Elucidating The Mechanism Of Dimethyl Ether Carbonylation In MOR Zeolites From First Principles

Ethanol is an important commodity chemical.Methyl acetate（CH₃COOCH₃,MA）was prepared from dimethyl ether（CH₃OCH₃,DME）through heterogeneous catalytic carbonylation reaction,and then ethanol was obtained by hydrogenation.This reaction process has high atomic economy,and the product is directly anhydrous ethanol,which can greatly reduce the separation energy consumption and production cost.Because of the fact that DME can be obtained from syngas which is derived from coal,biomass and natural gas through the mature process,the carbonylation of DME in series to prepare MA can construct an efficient and clean process path for the preparation of ethanol from non-petroleum carbon-based resources.The core difficulty of this path is how to achieve efficient and green synthesis of MA.Mercerite molecular sieve（MOR）shows excellent catalytic activity in carbonylation of DME due to its unique eight-membered ring channel,but its catalytic lifetime is very short due to the rapid carbon deposition in the twelve-membered ring channel.Therefore,it is very important to understand the carbonylation mechanism of dimethyl ether catalyzed by MOR molecular sieve for further improving its catalytic performance.In this paper,the differences of catalytic mechanism of active sites in different pore channels of MOR molecular sieve were analyzed in detail based on density functional theory.On this basis,B and Ga heteroatoms were introduced to construct B-MOR and Ga-MOR molecular sieves,and their effects on the catalytic activity of MOR were investigated.Finally,the carbonylation properties of different Ga species in MOR and Ga-MOR molecular sieves were investigated.The specific research conclusions are as follows:1.There are mainly two active centers in MOR molecular sieve,which are at the T3site of eight-membered ring channel and T4 site of twelve-membered ring channel.By comparing the change of energy barrier,it is found that the formation of acetyl group（Z-CH₃CO）is the reaction speed step.Compared with 12MR,8MR has a smaller energy span and a better catalytic effect.Acetyl group is more likely to exist in stable covalent form;Ionic acetyl groups formed at T4 are readily converted to ketene,the polymerization of which leads to rapid carbon deposition in 12MR,thereby inactivating the catalyst.2.T3 in the eight-membered ring channel is the active site.When the initial material is DME,acetyl group is more inclined to react with methanol generated after adsorption and activation of DME to produce methyl acetate;When the initial material is methanol,the acetyl group is more inclined to react with the water generated after adsorption and activation of methanol to produce acetic acid.3.Isocrystalline substitution at T3 and T4 sites was carried out with B and Ga atoms respectively.It was found that after B substitution,the reaction energy barrier of each elementary element of carbonylation reaction increased significantly,indicating that the reaction performance of MOR molecular sieve may be greatly inhibited after the introduction of B at T3.Because T4 is a carbon deposition deactivation site,the high energy barrier will reduce its deactivation rate.When Ga was introduced to the T3 site,the energy barrier of acetyl group was slightly higher than that of MOR,while the energy barrier of acetyl group reacting with dimethyl ether and methanol decreased,suggesting little effect on the acidity of T3 site.When Ga was introduced to T4 site,the reaction energy barrier was increased,and the increase was greater than that at T3 site.It was speculated that Ga could weaken the acid strength at T4 site to a certain extent,which was conducive to increasing stability.4.Using HMOR and Ga-MOR molecular sieve as the carrier,the differences of catalytic reaction of dimethyl ether in the presence of Ga⁺,[Ga O]⁺,[Ga H₂]⁺,[Ga（OH）₂]⁺in 8MR were compared.It was found that when Ga exists on MOR and Ga-Mor in two states,Ga+ and [Ga O]⁺,The energy barrier of acetyl group formation decreased significantly,and the energy barrier of Ga+ species decreased the most.It is speculated that the construction of Ga⁺/MOR and Ga⁺/ Ga-MOR catalytic system is helpful to further improve the catalytic performance.