11242
J. Phys. Chem. 1996, 100, 11242-11245
IR Spectroscopic Studies of Dealuminated and Realuminated Zeolite HY Jerzy Datka,*,† Bogdan Sulikowski,‡ and Barbara Gil† Faculty of Chemistry, Jagiellonian UniVersity, Ingardena 3, 30-060 Krako´ w, Poland, and Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 1, 30-239 Krako´ w, Poland ReceiVed: June 5, 1995; In Final Form: October 9, 1995X
Dealumination of zeolite NaHY with water vapor at 820 K results in the disappearance of the 3550 and 3640 cm-1 infrared bands from hydroxyl groups and the appearance of new bands at 3600 cm-1 (assigned by some authors to strongly acidic hydroxyls interacting with extraframework Al) and at 3690 cm-1 (from the AlOH groups). Sorption of benzene reveals that two kinds of Al-OH groups coexist in dealuminated zeolites: one accessible and another inaccessible to benzene molecules, corresponding to IR bands at 3681 and 3697 cm-1, respectively. NMR has shown that realumination using an aqueous solution of KOH at 350 K results in the reinsertion of most of Al into the framework and the reconstruction of the hydroxyls IR bands at 3550 and 3640 cm-1. These hydroxyl groups react with ammonia and pyridine in the same way as in the nondealuminated zeolite. Weak bands at 3600 and 3690 cm-1, characteristic of OH groups in dealuminated zeolite Y, are still present after realumination, which indicates that some extraframework Al remains in the sample. Pyridine sorption shows that in both the realuminated and the nondealuminated samples the lowfrequency hydroxyls situated inside the hexagonal prisms are inaccessible to the bulky molecule, indicating that realumination with water vapor at 820 K does not create vacancies which could facilitate access to these hydroxyls, as is the case with dealumination by H4EDTA. Benzene sorption measurements show that the acid strength of the 3640 cm-1 hydroxyls in the realuminated zeolite is higher than prior to dealumination. We attribute this to the changed relative populations of the Si(nAl) groupings in the course of the dealumination-realumination process. The contribution of the Si(1Al) sites, responsible for the creation of most of the acidic hydroxyls, increases upon realumination.
Introduction Because of their important chemical and catalytic properties, ultrastable zeolites have attracted a great deal of attention. Treatment of zeolite Y with water vapor at high temperatures results in the expulsion of some Al atoms from the framework into the interstitial space, which causes an increase in acidity and catalytic activity of the zeolite, attributed to the interaction of bridging hydroxyls with extraframework Al-OH groups which are strong Lewis acid sites.1,2 Dealumination proceeds in two stages:3,4 aluminum is first eliminated from the framework, leaving behind defect sites which are subsequently reoccupied by Si atoms migrating from elsewhere in the structure. While dealumination of zeolites was studied by many authors using a variety of methods,5 the process of realumination is much less well understood. Realumination using AlCl36 and Al2O37 is limited in extent and inconvenient. Hydrothermal treatment with aqueous solutions of alkali hydroxides is more successful, with most of the extraframework Al reinserted into the framework.8,9 This was demonstrated9 using 27Al and 29Si MAS NMR and IR spectroscopies, XRD, and sorption measurements. 29Si MAS NMR also shows that the dealuminationrealumination procedure changes the relative populations of the Si(nAl) groupings in the framework in comparison with the parent zeolite. The objective of the present study is to examine the effect of the dealumination-realumination procedure on the properties of OH groups in zeolite HY. It is particularly interesting to establish whether realumination can restore the “structural” Si-OH-Al groups present in the zeolite prior to dealumination and subsequently destroyed during the treatment with water vapor. We have studied the properties of OH groups †
Jagiellonian University. Polish Academy of Sciences. X Abstract published in AdVance ACS Abstracts, December 15, 1995. ‡
S0022-3654(95)01523-1 CCC: $12.00
