US2287935A - Treatment of hydrocarbons - Google Patents

Description

Patented June 30, 1942 TREATMENT or HYDROCARBONS Aristid V. Grosse and William J. Mattox, Chicago,

IlL, assignors to Universal Oil Products Company, Chicago, Ill., a corporation of Delaware No Drawing. Application October 9, 1939,

Serial No. 298,704

' 8 Claims.

This invention relates particularly to the conversion of straight chain hydrocarbons into closed chain or cyclic hydrocarbons.

More specifically it is concerned with a process involving (the use of particular catalysts and specific conditions of operation in regard to temperature, pressure, and time of reaction whereby aliphatic hydrocarbons can be converted efficiently into aromatic hydrocarbons.

In the straight pyrolysis of 'pure hydrocarbons or hydrocarbon mixtures, such as those encountered in fractions from petroleum or those oocurring naturally or produced synthetically, the

ness of catalytic materials and the art as a whole is in a more or less empirical state. using catalysts even in connection with conversion reactions among pure hydrocarbons and particularl in connection with the conversion of the relatively heavy distillates and residua which are available for cracking, there is a general tendency for the decomposition reactions to proceed at a very rapid rate, necessitating the use of extremely short time factors and very accurate control of temperature and pressure to avoid too extensive decomposition. There are further difiiculties encountered in maintaining the efiiciency 'of catalysts employed in pyrolysis since there is usually a rapid deposition of car'- bonaceous materials on their surfaces and their pores.

The foregoing brief review of the art of hydrocarbon pyrolysis is given to furnish a general background for indicating the improvement in such processes'which is embodied in the present invention, which may be applied to the treatment of pureparafiinor olefin hydrocarbons, hy-

drocarbon mixtures containing. substantial percentages of paraffin hydrocarbons such as relatively close out fractions producible by distilling petroleum, and analogous fractions which contain unsaturated as well as saturated straight chain hydrocarbons, such fractions resulting from cracking operations upon the heavier fractions of petroleum.

In one specific embodiment the present invention comprises a process for producing aromatic hydrocarbons from aliphatic hydrocarbons including parafiins and olefins which comprises contacting said aliphatic hydrocarbons under dehydrocycling conditions of temperature and pressure with a composite comprising essentially a major proportion of a substantially inert refractory carrier and a relatively minor proportion of an oxide of an'element selected from those occurring in the left-hand columns of Groups 4, 5 and 6 of the periodic table to produce a mixture containing aromatic and olefinic hydrocarbons; subjecting said mixture to contact with a hydrogen-transferring catalyst at a temperature in the approximate range of 400-800 F. under substantially atmospheric pressure to convert said olefinic hydrocarbons into aromatic and parafilnic hydrocarbons; separating aromatic hydrocarbons by solvent extraction; and recycling unconverted paraflins to further dehydrocyclization.

According to the present invention aliphatic or straight chain hydrocarbons having 6 or more carbon atoms in chain arrangement in their structure are specifically dehydrogenated in such away that the chain of carbon atoms undergoes ring closure with the production in the simplest case of benzene from n-hexane and in the case 'bon conversion reactions which are specifically accelerated under the preferred conditions by the present types of catalysts, the following structural equations are introduced:

CH2 CH. CH2 CHz CH OH I H CH1 CH3 CR: /CH I oH C11 n-hexanebenzene CH: C-CH: CH1 CHPCH: I fiH 4H 2 CH2 CH; CH CH CH: CH

n-heptane toluene on, on

(1H7 CH2CH3 CH C-CHa I I II CH1 CPU-CH: CH C-CH3 cm I CH n-octane o-xylene Ethyl benzene and m-x'ylene are also formed from n-octane by a combination of reactions involving dehydrogenation, cyclization, and isomerization, but no idea is offered as to the probable order in which these reactions occur.

In the foregoing table the structural formula of each of the primary paraffin hydrocarbons has been represented as a nearly closed ring instead 'ious amounts of hydrogen. It is not known at the present time whether ring closure occurs at. the loss of one hydrogen molecule or whether dehydrogenation of the chain carbons occurs so that the first ring compound formed is an aromatic such as benzene or one of its derivatives.

