Modeling of Silicon Substitution in SAPO-5 and ... - ACS Publications

© Copyright 1997 by the American Chemical Society

VOLUME 101, NUMBER 27, JULY 3, 1997

ARTICLES Modeling of Silicon Substitution in SAPO-5 and SAPO-34 Molecular Sieves German Sastre, Dewi W. Lewis,† and C. Richard A. Catlow* DaVy Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle St., London W1X 4BS, U.K. ReceiVed: NoVember 8, 1996; In Final Form: March 14, 1997X

We investigate the energetics of Si island formation in Si-substituted aluminophosphate molecular sieves. Lattice energy calculations confirm the stability of clusters (or islands) comprising Si-O-Si bridges within both SAPO-5 and SAPO-34. The influence of the framework topology is apparent in larger islands containing eight Si atoms, where effects of second-neighbor T shells become operative; the formation of the larger island is calculated to be energetically more favorable in SAPO-5 compared with SAPO-34 as observed experimentally. Estimates of deprotonation energies indicate greater acidity at the edges of islands and a correlation between acidity and island size, indicating that the acidity of these systems is directly related to the concentration of Si islands.

1. Introduction Crystalline aluminophosphate molecular sieves (AlPOs), first synthesized by workers at Union Carbide in 1982,1-4 not only exhibit properties characteristic of zeolites but also show novel physicochemical traits that are linked to their unique composition.5 The structure of these crystals is similar to that of the zeolites, but with the primary building units being formed by [Al-O-P] linkages instead of the [Si-O-Al] or [Si-O-Si] bridges of zeolites.6 The AlPO frameworks are neutral, and therefore compensating cations or anions are not necessary to provide charge compensation; thus, Brønsted acidity is not intrinsic to AlPOs, and their use as acid catalysts is therefore restricted.7 However, the introduction of silicon atoms into AlPO structures results in Brønsted acidity and thus the SAPOs (crystalline silicoaluminophosphates) can be used as acid catalysts.8-10 The SAPO maintains the same overall structure as the parent AlPO, although local structural changes are found as a consequence of the substitution.11 † Current address: Department of Materials Science and Metallurgy, University of Cambridge, Pembroke St., Cambridge CB2 3QZ, U.K. X Abstract published in AdVance ACS Abstracts, May 15, 1997.

S1089-5647(96)03736-4 CCC: $14.00

The primary building units of SAPOs are [Si-O-Al], [SiO-Si], and [Al-O-P]; no [Si-O-P] linkages have been observed to date.6,12,13 Indeed, our recent calculations have shown that such linkages are unstable.11 A number of different substitution mechanisms have been proposed for silicon incorporation into the AlPO structure:5,14 the first, denoted SM1, consists of the substitution of an aluminum atom by a silicon (AlfSi), although no experimental evidence has been found for such a mechanism;6 the second, denoted SM2, comprises the replacement of phosphorus by silicon and hydrogen (PfSi,H), the proton being attached to the oxygen on an Si-O-Al bridge, forming a Brønsted acid site; the third mechanism (SM3) is the double substitution of neighboring aluminum and phosphorus by two silicon atoms (Al,PfSi,Si). The structural and chemical properties of the SAPOs depend on the mechanisms of silicon incorporation.15 Silicon incorporation via the SM2 mechanism will give rise to the formation of one Brønsted acid center for each phosphorus replaced. Alternatively, islands (aggregates) of silicon can be formed in the SAPO structures by a combination of SM2 and SM3 mechanisms.16 The number and strength of the Brønsted acid centers generated by both mechanisms are © 1997 American Chemical Society

5250 J. Phys. Chem. B, Vol. 101, No. 27, 1997 different, and hence the catalytic properties of the SAPO depend, to some extent, on the relative occurrence of both processes.10,17,18 SAPO materials with low Si concentrations exhibit mild acidity, in contrast to aluminosilicate zeolites which are stronger Brønsted acids.19,20 The insertion of silicon into the framework in place of phosphorus leads to the formation of Brønsted sites of medium acid strength.4,21 However, increasing the amount of silicon in the framework can lead to high acidity in certain acid-catalyzed reactions.22,23 Thus, it is important to be able to control SAPO synthesis in order to form catalysts with high silicon content. Similarly, it may be desirable to control the synthesis so as to optimize the number of Brønsted acid sites of medium acid strength. If higher acidity is required, then it is necessary to maximize the formation of silicon islands in the structure since the acid centers at the border of the islands possess higher acid strength.24 Further discussion of the effect of silicon distribution on the acidity of SAPOs was given recently by Barthomouf.25 The compromise between the number and strength of the acid sites in SAPOs therefore determines their application as catalysts. Applications in petroleum refining and petrochemical conversion processes have already been found for SAPO materials,26 and many more can be devised, especially in those structures whose pore system is suitable for the diffusion of hydrocarbons involved in such processes. The relationship among the structure, chemical composition, and the catalytic properties of the SAPOs has been investigated, a particular focus being the distribution of silicon in the structure.27,28 Most of the research has been carried out on SAPO-5, since it has a high stability when reactions are performed in aqueous media,29 and to a lesser extent on SAPO-37 and SAPO-34.16 It is found that silicon islands in SAPO structures only start to form once the silicon content reaches a certain threshold value.30 The silicon distribution depends on at least two factors: the structural characteristics of the SAPO and the synthesis conditions.14,31 For example, island aggregates are formed in SAPO-5 at lower silicon content than in SAPO-34.16 However, the reasons for these differences are not yet fully understood. Moreover, we note that two structures often form from very similar gels: for example, Mg-containing SAPO-5 and SAPO-34 can be formed from almost identical gels, the phase formed being controlled by the amount of Mg included.17 In our previous work,11,32 mainly concerned with SAPO-5, we demonstrated how island formation is energetically favorable. Five- and eight-membered Si islands were shown to be more stable than two- and four-membered Si islands, a result which shows that [Si-O-P] linkages are energetically unfavorable. The aim of this paper is to develop models for silicon incorporation in SAPO-5 and SAPO-34, through lattice energy minimization calculations on the energetics of silicon islands and isolated Brønsted sites in the two SAPO structures. The results obtained will allow us to understand better the differences in the occurrence of silicon islands in SAPO-5 and SAPO-34 as the silicon content in the structure increases. We will also discuss the effect of Si environment on the acidity of the associated Brønsted sites, measured by both IR stretching frequency and proton affinity. 2. Methodology All the calculations in this work were performed using lattice energy minimization techniques, the GULP code33 being employed for this purpose. The code uses standard techniques based on the Ewald34 method for summation of the long-range

Sastre et al. Coulombic interactions and direct summation of the short-range interactions. In every case the original crystal structures were taken from the literature and all structural parameters; i.e., cell dimensions and atomic coordinates were optimized, without symmetry constraints under constant pressure conditions. The BFGS minimization method was used in all calculations with a convergence criterion of a gradient norm below 0.001 eV/Å.33 The Mott-Littleton methodology35 was employed to treat the incorporation of defects within the perfect lattice. This widely used method36 allows the full relaxation of atomic coordinates of an inner region (100-500 ions), surrounding a defect, to minimum energy, while more distant regions of the crystal are treated as a dielectric continuum. We note that these techniques have been successfully used for modeling framework substitution in zeolites,37,38 including providing insights into the avoidance of Al-O-Al links (Lowenstein’s rule)38 in aluminosilicates. Such results have proven to be consistent with experimental data. Furthermore, recent work has demonstrated their effectiveness in modeling AlPOs39 and SAPOs.11,39 The interatomic potentials used to model the interactions between the atoms in the structure included the following terms: Coulombic interactions, short-range pair potentials (described by a Buckingham function), and a three-body, bond bending term. The shell model40 was used to simulate the polarizability of the oxygen atoms. In the case of the Brønsted centers an additional Morse potential was included to model the O-H interaction. The model includes implicitly both shortrange repulsion and van der Waals interactions, although electron transfer is neglected. Such parameters should describe accurately the substitution of Si in an AlPO framework. Another important feature is the assumption that parameters are transferable between structures, allowing the construction of suitable (quantitative) thermochemical cycles; further discussions of the model and its features are given in refs 36 and 41. The potentials used for the AlPO structure42 were parametrized to reproduce the structure of Berlinite and have further been demonstrated to model successfully a number of AlPO structures.39 The Si-O potential43 and the O-H44 parameters have been extensively used for modeling the structures of zeolites (for example see ref 45). All the potential parameters used are listed in our previous work.11 A cutoff distance of 16 Å was applied to the short-range interactions. These same parameters were used in our previous studies on SAPO-511 and were shown to reproduce well experimental observations on Si incorporation. However, we note that recent results on the formation of isolated Brønsted acid sites in SAPO-3739 found that these potentials did not reproduce the experimentally observed occupancies46 of the different possible lattice sites. These authors have suggested, therefore, that further potential development would be advantageous. In this current paper we examine clusters of Si in different structures, and we consider that the relative energies of these clusters in the different structures will not be significantly influenced by the possible deficiencies in parametrization related to the problems discussed in ref 46. 3. Silicon Distribution in SAPO-5 and SAPO-34 Structures 3.1. Overview. There are many factors to consider in studying the process of silicon incorporation in AlPO and SAPO structures, and the comparison of theoretical results with experimental observations must be undertaken with care. The most important aspect of this work will be the relationship between silicon content and the formation of silicon islands. Here we provide a review of some of the features of SAPOs and of the experimental evidence relating to Si substitution, to provide a framework for discussing the results obtained later.

