14728
J. Phys. Chem. B 2006, 110, 14728-14735
Distribution of Aluminum and Boron in the Periodical Building Units of Boron-Containing β Zeolites 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: April 14, 2006; In Final Form: May 27, 2006
Various boron only ([B]-BEA) as well as aluminum- and boron-containing β zeolites ([Al,B]-BEA) have been prepared and modified by ion exchange of ammonium, sodium, and nickel ions. The zeolite samples have been characterized by 11B, 27Al, and 29Si MAS as well as three of them by 11B and 27Al 3Q-MAS NMR spectroscopy. The quantitative contributions of defect-free Si(nX) (n ) 2, 1, 0; X ) Al, B) and Si(OH)x (x ) 2, 1) sites to the NMR signal intensities were calculated from the various Si/(Al + B) ratios and relative 11B, 27Al, and 29Si NMR signal intensities using the special distribution of aluminum and boron in different periodical building units of the zeolite framework. The boron atoms are sitting exclusively in diagonal positions in the four-membered rings of [B]-BEA zeolites, while the aluminum atoms are situated both in diagonal and lone positions in the four-membered rings of [Al,B]-BEA zeolites. A higher part of boron atoms are positioned in framework-related deformed tetrahedral boron species than in lattice positions in the [B]-BEA than in the [Al,B]-BEA zeolites. All extraframework octahedral aluminum species are transformed back to lattice positions due to ion exchange from the protonated form to ammonium-, sodium-, or nickel-ions containing zeolites. Oppositely, trigonal boron leaves the zeolite structure completely during ion exchange.
Introduction New zeolitic materials showing specific catalytic properties can be gained by a partial isomorphous substitution of silicon by other tetrahedrally (T) coordinated heteroatoms such as boron and aluminum. Borosilicates of MFI structure having tunable acidic strength are new shape-selective industrial catalysts in oxidation and hydroxylation reactions.1,2 Boron-containing β ([B]-BEA) zeolites can be used as the precursor of other borosilicates ([B]-SSZ-31, [B]-SSZ-42, etc.).3,4 Boron can be extracted under extreme mild conditions, leaving thermally stable silanol nests, which can be reoccupied by other elements (e.g., by Al).5 The synthesis and possible usage of a number of new borosilicates is reviewed.6 The placement of aluminum (and boron) inside the zeolite framework has always been an intriguing question. The regular framework coordination of aluminum (and boron) in the zeolite framework is tetrahedral. The silicon-aluminum ordering in zeolites and the state of aluminum (and boron) incorporated either in the framework or out-of-lattice (extraframework) positions can be obtained by 29Si, 27Al, and 11B NMR spectroscopy. 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. But 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 * Corresponding author. Tel: +3614381100. Fax: +3614381143. Email:
[email protected]. † Chemical Research Center of the Hungarian Academy of Sciences. ‡ Facultes Universitaires Notre Dame de la Paix.
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.7 27Al NMR characterizes aluminum species of different (tetraor octahedral) coordination. The line (at ca. 55 ppm) of tetrahedral aluminum in the zeolitic lattice is well separated from the line (at 0 ppm) of out-of-lattice octahedral aluminum, following appropriate calibration their amounts can be determined quantitatively. But the amount of extraframework aluminum is often underestimated by this method, as not all aluminum is “NMR visible”. Broad signals of Al sites are subject to different quadrupolar interactions in the spectrum; their separation can be achieved by the two-dimensional triple quantum 2D 3Q-MAS NMR technique,8 or by ultra-high-field 27Al MAS NMR.9,10 11B NMR is able to distinguish the narrow lines of framework tetrahedral (B(OSi)4 at -3.5 ppm11) and framework-related deformed tetrahedral (HO-B(OSi)3- at -2 and -0.5 ppm11,12) and the broad line of out-of-framework trigonal (BO3 at 5-20 ppm1) boron. It is possible to distinguish tetrahedrally coordinated boron species located in well-ordered zeolite-like frameworks and in separate, randomly distributed, nonordered amorphous phases as well as trigonal boron. Corrections are necessary in the spectrum for BO3 line intensities for quantitative determination of trigonal boron due to quadrupolar interactions.1,13 Zeolite β has a three-dimensional channel system with 12membered ring apertures.14 The periodic building unit (PerBU) in tetragonal BEA equals the β layer (Scheme 1) composed of T16-units related by pure translations along the plane of paper.15 BEA is disordered in the plane normal direction; therefore, no ordered material has yet been produced, but its extreme disorder
10.1021/jp0623185 CCC: $33.50 © 2006 American Chemical Society Published on Web 07/12/2006
Distribution of Al and B in BEA Zeolites SCHEME 1: β Zeolite Periodical Building Units (PerBUs) in Thickened Lines with Tetrahedral T1-T9 Positionsa
J. Phys. Chem. B, Vol. 110, No. 30, 2006 14729 TABLE 1: Chemical Composition (mmol/g) of the Gloved β Zeolite Samples sample
a Possible X (Al, B) atom siting in “diagonal” PerBUs: X X , 1- 4 X2-X3, X3-X6, X4-X5, or X5-X6. In “lone” PerBUs: X1, X2, X3, X4, X5, or X6.
