A novel low-cost method of silica aerogel fabrication ...

Powder Technology 323 (2018) 310–322

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A novel low-cost method of silica aerogel fabrication using fly ash and trona ore with ambient pressure drying technique Xiaodong Wu a,b,d, Maohong Fan b, J. Fred Mclaughlin c, Xiaodong Shen d, Gang Tan a,⁎ a

Department of Civil and Architectural Engineering, University of Wyoming, Laramie, WY 82071, USA Department of Chemical and Petroleum Engineering, University of Wyoming, Laramie, WY 82071, USA Department of Senior Petrographer, Carbon Management Institute, University of Wyoming, Laramie, WY 82071, USA d Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, Nanjing Tech University, Nanjing, 210009, China b c

a r t i c l e

i n f o

Article history: Received 27 July 2017 Received in revised form 3 October 2017 Accepted 5 October 2017 Available online xxxx Keywords: Silica aerogel Fly ash Trona ore Ambient pressure drying Hydrophobic

a b s t r a c t Highly porous and hydrophobic silica aerogel is fabricated using fly ash and trona ore as the starting materials and the cost-effective ambient pressure drying technique. The optimal calcination parameters are determined as temperature of 850 °C, holding time of 2 h, and the trona ore/fly ash mass ratio of 1.4. The CO2 release mechanism during calcination is proposed and has been verified by the Thermogravimetric and Differential Scanning Calorimetry analysis (TG/DSC) and X-ray fluorescence (XRF) technique. The hydrogel is derived from reacting the calcination mixture with sulphuric acid solution, followed by filtration. The impurities can be effectively removed through water washing and solvent exchange processes. No ion exchange resin is used in this preparation method, and thus it is a safe, inexpensive and much more straightforward process. In order to minimize drying shrinkage, the hydrogel is first transformed into an alcogel by soaking in ethanol, after which the alcogel is surface modified with the hexane/ethanol/trimethylchlorosilane (hexane/ethanol/TMCS) mixture solution. It has been observed that the specific surface area, BJH desorption pore volume, and BJH desorption average pore diameter all first increase and then decrease so that reach to the maximum values once the heat treatment temperature approaches 500 °C due to the oxidation of\\CH3 groups on the silica skeleton surface. The final product presents special characteristics including: (1) high thermal stability for maintaining hydrophobicity up to 476 °C, (2) contact angle of the as-dried silica aerogel as high as 151°, (3) silica particle size of ca. 3–6 nm, agreeing well with the model as reported, (4) and the 500 °C heat treated sample possessing a large specific surface area of 856.2 m2/g, a large pore volume of 2.92cm3/g, and a BJH desorption average pore diameter of 17.1 nm. The proposed inexpensive approach produces silica aerogel with superior properties and is a scalable-manufacturing method for large-scale industrial production. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Aerogels are light-weight materials that are dried in such a way to avoid the skeleton collapse and leave the air inside the intact nanostructures. Aerogels generally have low thermal conductivity, low bulk density, high specific surface area, low refractive index, low sound velocity and high optical transmission [1–3]. Owing to these unique properties, aerogels find applications in a number of fields such as thermal insulations, acoustics, chemical sensors, catalyst barriers, particle detectors, adsorbents and electronics. Among all kinds of aerogels, silica aerogel has attracted great attention and has been studied for N80 years. However, silica aerogel faces a problem of hydrophilicity under humid environment due to \\OH on the surface that is caused by partially hydrolyzed alkoxy residues. Silica aerogels easily deteriorate with

⁎ Corresponding author. E-mail address: [email protected] (G. Tan).

https://doi.org/10.1016/j.powtec.2017.10.022 0032-5910/© 2017 Elsevier B.V. All rights reserved.

time and thus their use is limited due to their sensitivity to atmospheric moisture and water [4]. Therefore, increasing the hydrophobicity of silica aerogel via introducing non-polar groups (Si-R, where R represents Vinyl, Alkyl or Aryl groups) has been investigated over the past years [5–7]. It is found that this process often associates with surface modification on the wet gel skeleton, which is also a part of the cost-effective ambient pressure drying (APD) technique [2,6–7]. Silica aerogels are conventionally fabricated by the supercritical drying method (SCD). The products via this method have high purity, good stability and high porosity. However, SCD is always involved with high temperature and pressure, which causes operation risk and triggers relatively high cost because of the use of expensive organic silicon alkoxides (TMOS, TEOS). In addition, SCD method always costs much due to the adoption and maintenance of the SCD apparatus. Therefore, APD method has superior advantages over SCD method for silica aerogel fabrication in considering the fabrication cost and safety issues. Except for these two issues that SCD technique brings, the high cost of starting materials has also restricted silica aerogel's large-scale

