PROCESS FOR ISOMERIZATION OF ALKYLAROMATIC HYDROCARBONS

Abstract: A process for isomerizating a feed containing alkylaromatic hydrocarbons, e.g., monocyclic alkylaromatic hydrocarbons and/or bicyclic alkylaromatic hydrocarbons. The process is carried out by contacting the feed under conversion conditions with a zeolite bound zeolite catalyst which comprises first crystals of an intermediate pore size first zeolite and a binder which comprises second crystals of a second zeolite. The process finds particular application in isomerizing a feed containing ethylbenzene and less than equilibrium amounts of xylenes with ethylbenzene and can produce a product containing above equilibrium quantities of para-xylene with low aromatics ring loss and xylene loss.

Description: PROCESS FOR ISOMERIZATION OF ALKYLAROMATIC HYDROCARBONS FIELD OF THE INVENTION This invention relates to a process for the isomerization of alkylaromatic hydrocarbons using a zeolite bound zeolite catalyst. More particularly, but not by way of Limitation, this invention relates a process for the isomerization of ortho- and meta-xylenes to para- xylene, as well as the removal of ethylbenzene from a C» aromatics stream using a zeolite bound zeolite catalyst . BACKGROUND OF THE INVENTION Xylenes are valuable industrial chemicals. Sources of xylenes include catalytic reformate, pyrolysis gasoline, toluene disproportionation; C7-C9 aromatic transalkylation, and the like. For example, catalytic reforming hydrocarbon feeds such as naphtha using conventional aromatization catalysts produces a reformate which is richer in the content of Cβ-Cio aromatics than the feeds. Of these aromatics, significant quantities of Ca aromatics are produced which comprise a mixture of ethyl benzene, and mixed ortho-, meta- and para-xylene isomers. Typically, the product from the catalytic reformer (reformate) is fed to an aromatic extraction plant where the aromatics, e.g., Ce, C7 and Cβ aromatics, are separated from the paraffins and other non-aromatic products present in the reformate. The Cs aromatic fraction may then be separated from the lower boiling Cβ and C 7 aromatics by distillation. The Cβ aromatic fraction normally contains a mixture of ethyl benzene and the ortho-, para-, and meta-xylene isomers. The three xylene isomers are usually present in near thermodynamic equilibrium amounts, e.g., generally 52-53 wt.% meta-xylene, 23-24 wt.% para- xylene and 23.5 to 24.5 wt.% ortho-xylene. Of the xylene isomers, meta-xylene is typically the least desired product. Because para- xylene is of particular value as a chemical intermediate in a number of applications, it may be desirable to separate the para-xylene from the other isomers using conventional techniques such as crystallization, or by adsorption/desorption on zeolites. After such separation, the residual Cβ aromatic fraction contains non-equilibrium quantities of ethylbenzene and the mixed ortho- and meta-xylene isomers and is lean with respect to para-xylene content. The para-xylene lean residual product may be further upgraded by subjecting it to isomerization conditions wherein at least a portion of the ethylbenzene is converted to other products such diethylbenzene or benzene and ethane and a portion of the ortho- and meta-xylenes are isomerized to produce a mixture which once again approximates the equilibrium concentration of the ortho-, meta-, and para-xylene isomers. Typically such isomerization conditions comprise contacting the non-equilibrium Cβ aromatic feed with a suitable isomerization catalyst in a suitable reactor at temperatures above about 600°F and preferably at pressures sufficient to maintain the reaction in the vapor phase. A commercially viable xylene isomerization process must exhibit high xylene isomerization activity and, also, must produce the desired product without a significant loss of xylenes. The loss of xylene is a result of undesired side-reactions, involving hydrogenation of the aromatic ring, hydrogenolysis, demethylation, and particularly disproportionation and transalkylation. Another factor of importance in a xylene isomerization process is the effect that ethylbenzene has on the entire isomerization and xylene recovery loop. When ethylbenzene is present in appreciable quantities in the feed to the isomerization process, it will accumulate in the loop unless it is excluded from the feed or converted by some reaction in the loop to products which are separable from xylenes. Ethylbenzene can be separated from the xylenes by "sup erfr action ation", but this procedure is very expensive. A more desirable method of eliminating the ethylbenzene is through a conversion reaction taking place simultaneously with the isomerization reaction of the xylenes. One method of converting ethylbenzene is to isomerize the ethylbenzene to xylenes. It is often desirable that the ethylbenzene conversion reaction be a deethylation reaction producing benzene and ethane rather than a disproportionation reaction to benzene and diethylbenzene. The deethylation reaction preserves more xylenes and produces a high quality reaction product. Zeolites are comprised of a lattice silica and optionally alumina combined with exchangeable cations such as alkali or alkaline earth metal ions. Although the term "zeolites" includes materials containing silica and optionally alumina, it is recognized that the silica and alumina portions may be replaced in whole or in part with other oxides. For example, germanium oxide, tin oxide, phosphorous oxide, and mixtures thereof can replace the silica portion. Boron oxide, iron oxide, gallium oxide, indium oxide, and mixtures thereof can replace the alumina portion. Accordingly, the terms "zeolite", "zeolites" and "zeolite material", as used herein, shall mean not only materials containing silicon and, optionally, aluminum atoms in the crystalline lattice structure thereof, but also materials which contain suitable replacement atoms for such silicon and aluminum, such as gallosilicates, silicoaluminophosphates (SAPO) and aluminophosphates (ALPO). The term "aluminosilicate zeolite", as used herein, shall mean zeolite materials consisting essentially of silicon and aluminum atoms in the crystalline lattice structure thereof. Numerous processes have been proposed for the isomerization of xylene feeds using zeolite catalysts. For instance, U.S. Patent 4,312,790 involves a xylene isomerization process using an alumina bound zeolite catalyst. U.S. Patent 4,939,110 involves a xylene isomerization process using a zeolite catalyst such as a ZSM-5 which is bound by a binder material such as