The production of carbon materials by hydrothermal carbonization of cellulose
Introduction
The thermal treatment of water mixed with organic substances such as saccharides (glucose, sucrose or starch) or simpler compounds such as furfural at temperatures in the 150–350 °C range (autogenous pressure) gives rise to water-soluble organic substances and a carbon-rich solid product. This process, termed hydrothermal carbonization, has generated widespread interest in recent years [1], [2], [3], [4], [5], [6]. However, the hydrothermal treatment of saccharides is not a new process. Experiments involving the hydrothermal carbonization of different types of saccharides were carried out during the first half of the twentieth century in order to obtain information about the mechanism of natural coalification [7], [8], [9]. Later on, numerous papers dealing with the hydrothermal treatment of cellulose to obtain liquid fuel or chemicals appeared [10], [11], [12], [13], [14]. Not until recently, however, has any attention been paid to the solid residue resulting from this treatment. Wang et al. were the first to report on the hydrothermal treatment of sucrose as a way to produce carbonaceous microspheres (size ∼1.5 μm) [1]. Subsequently, Sun and Li reported on the preparation of carbonaceous microspheres of a tunable size (0.2–1.5 μm) loaded with noble-metal nanoparticles [3]. Other groups have taken advantage of the oxygen functionalities present on the outer surface of the carbonaceous microspheres and employed them as sacrificial templates to fabricate hollow spheres of inorganic materials (Ga2O3, GaN, WO3, SnO2, etc.) [3], [15], [16], [17], [18], [19]. We have previously investigated the use of carbonaceous microspheres produced by the hydrothermal carbonization of glucose, sucrose and starch as precursors for the production of graphitic carbon nanostructures and the application of these nanocarbons as electrocatalyst supports [20], [21].
Among the potential saccharides that can be employed to produce carbonaceous materials through hydrothermal carbonization, cellulose is the most promising material as it is by far the most abundant and inexpensive saccharide available. Although several authors have reported using cellulose for the production of carbon materials by means of pyrolysis at high temperatures [22], [23], [24], to the best of our knowledge, no one has employed cellulose to produce carbonaceous materials by means of hydrothermal carbonization. In the present work we investigate the potential of cellulose as precursor for the production of highly functionalized carbonaceous materials via hydrothermal carbonization. The chemical and structural characteristics of the hydrothermally carbonized products are also investigated.
Section snippets
Hydrothermal treatment of cellulose
The hydrothermal carbonization of cellulose was carried out according to the following procedure. Between 2 and 16 g of cellulose (Aldrich) was dispersed in water (50 mL) and stirred for 4 h. The ratio volume of solution/volume of autoclave is around 0.3. The mixture was then transferred to a stainless steel autoclave fitted with a stirring mechanism, heated up to a temperature in the 200–250 °C range and maintained at this temperature for 2 or 4 h. The resulting solid product (here denoted as
Structural properties
Fig. 1 shows SEM images of the cellulose (Fig. 1a) and products obtained by hydrothermal treatment at 210 °C (Fig. 1b), 220 °C (Fig. 1c), 230 °C (Fig. 1d) and 250 °C (Fig. 1e and f). It should be noted that, whereas the products from hydrothermal treatment at 210 °C exhibit an irregular morphology similar to that of pristine cellulose, the samples obtained at temperatures ⩾220 °C consist mainly of aggregates of microspheres with a diameter in the 2–10 μm range. The SEM images (Fig. 1) evidence that
Conclusions
In summary, we have presented a procedure for obtaining a carbonaceous product, denoted here as hydrochar, by means of the hydrothermal carbonization of cellulose. The hydrochar is made up of micrometer sized spheres (size ∼2–10 μm). The onset of carbonization under hydrothermal conditions takes place between 210 °C and 220 °C. From a chemical point of view, the hydrochar contains a high amount of oxygen (∼23–24 wt.% of oxygen), which is present both in the core and in the shell of the carbonaceous
Acknowledgments
The financial support for this research work provided by the Spanish MCyT (MAT2008-00407) is gratefully acknowledged. M.S. acknowledges the assistance of the Spanish MCyT for the award of a FPU grant.
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