Understanding the Nature of Water Bound to Solid Acid Surfaces. Ab Initio Simulation on HSAPO-34

Ab initio simulations of water adsorbed in the microporous silicoaluminum phosphate HSAPO-34 are reported. A neutron diffraction study by L. Smith et al., Science 1996, 271, 799, showed that both H 2 O and H 3 O + are present when this solid acid is loaded with water. The ab initio simulations support this; however, new details are revealed which are not accessible by diffraction experiments averaging over all sites. The acid−base reaction requires at least three water molecules per two nearby acidic sites in the micropores of the solid and leads to a protonated water cluster, H 3 O + (H 2 O) 2 .