US2294414A - Catalyst - Google Patents

Description

Patented Sept. 1, 1942 CATALYST Maryan P. Matuszak, Bartlesville, Okla, and Glen H. Morey, Terre Haute, Ind, assignors to Phillips Petroleum Company, a corporation of Delaware No Drawing. Application November 9, 1937,

Serial No. 173,708

I 21 Claims.

This invention relates to catalysts for use in catalytic processes and it has particular relation to catalysts that contain chromium oxide in substantial amount and that have been prepared by the thermal decomposition of ammonium-containing salts of chromic acid.

Catalysts consisting of or containing chromium oxide have been found usefulin various catalytic processes. One method of preparing chromium oxide-containing catalysts has been the ignition of ammonium-containing salts of chromic acid.

For example, Lazier in a, number of United States patents (for example, Nos. 1,746,783; 1,964,000; 1,964,001; 2,019,419) has described the preparation of catalysts by the, heating of a double chromate of a nitrogen base such as ammonia and a hydrogenating metal such as zinc, manganese, copper, nickel, and the like. Heretofore, however, the ignition conditions have not been considered to be of particular significance and therefore have not been subjected to definite control, with the consequence that thechromium oxide formed in this manner simultaneously underwent glowing or calorescence and thereby certain desirable properties were unwittingly destroyed in the catalyst. Thus, if a double chromate of ammonia and a hydrogenating metal is heated to a temperature at which a spontaneous exothermic decomposition takes place, in accordance with the teaching of the Lazier patents, generally at a temperature between about 200 and 400 (3., there results an evolution of sufiicient heat to leave a glowing residue. The resulting glowed or caloresced residue is non-coherent or finely divided and powdery in texture and therefore position 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 inthe cases of ammonium chromateand ammonium dichromate, with explosive violence.

*Chromium oxide catalysts prepared by ignition of ammonium dichromate have been described by Lazier and Vaughen inan article on The catalytic properties of chromium oxide, pub- Society, vol. 54, August, 1932, pp. 3080-3095. The ignition resulted in the formation of a flufiy oxide having a tea-leaf appearance'and exhibiting erratic catalytic behavior when tested for the hydrogenation of ethylene. It was non-homogeneous, as evidenced by the presence of particles of different color, that is, dark colored particles which appeared to possess. some catalytic activity and bright green'particles which were apparently completely inactive- The greenparticles, whose formation appeared to be favored by ignition in deep layers, did not exhibit the glow phenomenon but the dark and active material glowed feebly when heated to 500 C.- The best product obtained in this way by Lazier and Vaughen was prepared by warming one-gram portion of ammonium dichromate in a thin layer over a flame until ignition was initiated.- The resulting oxide was granulated by briquetting. A 20-00. portion of this best product, when tested at 400 C. with a 7-liter sample of an equimolar hydrogen-ethylene mixture passed over the catalyst in an hour, or at a spacevelocity of about 350, gave a conversion of 80 per cent of the ethylene into ethane. In preparing another sample ofcatalyst, Lazier and Vaughen heated ammoover the catalyst in an hour, 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 (1., gave a conversion of only 3 per cent.

Such hitherto available catalysts prepared by ignition or 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 lishedin the Journal 01 the American Chemical 55 also found limited application in the hydrogenation of certain organic materials and in the dehydrogenation oi alcohols to aldehydes. But although these hitherto available chromium oxidecontaining catalysts have been more or less sat-' i'sfactory for the conversion oi oxygenated organic compounds such as alcohols and the oxides of carbon, they have been entirely inadequate for the conversion of certain organic compounds art in catalytic and mechanical properties.

have been generally unsuited for the conversion of hydrocarbons by changing their carbon-tohydrogen ratio and particularly so for the dehydrogenation of paraflln hydrocarbons into monooleflns 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 tem-. perature 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. 1

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

It is a further object to effect the controlled and non-spontaneous thermal decomposition of ammonium-containing salts of chromic acidl Another object is to prepare catalytic materials containing 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 containing black 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 glow- Another object is to obtain such desirable lcatto be desirable or profitable, such as temperaalysts uniformly and at will, without the formadecomposition of ammonium-containing salts of chromic acid.

