DE2130732C3 - Ammonia synthesis catalyst - Google Patents

Description

The invention relates to a catalyst for ammonia synthesis and to its use.

In the synthesis of ammonia from nitrogen and hydrogen by known processes, an iron catalyst is usually used.Such a catalyst contains iron as its main component and a hardly reducible oxide as an auxiliary component, such as aluminum trioxide, thorium dioxide, zirconium oxide and the like, and an alkali metal or alkaline earth metal oxide such as potassium oxide , Calcium oxide and the like, distributed between the iron crystallites. Such a catalyst has the following composition, for example: 94% by weight Fe, 4% by weight Al ₂ O ₃ , 1% by weight CaO and 1% by weight K ₂ O. Disadvantageous However, such catalysts are that they do not always display the desired activity. For example, their catalytic activity at low temperatures, for example at temperatures below 300 ° C., is insufficient to promote the formation of ammonia in terms of chemical equilibrium. As a result, the usual industrial plants must necessarily be operated at high temperatures, for example at temperatures of 400 to 450 ^° C., in spite of the impairment of the chemical equilibrium. In order to eliminate the disadvantages associated with high temperatures, the known industrial plants had to work at high pressures, for example pressures of 300 to 500 kg / cm ² . Despite these high pressures, the direct conversion is limited to about 20% because of the adverse influence of the high temperature on the chemical equilibrium and because of the inhibiting effect of the ammonia formed. This results in an enormous increase in the amount of recycle gas, which furthermore leads to an increase in operating costs, such as, for example, the expenditure for operating power, heat transfer, ammonia production and the like.

The one from DT-AS 1300520 shows similar disadvantages known catalyst, the reduction of a potassium oxide, aluminum oxide, optionally lithium oxide and other oxides as well as granules containing mainly iron oxide and then Further treatment of the reduced granulate is produced. However, in the course of the reduction only the iron oxide is converted into the metallic state, while under the process conditions to be observed the alkali oxides also used in each case are not reduced.

The CH-PS 251380 deals with a catalyst for the synthesis of ammonia, which forms nitrides from a mixture of nitrogen and hydrogen Substances that form hydrides, which must be those substances which under the respective reaction conditions form a labile hydride and / or nitride. When using this known catalyst So, before the actual synthesis reaction, two more chemical reactions take place Make process management and the production of the catalyst more difficult.

Finally, from DT-OS 1939665 and from "Chemistry and Industry", 1960. page 130 ff. Catalysts known that contain an alkali metal in oxide form. Their activity in the synthesis of ammonia is unsatisfactory.

Another catalyst previously used in large-scale ammonia synthesis is essentially from a cyanate complex. This catalyst is simply not a suitable catalyst for the synthesis of ammonia, as its activity depends not only to a large extent on the purity of the feed gases, but also from practically the same factors as the activities of the catalysts mentioned at the beginning is dependent.

The invention is based on the object of providing a catalyst for the synthesis of ammonia which this allowed at lower temperatures and lower costs than the previously known catalysts and even at high ammonia concentrations a slight decrease in the reaction rate allows

The invention thus relates to a catalyst for the synthesis of ammonia, produced by vapor deposition an alkali metal on the surface of at least one present in the metallic state and by reduction a reducible transition metal compound

Preparation produced transition metal of groups VIB, VIIB or VIII of the periodic table in the presence of a support in vacuo at a temperature of 300 to 500 ⁰ C or by soaking at least one transition metal present in the metallic state of groups VIB, VIIB or VIII of the periodic table together with a support in a solution of an alkali metal in liquid ammonia and then evaporating the ammonia.

The catalyst according to the invention exhibits high activity and even at lower temperatures enables ammonia synthesis from nitrogen and hydrogen with far lower production costs than was previously the case. When using it is, at best, a slight decrease in the rate of reaction even at high ammonia concentrations to observe. With the optimal working conditions determined in each case, the inventive Catalyst excellent efficiency.

A catalyst according to the invention is based on the unique mode of action of the metallic state made optimal use of alkali metals It differs in its characteristic Properties of the customary catalysts previously used for ammonia synthesis are considerable. Through the Activation with an alkali metal present in the metallic state contains a transition metal component a 10 times higher specific catalytic activity (per surface area) than the transition metal prior to activation has essential to the invention is therefore the combination of a transition metal component with an alkali metal component each in a metallic state.

