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Preparation of silica powder with high whiteness from palygorskite
Applied Clay Science 50 (2010) 432–437
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Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y
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Preparation of silica powder with high whiteness from palygorskite Shiquan Lai ⁎, Li Yue, Xuefei Zhao, Lijuan Gao School of Chemical Engineering, University of Science and Technology Liaoning, Anshan, Liaoning 114051, China
a r t i c l e
i n f o
Article history: Received 24 December 2009 Received in revised form 5 August 2010 Accepted 21 August 2010 Available online 16 September 2010 Keywords: Silica Calcination Acid leaching Palygorskite
a b s t r a c t Silica powder with high whiteness was prepared from calcined and acid-activated palygorskite. The effect of calcination temperature, mass ratio of ammonium sulfate to palygorskite, acid concentration and leaching time on the chemical composition, the whiteness and the specific surface area (SBET) of the products were investigated. The suitable conditions for preparing silica powder with high whiteness from palygorskite were as follows: calcination temperature at 560 °C, mass ratio of ammonium sulfate to palygorskite of 3:1, H2SO4 concentration of 0.5 M and leaching time of 2 h. Under the proper conditions, the silica content, the whiteness and the specific surface area of the product were 85%, 92% and 308 m2/g, respectively. The solid products consisted mainly of amorphous silica with a small amount of quartz. The silica particles maintained the fibrous morphology of palygorskite and a few particles with 10–20 nm in diameter were observed. The length of the fibrous particles decreased from micrometer dimension to about 100–200 nm as the intensity of the modification reactions was increased. The silica products are promising materials as adsorbents, catalyst supports and fillers of plastics. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Because of its outstanding physicochemical properties, silica (SiO2) is widely used in electronics, ceramics, pharmaceuticals, catalysts, detergents, coatings and polymeric composite industries (Liou, 2004). At present, the techniques to prepare this material mainly include chemical vapor deposition, sol–gel, assembly and selective acid leaching (Okada et al., 2005; Liou, 2004). Acid leaching is a simple and cost-effective method, and usually used to prepare amorphous silica from rice husk (Zhang et al., 2010; Liou, 2004; Real et al., 1996), rice hull ash (Li and Wang, 2008; Kalapathy et al., 2000), coal fly ash (Misran et al., 2007) and various silicates including 1:1 type clay minerals such as kaolinite (Okada et al., 2008; Temuujin et al., 2001), metakolinite (Okada et al., 1998), antigorite (Kosuge et al., 1995) and chrysotile (Wypych et al., 2005), and 2:1 type clay minerals such as phlogopite (Okada et al., 2002), talc (Yang et al., 2006), vermiculite (Maqueda et al., 2007; Temuujin et al., 2003), montmorillonite (Temuujin, et al., 2004), saponite (Okada et al., 2007), chlorite (Okada et al., 2005), sepiolite (Aznar et al., 1996) and the layered alkali silicate mekatite (Tamura et al., 2007). Palygorskite is a 2:1 clay mineral characterized by a porous crystalline structure containing continuous two-dimensional tetrahedral sheets alloyed together by longitudinal sideline chains and has the theoretical
⁎ Corresponding author. Tel.: + 86 412 5929 263; fax: + 86 412 5929 627. E-mail address: [email protected] (S. Lai). 0169-1317/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2010.08.019
structural formula (Mg, Al and Fe)5(Si8O20)(OH)2(OH2)4·4H2O, with Mg preferentially located in octahedral sites (Brigatti et al., 2006; Barrios et al., 1995). Due to its fibrous morphology and higher specific surface area, palygorskite received a considerable attention as catalyst carriers (Miao et al., 2007; Lei et al., 2006) and adsorbents (Gan et al., 2009; Zhang et al., 2009; Chen et al., 2007; Álvarez-Ayuso and García-Sánchez, 2003). The adsorption properties of palygorskite may be improved by calcination or acid-activation. For instance, Melo et al. (2000, 2002) activated palygorskite by calcination and sequent HCl leaching, and prepared terbium- and lanthanum-palygorskite catalysts. Zhao et al. (2006) also used calcination combined with acid-activation to modify palygorskite and produced copper modified palygorskite catalysts (Cu2+–PG/TiO2). Frini-Srasra and Srasra (2010) modified palygorskite with HCl and investigated its adsorption behavior. Some researchers observed the formation of amorphous silica by acid leaching of palygorskite (Barrios et al., 1995; Vicente-Rodríguez et al., 1996; Cai et al., 2007). However, much less information is available on the preparation of silica powder with high whiteness from palygorskite. In our previous studies (Lai et al., 2004, 2006), we found that it was very difficult to prepare silica material with high whiteness from palygorskite only by acid-activation or calcination. We assumed that the micropores of the palygorskite fibers were not significantly affected by acid-activation or other treatments, even under strong conditions (concentrated solutions and/or high temperatures). In the present paper, we report an effective method of preparing silica powder with high whiteness from palygorskite and investigate the chemical compositions, the microstructures and morphologies and the thermal properties of the silica products.
