Langmuir 1997, 13, 747-750
747
IR Study of H2O Adsorbed on H-ZSM-5 Junko N. Kondo, Miho Iizuka, and Kazunari Domen* Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan
Fumitaka Wakabayashi Department of Science and Engineering, National Science Museum, 3-23-1 Hyakunin-cho, Shinjuku-ku, Tokyo 169, Japan Received July 30, 1996. In Final Form: November 18, 1996X Adsorption of H2O on H-ZSM-5 was studied by FT-IR in the presence of gaseous H2O in the temperature range between 293 and 423 K. Conversion of monomeric hydrogen-bonded H2O to dimeric species was clearly observed in IR spectra measured at 373 K by increasing the equilibrium pressure of H2O. Polymeric water was also formed by further increase of H2O pressure at the expenses of dimeric species. The frequencies and the relative intensities of IR bands due to dimeric species agree with the quantum-chemical results for H5O2+ species.
1. Introduction In recent years, adsorption of H2O on H-form zeolites has been extensively studied as one of the fundamental systems to understanding the Brønsted acidity of the zeolitic OH groups.1 Two models have been suggested for adsorption structure of H2O on H-form zeolites: a hydrogen-bonded model2 and a protonated model3 as depicted in Chart 1. Recent quantum chemical studies show that the neutral adsorption (hydrogen-bonded) structure gives the potential minimum and that the protonated ion pair is a transition state.4,5 Moreover, our infrared (IR) study using H218O directly demonstrates that the IR spectrum observed for the water adsorption on H-ZSM-5 is due to the hydrogen-bonded structure.6 The hydrogen-bonded model seems to be now widely accepted, although a recent neutron diffraction study suggests the coexistence of the neutral complex and the ion pair complex in the cage of HSAPO-34.7 Is it impossible to detect any protonated species by IR spectroscopy? Initially Jentys et al.8 and recently Zecchina and his co-workers reported that protonation was observed by further adsorption of H2O to 1:1 hydrogen-bonded structure to form dimeric H5O2+ and polymeric H5O2+‚ n(H2O) species.9,10 In this paper, we studied H2O adsorption at relatively high temperature by IR spectroscopy. H2O is reversibly adsorbed in the presence of gas molecules above 373 K, and the adsorption amount is successfully controlled by changing the equilibrium pressure of H2O. The observed spectra are carefully analyzed. X Abstract published in Advance ACS Abstracts, January 15, 1997.
(1) Sauer, J.; Ugliengo, P.; Garrone. E.; Saunders, V. R. Chem. Rev. 1994 94, 2095. (2) Pelmenschikov, A. G.; van Santen, R. A. J. Phys. Chem. 1993, 97, 10678. (3) Marchese,; Chen, L. J.; Wright, P. A.; Thomas, J. M. J. Phys. Chem. 1993, 97, 8109. (4) Krossner, M.; Sauer, J. J. Phys. Chem. 1996, 100, 6199. (5) Zygmunt, S. A.; Curtiss, L. A.; Iton, L. E.; Erhardt, M. K. J. Phys. Chem. 1996, 100, 6663. (6) Wakabayashi, F.; Kondo, J. N.;Domen, K.; Hirose, C. J. Phys. Chem. 1996, 100, 1442. (7) Smith, L.; Cheetham, A. K.; Morris, R. E.; Marchese, L.; Thomas, J. M.; Wright, P. A.; Chen, J. Science 1996, 271, 799. (8) Jentys, A.; Warecka, G.; Derewinski, M.; Lercher, J. A. J. Phys. Chem. 1989, 93, 4837. (9) Zecchina, A.; Buzzoni, R.; Bordiga, S.; Geobaldo, F.; Scarano, D.; Ricchiardi, G.; Spoto, G. Stud. Surf. Sci. Catal. 1995, 97, 213. (10) Buzzoni, R.; Bordiga, S.; Ricchiardi, G.; Spoto, G.; Zecchina, A. J. Phys. Chem. 1995, 99, 11937.
S0743-7463(96)00756-1 CCC: $14.00
Chart 1
2. Experimental Section The H-ZSM-5 (Si/Al ratio is about 50) sample was provided by Sumitomo Chemical Co., Ltd., which was pressed into a selfsupporting disk (20-30 mg, 20 mm in diameter). The sample was first treated by O2 at 673 K for 1 h in an IR cell connected to a closed gas circulation system, followed by evacuation at the same temperature for 30 min. Details of the experiment are described elsewhere.6 H2O was used after purification by degassing in evacuation. IR spectra were recorded on a JASCO FT/IR-7300 spectrometer with a MCT detector at 4 cm-1 resolution. Thirty two scans were averaged for one spectrum.
