MXPA00005755A - Catalyst based on palladium, cadmium, alkali and lanthanoids and a method for producing vinyl acetate - Google Patents

Abstract

The invention relates to a catalyst which contains palladium and/or compounds thereof, cadmium compounds, alkali metal compounds and at least one lanthanoid metal. The invention also relates to the utilization of the catalyst in order to produce vinyl acetate from acetic acid, ethylene and oxygen or gases containing oxygen.

Description

CATALYST BASED OF PALADIO, CADMIUM. ALKALINE METAL AND LANTANOIDS AND PROCEDURE FOR PREPARING ACETATE OF VINYL DESCRIPTIVE MEMORY The present invention relates to a catalyst comprising palladium and / or its compounds, cadmium compounds, alkali metal compounds and at least one lanthanoid metal compound, and its use to prepare vinyl acetate from acetic acid, ethylene and oxygen or gases that contain oxygen. It is known that ethylene can be reacted with acetic acid and oxygen or oxygen containing gases in the gas phase over fixed bed catalysts containing palladium / cadmium / alkali metal to produce vinyl acetate. According to US-A-4 902 823, US-A-3 939 199, US-A-4 668 819, the catalytically active metal salts are applied to the catalyst carrier by impregnation, spraying, vapor deposition, immersion or precipitation. The preparation of a catalyst containing palladium, cadmium and potassium is also known, comprising a carrier material which has been provided with a binder, for example an alkali metal or alkaline earth metal carboxylate, being washed before impregnation with an acid and then of the impregnation subjected to treatment with a base (EP-A-0 519 435).

EP-A-0 634 209 describes the preparation of catalysts containing palladium, cadmium and potassium by impregnation of carrier particles by intimately mixing them with a solution of palladium, cadmium and potassium salts and then drying them immediately, the dynamic viscosity of the solution being at least 0.003 Pa s and the volume of solution for impregnation being from 5 to 80% of the pore volume of the carrier particles. EP-A-0 634 208 describes the possibility of using a volume of solution that is more than 80% of the pore volume of the carrier particles for impregnation. However, with this method it is necessary to select a time before starting drying that is so short that, after the end of drying, a shell of 5 to 80% of the pore volume comprises said metal salts. The catalysts containing palladium, cadmium and potassium can also be prepared by the process described in EP-A-0 634 214 by spraying the carrier particles at the same time that they are 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 drying them immediately, the dynamic viscosity of the solution being at least 0.003 Pa s, and the volume of solution in the spray being of 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 compound and / or at least one zirconium compound. Therefore, a shell catalyst containing palladium, cadmium, potassium shows a space-time yield (gram of vinyl acetate formed per liter of catalyst and hour) of 922 (g / lh), at the same time observed an initial productivity of 950 g / lh after the addition of zirconium under conditions that are otherwise the same. It has now surprisingly been found that catalysts containing palladium, cadmium and potassium can be markedly improved if at least one lanthanide metal compound is added, ie they achieve a higher space-time yield with identical or greater selectivity for vinyl acetate. Also, the invention firstly relates to a process for preparing vinyl acetate in the gas phase from ethylene, acetic acid and oxygen or oxygen-containing gases on a catalyst comprising palladium and / or its compounds, compounds of cadmium and alkali metal compounds on a carrier, wherein the catalyst additionally comprises at least one lanthanide metal compound. Secondly, the invention relates to a catalyst comprising palladium and / or its compounds, cadmium compounds and alkali metal compounds on a carrier, wherein the catalyst additionally comprises at least one lanthanide metal compound.

