PROCESS FOR PREPARING A NOBLE METAL-CONTAINING ZEOLITE CATALYST

Abstract: A process for preparing a noble metal-containing zeolite comprises the steps of a) contacting the zeolite with a cationic noble metal complex; b) calcining the resulting material in the presence of molecular oxygen at 300 to 375°C for a time sufficient to cause decomposition of said complex; c) purging oxygen and water from the calcined material in an inert atmosphere at 200 to 350°C; and d) reducing the purged material with hydrogen at 200 to 350°C for a time sufficient to reduce the cationic noble metal to highly dispersed free metal.

Description: This invention relates to a process for preparing a noble metal-containing zeolite catalyst. Noble metal-containing zeolite catalysts and their methods of preparation are well-known in the prior art. For example, U. S. Patent No. 3,226,339 teaches a process for preparing platinum metal-crystalline zeolite catalysts having a pore size of 10 to 13 angstroms. The method comprises contacting a zeolite by ion-exchange with platinum metal containing ion from a solution of ionizable platinum metal compound followed by activating the resulting zeolite by thermal treatment in air at 120 to 600°C (250 to 1100°F), followed by reduction in hydrogen at 120 to 600°C (250 to 1100°F) in order to effect at least partial reduction of the platinum metal ion to the platinum metal. It is also known in the art that the gaseous environment in which some zeolites are thermally treated before reduction can affect the size of resulting platinum crystals in impregnated Pt-zeolite catalysts, probably owing to formation of Pt products such as PtO₂ and PtH which have different mobility and sensitivity to reduction. It is also known that pretreatment and reduction conditions such as temperature can affect the extent of metal agglomeration. See Zeolite Chemistry and Catalysis, Ed. J. A. Rabo, Chapter 10 - Catalytic Properties of Metal-Containing Zeolites, pp. 552 to 611, (pages 558 to 565) American Chemical Society, 1976. Despite these teachings, it has been difficult to prepare noble metal-containing zeolites which exhibit enhanced dispersion over their starting material zeolite after the reduction step because of the tendency of the noble metal to agglomerate during treatment with hydrogen, even at temperatures as low as 150 to 200°C. Other methods of preparing noble metal zeolites are set out in U.S. Patent Nos. 4,683,052 and 4,683,274. It has now been found that a noble metal-containing zeolite catalyst can be prepared by a process which comprises the steps of: a) introducing a cationic noble metal species on a zeolite having a Constraint Index of 1 to 12; b) heating the material of step (a) in the presence of an inert gas at 200 to 350°C to purge any oxygen therefrom; and c) heating the material of step (b) in the presence of hydrogen at 200 to 350°C for a period of time sufficient to reduce said cationic noble metal species to dispersed free metal. Step (a) includes contacting the zeolite with a cationic noble metal complex and then calcining the zeolite in the presence of oxygen at 300-375°C to convert a noble metal complex into said cationic species. Catalysts produced by the method of the present invention are particularly useful in any process or combination of processes which employ metal catalyst components such as platinum or palladium, as well as other noble metals. Examples of such processes include hydrogenation, dehydrogenation, dehydrocyclization, isomerization, hydrocracking, dewaxing, reforming, conversion of alkyl aromatics and oxidation. The catalysts produced by the present process have been found to be particularly useful in catalytic dewaxing of distillate fuel oils and gas oils wherein waxy components are isomerized by contact with a zeolite beta catalyst, according to the method disclosed in U.S. Patent 4,419,220. For the purpose of this invention, the term "zeolite" is meant to represent the class of porotectosilicates i.e., porous crystalline silicates that usually contain silicon and oxygen atoms as the major components. Other components may be present in minor amounts, usually less than 14 mole % and preferably less than 4 mole %. These components include aluminum, gallium, iron, boron, phosphorus and the like with aluminum being preferred and used herein for illustration purposes. The minor components may be present separately or in combination. Where the silica to alumina mole ratio referred to herein, it may be determined by conventional analysis. This