alumina, silica, or clay. Synthetic zeolites are normally prepared by the crystallization of zeolites from a supersaturated synthesis mixture. The resulting crystalline product is then dried and calcined to produce a zeolite powder. Although the zeolite powder has good adsorptive properties, its practical applications are severely limited because it is difficult to operate fixed beds with zeolite powder. Therefore, prior to using the powder in commercial processes, the zeolite crystals are usually bound. The zeolite powder is typically bound by forming a zeolite aggregate 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 the temperatures and other conditions, e.g., 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, that is, the formation of fines which are small particles, e.g., 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 a bound zeolite is used for xylene isomerization, the performance of the zeolite catalyst, e.g., activity, selectivity, activity maintenance, or combinations thereof, can be reduced because of the binder. For instance, since the binder is typically present in an amount of up to about 50 wt.% 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 of the zeolite or otherwise block access to the pores of the zeolite, or slow the rate of mass transfer to the pores of the zeolite which can reduce the effectiveness of the zeolite when used in xylene isomerization. Furthermore, when the bound zeolite is used in xylene isomerization, the binder may affect the chemical reactions that are taking place within the zeolite and also may itself catalyze undesirable reactions which can result in the formation of undesirable products. SUMMARY OF THE INVENTION The present is directed to a process for the isomerization of isomerizable alkylaromatic hydrocarbons utilizing a zeolite bound zeolite catalyst which comprises first crystals of a first intermediate pore size zeolite, and a binder comprising second crystals of a second zeolite. In another embodiment, the present invention provides a process for isomerizing a non-equilibrium feed mixture containing ethylbenzene and xylene isomers comprising contacting said feed mixture under xylene isomerization conditions with the zeolite bound zeolite catalyst. In a further embodiment of the invention, the present invention provides a process for isomerizating a non-equilibrium feed mixture containing ethylbenzene and xylene isomers comprising contacting said feed steam with the zeolite bound zeolite catalyst to partially convert the ethylbenzene and then contacting the resulting feed with a second catalyst to isomerize the xylene isomers. The isomerization of xylene streams in accordance with the process of the invention gives rise to an isomerization product which contains about equilibrium quantities or above of para-xylene with a very low percent of xylene loss. In addition, ethylbenzene present in the xylene streams is substantially converted, i.e., at least 30%, preferably greater than 50%, of the ethylbenzene is converted. DETAILED DESCRIPTION OF THE INVENTION The zeolite bound zeolite catalyst used in the process of the present invention comprises first crystals of a first intermediate pore size zeolite and a binder comprising second crystals of a second zeolite. The use of the second crystals of a second zeolite as a binder results in a catalyst which provides a means for controlling undesirable reactions taking place on or near the external surface of the first zeohte crystals and can have improved mass transfer of hydrocarbon molecules to and from the pores of the first zeohte. In addition, the second zeohte binding crystals, if desired, can also have catalytic activity, can function as a catalyst carrier, and/or can selectively prevent undesirable molecules from entering or exiting the pores of the first zeohte. Unlike typical zeolite catalysts used in hydrocarbon conversion processes which are normally bound with silica or alumina or other commonly used amorphous binders to enhance the mechanical strength of the zeolite, the zeohte catalyst of the present invention does not contain significant amounts of non-zeolitic binders. Preferably, the zeohte bound zeolite catalyst contains less than 10 percent by weight, based on the weight of the first and second zeohte, of non-zeolitic binders, more preferably contains less than 5 percent by weight, and, most preferably, the catalyst is substantially free of non-zeolitic binder. Preferably, the second zeohte crystals bind the first zeohte crystals by adhering to the surface of the first zeohte crystals thereby forming a matrix or bridge structure which also holds the first crystals particles together. More preferably, the second zeohte particles bind the first zeolite by intergrowing so as to form a coating or partial coating on the larger first zeohte crystals and, most preferably, the second zeohte crystals bind the first zeohte crystals by intergrowing to form an attrition resistant over-growth over the first zeohte crystals. Although the invention is not intended to be limited to any theory of operation, it is believed that one of the advantages of the zeohte bound zeohte catalyst of the present invention is obtained by the second zeohte crystals controlling the accessibility of the acid sites on the external surfaces of the first zeohte to reactants. Since the acid sites existing on the external surface of a zeohte catalyst are not shape selective, these acid sites can adversely affect reactants entering the pores of the zeohte and products exiting the pores of the zeohte. In line with this behef, since the acidity and structure type of the second zeohte can be carefully selected, the second zeohte does not significantly adversely affect the reactants exiting the pores of the first zeohte which can occur with conventionally bound zeohte catalysts and may beneficially affect the reactants exiting the pores of the first zeohte. Still further, since the second zeohte is not amorphous but, instead, is a molecular sieve, hydrocarbons may have increased access to the pores of the first zeohte during hydrocarbon conversion processes. Regardless of the theories proposed, the zeohte bound zeohte catalyst, when used in catalytic processes, has the improved properties which are disclosed herein. The terms "acidity", 'lower acidity" and "higher acidity" as apphed to zeohte are known to persons skilled in the art. The acidic properties of zeohte are well known. However, with respect to the present invention, a distinction must be made between acid strength and acid site density. Acid sites of a zeohte can be