NO127285B - - Google Patents

Description

The present invention relates to oxidation catalysts comprising oxides of uranium and arsenic, optionally also containing activators, and which are applicable for the catalytic oxidation of olefins to unsaturated aldehydes and acids, and for the oxidative dehydrogenation of olefins to conjugated dienes, and further for the catalytic ammonia oxidation of olefins to unsaturated nitriles. The expression "ammonia oxidation" used here shall be understood to include an oxidation in the presence of ammonia. Examples of catalytic oxidation reactions include the oxidation of propylene to acrolein and acrylic acid, the oxidation of isobutylene to methacrolein, the oxydehydrogenation of an olefin with 4-8 carbon atoms, such as, for example, the oxydehydrogenation of butene-1•or butene-2 to butadiene- 1,3, the amm oxidation of propylene to acrylonitrile and the amm oxidation of isobutylene to methacrylonitrile.

The basic catalyst according to the present invention is a mixture of oxides of uranium and arsenic. The ratio between the amounts of the elements in the base catalyst can vary within wide limits. Besides oxides of the basic elements uranium and arsenic, the catalyst can however contain more limited amounts of oxides of other elements, which promote the catalyst's effect, and which are called activators in the present. - The basic catalyst and its composition as well as the activator elements will be found in more detail in the requirements.

The preferred atomic ratio of uranium to arsenic in the base catalyst is from 6:1 to 1:6. In the catalyst according to the invention, the valence of uranium may vary from 4 to 6, and the valence of molybdenum may vary from 2 to 6. It is assumed that at least a part. of the oxides in the present catalysts are present as an active catalytic oxide complex. The catalyst according to the invention is distinguished by an insignificant loss of activity and selectivity over longer periods of time, despite the relatively high reaction temperatures that come into consideration in the catalytic processes in which the catalyst can be used.

Although the catalyst according to the invention gives good results

without the use of a carrier, then a preferred catalyst contains from 5 to 95% by weight of some known catalyst carrier, which is preferably silicic acid.

In the production of the base catalyst, the oxides of the elements can be mixed together, or they can be prepared separately and then mixed, or they can be prepared separately or together in situ.

In the production of the base catalyst, it is preferred to use water-soluble salts of the catalyst elements. A preferred source for uranium is thus uranyl nitrate, for arsenic ortho-arsenic acid and for molybdenum ammonium heptamolybdate.

It will be understood by those skilled in the art that various other salts, soluble at least to a certain extent in water, can be used to form the solutions, which when treated as described in more detail below will give the desired oxides for the base catalyst. In a similar way, it will be clear that the catalyst can also be produced by co-precipitation of the oxides or of salts that form oxides by heat treatment and impregnation of one or more of the metals that result in oxides by heat treatment.

Regardless of the method used to introduce the catalyst components into the base catalyst, it has been shown that the components that are present within the indicated areas exhibit unexpected and highly desirable properties for carrying out the above-mentioned method.

The catalytic activity of the base catalyst is increased by heating the catalyst to an elevated temperature. Preferably, the catalyst mixture is dried and heated at a temperature of 260-595°C, preferably at 315-425°C, for 2-24 hours. If the activity is insufficient, the catalyst can be further heat-treated at a temperature above approx. 425°C, but below a temperature which is harmful to the catalyst, preferably in the range 425-760°C for 1-48 hours in the presence of oxygen or an oxygen-containing gas, e.g. air.

It has also been shown that some of the present catalysts can be further activated by exposing the heat-treated catalyst to a reducing atmosphere for 4-48 hours at a temperature within the range 205-540°C. This reducing treatment is suitably carried out by letting a reducing gas, such as ammonia, hydrogen etc. flow over the catalyst. It turned out that catalysts treated with a reducing gas gave a higher degree of conversions after a short period of time, than when the same catalysts that were not subjected to the reducing treatment were used at the same time.

The activators that can be advantageously used are various metal oxides from the groups I-A, I-B, II-A, II-B, III-B, IV-A, IV-B, V-B, VI-B, VII-B and VIII in the Periodic Table. Particularly effective are the oxides of molybdenum, boron, vanadium, tin, nickel, bismuth, chromium, iron, manganese, zinc and tungsten in an amount corresponding to less than 1 atomic equivalent of either uranium or arsenic. The activator oxides can be introduced into the base catalyst by mixing it into the sludge or gel before calcination, or by mixing it into the oven-dried base catalyst before calcination.

A preferred way to introduce the activator element is to prepare an aqueous solution of its salt, mix this solution with the solutions of salts of the base catalyst elements and stir while continuously heating until the solution gels. Using a spoon, the gel is transferred to bowls and oven-dried at 120°C overnight. The dried catalyst is then calcined at 425°C. A further calcination at a higher temperature can also be used to increase the activity of the catalyst complex.

