Estudo de embriões de zeólitas como catalisadores básicos na condensação de Knoevenagel

Abstract

The accessibility to the catalytic sites is one of the main objectives involving the development of heterogeneous catalysts. In this context, several researches seek the reduction of crystal diameters or the creation of mesopores in zeolites. The present work presents a new way of increasing the accessibility to the sites through the synthesis of embryonic zeolites. This method consists of obtaining structures that have a short range order, which improves mass transfer and leads to better catalytic results, especially in the processing of larger molecules. For this, the study of formation of FAU, LTA and SOD structures was carried out, associated with the catalytic evaluation of Knoevenagel condensation between benzaldehyde and ethyl cyanoacetate. The synthesis of the microporous sieves was performed under various times at 60 °C and all materials obtained (with and without crystallinity) were evaluated as catalysts. The highest conversions of the condensation reagents were always achieved by the zeolite embryos, regardless of the structure that originated. This higher activity is related to the secondary units of construction of the zeolite structure, as observed by Raman spectroscopy, these units are formed during the induction period. While the zeolite structure is in formation all the catalytic sites are accessible to the reactants, as the crystallization process is initiated, some of the sites become inaccessible and consequently a reduction in catalytic activity is due. By means of the CO2 TPD technique, zeolites with high crystallinity have a higher number of sites compared to their homologues containing only the embryos, but the catalytic activity does not reflect this trend. That is, as verified by N2 physisorption, embryonic zeolites have no micro- or mesoporosity and hence bulky molecules of reagents have access to all sites. The presence of aluminum with tetrahedral coordination in all embryonic zeolites was evidenced by NMR technique. The zeolite embryos that gave rise to the LTA structure prove to be the most active catalyst of their FAU and SOD homologues. This higher activity is related to the fact that this catalyst has a higher content of aluminum in its network, that is, a larger number of basic sites. When performing XPS measurements and calculating activity by site (TOF0), it was found that the catalytic activity of the zeolite embryos is mainly dependent on the number of sites.