GB2055604A - Process for preparing phosphorus- vanadium-oxygen catalysts and the catalysts prepared by the process - Google Patents

Abstract

Phosphorus-vanadium-oxygen catalysts, highly resistant to attrition and dusting, are prepared by spheroidizing phosphorus-vanadium-oxygen catalyst precursors containing tetravalent vanadium 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.

Description

SPECIFICATION Process for preparing phosphorus-vanadium-oxygen catalysts and the catalysts prepared by the process 1. Field of the Invention This invention relates to a process for preparing 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 an hydride from non-aromatic hydrocarbons, especially n-butane, in excellent yields.

Maleic an hydride 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 resins. It is also a versatile intermediate for chemical synthesis, for example, it is a very reactive dienophile in Diels-Alder reactions. Significant quantities of maleic an hydride are produced each year to satisfy these varied needs.

2. 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 example, 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 catalysts. Mount et al, U.S. Patent 4,092,269 disclose a method for improving the yield of maleic anhydride from hydrocarbon feedstocks by adding a pore modification agent to a phosphorus-vanadium-oxygen catalyst precursor to provide a catalyst wherein the pore volume from pores having diameters between about 0.8 micron and about 10 microns is greater than 0.02 cubic centimeter/gram (cc/gram). Schneider, U.S.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 having a high surface area - from about 10 to 50 m2/gram. Harrison, U.S. 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 maleic anhydride under controlled temperature conditions in the presence of a phosphorus-vanadium-oxygen catalyst. In addition, numerous references are in the prior art relating to phosphorus-vanadium-oxygen catalysts containing a small amount of a promoting element to enhance the yield of maleic anhydride.

Although the prior art catalysts generally provide acceptable yields of maleic anhydride, they nevertheless suffer from various drawbacks. Typical phosphorus-vanadium-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, that is, have very low attrition resistance and (b) the phosphorus-vanadium-oxygen catalyst dust is toxic. Moreover, the catalyst structures are easily broken and such breakage can cause undesirable pressure drop difficulties during reactor operation. In attempts to alleviate such problems, high density forms have been employed using higher tabletting pressures. These high density forms, however, are less active than low density forms, due to a decrease in porosity.As a result, performance of such catalysts is adversely affected.

SUMMARY OF THE INVENTION This invention provides a process for preparing phosphorus-vanadium-oxygen catalysts having excellent resistance to attrition and dusting and having high performance characteristics to provide excellent yields of maleic anhydride.

The improved phosphorus-vanadium-oxygen catalysts of the invention are particularly suitable for converting n-butane to maleic anhydride.

The improved process disclosed herein for preparing phosphorus-vanadium-oxygen catalysts having a phosphorus to vanadium 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 (d) calcining the catalyst precursor at a temperature between about 300"C. and about 600"C.

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 friction and 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 the catalyst structures and the quotient multiplied by 1 00. The term "yield" means the ratio of the moles of maleic an hydride obtained to the moles of feed material introduced into the reactor. The term "space velocity" means the hourly volume of gaseous feed expressed in cubic centimeters (cc) at 15.5"C. and standard atmospheric pressure, divided by the catalyst bulk volume, expressed in cubic centimeters, the term expressed a cc/cc/hour.

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 extremely porous and have high total pore volume. Yet these catalysts are structurally sound in that they experience percent attrition less than 2% by weight, as determined by the attrition test described in detail in Example 1 2 hereinbelow and referred to hereinafter as "the attrition test". These characteristics distinguish these catalysts from prior art catalysts used in the manufacture of maleic anhydride and other dicarboxylic acid anhydrides, and the process by which the present catalysts are prepared causes these distinguishing characteristics.Details of the catalysts preparation, their distinguishing characteristics, and means by which such characteristics can be determined and the use of such catalysts to convert non-aromatic hydrocarbons to maleic anhydride and are hereinafter described.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 1. Catalyst Preparation Broadly described, the catalysts of this invention 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 state. The catalyst precursors are recovered and formed into structures by spheroidizing into spheroids for use in a maleic an hydride reactor. Thereafter, these spheroidal catalyst precursors are calcined at a temperature between about 300"C. and about 600"C. to form the catalyst.

