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 linear correlations were established between certain lattice vibration frequencies and the lattice aluminum concentration (Al/unit cell) of the Cu-Y preparation. A better fit of the data was obtained with the pore systems filled with H2O than with dry samples. These correlations held for the reparations both before reduction and after reoxidation following reduction with CO or H-2, showing that dealumination had not occurred during this cycle. Three frequency ranges were found to be significant, viz., those in the 560-600-cm-1 region stemming from vibrations of the double 6-rings, those in the 780-825-cm-1 region corresponding 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 associated with the extralattice oxygen introduced during base exchange with zeolites charge-balanced with bivalent, but not with monovalent, cations. This latter band disappeared when the oxygen was removed by reduction and reappeared on reoxidation. Four frequencies were observed in the 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 increased in intensity as the Si/Al ratio increased and the Al T-sites moved further apart. These same bands decreased in intensity on reduction 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 that 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 much of the extralattice oxygen is associated with copper-containing species.