XYLENE ISOMERIZATION PROCESS AND CATALYST THEREFOR

Abstract: Isomerization of xylene in admixture with ethylbenzene by contact with a zeolite catalyst such as ZSM-5 is improved by use of a zeolite having a catalyst particle size of at least 1 micron and having incorporated thereon two metals in the form of platinum and magnesium.

Description: XYLENE ISOMERIZATION PROCESS AND CATALYST THEREFOR This invention relates to a process and catalyst for effecting isomerization of an aromatic C8 mixture comprising ethylbenzene and xylene. Xylene isomerization in the presence of a catalyst is well known. For example, United States Patent 3,856,872 (Morrison) discloses a method of converting ethylbenzene over an active acid catalyst typified by zeolite ZSM-5. Ethylbenzene disproportionates to benzene and diethylbenzene which are readily separated from xylene by the distillation equipment needed in the loop to remove by-products. It is recognized that the rate of disproportionation of ethylbenzene is related to the rate of conversion of xylene into other compounds, e.g., by disproportionation. United States Patent 3,856,873 (Burress) also describes the reaction of C8 aromatics over ZSM-5 and shows the effect of performing the reaction at various temperatures up to 9500F (5100C) in the absence of metal catalyst and in the absence of hydrogen. In the presence of the catalyst and operating at these higher temperatures, the transalkylation route for xylene losses is reduced; however, xylene losses via the disproportionation route are increased. The xylene loss by-products are mostly trimethyl-benzene, ethyl-xylene, and Cg+ aromatics. United States Patent 4,098,837 (Chu) discloses a zeolite catalyst which incorporates phosphorus and one or more metals, including magnesium, in an effort to elicit a greater para-xylene content from the feed material. United States Patent 4,159,282 (Olson et al) discloses xylene isomerization with a crystalline zeolite catalyst having a crystal size of at least 1 micron. The original alkaline metal of the zeolite may be replaced by ion exchange with suitable ions of Groups 1B to 8. A second metal, e.g., magnesium, may be combined with the zeolite. United States Patent 4,163,028 (Tabak et al) discloses xylene isomerization and ethylbenzene conversion at temperatures of 8000F (4270C) or higher with ZSM-5 of very high silica/alumina ratio whereby the acid activity is reduced. United States Patent 4,278,565 (Chen et al) discloses a zeolite catalyst which has a crystalline size greater than 1 micron, e.g., HZSM-5C and which comprises three metals: a Group 2B metal, a Group 8 metal and magnesium incorporated into the catalyst in the order specified. In accordance with the invention it has now been found to be possible further to reduce xylene by-product formation by incorporating platinum and a Group 2A metal into a zeolite catalyst having a crystalline size greater than 1 micron. Accordingly, the invention resides in one aspect in a process for isomerizing an isomerization feed containing an aromatic C8 mixture of ethylbenzene and xylene which comprises contacting said feed, under conversion conditions, with a catalyst comprising a crystalline aluminosilicate zeolite having a crystal size of at least 1 micron, a silica to alumina ratio of at least 12 and a constraint index within the range 1 to 12, said zeolite containing platinum and a Group 2A metal component. The process of the invention utilizes a zeolite catalyst having a crystalline size of at least 1 micron, usually in the approximate range of 1 to 20 microns and preferably 1 to 6 microns. The zeolite catalyst has a silica to alumina ratio of at least 12, a constraint index within the approximate range of 1 to 12, and contains both platinum and a Group 2A metal, preferably magnesium. The present process preferably comprises isomerization of an aromatic C8 mixture in the presence of hydrogen such that the hydrogen to hydrocarbon ratio is in the range of 0.1 to 10, more preferably 1 to 5, at a temperature between 5000F and 10000F (2600C and 5380C), more preferably in excess of 6500F (3430C), at a pressure between 0 and 1500 psig (101 and 10444 kPa), utilizing a feed weight hourly space velocity (WHSV) between 0.5 and 50. The WHSV values are based on the weight of catalyst composition, i.e., total weight of active catalyst and binder thereof. In a further aspect, the invention resides in a catalyst comprising a crystalline aluminosilicate zeolite having a crystal size of at least 1 micron, a silica to alumina ratio of at least 12 and a constraint index within the range 1 to 12, said zeolite containing platinum and magnesium. It is contemplated that any aromatic C8 mixture containing ethylbenzene and xylene in which the para-xylene content is less than equilibrium may be used as the feed for the present process. Generally, the mixture will have an ethylbenzene content in the range of 5-50 wt.