posted on 2021-09-07, 19:34authored byBhavish Dinakar, Alexander C. Forse, Henry Z. H. Jiang, Ziting Zhu, Jung-Hoon Lee, Eugene J. Kim, Surya T. Parker, Connor J. Pollak, Rebecca L. Siegelman, Phillip J. Milner, Jeffrey A. Reimer, Jeffrey R. Long
Carbon capture at fossil fuel-fired
power plants is a critical
strategy to mitigate anthropogenic contributions to global warming,
but widespread deployment of this technology is hindered by a lack
of energy-efficient materials that can be optimized for CO2 capture from a specific flue gas. As a result of their tunable,
step-shaped CO2 adsorption profiles, diamine-functionalized
metal–organic frameworks (MOFs) of the form diamine–Mg2(dobpdc) (dobpdc4– = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate)
are among the most promising materials for carbon capture applications.
Here, we present a detailed investigation of dmen–Mg2(dobpdc) (dmen = 1,2-diamino-2-methylpropane), one of only two MOFs
with an adsorption step near the optimal pressure for CO2 capture from coal flue gas. While prior characterization suggested
that this material only adsorbs CO2 to half capacity (0.5
CO2 per diamine) at 1 bar, we show that the half-capacity
state is actually a metastable intermediate. Under appropriate conditions,
the MOF adsorbs CO2 to full capacity, but conversion from
the half-capacity structure happens on a very slow time scale, rendering
it inaccessible in traditional adsorption measurements. Data from
solid-state magic angle spinning nuclear magnetic resonance spectroscopy,
coupled with van der Waals-corrected density functional theory, indicate
that ammonium carbamate chains formed at half capacity and full capacity
adopt opposing configurations, and the need to convert between these
states likely dictates the sluggish post-half-capacity uptake. By
use of the more symmetric parent framework Mg2(pc-dobpdc)
(pc-dobpdc4– = 3,3′-dioxidobiphenyl-4,4′-dicarboxylate),
the metastable trap can be avoided and the full CO2 capacity
of dmen–Mg2(pc-dobpdc) accessed under conditions
relevant for carbon capture from coal-fired power plants.