MXPA98009943A - Methyling of toluene to para-xil - Google Patents

Abstract

A process for the production of para-xylene is provided by the methylation of toluene in the presence of a zeolite catalyst linked to zeolite. The catalyst comprises first crystals of zeolite which are agglutinated together by second crystals of zeolite. When used to methylate toluene to para-xylene. The zeolite catalyst bound to zeolite has a higher para-xylene selectivity than the thermodynamic equilibrium. This selectivity can be increased by subjecting the catalyst to a selective process

Description

METHYLATION OF TOLUENE TO PARA-XYLENE Field of the Invention This invention relates to a process for the production of xylenes by catalytic methylation of toluene in the presence of a zeolite catalyst linked to * zeolite. Furthermore, this invention relates to a process for the selective production of xylene by catalytic methylation of toluene in the presence of a zeolite catalyst bound to zeolite, selective. BACKGROUND OF THE INVENTION Of the xylene isomers, ie, ortho, meta and para-xylene, para-xylene is of particular value as an intermediate chemical in several applications, being useful in the manufacture of terephthalic acid., Which is an intermediary in the manufacture of synthetic fibers. A process to manufacture para-xylenes is through the disproportionation of toluenes in xylenes. One of the disadvantages of this process is that large amounts of benzene are also produced. Another process used to obtain para-xylene involves the isomerization of a feed stream containing non-equilibrium amounts of mixed ortho and meta-xylene isomers, and is poor with respect to the para-xylene content. A disadvantage of this process is that the separation of para-xylene from the other isomers is costly.

The zeolites comprise a lattice of silica and optionally alumina, combined with interchangeable cations such as alkali metal or alkaline earth metal ions. Although the term "zeolites" includes materials containing silica and optionally alumina, it is recognized that the portions of silica and alumina can be replaced in whole or in part with other oxides. For example, germanium oxide, tin oxide, phosphorus oxide, and mixtures thereof, can replace the silica portion. Boron oxide, iron oxide, gallium oxide, indium oxide, and their mixtures, can replace the alumina portion. Accordingly, the terms "zeolite", "zeolites" and "zeolite material", as used herein, will not only mean silicon-containing materials, and optionally aluminum atoms in their crystalline lattice structure, but also materials that they contain suitable replacement atoms for such silicon and aluminum, such as galosilicates, silicoaluminum-phosphates (SAPO), and aluminophosphates (ALPO). The term "aluminosilicate zeolite", as used herein, will mean materials consisting essentially of silicon and aluminum atoms in their crystalline lattice structure. Processes for the production of xylenes by methylation of toluene using a zeolite catalyst have been proposed. For example, U.S. Patent 3,965,207 involves the methylation of toluene using a zeolite catalyst such as ZSM-5. U.S. Patent 4,670,616 involves the production of xylenes by methylation of toluene using a molecular sieve of borosilicate that is bound by a binder such as alumina, silica, or alumina-silica. The synthetic zeolites are usually prepared by crystallization of zeolites from a mixture of supersaturated synthesis. The resulting crystalline product is then dried and calcined to produce a zeolite powder. Although zeolite powder has good adsorption properties, its practical applications are severely limited because it is difficult to operate fixed beds with zeolite powder. Therefore, before being used in commercial processes, the zeolite crystals are usually bound. The zeolite is typically bound to form an aggregate of zeolite such as a pill, sphere or extrudate. The extrudate is usually formed by extruding the zeolite in the presence of a non-zeolitic binder and drying and calcining the resulting extrudate. The binder materials used are resistant to temperatures and other conditions, for example mechanical attrition, which occur in various hydrocarbon conversion processes. Examples of binder materials include amorphous materials such as alumina, silica, titania and various types of clays. It is generally necessary that the zeolite be resistant to mechanical attrition, ie the formation of fine particles which are small particles, for example particles having a size of less than 20 microns.