ratio is meant to represent, as closely as possible, the ratio in the rigid anionic framework of the zeolite crystal and to exclude aluminum in the binder or in cationic or other forms within the channels. Although zeolites with a silica to alumina mole ratio in excess of 12 are useful, it is generally preferred to use zeolites having much higher silica to alumina mole ratios. Thus zeolites useful in the present invention preferably have silica to alumina mole ratios of at least 20 and in some instances at least 100 or even at least 200. The zeolites useful herein have a Constraint Index (C.I.), which is described in U.S. Patent No. 4,016,218, of 1 to 12. The Constraint Index (CI) values for some typical zeolites are: CI (at test temperature) ZSM-4 0.5 (316°C) ZSM-5 6-8.3 (371°C - 316°C) ZSM-11 5-8.7 (371°C - 316°C) ZSM-12 2.3 (316°C) ZSM-20 0.5 (371°C) ZSM-22 7.3 (427°C) ZSM-23 9.1 (427°C) ZSM-34 50 (371°C) ZSM-35 4.5 (454°C) ZSM-38 2 (510°C) ZSM-48 3.5 (538°C) ZSM-50 2.1 (427°C) TMA Offretite 3.7 (316°C) TEA Mordenite 0.4 (316°C) Clinoptilolite 3.4 (510°C) Mordenite 0.5 (316°C) REY 0.4 (316°C) Amorphous Silica-alumina 0.6 (538°C) Dealuminized Y 0.5 (510°C) Erionite 38 (316°C) Zeolite Beta 0.6-2.0 (316°C - 399°C) It should be noted that Constraint Index seems to vary somewhat with severity of operations (conversion) and the presence or absence of binders. Likewise, other variables, such as crystal size of the zeolite, the presence of occluded contaminants, etc., may affect the Constraint Index. Therefore, it will be appreciated that it may be possible to so select test conditions, e.g. temperature, as to establish more than one value for the Constraint Index of a particular zeolite. This explains the range of Constraint Indices for some zeolites, such as ZSM-5, ZSM-11 and Beta. For the purposes of the present invention, a zeolite is considered to have a Constraint Index of 1 to 12 if when tested at at least one temperature within the range of 550°F (290°C) to 950°F (570°C), it manifests a Constraint Index within indicated range of 1 to 12. Preferred zeolites for use in the process of the invention are zeolite beta, ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35 and ZSM-48, with ZSM-5, ZSM-11 and zeolite beta being particularly preferred. Zeolite beta is described in U.S. Patent No. 3,308,069. ZSM-5 is described in U.S. Patents No. 3,702,886 and Re. No. 29,949. ZSM-11 is described in U.S. Patent No. 3,709,979. ZSM-12 is described in U.S. Patent No. 3,832,449. ZSM-23 is described in U.S. Patent No. 4,076,842. ZSM-35 is described in U.S. Patent No. 4,016,245. ZSM-48 is described in U. S. Patent No. 4,234,231. As in the case of many catalysts, it is desirable to incorporate the zeolite with another material resistant to the temperatures and other conditions which will be experienced when the catalyst is in use. Such materials include active and inactive materials as well as inorganic materials such as clays, silica and/or metal oxides. Inactive materials suitably serve as diluents to control the amount of conversion in a given process so that products can be obtained economically without employing other means for controlling rate of reaction. Binders useful for compositing with the useful zeolite herein also include inorganic oxides, notably alumina, which is particularly preferred. In addition to the foregoing material, the zeolite catalyst can be composited with a porous matrix material such as silica-alumina, silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia, silica-titania, as well as ternary compositions such as silica-alumina-thoria, silica-alumina-zirconia, silica-alumina-magnesia and silica-magnesia-zirconia. The relative proportions of finely divided crystalline zeolite and inorganic oxide matrix may vary widely with the zeolite content ranging from 1 to 90 percent by weight, and more usually in the range of 10 to 50 percent by weight of the composite. The noble metal-containing catalyst of the present invention is prepared by bringing a suitable crystalline zeolite, of the nature described above, into contact with a solution of a cationic noble metal species for a sufficient period of time to effect deposition within the crystalline structure of the zeolite of a noble metal-containing cation derived from such a solution, drying the resulting composite and subjecting the same to an activating treatment. Suitable noble metals include platinum, palladium, ruthenium, osmium, iridium and rhodium, with platinum and palladium being preferred. Any