Síntese de peneiras moleculares MCM-41 ativas em catálise básica para a produção de chalconas

Abstract

Samples of MCM-41 mesoporous molecular sieves were synthesized from a gel presenting the following molar composition: 1,0 SiO2: 0,12 CTABr: 0,28 TMAOH: 100 H2O. The gel was hydrothermally treated or at 100°C for 24 hours or at 150°C for 5 days, forming Si-MCM-41 samples. After, calcining at 540°C, for removing surfactant, freeing the porosity, these samples were functionalized by reaction with two amines sources: 3-trimethoxysilylpropylamine and 3-trimethoxysilylpropylenediamine. The grafting process was shown to be very efficient, incorporing amounts of propylamine and etilenediaminepropyl groups near theoretical values. These samples were characterized by X ray diffraction (XRD), potentiometer titration, thermogravimetric analysis (TGA) and scanning electronic microscopy (SEM). From these analyses, it could be seen the samples presented well ordered MCM-41 structures with different pore size. The samples obtained from the 150°C for 5 days synthesis was formed by 2 μm particles while the one obtained from 100°C for 24 hours synthesis was formed by particles lower than 1 μm. The samples were catalytically evaluated by using the Claisen-Schmidt reaction, carried out at different temperatures (105, 140 and 165°C), to form chalcone and substituted chalconas. For the samples presenting propylamine groups grafted on their surface, it was observed that the activity increased with temperature increased. However, the etilenediaminepropyl grafted sample presented a maximum in activity at a temperature of 140°C. This reduction could be explained considering that these samples presented two basic sites very near each other, capable of promoting bimolecular degradation reaction, forming carbonaceous deposits in the sample surface, deactivating them. A dark color in the reaction medium evidenced secondary compounds formation. It was observed, also, that the samples presenting higher pore size were more catalytic actives and that this activity difference was more evident at higher temperatures.