US2408987A - Catalytic process - Google Patents

Description

Patented Oct. 8, 1946 UNITED STATES PATENT OFFICE CATALYTIC PROCESS Maryan l3. Matuszak, Bartlesville, Okla, and Glen H. Morey, Terre Haute, Ind., assignors to Phillips Petroleum Company, a corporation of Delaware No Drawing. Original application November 9,

1937, Serial No. 173,708. Divided and this application March 18, 1941, Serial No. 384,028

9 Claims. (Cl. 260-6833) tion from th hexavalent state present in chromates and dichromates to the trivalent state present in chromites. The spontaneous decomposition is generally very rapid and is always complete within a few minutes or at most within an hour or so and not infrequently it proceeds, as in the cases of ammonium chromate. and ammonium dichromate, with explosive violence.

Chromium oxide catalysts prepared by ignition hydrogen ratios, as in dehydrogenation and hi" 10 of ammonium dichromate have been described by drogenation, Lazier and Vaughen in an article, The catalytic This application is a division of our copending properties of chromium oxide, published in the application Serial No. 173,708, filed November 9, Journal of the American Chemical Society, vol. 1937, now U. S. Patent 2,294,414. 54, August, 1932, pp. 3080-3095. The ignition re- Catalysts consisting of or containing chromium sulted. in the formation of a fluffy oxide having a oxide have been found useful in various catalytic tea-leaf appearanc and exhibitin erratic cataprocesses. One method of preparing chromiiun lytic behavior when tested for the hydrogenation oxide-containing catalysts has been the ignition of ethylene. It was non-homogeneous, as eviof ammonium-containing salts of chromic acid. denced by the presence of particles of difierent For example, Lazier ina. number of United States colors, that is, dark colored particles which appatents (for example, Nos, 1,746,783; 1,964,000; peared to possess some catalytic activity and l fi 1019, 119) has described the preparabright green particles which were apparently tion of catalysts by the heating of a double chrocompletely inactive. The green particles, whose mate of a nitrogen base such as ammonia and a. formation appeared to be favored by ignition in y enating metal such as zinc, manganese, deep layers, did not exhibit the glow phenomenon copper, nickel, and the. like. Heretofore, howbut the dark and active material glowed feebly ever, the ignition conditions have not been conwhen heated to 500 C. The best product obsidered tov be of particular significance and theretained in this way by Lazier and Vaughen was fore have not been subjected to definite control, prepared by warming one-gram portions of amwith the consequence that the chromium oxide monium dichromate in a thin layer over a flame formed in this manner simultaneously underwent until ignition was initiated. The resulting oxide glowing or calorescence and thereby certain dewas granulated by briquetting. A 20-cc. portion sirable properties were unwittingly destroyed in of this best product, when tested at 400 C. with a the catalyst. Thus, if a double chromate of am- 7-Iiter sample of an equimolar hydrogen-ethylene monia and a hydrogenating metal is heated to a mixture passed over the catalyst in an hour, or temperature at which a spontaneous exothermic at. a space velocity of about 350,.gave a conversion decomposition takes place, in accordance with of 80 per cent of the ethylene into ethane. In the teaching of the Lazier patents, generally at preparing another sample of catalyst, Lazier and a temperature between about 200 and 400 0., Vaughen heated ammonium diohromate in avacthere results an evolution of suflicient heat to 49 uum at 200250 C, for4 or5hours. The product leave a glowing residue. The resultin glowed or was a glistening black residue which contained caloresced residue is non-coherent or finely dino ammonia, was slightly paramagnetic and was vided and powdery in texture and therefore must stable at. temperatures up to 400 C, but when generally be compressed or briquetted into suitfurther heated it suddenly glowed, leaving a light able form for use. It consists substantially of the green residue without catalytic activity for ethylat least partially chemically combined oxides of cue-hydrogenation. A 20-cc. portion of the unchromium and of the hydrogenating metal of the glowed material, when tested at 400 C. with a 7- original double salt in the form of a chromite, liter sample of an equimolar hydrogen-ethylene the chromium having been substantially commixture passed over the catalyst in an hour, pletely' reduced by the spontaneous decomposi- 50 caused a conversion of .25 per cent of the ethylene into ethane. A similar portion, which had undergone glowing by being heated in a vacuum to 500 0., gave a conversion of only 3 per cent.

