Study on the preparation of silica particles from chlorosilane residues

Study on the preparation of silica particles from chlorosilane residues

Journal of Non-Crystalline Solids 463 (2017) 72–79 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www...

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Journal of Non-Crystalline Solids 463 (2017) 72–79

Contents lists available at ScienceDirect

Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol

Study on the preparation of silica particles from chlorosilane residues Bugang Xu a, Liang Chen a,⁎, Yinguang Li a, Ping Luo b, Qikun Ma b a b

Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming, Yunnan 650504, China Kunming Metallurgical Institute New Materials Co., Ltd, Kunming, Yunnan, China

a r t i c l e

i n f o

Article history: Received 15 December 2016 Received in revised form 20 February 2017 Accepted 26 February 2017 Available online xxxx Keywords: Hydrolysis Chlorosilane residues Mechanism Silica particles Aqueous solution

a b s t r a c t In order to achieve resource utilization of chlorosilane residues, a facile method was proposed for the preparation of silica particles by the hydrolysis of chlorosilane residues in an aqueous solution. The synthesized silica powders were characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning electron microscope (SEM), dynamic light scattering (DLS) and infrared spectroscopy (IR). Additionally, the combination of thermogravimetry and mass spectrometry (TG-DSC-MS) was employed to study the variation of mass, energy and pyrolysis gas products of silica particles in the thermal decomposition process. Furthermore, to further understand hydrolysis process of chlorosilane residues, the hydrolysis and condensation mechanisms of chlorosilane residues were studied by analyzing the surface structure of the silica particles prepared from the hydrolysis of chlorosilane residues in an aqueous solution. At the same time, the three-dimensional network structures of the silica were proposed. The results shown that the prepared silica was an amorphous material, with high purity (exceed 96.46 wt% SiO2), low, and the median diameter of 17.2 to 33.8 μm, has the potential to be used as a rubber additive, which could lay the foundation for the commercialization of silica product produced by the hydrolysis of aqueous chlorosilane residues. © 2017 Elsevier B.V. All rights reserved.

1. Introduction During the production of polysilicon using the improved Siemens method, chlorosilane residues is inevitably generated from the processes of synthesis, purification and cold (or hot) hydrogenation of trichlorosilane. Generally, chlorosilane residues are mainly composed of silicon tetrachlorid (SiCl4), trichlorosilane (SiHCl3), dichlorosilane (Si2H2Cl2), hexachlorodisilane (Si2Cl6) and metal chlorides [1]. Some of these components (such as, SiCl4, Si2H2Cl2, and Si2HCl3) have a strong tendency to hydrolyze and are characterized by the release of hydrogen chloride and heat, which can easily cause damage to skin, eyes and mucous membrane to a certain extent [2,3]. In addition, both hydrogen chloride and heat can cause severe environmental pollution, if directly discharged into the atmosphere. For each ton of polysilicon produced, approximately 3–5 tons of chlorosilane residues are generated. In consideration of the hazards and resource utilization of chlorosilane residues, the study of an economic and environmental-benign approach for treating chlorosilane residues is of great significance, which can boost the sustainable development of polysilicon industry. A series of methods have been reported for the treatment of chlorosilane residues, including hydrolysis [1], combustion [4], filtration [5], drying [6] and crystallization [7]. Among these methods, hydrolysis

⁎ Corresponding author. E-mail address: [email protected] (L. Chen).

http://dx.doi.org/10.1016/j.jnoncrysol.2017.02.023 0022-3093/© 2017 Elsevier B.V. All rights reserved.

