MXPA01000941A - A process for producing terephthalic acid and/or dimethyl terephthalate - Google Patents

Abstract

A process for producing terephthalic acid and/or dimethyl terephthalate includes contacting a toluene-containing reaction stream with a first catalyst under toluene disproportionation conditions to produce an intermediate product stream of para-xylene with negligible ortho-xylene. The first catalyst includes a crystalline molecular sieve which has an ortho-xylene diffusion rate of at least 50 minutes. The first catalyst can be modified by selectivation with a silicon compound or carbon compound. The intermediate product stream, without need for para-xylene purification, is oxidized to terephthalic acid or dimethyl terephthalate.

Description

A PROCESS TO PRODUCE TEREFLABLY ACID AND / OR DIMETHYL TEREFTALATE Description The invention relates to a process that integrates the production of p-xylene with the production of terephthalic acid and / or dimethyl terephthalate, without the need for purification Intermediate p-xylene. Para-xylene is oxidized to produce terephthalic acid or dimethyl terephthalate, which is polymerized to produce fibers and polyester films. Processes for the production of terephthalic acid (1,4-benzenedicarboxylic acid) and dimethyl terephthalate 15 are well known and are described, for example, in U.S. Patent No. 2,833,816 and in the British patent specifications. Nos. 809,730 and 727,989. Of the three isomers of xylene, meta, ortho and para, only p-xylene is suitable for acid production Terephthalic and / or dimethyl terephthalate for polyester manufacture due to the ability of p-xylene to form straight polymer chains. Straight chains are necessary to give the polyester its fiber-forming or film-forming characteristics and high tensile strength. The p-xylene must be substantially pure to avoid undesirable side reactions when p-xylene is oxidized ? "Faith? to terephthalic acid. Para-xylene has typically been produced by methylation of toluene, for example by reaction of toluene with methanol, as generally described by Chen and co-workers, J. Am. Chem. Soc. 1979, 101: 6783, and disproportionation of toluene, for example as described by Pines in The Chemistry of Catalytic Hydrocarbon Conversions, Academic Press, New York, p.72. Such methods typically produce a mixture of C8 products including para-xylene, ortho-xylene, meta-xylene and ethylbenzene Para-xylene can be recovered from mixed C8 streams, followed by xylene isomerization of the remaining stream, as described, for example, in U.S. Patent Nos. 3,856,871 and Re 31,782. Terephthalic acid, an undesirable by-product is the result of the presence of o-xylene in the xylene feed Any o-xylene present is oxidized in orthophthalic acid and subsequently dehydrated in itlic anhydride or, interfering with the production of terephthalic acid and impacting the life of the oxidation catalyst. If ethylbenzene (EB), another C8 material, is present in the p-xylene feed for production of terephthalic acid, the EB can be oxidized to benzoic acid. The meta-xylene is oxidized to isophthalic acid. These oxidation products are more easily separated from purified terephthalic acid than the oxidation products of o-xylene. The quality of the polyester is affected by even small amounts of organic or inorganic impurities that cause difficulties in the process during polymerization and affect the color, thermal and photochemical stability, and long-term durability of the polyester product. The precursor for the production of polyester is purified terephthalic acid (PTA). In addition, the PTA produced in this manner can be esterified with an alcohol, for example the reaction of PTA with methanol yields dimethyl terephthalate which can also be used in the production of polyester. As disclosed, for example in British Patent Specification No. 727,989, if the initial p-xylene feed does not consist of pure para-xylene but also contains other xylene isomers and / or non-aromatic compounds, a purification is necessary. of food. Purification of the p-xylene feed is generally carried out using physical processes such as fractional distillation or crystallization, for example as described in US Pat. Nos. 3,177,255 and 3,467,724, absorption, for example as described in US