4148
J. Phys. Chem. 1996, 100, 4148-4153
Aluminum Distribution in Chabazite: An Experimental and Computational Study Duncan E. Akporiaye,* Ivar M. Dahl, Helle B. Mostad, and Rune Wendelbo SINTEF Chemistry, P.O. Box 124 Blindern, N-0314 Oslo, Norway ReceiVed: August 1, 1995; In Final Form: NoVember 30, 1995X
A set of synthetic and natural chabazite materials with compositions ranging from Si/Al ) 2.6-7.6 have been obtained. Detailed studies using multinuclear MAS NMR have been used to analyze the distribution of aluminum within the crystalline framework. Comparison of the observed 29Si NMR results to those predicted by various ordered and random models of the aluminum distribution indicates that, for the low-silica materials, the highest contribution from the ordered distribution to the spectrum originates from the configuration that has the least number of Al next nearest neighbor interactions. However, the results from all synthetic samples are also consistent with a random distribution, the one natural sample being less well described. Molecular modeling studies comparing the relative lattice energies of the different ordered distributions support these results.
Introduction For zeolites, the presence and distribution of aluminum in the crystalline aluminosilicate framework have important significance in terms of the degree of cation exchange available and the strength of the acidity when they are obtained in the hydrogen form. On the basis of Lo¨wenstein’s rule,1 specifying the avoidance of aluminum-aluminum nearest neighbor T sites, there have been a number of detailed studies2-6 directed at investigating the extent to which ordering of the Al distribution is present in the crystalline structure. These studies have primarily focused on the faujasite structure. In assessing the validity of various ordering schemes at different Si/Al ratios, the use of 29Si NMR has been an important means by which the relative populations of the Si(nAl) environments have been compared with the expected populations for proposed models. Recently, computer modeling techniques also have been used to study the theoretical validity of Lo¨wenstein’s rule7 and also to compare the relative energies of different ordering schemes of Al and the associated charge-balancing cations.8,9 In this communication we have embarked on a similar study applied to the zeolite chabazite. Chabazite is known as a mineral and may also be obtained in the synthetic form. Like faujasite, it is built up of a series of interconnected double-6-ring units (D6R) which form a single type of open cage. In its highest symmetry form, it is described by the rhombohedral R3hM space group,10 with a unit cell composition of MxAlxSi12-xO24 (M ) extraframework cation). This unit cell essentially encloses a single D6R unit. In the earliest reported synthesis,11 chabazite was prepared from a wholly inorganic gel system and was typically obtained with a Si/Al ratio of less than 3. A recent series of studies by Zones et al.12,13 have made use of an additional organic amine with a resultant product having Si/Al ratios as high as 7.5. The latter high-silica materials have been obtained by the use of specialized organic amines, the most notable being the N,N,N-trimethyladamantane ammonium iodide. The two preparative routes have been investigated by us and have resulted in a series of synthetic chabazite materials spanning the range of Si/Al ratios of 2.6-7.6. In addition to these synthetic materials, a naturally occurring chabazite sample was also obtained and included in the studies. These materials X
Abstract published in AdVance ACS Abstracts, February 1, 1996.
