Guenter H. Kuehl

On the nature of framework Brønsted and Lewis acid sites in ZSM-5

Abstract The acidity of aluminum in a nominally 70:1 SiO 2 /Al 2 O 3 , ratio ZSM-5 prepared by conventional means was characterized by TPAD, 27 Al and 29 Si n.m.r., XRD, and FT i.r. Discrepancies observed between the amount of tetrahedral “acidic” aluminum measured by TPAD and i.r. versus Al n.m.r. are due to the presence of Lewis aluminum. Furthermore, it is suggested that this Lewis aluminum may be located in the zeolite framework. These sites are not thought to be inherent to the ZSM-5 structure but are probably generated during calcination, resulting in partial hydrolysis of AlO bonds. A small percentage of octahedral aluminum was observed in hydrated HZSM-5, but was quantitatively converted to tetrahedral aluminum by treatment with ammonium nitrate solution at pH 8.

Acid sites in zeolite Beta: effects of ammonium exchange and steaming

Abstract Zeolite Beta samples that had undergone calcination and/or steam deactivation were examined by temperature-programmed ammonia desorption (TPAD), 27Al magic-angle spinning (MAS) nuclear magnetic resonance spectroscopy (NMR) and 29Si MAS NMR, in order to evaluate the nature and concentration of acid sites formed upon ammonium exchange or steaming. We observe octahedral aluminum species connected to the framework structure of zeolite Beta, and these aluminum sites exhibit characteristics of Lewis acid sites. Such aluminum sites are created by partial hydrolysis of framework SiOAl bonds; they convert reversibly to tetrahedral aluminum sites upon aqueous NH+4 exchange or reaction with NH3 gas. When zeolite Beta is steamed severely, a substantial portion of the framework aluminum is completely hydrolyzed, and non-framework aluminum species are generated. The high-temperature shoulder observed in some TPAD profiles is caused by liberation of ammonia from Lewis acid sites. This ammonia had previously been released from Bronsted acid sites and was temporarily re-adsorbed on Lewis acid sites. The evidence presented suggests that these Lewis sites are located in the zeolite framework.

EXAFS study of nickel exchanged into zeolite Y

EXAFS and near edge spectroscopy were used to monitor changes i n Ni coordination as a function of treatment conditions after aqueous exchange into zeolite Y. Our results suggest that after calcination and dehydration under the conditions of this study, major site occupancy for Ni appears to be in the tri-coordinate exchange sites , and not i n the hexagonal prisms as suggested by previous x-ray diffraction results.