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Long- versus short-range Si, Al ordering in zeolites X and Y
Volume 183, number 3,4
CHEMICAL PHYSICS LETTERS
30 August 199I
Long- versus short-range Si, Al ordering in zeolites X and Y Carlos P. Herrero, Luis Utrera and Rafael Ramirez Instituto de Ciencia de Materiales,
CSIC, Serrano,
115 dpdo., 28006 Madrid,
Spain
Received 14 May 1991; in final form I4 June 1991
A Monte Carlo method is employed to study the Si, Al distribution in zeolites X and Y. The simulation includes long-range Coulomb interactions and oxygen polarization effects. Both contributions to the lattice energy are found to be necessary for a realistic description of the tetrahedral cation ordering. The equilibrium Si, Al distributions show short-range order for all analyzed compositions ( I <.%/AI < 3), in good agreement with % NMR results. Long-range ordering is only found in the X region (Sij Al < I .4). Two order-disorder transitions are found at framework compositions for which earlier X-ray diffraction analyzes detected discontinuities in the lattice parameter as a function of Al loading.
1. Introduction The determination of both framework and extraframework cation ordering in aluminosilicate zeolites is a long-standing problem that has not yet been solved in a general fashion. The first indication for Si, Al ordering in faujasites was presented by Dempsey et al. [ 1] from the change of the lattice parameter a0 as a function of the Al content. This dependence is found to be linear, but it presents two discontinuities, which were assigned to possible changes in the Si, Al ordering pattern. During the eighties, 29Si nuclear magnetic resonance (NMR) spectroscopy has been used to detect different silicon environments in the faujasite framework, thus allowing one to investigate the short-range order of tetrahedral (T) cations in this structure [ 2-41. Nowadays, the most commonly accepted feature of Si, Al distributions in these materials is the avoidance of aluminium in contiguous tetrahedra (the so-called Loewenstein’s rule [ 5]), which was confirmed by NMR spectroscopy [ 6,7]. Moreover, the tendency of Al atoms to be dispersed beyond nearest neighbours has been addressed by several authors [ 4,8,9 1. From a theoretical point of view, Dempsey reported electrostatic energy calculations for different Si, Al ordering schemes in faujasite-like zeolites [ lo]. These energy calculations assumed implicitly the presence of long-range order in the T-atom distri-
bution, a fact that, in general, has not been demonstrated. Moreover, energy differences between certain ordering patterns are not high, and thermal effects can be non-negligible at the formation temperatures of these aluminosilicates. In fact, the term due to configurational entropy variations is of the same order as the energy changes found for several ordering schemes [ 9,111. Other recent calculations on the stability of various zeolitic frameworks for different %/Al ratios have neglected the contribution of Si, Al ordering to the lattice energy [ 121, which, however, can be significant in stabilizing the zeolite structures. In this Letter, the Si, Al ordering in zeolites X and Y is analyzed by a Monte Carlo method, aild special emphasis is laid upon the extent of order (short- versus long-range) in the distribution of tetrahedral cations. The results obtained are compared to *‘Si NMR and X-ray diffraction data.
2. Simulation method The unit formula of the faujasites studied in the present work is Na, (AI,Si,92-n0384)r with n in, the range 48-96. _u,, and Xsi will denote the Al and Si atomic fractions, respectively ( xAl = n/ 192 and Xsi= I -xAI). Atomic coordinates given by Eulenberger et al. [ 131 for dehydrated sodium faujasite
0009-2614/91/$ 03.50 0 1991 Elsevier Science Publishers B.V. All rights reserved.
199
Volume 183, number 3,4
CHEMICAL PHYSICS LETTERS
were employed in the calculations. Na+ cations are supposed to be randomly distributed over the available extra-framework sites. For each framework composition, 3x lo5 Si, Al configurations were generated by ,the Metropolis procedure [ 141 at the temperature of faujasite formation ( T, = 350 K). Longer sampling does not improve the statistics of the calculated structural variables. In order to “equilibrate” the system at 350 K, a “simulated annealing” process was carried out before each simulation run. From an initial temperature of about 2000 K, the system was slowly cooled down to the simulation temperature T,. This procedure reduces the risk that the simulation starts from a metastable state, as described by Kirkpatrick et al.