a Bronsted acid or a Lewis acid. The density of the acid sites and the number of acid sites are important in determining the acidity of the zeohte. Factors directly influencing the acid strength are (i) the chemical composition of the zeohte framework, i.e., relative concentration and type of tetrahedral atoms, (ii) the concentration of the extra-framework cations and the resulting extra-framework species, (hi) the local structure of the zeohte, e.g., the pore size and the location, within the crystal or at/near the surface of the zeohte, and (iv) the pretreatment conditions and presence of co-adsorbed molecules. The amount of acidity is related to the degree of isomorphous substitution provided, however, such acidity is hmited to the loss of acid sites for a pure Siθ2 composition. As used herein, the terms "acidity", 'lower acidity" and "higher acidity" refers to the concentration of acid sites irregardless of the strength of such acid sites which can be measured by ammonia absorption. The first zeohte used in the zeohte bound zeohte catalyst is an intermediate pore size zeohte. Intermediate pore size zeohtes have a pore size from about 5 to about 7 A and include, for example, MFI, MEL, MFS, MEI, MTW, EUO, MTT, HEU, FER, and TON structure type zeohtes. These zeohtes are described in "Atlas of Zeohte Structure Types", eds. W. H. Meier and D.H. Olson, Butter worth-Heineman, Third Edition, 1992, which is hereby incorporated by reference. Examples of specific intermediate pore size zeohtes include ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-34, ZSM-35, ZSM-38, ZSM- 48, ZSM-50, and ZSM-57. The term "average particle size" as used herein, means the arithmetic average of the diameter distribution of the crystals on a volume basis. The average crystal size of the crystals of the first zeohte is preferably from about 0.1 micron to about 15 microns, more preferably from about 1 to about 6 microns. Procedures to determine crystal size are know to persons skilled in the art. For instance, crystal size may be determined directly by taking a suitable scanning electron microscope (SEM) picture of a representative sample of the crystals. Intermediate pore size zeohtes suitable for use in the zeohte bound zeohte catalyst preferably comprise a composition having the following molar relationship: wherein X is a trivalent element, such as aluminum, boron and/or gallium, Y is a tetravalent element such as silicon, tin, and/or germanium; and n has a value greater than 10, said value being dependent upon the particular type of zeohte and the trivalent element present in the zeohte. When the intermediate pore size zeohte is a MFI structure type zeohte, n is preferably greater than 20. As known to persons skilled in the art, the acidity of a zeohte can be reduced using many techniques such as by steaming. In addition, the acidity of a zeohte is dependent upon the form of the zeohte with the hydrogen form having the highest acidity and other forms of the zeohte such as the sodium form having less acidity than the acid form. Accordingly, the mole ratios of silica to alumina and silica to galha disclosed herein shall include not only zeohtes having the disclosed mole ratios, but shall also include zeohtes not having the disclosed mole ratios but having equivalent catalytic activity. When the first zeohte is an aluminosihcate zeohte, the first zeohte will preferably have a silica to alumina mole ratio from 70:1 to 1000:1. When the first zeohte is a galhum silicate zeohte, the zeohte preferably comprises a composition having the following molar relationship: wherein y is between about 24 and about 500. The zeohte framework may contain only galhum and silicon atoms or may also contain a . combination of galhum, aluminum, and silicon. When the first zeohte is a MFI structure type galhum silicate zeohte, the second zeohte will preferably be an intermediate pore size zeohte having a silica to galha mole ratio greater than 100. The second zeohte can also have higher silica to galha mole ratios, e.g., greater than 200, 500, 1000, etc. The second zeohte will preferably be an intermediate pore size zeohte and have less acid activity than the first zeohte. For instance, when the second zeohte is an intermediate pore size aluminosilicate zeohte, the second zeohte will preferably have a silica to alumina mole ratio greater than 200: 1. The second zeohte can also have higher silica to alumina mole ratios, e.g., 500:1, 1,000:1 etc. The second zeohte can also be silicalite, i.e., a MFI type substantially free of alumina or silicalite 2, i.e., a MEL type substantially free of alumina. Preferably the second zeohte will have the same structure type as the first zeohte. The second zeohte is usually present in the zeohte bound zeohte catalyst system in an amount in the range of from about 10% to 60% by weight based on the weight of the first zeohte and, more preferably from about 20% to about 50% by weight. The second zeohte crystals preferably have a smaller size than the first zeohte crystals. The second zeohte crystals preferably have an average particle size of less than 1 micron, more preferably from about 0.1 to about 0.5 micron. The second zeohte crystals bind the first zeohte crystals. The second zeohte crystals preferably intergrow and form an over-growth which coats or partially coats the first zeohte crystals. Preferably, the coating will be resistant to attrition. When use to isomerize feeds containing ethylbenzene, the zeohte bound zeohte catalyst will preferably contain at least one hydrogenation metal. Examples of such metals include the oxide, hydroxide, sulfide, or free metal (i.e., zero valent) forms of Group VIII metals (i.e., Pt, Pd, Ir, Rh, Os, Ru, Ni, Co, and Fe), Group IVB metals (i.e., Sn and Pb), Group VB metals (i.e., Sb and Bi), and Group VILA metals (i.e., Mn, Tc, and Re). Noble metals (i.e., Pt, Pd, Ir, Rh, Os, and Ru) are preferred. Combinations of catalytic forms of noble or non- noble metals, such as combinations of Pt with Ni, may be used. The valence state of the metal is preferably in a reduced valence state, e.g., when this component is in the form of an oxide or hydroxide. The reduced valence state of this metal may be attained, in situ, during the course of a reaction, when a reducing agent, such as hydrogen, is included in the feed to the reaction. The amount of metal present in the zeohte bound zeohte catalyst will be an effective amount which will generally be from about 0.001 to about 10 percent by weight and, preferably 0.05 to 3.0 percent by weight. The amount will vary with the nature of the metal, less