Pat. U.S. Patent No. 2,985,589, or chemical processes, for example when contacted with formaldehyde in acid solution which removes m-xylene as a resin. A widely used method is the Parex® UOP process, described by J.J. Jeanneret, "UOP Parex Process", in Handbook of Petroleum Refining Processes, R.A. Meyers, editor, McGraw-Hill, New York, 1997. Other methods for separating p-xylene from an aromatic C8 mixture are described by U.S. Patent No. 4,705,909. In conventional xylene manufacturing, considerable costs are incurred for the production of p-xylene for PTA production. It is an object of the invention to improve the manufacture of terephthalic acid and / or dimethyl terephthalate by minimizing undesirable oxidation products. It is another object of the invention to eliminate the need for a purification process for p-xylene for use in the production of terephthalic acid and / or dimethyl terephthalate. According to the invention, a process for producing terephthalic acid and / or dimethyl terephthalate is provided, comprising the steps of: (a) contacting a reaction stream comprising toluene, under conditions of disproportionation of toluene, with a first catalyst, wherein the first catalyst comprises a crystalline molecular sieve having an ortho-xylene diffusion time of more than 50 minutes, said first contact producing a para-xylene product, and (b) contacting the product of ( a) with a second catalyst, under oxidation conditions, to produce terephthalic acid and / or dimethyl terephthalate. In the process of the invention, the first contacting step produces a stream of intermediate product containing p-xylene essentially without ortho-xylene or ortho-xylene ^^^^^ ^^^^^^ m? s. negligible, and essentially without ethylbenzene (EB). The intermediate product stream containing p-xylene is oxidized to terephthalic acid in a second contact with a second catalyst. There is no need for purification of the intermediate product stream to remove ortho-xylene or EB. The second catalyst is any catalyst that catalyzes the oxidation of p-xylene to terephthalic acid, for example a heavy metal catalyst such as cobalt and / or manganese, and which optionally may include a catalyst for esterification to dimethyl terephthalate. Advantageously, an expensive separation step of xylene is eliminated and the product stream of the first contact can be integrated directly with the oxidation process to terephthalic acid or pure dimethyl terephthalate. Catalyst System The catalysts useful for the production of p-xylene in this invention can comprise crystalline molecular sieves having the specified ortho-xylene diffusion rate, for example zeolites, ALP04's, SAPO's or their combinations. Examples of crystalline molecular sieves useful in this invention include zeolites of intermediate pore size ZSM-5 (U.S. Patent Nos. 3,702,886 and Re 29,948); ZSM-11 (U.S. Patent No. 3,709,979); ZSM-12 (U.S. Patent No. 3,832,449); ZSM-22 (U.S. Patent No. 4,556,477); ZSM-23 (U.S. Patent No. 4,076,842); ZSM-35 (patent of the States 3¿__ -, 3 **, íáü úÜd? TÉtÜta ja? M, g ^? ^ £ ^ ^ ^ United No. 4,016,245); ZSfí-48 (U.S. Patent No. 4,397,827); ZSM-57 (U.S. Patent No. 4,046,685); and ZSM-58 (U.S. Patent No. 4,417,780). Also useful are silicoaluminophosphates (SAPO's), particularly SAPO-5 and SAPO-11 (U.S. Patent No. 4,440,871) and aluminophosphates (ALP04's), particularly ALP04-5 and ALP04-11 (U.S. Pat. No. 4,310,440). Intermediate pore size zeolites generally have a constriction index in the approximate range of 1 to 12 (eg, zeolites having less than 7 7 Pore size angstroms, such as 5 to 77 Angles). Preferred intermediate pore size zeolites include ZSM-5, ZSM-11, ZSM-12, ZSM-35 and MCM-22. The most preferred is ZSM-5. When ZSM-5 is used as the toluene conversion catalyst of this invention, it may comprise a medium or large crystal size. If another intermediate pore size zeolite is used as a toluene conversion catalyst, the crystal size may need to be adjusted from the earlier data for better performance. The crystalline molecular sieve can be used in bound or unbound form, but preferably is bound with silica. In addition, the catalyst can be selected with silicon by impregnation methods or multiple impregnation ex si tu or selective methods in itself; or selective with