SAPO-5 and SAPO-34 Molecular Sieves

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3.1.1. Templates Used in the Synthesis. The template used in the synthesis procedure is one of the most important experimental parameters influencing the silicon distribution obtained in the SAPO structure. The information on these distributions is usually obtained from spectroscopic FTIR, XRD, EDX, 27Al, 29Si, and 31P MAS NMR measurements. The templates used in the synthesis of SAPOs not only play a structure-directing and space-filling role but also provide the charge compensation which allows Si to be incorporated into the framework.47 Depending on the template, at relatively high Si contents, Si can be incorporated forming isolated Si(4Al) species, which will be compensated on a 1:1 basis with the charge on the template. However, if Si islands are formed, the charge required to neutralize each Si is reduced since substitution of Al and P results in no net change in framework charge. Such behavior will be expected once a sufficiently high Si concentration is reached, as the framework will no longer be able to support isolated Si species. Thus, Si island formation is a mechanism by which more Si can be introduced into the framework than might otherwise be expected. We note that this mechanism is not normally found for other MeAlPOs or for the incorporation of Al into zeolites (Lowenstein’s rule). Nevertheless, the charge on the framework as a result of the introduction of Si cannot exceed the charge brought by the template. Recent theoretical work has demonstrated how the interactions between template and heteroatoms in the gel can have a dramatic effect on the material formed.48 We would therefore expect similar considerations to be important in the formation of SAPOs, since aggregation of silicon allows the incorporation of higher silicon concentrations. Recent work has tested the influence of the silicon distribution in SAPO-34 by varying the template used in the synthesis.47 From these results it can be seen that the use of two templates, morpholine and tetraethylammonium hydroxide (TEAOH), produced different silicon distributions. When using morpholine, at xSi ) 0.1, most of the silicon species are of the type Si(4Al), while with TEAOH species of the type Si(nAl) (n ) 4, 3, 2, 1, 0)sindicating island formationscan be detected at the same silicon content. Although both templates have similar size, the charge introduced in each SAPO-34 cage is +2 when using morpholine and +1 when TEAOH is used. Isolated acid sites need more charge than silicon islands, and the former cannot be formed so easily with TEAOH, which orients the silicon distribution to the island aggregates at lower silicon contents. However, if the effect of the template is not a limiting factor that forces the formation of silicon islands, they appear in SAPO-5 at lower silicon content than in SAPO-34.16 Clearly, Si island formation occurs before the long-range structure of the material has become established. However, here we will consider their formation only as a function of the final crystalline structure. Other work is underway to investigate the formation of precrystallization polymeric Si species in such systems. 3.1.2. Acid Strength. The number of protons required to neutralize the framework is higher when each silicon replaces one phosphorus (SM2) than when the silicon is replacing both aluminum and phosphorus, forming silicon islands (SM2+SM3). Take for instance the incorporation of five silicon atoms in the framework according to both procedures:

SM2: [AlPO4] + 5Si4+ + 5H+ f [5Si,5H]P + 5P5+

(1)

SM2+SM3: [AlPO4] + 5Si4+ + 3H+ f [5Si,3H]Al,4P + 4P5+ + Al3+ (2) Equation 1 describes the incorporation of five silicon atoms

forming five Brønsted sites through SM2 (5Pf5Si,5H). Equation 2 shows the formation of a five-silicon island which contains three Brønsted sites, through SM2+SM3 (3Pf3P,3H and P,AlfSi,Si). The notation used here, [5Si,3H]Al,4P, denotes that 1Al and 4P have been substituted in the original AlPO structure by 5Si and 3H. As the acidity generated by the centers at the border of silicon islands is higher than that due to centers formed by single substitution (SM2),22,25 strong acid centers are present in SAPOs where the silicon island formation is the dominant process, while mild acid centers are characteristic of SAPOs in which the silicon incorporation proceeds mostly through SM2. It should be stressed that the number of silicon atoms in the islands is controlled by the need to avoid [SiO-P] linkages, such linkages having been shown to be energetically unfavorable.11 This requirement limits the number of silicon atoms in the islands to 5, 8, 9, 12, etc., these numbers depending on the topology of the framework.28,49 Both processes of silicon incorporation are observed in SAPO-5 and SAPO-34,14,47 but the formation of silicon islands is noted at lower silicon contents in SAPO-5 than that in SAPO-34.16,47,50 3.1.3. Framework Composition and Mechanisms of Silicon Incorporation. The relative extent of the two mechanisms of silicon incorporation (SM2, SM3) can be calculated from the framework composition (Si molar fraction) of the SAPO structure.30 The silicon introduced in the framework by the SM2 mechanism (which is equal to the number of Brønsted acid sites) is calculated from

xSi(SM2) ) xAl - xP

(3)

and the silicon introduced by SM3 can be calculated from

xSi(SM3) ) 1 - 2xAl

(4)

However, due to uncertainties in elemental chemical analysis, viz., the possible presence of small amounts of silica, alumina, or other impurity phases, it is sometimes difficult to determine framework compositions with sufficient accuracy to establish acid site concentration by this procedure.50,51 Careful chemical compositions obtained by energy-dispersive X-ray spectrometry (EDX) on SAPO-34 have shown that, when the effect of the template does not force the formation of silicon islands, SM2 is practically the only mechanism in effect up to a silicon content of xSi ) 0.18.47 In these cases, the silicon content introduced by SM3 (eq 4) is always lower than 0.03. On the other hand, in SAPO-5, chemical analysis by atomic absorption showed that, at silicon contents of xSi ) 0.099, the fraction of silicon incorporated by SM3 according to eq 4 was xSi(SM3) ) 0.061.47 It is clear therefore that the relative importance of the two substitution mechanisms is influenced by the synthesis conditions and the structure formed. 3.1.4. Nuclear Magnetic Resonance Spectra. 29Si MAS NMR experiments have been successfully used to determine the silicon distribution in SAPO structures.52 As in zeolites, the strength of the acid sites depends on the environment of the silicon atoms, and the following order for the increase in the acid strength is expected:49

Si(4Al) < Si(3Al) < Si(2Al) < Si(1Al) The acid centers generated by means of SM2 are of the type Si(4Al), while the acid centers generated on the extremities of the silicon islands are of the type Si(3Al), Si(2Al), and Si(1Al). A quantitative calculation of the number of each type of environment can be carried out for each known structure depending on the silicon content, with the exact number of each type dependent on the size of the island and on the topology of

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TABLE 1: Definition of the Different Si Species Considered

a

name

silicon neighborsa

silicon environment

isolated Si distribution nonisolated Si distribution isolated defect (“infinitely diluted”)

Si(4Al)(nP) (1st and 2nd neigh) Si(4Al)(nSi) (1st and 2nd neigh) Si (AlPO neigh: Al, P, Al, P, ...)