1151. The lattice energy associated with a given cation distribution was calculated as a sum of three terms due to Coulomb interactions, polarization energy, and short-range repulsive potentials. The interactions that do not depend on the Si, Al ordering give a constant contribution to the lattice energy and do not need to be explicitly considered in the calculations. For instance, the contribution of the shortrange interatomic potential does not change as a function of tetrahedral cation ordering, since the number of Si-0, Al-O, and Na-0 bonds is a constant for each framework composition. Atomic contributions to the polarization energy were calculated as a function of the ionic polarizabilities and the electric field at each atomic center. Cation polarizabilities were not considered, since they are much lower than that of oxygen [ 121 ( ao2- = 1.984 A’, (I1si4+ =0.026 A3, ~*,3+ =0.049 A3, ~11Na+ =O. 128 A’). The only free parameter in our model is, therefore, the difference between the atomic charges of Si and Al in the lattice (Ss= qsi- qAl). This charge difference appears as a quadric factor in both electrostatic and polarization energies. Given the uncertainties in the effective atomic charges of Al and Si in these compounds, we have taken a value 6q=O.32 e (e, unit charge), which gives the best agreement between the local Si, Al ordering found in our simulations and that deduced from earlier NMR experiments, as described below. In order to compare the degree of aluminium dispersion in simulated distributions with that in real faujasites, we calculated the average number t, of Al200
30 August 199 I
Si-Al triplets per silicon atom (note that oxygen anions lying between tetrahedral cations are omitted to simplify the notation). This triplet concentration can be obtained from the 29SiNMR spectra by means of the expression, t, =I* + 31, f61, ,
(1)
where 1, is the intensity (normalized to unity) of the NMR signal associated with the Si-environment containing i Al and (4-i) Si. In general, t, t t2t t, = 6, where t2 and t3 are the average numbers of Al-Si-Si and Si-Si-Si triplets per silicon, respectively (i.e. each silicon is the center of six triplets). The short-range order present in the Si, Al distribution was quantified by means of order parameters showing pair correlations between tetrahedral sites. Given a site T,, we define the variable o’= ( t 1, - 1) if an atom (Al, Si) is at site Ti. Then, the pair correlation (c#} is given by the average of the product a’~#for all equivalent-site pairs (i, j) in the generated configurations [ 161. In particular, we are interested in the correlation .S,= ( o’cr3) between second-neighbour T-atoms, which can be deduced from NMR spectra. The correlation S, = (~‘a’) between nearest T-atoms is given by the AI-Al pair avoidance (S, = 1-4x*,), and does not provide any further information about the atom distribution. We have also investigated the presence of longrange order in the Si, Al distribution, which is known to appear in zeolite X for n near 96 [ 171. Although in faujasites, ail T-sites are actually crystallographitally equivalent [ 131, solely for the purpose of detecting the appearance of Si, Al long-range ordering, we classify the 192 T-sites in the unit cell in two subsets (T, and T2, each one containing 96 T-sites) that alternate in the network (i.e. the nearest neighbours of each T,-site are four T,-sites and vice versa). Calling n, and n2 the number of Al atoms on sites T, and TZ, respectively, one can quantify the long-range order of an atom distribution by the parameter L, defined as L=$n,-&)I
9
(2)
where the bars mean “absolute value”. This parameter takes its maximum value ( + 1) when all Al atoms are located at the same subset of T-sites (T, or
30 August 199I
CHEMICAL PHYSICS LETTERS
Volume 183, number 3,4
T, ), and it equals zero when aluminium occupies
both subsets with the same probability.
