2520
J. Phys. Chem. C 2007, 111, 2520-2524
Distribution of Aluminum in the Periodical Building Units of Faujasites Tama´ s I. Kora´ nyi*,†,‡ and Ja´ nos B.Nagy‡ Department of Molecular Spectroscopy, Institute of Structural Chemistry, Chemical Research Center of the Hungarian Academy of Sciences, P.O. Box 17, H-1525 Budapest, Hungary, and Laboratoire de R.M.N., Facultes UniVersitaires Notre Dame de la Paix, Rue de Bruxelles 61, B-5000 Namur, Belgium ReceiVed: October 6, 2006; In Final Form: NoVember 17, 2006
Four NH4-Y faujasites (FAU) with different Si/Al ratios were characterized by 27Al and 29Si magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. The quantitative contributions of the Al sites, defect-free Si(nAl), and silanol groups to the different double-six-ring (D6R) periodical building units (PerBUs) of the zeolite framework were calculated from the various Si/Al ratios and relative 27Al and 29Si NMR signal intensities. Four different PerBU systems were assumed in the FAU structure, but only the distribution of PerBUs containing three, two, or one Al atom pairs in body-diagonal positions of D6Rs followed the composition of the framework with appropriate precision in the studied composition range (Si/Al ) 2.619.5). Contrary to BEA, FER, and MOR zeolites, the PerBUs of FAU do not contain Al atoms in unpaired “lone” positions.
Introduction The presence and distribution of aluminum in zeolites have important significance in terms of the degree of cation exchange available and the strength of acidity in their hydrogen form. The Si, Al ordering must follow Loewenstein’s rule about the absence of Al-O-Al linkages1 and should be in accordance with Dempsey’s rule, which minimizes the number of Al, Al next-nearest Al-O-Si-O-Al neighbors.2 It is well-known that aluminum is nonuniformly distributed in different zeolites. A nonrandom distribution of Al atoms in some zeolites was revealed by ultrahigh-field 27Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR).3,4 At high silicon content, however, it has often been assumed that Si and Al are randomly distributed among the framework tetrahedral sites.5 The Si/Al ratio of zeolites and the number of crystallographically distinct sites for the five different Si(nAl) (n ) 0-4) configurations can be determined by 29Si NMR. However, the resulting Si/Al ratio may strongly underestimate the actual Si/ Al ratio as defect sites (Si(OH)x groups) are generally present in the zeolite framework. 1H-29Si cross-polarization (CP) makes possible the detection of silicon atoms to which one or more hydroxyl groups are attached. The line intensities of silicon atoms bearing OH groups are selectively and strongly enhanced, but the concentration of defect sites cannot be calculated directly from one single CP spectrum.6 The pioneering work of Lippmaa et al.7 has shown that the 29Si NMR spectrum of a NaY zeolite (Si/Al ) 2.5) exhibits five distinct signals which can be assigned to the above-mentioned five Si(nAl) (n ) 0-4) configurations. The frameworks of zeolite structures are built from periodic, structurally invariant periodic building units (PerBUs). The PerBUs are built from a limited number of tetrahedral (T) atoms by applying simple operations to these smaller units. The PerBU of faujasite (FAU) is either the sodalite cage or the double six * To whom correspondence should be addressed. Tel: +3614381100; fax: +3614381143; e-mail:
[email protected]. † Chemical Research Center of the Hungarian Academy of Sciences. ‡ Facultes Universitaires Notre Dame de la Paix.