-Si-Al-P-Al-Si-Al-P-Al-Si-Si-Al-Si-Al-Si-Al-Si-Al-Si-P-Al-P-Al-Si-Al-P-Al-P-

n ≡ total number of second neighbors.

the structure.28 For example, in SAPO-37 the order of occurrence of Si environment for small silicon islands (5-14 Si atoms) is Si(3Al) > Si(0Al) > Si(2Al) > Si(1Al). This results suggests a distribution of acid strengths with only a small number of strong acid sites, Si(1Al). A similar trend was also noted in SAPO-5.50 However, we must also consider the second-order neighbors when characterizing Si(4Al) species. It is important to distinguish between “isolated” silicon Si(4Al)(nP) and “nonisolated” silicon, Si(4Al)(xP,ySi), where x + y ) n, the number of next-nearest neighbors: Table 1 provides definitions of the various silicon species referred to in this work. These Si species present different 29Si MAS NMR peaks in zeolites and SAPOs, as has been experimentally observed in faujasite and SAPO-37, where, for example, the Si(4Al) signal moves from -84.7 to -89.1 ppm when the second T shell contains 9Si or 9P, respectively.53,54 In zeolites it was found that in order to have the strongest acidic sites, these sites must be separated by at least two T sites while in SAPOs the strongest acid sites are generated by the Si(1Al,3Si) environments.49,55 In SAPO-5, 29Si MAS NMR and CP NMR experiments have shown that under certain synthesis conditions the number of protons was equal to that of Si only up to xSi ≈ 0.007.14,30,56-58 Above this value, the measured number of protons falls from the 1:1 relationship. In other experiments it was found that at xSi ) 0.06, only around a half of the added Si result in Brønsted acid sites, which indicates the existence of silicon islands at such a silicon content.53 Thus, even at very low Si concentrations it is clear that structural and energetic factors can influence the mode of Si incorporation. 3.1.5. O-H Stretching Frequencies in SAPO-5 and SAPO34. IR spectroscopy is a widely used technique to characterize the acidity of SAPOs.59 Signals in the region 3520-3650 cm-1 are assigned to stretching of the Brønsted hydroxyls in bridging sites, tSiO(H)Alt. Since the extinction coefficient of the hydroxyl groups in SAPOs is unknown, IR spectroscopy suffers from the problem of obtaining reliable quantitative data on the number of different hydroxyls. This problem can be solved by using complementary techniques such as 29Si NMR. The study of the combination bands (stretching and out-of-plane bending vibrations) in the range 3900-4000 cm-1 has recently been shown to be useful in distinguishing the different kinds of hydroxyl in SAPO structures.60 A number of studies have addressed the influence of structure and chemical composition on the O-H frequencies.25,61-63 As the silicon content increases, up to a particular Si concentration, the O-H frequencies remain constant over a large range of silicon content.61 However, once silicon islands begin to form, the stronger acid centers located at the borders of these islands are created, as further demonstrated by hydrocarbon cracking experiments.25,62,63 It should be noted that the O-H frequency, although a useful guide, is not a true measure of the acidity,64 and therefore the effect of the chemical composition on the acidity cannot be always interpreted from the IR spectra. However, the effect of the structure on the vibrational properties can be more readily assessed and quantified. The frequency of the O-H stretch can be influenced by both the short- and longrange structural properties. The primary short-range effect is the influence of the SiO(H)Al angle, where, in aluminosilicates,

it is observed that hydroxyl frequencies decrease linearly as this angle increases.43 The long-range effect is related to the electrostatic potential generated inside the cavity in which the hydroxyl is located. It is believed that protons oriented into smaller cavities show a bathochromic effect,65 vibrating at lower frequencies as a result of interactions between the proton and the oxygens making up the cavity window. This shift occurs when hydrogen bonds are formed with oxygen atoms close to the proton, thus weakening the O-H bond. However, the relative importance of both long- and short-range contributions to the hydroxyl frequencies is not understood. In this paper we will address this problem and attempt to provide a better understanding of the acidity of the Brønsted acid centers in SAPOs. In particular, we will consider the effect of island formation on the acidity of these centers. 3.2. Energetic Considerations. Structural factors, both short and long range, have been shown to play an important role in explaining the catalytic activity of zeolite and SAPO structures.49,67,68 Incorporation of Si in an AlPO structure leads to some short-range structural deformations with respect to the original structure.11,39 These deformations depend on the flexibility of the structure and its topology and it is reasonable therefore to expect different energy changes on Si incorporation in different SAPO structures. In addition, some effect due to the ordering of the substituted silicon can be expected, as there must be an interaction between the substituted Si atoms, an effect which will differ depending on the relative distribution of Si throughout the structure. Finally, the energetics of substitution will be influenced by long-range ionic interactions which will be different in the different crystallographic positions of the structure and will differ from one structure to another. 3.3. Kinetic Considerations. The actual synthesis process will be controlled to a large extent by the kinetics of the reaction, rather than purely thermodynamic factors. The importance of kineticssand other variables such as choice of template and metal (or Si) concentrationsis clear from the large number of end products and compositional variants possible in such syntheses. However, the complex nature of the synthesis process makes characterization of the kinetics difficult. In our calculations, we assume that the thermodynamics of the system have a significant influence on the final structure of the SAPO, particularly relating to the location of the Si. We note that there is evidence for migration of protons and silicon within the SAPO structures on postsynthesis heat treatment,69 the effects of which are likely to be controlled by thermodynamic factors. Therefore, such a thermodynamically based treatment may still reveal many of the fundamental processes controlling both the final Si distribution in the SAPO structure and the chemical properties of the resultant Brønsted acid sites. 3.4. Topological Considerations. The topological density concept (TDC), developed by Barthomeuf,70 relates the number of isolated silicon atoms that can be found in a particular structure to the Si content. In this context, isolated silicon atom means a silicon tetrahedron that has no silicon neighbors either in the first or second coordination sphere of T atoms (see Table 1). This corresponds to an environment of the type Si(4Al)(nP), where n is the number of next-nearest neighbors; the value of n is characteristic of each structure. In SAPO-5 the isolated

SAPO-5 and SAPO-34 Molecular Sieves

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TABLE 2: Structural Properties of AlPO4-5 and AlPO4-34 AlPO4-5 coordination sequence (T1)a,b TD2-5c framework densitya (T/1000Å3) channelsa loop configuration of T atomsa

4, 11, 21, 35, 53, 77 0.2500 17.5 [001] 12, 7.3* see Figure 3

AlPO4-34

TABLE 3: Comparison of the Calculated and Experimental Structural and Cell Parameters in AlPO4-5 and AlPO4-34 (the Connectivity Is Shown in Figure 1)a

4, 9, 17, 29, 45, 64 0.2083 14.6 [001] 8, 3.8 × 3.8*** see Figure 4

a

Taken from ref 66. b Up to fifth coordination sphere. c Calculated from eq 6.

silicons are of the type Si(4Al)(11P), while in SAPO-34 they are of the type Si(4Al)(9P).66 According to TDC, there is a maximum limit to the Si content of a SAPO structure above which it is not possible to have all the Si isolated. This limit can be calculated according to the following equation:

xSi(limit) ) 2.26 × 10-2/TD2-5

(5)

where the constant factor is the same for all the SAPO structures. TD2-5 is the topological density calculated for the second to the fifth neighbor shell from the central T atom, as follows:

TD2-5 ) [number of tetrahedra in layers 2-5]/480 (6) the constant factor 480 being the theoretical maximum number of tetrahedra in layers 2-5. The lower the topological density, TD2-5, of the SAPO structure, the higher the silicon content where isolated Si environments are allowed to be formed. For SAPO-5 and SAPO-34 the numbers of tetrahedra in layers 2-5 are 120 and 100, respectively. This permits us to calculate using eq 5 the topological densities, TD2-5, and the maximum Si content at which all the Si can be present as isolated Si:

xSi(limit, SAPO-5) ) 0.090

(7)

xSi(limit, SAPO-34) ) 0.108

(8)

Of course, these limits do not preclude the possibility that Si can form nonisolated aggregates or silicon islands at lower values of silicon content. These values simply represent a topological limit above which some nonisolated SiO4 tetrahedra or silicon islands will unavoidably be found. In this paper we will consider the energetics involved in the incorporation of Si into the framework of a SAPO material. In particular, we will consider the effect of the different structures on the stability of small Si islands. We will also consider the influence of structure and Si concentration on the acidity of the Brønsted acid centers. 4. Results and Discussion 4.1. Minimization of AlPO Structures. The structures of AlPO4-5 and AlPO4-34 have been optimized using the GULP code. Some of the structural characteristics of these structures are summarized in Table 2. We should stress the fact that both structures have only one unique crystallographic cation position, and thus the geometry of a single TO4 unit characterizes the entire structure. The initial structures for the optimization were taken from crystallographic data,71,72 and a comparison between the experimental and calculated crystallographic parameters and geometries is shown in Table 3. The experimental and calculated P-O distances are in good agreement for AlPO4-5, although we note some deviation for AlPO4-34 where two (PO(3), P-O(4)) are overestimated by 0.02 Å and the other two are underestimated by around 0.09 Å. The calculated Al-O

distance (Å) P-O calc exp

O-Al calc exp

P-Al calc

angle (deg) POAl calc exp

PO(1)Al(1) PO(2)Al(2) PO(3)Al(3) PO(4)Al(4)

