Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
2000-07-26
2003-03-18
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, C252S520300
Reexamination Certificate
active
06534438
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a metal catalyst composition and in particular to a supported metal catalyst composition. The present invention also relates to a process for making such a catalyst composition
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 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 pant 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.
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 catalyst composition comprising 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.
The catalyst composition 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.
The present invention also provides a process for preparing a supported metal catalyst composition which process comprises impregnating support particles with a solution of least one catalytically active metal, or precursor thereof, such that the metal, or its precursor, is in a mobile state in the support particles and then treating the mobile metal, or precursor, in the support particles with at least one chemical reagent 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 composition comprising 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.
An advantage of the process of the present invention is that by treating a catalytically active metal, or its precursor, which is in a mobile state in the support particle with at least one chemical reagent which deposits and immobilizes it, 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, for example less than 25 microns. Preferably, the layer has an average thickness of greater than 0.1 microns.
The process for preparing the catalyst composition of the present invention may be used for the preparation of catalysts for use in fixed bed or preferably, fluid bed processes, for example, for the production of vinyl acetate monomer.
A suitable support material for use in a fluid bed process is a microspheroidal particulate material. When the catalyst composition is to be used in a fluid bed process, as is well known in the fluid bed art, the support particles must be small enough to be maintained in a fluid bed state under reaction conditions while keeping sufficient attrition resistance such that excessive amounts of catalyst composition need not be replenished during the process. Further, although typical particle sizes (as measured by mean particle diameters) should not be so large as to be difficult to keep in a fluid bed state, there should not be an excessive amount of very small particles (fines) which are difficult to remove from the system and may plug gas recycle lines. Thus, typically suitable fluid bed support particles have a distribution of larger to smaller particle sizes.
For example, in the fluid bed manufacture of vinyl acetate from ethylene, acetic acid and oxygen-containing gas, typically, at least 80% and preferably at least 90% of the support particles have mean diameters of less than about 300 microns.
A typical catalyst useful in this invention may have the following particle size distribution:
0 to 20 microns
0-30 wt %
0 to 44 microns
0-60 wt %
44 to 88 microns
10-80 wt %
88 to 106 microns
0-80 wt %
>106 microns
0-40 wt %
>300 microns
0-5 wt %
Persons skilled in the art will recognize that support particles sizes of 44, 88, and 300 micr
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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