Such hitherto available catalysts prepared by ignition of ammonium-containing salts of chromic acid have found considerable application in reactions such as the synthesizing of methanol from oxides of carbon and hydrogen. They have also found limited application in the hydrogenation of certain organic materials and in the dehydrogenation of alcohols to aldehydes. But although these hitherto available chromium oxidecontaining catalysts have been more or less satisfactory for the conversion of oxygenated organic compounds such as alcohols and the oxides of carbon, they have been entirely inadequate for the conversion of certain organic compounds such as hydrocarbons. For this reason, they have been generally unsuited for the conversion of hydrocarbons by changing their carbon-to-hydrogen ratios and particularly so for the dehydrogenation of paraflin hydrocarbons into mono-olefins of the same number of carbon atoms. This inadequacy is well illustrated by the aforementioned conversion figures obtained by Lazier and Vaughen when they are compared with thermodynamic data. Thus, if equilibrium had been attained under the stated conditions of temperature and gaseous composition, a conversion of 99 per cent of the ethylene to ethane should have been obtained whereas a maximum of only 80 per cent was actually reached, in spite of the fact that the relatively low space velocity used was very favorable for the attainment of equilibrium.

It is an object of our invention to overcome the hereinbefore mentioned defects and difficulties of the prior art in preparing and using catalysts which contain substantial proportions of chromium oxide.

It is a further object to efiect the controlled and non-spontaneous thermal decomposition of ammonium-containing salts of chromic acid.

Another object is to prepare catalytic materials in which the chromium oxide is substantially completely in the form of black and unglowed chromium oxide.

Another object is to prepare catalysts that are highly efiicient for use in catalytic processes such as they dehydrogenation of organic compounds and especially of paraiiin hydrocarbons into hydrogen-deficient hydrocarbons, for example, the corresponding mono-olefins, and the non-destructive hydrogenation of unsaturated or ethylenic linkages in organic compounds.

It is a further object to obtain chromium oxidecontaining catalytic materials in a dense but porous, coherent and granular, and mechanically strong pseudocrystalline and crystallomorphous form directly suitable for use without compression or briquetting.

It'is also an object to obtain catalysts containingblack unglowed chromium oxide with a high resistance to the glow phenomenon, so that they can be heated to temperatures above 400 or 500 C. without loss of catalytic activity due to glowing.

Another object is to obtain such desirable catalysts uniformly and at will, without the formation of substantial amounts of green and inactive chromium oxide.

Another object is to carry out catalytic processes by the use of these catalysts derived through the non-spontaneous and controlled thermal decomposition of ammonium-containing salts of chromic acid.

Further objects and advantages of our invention will be apparent to those skilled in the art from the following description.

We have found that it is possible to effect a slow thermal decomposition of ammonium-containing salts of chromic acid under controlled temperature conditions that do not cause spontaneous or explosive decomposition to take place, and that our procedure leads to the preparation of chromium oxide-containing catalysts that are definitely superior to the catalysts of the prior art in catalytic and mechanical properties. We have further found that the residue from such slow, controlled and non-spontaneous thermal decomposition, after being subjected to a controlled reduction retains its form and appearance at elevated temperatures, and is not readily subject to the glow phenomenon, which destroys the catalytic activity of such materials. For example, in the case of the non-spontaneous thermal decomposition of crystalline ammonium salts of chromic acid carried out in the light of our 'iscoveries, the product is a porous but dense and coherent. and fairly hard pseudocrystalline or crystallomorphous granular residue retaining the apparent or gross crystalline shape of the original ammonium salt. The salt granules or crystals shrink appreciably during the non-spontaneous decomposition, and the product retains a small proportion of ammonia and of water derived from oxidation of a part of the ammonium in the original salt. The total chromium oxide of the residue has an approximate empirical formula of C1O2, indicating that the chromium is, either actually or on the average, in a, tetravalent state. The presence of chromium having a valence greater than three is readily observable by dissolving a portion of the catalyst in hot dilute sulfuric acid, cooling, and adding potassium iodide, whereupon iodine is liberated. Under the same conditions, trivalent chromium does not cause any liberation of iodine.

Before the residue from the non-spontaneous thermal decomposition is used as a catalyst in reactions such as changing the carbon-hydrogen ratios of hydrocarbons it is reduced and it is generally preferable to subject it to a controlled reduction in the presence of an atmosphere consisting of or containing a reducing gas or gases. After reduction, advantageous conditions for which are hereinafter described, the black, unglowed and crystallomorphous residue from the non-spontaneous thermal decomposition of ammonium salts of chromic acid possesses a high catalytic efficiency for reactions such as dehydrogenation, non-destructive hydrogenation, hydrocarbon desulfurization, and the like. It can be used as a catalyst at all temperatures at which the conversion is thermo-dynamically high enough to be desirable or profitable, such as temperatures within the range 200-600 C. Any carbonaceous deposit formed on it during use can be burned off with air under suitable temperature conditions without destruction of its catalytic activity and it can thereafter be used again. Furthermore, it can be repeatedly used and reactivated without undergoing the glow phenomenon or calorescence that may accompany or follow the spontaneous decomposition of ammonium salts of chromic acid or which may often be in? duced in other chromium oxide-containing preparations by heating to a temperature above about 400 or 500 C.

