Simulations of Ammonia Adsorption for the Characterization of Acid Sites in Metal-Doped Amorphous Silicates

The catalytic activity of metal-doped amorphous silicates is directly related to their acidity. However, the local structure of the active sites is not well-known as it is difficult to probe it experimentally, which inhibits the systematic improvement of these mesoporous materials toward better catalytic activity. We use quantum mechanical simulations based on density functional theory to characterize Zr-, W-, and Nb-doped amorphous silicates, for which experimental data of acidity are available. We study the adsorption of one or two ammonia molecules on the metal (M) and various MOH and SiOH sites to model measurements of NH3 temperature-programmed desorption (NH3-TPD), a technique commonly used to determine the amount of acid sites in these materials. Our calculations reproduce the experimental trends of acidity strength across metals: Zr > W > Nb, where Zr sites are predominantly Lewis acids, while Nb and W sites are both Bronsted and Lewis acids. Metal atoms that are more grafted into the silica (i.e., have more M-O-Si bonds) exhibit stronger Lewis acidity. Furthermore, silyl oxoniums stabilized by a nearby metal atom exhibit the strongest affinity for ammonia, thus representing the most likely source of Bronsted acidity in these materials. On the basis of these results, we compile the Metal doped Amorphous Silicate Library (METASIL), comprising 70 cluster structures, which may be used for the simulation of the catalytic activity of real mesoporous silicates.