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Spectroscopic and Computational Characterizations of Alkaline-Earth- and Heavy-Metal-Exchanged Natrolites [ChemPlusChem](ChemPlusChem Via Acquire Media NewsEdge) Synchrotron infrared (IR) and micro-Raman spectra of natrolites containing alkaline-earth ions (Ca2+, Sr2+, and Ba2+) and heavy metals (Cd2+, Pb2+, and Ag+) as extra-framework cations (EFCs) were measured under ambient conditions. Complementing our previous spectroscopic investigations of natrolites with monovalent alkali metal (Li+, Na+, K+, Rb+, and Cs+) EFCs, we establish a correlation between the redshifts of the frequencies of the 4-ring and helical 8-ring units and the size of the EFCs in natrolite. Through ab initio calculations we have derived structural models of Ca2+ - and Ag+ -exchanged natrolites with hydrogen atoms, and found that the frequency shifts in the H-O-H bending mode and the differences in the O-H stretching vibration modes can be correlated with the orientations of the water molecules along the natrolite channel. Assuming that the members of a solid solution series behave as an ideal mixture, we will be able to use spectroscopy to probe compositions. Deviation from ideal behavior might indicate the occurrence of phase separation on various length scales. Keywords: alkaline earth metals · host-guest systems · micro-Raman spectroscopy · natrolites · synchrotron infrared spectroscopy Introduction Facile cation exchange and reversible hydration and dehydra- tion are the two major chemical properties of zeolites, forming the basis of their industrial uses.[1] An understanding of the host-guest interactions between the alumino-silicate framework and the adsorbed water mole- cules and extra-framework cations (EFCs) is, therefore, crucial for the estimation of their geochemical origin, stability, and reactivity, as well as for tailoring their physical and chemical properties for appli- cations. Although the small-pore zeolite natrolite (Na16Al16 Si24O80·16 H2O) has now been known since 1803,[2] it has often been overlooked as a "dense" zeolite because of its perceived limited cation-exchange capability.[3] However, we have shown that complete cation exchange can be achieved in natrolites by converting the natural sodium form into the potassium form, which can subsequently be exchanged by other isova- lent alkali metals (Li+ ,Rb+ ,andCs+ ), aliovalent alkaline earths (Ca2 + ,Sr2 + , and Ba2 + ), and selected metal cations (Cd2+ ,Pb2 + , and Ag +).[4, 5] The comparative crystal chemistry of these natro- lites has been established and related to the framework expan- sion or contraction under pressure or during hydration/dehy- dration, and has been found to be proportional to the size of the EFC. This size dependence also determines the topology of the EFCs and water molecules along the natrolite channels.[6] Furthermore, DFT calculations of natrolites with monovalent EFCs have been used to model the pressure-induced hydration and concomitant increase in unit-cell volume as well as the topological changes of the cation-water arrangements.[7] Spec- troscopic investigations on the alkali-metal series of natrolites have revealed that the frequency shifts are proportional to the ionic radius of the EFC. Furthermore the variations in the H2O stretching and bending modes are understood on the basis of structural changes.