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 be en investigated by density functional theory (DFT). Calculations were also performed to determine the thermodynamics of metal removal from t he zeolite. For all three forms of ZSM-5, water adsorbs by direct inte raction of the O atom with the cation and hydrogen bonding of one of t he two Il atoms with an oxygen atom in the zeolite framework. The magn itude 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 occ urs 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+(O H)(-)) > H-ZSM-5. At 800 K, demetalation to form a gaseous metal hydro xide species is unfavorable thermodynamically but will occur spontaneo usly if the final product is a metal oxide. Demetalation of Cu is proj ected to occur much more readily than demetalation of Co, in good agre ement with experimental observation.