to each of these species. More specifically, the IZM-2 zeolite present in the catalyst according to the invention was analyzed by NMR-MAS of the solid ²⁷ AI on a Brücker Avrom spectrometer of the 400 MHz Avance type using a 4 mm probe optimized for ²⁷ AI. . The speed of rotation of the sample is close to 12 kHz. The aluminum atom is a quadrupole nucleus whose spin is equal to 5/2. In so-called selective analysis conditions, namely a low radio frequency field equal to 30 kHz, a low pulse angle equal to π / 2 and in the presence of a sample saturated with water, the NMR technique of rotation at the magic angle (MAS), denoted RMN-MAS, is a quantitative technique. The decomposition of each NMR-MAS spectrum provides direct access to the quantity of the various aluminum species, namely tetracoordinated aluminum atoms AI ^IV , pentacoordinated Al ^v and hexacoordinated AI ^VI . Each spectrum is calibrated in chemical displacement relative to a molar solution of aluminum nitrate for which the aluminum signal is at zero ppm. The signals characterizing the tetacoordinated aluminum atoms AI ^IV are typically integrated between +50 ppm and +70 ppm which corresponds to the area 1, the signals characterizing the pentacoordinated aluminum atoms Al ^v are typically integrated between +20 ppm and +40 ppm which corresponds to area 2 and the signals characterizing the hexacoordinated aluminum atoms AI ^VI are typically integrated between -20 ppm and +10 ppm which corresponds to area 3. The weight percentage of each aluminum species is calculated from the ratio between its area and the total area. For example, the weight percentage of hexacoordinated aluminum atoms AI ^VI (denoted% RMNAI ^VI ) is calculated as follows:

% RMNAI ^VI = (area 3) * 100 / (area 1 + area 2 + area 3)

Advantageously, the IZM-2 zeolite used in step (a) according to the invention has a weight percentage of hexacoordinated aluminum atoms AI ^VI (denoted% RMNAI ^VI ) of less than 50%, preferably less than 40% , and very preferably less than 30%.

The number of moles of silicon per gram of IZM-2 zeolite is determined from the percentage (%) weight (wt) in silicon of the zeolite according to the formula:

n _Si = [(% wt%) / [MM (Si) * 100] with n _Si : moles of silicon per gram of zeolite, in mole / gram,% wt%: percentage by weight of aluminum in the zeolite (dry mass), measured by Fluorescence X by pearl dosing on an AXIOS brand PANalytical device working at 125 mA and 32 kV,

MM (Si): molar mass of silicon, in grams / mole.

The ratio of the number of moles of silicon divided by the number of moles of lattice aluminum of the zeolite IZM-2 (Si / AI) is calculated according to the formula:

Si / AI = n _Si / n _A i with n _Si / n _A i: ratio of the number of moles of silicon divided by the number of moles of lattice aluminum, in mole / mole, n _Si : moles of silicon per gram zeolite, in mole / gram, n _A : moles of lattice aluminum per gram of zeolite, in mole / gram.

The IZM-2 zeolite present in catalyst A used in the process according to the invention is very advantageously in its acid form, that is to say in protonated H ^{+ form} . In such a case, it is advantageous that the ratio of the number of moles of cation other than the proton per gram of IZM-2 zeolite divided by the number of moles of lattice aluminum per gram of IZM-2 zeolite is less than 0 , 9, preferably less than 0.6 and very preferably less than 0.3. To do this, the zeolite IZM-2 used in the composition of catalyst A used in the process according to the invention can for example be exchanged by at least one treatment with a solution of at least one ammonium salt so as to obtain the ammonium form of the IZM-2 zeolite which once calcined leads to the acid form of said IZM-2 zeolite. This exchange stage can be carried out at any stage of the preparation of the catalyst, that is to say after the stage of preparation of the zeolite IZM2, after the stage of shaping the zeolite IZM-2 with a matrix, or even after the step of introducing the hydro-dehydrogenating metal. Preferably the exchange step is carried out after the shaping step of the IZM-2 zeolite.

Matrix

Preferably, the catalyst A also comprises at least one matrix. Said matrix can advantageously be amorphous or crystallized. Preferably, said matrix is advantageously chosen from the group formed by alumina, silica, silica-alumina, clays, titanium oxide, boron oxide and zirconia, taken alone or as a mixture or else it is also possible to choose aluminates. Preferably, alumina is used as a matrix. Preferably, said matrix contains alumina in all its forms known to those skilled in the art, such as, for example, aluminas of the alpha, gamma, eta, delta type. Said aluminas differ in their specific surface and their pore volume. The mixture of the matrix and of the shaped IZM-2 zeolite constitutes the support for catalyst A.

Metallic phase

Preferably, catalyst A also comprises at least one metal from group VIII preferably chosen from iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum, preferably chosen from the noble metals of group VIII, very preferably chosen from palladium and platinum and even more preferably platinum is chosen.

The dispersion of the group VIII metal (s), determined by chemisorption, for example by H ₂ / O ₂ titration or by chemisorption of carbon monoxide, is between 10% and 100%, preferably between 20% and 100% and even more preferably between 30% and 100%. The gross distribution coefficient of the group VIII metal (s), obtained from its profile determined by the Castaing microprobe, defined as the ratio of the group VIII metal (s) concentrations to the core of the grain relative to the edge of this same grain, is between 0.7 and 1.3, preferably between 0.8 and 1.2. The value of this ratio, close to 1, testifies to the homogeneity of the distribution of the group VIII metal (s) in the catalyst A used in step a) according to the invention.

Said catalyst A can also advantageously comprise at least one additional metal chosen from the group formed by the metals of groups IIIA, IVA and VIIB of the periodic table of the elements and preferably chosen from gallium, indium, tin and rhenium. Said additional metal is preferably chosen from indium, tin and rhenium.