In this context, it should be noted that some have been used previously for ammonia synthesis Catalysts also contain alkali metals. However, the alkali metal is in the known catalysts as an oxide, e.g. as potassium oxide, and remains in the "operating state" of these catalysts Oxide form. The effect of such an alkali metal oxide used in conventionally known catalysts arises only in cooperation with the other oxides, such as aluminum trioxide, Silicon dioxide and the like. This is going to be special clearly if one realizes that when mixing an alkali metal oxide (alone) with iron, whose activity is rather severely impaired (see J.A.Almquist, and E.D. Crittenden in »Ind. Closely. Chem. «18, page 1307 - 1926 -). The mode of action of the alkali metal oxides in the known Catalysts are thus different from the effect of adding one in the metallic state present alkali metal quite considerably.

The combination of a transition metal with one contained in the catalyst according to the invention Alkali metal itself shows a sufficiently high catalytic activity in the synthesis of ammonia. To the further improve the catalytic! However, the catalyst according to the invention contains activity as third component or carrier substances, such as activated carbon or porous carbon, a hardly reducible oxide, a metal carbide and the like.

Transition metals effective in catalysts according to the invention are molybdenum (Mo), tungsten (W), Chromium (Cr) and uranium (U), from group VIl B of the periodic table manganese (Mn), rhenium (Re) and from group VIII of the periodic table iron (Fe), cobalt (Co), nickel (Ni), ruthenium (Ru), rhodium (Rh) and Osmium (Os). The most suitable transition metals are Fe, Co, Re, Ru, Os, Mo or Mixtures of these metals.

5 Although the content of transition metals in the catalysts according to the invention from 0.3 to 99% by weight should be sufficient in this regard between the two transition metal groups defined below A and B a difference can be made:

ίο A. rhenium, ruthenium or osmium,

B. other transition metals, namely iron, molybdenum or cobalt

The preferred content of group A transition metals in the catalysts according to the invention is 0.3 to 10% by weight and group B 5 to 99% by weight. Regarding the increasing suitability of the Alkali metals, that the activity with increasing Atomic number of the element increases; however, from an economic point of view, it is common as an alkali metal Potassium to use a

The amount of alkali metal required depends on the type of transition metal component and / or the third component. However, it is expedient to use the alkali metal in an amount of 0.1 up to 50% by weight. The appropriate weight ratio of alkali metal to transition metal in the catalysts According to the invention, group A is between 0.3 and 100, preferably between 2 and 10, and of group B between 0.01 and 3, preferably between 0.01 and 0.1 or 1 and 3.

Activated carbon, aluminum oxide, silicon dioxide, CeMt, diatomaceous earth, thorium dioxide, zirconium oxide, silicon carbide are suitable as the third component in catalysts according to the invention. Activated carbon, aluminum oxide, silicon dioxide, thorium dioxide and zirconium oxide are particularly suitable. The amount of the third ingredient is not critical. Even in the smallest amounts, it has a clearly positive effect. In some cases, the amount of the third ingredient depends on the type of transition metal. If larger amounts of the third component are used, an increase in the degree of efficiency can also be determined.
For combining the alkali metal with the transition metal, one can employ an adsorption method in which the alkali metal is evaporated in vacuo or entrained with an inert gas stream, or a soaking method using a solution of the alkali metal in liquid ammonia or a dispersion of the alkali metal in a hydrocarbon. Although the adsorption process, in which the alkali metal is entrained with an inert gas stream, has also proven to be suitable as a possibility of adding alkali metal to the catalyst located in a reactor, taking into account the achievable activity of the catalysts according to the invention and for reasons of economical mass production Soaking method in a solution of the alkali metal in liquid ammonia is preferred. The process conditions in carrying out the vacuum evaporation process change with the particular alkali metal; this method is carried out usually at a temperature between 300 and 500 ⁰ C.

