CA1140912A - Process for preparing phosphorus-vanadium-oxygen catalysts - Google Patents

Abstract

23-54-0076 PROCESS FOR PREPARING PHOSPHORUS-VANADIUM-OXYGEN CATALYSTS ABSTRACT OF THE DISCLOSURE Phosphorus-vanadium-oxygen catalysts, highly resistant to attrition and dusting, are prepared by spheroidizing phosphorus-vanadium-oxygen catalyst pre-cursors into spheroids prior to being subjected to calcination conditions. The resulting catalysts, after calcination, exhibit a percent attrition less than 2% by weight. The catalysts are useful for the conversion of non-aromatic hydrocarbons, particularly n-butane, to maleic anhydride.

Description

~14~91Z

PROCESS FOR PREPARING
PHOSPHORUS-VANA~I'UM-OXYGEN'CATALYSTS
BACKG~OUN~_~F THE'INVE'NTI'ON
S lo Pi'eld of'th'e''Invention This invention relates to a process for pre-paring catalysts useful in the manufacture of~maleic anhydride by the oxidation of non-aromatic hydrocarbons.
More particularly, it is directed to catalysts having excellent resistance to attrition and which are suitable for producing maleic anhydride from non-aromatic hydro-carbons, especially n-butane, in excellent yields.
Maleic anhydride is of significant commercial interest throughout the world. It is used alone or in combination with other acids in the manufacture of alkyd and polyester resinsO It is also a versatile intermediate for chemical synthesis, for example, it is a very reactive dienophile in Diels-Alder reactionsO Significant quanti-ties of maleic anhydride are produced each year to satis-fy these varied needs.20 Description of the Prior Art The prior art discloses a number of catalysts useful for the conversion of organic feedstocks to maleic anhydrideO As an axample, Mount et al, U.S. Patent 4,111,963 teach a method of increasing the productivity of phosphorus-vanadium-oxygen catalysts by the sequential order of the preparatory steps used to prepare such cata-lystsO Mount et al, UOSO Patent 4,092,269 disclose a method for impro~ing the yield of maleic anhydride from ll24U912 hydrocarbon feedstocks by adding a pore modification agent to a phosphorus-vanadium-oxygen catalyst precursor to pro~ide a catalyst wherein the pore volume from pores having diameters ~etween about 0.8 micron and about 10 microns is greater than 0.02 cu~ic centimeter/gram ~cc/-gram). Schneider, U.SO Patent 4,017,521 describes a process for oxidizing various hydrocarbon feed compounds to maleic anhydride in the presence of a phosphorus-vanadium-oxygen catalyst prepared by a method employing an organic solvent and ha~ing a high surface area --from about 10 to 50 m2/gram. Harrison, UOSO Patent 3,915,892 relates to the preparation of a phosphorus-vanadium-oxygen catalyst using a carefully controlled sequence of steps to heat the precursor to prepare the catalyst. Bergman et al, U.S. Patent 3,293,268 teach a process of oxidizing saturated aliphatic hydrocarbons to malelc anhydride under controlled temperature condi-tions inthe presence of a phosphorus-vanadium-oxrgen catalyst. In addition, numerous references are in the 20 prior art relating to phosphorus-vanadium-oxygen catalysts containing a small amount of a promoting element to en-hance the yield of maleic anhydride.
Although the prior art catalysts generally pro-vide acceptable yields of maleic anhydride, they never-25 theless suffer rom various drawbacks. Typical phosphor-f us-vanadiu~-oxygen catalysts are formed as pills, pellets, tablets, or extrusions. These structures generally require that precautionary measures be taken during reactor charging because (a) the catalyst structures are dusty, 30 that is, have very low attrition resistance and (b) the phosphorus-vanadium-oxygen catalyst dust is toxicO More-over, the catalyst structures are easily broken and such breakage can cause undeslrable pressure drop dificulties during reactor operation. In attempts to alleviate such 3s problems, high density forms have been employed using higher tabletting pressuresO These high density forms, however, are less active than lo~ density forms, due to a decrease in porosity. As a result, performance of such catalysts is adversely affected.

