Efficient synthesis of poly(oxymethylene) dimethyl ethers over PVP-stabilized heteropolyacids through self-assembly

A series of polyvinylpyrrolidone-stabilized heteropolyacids (PVP-HPAs) are generated by self-assembly of HPAs and PVP in methanol. The PVP-HPAs are then employed as catalysts for the synthesis of poly(oxymethylene) dimethyl ethers (DMM (n) , na (c) 3/41) by the methanolysis of trioxane. The results suggest that the acidity of PVP-HPAs is tunable by changing the ratio of PVP and HPAs, which is a key factor for the selectivity of the DMM (n) product. By optimizing the composition and reaction conditions, two types of PVP-HPA, PVP-phosphotungstic acid (PVP-HPW) in a PVP/HPW ratio of 1/4:1 and PVP-silicotungstic acid (PVP-HSiW) in a PVP/HSiW ratio of 1/4:3/4, respectively afford 52.4% and 50.3% yields of DMM2-5. The optimized catalysts are reusable for a minimum of 10 times without a significant drop in performance.