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Homogeneous OH groups in dealuminated HY zeolite studied by IR spectroscopy
Microporous and Mesoporous Materials 47 (2001) 61±66
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Homogeneous OH groups in dealuminated HY zeolite studied by IR spectroscopy Jerzy Datka *, Barbara Gil, Teresa Domagaøa, Kinga G ora-Marek Faculty of Chemistry, Jagiellonian University, 30-060 Cracow, Ingardena 3, Poland Received 9 January 2001; received in revised form 30 April 2001; accepted 7 May 2001
Abstract The analysis of IR spectra of free OH groups in strongly dealuminated zeolite HY (Si/Al 100) restoring on desorption of ammonia or pyridine provides spectroscopic evidence for the homogeneity of SiO1 HAl groups. The same conclusion is obtained by analysis of the IR spectra of OH and OD groups interacting (by hydrogen-bonding) with nitrogen at 170 K. The homogeneity of OH groups in strongly dealuminated HY is explained by the presence of only (SiO)3 SiO1 HAl(OSi)3 groups. By contrast, heterogeneous OH groups are found in non-dealuminated HY (Si/Al 2:56) containing three kinds of OH groups of various acid strength: (SiO)(AlO)2 SiO1 HAl(OSi)3 , (SiO)2 (AlO)SiO1 HAl(OSi)3 and (SiO)3 SiO1 HAl(OSi)3 . Our dealuminated sample is the only known example of a zeolite with strongly acidic homogeneous hydroxyls. It may be a model catalyst for strongly acidic homogeneous OH groups or a model system for spectroscopic studies (IR, NMR etc.). Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: IR spectroscopy; Zeolites; OH groups; Homogeneity
1. Introduction The heterogeneity of OH groups in zeolites has been studied in our laboratory for a long time [e.g. Refs. 1±5]. We started from the hypothesis that zeolitic SiOHAl groups would be homogeneous if all of them had the same bridge geometry and the same number of Al atoms close to the bridge. On the other hand, OH groups would be heterogeneous if hydroxyls of various bridge geometries or various numbers of associated Al atoms were present.
*
Corresponding author. Tel./fax: +48-12-634-0515.
Our IR studies showed that homogeneous OH groups are indeed found in zeolites NaHA and NaHX Si/Al 1:0 in which the geometry of all (AlO)3 SiO1 HAl(OSi)3 groups is the same [2,3]. On the other hand, the OH groups are heterogeneous in highly siliceous HZSM-5 in which the (SiO)3 SiOHAl(OSi)3 groups have various bridge geometries [1,3], and in HY [2,3] in which all SiO1 HAl groups have the same geometry, but hydroxyls with 1, 2 and 3 Al atoms close to the bridge exist. Heterogeneous OH groups were also found in H-mordenite Si/Al 6:5 [4] and H-Beta Si/ Al 10 [5] in which hydroxyls had 1 or 2 Al atoms close to the bridge. Known zeolites contain either homogeneous, weakly acidic hydroxyls (NaHA, NaHX) or more
1387-1811/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 1 ) 0 0 3 1 4 - 6
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J. Datka et al. / Microporous and Mesoporous Materials 47 (2001) 61±66
acidic but heterogeneous ones (HZSM-5, HY, HBeta, H-mordenite). No zeolite is known with strongly acidic homogeneous OH groups. Such a zeolite could be a model system for catalytic or spectroscopic studies. In the present study, we examined highly dealuminated zeolite HY (Si/Al 100) in the hope of ®nding strongly acidic and homogeneous hydroxyl groups. For comparison, we also studied OH groups in non-dealuminated zeolite NaHY Si/Al 2:56 with Na/H exchange degrees of 80% and 20%. The heterogeneity of OH groups was studied by comparing the half-width of the IR band of the free hydroxyls, following the position of the band restoring upon ammonia and pyridine desorption, and by comparing the half-width of the band of hydroxyls hydrogen-bonded to nitrogen at low temperature (170 K). Nitrogen is a useful probe molecule for studies of the properties of zeolitic hydroxyls [6,7].
were recorded using a Bruker MSL 500 spectrometer at a spinning rate of 4 kHz sample. 3. Results and discussion 3.1.
