Fe2O3 Mesoporous Composite Prepared with Activated

Temperature, pressure, and composite precursor ratio effects were studied. X-ray diffraction pattern proved that the product was composed of R-Al2O3 a...
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Ind. Eng. Chem. Res. 2006, 45, 5009-5012

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Al2O3/Fe2O3 Mesoporous Composite Prepared with Activated Carbon Template in Supercritical Carbon Dioxide Haijuan Fan,† Qun Xu,*,† Yiqun Guo,‡ and Yanxia Cao† College of Materials Science and Engineering, and Department of Chemistry, Zhengzhou UniVersity, Zhengzhou 450052, P.R. China

A Al2O3/Fe2O3 mesoporous composite has been prepared with a nanoscale casting process using activated carbon as template in supercritical carbon dioxide (SC CO2). The composite precursor of iron(III) acetylacetonate (Fe(acac)3) and aluminum acetylacetonate (Al(acac)3) and acetone were dissolved in supercritical CO2 and then coated on activated carbon in the desired supercritical conditions. After removal of activated carbon template by calcination in air at 600 °C, a Al2O3/Fe2O3 mesoporous composite was obtained. Temperature, pressure, and composite precursor ratio effects were studied. X-ray diffraction pattern proved that the product was composed of R-Al2O3 and γ-Fe2O3. Dendritic nanocrystals were observed from TEM graphs. SEM results show that the porous structure of activated carbon template was well replicated by composite product. The product pore structure information was calculated from the nitrogen sorption isotherms. 1. Introduction Porous materials have received much attention for their scientific interests and industrial applications in fields of catalysis, adsorption, separation, ion exchange, and chemical sensing.1 The traditional preparation method, such as the sol-gel technique,2 has proposed a potential environmental problem because of the extensive use of traditional organic solvents. In recent years, supercritical fluids (SCFs) have been widely used as a viable alternative to conventional liquid solvents.3,4 There are several specific reasons to consider SCFs as alternative solvents for the synthesis and processing of porous materials.5 Supercritical conditions refer to a state in which the critical temperature and pressure of a fluid have been exceeded. The most frequently used SCF is carbon dioxide (CO2). First, CO2 is inexpensive, environmentally benign, and nonflammable. And its mild critical conditions (Pc ) 73.8 bar; Tc ) 31.1 °C) allow CO2 to be used with safe laboratory and commercial operation conditions. Another advantage is that CO2 can be easily and completely removed from products and the porous structure can be obtained without collapse of the structure. In addition, the solvent can be easily recycled from gaseous CO2 after the pressure is diminished. A wide variety of nanoporous structures can be formed by templating both natural and synthetic materials.6-9 With use of this process, porous metals, oxides, and polymers have been prepared.10-13 Many researchers have paid enormous attention to developing new templates to prepare mesoporous materials with improved properties for practical applications. H. Wakayama and Y. Fukushima proposed a novel process to prepare porous materials replicating not only macroscopic shapes but also porous structures on a nanometer scale. With use of the nanoscale casting process, porous materials such as SiO2, TiO2, Al2O3, and Pt have been prepared.14-18 In this article, we describe the preparation of Al2O3/Fe2O3 porous composite in SC CO2. The hybrid composite ((Fe(acac)3 and Al(acac)3) mixed on a molecular scale may achieve a synergetic combination of the properties which cannot be obtained by either component. In supercritical conditions, effects * To whom correspondence should be addressed. Tel.:+86 371 67767827. Fax: +86 371 67763561. E-mail: [email protected]. † College of Materials Science and Engineering. ‡ Department of Chemistry.

