Pre-carburized molybdenum-modified zeolite catalyst and use thereof for the aromatization of lower alkanes

Abstract

The present invention relates to a method for producing a zeolite catalyst useful for aromatization of a lower alkane, a zeolite catalyst useful for aromatization of a lower alkane obtainable by said method and a process for aromatization of a lower alkane using the zeolite catalyst of the present invention.

Claims

1. A zeolite catalyst useful for aromatization of a lower alkane obtainable by the method comprising: contacting a medium pore zeolite catalyst precursor with a pre-carburizing gas stream comprising a pre-carburizing gas stream lower alkane and 50-90 more-% of an inert diluent gas at a temperature that is increased from 20-250 C./minute or less to a temperature useful for aromatization and keeping the temperature constant for 0-60 minutes at the temperature useful for aromatization to produce the zeolite catalyst; wherein the zeolite catalyst precursor comprises 2-10 wt % molybdenum (Mo) and 0-2 wt % of an additional element selected both Groups 6-11 of the Periodic Table; wherein the zeolite is de-aluminated and has a Si/Al ratio of 10-50.

2. The zeolite catalyst of claim 1, wherein the zeolite catalyst precursor is produced by the process comprising: (i) contacting a zeolite with a solution comprising molybdenum (Mo) and optionally a solution comprising the additional element selected from Group 6-11 of the Periodic Table; and (ii) drying and calcining the zeolite to provide a zeolite catalyst precursor.

3. The zeolite catalyst of claim 1, wherein the temperature is kept constant for 5-60 minutes at the temperature useful for aromatization after attaining said temperature useful for aromatization.

4. The zeolite catalyst of claim 1, further comprising, subsequent to keeping the temperature constant for 0-60 minutes at the temperature useful for aromatization, contacting the zeolite catalyst with a feedstream lower alkane, wherein the lower alkane is methane (CH.sub.4), ethane (C.sub.2H.sub.6) or a mixture thereof.

5. The zeolite catalyst of claim 1, wherein the inert diluent gas is selected from the group consisting of nitrogen (N.sub.2), helium (He) and argon (Ar).

6. A process for aromatization of a lower alkane comprising contacting the catalyst according to claim 1 with a feedstream comprising a lower alkane at conditions useful for aromatization.

7. The process of claim 6, wherein the temperature useful for aromatization is 700 C.-750 C.

8. The process of claim 6, wherein the pre-carburizing gas stream consists of a lower alkane and an inert diluent gas.

9. The process claim 6, wherein the lower alkane is methane (CH.sub.4), ethane (C.sub.2H.sub.6) or a mixture thereof.

10. A zeolite catalyst useful for aromatization of a lower alkane obtainable by the method comprising: contacting a medium pore zeolite catalyst precursor with a pre-carburizing gas stream comprising a pre-carburizing gas stream lower alkane and 50-90 mole-% of an inert diluent gas at a temperature that is increased from 20-250 C. at a rate of about 20 C./minute or less to a temperature useful for aromatization and keeping the temperature constant for 0-60 minutes at the temperature useful for aromatization to produce the zeolite catalyst; wherein the zeolite catalyst precursor comprises 2-10 wt % molybdenum (Mo) and 0-2 wt % of an additional element selected from Groups 6-11 of the Periodic Table; wherein the zeolite is de-aluminated.

11. The zeolite catalyst of claim 10, wherein the zeolite catalyst precursor is produced by the process comprising: (i) contacting a zeolite with a solution comprising molybdenum (Mo) and optionally a solution comprising the additional element selected from Group 6-11 of the Periodic Table; and (ii) drying and calcining the zeolite to provide a zeolite catalyst precursor.

12. The zeolite catalyst of claim 10, wherein the temperature is kept constant for 5-60 minutes at the temperature useful for aromatization after attaining said temperature useful for aromatization.

13. The zeolite catalyst of claim 10, wherein the zeolite catalyst precursor further comprises a binder.

14. The zeolite catalyst of claim 10, further comprising, subsequent to keeping the temperature constant for 0-60 minutes at the temperature useful for aromatization, contacting the zeolite catalyst with a feedstream lower alkane, wherein the lower alkane is methane (CH.sub.4), ethane (C.sub.2H.sub.6) or a mixture thereof.