X. Wu et al. / Powder Technology 323 (2018) 310–322

fabrication [8]. Therefore, researchers have attempted to use no or lowcost solid wastes and abundant natural resources for aerogel fabrication in recent years. This route of high-value aerogel production using a large amount of tailings and wastes, including, fly ash, coal gangue, kaolin, oil shale ash, rice husk ash, etc. instead of using critical natural resources is of great significance in considering both environment protection and energy saving. In previous research, the method to fabricating aerogels from wastes is called the solid-liquid reaction route [9–10]. During this route, the wastes are generally pretreated with high temperature to get rid of the impurities, followed by mixing with the alkaline solution, forming water glass, another conventional precursor for fabricating silica aerogels [11]. The ion exchange resin is then used to remove the Na+ inside, and then ammonia or NaOH solution is used for gelation, thus obtaining the silica wet gel. Aging, solvent exchange, surface modification and drying are then carried out to form the resulting silica aerogels. Fly ash is a kind of solid waste that is released by factories and thermal power plants, which contains mainly of silica-alumina mixture with little amount of iron, sodium, calcium and potassium [12]. As much as about 800 million tons fly ash has been generated in the world every year [13]. The disposal of fly ash has caused significant economic and environmental problems over the past years [14]. However, current use of fly ash mainly focuses on concrete, bricks, adsorbents, insulation panels and agricultural fertilizer. There lacks of high-value applications of fly ash. Some literatures have been published for aerogel fabrication using fly ash as the starting material via the solid-liquid reaction route [3,15]. In addition, it has been also proved that fly ash can be used for alumina extraction via calcination with solid alkaline at elevated temperatures [16]. Hu [17] presented that kaolin can be activated by adding analytical Na2CO3 at elevated temperatures. Trona ore is a kind of abundant and inexpensive natural resource with the main composition of sodium carbonate. Especially for Sweetwater County, Wyoming, which is a major contributor of trona to the world and supplies about 90% of the nation's soda ash [18]. Therefore, it is a feasible way to develop aerogel using fly ash and trona ore as the staring material. Herein, we present a novel solid-solid route combining with the cost-effective APD for synthesis of hydrophobic silica aerogel using industrial waste-fly ash and abundant natural resource trona ore as the starting materials. The newly proposed method uses large amount of abundant waste and natural resource instead of the conventional and expensive silica resource (TEOS and TMOS), and it avoids the usage of ion exchange resin which will decrease the complexity and total cost of fabrication. The resulting silica aerogel has large specific surface area and high thermal stability of hydrophobicity. The calcination temperatures on the physicochemical properties of resulting silica aerogels are also investigated in this paper.

311

corporation, USA. All of them were of analytical reagent grade and no further purification was carried out. The specific preparation route of silica aerogel was shown in Fig.1. Fly ash and trona ore were mixed after screening the grounded materials to a mesh of 100 using agate mortar. Then, the mixture was put into an muffle, and after that different calcination temperatures (750 °C–950 °C) with varying holding time lengths (1 h–5 h) were carried out on different samples. The calcining mixture was then immersed in 1 M H2SO4 solution with a certain ratio of 8 mL/g. The silica sol was then obtained after vacuum filtration, with the residues for further characterization. The silica sol with a certain volume was then put into the oven, and gelation occurred after ca. 12 h under 60 °C. The hydrogel was aged for 1 d, after which 1 M H2SO4 solution wash was performed to remove the impurities inside the skeleton at room temperature. The hydrogel was then immersed into EtOH for solvent exchange at room temperature, after which hexane was used to substitute the ethanol inside the pores. Surface modification was then carried out by soaking the wet gel in a mixture with a volume ratio of hexane:TMCS:EtOH = 8:2:1 for 24 h at 40 °C. After a complete surface modification, the silica wet gel was suspended in a liquor and bailed out. The modified silica wet gel was then dried under room temperature for 3 h, and then it was heated at 100 °C and 200 °C for 2 h, respectively, to obtain the super hydrophobic silica aerogel. 2.2. Characterizations` The X-ray diffraction (XRD) patterns were recorded using a Rigaku Smartlab X-ray diffraction analysis with CuKα1 radiation (λ = 0.15406 nm). The X-ray tube was operated at 40 kV and 40 mA. Thermal gravimetric analysis (TGA) and differential scanning calorimetry analysis (DSC) were performed by SDT Q600 Thermogravimetric analyzer under flowing air with a constant flowrate (100 mL/min) at a heating rate of 10 °C/min to 1000 °C and 800 °C, respectively. Scanning electron microscopy (SEM) equipped with an energy dispersive spectrum (EDS) was conducted using a FEI Quanta FEG 450 Scanning Electron Microscope. Transmission electron microscope (TEM) and selected area electron diffraction (SAED) images of the aerogels were taken with the FEI Technai G2 F20 S-Twin. The specific surface area, pore volume and pore distribution were measured using a QuadraSorb SI surface area and pore distribution analyzer. The FTIR spectra were taken with a Nicolet iS50 FT-IR with a Smart Orbit Diamond ATR attachment using a resolution of 4 cm−1 and 32 scans. The X-ray photoelectric spectroscopy (XPS) measurement was carried out using the ThermoScientifc ESCALAB 250 instrument. The contact angle was tested using a contact angle meter (Rame-hart instrument, USA).