3. Results and discussion
moving the continuous curve towards the dashed line in fig. 1. Pair correlations between second-neighbour T-sites are shown in fig. 2. Points are obtained from 29Si NMR spectra [6,7,18,19] by means of the equation, S~=l-(I,
In fig. 1, closed symbols represent values of t, derived from *‘Si NMR spectra of synthetic faujasites published by several authors [ 6,7,18,19]. The continuous line in this figure shows the dependence of the Al-Si-Al concentration on the framework composition, obtained by our Monte Carlo simulations for 6q=O.32 e. The contribution of oxygen polarization to the lattice energy is essential to obtain good agreement between simulated and experimental t, values. In fact, the dashed curve in fig. 1 was calculated without consideration of polarization effects, and no better agreement with the experimental points is found in this case for any choice of atomic charges. The estimated error bar for 6q is kO.05 e. Lower values of 6q yield significant amounts of AlAl pairs, in clear disagreement with experimental results (Loewenstein’s rule), whereas higher 6q values cause a decrease of the Al-Si-Al concentration,
t4Z*tZJ)X,i,
(3)
which follows from the definition given above for S, (the relationship between NMR line intensities and triplet concentrations is given in detail in ref. [ 9 ] ). Like for the Al-Si-Al concentration, an increase of the pair correlation .S, is found for increasing Al content in the region n ~64-96. For n< 64, however, a rather well-defined plateau is observed, which can be related to a specific short-range order pattern of the Si, Al distribution. To quantify short-range order differences between faujasite-like zeolites with different framework compositions, we calculated changes of the total number N, of triplets Al-Si-AI as a function of aluminium loading by using the equation, &=1+&N,-6n), T
which can be deduced from eqs. ( 1) and (3 ) (note
-48
64
80
96
Al atoms / cell
0
1 48
I
1
64
80
96
Al atoms / cell Fig. I. AI-Si-AI triplet concentration versus number of Al atoms per unit cell. Closed symbols correspond to values derived from NMR data of synthetic faujasites. Lines are obtained from Monte Carlo simulation: dashed curve, equilibrium distribution considering only long-range Coulomb interactions; continuous line, distribution simulated by including both Coulomb and polarization energy terms.
Fig. 2. Second-neighbour correlation S, versus number of Al atoms per unit cell. Closed symbols represent values deduced from *%i NMR spectra. The continuous line is obtained from simulations at temperature T, = 350 K. Vertical lines separate regions with different ordering schemes. Regions corresponding to X-and Y-type zeolites are indicated.
201
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CHEMICAL PHYSICS LETTERS
that N, =nrxsiti and nr is the number of T-sites per cell, &-= 192). Then, for Y-type zeolites with n < 64, one finds AN, JAn z 6, and for n > 64, MI JAn x 12. This result is in line with the X-ray diffraction data of Dempsey et al. [ 11, which show a discontinuity in the lattice parameter for an Al loading n x 64. The average number of Al atoms per hexamer ring in this zeolitic structure is given by n/32. Thus, the break at n=64 corresponds to a framework composition for which one has, on average, two Al atoms per hexamer ring. As suggested previously [ 1,7], the arrangement of Si and Al atoms on these rings is qualitatively different in the two ranges: n < 64 and n>64. Our results indicate that for hexamer rings containing two Al atoms, the “para” disposition of these atoms is favoured over the “meta” disposition for n<64, and the opposite is found for n> 64. According to ref. [ I], the zone between the dashed lines in fig. 2 (i.e. 64
66
72
78
84
90
96
30 August 1991
and consequently, that they will be crystallographitally equivalent. For n > 8 1, however, Al occupies basically only one subset of T-sites (T, or T,), which corresponds to a well-defined long-range order scheme. This crystal symmetry reduction is in agreement with the assignment of the space group Fd3 to zeolite X, versus the group Fdjm to zeolite Y [ 13,17 1. The rather steep order-disorder transition as a function of Al loading shown in fig. 3 coincides with the discontinuity in the lattice parameter observed by Dempsey et al. [ 1 ] between 80 and 81 Al atoms per unit cell. No other long-range order scheme has been found in our equilibrium distributions in the range n=48-80. “Maximum dispersion of aluminium” models, as those proposed previously [ 6,7] for faujasites in the Y region (n < SO), are not favoured due to oxygen polarization effects and configurational entropy contributions to the free energy at the temperatures of faujasite formation. Both effects contribute to increase the number of Al-Si-Al triplets found at T=O only for Coulomb interactions.
