关键词: 催化剂, 聚甲氧基二甲醚, 醇, 甲醛, 合成

Abstract:

Polyoxymethylene dimethyl ethers (PODE_n) are kind of diesel fuel additives with features of environment friendly and high efficiency. PODE_n were catalytically synthetized from methanol and formaldehyde by ZnCl₂. In this work, the mechanism of the synthesis reaction was discussed, and the effects of preparation conditions, such as temperatures, time, usage of catalyst and reactant mass ratio were investigated. The comparison of different catalysts of ZnCl₂, strong acid resin and ZnCl₂ modified resin was carried out. The concentrations of the PODE_n compounds and methanol were quantitatively analyzed using gas chromatography (GC), and formaldehyde was also determined by chemical titration method. Sulfuric acid standard solution was used to titrate sodium hydroxide formed by reaction of formaldehyde and excessive sodium sulfite. To obtain accurate experimental data, all the experiments were conducted strictly using the following procedures: poly-formaldehyde was de-polymerized to form anhydrous formaldehyde-methanol mixture solution; reaction device was heated to reaction temperatures (80, 95, 105 and 115℃); different proportions [2:(3—4)] of the mixture solution obtained, and ZnCl₂ were added into reaction vessel; then, the pressurized vessel was sealed quickly and fixed, and rotational speed of the reaction device was set for 20 r·min^-1; this moment was considered as the starting time of reaction; sampling time was placed after reacting 60—360 min. Reaction mechanism of synthetising PODE_n from methanol and formaldehyde catalyzed by ZnCl₂ was proposed as follows: formaldehyde was polarized firstly in the presence of catalyst, the first intermediate obtained came from the reaction of formaldehyde polarized and methanol, then other intermediates and PODE_n were produced from the reactions of the first intermediate and raw materials. The experimental results confirmed that ZnCl₂ could be used as catalyst for synthesis of PODE_n from methanol and formaldehyde. The yield and selectivity of PODE_2—6 reached 19.90%, 67.50%, respectively, when the better reaction conditions were used (mass ratio of methanol and formaldehyde 2:(3—4), catalyst usage 3.0% (mass), temperature 105℃ and reaction time 300 min). Content of the products was more than 10 percentage points higher for ZnCl₂ modified resin catalyst which had higher catalytic activity than for ZnCl₂ catalyst.

Key words: catalyst, polyoxymethylene dimethyl ethers, alcohol, formaldehyde, synthesis