3. Results and Discussions Hydrogen-bonded H2O is known to be formed on H-ZSM-5 at room temperature.2,4-7 IR spectra of H2O adsorbed on H-ZSM-5 at 293 K are shown in Figure 1. All spectra are presented as difference spectra. Pressure indicated in the figures is the equilibrium pressure of H2O. Since a background spectrum of H-ZSM-5 under evacuation was subtracted from each spectrum, a reverse band at 3608 cm-1 appeared due to hydrogen-bonding of Brønsted acid OH groups to H2O. As the assignments listed in Table 1, a 1:1 hydrogen-bonded complex shows IR bands at 3701, ∼3550, 2872, 2460, ∼1700, and 1350 cm-1.2 The sharp band at 1632 cm-1 is assigned to deformation (δ(HOH)) band of physisorbed water in microcavities.6 In addition to the bands observed in Figure 1, out-of-plane bending (γ(OH)) band of hydrogen-bonded Brønsted acid OH groups of H-ZSM-5 was observed at 875 cm-1 as reported by Zecchina et al.8 Initially, all of the introduced H2O was adsorbed on H-ZSM-5 and resulted in the equilibrium pressure at 0.0 Torr (lowest spectrum on Figure 1). Increasing the introducing pressure of H2O, hydrogen-bonded complex monotonously © 1997 American Chemical Society
748
Langmuir, Vol. 13, No. 4, 1997
Kondo et al.
Figure 1. IR spectra of H2O adsorbed under indicated pressures of gaseous H2O at 293 K. Table 1. Assignments of IR Bands of 1:1 Complex of Hydrogen-Bonded H2O on Acidic OH of H-ZSM-5
a
assignment
origina
observed band/cm-1
ν(OH) ν(OH) Ab Bb Cb δ(OH) γ(OH)
water water zeolite zeolite zeolite zeolite zeolite
3701 ∼3550 2872 2460 ∼1700 1350 875
Figure 2. Difference IR spectra of adsorbedH2O between indicated pressures of gaseous H2O at 293 K.
Reference 6. b (A, B, C) triplet.
increased, and change of the spectral appearance occurred when the equilibrium pressure was observed. The sharp band at 3701 cm-1 and two broad bands between 2200 and 3200 cm-1 as well as the band at 1350 cm-1 due to the 1:1 hydrogen-bonded complex markedly decreased by increasing the equilibrium pressure from 0.1 to 0.2 Torr, and they diminished at 1.0 Torr of H2O pressure. This indicates that the hydrogen-bonded H2O transformed to another type of complex by interaction with both additional H2O molecules and Brønsted acidic OH groups on H-ZSM5. Simultaneously, the generation of several bands between 2800 and 3800 cm-1 is noticed up to 0.4 Torr followed by further change in OH stretching region (above 3000 cm-1) at 1.0 Torr and above. From the fact that the new species observed in Figure 1 were produced by further adsorption of H2O after Brønsted acidic OH groups of H-ZSM-5 were fully covered by H2O, the formation of dimeric species is likely. Difference spectra between each pressure are shown in Figure 2 to clearly observe the stepwise change in IR spectra caused by the increase of the H2O pressure. In the lowest spectrum in Figure 2, hydrogen-bonded H2O was found to increase to 0.1 Torr, and the generation of the dimeric species was also noticed by the appearance of the band at around 3200 cm-1 and the different spectral feature between 3400 and 3800 cm-1 from that of pure hydrogen-bonded H2O (the lowest spectrum in Figure 1). Between 0.1 and 0.2 Torr, the formation of the dimeric species is noted by bands at 3598, 3342, 3207, and 1693 cm-1 at the expense of hydrogen-bonded H2O observed by reverse bands at 3704, 2861, 2462, and 1350 cm-1. However, the exact frequencies of bands assigned to the dimeric species were obscure due to the concurrent formation of
Figure 3. IR spectra of H2O adsorbed under indicated pressures of gaseous species at 423 K.
polymeric species as evidently observed in the difference spectrum between 1.0 and 0.4 Torr. Furthermore, it is not possible to make clear in Figure 2 whether the dimeric species is the precursor of polymeric species or not. Equilibrium adsorption of H2O was observed at higher temperatures to exclude the effect of the spectrum of polymeric species observed at 293 K on that of the dimeric species. Spectra measured at 423 K at various H2O pressure are shown in Figure 3, where hydrogen-bonded H2O is reversibly adsorbed. The adsorption of hydrogenbonded H2O saturated at 5.1 Torr above which it started to decrease accompanied by further adsorption and condensation of H2O in pores of the zeolite. The spectrum
IR Study of H2O Adsorbed on H-ZSM-5
Figure 4. IR spectra of H2O adsorbed under indicated pressures of gaseous H2O at 373 K.