of the highly active metals, particularly platinum, being required than of the less active metals. The hydrogenation metal may be incorporated into the zeohte bound zeohte catalyst by ion exchange and impregnation. Another way of incorporating the metal into the zeohte bound zeohte catalyst is to include at least a portion of the metal in the synthesis mixture of the second zeohte and then crystallize the metal with the second zeohte. Best results have been achieved by including the metal in the silica binder of a silica bound zeohte extrudate as the binder is being formed and then converting the silica to the second zeohte. The metal may be present on the surface of either or both zeohtes and may also be present in the intracrystalline matrix of either or both zeohtes. The zeohte bound zeohte catalyst is preferably prepared by a three step procedure whereby the hydrogenation metal is incorporated into the zeohte bound zeohte during the preparation of the amorphous binder which is subsequently converted to the second zeohte. The first step involves the synthesis of the intermediate pore size first zeohte. Process for preparing the first zeohte are known to persons skilled in the art. For example, with respect to the preparation of an aluminosilicate zeohte or a galhum silicate zeohte having a MFI structure type, one process comprises preparing a solution containing tetrapropyl ammonium hydroxide or bromide, alkali metal oxide, an oxide of aluminum or an oxide of galhum, an oxide of silicon and water, heating the reaction mixture to a temperature of 80°C to 200° for a period of from about four hours to eight days. The resulting gel forms sohd crystal particles which are separated from the reaction medium, washed with water and dried. The resulting product can be calcined in air at temperatures of 400°C - 550°C for a period of 10-40 hours to remove tetrapropylammonium (TPA) cations. In the second step, a silica-bound zeohte is prepared by mixing a mixture comprising the first zeohte crystals, a silica gel or sol, water, and optionally the hydrogenation metal, and optionally an extrusion aid, until a homogeneous composition in the form of an extrudable paste develops. The sihca binder used in preparing the sihca bound zeohte aggregate is preferably a silica sol and can contain various amounts of trivalent elements, e.g., aluminum or galhum. The amount of sihca used is such that the content of the zeohte in the dried extrudate at this stage will range from about 40 to 90% by weight, more preferably from about 50 to 80% by weight, with the balance being primarily sihca, e.g. about 20 to 50% by weight sihca. The resulting paste is then molded, e.g., extruded, and cut into small strands, e.g., approximately 2 mm diameter extrudates, which are dried at 100°C to 150°C for a period of 4-12 hours and then are calcined in air at a temperature of from about 400°C to 550°C for a period of from about 1 to 10 hours. Optionally, the sihca-bound aggregate can be made into very small particles which have apphcation in fluid bed processes such as catalytic cracking. This preferably involves mixing the zeohte with a sihca and metal containing matrix solution so that an aqueous solution of zeohte and silica binder is formed which can be sprayed dried to result in small fluidizable sihca-bound aggregate particles. Procedures for preparing such aggregate particles are known to persons skilled in the art. An example of such a procedure is described by Scherzer (Octane-Enhancing Zeohtic FCC Catalysts, Julius Scherzer, Marcel Dekker, Inc. New York, 1990). The fluidizable sihca-bound aggregate particles, hke the sihca bound extrudates described above, would then undergo the final step described below to convert the sihca binder to a second zeohte. The final step in the three step catalyst preparation process is the conversion of the sihca present in the sihca-bound zeohte to a second zeohte which binds the first zeohte crystals together. To prepare the second zeohte, the sihca-bound aggregate is first aged in an appropriate aqueous solution at elevated temperature. Next, the contents of the solution and the temperature at which the aggregate is aged are selected to convert the amorphous sihca binder into the desired second zeohte. The newly-formed second zeohte is produced as crystals. The crystals may grow on and/or adhere to the first zeohte crystals, and may also be produced in the form of new intergrown crystals, which are generally much smaller than the first crystals, e.g., of sub-micron size. These newly formed crystals may grow together and interconnect. The nature of the zeohte formed in the second synthesis conversion of the sihca to zeohte may vary as a function of the composition of the secondary synthesis solution and synthesis aging conditions. The secondary synthesis solution is preferably an aqueous ionic solution containing a source of hydroxy ions sufficient to covert the sihca to the desired zeohte. It is important, however, that the aging solution be of a composition which will not cause the sihca present in the bound zeohte extrudate to dissolve out of the extrudate. The first zeohte of the catalyst used in the process of the present invention is preferably at least partially in the hydrogen form. Preferably, hydrogen ions are incorporated into the zeohte by exchange of the alkali metal with intermediate ammonium, followed by calcination of the ammonium form to provide the acidic hydrogen form. The zeohte bound zeohte catalyst used in the process of the present invention can be further treated prior to use. For example, metal/acid activity of the catalyst can be modified by sulfiding. The sulfiding modification can be carried out by presulfiding the catalyst or by adding a sulfur-containing compound to the hydrocarbon feed. The aromatic hydrocarbon feed mixture employed in the process of the present invention will comprise isomerizable monocyclic alkylaromatic hydrocarbons that preferably contain from two to three alkyl group substituents on the ring, isomerizable bicychc alkylaromatic hydrocarbons that preferably contain from two to four alkyl group substituents on the rings. These hydrocarbons include: (A) monocychc alkylaromatic hydrocarbons represented by the formula: wherein: R 1 is a alkyl group having 1 to about 4 carbon atoms; and, X is integer of from 2 to 3 and equals the number of alkyl groups; (B) bicychc alkylaromatic hydrocarbons represented by the formula: wherein R 2 and R 3 are independently selected from an alkyl group having 1 to about 4 carbon atoms: Y is an integer of from 0 to 2; Z is an integer of