1~4~iZ

SUMMARY OF THE INVENTION
This invention provides a process for pre-paring phosphorus-vanadium-oxygen.catalysts having ex-cellent resistance to attrition and dusting and having high performance characteristics to provide excellent yields of maleic anhydride.
The improved phosphorus-vanadium-oxygen cata-lysts of the invention are particularly suitable for con-verting n-butane to maleic anhydride.
The improved process disclosed herein for pre-paring phosphorus-vanadium-oxygen catalysts having a - phosphorus to v~dium ratio in the range of about 1:2 to

2:1, comprises the steps of:
(a) contacting vanadium and phosphorus compounds under conditions which will provide a catalyst precursor wherein greater than 50% of the vanadium is in the tetravalent state;
(b) recovering the catalyst precursor;
(c) spheroidizing the catalyst precursor into spheroids; and (t) calcining the catalyst precursor at a temperature between about 300C. and about 600C.
For purposes of this invention, the term "spheroidizing" shall mean the forming of the catalyst precursor into generally "spherically" shaped structures under low pressure conditions. The term "attrition"
shall mean the act of wearing or grinding down by fric-tion ant breakage of the catalyst structures into dust and fines. The term "percent (or %) attrition" means the weight loss in grams by friction and breakage of the catalyst structures (initial weight, grams - subsequent weight, grams) divided by the initial weight in grams of i ~ 4~0 ~ 1Z

the catalyst structures and the quotient multiplied by 100. The term "yield" means the ratio of the moles of maleic anhydride obtained to the moles of feed material introduced into the reactorO The term "space velocity"
means the hourly volume of gaseous feed expressed in cubic centimeters ~cc) at 15.5 CO and standard atmos-pheric pressure, divided by the catalyst bulk volume, expressed in cubic centimeters, the term expressed as cc/cc/hourO
The catalysts of this invention are particularly useful for the conversion of n-butane to maleic anhydride.
The catalysts are characterized in that they are ex-tremely porous and have high total pore volume. Yet these catalysts are structurally sound in that they 15 experience percent attrition less than 2% by ~eight, as determined by the attrition test described in detail in Example 12 hereinbelow and referred to hereinafter as "the attrition test." These characteristics distinguish these catalysts from prior art catalysts used in the 20 manufacture of maleic anhydride and other dicarboxylic ~ acid anhydrides, and the process by which the present s, catalysts are prepared causes these distinguishing characteristics. Details of the catalysts preparation, their distinguishing characteristics, and means by which 25 such characteristics can be determined and the use of such catalysts to convert non-aromatic hydrocarbons to maleic anhydride are hereinafter described.
s DESCRIPTION OF THE
~ PREFERRED EMBODIMENTS
/
1. Catalyst Preparation Broadly described, the catalysts of this inven-tion are prepared by contacting a phosphorus compound and a vanadium compound under conditions which will pro-vide a catalyst precursor having a phosphorus to vanadium atom ratio between about 1:2 and about 2:1, and having greater than 50 atom percent of the vanadium in the tetra-valent state. The catalyst precursors are recovered and formed into structures by spheroidizing into spheroids for use in a maleic anhydride reactorO Thereafter, these 1~ ~

1 154~ ~ 1 2 spheroidal catalyst precursors are calcined at a tempera-ture bet~een about 300 C. and about 600 CO to form the cat alys t O ~
The vanadium compounds useful as a source of s vanadium in the catalyst precursors are those known in the art. Suitable, but non-limiting, vanadium com~ounds include: vanadium oxides, such as vanadium pentoxide, vanadium tetroxide, vanadium trioxide, and the like;
vanadium oxyhalides, such as vanad~l chloride, vanadyl dichloride, vanadyl trichloride, vanadyl bromide, vanadyl dibromide, vanadyl tribromide and the like; vanadium-containing acids, such as metavanadic acid, pyrovanadic acid, and the like; vanadium salts, such as ammonium metavanadate, vanadium sulfate, vanadium phosphate, vanadyl formate, vanadyl oxylate, and the like. Of these, however, ~anadium pentoxide is preferred.
The phosphorus compounds useful as a source of phosphorus in the catalyst precursors are also those ; known to the art. Suitable phosphorus compounds include:
phosphoric acids, such as orthophosphoric acid, metaphos-phoric acid, and the like; phosphorus oxides, such as phosphorus pentoxidé and the like; phosphorus halides, such as pho~phorus pentachlorlde, phosphorus oxybromide, phosphorus oxychloride, and the like; trivalent phosphorus compounts, such as phosphorous acid, phosphorus trihalides ~for example, phasphorus trichloride), organic phosphites ~for example, trimethyl phosphite), sometimes known as phosphonates, and the like. Of these, orthophosphoric acid and phosphorus pentoxide are preferred, with a mix-ture of orthophosphoric acid and phosphorous acid beingmost preferredO
To prepare the catalyst precursors by the pro-cess of the present invention, a suitable vanadium com-pound is contacted with a suitable phosphorus compound in an acid medium and the mixture is heated to dissolve the starting materialsO A reducing agent is used to reduce pentavalent vanadium to tetravalent vanadium and to maintain the vanadium in the tetravalent stateO As is well kno~n to those skilled in the art, hydrohalic :- 1140~1Z

