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Cation dynamics in zeolites studied by mas-NMR
1555
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.
CATION DYNAMICS IN ZEOLITES STUDIED BY MAS-NMR Jordan, E. and KoUer, H. Westfalische Wilhelms-Universitat Miinster, Schlossplatz 4/7, 48149 Miinster, Germany. Tel: +49 251 8323448. Fax: +49 251 8323409. E-mail: [email protected]
ABSTRACT The cationic motion in zeolites was investigated by solid-state NMR, namely the three cancrinite samples containing sodium sulfate, sodium carbonate and sodium chromate. The experiments show an interesting motional behaviour of the cations, depending on the presence of non-framework anions. The presence of alkali cations and complex anions suggests the existence of interactions, leading to cooperative motional properties. Temperature dependent ^''Na MAS NMR spectra show a considerably high local mobility of the sodium cations in both cancrinite samples that could be further characterised by high temperature spin echo NMR experiments. Conductivity measurements of the cancrinites show a long range ionic conductivity EXPERIMENTAL SECTION All cancrinite samples were prepared by hydrothermal synthesis in Teflon-lined steel autoclaves according to Sieger [3]. The phase purities of the cancrinites was confirmed by powder x-ray diffraction. All samples were carefully dehydrated under vacuum at 350°C. The MAS NMR spectra were acquired at a magnetic field of Bo = 9.4 T; additional static NMR spectra at Bo = 4.7 T and 7.05 T. The 2D exchange spectrum and some MQMAS spectra were recorded at 11.7 T. RESULTS AND DISCUSSION Chromate cancrinite Figure 1 shows the "^Na MAS NMR spectra of the chromate cancrinite with the composition |Na8Cr04|[Al6Si6024]. At 297 K two extremely narrow lines are observed, suggesting either a high local mobility or a highly symmetric coordination of the sodium cations at room temperature. The third signal is a characteristic quadrupolar powder pattern between -20 and -60 ppm with a large quadrupolar coupling constant of approximately 4.9 MHZ. For further details see Table 1. Table 1. The fit parameters for the "^"^Na MAS spin echo NMR spectrum of chromate cancrinite. Signal intensities are not corrected for quadrupolar effects. 1 Position I II III
5/s,; / ppm
1.4 -9.3 -6.67
Intensity / % 45 41 14
Co 1 MHz ^1 4.9 0.0
At temperatures above 410 K the broad quadrupolar line is further broadened, so that it is lost in the detection dead time of the probe head of the MAS NMR experiment. Above 350 K, the ^^Na MAS NMR spectra show an exchange signal between the two narrow lines indicating the onset of fast sodium cation dynamics between the two sites. The NMR characteristics between the two narrow lines does not show the expected typical coalescence phenomenon, where the two exchanging lines move towards the exchange signal at higher temperatures. These results suggest that there is a dynamic heterogeneity in the system, i.e. fast ions contribute to the exchange signal, and the separated lines originate from slow cations. Two-dimensional ~^Na MAS NMR exchange spectroscopy confirms these results (Fig. 2). The pulse sequence of this NMR experiment consists of three 7r/2-pulses: At first, the so-called preparation pulse creates transversal magnetisation. After an incrementable preparation time, Xp, in which each spin is labelled with its Larmor frequency C0|, the magnetisation is transferred in the +z-direction by the mixing pulse. The
1556 mixing pulse is followed by a constant mixing time im in which the chemical exchange between different cation positions can take place. The final reading pulse creates transversal magnetisation again. This signal is Fourier transformed in two dimensions (t and Xp) and thus gives a two-dimensional NMR spectrum. If the frequencies of a spin is the same during the times Xp and t, i.e. when it has not changed its chemical shift, then it appears as a line on the diagonal of the two-dimensional spectrum. The cross peaks in the exchange spectrum are due to sodium cations that changed their position, with corresponding different chemical shifts, in the mixing time [4]. To quantify the exchange process, experiments with different mixing times and temperatures were carried out, but for mixing times > 50 ms, there was too much signal loss due to spin-lattice relaxation. Above 360 K, the lines could not be resolved in the 2D spectra. So this experiment was limited to a very small range of mixing times and temperatures. Impedance measurements at temperatures up to 600 K show only a small long-range conductivity with an activation energy of 0.85 ±0.02 eV. Therefore, the cation dynamics observed by NMR is suggested to originate mainly from local motion.
