ARTICLE pubs.acs.org/JPCC
Evolution of Phases and Al Si Distribution during Na-4-Mica Synthesis María D. Alba,* Miguel A. Castro, Moises Naranjo, M. Mar Orta, Esperanza Pavon, and M. Carolina Pazos Instituto de Ciencia de Materiales de Sevilla, Consejo Superior de Investigaciones Científicas, Universidad de Sevilla, Avenida Americo Vespucio 49, 41092 Sevilla, Spain ABSTRACT: Na-4-mica, a highly charged fluorophlogopite, has recently attracted much attention because of its unique combination of high charge (four charges per unit cell) and its swelling and cationexchange properties. The ability to improve the properties of this mica depends on gaining knowledge about its phase evolution during the calcination process. For the synthesis, the stoichiometric powder mixture (4SiO2/2Al2O3/6MgF2/8NaCl) was heated to 900 °C for 0 600 h. The obtained solids were characterized by X-ray fluorescence (XRF); X-ray diffraction (XRD); scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) analysis; 29Si, 27Al, and 23 Na magic-angle-spinning MAS NMR spectroscopy; and thermogravimetric/differential thermal analysis (TG/DTA). The results showed that the precursors are rapidly (t < 3 h) transformed into sodalite, Al6Na8(SiO4)6Cl2, and a 2:1 phyllosilicate. For t e 7.5 h, the amount of 2:1 phyllosilicate increased as a result of the decomposition of sodalite, with a progressive incorporation of aluminum in the 2:1 phyllosilicate being observed. For t = 7.5 h, synthesis of Na-4-mica was considered to be complete, as the material remained essentially unaltered for the next 15 h. For t = 30 h, the mica started to decompose, and for very long reaction times (t g 300 h), only forsterite and a carnegierite phase were present.
1. INTRODUCTION Na-4-mica, a highly charged fluorophlogopite, has recently attracted much attention because of its unique combination of high charge (four charges per unit cell) and its swelling and cationexchange properties. A variety of studies have been published on the exchange and retention properties of Na-n-micas (n = 2, 3, 4), as well as their applications in the decontamination of soils.1,2 However, few studies have yet been devoted to clarifying key aspects such as the formation mechanism, the local order of the tetrahedral cations, and the arrangement of interlayer species.3 5 Since 1972, when small quantities of Na-4-mica were first synthesized by Gregorkiewitz,6,7 cheaper and easier synthesis r 2011 American Chemical Society
methods have been developed.8,9 In the first synthesis, Na-4-mica was obtained as one reaction product, among others, after augite powders had been melted in fluoride salts. Two decades later, Paulus et al.8 employed a multistep sol gel method, obtaining higher amounts of the silicate in each experiment, with mica being the main product. However, to leave the product free of insoluble salts, repeated washings in boric acid were necessary. Franklin and Lee9 succeeded in an “all-in-one” method in which Received: May 21, 2011 Revised: September 12, 2011 Published: September 12, 2011 20084
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several steps of the sol gel method were merged together, but the problem of impurities was not solved. Park et al.10 developed a new route, the so-called NaCl-melt method, obtaining a “pure” Na-4mica for which boric acid washings were not necessary. Studies of the long- and short-range order in Na-4-mica and its evolution with reaction time, as well as the reaction mechanism of the reactants, are essential for a better understanding of the properties of this material and, to the authors' knowledge, have not yet been published. It is well-known that the distribution of aluminum in the layered aluminosilicate structure, and thus the local distribution of the negative charge, can have a dramatic effect on the reactivity of aluminosilicates.11,12 The ability to tune the Al distribution in tailor-made aluminosilicates would allow the control of some of their properties. To achieve this ability, precise knowledge of the synthesis mechanism and of the Al Si distribution is necessary. The general goal of our work is to gain a deeper knowledge of the parameters that control the stability and cation distribution in Na-4-mica with the aim of optimizing the final product. Therefore, the particular objectives of this study were (a) to synthesize Na-4-mica products at reaction times in the range of 0 600 h, to determine the optimum reaction time for the process; (b) to identify the presence of intermediate species during the transformation that might shed light on the reaction mechanism; and (c) to analyze the changes following the synthesis process that influence the physicochemical properties and reactivity of the product.
