CC BY
7Scientific Notes of Taurida National V. I. Vernadsky University
Series : Physics and Mathematics Sciences. Volume 27 (66). 2014. No. 2. P. 70-78
UDK537.635 541.127
NMR OF 23Na IN NATROLITE
Paczwa M.1, Sapiga A. A.2, Olszewski M.1, Sergeev N. A.1, Sapiga A. V.2
1 Institute of Physics, University of Szczecin, Poland
2 Faculty of Physics, Taurida National V. I. Vernadsky University, Simferopol, Crimea, Russia
E-mail: ar mathematician@mail.ru
The temperature dependences of NMR and MAS NMR spectra of 23Na nuclei in Al-natrolite (Na2AkSi3Oio -2№O) and Ga-natrolite (Na2Ga2Si3Oio -2№O) have been studied. It has been shown that the diffusion of the sodium ions at T < 400 K is absent in Al- and Ga- natrolites. The temperature dependences of the spin-lattice relaxation times Ti in Al- and Ga- natrolite have been studied. The influence of the water molecular mobility in the nanochannels of natrolites on the spin-lattice relaxation times of 23Na has been discussed.
Keywords: NMR, magnetic relaxation, zeolite, natrolite, water mobility. PACS: 76.60.-k; 78.55.Mb
INTRODUCTION
The Al-natrolite (Na2Al2Si3Oi0 •2H2O) and Ga-natrolite (Na2Ga2Si3Oi0 •2H2O) are the typical channel-type compound with porous structure [i]. The natrolite framework consist of the chains AlO4 and SiO4 tetrahedra linked together via common oxygen atoms. The natrolite structure contains channels running both perpendicular and parallel to the c-axis. The water molecules and ions Na+ are located in the small nanochannels of natrolite in the form of zig-zag chains [i, 2] Each sodium ion is coordinated by two framework's oxygen atoms and by two water molecules. The sodium cations in the channels of natrolite, as well as water molecules, can possess the certain mobility. However unlike molecules of water they cannot be removed from a crystal if only to not resort by an ionic exchange [2].
The dynamics of water molecule in Al-natrolite have been studied by NMR in [2-4]. From temperature measurements of the spin-relaxation times (Ti h Tip) of iH nuclei in Al-natrolite it was concluded that in an temperature interval 330 K + 450 K takes place the reorientation of water molecules around of their pseudo-axes of second order symmetry (i80o flip motion), and in an interval 450 K + 540 K it has been assumed availability reorientation of water molecules around of the one hydrogen bonds [3]. In the subsequent it has been established, that this second type of water molecules mobility is connected with diffusion of water molecules in the channels of natrolite and the i80o flip motion take place simultaneously with diffusion along the c-axis [2, 4].
The powerful method for the study of zeolites is the NMR of quadrupole nuclei. The NMR of the quadrupole nuclei 23Na in a single crystal of Al-natrolite have been investigated at room temperature in [5, 6]. The obtained quadrupole coupling constant (QCC) and asymmetry parameter of the electric field gradient (EFG) tensor were determined to be: eqQ/h = i759 kHz; t] = 0,6427. From results represented in [4] it
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follows that the QCC of 23Na nuclei do not depend on the temperature at T < 500 K. However at T > 500 K the QCC are decreased and the observed decreasing of the QCC of 23Na nuclei is connected with diffusion of water molecules in the natrolite channels. From the observed temperature independent of NMR spectra of 23Na in Al-natrolite single crystal it was concluded that the translation diffusion of Na ions is absent in the natrolite cannels [4].
In the present paper we represent the results of the study of Al- and Ga-natrolites by the 23Na NMR. We investigate the temperature dependences of the 23Na NMR spectra in static and rotated (MAS) samples with and without :H decoupling as well as the temperature dependences of the spin-lattice relaxation times of the 23Na nuclei in Al- and Ga-natrolites.
