Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
2000-07-17
2003-06-03
Wood, Elizabeth D. (Department: 1755)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S008000, C502S325000, C502S326000, C502S327000, C502S328000, C502S329000, C502S527240
Reexamination Certificate
active
06573213
ABSTRACT:
INTRODUCTION AND BACKGROUND
The present invention relates to metal catalysts.
Activated metal catalysts are known in the field of chemical engineering as Raney catalysts. They are used, largely in powder form, for a large number of hydrogenation, dehydrogenation, isomerization and hydration reactions of organic compounds. These powdered catalysts are prepared from an alloy of a catalytically-active metal, also referred to herein as a catalyst metal, with a further alloying component which is soluble in alkalis. Mainly nickel, cobalt, copper, or iron are used as catalyst metals. Aluminum is generally used as the alloying component which is soluble in alkalis, but other components may also be used, in particular zinc and silicon or mixtures of these with aluminum.
These so-called Raney alloys are generally prepared by the ingot casting process. In that process a mixture of the catalyst metal and, for example, aluminum are first melted and casted into ingots. Typical alloy batches on a production scale amount to about ten to one hundred kg per ingot. According to DE 21 59 736 cooling times of up to two hours were obtained. This corresponds to an average rate of cooling of about 0.2/s. In contrast to this, rates of 102 to 106 K/s are achieved in processes where rapid cooling is applied (for example an atomizing process). The rate of cooling is affected in particular by the particle size and the cooling medium (see Materials Science and Technology edited by R. W. Chan, P. Haasen, E. J. Kramer, Vol. 15, Processing of Metals and Alloys, 1991, VCH-Verlag Weinheim, pages 57 to 110). A process of this type is used in EP 0 437 788 B 1 in order to prepare a Raney alloy powder. In that process the molten alloy at a temperature of 50 to 500° C. above its melting point is atomized and cooled using water and/or a gas.
To prepare a catalyst, the Raney alloy is first finely milled if it has not been produced in the desired powder form during preparation. Then the aluminum is entirely or partly removed by extraction with alkalis such as, for example, caustic soda solution. This activates the alloy powder. Following extraction of the aluminum the alloy power has a high specific surface area (BET), between 20 and 100 m
2
/g, and is rich in active hydrogen. The activated catalyst powder is pyrophoric and stored under water or organic solvents or is embedded in organic compounds which are solid at room temperature.
Powdered catalysts have the disadvantage that they can be used only in a batch process and, after the catalytic reaction, have to be separated from the reaction medium by costly sedimentation and/or filtration. Therefore a variety of processes for preparing molded items which lead to activated metal fixed-bed catalysts after extraction of the aluminum have been disclosed. Thus, for example, coarse particulate Raney alloys, i.e., Raney alloys which have only been coarsely milled, are obtainable and these can be activated by a treatment with caustic soda solution. Extraction and activation then occurs only in a surface layer the thickness of which can be adjusted by the conditions used during extraction.
A substantial disadvantage of catalysts prepared by these prior methods are the poor mechanical stability of the activated outer layer. Since only this outer layer of the catalysts is catalytically active, abrasion leads to rapid deactivation and renewed activation of deeper lying layers of alloy using caustic soda solution then leads at best to partial reactivation.
Patent application EP 0 648 534 B1 describes shaped, activated Raney metal fixed-bed catalysts and their preparation. These catalysts avoid the disadvantages described above, e.g., the poor mechanical stability resulting from activating an outer layer. To prepare these catalysts, a mixture of powders of a catalyst alloy and a binder are used. In the catalyst alloys each contain at least one catalytically active catalyst metal and an extractable alloying component. The pure catalyst metals or mixtures thereof which do not contain extractable components are used as binder. The use of the binder in an amount of 0.5 to 20 weight percent with respect to the catalyst alloy, is essential in order to achieve sufficient mechanical stability after activation.
After shaping the catalyst alloy and the binder with conventional shaping aids and pore producers, the freshly prepared items which are obtained are calcined at temperatures below 850° C. As a result of sintering processes in the finely divided binder, this produces solid compounds between the individual granules of the catalysts alloy. These compounds, in contrast to catalyst alloys, are non-extractable or only extractable to a small extent so that a mechanically stable structure is obtained even after activation.
However, the added binder has the disadvantage that it is substantially catalytically inactive and thus the number of active centers in the activated layer is reduced. In addition, the absolutely essential use of a binder means that only restricted range of amounts of pore producers can be used without endangering the strength of the shaped item. For this reason, the bulk density of these catalysts cannot be reduced to a value of less than 1.9 kg per liter without incurring loss of strength. This leads to a considerable economic disadvantage when using these catalysts in industrial processes.
In particular when using more expensive catalysts alloys, for example cobalt alloys, the high bulk density leads to a high investment per reactor bed, which is, however partly compensated for by the high activity and long-term stability of these catalyst. In certain cases, the high bulk density of the catalyst also requires a mechanically reinforced reactor structure.
An object of the present invention is therefore to provide activated base metal catalysts from hollow metallic forms which largely avoids the disadvantages of the above known fixed-bed catalysts.
SUMMARY OF THE INVENTION
The above and other objects of the invention can be achieved by producing hollow forms out of the desired alloys and activating then in order to make the catalyst. The major advantages of this invention are the low bulk density and high activity per gram of metal exhibited by these catalysts.
It is a feature of the present invention that the metal catalyst are hollow forms. Preferably the hollow forms are hollow spheres. These spheres can exhibit a diameter of 0.5 to 20 mm and a wall thickness of 0.1 to 5 mm.
The shell of the spheres can be impermeable or it can show an open porosity up to 80%.
The shell of the spheres can consist of different layers and/or the metal can be graduated.
The metal catalysts comprising hollow forms can be activated.
Another feature of the present invention is a process for the for the production of the metal catalysts comprising spraying of metal powders, optionally together with a binder on to forms consisting of a burnable or combustible material; i.e. polystyrene foam (styrofoam), and burning out the material to obtain the hollow form.
In another feature of the present invention, the process for the production of the metal catalysts is carried out by using metal powders consisting of a rapidly cooled alloy. The rapidly cooled alloy can be made according to commonly used methods such spray drying in various atmospheres as well as rapidly cooling in liquids such as water. The hollow form consisting of the alloy and optionally a binder can then be activated with an alkali solutions such as aqueous NaOH, to form the activated catalyst.
One of the metal powders can consist of a slowly cooled alloy. The hollow form consisting of the alloy and optionally a binder can then be activated with an alkali solution, such as an aqueous NaOH solution, to form the activated catalyst; i.e. the activated state. Prior to activation, the catalyst can be referred to as being in the unactivated form.
In the process for the production of the metal catalysts the alloy can consist of one or more catalytic metal such as nickel, iron, copper, palladium, ruthenium, and cobalt; an alkali soluble all
Berweiler Monika
Ostgard Daniel
Panster Peter
Rehren Claus
Schneider Lothar
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