Study On Direct Synthesis Of Dimethyl Ether From Syngas

Dimethyl ether is not only ideal clean energy source, but important chemical materials. The method for the production of dimethyl ether directly from syngas is not limited by thermodynamics like methanol, together with high conversion per pass and good processing route. The catalyst is called bifunctional catalyst which catalyzes both the synthesis dehydration of methanol. The catalyst will has good activity and selectivity if the synthesis and dehydration of methanol are synergic. Therefore, study on the bifunctional catalyst with good property is significant in academia as well as application.The synthesis and processing conditions of the bifunctional catalyst for dimethyl ether were studied by using fixed bed reactor in the paper. The equilibrium time of the synthesis and the optimal molar ratio of the components of methanol synthesis as well as that of methanol synthesis and methanol dehydration was got from the catalyst evaluation experiments. A series of catalysts were prepared by mechanical mixed method, coprecipitation sedimentation method, colloid deposition method and colloid impregnation mehtod, respectively. Additive agent Co, Fe and Cr were also used for modification. The characterizaton of the catalyst was performed by XRD diffractor and the influences of different synthesis methods and additive agents to activity and selectivity were discussed. Finally, effect of the factors such as temperature, pressure and space velocity on the activity and selectivity of the catalyst were studied, and the optimized processing conditions were obtained.The results indicated the best equilibrium time for the dimethyl synthesis was 3h and the catalyst avtivity was the highest when the molar ratio of copper, zinc and aluminum was 10:5:1, and the weight ratio 1:4 of the components of methanol synthesis and methanol dehydration, the conversion rate of CO was 73% and the selectivity of dimethyl ether was 51%. According to the best catalyst activity and selectivity, the preferential order of preparation was colloidal impregnation method > colloidal deposition method > coprecipitation sedimentation method > mechanical mixed method. Additive agent Co2O3, Fe2O3 and Cr2O3 were added in the mechanical mixed method, respectively. The function of auxiliary agent was given in a decreasing order: Co2O3 > Fe2O3 > Cr2O3. The same order of the additive agents was obtained in colloid impregnation method, i.e. Co(NO3)2 > Fe(NO3)3 > Cr(NO3)3. After different contents of additive agent Co were used in the four preparation method, the optimal content of Co is 5% with the conversion rate of CO 78% and the selectivity of dimethyl ether 68%. The optimal synthetic conditions of dimethyl ether bifunctional catalyst were: t=260℃, P=3.0MPa, n(H2)/n(CO)=2, space velocity 1500h-1.