The use of temperatures higher than 500 ° C is not advisable as they accelerate the crystallization of the transition metals. In the soaking process using liquid ammonia, the

third component impregnated with the transition metal, E.g. activated carbon impregnated with ruthenium in a solution of an alkali metal in liquid ammonia soaked; during the subsequent evaporation of the ammonia, the alkali metal remains with the transition metal united The process described can be carried out under atmospheric pressure or at elevated pressure execute. In the event that it is carried out at atmospheric pressure, it should expediently at a Temperatures below -34 ° C, the boiling point of liquid ammonia, can be carried out; is working on the other hand, at elevated pressure, the process according to the invention can be carried out at room temperature be performed. Basically, the ammonia solution contain the alkali metal up to saturation concentration; the loss of alkali metal increases however, with increasing concentration, so that a 1 to 10% strength by weight solution is preferred of the alkali metal in question in liquid ammonia

The catalysts according to the invention can be easily processed by the processes outlined produce. When using these catalysts, highly concentrated ammonia can be found at lower Synthesize temperatures.

It goes without saying that the use of low reaction temperatures and high reaction pressures from the thermodynamic point of view, useful when using a catalyst according to the invention is carried out conveniently at reaction temperatures of 150 to 450 ⁰ C and reaction pressures of 1 to 1000 kg / cm ² (absolute ) and preferably at reaction temperatures of 250 to 450 ° C and reaction pressures of 50 to 300 kg / cm ² (absolute).

When a catalyst according to the invention is used, the feed rate of the reaction gas is expediently from 2000 to 10000 OST " ¹ , preferably from 5000 to 30,000 hours.

The molar ratio of hydrogen to nitrogen can range between 0.1 and 4. In particular in the case of ruthenium-containing catalysts according to the invention, the hydrogen content is reduced to an increase in ammonia formation, resulting in an increase in direct hydrogen conversion has the consequence

If, for example, a catalyst according to the invention formed from ruthenium, activated carbon and potassium is used, when the reaction gas is fed in in a molar ratio of hydrogen to nitrogen of 1: 1, at a reaction ^{temperature of 350 °} C. and a reaction pressure of 100 kg / cm ² Take off 20% by volume of ammonia at the reactor outlet. If you use a catalyst formed from iron, aluminum trioxide and potassium according to the invention, you can with the supply of the reaction gas in a molar ratio of hydrogen to nitrogen of 3: 1, a reaction ^{temperature of 400 0} C and a reaction pressure of 100 kg / cm ² Take off 18% by volume of ammonia at the reactor outlet. These values clearly illuminate the technical progress which can be achieved when using catalysts according to the invention in the synthesis of ammonia. With conventional iron catalysts used for ammonia synthesis, which contain potassium oxide, calcium oxide, aluminum oxide and the like, only about 10% by volume of ammonia can be obtained under almost the same temperature and pressure conditions. It should be particularly pointed out that when using catalysts according to the invention, the influence of pressure on the rate of ammonia formation changes with the type of transition metal contained in the catalyst; the ruthenium-containing catalysts according to the invention show, compared to the iron-containing catalysts according to the invention, a better activity at relatively low pressure; however, the rate of ammonia formation increases

■ ο the iron-containing catalysts according to the invention with increasing pressure, the iron-containing catalysts ultimately having better activity than the ruthenium-containing catalysts.

The previous statements should be clear and unambiguous

■ s have shown that the catalysts of the invention have such a high activity, in particular at low temperatures, as they are at conventional known catalysts are never to be expected. However, to the activity of the catalysts according to To make optimal use of the invention, the process conditions must match each other as well as possible be matched.

When using catalysts according to the invention can be used as the reaction gas without any Purification a common ammonia synthesis gas can be used. In addition, the catalysts are according to the invention far less susceptible to catalyst poisons such as carbon monoxide and oxygen than the usual known catalysts.

The following examples are intended to illustrate the invention in more detail.

In Examples 1 to 5, 8 to 10 and in the comparative examples 1, 2 and 4 was implemented in a stainless steel, exterior heated reaction tube carried out with a clearance of 3 mm. Unless otherwise stated, was the synthesis gas, consisting of hydrogen and nitrogen in a molar ratio of 3: 1, into the reaction tube a speed of 51 / hour. fed in The amount of ammonia in the exhaust gas was determined by titration determined

In Examples 6 and 7 and in Comparative Example 3, the reaction was carried out in one with a catalyst charged glass tube with a clear width of 18 mm under a pressure of one atmosphere and at a flow rate of the synthesis gas, consisting of hydrogen and nitrogen in a molar ratio 3: 1, from 51 / h The amount of ammonia in the exhaust gas was carried out by condensation with liquid nitrogen charged cold trap determined

example 1

Using various catalysts, which by adding a given amount of one hardly reducible oxide as a third component to either the chloride or oxide of the respective super-

