J. Phys. Chem. 1993,97, 11853
COMMENTS Comment on “Free Radicals Formed in the Adsorption of Saturated Hydrocarbons on H-Mordenite” E. Roduner’ and R. Crockett Physikalisch- Chemisches Institut der Universitiit Zurich, Winterhurerstrasse 190, CH-8057 Ziirich, Switzerland Received: July 6, I993 Most EPR studies of organic radicals in zeolites involve either spontaneous oxidation of unsaturated adsorbate molecules or ionizing radiation. The recently reported observation by Chen and Fripiatl of EPR spectra upon adsorption of saturated hydrocarbons under mild conditions is therefore noteworthy and represents a significant achievement. There are, however, a number of serious errors and misinterpretations in this paper on which we wish to comment. Adsorption of 3-methylpentane on H-mordenite, followed by heating to 100 OC for 4 h, gave a nine equidistant line spectrum with 17.3-G hyperfine splitting and a g value of 2.003 (Figure 3 of ref 1, scale bar should be 35 G to comply with text). Knowing that activation of zeolites leads to Lewis sites which by virtue of their oxidizing nature initiate the formation of radical cations, it is not easy to fail to recognize that the characteristic spectrum belongs to the tetramethylethene radical cation. The latter has been observed many times in zeolites as well as in matrices2 and even in a 3-methylpentane glass.3 The g factor of 2.003 is typical but does not distinguish the cationic from the neutral species, and hyperfine couplings (hfcs) in the range 17.2 f 0.3 G have been reported. The lines are narrow as a consequence of the absence of unresolved small hfcs and of a-protons with their concomitant large hfc anisotropies. A spectrum with 13 much broader quasi-equidistant lines of again ca. 17 G average hfc and g = 2.003 was observed upon adsorption followed by heating to 65 OC of methylcyclohexane on H-mordenite.1 The spectrum was ascribed to the l-methylcyclohexyl radical with two pairs of inequivalent (at room temperature!) 8-protons with 17 and 35 G and with a 17 G methyl proton coupling. This gives only 10 lines, and in fact for their stick plot simulation in Figure 2b the authors’ have used a much less realistic methyl proton hfc of 35 G, in addition to the above pairs of methylene protons. The assignment disagrees with the literature values for the 1-methylcyclohexyl radical for which at 90 K reasonable hfcs of 23 G for the methyl protons and 46 and 9 G for the methylene protons have been reported.4 The observation by Chen and Fripiat’ is again much better explained by a radical cation: the 1,2-dimethylcyclopent- 1-ene radical cation which has six equivalent methyl protons of 16.7 G and at 157 K four equivalent methylene protons of 34.2 G.6 An isotropic simulation of this species, shown in Figure 1, is in excellent agreement with the experimental spectrum. The assignments of the two EPR spectra to radical cations rather than to neutral radicals changes the implications for the reaction mechanisms. The immediate observation of radical cations upon adsorption of unsaturated organic compounds which contrasts with the long induction period with saturated hydrocarbons as reported in ref 1 suggests that the unsaturated species, if not present, has to be formed first. In acidic zeolites, protonation occurs typically a t the tertiary carbon atom, and it is often followed
I Figure 1. Comparison of the experimental EPR spectrum observed by Chen and Fripiat’ with methylcyclohexane on H-mordenite (a) with an isotropic simulation (b) of the 1,2-dimethylcyclopent-1-ene radical cation using literature values for the hfc of the six methyl protons (16.7 G)and the four equivalent methylene protons (34.2 G).6
by Hz (as here) or low alkane elimination. The resulting alkylcarbenium ions undergo extensive isomerizations, probably via corner-protonated cyclopropane intermediates.5 The ring contraction of the methylcyclohexyl cation to the 1,Zdimethylcyclopentyl cation is also a known reaction in zeolites.5 Loss of a proton transforms the carbenium ion into an olefin. These processes are possible in zeolites in their H-form also at room temperature. Since they normally occur via diamagnetic intermediates, no radicals are observed. Only the presence of Lewis sites leads to oxidation of the unsaturated species, and the transients observed by EPR under these conditions are preferentially the radical cations generated from the olefin with the lowest ionization potential. EPR thus has a rather “selective perception” of the variety of molecules present in a zeolite.’ This may be regarded as a weakness of the method, but since it leads to clearly identifiable signatures of intermediates it is a t the same time a strength, and it is felt that the method has a significant future in the investigation of intermediates of catalytic processes in zeolites.
Acknowledgment. We thank the Swiss National Foundation for Scientific Research for financial support. References and Notes (1) Chen, F. R.; Fripiat, J. J. J. Phys. Chem. 1993, 97, 5796. (2) (a) Corio, P. L.; Shih, S . J. Phys. Chem. 1971, 75, 3475. (b) Shih, J. J . Catal. 1975, 36, 238. (c) Shida, T.; Egawa, Y.;Kubodera, H.; Kato, T. J . Chem. Phys. 1980, 73, 5963. (d) Fujisawa, J.; Sato, S.;Shimokmhi, K. Radiar. Phys. Chem. 1987, 29, 393. (e) Qin, X . 4 ; Trifunac, A. D. J . Phys. Chem. 1990, 94, 4751. (0 Barnabas, M. V.; Trifunac, A. D. Chem. Phys. Left. 1992, 293, 298. (8) Roduner, E.; Crockett, R.; Wu, L.-M. J . Chem. SOC.,Faraday Trans. 1993,89, 2101. (3) Ichikawa, T.; Ludwig, P. K. J . Am. Chem. SOC.1969, 91, 1023. (4) Chachaty, C. J . Chim. Phys. 1967, 64, 614. (5) Jacobs, P. A,; Martens, J. A. In Srudies in Surface Science and Catalysis; Van Bekkum, H., Flanigen, E. M., Jansen, J. C., Eds.;Elsevier: Amsterdam, 1991; Vol. 58, p 445. (6) Shiotani, M.; Lund, A. In Radical Ionic S y s t e m : Properties in CondensedPhases; Lund, A., Shiotani,M.,Eds.; Kluwer Academic: Dordrecht, 1991; p 151. (7) Crockett, R.; Roduncr, E. J . Chem. Soc.,Perkin Trans. 2 1993,1503.
0022-365419312097-11853%04.00/0 0 1993 American Chemical Society
