Patent Publication Number: US-3879486-A

Title: Conversion of non-cyclic C{HD 3{B -C{HD 5 {B alkanes and alkenes to aromatic hydrocarbons

Description:
United States Patent Mitchell, Jr.  
 [451 Apr. 22, 1975 CONVERSION OF NON-CYCLIC C -C ALKANES AND ALKENES TO AROMATIC HYDROCARBONS [75] Inventor: Maurice M. Mitchell, Jr.,  
 Wallingford, Pa.  
 [73] Assignee: Atlantic Richfield Company, Los Angeles, Calif.  
 [22] Filed: Sept. 24, 1973 [21] Appl. No.: 400,298  
 Related US. Application Data [63] Continuation-impart of Ser. No. 324.720, Jan. 18,  
 [52] US. Cl 260/673; 252/456 [5 1] Int. Cl. C07c 5/26 [58] Field of Search 260/673 [56] References Cited UNITED STATES PATENTS 2.378.209 6/1945 Fuller et al. 260/6735 Primary E.\&#39;uminerDelbert E. Gantz Assistant Examiner-Juanita M. Nelson Attorney, Agent, or Firnz--Delbert E. McCaslin [57] ABSTRACT Method for the conversion of non-cyclic C -C alkanes, alkenes or mixtures thereof to aromatic hydrocarbons in a single stage employing particles of bismuth oxide on an inert support physically admixed with particles of a chromia-aluminum catalyst having low coking and burning characteristics, the physical mixture being particularly characterized by the method of producing the supported bismuth oxide and by the fact that the chromia-alumina constitutes the major portion of the mixture.  
 5 Claims, No Drawings CONVERSION OF NON-CYCLIC C -C ALKANES AND ALKENES TO AROMATIC HYDROCARBONS CROSS REFERENCE TO RELATED APPLICATION This application is a continuation-in-part application of application Ser. No. 324,720, filed Jan. 18, 1973, to Maurice M. Mitchell, .lr., entitled CONVERSION OF NON-CYCLIC C -C ALKANES TO AROMATIC HYDROCARBONS.  
 BACKGROUND OF THE INVENTION This invention relates to a method for the conversion of non-cyclic C -C alkanes, alkenes or mixtures thereof to specific aromatic hydrocarbons in a single stage at high yields and selectivities employing particles of supported bismuth oxide physically admixed with particles of chromia-alumia catalyst characterized by having low coking and burning properties. The physical admixture is also characterized by the specific method of making the supported bismuth oxide and the fact that the chromia-alumina is the major component of the mixture. The C -C alkanes are also characterized by being capable of being dehydrogenated to an olefin having an allylic hydrogen atom, in particular propane and isobutane.  
  It is well-known to dehydrogenate alkanes to olefins, and the prior art is replete with methods for this conversion. It is also well-known that the acylic olefins of 3 to 4 carbon atoms, for example, can be dehydrodimerized with a compound containing oxygen bonded to bismuth and active for the removal of a hydrogen atom from the terminal methyl group of such olefins, for example, US. Pat. No. 3,631,216 shows sodium bismuthate and bismuth oxide as dehydrodimerization agents for C and C olefins. The products resulting from such dehydrodimerization reactions are predominantly the corresponding diolefin and it has also been proposed to cyclicize and dehydrogenate such diolefins to produce the corresponding aromatics.  
  Thus, although alkanes or alkenes could be converted to aromatics by a series of steps, it was shown in a recently issued patent, US. Pat. No. 3,644,550 (1972), that a mixture of particles of bismuth oxide and chromia-alumina could be used to accomplish the conversion of isobutane, or a mixture of isobutane and isobutylene, to para-xylene. In this patented process, however, the weight ratio of bismuth oxide particles to chromia&#39;alumina particles had to be in the range of from 19:1 to about 1:1 with examples showing a ratio of bismuth oxide to chromia-alumina of 4:1 weight ratio. In addition the process required long contact times with the result that a large quantity of catalyst was required per pass of hydrocarbon feed. The patentees demonstrated however, that the conversion could be accomplished in a single stage to give a good selectivity of para-xylene of high purity and thus accomplished a useful result. There are, however, certain inherent disadvantages in the patented process, namely, because of the long contact time required there would be a prohibitive amount of uncontrollable coking and burning of the charge and with the high amount of bismuth oxide required, which compound is relatively scarce, the process could not compete economically with existing commercial processes for the production of paraxylene. In addition the patented process shows the production of appreciable amounts, 5 per cent or more, of 2,5-dimethyl-2,4-hexadiene which is unconverted to aromatics and thus presents a problem of separation from the para-xylene because of the closeness of the boiling points of the two compounds.  