ring (D6R) consisting of 24 or 12 equiv T atoms, respectively. The D6R has two six- and six four-membered rings.8 From one to six aluminum atoms may sit in the D6R PerBUs of FAU. The connectivity-configuration matrix method of Takaishi9,10 allows only three Al atoms in meta positions in the sixmembered rings of D6R (Al atoms only in 1,3,5 (D6R-3, Scheme 1a) or 1,3,5-2′4′6′ (D6R-6, Scheme 1b) positions are possible). In other words, Al atoms are regularly distributed in the framework with a trigonal symmetry in Takaishi’s model. Besides this D6R-6 (Scheme 1b), Melchior et al.11 assume four or two aluminum arrangements in the D6Rs with Al atoms either in body (1,3-4′6′, Scheme 1c) or in face (1,4-2′5′ (Scheme 1d) and 1,4-3′6′ (Scheme 1e)) diagonal para positions. For the D6R of chabazite, Akporiaye et al.12 suggest another D6R-3 as the most probable (1,4-2′, Scheme 1f) position: two Al atoms are in the same six-membered ring in para position (1,4 face diagonal) and the third Al atom is in diagonal with one of them (2′) in the four-membered ring (or in the other six-membered ring, see Scheme 1f). All of these existing models in the literature studied low-silica materials up to 39-11 or 7.612 Si/Al ratios. We developed a new method to evaluate the distribution of aluminum in different PerBUs of MOR and BEA zeolites.13,14,15 Besides the two (“diagonal” PerBUs) or no (“allSi” PerBUs) Al atoms siting in the four-membered rings of periodic building units of these zeolites,16 we assumed that lone (“lone” PerBUs) Al atoms may also be situated in these rings.13,14 The existence of highly symmetric hydrated framework-related octahedral aluminum species was revealed by 27Al NMR13,14 in accordance with refs 3 and 4. Because of ion exchange of protons to cobalt ions, the conversion of octahedrally into tetrahedrally coordinated aluminum was observed in the 27Al NMR spectra of CoBEA zeolites.13,14 The aim of this work is to develop our PerBU model13-15 further over FAU zeolites. We try to explore the PerBU structure of FAU in a wide range of aluminum content ranging from 3 to 20 Si/Al ratios taking into account the suggested various Al atom arrangements.9-12
10.1021/jp066578y CCC: $37.00 © 2007 American Chemical Society Published on Web 01/24/2007
Distribution of Aluminum in Faujasites
J. Phys. Chem. C, Vol. 111, No. 6, 2007 2521
SCHEME 1: Possible T12 Periodic Building Units of Faujasites: D6R-3 (a), D6R-6 (b), Body-Diagonal (c), Face-Diagonals (d, e), and Triple-Face-Diagonal (f)a
Discussion The ratio of tetrahedral silicon to aluminum in the zeolite framework (Si/AlNMR) can be directly calculated from the line intensities in a 29Si MAS NMR spectrum (ISi(nAl) where n is the number of connecting Al atoms) by the following equation assuming that the Al-O-Al avoidance rule of Loewenstein1 is obeyed and Si(OH)x signals are not included in the bands:6
Si/AlNMR )
∑ISi(nX)/∑(n/4)ISi(nX)
summation is from n ) 0 to n ) 4 (1)
a
The resulting Si/AlNMR ratio may strongly underestimate the actual Si/Al ratio as defect sites (Si(OH)x groups) are generally present in the zeolite framework. If a characteristic difference between Si/Albulk and Si/AlNMR exists (Table 1), it clearly indicates the presence of defect silanol groups in the zeolite. The difference between Si/Albulk and Si/AlNMR (Table 1) is the quantitative degree of dealumination. As only tetrahedral framework aluminum (AlT) species were identified in the 27Al NMR spectra, the bulk and framework silicon to aluminum ratios are identical (Si/Albulk ) Si/Alfram.). Calculating the concentration of Si(OH)x sites, the silanol type defects should be eliminated from the 29Si NMR line intensities. Previously, it was shown that the 29Si NMR line at -101 ppm (“line1”) includes Si(1Al) and SiOH intensities and the line at around -95 ppm (“line2”) involves Si(2Al) and Si(OH)2 signals.13-15 Substituting these defect-free line intensity values (ISi(nAl) ) Iline(n) - ISi(OH)n where n ) 2 or 1) and the framework Si/Alfram. ratios to eq 1, the following 2ISi(OH)2 + ISiOH concentrations (Table 1) can be calculated by eq 2:
Full circles designate Al atom positions.
