Facile preparation of a SiO₂–Al₂O₃ aerogel using coal gangue as a raw material via an ambient pressure drying method and its application in organic solvent adsorption

Abstract

Because of its outstanding properties such as a large specific area and Brønsted acidity, the SiO₂–Al₂O₃ aerogel has been used for various applications such as adsorption, catalysis, and in synthetic chemistry. Regarding preparation of the SiO₂–Al₂O₃ aerogel, however, expensive raw materials are always used. In addition, the traditional preparation method is supercritical drying, which is complex and has safety issues. Here, we successfully prepare a SiO₂–Al₂O₃ aerogel utilizing coal gangue as a raw material, which is a zero-cost mining waste. More importantly, ambient pressure drying used in this work overcomes the drawbacks of supercritical drying. Characterizations show that the specific surface area of the SiO₂–Al₂O₃ aerogel prepared from coal gangue can be comparable to that prepared from relative costly raw materials such as Al(NO₃)₃·9H₂O or Al isopropoxide and tetraethoxysilane with supercritical drying conditions. In addition, adsorption tests show that the SiO₂–Al₂O₃ aerogel demonstrates good adsorbability of an organic solvent. Our work provides an effective route to synthesize a SiO₂–Al₂O₃ aerogel in mass quantities and opens a new direction for the comprehensive utilization of coal gangue.

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1. Introduction

After the discovery of the M41S family, mesoporous materials have attracted a large number of investigations because of their outstanding properties.^1–5 An aerogel, a typical mesoporous material, is a kind of nanoporous material with a continuous random network structure.⁶ Because of its excellent properties such as high porosity, high specific surface area and low density, the aerogel has attracted a growing number of studies and has been used in a wide range of applications such as adsorption, catalysis, environmental purification and in biochemistry.^7–12 As a unique kind of aerogel, the SiO₂–Al₂O₃ aerogel shows strong Brønsted acidity and is a promising material for absorption and catalysis, which is different from pure aerogels such as silica aerogel and alumina aerogel.^13,14 However, the production of SiO₂–Al₂O₃ aerogel has been restricted because of its expensive precursors, such as Al(NO₃)₃·9H₂O and tetraethoxysilane.^15,16 Therefore, it is highly desirable to find a kind of cheap material to synthesize the SiO₂–Al₂O₃ aerogel.

It is well known that coal gangue is one of the major solid wastes generated in the process of coal mining and washing.^17,18 Coal gangue accounts for 10–20% of the original coal production and production is also increasing every year with high speed in China.^19,20 Large accumulating stockpiles of coal gangue not only occupy farmland but also contaminate the groundwater, soil, and atmosphere if they are not treated timely and properly.^21,22 By far, the comprehensive utilization of coal gangue is mainly focused on its fundamental application, such as for power generation,^23,24 building material,²⁵ pavement,^26,27 and so on. Here, coal gangue is adopted as a starting material for synthesizing the SiO₂–Al₂O₃ aerogel, which not only allows for the production of the SiO₂–Al₂O₃ aerogel using cheap material, but provides a new direction for the comprehensive utilization of coal gangue.

In addition, the drying method is very important in the synthesis of an aerogel. Usually, three drying methods can be used: supercritical drying, freeze drying and ambient pressure drying.^28,29 The supercritical drying method can produce an aerogel with uniform particle distribution, high porosity and minimal shrinkage, however, issues including complicated processing, safety problems, high cost and equipment requirements restrict its application.³⁰ As a facile method, the freeze-drying method can obtain monolithic and high-quality aerogels. However, the gel network may eventually be destroyed by the nucleation and growth of solvent crystals, which tend to produce very large pores.³¹ Besides, the time-consuming process makes its commercial application unfeasible. Compared to the aforementioned drying methods, ambient pressure drying is a facile, relatively fast and very cheap way to realize the drying of aerogels.³² More importantly, the properties of the aerogel after ambient pressure drying could be comparable to those prepared using supercritical drying.^33,34 Herein, comparable properties of SiO₂–Al₂O₃ aerogels are assessed by taking advantage of the ambient pressure drying method.

