US2399496A - Desulphurization process - Google Patents

Description

. Patented Apr. 30, 1946 nnsunrnumza'rron raocnss Maryan P. Matuszak, Bartlesville, Okla and Glen H. Morey, Terre Haute, Ind., assignors to Phillips Petroleum Company, a corporation oi Delaware Noni-swing. Application March 18, 1941, Serial No. 384,028, which is division of application Serial No. 173,708, November 9, 1937. Divided and this application November 8, 1943, Serial 4 Claims.

This invention relates to catalysts for use in catalytic processes and it has particular relation to catalysts that contain chromium oxide in sub-'- stantial amount and that have been prepared by the thermal decomposition oi ammonium-containing salts of chromic acid. It further relates to the use or such catalysts for the treatment of hydrocarbons at elevated temperatures,- particularly sulphur-containing hydrocarbon mixtures under conditions such as to eflect desulphurization of such mixtures.

This application is a division of our copending application Serial No.- 384,028, filed March 18, 1941, which is in turn a division of our eopendin application Serial No. 173,708, filed November 9, 1937, that has issued as Patent No. 2,294,414.

Catalysts consisting of or containing chromium oxide have been found useful in 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 or 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. Heretoi'ore, however, the ignition conditions have not been considered to be 01' particular significance and there-- fore have not been subjected to definite control,

caioresced residue is noncoherent or finely divided and powdery in texture and therefore must generally be compressed or briquetted into suitable form foruse. It consists substantially of the at least partially chemically combined oxides of chromium and of the hydrogenating metal of the original double salt in the form of a chromite,

' the chromium having been substantially completely reduced by the spontaneous decomposition from the 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 oi ammonium chromate and ammonium dichromate, with explosiveviolence.

Chromium oxide catalysts prepared by ignition of ammonium dichromate have been described by Lazier and Vaughen in an article, The catalytic properties of chromium oxide, published in the Journal of the American Chemical Society, vol. 54, August, 1932, pp. 3080-3095. The ignition resulted in the formation of a fluffy oxide having a'tea-leaf app arance 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 colors, that is, dark colored particles which ap peared to possess some catalytic activity and bright green particles which were apparently completely inactive. The green particles, 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 Vaughan was prepared by warming one-gram portions of ammonium dichromate in a thin layer over a flame until ignition was initiated. The resulting oxide was granulated by briquetting. A 20-cc. portion of this best product, when tested at 400 C. with a l-liter sample of an equimolar hydrogen-ethylene mixture passed over the catalyst in an hour, or at a space velocity of about 350,'gave a conversion of 80 per cent of the ethylene into ethane. In preparing another sample of catalyst, Lazier and Vaughen heated ammonium dichromate in a vacuum at 200 to 250 C. for 4 or 5 hours. The

product was a glistening black residue which con tained no ammonia, was slightly paramagnetic and was stable at temperatures up to 400 C. but when further heated it suddenly glowed, leaving a light green residue without catalytic activity for ethyene-hydrogenation. A 20-cc. portion of the unglowed material, when tested at 400 C. with a 'i-liter sample of an equimolar hydrogen-ethylene mixture passed over 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 0., gave a conversion of only 3 per cent.

Such hitherto available catalysts prepared by ignition oiammonium-containing salts of chr0- mic 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 otcarconversion 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 dehydrogena-- tion of paraflin hydrocarbons into mono-olefins of the same number of carbon atoms. This inadequacy is well illustrated by theaiorementioned bon, they have been entirely inadequate for the conversion ilgures obtained by Lazier and Vaughen when they are compared with thermodynamic data. Thus, it equilibrium had been attained under the stated conditions or temperature and gaseous composition, a conversion of 90 have further found that the residue from such slow, controlled and nonspontaneous thermal decomposition, after being subjected to a controlled reduction retains its formand 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 nonspontaneous thermal decomposition of crystalline ammonium alts of chromic acid carried out in the light of our discoveries, 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 orig. inal ammonium salt. The salt granules or crystals shrink appreciably during the nonspontaneous decomposition, and the product retains a small proportion of ammonia and of water derived from oxidation of a part of the ammonium 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 difliculties oi. the prior art in preparing and using catalysts which contain substantial proportions of chromium oxide.