coke; or subjected to a combination of these, to result in the desired ortho-xylene diffusion rate. Methods for preparing silica-bound ZSM-5 are described in U.S. Patent Nos. 4,582,815; 5,053,374; and 5,182,242. A particular process for ligating ZSM-5 with a silica binder involves an extrusion process. The catalyst can be zeolite bound with zeolite, as described in U.S. Patent No. 5,665,325. There are several methods to increase the selectivity of the zeolite catalysts. One such method is to modify the catalyst by treatment with a "selective agent". For example, U.S. Patent Nos. 5,173,461; 4,950,835; 4,927,979; 4,465,886; 4,477,583; 4,379,761; 4,145,315; 4,127,616; 4,100,215; 4,090,981; 4,060,568; and 3,698,157, disclose specific methods for contacting a catalyst with a silicon-containing selective agent ("silicon compound"). Also, U.S. Patent Nos. 5,367,099; 5,382,737; 5,365,004; 4,503,800; 5,406,015; and 5,476,823 disclose methods for silicon selectiveization of catalysts and the use of those catalysts in disproportionation of toluene and ethylbenzene. According to a selective method (first method), the multiple impregnation method, the catalyst is selected by one or more treatments with a liquid organosilicon compound in a liquid carrier, each treatment being followed by calcination of the treated material in an atmosphere containing oxygen, for example air. More particularly, for example, with reference to the above-mentioned steps (i) - (iii), this first selective method may involve the additional steps of: (iv) contacting the calcined extruded step (iii) with a liquid comprising a liquid carrier and at least one organo-silicon selective agent having at least two silicon atoms per molecule under conditions sufficient to incorporate said organo-silicon selective agent into the extrudate; (v) calcining the extrudate from step (iv) under conditions sufficient to decompose said organo-silicon selective agent and to remove any residue of said liquid carrier from said extrudate; and, optionally, (vi) repeating the selection steps (iv) and (v) at least once. Another method (second method) for selecting the catalyst, selectivation by pruning, involves passing a feed stream comprising hydrogen and an aromatic compound (eg, toluene) or a paraffin (eg, hexane or decane) and a compound of organo-silicon on HZSM-5, for example ZSM-5 bound with silica, under conditions sufficient to deposit a residue of organo-silicon compound on ZSM-5. -..- > & The first aforementioned method for selecting zeolite, where zeolite, for example HZSM-5, is treated by multiple impregnation treatments, is referred to herein as the method of multiple impregnation. The aforementioned second method for selecting zeolite, where zeolite, for example HZSM-5, is treated under selective pruning conditions, is referred to herein as a pruning selection method. Another method (third method) for selecting the zeolite, described herein, which includes 0 decomposing an organic compound on and in the zeolite, is referred to herein as the coke-selective method. The present catalyst can be selected by any of the above selective methods or by more than one selective method used in combination. According to the multiple impregnation method, the zeolite, for example HZSM-5, is treated at least once, for example at least twice, for example three times or more, for example four to six times, with a liquid medium comprising a liquid carrier and at least one liquid organo-0 silicon compound. The organosilicon compound can be present in the form of a solute dissolved in the liquid carrier or in the form of droplets emulsified in the liquid carrier. For the purposes of the present disclosure, it will be understood that a normally solid organosilicon compound will be considered as a liquid (ie, in the liquid state) when v-faü-C dissolves or emulsifies in a liquid medium. The liquid carrier can be water, an organic liquid or a combination of water and an organic liquid. Particularly when the liquid medium comprises an emulsion of the organosilicon compound in water, the liquid medium may also comprise an emulsifying agent, such as a surfactant. As mentioned above, the zeolite can be bound with silica before the selectivation, after the selectivation, or between successive selective