15. The zeolite catalyst of claim 10, wherein the inert diluent gas is selected from the group consisting of nitrogen (N.sub.2), helium (He) and argon (Ar).

16. A process for aromatization of a lower alkane comprising contacting the catalyst according to claim 10 with a feedstream comprising a lower alkane at conditions useful for aromatization.

17. The process of claim 16, wherein the temperature useful for aromatization is 600 C.-850 C.

18. The process of claim 16, wherein the temperature useful for aromatization is 700 C.-750 C.

19. The process of claim 16, wherein the pre-carburizing gas stream consists of a lower alkane and an inert diluent gas.

20. The process claim 16, wherein the lower alkane is methane (CH.sub.4), ethane (C.sub.2H.sub.6) or a mixture thereof.

21. The process of claim 16, wherein the inert diluent gas is selected from the group consisting of nitrogen (N.sub.2), helium (He) and argon (Ar).

Description

MODE(S) FOR CARRYING OUT THE INVENTION

(1) The present invention will now be more fully described by the following non-limiting Examples.

Comparative Example 1

Non-Pre-Carburized 3.5% Mo H-ZSM-5

(2) In order to prepare Mo/H-ZSM-5 zeolite, 0.65 g ammonium molybdate tetrahydrate was dissolved in 15 ml demineralised water. 10 g of powder H-ZSM-5 having Si/Al ratio of 11.5 was added to the above solution. The resulting paste was thoroughly mixed and dried at 100 C. for 12 h. The dried catalyst mass was further heated up to 600 C. at a rate of 5 C./min followed by calcined at 600 C. for 2 h in the presence of moisture-free air.

(3) To prepare the binder, 0.19 g lanthanum nitrate hexahydrate was dissolved in 120 ml demineralised water. Subsequently, 6 g of powder kaolin was added to the above solution. The mixture was heated at 95-98 C. for 24 h under continuous stirring. Resulting La-exchanged kaolin was separated by filtration. The retained solid mass was washed with 2 liters of demineralised water and dried at 100 C. for 12 h. Dried La-exchanged kaolin was calcined in a muffle furnace at 650 C. for 4 h under flowing moisture-free air (flow: 100 ml/min) The solid mass was then cooled to room temperature. La content in the binder material was determined by AAS (Atomic Absorption Spectrometer) to be 1 wt. % La on Kaolin.

(4) The catalyst compositions comprising of Mo-containing H-ZSM-5 based zeolites and La-exchanged kaolin binder were prepared in particle form by mixing thoroughly the catalyst and the binder in the ratio of 2:1. The catalyst mixture was then pressed at 10 ton pressure to make pellets. The pressed catalyst compositions were crushed and sieved. The crushed solids containing particle sizes from 0.5 to 1.0 mm were selected for catalytic use.

(5) 2.0 g catalyst particles were loaded in a down flow fixed bed micro-catalytic reactor. The temperature of the reactor is heated under constant N.sub.2 flow. After attaining the temperature of the reactor to the desired reaction temperature (750 C.), N.sub.2 flow is stopped and a pure methane flow is fed to the catalyst bed (20 ml/min at 1 atm) and the reaction started. The Weight Hourly Space Velocity (WHSV) was 0.4 h.sup.1. Unconverted methane and the products formed were analysed by an on-line Gas Chromatograph equipped with Petrocol DH 50.2 column, using a Flame Ionization Detector. The obtained results are summarized in Table 1.

(6) TABLE-US-00001 TABLE 1 Methane Product distributionSelectivity (wt %) Time/h Conv./% C2-C5 Benzene BTX C9+ aromatics 1 14.5 11.4 64.8 67.5 21.1 2 13.5 13.0 71.6 74.7 12.3 3 12.7 14.2 73.8 77.2 8.6 4 11.4 16.4 74.6 78.1 5.5 5 10.2 18.4 75.7 79.0 2.6 6 8.6 21.0 75.7 78.5 0.5 7 7.5 24.0 73.4 75.7 0.3 8 6.8 27.4 70.3 72.4 0.2 9 6.1 30.1 67.7 69.7 0.2

Comparative Example 2

Non-Pre-Carburized 3.5% Mo/Dealuminated H-ZSM-5

(7) The experimental procedures for comparative Example 2 were identical to Comparative Example 1 with the exception that dealuminated H-ZSM-5 zeolite was used.

