Effect of aluminum and sodium on the sorption of water and methanol in microporous MFI-type zeolites and mesoporous SBA-15 materials

Zheng Li,Johannes Bender,Michael Hunger,Carolin Rieg,Michael Benz,Ann-Katrin Beurer,Michael Dyballa,Christof Schneck,Yvonne Traa

doi:10.1007/s10450-020-00275-8

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 aluminum-containing Na-ZSM-5, Na+ 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]SBA-15, 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.

Highlights

In the past decades, numerous studies were dealing with the adsorption properties of water and alcohols on porous solids
For siliceous Silicalite-1 as well as siliceous SBA15, the experimentally determined desorption enthalpies (38–42 kJ/mol) and the dominating 1H MAS NMR signals in the spectra of the samples saturated with water and methanol indicate, that a high content of these adsorbate molecules exist as mobile bulk clusters
By 29Si CPMAS NMR, the organophilicity of Silicalite-1 could be supported by stronger methanol than water interactions with surface SiOH groups of Q3-type

Summary

Introduction

Numerous studies were dealing with the adsorption properties of water and alcohols on porous solids. 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 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 oxidation to methanol (Dyballa et al 2019a, b; Pappas et al 2017; Ravi et al 2017). They do not allow a detailed insight into the properties of the adsorption sites and the nature of the adsorbate complexes

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Conclusion