soluble, preferably water soluble, noble metal salt can be used as the source of the cationic species but it is preferred to employ a complex cationic species, preferably the platinum tetrammine complex, Pt(NH₃) 2+ / 4 . It is contemplated that water will ordinarily be the solvent in the noble metal-containing solutions used. However, it will be understood that other solvents, although generally less preferred, may also be used. Thus, in addition to aqueous solutions, alcoholic solutions of the noble metal-containing compounds may be employed in the present process. The compounds of the noble metals undergo ionization in the particular solvent used. The concentration of the noble metal compound in the solution employed may vary widely depending on the amount of noble metal desired in the final catalyst composition and on the conditions under which contact between the crystalline zeolite and such solution is effected. The solution of the noble metal compound may be contacted with the crystalline zeolite in either the bound or unbound state. The volume of solution containing noble metal compound may be just sufficient to be adsorbed by the crystalline zeolite. Generally, however, an excess of solution is employed and such excess is removed from contact with the crystalline zeolite after a suitable period of contact and prior to drying of the zeolite. The time of contact between the solution of noble metal compound and crystalline zeolite is such as to effect deposition on the crystalline structure of the noble metal-containing ion derived from such solution. It will be appreciated that such period of contact may vary widely depending on the temperature of the solution, the nature of crystalline zeolite used, the particular noble metal compound employed, and the concentration of noble metal desired in the final catalyst. Thus, the time of contact may extend from a very brief period of the order of minutes for small particles to long periods of the order of days for large pellets. Generally, the time of contact will, depending on the various aforementioned factors, be within the range of 5 minutes to 10 days. The temperature of the solution will ordinarily be room temperature, but may be an elevated temperature below the boiling point of the solution. After the contact period, the crystalline zeolite is removed from the noble metal compound solution. Excess noble metal compound and foreign salt, if employed, are removed, suitably by washing with water. The resulting material is then dried, generally in air, to remove substantially all of the water therefrom. The dried material is then subjected to the activating treatment of the present invention. Where the noble metal is present as a complex cationic species, e.g. Pt(NH₃) 2+ / 4 , the actuation initially involves calcination of the dried material in air or oxygen at 300 to 375°C, preferably 320 to 360° for a time, generally 0.5 - 10 h, to decompose the complex and produce a simple cationic species, which may be at least partly an oxide form. The next step is to heat the material in an inert atmosphere (such as helium or nitrogen) at 200-350°C for 0.5 - 10 hours to purge any oxygen (and concomitantly water) from the zeolite. The purged material is then heated in contact with hydrogen at 200-350°C, preferably 200-300°C, for a time, conveniently 0.5 to 10 hours, to reduce the cationic species to the metal. The resultant catalyst comprises noble metal highly dispersed in the zeolite. The concentration of noble metal in the finished catalyst may vary depending on the use for which such catalyst is intended. The content of noble metal in the finished catalyst is generally within the approximate range of 0.001 to 5 percent by weight, preferably between about 0.05 and 2 weight percent. Noble metal-containing catalysts produced by the process of the present invention are particularly suited to catalytic dewaxing such as the isomerization dewaxing process disclosed in U.S. Patent No. 4,419,220. In this process a hydrocarbon feedstock such as distillate fuel oil is dewaxed by contact with a catalyst comprising zeolite beta having a silica to alumina ratio of at least 20 and a hydrogenation component such as platinum, under isomerization conditions. Isomerization conditions may include temperatures of 200 to 540°C (392 to 1005°F), preferably 400 to 450°C (752 to 842°F), pressures from atmospheric to 25,000 kPa, preferably 4,000 to 10,000 kPa, and a space velocity (LHSV) from 0.1 to 20, preferably from 0.2 to 5. Other suitable