4, Concluding remarks As for other aluminosilicates, Loewenstein’s rule is a natural consequence of the dilution of T-atoms with different charge. In contrast to earlier quantitative [lo] and qualitative [6,7] energetic considerations on the Si, Al distribution in zeolites X and Y, oxygen polarization is found to be important in defining the T-atom ordering. In this context, Coulomb interactions give only a rough approach to the distribution of T-atoms. Two order-disorder transitions are found as a function of Al concentration in the composition interval studied here (48-96 Al atoms per unit cell). The transition observed at n=64 takes place between two different short-range order schemes, but that detected at 80 Al atoms per cell is a transition between short- and long-range order. These order-disorder transitions are in agreement with structural information derived from 29SiNMR spectra and X-ray diffraction studies.
Al atoms / cell Fig. 3. Long-range order parameter L obtained from simulation for faujasites with different Al contents. An arrow indicates the framework composition at which a discontinuity is observed by X-ray diffraction in the lattice parameter a~.
202
Acknowledgement We thank E. Matesanz and C.E. Alonso for tech-
Volume 183, number 3,4
CHEMICAL PHYSICS LETTERS
nical support and J.M. Serratosa for critical comments on the manuscript. This work was supported by CICYT (Spain) under contract number MAT880166.
References [ I ] E. Dempsey, G.H. Kilhl and D.H. Olson, J. Phys. Chem. 73 (1969) 387. [2] E. Lippmaa, M. M%gi, A. Samoson, M. Tarmak and G. Engelhardt, J. Am. Chem. Sot. 103 (1981) 4992. [3] J. Klinowski, Progr. NMR Spectry. 16 (1984) 237. [ 41 G. Engelhardt and D. Michel, High resolution solid state NMR of silicates and zeolites (Wiley, New York, 1987). [ 51W. Loewenstein, Am. Mineral. 39 ( 1954) 92. [ 61G. Engelhardt, U. Lohse, E. Lippmaa, M. Tarmak and M. Magi, Z. Anorg. Allg. Chem. 482 ( I98 I ) 49. [7] J. Klinowski, S. Ramdas, J.M. Thomas, C.A. Fyfe and J.S. Hartman, J. Chem. Sot. Faraday Trans. II 78 ( 1982) 1025.
30 August 199 I
[ 81 A.J. Vega,Am. Chem. Sot. Symp. Ser. 218 ( 1983) 217. [ 91 C.P. Herrero, J. Phys. Chem. 95 (I991 ) 3282. [ IO] E. Dempsey, in: Molecularsieves, ed. R.M. Barrer (Society ofchemical Industry, London, 1968) p. 293.
[ I I ] C.P. Herrero, Chem. Phys. Letters 17I ( 1990) 369. [ 121 G. Ooms, R.A. van Santen, C.J.J. den Ouden, R.A. Jackson and C.R.A. Catlow, J. Phys. Chem. 92 ( 1988) 4462. [ 131 G.R. Eulenberger, D.P. Shoemaker and J.G. Keil, J. Phys. Chem. 71 (1967) 1812. [ 141 D.W. Heermann, Computer simulation methods (Springer, Berlin, 1986). [IS] S. Kirkpatrick, C.D. Gelatt Jr. and M.P. Vecchi, Science 220 (1983) 671. [ 161 W. Pitsch, U. Gahn and G. Inden, Z. Metallk. 75 (1984) 575. [ 171 D.H. Olson, .I. Phys. Chem. 74 ( 1969) 2758. [ 181 B. Sulikowski and J. Klinowski, J. Chem. Sot. Faraday Trans. 86 ( 1990) 199. [ 191S. Ramdas, J.M. Thomas, 1. Klinowski, CA. Fyfe and J.S. Hartman, Nature 292 ( 1981) 228.
