Fabrication of hierarchically structured carbon monoliths via self-binding and salt templating

https://doi.org/10.1016/j.micromeso.2006.05.024Get rights and content

Abstract

A new synthetic method is reported for the preparation of hierarchically structured carbon monoliths by self-binding and salt templating, which involves the mixing and shaping of SBA-15/poly(furfuryl alcohol) composites with small particle NaCl. After carbonization and leaching out the salt with water and the silica with NaOH solution, hierarchically structured carbon monoliths are obtained, which show the presence of macropores and mesopores derived from the removal of salt and silica, respectively. Such carbon monoliths can be further used as template to create monolithic CoAl2O4 spinel. The well preserved monolithic form of the spinel obtained from the hard template demonstrate the suitability of such hierarchically porous carbon monolith for templating other monolithic nano-structured materials.

Introduction

Monolithic structures can be interesting for applications in catalysis, separation, sensing, and others. In general, monoliths can be fabricated by extrusion or wet chemistry via sol–gel processes [1], [2]. In practice, monolithic materials are mostly produced by extrusion where a special mixture of binders and additives are required to create the monolith structure. This can lead to loss of porosity and dilution of the active material in the monolith. Generation of monolithic materials by sol–gel processes creates materials which typically have different textural properties than those obtained by extrusion. However, these procedures often require supercritical drying which is time consuming and expensive [3], [4], in order to avoid strong capillary pressures and cracking.

Synthesis of mesoporous carbons with adjustable structures and properties was a subject of intensive research over the past decade, as documented in various reviews [5], [6], [7]. Mesoporous carbons are of great interest in catalysis, electrodes, sensors and in separation. Very recently, a new application of mesoporous carbons was developed by us and other groups, i.e. the use of such materials as templates to nanocast other oxide materials [8], [9], [10], [11], [12], [13]. Most of these carbon materials are obtained in the form of powders, which limits their application potential, due to a difficult recovery, high-pressure drop and dust problem as well. It could therefore be interesting to develop a mechanically stable and self-supporting monolithic carbon materials. Unfortunately there are only few reports concerning the preparation of monolithic carbon materials at present [4], [14], [15], [16]. The recently developed synthetic strategies for the production of mesoporous carbon monoliths usually relied on the use of a silica monolith as a template in a nanocasting pathway [4], [14]. The final product often consists of a solid with ordered mesopores below 10 nm in size without larger textural pores, limiting their use in application where mass transfer is crucial. Moreover, the products often suffer from cracking and shrinkage, probably due to the lack of mechanical stability of the silica monolith template and the strong capillary forces. Thus, methods to produce stable monolithic carbons having high surface areas and large pores for efficient mass transfer are still needed.

In the present study, we have established a simple process to fabricate carbon monoliths with meso- and macropores. In this process, furfuryl alcohol (FA) impregnated into SBA-15 template is used both as carbon source and self-binding element to build up the monolith form. At the same time, sodium chloride, which is thermally stable, recyclable, and easily leachable [17] is employed as templating medium to construct the macropore structures. The feasibility of the resulting carbon monolith as template to further replicate other nano-structured monolithic materials was investigated. One ternary oxide, CoAl2O4 spinel was selected as an example. It was found that such carbon monolith is suitable to be used as templates to create monolithic materials.

Section snippets

Synthesis

The synthetic pathway is schematically shown in Scheme 1. The synthesis of the SBA-15 was performed using Pluronic surfactant P123 (BASF) as structure-directing agent and tetraethoxysilane (98%, Aldrich) as silica source. The detailed synthesis is described elsewhere [18]. The calcined SBA-15 was impregnated with furfuryl alcohol (98%, Fluka) as carbon precursor, containing oxalic acid (OxA), nFA/nOxA = 200–300 [19]. The volume of furfuryl alcohol used was twice the pore volume of the SBA-15

Results and discussion

As described above, the carbon monoliths can be fabricated via self-binding by a mechanical shaping technique in the precursor state and a subsequent carbonization step. This carbon monolith was named CM-0. In order to introduce macropores in the obtained carbon monoliths, NaCl powder as porogen was added to the SBA-15 template before the mechanical shaping. The mass ratios of NaCl to SBA-15 were chosen as 0.5, 1.0 and 2.0, and the obtained carbon monoliths were designated as CM-x, where x

Conclusion

We have shown that by a combination of self-binding and NaCl templating mechanically stable carbon monoliths with both meso- and macroporosity can be prepared. The advantages of the current synthesis is the use of furfuryl alcohol both as the carbon source and the binder, as well as the inexpensive, thermally stable and water soluble NaCl as macropore generator. This approach can be extended to prepare other carbon monoliths containing differently ordered mesopores and macropores, by judicious

Acknowledgments

We are grateful to Mr. Bongard for SEM measurements and Mr. Spliethoff for TEM measurements. The financial support from the DFG via the Leibniz-Program, in addition to the basic funding provided by the MPG, is gratefully acknowledged.

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