Cooperative Carbon Dioxide Capture in Diamine-Appended Magnesium–Olsalazine Frameworks

Diamine-appended Mg2(dobpdc) (dobpdc4– = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate) metal–organic
frameworks have emerged as promising candidates for carbon capture
owing to their exceptional CO2 selectivities, high separation
capacities, and step-shaped adsorption profiles, which arise from
a unique cooperative adsorption mechanism resulting in the formation
of ammonium carbamate chains. Materials appended with primary,secondary-diamines featuring bulky substituents,
in particular, exhibit excellent stabilities and CO2 adsorption
properties. However, these frameworks display double-step adsorption
behavior arising from steric repulsion between ammonium carbamates,
which ultimately results in increased regeneration energies. Herein,
we report frameworks of the type diamine–Mg2(olz)
(olz4– = (E)-5,5′-(diazene-1,2-diyl)bis(2-oxidobenzoate))
that feature diverse diamines with bulky substituents and display
desirable single-step CO2 adsorption across a wide range
of pressures and temperatures. Analysis of CO2 adsorption
data reveals that the basicity of the pore-dwelling aminein
addition to its steric bulkis an important factor influencing
adsorption step pressure; furthermore, the amine steric bulk is found
to be inversely correlated with the degree of cooperativity in CO2 uptake. One material, ee-2–Mg2(olz) (ee-2
= N,N-diethylethylenediamine), adsorbs
>90% of the CO2 from a simulated coal flue stream and
exhibits
exceptional thermal and oxidative stability over the course of extensive
adsorption/desorption cycling, placing it among top-performing adsorbents
to date for CO2 capture from a coal flue gas. Spectroscopic
characterization and van der Waals-corrected density functional theory
calculations indicate that diamine–Mg2(olz) materials
capture CO2 via the formation of ammonium carbamate chains.
These results point more broadly to the opportunity for fundamentally
advancing materials in this class through judicious design.