Process for the production of carboxylic anhydrides
REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of U.S. application Ser. No. 673,962, filed Mar. 22, 1991, herein incorporated by reference now U.S. Pat. No. 5,185,309. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a process for the production of carboxylic anhydrides from aromatic hydrocarbons. More particularly, it relates to use a specified fluid catalyst in the production of carboxylic anhydride from a hydrocarbon material such, especially, as o-xylene or naphthalene by means of gas phase oxidation. 
     BACKGROUND OF THE INVENTION 
     Carboxylic anhydrides are produced by gas phase oxidation of aromatic hydrocarbon materials, with a typical example being the production of phthalic anhydride from o-xylene or naphthalene. 
     For use in the production of phthalic anhydride from o-xylene by means of fixed bed gas phase oxidation, various catalysts have been reported (for example, Practical Catalysts for Respective Reactions, p. 358, 1970, edited by Kimio Tarama, published by Kagaku Kogyo-sha, Japan) including a combination of vanadium pentoxide with titanium oxide (anatase type), a combination of vanadium pentoxide with other active metal oxide such as tellurium oxide, molybdenum oxide, tungsten oxide, nickel oxide, niobium oxide, tin oxide, chromium oxide or the like, and a combination of vanadium pentoxide with an alkai metal salt such as of potassium, lithium, sodium or the like, supported on inert carriers such as Alundum, silicon carbide, quartz, pumice, α-alumina and the like. 
     In the case of a gas phase oxidation process in which a fixed catalyst bed is employed, excessively high heat generated by exothermic oxidation reaction is removed by employing a system in which a catalyst is uniformly packed into several thousands of reactor pipes, each having a small diameter of about 1 inch, and the exterior of the pipes is filled with a heat transfer medium for cooling use. This system, however, requires considerable labor and cost to complete uniform packing of a catalyst into each of such a massive number of reactor pipes, as well as great burdens of the cost of equipment and the operation management to maintain pressure loss and temperature of each of these reactor pipes at constant levels. This system also requires considerable labor and cost when deteriorated catalyst is replaced by fresh catalyst. 
     Also, a catalyst in which active components are coated on an inert carrier is apt to cause a run away reaction triggered by ununiformity of reaction due to channeling of reaction gas, formation of hot spots, increased pressure loss and the like caused by peeling and release cf the active components from the carrier during packing of the catalyst or at the time of the operation. In addition, high productivity cannot be attained by the use of the fixed bed process because concentration of a reaction gas must be maintained within its explosion limit and, therefore, the reaction gas can be supplied only at a low concentration level. 
     For the purpose of solving such problems, it is preferable to perform gas phase oxidation using a fluidized catalyst bed. 
     Compared to the aforementioned fixed bed process, such a fluidized bed process is markedly advantageous, because not only generated heat by the exothermic oxidation reaction can be removed easily but also channeling of the flow of material gas and formation of hot spots both of which are common in the case of the fixed bed process can be avoided. Another advantage of the fluidized bed process is that exchange and supplement of a catalyst can be made with less labor and cost. The fluid bed process still has a great advantage from a view point of productivity because of a possibility to increase concentration of a reaction material. 
     For use in the production of phthalic anhydride from o-xylene by means of gas phase oxidation, as well as the case of using naphthalene as the starting material, various fluid catalysts have been reported, for example in B.P 941,293 (1963) and U.S. Pat. No. 3,232,955 (1966), such as a combination of vanadium pentoxide with potassium sulfate and other combinations with molybdenum oxide, tungsten oxide, phosphorus oxide, boron oxide and the like supported on silica. The use of such silica-supported catalyst, however, causes excess oxidation reaction and side reactions which results in the formation of CO and CO 2 , thus making it difficult to obtain phthalic anhydride in a high yield. In order to improve the phthalic anhydride yield, attempts to mix the reaction gas with a halogen gas such as Br 2  have been reported for example in D.P 1,144,709 (1963) and U.S. Pat. No. 3,455,962 (1969), but the use of such a halogen gas causes corrosion of equipment and therefore results in operational troubles. 
     A number of catalysts in which titanium oxide is used as a carrier and vanadium pentoxide is supported on the carrier have been proposed for example in B.P 1,067,726 (1967) and Fr.P. 1,537,351 (1968). A catalyst having certain mechanical strength can be obtained by making a fused body of titanium oxide and vanadium pentoxide together with ammonium thiocyanate or an alkali compound. However, specific surface area and pore volume of the catalyst decrease by the formation of fused body, thus resulting in significant reduction of the catalytic activity. Because of the reduced activity, such a fused catalyst requires a high reaction temperature which causes excess oxidation and side reactions. In consequence, it is difficult or obtain phthalic anhydride with a high yield by the use of the fused catalyst. 
     In addition, since the formation of a fused body results in a catalyst having markedly high bulk density amplified by the high specific gravity of titanium oxide, it is difficult to perform efficient fluidized bed reaction using such a high bulk density catalyst. 
     Because of these reasons, production of phthalic anhydride from o-xylene by means of a fluidized bed gas phase oxidation reaction has not been put into practical use. 
     SUMMARY OF THE INVENTION 
     In view of the above, it therefore becomes an object of the present invention to provide a process for the production of carboxylic anhydride from o-xylene or naphthalene by means of a gas phase oxidation reaction in a fluidized bed process, a process which is greatly advantageous compared to a fixed bed process, as well as to provide a process for the production of such a fluid catalyst. More specific object of the present invention contemplates the provision of a process by using a high activity and high selectivity catalyst in a fluidized bed process, in which a mixture of titanium oxide silicon dioxide and boron oxide is used as a support or carrier and which has sufficient strength and appropriate bulk density. 
     The inventors of the present invention have conducted intensive studies on the development of a fluid catalyst containing titanium oxide as the main component, which is useful for the production of phthalic anhydride. This invention has been accomplished as a result of these efforts. 
     According to a first aspect of the present invention, there is provided a process for producing a carboxylic anhydride which comprises performing gas phase oxidation of an aromatic hydrocarbon in the presence of a fluid catalyst comprising 
     50 to 95% by weight, calculated as TiO 2  +SiO 2  +B 2  O 3 , of component (A) which comprises titanium oxide, silicon dioxide and boron oxide and 
     5 to 50% by weight, calculated as V 2  O 5  +M 2  O, wherein M is an alkali metal, +SO 3  of component (B) comprising vanadium oxide, an alkali metal oxide and sulfuric anhydride, 
     wherein the weight ratios of B 2  O 3  to TiO 2  and SiO 2  to TiO 2  in said component (A) are in the range of 0.02 to 0.5 and 0.25 to 1.0, respectively. 
     Other objects and advantages of the present invention will be made apparent as the description progresses. 
     DETAILED DESCRIPTION OF THE INVENTION 
     A fluid catalyst for use of the present invention in gas phase oxidation of aromatic hydrocarbons comprises: 
     50 to 95% by weight (calculated by TiO 2  +SiO 2  +B 2  O 3 ) of component (A) which comprises titanium oxide, silicon dioxide and boron oxide and 
