Síntese e caracterização de hidróxidos duplos lamelares de Mg/Fe com esferas de carbono magnéticas

Abstract

This research aimed to synthesize a stable magnesium/iron carbonate (Mg//Fe-CO3) lamellar double-hydroxide (LDH) compound with magnetic carbon spheres (MCS) in order to evaluate composite’s characteristics. LDHs are synthetic or natural anionic clays which structures are constituted of positively charged octahedral lamellas with intercalated di- and trivalent cations, and hydroxyls in the vertices. Water and anions are also in the inter-layered space. Spheric carbon products are known for a long time and the application of carbon nanomaterials is possible in many areas. The formation process of this material is hydrothermal, which does not result in singular products. Its results are big interconnected chains of carbon spheres (CS) attracted by Van der Waals forces. Chemical reagents of analytical purity were utilized in the synthesis. Mg/Fe-CO3 LDHs were synthetized by co-precipitation method with variable pH. And the hydrothermal method was applied to prepare the carbon spheres. The characterization of LDH, CS, MCS and LDH + MCS composite included analysis by X ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), infrared vibrational spectroscopy and Brunauer-Emmet-Teller method (BET) for specific surface area and pore size measurement. XRD analysis identified pyroaurite’s structure in the HDL samples and magnetite’s structure in the MCSs. The average values of pyroaurite’s unit cell, a = b parameters, interplanar distance (c) and interlamellar spacing thickness were 7,792 Å, 3,107 Å, 23,375 Å and 2,992 Å, respectively. Photomicrography was performed only in CS and MCS samples. It was possible to observe big agglomerates and chains of these materials, and the size variation between different samples and synthesis as well. Thermic decomposition of synthetic pyroaurite and the composite occurs in a similar way, which can be divided in five stages. Loss of adsorbed water and intercalated water elimination happen in the first (31-72 °C) and second (175-177 °C) stages, respectively. Then, between 284 °C and 395 °C, there is decarbonization and desidroxylation. Finally, the destruction of LDH’s structure happens at 746 – 786 °C. Intercalated anion in LDH’s structure was determined as a carbonate anion (1365 cm-2) by infrared vibrational spectroscopy. Carbonate anion and water fill the inter-layered spaces of LDH and LDH + MCS composite. Both LDH and LDH + MCS composite show infrared isotherms and type H3 hysteresis, according to IUPAC’s classification. Superficial area increased by 15-31%, while the size and volume of pores decreased by 44-49% and 26-42%, respectively.