acid or oxalic acid solutions, which are mild reducing agents, can serve not only as the acid medium, but also as the reducing agent for the pentavalent vanadiumO A
- trivalent phosphorus compound can also be used to pro-vide tetravalent vanadium, as well as serve as a source of phosphorus to provide a catalyst precursor. And, since, as noted hereina~ove, phosphorous acid is a pre-ferred compound, it is preferTed ~or use as the trivalent phosphorus compound which serves as an acid medium to pro-vide the tetravalent vanadium in the precursorsO Ifdesired, although not actually requlred, a surfactant may be added to the mixture to control particle size and prevent agglomeration of the catalyst precursors durlng the preparation thereof. Surfactants suitable for use in the present invention are described in Mount et al, U.S. Patent 4,149,992.
The amount of surfactant, when employed, suit-able for use in the process of the present invention can vary within wide limits. It has been found that the amount of surfactant should be at least about QO05~ by weight, based on the weight of the dry catalyst precursor, since at lower concentrations the effect of the surfactant is diminished considerably. On~the other hand, there is no upper limit as to the amount of surfactant that can be used, although there does not seem to be any advantage in using more than about 1.0~ by weight, and it is gener-ally preferred to use between about 0.1% and about 0.5 by weight, based on the dry weight of the catalyst pre-cursor.
The acid solution containing the phosphorus compound and the vanadium compound is heated until a blue solution is obtained, indicating that at least 50 atom percent of the vanadium is in the tetravalent stateO
The amount of time required to dissolve the phosphorus compound and the vanadium compound and to provide a substantial amount of vanadium in the tetravalent state and to provlde the catalyst precursors varies rom batch to batch, depending upon the compounds used as starting materials and the temperature at ~hich the compounds are V9lZ
` heated. In general, however, heating the solution to at least 100~ C. for a~out 4 hours is suficient. It will be apparent, however, to those skilled in the art that an aliquot of the so-lution can be analyzed to insure that at least 50 atom percent of the vanadium is in the tetra-valent state.
The atom ratio of phosphorus to vanadium in the starting material is important since it controls the phos-phorus to vanadium atom ratio in the final catalyst. When phosphorus-vanadium-oxygen catalyst precursors contain a phosphorus to vanatium atom ratio below about 1:2 or above 2:1, the yield of maleic anhydride using the cata-lyst prepared from these precursors is so low that it is not of commercial significance. It is preferred that phosphorus-vanadium-oxygen catalyst precursors have a phosphorus to vanatium atom ratio between about 1:1 and about 1.5:1. When the catalyst is used to convert a feed that is primarily n-butane to maleic anhydride, it is even more preferable that the catalyst precursors have a phosphorus to vanadium rario between about 1:1 and about 1.2:1.
After the vanadium and phosphorus compounds are contacted ant a substantial amount of the vanadium is in the tetravalent state, it is necessary to recover the ZS phosphorus-vanadium-oxygen catalyst precursors. Tech-niques for recovering the catalyst precursors are well known to those skilled in the art. Por example, the catalyst precursors can be deposited from aqueous solu-tion on a carrier, such as alumina or titania, or alternatively, the catalyst precursors can be recovered by gentle heating to dryness to provide solid phosphorus-vanadium^oxygen catalyst precursors. This latter tech-nique is preferred.
After the phosphorus-vanadium-oxygen catalyst precursors have been recovered as dry powders, it is critical in the process of the present invention to form the catalyst precursors into structures by spheroi-dizing the catalyst precursors into spheroids prior to subjecting them to calcining conditions. The performance -8- 114V~12 of the spheroid:iæing step provi.des a catalyst ~ater cal-cining) whi'ch'is cha'racterized by having a percent attri-tion less than 2~ by wei:ght as dete'rmined by t.he attrition test. Moreo:ver, after such catalysts: have b.een used for at least 16 hours to conver't non-aromatichydrocarbons to maleic anhydride,' that is, conditioned, they are further characterized by having ~a) a total pore volume greater ; than 0.400 cc!gram and ~b) porosities greater than 35%, and generally between about 55% and about 65% as deter-mined by a porosity test described hereinbelow.
The catalyst precursors can be spheroidized into spheroids by conventional techniques. Spheroids (spherical agglomerates) can be formed in balling devices such as rotating discs (disc pelletizers or pelletizing discs) or drums (drum pelletizers or pelletizing drums).
The finely divided catalyst precursors are fed into such a device at a constant rate, while being selectively wetted in the disc or drum with between about 20% and 45%
by weight water, based on the dry weight of the catalyst precur~or tor between about 17% and 31%, wet basis). The rotation of the unit produces a tumbling and cascading action forcing the dampened particles into intimate con-tact. 'l'he resulting capillary attraction of the particle surfaces and their molecular adhesion holds the particles together in the form of moist spheroids.
It will be noted, however, that the actual amount of water employed during the spheroidizlng step will depend on the nature of the material being spheroi-sized, particle size distribution, type and amount of additives present, size of spheroids desired, and the like. Thus the proper spheroidizing moisture for the production of catalysts as spheroids according to this invention is confined to a relatively narrow range for a given powder in that an excess of water reduces the capillary attraction of the particles, while insufficient water reduces the surface area over which the capillary forces can act.
In general, disc pelletizers are preferred for spheroidizing t.he catalyst precursors into spheroids in ,., : 114~1Z
g that due 'to the classification action of the discs, fines and smaller' or seed spheroids stratify to the bottom of the disc and are retained for further growing, whereas finished spheroids are'continuously discharged s within a very narrow size range. This, of course, reduces the need for further screening which is usually needed to a greater extent when drum pelletizers are employed, Suitable disc pelletizers with various disc sizes are available commercially from Dravo Corporation, Pittsburgh, Pennsylvania 15225.
The size of the spheroids of the present inven-tion is not narrowly critical. Suitable spheroid dia-meters can range from about 0.1 centimeter up to about l.0 centimeter, with a range between about 0.3 centi-meter and about 0.8 centimeter in general being preferred.Spheroids havin-g diameters smaller than 0.1 centimeter, while somewhat more active, experience other difficulties such as pressure drop for the hydrocarbon-air feed mix-. ture. Larger tiameter spheroids, particularly those having diameters larger than l.0 centimeter, while avoid-ing the pressure drop difficulty associated with spheroids having smaller diameters, tend to be somewhat less active.
It will be noted, however, that the actual size of the spheroids employed will vary depending on the reactor size and configuration.
The moist spheroids are dried by heating to temperatures between about 115 C. and 280 C. in an oven. 'l'he catalyst precursors, as -dry spheroids, are then calcined at temperatures between about 300 C. and about 600 C. for at least 2 hours in either an inert atmosphere such as nitrogen or a noble gas, or oxygen or an oxygen-containing gas such as air to convert the catalyst precursors to the catalysts of the present invention. When the calcination is carried out in an inert atmosphere, the catalyst precursor-to-catalyst conversion occurs without excessive oxidation of the tetravalent vanadium to pentavalent vanadium.
When a free-oxygen or oxygen-containing atmos-phere is~ employed', it is preferred to calcine the 114~ 1Z

