PROCESS FOR CONVERTING C2 TO C12 ALIPHATICS TO AROMATICS OVER A ZINC ACTIVATED ZEOLITE

Abstract: A catalytic process is provided for converting feedstocks containing C₂ to C₁₂ aliphatic hydrocarbons to aromatics by contacting said feedstocks, under conversion conditions, with a crystalline zeolite catalyst having a constraint index of about 1 to 12 and an alpha value of no higher than about 45, and a minor amount of an added metal consisting essentially of zinc, thereby converting the feedstock to aromatics.

Description: This invention relates to a process for the conversion of a gaseous feed containing a major proportion of C₂ to C₁₂ aliphatic hydrocarbons to aromatics in the presence of a crystalline zeolite catalyst containing an added metal consisting essentially of zinc. US-A-4,097,367 discloses the catalytic conversion of olefinic naphthas which contain diolefins over a special catalyst to yield a product stream which contains little or no non-aromatics boiling in the range of benzene, toluene and xylene. The catalyst is a combination of zinc and a metal from Groups IB and VIII of the Periodic Table with a crystalline aluminosilicate zeolite having a silica-alumina ratio greater than 12 and a Constraint Index of 1 to 12. US-A-4,120,910 discloses that aromatic compounds can be produced by contacting, in the absence of added air or oxygen under suitable conversion conditions, a gaseous, paraffinic feed stock containing a high percentage of ethane with a ZSM-5 type crystalline aluminosilicate zeolite catalyst having incorporated therein a minor amount of a metal or metal oxide from Group VIII, IIB or IB of the Periodic Table. Especially preferred is a zinc-­ copper mixture. US-A-4,350,835 discloses a catalytic process for converting a feedstock comprising a high percentage of ethane to aromatics employing as a catalyst a zeolite with a silica-alumina ratio of at least 12 and having incorporated therein a minor amount of gallium. It also discloses, for comparison purposes, a catalyst containing 1% zinc and 0.25% copper supported on a ZSM-5 zeolite having a silica to alumina ratio of 40. US-A-4,392,989 discloses the aromatization of paraffin feeds containing C₂-C₁₂ hydrocarbons using a catalyst comprising a zeolite containing zinc in combination with gallium or palladium. According to the present invention a process for producing aromatic compounds which comprises contacting a feed containing at least 50 weight percent of C₂ to C₁₂ aliphatic hydrocarbons under conversion conditions with a catalyst comprising a crystalline zeolite having a Constraint Index of 1 to 12 and an alpha value no higher than 45, said catalyst further comprising from 0.1 to 10 weight percent of zinc, and recovering said aromatic compounds. The zeolite advantageously has a silica/­ alumina mole ratio of at least 220, and indeed as high as 5000 or more. Its alpha value is preferably from 0.5 to 45. The conversion conditions suitably include a temperature of 482°C to 760°C, a pressure of 1×10⁵ to 28.6 × 10⁵ pascal and an LHSV of 0.1 to 5. The preferred zeolites are ZSM-5, ZSM-11, ZSM-12, ZSM-35, ZSM-38 or ZSM-­ 48, and the amount of zinc in the catalyst is preferably between 0.5 and 5 percent by weight. The zeolite may have been synthesized with an alpha value of over 45, or achieved such a value in post-­ synthesis processing, its alpha value then being reduced to below 45 by an after-treatment, suitably by steaming. It may be composited with a matrix material comprising at least one clay or inorganic oxide. In a particularly preferred embodiment of the invention the zeolite is ZSM-5, is composited with a porous matrix material and constitutes between 5 and 80 percent by weight of the dry composite, and the conversion conditions include a temperature of 482°C to 676.7°C, a pressure of 1×10⁵ to 7.9×10⁵ pascal and an LHSV of 0.3 to 3. The invention may be alternatively expressed as a process for producing aromatic hydrocarbons comprising bringing into contact at an elevated temperature a hydrocarbon feedstock containing a major proportion of C₂-­ C₁₂ aliphatic hydrocarbons with a catalyst composition comprising a zeolite having a low acid activity as indicated by an alpha value no greater than about 45, and an added metal consisting essentially of zinc deposited thereon and/or whose cations have been exchanged with ions of zinc. Preferably, the added metal consists solely of zinc. The significance, and manner of determination, of alpha value is described in Journal of Catalysis, Vol. VI, pp. 278-287, 1966. The term "added metal" is intended to cover those metallic elements not present in the zeolite as synthesized but added subsequently by means of a treatment such as impregnation, or ion-exchange. Such term thus does not include those metals such as aluminum or alkali metal, e.g. sodium, present as a result