Hydronium ion and water complexes vs. methanol on solid catalyst surfaces: how confinement influences stability and reactivity

Abstract

Water and alcohols like methanol are key substrates in catalysis. Thus, adsorption on catalysts plays a key role in chemical reactions. Herein we investigate how surface sites and confinement influence the formation of water surface species, their adsorption strength, and complex stability. We compare adsorption on microporous MFI zeolites, mesoporous SBA-15 materials and silica A200 as supports for silicotungstic acid (STA) in their isostructural silica, Na-, and H-forms. A systematic variation of confinements, surface sites and adsorbates enables a separation of confinement from other effects. In saturation, the quantity of bound water is determined by the available surface area and maximized for materials with high Si(OH) densities on the surface. The strong binding of water at Si(OH) explains why the Si(OH) density influences heterogeneous catalysis. Complexes between water and counter ions (Na⁺) or acid sites (H⁺) form under micropore confinement in Na- and H-ZSM-5. Complexes are absent or decompose quickly on aluminated SBA-15 and STA in Na- or H-forms. More methanol than water molecules bind to counter ions and acid sites on ZSM-5. This explains why acid sites in the methanol-to-olefin conversion are just partially blocked by water. We identify confinement as the key parameter for the formation of surface complexes and for adjusting the efficiency of their protonation. Our finding helps to rationalize reactions with water and/or alcohol reactants.