     5 to 50% by weight [calculated by V 2  O 5  +M 2  O (M represents an alkali metal) +SO 3  ] of component (B) comprising vanadium oxide, an alkali metal oxide and sulfuric anhydride, wherein weight ratios of B 2  O 3  to TiO 2  and SiO 2  to TiO 2  in the component (A) are in the range of from 0.02 to 0.5 and from 0.25 to 1.0, respectively. 
     Amounts of the component (A) as a carrier in the catalyst if smaller than 50% by weight may not be preferable, because a catalyst having an appropriate bulk density which is suitable for use in a fluidized bed process will not be obtained and the activity of the catalyst will decrease. If amounts of the component (A) are larger than 95% by weight, amounts of the component (B) which is the source of active components in the catalyst will conversely be reduced and, therefore, a desired catalytic activity will not be obtained. Preferable amount of the component (A) in the catalyst may be in the range of from 60 to 90% by weight, more preferably from 75 to 90% by weight. 
     In the component (A), a weight ratio of B 2  O 3  / TiO 2  may be in the range of from 0.02 to 0.5, preferably from 0.05 to 0.3, and that of SiO 2  / TiO 2  may be in the range of from 0.25 to 1.0, preferably from 0.3 to 0.9. 
     If the B 2  O 3  / TiO 2  weight ratio is smaller than 0.02, effects of the addition of boron oxide on the improvement of abrasion resistance and selectivity for reaction products of the catalyst will not be attained. The weight ratio if larger than 0.5 may not be preferable, because the catalytic activity will decrease markedly, though the effect on the improvement of abrasion resistance will not decrease. 
     If the SiO 2  / TiO 2  weight ratio is smaller than 0.25, bulk density of the resulting catalyst will increase and, therefore, a catalyst suitable for use in a fluidized bed process will not be obtained. The weight ratio if larger than 1.0 may not be preferable, because specific surface area of the resulting catalyst will become large and selectivity of the catalyst will decrease. 
     The component (B) as active component in the catalyst may be used in an amount of from 5 to 50% by weight, preferably from 10 to 40% by weight, more preferably from 10 to 30% by weight, calculated by a form of &#34;V 2  O 5  +M 2  O (M is an alkali metal)+SO 3  &#34;. 
     Amounts of the component (B) in the catalyst if smaller than 5% by weight would show no sufficient catalytic activity, and if larger than 50% by weight would reduce not only the catalytic activity but also fluidity because of considerable reduction of specific surface area of the resulting catalyst, deterioration of dispersion conditions of the active component and deposition of crystals in the catalyst. 
     Vanadium compound as a member of the component (B) may preferably be contained in the catalyst in an amount of from 1 to 30% by weight, more preferably from 1 to 15% by weight, calculated by a form of V 2  O 5 . 
     In the component (B), a molar ratio of SO 3  to M 2  O (SO 3  /M 2  O) may be in the range of from 0.1 to 6.0, preferably from 1.0 to 4.0, and a molar ratio of M 2  O to V 2  O 5  (M 2  O/V 2  O 5 ) may be in the range of from 0.1 to 5.0, preferably from 0.3 to 3.0. 
     The fluid catalyst for gas phase oxidation using in the present invention may further contain a third component such as a rare earth compound in addition to the components (A) and (B). 
     There is provided a process for the production of a fluid catalyst containing titanium oxide, silicon dioxide, vanadium oxide, an alkali metal oxide, sulfuric anhydride and boron oxide as main components, which comprises the steps of 
     (a) mixing compounds as respective sources of titanium oxide, silicon dioxide, boron oxide, vanadium oxide, alkali metal oxide, and sulfuric anhydride, 
     (b) spray drying the mixture of step (a) and 
     (c) calcinating spray-dried powder of step (b). 
     Preferably, the source of titanium oxide is a titanium hydroxide which is capable of forming titanium oxide having a crystallite diameter of not more than 30 nm when dried at a temperature of about 300° C. 
     The material for use in the production process of a catalyst is a solution or a suspension of respective sources of titanium oxide, silicon dioxide, vanadium oxide, an alkali metal oxide, sulfuric anhydride and boron oxide. 
     Preferably, the source of titanium oxide eligible for use in the catalyst is a titanium hydroxide which is capable of forming titanium oxide having a crystallite diameter of not more than 30 nm when dried at a temperature of about 300° C. The term &#34;crystallite diameter&#34; as used herein means a value calculated using the following formula, based on a half width of a diffraction peak of 2θ=25.3° (Cu Kα line) for anatase type titanium oxide in an X-ray diffraction pattern measured in accordance with the Debye-Scherrer&#39;s method. ##EQU1## 
     A catalyst containing titanium oxide which has a crystallite diameter of larger than 30 nm may have markedly low abrasion resistance compared to that having a crystallite diameter of not more than 30 nm. The use of such a catalyst containing large crystallite diameter titanium oxide in a fluidized bed system, therefore, will result in the discharge or scattering of the catalyst to a high degree which not only is uneconomical but also causes blocking tubes of a cyclone and a heat exchanger. Also, the use of such a catalyst will cause mixing of a large amount of the catalyst into a reaction product and also will make it difficult to keep a reaction gas in a proper state of fluidity. In addition, when crystallite diameter of titanium oxide in a catalyst exceeds 30 nm, specific surface area of titanium oxide conversely becomes small which will render uniform and sufficient distribution of an active component on the surface unattainable and therefore will result in markedly decreased catalytic activity. In consequence, satisfactory yield of a carboxylic anhydride may not be obtained when such a catalyst is used, even in the case of a fluidized bed gas phase oxidation process with a prolonged contact time. 
     The term &#34;a titanium hydroxide which is capable of forming titanium oxide having a crystallite diameter of not more than 30  nm when dried at a temperature of about 300° C.&#34; as used herein is intended to include not only wet (water containing) titanium compounds which are generally called titanium hydroxide, metatitanic acid, orthotitanic acid, titania sol, titania gel and the like but also powder preparations obtained by drying them at a low temperature. 
     A titanium oxide source (or titanium hydroxide) eligible for use in the catalyst may be obtained from any material or by any preparation method, provided that a crystallite diameter of titanium oxide in powder when dried at about 300° C. is not more than 30 nm, preferably 20 nm or smaller, more preferably 10 nm or smaller. 
     Examples of the source of titanium oxide (or titanium hydroxide) include a titanic acid which is obtained by thermal hydrolysis during an intermediate step of a process for the production of titanium oxide for pigment use, and a titania sol preparation which is obtained by adding an acid to the titanic acid. Also included are titanium hydroxides, titania sol preparations and the like which are obtained by means of a neutralization hydrolysis or an ion exchange deoxidation hydrolysis of titanium sulfate, titanyl sulfate, titanium tetrachloride and the like. Especially, a titanium hydroxide obtained from a solution of titanyl sulfate or the like by means of a neutralization hydrolysis at a low temperature of 40° C. or below may be most useful as a material for the production of a catalyst of the present invention, because its crystallite diameter when dried at about 300° C. is only several nm. 
     Examples of titanium oxide sources which form titanium oxide having a crystallite diameter of more than 30 nm when dried include: anatase, rutile and the like types of calcinated titanium oxide for pigment use obtained by means of thermal hydrolysis; calcinated powder of a titanium hydroxide or titania sol obtained by a neutralization method or an ion exchange method; and grown crystals of a titanium hydroxide obtained during a hydrolysis step or by autoclaving. 
