Síntese, caracterização e atividade catalítica do CU3(MOO4)2(OH)2 na esterificação do ácido oleico

Abstract

The use of petroleum-derived fuels as a major component in the global energy matrix has motivated the search for alternative sources of energy. In this context, biodiesel is a green fuel obtained from renewable sources, capable of totally or partially replacing petroleum diesel. Biodiesel is formed by a mixture of monoalkyl esters obtained mainly by transesterification in the presence of homogeneous catalysts. One of the main obstacles to biodiesel production is the total production cost, commonly associated with the price of the raw material, separation processes of the catalysts from the final product and the formation of soaps as by-products in transesterification. In this sense, in the present work the esterification of oleic acid with methanol was carried out using new heterogeneous catalysts, obtained by low cost methodologies. The [Cu3(MoO4)2(OH)2] catalyst was synthesized via sonochemistry at 50 °C temperature for 1 hour. The structural characterization by X-ray diffraction indicated the presence of lindgrenite with monoclinic structure (space group P21/n ) for the synthesized sample. The Rietveld refinement of diffraction data for this material revealed the cell parameters a = 5,3879 Å, b = 14,0078 Å, c = 5,6058 Å, β = 98,424° and V = 418,519 Å. The vibrational modes identified by Infrared and Raman spectroscopy corroborate the results obtained in the structural characterization. Scanning electron microscopy images of lindgrenite catalyst revealed the presence of “urchin-like” structures, composed by nanometric plates of [Cu3(MoO4)2(OH)2]. The catalytic performance of lindgrenite evaluated in the esterification of oleic acid with methanol resulted in conversions greater than 96 % using optimized experimental conditions: molar ratio oleic acid:methanol of 1:5, temperature of 120 °C, weight catalyst percent of 5 %. The activation energies of the esterifications carried out with and without catalyst were 22,1 kJ mol^-1 and 43,6 kJ mol^-1, respectively. The thermodynamic parameters of the oleic acid esterification reactions using lindgrenite were ΔG= -0,29 kJ mol^-1 (at 140 °C), ΔH= 67,05 kJ mol^-1 e ΔS= 0,165 kJ K mol^-1 . The proposed esterification mechanism suggests that the predominant action of Brønsted sites present on the surface of lindgrenite catalyst contributes substantially for the occurrence of oleic acid esterification with methanol.