Method for preparing 3,3',4,4'-benzophenonetetracarboxylic acid

Abstract

The present invention discloses a method for oxidizing 1,1-bis-(3,4-dimethylphenyl)-alkane with nitric acid in a pressure vessel to produce 3,3',4,4'-benzophenonetetracarboxylic acid and simultaneously form nitric oxide, wherein the nitric oxide is sent from the pressure vessel to an absorption vessel, and the nitric oxide in the absorption vessel reacts with molecular oxygen and water to generate an aqueous nitric acid solution, thereby preventing the emission of nitric oxide and avoiding the risk of oxygen inhibiting the oxidation of nitric acid. Furthermore, when the nitric acid from the absorption vessel is used to oxidize the 1,1-bis-(3,4-dimethylphenyl)-alkane, the consumption of nitric acid is reduced.

Description

Technical Field

[0001] The invention relates to a method for oxidizing 1,1-bis-(3,4-dimethylphenyl)-alkane to 3,3',4,4'-benzophenone tetracarboxylic acid. 3,3',4,4'-benzophenone tetracarboxylic acid can be used to produce 3,3',4,4'-benzophenone tetracarboxylic dianhydride. Background Art

[0002] 3,3',4,4'-Benzophenonetetracarboxylic dianhydride is used as a monomer for preparing aromatic polyimide. The polyimide prepared from this monomer has been commercialized under the trade name P84.

[0003] US 3,078,279 discloses the preparation of 3,3',4,4'-benzophenonetetracarboxylic dianhydride by oxidation of 1,1-bis-(3,4-dimethylphenyl)-ethane with 30% by weight nitric acid at 200° C. to 210° C. The reaction proceeds via the intermediates 3,3',4,4'-tetramethylbenzophenone and 3,3',4,4'-benzophenonetetracarboxylic acid, as described by JGD Schulz and A. Onopchenko in Ind. Eng. Chem., Prod. Res. Dev. 15 (1976) 152-156. US 3,479,400 discloses a two-step oxidation of 1,1-bis-(3,4-dimethylphenyl)-ethane, wherein only a small excess of nitric acid is used in a first oxidation step carried out at 105° C. to 160° C., a larger excess of more concentrated nitric acid is used in a second oxidation step carried out at about 175° C., and the aqueous filtrate obtained by filtering off 3,3',4,4'-benzophenonetetracarboxylic acid from the reaction mixture obtained in the second oxidation step is reused to provide nitric acid for the first oxidation step. A disadvantage of these processes is that large amounts of nitrogen monoxide are formed during the oxidation of 1,1-bis-(3,4-dimethylphenyl)-ethane to 3,3',4,4'-benzophenonetetracarboxylic acid and are discharged from the reactor in which the oxidation is carried out. Therefore, the commercial production of 3,3',4,4'-benzophenonetetracarboxylic dianhydride by this route requires costly waste gas treatment.

[0004] US 4,173,573 discloses the preparation of 3,3',4,4'-benzophenonetetracarboxylic dianhydride by oxidation of 1,1-bis-(3,4-dimethylphenyl)-ethane with 54 wt% nitric acid in a one-step feed-batch oxidation, wherein during the dosing of nitric acid the reaction temperature increases from 140° C. to 175° C. US 3,466,301 discloses a similar feed-batch oxidation with 70 wt% nitric acid, wherein during the dosing of nitric acid the reaction temperature increases from 140° C. to 170° C.