Said isomerization catalyst A used in step (a) according to the invention more particularly and preferably consists of:

from 1 to 90%, preferably from 3 to 80% and even more preferably from 4 to 60% by weight of zeolite IZM-2, from 0.01 to 4%, preferably from 0.05 to 2% by weight of '' at least one metal from group VIII of the periodic table, preferably platinum, optionally from 0.01 to 2%, preferably from 0.05 to 1% by weight of at least one additional metal chosen from the group formed by the metals of groups IIIA, IVA and VIIB, optionally a sulfur content, preferably such that the ratio of the number of moles of sulfur to the number of moles of metal (s) of group VIII is between 0 , 3 and 3, at least one matrix, preferably alumina, ensuring the complement to 100% in the catalyst.

Advantageously, the IZM-2 zeolite used in catalyst A has, on the one hand, a ratio between the number of moles of silicon and the number of moles of aluminum of between 60 and 150, preferably between 60 and 110, so very preferred between 60 and 100 and even more preferably between 60 and 95 and on the other hand has a weight percentage of hexacoordinated aluminum atoms less than 50%, preferably less than 40%, and very preferably less than 30%. Furthermore, in this zeolite, the ratio of the number of moles of cation other than the proton per gram of IZM-2 zeolite divided by the number of moles of lattice aluminum per gram of IZM-2 zeolite is preferably less than 0 , 9, preferably less than 0.6 and very preferably less than 0.3.

Step a) makes it possible to obtain an isomerized hydrocarbon effluent. Preferably, the isomerized hydrocarbon effluent produced at the end of step a) has a conversion of at least 10% of the ethylbenzene, preferably at least 15%, and very preferably d '' at least 20%.

Step b)

According to the invention, said method comprises a step b) of bringing at least part of the isomerized hydrocarbon effluent from step a) into contact with a second isomerization catalyst comprising at least one type zeolite structural EUO (B) at a temperature between 300 ° C and 500 ° C, a partial hydrogen pressure between 0.3 and 1.5 MPa, a total pressure between 0.45 and 1.9 MPa and a spatial feed rate, expressed in kilograms of aromatic fraction introduced per kilogram of catalyst and per hour, from 0.25 to 60 h ¹ to produce an isomerized effluent.

Advantageously, the isomerized effluent from step a) is fully sent in step b) according to the invention.

Preferably, said step b) is carried out at a temperature between 350 ° C and 450 ° C, a partial pressure of hydrogen generated between 0.4 and 1 MPa, a total pressure between 0.6 and 1, 2 MPa and a spatial feed speed, expressed in kilograms of aromatic fraction introduced per kilogram of catalyst and per hour, from 1 to 30 h ¹ .

Description of the isomerization catalyst B based on EUO zeolite

According to the invention, the isomerization catalyst B used in step b) of the process according to the invention comprises at least one zeolite of structural type EUO. Preferably said catalyst B also comprises at least one matrix and at least one metal from group VIII of the periodic table of the elements.

EUO zeolite

According to the invention, said catalyst B comprises a zeolite of structural type EUO. This zeolite presents a one-dimensional microporous network, the pore diameter of which is 4.1 × 5.4 Å (1 1 = 1 Angstrom = 10 ¹⁰ m) (“Atlas of Zeolite Framework Types”, Ch. Baerlocher, WM Meier and DH Oison, 5 ^th Edition, 2001). On the other hand, NA Briscoe et al taught in an article in the journal Zeolites (1988, 8, 74) that these one-dimensional channels have side pockets with a depth of 8.1 Â and a diameter of 6.8 x 5.8 Â . Said EUO structural type zeolite is chosen from EU-1, ZSM-50 and TPZ-3 zeolites, preferably EU-1 and TPZ-3, preferably it is EU-1 zeolite. The mode of synthesis of the EU-1 zeolite and its physicochemical characteristics have already been described in patent EP-B1-42,226. The patent US-4,640,829B describes the zeolite ZSM-50 and its preparation process. The TPZ-3 zeolite and its preparation process are disclosed in patent application EP-A1-51 318B.

EUO structural type zeolites generally have the following formula defined on an anhydrous basis and in terms of moles of oxides: XO ₂ : a 'T ₂ O ₃ : b' R ^ O. X represents silicon and / or germanium, T represents at least one element chosen from aluminum, iron, gallium, boron, titanium, vanadium, zirconium, molybdenum, arsenic, antimony , chromium and manganese, R represents a cation of valence n. In the formula provided above, a 'represents the number of moles of T ₂ O ₃ and b' represents the number of moles of R _{2 / n} O. Preferably the element X is silicon and the element T is aluminum.

The EUO structural type zeolite present in catalyst B used in the process according to the invention is very advantageously in its acid form, that is to say in protonated H ^{+ form} . In such a case, it is advantageous that the ratio of the number of moles of cation other than the proton per gram of zeolite of structural type EUO divided by the number of moles of element T per gram of zeolite of structural type EUO is less than 0.9, preferably less than 0.6 and very preferably less than 0.3. To do this, the zeolite of structural type EUO entering into the composition of catalyst B used in the process according to the invention can for example be exchanged by at least one treatment with a solution of at least one ammonium salt so as to obtaining the ammonium form of the EUO structural type zeolite which once calcined leads to the acid form of said EUO structural type zeolite. This exchange stage can be carried out at any stage of the preparation of the catalyst, that is to say after the stage of preparation of the zeolite of structural type EUO, after the stage of shaping of the zeolite of structural type EUO with a matrix, or even after the step of introducing the hydrodehydrogenating metal. Preferably the exchange step is carried out after the step of shaping the EUO structural type zeolite.

Matrix

Preferably, catalyst B also comprises at least one matrix. Said matrix can advantageously be amorphous or crystallized. Preferably, said matrix is advantageously chosen from the group formed by alumina, silica, silica-alumina, clays, titanium oxide, boron oxide and zirconia, taken alone or as a mixture or else it is also possible to choose aluminates. Preferably, alumina is used as a matrix. Preferably, said matrix contains alumina in all its forms known to those skilled in the art, such as, for example, aluminas of the alpha, gamma, eta, delta type. Said aluminas differ in their specific surface and their pore volume. The mixture of the matrix and the zeolite of structural EUO type formed forms the support for catalyst B.

