NL8300355A - PROCESS FOR THE DEHYDRATION CLOSURE OF ALKANES. - Google Patents

Description

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Process for dehydro ring closure of alkanes.

The present invention relates to a new catalyst and a new process for the dehydro ring closure of acyclic compounds, more particularly the dehydro ring closure of alkanes containing at least 6 carbon atoms, to form the corresponding aromatic hydrocarbons.

Catalytic reforming is known in the petroleum industry and relates to the treatment of naphtha fractions to improve the octane rating. More important hydrocarbon reactions that take place during the reforming operation include dehydrogenation of cyclohexanes to aromatics, dehydroisomerization of alkyl cydopentanes to aromatics, and dehydro ring closure of alkanes to aromatics. Hydrocracking reactions, which produce high yields of light gaseous hydrocarbons, eg, methane, ethane, propane and butane, should in particular be minimized during reforming as this will reduce the efficiency of gasoline boiling products decreases.

Dehydro ring closure is one of the main reactions in the reforming process. The usual methods for carrying out these dehydro ring closure reactions are based on the use of catalysts containing a noble metal on a support. Known catalysts of this type are based on aluminum oxide, which carries 0.2 to 0.8% by weight of platinum and preferably a second auxiliary metal.

The possibility of using supports other than alumina has also been studied and it has been proposed to use certain molecular sieves, such as X and Y zeolites, which have been found to be suitable provided that the molecules of the reagent and product are sufficiently small to pass through the pores of to pass the zeolite. However, catalysts based on the molecular sieves have not been successful.

In the conventional method of carrying out the aforementioned dehydro ring closure, convertible alkanes are passed over the catalyst in the presence of hydrogen at temperatures of the order of 500 ° C and pressures from 500 to 3000 kPa. Some of the alkanes are converted to aromatic hydrocarbons and the reaction is accompanied by isomerization and cracking reactions, which also convert the alkanes to isoalkanes and lighter hydrocarbons.

The conversion rate of the hydrocarbons to aromatic 8300355

N

2 * 4 hydrocarbons vary with reaction conditions and the nature of the catalyst.

The catalysts used hitherto have given moderately satisfactory results with heavy alkanes, but less satisfactory results with Cg-Cg alkanes, especially Cg alkanes. Zeolite type L catalysts are more selective with regard to the dehydrocirculation reaction and can be used to improve the conversion rate to aromatic hydrocarbons without requiring higher temperatures, which usually have a significant adverse effect on the stability of the catalyst and give excellent results with Cg-Cg alkanes. However, operating time and regenerability are problems and satisfactory regeneration methods are not known.

In one process for the dehydro ring closure of aliphatic hydrocarbons, hydrocarbons are contacted in the presence of hydrogen with a catalyst consisting essentially of a zeolite type L with exchangeable cations of which at least 90% are alkali metal ions selected from the group consisting of ions of sodium, lithium, potassium, rubidium and cesium and containing at least one metal 20 selected from the group consisting of metals of group VIII of the periodic table of elements, tin and germanium, which metal or which metals contain at least one metal of group VIII of the periodic table with a dehydrogenation effect to convert at least part of the feed into aromatic hydrocarbons.

A particularly effective embodiment of this process is a platinum / alkali metal / zeolite type L catalyst because of the excellent activity and selectivity for the conversion of hexanes and heptanes to aromatics, but operating time remains a problem.

The present invention overcomes the shortcomings of the prior art by using a catalyst containing a large pore zeolite, an alkaline earth metal and a Group VIII metal for hydrocarbon reforming at extremely high selectivity for the conversion of alkanes to aromatics. The hydrocarbons are contacted with a catalyst containing a 35-pore zeolite, at least one Group VIII metal (preferably platinum) and an alkaline earth metal selected from the group consisting of barium, strontium and calcium (preferably barium) . In one aspect of the present invention, the process conditions are controlled such that the selectivity to N-hexane dehydro ring closure is greater than 40%. In another aspect, the selectivity index of the catalyst is 8300355. >. f · s 3 sator greater than 60%. The catalyst provides a satisfactory operating time.

