Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
2001-07-16
2003-03-04
Silverman, Stanley S. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S327000, C502S328000, C502S330000, C502S331000, C502S332000, C502S333000, C502S339000, C502S527120, C502S527130, C502S104000
Reexamination Certificate
active
06528453
ABSTRACT:
BACKGROUND OF THE INVENTION
Supported metal catalysts are typically made by impregnating a suitable support material with a catalytically active metal or with its precursor. For example, catalysts for use in the production of vinyl acetate monomer (VAM) by the reaction of ethylene, acetic acid and oxygen are made by impregnating a support such as silica or alumina with a compound of a Group VIII noble metal such as palladium together with a gold compound and an alkali metal salt, typically in the form of an acetate, the palladium and gold compounds being converted to catalytically active state.
In early examples of fixed-bed catalysts for use in the production of VAM, palladium and gold were distributed more or less uniformly throughout the support, for example, U.S. Pat. No. 3,743,607. Since gaseous reactants do not diffuse significantly into the large fixed-bed catalyst particles, much of the expensive catalytic metal components in the interior of the catalyst were not useful. Subsequently, shell-impregnated, fixed-bed catalysts were developed in which most of the catalytic metals were deposited onto an outer shell of the support particle. For example, Great Britain Patent No. 1,500,167 describes a catalyst in which at least ninety percent of the palladium and gold is distributed in that part of the support particle which is not more than thirty percent of the particle radius from the surface. The palladium and gold being at
ear the surface are susceptible to loss through attrition.
In the preparation of shell-impregnated, fixed bed catalysts such as that described in GB 1,500,167 and EP-A-0 569 624, after impregnation of a support with a Group VIII noble metal solution, the noble metal is subsequently precipitated to the support by, for example, treatment with an aqueous solution of an alkali metal salt. Such precipitated noble metal has limited mobility.
U.S. Pat. No. 4,677,084 describes a process for preparing attrition resistant catalyst, catalyst precursor and catalyst support particles and in particular silica-containing vanadium/phosphorus oxide catalysts. The catalyst, catalyst precursor or catalyst support particles are slurried in a solution of an oxide such as silica. The slurry is then spray-dried and calcined to produce microspheres. The process results in the formation of an oxide-rich layer at the periphery of each calcined microsphere.
WO 99/62632 describes preparation of a vinyl acetate catalyst comprising palladium and gold in which it is said that the catalyst contains palladium and gold distributed in a thin shell at or near the surface of the catalyst support. The preparation involves a base fixing step for the palladium but not the gold.
GB 1,521,652 relates to a noble metal-containing catalyst which comprises a mixture of palladium and gold as noble metals, and a support material, and having (a) an outer layer of low or zero noble metal content, (b) an inner shell rich in noble metal and (c) a core having a low or zero noble metal content. According to GB 1,521,652 the geometrical dimensions of the support can, for example, be in the range 1-8 mm. In the preparation of these catalyst materials the carrier material is impregnated with a solution containing palladium salts and gold salts, dried and then an aqueous alkaline solution is added to convert the noble metal salts to water insoluble compounds. After an optional wash and/or drying the material is treated with a reducing agent.
U.S. Pat. No. 4,048,096 relates to catalysts for the preparation of vinyl esters and particularly to palladium-gold catalysts. According to U.S. Pat. No. 4,048,096 reproducing examples of U.S. Pat. No. 3,775,342 produced catalyst with an interior band of palladium-gold alloy deposited on the catalyst support. In the Comparative Example described therein, a catalyst was prepared having palladium and gold deposited in a narrow interior band of approximately 0.1 to 0.2 mm thickness, located approximately 0.5 mm below the surface. The preparation involved treatment with base. U.S. Pat. No. 3,775,342 also describes the use of base to form insoluble noble metal compounds as part of the catalyst preparation process.
There remains a need for an improved metal catalyst composition and in particular, a supported metal catalyst composition.
BRIEF DESCRIPTION OF THE INVENTION
Thus, according to the present invention there is provided a process for preparing a supported metal catalyst composition which process comprises impregnating microspheroidal support particles with a solution of at least one catalytically active metal, or precursor thereof, such that the metal, or its precursor, is in a mobile state in the support particles, drying the impregnated support particles and then treating the mobile metal, or precursor, in the support particles with a liquid comprising at least one reducing agent to deposit and immobilize the metal, or its precursor, in the support particles such that the metal, or its precursor, is distributed in the support particle in a layer below the surface of said support particle, said layer being between an inner and an outer region, each of said inner and outer regions having a lower concentration of said metal or precursor than said layer.
Also according to the present invention there is provided a catalyst composition comprising microspheroidal support particles having at least one catalytically active metal distributed therein, in which the metal is distributed in the support particle in a layer below the surface of said particle, said layer being between an inner and an outer region of said support particle, and each of said inner and outer regions having a lower concentration of said metal than said layer.
Also, according to the present invention there is provided a composition comprising microspheroidal support particles having at least one precursor of a catalytically active metal distributed therein, in which the precursor is distributed in the support particle in a layer below the surface of said particle, said layer being between an inner and an outer region of said support particle, and each of said inner and outer regions having a lower concentration of said precursor than said layer.
The process of the present invention prepares catalyst compositions which can provide high attrition resistance as well as high activity. The outer region of the catalyst composition may also provide some resistance to poisoning of the catalytically active metal.
An advantage of the process of the present invention is that by treating the dried microspheroidal support particles impregnated with a catalytically active metal, or its precursor, which is in a mobile state in the support particle, with a liquid comprising at least one reducing agent which deposits and immobilizes them, the metal, or its precursor, is distributed predominantly in a layer below the surface of the particle such that the catalyst composition so produced has high attrition resistance as well as high activity.
Preferably, the concentration of catalytically active metal or of its precursor in each of the inner and outer regions is less than half the concentration of the catalytically active metal or of its precursor in the layer.
In a preferred embodiment, the layer containing the catalytically active metal, or its precursor, has an outer edge which is at least 3% and no more than 75% of the particle radius from the surface of the support particle and preferably, at least 5%, and more preferably at least 10% of the particle radius from the surface of the support particle.
Depending upon the size of the support particles, alternatively or additionally, the layer containing the catalytically active metal, or its precursor, preferably has an outer edge which is at least 3 microns and no more than 20 microns below the surface of each support particle, and is more preferably 4 to 20 microns below the surface of each particle, and yet more preferably is 5 to 15 microns below the surface of each particle.
Typically, the layer has an average thickness which is less than half the radius of the particle,
Baker Michael James
Couves John William
Griffin Kenneth George
Johnston Peter
McNicol James Colin
BP Chemicals Limited
Nguyen Cam N.
Nixon & Vanderhye
Silverman Stanley S.
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