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J. Phys. Chem. B 2001, 105, 7878-7881
Simultaneous Observation of Alkenyl Carbenium Ions and Alkoxy Species on HZSM-5 Zeolite by Adsorption of 1-Methylcyclopentene and 1-Methylcyclopentanol Shuwu Yang, Junko N. Kondo, and Kazunari Domen* Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan ReceiVed: May 14, 2001; In Final Form: June 27, 2001
The adsorption and reaction of 1-methylcyclopentene (MCPE) and 1-methylcyclopentanol (MCPOH) on HZSM-5 were studied by in-situ FTIR spectroscopy at 150-295 K. For MCPE, weakly adsorbed species, i.e., alkyl group adsorption on the acidic OH group, were predominant below 196 K. At 196 K and above, the π-OH complex was formed followed by stepwise formation of stable alkenyl carbenium ions and alkoxy species, characterized respectively by an IR band at 1520 cm-1 and by the shift of C-H stretching frequencies. For MCPOH, dehydration occurs at 268 K and MCPE is produced. The latter further forms alkenyl carbenium ions and alkoxy species progressively with the increase in temperature.
Zeolites are probably the most important solid acid catalysts used in the transformation of hydrocarbons, and their hydroxyl groups play a critical role in the reactions requiring Brønsted acidity.1 Adsorption and reaction of hydrocarbons on acid sites of zeolites have been widely investigated by various techniques, such as IR and NMR. In the case of olefins, it was found that hydrogen-bonded adsorptions, both alkyl-OH interaction and π-OH interaction, occur at low temperatures.2,3 Increasing temperature causes the formation of oligomers4-6 or stable dimerized alkoxy species.7,8 In addition, several cyclic alkenyl carbenium ions were also detected to form on zeolites, mainly by Haw and co-workers using an in-situ NMR technique,9-13 and their recent results suggested that these cations are not only responsible for aromatic formation and coke deposition but also play a catalytic role in some reactions, e.g., MTO/MTG processes.14 The formation of alkenyl carbenium ions consumes a stoichiometric number of acid sites, therefore, it is of great importance to study their formation mechanism for a better understanding of the acidity, reactivity, and deactivation of zeolites. Since carbenium ions can be stabilized in cyclic structures,9 we systematically investigated the adsorption behavior of various cyclic precursors adsorbed on zeolites using in-situ infrared (IR) spectroscopy, a powerful technique that is widely used for studying zeolites. Very interestingly, dimerized alkenyl carbenium ions, characterized by an IR band at 1513 cm-1, were formed by adsorption of 1-methylcyclopentene on zeolite Y at temperatures as low as 150 K.15 Compared with zeolite Y, HZSM-5 has stronger acidity and smaller pore size (open apertures: 0.53 nm × 0.56 nm), therefore, it is of great interest to see the effect of acidity and pore size on this reaction. In this letter, the results of 1-methylcyclopentene and 1-methylcyclopentanol adsorbed on HZSM-5 are reported. Adsorptions were performed at temperatures below 150 K and followed by evacuation to avoid rapid oligomerization of olefins and to observe the formation of intermediates clearly. About 20-30 mg of HZSM-5 zeolite (JRC, Si/Al ) 27) was pressed into a self-supporting wafer (ca. 10 mg‚cm-2) and was placed in a quartz IR cell connected with a closed gas-circulation * Corresponding author. Tel: +81-45-924 5238. Fax: +81-45-924 5282. E-mail:
[email protected].
