6722
J. Phys. Chem. 1996, 100, 6722-6730
Structure and Stability of Silica Species in SAPO Molecular Sieves German Sastre, Dewi W. Lewis, and C. Richard A. Catlow* DaVy-Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London W1X 4BS, U.K. ReceiVed: NoVember 15, 1995; In Final Form: January 31, 1996X
We have investigated the energetics of silicon incorporation in AlPO4-5 using lattice simulation techniques. The energies of two silicon substitution mechanisms (Al, P f 2Si; P f Si, H) have been calculated. Silicon island formation by means of the first mechanism has been found to be energetically unfavorable, and thus 2Si and 4Si islands are not expected to form in SAPO structures. In contrast, the formation of silicon islands by means of a combination of both mechanisms has proved to be energetically favorable, leading to 5Si and 8Si islands being stable in the SAPO structure. These findings correlate well with experimental observation. In addition, the instability of the 2Si and 4Si islands has been related to the presence of [Si-O-P] linkages in the structure. On this basis, the absence of the [Si-O-P] units in SAPOs can be explained. We have also studied the interaction energy between Brønsted acid sites formed by means of one of the mechanisms of silicon substitution (P f Si, H). The calculations reveal a small binding energy for such centers at close separation.
1. Introduction The nature of the distribution of different atom types over the framework tetrahedral sites in microporous materials is among the most intriguing and difficult problems in the structural chemistry of these systems. In this paper we consider the question of the distribution of Si substituents in microporous aluminophosphates. Crystalline aluminophosphate molecular sieves (AlPO) that were first synthesised over 10 years ago1-4 have structures similar to those of zeolites, but with the primary building units being formed by [Al-O-P] units instead of the [Si-O-Si] or [Si-O-Al] bridges observed in zeolites. Some structural types are observed both as zeolites and AlPOs, but others are specific to AlPO materials.5 An important feature of AlPO structures is that no [Al-O-Al] or [P-O-P] units are observed;6 Al and P strictly alternate over the framework cation sites. Unlike aluminosilicate zeolites, the AlPO frameworks are neutral and no compensating cations or anions are necessary to neutralize the charge of the framework. Consequently, no Brønsted acidity is present in AlPOs, and their use as acid catalysts is therefore more restricted than for zeolites.7,8 However, the introduction of silicon atoms into the AlPO structure provides a mechanism for generating Brønsted acidity and changes dramatically the possibility of using these materials as acid catalysts.9-12 Silicon substitution does not lead to major structural changes although local changes in the geometry near the substituted atom are noted.13 The primary building units of SAPOs are [Al-O-P], [Si-O-Al], and [Si-O-Si], but no [Si-O-P] linkages have been observed.7 A number of different models for silicon incorporation in AlPO structures have been proposed:14,15 one is based on the formation of silicon islands; a second leads to the formation of dispersed silicon in the structure. The number and strength of the Brønsted acid centers generated in both mechanisms is different, and hence the catalytic properties of the SAPO depend in part on the relative extents of each mechanism.16 Temperature programmed desorption experiments show that, in SAPO5, the ratio between the silicon incorporated in the structure and the number of Brønsted sites is 1:1 at low Si concentrations X
Abstract published in AdVance ACS Abstracts, March 15, 1996.
0022-3654/96/20100-6722$12.00/0
Figure 1. Crystallographic structure of AlPO4-5. View across the [001] direction.
(xSi < 0.1);17 at higher concentrations the ratio of Brønsted sites to silicon content falls below one, as silicon islands are formed. These observations suggest that, at low Si concentrations, the substituted Si is dispersed; such models are supported by other techniques, including 29Si MAS NMR, where the spectra show only one peak around -90 ppm, corresponding to Si(4Al), when the silicon content is low. However, when the silicon content increases above xSi ≈ 0.1, peaks between -93 and -108 ppm indicate the presence of Si(3Al), Si(2Al), Si(1Al), and Si(0Al), consistent with the formation of silicon islands containing up to 14 Si atoms.18,19 In the present paper we investigate the energetics of Si substitution in SAPOs focusing on Si incorporation in the AlPO4-5, the structure of which is shown in Figure 1. We calculate the energies of different silicon island structures; and we provide an explanation in energetic terms of the absence of Si-O-P linkages. 2. Methodology All the calculations in this work were performed using lattice energy minimization techniques,20 the GULP code21 being employed for this purpose. The methodology uses standard techniques based on the Ewald22 method for summation of the © 1996 American Chemical Society
Silica Species in SAPO Molecular Sieves
J. Phys. Chem., Vol. 100, No. 16, 1996 6723
TABLE 1: Potential Parameters Used in the GULP Codea Buckingham potential 4+‚‚‚O2-
Si Al3+‚‚‚O2P5+‚‚‚O2O2-‚‚‚O2Si4+‚‚‚O1.4Al3+‚‚‚O1.4O2-‚‚‚O1.4O2-‚‚‚H0.4+
A (eV)
F (Å)
C (eV‚Å6)
1283.90 1460.30 877.34 22764.00 983.57 1142.68 22764.00 311.97
0.32052 0.29912 0.35940 0.14900 0.32052 0.29912 0.14900 0.25000
10.66 0.00 0.00 27.88 10.66 0.00 27.88 0.00
Morse potential
De (eV)
R (Å-1)
r0 (Å)
O1.4-‚‚‚H0.4+
7.05
2.1986
0.9485
three-body potential O-T-O
b
θ0 (deg)
k (eV/rad) 2.09724
109.47
core-shell potential
k (eV‚Å-2)
O2-
74.92
Coulombic charges P5+ Si4+ Al3+ +5 +4
O2-core
O2-shell
O1.4-
H0.4+
+3 +0.86902 -2.86902 -1.426 +0.426
a
A cutoff value of 10 Å was used for the short range Buckingham potentials. These parameters are used in the general energy expression given by the following: V ) ∑Vij(Buckingham) + ∑Vij(Coulombic) + ∑Vijk(three-body) + ∑Vij(core-shell) + ∑Vij(Morse), where Vij(Buckingham) ) Aij exp(-rij /F) - Cij /r6; Vij(Coulombic) ) (qiqj )/rij; Vijk(three-body) ) 1/2kijk(θijk - θ0ijk)2 with θ ) O-T-O; Vij(core-shell) ) 1/2kij (rij - r0ij)2; Vij(Morse) ) Deij{1 - exp[-Rij/(rij - rij0)]}2 Deij. b O ≡ O2-, O1.4-; T ≡ P5+, Si4+, Al3+.