Preferably, the large pore zeolite is a type L zeolite containing 0.1 to 5 wt% platinum and 0.1 to 35 wt% barium.

The hydrocarbons are contacted with the barium-exchanged type of zeolite at a temperature of 400 ° C to 600 ° C (preferably 430 ° C to 550 ° C) * a space velocity of 0.3 to 10 (preferably 0, 3 to 5), a pressure of 101.3 kPa to 3450 kPa (preferably from 345 to 2070 kPa) and an H2 / KW ratio of 1: 1 to 10: 1 (preferably from 2: 1 to 6 : 1).

In its broadest aspect, the present invention involves the use of a catalyst containing a large pore zeolite, an alkaline earth metal and a Group VIII metal in hydrocarbon reforming, especially the dehydro ring closure of alkanes, at extremely high selectivity for the conversion of hexanes to aromatics.

The term "selectivity" as used in the present invention is defined as the percentage of alkane molecules converted to aromatics with respect to the number of moles converted to aromatics and cracked products, 100 x number of moles of alkanes converted ie, selectivity * to aromatics_ number of moles of alkanes converted to aromatics and cracked products. Isomerization reactions and alkyl cyclopentane formation are not taken into account in determining the selectivity.

The term "selectivity to n-hexane," as used in the present invention, is defined as the percentage moles of n-hexane converted to aromatics with respect to the number of moles converted to aromatics and cracked products.

The selectivity for conversion of alkanes to aromatics is a measure of the efficiency of the process in the conversion of alkanes to the desired and valuable products: aromatics and hydrogen, as opposed to the less desirable hydrocracking products.

An inherent feature of any dehydro ring closure catalyst is its selectivity index. The selectivity index is defined as the "selectivity to n-hexane" using n-hexane as the 40 feed and proceeding at 490 ° C, 689.476 kPa, space velocity 3 and 3% / KW after 20 hours.

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Highly selective catalysts produce more hydrogen than less selective catalysts, because hydrogen is produced when alcanes are converted to aromatics and hydrogen is used when alkanes are converted to cracked products. Increasing the selectivity of the process increases the amount of hydrogen produced (more aromatization) and decreases the amount of hydrogen consumed (less cracking).

Another advantage of using highly selective catalysts is that the hydrogen produced by highly selective catalysts is purer than hydrogen generated by less selective catalysts. This greater purity arises because more hydrogen is produced, while less low-boiling hydrocarbons (cracked products) are produced. The purity of hydrogen generated in reforming is critical when, as is usually the case in an integrated refinery, the hydrogen generated is used in processes such as hydrotreating and hydrocracking, which require at least certain minimum hydrogen partial pressures. When the purity becomes too low, the hydrogen can no longer be used for this purpose and must be used in a less valuable manner, for example as fuel gas.

In the process of the invention, the feed hydrocarbons preferably contain non-aromatic hydrocarbons containing at least 6 carbon atoms. Preferably, the feed is substantially free of sulfur, nitrogen, metals and other poisons for reforming catalysts.

The dehydro ring closure is carried out in the presence of hydrogen at a set pressure to thermodynamically favor the reaction and limit unwanted hydrocracking reactions with genetic agents. The pressures used preferably range from 101.3 kPa to 3450 kPa, more preferably from 345 kPa to 2070 kPa, with the hydrogen to hydrocarbon mole ratio preferably from 1: 1 to 10: 1, more preferably from 2: 1 to 6: 1.

In the temperature range from 400 ° C to 600 ° C, the dehydro ring closure reaction takes place with acceptable rate and selectivity.

When the operating temperature is below 400 ° C, the reaction speed is insufficient and consequently the efficiency is too low for industrial purposes. Also, the dehydro ring closure equilibrium at low temperatures is unfavorable. When the operating temperature is above 600 ° C, obstructive secondary reactions such as hydrocracking and coking take place and significantly reduce the efficiency and increase the deactivation rate of the catalyst. Exceed 600 ° C.

