(307e) Role of Composites for Improving Hydrophobicity and Water Capacity of Zeolites in Adsorption Applications

Seeking an adsorbent with high water capacity and low regeneration temperature is crucial for systems involving water sorption, such as adsorption heat pumps/cooling systems and desiccant dehumidifiers. Water sorption systems may use a variety of adsorbent materials, such as silica gel, activated carbon, zeolite and metal organic framework (MOF). These materials mostly have high surface areas and large pore volumes, often resulting in high water adsorption capacities. Except aluminosilicate zeolites, they also have relatively low generation temperatures. On the other hand, zeolites generally have the advantage of being hydrothermally more stable and having faster adsorption kinetics.

Preparing composite materials may be useful to obtain adsorbents with enhanced adsorption performances. Improvement with respect to distinct adsorbents constituting the composite may be assured in case some limiting properties of the adsorbents are eliminated or favorable interactions exist between the different adsorbents. The latter signifies that the adsorption performance of the composite prepared by using a chemical process is superior to that of the composite obtained by simple mixing of the adsorbents. Both the regeneration temperature (hydrophobicity) and water capacity of the adsorbents may be affected.

In this study, firstly, water adsorption capacities of various adsorbents reported in the literature were investigated and a hydrophobicity index was defined. A power-law curve was proposed in order to describe the competency of water adsorption performances of various adsorbents, by taking into consideration the relation between water adsorption capacity and hydrophobicity. The situation for zeolites was highlighted and NaY zeolite was selected to investigate possible improvements that might occur when its composites were formed.

Then, some NaY zeolite composites were prepared and the materials obtained were mainly characterized by thermogravimetry (TG). The water capacities of the composites at relatively low and high temperatures as well as their hydrophobicity, as obtained by thermogravimtery, were compared to those of the pure materials. Some samples were also investigated by field emission gun scanning electron microscopy (FEGSEM) and energy dispersive X-ray spectroscopy (EDX). Salt impregnation was applied to the pores of NaY zeolite to obtain the first type of composites. The water adsorption capacities of many porous adsorbents are lower than those of hygroscopic salts, while salts are not stable, especially under high humidity ratios. Thus, composite adsorbents can make use of the superior sorption characteristics of hygroscopic salts and stability of zeolites. NaY-MgCl₂ and NaY-LiCl composites were prepared by immersing the zeolite in a MgCl₂ or LiCl solution (5 wt%), where it was kept for 24 h at room temperature. Then, the mixture was evaporated at 80 °C, so that the water could be removed and the residual MgCl₂ or LiCl could be induced into the zeolite. A second type of composite was prepared by immersing MIL-101 (MOF) particles in a clear zeolite synthesis mixture known to yield mainly zeolite NaY (molar composition: 42.5 Na₂O: 1 Al₂O₃: 17 SiO₂: 850 H₂O) [1]. After keeping the mixture at 80 °C for 24 h, the NaY-MIL-101 composite was obtained. Additionally, NaY-activated carbon composite was formed by immersing the activated carbon particles in the same clear zeolite synthesis mixture mentioned, using the same synthesis conditions. Sodium aluminate, sodium silicate, sodium hydroxide and deionized water were used to prepare the zeolite synthesis mixtures.

The power-law curves obtained by using literature adsorption data were proposed to be used as indicators of performance limits of adsorbents. In spite of their useful adsorption properties, zeolites generally exhibited quite low hydrophobicity, requiring the use of high regeneration temperatures. Improvements were obtained for NaY zeolite when its various composites were prepared. The water capacities of zeolite NaY composites at the relatively high temperature of 350 °C, as determined by thermogravimetry, were generally slightly higher than zeolite NaY, while the water capacity at 100 °C and the hydrophobicity index increased considerably up to about 50%. Some unfavorable interactions existed between the salt and impregnated zeolite and the positive impact of salt impregnation on the water capacity of the zeolite seemed to be limited by certain factors, such as partial pore blocking of the zeolite by the salt. The zeolite-MOF and zeolite-activated carbon composites exhibited favorable adsorption interactions and might be useful to improve the relatively slow adsorption/desorption kinetics of, especially, the MOF materials.

1. Tatlier, M., Atalay-Oral, C., Bayrak, A., MaraÅŸ, T., Erdem A., (2022), "Impact of ion exchange on zeolite hydrophilicity/hydrophobicity monitored by water capacity using thermal analysis. Thermochimica Acta, 713, 179240.