Oxidation of olefins with 3 carbon atoms in a straight chain is carried out at a reactor pressure in the range from 0.3 to 7 kg/cm 2 , a reactor temperature in the range from 260 to 595°C and apparent contact times that can vary from 0.1 to 50 seconds. A molar ratio of oxygen to olefin between 0.5:1 and 5:1 gives satisfactory results.

Olefins with 3 carbon atoms in a straight chain can be converted to the nitriles under similar conditions as described above, with the exception that ammonia is introduced into the reactor together with oxygen. A desired ammonia-to-olefin ratio is approx. 1:1, as it has been found that generally undesirable, olefinically unsaturated oxidized products are formed when this ratio is in the range above 0.15:1 to 0.75:1. Water is formed as a reaction product and additional water can be added both to improve the conversion and also to regulate the thermal conditions in the reactor.

Olefins with at least 4 and up to approx. 8 non-quaternary carbon atoms, of which at least 4 are arranged in sequence in a straight chain or in a ring, can be oxidatively dehydrogenated to diolefins and aromatic compounds. Preferred olefins are either normal straight chain or tertiary olefins, and both cis and trans isomers can be used. From approx. 0.3 to 3 mol of oxygen per moles of olefin can be used for oxydehydrogenation of the olefins, and a slight molar excess of oxygen is preferred. The reaction temperatures can vary from 325 to 1000°C, and care is taken to remove the exothermic heat of reaction, particularly when the reactor temperature exceeds 550°C. The pressure, temperature and contact times are in the same range as for the reactions described above.

The apparent contact time is defined as the length of time in seconds that the unit volume of gas measured under the reaction conditions is in contact with the apparent unit volume of the catalyst. The contact time is calculated from the apparent volume of the catalyst layer, the mean pressure and temperature in the reactor and the flow rates of the various components in the reaction mixture.

Percentage conversion per passage is defined as moles of unsaturated product recovered divided by moles of monoolefin introduced, multiplied by 100.

Any reaction apparatus suitable for carrying out vapor phase oxidation reactions can be used, preferably reactors of the fixed bed or fluidized bed type. Examples of base catalysts shall be given below, where all parts are based on weight unless otherwise stated.

Example 1.

In a typical preparation of an unactivated catalyst, 87.8 g of uranyl nitrate (U02)(N03)2.6H20 were dissolved in approx. 100 ml of hot water, 154.3 g of ortho-arsenic acid H3As04.1/2H20 were dissolved in approx.

400 ml warm water. The uranyl nitrate solution was added to the dilute ortho-arsenic acid and the mixture stirred. To this mixture was added 116.8 g of "Ludox" (colloidal solution of polymerized silicic acid). The mixture was continuously heated with constant stirring until it formed a gel. The gel was transferred to bowls using spoons, placed in an oven with air circulation at 120°C and dried overnight. The oven-dried catalyst was then heat-treated in an oven open to the atmosphere, starting with a temperature of 425°C and then increasing the temperature to 480°C over a period of 2 hours. The catalyst was calcined overnight at 480°C. The catalyst that was obtained had a composition that can be written as 82.5% UAs. q°ii 5" 17.5% SiO 2 .

The following table shows the results obtained by application

of the catalyst for the purposes stated above.

Examples no. 2 and 3 concern the formation of acrylonitrile by amm-oxidation of an olefin.

Examples 4 and 5 relate to the oxidation of an olefin to acrolein (smaller amounts of unsaturated acids which are also formed are not indicated).

Example No. 6 concerns the oxy-dehydrogenation of butenes to butadiene. The approximate mole ratio of butene to air was 1:12; as in the other examples in the table, smaller amounts of by-products formed are not indicated.

Heat treatment at 540°C and above as indicated in the table is generally to increase activation after calcination at 425°C for 16-24 hours.

Claims (3)

1. Catalyst mixture containing uranium and oxygen and which can be used for oxidation of olefins to unsaturated aldehydes and acids, for oxidative dehydrogenation of olefins to conjugated dienes and for amm oxidation of olefins to unsaturated nitriles, characterized in that it mainly consists of an activated catalytic oxide complex defined by the empirical formula As x U y 0 z, where "x" is a number from 1 to 10,"y" is a number from 1 to 25 and " z" is a number chosen so that it satisfies or corresponds to the valence of arsenic and uranium in the oxidation state in which they are present in the catalyst. 2. Catalyst mixture according to claim 1, characterized in that, as an activator, it also contains one or more oxides of molybdenum, boron, vanadium, tin, nickel, bismuth, chromium, iron, manganese, zinc and tungsten in an amount corresponding to less than 1 atomic equivalent in relation to uranium and arsenic. 3. Catalyst mixture according to claim 2, characterized in that it contains iron or molybdenum as an activator.