Protonation of water clusters in the cavities of acidic zeolites: (H2O)n·H-chabazite, n = 1–4

Abstract

Proton forms of zeolite chabazite (H-SSZ-13) loaded with 1 to 4 water molecules per acid site are examined by density functional theory with periodic boundary conditions. Equilibrium structures are determined by localizing minima on the potential energy surface and harmonic vibrational frequencies are calculated. Average structures, proton dynamics and anharmonic spectra at finite temperature (350 K) are determined by molecular dynamics (MD). The protonation state is found to depend on the number of water molecules per acid site (loading) following the trend of increasing proton affinity with increasing cluster size. Single water molecules are not protonated, the protonated water dimer is the most stable equilibrium structure with the PBE functional, but not with BLYP. MD shows that even with PBE, the protonated water dimer is not stable at finite temperature. The protonated water trimer may be formed as a short-lived species, but the protonated water tetramer is the smallest stable protonated cluster. For the same global loading (2 : 1), a heterogeneous distribution of adsorbed water molecules over the cells is more stable than a homogeneous one (1 : 1/3 : 1 vs. 2 : 1/2 : 1 for a double cell), i.e. non-protonated and protonated water clusters may exist simultaneously in polyhydrated H-SSZ13. Adsorption energies (0 K) per water molecule decrease from 71 to 51 kJ mol⁻¹ for n = 1 to n = 4.