is not important for catalytic applications. The PerBU contains nine T sites; T1-T2 and T3-T6 are positioned also in one and two four-membered rings, respectively, and T7-T9 are situated in fused 5- and 6-rings only (Scheme 1).16 Jia et al.17 claimed that the appearance of octahedral Al in the NMR spectra of β zeolites does not necessarily involve framework aluminum extraction. Kiricsi et al.18 assigned octahedral Al NMR resonance to transient-state aluminum species which are leaving the framework. Bokhoven et al.19 identified three-coordinate aluminum in MOR and BEA zeolites with in situ X-ray absorption near-edge spectroscopy (XANES). Abraham et al.20 observed octahedrally coordinated frameworkassociated aluminum atoms in BEA zeolites by triple-quantum 27Al MAS NMR. A nonrandom distribution of Al atoms in other zeolites was revealed by ultra-high-field 27Al MAS NMR.9,10 Scholle and Veeman21 reported that in boron-containing H-MFI zeolite (H-[B]-MFI) reversibly changes its coordination state to trigonal, as evidenced by the disappearance of the typical tetrahedral framework 11B NMR line at -3.5 ppm and by the appearance of a component the line-shape of which is dominated by second-order quadrupole interaction. Hwang et al.22 claimed that due to dehydration of H-[B]-BEA zeolites, besides SiOH....B(OSi) species, SiOH....B(OSi) (OH) and distorted B(OH) 3 2 3 species are also present, as revealed by 11B NMR. Koller et al.23 found two new lines in the 11B NMR spectra of H-[B]BEA zeolites upon dehydration, which were attributed to trigonal SiOH....B(OSi)3 and SiOH....B(OSi)(OH)2 species, but the abovementioned intermediate SiOH....B(OSi)2(OH) species was missing. In our preceding work24 we claimed that only H-[B]-BEA zeolites are sensitive to hydrolysis, and with Na+, NH4+, and Ni2+ cations, the tetrahedral state of boron is stabilized in these zeolites. Besides the above-mentioned species, we suggested the existence of a new terminal trigonal B(OSi)3 species at the end of SiOSiO chains in the vicinity of silanol nests in H-[B]BEA zeolites. We developed a new method to evaluate the distribution of aluminum in different PerBUs of MOR and BEA zeolites.25,26 Besides the two or no Al atoms siting in the four-membered rings of periodic building units of these zeolites,27 we assumed that lone Al atoms may also be situated in these rings.25,26 The existence of highly symmetric hydrated framework-related octahedral aluminum species was revealed by 27Al NMR25,26 in accordance with refs 9, 10, and 17-20. Due to ion exchange of protons to cobalt ions, the conversion of octahedrally into tetrahedrally coordinated aluminum was observed in the 27Al NMR spectra of Co-BEA zeolites.25,26 The aim of this work is to combine our PerBU method25,26 with our previous results on [B]-BEA zeolites24 in order to understand the distribution of B and Al sites over the crystal-
composition (mmol/g) NH4a Na Ni B Al
Si
[B]-BEA 0.01 1.07 0.06 15.97 NH4-[B] 0.89 0.77 0.06 16.15 Na-[B] 0.96 0.70 0.05 15.70 Ni-[B] 0.03 0.47 0.68 0.05 15.60 [Al,B]-BEAI 0.40 0.71 0.89 15.27 NH4-[Al,B]I 1.21 0.02 0.33 0.88 15.70 Na-[Al,B]I 1.18 0.28 0.89 15.11 Ni-[Al,B]I 0.42 0.54 0.33 0.87 14.83 [Al,B]-BEAII 0.72 1.12 0.66 15.07 NH4-[Al,B]II 0.99 0.23 0.61 0.71 15.57 Na-[Al,B]II 1.11 0.48 0.70 15.20 Ni-[Al,B]II 0.20 0.30 0.51 0.70 15.28
Si/Al Si/(Al + B) bulk bulk 290 294 291 289 17.1 17.8 16.9 17.0 23.0 21.9 21.8 21.9
14.2 19.6 20.8 21.3 9.5 13.0 12.9 12.4 8.5 11.8 12.9 12.7
a Measured by titration of ammonia evolved during NH -TPD, not 3 counted into the total composition.