The vanadium compounds useful as a source of vanadium in the catalyst precursors are those known in the art. Suitable, but non-limiting, vanadium compounds include: vanadium oxides, such as vanadium pentoxide, vanadium tetroxide, vanadium trioxide, and the like; vanadium oxyhalides, such as vanadyl 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, vanadium, pentoxide is preferred.

The phosphorus compounds useful as a source of phosphorus in the catalyst precursors are also those know to the art. Suitable phosphorus compounds include: phosphoric acids, such as orthophosphoric acid, metaphosphoric acid, and the like; phosphorus oxides, such as phosphorus pentoxide and the like; phosphorus halides, such as phosphorus pentachloride, phosphorus oxybromide, phosphorus oxychloride, and the like; trivalent phosphorus compounds, such as phosphorus acid, phosphorus trihalides (for example, phosphorus trichloride), organic phosphites (for example, phosphorus 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 mixture of orthophosphoric acid and phosphorous acid being most preferred.

To prepare the catalyst precursors by the process of the present invention, a suitable vanadium compound is contacted with a suitable phosphorus compound in an acid medium and the mixture is heated to dissolve the starting materials. A reducing agent is used to reduce pentavalent vanadium to tetravalent vanadium and to maintain the vanadium in the tetravalent state. As is well known 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 vanadium. A trivalent phosphorus compound can also be used to provide tetravalent vanadium, as well as serve as a source of phosphorus to provide a catalyst precursor.

And, since, as noted hereinabove, phosphorous acid is a preferred compound, it is preferred for use as the trivalent phosphorus compound which serves as an acid medium to provide the tetravalent vanadium in the precursors. 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 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, wher employed, suitab' for use in the process of the present invention can vary with wide limits. It has been -,ound 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 sufactant 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 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 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 state. 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 provide 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, however, heating the solution to at least 100"C. for about 4 hours is sufficient. 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 tetravalent state.

The atom ratio of phosphoric to vanadium in the starting material is important since it controls the phosphorus to vanadium atom ratio in the final catalyst. When phosphorus-vanadium-oxygen catalyst precursors contain a phosphorus to vanadium atom ratio below about 1:2 or above 2:1, the yield of maleic an hydride 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 1.5:1. 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 ratio between about 1:1 and about 1.2:1.

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-vanadiumoxygen catalyst precursors. Techniques 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, the catalyst precursors can be recovered by gentle heating to dryness to provide solid phosphorus-vanadium-oxygen catalyst precursors. This latter technique 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 spheroidizing the catalyst precursors into spheroids prior to subjecting them to calcining conditions. The performance of the spheroidizing step provides a catalyst (after calcining) which is characterized by having a percent attrition less than 2% by weight as determined by the attrition test.Moreover, after such catalysts have been used for at least 1 6 hours to convert non-aromatic hydrocarbons 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 determined 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 of 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 20% and 45% by weight water, based on the dry weight of the catalyst precursor (or between about 17% and 31%, wet basis). The rotation of the unit produces a tumbling and cascading action forcing the dampened particles into intimate contact.

The 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 spheroidizing step will depend on the nature of the material being spheroidized, 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 the catalyst precursors into spheroids in 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 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 1 5225.

The size of the spheroids of the present invention is not narrowly critical. Suitable diameters can range from about 0.1 centimeter up to about 1.0 centimeter, with a range between about 0.3 centimeter and about 0.8 centimeter in general being preferred. Spheroids having diameters smaller than 0.1 centimeter, while somewhat more active, experience other difficulties such as pressure drop for the hydrocarbon-air feed mixture. Larger diameter spheroids, particularly those having diameters larger than 1.0 centimeter, while avoiding 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 11 5"C. and 280"C.

in an oven. The 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 atmosphere 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 vanadium 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 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 beneficial than 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, suffering the attrition and dusting difficulties usually associated with prior art phosphorus-vanadium-oxygen catalysts and used to convert non-aromatic hydrocarbons to maleic anhydride. A mixture of hydrocarbon and free oxygen-containing gas, such as air, can be contacted 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 tile initial yield of maleic anhydride may be low, and if this indeed is the case, the catalyst can be "conditioned" by contacting the catalyst with low concentrations of hydrocarbon in air + vow space velocities for a period of time before product operations begin.

