| Synthetic zeolites with well-defined crystalline structures and catalytic properties have been widely used in the fields of petroleum refinery. However, the diffusion limit and fast catalyst deactivation are the main drawbacks of microporous zeolites with 0.5 nm micropores when processing bulky molecules. Moreover, heavy coke will also be easily formed due to low mass transport, which leads to faster catalyst deactivation. To overcome these limitations, researchers have focused on the design and synthesis of hierarchically structured zeolites with an improved catalytic performance.In this thesis, we designed and synthesized a series of novel hierarchical structured materials based on ZSM-5 zeolites to overcome the limitation of low mass transport and increase the stability of catalysts. We also aimed at simplifing the operation procedures, reducing the products cost and promoting the industrialization process of hierarchical materials.In the first part, the parent ZSM-5 zeolites, with regular crystal size of 1-2 um and Si/Al ratios of 15-500 nm, were first hydrothermally synthesized using inorganic silicon and aluminum sources and piperidine as structure directing agent with the assistance of active seeds. Controllable mesopores were successfully introduced into as-synthesized ZSM-5 crystals by alkaline-derived desilication with the addition of piperidine. The addition of organic amines shielded the zeolite crystals from extensive dissolving by NaOH attacking. However, piperidine molecules, which are smaller than the ZSM-5 micropore size, can enter into zeolites channels readily and they offer the same protection to the OH" attacking as achieved with TPA+or TBA+. Piperidine was employed as the structure-directing agents in the hydrothermal synthesis of ZSM-5 zeolites in this study. In post-treatment with NaOH solution, the same zeolite structure still could distinguish piperidine at the molecular level. Those accommodated amine molecules then acted as efficient pore-growth moderators during desilication. Possessing a lower hydrogen transfer activity, the alkali-treated ZSM-5 samples show a significantly higher propylene yield in the cracking of n-hexane than the parent ZSM-5. The mesopore formation suppresses effectively the secondary reactions and coke formation, leading to a longer catalyst life.In the second part, a series of well-defined, core-shell-structured composite materials comprising microporous/mesoporous ZSM-5 as core and mesoporous aluminosilicate as shell were synthesized by combining controlled desilication using sodium hydroxide solution with subsequent self-assembly using a triblock copolymer. The obtained composite zeolites exhibited a hierarchical porosity containing the original regular MFI micropores (ca.0.56 nm) and desilication-induced, randomly distributed mesopores (5-50 nm), both within the core ZSM-5 crystals, as well as relatively uniform mesopores (ca.6 nm) inside the shell part on the zeolite surface. The mesoporous aluminosilicate shell, self-assembled from the MFI zeolite fragments as a result of partially dissolved ZSM-5 crystals, demonstrated weak acidity and much higher hydrothermal stability in comparison to the shell synthesized by the additional silica source. Taking advantage of the confining effect of the mesopores, Pt nanoparticles were incorporated into the mesoporous shells, giving rise to bifunctional catalysts, which exhibited a higher selectivity of C5-C11 liquid products compared to the conventional Pt/ZSM-5 catalyst in the hydrocracking of n-hexadecane.In the third part, hierarchically porous zeolite supported Ni nanoparticles were successfully prepared through a base-assisted chemoselective interaction between the silicon species and metal salts. This process involved the selective and partial dissolving of SiO2 off the ZSM-5 crystals and then in situ interaction with the Ni2+ in solution to form the layered Ni3Si2O5(OH)4, then the nickel silicate precursors underwent decomposition and reduction to form Ni nanoparticles and S1O2. The pore size distribution showed that Ni/ZSM-5 possessed the mesopores with 5-30 nm diameters. The mesopores are mainly ascribed to the disordered assembly of the flexible SiO2 matrices. And the spherical Ni NPs with small sizes (5-6 nm), high loading (30 wt%) and high dispersion (5.0%) were dispersed uniformly on the SiO2 matrices, just like "diamond on the beach". The mesoporous SiO2 matrices effectively prevented the Ni NPs from sintering and lossing. The as prepared Ni/ZSM-5 catalysts exhibited excellent catalytic activity and stability in the hydrodeoxygenation of bioacids. Importantly, the synthetic strategy described above is quite general. We had successfully prepared other bifunctional catalysts, such as Ni/MOR, Ni/Beta, Ni/HY, Ni/MCM-22, Cu/ZSM-5 and Co/ZSM-5.In the fourth part, hierarchically porous zeolite supported Co nanoparticles are successfully prepared through a novel hydrothermal method. This process involved the selective and partial dissolving of S1O2 off the ZSM-5 crystals and then in situ interaction with the Co2+in solution to form the layered Co3(Si2O5)2(OH)2, then the precursors underwent decomposition and reduction to form Co nanoparticles and SiO2 under 600 ℃ in H2 atmosphere. The obtained core-shell structured materials of Co/ZSM-5 showed hierarchical pores, when the Co loading amount was 30 wt%, its mesopore volume and external surface area were 0.37 cm3 g-1 and 203 m2g-1, respectively. Co/ZSM-5 showed excellent activity and reusability in F-T synthesis and conversion of ethyl levulinate in ethanol to ethyl valerate. Especially, Co/ZSM-5 was first used as catalyst in the later reaction, the conversion for ethyl levulinate and the selectivity for ethyl valerate were 100% and 78% after reaction for 6 h at 250℃, respectively.In the fifth part, Magnesium was incorporated into the zeolite by a novel hydrothermal method. Flower-like Mg3Si4O9(OH)4 was coated on the outside of the zeolite by the interaction between the Mg2+ and SiO2 on the surface of zeolite under the alkaline condition to form core-shell structured materials of MgSiO2/ZSM-5. The resultant materials contained both acid (0.38 mmol g-1) and base active sites (0.25 mmol g-1). Compared with IM-MgO/ZSM-5 prepared by wetness impregnation method, MgSiO2/ZSM-5 contained large amounts of mesopores centered at 2-20 nm, its mesopore volume and external surface area were 0.26 cm3 g-1 and 193 m2 g-1, respectively. With the special properties, the MgSiO2/ZSM-5 showed excellent activity and stability during the tandem reaction of deacetalization-Knoevenagel condensation. |
