The Effect of Adsorbed Molecule Gas-Phase Deprotonation Enthalpy on Ion Exchange in Sodium Exchanged Zeolites: An In Situ FTIR Investigation

Abstract

Molecular-level understanding of the interactions between reactants and the surface of solid catalysts is of importance to the rational design of catalysts. Here, in situ transmission Fourier transform infrared spectroscopy is employed to investigate the ion exchange between the acidic hydrogen in organic molecules that have been adsorbed from the gas phase and sodium cations in zeolites. Organic compounds with functional groups common among key biomass-derived compounds are used as probe molecules. We demonstrate that ion exchange between acidic hydrogen in organic molecules and the sodium cations in zeolites with the FAU topology produces Bronsted acid sites and the corresponding adsorbed salt species by identifying signature spectroscopic bands. Furthermore, the gas-phase deprotonation enthalpy (GPDE) of the organic compounds is identified as a key descriptor in determining the feasibility and extent of the exchange process. Molecules with GPDE below 1462 kJ/mol, e.g., m-cresol (1462 kJ/mol), propanoic acid (1454), acetic acid (1457), acrylic acid (1440) and trifluoroacetic acid (1357), show clear vibrational bands for Bronsted acid sites and the corresponding sodium salts, while molecules with higher GPDE, such as trifluoroethanol (1513), ethanol (1586), and water (1622) do not. These data indicate that the degree of dissociation of the acidic hydrogen is a key element in the ion exchange. The generality of this process in zeolites is established by the observation of similar results on zeolites with differing topologies (FAU, MFI, *BEA, and MOR).