1.507 1.508 1.514 1.523

1.472 1.456 1.492 1.525

AlPO4-5b 1.721 1.729 1.696 1.700 1.722 1.726 1.722 1.683

3.101 3.202 3.120 3.128

147.6 176.3 149.1 149.0

150.2 178.1 148.8 151.0

PO(1)Al(1) PO(2)Al(2) PO(3)Al(3) PO(4)Al(4)

1.509 1.511 1.522 1.526

1.595 1.600 1.504 1.508

AlPO4-34c 1.724 1.722 1.731 1.676 1.720 1.691 1.724 1.752

3.107 3.073 3.128 3.122

147.8 142.8 149.5 147.6

144.3 146.1 151.2 147.4

AlPO4-5

cell parametersd

calc

exp71

a ) b (Å) c (Å) R (deg) β (deg) γ (deg)

13.83 8.55 89.98 89.99 119.98

13.71 8.43 90.00 90.00 120.00

AlPO4-34 calc exp72 9.29 9.29 94.84 94.84 94.84

9.37 9.37 94.67 94.67 94.67

a Lattice energies for the optimized structures: E (AlPO -5)/AlPO latt 4 4 unit ) -268.04 eV. Elatt (AlPO4-34)/AlPO4 unit ) -267.98 eV. b Oxygen atoms labeled as in ref 71. c Oxygen atoms labeled as in ref 72. d AlPO4-5, hexagonal system; AlPO4-34, rhombohedral system.

Figure 1. Connectivity of the unique crystallographic T position in AlPO4-5 and AlPO4-34 (primary building unit) showing the four crystallographically different oxygens in each structure.

distances are very close to the experimental values in both cases as are the POAl angles. The calculated cell parameters are slightly larger than the experimental values in the AlPO4-5 and slightly smaller in the AlPO4-34. The crystallographic angles R, β, and γ are very close to the experimental values in both structures. Although there are discrepancies in the bond distances in the case of AlPO4-34, we should note that the experimental data were collected on SAPO-34 since AlPO4-34 has not been prepared. We therefore expect that some of the discrepancy can be accounted for by this difference in composition. The lattice energies obtained are very similar to each other (as expected39,74), with AlPO4-5 being slightly more stable. We can rationalize the difference in stability in terms of framework density75 (Table 2), the more dense structure being more stable. 4.2. Silicon Incorporation in the Framework. 4.2.1. Formation of Single Brønsted Acid Sites. The formation of Brønsted acid sites proceeds through SM2 (PfSi,H), according to the equation

SM2:

[AlPO4] + Si4+ + H+ f [Si,H]P + P5+

(9)

Although there is only one crystallographically distinct site at which the Si will substitute, there are four distinct oxygens to which the proton can attach (Figures 1 and 2). In AlPO4-5, a proton attached to either O(1) or O(4) will point into the 12-

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Figure 2. Fragments of the structures of (a) AlPO4-5 and (b) AlPO434 showing the relative position in the structure of the four different crystallographic oxygens. Also shown are the T atoms considered in section 4.2.2 for the study of the interaction between two Brønsted acid sites (T1,T2,T3,T4); see also Table 6.

TABLE 4: Defect Energies (eV) for Silicon Incorporation According to the Mechanism PfSi,H (SM2) in AlPO4-5 and AlPO4-34; Proton Location at the Four Oxygen Sites Shown in Figure 2 E1Si-disp(O1) E1Si-disp(O2) E1Si-disp(O3) E1Si-disp(O4)

AlPO4-5a

AlPO4-34b

59.06 59.15 58.87 58.90

58.94 59.09 59.13 58.92

a Oxygen atoms labeled as in ref 71. b Oxygen atoms labeled as in ref 72.

membered ring (MR) and on O(2) or O(3) will point into the interior of the six MR (Figure 2a). In AlPO4-34, a proton attached to either O(1), O(2), or O(3) will point into the eight MR, while if attached to O(4) will point into a six MR (Figure 2b). The different defect energies are shown in Table 4. First we should stress that the magnitude of these energies which refer to the process given in eq 9 is a consequence of the fact that they form only part of a thermodynamic cycle. The entire (Born-Haber) thermodynamic cycle must consider, among other things, the source of silicon for the process.11,76 Construction of such cycles allow an expression to be formed in terms of the source of substitutional atoms, the structure formed, and the resultant formation of a suitable “sink” for the substituted atoms. This is difficult in this case due to the substitution of P, which would require the formation of a suitable phosphorus species, ideally in this case H3PO4. However, the force field parameters have been fitted to AlPO4 structures and are therefore inconsistent with the formation of the phosphoric acid species. For our present purposes it is, however, the differences in defect energy that give a reasonable estimate of the relative energetics of this process in the different structures and on the different oxygens. The results of the calculations (Table 4) show that there is some variance in substitution energy (PfSi,H), depending on which of the four different oxygens the proton is sited: these energies differ by 0.28 eV in the case of AlPO4-5 and 0.21 eV in the AlPO4-34. In terms of the thermodynamic stability of the structure, these differences are significant when compared to those expected from thermal fluctuations (kT is approximately 0.05 eV at room temperature) during synthesis and therefore may suggest that some preferential siting of protons in these systems is possible. We should note that the process of calcination, during which the template is removed and protons introduced are carried out at elevated temperatures (ca. 800 K). Furthermore, it is known that both Si and protons can migrate at such temperatures,69 which may further “average out” any preferential siting, and from absorption studies it has

Figure 3. Energy-minimized structure of SAPO-5 after substitution of a P atom with a Si and bridging proton: (a) distances in angstroms; (b) angles in degrees. Note that the oxygen atoms located between the Al, P, and Si sites are omitted for clarity. Each distance is given from the T atom to the bridging oxygen.

been concluded that protons oriented into the 12-T ring, O(1) and O(4), are equally populated with those which are orientated into the 6-T ring. Furthermore, as mentioned in the Methodology section, we note that in SAPO-37 the potential parameters used do not correctly predict the experimentally observed proton site populations.39,46 Thus, we are uncertain as to whether these calculations can give any indication of proton siting in these materials. An analysis of the geometries of the local acid site has been performed first at the level of the next-nearest neighbors of the substituted P atom. Reasonably similar geometries are observed for all the sites calculated. Here, we present results for the PO(4)Al f SiO(4)(H)Al in AlPO4-5 and the PO(2)Al f SiO(2)(H)Al in AlPO4-34. These two sites will be used in our later calculations on clustering and island formation. As can be seen from the comparison between the geometry of the final structures (Figures 3 and 4) and the initial AlPOs (Table 3), the geometry of the local vicinity of the acid sites changes remarkably as a consequence of the new Si-O-Al linkages, while the geometry in the next-nearest neighbors remains practically unchanged. Thus we can conclude that the incorporation of a single silicon into the framework affects only the first neighbors of the replaced P atom. The Si-O distances in the final structures are 1.55-1.57 and 1.77-1.78 Å in the case of Si-O(H), compared to P-O distances of 1.51-1.52 Å in the original structure. Similarly, the Al-O distances in the final structures are 1.66-1.69 and 1.82 Å in the case of O(H)-Al, compared to an average of 1.72 Å in the original AlPO. The same conclusions can be drawn with regard to the TOT angles, with distortions only being significant in the first neighboring shell of the substituted site (Table 3 and Figures 3 and 4). The

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Figure 5. Optimized angles around of the Si environment after a PfSi,H replacement in SAPO-5 (above) and SAPO-34 (below). The protons are located on (a) O(1), (b) O(2), (c) O(3) in SAPO-5 and (d) O(1), (e) O(3), (f) O(4) in SAPO-34. Also shown are Si-O-Al angles which can be compared with the original P-O-Al angles given in Table 3. O(4) in SAPO-5 and O(2) in SAPO-34 are shown in Figures 3 and 4.

Figure 4. Energy-minimized structure of SAPO-34 after substitution of a P atom with a Si and bridging proton: (a) distances in angstroms; (b) angles in degrees. Oxygen atoms are omitted as in Figure 3.

