The Catalytic Study On The Conversion Of CH₃Br Into Light Olefins

Light olefins(i.e. ethylene and propylene) are raw materials for the generation of a wide range of important chemicals, and traditionally they are produced from crude oil. With the oil crisis and the high demand for light olefins, light olefins production from nature gas is an attractive subject. Among the processes for natural gas utilization, methane transformation to light olefins via CH3 Br as intermediate is a potential and an alternative route, which could be realized under mild condition with less steps. In the present study, efficient catalysts for CH3 Br transformation to light olefins were developed, and the relationship between structure and performance of catalyst was disclosed.(1) A serious of SAPO-34 and Zn APSO-34 s zeolite were synthesized by hydrothermal method using triethylamine as template. When used as catalyst for the formation of light olefins from CH3 Br, ZnAPSO-34 s catalysts exhibit enhanced CH3 Br conversion and C2=+ C3=selectivity than SAPO-34, it is especially so over ZnAPSO-34 with a Zn/Si molar ratio of 0.02, giving a C2=+ C3=yield of 73.3% at a CH3 Br conversion of 92.0%. Based on the results of characterization, the relationship between particle size, acidity and catalytic efficiency was disclosed.Some smaller particles are generated with Zn incorporation. The crystal size decreases with the rise of Zn content in Zn APSO-34 s. The results indicate that the smaller particles of Zn APSO-34 s with a higher amount of acid sites showed enhanced conversion of CH3 Br and better stability, whereas a higher amount of weak acid sites causes better C2=+ C3=selectivity. In addition, a proper presence of water in CH3 Br improves the selectivity to C2=+ C3=.(2) Polycrystalline silicalite layers were formed on HZSM-5 and the obtained HZSM-5@silicalite-1 composites were served as catalysts for the generation of light olefins from CH3 Br. The HZSM-5@silicalite-1 composites showed high catalytic activity, stability and excellent selectivity of light olefins, and the results could be ascribed to the passivating acid sites of the external surface of HZSM-5 by the silicalite-1 shell. The enhancement of catalytic activites depends on the density and thickness of the silicalite-1 shell. The thicker the silicalite-1 shell is, the higher selectivity for light olefins and stability of HZSM-5@silicalite-1 composite achieved. The high stability of ZS-2 catalyst may be stem from the silicalite shell onthe surface which decreased the rate of coke formation of the catalyst.