[0005] JPS61-103851 discloses a two-stage oxidation of 1,1-bis-(3,4-dimethylphenyl)-ethane, wherein oxidation is carried out for 3 hours at 180°C in a closed autoclave with 30% by weight nitric acid, oxygen is then introduced into the autoclave, and the reaction is continued at the same temperature. The gas present in the autoclave at the end of the reaction contains only a small amount of nitrogen monoxide. Comparative Example 2 of the document shows that the introduction of oxygen from the beginning of nitric acid oxidation strongly inhibits the oxidation reaction. JPS62-198644 and US 4,684,738 disclose nitric acid oxidation of 1,1-bis-(3,4-dimethylphenyl)-ethane under reaction conditions similar to JPS61-103851, oxygen is continuously introduced into the autoclave, and a pressure of 20 bar is maintained by exhausting the gas. The exhaust gas has a low content of nitrogen monoxide. However, these documents do not describe how to prevent the inhibition of nitric acid oxidation by oxygen. Applicants have observed the same inhibitory effect of oxygen as described in JPS 61-103851 and have found that the introduction of oxygen into the pressure reactor during the oxidation of 1,1-bis-(3,4-dimethylphenyl)-methane by nitric acid completely inhibits the nitric acid oxidation.

[0006] Therefore, there is still a need for a process for preparing 3,3',4,4'-benzophenonetetracarboxylic acid by nitric acid oxidation of 1,1-bis-(3,4-dimethylphenyl)-alkanes, especially 1-bis-(3,4-dimethylphenyl)-methane, which can reduce or prevent the emission of nitrogen monoxide and avoid the risk of oxygen inhibiting the oxidation of nitric acid. Summary of the invention

[0007] The inventors of the present invention have now discovered that such a process can be achieved by passing nitrogen monoxide from a pressure vessel in which the nitric acid oxidation of 1,1-bis-(3,4-dimethylphenyl)-alkane is carried out to a separate absorption vessel and reacting the nitrogen monoxide in the absorption vessel with molecular oxygen and water to provide an aqueous nitric acid solution.

[0008] The subject of the present invention is therefore a process for oxidizing 1,1-bis-(3,4-dimethylphenyl)-alkane to 3,3',4,4'-benzophenonetetracarboxylic acid, comprising the steps of reacting 1,1-bis-(3,4-dimethylphenyl)-alkane or its partial oxidation product with nitric acid in a pressure vessel at a temperature of 120° C. to 220° C. to provide 3,3',4,4'-benzophenonetetracarboxylic acid with simultaneous formation of nitric oxide; conveying the nitric oxide from the pressure vessel to an absorption vessel; and reacting the nitric oxide in the absorption vessel with molecular oxygen and water to produce an aqueous nitric acid solution.

[0009] The method preferably comprises the following steps:

[0010] a) reacting 1,1-bis-(3,4-dimethylphenyl)-alkane with a first aqueous nitric acid composition at a temperature of 90° C. to 115° C. to provide a reaction mixture having two liquid phases: an aqueous phase, and an organic phase comprising 3,3′,4,4′-tetramethylbenzophenone,

[0011] b) separating the aqueous phase and the organic phase of step a), and

[0012] c) reacting the organic phase separated in step b) with a second aqueous nitric acid composition in a pressure vessel at a temperature of 120°C to 220°C.

[0013] In a preferred embodiment, the reaction mixture of step c) is cooled to crystallize 3,3′,4,4′-benzophenonetetracarboxylic acid, and the residual liquid separated from the crystallized 3,3′,4,4′-benzophenonetetracarboxylic acid is sent to step a) to provide all or part of the first aqueous nitric acid composition used in step a). DETAILED DESCRIPTION

[0014] The method of the present invention comprises the step of reacting 1,1-bis-(3,4-dimethylphenyl)-alkane or its partial oxidation product with nitric acid. The partial oxidation product is preferably the corresponding 1,1-bis-(3,4-dimethylphenyl)-alkanol or 3,3',4,4'-tetramethylbenzophenone, and most preferably 3,3',4,4'-tetramethylbenzophenone. The method of the present invention can also be carried out with a mixture comprising both 1,1-bis-(3,4-dimethylphenyl)-alkane and 3,3',4,4'-tetramethylbenzophenone. The alkane part in 1,1-bis-(3,4-dimethylphenyl)-alkane is preferably methane, ethane or propane, and most preferably methane, that is, the most preferred 1,1-bis-(3,4-dimethylphenyl)-alkane is 1,1-bis-(3,4-dimethylphenyl)-methane. Suitable 1,1-bis-(3,4-dimethylphenyl)-alkanes and methods for preparing them from o-xylene and aldehydes (e.g. formaldehyde, acetaldehyde or propionaldehyde) are known in the prior art. Preferably, 1,1-bis-(3,4-dimethylphenyl)-alkanes with a high isomeric purity are used, the isomeric purity being related to the position of the methyl groups in the dimethylphenyl moiety. Methods for increasing the isomeric purity by isomerization reactions or purification methods (e.g. fractional crystallization) are known in the prior art.