and combustion are the most commonly used methods to achieve a sound residue management. An aqueous lime slurry bath and contaminated chlorosilane residues are introduced into the reactor to carry out the hydrolysis reaction as described by Breneman et al. After drying the solids, silica content of 80–98% is obtained in this process [1]. The combustion of chlorosilane residues originated from the fumed silica technology [8]. The main objective of the technology is to transform chlorosilane residue into harmless silica by burning chlorosilane residues in a hydrogen-oxygen flame. The process of combustion produces almost no waste water and residues as compared to the hydrolysis process. However, the combustion technology is expensive due to the cost of fuel, is carried out at relatively complicated high temperature processing facilities and results in severe corrosion to equipment [1,9]. Furthermore, the alkaline hydrolysis may lead to complex post-processing requirements. Therefore, the authors focused mainly on acidic hydrolysis of chlorosilane residues in an aqueous solution. According to the best of our knowledge, studies on the hydrolysis mechanism and properties of silica produced by the hydrolysis of chlorosilane residue have never been reported previously. Based on the excellent reinforcing, thickening and thixotropic properties, silica particles have been widely used in the fields of elastomers [10], films [11], coating [12] and toothpaste [13]. In addition, silica particles have also been applied in the fields of catalysis [14], electrochemistry [15] and biomedical engineering [16]. As a result, it is necessary to study the properties of silica and its preparation process, which can provide theoretical basis for the resource utilization of chlorosilane residues.

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2. Experimental section 2.1. Materials Distilled water; sodium hydroxide (NaOH) (AR grade; Hangzhou Print-Rite Industry & Trade Co., Ltd., China), Chlorosilane residues (SiCl4:93%; SiHCl3:4.85%; SiHCl3:2.13% and HCl: 0.02%; from the Kunming Metallurgical Institute New Materials Co., Ltd).

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Table 1 Chemical composition of samples. Sample

Na2O Cl Fe2O3 SiO2 (wt%) (wt%) (wt%) (wt%)

CuO (wt%)

MnO (wt%)

Cr2O3 (wt%)

SiO2-152-H2O SiO2-73-H2O SiO2-47-H2O SiO2-152-NaOH-H2O SiO2-73-NaOH-H2O SiO2-47-NaOH-H2O

97.51 96.68 96.46 97.88 97.82 97.66

0.00078 0.00084 0.0011 0.00067 0.00070 0.00072

0.00064 0.00058 0.00062 0.00054 0.00051 0.00058

0.0058 0.0062 0.0071 0.0046 0.0053 0.0057

1.282 1.312 1.344

2.480 3.309 3.528 0.830 0.860 0.987

0.0022 0.0025 0.0028 0.0019 0.0014 0.0017

2.2. Preparation of silica 3. Results and discussion Fig. 1 shows the schematic of the experimental setup used to prepare the silica. Firstly, a certain amount of distilled water was poured into the three-necked flask (500 mL). Then, the chlorosilane residues were added to the three-necked flask by an injection pump (LSP011A) at a flow rate of 2 mL/min under mechanical stirring at a speed of 350 r/min and a temperature of 35 °C. A neck of the three-necked flask was connected to a portable multi gas detector (GX-2003-xhcy) to detect the composition of the mixed gases. The off-gas was moved to a conical flask containing 10% NaOH solution to absorb the HCl byproduct after the mixed gases were detected. The obtained suspension was kept aging for 2 h at 35 °C, and then filtered through a vacuum pump. The white precipitate was washed with distilled water or 1% sodium hydroxide solution, and then, with distilled water for several times. After the final wash, white solid material was dried in an electrothermal blowing dry box at 103 °C for 10 h. The molar ratio of distilled water to chlorosilane residues was set as 152:1, 73:1, 47:1 and 34:1 in various repeats of the hydrolysis process. Silica particles were obtained under different molar ratio and at different washing conditions. Thus, samples were given names according to their preparation conditions (for example, SiO2–152 for 152:1 M ratio of water to chlorosilane residues, respectively; SiO2-H2O for the sample which was washed with distilled water; SiO2-NaOH-H2O for the sample which was washed with NaOH solution and distilled water).

2.3. Characterization Crystal structure of the silica particles was characterized by using an XRD equipment (Empyrean) which used Cu radiation in the 2θ range of 5–80°, a current of 40 mA and voltage of 40 kV. The morphology of samples was observed by using an SEM (TESCAN VEGA3). Chemical components were detected using an XRF instrument (EDX8300). Chemical structure was determined by using infrared spectrometer (TENSOR 27) with a scan wave number ranging between 400 and 4000 cm−1. The DLS examination was performed with particles dispersed in the distilled water. In order to obtain a homogeneous suspension before the measurements, ultrasonic oscillation was used to treat the suspension for 60 min. The TG-DSC-MS (STA 449 F3 - QMS 403 C) was applied to determine the thermal stability of silica and the compositions of pyrolysis mixed gases. The TG-DSC-MS curves were obtained with a heating rate of 10 °C·min−1 from 50 °C to 1500 °C in an argon environment.