In the current work, the SiO₂–Al₂O₃ aerogel is successfully synthesized using coal gangue as the starting material via ambient pressure drying. To the best of our knowledge, few studies have reported the synthesis of a SiO₂–Al₂O₃ aerogel using coal gangue via ambient pressure drying. In addition, organic solvent absorption of the SiO₂–Al₂O₃ aerogel prepared from coal gangue has also been studied in detail.

2. Experimental

2.1 Materials

Commercial sodium carbonate (Na₂CO₃), hydrochloric acid (HCl), anhydrous ethanol (EtOH), cyclohexane, n-hexane and trimethylchlorosilane (TMCS) were purchased from Beijing, China. All of them are of analytical reagent grade and no further purification was carried out. The coal gangue sample used in the experiments was obtained from Shenmu, Xi’an, China. The chemical composition of the air dried basis (A_ad) of coal gangue was examined using a chemical method (Table 1).

Table 1 Chemical composition of the A_ad^a of coal gangue [wt%]

Composition	SiO2	Al2O3	Fe2O3	TiO2	CaO	MgO	K2O	Na2O	SO3	P2O5	P^b
a Air dried basis of coal gangue. b Ignition loss of coal gangue.
Coal gangue	66.31	21.04	4.08	0.65	1.13	1.31	3.38	1.30	0.30	0.06	20.31%

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2.2 Methods

The specific preparation procedure of the SiO₂–Al₂O₃ aerogel is shown in Fig. 1(a). Firstly, gangue samples and Na₂CO₃ were prepared by screening the ground raw materials to a mesh of 100. Then, coal gangue was mixed with Na₂CO₃ at different weight ratios and calcined at a certain temperature (870–930 °C). Subsequently, the sinter was blended with excess HCl solution (ensures that the pH of the solution would be less than 3, preventing the production of a precipitate) and stirred for 5–10 min at 100 °C. After that, the resulting mixture was filtered. The filtrate was composed entirely of silica–alumina sol, which could be demonstrated by the Tyndall phenomenon (Fig. 1(b)) and the insoluble solid was dried (the weight of the insoluble solid is set as m′). Silica–alumina sol was kept motionless to gelatinize at 80 °C. After gelation, the wet gel was aged for 3 h at 50 °C to strengthen the gel network. The aged gel was then washed with distilled water until the color of the product was transformed from yellow to white (Fig. 1(c) and (d)), which indicated that FeCl₃ trapped inside the system was eliminated completely. Next, the SiO₂–Al₂O₃ gel was immersed into EtOH for solvent exchange at 50 °C for 24 h. Surface modification was then carried out by soaking the gel in a mixture of TMCS: hexane with a volume ratio of 1 : 1 for 24 h. After a complete surface modification, the SiO₂–Al₂O₃ gel was suspended in a liquor and bailed out. The modified SiO₂–Al₂O₃ gel was dried at ambient pressure for 24 h and then heated at 100 °C and 200 °C for 1 h respectively. The final SiO₂–Al₂O₃ aerogel product can be obtained after cooling in an oven to room temperature.

Fig. 1 (a) Schematic diagram of the preparation of SiO₂–Al₂O₃ aerogel using coal gangue as the raw material, (b) pictures of the Tyndall phenomenon of silica–alumina (Si–Al) sol by comparing with water, (c) wet gel, and (d) SiO₂–Al₂O₃ aerogel.

For the absorption test of an organic solvent, 0.1 g of SiO₂–Al₂O₃ aerogel was added to a mixture solution of EtOH and cyclohexane (10 ml) with a volume ratio of 1 : 1. 20 minutes later, the resulting aerogel was fetched and weighed.