1. EXPERIMENTAL PART
The polycrystalline samples of natural Al-natrolite from Khibiny deposit (Kola Peninsula, Russia) were used in this study. The gallium form of natrolite was hydrothermaly synthesized as described in [7]. The 23Na NMR spectra were measured at V0 = 105.842 MHz frequency in 9.4 T magnetic field using a Bruker Avance-400 NMR spectrometer. The 23Na MAS NMR spectra were measured using the 4 mm diameter zirconia (ZrO2) rotor cells equipped with powdered sample rotated under magic angle with 10 kHz frequency. Classical direct acquisitions by single pulse excitations (free induction decay - FID) were used. The 23Na has I = 3/2 nuclear spin, and for selective excitation of the central (mi = +1/2 -1/2) transition the optimal pulse duration equals to the duration of a non-selective n/2 pulse divided by I + 1/2 = 2 [8]. In our experiments the radiofrequency (RF) puls rc/4 = 1.0 pi was used. The NMR spectra were obtained by Fourier transformation of FID signals. The spin-lattice relaxation time T1 for 23Na nuclei was measured by saturation-recovery method.
The Dmfit program [9] was used to simulate the 23Na spectra to extract the isotropic chemical shifts (8iSo), quadrupolar coupling constants and the asymmetry parameters (ne). The Dmfit model of MAS NMR spectrum includes an apodisation of the theoretical lineshape by Lorentzian or Gaussian curves with the broadening parameters Av, Avl or Avq, that indicate a distribution of slightly distinct environment of nuclei.
2. OBTAINED RESULTS AND DISCUSSION
The sodium neighbours in Al- and Ga-natrolites have a configuration of a distorted tetrahedron. In the tetrahedron corners there are two oxygen atoms belonging to a framework and two oxygen atoms of water molecules at an average distances of 2.37 A. Furthermore there are two oxygen atoms of a framework at an average distances of 2.5 A, four protons at an average distances of 2.8 A and atoms of silicon and aluminum at an average distances of 3.0 A.
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The experimental 23Na MAS NMR spectrum of Al-natrolite is shown in Fig. 1, a. The rotation of sample at magic angle (MAS NMR) leads to fully average of dipolar interaction of magnetic moments of 23Na with magnetic moments of other nuclei in natrolite :H, Si, Al. In this case the shape of NMR spectra is determined only by the second-order quadrupolar shift of the central transition [8]. Using program DMFit [9] we calculated the shape of MAS NMR spectrum of 23Na in polycrystalline natrolite. The result of theoretical calculations are shown in Fig. 1, b. The obtained theoretical values of the quadrupolar frequency vq and the asymmetry parameter ^ well coincide with experimental values obtained in [5].
In Fig. 2 is presented the 23Na MAS NMR spectra obtained in Al-natrolite at T = 300 K and T = 380 K. These spectra were obtained using the method of :H decoupling. NMR 1H decoupling is a special method used in NMR which allow to eliminate fully the effect of magnetic dipolar coupling between resonance nuclei (23Na in our case) and 1H nuclei. From the comparison of the 23Na NMR spectra, shown in Fig. 2, it appears that the 23Na NMR spectra have the same shape at T = 300 K and T = 380 K. The NMR shape of 23Na is determined by magnetic dipolar interactions with other magnetic nuclei and by electric quadrupolar interaction with the electric field gradient (EFG) on the 23Na sites. The interaction with magnetic moment of 1H nuclei give the main contribution to the magnetic dipolar interaction of 23Na nuclei. However the 1H decoupling, which was used at recording the NMR spectra of 23Na NMR (Fig. 2), leads to averaging of the dipolar interactions between the magnetic moments of 1H and 23Na nuclei and so, from Fig. 2, it follows that in the temperature region T < 380 K the electric field gradient (EFG) at the 23Na sites does not depend on the temperature.
H-'-1-'-T"
10 0 -10
ppm
Fig. 1. 23Na MAS NMR spectra of Al-natrolite at Q rot = 10 kHz: (a) experimental spectrum at T = 300 K; (b) theoretical spectrum with parameters Cq = 1759,2 kHz, ^ = 0,64, 5cSA = 8,19 ppm, AVGauss = 73,11 Hz.
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Fig. 2. 23Na NMR spectra with ^-decoupling in Al-natrolite at T = 300 K and T = 380 K.
