Elsevier

Carbon

Volume 37, Issue 12, 1999, Pages 2049-2055
Carbon

Preparation of mesoporous carbon by freeze drying

https://doi.org/10.1016/S0008-6223(99)00089-5Get rights and content

Abstract

Resorcinol–formaldehyde (RF) cryogels were synthesized by sol-gel polycondensation of resorcinol with formaldehyde and freeze drying with t-butanol. The cryogels were characterized by nitrogen adsorption and density measurements. Their porous properties were compared with those of RF aerogels prepared by supercritical drying with carbon dioxide. RF cryogels were mesoporous materials with large mesopore volumes >0.58 cm3/g. Although surface areas and mesopore volumes of the cryogels were smaller than those of the aerogels, the cryogels were useful precursors of mesoporous carbons. Carbon cryogels were obtained by pyrolyzing RF cryogels in an inert atmosphere. Carbon cryogels were mesoporous materials with high surface areas >800 m2/g and large mesopore volumes >0.55 cm3/g. When pyrolyzed, micropores are formed inside the cryogels more easily than inside the aerogels.

Introduction

Highly cross-linked and transparent gels are synthesized by sol-gel polymerization of metal alkoxides. If the gel is supercritically dried, the gel structure can be kept the same during drying, being called an aerogel[1]. Freeze drying is also useful to prevent drying shrinkage of gels. A freeze-dried gel is called a cryogel.

Resorcinol–formaldehyde (RF) aerogels were prepared by the sol-gel polycondensation of resorcinol (1,3-dihydroxybenzene) (C6H4(OH)2) (R) with formaldehyde (HCHO) (F) and supercritical drying with carbon dioxide (CO2) [2], [3]. Carbon aerogels were also obtained by pyrolyzing RF aerogels in an inert atmosphere [4], [5], [6]. RF and carbon aerogels have high porosity (>80%) and high surface areas (400–900 m2/g). Carbon aerogels are especially expected to be used as adsorbents, electric double layer capacitors, and materials for chromatographic separation.

Transmission electron microscopy (TEM), nuclear magnetic resonance (NMR), N2 adsorption, small-angle X-ray scattering (SAXS), mechanical testing, etc. have been used to characterize RF and carbon aerogels [2], [3], [4], [5], [6], [7], [8]. Especially, the analysis of mesoporous and microporous structures of the aerogels is very important from the viewpoint of their application to adsorption and catalysis. The control of mesoporous structures of RF and carbon aerogels is also indispensable to prepare adsorbents and catalysts. The authors have experimentally elucidated the influence of the amounts of resorcinol, water, and basic catalyst used in the sol-gel polycondensation on the porous structures of RF and carbon aerogels [6], [9]. They have also proposed a control procedure of porous structures of the aerogels [10].

Although carbon aerogels are unique materials with high surface areas and large mesopore volumes, the cost of supercritical drying is extremely expensive. Hence, it is useful to propose a novel method to prepare a mesoporous carbon. RF cryogels are synthesized by the polycondensation of resorcinol with formaldehyde and freeze drying. Carbon cryogels are obtained by pyrolyzing RF cryogels. The objective of the present work is to study the applicability of freeze drying to the preparation of mesoporous carbons.

In this article, RF and carbon cryogels are prepared and their porous structures are estimated by N2 adsorption and density measurements. Porous properties of the cryogels are compared with those of RF and carbon aerogels. Then the applicability of freeze drying to the preparation of mesoporous carbons is discussed.

Section snippets

Synthesis of RF cryogel

In the present work, the aqueous polycondensation of resorcinol (R) with formaldehyde (F) was proceeded through a sol-gel transition according to the method proposed by Pekala et al. [2], [3], [4]. Here, sodium carbonate (Na2CO3) (C) was used as a basic catalyst. RF solutions were prepared from resorcinol (Wako Pure Chemical Industries Inc., research grade), formaldehyde (Wako Pure Chemical Industries Inc., research grade, 37 wt% formaldehyde stabilized with 8 wt% methanol), sodium carbonate

Porous structure of RF cryogel

RF aerogels are unique mesoporous materials with high surface area and large pore volume [2], [3], [4], [5], [6]. RF cryogels are compared with the aerogels. RF cryogels (A1, A2, B1, and B2) were prepared under the conditions shown in Table 1. In the synthesis of RF cryogels, R/C, and R/W were changed at R/F=0.5. RF aerogels were prepared under the same conditions [9], [12].

Conclusion

RF cryogels were synthesized by sol-gel polycondensation of resorcinol with formaldehyde and freeze drying with t-butanol. Carbon cryogels were obtained by pyrolyzing RF cryogels in an inert atmosphere and the following conclusions were obtained:

  • 1.

    RF cryogels are mesoporous materials with large mesopore volumes >0.58 cm3/g. Although surface areas and mesopore volumes of RF cryogels are smaller than those of RF aerogels, they are larger than those of RF xerogels.

  • 2.

    Although RF cryogels are shrunk

Acknowledgements

This research was partially supported by the Ministry of Education, Science, Sports and Culture of Japan, Grant-in-Aid for Scientific Research on Priority Area (Carbon Alloys), No. 10137234 (1998) and by the Mazda Foundation’s Research Grant (1998).

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