Furtherobjects and advantages of our invention will be apparent to those skilled in the art from the following description. 7

We have found that it is possible to effect a means such as hydrocarbons. For this reason, they ance at elevated temperatures, and isnot 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 discoveries, the product m a porous but dense and coherent, and fairly hard meudocrystalline or crystallomorphous granular residue retaining the apparent or gross crystalline shape of the Original ammonium salt. The salt granules 0r crystals shrink appreciably during the non-spontan'eous 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 chromim oxide of the residue has an approximate empirical for. mula of ClOz, indicating that the chromium is, either actually or on the average, in a tetravalent state. Thepresence 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 ratio 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 oi chromic acid possesses a high catalytic eiiiciency 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 thermodynamically high enough tures wtihin the range 200-600 C. Any carbonaceous deposit formed on it during use can be burned ofl 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 induced in other chromium oxide-containing preparations by heating to a temperature .above about 400 0r 500 C.

Whether or not the chromium oxide of the residue obtained by the non-spontaneous-thermal slow, thermal decomposition of ammonium-comtaining 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 oi the prior We have further found that the residue from such slow, controlled and non-spontaneous thermal decomposition of our process is truly tetravalent, as the empirical formula CrO: implies, is not definitely known by us. We prefer to consider that the residue has a composition that may be expressed by the formula CraOgr CrOa, which has the same ratio oi chromium to oxygen as C102 and which implies that two-thirds of the chromium is trivalent and that one-third is hexavalent. Be-

- valence than three which is present in the residecomposition, after being subjected to a con-'- 1 due from the non-spontaneous thermal decomposition as being hexavalent, it being understood that we do not otherwise limit ourselves.

Advantage of v the presence oi this hexava-lent chromium in the residue may be taken for the purpose of determining when the controlled and non-spontaneous thermal decomposition has reached a satisfactory and point. We have foundthat 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 25-40 per cent, as determined by the method to be described directly.- In our best preparations the hexavalent chromium content has generally been within the range 27-35 per cent and we prefer that it be within the range 30-33 per cent. Before the decomposition, generally all of the chromium-is hexavalent, since a salt of chomic acid is the substance decomposed; Hence, the progress oi the non-spontaneous decomposition 1 may be readily followed by withdrawing samples from time to time and analyzing them for total chromiumand for hexavalent chromium. .The total chromium 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 01' mercurous nitrate, whereby all of the hexavalent chromium is reduced to the trivalent condition," then evaporatingthe 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, CrzOa. 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 suifuric a'cid, suitably with 500 cc. of 6-7 per cent sulfuric acid, which may require boiling for 20-30 minutes to efiect 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 pref;- erably 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 and harmful to the catalytic and physical properties of the product, it is essentiai 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 0., since such lower temperatures needlessly prolong the decomposition period. This is illusconditions are favorable; but generally it is felt that the gain in shortening the period of nonspontaneous decomposition does not compensate for the increased danger of occurrence of 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 found that, 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 in air at 200 C., in which a 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 45 hours the content of hexavalent chromium had increased to slightly over 40 per cent of the total chromium. Again, in the also aforementioned case of the decomposition of ammonium dichromate in air at 175 0., in which a minimum hexa mium in excess of approximately 35 per cent 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 Y the heating in an oxidizing atmosphere beyond the minimum content of hexavalent chromium,

to undergo a spontaneous thermal decomposition which causes a destruction of mechanical strength and catalytic activity. For example, if a preparation containing such hexavalent chromium is heated to a sumcientiy high temperature, such asv 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 mosphere.

