Synthesis of carbon aerogel material in atmosphere condition

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The experimental results indicated that carbon aerogels obtained by using ambient pressure drying showed BET surface area, density, average pore size, total pore volumes in the range of 459.7–522.1 m2 /g, 0.167–0.629 g/cm3 , 19.45 Å, and 0.259 cm3 /g, respectively. The activation process with CO2 was influence to properties of carbon aerogels and it can improve the surface area reach about 779 m 2 /g. Thus, depending on the applications, carbon aerogels can be synthesized at different conditions.

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Nội dung Text: Synthesis of carbon aerogel material in atmosphere condition

Nghiên cứu khoa học công nghệ SYNTHESIS OF CARBON AEROGEL MATERIAL IN ATMOSPHERE CONDITION Le Anh Kien1*, Le Khac Duyen1,2, Pham Quoc Nghiep1, Le Thi Kim Phung2 Abstract: Porous carbon aerogels were prepared from resorcinol–formaldehyde monomers using sodium carbonate catalyst by ambient pressure drying technique. Aerogels were characterized by using SEM, XRD, TGA and nitrogen adsorption methods. The experimental results indicated that carbon aerogels obtained by using ambient pressure drying showed BET surface area, density, average pore size, total pore volumes in the range of 459.7–522.1 m2/g, 0.167–0.629 g/cm3, 19.45 Å, and 0.259 cm3/g, respectively. The activation process with CO2 was influence to properties of carbon aerogels and it can improve the surface area reach about 779 m2/g. Thus, depending on the applications, carbon aerogels can be synthesized at different conditions. Keywords: Aerogel; Carbon aerogel; Ambient pressure drying; Resorcinol–formaldehyde; Microstructure. 1. INTRODUCTION Aerogels are highly porous materials with low density and large specific surface area, derived via sol–gel technique. The first aerogels were made from waterglass–derived silica gels, employing solvent exchange and using supercritical conditions to remove the pore liquid [1]. The aerogels can be obtained as monoliths, granulates, films, or powders. Most notably aerogels are known for their extreme low densities (0.0011–0.8 g/cm3), high porosity (80–99%), and large surface area (400–1000 m2/g) [2–4]. Due to these special properties, aerogels have attracted much attention for they have large potential to be applied in many fields. First carbon aerogel was obtained by resorcinol–formaldehyde organic aerogel synthesis, and pyrolysis in the inert atmosphere by Pekala [2] at the end of 1980s. The properties of carbon aerogels were depended on the amount of reactant, catalyst, and water used in the sol–gel polycondensation [5–7] and drying methods [8–10]. Because of their chemical and textural characteristics, they were expected to be materials for thermal and sonic insulation, chromatography packing in the research of Wu et al. [8], or the electrodes for electrochemical double layer capacitors by Pekala et al. and Kim et al. [11, 12], adsorbents in study of Meena et al. [13] and catalyst supports by research of Moreno– Castilla et al.[14]. In the research of Shariff et al., the effect of different catalysts, such as acetic acid, sodium carbonate and sodium hydroxide on the sol–gel process were tested. The results showed that the carbon aerogel that synthesized with acetic acid catalyst gave a very high surface area (619.2 m2/g), high pore–volume and microporous area comparing to the other samples in the same conditions [5]. All the different catalysts resulted in different surface morphology of the aerogels. The study of Liviu Cosmin et al. [15] with varieties of synthesis parameters such as R/C ratio and RF content indicated that when using ratio of R/C ≤ 100, mesoporous carbon aerogels with high BET surface area from 800 to 1100 m2/g were obtained, whereas for ratio of R/C > 100, the carbon aerogels were mostly macroporous. Drying