A ZEOLITE-CONTAINING HYDROCARBON CONVERSION CATALYST AND THE PREPARATION THEREOF

Abstract: The catalytic properties of a metal-containing acidic zeolite catalyst are significantly improved by converting said metal to an intermetallic compound. Thus, for example, zeolite beta containing the intermetallic component platinum silicide demonstrates improved catalytic properties for paraffin isomerization as compared to zeolite beta containing platinum metal alone.

Description: This invention relates to a zeolite-containing hydrocarbon conversion catalyst and the preparation thereof. Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversion. Certain zeolitic materials are ordered, porous crystalline silicates having a definite crystalline structure as determined by X-ray diffraction within which there are a number of smaller cavities which may be interconnected by a number of still smaller channels or pores. These cavities and pores are uniform in size within a specific zeolitic material. Since the dimensions of these pores are such as to accept for absorption molecules of certain dimensions while rejecting those of larger dimensions, these materials are said to be "shape selective" and are utilized in a variety of ways to take advantage of these properties. Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline aluminosilicates. These aluminosilicates can be described as a rigid three-dimensional framework of SiO₄ and AlO₄ in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total aluminum silicon atoms to oxygen atoms 1:2. The anionic nature of the tetrahedra containing aluminum gives rise to the acidity of such zeolites and is balanced by the inclusion in the crystal of a cation, for example an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of aluminum to the number of various cations, such as Ca/2, Sr/2, Na, K or Li, is equal to unity. One type of cation may be exchanged either entirely or partially with another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given aluminosilicate by suitable selection of the cation. Prior art techniques have resulted in the formation of a great variety of synthetic zeolites. The zeolites have come to be designated by letter or other convenient symbols, as illustrated by zeolite Z (U.S. Patent No. 2,882,243), zeolite X (U.S. Patent No. 2,882,244), zeolite Y (U.S. Patent No. 3,130,007), zeolite ZK-5 (U.S. Patent No., 3,247,195), zeolite beta, (U.S. Patent No. 3,308,069), zeolite XK-4 (U.S. Patent No. 3,314,752), zeolite ZSM-5 (U.S. Patent No. 3,702,886), zeolite ZSM-11 (U.S. Patent No. 3,709,979), zeolite ZSM-12 (U.S. Patent No. 3,832,449), zeolite ZSM-20 (U.S. Patent No. 3,972,983), ZSM-35 (U.S. Patent No. 3,016,245), ZSM-38 (U.S. Patent 4,046,859) and zeolite ZSM-23 (U.S. Patent No. 4,076,842). These and other zeolites have been widely investigated as catalysts for a variety of hydrocarbon conversion processes including isomerization of waxy paraffins. Thus, for example, U.S. Patent No. 4,419,220 describes a process for catalytically dewaxing a variety of feedstocks containing waxy normal and slightly branched paraffins over zeolite beta containing a hydrogenation component such as platinum or palladium. During the processing, the n-paraffins become isomerized to iso-paraffins and the slightly branched paraffins undergo isomerization to more highly branched aliphatics resulting in a product of reduced pour point. The modification of zeolites X and Y optionally containing a hydrogenating metal component with an organosilane capable of entering the pores, or channels, of these zeolites is known from U.S. Patent No. 4,451,572. The organosilane modified zeolite either before or after the deposition of the optional metal component, is heated, usually at above 300°C, in an inert or reducing atmosphere to form a stable surface resulting from condensation/polymerization of the silated surface. The amount of organosilane employed is such as to result in significant constriction of the pore diameter and a consequent reduction in pore volume. The modified zeolite is said to be useful in hydrodewaxing, selective paraffin cracking, paraxylene isomerization and the preferential acid catalyzed reactions of linear or slightly branched paraffins or olefins. It has now been discovered that the catalytic properties of an acidic zeolite containing a metallic component can be significantly improved by converting said metallic component to an intermetallic compound without, however, significantly reducing either the average pore diameter or average pore volume of the zeolite. Accordingly, the invention resides in one aspect in a hydrocarbon conversion catalyst comprising an acidic zeolite and a hydrogenation-dehydrogenation intermetallic catalytic component contained by the zeolite, said intermetallic catalytic component comprising a compound of a hydrogenation-dehydrogenation