2. Analysis of ffie Catalyst After the catalysts of the present invention have been conditioned for at least 1 6 hours to convert non-aromatic hydrocarbons to maleic 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 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 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 the number of milliliters of standardized 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 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 1 6 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. It is calculated from measurements using a mercury penetrometer of porisimeter. In this porosity test, a pure catalyst sample is weighed, and the apparent density (as gram/cc) is determined Dy measuring the volume occupied by the catalyst sample u ;ng mercury displ < cement at normal atmospheric pressure.

Thereafter, the total pore volume (as cc/gram) is determined by measuring the amount of mercury that is forced into the interstices oF the sample at about 10.55 X 106 kg/m2 [1 5,000 Ib./in2 (psi)] pressure. The porosity of the sample is then calculated by obtaining the product of the apparent density and the total pore volume of the catalyst sample as measured under 10.55 = 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 and about 1.54 X 105 kg/m2 (220.00 psi).The pore volume resulting from pores having diameters of less than 0.8 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 catalysts 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 for at least 1 6 hours, provide excellent yields of maleic anhydride.

It has also been found that contrary to teachings of the prior art, there is no direct correlation between the performance of the spheroidized catalyst and the surface area. The catalysts prepared according to the present process and conditioned for at least 1 6 hours exhibit relatively low but widely varying surface areas, usually in the range of from 7 to 1 5 m2/gram. The yield of maleic anhydride, however, remains high and is not adversely affected by such variations in surface area.

3. Preparation of Maleic Anhydride The spheroidized catalysts of the present invention are useful in a variety of reactors to convert non-aromatic hydrocarbons to maleic an hydride. Both fluidized bed reactors and fixedtube, 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 an hydride requires only contacting the hydrocarbons admixed with a free-oxygen containing gas, such as air or oxygen enriched air, with the catalysts 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 hydrocarbon 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 provide excellent yields of maleic anhydride. Maleic anhydride produced by using the spheroidized catalysts 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 purification of the anhydride.

A large number non-aromatic hydrocarbons having from 4 to 1 0 carbon atoms can be converted to maleic anhydride using the catalysts prepared according to the present process. It is only necessary that the hydrocarbon contain not less than 4 carbon atoms in a straight chain.

As an example, the saturated hydrocarbon n-butane is satisfactory, but isobutane (2-methylpropane) is not satisfactory for conversion to maleic anhydride although its presence is not harmful.

In addition to n-butane, other suitable saturated hydrocarbons include the pentanes, the hexanes, the heptanes, the octanes, the nonanes, the decanes, and mixtures of any of these, with or without n-butane.

Unsaturated hydrocarbons are also suitable for conversion to maleic anhydride using the spheroidized catalysts of this invention. Suitable unsaturated hydrocarbons include the butenes (1-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 cyclopentene, cyclopentane, 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 feedstocks.

It will be noted that the aforementioned feedstocks 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.

EXAMPLE 1 To a mixture of 340.0 grams (1.87 moles) of vanadium pentoxide, 11 50 milliliters of water, and 2.3 grams of Steroid NJ nonionic surfactant (nonylphenolethylene oxide condensate, molar ratio of about 1:10) were added 228.0 grams (1.09 moles) of 85% orthophorphoric acid and 173.0 grams (2.06 moles) of 97.6% phosphorous acid. The phosphorus to vanadium atom ratio was about 1.08:1. 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"C. and thereafter sealed.The mixture, while being stirred at 1,000 revolutions 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 i 10 minutes and opened. 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 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 (16 inch) inside diameter disc, and spheroidized 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 oven. 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 fizzed 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 conditioned for at least 1 6 hours to convert n-butane to maleic anhydride.

TABLE 1.

