Theoretical Insight into Gate-Opening Adsorption Mechanism and Sigmoidal Adsorption Isotherm into Porous Coordination Polymer

The
gate-opening adsorption mechanism and sigmoidal adsorption
isotherm were theoretically investigated taking CO2 adsorption
into porous coordination polymers, [Fe­(ppt)2]n (PCP-N, Hppt = 3-(2-pyrazinyl)-5-(4-pyridyl)-1,2,4-triazole)
and [Fe­(dpt)2]n (PCP-C, Hdpt = 3-(2-pyridinyl)-5-(4-pyridyl)-1,2,4-triazole) as examples,
where the hybrid method consisting of dispersion-corrected DFT for
infinite PCP and a post-Hartree–Fock (SCS-MP2 and CCSD­(T))
method for the cluster model was employed. PCP-N has
site I (one-dimensional channel), site II (small aperture to site
I), and site III (small pore) useful for CO2 adsorption.
CO2 adsorption at site I occurs in a one by one manner
with a Langmuir adsorption isotherm. CO2 adsorption at
sites II and III occurs through a gate-opening adsorption mechanism,
because the crystal deformation energy (EDEF) at these sites is induced largely by the first CO2 adsorption
but induced much less by the subsequent CO2 adsorption.
Interestingly, nine CO2 molecules are adsorbed simultaneously
at these sites because a large EDEF cannot
be overcome by adsorption of one CO2 molecule but can be
by simultaneous adsorption of nine CO2 molecules. For such
CO2 adsorption, the Langmuir–Freundlich sigmoidal
adsorption isotherm was derived from the equilibrium equation for
CO2 adsorption. A very complicated CO2 adsorption
isotherm, experimentally observed, is reproduced by combination of
the Langmuir and Langmuir–Freundlich adsorption isotherms.
In PCP-C, CO2 adsorption occurs only at site
I with the Langmuir adsorption isotherm. Sites II and III of PCP-C cannot be used for CO2 adsorption because
a very large EDEF cannot be overcome by
simultaneous adsorption of nine CO2 molecules. Factors
necessary for gate-opening adsorption mechanism are discussed on the
basis of differences between PCP-N and PCP-C.