2.3 Calculation and characterization

The decomposition rate of coal gangue (DG) and the organic solvent adsorption rate of the SiO₂–Al₂O₃ aerogel (AR) were calculated using the following equations:where m is the original weight of coal gangue, m₁ is set as 0.1 g in the current work, m₂ is the weight of the SiO₂–Al₂O₃ aerogel after absorbing the organic solvent, m′ is the weight of the insoluble solid from the reaction which produced the sinter (coal gangue and Na₂CO₃) and HCl solution (see the Method section in 2.2), and p is the ignition loss of coal gangue.

The density of the SiO₂–Al₂O₃ aerogel is determined by eqn (3) as follows:

where ρ, M, V are the density, weight and volume of the SiO₂–Al₂O₃ aerogel, respectively. For the specific procedure, we first screened the SiO₂–Al₂O₃ aerogel with a 200 mesh sieve. Then, the sieved SiO₂–Al₂O₃ aerogel was accurately weighed (M). Next, we loaded the weighed SiO₂–Al₂O₃ aerogel into a 5 ml measuring cylinder. After shaking the measuring cylinder 550 times, the volume of the SiO₂–Al₂O₃ aerogel could be obtained (V). The experiment was done three times and ρ is the average value from the three experiments.

Various experimental tools were used to characterize the as-prepared SiO₂–Al₂O₃ aerogel. Transmission electron microscopy (TEM) measurements were performed on a Tecnai G2 F20 S-TWIN apparatus. Images using field emission scanning electron microscopy (FESEM) were taken on a FESEM (Quanta 600 FEG). X-ray scattering patterns were obtained using an X-ray diffractometer (XRD) with Cu Kα radiation at 0.145 nm. Specific surface area data was collected using an ASAP 2020 HD88 analyzer by nitrogen adsorption and desorption. The hydrophobicity was measured using a hanging drop method on an optical contact angle & interface tension meter (SL200KB). The thermal stability was examined by thermogravimetric differential scanning calorimetry (TG-DSC) thermal analysis under an air atmosphere with a heating speed of 10 °C min⁻¹ from room temperature to 900 °C. The spectrum produced by Fourier transform infrared spectroscopy (FTIR) was recorded on a Bruker Tensor 27 spectrometer with a wavenumber range of 4000–400 cm⁻¹ at a resolution of 4 cm⁻¹ by averaging over 100 scans.

3. Results and discussions

3.1 Effects of different factors on DG

To obtain a high decomposition rate of coal gangue (DG), three experimental parameters are studied, including the weight ratio of coal gangue to Na₂CO₃ (m(coal gangue)/m(Na₂CO₃)), holding time, and calcination temperature. The first factor investigated for affecting the DG is m(coal gangue)/m(Na₂CO₃) by keeping a holding time of 2 h and a temperature of 900 °C (Fig. 2(a)). Reactions that happened at a high temperature are as follows:

3Al₂O₃·2SiO₂ + 3Na₂CO₃ = 2NaAlSiO₄ + 4NaAlO₂ + 3CO₂ (4)

SiO₂(liq) + NaAlO₂ = NaAlSiO₄ (5)

Al₂O₃(liq) + Na₂CO₃ = 2NaAlO₂ + CO₂ (6)

Al₂O₃(liq) + 2SiO₂(liq) + Na₂CO₃ = 2NaAlSiO₄ + CO₂ (7)

Fig. 2 Effects of different factors on the DG: (a) m(coal gangue)/m(Na₂CO₃), (b) holding time, and (c) calcination temperature. (d) XRD pattern of the insoluble substances from the reaction of coal gangue and HCl solution.

