Application of Micromixer in Synthesis of Methyl Mercaptoacetate

Application of Micromixer in Synthesis of Methyl Mercaptoacetate

Introduction

Methyl thioglycolate is an important intermediate for the synthesis of medicines, pesticides, food flavors and tobacco flavors. It can be used as a heat stabilizer for PVC organotin, a molecular weight regulator for polymers, and a polymercaptoacetic acid methyl ester can also be used as a low temperature curing agent for epoxy resins, optical lenses and manufacturing adhesives. However, our country has only begun to study the synthesis method of thioglycolic acid and its derivatives since the 1970s. Its research is relatively lagging behind, the production process is relatively backward, the environmental pollution is serious, and most of the products with high cost and high purity still need to be imported. There are some urgent problems to be solved.

Research progress of methyl thioglycolate

Anhui Fengle Agrochemical Co., Ltd. has successfully synthesized this product and used it to further synthesize the herbicide pesticide 3- (4-methoxy-6-methyl-1,3,5-triazine-2-ylaminosulfonyl) thiophene-2-carboxylic acid, namely thiophensulfuron. Guizhou Institute of Chemical Technology Yisheng Fine Chemical Co., Ltd. has also successfully synthesized this product and used it to further synthesize fragrance raw materials: 3-carbonyl-2-methylhydrothiophenecarboxylate methyl ester and 4-carbonyl-2-tetrahydrothiophenecarboxylate methyl ester [3].

The current production and synthesis methods of thioglycolate methyl ester are mainly batch synthesis methods using thioglycolic acid and methanol as raw materials and sulfuric acid as catalyst [4], but there are many problems such as low yield, long reaction time and serious equipment corrosion. Therefore, it is urgent to find an efficient synthesis process of thioglycolate methyl ester.

2. Advantages of micromixers in esterification reactions

A micromixer is a three-dimensional structural element fabricated on a solid substrate by means of special microfabrication techniques that can be used for chemical reactions. It is an important part in microfluidics [5]. Micromixers can overcome the problem that the laminar flow in the microchannel causes the fluid to be difficult to mix, and are generally divided into passive micromixers and active micromixers. Passive micromixers use the geometry of the channel and the characteristics of the fluid itself to mix, without external force and no moving parts [6].

In recent years, several researchers have applied micromixers to esterification reactions. Yao et al. [7] used p-toluenesulfonic acid as a catalyst to synthesize methyl acetate, ethyl acetate, butyl acetate and propyl acetate in a quartz capillary reactor. The results found that the highest ester yield was over 97.2% within a reaction time of 14.7 min. Brivio et al. [8] compared the esterification of 9-pyrene butyric acid with ethanol in a glass chip microstructure reactor and a conventional reactor, and found that higher ester yields were obtained in a shorter reaction time in a chip reactor. Benko-Lopez et al. [9] studied the esterification process of phthalic anhydride and methanol using an etched glass microstructure reactor. Compared with the traditional reactor, the reaction rate was increased by more than 50 times. Wiles et al. [10] successfully synthesized phenyl acetate and 4-nitrophenyl acetate using a borosilicate glass microstructure reactor. The conversion rate reached 100%, and the reaction time was significantly shortened. It can be seen that the use of a micromixer is conducive to improving the esterification reaction efficiency.

3. Application of Micromixer in Synthesis of Methyl Mercaptoacetate

Jia Shaoming et al. [11, 12] mixed thioglycolic acid with methanol and catalyst methylbenzenesulfonic acid, injected it into a capillary with a single injection pump for homogeneous catalytic synthesis of methyl thioglycolate. Results Compared with conventional reactors, the reaction time can be significantly shortened by microchannel reactor.

Niu Berlin et al. [13] improved the experimental device on the basis of Jia Shaoming's experiment, using p-toluenesulfonic acid as a catalyst, continuously feeding thioglycolic acid and methanol through a double pump, and using a micro-mixer to fully mix and then enter the capillary for thioglycolate methyl ester synthesis. The experimental device is shown in Figure 2.