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2. Experimental
2.3. Characterization
2.1. Materials
Elemental analyses of the solids were carried out by a WLY100-1 model inductively coupled plasma (ICP) atomic emission spectrometer made by Beijing Haiguang Instrument Company (China) and a PerkinElmer AAnalyst 200 model atomic absorption spectrometer (AAS). Test samples used for ICP and AAS analyses were prepared as follows: the silicas were mixed with anhydrous lithium tetraborate and fused at 1000 °C for 15 min in a platinum crucible. After cooling, the solids were dissolved with 5 mass% HCl solution at 80 °C under stirring. Whiteness of the solids was determined using a ZBD model whiteness measurement meter made by Guangzhong Chuangrui Industrial Test Equipment Co., Ltd. (China). FT-IR spectra were recorded in the region 4000– 400 cm− 1 in a WQF-200 model infrared Fourier transform spectrometer made by Beijing Optical Instrument Factory (China), using the KBr pellet technique (about 1 mg of sample and 300 mg of KBr were used in the preparation of the pellets). Thermal analyses were performed on a Perkin-Elmer Diamond TG-DTA 6300 thermogravimetric analyzer, and all measurements were carried out at a heating rate of 10 °C/min in a flow of air of 50 mL/min. X-ray powder diffraction patterns were obtained by using a Siemens D-5005 diffractometer, at 40 kV and 30 mA, and employing Cu Kα filtered radiation (λ = 0.1542 nm). The equipment was connected to a DACO-MP microprocessor and used DiffractAT software. Morphologic analyses of the silica samples were carried out on a JEOL JEM-2100 transmission electron microscope operating at an accelerating voltage of 200 keV, and all samples were ultrasonically dispersed in water for 10 min and then dropped onto a copper grid covered by a carbon film before observation. Nitrogen adsorption– desorption isotherms were measured at 77 K using a ST-03 A model N2 adsorption analyzer made by Beijing Beifen Instrument Company (China). The samples were degassed at 120 °C for 4 h under vacuum. The nitrogen adsorption data were obtained by measuring five adsorption points in the range of nitrogen partial pressure (P/P0) from 0.05 to 0.35. The specific surface area (SBET) was calculated by the Brunauer–Emmet–Teller (BET) method. The total pore volume was obtained from the maximum amount of nitrogen gas adsorbed at the partial pressure (P/P0) = 0.99 (Temuujin et al., 2003).
Palygorskite powder (designated as Paly) was obtained from Anhui Mingmei Minerals Co., Ltd. (China). Its chemical composition was SiO2, 52; MgO, 9.6; Al2O3, 11.9; Fe2O3, 5.9; TiO2, 0.6; CaO, 1.2; Na2O, 0.2; K2O, 1.7 and loss by ignition, 16.2 mass %. According to the ideal formula proposed by Bailey in 1980 (Frini-Srasra and Srasra, 2010), it had the chemical formula (Si7.49Al0.51)(Mg2.41Al1.86Fe0.73) (K0.36Na0.05Ca0.22)O20(OH)(OH2)4·4H2O. This formula was very close to the theoretical formula of palygorskite (Mg, Al and Fe)5Si8O20(OH) (OH2)4·4H2O, the differences being due to the small amount of impurities in the solid such as quartz. In addition, the whiteness of the palygorskite was 56%. Ammonium sulfate, sulfuric acid and hydrochloric acid were achieved from Shenyang Chemistry Reagent Co., Ltd. (China). Anhydrous lithium tetraborate was purchased from Shanghai Sinopharm Chemical Reagent Co., Ltd. (China). All reagents were used without further purification.
2.2. Preparation of silica powder Ten grams of palygorskite powder were thoroughly mixed with ammonium sulfate. The mass ratio of ammonium sulfate to palygorskite was 1:1, 2:1, 3:1, 4:1 and 5:1. These mixtures were calcined at 500, 520, 540, 560, 580, 600, and 700 °C for 1 h in a tube furnace under air atmosphere. The calcined solids were reacted with 200 mL of 0.5, 1, 2 and 3 M H2SO4 under reflux and stirring for 2, 6, 12 and 24 h. The precipitates were washed with deionized water till pH = 7. Finally, the products were ground into b75 μm after drying at 105 °C in vacuum. They were designated Paly-K–T–H2SO4–C–t, K (1, 2, 3, 4 or 5) denotes the mass ratio of ammonium sulfate to palygorskite, T (500, 520, 540, 560, 580, 600 or 700) denotes the calcination temperature, C (0.5, 1, 2 or 3) denotes the concentration of H2SO4 solution, t (2, 6, 12 or 24) denotes the time of treatment. A flow diagram of the preparation procedure is shown in Fig. 1.
3. Results and discussion 3.1. XRD observation The XRD pattern of palygorskite powder used in this study (Fig. 2) showed that palygorskite was the main component, with appreciable amounts of montmorillonite and quartz, and traces of illite and kaolinite (Alvarez-Puebla et al., 2004). The reflection at 2θ = 26.7° (d = 0.334 nm) was due to the presence of quartz (Melo et al., 2000, 2002). The XRD patterns of the calcined and acid-activated palygorskite showed broadened reflections (between 2θ = 15–30°) due to the formation of amorphous silica (Barrios et al., 1995), with sharp but less intense reflections of palygorskite and quartz. These differences implied that the calcination combined acid-activation removed the octahedral metal ions of the clay mineral. Reaction for 12 h showed a clear increase in the amount of amorphous silica (Paly-3–560–H2SO4– 0.5–12). 3.2. Chemical composition, specific surface area, total pore volume and whiteness
Fig. 1. A flow diagram of the procedure used to prepare SiO2 powder from palygorskite.
