This invention is directed to the production of nitrous oxide, N₂O, from ammonia and nitric oxide, NO, over a catalyst comprising molybdenum oxide that is supported on a Group IVB metal oxide support such as zirconia and that comprises a Group VIIIB metal oxide component such as iron oxide.

One commercial process for making nitrous oxide involves the thermal decomposition of ammonium nitrate when heated from 200 to 260°C according to the following equation:



        NH₄NO₃ - N₂O + 2H₂O



However, extreme caution must be used in handling ammonium nitrate, which may be highly explosive under extreme shock or elevated temperatures. Therefore, improved methods for controlling the thermal decomposition of ammonium nitrate have been disclosed. For example, the thermal decomposition of ammonium nitrate in the presence of a melt containing ammonium hydrogen sulfate and ammonium sulfate is described in US-A- 4,154,806 ; the thermal decomposition of ammonium nitrate into an aqueous, strongly acid reaction liquor containing chloride ions as a catalyst is described in US-A- 4,102,986 ; and the thermal decomposition of ammonium nitrate in a chloride-containing aqueous solution of nitric acid and the presence of catalytically active ions of manganese, copper, cerium, lead, bismuth, cobalt or nickel, is described in US-A- 3,656,899 .

Another commercial process for producing nitrous oxide involves the reaction of ammonia and air using Mn and Bi oxides as catalysts. For example, Japanese Patent No. 6122507 describes a process for preparing nitrous oxide by the oxidation of ammonia with oxygen in the presence of steam and a CuO/MnO₂ catalyst. Other methods for producing nitrous oxide include the reaction of a molten nitrate salt with ammonium chloride and the reaction of ammonia with at least one molten nitrate salt of an alkaline earth metal, as described in US-A- 4,720,377 .

The article "Selective oxidation of ammonia to nitrogen over SiO₂ supported MO₃ catalysts" by M. de Boer et al in Catalysis Today, 17 (1993) page 189, describes the testing of V₂O₅, MoO₃ and WO₃ catalysts on various supports in the selective catalytic reduction of NO. It is observed that minor amounts of N₂O are formed. The article proposes a reaction mechanism to explain the formation of N₂O.

US-A-5 401 478 provides a catalytic method for converting nitrogen oxides to nitrogen (i.e., N2) in the presence of a catalyst comprising an acidic solid component comprising a Group IVB metal oxide modified with an oxyanion of a Group VIB metal, such as zirconia modified with tungstate.

The present invention is directed to the conversion of ammonia with nitric oxide and/or oxygen into nitrous oxide using a catalyst comprising molybdenum oxide, Group VIII B metal oxide and Group IV B metal oxide. Accordingly, the present invention avoids the dangers associated with the use and handling of ammonium nitrate and is an alternative and novel nitrous oxide synthesis method that is inexpensive and safe.

The present invention is directed to a process for producing nitrous oxide comprising the step of reacting ammonia with nitric oxide and/or oxygen in the presence of a catalyst as specified in claim 1, to produce an effluent mixture comprising nitrous oxide.

The process of the present invention utilizes a catalyst that comprises the molybdenum oxide and Group VIIIB metal oxide, in combination with the Group IVB metal oxide as a support.

Sources of the molybdenum oxide may include pure molybdenum oxide MoO_3, ammonium heptamolybdate tetrahydrate (NH₄)₆Mo₇O₂₄ - 4H₂O, and molybdenum halides and oxyhalides.

The Group IVB metal oxide used in the process of the present invention may be an oxide of titanium, zirconium or hafnium. Preferably, the Group IVB metal oxide is zirconia. Sources of the Group IVB metal oxide may include zirconyl chloride and zirconyl nitrate. The weight ratio of the molybdenum oxide to Group IVB metal oxide is from 0.1:1 to 0.3:1.

The Group VIIIB metal oxide may be an oxide of iron. Preferably, the Group VIIIB metal oxide is iron oxide. Sources of the Group VIIIB metal oxide may include iron (II) sulfate heptahydrate FeSO₄ · 7H₂O, iron halides and iron nitrate. The molar ratio of the Group VIIIB metal oxide to the Group VIB metal oxide is preferably from 0.005:1 to 0.05:1.

