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Double interaction between ammonia and NaY zeolite and migration of Na+ upon adsorption of ammonia evidenced by neutron diffraction
Studies in Surface Science and Catalysis, volume 154 E. van Steen, L.H. Callanan and M. Claeys (Editors) © 2004 Elsevier B.V. All rights reserved.
1757
DOUBLE INTERACTION BETWEEN AMMONIA AND NaY ZEOLITE AND MIGRATION OF Na^ UPON ADSORPTION OF AMMONIA EVIDENCED BY NEUTRON DIFFRACTION Gilles, ¥.\ Blin, J . L . \ Mellot-Draznieks, C.^ Cheetham, A.K.^ and Su, B.L.^ 'Laboratoire de Chimie des Materiaux Inorganiques, The University of Namur (FUNDP), 61 Rue de Bruxelles, B-5000 Namur, Belgium. E-mail: [email protected] and [email protected] ^Institut Lavoisier, Universite de Versailles Saint-Quentin, 45 Avenue des Etats-Unis, F-78035 Versailles Cedex, France. Materials Research Laboratory, University of California Santa Barbara, CA 93106, USA.
ABSTRACT The crystal structure of NaY + ND3 has been determined at 20 K by Rietveld analysis of powder neutron diffraction data in space group Fd-3m. The cations were located at site 1' (8), site II (32) and at a third site (21) in the a-cage around (0.71, -0.83, 0.25). This reveals a significant migration of the cations from the site I, which is totally emptied, and the site F to large cages. The sites I and I' are known to be favorable cation sites in bare NaY. All the ND3 detected are located in the a-cage in electrostatic interaction with the Na^counter-ions and in hydrogen bonding interaction with the framework oxygen atoms. It is for the first time that this double interaction is experimentally evidenced on cationic zeolite with ammonia These interactions provoke a deformation of the Gt molecular symmetry of the ammonia molecule. INTRODUCTION Zeolites are a class of crystalline alumino-silicate minerals that are of great interest in catalysis, purification and separation processes due to their uniform pore size distribution and shape selectivity. Knowledge concerning the positions of the extra-framework cations and framework changes, with and without adsorbate, is crucial in understanding the adsorption, separation and catalytic properties of the zeolitic materials. Indeed, the positions of the cations can be influenced by adsorbed molecules such as water or polar molecules [1-3]. In this aim, ammonia adsorption behavior in zeolites and its effect on cations position have been studied because of its use in the synthesis of dimethylamine by reaction with methanol over zeolitic catalysts [4]. Our previous infrared and temperature programmed desorption studies [5-6] have evidenced the existence of an electrostatic interaction between the lone electron pair of the nitrogen atom of the ammonia molecule and the counter-ions of the zeolitic framework. Desorption experiments have revealed that the strength of this interaction increases with the Lewis acidity of the counter-ions of the zeolite (from Cs^ to Li^ in the alkali series). However, the shifts in wave-number of the ammonia molecule stretching vibration modes are contrary to the expectation. The highest shift content to the lower wavenumbers is present for the Cs^exchanged zeolite and the lowest for the Li^-exchanged one. On the basis of the Sanderson equalization electronegativity principle [7], Cs-exchanged zeolite possesses a higher negative charge on its oxygen atoms, thus more basic than Li-exchanged zeolite. As the shift to the lower wave-number can be correlated to the weakness of the N-H bond of the ammonia molecule, these observations suggests that the N-H bond is more affected by the interaction with the CsY than with the NaY and thus that the higher interaction is observed for the Cs^-exchanged zeolite. Therefore the higher the negative charge on the oxygen atoms, the higher the wave-number shift content. From this observation, it has been postulated that besides the interaction between the lone electron pair of the nitrogen atom of the ammonia molecule and the counter-ions, another interaction between the hydrogen atoms of this molecule and the framework oxygen atoms could exist. This postulated double cooperative interaction can explain the decomposition of the ammonia molecule even at room temperature. This property of cationic zeolites in the decomposition of ammonia is quite interesting and important since this can be exploited in the methylation of ammonia reaction to produce methylamines and for the hydrogen production using ammonia as hydrogen source for fuel cells uses.