long-range Coulombic interactions and direct summation of the short-range interactions which can be described by a number of standard formulations. The experimentally determined crystal structure of AlPO4-5 was input to the minimization calculations, and all structural parameters, i.e., cell dimensions and atomic coordinates, were optimized without symmetry constraints. The BFGS23 minimization method was used in all the cases with a convergence criterion of a gradient norm below 0.001 eV/Å. We employed the Mott-Littleton methodology24 to treat the incorporation of defects within the perfect lattice. This widely used method20 allows the full relaxation of atomic coordinates of an inner region (100-500 ions), surrounding the 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, yielding results which have been verified by comparison to experimental data.25,26 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 model27 was used to simulate the polarizability of the oxygen atoms. In the case of the Brønsted centres an additional Morse potential was included to model the O-H interaction. A cutoff distance of 10 Å was applied to the short-range interactions. The potentials used for the AlPO28 were parametrized to reproduce the structure of Berlinite and have further been demonstrated to successfully model a number of AlPO structures. The Si-O potential29 and the O-H30,31 parameters have been extensively used in the modeling of the structures of zeolites.32 All the parameters used are given in Table 1, which also lists the explicit form of the potential used. Further details of the methodologies employed20 and their application to the study of microporous solids33 are available elsewhere.
Figure 2. Different mechanisms proposed for silicon incorporation in AlPO structures. (a) mechanism 1, Al f Si replacement; (b) mechanism 2, P f Si, H replacement; (c) mechanism 3, Al, P f 2Si (separated); (d) mechanism 3, Al, P f 2Si (island); (e) mechanism 3, 2Al, 2P f 4Si (line); (f) mechanism 3, 2Al, 2P f 4Si (ring). The number of [SiO-Al], [Si-O-Si], and [Si-O-P] is indicated in the figure and the number of [Si-O-P] linkages is listed in Table 2.
TABLE 2: Number of Phosphorus Atoms in the First T-Coordination of the Silicon Substituted Atoms According to the Processes Described Schematically in Figure 1a Si(0P) 2Si-isl 2Si-sep 4Si-line 4Si-ring 1Si-disp 2Si-disp 5Si-isl 8Si-isl
1 1 2 2 1 2 5 8
Si(2P)
Si(3P)
Si(4P)
1 1 1 2
1
a The nomenclature used to describe the various Si containing structures is described in section 3.
3. Modeling of Silicon Incorporation in AlPO Structures 3.1. Mechanisms of Silicon Substitution in AlPOs. Three basic mechanisms for silicon incorporation in AlPOs have been proposed,13-15 as shown schematically in Figure 2. Before the substitution, all the linkages in the structure are [Al-O-P]. On silicon incorporation [Si-O-Al], [Si-O-P], and [Si-OSi] linkages can, in principle, be formed. The number of [SiO-P] linkages generated after the different substitutions is indicated in Table 2. We note that despite the two dimensional representation used in this table it is nevertheless valid for SAPO structures as the coordination of the T (Si, Al, P) atoms in the framework is always tetrahedral. We will now explore the energetics of these reactions in AlPO4-5. Mechanism 1. Al f Si Replacement. In this case a positively charged framework is generated. This mechanism has not been observed to date,6 and it will not be modeled in this paper. Positively charged frameworks will of course require anionic extraframework compensation, which is unlikely to be viable for this system given the reaction conditions and the cationic nature of many of the organic templates used.
6724 J. Phys. Chem., Vol. 100, No. 16, 1996
Sastre et al.