of pressure, temperature, and composite precursor ratio on coating ratio, i.e., the coating Al2O3/Fe2O3 on the activated carbon, have been studied. The product was characterized by X-ray diffraction (XRD), nitrogen sorption isotherms, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). SEM results shows that the pores structure of activated template have been well-replicated by the composite product of Al2O3/Fe2O3 composite product. From XRD, it indicates that the product is composed of R-Al2O3 and γ-Fe2O3. And dendritic nanocrystals can be observed from TEM results. 2. Experimental Section 2.1. Materials. Iron(III) acetylacetonate(Fe(acac)3) was offered by Peking Yili Fine Chemical Co. and was used after grinding. Aluminum acetylacetonate (Al(acac)3) was purchased from Alfa Aesar and was used as received. Activated carbon (granules) was offered by Tianjin Kermel Chemical Reagent Development Center. Acetone (A.R. grade) was received from Luoyang Chemical Reagent Plant. CO2 with a purity of 99.95% was provided by Zhengzhou Shuangyang Gas Co. and was used as received. 2.2. Activated Carbon Coating in SC CO2. In a typical experiment, a suitable amount of Fe(acac)3 and Al(acac)3, which was dissolved in 3 mL of acetone, was placed in the bottom of the stainless steel autoclave of 50-mL capacity (Hai’an Stainless Steel Autoclave Factory). Then 0.5 g of activated carbon was placed in a stainless steel cage fixed at the upper part of the autoclave. The autoclave temperature was adjusted to the desired experimental temperature and then the autoclave was filled with CO2 by a syringe pump (DB-80, Beijing Satellite Manufacturing Factory), until the desired pressure was obtained. After a suitable time, the pressure was released by venting. 2.3. Thermal Treatments. In the drying process, samples were heated at 105 °C for 12 h in a normal atmosphere. Then the mass of coated activated carbon was weighed and symbolized as mc. The coated activated carbon was calcinated at 600 °C for 12 h in air flow to remove the template. To study the experimental effects conveniently, the coating ratio was described as follows:

coating ratio )

10.1021/ie060076p CCC: $33.50 © 2006 American Chemical Society Published on Web 06/03/2006

mc - ma × 100% ma

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Figure 1. Effect of pressure on coating ratio. (All experiments were carried out at 40 °C, precursor ratio ) 1:1, both molar concentrations of precursor were 0.3 mol/L.)

Figure 2. Effect of temperature on coating ratio. (All experiments were carried out at 20 MPa, precursor ratio ) 1:1, both molar concentrations of precursor were 0.3 mol/L.)

where ma is the mass of activated carbon before coating and mc is the mass of activated carbon after coating. 2.4. Characterization. The calcinated products were characterized with nitrogen adsorption-desorption experiment and the isotherms at 77 K were collected on a Quantachrome NOVA 1000e surface area and pore size analyzer. Before this measurement, samples were heated at 473 K in 10-6 Torr for 1 h for degassing. The Barret-Joyner-Hallender (BJH) method was used to calculate the pore size distribution. X-ray diffraction (XRD) data were recorded on a Rigaku D/MAX-3B using Cu KR radiation at a scanning speed of 6°/min within the 2θ range of 10°-70°. Product morphology and microstructure were observed with a JEOL JSM-5600LV scanning electron microscope (SEM) at 15 kV acceleration voltage and with a FEI Tecnai transmission electron microscope (TEM).

Scheme 1. Fe(acac)3 and Al(acac)3 Coating in SC CO2

3. Results and Discussion 3.1. Effect of Experimental Conditions. To investigate the effect of different coating pressures on coating ratio, a series of experiments were performed at 40 °C and the results were shown in Figure 1. The precursor ratio (MFe(acac)3:MAl(acac)3) was 1:1, and their molar concentration in acetone was 0.30 mol/L. It can be seen that the coating ratio of the precursors on activated carbon was increased with increasing experimental pressure. When the experimental pressure increased, solvent power was enhanced, so more precursors were dissolved in the fluid phase and carried onto the activated carbon. Figure 2 shows the effect of different coating temperatures on coating ratio. A series of experiments were performed at 20 MPa. The precursor ratio was 1:1 and their molar concentration was 0.30 mol/L. From Figure 2, it can be seen that the coating ratio first increased with temperature and then reached a maximum at 70 °C, followed by a decrease at higher temperature. There are complex factors for temperature to have an effect on coating ratio in SC CO2 conditions. First, the amount of water molecule on activated carbon was determined by temperature and it is the key parameter for the coating ratio. The reaction mechanism can be illustrated in Scheme 1. For Fe(acac)3 and Al(acac)3 coating, because both the precursor molecules were surrounded by acetone, the interaction of acetones and the water molecules has an effect on the coating ratio. With the temperature increasing, the interaction is positive for the coating and more precursors can be coated onto the activated carbon. Temperature effect on SC CO2 property is the second decisive effect. With the temperature further increasing, the solvent power of SC CO2 reduced and only a limited

