CATALYST FOR AMMONIA SYNTHESIS WITH IMPROVED ACTIVITY

Abstract

An iron-containing catalyst for ammonia synthesis, characterized in that it contains the promoters potassium, calcium and aluminum, wherein the proportion of potassium, calculated as K.sub.2O, is 0.08% to 0.6% by weight, the proportion of calcium, calculated as CaO, is 0.8% to 2.2% by weight and the proportion of aluminum, calculated as Al.sub.2O.sub.3, is 1.0% to 2.3% by weight, is described. The invention further relates to the production of the catalyst according to the invention and to a process for ammonia synthesis using the catalyst according to the invention.

Claims

1. An iron-containing catalyst for ammonia synthesis, wherein it contains the promoters potassium, calculated as K.sub.2O, in the range from 0.8% to 0.6% by weight, calcium, calculated as CaO, in the range from 0.8% to 2.2% by weight and aluminum, calculated as Al.sub.2O.sub.3, in the range from 1.0% to 2.3% by weight, based on the total weight of the catalyst.

2. The catalyst as claimed in claim 1, wherein it contains potassium, calculated as K.sub.2O, in the range from 0.1% to 0.5% by weight, more preferably 0.15% to 0.4% by weight, most preferably 0.15% to 0.3% by weight, calcium, calculated as CaO, in the range from 0.8% to 2.0% by weight, more preferably 1.1% to 1.8% by weight, more preferably 1.2% to 1.6% by weight, most preferably 1.25% to 1.55% by weight, and aluminum, calculated as Al.sub.2O.sub.3, in the range from 1.2% to 2.0% by weight, more preferably 1.3% to 1.9% by weight, most preferably 1.35% to 1.75% by weight, based on the total weight of the catalyst.

3. The catalyst as claimed in claim 1, wherein the proportion of iron compounds is in the range from 80.0% to 100.0% by weight, preferably in the range from 80.0% to 99.9% by weight, more preferably in the range from 90% to 99.9% by weight, particularly preferably in the range from 90.0% to 97.0% by weight, based on the total weight of the catalyst.

4. The catalyst as claimed in claim 1, wherein the proportion of wuestite in the iron compounds in the catalyst is at least 50% by weight, preferably 80% by weight, more preferably 85% by weight, more preferably 90% by weight, very particularly preferably 100% by weight.

5. The catalyst as claimed in claim 1, wherein the catalyst contains a proportion of further promoters, calculated as oxides, of 0.1% to 20.0% by weight, preferably 0.1% to 10.0% by weight, particularly preferably 1.0% to 5.0% by weight, most preferably 1.5% to 2.5% by weight, based on the total weight of the catalyst.

6. A process for producing a catalyst according to any of the preceding claims, by: a) mixing elemental iron, an iron-containing compound, compounds of the promoters potassium, aluminum, calcium and optionally compounds of further promoters to obtain a mixture, b) melting the mixture obtained in step a), c) cooling the melt from step b) to obtain a solid of the catalyst d) comminuting the solid obtained in step c), wherein the compounds of the promoters potassium, calcium and aluminum are initially charged in step a) in such a way that the catalyst resulting from step d) contains potassium, calculated as K.sub.2O, in a proportion of 0.08% to 0.6% by weight, calcium, calculated as CaO, in a proportion of 0.8% to 2.2% by weight and aluminum, calculated as Al.sub.2O.sub.3, in a proportion of 1.0% to 2.3% by weight, based on the total weight of the catalyst.

7. The process as claimed in claim 6, wherein the melting in step b) is carried out in an electric arc furnace.

8. The process as claimed in claim 6, wherein the iron-containing compound is FeO, Fe.sub.2O or Fe.sub.3O.sub.4, preferably Fe.sub.3O.sub.4.

9. The process as claimed in claim 6, wherein the employed compounds of the promoters potassium, calcium and aluminum are the corresponding oxides, hydroxides, carbonates, hydrogencarbonates or nitrates, preferably the corresponding oxides, carbonates or nitrates.

10. The process as claimed in claim 6, wherein compounds of the promoters V, Co, Mg, the rare earths, or a combination thereof, preferably compounds of V or Mg or a combination thereof, are added in step a).

11. A process for ammonia synthesis using a catalyst as claimed in claim 1.

12. The process as claimed in claim 11, wherein the reaction fluid contains up to 100 ppmv, preferably 1 to 10 ppmv, of gaseous H.sub.2O.

13. The process as claimed in claim 11, wherein said process includes a preceding step of reducing the catalyst during which the concentration of H.sub.2O is in a range from 100 to 5000 ppmv based on the stream that has exited the reactor for a duration of 12-120 h.

14. The process as claimed in claim 13, wherein the concentration of H.sub.2O is 2000 to 5000 ppmv based on the stream that has exited the reactor for a duration of 10 minutes to 8 hours during the reduction.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0044] FIG. 1 shows the powder X-ray diffractograms of catalysts 1a to 1d, 1f to 1l and comparative catalyst 1e.

