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creased only to 60% of the original value. Thus, the ability of a molecular film on an electrode surface to alter double-layer capacitances or electrode kinetics may possibly rest on effecting changes in the outer monolayer of metal surface atoms as well as on the more commonly considered effect of providing a strong transport barrier between the metal and the electrolyte solution. In summary, the electrochemically formed PPO films exhibit permeabilities in acetonitrile and aqueous environments that can be controlled by details of the electrochemical polymerization and are relatively defect-free so as to act as permeant molecular volume-selective sieving barriers. Permeabilities are less in water
due presumably to poorer solvation of the film there. The permeabilities are comparable to those (less quantitatively) described for self-assembled alkylthiol monolayers and as such, the PPO films are the first example of a nonordered molecular layer combining extreme thinness with relative freedom from pinhole defects.
Acknowledgment. This research was supported in part by a Materials Chemistry Initiative grant from the National Science Foundation. R.L.M. acknowledges support from the Electrochemical Society (SS 1989).
Influence of Framework SVAI Ratio on the Raman Spectra of Faujasitic Zeolites Prabir K. Dutta* and Jen Twu Department of Chemistry, The Ohio State University, Columbus, Ohio 43210 (Received: August 20, 1990)
This paper examines the dependence of the Raman spectrum of the faujasitic family of zeolites on the Si/AI ratio. The vibrational spectrum of the completely siliceous faujasite is consistent with those of other silica polymorphs. Based on the correlation between the prominent Raman bands and the T O T angle, the strong, sharp bands at 488 and 510 cm-' in the completely siliceous faujasite correspond to average Si-0-Si angles of 141' and 147'. For the framework with Si/AI = 1 .O, the four bands in the 900-1 250-cm-' region are assigned to the S i 4 stretching vibrations of Si attached to the four different oxygens of the framework. At intermediate Si/AI ratios, bands in the 900-1250-cm-' region show an increase in frequency as the %/A1 ratio increases.
Introduction The faujasitic family of zeolites plays an important role in a wide variety of separation, chemical, and petrochemical processes.' Their thermal stability is important to their function as catalyst, and dealumination is an established procedure for the improvement of thermal stability.2 Direct synthesis of faujasitic-like zeolites is only possible for Si/Al ratios of the framework between 1 and 3.3 To achieve higher %/A1 ratios, secondary methods that remove AI from the framework are required. Many such dealumination techniques have been reported.2 Spectroscopic methods have played an important role in determining the framework structure upon dealumination. In particular, solid-state NMR spectroscopy has provided information about the framework composition, the nature of silanol defects, and the extra framework aluminum trapped in the zeolites.e6 Infrared spectroscopy, in both the mid-infrared and 0-H stretching regions, has been used to draw correlations with the Si/AI ratio, unit cell size, and silanol groups.+* In this paper, we present the first Raman spectroscopic study of the faujasitic zeolite as a function of Si/Al ratio, along the same lines as we did recently for zeolite A.9 The motivation behind this study is twofold. First, there is considerable interest in developing Raman spectroscopic data on zeolite frameworks, since this information complements the infrared data. This is especially true for faujasite, which because of its cubic symmetry is expected to have mutually exclusive infrared and Raman bands.1° Second, there are an (1) Ward, J. W. Appl. Ind. Catal. 1984, 3, 271. (2) Scherzer, J. ACS Symp. Ser. 1984, No. 248, 157. (3) Robson, H. ACS Symp. Ser. 1989, No. 398, 436. (4) Klinowski, J.; Thomas, J. M.; Fyfe, C. A.; Gobbi, G.C. Nature 1982, 296, 533. (5) Englehardt, G.;Lohse, U.; Samoson, A.; Magi, M.; Tarmak, M.; Lippmaa, E. Zeolites 1982, 2. 59. (6) Ray, G.J.; Nerheim, A. G.;Donohue, J. A. Zeolites 1988, 8, 458. (7) Pichat, P.;Beaumont, R.; Barthomeuf, D. J Chem. Soc., Faraday Trans. I 1974, 70, 1402. (8) Miecznikowski, A.; Honuza, J. Zeolites 1985, 5, 188. (9) Dutta, P. K.; Deibarco, B. J . Phys. Chem. 1988, 92, 354. ( 1 0 ) Maroni, V. A. Appl. Spectrosc. 1988, 42, 487.
0022-365419112095-2498$02.50/0
increasing number of calculations on zeolite framework vibrations, the veracity of which can only be verified by comparison with high-quality experimental data."J2 In this study, we have chosen to examine faujasitic zeolites with framework Si/Al ratios of 1, 1.3, 2.6,3.3,4.5, and m, as determined by X-ray fluorescence and NMR spectroscopy. This choice was also based on the relevance of these particular materials in separations and catalytic processes.
Experimental Section Faujasite with a Si/Al ratio of 1.0 was synthesized by following the procedure of Kuhl, using a mixed-cation system of NaOH and KOH." N o attempts were made to replace the K+ by Na+ in the framework, because of the susceptibility to hydrolysis. The frameworks with Si/Al ratios 1.3, 2.6, 3.5, and 4.5 were commercial samples. The first two samples were obtained from Union Carbide and the latter two from Katalistiks. The framework with Si/Al ratio was a gift from Dr. Jack Donohue of Amoco Chemical and prepared by SiC14dealumination as described by him and his co-workers.6 The Raman spectra were obtained from hydrated zeolite samples, using 457.9-nm radiation as the exciting source (Spectra Physics 171 laser), a Spex 1403 spectrometer, and a RCA C31034 GaAs PMT with photon counting. Slit widths were typically 6 cm-I, and a laser power of 20-50 mW was used. Sloping background in the Raman spectra were corrected for with the help of Spectra Calc programs. The curve deconvolution was also done with the help of this program. 0)
Results Figure 1 shows the Raman spectrum of completely siliceous faujasite. The spectrum can be divided into four regions: 200-400 (I), 450-550 (11), 750-900 (111), and 900-1250 cm-' (IV). Low-frequency torsional modes as well as cation-oxygen lattice (11) No, K. T.; Bae, D. H.; Jhon, M. S.J . Phys. Chem. 1986,90, 1772. (12) de Man, A. J. M.; van Beest, B. H. W.; Leslie, M.; van Santen, R. A. J . Phys. Chem. 1990, 94, 2524. (13) Kuhl, G.H. Zeolires 1987, 7 , 451.
0 1991 American Chemical Society
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Raman Spectra of Faujasitic Zeolites c- II ->
