A theoretical study of the role of (CTA+)(SiO-) ion pair on the catalytic activity of [CTA+]-Si-MCM-41

ALEGRE, C.; ZALAZAR, M. F; CAZULA, B.B.; ALVES,H.J.; PERUCHENA, N. M.

Santa Fe

Conferencia; VI San Luis School and Conference on Surfaces, Interfaces and Catalysis; 2018

INTEC-CONICET y UNL

Previous experimental works reported the use of MCM-41 without calcination, where the surfactant or the organic structure-directing agent (SDA) has been kept occluded, as a basic solid catalyst potentially usable for several reactions.1-3 In a recent study Ranucci et al.2 found good yields for the reaction of transesterification of vegetable oils using heterogeneous MCM-41 catalysts in the non-calcined form, denoted [CTA+]-Si-MCM-41 molecular sieves, where the cetyl-trimethylammonium cation (CTA+) from the structure-directing agent was not removed from the formed catalyst. Further results also showed that the catalyst without the occluded surfactant and the surfactant itself does not possess catalytic activity when were used separately, suggesting that the interaction between CTA+ and mesoporous silica is responsible for the catalytic activity in the transesterification reaction.4 Calcination caused a total loss in catalytic activity due to the removal of CTA+ cations from the mesoporous solids. Hence, such activity was associated with the formation of (CTA+)(SiO-) ion pair at the surface of the solid catalyst, and was postulated that the basic active sites resided at the pore mouth. But, despite good results, there is no further information available on the basic properties of the formed catalyst. Therefore, a theoretical study from ab initio calculations and the electron density analysis can be very useful to understand the role of the (CTA+)(SiO-) ion pair on the catalytic activity of non-calcinated form of MCM-41 catalyst. The study was performed in the framework of density functional theory (DFT) and atoms in molecules theory (QTAIM). The model surface was built using a CTA+ cation coupled on the silicate cluster. The silicate cluster models were built based on reported pure Si MCM-41 silanol-terminated surfaces.5 DFT calculations were performed at B3LYP/6-31G(d,p) level using the Gaussian 09 program. All stationary points were characterized by calculating the Hessian matrix and analyzing the vibrational normal modes. Electron density properties were calculated at B3LYP/6-31++G(d,p) level using the AIMAll software. In the present work we report the main results of our theoretical study of the (CTA+)(SiO-) ion pair used as a model surface of [CTA+]-Si-MCM-41 catalyst. DFT methods and electron density analysis have been applied in order to provide information that could be helpful in the interpretation of the experiments about transesterification reaction.