using IR spectroscopy, examining the IR bands from free hydroxyls, hydroxyls reacting with pyridine and ammonia, and hydroxyls interacting (by hydrogen bonding) with benzene. The aim of the experiments with the sorption of pyridine and ammonia was to determine whether the hydroxyls in the realuminated zeolite HY are accessible to molecules of different sizes. Benzene sorption provides information on the effect of the dealumination-realumination procedure on the acid strength of the OH groups. Experimental Section The parent zeolite NaY (Si/Al ) 2.47) was ion-exchanged into the ammonium form by treatment with 10% aqueous solution of NH4Cl at 350 K for 1 h. The degree of Na+/NH4+ exchange was 78%. Zeolite NaNH4Y was subsequently ultrastabilized by treatment with water at 820 K, injected at a rate of 12 mL/h for 18 h. The partial pressure of H2O during the treatment was ca. 1 atm. XRD shows that ultrastabilization reduces the lattice parameter from 2.466 to 2.451 nm. Realumination of the ultrastable (dealuminated) zeolite with KOH was performed as follows. Two grams of dealuminated zeolite was stirred in 0.25 M KOH solution at 353 K for 25 h, after which the sample was washed and dried. As shown by NMR9 and XRD (which reveals an increase of the lattice parameter from 2.451 to 2.463 nm), realumination resulted in the reinsertion of most of Al into the framework. In order to prepare samples with a high degree of ammonium exchange, the nondealuminated and the realuminated zeolites were treated five times with a 10% aqueous solution of NH4NO3 at 350 K. The parent zeolite, nondealuminated, dealuminated, and realuminated samples will be denoted in the present paper as sample 0, sample 1, sample 2, and sample 3, respectively. Ammonia (Union Carbide), pyridine, and benzene (POCh Gliwice, analytical © 1996 American Chemical Society
Hydroxyl Groups in Zeolite HY
J. Phys. Chem., Vol. 100, No. 27, 1996 11243 TABLE 1: Relative Populations (Normalized to 100%) of all Si(nAl) Species (n ) 0-4) and Corresponding Values of Si/Alframin Nondealuminated (Sample 1) and Realuminated (Sample 3) Zeolites9 (Upper Values); Relative Populations (Normalized to 100%) of Si(nAl) Species (n ) 1-4) Creating Bridging Hydroxyls Si(4Al) Si(3Al) Si(2Al) Si(1Al) Si(0Al) Si/Alfram nondeal. (sample 1) realumin (sample 3)
Figure 1. IR spectra of OH groups in zeolites: (a) nondealuminated (sample 1), (b) dealuminated (sample 2), and (c) realuminated (sample 3). The spectra of nondealuminated (sample 1) and realuminated (sample 3) zeolites are normalized to the same integrated intensity of the 3640 cm-1 band.
Figure 2. IR spectra of OH groups in dealuminated zeolite (sample 2) interacting with benzene: (a) activated zeolite, (b) after benzene sorption, and (c) difference spectrum (b - a).
grade) were used without further purification. For IR studies, the zeolites were pressed into thin wafers (5-10 mg cm-2) and activated in situ in an IR cell in vacuum (1 × 10-3 Torr) at 720 K for 1 h. The spectra were recorded by using a Bruker 48 PC spectrometer equipped with an MCT detector. Results and Discussion Hydroxyl Groups in the Dealuminated Zeolite. The spectra of OH groups in HY zeolites (Figure 1) show the presence of two distinct bands from Si-OH-Al groups at 3550 cm-1 (low frequency) and 3640 cm-1 (high frequency) in the nondealuminated zeolite (sample 1). Ultrastabilization with water vapor at 820 K results in the disappearance of both these bands and the appearance of new bands at 3600 and 3690 cm-1. The band at 3600 cm-1 was assigned to bridging hydroxyls projecting into supercage.1 The hydroxyls responsible were found to be very acidic, which was interpreted in terms of an interaction with neighboring extraframework Al atoms. The band at 3690 cm-1 is assigned to Al-OH groups on extraframework Al atoms. The spectra of hydroxyl groups in ultrastable zeolite interacting with benzene are given in Figure 2. A comparison of the spectrum of the activated zeolite (a), the spectrum recorded after benzene sorption (b), and the difference spectrum (c) shows that the 3690 cm-1 band is a superposition of bands at 3681 and 3697 cm-1 . The former corresponds to Al-OH