lographic tetrahedral sites and the realumination process of BEA zeolites in a deeper level. A new method will be presented for calculation of the distribution of boron and aluminum atoms in the different periodical building units of [B]-BEA and [B,Al]BEA zeolites, taking into account a deeper insight of possible boron and aluminum species with the help of 2D 3Q-MAS NMR spectroscopy. Assuming different formation mechanisms, we also try to reveal the origin of defect silanol groups. Experimental Section The preparation of [B]-BEA and [Al,B]-BEA zeolites is described in details in Ref.24 Ammonium and sodium forms of the prepared BEA zeolites were obtained by repeated ion exchange of the protonic form in aqueous chloride solutions, and the nickel forms of the Na-BEA and NH4-BEA zeolites were gained by ion exchange in a nickel acetate solution.24 The chemical compositions of the prepared zeolites, and their cationic ion exchanged forms were determined by atomic absorption spectroscopy (AAS) (boron by inductively coupled plasma (ICP)) and were partially verified by proton-induced gamma-ray emission (PIGE28). The chemical compositions and framework (bulk) Si/Al as well as Si/(Al + B) ratios are shown in Table 1. The difference between [Al,B]-BEA series I and II is the lower Si/Al ratio, the lower B content, and the somewhat higher Al content in the former than in the latter series (Table 1). The “X-ray crystallinity” (standard-related integrated XRD peak intensity) of [B]-BEA was close to 100%, that of [Al,B]BEA-I was 95%, and that of [Al,B]-BEA-II was about 70%. The thermogravimetric (TG) curves of the prepared zeolites showed about 3-5% water content with the exception of [Al,B]BEA-II (20%). More details are given elsewhere.24 The 1D NMR spectra were recorded either on a Bruker MSL 400, or Avance 500 spectrometer. 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) and 11B (160.4 MHz), a 1 µs (Θ ) π/12) pulse was used with a repetition time of 0.1 s, respectively. The 2D 3Q-MAS NMR experiments were carried out on a Bruker Avance 400WB spectrometer operating at 128.5 MHz for boron and 104.4 MHz for aluminum using a 4-mm HX MAS probe at a rotation speed of 10 kHz. Chemical shifts were referenced relative to TMS (Si), Al(NO3)3 (Al), and BF3‚Et2O (B). The decomposition of the 29Si NMR spectra into contributions of nonequivalent sites (Si(2X), Si(1X), and Si(0X), X ) Al, B) 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%.
14730 J. Phys. Chem. B, Vol. 110, No. 30, 2006
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TABLE 2: Framework and NMR Si/(Al + B) Ratios, Relative Tetrahedral AlT, BT and Si(nX) (n ) 2, 1, 0; X ) Al, B) Coordinations (%) of [B]-BEA and [Al,B]-BEA Zeolites Calculated from 27Al, 11B and 29Si NMR Spectra 27
zeolite
Al AlT
[B]-BEA NH4-[B] Na-[B] Ni-[B] [Al,B]-BEAI NH4-[Al,B]I Na-[Al,B]I Ni-[Al,B]I [Al,B]-BEAII NH4-[Al,B]II Na-[Al,B]II Ni-[Al,B]II
72.8 100 100 100 87.1 100 100 100 92.1 100 100 100
a
B BT
Si/(Al + B) frama
48.6 100 100 100 42.6 100 100 100 47.5 100 100 100
28.51 19.58 20.77 21.26 14.12 12.97 12.87 12.42 13.28 11.79 12.90 12.66
11
29Si
Si/(Al +
B)NMRb
11.85 18.47 19.84 20.17 10.79 12.69 12.45 11.75 10.81 11.66 11.08 11.01
Si(2X)
NMR Si(1X)
Si(0X)A
Si(0X)B
2Si(OH)2 + SiOHc
8.4 6.3 5.3 5.0 8.9 6.4 6.6 7.8 8.9 5.0 5.5 5.9
16.9 9.1 9.6 9.9 19.3 18.7 19.0 18.5 19.1 24.3 25.1 24.6
56.9 61.0 65.1 44.4 59.4 59.9 58.8 59.6 52.0 54.7 50.0 55.3
17.8 23.7 20.1 40.8 12.5 15.0 15.7 14.1 19.9 16.0 19.4 14.3
19.7 1.2 0.9 1.0 8.7 0.7 1.1 1.8 6.9 0.4 5.1 4.7
Calculated by eq 2. b Calculated by eq 1. c Calculated by eq 3.
