Identification of the rate-determining step for the synthesis of polyoxymethylene dimethyl ethers from paraformaldehyde and dimethoxymethane

Abstract

Polyoxymethylene dimethyl ethers (PODEn, CH3O(CH2O)nCH3, where n ≥ 1) are environmentally friendly diesel fuel additives because they can significantly reduce the emission of engine exhaust during combustion. It is an efficient way to synthesize PODEn from dimethoxymethane (DMM) and paraformaldehyde (PF) over solid acid catalysts. The synthesis process consisted of two steps: the depolymerization of PF and the polymerization of PODEn. In this work, the first step was proved to be the rate-determining step by analyzing the product distribution and the rate of reactions with and without PF. NKC-9 resins of different size were used to prove that the internal diffusion in catalyst was a key factor to affect the depolymerization process. The SO42−/TiO2 and SO42−/TiO2-ZrO2 catalysts were used to further prove that the catalysts with larger pores were more active for the synthesis of PODEn.