Experimental Section Four Y zeolites were prepared by different Si/Al ratios by repeatedly performed solid-state substitution of silicon for framework aluminum.17 The chemical compositions of the prepared zeolites and their ammonium ion exchanged forms (Si/ Albulk in Table 1) were determined by atomic absorption spectroscopy (AAS). The silicon and aluminum NMR spectra were recorded on a Bruker MSL 400 and an Avance 500 spectrometer, respectively. For 29Si (79.4 MHz), a 6 µs (Θ ) π/6) pulse was used with a repetition time of 6.0 s. For 27Al (130.3 MHz), a 1 µs (Θ ) π/12) pulse was used with a repetition time of 0.1 s. The decomposition of the 29Si NMR spectra into contributions of nonequivalent sites (Si(4Al), Si(3Al), Si(2Al), Si(1Al), and Si(0Al)) was carried out with a precision of about 5%. The effect of this fitting error on the proposed PerBU and Si site contributions was also about 5%.
Si/Alfram. ) 4/[4ISi(4Al) + 3ISi(3Al) + 2(Iline2 - ISi(OH)2) + (Iline1 - ISiOH)] (if
∑ISi(nAl) ) 1)
2ISi(OH)2 + ISiOH ) 4ISi(4Al) + 3ISi(3Al) + 2Iline2 + Iline1 4/Si/Alfram. (2)
Results The normal 29Si NMR spectra of faujasites (Figure 1) show six resonances at -85, -90, -95, -100, -106, and -110 ppm, which can be ascribed to Si(4Al), Si(3Al), Si(2Al), Si(1Al), Si(0Al)A, and Si(0Al)B sites, respectively (Table 1). Highly siliceous zeolites exhibit narrower lines with distinctly different intensity distributions of the three Si(nAl) lines. Splitting of the Si(0Al) signal reflects Si atoms in crystallographically nonequivalent T-sites.6 The 1H-29Si cross-polarized (CP) NMR spectra of all zeolites confirm the presence of some Si(OH)x groups in the bands assigned to Si(2Al) and Si(1Al) configurations (compare Figure 1a, 1c, 1e, and 1g with Figure 1b, 1d, 1f, and 1h, respectively), as their intensities compared to those of Si(0Al) bands are slightly higher in the CP than in the normal spectra. The 27Al NMR spectra of the FAU zeolites are not presented because they consist of a sharp single peak only, which is attributed to tetrahedral framework aluminum (AlT) species.
To get the concentration of defect-free Si(2Al) and Si(1Al) as well as defect Si(OH)2 and SiOH sites, the sum (2ISi(OH)2 + ISiOH) derived from eq 2 should be resolved. A simple assumption was taken to solve this problem; defect silanol groups originate with equal probability from the Si(2Al) and Si(1Al) configurations during the dealumination process:
Si(OH)2/Si(2Al) ) SiOH/Si(1Al)
(3)
To support this assumption, simulating the CP spectra compared to the single pulse spectra (Figure 1) would be useful. However, the CP intensity enhancements of proton-containing signals may be completely different for structurally different Si sites, which renders the quantitative evaluation of CP spectra difficult or quasi-impossible with the spectra taken with one contact time.6
TABLE 1: Bulk and NMR Si/Al Ratios, Relative Si(nAl) (n ) 4, 3, 2, 1, 0) Coordinations (%) of FAU Zeolites Calculated from 29Si NMR Spectra 29
Si NMR
FAU zeolite
Si/Al bulk
Si/AlNMRa
Si(4Al)
Si(3Al)
Si(2Al)
Si(1Al)
Si(0Al)A
Si(0Al)B
2Si(OH)2+ SiOHb
Y3 Y5 Y11 Y20
2.62 5.45 10.83 19.53
2.58 5.23 9.74 16.19
2.3
10.6 1.5
36.2 13.9 2.5
41.7 44.0 36.0 24.7
5.9 25.6 44.5 57.1
3.4 14.9 16.9 18.2
2.1 3.1 4.1 4.2
a
Calculated by eq 1. b Calculated by eq 2.
2522 J. Phys. Chem. C, Vol. 111, No. 6, 2007
Kora´nyi and Nagy
Figure 1. Normal and CP 29Si NMR spectra of Y3 (a, b), Y5 (c, d), Y11 (e, f), and Y20 (g, h) zeolites, respectively.