1758 The combination of infrared spectroscopy and TPD technique showed to be very efficient in the determination of interaction strength between adsorbate and adsorbant. However, the challenges arise in the understanding of the location of extraframework cations and the change in zeolite framework upon adsorption of ammonia. This paper deals thus with the neutron diffraction study of the adsorption of da ammonia in NaY zeolite in order to shed some light at an atomic level on the location of the guest molecules, their interaction and their effect on the counter-ions in the zeolitic structure. EXPERIMENTAL SECTION Starting samples NaY zeolite powders (provided by Union carbide) have a chemical formula of Na56Sii36Al560384 (Si/Al = 2.43) and particle size of 2-4 |xm. Adsorption of d3-ammonia on the dehydrated sample A defined amount of NaY zeolite was loaded in a quartz reactor which was connected to a vacuum line. A dried oxygen flow was introduced. The reactor was heated very slowly (2 K/min.) from room temperature to 723 K. this temperature was maintained for 6h. The vacuum was then realized. The sample was calcined under vacuum (10"^ Torr) for another 6h at the same temperature. Finally, the reactor was self-cooled down to room temperature. ~20 molecules of deuterated ammonia per unit cell were loaded on the sample at liquid nitrogen temperature. The sample was transferred in a glove box under argon atmosphere into a thin-walled vanadium can. The vanadium can was finally sealed and kept at room temperature until the neutron diffraction experiment was performed. Neutron data collection and refinement details The neutron powder diffraction data were collected using the high resolution powder diffractometer G42 at the Orphee reactor at the Laboratoire Leon Brillouin (Saclay, France) with a neutron wavelength of X = 2.343A and a temperature T = 20 K. The neutron diffraction patterns were recorded between 5 and 172° by steps of 0.1° (20). The data were analyzed by Rietveld method using the FULLPROF program [8]. The range > 140° (20) was excluded due to peak asymmetry. A total of 273 reflections were used. The diffraction peaks were shaped using a pseudo-Voigt function. The background was fixed manually. The framework atoms, i.e. silicium, aluminium and oxygen were firstly refined according to the known crystal structure of faujasite NaY, in the space group Fd-3m [9]. The parameters of the tetrahedral framework atoms were kept free to refine but were cheeked and reset for consistency. Then based on the inspection of the electron density obtained by the Fourier difference, the positions of the sodium counter-ions were introduced. Two sites were found for sodium cations. The first one situated at (x = y = z = 0.235) corresponding to sites II and the second one at (x = y = z = 0.045) characteristic of sites P. The reliability factors Rwp = 25.6 and RB = 20.9 presents relatively high values at this stage where the position and occupancy of the framework atoms and Na^ cations were refined. Numerous different Fourier maps were then calculated to locate the last sodium counter-ions and the sorbate ds-ammonia molecules. The maps reveal the presence of atoms in the a-cage near the framework. Some of these atoms were in van der Waals contact with the framework oxygen atoms and were identified as D of the ammonia molecule. The whole ammonia molecule was rebuilt using the different Fourier maps. The residual density peaks were further evidenced in the a-cage in the vinicity of the ammonia molecules. This position (0.71, -0.83, 0.25) was then introduced in the FULLPROF program fixing the position and the occupancy factors of the ammonia atoms. Finally, the asymmetry factors were refined and all the parameters were progressively kept free of refinement to reduce the difference between their refinement and neutron diffraction pattern. The reliability parameters Rwp and RB were decreased to 14 and 7.7, respectively. At the end, 61 Na^ per unit cell were found while the expected number is 56 per unit cell. 14 of-20 ammonia molecules were located. RESULTS AND DISCUSSION Details of the refinements are given in Table 1. Atomic parameters of NaY + ND3 are shown in Table2 with the isotropic temperature factors. Framework interatomic distances, angles and host-guest interaction distances are given in Table 3. The final observed, calculated and difference plots for the neutron diffraction data are shown in Figure 1.
1759 Table 1. Diffractogram information.
Data type Data collection temperature (K) 20 range (°) Step scan increment (°) Space group
NaY + NDs 20 5-146 0.1 Fd-3m 24.79 a (A) Profile R-factoT (R^f 0.148 0.140 Weighted profile i?-factor (R^f Structure i?-factor (Rff 0.131 0.077 Bragg i?-factor (R^f 0.044 Expected 7?-factor (RQXVY 10.3 t' Wavelength (A) 2.343 Zero correction -0.1561 Scale factor 0.0085 273 Number of reflections Profile function Pseudo-Voigt U VW 0.116-0.15 0.237 Background Manual Rietveld refinement was used to minimize Ewi(/o,i - /c,i)^, where /o,i and /c,i are the observed and calculated powder diffraction intensities for the zth point, respectively. Weigths Wi are l//o,i. ^ Weigthed and unweigthed profile 7?-factors are defined as T^wp = {[Ew,(/o,, - 4,)']/[2wi(/o,,)']}'^2 and R, = Il/o, - 4,1/2/o,. ' The structure /^-factor is defined as Rj: = ll(/o,0^^^ - (4i)^^^l/)']/ ^(/o,,)^^^.' The Bragg i?-factor is defined as RB = Sl/o,k - ^,kl/^ h,k^ where /k is the integrated intensity of the ^ h reflection. '^ The expected i?-factor (the statistically best possible value for R^) is defined as /?exp = [{NP)/(S Wj/oi^)]^^^, where A^ is the number of observed powder diffraction data point and P is the number of refined parameters. ^ X^ was calculated from (i?wp/ ^exp)^Table 2. Atomic parameters for NaY + ND3 at 20 K.