catalyst precursors until about 20 atom percent to about 90 atom percent o~ the vanadium has been converted to pentavalent vanadium. If more than about 90 atom percent of the vanadium is oxidized to pentavalent ranadium, usually caused by calcining too long, or at too high a temperature, the selectivity of the resultant catalysts and the yield of maleic anhydride decrease markedly.
On the other hand, oxidation of less than about 20 atom percent of vanadium during calcination in an oxygen-containing atmosphere does not seem to be more beneficialthan calcination in an inert atmosphere.
It will be apparent to those skilled in the art, of course, that the exact calcination conditions will depend on the method of preparing the catalyst precursors, the equipment configuration, additives to the catalyst precursors, and the like; however, it has been found that calcination at temperatures between about 400 C. and about 500 C. for about 4 hours is generally sufficient.
The phosphorus-vanadium-oxygen catalysts formed by calcining the catalyst precursors can be charged to a suitable reactor without, as indicated by the attrition test, sufering the attrition and dusting difficulties usually associated with prior art phosphorus-vanadium-oxygen catalysts and used to convert non-aromatic hydro-carbons to maleic anhydride. A mixture of hydrocarbon and free oxygen-containing gas, such as air, can be con-tacted with the catalyst at temperatures between about 350 C. and 600 C. at concentrations of from about 1 mole percent to about 10 mole percent hydrocarbon at a space velocity up to about 3,000 cc/cc/hour to produce maleic anhydride.
It will be noted, however, that the initial yield of maleic anhydride may be low, and if this indeed 3s is the case, the catalyst can be "conditioned" by con-tacting the catalyst wi~h low concentrations of hydro-carbon in air at low space velocities for a period of time before product operations begin.