methods were also affected on properties of carbon aerogels. The researches of Wu et al. carried out the drying techniques to convert the liquid gel to a solid gel, in the atmosphere condition drying [8], whilst the freeze drying [9] were chosen in the study of Yamamoto et al., supercritical drying were performed by Liu et al. and Tamon et al. [10, 16]. The results of these researches gave that the porous structure and surface area of the carbon aerogel depended on the initial conditions of synthesis, drying, and Tạp chí Nghiên cứu KH&CN quân sự, Số 42, 04 - 2016 155 Hóa học & Kỹ thuật môi trường carbonizing techniques. This also mentioned in the researches of Pekala et al., Czakkel et al., and Gallegos–Suarez et al. [2, 17, 18]. In this paper, carbon aerogels were synthesized via a sol–gel process by the aqueous polycondensation of resorcinol with formaldehyde, using sodium carbonate as a base catalyst under ambient drying technique. The preparing conditions such as the catalyst concentration, the ratio of reactant to solvents were investigated in this study, the structure and properties of the obtained products were characterized by scanning electron microscope (SEM), X–Ray diffraction (XRD), thermo gravimetric analysis (TGA) and nitrogen absorption measurements. The effects of activation process with CO2 was also tested on the product carbon aerogels. 2. EXPERIMENTAL 2.1. Preparation of carbon aerogel The resorcinol–formaldehyde wet gels were prepared using resorcinol (99.5% purity), formaldehyde (37% solution), Na2CO3 (99.8% purity), and deionized water. Appropriate amounts of resorcinol (R), formaldehyde (F), Na2CO3 (C) and water were mixed at room temperature with magnetic stirrer for in an hour. The mixture was transferred into plastic moulds (8 cm–length x 2 cm internal diameter) and cured: 1 day at room temperature, 1 day at 50oC, and 3 days at 80oC for gelation. The resorcinol–formaldehyde gels were then immersed in acetone for three days in order to completely exchange the water in the sol– gel structure. Subsequently, the samples were directly dried in air at room temperature for 2 days at first, and then further dried in an oven at 50oC for 2 days and 80oC for 1 day in ambient pressure. Finally, pyrolysis of the RF aerogel was performed at 800oC for 3 hours under N2 atmosphere (400 cm3min–1), resulting in carbon aerogels (CA). The pyrolysis product yield was calculated by dividing the weight of the carbon aerogels to the weight of as–prepared aerogel. Carbon aerogels was further activated at 800oC for 2 hours under a flow of CO2 to increase its properties. In this study, the ratio of R and F was kept at 1/2, the RF content was changed from 10% to 40%, the ratio of resorcinol and catalyst (R/C ratio) was changed from 500 to 3000. 2.2. Characterization methods Surface area and pore-size distribution of carbon aerogels samples were characterized by analysis of nitrogen absorption–desorption isotherms measured by ASAP 2020 analyzer (Micrometrics Instruments Corp.). The samples were degassed under vacuum (10–5 torr) to 110oC (for organic aerogels) and 200oC (for carbon aerogels) for at least 10 hours to remove all adsorbed species. Brunauer–Emmett–Teller (BET) method was used for total surface area measurements, and t–plot method was used for estimating mesopore surface area. Pore–size distribution was obtained by the Barret–Joyner–Halenda (BJH) method from desorption branch of the isotherms. Total pore–volume was calculated from the adsorbed volume of nitrogen at P/P0=0.99 (saturation pressure). The bulk densities of the samples were estimated by measuring the dimensions and the mass of each monolithic sample. Thermo gravimetric analysis was carried out to determine the weight loss in relation to temperature with a TA Instruments TGA Q500 V20.10 Build 36 thermal analyzer. The specimen was heated up to 150◦C for 1 h to ensure that the adsorbed water in the specimen to be removed. Then, the specimen was heated to 900◦C at a rate of 10◦C min−1 in air. Scanning electron microscopy was performed with a HITACHI S–4800 microscope. 