- Email: [email protected]
Charge-transfer molecular dynamics of protonated faujasite
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials
Studies in Surface Science and Catalysis, Vol. 105 1997 Elsevier Science B.V. All rights reserved.
2275
Charge-Transfer Molecular Dynamics of Protonated Faujasite Luis Javier Alvarez, ~ * Pedro Bosch Giral, b Claudio Zicovich Wilson c t and Jorge E. Ss163 a ~Laboratorio de Simulaci6n de Materiales, Direcci6n General de Servicios de CSmputo Acad~mico, Universidad Nacional Aut6noma de M6xico, Insurgentes Sur 3000, Zona Cultural, Ciudad Universitaria, Coyoacs 04510, M6xico, D. F., M6xico. bDepartamento de Quimica, Universidad Aut6noma Metropolitana-Iztapalapa, Av. Michoacs y Purlsima, Iztapalapa 09340, M~xico, D. F., M~xico. CGruppo di Chimica Te6rica, Dipartamento di Chimica Inorgs Chimica Fisica y Chimica dei Materiali, Univercita di Torino, 4 Giuria 5, 10125 Torino, Italia. Charge-transfer molecular dynamics simulations of protonated faujasite have been carried out in the microcanonical ensamble. The parameters of the pair interaction potentials of some species have been obtained from ab-initio calculations and are able to reproduce the bond-making and breaking of the Si-O pair. The simulated system consisted of 653 particles of which 165 were silicon atoms, 414 oxygen atoms, 47 aluminium atoms and 27 hydrogen atoms. Structural and dynamical properties of the system are well reproduced and some insight into the location and formation mechanisms of acid sites is obtained. 1. I N T R O D U C T I O N In a previous work[l], we have presented charge-transfer molecular dynamics simulations of sodium Y zeolite with extra-framework alumina where radial distribution functions, IR spectrum, and NMR experimental results were reproduced with this simulation scheme. Moreover, a mechanism of formation of extra-framework alumina was envisaged, and some of the physico-chemical aspects of alumina existence in the network were outlined. As a continuation of the previous study, sodium atoms were substituted by hydrogen atoms to simulate the protonated zeolite. Experimentally, the dehydration and thermal decomposition of ammonium-exchanged zeolite Y described as decationisation consists of a crystalline zeolite, from which the cations were removed by extensive NH + exchanged, followed by dehydration and removal of NH4 and NH3. The crystal structure *This work was partially supported by Cray Research Inc. under supercomputing grant DGAPASC-2095, and by the Commission of the European Communities under R+D grants CI1'-CT92-0016 and CI1'CT94-0064. This project was developed within the frame of Proyecto V.4 of Programa Iberoamericano de Ciencia y Tecnologia para el Desarrollo (CYTED). tOn leave from Instituto de Tecnologfa Qufmica, Universidad Polit~cnica de Valencia- CSIC., Av. Naranjos s/n 46022 Valencia. Spain. SOn leave from Departamento de Ffsica, Universidad Aut6noma Metropolitana-Iztapalapa, M~xico
2276 is known to remain unchanged at temperatures as high as 900 K with some structural changes occuring at 1000 K. Furthermore, if the zeolite is treated to obtain an ultrastable zeolite the hydrolisis of framework aluminium is expected. The obtained zeolite is a solid acid catalyst where Lewis sites are found in nanoparticles of non-framework alumina. The BrSnsted sites are constituted by OH bridging between framework silicon and aluminium atoms. Blumenfeld, et al [2] suggest that an important fraction of the acidic OH reacts with, and therefore is neutralised by non-framework alumina. In this work we present a first attempt to study the acidic Y zeolite using molecular dynamics simulations, paying special attention to the structural, dynamical and acidic properties of the system which compare fairly well with experimental data. 2. C A L C U L A T I O N S
2.1. Molecular Dynamics Simulations Molecular dynamics simulations involve the solution of the classical equation,s of motion of a set of N particles in a box of volume V with periodic boundary conditions, which interact through a pairwise additive potential. In our simulations an empirical charge transfer scheme was used to model the partially covalent Si-O interaction, whereas for all the other interactions a Pauling's type potential was employed. For the charge transfer potential we use a coulombic term plus a covalent term, coupled through a charge-transfer function, and allow charges on particles to change according to their position and local environment. The details of this empirical charge-transfer scheme are described elsewhere.