PROCESS FOR DISPERSING OR REDISPERSING A GROUP VIII NOBLE METAL SPECIES ON A POROUS INORGANIC SUPPORT

Abstract: In a process for dispersing or redispersing relatively large crystallites of Group VIII noble metal species on a porous inorganic support, the supported species is contacted with nitric oxide, either alone or in admixture with a source of halogen such as Cl₂, and thereafter any sorbed nitrogen oxide(s) and halogen are removed by purging with an inert gas. The treated metal-loaded catalyst demonstrates substantially increased benzene hydrogenation activity (BHA) compared to the same catalyst prior to treatment.

Description: This invention relates to a process for dispersing or redispersing a catalytically active noble metal species on a porous inorganic support which itself may or may not possess catalytic activity, e.g., dispersing or redispersing palladium or platinum on a zeolite catalyst such as ZSM-5 which has become deactivated due to the accumulation of carbonaceous material e.g., coke) during the course of its use in a hydrocarbon conversion operation such as catalytic dewaxing. Porous inorganic acidic oxides are used extensively in the petroleum and petrochemicals industry to catalyze a variety of hydrocarbon conversions, including catalytic cracking, hydrocracking, naptha reforming, benzene alkylation, xylene isomerization and catalytic dewaxing. During use the catalysts undergo progressive loss of catalytic activity and/or selectivity. The time required for the activity to decay to the point at which the catalyst is no longer useful may vary from as little as a few minutes, as in catalytic cracking, to several years, as with some versions of naptha reforming. Some of the factors which affect the aging rate include the nature of the feed, the nature of the catalyst and process conditions. In general, catalyst deactivation is accompanied by an accumulation of carbonaceous matter on the catalyst, and it is well known to regenerate deactivated catalysts by burning the carbonaceous matter in an oxygen-containing gas. In the case of metal-loaded catalysts, the severity of the burning operation often leads to agglomeration of the metal component into relatively large crystallites which, of course, are inherently less active than small crystallites which possess a greater surface area for an equivalent amount of metal. Reactivation of noble metal catalysts utilized in hydrocarbon processing procedures, such as reforming, is known in the art. For example, U.S. Patent No. 2,906,702 discloses a method of restoring the activity of an alumina-supported platinum catalyst, after deactivation occurring during the reforming of hydrocarbons, by contacting the catalyst with gaseous chlorine, fluorine, or other halogen or halogen-affording substance at an elevated temperature. U.S. Patent No. 3,134,732 describes a method for reactivating a noble metal catalyst supported on alumina by contacting the catalyst with halogen-containing gas, stripping excess halogen therefrom and subjecting the resulting catalyst to a reduction step with a hydrogen-containing gas. This treatment is intended to break up large noble metal crystallites into smaller crystallites. U.S. Patent No. 3,201,355 discloses reactivating a deactivated metal oxide-supported noble metal catalyst utilized in hydroforming processes. Reactivation is accomplished under anhydrous conditions employing a gaseous source of halogen such as chlorine (which is preferred) or nitrosyl chloride admixed with an oxygen-containing gas such as air or an inert gas such as nitrogen or carbon dioxide as the reactivating agent. U.S. Patent 3,625,860 discloses a process for activating and/or reactivating a platinum on alumina reforming catalyst by contacting the catalyst which has been previously subjected to a sequence of oxidative burn-off, oxygen treatment, purging and reducing operations with a nonmetallic chloride-containing compound, e.g., an organic chloride such as tertiary butyl chloride, propylene dichloride, carbon tetrachloride, etc., or an inorganic chloride such as hydrogen chloride. It is also known in the art to regenerate platinum group metal-containing zeolite catalysts. Regeneration of noble metal-loaded zeolite catalysts requires certain procedural modifications because the metal must be retained in a dispersed form within the zeolite pores. U.S. Patent No. 3,986,982 describes a procedure in which deactivated platinum group metal-loaded zeolite is contacted with a stream of inert gas containing fr m 0.5 to 20 percent volume of free oxygen and from 5 to 500 ppm volume of chloride as chlorine, HCl, or an organic chlorine-containing