amount of precursors can be dissolved in supercritical solvent, so the coating ratio decreased. Figure 3 shows the effect of the molar ratio of two precursors on coating ratio. A series of experiments were carried out at 70 °C and 20 MPa. The total molar concentration of precursors is 0.60 mol/L. From Figure 3, it can be seen that when the precursor is pure Al(acac)3, the coating ratio is only 10.6%. When the molar ratio mFe(acac)3:mAl(acac)3 ) 1:1, the coating ratio reached a maximum of 15.2%. When the molar ratio of mFe(acac)3: mAl(acac)3 increased to 2, the coating ratio of the composite precursor began to decrease. It can be concluded that the two precursors have a cooperating effect during the coating process; when the molar ratio of the precursors was 1:1, the cooperating

Figure 3. Effect of precursor ratio on coating ratio. (All experiments were carried out at 70 °C and 20 MPa, the total molar concentration of the precursor was 0.6 mol/L.)

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Figure 4. X-ray diffraction pattern of product (treated at 70 °C and 20 MPa, precursor ratio ) 1:1 followed by calcinations at 600 °C). Table 1. BET Surface Area of Products Prepared at Different Temperatures in SC CO2 sample

temp (°C)

surface area (m2/g)

1 2 3 4 5

40 50 60 70 80

162.34 155.93 147.45 139.37 150.59

Figure 5. Pore size distribution of two calcinated products which were treated at 20 MPa in SC CO2 and precursor ratio ) 1:1.

Table 2. Pore Microstructure Data of the Prepared Porous Composite: (A) Treated at 40 °C and 20 MPa in SC CO2, Precursor Ratio )1 :1; (B) Treated at 70 °C and 20 MPa in SC CO2, Precursor Ratio ) 1:1

sample A B

coating ratio (%)

surface area (m2/g)

pore volume (cm3/g)

average pore diameter (nm)

7.0 15.2

162.34 139.37

0.55 0.68

9.86 14.90

interactions were very strong so that the coating ratio reached a maximum. This mechanism needs to be further studied. 3.2. Characterizations. Figure 4 shows an X-ray diffraction pattern of the product which was treated in SC CO2 at 70 °C and 20 MPa followed by calcination in air at 600 °C. It confirmed that the product was composed of Al2O3 and Fe2O3. Alumina crystallized in R-Al2O3 form, and iron oxide crystallized in γ-Fe2O3 form. No other diffraction peak exists, which confirms that no activated carbon template is left in the product. BET surface area of different composite products calcinated in air at 600 °C after being treated in SC CO2 at different temperatures were characterized by nitrogen sorption experiments. The results are summarized in Table 1. It indicates that SC CO2-treated temperature has prominent influence on the surface area of the composite product. First, it decreased with the temperature increasing from 40 to 70 °C and then it began to increase when the temperature increased from 70 to 80 °C. Compared to the experimental results shown in Figure 2, it can be seen that there is no direct proportional dependence between coating ratio and surface area. Further, two composite products treated at temperatures of 40 and 70 °C separately in supercritical conditions were characterized for their pore size with nitrogen sorption experiment. The experimental results are shown in Table 2 and Figure 5. It can be explained here why there is no direct relation between coating ratio and surface area; it is due to another two parameters, that is, pore volume and diameters. Although the coating ratio is high for the composite obtained at 70 °C, because of the bigger pore size, the surface area for this sample is low. For the influence mechanism of temperature on the coating ratio and the pore size, the forming

Figure 6. SEM photos for (a) original activated carbon and (b) the product treated in SC CO2 at 70 °C and 20 MPa, precursor ratio ) 1:1, followed by calcinations at 600 °C.

rate of the composites under the different coating temperature should be the main factor for controlling the pore size and surface area. Figure 6 shows the SEM experimental results of the original activated carbon and the composite product which replicated activated carbon in SC CO2 at 70 °C and 20 MPa (precursor ratio ) 1:1). It can be seen that the product morphology was similar to the template activated carbon and it is in a loose structure. Figure 7 illustrated the TEM graphs of the composite product which was treated in SC CO2 at 70 °C and 20 MPa (precursor

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Figure 7. TEM graphs of product treated in SC CO2 at 70 °C and 20 MPa, precursor ratio ) 1:1, followed by calcinations at 600 °C.