(8) The Mo-modified dealuminated H-ZSM-5 zeolite of Comparative Example 2 was prepared as follows. 0.65 g ammonium molybdate tetrahydrate was dissolved in 15 ml demineralised water. 10 g of powder dealuminated H-ZSM-5 having Si/Al ratio of 12.6 was added to the above solution. The resulting paste was thoroughly mixed and dried at 100 C. for 12 h. The dried catalyst mass was further heated up to 600 C. at a rate of 5 C./min followed by calcined at 600 C. for 2 h in the presence of moisture-free air.

(9) 10 g parent H-ZSM-5 having Si/Al ratio of 11.5 was dispersed in 200 ml of aqueous 6 (N) nitric acid solution in a round bottom flask. The mixture was heated at 95-100 C. under stirring for 5 h. The solid mass was filtered out and washed thoroughly with 2 liters of demineralised water and dried at 100 C. for 12 h. The Si/Al ratio of the zeolite was determined by AAS (Atomic Absorption Spectrometer) to be 12.6.

(10) The same process conditions as in Comparative Example 1 were used. Unconverted methane and the products formed were analysed by an on-line Gas Chromatograph equipped with Petrocol DH 50.2 column, using a Flame Ionization Detector. The obtained results are summarized in Table 2.

(11) TABLE-US-00002 TABLE 2 Methane Product distributionSelectivity (wt %) Time/h Conv./% C2-C5 Benzene BTX C9+ aromatics 1 15.7 11.0 55.3 57.7 31.3 2 14.7 12.6 65.0 67.9 19.5 3 13.2 14.4 70.7 74.0 11.6 4 12.1 15.9 72.7 76.1 8.0 5 10.9 18.3 74.0 77.4 4.3 6 9.9 20.4 74.3 77.6 2.0 7 8.9 22.8 73.2 76.2 1.0 8 8.1 26.0 70.7 73.4 0.6 9 7.4 28.2 69.1 71.5 0.3

Comparative Example 3

(12) 3.5% Mo/dealuminated H-ZSM-5 pre-carburized with pure methane at constantly increasing temperature from 100-700 C. and holding time of 0.25 h at 700 C.

(13) The 3.5% Mo/dealuminated H-ZSM-5 of Comparative Example 3 was prepared as described under Comparative Example 2.

(14) In comparative Example 3, however, the catalyst was first subjected to pre-carburization using a pre-carburization gas stream consisting of pure methane and thus which does not comprise an inert diluent.

(15) Therefore 2.0 g catalyst particles were loaded in a down flow fixed bed micro-catalytic reactor and pre-carburized in the following way:

(16) Step 1: Exposed to the flowing moisture-free N.sub.2 (flow: 25 ml/min) at 100 C. for 0.25 h.

(17) Step 2: Exposed to the moisture-free stream consisting of pure methane (20 ml/min) under a constantly increasing temperature ramp of 5 C./min from 100 C. to 700 C., followed by holding at that temperature for 0.25 h at 700 C.

(18) Step 3: Exposed to a moisture-free N.sub.2 (flow: 50 ml/min) at 700 C., followed by increasing the temperature from the pre-carburization temperature to 750 C. using the ramp 5 C./min.

(19) After pre-carburization of catalyst and attaining the temperature of the reactor to the desired reaction temperature (750 C.), N.sub.2 flow is stopped and the methane flow fed to the catalyst bed is set at 20 ml/min at 1 atm and the reaction started. The Weight Hourly Space Velocity (WHSV) was 0.4 h.sup.1. Unconverted methane and the products formed were analysed by an on-line Gas Chromatograph equipped with Petrocol DH 50.2 column, using a Flame Ionization Detector. The obtained results are summarized in Table 3.

(20) TABLE-US-00003 TABLE 3 Methane Product distributionSelectivity (wt %) Time/h Conv./% C2-C5 Benzene BTX C9+ aromatics 1 14.2 12.4 63.6 66.2 21.4 2 13.6 14.0 69.8 72.6 13.4 3 13.0 14.4 70.9 74.0 11.6 4 12.2 15.7 71.4 74.8 9.5 5 11.3 16.9 71.1 74.6 8.5 6 10.4 19.1 71.2 74.5 6.4 7 9.6 22.1 69.4 72.4 5.5 8 8.8 25.3 68.9 71.8 2.9 9 8.1 28.4 68.0 70.6 1.0

Comparative Example 4

(21) 3.5% Mo/dealuminated H-ZSM-5 pre-carburized with methane and H.sub.2 at constantly increasing temperature from 100-700 C. and holding time of 0.25 h at 700 C.