from 0 to 2; wherein the sum of Y and Z is an integer in the range of from 1 to 4 and equals total the number of alkyl groups. R 1 , R 2 , and R 3 can be straight or branch chained alkyl groups. Examples of suitable alkyl groups include methyl, ethyl, n-propyl, n- butyl, isopropyl, isobutyl, or any combination thereof. The preferred group is methyl. Suitable monocychc alkylaromatic hydrocarbons include, for example, xylenes such as para-xylene, ortho-xylene, and meta-xylene, diethylbenzenes such as 1,4-diethylbenzene, 1,2-diethylbenzene, and 1,3-diethylbenzene, trimethylbenzenes such as mesitylene (1,3,5- trim ethylbenzene), hemimelhtene (1,2,3-trimethylbenzene), and pseudocumene (1,2,4-trimethylbenzene), ethyltoluenes, triethylbenzenes such as 1,3,5-triethylbenzene, methylpropylbenzenes, ethylpropylbenzenes, dipropylbenzenes, diisopropylbenzenes, triisopropylbenzenes, etc., and mixtures thereof. Suitable bicychc alkylaromatic hydrocarbons include 1-ethylnaphthalene, 2- methylnaphthalene, dialkylnaphthalenes such as 1,2- dimethylnaphthalene, 1,2-diethylnaphthalene 2,3-dimethylnaphtha- lene, 2,3-dipropylnaphthalene 2, 6 -dimethylnaphthalene, 2,6-dibutyl- naphthalene, and the like. The alkylaromatic hydrocarbon feed mixture may consist only of alkylaromatic hydrocarbons or may be a mixture of the alkylaromatic hydrocarbons with other aromatic hydrocarbons such as ethylbenzene and toluene. The present invention finds particular apphcation with aromatic Cβ mixtures containing ethylbenzene and xylenes. Such mixtures will usually have an ethylbenzene content in the range of 5 to 50 weight %, and ortho-xylene content in the range of 0 to 35 weight %, a meta- xylene content in the approximate range of 20 to 95 weight %, and a para-xylene content in the range of 0 to 15 weight %. The feed may also contain non-aromatic hydrocarbons, such as paraffins and naphthenes. The paraffins will generally comprise 0 to 10 weight percent of the feed and usually the paraffins will comprise CS-CΘ paraffins. In carrying out the process of the invention, the aromatic feeds are contacted with the zeohte bound zeohte catalyst under conversion conditions. These conversion conditions include a temperature in the range from about 400°-1000°F, preferably from 750°-925°F, and a pressure in the range of from about 0 to about 1000 (psig), preferably 50-500 (psig), a hydrogen/hydrocarbon molar ratio between about 0.1 to about 10, preferably 0.25 to 5, and a WHSV of between about 0.2 to about 100 and preferably between about 1 to about 10. Usually the xylene isomerization reaction is carried out in a fixed bed reactor containing the zeohte bound zeohte catalyst. In another embodiment, the xylene isomerization process is carried out in sequential beds using two catalysts. In this embodiment, each catalyst can be in a separate bed or one of the catalysts can form one part of a bed while the second catalyst will form the remaining part of the bed. In this embodiment, the zeohte bound zeohte catalyst will preferably comprise first crystals of an acidic first zeohte bound by second crystals of a second zeohte which has less acidity than the first zeohte and, more preferably, the second zeohte will be substantially nonacidic. The zeohte bound zeohte catalyst will be used primarily to convert ethylbenzene and preferably will be upstream with respect to the second catalyst. The second catalyst will be used to primarily isomerize xylene components in the Cβ aromatic feed. The second catalyst can be any catalyst which is suitable for use in xylene isomerization. Examples of catalysts suitable for xylene isomerization include zeohte bound zeohte catalysts, zeohte catalysts bound by amorphous material (sihca, alumina, etc.), and acidic non-zeohtic catalysts. In this embodiment, the zeohte bound zeohte catalyst used to convert ethylbenzenes present in the feed will preferably comprise from about 10 percent to about 90 percent of the bed volume. When a zeohte bound zeohte catalyst is used as the second catalyst, in a preferred embodiment, the first zeohte of the catalyst will have less acidity than the second zeohte and the average particle size of the first crystals will be less than the average particle size of the first crystals of the zeohte bound zeohte first catalyst. In this embodiment, the average particle size of the first crystals of the second catalyst will be from about 0.5 to about 6.0 microns and more preferably from about 0.5 to about 2.0 microns. By carrying out the process of the present invention, a resulting product is produced which contains p-xylene in an amount at least approaching thermodynamic equilibrium while ethylbenzene present in the feedstream is substantially converted, i.e., at least 30% conversion of ethylbenzene is converted. These conversions are carried out with low aromatics ring loss. The following examples illustrate the invention: EXAMPLE 1 Preparation of zeohte bound MFI type galhum silicate catalyst. I. Catalyst A - Platinum loaded during synthesis. MFI structure galhum silicate crystals were prepared as follows: The ingredients of Solution A were dissolved by boiling until a clear solution was obtained. Solution A was then cooled to ambient temperature and water loss from boiling was corrected. Solution B was poured into a 2 hter glass beaker. Solution C was poured into the contents of the beaker and mixed. Solution D was then poured into the contents of the beaker and the beaker content was mixed. The contents of the beaker were poured into a 2 hter stainless steel autoclave. Rinse Water was used to rinse the beaker and added to the autoclave. Solution A were added to the autoclave. The contents of the autoclave were mixed about 20 minutes. A smooth pourable gel was obtained. The gel had the following composition expressed in moles of pure oxide: 0.45 Na 2 0/0.90 TPA Br/0.125 Ga 2 0 3 /10Si0 2 /147 H 2 0 Next, the gel was seeded by adding 1.0 wt ppm of colloidal silicalite seeds. The autoclave was placed in an oven and heated to 150°C in 2 hours and maintained at 150°C at this temperature for 48 hours. The product was removed from the autoclave and divided into 3 portions. Each portion was washed 7 times with about 600 grams of water. The product was dried over night at 120°C. The amount of product recovered was 333.70 grams. The product was calcined in air at 475°C for 48 hours. The characteristics of the calcined product were the following: XRD: Pure MFI SEM: 4 micron size spherical crystals Elemental: Siθ2/Ga 2 0 3 = 80 A portion of the calcined product was formed into sihca bound 2 mm extrudates as follows: Components Used Quantity for Preparation (Grams) Sihca Sol (Nyacol 2034 Dl) 128.59 Sihca gel (aerosil 300) 12.26 H 2 PtClr, - 6H2θ 2.47 Water 35.01 Rinse Water 3.00 Galhum silicate MFI Crystals 130.00 Extrusion Aid 0.87 (hydroxypropyl methyl cellulose) The components were mixed in a food mixer in the order shown. After adding the extrusion aid and mixing for about 7 minutes, a thick and smooth paste was obtained. The paste was extruded into 2 mm extrudates and dried at ambient temperature for 3 hours. The extrudates were broken into smaller 5 mm pieces and dried in an oven at 120°C for 16 hours. The dried extrudates were calcined at 490°C for 8 hours in air. Composition of calcined sihca bound extrudate: Silica binder: 30.1 wt.