TABLE 5: T-T Distances (Å) of the First Neighbors of the Substituted Silicon after the Replacement PfSi,H (SM2) in SAPO-5 and SAPO-34 Structures Compared with the Original T-T Values (in AlPOs) before the Substitution

T-Al(1) T-Al(2) T-Al(3) T-Al(4) a

AlPO4-5 (T ) P)

SAPO-5a (T ) Si)

AlPO4-34 (T ) P)

SAPO-34b (T ) Si)

3.101 3.202 3.120 3.128

3.111 3.214 3.203 3.351

3.122 3.107 3.073 3.128

3.358 3.099 3.050 3.122

Hydrogen location on O(4). b Hydrogen location on O(2).

local distortions appear to have a greater effect on the TOT angles than on the T-O distances, as a consequence of the substituent atoms having to be accommodated in a constrained space. Deformations in the bond angles will require less energy than changes in bond lengths. This conclusion is further supported by the fact that the calculated Si-Al distances after the silicon incorporation are similar to the P-Al distances before the substitution (Table 5). Furthermore, we note that the geometry changes are very similar regardless of which of the four oxygens are protonated to compensate for the Si substitution as illustrated in Figure 5. As well as the influence of the local distortions on Si substitution, which are controlled by short-range interactions, we have also investigated the influence of the long-range electrostatic potential on the energy of the process. This factor can differ significantly between structures, and to investigate this effect the electrostatic site potentials in both structures have been calculated, using unit cells of each structure containing 12 and 24 T sites in AlPO4-5 and AlPO4-34, respectively. The results obtained (Figure 6) show only small differences in the potential at the different P and Al atoms because of the small

differences in the geometry of the TO4 units after optimization. The main conclusion of this calculation is that the electrostatic site potentials are practically the same in the two structures, AlPO4-5 and AlPO4-34. This result helps to explain the similarities between the defect energies of the PfSi,H process in the two structures considered. Furthermore, we can conclude that differences in substitution energies between the two structures will be primarily due to the ability of each structure to distort locally in order to accommodate the substituting silicon. These results are important in understanding the nature of Si incorporation from the point of view of the synthesis, as this result suggests that the energetics of silicon incorporation is not structure sensitive, at least in the cases of SAPO-5 and SAPO-34. 4.2.2. Interaction between Two Neighbor Brønsted Acid Centers. Here we consider the substitution of two P sites with Si according to SM2:

SM2: [AlPO4] + 2Si4+ + 2H+ f [2Si,2H]P + 2P5+ (10) When the silicon content in the SAPO structure is low, it is considered that acid centers will be homogeneously distributed throughout the framework. However, above a certain silicon concentration, the acid centers will be separated from each other by four TO4 tetrahedra, giving rise to the so-called “isolated silicon” (Si-Al-P-Al-Si units). This ordering in the structure can be detected by 29Si MAS NMR,12,18 and the Si concentration at which ordering occurs will depend on the topology of the structure.70 Further increases in the silicon content leads to the formation of either “nonisolated silicon” species (Si-Al-Si units) or silicon islands. Therefore, the investigation of the interaction energy between two acid centers can shed light on the feasibility of Si ordering in SAPO structures. We have therefore calculated the formation energy of two acid centers at different proximities. In these calculations, the protons were attached to O(4) in AlPO4-5 and to O(2) in AlPO4-34. In this way the differences in energy between the various configurations can be attributed to the interaction between centers and not to the type of oxygen to which the hydrogen is linked. In both sites the protons are directed into the largest cavity in the framework and as such are likely to be the most active catalytically. We have considered the cases where the acid sites are separated by two and four tetrahedra. The different distributions of pair acid centers considered in the calculations are shown in Figure 2, and the results of the defect energy calculations are given in Table 6. In AlPO4-5 it can be seen

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Figure 6. Electrostatic potential (in volts) in the T sites, (a) for Al and (b) for P, calculated in the optimized AlPO4-5 (solid line) and AlPO4-34 (dotted line) structures. The unit cells contain 12 and 24 T atoms in AlPO4-5 and AlPO4-34, respectively.

TABLE 6: Energies (eV) for the 2Si and 3Si Incorporation According to the Mechanism PfSi,H (SM2) in AlPO4-5 and AlPO4-34;a Positions for the Substituted Silicon Sites Are Shown in Figures 2 and 8 AlPO4-5

AlPO4-34

2Pf2Si,2H

E2Si-dispb

Eintc

E2Si-dispb

Eintc

T1-T2 T1-T3 T1-T4

117.75 117.65 117.63

-0.05 -0.15 -0.17

118.22 118.08 118.17

0.04 -0.10 -0.01

AlPO4-5

SM2: [AlPO4] + 3Si4+ + 3H+ f [3Si,3H]P + 3P5+ (12)

AlPO4-34

3Pf3Si,3H

E3Si-dispd

Einte

E3Si-dispd

Einte

T1-T2-T3 T1-T3-T4

176.71 176.99

0.01 0.29

177.40 177.31

0.13 0.04

a O(4) and O(2) proton sitings for SAPO-5 and SAPO-34, respectively. b See eq 10. c See eq 11. d See eq 12. e See eq 13.

that the three possibilities considered gave a similar defect energy. The highest energy results, not unexpectedly, when the two neighbor silicons are located in a four MR which is the most rigid ring structure present in the framework. The other defect energies considered are practically the same, corresponding to the two silicons separated by one and three TO4 tetrahedra, respectively. This suggests the possibility of a similar distribution of isolated (Si-Al-P-Al-Si) and nonisolated (Si-AlSi) silicon species in SAPO-5 at low silicon contents. The results for SAPO-34 offer similar conclusion, as the differences in the defect energies in the three cases are within 0.14 eV, suggesting again that no preferred silicon ordering occurs at low silicon content in the SAPO-34 structure. The interaction energy between the two sites was calculated according to the following equation:

Eint ) E2Si-disp - 2E1Si-disp

for Si clustering of at low silicon contents. Such clustering may be observable by 29Si NMR experiments. However, we should note that the energy differences are not much higher than thermal fluctuations, and hopping of the protons may occur at higher temperature resulting in a reduction in this clustering. 4.2.3. Interaction between Three Neighbor Brønsted Acid Centers. We now consider the interaction between three Si Brønsted acid centers:

(11)

The results obtained in Table 6 for E2Si-disp combined with the results for E1Si-disp in Table 4 (O(4) in AlPO4-5 and O(2) in AlPO4-34) allow us to calculate the interaction energy, Eint, which is generally negative (Table 6), which indicates a slightly higher stability when the two acid centers are close neighbors rather than being at infinite separation, suggesting a tendency

At low Si concentrations, before the formation of the smallest of the silicon islands (5Si island, containing three acid sites) in SAPO structures, it has been proposed30 that aggregations of, at least, three Brønsted sites (as shown in Figure 7) can occur, since it is proposed that the formation of Si-O-P linkages within the SAPO structure is energetically unfavorable, a conclusion supported by our recent calculation on SAPO-5.11 After such an aggregate is formed, further substitution of adjacent Al and P sites (SM3) at the boundaries of the (3Si,3H) aggregate (Figure 7) would result in a 5Si island. We have performed calculations on such three acid-site aggregates, allowing us to investigate their relative stability in both structures and hence comment on the feasibility of silicon island formation by this mechanism. Figure 8 illustrates the structure of the silicon configurations which lead to the formation of a 5Si islands by this mechanism in the two structures considered. The loop configuration of T atoms in both structures66 permits the arrangement of a three-silicon aggregate in two different ways around a central Al site, with the three silicon atoms being first neighbors of that Al. In the case of SAPO-5 (Figure 2a) either one or two silicons can be located in the 12 MR, of which the central Al is also a member, which is also the case in SAPO34, except here the ring is an 8 MR (Figure 2b). The results obtained for the defect energies (Table 6) show appreciable differences between the two different configurations: 0.28 and 0.09 eV in SAPO-5 and SAPO-34, respectively. These differences can be attributed to local contributions arising from the local deformation of the structure after the silicon incorporation. The corresponding interaction energies, calculated according to

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J. Phys. Chem. B, Vol. 101, No. 27, 1997 5257

Figure 8. Loop configuration scheme of SAPO-5 and SAPO-34 labeling the T atoms to be substituted by Si in the study of the interaction between three neighboring Brønsted sites. The results of that study are shown in Table 6. Figure 7. Schematic of the formation of a 5Si island in SAPO-5. (a) The island can be formed by further substitution into an environment containing three Brønsted sites arranged in the vicinity of a Al-P unit. (b) However, silicon islands cannot be formed when these Brønsted sites are absent due to the formation of Si-O-P linkages. The same considerations apply to SAPO-34.

