A density functional theory study of the interactions of H₂O with H-ZSM-5, Cu-ZSM-5, and Co-ZSM-5

The interactions of water with H-ZSM-5, Cu-ZSM-5, and Co-ZSM-5 have been investigated by density functional theory (DFT). Calculations were also performed to determine the thermodynamics of metal removal from the zeolite. For all three forms of ZSM-5, water adsorbs by direct interaction of the O atom with the cation and hydrogen bonding of one of the two H atoms with an oxygen atom in the zeolite framework. The magnitude of the energy of adsorption of one H₂O molecule decreases in the order Cu-ZSM-5 (Cu as Cu⁺) > Co-ZSM-5 (Co as Co²⁺(OH)^-) > H-ZSM-5 > Cu-ZSM-5 (Cu as Cu²⁺(OH)^-). Adsorption of a second H₂O molecule occurs with a smaller binding energy, which decreases in the order Cu-ZSM-5 (Cu as Cu⁺) > Cu-ZSM-5 (Cu as Cu²⁺(OH)^-) > Co-ZSM-5 (Co as Co²⁺(OH)^-) > H-ZSM-5. At 800 K, demetalation to form a gaseous metal hydroxide species is unfavorable thermodynamically but will occur spontaneously if the final product is a metal oxide. Demetalation of Cu is projected to occur much more readily than demetalation of Co, in good agreement with experimental observation.