Synthesis And Modification Of Hierarchical Zeolites And Their Catalytic Applications

The discovery of natural zeolites in the eighteenth century opened the era of studying on porous materials. Especially, in the middle of 1900's, the exploitation of synthesized zeolites of different Si/Al ratio strongly accelerated the development and application of zeolitic materials. Due to the uniform pore sizes, relatively high specific surface area, changeable porous structure, high thermal and hydrothermal stability, zeolites were widely used in the field of adsorption, separation and catalysis. As for catalysts, zeolites are the most widely used ones in petro chemistry and fine chemical synthesis.However, conventional microporous zeolites are small in pore size (less than 1.2 nm) so that they may face steric hindrance especially in the catalysis involving bulky molecules and the separation of bio-materials. Moreover, the restricted diffusion in micropores will also lead to coke formation in catalytic reactions which lowers the stability of catalysts. Hence, materials with larger pores caught the attention of researchers. In 1992, Scientists of Mobil Oil Cooperation first synthesized ordered mesoporous material M41S, using quaternary ammonium salts as the surfactant. The novel type of silica has high specific surface area, changeable pore systems and thick pore walls so that it seems to have good capability as catalysts. For example, owing to the larger pore size, the mesoporous materials enable bulky molecules to enter so that they exhibit well catalytic performance in the field of oil-refining and fine chemical synthesis involving bulky molecules.Anyway, compared with zeolites, the walls of traditional ordered mesoporous materials were amorphous, which leads to lower acidity and hydrothermal stability. This shortage restricted the industrial utilization of mesoporous materials. Hence, micro/meso zeolites (i.e hierarchical zeolites) were discovered recently to combine both the merits of mesoporous materials and microporous zeolites. The designed hierarchical zeolites will have strong acidity, well stability, high specific surface area and large pore volume. Nowadays, many research groups all over the world were exploring these kinds of materials through different routes.In this work, we utilized some of the synthesizing methods as experience for references to synthesis a series of hierarchical zeolites. We compared the textural and acid properties of hierarchical zeolites with conventional zeolites and amorphous mesoporous materials through different kinds of characterizations. The catalytic performances in some kinds of reactions were studied. Moreover, our aim was to set the inner relationship between "catalytic performance" and "material structure". The whole thesis can be divided into four parts.Part?:Synthesis of MSU-S, the mesoporous materials with zeolitic domains, and their catalytic performance in liquid phase reactions.(1) MSU-S with a wide range of Si/Al ratio (MSU-S(Y):5-50, MSU-S(BEA):33-150) were synthesized. The catalysts were characterized via X-ray diffraction (XRD), N2 adsorption/desorption, Fourier transformed infrared spectra (FT-IR),27Al MAS NMR, ammonia temperature programmed desorption (NH3-TPD) and liquid phase DTBPy adsorption, etc. Compared to conventional zeolite Y and Beta, MSU-S catalysts have larger surface area and pore diameter. MSU-S samples show no characteristic peaks of zeolites in XRD but the existence of zeolitic domains can be proved by FT-IR and 27Al MAS NMR.(2) Although the total acid sites of MSU-S were less than that of conventional zeolites, the accessible acid sites of MSU-S were far more than that of conventional zeolites. For the alkylation of hydroquinone and the isomerization of a-pinene, the activities of MSU-S catalysts are higher. MSU-S catalysts with Beta and Y seeds have close conversion (75% versus 60%) and yield for 2-TBHQ (54% versus 43%) for alkylation of hydroquinone. For isomerization of a-pinene, the difference between two kinds of zeolitic seeds was higher. The conversion of MSU-S (BEA) can reach 97% while at the same condition, the conversion of MSU-S(Y) is just 52%.(3) For alkoxylation of?-pinene, MSU-S showed quite low activity compared to conventional Beta and mesoporous Beta synthesized from polycation as hard template. Although this reaction also involves bulky molecules, the requirement for acid strength is severe. MSU-S catalysts are with low acidity so that for this kind of reaction, other routes to synthesized hierarchical zeolites were preferred.Part?:Swollen and delamination of MWW zeolites and their activities in liquid phase reaction.(1) MCM-22 and ITQ-2 catalysts with different Si/Al ratio were synthesized. In our static condition, MCM-22 samples with Si/Al ratio in the range of 15 to 50 were well crystallized using HMI as the surfactant. The structure were characterized by SEM, N2 adsorption/desorption and XRD. Starting from MCM-22 precursor with Si/Al ratio of 15 to 30, the delaminated zeolites, ITQ-2, were successfully obtained.(2) Acidity measurement such as NH3-TPD?liquid phase DTBPy adsorption and infrared spectra of pyridine adsorption (Py-IR) were employed. The delaminated zeolites, ITQ-2, have less acid sites and lower acidity compared to MCM-22 with similar Si/Al ratio, but the accessible acid sites on ITQ-2 catalysts were obviously higher. In addition, the ITQ-2 samples have higher ratio of Lewis to Br?nsted acid sites(B/L).(3) For alkylation of hydroquinone, a Frediel-Craft reaction with medium-strong acidity, ITQ-2 catalysts cannot improve the yield of the aimed product,2-TBHQ. For?-pinene isomerization, a liquid phase reaction with mild acid requirements, ITQ-2 showed higher activity. The distinct behavior in these two kinds of reactions was also discussed.Part?:Acidity modulation on MWW zeolites and their catalytic performance.MWW zeolites with Si/Al ratio of 20 were used as catalysts and catalyst supports. Acidity modulation via ion-exchange and Ga supporting were employed.(1) The acidity of MWW zeolites can be modulated by cation exchange. After ion-exchange with sodium cation, the Br?nsted acid sites on MCM-22, ITQ-2 and MCM-36 were suppressed while Lewis acid sites still preserving. For the Nopol synthesis from?-pinene with paraformaldehyde via Prins condensation, the surpression of Br?nsted acid sites reduced the side reaction of isomerization. The NaITQ-2 sample with Si/Al ratio of 20 has the best yield for Nopol (57%)(2) The length of ultrasonic treatment when delaminating from MCM-22 precursor is essential to the textural and acid properties of ITQ-2 products. With the increase of ultrasonic treatment time, ITQ-2 supports have larger surface areas, especially the external ones. After supporting Ga2O3 with wet-impregnation, Ga2O3/HITQ-2 showed higher propylene selectivity compared to Ga2O3/HMCM-22 and Ga2O3/HZSM-5. Firstly, the higher surface areas are favor of the diffusion of product propylene and suppress the by-products. Secondly, the Br?nsted acidity of the supports was reduced so that the extent of aromatization of propane was lower. Part?:Synthesis of core/shell microcomposite and their catalytic activities.The Core/Shell microcomposites, Beta/Silicalite-1, were synthesized. Compared to traditional pore modification like CVD and CLD, the pore system of Core/Shell microcomposites is more accessible. One could take advantage of the high adsorption capacity and the separation power to design shape-selective catalysts. The catalytic activities of Beta/Silicalite-1 samples on the dynamic kinetic resolution (DKR) of 1-phenylethanol were studied. Due to the smaller pore size of Silicalite-1, the shell Silicalite-1 performed as a sieve to prevent the racemization of product ester while enabled the reactant,1-phenylethanol to penetrate. The ee% value of the final product, 1-phenylethyl acetate, is higher while using Beta/Silicalite-1 Core/Shell microcomposites as catalyst.