% MFI: 69.4 wt.% Platinum 0.5 wt.% The sihca bound extrudates were converted into zeohte bound zeohte as follows: Solutions A and B were poured into a 1 hter autoclave and mixed. Finally, 70.0 grams of the sihca bound extrudates were added to the autoclave. The molar composition of the synthesis mixture was: 0.48Na 2 O/1.00TPABr/10SiO2/149H 2 O The autoclave was placed into an oven. The oven was heated from room temperature to 150°C in 2 hours and maintained at this temperature for 80 hours. The resulting product was washed at 60°C 4 times with 1700 ml of water. The conductivity of the last wash water was 49 micros Siemans/cm. The extrudates were dried at 120°C and calcined in air at 490°C for 16 hours. The product was analyzed by XRD and SEM with the following results: XRD: Excellent crystalhnity SEM: 4 micron size crystals coated with smaller size crystals. No visible amorphous sihca. Elemental: Core crystals:Siθ2/Ga2θs = 80 Binder crystals = silicalite Core crystals = 70 wt.% Platinum = .5 wt.% Platinum distribution and platinum particle size were determined by quahtatively examining a sample of the product by transmission electron microscopy (TEM). The platinum was distributed well. The major proportion of the platinum had a particle size of 5-10 nm. II. Catalyst B - Platinum loaded by pore filling. A portion of the calcined MFI structure type galhum silicate used to prepare Catalyst A was formed into sihca bound 2 mm extrudates as follows: Components Used Quality for Preparation (Grams) Gallium-silicate MFI crystals 130.05 Water 37.70 SlO 2 gel (aerosil 300) 45.26 Sihca Sol (NALCOAG 1034A) 128.57 Extrusion aid 0.89 (hydroxypropyl methyl cellulose) The above components were mixed in a food mixer in the order shown. After adding the extrusion aid and mixing for about 14 minutes, a thick and smooth paste was obtained. The paste was extruded into 2 mm extrudates. The extrudates were dried at 150°C for 7 hours and then calcined in air at 510°C for 8 hours. Composition of calcined sihca-bound extrudates: MFI: 70.0 wt.% S1O2 binder: 30.0 wt.% The sihca bound extrudates were converted into zeohte bound zeohte as follows: Solution A and B were poured into a 300 ml stainless steel autoclave and were mixed. Finally, 125.00 grams of the sihca-bound MFI extrudates were added to the autoclave. The molar composition of the synthesis mixture was: 0.48Na 2 0/0.99 TPA Br/Si0 2 /148H 2 0 In this mixture, the sihca is present in the binder is the extrudate. The autoclave was placed into an oven at room temperature, heated to 150°C within 2 hours, and maintained at 150°C for 72 hours. The resulting product was washed at 60°C with 7 portions of 2000 ml of water. The conductivity of the last wash water was 25 μS/cm. The product was dried at 150°C and calcined in air at 500°C for 16 hours. The resulting product was characterized by x-ray diffraction (XRD) and scanning electron microscopy (SEM) with the following results: XRD: Excellent crystalhnity SEM: 4 micron MFI crystals coated with smaller size crystals. No visible amorphous sihca. Elemental: Core crystals:Siθ2/Ga 2 θ3 = 80 Binder crystals = silicalite Core crystals = 70 wt.% Binder crystals = 30 wt.% An amount of 0.31 wt.% of platinum (based on the weight of product) was loaded into the catalyst. The process was carried out by first exchanging the catalyst at 65°C with a 1 normal NH4CI solution. The exchanged catalyst was washed with water, dried, and then calcined at 530°C for 8 hours. The loading of the platinum was done by the pore-filling method with an appropriate amount of Pt (NH 3 )4C1 2 dissolved in water. After loading, the catalyst was dried and calcined at 480°C for 8 hours. Platinum distribution and platinum particle size were determined by quahtatively examining a sample of the product by transmission electron microscopy (TEM). The platinum particle size was predominantly 10-30 nm and platinum was not as well distributed as Catalyst A. Example 2 A series of isomerization reactions were conducted using Catalyst A by passing an artificial feed through a fixed bed reactor. Catalyst A was pretreated in H2 for two (2) hours at 850°F and 250 psig. After the temperature had been lowered to 700°F, the catalyst was presulfided to breakthrough with about 500 ppm H2S in H2 at 250 psig. The tests were run at varying conditions. The conditions and results are shown in Table I below: * Negative values beheved due to minor gas chromatography variations. The percent, % EB reacted was determined by the formula: % EB Conv = lOOx [EB in-EB out] divided by EB in; Aromatics ring loss % was determined by the formula: 100 x (moles of aromatics in feed - moles of aromatics in product)/(moles of aromatics in feed). Loss of xylenes was determined by the formula: 100 x (moles of xylenes in feed - moles of xylenes in product)/(moles of xylenes in feed) and PX approach to equilibrium was determined by the formula: (Product PX/Xs-Feed PX/Xs)/(Equihbrium PX/Xs-FeedPX/Xs) X 100. Example 3 A series of isomerization reactions were conducted using Catalyst B by passing an artificial feed through a fixed bed reactor. Catalyst B was pretreated in H 2 and presulfided using the same procedure described in Example II. The tests were run at varying conditions. The conditions and results are shown in Table II below: The data in the Tables shows that both Catalysts A and B exhibit high EB removal activity and xylene selectivity with low aromatics ring loss and xylene loss. Both catalysts maintained high activity throughout the tests. Catalyst A was able to convert over 85% of the EB and produce a product containing greater than equilibrium amounts of para-xylene with very low aromatics ring loss. High EB activity and PX selectivity, low aromatics ring loss, and activity maintenance are several of the major criteria for selecting a xylene isomerization catalyst for a commercial operation.