Whether or not the chromium oxide of the residue obtained by the non-spontaneous thermal decomposition of our process is truly tetravalent, as the empirical formula. CrOz implies, is not definitely known by us. We prefer to consider that the residue has a composition that may be expressed by the formula CrzOaCrOs, which has th same ratio of chromium to oxygen as (H02 and which implies that two-thirds of the chromium is trivalent and. that one-third is hexavalent. Because of this preference and because of its convenience, we shall hereinafter refer, in the specification and claims, to the chromium of higher valence than three which is present in the residue from. the non-spontaneous thermal decomposition as. being hexavalent, it being understood that we do not Otherwise limit ourselves.

Advantage of the presence of this hexavalent chromium in the residue may be taken for the purpose of determining when the. controlled and non-spontaneous thermal decomposition has reached a satisfactory end point, We have found that the non-spontaneous decomposition should be continued to a point at which the content of hexavalent: chromium, as compared with the total chromium lies Within the range -40 per cent, as determined by the method to be described directly. In ourbest preparations the hexavalent chromium content has generally been within the range 27-35 per cent and we prefer that it be within the range -33" per cent. Before the decomposition, generally all of the chromium is hexavalent, since a salt of chromic acid is the substance decomposed. Hence, the progress of th non-spontaneous decomposition may be read ily followed by withdrawing samples from time to time and analyzing them for total chromium and for hexavalent chromium. The total chmmium is determined by taking a weighed portion of the sample, suitably about one. gram, gently heating it in an excess of a solution of mercurous nitrate, suitably in 5 cc. of a saturated solution of mercurous nitrate, whereby all of. the hexavalent chromium is reduced to the trivalent condition, then evaporating th solution to dryness and igniting the residue strongly to constant weight, whereby all but chromic oxide is volatilized and removed, and finally Weighing the residual chromic oxide, CrzOz. The hexavalent chromium is determined by dissolving it out from a second weighed portion, suitably 0.1-0.2 gram, of the sample with hot dilute sulfuric acid, suitably with 500 cc, of 6-7 per cent sulfuric acid, which may require boiling, for 20-30 minutes to effect dissolution, then cooling, adding an. excess of potassium iodide, suitably 1-2 grams, and titrating the liberated iodine with sodium thiosulfate solution of known strength, suitably 0.1 normal, with starch as indicator. From the data thus obtained the percentage of the total chromium found as hexavalent chromium may be readily calculated. The controlled non-spontaneous thermal decomposition is preferably continued until the hexavalent chromium has decreased to approximately one-third of the total chromium, more or less.

In order that the decomposition may not become of the spontaneous character, which We have found to be undesirable andharmful to the catalytic and physical properties of the product, it is essential that the temperature during decomposition be not permitted to exceed about 225 or 230 C. We prefer to carry out the decomposition at temperatures not exceeding about 200 0., since thereby the danger of spontaneous or explosive-decomposition. is minimized. But it is not desirable to use temperatures much: below 175" C.,. since such: lower, temperatures needlessly prolong the decomposition period. This is illustrated by the experimental facts that inthe-nonspontancous decomposition of a sample of ammonium dichromate in air in. an electric oven kept. at, 200 C. aminimum hexavalent chromium content of 32 per. cent-of the total chromium. was reached in a period of about: 15 hours, whereas when the-oven was kept at 175 C. thisperiod: became about 11 days. For these reasonsit is ad vantageous. and: preferable to carry out. the decomposition at: temperatures: between about 175 and 2.00 C.. Itais possible, however, to decompose successfully ammonium salts; of. chromic ac d: at temperatures somewhat: above this: preferred range, up. to about 225.. or230" C., ify-all conditions are favorable; but generally it is felt that the. gain in. shortening the period of non-spontaneous decomposition does not compensate for the increased danger of occurrenceof the undesired spontaneous decomposition and: its attendant: destruction of mechanical-"strength. and: catalytic activity- Another reason why it is preferable to use" a maximum decomposition temperature. not much in excess of 200 C; is that. We: have foun-dzthat, if the heating of the non-spontaneously thermally decomposed. material is continued in an oxidizing atmosphere such as air, the content of hexavalent chromium as defined. herein becomes a. minimum and then slowly increases again. For: example, in the aforementioned. case of the. decomposition of ammonium. dichromate air at 200 6., in whicha minimum hexavalent-chromium, content of 32. per cent of the total chromium: was reached in 15 hours, it was found that after a total of hours the, content of hexavalent chromium had increasedxt'o slightly over 40' per cent of thetotal chromium. Again, in the also aforementioned case of the decomposition of ammonium dichromate inair at. 0., in: which aminimum hexavalent-chromium content of 32 per-cent of the total chromium was reached in 11 days, the hexavalent chromium became 35 percent of the total chromium after a total period of 14 days; We have found that a content of hexaval'ent chromium in excess of approximately 35 percent of the total chromium is undesirable and disadvantageous because there exists a pronounced tendency of the oxide or oxides of such hexavalent chromium, which appear to be formed on continuing the heating in an oxidizing atmosphere beyond the minimum content of hexa alent chromium. to undergo aspontaneous thermal decomposition which causesa destruction of mechanical strength and catalytic activity. For ex ample, if a preparation containing such hexavalent' chromium is heated to a sufficiently high temperature, such as a temperature of about 300 C. or more, the higher oxides decompose, sometimes with explosive violence, and the product, if. it. has not disintegrated into dust, is so fragile-that it can be readily crushed or powdered between: the fingers and it is practicallyworthless as a catalyst for dehydrogenation of paramn: hy drocarbons to the corresponding mono-olefins and; likewise for hydrogenation reactions;