203
CHEMICAL PHYSICS LETTERS
30 August 199I
Long- versus short-range Si, Al ordering in zeolites X and Y Carlos P. Herrero, Luis Utrera and Rafael Ramirez Instituto de Ciencia de Materiales,
CSIC, Serrano,
115 dpdo., 28006 Madrid,
Spain
Received 14 May 1991; in final form I4 June 1991
A Monte Carlo method is employed to study the Si, Al distribution in zeolites X and Y. The simulation includes long-range Coulomb interactions and oxygen polarization effects. Both contributions to the lattice energy are found to be necessary for a realistic description of the tetrahedral cation ordering. The equilibrium Si, Al distributions show short-range order for all analyzed compositions ( I <.%/AI < 3), in good agreement with % NMR results. Long-range ordering is only found in the X region (Sij Al < I .4). Two order-disorder transitions are found at framework compositions for which earlier X-ray diffraction analyzes detected discontinuities in the lattice parameter as a function of Al loading.
1. Introduction The determination of both framework and extraframework cation ordering in aluminosilicate zeolites is a long-standing problem that has not yet been solved in a general fashion. The first indication for Si, Al ordering in faujasites was presented by Dempsey et al. [ 1] from the change of the lattice parameter a0 as a function of the Al content. This dependence is found to be linear, but it presents two discontinuities, which were assigned to possible changes in the Si, Al ordering pattern. During the eighties, 29Si nuclear magnetic resonance (NMR) spectroscopy has been used to detect different silicon environments in the faujasite framework, thus allowing one to investigate the short-range order of tetrahedral (T) cations in this structure [ 2-41. Nowadays, the most commonly accepted feature of Si, Al distributions in these materials is the avoidance of aluminium in contiguous tetrahedra (the so-called Loewenstein’s rule [ 5]), which was confirmed by NMR spectroscopy [ 6,7]. Moreover, the tendency of Al atoms to be dispersed beyond nearest neighbours has been addressed by several authors [ 4,8,9 1. From a theoretical point of view, Dempsey reported electrostatic energy calculations for different Si, Al ordering schemes in faujasite-like zeolites [ lo]. These energy calculations assumed implicitly the presence of long-range order in the T-atom distri-
bution, a fact that, in general, has not been demonstrated. Moreover, energy differences between certain ordering patterns are not high, and thermal effects can be non-negligible at the formation temperatures of these aluminosilicates. In fact, the term due to configurational entropy variations is of the same order as the energy changes found for several ordering schemes [ 9,111. Other recent calculations on the stability of various zeolitic frameworks for different %/Al ratios have neglected the contribution of Si, Al ordering to the lattice energy [ 121, which, however, can be significant in stabilizing the zeolite structures. In this Letter, the Si, Al ordering in zeolites X and Y is analyzed by a Monte Carlo method, aild special emphasis is laid upon the extent of order (short- versus long-range) in the distribution of tetrahedral cations. The results obtained are compared to *‘Si NMR and X-ray diffraction data.
2. Simulation method The unit formula of the faujasites studied in the present work is Na, (AI,Si,92-n0384)r with n in, the range 48-96. _u,, and Xsi will denote the Al and Si atomic fractions, respectively ( xAl = n/ 192 and Xsi= I -xAI). Atomic coordinates given by Eulenberger et al. [ 131 for dehydrated sodium faujasite
0009-2614/91/$ 03.50 0 1991 Elsevier Science Publishers B.V. All rights reserved.
199
Volume 183, number 3,4
CHEMICAL PHYSICS LETTERS
were employed in the calculations. Na+ cations are supposed to be randomly distributed over the available extra-framework sites. For each framework composition, 3x lo5 Si, Al configurations were generated by ,the Metropolis procedure [ 141 at the temperature of faujasite formation ( T, = 350 K). Longer sampling does not improve the statistics of the calculated structural variables. In order to “equilibrate” the system at 350 K, a “simulated annealing” process was carried out before each simulation run. From an initial temperature of about 2000 K, the system was slowly cooled down to the simulation temperature T,. This procedure reduces the risk that the simulation starts from a metastable state, as described by Kirkpatrick et al.