- transition metal, subsequent complete reduction of the respective transition metal chloride or oxide at a temperature of 300 to 600 ⁰ C and combination of the respective transition metal with 2 wt .-% potassium by evaporation of the same in a vacuum at a temperature of 400 ⁰ C were produced at a temperature of 400 ⁰ C and a pressure of 30 kg / cm ² ammonia is synthesized. The composition of the individual catalysts and the results obtained using the same are summarized in Table I below:

Table I.

No. transition metal Third component Percentage Ge
hold on to that
third component Educated
ammonia
crowd (ml *) / hour) 1 iron Alumina 2 411 2 iron Thorium dioxide 2 300 3 iron Zirconium oxide 2 358 4th iron Silicon dioxide 2 285 5 cobalt Alumina 2 50 6th nickel Alumina 2 7.5 7th Ruthenium Alumina 10 348 8th Ruthenium Thorium dioxide 5 320 9 rhenium Alumina 2 45 10 molybdenum Alumina 2 20th 11th tungsten Alumina 2 15th 12th osmium Alumina 10 170 13th Iron-
Ruthenium Alumina 2 375 14th Ruthenium ■
rhenium Alumina 2 60 15th Iron
molybdenum Alumina 2 34

*) At normal pressure and temperature

Example 2

Using various catalysts, obtained by adding a predetermined amount of a hardly reducible oxide as a third component to either the chloride or oxide of a transition metal, followed by complete reduction of the respective mixture at a temperature of 300 to 600 ⁰ C.

Table II

were and association of the respective transition metal with 2 wt .-% sodium by evaporating the same under vacuum at a temperature of 400 ⁰ C produced was measured at a temperature of 400 ⁰ C and synthesizes a pressure of 30 kg / cm ² ammonia. The composition of the same results obtained is summarized in the following table II:

No. *) Transition metal Third component Percentage Ge
hold on to that
third component Educated
ammonia
crowd (ml *) / hour) 1 iron Alumina 2 250 2 iron Thorium dioxide 2 210 3 iron Zirconium oxide 2 223 4th iron Silicon dioxide 2 201 5 cobalt Alumina 2 30th 6th Ruthenium Alumina 10 170 7th Ruthenium Thorium dioxide 5 155 8th rhenium Alumina 2 40 9 molybdenum Alumina 2 15th At normal pressure and temperature

ίο

continuation Transition metal Third component Percentage Ge
hold on to that
third component Educated
ammonia
crowd number 1 (ml *) / hour) tungsten Alumina 2 8th 10 osmium Alumina 10 88 11th Iron ruthenium Alumina 2 203 12th Ruthenium · rhenium Alumina H 46 13th Iron ■ molybdenum Alumina 2 24 14th

*) At normal pressure and temperature

Example 3

Using various catalysts which are similar to Example 1, but combining the respective transition metal with potassium after the soaking process using a 10 wt .-% - Solution of potassium in liquid ammonia

Table III

ammonia was synthesized at a temperature of 400 ° C. and a pressure of 30 kg / cm ^2. The composition of the individual catalysts and the results obtained using the same are listed in Table III below:

No. Transition metal Third component Percentage Ge
hold on to that
third component Educated
ammonia
crowd (ml *) / hour) i iron Alumina 2 450 2 iron Thorium dioxide 2 380 3 cobalt Alumina 2 50 4th Ruthenium Alumina 10 395 5 Ruthenium Thorium oxide 5 328 6th rhenium Alumina 2 56 7th molybdenum Alumina 2 23 8th tungsten Alumina 2 15th 9 osmium Alumina 10 191

*) At normal pressure and temperature

Example 4

Using various catalysts that according to Example 2, but with combining the respective transition metal with sodium according to the Soaking method using a 10% by weight solution of sodium in liquid ammonia

Table IV

ammonia was synthesized at a temperature of 400 ^° C. and a pressure of 30 kg / cm ^2. The composition of the individual catalysts and the results obtained using the same are summarized in Table IV below:

No.