29
Si MAS NMR spectra
The 29 Si MAS NMR spectra of the HYS zeolite and its precursor (industrial CBV 740) are shown in Fig. 1. The spectrum of CBV 740 which was steamed and acid-leached by the producer shows only two signals: Si(0Al) and a weaker one of Si(1Al). The signals at 94 and 90 ppm of Si(2Al) and Si(3Al) which are normally present in the spectra of non-dealuminated Y zeolite [8] are absent in the spectrum of CBV 740. The framework (Si/Al)fram ratio calculated from the NMR spec-
2. Experimental Zeolite HFAU with Si/Al 20 (CBV-740 from Zeolyst, steamed and acid-leached by the producer) was again steamed at 870 K for 3 h. After cooling down to room temperature, the zeolite was heated at 720 K in a nitrogen stream (temperature increase 2 K min 1 ). The resulting sample was denoted as HYS. We also studied non-dealuminated NaNH4 Y with Si/Al 2:56 (synthesized in the Institute of Industrial Chemistry, Warsaw) with exchange degrees of 20% and 80% (denoted as NaHY/20 and NaHY/80). For the IR studies the zeolites were activated at 720 K under a vacuum of 10 4 Pa for 1 h. In some experiments the properties of OD groups were studied. Deuteration was achieved by treating the activated zeolites with D2 O vapor at 370 K followed by evacuation at 770 K. The procedure was repeated three times, and 80% of the OH groups were replaced by OD. Ammonia was adsorbed at 320 K, pyridine at 420 K and N2 at 170 K. IR spectra were recorded using a Bruker 48 IFS spectrometer. 29 Si and 27 Al MAS NMR spectra
Fig. 1. 29 Si MAS NMR spectra of the parent CBV740 zeolite (A) and of the dealuminated HYS zeolite (B).
J. Datka et al. / Microporous and Mesoporous Materials 47 (2001) 61±66
63
trum is about 30. This value is higher than (Si/Al)bulk 20 (obtained from chemical analysis) indicating the presence of some extra-framework aluminium. The spectrum of HYS, obtained from CBV 740 by steaming (Fig. 1B), showed a less intense Si(1Al) signal indicating a further extraction of Al from the framework. The 27 Al MAS NMR spectra of the CBV 740 precursor and HYS (not shown) contain very weak signals of tetrahedral and octahedral Al indicating the presence of both kinds of Al species. The intensity of the Si(1Al) signal in the spectrum of HYS (Fig. 1B) is so small that it cannot be reliably measured. Therefore, the value of (Si/ Al)fram could not be estimated from the NMR data. Nor could it be estimated from the results of chemical analysis, because of the presence of extra-framework Al. We estimated (Si/Al)fram from quantitative IR studies of the concentration of acidic Si±OH±Al groups which should be equal to the concentration of framework Al (assuming that extra-framework Al species do not neutralize AlO4 tetrahedra). 3.2. OH bands in zeolites IR spectra of the steamed HYS zeolite show a high frequency (h.f.) band from SiO1 HAl groups in the supercages at 3631 cm 1 and a low frequency (l.f.) band from SiO3 HAl groups in the small cavities at 3567 cm 1 (Fig. 2A) as well as two bands of internal and external silanol groups at 3737 and 3748 cm 1 , respectively [9] (not seen in Fig. 2A). Other OH bands which are normally present in steamed zeolites, e.g. of strongly acidic SiO1 HAl and SiO3 HAl groups interacting with extra-framework Al, Al±OH or other non-acidic groups [10,11], were very weak in the spectra of our HYS sample. Both the h.f. and l.f. hydroxyls protonated ammonia which is typical for all HY zeolites, but unlike in non-dealuminated zeolites, both the h.f. and the l.f. hydroxyls were able to protonate pyridine. A similar situation was observed in H-mordenites [12] in which the hydroxyls in the 8-ring channels which are normally not accessible to pyridine were able to protonate it. It is possible that dealumination created a secondary
Fig. 2. IR spectra of OH groups in dealuminated HYS (A, D), non-dealuminated NaHY/80 (B, E), and NaHY/20 (C, F) restoring on ammonia (A±C) and pyridine (D±F) desorption at increasing temperatures (from bottom to top). Ammonia desorption temperature range from 450 to 550 K; pyridine desorption temperature range from 610 to 830 K.
pore system which made a reaction possible between hydroxyls in the narrow channels (mordenite) and in the small cavities (HY) with the bulky pyridine molecules. Similar results were obtained earlier by Neuber et al. [13]. The spectra of NaHY/80 and NaHY/20 (Fig. 2B and C) are typical for highly and moderately exchanged Y zeolites, respectively. The spectrum of NaHY/80 (Fig. 3B) showed the band of the h.f. SiO1 HAl groups in the supercages at 3651 cm 1 and the band of the l.f. SiO3 HAl groups in the small cages. The band of SiO3 HAl groups was absent in the spectrum of NaHY/20 (Fig. 2C). Deuteration resulted in a distinct decrease of the intensity of the OH bands and in the appearance of OD bands (Fig. 3B and D).