[8] Herein, we extend our spectroscopic characterizations of monovalent ion-exchanged natrolites to divalent alkaline-earth and heavy metal cations, and develop systematic correlations between structural and spectroscopic parameters as a function of these EFCs. To help interpret certain vibrational bands from synchrotron IR and micro-Raman spectroscopy experiments, we perform molecular dynamics simulations on selected natro- lites to complement the established X-ray structural models with H-positions. Results and Discussion The secondary structural building units of the natrolite (Na16Al16Si24O80·16 H2O) framework are T5O10 tetrahedral clusters (Al2Si3O10 or 4 =1 unit according to the International Zeolite Association code), which are built from alternating SiO4 and AlO4 tetrahedral 4-rings capped by another SiO4 tetrahedron (Figure 1). This 4= 1 unit repeats along the c axis to form so-called "fi- brous chains". These chains are then interconnected through Al^O^Si linkages by shifting the heights of the 4 =1 units rela- tive to one another by a quarter of its height (or the c-axis length). As a result, helical/elliptical 8-ring (if projected on the ab plane) channels are created along the c axis (Figure 1). Under ambient conditions, EFCs and water molecules are or- dered and placed along the center and minor-axis walls of the helical 8-ring channels, respectively.[9] It has recently been shown that various alkali, alkaline-earth, and heavy-metal cations can be exchanged into the natrolite channels, and that the unit cell volume and channel ellipticity of ion-exchanged natrolites have a nearly linear dependence on the cation radius.[4, 5] The larger the cation, the less elliptical the channel becomes, and hence, the unit cell volume expands by up to 25 % in the alkali metal series. For example, from Li- to Cs-natrolite, the chain rotation angle and unit cell volume vary from 25.58 and 2118.5 ^3 to 2.98 and 2658.8 ^3, respective- ly.[6] Extra-framework cations and adsorbed water molecules in the natrolite channels are arranged in two different ways, that is, either ordered or disordered, below or above a cation radius threshold of 1.3 ^. Therefore, EFCs and water molecules in Li + -, Na+-, Ca2+-, Ag+-, and Sr2+-exchanged natrolites (r < 1.3 ^) are ordered and distributed over well-separated sites with full site occupancies,[5] whereas those in K + -, Rb + -, and Cs +-ex- changed natrolites (r > 1.3 ^) reveal statistically disordered ar- rangements over closely separated sites.[4] The overall symme- try of the ordered cation forms is then dictated by the type of cation. Thus, all the alkaline-earth forms studied so far (Ca2 + -, Sr2 + -, and Ba2 + -exchanged natrolites) adopt a scolecite-like structure in space group Cc, whereas the alkali and heavy- metal forms (Li+-, Na+-, Cd2+-, Ag+-, and Pb2+ -exchanged na- trolites) crystallize in the original natrolite-like structure in space group Fdd2 (Figure 1). The Fdd2 !Cc symmetry reduc- tion also appears to be related to the level of hydration under ambient conditions, that is, the scolecite type has 24 H2O mol- ecules per 80 framework oxygen atoms, whereas the natrolite type contains close to 16 H2O molecules per 80 framework oxygen atoms. In Na-natrolite, four groups of phonon vibrational modes have been identified and assigned to the primary tetrahedral building units and adsorbed water molecules : the tetrahedral T^O stretching in the range 900-1100 cm^1 for Raman and 610-1320 cm^1 for IR ; O^T^O intra-tetrahedral bending in the range 420-900 cm^1 for Raman and 410-445 cm^1 for IR ; and the bending and stretching of O^H vibrational modes of the adsorbed water molecules in the range 1600-1670 cm^1 and 3300-3600 cm^1 for both Raman and IR spectra, respectively (Table 1).[10, 11] On the other hand, optical modes of the lattice vibrations are observed at low frequencies below 340 cm^1 in Raman and below 420 cm^1 in IR spectra. The strongest Raman band observed at around 530 cm^1 originates from the breath- ing mode of the 4-ring tetrahedral unit (^Al^O3^Si2^O4^Al^ O3^Si2^O4^), which is doubly capped by SiO4 tetrahedra to form a fibrous chain along the [001] direction (Figure 1).[12] An- other strong Raman band is observed at 443 cm^1 (corre- sponding to the IR band at 350 cm^1), which is assigned to a breathing vibrational mode of the helical 8-ring tetrahedral unit along the [001] direction (Figure 1).