(22) Comparative Example 4 is identical to Comparative Example 3, with the exception that the pre-carburizing gas stream consists of methane (10 ml/min) and H.sub.2 (10 ml/min) Accordingly, the pre-carburizing gas stream does not comprise an inert diluent.

(23) The same process conditions as in Comparative Example 3 were used. Unconverted methane and the products formed were analysed by an on-line Gas Chromatograph equipped with Petrocol DH 50.2 column, using a Flame Ionization Detector. The obtained results are summarized in Table 4.

(24) TABLE-US-00004 TABLE 4 Methane Product distributionSelectivity (wt %) Time/h Conv./% C2-C5 Benzene BTX C9+ aromatics 1 13.9 11.0 59.2 62.7 26.3 2 13.7 11.7 62.8 65.6 22.7 3 13.2 12.5 66.8 70.0 17.5 4 12.4 13.7 69.4 72.8 13.5 5 11.6 15.5 71.2 74.6 9.9 6 10.3 17.6 71.9 75.1 7.3 7 9.6 19.9 72.3 75.3 4.8 8 8.8 21.6 72.0 74.9 3.5 9 7.7 22.8 71.4 74.4 2.8

Comparative Example 5

(25) 3.5% Mo/dealuminated H-ZSM-5 pre-carburized at 750 C. for 15 min with methane and N.sub.2

(26) The 3.5% Mo/dealuminated H-ZSM-5 of Comparative Example 5 was prepared as described under Comparative Example 2.

(27) In Example 5, however, the catalyst was subjected to pre-carburization using a pre-carburization gas stream containing methane and N.sub.2. Therefore, 2.0 g catalyst particles were loaded in a down flow fixed bed micro-catalytic reactor and pre-carburized in the following way:

(28) Step 1: Exposed to the flowing moisture-free N.sub.2 (flow: 25 ml/min) at 100 C. for 0.25 h, followed by increase temperature to 750 C. under a constantly increasing temperature range of 5 C./min.

(29) Step 2: Exposed to the moisture-free stream containing methane (10 ml/min) and N.sub.2 (30 ml/min) at 750 C. for 0.25 h. Hence, the temperature was not gradually increased during pre-carburization.

(30) Step 3: Exposed to a moisture-free N.sub.2 (flow: 50 ml/min) at 750 C. for 0.1 h.

(31) After pre-carburization of catalyst and attaining the temperature of the reactor to the desired reaction temperature (750 C.), N.sub.2 flow is stopped and the methane flow fed to the catalyst bed is set at 20 ml/min at 1 atm and the reaction started. Pure methane was used as a feedstream for the reaction. The Weight Hourly Space Velocity (WHSV) was 0.4 h.sup.1. Unconverted methane and the products formed were analysed by an on-line Gas Chromatograph equipped with Petrocol DH 50.2 column, using a Flame Ionization Detector. The obtained results are summarized in Table 5.

(32) TABLE-US-00005 TABLE 5 Methane Product distributionSelectivity (wt %) Time/h Conv./% C2-C5 Benzene BTX C9+ aromatics 1 14.6 12.5 67.9 70.6 16.9 2 13.8 12.7 71.8 74.8 12.5 3 13.4 13.9 71.1 74.4 11.7 4 12.3 15.2 71.7 75.2 9.6 5 11.6 16.8 70.6 74.3 8.9 6 10.4 18.8 71.2 75.3 5.9 7 9.8 20.2 71.5 75.5 4.3 8 9.4 22.4 70.1 74.1 3.5 9 8.7 23.9 69.8 73.8 2.3

Example 1

(33) 3.5% Mo/dealuminated H-ZSM-5 pre-carburized with methane and N.sub.2 at constantly increasing temperature from 100-750 C. with no holding time at 750 C.

(34) Example 1 is identical to Comparative Example 5 with the exception that the catalyst was pre-carburized in the following way:

(35) Step 1: Exposed to the flowing moisture-free N.sub.2 (flow: 25 ml/min) at 100 C. for 0.25 h.