TABLE 2: Compositions of Possible FAU Double-Six-Ring (D6R) PerBUs with 12 T Sites AlT
Si(4Al)
Takaishi’s triple-only model
name
D6R-6 4-D6R-3 2-D6R-3 1-D6R-3
6 3 1.5 0.75
6
Akporiaye’s triple-facediagonal model
compact triple face-diagonal lone
6 3 2 1
6
Melchior’s facediagonal model
compact DFD sesqui SFD
6 4 3 2
6
Melchior’s bodydiagonal model
compact DBD sesqui SBD allSi
6 4 3 2
6
a
PerBU
Si(3Al)
Si(2Al)
Si(1Al)
Si(0Al)
Si/Al
2 0.25
2 1.5 0.75
2 2.25 1.5
3 6.5 9
1 3 7 15
0.5
4 2 0.5
2.5 4 3
2 4 7.5
1 3 5 11
2 0.5
4 3.5 2
2 3.5 4
1.5 4
1 2 3 5
2
4 4
2 4 8
1 2 12 - x - ya
1 2 3 5 ∞
x ) Si(OH)2, y ) SiOH contributions.
Four different models discussed in the Introduction section were taken into account: the “triple-only” model (D6R-6 (Scheme 1b) and D6R-3 (Scheme 1a) PerBUs) of Takaishi,9,10
the special “triple-face-diagonal” model (Scheme 1f, Al in 1,42′ positions) of Akporiaye et al.,12 as well as the “face- and body-diagonal” Al arrangements (Scheme 1c-1e) of Melchior
Distribution of Aluminum in Faujasites
J. Phys. Chem. C, Vol. 111, No. 6, 2007 2523
SCHEME 2: T12 PerBUs in the Body-Diagonal Model: Double-Body-Diagonal (DBD) (a), Mean of Two Sesqui Models (b), and Single-Body-Diagonal (SBD) (c)a
a Full circles designate Al atom positions, Si(n) (n ) 3-0) signs Si(nAl) coordinations, symbols before and after slash (/) mark compositions of the first and second sesqui configurations, respectively.
et al.11 As all of these models were applied for low-silica materials only, we extended the four different models also to high-silica-containing faujasites (Table 2). All model building starts with the “compact” or “D6R-6” structure (1,3,5-2′4′6′, Scheme 1b) at the possible lowest Si/Al ) 1 ratio. The mean of four, two, or one “triple” (1,3,5 (Scheme 1a) or 2′4′6′) D6Rs on a sodalite cage results in the 4-D6R-3, 2-D6R-3, and 1-D6R-3 PerBUs in Takaishi’s model (Table 2). (Three triple D6Rs or 3-D6R-3 is also possible but is not necessary for the final model.) The special “triple” (1,4-2′, Scheme 1f) PerBU in Akporiaye’s model changes to “face-diagonal” (mean of 1,4 and 1′4′) and “lone” (mean of 1 and 1′) PerBUs with decreasing Al content (Table 2). The compositions of “double-facediagonal” (DFD) and “double-body-diagonal” (DBD, Scheme 2a) PerBUs in both Melchior’s models are the same (Table 2), but their Al arrangements are different (1,4-2′5′ (Scheme 1d) and 1,4-3′6′ (Scheme 1e) for DFD and 1,3-4′6′ (Scheme 1c) for DBD). Decreasing their Al contents, one and a half (mean of two and one named “sesqui”, Scheme 2b) and one (“singleface-diagonal” (SFD) and “single-body-diagonal” (SBD, Scheme 2c)) diagonal Al-pairs remain in the structures (Table 2) with different positions (1,4 and 1-4′, respectively). Further decreas-
