(578f) Computational Investigation of Lewis and Brønsted Acidity in Metal-Doped Mesoporous Silicates

Mesoporous silicates are attractive materials for catalysis since they can host a variety of metals, and their adjustable pore size allows to control selectivity. Catalytic activity is enhanced by acidity around the metal centers, and the choice of metal influences both the type, i.e Lewis vs. Brønsted, and degree of acidity. However, understanding the correlation between metal and acidity is difficult due to the complexity of the amorphous structure of these materials. In this contribution, we focus on three metal dopants: Zr, Nb. and W. The first was reported to present only Lewis acidity, while the other two present both Lewis and Brønsted acidity[1-4]. Furthermore, studies of NH₃ adsorption showed that Zr-doped silicates present many more acid sites than Nb and W for the same metal loading (in mole fraction)[1-4]. We study these differences using multiscale quantum mechanical simulations based on density functional theory with the goal to understand the structure-property relations in doped mesoporous silicates. We consider multiple doping sites with varying metal coordination number and number of OH groups directly attached to the metal. Our results reproduce experimental trends, and allow us to correlate the Lewis/Brønsted acidity of these metals with their electronic structure and change in electronic density distribution in the presence of a base. Therefore, we can provide a molecular level understanding of these materials, which in turn may help improve experimental design towards increased catalytic activity and lifetime of these materials.

1. Ramanathan, et al. J Porous Mater 19, 961 (2012)
2. Pan, et al. Ind. Eng. Chem. Res. 52, 15481 (2013)
3. Ramanathan, et al. Microporous Mesoporous Mater. 167, 207 (2013)
4. Ramanathan, et al. Microporous Mesoporous Mater. 190, 240 (2014)