     For the catalyst, a source of silicon dioxide is used as a lightening agent (or a bulk density reducing agent) and as a binder. A desirable material as the source of silicon dioxide eligible for use in the present invention may be in the form of a solution or silica sol which has a good dispersion ability. Examples of such desirable material include for instance a silicic acid which is obtained by subjecting a sodium silicate or a potassium silicate to a neutralization or a cation exchange method, organic silicon compounds such as an ethyl silicate and acid hydrolyzates thereof, a quaternary ammonium silicate and an acid hydrolyzate thereof, colloidal silica and the like. 
     Of these, colloidal silica is most preferable because of its stability at a high concentration for a long period of time. When a colloidal silica preparation contains sodium as a stabilizer, it is preferable to remove sodium in advance by means of cation exchange, ultrafiltration or the like. A colloidal silica preparation having a particle size of 50 nm or larger may be useful as a bulk density reducing agent, but it has an inferior capacity as a binder. Gel-form materials, such as a hydrogel obtained by neutralizing a sodium silicate and powdered gel preparations including a white carbon and an aerogel, are not preferable as the source of silicon dioxide because such materials not only have poor dispersion ability and inferior capacity as a binder but also reduce the catalytic effect of simultaneously added boron compound (or boron oxide). 
     A source of vanadium oxide eligible for use in the catalyst may be selected from water soluble compounds which form vanadium oxide when calcinated in the air, such as ammonium metavanadate, vanadyl sulfate (or vanadium oxysulfate), vanadium formate, vanadium acetate, vanadyl oxalate, ammonium vanadium oxalate, vanadyl phosphate, a vanadium oxyhalide and the like. Of these, vanadyl sulfate, ammonium metavanadate and vanadyl oxalate may preferably be used. 
     Examples of the source of alkali metal oxide eligible for use in the catalyst include hydroxides, sulfates, carbonates, chlorides, nitrates, oxihalides, thiosulfates, nitrites, sulfites, hydrogensulfites, hydrogensulfates, oxalates, hydrogenoxalates and the like of potassium, cesium, rubidium and other alkali metals. Of these, hydroxides, sulfates and carbonates may be used preferably. 
     Examples of sulfuric acid compounds as the source of sulfuric anhydride include sulfuric acid, ammonium sulfate, ammonium hydrogensulfate and the like. Of these, sulfuric acid and ammonium sulfate may be used preferably. 
     For the catalyst, such a vanadium compound, an alkali metal compound and a sulfuric acid compound are used as active components. 
     A boron compound eligible for use in the catalyst as the source of boron oxide may be selected from soluble compounds such as boron oxide, boric acid, potassium tetraborate, potassium pentaborate, potassium metaborate, ammonium metaborate, ammonium tetraborate and the like, preferably from boric acid and ammonium metaborate because of their relatively high solubilities. A boron compound (or boron oxide) imparts not only an effect of improving strength of catalyst (abrasion resistance) markedly but also an effect of improving selectivity for the reaction products. In other words, a catalyst may have excellent activity, selectivity and abrasion resistance when it is produced by using active components and a titanium hydroxide which is capable of forming titanium oxide having a crystallite diameter of not more than 30 nm when dried at about 300° C. According to the present invention, strength of the catalyst and selectivity for the reaction products cf the catalyst are further improved by the effect of the use of a boron compound in spite of the accelerated formation of titanium oxide crystals at the time of calcination of the catalyst composition. 
     The process for the production of the catalyst comprises steps of mixing the foregoing component materials, spray drying the mixture and calcinating the spray-dried powder. Since mixing order of each component is optional, various mixing methods may be applicable such as simultaneous dissolving of two or more materials, dissolving of active components in a titanium hydroxide disperse liquid and the like. 
     Also, it is possible to use, as a source of titanium oxide, a mixture of a titanium hydroxide, which is capable of forming titanium oxide having a crystallite diameter of not more than 30 nm when dried, with another titanium compound such as a compound that forms titanium oxide having a crystallite diameter of larger than 30 nm. 
     If necessary, concentration of a slurry mixture thus prepared may be adjusted to an appropriate level by means of evaporation, prior to subjecting the mixture to the spray drying step to obtain spherical fine particles. The spray drying step may be effected by employing commonly used means. Preferably, spray drying may be carried out under such conditions that a weight average particle size of the resulting spherical fine 
     particles is controlled within the range of from 40 to 150 μm. The spherical fine particles thus obtained are then calcinated in the air preferably at a temperature of from 300° to 700° C., more preferably from 400° to 600° C., and preferably for 1 to 6 hours, more preferably for 2 to 4 hours. 
     By carrying through these steps of the process of the present invention, a catalyst having the following properties may preferably be obtained. 
     Bulk density (g/cm 3 ): 0.5 to 1.3 
     Specific surface area (m2/g): 5 to 60 
     Pore volume (cm 3/  g); 0.1 to 0.6 
     Abrasion rate (wt. %/15 Hr): 5 or less 
     Weight average particle size (μm): 40 to 150 
     In this instance, abrasion rate is measured by the ACC (American Cyanamid Co.) method described in B.P.737,429. 
     The catalyst can be preferably applied to a process of the present invention of producing a carboxylic anhydride from its corresponding aromatic hydrocarbon through a gas phase catalytic oxidation. Typical examples of the aromatic hydrocarbons which may be catalytically oxidized include benzene, xylene, naphthalene, cumene, pseudocumene, durene, tetraethylbenzene and mixtures thereof. Consequently, catalyst of the present invention may be used for the oxidation of benzene into maleic anhydride; o-xylene or naphthalene into phthalic anhydride; pseudocumene into trimellitic anhydride; and durene or tetraethylbenzene into pyromellitic anhydride. 
     The catalytic oxidation of an aromatic hydrocarbon into a carboxylic anhydride is hereinafter illustratively described by referring to non-limiting cases of the catalytic oxidation of o-xylene or naphthalene into phthalic anhydride; and durene or tetraethylbenzene into pyromellitic anhydride. 
     For the purpose of performing oxidation of o-xylene or naphthalene into phthalic anhydride, air may be used preferably as the source of oxygen taking various conditions into consideration, though not specifically restricted. A gas mixture which comprises oxygen and a dilution gas such as a nitrogen/carbon dioxide dilution gas, as well as an air system supplemented with oxygen, may also be useful as the source of oxygen. 
     The oxygen-containing gas source (air for example) may preferably be subjected to a preheating step (for example at 100 to 300° C.) prior to introduction into a reactor. 
     The oxidation reaction can be carried out at, over or below the atmospheric pressure. Generally, the reaction may preferably be carried out at an atmospheric pressure of from 0.5 to 3.0. Preferably, the oxidation reaction may be carried out at a reaction temperature of from 300° to 450° C. The catalyst may preferably be used in an amount of from 10 to 30 kg/(kg material gas/Hr). Preferable ratio of the air to the material gas in the reaction gas mixture may be in the range of from 5 to 15 kg/kg. 
     For the production of the pyromellitic anhydride, durene or tetraethylbenzene may be used as the source of the hydrocarbon. 
     The oxygen source and the oxygen preheating procedure employed may be the same as those described for the production of the phthalic anhydride. The reaction conditions may preferably include: 
     
