Synthesis, characterization and gas sorption properties of a molecularly-derived graphite oxide-like foam
Introduction
Graphite oxide is an oxygen-rich carbogenic material that is typically derived by strong oxidation of crystalline graphite and contains oxygen in the concentration range 30–40% w/w. The functional solid exhibits an extended lamellar structure with randomly distributed aromatic and aliphatic regions, as well as, a high amount of hydroxyl/carboxyl functional groups embedded on its layers. Accordingly, graphite oxide is endowed with interesting swelling, intercalation and ion exchange properties that enable the design and engineering processing of valuable reconstructed derivatives and thin films [1], [2], [3], [4], [5], [6]. In addition to these properties, the layered solid also exhibits a remarkable thermal behaviour: it easily decomposes at relatively low temperature (<200 °C) by releasing CO2 and H2O to afford lightweight carbogenic soot [7], [8]. Although the carbogenic soot lacks the colloidal properties of traditional graphite oxide, however, it still has a layered structure (i.e. bundles of graphene layers with d002 = 0.4 nm), it contains a residual amount of oxygen mainly at the periphery of the layers (5–10% w/w) and it is built up of aromatic (sp2-hybridized carbon) and aliphatic domains (sp3-hybridized carbon), similarly to conventional graphite oxide. As such, the lightweight graphite oxide-like derivative represents an alternative engineering form that holds promise in a range of important applications, including lithium batteries, insulators and absorbents [7], [8]. Nevertheless, the small surface area of the lightweight product (<50 m2 g−1) imposed by the large lateral dimensions of the graphite oxide precursor (>1 μm) combined with its low oxygen content, which provides binding sites to the surface, limits further potential uses.
In this respect, the design of lightweight and high surface area graphite oxide-like materials (i.e. foam-type) with oxygen content in the concentration range of traditional graphite oxide would be much recommended in order to give access to new carbon-based functional materials and reconstructed derivatives suitable for certain demands. To this aim we present here the synthesis, characterization and gas sorption properties of a molecularly-derived graphite oxide-like foam that is lightweight (dbulk ∼ 0.03 g cm−3), exhibits high specific surface area and porosity (510 m2 g−1, 67%), has enriched oxygen composition (39% w/w) and exhibits an unusually high spin concentration that may give rise to additional functionalities/reactivity. The synthetic graphite oxide-like foam is simply derived by mild pyrolysis under air of the crystalline molecular precursor chloranilate disodium salt. Of particular importance are the gas sorption properties of the material. The carbogenic foam selectively absorbs CO2, showing low affinity for N2, CH4 and H2. Worth noticing, the neat graphite oxide-like foam strongly sorbs CO in the absence of metal catalysts. Accordingly, the material holds promise in certain gas separation processes, like removal of CO2 and CO from reformer/shift reactor hydrogen mixtures (H2, CH4, N2, CO2, CO) and natural gas upgrade (CH4, CO2).
Overall, the method is advantageous in terms of facile synthesis and novel properties. In the context of functional carbogenic foams, a direct access through mild pyrolysis in air of commercially available, low cost and easy handled molecular precursors would be of great value since current methods employ more complex chemical or/and physical means, e.g. condensation polymerisation and carbonization, freeze drying or laser ablation [9], [10].
Section snippets
Experimental
In a typical procedure, 2.2 g chloranilic acid, disodium salt dihydrate (Aldrich) was calcined at 300 °C in air for 2 h (heating rate: 10 °C min−1). The obtained carbogenic foam was copiously washed with water and acetone prior drying at 65 °C for a day. The material is denoted as CA300. Yield: 5%. Color: black. Elemental analysis (w/w): C, 58.5%; H, 2.5%; O, 39%.
XRD patterns were recorded on a Siemens XD-500 diffractometer using CuKα radiation. The patterns were collected using background-free
Synthesis and texture
The synthetic graphite oxide-like foam is simply derived from chloranilate disodium salt via mild pyrolysis in air at 300 °C, i.e. near the decomposition temperature of the salt (290 °C) (Fig. 1). Elemental analysis of CA300 revealed the average composition C2OH, which corresponds well to that of traditional graphite oxide [6]. However, the material lacks the colloidal properties of the conventional solid. The foam can be rationalized as small bundles (crystallites) of graphene layers that are
Conclusions
Air pyrolysis of sodium chloranilate at 300 °C affords a graphite oxide-like foam that is lightweight, has high specific surface area and porosity, contains oxygen in the concentration range of traditional graphite oxide and possesses a high radical concentration. The abrupt thermal decomposition of the molecular precursor above 290 °C in conjunction with its layered-like packing in the bulk crystal favor the formation of a porous network of interconnected graphene bundles (crystallites) built up
Acknowledgement
Partial funding by the European Commission (FP6 contract SE56-518271/NESSHY) is gratefully acknowledged by the authors.
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