The catalyst used in the process of the present invention is prepared by combining a first liquid solution comprising a source of the molybdenum oxide, and a second liquid solution comprising a source of the Group IVB metal oxide and a third liquid solution comprising a source of the Group VIIIB metal oxide, under conditions sufficient to cause precipitation of the catalyst. Examples of the precipitating reagent include ammonium hydroxide, alkylammonium hydroxide and alkaline hydroxides such as sodium hydroxide. Water is a preferred solvent for these solutions.

The temperature at which the liquid solution is maintained during precipitation may be less than 200°C, e.g., from 0 to 200°C. A particular range of such temperatures is from 50 to 100°C. The pH range of the liquid solution during precipitation is from 4 to 11.

The catalyst may be recovered from the liquid solution by filtration, followed by drying. The catalyst may be subjected to a final calcination as described below to dehydrate the catalyst and to confer the required mechanical strength on the catalyst.

Calcination of the catalyst may be carried out, preferably in an oxidizing atmosphere, at atmospheric pressure to 6890 kPa (1000 psi); and at temperatures from 500 to 850°C. The calcination time may be up to 48 hours, e.g., for 0.5 to 24 hours, e.g., for 1.0 to 10 hours.

Nitric oxide and ammonia are converted to nitrous oxide in the present invention by a reaction which may be described by the following equation:



        4NH₃ + 4NO + 3O₂ - 4N₂O + 6H₂O,



although other reactions may also occur.

An additional reaction that occurs when oxygen is present is the oxidation of ammonia with oxygen to form nitrous oxide which may be described by the following equation:



        2NH₃ + 2O₂ - N₂O + 3H₂O

The molar feed ratio of nitric oxide to ammonia ranges from 0.01:1 to 10:1; the amount of oxygen fed to the reaction unit ranges from 0.01 to 35 molar %.

The method according to the present invention is carried out at a temperature ranging from 200 to 600°C, and preferably from 300 to 500°C; a pressure ranging from 10.1 kPa to 10.1 MPa (0.1 to 100 atmospheres), and preferably from 50.6 to 506 kPa (0.5 to 5 atmospheres). The gas hourly space velocity for the reaction ranges from 1,000 to 10,000,000 hr^-1, and preferably from 20,000 to 1,000,000 hr^-1.

The effluent mixture comprises nitrogen, oxygen, nitric oxide, ammonia and the desired product, nitrous oxide. The nitrous oxide may be separated and recovered from the effluent mixture by conventional means known to skilled artisans or the entire effluent may be used as a feed to a separate reactor. Alternatively, the nitrous oxide can be utilized in situ as a selective oxidant for various substrates. For example, benzene could be co-fed with the reactants and phenol product recovered.

The invention will now be more particularly described with reference to the Examples, each of which discloses the conversion of a gaseous mixture containing ammonia together with nitric oxide and/or oxygen, with the balance in each case being helium. Examples IV and XII are according to the invention as claimed.

Five hundred grams of ZrOCl₂.8H₂O were dissolved with stirring in 6.0 liters of distilled water. To this, a solution containing 33 grams of (NH₄)₆Mo₇O₂₄·4H₂O in 500 ml of H₂O was added. Finally, a solution containing 263 ml of conc. NH₄OH and 500 ml of distilled H₂O was added dropwise over a 30-45 minute period. The pH of the solution was approximately 9. This slurry was then placed in a steambox for 72 hours. The product formed was recovered by filtration, washed with excess water; and dried overnight at 85°C. The material was then calcined in air at 700°C for 3 hours. Chemical analysis of the dried materials showed a Mo/Zr ratio of 0.11 (weight basis); this corresponds to a water-free sample composition of approximately 7.3% Mo.