0022-3654/96/20100-4148$12.00/0
have now been the basis of an investigation of the distribution of Al in the chabazite structure as a function of Al concentration, comparing various ordered/random Al configurations. Computational studies have also been applied to assess their support of these configurations, including the possible positions for the associated charge-balancing cations. Experimental Section Synthesis. (i) Direct Synthesis without Organic Amine (CHA1). Reagents used were Ludox AS (DuPont), 30% colloidal silica in water; aluminum hydroxide (Aldrich), 54% Al2O3; potassium hydroxide (Fluka, 86.6%); and distilled water. Gel composition was Al2O3:5SiO2:2.5K2O:600H2O. KOH was dissolved in all the water; aluminum hydroxide was then added to this solution and finally the colloidal silica, followed by stirring for several minutes until a homogeneous gel was formed. The gel was heated under static conditions at 150 °C for 5 days. The crystalline product was filtered from the mother liquor, washed, and dried at 70 °C. (ii) Direct Synthesis with N,N,N-Trimethyladamantane Ammonium Iodide (CHA-3). Reagents used were NaOH (Aldrich), sodium silicate (Kebo), and sodium aluminate (Kebo). The organic amine was prepared according to a patented procedure,14 the quality of the product being confirmed by melting point analysis and NMR. A typical oxide ratio of Al2O3:32SiO2: 15Na2O:1100H2O:4R was employed (R ) trimethyladamantane ammonium iodide). The NaOH was first dissolved in the specified amount of water. To this was added sodium aluminate and then sodium silicate, and after stirring, the quaternary ammonium salt was carefully added to obtain a relatively homogeneous mixture. The whole mixture was transferred to 50 mL autoclaves rotating axially at 15 rpm in heated ovens. Crystallization was carried out at 150 °C over 120 h. (iii) Recrystallization of Zeolite Y with Trimethyladamantane Ammonium Iodide (CHA-4). The synthesis procedure was essentially similar to (ii) except that LZ-Y62 zeolite Y (Si/Al ) 2.47) was employed as alumina source instead of sodium aluminate. A typical oxide ratio of Al2O3:104SiO2:34Na2O: 734H2O:3.5R was employed. Characterization. The X-ray powder diffractograms (XRD) were obtained using a Siemens D5000 with Ni-filtered Cu KR radiation as the source. The 29Si MAS NMR spectra were obtained using a Varian VXR 300 S spectrometer operating at 300 MHz proton © 1996 American Chemical Society
Aluminum Distribution in Chabazite
J. Phys. Chem., Vol. 100, No. 10, 1996 4149
Figure 1. X-ray diffraction of high-silica chabazite (a) as-synthesized and (b) ion-exchanged to the ammonium form and calcined at 550 °C.
TABLE 1: Normalized Relative Peak Areas from the 29Si MAS NMR and Composition Results of the Four Chabazite Samples sample
Si/Al(anal)
Si/Al(NMR)
Al(NMR)/UCa
Si(4Al)
Si(3Al)
Si(2Al)
Si(1Al)
Si(0Al)
CHA-1 CHA-2 CHA-3 CHA-4
2.67 3.5 5.0 12.8
2.6 3.1 5.2 7.6
3.2 2.9 1.9 1.4
0.0(3)
0.1(1) 0.0(5) 0.0(3)
0.35(4) 0.33(0) 0.17(4) 0.11(1)
0.394(7) 0.48(4) 0.34(4) 0.30(1)
0.10(7) 0.13(7) 0.45(6) 0.58(7)
a
UC ) unit cell.
resonance frequency. The instrument was equipped with both Jakobsen and Doty MAS probes. The conditions for the acquisition of the spectra were as follows: 29Si frequency 59.6 MHz, sweep width 14 000 Hz, pulse width 8 µs (90° pulse, 8.3 µs), repetition time 5 s, acquisition time 1 s, number of scans 1000, MAS spinning speed 4.5 kHz, reference to Me4Si. 27Al NMR: 27Al frequency 78.157 MHz, sweep width 50 000 Hz, pulse width 0.5 µs, repetition time 2 s, acquisition time 0.05 s, MAS spinning speed 4.5 kHz, reference 1 M aluminum nitrate solution. Thermogravimetric analysis (TGA) was carried out on a Perkin-Elmer instrument. Approximately 10 mg of sample was analyzed using a heating rate of 20 °C/min under air purge. Scanning electron micrographs were taken using a JEOL JSM-840 instrument. Chemical analysis was carried out using a Perkin-Elmer 4000 AAS and ICP. The samples were dissolved in hydrofluoric acid in sealed polypropylene flags. Results Three synthetic routes were chosen for the preparation of synthetic chabazite; (i) direct synthesis in the absence of organic amine (CHA-1), (ii) direct synthesis in the presence of N,N,Ntrimethyladamantane ammonium iodide (CHA-3), (iii) recrystallization of a commercial zeolite Y in the presence of the ammonium salt (CHA-4). The recrystallization route (iii) essentially makes use of zeolite Y as an aluminum source, allowing the formation of a high-silica product. As initially reported by Zones et al.,13 variation in the gel composition was not able to increase the Si/Al of the product above a limiting value of about 7.6. In nearly all syntheses, a highly crystalline chabazite material was obtained, as confirmed by XRD, Figure 1. In contrast to the low-silica samples, the high-silica materials were stable to thermal treatment, even after ion-exchange to