Table 1 showed the chemical composition, specific surface area (SBET), total pore volume (Vtotal) and whiteness of palygorskite and the products obtained under different conditions. With increasing calcination temperature from 500 °C to 700 °C, the SiO2 content in the products showed a maximum of 85 mass% at 560 °C, compared to 52 mass% in palygorskite. Therefore, 560 °C was
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%. At strong conditions, as for Paly-5–560–H2SO4–0.5–2, Paly-3–560– H2SO4–0.5–24 or Paly-3–560–H2SO4–3–2, the water content was only 1–2 mass%. When the calcination temperature was increased from 500 °C to 700 °C, the whiteness of the products showed a maximum of 92% at 560 °C, but the yield reached a minimum of 70% (Table 1). The color of the solids became shallow or deep yellow when the calcination temperature was N600 °C. When the calcination temperature was maintained at 560 °C, the whiteness further increased and the yield further decreased with increasing mass ratio of ammonium sulfate to palygorskite, acid concentration or leaching time. The parent palygorskite had SBET of 139 m2/g and Vtotal of 0.216 cm3/g (Table 1). SBET and Vtotal of the products depended strongly on the reaction conditions and reached a maximum at 560 °C (Table 1). Increasing mass ratio of ammonium sulfate to palygorskite, H2SO4 concentration and leaching time further increased SBET and Vtotal of the products.
3.3. Thermal analyses The thermal behavior of the parent palygorskite (Fig. 3) proceeded in four steps (Vágvölgyi et al., 2008). The first step between 30 °C and 145 °C with a water loss of 6.9% was due to the loss of surfaceadsorbed and zeolitic water. The second step between 145 °C and 300 °C was corresponded to the loss of water (3.4% mass loss) coordinated to the edge-site Mg, Al or Fe cations of the octahedral sheet. The third step, between 300 °C and 560 °C, was also due to the loss of coordinated water molecules (4.5% mass loss) and overlapped with the water loss by dehydroxylation of the octahedral sheet (Barrios et al., 1995). The last dehydration step up to 560 °C with 1.4% mass loss indicated some OH groups or water molecules perhaps strongly fixed to coordinatively unsaturated Al, Mg or Fe ions (Belver et al., 2002). The DTG temperatures corresponding to the maximum mass loss rates of each step were 90 °C, 214 °C, 450 °C and 680 °C respectively. The thermal curves of the products were distinctly different from that of palygorskite. Only two dehydroxylation steps were observed for Paly-3–560–H2SO4–0.5–2 and Paly-3–560–H2SO4– 0.5–12. The first step, at b100 °C, was due to the loss of the water adsorbed by the free silica. The second step, a very modest slope from 100 °C to 800 °C, was attributed to the elimination of water from silanol groups (Barrios et al., 1995). As the intensity of
Fig. 2. XRD patterns of palygorskite, Paly-3–560–H2SO4–0.5–2 and Paly-3–560– H2SO4–0.5–12.
considered as an optimum calcination temperature. Correspondingly, the MgO, Al2O3, Fe2O3, TiO2, CaO, Na2O and K2O contents reached a minimum. At constant calcination temperature (560 °C), increasing mass ratio of ammonium sulfate to palygorskite, acid concentration or leaching time, the SiO2 content further increased and the content of MgO, Al2O3 and Fe2O3 further decreased, while the content of TiO2, CaO, Na2O and K2O changed moderately. The parent palygorskite contained 16.2 mass% water (Table 1). When mild conditions were used, as for Paly-3–500–H2SO4–0.5–2, Paly3–520–H2SO4–0.5–2, Paly-3–540–H2SO4–0.5–2, Paly-1–560–H2SO4– 0.5–2 or Paly-2–560–H2SO4–0.5–2, the water content was 8–11 mass
Table 1 Chemical compositions, specific surface area (SBET), total pore volume (Vtotal) and whiteness of palygorskite and the products. Sample
SBET (m2/g)
Vtotal (cm3/g)
Chemical compositions (mass%) SiO2
MgO
Al2O3
Fe2O3
TiO2
CaO
Na2O
K2O
H2O
Palygorskite Paly-3–500–H2SO4–0.5–2 Paly-3–520–H2SO4–0.5–2 Paly-3–540–H2SO4–0.5–2 Paly-3–560–H2SO4–0.5–2 Paly-3–580–H2SO4–0.5–2 Paly-3–600–H2SO4–0.5–2 Paly-3–700–H2SO4–0.5–2 Paly–1–560–H2SO4–0.5–2 Paly–2–560–H2SO4–0.5–2 Paly–4–560–H2SO4–0.5–2 Paly–5–560–H2SO4–0.5–2 Paly-3–560–H2SO4–0.5–6 Paly-3–560–H2SO4–0.5–12 Paly-3–560–H2SO4–0.5–24 Paly-3–560–H2SO4–1–2 Paly-3–560–H2SO4–2–2 Paly-3–560–H2SO4–3–2
139 228 256 289 308 301 273 163 216 279 328 337 321 346 369 345 366 379
0.22 0.28 0.29 0.30 0.33 0.32 0.26 0.23 0.26 0.31 0.33 0.33 0.32 0.34 0.35 0.35 0.35 0.36
52 73 76 80 85 83 80 70 64 77 89 91 87 89 92 91 92 93
9.6 4.6 4.2 3.0 2.2 3.0 4.1 7.2 7.5 4.5 1.9 1.6 2.0 1.8 1.6 1.6 1.5 1.3
11.9 5.8 5.0 3.5 3.0 4.1 4.8 9.2 10.0 5.2 2.2 2.1 2.3 2.1 2.0 2.0 1.7 1.4
5.9 2.5 2.0 1.5 1.1 1.4 2.5 4.5 4.4 2.2 1.0 1.0 1.1 1.1 1.1 1.1 1.0 0.8
0.6 0.2 0.1 0.1 0.1 0.1 0.4 0.5 0.5 0.3 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
1.2 0.3 0.4 0.2 0.2 0.3 0.4 1.1 1.0 0.5 0.3 0.2 0.3 0.2 0.2 0.2 0.2 0.2
0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.2 0.1 0.1 0.1
1.7 1.3 1.0 1.2 1.0 1.1 1.4 1.5 1.7 1.3 1.1 1.0 1.0 1.0 0.9 1.2 1.1 0.8
16.2 11.1 10.1 9.3 7.1 6.4 5.1 5.0 10.3 8.1 3.8 2.1 6.5 3.3 1.4 2.3 2.0 1.9
Whiteness (%)
Yield of silica (mass%)
56 87 88 90 92 91 86 65 78 86 94 96 94 94 95 94 95 97
– 73 73 72 70 72 79 83 83 78 69 67 70 69 66 61 59 56
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3.4. FT-IR spectra
Fig. 3. TGA and DTG curves of palygorskite, Paly-3–560–H2SO4–0.5–2 and Paly-3–560– H2SO4–0.5–12.