trated by the experimental facts that in the non-' v spontaneous decomposition'of a sample of ammonium dichromate in air in an electric oven kept at 200 C. a minimum hexavalent chromium content of 32 per cent of the total chromium was reached in a period of about '15 hours, whereascompose successfully'ammonium salts of chromic.

acid at temperatures somewhat above this preferred range, up to about 225 or 230 C., if all 78 it is practically worthless as a catalyst for dehydrogenation of paramn hydrocarbons to the corresponding mono-oleflns and likewise for hydrogenation reactions. I I

To avoid the reoxidation which has Just been described it is generally advantageous to carry out the controlled and nonspontaneous thermal decomposition in a non-oxidizing or reducing at- We have found that we can successfully decompose ammonium dichromate and ammonium chromate in atmospheres of hydrogen, nitrogen, ammonia, and carbon dioxide. :gen 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-sponta- -neous 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 spon taneous thermal decomposition of the still unreduced oxides of hexavalent chromium. For this reason, we prefer to carry out the nonspontaneous thermal decomposition and the reduction as separate and consecutive steps. During the non-spontaneous decomposition we further prefer to use a flowing atmosphere of an inert non-oxidizing and non-reducing gas of high Hydromolar heat such as carbon dioxide because such 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 the two extremes of the preferred range of l75200 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 hours for every degree centigrade that the temperature used lies below 200 C. At lower temperatures such as in the range ISO-175 0., the period would be of the order of two weeks or more and at higher temperatures up to about gases efficiently absorb the heat liberated by any I V catalyst.

and (NHQzCrOsZVOsCrOnGI-IZQ 'We have also used ammonium chromochromate,

m-no.oro2.o.cr.o.cro2.oNHi,

oxide, butane, propane, and the like,-or with an atmosphere containing such a reducing gas or gases. The reduction is preferably carried out asa 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 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, i

it is advantageous to use hydrogen or anatmos- ,phere 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 car bonaceous 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 solimiting the temperature of reduction is a part of our invention. I

Reduction can be carried out at as low a temperature as 175 C1. 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 the reducin as with an inert diluent gas such as nitrogen or carbon dioxide, as the diluentgas 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 of our invention. Completion of reduction can be readily determined by means neous 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 relawell. If v the original material available is in the formof crystals larger than desired, they may 7 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 is complete, the chromium oxide-containing residue is reduced. .This may be done with Example I.

As an example illustrating the practice of our invention, we cite the following facts. A quantity of ammonium dichromate crystals, screened to pass a 20-mesh sieve and to be retained-by a I 40-mesh sieve, was spread in a thin layer on a hot-plate and was then non-spontaneously de:

, composed by being heated to 200 C., and kept at this temperatur for 16 hours. At the end of any reducing gas, such as hydrogen, carbon menthistime 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 For example, in a. dehydrogenation the content of hexavalent chromium increased a as hexavalent chromium, as previously discussed and defined and as'determined by the analytical procedurehereinbefore 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 glasstube of about 8 mm. in internal diameter and provided with a concentric internal thermocouplewell. 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. Aftidly 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 bythe 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 slowgradual 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 C., whereby the carbona ceous deposit wasrbumed of! from the catalyst, it was reduced again by being heated in a stream of hydrogen while the temperature increased from room temperature to 450 0. Then, in a second run made with isobutane at atmospheric pressure and at a flow rate of 10 liters an hour and of 17 per cent.

Example If As a second example, a quantity of ammonium dichromate crystals, passing through a.j10-mesh and retained by a 20-mesh sieve, was non-spontaneously 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 100 per cent to 45 percent of the total chromium. After a total of 7 days, the content was per cent. After a total of 11 days, it reached a minimore hours. used repeatedly to, give the same'catalytic performance.

