CN102268687B - Method for preparing benzaldehyde/methylbenzaldehyde by oxidizing toluene/xylene - Google Patents

Abstract

A method for preparing benzaldehyde/methylbenzaldehyde by oxidizing toluene/xylene. The oxidation solution is prepared by placing an electrolytic cell without a diaphragm in an ultrasonic bath, adding electrolyte MnSO ₄ , selecting the PbO ₂ /Pb electrode as the anode, and Pb The electrode is the cathode, and the oxidizing solution is obtained by ultrasonic electrolysis, and its concentration is measured by ferrous titration; the preparation of benzaldehyde/methylbenzaldehyde is to put an appropriate amount of raw material toluene/xylene in the reactor, at a certain temperature and ultrasonic action Next, the prepared oxidation solution was added dropwise to carry out liquid-liquid two-phase oxidation reaction, and at the same time, an appropriate amount of sulfuric acid solution was added to adjust the acidity of the reaction solution. After the reaction, the upper organic phase was separated and distilled to obtain benzaldehyde/methyl Benzaldehyde. The method of the invention has simple and reliable reaction, high material utilization rate, current efficiency and product yield, greatly saves the raw materials used in the reaction and equipment space, and improves the reaction efficiency and product yield.

Description

A kind of method that oxidizes toluene/xylene to prepare benzaldehyde/methylbenzaldehyde

technical field

The invention relates to an ultrasonic indirect electrosynthesis method for synthesizing benzaldehyde compounds, more specifically, a method for preparing benzaldehyde/methylbenzaldehyde by oxidizing toluene/xylene.

Background technique

Benzaldehyde and its derivatives are an important class of organic compounds, which can be used in the synthesis of various chemical products such as drugs, fragrances, cosmetics, dyes and pesticides. Research and theoretical discussion on the method of preparing benzaldehyde compounds not only have good economic benefits, but also have high academic value.

At present, the preparation methods of benzaldehyde compounds mainly include chemical synthesis and electrolytic synthesis.

Chemical synthesis methods are mainly divided into toluene chloromethylation method, formylation method, selective oxidation method of toluene in gas phase or liquid phase, and carboxylic acid reduction method. This kind of method generally has relatively long synthetic route, strong corrosion, relatively large pollution, and low selectivity has always been an insurmountable problem.

The electrolytic synthesis method is a green chemical technology, which has the characteristics of high oxidation reaction selectivity, less side reactions, easy refining, and high product purity. Such as "Modern Chemical Industry", 1996, ₍ 6): "Synthesis of Benzaldehyde by Indirect Electrooxidation" reported on P _33-35 ; Electrooxidative synthesis of three kinds of methylbenzaldehyde"; the patent number is CN 95110505.1, and the title of the invention is "Electrosynthesis method of p-fluorobenzaldehyde" and so on. Such methods generally use redox couples such as Co(II)/Co(III), Ce(III)/Ce(IV), Mn(II)/Mn(III) to prepare a certain concentration of high-valent metals through electrolytic reactions. Ion oxidation solution, and then put the prepared oxidation solution and raw materials in a closed container for heating reaction, the oxidation solution oxidizes raw materials to generate products. After the reaction, the lower aqueous phase can be recycled as an oxidation solution after electrolysis, and the upper organic phase can be separated to obtain corresponding products through distillation, and the unreacted raw materials can be reused. Taking the Ce(III)/Ce(IV) redox pair as an example, the process of Ce(IV) oxidizing toluene to prepare benzaldehyde is:

Electrolysis reaction: anode reaction 2Ce(III)→2Ce(IV)+2e; cathode reaction 2H(I)+2e→H ₂

Chemical reaction: 4Ce(IV)+C ₆ H ₅ CH ₃ +2H ₂ O→C ₆ H ₅ CHO + 4Ce(III)+4H(I)

Although the method of indirect electrosynthesis is relatively mature, there are still some shortcomings, the most notable of which are the large reaction volume and low efficiency. Usually, in order to ensure that the product is not over-oxidized, it is often necessary to add a large amount of excess raw materials and to dilute the oxidation solution by a large multiple. Experimental results show that generating 1mL of benzaldehyde often requires the consumption of hundreds of milliliters of oxidizing solution (literature values are generally 170mL-300mL), which brings a lot of trouble to the separation of products and the reuse of oxidizing solution, which greatly restricts the The prospect of its industrial application.