2. EXPERIMENTAL SECTION 2.1. Synthesis Method. A single-step procedure described elsewhere,13 similar to the NaCl-melt method,10 was employed in the synthesis of Na-4-mica. A stoichiometric powder mixture with the molar composition 4SiO2/2Al2O3/6MgF2/8NaCl was used. The starting materials were SiO2 from Sigma (CAS no. 112945-52-5, 99.8% purity), Al(OH)3 from Riedel-de Ha€en (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 reactant mixtures were ground in an agate mortar, weighed, and subsequently heated in Pt crucibles at 900 °C for times in the range of 0 600 h. The 0-h calcination time means that the reactant mixture was heated to 900 °C and immediately cooled. After cooling, the solids were washed in deionized water and dried at room temperature. The solids were weighed before and after washing. The weight changes relative to the initial mass are expected to shed light on the synthesis and degradation processes of the synthesized mica. 2.2. Sample Characterization. The obtained solids were characterized by X-ray fluorescence (XRF); X-ray diffraction (XRD); scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) analysis; 29Si, 27Al, and 23Na solid-state magicangle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy; and thermogravimetric/differential thermal analysis (TG/DTA). TG/DTA experiments were carried out using a NETZSCH STA 409 PC/PG system, with alumina as the reference. The samples were placed in alumina crucibles and maintained under air throughout the heating period. The temperature was increased at a constant rate of 10 °C/min. XRF of powdered samples in borate flux was performed to obtain information about the chemical composition of the samples. XRF measurements were performed with an automated Philips PW1400 spectrometer at the Centro de Investigacion,
Figure 1. Total weight loss after (a) calcination and the total process and (b) washing process as a function of the calcination time.
Tecnología e Innovacion (CITIUS), Universidad de Sevilla, Sevilla, Spain. XRD patterns were obtained with a Bruker D8 instrument, at the CITIUS, using Cu Kα radiation at 40 kV and 40 mA. Diffractograms were obtained from 3° to 70° (2θ) at a scanning speed of 1° (2θ) min 1 and a scan step of 0.05° (2θ). Le Bail refinement, included in the TOPAS software package, was employed to index the XRD peaks and to refine the cell parameters of the synthesized micas and the other phases.14 Single-pulse (SP) MAS NMR experiments were recorded on 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 a frequency of 79.49 MHz, using a pulse width of 2.7 μs (π/2 pulse length = 7.1 μs) and delay times in the range of 3 60 s. 27Al MAS NMR spectra were recorded at 104.26 MHz with a pulse width of 0.92 μs (π/2 pulse length = 9.25 μs) and a delay time of 0.5 s. 23 Na MAS NMR spectra were recorded at 105.84 MHz with pulse widths of 2.0 μs (π/2 pulse length = 12.0 μs) and a delay time of 0.1 s. The chemical shift values are reported in parts per million (ppm) from tetramethylsilane for 29Si, from a 0.1 M AlCl3 solution for 27Al, and from a 0.5 M NaCl solution for 23 Na. A modified version of the WinFit program, which handles the finite spinning speed in MAS experiments, was used for the modeling of the 29Si MAS NMR spectra.15 The fit parameters were the chemical shift and full-width at half-maximum values (fwhm), as well as the areas under the curve of the different contributions. 20085
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Table 1. Temperature and Weight Loss of the Dehydration Reaction t (h)
TDTA (°C)
0
weight lossTG (%) 0
0.5 0.75
48.3
0 0.7
1
66.4
4.0
2
74.7
4.4
3
76.0
6.2
7.5
67.9
6.4
15
68.4
5.8
22.5
66.0
5.8
30 60
61.0 56.1
4.7 3.5
300
50.7
2.0
600
0
Scanning electron microscopy (SEM) was carried out in a JEOL JSM 5400 microscope equipped with a LINK Pentafet probe and ATW windows for EDX analysis.
