US3449461A - Dehydrocyclization of paraffins - Google Patents

Description

US. 'Cl. 260-6'73.5 8 Claims ABSTRACT OF THE DISCLOSURE A process for the dehydrocyclization of paraflin hydrocarbons having 6 to 20 carbon atoms using supported presulfided Group VIII noble metal catalyst. The reaction is preferably carried with a minor amount of a sulfur compound in the feed.

This invention relates to a process for the dehydrocyclization of paraffins to aromatics.

The commercial importance of aromatic hydrocarbons is well known and has been increasing in recent years. Although large quantities of aromatics are currently produced from coal distillation and from petroleum, such as the dehydrogenation of naphthenes in hydroforming naphtha fractions, the demand for aromatic hydrocarbons is increasing beyond the ability to produce them from such sources.

Aside from the above means to produce aromatic hydrocarbons, it is known to produce aromatics by thermal treatment of non-aromatic hydrocarbons, such as by severe thermal cracking conditions. (See Dulaney et al., US. 3,271,298, Sept. 6, 1966.) This method is generally impractical for the production of substantially pure aromatic hydrocarbons and has the concomitant disadvantage of low yields. Aromatics can be produced by a thermal treatment of light hydrocarbon gases at high temperatures according to the Fischer process. This process requires temperatures in the neighborhood of 1830 F. to 2370 F. and the use of a large number of expensive alloy reaction tubes of extremely small dimensions.

In the realm of more practical procedures, it is known that aromatic hydrocarbons may be synthesized by catalytic dehydrocyclization of open chain hydrocarbons. (See Catalysis VI, pp. 553542, edited by P. H. Emmett, Reinhold Publishing Co., New York, 1958.) This procedure allows aromatic hydrocarbons to be produced from relatively inexpensive and readily available hydrocarbons. Thus, paraflins which, in view of their poor octane number, are least desirable for gasoline fuels may advanta geously be utilized to obtain aromatic hydrocarbons, which can be used as a high octane gasoline component, solvent, or starting material for the chemical and plastic industry.

Widely known dehydrocyclization catalysts are the oxide catalysts such as molybdena-alumina and chrorniaalumina, the latter catalyst generally being considered about the best catalyst. Recently it has been proposed by Doelp, US 3,272,760, to use a platinum chromia-alumina catalyst for dehydrocyclization. Platinum catalysts, of course, have been known and used for years in the reforming of naphthenic fractions where aromatics are produced primarily by dehydrogenation of naphthenes. Although extensive Work has been done on catalytic dehydrocyclization, the present processes are not entirely nltfid States Patent 0 Patented June 10, 1969 satisfactory for one reason or another and elforts have been continued to provide a new or improved process.

It has now been discovered that parafiins can be dehydrocyclized to aromatics by means of certain noble metal catalysts which are active, stable, and selective. In accordance with the present invention, the parafiin to be converted is contacted under dehydrocyclization conditions with a sulfided Group VIII noble metal catalyst. Advantageously, the dehydrogenation is effected in the presence of minor amounts of sulfur. This is surprising since noble metals such as platinum are considered to be very strong hydrogenation-dehydrogenation catalysts when in the reduced form and sulfur is known to "be a noble-metal poison.

The catalyst employed in the process of the invention comprises a minor amount, e.g., 0.01-5 w. of one or more Group VIII noble metals, i.e., platinum, palladium, rhodium, ruthenium, osmium and iridium. The preferred metals are iridium, palladium, and platinum, with platinum being particularly preferred. If desired, other transition metals having dehydrogenation activity can be incorporated into the catalyst together with the noble metal.

The noble metal advantageously is supported on a suitable support such as activated carbon, refractory oxides and the like. Porous refractory oxides such as silica, alumina, magnesia and the like, e.g., substantially nonacidic, are highly suitable. These carriers are widely available commercially and their preparation is well known. Should a refractory oxide tend to be acidic, the acidity can be reduced by any suitable means, e.g., by the addition of alkali metal. Preferred carriers are alumina and silica, with silica being especially preferred. Advantageously, the silica has a high surface area, Le, a surface area of about 300 to 750 sq. m./g.