Mechanism 2. P f Si, H Replacement. This mechanism corresponds to the formation of silicon dispersed throughout the framework without island formation. Since no [P-O-P] units are observed in AlPO structures, no [Si-O-Si] linkages can be generated by this mechanism, and hence there is no silicon island formation. This mechanism therefore promotes a dispersed silicon distribution in the framework and results in four [Si-O-Al] bridges (Figure 1b). The proton is localized on one of the four oxygen atoms surrounding the replaced silicon. To investigate this mechanism we calculate the energy of the following replacement:
tAlOPt + Si4+ + H+ f tAlO(H)Sit + P5+ (AlPO structure) (SAPO structure) which we treat formally as the introduction of an Si4+ and H+ species from infinity and the displacement of a P5+ to infinity. Such energies must of course be used together with other terms, as part of a Born-Haber cycle, if physically significant quantities (such as heats of solution) are to be calculated. We represent the above process using the following short-hand notation:
[AlPO4] + Si4+ + H+ f [Si, H]P + P5+
Figure 3. Two possible arangements for the [2Si]Al,P substitution in AlPO4-5: (a) two silicon atoms separated (2Si-sep); (b) two silicon atoms forming an island (2Si-isl). The surroundings of the substituted silicon atoms are further illustrated in Figure 2c,d).
E1Si-disp
We can also calculate the energy of the double replacement process:
[AlPO4] + 2Si4+ + 2H+ f [2Si, 2H]2P + 2P5+
E2Si-disp
If the two defects are noninteracting, the energy of the second replacement will be simply
E2Si-disp ≈ 2E1Si-disp
(1)
Appreciable interaction between the defects will result in deviation from this relationship, the interaction energy Eint, being given by
∆Eint ) E2Si-disp - 2E1Si-disp
(2)
Mechanism 3. P,Al f Si, Si Replacement. This mechanism corresponds to the formation of silicon islands in the framework. After the P, Al f Si, Si replacement, two different environments are generated for T-site silicon, dependent on whether or not nearest neighbor T sites are substituted (Figure 2c,d). The first T coordination sphere of the silicons is either Si(3Al,1Si), Si(3P,1Si) (Figure 2d) or Si(4P), Si(4Al) (Figure 2c). Since no [Si-O-P] linkages are observed in SAPO structures, and in particular in SAPO-5,34-36 no 2Si islands are expected to be formed. However, calculations were undertaken in order to estimate the energetic penalty for the formation of this island. The smallest silicon island consistent with the absence of [SiO-P] linkages must contain 5 silicon atoms.19 This island can exist in all SAPO structures simply because each TO4 tetrahedron is necessarily linked to four others. If an Al(4P) unit is replaced by 5Si atoms, all of the linkages within and surrounding the atoms in the island will be [Si-O-Si] and [Si-O-Al], with no [Si-O-P] linkages being generated. Higher silicon islands can be formed, but the number of atoms in them now depends on the topology of the framework19 since the number of T atoms in the second coordination sphere of a particular T atom is structure dependent, lying usually between 9 and 12. We therefore model first the processes of 2Si and 4Si island formation, recalling that the instability of [Si-O-P] linkages in 2Si (Figure 3b) and 4Si islands (Figure 4) is claimed to be the
Figure 4. Two arrangements for the [4Si]2Al,2P substitution in AlPO45: four silicon atoms in a line, 4Si-line (top), and four silicon atoms arranged in a 4-T ring, 4Si-ring (bottom). The surroundings of the substituted silicon atoms are further illustrated in Figure 2e,f).
main reason for their absence in SAPO structures. The equations to describe the processes are the following:
[AlPO4] + 2Si4+ f [2Si]Al,P + Al3+ + P5+
E2Si
[AlPO4] + 4Si4+ f [4Si]2Al,2P + 2Al3+ + 2P5+
E4Si
The notation [2Si]Al,P represents two Si atoms replaced by Al and P, and similarly for the [4Si]2Al,2P. It is important to note that there are several valid arrangements of the 2Si and 4Si islands within the SAPO structure. For the 2Si arrangement we consider two possibilities: one corresponds to the atoms well separated; the corresponding defect energy is denoted E2Si-sep, (Figure 3a); the other to the island, in which we have an Si-O-Si bridge and the defect energy is denoted E2Si-isl (Figure 3b). In the case of the 4Si arrangement (Figure 4), only two possibilities will be considered: the linear arrangement, for which the defect energy is denoted E4Si-line, and the arrangement in a 4T-ring, for which the defect energy is denoted E4Si-ring. Next, we model the formation of 5Si islands (Figure 5) and 8Si islands (Figure 6). Each silicon island can be built by a
Silica Species in SAPO Molecular Sieves
J. Phys. Chem., Vol. 100, No. 16, 1996 6725 calculated for the substitution mechanisms described above may be used to evaluate the energetics of physically significant processes. We consider first the energetics of silica solution which we can calculate at least for the case of the formation of 2Si islands. 3.2. Dissolution Energy for SiO2 Corresponding to the 2Si Substitution Process. We first recall that the equations, given in the previous section, refer to the hypothetical process in which ions are incorporated from and displaced to infinity. As already noted, in order to use these quantities in the determination of solution energies, they must be combined with other terms in a Born-Haber cycle. To construct such a cycle, we must consider first the source of silicon used experimentally37 and secondly the “sink” for the displaced Al and P. We propose therefore the following simple procedure for the dissolution of SiO2 to give 2Si islands
[AlPO4] + 2SiO2 f [2Si]Al,P + AlPO4 giving the following expression for the dissolution energy, ∆Es2Si-isl Figure 5. 5Si island in AlPO4-5. This island is formed by three substitutions according to mechanism 2 (P f Si, H) and one substitution by mechanism 3 (Al, P f 2Si).