In natrolite there are two structurally nonequivalent 23Na and well-resolved fine structure of NMR spectra caused by the second-order qudrupolar effects is observed at some orientations of crystal in external magnetic field [6]. The experimental temperature dependencies of the quadrupolar second-order shift of the central NMR lines of the two structurally nonequivalent 23Na nuclei, for the case when the vector of the magnetic field B0 is parallel to [110] direction, are shown in Fig. 3 [4]. This result was obtained using the CW NMR spectrometer at a frequency of the of 11 MHz on Al-natrolite single crystals [4].
Fig. 3. The temperature dependencies of the second-order quadrupolar shifts and the linewidth (5v) of 23Na NMR spectra for the two structurally nonequivalent 23Na ions in the natrolite single crystal. Inset: the central part of NMR spectrum of 23Na in single crystal.
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The intensive diffusion of 23Na cations in natrolite pores should lead to the averaging of the second-order quadrupolar shifts of the two structurally nonequivalent 23Na nuclei. In reality, such effect is not observed (Fig. 3). So, from this result it follows that the diffusion of the sodium ions is absent in natrolite channels in the temperature region T < 500 K [4].
The EFG at the 23Na sites in the natrolite structure is determined by the electric charges of the ions of whole lattice and by the electric dipolar moments of the water molecules. According with 1H NMR data of Al-natrolite the water molecules at T < 380 K rotate about their quasi 2-fold axis [3] and the 180o flip motion take place simultaneously with diffusion along the c-axis [2]. These motions of the water molecules must lead to the averaging of the contributions of the electric dipolar moments of the water molecules to EFG at the 23Na sites. If contribution from the electric dipolar moments of water molecules to the EFG tensor at the 23Na sites is considerable the averaging of this contribution must be observable in the temperature dependence of 23Na NMR spectra. From our experimental temperature dependences of 23Na MAS NMR and NMR spectra represented in Fig. 2 and Fig. 3 it follows that this effect is not observed. From this fact we may conclude that the contributions of the electric dipolar moments of the water molecules to EFG at the 23Na sites are very small.
Fig. 4. shows the 23Na NMR spectra obtained without 1H decoupling at T = 300 K and T = 380 K. The difference between the NMR spectra represented in Fig. 2 and Fig. 4 are related to the dipolar interactions between the magnetic moments of 1H and 23Na nuclei. The shape of these NMR spectra is determined not only by the second-order quadrupolar shift of the central transition but also by the dipolar interaction between magnetic moment of 23Na and 1H nuclei. From Fig. 4 it follows that increasing of the sample temperature leads to thermal averaging of dipolar interactions between the magnetic moments of 1H and 23Na nuclei. Because from NMR data it follows that the 180o flip motion of water molecules take place simultaneously with diffusion along the c-axis [2] we may conclude that the averaging of dipolar interaction of magnetic moments of the 23Na with magnetic moments of protons is connected with the rotations of water molecules about their quasi 2-fold axis and with diffusion of water molecules across canals in natrolites [4].
f-1-1-1-1-1-.-1-1-1-1
-100 -50 0 50 100
ppm
Fig. 4. 23Na NMR spectra without ^-decoupling in Al-natrolite at T = 300 K and T = 380 K.
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The temperature dependence of the spin-lattice relaxation times T1 of 23Na in Ga-natrolite is shown in Fig. 5. The similar temperature dependence is observed in Al-natrolite.
In order to identify the main causes of the measured 23Na relaxation times in natrolite, we consider the theoretical calculations of T1 on the basis of different dynamical model. The physical mechanisms which could be induce the longitudinal relaxation of the 23Na nuclei are [10]:
(i) spin-phonon interactions - interactions of the quadrupolar electric moment of the 23Na nuclei with the crystal electric field gradient modulated by lattice vibrations;
(ii) dipolar interaction with paramagnetic impurities (for example with Fe3+);
(iii) magnetic dipolar interaction with absorbed ions or molecules and with magnetic moments of 27Al (71Ga, 69Ga), 29Si, :H and other 23Na nuclei of natrolite structure;
(iv) electric quadrupolar interactions with the crystal electric field gradients modulated by motion of charge cations or water molecules [10]. These motions are the "hopping" motions, i.e. the atoms or water molecules spend most of their time in potential well corresponding to equilibrium positions, and only a very small fraction move between these potential wells.