[0045] FIG. 2 shows the powder X-ray diffractograms of catalysts 2a and 2d.

[0046] FIG. 3 shows the yield of ammonia for catalyst 2a and for comparative catalyst 1e over the course of several cycles.

[0047] FIG. 4 shows the yield of ammonia for catalyst 2b and for comparative catalyst 1e as a function of reaction temperature.

[0048] FIG. 5 shows a representation of the H.sub.2O and NH.sub.3 concentrations generated by the catalyst 2a and the comparative catalyst 1e as a function of the temperature increase.

EXPERIMENTAL

Methods of Measurement

Powder X-Ray Diffraction

[0049] Determination of the crystal structures present in the catalyst and the weight fraction thereof was by X-ray diffractometry and Rietveld refinement. For this, the sample was measured in a Bruker D4 Endeavor instrument over a range from 5 to 90 2 (step sequence 0.020 2, 1.5 seconds measurement time per step). The radiation used was CuK1 radiation (wavelength 1.54060 , 40 kV, 35 mA). During the measurement, the sample stage was rotated about its axis at a speed of 30 revolutions/min. The obtained diffractogram of the reflection intensities was quantitatively analyzed by Rietveld refinement and the proportion of the respective crystal structure in the sample was determined. The proportion of the respective crystal structure was determined using TOPAS version 6 software from Bruker.

Elemental Analysis

[0050] Determination of chemical elements was by ICP analysis (inductively coupled plasma) according to DIN EN ISO 11885.

[0051] Determination of potassium was by AAS analysis (atomic absorption spectrometry) according to E13/E14 Deutsche Einheitsverfahren zur Wasser Abwasser and Schlammuntersuchung, volume 1, 1985.

Example 1: Catalysts 1a to 1d, 1f to 1l and Comparative Catalyst 1e

[0052] Catalysts 1a to 1d, 1f to 1l and comparative catalyst 1e were produced by mixing a mixture of magnetite and iron powder in a stoichiometric ratio of 1:1 with KNO.sub.3, Al.sub.2O.sub.3 and CaCO.sub.3 and further metal oxide-based promoters, homogenized and subsequently melted in an arc furnace, wherein for production of the catalysts 1a to 1d only the proportion of KNO.sub.3 was varied while for the comparative catalyst 1e the proportion of Al.sub.2O.sub.3 was additionally varied. The proportions of K.sub.2O, Al.sub.2O.sub.3 and CaO were varied for production of the catalysts 1f to 1l. Once the mixture was completely melted, the melt was cooled in a melt mold and the cooled material was converted into particles by crushing the material in a jaw crusher. The powder X-ray diffractograms of the individual catalysts are shown in FIG. 1 and show as the only iron oxide structure that of wuestite, whose reflections are likewise shown in the diffractogram for reference. The elemental compositions of the individual catalysts are shown in table 1.

TABLE-US-00001 TABLE 1 Content of promoters K, Al and Ca in the catalysts 1a to 1l Potassium content, Aluminum content, Calcium content, calculated as K.sub.2O, calculated as Al.sub.2O.sub.3, calculated as CaO, Catalyst in % by weight in % by weight in % by weight 1a 0.086 2.15 2.08 1b 0.173 2.12 2.08 1c 0.253 2.08 2.07 1d 0.506 2.12 2.03 1e 0.687 2.34 2.00 1f 0.113 1.80 1.65 1g 0.163 1.78 1.62 1h 0.218 1.68 1.57 1i 0.361 1.81 1.57 1j 0.434 1.25 1.19 1k 0.252 1.23 1.11 1l 0.410 1.19 1.05

Use Example 1

[0053] Inventive catalysts 1a to 1d, 1f to 1l and the comparative catalyst 1e were employed in a reaction for ammonia synthesis.

[0054] To this end, 7 g of catalyst sample in the form of the fraction having a particle diameter of 450 to 550 micrometers were charged into a reactor and at a reactor pressure of 90 bar a gas stream consisting of nitrogen (22.5% by volume), hydrogen (67.5% by volume) and argon (10% by volume) was passed therethrough. The temperature in the reactor interior was continuously increased to 520 C. and maintained at this temperature until reduction of the catalyst was complete. The pressure was then increased to 100 bar and the temperature reduced to 400 C. and these conditions were maintained for 22 hours. Once the 22 hours had elapsed the concentration of ammonia formed was detected and the temperature was subsequently increased to 520 C. and maintained for 14 hours to bring about accelerated deactivation of the catalyst. Thereafter, the procedure described above (maintaining the temperature at 400 C. for 22 h followed by increasing the temperature to 520 C. for 14 h) was repeated two further times. Ammonia concentration results are summarized in table 2.