, The above three equations are of a relatively simthe case of different types of dehydrogenation ple character indicating generally the type of re- I actions involved but in the case of n-paraffins or monoolefins of higher molecular weight than the octane shown and in the case of branch chain compounds which contain various alkyl substit uent groups in different positions along the 6-carbon atom chain, more complicated reactions will be involved. For example; in the case of such a primary compound as 2,3-dimethyl hexane the principal resultant product is apparently o-xylene although there are concurrently produced definite yields of suchcompounds as ethyl benzene indicating an isomerization of two substituent methyl groups. In the case of nonanes which are represented by the compound 2,3,4-trimethyl hexane, there is formation not only of mesitylene but also of such compounds as methyl ethyl benzene and various propyl benzenes.

It will be seen from the foregoing that the scope of the present-invention is preferably limited to the treatment of aliphatic hydrocarbons which contain at least 6 carbon atoms in straight chain arrangement. In the case of paramn hydrocarbons containing less than G-carbon atoms in linear arrangement, some formation of aromatics may take place due to primary isomerization reactions although obviously the extent of these will vary considerably with the type of compound and the conditions of operation. The process is readily applicable to ,parafiins from hexane up to dodecane and their corresponding olefins. With increase in molecular weight beyond this point the percentage of undesirable side reactions tends to increase and yields of the desired alkylated aromatics decrease in proportion.

While any suitable type of catalyst having dehydro'genating and cycling properties may be employed for converting parafilns into substantial yields of aromatics, together with a certain amount of simultaneously formed oleflnic hydrocarbons, according to this invention, a satisfactory catalytic material comprises a major proportion of a refractory spacing agent, carrier, or support and relatively minor proportions of an oxide of a member of the lefthand columns of Groups 4, 5, and 6 of the periodic table consisting of titanium, zirconium, cerium, hafnium, and thorium; vanadium,columbium, and tantalum; chromium, molybdenum, tungsten, and uranium.

The carriers or supports referred to above have relatively. low catalytic'activities, while the oxides 'of the elements mentioned are of relatively high catalytic activity -and furnish byl far th greater proportion of the observed catalytic effects; The oxides of these several elements vary somewhat in catalytic activity in any given reaction comprisedv within-the scope of the invention and this variation may be greater in and cyclization reactions.

In regard to the preparation of alumina which is generally preferable as a carrier or sup-- port for the preparation of dehydrocyclization catalysts, it may be stated that three hydrated oxides of aluminum occur in nature, to wit, hydrargillite or gibbsite having the formula Al2Os'.3I-Iz0, bauxite having the formula and diaspore, having the formula A12O3.H2 O. Of these three minerals the corresponding 'oxides from the trihydrated and dihydrated minerals are suitable for the manufacture of the present types of catalysts and these materials have furnished types of activated alumina which are entirely satisfactory. Precipitated trihydrates can also be dehydrated at moderately elevated temperatures to form satisfactory types of alumina. Crystallographically and X-ray spectroscopically, this most satisfactory type of alumina is referred to as gamma-alumina, crystallizing in the cubic system, the length of edge of the unit cube being about 7.90 angstrom units. Alumina in the form of powdered corrundum or prepared by dehydrating diaspore is not suitable.

It is best practice in the final steps of preparation of aluminum oxides for use in the catalyst composites to ignite them for some time at temperatures within the approximate range of 900-l050 F. This does not correspond to complete dehydration of th hydrated oxides but gives catalytic materials of good strength and porosity so that they are able to resist for a long period of time the deteriorating effects of the service and reactivation periods to which they are subjected.

The catalytic dehydrogenating efficiency of alumina is greatly improved by the presence of oxides of the preferred elements in relatively minor amounts. The oxides which constitute the principal active catalytic materials may be deposited upon the surface and in the pores of the activated alumina granules by several alternative methods, such as, for example, the ignition of nitrates which have been absorbed. or deposited from aqueous solution by evaporation or by a similar ignition of precipitated hydroxides; As an alternative method, though obviously less preferable, the finely divided oxides may be mixed a mechanically with the alumina granules either in the wet or the dry condition. The point of achieving the most uniform practical distribution of the oxides'on the alumina should constantly be bornein mind since the observed catalytic effects evidently depend principally upon' surface action.