The term "lantanoid metals" refers to the 14 elements of rare earths, cerium, praseodymium, neodymium, promised, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium, and the elements scandium, yttrium and lanthanum since its chemical behavior resembles that of rare earth elements. Suitable carriers are 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 m2 / g (measured by the BET method) and an average pore radius of 50 to 2000 Á (Angstrom) (measured by mercury porosimetry), especially silica (SiO2) and mixtures of SiO2 / AI2O3. These carriers can be used in any form such as, for example, in the form of spheres, tablets, rings, stars or particles of other figures, generally with a diameter or length and thickness of 3 to 9 mm. Carriers of these types can be prepared, for example, from aerogenic S¡O 2 or a mixture of aerogenic SiO 2 / AI 2 O 3 which can be prepared, for example, by instantaneous hydrolysis of silicon tetrachloride or a mixture of silicon tetrachloride / aluminum trichloride in an oxyhydrogen flame (US-A-3 939 199). Suitable solvents for the palladium, cadmium, alkali metal and lanthanide metal salts are all compounds in which the selected salts are soluble and can be easily removed after impregnation by drying. Suitable for the acetates, if used, are in particular the unsubstituted carboxylic acids having from 2 to 10 carbon atoms such as acetic acid, propionic acid, n- and isobutyric acid and the different Walloon acids. Among the carboxylic acids, acetic acid is preferred because of its physical properties and for economic reasons. Water is particularly suitable for chlorides and chlorine and acetate complexes. The additional use of another solvent is convenient if the salts are insufficiently soluble in acetic acid or in water. Thus, for example, palladium chloride can be better dissolved in an aqueous acetic acid than in a 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 lanthanide metal, but generally a salt of each of these elements is applied. It is possible to prepare the so-called "fully impregnated" catalysts wherein the catalytically active metal compounds have penetrated the carrier particles as far as the center, or also the so-called "shell catalysts" where the metal salts have only advanced to an external part, of variable size, of the carrier particles, that is to say the so-called "shell" of the particles, and not as far as the center. The elements palladium, cadmium, alkali metal and lanthanide metal which will be applied in each case can be applied in the form of individual salt solutions or in a suitable combination in any useful sequence, preferably using a single solution containing these elements to be applied in the salt form. Particularly it is preferred to use a single solution containing exactly one salt of each of these elements to be applied. This solution may further comprise a mixture of salts of at least two different lannarino metals, however this solution preferably contains a salt of a single lanthanide metal. When we usually talk about "the solution of salts", the same applies analogously to the case where a plurality of solutions are used in sequence, each of which contains only part of the totality of salts that will be applied, in which case the total of the individual parts equals the total amount of salts that will be applied to the carrier. The process for preparing fully impregnated catalysts is preferably as indicated below (US-A-4 902 803, US-A-3 393 199, US-A-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, when appropriate, the excess solution is poured or filtered. It is advantageous, with respect to solution losses, to use only the amount of solution corresponding to the integral pore volume of the catalyst carrier, and to mix this amount carefully so that the particles of the carrier material are uniformly wetted. It is convenient to carry out the impregnation step and mixing simultaneously, for example in a rotating drum or a dryer, in which case drying can follow immediately. It is also generally useful that the composition of the solution used to impregnate the catalyst carrier is such that the required amount of active substances is applied by a single impregnation. However, this amount can also be applied by a plurality of impregnations, in which case each impregnation is preferably followed by drying. The process for preparing shell catalysts is preferably by one of the following three methods, always using a solution of at least one salt of at least one of the elements palladium, cadmium, alkali metal and / or lanthanide metal with a dynamic viscosity. from at least 0.003 Pa s, preferably from 0.005 to 0.009 Pa s: 1. The carrier particles are sprayed one or more times at the same time as they are uniformly mixed with the solution of the salts in the form of droplets with an average diameter of at least 0.3 mm or in the form of liquid jets and, after each spray, dry immediately. "Immediate" drying means in this respect that the drying of the sprayed particles should start without delay. Generally it is sufficient that the drying of the particles begins no more than 30 minutes after the end of a spray. The volume of solution for a spray is 5 to 80% of the pore volume of the carrier particles. This method is described in more detail in EP-A-0 634 214, which is incorporated herein by reference. 2. The carrier particles are impregnated one or more times at the same time as they are uniformly mixed with the solution and dried immediately after each impregnation. "Immediate" drying means in this respect the same as in the first method, and the volume of solution for each impregnation is from 5 to 80% of the pore volume of the carrier particles. This method is described in greater detail in EP-A-0 634 209, which is likewise incorporated herein by reference. 3. The carrier particles are impregnated with the solution one or more times and dried after each impregnation but, unlike the second method, the solution volume does not have an upper limit. It is more than 80% of the pore volume for each impregnation. Because the volume of solution is greater, uniform mixing is not absolutely necessary although it is generally beneficial. Instead, it is now necessary that the duration of each impregnation and the time before the start of subsequent drying, ie the time of onset of each impregnation at the start of subsequent drying, be so short that, after the end of the last drying, a shell of 5 to 80% of the pore volume of the carrier particles contains the catalytically active elements. The duration of this short time that should be chosen for this purpose can be easily determined by preliminary tests. This method is described in more detail in EP-A-0 634, 208, which is incorporated herein by reference. The drying of the catalyst carrier impregnated or sprayed is preferably carried out under reduced pressure (0.01 to 0.08 MPa) both for fully impregnated catalysts and for shell catalysts. The temperature during drying should generally be 50 to 80 ° C, preferably 50 to 70 ° C. In addition, it is generally recommended to perform the drying in a stream of inert gas, for example in a stream of nitrogen and carbon dioxide. The residual solvent content after drying should preferably be less than 8% by weight, in particular less than 6% by weight. The finished catalysts containing palladium, cadmium, alkali metal and at least one lanthanide metal have the following metal contents: Palladium content: generally 0.6-3.5% by weight, preferably 0.8-3.0% by weight, in particular 1.0-2.5% by weight Cadmium content: generally 0.1-2.5% by weight, preferably 0.4-2.5% by weight, in particular 1.3 - 2% by weight Alkali metal content: generally 0.3 - 10% by weight, Potassium is preferably used.