Metallic phase

Preferably, catalyst B also comprises at least one metal from group VIII, preferably chosen from iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum, preferably chosen from the noble metals of group VIII, very preferably chosen from palladium and platinum and even more preferably platinum is chosen.

The dispersion of the group VIII metal (s), determined by chemisorption, for example by H ₂ / O ₂ titration or by chemisorption of carbon monoxide, is between 10% and 100%, preferably between 20% and 100% and even more preferably between 30% and 100%. The gross distribution coefficient of the group VIII metal (s), obtained from its profile determined by the Castaing microprobe, defined as the ratio of the group VIII metal (s) concentrations to the core of the grain relative to the edge of this same grain, is between 0.7 and 1.3, preferably between 0.8 and 1.2. The value of this ratio, close to 1, testifies to the homogeneity of the distribution of the group VIII metal (s) in catalyst B.

Said catalyst B may advantageously comprise at least one additional metal chosen from the group formed by the metals of groups IIIA, IVA and VIIB of the periodic table of the elements and preferably chosen from gallium, indium, tin and rhenium . Said additional metal is preferably chosen from indium, tin and rhenium.

Said isomerization catalyst B used in step (b) according to the invention more particularly comprises, and preferably consists of:

from 1 to 90%, preferably from 3 to 80% and even more preferably from 4 to 60% by weight of zeolite of structural type EUO, preferably of zeolite EU-1,

- from 0.01 to 4%, preferably from 0.05 to 2% by weight of at least one metal from group VIII of the periodic table of the elements, preferably platinum,

- optionally from 0.01 to 2%, preferably from 0.05 to 1% by weight of at least one additional metal chosen from the group formed by the metals of groups IIIA, IVA and VIIB,

- optionally a sulfur content, preferably such that the ratio of the number of moles of sulfur to the number of moles of group VIII metal (s) is between 0.3 and 3,

- at least one matrix, preferably alumina, ensuring the complement to 100% in the catalyst.

Preferably, the isomerized effluent produced at the end of step b) according to the invention has a higher para-xylene concentration in the xylenes than that obtained in the effluent from step a) according to l 'invention.

Preferably steps a) and b) are carried out successively. Preferably steps a) and b) are carried out in a fixed bed reactor, in which the isomerization catalyst A comprising an IZM-2 zeolite is located upstream of the isomerization catalyst B comprising a zeolite of structural type EUO. Preferably, the amount of catalyst A used in step a) is greater than or equal to the amount of catalyst B used in step b) in said isomerization process.

Preparation of catalysts A and B

The catalysts A and B according to the invention can advantageously be prepared according to all the methods well known to those skilled in the art. The catalysts A and B according to the invention are advantageously prepared according to the method described below.

Fitness

Advantageously, the various constituents of the support or of the catalyst can be shaped by kneading stage to form a paste and then extrusion of the paste obtained, or else by mixing of powders then tableting, or else by any other known process of agglomeration of 'a powder containing alumina. The supports thus obtained can be in different shapes and dimensions. Preferably, the shaping is carried out by kneading and extrusion.

During the shaping of the support by kneading and then extrusion, the zeolite of structural type EUO or IZM-2 can be introduced during the dissolution or suspension of alumina compounds or alumina precursors such as bohemite for example. Said zeolite can be, without being limiting, for example in the form of powder, ground powder, suspension, suspension having undergone a deagglomeration treatment. Thus, for example, said zeolite can advantageously be placed in an acidulous suspension or not at a concentration adjusted to the final content of zeolite targeted in the prepared catalyst. This suspension commonly called a slip is then mixed with the alumina compounds or alumina precursors.

Furthermore, the use of additives can advantageously be implemented to facilitate shaping and / or improve the final mechanical properties of the supports as is well known to those skilled in the art. By way of example of additives, mention may in particular be made of cellulose, carboxymethyl cellulose, carboxyethyl cellulose, tall oil (tall oil), xanthan gums, surfactants, flocculating agents such as polyacrylamides , carbon black, starches, stearic acid, polyacrylic alcohol, polyvinyl alcohol, biopolymers, glucose, polyethylene glycols, etc.

Advantageously, water can be added or removed to adjust the viscosity of the paste to be extruded. This step can advantageously be carried out at any stage of the kneading step.

To adjust the solid content of the dough to be extruded in order to make it extrudable, it is also possible to add a predominantly solid compound and preferably an oxide or a hydrate. Preferably, a hydrate is used and even more preferably an aluminum hydrate. The loss on ignition of this hydrate is advantageously greater than 15%.

The extrusion of the dough from the kneading stage can advantageously be carried out by any conventional tool, commercially available. The paste resulting from the kneading is advantageously extruded through a die, for example using a piston or a single-screw or double extrusion screw. The extrusion can advantageously be carried out by any method known to those skilled in the art.

The supports of catalyst A or B used in the process according to the invention are generally in the form of cylindrical or multi-lobed extrudates such as bilobed, three-lobed, multi-lobed in straight or twisted shape, but can optionally be manufactured and used in the form crushed powders, tablets, rings, balls and / or wheels. Preferably, these supports have the form of spheres or extrudates. Advantageously, the support is in the form of extrudates with a diameter between 0.5 and 5 mm and more particularly between 0.7 and 2.5 mm. The shapes can be cylindrical (which can be hollow or not) and / or twisted and / or multilobed cylindrical (2, 3, 4 or 5 lobes for example) and / or rings. The multilobed form is advantageously used in a preferred manner.