Fig. 1. Schematic of the experimental setup used to prepare silica particles. (A) Injection pump, (B) water bath, (C) three-necked flask (D) mechanical agitator, (E) portable multi gas detector, (F) NaOH solution.

3.1. Chemical composition and crystal structure of samples Because severe gelation occurred during the experiment of SiO2-34, no silica particles were prepared in this experiment, which resulted in SiO2-34 being not used to characterize and analyze. Except for H2O content, the chemical components of samples were determined by XRF (Table 1). The results indicated that all samples exceeded 96.46% silica. Very few impurities were detected, including Fe2O3, MnO, CuO. Additionally, the impurities might come from the chlorosilane residues because the content of these impurities were nearly equal in different samples. Samples SiO2-152(73, 47)-NaOH-H2O compared with samples SiO2-152(73, 47)-H2O, although the silica content was increased to a certain extent and Cl content was significantly decreased, impurity Na2O was introduced. No narrow and sharp diffraction peaks were observed in the XRD pattern of the silica powder (Fig. 2). A broad diffraction peak with a wide 2θ range confirmed that the silica powder produced by the experiments was amorphous. 3.2. Effect of the hydrolyzing water on silica particles Fig. 3 shows the size distribution of the silica particles prepared at different molar ratios of hydrolyzing water as determined by the dynamic light scattering. It can be seen that different molar ratio of distilled water to chlorosilane residues (152:1, 73:1 and 47:1, respectively) yielded 17.2, 27.4, and 33.8 μm median diameters respectively, whereas a polydispersity index (PDI) of 0.716 (for 152:1), 0.757 (for 73:1) and 0.852 (for 47:1), indicating a decrease in the particle size and polydispersity with the increase in the quantity of the water. 3.3. Properties of silica particles 3.3.1. Morphology The morphology of samples has been examined by SEM images (Fig. 4). The highly-aggregated structure of the particles was observed in the SEM micrographs, which is attributed to the strong polarity and high permittivity of water that allowed small silica particles with high surface energies to form hydrogen bonds more easily and to aggregate

Fig. 2. XRD pattern of the produced silica particles.

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Fig. 3. Size distribution of samples using Dynamic light scattering (DLS) method. (a) SiO2-152-H2O, (b) SiO2-73-H2O, (c) SiO2-47-H2O.

[17]. Furthermore, not homogeneous small silica particles could be seen on the SEM images of silica-152(73, 47)-H2O (Fig. 4(A), (C), (E)). One plausible explanation is that the concentration of the chlorosilane residues in the reactor was not homogeneous, so that the pH of the droplets was different. The pH of the droplets containing much silicon tetrachloride should be low enough to prepare the same shape of the particles [18]. However, sample SiO2-152(73, 47)-NaOH-H2O shown a more compact structure than sample SiO2-152(73, 47)-H2O, and the aggregation was more severe. 3.3.2. Thermal stability The thermal stability of samples was examined using TG-DSC-MS. Fig. 5 shows that the weight loss process of sample SiO2-73(47)-H2O

could be divided into three stages. The first stage (2.22% (2.38%) weight loss) happened between 55 and 200.3 °C, (55–175.6 °C), and corresponded to the precipitation peak of water (mass spectrum 8), indicating the precipitation of both physically absorbed and crystal water. Additionally, a small amount of absorbed hydrogen chloride was also released in this condition (mass spectrum 7). The second stage (2.84% (2.46%) weight loss) involved the loss of hydrogen, water and hydrogen chloride in the temperature range of 170.8–1030.3 °C (165.6–1020.6 °C) based on the analyses of mass spectra 6, 7 and 8. The release of hydrogen mainly originated from two sources: (i) absorbed hydrogen within the pores of silica; (ii) hydrogen produced due to the breakage of Si\\H bonds. The water precipitated at this stage includes the water formed by the cleavage of the silanols and crystal water. The hydrogen chloride

Fig. 4. Scanning electron micrographs of (A) SiO2-152-H2O, (B) SiO2-152-NaOH-H2O, (C) SiO2-73-H2O, (D) SiO2-73-NaOH-H2O, (E) SiO2-47-H2O, and (F) SiO2-47-NaOH-H2O.