.It is a further object to eflect'the controlled and nonspontaneous thermal decomposition of ammonium-containing salts of chromic acid."

Another object is to prepare catalytic mate rials in which the chromium oxide is substantiaily completelyin the form or black glowed chromium oxide. i

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 and unin the original salt. The total chromium oxide of the residue has an approximate empirical formula or CrOz, indicating that the chromium is, eitheractually 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 sulphuric acid, cooling, and adding potassium 40 conditions for which are hereinafter described,

the black, unglowedand crystallomorphous residue from the nonspontaneous thermal decomform 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 canbe heated to temperatures above 400 or 500 0. without loss of catalytic activity due to .50 p Another obiectis .to obtain suchdesirable'cataglowing.

lysts uniformly and at will, without the forms;-

tion or substantialamounts .oi' green and inactive chromium oxide. I r

, Another object is-to carry out, catalytic processes by the meet these catalysts derived through the nonspontaneous and controlled thermal de composition of ammonium containing salts" or chromic acid. V V r Another object is to provide a process for the treatment or a hydrocarbon material which con tains organic sulphur compounds to reduce the.

sulphur content thereof.

Further objects and advantages of our inven-i tion 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 oi 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 oi the prior art in catalytic and mechanical properties. We

* genation, hydrocarbon desulphurization, and the like.

tures at which the conversion isthermodynamically high enough to-be desirable or profitable, such as temperatures. within the range 200 to 600 C. Any carbonaceous deposit -',formed. on it during -use .canbe burned off with; air'under suitable position of ammonium salts of chromic acid possesses a. highcatalytic efllciencyior reactions such as dehydrogenation, nondestructive hydro It can be used as a catalyst at all temperat'emperature conditions without destruction'of its 4 catalytic activity and it can thereafter be used ;.again. Furthermore, it can berepeatedly used phenomenon orycalorescence that may acc0m-- and reactivated-without"undergoing the glow pany or follow the spontaneous decomposition-o1 ammonium salts of chromic acid orwhich may often be induced in other chromium oxide-containing preparations by heating to a temperatur 3 above about 400 or 500 C. i

Whether or-not the chromium oxide or the'residue obtainedby the nonspontaneous. thermal decomposition of our process is tru'ly-tetravalent, as

the empirical formula CrOz implies, is not defl- .nitely known by us. We prefer to consider that the residuehas a composition that may be expressed by the formula CrzOaCrOa, which has the same ratio of chromium to oxygen as CrO: 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 nonspontaneous thermal decomposition asbeing hexavalent, it being understood that we do not otherwise limit ourselves.

Advantage of the presence of this hexavalent.

scribed directly. In our best preparations the hexavalent chromium content has generally been within the range of 27 to 35 per cent and we prefer that it be within the range of 30 to 33 per cent. Before the decomposition, generally all of the chromium is hexavalent, since a salt of chromic acidis the substance decomposed, Hence, the progress of the nonspontaneous decomposition may be readily followed by withdrawing samples from time to time and analyzing them for total chromium and 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 cc. of a saturated solution of mercurous nitrate, whereby all of the hexavalent chromium is reduced to the trivalent condition, then evaporating the 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, ClzOs. The hexavalent chromium is determined by dissolving itout froma second weighed portion, suitably 0.1 to 0.2 gram, of the sample with hot dilute sulphuric acid, suitably with 500 cc. of 6 to '7 per cent sulphuric acid, which may require boiling for to minutes to effect dissolution, then cooling, adding an excess of potassium iodide, suitably 1 to 2 grams, and titrating the liberated iodine with sodium thicsulphate solution of known strength, suitably 0.1 normal, with starch asindicator. thus obtained the percentage of the total chromium found as hexavalent chromium may be readily calculated. The controlled nonspontaneous thermal decomposition is preferably continued until the hexavalent chromium has de- *creased to approximately one-third of the total chromium, more or less.