Results β (BEA) zeolites contain nine crystallographycally nonequivalent T-sites;14 therefore, a rather complex 29Si NMR spectrum is expected. The high-resolution spectrum of highly dealuminated BEA exhibits three groups of nine lines,29 but due
to the high concentration of stacking faults in the zeolite structure, generally three signals are observed.30 The relative integrated intensities of these three sets of lines around -111, -113, and -115 ppm were, respectively, around 50%, 25%, and 25%, but the presence of silanol groups and relatively high Al content (Si/Al < 100) decreased the resolution of the three components.29,30
Figure 1. 29Si NMR spectra of [B]-BEA (a), NH4-[B]-BEA (b), Na[B]-BEA (c), and Ni-[B]-BEA (d) zeolites.
Figure 2. 29Si NMR spectra of [Al,B]-BEAI (a), NH4-[Al,B]-BEAI (b), Na-[Al,B]- BEAI (c), and Ni,Na-[Al,B]-BEAI (d) zeolites.
Distribution of Al and B in BEA Zeolites
J. Phys. Chem. B, Vol. 110, No. 30, 2006 14731
Figure 4. 29Si CP NMR spectra of [B]-BEA (a), [Al,B]-BEAI (b), and [Al,B]-BEAII (c) zeolites.
Figure 3. 29Si NMR spectra of [Al,B]-BEAII (a), NH4-[Al,B]-BEAII (b), Na-[Al,B]-BEAII (c), and Ni,NH4-[Al,B]-BEAII (d) zeolites.
The 29Si NMR spectra of BEA zeolites (Figures 1-3) show four resonances at -100, -104, -110, and -114 ppm, which can be ascribed to Si(2X), Si(1X), Si(0X)A, and Si(0X)B sites (X ) Al, B), respectively (Table 2). Splitting of the signal assigned to Si(0X) configuration is due to two groups of different crystallographic sites;31 the presence of aluminum leads to a significant broadening of the bands and to a low field shift in the line positions.32 The CP spectra of some zeolites (Figure 4) confirm the presence of Si(OH)x groups in the bands assigned to Si(2Al) and Si(1Al) configurations (compare panels a, b, and c of Figure 4 with Figures 1a, 2a, and 3a, respectively), as their intensities compared to those of Si(0Al) bands are higher in the CP than in the normal spectra. The more regular line shape and the less noisy CP spectrum of [B]-BEA sample (Figure 4a) compared to the CP spectra of the other two original starting zeolites ([Al,B]-BEAI (Figure 4b) and [Al,B]-BEAII (Figure 4c)) suggests the presence of Si(OH)x groups in higher concentrations in the signals of the former than in the latter samples. The much lower Si/(Al + B)NMR than framework Si/(Al + B) ratio of the original [B]-BEA zeolite exhibits also the presence of defect silanol sites in a relatively high amount (Table 2). 27Al NMR spectra give information on the Al distribution in structurally distinct sites of the lattice. The 27Al NMR spectra of original H-BEA zeolites containing extraframework alumi-
num are shown in Figure 5. The signals at 55 ppm are attributed to aluminum in the zeolitic framework at tetrahedral coordination (AlT), and the lines around 0 ppm are assigned to extraframework aluminum in out-of-lattice octahedral positions (AlO). All other spectra exhibit the single narrow line of AlT only; therefore, they are not shown. The relative concentration of Al in tetrahedral positions (AlT in Table 2) was calculated from the relative integrated line areas of the 27Al NMR spectra (Figure 5). The 11B NMR spectra reveal the isomorphous replacement of aluminum by boron in zeolite frameworks. Boron may occur as tetrahedral BO4 in lattice position (BT), which shows a narrow and symmetric NMR line around -3.5 ppm.11,12,21-23,33 The trigonal BO3 exhibits a broad asymmetric NMR line above 10 ppm, which is characteristic of extraframework boron (BE). An additional signal assigned to tetrahedrally coordinated framework related boron (BF) species was observed at -211 and -0.512 ppm, respectively, where boron is still partially linked to the structure.1 Resolution of these lines determines their relative amounts present in the sample.7 The 11B NMR spectra of original H-BEA zeolites (Figure 6) show the narrow symmetric line of boron in lattice positions and the signal of trigonal out-of-lattice boron species (line around 17 ppm) in a relatively high intensity. All other spectra exhibit the single narrow line of BT only; therefore, they are not shown. The relative concentration of boron in tetrahedral position (BT in Table 2) was calculated from the relative integrated line areas of the 11B NMR spectra (Figure 6), taking into account that the measured intensities of BE were corrected by a factor of 1.3.13 To get information about the distribution of boron and aluminum in the crystal structure and to get a better resolution
14732 J. Phys. Chem. B, Vol. 110, No. 30, 2006
Figure 5. 29Al NMR spectra of [B]-BEA (a), [Al,B]-BEAI (b), and [Al,B]-BEAII (c) zeolites.