Atom
Site multiplicity
X
Si,Al 0(1) 0(2) 0(3) 0(4) Na(l) Na(2) Na(3) N(l) D(ll) D(12) D(13) N(2) D(21) D(22)
192i 96h 96g 96g 96g 32e 32e 192i 192i 192i 192i 192i 192i 192i 192i
-0.0545(4) -0.1085(3) -0.0043(2) -0.0370(4) 0.0750(3) 0.054(2) 0.2294(5) 0.711(2) 0.863(3) -0.107(2) 0.858(4) 0.898(3) 0.970(4) 0.96(1) 0.941(3)
y 0.0350(5) 0.0000 -0.0043(2) 0.0690(2) 0.0750(3) 0.054(2) 0.2294(5) -0.832(3) -0.099(2) 0.875(3) 0.943(3) 0.916(3) 0.970(4) 0.957(7) 0.933(3)
z 0.1238(4) 0.1085(3) 0.1431(3) 0.0690(2) 0.3175(3) 0.054(2) 0.2294(5) 0.247(2) 0.178(2) 0.219(2) 0.175(3) 0.183(3) 0.406(6) 0.37(1) 0.490(4)
B,so(A^) 3.9(1) 4.37(1) 4.37(1) 4.37(1) 4.37(1) 2.312(5) 2.312(5) 2.312(5) 4(1) 4(1) 4(1) 4(1) 4(1) 4(1) 4(1)
Occupancy 1 0.5 0.5 0.5 0.5 0.040(5) 0.167 0.11(1) 0.063(2) 0.063(2) 0.063(2) 0.063(2) 0.008(2) 0.008(2) 0.008(2)
1760 The adsorption of ammonia molecules in NaY has only small effects on the framework structure [9]. The average (T-O) bond lengths and (T-O-T) angles are hardly modified but the cation site occupancies are greatly influenced. In the bare NaY zeolite, the counter-ions are located at sites SI (x = y = z = 0) ; SV {x= y = z = 0.0061) and Sll (x = y = z = 0.224) with occupancy of 8, 19 and 30, respectively [9-11]. In our present study, 61 have been found while the number of cations presents in such a material is 56. Comparing to the values reported in Table 2, it is obvious that the adsorption of ammonia does not affect the position and the occupancy of the SII site which is fully occupied, i.e. 32 ions per unit cell. However, the cations located at SI (in hexagonal prism) and ST (in sodalite cage) sites are affected by the adsorption of ammonia molecules. The first one disappeared and the second one losses half cations to reach 8 cations per unit cell. All these cations are displaced into the a-cage.
29 O Figure 1. Plot showing the observed neutron diffraction pattern of NaY + ND3 at 20 K, with a loading on average of-20 molecules per unit cell (dots) overlaid with the calculated pattern from the crystal structure (plain line). The reflection positions (vertical lines) and the deviations between the observed and calculated results (full line), which are shown on the same scale as the observed pattern. Table 3. Selected bond lengths (A) and angles (°) for theframework,cations ant ND3 molecules in NaY zeolite T-O(l) T-0(2) T-0(3) T-0(4) Average T-0 T-0(1)-T T-0(2)-T T-0(3)-T T-0(4)-T Average T-O-T 0(l)-T-0(2) 0(l)-T-0(3) 0(l)-T-0(4) 0(2)-T-0(3) 0(2)-T-0(4) 0(3)-T-0(4) Average 0-T-O
1.638 1.652 1.657 1.644 1.648 131.1 143.6 139.9 153.4 142.0 111.8 107.1 109.8 109.8 106.2 112.3 109.5
N(l)-D(ll) N(l)-D(12) N(l)-D(13) D(l)-N(l)-D(2) D(l)-N(l)-D(3) D(2)-N(1).D(3) N(2)-D(21) N(2)-D(22) Na(3) •••• 0(4) Na(3) •••• 0(2) Na(3)--N(l) Na(3)--N(2) D(3)--0(l) D(2)--0(l) D(2)-0(l)
1.080 1.036 0.961 106.4 72.4 74.3 0.969 1.098 3.45 3.44 3.12 3.24 2.80 2.34 2.81
