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

The acidity of aluminum in a nominally 70:1 SiO₂/Al₂O₃, ratio ZSM-5 prepared by conventional means was characterized by TPAD, ²⁷Al and ²⁹Si n.m.r., XRD, and FTi.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.