2. Experimental section

3. Results and discussion

2.1. Materials and synthesis methods

Fig. 2 shows the photographs of fly ash, trona ore, calcining mixture and the resulting silica aerogel. The fly ash and trona ore exhibit grey color and light-yellow color, respectively. The mixture derived from calcining fly ash and trona ore at elevated temperature shows a light-green color, which indicates that some impurities, such as iron ion contained in the mixture. The solution also presents light-green color after immersing the mixture into H2SO4 solution, however, these impurities can be removed by water wash and solvent exchange process afterwards. Therefore, the resulting silica aerogel shows a semitransparent white appearance after the APD process.

The fly ash was from Wyoming, USA, which was firstly dried at 50 °C, and grounded to pass the 150 μm sieve, with the oxide composition listed in Table. 1. The fly ash contains 52.4% silica, 26.5% alumina and small amount of other oxides, such as Fe2O3, CaO, TiO2, etc. Trona ore was from Sweetwater county of Wyoming, USA, which was mainly composed of Na2CO3 with the purity of 85% (Table 2). Sulphuric acid (H2SO4), trimethyl chlorosilane (TMCS), n-hexane, ethanol (EtOH), and deionized water (H2O) were purchased from Sigma-Aldrich Table 1 Chemical composition of fly ash used for preparing silica aerogel. Elements

SiO2

Al2O3

Fe2O3

CaO

MgO

Na2O

K2O

SO3

TiO2

MnO

P2O5

SrO

BaO

LOI

(wt%)

52.4

26.5

5.58

7.95

3.00

0.15

0.73

1.04

0.68

0.10

0.49

0.09

0.21

1.02

Note: LOI refers to the ignition loss of fly ash.

312


Table 2 Chemical composition of trona ore used for preparing silica aerogel. Elements

SiO2

Al2O3

Fe2O3

CaO

MgO

Na2O

K2O

SO3

TiO2

P2O5

SrO

LOI

(wt%)

2.44

0.668

0.150

1.14

0.795

49.61

0.236

0.04

0.0135

0.0294

0.012

44.78

Fig. 3 and Fig. 4 show the SEM image, EDS analysis and typical XRD patterns of fly ash and trona ore, respectively. Table 1 also presents the oxide composition of fly ash which was measured by X-ray fluorescence (XRF) technique. It is found in Fig. 3(a) that fly ash mainly contains four phases: vitreous beads, ferrite, mullite and amorphous carbon, within which the mullite and ferrite are crystals while beads and carbon are amorphous. There are many spherical and smooth vitreous beads with different diameters [19], which are formed during rapid cooling and they compose of the majority of the composition within fly ash. The loose and porous carbon particles exist within fly ash, however, the amount of carbon (ignition loss) is very small as can be seen in Table. 1. It is observed from Fig. 3(b) and Fig. 3(c) that alumina silicate and ferrite are in the fly ash matrix. The brightness of ferrite in SEM image is much higher than others due to its relatively larger atom weight, which is consistent with the result shown in Table 1. In combination with Fig. 3(d), peaks corresponding to mullite and quartz are present via the XRD measurement, therefore it can be inferred that the alumina silicate can be assigned to mullite phase. In addition, the crystalline mullite and quartz are thermally stable, however, they can be effectively activated by calcination with alkaline at elevated temperatures [13,16]. The oxide composition of silica is as high as 52.4%, which is favorable to improve the silica extraction efficiency for aerogel fabrication. The chemical composition is also tested by XRF, showing that the purity of this mineral is ca. 84.8% as calculated by the content of Na2O. Trona ore (Fig. 4(a)) exhibits a laminar shape, and the impurities in trona ore are mainly alumina silicate and potassium silicate. This result can be verified by the EDS and XRD analysis.

The purpose of adding trona in fly ash is to transform the inert fly ash into reactive NaAlSiO4 and Na2SiO3. The addition of alkaline can be an effective way to activate coal gangue. Li et al. proposed that the increase of coal gangue activity can be attributed to the formation of 6coordinated aluminum and Q3 silica [20]. Guo [16] et al. also showed that Na2CO3 was an activation addtive of coal gangue, which can significantly improve the aluminum extraction due to the trasfromation of NaAlSiO4 that can be easily disolved in acid. The main mineral composition of coal gangue and fly ash are kaolinite and mullite, respectively, and they both belong to the aluminosilicate, therefore, the Si\\O\\Si and Si\\O\\Al bonds are broken by trona addition followed by calcination at high temperatures. In order to find an optimal condition for activation, the effectes of different mass ratio of fly ash to trona and different calcination temperatures, as well as the dwelling time, on the mass loss of the calcining process are carried out. Fig. 5 exhibits the mass loss during calcination of fly ash and trona ore, and the chemical reactions beween trona ore and quartz/mullite in the furnace follow formula (1)–(5): 3Na2 CO3 þ 3Al2 O3 :2SiO2 ðmulliteÞ → 2NaAlSiO4 þ 4NaAlO2 þ 3CO2 ↑

ð1Þ

NaAlO2 þ SiO2 ðliqÞ → NaAlSiO4

ð2Þ

Al2 O3 ðliqÞ þ Na2 CO3 → 2NaAlO2 þ CO2 ↑

ð3Þ

Na2 CO3 þ 2SiO2 ðliqÞ þ Al2 O3 ðliqÞ → 2NaAlSiO4 þ CO2 ↑

ð4Þ

Fig. 1. Schemaic diagram of silica aerogel preparation method from fly ash and trona ore 1-Muffle reactor for Na2SiO3 and NaAlSiO4 preparation, 2-Thermocouple, 3-Three-way valves, 4Gas analyzer, 5-Computer, 6-Power supply, 7-CO2 collect tank, 8-Mechanical stirrers, 9-Two-way valves, 10-Vacuum filtering flask, 11- Beaker.


