Physical Properties Prediction Of Polyoxymethylene Dimethyl Ethers And Study On Condensation Reaction Mechanism

Polyoxymethylene dimethyl ethers(DMMn)are considered to be a promising green diesel additive due to their similar physical properties to diesel.DMMn is added to diesel fuel in a certain proportion,which can improve combustion efficiency and reduce the content of nitrogen oxides and particulate matter in exhaust emissions.Existing research is mainly on the exploration of reactants,catalysts,reaction conditions,but less on the reaction mechanism.In this paper,four possible reaction mechanisms for the synthesis of polymethoxy dimethyl ether on HZSM-5 zeolite were theoretically calculated using density functional theory with dimethoxymethane(DMM)and trioxane(TOX)as reactants.The following conclusions are drawn:1.The TOX is decomposed to formaldehyde(FA)after two steps,and the generated FA is reacted with DMM.2.The reaction of FA with DMM is most likely to follow the reaction mechanism of the decomposition of DMM into carbocation and methanol.For the reaction of FA with DMM,DMM firstly decomposes into methanol and carbocation,and then one direction is that FA increases the carbocation chain,while the other direction is the reaction mechanism of methanol termination.Then we study the effect of water on the reaction mechanism.Water changes the path of the reaction.Follow the path below.In the first step,DMM reacts with water to form hemiacetal and methanol.In the second step,FA reacts with hemiacetal to increase the carbon chain of hemiacetal.In the third step,the hemiacetal with increased carbon chain reacts with methanol to block the end.After that,based on the most probable reaction mechanisms,we use ONIOM2 to calculate,and selects three zeolites of HZSM-5,HMOR and HMCM-22.The effect of pore structure on the reaction mechanism was studied,and the effect of HZSM-5 zeolite acid strength on the reaction mechanism was studied.The results show that the energy barrier to be overcome on the HMCM-22 zeolite is the smallest.The stronger the acidity of HZSM-5 zeolite,the-more conducive to the reaction.In the reaction processes,the active H in HZSM-5 zeolite plays an important role,but the structure of HZSM-5 zeolite remains basically unchanged.In addition,the physical parameters missing in this system are also estimated to provide a reference for process simulation.