Transition metal

Third component

Percentage educated
hold on to the ammonia

third ingredient quantity

(ml *) / hour)

1 iron Alumina 2 270 2 irons Thorium dioxide 2 230 *) At normal pressure and temperature

continuation Never transition Ui 11 Third component Percentage Ge
hold on to that
third component Educated
Ammonia-
crowd No. (ml *) / hour) cobalt Alumina 2 35 3 Ruthenium Alumina 10 178 4th Ruthenium Thorium dioxide 5 162 5 rhenium Alumina 2 45 6th molybdenum Alumina 2 18th 7th tungsten Alumina 2 8th 8th osmium Alumina 10 101 9 *) Normal pressure and temperature Example 5

Using different catalysts, which are the same as in Example 4, but with combining the respective transition metal with a mixture of equal parts potassium and sodium after the soaking process using 5 wt% readings each of metallic potassium and metallic

Table V

Sodium prepared in liquid ammonia was synthesized at a temperature of 400 ° C. and a pressure of 30 kg / cm ² ammonia. The composition of the individual catalysts and the results obtained using the same are shown in Table V below:

No. Transition metal Third component Percentage Ge Educated hold on to that ammonia third component crowd (ml *) / hour) 1 iron Alumina 2 330 2 iron Thorium dioxide 2 294 3 cobalt Alumina 2 48 4th Ruthenium Alumina 10 210 5 Ruthenium Thorium dioxide 5 178 6th rhenium Alumina 2 48 7th molybdenum Alumina 2 18th 8th tungsten Alumina 2 10 9 osmium Alumina 10 141 *) Normal pressure and temperature Example 6 Table VI

Using different catalysts, the impregnation of activated carbon with, based on the weight of the activated carbon, 10% by weight of various transition metals, subsequent reduction with hydrogen at a temperature of 350 to 400 ⁰ C and combining the respective transition metal with, based on the weight of activated carbon were 35Gew .-% potassium by evaporation of the same in vacuo at a temperature of 400 ⁰ C prepared was synthesized under atmospheric pressure at various temperatures ammonia. The composition of the individual catalysts and the results obtained using the same are summarized in Table VI below:

No.

Transition metal

Reaction temperature

( ⁰ C)

Amount of ammonia formed

(ml *) / hour)

1 iron 250 1.40 2 iron 200 0.40 3 cobalt 275 2.4 4th cobalt 235 0.33 5 nickel 310 0.86

*) Normal pressure and temperature

continuation Transitional Reaction Ucbiklclc Ληιιηο- No. mclall temperature niukmcngc ( ⁰ C) (ml *) / stci.) Ruthenium 165 0.60 6th Ruthenium 200 4.0 7th osmium 240 0.59 8th rhenium 239 0.51 9 molybdenum 254 1.7 10

*) Normal pressure and temperature

Example VII

Using various catalysts, the same as in Example 6, but replacing the potassium Made by sodium, was under atmospheric pressure at various temperatures Synthesized ammonia. The composition of the individual catalysts and the use of the same The results obtained are summarized in the following table VII:

Example X

Had been using a 2 wt .-% aluminum oxide containing iron catalyst, which incorporated by vaporizing potassium under vacuum at a temperature of 400 ⁰ C 2 wt .-% of potassium, was at a temperature of 400 ⁰ C and a pressure of 100 kg / cm ² synthesized ammonia. At normal pressure and temperature, 900 ml of ammonia were formed per hour. If the synthesis was carried out at a temperature of 300 ^° C. and a pressure of 100 kg / cm ² , 360 ml of ammonia were obtained per hour at normal pressure and temperature.

Comparative example 1

Using various catalysts which were produced by reducing either the chloride or oxide of various transition metals at a temperature of 300 to 600 ^° C. and without the addition of alkali metals, ammonia was synthesized ^{at a temperature of 400 °} C. and a pressure of 30 kg / cm ^2. The composition of the individual catalysts and the results obtained using the same are summarized in Table VIII below:

Reaction Educated ammo Table VIII 3 ° No. Transition metal Formed ammonia Tab" temperature niak amount crowd ( ⁰ C) (ml *) / hour) (ml *) / hour) No. 275 0.25 1 iron 35 200 0.40 2 cobalt very low -temperature 3 Ruthenium 15th 40 4 rhenium 8th Example 8 5 molybdenum 5 1 6th tungsten very low 2 7th osmium 15th *) ' ; ll VII Transition metal cobalt Ruthenium Normal pressure and

