Catalytic Performance Of Hierarchical ZSM-5 Zeolites Supported Cobalt Catalyst In Fischer-Tropsch Synthesis

As the environment of the international energy becomes more and more severe,searching and developing an alternative energy has become an increasingly important hot topic in the world.Fischer-Tropsch synthesis technology has become increasingly valued,for which can covert syngas molecules into clean liquid fules and chemicals.However,the convertional products of it follow ASF distribution,and have lower selectivity of C₅-C₂₀.Therefore,how to construct efficient catalysts to increase the selectivity of C₅-C₂₀ become the focus of current research.The contents of this thesis include as following:（a）The nano-spherical ZSM-5with particle diameter of 80 nm was synthesized by xerogel-steam assisted hydrothermal method.The effects of synthetic conditions on the morphology of ZSM-5 were studied.And then the cobalt particals were loaded to find out its appropriate loading.（b）The effects of morphology and the Na content in crystal on the catalytic performance of the catalysts were investigated.（c）The effects of aluminum content in ZSM-5 on the catalytic performance of cobalt in Fischer-Tropsch synthesis were compared.The results demonstrated as followed:（1）The concentration of structure-directing agent,nucleation temperature,nucleation time and the aluminum source affects the morphology of the ZSM-5crystallites.Higher temperature of nucleation was beneficial for bigger nucleus,which made some formed MFI structure dissolve under the influence of TPAOH,and then the silicon migrate from the inside to outside for recrystallization.The ZSM-5 with a surface porous structure synthesized under optimized condition.The catalysts had higher stability,and lower CH₄ selectivity when cobalt loaded was 15 wt%.（2）Lower the content of Na by ion-exchange was beneficial for improvement of catalysts activity of cobalt-base Fischer-Tropsch synthesis.Under hydrothermal reaction condition of FTS,Na migrated to the active site,covering part of the active center and reducing the activity of the catalyst.As morphology altered,ZSM-5 could anchor cobalt partical on the surface to improve the stability of catalyst and isoparaffin selectivity in products.（3）The appropriate mass percent of aluminum could improve the interaction between ZSM-5 and cobalt particals and the stability of catalysts.but when it was too lower or too high,the surface structure would be reconstructured,during reaction,resulting in stronger interaction and even formation of refractory cobalt silicate or cobalt aluminate,thus deactivating catalyst activity rapidly and changing C₅-C₂₀selectivity.Meanwhile,After the surface structure of ZSM-5 collapsed,a layer of film forming on cobalt particles,was an important part that leads to lower catalyst activity.