Controllable Preparation of SiO2 Nanoparticles Using a Microfiltration

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Controllable Preparation of SiO2 Nanoparticles Using a Microfiltration Membrane Dispersion Microreactor Le Du, Jing Tan, Kai Wang, Yangcheng Lu, and Guangsheng Luo* The State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China ABSTRACT: The study presents a new technology for controllable preparation of SiO2 nanoparticles using a membrane dispersion microreactor. A gas
liquid precipitation reaction system was suggested for economics. (NH4)2SiF6 aqueous solution, a byproduct from wet-process phosphoric acid technology, was supplied as the reactant. Pure ammonia (NH3) was used as the precipitation agent. A microfiltation membrane with 0.2 μm pore size was used as the microdispersion medium to control the bubble size. The effect of various operation parameters on nanoparticles was determined, and the process was optimized. The SiO2 nanoparticles were characterized by X-ray powder diffraction, transmission electron microscopy, and Brunauer
Emmet
Teller analysis. The results showed that the preparation process could be easily controlled. The SiO2 nanoparticles were of a pseudospherical shape, the average particle size of which could vary from 10 to 150 nm. The particle size distribution was relatively narrow, and a specific surface area of around 360 m2/g was reached.

1. INTRODUCTION With the special characteristics, such as surface effect and quantum effect, nanoparticles have been widely studied in recent decades. As an example, nanosize silica (SiO2) is an important multifunctional additive used primarily for silicone rubber, paint, paper, medicine, etc.1
6 The material plays a crucial role in extinction, thickening, and reinforcing processes.7 Silica nanoparticles are currently produced by several methods, including gas-phase chemical reaction8 and sol
gel9 and precipitation methods.10 For economic reasons, the precipitation method is widely used. Specifically, the precipitation process that requires fluoride or fluosilicic acid as the raw material is known to be effective. A byproduct of wet-process phosphoric acid technology, NH4F, is used to dissolve silica ore and obtain the (NH4)2SiF6 solution. Thus, a new process that requires (NH4)2SiF6 as a Si source has been investigated.11 This method makes great use of the byproduct and realizes recycling of NH3 gas. However, this process is not easily controlled for the reversible and exothermic synthesis reaction, which reduces the transfer efficiency and the controllability of reaction conditions. Furthermore, the exothermic phenomenon caused by dissolving NH3 also blocks the forward reaction 1 and leads to a reduction in the yield of SiO2. Microreactors, as new equipment for preparation nanoparticles, have exhibited excellent performance in high transfer efficiency, a uniform reaction environment, controllable operating conditions, a simple structure, and safety.12,13 The microstructured reactors have been gradually used for the preparation of nanoparticles to control the particle size and distribution, which is currently becoming a topic of wide concern and research. A variety of nanoparticles14
16 have been prepared using different microdevices. In our previous study, a membrane dispersion microreactor, with a microfiltration membrane as the dispersion media, has been developed and successfully used to prepare nanoparticles in homogeneous and heterogeneous mixing systems.17
21 r 2011 American Chemical Society

In this study, a new preparation technology of SiO2 nanoparticles using a membrane dispersion microreactor is developed. Ammonium silicofluoride ((NH4)2SiF6) solution and ammonia (NH3) gas were selected as the reaction agents to prepare SiO2 nanoparticles in this microreactor. The operation parameters were varied and their influence on the particle size and specific surface area was investigated. The process was optimized, and SiO2 nanoparticles with high surface area were successfully prepared. The research may provide some valuable information for the application of this byproduct of wet-process phosphoric acid technology.

2. EXPERIMENTAL SECTION The strategy for preparation of SiO2 nanoparticles contains presaturated, mixing and reaction, separation, and redissolved processes (Figure 1). The synthesis reaction can be described as follows, ðNH4 Þ2 SiF6 + 4NH3 ðgÞ + 2H2 O a 6NH4 F + SiO2 ðsÞ

ð1Þ

In this process, temperature control is important because of the exothermic phenomenon of the positive reaction. To reduce the effect of the exotherm and realize a high degree of supersaturation, a presaturated process was specially designed. The 3.5 wt % (NH4)2SiF6 solution was specially mixed with NH3 gas (99.999%, 0.3 MPa) to obtain a reactant solution before it was pumped into the microreactor. At the beginning, the pH of the system was 2.1. The mixing process was stopped when the pH value was 6.7, which implied the system was saturated with NH3. This pretreatment led to an enhancement in the yield of SiO2 and decreased the particle size. Received: February 18, 2011 Accepted: June 2, 2011 Revised: May 24, 2011 Published: June 02, 2011 8536

dx.doi.org/10.1021/ie2003363 | Ind. Eng. Chem. Res. 2011, 50, 8536–8541

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Figure 3. X-ray powder refraction patterns of SiO2 powders. Figure 1. Strategy of preparation of SiO2 nanoparticles via a circular process.

Figure 2. The experimental setup for preparing SiO2.

