UC Berkeley

Abstract

Enhanced CO2 Capture in Metal-Organic and Covalent-Organic Frameworks

by

Robinson W Flaig

Doctor of Philosophy in Chemistry

University of California, Berkeley

Professor Omar Yaghi, Chair

Global atmospheric carbon dioxide (CO2) levels are rising. It is widely acknowledged by the scientific community that this phenomenon is contributing to climate change. Therefore, the need for effective CO2 capture from both point sources (power plants, etc.) and direct air capture is urgent. Despite years of investigation, the development of scalable, cost-effective, and energy-efficient, carbon capture and sequestration (CCS) technologies remains an outstanding challenge. The most widely- employed technique to date is the use of aqueous monoethanolamine (MEA) solutions at CO2 point sources. Flue gas from point sources is pumped through these solutions which capture the CO2 by forming chemical bonds (e.g. ammonium carbamate formation) between the gas and the amine moieties in solution. Although a large amount of CO2 can be captured using this technology, it is energetically inefficient. With one of the highest specific heat capacities among common solvents, water requires a significant amount of energy to bring to temperatures at which the MEA molecules release the previously- captured CO2. The requisite energy is often produced at similar CO2-producing point sources, limiting the effectiveness of this technique when employed.

For this reason and others, (e.g. instability of aqueous MEA solutions to other contaminants present in flue gas) many are considering alternative methods of CO2 capture from point sources. One particularly potent method involves the use of amine- functionalized porous adsorbents in an analogous fashion to MEA solutions, as these materials frequently exhibit lower heat capacities, higher selectivities, and greater chemical tunability compared to their liquid-based counterparts.

Reticular chemistry, the process of assembling judiciously designed rigid molecular building blocks into predetermined ordered structures (networks) which are held together by strong bonds, has been widely applied to the synthesis of such porous adsorbents, most notably Metal-Organic Frameworks (MOFs) and Covalent Organic Frameworks (COFs). Much of the insight on the fundamental science which set the foundation for these materials as a potential next-generation of CO2 adsorbents was gleaned through development of many MOFs which demonstrate some of the highest CO2 uptake capacities of any porous material. Recent work has focused on elucidation of MOF systems with practical CO2 capture applications.

Recently, the Yaghi group designed a new MOF, IRMOF-74-III-CH2NH2 that selectively captures CO2 in the presence of water under flue gas conditions. I followed up on this study with an augmented material, IRMOF-74-III-(CH2NH2)2 which exhibited a 133% increase in CO2 uptake in the pressure regime of interest to this application (e.g. <150 mbar). Upon further investigation, it was concluded that this drastic increase in CO2 uptake was caused not only by the increase in the amine population in the framework, but also by a new chemical capture mechanism, the formation of elusive, relatively unstable, carbamic acid species upon reaction between framework-based amines and CO2 under dry (<10% relative humidity) conditions. Interestingly, I discovered that CO2 captured as carbamic acid moieties is removed with more facility than CO2 captured as ammonium carbamates. The understanding gleaned by these fundamental studies has necessitated the assessment of the applicability of such systems, while exploring the potential of additional, potentially more promising, systems.

Currently the use of COFs for CO2 capture is under investigation. Although only few have examined the behavior of COFs in such a manner, the potential advantages are quite apparent. Consisting of light atoms with high chemical and moisture stability, COFs have the potential to achieve high gravimetric uptake of CO2 with enhanced cyclability under humid, flue-gas analogous, conditions. Specifically, I synthesized a COF analogue (COF- 81) of IRMOF-74-III-(CH2NH2)2. This material has already exhibited modest CO2 adsorption capacity and formation of carbamic acid species under both dry and humid conditions. These observations warrant continued study of the material under flue gas conditions as the mechanistic tuning of the CO2 capture mechanism would allow this highly stable material to achieve high CO2 adsorption capacity accompanied by decreased regeneration energies.

Additionally, I developed a new, post-synthetically modified MOF material, MOF- 808-Glycine. The parent material, MOF-808-P can be synthesized on gram scales and rendered CO2-phillic by incorporation of an economical, abundant amino acid, glycine. Herein, I demonstrate that the framework remains crystalline and porous after each post- synthetic modification and that MOF-808 can be loaded with high quantities of glycine. Furthermore, I show that MOF-808-Glycine has significantly improved CO2 adsorption properties compared to MOF-808-P, and that the material chemisorbs CO2, forming a mixture of carbamic acid and ammonium carbamate under dry (0% RH) conditions, and ammonium bicarbonate under humid (95%) conditions. This work represents a significant step toward real-world, large scale CO2 adsorption using MOFs.