Two studies investigated the effect of stainless steel capillary diameters of φ 2mm and φ 0.6mm on the yield of thioglycolate methyl ester under the conditions of no micromixer and micromixer, respectively. The experimental results are shown in Figure 3 and Figure 4.

Under the same operating conditions, through the comparison of two sets of experiments, it is obvious that the use of micromixers has a significant promotion effect on the synthesis of methyl thioglycolate. From only partial comparison results, such as in 0.6 mm inner diameter stainless steel capillary, at 60 ° C, the yield of ester without micromixer was 86.2%, and the yield of ester increased to 91.0% after adding micromixer.

Further analyze the experimental results. At the macroscopic scale, two or more fluids can be mixed by turbulent flow. However, in microfluidics, the structure size of the system is in the order of microns, the Reynolds number of the fluid is very small, and it is in a laminar state, so turbulent mixing cannot be achieved. Therefore, the mixing mechanism of the fluid is molecular diffusion, and the process is quite slow. The micromixer has an effect on the flow friction coefficient in the flow medium channel, and the change curve of the friction coefficient with the Reynolds number can change the transition area of the flow, so as to achieve rapid mixing of multiple fluids, thus improving the reaction rate.

Conclusion

It can be seen from the above studies that the synthesis of methyl thioglycolate by micromixer technology can significantly improve the yield of the target product, so it is very valuable to study the synthesis of methyl thioglycolate with high content and high yield by micromixer.

References

[1] YU Yijun, Shi De. Encyclopedia of Pesticide Applications [M]. Agricultural Press, 2008.

[2] Shepherd. Robin Gerald. Intermediates for Drugs and Sweet-eners. EurPat. 344.983 GBAppl88/12,718.

ZHAO Aiming. Catalyst for methyl thioglycolate reaction [D]. Jiangnan University, 2009.

[4] Lin Changzhi. Synthesis of methyl thioglycolate [J]. Anhui Chemical Industry, 2004, 30 (3): 26-27.

[5] Ye Xingxing, Mansur E H A, Wang Yundong, et al. Research progress of micro-mixing technology [J]. Chemical Industry Progress, 2007, 26 (6): 755-761.

DU Dongxing. Effects of compressibility and roughness on flow and heat transfer characteristics in microtubes [D]. Beijing: Tsinghua University, 2000.

[7] Yao X, Yao J, Zhang L, et al. Fast Esterification of Acetic Acids with Short Chain Alcohols in Microchannel Reactors [J]. Catalysis Letters, 2009, 132 (1-2): 147-152.

[8] Brivio M, Oosterbroek R E, Verboom W, et al. Surface effects in the esterification of 9-pyrenebutyric acid within a glass micro reactor. [J]. Chemical Communications, 2003, 2003 (15): 1924-5.

[9] Benitolopez F, Tiggelaar R M, Salbut K, et al. Substantial rate enhancements of the esterification reaction of phthalic anhydride with methanol at high pressure and using supercritical CO2 as a co-solvent in a glass microreactor. [J]. Lab on A Chip, 2007, 7 (10): 1345-51.

[10] Wiles C, Watts P, Haswell S J, et al. Solution phase synthesis of esters within a micro reactor [J]. Tetrahedron, 2003, 59 (51): 10173-10179.

Jia Shaoming, Li Yafang, Zhao Jianhong, et al. Application of microstructure reactor in synthesis of methyl thioglycolate [J]. Advances in Chemical Engineering, 2011 (S1): 56-58.

Jia Shaoming. Application of microstructure reactor in synthesis of methyl thioglycolate [D]. Zhengzhou University, 2012.

[13] Niu Berlin, Jia Shaoming, Li Yafang, et al. Rapid synthesis of methyl thioglycolate in a microchannel reactor [J]. Journal of Nanjing University of Technology: Natural Science Edition, 2013, 35 (2): 36-40.