64
J. Datka et al. / Microporous and Mesoporous Materials 47 (2001) 61±66
from the values 840 and 3460 lmol g from the composition of the zeolites.
1
calculated
3.4. Homogeneity of the OH groups in HYS
Fig. 3. IR spectra of OH (A, B) and OD (C, D) groups in HYS (A, B) and NaHY/20 (C, D) interacting at 170 K with N2 : a: activated zeolite, b: upon N2 sorption, c: dierence spectra (b minus a).
3.3. Concentration of OH in HYS, NaHY/20 and NaHY/80 The concentration of acidic hydroxyls (both h.f and l.f) in HYS was determined (in quantitative IR experiments of ammonia adsorption) in order to estimate the value of the Si/Al ratio. Excess ammonia was adsorbed at 320 K, and physisorbed ammonia was removed by evacuation at the same temperature. The spectrum of chemisorbed ammonia showed practically only the bands of ammonium ions (at 1450 cm 1 ). The band of ammonia bonded to Lewis acid sites 1622 cm 1 is very weak, indicating a low concentration of Lewis sites in our HYS sample. The concentration of hydroxyls was calculated from the intensity of the 1450 cm 1 band and its extinction coecient determined in a previous study [8]. The concentration of all hydroxyls (both h.f. and l.f.) was 140 lmol g 1 which corresponds to Si/Al 100 (assuming that extra-framework Al did not neutralize the charge of AlO4 tetrahedra). The concentration of acidic hydroxyls in NaHY/20 and NaHY/80 was 660 and 3750 lmol g 1 , respectively, which is not very dierent
The main results concern the problem of homogeneity of OH groups in dealuminated HYS. Early conclusions came from the comparison of the half-width of the bands of free hydroxyls. The fact that the band is narrower in dealuminated HYS (Fig. 2A) than in NaHY/80 and NaHY/20 (Fig. 2B and C) suggests that the OH groups in HYS are more homogeneous than in non-dealuminated zeolites. The same conclusion can be drawn from the comparison of the half-width of the OD bands (Fig. 3B and D). More important spectroscopic evidence that the OH groups in dealuminated HYS are homogeneous was obtained by following the frequency of OH band restoring upon desorption of previously adsorbed ammonia (or pyridine) at increasing temperatures (Fig. 2A±F). For heterogeneous OH groups in NaHY/80 the restoring band shifted to lower frequencies (Dm 11 cm 1 for ammonia and 14 cm 1 for pyridine ± Fig. 2B and E) because less acidic hydroxyls of higher stretching frequency release ammonia at lower temperatures than more acidic hydroxyls of lower stretching frequency. Similar but smaller frequency shifts (Dm 8 cm 1 for ammonia and 5 cm 1 for pyridine ± Fig. 2C and F) were also observed for NaHY/20 in which the concentration of OH groups is lower and some of the most acidic hydroxyls were still replaced by sodium ions. According to the data presented in Fig. 2A and D, practically no frequency shift was observed in dealuminated HYS neither for ammonia Dm 0 cm 1 nor for pyridine Dm 1 cm 1 indicating that all hydroxyls are homogeneous. Another important spectroscopic evidence for the homogeneity of the OH groups in dealuminated HYS zeolite was obtained in experiments of nitrogen sorption at 170 K. The spectra recorded before and after nitrogen sorption in HYS and NaHY/20, as well as dierence spectra, are presented in Fig. 3A and B. NaHY/80 could not be used in nitrogen adsorption experiments, because
J. Datka et al. / Microporous and Mesoporous Materials 47 (2001) 61±66
the band of the h.f. hydroxyls interacting with nitrogen overlapped with the band of the l.f. hydroxyls. In dealuminated HYS in which both h.f. and l.f. hydroxyls are present, only the h.f. ones (in the supercages) interacted via hydrogen-bonding with nitrogen at low temperature. The fact, that the l.f. hydroxyls in the small cages do not interact with nitrogen, whereas they do react with the bulkier pyridine may be due to the lower N2 adsorption temperature i.e., 170 K for N2 and 420 K for pyridine, and the less intense framework and molecule vibrations. According to the data presented in Fig. 3A and C, in dealuminated HYS, the band of the