processes for the catalysts produced by the present process include dealkylation and isomerization of alkyl aromatics, particularly xylene isomerization and hydroisomerization of ethylbenzene to xylene. Suitable conditions for the latter include a temperature of between 260°C (500°F) and 600°C (1112°F), preferably between 320°C (608°F) and 500°C (932°F), a pressure of between 240 kPa (35 psia) and 7000 kPa (1015 psia), preferably between 275 kPa (40 psia) and 2860 kPa (415 psia), a hydrogen/feed mole ratio of between 1 and 20, preferably between 2 and 8 and a WHSV of between 1 and 50, preferably between 5 and 15. The invention will now be further described by reference to the following Example. EXAMPLE A. A 65 weight percent zeolite beta/35 weight percent Al₂O₃ catalyst containing 0.6 weight percent platinum was prepared by wetting a mixture of 65 weight percent sodium tetraethylammonium zeolite beta having a SiO₂/Al₂O₃ molar ratio of 37 and 35 weight percent Kaiser SA alumina in order to form 0.16cm. diameter extrudate. The extrudate was dried at 149°C (300°F), and then calcined at 540°C (1000°F) in 30 volume/volume/minute N₂. The calcined product was cooled to room temperature and exchanged twice at room temperature in 1N NH₄NO₃, washed with deionized water and then dried at 138°C (280°F). The resulting ammonium exchanged extrudate was steamed for 10.5/h at 540°C (1000°F) in 1 atm steam (27.8 volume/volume/minute flow). The catalyst was heated at 1.4°C/min (2.5°F/min) in air to the steaming temperature of 540°C (1000°F). The alpha value was measured as 47. The steamed extrudate was exchanged for 12 h at room temperature with Pt(NH₃)₄Cl₂ in 7 cm³/g of circulating water and then washed three times with deionized water, drained and then dried at 138°C (280°F). The dried material was calcined for 3 hours at 349°C (660°F) in a mixture of 40% air/60% N₂ (6 volume/volume/minute) having a dewpoint of -2°C (29°F). The material was heated to 349°C (660°F) at a rate of 1°C/minute (2°F/minute). The resulting catalyst showed a C content of 0.1% by weight and less than 10 ppm of N. The catalyst was H₂ reduced using various procedures (see Table), and then characterized using XRD and chemisorption techniques. XRD represents, upon a careful 200°C H₂ reduction of the resulting catalyst, the fraction of Pt whose particle sizes are greater than 50 Angstrom. H/Pt and O/Pt ratios represent the exposed Pt surface area upon which H and O atoms, respectively, chemisorb at 25°C. The sample preparation steps for chemisorption measurements were as follows: Catalysts were first evacuated (0.01 Torr) at ambient temperature for 5 minutes, heated at 10°C/min to 150°C under 0.01 Torr vacuum, then held at 150°C for 5 minutes, and finally evacuated for 20 minutes at 10⁻⁶ Torr. About 1/3 atm of H₂ was introduced at 150°C, and then heated to 200°C where it was kept for 0.5 hours. These treatments were sufficiently mild such that the platinum state was essentially unchanged. Afterwards hydrogen chemisorption isotherms were obtained with the maximum equilibrium hydrogen pressure of 400 Torr. The linear region of the isotherm normally between 40 to 400 torr was extrapolated to 0 Torr. The resulting intercept is the H/Pt ratio. After H chemisorption, catalysts were evacuated (10⁻⁶ Torr) at 200°C to remove chemisorbed H₂. Then the same technique was used to measure oxygen chemisorption. The same extrapolation scheme was subsequently applied to yield the O/Pt ratio. The Table exhibits the hydrogen reduction conditions for the catalyst. During the heat-up, the gas used was either H₂ or N₂, and the gas flows varied from 5 to 15 cm³/min/g catalyst. In the holding period, gas flow was set the same as in the heating period, unless otherwise specified. At ambient temperature H₂ reduction did not alter the state of the small Pt particles, since a sample treated by procedure A (25°C, 40 h) gave the same chemisorption results as the fresh sample. However, at temperatures above 200°C, H₂ reduction started to agglomerate small Pt particles. Thus procedures B (200°C, 2 h) and C (450°C, 2 h) yielded lower chemisorption values (H/Pt 0.64, O/Pt 0.44) and (H/Pt 0.62, O/Pt 0.46), than the fresh catalyst, (H/Pt 1.04, O/Pt 0.51). It was also noticed that the H/Pt ratio was more strongly affected by these treatments than the O/Pt ratio. Chemisorption data from procedures B and C were essentially identical, indicating that the changes in Pt size distribution had occurred at 200°C and no further deterioration was observable after