1151. The lattice energy associated with a given cation distribution was calculated as a sum of three terms due to Coulomb interactions, polarization energy, and short-range repulsive potentials. The interactions that do not depend on the Si, Al ordering give a constant contribution to the lattice energy and do not need to be explicitly considered in the calculations. For instance, the contribution of the shortrange interatomic potential does not change as a function of tetrahedral cation ordering, since the number of Si-0, Al-O, and Na-0 bonds is a constant for each framework composition. Atomic contributions to the polarization energy were calculated as a function of the ionic polarizabilities and the electric field at each atomic center. Cation polarizabilities were not considered, since they are much lower than that of oxygen [ 121 ( ao2- = 1.984 A’, (I1si4+ =0.026 A3, ~*,3+ =0.049 A3, ~11Na+ =O. 128 A’). The only free parameter in our model is, therefore, the difference between the atomic charges of Si and Al in the lattice (Ss= qsi- qAl). This charge difference appears as a quadric factor in both electrostatic and polarization energies. Given the uncertainties in the effective atomic charges of Al and Si in these compounds, we have taken a value 6q=O.32 e (e, unit charge), which gives the best agreement between the local Si, Al ordering found in our simulations and that deduced from earlier NMR experiments, as described below. In order to compare the degree of aluminium dispersion in simulated distributions with that in real faujasites, we calculated the average number t, of Al200
30 August 199 I
Si-Al triplets per silicon atom (note that oxygen anions lying between tetrahedral cations are omitted to simplify the notation). This triplet concentration can be obtained from the 29SiNMR spectra by means of the expression, t, =I* + 31, f61, ,
(1)
where 1, is the intensity (normalized to unity) of the NMR signal associated with the Si-environment containing i Al and (4-i) Si. In general, t, t t2t t, = 6, where t2 and t3 are the average numbers of Al-Si-Si and Si-Si-Si triplets per silicon, respectively (i.e. each silicon is the center of six triplets). The short-range order present in the Si, Al distribution was quantified by means of order parameters showing pair correlations between tetrahedral sites. Given a site T,, we define the variable o’= ( t 1, - 1) if an atom (Al, Si) is at site Ti. Then, the pair correlation (c#} is given by the average of the product a’~#for all equivalent-site pairs (i, j) in the generated configurations [ 161. In particular, we are interested in the correlation .S,= ( o’cr3) between second-neighbour T-atoms, which can be deduced from NMR spectra. The correlation S, = (~‘a’) between nearest T-atoms is given by the AI-Al pair avoidance (S, = 1-4x*,), and does not provide any further information about the atom distribution. We have also investigated the presence of longrange order in the Si, Al distribution, which is known to appear in zeolite X for n near 96 [ 171. Although in faujasites, ail T-sites are actually crystallographitally equivalent [ 131, solely for the purpose of detecting the appearance of Si, Al long-range ordering, we classify the 192 T-sites in the unit cell in two subsets (T, and T2, each one containing 96 T-sites) that alternate in the network (i.e. the nearest neighbours of each T,-site are four T,-sites and vice versa). Calling n, and n2 the number of Al atoms on sites T, and TZ, respectively, one can quantify the long-range order of an atom distribution by the parameter L, defined as L=$n,-&)I
9
(2)
where the bars mean “absolute value”. This parameter takes its maximum value ( + 1) when all Al atoms are located at the same subset of T-sites (T, or
30 August 199I
CHEMICAL PHYSICS LETTERS
Volume 183, number 3,4
T, ), and it equals zero when aluminium occupies
both subsets with the same probability.