To avoid the reoxidation which. has just been described it is generally advantageous to carry out the controlled and non-spontaneous thermal decomposition ina non-oxidizing or reducing. at mosphere. We have found that we can successfully decompose ammonium dichromate and: ammonium chromate in. atmospheres of hydrogen, nitrogen, ammonia, and carbon dioxide. Hydrogen has the advantage that the reduction of the catalyst, which is necessary before it is ready to be used in a catalytic conversion process, can be carried on simultaneously with the non-spontaneous thermal decomposition. However, great care must be exercised that heat liberated by the reduction, which is highly exothermic, does not raise the temperature high enough to cause spontaneous thermal decomposition of the still unreduced oxides of hexavalent chromium. For this reason, we prefer to carry out the non-spontane ous thermal decomposition and the reduction as separate and consecutive steps. During the nonspontaneous decomposition we further prefer to use a flowing atmosphere of an inert non-oxidizing and non-reducing gas of high molar heat such as carbon dioxide because such gases efficiently absorb the heat liberated by any incipient spontaneous decomposition and thus minimize or inhibit the tendency for such undesirable spontaneous decomposition to occur or to continue.

The time required for the non-spontaneous thermal decomposition depends upon the temperature. We have hereinbefore given specific directions for determining when the decomposition is completed and have cited the lengths of representative periods at thetwo extremes of the preferred range of 175-200" C. Within this preferred range a generally suitable period of time for carrying out the controlled and non-spontaneous thermal decomposition may be found by adding to a period of 15 hours an additional period of 10 hours for every degree centigrade that the temperature used lies below 200 C. At lower temperatures such as in the range 150-175" C., the period would be of the order of two Weeks or more and at higher temperatures up to about 230 C. it would be of the order of several hours or less, depending on the extent that the temperature used differed from the preferred range.

By similar controlled and non-spontaneous thermal decomposition of mixed or double ammonium-containing salts of chromic acid we may obtain homogeneously commingled chromium oxide-containing catalysts that have as other 5' We have also used ammonium chromochromate,

NH4O.CrO2.O.Cr.O.CrOz.ONI-I4 which is an ammonium-containing salt of chromic acid containing also divalent chromium. Due, however, to greater difficulties of preparing these various ammonium-containing salts of chromic acid, their higher cost, and their tendency to form very small crystals or none at all, we do not consider such materials to be as advantageous for making catalysts by non-spontaneous decomposition as simple ammonium salts of chromic acid, such as ammonium chromate or ammonium dichromate. Of the advantageous materials we have found ammonium dichromate to be the more advantageous because of its relatively more desirable, because less elongated, crystalline shape.

With respect to the size of crystals, we prefer a size that passes through a. 10-mesh and is retained by a 20-mesh screen; but we do not wish to have our invention limited to this particular size, as other sizes operate almost or equally as well. If the original material available is in the form of crystals larger than desired, they may be broken or crushed to granules of the desired size, preferably but not necessarily before the non-spontaneous thermal decomposition. If it is in the form of crystals smaller than desired,- it may be recrystallized to yield larger crystals.

