  Greenhouse gas emissions originating from fossil fuel combustion contribute
significantly to global warming, and therefore the design of novel materials
that efficiently capture CO$_2$ can play a crucial role in solving this
challenge. Here, we show that reducing the dimensionality of bulk crystalline
portlandite results in a stable monolayer material, named portlandene, that is
highly effective at capturing CO$_2$. Based on theoretical analysis comprised
of ab-initio quantum mechanical calculations and force-field molecular dynamics
simulations, we show that this single-layer phase is robust and maintains its
stability even at high temperatures. The chemical activity of portlandene is
seen to further increase upon defect engineering of its surface using vacancy
sites. Defect-containing portlandene is capable of separating CO and CO$_2$
from a syngas (CO/CO$_2$/H$_2$) stream, yet is inert to water vapor. This
selective behavior and the associated mechanisms have been elucidated by
examining the electronic structure, local charge distribution and bonding
orbitals of portlandene. Additionally, unlike conventional CO$_2$ capturing
technologies, the regeneration process of portlandene does not require high
temperature heat treatment since it can release the captured CO$_2$ by
application of a mild external electric field, making portlandene an ideal
CO$_2$ capturing material for both pre- and post-combustion processes.
