Experimental and theoretical adsorption studies in tuneable organic-inorganic materials

Abstract

Adsorption processes are widely used for the storage and separation of gases in many industrial and environmental applications. The performance of the process depends strongly on the adsorbent and its interaction with the gases. Therefore, the idea of tailoring the adsorbent to the application by adapting the pore size and/or the chemical composition is very attractive. This work focuses on two groups of customizable hybrid materials: Firstly, in crystalline metal-organic frameworks (MOFs) the chemical and structural properties can be modified by changing the metal-oxide corner or the organic linker. Secondly, periodic mesoporous silica materials can be prepared with different pore sizes and geometries depending on the surfactant and its concentration and additionally modified with organic surface groups. The adsorption behaviour of the materials can be predicted by molecular simulation and thus the influence of modifications can be studied without the need of synthesising the material. For MOFs, the coordinates of the atoms can be obtained from XRD measurements. The quality of the predicted adsorption results was investigated for pure gas (methane, ethane, propane, nitrogen and carbon dioxide) and gas mixture (methane – carbon dioxide) adsorption on the metal-organic framework CuBTC. The comparison showed a good agreement between experimental and simulated results especially at low pressures. In order to create atomistic models for the mesoporous silica structures that are amorphous on the atomistic level, two existing simulation methods to model MCM-41-type materials were combined: micellar structures from coarse grained simulations that capture the phase separation in the surfactant/silica/solvent mixtures were used as input in kinetic Monte Carlo simulation that created the pore model on the atomistic level. The model created with this new methodology showed similar adsorption behaviour compared with a model created only with the kMC method using an ideal geometrical structure as micelle. The influence of modifications of the MOF structures (exchange of metal, linker length/composition and catenation) was investigated by Grand Canonical Monte Carlo simulations for hydrogen adsorption at low temperature and temperature controlled desorption. The peaks in the desorption spectra could be related to steps in the adsorption isotherms at 20 K.