After the non-spontaneous thermal decomposition i complete, the chromium oxide-containing residue is reduced. This may be done with any reducing gas, such as hydrogen, carbon monoxide, butane, propane, and the like, or with an atmosphere containing such a reducing gas or gases. The reduction is preferably carried out as a separate step, but in many cases it may be incorporated as a part of the starting up of a run in which this material is to be used as a catalyst. For example, in a dehydrogenation procedure the material to be dehydrogenated may be passed over the chromium oxide-containing material while the catalyst chamber is in the warm-up period, the material to be dehydrogenated thus acting as the reducing gas. However, it is advantageous to use hydrogen or an atmosphere containing hydrogen as the reducing gas, as therewith the reduction can be carried out at the lowest possible temperature and the possibility of the simultaneous formation of a carbonaceous deposit on the catalyst is avoided. The

- temperature should not in any case be allowed to rise above about 300 C., and preferably not above about 250 C., before the reduction is complete, since above this temperature thermal decomposition of unreduced higher oxides of chromium is rapid and the catalyst particles may consequently disintegrate into dust and simultaneously lose much or all of their catalytic activity, and so limiting the temperature of reduction is a part of our invention.

Reduction can be carried out at as low a temperature as 175 C. or lower and we prefer to carry out the reduction with the temperature slowly and gradually increasing from below or about this value to one of about 250 C. If desired, room temperature may be the starting point. We further prefer to dilute th reducing gas with an inert diluent gas such as nitrogen or carbon dioxide, as the diluent gas advantageously tends to prevent or minimize any local rise in temperature caused by the reduction, which is highly exothermic in nature. Due to its higher molar heat and its greater tendency to be adsorbed on the catalyst, carbon dioxide is somewhat superior to nitrogen as a diluent gas, as it appears to be capable of absorbing more energy arising from the reduction than is a gas such as nitrogen, probably not only in the form of molecular and intramolecular energy but also in the form of latent heat of desorption. However, the lack of such dilution is not to be considered as going outside the scope of our invention. Completion of reduction can be readily determined by means that are well known to workers in the art, as by determining if water is being formed or by determining if any hydrogen is being consumed. Analyses of the oxide content may also be used for control purposes.

The following examples will further illustrate the nature of this invention but the invention is not to be restricted thereby.

9 Example I to pass a 20-mesh sieve and to be retained by a- 40-mesh sieve, was spread in a thin layer on a hot-plate and was then non-spontaneously decomposed by being heated to 200 C., and kept at this temperature for 16 hours. At the end of this time the residue consisted of homogeneously black, porous but dense and coherent, and mechanically strong crystallomorphous granules that retained the apparent or gross crystalline shape of the original ammonium dichromate. It contained 33.8 per cent of the total chromium as hexavalent chromium, as previously discussed and defined and as determined by the analytical procedure hereinbefore described. The residue contained considerable ammonia and water which were evolved in the subsequent treatment now to be described. A -cc. sample was placed in a vertical catalyst chamber made from a piece of heat-resistant glass tube of about 8 mm. in internal diameter and provided with a concentric internal thermocouple well. It was then slowly heated by an electric resistance furnace in the presence of a downwardly flowing atmosphere of hydrogen to a temperature of about 200 C. After reduction was complete at this temperature, the temperature was gradually increased to 450 C. After about an hour at 450 C., the hydrogen Was replaced by a, stream of isobutane at atmospheric pressure and flowing at the rate of 10 liters per hour, whereby a space velocity of about 2000 was established, calculated without regard to any shrinkage of the catalyst. Conversion of isobutane to isobutylene began at once, increased rapidly to the equilibrium value which is about 17 per cent for this temperature, maintained this value for about 2 hours, and then decreased gradually to 10 per cent in about 6 more hours. The heating of the furnace was automatically regulated to maintain a constant temperature of 450 C. at the bottom of the catalyst bed; above this point the temperature was somewhat below 450 C. because of removal of heat by the dehydrogenation reaction, which is strongly endothermic. After a total period of 20 hours, the conversion had decreased to a value of 3 per cent. The decrease in activity during the test was caused by a slow gradual deposition of carbonaceous matter on the catalyst. The catalyst was now removed and its volume was found to be 3.4 cc.; the shrinkage in volume of about 1.6 cc. during the reduction and heating to the operating temperature of 450 C. had probably occurred because of expulsion of ammonia and water. After reactivation overnight in a current of air at temperatures gradually increasing from room temperature to about 285 0., whereby the carbonaceous deposit was burned off from th catalyst, it was reduced again by being heated in a stream or hydrogen while the temperature increased from room temperature to 450 C. Then, in a second run made with isobutane at atmospheric pressure and at a flow rate of 10 liters an hour and with the temperature gradually raised from 450 to 515 C., in order to compensate for the gradual deactivation that was probably caused by deposition of carbonaceous matter, the catalyst caused a substantially constant conversion of 17 per cent of the isobutane into isobutylene for a period of 9 hours. The performance of the catalyst in these two runs was many times repeated in subsequent 10 runs made either at a constant temperature of 450 C. or at a constant conversion of 1'? per cent.

