Role of framework on NH4NO3 occlusion in zeolite pores

Salt occlusion in zeolites is a unique phenomenon and plays an important ro le in solid and molten state reactions. To elucidate its mechanism, the NH4 NO3 occlusion was studied with various zeolites that exhibit diversity in p ore shape (channel and cavity), window size, and composition and charge of framework. We also attempted to classify NH4NO3 introduced into zeolite por es by characterizing their properties such as stability against washing wit h water, thermal behavior, and spectroscopic characteristics. Based on the geometric consideration between zeolite pores and salts, the salts introduc ed into zeolite pores could be classified as 'dispersed', 'occluded', and ' trapped' salts. This classification was based on their stability against wa shing with water. Occluded NH4NO3 salts were present in the pores of zeolit es such as AlPO4-18, Na-P1 and 4A, while dispersed ones were observed in th ose of zeolites such as ZSM-5 and 13X. Framework charge was not essential i n salt occlusion, although it affected the occluded amount and stability of occluded NH4NO3. It is clear from these results that the requirement for s alt occlusion is the proper size fit between the window size of zeolite por e and the size of each ion pair of salt. Occluded salts were stabilized thr ough geometric fit and/or electrostatic interactions with negatively charge d frameworks that led to increase in their stabilities against washing with water and thermal treatment. Therefore, the geometric relationship of zeol ite pore to salt plays more crucial role in salt occlusion than framework c harge. (C) 2001 Elsevier Science B.V. All rights reserved.