Using a catalyst which is produced by impregnating activated carbon with, based on the weight of the activated carbon, 10% by weight ruthenium, subsequent reduction with hydrogen at a temperature of 350 ^° C. and combining the ruthenium with, based on the weight of the activated carbon, 20% by weight of potassium had been prepared by the soaking method using a 5% by weight solution of potassium in liquid ammonia, ammonia was synthesized at a temperature of 375 ° C. and a pressure of 100 kg / cm ² . At normal pressure and temperature, 1000 ml of ammonia were obtained per hour.

Example 9

60

Using the catalyst prepared according to Example 8, ammonia was synthesized from a synthesis gas consisting of hydrogen and nitrogen in a molar ratio of 1: 1, at a temperature of 350 ^° C. and a pressure of 100 kg / cm ² Hours 1000 ml of ammonia synthesized.

*) Normal pressure and temperature

The specific surface area of the iron catalyst used in Experiment 1, determined by the BET method, was 2 nr / g; the activity per surface unit was 17.5 ml / (normal pressure and temperature) / hour. ■ m ² ■ catalyst.

Comparative example 2

Using various catalysts which had been prepared by adding a given amount of a hardly reducible oxide as a third component to either the chloride or oxide of a transition metal, followed by complete reduction of the respective mixture at a temperature of 300 to 600 ^° C. and without the addition of alkali metals cm ² synthesized ammonia at a temperature of 400 ⁰ C and a pressure of 30kg /. The composition of the individual catalysts and the results obtained using the same are shown in Table IX below:

Table IX

No. Oversized metal Third component Percentage Ge Educated hold on to that ammonia third component crowd (ml *) / hour) 1 iron Alumina 2 90 2 iron Thorium dioxide 2 165 3 iron Zirconium oxide 2 153 4th iron Silicon dioxide 2 110 5 cobalt Alumina 2 very low 6th nickel Alumina 2 very low 7th Ruthenium Alumina 10 7th 8th Ruthenium Thorium dioxide 5 4th 9 rhenium Alumina 2 very low 10 molybdenum Alumina 2 very low 11th tungsten Alumina 2 very low 12th osmium Alumina 10 15th

*) Normal pressure and temperature

Comparative example 3

Atmospheric pressure was examined for its activity in the ammonia synthesis, an ammonia synthesis could not be found in any of the transition metals mentioned in Table XI in the temperature range from 350 to 400 ^{0 C.}

A catalyst made by impregnating activated carbon with, based on the weight of the activated carbon,
10% by weight of a transition metal and no alkali metal addition was produced before its
Use at a temperature of 400 ⁰ C with
Reduced water pressure. This catalyst was taking

Comparative example 4

Was a conventional, large-scale 40 and temperature per hour 550 ml of ammonia synthesized

Iron catalyst with potassium oxide, calcium oxide and be sated. In the event that at a reaction temperature

Alumina at a temperature of 45O ⁰ C temperature complete 300 ⁰ C and a reaction pressure of

constantly reduced and then worked to its activity 100 kg / cm ² , could at normal pressure

at a temperature of 400 ^° C. and a pressure and temperature per hour, only 100 ml of ammonia

of 100 kg / cm ² examined, 45 could be synthesized at normal pressure.

Claims (4)

Patent claims: 1. Catalyst for ammonia synthesis, produced by vapor deposition of an alkali metal on the surface of at least one transition metal from groups VIB, VIIB or VIII of the periodic table, which is present in a metallic state and produced by reduction of a reduced transition metal compound, in the presence of a support in vacuo at a temperature of 300 to 500 ⁰ C or by soaking at least one transition metal from groups VIB, VIIB or VIII of the periodic table in the metallic state together with a carrier in a solution of an alkali metal in liquid ammonia and then evaporating the ammonia. 2. Catalyst according to claim 1, characterized in that that it contains 0.3 to 99% by weight of transition metal 3. Catalyst according to one of claims 1 and 2, characterized in that it contains 0.1 to 50% by weight Contains alkali metal 4. Use of a catalyst according to one or more of claims 1 to 3 for synthesis of ammonia from hydrogen and nitrogen.