The oxide of vanadium which results from the ignition of the nitrate, the hydroxide, or the carbonate is principally the pentoxide V205 which is reduced by hydrogen to form the tetroxide V204, or the corresponding dioxide V0: and then to the sesquioxide V203. In any case the primary deposition of vanadium compounds upon alumina granules may be made by the use of the soluble vanadyl sulfate or .the nitrate and also solutions of ammonium and alkali metal vanadates may be employed, which furnish alkaline residues on ignition. It is probable that the sesquioxide isthe principal compound which accounts for the catalytic activity observed with vanadium catalysts in reactions of the present character.

The element chromium has three oxides, the

trioxide CrOa, the dioxide CrOz, and the sesquioxide CI2O3, the last named being readily produced by heating the trioxide in hydrogen or hydrocarbon vapors at a temperature of 480 F, The dioxide has been considered to be an equimolecular mixture of the trioxide and the sesquioxide. The oxides are readily developed on the surfaces and pores of alumina or other carrier granules by utilizing primary solutions of chromic acid H2C1O4' or chromium. nitrate Cr(NO3)s. The ignition of the chromic acid, the nitrate, or the precipitated trihydroxide produces primarily the trioxide which is then reduced to the sesquioxideto furnish an activecatalyst for use in reactions of the present character.

The two most important oxides of molybdenum which are employed alternatively in the production of dehydrocyclization catalystS aCcording to the present invention, are the dioxide M002 and the sesquioxide M0203. Since the reduction of the trioxide by hydrogen begins at about 570 F. and the reduction is rapid at 840 F. the effec- "essentially composites of precipitated silica with tive catalytic material is principally the sesquioxide. The trioxide may be added to theactive alumina carrier from a solution in aqueous ammonia or from a solution of ammonium molybdate which is added in amounts just requisite to wet the carrier granules uniformly and the mass is then dried and calcined.

It has been found essential in the production of high yields of aromatics from parafiinic hydrocarbons when using the preferred types of I dehydrocyclization catalysts that, depending upon the parafiinic hydrocarbon or mixture of hydrocarbons being treated, temperatures from 850 v to 1200 F. should be employed, contact times of approximately 0.1 to 60 seconds, and pressures approximating atmospheric. The use of subatmospheric pressure of the order of /4 atmosphere may be beneficial in that reduced pressure generally favors selective dehydrogenation reactions, but on the other hand moderately superatmospheric pressure, usually of the order of less than 100 pounds per square inch, increases the capacityof commercial plant equipment,.so that in practice a balance is struck between these two factors.

It will be appreciated by those familiar with the art of hydrocarbon conversion in the presence of catalysts that the factors of temperature, pressure, and time will frequently need to be adjusted upon the basis of preliminary experiments fins and diolefins will predominate over those of I aromatics.

I Catalysts which are used according to the p esent invention for effecting hydrogen-transfer reactions and converting the olefinic content of the d'ehydrocyclization product into a mixture of arcmatic and paraffinic' hydrocarbons from which the aromatics maybe separated by solvent exprecipitated alumina, zirconia, and alumina-zirconia mixtures. Prior to use these precipitated composites are washed to remove water soluble timum activity, based on, yields and quality of gasoline, will correspond to silica-alumina, and

silica-zirconia ratios' of the order of about 30 to Inmanufacturing the preferred catalysts in accordance with the present process it is necessary to employ silica, which has been prepared by precipitation from solution as a hydrogel within and/or upon which alumina, zirconia, or a mixture of alumina and zirconia, 1s deposited The most also by precipitation as a hydrogel. convenient and ordinary method of preparation of a satisfactory silica gel is to acidify an aqueous solutionof sodium silicate by the addition of the required amount of hydrochloric acid. The excess of acid and the concentration of the solution in which the precipitation is brought about will determine the eventual primary activity of the silica and its suitability for compositing with the alumina to produce a catalyst of increased high activity. In general, the most active silica is produced by adding only enough acid to cause gel formation to occur in the sodium silicate, but the material formed at such a point is incompletely coagulated, rather gelatinous and filtrable only with difficulty. By adding a moderate excess of acid after the gel has formed, the more desirable physical characteristics in regard to catalyst activity are conserved,