treatment was increased, the intensity of the two dehydration reactions decreased. The water contents of the products determined by TGA were given in Table 1.
The FT-IR spectrum of palygorskite displayed the typical bands of this clay material (Fig. 4). The band at 3686 cm− 1 was attributed to the OH stretching vibration in Mg3–OH, the bands at 3618, 3583 and 3556 cm− 1 were assigned to Al2–OH, Mg2–OH and (Fe, Mg)–OH (Suárez and García-Romero, 2006). The bands at 3406 and 3272 cm − 1 belonged to the OH stretching vibration of adsorbed and zeolitic water. The band at 1656 cm− 1 was due to the OH bending vibration of coordinated and adsorbed water (Augsburger et al., 1998; Barrios et al., 1995). In the low frequency region, there were seven characteristic bands: 1201, 1025 and 984 (Si–O stretching vibration), 783 (Si–O–Si symmetric vibration), 688 (Si–O out-of-plane bending vibration), 647 (Si–O–Al bending vibration), and 477 cm− 1 (Si–O–Si in-plane bending vibration) (Cai et al., 2007). The products showed simple spectra different from palygorskite. A wide band at 3420–3440 cm− 1 was assigned to water adsorbed on the surface of the silica (Barrios et al., 1995). The bending band of water molecules was found at 1632 cm− 1, but its intensity was much lower than that of the corresponding band of palygorskite. Three bands at 1087, 795 and 464 cm− 1 were assigned to the characteristic vibrations of silica and their intensity was distinctly higher than that of the corresponding bands in palygorskite (Cai et al., 2007). In comparison with Paly-3–500– H2SO4–0.5–2 and Paly-3–600–H2SO4–0.5–2, Paly–3–560–H2SO4– 0.5–2 showed the maximum intensity of this band, which indicated that the product had the highest SiO2 content being consistent with the ICP analyses.
3.5. Electron microscopy Palygorskite (Fig. 5a) showed the typical fibrous morphology (Barrios et al., 1995; Cao et al., 1998). The average length of the fibers was about 1 μm and its width was 10–25 nm (Lai et al., 2004). The products maintained the fibrous morphology of palygorskite. The width of the fibrous particles was hardly changed, but its length was reduced compared to palygorskite. With the increasing intensity of modification reactions, the length of the fibrous particles decreased from micrometers to several hundred nanometers. For example, the length of the fibrous particles of Paly-3–560– H2SO4–0.5–24 (Fig. 5d) was 100–200 nm. In addition, Paly-3–560– H2SO4–0.5–12 (Fig. 5c) and Paly-3–560–H2SO4–0.5–24 (Fig. 5d) contained a few spherical particles with diameters of 10–20 nm. Thus, the calcination followed by acid-activation did not destroy the fibrous morphology of palygorskite in contrast to the report of Barrios et al. (1995).
4. Summary
Fig. 4. FT-IR spectra of palygorskite, Paly-3–500–H2SO4–0.5–2, Paly-3–560–H2SO4–0.5– 2 and Paly-3–600–H2SO4–0.5–2.
Silica powder with high whiteness was prepared by calcination and acid-activation of palygorskite. 560 °C was the optimum calcination temperature. When palygorskite was calcined at 560 °C in the presence of ammonium sulfate and reacted with 0.5 M H2SO4 for 2 h, 80% of the octahedral metal ions (Mg2+, Al3+ and Fe3+) were removed and amorphous silica was obtained with a whiteness of 92%, a specific surface area of 308 m2/g, and a total pore volume of 0.33 cm3/g. The silica still maintained the fibrous morphology of palygorskite besides a few spherical particles of 10–20 nm in diameter. The length of the fibrous particles decreased from micrometer scale to 100–200 nm. The proposed method provides a cost-effective way for preparing silica powder with high whiteness from palygorskite, and might also be suitable for producing silica from other silicates.
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Fig. 5. TEM micrographs of palygorskite (a), Paly-3–560–H2SO4–0.5–2 (b) Paly-3–560–H2SO4–0.5–12 (c) and Paly-3–560–H2SO4–0.5–24 (d).
