METHOD FOR CATALYTICALLY PRODUCING UREA

Abstract

A process for preparing urea comprises preparing formamide based on carbon dioxide, hydrogen, and ammonia, forming methyl formate or ammonium formate as an intermediate in a catalytic reaction, and preparing urea by reacting the formamide and possibly ammonia in the presence of a catalyst. The source of carbon dioxide is a liquid laden with chemically and/or physically bound carbon dioxide and selected from a methanol phase or an aqueous ammonia solution obtained by gas scrubbing of a syngas for removing carbon dioxide using a scrubbing fluid. The scrubbing fluid can be a methanol phase, or carbon dioxide is desorbed from the scrubbing fluid and absorbed into a methanol phase to give a carbon dioxide-laden methanol phase that is then reacted as carbon dioxide-containing stream with a hydrogen-containing stream in the presence of a catalyst to form methyl formate. The methyl formate is reacted with an ammonia-containing stream to form formamide.

Claims

1.-20. (canceled)

21. A process for preparing urea comprising: preparing formamide based on carbon dioxide, hydrogen, and ammonia, forming methyl formate or ammonium formate as an intermediate in a catalytic reaction; and preparing urea by reacting the formamide or the formamide with ammonia in the presence of a catalyst, wherein a source of carbon dioxide is a liquid laden with chemically and/or physically bound carbon dioxide and selected from a methanol phase or an aqueous ammonia solution that is obtained by gas scrubbing of a syngas for removal of carbon dioxide using a scrubbing fluid, wherein either: the scrubbing fluid is a methanol phase, or carbon dioxide is desorbed from the scrubbing fluid laden with chemically and/or physically bound carbon dioxide and absorbed into a methanol phase to give a carbon dioxide-laden methanol phase, wherein the carbon dioxide-laden methanol phase is reacted as a carbon dioxide-containing stream with a hydrogen-containing stream in the presence of a catalyst to form the methyl formate, and the methyl formate is reacted with an ammonia-containing stream to form the formamide, or the scrubbing fluid is an aqueous ammonia solution and carbon dioxide is bound at least partly in the form of carbonates in the scrubbing fluid, wherein the scrubbing fluid laden with chemically and/or physically bound carbon dioxide is reacted as a carbon dioxide-containing stream with a hydrogen-containing stream in the presence of a catalyst to form ammonium formate or to form ammonium formate and formamide, wherein the ammonium formate is converted into the formamide by heat treatment.

22. The process of claim 21 wherein the syngas is a syngas for ammonia synthesis and/or the gas scrubbing is performed on a syngas obtained from steam reforming and/or a subsequent water-gas shift reaction.

23. The process of claim 21 wherein the syngas comprises a gas from a coke oven gas, a blast furnace gas, a converter gas, or an offgas from cement works.

24. The process of claim 21 wherein methanol or an aqueous ammonia solution is used as the scrubbing fluid for the gas scrubbing for removing carbon dioxide.

25. The process of claim 21 wherein either: the gas scrubbing of the syngas for removing carbon dioxide is performed with the aqueous ammonia solution as the scrubbing fluid at a pressure of 20 to 50 bar and/or at a temperature of below 100° C.; or the gas scrubbing of the syngas for removing carbon dioxide is performed with methanol as the scrubbing fluid at a pressure of 20 to 50 bar and/or with methanol cooled to a temperature of −20° C. or below.

26. The process of claim 22 wherein the hydrogen-containing stream comprises: a substream of the syngas after the gas scrubbing; a substream of the syngas before the gas scrubbing; and hydrogen obtained from the processing of products of the ammonia synthesis and/or urea synthesis.

27. The process of claim 21 wherein the hydrogen-containing stream comprises hydrogen obtained from processing products of urea synthesis.

28. The process of claim 21 wherein ammonia in the ammonia-containing stream and/or ammonia for the aqueous ammonia solution is obtained from the syngas via ammonia synthesis.