[0015] 1,1-bis-(3,4-dimethylphenyl)-alkane or its partial oxidation product is reacted with nitric acid in a pressure vessel at a temperature of 120°C to 220°C. In order to maintain a liquid reaction mixture, the reaction is preferably carried out at a pressure above the vapor pressure of water at the reaction temperature. The reaction is most preferably carried out under autogenous pressure, i.e., the pressure generated by steam generated by water or aqueous nitric acid that has been added to the pressure vessel before the pressure vessel is heated to the reaction temperature. The reaction is carried out for a time sufficient to oxidize at least part of the starting material to 3,3',4,4'-benzophenonetetracarboxylic acid. The reaction time is preferably selected to oxidize at least 50% of the starting material, preferably at least 80% of the starting material, more preferably greater than 95% of the starting material to a product in which the methyl groups in the 3,4-dimethylphenyl moiety are oxidized to carboxylic acid groups. Preferably, during the reaction of 1,1-bis-(3,4-dimethylphenyl)-alkane or its partial oxidation product with nitric acid, no oxygen is introduced into the pressure vessel to prevent oxygen from inhibiting the nitric acid reaction. In a preferred embodiment, the pressure vessel is flushed with an inert gas, preferably nitrogen, to remove oxygen from the interior thereof before the pressure vessel is heated to the reaction temperature.

[0016] The oxidation of 1,1-bis-(3,4-dimethylphenyl)-alkane with nitric acid produces a large amount of nitric oxide as a byproduct. The stoichiometric equation for the oxidation of 1,1-bis-(3,4-dimethylphenyl)-ethane (xylylethane, DXE) to 3,3',4,4'-benzophenonetetracarboxylic acid (BTA) is

[0017] 3DXE+34HNO3→3BTA+3CO2+34NO+35H2O,

[0018] The chemical dosage equation for the oxidation of 1,1-bis-(3,4-dimethylphenyl)-methane (xylylmethane, DXM) to 3,3',4,4'-benzophenonetetracarboxylic acid (BTA) is:

[0019] 3DXM+28HNO3→3BTA+28NO+29H2O.

[0020] Preferably, the amount of nitric acid used is 0.9 to 2.0 times the stoichiometric amount, preferably 1.1 to 1.5 times the stoichiometric amount. Nitric acid preferably uses an aqueous solution containing 30% to 70% by weight of nitric acid, and more preferably uses an aqueous solution containing 50% to 70% by weight of nitric acid. It is preferred to react under the condition that the nitric acid concentration in the reaction mixture does not exceed 30% by weight to prevent nitration of the phenyl group. In a preferred embodiment, the reaction is carried out in a batch feed reaction: 1,1-bis-(3,4-dimethylphenyl)-alkane and water or an aqueous nitric acid solution are added to a pressure vessel so that the nitric acid content in the resulting mixture does not exceed 15% by weight, and an aqueous nitric acid solution containing 30% to 70% by weight of nitric acid is fed into the pressure vessel within 1 hour to 10 hours. Preferably, during the feeding of nitric acid, the reaction temperature is increased from an initial value in the range of 90°C to 110°C to a final value in the range of 140°C to 180°C. The low initial nitric acid concentration in the reaction mixture, and the batch feed reaction with the starting temperature of the temperature gradient below the temperature required to oxidize the methyl groups of the dimethylphenyl moiety, reduce the formation of by-products and provide 3,3',4,4'-benzophenonetetracarboxylic acid with better yield and purity.