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 connection with our first example. 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 6 more 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 crystals 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 our first example, 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 The catalyst was reactivated and Example I V As an example which shows that the control of decomposition conditions is important, a quart-- tity of 10-40 mesh crystals of ammonium dichromate was non-spontaneously decomposed by he- .ing heated at 175 C. for. 12 days.

residue was heated in an electric drying oven at three cited examples. It'contained only l:5' per Y with the temperature gradually raised from 450 cent of the total chromium as hexavalent chromium. On being subjected to the usualreduc tion procedure with hydrogen and then tested for the dehydrogenation'of isobutane at 450 C. un-

der the conditionsdescribed for our first example,

a 5-cc. portion of this materlal 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 The final volume of this sample was 4.9 s

hour. cc., 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 mium having a valence greater than three when the temperature gets out of control and is per-' 'mitted to become too high. We have repeatedly observed similar effects 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 mum of 31.9 per cent. On continued heating, reduction raised theitemperature of the still'un- Then the.

The material became dull black treatment with such catalysts.

reduced higher oxides to such a point that the 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-isobutyle'ne 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 efiluent 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 showing eflicient 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 dehydration to the dehydrogenation of isobutane. Thus we have been able to show that we can reproduce our results consistently, and that we can produce similar 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, paraflinic motor fuels such'as straight run gasoline may be improved in combustion characteristics by being subjected to Such catalysts are also valuable in the production of cyclo-olefin it has been shown that this may be further rep resented by a simple mixture or combination of chromium oxides such as Cr203.CrO:. 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 mention made herein or in the claims which follow of hexavalent chromium in chromium oxide or oi 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-spontane-v ous 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 usecl'as raw material. It is probable, but not definitely known, that the atoms in the product are definitely arranged and aromatic hydrocarbons, such as in the formation of'benzene from cyclohexane. The production of diolefins from olefins may also be accomplished using these catalysts. These catalysts are also quite eflicient in proas 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 oi ammonium-containing chromates or dichromates has an empirical forand spaced and thus in this respect resemble the atoms in a true crystal; this may possibly contribute to the high catalytic efiiciency'of the product.

The term coherent as used herein is to be understood to imply that the residue from the nonspontaneous 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 iikewhich we have cited in our illustrative examples. Hence, we desire to have it understood that, within the scope of the ap pended claims, our invention is as extensive in scope and equivalents as the prior art allows.

We claim:

1. The process of preparing a granular catalyst, which comprises subjecting a granular nonpowdery crystalline ammoniurn-containing salt of chromic acid the particles of which are suiiiciently large to be retained on a 40-mesh sieve, to controlled heating at an elevated temperature belowthe temperature at which said salt decomposes with incandescence, until the salt is substantially completely decomposed homogeneously throughout the individual gran= uleswhile retaining its crystallic shape without substantial disruption of the granules.

2. The process of claim 1, in which said salt is in the form of crystals small enough to pass through a IO-mesh sieve and large enough to beretained one. ZO-mesh sieve.

3, I'he process of claim 1, in which saidheat- I ing is efiected in an inert atmosphere.

mule which is closely approximate to Cr'Oz, and

d. The process of, claim 1, in which said heating is efiected in an inert atmosphere comprising a gas of high molar heat.

5. The process of claim 1, in which said elevated temperature is within a range of about 75 C. below and adjacent to the temperature at which said salt decomposes with incandescence.

6. The process of claim 1, in which the resulting granular residue is subsequently subjected to the action of "a reducing atmosphere at an elevated temperature below about 300C. until reduction is substantially complete.

7. The process of claim 1, in which-the resulting granular residue is subsequently subjected to the action of a reducing atmosphere at an elevated temperature in the range of about 1'75 to 250 C. until reduction is substantially complete.

