MOF-808: Zirconium MOF & Monolith Mechanics

• MOF-808 is a zirconium-based framework constructed with BTC linkers and Zr6-oxo clusters that form tetrahedral cages and adamantane-like pores.
• Gelation and sol–gel processing enable MOF-808 to form binder-free, monolithic xerogels with hierarchical porosity for improved catalytic and adsorptive applications.
• Mechanical testing reveals that MOF-808 monoliths resist contact-induced cracking through a shear-accommodating nanograin structure and effective grain-boundary sliding.

Searching arXiv for recent MOF-808 papers to ensure citations are current and relevant. MOF-808 is a zirconium-based metal-organic framework constructed from 1,3,5-benzenetricarboxylate (BTC) linkers and Zr$_6$ metal-oxide secondary building units (SBUs). In the Zr–carboxylate family, it is distinguished in the available arXiv literature by two closely related features: it readily forms gels under concentrated synthetic conditions and can therefore be processed into binder-free monolithic xerogels with hierarchical pore systems, and, when prepared as a monolith, it shows unusually high resistance to contact-induced cracking relative to several prototypical MOF monoliths. The current picture of MOF-808 is therefore not limited to framework topology or adsorption-oriented morphology; it also includes a shear-accommodating, nanograin-consolidated mechanical response under indentation and scratching (Bueken et al., 2016, Tricarico et al., 2022).

1. Framework chemistry and pore architecture

MOF-808 belongs to the class of Zr$^ {4+}$-based MOFs assembled from the hydrolyzed cluster $\mathrm{[Zr}_6\mathrm{O}_4(\mathrm{OH})_4]^{12+}$ and carboxylate linkers. In MOF-808, the organic linker is BTC, and the topology deviates from the face-centered cubic UiO-66 family because the tricarboxylate linker produces a lower connectivity at the Zr$_6$ node than the 12-connected UiO-66 case. In gel syntheses of MOF-808, formic acid functions as a modulator or terminal ligand, consistent with the chemistry typically associated with this framework (Bueken et al., 2016).

For monolithic MOF-808, the framework is described as comprising zirconium-oxide SBUs, specifically clusters of six octahedrally coordinated Zr atoms linked by $\mu_3$-oxo atoms, connected by BTC linkers into tetrahedral cages of approximately 4.8 Å and an adamantane-like pore of approximately 18.4 Å. The robust Zr–O connectivity and multi-linker coordination are associated with a stiff but energy-dissipative network, while the large pore architecture is implicated in permanent deformation under concentrated contact loading (Tricarico et al., 2022).

This structural combination is central to later processing and mechanics. The Zr$_6$-cluster chemistry enables gelation through hydrolysis-driven nucleation, and the large-pore, Zr-connected framework persists in monolithic specimens that exhibit significant non-brittle accommodation under sharp contact. A plausible implication is that MOF-808 should be considered not only as a porous crystal but also as a framework whose mesoscale assembly state strongly conditions its observed properties.

2. Gelation, sol–gel processing, and monolith formation

A defined gelation route for MOF-808 was reported using 4.8 mmol (1.02 g) BTC and 3.3 mmol (1.07 g) zirconyl chloride octahydrate in 10 mL N,N-dimethylformamide (DMF), 10 mL formic acid (98%), and 0.3 mL distilled water, followed by reaction at 100 °C for 2 h. Under these relatively concentrated conditions, with added water to promote hydrolysis of the Zr precursor to the Zr$_6$ cluster, the reaction mixture gelates to yield a MOF-808 gel. More generally, high reactant and water concentrations were identified as the trigger for gel formation across several Zr-MOFs, and MOF-808 was specifically noted as a system for which gels were readily prepared by increasing reactant concentration and including additional water relative to the original synthesis (Bueken et al., 2016).

The gel-to-monolith conversion route involved solvent exchange and drying. MOF-808 gels were dispersed and homogenized in fresh DMF, then exchanged with ethanol through iterative vortex mixing and centrifugation-driven syneresis, with supernatant decanted between exchanges. After the final exchange, an ethanol-containing, non-flowing gel with controlled volume was obtained. Air drying of the ethanol-exchanged gel at 200 °C for 2 h yielded monolithic xerogels; for MOF-808, xerogel monoliths were explicitly reported, whereas aerogels were not explicitly reported in that study (Bueken et al., 2016).

A later study employed a sol–gel-like route specifically to prepare MOF-808 monoliths for mechanical testing. In that synthesis, 210 mg BTC and 970 mg zirconyl chloride octahydrate were dissolved in DMF/formic acid (30 mL + 30 mL) and heated at 130 °C for 2 days. Post-synthesis processing comprised centrifugation, four DMF washes, acetone exchange with 100 mL acetone for 4 days with solvent replaced twice per day, evacuation at room temperature for 24 h, and activation at 150 °C for 24 h to yield the monoliths. Powder XRD confirmed the expected MOF-808 phase. The resulting bulk solids were millimeter-scale and suitable for mechanical testing; before indentation, they were cold-mounted in epoxy (Struers Epofix), then ground and polished to obtain a flat surface (Tricarico et al., 2022).

