Additives assisted catalytic cyclo-dehydration of diethylene glycol in near-critical water

The additives assisted the cyclo-dehydration of diethylene glycol (DEG) reaction was studied in near-critical water (NCW). Zinc chloride (ZnCl₂) and sodium carbonate (Na₂CO₃) were selected to investigate their effects on the cyclo-dehydration of diol. The influences of reaction temperature, time, pressure, reactant/water ratio (r / w) and the concentration of additives on the product yield of the cyclo-dehydration of DEG were examined. The results showed that the final obtained product was primarily 1,4-dioxane, resulting from the cyclo-dehydration of DEG in NCW. The yield of 1,4-dioxane was only 9.84 wt.% in pure water by reacting at 340 °C for 240 min, but the maximum yield of 1,4-dioxane could reach as high as 50.89 wt.% in the solution of 0.50 wt.% ZnCl₂ at 340 °C for 120 min, and the conversion of DEG was 91.94 wt.%. The complete conversion of DEG was obtained in 1.00 wt.% ZnCl₂ at 340 °C for 120 min, but the yield of 1,4-dioxane was only 10.18 wt.%. In the case of Na₂CO₃, it did not have significant promotion effect on the cyclo-dehydration of DEG reaction. All these experimental results demonstrated that ZnCl₂ had the positive effects on the dehydration of DEG while Na₂CO₃ depressed the reaction. Based on these results, a possible reaction mechanism and pathway was proposed in NCW.