The experimental setup is shown in Figure 2. A membrane dispersion microreactor was used in which a 0.2 μm Ni microfiltration membrane (purchased from the Central Iron & Steel Research Institute, Beijing, P. R. China) with an active membrane area of 12.5 mm2 was applied. The presaturated (NH4)2SiF6 solution as the continuous phase and NH3 as the dispersed phase were mixed in the microreactor. The pressure difference between the two sides of the membrane was applied as the driving force to disperse the NH3 gas in the form of bubbles into the continuous phase. SiO2 precipitates were synthesized when the two phases contacted each other in the mixing chamber. After an aging treatment for 1 h, SiO2 precipitates were separated from the reaction system by a centrifugal separator (LD5
2A, Beijing Medical Centrifugal Separator Factory), then the separated liquid phase could be reused to prepare new (NH4)2SiF6 solution. The SiO2 precipitates were washed three times with distilled water and once with ethanol and dried in a drying cabinet at 100 °C for 24 h. Finally, the product SiO2 was obtained. The morphology of the SiO2 was recorded by transmission electron microscopy (TEM; JEOL-2010 120 kV) images. The crystal form of the nanoparticles was characterized by X-ray diffraction analysis (XRD; Rigaku Corporation D/max-RB). The specific surface area and pore size of the nanoparticles were determined by the Brunauer
Emmet
Teller (BET) (Quanttachrome autosorb-1) method. The weight loss was measured by thermogravimetry (STA 409 PC).

Figure 4. TEM images of SiO2 nanoparticles: (a) FC = 50 mL/min, FD = 1200 mL/min, 25 °C; (b) FC = 15 mL/min, FD = 1200 mL/min, 25 °C; (c) FC = 50 mL/min, FD = 1200 mL/min, 0 °C; (d) FC = 15 mL/ min, FD = 1200 mL/min, 0 °C; (e) FC = 50 mL/min, FD = 1200 mL/ min, 50 °C; (f) FC = 15 mL/min, FD = 1200 mL/min, 50 °C. The scale bar is 100 nm.

3. RESULTS AND DISCUSSION Morphology and Size Distribution. The result of XRD analysis is shown in Figure 3, which indicates that the SiO2 particles are amorphous. Crystallographic identification of the powders was accomplished by comparing the patterns to the Joint Committee on Powder Diffraction Standards (JCPDS), which is consistent with JCPDS Card No. 29-0085. In addition, the absence of the characteristic peaks of crystalline silica also implies the amorphous structure. Figure 4 exhibits the TEM images of SiO2 particles, indicating that the SiO2 particles are of spherical morphology. The particle size ranges from 10 to 150 nm. The particle size distributions under different operating conditions are shown in Figure 5, where “num” refers to the number fraction. These results show that the size distribution is narrow. Furthermore, the SiO2 nanoparticles exhibit a special characteristic of pore size for the framework structure composed of
O
Si
O
. The pore size distribution calculated by the BJH method using the adsorption branch shows obvious peaks at 10 and 40 nm, verifying that mesopores exist in the powders. The pore size was apparently affected by the particle size and framework structure. Combining the results from Figure 6 (a
d), we can clearly see the pore structures in the silica nanoparticles of various sizes. 8537

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Figure 5. Particle size distributions of SiO2 nanoparticles: (a) FC = 50 mL/min, FD = 1200 mL/min, 25 °C; (b) FC = 5 mL/min, FD = 1200 mL/min, 0 °C.

Figure 6. TEM images and pore size distributions calculated by the BJH method: (a, c) FC = 15 mL/min, FD = 1200 mL/min, 25 °C; (b, d) FC = 25 mL/min, FD = 1200 mL/min, 25 °C.

Figure 7. The thermogravimetric curve of SiO2 particles.

In addition, the weight loss of SiO2 particles obtained from thermogravimetric analysis (STA 409 PC) is only 5.18% at 1000 °C (Figure 7), which is smaller than the common industry standard (7%). A weight loss of ∼2% occurs significantly in the range of 100
110 °C, indicating the bound water is driven off. Another obvious thermal degradation implies the
Si
OH decomposition beyond 800 °C. Effects of Operating Conditions on the Particle Size. For controllable preparation of SiO2 nanoparticles, a series of experiments has been designed to examine the effect of operating conditions on the process. The results show that the flow rates of reactants have a great influence on the mixing performance. The average particle sizes under different continuous phase flow rates 8538

dx.doi.org/10.1021/ie2003363 |Ind. Eng. Chem. Res. 2011, 50, 8536–8541

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Figure 8. (a) Effect of continuous phase flow rate on particle size; (b) effect of continuous phase flow rate on the specific surface area; (c) effect of dispersed phase flow rate on particle size.