%, an ortho-xylene content in the range of 0-15 wt.%, and a meta-xylene content in the range of 0-70 wt.%. The feed may also contain non-aromatic hydrocarbons, i.e., naphthenes and paraffins. In a preferred embodiment, a mixture of C8 aromatic is processed so as to reduce the ethylbenzene content of the mixture and increase its content of para-xylene. The feed is brought into contact, under conversion conditions, with a catalyst comprising a crystalline aluminosilicate zeolite having a crystal size of at least 1 micron, a silica to alumina ratio of at least 12 and a constraint index within the range of 1 to 12. sUch zeolites are well known to the art and suitable descriptions may be found in United States Patents 4,159,282; 4,163,028; and 4,278,565. The preferred crystalline aluminosilicate zeolites are ZSM-5, ZSM-ll, ZSM-12, ZSM-355 and ZSM-38, with ZSM-5 being particularly preferred. The crystal size of the crystalline aluminosilicate zeolite employed herein is at least 1 micron, more preferably in the range of 1 to 20 microns and most preferably in the range of 1 to 6 microns. The longer diffusional paths of zeolite crystals in excess of 1 micron further reduce by-product formation thus decreasing xylene loss compared to zeolite crystals in the 0.5 micron range. Further, with the use of crystals within such a size range distinctly higher selectivity for production of para-xylene has been observed as compared with comparable use of smaller size crystals. The preferred catalyst contains from 0.5% to 10%, more preferably 1% to 5%, by weight of the Group 2A component, most preferably magnesium, and 0.01% to 1%, more preferably 0.05% to 1%, by weight of platinum. The catalyst is prepared by conventional techniques including impregnation, base exchange, drying and air calcination. If desired the catalyst can be steamed for 1 or more hours at temperatures upwards of 3000F (1490C), the time, pressure and temperature being inter-related such that less time is required at higher temperatures and/or pressures. The feed is contacted with the above-described catalyst at a temperature between 5000F and 9500F (2600C and 5100C);- a contact time equivalent to or the same as a weight hourly space velocity (WHSV) of 0.5 to 50, preferably 5 to 25, and a pressure of O to 1500 Dsig (101 to 10444 kPa), preferably between 20 and 400 psig (239 and 2859 kPa). The reaction product effluent contains ethane, benzene, toluene and other aromatic hydrocarbons with high selectivity for para-xylene. The combination of the larger-size ZSM-5 crystal with two metals, e.g., platinum and magnesium, has been found to reduce the formation of by-products, such as trimethyl-benzene, ethyl-xylene, C9+ aromatics, and increase benzene formation. The following example serves to illustrate the process of this invention with reference to the accompanying drawin3 which compares graphically the amounts of ethylbenzene conversion and xylene loss for three isomerization processes. EXAMPLE Catalyst Preparation HZSM-5 having an average crystal size of 2 microns was conventionally produced using tetra-propylammonium and sodium cations or tetra-methylammonium and sodium cations. Examples of the production of such zeolites are given in British Patent No. 2,002,733. The resultant HZSM-5 crystals were ion exchanged with ammonium nitrate solution, dried and then introduced into a reactor through which a Mg(N03)2 solution was circulated until about 2.4% Mg was incorporated onto the catalyst. The catalyst was then dried at 2500F (121oC) and subsequently calcined by heating to 8500F (4550C) in N followed by heating to 10000F (5380C) for 2 hours. The resultant Mg ZSM-5 was then agitated with an aqueous solution of (Pt(NH3)4)Cl containing the equivalent of 0.1 wt% Pt, dried at 2500F (121 C) and then air calcined at 10000F (5380C) for 3 hours. This catalyst will hereinafter be referred to as Catalyst A. Isomerization A feed comprising a blend of ethylbenzene and mixed xylenes and having the composition set forth in Table I below, was tested in three different pressurized xylene isomerization units. In Run 1, the feed was passed over a catalyst comprising a platinum impregnated HZSM-5 (no magnesium) having an average crystal size of 0.1 microns, hereinafter referred to as Catalyst B, at a temperature of 8530F (4560C), a pressure of 100 psig (791 kPa) and WHSV of 14.7. In Run 2, the feed was passed over a catalyst comprising a platinum-impregnated HZSM-5 (no magnesium) having an average crystal size of 0.1 microns, hereinafter referred