species accessible to benzene (note the 120 cm-1 frequency shift) and the latter to hydroxyls inaccessible to sorbed molecules. It is possible that there are two kinds of extraframework Al (as suggested in refs 10-13) and therefore two kinds of Al-OH groups: one situated more deeply inside the channels and another close to the surface of the crystallite. The stretching frequency of the inaccessible Al-OH groups (3681 cm-1) is lower than that of the accessible groups (3897 cm-1) which may be due to the (Al-OH‚‚‚O) interaction of Al-OH with the neighboring oxygen atoms. The comparison of the intensity of the minimum at 3681 cm-1 (Figure 2) and
1.4 (1.5) 3.1 (3.7)
11.6 (12.6) 9.3 (11.3)
41.8 (45.6) 32.6 (38.9)
36.9 (40.3) 38.9 (46.3)
8.4
2.47
16.0
2.77
the maximum at 3697 cm-1 leads to the conclusion that most of the extraframework Al is inaccessible to benzene and thus hidden deeply inside the channels. The poor accessibility such sites in the channels of ultrastable zeolite Y suggests that the channels are blocked by extraneous material. We note that N2 sorption at 77 K on the dealuminated sample9 shows a ca. 30% decrease of sorption capacity upon ultrastabilization. Hydroxyl Groups in the Realuminated Zeolite. Realumination of ultrastable zeolite Y with a solution of KOH results in the reinsertion of most of Al into the framework. The IR spectrum of OH groups in realuminated zeolite (sample 3) after ion exchange and decomposition of the ammonium ions is shown in Figure 1c. Both spectra are normalized to the same integrated intensity of high-frequency band for better comparison with the spectrum of the nondealuminated sample (sample 1). Realumination and the subsequent K+/H+ exchange resulted in the reappearance of both the high- and low-frequency hydroxyl bands. The stretching frequencies of these bands are practically the same as in the nondealuminated zeolite (spectrum a). The intensity ratio of the high-frequency and low-frequency OH bands is also the same, suggesting that the properties and populations of Si-OH-Al groups appearing upon dealumination and subsequent realumination and hydroxyls which were present initially in the nondealuminated zeolite (sample 1) are similar. The presence of the band at 3690 cm-1 from Al-OH groups and the increased spectral background at ca. 3600 cm-1 suggest that the realuminated zeolite still contains some extraframework Al which behaves in the same way as in the dealuminated sample. Thus, not all of the extraframework Al atoms are reinserted. This is also reflected in the higher value of Si/Alfram for the realuminated zeolite than for the parent sample (2.77 and 2.47, respectively), as calculated from the 29Si MAS NMR spectra (see Table 1). N2 sorption showed9 a 22% decrease of sorption capacity of the realuminated zeolite (sample 3) in comparison with the parent sample, again suggesting that some extraneous material survives realumination. Sorption of Pyridine and Ammonia. Sorption of pyridine and ammonia was studied in order to determine whether the hydroxyls in the realuminated zeolite are accessible to molecules of different sizes. Our earlier study has shown that, in zeolite HY dealuminated using H4EDTA at 370 K, some low-frequency hydroxyls located inside the hexagonal prisms, normally inacessible to pyridine, formed pyridinium ions.14 At low temperatures (370 K) H4EDTA removes Al from the framework without filling the vacancies.15 It is also known15,16 that a secondary mesopore system is formed during the process via the removal of the entire sodalite cages from the structure. The presence of vacancies created by the ejection of Al and of the secondary mesopore system make some hydroxyls in the hexagonal prisms accessible to bulky molecules. It is interesting to see whether similar effects can be observed in zeolite HY dealuminated with water vapor at high temperatures and subsequently realuminated using KOH. Pyridine was sorbed in both nondealuminated (sample 1) and realuminated (sample
11244 J. Phys. Chem., Vol. 100, No. 27, 1996
Datka et al.
Figure 3. Ammonia sorption in realuminated zeolite (sample 3): (a) activated zeolite; (b) after ammonia sorption.