The phonon-based relaxation mechanism could not be the cause of spin-lattice relaxation of the quadrupolar nuclei in natrolites. From obtained estimations it follows that if relaxation were to proceed by this mechanism the values of T1 for quadrupolar nuclei in zeolites at room temperature should be 4-5 orders of magnitude larger than the experimental values [10]. The spin-lattice via paramagnetic impurities may be significant only at very low temperature [10]. The magnetic dipolar interaction could not be also the cause of the spin-lattice relaxation of the 23Na nuclei in natrolite. From our estimations we obtain that the dipolar interactions of the 23Na with magnetic moments of the proton magnetic moment give the minimal value of the spin-relaxation time T1mm equals ~ 5 s, which much larger than experimental values 0,04 s (Fig. 5). So only one mechanism (iv) need to be considered.
It is known that the relaxation of the quadrupolar nuclei may be multiexponential [11, 12]. However, for selective saturation of the central transition [8], the relaxation is well described by single exponential [13]. In our case the translational and reorientationa jumps of water molecules modulate only the part of EFG tensor concerned with electric dipolar moments of water molecules. The remaining part of the EFG tensor given by the electric charges of the ions of whole lattice is not changed. So the quadrupolar relaxation concerned with modulation of the part of EFG at the site of the quadrupolar nuclei as a result of the activated translational and reorientational jumps of electric dipoles of water molecules may be described (I = 3/2) by equation [10, 12, 13]
T 1 = 9
1Q ^
1 +
3
ACQ -^TT , (2)
Q 1 + ®o*c2
where m is the Larmor frequency of the quadrupole nucleus; ^ - asymmetry parameter of EFG tensor and
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Fig. 5. The temperature dependences of the spin-lattice relaxation time of the 23Na nuclei in Ga - natrolite.
e2 AqQ
Q ~ h
(3)
Here eAq describes the part of EFG at the site of the 23Na nuclei connected with the electric dipoles of water molecules.
From Eq. (2) it follows that minimal value of Tiq for selective saturation and detection of the central transition is observed at m^c = 1 and is equal
T = . 4V
1min
3(3 + n2)acQ '
(4)
For the 23Na nuclei in the natrolite Timm = 0.04 s. Using ^ = 0,64 and vo = 105,542 MHz we obtain from Eq. (4)
AC » 56,93 kHz.
(5)
The full constant of quadrupolar interaction Cq = 1759,3 kHz and so from our result it follows that the contributions of the electric dipolar moments of the water molecules to full EFG at the 23Na sites are 3.2% only.
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CONCLUSION
From analysis of the temperature dependencies of NMR and MAS NMR spectra of
23
Na nuclei (with and without 1H-decoupling) it follows that the diffusion of the sodium ions at T < 400 K absents in Al- and Ga- natrolites. From analysis of NMR spectra of 23Na nuclei (with and without :H-decoupling) it follows that in Al- and Ga-natrolites the 180o flip motion of water molecules take place simultaneously with diffusion of the water along the Schottky defects. The obtained from 23Na MAS NMR spectrum theoretical values of the quadrupolar frequency Vq and the asymmetry parameter ^ well coincide with experimental values obtained early. The spin-lattice relaxation of the 23Na is governed by the electric quadrupole interaction with the crystal electric field gradients modulated by translational motion of H2O molecules in the natrolite pores. The dipolar interactions with paramagnetic impurities become significant as a relaxation mechanism of the 23Na nuclei only at low temperature (< 270 K).
References
1. G. Engelhardt and D. Michel, High-Resolution Solid-State NMR of Silicates and Zeolites (John Wiley&Sons, Chichester-New York-Brisbane-Toronto-Singapore, 1987).
2. A. V. Sapiga, "The shape of NMR spectra and study of structure and molecular mobility in natrolite", Thesis, (Taurida National V. I. Vernadsky University, Simferopol, 2003).