Although such bound zeolite aggregates have much better mechanical strength than the zeolite powder, when such bound zeolite is used for toluene methylation, the performance of the catalyst, for example activity, selectivity, maintenance of activity, or combinations thereof, It can be reduced due to the binder. For example, since the amorphous binder is typically present in an amount of up to about 50% by weight of zeolite, the binder dilutes the adsorption properties of the zeolite aggregate. In addition, since the bound zeolite is prepared by extruding or otherwise forming the zeolite with the binder and subsequently drying and calcining the extrudate, the amorphous binder can penetrate the pores or otherwise block access to the pores of the zeolite, or reduce the rate of mass transfer to the pores of the zeolite, which can reduce the effectiveness of the zeolite when it is used in hydrocarbon conversion processes. Furthermore, when such bound zeolite is used in catalytic conversion processes, such as toluene methylation, the binder can affect the chemical reactions that are taking place within the zeolite and can itself catalyze undesirable reactions which can result in the formation of undesirable products. SUMMARY OF THE INVENTION The present invention is directed to a process for producing para-xylene by the reaction of toluene and a methylating agent under conversion conditions using a zeolite catalyst linked to zeolite, comprising first crystals of a first size zeolite of intermediate pore and a binder comprising second crystals of a second zeolite. In another embodiment, a process for selectively producing para-xylene in preference to meta or ortho-xylene is provided by the reaction of toluene and a methylating agent under conversion conditions in the presence of zeolite catalyst bound to zeolite which has been selective by depositing a selective agent on it. In comparison with a mixture of xylenes in conventional thermodynamic equilibrium in which the ratio for: meta: ortho is approximately 1: 2: 1, the process can achieve a xylene product in which the para-xylene content can exceed 70% The improved para-xylene yields reduce the cost of separation of para-xylene from the other xylene isomers. Detailed Description of the Invention The zeolite catalyst linked to zeolite used in the process of the present invention comprises first crystals of a first zeolite of intermediate pore size, acidulated and a binder comprising second crystals of a second zeolite. The use of second crystals of zeolite as agglutinate-gives you as a result a catalyst that provides means to control undesirable reactions that take place at or near the surface of the first crystals of zeolite and can have improved reagent mass transfer and greater access and from the pores of the zeolite. Unlike zeolite catalysts bound with amorphous material such as silica or alumina to improve the mechanical strength of the zeolite, the zeolite catalyst bound to zeolite using in the process of the present invention does not contain significant amounts of non-zeolitic binders. Preferably, the zeolite catalyst bound to zeolite contains less than 10% by weight based on the total weight of the first and second zeolites of the non-zeolitic binder, more preferably it contains less than 5% by weight, and with the greatest preference , the first and second zeolites are substantially free of the non-zeolitic binder. Preferably, the second crystals of zeolite bind the first crystals of zeolite by adhering to the surface of the first crystals of zeolite, thereby forming a matrix or bridge structure which also holds together the particles of the first crystals. More preferably, the second crystals of zeolite bind the first zeolite by inter-development to form an over-development resistant to attrition on the first crystals of zeolite. Although the invention is not intended to be limited by any theory of operation, it is believed that one of the advantages of the zeolite catalyst linked to zeolite, when used in the process of the present invention, is obtained by the fact that the latter Zeolite crystals control the accessibility of the acid sites on the outer surfaces of the first zeolite to the reactants. As the acid sites that exist on the outer surface of a zeolite catalyst are not selective in form, these acidic sites can adversely affect reagents entering the pores of the zeolite and products leaving the pores of the zeolite. In line with this belief, as the acidity of the second zeolite can be carefully selected, the second zeolite does not significantly negatively affect reagents coming out of the pores of the first zeolite, which can occur with conventionally bound zeolite catalysts and it can bene fi cially affect aromatic selectivity of a dehydrogenation process, and also reagents coming out of the pores of the first zeolite. Furthermore, since the second zeolite is not amorphous but instead is a molecular sieve, the hydrocarbons have increased access to the pores of the first zeolite during the aromatization process. The terms "acidity", "lower acidity" and "high acidity", as applied to zeolite, are known to those skilled in the art. The acidulated properties of zeolite are well known. However, with respect to the present invention, a distinction must be made between acid intensity and acid site density. The acid sites of a zeolite can be a Bronstead acid or a Lewis acid. The density of the acid sites and the number of acid sites are important to determine the acidity of the zeolite. The factors that directly influence the acid intensity are (i) the chemical composition of the zeolite framework, ie the relative concentration and type of tetrahedron atoms, (ii) the concentration of the extra-framework cations and the species extra-framework resulting, (iii) the local structure of the zeolite, for example, the pore size and location, within the crystal and at or near the surface of the zeolite, and (iv) the pre-treatment and the presence of co-adsorbed molecules. The magnitude of the acidity is related to the degree of isomorphic substitution provided; however, such acidity is limited to the loss of acid sites for a pure SiO2 composition. As used herein, the terms "acidity", "lower acidity" and "higher acidity" refer to the concentration of acidic sites, regardless of the intensity of such acidic sites, which can be measured by adsorption with ammonia. The first zeolite used in the zeolite catalyst linked to zeolite is a zeolite of intermediate pore size. Intermediate pore size zeolites have a pore size of about 5 to about 7 A and include, for example, zeolite type AEL structure, MFI, MEL, MFS, MEI, MTW, EUO, MTT, HEU, FER