These reactions testify that Na₂CO₃ mainly reacts with Al₂O₃ and SiO₂ inside coal gangue. The main product is NaAlSiO₄ that is easily dissolved by the HCl solution, which causes the decomposition of coal gangue. Fig. 2(a) demonstrates that the DG increases first and then reduces along with the decrease of m(coal gangue)/m(Na₂CO₃). When m(coal gangue)/m(Na₂CO₃) is 1 : 0.6, the DG reaches its maximum (72.5%). Therefore, with regards to the weight ratio of coal gangue to Na₂CO₃, 1 : 0.6 is optimal to obtain a high DG.

In addition, the experimental parameter, holding time, was studied. As can be seen from Fig. 2(b), the DG increases along with an increase of the holding time when the temperature is set to 900 °C and m(coal gangue)/m(Na₂CO₃) is set to 1 : 0.6. When the holding time is 3 h, the DG is 85% and only increases by 0.41% at the sacrifice of extending the holding time by 1 h. It can be seen from Table 1 that Al₂O₃ plus SiO₂ account for 87.35% of the A_ad of coal gangue, so the DG will not increase significantly even if the holding time is further prolonged. That is to say, almost all SiO₂ and Al₂O₃ have been extracted from coal gangue. Therefore, 3 h is the optimal holding time to obtain a high DG.

The third experimental parameter is the calcination temperature. With a holding time of 3 h and m(coal gangue)/m(Na₂CO₃) set to 1 : 0.6, the effect of the calcination temperature on the DG is studied. As shown in Fig. 2(c), the optimal calcination temperature is 900 °C to obtain a higher DG.

XRD analysis is carried out to identify the crystal structure of insoluble impurities from the reaction producing the calcination mixture and the HCl solution. As illustrated in Fig. 2(d), the XRD pattern is a precise match for NaAlSi₂O₆ (standard pdf card 46-0012). That is to say, at high temperature, the reaction of coal gangue with Na₂CO₃ produces NaAlSi₂O₆ (eqn (8)). Unlike NaAlSiO₄ (eqn (4), (5) and (7)), NaAlSi₂O₆ cannot react with an acid. Thus, this causes the decline of the DG.

Al₂O₃(liq) + 4SiO₂(liq) + Na₂CO₃ = 2NaAlSi₂O₆ + CO₂ (8)

In summary, the optimal parameters to maximize the DG are as follows: m(coal gangue)/m(Na₂CO₃) set to 1 : 0.6, a holding time of 3 h, and a calcination temperature of 900 °C.

3.2 Characteristics of the SiO₂–Al₂O₃ aerogel

The final product is synthesized using an ambient pressure drying method under the optimal parameters. Firstly, to study the crystal structure of the final product, the XRD pattern is used. According to the XRD pattern (Fig. 3(a)), a strong diffuse peak around 25° of 2θ is due to amorphous SiO₂. As observed between 30° and 80° of 2θ in Fig. 3(a), the XRD pattern is a precise match with Al₂O₃ (standard pdf card 75-0278). Therefore, the XRD result demonstrates that the as-prepared product is the SiO₂–Al₂O₃ aerogel. Furthermore, the morphology of the SiO₂–Al₂O₃ aerogel has been studied. Fig. 3(b) is the SEM image of the final product. As can be seen from the image, the sample mainly consists of nanoparticles. Particles of the sample are uniformly distributed and the size is approximately less than 50 nm. Importantly, the inter-particle connection forms clusters, and then the clusters are linked together with each other. As a result, a large quantity of pores are formed because of the continuous inter-particle connection, as shown in the TEM image in Fig. 3(c). It is noted that the ambient pressure drying method is adopted in the preparation of the SiO₂–Al₂O₃ aerogel. Ambient pressure drying is a facile, relatively fast and very cheap way to realize the drying of an aerogel. Based on Fig. 3(b) and (c), the pore size and particle distribution of the as-prepared SiO₂–Al₂O₃ aerogel using coal gangue as the raw material via ambient pressure drying is comparable to that of a SiO₂–Al₂O₃ aerogel prepared using the supercritical drying method (Fig. 3(d)), which is complex and high-cost.³⁵

Fig. 3 (a) XRD pattern of the SiO₂–Al₂O₃ aerogel, (b) SEM image of the SiO₂–Al₂O₃ aerogel, and (c) TEM image of the SiO₂–Al₂O₃ aerogel. (d) SEM image of a SiO₂–Al₂O₃ aerogel prepared using a supercritical drying method.³⁵ All of the scale bars indicate 1 µm. Reproduced with permission from ref. 35. Copyright 2005 Elsevier Inc.