29. The process of claim 21 comprising using a ruthenium-phosphine complex as a catalyst for at least one of: a reaction of the formamide to form urea or for a reaction of the formamide with ammonia to form urea; a reaction of the carbon dioxide-laden methanol phase and the hydrogen-containing stream to form the methyl formate; or a reaction of the aqueous ammonia solution and the hydrogen-containing stream to form the ammonium formate or the ammonium formate and the formamide.

30. The process of claim 29 wherein the ruthenium-phosphine complex comprises at least one monophosphine, one diphosphine, one triphosphine, or one compound having more than three phosphine groups, the phosphine having the formula PR.sup.1R.sup.2R.sup.3, in which R.sup.1, R.sup.2, and R.sup.3 independently of one another are in each case substituted or unsubstituted alkyl, substituted or unsubstituted aryl, or substituted or unsubstituted heteroaryl.

31. The process of claim 29 wherein the ruthenium-phosphine complex is a ruthenium-triphosphine complex, with the triphosphine having a general formula I: ##STR00020## in which R.sup.1 to R.sup.6 independently of one another are substituted or unsubstituted aryl or substituted or unsubstituted heteroaryl, wherein R.sup.7 is hydrogen, alkyl, cycloalkyl, or aryl.

32. The process of claim 29 wherein the ruthenium-phosphine complex has a general formula II, (A)Ru(L).sub.3, in which A is a triphosphine of the general formula I ##STR00021## wherein R.sup.1 to R.sup.6 independently of one another are substituted or unsubstituted aryl or substituted or unsubstituted heteroaryl and R.sup.7 is hydrogen, alkyl, cycloalkyl, or aryl, wherein L in each case independently of one another are monodentate ligands, wherein two monodentate ligands L are replaceable by one bidentate ligand or wherein three monodentate ligands L are replaceable by one tridentate ligand.

33. The process of claim 21 comprising performing the reaction of the carbon dioxide-laden methanol phase and the hydrogen-containing stream to form the methyl formate and/or the reaction of the carbon dioxide-laden aqueous ammonia solution and the hydrogen-containing stream to form the ammonium formate or the ammonium formate and the formamide using a ruthenium-phosphine complex as the catalyst.

34. The process of claim 21 comprising performing a catalytic reaction of the formamide to form urea or a catalytic reaction of the formamide with ammonia to form urea at a temperature in a range from 50 to 250° C. and/or at a pressure in a range from ambient pressure to 150 bar.

35. The process of claim 34 comprising performing the catalytic reaction of the formamide to form urea or the catalytic reaction of the formamide with ammonia to form urea in a nonpolar or polar aprotic organic solvent or in liquid or supercritical ammonia.

36. The process of claim 21 wherein at least one of: a catalytic reaction of the carbon dioxide-laden methanol phase and the hydrogen-containing stream to form the methyl formate is performed at a temperature in a range from 20 to 150° C. and/or at a pressure in a range from 40 bar to 220 bar; a reaction of the methyl formate with ammonia to form the formamide is performed at a temperature in a range from 20° C. to 100° C. and/or at a pressure in a range from atmospheric pressure to 70 bar; a catalytic reaction of the aqueous ammonia solution laden with chemically bound carbon dioxide and the hydrogen-containing stream to form the ammonium formate or the ammonium formate and the formamide is performed at a temperature in a range from 60 to 180° C. and/or at a pressure in a range from 35 bar to 210 bar; or a heat treatment of the ammonium formate to form the formamide is performed at a temperature in a range from 100° C. to 185° C.

37. The process of claim 21 comprising reusing methanol formed in a reaction of the methyl formate with the ammonia-containing stream to form the formamide for the scrubbing fluid or a methanol-containing liquid.

38. The process of claim 21 coupled with ammonia synthesis, the ammonia synthesis comprising preparation of the syngas by steam reforming with a water-gas shift reaction, gas scrubbing the syngas with the scrubbing fluid for removing carbon dioxide, methanization of the syngas as scrubbed, and preparation of ammonia with the syngas, wherein the carbon dioxide-laden scrubbing fluid is used as a source of carbon dioxide for the preparation of urea.