of the initial synthesis of the zeolite. The term "consisting essentially" is intended to exclude any added metal other than zinc which would have the effect of changing the basic and novel characteristics of the processes and catalysts disclosed and claimed herein, or which would change the nature and intent of such processes and catalysts. The zinc in the catalyst composition may be present as cations introduced into the zeolite in known manner, for example by exchange with an aqueous solution of zinc-containing metal salts such as zinc sulfate, nitrate or chloride. Before exchange the zeolite may be acid treated. The process of the present invention may also be carried out using catalysts in which zinc is impregnated on the surface of the zeolite. Where the impregnation employs a compound of zinc which ionizes in aqueous solution, for example zinc nitrate, some of the zinc ions are generally exchanged with the cations in the zeolite even though the procedure is directed to impregnation of the zeolite. Whichever method of catalyst preparation is used, the amount of zinc present in the catalyst composition (zinc plus zeolite) may vary between about 0.1 and 10%, preferably between about 0.5 and 5% by weight. Although the zeolites utilized in the process may have extremely low alumina contents, e.g. silica to alumina mole ratios exceeding about 220, they are nevertheless very active. The activity is surprising since catalytic activity is generally attributed to framework aluminum atoms and/or cations associated with these aluminum atoms. These zeolites retain their crystallinity for long periods in spite of the presence of steam at high temperatures which induces irreversible collapse of the framework of other zeolites, e.g. of the X and A type. Furthermore, carbonaceous deposits, when formed, may be removed by burning at higher than usual temperatures to restore activity. These zeolites, used as catalysts, generally have low coke-forming activity and therefore are conducive to long times on stream between regenerations by burning carbonaceous deposits with oxygen-containing gas such as air. The low alpha value (not greater than about 45) catalyst may be prepared as-synthesized, or by decreasing the alpha value of a high alpha value catalyst, e.g. by steaming, dealuminizing or framework exchange procedures. The alpha value decreases with a rise in the silica/alumina ratio of the zeolite and, in the absence of any specific treatment to reduce its acidity, the protonated or hydrogen form of the zeolite having an alpha value within the limits of this invention will generally have a silica/alumina ratio of at least 220. However, the acidity and thus the alpha value of a zeolite having a silica-alumina ratio below 220 may be reduced to the desired level by the means mentioned previously, e.g. steaming, dealuminizing or framework exchange procedures. If such means are used to reduce below 45 the alpha value of a zeolite having a silica-alumina ratio of below 220, then such zeolite is contemplated for use in this invention. It should be noted also that since some acidity of the catalyst is necessary in order to obtain sufficient conversion of the feed hydrocarbons, the alpha value of the catalyst will in most cases be at least about 0.5. The techniques of steaming zeolites to obtain desired alpha values are known to those skilled in the art. When the treating atmosphere is steam, temperatures may be used extending from about 500°F to about 1800°F depending on the steam pressure, with the use of higher pressure requiring a lower temperature. This treatment is carried on for a period of time sufficient to effect the desired reduction in alpha value. Generally, such period will be between about 1/2 hour and 100 hours. A steam treating atmosphere may be employed which is substantially inert with respect to the zeolite. It is contemplated that the treatment will generally be effected at atmospheric pressure, but pressures ranging from sub-­ atmospheric pressure to several hundred atmospheres may be employed. With the use of elevated pressures, temperatures in the lower region of the above-specified range will usually be applicable in achieving the desired reduction in alpha value of the zeolite under treatment. Thus, it as been found, that at elevated steam pressure, the temperature of treatment can be reduced substantially to achieve the same degree of modification. Although zeolites with silica to alumina mole ratios of at least about 220 are useful, it is preferred in some instance to use zeolites having substantially higher silica/alumina ratios, e.g. 880 and above. The members of the class of zeolites useful herein have an effective pore size of generally from about 5 to about 8 angstroms, such as to freely sorb normal hexane, consistent with their possession of a constraint index of 1 to 