Claims: CLAIMS 1. A process for isomerizing a feed containing isomerizable monocychc alkylaromatic hydrocarbons, isomerizable bicychc alkylaromatic hydrocarbons, or mixtures thereof comprising: contacting the feed under isomerization conversion conditions with a zeohte bound zeohte catalyst which does not contain significant amounts of non-zeohtic binder and comprises: (a) first crystals of a first zeohte of intermediate pore size; and (b) a binder comprising second crystals of a second zeohte. 2. The process recited in Claim 1 wherein the second crystals are intergrown and form at least a partial coating on the first crystals. 3. The process recited in Claim 1 or 2 wherein the first crystals have an average particle size of at least 0.1 micron and the second crystals have an average particle size less than that of the first crystals. 4. The process recited in Claim 3 wherein the first crystals have an average particle size of from 1 to 6 microns and the second crystals have an average particle size of from 0.1 to 0.5 microns. The process recited in any preceding Claim wherein the structure types of the first zeohte and the second zeohte are independently MFI, MEL, MTW, MTT, FER, EUO or TON. The process recited in Claim 5 wherein the first zeohte has an MFI structure. 7. The process recited in Claim 5 or 6 wherein the second zeohte has an MFI or MEL structure. 8. The process recited in any preceding Claim wherein the first zeohte is at least partially in the hydrogen form. 9. The process recited in any preceding Claim wherein the first zeohte is an aluminosilicate zeohte or a galhum silicate zeohte. 10. The process recited in Claim 9 wherein the first zeohte has a silica to alumina mole ratio of from 70:1 to 700:1 or a sihca to galha mole ratio from 24:1 to 500:1. 11. The process recited in any preceding Claim wherein the first zeohte contains catalytically active sites. 12. The process recited in any preceding Claim wherein the second zeohte has less acidity than the first zeohte, and is preferably substantially nonacidic. 13. The process recited in any preceding Claim wherein the second zeohte is an aluminosilicate zeohte or a galhum silicate zeohte. 14. The process recited in Claim 13 wherein the second zeohte has a sihca to alumina mole ratio greater than 200: 1 or a sihca to galha mole ratio greater than 100:1, preferably greater than 200:1. 15. The process recited in Claim 13 or 14 wherein the second zeohte is silicalite or silicalite 2. 16. The process recited in any preceding Claim wherein the zeohte bound zeohte catalyst further comprises at least one catalytic hydrogenation metal. 17. The process recited in Claim 16 wherein the catalytic hydrogenation metal is a Group VIII metal. 18. The process recited in Claim 16 or 17 wherein the zeohte bound zeohte catalyst is prep arable by a method that comprises providing a sihca-bound aggregate comprising the first crystals of the first zeohte and at least a portion of the catalytic hydrogenation metal and converting same to the second zeohte. 19. The process recited in any preceding Claim wherein the zeohte bound zeohte catalyst is preparable by a method that comprises aging at elevated temperatures a sihca-bound aggregate containing the first crystals of the first zeohte in an aqueous ionic solution containing hydroxy ions. 20. The process recited in any preceding Claim wherein the zeohte bound zeohte catalyst contains less than 5% by weight of non- zeohtic binder based on the combined weight of first and second zeohte. 21. The process recited in any preceding Claim wherein the alkylaromatic hydrocarbons are selected from the group consisting of: (a) monocychc alkylaromatic hydrocarbons represented by the formula: wherein: each R 1 is independently an alkyl group having 1 to 4 carbon atoms; and x is 2 or 3; and (b) bicychc alkylaromatic hydrocarbons represented by the formula: wherein R 2 and R 3 is each independently an alkyl group having 1 to 4 carbon atoms; and y and z is each independently 0, 1 or 2, provided that the sum of y and z is 2, 3, or 4. 22. The process recited in Claim 21 wherein the feed comprises monocychc alkylaromatic hydrocarbons of formula I wherein R 1 is methyl or ethyl and x is 2. 23. The process recited in Claim 21 or 22 wherein the feed comprises bicychc alkylaromatic hydrocarbons of formula II wherein each of R 2 and R 3 is methyl, y is 1 and z is 1. 24. The process recited in any preceding Claim wherein the feed comprises an aromatic Cs mixture of ethylbenzene and xylenes in which the paraxylene concentration is at less than thermodynamic equilibrium. 25. The process recited in Claim 24 wherein in addition to being contacted with the zeohte bound zeohte catalyst, the feed is also contacted under conversion conditions with a second catalyst that is a xylene isomerization catalyst; and wherein the zeohte bound zeohte catalyst comprises an acidic first zeohte, a second zeohte having less acidity than the first zeohte, and an effective amount of a catalytic hydrogenation metal. 26. The process recited in Claim 25 wherein the feed is contacted with the zeohte bound zeohte catalyst before it is contacted with the second catalyst. 