[0021] In the method of the present invention, the nitric oxide formed from the nitric acid during the reaction is sent from the pressure vessel to the absorption vessel and reacts with the molecular oxygen and water in the absorption vessel to produce an aqueous nitric acid solution. The stoichiometric equation for the reaction of nitric oxide with the oxygen and water in the absorption vessel is:

[0022] 4NO+3O2+2H2O→4HNO3.

[0023] Preferably, the nitric oxide reacts with the oxygen and water in the absorption vessel at a temperature of 10°C to 40°C and a pressure of 1 to 10 bar. The oxygen required for the reaction is preferably introduced into the absorption vessel by a gas containing at least 98% by volume of oxygen. Any device suitable for contacting a gas with a liquid can be used as an absorption vessel. Preferably, a stirred tank reactor or an absorption tower is used as an absorption vessel. Preferably, the nitric oxide is sent from the pressure vessel to the absorption vessel through a conduit equipped with a pressure regulating valve, the pressure set by the pressure regulating valve being higher than the vapor pressure at the temperature used for the reaction in the pressure vessel. The set point of the pressure regulating valve is preferably 0.1 bar to 5 bar higher than the vapor pressure at the highest temperature used for the reaction in the pressure vessel. When the reaction of 1,1-bis-(3,4-dimethylphenyl)-alkane is completed, the pressure setting of the pressure regulating valve can be reduced, so that more nitric oxide is purged from the pressure vessel to the absorption vessel with water vapor evaporated from the reaction mixture in the pressure vessel. Preferably, oxygen is introduced into the absorption vessel through a pressure regulating valve, and the pressure valve is set to the working pressure of the absorption vessel. Preferably, after all the nitrogen monoxide reacted in the absorption vessel from the pressure vessel, a certain amount of water is added to the absorption vessel to provide a nitric acid concentration of 30% to 70% by weight. Before the nitrogen monoxide is sent to the absorption vessel from the pressure vessel, the absorption vessel is preferably flushed with oxygen. The nitrogen monoxide formed during the oxidation of 1,1-bis-(3,4-dimethylphenyl)-alkane or its partial oxidation product is sent to the absorption vessel from the pressure vessel, and the nitrogen monoxide in the absorption vessel is reacted with molecular oxygen and water, which significantly reduces the amount of nitrogen monoxide discharged in the process, while safely preventing molecular oxygen from inhibiting the oxidation reaction in the pressure vessel.

[0024] The nitric acid solution produced in the absorption vessel can be used in the step of oxidizing 1,1-bis-(3,4-dimethylphenyl)-alkane or its partial oxidation product, which reduces the consumption of nitric acid compared to the prior art method that emits nitrogen monoxide in the process. When the oxidation of 1,1-bis-(3,4-dimethylphenyl)-alkane or its partial oxidation product is carried out in a batch feed reaction mode, the nitric acid solution produced in the absorption vessel is preferably used in subsequent batches. When 1,1-bis-(3,4-dimethylphenyl)-methane is used as 1,1-bis-(3,4-dimethylphenyl)-alkane and oxygen is supplied to the absorption vessel by a gas containing at least 98% by volume of oxygen, the method of the present invention can be carried out without discharging gas from the assembly of the pressure vessel and the absorption vessel during the reaction. In this case, it is not necessary to purify the exhaust gas before discharging it into the atmosphere.

[0025] Preferably, 3,3',4,4'-benzophenonetetracarboxylic acid obtained by reacting 1,1-bis-(3,4-dimethylphenyl)-alkane or a partial oxidation product thereof with nitric acid is separated from the reaction mixture obtained in the pressure vessel, preferably by cooling the reaction mixture to crystallize 3,3',4,4'-benzophenonetetracarboxylic acid and separating the crystallized product from the residual liquid by filtration or centrifugation. The crystallized product may be washed with water to remove by-products.