material. The resulting catalyst is purged to remove residual oxygen and chlorine and then reduced in a stream of hydrogen at 200o to 600°C. Other processes for regenerating or otherwise treating metal-loaded zeolites, most of which feature the use of molecular chlorine or other source of chlorine, are described in U.S. Patent Nos. 3,943,052; 4,444,895; 4,444,897; 4,447,551; 4,517,076; 4,518,708; 4,600,700; 4,645,751; and, 4,657,874; U.K. Patent Publication No. 2,106,413 and European Patent Publication No. 142,352. Processes for treating catalysts with an oxide of nitrogen are also known. U.S. Patent No. 3,243,383 discloses a process for regenerating a spent cobalt oxide on carbon catalyst, useful in olefin polymerization, wherein the spent catalyst, following heating at 250-1000°C in an inert atmosphere and cooling, is treated with nitric acid, nitric oxide (NO), nitrogen dioxide gas (NO2) or mixtures thereof and thereafter ammoniated, if desired, and finally heated to reactivation temperature. U.S. Patent No. 3,451,942 describes a process for the rejuvenation of a deactivated hydrocracking catalyst containing a hydrogenation-dehydrogenation component present in the form of large crystallites on an acidic cracking component, e.g., an arsenided-nickel-on-fluorided-silica-alumina catalyst, in which the deactivated catalyst, following removal of at least a major part of the accumulated carbonaceous matter therefrom, is treated with a nitrogen oxide, optionally in aqueous nitric acid solution, under conditions causing the material to react with the hydrogenation-dehydrogenation component. Thereafter, the catalyst is treated with oxygen and then reduced with hydrogen. It is hypothesized that the hydrogenation-dehydrogenation component is sequentially converted in this series of operations to a salt, possibly a nitrate, then, following the treatment with oxygen, into an ionic form and finally, following reduction with hydrogen, into small crystallites. According to the process for regenerating a supported tellurium and/or tellurium compound-containing catalyst disclosed in U.S. Patent No. 3,536,631, the deactivated catalyst is treated at 50-400°C with a gaseous nitrogen oxide of the formula NOx in which x is 1, 1.5, 2 or 2.5 and/or a gaseous oxyacid of nitrogen of the formula HNOy in which y is 2 or 3. Deactivated phosphomolybdic acid based catalysts which are used for the conversion of saturated and unsaturated aldehydes to acids are reactivated according to the process of U.S. Patent No. 4,471,062 by feeding an oxide of nitrogen, preferably nitric oxide (NO), over the deactivated catalyst at 100-400°C. Che et al., "A Study of the Chemisorption of Nitric Oxide on pdY Zeolite. Evidence For a Room Temperature Oxidative Dissolution of Pd Crystallites", J. Phys. Chem. 80, 2371-2381 (1976) describes the redispersion of mildly agglomerated palladium, i.e., crystallites of 2.0nm (20 Angstroms), supported on zeolite Y with nitric oxide at room temperature. Foger et al., "The Redispersion of Iridium on SiO2 and Gamma-A1203 Supports with Chlorine-containing Gases", J. Catalysis, 96, 154-169 (1985) discloses that while a gaseous mixture containing a major amount of chlorine and a minor amount of nitric oxide is effective for redispersing iridium on alumina, the mixture is not effective for redispersing iridium on silica. Similarly, Foger et al., "Redispersion of Pt-Ir Supported on Gamma -A1203 and SiO2 in Chlorine-Containing Gases" discloses that the aforesaid chlorine-nitric oxide mixture is not effective for redispersing bimetallic Pt-Ir on silica. In accordance with the present invention, a process is provided for dispersing or redispersing a Group VIII noble metal species present on a porous inorganic support which comprises: a) contacting a supported Group VIII noble metal species possissing an average crystallite diameter of at least 2.5 nm (25 Angstroms) with gaseous nitric oxide (NO) at a temperature of 40-500°C, the support being a porous inorganic material from which at least a major amount of any carbonaceous matter which may have been present thereon has been removed prior to contacting with nitric oxide b) removing sorbed nitrogen oxide(s) from the supported dispersed/redispersed noble metal species. The process of this invention can be used to initially disperse the noble metal species when preparing the fresh catalyst as well as to redisperse the agglomerated noble metal species component of a "deactivated catalyst", i.e., a catalyst of diminished activity compared to that of the freshly prepared catalyst, resulting from the use of the catalyst in a chemical conversion process. Dispersion