J. Phys. Chem. 1996, 100, 18883
Reply to Comments on “N2 Adsorption at 77 K on H-Mordenite and Alkali-Metal-Exchanged Mordenites: An IR Study” A. Zecchina,* F. Geobaldo,† C. Lamberti, G. Ricchiardi, and S. Bordiga Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, UniVersita` di Torino, I-10125 Via P. Giuria 7, Torino, Italy
G. Turnes Palomino and C. Otero Area´n Departamento de Quı´mica, UniVersidad de las Islas Baleares, 07071 Palma de Mallorca, Spain ReceiVed: July 10, 1996 Regarding low-temperature dinitrogen adsorption on Hmordenites, the main difference between our interpretation1 and that of Wakabayashi et al.2 concerns the access of N2 molecules to the side pockets of the main zeolite channels. We have argued that dinitrogen has free access to these pockets, while Wakabayashi et al.2 stated that such an access is Virtually impossible. This argument is now settled, according to their present comments. The second point raised by the authors of the Comments concerns the extent of the interaction between N2 molecules and hydroxy groups located in the side pockets. An interpretation given in our article1 was that interaction of N2 with Brønsted acid groups situated in the main channel leads to a bathochromic shift of the O-H stretching frequency which amounts to -109 cm-1, while the corresponding shift for OH groups located in the side pockets is only of -14 cm-1. This implies that † Current address: Dipartimento di Scienza dei Materiali ed Ingegneria Chimica, Corso Duca degli Abruzzi 24, I-10129 Torino, Italy.
S0022-3654(96)02063-1 CCC: $12.00
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interaction of N2 with the latter hydroxy groups could be, in any case, very small. It does not seem to be a great difference between this conclusion and the present statement by Wakabayashi et al.: it is reasonable to conclude that the OH groups in the side pockets remain almost unaffected by N2 adsorption. However, before discarding the existence of a weak (specific) interaction between N2 molecules and OH groups in the side pockets, the following arguments should be carefully considered. (i) Bathochromic shifts of the O-H stretching frequency upon N2 adsorption were obtained from band deconvolution. Our quoted value1 is -14 cm-1, while Wakabayashi et al. report now -5 cm-1. It should be admitted that some uncertainty remains on the magnitude of this shift, and an average value of -10 cm-1 would still be significant. (ii) Our reported IR spectra in the N-N stretching region (Figure 8 in ref 1) show clear evidence for two peaks: 2330 and 2333 cm-1, which should both correspond to interaction of N2 with Brønsted acid sites, so that the probe molecule appears to be interacting with both types of OH groups (main channel and side pockets). This argument, which was developed in our paper,1 seems to have been ignored by Wakabayashi et al. (iii) The argument put forward by these authors when they state that xenon and alkanes (which do not enter the side pockets) cause a shift similar to N2 (which does enter) appears to involve some contradiction; even if nonspecific interactions are admitted in all cases, it is unlikely that long-range effects should cause the same quantitative response as short-range interactions. References and Notes (1) Geobaldo, F.; Lamberti, C.; Ricchiardi, G.; Bordiga, S.; Zecchina, A.; Turnes Palomino, G.; Otero Area´n, C. J. Phys. Chem. 1995, 99, 11167. (2) Wakabayashi, F.; Kondo, J.; Wada, A.; Domen, K.; Hirose, C. J. Phys. Chem. 1993, 97, 10761.
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© 1996 American Chemical Society