1761 These displaced counter-ions are located at (0.71, -0.83, 0.24) in the super-cage. This position is above six membered ring windows of the super-cage with typical Na(3)'-0(2) and Na(3)--0(4) distances of 3.44 and 3.45 A. The absence of sodium ions at sites I and the decreasing of the occupancy of the sites F, together with the detection of sodium ions close to the six-ring windows in the a-cage, may be regarded as a result of a cation migration from sites SI and ST upon adsorption of ammonia molecule. Such a cation migration of the counter-ions from their dehydrated position in bare NaY to the super-cage, has been evidenced with water or chlorofluorocarbons molecules by C. P. Grey and C. Mellot-Draznieks [1-3], respectively and has never been evidenced upon adsorption of ammonia molecule. This kind of displacement did not take place in NaY system upon adsorption of benzene although a migration of sodium ions from small to large cages was revealed upon adsorption of benzene in Na-EMT system [12, 13]. The Rietveld refinement allows to detect 14 ammonia molecules that are all located in the super-cage of the zeolitic framework near sodium ions. These ammonia molecules can be classified into two groups. The first one with 12 molecules, noted as N(l), are located at 3.12 A of Na(3) sodium ions. For these molecules, the Dammonia'"Oframework distanccs ranging from 2.34 to 2.81 A are consistent with van der Waals interactions. In the gazeous ammonia, molecule has a tetrahedral Csv symmetry with bond lengths and angles of 1.01 A and 106.7°, respectively [ref|. These values are affected by the double interaction with the zeolite. Indeed, the N(l)-D bond lengths are then situated between 0.96 and 1.08A and the D-N(l)-D angles between 72.4 and 106.4°. These values shows that the double interaction affects strongly the symmetry of the ammonia molecule. However, for the two last ammonia molecules, noted as N(2), only an interaction with sodium ions (3.24 A) is evidenced. In this case, the symmetry of the molecule is also highly affected by the interaction (Table 3). The location of sodium ions and ammonia molecules in the super-cage is illustrated at Figure 2. Such an interaction has experimentally and computationally been evidenced by A. K. Cheetham et al [14] between chloroform and NaY faujasite zeolite. This polar molecule can electrostatically interact with the Na^ counter ions via its chlorine atoms and also with the framework oxygen atoms via its hydrogen atoms. This kind of hydrogen bonding interaction has already been postulated by Brandel et al [15] on the basis of computational calculations between protonic zeolites and ammonia. However, this is the first time that this interaction is evidenced in a cationic zeolite with ammonia.
Figure 2. Adsorption of ND3 in NaY : adsorption site geometry obtained from the powder pattern refinement. The molecule is bound to the framework, with D---Ozeoiite interactions within typical van der Waals distances.
1762 CONCLUSIONS The neutron diffraction study confirms the existence of a double interaction between cationic zeoHtes and ammonia molecule postulated on the basis of our previous infrared observations. The present study reveals a migration of the cation situated at SI sites and of a part located at ST sites to the super-cage upon adsorption of ammonia. The observations from the present work are therefore of a great interest in designing the new adsorbents and catalysts with advanced performances. ACKNOWLEDGEMENTS F. Gilles thanks FNRS (Fonds National de la Recherche Scientifique, Belgium) for a FRIA scholarship. The authors thank Dr. J. Rodriguez-Carvajal of the Laboratoire Leon Brillouin (CEA, Saclay, France) for supervision of the neutron diffraction experiment.
