Syntheses And Performance Of Nanosized/hierarchical ~*BEA And MFI-type Zeolites

Zeolites with regular pore systems have occupied an indispensable position in the application fields of catalysis,adsorption and separation,which is due to their high specific surface area,unique catalytic shape selectivity,tunable acidities and outstanding thermal/hydrothermal stability.Particularly,*BEA-type zeolites with 3-dimensional 12-ring channel system（Beta zeolites）and MFI-type zeolites with 2-dimensional 10-ring intersecting channel system（e.g.,ZSM-5 zeolites）,are two types of important industrial catalysts,which are applied in traditional petrochemical industry and play a crucial role in sustainable development fields such as biomass conversion and environmental chemistry.Ai-rich Beta zeolites can be applied for the efficient catalytic conversion of lactic acid to lactide,which is used for further production of biodegradable polylactic acid.Meanwhile,high-silica Beta zeolites exhibit excellent performance in the methanol-to-propene（MTP）reaction,gas separations and adsorption of volatile organic compounds（VOCs）.Nanosized and hierarchical zeolites,possessing large surface area and short diffusion path,could significantly improve their catalytic and adsorption efficiency.However,the prepared high-silica Beta zeolites generally possess large crystal size（>1μm）and the range of Si/Al ratios is limited（Si/Al<300）.Expanding the range of Si/Al ratios of Beta zeolites while ensuring their nano sizes remains a great challenge and is urgent to be solved.Using solvent-free route to prepare Beta zeolites can reduce the amount of template and solvent,further reducing cost and pollutants.However,the as-prepared zeolites present poor product uniformity and texture properties,which is not conducive to their catalytic application.Therefore,the development of a facile strategy based on solid raw materials to prepare highly dispersed nanosized or hierarchical zeolites is helpful to achieve green synthesis of high-performance zeolites and their application in efficient catalytic conversion and adsorption/separation.On the other hand,metal–supported zeolites could construct efficient multifunctional catalytic materials.In recent years,MFI-type zeolites have been gradually developed as an ideal support for the confinement of metal species such as metal single sites,clusters and nanoparticles.Among them,the studies on the application of Ni-based MFI catalysts for dry reforming of methane（DRM）has been widely reported.However,the catalytic stability of these catalysts is generally low.Adjusting the structure of zeolites and optimizing the metal loading method to enhance the metal–support interaction,are expected to improve the DRM performance,which is of great significance for the efficient utilization of resources and environmental protection.This thesis mainly focuses on the development of synthesis strategies and performance optimization of nanosized/hierarchical*BEA and MFI-type zeolites.Nanosized high-silica Beta zeolites and nanosized hierarchical Beta/ZSM-5 zeolites have been synthesized by microwave-assisted hydrothermal strategy and steam-assisted crystallization from ball-milled solid raw materials,which exhibit superior performance in the MTP reaction,adsorption of VOCs and conversion of lactic acid to lactide.In addition,nanosized Ni@S-1 zeolites have been synthesized through dispersing the metal in all-silica MFI zeolite silicalite-1 by ethanol-assisted impregnation,which exhibit superior catalytic stability in the dry reforming of methane.This thesis makes an effort to reveal the crystallization mechanism of nanosized high-silica/hierarchical Beta and establishes the structure-activity relationship between the size and distribution of Ni species and catalytic activity.This work will provide useful guidance for the designed synthesis and performance optimization of*BEA zeolites and Ni-based MFI zeolite catalysts with superior catalytic properties.The main results of this thesis are as follows:1.A series of nanosized high-silica Beta zeolites（Si/Al=9–∞）were synthesized by a microwave-assisted hydrothermal strategy.The rapid synthesis of Beta zeolites with a wide range of Si/Al ratios（Si/Al=9–338）while maintaining the nano sizes（25–100nm）have been achieved,in which the crystallization only costs 10–14 h.By comparison,conventional hydrothermal synthesis of high-silica Beta usually takes at least 96 hours.Notably,the pure-silica Beta zeolite with a crystal size of 180 nm was successfully synthesized through the hydrothermal synthesis in the absence of fluoride medium and seed.As-prepared Beta zeolites show outstanding performance in the MTP reaction and the adsorption of VOCs.By means of X-ray diffraction,transmission electron microscopy and electron paramagnetic resonance analysis,we reveal the two key roles of microwave irradiation:1)accelerating the nucleation by a rapid heating of the precursor suspension,thus increasing the number of nucleation sites and reducing the crystal size.2)inducing the production of hydroxyl free radicals（·OH）,thus promoting the formation of Si-O-Si bond during the nucleation process and then increasing the Si/Al ratio of zeolites.In this work,the mechanism of microwave irradiation on the synthesis of high-silica zeolites has been studied for the first time,which opens a new avenue to the synthesis of nanosized high-silica Beta zeolites and provides useful guidance for the synthesis of nanosized high-silica zeolites in other structure types.2.Highly dispersed nanosized hierarchical Beta zeolites with varied Si/Al ratios（Si/Al=10–40）were prepared via steam-assisted crystallization from ball-milled solid raw materials.The as-prepared zeolites present mulberry-like zeolite aggregates in size of ca.200 nm that are assembled from uniform 15 nm crystallites,and possess abundant interconnected intraparticle mesopores（6–15 nm）.As-prepared Beta zeolites show excellent catalytic activity and stability in the conversion of lactic acid to lactide.The crystallization process and mesoporous evolution mechanism are investigated through low-voltage high-resolution scanning electron microscopy,high-resolution transmission electron microscopy and X-ray diffraction analysis.Sufficient mixing of solid raw materials under ball milling and favorable migration of solid mixture under steam condition contribute to the construction of uniform nanoparticles and interconnected intraparticle mesopores.Furthermore,this strategy is extended to synthesize nanosized hierarchical ZSM-5 zeolites.Such an approach exhibits lower cost and simpler synthesis procedure than traditional steam-assisted conversion and the products present higher dispersion and uniformity of crystals than those obtained in the solvent-free route.This strategy provides a new and green synthesis method for the simple and cost-efficient synthesis of highly dispersed nanosized hierarchical zeolites.3.Using nanosized MFI zeolites silicalite-1 as support,the Ni@S-1 zeolite catalyst with highly dispersed Ni nanoparticles（NPs）was synthesized by ethanol-assisted impregnation method.Compared with Ni/S-1 catalyst prepared by deposition precipitation,the sizes of Ni NPs in Ni@S-1 maintain smaller and present more uniform dispersion,endowing its excellent catalytic activity/stability in the dry reforming of methane.The CH₄ and CO₂ conversions over Ni@S-1 catalyst can still reach 89%and94%after 200 h,respectively,which are superior than most reported Ni-based zeolite catalysts and other Ni-based catalysts.The structure-activity relationship of the size and distribution of Ni NPs and the catalytic activities are investigated by combining X-ray diffraction,scanning transmission electron microscopy,H₂ temperature-programmed reduction and Raman analysis.The Ni NPs with small size（1.5 nm）are uniformly encapsulated in Silicalite-1 zeolite,and they slightly increase to 2.5 nm but remain well dispersed.The strong metal–support interaction between Ni species and Silicalite-1zeolite endowed Ni@S-1 catalyst high sintering-and coking-resistant properties.This work provides guidance for the synthesis of Ni-based zeolite catalysts with high performance.