CA1148527A - Phosphorus-vanadium-oxygen catalyst precursors and catalysts - Google Patents

Abstract

C23-54-0077A PHOSPHORUS-VANADIUM-OXYGEN CATALYST PRECURSORS AND CATALYSTS ABSTRACT OF THE DISCLOSURE Heat treatment of phosphorus-vanadium-oxygen catalyst precursors at temperatures between about 150° C. and about 300° C. for a period of about 1 hour to about 6 hours prior to being subjected to calcination conditions yields catalyst precursors having increased crush strength and attrition resistance. The catalyst precursors, upon calcination, are converted to phosphorus-vanadium-oxygen catalysts which are useful for the conversion of non-aromatic hydrocarbons, particularly n-butane, to maleic anhydride.

Description

~148527 PHOSPHORUS-VANADIUM-OXYGEN CATALYST
PRECURSORS AND CATALYSTS
BACRGROUND OF THE INVENTION
1. Field of the Invention This invention relates to a process for pre-paring catalysts useful in the manufacture of maleic anhydride by the oxidation of non-aromatic hydrocarbonsO
More particularly, it is directed to catalyst precursors having improved crush strength and resistance to attri-tion and which, upon calcination, are converted to cata-lysts which are suitable for producing maleic anhydride from non-aromatic hydrocarbons, especially n-butane, in excellent yieldsO
Maleic anhydride is of significant commercial interest throughout the worldO 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 ch0mical synthesis, for example, it is a very reactive dienophile in Diels-Alder reactionsO Significant quanti-ties of maleic anhydride are produced each year to satisfy these varied needs.
Description of the Prior Art The prior art discloses a number of catalysts useful for the conversion of organic feedstocks to maleic anhydride. As an exa~ple, Mount et al, U~SO 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 catalysts.

