Molecular size reforming of undersized and oversized polyoxymethylene dimethyl ethers

Abstract

Polyoxymethylene dimethyl ethers (CH₃–O–(CH₂O)_n–CH₃, PODE_n) are potential environmentally benign coal-based diesel fuel blending compounds. They are synthesized from dimethoxymethane (DMM) and paraformaldehyde (PF). Among the PODE_n homologues, PODE_3–4 have a good match with diesel as a fuel, while the undersized PODE_1–2 and oversized PODE_n>4 have unsuitable properties. This work studied the molecular size reforming of undersized PODE_1–2 and oversized PODE_n>4 by two different methods, namely, self-reforming and reacting with DMM over an acidic ion exchange resin. The molecular size reforming of PODE_1–2 and PODE_n>4 by self-reforming gave a high concentration of formaldehyde, which shifted the distribution to longer chain PODE_n and formed solid PF. In contrast, PODE_1–2 and PODE_n>4 were mainly converted to PODE_3–4 by the reaction with DMM and the high concentration of formaldehyde was also diminished. The equilibrium reorganized molecular size distribution of PODE_1–2 and PODE_n>4 followed the Schulz–Flory distribution. A proposed kinetic model described well the molecular size reforming pathways of PODE_1–2 and PODE_n>4. A methanol-to-PODE_n close-loop process was proposed to enhance atom-economy by recycling the PODE_1–2 and PODE_n>4 streams.