A Series of Metal–Organic Frameworks Built of Triazolate-Trinuclear and Paddlewheel Units: Solid-Solution Framework Approach for Optimizing CO₂ Adsorption and Separation

Metal–organic
frameworks with tunable functional groups
like solid-solution have received considerable interest. In this paper,
a crs-type structure built of [Zn3(OH)­(dmtrz)3] triazolate-trinuclear and [Zn2(COO)4] paddlewheel units, linked by 3,5-dimethyl-1,2,4-triazolate (dmtrz)
and isophthalate (ipa) ligands (named as MAC-4-A), has been used as
a prototype framework. Four types of functional groups (B: 5-hydroxyisophthalate,
C: 5-aminoisophthalate, D: 5-ethoxyisophthalate, and E: 5-acetamidoisophthalate)
have been integrated into the framework, giving isostructures of MAC-4-B
to E, respectively. Then, by the solid-solution framework approach,
four group samples are prepared, which are MAC-4-AB-x, MAC-4-AC-x, MAC-4-AD-x, and MAC-4-AE-x (x denoted as the ratio of functional
ligands in the framework, x = 0.3, 0.5, 0.7), and
their CO2 and N2 adsorptions have been studied.
Our results revealed that the CO2 capacity is enhanced
with the increase of functional groups and then decreases, showing
a maximum uptake amount on MAC-4-AB-0.5, MAC-4-AC-0.5, MAC-4-AD-0.7,
and MAC-4-AE-0.5 at 298 K and 1 bar, respectively. On the other hand,
the calculated selectivity of CO2 over N2 gradually
increases, giving the highest selectivity after the pore surface is
completely functionalized.