A Comparative Spectroscopic Study on the Location of Benzene and Cations in a Series of Si-Rich NaY Zeolites

Abstract

Benzene adsorption in two Si-rich NaY (NaYs and NaYd) zeolites (Si/Al ratio ≈ 3.5), prepared from different methods, has been investigated using FTIR spectroscopy and compared with that in NaY (Si/Al ratio = 2.3) and NaEMT (Si/Al ratio = 3.6) zeolites. The interaction strength of benzene with one site or another in these zeolites has been evaluated by a successive desorption of benzene at different temperatures. The location of cations in these two different Si-rich zeolites has been discussed. The quantitative analysis of the changes of the absorbance of the C−H out-of-plane vibrations with benzene loadings gives access to total benzene adsorption capacity and the amounts of benzene adsorbed on one site or another. The adsorption behavior of benzene in these zeolites has been then correlated with the Al distribution, the location of Na+ ions, the average negative charge of oxygen atoms, the zeolite structure, and the interaction strength of benzene with one site or another. As observed in NaY zeolite, two adsorption sites for benzene, Na+ ions and 12R windows, have been evidenced in two Si-rich NaY, whereas benzene molecules sit mainly on Na+ ions in NaEMT. No significant influence of the Al distribution on the benzene location and on the distribution of cations in the supercages of these two different Si-rich NaY zeolites is observed. A new pair of shoulders at high wavenumbers (2050−1913 cm-1) at high benzene loadings is detected for the first time. In all the NaY zeolites studied here, the interaction of benzene with the Na+ ions is stronger than that with the 12R windows. This explains the observation that the number of benzene molecules adsorbed on the 12R windows is lower than that on the Na+ ions. The low adsorption capacity for a supercage of NaYs and NaYd zeolites is most likely related to the low number of Na+ ions in a supercage and consequently to the high mobility of benzene molecules in the zeolite structures. It reveals that there are the diverse strengths of interaction of benzene with Na+ ions in NaEMT due to the migration of Na+ ions from the small cages to the large cages in the presence of benzene.