The preferred temperature range of dehydro ring closure (430 ° C to 550 ° C) is that with the process being optimal in terms of activity, selectivity and stability of the catalyst. sator.

The space velocity of the hydrocarbons is preferably between 0.1 and 10.

The catalyst of the invention is a large pore zeolite loaded with one or more dehydrogenation components. The term "large pore zeolite" is defined as a zeolite having an effective pore diameter of 0.6 to 1.5 nanometers.

Among the large pore crystalline zeolites that have been found to be suitable in the practice of the present invention, type I zeolites and faujasite structure synthetic zeolites such as zeolite X and zeolite are the most important and these have apparent pore sizes on the order of 0.7 to 0.9 nanometers.

A zeolite type L composition, expressed in molar oxides ratios, can be represented as follows: (0.9-1.3) M2 / nO: Al2 O3 (5.2-6.9) SiO2: yH20 where M means a cation, n represents the valence of N and y can be any value from 0 to about 9. Zeolite L, the X-ray diff-25 fraction pattern, the properties and the process for its preparation are described in detail in U.S. Pat. No. 3,216,789. The actual formula may vary without changing the crystal structure; for example, the silicon to aluminum (Si / Al) molar ratio can range from 1.0 to 3.5.

The chemical formula for zeolite Y expressed in mole ratios of oxides can be written as: (0.7-1.1) Na 2 O: Al 2 O 3: SiO 2: yH 2 O where x is a value greater than 3 to about 6 and y can be a value up to about 9 to be. Zeolite Y has a characteristic X-ray powder diffraction pattern which can be used with the above formula for identification. Zeolite Y is described in more detail in U.S. Patent 3,130,007.

Zeolite X is a synthetic crystalline zeolite-like molecular sieve, which can be represented by the formula:

40 (0.7-1.1) M2 / nO: Al2 O3: (2.0-3.0) SiO2: yHQ

8300355 6 wherein Μ represents a metal, especially an alkali or alkaline earth metal, n is the valence of M and y may have a value up to about 8 depending on the identity of M and the degree of hydration of the crystalline zeolite. Zeolite X, the X-ray diffraction pattern, the properties and method of preparing it are described in detail in U.S. Pat. No. 2,882,244.

The preferred catalyst of the invention is a type L zeolite loaded with one or more dehydrogenation components.

An essential element of the present invention is the presence of an alkaline earth metal in the large pore zeolite. That alkaline earth metal should be barium, strontium or calcium, preferably barium. The alkaline earth metal can be incorporated into the zeolite by synthesis, impregnation or ion exchange. Barium is preferred over the other alkaline earth metals because it results in a slightly less acidic catalyst. Strong acidity is undesirable in the catalyst as it promotes cracking resulting in lower selectivity.

In one embodiment, at least a portion of the alkali metal is exchanged with barium, using techniques known for ion exchange of zeolites. This involves contacting the zeolite with a solution containing excess Ba "* - * · ions The barium should make up from 0.1 to 35% by weight of the zeolite.

The dehydro ring closure catalysts of the invention are charged with one or more Group VIII metals, for example, nickel, ruthenium, rhodium, palladium, iridium or platinum.

The preferred Group VIII metals are iridium, and especially platinum, which are more selective with regard to dehydroring closure and are also more stable under the dehydroring closure reaction conditions than other Group VIII metals.

The preferred percentage of platinum in the catalyst is between 0.1 and 5%.

Group VIII metals are introduced into the large pore zeolite by synthesis, impregnation or exchange in an aqueous solution of a suitable salt. When it is desired to introduce two Group VIII metals into the zeolite, the treatment can be carried out simultaneously or sequentially.

For example, platinum can be introduced by impregnating the zeolite with an aqueous solution of tetrammineplati-40 na (II) nitrate, tetrammineplatin (II) hydroxide, dinitrodiamino-platinum 8300355

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* 7 or tetramrin platinum (II) chloride. In an ion exchange process, platinum feed can be introduced using cationic platinum complexes, such as tetraimain platinum (II) nitrate.