Drying

The support thus obtained can then be subjected to a drying step. Said drying step is advantageously carried out by any technique known to a person skilled in the art. Preferably, the drying is carried out under air flow. Said drying can also be carried out under the flow of any oxidizing, reducing or inert gas. Preferably, the drying is advantageously carried out at a temperature between 50 and 180 ° C, preferably between 60 and 150 ° C and very preferably between 80 and 130 ° C.

Calcination

Said support, optionally dried, then preferably undergoes a calcination step. Said calcination step is advantageously carried out in the presence of molecular oxygen, for example by carrying out an air sweep, at a temperature advantageously greater than 200 ° C. and less than or equal to 1100 ° C. Said calcination step can advantageously be carried out in a crossed bed, in a licked bed or in a static atmosphere. For example, the oven used can be a rotary rotary oven or a vertical oven with radial traversed layers. Preferably, said calcination step is carried out between more than one hour at 200 ° C to less than one hour at 1100 ° C. The calcination can advantageously be carried out in the presence of water vapor and / or in the presence of acidic or basic vapor. For example, calcination can be carried out under partial pressure of ammonia.

Post-calcination treatments

Post-calcination treatments can optionally be carried out, so as to improve the properties of the support, in particular the textural properties. Thus, the support of catalyst A or B used in the process according to the present invention can be subjected to a hydrothermal treatment in a confined atmosphere. Hydrothermal treatment in a confined atmosphere is understood to mean an autoclave treatment in the presence of water at a temperature above room temperature, preferably above 25 ° C., preferably above 30 ° C. During this hydrothermal treatment, the support can advantageously be impregnated, before it is autoclaved (the autoclaving being carried out either in the vapor phase or in the liquid phase, this vapor or liquid phase of the autoclave being able to be acidic. or not). This impregnation, prior to autoclaving, can advantageously be acidic or not. This impregnation, prior to autoclaving, can advantageously be carried out dry or by immersion of the support in an acidic aqueous solution. By dry impregnation means contacting the support with a volume of solution less than or equal to the total pore volume of the support. Preferably, the impregnation is carried out dry. The autoclave is preferably an autoclave with a rotary basket such as that defined in patent application EP0387109 A. The temperature during autoclaving can be between 100 and 250 ° C. for a period of time between 30 minutes and 3 hours.

Deposition of the metallic phase

For the deposition of the metal from group VIII of the periodic table of the elements, all the deposition techniques known to those skilled in the art and all the precursors of such metals may be suitable. Deposition techniques can be used by dry impregnation or in excess of a solution containing the metal precursors, in the presence of competitors or not. The metal can be introduced at any stage of the preparation of the catalyst: on the zeolite and / or on the matrix, in particular before the shaping step, during the shaping step, or after the 'shaping step, on the catalyst support. Preferably, the metal is deposited after the shaping step.

The control of certain parameters implemented during the deposition, in particular the nature of the precursor of the metal (s) of group VIII used, makes it possible to direct the deposition of the said metal (s) ( ux) mainly on the matrix or on the zeolite.

Thus, to introduce the metal (s) (group) of group VIII, preferably platinum and / or palladium, mainly on the matrix, it is possible to use an anion exchange with hexachloroplatinic acid and / or hexachloropalladic acid, in the presence of a competing agent, for example hydrochloric acid, the deposit being generally followed by calcination, for example at a temperature between 350 and 550 ° C and for a period of time between 1 and 4 hours. With such precursors, the group VIII metal (s) is (are) deposited (s) mainly on the matrix and the said (s) metal (s) has (s) good dispersion and good macroscopic distribution across the grain of catalyst.

It is also possible to envisage depositing the group VIII metal (s), preferably platinum and / or palladium, by cation exchange so that the said metal (s) itself (in ) t deposited (s) mainly on the zeolite. Thus, in the case of platinum, the precursor can for example be chosen from:

- ammonia compounds such as platinum (II) tetramine salts of formula Pt (NH ₃ ) ₄ X ₂ , platinum (IV) hexamine salts of formula Pt (NH ₃ ) ₆ X ₄ ;

the platinum (IV) halopentamine salts of formula (PtX (NH ₃ ) ₅ ) X ₃ ; platinum N-tetrahalogenodiamine salts of formula PtX ₄ (NH ₃ ) ₂ ; and

- halogenated compounds of formula H (Pt (acac) ₂ X);

X being a halogen chosen from the group formed by chlorine, fluorine, bromine and iodine, X being preferably chlorine, and acac representing the acetylacetonate group (of crude formula C ₅ H ₇ O ₂ ), derived from acetylacetone. With such precursors, the metal (s) of group VIII is (are) deposited (s) mainly on the zeolite and the said (s) metal (s) has (s) good dispersion and good macroscopic distribution across the grain of catalyst.

In the case where the catalyst A or B used in the process of the invention also contains at least one metal chosen from the metals of groups IIIA, IVA and VIIB, all the techniques for depositing such a metal known to man of the trade and all the precursors of such metals may be suitable.

The metal (s) of group VIII and that (s) of groups IIIA, IVA and VIIB can be added, either separately or simultaneously in at least one unitary step. When at least one group IIIA, IVA and VIIB metal is added separately, it is preferable that it is added after the group VIII metal.

The additional metal chosen from the metals of groups IIIA, IVA and VIIB can be introduced via compounds such as, for example, the chlorides, bromides and nitrates of the metals of groups IIIA, IVA and VIIB. For example in the case of indium, nitrate or chloride is advantageously used and in the case of rhenium, perrhenic acid is advantageously used. The additional metal chosen from the metals of groups IIIA, IVA and VIIB can also be introduced in the form of at least one organic compound chosen from the group consisting of the complexes of said metal, in particular the polyketonic complexes of the metal and the hydrocarbylmetals such than alkyls, cycloalkyls, aryls, alkylaryls and arylalkyls of metals. In the latter case, the introduction of the metal is advantageously carried out using a solution of the organometallic compound of said metal in an organic solvent. Organohalogenated metal compounds can also be used. As organic metal compounds, mention may be made in particular of tetrabutyltin, in the case of tin, and triphenylindium, in the case of indium.

