Study On Formation Kinetics Of Polyoxymethylene Dimethyl Ethers

Polyoxymethylene dimethyl ethers(PODEn)are oligomers with a different number of CH2O units and two ending groups of-O-CH3(methoxyl)and-CH3(methyl),and the degree of polymerization(n)is generally less than 8.PODE_n with a degree of polymerization of 3-5 are an excellent diesel additive,which can effectively reduce the air pollution caused by insufficient diesel combustion and greatly reduce the emission of NOx and soot particles.At present,the route of production using anhydrous trioxane as educts is not economical;while the production route using dilute formaldehyde as educts has complicated reaction processes,many by-products,difficult separation and high energy consumption.Our team has developed a production technology using methylal and trioxane with little water as educts,which can solve these problems.In this paper,for the purpose of scaling up of condensation reaction,the formation kinetics of PODE_n from methylal and trioxane with little water catalyzed by ZSM-5 is studied.The specific research contents are as follows:Based on the analysis of the experimental results of the synthesis of polymethoxy dimethyl ether,combined with relevant literature,the possible reactions in the reaction system were analyzed,and the kinetics of formation of PODE_n from methylal and trioxane with little water catalyzed by ZSM-5 was established.The combination of the genetic algorithm and the fmincon optimization function is used to estimate parameters in the kinetic model,and finally,a reliable kinetic model is obtained.In addition,through the fitting of ln(w_n/(n+1.533))and the degree of polymerization(n)of the PODE_n analysis results obtained at any time during the reaction,it was found that the molecular size distribution of PODE_n follows the S-F distribution at any time during the reaction,although this model is derived from the equilibrium state.In order to investigate the effect of water on the synthesis of polymethoxy dimethyl ether,the formation kinetics of polyoxymethylene dimethyl ethers from methylal and trioxane was established.It was found that the influence of water on the synthesis of polymethoxy dimethyl ether is mainly in two aspects:First,water as a reactant participated in a series of reactions in the reaction system;Second,the presence of water affects the diffusion of reactants to the active sites in the catalyst,which affects the reaction rate.