Contents of the invention

The purpose of the present invention is to seek a kind of practical and efficient synthetic method for preparing benzaldehyde/methylbenzaldehyde by optimizing and improving the method for existing indirect electrosynthesis of benzaldehyde/methylbenzaldehyde, and further solve the problems in existing similar methods. The invention solves the problem of low reaction efficiency caused by too large volume of reaction solution, and provides a method for preparing benzaldehyde/methylbenzaldehyde by oxidizing toluene/xylene.

A kind of ultrasonic indirect electrosynthesis method for preparing benzaldehyde/tolylbenzaldehyde provided by the present invention is divided into two steps, and concrete method is as follows:

(1) Preparation of oxidizing solution

Place the diaphragmless electrolytic cell in an ultrasonic bath, add 0.66mol/L MnSO ₄ solution as the electrolyte, wherein the sulfuric acid concentration is 5.5mol/L; select the PbO ₂ /Pb electrode as the anode, and the Pb electrode as the cathode, and the anode and cathode The area ratio is 5:1, the current density is 950~1100A m ^-2 and the reaction time is 1.84h; at the same time, ultrasonic waves with a frequency of 59KHz and a power of 175W are applied for heterogeneous constant current electrolysis. After the electrolysis is completed, the oxidation solution is obtained It is a mixture of Mn(III) and a small amount of MnO ₂ , uniformly sampled and analyzed, and its concentration (calculated as Mn(III)) is determined by ferrous titration, that is, accurately pipette 5.00mL of the oxidizing solution and place it in a In a 50mL beaker, titrate with the prepared 0.30 mol/L _FeS04 standard solution until the pink color of the solution fades as the end point; according to the volume of the consumed _FeS04 standard solution, calculate according to the formula C _Mn(Ⅲ) =0.30×V _Fe /0.005 The concentration of the oxidizing solution; where C _Mn(Ⅲ) is the concentration of the oxidizing solution (mol/L); V _Fe is the volume of FeS0 ₄ standard solution consumed by titration (L). After the detection is completed, the oxidizing solution is ready for use.

(2) Preparation of Benzaldehyde/Methylbenzaldehyde

Remove the oxidizing solution prepared in the above step (1), rinse the electrolytic cell with 30mL of 7.0mol/L or 6.5mol/L sulfuric acid solution, put the rinse solution in the reactor together with the raw material toluene or xylene, add the raw material The molar ratio of the amount of Mn(III) to the measured oxidizing solution Mn(III) is 4:1~2:1. Under the action of 59KHz and 175W ultrasonic waves at a temperature of 60 ^o C, the prepared oxidizing solution is added dropwise to the reactor for Liquid-liquid two-phase oxidation reaction, while continuously adjusting the acidity of the reaction solution, when the color of the water phase fades, the upper organic phase is separated and distilled to obtain benzaldehyde/methylbenzaldehyde. The product was analyzed by reverse-phase high-performance liquid chromatography. The chromatographic conditions were Kromasil C ₁₈ chromatographic column (5 μm, 250mm×4.6mm id), methanol was the mobile phase, the flow rate was 0.8 mL/min, the column temperature was 40°C, and the detection wavelength was 250 nm, the injection volume is 10.00 μL.

In the above technical solution, the oxidizing solution is added dropwise under the action of ultrasonic waves, and the oxidizing solution is added dropwise in 0.5 s. For every 20 mL of oxidizing solution added, 5 mL of sulfuric acid solution is added, and the oxidizing solution is added dropwise at least twice. , to keep the concentration of sulfuric acid in the reaction solution constant.

In the above technical solution, the oxidizing solution is added dropwise under the action of ultrasonic waves, and the oxidizing solution is added dropwise at an average speed of 0.65mL/min. After adding 20mL of oxidizing solution, add 5mL of sulfuric acid solution, at least twice Add the oxidizing solution dropwise to keep the acidity of the reaction solution basically unchanged.