(36) Step 2: Exposed to the moisture-free stream containing methane (10 ml/min) and N.sub.2 (30 ml/min) under a constantly increasing temperature ramp of 5 C./min from 100 C. to 750 C. Hence, the temperature was gradually increased during pre-carburization.

(37) Step 3: Exposed to a moisture-free N.sub.2 (flow: 50 ml/min) at 750 C. for 0.1 h.

(38) Subsequently, the catalyst is switched to the methane feedstream. The same process conditions as in the comparative Example 1 were used. Unconverted methane and the products formed were analysed by an on-line Gas Chromatograph equipped with Petrocol DH 50.2 column, using a Flame Ionization Detector. The obtained results are summarized in Table 6.

(39) TABLE-US-00006 TABLE 6 Methane Product distributionSelectivity (wt %) Time/h Conv./% C2-C5 Benzene BTX C9+ aromatics 1 15.0 11.6 65.1 67.9 20.5 2 14.4 12.4 70.7 73.7 13.9 3 13.3 13.6 75 78.2 8.2 4 12.3 15.4 76 79.2 5.4 5 11.4 17.0 75.8 78.9 4.1 6 10.6 17.5 76.7 79.8 2.7 7 9.8 19.1 75.4 78.7 2.2 8 9.5 21.1 73.9 77.1 1.8 9 8.8 23.0 72.4 75.6 1.4

Example 2

(40) 3.5% Mo/dealuminated H-ZSM-5 pre-carburized with methane and N.sub.2 at constantly increasing temperature from 100-750 C. with holding time of 0.25 h at 750 C.

(41) Example 2 is identical to Example 1 with the exception that the catalyst was pre-carburized in the following way:

(42) Step 1: Exposed to the flowing moisture-free N.sub.2 (flow: 25 ml/min) at 100 C. for 0.25 h.

(43) Step 2: Exposed to the moisture-free stream containing methane (10 ml/min) and N.sub.2 (30 ml/min) under a constantly increasing temperature ramp of 5 C./min from 100 C. to 750 C. followed by holding at 750 C. for 0.25 h.

(44) Step 3: Exposed to moisture-free N.sub.2 (flow: 50 ml/min) at 750 C. for 0.1 h.

(45) The same process conditions as in Example 1 were used. Unconverted methane and the products formed were analysed by an on-line Gas Chromatograph equipped with Petrocol DH 50.2 column, using a Flame Ionization Detector. The obtained results are summarized in Table 7.

(46) TABLE-US-00007 TABLE 7 Methane Product distributionSelectivity (wt %) Time/h Conv./% C2-C5 Benzene BTX C9+ aromatics 1 15.0 11.9 67.8 70.9 17.2 2 14.5 12.4 71.4 74.7 12.9 3 13.6 14.2 72.3 75.7 10.1 4 13.1 15.3 72.9 76.3 8.4 5 12.4 16.8 73.3 76.8 6.4 6 11.6 18.1 73.1 76.7 5.2 7 10.9 19.2 72.7 76.4 4.4 8 10.4 20.7 71.9 75.6 3.7 9 10.1 21.9 71.5 75.2 2.9

Example 3

(47) 3.5% Mo/dealuminated H-ZSM-5 pre-carburized with methane and N.sub.2 at constantly increasing temperature from 200-750 C. with holding time of 0.25 h at 750 C.

(48) Example 3 is identical to Example 1 with the exception that the catalyst was pre-carburized in the following way:

(49) Step 1: Exposed to the flowing moisture-free N.sub.2 (flow: 25 ml/min) at 200 C. for 0.25 h.

(50) Step 2: Exposed to the moisture-free stream containing methane (10 ml/min) and N.sub.2 (30 ml/min) under a constantly increasing temperature ramp of 5 C./min from 200 C. to 750 C. followed by holding at 750 C. for 0.25 h.

(51) Step 3: Exposed to moisture-free N.sub.2 (flow: 50 ml/min) at 750 C. for 0.1 h.

(52) The same process conditions as in Example 1 were used. Unconverted methane and the products formed were analysed by an on-line Gas Chromatograph equipped with Petrocol DH 50.2 column, using a Flame Ionization Detector. The obtained results are summarized in Table 8.