39. The process of claim 38 wherein the hydrogen-containing stream comprises a substream of the syngas after the scrubbing, wherein hydrogen is obtained from processing products of the ammonia synthesis and/or urea synthesis, and/or a source of ammonia comprises ammonia formed in the ammonia synthesis.

40. The process of claim 39 wherein the ammonia synthesis comprises a steam reforming in a primary reformer and a downstream secondary reformer and/or a two-stage water-gas shift reaction with a high-temperature shift stage and a low-temperature shift stage.

Description

[0206] Below, the invention is described with reference to exemplary embodiments, which are elucidated in more detail with reference to the figures. The specific exemplary embodiments are not intended in any way to limit the scope of the claimed invention. In these figures:

[0207] FIG. 1 shows a block diagram of a specific example of a process of the invention for producing urea, coupled with an ammonia synthesis;

[0208] FIG. 2 shows a reaction scheme of the variant of the formamide synthesis via the methanol route (methyl formate);

[0209] FIG. 3 shows a reaction scheme of the variant of the formamide synthesis via the aqueous route (ammonium formate);

[0210] FIG. 4 shows a block diagram of an example of the processing of the urea.

[0211] FIG. 1 shows a block diagram of an example of a process of the invention for producing urea, which is coupled with an ammonia synthesis. A syngas, which in this case comes from steam reforming and is intended for the ammonia synthesis, comprises hydrogen, nitrogen, and carbon monoxide. The syngas is subjected to a water-gas shift reaction (water-gas conversion reaction) in order to convert carbon monoxide into carbon dioxide. After that a gas scrub is performed in order to remove carbon dioxide from the syngas. The gas is scrubbed by means of a scrubbing fluid, with the scrubbing fluid becoming laden with chemically and/or physically bound carbon dioxide during the gas scrubbing.

[0212] In this exemplary embodiment, the gas scrubbing takes place with a scrubbing fluid which is methanol (Rectisol). During the gas scrubbing, the methanol becomes laden with carbon dioxide. The resultant stream is used as a CO.sub.2-containing stream for the catalytic formamide synthesis (methyl formate intermediate). The catalytic formamide synthesis is shown only schematically in FIG. 1; as described, it may be carried out in one stage or two stages (with or without isolation of the intermediate stage) to form methyl formate as intermediate and then to form formamide as the end product. A reaction scheme for this variant is shown in FIG. 2. The resultant formamide is passed as a reactant to the urea synthesis.

[0213] An alternative embodiment is represented with dashed lines. In this case the gas is scrubbed with an aqueous solution of methyldiethanolamine with piperazine as scrubbing fluid (aMDEA scrubbing). Alternatively, any conventional scrubbing fluid known to the skilled person may be used, an example being aqueous potassium carbonate (Benfield scrubbing). CO.sub.2 is desorbed by CO.sub.2 desorption from the carbon dioxide-laden scrubbing fluid, and the CO.sub.2 released is absorbed into methanol, to give a CO.sub.2-laden methanol phase. In analogy to the Rectisol scrubbing described above, the resulting stream is used as a CO.sub.2-containing stream for the catalytic formamide synthesis, for which it is necessary to increase the pressure of the CO.sub.2 before or after the absorption into MeOH.

[0214] In a further, alternative embodiment, not shown, the gas is scrubbed with an aqueous ammonia solution (NH.sub.3—H.sub.2O scrubbing). In this case, CO.sub.2 is bound physically and in the form of carbonates and carbamate in the scrubbing fluid and is used directly as a CO.sub.2-containing stream for the catalytic formamide synthesis (ammonium formate intermediate). In the case of this variant, the catalytic formamide synthesis may be carried out as described in one stage or two stages (with or without isolation of the intermediate) to form ammonium formate as intermediate, and subsequently to form the formamide as the end product. A reaction scheme for the alternative variant based on the aqueous ammonia solution with the subsequent urea synthesis is shown in FIG. 3.