114~9~2 2. ' An'a'lysis 'of 't'he''C'a't'al'y'st After the 'catalysts of the pres'ent invention have been conditioned for at lea'st 16 hours to convert non-aromatic hydrocarbons to malei'c anhydride, the catalysts have a tetravalent vanadium content between about 20 atom percent and 100 atom percent.
The atom percent tetravalent vanadium (in total vanadium) can be determined by the "tetravalent vanadium test." In this test, a sample of the catalyst is dis-solved-in dilute sulfuric acid, and thereafter the te~ravalent vanadium is titrated with a standardized permanganate solution in a first titration. The penta-valent vanadium is then reduced to the tetravalent state by the addition of sodium sulfite and the tetravalent vanadium is titrated with the standardized permanganate solution in a second titration. The percent tetravalent vanadium can be calculated by dividing t~e number of milliliters of standardized permanganate solution from the first titration by the number of milliliters of standardized permanganate solution from the second titra-tion and multiplying the quotient by 100 to obtain a percentage figure.
As noted hereinabove, the catalysts prepared according to the process of the present invention exhibit a percent attrition less than 2~ by weight. In addition, after such catalysts have been conditioned for at least 16 hours to convert non-aromatic hydrocarbons to maleic anhydride, they are further characterized by having ~a) a total pore volume greater than 0.400 cc/gram and (b) porosities greater than 35%, and generally between about 55% and about 65%.
The porosity of the catalysts is determined after they have been conditioned for at least 16 hours to convert non-aromatic hydrocarbons to maleic anhydride.
3s It is calculated from measurements using a mercury pene-trometer or porisimeter. In this porosity test, a pure catalyst sample is weighed, and the apparent density (as gram/cc) is determined by measuring the volume occu-pied by the catalyst sample using mercury displacement ~ O ~ 1 2 at normal atmospheric pressure. Thereater, the total pore volume (as cc!gram) is determinea by measuring the amount of mercury that i$ forced into the interstices of the sample at about 10.55 x 106 kglm2 [15,000 lb./-s in. (psi)] pressure. The porosity of the sample is then calculated by obtaining the product of the apparent density and the total pore yolume of the catalyst sample as measured under 10.55 x 106 kg/m2 mercury pressure.
This product is multiplied by 100 to obtain a percentage figure for the porosity.
The pore volume distribution -- that is, the pore volume resulting from pores having various size diameters -- can be determined during the pore volume measurements by measuring the amount of mercury that can be foreced into the interstices of the catalyst sample at different pressures. For example, the pore volume of a sample from pores having greater than 10 microns in diameter can be determined by measuring the amount of mercury that can be forced into the interstices of the sample up to about 1.23 x 104 kg/m2 (17.50 psi). The pore volume of a sample from pores having diameters between about 0.8 and 10 microns can be determined by measuring the amount of mercury that can be forced into the interstices of the sample at pressures between about 1.23 x 104 kg/m2 ant about 1.54 x 105 kg/m2 (220.00 psi).
The pore volume resulting from pores having diameters of less than ~.8 micron can be determined by measuring the amount of mercury that can be forced into the interstices of the sample at pressures between about 1.54 x 105 kg/m2 and about 10.55 x 106 kg/m2.
It has been found that the spheroidized cata-lysts prepared in accordance with the present process are highly porous, with in general greater than 50~ of the total pore volume of such catalysts resulting from pores having diameters between about 0.1 micron and about 0.6 micron. Surprisingly, such catalysts have been found to exhibit increased resistance to attrition and dusting when compared to catalysts of the prior art.
;~ At the same time, such catalysts, after being conditioned - -13- 1 1 4'~ ~ 1 Z
- for at least 16 hours, provide excellent yields of maleic anhydride.
' It has also been found that contrary to teach-'s ings of the prior art, ther'e is no direc~ correlation ; 5 between the performance of the spheroidizea catalyst and the surface area'. The'catalysts prepared according to ' the pres'ent proces's and conditioned for at least 16 hours ' exhibit rel'atively low but widely varying surface areas, usually in the range of from 7 to 15 m2/gram. The yield of maleic anhydride, however, remains high and is not ' adversel'y affected by such variations in surface area.
: 3. Pre~__at n o'f Mal'eic'Anhydride ., 'l'he spheroidized catalysts of the present invention are useful in a variety of reactors to convert non-aromatic hydrocarbons to maleic anhydride. Both 1uidized bed reactors and fixed-tube, heat-exchanger type reactors are satisfactory, and the details of the operation of such reactors are well known to those skilled in the art. The reaction to convert non-aromatic hydrocarbons to maleic anhydride requires only contacting the hydrocarbons admixed with a free-oxygen containing gas, such as air or oxygen enriched air, with the cata-lysts at elevated temperatures. The hydrocarbon/air mixture is contacted with the spheroidized catalyst at a concentration of about 1 mole percent to about 10 mole percent hytrocarbon at a space velocity of about 100 cc/cc/hour to about 3,000 cc/cc/hour at temperatures between about 300 C. and about 600 C. to pro~ide excel-lent yields of maleic anhydride. Maleic anhydride producet by using the spheroitized catalysts of this invention can be recovered by any number of means well known to those skilled in the art. For example, maleic anhytride can be recoveret by direct condensation or by absorption in suitable media with subsequent separation and purification of the anhydride.
A large number of non-aromatic hydrocarbons ha~ing from 4 to 10 carbon atoms can be converted to maleic anhydride using the catalysts prepared according to the present process. It is only necessary that the ' ' -14- 1 ~ 4~ ~ 1 2 hydrocarbon contain not less than 4 car~n atoms in a ,' straight' chain. As an example, the saturated hydrocar-bon n-butane is satisfactory, ~ut isobutane ~2-methyl-propane) is not satisfactory for conversion to'maleic anhydride although its presence'is not harmful. In addition to n-butane, other suitable saturated hydro-carbons include the pentanes, the hexanes, the heptanes, the 'octanes, the nonanes, the 'decanes, and mixtures of any of these,' with or wit~out n-butane.
:, 10 Unsaturated hydrocarbons are also suitable for ' conversion to maleic anhydride using the'spheroidized catalysts af this invention. Suitable unsaturated hydro-' carbons include the butenes (l-butene and 2-butene), 1,3-butadiene, the pentenes, the hexenes, the heptenes, the octenes, the nonenes, the decenes and mixtures of any of these, with or without the butenes.
Cyclic compounds such as cyclopentane, cyclo-pentene, oxygenated compounds such as furan, tihydro-furan, or even tetrahydrofurfural alcohol are also satisfactory.
Of the aforementioned feedstocks, n-butane is the preferred saturated hydrocarbon and the butenes are the preferred unsaturated hydrocarbons, with n-butane being most preferred of all feedstocks.
It will be noted that the aforementioned feed-stocks need not necessarily be pure substances, but can be technical grade hydrocarbons.
The principal product from the oxidation of the above feed materials is maleic anhydride, although small amounts of citraconic anhydride ~methylmaleic anhydride) may also be produced when the feedstock is a hydrocarbon containing more than 4 carbon atoms.
The following examples illustrate the invention.
They are not to be construed as limitive upon the overall scope thereof.
'EXAMPLB'l To a mixture of 340.0 grams (1.87 moles) of vanadium pentoxide, 1150 milliliters o~ water, and 2.3 grams of Sterox~ NJ nonionic surfactant (nonylphenol-. .