3. RESULTS AND DISCUSSION 3.1. Gravimetric Results. Figure 1a shows the weight changes of the products in the calcination step and in the total process (calcination and washing). In the calcination step, represented by square symbols in the figure, a progressive increase in the weight loss was observed from t = 0 h to t = 30 h. For t > 30 h, no significant decrease in weight was observed. In the total process (circle symbols), a more complicated trend was observed, and by comparison of the two experimental plots, several stages can be distinguished. To clarify this aspect, Figure 1b includes the difference between the two experimental weights. Three different regions, marked with different shades, can be observed. In the early stages, a net weight loss is observed that decreases with calcination time. No weight change is observed for t = 15 h. For calcination times between t = 15 h and t = 60 h, a net gain of mass in the washing is evident, reaching a maximum for t = 22.5 h. For t g 60 h, the observed weight change is virtually zero. The processes involved in the gain or loss of weight can be summarized as follows: (i) In the calcination step, evaporation of volatile products (H2, Cl2, F2, HF, or HCl) is the only expected process leading to a weight loss, whereas weight gain is expected only through hydration of the products. (ii) In the washing step, weight loss is the result of dissolution of soluble products, such as NaCl, whereas weight gain is the result of hydration of the layer silicate. As observed in Figure 1, the monotonic plot of weight loss versus time in the calcination step suggests that the evaporation of volatile species is the only relevant process. The shape of the curve describing the washing step, with increasing and decreasing ranges separated by a maximum, supports the occurrence of both processes described above. At reaction times shorter than 15 h, a considerable fraction of the total weight loss was observed in the washing process, because of the small amount of volatile products, as well as the high level of dissolution of remaining precursors. The process of hydration was much less important in these products, as the amount of layer silicate formed was still, presumably, relatively
Figure 2. XRF element composition as a function of the calcination time. The compositions at time 60 h, it was virtually zero. This is in good agreement with the gravimetric results, thus suggesting that the evaporation of volatile species containing F and Cl was relevant only during the early stages of the process. 3.4. X-ray Diffraction and Scanning Electron Microscopy. Figure 3 shows the X-ray diffractograms obtained for the different products synthesized and the stick patterns of a set of crystalline phases accounting for the experimental data. The first product of the series (t = 0 h) exhibited an XRD pattern that can be described as the result of reflections arising from anhydrous mica (d001 = 9.50 Å)13 and three other crystalline phases: MgF2 (PDF 41-1443), neighborite (NaMgF3, PDF 82-1226), and sodalite (PDF 72-0029). The layer silicate became hydrated from t = 0.75 h, showing a new 001 reflection at 7.35° (2θ) (d001 = 12.0 Å), corresponding to interlayer monovalent cations surrounded by a single water sheet.17 For t g 1 h, the silicate was completely hydrated, and the 001 reflection at 9.30° (2θ) was no longer observed. The presence of hydrated mica was responsible for the weight loss observed in these products by thermogravimetric analysis (Table 1). The intensity of the reflections from other crystalline phased diminishes with reaction
