Interaction of hydrated cations with mica-n (n = 2, 3 and 4) surface

1  Interaction of Hydrated Cations with Mica-n (n=2, 3 and 4) Surface. Esperanza Pavón1,*, Miguel A. Castro2, Agustín Cota3, Francisco J. Osuna2, M. Carolina Pazos4, María D. Alba2 1 Unité de Catalyse et de Chimie du Solide, UCCS, CNRS, UMR8181, Université Lille Nord de France, 59655 Villeneuve d’Ascq, France 2Instituto Ciencia de Materiales de Sevilla (CSIC-Universidad de Sevilla). Avda. Américo Vespucio, 49. 41092 Sevilla, Spain. 3Laboratorio de Rayos-X. CITIUS. Universidad de Sevilla. Avda. Reina Mercedes, 4b. 41012-Sevilla, Spain. 4 Escuela de Ciencias Químicas, Universidad Pedagógica y Tecnológica de Colombia UPTC. Avda. Central del Norte, Vía Paipa, Tunja, Boyacá, Colombia. 2  ABSTRACT.High charged swelling micas, with layer charge comprised between 2 and 4, has been found to readily swell with water and that complete cation exchange (CEC) can be achieved. Due to their high CEC, applications like radioactive cation fixation or removal of heavy metal cations from waste water were proposed. Their applicability can be controlled by the location of the interlayer cation in a confined space with a high electric field. In synthetic brittle micas, the interlayer cation has a low water coordination number; therefore their coordination sphere would be completed by the basal oxygen of the tetrahedral layer as innersphere complexes (ISC). However, no direct evidence of these complexes formation in brittle micas has been reported yet. In this contribution, we mainly focus on the understanding the mechanisms that provoke the formation of ISC in high charge swelling micas, Mica-n. A whole series of cations (X) were used to explore the influence of the charge and size of the interlayer cation. Three brittle swelling micas, Mica-n (n=4, 3 and 2), were selected in order to analyze the influence of the layer charge in the formation of ISC. The contribution of the ISC has been analyzed thorough the evolution of the 060 reflection and the changes in the short-range order of the tetrahedral cations will be followed 29Si and 27Al MAS-NMR.The results showed that ISC was favored in X-Mica-4 and that provoked a high distortion angle between the Si-Al tetrahedra. When the content of aluminum decreases, the electrostatic forces between the layers are relaxed, and the hydrated cations did not interact so strongly with the tetrahedral sheet, having the opportunity to complete their hydration sphere. KEYWORDS. High-charged micas, inner sphere complex, hexagonal cavity, tetrahedra distortion, NMR, b-parameter. 3  TOC GRAPHIC  4  INTRODUCTION High charge swelling micas, with layer charge comprised between 2 and 4, has been found to readily swell with water and that complete cation exchange (CEC) can be achieved.1,2,3,4,5,6,7 Due to their high CEC (theoretically up to 468 meq/100g), applications like radioactive cation fixation or removal of heavy metal cations from waste water were proposed.8,9,10,11,12,13 In those high charged synthetic micas, the layers bear a high permanent negative charge compensated by counterions located between them (interlayer space). These counterions are the origin of two interesting features: mica swelling and cationic exchange. The former refers to the uptake of water into the interlayer space, while the latter involves the replacement of original counterions Na+ by cations of aqueous solution.14,15,16,17 Both processes will be controlled by the location of the interlayer cation in a confined space with a high electric field. Recently, two important items related to these swelling brittle micas have been reported 1) the interlayer cations do not exchange completely.18 2) The water coordination of the cations is small to full fill their hydration sphere.19 The hydration state of the interlayer cations has been shown to depend on both the layer charge of the aluminosilicate and the nature of the interlayer cation.19 In synthetic brittle