313

Fig. 2. Photographs of (a) fly ash, (b) trona ore, (c) calcination mixture and (d) the resulting silica aerogel.

Na2 CO3 þ SiO2 → Na2 SiO3 þ CO2 ↑

ð5Þ

It is noted that the reaction (4) is preferential when compared with reaction (5), which means that if the trona ore is excessive after reaction (4), it will then further react with silica as seen in reaction (5), thus

forming Na2SiO3, another reactive mineral which can be dissolved in acid afterwards [16]. According to reaction (4) and reaction (5), the mass loss will increase due to the release of CO2 with the increase of trona ore, while it reaches the maximum when sufficient trona is introduced, and then the mass loss percentage will begin to decrease due to the excessive trona remains inert. It is consistent with the results

Fig. 3. (a) SEM image, (b, c) EDS analysis and (d) typical XRD pattern of fly ash as starting material.

314


Fig. 4. (a) SEM image, (b, c) EDS analysis and (d) typical XRD pattern of trona ore as starting material.

that are shown in Fig. 5, which shows that the mass loss peaks at the mass ratio of trona/fly ash 1.4. This indicates that when the experimental condition is set as 850 °C with mass ratio of 1.4, the reactions leading to the formation of NaAlSiO4 and Na2SiO3 have been sufficiently completed. It is also found that the mass losses of calcining temperature at 850 °C are much higher than those calcining at 750 °C, while a little lower than those derived from 950 °C calcination. This further indicates that the optimal temperature and mass ratio can be determined at 850 °C and 1.4, respectively. Fig. 6 presents the typical XRD patterns of samples calcining at different mass ratios and different temperatures. It is observed in Fig. 6(a) that the intense of peaks at 37° and 48° corresponding to the

Fig. 5. Mass loss of fly ash and trona after calcining at different temperatures with varying mass ratio of trona to fly ash.

condition that Na2SiO3 phase increases first and then decreases, and when the mass ratio is at 1.4, the intensity reaches the maximum. In addition, the peak at ca. 62°, which is assigned to NaAlSiO4, also shows similarity as Na2SiO3 phase. It is well known that NaAlSiO4 and Na2SiO3 are both soluble in acid solution, which can be used for silica sol preparation. Therefore, in order to increase the silica extraction from fly ash, maximizing of both NaAlSiO4 and Na2SiO3 is preferred. The optimal mass ratio is then determined as 1.4 in considering results in Fig. 6(a). The optimal temperature is then further determined by detecting the phase changes during heating process. It is observed in Fig. 6(b), when the calcining temperature is carried out at 750 °C, some amount of quartz and mullite still remain in the mixture, which indicates that 750 °C is not high enough for fly ash activation. However, with calcining at 850 °C, the peaks corresponding to mullite and quartz disappear, while peaks assigned to NaAlSiO4 and Na2SiO3 begin to emerge. With a further calcining temperature at 950 °C, the intense of these peaks begins to decrease; therefore, the optimal temperature can be confirmed at 850 °C. These results agree well with the mass loss analysis in Fig. 5. It is found that SiO2, Al2O3 and Na2O occupy the majority of the sample, whose ratio is 31.36%, 14.33% and 31.51%, respectively (Table 3). Therefore it can be calculated that the mass percentage of NaAlSiO4 and Na2SiO3 formed in this process can be at 40.0% and 44.9%, respectively. The reaction mechanism is then investigated by the TG/DSC technique (Fig. 7). There are three significant mass losses at 25–100 °C, 580 °C–720 °C and 720 °C–850 °C, respectively. There are two intense endothermic peaks at 100 °C and 835 °C, which are assigned to the release of water adsorbed in trona ore (there is almost no mass loss for fly ash during calcination) and the release of CO2 at elevated temperatures, respectively. The mass loss during the whole process is 34%, and no mass loss can be observed with further calcination, which agrees well with the mass loss results in Fig. 5. It is noted there exists a turning point at ca. 720 °C, at which the CO2 release mechanism can be changed. The reaction process is proposed as the following reactions (6) and (7):


315

Fig. 6. XRD patterns of samples calcining at (a) different mass ratios and (b) different temperatures.

