Experimental And Theoretical Calculation Study On The Catalytic Synthesis Of Polyoxymethylene Dimethyl Ethers By Modified MCM-41 Zeolite

Polyoxymethylene dimethyl ethers(PODE_n)is a general term for a class of polymers.It can not only reduce the pollutants emitted by the diesel engine during combustion,but also improve the thermal efficiency of the diesel engine.It has a broad application prospect.This paper has carried out the research work on the synthesis of PODE_n zeolites catalyst.Six different metal-modified MCM-41 mesoporous zeolites,such as Fe and Al,were prepared by the primary crystallization method and the secondary crystallization method as the catalyst.Synthesize PODE_n in an autoclave with inexpensive methanol and paraformaldehyde(PF),the Al-modified zeolite was selected as a better catalyst,and the selected catalyst modification method was the primary crystallization method.Al-MCM-41(X)mesoporous zeolites with different Si/Al were prepared(X is the value of Si/Al),and the performance of Al-MCM-41(X)zeolites for the synthesis of PODE_n was investigated.XRD,etc.were used for analyzing the effects of Al incorporation and Si/Al changes on the structure and acidity of MCM-41 zeolite are discussed.The results show that different Si/Al zeolites all have the unique ordered mesoporous structure of MCM-41zeolites.The incorporation of Al will cause partial collapse of the surface of the MCM-41zeolites,and the specific surface area,pore volume and pore diameter have been reduced.However,the incorporation of an appropriate amount of Al can effectively increase the amount of medium-strong acid in the MCM-41 zeolite,it can further promotes the formation of high polymerization degree product PODE_3-5 in synthetic PODE_n.Al-MCM-41(40)zeolite has the best performance in the synthesis of PODE_n.The synthetic path and thermodynamic law of PODE_n from methanol and polyformaldehyde condensation catalyzed by Al-MCM-41 zeolite were studied by means of density functional theory(DFT)calculation.Three synthetic paths are designed,and optimized the structure of intermediates and transition states.By comparing the size of the Gibbs free energy barrier of each reaction under three paths and combining the product distribution in the experiment,the synthesis is proposed as a direct chain extension path of PODE_n-1.Paraformaldehyde is first decomposed into formaldehyde.The latter reacts with methanol to form dimethoxymethane(PODE₁)through a hemiacetal intermediate.Long-chain PODE_n is gradually formed by chain extension of PODE_n-1 and formaldehyde.Among them,the free energy barrier of the reaction between formaldehyde and methanol to form methylal is 121.2 k J·mol^-1,which is the rate control step of the PODE_n synthesis.After determining the synthetic path,the thermodynamic change law of each step in the path was investigated.In the range of 298 K to 453 K,the decomposition of paraformaldehyde is an endothermic reaction,and the synthesis reactions of PODE_1-7 are all exothermic reactions,and the amount of heat released decreases with the increase of the PODE_n chain length.The changing trend of the?_rH_m of all reactions with temperature is not obvious.The?_rG_m of the synthesis reaction of PODE_1-7 gradually increases with the increase of temperature,the equilibrium constant K gradually decreases,the spontaneous degree of the synthesis reaction decreases,and the synthesis of PODE_n exists an optimal reaction temperature.Finally,the influence of reaction conditions on the synthesis of PODE_n was investigated,and the optimal process conditions of PODE_n were screened out.And under this condition,the usage times of Al-MCM-41(40)zeolite catalyst were investigated.The results showed that the best synthesis conditions for PODE_n were:135?,5 h,methanol to paraformaldehyde molar ratio of 1.5:1,and catalyst addition of 2%(wt.).Under these conditions,the methanol conversion rate reached 74.5%,and the selectivity of PODE_1-7and PODE_3-5 are 76.4%and 31.3%,respectively.Al-MCM-41(40)zeolite catalyst has a good catalytic activity after repeated use 5 times.