470 K
0
-20
6/ppm Figure 1. ^^Na MAS NMR spectra of chromate cancrinite in the temperature range between 297K and 470K.
8 -10 -12 ppm Figure 2. ^^Na Exchange spectrum at 350K (Mixing time x = 50ms).
Additional static spin echo NMR spectra were recorded between room temperature and 540°C (Figure 3). The static spectra are in good agreement with the MAS NMR spectra showing one Lorentzian line and one broad static quadrupolar powder pattern. At 180°C, motional narrowing begins to reduce the width of the Lorentzian line. The second powder pattern is broadened to an extent beyond detection until it reappears at 340 °C, but now with a remarkably reduced quadrupolar coupling constant of 2.36 MHz. Upon further increasing the temperature, the asymmetry parameter r| of the quadrupolar line decreases and the singularities of the powder pattern become sharp. Additionally, the Lorentzian line is broadened again. This broadening of the line is explained by an overlap of central (m = ± Vi
1557
400 300 200 100 0 -100 -200 -300 -CO 5^ppm
300 200 100 0 -100-200-3005/ppm
200 100
0
.100 -200
Figure 3. The temperature dependence of the static ^''Na spin echo NMR spectra of chromate cancrinite Together with the results from the MAS NMR spectra, this leads to the conclusion that the Lorentzian line in the static spectra (corresponding to the two narrow signals in the MAS NMR spectra) is due to highly mobile sodium cations in the channels of the cancrinite structure, whereas the broad quadrupolar patterns in both MAS and static NMR spectra are caused by sodium cations in the cancrinite s-cages. This assumption is supported by the fact that each 8-cage has five aluminosilicate six rings. Two of them are oriented perpendicular and the other three parallel to the c axis. For Sodium cations located in front of zeolitic six rings, the electric field gradient tensor is always perpendicular to the six ring plane with an asymmetry parameter r| close to zero. An exchange process between six rings perpendicular and parallel to the c axis would cause a jump angle of the electric field gradient of 90°. For this jump angle, the quadrupolar interaction is predicted to be reduced to 50% of its value in the static case [4]. Additionally, in the range of 90° two-site jumps, the asymmetry parameter r| of the quadrupolar interaction is extremely sensitive to small changes of the jump angle. The motionally averaged quadrupole coupling constant at high temperatures is one half of this interaction at room temperature which is consistent with a 90° two-site jump. The asymmetry parameter of the quadrupolar pattern at room temperature as well as of that one above 500°C is below 0.1. These facts indicate an exactly defined jump process of the sodium cations between the different six ring sites in the s-cages. Carbonate cancrinite The ^^Na MAS NMR spectrum of the carbonate cancrinite |Na8C03|[Al6Si6024], shows four different cation sites (Figure 4) which are explicitly described in Table 2. Table 2. The fit parameters for the ^"^Na MAS spin echo spectrum of carbonate cancrinite. 1 Position
I II III IV
13,so 1 ppm
1 -0.9 -12.3 0.6 12.7
Intensity / % 59.78 20.31 18.37 1.54
Co 1 MHz 2.32 4.48 1.87 1.11
^n
1
0.43 0.10 0.06 0.5
The lines I and III are assigned sodium positions in the cancrinite channels together with the carbonate anions according to the structure refinement by Buhl and Fechtelkord [5]. The large quadrupolar coupling constant of the sodium position II is characteristic for sodium cations solely coordinated to an aluminosilicate six ring. Thus, this line can be assigned to the sodium cations inside the s-cages. ^^Na MAS NMR spectra at temperatures up to 550 K show a motional averaging of the lines leading to one single broad and featureless NMR signal. As for the chromate cancrinite system, static ^"^Na spin echo NMR spectra were recorded between room temperature and 540°C. The behaviour of the carbonate containing sample is quite similar: At low temperatures, the spectra consist of one broad Lorentzian line and of a second quadrupolar powder pattern in good agreement with the MAS NMR spectra. In contrast to the aforementioned system, the broad quadrupolar line stays almost unchanged until it is motionally averaged at 340°C. Above 420°C, a quadrupolar signal reappears with a quadrupolar coupling constant of CQ = 2.16 MHz. Upon further increasing the temperature, the intensity of the Lorentzian line is reduced strongly. These observations are
1558 quite unusual, and it may have to do with a change in the mechanisms of motion at the highest temperatures investigated. This will be subject for further investigations.
