Aplicação de peneiras moleculares contendo cátions orgânicos em catálise básica

Abstract

This thesis was aimed at studying novel basic catalysts based on molecular sieves containing alkylammonium cations. The studied molecular sieves were faujasite zeolites (FAU) and mesoporous silicate (like MCM-41). The following parameters were evaluated: (1) different alkylammonium cations, (2) different amount of alkylammonium cations and (3) application of these catalysts in base catalyzed reactions. The study and application of these novel basic catalysts was a completely unexplored field, however one expects that the outstanding properties of these materials, recently revealed, may drive their application in the production of chemical specialties. The novel catalysts were compared in of Knoevenagel and Claisen-Schmidt condensation reactions and in the addition reaction of propylene oxide with methanol. During the preparation of the FAU zeolite containing alkylammonium cations the exchange equilibrium was readily achieved, even when using the most voluminous cation (CH3)4N+. For spatial reasons none of the alkylammonium cations exchanged all sodium cations from FAU zeolite, since the ion exchange was restricted to the super cage. Additionally, all cations were verified to be located in ion exchange sites, therefore the basic character arises essentially from the oxygen anions belonging to the zeolite framework. The novel catalysts showed a superior activity in the reactions in comparison to the zeolite containing cesium. Cesium, in the cationic form Cs+ exchanged in zeolites or dispersed as Cs2O, is the most studied element in the literature to generate strong basic sites, however under high costs. On the other hand, alkylammonium cations are of low cost. The mesoporous molecular sieve [CTA+]-SiMCM-41 (CTA+ is the structure directing agent) was used as synthesized and showed much higher performance. In this catalyst, basic sites were identified as siloxy anions in association to CTA+ cations (≡SiO-CTA+). Nonetheless, as pores were still occluded by the structure directing agent reaction took place necessarily in the pore mouth. These novel results open new perspectives of application of these molecular sieves, highly basic and of low cost.