Continuous Ammonium Silicofluoride Ammonification for SiO2

Article pubs.acs.org/IECR

Continuous Ammonium Silicofluoride Ammonification for SiO2 Nanoparticles Preparation in a Microchemical System Tongbao Zhang, Yangcheng Lu,* Jianquan Liu, Kai Wang, and Guangsheng Luo State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, 10084, Beijing, China S Supporting Information *

ABSTRACT: The ammonium silicofluoride ammonification process is a potential atomic economical process for nanosized silica preparation. In this work, this process was continuously conducted in a micropores dispersion microreactor and specially investigated. The exploration on the evolution of the yield and the specific surface area of product with reaction proceeding under various feeding conditions indicated that both of two consequent stages of fast precipitation and precipitate aging may occur remarkably in the microchemical system. However, their overlap could be reduced by enhancing mixing and strictly controlling the residence time. Correspondingly, this process could stably preparing high-quality SiO2 nanoparticles (specific surface area >400 m2/g) at equilibrium yield. Furthermore, optimization on reaction temperature was discussed, and the adaptability of high concentration (NH4)2SiF6 as feed was tested as well. This research provided fundamentals and a guide for developing and designing an industrial production technology of nanosized silica based on the ammonium silicofluoride ammonification process.

1. INTRODUCTION The rapid development of nanosciences and nanotechnologies has boosted wide applications of nanomaterials in various areas. Many of them have now been commercial products frequently used in industrial production and people’s daily life, among which nanosized silica (SiO2) is a typical example. Because of a lot of outstanding properties with respect to good stability, high surface area, high chemical purity, well dispersion, and ease of modification, nanosized silica has become one of the largest output nanomaterials. It has been widely used in a variety of industries including painting, cosmetics, paper, and food, etc.1−5 In addition, it also plays an important role in medicine and has been developed for a host of biomedical and biotechnological applications such as cancer therapy, DNA transfection, drug delivery, and enzyme immobilization.6−10 Several methods have been successfully used for fabricating nanosized silica, such as gas-phase chemical reaction,11 sol−gel method,12 and precipitation method.1 Because of avoiding too high of a cost or a complicated process compared with other two methods, the precipitation method is economically preferred in an industrial production. The precipitation processes for nanoparticle preparation are usually efficient and contain prenucleation stage, nucleation stage, and growth stage. On the basis of classic precipitation theory and a population balance model, Li et al.13 modeled the process of nanoparticle preparation in the membrane dispersion microstructured reactor by introducing a parameter of mixing scale. They indicated the mixing scale had great effect on the evolution of supersaturation ratio and then determined the particle size. It is common sense that high performance mixing and sufficient supersaturation ratio are required for meeting the criterion of ensuring nanosize: explosive nucleation and wellconstrained growth. Herein, the micropores dispersion microstructured reactor is just a representative of varieties of microreactors which have © 2013 American Chemical Society

being gaining numerous research interests since the 1990s due to their excellent performance in mass transfer, precise control of reaction conditions, and improved safety14−19 compared with a conventional batch reactor. Using microreactor fabricating nanoparticles has become an attractive topic and proved successful for varieties of materials like Ag, Ca10(PO4)6(OH)2, CaCO3, BaSO4, Al2O3, FePO4, and etc.20−25 In our previous work,26 using a microstructured microreactor, we successfully prepared SiO2 nanoparticles with a surface area around 200 m2/g and a proper size distribution by the ammonium silicofluoride ((NH4)2SiF6) ammonification process. This process is a potential atomic economical process, since theoretically only silicon converts to nanosized silica while all the nitrogen and fluorine recycles as NH4F and NH3. Besides, the silicon can come from the gas byproduct in wetprocess phosphoric acid production, which means simple, almost zero cost of raw materials and additional environmental benefit. We carried out the preparation process in a semibatch mode, i.e., the gas containing N2 fed continuously, aqueous solution/slurry recycled in system before the pH value reaching a set point. A contour was drawn for the industrial application of this process. However, unsteady character of the operating mode may bring unexpected/uncontrollable particle growth, which will aggregate particularly since nanosized silica has a strong tendency to aggregate due to plenty of hydroxyl groups. As is well-known, continuous production is widely adopted in industry due to well controllability and high productivity. It is worth considering for the nanosized silica preparation. For the ammonium silicofluoride ammonification process, although semibatch production and continuous production have similar Received: Revised: Accepted: Published: 5757