Potassium content: generally 0.5-4.0% by weight, preferably 1.0-3.0% by weight, in particular 1.5-2.5% by weight Metal content generally 0.01 - 1% by weight, lanthanoid: preferably 0.05 - 0.5% by weight, in particular 0.2 - 0.5% by weight.

If more than one lanthanide metal is used to contaminate catalysts containing palladium, cadmium and alkali metal, the term "lanthanoid metal content" refers to the total content of all lanthanoid metals present in the finished catalyst. The percentages established are always related to the quantities of the elements palladium, cadmium, alkali metal and lanthanide metal present in the catalyst, based on the total weight of catalyst (active elements plus anions plus carrier material). Suitable salts are all salts of palladium, cadmium and alkali metal and a lanthanoid element which are soluble; acetates, chlorides, and acetate and chlorine complexes are preferred. However, in the case of interference anions such as, for example, in the case of chlorides, it should be ensured that these anions are removed substantially before using the catalyst. This is carried out by washing the doped carrier, for example with water after the metals have been converted to an insoluble form, for example by reduction and / or by reaction with compounds having an alkaline reaction.

Particularly suitable salts of palladium are the carboxylates, preferably the salts of aliphatic monocarboxylic acids having from 2 to 5 carbon atoms, for example acetate, propionate or butyrate. Other suitable examples are nitrate, nitrite, hydrous oxide, oxalate, acetylacetonate or acetoacetate. Due to its good solubility and availability, the palladium salt particularly preferred is palladium acetate. Particularly suitable as a cadmium compound is acetate. The alkali metal compound preferably employed is at least one compound of sodium, potassium, rubidium or cesium, in particular at least one potassium compound. Particularly suitable compounds are the carboxylates, in particular acetates and propionates. Compounds that are converted under the reaction conditions into alkali metal acetate, such as, for example, hydroxide, oxide or carbonate, are also suitable. Particularly suitable as a lanthanide metal compound are chlorides, nitrates, acetates and acetylacetonates. If a reduction of the palladium compounds is made, which is sometimes beneficial, it is possible to use a gaseous reducing agent for this purpose. Examples of suitable reducing agents are hydrogen, methanol, formaldehyde, ethylene, propylene, isobutylene, butylene or other olefins. The reduction temperature is generally between 40 and 260 ° C, preferably between 70 and 200 ° C. It is generally convenient to use a reducing agent which is diluted with inert gas and which contains from 0.01 to 50% by volume, preferably from 0.5 to 20% by volume, of reducing agent for the reduction. Nitrogen, carbon dioxide or a noble gas, for example, are suitable for use as inert gases. The reduction can also be carried out in the liquid phase at a temperature of 0 ° C to 90 ° C, preferably 15 to 25 ° C. Examples of reducing agents which can be used are aqueous solutions of hydrazine, formic acid or alkali metal borohydrides, in particular sodium borohydride. The amount of the reducing agent depends on the amount of palladium; the reduction equivalent must be at least equal to the quantity oxidation equivalent, however greater amounts of reducing agent are not harmful. The reduction is done after drying. Vinyl acetate is generally prepared by passing acetic acid, ethylene and oxygen containing gases at temperatures of 100 to 220 ° C, preferably 120 to 200 ° C, under pressures of 0.1 to 2.5 MPa, preferably 0.1 to 2.0 MPa, the finished catalyst, it being possible to circulate unreacted components. It is also advantageous under some circumstances to dilute with inert gases such as nitrogen or carbon dioxide. Carbon dioxide is particularly suitable for dilution since it is formed in small amounts during the reaction. With the aid of the novel catalysts, under the same reaction conditions, it is possible to prepare more vinyl acetate per reactor volume and time which, at the same time, improves the selectivity in comparison with known catalysts. This facilitates the treatment of the resulting crude vinyl acetate because the content of vinyl acetate in the gas discharged from the reactor is higher, which also results in an energy saving in the treatment part. A suitable treatment is described, for example in US-A-5 066 365. If, on the other hand, it is desired to maintain the constant space-time yield, it is possible to reduce the reaction temperature and thus carry out the reaction more selectively , with the same total productivity, in which case there is a saving of precursors. This is also associated with a decrease in the amount of carbon dioxide, which is formed as a byproduct and therefore must be removed, and in the ethylene input loss associated with this removal. In addition, this procedure results in an increase in the useful life of the catalyst. The following examples attempt to illustrate the invention but do not limit it. The percentages of the elements palladium, cadmium, potassium and the lanthanide metal are percentages by weight based on the total weight of the catalyst. SiO2 was used as a material of the catalyst carrier, from which tablets with a diameter and a height of 6 mm each were produced as described in DE-A 3 912 504. These tablets were used as the catalyst carrier. The pore volume of 1 I of carrier was 392 ml.

