Hydrolysis of zeolite framework aluminum and its impact on acid catalyzed alkane reactions

• Published in: Journal of catalysis Vol. 365; pp. 359 - 366
• Main Authors: Xue, Nianhua, Vjunov, Aleksei, Schallmoser, Stefan, Fulton, John L., Sanchez-Sanchez, Maricruz, Hu, Jian Zhi, Mei, Donghai, Lercher, Johannes A.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01.09.2018
• Subjects: alkanes; aluminum; aluminum oxide; Bronsted acids; Butane cracking; catalytic activity; Confinement effect; crystallites; dehydrogenation; entropy; H/D exchange; hydrogen; hydrolysis; HZSM-5; kinetics; Mild steaming; nuclear magnetic resonance spectroscopy; solids; stable isotopes; steaming; temperature; water vapor; zeolites; Mild steaming; H/D exchange; HZSM-5; Confinement effect; Butane cracking
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1016/j.jcat.2018.07.015

[Display omitted] •Mild steaming leads to partially framework-bound aluminum oxide species.•These aluminum oxide clusters are in the vicinity of Brønsted acid sites.•Site proximity leads to an increase of activation entropies for alkane cracking.•n-Butane cracking and D2/OH exchange reaction rates are also enhanced. Short exposure of HZSM-5 zeolites to water vapor at high temperatures (mild steaming) enhances the rates of alkane cracking and dehydrogenation as well as the rates of exchange between H2 and D2. Solid state 27Al MAS NMR results show that mild steaming leads to metastable aluminum oxide species, hypothesized to be partially framework-bound. Combining double quantum magic-angle spinning nuclear magnetic resonance experiments and kinetic analysis of cracking reactions allows us to conclude that the presence of these aluminum oxide clusters in the vicinity of Brønsted acid sites leads to an increase in the activation entropies during alkane cracking, induced by increasing steric constraints. Prolonged steaming results, in contrast, in the extraction of framework Al, which subsequently forms aggregated extra-framework Al oxide species partly blocking Brønsted acid sites and partly deposited at the outer surface of the crystallites.