         ______________________________________                                    
reaction pressure   0.5 to 3.0 atm.                                       
reaction temperature                                                      
                    280 to 380° C.                                 
amount of the catalyst used                                               
                    10 to 40 kg catalyst/kg                               
                    material gas/hr                                       
air/material gas ratio                                                    
                    10 to 20 kg/kg                                        
______________________________________                                    
 
    
     For the catalytic fluidized oxidization of other aromatic hydrocarbons into their corresponding carboxylic anhydrides using the catalyst of the present invention, the reaction conditions may be determined by referring to the above-described cases. 
    
    
     EXAMPLES 
     The present invention will now be described by way of the following examples which should be regarded as illustrative rather than restrictive. 
     EXAMPLE 1 
     A 600 kg portion of a titanyl sulfate aqueous solution containing 5% by weight of titanium as TiO 2  was cooled to 12° C. and then neutralized by gradually adding 140 kg of 15% by weight aqueous ammonia spending about 10 minutes with thorough stirring to obtain a titanium hydroxide gel. The thus obtained gel showed a pH value of 8.5 and a temperature of 25° C. 
     A 20 kg portion of the gel slurry was subjected to dehydration under a reduced pressure using a plate filter, and 300 dm 3  of pure water was gradually added to the resulting gel cake in order to remove ammonium sulfate formed by the foregoing neutralization. Concentration of TiO 2  in the thus purified titanium hydroxide gel was found to be 11.8% by weight. 
     A portion of the purified gel was dried at a temperature of 300° C. and the resulting powder was subjected to X-ray diffraction. The crystallite diameter of anatase type titanium oxide in the powder was calculated to be 5.1 nm from a diffraction peak at an angle of 2θ=25.3° in the diffraction pattern. 
     To 100 kg of the aforementioned gel containing 11.8% by weight of TiO 2  obtained by the repeated washing using the plate filter was added 140 kg of pure water with thorough stirring to obtain a gel slurry containing 5.0% by weight of TiO 2 . 
     A silica sol article having a mean particle size of 5 nm (Cataloid SI-550, manufactured by Catalysts &amp; Chemicals Industries Co , Ltd.; consult Table 1 for its properties) was passed through a cation exchange resin layer to obtain 200 kg of Na-removed silica sol. The thus obtained Na-removed silica sol contained 10.3% by weight of SiO 2  with a Na 2  O/SiO 2  weight ratio of 0.002. 
     A 600 dm 3  capacity stainless steel vessel equipped with a steam jacket was charged with the whole amount of the previously prepared gel slurry containing 5.0% by weight of TiO 2  and, with stirring, charged further with 97.5 kg of the just described Na-removed silica sol, 7.82 kg of a vanadyl sulfate aqueous solution containing 19.3% by weight of vanadium as V 2  O 5  and 2.36 kg of a cesium sulfate aqueous solution containing 50.0% by weight of cesium as Cs 2  SO 4  in that order. To the resulting mixture was added 0.97 kg of ammonium sulfate crystals and, finally, 80 kg of a previously prepared boric acid aqueous solution (2.8% by weight as H3B03 concentration). This mixture showed a pH value of 2.5. 
     The thus obtained slurry mixture was heated with thorough stirring to evaporate water and concentrate the slurry to a level of 20% by weight as &#34;TiO 2  +SiO 2  +V 2  O 5  +Cs 2  SO 4  +SO 3  +B 2  O 3  &#34;. Thereafter, the concentrated slurry was dispersed thoroughly using a homogenizer and applied to a disk type spray dryer, and the resulting spray-dried powder was further dried overnight at 150° C. and then calcinated at 570° C. for 3 hours to obtain a catalyst which was named catalyst A. Chemical composition and physical properties of the catalyst are shown in Table 2. 
     EXAMPLE 2 
     The process of Example 1 was repeated except that the amount of boron oxide was changed to obtain catalyst B, with its chemical composition and physical properties shown in Table 2. 
     EXAMPLE 3 
     The process of Example 1 wa repeated except that the amount of boron oxide was changed to obtain catalyst C, with its chemical composition and physical properties shown in Table 2. 
     COMPARATIVE EXAMPLE 1 
     The process of Example 1 was repeated except that the source of B 2  O 3  was not used to obtain catalyst D, with its chemical composition and physical properties shown in Table 2. 
     COMPARATIVE EXAMPLE 2 
     The process of Example 1 was repeated except that the amount of boron oxide was changed to the highest level to obtain catalyst E, with its chemical composition and physical properties shown in Table 2. 
     EXAMPLE 4 
     A metatitanic acid slurry, which is obtained as an intermediate product of a process for the production of titanium oxide for pigment use, was diluted and subjected to neutralization and washing steps in the same manner as in Example 1 to obtain a titanium hydroxide gel from which sulfuric acid was removed. Concentration of TiO 2  in the gel was found to be 29.0% by weight. When a portion of the thus prepared gel was dried at 300° C., the resulting powder showed a crystallite diameter of 16 nm. A powder preparation containing silicon dioxide, active components and boron oxide was prepared from the gel by similar means to the case of catalyst A, and the powder was calcinated at 600° C. for 3 hours to obtain catalyst F. Chemical composition and physical properties of this catalyst are shown in Table 2. 
     EXAMPLE 5 