∆Es2Si-isl ) E2Si-isl + Elatt(AlPO4) - 2Elatt(SiO2)
(5)
in which the three terms on the right hand side refer to the following processes:
2SiO2 f 2Si4+ + 4O2-
-2Elatt(SiO2)
[AlPO4] + 2Si4+ f [2Si]Al,P + P5+ + Al3+ Al3+ + P5+ + 4O2- f AlPO4
E2Si-isl
Elatt(AlPO4)
3.3. Estimation of the [Si-O-P] Energetic Penalty in SAPO Structures. As discussed above, [Si-O-P] linkages are not observed in SAPO structures and are believed to destabilize the framework significantly. To investigate the energetics of this process, we consider reactions in which we create 2-silicon islands (which necessarily include Si-O-P linkages) from larger aggregates which contain no such linkages. The following three cases have been considered for which we give the corresponding energy expression:
Figure 6. 8Si island in AlPO4-5. This island is formed by four substitutions according to mechanism 2 (P f Si, H) and two substitutions by mechanism 3 (Al, P f 2Si).
different sequence of component substitutions, each of which corresponds to either mechanisms 2 or 3. The 5Si island requires three substitutions by mechanism 2 (P f Si, H) and one substitution by mechanism 3 (Al,P f Si, Si). The 8Si island requires four substitutions by mechanism 2 and two substitutions by mechanism 3. The specific processes involved for 5 and 8 island formation for which energies are calculated are therefore the following:
[AlPO4] + 5Si4+ + 3H+ f [5Si, 3H]1Al,4P + 4P5+ + Al3+
E5Si-isl (3)
[AlPO4] + 8Si4+ + 4H+ f [8Si, 4H]2Al,6P + 6P5+ + 2Al3+
E8Si-isl (4)
In the following sections we consider how the defect energies
[5Si,3H]Al,4P f [2Si]Al,P + 3[Si, H]P 5 island 2 island 3 dispersed Si ∆E1 ) E2Si-isl + 3E1Si-disp - E5Si-isl (6) [8Si,4H]2Al,6P f [5Si,3H]Al,4P + [2Si]Al,P + [Si, H]P 8 island 5 island 2 island dispersed Si ∆E2 ) E5Si-isl + E2Si-isl + E1Si-disp - E8Si-isl (7) [8Si,4H]2Al,6P f 2[2Si]Al,P + 4[Si, H]P 8 island 2 island 4 dispersed Si ∆E3 ) 2E2Si-isl + 4E1Si-disp - E8Si-isl (8) These equations establish the relative stability of the 5Si and 8Si islands with respect to other possible arrangements of the silicon and hydrogen atoms. In these three cases, the energy expressions have been obtained by appropriate combinations of the equations given above, in section 3.1, where the basic equations of the silicon incorporation by mechanisms 2 and 3 were established. We note that if the energies ∆E1, ∆E2, and ∆E3 are positive, the islands are more stable than the final products. If this is the
6726 J. Phys. Chem., Vol. 100, No. 16, 1996
Sastre et al. TABLE 5: Lattice Energies for AlPO4-5 and Quartz, and Defect Energies for the Different Silicon Incorporation Reactions in AlPO4-5a Elatt(AlPO4)/(mol of AlPO4) Elatt(SiO2-quartz)/(mol of SiO2) E1Si-disp E2Si-disp(1) E2Si-disp(2) E2Si-isl
-268.04 -128.70 58.90 117.75 117.63 12.22
E2Si-sep E4Si-line E4Si-ring E5Si-isl E8Si-isl
12.89 23.90 23.76 188.46 258.13
a The nomenclature corresponds to that used in the section 3. All values are in electronvolts.
Figure 7. Geometry of the unique crystallographic T(Al,P) position in AlPO4-5 (primary building unit) optimized by GULP. The optimized distances and angles are given in Table 3. When the P f Si, H substitution takes place, the hydrogen atom can be located in one of the four O(i) atoms (i ) 1, 2, 3, 4).
