Syntheses Of Single-crystalline Nanosized/Hierarchical ZSM-5 And Beta Zeolites With Excellent Catalytic Performance

Zeoliteswithuniformmicropores,tunableaciditiesandhigh thermal/hydrothermal stability are important shape-selective catalysts in the petroleum refining,petrochemical processes and fine chemicals production.Particularly,ZSM-5?MFI-type?with 2-dimensional 10-ring intersecting channel systems and Beta?^*BEA-type?zeolites with 3-dimensional 12-ring channel systems are two types of important industrial catalysts,which are extensively applied in various important catalytic processes such as isomerization,alkylation,methanol to olefin and biomass conversion.Nowadays,the fossil energy crisis is an urgent problem all over the world.The efficient utilization of fossil sources is an effective method to solve this problem,which highly depends on the fabrication of high-performance zeolite catalysts.However,the sole microporosity of conventional zeolites restricts their catalytic performance especially when bulky reactants/products are involved.Fabrication of nanosized and hierarchical zeolites with larger surface area,shorter diffusion path length and more exposed active sites has proven to be effective strategies to overcome the limitations of accessibility,diffusion and mass transportation,thus improving the catalysis efficiency.It is noted that the current synthesis approaches for nanosized and hierarchical zeolites generally cause a polycrystalline feature,poor crystallinity and hydrothermal stability,low product yield and high synthetic cost.In this thesis,we present a facile strategy affording high-quality single-crystalline nanosized/hierarchical ZSM-5 and Beta zeolites with good monodispersity,high crystallinity,high product yield and controllable Si/Al ratios as well as mesopore sizes.They are prepared by synergistically using an L-lysine-assisted approach and a two-step crystallization process in a concentrated gel system.By utilizing C_s-corrected scanning transmission electron microscopy?C_s-corrected STEM?,low-voltage high-resolution scanning electron microscopy?LV-HR-SEM?and high-resolution transmission electron microscopy?HRTEM?analysis,we systematically investigate the effects of synthetic parameters on the size and morphology for zeolite crystals,as well as the evolution process of single-crystallinenanosized/hierarchicalnanocrystals.Theas-prepared single-crystalline nanosized/hierarchical catalysts exhibit superior catalytic performance in methanol-to-propylene?MTP?reactions,diesel production by hydrodeoxygenation of palm oil and lactide production from lactic acid.The main research results of this thesis are shown as follows:1.A facile strategy has been developed,which affords high-quality single-crystalline MFI-type nanozeolites?10-55 nm?with hexagonal prism morphology,good monodispersity,high crystallinity and high product yield?above97%?.This is achieved by synergistically using an L-lysine-assisted approach and a two-step crystallization process in a concentrated gel system?H₂O/Si=9?.The morphological evolution of single-crystalline nanosized silicalite-1 is monitored by high-resolution transmission electron microscopy?HRTEM?and X-ray diffraction analysis.In this process,metastable irregular nanoparticles are initially obtained at80°C in the first step.Consequently,the single crystal nanoparticles with irregular morphology gradually transformed into nanozeolites with regular morphology and high crystallinity at 170°C in the second step.Throughout the whole process,L-lysine acts as an inhibitor to effectively limit the crystal growth of zeolites.Concentrated gel system contributes to increasing the product yield.Compared with the micron-sized counterparts,the as-prepared nanosized ZSM-5 catalysts exhibit enhanced performance in methanol-to-propylene?MTP?.2.In the synthesis gel,excess L-lysine is added to induce a kinetic-modulated crystallization process for constructing single-crystalline and defect-free hierarchical ZSM-5 nanocrystals.By utilizing C_s-corrected scanning transmission electron microscopy?C_s-corrected STEM?,low-voltage high-resolution scanning electron microscopy?LV-HR-SEM?and solid NMR analysis,we investigate the evolution process of single-crystalline hierarchical ZSM-5 zeolites.Under low temperature conditions,protozeolitic primary nanoparticles are obtained and then undergo an oriented aggregation in a non-compact manner,forming single-crystalline hierarchical nanozeolites with isolated mesopores;Under high temperature conditions,primary nanoparticles coalesce into larger ones through intraparticle ripening,consequently leading to the evolution of pregenerated isolated mesopores to interconnected mesopores.Via this strategy,we have successfully synthesized single-crystalline and defect-free hierarchical ZSM-5 nanocrystals?155 nm?with high crystallinity,high product yield and high hydrothermal stability.After loading the Ni component,the Ni/ZSM-5 catalysts exhibits superior performance in converting stearic acid and palm oil to diesel.3.Nanocrystalline Beta zeolites with a broad Si/Al ratio?SiO₂/Al₂O₃=18-600?,good crystallization and high product yield?above 80%?have been successfully prepared by hydrothermal synthesis route which efficiently combines L-lysine-assisted approach confining the growth of zeolite,a two-step crystallization process endowing zeolite crystals with good crystallinity and a concentrated gel system?H₂O/Si=6?ensuring high product yield.The as-prepared ultrasmall Beta sample possesses the smallest particle size with a diameter of ca.9.5 nm.The crystallization process is monitored by high-resolution transmission electron microscopy?HRTEM?and X-ray diffraction?XRD?analysis.In the first step?80°C?,amorphous gel nanoparticles?10-20 nm?are obtained,in which L-lysine is used as a growth inhibitor.Consequently,in the second step?140°C?,nucleation occurrs in the amorphous gel nanoparticles,resulting in tiny embedded crystallites in gel nanoparticles.The gel nanoparticles are consumed and the gel-zeolite intermediates maintain their average size throughout the whole process of the complete conversion into nanocrystalline Beta zeolite?10-20nm?.The catalytic performance of nanocrystalline Beta zeolite is evaluated through a biomass conversion yielding lactide?LT?from lactic acid?LA?.The nanocrystalline Beta zeolites perform the best in converting L₃A to LT compared with their micron-sized counterparts and other commercially available nanosized Beta zeolites.We demonstrated for the first time that LA trimer?L₃A?,which is observed in the crude reactants and also detected as an intermediate,could be converted to LT over the as-prepared Beta zeolites.The reaction mechanism over Beta is further investigated by DFT calculation,and the reaction pathway from L₃A to lactide and from L₂A to LT is proposed,which gives a good explanation for the catalytic results.4.High-quality aluminum-rich nanosized ZSM-5 and Beta zeolites have been successfully prepared by synergistically using two-step crystallization and amino acid-assisted approach in concentrated gel system.Aluminum-rich ZSM-5?SiO₂/Al₂O₃=18?zeolite possesses only 4-coordinated framework aluminum atoms and single-crystalline feature.Aluminum-rich Beta zeolite?SiO₂/Al₂O₃=12?possesses nanosize and high crystallinitye.In this synthesis strategy,amino acids play an important role in introducing framework aluminum species into the zeolites.Liquid NMR measurement reveals that amino acids can chelate with aluminum and silicon in the initial gel.Such Si-amino acid and Al-amino acid complexes serve as new precursor sources nourishing the growth of zeolites,which is favorable for incorporation of more Al species into zeolite framework and prevent the emergence of amorphous and impurity phase.