Salt occlusion in zeolites is a unique phenomenon and plays an important role in solid and molten state reactions. To elucidate its mechanism, the NH4NO3 occlusion was studied with various zeolites that exhibit diversity in pore shape (channel and cavity), window size, and composition and charge of framework. We also attempted to classify NH4NO3 introduced into zeolite pores by characterizing their properties such as stability against washing with water, thermal behavior, and spectroscopic characteristics. Based on the geometric consideration between zeolite pores and salts, the salts introduced into zeolite pores could be classified as 'dispersed', 'occluded', and 'trapped' salts. This classification was based on their stability against washing with water. Occluded NH4NO3 salts were present in the pores of zeolites such as AlPO4-18, Na-Pl and 4A, while dispersed ones were observed in those of zeolites such as ZSM-5 and 13X. Framework charge was not essential in salt occlusion, although it affected the occluded amount and stability of occluded NH4NO3. It is clear from these results that the requirement for salt occlusion is the proper size fit between the window size of zeolite pore and the size of each ion pair of salt. Occluded salts were stabilized through geometric fit and/or electrostatic interactions with negatively charged frameworks that led to increase in their stabilities against washing with water and thermal treatment. Therefore, the geometric relationship of zeolite pore to salt plays more crucial role in salt occlusion than framework charge.
All Science Journal Classification (ASJC) codes
- Materials Science(all)
- Condensed Matter Physics
- Mechanics of Materials