[0026] In a preferred embodiment, the process of the present invention is carried out in two subsequent reaction steps. In the first reaction step, 1,1-bis-(3,4-dimethylphenyl)-alkane is reacted with a first aqueous nitric acid composition at a temperature of 90° C. to 115° C. The initial nitric acid content in the reaction mixture is preferably maintained in the range of 10% to 30% by weight to prevent side reactions, such as nitration of the phenyl group. The first reaction step provides a reaction mixture having two liquid phases: an aqueous phase and an organic phase comprising 3,3',4,4'-tetramethylbenzophenone. The reaction time of the first reaction step is preferably selected to provide a conversion of 1,1-bis-(3,4-dimethylphenyl)-alkane of greater than 50%, preferably greater than 80%. The first reaction step is preferably carried out in a stirred reactor, by stirring 1,1-bis-(3,4-dimethylphenyl)-alkane dispersed in the first aqueous nitric acid composition. The density of the organic phase increases during the first reaction step and in case of sufficient conversion of 1,1-bis-(3,4-dimethylphenyl)-alkane, the density of the organic phase will increase to a higher density than the aqueous phase, thus becoming the heavy phase in the reaction mixture. In a subsequent step, the aqueous and organic phases of the reaction mixture formed in the first reaction step are separated, preferably by settling and recovering the organic phase as the bottom phase after settling. The settling can be carried out in the same vessel used to carry out the first reaction step, in a separate settler vessel or in any other device suitable for separating two liquid phases. The separation of the organic phase from the aqueous phase is preferably carried out at a temperature above 50°C, more preferably at a temperature of 60°C to 80°C, in order to prevent precipitation of solid 3,3',4,4'-tetramethylbenzophenone. Then, in a second reaction step carried out in a pressure vessel, the organic phase separated from the reaction mixture of the first reaction step is reacted with a second aqueous nitric acid composition. The second reaction step is carried out at a temperature of 120°C to 220°C. The second aqueous nitric acid composition is preferably added in an amount such that the final concentration of nitric acid in the reaction mixture is 10% to 30% by weight to prevent side reactions. The reaction time of the second reaction step is preferably selected so that the conversion of 3,3',4,4'-tetramethylbenzophenone contained in the organic phase of the reaction mixture formed in the first reaction step is greater than 50%, preferably greater than 80%. The nitric oxide formed in the second reaction step is sent from the pressure vessel to an absorption vessel and reacted with molecular oxygen and water in the absorption vessel to produce an aqueous nitric acid solution, as further described above. Preferably, the nitric oxide formed in the first reaction step is also sent to the absorption vessel.

[0027] The second reaction step is preferably carried out as a batch feed reaction, wherein an aqueous nitric acid solution containing 30% to 70% by weight of nitric acid is fed into the pressure vessel over a period of 1 to 10 hours. The reaction temperature is preferably increased from an initial value in the range of 110°C to 130°C to a final value in the range of 140°C to 180°C during the feeding period. Dosing concentrated nitric acid and increasing the reaction temperature during the dosing process allows for good temperature control of the exothermic oxidation reaction, can reduce the formation of by-products, and keep the reaction volume low, thereby increasing productivity and increasing the yield of 3,3',4,4'-benzophenonetetracarboxylic acid recovered by crystallization.

[0028] The reaction mixture obtained in the second reaction step is preferably cooled to crystallize 3,3',4,4'-benzophenonetetracarboxylic acid, the crystallized 3,3',4,4'-benzophenonetetracarboxylic acid is separated from the residual liquid, preferably by filtration or centrifugation, and the separated residual liquid is sent to the first reaction step to provide at least a portion of the first aqueous nitric acid composition, preferably the entire first aqueous nitric acid composition. The reuse of the residual liquid from the second reaction step as an oxidant in the first reaction step reduces the amount of nitric acid discharged with the effluent of the process and reduces the overall consumption of nitric acid.