     The titanium hydroxide gel obtained in Example 4 was diluted with pure water to a gel concentration of 15% by weight, and the diluted gel wa adjusted to pH 2 with nitric acid. The resulting slurry was heated at 180° C. for 100 hours with stirring using an external heating type autoclave. When a portion of the thus heated slurry was dried at 300° C., the resulting powder showed a crystallite diameter of 35 nm. In the same manner as in Example 1, catalyst G having properties as shown in Table 2 was obtained from the thus prepared titanium hydroxide slurry. 
     EXAMPLE 6 
     A commercial article of anatase type titanium oxide powder (A#200, manufactured by Teikoku Kako Co , Ltd.) was suspended in pure water to prepare a slurry containing 30% by weight of titanium as TiO 2 , and catalyst H having properties as shown in Table 2 was obtained from the thus prepared slurry in the same manner as in Example 1. 
     EXAMPLE 7 
     A 200 dm 3  capacity stainless steel vessel equipped with a steam jacket was charged with compounds in the following order: 53.5 kg of the titanium hydroxide gel used in Example 1, the same amount of pure water, 9.5 kg of a silica sol article as shown in Table 1 having a mean particle size of 17 nm (Cataloid S-20L, manufactured by Catalysts &amp; Chemicals Industries Co., Ltd.; SiO 2  contents, 20.5% by weight), 2.95 kg of a vanadyl sulfate aqueous solution containing 19.3% by weight of vanadium as V 2  O 5 , 0.45 kg of potassium sulfate crystals (purity, 99% by weight), 0.37 kg of ammonium sulfate crystals and 30 kg of a boric acid aqueous solution containing 1 6% by weight of boron as B 2  O 3 . The resulting slurry was concentrated to a level of 16% by weight as &#34;TiO 2  +SiO 2  +V 2  O 5  +K 2  SO 4  +SO 3  +B.sub. 2 O 3  &#34;, and the concentrated slurry was subjected to spray drying and drying/calcination steps in the same manner as in Example 1 to obtain catalyst I, with its chemical composition and physical properties shown in Table 2. 
     EXAMPLE 8 
     A tank charged with 53.5 kg of the above titanium hydroxide gel was further charged with 5.6 kg of a silica sol article having a mean particle size of 26 nm (Cataloid SI-50; cf. Table 1 for its properties) which has been passed through a cation exchange resin layer to remove Na, 2.7 kg of a vanadyl sulfate aqueous solution, 1.0 kg of cesium sulfate, 0.35 kg of ammonium sulfate crystals and 30.5 kg of a boric acid aqueous solution (contents as B 2  O 3 , 1.6% by weight), and the resulting mixture was treated in the same manner as in Example 1 to obtain catalyst J, with its chemical composition and physical properties shown in Table 2. 
     EXAMPLE 9 
     The same titanium hydroxide gel containing tank as described above was charged with 17.5 kg of a silica sol article as shown in Table 1 (Cataloid S 20L), 6.4 kg of a vanadyl sulfate aqueous solution, 0.9 kg of cesium sulfate, 0.38 kg of ammonium sulfate crystals and 30.5 kg of a boric acid aqueous solution, the resulting slurry was concentrated with stirring to a slurry concentration of 21% by weight, the thus concentrated slurry was subjected to spray drying and, thereafter, the resulting powder was treated in the same manner as in Example 1 to obtain catalyst K. Chemical composition and physical properties of this catalyst are shown in Table 2. 
     COMPARATIVE EXAMPLE 3 
     Catalyst L was prepared by repeating the process for the production of catalyst A in Example 1 using the same titanium hydroxide gel except that the silica sol article was not used. Chemical composition and physical properties of this catalyst are shown in Table 2. 
     EXAMPLE 10 
     A mixture of 500 kg of a titanyl sulfate aqueous solution containing 5% by weight of titanium as TiO 2  with 80.6 kg of an Na-removed silica sol having an SiO 2  concentration of 10.3% by weight obtained from a silica gel article (Cataloid SI-550; cf. Table 1) was cooled to 10° C. and then neutralized by gradually adding 128 kg of 15% by weight aqueous ammonia spending about 13 minutes with thorough stirring to obtain a coprecipitation gel of titanium hydroxide and silica. The thus obtained gel showed a pH value of 9.1 and a temperature of 22° C. The gel slurry thus obtained was subjected to dehydration and washing using a plate filter. Concentration of solid contents in the thus purified coprecipitation gel was found to be 9.5% by weight. 
     When a portion of the purified gel was dried at a temperature of 300° C., the crystallite diameter of anatase type titanium oxide in the resulting powder was found to be 4.0 nm. With stirring, pure water was added to 350 kg of the coprecipitation 9el to obtain a gel slurry containing 5% by weight of solid contents. 
     The resulting slurry was mixed thoroughly with 120 kg of the aforementioned Na-removed silica sol, 15.0 kg of a vanadyl sulfate aqueous solution, 5.0 kg of a Cs 2  SO 4  aqueous solution, 2.0 kg of ammonium sulfate crystals and 175 kg of a 2.8% by weight boric acid aqueous solution. The thus obtained slurry mixture was heated to evaporate water and concentrate the slurry to a level of 17.8% by weight as &#34;TiO 2  +SiO 2  +V 2  O 5  +Cs 2  SO 4  +SO 3  +B 2  O 3  &#34;. Thereafter, the concentrated slurry was subjected to spray drying and calcination in the same manner as in Example 1 to obtain a catalyst which was named catalyst M. Chemical composition and physical properties of this catalyst are shown in Table 2. 
     EXAMPLE 11 
     In the same manner as in the case of catalyst A, a mixture was prepared from the titanium hydroxide gel, silica sol, vanadyl sulfate aqueous solution, cesium sulfate aqueous solution, ammonium sulfate crystals and boric acid aqueous solution. The mixture was further mixed with a lanthanum nitrate aqueous solution and then subjected to spray drying and calcination to obtain catalyst N, with its chemical composition and physical properties shown in Table 2. 
     EXAMPLE 12 
     Catalysts A to N obtained in Examples 1 to 11 and Comparative Examples 1 to 3 were subjected to a production test of phthalic anhydride from o-xylene by means of a gas phase oxidation using a fluidized bed reactor (made of SUS 304  stainless steel, 83 mmφ×3400 mmL), with the results shown in Table 3. Unreacted o-xylene and reaction products including phthalic anhydride, phthalides, trialdehydes, maleic anhydride and CO+CO 2  were analyzed by gas chromatography. 
     