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Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y
Note
Preparation of silica powder with high whiteness from palygorskite Shiquan Lai ⁎, Li Yue, Xuefei Zhao, Lijuan Gao School of Chemical Engineering, University of Science and Technology Liaoning, Anshan, Liaoning 114051, China
a r t i c l e
i n f o
Article history: Received 24 December 2009 Received in revised form 5 August 2010 Accepted 21 August 2010 Available online 16 September 2010 Keywords: Silica Calcination Acid leaching Palygorskite
a b s t r a c t Silica powder with high whiteness was prepared from calcined and acid-activated palygorskite. The effect of calcination temperature, mass ratio of ammonium sulfate to palygorskite, acid concentration and leaching time on the chemical composition, the whiteness and the specific surface area (SBET) of the products were investigated. The suitable conditions for preparing silica powder with high whiteness from palygorskite were as follows: calcination temperature at 560 °C, mass ratio of ammonium sulfate to palygorskite of 3:1, H2SO4 concentration of 0.5 M and leaching time of 2 h. Under the proper conditions, the silica content, the whiteness and the specific surface area of the product were 85%, 92% and 308 m2/g, respectively. The solid products consisted mainly of amorphous silica with a small amount of quartz. The silica particles maintained the fibrous morphology of palygorskite and a few particles with 10–20 nm in diameter were observed. The length of the fibrous particles decreased from micrometer dimension to about 100–200 nm as the intensity of the modification reactions was increased. The silica products are promising materials as adsorbents, catalyst supports and fillers of plastics. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Because of its outstanding physicochemical properties, silica (SiO2) is widely used in electronics, ceramics, pharmaceuticals, catalysts, detergents, coatings and polymeric composite industries (Liou, 2004). At present, the techniques to prepare this material mainly include chemical vapor deposition, sol–gel, assembly and selective acid leaching (Okada et al., 2005; Liou, 2004). Acid leaching is a simple and cost-effective method, and usually used to prepare amorphous silica from rice husk (Zhang et al., 2010; Liou, 2004; Real et al., 1996), rice hull ash (Li and Wang, 2008; Kalapathy et al., 2000), coal fly ash (Misran et al., 2007) and various silicates including 1:1 type clay minerals such as kaolinite (Okada et al., 2008; Temuujin et al., 2001), metakolinite (Okada et al., 1998), antigorite (Kosuge et al., 1995) and chrysotile (Wypych et al., 2005), and 2:1 type clay minerals such as phlogopite (Okada et al., 2002), talc (Yang et al., 2006), vermiculite (Maqueda et al., 2007; Temuujin et al., 2003), montmorillonite (Temuujin, et al., 2004), saponite (Okada et al., 2007), chlorite (Okada et al., 2005), sepiolite (Aznar et al., 1996) and the layered alkali silicate mekatite (Tamura et al., 2007). Palygorskite is a 2:1 clay mineral characterized by a porous crystalline structure containing continuous two-dimensional tetrahedral sheets alloyed together by longitudinal sideline chains and has the theoretical
⁎ Corresponding author. Tel.: + 86 412 5929 263; fax: + 86 412 5929 627. E-mail address: [email protected] (S. Lai). 0169-1317/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2010.08.019
structural formula (Mg, Al and Fe)5(Si8O20)(OH)2(OH2)4·4H2O, with Mg preferentially located in octahedral sites (Brigatti et al., 2006; Barrios et al., 1995). Due to its fibrous morphology and higher specific surface area, palygorskite received a considerable attention as catalyst carriers (Miao et al., 2007; Lei et al., 2006) and adsorbents (Gan et al., 2009; Zhang et al., 2009; Chen et al., 2007; Álvarez-Ayuso and García-Sánchez, 2003). The adsorption properties of palygorskite may be improved by calcination or acid-activation. For instance, Melo et al. (2000, 2002) activated palygorskite by calcination and sequent HCl leaching, and prepared terbium- and lanthanum-palygorskite catalysts. Zhao et al. (2006) also used calcination combined with acid-activation to modify palygorskite and produced copper modified palygorskite catalysts (Cu2+–PG/TiO2). Frini-Srasra and Srasra (2010) modified palygorskite with HCl and investigated its adsorption behavior. Some researchers observed the formation of amorphous silica by acid leaching of palygorskite (Barrios et al., 1995; Vicente-Rodríguez et al., 1996; Cai et al., 2007). However, much less information is available on the preparation of silica powder with high whiteness from palygorskite. In our previous studies (Lai et al., 2004, 2006), we found that it was very difficult to prepare silica material with high whiteness from palygorskite only by acid-activation or calcination. We assumed that the micropores of the palygorskite fibers were not significantly affected by acid-activation or other treatments, even under strong conditions (concentrated solutions and/or high temperatures). In the present paper, we report an effective method of preparing silica powder with high whiteness from palygorskite and investigate the chemical compositions, the microstructures and morphologies and the thermal properties of the silica products.