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Fig. 5. TG-DSC-MS spectra of SiO2-73-H2O, SiO2-73-NaOH-H2O, SiO2-47-H2O, and SiO2-47-NaOH-H2O. (1) Thermogravimetry curve (green), (2) Differential Scanning Calorimetry curve (orange), (3) SiCl4 mass spectrum (black), (4) SiH2Cl2 mass spectrum (black), (5) SiHCl3 mass spectrum (black), (6) H2 mass spectrum (red), (7) HCl mass spectrum (Magenta), (8) H2O mass spectrum (blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

released at this stage includes adsorbed and binding-hydrogen chloride. The reason for the presence of binding-hydrogen chloride may be the silicic acid cations have to be combined with a chlorine atom to meet the electrically neutral requirements in the absence of aqueous solution, which leads to the formation of binding-hydrogen chloride. The last stage (1.88% (2.06%) weight loss) was mainly a result of the losses of binding-hydrogen chloride and isolated silanols. Surprisingly, the TG curve of sample SiO2-73-H2O showed a peak at high temperature above 1280 °C with an increase of 0.38%, which can be due to the reason that silica has strong adsorption for the protective gas (argon) under the prevailing conditions. Because the increase of other samples was very small, they were not marked in Fig. 7. The mass spectra 3, 4, and 5 showed that almost no SiCl4, SiH2Cl2 and SiHCl3 were released in the pyrolysis process for sample SiO2-73(47)-H2O. Since the ionic strength values of SiCl4, SiH2Cl2 and SiHCl3 have a very small difference among them, their mass spectra lie on the same curve. The differential scanning calorimetry (DSC) curve 2 of sample SiO273(47)-H2O shows that that there are endothermic and exothermic reactions occurring during the heating process (Fig. 5). The endothermic peak at the temperature of 130 °C (112 °C) was a result of the precipitation of adsorbed water and crystal water. The broader exothermic reaction peak was also observed in the temperature range of 985.2– 1281.1 °C (1025.2–1297.8 °C), whereas the peak area was −111.3 J/g (−92.6 J/g). During this stage, the amorphous silica was converted to a crystalline state structure. The biggest difference between sample SiO2-73(47)-H2O and sample SiO2-73 (47)-NaOH-H2O was that no HCl, H2, SiCl4, SiH2Cl2 and SiHCl3 gases were released throughout the heating process.

Table 2 Components and release amount of the pyrolysis gas of the samples. Sample

H2O (wt%)

HCl (wt%)

H2 (wt%)

SiH2Cl2 (wt%)

SiHCl3 (wt%)

SiCl4 (wt%)

SiO2-73-H2O SiO2-47-H2O SiO2-73-NaOH-H2O SiO2-73-NaOH-H2O

2.77 2.75 4.22 4.45

3.41 3.60 – –

0.42 0.47 – –

0.01 0.01 – –

0.01 0.01 – –

0.01 0.01 – –

The mass spectra can reflect the precipitation rate of pyrolysis gases, so that the relative accumulation of various gases can be obtained after normalizing and integrating the mass spectra (Fig. 5). Table 2 shows the calculation results of samples SiO2-73(47)-H2O and SiO2-73(47)-NaOHH2O.