coatings. The organosilicon compound which is used to selectively select the zeolite can be a silicone, siloxane or a silane. Silicones are defined herein as those compounds where silicon atoms are linked together via oxygen atoms. The silanes are defined herein as those compounds where the silicon atoms are directly bonded together. The pre-selectivation agent of the organosilicon compound can be, for example, a silicone, a siloxane, a silane, or mixtures thereof. These organosilicon compounds can have at least two silicon atoms per molecule. These organosilicon compounds can be solid in pure form, with the proviso that they are soluble or otherwise convertible to the liquid form upon combination with the liquid carrier medium. The molecular weight of the silicone, siloxane or silane compound used as the pre-selection agent may be between 80 and 20,000, and preferably within the approximate range of 150 to 10,000. The kinetic diameter of the selective agent is preferably larger than the pore diameter of zeolite, in order to prevent the entry of the selective agent into the zeolite pores and any concomitant reduction in the internal activity of the zeolite. The silicone compound that can be used to selectively select the present zeolite can be considered to be constructed of a siloxy backbone structure topped with end groups. This siloxy backbone structure can be a chain structure represented by the formula: where p is from 1 to 100, for example 1 to 25, for example 1 to 9. This siloxy backbone structure can also be a cyclic structure represented by the formula: where q is from 2 to 10. Branched chain structures and chain / cyclic structures are also possible for the siloxy backbone of the silicone selective agent.

The hydrocarbyl groups that cap off the available bonds of the siloxy backbone can have from 1 to 10 carbon atoms. Examples of such hydrocarbyl groups are methyl and phenyl. Examples of silicone compounds having a chain siloxy backbone structure include those of the formula: where Rj. and R6 are independently hydrogen, methyl or phenyl; R2, R3, R4 and R5 are independently methyl or phenyl; and m is from 1 to 100, for example from 1 to 25, for example from 1 to 10, for example from 1 to 4. Preferably, not more than one phenyl group is bonded to each silicon atom. Particular examples of such a silicone compound having a siloxy chain backbone structure include hexamethyldisiloxane, decamethyltetra-siloxane, and diphenyltetramethyldisiloxane. Particular examples of silicone compounds having a cyclic siloxy backbone structure include octamethylcyclotetrasiloxane and decamethylcyclopentasiloxane. Particular examples of silicone compounds having a branched siloxy backbone structure are tris- (trimethylsiloxy) -phenylsilane and tris- (trimethylsiloxy) -silane.

-"TV The silane compounds, useful as selective agents according to the present method, can have structures corresponding to the aforementioned silicone compounds, where the silicon atoms are directly linked to one another instead of via oxygen atoms. Examples of silanes having a chain backbone structure include those of the formula: where R and R6 are independently hydrogen, methyl or phenyl; R2, R3, R4 and R5 are independently methyl or phenyl; and m is from 1 to 100, for example from 1 to 25, for example from 1 to 10, for example from 1 to 4. An example of such a silane compound is hexamethyldisilane. Representative silicone compounds for pre-selection include dimethyl silicone, diethyl silicone, phenylmethyl silicone, methylhydrogen silicone, ethylhydrogen silicone, phenylhydrogen silicone, methylethyl silicone, phenylethylsilicon, diphenyl silicone, methyltrifluoropropyl silicone, ethyltrifluoropropyl silicone, polydimethyl silicon, tetrachlorophenylmethyl silicon, tetrachlorophenylethyl silicone, tetrachlorophenylhydrogen silicone, tetrachlorophenylphenyl silicon, methylvinyl silicone, and ethylvinyl silicone. The silicone, siloxane or silane compound of Selectivation does not need to be linear, but can be cyclic, for example hexamethyl cyclotrisiloxane, octamethyl cyclotetrasyl-xano, hexaphenyl cyclotrisiloxane and octaphenyl cyclotetrasiloxane. Mixtures of these compounds can also be used as pre-selection agents, as can silicones with other functional