Preparation Of ZrO₂-CeO₂/NaZSM-5 Composite Catalyst And Its Catalytic Effect On Methanol To Propylene

Based on the resource distribution status of rich coal,poor oil and little gas in China,the dependence of propylene preparation on oil has been greatly reduced by methanol to propylene（MTP）.In this paper,aiming at the problem that the strong acidity and microporous structure of molecular sieve will lead to the decline of methanol conversion,propylene selectivity and propylene/ethylene（P/E）ratio in MTP reaction,the surface acidity and pore structure are adjusted by composite metal oxide,so as to improve the catalytic performance of MTP.CeO₂/NaZSM-5 composite catalyst was constructed by compounding mesoporous CeO₂ with NaZSM-5molecular sieve.Because bimetallic oxide will improve the catalytic performance to a greater extent than single metal oxide composite modified molecular sieve,this paper further proposes to use mesoporous ZrO₂-CeO₂ hydrothermal composite NaZSM-5molecular sieve with rich acidity and alkalinity to construct ZrO₂-CeO₂/NaZSM-5composite catalyst.The preparation conditions of CeO₂/NaZSM-5 and ZrO₂-CeO₂/NaZSM-5 composite catalysts and MTP process conditions were optimized.By means of XRD,XRF,XPS,NH₃-TPD,CO₂-TPD,FT-IR,N₂adsorption/desorption,SEM,TEM and EDX-Mapping,the effects of preparation method,mass ratio of CeO₂ to NaZSM-5,silicon aluminum ratio of NaZSM-5 and calcination temperature on the structure,properties and catalytic performance of CeO₂/NaZSM-5 composite catalyst were investigated.The composite catalyst with the best performance was obtained by hydrothermal method when the mass ratio of CeO₂ to NaZSM-5 was 1:4,the silicon aluminum ratio of NaZSM-5 was 250 and the calcination temperature was 550℃.In addition,the effects of reaction temperature,heavy time space velocity,nitrogen flow rate and water alcohol molar ratio on the catalytic performance of CeO₂/NaZSM-5 composite catalyst were investigated.When the reaction temperature was 480℃,the heavy space-time velocity was 2.6 h^-1,the nitrogen flow rate was 100 m L·min^-1,and the pure methanol was fed,the methanol conversion,propylene selectivity and P/E ratio were 99.90%,42.78%and 6.30respectively.Under these conditions,when the reaction temperature is 400℃,the methanol conversion,propylene selectivity and P/E ratio are 95.05%,31.43%and3.51 respectively.In order to further improve the catalytic performance of the catalyst,the design idea of bimetallic oxide composite molecular sieve was proposed,and the NaZSM-5molecular sieve was hydrothermally modified with ZrO₂-CeO₂.Through the investigation of ZrO₂-CeO₂ composite mode and ZrO₂-CeO₂mass ratio,it is found that under the same reaction conditions,the methanol conversion,propylene selectivity and P/E ratio of ZrO₂-CeO₂/NaZSM-5 catalyst prepared by ZrO₂-CeO₂composite by coprecipitation method and two-phase mass ratio of 1:1 are 96.75%,36.33%and 3.92 respectively at 480℃.The methanol conversion and propylene selectivity at the reaction temperature of 400℃were not different from that at the reaction temperature of 480℃,but the P/E ratio increased by 0.72 times.Therefore,the process conditions were further optimized.When the reaction temperature was400℃,the heavy space-time velocity was 2.6 h^-1,the nitrogen flow rate was 100m L·min^-1,and the pure methanol was fed,the methanol conversion,propylene selectivity and P/E ratio were 96.17%,35.82%and 6.75 respectively.Compared with CeO₂/NaZSM-5 composite catalyst at 400℃,methanol conversion,propylene selectivity and P/E ratio were increased by 1.12%,4.39%and 3.24%,respectively.It shows that the introduction of an appropriate amount of ZrO₂ is conducive to improve the catalytic performance of MTP reaction and reduce the reaction energy consumption at lower reaction temperature.