テンプレートフリーゼオライトナノクリスタル合成法

抄録

In hydrothermal synthesis of zeolites, organic-structure-directing agents (OSDA) are employed to form zeolite structure, in addition to alkali metals, and Si and Al sources. Recently, there has been growing interest in OSDA-free synthesis of zeolites, such as MFI and MOR. Since formation of zeolite nuclei and crystals proceeds in a strong alkaline solution, however, nucleation/crystallization and resolution of zeolites simultaneously occurs, leading to low yield of zeolite and poor crystallinity due to deposition of amorphous silica. In contrast, we successfully prepared MFI and MOR zeolite nanocrystals via hydrothermal synthesis in a water/surfactant/organic solvent, where the non-ionic surfactants adsorbed on the surface of the zeolite precursors likely induced the formation of zeolite nuclei. Moreover, it is considered that the adsorbed surfactant inhibit the zeolite nuclei and crystal from their resolution. Main objective of this study is development of OSDA-free synthesis of MFI zeolite at high zeolite yield.

In order to investigate the effect of surfactant properties on the crystallinity, OSDA-free MFI zeolite synthesis is carried out using the surfactants with different oxyethylene-chain length. Addition of surfactant into the synthetic solution is effective in improving the crystallinity of obtained MFI zeolite. Moreover, the HLB (Hydrophile-Lipophile Balance) vales of surfactant affect the amount of N2 adsorbed on micropore. Since zeolites possess hydrophilic-hydrophobic properties on their surface, there exists the affinity between the zeolite surface and surfactant, depending on HLB values. As compared with the sample without surfactant, the yield of MFI zeolite at Si/Al ratio of 12.5 increased from approximately 60% to 90% in OSDA-free synthesis using surfactant O-15. The high yield of zeolite in OSDA-free synthesis is ascribed to the adsorption of surfactant on the zeolite surface, where the resolution of Si and Al atoms in zeolite framework are inhibited.