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Published May 2015 | metadata_only
Journal Article

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


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 Brønsted 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 Brønsted 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).

Additional Information

© 2015 Springer Science+Business Media. We acknowledge support from the Catalysis Center for Energy Innovation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award No. DE-SC0001004. BX and MED acknowledge that preliminary results of this work were obtained at Caltech with financial support provided by a donation from Mr. and Mrs. Lewis W. F Amerongen. The authors would also like to acknowledge Dr. Sonjong Hwang at the California Institute of Technology for his assistance in performing the ^(29)Si solid-state NMR measurements. This article is dedicated to Prof. Mark E. Davis, the recipient of the prestigious Gabor A. Somorjai Award for Creative Research in Catalysis. Mark's pioneering work in the synthesis of zeolites with novel structures and unprecedented control of catalytic sites has to a significant extent shaped the field of microporous materials and profoundly influenced both academic and industrial researchers in heterogeneous catalysis on the rational design of solid acid catalysts. Aside from his achievements in heterogeneous catalysis, Mark's creativity has also benefited several other fields, such as cancer therapy and thermochemical cycles. During my postdoctoral stay in his group (2011–2013), we developed a low temperature manganese-oxide based thermochemical cycle for water splitting [39, 40], a topic Mark explored very early on in his academic career. Another key aspect of Mark's contribution to the scientific community in general is via mentoring and inspiring students and junior researchers around him to take on important scientific challenges in a rational and creative manner, from which I benefited enormously. In this contribution, we show that gas-phase deprotonation enthalpy of organic molecules adsorbed from gas phase on sodium exchange zeolites is a reliable predictor for the ion exchange reaction, creating in situ generated Brønsted acid sites. Some preliminary experiments for this work were conducted in Mark's lab at Caltech towards the end of my stay. Finally, I would like to congratulate Mark for receiving this prestigious and well-deserved award.

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August 22, 2023
August 22, 2023