the ammonium form. The compositions of the three synthetic samples and the natural sample obtained from Bowie Arizona (CHA-2) used in this study are presented in Table 1. For the sample with a Si/Al ratio of 7.6, the compositions obtained from chemical analysis and calculated from the NMR spectra were not in agreement. This was probably due to the presence of some amorphous silica in the products, since the Si/Al ratio in the initial gel was normally quite high in an attempt to maximize the silicon concentration in the final material. The results of chemical and NMR analysis for the samples CHA-1, CHA-2, and CHA-3 were, however, in reasonable agreement. Thermal gravimetric analysis of the material containing the organic amine was also studied to check the conditions for template removal. As shown in Figure 2, an extremely welldefined weight loss is present in the range 750-800 K. This must be associated with the decomposition of the organic, since desorption is not possible due to the small size of the 8-ring pores compared to the size of the organic. The very sharp transition reflects the very rapid evolution of the organic material as soon as the temperature for its decomposition is reached. The additional high-temperature weight changes at 800-1000 K are possibly due to oxidation of residual organic fragments. The 29Si MAS NMR spectra of the as-synthesized samples are presented in Figure 3 along with the one natural sample. From these results, it is evident that up to five different resonances may be observed, the relative intensities varying with the Si/Al ratio. The chemical shifts observed for the synthetic chabazite are in general agreement with the values observed for faujasite and other high-silica materials. Countless studies of zeolites have now established the relationship between the Si chemical environment and the chemical shift. In purely siliceous materials, individual resonances characterizing unique crystallographic sites have been identified.15 In the aluminosilicate forms, different resonances are associated with the
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Akporiaye et al.
Figure 2. Thermal gravimetric analysis (air) results of sample CHA-4.
crystalline framework may be readily calculated from the relationship
Si/Al ) ∑ISi(nAl)/0.25∑nISi(nAl) where n ) 0-4; ISi(nAl) ) area of the peak for this Si environment. The values of the framework composition presented in Table 1 have been calculated from the relative peak areas of the deconvulated spectra. The deconvolution has been carried out by use of an unconstrained curve-fitting procedure making use of Gaussian peak shapes. The normalized peak areas from the spectra are also presented in Table 1. The 27Al spectra of the three representative samples CHA-1, CHA-2, and CHA-4 are presented in Figure 4. The spectra of all the materials exhibit resonances in the range 55-58 ppm, which are typical for aluminum in a tetrahedral environment within the framework of the zeolite. Aluminum Distribution
Figure 3. 29Si MAS NMR of as-synthesized chabazite materials: (a) CHA-1, (b) CHA-2, (c) CHA-3, and (d) CHA-4.
different Si(nAl) environments. For materials such as faujasite and chabazite with a single unique crystallographic T site, on the basis of Lo¨wenstein’s rule, the Si/Al composition of the
The distribution of aluminum over a wide compositional range has been extensively studied in the faujasite structure.2-4 It appears to be conclusive that at each level of aluminum concentration a degree of aluminum ordering is present. The basis for this ordering has been expressed earlier by Dempsey16 as minimizing the electrostatic interactions between next nearest neighbor aluminum sites (NNN). Considering the case of the chabazite structure, an analysis of the extent to which the 29Si NMR data are consistent/inconsistent with some degree of aluminum ordering has been carried out. Using D6R as the basic repeating unit, Lo¨wenstein’s rule limits the maximum number of aluminum atoms in the structure to 6/unit cell, equivalent to a Si/Al ratio of 1 and full alternation of Al and Si in the framework. With a decrease of the numbers of Al per unit cell, there is a concomitant increase in the number of unique ways in which the Al may be distributed in the D6R unit. In two recent studies,17,18 a new method for examining the Si/Al distribution in zeolites was outlined. This was based on
Aluminum Distribution in Chabazite
Figure 4. CHA-4.