If the additional metal chosen from the metals of groups IIIA, IVA and VIIB is introduced before the metal of group VIII, the metal compound IIIA, IVA and / or VIIB used is generally chosen from the group consisting of the halide, the nitrate , acetate, tartrate, carbonate and oxalate of the metal. The introduction is then advantageously carried out in aqueous solution. However, it can also be introduced using a solution of an organometallic compound of the metal, for example tetrabutyltin. In this case, before proceeding with the introduction of at least one group VIII metal, a calcination in air will be carried out.

In addition, intermediate treatments such as for example calcination and / or reduction can be applied between the successive deposits of the different metals.

Before its use in the process according to the invention, the catalyst is preferably reduced. This reduction step is advantageously carried out by treatment under hydrogen at a temperature between 150 ° C and 650 ° C and a total pressure between 0.1 and 25 MPa. For example, a reduction consists of a two-hour plateau at 150 ° C, then a temperature rise to 450 ° C at a speed of 1 ° C / min, then a two-hour plateau at 450 ° C; saying this whole reduction step, the hydrogen flow rate is 1000 normal m ³ of hydrogen per ton catalyst and the total pressure kept constant at 0.2 MPa. Any ex situ reduction method can advantageously be considered. A prior reduction of the final catalyst ex situ, under a stream of hydrogen, can be implemented, for example at a temperature of 450 ° C to 600 ° C, for a period of 0.5 to 4 hours.

Advantageously, said catalyst A or B can also comprise sulfur. In the case where catalysts A or B contain sulfur, this can be introduced at any stage of the preparation of the catalyst: before or after shaping stage, and / or drying and / or calcination, before and / or after the introduction of the metal or metals mentioned above, or also by in situ sulfurization and / or ex situ before the catalytic reaction. In the case of in situ sulfurization, the reduction, if the catalyst has not been previously reduced, takes place before the sulfurization. In the case of ex situ sulfurization, the reduction is also carried out followed by the sulfurization. The sulfurization is preferably carried out in the presence of hydrogen using any sulphurizing agent well known to the skilled person, such as for example dimethyl sulphide or hydrogen sulphide.

The catalyst A or B used in the process according to the invention comes in different forms and dimensions. It is generally used in the form of cylindrical and / or multi-lobed extrudates such as bilobed, three-lobed, multi-lobed in a straight and / or twisted shape, but can optionally be manufactured and used in the form of crushed powders, tablets, rings, balls and / or wheels. Preferably, the catalyst A or B used in the process according to the invention has the form of spheres or extrudates. Advantageously, the catalyst A or B is in the form of extrudates with a diameter between 0.5 and 5 mm and more particularly between 0.7 and 2.5 mm. The shapes can be cylindrical (which can be hollow or not) and / or twisted and / or multilobed cylindrical (2, 3, 4 or 5 lobes for example) and / or rings. The multilobed form is advantageously used in a preferred manner. The deposition of the metal does not change the shape of the support.

The following examples illustrate the invention without, however, limiting its scope.

Examples

Example 1 (according to the invention): preparation of the isomerization catalyst A

Catalyst A is a catalyst comprising an IZM-2 zeolite, platinum, and an alumina matrix. This zeolite IZM-2 was synthesized in accordance with the teaching of patent FR 2 918 050 B. A colloidal suspension of silica known under the commercial name Ludox HS-40 marketed by Aldrich, is incorporated into a solution composed of sodium hydroxide (Prolabo) , structuring dibromide of 1.6bis (methylpiperidinium) hexane, sodium aluminate (Carlo Erba) and deionized water. The molar composition of the mixture is as follows: 1 SiO ₂ ; 0.0042 AI ₂ O ₃ ; 0.1666 Na ₂ O; 0.1666 1.6bis (methylpiperidinium) hexane; 33.3333 H ₂ O. The mixture is stirred vigorously for half an hour. The mixture is then transferred, after homogenization, to a PARR type autoclave. The autoclave is heated for 5 days at 170 ° C. with stirring in a spindle burne (30 rpm). The product obtained is filtered, washed with deionized water to reach a neutral pH and then dried overnight at 100 ° C in an oven. The solid is then introduced into a muffle furnace to be calcined therein in order to remove the structuring agent. The calcination cycle includes a rise in temperature up to 200 ° C, a plateau at this temperature of two hours, a rise in temperature up to 550 ° C followed by a plateau of eight hours at this temperature and finally a return at room temperature. The temperature rises are carried out with a ramp of 2 ° C / min. The solicte thus obtained is then refluxed for 2 hours in an aqueous solution of ammonium nitrate (10 ml of solution per gram of solid, concentration of ammonium nitrate of 3 M) in order to exchange the sodium alkaline cations by ammonium ions. This refluxing step is carried out six times with a fresh solution of ammonium nitrate, then the solid is filtered, washed with deionized water and dried in an oven overnight at 100 ° C. Finally, to obtain the zeolite in its acid form (protonated H ⁺ ) a calcination step is carried out at 550 ° C for ten hours (temperature rise ramp of 2 ° C / min) in a bed crossed under dry air (2 normal liters per hour and per gram of solid). The solid thus obtained was analyzed by X-ray diffraction and identified as being constituted by the zeolite IZM-2. Characterizations using NMR methods of ²⁷ AI, X-ray fluorescence (in particular by pearl assay on an AXIOS device of PANalytical brand working at 125 mA and 32 kV) and ICP (in particular on a SPECTRO ARCOS ICP-OES device of SPECTRO according to the ASTM D7260 method) give access to the following results for ΙΊΖΜ-2:

- weight percentage of hexacoordinated aluminum atoms AI ^VI : 5%,

- ratio of the number of moles of silicon divided by the number of moles of lattice aluminum, in mole / mole, Si / AI: 81,

- Ratio of the number of moles of sodium divided by the number of moles of lattice aluminum, in mole / mole, Na / Al: 0.03.