85;~7 Mount et al, U.SO Patent 4,092,269 disclose a method for improving the yield of maleic anhydride from hydrocarbon feedstocks by adding to a phosphorus-vanadium-oxygen catalyst precursor a pore modification agent to provide a catalyst wherein the pore volume from pores having dia-meters between about 0.8 micron and about lO microns is greater than 0.02 cubic centimeter/gram (cc/gram).
Schneider, UOSo 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 sol-vent and having a high surface area -- from about 10 to 50 square meters/gram (m2/gram)O Harrison, U.SO 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 catalystO Bergman et al, U.S. Patent 3,293,268 teach a process of oxidizing saturated aliphatic hydrocarbons to maleic anhydride under controlled temperature conditions in the presence of a phosphorus-vanadium-oxygen catalystO
In addition, numerous references are in the prior art relating to phosphorus-vanadium-oxygen catalysts con-taining a small amount of a promoting element to enhance the yield of maleic anhydrideO
Although the prior art catalysts generally pro-vide acceptable yieLds of maleic anhydride, they neverthe-less suffer from various drawbacks~ In preferred pro-cesses of the prior art, phosphorus-vanadium-oxygen cata-lysts are formed as pills, pellets, tablets, or extrusions prior to calcination -- that is, while still in the catalyst precursor sta~e of preparation. These uncalcined struc-tures generally require that precautionary measures be taken during handling and storage because such structures exhibit very low crush strength and attrition resistanceO
The low crush strength and attrition resistance in turn result in severe dusting, breaking, and attriting problems, Moreover, the catalyst precursor, as well as the catalyst dust is toxicO In attempts to improve the crush strength and attrition resistance, and thereby alleviate the dusting, 85~7 .. -3-breaking and attriting problems, and losses due thereto, high density forms have been employed using higher forming pressuresO These high density forms, however, after calcination to provide the catalyst, are less active than the low density forms due to a decrease in porosity~ As a result, performance of such catalysts is adversely affectedO
Therefore, a catalyst precursor exhibiting improved crush strength and attrition resistance and whose performance, when converted to the active catalyst, i.s not adversely affected would be a decided advance in the state of the art.
SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide a process for preparing phosphorus-vanadium-oxy-gen catalsyt precursor structures having improved crush strength and attrition resistanceO
Another objoct of this invention is to provide a process for strengthening the phosphorus-vanadium-oxygen catalyst precursor structures to make handling and storage prior to calcining possible with minimal losses due to dusting, breaking, and attritingO
Still another object of this invention is to provide a process for preparing phosphorus-vanadium-oxygen catalyst precursor structures having improved resistance to dusting, breaking, and attritingO
An additional object of this invention is to pro-vide a process for preparing phosphorus-vanadium-oxygen catalyst precursor structures having crush strength and attrition resistance sufficient to permit direct charging of the uncalcined structures to maleic anhydride reactors for in situ calcining.
A further object of this invention is to provide a process for preparing phosphorus-vanadium-oxygen catalysts having high performance characteristics to provide excel-lent yields of maleic anhydrideO
A still further object of this invention is to provide a process for preparing phosphorus-vanadium-oxygen catalysts particularly suitable for converting n-butane to maleic anhydrideO
These and other objects are achieved by the improved process disclosed herein for preparing phos-phorus-vanadium-oxygen 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 precurser wherein greater than 50 atom per-cent of the vanadium is in the tetravalent state;
~b) recovering the catalyst precursor;
~c) forming the catalyst precursor into lS structures; and (d) calcining the catalyst precursor structures at a temperature between about 300 CO and ; about 600 CO;
wherein the improvement comprises heating the catalyst precursor structures prior to step ~d) to a temperature between about 150 CO and about 300 CO for a period of about 1 hour to about 6 hoursO
For purposes of this invention, the term "average crush strength" shall mean the average of a number of deter-minations ~usually 10 or more) of the crush strength ofthe structures of a given catalyst precursor sample. The term "crush strength" shall mean the maximum force that can be applied to a structure prior to its losing its nominal, geometric integrity. The term "attrition" shall mean the act of wearing ~r grinding down by friction and breakage of the structures into dust and fines. The term "percent attrition" means the weight loss in grams by friction and breakage of the structures di~ided by the weight in grams of the original structures and the quo-tient multiplied by lOOo The term "yield" means the ratioof the moles of maleic anhydride obtained to the moles of feed introduced into the reactor. The term "space velocity"
means the hourly volume of gaseous feed expressed in cubic centimeters (cc) at 1505 C~ and standard atmospheric t i ~ ~35~7 pressure, divided by the ~atalyst bulk volume, e~pressed in cubic centimeters, the term expressed as cc/cc/hourO
The catalysts prepared according to this inven-tion are particularly useful for the conversion of n-butane to maleic anhydrideO The catalyst precursors arecharacterized in that they show improved average crush strength and percent attritionO Moreover, when the catalyst precursors are converted to the active catalyst, performance is not adversely affectedO These distin-guishing characteristic properties are caused by theprocess by which the catalyst precursors and catalysts are preparedO Details of the preparation process, the distinguishing characteristics of the catalyst precursors and catalysts and means by which such characteristics can be determined, and the use of such catalysts to convert non-aromatic hydrocarbons to maleic anhydride are herein-after describedO
DESCRIPTION OF THE PREFERRED EMBODIMENTS
lo 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 provide 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 tetravalent stateO The catalyst precursors are recovered and formed into any of a number of suitable structures -- tablets, pills, pellets, extrusions, for example --- for use in a maleic anhydride reactorO If desired, a pore modification agent may be added prior to structure formation, There-after, the structured catalyst precursor is subjected to heat treatment at a temperature between about 150 CO and about 300 C~ for a period of about 1 hour to about 6 hours to improve both the average crush strength and the attri-tion resistanceO Following the heat treatment, the heat-hardened catalyst precursors are calcined at a temperature between about 300 C. and about 600 C. to form the cata-lyst~

