A kind of preparation method of p-nitrobenzoic acid

Abstract

The present application discloses a method for preparing p-nitrobenzoic acid, which comprises: a raw material liquid containing p-nitrotoluene is contacted with an oxidant and an oxidation reaction catalyst to generate an oxidation reaction to obtain p-nitrobenzoic acid; wherein the oxidant comprises methyl nitrite and oxygen. The method can fully utilize methyl nitrite, an important intermediate in the coal-to-ethylene glycol process. Using methyl nitrite and oxygen as oxidants, p-nitrotoluene can be efficiently oxidized to prepare p-nitrobenzoic acid under low temperature and bromine-free conditions.

Description

Technical Field

[0001] The present application relates to a method for preparing p-nitrobenzoic acid, and belongs to the field of fine chemicals. Background Art

[0002] As an important organic synthesis intermediate, p-nitrobenzoic acid is mainly used in the production of medicines, dyes and pesticides. For example, in the pharmaceutical field, it is used to synthesize local anesthetics benzocaine (ethyl p-aminobenzoate) and procaine hydrochloride [ethyl p-aminobenzoate-2-(diethylamino) ester] as well as folic acid and tranexamic acid. It is also an important intermediate for the production of pesticides, dyes (azo dyes, color film) and other chemical products, and has a large market demand at home and abroad. The preparation of p-nitrobenzoic acid has a high technical content and broad application prospects.

[0003] p-Nitrobenzoic acid is an important intermediate for medicine and pesticides. It is the basic raw material for the production of benzocaine, vitamin B and some dyes. In industrial production, p-nitrotoluene is generally used as a raw material and is obtained through oxidation reaction. At present, there are several reasonable oxidation processes at home and abroad: 1) chemical oxidation, 2) catalytic oxidation, 3) electrolytic oxidation, and 4) photooxidation.

[0004] (1) Chemical oxidation method

[0005] Chemical oxidation method refers to the method of preparing p-nitrobenzoic acid by oxidizing p-nitrotoluene with an oxidant other than air or oxygen. Commonly used oxidants are: KMnO4, Na2Cr2O7, MnO2, HNO3, O3, H2O2, etc.

[0006] 1) KMnO4 oxidation method

[0007] KMnO4 is a very strong chemical oxidant that can oxidize in acidic, neutral and alkaline media. The early KMnO4 oxidation method mainly used water as the medium. In water, the yield of p-nitrobenzoic acid was 91% when p-nitrotoluene was oxidized by KMnO4. In recent years, the PTC-KMnO4 oxidation method has developed rapidly. CTAB-KMnO4 is used to oxidize p-nitrotoluene to synthesize p-nitrobenzoic acid. - The reaction rate is greatly improved when the oxidant is transferred into the organic phase, but the yield is low, only 75%. The advantages of this method are mild conditions and high yield, but the oxidant is expensive and the post-processing is troublesome.

[0008] 2) Na2Cr2O7 oxidation method

[0009] Na2Cr2O7 is the most commonly used oxidant. In H2SO4 medium, Na2Cr2O7 oxidizes p-nitrotoluene, and the yield of p-nitrobenzoic acid is 85%. Adding a small amount of Mn salt to the oxidation solution can increase the yield of p-nitrobenzoic acid. For example, adding a catalytic amount of MnSO4 to the Na2Cr2O7 solution increases the yield of p-nitrobenzoic acid to 95%. Compared with the KMnO4 oxidation method, this method has a much lower cost because the by-products can be recycled, but the pollution is still serious.

[0010] 3) MnO2 oxidation method

[0011] MnO2 has a moderate oxidizing ability in an acidic solution. In a 58% H2SO4 medium, p-nitrotoluene is oxidized with MnO2, and the yield of p-nitrobenzoic acid is 87%. Using pyrolusite containing MnO2 ≥ 80% as a catalyst, p-nitrotoluene is oxidized in a 7% H2SO4 medium, and the yield of p-nitrobenzoic acid is 84%. The advantage of this method is that the oxidant cost is low, but the reaction time is fast and the yield is low.