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2. Experimental
2.3. Characterization
2.1. Materials
Elemental analyses of the solids were carried out by a WLY100-1 model inductively coupled plasma (ICP) atomic emission spectrometer made by Beijing Haiguang Instrument Company (China) and a PerkinElmer AAnalyst 200 model atomic absorption spectrometer (AAS). Test samples used for ICP and AAS analyses were prepared as follows: the silicas were mixed with anhydrous lithium tetraborate and fused at 1000 °C for 15 min in a platinum crucible. After cooling, the solids were dissolved with 5 mass% HCl solution at 80 °C under stirring. Whiteness of the solids was determined using a ZBD model whiteness measurement meter made by Guangzhong Chuangrui Industrial Test Equipment Co., Ltd. (China). FT-IR spectra were recorded in the region 4000– 400 cm− 1 in a WQF-200 model infrared Fourier transform spectrometer made by Beijing Optical Instrument Factory (China), using the KBr pellet technique (about 1 mg of sample and 300 mg of KBr were used in the preparation of the pellets). Thermal analyses were performed on a Perkin-Elmer Diamond TG-DTA 6300 thermogravimetric analyzer, and all measurements were carried out at a heating rate of 10 °C/min in a flow of air of 50 mL/min. X-ray powder diffraction patterns were obtained by using a Siemens D-5005 diffractometer, at 40 kV and 30 mA, and employing Cu Kα filtered radiation (λ = 0.1542 nm). The equipment was connected to a DACO-MP microprocessor and used DiffractAT software. Morphologic analyses of the silica samples were carried out on a JEOL JEM-2100 transmission electron microscope operating at an accelerating voltage of 200 keV, and all samples were ultrasonically dispersed in water for 10 min and then dropped onto a copper grid covered by a carbon film before observation. Nitrogen adsorption– desorption isotherms were measured at 77 K using a ST-03 A model N2 adsorption analyzer made by Beijing Beifen Instrument Company (China). The samples were degassed at 120 °C for 4 h under vacuum. The nitrogen adsorption data were obtained by measuring five adsorption points in the range of nitrogen partial pressure (P/P0) from 0.05 to 0.35. The specific surface area (SBET) was calculated by the Brunauer–Emmet–Teller (BET) method. The total pore volume was obtained from the maximum amount of nitrogen gas adsorbed at the partial pressure (P/P0) = 0.99 (Temuujin et al., 2003).
Palygorskite powder (designated as Paly) was obtained from Anhui Mingmei Minerals Co., Ltd. (China). Its chemical composition was SiO2, 52; MgO, 9.6; Al2O3, 11.9; Fe2O3, 5.9; TiO2, 0.6; CaO, 1.2; Na2O, 0.2; K2O, 1.7 and loss by ignition, 16.2 mass %. According to the ideal formula proposed by Bailey in 1980 (Frini-Srasra and Srasra, 2010), it had the chemical formula (Si7.49Al0.51)(Mg2.41Al1.86Fe0.73) (K0.36Na0.05Ca0.22)O20(OH)(OH2)4·4H2O. This formula was very close to the theoretical formula of palygorskite (Mg, Al and Fe)5Si8O20(OH) (OH2)4·4H2O, the differences being due to the small amount of impurities in the solid such as quartz. In addition, the whiteness of the palygorskite was 56%. Ammonium sulfate, sulfuric acid and hydrochloric acid were achieved from Shenyang Chemistry Reagent Co., Ltd. (China). Anhydrous lithium tetraborate was purchased from Shanghai Sinopharm Chemical Reagent Co., Ltd. (China). All reagents were used without further purification.
2.2. Preparation of silica powder Ten grams of palygorskite powder were thoroughly mixed with ammonium sulfate. The mass ratio of ammonium sulfate to palygorskite was 1:1, 2:1, 3:1, 4:1 and 5:1. These mixtures were calcined at 500, 520, 540, 560, 580, 600, and 700 °C for 1 h in a tube furnace under air atmosphere. The calcined solids were reacted with 200 mL of 0.5, 1, 2 and 3 M H2SO4 under reflux and stirring for 2, 6, 12 and 24 h. The precipitates were washed with deionized water till pH = 7. Finally, the products were ground into b75 μm after drying at 105 °C in vacuum. They were designated Paly-K–T–H2SO4–C–t, K (1, 2, 3, 4 or 5) denotes the mass ratio of ammonium sulfate to palygorskite, T (500, 520, 540, 560, 580, 600 or 700) denotes the calcination temperature, C (0.5, 1, 2 or 3) denotes the concentration of H2SO4 solution, t (2, 6, 12 or 24) denotes the time of treatment. A flow diagram of the preparation procedure is shown in Fig. 1.
3. Results and discussion 3.1. XRD observation The XRD pattern of palygorskite powder used in this study (Fig. 2) showed that palygorskite was the main component, with appreciable amounts of montmorillonite and quartz, and traces of illite and kaolinite (Alvarez-Puebla et al., 2004). The reflection at 2θ = 26.7° (d = 0.334 nm) was due to the presence of quartz (Melo et al., 2000, 2002). The XRD patterns of the calcined and acid-activated palygorskite showed broadened reflections (between 2θ = 15–30°) due to the formation of amorphous silica (Barrios et al., 1995), with sharp but less intense reflections of palygorskite and quartz. These differences implied that the calcination combined acid-activation removed the octahedral metal ions of the clay mineral. Reaction for 12 h showed a clear increase in the amount of amorphous silica (Paly-3–560–H2SO4– 0.5–12). 3.2. Chemical composition, specific surface area, total pore volume and whiteness
Fig. 1. A flow diagram of the procedure used to prepare SiO2 powder from palygorskite.
