Controlling Cooperative CO₂ Adsorption in Diamine-Appended Mg₂(dobpdc) Metal–Organic Frameworks

In the transition
to a clean-energy future, CO2 separations
will play a critical role in mitigating current greenhouse gas emissions
and facilitating conversion to cleaner-burning and renewable fuels.
New materials with high selectivities for CO2 adsorption,
large CO2 removal capacities, and low regeneration energies
are needed to achieve these separations efficiently at scale. Here,
we present a detailed investigation of nine diamine-appended variants
of the metal–organic framework Mg2(dobpdc) (dobpdc4– = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate)
that feature step-shaped CO2 adsorption isotherms resulting
from cooperative and reversible insertion of CO2 into metal–amine
bonds to form ammonium carbamate chains. Small modifications to the
diamine structure are found to shift the threshold pressure for cooperative
CO2 adsorption by over 4 orders of magnitude at a given
temperature, and the observed trends are rationalized on the basis
of crystal structures of the isostructural zinc frameworks obtained
from in situ single-crystal X-ray diffraction experiments.
The structure–activity relationships derived from these results
can be leveraged to tailor adsorbents to the conditions of a given
CO2 separation process. The unparalleled versatility of
these materials, coupled with their high CO2 capacities
and low projected energy costs, highlights their potential as next-generation
adsorbents for a wide array of CO2 separations.