[0215] As a hydrogen-containing stream, a substream of the syngas, containing hydrogen and nitrogen, can be diverted before or after the gas scrubbing and used for the formation of formamide (in FIG. 1, after the gas scrubbing and before the methanization). The rest of the syngas is subjected to methanization, in order to remove further carbon monoxide and/or carbon dioxide from the syngas, by formation of methane. After compression, the syngas is then used for the ammonia synthesis. The reaction mixture recovered from the ammonia synthesis is worked up for recovery of NH.sub.3 and H.sub.2. The resultant ammonia and the resultant hydrogen may also be used for the formamide synthesis. The resultant hydrogen may be returned to the NH.sub.3 synthesis or used as a make-up stream for the formamide synthesis, if small amounts of H.sub.2 are lost in the formamide synthesis. Other variants of the syngas withdrawal have been described above.

[0216] As elucidated above, the hydrogen from the urea synthesis is preferably used as a hydrogen-containing stream for the formation of formamide (shown as a dashed arrow in FIG. 1), and the substream of the syngas may be utilized in the manner of a supplement if the urea synthesis is not running and/or in order to compensate any hydrogen losses in the urea synthesis.

[0217] The methanol likewise formed in the reaction of methyl formate with ammonia to form formamide is reused in the process for the scrubbing fluid or the methanol phase. It is reprocessed together with excess MeOH from the scrubbing fluid, and is returned to the scrubbing.

[0218] Specific examples of the reactions are given later on below.

[0219] The urea synthesis is followed by the processing of the resultant mixture comprising urea, formamide, solvent, catalyst (CAT), ammonia (NH.sub.3) and hydrogen (H.sub.2). Details of the possible processing of the product obtained in the urea synthesis are elucidated below in FIG. 4. Hydrogen obtained from the processing of the products of the urea synthesis is used for the syngas after H.sub.2 and NH.sub.3 recovery and optional processing of H.sub.2/N.sub.2. In other words, the hydrogen formed in the production of the urea from formamide and ammonia can be returned into the operation. The ammonia obtained in the H.sub.2/NH.sub.3 recovery may likewise be reused at numerous points in the operation.

[0220] FIG. 2 shows a schematic reaction scheme of the variant of the formamide synthesis via the “methanol route” to give formamide via methyl formate, and the reaction of the formamide to give urea, corresponding to equations eq. 4, eq. 5 and eq. 6, with the reactant circuit and product circuit. The reactions shown have been described in detail above.

[0221] FIG. 3 shows a schematic reaction scheme of the variant of the formamide synthesis via the “aqueous route” to give formamide via ammonium formate, and the reaction of the formamide to give urea, corresponding to equations eq. 8, eq. 9 and eq. 6, with the reactant circuit and product circuit. The reactions shown have been described in detail above.

[0222] FIG. 4 shows a block diagram of an example for the possible processing of the product obtained in the urea synthesis. In FIG. 4, illustratively, a pressure of 8 bar and a temperature of 150° C. are specified for the urea synthesis. The temperature may be significantly lower, but the pressure may also be significantly higher. Following production of the urea, the temperature may possibly be lowered (heat integration); otherwise a high-pressure flash is produced. A high temperature is advantageous, though, since more hydrogen and ammonia are separated off, even at high pressure. A change in pressure is to be avoided as far as possible. As described above, hydrogen and ammonia are stripped in gas form from the mother liquor, optionally using nitrogen as the stripping gas. This gas mixture is processed in a manner similar to that customary in the ammonia synthesis. The liquid reaction residue, comprising urea, solvent, catalyst, formamide and traces of ammonia, is processed further in accordance with customary methods. The mother liquor is cooled down to about −30° C. and subjected to filtration. In the course of cooling, the predominant fraction of the urea is precipitated from the mother solution. The residue contains urea and traces of solvent, formamide and catalyst. These traces are removed by washing with fresh, optionally cold solvent, and the residue, containing urea and traces of the solvent, is subjected to granulation in order to obtain the urea.

