EFFECTS OF REDUCTION AND REOXIDATION ON THE INFRARED-SPECTRA FROM CU-Y AND CU-ZSM-5 ZEOLITES

Several zeolite preparations of varying Si/Al ratios were studied with their base-''change cations in the Cu2+ and in the Cu+ states. Good l inear correlations were established between certain lattice vibration frequencies and the lattice aluminum concentration (Al/unit cell) of t he Cu-Y preparation. A better fit of the data was obtained with the po re systems filled with H2O than with dry samples. These correlations h eld for the reparations both before reduction and after reoxidation fo llowing reduction with CO or H-2, showing that dealumination had not o ccurred during this cycle. Three frequency ranges were found to be sig nificant, viz., those in the 560-600-cm-1 region stemming from vibrati ons of the double 6-rings, those in the 780-825-cm-1 region correspond ing to T-0 vibrations of the external linkages between tetrahedra, and a band at about 905 cm-1 (at 935 cm-1 for Cu-ZSM-5) which was associa ted with the extralattice oxygen introduced during base exchange with zeolites charge-balanced with bivalent, but not with monovalent, catio ns. This latter band disappeared when the oxygen was removed by reduct ion and reappeared on reoxidation. Four frequencies were observed in t he OH stretching region: at around 3745 cm-1 (the invariant ubiquitous SiOH vibration), the Bronsted acid bands at 3640 and 3550 cm-1, and a band near 3675 cm-1 assignable to a CuOH stretch. This latter band in creased in intensity as the Si/Al ratio increased and the Al T-sites m oved further apart. These same bands decreased in intensity on reducti on with CO, but increased when H-2 was used. Isotopic substitution of OH by OD effected the expected isotopic shift in the OH bands, but did not affect the 907-cm-1 band. Substitution of O-18 for O-16 produced a shift from 909 to 895 cm-1 (DELTAnu = 14 cm-1). The data suggested t hat the species responsible for the 3675-cm-1 band is convertible into that for the 907-cm-1 species. These results support the view that mu ch of the extralattice oxygen is associated with copper-containing spe cies.