3.3 Properties of the SiO₂–Al₂O₃ aerogel

Considering the fact that the hydrophilic–hydrophobic property plays an important role in organic solvent absorption, this property is studied first. As shown in Fig. 4(a), the contact angle is 134°, which demonstrates that the as-prepared SiO₂–Al₂O₃ aerogel shows strong hydrophobicity. Thus, the SiO₂–Al₂O₃ aerogel has a potential application in organic solvent adsorption. To further clarify the hydrophobicity, the surface functional groups of the SiO₂–Al₂O₃ aerogel are studied using FTIR, as shown in Fig. 4(a). A peak around 3480 cm⁻¹ is attributed to the antisymmetric stretching vibration of –OH, which is caused by the physical absorption of H₂O in air. Peaks at 2960 cm⁻¹ and 1260 cm⁻¹ are due to the stretching and bending vibration of C–H respectively, which demonstrate that the SiO₂–Al₂O₃ aerogel surface is connected with –CH₃.³⁶ The hydrophobicity is caused by the –CH₃ group connected with the SiO₂–Al₂O₃ aerogel. Because of the rotational vibration of H–O–H, an absorption peak at 1640 cm⁻¹ appears.³⁷ On the other hand, other peaks are also observed. Peaks around 1095 cm⁻¹ and 460 cm⁻¹ are ascribed to Si–O–Si bonds. A low intensity absorption peak of Si–OH near 960 cm⁻¹ shows that the SiO₂–Al₂O₃ aerogel is relatively uniform.³⁸ More importantly, the peak at 846 cm⁻¹ is due to the vibration of the Al–O bond, which proves that the Si–O–Al bond exists in the sample as well as the successful preparation of the SiO₂–Al₂O₃ aerogel using coal gangue.

Fig. 4 (a) FTIR spectrum of the SiO₂–Al₂O₃ aerogel. Inset of (a) is the contact angle of the SiO₂–Al₂O₃ aerogel with water. (b) TG-DSC curve of the SiO₂–Al₂O₃ aerogel in air. (c) N₂ adsorption–desorption isotherms of the SiO₂–Al₂O₃ aerogel. (d) Pore size distribution of the SiO₂–Al₂O₃ aerogel.

To study the thermal stability of the SiO₂–Al₂O₃ aerogel, TG-DSC is performed. Fig. 4(b) is the TG-DSC curve of the SiO₂–Al₂O₃ aerogel with a heating rate of 10 °C min⁻¹ under an air atmosphere. From 0 °C to 431 °C, the mass loss is approximately 4%, which is primarily ascribed to the removal of a small amount of physically absorbed water trapped in the sample. Obviously, there is a rapid weight loss (about 4.5%) at 431 °C, which is attributed to the oxidation of –CH₃ connected to the surface of the SiO₂–Al₂O₃ aerogel. With the increase of temperature, the weight of the sample continues to decrease. The mass loss from 431 °C to 900 °C results from a crystallization change of SiO₂. As a result, the SiO₂–Al₂O₃ aerogel prepared from coal gangue can be maintained up to 431 °C in air, which implies that the coal gangue based SiO₂–Al₂O₃ aerogel possesses high thermal stability.