h.f. OH hydrogen-bonded to nitrogen was narrower than in non-dealuminated NaHY/20 (even though Dm was higher). Similar results were obtained when nitrogen was sorbed in deuterated zeolites (Fig. 3B and D). The band of the h.f. OD groups interacting with nitrogen in HYS was narrower than in the non-dealuminated zeolite. This is additional evidence that the hydroxyls in HYS are homogeneous. By contrast, they are heterogeneous in the non-dealuminated zeolite, and the band of OH groups interacting with nitrogen is broader due to overlapping of several sub-maxima of hydroxyls of various acid strengths and, consequently, of various Dm. As mentioned in the introduction, three kinds of hydroxyls of various acid strength (SiO)(AlO)2 SiO1 HAl(OSi)3 , (SiO)2 (AlO)SiO1 HAl(OSi)3 and (SiO)3 SiO1 HAl(OSi)3 corresponding to Si(3Al), Si(2Al) and Si(1Al) are found in the non-dealuminated zeolite. In our dealuminated zeolite (Si/ Al 100), only the most acidic (SiO)3 SiO1 HAl(OSi)3 groups are present. Therefore, the dealumination procedure applied in this study, i.e., steaming at 870 K of HY zeolite (of Si/Al 20) which was previously steamed and acid-leached, resulted in such a rearrangement of framework Si and Al that only Si(0Al) not forming bridging hydroxyls and Si(1Al) forming the most strongly acidic (SiO)3 SiO1 HAl(OSi)3 are present. This is supported by the 29 Si MAS NMR spectra (Fig. 1A and B): there were neither Si(2Al) nor Si(3Al) signals in the spectra of our dealuminated HYS and its CBV 740 precursor. Our dealuminated HYS zeolite is the only known example for a zeolite containing homoge-
65
neous strongly acidic OH groups. Other zeolites contain either homogeneous weakly acidic hydroxyls (NaHA, NaHX) or strongly acidic but heterogeneous ones (HZSM-5, H-mordenite, HBeta). The strongly dealuminated zeolite may serve as model catalyst for homogeneous strongly acidic OH groups. It may also be used for spectroscopic studies (e.g. IR, NMR etc.) of the interaction between hydroxyls and reactant or probe molecules. Acknowledgements This study was sponsored by the State Committee for Scienti®c Researches (grant no. 3 T09A 010 17). We thank Doz. B. Staudte and Dr. U. Seidel from Leipzig University who performed MAS NMR experiments. References [1] J. Datka, B. Boczar, P. Rymanowicz, Catal. 114 (1988) 368. [2] J. Datka, B. Gil, J. Catal. 145 (1994) 372. [3] J. Datka, M. Boczar, B. Gil, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 105 (1995) 1. [4] M. Guisnet, P. Ayrault, J. Datka, Polish J. Chem. 71 (1997) 1455. [5] M. Guisnet, P. Ayrault, J. Datka, J. Chem. Soc. Faraday Trans. 93 (1997) 1661. [6] F. Wakabayashi, J. Kondo, K. Domen, C. Hirose, in: H. Hattori, M. Misono, Y. Ono (Eds.), Studies in Surface Science and Catalysis, vol. 90, Elsevier, Amsterdam, 1994, p. 157. [7] A. Zecchina, C. Otero Arean, C. Lamberti, G. Turnes Palomino, G. Spoto, D. Scarano, S. Bordiga, F. Geobaldo, in: M.M.J. Treacy, B.K. Marcus, M.E. Bisher, J.B. Higgins (Eds.), Proceedings of 12th International Zeolite Conference, Baltimore 1998, MRS, 1999, p. 317. [8] J. Datka, B. Gil, J. Fraissard, P. Massiani, P. Batamack, Polish J. Chem. 73 (1999) 1535. [9] I. Kiricsi, C. Flego, G. Pazzuchini, W.O. Parker Jr., R. Millini, C. Perego, G. Bellussi, J. Phys. Chem. 89 (1994) 4627. [10] S. Khabtou, T. Chevreau, J.C. Lavalley, Micropor. Mater. 3 (1994) 133. [11] M.A. Makarova, A. Garforth, V.I. Zholobenko, J. Dwyer, G.J. Earl, D. Rowlence, in: J. Weitkamp, H.G. Karge, H. Pfeifer, W. H olderich (Eds.), Zeolites and Related
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Microporous Materials: State of the Art. 1994, Studies in Surface Science and Catalysis, vol. 84, Elsevier, Amsterdam, 1994, p. 356. [12] M. Guisnet, P. Ayrault, J. Datka, Polish J. Chem. 71 (1997) 1445.
[13] N. Neuber, V. Dondur, H.G. Karge, L. Pacheco, S. Ernst, J. Weitkamp, in: P.J. Grobet, W.J. Mortier, E.F. Vasant, G. Schulz-Eklo, (Eds.), Innovation in Zeolite Materials Science, Studies in Surface Science and Catalysis, vol. 37, Elsevier, Amsterdam, 1988, p. 461.