Table 1 shows the activity and selectivity of this sample for the conversion of 500 ppm each of ammonia and nitric oxide into N₂O at a space velocity of approximately 600,000 hr^-1. At 500°C, 60% of the inlet ammonia and nitric oxide is converted. The selectivity to N₂O is approximately 61 %; the remaining 39% of the material is converted to N₂. inlet concentrations outlet concentrations selectivity to T(°C) O₂(%) NH₃(ppm) NO(ppm) NH₃(ppm) NO(ppm) N₂(ppm) N₂O(ppm) N₂O 300 1 500 500 448 447 45 5 9.9% 350 1 500 500 399 397 84 14 14.4% 400 1 500 500 345 345 105 44 29.6% 450 1 500 500 273 288 109 107 49.7% 500 1 500 500 173 230 115 180 61.1%

This catalyst was prepared in a manner identical to Example I except that it was calcined in air at 830°C for 3 hours. Table 2 shows that calcination of this material at a higher temperature gives higher selectivity to N₂O at temperatures of 400°C and below than the catalyst of Example I, but a lower selectivity at 500°C. The selectivity of this sample is much less affected by temperature than that of the catalyst used in Example I. inlet concentrations outlet concentrations selectivity to T(°C) O₂(%) NH₃(ppm) NO(ppm) NH₃(ppm) NO(ppm) N₂(ppm) N₂O(ppm) N₂O 300 1 500 500 465 473 14 16 52.7% 350 1 500 500 397 432 37 47 55.7% 400 1 500 500 291 397 70 85 54.8% 450 1 500 500 193 378 101 110 52.2% 500 1 500 500 132 378 124 117 48.5%

Five hundred grams of ZrOCl₂·8H₂O were dissolved with stirring in 6.0 liters of distilled water. To this, a solution containing 66 grams of (NH₄)₆Mo₇O₂₄·4H₂O in 500 ml of H₂O was added. Finally, a solution containing 263 ml of conc. NH₄OH and 500 ml of distilled H₂O was added dropwise over a 30-45 minute period. The pH of the solution was adjusted to approximately 9 by the addition of 146 grams of concentrated NH₄OH. This slurry was then placed in a steambox for 72 hours. The product formed was recovered by filtration, washed with excess water, and dried overnight at 85°C. The material was then calcined in air at 600°C for 3 hours. Chemical analysis of the dried materials showed an Mo/Zr ratio of 0.19 (weight basis); this corresponds to a water-free sample composition of approximately 11.4% Mo. Table 3 shows the activity and selectivity of this sample for N₂O formation. The higher concentrations of molybdenum gives both higher activities and selectivities than observed in Example I. inlet concentrations outlet concentrations selectivity to T(°C) O₂(%) NH₃(ppm) NO(ppm) NH₃(ppm) NO(ppm) N₂(ppm) N₂O(ppm) N₂O 300 1 500 500 385 385 91 15 14.5% 350 1 500 500 322 321 121 50 29.0% 400 1 500 500 252 269 109 125 53.5% 450 1 500 500 167 228 95 205 68.4% 500 1 500 500 92 211 99 250 71.7%

Five hundred grams of ZrOCl₂·8H₂O were dissolved with stirring in 6.0 liters of distilled water. To this, a solution containing 66 grams of (NH₄)₆Mo₇O₂₄·4H₂O in 500 ml of H₂O was added. Another solution containing 7.5 grams of FeSO₄·7H₂O in 500 ml of H₂O was added. Finally, a solution containing 263 ml of conc. NH₄OH and 500 ml of distilled H₂O was added dropwise over a 30-45 minute period. The pH of the solution was adjusted to approximately 9. This slurry was then placed in a steambox for 72 hours. The product formed was recovered by filtration, washed with excess water, and dried overnight at 85°C. The material was then calcined in air at 600°C for 3 hours. Chemical analysis of the dried materials showed an Mo/Zr ratio of 0.20 (weight basis) and an Fe/Zr ratio of 0.012; this corresponds to a water-free sample composition of approximately 12.1% Mo and 0.72% Fe. Table 4 shows the activity and selectivity of this iron-containing sample. inlet concentrations outlet concentrations selectivity to T(°C) O₂(%) NH₃(ppm) NO(ppm) NH₃(ppm) NO(ppm) N₂(ppm) N₂O(ppm) N₂O 300 1 500 500 387 382 90 19 17.2% 350 1 500 500 310 314 117 62 34.4% 400 1 500 500 220 259 110 145 56.9% 450 1 500 500 132 226 101 216 68.1% 500 1 500 500 70 222 106 246 69.8%