-15- 114~12 "
ethylene oxide condensate, molar ratio of about 1:10) were added 228.0 grams (1.09 moles) of 85% orthophor-phoric acid and 173.0 grams (2.06 moles) of 97.6%
phosphorous acid. The phosphorus to vanadium atom ratio was about 1.08:1. 'l'he aqueous mixture of vanadium and ' phosphorus compounds was charged to a 2-liter Parr auto-clave, fitted with a thermowell, two 6-bladed stirrers, ' and a vent, and heated to about 100 C. and thereafter sealed. The mixture, while being stirred at 1,000 revol-utions per minute ~rpm), was heated to about 150 C. in about 50 ~ 10 minutes and held at this temperature for about 4 hours. After the hold period, the autoclave was - cooled to about 80 C. in 50 ~ 10 minutes and opened.
The aqueous phosphorus-vanadium-oxygen catalyst precursor slurry was placet in an open dish casserole and evapor-ated to tryness in an oven at 120 C. The resultant phosphorus-vanadium-oxygen catalyst precursor powder was ground to pass an 18 mesh sieve (U.S. Standard Sieve Size), placed in a disc pelletizer equipped with a 40.64 centimeter tl6 inch) inside diameter.disc, and spheroi-dized into spheroids having a diameter between about 0.47 centimeter and about 0.67 centimeter. The moist spheroids were collected and dried by heating to 120 C.
in an ovon. Thereafter, the spheroids were calcined in air at about 450 C. for about 4 hours to convert the catalyst precursor to the active catalyst.
The catalyst was tested by placing the spheroids in a fixed tube reactor having the dimensions listed in Table 1 under the heading "REACTOR." Such a reactor gives results comparable to those obtained in a production reactor. At a temperature of about 400 C., using a feed stream containing 1.5 mole percent n-butane-in-air at a space velocity of about 1,450 cc/cc/hour, the n-butane was converted to maleic anhydride. The total pore volume, porosity, and surface area of the catalyst, and the yield of maleic anhydride, which results are tabulated in Table 1, were obtained after the catalyst had been con-ditioned for at least 16 hours to con~ert n-butane to maleic anhydride.