[1,3] The original structure of the framework was set up using a total of 576 particles of which 192 were silicon atoms and 386 were oxygen atoms. Then 27 aluminium atoms were placed randomly in sites originally occupied by silicon atoms, taking into account the Lowenstein avoidance rule. Then, extra aluminium and oxygen atoms were added in such a way as to decrease the Si/A1 ratio from the original 6.11 to 3.51. Finally 27 H atoms were introduced in the structure in the vicinity of substitutional aluminium atoms. The Si/A1 ratios considering framework and extra-framework aluminium atoms, roughly correspond to those experimentally found by NMR and chemical analysis respectively. [4] The simulation procedure was as follows: First an equilibration run was performed for 110 ps to set the system at 300 K, followed by a 10 ps run without temperature control to relax the system. A further production run was carried out for 5 ps in order to obtain the statistics of the simulation. Radial distribution and velocity autocorrelation functions were calculated and vibrational spectra were obtained to be compared with experimental IR data. The calculations were performed using SIMULA code.[5] The overall simulation took about 50 hours on the CRAY Y-MP/464 of the supercomputing center of the Universidad Nacional AutSnoma de M~xico. 3. R E S U L T S A N D D I S C U S S I O N
3.1. Structure Figure 1 is a photo of the simulated H-Y zeolite. Figure 2 shows only the wire structure so the crystallinity can be observed. Hydrogen atoms, represented by the very small spheres are regularly distributed in the network and in the vicinity of aluminium atoms.
2277
Figure 1. Photo of the simulated H - Y zeolite.
Figure 2. Wire structure of the simulated H- Y zeolite.
These in turn may be part of the zeolite framework, or part of the nanoparticles of extra lattice alumina. Figure 3 is a zoom view of a section of the large cavity, where a nanoparticle of alumina can be observed bridged to the network by a hydrogen atom. Figure 4 represents the relative general position of hydrogen atoms linked to a dangling oxygen, characterised in our previous work. These two kinds of OH bridges can be correlated with the results of Blumenfeld et al,[2] who distinguished bridging OH, first, as those which can donate its proton to NH3, forming NH + which can reorient isotropically, and second, OH operating as a proton donor to a strongly hydrogen bonded amonia. The calculated partial radial distribution functions ploted in figure 5 reproduce the experimental results of other authors. The Si-O first neighbour peak appears in the simulation at 1.642 ,~, whereas the experimental value is 1.684 ~. Our results for the H-O and H-A1 first neighbour peaks are at 1.570 and 2.84 ]k respectively. The corresponding experimental values are 1.94 and 2.38 A[6,71 The average H-O-Si angle was calculated from the simulations to compare it with the one obtained from ab-initio calculations to be reported elsewhere. The simulation value yields 125.03 degrees with a standard deviation of 28 degrees, whereas the theoretical one is of 119.67 degrees. Our simulations reveal that this angle is strongly dependent on the local environment of hydrogen atoms.
3.2. Spectrum Figure 6 shows the vibrational spectra of each species obtained by Fourier transforming the velocity autocorrelation functions. The bands which can be associated with previous results are tabulated in Table 1 and the wavenumbers of the main peaks are summarized in Table 2. The main features of all spectra agree fairly well with experimental data, [8-10] however it is worth noting some characteristics of the calculated spectra: The low frequency band below 250 cm -1 in the present simulation is due to the extraframework aluminium atoms, and not to compensation cations, since they are not present in the system. In our previous simulation of the NaY zeolite this band is due to both compensation cations and extra framework aluminium. The assignment of the band around 730 cm -1 has been a matter of debate in the zeolite literature. In our simulations, hydrogen vibrations are the main contributors to
2278
Figure 3.
large cavity.
Nanoparticle of alumina at the
Figure 4. Relative general position of hy-
drogen atoms linked to a dangling oxygen.
the intensity in that band of the specttum. However there are contributions of aluminium and oxygen intensities. This strongly suggests that this vibrational mode is possibly due to three or four coordinated extra-framework aluminiums with hydrogen atoms associated to them. The peaks a t 2500 and 3200 cm -1 in our spectra can be associated to the LF and HF bands of hydroxyls, whereas the one at 2850 cm -1 would correspond to the strong acid band of hydroxyls. The obvious shift of the simulated signals with respect to the experimental ones can be explained by the absence of water in the simulation. This can be better understood thinking of the closest confinement of extra-framework species when water molecules are present in the system. Another possible reason for this shifting is that our H-O pair potential may be under-estimating the force constant and therefore yielding a lower vibrational frequency. Indeed a larger force constant would reduce the H-O and H-A1 interatomic distances and would increase the vibrational frequency of hydroxils and their shift with respect to the corresponding experimental data. Finally, we attribute the peaks of silicon and oxygen at 1650 cm -1 to the vibrational
2279
60.0
T
I
T
...........
