Nanoengineering of Metal-Organic Frameworks (MOFs) for Carbon Capture

Abstract

Metal organic frameworks (MOFs), a new class of crystalline nanoporous materials built of metal atoms bridged by organic ligands, have been spotlighted recently as efficient CO2 capture adsorbents that can lower the carbon footprints below current level, due to their unique characteristics including uniform pore structure, high surface areas, and framework flexibility. Among this emerging class of nanoframeworks, Mg/DOBDC reports the highest and competitive adsorption capacity of 0.38 g CO2/g sorbent at low CO2 partial pressures simulating the CO2 capture conditions from flue gas. However, this material may become structurally unstable under humid conditions and after repetitive cycles of adsorption and desorption operations. For instance, due to the hydrophilic nature of MOFs, exposure to humidity could lead to such degradation where H2O molecules could replace framework ligands causing defects in crystal lattice. Since practical operating conditions of carbon capture process from flue gas involve some degree of moisture, stability enhancement of MOFs under humid atmosphere is critical. Functionalization of open metal sites with organic moieties would be one way to enhance its structural stability and carbon capture capability. In this project, molecules containing amine groups such as ethylene diamine (ED) were introduced into the pore space of Mg/DOBDC as a grafting agent to the open metal sites. Preliminary experiments showed that CO2 uptake stayed stable at 1.5 mmol/g after four adsorption/desorption cycles, which implies improved working capacity and overall stability/regenerability of the material after functionalization. Accelerated steam stripping experiments are currently being performed to demonstrate enhanced stability of functionalized Mg/DOBDC.