(703b) Simulations of Ammonia Adsorption for the Characterization of Acid Sites in Metal-Doped Amorphous Silicates

The acidity of amorphous mesoporous silicates is directly related to the catalytic activity of these materials, and metal doping increases the acidic behavior. However, the local structure of the active sites is not well known as it is difficult to probe it experimentally. Additionally, it is not clear what drives the formation of specific acid sites for a specific metal dopant, i.e. Brønsted and/or Lewis, and their relative strength. All of this inhibits the systematic improvement of the catalytic activity of these materials. We use quantum mechanical simulations based on density functional theory to characterize the local structure of Zr-, W-, and Nb-doped amorphous silicates, for which experimental data of acidity is available. We consider a different number of hydroxyl groups directly bonded to the metal centers and of nearby silanol groups, and their effect on the Lewis and Brønsted acidity of the active site. The acidity is studied via adsorption of one or two ammonia molecules on the metal, M-OH, and Si-OH sites, respectively, to model measurements with the NH₃ temperature controlled desorption (NH₃-TCD) technique. Our calculations reproduce the experimental trends of acidity strength across metals as seen by Ramanathan (2012, 2013, 2014): Zr > W > Nb, where Zr mostly shows Lewis acidity, while Nb and W showing both Brønsted and Lewis sites. Metal atoms that are more grafted into the silica (have more M-O-Si interactions) exhibit the strongest Lewis acidity. Furthermore, silyl oxoniums stabilized by a nearby metal atom exhibit the strongest affinity for ammonia, and represent the most likely source of Brønsted acidity in these materials.