Table 3 Chemical composition of the derived calcination mixture from trona and fly ash. Elements

SiO2

Al2O3

Fe2O3

CaO

MgO

Na2O

K2O

SO3

TiO2

MnO

P2O5

BaO

LOI

(wt%)

31.36

14.33

4.22

4.75

1.72

31.51

0.40

0.46

0.41

0.06

0.25

0.12

10.24

there is an intermediate product of NaAlSiO4 (sodium aluminum silicate) formed at temperatures under 720 °C, and then it is transformed to NaAlSiO4 (nepheline) at temperatures over 720 °C as well as the formation of Na2SiO3. The mass ratio of NaAlSiO4 to Na2SiO3 can be calculated as 1:1 by the CO2 release rates during these two different stages. This result is consistent with the aforementioned result that XRF has provided. Guo [13] et al. has also mentioned the different stages during CO2 release; however, the turning point in their work was around 700 °C. Stage 1: Decomposition of mullite (b720 °C) 3Na2 CO3 þ 4SiO2 ðquartzÞ þ 3Al2 O3 :2SiO2 ðmulliteÞ→ 6NaAlSiO4 ðsodium aluminum silicateÞ þ CO2 ↑

ð6Þ

The calcining time is then studied based on the optimal mass ratio (1.4) and calcining temperature (850 °C). As can be seen in Fig. 8, the mass loss increases dramatically from 24.2% to 35.2% with the holding temperatures increase from 1 h to 2 h, while it almost remains unchanged with further increasing the holding temperature. The SEM image and EDS images in the bottom right inlet of Fig. 7 show that the mixture after calcining possesses loose and coarse morphologies, which is favorable to improving the reactivity with acid afterwards. In addition, the EDS shows that it mainly contains elements of Na, Al, Si, O, which further confirms the formation of NaAlSiO4 and Na2SiO3. Finally, the optimal calcining condition for fly ash and trona can be determined of mass ratio at 1.4, calcining temperature at 850 °C and holding time at 2 h. H2SO4 solution is then used to dissolve the calcining mixture, in which the chemical reactions happen as follows:

Stage 2: Formation of NaAlSiO4 (nepheline): (720 °C–850 °C) Na2 CO3 þ 3NaAlSiO4 ðsodium aluminum silicateÞ→ NaAlSiO4 ðnaphelineÞ þ 2Na2 SiO3 þ Al2 O3 þ CO2 ↑

ð7Þ

2NaAlSiO4 þ 4H2 SO4 → 2H4 SiO4 þ Al2 ðSO4 Þ3 þ Na2 SO4

ð8Þ

Na2 SiO3 þ H2 SO4 → H2 SiO3 þ Na2 SO4

ð9Þ

The reaction (7) can be used for further explaining the phenomenon of intensity decline of NaAlSiO4 (Fig. 6(a)), since trona can further react with the formed NaAlSiO4 phase at a high mass ratio of trona to fly ash.

Fig. 7. TG/DSC curves of fly ash mixed with trona ore during calcination.

Fig. 8. Mass loss of the calcining mixtures with different holding time and the (a) SEM and (b) EDS images in bottom right inlet.

316


Fig. 9. (a) XRD pattern and (b) EDS analysis of the resulting silica aerogel (top right inlet).

H4 SiO4 → H2 SiO3 þ H2 O

ð10Þ

mH2 SiO3 þ H2 O → mSiO2 ðm þ 1ÞH2 O ðamorphousÞ

ð11Þ

The filtration containing amorphous silica is collected as silica sol, and gelation happens when it is put into an oven at 60 °C for 12 h. Many researchers [4,6,21,22] have used ion exchange resin to remove the impurities, such as Na+, Al3+, Ca2+ inside the sol, before gelation. However, the process is complicated and not appropriate for largescale manufacturing. Instead, in this work, no resin is used for impurity removal, and the impurity removal is achieved via water wash and solvent exchange. Fig. 9 shows the typical XRD pattern of the silica aerogel, which has a wide and broad peak at ca. 22°. It is found that the peaks at ca. 32° and 46° (assigned to NaCl) also immerge in the pattern. This is caused by the Na+ brought in by trona ore and Cl− introduced in by TMCS during the surface modification process. The SEM image in top right inlet further confirms the formation of NaCl in the aerogel matrix.