         ______________________________________                                    
&lt;Reaction conditions&gt;                                                     
______________________________________                                    
Feed rate of o-xylene  230 g/Hr (sulfur)                                  
contents, 1000 ppm; nitrogen contents,                                    
500 ppm)                                                                  
Air volume             2200 standard                                      
                       state dm.sup.3 /Hr                                 
Linear velocity (empty reactor basis)                                     
                       17 cm/sec                                          
Air/o-xylene ratio     12.2 kg/kg                                         
Pressure               1.0 kg/cm.sup.2 · G                       
Reaction temperature   340° C.                                     
Catalyst volume        5000 cm.sup.3                                      
______________________________________                                    
 
    
     Conversion ratio of o-xylene, selectivity for phthalic anhydride and yield of phthalic anhydride were calculated based on the following formulae. ##EQU2## 
     EXAMPLE 13 
     A production test of phthalic anhydride from naphthalene by means of a gas phase contact oxidation was carried out using the same fluidized bed reactor used in Example 12, with the results shown in Table 3. Analysis and calculation of the results were carried out in the same manner as in the case of Example 12. 
     
         ______________________________________                                    
&lt;Reaction conditions&gt;                                                     
______________________________________                                    
Feed rate of naphthalene                                                  
                       275 g/Hr (sulfur)                                  
contents, 1000 ppm; nitrogen contents,                                    
500 ppm)                                                                  
Air volume             2200 standard                                      
                       state dm.sup.3 /Hr                                 
Linear velocity (empty reactor basis)                                     
                       16 cm/sec                                          
Air/naphthalene ratio  10.3 kg/kg                                         
Pressure               1.0 kg/cm.sup.2 · G                       
Reaction temperature   330° C.                                     
Catalyst volume        5000 cm.sup.3                                      
______________________________________                                    
 
    
     EXAMPLE 14 
     Of the catalysts evaluated in Example 12 and 13 with regard to their catalytic activity for oxidizing o-xylene and naphthalene into phthalic anhydride, catalysts A, D, E and N, which were produced in Example 1, Comparative Examples 1 and 2 and Example 11, respectively, were evaluated for their catalytic activity in a gas phase oxidation of durene into pyromellitic anhydride by using the same fluidized bed reactor as Example 12 under the following reaction conditions. 
     