TABLE 3: Comparison of the Calculated and Experimental Structural Parameters in AlPO4-5 (Also Shown Graphically in Figure 7) distance Å P-O calc expt 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
O-Al calc expt 1.721 1.696 1.722 1.722
1.729 1.700 1.726 1.683
P-Al calc 3.101 3.202 3.120 3.128
angle (deg) POAl calc expt 147.6 176.3 149.1 149.0
150.2 178.1 148.8 151.0
Cell Parameters a ) b (Å) c (Å) R (deg)
calc
expt38
13.83 8.55 89.98
13.71 8.43 90.00
β (deg) γ (deg)
calc
expt38
89.99 119.98
90.00 120.00
TABLE 4: Defect Energies (eV) for the Mechanism P f Si, H in AlPO4-5 for the Proton Substitution at the Four Oxygen Sites Shown in Figure 7 E1Si-disp(O1) E1Si-disp(O2)
59.06 59.15
E1Si-disp(O3) E1Si-disp(O4)
58.87 58.90
case, we will have demonstrated that the formation of [Si-OP] linkages are energetically unfavorable. 4. Results and Discussion The structure of AlPO4-538 contains only one crystallographic equivalent position, and consequently when a substitution of a T (Al, P) atom is considered, no reference to the crystallographic position needs to be given in the calculations. We recall that all of the substitution processes to be studied are based on mechanism 2 (P f Si, H) and mechanism 3 (Al, P f Si, Si). That is, in all of the calculations, the substitutions will be described as a combination of these two mechanisms in different proportions, giving either islands or dispersed silicon. To evaluate the energies of the processes described in the previous section, we require calculations of both defect formation energies and lattice energies. 4.1. Lattice Energies. The initial AlPO structure was taken from the crystallographic data34 and was used as the starting point of the optimisation procedure within the GULP code. The geometry obtained is shown in Table 3 and Figure 7. We note with interest that the PO(2)Al(2) angle remains almost linear, as determined in the experimental crystal solution. Much
speculation exists over whether this oxygen position corresponds to a true energy minimum or if it is an average position. Here, we find an energy minimum with this near linear configuration. However, we are investigating this problem further to determine if any other local minima are present with more usual AlOP angles. The quartz structure of SiO2 was optimized in a similar way; the latter structure was chosen since this is a typical Si source used during synthesis of SAPOs.33 Both lattice energies will be used later to obtain an estimate of the dissolution energy process calculated according to eq 5. The calculated lattice energies are given in Table 5. 4.2. Hydrogen Location after the P f Si, H Substitution. In the case of the substitutions involving mechanism 2, the hydrogen atom will be located on one of the four possible [SiO-Al] linkages (Figure 7). The four oxygens are crystallographically distinct (Figure 7, Table 3), and some small differences in the defect energy can therefore be expected. The calculations performed (Table 4) by placing the hydrogen atom on the four possible oxygens show differences within 0.28 eV, which could be significant in explaining differences in the acid strength of the Brønsted acid sites of the structure. The question of the acidity of the different configurations will be considered in a future paper. However, in the present work, the proton will be considered as being bonded to the O(4) oxygen (Figure 7) for all of the calculations involving P f Si, H substitutions. This position corresponds to the protons pointing into the large cavities, which are located on the 4-T membered rings. We require a consistent value for the energy of the P f Si,H substitution, and O(4) was chosen since this oxygen site is present in all of the islands considered. 4.3. Si Substitution Processes. 4.3.1. P f Si, H. This substitution corresponds to mechanism 2 described in the previous section, for which the defect energy is E1Si-disp. The resulting defect energy (Table 5) is highly positive, which is to be expected since the charges of both the T-site and O site species are reduced. However, we note that the high value of the defect energy E1Si-disp is not itself significant, as the defect energy is only a component of a thermochemical cycle. Another factor contributing to the defect energy is the relaxation of the framework. The geometries derived from the calculations are shown in Figure 8. After the replacement, the SiO(H)Al angle (Figure 8) has a value markedly distinct from that in the original structure, producing a significant local distortion in the structure. We shall see later how island formation reduces this distortion. 4.3.2. Al, P f 2Si. In this case, two different cations in the AlPO structure are replaced by silicon. We recall that the aluminium and phosphorus cations selected to be replaced can be located on nearest neighbor T sites or separated by other T (Al, P) atoms (Figure 3). The defect energies for the two possible arrangements, [2Si]2Si-sep and [2Si]2Si-isl, are given in Table 5. The energies are positive because the equilibrium distance for Si-O is different from that of Al-O and P-O and a complete relaxation of these distances is not possible due to framework constraints. The energy difference between the two
Silica Species in SAPO Molecular Sieves
J. Phys. Chem., Vol. 100, No. 16, 1996 6727
Figure 8. Differences in geometry in the TOT units in AlPO4-5 (top) and the same structure upon P f Si, H substitution (bottom). Distances in angstroms; angles in degrees.
TABLE 6: Dissolution Energy for the Al, P f Si, Si Substitution Processa and an Estimate of the Interaction Energyb between Two Brønsted Acid Centers in AlPO4-5 (All Values Are in Electronvolts) ∆Es2Si-isl ∆Eint(1) ∆Eint(2)
1.58 -0.05 -0.17
a The equation and expression for the dissolution energy process are indicated in eq 5. b The two configurations (1) and (2) are displayed in Figure 9, and the interaction energy is derived according to eq 2. The values for E2Si-disp(i) and E1Si-disp are taken from Table 5.
cases E2Si-sep, and E2Si-isl (0.67 eV) can be understood in terms of the [T-O-T] units surrounding the Si atoms. The first structure ([2Si]2Si-sep) contains four [Si-O-P] linkages, while the second ([2Si]2Si-isl) contains three [Si-O-P] linkages (Figure 2c,d and Table 2). The greater number of these linkages in the first structure results in a higher defect energy. We can also consider this effect in terms of the Coulombic interactions involved. The two isolated species are oppositely charged, and thus the formation of the 2Si island leads to increased favorable Coulombic interactions. Using these calculated defect energies, we can obtain the dissolution energy of this process, according to eq 5. The results (Table 6) show that the dissolution energy is positive, although the relatively low value of 0.8 eV per Si suggests that some appreciable dissolution will occur as is experimentally observed; the AFI structure is accessible to molar fractions of Si up to 1.0.39 Furthermore, our later calculations suggest that this silica solution is promoted by the formation of larger aggregates of Si. We therefore expect that the solution energy will be lower by several tenths of an electronvolt and thus that the experimentally observed solubilities are attainable by this mechanism. 4.3.3. Substitution 2P f 2Si, 2H. The relative positions of the two silicon atoms generated after the substitution are shown in Figure 9. In order to investigate the interaction between these species, two configurations have been considered. The interaction energy is expected to decrease as the distance between the two centers increases. The defect energies E2Si-disp(1), and E2Si-disp(2), are given in Table 5, and the interaction energy, ∆Eint ) E2Si-disp - 2E1Si-disp, in Table 6. We find favorable interaction between Brønsted acid sites at next nearest neighbor sites. Thus, although the Si is dispersed at low Si concentrations, with no Si islands, there is a tendency for the substituent Si to aggregate.