3.3.3. Analyses of surface structure of SiO2-H2O and SiO2-NaOH-H2O The IR spectra of SiO2-47(73,152)-H2O and SiO2-47(73,152)-H2ONaOH are shown in Fig. 6. In the case of SiO2-47(73,152)-H2O-NaOH, a clear experimental phenomenon was observed showing the release of many bubbles, whereas the gas was found to be hydrogen after detection by the portable multi-gas detector. Several transmission peaks of SiO2-47(73,152)-H2O could be observed in the IR spectra (Fig. 6). According to theoretical inference and related literatures [19–23], the absorption bands at 453 cm− 1 (457 cm−1, 453 cm−1), 760 cm−1 (795 cm− 1, 765 cm− 1) and 1074 cm− 1 (1080 cm− 1, 1073 cm− 1) were assigned to the rocking vibration, symmetric stretching vibration and asymmetric stretching vibration of the Si\\O\\Si bond, respectively. The weak peak at 880 cm−1 (952 cm− 1, 879 cm− 1) revealed the bending vibration of Si\\OH bond. The peak at 3449 cm−1 (3441 cm−1, 3448 cm−1) was ascribed to the stretching vibration of \\OH bond of silanols and bound water. The peak at 1638 cm− 1 (1641 cm−1, 1639 cm− 1) was the bending vibration of H\\O\\H bond of the physically absorbed water. Additionally, an overtone of Si\\H stretching vibration, corresponded to the band of 2262 cm−1 (2356 cm−1, 2260 cm− 1) was also detected. The presence of these bands proved that the silica was not a single silicon atom compound, and was a polymer with surface hydroxyls and hydrogen groups. Compared with SiO2-47(73,152)-H2O, the IR spectra of SiO2-47(73,152)H2O-NaOH has a significant discrepancy in that the peak at 2262 cm−1 (2356 cm−1, 2260 cm−1) disappeared completely, indicating that the Si\\H bonds located on the silica were fully hydrolyzed. This result can be explained by the fact that the hydrolysis of Si\\H bond was a nucleophilic substitution reaction. However, the nucleophilic ability of hydroxyl is much higher than that of water. The hydrolysis of Si\\H bonds should be responsible for the release of hydrogen, which was consistent with the experimental phenomenon.

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Fig. 6. IR spectra of silica samples for hydrolysis (a) SiO2-47-NaOH-H2O, (b) SiO2-47-H2O, (c) SiO2-73-NaOH-H2O, (d) SiO2-73-H2O, (e) SiO2-152-NaOH-H2O, (f) SiO2-152-H2O.

3.4. Formation mechanism of silica particles leading to bulk silica 3.4.1. Hydrolysis mechanism Since the electronegativity difference of Si\\Cl bond (Δx = 1.2) is large, which leads to it being readily hydrolyzed [24], the Si\\Cl bond was not seen in the IR spectra of silica samples (Fig. 6). However, hydrogen was detected during the hydrolysis of chlorosilane residues (all experiments), whereas Si\\H bond could also be observed in the IR spectra, which revealed that only a portion of Si\\H was slowly hydrolyzed during the reaction. The hydrolysis mechanisms of chlorosilane residues (SiCl4 + SiH2Cl2 + SiHCl3) can be proposed according to the above conclusions and relevant literature [25,26]. The commonly accept mechanism for the hydrolysis of SiCl4 is shown in Scheme 1 (Eq. (1)), which belongs to a nucleophilic substitution reaction, where empty d orbital of the central Si atom is attacked by lone pair of electrons of the oxygen atom from the water molecules. Thus, an intermediate ligand (SiCl4(OH2) is formed after the hydrolysis of SiCl4 to accept the lone pair of electrons, followed by the hybridization state of the central Si atom which change from sp3 to sp3d [27]. In addition, the Si\\Cl covalent bond is completely destroyed and converted to Si\\OH covalent bond. Furthermore, the hybridization state of the transition state (SiCl3OH) is transformed into sp3. The process is

repeated for the formation of orthosilicate Si (OH)4. Similarly, the hydrolysis mechanisms of SiHCl3 and SiH2Cl2 can be deduced as Eqs. (2) and (3) in Scheme 1. The hydrolysis mechanism of Si\\H bond in SiH2(OH) 2 and SiH(OH)3 is similar to the one illustrate in Eq. (4) (Scheme 1), however the process is carried out very slowly.