In this method, under the condition of applying ultrasonic waves, the oxidizing solution with a higher concentration is prepared by the heterogeneous constant current electrolysis method, and the oxidizing solution is added dropwise to oxidize toluene/xylene to prepare benzaldehyde/methylbenzaldehyde, which has a reaction Simple and reliable, with relatively high material utilization rate, current efficiency and product yield.

Compared with the existing technology, the traditional out-of-tank indirect electrosynthesis method prepares benzaldehyde/methylbenzaldehyde, and the current efficiency of the electrolytic oxidation solution is usually between 80% and 85%, and the preparation of 1mL benzaldehyde/methylbenzaldehyde Formaldehyde needs to consume 170～300mL oxidation solution, the yield of benzaldehyde is generally 65～78%, and the yield of methylbenzaldehyde is then 75～80%; Apply the method of the present invention, the current efficiency of electrolysis oxidation solution can reach 89%. % or more, the preparation of 1mL benzaldehyde/methylbenzaldehyde only needs to consume 40-60mL of oxidation solution, and the yield of benzaldehyde can reach more than 78.0%, and the yield of methylbenzaldehyde can reach more than 92.0%. Not only the raw materials and equipment space used in the reaction are greatly saved, but also the reaction efficiency and the yield of the product are improved to a certain extent.

Detailed ways

Further details are given below.

Example 1

Implement the preparation method of benzaldehyde as follows:

The first step is to place the diaphragmless electrolytic cell in an ultrasonic bath, apply ultrasonic waves of 59kHz and 175W, use the PbO ₂ /Pb electrode as the anode, and the Pb electrode as the cathode, and the area ratio of the anode and the cathode is 5:1; Add 9.5g of manganese sulfate monohydrate, dissolve it with an appropriate amount of water, then add 26mL of analytically pure sulfuric acid, adjust the volume of the solution to 85mL with pure water (during this process, manganese sulfate powder will precipitate out), and then apply electrolytic current 0.95A (that is, the current density is 1086A·m ^-2 ) for constant current electrolysis for 1.84h. After electrolysis, take 5.00mL of oxidizing solution, and use ferrous titration to measure its concentration. The concentration is generally 0.60-0.70mol/L in terms of Mn(III).

The function of using the heterogeneous constant current electrolysis technology in this method is that during the electrolysis process, the Mn(II) in the solution is converted into Mn(III), and the manganese sulfate at the bottom of the solution can gradually dissolve into the solution according to the precipitation and dissolution balance, and at the same time As the reaction time increases, a small amount of Mn(III) in the solution will be transformed into MnO ₂ , which can oxidize Mn(II) in the solution to Mn(III). Therefore, this method can prepare a higher concentration of oxidizing solution. The concentration of the oxidizing solution prepared by the usual electrolysis method is generally below 0.4mol/L, but it can reach 0.60-0.70mol/L by this method, and the current efficiency is kept above 89%.

In the second step, transfer about 80 mL of the upper layer solution of the oxidation solution into a constant pressure titration funnel, rinse the electrolytic cell in batches with 30 mL of 7.0 mol/L sulfuric acid solution, pour the washing liquid into a three-necked grinding-mouth flask, and add an appropriate amount of raw material toluene , so that the molar ratio of the measured oxidizing solution Mn(III) is 2:1, then place the ground-mouth flask in an ultrasonic bath, control the temperature of the water bath at 60 ^o C, and apply ultrasonic waves of 59 kHz and 175 W until the color of the bottom liquid Basically fade, and then add the oxidizing solution in the constant pressure titration funnel dropwise into the ground flask four times, add 5mL of sulfuric acid solution for every 20mL of oxidizing solution added, to keep the acidity of the reaction solution basically unchanged, add the oxidizing solution dropwise Observe at the speed of adding one drop until the color of the reaction solution fades and then add another drop. When the oxidation solution is added dropwise, add 5mL of sulfuric acid solution to adjust the acidity of the reaction solution, and make it continue to react until the color of the water phase completely fades, which is the end point. Stop heating and applying ultrasonic waves, and use a separatory funnel to separate the aqueous phase from the organic phase; the aqueous phase is extracted several times with about 10 mL of toluene, and then reserved as a circulating electrolyte for the next reaction; Washing with sodium bicarbonate solution and distilled water until neutral, the product benzaldehyde can be separated by distillation, and the unreacted toluene in the organic phase can be returned to the chemical reaction system for reuse.

