Zr- and Hf-Based Metal–Organic Frameworks: Tracking Down the Polymorphism

Six
novel Zr­(IV)- and Hf­(IV)-based MOFs, namely DUT-67, DUT-68,
and DUT-69 (DUT, Dresden University of Technology) were obtained using
a modulated synthesis approach with the acetic acid as a modulator
and the bent 2,5-thiophenedicarboxylate (tdc2–)
as a ligand. The modulator not only increases the size of the MOF
crystallites but also plays a role of a structure directing agent,
affecting both the secondary building unit (SBU) connectivity and
topology of the resulting frameworks. The structure of DUT-67 is based
on the reo underlying net, characteristic for its cuboctahedral
and octahedral pores and is therefore isoreticular to DUT-51. The
DUT-68 material has a more complicated hierarchical pore system including
rhombicuboctahedral mesopore, surrounded by cuboctahedral, square-antiprismatic
and octahedral microcages. DUT-69 is the first example of Zr-based
MOF containing 10-connected SBU. DUT-69 has bct topology,
possessing octahedral cages and channels running along one crystallographic
direction. In accordance with X-ray single crystal analysis, the pores
of DUT-67 and DUT-68, which were obtained at high modulator concentrations,
are partially occupied by additional clusters. All novel materials
are found to be robust, hydrophilic, chemically, and thermally stable.
The BET specific surface area amounts to 1064 and 810 m2·g–1 for DUT-67­(Zr) and DUT-67­(Hf), 891 and
749 m2·g–1 for DUT-68­(Zr) and DUT-68­(Hf),
and 560 and 450 m2·g–1 for DUT-69­(Zr)
and DUT-69­(Hf), respectively.