Insight into the reaction mechanism for condensation reaction of trioxane and dimethoxymethane over HZSM-5 zeolite

Abstract

Polyoxymethylene dimethyl ethers (DMMn) are considered to be a promising green diesel additive due to their similar physical properties to diesel. At present, the studies are mainly focused on the catalysts and reaction conditions of DMMn condensation reaction (i.e., DMMn was obtained by trioxane (TOX) reacting with dimethoxymethane (DMM)), but little research has been carried out on the reaction mechanism. In this work, four possible reaction mechanisms on the DMMn synthesis by TOX reacting with DMM on HZSM-5 molecular sieve were explored by the density functional theory (DFT) using Gaussian 09 package. Our calculations showed that the most likely reaction mechanism is that TOX is first decomposed into formaldehyde (FA), and then DMMn are obtained by FA reacting with DMM. 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. In the reaction processes, the active H in HZSM-5 molecular sieve plays an important role, but the structure of HZSM-5 molecular sieve remains basically unchanged.