Example II As a second example, a quantity of ammonium dichromate crystals, passing through a ill-mesh and retained by a 20-mesh sieve, was non-spon taneously decomposed at a temperature of 175 C. After 3 days at this temperature, the content of hexavalent chromium, as determined by the method hereinbefore described, was found to have decreased from the original value of per cent to 45 per cent of the total chromium. After a total of 7 days, the content was 35 per cent. After a total of 11 days, it reached a minimum of 31.9 per cent. On continued heating, the content of hexavalent chromium increased gradually to 34.9 per cent after a total of 14 days. A 5-cc. portion was then reduced with hydrogen and used to dehydrogenate isobutane under the same conditions that have been described in Example I. At a constant temperature of 450 0., measured at the bottom of the catalyst bed, it caused an equilibrium conversion of 17' per cent of the isobutane into isobutylene for 2 hours and then the conversion dropped to 10 per cent in Gmore hours. The final volume of the catalyst was 3.6 cc. It was repeatedly reactivated and used without appreciable diminution of its catalytic activity.

Example III As a third specific example, a quantity of crystale of ammonium dichromate, crushed to 20-40 mesh size was non-spontaneously decomposed by being heated for 20 hours at 200 C. in an atmosphere of ammonia gas. A 5-cc. portion, which shrank to 3.5 cc. during the reduction with hydrogen and the bringing to a temperature of 450 C. as described in Example I, was used at 450 C. to dehydrogenate isobutane at a flowrate of 10 liters per hour. Equilibrium conversion of 17 per cent of isobutane to isobutylene was maintained for 2 hours and then the conversion gradually decreased to 10 per cent in 6 more hours. The catalyst was reactivated and used repeatedly to give the same catalytic performance.

Example I V As an example which shows that the control of decomposition conditions is important, a quan-' tity of 10-40 mesh crystals of ammonium dichromate was non-spontaneously decomposed by being heated at C. for 1'2'days. Then the residue was heated in an electric drying oven at 200-230 C. for some time. By accident the temperature of the oven increased to about 300 C., more or less. The material became dull black and powdery in appearance and very weak in mechanical strength, differing strikingly from the glistening black and mechanically strong crystallo-morphous granules'obtaine'd in the preceding three examples. It contained only 1.5 per cent of the total chromium as hexavalent chromium. On being subjected to the usual reduction procedure with hydrogen and then tested for the dehydrogenation of isobutane at 450 C. under the conditions described in Example I, a 5-cc. portion of this material gave an initial maximum conversion of only 13 per cent of the isobutane into isobutylene and this conversion rapidly decreased to 10 per cent in a little over an hour. The final volume of this sample was 4.9 00., indicating that, within the limits of experimental error, all shrinkage in volume had occurred during the accidental rise of temperature in the oven prior to reduction. In spite of the larger final volume of material, the performance of this material was definitely much inferior as a catalyst to that obtained in the three preceding examples. This illustrates the deleterious efiect caused by spontaneous thermal decomposition of oxides of hexavalent chromium or of chromium having a valence greater than three when the temperature gets out of control and is permitted to become too high. We have repeatedly observed similar efiects when a portion of a batch which under other conditions produced a good catalyst was reduced too rapidly, or at too high a temperature, whereby the heat liberated by the reduction raised the temperature of the still unreduced higher oxides to such a point that harmful spontaneous decomposition took place.

Example V As an example of the use of our catalyst for the addition of hydrogen to an unsaturated linkage between carbon atoms, we may hydrogenate octenes to octanes in the following manner: Pure di-isobutylene was passed with an excess of hydrogen over our catalyst at 250 C. and at a total pressure slightly in excess of atmospheric. The catalyst was similar to that described in Example I. The flow rate was equal to four volumes of liquid hydrocarbon per volume of catalyst per hour. The effluent hydrocarbons were 99 per cent saturated, as determined by titration at about C. with a one per cent solution of bromine in carbon tetrachloride.

Example VI Gasoline-boiling-range hydrocarbons which were prepared by the catalytic polymerization of cracking-still gases and which contained about 3 per cent of sulfur in the form of sulfur-containing organic compounds were treated in a manner similar to that of Example V, except that the operating temperature was300 C. and the pressure was 250 pounds per square inch. The resultant gasoline, after being washed with an alkali solution, was sweet to the doctor test, showing efiicient desulfurization, and was 98.5 per cent saturated, showing excellent hydrogenation.