Spsctmm. Pit
50
-50 G/ppm.
-100
-150
400
-200
Figure 4. The ^^Na MAS NMR spectrum of carbonate cancrinite at room temperature.
200
0
-200
-400
Figure 5. The static ^^Na spin echo spectra of carbonate cancrinite between room temperature and 540 K.
Sulfate cancrinite The ^^Na MAS NMR spectrum of the sulfate cancrinite, |Na8S04|[Al6Si6024], shows two quadrupolar broadened lines (Figure 6) as also confirmed by ^^Na MQMAS NMR. Above 450 K, the observed signals become more and more broadened until they collapse into a single Gaussian line above 500 K (Figure 7). Conductivity measurements showed a similar long-range conductivity as for the chromate cancrinite, but with slightly lower activation energy (0.78 ± 0.02 eV).
a']t5s><.= 1.4 ppm r(=0.99
Fit
30
20
10
0
-10
-20
-30
40
S/pprn Figure 6. The ^^Na MAS NMR spectrum of sulfate cancrinite together with the line shape simulation.
40
20
-20 0 S^ppm
-40
Figure 7. The temperature dependent ^^Na MAS NMR spectra of sulfate cancrinite.
CONCLUSIONS ^^Na solid state NMR techniques provide interesting information on the local ionic motion in different cancrinite types. In the case of the chromate cancrinite, two local motional processes could be identified. One restricted to the cancrinite channels with apparently low activation energy as is of these processes
1559 indicated by the onset of motion at only slightly elevated temperatures. The second process seems to be higher activated. This second process could be further characterised by the detailed evaluation of the quadrupolar coupling constant and the local symmetry of the involved sodium sites. This second process was assigned to a two-site jump of sodium cations in the s-cages of the cancrinite structure. The different activation energies of both processes support the assumption that the chromate anions in the channel promote the sodium mobility by additional coordination to the cations or by cooperative motional processes. Conductivity measurements indicate that there is also certain long range cation mobility in the sample. In the case of carbonate cancrinite, the temperature dependent MAS NMR spectra indicate some ion mobility inside the channels whereas the static spin echo spectra show an unusual broadening of the spectrum again which needs to be further investigated. Additional experiments are underway to investigate the ion dynamics of the sulfate cancrinite.
REFERENCES 1. 2. 3. 4.
R. M. Barrer, J. F. Cole, H. Villiger, J. Chem. Soc. (A), 1523 (1970). R. M. Barrer, J. F. Cole, J. Chem. Soc, (A), 1516 (1970). P. Sieger, PhD Thesis, Konstanz (1992). K. Schmidt-Rohr, H. W. Spiess, Multidimensional Solid-State NMR and Polymers, Academic Press, 1994. 5. K. Hackbarth, Th. M. Gesing, M. Fechtelkord, F. Stief, J.-Ch. Buhl, Microporous and Mesoporous Materials 30, 347-358, (1999).
