Avaliação de modificações pós-síntese em zeólita ZSM-5 comercial aplicada na esterificação de ácido oleico com acetato de metila

Abstract

Zeolites are considered the most important catalysts in industry due to their unique properties. Among these materials, ZSM-5 highlights because of its stability and versatility, presenting a wide range of applications. Usually employed in processes related to petrochemical industry, ZSM-5 is also a potential catalyst of esterification reactions, within the context of researching alternative routes for biodiesel production. In this scenario, the realization of present work had as objective to promote different post-synthesis treatments over commercial ZSM-5 zeolite with high Si/Al ratio, evaluating the influence of such treatments on zeolite physicochemical properties and on its catalytic activity in esterification reaction of oleic acid with methyl acetate. The treatments employed were ion exchange, desilication, acidification and active phase impregnation (tungsten oxide) via mechanical stirring and ultrasound-assisted. The prepared catalysts had their textural, morphological and acidity properties characterized through techniques of X ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electronic microscopy (SEM) and characterization of acidity properties via infrared spectroscopy after adsorption of liquid pyridine. Esterification reactions were carried out at 240 °C, molar ratio equal to 1:10 (oleic acid:methyl acetate) and 10% (m/m) of catalyst, in relation to mass of oleic acid. The treatments performed were responsible for changing catalysts properties, as it was verified by characterization results. Besides that, post-synthesis modifications have influenced results of esterification reactions, being responsible for different yields and distribution of cis and trans isomers of oleic acid methyl ester (methyl oleate and methyl elaidate, respectively). The highest yield, equal to 66.71%, was achieved with zeolite subjected to ion exchange. With this catalyst, an egalitarian distribution between both isomers products was observed. From quantification of the two isomers and evaluation of their formation kinetics, it was possible to propose a reaction mechanism based on oleic acid isomerization, which is converted into elaidic acid, followed by esterification of both fatty acids to form the correspondent esters.