 SUMMARY OF THE INVENTION In accordance with the instant invention a C to C non-cyclic alkane which can be dehydrogenated to an olefin having allylic hydrogen atom such as propane, n-butane, isobutane, Z-methylbutane and the like or a C to C alkene such as propylene, butene-l, butene-Z, isobutylene, 3-methylbutene-l, 2-methylbutene2, and the like, or a mixture of such alkane and alkene is contacted with a dehydrodimesization compound containing oxygen bonded to bismuth and active for the removal of a hydrogen from the terminal methyl group of the olefin corresponding to the dehydrogenated aforementioned alkanes or from the aforementioned alkenes with particles of chromia-alumina catalyst which has been treated such that it has low coking and burning properties. The dehydrodimerization compound such as bismuth oxide, Bi O- is supported on an inert support such as silica, such that the particles will have approximately the same density as the chromia-alumina particles and in addition the particles of each material are in the same particle size range. The conversion is carried out in the range of from 400 C. to 650 C. at about atmospheric pressure or slightly above. The products are principally benzene, from the C hydrocarbons and ortho-xylene, para-xylene and ethyl benzene from the C hydrocarbons while paradiethylbenzene is obtained from the 2-methylbutane, but mixtures of a large number of aromatic compounds are obtained from the C alkenes.  
  An additional feature of the invention is that a specific method of producing the dehydrodimerization agent is required in order that it has the desired density to match that of the chromia-alumina and that it has an activity such that a short contact time is employed and with a weight of chromia-alumina being in considerable excess over the weight of the bismuth oxide. In addition since the bismuth oxide enters into the reaction with loss of oxygen such loss must be terminated at a specific point and the compound reactivated in a specific manner.  
  It is an objet of this invention therefore to provide a method for the single stage conversion of non-cyclic C -C alkanes, alkenes or mixtures thereof to aromatic hydrocarbons having twice the number of carbon atoms as the starting C C hydrocarbon.  
  It is another object of this invention to provide a mixture of particles in a specific size range of bismuth oxide on a support with particles in the same size range of chromia-alumina catalyst in a weight ratio that will convert C -C alkanes, alkenes or mixtures thereof to the corresponding aromatic hydrocarbons having twice the number of carbon atoms as the specific starting hydrocarbon at short contact times with high conversion and selectivities.  
  It is another object of this invention to provide a method for the production of a supported bismuth oxide dehydrodimerization agent in particle form in a specific size range and having an activity such that said particles can be combined with particles of chromiaalumina catalyst to convert C -C alkanes, alkenes or mixtures thereof to their corresponding aromatics having twice the number of carbon atoms as the starting hydrocarbon.  
  Additional objects of this invention will be apparent from the description of the invention which follows and from the claims.  
 DESCRIPTION OF THE INVENTION In C -C alkanes which can be used in the instant invention are those which can be dehydrogenated to an olefin having an allylic hydrogen atom, in particular propane for the production of benzene and isobutane for the production of para-xylene. In addition, however, n-butane can be used for the production of orthoxylene and Z-methylbutane can be used in the production of para-diethylbenzene. Although n-pentane can be used to produce principally l,2,4,5-tetramethyl benzene (durene) the yields and selectivities are less attractive because of competing side reactions. Neopentane (2,2-dimethylpropane) cannot be employed in the instant invention since it cannot be dehydrogenated to a C olefin having an allylic hydrogen or in fact be converted to a C olefin of any kind, and thus it cannot be converted to a C aromatic.  
  The C -C alkenes which can be used in the instant invention include propylene for the production of benzene and isobutylene for the production of para-xylene, the latter being particularly suitable. Other C and C alkenes include butene-l cis-butene&#39;2, or transbutene-2 which produce a mixture of para-xylene, ethyl benzene and ortho-xylene with the latter being in the largest amount; 3-methylbutene-l, 2-methylbutene-2 or 2-methylbutene-l which produce para-diethyl benzene, meta and para-methyl isopropyl benzene as well as other aromatics such as durene. Thus these hydrocarbons are less attractive since they are less selective for the production of a single aromatic although all are convertible to aromatics in accordance with this invention. A particularly useful mixture of alkane and alkene is isobutane and isobutylene for the production of paraxylene.  
