Molecular size reforming of undersized and oversized polyoxymethylene dimethyl ethers

摘要

Polyoxymethylene dimethyl ethers (CH _(3) –O–(CH _(2) O) _( n ) –CH _(3) , 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.