groups. Preferred organosilicon pre-selectivation agents, particularly when the pre-selection agent is dissolved in an organic carrier or emulsified in an aqueous carrier, include dimethylphenyl methyl polysiloxane (e.g., Dow-550) and phenylmethyl polysiloxane (e.g. , Dow-710). Dow-550 and Dow-710 are available from The Dow Chemical Co., of Midland, Michigan, United States. When the organosilicon pre-selectivation agent is present in the form of a water-soluble compound in an aqueous solution, the organosilicon can be substituted with one or more functional groups or hydrophilic fractions, which serve to promote solubility overall in water of the organo-silicon compound. These hydrophilic functional groups may include one or more organoamine groups, such as -N (CH3) 3, -N (C2H5) 3, and -N (C3H7) 3 groups. A preferred water-soluble organo-silicon pre-selective agent is an n-propylamine silane, available as Hydrosil 2627 from Huís America. Particular water-soluble organosilicon compounds, which can be used for multiple impregnations of the present catalyst, are referred to as amino silane polymers in U.S. Patent No. 5,371,312. As previously mentioned herein, aqueous emulsions of organosilicon compounds comprising surfactants can be used for the impregnation of the present catalyst. Stable aqueous emulsions of organosilicon compounds (e.g., silicone oil) are described in U.S. Patent No. 5,726,114. The first catalyst can be selected by more than one selective method. In particular, before use in the present process, the crystalline molecular sieve can be contacted with an organosilicon compound, followed by calcination in an oxygen-containing atmosphere. Such pretreatment of the molecular sieve can also be referred to herein as a pre-selective treatment. According to an example of a pre-selective method, the catalyst is pre-selected by a single treatment or multiple treatments with a liquid organosilicon compound in a liquid carrier, each treatment being followed by calcination of the treated material in a atmosphere containing oxygen, for example air. When the catalyst is pre-selected by a single or multiple impregnation technique, the catalyst is calcined after each impregnation to remove the carrier and to convert the liquid organosilicon compound to a solid residue material thereof. This waste material solid is referred to in 1 present as a solid siliceous material, as long as it is believed that the material is a polymeric species having a high content of silicon atoms in its various structures. Without ^ argo, this solid siliceous residue material can also comprise carbon atoms in its structure, which are a result of the residue of the organ portion of the organosilicon compound used to impregnate the catalyst. After each impregnation, the catalyst can be calcined at a rate of 0.2 to 5 ° C / minute at a temperature higher than 200 ° C, but below the temperature at which the crystalline molecular sieve is adversely affected. This calcination temperature may be below 700 ° C, for example within the approximate range of 350 to 550 ° C. The duration of calcination in the calcination temperature can be from 1 to 24 hours, for example from 2 to 6 hours. The impregnated catalyst can be calcined in an inert or oxidizing atmosphere. An example of such an inert atmosphere is nitrogen, ie an atmosphere of N2. An example of an oxidizing atmosphere is an oxygen-containing atmosphere, such as air. The calcination may take place initially in an inert atmosphere, for example of N2, followed by calcination in an oxygen-containing atmosphere, such as air or a mixture of air and N2. The calcination must be carried out in an atmosphere substantially free of water vapor to avoid formation undesired and uncontrolled water vapor from the catalyst. The catalyst can be calcined once or more than once after each impregnation. The various calcinations that follow each impregnation need not be identical, but may vary with respect to temperature, the rate of temperature rise, the atmosphere, and the duration of calcination. The amount of siliceous residue material that is deposited on the molecular sieve or the bound molecular sieve depends on several factors, including the temperatures of the impregnation and calcination steps, the concentration of