An inorganic oxide can be used as a support to bind the large pore zeolite containing the Group VIII metal and alkaline earth metal. The support can be a natural or a synthetically prepared inorganic oxide or combination of inorganic oxides. Conventional inorganic oxide supports which can be used include clays, alumina and silica, in which acidic sites are preferably exchanged for cations which do not impart strong acidity (such as Na, K, Rb, Cs, Ca, Sr or Ba).

The catalyst can be used in any of the known types of equipment known in the art. The catalyst can be used in the form of pellets, granules, granules, broken pieces or various special shapes arranged as a fixed bed within a reaction zone and the fill material can be passed through it in the liquid, vapor or mixed phase and upward or downward flow. Also, the catalyst can be prepared in a suitable form for use in moving beds or in fluidized solid processes, the fill material being passed upward through a moving bed of finely divided catalyst.

After the desired metal or metals are introduced, the catalyst can be treated in air at about 260 ° C and then reduced in hydrogen at temperatures from 200 ° C to 70 ° C, preferably 400 ° C to 620 ° C.

At this step, the catalyst is ready for use in the dehydroring closure process. In some cases, however, for example, when the metal or metals have been introduced by an ion-exchange process, it is preferable to remove any residual acidity from the zeolite by adding the catalyst with an aqueous solution of a salt or hydroxide of a suitable alkali metal or alkaline earth metal metal element to neutralize any hydrogen ions formed by hydrogen during the reduction of metal ions.

In order to obtain optimum selectivity, the temperature should be controlled so that the reaction rate is noticeable, but the conversion is less than 98%, since excessive temperature and excess reaction may adversely affect selectivity.

The pressure should also be controlled within a suitable range.

Too high a pressure will place a thermodynamic (equilibrium) limit on the desired reaction, especially for aromatization of hexane, and too low a pressure may result in coking and deactivation.

While the primary advantage of the present invention lies in improving the selectivity for conversion of alkanes (especially Cg-Cg alkanes) to aromatics, it has also surprisingly been found that the selectivity for conversion of methylcydopentane to benzene is excellent. This reaction, which involves an acid-catalyzed isomerization step on conventional chloride-treated alumina reforming catalysts, occurs on the catalyst of the present invention with as good or better selectivity than prior art catalysts based on the prior art. of chloride-treated alumina. Therefore, the present invention can also be used to catalyze the conversion of stocks of high 5-ring alkylnaphthenes to aromatics. Another advantage of the present invention is that the catalyst of the present invention is more stable than prior art zeolite-like catalysts. The stability of the catalyst, or the resistance to deactivation, determines its useful life. Longer operating times result in less downtime and costs when regenerating or replacing the catalyst charge.

In one embodiment of the present invention, a hydrocarbon feed is contacted with a first catalyst which is a conventional reforming catalyst and a second catalyst which is a dehydro ring closure catalyst containing a 25 pore zeolite, an alkaline earth metal and a Group VIII metal.

The use of a reforming catalyst containing an alumina support, platinum and rhenium has been fully discussed in U.S. Pat. No. 3,415,737, which shows the use of an effective conventional reforming catalyst. Other effective two metal containing catalysts include platinum tin, platinum germanium, platinum lead and platinum iridium.

The hydrocarbons can be contacted with the two catalysts in series, the hydrocarbons being first in contact with the first (conventional) reforming catalyst and then with the second (dehydro ring closure) catalyst or with the hydrocarbons first in contact with the second catalyst and then with the first catalyst. Also, the hydrocarbons can be contacted in parallel with one fraction of the hydrocarbon in contact with the first catalyst and another fraction of the hydrocarbons in contact with the second catalyst. The hydrocarbons can also be used with 8300355. : Both catalysts were simultaneously contacted in the same reactor.

Example I

A directly obtained arabic light oil, which was hydrorefined to remove sulfur, oxygen and nitrogen, was reformed with three different catalysts at 690 kPa, a space velocity of 2 and 6 H 2 / KW. The feed contained 80.2 vol% alkanes, 16.7 vol% naphthenes and 3.1 vol% aromatics and contained 21.8 vol% C5, 52.9 vol% Cg, 21.3 vol% C7 and 3.2 vol.% Cg.