[10, 13] Micro-Raman and synchrotron far- and mid-IR spectra of var- ious samples of natrolites (as-prepared, hydrated Ca2 + -, Sr2 + -, Ba2 + -, Cd2 +-, Pb2 + -, and Ag + - natrolites) are shown in Figures 2 and 3, respectively. The intensity of each spectrum was normal- ized relative to its strongest band. The refined peak positions of all observed IR vibrational modes in this study are compiled in Table 1. The intensity of the helical 8-ring band relative to that of the 4-ring band in both the Raman and IR spectra for Ag-NAT is similar to that of Na-NAT, but very different from the rest of the ion-exchanged natrolites studied here. This could be because of the different electrical field within the helical 8- ring unit owing to the different electric charges in monovalent (Na+ -, Ag + -) and divalent (Ca2 + -, Sr2+-, Ba2+ -, Cd2+-, Pb2+-) natro- lites. In accordance with previ- ous spectroscopic studies on alkali-metal forms of natrolites,[8] we confirm that the systematic redshift of the frequency related to the helical 8-ring vibrational mode in both the Raman and IR spectra depends on the cation size (Figure 4 a,b): the peak posi- tion of this band shifts progres- sively to lower frequencies as the size of the EFC increases. These systematic redshifts can be approximated by linear func- tions with slopes of ^54.6 for the Raman and ^18.5 for the IR active modes (solid curves as shown in Figure 4 a,b). The data points obtained from the alkaline-earth- (Ca2+ ,Sr2 + , and Ba2 + ) and heavy-metal- (Cd2+ and Pb2+ ) containing natrolites show a similar but distinct trend compared to the alkali-metal-ex- changed natrolites. We argue that this is because of the stron- ger and localized interactions between the divalent cations and the bridging O2 oxygen atom. As the ellipticity of the helical 8-ring unit determines the pore opening of the natrolite channel, the systematic changes in the frequency of the helical 8-ring vibrational mode can be correlated to the pore opening, for example, to the chain rota- tion angle y or the chain bridging angle T^O2^T in all the na- trolites studied to date (Figure 5a,b). Here, the T^O2^T angular dependence on the frequency of the helical 8-ring vibrational mode is plotted for all the ion-exchanged natrolites studied so far (M-NAT-hyd; M =Li+,Na+,K+,Rb+,Cs+,Ca2+,Sr2+,Cd2+, Pb2+,Ag+). Clearly, the charge of the EFC plays an important role, as the data point of Ag + -NAT fits onto the curve derived from the monovalent alkali-metal-exchanged natrolites, where- as those from the alkaline-earth/heavy-metal-exchanged natro- lites clearly deviate from this curve. The dynamical behavior of the water molecules in the pores of zeolites has attracted particular attention because the trans- port and mobility of water is a subject of great scientific and technological interest.[14] Spectroscopic detection on the O^H bending and stretching vibration modes has been shown to be the most useful and versatile tool in determining the dy- namics of guest constituents such as H2O in zeolites.[15] Spec- troscopic observations of guest water molecules can be inter- preted correctly on the basis of accurate structural parameters derived from crystallographic studies. X-ray diffraction, howev- er, relies on scattering based on the electron density of the constituent atoms, and hence, is not suitable for deriving direct information about low-Z hydrogen atoms. To comple- ment our previously established X-ray structural models and understand our spectroscopic observations with respect to hy- drogen, we performed ab initio calculations using selected crystallographic models of ion-exchanged natrolites. In Table 2, the geometrically optimized models for hydrated Ca2 + - and Ag + -NAT are compared to their respective X-ray-diffraction- based models.