Synthesis Of Mesoporous Zeolite Via Hard-Templating Route

Zeolites are porous crystalline solids whose pores are of molecular dimensions thereby providing size and shape selectivity for guest molecules. Zeolites are widely used in catalysis as well as in the separation and purification fields due to their uniform, small pore size, high internal surface area, flexible frameworks, and controlled chemistry. The major drawback of zeolites is that the small size of the channels (less than <0.8 nm) and cavities (typically <1.5 nm) imposes diffusional limitations on reactions that can cause high back pressure on flow systems. Compared to the dimensions of the zeolite micropores (<2 nm), mesopores (2-50 nm) permit entrance in and out of large molecules and fast mass transfer of the guest molecules, which overcomes the diffusional limitations of zeolites. However, compared to conventional microporous zeolites, hydrothermal stability and catalytic activity is relatively low due to the amorphous silica framework. The concept of infusing mesopores into zeolite particles to possess microporous zeolites framework and mesopores together has attracted much attention. The goal of this study is to synthesize mesoporous zeolite by using proper carbon template. The ordered mesoporous carbon with pores smaller than 4 nm was not suitable for the template synthesis of mesoporous zeolite. We have synthesized mesoporous zeolites by choosing porous carbon with large pores replicated from the colloidal silica particles with different size (10-100 nm).In chapter 2, ordered mesoporous carbons are prepared by using ordered mesoporous silicas as templates: 1) to introduce organosilican in the co-condensation synthesis of bi-continuous ordered mesoporous silica, the pore size decreased with the increasing amount of organosilica added; the thickness of silica framework increased correspondingly while cell parameter varied little. The replica of ordered mesoporous carbon would possess large pore size originated from thick walls. 2) 3-D cage-like ordered mesoporous carbons were synthesized by using small-pore 3-D cage-like ordered mesoporous silicas as templates. The entrances to the cages were too small for carbon source to fulfill the pore system and the resultant carbon bridges were not strong enough to preserve the mesophase. After post-treatment of refluxing ordered mesoporous silica templates with surfactants in 1.0 M HCl acid solution, the pore size was enlarged without destruction of mesophase. The modified templates could successfully synthesize ordered mesoporous carbons.In chapter 3, large porous carbons were successfully prepared by using colloidal silica with different particle size and pH value as templates. There are two different methods before carbonization: (1) the solution of sucrose and sulfuric acid was added into the templates composed of dried colloidal silicas; (2) the sucrose was directly added into the colloidal silica solution, then evaporation of water. Large porous carbons were obtained after carbonization of the colloidal silica/sucrose composites and removal of silica particles. The BET surface area、pore volume and mean pore size of porous carbons were affected not only by the size of silica particles but also the pH value of colloidal silica solution in method 1. Porous carbons from low pH solution shows larger BET surface area, lower pore volume and lower mean pore size with narrower pore size distribution than that from high pH solution with the same particle size. In method 2, the single silica particles played the true template role in the synthesis of large porous carbons especially for porous carbon replicated from smallest particle size.In chapter 4, mesoporous zeolites were prepared by using large porous carbon originated from the colloidal silicas with different particle size. The hot clear zeolites precursor solution fulfilled the pore system of large porous carbon, nucleation and crystallization at high temperature. Mesoporous zeolites were obtained after calcination in air at high temperature 600oC. Every MFI zeolite crystals show a large amount of pores inside. More interesting, the shapes of mesopores in zeolite crystals turned out to be regular after refluxing in boiling water for a certain time. The shapes of mesopores showed hexagonal and rectangular when observed from the [010] and [100] crystal axis of MFI zeolite, respectively. Furthermore, the frameworks of the mesopores were composed of the specific zeolite planes in single crystal states, and mesoporous zeolites showed highly thermal and hydrothermal stability. Mesoporous MFI zeolite templated from small pore ordered mesoporous carbon showed little mesopores in crystals.