In the first run, the directly obtained arabic light oil was reformed at 499 ° C using a sulfided platinum rhenium alumina catalyst described in U.S. Pat. No. 3,415,737.

In the second run, the directly obtained arabic light oil was reformed at 493 ° C using a platinum-potassium zeolite type L catalyst formed by: (1) impregnating a potassium zeolite type L by 0.8% platinum using tetrammineplatin (II) nitrate, (2) drying the catalyst, (3) calcining the catalyst at 260 ° C and (4) reducing the catalyst for 1 hour at 480 ° C to 500 ° C.

In the third test, the process of the present invention, the directly obtained arabic light oil was reformed at 493 ° C using a platinum barium zeolite type L catalyst formed by: (1) ion exchange of a potassium zeol type L with a sufficient volume of a 0.17 molar barium nitrate solution to contain excess barium compared to the ion exchange capacity of the zeolite, (2) drying the resulting barium-exchanged zeolite type L catalyst, (3) calcining the catalyst at 590 ° C. (4) impregnating the catalyst with 0.8% platinum using tetrammine platinum (II) nitrate, (5) drying the catalyst, (6) calcining the catalyst at 260 ° C and (7) reducing the catalyst in hydrogen for 1 hour at 480 to 500 ° C.

The results of the three tests are shown in Table A.

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Table A

- 499 ° C

Pt / Re

5 aluminum 493 ° C 493 ° C

Oxide supply Pt / K / l · Pt / Ba / L

Ci wt.% Feed 2.8 5.5 3.6 C2 6.6 2.5 1.3 C3 9.3 3.2 1.5 10 iC4 0.1 5.8 0.9. 0.5 n-C4 0.5 6.8 3.8 2.4 iC5 5.1 13.6 6.7 5.6 n-C5 11.3 9.8 12.6 12.6 C $ + PMST 81.3 13.4 7.8 9.3 15 benzene 1.5 15.1 40.6 43.8 C7 + aromatics 0.8 15.8 12.7 15.0 C5 + LV Z yield 63 69.9 74. 4 hydrogen, Nm ^ / hl 8.37 29.5 36.5 selectivity, mol% 20 72 87 20 C5 + P -> aromatics

This series of tests shows that the use of a platinum-barium zeolite type 1 catalyst in reforming gives a selectivity for the conversion of hexanea to benzene that is markedly better than that of the prior art. It is noted that in connection with this better selectivity there is an increase in the hydrogen gas product which can be used in other processes. It is also noted that the purity of hydrogen is greater for the Pt / Ba / L test, as more hydrogen is produced and less Ci + C2 are produced.

Example II

A second series of tests was performed to demonstrate that the present invention will work other larger pore zeolites besides zeolite type L. The selectivity index was measured for four catalysts.

This series of tests was performed using n-hexane as the feed. All tests in this series were done at 490 ° C, 690 kPa, a space velocity of 3 and 3 H2 / KW.

The first run used a platinum-potassium zeolite L 40 prepared by the methods described in the second process of Example I.

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The second run used a platinum barium zeolite type L prepared according to the methods of the third process of Example 1 except that the barium nitrate solution was 0.3 molar instead of 0.17 molar.

The third run used a platinum sodium zeolite Y prepared by impregnating a sodium zeolite Y with Pt (SH3) 4 (NO3> 2) to obtain 0.8% platinum, after which the catalyst was dried and calcined at 260 ° C and reduced in hydrogen at 480-500 ° C.

In the fourth run, a platinum barium zeolite Y was used, which was prepared by ion exchange of a sodium zeolite Y with 0.3 molar barium nitrate at 80 ° C, drying and calcining at 590 ° C, followed by impregnation of the zeolite with Pt (NH3) 4 (NO3) 2 to obtain 0.8% platinum, followed by drying, calcining the catalyst at 260 ° C and reducing in hydrogen at 480-500 ° C. The results of these tests are shown in Table B.