[5,16] The equilibrium lattice constants and atomic coordinates of Ca2 + - and Ag + -NAT-hyd are in good agreement with the experimental models, and the optimized H-positions suggest hydrogen bonding toward the framework oxygen atoms, as observed in mineral natrolite by neutron diffrac- tion.[17] Guest water molecules in a crystalline matrix are known to have larger H^OW^H angels than free water, for example, 107.98 in natrolite[17] and 107.28 in crystalline hydrates[18] com- pared with 104.528 in water vapor.[19] This is because the oxygen atom in water acts as an acceptor in hydrogen bond- ing. This implies that the hydrogen bonding formed inside the natrolite channels should impact the geometry of the channel as well as the distribution of the EFCs. It has been shown that the EFC-water arrangement in Ag- NAT-hyd is similar to that of Na-natrolite with space group Fdd2, and the water molecules occupy a single site along the screw diad axis, forming a zigzag chain along the elliptical channel.[5] On the other hand, three separate water sites are lo- cated in the channels of Ca2 + -NAT-hyd, which is analogous to that of mineral scolecite with space group Cc. In all cases, the water molecules in the channels of Ca2 + - and Ag + -NAT-hyd fully occupy general crystallographic positions and have typical OW···O bond lengths and H^O^H bond angles (Table 3). Here, three internal modes (stretching of O^H bonds and H^O^H bending) and six external modes of the guest water molecule in the natrolite channel are active in both Raman and IR spec- troscopy. We observed two characteristic absorption bands for H^O^H bending in the range 1580-1670 cm^1 and O^H anti- symmetric and symmetric stretching bands around 3400 cm^1 (Figure 3 b). Previously, the degree of blueshift of the H^O^H bending band has been correlated with the bonding behavior of the water molecules.[11, 15] Using our calculated structural models including hydrogen, we can describe the dependence of the observed H^O^H bending band on the H^O^H bond angle (Figure 6). We find an inverse proportional relationship between them, that is, a gradual increase in the frequency of the H^O^H bending band from 1585 to 1665 cm^1 corre- sponding to a gradual decrease in the H^O^H bond angle from 111.28 to 106.68 (Figure 6). The O-H stretching vibrational modes reveal the differences in the hydrogen-bond configurations of different natrolites. For Ca2 + -, Sr2+ -, and Ba2 + -NAT-hyd, one should observe six IR active modes of O-H stretching vibrations, because there are three separate water sites. As expected, the measured IR spec- tra of Ca2 + -NAT-hyd show four distinguishable O-H stretching bands at 3232.6, 3328.7, 3407.9, and 3580 cm-1, and the other two are overlapping bands at 3498 and 3503 cm-1 (Figure 3). Here, the bands at 3580 and 3328.7 cm-1 are assigned to stretching vibrations of OW1···O interactions with interatomic distances of 3.306(1) and 2.741(1) -, respectively (Table 3). Then, the bands at 3503 and 3407.9 cm-1 are identified as the vibrations of OW3···O interactions, which are modeled at in- teratomic distances of 3.111(1) and 2.876(1) -, respectively, and those at 3498 and 3232.6 cm-1 are assigned to the vibrations of OW2···O with interatomic distances of 3.412(1) and 2.706(1) -, respectively. As discussed above, the EFC-water ar- rangements in Sr2+ - and Ba2 + -NAT-hyd are similar to those of Ca2 + -NAT-hyd. Accordingly, in Sr2+ -NAT-hyd, we observe six O- H stretching vibrational bands at 3264.8, 3342.7, 3394.6, 3452.6, 3523.0, and 3596.3 cm-1. On the other hand, Ba2+ -NAT- hyd reveals broad bands of O-H stretching vibrations located between 3080 and 3680 cm-1 (Figure 3). This may arise from the disordered water distribution with closely separated OW- H···O distances (rBa2+ > 1.3 - disorder threshold). For Ag+-NAT- hyd, which possesses one fully occupied water site, one O-H stretching vibration band is observed clearly as a sharp peak at 3504.1 