Si-Si
40.0
Si-O
g(r)
O-O 20.0
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.
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~/~" .' '..\
/
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'-
-
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.
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.
, ,
.
.
.
.
.
.
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?
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/"
.
.
.
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.
.' '
i
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.
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i
,
;,
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:.
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\
',
\
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\
.....~.,~~:::5",,"=~.-
^ -
200.0 gt(r)
150.0 100.0 50.0 0.0 1.0
2.0
3.0 r,.~
4.0
5.0
Figure 5. Partial radial distribution functions.
modes of the dangling oxygens in the network which have not been reported from experimental studies, perhaps because of their very low number. However these could be related to the water vibration whenever it is present. In fact this frequency in the IR spectrum is associated to crystalisation water. We believe that these dangling oxygens which appear only in the simulated structure may be responsible for the signal associated to water since they would serve as anchors for water molecules. Figure 6 shows a local configuration where this effect can be clearly observed even though in our simulation water is not explicitly introduced. In the real system the IR signals due to water and dangling oxygens may be located in the same frequency band. However the presence of water in the system would certainly produce new local configurations .~.t the surface of cavities.
2280 8.0
[
9
,
9
,
9
,
.
,
9
,
: 6"0[I 1 I(r
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,
i AI
o ll,
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..
:
Ji
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-
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"
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~ -^_it"l,
0.0 0.0
500.0
1000.0
1500.0
2000.0
2500.0
3000.0
-1
CO, c m
Figure 6. Vibracional spectra of all species.
3.3. Acid sites In the present simulation Lewis acid sites appear located at extra-framework alumina (EF) nanoparticles as well as associated to framework aluminium atoms. In general, EF aluminium atoms appear coordinated to two oxygen atoms because the alumina nanoparticles are constituted strictly by two aluminium atoms and three oxygen atoms. This is due to the absence of water molecules in the system. Upon addition of water in the simulation it is to be expected that these aluminium atoms increase their coordination number giving place to a strong Lewis acid site. The framework alllminium atoms show coordination numbers of three and four, and therefore would have a lesser degree of acidity. This result agrees with experimental data reported by Yong et a/.[ll] BrSnsted sites appear mainly associated to framework bridging hydroxyls, although they also are present as EF alumina terminating hydroxyls. Both kinds of acid sites are therefore present in
2281
Table 1 A comparison between previous results and this work. Structure insensitive vibrations (cm -1) Experimental MD results Previous work Present work maximum Asym. stretch 950-1250 950-1250 1146 950-1150 Sym. stretch 650-720 625-755 729 625-800 T-O bending 420-500 416-500 443 400-500 Structure sensitive vibrations (cm -1) Asym.stretch 1050-1150 1050-1150 1068 1020-1160 Sym. strech 750-820 751-807 781 Double ring 500-650 495-625 573 580-680 Pore opening 300-420 300-416 364 340-420
maximum 1027 755 455 1035 625 354
the simulated zeolite. Some of them are shown in figures 1, 3 and 4.
4. C O N C L U S I O N S The simulated protonated-Y zeolite shows some features which were not present in our previous simulated cationic-Y zeolite, such as the hydrogen bridging of extra-framework alumina. Even with an empirical pair potential for the O-H interaction it is possible to observe the formation of some water molecules, although their characteristic angles and bond distances are not exactly the experimental ones. Acid sites of both BrSnsted and Lewis types can be identified throughout the structure. The assignment of some bands of the vibrational spectrum is possible since the molecular dynamics calculations give the individual spectrum of each species of the system. Computer simulation of the structure and dynamics of Y zeolite have been performed using a combination of ab initio and empirical interaction potentials. Even though the potentials used in our simulations are not all obtained from quantum mechanical calculations, some results on both the structure and vibrational spectra are in good agreement with experimental data, and some insight into the formation of acid sites can be obtained. The overall physico-chemical properties of the system are thus well represented by the interaction models we used and allow for a better understanding of the protonated structure which includes extra-framework alumina, as well as a new possible interpretation of infra-red spectra. Further work is undertaken to improve the quality and representativeness of the interaction potentials as well as to simulate more realistic zeolitic systems such as hydrated protonated zeolites. With these in hand the reproduction of known physicochemical properties of this important catalyst will enhance the prediction capabilities of numerical simulation.
