CO₂-promoted Hydration Of Ethylene Oxide Catralyzed By Ionic Liquids/Organic Bases

Monoethylene glycol（MEG）is a widely used chemical raw material with global production exceeding 30 million tons/year.The main production process of ethylene glycol is direct hydration of ethylene oxide（EO）.However,this process has the problems of high hydration ratio and low selectivity of ethylene glycol,which increased the cost and energy consumption of distillation and dewatering in the post-treatment process.The development of efficient and low-cost strategy for production of MEG through hydration of EO at low H₂O/EO molar ratios is an important industrial challenge.CO₂ as the reaction atmosphere,hydration of EO can be achieved at a low H₂O/EO ratio of 1.5:1.The different ionic liquids（1-vinyl-3-butyl-imidazole bromine（VBIm Br）,1-butyl-3-methyl imidazolium bromide（BMIm Br）,1-benzyl-3-butyl-imidazolium bromide（Bn BIm Br）,1-butyl-2,3-dimethyl-imidazolium bromide（Bd MIm Br）and tetrabutylammonium bromide（Bu₄NBr））and different organic bases（vinylimidazole（VIM）1,8-diazacyclic undecylene-7-ene（DBU）,4-dimethylaminopyridine（DMAP）,diallylamine（DAA）,triallylamine（TAA）and 4-vinylpyridine（4-VP））as catalysts under the conditions of CO₂ or N₂ with the hydration ratio as low as 1.5:1were performed along with high yields（88-94%）and selectivities（91-97%）of MEG.The yield of MEG is only 11-21%and the selectivity is only 16-32%when other reaction conditions remain unchanged.Compared with N₂,CO₂ atmosphere could assist the hydration of ethylene oxide and improve the selectivity of ethylene glycol significantly.Isotope labeling experiments was performed with the hydration of EO with H₂¹⁸O under CO₂ or N₂ atmosphere.The direct hydration of EO with H₂¹⁸O under N₂ afforded the isotope-labeled MEG（m/z 64）as the main product.In contrast,the hydration of EO with H₂¹⁸O under CO₂ atmosphere gave predominantly the unlabeled MEG（m/z 62）,while ¹⁸O was found transferred from H₂¹⁸O to C¹⁸OO（m/z 46）.The proposed two-step mechanism for the catalyzed reaction under CO₂,in which the cycloaddition of EO with CO₂ occurs first followed by hydrolysis of ethylene carbonate（EC）to generate MEG and recover CO₂.DFT calculations was used to calculate the transition state and energy barrier of the hydration reaction of ethylene oxide in the atmosphere of CO₂ and N₂.It was found that the energy barrier of the direct hydration reaction of ethylene oxide in the atmosphere of N₂ was 39.1 kcal mol-1,and the energy barrier of the assisted hydration reaction of CO₂was 32.3 kcal mol-1.The energy barrier of the hydration reaction in nitrogen atmosphere is 6.8 kcal mol-1 higher than that in carbon dioxide atmosphere,indicating that the carbon dioxide assisted reaction path is more favorable in kinetics.The organic base or ionic liquid was used as single catalyst for CO₂-promoted hydration of EO,it has been proved that the ionic liquids and organic bases synergistically catalyzed the one-pot two-step reaction,in which the ionic liquid catalyzed the cycloaddition of EO and the organic base catalyzed the hydrolysis of EC.The CO₂-promoted hydration of EO catalyzed by ionic liquid/organic base developed an efficient and simple catalytic system for the green production technology of MEG with low energy consumption.