as MILs, more notable for their flexibility on uptake of guest species [179]. Interestingly, since this transition was published, others have since shown that the transition
is also crystallite size dependent, with larger nanosized crystals not undergoing the
same transition on exposure to N 2 gas [180]. In 2016, in a study by Hobday et al.
[181], the crystallite size dependence was overcome by loading N 2 as a PTM at
2,000 bar, inducing the transition to the ‘open’ structure. Crystallographic measurements were also accompanied by complementary DFT and Grand Canonical Monte
Carlo (GCMC) simulations, which showed that the transition was driven by the
formation of a new favourable framework binding site for the N 2 gas molecules
inside the pores.
The ability to probe the effect of PTM guest uptake in MOFs and explore guestdependent behaviour under pressure has resulted in a number of studies for the
MOFs HKUST-1 [182, 183], MOF-5 [184], Sc 2 BDC 3 [185], ZIF-7 [186] and
MIL-47 [187], though the latter study here on MIL-47 is conducted at MPa rather
than GPa pressures utilising mercury intrusion experiments. The review by
McKellar et al. [50] covers the majority of behaviour on these MOFs; however, a
significant body of work has been conducted on the isoreticular UiO-MOFs [188]
(Universitetet i Oslo) since 2015.
UiO-MOFs comprise hexanuclear metal nodes, M 6 O 4 (OH) 4 (where M ¼ Zr or
Hf) linked by carboxylate bridges. Currently, there are 11 unique native UiO-type
structures in the CSD [97]. High-pressure studies have focussed on the isostructural
series, UiO-66 [189], UiO-67 [190], UiO-68 [191] and UiO-abdc, which feature
1,4-benzendicarboxylate (BDC),biphenyl-4,4
0 -dicarboxylate (BPDC), p-terphenyl4,4
0 -dicarboxylic acid (TPDC) and azobenzene-4,4
0 -dicarboxylate ligands, respectively (ABDC). Each crystallises in the cubic space group Fm-3 m and contains a
large central octahedral pore that shares faces with eight smaller tetrahedral voids.
Thermal and mechanical stability is provided to the framework by the highly
coordinated metal ions and the 12-connected paddle-wheel nodes. These connections limit the flexibility of the framework, making the elastic properties of
UiO-frameworks amongst the highest reported for porous MOFs (E ~ 20–59 GPa,
K ~ 8–65 GPa) [192–195].
The framework resilience of UiO-66 and its amine-functionalised derivative,
UiO-66-NH 2 , was demonstrated by Yot et al. [192] and Peterson et al. [196, 197],
who reported no structural degradation during isotropic compression up to 2 GPa or
pelletisation up to 0.17 GPa. Functionalisation with –NH 2 groups provided
improved structural stability and higher incompressibility to the native framework,
increasing its bulk modulus from 17 GPa (UiO-66) to 25 GPa (UiO-66-NH 2 ).
Corresponding behaviour was reported for nitro-functionalised scandium terephthalate, Sc 2 BDC 3 -NO 2 , which resisted amorphisation compared to the parent material
[185]. It must be noted that on increasing pressure on MOFs, crystalline-amorphous
transitions with a resulting loss in Bragg scattering appear to be frequently observed.
Recently, the mechanical properties of UiO-66, UiO-67 and UiO-abdc have been
examined by compression in a hydraulic piston press, TEM punch experiments and
first-principles calculations. Under anisotropic pressure, UiO-66 loses crystallinity at
only 0.4 GPa. Similar behaviour was predicted under hydrostatic compression from
Crystallography Under High Pressures
187
is also crystallite size dependent, with larger nanosized crystals not undergoing the
same transition on exposure to N 2 gas [180]. In 2016, in a study by Hobday et al.
[181], the crystallite size dependence was overcome by loading N 2 as a PTM at
2,000 bar, inducing the transition to the ‘open’ structure. Crystallographic measurements were also accompanied by complementary DFT and Grand Canonical Monte
Carlo (GCMC) simulations, which showed that the transition was driven by the
formation of a new favourable framework binding site for the N 2 gas molecules
inside the pores.
The ability to probe the effect of PTM guest uptake in MOFs and explore guestdependent behaviour under pressure has resulted in a number of studies for the
MOFs HKUST-1 [182, 183], MOF-5 [184], Sc 2 BDC 3 [185], ZIF-7 [186] and
MIL-47 [187], though the latter study here on MIL-47 is conducted at MPa rather
than GPa pressures utilising mercury intrusion experiments. The review by
McKellar et al. [50] covers the majority of behaviour on these MOFs; however, a
significant body of work has been conducted on the isoreticular UiO-MOFs [188]
(Universitetet i Oslo) since 2015.
UiO-MOFs comprise hexanuclear metal nodes, M 6 O 4 (OH) 4 (where M ¼ Zr or
Hf) linked by carboxylate bridges. Currently, there are 11 unique native UiO-type
structures in the CSD [97]. High-pressure studies have focussed on the isostructural
series, UiO-66 [189], UiO-67 [190], UiO-68 [191] and UiO-abdc, which feature
1,4-benzendicarboxylate (BDC),biphenyl-4,4
0 -dicarboxylate (BPDC), p-terphenyl4,4
0 -dicarboxylic acid (TPDC) and azobenzene-4,4
0 -dicarboxylate ligands, respectively (ABDC). Each crystallises in the cubic space group Fm-3 m and contains a
large central octahedral pore that shares faces with eight smaller tetrahedral voids.
Thermal and mechanical stability is provided to the framework by the highly
coordinated metal ions and the 12-connected paddle-wheel nodes. These connections limit the flexibility of the framework, making the elastic properties of
UiO-frameworks amongst the highest reported for porous MOFs (E ~ 20–59 GPa,
K ~ 8–65 GPa) [192–195].
The framework resilience of UiO-66 and its amine-functionalised derivative,
UiO-66-NH 2 , was demonstrated by Yot et al. [192] and Peterson et al. [196, 197],
who reported no structural degradation during isotropic compression up to 2 GPa or
pelletisation up to 0.17 GPa. Functionalisation with –NH 2 groups provided
improved structural stability and higher incompressibility to the native framework,
increasing its bulk modulus from 17 GPa (UiO-66) to 25 GPa (UiO-66-NH 2 ).
Corresponding behaviour was reported for nitro-functionalised scandium terephthalate, Sc 2 BDC 3 -NO 2 , which resisted amorphisation compared to the parent material
[185]. It must be noted that on increasing pressure on MOFs, crystalline-amorphous
transitions with a resulting loss in Bragg scattering appear to be frequently observed.
Recently, the mechanical properties of UiO-66, UiO-67 and UiO-abdc have been
examined by compression in a hydraulic piston press, TEM punch experiments and
first-principles calculations. Under anisotropic pressure, UiO-66 loses crystallinity at
only 0.4 GPa. Similar behaviour was predicted under hydrostatic compression from
Crystallography Under High Pressures
187