February 20, 2013 March 27, 2013 April 1, 2013 April 1, 2013 dx.doi.org/10.1021/ie400547z | Ind. Eng. Chem. Res. 2013, 52, 5757−5764

Industrial & Engineering Chemistry Research

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should be designed as high as possible with obtaining a high quality SiO2 product as a prerequisite. The difference between a continuous and semibatch ammonium silicofluoride ammonification process for SiO2 nanoparticle preparation is illustrated in Figure 2. In the

original nucleation stage as the continuous feed and the disperse feed contact in the microreactor, we could find some difference in the mixing state profile determining the final silica quality between them clearly. In details, the semibatch production needs at least one tank for stock recycling, and the mixing performance in the tank, usually poor compared with the microreactor, plays an important role in silica morphology evolution. Dissimilarly, the continuous production without a secondary mixing can keep the good mixing state provided by the microreactor during the whole process. Furthermore, after the original nucleation stage, the mixing state can influence not only the generation of follow-up precipitates but also the aggregation of existing nanoparticles within the confined space. Therefore, what happens in the continuous production is perhaps complicated. In this work, continuous production of SiO2 nanoparticles in a micropores dispersion microreactor was specially investigated. Through exploring the evolution of the yield and the specific surface area of product with reaction proceeding under various feeding conditions, the mechanisms determining the conversion or transformation of silica were clearly revealed. Optimization on operation parameters including residence time and reaction temperature were discussed for obtaining high-quality SiO2 product with better economical competence as a prerequisite. The adaptability of high concentration (NH4)2SiF6 as feed was also tested. This research was conducted to provide fundamentals and a guide for developing and designing an industrial production technology of nanosized silica based on the ammonium silicofluoride ammonification process.

Figure 2. Illustration for the difference between the continuous and semibatch ammonium silicofluoride ammonification processes.

semibatch process, a stirred storage tank is commonly used for solution/slurry recycling. Compared with the continuously operated system composed of a microreactor, delay loop and separator, it leads to poor mixing performance and long and wide distributed residence time, which is unfavorable for SiO2 product sustaining the high specific surface area as originally generated. Vice versa, the continuous production mode is evidently efficient and has better potential in controllability, although the various operation parameters including residence time, reaction temperature, and mixing state need careful consideration.

2. PROCESS THEORY AND CONFIGURATION The process for SiO2 nanoparticle preparation via the ammonium silicofluoride ammonification strategy is illustrated in Figure 1. It basically contains two processes: the ammonium

3. EXPERIMENTAL SECTION NH3 gas (99.999%, 0.3 MPa) and N2 gas (99.999%, 0.3 MPa) were purchased from Beijing Hua Yuan Gas Chemical Industry Co., Ltd. and used directly without further treatment. (NH4)2SiF6 solution was obtained from Wengfu Group by using NH4F solution dissolving crude silica. Before using as the continuous phase, (NH4)2SiF6 solution is presaturated to weaken the effect of dissolution heat on the reaction proceeding and avoid suck-back caused by NH3 rapidly dissolving in operation. In detail, NH3 gas was fed into the (NH4)2SiF6 solution under continuous stirring until the pH value of the solution reached 6. Typically, the presaturated (NH4)2SiF6 solution with 80 mL/ min flow rate was used as a continuous phase, while the gas mixture of NH3 and N2 was used as the dispersion phase. The flow rate of NH3 gas was 1.3 times the stoichiometric value and N2 was 10% of the NH3 amount. Under a pressure difference between two sides of the dispersion medium, the dispersion phase (gas) was pressed through the micropores in the form of bubbles into the microchannel to mix with the continuous phase coming from the continuous feed inlet. SiO2 precipitate was generated immediately when the two phases were in contact. The precipitate was filtered and washed with distilled water three times and anhydrous ethanol once at room temperature. Finally, the SiO2 product was obtained after being dried at 105 °C in air overnight. The yield of the SiO2 product was defined as the ratio of actual precipitate amount to

