Estudos sobre a acessibilidade aos sítios de zeólitas para aplicação em catálise básica

Abstract

The application of zeolites in catalysis is restricted when bulkier molecules are involved, for example, in the manufacture of fine chemicals, where the reactant molecules are unable to access their micropores. A solution to this problem could be the synthesis of zeolites containing mesopores to improve accessibility to sites. Zeolites have intrinsic basicity, due to residual negative charges in their framework, which are generated by the presence of trivalent aluminum atoms with tetrahedral coordination. The functionalization of the surface of zeolites with amine groups, through the anchoring of silanes containing basic nitrogenous species, is one strategy to improve their basicity. In this context, this work studied the synthesis of 4A zeolites through the constructive method (bottom-up) using the organosilane surfactant [3 (trimethoxysilyl) propyl] octadecyldimethyl ammonium chloride (TPOAC) as mesopores generator at different concentrations in the synthesis mixture (TPOAC/Al2O3= 0-0.09). These zeolites were evaluated in the Knoevenagel condensation between benzaldehyde and ethyl cyanoacetate, which is influenced by the accessibility to the catalytic sites, due to the volume of the molecules involved. This study also evaluated the improvement of the basicity of zeolites through the functionalization of their surface with amine groups, for this, the zeolite containing the highest amount of mesopores, and the conventional zeolite were treated with aminopropil-trimetoxisilano (APTMS). 4A zeolites with mesopores were formed in the presence of TPOAC at the different concentrations evaluated, showing an increase in the external surface. The benzaldehyde conversions were higher when using zeolites with mesopores instead of conventional 4A, due to the improved accessibility of the to the catalytic sites. In addition, the reaction catalyzed by zeolite with mesopores has a lower activation energy than that catalyzed by conventional 4A zeolite. It was not possible to anchor APTMS on the surface of the conventional zeolite, probably due to the low external surface and consequent lack of available silanol groups that can react with the silane. On the other hand, the functionalization of zeolite with mesopores was possible, due to its high surface that provides a higher amount of silanol groups compared to the external surface of the conventional zeolite crystal. Consequently, zeolite with mesopores treated with APTMS has superior catalytic activity in Knoevenagel condensation, due to improved accessibility and addition of catalytic sites.