3.4.2. Condensation mechanism The proposed hydrolysis mechanism of the chlorosilane residues indicates the presence of hydroxyl groups on the Si atom surface leading to condensation reactions between silanols creating Si\\O\\Si bridges which form the silica structure. The hydrolysis of chlorosilane residues was fulfilled under strongly acidic conditions, so that the orthosilicate (Si(OH)4) could combine with the hydrogen ions to form orthosilicate + cations (such as H5SiO+ 4 or Si(OH)4H ), whereas the presence of the substance was confirmed through electrophoresis [28]. The formation process of the orthosilicate cations is shown in Scheme 2 (Eq. (4)). Similarly, the synthesis of HSi(OH)3H+ and H2Si(OH)2H+ can also be represented using Eqs. (5) and (6). The following seven possibilities are presumed to be the main condensation reactions between the translation states and water molecules (Scheme 3) to form dimers in the hydrolysis system. The condensation

Scheme 1. Hydrolysis mechanisms of SiCl4, SiHCl3, SiH2Cl2, SiH2(OH)2 and SiH(OH)3.

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Scheme 2. Formation of three kinds of silicic acid cations during the hydrolysis of the chlorosilane residues.

reaction (7) has been proposed by Anbang et al. [29], while the other six condensation reactions are inferred based on Reaction 7. 3.4.3. Synthesis mechanism of secondary particles 3.4.3.1. Condensation and nucleation. Further condensation between the + + monomers (for example, dimers, H5SiO+ 4 , H3Si(OH)2 , H2Si(OH)3 and Si(OH)4)) will yield oligomers and polymeric structures. The crystal nucleus of the primary particles is formed when the condensation products reach a critical supersaturation [30–32]. At that moment, a certain size and degree of cross-linking are achieved for the polymer, where they are no longer soluble [32]. Subsequently, silica primary particles are formed by a way of rapid condensation between the aggregates. 3.4.3.2. Aggregation and growth. Because the primary particles are unstable in the reaction environment, aggregation and formation of larger particles (secondary silica particles) are an unavoidable process throughout the reaction, until the stability of the colloid is reached [33,34]. However, newly formed hydrolyzed species and primary particles can be consumed quickly in the secondary growth process, which results in the aggregation process to form the secondary particles, thus making it is a transient event [35]. Collision and coalescence of the silica particles are the main factors to control the degree of agglomeration in the reaction system [34]. Particles growth mainly occurs by addition of small aggregates and singer primary particles at higher concentrations of monomers [36]. When the majority of the monomers have been consumed, the nucleation of primary particles would not occur again. At this point, the particle growth simply depends on the addition of monomer on the surface of the particles [37]. 3.4.4. Silica structure The amorphous inorganic polymer silica can be described as an oxide composed of siloxane groups, forming the internal part of the inorganic backbone of the polymer, which is normally coated by silanols and distributed on the entire surface. The permanent droplet on the silanols results in the formation of hydrogen bond with silanols and water present in the silica [38]. De Farias and Airoldi [39] found that

there were three types of hydroxyl groups on the surface of silica particles, namely germinal, vicinal and isolated silanols. When two hydroxyl groups are present in a single silica atom belonging to the geminal groups, the two hydroxyl groups present in two adjacent silica atoms are termed as vicinal. Isolated hydroxyl is present in a single silica atom with no hydrogen bonds formed with other silanols of the same silica atom. Hydrogen bonds can be formed between the germinal groups or geminal hydroxyl groups with other vicinal or germinal silanols [19]. Similarly, it is predicted that the surface of the silica prepared in the current work should also exist in germinal, vicinal and isolated hydrogen groups. These hydrogen groups may be formed due to the condensation reactions of H2Si(OH)2, HSi(OH)3 and HSi(OH)3 with other monomers or polymers, respectively. Hence, the three-dimensional network structures of the silica prepared in this study can be shown in Fig. 7. 4. Conclusions The mechanisms of hydrolysis and condensation of chlorosilane residues are first proposed, which is of great significance for future research on the hydrolysis of chlorosilane residues to produce highquality silica. The three-dimensional network structures of the silica prepared in this study were deduced. The particle size and size distribution of the silica decreased with the increase in the quantity of hydrolyzing water. Additionally, the chemical composition, crystal and chemical surface of the resulting silica were elucidated using XRF, XRD, IR and SEM, respectively. The XRF and XRD results indicated that the product was amorphous silica with a purity of more than 96.46%, and after samples were washed with NaOH solution, the silica content in the sample would be increased. The IR detection confirmed the presence of Si\\H, Si\\OH, Si\\O\\Si, and H\\O\\H bonds on the silica surface, while the Si\\H bonds could be removed through washing with NaOH solution. The SEM and DLS results revealed that highly aggregated silica particles with diameters ranging from 17.2 μm to 33.8 μm could be prepared and the more compact structures more compact silica structure were observed after the samples were washed with NaOH solution. Thermal analysis showed that the weight losses of samples washed without

Scheme 3. Condensation reactions for the formation of dimers in the reaction system.