12. The significance and manner of determination of constraint index is described in our GB-A-1,446,522. We do however draw attention to the fact that testing parameters to some degree determine the precise value of constraint index yielded for a given zeolite by an individual test, and that any zeolite which is capable, upon testing within the prescribed conditions, of manifesting a constraint index of 1 to 12 is considered for the purposes of the present specification to be a zeolite having a constraint index of 1 to 12. The zeolites useful herein are exemplified by ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-38, ZSM-48, defined respectively by the x-ray diffraction data set forth in US-A-3,702,886 and US-E-29948; 3,709,979; 3,832,449; 4,076,842; 4,016,245; 4,046,859; and 4,350,835. The present invention contemplates utilization of such zeolites having an alpha value no higher than 45 and as low as 0.5, and wherein the mole ratio of silica to alumina of the as synthesized catalyst in the absence of treatment to reduce its alpha value is at least about 220 and may be as high as 5000, even as high as about 26,000. Reference to the above identified specifications therefore does not limit the crystalline zeolites useful in the present invention to those having the specific alpha values and silica-alumina mole ratios discussed in those specifications, since such zeolites may have much lower alpha values and higher silica-alumina ratios and yet, having the same crystal structure as the disclosed materials, may be useful or even preferred in some applications. The specific zeolites described, when prepared in the presence of organic cations, are substantially catalytically inactive, possibly because the intra­ crystalline free space is occupied by organic cations from the forming solution. They may be activated by heating in an inert atmosphere at about 540°C for one hour, for example, followed by base exchange with ammonium salts followed by calcination at about 540°C in air. The presence of organic cations in the forming solution may not be absolutely essential to the formation of this type zeolite; however, the presence of these cations does appear to favor the formation of this special class of zeolite. In many cases, it is desirable to activate this type catalyst by base exchange with ammonium salts followed by calcination in air at about 540°C for from about 15 minutes to about 24 hours. Natural zeolites may sometimes be converted to zeolite structures of the class herein identified by various activation procedures and other treatments such as base exchange, steaming, alumina extraction and calcination, alone or in combinations. Natural minerals which may be so treated include ferrierite, brewsterite, stilbite, dachiardite, epistilbite, heulandite, and clinoptilolite. When synthesised in the alkali metal form, the zeolite is conveniently converted to the hydrogen form, generally by intermediate formation of the ammonium form as a result of ammonium ion exchange and calcination of the ammonium form to yeild the hydrogen form. In addition to the hydrogen form, other forms of the zeolite wherein the original alkali metal has been reduced to less than about 1.5 percent by weight may be used. Thus, the original alkali metal of the zeolite may be replaced by ion exchange with zinc as mentioned previously. In practicing a particularly desired chemical conversion process, it may be useful to composite the above-described crystalline zeolite with a matrix comprising another material resisitant to the temperature and other conditions employed in the process. Such matrix material is useful as a binder and imparts greater resistance to the catalyst for the severe temperature, pressure and reactant feed stream velocity conditions encountered in many conversion processes. Useful matrix materials include both synthetic and naturally occurring substances, as well as inorganic materials such as clays, and inorganic oxides such as silica and/or metal oxides and mixtures thereof, and are fully described in our EP-A-­ 1695. The feed stream to the process of this invention contains at least 50% by weight of at least one aliphatic hydrocarbon containing 2 to 12 carbon atoms. The hydrocarbon may be straight chain, open chain or cyclic and may be saturated or unsaturated. some contemplated hydrocarbons are ethane, propane, propylene, n-butane, n-­ butenes, isobutane, isobutene, straight-and branch chain hexanes, hexenes, octanes, octenes, decanes and decenes, cyclohexane and cyclohexene. The process of this invention is conducted so that a feed containing a high percentage, i.e. at least 50 wt.