         ______________________________________                                    
Feed rate of durene                                                       
                 160 g/hr                                                 
Air volume       2200 dm.sup.3 at standard state/hr                       
Linear velocity (empty                                                    
                 17 cm/sec                                                
reactor basis)                                                            
Air/durene ratio 17.7 kg/kg                                               
Pressure         1.0 kg/cm.sup.2 · G                             
Reaction temperature                                                      
                 300° C.                                           
Catalyst volume  5,000 cm.sup.3                                           
______________________________________                                    
 
    
     The obtained reaction gas was contacted with steam, then, unreacted durene, pyromellitic anhydride, methyl trimellitic anhydride, dimethyl phthalic anhydride were absorbed in stream and recovered as aqueous solution After evaporation of the thus obtained aqueous solution, the reaction products were esterified by using boron trifluoride-methanol complex and analyzed by gas chromatography. CO and CO 2  were collected and analyzed also by gas chromatography. 
     The conversion and the selectivity were calculated as described in Example 12. The results are shown in Table 4, below. 
     
                       TABLE 4                                                     
______________________________________                                    
                             Selectivity                                  
                   Conversion                                             
                             (mol. %) of                                  
                   (mol. %)  pyromellitic                                 
Catalyst           of durene anhydride                                    
______________________________________                                    
A      (E. 1)      73.1      51.1                                         
N      (E. 11)     70.5      55.0                                         
D      (C.E. 1)    78.2      38.3                                         
E      (C.E. 2)    58.9      52.7                                         
______________________________________                                    
 
    
     
                       TABLE 1                                                     
______________________________________                                    
Properties of silica sol materials                                        
       SiO.sub.2                                                          
               Na.sub.2 O                                                 
                        Specific Particle                                 
       conc.   conc.    surface area                                      
                                 size   Catalyst                          
Name   %       %        m.sup.2 /g                                        
                                 nm     applied                           
______________________________________                                    
Cataloid                                                                  
       20.2    0.75     530       5.1   A, B, C,                          
SI-550          (0.002)                 D, E, F,                          
                                        G, H, M,                          
                                        N                                 
Cataloid                                                                  
       20.5    0.04     146      18.7   I, K                              
S-20L                                                                     
Cataloid                                                                  
       47.8    0.61     103      26.5   J                                 
SI-50           (0.002)                                                   
______________________________________                                    
 (Note)                                                                   
 SI550 and SI50 were subjected to Naremoval using an ion exchange resin   
 prior to their use, while S20L was used per se. Parenthesis indicates    
 Na.sub.2 O/SiO.sub.2 weight ratio after Na removal.                      
 
    
     
                                           TABLE 2                                 
__________________________________________________________________________
Chemical compositions and physical properties of catalysts                
__________________________________________________________________________
           Chemical compositions                                          
                                         SO.sub.3 /                       
                                              M.sub.2 O/                  
                                                  SiO.sub.2 /             
                                                      B.sub.2 O.sub.3     
                                                          V.sub.2 O.sub.5 
                                                          +               
       Cata-                             M.sub.2 O                        
                                              V.sub.2 O.sub.5             
                                                  TiO.sub.2               
                                                      TiO.sub.2           
                                                          M.sub.2 O +     
       lyst                                                               
           V.sub.2 O.sub.5                                                
               M.sub.2 SO.sub.4                                           
                    SO.sub.3                                              
                        La.sub.2 O.sub.3                                  
                            TiO.sub.2                                     
                                 B.sub.2 O.sub.3                          
                                     SiO.sub.2                            
                                         molar                            
                                              molar                       
                                                  weight                  
                                                      weight              
                                                          SO.sub.3        
       No. wt %                                                           
               wt % wt %                                                  
                        wt %                                              
                            wt % wt %                                     
                                     wt %                                 
                                         ratio                            
                                              ratio                       
                                                  ratio                   
                                                      ratio               
                                                          wt              
__________________________________________________________________________
                                                          %               
Example                                                                   
No.                                                                       
1      A   5.7 4.5  2.2 --  44.8 4.8 38.1                                 
                                         3.2  0.40                        
                                                  0.85                    
                                                      0.11                
                                                          11.4            
2      B   5.9 4.6  2.3 --  46.2 1.8 39.3                                 
                                         3.3  0.39                        
                                                  0.85                    
                                                      0.04                
                                                          11.8            
3      C   5.0 4.0  1.9 --  39.5 16.0                                     
                                     33.6                                 
                                         3.1  0.40                        
                                                  0.85                    
                                                      0.41                
                                                          10.0            
Comparative                                                               
       D   6.0 4.7  2.3 --  47.0 --  40.0                                 
                                         3.2  0.39                        
                                                  0.85                    
                                                      0   12.0            
Comparative                                                               
       E   4.6 3.6  1.8 --  36.2 23.0                                     
                                     30.8                                 
                                         3.3  0.39                        
                                                  0.85                    
                                                      0.64                
                                                           9.2            
2                                                                         
4      F   5.7 4.4  2.2 --  45.0 4.9 37.8                                 
                                         3.3  0.39                        
                                                  0.84                    
                                                      0.11                
                                                          11.3            
5      G   5.8 4.5  2.2 --  43.0 8.2 36.3                                 
                                         3.2  0.39                        
                                                  0.85                    
                                                      0.19                
                                                          11.5            
6      H   5.7 4.5  2.1 --  38.5 17.1                                     
                                     32.1                                 
                                         3.1  0.40                        
                                                  0.83                    
                                                      0.44                
                                                          11.3            
7      I   5.7 K.sub.2 SO.sub.4                                           
                    2.2 --  63.3 4.8 19.5                                 
                                         2.1  0.82                        
                                                  0.31                    
                                                      0.08                
                                                          10.3            
               4.5                                                        
8      J   5.2 4.9  2.1 --  55.9 4.9 27.0                                 
                                         2.9  0.61                        
                                                  0.48                    
                                                      0.09                
                                                          11.1            
9      K   12.3                                                           
               4.4  2.3 --  40.3 4.9 35.8                                 
                                         3.4  0.96                        
                                                  0.89                    
                                                      0.12                
                                                          18.0            
Comparative                                                               
       L   6.0 4.6  2.4 --  82.0 5.0 --  3.4  0.39                        
                                                  0   0.06                
                                                          12.0            
3                                                                         
10     M   5.3 4.5  2.2 --  45.1 5.0 37.9                                 
                                         3.2  0.43                        
                                                  0.84                    
                                                      0.11                
                                                          11.0            
11     N   5.6 4.4  2.1 2.6 43.6 4.6 37.1                                 
                                         3.2  0.51                        
                                                  0.85                    
                                                      0.11                
                                                          11.1            
__________________________________________________________________________
                  Physical properties                Crystallite          
                  Specific                           diameter             
                  surface         Bulk                                    
                                      Abrasion Crystallite                
                                                     after                
         Catalyst area Pore volume                                        
                                  density                                 
                                      rate     diameter                   
                                                     drying*              
         No.      m.sup.2 /g                                              
                       cm.sup.3 /g                                        
                                  g/cm.sup.3                              
                                      wt %     nm    nm                   
__________________________________________________________________________
Example                                                                   
No.                                                                       
1        A        23   0.39       0.86                                    
                                      0.8      40    5.1                  
2        B        46   0.55       0.70                                    
                                      2.2      34    5.1                  
3        C         7   0.17       1.25                                    
                                      0.2      58    5.1                  
Comparative                                                               
         D        120  0.71       0.60                                    
                                      7.9      23    5.1                  
Comparative                                                               
         E         3   0.08       1.53                                    
                                      0.1      71    5.1                  
2                                                                         
4        F        11   0.42       0.82                                    
                                      2.7      55    16.0                 
5        G        10   0.29       0.67                                    
                                      4.9      52    35.0                 
6        H         2   0.27       0.55                                    
                                      5.0      93    89.0                 
7        I        13   0.20       1.12                                    
                                      0.7      50    5.1                  
8        J        19   0.34       0.99                                    
                                      0.6      44    5.1                  
9        K        14   0.23       1.10                                    
                                      0.5      48    5.1                  
Comparative                                                               
         L         2   0.05       1.68                                    
                                      0.1      81    5.1                  
3                                                                         
10       M        40   0.48       0.74                                    
                                      0.6      35    4.0                  
11       N        30   0.40       0.83                                    
                                      0.9      37    5.1                  
__________________________________________________________________________
 *Crystallite diameter of titanium hydroxide after drying at 300° C
 
