Abstract
Monolithic Fe2O3-Al2O3 composite aerogels have been prepared successfully via organic solvent sublimation drying method. The results show that a new phase forms when the right amount of ferric oxide is added to the alumina aerogel. From the TEM pictures we can see a shuttle-type structure with the length of about 15 nm forms, which leads to the high surface areas of composited aerogel.
1. Introduction
Aerogel is an extremely porous material prepared by using air replacing the liquid component of the wet gel [1–3]. Its unique structure makes it have extraordinary properties like low density, large porosity, low thermal conductivity, and high specific surface area, which lead it to receive much attention in a wide range of applications [4–8]. In particular alumina aerogel, which provides thermal insulation over a large temperature range and large specific surface area, would be a very useful material [9–12]. In order to make it better applied to some specific areas, we need to add other special materials to improve the characteristics of alumina aerogel. After adding iron oxide, alumina aerogel shows high magnetic strength and porosity. Thus Fe2O3-Al2O3 composite aerogel can be used as an absorbent, magnetic carrier, and high efficient reaction catalyst [13, 14]. In addition, this composite aerogel has been used for selective hydrogenation reaction of sulfur dioxide because of good heat insulation performance [15]. But preparation of monolithic narrow pore size distribution mesoporous composite aerogel is very difficult. Recently we developed a new drying method to prepare inorganic oxide aerogels. That is organic solvents sublimation drying (OSSD) method [16]. This method can avoid the influence of surface tension of gas-liquid phase transformation during the drying process.
In this paper we applied it to prepare stable Fe2O3-Al2O3 composite aerogels with different ratio of iron oxide and investigate the effect of iron oxide on alumina aerogel through X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, nitrogen sorption isotherms, and transmission electron microscopy (TEM).
2. Experimental
2.1. Preparation
First, appropriate amount of ethanol and water was mixed and heated to 80°C in a beaker, and a certain amount of aluminum chloride hexahydrate (Sinopharm Chemical Reagent Co., Ltd.) was added with vigorous stirring until being dissolved. Then certain amount of hydrochloric acid (Shanghai Lingfeng Chemical Reagent Co., Ltd.) was added with stirring and reflux condensation for 4 h. At last the sample was cooled to room temperature and the alumina clear sol was obtained.
The second step was the preparation of clear iron oxide sol. The detailed procedures are as follows: a certain amount of iron chloride (Sinopharm Chemical Reagent Co., Ltd.) was dissolved in anhydrous ethanol and then stirred at room temperature for 4 hours until getting Fe alcohol sol.
Then alumina sol and iron oxide sol were mixed according to the desired mole ratio of Fe and Al and reacted at room temperature for 5 h to get composite sol. A certain amount of epoxy propane (Shanghai Lingfeng Chemical Reagent Co., Ltd.) as a gelation agent was slowly added into the alumina, iron oxide, and composite sols, respectively, and then sealed at 25°C and let them turn into gels.
The obtained wet gel was dried through organic solvent sublimation drying technology (OSSD) [16]. The detailed procedure is as follows: the samples were aged in EtOH for 48 h at 50°C and then washed in 50%, 80%, and 100% acetonitrile/EtOH (v/v) exchanged solvents for 24 h at 50°C, respectively. After that, the wet gel was stored in a beaker and dried under the low vacuum condition.
2.2. Characterization
The pore size distribution and specific surface area of the samples were measured using a N2 adsorption analyzer (ASAP 2020). Before test, the samples were heated at 473 K in 10−6 Torr for 1 h for degassing. The Barrett-Joyner-Hollander (BJH) method was used to calculate the pore size distribution. The morphology and microstructures of the samples were characterized by a transmission electron microscope (TEM, JEOL-1230). Organic groups were investigated by a Fourier transform infrared spectroscope (FT-IR, TENSOR27). X-ray diffraction (XRD) data were recorded on D8-Discover (Bruker) using Cu Kα radiation at a scanning speed of 2°/min within the 2θ range of 10°–80°.