Figure 1. Process illustration for SiO2 nanoparticle preparation via the ammonium silicofluoride ammonification strategy.

silicofluride preparation process and the precipitation reaction process. In the ammonium silicofluride preparation process, high temperature is required for improving NH3 release and dissolving of crude silica. While in the precipitation reaction process that SiO2 nanoparticles were generated through the precipitation reaction between NH3 presaturated (NH4)2SiF6 solution and NH3 gas, low temperature is preferred since both the NH3 dissolving and precipitation reactions are exothermic. With the consideration on energy saving, the temperature difference between the two recycled process should be as small as possible, and the temperature for the precipitation reaction 5758

dx.doi.org/10.1021/ie400547z | Ind. Eng. Chem. Res. 2013, 52, 5757−5764


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4. RESULTS AND DISCUSSION It is generally appreciated that the microchemical system has difficulties in handling of solid generating reactions because irreversible clogging of the microchannels is easy to occur with probably a bridging and constriction mechanism.27 Despite that many efforts have been devoted to address the problem and some gratifying achievements have been obtained,28−31 there still lack of a simple and universal method to overcome the challenge in all situations. In continuous ammonium silicofluoride ammonification process reported here, three strategies were used to solve the problem. One is to appropriately increase the outlet size of the microreactor since it has a higher possibility to be clogged. The other is introducing inert N2 gas or extra gas reactant in the gas feed to help keep a certain pressure of the gas side to avoid suck-back of the (NH4)2SiF6 solution and followed by formation of precipitate on the micropores to clog the inlet of the gas mixture. The last is strictly following the sequence of operations, i.e., gas feeding always opened first and shut down last to let the gas be taken out of the deposited nanoparticles. Moreover, mixing performance was greatly influenced by the flow rates of continuous and disperse phase in the microchemical system.32 In the process reported here, flow rates of both (NH 4 ) 2 SiF 6 solution and gas was sufficient for guaranteeing a good mixing performance according to our previous work.26 Concentration of (NH4)2SiF6 solution was 3 wt % except for special declaration. 4.1. Time Profile for Silica Preparation. In order to reveal the reaction proceeding of the continuous ammonium silicofluoride ammonification process in the microchemical system, several experiments were designed under different residence time. Concerning the compressibility and reaction of the gas mixture, residence time could not be calculated accurately. Thus, different tube length was used as an intuitive index representation of residence time. The results of yield with reaction proceeding for SiO2 product were shown in Figure 4. As seen in Figure 4a, the yield of SiO2 product sharply increased to equilibrium value and kept constant when residence time increased. Similar results were observed under different reaction temperatures. However, some unexpected results occurred with regard to the specific surface area of the SiO2 product with the reaction proceeding, as illustrated in Figure 5a. Both of the specific surface area was low in an extreme residence time situation of being too short or too long. There existed an optimal residence time for obtaining a high specific surface area.

theoretically calculated precipitate amount. During all the experiments, about 2 g of precipitate was collected to determine the yield, and the density of the precipitation in the liquid stream outlet is 1% wt−3% wt. The experimental setup was shown in Figure 3. The microreactor mainly consisted of two stainless steel sample

Figure 3. Experimental setup for continuous preparation of SiO2 nanoparticles.

