Probing the pore structure and dehumidifying characteristics of chemically modified zeolite Y

This study was aimed at modifying the pore structure and surface chemistry of a zeolite to achieve an ideal material for gas-fired dehumidifying and cooling system application. Zeolite Y was chemically modified with various amounts of ethylenediaminetetraacetic acid (H₄EDTA) corresponding to H₄EDTA/Al (zeolite) molar ratio, X=0.5, 0.6, 0.75, 0.9 and 1.0 under reflux condition to achieve various degrees of dealumination. Sample treated with X=0.5 remained highly crystalline, but crystallinity of the samples decreased as the X increased. Although the sample treated with X=0.9 was x-ray amorphous, SEM micrographs indicated that the original crystal morphology was largely preserved. Chemical analysis and ²⁹Si showed a progressive extraction of Al and Na with increased X. ²⁷Al MASNMR indicated that Al^IV was the only Al species up to X=0.75, whereas the x-ray amorphous phase (X=0.9) exhibited three peaks at 60, 52 and 38 ppm indicating the distortion of and change in Al^IV coordination. Adsorption analyses using H₂O, N₂ or Ar as probe molecules indicated that the X-ray amorphous product remained essentially microporous having a broad micropore size distribution and a reduced hydrophilicity. Consequently, water adsorption isotherm changed from an extreme Type I to a moderate Type I. The moderate Type I isotherm with water is the key for its suitability as a desiccant in gas-fired dehumidifying and cooling system. These materials are also potentially useful in solar heating and cooling applications.