Taken together, these reports establish that MOF-808 can be processed by gel-derived and sol–gel-like routes into self-supporting, binder-free bulk forms. The processing emphasis is not incidental: monolith formation is the route by which hierarchical porosity and contact-mechanical resilience become experimentally accessible in this system.

3. Hierarchical porosity and consolidated nanostructure

The gel-based morphological model for MOF-808 is nanoparticle-driven. Gel formation arises from rapid nucleation and aggregation of MOF nanoparticles into a sample-spanning network, and broad X-ray diffraction reflections in the MOF-808 xerogel were taken as evidence of nanosized crystallites. Across the Zr-MOF monoliths studied, irregular nanoparticle packing was identified as the origin of hierarchical pore systems comprising intraparticle micropores from the framework itself and interparticle mesopores generated by the packing of nanoparticles. For MOF-808 specifically, separate N$_2$ physisorption curves, BET areas, pore volumes, and pore size distributions were not reported, but the study explicitly generalized the hierarchical pore interpretation to MOF-808 monoliths derived from gels (Bueken et al., 2016).

The later mechanical study placed MOF-808 within this sol–gel-derived class of monoliths and likewise associated hierarchical porosity with slow solvent removal from gels to form polycrystalline bulk solids. In that account, hierarchical porosity was explicitly linked to improved volumetric adsorption capacity relative to powders. The same study also provided a finer microstructural description: AFM phase and height images showed that MOF-808 consists of nanograins that aggregate into polycrystalline “lumps,” with grain size approximately three times smaller than the “nanoplate stacks” observed in ZIF-8 and HKUST-1 (Tricarico et al., 2022).

This finer nanograin size implies a higher grain-boundary volume fraction. The authors used this as evidence for a consolidated nanostructure, and they explicitly connected that consolidated nanostructure to MOF-808’s high resistance to crack initiation and propagation. In this microstructural picture, grain boundaries are not merely defects or interfaces; they are the loci that enable substantial grain boundary sliding (GBS) under contact loading and subsequently deflect or arrest cracks when cracks do form. This suggests that, for monolithic MOF-808, hierarchical porosity and crack resistance are both outcomes of the same nanoparticle-assembled architecture rather than mutually exclusive attributes.

4. Nanoindentation methodology and quantitative response

The indentation response of monolithic MOF-808 was measured using a KLA-Tencor iMicro nanoindenter equipped with a Berkovich tip and operated in Continuous Stiffness Measurement (CSM) mode. The loading protocol was displacement-controlled to a maximum surface penetration depth of 1000 nm at an indentation strain rate of 0.2 s$^{-1}$. The maximum load was held for 1 s to assess creep, and during unloading the load was held at 10% of maximum to quantify thermal drift; the corresponding horizontal segment was reported as less than 2 mN. For each sample, 32 indents were collected, and values were computed by averaging CSM data between 500 and 1000 nm. Data reduction followed the Oliver–Pharr method (Tricarico et al., 2022).

The quantities used in that analysis were defined as follows:

$S=\frac{dP}{dh}$

for the contact stiffness, where $^ {4+}$0 is the slope of the unloading curve at maximum load;

$^ {4+}$1

for hardness, where $^ {4+}$2 is the maximum load and $^ {4+}$3 is the calibrated area function;

$^ {4+}$4

for the reduced indentation modulus; and

$^ {4+}$5

for the relation between reduced modulus and sample or indenter elastic constants. The fraction of elastic work was written as

$^ {4+}$6

The measured mechanical quantities for MOF-808 monoliths are summarized below.

Quantity	Value for MOF-808 monolith	Note
Reduced indentation modulus $^ {4+}$7	4.61 ± 0.32 GPa	True $^ {4+}$8 not computed because $^ {4+}$9 was unavailable
Hardness $\mathrm{[Zr}_6\mathrm{O}_4(\mathrm{OH})_4]^{12+}$0	122 ± 14 MPa	Low relative to several comparison monoliths
Elastic recovery $\mathrm{[Zr}_6\mathrm{O}_4(\mathrm{OH})_4]^{12+}$1	13.6 ± 1.1%	Very low elastic recovery
$\mathrm{[Zr}_6\mathrm{O}_4(\mathrm{OH})_4]^{12+}$2 ratio	approximately 0.026	Placed in a low elastic recovery/low $\mathrm{[Zr}_6\mathrm{O}_4(\mathrm{OH})_4]^{12+}$3 regime
Density	1.522 ± 0.104 g cm$\mathrm{[Zr}_6\mathrm{O}_4(\mathrm{OH})_4]^{12+}$4	Reported for the monolith
Residual indentation depth, immediate	948 ± 13 nm	From the load–depth curve
Residual imprint depth by AFM, later	538 ± 5 nm	Indicates strong time-dependent recovery

The difference between the immediate residual indentation depth and the later AFM residual imprint depth corresponds to approximately 43% recovery. The authors suggested that this viscoelastic or time-dependent recovery may contribute to crack closure and suppression of radial crack propagation during unloading. Because Poisson’s ratio for MOF-808 was not available, the true Young’s modulus was not extracted from $\mathrm{[Zr}_6\mathrm{O}_4(\mathrm{OH})_4]^{12+}$5. Even without that conversion, the measured response was interpreted as consistent with a ductile, shear-accommodating contact behavior (Tricarico et al., 2022).