Table 1 showed the chemical composition, specific surface area (SBET), total pore volume (Vtotal) and whiteness of palygorskite and the products obtained under different conditions. With increasing calcination temperature from 500 °C to 700 °C, the SiO2 content in the products showed a maximum of 85 mass% at 560 °C, compared to 52 mass% in palygorskite. Therefore, 560 °C was
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%. At strong conditions, as for Paly-5–560–H2SO4–0.5–2, Paly-3–560– H2SO4–0.5–24 or Paly-3–560–H2SO4–3–2, the water content was only 1–2 mass%. When the calcination temperature was increased from 500 °C to 700 °C, the whiteness of the products showed a maximum of 92% at 560 °C, but the yield reached a minimum of 70% (Table 1). The color of the solids became shallow or deep yellow when the calcination temperature was N600 °C. When the calcination temperature was maintained at 560 °C, the whiteness further increased and the yield further decreased with increasing mass ratio of ammonium sulfate to palygorskite, acid concentration or leaching time. The parent palygorskite had SBET of 139 m2/g and Vtotal of 0.216 cm3/g (Table 1). SBET and Vtotal of the products depended strongly on the reaction conditions and reached a maximum at 560 °C (Table 1). Increasing mass ratio of ammonium sulfate to palygorskite, H2SO4 concentration and leaching time further increased SBET and Vtotal of the products.
3.3. Thermal analyses The thermal behavior of the parent palygorskite (Fig. 3) proceeded in four steps (Vágvölgyi et al., 2008). The first step between 30 °C and 145 °C with a water loss of 6.9% was due to the loss of surfaceadsorbed and zeolitic water. The second step between 145 °C and 300 °C was corresponded to the loss of water (3.4% mass loss) coordinated to the edge-site Mg, Al or Fe cations of the octahedral sheet. The third step, between 300 °C and 560 °C, was also due to the loss of coordinated water molecules (4.5% mass loss) and overlapped with the water loss by dehydroxylation of the octahedral sheet (Barrios et al., 1995). The last dehydration step up to 560 °C with 1.4% mass loss indicated some OH groups or water molecules perhaps strongly fixed to coordinatively unsaturated Al, Mg or Fe ions (Belver et al., 2002). The DTG temperatures corresponding to the maximum mass loss rates of each step were 90 °C, 214 °C, 450 °C and 680 °C respectively. The thermal curves of the products were distinctly different from that of palygorskite. Only two dehydroxylation steps were observed for Paly-3–560–H2SO4–0.5–2 and Paly-3–560–H2SO4– 0.5–12. The first step, at b100 °C, was due to the loss of the water adsorbed by the free silica. The second step, a very modest slope from 100 °C to 800 °C, was attributed to the elimination of water from silanol groups (Barrios et al., 1995). As the intensity of
Fig. 2. XRD patterns of palygorskite, Paly-3–560–H2SO4–0.5–2 and Paly-3–560– H2SO4–0.5–12.
considered as an optimum calcination temperature. Correspondingly, the MgO, Al2O3, Fe2O3, TiO2, CaO, Na2O and K2O contents reached a minimum. At constant calcination temperature (560 °C), increasing mass ratio of ammonium sulfate to palygorskite, acid concentration or leaching time, the SiO2 content further increased and the content of MgO, Al2O3 and Fe2O3 further decreased, while the content of TiO2, CaO, Na2O and K2O changed moderately. The parent palygorskite contained 16.2 mass% water (Table 1). When mild conditions were used, as for Paly-3–500–H2SO4–0.5–2, Paly3–520–H2SO4–0.5–2, Paly-3–540–H2SO4–0.5–2, Paly-1–560–H2SO4– 0.5–2 or Paly-2–560–H2SO4–0.5–2, the water content was 8–11 mass
Table 1 Chemical compositions, specific surface area (SBET), total pore volume (Vtotal) and whiteness of palygorskite and the products. Sample
SBET (m2/g)
Vtotal (cm3/g)
Chemical compositions (mass%) SiO2
MgO
Al2O3
Fe2O3
TiO2
CaO
Na2O
K2O
H2O
Palygorskite Paly-3–500–H2SO4–0.5–2 Paly-3–520–H2SO4–0.5–2 Paly-3–540–H2SO4–0.5–2 Paly-3–560–H2SO4–0.5–2 Paly-3–580–H2SO4–0.5–2 Paly-3–600–H2SO4–0.5–2 Paly-3–700–H2SO4–0.5–2 Paly–1–560–H2SO4–0.5–2 Paly–2–560–H2SO4–0.5–2 Paly–4–560–H2SO4–0.5–2 Paly–5–560–H2SO4–0.5–2 Paly-3–560–H2SO4–0.5–6 Paly-3–560–H2SO4–0.5–12 Paly-3–560–H2SO4–0.5–24 Paly-3–560–H2SO4–1–2 Paly-3–560–H2SO4–2–2 Paly-3–560–H2SO4–3–2
139 228 256 289 308 301 273 163 216 279 328 337 321 346 369 345 366 379
0.22 0.28 0.29 0.30 0.33 0.32 0.26 0.23 0.26 0.31 0.33 0.33 0.32 0.34 0.35 0.35 0.35 0.36
52 73 76 80 85 83 80 70 64 77 89 91 87 89 92 91 92 93
9.6 4.6 4.2 3.0 2.2 3.0 4.1 7.2 7.5 4.5 1.9 1.6 2.0 1.8 1.6 1.6 1.5 1.3
11.9 5.8 5.0 3.5 3.0 4.1 4.8 9.2 10.0 5.2 2.2 2.1 2.3 2.1 2.0 2.0 1.7 1.4
5.9 2.5 2.0 1.5 1.1 1.4 2.5 4.5 4.4 2.2 1.0 1.0 1.1 1.1 1.1 1.1 1.0 0.8
0.6 0.2 0.1 0.1 0.1 0.1 0.4 0.5 0.5 0.3 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
1.2 0.3 0.4 0.2 0.2 0.3 0.4 1.1 1.0 0.5 0.3 0.2 0.3 0.2 0.2 0.2 0.2 0.2
0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.2 0.1 0.1 0.1
1.7 1.3 1.0 1.2 1.0 1.1 1.4 1.5 1.7 1.3 1.1 1.0 1.0 1.0 0.9 1.2 1.1 0.8
16.2 11.1 10.1 9.3 7.1 6.4 5.1 5.0 10.3 8.1 3.8 2.1 6.5 3.3 1.4 2.3 2.0 1.9
Whiteness (%)
Yield of silica (mass%)
56 87 88 90 92 91 86 65 78 86 94 96 94 94 95 94 95 97
– 73 73 72 70 72 79 83 83 78 69 67 70 69 66 61 59 56
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3.4. FT-IR spectra
Fig. 3. TGA and DTG curves of palygorskite, Paly-3–560–H2SO4–0.5–2 and Paly-3–560– H2SO4–0.5–12.