or redispersion effects a reduction in the average diameter, or size, of the noble metal crystallites. For example, when applied to a supported noble metal species possessing an initial average crystallite size of from 10 to 100 nm (100 to 1000 Angstroms), i.e., a relatively highly agglomerated metal component, the process of this invention can readily provide a dispersed/redispersed metal species whose average crystallite size has been reduced by a factor of up to 10. The dispersion/redispersion process of this invention is carried out upon a supported Group VIII noble metal species, e.g., a metal such as platinum, palladium, iridium, osmium, rhodium or ruthenium. The noble metal species may be present in elemental form or as a compound, e.g., as an oxide or sulfide. Alloys of two or more noble metal species can also be used. The noble metal species can also be associated with a non-noble metal component present as a promoter. The latter can be selected from Group IB, Group IVB, Group VIIA, and non-noble Group VIII metals of the Periodic Table. Generally, the catalyst treated by the present invention possesses a noble metal content ranging from 0.01 to 10, and preferably 0.1 to 3, weight percent. The porous inorganic support can be any of those encountered in the catalyst art and include materials such as alumina and silica and other highly siliceous materials, both of the amorphous and crystalline variety. Included among the latter are the crystalline zeolites, which may or may not be composited with one or more inorganic catalytically active or inactive refractory binders. The zeolites constitute an especially prominent class of catalytically active support which are suitable for the practice of this invention. Included among the useful zeolites are zeolite A (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; Re. 28,341), zeolite XK-4 (U.S. Patent No. 3,314,752), zeolite ZSM-5 (U.S. Patent No. 3,702,886; Re. 29,948), 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) and zeolite ZSM-23 (U.S. Patent No. 4,076,842), merely to name a few. When, in accordance with the process of this invention, it has become apparent that the supported noble metal catalyst composition has become deactivated, e.g., as a result of its use in a hydrocarbon processing operation, the operation is terminated by discontinuing the flow of feedstock to the reactor. It is preferred that the reactor be purged of feedstock by feeding a gaseous reducing agent, preferably dry hydrogen, therethrough. It is contemplated that temperatures of from 250°C to 550°C and pressures ranging from atmospheric to the operating pressure of the process, e.g., up to 7000 kPa (70 atmospheres), can be used in this operation. Purging the reactor with hydrogen under these conditions simultaneously effects stripping of the catalyst and reduction of any oxidized metal component thereon to the zerovalent state. Of course, reduction of oxidized metal component can be accomplished apart from any purging of the reactor. The reactor can also be purged subsequently from an inert gas in order to remove the hydrogen. Suitable purge conditions include temperatures of from 25 to 450°C and pressures of 100 to 4000 kPa (1 to 40 atm), using a stream of an inert gas such as nitrogen. The catalyst is then treated in an oxidizing atmosphere in order to burn off at least a major amount of any carbonaceous deposits which may be present, e.g., coke as well as nitrogen or sulfur compounds. This burning operation is not narrowly critical and suitable conditions range from temperatures of 260°C to 538°C. (500 to 1000°F.) or even higher and oxygen concentrations range from 0.10 to 10 mol percent. The duration of the coke burning step is also not narrowly critical and will obviously vary depending on the temperature, oxygen concentration and the amount of coke on the catalyst. Preferred operation of the coke burning step includes treating a spent catalyst with 0.2 to 7 mol percent of oxygen at temperatures of 370° to 454°C. (700 to 850°F.) and pressures of from 100 to 7000 kPa (1 to 70 atmospheres). Where the support component of the catalyst is a zeolite, the burn-off conditions should not be so severe as to cause any substantial loss in zeolite crystallinity. At this point, any remaining carbon dioxide can be purged from the reactor with an inert gas such as helium or nitrogen. Following any coke burn-off operation, the metal-loaded catalyst is contacted with gaseous nitric oxide (NO), preferably, in combination with a relatively minor amount of a source of gaseous halogen, preferably, chlorine gas. An inert diluent such as helium or nitrogen can