time, and only reflections from pure hydrated mica were observed for t g 3 h. The XRD patterns of the products heated between 3 and 15 h showed reflections that can be indexed as hydrated mica with 12.0-Å basal spacing. There was no change of the position and intensity of the 001 reflection that could explain the increase of the weight loss observed by TG. Hydrated mica was found to be stable throughout the intermediate stages (t = 3 60 h), although a new 001 reflection at 9.30° (2θ), attributable to dry mica,13 appeared for t = 60 h. Additionally, for t g 22.5 h, a new set of reflections was observed, indexed as forsterite (Mg2SiO4, PDF 34-0189) and a carnegieite phase (Na1.75Al1.75Si0.25O4, PDF 49-0004). The scanning electron micrograph of Na-4-mica that had been heated for 15 h (Figure 4) reveals hexagonal crystallites typical of the mica morphology with a mean crystal size 2 μm that conform with the observation that larger amounts of NaCl led to smaller crystal sizes.10 The EDX analysis of the lamellar particles showed that the composition was in good agreement with the nominal composition of Na-4-mica.10 For the longest calcination times (t = 300 600 h), the mica reflections were absent, and the firing products forsterite and carnegieite were the only crystalline structures. The presence of these nonhydratable phases explains the absence of mass change after washing (Figure 1). This agrees with the absence of fluorine found by XRF analysis for these products. 20087
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Table 2. 29Si Chemicals Shifts (δ), Line Widths (fwhh), and Quantification (%) Obtained by Fitting the 29Si MAS NMR Spectra of the Products after the Shortest Calcination Time (0 e t e 2 h) t (h) 0
0.5
0.75
1
2
δ (ppm)
fwhh (Hz)
%
84.6
396.4
26.9
88.7
362.5
45.8
93.1
279.2
10.5
100.2
1066.7
16.7
79.6
224.2
3.2
82.9
181.5
10.5
84.7 88.2
205.5 303.5
21.7 54.1
92.8
234.5
10.5
79.2
194.4
1.9
82.7
181.5
9.3
84.4
205.5
21.0
88.2
321.4
57.8
92.9
234.4
10.0
78.3 82.5
197.7 200.0
7.4 13.5
85.0
198.7
11.3
88.9
273.0
58.2
93.7
168.9
9.6
78.0
182.9
9.3
82.3
228.0
19.2
85.0
198.7
11.1
88.5 93.3
297.6 153.6
56.0 4.6
3.5. MAS NMR Spectroscopy. Figure 5 shows the 29Si, 27Al,
and 23Na MAS NMR spectra obtained for products in the calcination time range of 0 2 h. The 29Si MAS NMR spectrum for the t = 0 h product (Figure 5a, Table 2) is characterized by four peaks. The three peaks at higher frequencies ( 84.6, 88.7, and 93.1 ppm) are due to the Q3(mAl), 2 g m g 0, sites of layered aluminosilicates, where the peak at ca. 84.6 ppm is an overlapping of the signal of Q3(3Al) site and the sodalite phase,18 detected by XRD. The fourth broad peak is centered at 100.2 ppm and corresponds to the Q4 environment.19 For 0 h < t e 2 h, the 29Si MAS NMR spectra are characterized by the absence of the broad peak at ca. 100 ppm (Table 2) and a set of five peaks. The peaks at ca. 78, 83, 88, and 93 ppm correspond to the Q3(mAl), 3 g m g 0, sites of layered aluminosilicates, where the peak at ca. 84.6 ppm is due to sodalite, marked with asterisk in Figure 5a.18 A detailed analysis of the layered aluminosilicate peaks shows that the intensity of the peak due to Q3(3Al), at ca. 78 ppm, increased with reaction time [from 4.1% to 10.5% relative intensity of the Q3(mAl)], which can be interpreted as the result of aluminum enrichment of the layered aluminosilicate as the reaction proceeded. It is remarkable that the spectra for t = 1 h and t = 3 h resemble the already-published spectra for Na-2-mica and Na-3mica, respectively.13 The presence of sodalite decreased with calcination time (from 21.7% to 11.1%, Table 2). The 27Al MAS NMR spectra for 0 h e t e 2 h (Figure 5b) contain one minor signal at ca. 11 ppm due to octahedral aluminum, with its highest intensity at t = 0.75 h. For longer
Figure 6. (a) 29Si, (b) 27Al, and (c) 23Na MAS NMR spectra of the reaction products at the intermediate stage. (* indicates forsterite, and + indicates Q2 Si environment.)
