Soft Porous Crystal Based upon Organic Cages That Exhibit Guest-Induced Breathing and Selective Gas Separation

Soft porous crystals
(SPCs) that exhibit stimuli-responsive dynamic
sorption behavior are attracting interest for gas storage/separation
applications. However, the design and synthesis of SPCs is challenging.
Herein, we report a new type of SPC based on a [2 + 3] imide-based
organic cage (NKPOC-1) and find that it exhibits guest-induced
breathing behavior. Various gases were found to induce activated NKPOC-1 crystals to reversibly switch from a “closed”
nonporous phase (α) to two porous “open”
phases (β and γ). The net effect
is gate-opening behavior induced by CO2 and C3 hydrocarbons.
Interestingly, NKPOC-1-α selectively adsorbs propyne
over propylene and propane under ambient conditions. Thus, NKPOC-1-α has the potential to separate binary and ternary C3 hydrocarbon
mixtures, and the performance was subsequently verified by fixed bed
column breakthrough experiments. In addition, molecular dynamics calculations
and in situ X-ray diffraction experiments indicate that the gate-opening
effect is accompanied by reversible structural transformations. The
adsorption energies from molecular dynamics simulations aid are consistent
with the experimentally observed selective adsorption phenomena. The
understanding gained from this study of NKPOC-1 supports
the further development of SPCs for applications in gas separation/storage
because SPCs do not inherently suffer from the recyclability problems
often encountered with rigid materials.