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Homogeneous OH groups in dealuminated HY zeolite studied by IR spectroscopy Jerzy Datka *, Barbara Gil, Teresa Domagaøa, Kinga G ora-Marek Faculty of Chemistry, Jagiellonian University, 30-060 Cracow, Ingardena 3, Poland Received 9 January 2001; received in revised form 30 April 2001; accepted 7 May 2001
Abstract The analysis of IR spectra of free OH groups in strongly dealuminated zeolite HY (Si/Al 100) restoring on desorption of ammonia or pyridine provides spectroscopic evidence for the homogeneity of SiO1 HAl groups. The same conclusion is obtained by analysis of the IR spectra of OH and OD groups interacting (by hydrogen-bonding) with nitrogen at 170 K. The homogeneity of OH groups in strongly dealuminated HY is explained by the presence of only (SiO)3 SiO1 HAl(OSi)3 groups. By contrast, heterogeneous OH groups are found in non-dealuminated HY (Si/Al 2:56) containing three kinds of OH groups of various acid strength: (SiO)(AlO)2 SiO1 HAl(OSi)3 , (SiO)2 (AlO)SiO1 HAl(OSi)3 and (SiO)3 SiO1 HAl(OSi)3 . Our dealuminated sample is the only known example of a zeolite with strongly acidic homogeneous hydroxyls. It may be a model catalyst for strongly acidic homogeneous OH groups or a model system for spectroscopic studies (IR, NMR etc.). Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: IR spectroscopy; Zeolites; OH groups; Homogeneity
1. Introduction The heterogeneity of OH groups in zeolites has been studied in our laboratory for a long time [e.g. Refs. 1±5]. We started from the hypothesis that zeolitic SiOHAl groups would be homogeneous if all of them had the same bridge geometry and the same number of Al atoms close to the bridge. On the other hand, OH groups would be heterogeneous if hydroxyls of various bridge geometries or various numbers of associated Al atoms were present.
*
Corresponding author. Tel./fax: +48-12-634-0515.
Our IR studies showed that homogeneous OH groups are indeed found in zeolites NaHA and NaHX Si/Al 1:0 in which the geometry of all (AlO)3 SiO1 HAl(OSi)3 groups is the same [2,3]. On the other hand, the OH groups are heterogeneous in highly siliceous HZSM-5 in which the (SiO)3 SiOHAl(OSi)3 groups have various bridge geometries [1,3], and in HY [2,3] in which all SiO1 HAl groups have the same geometry, but hydroxyls with 1, 2 and 3 Al atoms close to the bridge exist. Heterogeneous OH groups were also found in H-mordenite Si/Al 6:5 [4] and H-Beta Si/ Al 10 [5] in which hydroxyls had 1 or 2 Al atoms close to the bridge. Known zeolites contain either homogeneous, weakly acidic hydroxyls (NaHA, NaHX) or more
1387-1811/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 1 ) 0 0 3 1 4 - 6
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J. Datka et al. / Microporous and Mesoporous Materials 47 (2001) 61±66
acidic but heterogeneous ones (HZSM-5, HY, HBeta, H-mordenite). No zeolite is known with strongly acidic homogeneous OH groups. Such a zeolite could be a model system for catalytic or spectroscopic studies. In the present study, we examined highly dealuminated zeolite HY (Si/Al 100) in the hope of ®nding strongly acidic and homogeneous hydroxyl groups. For comparison, we also studied OH groups in non-dealuminated zeolite NaHY Si/Al 2:56 with Na/H exchange degrees of 80% and 20%. The heterogeneity of OH groups was studied by comparing the half-width of the IR band of the free hydroxyls, following the position of the band restoring upon ammonia and pyridine desorption, and by comparing the half-width of the band of hydroxyls hydrogen-bonded to nitrogen at low temperature (170 K). Nitrogen is a useful probe molecule for studies of the properties of zeolitic hydroxyls [6,7].
were recorded using a Bruker MSL 500 spectrometer at a spinning rate of 4 kHz sample. 3. Results and discussion 3.1.