[0029] The 3,3',4,4'-benzophenonetetracarboxylic acid separated from the reaction mixture obtained in the pressure vessel is preferably dehydrated to form 3,3',4,4'-benzophenonetetracarboxylic dianhydride. The dehydration can be carried out by any method known in the prior art, for example by heating under reduced pressure or by heating in an inert solvent, as described in US 4,684,738.

[0030] Example

[0031] The oxidation of 1,1-bis-(3,4-dimethylphenyl)-methane was carried out in an oil-heated jacketed 250 ml glass autoclave equipped with a Hastelloy propeller stirrer and a Hastelloy temperature sensor. The autoclave was connected to a stainless steel absorption vessel equipped with a magnetic stirrer and filled with 40 ml of water via a conduit and a pressure regulating valve set to 6.9 bar. 28 g of 1,1-bis-(3,4-dimethylphenyl)-methane, 40 g of water and 10 g of 65 wt % nitric acid were added to the glass autoclave. 150 g of 65 wt % nitric acid was added to a feed container connected to the glass autoclave via a high-pressure pump. The glass autoclave was closed and heated to 100°C with stirring, and then nitric acid was quantitatively added from the feed container at a feed rate of 0.5 ml / min. The reaction temperature was maintained at 110°C to 115°C for 45 minutes, then increased to 145°C within 30 minutes, and the feed rate was increased to 1 ml / h after about 30 minutes. After all the nitric acid in the feed container was added, the temperature was raised to 160°C and the reaction mixture was stirred at this temperature for another 130 minutes and then cooled to room temperature. During the oxidation, oxygen was supplied from a pressure bottle to the absorption container through a manually operated valve to maintain the pressure in the absorption container at a pressure greater than 1 bar and less than 6.9 bar. The solid precipitated after cooling was filtered out, washed with water and dried to obtain 35.8 g of 3,3',4,4'-benzophenonetetracarboxylic acid.

Claims

1. A method for oxidizing 1,1-bis-(3,4-dimethylphenyl)-alkane to 3,3',4,4'-benzophenonetetracarboxylic acid, comprising the steps of: reacting the 1,1-bis-(3,4-dimethylphenyl)-alkane or a partial oxidation product thereof, or a mixture comprising both 1,1-bis-(3,4-dimethylphenyl)-alkane and 3,3',4,4'-tetramethylbenzophenone, with nitric acid in a pressure vessel at a temperature of 120° C. to 220° C., wherein the partial oxidation product is selected from the corresponding 1,1-bis-(3,4-dimethylphenyl)-alkanol and 3,3',4,4'-tetramethylbenzophenone to provide 3,3',4,4'-benzophenonetetracarboxylic acid and simultaneously forming nitric oxide; conveying the nitric oxide from the pressure vessel to an absorption vessel; and reacting the nitric oxide in the absorption vessel with molecular oxygen and water to produce an aqueous nitric acid solution, in, No molecular oxygen was introduced into the pressure vessel. 2 . The method according to claim 1 , wherein the 1,1-bis-(3,4-dimethylphenyl)-alkane is bis-(3,4-dimethylphenyl)-methane.

3. The method according to claim 1 or 2, wherein: The method includes the step of using the aqueous nitric acid solution produced in the absorption vessel to react the 1,1-bis-(3,4-dimethylphenyl)-alkane or a partial oxidation product thereof with nitric acid.

4. The process according to claim 1 or 2, comprising separating 3,3',4,4'-benzophenonetetracarboxylic acid from the reaction mixture obtained in the pressure vessel.

5. The process according to claim 3, comprising separating 3,3',4,4'-benzophenonetetracarboxylic acid from the reaction mixture obtained in the pressure vessel.

6. The method according to claim 1 or 2, wherein: The molecular oxygen is introduced into the absorption vessel via a gas containing at least 98% by volume oxygen.

7. The method according to claim 3, wherein: The molecular oxygen is introduced into the absorption vessel via a gas containing at least 98% by volume oxygen.

