Influence of Hydronium Ions in Zeolites on Sorption

In the presence of sufficient concentrations of water, stable, hydrated hydronium ions are formed in the pores and at the surface of solid acids such as zeolites. For a medium‐pore zeolite, such as zeolite MFI, hydrated hydronium ions consist of eight water molecules and have an effective volume of...

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Bibliographic Details
Published in:Angewandte Chemie 2019-03, Vol.131 (11), p.3488-3493
Main Authors: Eckstein, Sebastian, Hintermeier, Peter H., Zhao, Ruixue, Baráth, Eszter, Shi, Hui, Liu, Yue, Lercher, Johannes A.
Format: Article
Language:English
Subjects:
Activity coefficients
Adsorption
Charge density
Chemistry
Cyclohexanol
Hydronium ions
Hydroniumion
Hydrophobicity
Ionic strength
Ions
Organic chemistry
Substrates
Water chemistry
Zeolites
Zeolithe
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Summary:In the presence of sufficient concentrations of water, stable, hydrated hydronium ions are formed in the pores and at the surface of solid acids such as zeolites. For a medium‐pore zeolite, such as zeolite MFI, hydrated hydronium ions consist of eight water molecules and have an effective volume of 0.24 nm3. In their presence, larger organic molecules can only adsorb in the portions of the pore that are not occupied by hydronium ions. As a consequence, the available pore volume decreases proportionally to the concentration of the hydronium ions. The higher charge density (the increasing ionic strength) that accompanies an increasing concentration of hydronium ions leads to an increase in the activity coefficients of the adsorbed substrates, thus, weakening the interactions between the organic part of the molecules and the zeolite and favoring the interactions with polar groups. The quantitative understanding of these interactions makes it possible to link a collective property such as hydrophilicity and hydrophobicity of zeolites to specific interactions on molecular level. Ein hydratisiertes Hydroniumion in Zeolith H‐MFI besteht aus acht Wassermolekülen und nimmt ein Volumen von 0.24 nm3 ein, das für unpolare Substrate nicht zugänglich ist. Hydratisierte Hydroniumionen induzieren lokal eine hohe Ionenstärke, was zu einer stark nichtidealen Umgebung führt. Diese Bedingungen destabilisieren die Adsorption von Substraten, indem die Adsorptionswärme reduziert wird und die Aktivitätskoeffizienten des Substrats ansteigen.
ISSN:0044-8249
1521-3757
DOI:10.1002/ange.201812184