REFERENCES 1. Grey, C. P., Poshni, P. I., Gualtieri, A. F., Norby, P., Hanson J. C , Corbin, D. R., J. Am. Chem. Soc, 119(1997), 1981. 2. Mellot-Draznieks, C , Rodriguez-Carvajal, J., Cox, D. E., Cheetham, A. K., Phys. Chem. Chem. Phys., 5 (2003), 1882. 3. Hunger, M., Engelhardt, G., Weitkamp, J., Microporous Materials, 3 (1995), 497. 4. Corbin, D. R., Schwarz, S., Sonnichsen, G. C. Catalysis Today, 37 (1997), 71. 5. F. Gilles, F., Docquir, F., Su., B. L., Adsorption Science and Technology, World Scientific, Sidney, 2000, 578. 6. Gilles, F., Blin, J. L., Toufar, H., Su, B. L., Stud. Surf Sci. Catal., 142 (2002), 1687. 7. Mortier, W. J., J. Catal., 55 (1978), 138. 8. Roisnel, T., Rodriguez-Carvajal, J., WinPLOTR: a Windows Tool for Powder Diffraction Pattern Analysis, Material Science Forum, Proceeding of the European Powder Diffraction Conference (EPDIC 7),vol. 378,(2001), 118. 9. Fitch, A. N., Jobic, H., Renouprez, A., J. Phys. Chem., 90 (1986), 1311. 10. Eulenberger, G. R., Shoemaker, D. P., Keil, J. G., J. Phys. Chem., 71(6) (1967), 1812. 11. Mortier, W. J., Van den Bosshe, E., Uytterhoeven, J. B., Zeolites, 4 (1984), 41. 12. Auerbach, S. M., Saravanan, C , Jousse, F., Proceedings of the 12* Interational Zeolite Conference, vol. 2 (1998), 357. 13. Su, B. L., Manoli, J. M., Potvin, C , Barthomeuf, D., J. Chem. Soc, Faraday Trans., 89(5) (1993), 857. 14. Eckert, J., Mellot-Draznieks, C , Cheetham, A. K., J. Am. Chem. Soc, 124(2) (2002), 170. 15. Brandel, M., Sauer, J., J. Mol. Catal. A : Chemical, 119 (1997), 19.
1757
DOUBLE INTERACTION BETWEEN AMMONIA AND NaY ZEOLITE AND MIGRATION OF Na^ UPON ADSORPTION OF AMMONIA EVIDENCED BY NEUTRON DIFFRACTION Gilles, ¥.\ Blin, J . L . \ Mellot-Draznieks, C.^ Cheetham, A.K.^ and Su, B.L.^ 'Laboratoire de Chimie des Materiaux Inorganiques, The University of Namur (FUNDP), 61 Rue de Bruxelles, B-5000 Namur, Belgium. E-mail: [email protected] and [email protected] ^Institut Lavoisier, Universite de Versailles Saint-Quentin, 45 Avenue des Etats-Unis, F-78035 Versailles Cedex, France. Materials Research Laboratory, University of California Santa Barbara, CA 93106, USA.
ABSTRACT The crystal structure of NaY + ND3 has been determined at 20 K by Rietveld analysis of powder neutron diffraction data in space group Fd-3m. The cations were located at site 1' (8), site II (32) and at a third site (21) in the a-cage around (0.71, -0.83, 0.25). This reveals a significant migration of the cations from the site I, which is totally emptied, and the site F to large cages. The sites I and I' are known to be favorable cation sites in bare NaY. All the ND3 detected are located in the a-cage in electrostatic interaction with the Na^counter-ions and in hydrogen bonding interaction with the framework oxygen atoms. It is for the first time that this double interaction is experimentally evidenced on cationic zeolite with ammonia These interactions provoke a deformation of the Gt molecular symmetry of the ammonia molecule. INTRODUCTION Zeolites are a class of crystalline alumino-silicate minerals that are of great interest in catalysis, purification and separation processes due to their uniform pore size distribution and shape selectivity. Knowledge concerning the positions of the extra-framework cations and framework changes, with and without adsorbate, is crucial in understanding the adsorption, separation and catalytic properties of the zeolitic materials. Indeed, the positions of the cations can be influenced by adsorbed molecules such as water or polar molecules [1-3]. In this aim, ammonia adsorption behavior in zeolites and its effect on cations position have been studied because of its use in the synthesis of dimethylamine by reaction with methanol over zeolitic catalysts [4]. Our previous infrared and temperature programmed desorption studies [5-6] have evidenced the existence of an electrostatic interaction between the lone electron pair of the nitrogen atom of the ammonia molecule and the counter-ions of the zeolitic framework. Desorption experiments have revealed that the strength of this interaction increases with the Lewis acidity of the counter-ions of the zeolite (from Cs^ to Li^ in the alkali series). However, the shifts in wave-number of the ammonia molecule stretching vibration modes are contrary to the expectation. The highest shift content to the lower wavenumbers is present for the Cs^exchanged zeolite and the lowest for the Li^-exchanged one. On the basis of the Sanderson equalization electronegativity principle [7], Cs-exchanged zeolite possesses a higher negative charge on its oxygen atoms, thus more basic than Li-exchanged zeolite. As the shift to the lower wave-number can be correlated to the weakness of the N-H bond of the ammonia molecule, these observations suggests that the N-H bond is more affected by the interaction with the CsY than with the NaY and thus that the higher interaction is observed for the Cs^-exchanged zeolite. Therefore the higher the negative charge on the oxygen atoms, the higher the wave-number shift content. From this observation, it has been postulated that besides the interaction between the lone electron pair of the nitrogen atom of the ammonia molecule and the counter-ions, another interaction between the hydrogen atoms of this molecule and the framework oxygen atoms could exist. This postulated double cooperative interaction can explain the decomposition of the ammonia molecule even at room temperature. This property of cationic zeolites in the decomposition of ammonia is quite interesting and important since this can be exploited in the methylation of ammonia reaction to produce methylamines and for the hydrogen production using ammonia as hydrogen source for fuel cells uses.