cm-1, and the other is a broad band at 3291.5 cm-1. This is in good agreement with our crystallographic and com- putational model for well-separated OW···O interatomic distan- ces of 3.137(1) and 2.764(1) -, respectively (Table 3). The three broad O-H stretching vibration bands (3214, 3385.5, and 3563.4 cm-1)inCd2 +-NAT-hyd and four O-H stretching vibra- tion bands (3215.7, 3354.2, 3428.3, and 3545.8 cm-1)inPb2 + -NAT-hyd correspond to their respective guest water models with almost fully occupied water sites showing water-oxygen to framework-oxygen distances in the range 3.03(1)-3.15(1) - in Cd2 +-NAT-hyd and 2.66(1)-3.16(1) - in Pb2 + -NAT-hyd.[5] Conclusion In conclusion, we have extended our spectroscopic characteri- zation of natrolite to the alkaline-earth (M-NAT-hyd, M = Ca2+, Sr2+,andBa2+) and some heavy-metal forms (M-NAT-hyd, M = Cd2 + ,Pb2+ , and Ag + ) to complete the systematic correlations between the average structural and spectroscopic parameters as a function of the extra-framework cation substitution. Our results now establish the correlation between the systematic red-shifts in the frequencies of the 4-ring and helical 8-ring units of the natrolite framework and the radius of the extra- framework cations in the channel. To better analyze the ob- served spectroscopic data, we have carried out ab initio calcu- lations to complete the structural models of Ca- and Ag-ex- changed natrolites with the hydrogen atoms. We find that the frequency shifts in the H-O-H bending mode and the differen- ces in the O-H stretching vibration modes could be correlated with the water molecular orientations along the natrolite chan- nel. Experimental Section Synchrotron infrared spectroscopy The samples of the divalent alkaline-earth- (Ca2+ ,Sr2 + , and Ba2 +) and heavy-metal- (Cd2 +,Pb2 + , and Ag + ) exchanged natrolites used in this work were all prepared according to procedures described elsewhere.[5] Synchrotron IR experiments were performed at the U2A beamline at the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (BNL). The far-IR (100-700 cm-1) spectra were obtained with a Bruker IFS 66v/S Fourier-transform in- frared (FTIR) spectrometer in combination with a custom-made vacuum microscope system equipped with a Si bolometer detector (Infrared Laboratories) and a Mylar beam splitter (3.5 mm thick). The mid-IR (650-5000 cm-1) spectra were then collected in trans- mission mode using a Bruker Vertex 80v FTIR spectrometer and a Hyperion 2000 IR microscope with a nitrogen-cooled MCT detec- tor at U2A side station. The spectral resolution for all measure- ments was 4 cm-1, and all data were collected under ambient con- ditions. More details of the optical layout of the U2A beamline are described elsewhere.[9, 20] Raman spectra The Raman spectra were recorded using a custom-built micro- Raman system including a spectrograph with 0.5 meter focal dis- tance, and a CCD detector (Princeton Instruments) at Yonsei Uni- versity. A diode-pumped solid-state (DPSS) laser (Spectra Physics Excelsior CW laser with 532 nm wavelength and 150 mW power) was used as an excitation source in back-scattering geometry. The laser power on the sample was kept at approximately 15 mW to avoid laser-induced sample decomposition. The average acquisi- tion time for a single spectrum was 30 s. Computational simulation For the interpretation of our spectroscopic observations on the basis of our previously established X-ray structural models of ion- exchanged natrolites, computational simulations were performed on selected ion-exchanged natrolites. The structural models of Ca2 + - and Ag + -exchanged natrolites were chosen, as these materi- als are analogous to mineral scolecite (Ca8Al16Si24O80·24 H2O) and natrolite (Na16Al16Si24O80·16 H2O) and exhibit an ordered distribution of EFCs and water molecules in the Cc and Fdd2 space groups, re- spectively.