Direct synthesis of magnesium doped mesoporous silica: Their catalytic activity in Knoevenagel Condensation

Abstract

Mesoporous magnesium silicate has been synthesized by co-condensation method using unconventional source of magnesium, namely magnesium ethoxide, in the presence of supramolecular assembly of a mixture of surfactants. Three different samples have been synthesized with magnesium to silicon ratio of 1:100, 1:50 and 1:10. The samples produced had high surface area and uniform pore size distribution and the loading of magnesium in each case was in conformity with the amount of magnesium introduced in the reaction gel during the syntheses of the respective samples. The nanostructures of the materials have been characterized by powder X-ray diffraction, transmission electron microscopy and nitrogen adsorption-desorption studies. The insertion of magnesium into the silica framework has been confirmed by UV-visible spectroscopy whereas the magnesium content of the samples has been estimated by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Catalytic studies revealed high activity of these magnesium-doped mesoporous silica in Knoevenagel condensation of benzaldehyde, 1-naphthaldehyde and 2-hydroxy naphthaldehyde with ethyl cyanoacetate. Knoevenagel reaction takes place in presence of a base and in this case the alkaline earth metal sites act as those basic centres. Such high catalytic efficiency cannot be achieved over basic sites in conventional microporous zeolites due to space constraint. The catalysts were found to be reusable for up to three reaction cycles and their efficiency can be attributed to the improved mass transfer rate which arises due to the presence of mesopores. Hot filtration test carried out for a representative reaction further established the heterogeneity of the catalytic reaction.