Generally, EPDA is produced through the chemical reactions of epichlorohydrin (ECH) with neodecanoic acid (NDA), and three typical synthetic processes have been developed according to the published literatures and patents (seeScheme 1 ).10 The first preparation method is carried out by using the alkali metal salt of NDA. The specific operations can be summarized as: (1) NDA reacts with sodium hydroxide aqueous solution to generate neodecanoate, (2) then the salt solution is dripped into the reactor with ECH heated reflux in it. This organic-aqueous reaction system has high viscosity and low interfacial tension because the neodecanoate, whose chemical structure consists of a polar head and a hydrophobic tail, can function as a surfactant. Hence, the emulsification can be rather serious for this system. Besides, the alkaline environment can cause the hydrolysis of ECH to produce dichloropropanol and glycerol, leading to the waste of reactant and posing difficulties for waste water treatment. The second synthetic process is based on the largely excessive use of ECH, the by-product dichloropropanol can be converted into ECH and recycled through the reaction with alkalis. However, the atom economy is poor, and the relatively low purity of EPDA increases the cost in the downstream purification processes.
At present, the two-step process is the most widespread path for EPDA production, including the initial acidolysis ring-opening reaction of ECH with neodecanoic acid to produce chlorohydrin ester intermediate (EPDA-M), and the following ring-closure step by alkali treatment. The acidolysis reactions of ECH with organic acids can be catalyzed by a series of nitrogenous catalyst11-13 and complexes of transition metal14, 15. Tetramethylammonium chloride (TMAC) has been recognized as a high-performance catalyst because of its high catalytic activity and ease of separation and recovery.16 The significant advances have been made to explore the catalytic mechanism and the interaction effect between TMAC and the reactants17, providing theoretical support for process optimization. The following ring-closure reaction by dehydrochlorination is a relatively mature process involved in some practical industrial applications, such as the production of propylene oxide through the chlorohydrin approach18, 19 and the production of ECH from dichloropropanol (DCP)20, 21.
The advantages, including the mild conditions applied in the whole process, the high conversion of neodecanoic acid and high selectivity towards EPDA, determine the great applicability of this two-step process. However, there are few researches to optimize the process conditions. Besides, the complete catalytic reaction pathways and the mechanism to explain the formation of the side product dichloropropanol (DCP) are still not clear. The lack of these studies leads to the unclarity of the strategies to improve the effective utilization rate of ECH and reduce the organic waste.
The essential objective of this work is to optimize the two-step production process of EPDA catalyzed by TMAC in batch condition. Moreover, some reliable experiments are conducted to verify the catalytic mechanism of the main acidolysis reaction and demonstrate the side reactions, as well as its occurrence conditions and control methods. Hopefully, the study methods and the obtained mechanisms can be successfully applied for other production processes of glycidyl eaters, such as glycidyl acrylate and glycidyl methacrylate.
Experiment
Reagents and materials
All reagents used in the experiments were classified as analytical grade and purchased from commercial reagent company. Neodecanoic acid was purchased from Shanghai Ethyl Chemical Co., LTD, with the purity of 99% tested by titration. The rest of chemicals were routinely used in our laboratory, and all purchased from Aladdin Chemistry Co., LTD. The purities of all reagents could meet the experimental requirements, hence the further purification steps were not required.
Experimental procedures