Any suitable method for adding the metal component to the support can be used. Highly suitable catalysts can be prepared by impregnating the noble metal on the support, or ion-exchanging metal with an appropriate support such as a refractory oxide. Any suitable metal compound, preferably Water soluble, can be used. For example, to prepare the preferred platinum catalysts, suitable platinum compounds which can be used include chloroplatinic acid, platinous tetrammine compounds such as platinous tetrammine chloride, platinuous tetrammine nitrate or platinous tetrammine hydroxide. Alternatively, the metal component can be provided by mixing an aqueous dispersion of a sulfide such as platinum sulfide with the refractory oxide. After addition of the metal compound, the catalyst is usually dried and preferably is calcined in air. Calcination in air, of course, decomposes the metal salt and converts the metal to the oxide form. A calcin-ation temperature in the range from about 600-900 F. is highly advantageous, although higher or lower temperatures can be used if desired.

The catalyst is sulfided to provide high activity, stability and selectivity. Sulfiding can be carried out in a known manner, such as by passing a mixture of hydrogen and hydrogen-sulfide over the catalyst at a temperature of about 500750 F. for a suitable length of time. Alternatively, the catalyst can be sulfided by adding sulfur, hydrogen sulfide, or a decomposable sulfur compound such as mercaptans, disulfides, thiophene and the like to the feed. Suitable concentrations of sulfur range from about 50 to 10,000 ppm. by weight sulfur based on the hydrocarbon. Preferably, rather high concentrations of sulfur,

e.g., from about 1,000 p.p.m. to 6,000 p.p.m. are used. A convenient and preferred method is to sulfide the catalyst as it is being heated to the desired operating temperature. Advantageously, the paraffin dehydrogenation is effected in the presence of similar concentrations of sulfur. After the initial sulfiding operatin, it is preferred to adjust the concentration of sulfur to a level of about 100 to 2,000 p.p.m. by Weight.

The catalyst is ordinarily used in granular or pelleted form in fixed beds. Fairly uniform particles of about inch to about inch in size are satisfactory. If desired, the dehydrogenation may be effected with finely divided catalyst to provide a fluidized catalytic process.

Dehydrocyclization of paraffins to aromatics is carried out at a temperature in the range from about 750-1200 F. and preferably from about 9001100 F. In general, the pressure is relatively low and can be in the range from a subatmospheric pressure of about 2 p.s.i.a. to an elevated pressure of about 100 p.s.i.a. or more. Preferred pressure are in the range from about 5 p.s.i.a. to 50 p.s.i.a. Low pressure tends to favor dehydrocyclization. Weight hourly space velocity can vary over a considerable range, such as from about 0.1 to about and preferably from about 0.25 to 5.

There is a net production of hydrogen in the dehydrocyclization reaction. Nevertheless, it appears that the presence of added hydrogen is beneficial to catalyst stability. The molar ratio of hydrogen to hydrocarbon can vary from as low as about 0.1:1 to as high as 5:1, although lower and higher ratios can be used if desired. Low hydrogen/oil ratios tend to favor dehydrocyclization. Recycle hydrogen can be used.

While paraffins which can be dehydrocyclized by the process of the invention comprise paraffins ranging from C C individually or in a mixture, the process of the invention is particularly suitable for lower paraffins, e.g., C C paraflins. The lower parafiins are generally considered to be more difficult to dehydrocyclize than higher molecular weight paraffins. The parafiin can also be in admixture with other hydrocarbons. Suitable feeds can range from hexane, heptane, or other individual hydrocarbon fractions available in a petroleum refinery to mixed hydrocarbon fractions comprising 60% v., preferably 70% v. or more, paraffins. Such fractions can be straight-run fractions, raflinates, e.g., C -C raffinates resulting from solvent extraction of aromatic hydrocarbons, molecular sieve or other processes for the separation of normal paraffins from non-normal paraflins and the like.

The hydrocarbon feed can contain cyclic paraffins. Unlike chromia-alumina, catalyst stability is little affected by cyclopentane structures. Indeed, methylcyclopentane is a significant product of normal hexane dehydrocyclization in the present process. Also, in contrast, it may be noted that in conventional catalytic reforming of naphthas with a catalyst of platinum on halogenated alumina, there is some cyclization to five membered rings followed by dehydroisomerization to aromatics. However, since the ratio of ring opening of five-membered rings to ring closure to five-membered rings is governed by an equilibrium constant, any increase in cyclization rate automatically increases ring opening rate. The net result, then, with practical reforming feeds containing methylcyclopentane is that five-membered rings are usually destroyed faster than they are produced.