organosilicon compound in the carrier medium, the degree in which the catalyst has been dried before contact with the organosilicon compound, the atmosphere used in the calcination and the duration of the calcination. An adequate amount of silicon on the catalyst is greater than 9% by weight, for example greater than 12% by weight, excluding the silica present in the binder or on the crystalline molecular sieve itself. According to the pruning selective method described herein, the first catalyst can be contacted with a feed stream that typically comprises hydrogen and an aromatic compound (e.g., toluene) or a paraffinic compound (e.g., hexane). or dean), with the organosilicon compound under suitable pruning selective conditions. These conditions may include a temperature ranging from 100 to 600 'C, for example from 300 to 500 ° C, a gauge pressure ranging from 0 to 2,000 psi, for example from 15 to 800 psi, a molar ratio of hydrogen to hydrocarbons (for example, toluene) from 0.1 to 20, for example from 0.25 to 10, for example from 1 to 4, and a space speed hourly in weight (HSV) from 0.1 to 100 hr "1, for example from 0.1 to 10 hr" 1. The toluene may comprise 5.0 to 100% by weight, for example at least 80% by weight of the hydrocarbons in the feedstock. Other hydrocarbons, such as benzene, xylenes and trimethylbenzenes, may be present in the pruning selective feed material. The presence of a sufficient amount of hydrogen in the pruning selective feedstock is useful to prevent rapid aging of the catalyst during the selective process, and a small amount of a carbonaceous deposit can be formed on the catalyst. As a result of this carbonaceous deposit, elemental analysis of the pruned selective catalyst can reveal a carbon content considerably greater than the carbon content of the fresh catalyst prepared by the multiple impregnation method described herein. More particularly, the pruning-selective catalyst can contain at least 2% by weight, for example at least 4% by weight, of carbon by elemental analysis, while the catalyst prepared by the multiple impregnation method can contain less than 0.5. % by weight of carbon as measured by elemental analysis. These percentages by weight they are expressed in terms of the weight of the entire catalyst, including the crystalline molecular sieve, the binder, and optional components, such as hydrogenation / dehydrogenation components. The first catalyst can also be subjected to controlled coke formation. This process of controlled coke formation is also referred to herein as coke selective. This optional coke selectivation may involve contacting the catalyst with a compound Organic material capable of thermally decomposing at an elevated temperature in excess of the decomposition temperature of said compound but below the temperature at which the crystallinity of the zeolite is negatively affected. This contact temperature can be, for example, less than 650 ° C. The catalyst can be coked in a reactor or other vessel different from that used for the conversion of toluene, followed by transport of the coked catalyst to the toluene conversion reactor. The organic materials that can be used for this coke selective process encompass a wide variety of compounds, including, by way of example, hydrocarbons, such as paraffins, cyclo-paraffins, olefins, cyclo-olefins and aromatics.; oxygen-containing organic compounds, such as alcohols, aldehydes, ethers, ketones and phenols; and heterocyclics, such as furans, thiophenes, pyrroles TS ¿* ÜJ. ^^ and pyridines. A co-feed of hydrogen can be used to prevent excessive accumulation of coke. Additional details regarding coke selective techniques are provided in U.S. Patent Nos. 4,117,026 and 5,476,823. Optionally, an organo-silicon co-ordination may be included together with the feed of organic material used for coke selectivation. This organosilicon material may be selected from the aforementioned organosilicon compounds herein for use in the catalyst selectivation. Although not intended to be limited by any particular theory, it is possible that the selectivity of the present catalyst is obtained by producing changes in the diffusion properties of the zeolite which favor the desired reactions and inhibit the undesired reactions. The crystalline