metal and boron, aluminum, gallium, indium, silicon, germanium and/or tin. In a further aspect, the invention resides in a process for preparing a hydrocarbon conversion catalyst comprising reacting a hydrogenation-dehydrogenation metal-containing acidic zeolite catalyst with a volatile inorganic metal compound or a volatile organometallic compound under anhydrous conditions in a reducing atmosphere. The intermetallic compound-containing zeolite catalyst of the present invention exhibits greater useful life due to an enhanced ability to resist agglomeration of the catalystically active metal species. In addition, the catalysts possess increased catalytic activity for a variety of hydrocarbon conversion processes such as the isomerization process specifically illustrated below. The acidic zeolite which is employed in the present invention preferably possesses a silica to alumina molar ratio of at least 12, a constraint index of 1 to 12 and an acid craking activity, alpha, of 50-300. In this respect it is to be noted that the determination of the constraint index of zeolites is described in U.S. Patent No. 4,016,218, whereas the determination of zeolite alpha values is described in J. Catalysis 61 , 395 (1980) and J. Catalysis 6 , 278 (1966). Representative of suitable zeolites are zeolite beta, ZSM-5, ZSM-11, ZSM-12, ZSM-23 and ZSM-35. Zeolite beta is disclosed in U.S. Patent No. 3,308,069; ZSM-5 is disclosed in U.S. Patent No. 3,702,886 and U.S. Patent No. Re. 29,948; ZSM-11 is disclosed in U.S. Patent No. 3,709,979; ZSM-12 is disclosed in U.S. Patent No. 3,832,449; ZSM-23 is disclosed in U.S. Patent No. 4,076,842 and ZSM-35 is disclosed in U.S. Patent No. 4,016,245. Other zeolites which can undergo modification as described herein include a variety of medium pore (about 5 to 9 angstroms) siliceous materials such as borosilicates, ferrosilicates, and/or aluminosilicates such as are disclosed in U.S. Patent Nos. 4,414,423 and 4,417,088. Optionally, the surface activity of the zeolite can be modified by pretreatment, e.g., with a neutralizing base as disclosed in U.S. Patent No. 4,520,221 which reduces the surface acidity of the zeolite, or by any of a variety of procedures involving removal of framework aluminum, e.g., the dealuminizing methods described in U.S. Patent No. 3,442,795 and U.K. Patent No. 1,058,188 (hydrolysis and removal of aluminum by chelation); U.K. Patent No. 1,061,847 (acid extraction of aluminum); U.S. Patent No., 3,493,519 (aluminum removal by steaming and chelation); U.S. Patent No. 3,591,488 (aluminum removal by steaming); U.S. Patent No. 4,273,753 (dealuminization by silicon halides and oxyhalides): U.S. Patent No. 3,691,099 (aluminum extraction with acid); U.S. Patent No. 4,093,560 (dealuminization by treatment with salts): U.S. Patent No. 3,937,791 (aluminum removal with Cr(III) solutions); U.S. Patent No. 3,506,400 (steaming followed by chelation); U.S. Patent No. 3,640,681 (extraction of aluminum with acetylacetonate followed by dehydroxylation); U.S. Patent No. 3,836,561 (removal of aluminum with acid); U.S. Patent No. 4,503,023 and German Patent No. 2,510,740 (treatment of zeolite with chlorine or chlorine-containing gases at high temperatures); and J. Catalysis , 54 , 295 (1978) (hydrothermal treatment followed by acid extraction). The zeolite to undergo modification in accordance with the process of this invention is associated with one or more hydrogenation-dehydrogenation metals A, e.g., a noble metal such as platinum, palladium or rhodium. Combinations of noble metals such as platinum, rhenium, platinum-palladium, platinum-iridium or platinum-iridium-rhenium together with combinations with non-noble metals, particularly of Groups VIA and VIIIA are of interest, particularly with metals such as cobalt, nickel, vanadium, tungsten, titanium and molybdenum; for example, platinum-tungsten, platinum-nickel or platinum-nickel-tungsten. The metal can be incorporated into the catalyst by any suitable method such as impregnation or exchange onto the zeolite. The metal can be incorporated in the form of a cationic, anionic or neutral complex such as Pt(NH₃)₄2+ and cationic complexes of this type will be found convenient for exchanging metals onto the zeolite. Anionic complexes such as the vanadate or metatungstate ions are useful for impregnating metals into the zeolites. The amount of metal component is suitably from 0.01 to 10 percent by weight, normally 0.1 to 5 percent by weight, although this will, of course, vary with the nature of the component, less of the highly active noble metals, particularly platinum, being required than of the less active base metals. Base metal components such as cobalt, nickel, molybdenum and tungsten can be subjected to a pre-sulfiding treatment with a sulfur-containing gas such as hydrogen sulfide in order to convert the oxide forms of the metal to