Effect of aluminum and sodium on the sorption of water and methanol in microporous MFI-type zeolites and mesoporous SBA-
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Effect of aluminum and sodium on the sorption of water and methanol in microporous MFI‑type zeolites and mesoporous SBA‑15 materials Zheng Li1 · Carolin Rieg1 · Ann‑Katrin Beurer1 · Michael Benz1 · Johannes Bender2 · Christof Schneck3 · Yvonne Traa1 · Michael Dyballa1 · Michael Hunger1 Received: 6 May 2020 / Revised: 27 July 2020 / Accepted: 26 September 2020 © The Author(s) 2020
Abstract The interaction and nature of surface sites for water and methanol sorption on MFI-type zeolites and mesoporous SBA-15 were investigated by solid-state NMR spectroscopy and correlated with the desorption enthalpies determined via TGA/DSC. For siliceous Silicalite-1, 29Si CPMAS NMR studies support stronger methanol than water interactions with SiOH groups of Q3-type. On siliceous SBA-15, SiOH groups of Q2-type are accompanied by an enhanced hydrophilicity. In aluminumcontaining Na-ZSM-5, N a+ cations are strong adsorption sites for water and methanol as evidenced by 23Na MAS NMR in agreement with high desorption enthalpies of ΔH = 66–74 kJ/mol. Solid-state NMR of aluminum-containing Na-[Al]SBA15, in contrast, has shown negligible water and methanol interactions with sodium and aluminum. Desorption enthalpies of ΔH = 44–60 kJ/mol hint at adsorption sites consisting of SiOH groups influenced by distant framework aluminum. On H-ZSM-5, Brønsted acidic OH groups are strong adsorption sites as indicated by partial protonation of water and methanol causing low-field shifts of their 1H MAS NMR signals and enhanced desorption enthalpies. Due to the small number of Brønsted acid sites in aluminum-containing H-[Al]SBA-15, water and methanol adsorption on this material is suggested to mainly occur at SiOH groups with distant framework aluminum species, as in the case of Na-[Al]SBA-15.
Keywords Water adsorption · Methanol adsorption · MFI-type zeolites · Mesoporous SBA-15 · Solid-state NMR spectroscopy · Desorption enthalpies
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10450-020-00275-8) contains supplementary material, which is available to authorized users. * Michael Dyballa [email protected]‑stuttgart.de Extended author information available on the last page of the article
1 Introduction In the past decades, numerous studies were dealing with the adsorption properties of water and alcohols on porous solids. Often, these investigations focused on water and ethanol due to the application of porous solids with hydrophobic and
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organophilic properties in separation processes (Caro and Noack 2008). Because of the raising industrial application of the methanol to hydrocarbon conversion on acidic zeolite catalysts (Olsbye et al. 2012; Tian et al. 2015), which is accompanied by the formation of water, it is also interesting to clarify the adsorption properties of methanol and to compare them with those of water. Another application is the desorption of surface-bound methanol from zeolites in the presence of water, for example after methane oxi
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