micas, the interlayer cation has a low water coordination number; therefore their coordination sphere would be completed by the basal oxygen of the tetrahedral layer as inner-sphere complexes (ISC). This fact has been found as well in the hectorite surface, where weakly solvated ions, such as K+, are able to form ISC.20 However, no direct evidence of these complexes formation in brittle micas has been reported yet. In this contribution, we mainly focus on the understanding the mechanisms that provoke the formation of ISC in high charge swelling micas, Mica-n. In terms of methodology, we draw attention to the relation between the hexagonal hole in the tetrahedral layer and the interlayer cation, which allows detailed understanding of both the confined cation and the clay surface structure. Then, a whole series of cations will be used to explore the influence of the charge and 5  size of the interlayer cation. Three brittle swelling micas, with different Si/Al ratio, were selected in order to analyze the influence of the layer charge in the formation of ISC. The contribution of the ISC will be analyzed thorough the evolution of the 060 reflection and the changes in the shortrange order of the tetrahedral cations will be followed 29Si and 27Al MAS-NMR EXPERIMENTAL Synthesis Method A procedure similar to that described by Alba et al.21 was employed. Nearstoichiometric powder mixtures with the molar compositions (8 - n) SiO 2, (n/2) Al2O3, 6 MgF2, and (2n) NaCl were used to synthesize Na-Mica-n (n = 2, 3, 4). The starting materials were SiO2 from Sigma (CAS no. 112945-52-5, 99.8% purity), Al(OH)3 from Riedel-de Haën (CAS no. 21645-51-2, 99% purity), MgF2 from Aldrich (CAS no. 20831-0, 98% purity), and NaCl from Panreac (CAS no. 131659, 99.5% purity). All reagents were mixed and vigorously grounded before heating up to 900 °C in a Pt crucible for 15 h. After cooling, the solids were washed with deionized water and dried at room temperature. The as-synthesized samples are named Na-Mica-n (n ranging between 2 and 4). XRF analyses were carried out in order to check the chemical composition of these samples (see “Sample Characterization” session for acquisition details). The results are displayed in Table 1, as well as the Si/Al ratio achieved compared with the theoretical value. A good accurate is obtained and allows us affirming that chemical compositions of these samples are close to the theoretical one. The as-made solids were analyzed by X-ray Diffraction (XRD) to evaluate their purity (Figure 1S, Supporting Information). Cation-Exchange Process The as-synthesized Na-Mica-n were exchanged with solutions of Li+, K+, Mg+2 and Al+3 salts at concentrations that ensured that the molar amount of cation was 10 times the cation-exchange capacity (CEC) of the mica.19 The most important characteristics of these 6  ions in solution are displayed in Table 2. The reagents used were MgCl2 from Sigma-Aldrich (CAS n° 7786-30-6, 99.99% purity), KCl from Fluka (CAS n° 7447-40-7, >99% purity), AlCl3 from Fluka (CAS n° 7784-13-6, >99.0 % purity), and LiCl from Fluka (CAS n° 7447- 41-8, >99.0% purity). Ion-exchange process was described elsewhere.19 The resulting solids were analyzed by X-ray Diffraction (XRD) to evaluate the purity of the samples (Figure 2S, Supporting Information). This exchange method prevents the modification of the silicate framework and consequently, XRF analyses were not needed. The extent of the cation exchange reaction was monitored by 23Na MASNMR. These solids are referred to as X- Mica-n, where X=Na+, Li+, K+, Mg+2, or Al+3 and n=2, 3, or 4. Sample Characterization. X-ray diffraction (XRD) patterns were measured at the CITIUS X-ray laboratory (University of Seville, Spain) using a Bruker D8 Advance instrument equipped with a Cu Kα radiation source, operating at 40 kV and 40 mA, and with a Ni filter. The powder XRD patterns were registered in the 2θ-range 58–62° with a step size of 0.05° and a time step of 20 s. The analysis of the peaks were carried out using TOPAS© from Bruker© and Pseudovoight functions. XRF of powdered samples in borate flux was performed to obtain information about the chemical composition of the samples. XRF measurements were made with an automated Philips PW1400 spectrometer at the CITIUS, Universidad de Sevilla. 