reaction times, the intensity remained constant. The main signals of the spectra are due to aluminum in tetrahedral coordination (67.1 and 64.4 ppm). The narrow signal at 64.4 ppm has been assigned to sodalite,20 and the broad signal at 67.1 ppm accounts for Al(OSi)3 environment of layered silicates.21 The signal from the Al(OSi)3 environment of layered silicates increased with reaction time. The 23Na MAS NMR spectra of the early stage (Figure 5c) are characterized by an asymmetric narrow peak at 5.6 ppm due to sodalite20 and a symmetric broad peak at 9.3 ppm. A signal at negative chemical shift has previously been observed from Na+ in the interlayer space of swelling mica, but our signal has a more negative chemical shift than expected for the one-layer hydrated state (δ = 7 ppm).22 Casal et al.23 observed a signal at 9 ppm in hydrated samples that was assigned to Na+ cations simultaneously associated with a monolayer of water and with oxygens belonging to one of the silicate layers. The intensity of our signal increased with reaction time, in good agreement with XRD results, showing a reduction in the presence of sodalite and an increase in the amount of hydrated mica. Signals from the other Na-containing phase, NaMgF3, observed by XRD, were not detected in the 23Na spectra. For 2 h < t e 3 h, dramatic changes in the 29Si, 27Al, and 23Na MAS NMR spectra (Figure 6) were observed, as explained below. The 29Si MAS NMR spectra (Figure 6a) show a set of four signals due to the Q3(mAl), 3 g m g 0, sites of layered aluminosilicate where the peak at ca. 81.6 ppm can be explained as the result of contributions from the Q3(2Al) site and from 29Si environments in the carnegieite phase (Table 3).24 It is remarkable that the largest contribution is the Q3(3Al) environment, as observed in brittle micas. The presence of the other environments indicates that the distribution charge layer was not homogeneous.13 The spectrum for t = 7.5 h is very much the same as the already-published spectrum for Na-4-mica.13 It is noteworthy that micas with quite similar Al Si distributions 20088
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Table 3. 29Si Chemicals Shifts (δ), Line Widths (fwhh), and Quantification (%) Obtained by Fitting the 29Si MAS NMR Spectra of the Products after Intermediate Calcination Times (3 e t e 60 h) t (h) 3
7.5
15
22.5
30
δ (ppm)
%
61.2
376.8
1.2
77.8
254.8
32.7
81.9
273.9
23.0
86.1
560.4
42.8
93.5
140.6
0.4
61.9
156.1
1.0
74.7 77.6
119.1 139.3
0.4 38.5
81.7
271.1
47.2
86.4
286.2
12.0
90.5
223.7
0.9
61.9
298.6
1.2
75.1
147.3
5.0
77.5
124.3
44.3
81.5 85.9
221.2 184.7
40.5 7.7
Figure 7. (a) 29Si, (b) 27Al, and (c) 23Na MAS NMR spectra of the reaction products after the longest calcination times.
90.3
228.0
1.2
74.9
215.1
10.3
77.6
143.9
44.1
81.4
286.6
41.8
86.0
169.1
3.4
89.7
96.1
0.5
61.6 73.2
258.8 220.6
2.9 2.0
t (h)
77.5
255.7
47.3
300
81.6
60
fwhh (Hz)
440.1
Table 4. 29Si Chemicals Shifts (δ), Line Widths (fwhh), and Quantification (%) Obtained by Fitting the 29Si MAS NMR Spectra of the Products after the Longest Calcination Times (300 e t e 600 h) δ (ppm)
fwhh (kHz)
%
61.8
43.0
29.0
45.5
74.0
248.2
8.6
78.1
334.5
28.5
81.7
460.9
33.9
61.8
50.4
39.0
86.3
121.3
0.8
89.2
369.0
1.5 600
61.9
190.5
4.1
75.0
204.9
38.3
74.2
231.2
6.9
78.5 82.9
406.9 464.1
36.9 17.2
78.0 81.1
284.3 529.9
22.8 33.0
85.8
186.4
1.3
88.4
131.4
0.6
were stable over a wide range of calcination times, although the percentage Q3(3Al) contribution to the Q3(mAl) environments increased slightly from 39.0% to 49.0% . The decomposition of the mica was detected at t = 30 h. Additionally, the spectra (Figure 6a) show a signal centered at ca. 61.5 ppm due to forsterite, marked with an asterisk,25 and a signal at ca. 75.0 ppm that is in the range of Q2 Si environments, marked with a plus mark sign.26 The sodalite phase is absent, the carnegieite and forsterite phases increase with reaction time, and the intensity of the peak at ca. 75 ppm has a maximum at 22.5 h. At the intermediate stage, the shape of the 27Al spectra changed dramatically: (i) The Al signal from sodalite (the sharp peak at 64.4 ppm) disappeared, and (ii) the tetrahedral range of the spectra showed only a broad signal. In the range from 3 to 22.5 h, the spectra exhibited a broad signal at 66.5 ppm, as already observed for the Na-4-mica phase,13,25 and a shoulder