2282 Table 2 Positions of the main peaks Range 0000-0200 0200-0400
Silicon 116, 156, 182 246, 286, 352
0400-0600
417, 455, 495 ,520, 560. 626, 691,742 ,795. 860, 900, 938 ,978. 1043
0600-0800 0800-1000 1000-1200 1200-1400 1400-1800 1800-3300
1315,1368 1668
(cm-1) Oxygen 117, 155 246, 312, 352 ,390. 417, 455, 495 ,520, 560. 626, 691 854, 900, 951
Aluminum 117, 156 235, 286, 352 ,390. 430, 455, 467 ,495, 520, 560. 626, 665, 716 755. 860, 900, 990
1029
1029,] 133
1315,1368 1668
Hydrogen 182 352, 390 422, 455, 467 ,520, 560. 638, 677, 755 820, 860, 900 ,939, 978. 1029,1068,1133 ,1174. 1368 1446,1541 2144,2370,2490 ,2860,3222.
REFERENCES
1. L.J. Alvarez, A. Ramirez Solls and Pedro Bosch Giral, Zeolites (1995) submitted. 2. A.L. Blumenfeld, D. Coster, and J. J. Fripiat, J. Phys. Chem. 99 (1995) 15181-15191. 3. A. Alavi, L. J. Alvarez, S. R. Elliott, and I. R. McDonald, Phil. Mag. B, 65 (1992) 489 4. D. Coster, A. Blumenfeld, and J. J. Fripiat, J. Phys. Chem. 98 (1994) 6201. 5. SIMULA is a molecular dynamics and visualisation software developed by L. J. Alvarez at Laboratorio de Simulacidn de Materiales, DGSCA, Universidad Nacional Aut6noma de M6xico. 6. D. Freude, J. Klinowski and H. Hamdan, Chem. Phys. Lett. 149 (1988) 355. 7. H. Pfeifer, D. Freude and M. Hunger, Zeolites 5 (1985) 274. 8. G. Garraldn, A. Corma, and V. Forn~s, Zeolites 9 (1989) 84-86. 9. R.A. van Santen and D. L. Vogel, Advances in Solid State Chemistry, Volume 1 (1989) 151-224. 10. W. P. J. H. Jacobs, J. H. M. C. van Wolput, and R. A. van Santen, Zeolites 13 (1993) 170-182. 11. H. Yong, D. Coster, F. R. Chen, J. G. Davis, and J. J. Fripiat, New Frontiers in Catalysis, Guczi, L. et al (Editors), Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary.
Studies in Surface Science and Catalysis, Vol. 105 1997 Elsevier Science B.V. All rights reserved.
2275
Charge-Transfer Molecular Dynamics of Protonated Faujasite Luis Javier Alvarez, ~ * Pedro Bosch Giral, b Claudio Zicovich Wilson c t and Jorge E. Ss163 a ~Laboratorio de Simulaci6n de Materiales, Direcci6n General de Servicios de CSmputo Acad~mico, Universidad Nacional Aut6noma de M6xico, Insurgentes Sur 3000, Zona Cultural, Ciudad Universitaria, Coyoacs 04510, M6xico, D. F., M6xico. bDepartamento de Quimica, Universidad Aut6noma Metropolitana-Iztapalapa, Av. Michoacs y Purlsima, Iztapalapa 09340, M~xico, D. F., M~xico. CGruppo di Chimica Te6rica, Dipartamento di Chimica Inorgs Chimica Fisica y Chimica dei Materiali, Univercita di Torino, 4 Giuria 5, 10125 Torino, Italia. Charge-transfer molecular dynamics simulations of protonated faujasite have been carried out in the microcanonical ensamble. The parameters of the pair interaction potentials of some species have been obtained from ab-initio calculations and are able to reproduce the bond-making and breaking of the Si-O pair. The simulated system consisted of 653 particles of which 165 were silicon atoms, 414 oxygen atoms, 47 aluminium atoms and 27 hydrogen atoms. Structural and dynamical properties of the system are well reproduced and some insight into the location and formation mechanisms of acid sites is obtained. 1. I N T R O D U C T I O N In a previous work[l], we have presented charge-transfer molecular dynamics simulations of sodium Y zeolite with extra-framework alumina where radial distribution functions, IR spectrum, and NMR experimental results were reproduced with this simulation scheme. Moreover, a mechanism of formation of extra-framework alumina was envisaged, and some of the physico-chemical aspects of alumina existence in the network were outlined. As a continuation of the previous study, sodium atoms were substituted by hydrogen atoms to simulate the protonated zeolite. Experimentally, the dehydration and thermal decomposition of ammonium-exchanged zeolite Y described as decationisation consists of a crystalline zeolite, from which the cations were removed by extensive NH + exchanged, followed by dehydration and removal of NH4 and NH3. The crystal structure *This work was partially supported by Cray Research Inc. under supercomputing grant DGAPASC-2095, and by the Commission of the European Communities under R+D grants CI1'-CT92-0016 and CI1'CT94-0064. This project was developed within the frame of Proyecto V.4 of Programa Iberoamericano de Ciencia y Tecnologia para el Desarrollo (CYTED). tOn leave from Instituto de Tecnologfa Qufmica, Universidad Polit~cnica de Valencia- CSIC., Av. Naranjos s/n 46022 Valencia. Spain. SOn leave from Departamento de Ffsica, Universidad Aut6noma Metropolitana-Iztapalapa, M~xico