molecular sieve component of the catalysts suitable for use as the first catalyst may be characterized by different xylene diffusion properties or xylene sorption capacities. In particular, it has been found that the first catalyst must have an equilibrium xylene sorption capacity, which can be either for, meta, ortho or a mixture of these, frequently para-xylene, since this isomer reaches equilibrium within the shorter time, of at least 1 g per 100 g of zeolite, measured at 120"C and a xylene pressure of 4.5 ± 0.8 mm of mercury, and a time of ortho-xylene sorption for 30% of the xylene sorption capacity of more than 50, preferably more than 200, more preferably more than 1,200 (at the same conditions of temperature and pressure) in order to achieve the desired level of para-xylene selectivity to an appropriate toluene conversion. The sorption measurements can be carried out gravimetrically in a thermal balance. It has been found that zeolites that exhibit extremely high selectivity to para-xylene while minimizing ortho-xylene require an extremely long time, preferably up to and exceeding 1,200 minutes, to sorbit ortho-xylene in an amount of 30% capacity. total xylene sorption. For those materials, it may be more convenient to determine the sorption time for a lower degree of sorption, such as 5, 10 or 20% of the capacity, and then estimate the sorption time of 30% by applying the following multiplication factor F, as illustrated for 5% sorption: t? 3 = £ t0 05% Sorption Capacity Factor F, to estimate sorption time of 30%, t0 3 5 36 10 9 20 2.25 Alternatively, t03 can be calculated for other sorption times to less than 30% of the xylene capacity, using the following relationship: t0.3 = t0.x (0.3 / Ox) 2 where t03 - sorption time for 30% of the total xylene capacity, t0 x - sorption time for x% of the total xylene capacity, 0.x - quantity fractional sorption of ortho-xylene relative to the total capacity of xylene According to the invention, the crystalline molecular sieve component of the catalyst which is effective for conversion of toluene may have a value t03 (in minutes) for ortho-xylene in 50 excess, for example more than 200, for example more than 1,200 minutes. Disproportionation of Toluene The first catalyst may be contacted with a toluene feedstock under conditions to effect disproportionation. Effective conditions for achieving high selectivity to p-xylene and acceptable levels of toluene conversion include a reactor inlet temperature of 200 to 550 'C (392 to 1,022'F), preferably 312 to 532 ° C (600 to 1,000). F); an atmospheric pressure at 5,000 psi, preferably a gauge pressure of 20 to 1,000 psi; a WHSV from 0.1 to 20, preferably from 0.5 to 10; and a molar ratio of H2 / hydrocarbon from 0 to 20, preferably from 0 to 10. This process can be conducted in continuous flow, batch or fluid bed operation. The catalyst can be further modified to • «^ t. ^ reducing undesirable amounts of side products, particularly ethylbenzene, by incorporating a hydrogenation / dehydrogenation function within the catalyst, such as by the addition of a metal compound such as platinum or other metals of groups 4 to 13 of the Periodic Table, as platinum, palladium, silver, gold, copper, zinc, nickel, gallium, cobalt, molybdenum, rhodium, ruthenium, manganese, rhenium, tungsten, chromium, iridium, osmium, iron, cadmium and their mixtures (combinations). The metal can be added by exchange of cations or by impregnation by known methods in amounts of 0.01 to 10%, typically 0.05 to 10%. The disproportionation of toluene on the selective catalyst showed selectivity towards a p-and m-xylene product of high purity. It is possible to achieve a xylene product virtually no o-xylene at toluene conversion levels as high as 27%. A C8 stream can be produced without o-xylene, physically mixed to reduce o-xylene, or a current can be taken through an o-xylene splitter to prepare a C8 feed for a purified terephthalic acid unit. The xylene product preferably includes essentially no o-xylene or negligible o-xylene. By negligible it is meant less than 0.2%, preferably less than 0.1%, more preferably less than 0.05%. It has been found that such a xylene product can be fed directly to step 5 of production of terephthalic acid without intermediate purification.