[0012] 4) HNO3 oxidation method

[0013] HNO3 is also a commonly used oxidant, and the oxidizing ability of dilute HNO3 is stronger than that of concentrated HNO3. 7 Pa, 40% to 50% HNO3 was used to oxidize p-nitrotoluene, and the yield of p-nitrobenzoic acid was 90%. After optimization, 35% HNO3 was used at 200℃ and 2.026×10 6 Under Pa, the yield of p-nitrotoluene by oxidation can reach 99%. The advantages of this method are high yield, and because the reduction product of dilute HNO3 is NO, it is easy to remove, so the product purity is high and the post-processing is simple. However, the equipment is severely corroded.

[0014] 5) O3 oxidation method

[0015] Using Co(OAc)2-KBr as catalyst, p-nitrotoluene is oxidized by O3-air to synthesize p-nitrobenzoic acid with a yield of 96.6%. The advantages of this method are high product purity and yield, but the catalyst cost is high, the equipment requirements are high and the preparation is relatively complicated.

[0016] 6) H2O2 oxidation method

[0017] In an autoclave, 5% HBr solution was used as solvent, and the reaction temperature was 200°C. The yield of p-nitrobenzoic acid was 84%. The reaction conditions of this method are basically the same as those of the HNO3 oxidation method, but the yield is not as good as that of the HNO3 oxidation method.

[0018] (2) Catalytic oxidation method

[0019] Catalytic oxidation method refers to the method of synthesizing p-nitrobenzoic acid by oxidizing p-nitrotoluene with air or O2 under the action of a catalyst. It can be divided into air oxidation method and O2 oxidation method.

[0020] 1) Air oxidation method

[0021] In acetic acid medium, with Co(OAc)2.4H2O as catalyst, at 140℃, 2.532×10 6 Under the pressure of Pa, p-nitrotoluene is oxidized by air, and the yield of p-nitrobenzoic acid is 93.6%. Adding bromide, such as KBr or NH4Br, to CCo(OAc)2.4H2O can effectively improve the catalytic ability of the catalyst. Co(OAc)2-CoBr2-Mn(OAc)2-Cr(OAc)3 replaces Co(OAc)2.4H2O as a catalyst, which significantly reduces the reaction temperature. By changing the acidic medium to an alkaline medium and using tetraphenylporphyrin iron as a catalyst, the reaction temperature can be reduced to room temperature, but the yield is only 80%. Replacing the traditional transition metal complex catalyst with an organic solvent is a major improvement in the air oxidation method. Using (CH3O)2CHCH3 or C6H6 as a solvent, under alkaline conditions, p-nitrotoluene is oxidized by air, and the yield of p-nitrobenzoic acid is 80%. The advantages of this method are that the oxidant is cheap and easy to obtain, and the yield is high, but the post-processing is more troublesome and the equipment is severely corroded.

[0022] 2) O2 oxidation method

[0023] In acetic acid medium, with CoBr2.6H2O as catalyst, at 140℃, 1.823×10 6 Pa pressure, using O2 + N2 (7 + 93) mixed gas to oxidize p-nitrotoluene, the yield of p-nitrobenzoic acid is 90%. The catalyst was improved by using CoBr2.6H2O-Co(OAc) 2.4 The mixed catalyst of H2O-Ce(OAc)4-Zr(OAc)4 can significantly reduce the amount of catalyst used. When CoCl2 is used as a catalyst and p-nitrotoluene is oxidized with O2, adding a small amount of PTC, such as hexadecyltrimethylammonium bromide, can accelerate the reaction rate and increase the yield of p-nitrobenzoic acid to 99%. In an alkaline organic solvent, such as C6H6-NaOH or CH3OH-NaOH, p-nitrotoluene is directly oxidized with O2, and the yield of p-nitrobenzoic acid is 85%. Compared with the air oxidation method, this method has a higher cost.