the corresponding sulfides. The metal-containing zeolite, preferably substantially in the hydrogen form (i.e., with an alkali metal content of less than about 4 wt. percent and preferably less than about 1.5 wt. percent), is treated with a reactive organometallic composition or inorganic metal compound, e.g., a metal hydride, which is volatile, e.g., one having a vapor pressure of at least 0.01 Torr at the reaction temperature which is to be employed. The reactive composition should contain atoms(s) B selected from boron, aluminum, gallium, indium, silicon, germanium, tin, or combinations thereof. It is possible to form a large variety of intermetallic compounds utilizing metal(s) A and a reactive gas containing metal(s) B. Not all combinations of A and B necessarily form supported intermetallic compositions. However, if an intermetallic composition of the form A x B y exists in bulk, then a supported intermetallic of the form A z B w (where z and w are not necessarily equivalent to x and y, respectively) is producible by forming the zeolite-supported metal(s) A and then reacting it with a gas or gases having the desired metal(s) B. (The existence of a particular intermetallic bulk compound is easily determined by consulting the extensive phase diagrams published in, for example, W. G. Moffatt, The Handbook of Binary Phase Diagrams , General Electric Company, Schenectady, N.Y. 1978) To ensure that a desired intermetallic compound is formed, the reactive compound should satisfy certain criteria. First, the reactive compound should be volatile, i.e., have a vapor pressure (at the reaction temperature employed to produce the intermetallic compound) of greater than 0.01 Torr, preferably greater than 0.1 Torr. If a compound with a lower volatility is employed, it is difficult to supply sufficient gas for reaction with the supported Group VIII metal. Generally, for adequate reaction to occur, partional pressures above 0.01 Torr, preferably above 0.1 Torr, are utilized. The reactive gas should react sufficiently with the supported metal so that sufficient intermetallic composition is formed to yield catalytic activity which differs from that of metal A. Generally very small amounts of intermetallic composition do afford useful catalytic activity. In general, an amount of reactive gas not significantly exceeding a stoichiometric amount should be used. Stated another way, the amount of reactive gas employed should not be such as to result in an appreciable reduction in the diameter of the zeolite pore or pore volume. The reactivity of the gas depends not only on its specific composition but also on the temperature employed for reaction, the concentration of the reactive gas, and the presence of other agents, such as hydrogen, carbon monoxide or other reducing gas in the reaction stream. Gases with relatively high reactivity are easily obtainable for boron, aluminum, silicon, germanium, gallium, indium and tin. Volatile inorganic and organic silanes and germanes are preferred. Inorganic silane compounds that are useful include silane, disilane, trisilane, tetrasilane, bromosilane, bromotrichlorosilane, dibromosilane, dibromodichlorosilane, trichlorosilane, triflourosilane, silicontetrachloride and hexafluorodisilane. Examples of organosilanes which can be used are hexamethyldisilane, tetramethylsilane, methylsilane, dimethylsilane, trimethylsilane, ethyltrimethylsilane, ethyl-dimethylsilane, and other short chain alkyl silanes. Examples of useful inorganic germanium compounds are germane, digermane, trigermane, germane imide, germanium tetrabromide, germanium dichloride, germanium tetrachloride, and germanium oxychloride. Examples of useful organogermanium compounds are hexamethyldigermane, tetramethylgermane, methylgermane, dimethylgermane, trimethylgermane, ethyltrimethylgermane, ethyldimethylgermane, and other short chain alkyl germanes. For gases which upon reaction liberate carbon containing entities, e.g., methyl groups from hexamethyldigermane, certain precautions should be taken. In particular, a reducing gas should be present to prevent the formation of carbonaceous deposits on the catalyst body. Typically reducing agent partial pressures, e.g., hydrogen pressures greater than 1 Torr, are suitable. In a preferred embodiment, a gas having a metal-metal or metal-hydrogen bond, such as silane or disilane, is employed. These bonds are readily cleaved on the supported metal surface resulting in a rapid reaction between the reactive metal and the supported metal. A control sample is easily utilized to determine an appropriate temperature, pressure, and reaction time for a given reactive gas to yield the desired intermetallic composition. Generally, temperatures in the range 50 to 600°C employed in conjunction with typical partial pressures greater than 0.01 Torr, preferably greater than 0.1 Torr, yield suitable