The adsorption of carbon monoxide on H-ZSM-5 and hydrothermally treated H-ZSM-5

The adsorption of CO was studied on the H-form and hydrothermally treated H-form of ZSM-5 by in situ FTIR spectroscopy. As seen in other studies the interaction of Bronsted acid sites with the CO molecules shifted the bridging Si(OH)Al stretching frequency from 3616 cm(-1) down to 3310 cm(-1). Concomitantly, the C-O stretching frequency of adsorbed, H-bonded CO appeared at 2173 cm(-1) blue-shifted from the gas phase value of 2143 cm(-1). From quantitative measurements of the CO stretching vibrational band intensities the isosteric heat of adsorption oi CO was measured to be 32.2 kJ mol(-1) over the temperature range 175 to 285 K and at pressures from 0.1 to 100 Torr. Alternatively, a heat of adsorption of 33.4 kJ mol(-1) was estimated from a Van't Hoff plot of -1nK(eq) versus 1/T (where K-eq describes the adsorption-desorption equilibrium). The effect of steaming on the CO adsorption properties of H-ZSM-5 was studied at two different steaming temperatures (873 and 1073 K). Hydrothermal treatment of the H-form of the ZSM-5 for 2, 4, and 8 h at 873 K gradually decreased the spectral signature of the bridging Si(OH)PII and resulted in the growth of features corresponding to extra-lattice alumina and/or Al-OH species which are formed during the hydrolysis of the Si-O-Al bridge but are still in the zeolite framework. The apparent heat of adsorption of CO measured in the steamed H-ZSM-5 materials was approximately 4 kJ mol(-1) lower than that for the parent H-ZSM-5 zeolite. For the higher temperature steaming conditions the H-ZSM-5 was observed to dealuminate rapidly with nearly complete loss of framework aluminum in 15 min or less. These hydroxylated Al-containing intermediate species present under mild hydrothermal conditions are transformed into stable forms of Al-oxide species. These Al-oxides present in extra-framework positions also act as Lewis acid centers for the adsorption of CO.