Synthesis And Modification Of Hierarchical High-silica Pentasil Zeolites And Its Catalytic Performance In Hydrocarbons Cracking And Hydrocarbons Synthesis

ZSM-5,a typical member in the pentasil family,has been widely used in industrial processes such as petroleum-chemistry and fine chemistry due to its superior ion-exchange properties,high catalytic activity and unique shape-selectivity.Hierarchical zeolites,possessing high thermal/hydrothermal stability of microporosity and the difusion advantage of mesoporous materials,are playing an important role in heterogeneous catalysis.Preparation of hierarchical zeolites is of great importance both industrially and scientifically.This dissertation focused on the preparation of hierarchical pentasil zeolites and their uses in hydrocarbon cracking and methanol to hydrocarbons(MTH).The details of the dissertation include:Firstly,synthesis of microspherical ZSM-5 nanocrystalline aggregates and long-sheet ZSM-5 with hierarchical porosity was systematically investigated.N-hexamine was directly used as the single template.A structure-directing mechanism of the primary C6 amines for the assembly of the microspherical ZSM-5 was proposed based on the intensive exploration.of growth behaviors.The accurate control of morphology and mesopore-size was achieved by using this cost-effective and facile method.Hierarchically porous high-silica microspherical ZSM-5,which could hardly be obtained by the direct synthesis,was obtained through dealumination and reinsertion of Si to Al on the prepared ZSM-5.Furthermore,seed-assisted synthesis of long-sheet ZSM-5 with decreased diffusion length along b axis was investigated by using n-hexamine as template.The obtained zeolites exhibited different morphologies and textural properties and performed distinctly in hydrocarbon cracking and MTH process,showing different selectivity for C2 or C3 olefins although both with prolonged catalytic lifetime.Secondly,combined post-treatment with controllable desilication and phosphorus modification was carried out on industrial high-silica ZSM-5,resulting in mesoporosity and phosphorus introduction.The combined treatment reduced the diffusion limitation and simultaneously stabilized the zeolite structure by improving the hydrothermal stability and preserving the acidic properties,leading to the more stable catalytic performance with lower deactivation rate and higher selectivity for C2 or C3 olefins in hydrocarbon cracking and MTH process.Furthermore,to reduce the complexity of traditional preparation,a novel one-step method for preparation of hierarchical ZSM-5 with high hydrothermal stability was developed.Organic quaternary phosphonium hydroxide(TEPOH)was chosen as phosphorus resource to enhance the accessibility/interaction with framework aluminum.Phosphorus was rationally inserted into zeolitic framework and the easy leaching of the phosphorus from the channel was effectively avoided.The material prepared by the newly developed post-treatment exhibited much higher reactivity and more pronounced deactivation resistance property in the acid-catalysis,owing to the simultaneous mesoporosity and phosphorus introduction.In the third part,the synthesis of high-silica MEL zeolite,another member in the pentasil family,was investigated.Herein,we proposed a facile procedure to obtain the hierarchical ZSM-11 aggregates with large external surface area(>300 m~2g-1)by a two-step synthesis strategy instead of the traditional direct synthesis.This strategy was consisted with the synthesis of a non-alumina silicalite-2 with larger macro-size in the TBAOH-TEOS-H2O system firstly and then a subsequent alumination.It avoided the difficulty on direct synthesis of hierarchical ZSM-11 aggregates in Al-sensitive synthetic system.The obtained hierarchically porous ZSM-11 aggregates exhibited the prolonged catalytic lifetime and higher desirable selectivity in hydrocarbon cracking and MTH process.The outstanding catalytic performance demonstrated the efficiencies of this newly-developed modification in reverse thinking.