Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
2000-07-20
2002-02-12
Geist, Gary (Department: 1623)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S302000, C502S326000, C502S330000, C502S339000, C502S340000, C502S522000, C502S245000
Reexamination Certificate
active
06346501
ABSTRACT:
The present invention relates to a catalyst which comprises palladium and/or its compounds, cadmium compounds, alkali metal compounds and at least one lanthanoid metal compound, and to its use for preparing vinyl acetate from acetic acid, ethylene and oxygen or oxygen-containing gases.
It is known that ethylene can be reacted with acetic acid and oxygen or oxygen-containing gases in the gas phase on palladium/cadmium/alkali metal-containing fixed bed catalysts to give vinyl acetate. According to U.S. Pat. No. 4,902,823, U.S. Pat. No. 3,939,199, U.S. Pat. No. 4,668,819, the catalytically active metal salts are applied to the catalyst carrier by impregnation, spraying on, vapor deposition, immersion or precipitation. The preparation of a palladium, cadmium and potassium-containing catalyst is also known, entailing a carrier material which has been provided with a binder, for example an alkali metal or alkaline earth metal carboxylate, being washed before the impregnation with an acid and being treated after the impregnation with a base (EP-A-0 519 435).
EP-A-0 634 209 discloses the preparation of palladium, cadmium and potassium-containing catalysts by the carrier particles being impregnated by being intimately mixed with a solution of palladium, cadmium and potassium salts and then being immediately dried, the dynamic viscosity of the solution being at least 0.003 Pa·s and the solution volume for the impregnation being 5 to 80% of the pore volume of the carrier particles. EP-A-0 634 208 discloses the possibility of using a solution volume which is more than 80% of the pore volume of the carrier particles for the impregnation. However, with this procedure it is necessary to select a time before starting the drying which is so short that, after the end of the drying, a shell of 5 to 80% of the pore volume comprises said metal salts.
Palladium, cadmium and potassium-containing catalysts can also be prepared by the process disclosed in EP-A-0 634 214 by spraying the carrier particles while being intimately mixed with a solution of palladium, cadmium and potassium salts in the form of drops with an average diameter of at least 0.3 mm or in the form of liquid jets, and then immediately drying them, the dynamic viscosity of the solution being at least 0.003 Pa·s, and the solution volume in the spraying being 5 to 80% of the pore volume of the carrier particles.
The PCT application WO 96/37455 discloses that catalysts of this type can be considerably improved by adding at least one rhenium and/or at least one zirconium compound. Thus, a palladium, cadmium, potassium-containing shell catalyst shows a space-time yield (gram of vinyl acetate formed per liter of catalyst and hour) of 922 (g/l·h), whereas an initial productivity of 950 g/l·h is observed after addition of zirconium under conditions which are otherwise the same.
It has now been found, surprisingly, that palladium, cadmium and potassium-containing catalysts can be distinctly improved by adding at least one lanthanoid metal compound, i.e. they afford a higher space-time yield with identical or greater selectivity for vinyl acetate.
The invention accordingly relates firstly to a process for preparing vinyl acetate in the gas phase from ethylene, acetic acid and oxygen or oxygen-containing gases on a catalyst which comprises palladium and/or its compounds, cadmium compounds and alkali metal compounds on a carrier, wherein the catalyst additionally comprises at least one lanthanoid metal compound.
The invention relates secondly to a catalyst which comprises palladium and/or its compounds, cadmium compounds and alkali metal compounds on a carrier, wherein the catalyst additionally comprises at least one lanthanoid metal compound.
The term “lanthanoid metals” means the 14 rare earth elements cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium, and the elements scandium, yttrium and lanthanum because their chemical behavior resembles that of the rare earth elements.
Suitable carriers are the known inert carrier materials such as silica, alumina, aluminosilicates, silicates, titanium oxide, zirconium oxide, titanates, silicon carbide and carbon. Particularly suitable carriers of this type are those with a specific surface area of 40 to 350 m
2
/g (measured by the BET method) and an average pore radius of 50 to 2000 Å (Angstrom) (measured by mercury porosimetry), especially silica (SiO
2
) and SiO
2
/Al
2
O
3
mixtures. These carriers can be used in any form such as, for example, in the form of beads, tablets, rings, stars or particles of other shapes, with a diameter or length and thickness generally of 3 to 9 mm.
Carriers of these types can be prepared, for example, from aerogenic SiO
2
or an aerogenic SiO
2
/Al
2
O
3
mixture which can be prepared, for example, by flash hydrolysis of silicon tetrachloride or a silicon tetrachloride/aluminum trichloride mixture in an oxyhydrogen flame (U.S. Pat. No. 3,939,199).
Suitable solvents for the palladium, cadmium, alkali metal and lanthanoid metal salts are all compounds in which the selected salts are soluble and which can easily be removed again after the impregnation by drying. Suitable for the acetates, if they are used, are in particular, unsubstituted carboxylic acids having 2 to 10 carbon atoms such as acetic acid, propionic acid, n- and iso-butyric acid and the various valeric acids. Among the carboxylic acids, acetic acid is preferred because of its physical properties and for economic reasons. Water is particularly suitable for the chlorides and chloro and acetato complexes. Additional use of another solvent is expedient if the salts are insufficiently soluble in acetic acid or in water. Thus, for example, palladium chloride can be dissolved considerably better in an aqueous acetic acid than in glacial acetic acid. Suitable additional solvents are those which are inert and are miscible with acetic acid or water. Those which may be mentioned as additions for acetic acid are ketones such as acetone and acetylacetone, also ethers such as tetrahydrofuran or dioxane, but also hydrocarbons such as benzene.
It is possible to apply a plurality of salts of palladium, cadmium, alkali metal and the particular lanthanoid metal, but generally exactly one salt of each of these elements is applied.
It is possible to prepare either so-called “fully impregnated” catalysts where the catalytically active metal compounds have penetrated into the carrier particles as far as the core, or else so-called “shell catalysts” where the metal salts have advanced only into an outer part, of variable size, of the carrier particles, i.e. the so-called “shell” of the particles, and not as far as the core.
The elements palladium, cadmium, alkali metal and lanthanoid metal to be applied in each case can be applied in the form of salt solutions singly or else in any suitable combination in any suitable sequence, preferably using a single solution which contains these elements to be applied in the form of salts. It is particularly preferred to use a single solution which contains exactly one salt of each of these elements to be applied. This solution may moreover contain a mixture of salts of at least two different lanthanoid metals, but this solution preferably contains one salt of only one lanthanoid metal.
Where the following speaks generally of “the solution of the salts”, the same applies analogously to the case where a plurality of solutions are employed in sequence, each of which contains only part of the totality of salts to be applied, in which case the total of the individual parts amounts to the total quantity of salts to be applied to the carrier.
The procedure for preparing fully impregnated catalysts is preferably as follows (U.S. Pat. No. 4,902,823, U.S. Pat. No. 3,393,199, U.S. Pat. No. 4,668,819):
The catalyst carrier is impregnated with the solution of the active components in such a way that the carrier material is covered with the solution and, where appropriate, excess solution is then poured off
Herzog Bernhard
Nicolau Ioan
Wang Tao
Bierman, Muserlian and Lucas
Celanese Chemicals Europe GmbH
Deemie Robert W.
Geist Gary
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