29Si and 27Al (SP) MAS-NMR spectra were recorded at the Spectroscopy Service of ICMS (CSIC-US, Seville, Spain) using a Bruker DRX400 spectrometer equipped with a multinuclear probe. Powdered samples were packed in 4-mm zirconia rotors and spun at 10 kHz. 29Si MAS-NMR spectra were acquired at 79.49 MHz, using a pulse-width of 2.7 μs (π/2=7.1 μs) and a pulse space of 3 s. 27Al (SP) MAS-NMR spectra were recorded at 104.26 7  MHz, using a pulse width of 0.92 μs (π/2=9.25 μs) and a pulse space of 0.1s. The chemical shift values are reported in ppm with respect to tetramethylsilane for 29Si and AlCl3 0.1M for 27Al. RESULTS AND DISCUSSION Hexagonal cavity distortion: b parameter In all the samples, the 060 reflections (Figure 1) are in the 2θ range between 59.5 to 60.2º which is typical of trioctahedral clays22 and the 2θ value depend on the interlayer cation (Table 2). The distance associated to this reflection, d(060), (Table 2) depends on the composition of the octahedral sheet, the amount of Al in tetrahedral sheet and the degree of tetrahedral tilt angle.22 For these reasons, trioctahedral clays like saponite and vermiculite exhibit a higher value (1.52 and 1.54 Å, respectively) than dioctahedral clay such as montmorillonite (d(060) =1.49-1.50 Å). The values obtained for our samples ranges between 1.50 to 1.56 Å. The biggest values, found for the highest charge micas (n=4 and 3), are closer to the values reported for vermiculites, whereas for n=2, d(060) is in the reported value for trioctahedral micas, like Biotite (1.538 Å). Geometry considerations can be derived from the 060 reflection (Table 2). For most of the layered silicate, the ideal hexagonal cavity (Figure 2a) is distorted to a ditrigonal symmetry by the oppose rotation of alternate tetrahedron (Figure 2.b). The amount of this rotation varies from a few degrees to near the theoretical maximum of 30º. This tetrahedral rotation has been attributed to the difference in size between the tetrahedral and octahedral sheets (the first one is higher than the second).23,24 The average tetrahedral rotation from 8  hexagonal symmetry, α, may be predicted from the observed b axis and the known Al-for-Si substitution as:22 cos   (1) where btheor is the value obtained from25 󰇛󰇜9.150.74,󰇗, (x is the theoretical grade of substitution of Si4+ for Al3+ in the tetrahedral sheet) and bobs is the experimental value calculated form d(060) distance, taking into account that in monoclinic cells, b=6· d(060), Å. From the experimental d(060) values and the sample chemical compositions, the distortion angle, α and b parameter were calculated (Table 2) and have been correlated to the interlayer cation size (Figure 3). The maximum distortion is found for the samples with the highest isomorphical substitution Si/Al and hence, highest layer charge (upper blank triangle, Figure 3). This behavior is a consequence of the inhibition of the incorporation of the water molecules to the interlayer space when layer charge increase.19 In such case, the interlayer cations are less hydrated and increase the contribution of inner sphere complex. As observed in the Figure, the distortion created increases with the increase in the ionic radii of the interlayer cation, meaning that in this case, where ISC are favored to occur, the radii of the interlayer cation is the parameter that influence in the grade of distortion produce in the layer. The sample homoinized by K+ is the only exception in this behavior. This case will be analyzed further in the following section (see 29Si NMR). On the opposite site, when layer charge decreases, the electrostatic interaction