results. (Although the discussion has dealt with the use of one reactive gas, combinations of intermetallic compounds or higher intermetallic compounds, e.g., ternaries, are producible if the supported metal(s) are reacted with a mixture of gases.) The reaction of the gas with the zeolite-supported metal is performed in conventional equipment. The particular conditions employed to yield a specific composition of intermetallic compound is easily determined by utilizing a control sample. (The composition of the intermetallic compound produced is easily tested utilizing conventional analytical techniques such as X-ray fluorescence and quantitative elemental analysis.) The intermetallic compound-containing zeolites of the present invention are useful as catalysts for a variety of hydrocarbon conversion processes, particularly dewaxing of lube oils and isomerization of gas oils to low pour point distillates and fuel oils. In practice, it may be desirable to incorporate the catalyst of the present invention with another material resistant to the temperature and other conditions employed in hydrocarbon conversion processes. Such matrix materials include synthetic or natural substances as well as inorganic materials such as clay, silica and/or metal oxides. The latter may be either naturally occurring or in the form of gelatinous precipitates or gels including mixtures of silica and metal oxides. Naturally occurring clays which can be composited with the catalyst include those of the montmorillonite and kaolin families. These claims can be used in the raw state as originally mined or initially subjected to calcination, acid treatment or chemical modification. The catalysts can also be composited with a porous matrix material, such as alumina, silica-alumina, silica-magnesia, silica-zirconia, silica-thoria, silica-berylia, silica-titania as well as ternar compositions, such as silica-alumina-thoria, silica-alumina-zirconia, silica-alumina-magnesia, and silica-magnesia-zirconia. The matrix may be in the form of a cogel with the zeolite. The relative proportions of zeolite component and inorganic oxide gel matrix may vary widely with the zeolite content ranging from between 1 to 99, more usually 5 to 80, percent by weight of the composite. The matrix may itself possess catalytic properties, generally of an acidic nature. The invention will now be more particularly described with reference to the following Examples and the accompanying drawings, in which: Figure 1 is graph plotting the rate conversion of hexamethyldisilane when used to modify Pt-zeolite beta in accordance with Example 1; Figures 2a - 2d are partial X-ray diffraction patterns showing the effect of steaming on Pt-beta, both with and without treatment in accordance with Example 1; Figure 3 is a graph comparing the n-heptane isomerization activity of Pt-beta, with and without treatment in accordance with Example 1; and Figure 4 is a graph similar to Figure 3 but comparing the selectivity of the catalysts to the production of iso-heptane. EXAMPLE 1 A platinum-containing zeolite beta was prepared in the manner described in U.S. Patent No. 3,308,069 and the as-crystallized zeolite was then composited with alumina, calcined in nitrogen to 538°C for 3 hours and exchanged with NH₄NO₃ under anhydrous conditions. The base-exchanged zeolite was then calcined in air to 538°C for 3 hours, exchanged with Pt(NH₃)₄2⁺ to provide 0.6 weight percent platinum and calcined to 350°C in air for 3 hours. This catalyst was then treated with hexamethyldisilane (CH₃)₆Si₂, by first heating the catalyst to 300°C in hydrogen to reduce the platinum and then passing 0.12 g hexamethyldisilane/g catalyst over the catalyst in H₂ at 300°C. The silane compound was introduced by injecting the compound upstream of the catalyst over the course of 30 minutes. Throughout the treatment, the hydrogen flow rate was maintained at 400 cc/min. The conversion of hexamethyldisilane was monitored by sampling the effluent stream for decomposition products. Figure 1 charts the conversion of hexamethyldisilane over the course of the treatment as determined by gas chromotoraphic analysis of the effluent stream. Approximately 55 mg/g of the compound was converted over the catalyst before breakthrough of the hexamethyldisilane occurred. This corresponds to a mole Si/mole Pt ratio of 20.3 or a mole Si/mole exposed Pt ratio of 22.5 based on a previous Pt dispersion measurement of 90%. Elemental analysis of the treated and untreated catalysts showed that the silicon content increased from 29.09 wt. % to 31.11 wt.