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Fig. 7. The three-dimensional network structures of the silica prepared by the hydrolysis of chlorosilane residues under different washing condition, (a) hydrogen bond, (b) germinal hydroxyl, (c) isolated hydrogen, (d) vicinal hydroxyl, (e) vicinal hydrogen, (f) isolated hydroxyl, (g) silicon oxygen group, (h) adsorbed water, (i) germinal hydrogen.

NaOH solution were mainly caused by the loss of adsorbed water, crystal water, adsorbed hydrogen, adsorbed hydrogen chloride, different groups (such as,\\OH, and\\H) and binding-hydrogen chloride. However, no hydrogen chloride and hydrogen were released for samples washed with NaOH solution. Furthermore, the total weight losses of the analyzed samples were lower than 6.94%. The above results (such as the purity, impurity content, weight losses and particle sizes of samples) indicated that the silica prepared in the current work meets the national Chinese standards of rubber additives, thus showing the feasibility of producing valuable silica by the hydrolysis of chlorosilane residues in aqueous solution. Conflict of interest The final manuscript has been read by all the authors, who do not declare any conflict of interest. Acknowledgments This project was financed by the National Nature Science Foundation of China (Grant No. 41261079) and Kunming Metallurgical Institute New Materials Co., Ltd., China (2014KF261), which is greatly acknowledged. References [1] W.C. Breneman, D.M. Reeser, Disposal Process for Contaminated Chlorosilanes(US: 4690810) 1987. [2] Y.P. Moreno, M.B. Cardoso, M.F. Ferrão, E.A. Moncada, J.H.Z.D. Santos, Effect of SiCl4 on the preparation of functionalized mixed-structure silica from monodisperse sol–gel silica nanoparticles, Chem. Eng. J. 292 (2016) 233–245. [3] S. López, J. Bergueiro, J. Fidalgo, Silicon tetrachloride, Ency. Toxicology. J. 41 (2014) 916–918. [4] L.M. Coleman, W. Tambo, Waste Treatment in Silicon Production Operations(US: 4519999) 1985. [5] B. Kohler, E. Schulz, B. Vendt, Separation of Metal Chlorides From Their Suspensions in Chlorosilanes(US:6602428) 2003. [6] W.C. Breneman, Process for the Treatment of Waste Metal Chlorides(US:0193958) 2006. [7] S.M. Lord, Process for Removing Aluminum and Other Metal Chlorides From Chlorosilanes(US:7736614) 2010. [8] G.D. Ulrich, Theory of particle formation and growth in oxide synthesis flames, Combust. Sci. Technol. 4 (1971) 47–57. [9] X. Chen, J. Jiang, F. Yan, S. Tian, K. Li, A novel low temperature vapor phase hydrolysis method for the production of nano-structured silica materials using silicon tetrachloride, RSC Adv. 4 (2014) 8703–8710. [10] T.A. Okel, Microporous Precipitated Silica(US: 8114935) 2012. [11] G. Wu, J. Wang, J. Shen, et al., Properties of sol–gel derived scratch-resistant nanoporous silica films by a mixed atmosphere treatment [J], J. Non-Cryst. Solids 275 (2000) 169–174. [12] C.C. Chang, T.Y. Oyang, F.H. Hwang, et al., Preparation of polymer/silica hybrid hard coatings with enhanced hydrophobicity on plastic substrates [J], J. Non-Cryst. Solids 358 (2012) 72–76. [13] K.H. Muller, M. Neumuller, G. Turk, Dental-care Composition Using Precipitated Silica(US:4753791) 1988.

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