This zeolite is kneaded with an alumina gel of the GA7001 type supplied by the company Axens. The kneaded dough is extruded through a 1.6 mm diameter cylindrical die. After drying in an oven overnight at 110 ° C., the extrudates are calcined at 550 ° C. for two hours (temperature rise ramp of 5 ° C./min) in a bed traversed under dry air (2 normal liters per hour and per gram of solid). The quantity of zeolite used is chosen so as to obtain approximately 25% by weight of zeolite in the extrudates after calcination. The platinum is then deposited in the extrudates by dry impregnation in a bezel with an aqueous solution of platinum chloride tetramine Pt (NH ₃ ) ₄ CI ₂ . The platinum content in the impregnation solution is adjusted so as to obtain approximately 0.3% by weight of platinum on the catalyst after calcination. After impregnation, the extrudates are left to mature for five hours in laboratory air and then allowed to dry overnight in an oven at 110 ° C. The extrudates are then calcined under a flow of dry air in a crossed bed (1 normal liter per hour and per gram of solid) under the following conditions:

- rise in ambient temperature to 150 ° C to 5 ° C / min,

- one hour stop at 150 ° C,

- rise from 150 to 450 ° C at 5 ° C / min,

- one hour level at 450 ° C,

- ambient descent.

Characterizations by X-ray fluorescence, Castaing microprobe and H ₂ / O ₂ titration provide access to the following results for catalyst A:

- percentage of IZM-2 zeolite (dry mass): 24% by weight,

- percentage of platinum (dry mass): 0.31% by weight,

- platinum dispersion: 66%

- platinum distribution coefficient: 0.85.

Example 2 (according to the invention): preparation of the isomerization catalyst B

Catalyst B is a catalyst comprising an EU-1 zeolite, platinum, and an alumina matrix. The EU-1 zeolite is synthesized in accordance with the teaching of patent EP-B1-042.226 using the organic structuring agent 1.6 N, N, N, Ν ', Ν', N'hexamethylhexamethylene diammonium, bromine being the counter- ion (Hexa-Br ₂ ). For the preparation of such a zeolite, the reaction mixture has the following molar composition: 1 SiO ₂ ; 0.025 AI ₂ O ₃ ; 0.1766 Na ₂ O; 0.3250 Hexa-Br ₂ ; 0.0878 NaBr; 46.2833 H ₂ O. The reaction mixture is placed in an autoclave with stirring (300 rpm) for 5 days at 180 ° C. The product obtained is filtered, washed with deionized water to reach a neutral pH and then dried overnight at 100 ° C in an oven. In order to remove the organic structuring agent, the solid is then calcined under a flow of dry air in a crossed bed (2 NL / h / gram of solid) under the following conditions:

- rise from room temperature to 150 ° C to 5 ° C / nin,

- one hour stop at 150 ° C,

- rise from 150 ° C to 250 ° C at 5 ° C / min,

- one hour stop at 250 ° C,

- rise from 250 ° C to 350 ° C at 5 ° C / min,

- one hour increments at 350 ° C,

- rise from 350 ° C to 450 ° C at 5 ° C / min,

- one hour stop at 450 ° C,

- rise from 450 ° C to 520 ° C at 1 ° C / min,

- twenty hour stop at 520 ° C,

- cool down to room temperature.

The solid is then refluxed for 4 hours in an ammonium nitrate solution (10 ml of solution per gram of solid, concentration of ammonium nitrate of 10 M) in order to exchange the alkali cations with ammonium ions. This exchange step is carried out four times. Finally, to obtain the zeolite in its acid form (protonated H ⁺ ), a calcination step is carried out at 520 ° C durart four hours (temperature rise ramp of 5 ° C / min) in a bed crossed under dry air (1 normal liters per hour and per gram of solid). The X-ray diffraction analysis of the solid confirms that the EU-1 zeolite has been obtained. Characterizations using NMR methods of ²⁷ AI, X-ray fluorescence (in particular by pearl assay on an AXIOS device of PANalytical brand working at 125 mA and 32 kV) and ICP (in particular on a SPECTRO ARCOS ICP-OES device of SPECTRO according to the ASTM D7260 method) give access to the following results for the EU-1:

- weight percentage of hexacoordinated aluminum atoms AI ^VI : 11%,

- ratio of the number of moles of silicon divided by the number of moles of lattice aluminum, in mole / mole, Si / AI: 16,

- Ratio of the number of moles of sodium divided by the number of moles of lattice aluminum, in mole / mole, Na / Al: 0.006.

This zeolite is kneaded with an alumina gel of the Pural SB3 type supplied by the company Sasol. The kneaded dough is extruded through a 1.6 mm diameter cylindrical die. After drying in an oven overnight at 110 ° C, the extrudates are calcined at 550 ° C for two hours (temperature rise ramp of 5 ° C / min) in a bed traversed under dry air (2 normal liters per hour and per gram solid). The amount of zeolite used is chosen so as to obtain approximately 10% by weight of zeolite in the extrudates after calcination. The platinum is then deposited in the extrudates by an excess impregnation method. First of all, the extrudates are saturated by filling the porous volume with distilled water (dropwise addition) then wetting for typically 30 minutes. The extrudates are then acidified with an aqueous hydrochloric acid solution up to 5% by weight in chlorine relative to the dry mass of extrudates (4 ml of solution per gram of solid) and the mixture is left stirring for one hour on stirring table then the solution is drawn off. An excess impregnation is then carried out with an aqueous solution of hexachloroplatinic acid up to 0.31% by weight of platinum relative to the dry mass of extrudates (4 ml of solution per gram of solid) and then left to stir on stirring table for 24 hours then the solution is drawn off and the extrudates are rinsed with twice the exchange volume of distilled water. The extrudates are then dried overnight in an oven at 110 ° C. and then calcined under a flow of dry air in a crossed bed (1 normal liter per hour and per gram of solid) under the following conditions:

- rise in ambient temperature to 150 ° C to 5 ° C / min,

- one hour stop at 150 ° C,

- rise from 150 to 250 ° C at 5 ° C / min,

- one hour stop at 250 ° C,

- rise from 250 to 350 ° C at 5 ° C / min,

- one hour level at 350 ° C,

- rise from 350 to 520 ° C at 5 ° C / min,

- two hour stop at 520 ° C,

- ambient descent.