3. Results and discussion
moving the continuous curve towards the dashed line in fig. 1. Pair correlations between second-neighbour T-sites are shown in fig. 2. Points are obtained from 29Si NMR spectra [6,7,18,19] by means of the equation, S~=l-(I,
In fig. 1, closed symbols represent values of t, derived from *‘Si NMR spectra of synthetic faujasites published by several authors [ 6,7,18,19]. The continuous line in this figure shows the dependence of the Al-Si-Al concentration on the framework composition, obtained by our Monte Carlo simulations for 6q=O.32 e. The contribution of oxygen polarization to the lattice energy is essential to obtain good agreement between simulated and experimental t, values. In fact, the dashed curve in fig. 1 was calculated without consideration of polarization effects, and no better agreement with the experimental points is found in this case for any choice of atomic charges. The estimated error bar for 6q is kO.05 e. Lower values of 6q yield significant amounts of AlAl pairs, in clear disagreement with experimental results (Loewenstein’s rule), whereas higher 6q values cause a decrease of the Al-Si-Al concentration,
t4Z*tZJ)X,i,
(3)
which follows from the definition given above for S, (the relationship between NMR line intensities and triplet concentrations is given in detail in ref. [ 9 ] ). Like for the Al-Si-Al concentration, an increase of the pair correlation .S, is found for increasing Al content in the region n ~64-96. For n< 64, however, a rather well-defined plateau is observed, which can be related to a specific short-range order pattern of the Si, Al distribution. To quantify short-range order differences between faujasite-like zeolites with different framework compositions, we calculated changes of the total number N, of triplets Al-Si-AI as a function of aluminium loading by using the equation, &=1+&N,-6n), T
which can be deduced from eqs. ( 1) and (3 ) (note
-48
64
80
96
Al atoms / cell
0
1 48
I
1
64
80
96
Al atoms / cell Fig. I. AI-Si-AI triplet concentration versus number of Al atoms per unit cell. Closed symbols correspond to values derived from NMR data of synthetic faujasites. Lines are obtained from Monte Carlo simulation: dashed curve, equilibrium distribution considering only long-range Coulomb interactions; continuous line, distribution simulated by including both Coulomb and polarization energy terms.
Fig. 2. Second-neighbour correlation S, versus number of Al atoms per unit cell. Closed symbols represent values deduced from *%i NMR spectra. The continuous line is obtained from simulations at temperature T, = 350 K. Vertical lines separate regions with different ordering schemes. Regions corresponding to X-and Y-type zeolites are indicated.
201
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CHEMICAL PHYSICS LETTERS
that N, =nrxsiti and nr is the number of T-sites per cell, &-= 192). Then, for Y-type zeolites with n < 64, one finds AN, JAn z 6, and for n > 64, MI JAn x 12. This result is in line with the X-ray diffraction data of Dempsey et al. [ 11, which show a discontinuity in the lattice parameter for an Al loading n x 64. The average number of Al atoms per hexamer ring in this zeolitic structure is given by n/32. Thus, the break at n=64 corresponds to a framework composition for which one has, on average, two Al atoms per hexamer ring. As suggested previously [ 1,7], the arrangement of Si and Al atoms on these rings is qualitatively different in the two ranges: n < 64 and n>64. Our results indicate that for hexamer rings containing two Al atoms, the “para” disposition of these atoms is favoured over the “meta” disposition for n<64, and the opposite is found for n> 64. According to ref. [ I], the zone between the dashed lines in fig. 2 (i.e. 64
66
72
78
84
90
96
30 August 1991
and consequently, that they will be crystallographitally equivalent. For n > 8 1, however, Al occupies basically only one subset of T-sites (T, or T,), which corresponds to a well-defined long-range order scheme. This crystal symmetry reduction is in agreement with the assignment of the space group Fd3 to zeolite X, versus the group Fdjm to zeolite Y [ 13,17 1. The rather steep order-disorder transition as a function of Al loading shown in fig. 3 coincides with the discontinuity in the lattice parameter observed by Dempsey et al. [ 1 ] between 80 and 81 Al atoms per unit cell. No other long-range order scheme has been found in our equilibrium distributions in the range n=48-80. “Maximum dispersion of aluminium” models, as those proposed previously [ 6,7] for faujasites in the Y region (n < SO), are not favoured due to oxygen polarization effects and configurational entropy contributions to the free energy at the temperatures of faujasite formation. Both effects contribute to increase the number of Al-Si-Al triplets found at T=O only for Coulomb interactions.