of condensed (or polymerized) H2O appeared differently at 423 K from that observed at 293 K (top spectrum in Figure 1) but similarly to that of H2O in liquid phase11 or on SiO2.12 The effect of the complicated spectrum of polymeric H2O observed at 293 K was avoided. However, the bands of only hydrogen-bonded and polymeric H2O were observed while those due to the dimeric species were not in any difference spectra (not shown). The same experiments were then conducted at 373 K where considerable amount of the dimeric species is expected to be observed. IR spectra of H2O on H-ZSM-5 under various H2O pressures at 373 K are shown in Figure 4. Hydrogen-bonded H2O increased in the same manner at any pressures during increase up to 1.0 Torr. At 1.0 Torr, all the isolated Brønsted acidic OH groups was confirmed to be hydrogen-bonded with adsorbed H2O. Increasing the pressure above 1.0 Torr, all the bands due to hydrogen-bonded H2O were attenuated as observed at 293 and 423 K. The change of the OH stretching (ν(OH)) bands was not clearly observed due to overlapping of several bands. Concurrently, strong features appeared between 3600 and 2800 cm-1. When the gas phase H2O was evacuated, only negligible amount of hydrogen-bonded H2O remained, indicating that both hydrogen-bonded and the dimeric species exist in the equilibrium condition at this temperature. In order to extract the change in spectra of adsorbed H2O at pressures above 1.0 Torr, difference spectra are shown in Figure 5 using the spectrum obtained at 1.0 Torr of H2O as a background. Decrease of the bands due to hydrogen-bonded H2O is clearly observed simultaneously with the appearance of the bands at 3600, 3366, 3213, 1621, and ca. 1700 cm-1. The γ(OH) band at 875 cm-1 also decreased (not shown for simplicity). At much higher H2O pressure, the polymerized species were observed. In Figure 6, difference spectra are shown after subtracting the spectrum measured at 15.6 Torr of H2O pressure where the dimeric species is regarded as being saturated. Reverse bands at 3661 and ca. 1730 cm-1 is considered to be due to transformation of dimeric species to polymeric water (3429 and ca. 1659 cm-1). Thus, the 3661- and ca. 1730-cm-1 bands are clearly attributed to the dimeric species. Two bands at 3366 and 3213 cm-1 observed in Figure 5 were obscure due to bands of the polymeric species. The existence of the dimeric species was initially proposed by Jentys et al.8 and recently suggested by (11) Pouchert, C. J., Ed. The Aldrich Library of FT-IR Spectra; Aldrich Chemical Co. Inc.: Milwaukee, WI. (12) Little, L. H. Infrared Spectra of Adsorbed Species; Academic Press: London, 1966.
Langmuir, Vol. 13, No. 4, 1997 749
Figure 5. Difference IR spectra H2O adsorbed under indicated pressures of gaseous H2O at 373 K. The spectrum observed under 1.0 Torr of H2O was used as a background.
Figure 6. Difference IR spectra H2O adsorbed under 18.1 and 19.2 Torr of gaseous H2O at 373 K. The spectrum observed under 15.6 Torr of H2O was used as a background.
Zecchina et al.9,10 both by IR observation. Zecchina et al. proposed that the H5O2+ species were formed by the adsorption of second H2O molecules upon the protonation of the initially hydrogen-bonded H2O9 based on the comparison of the characteristic features in the IR spectra observed for concentrated acids.10 Smith et al. also suggested that a similar band at 3650 cm-1 observed for the H2O adsorption on HSAPO-34 is due to the H5O2+ species.7 In this way, several studies were reported on the formation of the dimer species and on the sttribution of them to the H5O2+. However, clear IR spectra and bands due to the dimeric species were demonstrated for the first time in the present study by observing adsorption of H2O in the presence of gaseous H2O in the temperature range between 293 and 423 K. The frequencies and relative intensities of the observed bands enabled us to compare with the quantum-chemical results in order to confirm the structure of the dimeric species. Adsorption structure and vibrational frequencies of complex of two water molecules were recently reported by ab initio calculations by Sauer et al.4 They assumed both neutral and ion pairs
750
Langmuir, Vol. 13, No. 4, 1997
for the structure of the complex and used three different calculation methods and reported that the ion pair became more stable by the adsorption of the second molecule. The calculated frequencies and relative intensities of some IR bands due to the H5O2+ species agree with the dimeric species observed in this study. Therefore, the equilibrium of hydrogen-bonded H2O and dimeric H5O2+ was confirmed. For the polymeric form, the H5O2+‚n(H2O) structure was proposed on the basis of the observation of a broad band between 2000 and 1300 cm-1 by Zecchina et al.8 In the present study, polymeric water observed above 423 K gave spectra similar to that of H2O in liquid phase11 or on SiO2,12 and it is rather assigned to the neutral water polymer (nH2O). The ionic H5O2+‚n(H2O) species is rather likely to be formed at 293 K, if exist, showing the characteristic IR spectrum as observed in Figure 1 or 3. The observed vibrational frequencies of which on H-ZSM-5 are summarized in Table 2. Summary Adsorption of H2O on H-ZSM-5 in the presence of gaseous molecules was studied by FT-IR in the temper-
Kondo et al. Table 2. Observed IR Bands Due to Adsorbed Water on H-ZSM-5 assignment ν(OH)
δ(HOH) δ(OH) γ(OH) a
monomer
dimer
3701 ∼3550 2872a 2460a
3600 3366 3213
∼1700a
∼1700
polymer 3429
1659 1350 875
(A, B, C) triplet.
ature range between 293 and 423 K. The equilibrium between gaseous, hydrogen-bonded, dimeric, and polymeric water species was found to exist on H-ZSM-5 above 373 K. IR spectra of the dimeric species were clearly observed, and the dimeric water was assigned to the ionic H5O2+ by comparison with the reference of the recent result of the ab initio calculation. LA9607565