while the filtrabilityv is greatly improved and the silica hydrogel is more completely precipitated. Fairly good hydrated silica for present catalytic purposes may be made by employing as high as a 20% excess of hydrochloric acid, but

beyond this point part of the more desirable properties are lost.

After precipitation the silica gel is washed, preferably until substantially free from salts by using several alternative'reagents, which-will be described later. In one mode of preparing the activated form, the silica gel may be boiled either with a separately precipitated alumina, zirconia, or alumina-zirconiagel, which is added in the wet condition to the silica suspension, or the silica gel may be suspended in and boiled with a solution of analuminum or zirconium salt or a mixture of aluminum and zirconium salts, for example, an aqueous solution of aluminum chloride. In either case the final precipitate, comprising the hydrated silica and alumina, zirconia, or a mixture of alumina and zirconia, is

' finally washed to substantially complete removal of water soluble materials and then driedat about 300 F. to produce a granular material, which may be ground and sized to produce particles of catalyst which are then calcined at a temperature of the order of 850-1200 F'., but preferably within the range of 9004000" F.

The necessary alumina, zirconia, or mixture of alumina and zirconia gel is preferably deposited in or upon the washed alkali metal-free silica gel by adding an alkaline precipitant, such as ammonium hydroxide, ammonium carbonate, or

ammonium sulfide to aqueous solutions of aluminum and/or zirconium salts, followed by suitable washing to remove impurities. The alumina, zirconia, or mixture of alumina and zirconia may be precipitated from such solution in which previously prepared and washed hydrated silica is suspended, following by washing of the total composite of the precipitate. Similarly, purified silica may be suspended in a solution of an aluminate, such as sodium aluminate and alumina precipitated by the addition of an aluminum salt of a mineral acid, or by the requisite quantity of the acid itself. As a further alternative method of producing the desired catalyst components, salts of aluminum or zirconium, or a mixture of aluminum and zirconium salts may be added to a solution of an alkali metal silicate to jointly precipitate silica and alumina, zirconia, or an alumina-zirconia mixture; and further amounts of silica may then be precipitated by the addition of acid.

It should be emphasized in the present connection that the catalyst utilized for converting olefins into a mixture of aromatic and parafiinic hydrocarbons, according to this invention, are essentially composites of substantially pure amorphous silica with amorphous alumina, zirconia, or an alumina and zircom'a mixture. Ex-

to approximately 500 pounds per st uare inch with a contact time of the order of 0.5 to 60 seconds. The olefin-free aromatized product is then separated from the relatively small amount of gas formed during the hydrogen-shifting treatment and subjected to solvent extraction, distillation, or other means of aromatic separation, or the aromatics may be subjected to chemical .treatment such as nitration, etc., in the presence of the remaining paraflins. Thus by hydrogentransfer reactions one molecule of an olefin containing a straight chain of 6 to 12 carbon atoms may undergo dehydrogenation and cycling to produce a molecule of an aromatic hydrocarbon and provide sufiicient hydrogen for thesimultaneous conversion of 3 olefin molecules into 3 paramn molecules. Aromatics so derived from olefins may boil within or outside of the boiling range of the aromatics formed by dehydrocyclization of the original parafiin fraction in the presence of the dehydrocyclization catalyst.

The process of this invention makes possible a the production of aromatic concentrates or aroperiments have indicated definitely that distinctly inferior catalytic materials are obtained when any of the constituents in the cracking catalyst composites has any crystalline characteristics.