Fig. 10 exhibits the SEM and EDS images of the resulting silica aerogel and the solid residues after acid leaching. It is found that the silica aerogel presents an interpenetrating and homogeneous structures that are composed of spherical nanoparticles with diameters approximately 20– 30 nm. Uniformly distributed silica building blocks are assembled to form the three-dimentional network with pores ranging from several nanometers to over 100 nm [23,24]. It is noted that in the EDS image, only four elements of C, Si, O and Au can be detected. The Au element is induced by the spay gold, while the amount of carbon is little. The EDS in top right inlet further confirms the main composition of SiO2 in the aerogel matrix. Since no other elements are detected, this indicates that the impurities have been effectively removed during solvent exchange and surface modification process. Na+ and Cl− both don't appear in the EDS figure, indicating that the amount of NaCl inside the aerogel matrix is rather low. In addition, the residue after filtration shows a needle-like mophology. It contains many kinds of different metal ions including Mg2+, Na+, Al3+, Fe3+, Ca2+, and Ti4+. This indicates that the impurities have been partially removed during acid leaching process and the they can be further removed during the following processes. A detailed characterization of the silica aerogel under TEM image (Fig. 11) verifies a hierarchical porous structures with a typical pearl necklace structure which is composed of silica particles mostly in a range of 3–6 nm, combined with a typical mesoporous srtuctures of which the interparticle pores are in the range of 2–50 nm. This is consistent with that of many other silica aerogels fabricated from alkoxides and water glass as the starting materials [7,25,26,27]. The SAED pattern shows no apparent diffraction spots, a typical characteristic of amorphous material. The resulting aerogel via ambient pressure drying shows an excellent hydrophobic property. The proposed mechanism of the surface modification process is described in Fig. 12. When TMCS/EtOH is introduced into alcogel soaked in haxane, several chemical reactions occur spontaneously at the interface of the alcogel and main hexane phase. EtOH and TMCS will partially react (reaction 12) with each other, forming (CH3)3Si\\OC2H5 (TMES), which is also a modification agent as is seen in reaction (18). ðCH3 Þ3 Si‐Cl þ CH3 CH2 OH → ðCH3 Þ3 Si‐OC2 H5 þ HCl

Fig. 10. SEM and EDS images of the resulting silica aerogel (a, b and c) and the residue (d, e and f) after H2SO4 leaching the calcination mixture.

ð12Þ


317

Fig. 11. TEM images (a, b and c) and SAED pattern (d) of the resulting silica aerogel.

The reaction (12) effectively slows the reaction between TMCS and water, favorable to form uncracked aerogel [24,28]. During the surface modification process, the reactions can be categorized into two types. One type is for reaction (13) and reaction (14), which can remove water from the pore channels of the alcogel.

It should be mentioned that the reaction (12) and reaction (13) can be reversible as reactions (15) and (16). ðCH3 Þ3 Si‐O‐SiðCH3 Þ3 þ 2HCl → 2ðCH3 Þ3 Si‐Cl þ H2 O

ð15Þ

ðCH3 Þ3 Si‐OC2 H5 þ HCl → ðCH3 Þ3 Si‐Cl þ CH3 CH2 OH

ð16Þ

2ðCH3 Þ3 Si‐Cl þ H2 OðporewaterÞ → ðCH3 Þ3 Si‐O‐SiðCH3 Þ3 þ 2HCl

ð13Þ

2ðCH3 Þ3 Si‐OC2 H5 þ H2 OðporewaterÞ → ðCH3 Þ3 Si‐O‐SiðCH3 Þ3 þ 2CH3 CH2 OH

The other type is silylation reactions including reaction (17) and reaction (18), which modify the initial silanol surface groups into\\Si(CH3)3 groups.

ð14Þ

ðCH3 Þ3 Si‐Cl þ ≡ Si‐OH → ≡ Si‐O‐SiðCH3 Þ3 þ HCl

Fig. 12. Schematic diagram of the surface modification process using the TMCS/hexane/EtOH agent.

ð17Þ

318


Fig. 13. TG/DSC curves of the resulting silica aerogel under air environment.

ðCH3 Þ3 Si‐OC2 H5 þ ≡ Si‐OH→ ≡ Si‐O‐SiðCH3 Þ3 þ CH3 CH2 OH

ð18Þ

In surface modification process, the hydrophilic property of the alcogel network is transformed to hydrophobic. As the hydrophobic areas grow into the surface of the alcogel, a yellow-color aqueous water/EtOH/HCl phase can be bailed out from the pores. This phase is not miscible with the main phase (HMDSO/hexane, rich in hexane), and the density of the aqueous phase is higher. Therefore, the silylated wet gel suspends in the interface of main hexane and the yellow aqueous solution. This phenomenon can be an indication of the accomplishment of surface modification. However, since EtOH is miscible with both water and hexane, it can coexist in the aqueous phase and the main phase. Finally, the wet gel is filled with hexane as the pore liquid, which can be further dried.

Fig. 13 shows the TG/DSC curves of the resulting silica aerogel under air environment, which is used to detect the thermal stability of the resulting aerogel. The mass loss can be separated as three regions: 25 °C to 150 °C, 150 °C to 420 °C and 420 °C to 800 °C. The first region mass loss is 4%, caused by the release of residual solvent and water adsorbed on the surface of the aerogel [29]. The second region with a mass loss of ca. 2% is caused by the oxidation of the residual organics in the silica aerogel [30]. The third region with the corresponding mass loss of 9% occupies the most significant mass loss during calcination, which can be attributed to the oxidation of Si\\CH3 developed from the modification process. In addition, there is an evident exothermic peak during this region with a peak temperature at around 476 °C, which means that after calcining at temperatures over 476 °C, the silica aerogel changes from hydrophobic to hydrophilic. This temperature is higher than those reported in literature [7,31,32], indicating the silica aerogel possesses an excellent thermal stability. The quantitative information of the relative surface modification composition and the thermal stability of the resulting silica aerogel is evaluated by obtaining high-resolution XPS spectra of Si 2p, C 1 s and O 1 s (Fig. 14 and Fig. 15). The possible surface species with the particular binding energy are based on reference [33]. The results obtained using the Lorentzian-Gaussian function with varying contribution are summarized in Table 4. The resulting aerogel primarily have elements of O, C and Si with little amount of Na. It is observed that the Si\\C(\\H) bond with binding energy of 102.4 eV occupies 28.8% due to the surface modification process, and this value decreases to 0 after 500 °C calcination. It further confirms that with 500 °C heat treatment, the silica aerogel transforms from hydrophobic to hydrophilic. At the same time, after surface modification, the C\\C(\\H) bond at 284.6 eV occupies 78.8%, while the ratio of C\\O(\\Si) bond is almost 0. It is demonstrated that with the addition of TMCS, considerable amount of \\OR groups on the surface are further hydrolyzed to form \\Si\\OH, and the \\Si\\OH groups on both inner and outer surfaces of silica skeleton are then silylated by Si\\CH3, as is shown in reaction (17) and reaction (18).