2276 is known to remain unchanged at temperatures as high as 900 K with some structural changes occuring at 1000 K. Furthermore, if the zeolite is treated to obtain an ultrastable zeolite the hydrolisis of framework aluminium is expected. The obtained zeolite is a solid acid catalyst where Lewis sites are found in nanoparticles of non-framework alumina. The BrSnsted sites are constituted by OH bridging between framework silicon and aluminium atoms. Blumenfeld, et al [2] suggest that an important fraction of the acidic OH reacts with, and therefore is neutralised by non-framework alumina. In this work we present a first attempt to study the acidic Y zeolite using molecular dynamics simulations, paying special attention to the structural, dynamical and acidic properties of the system which compare fairly well with experimental data. 2. C A L C U L A T I O N S
2.1. Molecular Dynamics Simulations Molecular dynamics simulations involve the solution of the classical equation,s of motion of a set of N particles in a box of volume V with periodic boundary conditions, which interact through a pairwise additive potential. In our simulations an empirical charge transfer scheme was used to model the partially covalent Si-O interaction, whereas for all the other interactions a Pauling's type potential was employed. For the charge transfer potential we use a coulombic term plus a covalent term, coupled through a charge-transfer function, and allow charges on particles to change according to their position and local environment. The details of this empirical charge-transfer scheme are described elsewhere.[1,3] The original structure of the framework was set up using a total of 576 particles of which 192 were silicon atoms and 386 were oxygen atoms. Then 27 aluminium atoms were placed randomly in sites originally occupied by silicon atoms, taking into account the Lowenstein avoidance rule. Then, extra aluminium and oxygen atoms were added in such a way as to decrease the Si/A1 ratio from the original 6.11 to 3.51. Finally 27 H atoms were introduced in the structure in the vicinity of substitutional aluminium atoms. The Si/A1 ratios considering framework and extra-framework aluminium atoms, roughly correspond to those experimentally found by NMR and chemical analysis respectively. [4] The simulation procedure was as follows: First an equilibration run was performed for 110 ps to set the system at 300 K, followed by a 10 ps run without temperature control to relax the system. A further production run was carried out for 5 ps in order to obtain the statistics of the simulation. Radial distribution and velocity autocorrelation functions were calculated and vibrational spectra were obtained to be compared with experimental IR data. The calculations were performed using SIMULA code.[5] The overall simulation took about 50 hours on the CRAY Y-MP/464 of the supercomputing center of the Universidad Nacional AutSnoma de M~xico. 3. R E S U L T S A N D D I S C U S S I O N
3.1. Structure Figure 1 is a photo of the simulated H-Y zeolite. Figure 2 shows only the wire structure so the crystallinity can be observed. Hydrogen atoms, represented by the very small spheres are regularly distributed in the network and in the vicinity of aluminium atoms.
2277
Figure 1. Photo of the simulated H - Y zeolite.
Figure 2. Wire structure of the simulated H- Y zeolite.