~85~7 The vanadium compounds useful as a source of Yanadium in the catalyst precursors are those known in the artO Suitable, but non-limiting, vanadium compounds include: vanadium oxides, such as vanadium pentoxide, vanadium tetroxide, vanadium trioxide, and the like;
vanadium oxyhalides sucn as vanadyl chloride, vanadyl dichloride, vanadyl trichloride, vanadyl bromide, vanadyl dibromide, vanadyl tribromide, andthe like; vanadium-containing acids such as metavanadic acid, pyrcvanadic acid, and the like; vanadium salts, such as ammonium metavanadate, vanadium sulfate, vanadium phosphate, vanadyl formate, vanadyl oxylate, and the likeO Of these, however, vanadium pentoxide is preferredO
The phosphorus compounds useful as a source of phosphorus in the catalyst precursors are also those known to the artO Suitable phosphorus compounds include: phos-phoric acids, sucn as orthophosphoric acid, metaphosphoric acid, and the like; phosphorus oxides, such as phosphorus pentoxide, and the like; phosphorus halides, such as phos-phorus pentachloride, phosphorus oxybromide, phosphorusoxychloride, and the like; trivalent phosphorus compounds, such as phosphorous acid, phosphorus trihalides (for example, phosphorus trichloride), organic phosphites (for example, trimethyl phosphite), sometimes known as phos-phonates, and the likeO Of these, orthophosphoric acidand phosphorus pentoxide are preferred, with a mixture of orthophosphoric acid and phosphorous acid being most pre-ferredO
To prepare the catalyst precursors by the pro-cess of the present invention, a suitable vanadium compound is contacted with a suitable phosphorus compound in an acid medium and the mixtur~ is heated to dissolve the starting materials. A reducing agent is used to reduce pentavalent vanadium to tetravalent vanadium and to main-tain the vanadium in the tetravalent stateO As is wellknown to those skilled in the art, hydrohalic 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 _7~ 5~7 phosphorus compound can also be used as a reducing agent for the pentavalent vanadium, as well as serve as a source of phosphorus to provide the catalyst precursors. Phos-phorous acid is the trivalent phosphorus compound of choice for use in the preparation-of-the catalyst pre-cursors in that, as noted hereinabove, it is a preferred compound, and, in addition, can serve as an acid medium for carrying out the desired reduction of the pentavalent vanadium to the tetravalent vanadium. If desired, although not actually required, a surfactant may be added to the mixture to control particle size and prevent agglomeration of the catalyst precursors during the prep-aration theseof. 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, suitable 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 0.05~ by weight, based - - on the weight of the dry catalyst precursor, since at lower concentrations the effect of the surfactant is diminished considerablyO 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 atvantage in using more than about 1.0~ by weight, and it.is generally preferred to use between about 0.1% and about 0.5% by weight, based on the dry weight of the catalyst precursor.
The acid solution containing the phosphorus com-pound 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 state. The amount of time required to dissolYe the phosphorus com-pound and the vanadium compound and to provide a substan-tial amount of vanadium in the tetravalent state and toprovide the catalyst precursors varies from batch to batch, depending upon the compounds used as starting materials and the temperature at which the compounds are heated.
In general, ho~ever, heating the solution to at least 5~7 100 C0 for about 4 hours is sufficientO It will be apparent, however, to those skilled in the art that an aliquot of the solution can be analyzed to insure that at least 50 atom percent of the vanadium is in the tetra-5 valent stateO
. The atom ratio of phosphorus to vanadium in the starting material is important since it controls the phosphorus to vanadium atom ratio in the final catalystO
When phosphorus-vanadium-oxygen catalyst precursors con-tain a phosphorus to vanadium atom ratio below about 1:2or above 2:1, the yield of maleic anhydride using the catalyst 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 vanadium atom ratio between about 1:1 and about 1.5:10 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 atom ratio between about 1:1 and about 102:10 After the vanadium and phosphorus compounds are contacted and a substantial amount of the vanadium is in the tetravalent state, it is necessary to recover the phosphorus-vanadium-oxygen catalyst precursorsO Tech-niques for recovering the catalyst precursors are well known to those skilled in the art. For example, the catalyst precursors can be deposited from aqueous solution on a carrier, such as alumina or titania, or alternatively, catalyst precursors can be recovered by gentle heating to dryness to provide solid phosphorus-vanadium-oxygen cata-lyst precursors. This latter technique is Preferred.
As noted hereinabove, pore modification agents, which are useful to increase the overall porosity of a given catalyst structure, can be employed, if desiredO
Suitable procedures concerning pore modification agents and their use are described in ~ount et al, U~S. Patent 4,092,2690 After the phosphorus-vanadium-oxygen catalyst precursors are recovered, regardless of whether they ~ 852~7 contain a pore modification agent or not, they are formed into structures suitable for use in a maleic anhydride reactor, Techniques for forming appropriate structures from precursors for use in a fluidized bed reactor or in a fixed-tube, heat-exchanger type reactor are well known to those skilled in the artO For example, the catalyst precursors can be structured for use in a fluidized bed reactor by depositing the phosphorus-vanadium-oxygen catalyst precursor on a carrier. Alter-natively, driet catalyst precursors can be comminutedfor use in the fluidized bed reactorO On the other hand, catalyst precursors can be structured for use in a fixed-tube, heat-exchanger type reactor by extruding a paste of the precursors through an orifice, or by prilling, by lS tabletting, and the like the precursorsO And as noted hereinabore, the catalyst precursors may contain a pore modification agent, if desired.
After the phosphorus-vanadium-oxygen catalyst precursors are formed into structures which will be used in a maleic anhydride reactor, the precursors are heated under conditions and ~or a time sufficient to strengthen the structures ~ithout inducing undesirable phase changes or transitions associated with calcination. The unde-sirability of effecting calcination of the catalyst pre-cursors during the heat treatment is due to the tendencyof the calcined, non-conditioned (discussed hereinbelow) catal~st to adsorb water, thereby causing a substantial reduction in catalyst acti~ity.
The heat treatment can be carried out in either an inert atmosphere or an ox~gen or oxygen-containing atmosphere such as those discussed hereinbelow for the calcination step.
The temperature conditions utilized during the heat-treatment step of the present process can range within fairly wide limits. In general, temperatures between about 150 C. and about 300 C~ are suitable, with the preferred temperature being between about 200 C~ and about 280 C~, and usually between about 22ao ~ and about 260 C~ At the preferred temperatures, the desired .~ ..