[0223] Alternatively, following removal of urea, the filtrate can be admixed with fresh components NH.sub.3 and formamide (in amounts which have been consumed) and returned directly to the urea synthesis. Ideally, the solvent which remains when the urea isolated by filtration is washed is combined with the filtrate and concentrated, in order to avoid losses of catalyst. The solvent removed by distillation can be reused for the wash, and the concentrated solution can be returned to the urea synthesis.

[0224] The processing of the residue (washing with fresh cold solvent, removal of the catalyst, etc.) may take place alternatively in a separate circuit, which is also closed. The components recovered may be admixed to the main streams (e.g. urea, catalyst or formamide; dashed arrow in FIG. 4) at intervals of time. The wash solution used for the washing is combined with the filtrate. The resultant mixture contains solvent, catalyst, formamide and traces of urea. The mixture is concentrated by evaporation, optionally under reduced pressure. The solvent removed by distillation and the concentrated solution may be reused as elucidated above, with this being the most judicious approach. Alternatively, given sufficient quality, formamide, solvent and distillate may be returned to the process. The residue is recrystallized in order to separate urea and catalyst from one another. The catalyst can be reused in the process.

EXAMPLES

Synthesis of [Ru(triphos)(tmm)]

[0225] A 35 mL Schlenk tube was filled with 319 mg (1.00 mmol) of [Ru(cod)(methylallyl)] (cod=1,5-cyclooctadiene) and 624 mg (1.17 mmol) of 1,1,1-tris(diphenyl-phosphinomethyl)ethane in 20 mL of toluene. The reaction mixture was stirred and was heated at 110° C. for 2 h, cooled to room temperature and concentrated under reduced pressure. Following treatment with 15 mL of pentane, the precipitating complex was isolated, washed with pentane (3×10 mL) and dried under reduced pressure overnight, to give [Ru(triphos)(tmm)] as a pale yellow powder (0.531 g, 0.678 mmol, 68% yield). The identity was confirmed by .sup.1H, .sup.13C APT and .sup.31P NMR spectra.

Examples 1-9

Synthesis of urea from formamide and ammonia with Ru(triphos)(tmm)

[0226] The urea was synthesized in accordance with the following equation:

##STR00009##

[0227] High-pressure batch experiments were performed in a 10 mL autoclave fitted with a glass insert and a magnetic stirring rod. When 2 mL of 1,4-dioxane and 0.6 g of NH.sub.3 were used, the reaction pressure was about 30 bar in the hot state (reaction temperature) and the pressure in the cold state (room temperature) was about 8-10 bar. Before being used, the autoclave was evacuated for at least 30 minutes and filled repeatedly with argon. The catalyst [Ru(triphos)(tmm)] (7.8 mg, 0.01 mmol) was weighed under an argon atmosphere into a Schlenk tube and dissolved in 1,4-dioxane (2.0 mL). Following addition of formamide (40 μL, 1.00 mmol), the reaction mixture was transferred to the autoclave with a canula under an argon countercurrent. Liquid NH.sub.3 (between 0.5 and 1.0 g) was introduced into the autoclave, and the autoclave was sealed. The reaction mixture was stirred and was heated to the respective reaction temperature in an aluminum cone for the respective reaction time. After cooling to room temperature, the autoclave was cautiously let down with air. Following removal of the solvent under reduced pressure, the reaction solution obtained was analyzed by .sup.1H and .sup.13C NMR spectroscopy, using mesitylene as internal standard, and the yield was determined.

[0228] The experiment was repeated a number of times, with the catalyst loading, solvent, reaction temperature and reaction time being varied as shown in table 1 below. Table 1 also shows the yield of urea obtained.

[0229] The catalyst loading is the amount of catalyst used in mol %, relative to the amount of formamide used (in mol).

