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Y. Ono, K. Suzuki, and T. Keii
Electron Spin Resonance Study of the Formation of Anion Radicals over Titanium Exchanged Y-Zeolite Yoshio Ono,* Kenji Suzuki, and Tominaga Keii Department of Chemical Engineering, Tokyo lnstltute of Technology. Ookayama, Meguro-Ku, Tokyo, Japan (Received July 30, 7973) Publication costs assisted by the Tokyo lnstltute of Technology
The state and the chemical reactivity of Ti3+ ions in Y-zeolite are characterized with electron spin resonance. Ti3* ions tumble in the cavities of the hydrated-zeolite, but do not in the dehydrated one. Oxygen or sulfur dioxide adsorbs on TiY to form the anion radicals ( 0 2 - or SOz-) and the adsorption centers are Ti3+ ions. The reactivity of 0 2 - toward various molecules is examined. The formation of 0 radicals upon adsorption of N2O is also suggested.
Introduction Synthetic zeolites have been used for catalysts in many reactions. Their catalytic activity depends strongly on the nature of exchangeable metal cations. Thus, it is no doubt that the metal cations play an important role in the development of the catalytic activity. In many cases, especially in carbonogenic reactions, Brqinsted acid sites, which are raised with the introduction of the multivalent metal cations, are responsible for the catalytic activity.l Yet, in some cases, the metal cations themselves are believed to be active centers.1,2 Although the structure of the aluminosilicate framework has been determin,ed, the chemical properties of metal cations and, more importantly, the role of metal cations in determining catalytic behavior have not been completely resolved. The spectroscopic methods are suitable for this purpose since they give us direct information on the behavior of metal cations. Electron spin resonance and electronic spectral studies for several metal cations have been reported so far,3-6 but no work has been performed on titanium exchanged zeolites. Oxygen or sulfur dioxide adsorbs on partially reduced titanium dioxide to form the corresponding anion radicals and Ti3+ ions are postulated active centers for the charge transfer.7-ll Thus, it seems interesting to know the behavior of Ti3+ ions in zeolites. In this work, electron spin resonance of Ti3+ ions is studied and the physical and chemical properties of Ti3+ ions will be discussed. Experimental Section Titanium ion-exchanged zeolites (Ti-Y) were prepared by the conventional ion exchange of sodium form of Yzeolites (SK-40) with Ti3+; Na-Y was immersed in an aqueous solution of Tic13 (supplied by Toho-Titanium Co.) for several days and then filtered and washed thoroughly with water. Ti-Y thus obtained was stored under a nitrogen atmosphere. All procedures were carried out in a nitrogen atmosphere since Ti-Y is readily oxidized in air. The degree of exchange was determined by the gravimetric analysis of eluted sodium ions. Two samples with 4.2 and 10 Ti3+ ions per unit cell were prepared. Adsorption of gases was carried out with a conventional vacuum apparatus. The sample tubes for esr measurements ate quartz tubes 5 mm in diameter. Esr apparatus used was a The Journal of Physical Chemistry, Vol. 78, No. 3 , 1974
X-band spectrometer IJapan Electron Optics Laboratory JES-3BX) with the field modulation of 100 kHz. Results and Discussion Effect of Dehydration. Ti-Y evacuated for 30 min a t room temperature shows an isotropic spectrum with a g value of 1.945 and a line width of 35 Oe a t room temperature (Figure l a ) . The same sample shows an anisotropic spectrum when measured a t 77°K (Figure lb). The variation in line shape with measurement temperature indicates that Ti3+ ions in Y-zeolites have some freedom of tumbling at room temperature and that the freedom is lost a t 77°K. T i ( H 2 0 ) ~ in ~ +aqueous solution does not give esr absorption a t room temperature since complete octahedral configuration leads to very rapid spin lattice relaxation. Since the esr of the Ti3+ ions in Y-zeolite is readily observable a t room temperature as well as 77"K, it is clear that the configuration of ligands deviates from octahedral. The number of water molecules coordinated to Ti3+ ions may be less than six in zeolites. When Ti-Y is evacuated a t higher temperature (above 120°), it shows an anisotropic spectrum even a t room temperature and the line shape measured at room temperature is same as that measured at 77°K (Figure 2a). This implies that Ti3+ ions do not tumble in dehydrated zeolites. This behavior of Ti3+ ions to adsorbed water is quite ~ ~ samples similar to that of Cu2+ ions in C U - Y . ~The evacuated over 120" showed a small peak at g = 2.002 with a line width of 7 Oe. The origin of this absorption is unknown. Adsorption of Oxygen. When Ti-Y was evacuated at 500" and contacted with oxygen of 10 mm pressure for 30 min and then evacuated a t room temperature, the absorption signals with three g values (gl = 2.0196, g2 = 2.0089, g3 = 2.0031) appeared as shown in Figure 2. Since the g values agree well with those of the oxygen species on partially reduced titanium dioxide7-l0 and the extent of anisotropy in 02-radicals is known to depend on the cationic charge of metal cations to which 0 2 - radicals are attached,s we can conclude these new signals are caused by 0 2 - ions attached to T i nuclei. The reaction can be written as
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Formation of Anion Radicals over Titanium Exchanged Y-Zeolite
Figure 3. Energy levels of 02-
Figure 1. Esr spectra of hydrated Ti-Y measured at room temperature (a) and at 77°K ( b ) .
1
0.2
25
100
200
300
400
Adsorption temperature, "c
500
Figure 4. Effect of adsorption temperature on the esr intensity of 02-on Ti-Y containing 10 (0) and 4.2 (A)Ti3+ ions per unit
cell.
Figure 2. Esr spectrum of oxygen adsorbed on Ti-Y evacuated at 500" (a) and after 02 adsorption ( b ) .
The same type of superoxide ion formation has been demonstrated in the case of several cobalt complexes in solution .I2 Co(I1)
+
0, = Co(II1) -0-o-
Many works have been reported on the formation of 0 2 radicals in X- and Y - ~ e o l i t e s , l ~but - ~ ~the formation was always assisted by y-ray or ultraviolet irradiation. This is the first evidence of 0 2 - radical formation in zeolite by a purely chemical reaction. Kanzig and Coh'en17 have derived a theoretical expression for the g values of 0 2 - radicals
Figure 5. Esr spectrum of S02- on Ti-Y.
relative intensity of 0 2 - and Ti3+ decreased. At 500", both 0 2 - and Ti3+ signals completely disappeared. This may be caused by the further oxidation of Ti4+-O-Ocomplexes to titanium dioxide. Ti4+-O-O-
where X is the absolute value of the spin-orbital interaction constant for the 0 2 - radical, g, is the g value of the free electron, and A and E are the energies defined in Figure 3 . Assuming X/A