plates (30 mm × 30 mm × 12 mm). Another stainless steel plate fabricated by the laser etching technique (30 mm × 30 mm × 0.6 mm) was installed in the microreactor with some 600 μm pores on it and acted as the dispersion medium. The geometric size of the microchannel was 10 mm × 1.2 mm × 1.2 mm (length × width × height). The detailed schematic picture of the microreactor could be found in Figure S1 in the Supporting Information. The inner diameter of the outlet tubes was 3 mm. The morphology of the SiO2 nanoparticles was observed by transmission electron microscopy (JEM-2010, 120 kV). The specific surface area of the as prepared samples was measured by a surface area analyzer (Quantachrome Autosorb-1-C chemisorptions/physisorption analyzer). The surface area was calculated from the adsorption branch in the relative pressure ranging from 0.05 to 0.30.

Figure 4. Changing of the yield of SiO2 product with the tube length under various reaction temperatures, NH3 containing 10% N2 (a) or pure NH3 (b) as gas feeding. 5759



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Figure 5. Changing of the specific surface area of SiO2 product with the tube length under various reaction temperatures, NH3 containing 10% N2 (a) or pure NH3 (b) as gas feeding.

The variation trend of the specific surface area was also supported by particle size directly observed by transmission electron microscopy (JEM-2010, 120 kV). Corresponding TEM images are illustrated in Figure 6. As told by images a− d in Figure 6, a considerable part of the product is of particles aggregates when the residence time is too short (corresponding to tube length 0.6 m). The average size of the aggregates is about 130 nm. When the tube length increased to 1.0 or 1.5 m, separate particles could be found easily in TEM images, which indicated that the dispersion of the product was remarkably improved. The average particle sizes are about 11 and 13 nm, respectively. The size distribution of them, shown in images e and f, demonstrated that product produced under two such conditions both owned a narrow size distribution. However, the particles were prone to grow and aggregate again with the average size of them increased to about 25 nm as the tube length further increased to 2 m. Similar process was also observed for product generated under different temperature, shown in images a′−d′ in Figure 6. On the basis of the results revealed above, the reaction proceeding with a residence time for the continuous ammonium silicofluoride ammonification process could be understood in detail. In the gas−liquid reaction system discussed here, NH3 gas was not only a reactant participating in the precipitation reaction but also a pH regulator of the aqueous solution during the whole process. When the gas fed first mixed with (NH4)2SiF6 solution in the microchamber, mass transfer for NH3 from the gas to liquid phase was not sufficient for fulfilling the two roles and was mainly consumed as a reactant. Accordingly, the pH value of the reaction solution hardly fluctuated in the initial reaction proceeding and kept acidic. On account of SiO2 being solvable in containing fluoride ions acid solution, the solution environment was unfavorable for them, and they completely precipitated out and resulted in the low yield. However, the NH3 gas interphase mass transfer was sustained with the reaction proceeding, as well of the good mixing state produced in the microreactor, the transformation of the reaction solution to alkaline could be achieved in seconds for providing a desired situation and reaching an equilibrium yield. In conclusion, the pH value of the reaction solution was directly related to residence time, which would significantly affect the SiO2 product yield. The situation was complicated when talking about the relevance between specific surface area and reaction proceeding. It is well understood that the specific surface area of the SiO2 product was low in initial reaction proceeding due to the unsuitable solution environment (referred to pH value) for precipitation. Also the SiO2 product behaved like a gel generated at low pH. After that, fast

Figure 6. TEM images of SiO2 products produced under various preparation conditions, NH3 containing 10% N2 as gas feeding. Images a, b, c, and d correspond to a tube length of 0.6 m, 1 m, 1.5 m, and 2 m, respectively. The apostrophe represents T = 40 °C, otherwise T = 30 °C. Images e and f indicate the particle size distribution of corresponding samples.

precipitation and well dispersed nanoparticles generation was dominant with the pH value increasing. A good mixing state was highly appreciated during the period for obtaining a high 5760



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Figure 7. Influence of temperature on SiO2 yield and specific surface area under various residence times.

