Issue 24/2014 - Advances in cyclohexene catalytic oxidation synthesis of adipic acid

Adipic acid is an important raw material for the production of nylon 66 fiber, nylon 66 engineering plastics, polyurethane foam, etc.
, at present, almost all foreign adipic acid manufacturers are using cyclohexanol and cyclohexanone mixture as raw materials nitric acid oxidation process route, its yield and selectivity are high, but the equipment corrosion is serious, and the production of 1 ton of products consumes 68% of nitric acid 1.
3 tons, the production of nitric oxide and nitric acid vapor and waste acid liquid seriously pollute the environment
。 Hydrogen peroxide (H2O2) is an ideal clean oxidant, the only expected by-product of its reaction is water, and post-reaction treatment is easy; On the other hand, the molecular structure of cyclohexene contains active double bonds, which can be directly generated after opening, and cyclohexene can be mass-produced
with the development of benzene partial catalytic hydrogenation process.
Therefore, with hydrogen peroxide as the oxidant, the direct oxidation of cyclohexene to produce adipic acid has become a hot spot, and the key is the research and development
of catalytic systems.
The current catalysts mainly include tungstic acid, tungstate, tungsten trioxide and heteropoly acid
.

Tungstic acid and tungstate catalyze the synthesis of adipic acid

It was found that tungstic acid alone was used as a catalyst for low activity, while tungstic acid/inorganic acid ligands (such as phosphoric acid, boric acid, etc.
) were used as catalytic systems under the same conditions, and the yield of adipic acid was higher
without organic solvents and phase transfer agents.
Cao Fabin of Liaoning Shihua University et al.
catalyze the synthesis of adipic acid from 30% cyclohexene peroxide without organic solvents and phase transfer agents using tungsten acid/organic acid
additives as a catalytic system.
The results showed that the catalytic effect of catalyzing the oxidation of cyclohexene by tungstic acid/resorcinol was the best, and the separation yield of adipic acid was 90.
9% and the purity was close to 100% within 8h.
Without the use of organic acid additives, the separation yield of adipic acid is only 72.
1%, and the purity of the product is 96.
2%.

When sulfonic acid salicylic acid is used as an organic acid additive, adipic acid separation yield and purity are high
.

Because cyclohexene is the oil phase, the oxidant H2O2 and the catalyst tungstate are in the aqueous phase, and the water-soluble perate must enter the oil phase and be in full contact with the substrate to react
.
Therefore, in the reaction, the method of adding phase transfer catalyst can generally improve the activity
of tungstic acid compounds.
Commonly used phase transfer catalysts are [CH3(n-C8H17)3N]HSO4, benzyltriethylammonium chloride, cetyl trimethylammonium bromide, oxalic acid, salicylic acid, ascorbic acid and 8-hydroxyquinoline
.
Jia Zhijian et al.
, School of Chemical Engineering and Environment, North University of China, used the complexes formed by sodium tungstate dihydrate and different ligands as catalysts, and synthesized adipic acid
with 30% cyclohexene hydroxide under the action of phase transfer agent.
In phosphoric acid as ligand, PEG600 as phase transfer catalyst, 220mmol 30% and H2O2 oxidation 50mmol cyclohexene, and n(Na2WO4·2H2O): n(H3PO4): n(cyclohexene)=1∶2∶20, conventional reflux 3.
0h, adipic acid yield of 67.
4%.

After repeated use of sodium tungstate dihydrate and hydrogen peroxide catalytic oxidation system for 5 times, the separation yield of adipic acid was still high
.

It was found that when cyclohexene is directly oxidized to prepare adipic acid by using quaternary ammonium decametungstate as a catalyst, the catalyst used is insoluble in water, but under the action of hydrogen peroxide, it can participate in the reaction of reactive oxygen species transfer and dissolve in the reaction system
.
When the hydrogen peroxide is consumed, the catalyst precipitates again
.
Zhang Yingwei et al.
of Beijing Xuyang Chemical Technology Research Institute Co.
, Ltd.
have developed a method
involving silica or polystyrene resin solidified decacanic tungstate catalyst and its catalytic oxidation of cyclohexene to synthesize adipic acid.
The catalyst can use hydrogen peroxide as an oxidizing agent under certain conditions to catalyze the oxidation of cyclohexene to synthesize adipic acid
.
The main features of this method are that the organic solvent is not used as the medium, the catalyst used has good stability, easy to separate, reduces the corrosion of the equipment, the yield of adipic acid can reach 90%~95%, the purity is greater than 99%, and the recovery rate of the catalyst is greater than 90%.

In recent years, room temperature ionic liquids have shown good application prospects
in catalyzing organic reactions due to their advantages of low toxicity, good thermal stability, unique solubility and simple separation of reaction products.
At present, imidazolyl acidic ionic liquids have been successfully used in
the synthesis of adipic acid.
Under the condition of no organic solvents and halogens, Yang Xuegang, Beijing Xuyang Chemical Industry Technology Research Institute Co.
, Ltd.
uses hydrogen peroxide as oxidant, Na2WO4·2H2O as catalyst, and synthetic amino acid ionic liquid as cocatalyst to oxidize cyclohexene to synthesize adipic acid
.
The results showed that phenylalanine bisulfate ionic liquid had the best
catalytic effect when used as a ligand.
Under the optimal process conditions, the conversion rate of cyclohexene is 100%, and the total yield of adipic acid is 87.
18%.