  The dehydrodimerization agent, i.e. bismuth oxide must be produced in a specific manner in order to provide the particle size and activity required for the method of this invention. Although the bismuth oxide can be deposited on a variety of inert supports or slightly basic support materials such as magnesia, zirconia, and the like, the preferrred support is silica gel. The most preferred method of producing the supported bismuth oxide is by the oxy nitrate method. Reagent grade bismuth nitrate is dissolved in dilute nitric acid, i.e. from 3 to volumes of reagent grade concentrated nitric acid is combined with sufficient water to make 100 volumes of dilute acid, with 6 to 8 volumes concentrated nitric acid combined with sufficient water to make 100 volumes being a convenient dilute solution. The exact dilution is not critical and the aforementioned dilutions are preferred primarily for their convenience in the preparation. Concentrated solutions of nitric acid, however, should be avoided. A convenient weight proportion of the bismuth nitrate in the nitric acid solution is 2 parts by weight of bismuth nitrate to one part by weight of the nitric acid solution although this is not critical and can be varied from the solubility limit of the bismuth nitrate in the nitric acid solution to 1 part by weight of the bismuth nitrate to 10 parts by weight of the nitric acid solution. Extremes, however, in either direction simply are less convenient since on the one hand the bismuth nitrate solution is so concentrated it cannot be deposited evenly on the silica gel while on the other hand if very dilute too much solution must be handled. The bismuth nitrate in nitric acid solution is then contacted with silica gel of from 20 to 50 mesh, i.e. particles which pass through the 20 mesh screen and are retained on the 50 mesh screen of a US. Sieve.  
  The silica gel can be any one of a number of commercial grades which are prepared in a manner such that they retain their pore structure and do not decrepitate (fracture into pieces) in aqueous solutions such that the particles can be impregnated with any desired salt contained in the aqueous solution. In general such commercial materials have surface areas of the order of 300 square meters per gram as measured by conventional techniques.  
  The silica gel slurry in the bismuth nitrate solution is evaporated with constant stirring to deposit the bismuth nitrate on the silica surface. The solids thus obtained are air dried and quenched with sufficient dilute aqueous ammonia, for example, 1 volume concentrated aqueous ammonium hydroxide and 99 volumes of water, such that the final pH of the wash solution is of the order of about 10. Dilute ammonia is used to neutralize the nitric acid to prevent excessive heat of neutralization which would disintegrate the solid particles. The solids are recovered by filtration and dried at an elevated temperature, for example, of from about 100 C. to C., relatively slowly such as for about 5 hours. Although somewhat shorter times or longer times can be used, socalled flash-drying is to be avoided. It is desired to obtain the free-flowing solids without disintegrating the solid particles.  
  The method of calcination of the dried solids thus obtained is a critical feature of the invention. It is preferred the calcination be in air and it is necessary in calcining that the calcination temperature beincreased slowly so that the final calcination temperature is reached only after about 6 to 8 hours or longer. In genera] the final calcination temperature should be in the range of from about 575 C. to 625 C. with the preferred range being from 600 C. to 610 C. The solids are held at this temperature for an additional hour. One method of accomplishing this method of calcination is to calcine in temperature stages holding the solids at a particular temperature stage for from A to 1 hour with the increments between stages being from 35 C. to 50- C. preferably. For example, a quantity of solids produced as described above were calcined at 149 C., 187 C., 225 C., 263 C., etc., up to 605 C., i.e. 38  
 C. increments throughout, with the time at each stage being about V2 hour and with the solids being held at 605 C. for 1 hour. Thus, it required approximately 6 hours to raise the catalyst to the final calcination temperature and it was held at this temperature for the additional hour. If calcination is carried out in stages in this manner, which is convenient in small scale or small plant scale procedures, the time should be at least /2 hour at each stage and preferably if the temperature increments are toward the larger end of the range then longer times should be used, for example up to 1 hour. Increments of temperature larger than about 50 C. are to be avoided since these tend to cause disintegration of the solid particles. On a commercial scale where close temperature control and moving gas streams such as air, preferably, can be employed to aid the decomposition of the nitrate it is unnecessary to use the stage calcination since the temperature can be raised gradually over the desired 6 to 7 hour period and held at the desired upper calcination temperature for the required time. After cooling the solids are ready for use.  
  It has been found that the amount of bismuth oxide on the silica gel should be in the range of from about 12 weight per cent to about 22 weight per cent and preferably from about 14 to 16 weight per cent based on the total weight of the agent. In order to obtain this amount of loading on the silica gel it is necessary to adjust the ratio of bismuth nitrate in the nitric acid solution to the amount of silica gel to provide these proper proportions. It has been found that when the bismuth oxide is deposited on the silica gel in the manner described that the surface area of the resulting composition is in the range of from 200 to 250 square meters per gram showing only a nominal loss from the original surface area of the silica gel.  