250 grams of ZrOCl₂·8H₂O were dissolved with stirring in 1.5 liters of distilled water. This solution was heated to approximately 60°C. To this, a solution containing 33 grams of (NH₄)₆Mo₇O₂₄·4H₂O and 130 grams of conc. NH₄OH in 1250 ml of H₂O was added. This solution was also heated to approximately 60°C. The solution containing the zirconyl chloride was slowly added to the second solution with mixing. The pH of the solution was approximately 7. This slurry was then placed in a steambox for 72 hours. The product formed was recovered by filtration, washed with excess water, and dried overnight at 85°C. The material was then calcined in air at 600°C for 3 hours. Chemical analysis of the dried materials showed a Mo/Zr ratio of 0.24 (weight basis); this corresponds to a water-free sample composition of approximately 14.1% Mo. Table 5 shows that catalyst synthesis at a higher temperature results in a sample with slightly higher selectivity to N₂O, especially at temperatures of 400°C and below, as compared to Example III. inlet concentrations outlet concentrations selectivity to T (°C) O₂(%) NH₃(ppm) NO(ppm) NH₃(ppm) NO(ppm) N₂(ppm) N₂O(ppm) N₂O 300 1 500 500 371 362 90 30 25.3% 350 1 500 500 272 279 117 93 44.2% 400 1 500 500 162 223 112 185 62.2% 450 1 500 500 78 202 106 246 69.9% 500 1 500 500 32 208 108 265 71.1%

This catalyst was prepared in a manner identical to Example V except that it was calcined in air at 700°C for 3 hours. Table 6 shows that calcining at 700°C lowers overall activity somewhat compared to Example V; selectivities to N₂O are higher at 400°C and below but are lower at 450 and 500°C. inlet concentrations outlet concentrations selectivity to T (°C) O₂(%) NH₃(ppm) NO(ppm) NH₃(ppm) NO(ppm) N₂(ppm) N₂O(ppm) N₂O 300 1 500 500 402 403 62 30 32.8% 350 1 500 500 306 330 89 85 49.0% 400 1 500 500 197 280 98 157 61.7% 450 1 500 500 111 260 106 204 65.9% 500 1 500 500 57 261 115 220 65.8%

This catalyst was prepared in a manner identical to Example V except that it was calcined in air at 830°C for 3 hours. Table 7 shows that calcining at 830°C lowers both activity and selectivity to N₂O. inlet concentrations outlet concentrations selectivity to T(°C) O₂(%) NH₃(ppm) NO(ppm) NH₃(ppm) NO(ppm) N₂(ppm) N₂O(ppm) N₂O 300 1 500 500 449 453 29 21 41.7% 350 1 500 500 376 401 54 57 51.1% 400 1 500 500 276 361 77 102 57.0% 450 1 500 500 183 336 97 140 59.2% 500 1 500 500 115 327 112 161 59.0%

A solution containing 41.4 grams of (NH₄)₆Mo₇O₂₄·4H₂O and 200 grams of H₂O was prepared. The solution was slowly added to 150 grams of silica (HiSil 233) and placed in a polypropylene bottle. This mixture was mixed using a roller overnight. The product formed was recovered by filtration, washed with minimum water, and dried overnight at 85°C. The material was then calcined in air at 540°C for 3 hours. From the preparation we expect this sample to contain approximately 15% Mo by weight. Table 8 shows that this Mo/SiO₂ catalyst can also convert nitric oxide and ammonia into N₂O, although in this case with a lower selectivity than most of the examples described above. inlet concentrations outlet concentrations selectivity to T(°C) O₂(%) NH₃(ppm) NO(ppm) NH₃(ppm) NO(ppm) N₂(ppm) N₂O(ppm) N₂O 300 1 500 500 471 472 21 10 31.1% 350 1 500 500 442 442 40 18 31.3% 400 1 500 500 392 401 66 32 32.8% 450 1 500 500 323 351 100 54 35.2% 500 1 500 500 205 280 150 92 38.1%