and TON These zeolites are described in "Atlas of Zeolite Structure Types", W.H. Meier and D.H. Olson, editors, Butterworth-Heineman, third edition, 1992, which is incorporated herein by reference. Examples of intermediate pore size specific zeolites include ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-34, ZSM-35, ZSM-38, ZSM-48, ZSM-50 Y ZSM-57. First preferred zeolites are gallosilicate zeolites having an MFI structure and aluminosilicate zeolites having an MFI structure. The term "average particle size", as used herein, means the average diameter of the crystals, for example the numerical average of the major axis and the minor axis. The average crystal size of the crystals of the first zeolite is preferably from about 0.1 to about 15 microns, more preferably from about 1 to about 6 microns. The methods for determining crystal size are known to those skilled in the art. For example, the crystal size can be determined directly by taking a suitable scanning electron microscope (SEM) image of a representative sample of the crystals. The first intermediate pore size zeolites will generally comprise a composition having the following molar ratio: X203: (n) Y02, wherein X is a trivalent element such as aluminum and gallium and Y is a tetravalent element such as silicon, tin and / or germanium; and n has a value greater than 12, said value being dependent on the particular type of zeolite. When the intermediate pore size zeolite is a structure-type zeolite MFI, n is preferably greater than 20. As is known to those skilled in the art, the acidity of a zeolite can be reduced by using many techniques, such as by formation of water vapor. In addition, the acidity of a zeolite depends on the shape of the zeolite, the hydrogen form having the highest acidity and other forms of the zeolite, such as the sodium form, having lower acidity than the acid form. Accordingly, the molar ratios of silica to alumina and silica to galia disclosed herein include not only the zeolites having the reported molar ratios, but will also include zeolites that do not have the disclosed molar ratios but have equivalent catalytic activity. When the first zeolite is an aluminosilicate zeolite, the first zeolite will preferably have a molar ratio of silica to alumina of 10: 1 to 300: 1. When the first zeolite is a gallosilicate zeolite, the zeolite preferably comprises a composition having the following molar ratio: Ga203: yS102, where y is between about 10 and about 150. The framework of the zeolite may contain only gallium and silicon atoms or may also contain a combination of gallium, aluminum and silicon. When the first zeolite is a galosilicate zeolite with MFI type structure, the second zeolite will preferably be a zeolite of intermediate pore size having a silica to gallium molar ratio greater than 100. The second zeolite may also have higher silica molar ratios to galia, for example greater than 200, 500, 1,000, etc. The second zeolite will usually have an intermediate pore size and will have less activity than the first zeolite. Preferably, the second zeolite will be substantially non-acidic and will have the same type of structure as the first zeolite. The second preferred zeolites are aluminosilicate zeolites having a molar ratio of silica to alumina greater than 100, such as ZSM-5 of low acidity. If the second zeolite is an aluminosilicate zeolite, the second zeolite will generally have a molar ratio of silica to alumina greater than 200: 1, for example 500: 1, 1,000: 1, etc., and in some applications it will contain no more than trace of alumina. The second zeolite can also be silicalite, ie an MFI-like structure substantially free of alumina, or silicalite 2, a MEL-like structure substantially free of alumina. The second zeolite is usually present in the zeolite catalyst bound to zeolite in an amount in the range of about 10 to 60% by weight, based on the weight of the first zeolite, and more preferably around 20 to about 50% by weight. The second zeolite crystals preferably have a smaller size than the first zeolite crystals, and more preferably they will have an average particle size of less than 1 miera, and most preferably they will have an average particle size of about 0.1. around 0.5 microns. The second crystals of zeolite, in addition to ligating the first zeolite particles and maximizing the performance of the catalyst, will preferably inter-develop and form an over-development that coats or partially coats the first crystals of zeolite. Preferably, the crystals will be resistant to attrition. The zeolite catalyst bound to zeolite used in the process of the present invention is preferably prepared by a three-step process. The first step involves the synthesis of the first crystals of zeolite before converting them into the zeolite catalyst bound to zeolite. Processes for preparing the first zeolite are known in the art. For example, with respect to the preparation of an MFI type aluminosilicate zeolite, a preferred process comprises preparing a solution containing tetrapropyl ammonium hydroxide or bromide, alkali metal oxide, aluminum oxide, silicon oxide and water. , and then heating the reaction mixture to a temperature of 80 to 200 ° C for a period of about four hours to eight days. The resulting gel forms solid crystal particles that are separated from the reaction medium, washed with water and dried. The resulting product can be optionally calcined in air at temperatures of 400-550 ° C for a period of 10-40 hours to remove tetrapropyl ammonium cations (TPA). Next, a silica-bound aluminosilicate zeolite can be prepared, preferably by mixing a mixture comprising crystals of aluminosilicate zeolite, a silica gel or sol, water and optionally an extrusion aid and, optionally, the metal component, until a homogeneous composition develops in the form of an extruded paste. The silica binder used in the preparation of the silica-bound zeolite aggregate is preferably a silica sol and preferably only contains very minor amounts of alumina or gallium, for example less than 2,000 ppm. The amount of silica used is such that the content of the zeolite in the dry extrudate varied from about 40 to 90% by weight, more preferably from about 50 to 80% by weight, the remainder being mainly silica, for example around 20 to 50% by weight of silica. The resulting paste can be molded, for example extruded, and cut into small filaments, for example extrudates of approximately 2 mm in diameter, which can be dried at 100-150 ° C for a period of 4-12 hours and then calcined in air at a temperature of about 400 to 550 ° C for a period of about 1 to 10 hours.