the equation

Eint ) E3Si-disp - 3E1Si-disp

(13)

are listed in Table 6. We find that this interaction energy is positive in each case, suggesting that three separated Brønsted sites are more stable than the cluster configuration. From this result we conclude that these “preisland” aggregates will only form at relatively high silicon contents, that is, when the formation of more, energetically favorable, isolated species can no longer occur. The reason for the instability of the 3Si clusters may be the increased strain in the structure on the clustering of additional Si: the framework is flexible enough to accommodate 2Si at adjacent P sites but not three. The formation of islands will reduce the strain due to the substitution of the smaller Al site, with the resulting Si-O bonds being close to an average of the Al-O and P-O bond lengths. We will now consider the formation of such small Si islands for whose existence there is experimental evidence. 4.2.4. Formation of 5Si and 8Si Islands. When the Si concentration is sufficient, Si islands containing five and eight Si species can form, through a combination of SM2 and SM3 mechanisms:

SM2+SM3: [AlPO4] + 5Si4+ + 3H+ f [5Si,3H]Al,4P + 4P5+ + Al3+ (14)

SM2+SM3: [AlPO4] + 8Si4+ + 4H+ f [8Si,4H]2Al,6P + 6P5+ + 2Al3+ (15) The smallest silicon island that can be formed contains five Si atoms (Figure 9), which can nucleate from a 3Si aggregate as shown in Figure 8a. From the two possible configurations for the 3Si aggregates considered above (Figure 7), two different 5Si islands can be considered. In one, the acid sites are located close to the T1, T2, and T3 atoms, while in the other case the acid sites are close to the T2, T3, and T4 atoms (the T site notation being the same as that used in Figures 2 and 8). The defect energy calculations show (Table 7) that the two types of 5Si island have similar energies, and hence the location of the acid centers does not significantly affect the stability of the island. Moreover, the 5Si island has the same energy in SAPO-5 and SAPO-34. However, when we consider the 8Si island (Figure 9), we calculate different defect energies in the two structures (Table 7) with the energy to form the 8Si island being higher in SAPO-34. This important result suggests that it is easier to form Si islands in SAPO-5 than in SAPO-34 provided that the concentration of 8Si islands is significant. This appears to be the case experimentally where small islands appear to be favored.46 We conclude that the relative energetic stability of an 8Si island, with respect to isolated Si, is higher in SAPO-5 than in SAPO-34 and consequently that the formation of Si islands will occur at a lower Si content in SAPO-5 than in SAPO-34. This conclusion is fully supported by a number of experimental results.16,24,47,50,58 Analysis of the structures of both materials after the formation of an 8Si island shows that the strain in the 4Si ring is more significant in SAPO-34 than in SAPO-5. The flexibility of the 4Si ring in SAPO-5 allows the formation of the island to be more energetically favorable than in SAPO-34. A further conclusion that we can draw from these results is

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Figure 10. Two-dimensional representation of a SAPO with all the Si atoms being “isolated silicon”, giving an “all P environment” in the next-nearest-neighbor shell of a given Si atom.

4Si around a previously included Si. Therefore, we are considering 5Si island formation by the following process:

SM2: [AlPO] + nSi4+ + nH+ f [SAPO]

Figure 9. Scheme representing the Si environments after the formation of a 5Si (left) and 8Si (right) islands in SAPO-5 and SAPO-34.

TABLE 7: Energies (eV) Corresponding to the Formation of Silicon Islands and “Isolated Si Distributions” (Eqs 14-17) in AlPO4-5 and AlPO4-34 Si conformation

AlPO4-5

AlPO4-34

(T1,T2,T3)a

188.46 188.46 258.13 589.99 59.00 (n ) 10) 129.63

188.42 188.42 259.39 708.42 59.04 (n ) 12) 129.52

E5Si-isl E5Si-isl (T2,T3,T4)a E8Si-isl (Figure 9)b Eisol-Si-dist (Figure 10)c (Eisol-Si-dist)/n (Figure 10)c E#5Si-isld

a Location of the Brønsted acid sites indicated in Figure 8. Equation 14. b Equation 15. c Equation 16. d Equation 17. Note the difference between E#5Si-isl and E5Si-isl: the former is the energy to form a 5Si island in a lattice which already has a single Si subsitutional (i.e., four Si are added), while the latter is the formation of a 5Si island in a pure AlPO structure.

that topological differences are only significant if the island itself extends beyond the first Si neighbor shell. Thus, while the stability of 5Si islands does not appear to be influenced by different topologies, 8Si islands exhibit different stabilities in different topological environments. A more sophisticated approach should take into account the fact that these islands, as noted in section 4.2.3, are formed only after a certain silicon content is reached in the framework. That is, we should consider the addition of Si to a framework which is already Si-substituted. We have therefore modeled island formation in a SAPO structure which already contains isolated Si environments (P-Al-Si-Al-P) (Table 1). A twodimensional schematic representation of this environment is shown in Figure 10. Silicon was initially introduced into AlPO4-5 and AlPO4-34 supercells containing 96 T (Al,P) atoms, so that the maximum amount of silicon was incorporated without forming Si-Al-Si environments. Ten Si atoms were introduced in AlPO4-5 and 12 Si into AlPO4-34, giving silicon molar fractions of 0.104 (10/96) and 0.125 (12/96), respectively. The Si incorporated was introduced according to SM2 with the electroneutrality of the framework being maintained by the formation of acid centers. Consequently, the formation of 5Si islands can now be considered as the incorporation of a further

SM2+SM3: [SAPO] + 4Si4+ + 2H+ f [4Si,2H]Al,3P + 3P5+ + Al3+

Eiso-Si-dist (16) E#5Si-isl (17)

First, we consider the first stage (eq 16), forming SAPOs with an “isolated silicon distribution” (as shown in Figure 10). The difference in energy between the SAPO and parent AlPO structure, Eiso-Si-dist, is a measure of the energy necessary to incorporate silicon into the AlPO framework, and if we normalize this value to the number of silicons incorporated, we have an estimate of the average energy required per silicon atom to create an “isolated silicon distribution” (as defined in Table 1) which can be compared in different structures. The average energy per silicon atom to create the isolated Si SAPO (Table 7) is practically the same in the two structures. Two conclusions can be drawn from these results. First, we see once more that the structural differences do not play significant role in the energy required to incorporate Si at these levels. Second, since the average substitution energy per Si is very similar to the defect energies calculated for infinitely dilute Si substitution (Table 4), we conclude that medium-range interactions are not significant, thus supporting our previous conclusion (section 4.2.1) that little structural deformations of the next-nearest neighbors of the substituted atom is required to incorporate Si into a framework site. In the SAPOs generated for these calculations the Si atoms are separated by three T (T ) Al, P) atoms (Figure 10), and thus the structural deformations do not interfere with each other, each Si site having little structural influence on its neighbor. We now consider the second stage of island formation, shown in eq 17, where the SAPO supercells with an “isolated Si distribution” were used as an initial structure to generate 5Si islands in SAPO-5 and SAPO-34. Here, as the initial structure contains Si, we incorporate another four Si atoms to generate the 5Si island. The defect energy is denoted as E#5Si-isl to differentiate it from the energy to form a 5Si island in an AlPO, E5Si-isl. The results in Table 7 show that the energy required to form a 5Si island by this mechanism is almost the same in both structures. Thus, we draw the same conclusion as for the previous calculations (eq 14) for the formation of a 5Si island in the AlPO structure. We further conclude that there will be no difference in the energy of forming 5Si islands in the two

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TABLE 8: O-H Stretching Frequencies (cm-1) and SiO(H)Al Angles (deg) of Different Brønsted Sites in SAPO-5 and SAPO-34 SAPO-5 Si sites O(1)-H O(2)-H O(3)-H O(4)-H

ν(O-H)

SiO(H)Al

SAPO-34 ν(O-H)

Isolated Brønsted Site (Eq 9, Figure 1) 3739.0 142.2 3764.5 3723.3 152.8 3753.0 3767.5 136.2 3780.7 3786.4 136.9 3758.1

SiO(H)Al 138.5 138.4 135.2 139.4

Two Brønsted Sitesa (Eq 10, Figure 1) T1-T2 ν1 ν2 T1-T3 ν1 ν2 T1-T4 ν1 ν2

3787.2 3801.0

149.4 149.0

3747.8 3748.2

138.4 138.5

3776.2 3796.6

138.1 134.7

3775.0 3792.2

140.3 137.2

3797.3 3810.0

136.9 134.5

3762.8 3769.7

138.4 138.4

ν1 ν2 ν3

5Si Islanda (Eq 14, Figure 9) 3723.6 133.5 3735.2 3746.1 133.7 3737.0 3756.8 140.3 3738.7

138.2 140.1 140.0

ν1 ν2 ν3 ν4

8Si Islanda (Eq 15, Figure 9) 3675.3 138.8 3725.2 3686.6 138.9 3725.7 3702.9 139.9 3726.4 3708.2 139.6 3727.8

