Research on the catalytic cyclo-dehydration of diethylene glycol assisted with additive in near-critical water

In near-critical water (NCW) system, the cyclo-dehydration of diethylene glycol (DEG) assisted with additive to form 1, 4-dioxane was investigated. The yield of 1, 4-dioxane was measured under different reaction times, temperatures, pressures, reactant/water ratios (r/w) and concentrations of additive. The results demonstrate that the 1, 4-dioxnane can be formed from the cyclo-dehydration of DEG in NCW system without any additive; however the yield of 1, 4-dioxane is low; at 340°C and 240 min, the yield is only 9.84%(wt) in NCW system of pure water. The yield of 1, 4-dioxane increases remarkably with the assist of additive ZnCl₂; the maximum yield of 50.89%(wt) could be achieved in the NCW system of 0.5%(wt) ZnCl₂ solution at 340°C and 120 min. The yield of 1, 4-dioxane is 36.18%(wt) in the NCW system of 1.0%(wt) ZnCl₂ solution at the above reaction temperature and time, and there is no 1, 4-dioxane can be detected when the cyclo-dehydration of (DEG) was conducted in the NCW system of Na₂CO₃ solution under above reaction conditions. It indicates that the additive Na₃CO₃ has no promotion effect for the cyclo-dehydration of (DEG) in NCW system. Based on the results found, the possible reaction mechanism and pathways of cyclo-dehydration of DEG in NCW system with the assist of additive were proposed.