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.
CATION DYNAMICS IN ZEOLITES STUDIED BY MAS-NMR Jordan, E. and KoUer, H. Westfalische Wilhelms-Universitat Miinster, Schlossplatz 4/7, 48149 Miinster, Germany. Tel: +49 251 8323448. Fax: +49 251 8323409. E-mail: [email protected]
ABSTRACT The cationic motion in zeolites was investigated by solid-state NMR, namely the three cancrinite samples containing sodium sulfate, sodium carbonate and sodium chromate. The experiments show an interesting motional behaviour of the cations, depending on the presence of non-framework anions. The presence of alkali cations and complex anions suggests the existence of interactions, leading to cooperative motional properties. Temperature dependent ^''Na MAS NMR spectra show a considerably high local mobility of the sodium cations in both cancrinite samples that could be further characterised by high temperature spin echo NMR experiments. Conductivity measurements of the cancrinites show a long range ionic conductivity EXPERIMENTAL SECTION All cancrinite samples were prepared by hydrothermal synthesis in Teflon-lined steel autoclaves according to Sieger [3]. The phase purities of the cancrinites was confirmed by powder x-ray diffraction. All samples were carefully dehydrated under vacuum at 350°C. The MAS NMR spectra were acquired at a magnetic field of Bo = 9.4 T; additional static NMR spectra at Bo = 4.7 T and 7.05 T. The 2D exchange spectrum and some MQMAS spectra were recorded at 11.7 T. RESULTS AND DISCUSSION Chromate cancrinite Figure 1 shows the "^Na MAS NMR spectra of the chromate cancrinite with the composition |Na8Cr04|[Al6Si6024]. At 297 K two extremely narrow lines are observed, suggesting either a high local mobility or a highly symmetric coordination of the sodium cations at room temperature. The third signal is a characteristic quadrupolar powder pattern between -20 and -60 ppm with a large quadrupolar coupling constant of approximately 4.9 MHZ. For further details see Table 1. Table 1. The fit parameters for the "^"^Na MAS spin echo NMR spectrum of chromate cancrinite. Signal intensities are not corrected for quadrupolar effects. 1 Position I II III
5/s,; / ppm
1.4 -9.3 -6.67
Intensity / % 45 41 14
Co 1 MHz ^1 4.9 0.0
At temperatures above 410 K the broad quadrupolar line is further broadened, so that it is lost in the detection dead time of the probe head of the MAS NMR experiment. Above 350 K, the ^^Na MAS NMR spectra show an exchange signal between the two narrow lines indicating the onset of fast sodium cation dynamics between the two sites. The NMR characteristics between the two narrow lines does not show the expected typical coalescence phenomenon, where the two exchanging lines move towards the exchange signal at higher temperatures. These results suggest that there is a dynamic heterogeneity in the system, i.e. fast ions contribute to the exchange signal, and the separated lines originate from slow cations. Two-dimensional ~^Na MAS NMR exchange spectroscopy confirms these results (Fig. 2). The pulse sequence of this NMR experiment consists of three 7r/2-pulses: At first, the so-called preparation pulse creates transversal magnetisation. After an incrementable preparation time, Xp, in which each spin is labelled with its Larmor frequency C0|, the magnetisation is transferred in the +z-direction by the mixing pulse. The
1556 mixing pulse is followed by a constant mixing time im in which the chemical exchange between different cation positions can take place. The final reading pulse creates transversal magnetisation again. This signal is Fourier transformed in two dimensions (t and Xp) and thus gives a two-dimensional NMR spectrum. If the frequencies of a spin is the same during the times Xp and t, i.e. when it has not changed its chemical shift, then it appears as a line on the diagonal of the two-dimensional spectrum. The cross peaks in the exchange spectrum are due to sodium cations that changed their position, with corresponding different chemical shifts, in the mixing time [4]. To quantify the exchange process, experiments with different mixing times and temperatures were carried out, but for mixing times > 50 ms, there was too much signal loss due to spin-lattice relaxation. Above 360 K, the lines could not be resolved in the 2D spectra. So this experiment was limited to a very small range of mixing times and temperatures. Impedance measurements at temperatures up to 600 K show only a small long-range conductivity with an activation energy of 0.85 ±0.02 eV. Therefore, the cation dynamics observed by NMR is suggested to originate mainly from local motion.