29
Si MAS NMR spectra
The 29 Si MAS NMR spectra of the HYS zeolite and its precursor (industrial CBV 740) are shown in Fig. 1. The spectrum of CBV 740 which was steamed and acid-leached by the producer shows only two signals: Si(0Al) and a weaker one of Si(1Al). The signals at 94 and 90 ppm of Si(2Al) and Si(3Al) which are normally present in the spectra of non-dealuminated Y zeolite [8] are absent in the spectrum of CBV 740. The framework (Si/Al)fram ratio calculated from the NMR spec-
2. Experimental Zeolite HFAU with Si/Al 20 (CBV-740 from Zeolyst, steamed and acid-leached by the producer) was again steamed at 870 K for 3 h. After cooling down to room temperature, the zeolite was heated at 720 K in a nitrogen stream (temperature increase 2 K min 1 ). The resulting sample was denoted as HYS. We also studied non-dealuminated NaNH4 Y with Si/Al 2:56 (synthesized in the Institute of Industrial Chemistry, Warsaw) with exchange degrees of 20% and 80% (denoted as NaHY/20 and NaHY/80). For the IR studies the zeolites were activated at 720 K under a vacuum of 10 4 Pa for 1 h. In some experiments the properties of OD groups were studied. Deuteration was achieved by treating the activated zeolites with D2 O vapor at 370 K followed by evacuation at 770 K. The procedure was repeated three times, and 80% of the OH groups were replaced by OD. Ammonia was adsorbed at 320 K, pyridine at 420 K and N2 at 170 K. IR spectra were recorded using a Bruker 48 IFS spectrometer. 29 Si and 27 Al MAS NMR spectra
Fig. 1. 29 Si MAS NMR spectra of the parent CBV740 zeolite (A) and of the dealuminated HYS zeolite (B).
J. Datka et al. / Microporous and Mesoporous Materials 47 (2001) 61±66
63
trum is about 30. This value is higher than (Si/Al)bulk 20 (obtained from chemical analysis) indicating the presence of some extra-framework aluminium. The spectrum of HYS, obtained from CBV 740 by steaming (Fig. 1B), showed a less intense Si(1Al) signal indicating a further extraction of Al from the framework. The 27 Al MAS NMR spectra of the CBV 740 precursor and HYS (not shown) contain very weak signals of tetrahedral and octahedral Al indicating the presence of both kinds of Al species. The intensity of the Si(1Al) signal in the spectrum of HYS (Fig. 1B) is so small that it cannot be reliably measured. Therefore, the value of (Si/ Al)fram could not be estimated from the NMR data. Nor could it be estimated from the results of chemical analysis, because of the presence of extra-framework Al. We estimated (Si/Al)fram from quantitative IR studies of the concentration of acidic Si±OH±Al groups which should be equal to the concentration of framework Al (assuming that extra-framework Al species do not neutralize AlO4 tetrahedra). 3.2. OH bands in zeolites IR spectra of the steamed HYS zeolite show a high frequency (h.f.) band from SiO1 HAl groups in the supercages at 3631 cm 1 and a low frequency (l.f.) band from SiO3 HAl groups in the small cavities at 3567 cm 1 (Fig. 2A) as well as two bands of internal and external silanol groups at 3737 and 3748 cm 1 , respectively [9] (not seen in Fig. 2A). Other OH bands which are normally present in steamed zeolites, e.g. of strongly acidic SiO1 HAl and SiO3 HAl groups interacting with extra-framework Al, Al±OH or other non-acidic groups [10,11], were very weak in the spectra of our HYS sample. Both the h.f. and l.f. hydroxyls protonated ammonia which is typical for all HY zeolites, but unlike in non-dealuminated zeolites, both the h.f. and the l.f. hydroxyls were able to protonate pyridine. A similar situation was observed in H-mordenites [12] in which the hydroxyls in the 8-ring channels which are normally not accessible to pyridine were able to protonate it. It is possible that dealumination created a secondary
Fig. 2. IR spectra of OH groups in dealuminated HYS (A, D), non-dealuminated NaHY/80 (B, E), and NaHY/20 (C, F) restoring on ammonia (A±C) and pyridine (D±F) desorption at increasing temperatures (from bottom to top). Ammonia desorption temperature range from 450 to 550 K; pyridine desorption temperature range from 610 to 830 K.
pore system which made a reaction possible between hydroxyls in the narrow channels (mordenite) and in the small cavities (HY) with the bulky pyridine molecules. Similar results were obtained earlier by Neuber et al. [13]. The spectra of NaHY/80 and NaHY/20 (Fig. 2B and C) are typical for highly and moderately exchanged Y zeolites, respectively. The spectrum of NaHY/80 (Fig. 3B) showed the band of the h.f. SiO1 HAl groups in the supercages at 3651 cm 1 and the band of the l.f. SiO3 HAl groups in the small cages. The band of SiO3 HAl groups was absent in the spectrum of NaHY/20 (Fig. 2C). Deuteration resulted in a distinct decrease of the intensity of the OH bands and in the appearance of OD bands (Fig. 3B and D).
