Published January 26, 2021 | Version v1
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Confinement effects and acid strength in Zeolites

  • 1. Department of Chemistry and Applied Biosciences, ETH Zurich, c/o USI Campus, Via Giuseppe Buffi 13, CH-6900 Lugano, Ticino, Switzerland.
  • 2. Institute of Computational Science, Università della Svizzera italiana (USI), Via Giuseppe Buffi 13, CH-6900, Lugano, Ticino, Switzerland.
  • 3. Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland Washington, USA.
  • 4. Department Chemie and Catalysis Research Center, TU München, Lichtenbergstr. 4, 85747 Garching, Germany
  • 5. Italian Institute of Technology, Via Morego 30, 16163 Genova, Italy.

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Description

Chemical reactivity and sorption in zeolites are coupled to confinement and - to a lesser extent- to the acid strength of Brønsted acid sites (BAS). In presence of water the zeolite Brønsted acid sites eventually convert into hydronium ions. The gradual transition from zeolite Brønsted acid sites to hydronium ions conversion in zeolites of varying pore size is examined by ab initio molecular dynamics combined with enhanced sampling based on well-tempered metadynamics and a recently developed set of collective variables. While at low water content (1-2 water/BAS) the acidic protons prefer to be shared between zeolites and water, higher water contents (n>2) invariably lead to solvation of the protons within a localized water cluster adjacent to the BAS. At low water loadings the standard free energy of the formed complexes is dominated by enthalpy and is associated with the acid strength of the BAS and the space around the site. Conversely, the entropy increases linearly with the concentration of waters in the pores, favors proton solvation and is independent of the pore size/shape.

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References

Preprint
E. Grifoni, G. Piccini, J. Lercher, V.-A. Glezakou, R. Rousseau, M. Parrinello, Confinement Effects and Acid Strength in Zeolites. Res. Sq. 2020., doi: 10.21203/rs.3.rs-79664/v1