ing the aluminum content “lone” PerBUs with only one Al atom should follow. Finally, the Al-free, only Si containing “allSi” PerBU remains (Table 2). Following calculation of the concentrations of defect-free Si(nAl) (n ) 4-0) and Si(OH)x silanol sites by eqs 1-3, the compositions presented in Table 2 were used computing the possible PerBU contributions of faujasites. The goodness of the calculations is expressed with one number; with the calculated Si/Al ratios from the different models divided by the bulk Si/ Al ratios (Table 3). To increase the precision of model calculations, the theoretical relative Si(nAl) coordinations computed from Melchior’s model18 at Si/Al ) 2.60 ratio (Th2.6) were also calculated with the different models. It is clearly seen in Table 3 that only the body-diagonal model extended to highsilica-containing faujasites describes correctly the composition of these zeolites in the whole examined Si/Al ratio range. The main reason of the exclusivity of this model is that Si(3Al) and Si(2Al) contributions are not required at relatively high Si/Al ratios (Table 2) in agreement with the data calculated from the 29Si NMR spectra (Table 1). For example, the triple-only model needs remarkable Si(3Al) and Si(2Al) contributions at Si/Al ) 7 and 15 compositions, respectively (Table 2), while these coordinations are present in small amounts only in the Y5 and Y11 zeolites (Table 1). All except the body-diagonal models require some Si(2Al) coordinations at their highest Si/Al ratios (compare the 0.75, 0.5, 2, and none values in Table 2), but its contribution is only 2.5% and none, calculated from the 29Si NMR spectra of the Y11 and Y20 samples, respectively (Table 1). The PerBU contributions in the theoretical sample (Th2.6) and the studied four FAU zeolites are presented in Table 4. Because the compact PerBU can be regarded as three Al atom pairs in body-diagonal positions (Scheme 1b), the possible PerBU compositions of faujasites consist of 3, 2, or 1 Al atom
TABLE 3: Calculated Si/Al Ratios from the Different Models Divided by the Bulk Si/Al Ratios (%) model FAU
Takaishi’s triple-only
Akporiaye’s triple-face-diagonal
Melchior’s face-diagonal
Melchior’s body-diagonal
Th2.6 Y3 Y5 Y11 Y20
111.9 102.8 120.2 114.0 94.9
127.5 120.7 80.7 96.5 86.1
107.9 112.7 90.1 75.1 68.4
100.0 100.0 100.0 100.0 100.0
TABLE 4: FAU Zeolite PerBU Contributions (%) Assuming Perfect, Theoretical PerBUs According to the Body-Diagonal Modela PerBU
Σ atoms
AlT
Si(4Al)
compactth DBDth sesquith SBDth allSith
0.8 42.5 49.6 4.6 2.4
0.4 14.2 12.4 0.8
0.4
compact3 DBD3 sesqui3 SBD3 allSi3
3.3 45.8 31.4 17.2 2.3
1.6 15.3 7.8 2.9
1.6
DBD5 sesqui5 SBD5 allSi5
7.7 26.1 38.4 27.7
2.6 6.5 6.4
sesqui11 SBD11 allSi11
6.2 41.4 52.4
1.6 6.9
SBD20 allSi20
29.2 70.8
4.9
a
Si(3Al)
Si(2Al)
Si(1Al)
7.1
14.2 16.5
7.1 16.5 3.0
15.3 10.5
7.6 10.5 11.5
2.6 8.7
1.3 8.7 25.6
2.1
7.6
1.3
Si(0Al)
Si(OH)2
SiOH
2.6 2.9 1.2
0.5
0.6
2.2 6.4 25.6
0.5
1.6
2.1 27.6
0.5 6.9 48.8
0.2
3.3
19.5
4.9 66.8
0
4.0
4.1 0.8 2.4
Th2.6 ) subscript th, Y3 ) subscript 3, Y5 ) subscript 5, Y11 ) subscript 11, and Y20 ) subscript 20.