1758 The combination of infrared spectroscopy and TPD technique showed to be very efficient in the determination of interaction strength between adsorbate and adsorbant. However, the challenges arise in the understanding of the location of extraframework cations and the change in zeolite framework upon adsorption of ammonia. This paper deals thus with the neutron diffraction study of the adsorption of da ammonia in NaY zeolite in order to shed some light at an atomic level on the location of the guest molecules, their interaction and their effect on the counter-ions in the zeolitic structure. EXPERIMENTAL SECTION Starting samples NaY zeolite powders (provided by Union carbide) have a chemical formula of Na56Sii36Al560384 (Si/Al = 2.43) and particle size of 2-4 |xm. Adsorption of d3-ammonia on the dehydrated sample A defined amount of NaY zeolite was loaded in a quartz reactor which was connected to a vacuum line. A dried oxygen flow was introduced. The reactor was heated very slowly (2 K/min.) from room temperature to 723 K. this temperature was maintained for 6h. The vacuum was then realized. The sample was calcined under vacuum (10"^ Torr) for another 6h at the same temperature. Finally, the reactor was self-cooled down to room temperature. ~20 molecules of deuterated ammonia per unit cell were loaded on the sample at liquid nitrogen temperature. The sample was transferred in a glove box under argon atmosphere into a thin-walled vanadium can. The vanadium can was finally sealed and kept at room temperature until the neutron diffraction experiment was performed. Neutron data collection and refinement details The neutron powder diffraction data were collected using the high resolution powder diffractometer G42 at the Orphee reactor at the Laboratoire Leon Brillouin (Saclay, France) with a neutron wavelength of X = 2.343A and a temperature T = 20 K. The neutron diffraction patterns were recorded between 5 and 172° by steps of 0.1° (20). The data were analyzed by Rietveld method using the FULLPROF program [8]. The range > 140° (20) was excluded due to peak asymmetry. A total of 273 reflections were used. The diffraction peaks were shaped using a pseudo-Voigt function. The background was fixed manually. The framework atoms, i.e. silicium, aluminium and oxygen were firstly refined according to the known crystal structure of faujasite NaY, in the space group Fd-3m [9]. The parameters of the tetrahedral framework atoms were kept free to refine but were cheeked and reset for consistency. Then based on the inspection of the electron density obtained by the Fourier difference, the positions of the sodium counter-ions were introduced. Two sites were found for sodium cations. The first one situated at (x = y = z = 0.235) corresponding to sites II and the second one at (x = y = z = 0.045) characteristic of sites P. The reliability factors Rwp = 25.6 and RB = 20.9 presents relatively high values at this stage where the position and occupancy of the framework atoms and Na^ cations were refined. Numerous different Fourier maps were then calculated to locate the last sodium counter-ions and the sorbate ds-ammonia molecules. The maps reveal the presence of atoms in the a-cage near the framework. Some of these atoms were in van der Waals contact with the framework oxygen atoms and were identified as D of the ammonia molecule. The whole ammonia molecule was rebuilt using the different Fourier maps. The residual density peaks were further evidenced in the a-cage in the vinicity of the ammonia molecules. This position (0.71, -0.83, 0.25) was then introduced in the FULLPROF program fixing the position and the occupancy factors of the ammonia atoms. Finally, the asymmetry factors were refined and all the parameters were progressively kept free of refinement to reduce the difference between their refinement and neutron diffraction pattern. The reliability parameters Rwp and RB were decreased to 14 and 7.7, respectively. At the end, 61 Na^ per unit cell were found while the expected number is 56 per unit cell. 14 of-20 ammonia molecules were located. RESULTS AND DISCUSSION Details of the refinements are given in Table 1. Atomic parameters of NaY + ND3 are shown in Table2 with the isotropic temperature factors. Framework interatomic distances, angles and host-guest interaction distances are given in Table 3. The final observed, calculated and difference plots for the neutron diffraction data are shown in Figure 1.