  Thus the critical features of the dehydrodimerization agent preparation are that the particle size be in the range of from 20 to 50 mesh U.S. Sieve, that the loading of the bismuth oxide on the silica gel be within the range described in order to provide (1) the desired activity and surface area and, (2) a particle having essentially the same density as the chromia-alumina particles to be described hereinafter. In addition to having the desired loading of bismuth oxide on silica gel particles in the 20 to 50 mesh range, after depositing the bismuth nitrate on the silica gel from the nitric acid solution, the air dried particles are washed with a sufficient amount of dilute aqueous ammonia to give a final pH of the wash solution in the range of about and thereafter the particles are dried slowly to give a free-flowing material. It is then critical that these solids be calcined, in air, at the slow rate described in order to develop the maximum activity of the bismuth oxide as it is produced by the decomposition of the nitrate and so that the final calcined composition retains a large proportion of its original surface area and at the same time does not disintegrate the particles, which decomposition occurs with too rapid calcining.  
  As will be shown in detail hereinafter the amount of bismuth oxide on the silica gel should be within the desired range since if amounts lower than about 12 percent are employed the composition is not sufficiently active and would require inordinately large amounts of the material, contrary to the objects of the invention, while if too large amounts of bismuth oxide are deposited on the silica gel and density is higher than the density of the chromia-alumina particles, the activity of the agent is too high and too much bismuth oxide is required thus again defeating the objects of the invention.  
  The chromia-alumina catalyst for use in this invention can be any chromia-alumina catalyst, but preferably is the commercial material produced by conventional manufacturing methods and having conventional composition. In general, chromia-alumina catalysts can be prepared by treating preformed alumina particles with a solution of a chromia compound and, thereafter, the impregnated particles are pelleted, dried and calcined to produce the desired chromia-alumina catalyst. Although chromia-alumina compositions containing from about 10 to about 25 mole percent chromia can be used, the preferred chromia-alumina catalysts are those containing from about 17 to 18 mole per cent chromia since such catalyst has the optimum density and activity. Irrespective of the method of manufacture or source of the chromia-alumina such as catalysts generally contain acid sites.  
  Acid sites are well-known to catalyze coking and cracking of hydrocarbons and in the instant invention they also promote burning reactions. It has been found that if the chromia-alumina catalyst&#39;is treated with a dilute, e.g. 1 weight per cent, aqueous solution of a base, sodium hydroxide, preferably, for a number of hours, for example, about 16 hours such that the catalyst is completely penetrated by the hydroxide, the acid sites are substantially completely neutralized. The catalyst is washed thereafter until the wash water has a pH of 7 showing that neutrality has been achieved and any excess hydroxide has been removed. If, however, the chromia-alumina catalyst is not in the 20 to 50 mesh range, U.S. Sieve, it is crushed and sieved to produce a fraction in this range and thereafter is neutralized as described.  
  It has been found, as has been pointed out, that when the silica gel is loaded with 14 to 16 weight per cent bismuth oxide the density of these particles are substantially the same as the density of the chromia-alumina particles and, since they have the same particle size range they can be admixed in any desired proportion without danger of segregation, i.e. there is produced a non-segregating admixture. This permits mixing the particles in a fixed bed operation or in a moving bed system.  
  In a fixed bed process the single reactor containing the mixed solids can be operated either up-flow or down-flow.  
  The solids can be in the form of a single bed or can be supported on trays or similar conventional means. In the moving bed system the solids are passed thorugh a reactor zone and then into a regeneration zone for regeneration with air.  
  The bismuth oxide dehydrodimerization agent reacts with the allylic hydrogen on each of two molecules of the olefin produced from the dehydrogenation of the alkane, or from the alkene itself if such is used as the original hydrocarbon charge material. The dehydrodimerization reaction thereby produces water and a di-allylic dimer of the olefin, for example, if isobutane is utilized as the alkane isobutylene is produced which in turn is converted to the 2,5-dimethylhexadicue-1,5 and the bismuth oxide will be reduced while water is also produced. In operation the oxygen is extracted from the crystal lattice of the bismuth oxide supported on the silica gel support and if more than from about 50 to percent of oxygen is removed there is danger that the crystal lattice may collapse.  
