| Hierarchically porous zeolite microspheres combining the advantages of desirable mass transport of nanozeolites and easy separation and handling of bulky zeolites are excellent candidates in adsorption and catalytic applications.Facile routes for the assembly of hierarchical zeolite microspheres with controllable morphologies and robust mechanical strength are much expected.Herein,based on a microfluidic jet spray drying technology,we explored the controllable synthesis of hierarchical zeolite microspheres and their formation mechanisms.The applications in acid catalysis were also explored.The main results of the thesis are shown as follows:1.Based on spray drying,a ’"chemical crosslinking" strategy is proposed to synthesize hierarchical zeolites microspheres(SD-ZSM-5-190-12-M)by directly using the as-synthesized nanozeolite suspension as the precursor.This route not only avoids the energy-intensive centrifugal separation process of nanozeolites,but also significantly increases the uniformity and mechanical strength of the obtained zeoloite microspheres.The soluble aluminosilicate species in the precursor:first act as a stabilizer to improve the droplet stability during the spray drying process,and then as a "cross linker" to chemically bind and interconnect zeolite nanoparticles to form robust bodies after drying and calcination.The obtained zeolites show a spherical buckled morphology,uniform and controllable sizes(from 97～108μm).They possess emerged uniform mesopores(～6 nm)and well-maintained high surface area(509 m2/g),large pore volume(0.51 cm3/g)and high microporosity as originated from nanozeolites.As a result,they exhibit excellent performance in acetolysis of epichlorohydrin and catalytic cracking of low-density polyethylene.2.The influnences of the precursor compositions and spray drying conditions(solid content,drying temperature,diameter of nanozeolite,nozzle size and the precursor pH)on the morphology of the spray-drying assembled zeolite microspheres were investigated.The particle formation and morphology transition mechanism were explored.The results shows that without pH adjustment,the solid content,drying temperature,diameter of nanozeolite and nozzle size have a certain capability towards controlling the morphology and the particle sizes of obtained zeolite microspheres.However,such a capability is limited by the drying ability of the equipment or the working state of the nozzle,which makes the synthesis of non-buckled zeolite microspheres quite difficult.The precursor pH plays a decisive role in the regulation of the morphology of the zeolite microspheres.Combined with single-drop drying experiment results and the droplets drying theory,the mechanism of pH mediated morphology transformation was studied.Because of the reversible agglomeration induced by reduced surface charge of the nanozeolite at slightly lower pH,the packing density of the formed shell can be reduced,as a consequence,the capillary force acting on the formed shell during the drying process can be significantly weakened,which results in the isotropical shrinkage of the droplet,leading to the formation of perfect spherical zeolites.3.Based on spray drying,a "deposition" strategy is proposed to synthesize metal oxide loaded hierarchical zeolite microspheres by using Fe(NO3)3·9H2O as a metal source and Nano-ZSM-5 as support.The distribution α-Fe2O3 is affected by the loading as well as the precursor pH.When the precursor pH<1.58,α-Fe2O3 is well dispersed in the form of nanoparticles(12.50±3.67 nm)at the intercrystalline voids originated from the stacking of zeolite nanoparticles.With the increase of Fe loading amounts,plenty of α-Fe2O3 sub-microparticles appeared on the surface of the Fe-ZSM-5 microparticles,and their diameters gradually increase.When the loading reaches 3.0 wt%,with the increase of precursor pH,those α-Fe2O3 sub-microparticles gradually disappear,along with the morphology transformed from red-blood cell to perfect spherical morphology.Sucn transformation is attributed to the reduced diffusion rate of Fe species and the interaction between nanozeolites.These Fe-ZSM-5 microparticles exhibit certain activity in the photocatalytic decolorization of methylene blue.4.Based on spray drying,an ecapsulation-transformation strategy is proposed to synthesize hierarchial zeolite microsperes.Amorphous aluminosilicate microspheres with tetrapropylammonium hydroxide(TPAOH,the template for zeolite growth)encapsulated as the dry gel precursors are first synthesized by spray drying method.Then,they are successfully transformed into robust hierarchical zeolite microspheres by using a steam-assisted crystallization(SAC)process.The drying temperature in the synthesis of the dry gels plays a decisive role in determining the preservation of TPAOH guiding zeolite crystallization,as well as the morphology and sizes of the obtained zeolites.The morphology,structure and crystallinity of the resulted zeolites can also be controlled by adjusting the water/dry gel mass ratio and time adopted in the SAC process.The detailed structure and crystallization evolution over the SAC transformation process is studied and discussed.The typical obtained zeolite microspheres possess unique surface buckled morphologies,uniform large particle sizes(53.7±2.0 μm),excellent crystallinity(95.2%),hierarchical micro/meso/macro porosities,good mechanic strength and intrinsic strong acid sites.They demonstrate enhanced catalytic activity and stability,as well as easy catalyst separation,in acid catalytic aldol condensation. |
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