    
     
                                           TABLE 3                                 
__________________________________________________________________________
Results of the gas phase oxidation reaction of o-xylene and naphthalene   
Naphthalene as starting material                                          
                               o-Xylene as starting material              
             Selectivity             Selectivity                          
      Conversion                                                          
             for   Yield of                                               
                         Yield of                                         
                               Conversion                                 
                                     for   Yield of                       
                                                 Yield of                 
Example                                                                   
      rate of                                                             
             phthalic                                                     
                   phthalic                                               
                         phthalic                                         
                               rate of                                    
                                     phthalic                             
                                           phthalic                       
                                                 phthalic                 
No.   naphthalene                                                         
             anhydride                                                    
                   anhydride                                              
                         anhydride                                        
                               o-xylene                                   
                                     anhydride                            
                                           anhydride                      
                                                 anhydride                
(Catalyst)                                                                
      (mol %)                                                             
             (mol %)                                                      
                   (mol %)                                                
                         (wt %)                                           
                               (mol %)                                    
                                     (mol %)                              
                                           (mol %)                        
                                                 (wt %)                   
__________________________________________________________________________
1  (A)                                                                    
      65.4   87.2  83.2  72.0  92.7  74.9  69.4  96.9                     
2  (B)                                                                    
      96.1   82.7  79.5  68.8  94.3  64.2  60.5  84.5                     
3  (C)                                                                    
      80.3   84.3  67.7  58.6  70.2  79.9  56.1  78.3                     
CE1                                                                       
   (D)                                                                    
      98.0   47.0  46.1  39.9  98.2  39.4  38.7  54.0                     
CE2                                                                       
   (E)                                                                    
      65.2   80.4  52.4  45.3  62.6  67.1  42.0  58.6                     
4  (F)                                                                    
      85.2   86.1  73.4  63.5  79.7  56.0  44.6  62.3                     
5  (G)                                                                    
      90.2   75.4  68.0  58.8  86.9  65.2  56.7  79.2                     
6  (H)                                                                    
      60.5   75.2  45.4  39.4  53.7  70.6  37.9  52.9                     
7  (I)                                                                    
      90.3   80.7  72.9  63.1  88.6  70.8  62.7  87.5                     
8  (J)                                                                    
      95.7   86.1  82.4  71.3  94.0  67.4  63.4  88.5                     
9  (K)                                                                    
      100    65.3  65.3  56.5  97.2  51.0  49.6  69.2                     
CE3                                                                       
   (L)                                                                    
      78.4   82.6  64.8  56.1  72.5  59.6  43.2  60.3                     
10 (M)                                                                    
      95.4   87.3  83.3  72.1  91.6  74.0  67.8  94.6                     
11 (N)                                                                    
      98.3   90.4  88.9  76.9  94.7  78.7  74.5  109.8                    
__________________________________________________________________________
 
    
     It is evident from the above results that catalyst L (Comparative Example 3) which does not contain SiO 2  in its carrier cannot be used suitably as a fluid catalyst because of its markedly high bulk density. It is evident also that catalyst D (Comparative Example 1) which contains no B 2  O 3  in its carrier has extremely large specific surface area and markedly high abrasion rate (low abrasion resistance or weak strength), thus showing considerably low product selectively. Too much content of B 2  O 3  (catalyst E or Comparative Example 2), however, showed a high bulk density and a low pore volume and resulted in a low product yield. 
     Thus, it is apparent that there has been provided, in accordance with the present invention, a catalyst which is possessed of not only high activity, high product selectivity and high abrasion resistance (strength) but also suitable bulk density for fluidization, while these excellent properties cannot be found in commonly used catalysts for use in the production of carboxylic anhydrides from hydrocarbons such as o-xylene and naphthalene by means of fluidized bed gas phase oxidation. 
     Although some preferred embodiments have been described, many modifications and variations may be made thereto in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.