S~7 strengthening of the catalyst precursor struc~uFes occurs in the absence of calcination phase transitionsO It will be recognized, however, that the actual temperature em-ployed will depend to some extent on the rate of heating to the desired temperatures and the length of time during which the temperature is maintainedO
The time required for the desired strengthening of the catalyst precursor structures to occur via heat treatment is not criticalO Suitable heating times can vary from about 1 hour to about 6 hours, with about 2 hours usually being sufficient. The actual time period employed, ho~ever, will vary to some extent with the temperatures employedO Moreover, since at temperatures of about 300 CO or higher, the realm of calcination is entered, and since, as noted hereinabove, calcination is to be avoided during the heat-treatment step, prolonged heating at such temperatures simply exacerbates the prob-lem. Thus the higher the temperatures employed, particu-larly at about 300 C. and h;gher, generally, the shorter the heat-treatment timeO
Following the heat treatment, the catalyst pre-cursors are calcined at temperatures between about 300 CO
and about 600 C. for a suitable period of time, usually 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 inventionO When the calcination is carried out in an inert atmosphere, the catalyst pre-cursor-to-catalyst conversion occurs without excessive oxidation of the tetravalent vanadium to pentavalent vanadiumO
When a free-oxygen or oxygen-containing atmos-phere is employed, it is preferred to calcine the catalyst precursors until about 20 atom percent to about 90 atom percent of the vanadium has been converted to pentavalent vanadium. If more than about 90 atom percent of the vana-dium is oxidized to pentavalent vanadium usually caused by calcining too long, or at too high a temperature, the selectivity of the resultant catalysts and the yield of 85~7 maleic anhydride decrease markedlyO On the other hand, oxidation of less than about 20 atom percent of ~anadium during calcination in an oxygen-containing atmosphere does not seem to be more beneficial than calcination in an inert atmosphereO
It will be apparent to those skilled in the art as a result of the strengthening of the catalyst precur-sor structures by heat treatment, the uncalcined structures alternatively may be charged directly to a suitable maleic anhydride reactor prior to calcination without suffering the dusting, breaking, and attriting difficulties usually associated with phosphorus-vanadium-oxygen catalyst pre-cursor (and catalyst) structuresO The precursor struc-tures may then be calcined in the reactor, that is, ln lS situ, under appropriate conditions as described herein-.
above to provide the active catalystO This, o cours.e, could be highly advantageous in that water adsorption by the calcined and non-conditioned catalyst, as well as handling of the calcined catalyst, is aYoidedO
It will be recognized, of course, that the exact calcination conditions will depend on the method of pre-paring the catalyst precursors, the equipment configura-tion, additives to the catalyst precursors, and the like;
however, it has been found that calcination at temperatures between about 400 C~ and about 5000 C. for about 4 hours is generally sufficient, regardless of whether the cata-lyst precursor structures are calcined prior to or after being charged to the maleic anhydride reactorO
The phosphorus-vanadium-oxygen catalyst formed by calcining the catalyst precursors can be used ~in a suitable reactor) to convert non-aromatic hydrocarbons to maleic anhydride. A mixture of hydrocarbon and free oxy-gen-containing gas such as air, can be contacted with the catalyst at temperatures between about 350 CO and 600 C.
at concentrations of from about one 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 is the case, the catalyst, as will occur to those skilled 5~7 in the art, can be "conditioned" by contacting the cata-lyst, with low concentrations of hydrocarbon and air at low space velocities for a period of time before product operations beginO
s 2. Analysis of the Catalyst After the catalysts, which are prepared by cal-cination of the heat-treated catalyst precursors of the present invention, have been conditioned for at least 16 hours to convert non-aromatic hydrocarbons to maleic anhydride, the catalysts have a tetravalent vanadium con-tent between about 20 atom percent and 100 atom percentO