ratio ) 1:1) followed by calcinations at 600 °C. Ddendrite-like nanocrystal structure (see Figures 7a and 7b) can be observed for this composite at different places. Further, from the highresolution transmission electron micrograph (HRTEM) shown in Figures 7c and 7d, it illustrates that single nanocrystals can be separated out. And the lattice plane distance is 0.48 nm, which matches the (111) plane of Fe2O3. According to the inset of Fourier transforms of the image, it is deduced that there exists the second lattice plane distance which is 0.24 nm and matches the (222) plane. So it is cubic γ-Fe2O3 and this result is in accordance with our observation of Figure 7c. The experimental results reveal that the formation of γ-Fe2O3 dendritic nanocrystals may be attributed to the preferred orientated aggregation. And growth of Fe2O3 nanoparticles occurs for two reasons: one is the intrinsically inclined crystalline anisotropy and the other is the coordination precursor of Al(acac)3 assistance. And in reference to the SC CO2-treated process, it is first reported by us for the production of dendritic nanocrystals. 4. Conclusions Al2O3/Fe2O3 mesoporous composites were prepared with nanoscale casting process using activated carbon as template in SC CO2. The effects of the experimental conditions were studied on the coating ratio. Nitrogen sorption experimental results show that the porous structure of activated carbon template was well-replicated by the composite product. The BET surface area and product pore structure information were calculated from the nitrogen sorption isotherms and it was found that SC CO2-treated conditions have a prominent effect on the pore microstructure of the composite product. Further, from TEM graphs, dendritic nanocrystals were observed. This can give us a new clue on preparing nanocrystal materials and at the same time it is in the porous structure for the bulk. This kind of material may have potential application in further building functional materials such as catalysts, sorbents, and sensors.

Acknowledgment We are grateful for the financial support from the Prominent Research Talents in University of Henan Province, the National Natural Science Foundation of China (No. 20404012), and the Prominent Youth Science Foundation of Henan Province (No. 0512001200). Literature Cited (1) Poliakoff, M.; Fitzpatrick, J. M.; Farren, T. R.; Anastas, P. T. Science 2002, 297, 807. (2) Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Academic Press: San Diego, 1990. (3) Supercritical Fluid Extraction: Principles and Practice; McHugh, M., Krukonis, V., Eds.; Butterworth: London, 1986. (4) Larson, A.; King, M. L. Biotechnol. Prog. 1986, 2, 73. (5) Cooper, A. I. AdV. Mater. 2003, 13, 1049. (6) Zhang, B. J.; Davis, S. A.; Mann, S. Chem. Mater. 2002, 14, 1369. (7) Xia, Y.; Gates, B.; Z. Y. Li. AdV. Mater. 2001, 13, 409. (8) Caruso, R. A.; Giersig, M.; Willig, F.; Antonietti, M. Langmuir 1998, 14, 6333. (9) Velev, O. D.; Kaler, E. W. AdV. Mater. 2000, 12, 531. (10) Templin, M.; Franck, A.; Chesne, A. D. Science 1997, 278, 1795. (11) Johnson, S. A.; Brigham, E. S.; Ollivier, P. J.; Mallouk, T. E. Chem. Mater. 1997, 9, 2448. (12) Wakayama, H.; Fukushima, Y. Chem. Mater. 2000, 12, 756. (13) Wakayama, H.; Itahara, H.; Tatsuada, N.; Inagake, S.; Fukushima, Y. Chem. Mater. 2001, 13, 2392. (14) Wakayama, H.; Fukushima, Y. Chem. Commun. 1999, 4, 391. (15) Wakayama, H.; Fukushima, Y. Ind. Eng. Chem. Res. 2000, 39, 4641. (16) Fukushima, Y.; Wakayama, H. J. Phys. Chem. B 1999, 103, 3062. (17) Wakayama, H.; Inagaki, S.; Fukushima, Y. J. Am. Ceram. Soc. 2002, 85, 161. (18) Wakayama, H.; Setoyama, N.; Fukushima, Y. AdV. Mater. 2003, 15, 742.

ReceiVed for reView January 17, 2006 ReVised manuscript receiVed April 27, 2006 Accepted May 8, 2006 IE060076P