Synthesis of polyoxymethylene dimethyl ethers from methanol and trioxymethylene with molecular sieves as catalysts

Polyoxymethylene dimethyl ethers (PODEn or DMMn) were synthesized by the condensation of methanol and trioxymethylene over the catalysts of several molecular sieves like HY, HZSM-5, Hβ, and HMCM-22; the effect of their acidic properties on product distribution was investigated. The results indicated that the acidic molecular sieves, especially HMCM-22, are catalytically active for the condensation of methanol and trioxymethylene to form DMMn. Over HY, the main product is dimethoxymethane (DMM), with a selectivity of 92.87%. Over HZSM-5 and Hβ, the main products turn out to be DMM1-3 and the yields of DMM3-8, which are ideal additives for diesel fuel, reach 6.40% and 13.78%, respectively. With HMCM-22 as the catalyst, the formation of long chain DMMn products is further enhanced and the yield of DMM3-8 attains 29.39%. The results of NH3-TPD demonstrated that the product distribution is related to the surface acidic properties of the catalyst used; short chain DMM may be primarily formed on weak acidic sites, while the acidic sites of medium strength can enhance the formation of diesel fuel additive components DMM3-8.