2524 J. Phys. Chem. C, Vol. 111, No. 6, 2007 pairs (sesqui is the mean of 2 and 1 pairs (Scheme 2b)), which are situated in body-diagonal positions in the double-six-rings. The difference between Th2.6 and Y3 samples is not significant in the PerBU, AlT, and Si(nAl) compositions. The amount of DBD and sesqui PerBUs is remarkable in both high-Alcontaining zeolites. The quantity of high-Al-containing PerBUs (compact, DBD, sesqui) decreases, and in a parallel way, the concentration of other PerBUs (SBD, allSi) increases with increasing Si/Al ratios, in accordance with the expectations. As the studied faujasite samples do not contain dealuminated species (because only framework AlT species were identified in the 27Al NMR spectra), all defected sites, namely, the silanol groups, are present in the Si only containing “allSi” PerBUs (Table 4). It is quite interesting that contrary to the PerBU structure of other (BEA,13-15 FER,19 MOR13,14,19,20) zeolites, the faujasites are the first microporous materials which do not require the existence of “lone” PerBUs in their structure (besides only boron-containing [B]-BEA zeolites15). The FAU framework also differs from other structures in the respect that all PerBUs in the body-diagonal model fulfills Dempsey’s rule2 at a maximal extent. It is quite different from, for example, the PerBU structures of mordenites and beta zeolites, where Al atoms sit preferentially in pairs in the four-membered rings,13-15,20 that is, as close to each other as possible, contrary to Dempsey’s rule. Conclusions The extension of the PerBU model to faujasites proved to be successful. The distribution of tetrahedral aluminum and different defect-free Si(nAl) (n ) 4-0) species in the FAU structure was characterized by four different periodical building unit models. The application of all existing models was developed to high-silica-containing frameworks. Only one of them, the body-diagonal model, describes perfectly the distribution of tetrahedral Al, defect-free Si(nAl), and defected silanol sites in the Si/Al ) 2.6-19.5 composition range. Contrary to
Kora´nyi and Nagy the PerBU model applied to other zeolites, the body-diagonal PerBU model fulfills Dempsey’s rule at a maximal extent. It means that all Al atoms are situated in three, two, or one pairs in the double-six-ring PerBU structure as far as possible from each other in body-diagonal positions. Acknowledgment. T.I.K. is indebted to F.N.R.S. (Belgium) for the financial support. References and Notes (1) Loewenstein, W. Am. Mineral. 1954, 39, 92. (2) Dempsey, E. J. Phys. Chem. 1969, 73, 3660. (3) Kennedy, G. J.; Afeworki, M.; Hong, S. B. Microporous Mesoporous Mater. 2002, 52, 55. (4) Han, O. H.; Kim, C.-S.; Hong, S. B. Angew. Chem., Int. Ed. 2002, 41, 469. (5) Smith, J. V. In Molecular SieVe Zeolites-1; Flanigen, E. M., Sand, L. B., Eds.; American Chemical Society: Washington, DC, 1971; p 171. (6) Engelhardt, G.; Michel, D. High-Resolution Solid-State NMR of Silicates and Zeolites; Wiley: New York, 1987. (7) Lippmaa, E.; Ma¨gi, M.; Samoson, A.; Tarmak, M.; Engelhardt, G. J. Am. Chem. Soc. 1981, 103, 4992. (8) International Zeolite Association, Structure Commission. http:// www.iza-structure.org (accessed 2006). (9) Takaishi, T. J. Phys. Chem. 1995, 99, 10982. (10) Takaishi, T. Zeolites 1996, 17, 389. (11) Melchior, M. T.; Vaughan, D. E. W.; Jacobson, A. J. J. Am. Chem. Soc. 1982, 104, 4859. (12) Akporiaye, D. E.; Dahl, I. M.; Mostad, H. B.; Wendelbo, R. J. Phys. Chem. 1996, 100, 4148. (13) Kora´nyi, T. I.; Fo¨ttinger, K.; Vinek, H.; B.Nagy, J. Stud. Surf. Sci. Catal. 2005, 158, 765. (14) Kora´nyi, T. I.; B.Nagy, J. J. Phys. Chem. B 2005, 109, 15791. (15) Kora´nyi, T. I.; B.Nagy, J. J. Phys. Chem. B 2006, 110, 14728. (16) Bodart, P.; B.Nagy, J.; Debras, G.; Gabelica, Z.; Jacobs, P. A. J. Phys. Chem. 1986, 90, 5183. (17) Pa´l-Borbe´ly, G.; Beyer, H. K. Phys. Chem. Chem. Phys. 2003, 5, 5544. (18) Melchior, M. T.; Vaughan, D. E. W.; Pictroski, C. F. J. Phys. Chem. 1995, 99, 6128. (19) Kora´nyi, T. I.; B.Nagy, J. in preparation. (20) Kora´nyi, T. I.; B.Nagy, J. In Sampling Catalysis Research in the Pannonian Region, Proc. 8th Pannonian Int. Symp. Catal. (ISBN 963 06 0138 9); Pa´linko´, I., Ed.; The Hungarian Zeolite Association: Szeged, Hungary, 2006; p 150.