Optionally, the aggregate bonded to silica can be made into extremely small particles that have application in fluid bed processes such as catalytic cracking. This preferably involves mixing the zeolite with a silica-containing matrix solution so as to form an aqueous solution of zeolite and silica binder which can be spray dried to result in small, fluidizable, silica-bonded aggregate particles. The processes for preparing such aggregate particles are known to those skilled in the art. An example of such a procedure is described by Scherzer (Octane-Enhancing Zeolitic FCC Catalysts, Julius Scherzer, Marcel Dekker, Inc., New York, 1990). The fluidizable silica bonded aggregate particles, such as the silica-bonded extrudates described above, would then undergo the final step described below to convert the silica binder into a second zeolite. The final step in the three-step catalyst preparation process is the conversion of the silica present in the silica-bound catalyst into a second zeolite, which serves to ligate the first zeolite crystals together. The first zeolite crystals are thus held together without the use of a significant amount of non-zeolite binder. To prepare the zeolite catalyst linked to zeolite, the aggregate bound to zeolite can first be aged in an appropriate aqueous solution at an elevated temperature.

Next, the content of the solution and the temperature at which the aggregate is aged should be selected to convert the amorphous silica binder into the second zeolite. It is preferred that the second zeolite be of the same type as the first zeolite. The newly formed zeolite is produced as crystals. The crystals can develop in and / or adhere to the initial crystals of zeolite, and can also be produced in the form of new inter-developed crystals, which are generally much smaller than the initial crystals, for example of sub-micron size. These newly formed crystals can develop and interconnect together. The nature of the aluminosilicate zeolite formed in the conversion of secondary synthesis of silica into zeolite can vary as a function of the composition of the secondary synthesis solution and the aging conditions of synthesis. The secondary synthesis solution is preferably an aqueous ionic solution containing a sufficient hydroxyl ion source to convert the silica into the desired zeolite. The zeolite catalyst bound to zeolite is usually in the acid or acid form, partially neutralized. In order to obtain the acid form, the zeolite is subjected to ion exchange to produce the ammonium salt form. As a result of the calcination, the acid form of the zeolite catalyst bound to zeolite is produced.

