The location and stability of Brønsted
acid sites catalytically active in
zeolites during aqueous phase
dehydration of
alcohols were studied on the example of
cyclohexanol. The catalytically active hydronium
ions originate from Brønsted
acid sites (BAS) of the
zeolite that are formed by framework tetrahedral Si atom substitution by Al. Al K-edge extended X-ray absorption fine structure (EXAFS) and (27)Al magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopies in combination with density functional theory (DFT) calculations are used to determine the distribution of tetrahedral Al sites (Al T-sites) both qualitatively and quantitatively for both parent and HBEA catalysts aged in water prior to catalytic testing. The aging procedure leads to partial degradation of the
zeolite framework evidenced from the decrease of material crystallinity (XRD) as well as sorption capacity (BET). With the exception of one commercial
zeolite sample, which had the highest concentration of framework
silanol-defects, there is no evidence of Al coordination modification after aging in water. The catalyst weight-normalized
dehydration rate correlated best with the sum of strong and weak Brønsted acidic
protons both able to generate the hydrated hydronium
ions. All hydronium
ions were equally active for the
acid-catalyzed reactions in water.
Zeolite aging in hot water prior to catalysis decreased the weight normalized
dehydration reaction rate compared to that of the parent HBEA, which is attributed to the reduced concentration of accessible Brønsted
acid sites. Sites are hypothesized to be blocked due to reprecipitation of
silica dissolved during framework hydrolysis in the aging procedure.