To explore water adsorption in zeolite pores, micrometer-sized crystals of all-silica CHA, DDR, and MFI zeolites as well as NaA (Si/Al = 1) zeolite were synthesized. For comparison, nanocrystals of all-silica MFI zeolite were also prepared. Water adsorption isotherms were recorded at 293 K and a model based on the Langmuir, BET and Kelvin equations was fitted to the data. The fitted model showed that for all-silica CHA, MFI, and DDR zeolites, as little as 14%, 3.1%, and 0.92% of the zeolite pore volume was occupied by water at saturation, respectively. This indicates that the pores in these zeolites are quite nonpolar, although the zeolites were synthesized in hydroxide media. However, for the polar aluminosilicate NaA zeolite, 100% of the zeolite pore volume was filled by water at saturation. The model also showed that only a fraction of the adsorbed water is located inside the pores, while a substantial amount is adsorbed as a multilayer on the external surface of the micrometer-sized zeolite crystals, and even as condensed water in the capillaries between nanocrystals. Consequently, the water adsorption capacities of the all-silica zeolite pores at saturation determined in the present work are much lower than previously reported ones. This study demonstrated the limited water adsorption in all-silica zeolite pores and the effect of multilayer adsorption and even condensation at the external surface, which is important for understanding and improving the performance of zeolite materials in various applications.