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~ ' EX~MPLE'S~'Z'-9 ,' The procedure describ,ed in Example 1 above was ,, employed with the results as shown in Table 1.
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', ''EXAMPL'E`l'O
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,' This Example illustrates the properties of the ' 5 spheroids of Example 1 above in comparison with tablets and tablet`s prepared with 10~' methylcel'lulose pore modi-fication agent.
The catalyst precursor was prepared as des-cribed in Example 1. The s'amples were divided into approximately equal portions and treated as follows:
Procedure A-- One portion of catalyst precursor powder was formed into 0.48 centimeter diameter tablets using 1% by weight graphite as a pelle-tizing lubricant. For convenience, these tablets were designated as Tablets lOA.
' P'ro~ced'u'r'e'B - To the remaining portion of the catalyst precursor powder was added 10% by weight methylcellulose, and the powder al~d ~ methylcellulose were blended. The blended mixture was formed into 0.48 centimeter diameter tablets using 1% by weight graphite as a pelletizing lubricant., For convenience, these tablets were designated as Tablets lOB.
The tablets (lOA and lOB) were calcined or about 4 hours at about 450 C. to convert the catalyst precursor tablets into active catalysts. The methyl-cellulose pore modiication agent was removed during the calcination.
The total pore rolume and total surface area were measured using a mercury penetrometer at 10.55 x 106 kg/m2 mercury pressure after the catalysts had been conditioned for at least 16 hours to convert n~butane to maleic anhydride. The results obtained are outlined in Table 2 hereinbelow.
' EXAMPL~ '11 The catalyst tablets were prepared as described in Example 10 except that 7.5% by wei'ght' methylcellulose ` 114V91 ~ _~
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-19~ V 9 1 2 was blended with the catalyst prçcursor powder in Pro-cedure B. The's'e'tablets, designated for con~enience as Tablet's llA and llB, were 'used in the attrition test described in Example 12.