TEST RESULTS FROM PHOSPHORUS-VANADIUM-OXYGEN CATALYSTS AS SPHEROIDS REACTOR TEST CONDITIONS TOTAL FEED SPACE PORE LENGTH DIAMETER CONCENTRATION VELOCITY YIELD POROSITY VOLUME SURFACE AREA EXAMPLE cm cm MOLE % cc/cc/Hour % % cc/gram m/gram 1 335.28 2.54 1.5 1450 55.0 61.2 0.472 7.4 2 335.28 2.54 1.5 1450 55.0 62.0 0.488 8.7 3 335.28 2.54 1.5 1450 54.0 60.1 0.478 6.4 4 335.28 2.54 1.5 1450 54.0 59.4 0.444 11.6 5 335.28 2.54 1.5 1450 51.0 56.6 0.478 13.2 6 335.28 2.54 1.5 1450 55.0 64.4 0.548 10.7 7 335.28 2.54 1.5 1450 53.0 58.5 0.464 10.2 8 121.92 2.54 1.5 1450 53.0 62.2 0.482 15.9 9 60.96 2.54 1.5 1450 52.7 59.7 0.471 14.3 TABLES 2-9 The procedure in Example 1 above was employed with the results as shown in Table 1.

EXAMPLE 10 This example illustrates the properties of the spheroids of Example 1 above in comparison with tablets and tablets prepared with 10% methylcellulose pore modification agent.

The catalyst precursor was prepared as described in Example 1. The samples 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 pelletizing lubricant. For convenience, these tablets were designated as Tablets 1 or.

Procedure B - To the remaining portion of the catalyst precursor powder was added 10% by weight methylcellulose, and the powder and 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 1 or.

The tablets (1 0A and 1 0B) were calcined for about 4 hours at about 450"C. to convert the catalyst precursor tablets into active catalysts. The methylcellulose pore modification agent was removed during the calcination.

The total pore volume 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 1 6 hours to convert n-butane to maleic anhydride. The results obtained are outlined in Table 2 hereinbelow.

EXAMPLE II The catalyst tablets were prepared as described in Example 10 except that 7.5% by weight methylcellulose was blended with the catalyst precursor powder in Procedure B. These tablets, designated for convenience as tablets 1 IA and 11 B, were used in the attrition test described in Example 1 2.

TABLE 2.

PROPERTIES OF PHOSPHORUS-VANADIUM-OXYGEN CATALYST STRUCTURES TABLETS 10A1 TABLETS lOB2 SPHEROIDS3 APPARENT DENSITY, gram/cc 1.86 1.48 1.30 POROSITY, % 42.4 49.9 61.2 MEAN PORE DIAMETER, micron 0.12 0.11 0.25 TOTAL PORE VOLUME, cc/gram 0.2281 0.3374 0.4716 TOTAL SURFACE AREA, m2/gram 7.565 12.781 7.439 'Zero (0.0) percent methylcellulose employed.

2Ten (10.0) percent by weight methylcellulose employed as a pore modification agent to provide a catalyst wherein the pore volume of the catalyst from pores having diameters between about 0.8 micron and about 10 microns is greater than 0.02 cc/gram.

3Taken from example 1.

EXAMPLE 12 This Example illustrates the attrition test used to determine percent attrition of phosphorusvanadium-oxygen catalysts.

A 17.78 centimeter (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 catalysts 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 1 60 + 5 revolutions per minute (rpm) for 1 5 minutes. The catalysts were then removed from the jar, screened on a 10 mesh sieve and weighed (subsequent weight) to determine the amount attrited material which passed through the mesh sieve.The percent attrition was calculated as follows: Initial weight, 9, dms-Subseque. weight, grams % Attrition = X 100 Initial weight, grams The results were as follows: Initial Subsequent Weight Weight Attrited Attrition Catalyst' (grams) (grams) (grams) (%) Tablets 11A 50.02 38.27 11.75 23.49 Tablets 11B 50.00 47.60 2.40 4.80 Spheroids 50.01 49.79 0.22 0.44 1The tests were made using calcined catalysts such as would be charged to a maleic an hydride 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.

Thus, it is apparent that there has been provided, 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 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 description.

Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the invention.

Claims (9)

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 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 phosphorus-vanadium-oxygen catalyst prepared according to the process of Claim 1.

6. The catalyst of Claim 5 characterized in that the percent attrition is less than 2% by weight as determined by the attrition test.

7. A process according to Claim 1 and substantially as herein described with reference to any of Examples 1 to 9.

8. A catalyst according to Claim 5 and substantially as herein described with reference to any of Examples 1 to 9.

9. A process for the preparation of maleic anhydride by contacting a mixture of a nonaromatic hydrocarbon and a free-oxygen containing gas with a catalyst according to Claim 5 at elevated temperature.