470 K
0
-20
6/ppm Figure 1. ^^Na MAS NMR spectra of chromate cancrinite in the temperature range between 297K and 470K.
8 -10 -12 ppm Figure 2. ^^Na Exchange spectrum at 350K (Mixing time x = 50ms).
Additional static spin echo NMR spectra were recorded between room temperature and 540°C (Figure 3). The static spectra are in good agreement with the MAS NMR spectra showing one Lorentzian line and one broad static quadrupolar powder pattern. At 180°C, motional narrowing begins to reduce the width of the Lorentzian line. The second powder pattern is broadened to an extent beyond detection until it reappears at 340 °C, but now with a remarkably reduced quadrupolar coupling constant of 2.36 MHz. Upon further increasing the temperature, the asymmetry parameter r| of the quadrupolar line decreases and the singularities of the powder pattern become sharp. Additionally, the Lorentzian line is broadened again. This broadening of the line is explained by an overlap of central (m = ± Vi
1557
400 300 200 100 0 -100 -200 -300 -CO 5^ppm
300 200 100 0 -100-200-3005/ppm
200 100
0
.100 -200
Figure 3. The temperature dependence of the static ^''Na spin echo NMR spectra of chromate cancrinite Together with the results from the MAS NMR spectra, this leads to the conclusion that the Lorentzian line in the static spectra (corresponding to the two narrow signals in the MAS NMR spectra) is due to highly mobile sodium cations in the channels of the cancrinite structure, whereas the broad quadrupolar patterns in both MAS and static NMR spectra are caused by sodium cations in the cancrinite s-cages. This assumption is supported by the fact that each 8-cage has five aluminosilicate six rings. Two of them are oriented perpendicular and the other three parallel to the c axis. For Sodium cations located in front of zeolitic six rings, the electric field gradient tensor is always perpendicular to the six ring plane with an asymmetry parameter r| close to zero. An exchange process between six rings perpendicular and parallel to the c axis would cause a jump angle of the electric field gradient of 90°. For this jump angle, the quadrupolar interaction is predicted to be reduced to 50% of its value in the static case [4]. Additionally, in the range of 90° two-site jumps, the asymmetry parameter r| of the quadrupolar interaction is extremely sensitive to small changes of the jump angle. The motionally averaged quadrupole coupling constant at high temperatures is one half of this interaction at room temperature which is consistent with a 90° two-site jump. The asymmetry parameter of the quadrupolar pattern at room temperature as well as of that one above 500°C is below 0.1. These facts indicate an exactly defined jump process of the sodium cations between the different six ring sites in the s-cages. Carbonate cancrinite The ^^Na MAS NMR spectrum of the carbonate cancrinite |Na8C03|[Al6Si6024], shows four different cation sites (Figure 4) which are explicitly described in Table 2. Table 2. The fit parameters for the ^"^Na MAS spin echo spectrum of carbonate cancrinite. 1 Position
I II III IV
13,so 1 ppm
1 -0.9 -12.3 0.6 12.7
Intensity / % 59.78 20.31 18.37 1.54
Co 1 MHz 2.32 4.48 1.87 1.11
^n
1
0.43 0.10 0.06 0.5
The lines I and III are assigned sodium positions in the cancrinite channels together with the carbonate anions according to the structure refinement by Buhl and Fechtelkord [5]. The large quadrupolar coupling constant of the sodium position II is characteristic for sodium cations solely coordinated to an aluminosilicate six ring. Thus, this line can be assigned to the sodium cations inside the s-cages. ^^Na MAS NMR spectra at temperatures up to 550 K show a motional averaging of the lines leading to one single broad and featureless NMR signal. As for the chromate cancrinite system, static ^"^Na spin echo NMR spectra were recorded between room temperature and 540°C. The behaviour of the carbonate containing sample is quite similar: At low temperatures, the spectra consist of one broad Lorentzian line and of a second quadrupolar powder pattern in good agreement with the MAS NMR spectra. In contrast to the aforementioned system, the broad quadrupolar line stays almost unchanged until it is motionally averaged at 340°C. Above 420°C, a quadrupolar signal reappears with a quadrupolar coupling constant of CQ = 2.16 MHz. Upon further increasing the temperature, the intensity of the Lorentzian line is reduced strongly. These observations are
1558 quite unusual, and it may have to do with a change in the mechanisms of motion at the highest temperatures investigated. This will be subject for further investigations.