system. The pretreatment of the sample consisted of evacuation at 773 K for 1 h, oxidation by circulating O2 (100 Torr, 1 Torr ) 133.3 Pa) at 773 K for 1 h, and evacuation at the same temperature for a further 15 min to remove residual contaminants. Subsequently, the sample was treated with 100 Torr H2 at 673 K for 1 h, and followed by evacuation at room temperature. The treatment conditions were chosen to avoid the production of Lewis acid sites on the sample, as confirmed by CO adsorption.2 A small amount (ca. 13 µmol) of 1-methylcyclopentene (MCPE) or 1-methylcyclopentanol (MCPOH) was introduced to the IR cell at below 150 K and was immediately evacuated unless otherwise indicated. The IR cell was then gradually warmed at a rate of ca. 6 K‚min-1 during evacuation. All IR spectra were recorded on a JASCO 7000 FTIR spectrometer equipped with an MCT detector at a resolution of 4 cm-1, and 64 scans were collected for each spectrum. Only differential spectra are given by subtracting the spectra taken at different temperatures before adsorption. Figure 1 shows the IR spectra of MCPE adsorbed on HZSM5. The spectra of MCPE adsorbed on SiO2 (Aerosil Nippon, partially deuterated) and MCPOH adsorbed on ZrO2 (to form 1-methylcyclopentoxy) are also given as references. On SiO2 (Figure 1A-a), MCPE is only adsorbed molecularly via hydrogen bonds, indicated by the two negative bands at 3750 and 2765 cm-1 due to OH and OD silanols, respectively. The hydrogenbonded OH and OD bands appear at the lower frequency side as broad bands at 3495 and 2592 cm-1, respectively. The spectrum of adsorbed MCPE on SiO2 is identical to that of MCPE molecules in liquid phase.16 With increasing temperature, the adsorbed molecules are simply desorbed from SiO2 under evacuation. When MCPE was adsorbed on HZSM-5, it is also molecularly adsorbed at temperatures below 196 K, giving the νdC-H and νCdC bands at 3048 and 1660 cm-1 respectively. The latter band is at the same frequency as that of free MCPE molecules,16 which represents a weakly adsorbed species, i.e., the so-called alkyl-OH complex.2,3 Also an additional νCdC band appeared at 1634 cm-1 at 183 K. This band is 26 cm-1 lower than that of free molecules,16 suggesting the formation of a π-OH complex.2,3 However, several changes occurred by warming the sample to 196 K and above. The overall absorption
10.1021/jp011848s CCC: $20.00 © 2001 American Chemical Society Published on Web 08/01/2001
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Figure 1. IR spectra of 1-methylcyclopentene adsorbed on SiO2 at 215 K and on HZSM-5 at various temperatures: (A) 4000-1300 cm-1 region and (B) 3200-2700 cm-1 region. The spectra of 1-methylcyclopentene adsorbed on DY at 298 K (to form dimerized alkenyl carbenium ions) and 1-methylcyclopentanol adsorbed on ZrO2 at 398 K (to form 1-methylcyclopentoxy) are given as references.
gained intensity in the νC-H region. The νdC-H and νCdC bands at 3048 and 1660 cm-1 reduced in intensity with the increase in temperature. In the low-frequency region (1600-1300 cm-1), a new band appeared at 1520 cm-1. This band increased in intensity with increasing temperature, and reached the maximum upon heating to 373 K (not shown for the sake of brevity). At 473 K, it shifted to 1510 cm-1 and then disappeared after evacuation at 623 K. Because an IR band at 1490-1530 cm-1 has been attributed to the formation of alkenyl carbenium ions,13,15 the results suggest that alkenyl carbenium ions have formed at temperatures as low as 196 K, and are particularly stable. As shown in Figure 1B, more interesting phenomena were observed by enlarging the C-H stretching region. For free MCPE molecules, five bands exist in 3000-2800 cm-1, and the main bands are at 2969, 2940, and 2856 cm-1. After MCPE was adsorbed on HZSM-5, the 2969 and 2940-cm-1 bands shifted downward slightly. Meanwhile, the 2856-cm-1 band reduced in intensity with increasing temperature, and finally disappeared at 295 K. At 227 K, a new band at 2872 cm-1, which is 16 cm-1 higher than the 2856-cm-1 band, can be clearly observed. It grew in intensity and replaced the 2856cm-1 band at 295 K. It is worth noting that the 2960-cm-1 band is the strongest one all the time. Formation of alkenyl carbenium ions on HY zeolite gives