[0024] (3) Electrolytic oxidation method

[0025] Electrolytic oxidation refers to a method of oxidizing p-nitrotoluene to generate p-nitrobenzoic acid under the action of electric current. It can be divided into direct electrolytic oxidation and indirect electrolytic oxidation.

[0026] 1) Direct electrolytic oxidation method

[0027] Direct electrolytic oxidation is a method of synthesizing p-nitrobenzoic acid by directly oxidizing p-nitrotoluene at the anode in the presence of an electrolyte by selecting an appropriate material as the anode. Using 20% ​​to 30% HNO3 as the electrolyte, Pt as the anode, and a current density of 4A / dm 2 When the electrolysis temperature is 100℃, the yield of p-nitrobenzoic acid is 40% by electrolytic oxidation of p-nitrotoluene. The yield of p-nitrobenzoic acid can be increased to 55% by adding some special media such as HSO3F to the electrolyte.

[0028] In recent years, direct electrolytic synthesis of p-nitrobenzoic acid has been explored in some new areas, such as O2 - The electrolytic formation and oxidation mechanism of O2 were studied. Cyclic voltammetry and controlled potential method were used in N,N-dimethylformamide (DMF) medium to confirm the electrolytic generation of O2 - Oxidizes p-nitrotoluene. The introduction of the nitro group activates the methyl group on the benzene ring, making it easy to oxidize. Another view is that p-nitrotoluene is not easily oxidized due to the inhibitory effect of the nitro group. - The oxidation becomes easy when there is. The advantages of this method are simple equipment and easy operation, but the electrode is seriously polluted and the yield is low.

[0029] 2) Indirect electrolytic oxidation method

[0030] Indirect electrolytic oxidation refers to the use of appropriate variable valence ions as the medium for transferring electrons, and high-valence ions as oxidants, so that the low-valence ions generated after the oxidation of p-nitrotoluene return to the anode for regeneration, thereby achieving the purpose of recycling the oxidant. The most commonly used indirect electrolytic oxidation ion pair is Cr2O7 2- / Cr 3+ . When using a lower cell voltage (3.5V) and a higher Cr2(SO4)3 concentration (0.2mol / L), the current efficiency can reach 94.1%. Adding a small amount of phase transfer catalyst (PTC), such as tetrabutylammonium sulfate, to the oxidizing medium greatly increases the reaction rate. The advantages of this method are simple process, high current efficiency and low pollution. However, due to its high power consumption, its development is greatly restricted in areas where power resources are scarce.

[0031] (4) Photooxidation method

[0032] Tetrachlorobenzoquinone (TCBQ) sensitizes the electron transfer oxidation reaction of p-nitrotoluene. In CH3CN solution, using TCBQ as a sensitizer and adding biphenyl (BP) in an equal molar amount to the reactant into the reaction system, p-nitrotoluene can be smoothly oxidized to form p-nitrobenzoic acid. Through fluorescence quenching and photosensitizer BP, anhydrous Mg(ClO4)2, O2 - The effects of the scavenger p-benzoquinone and the electron donor p-dimethoxybenzene on the photooxidation were discussed, and the reaction process was discussed. It is believed that BP plays a co-sensitizing role in the electron transfer between TCBQ and p-nitrotoluene. The advantages of this method are mild conditions and easy control, but the yield is low. Summary of the invention

[0033] Ethylene glycol is a major petrochemical product, mainly used in antifreeze and the preparation of PET polyester. Ethylene glycol is currently mainly produced using a petrochemical technology route: ethylene is used as a raw material to prepare ethylene oxide through epoxidation, and then hydrated to prepare ethylene glycol; this technology consumes a large amount of petroleum resources and consumes a lot of energy for dehydration and separation. The development and application of coal chemical technology has provided a new technical route for the production of ethylene glycol through non-petroleum routes. In the coal-to-ethylene glycol process, methyl nitrite is an important intermediate, which exists in large quantities in the ethylene glycol preparation process and the by-product dilute nitric acid treatment process. It has the characteristics of wide sources, low prices and easy availability. Therefore, the stoichiometric oxidation of p-nitrotoluene to p-nitrobenzoic acid through the oxidizing properties of methyl nitrite is a new way to prepare p-nitrobenzoic acid. It has very important practical significance for reducing the preparation cost of p-nitrobenzoic acid and improving the comprehensive utilization of coal-to-ethylene glycol intermediates.