The atom percent tetravalent vanadium (in total vanadium) can be determined by the "tetravalent vanadium testO" In this test, a sample of the catalyst is dissolved in dilute sulfuric acid, and thereafter the tetravalent vanadium is titrated with a standardized permanganate solution in a first titration. The pentavalent vanadium is then reduced to a 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 calcu-lated by dividing the number of milliliters of standard-ized permanganate solution from the first titration by the number of milliliters of standardized permanganate solution from the second titration and multiplying the quotient by 100 to obtain a percentage figure.
As noted hereinabove, the phosphorus-vanadium-oxygen catalyst precursor structures can be substantially strengthened by heating such structures to a temperature between about 150 C~ and about 300 CO for a period of between about 1 hour and about 6 hours. That is, the structures show increased average crush strength and attrition resistance, thereby facilitating the handling of such sturctures without experiencing the dusting, breaking, and attriting difficulties usually associated with phosphorus-vanadium-oxygen catalyst precursor struc-turesO
The attrition resistance, calculated as percent attrition, ls determined by an attrition test wherein the 5~7 amount in grams of dust and fines generated from a speci-fied weight of a sample of~~the bulk structures by friction and breakage under stated conditions is measuredO The percent attrition can then be calculated by dividing the weight in grams of the dust and fines (initial weight, grams minus subsequent weight, grams) by the initial weight in grams of the sample and multiplying the quo-tient by 100 to obtain a percentage figure.
The test is described in detail in Example 3, with results tabulated in Tables 1 and 20 As can readily be seen, the heat treatment of uncalcined catalyst pre-cursor structures results in decreasing percent attrition with increasing temperatures (up to the preferred tempera-tures)O
The average crush strength is determined by a crush strength test wherein the maximum force that can be applied to the structure in question, whether tablet, pill, extrusion and the like, prior to its losing its nominal, geometric integrity is measuredO Generally, ten or more determinations are made from a given sample of structures and the values obtained for the crush strength are averaged to provide an average crush strengthO
The crush strength test is described in detail in Example 4, with results tabulated in Tables 1 and 2.
And as can readily be seen, the heat treatment of uncal-cined catalyst precursor structures results in an increase in the average crush strength with increasing temperatures ~up to the preferred temperatures)O
3O Preparation of Maleic Anhydride The catalysts prepared by calcination of the heat-treated catalyst precursors of the present invention are useful in a variety of reactors to convert non-aromatic hydrocarbons to maleic anhydride~ Both fluidized bed reac-tors and fixed-tube, heat-exchanger type reactors are satis-factory, and de~ails of the operation of such reactors are well known to those skilled in the artO 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 catalyst at elevated temperatures The hydrocarbon/air mixture is contacted with the catalyst at a concentration of about 1 mole percent to about lO
mole percent hydrocarbon at a space velocity of about 100 cc/cc/hour to about 3,000 cc/cc/hour at temperatures between about 300 CO and about 600 CO to provide excel-lent yields of maleic anhydrideO Maleic anhydride pro-duced by using the heat-treated catalyst of this invention can be recovered by any number of means well known to those skilled in the art. For example, maleic anhydride can be recovered by direct condensation or by absorption in suitable media with subsequent separation and purifi-cation of the anhydrideO
A large number of non-aromatic hydrocarbons having from 4 to 10 carbon atoms can be converted to maleic anhydride using the catalyst prepared according to the present processO It is only necessary that the hydrocarbon contain not less than fourcarbon atoms in a straight chain. As an example, the saturated hydro-carbon n-butane is satisfactory, but isobutane (2-methyl-propane) is not satisfactory for conversion to maleic anhydride although its presence is not harmfulO 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 without n-butaneO
Unsaturated hydrocarbons are also suitable for conversion to maleic anhydride using the heat-treated catalyst of this inventionO 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, dihydrofuran,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 feedstocksO