3) zeolites at 420 K. In both cases only high-frequency hydroxyls (inside the supercages) could react with pyridine and form pyridinium ions (the IR band at 1545 cm-1). This indicates that dealumination with water vapor and the subsequent realumination do not generate vacancies which could make the low-frequency hydroxyls accessible to bulky molecules and that in the realuminated zeolite only the high-frequency hydroxyls can act as potential active sites for catalytic reactions. Sorption of ammonia at 320 K on nondealuminated (sample 1) and realuminated (sample 3) zeolites shows that, unlike pyridine, ammonia reacts with both high- and low-frequency hydroxyls to form ammonium ions (IR band at 1450 cm-1). The spectrum of the realuminated zeolite recorded upon ammonia sorption (Figure 3) shows OH bands at 3600 and 3670 cm-1, the same as in the dealuminated zeolite (sample 2), but with lower intensity. Prior to sorption, these bands are less pronounced (Figure 1, spectrum b), because they strongly overlap with low- and high-frequency bands of the bridging hydroxyls. Sorption of Benzene. The aim of benzene sorption experiments was to determine the acid strength of high-frequency OH groups in nondealuminated (sample 1) and realuminated (sample 3) zeolites and to follow the effect of dealumination and realumination on the acid strength. The spectra of highfrequency OH groups interacting by hydrogen bonding with benzene are given in Figure 4. In the case of the realuminated (sample 3) zeolite, the band shift accompanying hydrogen bonding was higher than in nondealuminated one (sample 1). This effect can be rationalized by assuming the heterogeneity of OH groups in HY zeolites. According to our previous studies, the heterogeneity of OH groups in zeolite HNaY can be related to the presence of Si with various numbers of Al in the second coordination sphere. The presence of Si(nAl) n ) 0-4, has been demonstrated by 29Si MAS NMR spectra. As Si(0Al) cannot create bridging hydroxyls, only four kinds of hydroxyls of various acid strength are present: (SiO)3Si-OHAl(OSi)3, (SiO)2(AlO)Si-OH-Al(OSi)3, (SiO)(AlO)2Si-OHAl(OSi)3, and (AlO)3Si-OH-Al(OSi)3. Our studies of zeolites HNaX and HNaY with Si/Al ) 1.06-7.02 (ref 20) have shown that the population ratio of hydroxyls with various acid strengths (as determined by IR spectroscopy) depended on the Si/Al ratio in the same way as the population ratios of the corresponding Si(nAl) species (as determined by NMR). NMR derived relative populations of Si(nAl) with n ) 0-4, and with n ) 1-4 for nondealuminated (sample 1) and realuminated (sample 3) zeolites are given in Table 1. Only Si(nAl) with n ) 1-4 can be associated with bridging hydroxyls. The contribution of Si(1Al) groups associated with the most acidic (SiO)3Si-OHAl(OSi)3 hydroxyls in realuminated zeolite (sample 3) is higher than in nondealuminated one (sample 1). We believe that this is responsible for the increased acid strength of high-frequency hydroxyls found by benzene sorption.
Figure 4. OH groups in (A) nondealuminated (sample 1) and in (B) realuminated (sample 3) zeolites interacting with benzene: (a) activated zeolite, (b) after benzene sorption, (c) difference spectrum. (C) The spectra of OH groups interacting with benzene: (a) nondealuminated zeolite (sample 1); (b) realuminated zeolite (sample 3). Spectra are normalized to the same integrated intensity.