27Al
J. Phys. Chem., Vol. 100, No. 10, 1996 4151
MAS NMR spectra of (a) CHA-1, (b) CHA-2, and (c)
the node index, taking into account the connectivity of the framework, and was used to examine different Al distribution schemes as well as examine the validity of Dempsey’s rule. According to this method, as associated substitution matrix can be built from this connectivity which allows the number of different distribution patterns for each Al loading to be readily calculated. The allowed ordering schemes for CHA are shown in Figure 5, and the calculated 29Si patterns presented in Table 2. The scheme used in Figure 5 presents a simplified view of the CHA structure, normal to the D6R along the 3-fold axis. Comparison of the simulated patterns to the observed 29Si results indicates that none of the experimental results are adequately reproduced by any single ordered distribution. It is also possible to take into account the presence of some degree of inhomogeneity in the ordering and concentration of the Al within the zeolite crystals by calculating the relative contributions of the different ordering schemes to the observed patterns, for the complete range of aluminum loading (0-6/ unit cell). For most zeolite structures this provides a prohibitive number of combinations for analysis, but for the chabazite structure as described here, this can be realistically carried out, since there are only 13 unique spectra that can be obtained from all the ordered configurations in the composition range of Si/ Al ) 1 to infinity. For all the samples a least squares fit to the observed spectrum was carried out of the relative contributions of each unique spectra from each aluminum loading. To ensure that all possible solutions were sampled, this procedure was repeated many times with different sets of randomly generated initial conditions, and the results were averaged out. The results from this analysis are presented in Table 3. It can thus be seen that the maximum contribution to the Si spectrum for each material corresponds
Figure 5. (a, upper) Simplified representation of the CHA structure. The relationship of the 8-ring window and the cationic sites to the D6R unit is indicated. The dashed line shows the position of the fourmembered rings. Filled and open circles indicate atomic positions above and below the plane of the two surfaces of the D6R unit. (b, lower) Unique aluminum ordering schemes for the compositional range 1-4 Al/unit cell. Filled and empty circles represent atoms below and above the D6R unit.
TABLE 2: List of the Unique Simulated 29Si NMR Spectra Calculated from All Possible Aluminum Configurations at Each Level of Aluminum Content in the CHA Structure normalized intensity of calculated 29Si NMR spectra Al/UC Al0 Al1 Al2-1 Al2-2, Al2-4 Al2-3, Al2-5 Al3-1 Al3-2 Al3-3 Al3-4 Al4-1 Al4-2, Al4-3 Al5 Al6
Si(4Al)
Si(3Al)
Si(2Al)
0.300 0.200
0.125 0.290 1.000
0.110 0.222 0.250 0.500 0.570
0.333 0.667 0.440 0.222 0.375 0.250
Si(1Al) 0.333 0.200 0.400 0.800 0.667 0.110 0.222
Si(0Al) 1.000 0.667 0.500 0.400 0.200 0.333 0.330 0.333 0.250 0.250 0.140
well with the expected aluminum loading calculated from the deconvoluted NMR spectra. For the high-silica materials (CHA-3 and CHA-4), no particular ordering scheme is significantly favored. However, the low-silica materials (CHA-1 and CHA-2) show the highest contribution of only one particular ordering scheme of the four possible for 3 Al/unit cell, that is AL3-1 in Figure 5.
