Structure and dynamics of a Bronsted acid site in a zeolite: An ab initio study of hydrogen sodalite

We present the results of ab initio molecular dynamics simulations on the structural and dynamical properties of the Bronsted acid site in a zeolitic cage. The protonated sodalite H[AlSi11O24] is the case studied, and the sodium sodalite is the reference unprotonated zeolite. The comparison between the geometrical parameters calculated for these two sodalites shows that the formation of an O-H bond gives rise to a significant local distortion of the framework geometry. Moreover, an equilibrium between partially ionic and covalent structures in the T-O bonds of the acid site is deduced from our calculations. We have found that the OH bond oscillates in and out of the plane of the 6-T-ring. The analysis of the simulated vibrational spectra has allowed us to assign the bands relative to the O-H in-plane and out-of-plane bending modes and confirms that these bands overlap with the framework modes.