Characterizations by X-ray fluorescence (in particular by pearl assay on an AXIOS device of the PANalytical brand working at 125 mA and 32 kV), Castaing microprobe and H ₂ / O ₂ titration provide access to the following results for catalyst B:

- percentage of EU-1 zeolite (dry mass): 10% by weight,

- percentage of platinum (dry mass): 0.3% by weight,

- dispersion of platinum: 81%

- platinum distribution coefficient: 0.97.

Example 3: Evaluation of the catalytic properties of catalysts A alone (non-compliant), B alone (non-compliant), sequence of catalyst B then catalyst A (B + A non-compliant), sequence of catalyst A then catalyst B (A + B compliant) and mixture of catalysts A and B (non-conforming) in isomerization of a C ₈ aromatic cut

The catalysts or chains of catalysts were tested in isomerization of an aromatic C ₈ cut composed of ethylbenzene (19% by weight), orthoxylene (16% by weight), meta-xylene (58% by weight) and ethylcyclohexane (7% by weight). The tests were carried out in a micro-unit implementing a fixed bed reactor and working in downdraft without recycling. When catalysts A alone and B alone are tested in the unit, 1.5 grams of catalyst are loaded. When the sequence of catalyst B then catalyst A (B + A nonconforming) is produced, 0.45 grams of catalyst B and 1.05 grams of catalyst A are loaded at the bottom of the reactor. The aromatic C ₈ cut is therefore transformed on catalyst B then on catalyst A. When the sequence of catalyst A then catalyst B (A + B conform) is carried out, the reactor head is charged with 1.05 grams of catalyst A and 0.45 grams of catalyst B at the bottom of the reactor. The C ₈ aromatic cut is therefore transformed in this case onto catalyst A and then onto catalyst B. When the mixture of catalyst A and B is produced, 0.45 grams of catalyst B and 1.05 grams of Catalyst A and this mixture is then loaded into the unit. The analysis of the hydrocarbon effluents is carried out online by gas chromatography. Before loading into the unit, the catalyst or chain of catalysts is dried beforehand for at least one night in an oven at 110 ° C. Once loaded into the unit, the catalyst or chain of catalysts undergoes a first stage of drying under nitrogen under the following conditions:

- nitrogen flow rate: 5 normal liters per hour and per gram of catalyst or chain of catalysts,

- total pressure: 1.3 MPa,

- ramp up to ambient temperature at 150 ° C: 10 ° C / min,

- level at 150 ° C for 30 minutes.

After drying, the nitrogen is replaced by hydrogen and a reduction step under flow of pure hydrogen is then carried out under the following conditions:

- hydrogen flow: 4 normal liters per hour and per gram of catalyst or chain of catalysts,

- total pressure: 1.3 MPa,

- temperature rise ramp from 150 to 480 ° C: 5 ° Cmin,

- stop at 480 ° C for 2 hours.

The temperature then dropped to 425 ° C., then the catalyst or chain of catalysts was stabilized for 24 hours under a stream of hydrogen and hydrocarbons (mixture of ethylbenzene at 20% by weight and ortho-xylene at 80% by weight) , under the following operating conditions:

- space feeding speed of 5 grams of hydrocarbons per hour and per gram of catalyst or chain of catalysts,

- partial hydrogen pressure of 1.04 MPa,

- total pressure of 1.3 MPa.

After the stabilization step, the temperature then dropped to 385 ° C., and the catalyst is brought into contact with the aromatic C ₈ cut mentioned above under the following conditions:

- space feeding speed of 3.5 grams of the C ₈ aromatic cut per hour and per gram of catalyst or chain of catalysts,

- partial pressure of hydrogen of 0.69 MPa,

- total pressure of 0.86 MPa.

The catalyst or chain of catalysts is maintained for 7 hours under these operating conditions then the catalytic performances are evaluated according to the different operating conditions which are summarized in Table 2 below.

The variation in the space feeding speed makes it possible to vary the levels of conversion to ethylbenzene and of isomerization of the xylenes and therefore the production of para-xylene. At each operating condition two analyzes by chromatography are carried out in order to measure the performance of the catalyst or chain of catalysts.

Table 2: operating conditions implemented for the catalytic evaluation

conditions 1 2 3 4 5 6 Duration at each condition (h) 2.8 2.1 2.1 2.1 2.1 2.1 Temperature (° C) 385 385 385 385 385 385 Spatial feeding speed (h ¹ ) 3.5 4.5 6.0 9.0 12.0 20.0 Total pressure (MPa) 0.86 0.86 0.86 0.86 0.86 0.86 Partial pressure H ₂ (MPa) 0.69 0.69 0.69 0.69 0.69 0.69

The yield of para-xylene (PX) in the hydrocarbon effluent obtained at the space feeding speed of 20 h ¹ makes it possible to evaluate the activity of the catalyst or chain of catalysts for the production of para-xylene:

PX =% by weight of para-xylene in the hydrocarbon effluent, where PX is the yield of para-xylene in% by weight.

The evolution of the yield in net losses (PN) as a function of the yield of paraxylene makes it possible to evaluate the selectivity of the catalyst or chain of catalysts. All hydrocarbon molecules other than cyclic molecules with eight carbon atoms are considered to be net losses:

PN = 100-PX-EB-OX-MX-N8 with:

PN: yield in net losses in the hydrocarbon effluent, in% by weight,

PX:% by weight of para-xylene in the hydrocarbon effluent,

EB:% by weight of ethylbenzene in the hydrocarbon effluent,

OX:% by weight of ortho-xylene in the hydrocarbon effluent,

MX:% by weight of meta-xylene in the hydrocarbon effluent,

N8:% by weight of naphthenes containing eight carbon atoms in the hydrocarbon effluent.