  It is preferred to discontinue the reaction, however, when from about 20 to 50 per cent of the oxygen has been removed from the bismuth oxide. After the reaction is stopped the mixture of supported bismuth oxide and chromia-alumina is purged with an inert gas such as steam, carbon dioxide or nitrogen preferably while in the reactor and then the mixture is regenerated with a dilute molecular oxygen-containing gas such as air at temperatures preferably between 600 C. and 650 C. The regeneration accomplishes two objectives, it replaces the oxygen lost from the bismuth oxide and also burns off any coke or hydrocarbonaceous deposits on the chromia-alumina catalyst. It is preferred to discontinue the regeneration before all of the oxygen has been replaced in the bismuth oxide, i.e. the oxide content has been replaced to the extent of from about to 98 per cent of that originally present. lt has been found that if it is attempted to replace all of the oxygen content of the bismuth oxide, it becomes over-activated and promotes burning instead of the desired reactions.  
  After regeneration the mixture is again purged and the hydrocarbon reactant is again passed over in contact with the mixture.  
  As has been pointed out the particle size range of the two components must be in the range of from 20 to 50 mesh. U.S. Sieve, in order to achieve the objects of this invention namely that two components have the desired activity to give the optimum selectivity for the production of the desired aromatic and in addition have the same density such that the mixture is nonsegregating. It is theorized also that if the particle size is larger than about 20 mesh, the particles of the supported bismuth oxide dehydrodimerization agent and the particles of the chromia-alumina catalyst are so far apart that when the dehydrogenation reaction occurs before the dehydrodimerization reaction, or alternatively, after the dehydrodimerization reaction there is sufficient time for side reactions to occur before the aromatization reaction. Thus the particles should not be larger than the 20 mesh in order to minimize the time for the reaction molecules to move from one active particle to the other.  
  In support of this hypothesis it has been found that with particles larger than about 20 mesh the selectivity for the production of para-xylene from isobutane or isobutylene, for example, decreases very markedly, that is the particles are too far apart to give the desired selectivity for the production of the stable aromatic and instead other reactions have time to occur.  
  Likewise if the particles are smaller than about 50 mesh they are so close together that apparently some sintering&#34; or similar phenomenon occurs with the result the mixture loses its selectivity for the production of the aromatic and instead forms a complex which promotes burning, i.e. CO production, instead of promoting the desired reactions.  
  The additinal critical feature of this invention is that the weight ratio of the chromia-alumina to the bismuth oxide itself, i.e. not including the weight of the support should be greater than 1:1. It has also been found preferable to use about a 1:1 weight ratio of chromiaalumina to supported bismuth oxide. It has also been found that chromia-alumina particles having from to 25 weight per cent chromia (Cr O and bismuth oxide on silica gel having from 12 to 22 weight per cent bismuth oxide (Bi O are sufficiently near the same density so that in the same particle size range these two components do not segregate to the extent to render the process inoperable. The commercial chromiaalumina catalyst contains about 17.5 weight per cent Cr O and its density matches almost exactly the density of a bismuth oxide-silica gel composition wherein the bismuth oxide amounts to from 14 to 16 weight per cent. Thus in general it can be stated that with the desired weight ratio of supported bismuth oxide to chromia-alumina of about 1:1 the chromia-alumina weight ratio to the bismuth oxide alone (i.e. it can range from 12 to 22 weight per cent of the supported composition) will be in the range of from 8.4:l to 4.6:1 and at the preferred range of 14-16 weight per cent bismuth oxide on silica gel the ratio is about 7.221 to 6.2: l. The most preferred ratio of chromia-alumina to silica gel supported bismuth oxide (14.5 Bi O is about 7:1.  
  The reaction can be carried out in the range of from 400 C. to 650 C. If pure alkanes are employed or a mixture of alkanes and alkenes are employed temperatures at the upper end of the range are preferred, e.g. from 575 C. to 650 C., or more preferably 600 C. to 625 C. A specific temperature of about 610 C. has been found satisfactory for both pure alkanes and mixtures of alkanes and alkenes such as in a recycle stream. If pure alkenes are being processed it is unnecessary that they be dehydrogenated and consequently temperatures at the lower end of the range are preferred, e.g. from 400 C. to 600 C. or more preferably from 500 C. to 550 C. with the most preferred range being from 525 C. to 550 C.  
  The reaction can be carried out either at subatmospheric, atmospheric or super-atmospheric pressures, however, slightly super-atmospheric pressures are preferred, i.e. in the range of 50 psia to psia, with 100 psia being a convenient pressure.  