1759 Table 1. Diffractogram information.
Data type Data collection temperature (K) 20 range (°) Step scan increment (°) Space group
NaY + NDs 20 5-146 0.1 Fd-3m 24.79 a (A) Profile R-factoT (R^f 0.148 0.140 Weighted profile i?-factor (R^f Structure i?-factor (Rff 0.131 0.077 Bragg i?-factor (R^f 0.044 Expected 7?-factor (RQXVY 10.3 t' Wavelength (A) 2.343 Zero correction -0.1561 Scale factor 0.0085 273 Number of reflections Profile function Pseudo-Voigt U VW 0.116-0.15 0.237 Background Manual Rietveld refinement was used to minimize Ewi(/o,i - /c,i)^, where /o,i and /c,i are the observed and calculated powder diffraction intensities for the zth point, respectively. Weigths Wi are l//o,i. ^ Weigthed and unweigthed profile 7?-factors are defined as T^wp = {[Ew,(/o,, - 4,)']/[2wi(/o,,)']}'^2 and R, = Il/o, - 4,1/2/o,. ' The structure /^-factor is defined as Rj: = ll(/o,0^^^ - (4i)^^^l/)']/ ^(/o,,)^^^.' The Bragg i?-factor is defined as RB = Sl/o,k - ^,kl/^ h,k^ where /k is the integrated intensity of the ^ h reflection. '^ The expected i?-factor (the statistically best possible value for R^) is defined as /?exp = [{NP)/(S Wj/oi^)]^^^, where A^ is the number of observed powder diffraction data point and P is the number of refined parameters. ^ X^ was calculated from (i?wp/ ^exp)^Table 2. Atomic parameters for NaY + ND3 at 20 K.
Atom
Site multiplicity
X
Si,Al 0(1) 0(2) 0(3) 0(4) Na(l) Na(2) Na(3) N(l) D(ll) D(12) D(13) N(2) D(21) D(22)
192i 96h 96g 96g 96g 32e 32e 192i 192i 192i 192i 192i 192i 192i 192i
-0.0545(4) -0.1085(3) -0.0043(2) -0.0370(4) 0.0750(3) 0.054(2) 0.2294(5) 0.711(2) 0.863(3) -0.107(2) 0.858(4) 0.898(3) 0.970(4) 0.96(1) 0.941(3)
y 0.0350(5) 0.0000 -0.0043(2) 0.0690(2) 0.0750(3) 0.054(2) 0.2294(5) -0.832(3) -0.099(2) 0.875(3) 0.943(3) 0.916(3) 0.970(4) 0.957(7) 0.933(3)
z 0.1238(4) 0.1085(3) 0.1431(3) 0.0690(2) 0.3175(3) 0.054(2) 0.2294(5) 0.247(2) 0.178(2) 0.219(2) 0.175(3) 0.183(3) 0.406(6) 0.37(1) 0.490(4)
B,so(A^) 3.9(1) 4.37(1) 4.37(1) 4.37(1) 4.37(1) 2.312(5) 2.312(5) 2.312(5) 4(1) 4(1) 4(1) 4(1) 4(1) 4(1) 4(1)
Occupancy 1 0.5 0.5 0.5 0.5 0.040(5) 0.167 0.11(1) 0.063(2) 0.063(2) 0.063(2) 0.063(2) 0.008(2) 0.008(2) 0.008(2)
1760 The adsorption of ammonia molecules in NaY has only small effects on the framework structure [9]. The average (T-O) bond lengths and (T-O-T) angles are hardly modified but the cation site occupancies are greatly influenced. In the bare NaY zeolite, the counter-ions are located at sites SI (x = y = z = 0) ; SV {x= y = z = 0.0061) and Sll (x = y = z = 0.224) with occupancy of 8, 19 and 30, respectively [9-11]. In our present study, 61 have been found while the number of cations presents in such a material is 56. Comparing to the values reported in Table 2, it is obvious that the adsorption of ammonia does not affect the position and the occupancy of the SII site which is fully occupied, i.e. 32 ions per unit cell. However, the cations located at SI (in hexagonal prism) and ST (in sodalite cage) sites are affected by the adsorption of ammonia molecules. The first one disappeared and the second one losses half cations to reach 8 cations per unit cell. All these cations are displaced into the a-cage.