Fig. 14. A survey XPS spectrum (a) and O 1 s (b), Si 2p (c) and C1s (d) spectrum of the resulting aerogel.


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Fig. 15. A survey XPS spectrum (a) and O 1 s (b), Si 2p (c) and C1s (d) spectrum of the aerogel heat treatment at 500 °C.

Table 4 Surface chemical composition of the as-dried silica aerogel and aerogel with 500 °C heat treatment from XPS analysis. Samples

Si 2p/binding energy/amount/%

as-dried

Si\ \O(\ \Si) Si\ \O(\ \C) Si\ \C(\ \H) Si\ \O(\ \Si) Si\ \O(\ \C) Si\ \C(\ \H)

500 °C heat treatment

103.4 104.4 102.4 103.4 104.4 102.4

C 1s/binding energy/amount/% 27.0 44.2 28.8 65.1 34.9 0

C\ \Si(\ \O) C\ \O(\ \Si) C\ \C(\ \H) C\ \Si(\ \O) C\ \O(\ \Si) C\ \C(\ \H)

In order to understand the pore structures' changes during heat treatment, the nitrogen sorption isotherms and BJH pore size distribution tests (shown as inlet in Fig. 16) of silica aerogel are carried out

285.4 286.6 284.6 285.4 286.5 284.6

O 1s/binding energy/amount/% 21.2 0 78.8 12.8 0 87.2

(Si\ \)O\ \Si (Si\ \)O\ \C

532.6 533.6

51.9 48.1

(Si\ \)O\ \Si (Si\ \)O\ \C

532.6 533.6

53.0 47.0

after heat treatment at different temperatures (Fig. 16). The curves all exhibit type IV curves with type H1 hysteresis loop in the IUPAC classification, which are generally observed in the mesoporous materials with cylindrical pores [34,35]. The desorption cycles of the isotherms show a hysteresis loop for the all the samples, which is generally attributed to the capillary condensation in the mesopores. Macropores are almost absent in all the samples as indicated by the immerge of plateau shape of the adsorption curves which can be associated with multilayer adsorption on the external surface of the samples [36]. The most probable pore diameter of the as-dried silica aerogel is at 11 nm, while it shifts to the larger value at first and then decreases to the smaller one during the increase of heat treatment temperatures. Table 5 prensents the pore Table 5 Pore structures of the silica aerogels with different temperatures of heat treatment.

Fig. 16. Nitrogen sorption isotherms and BJH pore size distribution (inlet) of SiO2 aerogel with heat treatment at different temperatures.

Heat treatment temperature

SF micropore volumes (cm3/g)

BET surface areas (m2/g)

BJH adsorption pore diameters (nm)

BJH desorption volumes (cm3/g)

as-dried 500 600 700 800 900

0.1021 0.2235 0.2097 0.2008 0.1897 0.1147

704.2 856.2 776.5 785.6 776.3 516.3

11.0 17.1 14.7 13.7 13.3 8.3

2.60 2.92 2.44 2.18 2.20 1.24

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Fig. 17. TEM images of SiO2 aerogel heat treatment at different temperatures (a) as-dried, (b) 500 °C, (c) 700 °C, and (d) 900 °C.

structure parameters of the silica aerogel during heat treatment. It is found that the micropore volumes and BET specific surface ares all increase first and then decrease, and they reach the maxmium values when heat treatment temperature approaches 500 °C. The BET sepecific surface area of the 500 °C heat treatment sample has been found as high as 856.2 m2/g, which is much higher than what references have reported [2,17,22,37]. The BJH pore volumes also increase to maximum after 500 °C heat treatment, which has a large total pore volume of 2.92 cm3/g. The increase of the pore structure values can be explained by the oxidation of –CH3 at 476 °C, which creates many mesopores as well as small amount of micropores. In addition, the decrease of such

values afterwards can be attributed to the shrinkage of the silica aerogel skeleton under higher temperatures. However, the BET specific surface can be maintained at 516.3 m2/g, even after heat treatment at temperature as high as 900 °C. The TEM measurement is then carried out for measuring the textural structures of the silica aerogel. As seen in Fig. 17 (a), some agglomerations occur in the as-dried silica aerogel, though silica particles as well as the mesopores occupy majority of the aerogel. This is caused by the attraction of Si\\CH3 on the surface of silica particle; however, after heat treatment at 500 °C, the \\CH3 groups disappear due to oxidation, which brings many mesopores due to the formation of CO2 gas inside the aerogel matrix. The structures become

Fig. 18. Silica particle size with different bulk densities and varying contact ratios (specific surface area as a function of bulk densities in bottom inlet).