TABLE-US-00001 TABLE 1 Ru-catalyzed synthesis of urea from formamide and ammonia* Catalyst Reaction Reaction loading temperature time Yield Ex. [mol %] Solvent [° C.] [hours] [%] 1 1.00 1,4-Dioxane 150 5 44 2 1.00 1,4-Dioxane 150 10 64 3 1.00 1,4-Dioxane 150 15 57 4 1.00 1,4-Dioxane 130 10 12 5 1.00 1,4-Dioxane 110 10 1 6 0.50 1,4-Dioxane 150 10 26 7 0.25 1,4-Dioxane 150 10 14 8 1.00 Toluene 150 10 53 9 1.00 THF 150 10 47 *Reaction conditions: [Ru(triphos)(tmm)], 1 mmol formamide, 2 mL solvent, 0.5-1.0 g NH.sub.3

Example 10

Preparation of Ru(Triphos)(Tmm) In Situ for Synthesis of Urea

[0230] The catalyst Ru(triphos)(tmm) was formed in situ from the catalyst precursor [Ru(cod)(methylallyl).sub.2] and triphos.

[0231] For this, 1 mol % of [Ru(cod)(methylallyl).sub.2], 1.3 mol % of triphos, 1 mmol of formamide, 2 mL of 1,4-dioxane and 0.6 g of NH.sub.3 were reacted at 150° C. for 10 h. The pressure was about 8 bar in the cold state and about 30 bar at 150° C. The yield of urea was 51%.

Example 11

Synthesis of Urea from Formamide in the Absence of Ammonia

[0232] 1 mol % of [Ru(Triphos)tmm], 1 mmol of formamide and 2 mL of 1,4-dioxane were reacted at 150° C. and 15 bar for 10 h. The yield of urea was 7%.

Examples 12 to 18

Catalytic Activity of Ru-Phosphine Complexes as a Function of the Ligands on the Phosphorus

[0233] The catalytic activity of various Ru-phosphine complexes in the synthesis of urea from formamide and ammonia was tested as a function of the ligands on the phosphorus. Table 2 indicates the complexes (catalysts) studied, the reaction conditions and the yields obtained. In the experiments the reaction pressure was about 30 bar at the reaction temperature and the pressure in the cold state was about 8 bar, except in ex. 15.

[0234] Ruthenium-triphosphine complexes with the following structure were studied:

##STR00010##

[0235] The nature of the substituent R is shown in table 2 below; where not all of the substituents R on the three phosphorus atoms are the same, the substituents R on a first P atom are identified as R.sup.1, on a second P atom as R.sup.2, and on a third P atom as R.sup.3. For example, the complex of ex. 16 has two phenyl groups on two phosphine groups, and the third phosphine group has two isopropyl groups.

[0236] The ruthenium-triphosphine complex additionally possesses the tridentate ligand trimethylenemethane.

[0237] The pressures reported in the table relate to room temperature (about 23° C.). The autoclave was charged at room temperature and then brought to reaction temperature and reaction pressure.

TABLE-US-00002 TABLE 2 Ex. R = Reaction conditions Urea yield 12 [00011] 1 mol % cat., 1 mmol formamide, 2 mL 1,4- dioxane, 0.6 g NH.sub.3, 150° C., 10 H 64% 13 [00012] 0.5 mol % cat., 1 mmol formamide, 2 mL 1,4- dioxane, 8 bar NH.sub.3, 150° C., 20 h  8% 14 [00013] 0.5 mol % cat., 1 mmol formamide, 2 mL 1,4- dioxane, 8 bar NH.sub.3, 150° C., 20 h  8% 15 [00014] 1 mol % cat., 2 mol % B(C.sub.6F.sub.5).sub.3, 1 mmol formamide, 2 mL 1,4- dioxane, 4 bar NH.sub.3, 150° C., 20 h 16% 16 [00015] 0.5 mol % cat., 1 mmol formamide, 2 mL 1,4- dioxane, 8 bar NH.sub.3, 150° C., 20 h  2% 17 [00016] 1 mol % cat., 1 mmol formamide, 2 mL 1,4- dioxane, 8 bar NH.sub.3, 150° C., 20 h 30% 18 [00017] 0.2 mol % cat., 5 mmol formamide, 4.0 g NH.sub.3, 150° C., 20 h  6%

Examples 19 to 21

Catalytic Activity of Ru-Phosphine Complexes as a Function of the Additional Ligands on Ruthenium (Nonphosphine Ligands)

[0238] The catalytic activity of various Ru-phosphine complexes in the synthesis of urea from formamide and ammonia was tested as a function of the nonphosphine ligands on the ruthenium. Table 3 indicates the complexes (catalysts) studied, the reaction conditions and the yields obtained. In the experiments the pressure was about 30 bar at the reaction temperature and the pressure in the cold state (room temperature) was about 8-10 bar. Example 19 corresponds to example 12.

