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

Abstract Polyoxymethylene dimethyl ethers (PODE n , CH 3 O(CH 2 O) n CH 3 , 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 PODE n 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 PODE n . 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 SO 4 2− /TiO 2 and SO 4 2− /TiO 2 -ZrO 2 catalysts were used to further prove that the catalysts with larger pores were more active for the synthesis of PODE n .