(510b) Post-Synthetic Functionalization of Mg-MOF-74 with Tetraethylenepentamine: Structural Characterization and Enhanced CO2 Adsorption

Authors: 
Martis, V., Surface Measurement Systems
Su, X., Massachusetts Institute of Technology
Bromberg, L., Massachusetts Institute of Technology
Huq, A., Oak Ridge National Laboratory
Hatton, T. A., Massachusetts Institute of Technology
Metal-organic frameworks (MOFs) are an emerging class of micro- and mesoporous materials with great potential in gas adsorption and separation. MOFs are composed of organic linkers coordinated to metal centers, possess open-framework structures that rank among the materials with the highest surface areas known. Among the MOFs used for carbon dioxide adsorption, Mg-MOF-74 is an interesting option to study due to the ease of synthesis, very high equilibrium and dynamic gas uptake, the economic feasibility of its reagents. The materialâ??s cylindrical pores and unsaturated metal centers can adsorb between 21-26% CO2, which places it among adsorbers with the highest capacity for CO2. In an effort to improve the adsorption behavior of MOFs under different conditions, we grafted the Mg-MOF-74 with tetraethylene pentamine (TEPA), in order. Multi-unit amines (similar to polyethyleneimines) have been shown to provide high CO2 uptake capacity as well as regenerability over multiple cycles. We expect that through functionalization by longer-chain amines such as TEPA the MOF adsorption behavior can be enhanced. Gas adsorption testing on our MOF/TEPA materials suggests that CO2 total uptake capacity is dependent on the amine loading, with the adsorption increase dependent on the functionalization degree. CO2 uptake is reduced over the parent MOF when amines are loaded in excess, but improved when limited amine fraction is added. The crystal structure from XRD is identical pre- and post-functionalization, as well as pre-post-capture. Neutron diffraction was performed on the functionalized structures to understand the conformation of amines within the pores and draw structure-activity relations. Finally, Dynamic Vapor Sorption (DVS vacuum) is employed to perform competitive co-adsorption of CO­2 with H2O on Mg-MOF-74. The study shows how the competition of probe molecules for specific adsorption sites affects the efficiency and selectivity of the process. Water vapor is a major molecular competitor and affects CO­2 separation performance for porous materials in almost all industrial processes. This is one of the few multi-component H2O/CO2studies existent on post-synthetic functionalized Mg-MOFs, and is expected to provide guidelines for future materials design.