Table B

20 Conversion Selectivity 5 h. 20 h. index_

Pt / K / L 70 59 79

Pt / Ba / L 85 85 92

Pt / Na / Y 82 79 54 25 Pt / Ba / Y 74 68 66

Thus, in operation, the incorporation of barium into a large pore zeolite such as a zeolite type Y greatly improves selectivity to n-hexane. It is noted that the stability of the platinum-barium-zeolite type L is excellent. After 20 hours, there was no drop in conversion when using platinum-barium-zeolite type L catalyst.

Example XII

A third series of tests was performed to demonstrate the effect of addition of additional ingredients to the catalyst *

This third series of tests was done using a feed which had been hydrorefined to remove sulfur, oxygen and nitrogen, containing 8.9% by volume of alkanes, 16.8% by volume of naphthenes and 1.7% by volume of aromatics contained. The feed also contained 2.6% by volume C5, 47.6% by volume 40 Cg, 43.4% by volume C7 and 6.3% by volume Cg. All tests in this series 8300355 12 were performed at 490 ° C, 690 kPa, a space velocity of 2.0 and 6.0 H2 / KW.

In the first run, a platinum sodium zeolite Y was prepared according to the methods outlined in the third process of Example IX.

In the second run, a platinum-barium-zeolite Y was prepared according to the methods indicated in the fourth process of Example II.

In the third run, a platinum rare earth metal zeolite Y was prepared by a rare earth metal zeolite Y obtained from Strem Chemicals Ine. impregnated to obtain 0.8% platinum using Pt (NH3) 4 (NO3) 2, then the zeolite was dried, calcined at 260 ° C and reduced at 480-500 ° C.

In the fourth run, a platinum rare earth metal barium zeolite Y was prepared by ion exchange of a rare earth metal ze olite Y, commercially available from Strem Chemicals Inc., with a 0.3 molar Ba (NO3) 2 solution at 80 ° C, drying and calcining the zeolite at 590 ° C, impregnating the zeolite with Pt (NH3) 4 (NO3) 2 to obtain 0.8% platinum followed by drying, calcining the zeolite at 260 ° C and reducing at 480-500 ° C. The results of these tests are given in Table C.

Table C

Activity aromatics at 3 h, C5 + Selectivity, mol% of feed% at 3 h

Pt / Na / Y 36 46

Pt / Ba / Y 54 68 30 Pt / rare earth / Y 22 (too low to measure)

Pt / Ba / rare earth metal / Y 36 27

This series of tests shows that the addition of rare earth metal to the catalyst has an adverse effect on selectivity.

Example IV

An arabic naphtha, which had been hydrorefined to remove sulfur, oxygen and nitrogen, was reformed at 690 kPa, a space velocity of 3 and 3 H2 / KW to prepare a C5 + product with an aromatics content of 82% by weight according to two different processes.

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The feed was a hydrorefined Arabian naphtha containing 67.9% alkanes, 23.7% naphthenes and 8.4% aromatics. Distillation results according to the D86 method were: initial - 95.0 ° C, 5% -103.9, 10% -106.7, 30% -120.0, 50% -129.4, 70% -143.9, 90% -160.6, 95% -169.4, bpt. 187.8 ° C.

In the first process, the arabic naphtha was reformed at 516 ° C in a reactor using a conventional reforming catalyst, containing 0.3 wt% Ft, 0.6 wt% Re and 1.0 wt. % Cl on alumina. Separation was sulfided beforehand.

In the second process, the Arabian naphtha was reformed at 493 ° C in the same reactor, with the top half of the reactor containing the same type of catalyst as the first process and the bottom half of the reactor containing a platinum-barium zeolite type L catalyst formed according to the methods of Example I.

The results of these two tests are shown in Table D.

