FTIR study of low-temperature CO adsorption on Y zeolite exchanged with Be 2+, Mg 2+, Ca 2+, Sr 2+ and Ba 2+ cations

Abstract

Low-temperature CO adsorption on BeNaY and MgNaY zeolites results in formation of several species (in addition to the carbonyls of Na +): two types of Be 2+–CO carbonyls (a band at 2222 cm −1 shifted to 2212 cm −1 at high coverages and a weak band at 2198 cm −1) and Mg 2+–CO (coverage independent band at 2215 cm −1). However, low-temperature adsorption of CO on CaNaY, SrNaY and BaNaY leads to the simultaneous coordination of three CO molecules to one cationic site, the frequencies characterizing tricarbonyls being at 2185, 2182.5 and ca. 2177 cm −1 for Ca 2+, Sr 2+ and Ba 2+, respectively. Decreasing the equilibrium CO pressure leads to a stepwise loss of one and two CO ligands, the monocarbonyls being quite stable at 85 K. The frequencies of the Ca 2+(CO) 2, Sr 2+(CO) 2 and Ba 2+(CO) 2 dicarbonyls are observed at 2201, 2193 and 2181 cm −1, respectively. The linear species, Ca 2+–CO, Sr 2+–CO and Ba 2+–CO are characterized by bands at 2201, 2193 and 2181 cm −1, respectively, which are slightly red-shifted at room temperature. CO adsorption results also in formation of Ca 2+–OC (2094 cm −1), Sr 2+–OC (2097 cm −1) and Ba 2+–OC (2100 cm −1) species, their concentration increasing in the same order. Mixed complexes containing both C-bonded and O-bonded CO are also detected. The stability of monocarbonyls decreases in the sequence from Be to Ba. The reasons for the formation of polycarbonyls are discussed. It is proposed that these species are formed on cations with sufficiently large ionic radius.