(53) TABLE-US-00008 TABLE 8 Methane Product distributionSelectivity (wt %) Time/h Conv./% C2-C5 Benzene BTX C9+ aromatics 1 14.1 11.2 67.8 71.0 17.8 2 13.8 12.3 71.3 74.3 13.4 3 12.8 13.8 74.1 77.2 9.0 4 12.4 14.4 72.3 75.5 10.1 5 11.5 15.6 73.3 76.6 7.8 6 10.7 17.6 72.5 75.9 6.5 7 10.2 18.2 72.1 75.7 6.1 8 9.8 19.7 70.8 74.6 5.7 9 9.5 21.5 69.3 73.2 5.3

Example 4

(54) 3.5% Mo/dealuminated H-ZSM-5 pre-carburized with methane and N.sub.2 at constantly increasing temperature from 100-700 C. with holding time of 0.25 h at 700 C.

(55) Example 4 is identical to Example 1 with the exception that the catalyst was pre-carburized in the following way:

(56) Step 1: Exposed to the flowing moisture-free N.sub.2 (flow: 25 ml/min) at 100 C. for 0.25 h.

(57) Step 2: Exposed to the moisture-free stream containing methane (10 ml/min) and N.sub.2 (30 ml/min) under a constantly increasing temperature ramp of 5 C./min from 100 C. to 700 C. followed by holding at 700 C. for 0.25 h.

(58) Step 3: Exposed to moisture-free N.sub.2 (flow: 50 ml/min) at 700 C., followed by increasing the temperature from 700 C. to 750 C. using the ramp 5 C./min.

(59) The same process conditions as in Example 1 were used. Unconverted methane and the products formed were analysed by an on-line Gas Chromatograph equipped with Petrocol DH 50.2 column, using a Flame Ionization Detector. The obtained results are summarized in Table 9.

(60) TABLE-US-00009 TABLE 9 Methane Product distributionSelectivity (wt %) Time/h Conv./% C2-C5 Benzene BTX C9+ aromatics 1 15.1 11.4 63.1 65.8 22.8 2 14.5 12.6 70.2 73.3 14.1 3 13.6 13.6 73.9 77.1 9.3 4 12.1 15.8 76.5 79.8 4.4 5 11.4 17.5 76.9 80.1 2.4 6 10.6 18.5 77.4 80.5 1.0 7 10.2 20.1 76.3 79.3 0.6 8 9.4 21.9 74.7 77.7 0.4 9 8.9 23.6 73.3 76.2 0.2

Comparative Example 5

(61) 3.5% Mo/dealuminated H-ZSM-5 pre-carburized with methane and H.sub.2 at constantly increasing temperature from 100-750 C. and holding time of 0.25 h at 750 C.

(62) Comparative Example 5 is identical to Example 2, with the exception that the pre-carburizing gas stream consists of methane (10 ml/min) and H.sub.2 (30 ml/min) Accordingly, the pre-carburizing gas stream does not comprise an inert diluent.

(63) The same process conditions as in Example 2 were used. Unconverted methane and the products formed were analysed by an on-line Gas Chromatograph equipped with Petrocol DH 50.2 column, using a Flame Ionization Detector. The obtained results are summarized in Table 10.

(64) TABLE-US-00010 TABLE 10 Methane Product distributionSelectivity (wt %) Time/h Conv./% C2-C5 Benzene BTX C9+ aromatics 1 14.1 15.6 63.1 65.4 19.0 2 13.9 16.1 68.0 70.1 13.8 3 13.5 16.3 71.9 73.1 10.6 4 12.9 16.7 72.3 74.0 9.3 5 12.4 17.5 73.7 74.9 7.6 6 11.5 18.1 73.8 75.8 6.1 7 10.5 18.8 74.6 76.5 4.7 8 9.6 19.8 74.0 76.4 3.8 9 8.8 21.3 73.7 76.0 2.7

(65) By comparing the results in Tables 1-5 with Table 6 and by comparing the results in Table 10 with Table 7, it is clear that the pre-carburization of Mo-loaded H-ZSM-5 zeolite catalyst precursor with a combined stream of the lower alkane methane and inert diluent gas like nitrogen at a constantly increasing temperature from e.g. 100 to 750 C. remarkably improves the stability/performance of the catalyst for methane aromatization. Catalyst performance is even further improved in case the catalyst precursor is pre-carburized under the combined stream of methane and nitrogen at a constantly increasing temperature to the temperature useful for aromatization and is subsequently kept for e.g. 15 minutes at the temperature useful for aromatization; see Tables 6-7.