EXAMPLE 1 At 65 ° C, 25.3 g (0.11 mole) of palladium acetate, 25 g (0.09 mole) of cadmium acetate, 25.3 g (0.26 mole) of potassium acetate and 6.82 g (0.016 mole) of cerium acetylacetonate were dissolved. , in 130 ml of glacial acetic acid (volume of solution = 33% of the pore volume), and the highly viscous solution was introduced in a receiver preheated to 65 ° C. 1 I of catalyst carrier was also heated to 65 ° C and placed in a flask. All of the impregnation solution was poured over the carrier particles and uniformly mixed until the complete impregnation solution had been absorbed by the catalyst carrier. This step was concluded after 3 minutes. The drying took place in a stream of nitrogen at 65 ° C and 0.02 MPa at constant weight. The finished catalyst contained 2.0% by weight of palladium, 1.7% by weight of cadmium, 1.7% by weight of potassium and 0.38% by weight of cesium.

COMPARATIVE EXAMPLE 1a The procedure was as in Example 1, but no lanthanide metal salts were added to the impregnation solution containing palladium acetate, calcium acetate and potassium acetate. The finished catalyst contained 2.0% by weight of palladium, 1.7% by weight of cadmium and 1.7% by weight of potassium.

The method used to test the novel catalyst prepared in Example 1 and the catalyst prepared in Comparative Example 1a was as indicated below. 225 ml of the particular catalyst were introduced into a reaction tube with an internal diameter of 20 mm and a length of 65 cm. Then the gas to be reacted was passed over the catalyst under a pressure of 0.8 MPa (reactor inlet) and at a catalyst temperature of 150 ° C for 5 days. This gas consisted of 58% by volume of ethylene, 25% by volume of nitrogen, 12% by volume of acetic acid and 5% by volume of oxygen; the results are evident from the following table.

The space-time yield in grams of vinyl acetate per liter of catalyst per hour. The selectivity of CO2 in% based on the amount of ethylene reacted. It was found, surprisingly, that even small additions of lanthanide metal compounds to catalysts containing potassium, cadmium and palladium markedly improved the selectivity of CO2 and productivity (space-time yield) of these catalysts to prepare vinyl acetate .

Claims (8)

NOVELTY OF THE INVENTION CLAIMS

1. - A process for preparing vinyl acetate in the gas phase from ethylene, acetic acid and oxygen or oxygen-containing gases on a catalyst comprising palladium and / or its compounds, cadmium compounds and alkali metal compounds on a carrier , wherein the catalyst additionally comprises at least one lanthanide metal compound.

2. The process according to claim 1, further characterized in that the catalyst comprises at least one potassium compound.

3. The process according to claim 1 or 2, further characterized in that the catalyst comprises 0.01% by weight to 1% by weight of lanthanide metal based on the total weight of the catalyst.

4. The process according to any of claims 1 to 3, further characterized in that the catalyst comprises 0.05% by weight to 0.5% by weight of lanthanide metal based on the total weight of the catalyst.

5. A catalyst comprising palladium and / or its compounds, cadmium compounds and alkali metal compounds on a carrier, wherein the catalyst additionally comprises at least one lanthanide metal compound.

6. A catalyst according to claim 5, further characterized in that the catalyst comprises at least one potassium compound.

7. A catalyst according to claim 5 or 6, further characterized in that the catalyst comprises 0.01% by weight to 1% by weight of lanthanide metal based on the total weight of the catalyst.

8. A catalyst according to any of claims 5 to 7, further characterized in that the catalyst comprises 0.05% by weight to 0.5% by weight of lanthanide metal based on the total weight of the catalyst.