These in turn may be part of the zeolite framework, or part of the nanoparticles of extra lattice alumina. Figure 3 is a zoom view of a section of the large cavity, where a nanoparticle of alumina can be observed bridged to the network by a hydrogen atom. Figure 4 represents the relative general position of hydrogen atoms linked to a dangling oxygen, characterised in our previous work. These two kinds of OH bridges can be correlated with the results of Blumenfeld et al,[2] who distinguished bridging OH, first, as those which can donate its proton to NH3, forming NH + which can reorient isotropically, and second, OH operating as a proton donor to a strongly hydrogen bonded amonia. The calculated partial radial distribution functions ploted in figure 5 reproduce the experimental results of other authors. The Si-O first neighbour peak appears in the simulation at 1.642 ,~, whereas the experimental value is 1.684 ~. Our results for the H-O and H-A1 first neighbour peaks are at 1.570 and 2.84 ]k respectively. The corresponding experimental values are 1.94 and 2.38 A[6,71 The average H-O-Si angle was calculated from the simulations to compare it with the one obtained from ab-initio calculations to be reported elsewhere. The simulation value yields 125.03 degrees with a standard deviation of 28 degrees, whereas the theoretical one is of 119.67 degrees. Our simulations reveal that this angle is strongly dependent on the local environment of hydrogen atoms.
3.2. Spectrum Figure 6 shows the vibrational spectra of each species obtained by Fourier transforming the velocity autocorrelation functions. The bands which can be associated with previous results are tabulated in Table 1 and the wavenumbers of the main peaks are summarized in Table 2. The main features of all spectra agree fairly well with experimental data, [8-10] however it is worth noting some characteristics of the calculated spectra: The low frequency band below 250 cm -1 in the present simulation is due to the extraframework aluminium atoms, and not to compensation cations, since they are not present in the system. In our previous simulation of the NaY zeolite this band is due to both compensation cations and extra framework aluminium. The assignment of the band around 730 cm -1 has been a matter of debate in the zeolite literature. In our simulations, hydrogen vibrations are the main contributors to
2278
Figure 3.
large cavity.
Nanoparticle of alumina at the
Figure 4. Relative general position of hy-
drogen atoms linked to a dangling oxygen.
the intensity in that band of the specttum. However there are contributions of aluminium and oxygen intensities. This strongly suggests that this vibrational mode is possibly due to three or four coordinated extra-framework aluminiums with hydrogen atoms associated to them. The peaks a t 2500 and 3200 cm -1 in our spectra can be associated to the LF and HF bands of hydroxyls, whereas the one at 2850 cm -1 would correspond to the strong acid band of hydroxyls. The obvious shift of the simulated signals with respect to the experimental ones can be explained by the absence of water in the simulation. This can be better understood thinking of the closest confinement of extra-framework species when water molecules are present in the system. Another possible reason for this shifting is that our H-O pair potential may be under-estimating the force constant and therefore yielding a lower vibrational frequency. Indeed a larger force constant would reduce the H-O and H-A1 interatomic distances and would increase the vibrational frequency of hydroxils and their shift with respect to the corresponding experimental data. Finally, we attribute the peaks of silicon and oxygen at 1650 cm -1 to the vibrational
2279
60.0
T
I
T
...........
Si-Si
40.0
Si-O
g(r)
O-O 20.0
/ I t,\,
.
'
~/~" .' '..\
/
0.0
A1-O Si-AI
"
" ~ - ' -
......
'-
-
"
"='---'-
H - S i
20.0
.
g(r)
. .
."".
.
, ,
.
.
.
.
.
.
' " i
?
0.0
/"
.
.
.
H-O
.
.' '
i
10.0
.
/
i
,
;,
/
:.
H-A1 H-H
\
',
\
/
\
"...................
\
.....~.,~~:::5",,"=~.-
^ -
200.0 gt(r)
150.0 100.0 50.0 0.0 1.0
2.0
3.0 r,.~
4.0
5.0
Figure 5. Partial radial distribution functions.
modes of the dangling oxygens in the network which have not been reported from experimental studies, perhaps because of their very low number. However these could be related to the water vibration whenever it is present. In fact this frequency in the IR spectrum is associated to crystalisation water. We believe that these dangling oxygens which appear only in the simulated structure may be responsible for the signal associated to water since they would serve as anchors for water molecules. Figure 6 shows a local configuration where this effect can be clearly observed even though in our simulation water is not explicitly introduced. In the real system the IR signals due to water and dangling oxygens may be located in the same frequency band. However the presence of water in the system would certainly produce new local configurations .~.t the surface of cavities.