[0034] According to one aspect of the present application, a method for preparing p-nitrobenzoic acid is provided, which can fully utilize methyl nitrite, an important intermediate in the process of coal-to-ethylene glycol. By using methyl nitrite as an oxidant, p-nitrotoluene can be efficiently converted into p-nitrobenzoic acid under mild conditions using a non-precious metal catalyst.

[0035] Optionally, the preparation method of p-nitrobenzoic acid at least includes: a raw material liquid containing p-nitrotoluene is contacted with an oxidant and an oxidation reaction catalyst to cause an oxidation reaction to produce p-nitrobenzoic acid; wherein the oxidant includes methyl nitrite and oxygen.

[0036] The method can make full use of the intermediates in the coal-to-ethylene glycol process and realize the co-production with the coal-to-ethylene glycol process. The method can realize the oxidation of p-nitrotoluene by methyl nitrite to prepare p-nitrobenzoic acid, and the conversion rate of p-nitrobenzoic acid is high.

[0037] Optionally, the methyl nitrite gas comes from a coal-to-ethylene glycol process.

[0038] Optionally, the molar ratio of methyl nitrite to oxygen is 1 to 10;

[0039] Optionally, the molar ratio of methyl nitrite to oxygen is 2 to 5;

[0040] Optionally, the upper limit of the molar ratio of methyl nitrite to oxygen can be independently selected from 2:1, 5:1, 10:1, and the lower limit can be independently selected from 1:1, 2:1, 5:1.

[0041] Optionally, in the raw material solution, the solvent is a mixture of acetic acid and butyraldehyde, wherein the molar ratio of acetic acid to butyraldehyde is 1 to 5;

[0042] Optionally, the molar ratio of acetic acid to butyraldehyde is 2 to 4;

[0043] Optionally, the upper limit of the molar ratio of acetic acid to butyraldehyde can be independently selected from 2:1, 3:1, 4:1, 5:1, and the lower limit can be independently selected from 1:1, 2:1, 3:1, 4:1.

[0044] Optionally, in the raw material liquid, the concentration of p-nitrotoluene is 10 to 40 wt %;

[0045] Optionally, the concentration of p-nitrotoluene is 25 wt %;

[0046] Optionally, the upper limit of the concentration of p-nitrotoluene can be independently selected from 20wt%, 25wt%, 30wt%, 35wt%, 40wt%, and the lower limit can be independently selected from 10wt%, 20wt%, 25wt%, 30wt%, 35wt%.

[0047] Optionally, the oxidation reaction catalyst is a transition metal salt.

[0048] Optionally, the transition metal salt is selected from at least one of Co(OAC)2, Mn(OAC)2, Cu(OAC)2, Zn(OAC)2, and Cr(OAC)3.

[0049] Optionally, the mass ratio of the oxidation reaction catalyst to the raw material liquid is 500 to 10000 ppm;

[0050] Optionally, the mass ratio of the oxidation reaction catalyst to the raw material liquid is 1500 to 8000 ppm;

[0051] Optionally, the upper limit of the mass ratio of the oxidation reaction catalyst to the raw material liquid can be independently selected from 1000ppm, 1500ppm, 3000ppm, 5000ppm, 8000ppm, and 10000ppm, and the lower limit can be independently selected from 500ppm, 1000ppm, 1500ppm, 3000ppm, 5000ppm, and 8000ppm.