Table 9 shows that bulk MoO₃, obtained from the Aldrich Chemical Company, is also an effective catalyst for converting nitric oxide and ammonia into N₂O. Compared to the supported catalysts, selectivities at 350°C and below are higher, but fall steadily as temperature increases. inlet concentrations outlet concentrations selectivity to T(°C) O₂(%) NH₃(ppm) NO(ppm) NH₃(ppm) NO(ppm) N₂(ppm) N₂O(ppm) N₂O 300 1 500 500 462 466 10 33 76.0% 350 1 500 500 386 415 29 73 71.4% 400 1 500 500 243 353 72 128 63.9% 450 1 500 500 32 340 151 148 49.5% 500 1 500 500 0 449 194 61 23.8%

The data shown in Table 10 were collected using the catalyst described in Example III. They show that selectivity to N₂O is not a strong function of inlet nitric oxide and ammonia concentration when nitric oxide and ammonia are fed in equimolar amounts. There is no reason to believe that feeding much higher concentrations of nitric oxide and ammonia would not also result in the formation of correspondingly high concentrations of N₂O. inlet concentrations outlet concentrations selectivity to T(°C) O₂(%) NH₃(ppm) NO(ppm) NH₃(ppm) NO(ppm) N₂(ppm) N₂O(ppm) N₂O 500 1 500 500 242 311 63 157 71.4% 500 1 200 200 84 132 25 68 73.4% 500 1 350 350 163 220 45 114 71.7% 500 1 750 750 387 463 95 230 70.9% 500 1 907 907 481 560 115 277 71.6%

The data shown in Table 11 were collected using the catalyst described in Example V. They show that significant amounts of N₂O are formed even if nitric oxide and ammonia are not fed in equimolar amounts. inlet concentrations outlet concentrations selectivity to T(°C) O₂(%) NH₃(ppm) NO(ppm) NH₃(ppm) NO(ppm) N₂(ppm) N₂O(ppm) N₂O 500 1 500 500 32 208 108 265 71.1% 500 1 625 375 54 108 148 260 63.7%

The data shown in Table 12 were collected using the catalyst described in Example IV. They show that some N₂O is formed even when no nitric oxide is fed, i.e., when only ammonia and oxygen are fed. Selectivities are lower, however, than when equimolar amounts of nitric oxide and ammonia are fed. inlet concentrations outlet concentrations selectivity to T(°C) O₂(%) NH₃(ppm) NO(ppm) NH₃(ppm) NO(ppm) N₂(ppm) N₂O(ppm) N₂O 445 1 500 0 211 0 119 17 12.5% 445 2 500 0 212 0 124 20 13.9% 445 3 500 0 214 0 127 22 14.5% 550 1 500 0 48 18 149 61 29.0% 550 3 500 0 51 19 143 78 35.3% 550 0.6 500 0 54 14 148 58 28.5% 550 0.6 1000 0 145 12 318 90 22.2% 550 0.8 1840 0 337 9 606 125 17.1%

The data shown in Table 13 were collected using the catalyst described in Example III calcined at 600°C. They show that the conversion and N₂O selectivity are increased as increasing amounts of oxygen in the 1-4.5% range are fed Outlet concentrations percent selectivity to T (°C) inlet O₂(ppm) NH₃(ppm) NO(ppm) N₂(ppm) N₂O(ppm) conversion* N₂O† 450 1 319 347 52 111 32.5 68.3 450 4.5 319 342 53 116 33.8 68.6 475 1 278 326 58 137 38.9 70.4 475 4.5 276 320 57 145 40.4 71.8 500 1 247 310 62 159 44.2 72.0 500 2 240 306 61 167 45.6 73.5