8. The method according to claim 4, wherein: The molecular oxygen is introduced into the absorption vessel via a gas containing at least 98% by volume oxygen.

9. The method according to claim 1 or 2, wherein: The reaction is carried out in a batch feed reaction mode, the 1,1-bis-(3,4-dimethylphenyl)-alkane and water or aqueous nitric acid solution are charged into the pressure vessel so that the nitric acid content in the obtained mixture does not exceed 15% by weight, and an aqueous nitric acid solution containing 30% by weight to 70% by weight of nitric acid is fed into the pressure vessel within 1 hour to 10 hours, and the reaction temperature is increased from an initial value in the range of 90°C to 110°C to a final value in the range of 140°C to 180°C during the feeding.

10. The method according to claim 3, wherein: The reaction is carried out in a batch feed reaction mode, the 1,1-bis-(3,4-dimethylphenyl)-alkane and water or aqueous nitric acid solution are charged into the pressure vessel so that the nitric acid content in the obtained mixture does not exceed 15% by weight, and an aqueous nitric acid solution containing 30% by weight to 70% by weight of nitric acid is fed into the pressure vessel within 1 hour to 10 hours, and the reaction temperature is increased from an initial value in the range of 90°C to 110°C to a final value in the range of 140°C to 180°C during the feeding.

11. The method according to claim 4, wherein: The reaction is carried out in a batch feed reaction mode, the 1,1-bis-(3,4-dimethylphenyl)-alkane and water or aqueous nitric acid solution are charged into the pressure vessel so that the nitric acid content in the obtained mixture does not exceed 15% by weight, and an aqueous nitric acid solution containing 30% by weight to 70% by weight of nitric acid is fed into the pressure vessel within 1 hour to 10 hours, and the reaction temperature is increased from an initial value in the range of 90°C to 110°C to a final value in the range of 140°C to 180°C during the feeding.

12. The method according to claim 1 or 2, comprising the steps of: a) reacting 1,1-bis-(3,4-dimethylphenyl)-alkane with a first aqueous nitric acid composition at a temperature of 90° C. to 115° C. to provide a reaction mixture having two liquid phases: an aqueous phase and an organic phase comprising 3,3′,4,4′-tetramethylbenzophenone, b) separating the aqueous phase and the organic phase of step a), and c) reacting the organic phase separated in step b) with a second aqueous nitric acid composition in the pressure vessel at 120° C. to 220° C.

13. The method according to claim 3, comprising the steps of: a) reacting 1,1-bis-(3,4-dimethylphenyl)-alkane with a first aqueous nitric acid composition at a temperature of 90° C. to 115° C. to provide a reaction mixture having two liquid phases: an aqueous phase and an organic phase comprising 3,3′,4,4′-tetramethylbenzophenone, b) separating the aqueous phase and the organic phase of step a), and c) reacting the organic phase separated in step b) with a second aqueous nitric acid composition in the pressure vessel at 120° C. to 220° C.

14. The method according to claim 4, comprising the steps of: a) reacting 1,1-bis-(3,4-dimethylphenyl)-alkane with a first aqueous nitric acid composition at a temperature of 90° C. to 115° C. to provide a reaction mixture having two liquid phases: an aqueous phase and an organic phase comprising 3,3′,4,4′-tetramethylbenzophenone, b) separating the aqueous phase and the organic phase of step a), and c) reacting the organic phase separated in step b) with a second aqueous nitric acid composition in the pressure vessel at 120° C. to 220° C.

15. The process according to claim 12, wherein the reaction mixture of step c) is cooled to crystallize 3,3',4,4'-benzophenonetetracarboxylic acid, and the residual liquid separated from the crystallized 3,3',4,4'-benzophenonetetracarboxylic acid is sent to step a) to provide at least a portion of the first aqueous nitric acid composition.