139.7 139.7 140.9 140.9

a The protons are bonded to O(4) and O(2) in SAPO-5 and SAPO34, respectively.

structures at Si concentrations ranges of 0-10/96 and 0-12/ 96 in SAPO-5 and SAPO-34, respectively. 4.2.5. Effect of Si Incorporation on the Acidity of the Brønsted Acid Sites. We now consider the influence of Si concentration and structure on the acidity of the resulting Brønsted acid sites in the SAPO materials. We will consider two measures of acidity: first the stretching frequency of the O-H and second the proton affinity of the framework oxygen. Two ν(O-H) stretching frequencies are normally observed in SAPO structures. In SAPO-577,78 the bands are observed at 3620 and 3525 cm-1 (and sometimes an additional band is observed at 3618 cm-1), and in SAPO-3424 they appear at 3626 and 3605 cm-1. The bands near 3620 cm-1 are normally assigned to undisturbed SiO(H)Al groups while the bands close to 3520 cm-1 are assigned to O-H groups interacting with nearby framework oxygens.24 No correlation between the local deformations of the SiO(H)Al bridge and the hydroxyl stretching frequency has been established to date in the experimental literature although some theoretical work39,43 considered such a correlation. In other words, the differences in the O-H stretching frequencies are classically interpreted in terms of medium-long-range interactions between the proton site and the surroundings. Here we will attempt to determine the effect of both short- and long-range structural properties on the acidity of both SAPO-5 and SAPO-34 in terms of their ν(O-H) stretching frequency. We consider first the vibrational properties of a single Brønsted site. The four proton sites (attached to the four crystallographically distinct oxygens) were considered in SAPO-5 and SAPO-34. The results for the defect calculations corresponding to the SM2 process described in eq 9 are shown in Table 8. We note first that the absolute values of the O-H stretching frequencies, ν(O-H), are too high in all cases. The shift in frequency is a consequence of the use of the harmonic model to calculate the vibrational spectra. A previous study,43 using the same potentials as those used here, determined that

there is a shift of around 150 cm-1, in aluminosilicates, as a consequence of this approximation. If the same anharmonicity correction is applied to the frequencies calculated here, they are in reasonably good agreement with experiment, although the need to refine the current force field is clear (as previously noted by Henson et al.39). However, despite errors in the absolute values calculated, relative differences in frequency between different environments should be accurate. Our calculations show (Table 8) a range of 63 cm-1 in the O-H frequencies in SAPO-5 and 28 cm-1 in SAPO-34, which qualitatively resembles the range observed experimentally (95 and 20 cm-1, respectively).24,77,78 More importantly, the frequencies correlate well with the value of the SiO(H)Al angle in both SAPO-5 and SAPO-34, decreasing as the angle increases. In SAPO-5, if the medium-long-range effects contributed significantly to the value of ν(O-H), we would expect ν(O2-H) and ν(O3-H) to be at the lower frequencies since these protons will interact more strongly with neighboring framework oxygens, being in the smaller cavities. Although ν(O2-H) is low, ν(O3-H) (which also vibrates inside the 6-T ring) is higher than expected, suggesting that hydroxyls in the small cavities do not necessarily result in low stretching frequencies. Absorption studies also suggest that the proton vibrating in the 12-T ring are those at the higher wavenumber.23 Although the average wavenumber for the 12-T ring protons is higher than for the 6-T protons, there again does not appear a strict correlation between ring size and subsequent vibrational frequency. It is clear, therefore, that we cannot unambiguously separate the short- and long-range interactions, and therefore the value of SiO(H)Al will also contribute to this result. The results for SAPO-34 show a narrower range in hydroxyl frequencies which correlates with the narrower range of SiO(H)Al angles in this structure. Again, no bathochromic effect is observed (in SAPO34) for ν(O4-H), which we would expect to vibrate at lower frequencies due to its orientation into a double 6-T ring. We conclude from these results that the short-range (i.e., the bond angle) effects are the dominant factor in determining the relative stretching frequencies of isolated acidic protons in the SAPO environment. However, this will of course be influenced to a degree by the topology surrounding the Si site (i.e., type of ring size). We will consider below the influence of long-range electrostatic interactions below. We next calculated the vibrational spectra for two Brønsted sites generated according to the SM3 process described in eq 10. The various possibilities are shown according to the nomenclature used in Figure 2. These calculations were aimed at studying the influence of the vicinity of acid sites on their stretching frequencies. Such an influence has been demonstrated for zeolites61 and has been related to the variation in the strength of the acid sites with the aluminum content. In SAPOs, although, as discussed above, acid sites located at the borders of silicon islands appear to be stronger,25,62,63 there is little discussion of the variation of hydroxyl frequencies with the silicon content. Table 8 gives the vibrational frequencies for the O-H stretch when two adjacent T-sites are substituted with Si, the charge-compensating protons being attached to O(4) and O(2) in SAPO-5 and SAPO-34, respectively. In both structures, the correlation between O-H frequencies and SiO(H)Al angle is less clear than for single acid sites, and other factors appear to influence the frequency shift with respect to the isolated Brønsted sites. In SAPO-34, when the silicons are in the sites T1 and T2, the two acid sites are located in different sized cavities. Since the O-H frequencies are very similar to those for the proton in the isolated site, it would therefore appear that

5260 J. Phys. Chem. B, Vol. 101, No. 27, 1997 medium-long-range interactions are not significant. On the other hand, when the two protons are located in the same cavity (T1-T3 and T1-T4 calculations), there is a shift to higher frequencies which may be due to changes in the electrostatic potential in the cavity when two hydroxyl groups, instead of one as in the previous case (single acid site), are present. Such a modification may effectively shield some of the protonframework oxygen interactions, resulting in a strengthening of the O-H bond. In the case of SAPO-5 the shielding effect is not as apparent, probably as a result of the protons being present in larger cavities (12-rings), with consequent weaker protonframework oxygen interactions. The frequency shifts, in this case, are more difficult to interpret, partly as a result of a combination of both short- and long-range interactions. The balance between short- and medium-long-range contributions to the ν(O-H) shifts is difficult to estimate quantitatively although the significant contribution of the short-range effects, particularly SiO(H)Al angles, is clear form these results. Next we considered the O-H frequencies of acid sites at the borders of silicon islands. The results shown in Table 8 correspond to the processes described in eqs 14 and 15 which give rise to 5Si and 8Si islands. A previous analysis of the geometrical variations in the structure induced upon the formation of silicon islands showed us that the structural strain is reduced by the formation of silicon islands, due to the fact that SiOSi bridges (present in silicon islands) have an equilibrium bond angle that is closer to that in the original AlOP bridge than that in SiOAl and SiO(H)Al. However, the acid sites at the borders of silicon islands have typical AlO(H)Si angles of ca. 140° and are thus in a more distorted environment. The results (Table 8) show that the stretching frequencies shift to a considerably lower value than those of the corresponding isolated acid sites, as is observed experimentally.25,62,63 Thus, if we take these frequencies as a relative measure of acidity, the experimental observation25,62,63 regarding the stronger acidity of the protons at the border of the islands is reproduced. Furthermore, although the range of islands considered is small, there appears to be a correlation of acid strength and the size of the island. We would therefore expect that larger islands to shift the O-H stretching frequencies further, tending toward the values found in aluminosilicates. We have seen that the exact nature of the silicon sites considered has a major influence on the acidity (as measured by ν(O-H)) with an apparent correlation with bond angle for isolated sites, while there is little correlation with bond angle in islands. Such a discrepancy may be a result of the changes in electrostatic potentials which will be found when islands are formed. To estimate the effect of these long-range electrostatic interactions on the acidity, we have determined the electrostatic potential at the proton site in both isolated and island configurations (Table 9). Considering first the potential at isolated Brønsted acid sites, we find little correlation between potential and O-H stretching frequency. We can therefore conclude that for such isolated sites it is, as discussed above, the bond angle which is the dominant influence on the frequency. Furthermore, given that the electrostatic potential will depend critically on the topology of the structure, we conclude that little correlation can be expected between the cavity size and the resulting frequency. Considering next the change in potential on formation of islands, we find, in general (Table 9), that the formation of islands leads to a decrease in the potential which is consistent with the increased acidity of the island protons. Furthermore, the potential is reduced further with an increase from 5Si to 8Si islands, again correlating with experimental evidence that larger islands give rise to more acidic centers. Summarizing,