[0052] As a specific embodiment, the method for preparing p-nitrobenzoic acid at least comprises: contacting a raw material liquid containing an oxidant of methyl nitrite and oxygen and p-nitrotoluene with an oxidation reaction catalyst in a reactor to carry out an oxidation reaction to obtain p-nitrotoluene.

[0053] Optionally, the reactor is a tank reactor;

[0054] Optionally, the reactor is a pressure-resistant autoclave reactor with mechanical stirring.

[0055] Optionally, the reaction temperature of the oxidation reaction is 60-120° C., the reaction pressure is 0.2-2.0 MPa, and the reaction time is 1.0-5.0 h.

[0056] Optionally, the upper limit of the reaction temperature can be independently selected from 80°C, 100°C, 110°C, 120°C, and the lower limit can be independently selected from 60°C, 80°C, 100°C, 110°C;

[0057] Optionally, the upper limit of the reaction pressure can be independently selected from 0.4Mpa, 0.6Mpa, 0.8Mpa, 1Mpa, 1.2Mpa, 1.5Mpa, 2.0Mpa, and the lower limit can be independently selected from 0.2Mpa, 0.4Mpa, 0.6Mpa, 0.8Mpa, 1Mpa, 1.2Mpa, 1.5Mpa;

[0058] Optionally, the upper limit of the reaction time can be independently selected from 2h, 3h, 4h, 5h, and the lower limit can be independently selected from 1h, 2h, 3h, 4h;

[0059] Optionally, the reaction temperature of the oxidation reaction is 80-110° C., the reaction pressure is 0.4-1.5 MPa, and the reaction time is 2.0-4.0 h.

[0060] The beneficial effects of this application include:

[0061] 1) The present application provides a method for preparing p-nitrobenzoic acid, which uses methyl nitrite and oxygen as oxidants to oxidize a raw material containing p-nitrotoluene to p-nitrobenzoic acid through catalytic oxidation. The method is environmentally friendly, easy to operate, has a high conversion efficiency of p-nitrobenzoic acid, and p-nitrobenzoic acid is easily separated from the reaction raw materials. The oxidant used in the method can be methyl nitrite, an intermediate product in the process of coal-to-ethylene glycol, which is convenient for large-scale industrial production in conjunction with existing coal-to-ethylene glycol.

[0062] 2) The method for preparing p-nitrobenzoic acid provided in the present application adopts intermittent reaction and uses a kettle reaction device, which is conducive to reducing production costs and is easy to realize industrial production. The entire reaction process is carried out under mild conditions and does not require the addition of halogen. The requirements for equipment in the reaction are greatly reduced, and it has broad industrial application prospects.

[0063] 3) As a nitrogen oxide, methyl nitrite has a more efficient oxidation performance than molecular oxygen. DETAILED DESCRIPTION

[0064] The present application is described in detail below with reference to embodiments, but the present application is not limited to these embodiments.

[0065] Unless otherwise specified, the raw materials and catalysts in the examples of the present application were purchased from commercial sources, and all reagents used were of analytical grade.

[0066] In the embodiment, methyl nitrite gas comes from a coal-to-ethylene glycol unit; acetic acid and butyraldehyde are purchased from Tianjin Komio Reagent Company; p-nitrotoluene is purchased from Shanghai Aladdin Reagent Company; Co(OAC)2, Mn(OAC)2, Cu(OAC)2, Zn(OAC)2, and Cr(OAC)3 are purchased from Shanghai Sinopharm Reagent Company.

[0067] In the examples, the oxidation catalyst was dried with air at 200° C. for 2 to 12 hours before use.

[0068] In the embodiment, the catalytic p-nitrotoluene oxidation to p-nitrobenzoic acid reaction is carried out in a pressure-resistant autoclave reactor, and the catalyst, p-nitrotoluene, and reaction solvent are added to the autoclave reactor in proportion. The reaction device is heated to a certain temperature, and a mixed gas of methyl nitrite and oxygen is introduced to the reaction pressure for oxidation reaction. After the reaction is completed, the autoclave is vented and opened. The obtained solid-liquid two-phase product is completely dissolved in ethanol to a fixed volume. The contents of p-nitrotoluene and p-nitrobenzoic acid are analyzed by gas chromatography equipped with a thermal conductivity detector, and the conversion rate of p-nitrotoluene and the yield of p-nitrobenzoic acid are calculated by the internal standard method.

