Mechanisms of Gas Storage in Metal-Organic Frameworks

Seminars
02.04.2017
14:30
David Wang Auditorium, 3rd floor Dalia Maydan Bldg.
Dr. Ehud Tsivion

Dr. Ehud Tsivion
Lawrence Berkeley National Laboratory, Materials Sciences Division and UC Berkeley, Department of Chemistry

Hydrogen (H2) and Natural gas (NG) are alternatives to gasoline as fuels for vehicles. However, despite having several advantages over gasoline, they both suffers from low volumetric density at ambient temperature and pressure such that storage of sufficient quantities requires special and bulky equipment for their cooling or compression. In principle, storage of gases by adsorption in a high surface area, nano-porous, material is a promising alternative approach: the adsorbed gas takes a much smaller volume compared to its standard state, enabling storage at lower pressures with considerably less storage volume.
Due to their high surface area and tunable properties, metal-organic frameworks (MOFs) are considered as promising materials for gas storage applications. To be able to design new gas adsorbing MOFs, it is important to have a detailed understanding of the atomistic factors that drive the adsorption process. Particularly, identifying specific chemical or structural elements that govern optimal adsorption is of critical importance, as these could potentially be post-synthetically introduced as a component in a specifically designed adsorption system.

The first part of the seminar will discuss the challenge of designing materials with sufficient interaction with H2, where it is commonly considered that coordinatively unsaturated (“open”) metal-sites required for significant H2 interaction. This notion is challenged by studying several types of these sites and demonstrating that only metal-sites with specific arrangements of charges can exert sufficient polarization in the H2 molecule and enable its storage.

The second part of the seminar will discuss the adsorption of methane (CH4) in MOF-5. By combining our own cluster-model DFT calculations with experimentally measured adsorption isotherms the factors which control the adsorption process are identified. It is found that CH4 adsorption is dominated by electrostatic interactions with the negatively charged oxygens on the metal-coordinating linkers. Additionally, the formation of a nano-structured surface enables additional adsorption of CH4 molecules at favorable free-energy thanks to site-localized diffusion processes.