If alkali metal salts are present in sufiicient quantities in either the originally precipitated silica gel or in the final catalyst composite, catalytic materials are obtained with relatively low activity, or they may lose activity rapidly after repeated periods of use and reactivation. For these reasons, special washes are used which are capable of removing alkali metal compounds from the catalyst so that only minute amounts of these impurities remain, since it has been found that these alkali metals could not be washed out entirely with water alone. The washes developed are dilute hydrochloric acid, ammonium chloride, and aluminum chloride solutions which serve to displace the alkali metals so that additional water washes canremove them from the catalyst. For economic reasons hydrochloric acid and/or aluminum chloride seem to be both preferable to ammonium chloride, although ammonium chloride is apparently slightly inferior in its effectiveness.

Thus by the process of this invention a paraffinic gasoline, naphtha, or other hydrocarbon fraction may be subjected to contact witha dehydrocyclization catalyst so as to produce a 20-50% yield of aromatic hydrocarbons per pass admixed with relatively small proportions of simultaneously formed olefins and unconverted paraffinic hydrocarbons. In this cyclization treatment the parafiinic fraction is contacted, for example, with a material comprising granular activated alumina supporting approximately 4-25% by weight ,of chromium sesquioxide at 350-1200 F. under substantially atmospheric pressure using a contact time varying from 0.1

- fins, although neither section is intended to be or silica-alumina-zirconia mixtures at a temperature of the order of-400800 F. under a pressure in the range of substantially atmospheric matic-parafiin hydrocarbon mixtures which are free from olefins and suitable for solvent extraction or other methods of separating pure aromatics, or which may be used directly for nitration or for other chemical reactions. By this process the olefins formed incidental to the cyclization reaction are partially converted into aromatics thereby increasing further the yield of these desirable hydrocarbons.

The following example is introduced to show results obtainable in the operation of the process, although these data are not presented with the intention of unduly limiting the broad scope of the invention:

A parafiinic hydrocarbon fraction boiling with- I in the range of 175-390 F. was passed at 1022 F. under atmospheric pressure with a liquid space velocity of 3.8 through a chamber containing a granular composite consisting of 8% chromium sesquioxide supported by activated alumina. The product from this treatment containing 72% aromatics, 23% olefins, and 5% parafiins was contacted at 660 F. and atmospheric pressure using a liquid space velocity of 0.5 per hour with 3 x 3 mm. pellets formed by calcining at 950 F. for 6 hours a precipitated hydrogel composite consisting of mol per cent of silica and 10 mol per cent of alumina. No gas was formed by this treatment with the silica-a'lumina-catalyst and the hydrocarbon recovery amounted to 95.3% by weight of-the charge. As produced the recovered hydrocarbon contained 1% of olefins, which were eliminated entirely by removing by distillation 3% of the light hydrocarbons boiling below F.

The foregoing specification and example show clearly the character of the invention and the results to be expected in its application to allphatic hydrocarbons, including parafiins and ole-.

unduly limiting.

We claim as our invention:

- 1. A process for producing a substantially olefin-freearomatic product from aliphatic hydrocarbons which comprises subjecting the allphatics to dehydrocyclization, thereby forming a reaction mixture containing aromatics andolefins, contacting said mixture with a hydrogentransferring catalystunder conditions adequate to convert the olefins toaromatic and parafllnic hydrocarbons, separating the parafiins from the aromatics and supplying the same to the dehydrocyclization step, and recovering the separated aromatic hydrocarbons.

2. The process as defined in claim 1 further characterized in that said catalyst comprisesailica and alumina.

3. The process as defined in claim 1 further characterized in that said catalyst compriaes silica and zirconia.

contacting said mixture with'ahydrogen-transrel-ring catalyst at a temperatureiinthe approximate range of 1400-800 F. whereby to convert the oleflns to aromatic andparamnic hydrocarbons, separating. the paraflins from the aromatics and applyin theme to the dehydrocyclization'atep,--andrecovering-.the separated aromatic hydrocarbons.

6. '1'he process-as deflned'in claim 5 further :characterized ,gin that said catalyst comprises silica and-alumina.

characterizedN-inthat said catalysis comprises silica and-.zirconia.

" '.J8.V'1'hecprocess:as defined in' claim 7 further characterized-in that said catalyst comprises silica; alumina-and zircon'ia.