Figure 9. Two different 2P f 2Si, 2H substitutions considered in AlPO4-5 calculations. In the first calculation, 2Si-disp(1), the second silicon atom is at the first neighboring P site of the Si(1). In the second calculation, 2Si-disp(2), the second silicon is at the second neighboring P site of the Si(1). Results are given in Table 5.
However, this aggregation does not occur by the same mechanism which leads to Si island formation at higher Si concentrations. 4.3.4. Substitution 2P, 2Al f 4Si. Following the approach outlined in section 4.3.2, two possibilities (Figure 4) have been studied, [4Si](4Si-line) and [2Si] (4Si-ring). The number of [Si-OP] linkages is different in the two cases, being five in the first and four in the second case (Figure 2e,f and Table 2). Again, the defect energy is lower in the arrangement with a smaller number of [Si-O-P] linkages (E4Si-ring < E4Si-line), which adds further support to our previous calculations which show that these linkages are energetically unfavorable. The results, E4Si-ring and E4Si-line, (Table 5), show a difference of 0.69 eV per Si-O-P linkage, which is very similar to that obtained previously for the 2Si substitution case. 4.3.5. Substitution Al,4P f 5Si, 3H. Formation of a 5Si Island. This process corresponds to the formation of a 5Si island as described in eq 3. The atomic distribution is shown in Figure 5. The difference between this process and that of 2-silicon island formation is that the 5Si island results in the formation of no [Si-O-P] linkages. The value obtained for E5Si-isl, (Table 5) shows a positive defect energy as in the previous cases because of the positive value of the defect energies of the component processes (mechanisms 2 and 3) involved in the formation of the island. The relative stability of the island was tested by means of the reaction given in eq 6. The energy of this process (Table 7) is positive, showing that the 5-silicon island is more stable than the final products, demonstrating the stability of this 5Si islandsa result which is fully consistent with the absence of [Si-O-P] linkages in the final products of the silica dissolution reaction.
6728 J. Phys. Chem., Vol. 100, No. 16, 1996
Sastre et al.
TABLE 7: Calculated Dissociation Energies (eV) of Si Island Clustersa ∆E1 (see eq 6) ∆E2 (see eq 7) ∆E3 (see eq 8) a
0.46 1.45 1.91
The nomenclature corresponds to that used in the section 3.
4.3.6. Substitution 2Al, 6P f 8Si, 4H. Formation of an 8Si Island. This process corresponds to the formation of the 8Si island (Figure 6) as described in eq 4, for which the defect energy is given in Table 5. In this case the stability of the island was tested by means of two possible reactions given in eqs 7 and 8. In the first case, the 8Si island dissociates into a 5Si island and the smaller arrangements described in mechanisms 2 and 3. In the second case, the 8Si island transforms directly into simpler arrangements (mechanisms 2 and 3). The positive dissociation energy of both processes (Table 7) demonstrates the relative stability of the 8-Si islands with respect to these final aggregates, indicating that such islands will be stable and emphasizing once more that formation of [Si-O-P] linkages in SAPO structures is energetically unfavorable. 4.3.7. Structural Changes on Si Substitution. The effect of Si substitution on the structure of AlPO4-5 have been also been considered. We generally note changes in geometry up to the second coordination sphere of the substituted T atom, and thus geometric changes are local in nature. Two cases are discussed here: the formation of an acid center by means of mechanism 2 and the formation of the 5Si island.
In the first case, isolated Si produces significant distortion as a result of the differences between the Si-O and P-O bond lengths. Comparison of Figures 10a and 11a shows that P-O distances around the central atom in the original AlPO structure (optimized by GULP) take the values 1.51, 1.51, 1.51, and 1.52 Å, while after the silicon incorporation the new distances, now for Si-O, increase to 1.54, 1.55, 1.55, and 1.78 Å. The latter value is due to the presence of the proton attached to the corresponding oxygen. A decrease in the O-Al distances corresponding to the first shell around the central atom is also observed as a consequence of the P f Si replacement, changing from 1.70, 1.72, 1.72, and 1.72 Å to 1.66, 1.67, 1.67, and 1.82 Å. Again the latter value corresponds to the local distortion introduced by the acid center formed. Thus, some noticeable change in the first coordination sphere of the central substituted atom is observed after the introduction of the silicon, with the local distortion in the tSiO(H)Alt bridge being significant. The same conclusions can be applied to the local deformations observed in the TOT angles (Figures 10b and 11b) after silicon incorporation, the major change being in the TOT angle which is substituted in the SAPO, which changes from 149.0° in AlPO to 136.9° following replacement by the SiO(H)Al bridge. In the second case, substitution by both mechanisms 2 and 3 results in the formation of the 5Si island. The local distortions produced by the formation of the Brønsted acid sites are similar to those found for the isolated substitutions discussed above. However, in addition to these SiO(H)Al bridges we now have
Figure 10. Optimized AlPO4-5 structure showing the geometry (T-O distances and TOT angles) and connectivity of the next nearest neighbors from a central P atom. (a) Distances in angstroms; (b) angles in degrees.
