A Synthesis, Process Optimization, and Mechanism Investigation for the Formation of Polyoxymethylene Dimethyl Ethers

Abstract

Polyoxymethylene dimethyl ethers (DMM _n) are promising diesel additives. The synthesis of DMM _n from methylal (DMM) and paraformaldehyde over the NKC-9 acidic ion-exchange resin catalyst was investigated. Many unrecyclable by-products such as methyl formate, dimethyl ether and formic acid were produced in the reaction. To increase the selectivity of the desired products DMM_3‒6 and reduce the amount of unrecyclable by-products, the effects of reaction temperature, time, pressure and the molar ratio of the raw materials were evaluated through a series of single factor experiments. Experiments revealed that trace amount of water could suppress the formation of unrecyclable by-products, and the optimum initial water content (less than 2 wt%) was investigated. In addition, the synthetic process needs to go through the polyoxymethylene hemiformals intermediate stage, and then the DMM _n were obtained when polyoxymethylene hemiformals reacted with methanol. Ultimately, a possible mechanism is proposed to describe the formation of DMM _n from polyoxymethylene hemiformals in detail, in which it is revealed that the formation of carbocation intermediates is important in the reaction processes.

Keywords

Polyoxymethylene dimethyl ethers / Paraformaldehyde / Methylal / Diesel additive / Polyoxymethylene hemiformals

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