between the layers is relaxed and allows the hydration of the interlayer cations.19 This fact hinders the contribution of inner sphere complexes and consequently, smaller distortion in the tetrahedral 9  sheet are expected as occurred for samples n=3 and n=2. In this case, the influence of the ionic radii in the distortion is smaller. In X-Mica-n, the interlayer cations are housed in the hexagonal cavities formed by the O2- anions of the opposite tetrahedral sheets (Figure 2c). Therefore, disregarding distortions, these cations have 12-fold coordination, and the bond length between oxygens and interlayer cations, d, can be calculated through the equation:   (2) where h is the interlayer space, h=d001-9.4, Å and r is de distance from the centre of the hexagonal cavity to the oxygens of the plane in a tetrahedral sheet. In case of the tetrahedral sheets deformation by rotation of angle α, r varies as follows:26     √3tan (3) Bond distance between the basal oxygens and the interlayer cations calculated using eq. 2 are also displayed in Table 2. To determine these parameters the interlayer space is required (calculated using the d001 distance, Table 2).19 The distance between the basal oxygens and the centre of the hexagonal cavity, r, slightly differs from the expected range (2.6 Å)27. Shortest distances are obtained when n=4, and the presence of ISC is encouraged. However, when layer charge decreases, this distance increases reflecting the smaller distortion provoked by the interlayer cations, although it never arrives to the ideal value. The distance between the interlayer cation and the basal oxygens (d) when a monolayer of water is considered24 (d001≈12Å) range between 2.6 and 3.4 Å. In the case n=4, the distance is smaller (between 2.6 and 3.08 Å) indicating that hexagonal cavities are 16   (8) Kodama, T.; Komarneni, S. Na-4-mica: Cd2+, Ni2+, Co2+, Mn2+ and Zn2+ ion exchange. J. Mater Chem. 1999, 9, 533-539. (9) Paulus, W.J.; Komarneni, S.; Roy, R. Bulk synthesis and selective exchange of strontium ions in Na4Mg6Al4Si4O20F4 Mica. Nature. 1992, 357, 571-573. (10) Kodama, T.; Komarneni, S. Alkali metal and alkaline earth metal ion exchange with Na- 4-mica prepared by a new synthetic route from kaolinite. J. Mater Chem. 1999, 9, 2475-2479. (11) Brigatti, M.F.; Colonna, S.; Malferrari, D.; Medici, L.; Poppi, L. Mercury adsorption by montmorillonite and vermiculite: a combined XRD, TG-MS, and EXAFS study. Appl. Clay Sci. 2005, 28, 1-8. 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Acta Crystal. B. 1969, 25, 925-946. (34) Thompson, J.G. 29Si and 27Al nuclear magnetic resonance spectroscopy of 2:1 clay minerals. Clay Miner. 1984, 19, 229-236. (35) Pearson, R.G. Hard and soft acids and bases. J. Am. Chem. Soc. 1963, 85, 3533-3539. (36) Pearson, R.G. Hard and soft acids and bases, HSAB, part I: Fundamental principles. J. Chem. Educ.1968, 581-586. (37) Pearson, R.G. Hard and soft acids and bases, HSAB, part II: Underlying theories. J. Chem. Educ.1968, 643-648. (38) Shainber, I.; Kemper, W.D. Ion exchange equilibria on montmorillonite. Soil Sci. 1967, 103, 4-9. (39) Bolt, G.H.; Kemper, W.D.; Shainber, I. Discussion of paper by Shainberg, I and Kemper, WD entitled ion exchange equilibria on montmorillonite. Soil Sci. 1967, 104, 444-453. (40) Komarneni, S.; Pidugu, R.; Amonett, J.E. Synthesis of Na-4-Mica from metakaolinite and MgO: characterization and Sr+2 uptake kinetics. J. Mater. Chem. 1988, 8, 205-208. 19   (41) Trillo, J.M.; Alba, M.D.; Alvero, R.; Castro, M.A. Rexpansion of collapsed Li- Montomorillonites. Evidence of the location of Li+ ions. J. Chem. Soc. Chem. Comm. 1993, 24, 1809-1811. (42) Trillo, J.M.; Alba, M.D.; Alvero, R.; Castro, M.A. Reversible migration of lithium in montmorillonites. J. Phys. Chem. 1994, 98, 7484-7853. 