Sastre et al. TABLE 9: Electrostatic Potential in the Proton Site of Different Si Environments; Protons Are Sited on O(4) and O(2) Sites in SAPO-5 and SAPO-34, Respectively, in the Islands electrostatic potential (V) Si site isolated Brønsted site O(1) O(2) O(3) O(4) 5Si island site 1 site 2 site 3 8Si island site 1 site 2 site 3 site 4

SAPO-5

SAPO-34

-10.62 -10.49 -11.00 -10.88

-10.72 -10.61 -10.53 -10.73

-10.59 -10.64 -10.92

-10.28 -10.20 -10.34

-10.03 -10.06 -10.46 -10.48

-9.79 -9.79 -10.15 -10.15

TABLE 10: Proton Affinities of Acid Sites in SAPO-5 and SAPO-34; Calculated According to Eq 18 Ep (kJ mol-1) species

SAPO-34

SAPO-5

infinitely dilute Si 5-island 8-island

923 906 819

921 907 782

therefore, it is apparent that for isolated acid centers in SAPOs that the acidity is correlated to the Al-O(H)-Si angle, while in islands it is the change in electrostatic potential which has the major influence. However, the bond angle, the electrostatic potential, and the topology of the structure are all correlated, and we cannot unambiguously assign frequency shifts to particular geometry changes. We can also consider the proton affinity as a measure of the acidity of bridging hydroxyl. The proton affinity can be estimated by calculating the energy required to remove a proton from the framework oxygen to the gas phase

OH(SAPO) f -OH(g) f O2-(g) + H+(g) f O2-(SAPO) + H+(g)

-

which can be expressed as

EP(SAPO) ) E1p - Edef

(18)

where Edef is the change in the lattice energy on removing a proton from the lattice, and E1P is the first proton affinity of O2- which has been estimated from thermochemical data to be 26.29 eV.79 Table 10 shows the results for isolated and island acid centers in both SAPO-5 and SAPO-34. For both materials these results mirror those obtained from vibrational frequency calculations, with the proton affinity decreasing, indicating that the proton is more readily removed, i.e., being more acidic in nature, with increasing island size. With 5Si islands we note that the proton affinity is similar in the two structures (as is the case for isolated Si), which correlates with the fact that the islands are of similar stability in the two structures (Table 7). However, we note a larger difference between the proton affinity in the two materials for the 8Si island, with SAPO-5 being more acidic, again correlating with the variations in the calculated vibrational frequencies. Although we have discussed the greater stability of the 8Si island in SAPO-5, this result in itself does not explain the differences in proton affinity since the stability of the deprotonated species is also involved. In the case of the 8Si island, we find that the anionic species (the 8Si island after a proton is removed) is stabilized to a greater extent in SAPO-5

SAPO-5 and SAPO-34 Molecular Sieves than in SAPO-34. The difference in energy between the 8Si island in SAPO-5 and SAPO-34 is 122 kJ mol-1 while for the anionic island species it is 160 kJ mol-1. In contrast, there is little difference in the stability of the deprotonated 5Si island in the two structures. However, we should note that these calculations are at the limit of our current computational capacity. In particular, we have not been able to increase the size of both region I and region II in these Mott-Littleton calculations (see Methodology section) to confirm that the calculations are fully converged with respect to these region sizes. Nevertheless, although the absolute magnitude of the proton affinities may change with increasing region size, we do not expect the trends noted to be altered. Future calculations with larger computational facilities will resolve the uncertainties in these values. Both measures of acidity considered here correlate well with experimental observations of acidic properties. Furthermore, we show that it is the long-range electrostatic contribution which is the dominant factor in the increasing acidity noted on island formation. Therefore, it is clear that topological differences have a major influence on the acidic properties of the SAPO materials. 5. Conclusions We have calculated the energy of the formation of isolated Brønsted acid centers according to the mechanism SM2 (PfSi,H). The deformations produced in the structure after the substitution have proved to be of short range, up to the first T coordination sphere, leaving the rest of the structure practically unperturbed. For this reason the structural differences between SAPO-5 and SAPO-34 do not influence this energy, and the formation energies of isolated Brønsted acid sites are the same in both structures. Furthermore, we have shown that the electrostatic site potentials, which comprises a major proportion of the substitution energy, are almost identical in the AlPO4-5 and AlPO4-34 structures. The preferred conformation of two neighboring Brønsted acid sites has been calculated in a range of different configurations. The highest energy results when the two neighbor silicons are located in a 4 MR. The energies of isolated Si (Si-Al-PAl-Si) and nonisolated Si distributions (Si-Al-Si) are very similar and in both SAPO-5 and SAPO-34. Moreover, these Si configurations are lower in energy than those for a distribution in which the two Si atoms are widely separated, suggesting that a certain clustering tendency will be observed in both structures at low Si contents. Turning to three neighbor Brønsted acid centers, we considered two different arrangements; the first is denoted the “preisland” conformation because it corresponds to the necessary configuration of Si atoms to produce an island if more Si is incorporated into the structure. The results show that these conformations are less stable than a more separated distribution of these three Brønsted sites which seems to indicate that the “pre-island” will only form if no other, more dispersed, configuration is possible due to the amount of Si present in the structure. The strain on the framework as a result of the formation of such a “pre-island” configuration cannot be accommodated as readily by the framework, contrary to that for the two-site cluster. This result can help to explain experimental data which show that silicon islands are only formed after a certain silicon content is reached in the SAPO. The formation of 5Si and 8Si islands has been simulated, and our results show that the same energy is required to form the 5Si island in SAPO-5 and SAPO-34. On the other hand, the 8Si island gives a considerably different energy in both structures being more favorable in SAPO-5 than in SAPO-34,

J. Phys. Chem. B, Vol. 101, No. 27, 1997 5261 thus explaining why it is observed experimentally that the silicon islands start to form at lower silicon contents in SAPO-5 than in SAPO-34. The main source of the difference is, we believe, the strain introduced into the structure of SAPO-34 as a consequence of the substitution of four Si atoms into a fourmembered ring. Thus, we believe that 8Si islands are less likely to form in SAPO-34 than in SAPO-5. These results also indicate that topological differences become important once the island extends beyond the first Si shell, that is, for clusters larger than a 5Si island. Thus, low Si SAPOs of different structures will possess similar acidic properties, but if the Si content is such that island formation is promoted, then the acidic characteristics will become strongly dependent on the topology of the material. Entropic terms will further stabilize larger islands due to the increased number of possible island configurations; again, topological considerations will be a major influence.28 Finally, we should note that experimentally, Si migration, forming islands from more dispersed configurations, occurs at elevated temperatures.69 Such migration would be driven by the thermodynamic stability of the islands formed which is demonstrated in the present calculations. Thus, although our calculations offer a simplified picture of Si island formation compared with the reality of crystallization from a gel medium, they are in agreement with experimental evidence that thermodynamics favors the formation of islands. From our study of the acidity of the isolated Brønsted acid sites we conclude that the Al-O(H)-Si angle has a significant effect on the resulting acidity. Furthermore, there appears little correlation between acidity and the topological environment (ring size). However, since these two properties are intrinsically linked, it has proved difficult to isolate short-range (bond angle) from medium-range (topology) interactions. Our calculations also support the experimental evidence that the formation of Si islands promotes the acidity of SAPO materials and that the acidity of acid centers at the edges of such islands will increase with island size. The increase in acidity in islands is primarily the result of changes in the electrostatic potential bought about by the substitution of Si for Al and P. We again conclude that it is the formation of islands which leads to the differences in acidity noted between SAPO structures. The work presented here demonstrates how the differences in the stability of silicon islands is influenced by the structure of the parent aluminophosphate. Furthermore, our results show how the resulting geometry of the substituted island is responsible for the variation in acidic properties of the resulting material. Although the formation of silicon species in SAPOs will be significantly affected by synthesis conditions and gel chemistry, the excellent correlation between these calculations on the final structure and experimental data indicates that such calculations can be profitable in understanding the behavior of such materials. Acknowledgment. G.S. thanks MEC of Spain for financial support. The EPSRC is acknowledged for the provision of a ROPA award to D.W.L. and for general financial support. MSI is also thanked for providing the InsightII software as part of the Catalysis and Sorption Consortium. We are indebted to Prof. D. Barthomeuf for useful comments during the course of this work. Dr. J. D. Gale is also thanked for useful discussions regarding this work. Finally, we thank the referees for making many useful suggestions during the reviewing process. References and Notes (1) Wilson, S. T.; Lok, B. M.; Messina, C. A.; Cannan, T. R.; Flanigen, E. M. J. Am. Chem. Soc. 1982, 104, 1146.

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