156 Le Anh Kien,…, “Synthesis of carbon aerogel material in atmosphere condition.” Nghiên cứu khoa học công nghệ X–ray diffractogram was recorded on Bruker D8 Advance diffractometer with Cu–Kα radiation (λ=1.54060 Å) operated at the voltage and current values of 40 kV and 40 mA respectively for the 2θ values in the range 5–70° at a scan speed of 1.2°/min. 3. RESULTS AND DISCUSSION 3.1. Evaluation of the synthesis process In this study, the RF aerogels were synthesized via a sol–gel process by the aqueous polycondensation of resorcinol with formaldehyde, using sodium carbonate as a base catalyst [2]. The R and F monomers were polymerized in a plastic container and a monolithic product was obtained by ambient pressure drying. The RF aerogel samples with dimension of 140x120x5 of length x width x thickness were prepared as in Figure 1. Figure 1. RF aerogel and carbon aerogel samples with R/C = 1000, RF content = 40%. The varieties of catalyst concentration (R/C ratio) and RF content (RF%) were investigated to determine the optimal condition for synthesis RF aerogel by ambient pressure drying. The results showed that, all RF samples with R/C ratio from 1500 to 3000 were not completely gelation, they all were coagulated in the solution and precipitated at the bottom of container. This might be because of the concentration of catalyst was not enough to form a cross–linked network. When the R/C ratio was decreased, the sol–gel process was improved. RF samples with R/C ratio less than 1000 were easily gelation. In the gelation process, catalyst concentration was much affected. This was because of that: (1) the formation of monomer from resorcinol and formaldehyde, (2) the condensation of monomer with each other to form nanometer–sized clusters; and (3) crosslinking of clusters through their surface groups to form a gel [1]. The size of clusters was regulated by the catalyst concentration in RF mixture. Figure 2 showed a schematic diagram of the reaction of resorcinol with formaldehyde under used conditions in this study. Figure 2. A schematic diagram of the reaction of resorcinol with formaldehyde. Tạp chí Nghiên cứu KH&CN quân sự, Số 42, 04 - 2016 157 Hóa học & Kỹ thuật môi trường In this results, densities of RF samples ranged of 0.254–0.853 g/cm3, and surface area were in the range of 176.08–204.08 m2/g. The RF samples with R/C = 1000 and RF content = 40% show the best properties. The density of those samples were minimum value of all types because of lowest volume shrinkage taken place (Table 1). Table 1. Physical properties of RF aerogel at different conditions. Samples Mass shrinkage (%) Volume shrinkage (%) Density (g/cm3) RF–40–1000 79.47 18.34 0.254 RF–40–500 79.40 52.25 0.436 RF–30–500 84.65 67.33 0.475 RF–20–500 95.21 90.04 0.485 RF–10–500 94.73 90.36 0.552 RF aerogels were then pyrolysized to form CA samples. The pyrolysis yields of aerogels under different conditions were all the same, ranged from 41% to 50%. The densities of the CA samples were in the range of 0.167 – 0.629 g/cm3, and the surface area of CA were of 459.76 – 522.13 m2/g. They all were close to the values of monolithic CAs in the literature [8]. During the pyrolysis process, the organic groups in the structure of RF aerogel were volatile to form carbon network. The mass loss of the samples due to pyrolysis was in range of 51–56%, similar to the study of Yamashita et al. [19]. 