Hard And Soft Template Method For The Synthesis Of Mesoporous Zeolite And Its Catalytic Performance

The zeolite-based hierarchical porous materials with two or more levels of porosity have drawn much attention in the field of porous materials, because of their potential applications in the industrial fields. These materials have been proven to be the promising catalysts that combine the advantage of shape selectivity with the efficient mass transport. The presence of mesopores in zeolite has improved catalytic activity and selectivity significantly in comparison with conventional zeolite catalysts. Most importantly, some zeolites with mesoporosity have been used in a number of industrial processes including the cracking of heavy oil fraction over zeolite Y, the production of cumene, hydro-isomerization of alkanes over mordenite, and the synthesis of fine chemicals over Y, ZSM-5, and beta. The enhanced catalytic activities are due to the reduction of diffusion path and easier access to the active site as the result of the introduction of mesopores into the zeolite framework. In this dissertation we used nano-sized CaCO₃ and polyvinyl butyral gel as the hard mesopore directing agent for the synthesis of mesoporous zeolite. Meanwhile, we investigated the zeolite crystallization in the polyvinyl alcohol system, and it was found that the polyvinyl alcohol in zeolite synthesis could produce mesopores within the zeolite. Lastly, the catalytic performance of mesoporous zeolite was studied in the tri-methyl benzene cracking reaction as well as toluene disproportionation and transalkylation with C₉ and C₁₀ aromatics. The main results in this dissertation are as follows:1. The silicalite-1 and ZSM-5 crystal with secondary pores in the range of 50-100 nm was synthesized by using the nanosized CaCO₃ as a hard template. The nanosized CaCO₃ can be trapped into the MFI zeolite crystals during the crystallization process. By means of acid dissolution, the encapsulated nanoparticles were removed, giving rise to the intracrystal pores within the zeolite crystal. Characterization techniques including XRD, TEM, SEM, and N2 adsorption provided the detailed information on this hierarchical pore structure. The hydroxyl groups on the surface of CaCO₃ are essential to taking the hard template effect. The secondary pores within zeolite correspond well to the morphology of the nanosized CaCO₃, which confirms the template effect of nanosized CaCO₃. These results suggest that using CaCO₃ as a hard template may be a useful approach for the synthesis of hierarchical porous materials.2. The zeolite crystallized in the presence of polyvinyl alcohol solution was systematically studied. The existence of polyvinyl alcohol in zeolite synthesis did not exert negative effect on the crystallization of zeolite, meanwhile ZSM-5 and ZSM-11 could be synthesized in the polyvinyl alcohol system. The characterization technique indicated that the zeolites obtained from polyvinyl alcohol not only possessed micropore but also had additional mesopore. The mesopore volume can be controlled by the amount of polyvinyl alcohol in the synthesis.3. The mesoporous zeolites with BEA, MFI, and MEL topology were synthesized by using polyvinyl butyral gel as the mesopore directing agent. The PVB/silica composite was readily prepared by sol-gel technique and then used as precursor for the synthesis of mesoporous zeolites. Upon the direct hydrothermal crystallization of PVB/silica composite, the silica was transformed into zeolite, whereas the PVB gel was incorporated in the zeolite crystals. Calcination of the obtained PVB/zeolite composite yielded the opening of micropores and mesopores in zeolites, respectively. The characterization techniques confirmed that the obtained porous materials consisted of well-defined microporosity and irregular mesoporosity. Raising the ratio of PVB/SiO2 in the synthesis leaded to the increase in mesopore volume of mesoporos zeolite, indicating the mesopore volume in mesoporous zeolite is controllable. 4. The NMR analyses on the mesoporous zeolites suggested that the mesoporosity in zeolite had little influence on the Si and Al species in the framework. Furthermore, the characterization data from the temperature-programmed desorption of ammonia (NH₃-TPD) on the H-form conventional zeolites and mesoporous zeolites confirmed that the mesoporous zeolites shown the same acidity as that of conventional zeolites. The mesoporous zeolites exhibited better catalytic activity and enhanced resistance toward deactivation in the cracking of 1,3,5-trimethylbenzene and 1,2,4-trimethylbenzene, as compared with conventional zeolites. Also, the mesoporous zeolites show improved catalytic activity in the toluene disproportionation and transalkylation with C9 and C10 aromatics. The enhanced catalytic performance is due to the presence of mesopores for increasing the diffusion rate of the reactant. Bimodal porosity as well as strong acidity will enable these materials to be a promising catalyst in a wide range of chemical industry.