Figure 11. Calculated geometry of the SAPO-5 structure after a single replacement P f Si, H (mechanism 2) showing the geometry (T-O distances and TOT angles) and connectivity of the next nearest neighbors of the central Si atom. (a) Distances in angstroms; (b) angles in degrees.
Silica Species in SAPO Molecular Sieves
J. Phys. Chem., Vol. 100, No. 16, 1996 6729
Figure 12. Calculated geometry of the SAPO-5 structure after the formation of a 5Si island showing the geometry (T-O distances and TOT angles) and connectivity of the next nearest neighbors of the central Si atom. (a) Distances in angstroms; (b) angles in degrees.
to consider the changes in the SiOSi units surrounding the central Si atom in the island. It can be observed, after the comparison between the Figures 10 and 12, that the local distortion introduced by the new tSiOSit unit is significantly smaller than in the previous case. The two Si-O bonds formed have a mean value similar to the mean of the Al-O and P-O bonds in the AlPO structure; thus the T-T distance is similar. Therefore the formation of Si islands, with the subsequent formation of tSiOSit units would seem to be favorable from a geometric standpoint. The AlPO lattice can accommodate such substitutions with only small distortions to the lattice, a process which will be energetically less expensive than those resulting in major structural modifications. The local distortions in the AlPO structures after the silicon incorporation in these two cases will be an important factor in explaining the variation in the cell parameters and pore dimensions of the SAPO structures with increasing silicon content.35 5. Conclusions The first main conclusion of this paper is that there is a significant driving force for aggregation of Si substituted into SAPOs; a second and related general point to emerge is that there is an appreciable energy penalty for the formation of [SiO-P] bridges. Thus, we have shown the relative instability of the 2Si and 4Si islands with respect to the 5Si and 8Si islands. Indeed our calculations suggest that the smallest silicon islands observed in SAPO structures will contain five silicon atoms. Moreover, we have shown that the instability of the 2Si and 4Si islands is related to the presence of [Si-O-P] linkages in these structures, linkages which are not observed in SAPOs; and in general, we find that the formation energy of the different arrangements increases as the number of [Si-O-P] linkages increases. The tendency for clustering of the substituted Si is also shown by Brønsted acid centers formed by the silicon incorporation mechanisms (P f Si,H), where the calculations show an energetic stabilization for closely spaced centers. These conclusions, drawn from our study of SAPO-5, can almost certainly be extended to other SAPO structures. The strong driving force for aggregation of Si in SAPOs may result in the reduction in the corresponding acidity. The question of the different acidities of the various centers described in this paper will be the topic of a future investigation. Acknowledgment. G.S. thanks Ministerio de Educacion y Ciencia of Spain for a postdoctoral research grant. D.W.L.
acknowledges EPSRC for providing a ROPA grant. We thank Dr. J. D. Gale and Prof. A. Corma for useful discussions. Note Added in Proof: Further to our comments (section 4.1) regarding the linear PO(2)Al(2) bond angle, our recent calculations on the structure of AlPO4-5 have indicated that lowering the symmetry from the experimentally determined P6cc space group allows the PO(2)Al(2) bond angle to take up a value more typical for T-O-T angles in AlPO structures. This work will be reported elsewhere. 