^ Mfe? Production of Terephthalic Acid Several processes are used commercially in the production of terephthalic acid. One is the Amoco process described, for example, in U.S. Patent No. 2,833,816. This process involves oxidation in liquid phase air of p-xylene using multivalent (heavy) metals, particularly cobalt and manganese, as a catalyst in an acetic acid solvent and with bromine as a renewable source of free radicals. The crystals of the terephthalic acid product are recovered, for example by centrifugation, and purified by dissolving the crystals in water in contact with a hydrogenation catalyst, for example noble metal on a carbon support, and again recovering the crystals. Dimethyl terephthalate can be produced by liquid phase esterification of the terephthalic acid using metal catalysts such as zinc, molybdenum, antimony and tin with a large excess of methanol. In another process, four steps are used, alternating oxidation and esterification to produce dimethyl terephthalate, as described, for example, in British patent specifications Nos. 727,989 and 809,730. First, p-xylene is oxidized with a gas containing molecular oxygen (air) in a liquid phase in the presence of a heavy metal catalyst such as cobalt, manganese or a mixture of both, to produce p-toluic acid (PTA), which is esterified with methanol to produce methyl p-toluate (MPT). A second oxidation of MPT with the same catalyst and molecular oxygen yields, in liquid phase, monomethyl terephthalate which is esterified to the diester dimethyl terephthalate. Both terephthalic acid and dimethyl terephthalate are used in the production of polyethylene terephthalate (PET) or other polyesters by reaction with a glycol, for example ethylene glycol or tetramethylene glycol. The invention is illustrated by the following non-limiting example. EXAMPLE A catalyst was made by multiple silica excision of a ZSM-5 containing extruded (65% zeolite / 35% silica). Selectivation of the catalyst was carried out using the pore-filling technique by contacting the catalyst four times with fluid 7.8% organosilicon (Dow-550, The Dow Chemical Co., of Midland, Michigan, United States) dissolved in decane and twice more with 2% fluid organo-silicon, followed by hybrid calcination in nitrogen / air. Catalyst evaluations were carried out by charging 2 g of extrudate, mixed with sand as packing material, into a reactor tube with 3/8"external diameter, the catalyst was then heated to a reaction temperature under nitrogen, when The mixed hydrogen / toluene feed is introduced, and sample analyzes were acquired via on-line gas chromatography (GC). * m & aAa *? * ~ *? ± .. * < *. ^^ .. «.» ... M a » operation and yields are shown later in Table 1. Table 1 * Where a precise measurement of the concentration in the present gas chromatography technique can not be obtained, a value of less than 100 ppm is estimated. An optimum level of toluene conversion is selected and a piece of product is selected in which the C8 moiety is essentially all p-xylene. This current can be used directly (without requiring a crystallization or sorption process to purify the p-xylene) in a unit of purified terephthalic acid (PTA) for oxidation and subsequent esterification.

Claims (8)

1. CLAIMS 1. A process for producing terephthalic acid or dimethyl terephthalate, comprising: (a) contacting a reaction stream comprising toluene, under conditions of disproportionation of toluene, with a first catalyst, wherein the first catalyst comprises a sieve crystalline molecular having an ortho-xylene diffusion time of more than 50 minutes, said first contact producing a para-xylene product, and (b) contacting the product of (a) with a second catalyst, under conditions of oxidation, to produce terephthalic acid and / or dimethyl terephthalate.
2. 2. The process of claim 1, wherein the first catalyst comprises an intermediate pore zeolite, a SAPO, an ALPO, or combinations thereof.
3. 3. The process of claim 2, wherein the intermediate pore zeolite has a constriction index ranging from 1 to 12.
4. The process of claim 1, wherein the disproportionate conditions of toluene comprise a temperature of 600 to 1,000. 'F, a pressure of 20 to 1,000 psi, a WHSV of 0.
5. 5 to 20, and a hydrogen / hydrocarbon molar ratio of 0 to 10. The process of claim 1, wherein the first catalyst has been subjected to modification by exposure to a Composed of silicon or a carbon compound.
6. 6. The process of claim 5, wherein the modification is ex situ silicon selectivity by impregnation or multiple impregnation, silicon pruning selective in situ, carbon selective, or combinations thereof. The process of claim 1, wherein the contact step (b) comprises oxidation in the liquid phase. The process of claim 1, wherein the para-xylene product of step (a) contains negligible ortho-xylene and is fed directly to step (b) without intermediate purification.