20          Table 1. Elemental quantification obtained by XRF analysis for the three initial samples. % Na (±0.09) Al (±0.4) Mg (±0.06) Si (±0.09) F (±0.3) Si/Al Si/Al theor Na-Mica-4 7.95 6.1 17.10 18.95 13.5 1.04 1 Na-Mica-3 9.46 9.0 16.75 15.67 14.3 1.74 1.68 Na-Mica-2 9.46 12.4 17.30 12.91 11 3.1 3   21                 Table 2. Physicochemical properties of cations. r represents the Pauling ionic radius, q/r is the relation between the ionic charge and ionic radius. ΔH0hyd is the hydration enthalpy and pKa is the acidity constant. K+ Na+ Li+ Mg+2 Al+3 r (Å) 1.33 0.95 0.6 0.65 0.5 q/r 0.75 1.05 1.70 3.08 6.00 ΔH0hyd(KJ/mol) -305 -406 -519 -1922 -4660 pKa 14.5 14.2 13.6 11.4 5.0  22       23        Table 3. 060 and 001 planar distance, b-parameter, distortion tetraedra angle (α) and cation-basal plane distance of X-Mica-n. Cation (X) 060/2θ d060 (Å) d001(Å) bobs(Å) α r(Å) d(Å) X-Mica-4 Li+ 59.75±0.01 1.55±0.01 11.96±0.02 9.28±0.05 13.92±0.02 2.30±0.05 2.63±0.05 Na+ 59.99±0.01 1.54±0.01 12.17±0.01 9.25±0.04 14.71±0.02 2.26±0.03 2.65±0.03 K+ 59.33±0.02 1.56±0.03 12.83±0.02 9.30±0.20 12.33±0.08 2.40±0.20 2.90±0.10 Mg+2 59.84±0.02 1.54±0.03 12.06±0.02 9.30±0.20 14.23±0.09 2.30±0.20 2.60±0.20 Al+3 59.69±0.01 1.55±0.02 13.50±0.02 9.30±0.10 13.50±0.02 2.30±0.10 3.08±0.08 X-Mica-3 Li+ 59.80±0.01 1.55±0.02 12.13±0.02 9.27±0.09 10.60±0.04 2.29±0.09 2.80±0.30 Na+ 59.97±0.01 1.54±0.02 12.17±0.01 9.30±0.10 11.31±0.04 2.36±0.09 2.74±0.08 K+ 59.45±0.06 1.55±0.08 12.80±0.04 9.32±0.50 8.80±0.20 2.50±0.50 3.00±0.40 Mg+2 60.06±0.03 1.54±0.05 14.27±0.02 9.20±0.30 11.80±0.10 2.40±0.30 3.40±0.20 Al+3 60.06±0.05 1.54±0.07 14.05±0.04 9.20±0.40 11.70±0.20 2.40±0.40 3.30±0.30 X-Mica-2 Li+ 60.34±0.02 1.53±0.03 12.11±0.03 9.20±0.20 8.80±0.10 2.40±0.30 3.40±0.20 Na+ 60.36±0.01 1.53±0.02 12.17±0.01 9.20±0.20 8.90±0.07 2.40±0.10 2.80±0.10 K+ 60.02±0.07 1.50±0.10 12.86±0.02 9.20±0.60 6.70±0.60 2.00±0.10 3.10±0.10 Mg+2 60.13±0.02 1.54±0.03 14.34±0.03 9.20±0.30 7.20±0.20 2.50±0.30 3.50±0.20 Al+3 60.21±0.06 1.50±0.10 13.94±0.02 9.20±0.30 8.00±0.40 2.40±0.70 3.30±0.50 24  FIGURE CAPTIONS Figure 1. X-ray diffraction diagrams in the range of the 060 reflection. Asterisks represent the peak used in the following calculations (Table 2). Figure 2. a) Projection of the ab plane of the tetrahedral sheet. Continuous line represents the Bravais unit cell and the discontinuous line represents the primitive cell. b) Tetrahedral sheet distortion due to the α-rotation of adjacent tetrahedra. c) Distance between the apical oxygens and the interlayer cations. Figure 3. Distortion angles calculated from the b parameter obtained from the 060 reflection versus the interlayer cation radius. Squares= X-Mica-4, circles = X-Mica-3 and triangles= X- Mica-2. Figure 4. 29Si MAS-NMR spectra of X-Mica-n as a function of the layer charge (n) for the alkaline interlayer cations: a) Li+, b) Na+, and, c) K+. Q3(mAl) environments are marked with * Figure 5. 29Si MAS-NMR spectra of X-Mica-n as a function of the layer charge (n) for the third period interlayer cations: a) Na+, b) Mg2+, and, c) Al3+. Q3(mAl) environments are marked with * Figure 6. 29Si chemical shift values obtained from the fit of the spectra of the Figures 4 and 5 in function of the interlayer cation radius. Q3(mAl) m=3, 2, 1 and 0 are represented by squares, circles, upper triangle and downer triangle respectively, and are separated by dot lines. Solid symbols=X-Mica-4, crossed=X-Mica-3 and open=X-Mica-2. Figure 7. 27Al MAS-NMR spectra of X-Mica-n.  25  Figure 1  32  EXPERIMENTAL Sample Characterization. X-ray diffraction (XRD) patterns were measured at the CITIUS X-ray laboratory (University of Seville, Spain) using a Bruker D8 Advance instrument equipped with a Cu Kα radiation source, operating at 40 kV and 40 mA, and with a Ni filter. The powder XRD patterns were registered in the 2θ-range 3–70° with a step size of 0.05° and a time step of 0.03 s.    Figure 1S: X-ray diffractograms obtained for the starting samples- Na-mica-n                    33  Figure 2S: X-ray diffractograms obtained for the starting samples homoionized with Li+, K+, Mg+ 2and Al+3 cations.    