of the spectra without quadrupolar distortions, preliminary twodimensional triple-quantum (2D 3Q) 11B and 27Al NMR experiments were carried out on the [B]-BEA, [Al,B]-BEAI, and Ni-[Al,B]-BEAI samples. Only the F1 projections are shown, which exhibit purely isotropic dimensions. The F1 projection of 11B 3Q-MAS NMR spectrum of [B]-BEA zeolite (Figure 7a) resolves the singlet narrow line at -2.5 ppm in the 1D spectrum (Figure 6a) into two well-separated lines at -1.7 and -3.4 ppm with 9:1 relative intensities. The same line in the 1D spectrum of [Al,B]-BEAI (Figure 6b) at -2.9 ppm splits into a doublet (Figure 7b) at -1.7 and -3.5 ppm with 1:1 relative intensities. The distribution of boron in [B]-BEA and [Al,B]-BEA zeolites is debated in the literature. It is claimed that boron has only one type of tetrahedral position in [Al,B]-BEA zeolites;34 others found a tetrahedral doublet for 11B at -3.8 and -4.5 ppm with a one-to-one relative ratio, suggesting two different boron environments in borosilicate SSZ-42 (IFR).4 Our boron doublet found in the 11B 3Q-MAS NMR spectra (Figure 7a,b) can also be interpreted as boron species in two different tetrahedral environments similar to Al (Figure 7c,d), but it is more probable that due to our different boron chemical shift values (-1.7 and -3.4 ppm) the first signal should be assigned to BF species and the latter line is attributed to BT species. The line of AlT in the 1D 27Al spectra also resolves into two components in the 3Q-MAS spectra at 60.5 and 55.4 ppm, with 9:1 and 1:1 relative intensities ([Al,B]-BEAI (Figure 7c) and Ni-[Al,B]-BEAI (Figure 7d)), respectively. The line at 60 ppm is assigned to aluminum atoms positioned in T3-T9 sites (Scheme 1), while the line at 55 ppm corresponds to Al atoms on sites T1 and T2.35 It means that the distribution of boron and aluminum atoms is different in the three studied zeolites: only
Kora´nyi and Nagy
Figure 6. 11B NMR spectra of [B]-BEA (a), [Al,B]-BEAI (b), and [Al,B]-BEAII (c) zeolites.
a small fraction of boron species are situated in BT sites in the [B]-BEA zeolite, about the same amount of boron atoms are sitting in BT and BF positions, and more aluminum atoms are situated in the T3-T9 sites than the random (3:1) distribution in the [Al,B]-BEAI samples, and the distribution of Al atoms between T1-T2 and T3-T9 sites is roughly equal in the Ni[Al,B]-BEAI zeolite. Discussion The periodic building unit (PerBU) of BEA (Scheme 1) is a tetragonal β layer composed of 16-membered units connected through four-membered rings.15 This PerBU contains nine different T-atoms, from which T1-T6 positions (12 from all 16 atoms) are also situated in the four-membered rings (Scheme 1).15 We previously suggested that either two aluminum atoms are sitting in the four-membered rings of the BEA structure in diagonal positions (twin2BEA and twin1BEA), or only one Al atom occupies a single position (loneBEA) in the PerBUs of β zeolites.25 The relative abundance of T1-T6 sites is 3/4 in the BEA structure; consequently, all aluminum (and boron) atoms should sit in the four-membered rings of the BEA zeolites in T1-T6 positions (Scheme 1), in agreement with theoretical calculations.36,37 Two Al (or B) atoms are connected in diagonal T1-T4, T2-T3, T3-T6, T4-T5, or T5-T6 sites with adjacent two Si(2X) and four Si(1X) positions in the “diagonal“ PerBUs, Si(0X) species occupy the remaining eight sites (Scheme 1). The one Al (or B) atom sits in T1-T6 positions with adjacent four Si(1X) sites and 11 Si(0X) species in the “lone” PerBU (Scheme 1). A third kind of “allSi” PerBU is also suggested with 16 Si(0X) positions. The ratio of tetrahedral silicon to aluminum (and/or boron) in the zeolite framework (Si/(Al + B)NMR) can be directly
Distribution of Al and B in BEA Zeolites
J. Phys. Chem. B, Vol. 110, No. 30, 2006 14733
Figure 7. F1 projections of 11B (a, b) and 27Al (c, d) 3Q-MAS NMR spectra of [B]-BEA (a), [Al,B]-BEAI (b, c), and Ni-[Al,B]-BEAI (d) zeolites.
