Efeito da acessibilidade aos sítios ácidos da Zeólita MFI na esterificação catalítica

Abstract

In the context of increasing ethanol production through fermentation, substantial amounts of fusel oil are produced. Consequently, there is a growing interest in exploring methods to reuse fusel oil by transforming its major component, isopentanol, into valuable industrial products. One potential approach involves the conversion of isopentanol into isopentyl acetate, an ester widely utilized in the food industry for flavor enhancement. Viable mechanisms for effecting this conversion is to apply esterification reactions, facilitated by a protonic catalyst. Zeolites, noted for their acidity arising from Brønsted sites, have been recognized as effective catalysts for this purpose. Recent research has revealed that the main factor that enhances catalytic efficacy in esterification reactions is the expansion of the external surface area of zeolites due to constraints associated with molecular diffusion within the micropores of MFI zeolite. In this context, the present study aim to investigate how the accessibility to acidic sites of MFI zeolite can be improved by reducing particle size to obtain isopentyl acetate. In order of this, several parameters were explored, including the aging time of the reaction mixture during the synthesis of MFI zeolite with n-butylamine (BTA) and tetrapropylammonium cations (TPA+), Si/Al ratio, and the hydrothermal treatment time. The analyses, including N2 physisorption and SEM results, demonstrated that prolonging the aging time in both BTA and TPA+ syntheses led to smaller particle sizes while maintaining the chemical composition of the zeolites relatively constant. Additionally, the research established that TPA+ cations were more effective in producing smaller particle sizes, and lowering the Si/Al ratio from 25 to 15 also resulted in smaller particle diameters. Thus, adjusting these synthesis parameters permits the variation of the particle size of the zeolites. Regarding the observed catalytic activity, we observed that zeolites synthesized with TPA+ and smaller particle sizes, resulting in a larger external surface, achieved higher conversion rates. It occurs due to molecules having better access to catalytic sites. Conversely, samples synthesized with BTA, having a smaller external surface area, achieved similar conversion rates. Nevertheless, a comparative analysis of samples from BTA and TPA+ syntheses has revealed that those derived from BTA synthesis exhibit superior TOF0, despite their lower aluminum content. Therefore, it demonstrates that the performance of the reaction is mainly affected by the external surfaces acidity profile.