This Example illustrates the attrition test used to determine percent attrition of phosphorus-vanadium-oxygen catalysts.
A 17.78 cent'imeter (7,0 inch) high x 9.525 centimeter (3.75 inch) outside diameter 0.946 liter (1 quart) round jar equipped with a screw cap and two 1.27 centimeter (0.5 inch) high x 8.89 centimeter (3.5 inch) long stainless steel baffles cemented lengthwise to the inner sides at 180 opposed angles was employed.
The catalysts were screened, using a 10 mesh sieve (U.S. Standard Sieve Size) to remove any dust and fines. Approximately 50.00 grams of the screened cata-lysts were accurately weighed ~initial weight) and charged to the apparatus described above. The baffled jar was capped and placed on a roller mill and rolled at 160 ' 5 revolutions per minute (rpm) for 15 minutes.
The catalysts were then removed from the jar, screened on a 10 mesh sieve, and weighed ~subsequent weight) to tetermine the amount of attrited material which passed through the mesh sieve. The percent attrition was cal-culated as follows:
Attrition ,''In'iti'al we'i~ht',''~'ram's'-'Sub's'eq'u'ent''we'i'~h't',' ~rams Initial weig t, grams x 100 The results were as follows:
Initial Subsequent l Weight Weight Attrited Attrition Ca'taly's't' '('gr'ams) ' _(grams') '' (grams) (~) ' Tablets llA S0,02 38.27 11.75 Z3.49 Tablets llB 50.00 47.60 2.40 4.80 Spheroids 50.01 49.79 0.22 0.44 The tests were made using calcined catalysts such as would be charged to a maleic anhydride reactor.
The test results clearly show the spheroids to be superior to tablets in resistance to attrition. The ~ usual dusting problems associated with phosphorus-vanadium-oxygen catalysts are 'therefore substantially eliminated by spheroidizing the catalyst precursors into spheroids prior to calcination.
S Thus, it is apparen't that there has been pro-vided, in accordance with the present invention, a process that fully satisfies the objects and advantages set forth hereinabove.' While the invention has been described with respect to various specific examples and embodiments thereo'f, it is understood that the inrention is not limited thereto and that many alternatives, modi-fications and variations will be apparent to those skilled in the art in light of the foregoing description.
Accordingly, it is intended to embrace all such alterna-tives, modifications, and variations as fall within thespirit and broad scope of the invention.

Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for preparing phosphorus-vanadium-oxygen complex catalysts having a phosphorus to vanadium atom ratio in the range of about 1:2 to about 2:1, comprising the steps of:
(a) contacting vanadium and phosphorus compounds under conditions which will provide a catalyst precursor wherein greater than 50 atom percent of the vanadium is in the tetravalent state;
(b) recovering the catalyst precursor;
(c) forming the catalyst precursor into structures;
and (d) calcining the catalyst precursor structures at a temperature between about 300°C. and 600°C.;
characterized by spheroidizing the catalyst precursor into spheroids in step (c).

2. The process of claim 1 characterized in that between about 20% by weight and 45% by weight of water, based on the weight of the dry precursor, is admixed with the catalyst precursor during the spheroidization step.

3. The process of claim 1 characterized in that the percent attrition of the catalyst is less than 2% by weight as determined by the attrition test.

4. The process of claim 1 characterized in that the spheroids' diameters range from about 0.1 centimeter to about 1.0 centimeter.

5. A spheroidal shaped phosphorus-vanadium-oxygen complex catalyst having a phosphorus-vanadium atom ratio between 1:2 and 2:1, greater than 50% vanadium in a tetravalent state, and at least 50% of the total pore volume resulting from pores having diameters between 0.1 micron and about 0.6 micron, and a percent attrition less than 2% by weight as determined by the attrition test.