Synthesis Technique of Dimethylacetamide

DMAC is a fine chemical product, the production is relatively small, the annual production of hundreds of thousands of tons. However, from the perspective of the application development of DMAC and the future market demand, the advantages of DMAC will be more and more prominent, and the use will be more and more. The demand for DMacs will grow. At present, there are many synthetic routes, the traditional methods mainly include acetic anhydride, acetyl chloride, acetic acid, trimethylamine carbonylation and so on. In recent years, there are also some new methods for DMAC production research.

Acetic anhydride method

Acetic anhydride reacts with dimethylamine to give dimethylacetamide, the reaction equation is as follows:

Under normal pressure, pure dimethylamine gas is passed into acetic anhydride liquid at room temperature for acylation reaction, the reaction is exothermic reaction, the temperature of the acylation liquid is controlled not to exceed 20℃, and after complete acylation, sodium hydroxide solution is added for neutralization, and sodium acetate is generated. When the PH value of the solution is adjusted to 8~9, sodium acetate is separated, and the remaining neutralizing liquid is washed again with alkali. Add ethyl acetate, azeotrope dehydration to get the crude product, the crude product for rectification, collect the distillate of 164~166.5℃, that is, the pure product DMAC. The production process needs to consume 1.16~1.20 tons of acetic anhydride (95% content) for each ton of DMAC products, while consuming 0.756~0.76 tons of dimethylamine gas. The advantages of this production method are high purity of the product, but its process route is long and the production cost of the product is high.

Acetyl chloride process

Dimethylacetamide is obtained by the reaction of dimethylamine with acetyl chloride:

Firstly, it is ensured that dimethylamine gas is passed into the ether at low temperature, and then the mixture of ether and acetyl chloride is slowly added, slowly added and quickly stirred. The mixed solution will immediately precipitate the white solid of dimethylamine hydrochloride, and the solid will be filtered out until it is completely precipitated. The remaining filtrate was recovered by water bath with incomplete reaction of the ether, the solution was rectified, and the distillate of 164~166.5 ℃ was collected, and the collected product was dimethylacetamide. The acetyl chloride method has the advantage of high yield, but with acetyl chloride as raw material, the production cost is high, and with ether as the reaction solvent, it is difficult to control the use and recovery of solvent.

Acetic acid method

The acetic acid method is a method to produce DMAC by the reaction of acetic acid and dimethylamine. The acetic acid method includes two kinds of catalytic shrinkage method and high compression method, and these two methods are also the main methods for the production of dimethylacetamide in the world. Using acetic acid and dimethylamine as raw materials, the DMAC crude product was synthesized, and then poured into lye for neutralization. After a series of processes such as filtration and distillation, the DMAC product with required purity was obtained. The reaction equation is as follows:

In the acetic acid process, there is a large amount of unreacted acetic acid in the solution after the reaction, resulting in low yield of dimethylacetamide. Because the reactant acetic acid can form a high boiling azeotrope with the product dimethylacetamide (containing 84.19% mass fraction of dimethylacetamide and 15.11% acetic acid), the product cannot be purified by conventional distillation after the reaction. Acetic acid method has the advantages of low production cost and stable process, but due to the presence of acetic acid in the product, it leads to cumbersome post-processing. But this method is suitable for continuous production.

(1) Catalytic shrinkage method

Catalytic shrinkage method is to add catalyst in the reaction to speed up the reaction speed, so as to improve the yield of DMAC and shorten the reaction time. Commonly used catalysts are molybdenum oxide, molybdenum silicate, phosphotungstic acid, tungsten trioxide, sodium metavanadate and so on. Japanese patent JP-B-53-82716 proposed to prepare dimethylacetamide by adding molybdenum oxide to acetic acid and dimethylamine system as catalyst. However, the DMAC yield of this method is still not very high, and the reaction time is not significantly shortened compared with the high compression method [18]. The Chinese patent CN1865233A proposes to use Lewisate as catalyst to catalyze the reaction. The catalyst price is relatively low and the reaction conversion rate is very good.

(2) High compression method

High pressure method is acetic acid and excess dimethylamine reaction at high temperature and high pressure, after the reaction to remove the generated water, to prepare dimethylacetamide. Japanese patent JP-A-6-279375 refers to a method of preparing carboxylic amides by reacting carboxylic acids and amines under high pressure to remove water. The high compression method reduces the separation steps of the catalyst. However, due to the high pressure reaction, the equipment requires acid-resistant materials, and the equipment investment cost is high.

Carbonylation of trimethylamine with carbon monoxide

Relevant materials have been introduced to the production of DMAC by trimethylamine and carbon monoxide, and foreign countries have also studied the high-pressure catalytic method of carbonylation of high purity trimethylamine and carbon monoxide to synthesize dimethylacetamide. N-methylpyrrolidone was used as an additive in the reaction. The conversion rate of trimethylamine is more than 70%, and the selectivity of N, N-dimethylacetamide is as high as 77.7%. The reaction equation is as follows:

This method has actually been proposed for a long time, but there have been no reports of industrialization. As early as 1968, a US patent reported that trimethylamine and CO were used in the presence of cobaltous octacarbonyl catalyst for direct reaction at 175 ~ 250℃, and after separating unreacted trimethylamine, it was almost entirely dimethylacetamide. The production process of this method is simple, the reaction material is cheap and easy to obtain, and the cost is greatly reduced compared with the existing production process. And from the point of view of the reaction itself, this is an interpolated carbonyl reaction, no other products are generated, all atoms are used to generate products, in line with the principle of atomic economy. If this method is industrialized, it will greatly reduce production costs.

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