Post-Synthetic Functionalization of Magnesium Metal-Organic Framework With Tetraethylene Pentamine for CO2 Adsorption

Metal-organic frameworks (MOFs) are a class of microporous and mesoporous materials with great potential in gas adsorption and separation. MOFs are composed of organic linkers coordinated to metal centers, and produce open-framework structures that rank among the materials with the highest surface areas currently available. Amongst the metal-organic frameworks used for carbon dioxide adsorption, Mg-MOF-74 is an interesting option to study due to its very high equilibrium and dynamic uptake, the economic feasibility of its reagents as well as its ease of synthesis.^1-2 Its cylindrical pores and unsaturated metal centers can adsorb between 21-26% CO₂, and represents one of the solid-state adsorbents with the highest capacity for CO₂.

Post-synthetic functionalization of MOFs is a technique to impart properties inaccessible by direct synthesis. For gas adsorption applications, functionalization with amines plays an interesting role due to their ability to improve adsorption behavior of MOFs under different conditions.³ We grafted the Mg-MOF-74 with tetra-ethylene pentamine (TEPA, a multi-unit amine), in order to study its interaction with the framework and its effect on the CO₂ adsorption. Multi-unit amines (similar to poly-ethylene-imines) have been shown recently to provide high carbon dioxide uptake capacity as well as regenerability over multiple cycles.⁴ We expect that through functionalization of longer-chain amines, to improve adsorption over current ethylene diamine studies, since one amine and one Mg-site are lost during the grafting process, whereas for TEPA, we gain one primary amine and 3 secondary amines.

Gas adsorption testing on our MOF/TEPA materials suggests that CO₂ total uptake capacity is dependent on amine loading. CO₂ uptake is reduced over the parent MOF when amines are put in excess, but improved when limiting amine is added. The crystal structure from XRD seems to be identical pre- and post-functionalization, as well as pre-post-capture.  After functionalization in excess amine, the surface elemental analysis through XPS gives C_55.98%N_16.82%O_22.5%Mg_4.71% and C_54.03%N_19.38%Mg_4.52%O_21.7%. The ratio of N/Mg is around 5, which indicates that each surface Mg-site has a tetra-ethylene pentamine grafted onto it. We speculate that the surface Mg-sites are all grafted, but the large amount of amines induces pore-blocking which reduces bulk CO₂ mass transfer. We grafted the equivalent of 1% of the excess amount of amines, and the surface composition became C_54.03%N_10.77%Mg_7.15%O_28.05%, with improved CO₂ uptake over Mg-MOF-74_.

Neutron powder diffraction studies are currently being carried out to investigate the crystal structure of the post-synthetically functionalized MOF. We attempt to identify the location within the unit cell of the amines and their conformation, as well as potential sites for CO₂ adsorption. We expect detailed diffraction studies will allow us to draw structure-performance relationships, and gain understanding on the adsorption mechanisms behind adsorption of CO₂ on amine-functionalized MOFs.

 

 

References

1.      Caskey, S. R.; Wong-Foy, A. G.; Matzger, A. J., Dramatic tuning of carbon dioxide uptake via metal substitution in a coordination polymer with cylindrical pores. Journal of the American Chemical Society 2008, 130 (33), 10870-1.

2.      Britt, D.; Furukawa, H.; Wang, B.; Glover, T. G.; Yaghi, O. M., Highly efficient separation of carbon dioxide by a metal-organic framework replete with open metal sites. Proceedings of the National Academy of Sciences of the United States of America 2009, 106 (49), 20637-40.

3.      Choi, S.; Watanabe, T.; Bae T.; Sholl, D.; Jones, C. W. Modification of Mg / DOBDC MOF with Amines to Enhance CO 2 Adsorption from Ultra-Dilute Gases. Journal of Physical Chemistry Letters 2012, 3, 1136-1141.

4.      Zhao, J.; Simeon, F.; Wang, Y.; Luo, G.; Hatton, T. A., Polyethylenimine-impregnated siliceous mesocellular foam particles as high capacity CO2 adsorbents. RSC Advances 2012, 2 (16), 6509-6509.