A Soft Porous Crystal based upon Organic Cages that Exhibits Guest-Induced Breathing and Selective Gas Separation.

Wang Z , Sikdar N , Wang SQ , Li X , Yu M , Bu XH , Chang Z , Zou X , Chen Y , Cheng P , Yu K , Zaworotko MJ , Zhang Z
Journal of the American Chemical Society
Soft porous crystals (SPCs) that exhibit stimuli-responsive dynamic sorption behavior are attracting interest for gas stor-age/separation applications. However, 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 at ambient conditions. Thus, NKPOC-1-α has the potential to separate binary and ternary C3 hydrocarbon mixtures and 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 separa-tion/storage since SPCs do not inherently suffer from the recyclability problems often encountered with rigid materials.

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