treatment was increased, the intensity of the two dehydration reactions decreased. The water contents of the products determined by TGA were given in Table 1.
The FT-IR spectrum of palygorskite displayed the typical bands of this clay material (Fig. 4). The band at 3686 cm− 1 was attributed to the OH stretching vibration in Mg3–OH, the bands at 3618, 3583 and 3556 cm− 1 were assigned to Al2–OH, Mg2–OH and (Fe, Mg)–OH (Suárez and García-Romero, 2006). The bands at 3406 and 3272 cm − 1 belonged to the OH stretching vibration of adsorbed and zeolitic water. The band at 1656 cm− 1 was due to the OH bending vibration of coordinated and adsorbed water (Augsburger et al., 1998; Barrios et al., 1995). In the low frequency region, there were seven characteristic bands: 1201, 1025 and 984 (Si–O stretching vibration), 783 (Si–O–Si symmetric vibration), 688 (Si–O out-of-plane bending vibration), 647 (Si–O–Al bending vibration), and 477 cm− 1 (Si–O–Si in-plane bending vibration) (Cai et al., 2007). The products showed simple spectra different from palygorskite. A wide band at 3420–3440 cm− 1 was assigned to water adsorbed on the surface of the silica (Barrios et al., 1995). The bending band of water molecules was found at 1632 cm− 1, but its intensity was much lower than that of the corresponding band of palygorskite. Three bands at 1087, 795 and 464 cm− 1 were assigned to the characteristic vibrations of silica and their intensity was distinctly higher than that of the corresponding bands in palygorskite (Cai et al., 2007). In comparison with Paly-3–500– H2SO4–0.5–2 and Paly-3–600–H2SO4–0.5–2, Paly–3–560–H2SO4– 0.5–2 showed the maximum intensity of this band, which indicated that the product had the highest SiO2 content being consistent with the ICP analyses.
3.5. Electron microscopy Palygorskite (Fig. 5a) showed the typical fibrous morphology (Barrios et al., 1995; Cao et al., 1998). The average length of the fibers was about 1 μm and its width was 10–25 nm (Lai et al., 2004). The products maintained the fibrous morphology of palygorskite. The width of the fibrous particles was hardly changed, but its length was reduced compared to palygorskite. With the increasing intensity of modification reactions, the length of the fibrous particles decreased from micrometers to several hundred nanometers. For example, the length of the fibrous particles of Paly-3–560– H2SO4–0.5–24 (Fig. 5d) was 100–200 nm. In addition, Paly-3–560– H2SO4–0.5–12 (Fig. 5c) and Paly-3–560–H2SO4–0.5–24 (Fig. 5d) contained a few spherical particles with diameters of 10–20 nm. Thus, the calcination followed by acid-activation did not destroy the fibrous morphology of palygorskite in contrast to the report of Barrios et al. (1995).
4. Summary
Fig. 4. FT-IR spectra of palygorskite, Paly-3–500–H2SO4–0.5–2, Paly-3–560–H2SO4–0.5– 2 and Paly-3–600–H2SO4–0.5–2.
Silica powder with high whiteness was prepared by calcination and acid-activation of palygorskite. 560 °C was the optimum calcination temperature. When palygorskite was calcined at 560 °C in the presence of ammonium sulfate and reacted with 0.5 M H2SO4 for 2 h, 80% of the octahedral metal ions (Mg2+, Al3+ and Fe3+) were removed and amorphous silica was obtained with a whiteness of 92%, a specific surface area of 308 m2/g, and a total pore volume of 0.33 cm3/g. The silica still maintained the fibrous morphology of palygorskite besides a few spherical particles of 10–20 nm in diameter. The length of the fibrous particles decreased from micrometer scale to 100–200 nm. The proposed method provides a cost-effective way for preparing silica powder with high whiteness from palygorskite, and might also be suitable for producing silica from other silicates.
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Fig. 5. TEM micrographs of palygorskite (a), Paly-3–560–H2SO4–0.5–2 (b) Paly-3–560–H2SO4–0.5–12 (c) and Paly-3–560–H2SO4–0.5–24 (d).
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