15

Table D Pt / Re aluminum- 1/2 Pt / Re / aluminum oxide 20 oxide 1/2 Pt / Ba / L_

Deactivation rate 2.0 1.9 C5 + yield, LV% yield 68.9 71.0 hydrogen, Nm ^ / hl 16.9 18.7 * 8300355

Claims (23)

Process for hydrocarbon reforming characterized in that the hydrocarbons are contacted with a large pore zeolite catalyst containing: (a) at least one Group VIII metal of the periodic table and (b) an alkaline earth metal selected from the group consisting of barium, strontium and calcium, the catalyst selectivity index being greater than 60%. 2. Process according to claim 1, characterized in that the alkaline earth metal used is barium and the group VIII metal platinum. Process according to claim 2, characterized in that a catalyst is used which contains 0.1 to 35% by weight barium and 0.1 to 5% by weight platinum. 4. Process according to claims 1 to 3, characterized in that a large pore zeolite is used with an apparent pore size of 0.7 to 0.9 nanometer. Process according to claims 1 to 4, characterized in that a large pore zeolite is used selected from the group consisting of zeolite X, zeolite Y and zeolite type L. 6. Process according to claim 5, characterized in that zeolite Y is used as the zeolite with large pores. Process according to claim 5, characterized in that zeolite type L is used as the large pore zeolite. 8. Process according to claims 1 to 7, characterized in that the contacting of the hydrocarbons with the catalyst is carried out at a temperature of 400 ° C to 600 ° C, a space velocity of 0.3 to 5, one- pressure from 100 kPa to 3500 kPa and an H2 / KW ratio from 1: 1 to 10: 1. 9. Process according to claim 8, characterized in that a temperature of 430 ° C to 550 ° C, a pressure of 350 to 2100 kPa and an H2 / KW ratio of 2: 1 to 6: 1 are used. 10. Process for hydrocarbon reforming, characterized in that the hydrocarbons are contacted with a large pore zeolite catalyst containing: 35 (a) at least one Group VIII metal of the periodic table and (b) an alkaline earth metal selected from the group consisting of barium, strontium and calcium, the process conditions being adjusted such that the selectivity for n-hexane dehydration closure is greater than 60%. 11. Process according to claim 10, characterized in that as 8300355 • w 'Λ ____ V * · .- «. Alkaline earth metal uses barium and as Group VIII platinum metal. 12. Process according to claim 11, characterized in that a catalyst is used which contains 0.1 to 35% by weight barium and 0.1 to 5% by weight platinum. Process according to claims 10 to 12, characterized in that a large pore zeolite is used selected from the group consisting of zeolite X, zeolite Y, - zeolite type L. 14. Process according to claim 13, characterized in that zeolite Y is used as the zeolite with large pores. Process according to claim 13, characterized in that zeolite type L is used as the large pore zeolite. 16. Process according to claims 10 to 15, characterized in that the contacting is carried out at a temperature of 430 ° C to 550 ° C, a pressure of 350 to 2100 kPa and an H2 / KW ratio of 2: 1. to 6: 1. 17. Process for converting hydrocarbons, characterized in that (a) the hydrocarbons are contacted, under reforming conditions and in the presence of hydrogen, with a first catalyst containing a metal oxide support containing platinum and rhenium in intimate mixture, and (b) contacting the hydrocarbons with a large pore zeolite-containing second catalyst containing at least one Group VIII metal of the periodic table and an alkaline earth metal selected from the group consisting of barium, strontium and calcium. Process according to Claim 17, characterized in that the alkaline earth metal used is barium and the group VIII metal platinum. 19. Process according to claim 18, characterized in that a dehydro ring closure catalyst is used which contains 0.1 to 35 wt.% Barium and 0.1 to 5 wt.% Platinum. Process according to claims 17 to 19, characterized in that a large pore zeolite is used selected from the group consisting of zeolite X, zeolite Y and zeolite type L. 21. Process according to claim 20, characterized in that zeolite Y is used as the zeolite with large pores. Method according to claim 20, characterized in that a zeolite of type L is used as the zeolite with large pores. Method according to claims 17 to 22, characterized in that the contact in step (b) is carried out at a temperature of 430 ° C to 40 550 ° C, a pressure of 350 to 2100 kPa and an H2 / KW ratio of 2: 1 to 6: 1. λ-τλλ— 8300355 _