Effect of aluminum and sodium on the sorption of water and methanol in microporous MFI-type zeolites and mesoporous SBA-15 materials
Zheng Li, Carolin Rieg, Ann‐Katrin Beurer, Michael Benz, Johannes Bender, Christof Schneck, Yvonne Traa, Michael Dyballa, Michael Hunger
Abstract
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, 29 Si CPMAS NMR studies support stronger methanol than water interactions with SiOH groups of Q 3 -type. On siliceous SBA-15, SiOH groups of Q 2 -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 23 Na MAS NMR in agreement with high desorption enthalpies of Δ H = 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 Δ H = 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 1 H 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.