Transport of Polar Molecules Across Solid/Gas Interfaces

All chemical and industrial processes somehow involve transport. Transport of heat, mass, charge, or various combinations of these are of paramount importance for the inner workings of distillation columns, chemical reactors, and membrane units. To optimize the performance of such units, a thorough understanding of transport processes on a molecular scale are in order. In this project we will focus on the separation and transport properties at the external surface of metal-organic frameworks. Metal-organic frameworks (MOFs) are a new group of materials with microporous structure. The vast number of available structures, along with easy routes to synthesize, or modify the framework has ensured much attention on these materials. We are more specifically focusing on the structure known as MOF-74 and the amine-enhanced extended MOF-74. This structure has hexagonal straight channels, with open metal-sites. These sites strongly interact with CO2, and other polar molecules. It can be synthesized with magnesium, zinc, cobalt, nickel, and iron as the metals, resulting in a diverse type of material. The enhanced material uses amine to capture CO2 using a novel phase-change mechanism, in which the amines attached to the metal-sites organize in patterns that favors adsorption of CO2. These results were recently published in a Nature paper, building on work from this project. This material has unique properties, with an extremely high affinity for CO2, and no affinity for nitrogen, and is one of the best current candidates for CO2-capture in porous materials. To enable more accurate modeling, and understanding of the adsorption and transport properties across the gas/MOF interface, we will describe the external surface using molecular modeling. We will further strive to understand why the amine MOF-74 structure has so high selectivity for CO2 over nitrogen. This can be a purely kinetically determined difference, determined by the interactions at the external surface.

Interfaces are important in many transport- and adsorption processes, separating the gas phase, from the adsorbate phase. Since the interface is on a molecular scale, different methods must be applied to study transport at this level. Important details at the interface is usually not captured by experiments, as the spatial and temporal resolution of the equipment is too low. In this project we will study thermodynamics and transport at the solid/gas interface. This will be done using molecular simulations with classical force fields. Quantum mechanical simulations will be used to determine interaction parameters of the solid/gas interface. With a force field describing the interaction at the interface, non-equilibrium molecular dynamics simulations can be used to obtain transport coefficients, and determine the resistance to the transport of heat and mass across the solid/gas interface. The molecules whose transport properties will be studied, CO2 and water, have a quadrupole moment and a dipole moment re spectively. Electrostatic interactions at the solid/gas interface are expected to be important. The solid interface will be that of selected metal-organic frameworks (MOFs), which are nanoporous materials with great interest for the next generation Carbon Capture and Sequestration (CCS). The structure of MOFs allow molecules to adsorb both on the external surface and inside the channels, cages and/or intersections in the material. Transport mechanisms across the solid/gas interface depend on molecular sca le properties, and to get a detailed understanding of these mechanisms, numerical simulations are necessary.