humidity.
3. Results and discussion
3.1. Physicochemical characterization of silicas
The results of surface area, pore volume, mean pore diameter and pore size distribution are shown in Table 2 and Fig. 1. An
adsorbent material must have high specific surface area to enable the adsorption of greater amounts of impurities and a suitable pore structure to allow the access of all impurities to its internal surface without difficulty. This latter topic is critical for large molecules (soaps, mono, di and triglycerides) with sizes similar to those of the pores of the adsorbent.
Isotherms obtained for the three silicas (not shown) can be classified as type IV, according to IUPAC nomenclature [17]. This type of isotherm is characteristic of mesoporous solids (pore diameters between 20 and 200 Å) [18]. Silicas have high specific surface area values (approximately 600 m2 g1) and silica Trisyl 3000 presents the highest value of surface area. The pore size distribution is unimodal for all samples (see Fig. 1) with pore diameters between 25 and 125 Å. Therefore the structures are purely mesoporous. The wide pore network of the silica was thought to present no problems for diffusion of the adsorbates. The approximate length of some molecules was included in Fig. 1. It can be seen that the sizes of soaps and TG are smaller than the average pore diameter of the network (Table 2). Then, it can be concluded that the silicas have high specific surface area and a suitable porous structure for use in biodiesel purification processes by selective adsorption.
The results of TGA are shown in Fig. 2. A significant mass loss can be seen in the three cases, which starts at temperatures near 40 C and ends at temperatures of 110–120 C. The loss of mass is greater in samples 450 and 300B (40 and 50% respectively) and lower in sample Trisyl 3000 (10%). This loss is attributed to the release of water adsorbed on the surface. A subsequent