% of C₂-C₁₂ aliphatic hydrocarbons is contacted with a suitable zeolite catalyst in a reaction zone, such as, for example, a fixed bed of catalyst composition under effective conversion conditions. In a typical embodiment of the process of this invention, the feed stream is introduced into the reaction zone at a temperature within the range of 482°C (900°F) and about 760°C (1400°F), a pressure within the range of 1×10⁵ pascal (atmospheric pressure) to 26.6×10⁵ pascal (400 psig) and a LHSV of 0.1 to 5. Preferred temperatures for the process of this invention fall within the range of about 482°C (900°F) to about 676.7°C (1250°F) and preferred pressures fall within the range of 1×10⁵ pascal (one atmosphere) to 7.9×10⁵ pascal (100 psig). A preferred LHSV is between 0.3 and 3. The effluent from the reaction zone is separated and distilled to remove the desired aromatic product and the remainder is recycled for further reaction. The following examples further illustrate the invention. Example 1 is an embodiment of processes carried out within the invention, while Example 2 (Comparative) is a similar embodiment which shows the results obtained when a high acid activity catalyst (alpha value = 150) is used which is outside the scope of the invention. EXAMPLE 1 This example illustrates the preparation and use of a zinc-containing zeolite catalyst under the invention wherein the zinc is deposited on the zeolite by impregnation. Five grams of a hydrogen or acid-base ZSM-5 zeolite having an alpha value of about 5 and a silica-­ alumina ratio of about 1000, and 0.23 grams of Zn(NO₃)₂ 6H₂O, were mixed with 10cc of water. The mixture was then dried and calcined at 538°C (in air) for 6 hours. The final catalyst contained about 1.0 wt% of added zinc. This catalyst was tested for aromatization activity by loading it into a reactor and introducing n-­ hexane at a temperature of 538°C (1000°F), atmospheric pressure, and an LHSV of 0.59. After various times on stream, the product was analyzed. Hydrogen and light gases (C₁-C₃) were analyzed by a refinery gas analysis GC, C₃+ gases were analyzed on a n-octane-porasil C column, and liquid products were analyzed on a DB-1 Durabond capillary column. The results are shown in the table where the numbers all signify weight percent of product except as indicated, "C₁+C₂" is the total of methane and ethane, "C₂=+" is the total of ethylene and higher aliphatics and "Aromatics Selectivity" is defined as follows. EXAMPLE 2 (Comparative) The procedure of Example 1 was followed except that the ZSM-5 zeolite has an alpha value of about 150 and a silica/alumina ratio of 70. The result are shown in the table. As shown in the table, the catalyst and process of this invention utilizing a zeolite with a relatively low alpha value and high silica/alumina ratio resulted in higher aromatics selectivity and less undesirable by-­ products such as methane and ethane than when a more conventional zeolite having a higher alpha value and lower silica/alumina ratio outside the scope of the invention was employed.

Claims: 1. A process for producing aromatic compounds which comprises contacting a feed containing at least 50 weight percent of C₂ to C₁₂ aliphatic hydrocarbons under conversion conditions with a catalyst comprising a crystalline zeolite having a Constraint Index of 1 to 12 and an alpha value no higher than 45, said catalyst further comprising from 0.1 to 10 weight percent of zinc, and recovering said aromatic compounds. 2. The process of claim 1 wherein said zeolite has a silica/alumina mole ratio of at least 220. 3. The process of claim 1 or claim 2 wherein said alpha value is from 0.5 to 45. 4. The process of any preceding claim wherein said zeolite has a silica/alumina mole ratio of 220 to 5000. 5. The process of any preceding claim wherein the conversion conditions include a temperature of 482°C to 760°C, a pressure of 1×10⁵ to 28.6 × 10⁵ pascal and an LHSV of 0.1 to 5. 6. The process of any preceding claim wherein said crystalline zeolite is ZSM-5, ZSM-11, ZSM-12, ZSM-35, ZSM-38 or ZSM-48. 7. The process of any preceding claim wherein the amount of zinc in said catalyst is between 0.5 and 5 percent by weight. 8. The process of any preceding claim wherein said zeolite as synthesized as an alpha value of over 45 which is reduced to below 45 by an after-treatment. 9. The process of claim 8 wherein said after-­ treatment is steaming. 10. The process of any preceding claim wherein said zeolite is composited with a matrix material comprising at least one clay or inorganic oxide. 11. The process of claim 10 wherein said zeolite is ZSM-5, is composited with a porous matrix material and constitutes between 5 and 80 percent by weight of the dry composite, and the conversion conditions include a temperature of 482°C to 676.7°C, a pressure of 1×10⁵ to 7.9×10⁵ pascal and an LHSV of 0.3 to 3.

Inventors: CHU, CYNTHIA TING-WAH
Applicants: MOBIL OIL CORPORATION
Application year: 1987
Main classification: C07C5
IPC codes: B01J27, C07C2, C07C1, C07C5, C07C15, C10G35, B01J29, C07C67