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J. Phys. Chem. 1996, 100, 18882
COMMENTS Comments on “N2 Adsorption at 77 K on H-Mordenite and Alkali-Metal-Exchanged Mordenites: An IR Study” Fumitaka Wakabayashi,*,† Junko N. Kondo,‡ Akihide Wada,‡ Kazunari Domen,‡ and Chiaki Hirose‡ Department of Science and Engineering, National Science Museum, 3-23-1 Hyakunin-cho, Shinjuku-ku, Tokyo 169, Japan, and Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan ReceiVed: May 29, 1996 In 1993, we reported an FT-IR study of the N2 adsorption on H-MOR.1 We revealed that N2 serves as a probe of acid sites in zeolites and also concluded that there is an OH band at ∼3590 cm-1 that remains almost unaffected by the N2 adsorption; the band has been attributed to the bridging OH groups located in the side pockets, where N2 cannot approach and thus cannot interact with the OH groups. Recently, Geobaldo et al. reexamined the N2 adsorption on MORs in more detail.2 Most of their results, including the existence of the ∼3590 cm-1 band, are in agreement with ours. However, they claimed that N2 can approach the OH groups in the side pockets and interact with them; the remaining band at ∼3590 cm-1 is not due to the unperturbed OH groups but to the Al-OH groups perturbed by N2. One of their main reasons is as follows: since CO, which has a molecular dimension similar to N2, interacts with the OH groups, N2 should be able to approach the sites and interact with them. We admit that N2 can approach the sites, considering the fact that its kinetic diameter (0.36 nm) is smaller than the open aperture of the side pockets (0.37 × 0.48 nm2). However, it does not necessarily mean that N2 interacts with the OH groups in the side pockets. We would like to reconfirm in the present Comments that N2 is not adsorbed on the OH groups in the side pocket. Since N2 is a much weaker base than CO, the interaction of N2 with the acid sites should be weaker than that of CO. In fact, such examples have recently been reported.3,4 Marchese et al. revealed that N2 does not break the Co-O(H) bond of CoAPO-18 but CO does.3 Area´n et al. found that not N2 but CO interacts with the OH groups that are hydrogen bonded to lattice oxygens in the case of H-GaZSM-5; N2 probes only free acidic OH groups.4 Since the OH groups in the side pockets of MOR are known to be forming hydrogen bonds with the lattice oxygens,5 it is very likely that N2 cannot interact with the OH groups in the side pockets. A second reason of Geobaldo et al. is that the ∼3590 cm-1 band has shifted to lower frequency by ∼14 cm-1, and its bandwidth has distinctly increased by N2 adsorption; it is hard * To whom correspondence should
[email protected]. † National Science Museum. ‡ Tokyo Institute of Technology.
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to consider that the ∼3590 cm-1 band remains unperturbed by N2. Then, we reexamined the band deconvolution of the OH bands using a commercial curve-fitting program, GRAMS/386. Voigt functions were used as a shape of the component bands: a much better curve fitting than the previous one was obtained (Figure 1S). It has been found that the OH band of the side pocket OH groups shifted by only 5 cm-1 (from 3597.4 to 3592.2 cm-1) by N2 adsorption (saturation coverage at 3.7 kPa) while the broadening of the band was seen. A similar slight shift and broadening of the ∼3590 cm-1 band have also been observed for the adsorptions of Xe6 and alkanes7 on H-MOR. Since the kinetic diameters of Xe (0.40 nm) and alkanes (∼0.45 nm) are larger than the open aperture of the side pockets, it is hard to think that these molecules can enter the pockets. Hence, there are cases where this minor shift is not caused by the direct interaction with the probe molecules but by other long-range effects, such as an electrostatic one: the band broadening would be caused by the electrostatic effect. Thus, it is acceptable that N2 is not adsorbed on the OH groups in the side pockets in spite of the slight shift of the OH band. Geobaldo et al. attributed the ∼3575 cm-1 band that remained at the saturation coverage of the N2 adsorption to the Al-OH groups perturbed by N2. However, this assertion is qualitative and seems to be of less evidence. They claimed that Figure 7 in their article shows that the 3575 cm-1 band is formed at the expense of the 3675 cm-1 band (due to the Al-OH groups). However, it also seems that the 3575 cm-1 band become apparent only by the disappearance of the 3609 cm-1 band. We concluded that the perturbed Al-OH group is not the origin of the ∼3590 (3575) cm-1 band because although our sample has much less amount of the Al-OH groups, which is evidenced by the fact that the 3675 cm-1 band is hardly observed, the remaining ∼3590 cm-1 band is relatively large. Consequently, it is reasonable to conclude that the OH groups in the side pockets remain almost unaffected by the N2 adsorption and thus that N2 is not adsorbed on the OH groups in the side pockets although N2 can approach the sites. Supporting Information Available: Figure of band deconvolution (1 page). Ordering information is given on any current masthead page. References and Notes (1) Wakabayashi, F.; Kondo, J.; Wada, A.; Domen, K.; Hirose, C. J. Phys. Chem. 1993, 97, 10761. (2) Geobaldo, F.; Lamberti, C.; Ricchiardi, G.; Bordiga, S.; Zecchina, A.; Palomino, G. T.; Area´n, C. O. J. Phys. Chem. 1995, 99, 11167. (3) Marchese, L.; Gianotti, E.; Damilano, N.; Colluccia, S.; Thomas, J. M. Catal. Lett. 1996, 37, 107. (4) Area´n, C. O.; Palomino, G. T.; Geobaldo, F.; Zecchina, A. J. Phys. Chem. 1996, 100, 6678. (5) Bonn, M.; Brugmans, M. J. P.; Kleyn, A. W.; van Santen, R. A. J. Chem. Phys. 1995, 102, 2181. (6) Wakabayashi, F.; Fujino, T.; Kondo, J. N.; Domen, K.; Hirose, C. J. Phys. Chem. 1995, 99, 14805. (7) Lercher, J. A.; Gru¨ndling, C.; Eder-Mirth, G. Catal. Today 1996, 27, 353.
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© 1996 American Chemical Society