3. R. T. Thompson, R. R. Knispel, H. E. Petch, Can. J. Phys. 52, 2164 (1974).
4. A. V. Sapiga and N. A. Sergeev, Cryst. Res. Technol. 36, 8 (2001).
5. H. E. Petch, K. S. Pennington, J.Chem.Phys. 36, 1261 (1962).
6. V. N. Szczerbakov, "The NMR study of electric field gradients in zeolites", Thesis (Institute of Physics, Krasnojarsk, USSR, 1972).
7. A. A. Sapiga, M. Olszewski, M. Paczwa, A. V. Sapiga, N. A. Sergeev, Functional Materials 21, 181 (2014).
8. D. Freude, "Quadrupolar Nuclei in Solid-State Nuclear Magnetic Resonance", in Encyclopaedia of Analytical Chemistry, Ed. By R. A. Meyers (2000), p. 12188.
9. D. Massiot, F. Fayon, M. Capron, I. King, S. Le Calve, B. Alonso, J.-O. Durand, B. Bujoli, Z. Gan & G. Hoatson, Magnetic Resonance in Chemistry 40, 70 (2002).
10. J. Haase, H. Pfeifer, W. Oehme, J. Klinowski, Chem. Phys. Letters 150, 189 (1988).
11. P. S. Hubbard, J. Chem. Phys. 53, 985 (1970).
12. A. Abragam, The Principles of Nuclear Magnetism (Oxford U.P., London, 1961).
13. J. Haase, K.D. Park, K. Guo, H. K. C. Timken, E. Oldfield, J. Phys. Chem. 95, 6996 (1991).
Пачва M. ЯМР 23Na в натролт / M. Пачва, О. О. Сапига, M. Ольшевски, М. A. Сергеев, О. В. Сашга // Вчеш записки Тавршського национального ушверситету iMeHi В. I. Вернадського. Серiя : Фiзико-математичнi науки. - 2014. - Т. 27 (66), № 2. - С. 70-78.
В Al-натролт (Na2AhSi3O10 • 2H2O) та Ga-натролгге (Na2Ga2Si3O10 • 2H2O) вивчен температуры залежноста спекав ЯМР i MAS ЯМР ядер 23Na. Було показано, що в Al-натролт та Ga-натролт при Т < 400 К вщсутня дифузш юшв натрта. Отримано температурш залежност часу стн-граткових релаксаци T1 в Al-натролт i Ga-натролт. Обговорюсться вплив молекулярно! рухливосл води в наноканалах структури натролгга на спин-граткову релаксащю юшв 23Na. Kmnoei слова: ЯМР, магштна релаксацiя, цеолгш, рухливють води, натролп\
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Пачва M. ЯМР 23Na в натролите / M. Пачва, A. A. Сапига, M. Ольшевски, Н. A. Сергеев, A. В. Сапига // Ученые записки Таврического национального университета имени В. И. Вернадского. Серия : Физико-математические науки. - 2014. - Т. 27 (66), № 2. - С. 70-78.
В Al-натролите (Na2AkSi3Oio • 2H2O) и Ga-натролите (Na2Ga2Si3Oio • 2H2O изучены температурные зависимости спектров ЯМР и MAS ЯМР ядер 23Na. Было показано, что в Al-натролите и Ga-натролите при Т < 400 К отсутствует диффузия ионов натрия. Получены температурные зависимости времени спин-решеточной релаксации времени Ti в Al-натролите и Ga-натролите. Обсуждается влияние молекулярной подвижности воды в наноканалах структуры натролита на спин-решеточную релаксацию ионов 23Na.
Ключевые слова: ЯМР, магнитная релаксация, цеолиты, подвижность воды, натролит.
Список литературы
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2. Сапига Алексей Владимирович. Исследование структуры и молекулярной подвижности в натролите по форме спектра ЯМР : Дис... канд. физ.-мат. наук : 01.04.07 / Таврический национальный ун-т им. В.И.Вернадского. - Симф., 2003. - 156л. : рис. - Библиогр.: л. 144-156.
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Received 21 September 2014.
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