  Conventional approaches to the capture of CO_2 by metal-organic frameworks
focus on equilibrium conditions, and frameworks that contain little CO_2 in
equilibrium are often rejected as carbon-capture materials. Here we use a
statistical mechanical model, parameterized by quantum mechanical data, to
suggest that metal-organic frameworks can be used to separate CO_2 from a
typical flue gas mixture when used under {\em nonequilibrium} conditions. The
origin of this selectivity is an emergent gas-separation mechanism that results
from the acquisition by different gas types of different mobilities within a
crowded framework. The resulting distribution of gas types within the framework
is in general spatially and dynamically heterogeneous. Our results suggest that
relaxing the requirement of equilibrium can substantially increase the
parameter space of conditions and materials for which selective gas capture can
be effected.