[0239] Ruthenium-triphosphine complexes with the following structure were studied:

##STR00018##

[0240] The three ligands L are shown in table 3 below, with one ligand L being designated L.sup.1, a second ligand L L.sup.2, and a third ligand L L.sup.3. In example 19 the three ligands L are formed together by the tridentate ligand trimethylenemethane (tmm). The pressures reported in the table relate to room temperature (about 23° C.). The autoclave was charged at room temperature and then brought to reaction temperature and reaction pressure.

TABLE-US-00003 TABLE 3 Ex. L = Reaction conditions Urea yield 19 [00019] 1 mol % cat., 1 mmol formamide, 2 mL 1,4-dioxane, 0.6 g NH.sub.3, 150° C., 10 h 64% 20 L.sup.1 = CO; L.sup.2 = H; L.sup.3 = Cl 1 mol % cat., 1 mmol 18% formamide, 2 mL 1,4-dioxane, 8 bar NH.sub.3, 150° C., 20 h 21 L.sup.1 = CO; L.sup.2, L.sup.3 = H 1 mol % cat., 1 mmol 24% formamide, 2 mL 1,4-dioxane, 0.l7 g NH.sub.3, 150° C., 10 h

Examples 22 to 28

Catalytic Activity of Ru-Phosphine Complexes as a Function of Catalyst Concentration

[0241] The catalytic activity as a function of the catalyst concentration was tested for the following reaction conditions:

[0242] Catalyst: [Ru(triphos)(tmm)], 1 mmol formamide, 2 mL 1,4-dioxane, 0.6 g NH.sub.3, 150° C., 10 h, with the catalyst concentration being varied. The reaction pressure was about 30 bar at the reaction temperature and the pressure in the cold state was about 8-10 bar.

[0243] Table 4 indicates the catalyst concentration (in mol % based on formamide) used under these reaction conditions, and the yields obtained.

TABLE-US-00004 TABLE 4 Ex. c(cat.) [mol %] Urea yield [%] 22 0.05 <1 23 0.25 16 24 1 40 25 5 63

[0244] The catalytic activity as a function of the catalyst concentration was additionally tested for the following reaction conditions:

[0245] Catalyst: [Ru(triphos)(tmm)], 1 mmol formamide, 2 mL 1,4-dioxane, 4 bar NH.sub.3 at room temperature (around 23° C.), 150° C., 20 h, with the catalyst concentration being varied.

[0246] Table 5 indicates the catalyst concentration (in mol % based on formamide) used under these reaction conditions, and the yields obtained.

TABLE-US-00005 TABLE 5 Ex. c(cat.) [mol %] Urea yield [%] 26 0.25 25 27 0.5 30 28 2 33

Examples 29 to 35

Catalytic Activity of Ru-Phosphine Complexes as a Function of the Solvent Concentration

[0247] The catalytic activity as a function of the solvent concentration was tested for the following reaction conditions:

[0248] Catalyst: 1 mol % [Ru(triphos)(tmm)], 1 mmol formamide, 0.6 g NH.sub.3, 150° C., 10 h, with the solvent concentration being varied. The reaction pressure was about 30 bar at the reaction temperature and the pressure in the cold state was about 8-10 bar. The solvent was 1,4-dioxane.

[0249] Table 6 indicates the amount of 1,4-dioxane used under these reaction conditions, in ml (V(1,4-dioxane) [mL]), and the yields obtained.