  The fresh feed is preferably the pure alkane or alkene since it is desired to produce the pure aromatic as has been described, however, since one of the byproducts of the reactoin is the mono-olefin corresponding to the alkane this can be recycled to the reaction together with the fresh feed. Thus, for example, if isobutane is the alkane feed some isobutylene will be produced which is not converted to the aromatic and this can be recycled to the reactor along with the fresh feed isobutane. Likewise, of course, mixtures of the alkane and the corresonding alkene can be employed as fresh feed, for example, propane and propylene, isobutane and isobutylene and the like.  
  The contact time can range from 0.1 seconds to 3.0 seconds with from 0.3 to 2.6 seconds being preferred and from 0.5 to 2.0 being more preferred, with a time of about 1.3 seconds being used for comparative purposes in many of the runs shown in the examples which follow. These times are at reaction conditions, i.e. taking into account reaction temperature and void space in the reactor. When this is converted to space time units as described in the prior art (wherein a mixture of bismuth oxide and chromia alumina is employed) it is less than 4 seconds, i.e. less than the minimum and far less than the most preferred range.  
  As has been pointed out the instant invention uses a supported bismuth oxide-chromia-alumina mixture wherein the weight of the chromia-alumina is greatly in excess of the bismuth oxide. Consequently, taking this into account along with the shorter contact time required in the instant method it can be shown that for commerical plants of the same capacity the plant using the supported bismuth oxide and chromia oxide in the desired weight ratio and particle size range with the density of the particles being substantially the same amount of bismuth oxide required is less than about 1 per cent of that required in the plant using the method of the prior art wherein bulk bismuth oxide is the major component and the particle size of the components are larger, i.e. the smallest prior art particles are retained on a 20 mesh sieve whereas the largest particles of the components of the instant invention pass through the 20 mesh sieve.  
  In the following examples the bismuth oxide supported on silica gel was produced by the oxy nitrate method described in detail hereinbefore. The bismuth nitrate solution was admixed with the diluted nitric acid as described and then deposited on commercial 20-50 mesh U.S. Sieve silica gel having a surface area of about 300 square meters per gram. After drying, quenching with aqueous ammonia until the final solution had a pH of 10 the solids were recovered and dried at about 130 C. for hours. The solids were calcined in stages at 149 C., 187 C., 225 C., etc. up to 605 C. (38 C. increments throughout) with the time at each stage being about /2 hour. The 605 C. temperature was held for one hour. In a typical product the bismuth oxide-silica supported material had a surface area of 223 square meters per gram showing only a nominal reduction of the original surface area. In each production run the amount of bismuth oxide to silica gel was adjusted to give the desired loading of the bismuth oxide on the silica gel support. The solids after cooling were admixed with the chromia-alumina catalyst.  
  The chromia-alumina catalyst employed in the runs of the examples which follow was a commercial material which had been reduced in particle size and sieved to give a 20-50 mesh U.S. Sieve fraction. The catalyst contained 17.5 weight percent chromia (Cr O Since the catalyst as received always contained acidic sites it was treated to neutralize those sites. This treatment consisted of contacting the catalyst with a 1 weight per cent sodium hydroxide solution, washing until essentially neutral wash was obtained and drying. The base may also be potassium or ammonium hydroxide, for example, since this is not critical nor is the exact concentration critical, however, dilute solutions are preferred in order that washing is facilitated. The neutralization step is a critical step in the chromia-alumina catalyst The following examples are provided to illustrate the invention further and are not to be construed as limiting.  
 EXAMPLE 1 In order to demonstrate the effect of particle size, a number of runs were carried out using various mesh size particles. It is, of course, obvious that one cannot simply coat large particles and reduce them to the finer mesh sizes since the coating is on the outside of the particles and grinding would simply expose uncoated surfaces. Moreover, if various size particles were coated the surface to weight of bismuth oxide ratio would not be comparable for the various mesh size ranges consequently bulk bismuth oxide particles were prepared by slurrying bismuth oxide powder in a nitric acid solution prepared by mixing 25 volumes reagent grade concentrated nitric acid with 75 volumes of water. After filtering, drying and calcining the filter cake was crushed into granules of the desired mesh size ranges shown in Table 1. One volume of bismuth oxide granules was mixed with 2 volumes of neturalized chromia-alumina catalyst. In the first run the bismuth oxide granules were 10 to 12 mesh U.S. Sieve while the chromiaalumina was 12 to 20 mesh. This prevented to some degree segregation of the particles. In the other runs the particles were in the same mesh range. The runs were made on isobutane at 610 C. the contact times were varied to give constant conversion for strict comparison purposes. The results are shown in Table I.  
