ON THE NATURE OF FRAMEWORK BRONSTED AND LEWIS-ACID SITES IN ZSM-5

The acidity of aluminum in a nominally 70:1 SiO2/Al2O3 ratio ZSM-5 pre pared by conventional means was characterized by TPAD, Al-27 and Si-29 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 su ggested that this Lewis aluminum may be located in the zeolite framewo rk. These sites are not thought to be inherent to the ZSM-5 structure but are probably generated during calcination, resulting in partial hy drolysis of Al-O bonds. A small percentage of octahedral aluminum was observed in hydrated HZSM-5, but was quantitatively converted to tetra hedral aluminum by treatment with ammonium nitrate solution at pH 8. ( C) Elsevier Science Inc. 1997.