TABLE-US-00002 TABLE 2 Relative ammonia space-time yields for catalysts 1a to 1l Relative ammonia space-time yield per cycle [%] Catalyst cycle 1 cycle 2 cycle 3 1a 91.74 93.98 95.70 1b 97.76 99.31 100.34 1c 98.45 97.25 97.76 1d 100.00 96.21 96.04 1e 96.39 93.29 92.60 1f 91.76 97.01 98.86 1g 99.71 100.93 100.93 1h 100.81 102.30 102.27 1i 101.91 100.34 98.95 1j 99.42 99.72 98.42 1k 90.59 96.34 98.52 1l 98.80 101.81 102.32

[0055] It is apparent from table 2 that at the latest in the 2nd cycle the inventive catalysts bring about a higher yield of ammonia than the comparative catalyst and in the case of catalysts 1a, 1b, 1c, 1f, 1g, 1h, 1j, 1k and 1l activity even increases with increasing cycle duration.

Example 2: Catalyst 2a and 2b

[0056] Catalysts 2a and 2b were produced according to the procedure in example 1, wherein the amounts of potassium, aluminum and calcium compounds were chosen such that the resulting catalysts had the following elemental composition based on the corresponding oxides: [0057] Catalyst 2a: 0.25% by weight K.sub.2O, 1.46% by weight CaO, 1.64% by weight Al.sub.2O.sub.3 [0058] Catalyst 2b: 0.31% by weight K.sub.2O, 1.48% by weight CaO, 1.70% by weight Al.sub.2O.sub.3

[0059] Again, wuestite was identified as the only iron oxide structure, as shown in FIG. 2. The reflections of the wuestite are likewise shown in the diffractogram for reference.

Use Example 2

[0060] Inventive catalyst 2a and comparative catalyst 1e were employed in a reaction for ammonia synthesis.

[0061] To this end, 120 g of catalyst sample in the form of a granulate having diameters of 1.5-3.0 mm were charged into a reactor and at a reactor pressure of 90 bar a gas stream consisting of nitrogen (22.5% by volume), hydrogen (67.5% by volume) and argon (10% by volume) was passed therethrough. The temperature in the reactor interior was continuously increased to 520 C. and maintained at this temperature until reduction of the catalyst was complete. The pressure was then increased to 100 bar and the temperature reduced to 400 C. and these conditions were maintained for 19 hours. Once the 19 hours had elapsed the concentration of ammonia formed was detected and the temperature was subsequently increased to 520 C. and a pressure of 150 bar and maintained for 10 hours to bring about accelerated deactivation of the catalyst. Thereafter, the procedure described above (maintaining the temperature at 400 C. and 100 bar for 19 h followed by increasing the temperature to 520 C. and 150 bar for 10 h) was repeated eleven further times for catalyst 2a and seven further times for comparative catalyst 1e. Ammonia concentration results are summarized in FIG. 3.

Use Example 3

[0062] Catalyst 2b and comparative catalyst 1e were tested in a process for ammonia synthesis in which the employed gas mixture additionally contained H.sub.2O. To this end, 120 g of catalyst sample in the form of a granulate having diameters of 1.5-3.0 mm were charged into a reactor and at a reactor pressure of 90 bar a gas stream consisting of nitrogen (22.5% by volume), hydrogen (67.5% by volume), 80 ppmv of H.sub.2O and argon (balance to 100% by volume) was passed therethrough. The temperature in the reactor interior was continuously increased to 520 C. and maintained at this temperature until reduction of the catalyst was complete. The pressure was then increased to 100 bar and the temperature reduced to 400 C. and these conditions were maintained for 24 hours. Once the 24 hours had elapsed the concentration of ammonia formed was detected. This test was repeated for different reaction temperatures, wherein each temperature stage was held for 8 h. Ammonia concentration results are summarized in FIG. 4.

Use Example 4

[0063] Catalyst 2b and comparative catalyst 1e were tested in respect of their reduction behavior. To this end, 120 g of catalyst sample in the form of a granulate having diameters of 1.5-3.0 mm were charged into a reactor and at a reactor pressure of 90 bar a gas stream consisting of nitrogen (22.5% by volume), hydrogen (67.5% by volume) and argon (10% by volume) was passed therethrough. The temperature in the reactor interior was continuously increased to 520 C. and maintained at this temperature until reduction of the catalyst was complete. The progress of the reduction is shown in FIG. 5. This shows a plot of water concentration and ammonia concentration as a function of the temperature inside the catalyst bed. It is apparent that the inventive catalyst 2a is reduced to the metallic state more quickly than the comparative catalyst 1e, as apparent from an earlier increase in the water concentration. Since the reduced state is achieved more quickly, catalyst 2a can also achieve earlier conversion of a portion of the nitrogen and hydrogen present in the gas stream into ammonia. Due to the improved reduction behavior of the inventive catalyst, ammonia synthesis may be performed with considerable time savings on the industrial scale too.