also be present. The conditions of this operation can vary widely and include a concentration of nitric oxide of at least about 0.1 volume percent, preferably at least about 1 volume percent and still more preferably at least about 10 volume percent, a concentration of gaseous halogen or source of halogen where utilized, of from 0.01 to 0.5 volume percent, preferably from 0.05 to 4 volume percent and still more preferably from 0.2 to 2 volume percent and a concentration of inert diluent gas, e.g., nitrogen, where utilized representing the balance of the nitric oxide-containing atmosphere. Contacting is carried out at a temperature of from 40 to 500°C, preferably 100-400°C and still more preferably 200-400°C, a pressure ranging 100 to 10000 kPa (1 to 100 atmospheres), preferably from 100 to 5000 kPa (1 to 50 atmospheres) and still more preferably from 100 to 1000 kPa (1 to 10 atmospheres) at a volumetric flow rate of from 100 to 15,000 gaseous hourly space velocity (GHSV), preferably at from 200 to 10000 GHSV and still more preferably at from 600 to 6000 GHSV. In general, contact times of from 10 minutes to 24 hours or more, preferably from 30 minutes to 20 hours and still more preferably from 1 to 16 hours provide acceptable results. Following contact with NO and prior to its use as a catalyst, the supported, dispersed/redispersed noble metal species is purged of sorbed NOx wherein x is 0.5 to 2. This can be readily accomplished by sweeping the catalyst composition with a suitable gaseous purge medium, nitrogen being advantageously employed for this purpose. The purge operation will normally be carried out under conditions effecting the removal of at least a substantial part, and preferably essentially all, of such sorbed nitiogen oxide(s). Helium purge conducted at elevated temperature, e.g., from 90 to 540°C (200 to 1000°F) and preferably from 260 to 480°C (500 to 900°F) for from 10 minutes to 10 hours or more, and preferably from 30 minutes to 2 hours generally provides good results. ' The following examples are illustrative of the process for dispersing/redispersing metal on a porous'inorganic acidic catalyst in accordance with the present invention. In all examples, the treated catalysts were purged with helium at 454°C (850°F) for 1 hour. EXAMPLE 1 A metal-loaded catalyst comprising 0.4 weight percent palladium on a bound ZSM-5 catalyst containing 65 weight percent ZSM-5 and 35 weight percent alumina (Catalyst A) was prepared. The properties of this catalyst are set forth in Table 1 as follows: Table 1: Properties of Fresh Catalyst A Palladium, weight percent 0.35 Sodium, weight percent 0.027 Chlorine, weight percent 0.21 Density, g/cc Packed 0.543 Particle 0.890 Real 2.611 Pore Volume, cc/g 0.741 Surface Area, m²/g 339 A portion of Catalyst A was deactivated in an atmosphere of 80 volume percent hydrogen and 20 volume percent steam at 450°C (840°F) and 100 kPa (1 atm) for 1 hour and purged with helium at the same temperature for 0.5 hour (Catalyst B). Another portion of Catalyst A was used in hydrodewaxing 343°C + (650°F +) petroleum chargestocks at conditions of 2860 kPa (400 psig)H₂, 1 LHSV, 445 Nm³/m³ (2500 scf) H₂/bbl, 282°C (540°F) start-of-cycle temperature and 357°C (675°F) end-of-cycle temperature. The duration of this test was 63 days after which the coked catalyst was removed from the reactor. A sample of the coked catalyst was hydrogen reactivated at 482°C (900°F) for 18 hours and oxygen regenerated, both operations being carried out at atmospheric pressure, in a glass reactor from 400°C (750°F) to a maximum temperature of 454°C (850°F) in increasing oxygen concentration. Elemental analyses of the coked, reactivated, and regenerated catalyst (Catalyst C) are shown in Table 2 as follows: Another portion of the coked catalyst was regenerated at 790 kPa (100 psig), 454°C (850°F) maximum temperature, 7% maximum oxygen concentration and in the presence of 9.3 kPa (70 torr) water vapor. Analysis of the regenerated catalyst (Catalyst D) by scanning scanning transmission electron microscopic (STEM) analysis showed the average size of the agglomerated palladium crystallites to be about 10.0 nm (100 Angstroms) or larger. In still another example, a commercial 0.5% Pd/Al₂O₃ catalyst was deactivated in an atmosphere of 80% hydrogen and 20% steam at 450°C (840°F) for 1 hour (Catalyst E). Catalysts B-E were contacted with 75 volume percent NO and 25 volume percent nitrogen or helium under varying conditions of temperature and time. The redispersion of palladium on Catalysts B-E was monitored by benzene hydrogenation activity (BHA) and STEM analysis. The hydrogenation of benzene to