In a more preferred embodiment, the zeolite catalyst bound to zeolite is selective to improve its selectivity to para-xylene. The processes for selecting the catalyst are known to those skilled in the art. For example, the selectivation can be achieved by exposing the catalyst in a reactor bed to a thermally decomposable organic compound, for example toluene, at a temperature in excess of the decomposition temperature of said compound, for example from about 480 to about 650 ° C, more preferably 540 to 650 ° C, to a WHSV in the range of about 0.1 to 20 pounds of feed per pound of catalyst per hour, at a pressure in the range of about 1 to 100 atmospheres, and in presence of from 0 to about 2 moles of hydrogen, more preferably from about 0.1 to about 1 mole of hydrogen per mole of the organic compound, and optionally in the presence of 0.10 mole of nitrogen or other inert gas per mole of organic compound . This process is conducted for a period of time until a sufficient amount of coke has been deposited on the catalyst surface, generally at least about 2% by weight and more preferably from about 4 to about 40% by weight of coke. In a preferred embodiment, such selective process is conducted in the presence of hydrogen in order to prevent rampant coke formation on the catalyst. The initial molar ratio of hydrogen gas to toluene present in the toluene feed stream can be reduced during the selectivation process after a considerable amount of coke has been deposited on the catalyst surface. The selectivity of the catalyst can also be achieved by using organosilicon compounds. The silicon compounds may comprise a polysiloxane, including silicones, a siloxane, and a silane, including disilanes and alkoxysilanes. The silicon compounds that can be used in the present invention can be characterized by the general formula: where R is hydrogen, fluoride, hydroxy, alkyl, aralkyl, alkaryl or fluoroalkyl. The hydrocarbon substituents generally contain from 1 to 10 carbon atoms, and preferably are methyl or ethyl groups. R2 is selected from the same group as Rlf and n is an integer of at least 2 and generally in the range of 2 to 1,000. The molecular weight of the silicon compound generally used is between 80 and 20,000, and preferably 150 to 10,000. Representative silicon compounds include dimethylsilicon, diethyl silicon, phenylmethylsilicon, methylhydrogen silicone, ethylhydrogensilicon, phenylhydrogensilicon, methylethyl silicone, phenylethylsilicon, diphenylsilicon, methyltrifluoropropyl silicone, ethyltrifluoropropylsilicon, tetrachlorophenyl methyl silicon, tetrachlorophenylethyl silicon, tetrachloro phenylhydrogensity, tetrachlorophenylphenyl silicon , methyl vinyl silicon, and ethyl vinyl silicon. The silicon compound does not need to be linear but can be cyclic, such as, for example, hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, hexaphenylcyclotrisiloxane and octaphenylcyclotetrasiloxane. Mixtures of these compounds as well as silicones can be used with other functional groups. Useful siloxanes or polysiloxanes include, as non-limiting examples, hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, decamethyl cyclopentasiloxane, hexamethyldisiloxane, octamethyltrisiloxane, decamethyltetrasiloxane, hexaethylcyclotris-loxane, octaethylcyclotetrasiloxane, hexaphenylcyclotrisiloxane, and octaphenyl cyclotetrasiloxane. Useful silanes, disilanes or alkoxysilanes include organic-substituted silanes having the general formula: R2 wherein R is a reactive group such as hydrogen, alkoxy, halogen, carboxy, amino, acetamide, trialkylsilioxy, R1 R2 and R3 may be the same as R or may be an organic radical, which may include alkyl of 1 to 40 carbon atoms, alkyl or aryl carboxylic acid, where the organic portion of the alkyl contains 1 to 30 carbon atoms and the aryl group contains from 6 to 24 carbon atoms, which can be further substituted, alkylaryl and arylalkyl groups containing 7 to 30 carbon atoms. Preferably, the alkyl group for an alkylsilane is between 1 and 4 carbon atoms of chain length. Mixtures can also be used. The silanes or disilanes include, as non-limiting examples, dimethylphenylsilane, phenyltrimethylsilane, triethylsilane and hexamethyldisilane. Useful alkoxysilanes are those with at least one silicon-hydrogen bond. The methylation process can be carried out as a load-type operation, semi-continuous or continuous, using a fixed or moving bed catalyst system. Multiple injection of the methylating agent can be employed. The toluene and the methylating agent are usually pre-mixed and fed together to the reaction vessel to maintain the desired ratio between them, without local concentration of any of the reactants to disturb the kinetics of the reaction. However, individual feeds may be employed, although care must be taken to ensure good mixing of the vapors of the reactants in the reaction vessel. The instantaneous concentration of the methylating agent can be kept low by additions in stages thereof. By stepwise additions, the toluene / methylation agent concentrations can be maintained at optimum levels, to give good conversions of toluene. Hydrogen gas can be supplied to the reaction as an anti-coke and diluent agent. By carrying the process into practice, the catalyst and reagents can be heated to the reaction temperature separately or together. The reaction temperatures are from about 300 to about 700 ° C, and preferably from about 400 to about 700 ° C. The reaction is preferably carried out at a gauge pressure of about 1 to 1,000 psi, a space velocity hour by weight (WHSV) of between about 1 and about 2., 000, a molar ratio of methylating agent to toluene of between about 0.05 and about 5 and a WHSV of between about 1 and about 200, and preferably between about 5 and about 150 units of charge weight per unit weight of catalyst per hour. The reaction product can be separated by any suitable means. Typical methylation agents include methanol, dimethyl ether, methyl chloride, methyl bromide and dimethyl sulfide. A person skilled in the art will know that other methylating agents can be employed in the process of this invention, based on the description provided herein. Preferred methylation agents are methanol and dimethyl ether. Methanol is the most preferred. The following examples illustrate the invention: Example 1 I. Catalyst A Catalyst A comprised 70% by weight of core crystals of H-ZSM-5 (average particle size 3.5 microns), having a molar ratio of silica to alumina of 75: 1, and 30% by weight of crystals ZSM-5 binders, having a molar ratio of silica to alumina of about 900: 1. The catalyst was first prepared by mixing the core crystals of ZSM-5 with amorphous silica containing traces of alumina and then extruding the mixture in a silica-bonded extrudate. Next, the silica binder of the extrudate was converted into the second zeolite by aging the aggregate at elevated temperatures in an aqueous solution containing a template and sufficient hydroxyl ions to convert the silica into the binding crystals. The resulting zeolite bound to zeolite was then washed, dried, calcined, and subjected to ion exchange to the hydrogen form. II. Catalyst B Catalyst B comprised 70% by weight of H-ZSM-5 (average particle size 3.5 microns) having a mole ratio of silica to alumina of 75: 1 and 30% by weight of amorphous silica binder.