at lower frequencies due to Al(OSi)4 of sodium aluminosilicate.27 The intensity of the shoulder increased at longer reaction times. The XRD and 29Si MAS NMR results indicate that, for t = 30 h, the layered silicate started to decompose, which is also evident from the 27Al MAS NMR spectra. They showed quite a different profile, characterized by two broad signals at ca. 66 and ca.60 ppm that shifted to higher frequencies for t g 60 h. Aluminum in octahedral coordination remained constant along the intermediate stages. All of the spectra showed a signal of aluminum in octahedral coordination. For t = 3 h, the 23Na MAS NMR spectrum contained a signal corresponding to hydrated Na+ in the interlayer space of mica, which remained up to 30 h. For t = 60 h, the main signal shifted to 10 ppm and showed an asymmetry at lower frequencies, suggesting a certain fraction of nonhydrated Na+.22,23 The presence of nonhydrated mica after t = 60 h was already observed by XRD. For t > 60 h, the mica decomposed, and only recrystallized phases were observed in the 29Si, 27Al, and 23Na MAS NMR spectra (Figure 7, Table 4). At this stage, two contributions to the 20089
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Si spectra were present. The most intense signal was a narrow peak at ca. 61.8 ppm due to forsterite, and the other was a broad and small signal in the range of carnegieite.24 The intensity of the resonance from forsterite increased with time, from 29.0% to 39.0% (see Figure 7a and Table 3). The 27Al MAS NMR spectra for t > 60 h (Figure 7b) showed three signals: one at ca. 7 ppm due to octahedral aluminum and two at 57.5 and 74.7 ppm due to tetrahedral aluminum in Al(OSi)4 and Al(OAl)4 sites, respectively,27 as can be expected from the presence of carnegieite. As the reaction time increased from 300 to 600 h, a reduction in octahedral aluminum and an increase in the Al(OAl)4 environment were observed. Finally, the 23Na MAS NMR spectra for t g 300 h (Figure 7c) contained a broad symmetric signal at 5.2 ppm due to carnegieite.
4. CONCLUSIONS The results of this work have helped clarify the synthesis mechanism of high-charge swelling micas and their range of stability. It was found that the precursors are rapidly (t < 3 h) transformed into sodalite, Na8(AlSiO4)6Cl2, and a 2:1 phyllosilicate. For t e 7.5 h, the amount of 2:1 phyllosilicate increases as a result of the decomposition of sodalite, with the progressive incorporation of aluminum in the 2:1 phyllosilicate framework being observed. For t = 7.5 h, synthesis of Na-4-mica can be considered to be complete, with the material remaining basically unaltered for the next 15 h. For t = 30 h, mica starts to decompose, and for very long reaction times (t g 300 h), only the firing products, forsterite and carnegieite, are present. By comparison with published work on Na-4-mica synthesis, it can be concluded that the formation of secondary phases and the particle size and stability of the mica depend strongly on the starting materials, the stoichiometry of the precursors, the vessel, and the time and temperature of heating.1,7 10,28 For example, when metakaolin was used as the aluminosilicate source, some insoluble phases remained, and the Na-4-mica showed a small portion of anhydrous form.1 The so-called NaCl-melt method was used to obtain pure Na-4-mica, with anhydrous phases being observed only when the temperature was higher or lower than 900 °C. When silica gel was used as the silicon source with NaOH as a mineralizer, the synthesized hydrated Na-4-mica phase quickly degenerated under open-air environment within a few days.10 When the stoichiometric fluorine content method for synthesizing high-charge-density swelling mica was employed, hydrated mica phase was obtained in all cases but with a small particle size (