[5] The geometrical optimizations were performed through density-functional theory (DFT) calculations using the pseudo-potential plane-wave method. We employed the all-elec- tron projector augmented wave (PAW) approach[21] within the Perdew-Burke-Ernzerhof (PBE) parameterization of the generalized gradient approximation (GGA), as implemented in the Vienna ab i- nitio simulation package (VASP) code.[22] The kinetic energy cutoff of the plane wave was set at 500 eV. The k-point meshes for the Brillouin zone sampling were constructed using the Monkhorst- Pack scheme.[23] We used 1 -1 -4 and 4 -4 -3 k-point meshes for the orthorhombic (Fdd2) Ag + -natrolite and monoclinic (Cc)Ca2 + -natrolite unit cells, respectively. Such parameters were found to be sufficient to obtain fully converging results. For computational convenience, we performed energy calculations at T =0 K, and thus, the free energy reduces to the enthalpy. Acknowledgements This study was supported by the Global Research Laboratory Pro- gram of the Korean Ministry of Science, ICT and Planning (MSIP). The use of beamline U2A at NSLS was supported by COMPRES, the Consortium for Materials Properties Research in Earth Scien- ces under NSF Cooperative Agreement EAR 11-57758 and by DOE/NNSA (DE-FC-52-08NA28554, CDAC). Research carried out in part at the NSLS at BNL was supported by DOE (DE-AC02- 98CH10886). [1] R. M. Barrer, Hydrothermal Chemistry of Zeolites, Academic Press, London, 1982. [2] M. H. Klaproth, Ges. Naturforsch. Freunde Berlin, Neue Schrifl 1803, 4, 243-248. [3] A. Dyer, H. Faghihian, Microporous Mesoporous Mater. 1998, 21, 27 - 38. [4] Y. Lee, Y. Lee, D. Seoung, Am. Mineral. 2010, 95, 1636 - 1641. [5] Y. Lee, D. Seoung, Y. Lee, Am. Mineral. 2011, 96, 1718 - 1724. [6] D. Seoung, Y. Lee, C.-C. Kao, T. Vogt, Y. Lee, Chem. Eur. J. 2013, 19, 10876-10883. [7] A. Kremleva, T. Vogt, N. Roesch, J. Phys. Chem. C 2013, 117, 19020 - 19030. [8] D. Liu, Z. X. Liu, Y. M. Lee, D. H. Seoung, Y. Lee, Am. Mineral. 2012, 97, 419-424. [9] W. M. Meier, Z. Kristallogr. 1960, 113, 430 - 444. [10] F. Pechar, D. Rykl, Can. Mineral. 1983, 21, 689 - 695. [11] C. M. B. Line, G. J. Kearley, Chem. Phys. 1998, 234, 207 - 222. [12] S. V. Goryainov, M. B. Smirnov, Eur. J. Mineral. 2001, 13, 507 - 519. [13] D. W. Breck, Zeolite Molecular Sieves,Wiley, London, 1974. [14] C. M. B. Line, G. J. Kearley, J. Chem. Phys. 2000, 112, 9058 - 9067. [15] B. A. Kolesov, C. A. Geiger, Am. Mineral. 2006, 91, 1039 - 1048. [16] W. H. Baur, D. Kassner, C. H. Kim, N. H. W. Sieber, Eur. J. Mineral. 1990, 2, 761-769. [17] G. Artioli, J. V. Smith, -. Kvick, Acta Crystallogr. Sect. C 1984, 40, 1658 - 1662. [18] G. Chiari, G. Ferraris, Acta Crystallogr. Sect. B 1982, 38, 2331 -2341. [19] K. Kuchitsu, Bull. Chem. Soc. Jpn. 1971 , 44, 96 - 99. [20] Z. Liu, J. Hu, H. K. Mao, R. J. Hemley, J. Phys. Condens. Matter 2002, 14, 10641-10646. [21] P. E. Blçchl, Phys. Rev. B 1994, 50, 17953 - 17979. [22] G. Kresse, D. Joubert, Phys. Rev. B 1999, 59, 1758 -1775. [23] H. J. Monkhorst, J. D. Pack, Phys. Rev. B 1976, 13, 5188 -5192. Received : April 16, 2014 Published online on June 17, 2014 Dan Liu,[a, d] Xin Chen,[b] Yanming Ma,[b] Zhenxian Liu,[c] Thomas Vogt,[e] and Yongjae Lee*[a] [a] Dr. D. Liu, Prof. Y. Lee Department of Earth System Sciences Yonsei University, Seoul 120-749 (Korea) Fax: (+ 82) 2-2123-5667 E-mail : [email protected] [b] Dr. X. Chen, Prof. Y. Ma State Key Laboratory of Superhard Materials Jilin University, Changchun 130012 (P. R. China) [c] Dr. Z. Liu Geophysical Laboratory Carnegie Institution of Washington, Washington, DC 20015 (USA) [d] Dr. D. Liu Institute for Frontier Materials, Geelong Technology Precinct Deakin University, Waurn Ponds, Victoria 3217 (Australia) [e] Prof. T. Vogt Nan°Center & Department of Chemistry and Biochemistry University of South Carolina, Columbia, SC 29208, (USA) (c) 2014 Blackwell Publishing Ltd. |