Obtenção do aluminato de zinco e óxido de zinco através do método hidrotérmico assistido por micro-ondas para aplicação como catalisador na síntese de biodiesel.

Abstract

The transesterification of vegetable oils for the production of alkyl esters (biodiesel) can be used with a potential substitute for exhaustible diesel oil. Several communications has focused on the development process eco-friendly involving heterogeneous catalysts, they suppress neutralization and separation steps. An effective methodology to obtain catalysts powder with different properties with respect the structural, textural and morphological characteristics is the hydrothermal method assisted by microwave energy (HMO). The microwaves energy couple directly with the material, resulting in a volumetric heating of the material more quickly. Thus, the aim of this work was to prepare and characterize the nanostructures of zinc aluminate and zinc oxide using the new HMO method, varying mineralizing agents, time, temperature and PEG assisted or not, as well as catalytic tests with these materials in bench scale in the transesterification reaction of commercial soybean oil (biodiesel). Under the conditions of synthesis evaluated, only using the mineraliser urea was possible to obtain the zinc aluminate. This was first synthesized at a temperature of 180 °C in treatment times of 12, 8, 2, 1 and 0.5 hours, and it was observed that the time did not affect the structural properties, textural and morphology of this material. The ZnAl2O4 was obtained at a lower temperature in the literature (150 °C) in time 0.5 and 0.25 hours, with or without addition of PEG. PEG did not affect the structural, morphological and textural samples ZnAl2O4. However, all samples of the ZnAl2O4 exhibited high surface areas, varying from 226 to 266m2.g-1. The another material, zinc oxide was synthesized at a temperature of 100°C in periods of 30 minutes varying the agent mineraliser without cooling the system and within five minutes with cooling hydrothermal system assisted by microwave, by varying the agent mineraliser and the use of PEG in the precursor solution. The materials were characterized by XRD, SEM and textural analysis. It was observed that depending on the mineralization agent, it was possible to control the morphology of the powder. These materials shows a lamellar aggregates having a morphology resulting in nearly spherical when the was used mineraliser sodium hydroxide, and morphology of agglomerates which result in nanoflowers when the ammonium hydroxide was used. In the case of ZnO, the addition of PEG resulted in a decrease in surface area of the samples obtained in five minutes (super rapid synthesis). It can be concluded that the use of hydrothermal system assisted by microwave energy, was possible to obtain ZnAl2O4 with high surface area and ZnO with differentiated morphology, both single phase at low temperatures and shorter times synthesis, showing that the system is efficient for obtaining these materials. Both materials present a catalytic activity in the methyl transesterification of soybean oil.