Adsorption of carbon monoxide on ZSM-5 zeolites: infrared spectroscopic study and quantum-chemical calculations

Low temperature adsorption of CO was studied on H-ZSM-5 zeolites modified by dehydroxylation, ionic exchange with Al/sup 3 +/, and impregnation with Al/sub 2/O/sub 3/ and on Na-ZSM-5 and CaH-ZSM-5 zeolites. It was found that interaction of CO with framework OH groups results in the formation of a hydrogen-bonded CO complex whose OH bond frequency is decreased by 310-320 cm/sup -1/ compared with that of free hydroxyls. For the less acidic framework hydroxyls in large cavities of H/sub 70/Na/sub 30/-Y zeolite the observed shift is 275 cm/sup -1/. With ZSM-5 zeolites, at least six types of electron-accepting sites are observed originating from nonframework Al species (band of CO in the interaction complex: 2132, 2222, 2202, 2195, and 2198 cm/sup -1/) and the Al/sub 2/O/sub 3/ microcrystalline phase (CO band at 2153 cm/sup -1/). The CO bond orders calculated by the CNDO/2 method for the CO interaction complexes with models of surface sites increase in the following order: > O-CO < > OH-CO approx. ..-->.. Al-CO approx. = Na-CO < alumina-CO approx. = Ca-CO < ..-->.. Si-CO < Al(cationic)-CO. A correlation between the calculated bond orders of CO and the observed vibrational frequencies of CO-forming interaction complexes is drawn.