Table 3 thus reports the para-xylene yield at a space speed of 20 h ^{1 as} well as the estimated net losses for a para-xylene yield of 18% for the catalysts A alone, B alone and the sequences of catalyst B then catalyst A, and catalyst A then catalyst B according to the invention. The net losses (PN) at 18% yield in para-xylene are estimated by linear interpolation of the experimental data of the evolution of the yield in net losses as a function of the yield in para-xylene.

The results obtained show that catalyst A produces fewer net losses at 18% yield of para-xylene than catalyst B but that it is less active for the production of para-xylene. Surprisingly, the sequence of catalyst A and then of catalyst B makes it possible to obtain a higher activity than that of catalyst A alone while retaining a production of net losses comparable to that of catalyst A. It is also observed that the sequence of catalyst A then catalyst B according to the invention makes it possible to reduce the net losses for the same yield of para-xylene compared to catalyst B used alone. The sequence of catalyst B and catalyst A leads to a production of net losses at 18% of para-xylene yield higher than that obtained with the sequence of catalyst A then catalyst B while the activity of the two sequences for production of para-xylene is comparable. The combination of catalyst A and then catalyst B therefore has higher performance than the combination of catalyst B and then catalyst A. In the same way, the superiority of the combination of catalyst A and catalyst B is manifest compared to the mixture of the two catalysts.

Table 3: catalytic performances of catalysts A alone, B alone, and of sequences of catalysts B then A (B + A), A then B (A + B) and the mixture of catalysts A and B

Catalyst or chain of catalysts Alone (no compliant) B alone (no compliant) B + A (no compliant) A and B mixed (no compliant) A + B (compliant) Para-xylene yield at 20 h ¹ space speed 10.3% 12.4% 11.3% 10.2% 11.5% Net loss yield at 18% para-xylene yield 2.9% 4.5% 3.2% 3.2% 2.8%

Claims (12)

1. Process for the isomerization of an aromatic cut comprising at least one aromatic compound with eight carbon atoms per molecule, said process using two isomerization catalysts and comprising the following steps: (a) bringing said aromatic cut into contact with an isomerization catalyst comprising at least one IZM-2 zeolite at a temperature between 300 ° C. and 500 ° C., a partial hydrogen pressure ranging between 0.3 and 1 , 5 MPa, a total pressure between 0.45 and 1.9 MPa and a spatial feed speed, expressed in kilograms of aromatic fraction introduced per kilogram of catalyst and per hour, from 0.25 to 60 h ¹ to produce an isomerized hydrocarbon effluent, (b) contacting at least part of the isomerized hydrocarbon effluent from step (a) with a second isomerization catalyst comprising at least one zeolite of structural type EUO at a temperature between 300 ° C and 500 ° C, a partial hydrogen pressure between 0.3 and 1.5 MPa, a total pressure between 0.45 and 1.9 MPa and a space velocity of supply, expressed in kilogram of aromatic cut introduced per kilogram of catalyst e t per hour, from 0.25 to 60 h ¹ to produce an isomerized effluent. 2. Method according to claim 1 wherein the feedstock treated is a hydrocarbon fraction comprising aromatic molecules with 8 carbon atoms resulting from catalytic reforming and / or steam cracking. 3. Method according to claim 1 or 2 wherein the treated filler contains ethylbenzene and / or a mixture of xylenes poor in para-xylene or free of para-xylene. 4. Method according to one of the preceding claims, in which step (a) is carried out at a temperature between 350 ° C and 450 ° C, a partial pressure of hydrogen between 0.4 and 1 MPa, a total pressure of between 0.6 and 1.2 MPa and a spatial feeding speed, expressed in kilograms of aromatic fraction introduced per kilogram of catalyst and per hour, from 1 to 30 h -1. 5. Method according to one of the preceding claims, in which step (b) is carried out at a temperature between 350 ° C and 450 ° C, a partial pressure of hydrogen between 0.4 and 1 MPa, a total pressure of between 0.6 and 1.2 MPa and a spatial feeding speed, expressed in kilograms of aromatic fraction introduced per kilogram of catalyst and per hour, from 1 to 30 h ¹ . 6. Method according to one of the preceding claims wherein the amount of catalyst A used in step (a) is greater than or equal to the amount of catalyst B used in step (a). 7. Method according to one of the preceding claims in which the isomerized effluent from step (a) is sent entirely in step (b). 8. Method according to one of the preceding claims in which steps (a) and (b) are carried out in a fixed bed reactor. 9. Method according to one of the preceding claims, in which the isomerization catalyst A used in step (a) comprises: - from 1 to 90%, preferably from 3 to 80% by weight of IZM-2 zeolite, - from 0.01 to 4% by weight of at least one metal from group VIII of the periodic table, - optionally from 0.01 to 2% by weight of at least one additional metal chosen from the group formed by the metals of groups IIIA, IVA and VIIB, - possibly a sulfur content, - at least one matrix ensuring the complement to 100% in the catalyst. 10. The method of claim 9 wherein the ratio between the number of moles of silicon and the number of moles of aluminum of the zeolite IZM-2 used in step (a) is between 60 and 150. 11. Method according to one of the preceding claims, in which the isomerization catalyst B used in step (b) comprises: from 1 to 90% by weight of EUO structural type zeolite, preferably EU-1 zeolite, 5 - from 0.01 to 4% by weight of at least one metal from group VIII of the periodic table, - optionally from 0.01 to 2% by weight of at least one additional metal chosen from the group formed by the metals of groups IIIA, IVA and VIIB, - possibly a sulfur content, 10 - at least one matrix. 12. The method of claim 9 or 11 wherein the matrix is alumina.