In general, aromatic hydrocarbons produced in the dehydrocyclization reaction comprise at least about 15% by weight and usually at least about 25% by Weight of the liquid hydrocarbon product. The aromatics can be recovered by any suitable separation process, e.g., extraction by means of an aromatic selective solvent.

The following examples illustrate the process of the invention and its advantages.

EXAMPLE I A catalyst comprising 2% w. platinum impregnated on silica gel (Davison grade was tested with and without sulfur for the dehydrocyclization of n-dodecane. For the test with sulfur, the catalyst was presulfided by passing hydrogen and n-dodecane with added dimethyl disulfide over the catalyst While slowly heating the catalyst to operating temperature, and conducting the conversion with 1360 p.p.m. w. sulfur as dimethyl disulfide added to the feed. Operation was conducted at 860 F., 15 p.s.i.g., 2 WHSV, and 3 H /oil mol ratio. Results are given below for an interval between the second and third hour of the process period.

EXAMPLE II Various noble metal catalysts were prepared by impregnating silica gel (Davison grade 950) with an aqueous solution of an appropriate salt of the particular metal. The catalystmetal salt solution mixture was dried at 250 F. with frequent agitation for at least 5 hours and further dried at 700 F. for 1 hour.

Each catalyst was sulfided with n-hexane containing 1000 p.p.m. S as dimethylsulfide at 700 F. for 2 hours. The sulfided catalyst was tested for dehydrocyclization of n-hexane at 20 p.s.i.g. pressure, 1 WHSV, 1 H /oil molar ratio, with 1000 p.p.m. w. sulfur as dimethyl disulfide added to the feed. Conversion of n-hexane to benzene represents the most difficult dehydrocyclization case and thus provides a good test of catalyst activity. Results are given below at a catalyst age of 25 hours.

TABLE II Active Metal 1 percent w. 1 percent w. 1 percent w. 1 crcent w. Rhodium Palladium Iridium Platinum Metal Source B11013 Pd(NHa) 201 HzIl'CisfiHzO HzPtClz-GHaO Temperature, F.

Yields, percent w.:

5 EXAMPLE III This example demonstrates dehydrocyclization of various hydrocarbons. The catalyst employed was previously used for 125 hours in the dehydrocyclization of n-hexane which contained 1000 p.p.m. sulfur and comprised 1% w. platinum on silica (Davison grade 950). Reaction conditions were 1000 F., 20 p.s.i.g., 1 WHSV, 1 H /oil molar ratio and 1000 ppm. sulfur in the feed. Aromatics yield for each feed hydrocarbon is given below.

TAB LE III 2,5-dimethyl- Feed nCe n RC3 RC9 hexane Aromatics Yield, percent w. (B asis Feed) Benzene 9. 2

Toluene 31.

o-Xylene. 24. 8 0.3

m-Xylene. 0. 4 0. 8

p-Xylene 0. 2 20. 7

Ethylbenzene 18. 6 0. 2

1-methyl-2-ethylbenzene 27. 4

n-Propylbenzene 10. 5

Other 0,; Aromatics 6. 2 Total Aromatics 9. 2 31. 5 44. 0 44. 1 22.0

EXAMPLE IV TAB LE IV Toluene Ralfinate (30-7 Naphtha Feed Product Composition, percent w. Feed Product H2 3. 1 4. 9 Urea 3. 6 3. 1 C4 2. 1 1. 2 C5 2. 5 1. 0 Ce;

Acyclic 4. 3 4. 6 18. 9 14. 3 CYOlO-Os 3. 4 3. 5 9. 2 7. 9 Cycle-Ct I- 0.5 0. 0 10. 4 0. 0 C Benzene 0.0 1.6 3. 6 16.2

Acyclic 63.0 27. 8 32.3 10.8 Cyc1o'C 6. 9 9. 9 14. 0 10.1 Cyclo-Cs 1.0 0. 0 8.6 0.0 Toluene 1. 8 25. 5 2. 4 29. 7 32 Aeyclic 18.8 6.0 Cycle-0 0.3 0.9 Cyclo-Ct 0. 0 0. 0 1. 0 0. 9 Aromatics. 0. 0 8. 9 0 100.0 88. 7 100.0 89. 9 Total Aromatics-. 1. 8 36.0 6.0 46. 0