Toluene methylation tests were carried out using catalyst A and catalyst B. Before the start of the tests, each catalyst was shredded and sized between U.S. 30 and +40. Then, an amount of 1.5 g of catalyst was mixed with 3 g of quartz chips of size 14/20 mesh and packed in a tubular reactor. Next, a 3.8 / 1 molar mixture of toluene was vaporized to methanol and fed to the reactor. The test conditions included a WHSV of 12, a temperature of 400 ° C, 450 ml / min of N2, and a total gauge pressure of 6.0 psi. The results are shown below in Table I: Table I Selectivity at PX = (PX [PX + MX + OX]) x 100 The data shows that catalyst A had a selectivity to para-xylene considerably greater than the thermodynamic equilibrium and also higher than the catalyst bound to amorphous silica. Catalyst A was selective with hexamethyldisiloxane (HMDS). The feed comprised 4.7% by weight of hexamethyldisiloxane, 1.0% by weight of n-propylmercaptan, and a molar ratio of toluene to methanol of 3.8: 1. The feed was pumped into a hot line where it was vaporized at 325 ° C and fed to the tubular reactor. The conditions of the test included a temperature of 400 ° C, a WHSV of 12, 450 ml / min of N2, and a total gauge pressure of 6.0 psi. The feed used to select the catalyst had a ratio of toluene to methanol and contained 4.7% by weight of hexamethyldisiloxane and 1.0% by weight of n-propyl mercaptan. The feed was vaporized and then fed to 1.5 g of catalyst A, which was packed with 3.0 g of quartz chips in the tubular reactor. The conditions for selection were 400 ° C, a WHSV of 7.9, 8 ml / min of 54% H2 in N2, and a total gauge pressure of 0.3-0.5 psi. The products were analyzed by gas chromatography online. The results are shown in Table II. Table II After the selectivation, catalyst A was subjected to toluene methylation test, using the same procedure as Example 1, except that the WHSV was 7.9 and the total gauge pressure was 5.3-5.6 psi. The results are shown below in Table III: Table III The data shows that the activity and selectivity to para-xylene of catalyst A were improved by the selectivation. Example 2 A zeolite catalyst bound to zeolite, calcined, comprising core crystals of H-ZSM-5 (molar ratio of silica to alumina of about 900) was selected by feeding toluene through the catalyst under the conditions indicated in Table IV then: Table IV Selectivation Conditions After the selectivation, the toluene was alkylated with methanol using the selective catalyst. The test conditions and the performance of the catalyst in oil are shown in Table V.