Acknowledgment. We are especially grateful to Dr. J. Klinowski, University of Cambridge, whose pioneering work on realumination of zeolites inspired us to study the process further. We also thank Mrs. Irena Szpyt for help in IR studies. This study was sponsored by the Polish Komitet Badan Naukowych. The IR studies were supported by Grant 0634 P3 94 07 and the synthesis and characterization of samples by Grant 2 P303 149 04. References and Notes (1) Makarova, M. A.; Garforth, A. ; Zholobenko, V. L.; Dwyer, J.; Earl, G. J.; Rawlence, D. In Zeolites and Microporous Matherials: State of the Art 1994; Weitkamp, J., Karge, H. G., Pfeifer, H., Ho¨lderich, W., Eds.; Stud. Surf. Sci. Catal. 84; Elsevier: Amsterdam, 1994; p 365. (2) Sauer, J.; Schirmer, W. In InnoVation in Zeolite Material Science; Grobet, P. J., Mortier, W. J., Vansant, E. F., Schulz-Eckloff, G., Eds.; Stud. Surf. Sci. Catal. 34; Elsevier: Amsterdam, 1988; p 323. (3) Klinowski, J.; Thomas J. M.; Fyfe, C. A.; Gobbi, G. C. Nature (London) 1982, 296, 533. (4) Maxwell, I. E.; van Erp, W. A.; Hays, G. R.; Couperus, T.; Huis, R.; Clague, A. D. H. J. Chem. Soc., Chem. Commun. 1982, 523. (5) Scherzer, J. In Catalytic Materials: Relationship between Structure and ReactiVity; Whyte Jr., T. R., Dalla Betta, R. A., Derouane, E. G., Beker, R. T. K., Eds.; ACS Symp. Ser. No. 284; American Chemical Society: Washington, DC, 1984; p 157. (6) Anderson, M. W.; Klinowski, J.; Liu, X. J. Chem. Soc., Chem. Commun. 1984, 1596. (7) Chang, C. D.; Hellring, S. D.; Miale, J. N.; Schmitt, K. D.; Brigandi, P. W. J. Chem. Soc., Faraday Trans. 1985, 81, 2215. (8) Liu, X. J.; Klinowski, J.; Thomas, J. M. J. Chem. Soc., Chem. Commun. 1986, 582. (9) Hamdan, H.; Sulikowski, B.; Klinowski, J. J. Phys. Chem. 1989, 93, 350. (10) Engelhardt, G.; Lohse, U.; Samoson, A.; Ma¨gi, M.; Tarmak, M.; Lippmaa, H. Zeolites 1982, 2, 59.
Hydroxyl Groups in Zeolite HY (11) Engelhardt, G.; Lohse, U.; Patzelova, V.; Ma¨gi, M.; Lippmaa, H. Zeolites 1983, 3, 233. (12) Lohse, U.; Stach, H.; Thamm, H.; Schirmer, W.; Isirikjan, A. A.; Regent, N. I.; Dubinin, M. M. Z. Anorg. Allg. Chem. 1980, 460, 179. (13) Engelhardt, G.; Lohse, U.; Patzelova, V.; Ma¨gi, M.; Lippmaa, H. Zeolites 1983, 3, 329. (14) Bielan´ski, A.; Berak, J. M.; Czerwin´ska, E.; Datka, J.; Drelinkiewicz, A. Bull. Acad. Pol. Sci., Ser. Sci. Chim. 1975, 23, 455. (15) Datka, J.; Kołodziejski, W.; Klinowski, J.; Sulikowski, B. Catal. Lett. 1993, 19, 159. (16) Ciembroniewicz, A.; Z˙ otcin´ska-Jezierska, J.; Sulikowski, B. Pol. J. Chem. 1979, 53, 1325.
J. Phys. Chem., Vol. 100, No. 27, 1996 11245 (17) Lohse, U.; Mildebrath, M. Z. Anorg. Allg. Chem. 1981, 476, 126. (18) Datka, J.; Boczar, M.; Gil, B. Langmuir 1993, 9, 2496. (19) Datka, J.; Gil, B. J. Catal. 1994, 145, 372. (20) Gil, B.; Brocławik, E.; Datka, J.; Klinowski, J. J. Phys. Chem. 1994, 98, 930. (21) Datka, J.; Brocławik, E.; Gil, B. J. Phys. Chem. 1994, 98, 5622. (22) Klinowski, J. Progr. Nucl. Magn. Reson. Spectrosc. 1984, 16, 237. (23) Engelhardt, G.; Michel, D. In High-Resolution Solid State NMR of Silicates and Zeolites; Wiley: New York, 1987.
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