29 O Figure 1. Plot showing the observed neutron diffraction pattern of NaY + ND3 at 20 K, with a loading on average of-20 molecules per unit cell (dots) overlaid with the calculated pattern from the crystal structure (plain line). The reflection positions (vertical lines) and the deviations between the observed and calculated results (full line), which are shown on the same scale as the observed pattern. Table 3. Selected bond lengths (A) and angles (°) for theframework,cations ant ND3 molecules in NaY zeolite T-O(l) T-0(2) T-0(3) T-0(4) Average T-0 T-0(1)-T T-0(2)-T T-0(3)-T T-0(4)-T Average T-O-T 0(l)-T-0(2) 0(l)-T-0(3) 0(l)-T-0(4) 0(2)-T-0(3) 0(2)-T-0(4) 0(3)-T-0(4) Average 0-T-O
1.638 1.652 1.657 1.644 1.648 131.1 143.6 139.9 153.4 142.0 111.8 107.1 109.8 109.8 106.2 112.3 109.5
N(l)-D(ll) N(l)-D(12) N(l)-D(13) D(l)-N(l)-D(2) D(l)-N(l)-D(3) D(2)-N(1).D(3) N(2)-D(21) N(2)-D(22) Na(3) •••• 0(4) Na(3) •••• 0(2) Na(3)--N(l) Na(3)--N(2) D(3)--0(l) D(2)--0(l) D(2)-0(l)
1.080 1.036 0.961 106.4 72.4 74.3 0.969 1.098 3.45 3.44 3.12 3.24 2.80 2.34 2.81
1761 These displaced counter-ions are located at (0.71, -0.83, 0.24) in the super-cage. This position is above six membered ring windows of the super-cage with typical Na(3)'-0(2) and Na(3)--0(4) distances of 3.44 and 3.45 A. The absence of sodium ions at sites I and the decreasing of the occupancy of the sites F, together with the detection of sodium ions close to the six-ring windows in the a-cage, may be regarded as a result of a cation migration from sites SI and ST upon adsorption of ammonia molecule. Such a cation migration of the counter-ions from their dehydrated position in bare NaY to the super-cage, has been evidenced with water or chlorofluorocarbons molecules by C. P. Grey and C. Mellot-Draznieks [1-3], respectively and has never been evidenced upon adsorption of ammonia molecule. This kind of displacement did not take place in NaY system upon adsorption of benzene although a migration of sodium ions from small to large cages was revealed upon adsorption of benzene in Na-EMT system [12, 13]. The Rietveld refinement allows to detect 14 ammonia molecules that are all located in the super-cage of the zeolitic framework near sodium ions. These ammonia molecules can be classified into two groups. The first one with 12 molecules, noted as N(l), are located at 3.12 A of Na(3) sodium ions. For these molecules, the Dammonia'"Oframework distanccs ranging from 2.34 to 2.81 A are consistent with van der Waals interactions. In the gazeous ammonia, molecule has a tetrahedral Csv symmetry with bond lengths and angles of 1.01 A and 106.7°, respectively [ref|. These values are affected by the double interaction with the zeolite. Indeed, the N(l)-D bond lengths are then situated between 0.96 and 1.08A and the D-N(l)-D angles between 72.4 and 106.4°. These values shows that the double interaction affects strongly the symmetry of the ammonia molecule. However, for the two last ammonia molecules, noted as N(2), only an interaction with sodium ions (3.24 A) is evidenced. In this case, the symmetry of the molecule is also highly affected by the interaction (Table 3). The location of sodium ions and ammonia molecules in the super-cage is illustrated at Figure 2. Such an interaction has experimentally and computationally been evidenced by A. K. Cheetham et al [14] between chloroform and NaY faujasite zeolite. This polar molecule can electrostatically interact with the Na^ counter ions via its chlorine atoms and also with the framework oxygen atoms via its hydrogen atoms. This kind of hydrogen bonding interaction has already been postulated by Brandel et al [15] on the basis of computational calculations between protonic zeolites and ammonia. However, this is the first time that this interaction is evidenced in a cationic zeolite with ammonia.
Figure 2. Adsorption of ND3 in NaY : adsorption site geometry obtained from the powder pattern refinement. The molecule is bound to the framework, with D---Ozeoiite interactions within typical van der Waals distances.
1762 CONCLUSIONS The neutron diffraction study confirms the existence of a double interaction between cationic zeoHtes and ammonia molecule postulated on the basis of our previous infrared observations. The present study reveals a migration of the cation situated at SI sites and of a part located at ST sites to the super-cage upon adsorption of ammonia. The observations from the present work are therefore of a great interest in designing the new adsorbents and catalysts with advanced performances. ACKNOWLEDGEMENTS F. Gilles thanks FNRS (Fonds National de la Recherche Scientifique, Belgium) for a FRIA scholarship. The authors thank Dr. J. Rodriguez-Carvajal of the Laboratoire Leon Brillouin (CEA, Saclay, France) for supervision of the neutron diffraction experiment.
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