EXAMPLE V This example demonstrates the stability of the catalyst. A catalyst of 1% w. platinum impregnated on silica (Davison grade 950) was presulfided and tested for the dehydrocyclization of hexane with dimethyl disulfide added to provide 1000 p.p.m. by weight sulfur.. Reaction conditions were 20 p.s.i.g., 1 WHSV, 1 H /oil mol ratio, and temperature adjusted as necessary to provide a benzene yield of 32% w., basis feed. Temperature increase was substantially linear throughout the 100 hour test period from 978% F. to 1008 R, an activity decline rate of 0.3 F. per hour.

In a similar experiment, the sulfur level was reduced to 50 p.p.m. by weight at the end of 24 hours, after which a new, steeper decline rate of 07 F. per hour was observed.

6 EXAMPLE v1 This example demonstrates regenerability of the catalyst. A catalyst comprising 1% w. Pt on silica (Davison grade 40) was used, with and without sulfur, for dehydrocyclization of normal hexane at 20 p.s.i.g., 1 WHSV and 1 H /oil mol ratio. Temperature was adjusted as necessary to provide .a benzene yield of 30% w.

Without sulfur, catalyst decline rate was substantially constant at slightly over 1 F/ hour up to about hours (at which time the temperature demand was 951 F.) and began to accelerate thereafter. The catalyst was subjected to a simple carbon burn-off with dilute air and tested again without sulfur. The regenerated catalyst was very unstable, temperature demand increasing very rapidly from about 900 F. at the end of the two hours to 1000 F. at the end of twenty-two hours.

A fresh portion of the catalyst (presulfided at 700 F. with n-hexane containing 1000 p.p.m. S as dimethyl disulfide) was tested under similar conditions but with dimethyl disulfide added to the feed to provide 1000 p.p.m. by weight sulfur. The catalyst was quite stable, as indicated by a gradually decreasing temperature demand in the period from 10 hours (975 F.) to 84 hours (987 F.). The catalyst was subjected to a simple carbon burn identical to that for the unsulfided catalyst, and was tested again with normal hexane containing 1000 p.p.m. sulfur. Again good stability was demonstrated, the temperature demand curve being substantially the same as in the first cycle but at a 35 F. higher level. Thus, the sulfur is not only beneficial to catalyst stability but is also beneficial to regenerability of the catalyst.

I claim as my invention:

1. A process for dehydrocyclization of a parafiin having from 6 through 20 carbon atoms which comprises contacting said paraffin at a temperature of about 750 F. to 1200 F. with a sulfided Group VIII noble metal catalyst, the catalyst being in the sulfided state prior to contact with the paraffin.

2. The process according to claim 1 wherein the catalyst comprises from about 0.01 to 5% by weight noble metal on a substantially non-acidic porous refractory oxide.

3. The process according to claim 2 wherein the dehydrocyclization is carried out in the presence of from about 50 to 10,000 parts per million by weight sulfur, basis hydrocarbon.

4. The process according to claim 2 wherein the dehydrocyclization is carried out at a hydrogen to hydrocarbon mol ratio of about 0.1:1 to 5:1 and a pressure from about 2 to 100 pounds per square inch absolute.

5. The process according to claim 2 wherein the refractory oxide is silica having a surface area of about 300 to 750 square meters per gram.

6. The process according to claim 2 wherein the paraflin has from 6 through 10 carbon atoms.

7. The process according to claim 6 wherein the dehydrocyclization is effected at about 900 to 1100 F. with a sulfided catalyst comprising from about 0.015% w. platinum on silica.

8. The process according to claim 7 wherein the dehydrocyclization is carried out in the presence of from about 100 to 2000 parts per million by weight sulfur.

References Cited UNITED STATES PATENTS 2,378,209 6/ 1945 Fuller et al. 260-673.5

2,317,683 4/ 1943 Greenfelder 260668 FOREIGN PATENTS 1,024,326 3/ 1966 Great Britain.

DELBERT E. GANTZ, Primary Examiner.

J. D. MYERS, Assistant Examiner.