-15~ 5~7 It will be noted that the aforementioned feed-stocks need not necessarily be pure substances, but can be technical grade hydrocarbons.
The principle product from the oxidation of the above feed materials is maleic anhydride, although small amounts of citraconic anh~dride (methylmaleic anhydride) may also be produced when the feedstock is a hydrocarbon containing more than four carbon atomsO
The following examples illustrate the invention.
They are not to be construed as limitive upon the overall scope thereofO

To a mixture of 340O0 grams ~1.87 moles) of vanadium pentoxide, 1150 milliliters of water, and 2.3 grams of Sterox NJ nonionic surfactant ~nonylphenol-ethylene oxide condensate, molar ratio of about 1:10) were added 228.0 grams ~1098 moles) of 85% orthophosphoric acid and 17300 grams (2006 moles) of 97.6~ phosphorous acidO The phosphorus to vanadium atom ratio was about 1~08:10 The aqueous mixture of vanadium and phosphorus compounds was charged to a 2-liter Parr autoclave, fitted with a thermowell, two 6-bladed stirrers, and a vent, and heated to about 100 CO The autoclave was the~eafter sealedO The mixture, while being stirred at 1,000 revolu-tions per minute (rpm), was heated to about 150 CO in about 50 + 10 minutes and held at this temperature for about 4 hoursc After the hold period, the autoclave was cooled to about 80 CO in 50 +10 minutes and openedO
The aqueous phosphorus-vanadium-oxygen catalyst precursor slurry was placed in an open dish casserole and evaporated to dryness in an oven at 120 C. The remaining solids were ground to pass an 18 mesh sieve (U~SO Standard Sieve Size) and formed into 0,48 centimeter diameter tablets using 1 weight percent graphite as a pelletizing lubri-3s cantO Low tabletting pressures were employed as indicatedby the 8054 Newton (1092 pounds) average crush strength ~Table 1) for the non-heat-treated tabletsO Samples of the tablets were placed in 116-millimeter porcelain evap-orating dishes and, starting at room temperatu~e, heated -16- ~1~8527 in an oven to temperatures ranging, at regular intervals, from 100 C0 to 330 C0 The indicated temperature was maintained for 4 hoursO The results are shown in Table 1 o The procedure described in Example 1 was repeated except that higher tabletting pressures were employed as evidenced by the 28017 Newton ~6.33 pounds) average crush strength (Table 2) for the non-heat-treated tablets. The results are shown in Table 20 This Example illustrates the attrition test used to determine percent attrition of the phosphorus-vanadium-oxygen catalyst precursors ~or catalysts)0 A 17078 centimeter (700 inch) high x 9.525 centi-meter ~3075 inch) outside diameter 00946 liter (loO quart) jar equipped with a screw-on cap and two 1027 centimeter ~0.5 inch) high x 8.89 centimeter (305 inch) long stain-less steel baffles cemented lengthwise to the inner sides at 180 opposed angles was employedJ
The catalyst precursor samples from Example 1 and Example 2 were separately screened, using a 10 mesh sieve (UOSo Standard Sieve Size) to remove any dust and finesO Approximately 50000 grams of each of the screened samples were accurately weighed (initial weight in grams) and charged to the apparatus described above. The baffled jar containing the catalyst precursor was placed on a roller mill and rolled at 160 + 5 revolutions per minute (rpm) for 15 minutes. The sample was then removed from the jar, screened, and weighed (subsequent weight in grams) to determine the amount of attrited material which passed through the 10 mesh sieveO The percent attrition was cal-culated as follows:
% A iti Initial weight~ ~rlams-Subsequent weight, ~r nltla welg t, grams x 100 The results for Examples 1 and 2 are shown, respectively, in Tables 1 and 2 under the columns headed "% AttritionO"