calculated from the line intensities in a 29Si MAS NMR spectrum (ISi(nX) where X ) Al, B and n is the number of connecting X atoms) by the following equation assuming that the Al-O-Al (as well as B-O-B and B-O-Al) avoidance rule of Loewenstein is obeyed and Si(OH)x signals are not included in the bands:7
configurations during the dealumination (deboronation) process:
∑ISi(nX)/∑(n/4)ISi(nX)
Si(OH)2/Si(2X) ) SiOH/Si(1X) ) XO/XT
Si/(Al + B)NMR )
summation is from n ) 0 to 4 (1)
The resulting Si/(Al + B)NMR ratio may strongly underestimate the actual Si/(Al + B) ratio as defect sites (Si(OH)x groups) are generally present in the zeolite framework. Framework Si/ (Al + B)fram ratios were calculated from the elemental compositions (Table 1) and tetrahedral aluminum (AlT) and boron (BT) fractions (Table 2) by the following equation:
Si/(Al + B)fram ) Si/(Al*AlT + B*BT)
(2)
If a characteristic difference between Si/(Al + B)fram and Si/ (Al + B)NMR exists (Table 2), it clearly indicates the presence of defect silanol groups in the zeolite. The difference between Si/(Al + B)bulk (Table 1) and Si/(Al + B)fram (Table 2) is the quantitative degree of dealumination/deboronation. 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 -104 ppm (“line1”) includes Si(1X) and SiOH intensities and the line at around -100 ppm (“line2”) involves Si(2X) and Si(OH)2 signals.25,26 Substituting these defect-free line intensity values (ISi(nX) ) Iline(n) - ISi(OH)n, where n ) 2 or 1) and the real Si/(Al + B)fram ratios (Table 2) to eq 1, the following 2ISi(OH)2 + ISiOH concentrations (Table 2) can be calculated by eq 3:
Si/(Al + B)fram ) 4/[2(Iline2 - ISi(OH)2) + (Iline1 - ISiOH)] (if
∑ISi(nAl) ) 1)
2ISi(OH)2 + ISiOH ) 2Iline2 + Iline1 - 4/Si/(Al + B)fram
(3)
To get the concentration of defect-free Si(2X) and Si(1X) as well as defect Si(OH)2 and SiOH sites, the sum (2ISi(OH)2 + ISiOH) derived from eq 3 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)
“diagonal” PerBU: XT + Si(2X) + 2Si(1X) f XO + Si(OH)2 + 2SiOH “lone” PerBU:XT + 4Si(1X) f XO + 4SiOH (4)
The formulas used in calculation of the concentration of Si (Si(nX), Si(OH)x, Si(0X)), and X (XT, XO) sites as well as PerBU (X and X free (deX)) distributions are presented in Table 3. The concentrations in “diagonal” PerBUs are determined by the calculated defect-free Si(2X) and defected Si(OH)2 contents (p and q, respectively, in Table 3). The “lone” PerBU distributions are computed from the calculated defect-free Si(1X) and defected SiOH compositions subtracting the “diagonal” compositions of the same species (r - 2p and s - 2q, respectively, in Table 3). The “allSi” PerBUs are composed of the residual Si(0X) (calculated Si(0X) subtracted Si(0X) content of “diagonal” and “lone” PerBUs), as well as the original Si(OH)2 and SiOH concentrations (t + 3{p + q}/2 - 11{r + s}/4, w, and v, respectively, in Table 3). The original Si(OH)2 and SiOH sites are not produced in the dealumination (deboronation) process, because in parallel XO species are not generated; they are products of the possible disruption of SiOSi or SiOSiOSi bonds.25,26 The PerBU compositions computed with formulas shown in Table 3 are presented in Table 4. All “diagonal” PerBUs