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TABLE 3: Results of Fitting the Complete Set of Spectra from Table 2 to the Observed 29Si NMR Spectra Al/UC
no. of NNN interactions
Al0 Al1 Al2-1 Al2-2, Al2-4 Al2-3, Al2-5 Al3-1 Al3-2 Al3-3 Al3-4 Al4-1 Al4-2, Al4-3 Al5 Al6
0 0 3 3 0 3 9 8 9 17 17 28 43
a
weighting factors from fitting to spectraa CHA-1
0.60 0.12 0.13 0.11 0.04 0.01
CHA-2
0.58 0.30
0.22 0.01
CHA-4
aluminum ordering
Al2
Al3
Al4
0.12 0.20 0.12 0.16 0.13 0.10 0.05 0.07 0.04
0.27 0.26 0.12 0.15 0.10 0.06 0.04
-1 -2 -3 -4 -5
-1423.912 -1424.824 -1424.988 -1424.901 -1424.986
-1443.035 -1442.800 -1442.759 a
-1366.737 -1366.949 -1366.700
0.02
TABLE 4: List of the Unique Simulated 29Si Spectra Derived from a Random Distribution of the Aluminum in the Framework normalized intensity of the calculated 29Si NMR spectra sample
Si/Al(NMR) Si(4Al) Si(3Al) Si(2Al) Si(1Al) Si(0Al) 2.6 3.1 5.2 7.6
0.02 0.01
0.15 0.09 0.02 0.01
0.34 0.29 0.14 0.08
lattice energy/eV
CHA-3
Errors less than 0.01.
CHA-1 CHA-2 CHA-3 CHA-4
TABLE 5: Results of the Lattice Energy Minimization of the Different Aluminum Configurations
0.35 0.40 0.41 0.34
0.14 0.21 0.43 0.57
In addition to considering the distribution of the aluminum in the framework on the basis of a set of ordered configurations, we have also examined a random model assuming a binomial distribution. The predicted spectra for the four sample compositions are presented in Table 4. It is apparent from these results that the observed intensities for the three synthetic samples are to a significant degree consistent with the data from the random model, whereas the natural sample (CHA-2) is less well described. However, as has been indicated for faujasite,3 the close similarity between the results of the random model and the observed results does not provide proof of a random distribution of aluminum in the synthetic samples, although the results from Table 3 indicate that the high-silica samples are not well described by the ordered configurations. In contrast, the results of the natural sample are not well described by the random model, but are better described by the ordered scheme of AL3-1. It is interesting to note that this is also the one scheme at this composition which has the minimum number of NNN interactions. Modeling To obtain additional information on the likely preferred ordering schemes of Al in the CHA framework, a computational study of the relative energies of these various configurations was also carried out. This follows earlier work on aluminum in zeolite A,7 faujasite,8 and ZSM-189 and iron in ZSM-5.19 The application of computer modeling in this way involves the use of a potential model that effectively describes the interactions between the species in the structure. On the basis of these interactions, the energy of the system may then be calculated and minimized at constant pressure with respect to the atomic coordinates. The lattice energy derived from this minimized configuration can then be compared between the various configurations at the same Al substitution, the most stable scheme having the most negative lattice energy. The potentials used in these calculations8 have been extensively applied to silicate and aluminosilicate systems.
a
Convergence could not be obtained.
To have the most realistic representation of the studied system, it was also important to include the charge-balancing cations associated with each aluminum. The available cationic sites in the chabazite framework have been well characterized and are shown in Figure 5a. Of the three possible locations, the site inside the D6-ring was shown to have the lowest occupancy and may be considered to be the least favored, even at high cation loadings.20 Therefore, in this study, the primary positions considered for the cations were in the 8-ring sites and above/below the D6-ring. The relative stabilities of the various ordering schemes were assessed at the range of Al substitution 2-4, covering the composition of the synthesized materials and of dehydrated NaCHA.20 For each aluminum loading, a number of unique Al configurations were already identified, Figure 5. The effect of the different cation distributions on the relative energies of each aluminum distribution was studied in detail and will be reported elsewhere.21 However, the results of the calculations for the lowest energy configuration for the various systems are presented in Table 5, in terms of the total lattice energy of the system. At an aluminum loading of 2/unit cell (equivalent to CHA-3), the lowest energy aluminum configurations were Al2-3 and Al2-5, although the energy differences were small (
