The Role of Silanols in the Interactions between Methyl <i>tert</i>-Butyl Ether and High-Silica Faujasite Y: An Infrared Spectroscopy and Computational Model Study

Abstract

It is generally believed that the retention of methyl <i>tert</i>-butyl ether (MTBE) by zeolites is positively correlated to the silica content of these materials. Nevertheless, highly dealuminated zeolites can contain relevant amounts of silanol groups. In this study, the effect of these point defects in a zeolite Y (SiO₂/Al₂O₃ = 200) on the adsorption of MTBE was evaluated by means of infrared spectroscopy, supported by DFT calculations. The adsorption process is found to occur in two steps, involving isolated silanol sites and the siloxane network of the zeolite, respectively, with an average loading of 1.3 molecules per cage in the first and 1.3 molecules in the second stage. Both external and internal isolated silanol groups (stretching at 3746 and 3738 cm^–1, respectively) are involved in the MTBE adsorption process with the formation of H-bonded complexes and associated shifts (516 and 358 cm^–1, respectively), consistent with a H-bonding strength higher for external than for the internal ones. However, MTBE is more tightly adsorbed on the internal silanols as a result of the cage confinement effect. The band assigned to the methyl symmetric stretching of the CH₃O– group can be used to discriminate between H-bond and van der Waals MTBE–zeolite interactions (2843 and 2828 cm^–1, respectively). Ab initio models were used to compute the harmonic frequencies of different MTBE–zeolite models and to simulate the cage confinement of one and three ether molecules