are “twin2” units in the previous methodology,25,26 which means two Al (B) atoms in one PerBU unit (Scheme 1). Only “diagonal” and “allSi” units are present in the [B]-BEA series, and the ratio of “lone” units is very low in the protonic form of [Al,B]-BEAI and [Al,B]-BEAII zeolites. The ratio of “diagonal” PerBUs always exceeds the concentration of “lone” units in the [Al,B]-BEAI and [Al,B]-BEAII series (Table 4). It means a distinct difference between the only boron-containing [B]-BEA and the two mixed, aluminum and boron-containing [Al,B]BEA series; boron does not seem to be in “lone” position as part of aluminum, but always likes to be situated in pairs, in “diagonal” positions in the four-membered rings of the zeolite lattice. Dealuminated (deboronated) PerBUs and defect silanol groups are present in the original, protonated zeolites ([B]-BEA, [Al,B]-
14734 J. Phys. Chem. B, Vol. 110, No. 30, 2006
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TABLE 3: Compositions Calculated for T16 Periodic Building Units (PerBUs) of β Zeolites (X ) Al, B)a ∑ PerBU
X + Si
diagonal/atom diag.deX/atom lone/atom lonedeX/atom allSi/atom
8p 8q 4(r - 2p) 4(s - 2q) t + 3(p + q)/2 -11(r + s)/4 w+v
∑
Si site contribution XT
XO
Si(2X)
p
Si(OH)2
Si(1X)
p
(r - 2p)/4
SiOH
q
q
(s - 2q)/4
(2q + s)/4
q+w
p
4p 4q 11(r - 2p)/4 11(s - 2q)/4 t+3(p + q)/2 -11(r + s)/4
2q
r - 2p
s - 2q v
w (2p + r)/4
Si(0X)
2p
s+v
r
t
Diag.deX ) diagonal dealumi{deboro}nated, lonedeX ) lone dealumi{deboro}nated, p ) Si(2X), q ) Si(OH)2 (dealumi{deboro}nated), r ) Si(1X), s ) SiOH (dealumi{deboro}nated), t ) Si(0X), v ) SiOH (original), w ) Si(OH)2 (original) concentrations. a
TABLE 4: [B]-BEA and [Al,B]-BEA Zeolite Compositions (%) Assuming Perfect, Theoretical Diagonal, Lone and AllSi PerBUsa PerBU zeolite [B]-BEA NH4-[B] Na-[B] Ni-[B] [Al,B]-BEAI NH4-[Al,B]I Na-[Al,B]I Ni-[Al,B]I [Al,B]-BEAII NH4-[Al,B]II Na-[Al,B]II Ni-[Al,B]II
a
Si site contribution
PerBU
X
deX
XT
XO
Si(2X)
Si(OH)2
Si(1X)
SiOH
Si(0X)
diagonal lone allSi diagonal lone allSi diagonal lone allSi diagonal lone allSi diagonal lone allSi diagonal lone allSi diagonal lone allSi diagonal lone allSi diagonal lone allSi diagonal lone allSi diagonal lone allSi diagonal lone allSi
26.2 0 48.9 41.7 0 58.3 37.6 0 62.4 36.0 0 64.0 48.0 4.3 24.8 46.6 21.4 32.1 47.1 21.2 31.7 54.5 10.3 35.3 49.4 3.5 20.3 36.5 25.3 38.2 35.1 21.8 43.1 38.0 19.9 42.1
24.7 0
3.3 0
3.1 0
3.3
3.1
6.6 0
0 0
5.6 0
0 0
5.6
1.5 0
0 0
4.8 0
0 0
4.8
0.3 0
0 0
4.5 0
0 0
4.5
0.2 0
21.0 1.9
6.0 0.3
2.6 0.1
6.0
0.2 2.6
0 0
5.8 1.3
0 0
5.8
0.0 0
0 0
5.9 1.3
0 0
5.9
0.1 0
0 0
6.8 0.6
0 0
6.8
0.2 0
25.0 1.8
6.2 0.2
3.1 0.1
6.2
0.4 3.1
0 0
4.6 3.3
0 0
4.6
0.0 0
0 0
4.4 2.8
0 0
4.4
0.0 0
0 0
4.7 2.6
0 0
4.7
0.7 0
6.2 0 3.0 0 0 0.5 0 0 0.4 0 0 0.5 5.3 0.5 0.0 0 0 0.4 0 0 0.6 0 0 0.9 6.2 0.4 0.0 0 0 0.2 0 0 3.3 0 0 3.0
25.5 0.1 44.4 22.4 0 57.5 19.3 0 61.8 18.0 0 63.3 34.5 4.3 24.8 23.3 14.7 31.6 23.5 14.6 31.0 27.2 7.1 33.9 37.2 3.6 20.3 18.2 9.0 38.0 17.5 7.7 39.1 19.0 7.1 38.4
8.1 0 8.7 0 9.0 0 12.0 1.1 11.6 5.3 11.8 5.3 13.6 2.6 12.4 0.9 9.1 13.0 8.8 11.2 9.5 10.3
0.7
X ) relative concentration of Al, B containing PerBU, deX ) relative concentration of dealumi{deboro}nated PerBU.