TABLE-US-00006 TABLE 6 Ex. V(1,4-dioxane) [mL] Urea yield [%] 29 0.5 53 30 0.8 45 31 1.1 50 32 1.4 60 33 1.7 42 34 2.3 37 35 2.6 31

Example 36

[0250] A syngas generated for an ammonia synthesis is subjected to gas scrubbing, using an aqueous ammonia solution having an ammonia fraction of about 5 to 60 wt %, preferably about 5 to 30 wt %, for removing the carbon dioxide from the syngas at the syngas generation pressure of around 36 bar and at a temperature of about 40° C. to 60° C., in an absorber. CO.sub.2 removal from crude syngas with aqueous ammonia is a commonplace process and is also described, for example, in US 2018/0282265 A1.

[0251] Without further processing, the resulting solution or suspension of chemically and physically bound carbon dioxide is mixed in a separate reactor with a substream of the syngas purified by gas scrubbing or with a substream of the unpurified syngas (crude syngas) as hydrogen source. Mixing takes place so as to maintain, at least approximately, a ratio p(NH.sub.3):p(CO.sub.2):p(H.sub.2) in the mixture of 8:32:80 (p=partial pressure at room temperature (23° C.)).

[0252] A catalyst solution composed of the ruthenium-phosphine complex [Ru(Triphos)(tmm)] in solution in toluene at a concentration of 0.7 pmol of catalyst per mL of toluene and 25 equivalents of Al(OTf).sub.3, based on the amount of substance of the catalyst, or Nafion as acid are added to the mixture. The resultant mixture forms a two-phase system (water/toluene) and is reacted at a temperature of about 100° C. and at a pressure of about 180 bar for 12 h. The TON based on ammonium format is 6941, for example.

[0253] Following the reaction, the aqueous phase is isolated. The aqueous phase is then subjected to a thermal decomposition treatment at 176° C. and ambient pressure, which converts the ammonium formate contained therein to an extent of 83% into formamide. Water is removed by distillation during or after the thermal decomposition treatment. The resultant formamide product may be subsequently rectified for purification.

Example 37

[0254] A syngas generated for an ammonia synthesis is subjected to gas scrubbing for removal of CO.sub.2. For this purpose the carbon dioxide is removed from the syngas using what is called a Rectisol process, a physical gas purification process with methanol as scrubbing medium, at the prevailing syngas generation pressure of around 36 bar and at a temperature of about −40° C. to −20° C. For the absorber temperature, a figure of between −40° C. and −30° C. is preferred here. The resulting methanol solution of physically bound carbon dioxide is brought to an increased pressure without further processing, with optional subsequent low-temperature integration and with the aid of a pump. The laden methanol solution is subsequently mixed in a separate reactor with a substream of the syngas purified by gas scrubbing, or with a substream of the unpurified syngas (crude syngas), as a hydrogen source, and admixed with the catalyst in solution in methanol, in order to carry out the reaction sequence according to eq. 4 and eq. 5. To imitate the industrial operation, the ruthenium-phosphine complex [Ru(Triphos)(tmm)] (0.5 μmol, 0.001 mol % based on methanol) and aluminum triflate (Al(OTf).sub.3 (12.5 μmol, 0.025 mol % based on methanol) as Lewis acid in 2 mL of methanol were admixed with the CO.sub.2 in a 10 mL autoclave at room temperature, and so the pressure of the reaction mixture was around 40 bar (syngas generation pressure). Hydrogen or a hydrogen-nitrogen mixture (molar ratio 3:1, imitating the ammonia syngas) was passed into the autoclave, so that the total pressure in the autoclave was brought to 120 bar. The mixture was reacted at a temperature of 100° C. and at a resultant pressure of about 180 bar for 18 hours, to form a reaction mixture comprising methyl formate. Depending on catalyst loading, a turnover number (TON) of 4000 is attained for methyl formate.

[0255] The reaction mixture was cooled, the pressure was lowered, and the gaseous reactants were removed. The mixture comprising methyl formate was admixed with NH.sub.3 (8 bar total pressure at room temperature) and stirred at a temperature of 60° C. (reaction pressure of around 27 bar) for an hour. The methanol released can be subsequently removed by distillation from the formamide product. The reaction of methyl formate with ammonia to give the formamide is virtually quantitative.