64
J. Datka et al. / Microporous and Mesoporous Materials 47 (2001) 61±66
from the values 840 and 3460 lmol g from the composition of the zeolites.
1
calculated
3.4. Homogeneity of the OH groups in HYS
Fig. 3. IR spectra of OH (A, B) and OD (C, D) groups in HYS (A, B) and NaHY/20 (C, D) interacting at 170 K with N2 : a: activated zeolite, b: upon N2 sorption, c: dierence spectra (b minus a).
3.3. Concentration of OH in HYS, NaHY/20 and NaHY/80 The concentration of acidic hydroxyls (both h.f and l.f) in HYS was determined (in quantitative IR experiments of ammonia adsorption) in order to estimate the value of the Si/Al ratio. Excess ammonia was adsorbed at 320 K, and physisorbed ammonia was removed by evacuation at the same temperature. The spectrum of chemisorbed ammonia showed practically only the bands of ammonium ions (at 1450 cm 1 ). The band of ammonia bonded to Lewis acid sites 1622 cm 1 is very weak, indicating a low concentration of Lewis sites in our HYS sample. The concentration of hydroxyls was calculated from the intensity of the 1450 cm 1 band and its extinction coecient determined in a previous study [8]. The concentration of all hydroxyls (both h.f. and l.f.) was 140 lmol g 1 which corresponds to Si/Al 100 (assuming that extra-framework Al did not neutralize the charge of AlO4 tetrahedra). The concentration of acidic hydroxyls in NaHY/20 and NaHY/80 was 660 and 3750 lmol g 1 , respectively, which is not very dierent
The main results concern the problem of homogeneity of OH groups in dealuminated HYS. Early conclusions came from the comparison of the half-width of the bands of free hydroxyls. The fact that the band is narrower in dealuminated HYS (Fig. 2A) than in NaHY/80 and NaHY/20 (Fig. 2B and C) suggests that the OH groups in HYS are more homogeneous than in non-dealuminated zeolites. The same conclusion can be drawn from the comparison of the half-width of the OD bands (Fig. 3B and D). More important spectroscopic evidence that the OH groups in dealuminated HYS are homogeneous was obtained by following the frequency of OH band restoring upon desorption of previously adsorbed ammonia (or pyridine) at increasing temperatures (Fig. 2A±F). For heterogeneous OH groups in NaHY/80 the restoring band shifted to lower frequencies (Dm 11 cm 1 for ammonia and 14 cm 1 for pyridine ± Fig. 2B and E) because less acidic hydroxyls of higher stretching frequency release ammonia at lower temperatures than more acidic hydroxyls of lower stretching frequency. Similar but smaller frequency shifts (Dm 8 cm 1 for ammonia and 5 cm 1 for pyridine ± Fig. 2C and F) were also observed for NaHY/20 in which the concentration of OH groups is lower and some of the most acidic hydroxyls were still replaced by sodium ions. According to the data presented in Fig. 2A and D, practically no frequency shift was observed in dealuminated HYS neither for ammonia Dm 0 cm 1 nor for pyridine Dm 1 cm 1 indicating that all hydroxyls are homogeneous. Another important spectroscopic evidence for the homogeneity of the OH groups in dealuminated HYS zeolite was obtained in experiments of nitrogen sorption at 170 K. The spectra recorded before and after nitrogen sorption in HYS and NaHY/20, as well as dierence spectra, are presented in Fig. 3A and B. NaHY/80 could not be used in nitrogen adsorption experiments, because
J. Datka et al. / Microporous and Mesoporous Materials 47 (2001) 61±66
the band of the h.f. hydroxyls interacting with nitrogen overlapped with the band of the l.f. hydroxyls. In dealuminated HYS in which both h.f. and l.f. hydroxyls are present, only the h.f. ones (in the supercages) interacted via hydrogen-bonding with nitrogen at low temperature. The fact, that the l.f. hydroxyls in the small cages do not interact with nitrogen, whereas they do react with the bulkier pyridine may be due to the lower N2 adsorption temperature i.e., 170 K for N2 and 420 K for pyridine, and the less intense framework and molecule vibrations. According to the data presented in Fig. 3A and C, in dealuminated HYS, the band of the h.f. OH hydrogen-bonded