and a specific surface area SiO2 nanoparticle generation as a prerequisite. The experimental results showed that temperature control was of great importance for the yield and specific surface area of the product, as illustrated in Figure 7. The yield of SiO2 product gradually decreased when the reaction temperature increased, which was confirmed under different residence time conditions. The result was ascribed to the synergistic effect of reduced solubility of NH3 gas and (NH4)2SiF6 and an equilibrium constant of the reaction as the reaction temperature is increased. The specific surface area of SiO2 nanoparticles produced at explored temperatures did not change much and could stably reach to about 400 m2/g under a proper residence time, which had far exceeded 190 m2/ g commonly required in industry standards. Thus, temperature set in the range between 30 and 40 °C was suitable for the continuous ammonium silicofluoride ammonification process by comprehensively considering the product yield, specific surface area, and energy saving as well. 4.3. Adaptability on High Concentration (NH4)2SiF6. During above-mentioned experiments, the concentration of reactant (NH4)2SiF6 solution was 3% wt. However, production efficiency greatly benefited from increasing the (NH4)2SiF6 concentration participating in the precipitation reaction. Thus, the adaptability of the high concentration of (NH4)2SiF6 in the continuous ammonium silicofluoride ammonification process was of significant importance for industrial production. The results of the influence of (NH4)2SiF6 concentration on SiO2 product yield and specific surface area was investigated and shown in Figure 8. As seen, both of the yield and specific

specific surface area product. However, the precipitates aging process was consequently accompanied and the overlap between fast precipitation and precipitates aging was aggravated by the good mixing state with reaction proceeding. The existing and follow-up generated nanoparticles were aggregated and grew in the confined space due to plenty of surface hydroxyl groups at a relatively high pH value when the residence time further increased. Therefore, the specific surface area would first grow and then fall with the reaction proceeding. In order to certify the process we revealed, we designed another experiment by using pure NH3 as the gas feed instead of the NH3 and N2 gas mixture. The most significant difference between the two situations was residence time. The results of evolution for yield and specific surface area of the product with reaction proceeding were shown in Figures 4b and 5b. As seen, in both the SiO2 yield reached an equilibrium value due to the extension of the residence time. Moreover, the specific surface area of the SiO2 product was higher and more evident for extreme residence time, such as too short or too long. For the former situation, the increased specific surface area also benefits from the extension of the residence time for locating the reaction proceeding in the fast precipitation region. While for the latter, the increased specific surface area was mainly beneficial from the decreased mixing state due to lack of turbulence caused by N2 gas in the late proceeding reaction. As a result, the overlap between the fast precipitation and precipitates aging could be reduced to some extent for avoiding the growth and aggregation of existing nanoparticles. All in all, the SiO2 quality generated in the continuous ammonium silicofluoride ammonification process is highly dependent on the reaction proceeding. Both the yield and specific surface area could be highly qualified by strictly controlling the residence time and mixing state to locate the reaction in the fast precipitation region in a proper solution environment and reduce the overlap between fast precipitation and precipitate aging. 4.2. Reaction Temperature Selection. The reaction temperature is one of the most important factors for the continuous ammonium silicofluoride ammonification process discussed here, which would not only determine the economics of the process revealed before but also give a complex influence on the precipitation reaction through directly determining the solubility of reactants of NH3 gas and (NH4)2SiF6 as well as mass transfer and diffusion coefficient. Moreover, the equilibrium constant of the reversible exothermic precipitation reaction was also greatly affected by the reaction temperature. Herein, a temperature range was carefully selected by judging both SiO2 product quality and energy saving, namely, designing the reaction temperature as high as possible with a high yield

Figure 8. Influence of (NH4)2SiF6 concentration on the SiO2 product yield and surface area. All products were produced at 30 °C with 1 min residence time.

surface area of the product decreased when (NH4)2SiF6 concentration increased. Corresponding TEM images of various concentrations of (NH4)2SiF6 feeding conditions were also illustrated in Figure 9, which clearly showed that particle size increased as (NH4)2SiF6 concentration increased. The 5761



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silicon converts to nanosized silica while all the nitrogen and fluorine recycles. Besides, the silicon can come from the gas byproduct in wet-process phosphoric acid production, which means the method is more environmental friendly. The technique could also be easily carried out in industrial production by the numbering up of microreactors.