In the process of chemical oxidation, the reactivity of Mn(III) in the oxidation solution is relatively high, while the oxidation ability of _MnO2 is relatively strong. In order to ensure that the target product benzaldehyde will not be excessively oxidized into acid, this experiment uses online ultrasonic addition dropwise Adding the oxidizing solution by method not only keeps the Mn(III) concentration in the chemical reaction tank at a low level, but also makes the Mn(II) in the bottom solution react with the high-valent manganese in the oxidizing solution to form Mn( III), so it can not only effectively avoid overoxidation caused by various factors, but also make full use of the oxidation property of high-valent manganese. Moreover, the method does not need to dilute the oxidizing solution at all, thereby solving the problem of low reaction efficiency caused by the excessive volume of the reaction solution, and achieving unexpected effects. The product is analyzed by reverse-phase high-performance liquid chromatography, and its purity can reach more than 99.0%, and the yield of benzaldehyde is 78.39%.

Example 2

Preparation of tolualdehyde: change the raw material of the second step in Example 1 to mixed xylene, adjust the concentration of sulfuric acid to 6.5mol L ^-1 , the molar ratio of xylene to Mn(III) is 2:1, and the rest Each step is the same as example 1, and the yield of final target product tolualdehyde is 92.5%, and its purity can reach more than 99.0%.

Example 3

Recycling of the oxidizing solution: the aqueous phase that is reserved as the circulating electrolyte in the second step of Example 1, by adding an appropriate amount of pure water and manganese sulfate monohydrate, the acidity of the solution and the concentration of manganese sulfate are adjusted so that it is the same as that of the solution in Example 1. The relevant data are consistent; get its 85mL again as electrolyte, and all the other steps are the same as in Example 1. The experimental results show that the current efficiency of the electrolytic oxidation solution is 82.8%, and the current efficiency is 82.0%, 83.9% and 82.8% after being reused three times, with a difference of less than 2%. , The yield of available benzaldehyde is 76.3%.

Claims (1)

1. a kind of method that oxidized toluene/xylene prepares benzaldehyde/methyl benzaldehyde, and its preparation method is as follows: (1) Preparation of oxidizing solution Place the diaphragmless electrolytic cell in an ultrasonic bath, add 0.66mol/L MnSO ₄ solution as the electrolyte, and the concentration of sulfuric acid is 5.5mol/L; select the PbO ₂ /Pb electrode as the anode, and the Pb electrode as the cathode, and the current density is 950 ~1100A·m ^-2 and reaction time 1.84h; at the same time, apply ultrasonic waves with a frequency of 59KHz and a power of 175W for heterogeneous constant current electrolysis. After electrolysis is completed, the oxidizing solution is a mixture of Mn(III) and a small amount of MnO ₂ Liquid, sample analysis, adopt ferrous titration method to measure its concentration, standby; (2) Preparation of Benzaldehyde/Methylbenzaldehyde Remove the oxidizing solution prepared in the above step (1), rinse the electrolytic cell with 30mL of 7.0mol/L or 6.5mol/L sulfuric acid solution, put the rinse solution in the reactor together with the raw material toluene or xylene, add the raw material The molar ratio of the amount of Mn(III) to the measured oxidizing solution Mn(III) is 4:1~2:1. Under the action of 59KHz and 175W ultrasonic waves at a temperature of 60 ^o C, the prepared oxidizing solution is added dropwise to the reactor for Liquid-liquid two-phase oxidation reaction, while continuously adjusting the acidity of the reaction solution, when the color of the water phase fades, the upper organic phase is separated and distilled to obtain benzaldehyde or methylbenzaldehyde; The dropwise addition of the oxidizing solution is under the action of ultrasonic waves, and the oxidizing solution is added dropwise at an average speed of 0.65mL/min. After adding 20mL of the oxidizing solution, 5mL of sulfuric acid solution is added, and the oxidizing solution is added dropwise at least twice. Keep the acidity of the reaction solution substantially constant.