to nitrogen was narrower than in non-dealuminated NaHY/20 (even though Dm was higher). Similar results were obtained when nitrogen was sorbed in deuterated zeolites (Fig. 3B and D). The band of the h.f. OD groups interacting with nitrogen in HYS was narrower than in the non-dealuminated zeolite. This is additional evidence that the hydroxyls in HYS are homogeneous. By contrast, they are heterogeneous in the non-dealuminated zeolite, and the band of OH groups interacting with nitrogen is broader due to overlapping of several sub-maxima of hydroxyls of various acid strengths and, consequently, of various Dm. As mentioned in the introduction, three kinds of hydroxyls of various acid strength (SiO)(AlO)2 SiO1 HAl(OSi)3 , (SiO)2 (AlO)SiO1 HAl(OSi)3 and (SiO)3 SiO1 HAl(OSi)3 corresponding to Si(3Al), Si(2Al) and Si(1Al) are found in the non-dealuminated zeolite. In our dealuminated zeolite (Si/ Al 100), only the most acidic (SiO)3 SiO1 HAl(OSi)3 groups are present. Therefore, the dealumination procedure applied in this study, i.e., steaming at 870 K of HY zeolite (of Si/Al 20) which was previously steamed and acid-leached, resulted in such a rearrangement of framework Si and Al that only Si(0Al) not forming bridging hydroxyls and Si(1Al) forming the most strongly acidic (SiO)3 SiO1 HAl(OSi)3 are present. This is supported by the 29 Si MAS NMR spectra (Fig. 1A and B): there were neither Si(2Al) nor Si(3Al) signals in the spectra of our dealuminated HYS and its CBV 740 precursor. Our dealuminated HYS zeolite is the only known example for a zeolite containing homoge-
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neous strongly acidic OH groups. Other zeolites contain either homogeneous weakly acidic hydroxyls (NaHA, NaHX) or strongly acidic but heterogeneous ones (HZSM-5, H-mordenite, HBeta). The strongly dealuminated zeolite may serve as model catalyst for homogeneous strongly acidic OH groups. It may also be used for spectroscopic studies (e.g. IR, NMR etc.) of the interaction between hydroxyls and reactant or probe molecules. Acknowledgements This study was sponsored by the State Committee for Scienti®c Researches (grant no. 3 T09A 010 17). We thank Doz. B. Staudte and Dr. U. Seidel from Leipzig University who performed MAS NMR experiments. References [1] J. Datka, B. Boczar, P. Rymanowicz, Catal. 114 (1988) 368. [2] J. Datka, B. Gil, J. Catal. 145 (1994) 372. [3] J. Datka, M. Boczar, B. Gil, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 105 (1995) 1. [4] M. Guisnet, P. Ayrault, J. Datka, Polish J. Chem. 71 (1997) 1455. [5] M. Guisnet, P. Ayrault, J. Datka, J. Chem. Soc. Faraday Trans. 93 (1997) 1661. [6] F. Wakabayashi, J. Kondo, K. Domen, C. Hirose, in: H. Hattori, M. Misono, Y. Ono (Eds.), Studies in Surface Science and Catalysis, vol. 90, Elsevier, Amsterdam, 1994, p. 157. [7] A. Zecchina, C. Otero Arean, C. Lamberti, G. Turnes Palomino, G. Spoto, D. Scarano, S. Bordiga, F. Geobaldo, in: M.M.J. Treacy, B.K. Marcus, M.E. Bisher, J.B. Higgins (Eds.), Proceedings of 12th International Zeolite Conference, Baltimore 1998, MRS, 1999, p. 317. [8] J. Datka, B. Gil, J. Fraissard, P. Massiani, P. Batamack, Polish J. Chem. 73 (1999) 1535. [9] I. Kiricsi, C. Flego, G. Pazzuchini, W.O. Parker Jr., R. Millini, C. Perego, G. Bellussi, J. Phys. Chem. 89 (1994) 4627. [10] S. Khabtou, T. Chevreau, J.C. Lavalley, Micropor. Mater. 3 (1994) 133. [11] M.A. Makarova, A. Garforth, V.I. Zholobenko, J. Dwyer, G.J. Earl, D. Rowlence, in: J. Weitkamp, H.G. Karge, H. Pfeifer, W. H olderich (Eds.), Zeolites and Related
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Microporous Materials: State of the Art. 1994, Studies in Surface Science and Catalysis, vol. 84, Elsevier, Amsterdam, 1994, p. 356. [12] M. Guisnet, P. Ayrault, J. Datka, Polish J. Chem. 71 (1997) 1445.
[13] N. Neuber, V. Dondur, H.G. Karge, L. Pacheco, S. Ernst, J. Weitkamp, in: P.J. Grobet, W.J. Mortier, E.F. Vasant, G. Schulz-Eklo, (Eds.), Innovation in Zeolite Materials Science, Studies in Surface Science and Catalysis, vol. 37, Elsevier, Amsterdam, 1988, p. 461.