Computational Modeling of the Structure and Properties of Zr(OH)4

Ab initio molecular dynamics (AIMD), based on density functional theory, has been used to develop and test a model for amorphous Zr­(OH)4, which is of interest as an agent for the adsorption of toxic gases. Beginning with an idealized and highly ordered structure based on two-dimensional layers of polymeric Zr­(OH)4, AIMD at 300 K and above yields an amorphous structure. Disordering is seen to occur concomitantly with a reaction between acidic bridging OH sites and basic terminal OH to produce H2O and Zr–O–Zr bridges. The modeling also shows that Zr­(OH)4 can be maintained in an artificial ordered state either by a regular array of hydrogen bonds within layers or by interaction between layers. Radial distribution functions and distributions of hydrogen-bond parameters are calculated for both the ordered and the amorphous structures, and a comparison between the two sets of results provides insight regarding the nature of the amorphous state. Infrared and proton nuclear magnetic resonance spectra are computed for the amorphous model, and chemical properties are evaluated by examining the acidity of different types of OH groups and also the reactions with SO2 and CO2. In particular, various mechanisms leading to sulfite formation during exposure to SO2 are examined in some detail. In all cases, good agreement with experiment is found, which indicates that the model is a suitable basis for analyzing the adsorption properties of amorphous Zr­(OH)4.