3.2. Morphology and nanostructure of the carbon aerogel (a)-CA500 (b)-CA1000 (c)-ACA1000 Figure 3. SEM photographs of aerogel samples. (a) carbon aerogel R/C=500 (CA500); (b) carbon aerogel R/C=1000 (CA1000) và (c) activated carbon aerogel R/C=1000 (ACA1000). Figure 3 showed the morphology of CA samples with different R/C ratio. The morphologies of CA particles were found to be nearly spherical in shape. All types of structures were formed by a three–dimensional network of interconnected nano–sized primary particles more or less fused. It can be seen that the product CA particles at 158 Le Anh Kien,…, “Synthesis of carbon aerogel material in atmosphere condition.” Nghiên ccứu ứu khoa học công nghệ CA1000 were larger than those at CA500. These particles were very difficult to distinguish in CA500 sample, where a continuous and hhighly ighly cross–linked cross linked porous structure has been formed. The carbon nano–particles nano particles in obtained aerogels stacked into grape–like grape like aggregates and then these aggregates interconnect in different directions into a network. When the R/C ratio was controlled at 1000 (see Figure 3a), the size of the carbon particles of the CA1000 sample was in the range of about 40 40–50 50 nm, similar to the nano–structures nano structures of monolithic CA reported by Wu et al. [88].. The CA prepared in this work have the nano–nano– particle structures typical of the samples prepared with the CO2 supercritical drying technique in the studies of Al–Muhtaseb Al et al. and Qin et al. [6, 20]. 20 . Additionally, the size of the carbon particles was hardly affected by activation with CO2; Figure 3c indicated that the particles size was decreased into 20– 20–30 30 nm due to the reaction of CO2 with carbon network and the abrasion of carbon structure during activation. The XRD diagram of the synthesized CA samples were shown in Figure 4. It presented two large peaks at about 2θ = 24ο and 44o, similar to the diffraction peaks of C(002) and C(101) and it was in agreement with the literature data [7].. The first peak indicated that CA samples contained a proportion of highly disordered materials in the form of amorphous carbon. In addition, the samples also con contained tained some graphite–like graphite like structures (crystalline carbon) indicated by the presence of a clear (002) band at ~ 24o and (101) weak band at ~ 44o. Moreover, the presence of a peak at ~10o showed that there was impurity in the CA samples after pyrolysis becau because se its intensity was decreased after activation with CO2. The activation process removed the impurities; surface functional groups and carbon in the structure of CA by abrasion and reaction of CO2. These observations suggested that the crystallites in all the CA samples have intermediate structures between graphite and amorphous state called turbostratic structure or random layer lattice structure. Figure 4. X–Ray Ray diffraction pattern of carbon aerogel (AC1000) and activated carbon aerogel (ACA1000). TGA experiments showed that a first slight weight loss from 50 50–90oC was due to the desorption of physical moisture from the aerogel structure. Further heating in the temperature range of 550o–650650oC resulted in about 81% weight lo loss. ss. This weight loss can be attributed to the destruction of CA structure and the combustion in air. This results showed that CA samples easily absorbed moisture during storage and were destroyed in the air at the temperature of higher 550oC. Tạp ạp chí Nghi Nghiên cứu ứu KH&CN quân uân sự, s Số 422, 04 - 2016 159 Hóa học & Kỹ thuật môi trường Figure 5. Thermo gravimetric analysis data for activated carbon aerogel (ACA1000). 