3. Results and Discussions
Through OSSD drying method, we successfully prepared monolithic alumina, Fe2O3, and Fe2O3-Al2O3 aerogels, as shown in Figure 1. All of the three aerogels are shiny and have no crack. The colour of Fe2O3-Al2O3 composite aerogel is shallower than pure iron oxide. We synthesized a series of Fe2O3-Al2O3 aerogels with different molar ratio of Fe and Al to investigate the effect of iron oxide on alumina aerogel.
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Table 1 shows the BET surface area and pore structure parameters for different aerogels. From Table 1, we can find the pore volume and average pore diameter for Fe2O3-Al2O3 aerogels are between those of the Al2O3 and Fe2O3 aerogels, in addition to the BET surface area. When a little of iron oxides are added into Al2O3 aerogel, the surface area has a little change. With the increase of content of Fe2O3, the surface area suddenly increases to 545.66 m2·g−1. Then as the further increase of content of Fe2O3, the specific surface areas of composite aerogels begin to decrease little by little. So for Fe2O3-Al2O3 composite aerogels, appropriate proportion can make it have the largest specific surface area.
Figure 2 shows the nitrogen adsorption-desorption isotherms and pore size distribution of alumina aerogel, Fe2O3 aerogel, and Fe2O3-Al2O3 aerogel with 1 : 2 molar ratio of Fe to Al, respectively. The pore structure and the BET surface area of Fe2O3-Al2O3 composite aerogel were close to those of pure alumina aerogel, rather than iron oxide which gave priority to micropores. The Fe2O3-Al2O3 composite aerogel and alumina aerogel had pronounced mesoporosity in the rage of 3–15 nm with a narrow pore size distribution, which made them have high internal surface areas.
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Figure 3 shows the TEM pictures of three aerogels. From Figure 3 we can see that when floc structure Fe2O3 aerogel is added into thread-like structure alumina aerogel, shuttle-type structure with the length of about 15 nm forms, which leads to the composite aerogel having the high surface area. At the same time, the figure also hints that a new form appears at composite aerogel, which is different from the pure alumina and Fe2O3 aerogel.
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In order to further investigate the aerogels’ composition, we carried out the FT-IR experiment, as shown in Figure 4. For alumina aerogel, boehmite is the main component for its infrared absorption peak at 1050 cm−1 and 616 cm−1. For Fe2O3 aerogel, the main composition should be FeOOH for its infrared absorption peak at 2920 cm−1, 1105 cm−1, and 1404 cm−1. When Fe2O3 is added into alumina aerogel, the IR spectra characteristic absorption peaks of boehmite and FeOOH are both reflected. In addition, there is an absorption peak at 496 cm−1, caused by the presence of the Al-O stretching vibration in FeAl2O4. Figure 5 shows a set of XRD patterns of above samples. The XRD patterns reveal the same information with the results of FT-IR experiment. For Fe2O3-Al2O3 aerogel, the peaks corresponding to FeAl2O4 appear obviously.
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We think that FeAl2O4 is formed after mixing two kinds of sol. Hydrochloric acid was used in the preparation of alumina sol. When alumina sol was mixed with iron oxide sol, acidic condition would lead to some FeAl2O4 formation. Therefore, FeAl2O4 can be found in the characterization.
4. Conclusions
We successfully synergized monolithic alumina, Fe2O3, and Fe2O3-Al2O3 aerogels through OSSD drying method. Combined with IR spectra, TEM images, and pore structure parameters of aerogels, we found that the Al2O3-Fe2O3 composite aerogel with the appropriate proportion would form a new structure, which made it have the higher specific surface area than the pure Al2O3 and Fe2O3 aerogels. And its particle size focused on about 15 nm and average pore diameter was about 4 nm.
Competing Interests
The authors declare that they have no competing interests.
Acknowledgments
This work was partially supported by Nature Science Foundation of Jiangsu Province of China (BK20151408) and National Nature Science Foundation of China (21206018).