For the sake of being able to make direct comparisons we have limited our specific illustrative examples on dehydrogenation to the dehydrogenation of isobutane. Thus we have been able to show that we can reproduce our results consistently, and that we can producesimilar results using several different modifications. Catalysts prepared in the manner herein disclosed may be used for the dehydrogenation of many other paramn hydrocarbons, from ethane through heavy oils and waxes. Thus, parafiinic motor fuels such as straight-run gasoline may be improved in combustion characteristics by being subjected to treatment with such catalysts. Such catalysts are also valuable in the production of cycloolefin and aromatic hydrocarbons, such as in the formation of benzene from cyclohexane. The production of diolefins from olefins may also be accomplished by the use of these catalysts.

These catalysts are also quite efiicient in promoting the addition of hydrogen to unsaturated linkages between carbon atoms, and especially in the non-destructive addition of hydrogen to olefin hydrocarbons. They are further efficient because they are not readily poisoned by the usual poisons for non-destructive hydrogenation catalysts. In fact, we have found that in the case of the most common poison, sulfur, our catalysts may be used for desulfurizing organic materials by converting the organic sulfur contained therein substantially completely into hydrogen sulfide, which can then be removed by well-known means, as by an alkali wash. They may also be used for the production of hydrogen from steam and carbon monoxide and for other reactions known for this type of catalytic material.

Mention has often been made herein that the chromium oxide or oxides in the most desirable residue from the controlled and non-spontaneous thermal decomposition of ammonium-containing chromates or dichromates has an empirical formula which closely approximates C102, and it has been shown that this may be further represented by a simple mixture or combination of chromium oxides such as CI2O3.CIO3. For this reason, and as a matter of convenience, the chromium with a valence higher than three has been spoken of as a certain amount of hexavalent chromium, and a method for determining this higher-valent chromium has been given. The true chemical formula of the residue has not been definitely established; but it is immaterial whether the higher-valent chromium is considered as being truly tetravalent, as in CrOz, or as being partly truly hexavalent and partly truly trivalent, as in CI'203.C1O3, since the chemical formula has no bearing on the invention other than as discussed herein. Therefore, any mention made herein or in the claims which follow of hexavalent chromium in chromium oxide or of chromium oxide of any particular content of hexavalent chromium, is to be considered in the light of this discussion and disclosure.

The terms pseudocrystalline and "crystallomorphous used in this specification and in the accompanying claims are taken to mean that the granules of the product from the non-spontaneous thermal decomposition have the same apparent or gross shape or physical form as the original crystals or granules of ammonium-containing salt of chromic acid used as raw material. It is probable, but not definitely known, that the atoms in the product are definitely arranged and spaced and thus in this respect resemble the atoms in a true crystal; this may possibly contribute to the high catalytic efliciency of the product.

The'term coherent as used herein is to be understood to imply that the residue from the non-spontaneous thermal decomposition persists in the form of the original granules or crystals instead of readily falling into powder; furthermore, it is not to be taken as implying a coalescence of granules into a larger mass or masses. We do not wish to exclude from our invention certain modifications or alternatives which will be obvious to those skilled in the art. Furthermore, we do not wish to limit our invention to the details of materials, temperatures, pressures, times, and the like which we have cited in our illustrative examples. Hence, we desire to have it understood that, within the scope of the appended claims, our invention is as extensive in scop'eand equivalents as the prior art allows.

We claim:

l. A process for catalytically dehydrogenatlng a hydrocarbon, which comprises contacting said hydrocarbon at a dehydrogenating temperature with an unglowed metal chromate catalyst which has been prepared by subjecting a complex crys talline compound having the general formula (NH4)zM(CrO4')2-2NH3, where M represents one or more metals selected from the group consisting of copper and cadmium, to controlled heating at an elevated temperature below the temperature at which said compound decomposes with incandescence, until the compound is substantiallydeoomposed.

2. A process'for the treatment of a hydrocarbon material to effect a change in the carbon-hydrogen ratio thereof, which comprises contacting said hydrocarbon material at a reaction temperature within the range of 200 to 600 C. with a granular catalyst prepared by subjecting a crystalline ammonium-containing salt of chromic acid, which comprises at least one metal other than the chromium in said chromic acid, to controlled heating at an elevated temperature below the temperature at which said salt decomposes with incandescence, until the salt is substantially completely decomposed while retaining its crystallic shape.

3. The process of dehydrogenating a hydrocarbon having at least two carbon atoms per molecule, which comprises contacting said hydrocarbon in the vapor phase at a dehydrogenating temperature below about 600 C. with a granular catalyst prepared by subjecting a crystalline ammonium-containing salt of chromic acid which comprises at least one metal other than the chromium in said chromic acid to controlled heating at an elevated temperature below the temperature at which said salt decomposes with incandescence, until the salt is substantially completely decomposed while retaining its crystallic shape, and subsequently subjecting the resultant material to the action of a reducing atmosphere at an elevated temperature not greater than about 300 C. until reduction is substantially complete.

