Thermodynamically Guided Synthesis of Mixed-Linker Zr-MOFs with Enhanced Tunability

Guided by thermodynamics, we have
synthesized two mixed-linker
zirconium-based metal–organic frameworks (Zr-MOFs), namely,
PCN-133 and PCN-134. Both of them possess a layer-pillar structure,
in which the connection between Zr6 clusters and primary
BTB linkers form a (3,6)-connected kdg layer that is
further extended into 3D frameworks by auxiliary DCDPS/TCPP linkers
(BTB = benzene tribenzoate, DCDPS = 4,4′-dicarboxydiphenyl
sulfone, TCPP = tetrakis­(4-carboxyphenyl)­porphyrin). PCN-134 demonstrates
high porosity (N2 uptake of 717 cm3·g–1 and BET surface area of 1946 cm2·g–1) and excellent chemical stability in aqueous solutions
with pH values ranging from 0 to 13. More importantly, PCN-134 tolerates
the partial absence of auxiliary linkers leading to structural defects
during the assembly process while preserving its framework integrity.
Furthermore, the defect density can be systematically controlled by
tuning the occupancy of the auxiliary linker, which in turn affects
the MOF properties. For instance, the dichromate uptake of PCN-134
is tuned by adjusting the BTB/TCPP ratios, which gives rise to an
efficient dichromate absorbent when the TCPP molar ratio in linkers
is set as 22%. In addition, the photocatalytic reduction of Cr­(VI)
in aqueous solution was also performed by PCN-134–22%TCPP which
exhibits excellent catalytic activity. This work not only opens up
a new synthetic route toward mixed-linker MOFs, but also provides
tunable control of MOF defects and, in turn, the properties.