Wang Xiaodan of Shenyang University of Chemical Technology et al.
catalyzed the oxidation of cyclohexene to synthesize adipic acid under the condition of no organic solvents and halogens, using tungsten acid/acid ionic liquid as the catalytic system and hydrogen peroxide with a mass fraction of 30% as the oxidant, when N (cyclohexene): n (tungsten acid): n (N-methylimidazolium bisulfate): n (hydrogen peroxide) = 50: 1: 5: 5: 220 (ionic liquid dosage is 10mmol), the reflux reaction is 7.
5h, and the separation yield of adipic acid can reach 90.
5
。 The ionic liquid is cheap and easy to prepare, environmentally friendly and reusable
.

Tungstic acid trioxide catalyzes the synthesis of adipic acid

Tungsten trioxide is neither soluble in water nor in acid, but can react with hydrogen peroxide to generate tungsten peroxide acid, which brings active oxygen species to the active component while bringing acidity to the solution, and it can also receive better results
in the synthesis of adipic acid catalyzed by cyclohexene as a catalyst.
For example, Yan Song of Liaoning Shihua University and others reacted under the condition of only using tungsten trioxide as a catalyst, the yield of adipic acid was 75.
4%, the catalyst was reused 4 times, and the separation yield of adipic acid could still reach more than
70%.
This method not only overcomes the separation problem of catalyst, but also clean, cheap and easy to obtain raw materials, green environmental protection, simple process and easy operation
.

Heteropoly acids and their salts

Catalytic synthesis of adipic acid

Among the catalysts, because heteropoly acid has unique quasi-liquid phase behavior, strong thermal stability and other advantages, more and more attention, among which phosphotungstic acid, phosphotungstic acid quaternary ammonium salt and supported phosphotungstic acid as catalysts, cyclohexene hydroxide hydroxide can obtain adipic acid
with higher yield.
For example, phosphotungstic acid has a unique "false liquid phase" behavior, the existence of this behavior allows the reaction to be carried out not only on the surface of the catalyst, but also in the entire catalyst phase, so that phosphotungstic acid has high catalytic activity
.
If oxalic acid is added, the yield can be further increased
.
This is because the addition of oxalic acid enhances the catalytic activity of phosphotungstic acid
.
However, the use of heteropoly acid alone can not achieve a good catalytic effect, and the system also needs to add cocatalysts, such as surfactants, various acidic ligands or phase transfer catalysts
.

Wang Xiaodan of Shenyang University of Chemical Technology et al.
synthesized glutamic acid heteropolyacid ([HGlu]PTA) catalyst by reacting L-glutamic acid and phosphotungstic acid, and used it to catalyze the oxidation of cyclohexene to synthesize adipic acid
.
The results showed that the separation yield of adipic acid could reach 94.
76%
under the conditions of cyclohexene being 100mmol, 30% hydrogen peroxide 44.
5mL, [HGlu]PTA 5mmol, reflux temperature 90°C, and reaction time of 9h.
[HGlu] After 4 repeated reuses of PTA catalyst, the yield of adipic acid can still reach more than
80%.

Tang Qin of Jiangsu Yangnong Chemical Group Co.
, Ltd.
used the synthetic compound quaternary ammonium phosphotungstate as the catalyst and 35% H2O2 aqueous solution as the oxygen source to catalyze the synthesis of adipic acid
from cyclohexene.
Under the conditions of reaction temperature 95 °C, reaction time 6h, cyclohexene dosage 8.
2g, 35% H2O2 dosage 42.
7g and catalyst dosage 1.
33g, adipic acid yield of 85.
0%, purity of 99.
6%, catalyst recovery rate of 88%.

Jiang Zhaobo of Dongying Vocational College of Shandong Province prepared a supported heteropoly acid catalyst with phosphotungstic acid as the catalytic activity center and lanthanum-modified quasi-thin baux-gzaesite as the carrier, which has good solid strength and repeated use performance, and the yield of catalytic oxidation of cyclohexene to synthesize adipic acid is close to that of the liquid phosphotungstic acid catalytic system (about 75%)
.

Li Huiyun, Department of Chemistry, Henan Anyang Normal University, et al.
used phosphotungstic acid to catalyze the synthesis of cyclohexene hydroxide hydroxide under the condition of phase-free transfer agent
.
In cyclohexene: hydrogen peroxide: phosphotungstic acid: oxalic acid = 1∶4.
0∶0.
002∶0.
0315 (molar ratio), the reaction at 80 °C for 8h, the yield can reach 87.
7%.

The addition of oxalic acid significantly increased
the yield of adipic acid.

epilogue

Using tungstic acid and tungstate, tungsten trioxide and heteropoly acid as catalysts, adipic acid products obtained by cyclohexene peroxide are of high purity, do not need further purification, and will not produce waste acid and smoke that pollute the environment, and the reaction conditions are mild and easy to control, so it is a good way to
clean and synthesize adipic acid.
However, because the nature of hydrogen peroxide is not very stable, decomposition may occur under reaction conditions, and the price is relatively high, when used for industrial generation, the generation cost must be considered, so how to reduce the amount of hydrogen peroxide and improve its utilization rate is a key problem
that needs to be solved in the future.

For specific catalysts, different catalysts have their own characteristics
.
In the absence of additives, heteropoly acid catalysts such as phosphotungstic acid are better catalysts, with less dosage and high adipic acid yield, but their price is expensive
.
The relatively inexpensive tungstate (sodium tungstate, etc.
) as a catalyst requires the presence of additives, phase transfer agents and a variety of acidic ligands are good additives
.
In addition, ionic liquids are also a direction that
should be developed in the future.