3.3. Porous properties of the CA samples Physical properties, especially the surface characteristics of aerogels were evaluated by nitrogen gas sorption studies using BET analyzer at 77 K (Figure 6). Results of BET analysis were given in Table 2. The isotherms of the RF aerogels have been observed to be of Type IVa following the IUPAC classification, indicating multilayer absorption on the surface of the RF aerogels. This type of isotherm was characteristic of mesoporous adsorbents. Pore–diameter of RF aerogel was distributed from 10 to 150 Å (Figure 7), with mesopore–volume percentage was 42.5% of total pore–volume. BET surface area of RF aerogels was in range of 176–208 m2/g. The isotherms of carbon aerogel were type Ia, indicating the microporous adsorbents. Microporous volume was about 98 percent of total pore–volume of carbon aerogel, with the pore–size was in range of 5–11 Å. The surface area of carbon aerogels increased dramatically to 459–522 m2/g after pyrolysis, which was due to the creation of micropores as a result of evaporative loss of organic moieties. Table 2. Porous properties of RF aerogel, carbon aerogel and activated carbon aerogel. Activated carbon Properties RF aerogel Carbon aerogel aerogel Density (g/cm3) 0.254–0.853 0.167–0.629 0.150–0.510 SBET (m2/g) 176.08–204.08 459.76–522.13 637.13–779.06 Average pore size (Å) 31.77 19.45 22.24 Median pore width (Å) 20.17 5.66 6.11 Average particle size (Å) 294.00 114.91 77.02 Vtotal (cm3/g) 0.1658 0.2591 0.4408 Vmic(cm3/g) 0.0879 0.2540 0.3173 Vmes(cm3/g) 0.0705 0.0033 0.0547 Vmac(cm3/g) 0.0074 0.0051 0.0688 Vmic (%) 53.01 98.04 71.98 Vmes (%) 42.52 1.27 12.40 Vmac (%) 4.47 1.96 15.62 160 Le Anh Kien,…, “Synthesis of carbon aerogel material in atmosphere condition.” Nghiên ccứu ứu khoa học công nghệ Figure 6. Adsorption–desorption Adsorption desorption isotherms of RF aerogel (RF (RF1000 1000); ); Carbon aerogel (CA1000)) and Activated carbon aerogel (ACA (ACA1000 1000). After activation with CO2, the characteristics of carbon aerogel were improved. The highest measured BET surface area of activated carbon aerogel reached 779 m2/g. The mesopore volumes and macropore volume of the samples were increased, indicated by the adsorption sorption isotherm type IIb. The pore– pore–sizes sizes were in the range from 8 to 200 Å, with average pore pore–diameter diameter was 22.2 Å, similar to the study of Gallegos Gallegos–Suárezet Suárezet et al. [18].. On the other hand, the size of aerogels particles was decreased by act activation ivation with CO2 (Table 2); this suggested that the effect of activation on structure of carbon aerogel was creation more pore and surface area. Figure 7. Pore–size size distributions of RF aerogel (RF1000); (RF1000); carbon aerogel (CA1000) and nd activated carbon aerogel (ACA1000). (ACA1000) 4. CONCLUSION The carbon aerogels were synthesized from resorcinol resorcinol–formaldehyde formaldehyde monomers using base catalyst via sol–gel sol gel process under ambient pressure drying technique. The R/C ratio and RF content of organic matrix were greatly influence to the shrinkage of gel–network gel network of the obtained materials. Characteristics of obtained aerogels were evaluated and compared with period studies. The results esults showed that carbon aerogels have intermediate structures between graphite and amorphous state and were destroyed in the air at the temperature of higher 550oC. C Surface area, density, average pore pore–size size and total pore– pore– volume of carbon aerogels 459.7 522.1 m2/g, 0.167–0.629 aerogel were in the range of 459.7–522.1 0.629 g/cm3, Tạp ạp chí Nghi Nghiên cứu ứu KH&CN quân uân sự, s Số 422, 04 - 2016 161 Hóa học & Kỹ thuật môi trường 19.45Å, and 0.259 cm3/g, respectively. The activation process affected on the properties of carbon aerogels. The highest surface area of activated carbon aerogel reached 779 m2/g. REFERENCES [1]. M. A. Aegerter et al., “Aerogels handbook,” Springer Science & Business Media (2011). [2]. R. Pekala, "Organic aerogels from the polycondensation of resorcinol with formaldehyde," J. of Materials Science, Vol. 24 (1989), pp. 3221–3227. [3]. S. He et al., "Synthesis and characterization of silica aerogels dried under ambient pressure bed on water glass," J. of Non–Crystalline Solids, Vol. 410 (2015), pp. 58– 64. [4]. P. Aravind et al, "Ambient pressure drying: a successful approach for the preparation of silica and silica based mixed oxide aerogels," J. of