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

Polyoxymethylene dimethyl ethers (PODE_n or DMM_n) 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 DMM_n. Over HY, the main product is dimethoxymethane (DMM), with a selectivity of 92.87%. Over HZSM-5 and Hβ, the main products turn to be DMM_1~3 and the yields of DMM_3~8, 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 DMM_n products is further enhanced and the yield of DMM_3~8 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 DMM_3~8.