The specific surface area also plays a key role in organic solvent absorption. Therefore, the characterization of the specific surface area is performed. N₂ adsorption–desorption isotherms and the Barrett–Joyner–Halenda (BJH) pore size distribution curve are shown in Fig. 4(c). The isotherm is type IV and the hysteresis loop belongs to H1, which agree well with the characteristics of mesoporous materials.^39,40 The SiO₂–Al₂O₃ aerogel has a specific surface area as large as 493.5 m² g⁻¹ with a pore volume over 0.59 cm³ g⁻¹ and the average pore diameter is 4.8 nm. In addition, the calculation result of eqn (3) shows that the density of the SiO₂–Al₂O₃ aerogel is 0.3475 g cm⁻³. The low density of the SiO₂–Al₂O₃ aerogel agrees well with a large specific surface area. As shown in Fig. 4(d), two pore size distribution ranges are observed: 2–10 nm and 10–50 nm. The former has the overwhelming majority of the distribution compared to the latter, explaining the small average pore diameter. On one hand, a large number of inter-particle links forming small pores causes the 2–5 nm pore size distribution. On the other hand, connections of clusters composed of nanoparticles are inclined to form large pores, which explain the 10–50 nm pore size distribution. The above results agree well with the characteristics of the TEM image. Therefore, the SiO₂–Al₂O₃ aerogel prepared from coal gangue has a large specific surface area, small pore size and low density. The specific surface area also can be comparable to SiO₂–Al₂O₃ aerogels prepared under supercritical drying conditions. For instance, using supercritical drying conditions, C. Hernandez et al. obtained SiO₂–Al₂O₃ aerogels with a specific surface area range of 253–729 m² g⁻¹.⁴¹

3.4 Application in the absorption of an organic solvent

The application of a SiO₂–Al₂O₃ aerogel for the treatment of organic solvents is a significant aspect. Its absorption rate of an organic solvent (mixture of EtOH and cyclohexane) was examined following exposure to atmosphere for 0, 10, 20, and 30 days. As can be seen from Fig. 5, the freshly produced SiO₂–Al₂O₃ aerogel can absorb an organic solvent three times its own weight, which is in accordance with the BET data. With the extension of time, the adsorption capacity will slightly decline. However, though exposed to air for 30 days, the absorption rate of the SiO₂–Al₂O₃ aerogel for an organic solvent only decreases by 40%. Hence, the SiO₂–Al₂O₃ aerogel prepared from coal gangue not only exhibits good adsorbability, but can maintain high stability in air, which also agrees with the TG-DSC result.

Fig. 5 Absorption rate tendency of the SiO₂–Al₂O₃ aerogel to an organic solvent over time.

4. Conclusions

In summary, we synthesized a SiO₂–Al₂O₃ aerogel using coal gangue as the raw material via ambient pressure drying. The optimal parameters to obtain a maximum DG are: m(coal gangue)/m(Na₂CO₃) set to 1 : 0.6, a holding time of 3 h, and a calcination temperature of 900 °C. Under these conditions, almost all SiO₂ and Al₂O₃ can be extracted from coal gangue (the DG reaches up to 85%). Residues from the reaction of the HCl solution and calcination mixture are detected by XRD. Results show that the residue is NaAlSi₂O₆, which cannot react with an acid and should be responsible for the decrease of the DG. More importantly, the final product has strong hydrophobicity (contact angle with water is 134°), high thermal stability that cannot be oxidized before 431 °C in air, a large specific surface area (493.5 m² g⁻¹), a small pore size (average pore diameter 4.8 nm) and low density (0.3475 g cm⁻³). In addition, adsorption tests show that the SiO₂–Al₂O₃ aerogel demonstrates good adsorbability of an organic solvent. Our work can provide an effective route to synthesize SiO₂–Al₂O₃ aerogels in mass quantities and open a new direction for the comprehensive utilization of coal gangue.

Acknowledgements

This research is supported by the National Natural Science Foundation of China (No. 51472204, 51571166, and 61505167). We are also thankful for the support of Key Scientific and Technological Team from Shaanxi Province (No. 2015KCT-12) and start-up funds from Northwestern Polytechnical University.

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