to as Catalyst C, at a temperature of 6770F (3580C), a pressure of 100 psig (791 kPa) and a WHSV of 7.0. In Run 3, the feed was passed over Catalyst A at a temperature of 7000F (3710C), a pressure of 100 psig (791 kPa) and a WHSV of 6.5. The product compositions and selectivities obtained in each instance are given in Table I, in which: Xylene Loss (mole %) is calculated by: (T) P-F + ±TOMB) P-F + EEX3, Xylene loss = x 100 (P-X + M-X + 0-x) F where T = Toluene, TMB = trimethylbenzene, EX = ethylxylene, P-X, M-X and O-X = para-, meta-, and ortho-xylene, respectively, P = product, F = feed, C ] = mole % H2/HC Molar Ratio is the molar ratio of H2 and hydrocarbons in the feed. EB Conv. Wt. % is the weight percent conversion of ethylbenzene. P-Xylene Equil. Approach % is the weight percent equilibrium approach of P-xylene. C2=/C2 is the molar ratio of ethylene and ethane in the product. # EB is the amount of ethylbenzene converted. BZ is the amount (moles) of benzene formed in the product. C2/iEB is the molar ratio of ethane produced per mole of ethylbenzene converted. BZ/#EB is the molar ratio of benzene produced per mole of ethylbenzene converted. C1-C4 and C5-Cg N. A. designates the C1-C4 and C5-Cg non-aromatics respectively formed in the product. TABLE I Run No. 1 2 3 Catalyst B C A Temp., F ( C) 853 (456) 677 (358) 700 (371) Pressure, psig (kPa) 100 (791) 100 (791) 100 (791) WHSV 14.7 7.0 6.5 H2/HC Molar Ratio 2.2 2.0 2.0 Product Analysis (wt. %) feed feed feed C1-C4 N.A. \ 0.7 0.7 1.4 C5-Cg N.A. 0.0 0.1 0.7 BZ 2.2 1.6 2.5 Toluene 1.5 2.1 1.5 2.1 1.5 1.9 Ethylbenzene 8.1 5.2 8.6 5.7 9.0 5.1 P-Xylene 9.2 21.3 9.4 21.5 8.8 21.4 M-Xylene 61.3 46.8 60.8 47.1 61.4 47.8 O-Xylene 19.9 20.7 19.7 19.9 19.3 18.6 Ethyl-toluene 0.1 0.3 0.2 Tri-methylbenzene 0.7 0.5 0.3 Di-ethylbenzene > o 0 0.2 0.2 Ethyl-xylene 0.1 0.3 C10 - - - EB Conv. Wt.% 35 33 43 Xylene Loss, Mole % 1.4 1.5 0.9 P-Xylene Equil. Approach % 105 105 105 C2=/C2 0 0 0 2 2 C2/2SEB 0.8 0.5 0.9 BZ/ EB 1.1 0.8 0.9 A comparison of the xylene selectivities of the three isomerization processes utilized in the above Example is shown in the drawing. Referring more particularly to this Figure, selectivity is expressed in terms of the ratio of xylene loss to relative ethylbenzene conversion. It will be evident that there is a marked improvement in selectivity obtained utilizing Catalyst A over that obtained with conventional catalysts exemplified by Catalysts B and C. At about 50% ethylbenzene conversion, the xylene loss advantage for the improved Catalyst A is about 0.9% per pass.

Claims: CLAIMS: 1. A process for isomerizing an isomerization feed containing an aromatic C8 mixture of ethylbenzene and xylene which comprises contacting said feed, under conversion conditions, with a catalyst comprising a crystalline aluminosilicate zeolite having a crystal size of at least 1 micron, a silica to alumina ratio of at least 12 and a constraint index within the range 1 to 12, said zeolite containing platinum and a Group 2A metal component. 2. The process of claim 1 wherein said conversion conditions include a temperature of from 5000F to 10000F (2600C to 5380C), a pressure of O to 1500 psig (101 to 10444 kPa) and a weight hourly space velocity of between 0.5 and 100. 3. The process of claim 1 or claim 2 wherein the Group 2A metal is magnesium. 4. The process of any preceding claim wherein the zeolite is ZSM-5, ZSM-ll, ZSM-12, ZSM-35, or ZSM-38. 5. The process of any preceding claim wherein said crystalline aluminosilicate zeolite is ZSM-5. 6. The process of any preceding claim wherein said crystalline aluminosilicate zeolite is ZSM-5 predominately in the hydrogen form. 7. The process of any preceding claim wherein the crystal size of said crystalline aluminosilicate zeolite is within the range of 1 to 20 microns. 8. The process of any preceding claim wherein the crystal size of said crystalline aluminosilicate zeolite is within the range of 1 to 6 microns. 9. A catalyst for use in the process as claimed in any preceding claim comprising a crystalline aluminosilicate zeolite having a crystal size of at least 1 micron, a silica to alumina ratio of at least 12 and a constraint index within the range of 1 to 12, said zeolite containing platinum and magnesium. 10. A catalyst as claimed in claim 9 wherein the zeolite has incorporated therein platinum and magnesium alone, with the platinum constituting between 0.01% and 1% by weight of the catalyst and the magnesium constituting between O.SX and 10% by weight of the catalyst.

Inventors: CHESTER, ARTHUR WARREN, CHU, YUNG-FENG, SMITH, FRITZ ARTHUR
Applicants: MOBIL OIL CORPORATION
Application year: 1983
Main classification: C07C5
IPC codes: C07C1, C07C5, C07C15, B01J29, C07C67, C07B61