 TABLE 1 Particle Size 10 12* 20 50 100*** 100 200* Range, U.S. Sieve l2 20** Conversion of isobutane, wt. 20 20 20 20 Para-xylene selectivity wt. 7: based on isobutane converted 18 42.5 34.5 7 C0 Selectivity based on isobutane converted 6 14.0 44.5 82.5  
 Bismuth oxide chromia-alumina Both Bismuth oxide and Chromiu-uluminu preparation, i.e. in addition to the sizing and sieving to the desired mesh range, since the unneutralized chromia-alumina has been found to effect markedly the selectivity of the process in particular the purity of the desired aromatic and also increases the buring tendencies of the catalyst.  
  In the runs which follow a tubular electrically heated reactor was employed. In general 25 cc of the silica gel supported bismuth oxide-chromia-alumina catalyst particle mixture was employed. The particles were maintained at the desired reaction temperature and the hydrocarbon flow over the particles was maintained for about 4 minutes using a flow rate to provide the desired residence (contact) time. In this length of time approximately 15-20 per cent of the oxygen in the bismuth oxide was consumed. The reactor was flushed with an inert gas (argon, although any inert gas could be employed commercially) regenerated with air, flushed again with inert gas (argon) and then the flow of hydrocarbon was resumed. As has been pointed out the regeneration of the bismuth oxide was halted shortly before completion to preserve its selectivity and prevent over-activation.  
  It will be apparent that the 20-50 mesh particles size range gives the optimum selectivity for para-xylene production and also minimizes the burning reaction since the ratio of CO selectivity to para-xylene selectivity is the same for the 20-50 mesh particles as for the 10 to 20 mesh particles but the para-xylene selectively is greatly increased for the 20 to 50 mesh range.  
  Having demonstrated the criticality of particle size it was next necessary to show the criticality of the amount of bismuth oxide on the support and the need for a high ratio of chromia-alumina to the bismuth oxide. This is shown in Example 11.  
 EXAMPLE II In order to show the above described effects of varying the amount of bismuth oxide on the silica gel support, various supported bismuth oxide agents were produced in the 20-50 mesh U.S. Sieve range in accordance with the procedure described herein. These particles were admixed with an equal volume of the neutralized chromia-alumina particles in the same 20-50 mesh range. The reaction was carried out with isobutane at 600 C. with a contact time of 1.3 seconds. The conditions and results are shown in Table 11.  
  The most interesting features of the data are an increase in isobutylene, a decrease in propylene and no loss of xylene as a result of using mixed feed of single- TABLE 11 pass-product composition. Likewise no wholesale wt Bizox burning of isobutylene to CO occurred under the more silica gel vigorous conditions required for isobutane conversion &#34;FP 5 20 40 compared with the results one obtains for pure isobu- Convcrsmn of isobumnc wt 1;; 23 26 3O 36 28 tylene. Further, a slight but real increase in overall cony version is obtained reflecting mostly the improved isose ectivtty wt. based on 10 butylene yield. isobutane Since as pointed out in Example 111 when pure lSObU- gfg&#39; iz fg .8 8 9 g 6 &#39;3 tylene is run under conditions optimum for pure isobu- Para-xylene urity7&#39; 861, 35, 99+ 9 9 tane considerable conversion to CO occurs, the folcozsehct&#39;vly lowing Example is provided to show the results obwt. 71 based on isobutane tamed when pure isobutylene is run under conditions converted 15.9 9.2 3.6 5 2 6.6 preferred for pure alkenes.  