2280 8.0
[
9
,
9
,
9
,
.
,
9
,
: 6"0[I 1 I(r
9
,
i AI
o ll,
A
4.0
A
..
:
Ji
- -
-
"'
I
"
30 2.0
'
' '
1.0
nj
~ -^_it"l,
0.0 0.0
500.0
1000.0
1500.0
2000.0
2500.0
3000.0
-1
CO, c m
Figure 6. Vibracional spectra of all species.
3.3. Acid sites In the present simulation Lewis acid sites appear located at extra-framework alumina (EF) nanoparticles as well as associated to framework aluminium atoms. In general, EF aluminium atoms appear coordinated to two oxygen atoms because the alumina nanoparticles are constituted strictly by two aluminium atoms and three oxygen atoms. This is due to the absence of water molecules in the system. Upon addition of water in the simulation it is to be expected that these aluminium atoms increase their coordination number giving place to a strong Lewis acid site. The framework alllminium atoms show coordination numbers of three and four, and therefore would have a lesser degree of acidity. This result agrees with experimental data reported by Yong et a/.[ll] BrSnsted sites appear mainly associated to framework bridging hydroxyls, although they also are present as EF alumina terminating hydroxyls. Both kinds of acid sites are therefore present in
2281
Table 1 A comparison between previous results and this work. Structure insensitive vibrations (cm -1) Experimental MD results Previous work Present work maximum Asym. stretch 950-1250 950-1250 1146 950-1150 Sym. stretch 650-720 625-755 729 625-800 T-O bending 420-500 416-500 443 400-500 Structure sensitive vibrations (cm -1) Asym.stretch 1050-1150 1050-1150 1068 1020-1160 Sym. strech 750-820 751-807 781 Double ring 500-650 495-625 573 580-680 Pore opening 300-420 300-416 364 340-420
maximum 1027 755 455 1035 625 354
the simulated zeolite. Some of them are shown in figures 1, 3 and 4.
4. C O N C L U S I O N S The simulated protonated-Y zeolite shows some features which were not present in our previous simulated cationic-Y zeolite, such as the hydrogen bridging of extra-framework alumina. Even with an empirical pair potential for the O-H interaction it is possible to observe the formation of some water molecules, although their characteristic angles and bond distances are not exactly the experimental ones. Acid sites of both BrSnsted and Lewis types can be identified throughout the structure. The assignment of some bands of the vibrational spectrum is possible since the molecular dynamics calculations give the individual spectrum of each species of the system. Computer simulation of the structure and dynamics of Y zeolite have been performed using a combination of ab initio and empirical interaction potentials. Even though the potentials used in our simulations are not all obtained from quantum mechanical calculations, some results on both the structure and vibrational spectra are in good agreement with experimental data, and some insight into the formation of acid sites can be obtained. The overall physico-chemical properties of the system are thus well represented by the interaction models we used and allow for a better understanding of the protonated structure which includes extra-framework alumina, as well as a new possible interpretation of infra-red spectra. Further work is undertaken to improve the quality and representativeness of the interaction potentials as well as to simulate more realistic zeolitic systems such as hydrated protonated zeolites. With these in hand the reproduction of known physicochemical properties of this important catalyst will enhance the prediction capabilities of numerical simulation.
2282 Table 2 Positions of the main peaks Range 0000-0200 0200-0400
Silicon 116, 156, 182 246, 286, 352
0400-0600
417, 455, 495 ,520, 560. 626, 691,742 ,795. 860, 900, 938 ,978. 1043
0600-0800 0800-1000 1000-1200 1200-1400 1400-1800 1800-3300
1315,1368 1668
(cm-1) Oxygen 117, 155 246, 312, 352 ,390. 417, 455, 495 ,520, 560. 626, 691 854, 900, 951
Aluminum 117, 156 235, 286, 352 ,390. 430, 455, 467 ,495, 520, 560. 626, 665, 716 755. 860, 900, 990
1029
1029,] 133
1315,1368 1668
Hydrogen 182 352, 390 422, 455, 467 ,520, 560. 638, 677, 755 820, 860, 900 ,939, 978. 1029,1068,1133 ,1174. 1368 1446,1541 2144,2370,2490 ,2860,3222.
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