% following the treatment. This is equivalent to the deposition onto the catalyst of 72% of the charged silcon largely in the form of the intermetallic compound platinum silicide. EXAMPLE 2 This example demonstrates the superior stability of a platinum silicide-containing zeolite beta prepared as in Example 1 compared to that of essentially the same zeolite beta which has not undergone modification. Well dispersed platinum is known to agglomerate upon treatment with even low partial pressures of water (>50 Torr) at elevated temperatures (>350°C). To evaluate the stability of platinum-containing catalysts the x-ray diffraction (XRD) spectra of the platinum located on a series of catalysts prepared from the same 0.6% platinum on alumina bound zeolite beta described above were compared. Areas under the XRD spectra in the 37° to 43° 2ϑ range are known to be a quantitive indicator of large agglomerate and therefore poorly dispersed platinum. Catalyst A was prepared as described above Example 1. Catalyst B was prepared by omitting the hexamethyldisilane treatment step but following the same reduction procedure described above. Catalysts C and D were prepared by treating Catalysts A and B respectively, with pure steam (101 kPa H₂O) at 538°C for 17 hours. The results are shown in Table 1 and Figures 2a - 2d. It will be seen that the treatment with hexamethyldisilane did not alter the original platinum dispersion of the catalyst, but while the steaming had no effect on the platinum in the hexamethyldisilane treated sample, steaming increased the amount of large crystal platinum (>75A) in the untreated sample by a factor of 2.5. Thus, the effect of the treatment with the silane compound was to stabilize the platinum under even very severe hydrothermal conditions. EXAMPLE 3 This example demonstrates the use of Catalyst A of Example 2 in the isomerization of n-heptane and compares the results obtained with those achieved employing Catalyst B of Example 2 (unmodified catalyst). Conditions of the isomerization were as follows: The results of these runs are shown in Figures 3 and 4. Figure 3 plots the isoheptane yield against temperature for the disilane treated catalyst (Catalyst A) and the untreated catalyst (Catalyst B). The maximum isomer yield of Catalyst A was higher than that of Catalyst B (43% compared to 25%). Figure 4 compares the isomerization selectivities of the two catalysts. Isomerization selectivity is defined herein as: Selectivity = % Conversion to Isoheptanes x 100 % Conversion to Isoheptane and Other Products These data show that over the entire range of n-heptane conversion, the hexamethyldisilane treated catalyst was significantly more selective than the untreated catalyst for the isomerization reaction.

Claims: 1. A hydrocarbon conversion catalyst comprising an acidic zeolite and a hydrogenation-dehydrogenation intermetallic catalytic component contained by the zeolite, said intermetallic catalytic component comprising a compound of a hydrogenation-dehydrogenation metal and boron, aluminum, gallium, indium, silicon, germanium and/or tin. 2. The catalyst of claim 1, wherein the intermetallic catalytic component comprises a compound of a hydrogenation-dehydrogenation metal and silicon or germanium. 3. The catalyst of claim 1 or 2, wherein said hydrogenation-dehydrogenation metal includes a noble metal. 4. The catalyst of any preceding claim, wherein said hydrogenation-dehydrogenation metal is selected from platinum, palladium, rhenium, platinum-palladium, platinum-iridium and platinum-iridium-rhenium. 5. The catalyst of any preceding claim, wherein said hydrogenation-dehydrogenation intermetallic catalytic component includes platinum silicide. 6. The catalyst of any preceding claim, wherein said zeolite has a silica to alumina molar ratio of at least 12, a constraint index from 1 10 12, and an alpha value from 50 to 300. 7. A process for preparing a hydrocarbon conversion catalyst comprising reacting a hydrogenation-dehydrogenation metal-containing acidic zeolite catalyst with a volatile inorganic metal compound or a volatile organometallic compound under anhydrous conditions in a reducing atmosphere. 8. The process of claim 7, wherein said inorganic metal compound or organometallic compound includes silicon or germanium. 9. The process of claim 7, wherein said inorganic metal compound is selected from silane, disilane, trisilane, tetrasilane, bromosilane, bromotrichorosilane, dibromosilane, dibromodichlorosilane trichlorosilane, germane, digermane, trigermane, germane imide, germanium tetrabromide, germanium dichloride, germanium tertrachloride and germanium oxychloride. 10. The process of claim 7, wherein said organometallic compound is selected from hexamethyldisilane, tertramethylsilane, methylsilane, dimethylsilane, trimethylsilane, ethyltrimethylsilane, ethyldimethylsilane, hexamethyldigermane, tetramethylgermane, methylgermane, dimethylgermane, trimethylgermane, ethyltrimethylgermane, and ethyldimethylgermane.

Inventors: CHEN, NAI YUEN, DEGNAN, THOMAS FRANCIS, JR., WEISZ, PAUL BERG
Applicants: MOBIL OIL CORPORATION
Application year: 1988
Main classification: C10G45
IPC codes: C10G45, B01J29