that. Even when the hydrogen partial pressure was reduced from 100 kPa (1 atm) to 10 kPa (0.1 atm) at 200°C in procedure D no improvement was observed. However, procedure E with higher hydrogen partial pressure (30 kPa, 0.3 atm), and higher flow rate gave higher chemisorptions (H/Pt 0.88, O/Pt 0.49). There was no reason that 30 kPa (0.3 atm) H₂ partial pressure was capable of retaining higher chemisorptions than those from 10 kPa (0.1 atm) and 100 kPa (1 atm) H₂ treatments, if Pt agglomeration was hydrogen partial pressure dependent. Thus the only explanation that procedure E gave higher values was its threefold gas flow rate. It also implied that some active species was purged out of catalyst more quickly during the heating period by the high flow rate of procedure E. Since water is a common entrapped species in zeolite containing catalysts, it was decided to investigate whether the presence of water during heat-up of the sample had a strong effect on the platinum size distribution. Procedure F was the same as E except that 2% of water was added to the gas mixture. Indeed, it brought down the chemisorptions to (H/Pt 0.66, O/Pt 0./42), and was similar to those from B through D. Since water seems to promote Pt agglomeration, the next thing to prove was that its absence would stop Pt agglomeration. Procedure G was designed for this purpose. The catalyst was heated to 200°C in N₂ using a high gas flow rate, 12.5 cm³/min/g catalyst, and then switched over at temperature to H₂. It gave very high values (H/Pt 1.33, O/Pt 0.63). In fact, these values were even higher than those from fresh catalyst, (H/Pt 1.04, O/Pt 0.51). Procedure H was a repeat of G, and gave the same results. Thus it appeared that platinum agglomeration was not dependent on the hydrogen partial pressure in the range of 10-100 kPa (0.1 to 1 atm) but may be dependent on the water partial pressure. Procedure I was the same as G except that the H₂ reduction temperature was 450°C. At this temperature H₂ reduction caused some Pt agglomeration even after the removal of physisorbed water in the heat-up. The chemisorption values were H/Pt 0.81 and O/Pt 0.52; but the degree of agglomeration was still lower than that caused by the presence of water. The agglomeration of Pt in procedure I appeared irreversible. A redox treatment (350°C O₂, 200°C H₂), as commonly applied did not recover H/Pt and O/Pt ratios back to 1.33 and 0.63, respectively. After redox treatment, the sample from procedure I gave H/Pt 0.90, and O/Pt 0.46. Procedure J was the same as I except that the H₂ reduction temperature was 350°C. It gave slightly higher values (H/Pt 1.18, O/Pt 0.57) than the catalyst of I, but still lower than the catalyst of G which was reduced at 200°. It was found that the noble metal states, particle size, and locations of fresh noble metal-containing zeolite catalysts were heavily influenced by their pretreatments. In the presence of trace amounts of water, hydrogen reduction agglomerated Pt particles even at temperature as low as 200°C. Cationic Pt located inside the zeolite was highly sensitive to hydrogen reduction. Without careful removal of entrapped water from such catalysts, they can easily lose 50% of initial Pt dispersion.

Claims: A process for preparing a noble-metal-containing zeolite catalyst comprising contacting a zeolite having a constraint index of 1 to 12 with a cationic noble metal complex, calcining the thus contacted zeolite in the presence of oxygen at 300 to 375°C to convert said complex into a cationic noble metal species and contacting the calcined zeolite with hydrogen at 200 to 350°C for a time sufficient to reduce said cationic species to dispersed free metal, characterised in that before the contacting with hydrogen, oxygen is purged from the calcined zeolite by heating the zeolite in the presence of an inert gas at 200 to 350°C. The process of claim 1 wherein said calcining is carried out at 320 to 360°C. The process of any preceding claim, wherein the heating in the presence of an inert gas is carried out at 200-300°C. The process of any preceding claim wherein said zeolite is selected from zeolite beta, ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35 and ZSM-48. The process of any preceding claim wherein the noble metal is selected from platinum, palladium, rhodium, ruthenium, osmium and iridium.

Inventors: TSAO, YING-YEN PETER
Applicants: MOBIL OIL CORPORATION
Application year: 1988
Main classification: B01J29
IPC codes: C10G45, B01J29, C07C1, C07B61, C07C67, C07C15, C07C4, C07C5