[0069] The conversion rate and yield in the examples of this application are calculated as follows:

[0070] Conversion rate of p-nitrotoluene = (number of moles of p-nitrotoluene before reaction - number of moles of p-nitrotoluene after reaction) / number of moles of p-nitrotoluene before reaction * 100%

[0071] Yield of p-nitrobenzoic acid = moles of p-nitrobenzoic acid / moles of p-nitrotoluene converted * 100%

[0072] Example 1 Reaction of p-nitrotoluene to p-nitrobenzoic acid catalyzed by different transition metal catalysts

[0073] Different transition metal catalysts were added into the reactor to verify the catalytic activity.

[0074] 20g of p-nitrotoluene, 80g of solvent (molar ratio of acetic acid to butyraldehyde is 3:1); amount of catalyst added: the mass ratio of p-nitrotoluene solution (the sum of the mass of p-nitrotoluene and solvent) is 5000ppm; the oxidant composition is methyl nitrite and oxygen, the molar ratio is 5:1; the reaction temperature is 100°C, the reactor pressure is 1.0MPa; the reaction time is 2 hours.

[0075] The results obtained on different transition metal catalysts are shown in Table 1.

[0076] Table 1 Oxidation of p-nitrotoluene to p-nitrobenzoic acid over different transition metal catalysts

[0077]

[0078] As can be seen from Table 1, among different transition metal catalysts, Co(OAC)2 exhibits higher p-nitrotoluene oxidation conversion rate and p-nitrobenzoic acid yield.

[0079] Example 2 Effect of different catalyst dosages on the catalytic oxidation of p-nitrotoluene to p-nitrobenzoic acid

[0080] Different masses of Co(OAC)2 were added into the reactor to verify the catalytic activity.

[0081] 20g of p-nitrotoluene, 80g of solvent (acetic acid and butyraldehyde in a molar ratio of 3:1); the oxidant composition is methyl nitrite and oxygen in a molar ratio of 5:1. The reaction temperature is 100°C, the reactor pressure is 1.0MPa, and the reaction time is 2 hours.

[0082] The results obtained with different catalyst dosages (mass ratio to p-nitrotoluene solution) are shown in Table 2.

[0083] Table 2 Effect of catalyst dosage on catalytic oxidation of p-nitrotoluene to p-nitrobenzoic acid

[0084]

[0085]

[0086] As can be seen from the results in Table 2, in the reaction of catalytic oxidation of p-nitrotoluene to p-nitrobenzoic acid using Co(OAC)2 catalyst, increasing the amount of catalyst can significantly improve the reaction conversion rate. When the catalyst feed amount is greater than 3000ppm, good results are obtained, the conversion rate of p-nitrotoluene is above 80%, and the yield of p-nitrobenzoic acid is above 85%.

[0087] Example 3 Effect of methyl nitrite and oxygen content in the oxidant on the reaction

[0088] Mixed gases of methyl nitrite and oxygen (ester-oxygen ratio) in different molar ratios were added into the reactor to verify the catalytic activity.

[0089] 20g of p-nitrotoluene, 80g of solvent (molar ratio of acetic acid to butyraldehyde is 3:1); the catalyst is Co(OAC)2, the added amount is 5000ppm relative to the mass of p-nitrotoluene solution (the sum of the mass of p-nitrotoluene and the solvent); the reaction temperature is 100°C, the reactor pressure is 1.0MPa, and the reaction time is 2 hours.

[0090] The reaction results of different ester-oxygen ratios are shown in Table 3.

