A Carbon Macro-network From the Controlled Pyrolysis of Saccharose

Abstract

Well de®ned, molecular networks such as fullerenes and nanotubes have been some of the most remarkable discoveries in the area of carbon research in recent years. Another non-well de®ned carbon network, such as that represented by the microporous structure of activated carbons, a material with many applications [1], is assumed to be constituted by polyhexagon sheets of carbon forming micropores and mesopores of widths around 2 nm [2, 3]. In addition, other networks have been observed by microscopy in activated carbons [4]. In this case, the macropore (. 50 nm width) structure observed is originated from the preservation after carbonization of the biological tissue skeleton of the raw material (wood, coconut shell, etc.). Accordingly, three carbonaceous network structures can be considered according to the scale: the molecular (fullerenes, nanotubes), and micro (microporous carbon), and the macro (macroporous carbon). Although in the case of carbon materials, synthetic routes for both molecular-networks and micro-networks have been extensively investigated, few studies exist regarding the synthesis and characterization of macronetworks. This communication presents the synthesis and de®nition of the topological organization of a carbonaceous macronetwork obtained from a step-wise heat treatment of saccharose. A solution composed of 1 ml of water plus 2 g of D(‡)-saccharose (Merck, bacteriology grade) was placed inside a common pyrex 100 ml beaker. This was submitted to two consecutive heat treatments: First, a low temperature stabilization inside an oven at the following temperatures: 60, 90, 110, and 130 8C, for 10 min at each temperature, using heating rates of about 3 8C miny1. Secondly a heat treatment consisting of a carbonization under an inert (nitrogen) ow using the following step temperatures: 50, 100, 150, 200, 250, 350 8C, leaving 30 min at each temperature, and ®nally 1 h at 450 8C, using heating rates of about 5 8C miny1. This ®nal temperature was chosen as this had been found to be the optimun for the preparation of lowtemperature (L-type) activated carbon from lignocellulosic raw materials [5, 6]. A statistical analysis of the macronetwork obtained was carried out as described in previous studies for other cell structural systems [7]. The macronetwork material was submitted to other measurements: N2 adsorption at 77 K to obtain the BET surface area, employing a Micromeritics ASAP-2000 apparatus; temperature-programmed reduction (TPR) with H2 employing a Micromeritics TPR=TPD 29000 apparatus; and elemental analysis with an Eager 200 apparatus. After carbonization, a macronetwork ®lm was obtained that adhered both to the bottom of the beaker and around the cylindrical beaker wall, covering approximately one ®fth of the beaker height. This network was constituted by a net of cells in the form of polygonal rings (Fig. 1), each ring formed by grains of carbonaceous material. Polygons from three up to eight sides were identi®ed. In addition, an irregular sponge ball featuring a similar structure to that seen in the ®lm was also obtained in the bottom of the beaker. This was apparently loosely connected to the ®lm network, i.e. it changed position when the beaker was shaken. Therefore, both twoand three-dimensional networks were obtained simultaneously. Also, the noticeable separation of the grains constituting the polygons (Fig. 1) suggest weak linking, which most probably caused the easy disconnection of the sponge ball from the ®lm adhered to the wall. During carbonization, there was a remarkable