different IR spectra in C-H stretching region (e.g., see Figure 1B-i).15 When MCPE is adsorbed on HY, alkenyl carbenium ions are formed as characterized by an IR band at 1513 cm-1 and a UV band at 323 nm. In such cases, only two bands exist at 2930 and 2860 cm-1 in C-H stretching region, and the former one is stronger. Also, our theoretical results did not predict a strong band at around 2960 cm-1 for 1-methylcyclopentenyl cations. Therefore, other adsorbed species should be considered to coexist with alkenyl carbenium ions on HZSM-5. Alkoxy species were suggested to be more stable than ionic protonated species theoretically17,18 and experimentally.6-8,10,19 They were found to be important long-live intermediates in the
oligomerization reactions of propene on zeolite HY10 and in the dehydration reaction of tert-butyl alcohol on HZSM-5.19 They were also reported to represent the main adsorption state of dimer7,8 or oligomer,6 which are actually very stable.7 Alkoxy species can be produced by adsorption of alcohols on ZrO2 followed by evacuation at above 373 K.20 Therefore, the IR spectrum of the 1-methylcyclopentoxy species, which was formed by adsorption of MCPOH on ZrO2 at 398 K, is compared in Figure 1A-h. The alkoxy species shows similar features as its corresponding alcohol, that is, in the 3000-2800 cm-1 region, two main bands can be observed at 2970 and 2879 cm-1, and the former is very strong. Comparing the spectrum of 1-methylcyclopentoxy species with that of MCPE adsorbed on HZSM-5 at 295 K (Figure 1A-g), it can be found that the two spectra are similar in C-H stretching region, with the exception that the 2935-cm-1 band in Figure 1A-g is stronger. Because the 2935-cm-1 band is associated with the formation of alkenyl carbenium ions,15 it is safe to say that alkoxy species are also formed together with alkenyl carbenium ions by the adsorption of MCPE on HZSM-5. Both species gave contributions to the whole spectrum. Figure 2 shows the IR spectra of MCPOH adsorbed on HZSM-5. At temperatures lower than 268 K, MCPOH is only adsorbed molecularly. Three bands appear at 2970, 2930 (shoulder), and 2882 cm-1 in the νC-H region, and the 2970cm-1 band is the most intense. At 268 K, MCPOH undergoes dehydration to form MCPE, as evidenced by the appearance of two bands at 3051 and 1622 cm-1, which are attributed to olefinic C-H stretching and adsorbed water, respectively. At the same time, the band at 1520 cm-1 appears. With increasing temperature, the two bands at 1622 and 1520 cm-1 grow progressively. From Figure 2B, the changes of the νC-H can be clearly observed, that is, MCPOH first undergoes dehydration and gives MCPE, and then MCPE forms alkenyl carbenium ions and alkoxy species. Ethene was used to calibrate the acid sites of HZSM-5, in order to identify the structure of the species formed on HZSM-5
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Figure 2. IR spectra of 1-methylcyclopentanol adsorbed on HZSM-5 at various temperatures and on ZrO2 at 398 K: (A) 4000-1300 cm-1 region and (B) 3200-2700 cm-1 region. The spectrum of 1-methylcyclopentene adsorbed on SiO2 at 215 K is given for comparison.
Figure 3. Calibration of acid sites of HZSM-5 using ethene at 213 K. For comparison, 3.35 µmol 1-methylcyclopetene (0.1 mmol‚g-1 cat.) is also introduced on HZSM-5 at 298 K. If monomeric species were formed, the consumption of the acid sites should be situated on the dotted line.
by measuring the number of adsorption sites. At 213 K, ethene is only monomolecularly adsorbed on the acid sites of HZSM-5 (forming a 1:1 π-OH complex) without oligomerization.3 Therefore, ethene was introduced to the IR cell at this temperature with increasing the dosage amount. By measuring the decrease of the integrated intensity of the 3616 cm-1 band (due to the isolated acidic OH groups), the correlation between OH band intensity (represents the amount of acid sites) and dosing amount of ethene was obtained (Figure 3). In separate experiments, 3.35 µmol MCPE was dosed to the sample at 213 K or at room temperature, and the decrease of the 3616 cm-1 band intensity was plotted versus adsorption time. If adsorbed MCPE species is monomeric, the consumption of acid sites should be the same as dosing 3.35 µmol ethene, as indicated by the dotted line in Figure 3. However, the introduced MCPE consumes fewer acid sites, indicating that oligomerization of MCPE has already occurred. Assuming that the oligomerized products are dimer