[0091] Table 3 Results of oxidation of p-nitrotoluene to p-nitrobenzoic acid under different ester-oxygen ratios

[0092]

[0093] As can be seen from the results in Table 3, the molar ratio of methyl nitrite to oxygen has a significant effect on the conversion rate of p-nitrotoluene and the yield of p-nitrobenzoic acid. As the ester-oxygen ratio increases, the conversion rate of p-nitrotoluene gradually decreases. The reason may be that the reduction in oxygen content affects the activation process of the substrate in the oxidation reaction, thereby reducing the conversion rate. In addition, as the ester-oxygen ratio increases, the yield of p-nitrobenzoic acid increases significantly, which shows that compared with oxygen, methyl nitrite has higher reaction activity, is more conducive to the production of the target product, and reduces the content of intermediate products.

[0094] Example 4 Effect of acetic acid and butyraldehyde content on the reaction

[0095] Acetic acid and butyraldehyde in different molar ratios were added into the reactor as solvent to verify the catalytic activity.

[0096] 20g of p-nitrotoluene and 80g of solvent; the catalyst is Co(OAC)2, and the added amount is: the mass ratio of p-nitrotoluene solution (the sum of the mass of p-nitrotoluene and the solvent) is 5000ppm; the oxidant composition is methyl nitrite and oxygen, with a molar ratio of 5:1; the reaction temperature is 100°C, the reactor pressure is 1.0MPa, and the reaction time is 2 hours.

[0097] The reaction results of acetic acid and butyraldehyde in different molar ratios as solvents are shown in Table 4.

[0098] Table 4 Different molar ratios of acetic acid and butyraldehyde as solvents for catalytic oxidation of p-nitrotoluene to p-nitrobenzoic acid

[0099]

[0100] From the results in Table 4, it can be seen that different molar ratios of acetic acid and butyraldehyde have a significant effect on the conversion rate of nitrotoluene. A too small acid-aldehyde ratio will affect the dispersion of the catalyst and reactants in the system, while a too large acid-aldehyde ratio will affect the activation of the substrate and the initiation of the oxidation reaction.

[0101] Example 5 Effect of reaction temperature

[0102] The catalytic activity was verified by changing the reaction temperature.

[0103] 20g of p-nitrotoluene, 80g of solvent (molar ratio of acetic acid to butyraldehyde is 3:1); the catalyst is Co(OAC)2, the addition amount is 5000ppm relative to the mass of p-nitrotoluene solution (the sum of the mass of p-nitrotoluene and the solvent); the oxidant is composed of methyl nitrite and oxygen, with a molar ratio of 5:1; the reactor pressure is 1.0MPa, and the reaction time is 2 hours.

[0104] The reaction results at different temperatures are shown in Table 5.

[0105] Table 5 Oxidation of p-nitrotoluene to p-nitrobenzoic acid at different reaction temperatures

[0106]

[0107]

[0108] From the results in Table 5, it can be seen that the reaction temperature has a significant effect on the conversion rate of p-nitrotoluene. As the reaction temperature increases, the conversion rate of p-nitrotoluene gradually increases and then remains constant. The yield of p-nitrobenzoic acid decreases as the temperature increases. The optimal reaction temperature is 100°C.

[0109] Example 6 Effect of reaction pressure

[0110] The catalytic activity was verified by changing the reaction pressure.

[0111] 20g of p-nitrotoluene, 80g of solvent (molar ratio of acetic acid to butyraldehyde is 3:1); the catalyst is Co(OAC)2, the addition amount is 5000ppm relative to the mass of p-nitrotoluene solution (the sum of the mass of p-nitrotoluene and the solvent); the oxidant is composed of methyl nitrite and oxygen, with a molar ratio of 5:1; the reaction temperature is 100°C, and the reaction time is 2 hours.

[0112] The reaction results under different pressures are shown in Table 6.

[0113] Table 6 Oxidation of p-nitrotoluene to p-nitrobenzoic acid under different reaction pressures

[0114]

[0115] It can be seen from the results in Table 6 that the reaction pressure has a significant effect on the conversion rate of p-nitrotoluene. With the increase of reaction pressure, the conversion rate of p-nitrotoluene increases significantly. When the pressure exceeds 1.0 MPa, the yield of p-nitrotoluene decreases significantly with the increase of pressure, which may be due to the increase of by-products caused by the increase of oxidant content.