Fig. 19. FTIR spectra of SiO2 aerogel with heat treatment at different temperatures.

more homogeneous in comparison with the as-dried sample. This result is consistent with the N2 adsorption/desorption testing results. According to Zeng [38] et al., the porosity and specific surface area of silica aerogels can be calculated as Eq. (19) and (20): Y

pffiffiffiffiffiffiffiffiffiffiffiffii h 2 ¼ 1‐πd 2 þ a2 2 3D‐2d 1‐a2 2 D‐3 =12 ¼ 1‐ρ=ρSiO2

h pffiffiffiffiffiffiffiffiffiffiffiffii S ¼ πd 3D‐2d 1‐a2 2 = ρD3

ð19Þ ð20Þ

Where Π, d, a2, D, ρ and ρSiO2 refer to porosity, particle size, contact ratio (a2 = a/d), pore size, bulk density and skeletal density (2270 kg/m3) of silica aerogels. Therefore, the particle size can be defined as Eq. (21): d¼

Q 12ð1‐ Þ 12 ¼ ð2 þ a2 2 ÞρS ð2 þ a2 2 ÞρSiO2 S

ð21Þ

In addition, according to Dan et al. [39], the specific surface area of silica aerogel can be calculated as Eq. (22): S ¼ ð324:3=ρ þ 5:03Þ 105

ð22Þ

The bulk density of the silica aerogel heat treated at 500 °C is ca. 99 kg/m3/. The specific surface area has been calculated as 830 m2/g, consistent with the aforementioned BET result. Fig. 18 also shows the silica particle size with different bulk densities and varying

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contact ratios (a2 = 0.1–0.9). Therefore, when the bulk density is in the range of 50 kg/m3 to 200 kg/m3, the particle size of silica particles can be caculated at 1.6–4 nm, which agrees well with the TEM results shown in Fig. 17 (b). After heat treatment at higher temperatures (Fig. 17 (c) and Fig. 17 (d)), it can be clearly seen that the pore shrinkages begin to occur, which therefore decreases the specific surface area of the silica aerogel. Fig. 19 shows the FTIR analysis of the as-dried sample and the spectra of samples which are heat-treated at different temperatures. In the as-dried sample, the adsorption peaks at 452 cm− 1 and 1078 cm− 1 are due to bending and asymmetric modes of Si\\O\\Si vibrations [40]. It can be observed that it has three extra absorption peaks at 759 cm− 1 , 847 cm − 1 and 1259 cm− 1 [6,41,42], and these three peaks are caused by the Si\\C, which are brought in from surface modification process. It is well known that the super hydrophobicity of silica aerogels is favorable to keep its outstanding properties such as thermal stability and durability. These two peaks appear after heat treatment temperatures over 500 °C, indicating that it has been transformed form hydrophobic to hydrophilic after 500 °C calcination. In addition, it is observed that the intensity of the peak around 807 cm− 1 assigned to the Si\\O\\Si symmetric bond stretching vibration is not apparent for the as-dried sample, while dramatically increases after calcining at 500 °C [43]. This can be a further confirmation of the transformation of Si\\CH3 to Si\\OH, followed by forming a more stable structure with Si\\O\\Si skeleton. Silica aerogel is fragile, therefore in order to enhance the mechanical strength of silica aerogel, fiber mat is always used as seen in Fig. 20 (a). A drop of water is standing on its surface, though the fiber itself is hydrophilic. The presence of hydrolytically stable methyl group is responsible for the hydrophobicity in the silica aerogel blanket. The contact angle is measured as high as 151°, confirming a super hydrophobic property of the obtained silica aerogel composite. 4. Conclusions Highly porous and hydrophobic silica aerogel is fabricated using fly ash and trona ore as the starting materials, combined with the costeffective ambient pressure drying technique. The optimal calcination parameters have been determined at 850 °C with the holding time of 2 h at the trona ore/fly ash mass ratio of 1.4. NaAlSiO4 is first formed during calcination and Na2SiO3 is formed afterwards. The acid soluble minerals of NaAlSiO4 and Na2SiO3 occupy 40.0% and 44.9% of the calcination product respectively. The impurities in silica aerogel matrix can be effectively removed by water wash and solvent exchange processes. The final product is observed to have a rather high thermal stability, which can maintain its hydrophobicity at around 476 °C. The contact angle of the as-dried silica aerogel is as high as 151°. The silica particle size is ca. 3–6 nm, which agrees well with the theoretical model. The

Fig. 20. (a) Photograph and (b) contact angle measurement of the fiber reinforced silica aerogel.

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