Supported Heteropoly Acids Catalysts For The Carbonylation Of Dimethyl Ether

Ethanol is one of the most promising alternatives to fossil fuels.Combined with the energy structure characteristics of our country,it is of great value and significance to develop the new green technology of syngas to ethanol.The path for ethanol production from syngas via dimethyl ether carbonylation to methyl acetate,and then methyl acetate hydrogenation to ethanol is simple and conducted under mild reaction conditions.Heteropoly acids as a kind of solid acid catalyst can catalyze the reaction of dimethyl ether carbonylation,whereas the low surface area and poor porosity limit the participation of active component in the reaction.The active sites of heteropoly acids can be dispersed in virtue of the mesoporous molecular sieves with large surface area and porosity,thus improving the accessibility to the reaction species and the catalytic performance.This work focus on the relation between acid property of Br?nsted acid sites and the reaction of dimethyl ether carbonylation,based on the supported heteropoly acids catalysts on SBA-15.Taking the SBA-15 as supporter,a series of supported phosphotungstic acid（HPW）catalysts with different loading were prepared by impregnation.The impact of HPW loading on the catalyst structure,acid properties and catalytic performance was discussed.Combined with the characterizations like TEM,Pyridine-FTIR,Pyridine-TPD and so on,results were obtained.After supported,the dispersion of HPW is improved,which was more easily accessible to reactions,thus increasing the yield of methyl acetate by the unit mass HPW.However,overloads can cause the aggregation of HPW on SBA-15,resulting in the decline of utilization efficiency.The interaction between the active component and the supporter reduced the acid strength of the catalysts to some degree.The change of acid strength affects the distribution of the product.With the increase of acid strength,the selectivity of main product methyl acetate was improved,accordingly,the selectivity of by-products（methanol,methyl formate,acetic acid）was reduced.Considering the catalyst structure,acid properties and catalytic performance,the 25 wt.%was considered as the best loading.To explore the affect of heteropoly acids type on the catalytic performance,supported three kinds of heteropoly acids with different intensity,HPW,silicotungstic acid（HSiW）and phosphomolybdic acid（HPMo）catalysts were prepared at the load of25 wt.%.Under the premise that the loading,physical structure and property are close,and the structure of active component is intact,the acid strength and its impact on the carbinylation of dimethyl ether were investigated.The results demonstrated that the acid strength order after supported is 25 wt.%HPW/SBA-15>25 wt.%HSiW/SBA-15>25 wt.%HPMo/SBA-15,moreover,the greater acid strength contributes to the formation of methyl acetate while at the same time reduces the selectivity of by-products.According to the DFT,the adsorption energy of all species,activation barriers and reaction energies were calculated in the process of dimethyl ether carbonylation catalyzed by HPW,HSiW and HPMo,the reaction path:CH₃OCH₃→CH₃^*→CH₂CO+H→CH₃CO→CH₃COOCH₃ was presented and the effect mechanism of Br?nsted acid on the reaction was investigated.The results showed that the increase of heteropoly acids has a significant effect on the dissociation of dimethyl ether and the insertion of CO,thus accelerating the reaction.