composition at temperatures between about 175 and 200C. It is possible, however, to decompose successfully ammonium salts of chromic acid at temperatures somewhat above this preferred range, up to about 225 or 230 0., if all conditlons 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 nonspontaneously thermally decomposed material is continued in an oxidizing atmospher 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 dichron' ate in air at 200 (7., in which a minimum hexayalent-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 avalent chromium in excess of approximately 35 From the data 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 essential that the temperature during decomposition be not permitted to exceed about 225 or 230 C. We prefer to carry out the decomposition at vtemperatures'not exceeding about 200 (2., since thereby th danger of spontaneous or explosive decomposition is minimized. But it is not desirable to use temperatures much below 175 C., since such lower temperaturesneedlessly prolong the decomposition period. This is illustrated by the experimental facts that in the nonspontaneous 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 periodof about 15 hours, whereas when the oven was kept at C. this period 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 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 hex avalent chromiumis heated to a sufficiently high temperature, such as a temperature of about 300 I fully 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 nonspontaneans thermal decomposition and the reduction as separate and consecutive steps. During the nonaaeasee it is in the form of crystals smaller than desired,

spontaneous decomposition we further prefer to use a flowing atmosphere of an inert nonoxldizing and nonreducing gas of high molar heat such as carbon dioxide because such gases efiiciently absorb the heat liberated by any incipient spontaneous decomposition and thus minimize or inhibit the tendency for such undesirable spon--.

taneous decomposition to occur or to continue.

The time required for the nonspontaneous thermal decomposition dependsupon the temperature. We have hereinbefore given specific directions for determining when the decomposition is completed and have clte'dthe lengths of representative periods at'the two extremes of the preferred range of H5 to 200 C. Within this preferred range a generally suitable period of time for carrying out the controlled and nonspontaneous thermal decomposition may be found byadding to a period of 15 hours an additional period of 10 hours for every degree centigra'de that the temperature used lies below 200C. At lower temperatures, such as in the range of 150 .to 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.

B similar controlled and nonspontaneous thermal decomposition of mixed or double ammonium-containing salts of chromic acid we may obtain homogeneously commingled catalysts con-- taining chromium oxide that have as other constituents one or more metals or oxides of metals other than chromium. We have, for example, obtained catalytic preparations by slow controlled and nonspontaneous decomposition of the following compounds:

and NHOdCrOLZVOLCIQLBHzO. we have also.

used ammonium chromochromate,

'nmo.crol.o.cr. o.croi.onni

1 which is an ammonium containing salt ofchro, -mic' acid containingv also divalent chromium.

Due, however, to greater diiiiculties of preparing to be the more advantageous because of its relatively more desirable, because less elongated, crystalline shape. I

With respect to the size of crystals, we prefer a size that passes through a 10-mesh and is re tained 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 nonspontaneous thermal decomposition. If

' it may be recrystallized to yield larger crystals.

After the nonspontaneous thermal decomposition is 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 areducing gas or gases.

The reduction is preferably carried out as a sepa-' rate 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 0., and preferably not above about 250 0., before the reduction is complete, since above this temperature thermal decomposition of unreduccd 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 limitingthe 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 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 as nitrogen, probably not only inthe form of molecular and intramolecular energy but 4 also in the form of latent heat of desorption. How

ever, the lack of such dilution is not to be con-.

sidered' asgoing outside the scope of our invention. Completion of reduction can be readily -'determined by means that are-well vknown to Example As an example illustrating the practice of our invention, we cite the following: A quantity of ammonium 'dichromate crystals, screened to pass 1 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 nonspontaneously decomposed by being heated to 200 0.. and kept at this temperature for 16 hours. At the end of this time the 'reddue 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 chrom um as hexavalent chromium, as previously discussed and defined and as determined by the analytical procedure hereinbefore described. The residue contained con temperature of about 260 C. After reduction was complete at this temperature, the temperature was gradually increased to 450 C.

A mixture of hydrocarbons having a boiling range within the gasoline boiling range which contained 3 per cent of sulphur in the form or for desulphurizing organic materials by converting the organic sulphur contained therein substantially completely into hydrogen sulphide, which can then be removed by well-known means, as by an alkali wash.