-17- 11'~8527 This Example illustrates the crush strength test used to determine the average crush strength (measured as longitudinal, side crush strength for tablet~) of the phosphorus-vanadium-oxygen catalyst precursor structuresO
A John Chatillon and Sons Universal Test Stand, Model LTCM (motorized, variable speed unit) equipped with Chatillon Dial Push/Pull Guages, either Model DPP-10 [10 pounds (44050 Newtons) maximum] or Model DPP-50 [50 pounds (222050 Newtons) maximum], depending on the anticipated range of crush strength, was used as the test equipmentO
The tablet was placed on its side under the center of the plunger and the instrument driven in the automatic mode at a setting of 20 The maximum force exerted during the test was taken as the crush strength for the tablet. Generally 10 or more determinations were made from a given sample of tablets and the crush strength values averaged to give an average crush strengthO The results are reported in Tables 1 and 20 EXA~IPLE S
This Example illustrates the effect of time/-temperature on the strengthening of the phosphorus-vana-dium-oxygen catalyst precursor structuresO
The catalyst precursor samples (tablets pre-pared according to the procedure described in Example 2) were screened, using a 10 mesh sieve CU.S0 Standard Sieve Size), to remove any dust and finesO Approximately 75.00 grams of each of the screened samples were accurately weighed and placed in porcelain evaporating dishes in a large forced draft oven at room temperature (23 C0)O
The temperature of the oven was increased over a 67-minute period to 250 C0 and maintained throughout the experimentO
Samples were removed from the oven at various time inter-vals and the percent attrition and average crush strengthdetermined to measure the effect of time at a specified temperature on the strengthening of the structures during the heat treatmentO The results were as indicated in Table 3 following this ExampleO

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U~ o The tabulated results (Table 3) indicate that most of the observed improvement in percent attrition and average crush strength occurs during the approximately l-hour heat up to 250 CO and that only marginal improve-ment occurs beyond the l-hour heat up to, and the 2-hour hold at, 250 CO Thus, the desired strengthening of the catalyst precursor structures can be effected by heating the structures at the preferred temperatures for about 2 hours.

The procedure described in Example 1 above was repeated except that the Sterox~ NJ nonionic surfactant was omitted and the tablets, starting at room temperature, were heated in a forced draft oven over a l-hour heat-up period to 300 C., which temperature was maintained for 3 hours. The heat-treated tablets were charged to a 2.1-centimeter (0.83 inch) inside diameter x 335.28-centimeter (11 foot) long fixed-tube reactor and calcined in situ by slowly raising the temperature from about 170 CO to about 370 C. over a 24-hour period to convert the cata-lyst precursor to the active catalyst.
The catalyst was conditioned for conversion of n-butane to maleic anhydride for at least 16 hours at temperatures between about 370 C~ and about 450 C., using a feed stream containing lo5 mole percent n-butane-in-air at a space velocity of about 1450 cc/cc/hourJ
Following this period, the temperature was adjusted to about 400 Cc and, after about 100 hours of operation, the yield of maleic anhydride determinedO The yield of maleic anhydride for the conditioned catalyst and the average crush strength of the non-heat-treated catalyst precursor tablets and the heat-treated catalyst precursor tablets were as follows:
Average Crush Strength, Newtons Male~c Anhydrlde Non-heat-treated Tablets Heat-~eated Tablets Yield, Mole %
12.91 62~30 52 0 The results clearly show the heat-treated struc-tures to be superior to ~on-heat-treated structures in both average crush strength and resistance to attrition~

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The usual dusting, breaking, and attriting problems associated with phosphorus-vanadium-oxygen catalyst pre-cursors are therefore substantially eliminated by heating the catalyst precursor structures to a temperature between about 150 C. and about 300 C~ for a period of about 1 hour and about 6 hours prior to calcination.
Thus, it is apparent that there has been pro-vided, in accordance with the present invention, a pro-cess that fully satisfies the objects and advantages set forth hereinabove. While the invention has been described with respect to various specific examples and embodiments thereof, it is understood that the invention is not limited thereto and that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing descriptionO Accordingly, it is intended to embrace all such alternatives, modifica-tions, and variations as fall within the spirit and broad scope of the inventionO

i^ r

Claims (3)

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

1. A process for preparing a phosphorus-vanadium-oxygen complex catalyst 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 about 600°C.;
characterized by heating the catalyst precursor structures prior to step (d) to a temperature between about 150°C. and about 300°C. for a period of about 1 hour to about 6 hours.

2. The process of claim 1 characterized in that the catalyst precursor structures are heated to a temperature between about 220°C. and about 260°C. for a period of about 2 hours prior to step (d).

3. The process of claim 1 characterized in that the heat-treated catalyst precursor structures are charged directly to a maleic anhydride reactor and calcined in situ.