cyclohexane is commonly used to determine catalytic activity of noble metal catalysts. The reaction is generally observed as structure insensitive which means that the specific metal activity is not a function of the size geometry, and orientation of the metal particles on the catalyst. Hence, the test can be used as a measure of the overall extent of metal dispersion. In this test, a gaseous mixture containing 100:1 molar ratio of hydrogen and'benzene is passed through a vertical vycor tubular reactor, having an internal diameter of 0.6 cm (0.25 inch) and a length of 12.7 cm (5 inch) containing about 250 mg of the palladium catalyst at a hydrogen flow rate of 200 cc/min and a total pressure of 100 kPa (1 atm). The temperature range is from 24-150°C (75-300°F) depending on the activity of the catalyst. Before the introduction of benzene, the catalyst is reduced in flowing hydrogen from ambient temperature to a final temperature of 204°C (400°F). The benzene hydrogenation activity of fresh, deactivated, and nitric oxide rejuvenated catalysts are shown in Table 3 below. In all cases, the rejuvenated catalyst samples have higher activity than the deactivated samples. EXAMPLE 2 In place of contacting deactivated Catalysts B-D with NO alone as in Example 1, the catalysts were contacted with a gaseous mixture of 50 volume percent NO, 2 volume percent Cl₂ and 48 volume percent N₂ (diluent) at 200°C (390°F) and a total flow of 100 cc/min for 2.5 hrs. The extent of redispersion as measured by the BHA test is set forth in Table 4 as follows: It may be noted that the BHA, and therefore the extent of palladium dispersion, of the deactivated catalysts exceeded that of the freshly prepared catalyst. EXAMPLE 3 This example compares the effectiveness of NO for redispersing palladium on ZSM-5 with that of NO₂. The conditions of treating regenerated Pd ZSM-5 catalysts with NO, NO₂ or NO/Cl₂ and the BHA levels (mole/mole Pd/hr) of the treated catalysts are set forth in the following table. These data show that under essentially identical treatment conditions employing essentially identical catalysts, nitric oxide, and especially nitric oxide in admixture with chlorine, resulted in significantly greater BHA levels than nitrogen dioxide. EXAMPLE 4 A Pd on Al₂O₃ catalyst had a fresh BHA activity of 57.1 mole/mole Pd/hr. When deactivated in an atmosphere of 80% hydrogen and 20% steam at 426°C (800°F) for 1 hr, the BHA activity diminished to zero. Treatment with oxygen at 593°C (1100°F) for 2.5 hr increased the activity to 14.6 mole/mole Pd/hr. However, treatment with NO at 200°C (390°F) for 2.5 hr followed by helium purge restored the BHA activity to 23.5 mole/mold Pd/hr. EXAMPLE 5 A variety of zeolite-supported platinum catalysts were prepared and treated under the conditions and with the observed BHA levels (mole/mole Pt/hr) shown in the following table.

Claims: 1. A process for dispersing or redispersing a Group VIII noble metal species present on a porous inorganic support which comprises: a) contacting a supported Group VIII noble metal species possessing an average crystallite diameter of at least 2.5nm (25 Angstroms) with gaseous nitric oxide (NO) at a temperature of 40-500°C, the support being a porous inorganic material from which at least a major amount of any carbonaceous matter which may have been present thereon has been removed prior to contacting with nitric oxide; and b) removing nitrogen oxide(s) from the supported dispersed/redispersed noble metal species. 2. The process of Claim 1 wherein the noble metal species is at least one metal, oxide or sulfide of platinum, palladium, iridium, osmium, rhodium or ruthenium. 3. The process of Claim 2 wherein there is additionally present at least one non-noble metal selected from Group IB, Group IVB, Group VIIA or Group VIII of the Periodic Table of the Elements. 4. The process of any preceding claim wherein the porous inorganic support is a zeolite. 5. The process of Claim 4 wherein the zeolite is zeolite L, X, Y, Beta, ZSM-5, ZSM-11, ZSM-23, ZSM-35, or ZSM-48. 6. The process of any preceding claim wherein step(a) is effected with a mixture comprising nitric oxide and a minor amount of a gaseous halogen. 7. The process of Claim 6 wherein the mixture contains 0.01-5 volume percent chlorine. 8. The process of Claim 6 wherein the mixture contains 0.1-2 volume percent chlorine. 9. The process of any preceding Claim wherein said temperature is 200-400°C.

Inventors: HUANG, YUN-YANG, LAPIERRE, RENE BERNARD, MCHALE, WILLIAM DEPAUL
Applicants: MOBIL OIL CORPORATION
Application year: 1988
Main classification: B01J23
IPC codes: B01J29, C10G45, B01J23, B01J38, B01J37