[0116] Example 7 Effect of reaction time

[0117] The catalytic activity was verified by varying the reaction time.

[0118] 20g of p-nitrotoluene, 80g of solvent (molar ratio of acetic acid to butyraldehyde is 3:1); the catalyst is Co(OAC)2, and the added amount is 5000ppm relative to the mass of p-nitrotoluene solution (the sum of the mass of p-nitrotoluene and the solvent); the oxidant is composed of methyl nitrite and oxygen, with a molar ratio of 5:1; the reaction temperature is 100°C, and the reaction pressure is 1.0MPa.

[0119] The reaction results under different pressures are shown in Table 7.

[0120] Table 7 Oxidation of p-nitrotoluene to p-nitrobenzoic acid at different reaction times

[0121]

[0122] As shown in Table 7, prolonging the reaction time can improve the conversion rate of p-nitrotoluene to a certain extent. However, for the yield of p-nitrobenzoic acid, there is a process of first increasing and then decreasing. This may be caused by the competition between the conversion of the intermediate product and the side reaction.

[0123] The above are only a few embodiments of the present application and do not constitute any form of limitation to the present application. Although the present application is disclosed as above with preferred embodiments, it is not intended to limit the present application. Any technician familiar with the profession, without departing from the scope of the technical solution of the present application, using the technical content disclosed above to make slight changes or modifications are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. A method for preparing p-nitrobenzoic acid, characterized in that: The preparation method comprises: a raw material liquid containing p-nitrotoluene is contacted with an oxidant and an oxidation reaction catalyst to generate an oxidation reaction to obtain p-nitrobenzoic acid; Wherein, the oxidant comprises methyl nitrite and oxygen; The oxidation reaction catalyst is a transition metal salt; The transition metal salt is selected from at least one of Co(OAC)2, Mn(OAC)2, Cu(OAC)2, Zn(OAC)2, and Cr(OAC)3.

2. The preparation method according to claim 1, characterized in that: The molar ratio of the methyl nitrite to the oxygen is 1-10.

3. The preparation method according to claim 1, characterized in that: The molar ratio of the methyl nitrite to the oxygen is 2-5.

4. The preparation method according to claim 1, characterized in that: The methyl nitrite gas comes from the coal-to-ethylene glycol process.

5. The preparation method according to claim 1, characterized in that: In the raw material liquid, the solvent is a mixture of acetic acid and butyraldehyde, wherein the molar ratio of acetic acid to butyraldehyde is 1-5.

6. The preparation method according to claim 5, characterized in that: In the raw material liquid, the molar ratio of acetic acid to butyraldehyde is 2-4.

7. The preparation method according to claim 1, characterized in that: In the raw material liquid, the concentration of p-nitrotoluene is 10-40wt%.

8. The preparation method according to claim 1, characterized in that: In the raw material liquid, the concentration of p-nitrotoluene is 25wt%.

9. The preparation method according to claim 1, characterized in that: The mass ratio of the oxidation reaction catalyst to the raw material liquid is 500 to 10000 ppm.

10. The preparation method according to claim 1, characterized in that: The mass ratio of the oxidation reaction catalyst to the raw material liquid is 1500 to 8000 ppm.

11. The preparation method according to claim 1, characterized in that: The oxidation reaction is carried out in a tank reactor.

12. The preparation method according to claim 1, characterized in that: The reaction temperature of the oxidation reaction is 60-120° C., the reaction pressure is 0.2-2.0 MPa, and the reaction time is 1.0-5.0 h.

13. The preparation method according to claim 1, characterized in that: The reaction temperature of the oxidation reaction is 80-110° C., the reaction pressure is 0.4-1.5 MPa, and the reaction time is 2.0-4.0 h.

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