4. A process for the treatment of a hydrocarbon material to effect a change in the carbonhydrogen ratio thereof, which comprises contacting said hydrocarbon material at a reaction temperature within the range of 200 to 600 C. with a granular catalyst prepared by subjecting a granular non-powdery crystalline ammoniumcontaining salt of chromic acid, the particles of which are sufliciently large to be retained on a 40-mesh sieve, to controlled heating at an elevated temperature below the temperature at which said salt decomposes with incandescence, until the salt is substantially completely decomposed homogeneously throughout the individual granules while retaining its crystallic shape without disruption of the granules.

5. A process for the treatment of a hydrocarbon material to effect a change in the carbonhydrogen ratio thereof, which comprises contacting said hydrocarbon material at a reaction temperature within the range of 200 to 600 C. with a granular catalyst prepared by subjecting a granular nonpowdery crystalline ammoniumcontaining salt of chromic acid, the particles of which are sufficiently large to be retained on a LO-mesh sieve, to controlled heating at an elevated temperature below the temperature at which said salt decomposes with incandescence, until the salt is substantially completely decomposed homogeneously throughout the individual granules while retaining its crystallic shape Without disruption of the granules, and subsequently subjecting the resulting granular residue to the action of a reducing atmosphere at an elevated [ill temperature below about 300 C. untilreduction is substantially complete.

'6. The process of dehydrogenating a paramn hydrocarbon having at least two carbon atoms per molecule, which comprises contacting said hydrocarbons in thevapor phase at a. dehydrogenating temperature below about 600 C. with a granular catalyst prepared by subjecting a granular nonpowdery crystalline ammonium containing salt of chromic acid, the particles of which are sufiiciently large to be retained on a lo-mesh sieve, to controlled heating at an elevated temperature below the temperature at which said salt decomposes with incandescence, until the salt is substantially completely decomposed homogeneously throughout the individual granules while retaining its crystallic shape without disruption of the granules, and subsequently subjecting the resulting granular residue to the action of a reducing atmosphere at an elevated temperature below about 300 C. until reduction is substantially complete.

'7. The process of hydrogenating an unsaturated hydrocarbon which comprises passing said hydrocarbon together with free hydrogen under a hydrogenating pressure and at a hydrogenation temperature not in excess of about 600 C. with a granular catalyst prepared by subjecting a granular nonpowdery crystalline ammoniumcontaining salt of chromic acid, the particles of which are sufiiciently large to be retained on a 40-mesh sieve, to controlled heating at an elevated temperature below the temperature at which said salt decomposes with incandescence, until the salt is substantially completely decomposed homogeneously throughout the individual granules while retaining its crystallic shape without disruption of the granules, and subsequently subjecting the resulting granular residue to the action of a reducing atmosphere at an elevated temperature below about 300 C. until reduction is substantially complete.

8. The process of dehydrogenating a paraffin hydrocarbon having at least two carbon atoms per molecule, which comprises contacting said hydrocarbon in the vapor phase at a dehydrogenating temperature below about 600 C. with an unglowed chromium oxide catalyst composed of coarse granules and prepared by subjecting a crystalline ammonium chromate, the particles of which are sufiiciently large to be retained on a lo-mesh sieve, to controlled heating at an elevated temperature in the range of about to 200 C. for a time of 15 hours plus an additional time of 10 hours for every degree centrigrade the said temperature lies below 200 C., the said particles undergoing decomposition substantially homogeneously throughout while retaining their crystallic shape and undergoing said treatment substantially without disintegration.

9. The process of dehydrogenating a paraffin hydrocarbon having at least two carbon atoms per molecule, which comprises contacting said hydrocarbon in the vapor phase at a dehydrogenating temperature below about 600 C. with an unglowed chromium oxide catalyst composed of coarse granules and prepared by subjecting a crystalline, granular and nonpowdery, ammonium-containing salt of chromic acid, the particles of which are suificiently large to be retained on a 40-mesh sieve, to a controlled heating in an oxidizing atmosphere containing free oxygen at an elevated temperature within a range of 75 C. below and adjacent to the spontaneous thermal decomposition temperature of said salt for 15. a time sufllcient to effect a substantially complete controlled decomposition of said salt homogeneously throughout said granules to an unglowed dark residue with a content of chromlum having a valence higher than three within a range of about 25 to 40 per cent of the total chromium and substantially at a minimum, said salt retaining its orystallicshape without substantial disruption of the granules thereof, and

subsequently subjecting said decomposed salt to the action of a reducing atmosphere at a temperature within the range of about 1'75 to 250 C. for a period of time sufilcient to efiect substantially complete reduction while substantially maintaining the unglowed and granular condition of said material.

MARYAN P. MA'I'USZAK.

GLEN H. MOREY.