sol–gel science and technology, Vol. 54 (2010), pp. 105–117. [5]. A. M. Shariff et al., "Some studies on the synthesis and characterization of carbon aerogel," Transactions of the Indian Ceramic Society, Vol. 69 (2010), pp. 83–88. [6]. S. A. Al‐Muhtaseb and J. A. Ritter, "Preparation and properties of resorcinol– formaldehyde organic and carbon gels," Advanced Materials, Vol. 15 (2003), pp. 101–114. [7]. K. S. Rejitha et al., "Role of catalyst on the formation of resorcinol–furfural based carbon aerogels and its physical properties," 2013. [8]. D. Wu et al., "Preparation of low–density carbon aerogels by ambient pressure drying," Carbon, Vol. 42 (2004), pp. 2033–2039. [9]. T. Yamamoto et al., "Control of mesoporosity of carbon gels prepared by sol–gel polycondensation and freeze drying," J. of non–crystalline solids, Vol. 288 (2001), pp. 46–55. [10]. N. Liu et al., "Carbon aerogel spheres prepared via alcohol supercritical drying," Carbon, Vol. 44 (2006), pp. 2430–2436. [11]. R. Pekala et al., "Carbon aerogels for electrochemical applications," J. of non– crystalline solids, Vol. 225 (1998), pp. 74–80. [12]. S. Kim et al., "Preparation of carbon aerogel electrodes for supercapacitor and their electrochemical characteristics," J. of materials science, Vol. 40 (2005), pp. 725– 731. [13]. A. K. Meena et al., "Removal of heavy metal ions from aqueous solutions using carbon aerogel as an adsorbent," J. of Hazardous Materials, Vol. 122 (2005), pp. 161–170. [14]. C. Moreno–Castilla and F. Maldonado–Hódar, "Carbon aerogels for catalysis applications: An overview," Carbon, Vol. 43 (2005), pp. 455–465. [15]. L. C. Coteţ et al., "Synthesis of meso–and macroporous carbon aerogels," Revue Roumaine de Chimie, Vol. 52 (2007), pp. 1077–1081. [16]. H. Tamon et al., "Control of mesoporous structure of organic and carbon aerogels," Carbon, Vol. 36 (1998), pp. 1257–1262. [17]. O. Czakkel et al., "Influence of drying on the morphology of resorcinol– formaldehyde–based carbon gels," Microporous and Mesoporous Materials, Vol. 86 (2005), pp. 124–133. [18]. E. Gallegos–Suárez et al., "On the micro–and mesoporosity of carbon aerogels and xerogels. The role of the drying conditions during the synthesis processes," Chemical Engineering J., Vol. 181 (2012), pp. 851–855. 162 Le Anh Kien,…, “Synthesis of carbon aerogel material in atmosphere condition.” Nghiên cứu khoa học công nghệ [19]. J. Yamashita et al., "Organic and carbon aerogels derived from poly (vinyl chloride)," Carbon, Vol. 41 (2003), pp. 285–294. [20]. G. Qin and S. Guo, "Preparation of RF organic aerogels and carbon aerogels by alcoholic sol–gel process," Carbon, Vol. 39 (2001), pp. 1935–1937. TÓM TẮT TỔNG HỢP VẬT LIỆU CARBON AEROGEL TRONG ĐIỀU KIỆN KHÍ QUYỂN Vật liệu xốp carbon aerogels được tổng hợp từ các monomer resorcinol- formaldehyde với xúc tác natri carbonate trong điều kiện sấy khí quyển. Các tính chất đặc trưng của aerogels được xác định bằng các phương pháp SEM, XRD, TGA và hấp phụ nitrogen. Kết quả nghiên cứu cho thấy carbon aerogel thu được có các tính chất diện tích bề mặt, khối lượng riêng, kích thước lỗ xốp trung bình, tổng thể tích lỗ xốp lần lượt là 459.7–522.1 m2/g, 0.167–0.629 g/cm3, 19.45 Å, and 0.259 cm3/g. Hoạt hóa vật liệu carbon aerogel bằng khí CO2 sẽ ảnh hưởng đến các tính chất của vật liệu, sau quá trình hoạt hóa diện tích bề mặt riêng carbon aerogel tăng đến 779 m2/g. Vì thế, tùy thuộc vào các lĩnh vực ứng dụng, carbon aerogel có thể được tổng hợp trong các điều kiện khác nhau để đạt tính chất mong muốn. Từ khóa: Aerogel; Carbon aerogel; Sấy khí quyển; Resorcinol–formaldehyde; Cấu trúc micro. Nhận bài ngày 22 tháng 02 năm 2016 Hoàn thiện ngày 08 tháng 4 năm 2016 Chấp nhận đăng ngày 20 tháng 4 năm 2016 1 Địa chỉ: Viện Nhiệt đới môi trường, Viện KHCN quân sự; 2 Khoa Kỹ thuật Hóa học – Trường Đại học Bách Khoa TP.HCM; * Email: leanhkien@vnn.vn Tạp chí Nghiên cứu KH&CN quân sự, Số 42, 04 - 2016 163