5. CONCLUSION The continuous ammonium silicofluoride ammonification process for SiO2 nanoparticles preparation was achieved in a micropores dispersion microreactor. The reaction proceeding of the process was deeply investigated and clearly revealed by detailed exploration on the evolution of the yield and the specific surface area of product under various feeding conditions. The results showed that yield and specific surface area of the product were in direct respect of both the residence time and mixing state, suggesting that the process is combined of the preprecipitation period, the explosive precipitation period, and the follow-up precipitates aging period in the microchemical system. However, the quality of the SiO2 product could be well determined by carefully controlling the residence time and mixing state. Correspondingly, this process could stably prepare high-quality SiO2 nanoparticles (specific surface area >400 m2/g) at equilibrium yield. The reaction temperature was then optimized basically in consideration of economical competence. Moreover, experiments on the effect of (NH4)2SiF6 concentration confirmed that the technique was well adaptable for high concentration reactants and would be highly favorable in industrial production for further improving the production efficiency on the premise of maintaining high qualified product and good controllability and stability.

Figure 9. TEM images of products produced under different (NH4)2SiF6 concentration (wt): 3% (a), 5% (b), and 8% (c) and their average particle size (d).

average particle size was 11 nm, 15 nm, and 33 nm, respectively, as plotted in Figure 9d. The results could be explained by the reaction proceeding we revealed before. When the concentration of continuous (NH4)2SiF6 feeding increased, the corresponding amount of the dispersed gas mixture of NH3 and N2 also sharply increased, which would directly lead to the reduction of effective residence time. Under the conditions, the precipitation reaction was in the initial stage and the unfavorable solution environment was responsible for both the decreased product yield and the specific surface area. The results were well accordant with the reaction proceeding, suggesting the continuous ammonium silicofluoride ammonification process for SiO2 nanoparticles preparation in the microchemical system was highly adaptable for high concentration (NH4)2SiF6 feeding as well. Through accurately controlling the residence time, the production efficiency of the process could be further improved by using a high concentration (NH4)2SiF6 solution as a continuous feed also with highly qualified SiO2 nanoparticles produced. 4.4. Comparison of Various Nanosized Silica Preparation Techniques. In order to declare the features of the technique we reported here, a comparison with reported techniques was made from efficiency, cost, and issues with respect to economics and environmental impact. The results were shown in Table 1. In summary, the ammonium silicofluoride ammonification process for preparation of SiO2 nanoparticles in a microchemical system reported here is better in production efficiency because of the continuous production mode and is highly adaptable for high concentration (NH4)2SiF6 feeding as well. The process is also superior in atomic economics since only

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ASSOCIATED CONTENT

S Supporting Information *

Detailed schematic picture of microreactor used during the experiments. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86 10 62773017. Fax: +86 10 62770304. E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

Table 1. Results of Comparison of the Advance with Reported Techniques techniques sol−gel gas-phase chemical reaction traditional precipitation our technique

advantages

disadvantages

uniformity of size, well dispersity high purity, well control on crystal structure simple process, relatively high efficiency low cost, high production efficiency, high atom economic, good environmental friendship

large amount of wastewater, low production efficiency low silica output, large consumption of H2 and O2, formation of the corrosive byproducts HCl and Cl2 low atom economic, poor controllability, large consumption of mineral acids robustness of microreactor to be determined

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ACKNOWLEDGMENTS The authors are gratefully thankful for the support of the National Natural Science Foundation of China (Grants 20876084, 21036002, 21176136) and the National Basic Research Program of China (Grant 2007CB714302) on this work.

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