  It will be seen from these data that a loading of from about 12 to 22 weight per cent Bi O on the silica EXAMPLE 1V gel produces optimum selectivity, yields the purity of para-xylene while minimizing CO production. It is also The catalyst and method are the Same as described apparent from these data that a preferred range is from In Example cept flo of pure isobutylene IS 14 to 16 weight pe t Bi O on th ili passed through the catalyst bed at a rate corresponding A number f th runs were i d out on other b 2 to an average residence time of 0.5 seconds and at a O drocarbons and mixtures thereof such as propane, n- 5 temperature of 550 butane, propane and isobutane, isobutane and isobutyl- Under these chhdltlohs, In a yp run, 14 We1ght ene to demonstrate that the desired aromatics could be P Cent of the lsohutylehe fed 10 the reahtor 1S produced. The isobutane-isobutylene mixture simu- Verted t0 Product, 80 Percent of Whlch 1S aromatic lated recycle of xylene-free product, as shown in the Products of aromatlc Precursors (dlmeihylhexadlenesl following example. 30 About 60 percent of the product is para-xylene at a purity of 98 percent in the meta-para isomer mixture. Re- EXAMPLE cycle of the dienes improves the para-xylene yield to 71 In order to simulate recycle of xylene-free product a Percent Of the Product Selectivity 2 15 5 Percent mixture f i ob t a d i b l was passed over of the converted isobutylene. Use tlme for the reactor th abov d s ib d t l wh i [h Optimum bed mixture is about two minutes before regeneration loading of B50, on silica gel (sized to 2050 mesh) w is necessary. After use, the system is purged with nitroadmixed with the alkali-neutralized commerical gen and regenerated with air. chromia&#39;alumina also sized to 20-50 mesh. The weight The critical features of this invention have been demratio of the supported Bi O to chromia alumina was 40 onstrated, namely that there is a critical particle size 1:]. The runs were carried out in the same manner as range for the two components, that the bismuth oxide described for Example l and 11. The concentration of must be supported and prepared in a critical manner in isobutylene in the mixture was 9.1 weight per cent order to provide the desired selectivity and density which approximates the concentration of isobutylene characteristics such that the two components are nonobtained from isobutane operating at optimum condisegregating and that the chromia-alumina component tions. For comparison purposes runs on pure isobutane must be in excess over the bismuth oxide in order to and pure isobutylene were also made. The conditions provide the advantages over prior art processes. and results are shown in Table 111.  
 TABLE 111 1007: 9.09 Wt.% Isobutylcne lsobutane lsobutylene 90.91 Wt.% lsobutane 610C. 610&#34;C. 610C. 1.3 sec. Contact Time 1.3 sec. Contact Time 1.2 see. Contact Time Yield Select. Yield Select Yield Select. C, C2 hydrocarbons 2.75 1 1.4 nil 1.94 5.39 C&#34; olefin 5.40 22.4 nil 3.53 9.82 lsobutylene 9.95 41.3 23.64 65.74 Hexadiencs 0.12 0.5 1.52 6.5 0.11 0.31 Benzene 0.19 0.8 0.47 2.0 0.21 0.53 Toluene 0.67 2.8 0.47 2.0 0.60 1.67 P-xylene 3.35 13.9 9.22 39.4 3.95 10.98 Others 0.19 0.8 1.99 8.5 0.24 0.67 C02 1.47 6.1 9.73 41.6 1.74 4.84 TOTAL 24.1 100 0 23.40 100.00 35.96 100.00 Unconvcrtcd 75.9 76.6 64.04  
 Yield (Wt. of product/Wt. of feed converted) X 101) Selectivity (Wt. 01&#39; product/Wt. of feed converted) X 100 Sum 01&#39; all yields conversion Sum 01&#39; all seleclivilics 100% Wt. of product Wt. of feed which becomes each product.  
 I claim:  
  1. In the method for the single stage conversion of non-cyclic C -C alkenes or mixtures of alkenes and alkanes to aromatic hydrocarbons having twice the number of carbon atoms as the alkene or mixtures of alkene and alkane which is converted wherein said alkene or mixture of alkene and alkane is contacted with a physical admixture of particles of supported bismuth oxide dehydrodimerization agent and particles of a chromiaalumina catalyst said contacting being at a temperature in the range of from 400C. to 650C., the improvement comprising employing an essentially nonsegregating particulate admixture of said supported bismuth oxide and said chromia-alumina at a weight ratio of supported bismuth oxide to chromia-alumina of about 1:1 said supported bismuth oxide particles and chromia-alumina particles being of substantially the same density and having a mesh size of from 20 to 50 and wherein the weight ratio of chromia-alumina to bismuth oxide alone is about 8.4:1 to 4.611.  
 2. The method according to claim 1, wherein said bismuth oxide is supported on silica gel in an amount ranging from 12 weight per cent to 22 weight per cent based on the total weight of bismuth oxide and silica gel and has been calcined to a final calcination temperature in the range from about 575C. to 625C. and the chromia-alumina catalyst contains from l0 to 25 mole per cent chromia and has been treated to neutralize substantially all of the acid sites of said catalyst.  
  3. The method according to claim 2, wherein the bismuth oxide ranges from about 14 to 16 weight per cent, the final calcination temperature of the bismuth oxidesilica gel is in the range of 600C. to 610C and the chromia ranges from 17 to 18 mole per cent of the chromia-alumina catalyst.  
  4. The method according to claim 1, wherein said alkene is isobutylene and the aromatic produced is paraxylene.  
 5. The method according to claim 4, wherein isobutane is admixed with the isobutylene.