and/or trimer, one can estimate that ca. 20-30% MCPE molecules undergo oligomerization at 298 K. Xu and Haw12 studied the adsorption of cyclopentene and cyclopentanol on HZSM-5 by using NMR. They found that trimerized carbenium ions were formed at 373-433 K when cyclopentanol was adsorbed, and at 393 K when cyclopentene was adsorbed (the oligomerization of cyclopentene was nearly completed at this temperature). In our case, because of the existence of a tertiary carbon, both formation of alkyl carbenium ions and oligomerization are probably facilitated. As a result, the formation of alkenyl carbenium ions can be observed at much lower temperature. Currently, it is not possible to give a clear structure for the formed carbenium ions; however, from the NMR results of cyclopentene adsorbed on HZSM-5,12 one can deduce that the formed alkenyl carbenium ions are probably not monomeric, but dimerized or trimerized ones formed from MCPE oligomers. The formation of oligomers and dimerized or trimerized carbenium ions is responsible for the release of the isolated OH groups as shown in Figure 3. As mentioned above, alkoxy species were also formed on the sample. At the present time, it is hard to say if they are monomeric (i.e., 1-methylcyclopentoxy) or not. Theoretical calculations are in progress to evaluate the stability of 1-methylcyclopentoxy and its relation with alkyl and alkenyl carbenium ions. In summary, simultaneous formation of alkenyl carbenium ions as well as alkoxy species was observed by adsorption of 1-methylcyclopentene and 1-mentylcyclopentanol on HZSM-5 at 196 K and above. At room temperature, about 20-30% adsorbed 1-methylcyclopentene molecules underwent oligomerization to form oligomers, alkenyl carbenium ions, and/or alkoxy species. Acknowledgment. This work has been carried out as a research project of the Japan Petroleum Institute commissioned by the Petroleum Energy Center with the subsidy of the Ministry of International Trade and Industry. Grant-in-aid for Scientific Research (C) (No. 11640600) from the Ministry of Education,
Letters Science and Culture is also acknowledged. S.Y. thanks JSPS for providing fellowship and Grant-in-aid under project No. P99286. References and Notes (1) Corma, A. Chem. ReV. 1995, 95, 559. (2) Kondo, J. N.; Shao, L.; Wakabayashi, F.; Domen, K. J. Phys. Chem. B 1997, 101, 9314. (3) Kondo, J. N.; Wakabayashi, F.; Domen, K. J. Phys. Chem. B 1998, 102, 2259. (4) Spoto, G.; Bordiga, S.; Ricchiardi, G.; Scarano, D.; Zecchina, A.; Borello, E. J. Chem. Soc., Faraday Trans. 1994, 90, 2827. (5) Kondo, J. N.; Domen, K.; Wakabayashi, F. Catal. Lett. 1998, 53, 215. (6) Stepanov, A. G.; Luzgin, M. V.; Romannikov, V. N.; Sidelnikov V. N.; Paukshtis, E. A. J. Catal. 1998, 178, 466. (7) Ishikawa, H.; Yoda, E.; Kondo, J. N.; Wakabayashi, F.; Domen, K. J. Phys. Chem. B 1999, 103, 568. (8) Kondo, J. N.; Ishikawa, H.; Yoda, E.; Wakabayashi, F.; Domen, K. J. Phys. Chem. B 1999, 103, 8538.
J. Phys. Chem. B, Vol. 105, No. 33, 2001 7881 (9) Haw, J. F.; Nicholas, J. B.; Xu, T.; Beck L. W.; Ferguson, D. B. Acc. Chem. Res. 1996, 29, 259, and references therein. (10) Haw, J. F.; Richadson, B. R.; Oshiro, I. S.; Lazo, N. D.; Speed, A. J. J. Am. Chem. Soc. 1989, 111, 2052. (11) Oliver, F.; G. Munson, F. J.; Haw, J. F. J. Phys. Chem. 1992, 96, 8106. (12) Xu, T.; Haw, J. F. J. Am. Chem. Soc. 1994, 116, 7753. (13) Kirisci, I.; Fo¨rster, H.; Tasi, G.; Nagy, J. B. Chem. ReV. 1999, 99, 2085, and references therein. (14) Haw, J. F.; Nicholas, J. B.; Song, W.; Deng, F.; Wang, Z.; Xu, T.; Heneghan, C. S. J. Am. Chem. Soc. 2000, 122, 4763. (15) Yang, S.; Kondo, J. N.; Domen, K. Stud. Surf. Sci. Catal. 2001, 135, 217. (16) Durig, J. R.; Shing, A. C.; Bucy, W. E.; Wurrey, C. J. Spectrochim. Acta 1978, 34A, 525. (17) Kazansky, V. B. Acc. Chem. Res. 1991, 24, 379. (18) van Santen, R. A. Catal. Today 1997, 38, 377. (19) Stepanov, A. G.; Zamaraev, K. I.; Thomas, J. M. Catal. Lett. 1992, 13, 407. (20) Ouyang, F.; Kondo, J. N.; Maruya, K.; Domen, K. J. Chem. Soc., Faraday Trans. 1997, 93, 169, and references therein.