Table V Selectivity to PX = (PX [PX + MX + OX]) x 100 The data in Table V show that the catalyst has a high selectivity to para-xylene and that the xylenes: benzene ratio increased with time in current.

Claims (20)

1. CLAIMS 1. A process for making para-xylene by reaction of toluene with a methylating agent under methylation conditions in the presence of a zeolite catalyst bound to zeolite which does not contain significant amounts of non-zeolitic binder and which comprises: (a) first crystals of a first zeolite of intermediate pore size; and (b) a binder comprising second crystals of a second zeolite.
2. 2. The process defined in claim 1, wherein the second crystals are inter-developed and form at least a partial coating on the first crystals.
3. 3. The process defined in claim 1 or 2, wherein the catalyst contains less than 5% by weight non-zeolitic binder based on the combined weight of the first zeolite and the second zeolite.
4. 4. The process defined in any of the preceding claims, wherein the first crystals have an average particle size greater than 0.1 microns and the second crystals have an average particle size smaller than that of the first crystals. The process defined in claim 4, wherein the first crystals have an average particle size of 1 to 6 microns and / or the second crystals have an average particle size of 0.1 to 0.5 microns. 6. The process defined in any of the preceding claims, wherein the first zeolite and / or the second zeolite is an aluminosilicate zeolite or a gallosilicate zeolite. The process defined in claim 6, wherein the first zeolite is an aluminosilicate with a molar ratio of silica to alumina of 10: 1 to 300: 1 or a gallosilicate with a molar ratio of silica to galia of 40: 1 to 500 :1. The process defined in claim 6 or 7, wherein the second zeolite is an aluminosilicate with a molar ratio of silica to alumina greater than 200: 1 or a gallosilicate with a molar ratio of silica to galia greater than 100: 1. 9. The process defined in any of the preceding claims, wherein the type of structure of the first zeolite and the second zeolite is independently selected from the group consisting of AEL, MFI, MEL, MTW, MTT, FER, TON and EUO The process defined in claim 9, wherein the first zeolite has an MFI structure and / or the second zeolite has an MFI or MEL structure. 11. The process defined in claim 10, wherein the second zeolite is silicalite or silicalite 2. 12. The process defined in any of the preceding claims, wherein the catalyst is capable of being prepared by aging at high temperature an aggregate bound to silica. which contains the first crystals of the first zeolite in an aqueous ionic solution containing a sufficient source of hydroxy ions to convert the silica into the aggregate in the second zeolite. 13. The process defined in any of the preceding claims, wherein the catalyst is selectivated. The process defined in claim 13, wherein the catalyst is selective with a selective agent comprising a silicon compound, preferably hexadimethylsi-loxane. 15. The process defined in claim 13 or 14, wherein the catalyst is pre-selectivated by contacting the catalyst with a stream of toluene at a temperature of 482 to 649 ° C (900 to 1,200 ° F), a pressure of 0.10. at 10.1 MPa (1 to 100 atmospheres), and a time-space velocity by weight of 0.1 to 20, and where the toluene stream optionally also contains hydrogen at a molar ratio of H2 / toluene of up to 2. 16. The process defined in claim 15, wherein the molar ratio of hydrogen to toluene in the toluene stream is from 0.1 to 2. 17. The process defined in any of claims 13 to 16, wherein the catalyst contains at least 2% by weight of coke. . 18. The process defined in any of the preceding claims, wherein the methylating agent is methanol, methyl chloride, methyl bromide, dimethyl ether or dimethyl sulfide. 19. The process defined in any of the preceding claims, wherein the methylation conditions include a temperature of 250 to 750 ° C and / or a pressure of 0.10 MPa to 6.9 MPag (1 atmosphere to 1,000 psig) and / or a space-speed in weight from 1 to 200 and / or a molar ratio of methylation agent to toluene from 0.05 to
5. 5. The process defined in any of the preceding claims, which produces a product stream containing larger amounts than the para-xylene equilibrium.