16. The process according to claim 13, wherein the reaction mixture of step c) is cooled to crystallize 3,3',4,4'-benzophenonetetracarboxylic acid, and the residual liquid separated from the crystallized 3,3',4,4'-benzophenonetetracarboxylic acid is sent to step a) to provide at least a portion of the first aqueous nitric acid composition.

17. The process according to claim 14, wherein the reaction mixture of step c) is cooled to crystallize 3,3',4,4'-benzophenonetetracarboxylic acid, and the residual liquid separated from the crystallized 3,3',4,4'-benzophenonetetracarboxylic acid is sent to step a) to provide at least a portion of the first aqueous nitric acid composition.

18. The method of claim 15, wherein the amount of the second aqueous nitric acid composition added is such as to provide a final concentration of 10 to 30 weight percent nitric acid in the reaction mixture obtained from step c).

19. The method according to claim 12, wherein: Additionally, the nitric oxide formed in step a) is fed to the absorption vessel.

20. The method according to claim 13, wherein: Additionally, the nitric oxide formed in step a) is fed to the absorption vessel.

21. The method according to claim 14, wherein: Additionally, the nitric oxide formed in step a) is fed to the absorption vessel.

22. The method according to claim 12, wherein: Step c) is carried out in a batch feed reaction mode, wherein an aqueous nitric acid solution containing 30 wt% to 70 wt% of nitric acid is fed into the pressure vessel within 1 to 10 hours, and during the feeding, the reaction temperature is increased from an initial value in the range of 110°C to 130°C to a final value in the range of 140°C to 180°C.

23. The method according to claim 13, wherein: Step c) is carried out in a batch feed reaction mode, wherein an aqueous nitric acid solution containing 30 wt% to 70 wt% of nitric acid is fed into the pressure vessel within 1 to 10 hours, and during the feeding, the reaction temperature is increased from an initial value in the range of 110°C to 130°C to a final value in the range of 140°C to 180°C.

24. The method according to claim 14, wherein: Step c) is carried out in a batch feed reaction mode, wherein an aqueous nitric acid solution containing 30 wt% to 70 wt% of nitric acid is fed into the pressure vessel within 1 to 10 hours, and during the feeding, the reaction temperature is increased from an initial value in the range of 110°C to 130°C to a final value in the range of 140°C to 180°C.

25. The process according to claim 1 or 2, wherein 3,3',4,4'-benzophenonetetracarboxylic acid is separated from the reaction mixture obtained in the pressure vessel and dehydrated to form 3,3',4,4'-benzophenonetetracarboxylic dianhydride.

26. The process according to claim 3, wherein 3,3',4,4'-benzophenonetetracarboxylic acid is separated from the reaction mixture obtained in the pressure vessel and dehydrated to form 3,3',4,4'-benzophenonetetracarboxylic dianhydride.

27. The process according to claim 4, wherein 3,3',4,4'-benzophenonetetracarboxylic acid is separated from the reaction mixture obtained in the pressure vessel and dehydrated to form 3,3',4,4'-benzophenonetetracarboxylic dianhydride.

28. The process according to claim 5, wherein 3,3',4,4'-benzophenonetetracarboxylic acid is separated from the reaction mixture obtained in the pressure vessel and dehydrated to form 3,3',4,4'-benzophenonetetracarboxylic dianhydride.

29. The method of claim 12, wherein 3,3',4,4'-benzophenonetetracarboxylic acid is separated from the reaction mixture obtained in the pressure vessel and dehydrated to form 3,3',4,4'-benzophenonetetracarboxylic dianhydride.

30. The process of claim 13, wherein 3,3',4,4'-benzophenonetetracarboxylic acid is separated from the reaction mixture obtained in the pressure vessel and dehydrated to form 3,3',4,4'-benzophenonetetracarboxylic dianhydride.

31. The process of claim 14, wherein 3,3',4,4'-benzophenonetetracarboxylic acid is separated from the reaction mixture obtained in the pressure vessel and dehydrated to form 3,3',4,4'-benzophenonetetracarboxylic dianhydride.