Hidróxidos duplos lamelares e sílica gel à base de escória de alto forno, com ênfase na aplicação do HDL na catálise do óleo essencial de Conobea scoparioides da região Amazônica

Abstract

Lamellar double hydroxides (LDHs) and silica gel were obtained from the residue steel blast furnace (SBF) from the municipality of Maraba-PA. The SBF when digested in HCl resulted in two phases. The soluble phase (solution SBF) was used to synthesize the LDHs, while the solid phase is characterized by different techniques. The LDHs were synthesized by coprecipitation with increasing pH by mixing an aqueous NaOH solution with SBF and MgCl2.6H2O, establishing a theoretical molar ratio Mg/Al = 3.5. The variables investigated were the synthesis temperature (30 to 45 °C) and pH (9 and 12). The HDLs showed a hydrotalcite like structure. The LDHs synthesized at 30 °C showed characteristics of the system Mg-Al-CO3 while synthesized at 45 °C were belonging to the system Mg-Al-CO3-Cl. The pH values directly influenced the crystal structure of LDHs. The LDHs synthesized at higher pH values shown greater degree of structural ordering and highest values of the unit cell parameters a and c and basal spacing d (003), besides having higher Mg content. The resulting solid phase from the acid digestion of SBF, silica-coded EAF, showed amorphous character, typical of amorphous silica. The EAF-silica showed similar properties as those of commercial high purity silica, with a 99.65%-weight of SiO2, moisture free. It showed a high specific surface area (282 m2/g), high affinity for physio-sorbed water (12,27%) and high amount of water content (6.18%). It has extremely fine particle sizes, forming agglomerates of varying sizes, some of them smaller than 1 μm with morphology typical of porous material. The particles of EAF-silica exhibited unimodal size distribution and good uniformity with a mean diameter D50 equal to 7,0 μm. With the intent to find an application for LDHs based SBF, the EAF hydrotalcite-HDL-4 belonging to the system Mg-Al-CO3-Cl, with a molar ratio Mg/ Al: 3.2 and porous morphology was evaluated as a catalyst in the conversion between the major constituents of the essential oil of C. scoparioides, an aromatic herb from the Amazon region known as “pataqueira”. The EAF-4-HDL showed a strong influence on the conversion of α-phellandrene, ρ-cymene, methyl ether of thymol and thymol, with high selectivity to the formation of thymol. The EAF-HDL-4 in contact with the oil sample in EtOH showed no significant catalytic activity. However, in the samples of the oil in EtOH: water showed a decreasing trend in the percentage of α-phellandrene, ρ-cymene and thymol methyl ether, with increasing water content, while the percentage of thymol has been increased. It was found that, in the samples analyzed after contact with the EAF-HDL-4, the percentage of thymol was above those in the respective control samples, with values increased between 2.69% to 86.75%. Based on these data, we suggest that the conversion between the major constituents was favored by water and catalyzed by EAF-HDL-4. We propose that both the α-phellandrene and ρ-cymene and thymol methyl ether were converted to thymol, increasing it percentage in the essential oil of C. scoparioides that was 41.62% and after contact with the HDL went up to a percentage of 95.02% thymol.