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Chem. Mater. 2004, 16, 100-103
Synthesis of Polyacrylonitrile-Based Ordered Mesoporous Carbon with Tunable Pore Structures Anhui Lu, Andreas Kiefer, Wolfgang Schmidt, and Ferdi Schu¨th* Max-Planck-Institut fu¨ r Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mu¨ lheim an der Ruhr, Germany Received June 12, 2003. Revised Manuscript Received October 15, 2003
Analogous to the synthesis of CMK-3, ordered mesoporous carbons (OMC) containing nitrogen groups were synthesized using mesoporous silica SBA-15 as a template, polyacrylonitrile (PAN) as a carbon source, and 2,2′-azobisisobutyronitrile as an initiator. Before carbonization, PAN/SBA-15 composites were treated in air at low temperature to stabilize the PAN to increase the carbon yield. After carbonization and removal of the silica template, PAN-based OMC can be obtained. Samples were characterized by XRD, TEM, nitrogen adsorption, and elemental analysis. The results show that the pore structures of PAN-based OMC are tunable over a certain range. When the stabilization temperature and carbonization temperature are changed, PAN-based OMC with either monomodal or bimodal pore size distributions can be prepared.
Introduction Ordered mesoporous materials are of high interest both in practical and in fundamental research due to their high surface areas, regular frameworks, and narrow pore size distributions, leading to promising applications, for example, catalysis, optics, or electrode materials. Mesoporous carbons, namely, the CMK family, have been synthesized by using mesoporous silica as a template.1-4 With the different mesoporous silicas MCM-48,1 SBA-1,2 and SBA-15,3,4 ordered mesoporous carbon with cubic (cubic I132 and Pm3n) and hexagonal (P6m) symmetry can be obtained. Even using the same template, ordered mesoporous carbons consisting of interconnected carbon rods or interconnected carbon tubes can be achieved just by using different amounts of carbon.3,5,6 This demonstrates that the nanostructures of CMK materials can be varied by carefully tuning the synthesis conditions. To date, the reported CMK materials were synthesized using sucrose, furfuryl alcohol, or acetylene as carbon source. These kinds of carbon only possess oxygen-containing groups.7 However, for some applications, mesoporous carbon with other specific functional groups are desirable. Previous studies showed the nanostructures of meosporous carbon to depend highly on the precursors and synthesis conditions. Therefore, * To whom correspondence should be addressed. Fax: 0208-3062995. E-mail:
[email protected]. (1) Ryoo, R.; Joo, S. H.; Jun, S. J. J. Phys. Chem. B 1999, 103, 7743. (2) Ryoo, R.; Joo, S. H.; Kruk, M.; Jaroniec, M. Adv. Mater. 2001, 13, 677. (3) Joo, S. H.; Choi, S. J.; Oh, I.; Kwak, J.; Liu, Z.; Terasaki, O.; Ryoo, R. Nature 2001, 412, 169. (4) Zhang, W. H.; Liang, C.; Sun, H.; Shen, Z.; Guan, Y.; Ying, P.; Li, C. Adv. Mater. 2002, 14, 1776. (5) Lu, A. H.; Schmidt, W.; Spliethoff, B.; Schu¨th, F. Adv. Mater. 2003, 15, 1602. (6) Joo, S. H.; Ryoo, R.; Kruk, M.; Jaroniec, M. J. Phys. Chem. B 2002, 106, 4640. (7) Darmstadt, H.; Roy, C.; Kaliaguine, S.; Choi, S. J.; Ryoo, R. Carbon 2002, 40, 2673.
it is expected that using different precursors and synthesis conditions might result in mesoporous carbons with novel nanostructures. To the best of our knowledge, there is no report on the synthesis of CMK-n exhibiting functional nitrogen groups so far. The nitrile group containing polyacrylonitrile (PAN) is usually used to synthesize PAN fibers,8 which are empoyed widely as textile fibers and as reinforcement fibers. Moreover, PAN-based activated carbon fibers are considered to be excellent catalysts for dehydrochlorination9 and deSOx10 reactions due to their functional groups. Therefore, the synthesis of PANbased ordered mesoporous carbon (OMC) is highly desirable, not only due to its exposed nitrogen-containing groups but also due to its mesopore structure facilitating the diffusion of gaseous molecules within the pore systems. Hence, in the present work, we investigated the use of PAN as a carbon source and 2,2′-azobisisobutyronitrile (AIBN) as an initiator to synthesize a PAN-based material analogous to CMK-3. Similar to the synthesis of PAN-based carbon fiber, an additional stabilization step, namely, a low-temperature heat treatment in air after the polymerization, seemed to be promising, to increase the carbon yield of PAN.8,11 Experimental Section In a typical preparation of SBA-15,12 4.0 g of Pluronic P123 block copolymer was dissolved in 105 mL of distilled water, and then 20 mL of an aqueous 37% HCl solution was added to the solution at 40 °C under vigorous stirring. After 8.5 g of (8) Ko, T.; Huang, L. J. Appl. Polym. Sci. 1998, 70, 2409. (9) Sotowa, C.; Watanabe, Y.; Yatsunami, S.; Korai, Y.; Mochida, I. Appl. Catal., A 1999, 180, 317. (10) Muniz, J.; Herrero, J. E.; Fuertes, A. B. Appl. Catal., B 1998, 18, 171. (11) Hinrichsen, G. J. Appl. Polym. Sci. 1973, 17, 3305. (12) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548.
10.1021/cm031095h CCC: $27.50 © 2004 American Chemical Society Published on Web 12/05/2003
Polyacrylonitrile-Based Ordered Mesoporous Carbon
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Table 1. Synthesis Parameters and Properties of the Different Samples (Tstab, Stabilization Temperature; Tcarb, Carbonization Temperature; a, Unit Cell Parameter; SBET, Specific Surface Area According to BET; DBJH, Pore Diameter According to BJH (Adsorption Branch); Vpore, Total Pore Volume; C/N, Atomic Ratio of Carbon and Nitrogen; C/H, Atomic Ratio of Carbon and Hydrogen; C/O, Atomic Ratio of Carbon and Oxygen) sample SBA-15 NCC-2-a NCC-2-b PAN-CMK-3 NCC-2-c
Tstab (°C) 200 230 250 250
Tcarb (°C)
a (nm)
SBET (m2 g-1)
DBJH (nm)
Vpore (cm3 g-1)
850 850 850 1100
10.9 9.8 10.3 10.0 9.3
816 800 479 644 690
6.2 6.5 4.5 3.4 4.1
1.03 1.20 0.55 0.56 0.76
TEOS was added to the solution, a milky mixture was maintained at 40 °C for 4 h, followed by aging at 90 °C for 2 days. The solid products were filtered off and dried at 80 °C overnight. SBA-15 was obtained by calcination of the powders at 550 °C in air. The PAN-based OMC was synthesized with acrylonitrile and SBA-15. The SBA-15 was evacuated at 150 °C and then infiltrated with acrylonitrile solution in the presence of an AIBN initiator by the incipient wetness technique. The mixture was maintained at 50 °C for 12 h and then heated at 60 °C for 8 h under static conditions in an argon atmosphere. In this way, a PAN homopolymer was synthesized by a conventional radical polymerization. The stabilization step was carried out under air in the temperature range of 200-250 °C. Subsequently, the stabilized PAN was carbonized at a temperature of 850 or 1100 °C. PAN-based OMC was obtained by dissolving the silica in an aqueous solution of hydrofluoric acid. Thermogravimetric analysis showed the silica content in the product being