4, Concluding remarks As for other aluminosilicates, Loewenstein’s rule is a natural consequence of the dilution of T-atoms with different charge. In contrast to earlier quantitative [lo] and qualitative [6,7] energetic considerations on the Si, Al distribution in zeolites X and Y, oxygen polarization is found to be important in defining the T-atom ordering. In this context, Coulomb interactions give only a rough approach to the distribution of T-atoms. Two order-disorder transitions are found as a function of Al concentration in the composition interval studied here (48-96 Al atoms per unit cell). The transition observed at n=64 takes place between two different short-range order schemes, but that detected at 80 Al atoms per cell is a transition between short- and long-range order. These order-disorder transitions are in agreement with structural information derived from 29SiNMR spectra and X-ray diffraction studies.
Al atoms / cell Fig. 3. Long-range order parameter L obtained from simulation for faujasites with different Al contents. An arrow indicates the framework composition at which a discontinuity is observed by X-ray diffraction in the lattice parameter a~.
202
Acknowledgement We thank E. Matesanz and C.E. Alonso for tech-
Volume 183, number 3,4
CHEMICAL PHYSICS LETTERS
nical support and J.M. Serratosa for critical comments on the manuscript. This work was supported by CICYT (Spain) under contract number MAT880166.
References [ I ] E. Dempsey, G.H. Kilhl and D.H. Olson, J. Phys. Chem. 73 (1969) 387. [2] E. Lippmaa, M. M%gi, A. Samoson, M. Tarmak and G. Engelhardt, J. Am. Chem. Sot. 103 (1981) 4992. [3] J. Klinowski, Progr. NMR Spectry. 16 (1984) 237. [ 41 G. Engelhardt and D. Michel, High resolution solid state NMR of silicates and zeolites (Wiley, New York, 1987). [ 51W. Loewenstein, Am. Mineral. 39 ( 1954) 92. [ 61G. Engelhardt, U. Lohse, E. Lippmaa, M. Tarmak and M. Magi, Z. Anorg. Allg. Chem. 482 ( I98 I ) 49. [7] J. Klinowski, S. Ramdas, J.M. Thomas, C.A. Fyfe and J.S. Hartman, J. Chem. Sot. Faraday Trans. II 78 ( 1982) 1025.
30 August 199 I
[ 81 A.J. Vega,Am. Chem. Sot. Symp. Ser. 218 ( 1983) 217. [ 91 C.P. Herrero, J. Phys. Chem. 95 (I991 ) 3282. [ IO] E. Dempsey, in: Molecularsieves, ed. R.M. Barrer (Society ofchemical Industry, London, 1968) p. 293.
[ I I ] C.P. Herrero, Chem. Phys. Letters 17I ( 1990) 369. [ 121 G. Ooms, R.A. van Santen, C.J.J. den Ouden, R.A. Jackson and C.R.A. Catlow, J. Phys. Chem. 92 ( 1988) 4462. [ 131 G.R. Eulenberger, D.P. Shoemaker and J.G. Keil, J. Phys. Chem. 71 (1967) 1812. [ 141 D.W. Heermann, Computer simulation methods (Springer, Berlin, 1986). [IS] S. Kirkpatrick, C.D. Gelatt Jr. and M.P. Vecchi, Science 220 (1983) 671. [ 161 W. Pitsch, U. Gahn and G. Inden, Z. Metallk. 75 (1984) 575. [ 171 D.H. Olson, .I. Phys. Chem. 74 ( 1969) 2758. [ 181 B. Sulikowski and J. Klinowski, J. Chem. Sot. Faraday Trans. 86 ( 1990) 199. [ 191S. Ramdas, J.M. Thomas, 1. Klinowski, CA. Fyfe and J.S. Hartman, Nature 292 ( 1981) 228.
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