7336 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA J. Phys. Chem. zyxwvu 1990, 94, zyxwvu 7336-1331 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJ lumnar conduction discussed in ref zyxwvutsr 5). On the other hand, the rise of c',~ from the isotropic-discotic transition temperature to its maximum value observed at 407 K could be attributed to a gradual "arrangement" of the columnar phase; Le., the lengths of the columns increase with decreasing temperature. Such a gradual increase in the columnar and ori- entational order with decreasing temperature has already been observed by Vilfan et al.I9 in their study of hexapentoxytri- phenylene. Below this temperature (407 K), zyxwvutsrq dlI decreases down to the discotic-solid transition. This behavior may be explained by the decrease of the thermally activated charge carriers con- centration and their mobility. On the contrary, the slight variation of dl indicates that the induced dipolar contribution in the molecular plane is much lower and independent of temperature (hence, of charge carriers concentration) but it depends only on the electronic polarizability of the molecules in their planes. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Conclusion We have measured the dielectric constant and conductivity of [(C,20),Pc]2Lu in the homeotropic and homogeneous orientation obtained with a relatively low magnetic field. The dielectric anisotropy is positive and its value may be compared to that found for nematic liquid crystals. We have proposed an explanation based on the intracolumnar conduction which generates an im- portant induced dipolar moment along the columnar axis. We may thus conclude that the columnar phase possesses a negative diamagnetic anisotropy. These results are in agreement with what is actually observed: the major contribution to the conduction occurs along the columnar axis. The validity of such an argument is to be tested by further dc and ac investigations under magnetic and/or electric fields, which are in progress and will be reported in a future publication. (19) Vilfan, M.; Lahajnar, G.; Rutar, V.; Blinc, R.; Topic, B.; Zann, A,; Dubo~s, J. C. J. Chem. Phys. 1981, 75, 5250. Rutar, v.; Blinc, R.; Vilfan, M.; Zann. A.; Dubois, J. C. J. Phys. 1982, 43, 761. Acknowledgment. I thank the members of the GRIMM and especially Prof. J. Simon for providing the compound and for his precious remarks. Characterizatlon of Vanadium Oxide Catalysts Supported on SnO, by 51V and 'H SoHd-State NMR Spectroscopy K. Narsimha, B. Mahipal Reddy,* P. Kanta Rao,* Catalysis Section, Indian Institute zyxwvut of Chemical Technology, Hyderabad 500 007, India and V. M. Mastikhin Institute of Catalysis, Novosibirsk 630 090, USSR (Received: May 24, 1990) The solid-state 51V and 'H NMR spectra of V205/Sn02 catalysts reveal the existence of two types of vanadia species on the surface: one due to a dispersed vanadia phase at lower vanadium loadings and the other due to crystalline vanadia phase at higher vanadium contents. Vanadia-based catalysts are well-known for catalyzing a great variety of industrial reactions.] Recent studies have shown that V2Os supported on a metal oxide support such as A1203, S O z , and TiO, is a superior catalyst to unsupported crystalline V205 for the selective oxidation and ammoxidation of many hydro- carbons.2 Thus the support metal oxide plays a major role in determining the dispersion and activity of the V2O5 when sup- ported. In recent years attention has been focused mainly on the study of the dispersion as well as the interaction of vanadia species with the supported oxides. The availability of new surface sensitive techniques such as XPS,3-5 EXAFS,6 ESR? and Raman spec- troscopy5+* have made possible a number of recent careful scientific studies. The technique of solid-state 5'V NMR represents a promising approach to these systems. Owing to a large magnetic moment, high natural abundance (99.76%), and favorable re- laxation characteristic, this nucleus is very amenable to solid-state N M R investigation? In addition, the recent development of the magic angle spinning (MAS) technique has afforded the high- resolution N M R spectra of 'H nuclei in solid samples.I0 However, the applications of the J;'V and 'H MAS NMR techniques to the characterization of V,O,/SnO, catalysts have not been previously reported. It is only recently that the importance of tin oxide-based catalytic systems has begun to be recognized." In this study the SnO,-supported catalysts containing 0.5-5.5 wt 7% V205 were prepared by the standard wet impregnation technique with the required amounts of aqueous ammonium Address correspondence to these authors metavanadate solutions. The impregnated catalysts were dried at 120 OC for 12 h and calcined at 500 'C for 6 h. The Sn02 support (N, BET surface area 30 m2 g-l) was prepared by pre- cipitating stannic hydroxide from stannic chloride with dilute ammonia solution. The chloride free precipitate was dried at 120 OC for 16 h and calcined at 600 OC for 6 h in air. The solid-state 51Vand IH NMR spectra with MAS technique have been re- corded on a Bruker CXP-300 spectrometer. 51V NMR spectra were recorded at a frequency of 78-86 MHz in the frequency range of 150 kHz, using 1-ps radio frequency pulses with repetition rate of 10 Hz. Chemical shifts were measured relative to external V0Cl3. The 'H N M R spectra were recorded at a frequency of 300.090 MHz. The frequency range was 50 kHz, (a/2) pulse (1) Gellings, P. J. Curulysis; Royal Society of Chemistry: London, 1985; (2) Bond, G. C.; Flamerz, S.; Shukri, R. Furaday Discuss. Chem. Soc. (3) Anderson, S. L. T. J. Chem. Soc., Faraday Trans. 1 1919.75, 1357. (4) Bond, G. C.; Zurita, J. P.; Flamerz, S. Appl. Caral. 1986, 27, 353. (5) Saleh, R. Y.; Wachs, 1. E.; Chan, S. S.; Chersich, C. C. J. Cutal. 1986, (6) Kozlowski, R.; Pettifer, R. F.; Thomas, J. M. J. Phys. Chem. 1983,87, (7) Chary, K. V. R.; Reddy, B. M.; Nag, N. K.; Sunandana, C. S.; Su- (8) Oyama, S. T.; Went, G. T.; Lewis, K. B.; Bell, A. T.; Somorjai, G. A. (9) Zamaraev, K. I.; Mastikhin, V. M. Colloids Surf. 1984, 12, 401. (10) Clague, A. D. H. Cafalysis; Royal Society of Chemistry: London, (I 1) Tanaka, K.; Sasaki, M.; Toyoshima, 1. J. Phys. Chem. 1988,92,4730. Vol. 7, p 165. 1989, 87, 225 and references therein. 98, 102. 5176. brahmanyam, V. S. J. Phys. Chem. 1984,88, 2622. J. Phys. Chem. 1989, 93, 6786, 1985; Vol. 7, p 61. 0022-3654/90/2094-7336$02.50/0 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 0 1990 American Chemical Society