- Mention has been made herein that the chromium oxide or oxides in the most desirable residue from the controlled and nonspontaneous thermal decomposition of ammonium-containing chromates or dichromates has an empirical formula.

which closely approximates CrOz, and it has been shown that this may be further represented by a simple mixture or combination of chromium oxides such as CrzO3.CI'O3. 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 chro. mium 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 C1203.Cl03, 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 "pseudo'crystalline and "crystallomorphous used in-this specification and in the accompanying claims are taken to mean that the granules of the product from the nonspontaneous 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 efficiency of the product.

The term coherent as used herein is to be understood to imply that the residue irom 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, pressurea' times, and the like which we have cited in our illustrative example. Hence, we desire to have it understood that, within the scope of the appended claims, our invention is as extensive in r V scope and equivalents as the We claim:

1. A process for the desulphurization of a hydrocarbon material which contains organic sulphur compounds to reduce the sulphur content thereof, which comprises contacting such a hydrocarbon material in admixture with hydrogen at a reaction temperature of approximately 300 C. and a pressure of approximately 250 pounds per square inch with an unglowed chromium oxide catalyst composed of coarse granules and prepared by sub- Jecting a crystalline, granular and nonpowdery, ammonium-containing salt of chromic acid,the particlesof which are sufliciently large to be retained on a 40-mesh sieve, to controlled heating prior art allows.

in an oxidizing atmosphere containing free oxygen at an elevated temperature within a range of C. below and adjacent to the spontaneous thermal decomposition temperature of said salt for a time suflicient to eflect a. substantially complete controlled decomposition of said salt homogeneously throughout said granules to an unglowed dark residue with a. content of chromium having a. valence higher than three within a range of approximately 25 to approximately 40 per cent of the total chromium and substantially at a minimum, said salt retaining its crystallic shape 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 approximately to approximately 250 C. for a period of time suflicient to eflect substantially complete reduction while substantially maintaining the unglowed and granular condition of said material.

2. The process for the desulphurization of a hydrocarbon material which contains organic sulphur compounds to reduce the sulphur content thereof, which comprises contacting such a hydrocarbon material in the presence of free hydrogen at a reaction temperature not in substantial excess of approximately 600 C. with a granular catalyst prepared by subjecting a granular, nonpowdery crystalline ammonium-containing salt'oi chromic acid, the particles of which are sufflciently 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 issubstantially completely decomposed homogeneously throughout the individual granules while retaining its crystallic shape and without drocarbon material which contains organic sulphur compounds to reduce the sulphur content thereoi', which comprises contacting such a hydrocarbon material in admixture with free bydrogen at a reaction temperature of approximately 300 C. and a pressure of approximately 250 pounds per square inch with a granular catalyst prepared by subjecting a granular, nonpowdery crystalline ammonium-containing salt of chromic acid, the particles oil which are sufilciently 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 salt is substantially completely decomposed homogeneously throughout the individual granule while retaining its crystallic shape and without substantial disruption of the granules, and subsequently subjecting the resultant material to the action of a reducing atmosphere at an elevated temperature below approximately 300 C., until reduction is substantially complete.

4 A process for the desulphurization of a bydrocarbon material which contains orsanic sulphur compounds to reduce the sulphur content thereof, which comprises contacting such a hydrocarbon material in admixture with free hydrogen at a reaction temperature of approximately 300 C. and a pressure of approximately 250 pounds per square inch with a granular unalowed chromium oxide catalyst prepared by subjecting crystals of ammonium dlchromate, large enough to be retained on a -mesh sieve, to controlled heating at an elevated temperature in the range of approximately to approximately 225 C., until said crystals are substantially completely transformed homogeneously throughout and without substantial disruption into granules 0! black chromium oxide having the same crystallic shape as the original crystals and having a content of chromium with a valence greater than three that is equivalent to a hexavalent chromium content in the range of approximately 25 to approximately 40 per cent of the total chromium, and subsequently subjecting said granules to the action or a reducing atmosphere at an elevated temperature in the range of approximately to approximately 250 C. until reduction is substantially complete; v

MARYAN P. MATUSZAK. GLEN H. MOREY.