8. The process of claim 1, in which 'the resulting granular residue is subsequently subjected to the action of a reducing atmosphere comprising free hydrogen and an inert gas of high molar heat, at an elevated temperature below about 300 C., until reduction is substantially complete. I

9. The process of claim 1, in which said salt is an ammonium salt of chromic acid.

10. The process of claim 1, in which said salt is ammonium dichromate.

11. The process of claim 1, in which saidsalt is ammonium chromate.

granules to the action of a reducing'atmosphere at an elevated temperature in the range of about 175 to 250 C. until reduction is substantially complete.

16. A catalyst which consists of the granular residue produced by subjecting a granular nonpowdery crystalline ammonium-containing salt of chromic acid, the particles of which are suili-.

ciently 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 chromium in said chromic acid, to controlled 12. The process of preparing a granular catalyst, which comprises subjecting a granular nonpowdery crystalline ammonium-containing salt of chromic acid the particlesfof which are sufficiently large to be retained on a IO-mesh sieve, to controlled heating at an elevated temperature below the temperature at which said salt decomposes with incandescence, until the content of chromium having a valence higher than three becomes decreased to a value equivalent to a hexavalent-chromium content within the range of about 25 to 40 per cent of the total chromium, the said salt undergoing a decomposition homogeneously throughout its granules and retaining its crystallic shape without substantial disruption of the granules.

13. The process of claim 12, in which said hexavalent-chromium content is within the range of-about 27 to per cent of the total chromium.

14. The process of preparing a granular unglowed chromium oxide catalyst, which comprises subjecting a granular nonpowdery crystalline ammonium salt of chromic acid, particles of which are sufficiently large to be retained on a lo-mesh sieve, to controlled heating at an ele- 'large enough to be retained on a IO-mesh sieve,

to controlled heating at an elevated temperature in the range of about 150 to 225 C., until said crystals. are substantially completely transformed heating at an elevated temperature below the temperature at which said salt decomposes with incandescence, until said salt is substantially completely decomposed while retaining its crystallic shape.

18. A process for preparing an unglowed chromium oxide catalyst composed of coarse granules, which comprises subjecting a crystalline ammonium chromate, the particles of which are sufficiently large to be retained on a 40-mesh sieve, to controlled heating at an elevated temperature in the range of about 175 to 200 C. for at time of 15 hours plus an additional time of 10 hours for every degree centrigrade the said temperature lies below 200 C., the said p'articles undergoing decomposition substantially homoge'heously throughout while retaining their crystallic shape and undergoing said treatment substantially without disntegration.

19. A process for preparing an unglowed chromium oxide catalyst composed of coarse granules,

which comprises subjecting a crystalline, granular and nonpowdery, ammonium containing salt of chromic acid, the particles of which are sumciently large to be retained on a 40-mesh sieve, to a controlled heating inan 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 a time suflicient to effect a substantially complete controlled decomhomogeneously throughout and without substantial disruption into granules of black chromium oxide having the same crystallic shape as the .original crystals andhaving a content of chromium with a valence greater than three that is equivalent to a hexavalent-chromium content in the range of about 25 to 40 per cent of the-total chromium, and subsequently subjecting said 75 said granules to an unglowed dark residue with a content of chromium 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 crystallic shape without substantial disruption of the gran ules 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 suflicient to effect substantially complete reduction while substantially maintaining the unglowed and granular condition of said material.

20. .A process for preparing an unglowed'chromium oxide catalyst composed of coarse granules,

which comprises subjecting crystalline, granular 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 C. below and adjacent t'o'the spontaneous thermal decomposition temperature of said salt for a time suficient to efiect a substantially complete controlled decomposition of said salt homogeneously through said granules to an unglowed dark residue with a content of chromium 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 crystallicshape without substantial disruption of the-granules thereof, and subsequently subjecting said decomposed salt to the complete reduction while substantially maintain- 15 1 ing the unglowed and granular condition of said material.

21. The process of preparing an unglowed metal chromate catalyst which comprises subjecting a complex crystalline compound having the general formula (N114) 2M(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 substantially decomposed.

YAN P. MATUSZAK. GLEN H. MOREY.