Figure 9. Effect of reaction temperature on (a) particle size and (b) specific surface area.

are presented in Figure 8a. With the increase in the continuous phase flow rate, the particle size markedly decreases until the continuous phase flow rate is larger than 30 mL/min. The increased continuous phase flow rate provides a strong cross-flow drag force, which not only reduces the dispersed phase size but also provides a large mass transfer area between the two reactants. Therefore, nucleation costs plenty of material to generate a large number of small nuclei and lead to the decrease in the particle size. The specific surface area of SiO2 particles increases with the increase in the continuous phase flow rate (Figure 8b), which is due to the reduction of SiO2 particle size. In addition, it is well known that common industry standards require that the specific surface area of SiO2 particles be larger than 190 m2/g. Thus, we can conclude that the SiO2 particles prepared using the microreactor are well qualified.

The influence of the dispersed phase flow rate on the average SiO2 particle size is shown in Figure 8c. The dispersed phase flow rate does not obviously influence the particle size. With the increase in the dispersed phase flow rate, the disturbance is enhanced, and mass transfer has also been strengthened, but the disperse phase size increases at the same time, which may decrease the mass transfer rate. Temperature is one of the most important factors for the gas
liquid reaction system. Not only the property of NH3 gas, but also the solubility of (NH4)2SiF6 is affected by temperature. In general, the reaction temperature influences the solubility of reactants, the diffusion coefficient, and supersaturation. Furthermore, a reversible exothermic reaction occurs in the process, and NH3 gas's dissolving leads to large amount of generated heat, which reduces the supersaturation 8539

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Figure 10. Effect of pH value on (a) particle size, (b) specific surface area, and (c) yield of SiO2.

Figure 11. TEM images of SiO2 particles prepared using the microreactor (a, b) and stirred tank reactor (c, d). The scale bar is 100 nm.

for the low concentration of NH4+ and the controllability of reaction conditions. The experimental results showed that the temperature control was of great importance, and the particle size was decreased significantly with the decrease in the temperature, as shown in Figure 9a. The specific surface area of the SiO2 particles increases markedly with the decrease in the temperature (Figure 9b) as a result of the particle size reduction. In this reaction system, the pH has a great effect on the concentration of NH4+ and OH
. Apparently, the partial pressure of the NH3 gas has to be enhanced to promote the positive reaction; however, the dissolved NH3 will produce more OH
, which could react with SiO2. Hence, the pH is at the

optimal value at the greatest supersaturation in the system, which leads to the smaller particle size, narrow size distributions, and increase in the yield. The yield of SiO2 powders was defined as the ratio of the actual production yield to the theoretical production yield calculated when the precipitation was completed. The particle size increases with the increase in the pH value, as shown in Figure 10a. The specific surface area of SiO2 particles decreases quickly with the increase in pH value; the results are shown in Figure 10b. Figure 10c shows the influence of the pH value on the yield of SiO2. The yield is enhanced with increasing pH value until it reaches 9.23, and then it decreases markedly. Hence, it is the optimal pH value that makes the system of the largest supersaturation. Comparison with the Particles Prepared in the Microreactor and Stirred Tank Reactor. SiO2 particles were prepared separately using the microreactor and stirred tank reactor to compare the qualities of the particles. The experiments were carried out under the same conditions. Figure 11 shows TEM images of the SiO2 particles prepared using the microreactor (a, b) and the stirred tank reactor (c, d). Apparently, the SiO2 particles' average diameters are uniform. In contrast, SiO2 particles prepared in the stirred tank reactor are of low quality. The particles were hardly of small particle size and uneven in average diameter. In the stirred tank reactor, the process of reactants' mixing requires a great deal of time to obtain a uniform concentration in the system. Therefore, the uneven concentration distribution in the system leads to the large particle size and wide size distribution. An efficient mixing and fast mass transfer rate of reactants can be achieved in the microfiltration membrane microreactor within a short time. Thus, a uniform reaction environment is achieved with high supersaturation, which is crucial for the size and distribution of SiO2 particles. 8540

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4. CONCLUSION An efficient mixing and fast mass transfer rate were achieved in the microfiltration membrane microreactor. Both a high supersaturation and uniform reaction environment were obtained to prepare SiO2 nanoparticles. The particles were controllable and prepared with an average size ranged from 10 to 150 nm, and the specific surface area ranged from 190 to 360 m2/g. The effects of operation conditions on the particle size were investigated, confirming that the particle size could be controlled by a continuous phase flow rate, dispersed phase flow rate, temperature, pH value, etc. Furthermore, the pH was crucial for the yield of SiO2. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We gratefully acknowledge support of the National Natural Science Foundation of China (21036002, 20876084) and National Basic Research Program of China (2007CB714302) for this work. ’ NOTATION d = diameter of SiO2 particles (nm) FC = flow rate of continuous phase (mL min
1) FD = flow rate of disperse phase (mL min
1) num = number fraction of SiO2 particle size SBET = the specific surface area of SiO2 particles (m2 g
1) T = reaction temperature (°C) ’ REFERENCES

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dx.doi.org/10.1021/ie2003363 |Ind. Eng. Chem. Res. 2011, 50, 8536–8541