Triptycene-Based Porous Metal-Assisted Salphen Organic Frameworks: Influence of the Metal Ions on Formation and Gas Sorption

Porous
organic polymers (POPs) are chemically and thermally robust
materials and have been often investigated for their gas sorption
properties. From the related field of metal–organic frameworks
(MOFs) it is known that open ligation sites at metal centers can enhance
the performance of gas sorption significantly, especially the selectivity
toward one gas of a binary mixture, such as CO2/N2 or CO2/CH4. POPs that contain metal centers
are rarer. One possibility to introduce metals into POPs is by the
synthesis of metal-assisted salphen organic frameworks (MaSOFs), where
the framework development is associated with the formation of the
metal–salphen pockets. Based on a hexakissalicylaldehyde, a
variety of three-dimensional isostructural porous MaSOFs with different
metal ions (Zn2+, Ni2+, Cu2+, Pd2+, and Pt2+) are introduced. All compounds show
a very similar pore structure and comparable specific surface areas,
which make these MaSOFs ideal candidates to study the influence of
the nature of the incorporated metal center on gas sorption selectivity.
Due to the environmental importance, the adsorption of CO2 in comparison to N2 and CH4 was extensively
studied. Depending on the metal ions, the heat of adsorption was different
as well as the Henry and IAST selectivities. Cu–MaSOF100 for instance shows a high Qst of 31.2
kJ mol–1 for CO2 and an uptake of 14.9
wt % at 1 bar and 273 K. The IAST selectivity of CO2/N2 for an 80/20 mixture is with SIAST = 52 very high for a metal containing POP and even comparable to
some of the best performing MOFs. The MaSOFs are stable even in boiling
water. This, as well as the simple synthesis, makes them potential
good candidates for CO2 removal of binary mixtures.