Spsctmm. Pit
50
-50 G/ppm.
-100
-150
400
-200
Figure 4. The ^^Na MAS NMR spectrum of carbonate cancrinite at room temperature.
200
0
-200
-400
Figure 5. The static ^^Na spin echo spectra of carbonate cancrinite between room temperature and 540 K.
Sulfate cancrinite The ^^Na MAS NMR spectrum of the sulfate cancrinite, |Na8S04|[Al6Si6024], shows two quadrupolar broadened lines (Figure 6) as also confirmed by ^^Na MQMAS NMR. Above 450 K, the observed signals become more and more broadened until they collapse into a single Gaussian line above 500 K (Figure 7). Conductivity measurements showed a similar long-range conductivity as for the chromate cancrinite, but with slightly lower activation energy (0.78 ± 0.02 eV).
a']t5s><.= 1.4 ppm r(=0.99
Fit
30
20
10
0
-10
-20
-30
40
S/pprn Figure 6. The ^^Na MAS NMR spectrum of sulfate cancrinite together with the line shape simulation.
40
20
-20 0 S^ppm
-40
Figure 7. The temperature dependent ^^Na MAS NMR spectra of sulfate cancrinite.
CONCLUSIONS ^^Na solid state NMR techniques provide interesting information on the local ionic motion in different cancrinite types. In the case of the chromate cancrinite, two local motional processes could be identified. One restricted to the cancrinite channels with apparently low activation energy as is of these processes
1559 indicated by the onset of motion at only slightly elevated temperatures. The second process seems to be higher activated. This second process could be further characterised by the detailed evaluation of the quadrupolar coupling constant and the local symmetry of the involved sodium sites. This second process was assigned to a two-site jump of sodium cations in the s-cages of the cancrinite structure. The different activation energies of both processes support the assumption that the chromate anions in the channel promote the sodium mobility by additional coordination to the cations or by cooperative motional processes. Conductivity measurements indicate that there is also certain long range cation mobility in the sample. In the case of carbonate cancrinite, the temperature dependent MAS NMR spectra indicate some ion mobility inside the channels whereas the static spin echo spectra show an unusual broadening of the spectrum again which needs to be further investigated. Additional experiments are underway to investigate the ion dynamics of the sulfate cancrinite.
REFERENCES 1. 2. 3. 4.
R. M. Barrer, J. F. Cole, H. Villiger, J. Chem. Soc. (A), 1523 (1970). R. M. Barrer, J. F. Cole, J. Chem. Soc, (A), 1516 (1970). P. Sieger, PhD Thesis, Konstanz (1992). K. Schmidt-Rohr, H. W. Spiess, Multidimensional Solid-State NMR and Polymers, Academic Press, 1994. 5. K. Hackbarth, Th. M. Gesing, M. Fechtelkord, F. Stief, J.-Ch. Buhl, Microporous and Mesoporous Materials 30, 347-358, (1999).