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. (2) Wilson, S. T.; Lok, B. M.; Flanigen, E. M. U.S. Pat. 4 310 440, 1982. (3) Lok, B. M.; Messina, C. A.; Patton, R. L.; Gajek, R. T.; Cannan, T. R.; Flanigen, E. M. U.S. Pat. 4 440 871, 1984. (4) Lok, B. M.; Messina, C. A.; Patton, R. L.; Gajek, R. T.; Cannan, T. R.; Flanigen, E. M. J. Am. Chem. Soc. 1984, 106, 6092. (5) Meier, W. M.; Olson, D. H. Atlas of Zeolite Structure Types, Butterworth-Heinemann, Stoneham, MA, 1992. (6) Flanigen, E. M.; Lok, B. M.; Patton, R. L.; Wilson, S. T. In New DeVelopments in Zeolite Science and Technology; Proceedings of the 7th International Zeolite Conference; Murakami, Y., Iijima, A., Ward, J. W., Eds.; Elsevier: New York, 1986; 103. (7) Flanigen, E. M.; Patton, R. L.; Wilson, S. T. In InnoVation in Zeolite Material Science; Grobet, P. J., Mortier, W. J., Vansant, E. F., SchulzEkloff, G., Eds.; Studies in Surfurface Science Catalysis; Elsevier: Amsterdam, 1988; Vol. 37, p 13. (8) Tapp, N. J.; Milestone, N. B.; Bibby, D. M. In InnoVation in Zeolite Material Science; Grobet, P. J., Mortier, W. J., Vansant, E. F., SchulzEkloff, G., Eds.; Studies in Surface Science and Catalysis; Elsevier: Amsterdam, 1988, Vol. 37, p 393. (9) Chen, J.; Wright, P. A.; Natarajan, S.; Thomas, J. M. Zeolites and Related Microporous Materials: State of the Art 1994; Weitkamp, J., Karge, H. G., Pfeifer, H., Holderich, W., Eds.; Studies in Surface Science and Catalysis; Elsevier: Amsterdam, 1994; Vol 84, p 1731. (10) Martens, J. A.; Grobet, P. J.; Jacobs, P. A. J. Catal. 1990, 126, 299. (11) Rabo, J. A.; Pellet, R. J.; Coughlin, P. K.; Shamshoun, E. S. Stud. Surf. Sci. Catal. 1989, 46, 1. (12) Gielgens, L. H.; Veenstra, I. H. E.; Ponec, V.; Haanepen, M. J.; van Hoof, J. H.C. Catal. Lett. 1995, 32, 195. (13) Prasad, S.; Vetrivel, R. J. Mol. Catal. 1993, 84, 299. (14) de Saldarriaga, L. S.; Saldarriaga, C.; Davies, M. E. J. Am. Chem. Soc. 1987, 109, 2686. (15) Freude, D.; Ernst, H.; Hunger, M.; Pfeifer, H.; Jahn, E. Chem. Phys. Lett. 1988, 143, 477. (16) Martens, J. A.; Janssens, C.; Grobet, P. J.; Beyer, H. K.; Jacobs, P. A. In Zeolites: Facts, Figures, Future; Jacobs, P. A.; Van Santen, R. L., Eds.; Studies in Surface Science and Catalysis; Elsevier: Amsterdam, 1989; Vol. 49A, p 215. (17) Makarova, M. A.; Ojo, A. F.; Al-Ghefali, K. M.; Dwyer, J. In Proceedings of the 9th International Zeolite Conference; Von Ballmoos,
6730 J. Phys. Chem., Vol. 100, No. 16, 1996 R., Higgins, J. B., Treacy, M. M. J., Eds.; Butterworth-Heinemann: Boston, 1993; Vol. II, p 259. (18) Mertens, M.; Martens, J. A.; Grobet, P. J.; Jacobs, P. A. In Guidelines for Mastering the Properties of Molecular SieVes; Barthomeuf, D., Derouane, E. G., Holderich, W. Eds.; NATO ASI Series B, 221; Plenum Press: New York, 1990, p 1. (19) Barthomeuf, D. Zeolites 1994, 14, 394. (20) Catlow, C. R. A., Mackrodt, W. C., Eds. Computer Simulation in Solids; Lecture Notes in Physics No. 166; Springer: Berlin, 1982. (21) Gale, J. D. General Utility Lattice Program (GULP), The Royal Institution of Great Britain/Imperial College, 1992-1996. (22) Ewald, P. P. Ann. Phys. 1921, 64, 253. (23) Shannon, D. F. Math. Comput. 1970, 24, 647. (24) Mott, N. F.; Littleton, M. J. Trans. Faraday Soc. 1938, 34, 485. (25) Lewis, D. W.; Catlow, C. R. A.; Sankar, G.; Carr, S. W. J. Phys. Chem. 1995, 99, 2377. (26) Jentys, A.; Catlow, C. R. A. Catal. Lett. 1993, 22, 251. (27) Dick, B. G.; Overhauser, A. W. Phys. ReV. 1958, 112, 90. (28) Gale, J. D.; Henson, N. J. J. Chem. Soc., Faraday Trans. 1994, 90, 3175. (29) Winkler, B.; Dove, M. T.; Leslie, M. Am. Mineral. 1991, 76, 4462. (30) Saul, P.; Catlow, C. R. A.; Kendrick, J. Philos. Mag. B 1985, 51, 107.
Sastre et al. (31) Schroder, K.-P.; Sauer, J.; Leslie, M.; Catlow, C. R. A.; Thomas, J. M. Chem. Phys. Lett. 1992, 188, 320. (32) Jackson, R. A.; Catlow, C. R. A. Mol. Simul. 1988, 1, 207. (33) C. R. A. Catlow, Ed. In Modelling of Structure and ReactiVity in Zeolites; Academic Press: London, 1992. (34) Wang, X.; Liu, X.; Song, T.; Hu, J.; Qiu, J. Chem. Phys. Lett. 1989, 157, 87. (35) Martens, J. A.; Mertens, M.; Grobet, P. J.; Jacobs, P. A. In InnoVation in Zeolite Material Science; Grobet, P. J., Mortier, W. J., Vansant, E. F., Schulz-Ekloff, G. Eds.; Studies in Surface Science and Catalysis; 1988, 37, 97. (36) Ojo, A. F.; Dwyer, J.; Dewing, J.; O’Malley, P. J.; Nabhan, A. J. Chem. Soc. Faraday Trans. 1992, 88 (1), 105. (37) Young, D.; Davis, M. E. Zeolites 1991, 11, 277. (38) Bennet, J. M.; Cohen, J. P.; Flanigen, E. M.; Pluth, J. J.; Smith, J. V. In Intrazeolite Chemistry, Stucky, G. D., Dwyer, F. G., Eds.; ACS Symposium Series 218; American Chemical Society: Washington, DC, 1983; p 109. (39) Zones, S. I. Eur. Pat. Appl. 231 019, 1987.
JP953362F
