Metal–organic
frameworks are among the most promising materials
for industrial gas separations, including the removal of carbon dioxide
from natural gas, although substantial improvements in adsorption
selectivity are still sought. Herein, we use equilibrium adsorption
experiments to demonstrate that the flexible metal–organic
framework Co(bdp) (bdp2– = 1,4-benzenedipyrazolate)
exhibits a large CO2 adsorption capacity and approaches
complete exclusion of CH4 under 50:50 mixtures of the two
gases, leading to outstanding CO2/CH4 selectivity
under these conditions. In situ powder X-ray diffraction
data indicate that this selectivity arises from reversible guest templating,
in which the framework expands to form a CO2 clathrate
and then collapses to the nontemplated phase upon desorption. Under
an atmosphere dominated by CH4, Co(bdp) adsorbs minor amounts
of CH4 along with CO2, highlighting the importance
of studying all relevant pressure and composition ranges via multicomponent
measurements when examining mixed-gas selectivity in structurally
flexible materials. Altogether, these results show that Co(bdp) may
be a promising CO2/CH4 separation material and
provide insights for the further study of flexible adsorbents for
gas separations.