27. The process recited in Claim 25 or 26 wherein the zeohte bound zeohte catalyst and the second catalyst are located in separate catalyst beds. 28. The process recited in Claims 25 or 26 wherein the zeohte bound zeohte catalyst and the second catalyst are located in the same catalyst bed. 29. The process recited in any of Claims 25 to 28 wherein the zeohte bound zeohte catalyst and the second catalyst are located in the same reactor. 30. The process recited in any of Claims 24 to 29 wherein at least 30 percent of the ethylbenzene is converted. 31. The process recited in Claim 30 wherein at least 50% of the ethylbenzene is converted. 32. The process recited in any of Claims 25 to 31 wherein the second catalyst is a zeohte bound zeohte catalyst which does not contain significant amounts of non-zeohtic binder and comprises: (a) first crystals of an intermediate pore size first zeohte; and (b) binder comprising second crystals of a second zeohte having higher acidity than the first zeohte. 33. The process recited in Claim 32 wherein the second catalyst comprises an MFI structure type first zeohte; and an MFI or MEL structure type second zeohte. 34. The process recited in Claim 32 or 33 wherein the second catalyst further comprises an effective amount of at least one catalytic hydrogenation metal. 35. The process recited in any of Claims 25 to 31 wherein the second catalyst is an acidic non-zeohtic catalyst. 36. The process recited in any of Claims 25 to 31 wherein the second catalyst is a zeohte catalyst bound by amorphous material. 37. The process recited in any preceding Claim wherein the isomerization conditions comprise a temperature of from 204° to 538°C (400° to 1000°F) and/or a pressure of from 0.34 to 6.9 MPag (50 to 1000 psig) and/or a weight hourly space velocity of from 0.5 to 100 and/or a hydrogen/hydrocarbon molar ratio of from 0.1 to 10. AMENDED CLAIMS [rece ived by the Internat ional Bureau on 7 November 1997 (07. 1 1 .97 ) ; orig inal claims 9 and 18 amended ; remain ing c laims unchanged (2 pages ) ] 5. The process recited in any preceding Claim wherein the structure types of the first zeohte and the second zeohte are independently MFI, MEL, MTW, MTT, FER, EUO or TON. 6. The process recited in Claim 5 wherein the first zeohte has an MFI structure. 7. The process recited in Claim 5 or 6 wherein the second zeohte has an MFI or MEL structure. 8. The process recited in any preceding Claim wherein the first zeohte is at least partially in the hydrogen form. 9. The process recited in any preceding Claim wherein the first zeohte and said second zeohte are each independently a composition having the following molar relationship: wherein X is aluminum, boron, titanium, and/or galhum, Y is silicon, tin, and/or germanium, and n has a value greater than 10. 10. The process recited in Claim 9 wherein the first zeohte has a sihca to alurnina mole ratio of from 70:1 to 700:1 or a sihca to galha mole ratio from 24:1 to 500:1. 11. The process recited in any preceding Claim wherein the first zeohte contains catalytically active sites. 12. The process recited in any preceding Claim wherein the second zeohte has less acidity than the first zeohte, and is preferably substantially nonacidic. 13. The process recited in any preceding Claim wherein the second zeohte is an aluminosilicate zeohte or a galhum silicate zeohte. 14. The process recited in Claim 13 wherein the second zeohte has a sihca to alumina mole ratio greater than 200:1 or a sihca to galha mole ratio greater than 100:1, preferably greater than 200:1 15. The process recited in Claim 13 or 14 wherein the second zeohte is sihcahte or silicalite 2. 16. The process recited in any preceding Claim wherein the zeohte bound zeohte catalyst further comprises at least one catalytic hydrogenation metal. 17. The process recited in Claim 16 wherein the catalytic hydrogenation metal is a Group VIII metal. 18. The process recited in Claim 16 or 17 wherein the zeohte bound zeohte catalyst is preparable by a method that comprises providing a sihca-bound aggregate comprising at least a portion of the catalytic hydrogenation metal and the first crystals of the first zeohte and converting same to the second zeohte. 19. The process recited in any preceding Claim wherein the zeohte bound zeohte catalyst is preparable by a method that comprises STATEMENT UNDER ARTICLE 19 Under the provisions of Article 19 and Rule 46 of the PCT, the Applicant requests that Claims 9 and 18 for the subject application be amended. All other claims [1-8, 10-17, and 19-37] are unchanged. The mailing date for the international search report was September 9, 1997. The amendments do not go beyond the disclosure of the application as filed. Enclosed are replacement pages 32 and 33 with amended Claims 9 and 18. Support for amended Claim 9 is found on page 10, hnes 1-14. The amendment to Claim 18 is supported by original Claim 18 as the change is merely editorial. It is respectfully requested that the amendments to Claims 9 and 18 be entered. Lf there are any questions, it is respectfully requested that the undersigned be called at telephone number (281) 834-5933.

Inventors: MOHR GARY D, VERDUIJN JOHANNES P, MOHR, GARY, D., VERDUIJN, JOHANNES, P.
Applicants: EXXON CHEMICAL PATENTS INC, EXXON CHEMICAL PATENTS INC.
Application year: 1997
Main classification: C07C5
IPC codes: B01J29, C07C5, C07C15