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.