BEAI, and [Al,B]-BEAII) only, and they disappear following ion exchange (Table 4). During ion exchange, only boron leaves the zeolite structure based on the chemical compositions (Table 1). Comparing the sum of dealuminated (deboronated) PerBUs in the original protonated zeolites (from Table 4) with the ratio of leaving boron plus aluminum during ion exchange (from Table 1), the values of protonated and NH4-exchanged forms are identical within the experimental error (Table 5). This means that trigonal boron leaves the zeolite structure completely; in parallel octahedral aluminum converts back to tetrahedral framework aluminum during the first ion exchange from the H form to the NH4 form. During the subsequent exchanges (Na, Ni forms), some further tetrahedral boron leaves the zeolite
TABLE 5: Sum of Dealuminated (deboronated) PerBUs in the Original Protonated Zeolites (∑DeX (%)) and Ratio of Leaving Boron Plus Aluminum (1-(B+Al)IE/ (B+Al)OR (%)) in the Ion-Exchanged Zeolites (IE ) ion-exchanged, OR ) original) zeolite HNH4NaNi-
[B]-BEA [Al,B]-BEAI [Al,B]-BEAII ∑deX (%) 1-(B+Al)IE/(B+Al)OR 1-(B+Al)IE/(B+Al)OR 1-(B+Al)IE/(B+Al)OR
24.8 26.7 33.0 34.8
22.9 24.6 26.8 25.5
26.7 25.6 33.6 32.0
structure in the [B]-BEA and [Al,B]-BEAII zeolites (Tables 1 and 5). This result can be regarded as a quantitative argument for the conversion of octahedrally coordinated frameworkassociated aluminum species into tetrahedral lattice positions
Distribution of Al and B in BEA Zeolites during ion exchange referred several times but without this stoichiometric evidence in the literature.17-20,38 Comparing [B]-BEA with [Al,B]-BEAI and [Al,B]-BEAII zeolites, besides the lack of “lone” PerBUs, a much higher amount of silanol groups exists in the former (Table 2). Another distinct difference is the roughly similar “diagonal” to deboronated diagonal PerBU ratio in the [B]-BEA, compared to the two to one (in [Al,B]-BEAII) or higher (in [Al,B]-BEAI) diagonal to dealuminated (deboronated) diagonal PerBU ratio (Table 4). This finding is supported by the 11B 3Q-MAS NMR spectra: the overwhelming majority of boron atoms are in BF positions in [B]-BEA (Figure 7a), and a marked minority of the same positions is situated in [Al,B]-BEAI (Figure 7b) zeolites, which is also indicated in the difference in the amount of defect silanol Si(OH)x groups. The high concentration of stable silanol nests and BF speciesswhich can also be connected to OH groupssin [B]-BEA zeolites gives also a stoichiometric argument for the forecasts in the literature,5 accordingly boron is located mainly in the tetrahedral framework position. Another interesting finding is that following ion exchange of the protonic form of [Al,B]-BEAI and [Al,B]-BEAII zeolites, the back building of AlO species becomes mainly into “lone” lattice positions, while the conversion of BF species presumably takes place always into “diagonal” sites (Table 4). The site distribution of the ion-exchanged (NH4, Na, Ni) forms of all zeolites is surprisingly uniform, and within a series there are no characteristic differences in the PerBU ratios. This means that the type of the cation does not play a role in the conversion of framework related Al (B) to lattice heteroatom (Al, B) process. Conclusions Our original PerBU method25,26 determining aluminum siting as well as dealumination and silanol formation mechanisms was developed further. We clarified that two B atoms are sitting in diagonal positions in the four-membered rings of periodical building units of [B]-BEA zeolites, and beside these positions we assumed that lone Al atoms may also be situated in these rings in [Al,B]-BEAI and [Al,B]-BEAII samples. The existence of lone B atoms is not probable. Dealuminated octahedral Al and trigonal (and framework related tetrahedral) B species as well as defected silanol groups are situated in the protonic forms of the zeolites only. Following ion exchange to ammonium, sodium and/or nickel ions affected the full conversion of octahedrally into tetrahedrally coordinated framework aluminum and presumably framework-related tetrahedral to tetrahedral lattice boron species in BEA zeolites, in parallel trigonal boron, completely left the zeolite lattice. Stoichiometric arguments were found for the high concentration of stable silanol nests and tetrahedrally coordinated framework related B speciesswhich can also be connected to OH groupssin [B]-BEA compared to [Al,B]-BEA zeolites. A detailed theoretical structure analysis of [B]-BEA and [Al,B]-BEA lattices enabled us to a deeper understanding of the origin of defect sites and the mechanism of back building of dealuminated (deboronated) species to lattice positions during ion exchange through the 29Si and 27Al NMR analysis of Si and Al site contribution. We conclude that we are able to distinguish the Si(OH)x groups, which are original defect sites or produced in a dealumination (deboronation) or calcination process.
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