5. Indentation fracture, shear faulting, and nanoscratch behavior

The fracture response of MOF-808 monoliths departs from the classical picture of corner-originating radial cracking at moderate nanoindentation loads. Under cube-corner nanoindentation up to 50 mN, employed specifically to encourage crack formation through high stress concentration, MOF-808 showed no radial cracks from the indent corners. This contrasted with ZIF-8 and HKUST-1, which cracked under the same condition (Tricarico et al., 2022).

At higher Vickers microindentation loads, the same material displayed a different failure sequence. At HV0.05 (50 gf, approximately 0.49 N), MOF-808 exhibited layered shear faults within the contact area but no radial cracks. At HV0.3 (300 gf, approximately 2.9 N), radial cracks were observed. Crucially, these cracks did not initiate at the indent corners; instead, they originated from shear faults inside the contact area and then propagated in a catastrophic but deflecting manner along low-energy grain boundaries. The dominant failure mode was therefore identified as shear faults inside the contact area, consistent with sub-surface median cracking and shear activation in the Lawn–Evans–Marshall ceramic indentation fracture framework (Tricarico et al., 2022).

This point addresses a common oversimplification in interpreting MOF-808 fracture. The available evidence does not support treating the material as one that forms conventional corner-originating radial cracks under standard sharp-contact loading. Rather, the low elastic recovery and consolidated nanostructure enable plastic accommodation via GBS, reducing the effectiveness of elastic-driven radial crack propagation during unloading. When cracks finally appear at higher loads, they emanate from shear-fault bands and are redirected along grain boundaries instead of following the canonical corner-radial morphology (Tricarico et al., 2022).

The same shear-accommodating character was observed in nanoscratch tests using a Berkovich tip in both ploughing mode, with the sharp end forward, and pushing mode, with the flat end forward. In both modes, the normal load was ramped linearly from 0 to 50 mN over a scratch length of 100 μm at 10 μm s$\mathrm{[Zr}_6\mathrm{O}_4(\mathrm{OH})_4]^{12+}$6, and pre-scratch, scratch, and post-scratch profiles were recorded. MOF-808 exhibited the largest scratch critical depths among the four monoliths studied: 9255 ± 573 nm in ploughing mode and 10549 ± 467 nm in pushing mode. The maximum residual depths from the mid-length cross-profiles were 3266 nm and 3762 nm, respectively. No cracking was observed in either mode up to 50 mN, and the absence of pile-up around the scratches suggested that plastic flow was accommodated by stepwise shear-activated failure within the scratch track, contained by the small nanograin size and extensive grain-boundary network. Friction coefficient $\mathrm{[Zr}_6\mathrm{O}_4(\mathrm{OH})_4]^{12+}$7 and critical load $\mathrm{[Zr}_6\mathrm{O}_4(\mathrm{OH})_4]^{12+}$8 were not reported (Tricarico et al., 2022).

Because radial cracks were not obtained for MOF-808 under the standard 50 mN cube-corner protocol, no fracture toughness $\mathrm{[Zr}_6\mathrm{O}_4(\mathrm{OH})_4]^{12+}$9 was reported. The empirical cube-corner pathway used for other materials in the study was therefore not applicable to MOF-808 under those conditions. In addition, the later microindentation-induced cracks originated from shear faults and deflected along grain boundaries, which complicates toughness extraction using conventional corner-crack models (Tricarico et al., 2022).

6. Comparative context, implications, and limitations

Within the comparative set of ZIF-8, HKUST-1, MIL-68, and MOF-808 monoliths, MOF-808 and MIL-68 were identified as the two systems showing a remarkably high resistance to cracking. MOF-808 never developed radial cracks under 50 mN cube-corner nanoindentation, cracked only at the higher Vickers load of approximately 2.9 N, and remained crack-free in nanoscratch experiments up to 50 mN in both ploughing and pushing modes. By contrast, ZIF-8 and HKUST-1 cracked under the 50 mN cube-corner condition, and ZIF-8 was the only monolith showing scratch-induced cracking. MOF-808 also had the highest scratch critical depths in both scratch modes (Tricarico et al., 2022).

The proposed explanation for this comparative behavior is explicitly microstructural. MIL-68 and MOF-808 possess significantly smaller nanograins, approximately three times smaller than those in ZIF-8 and HKUST-1, and these nanograins assemble into polycrystalline lumps with numerous grain boundaries. That consolidated nanostructure facilitates GBS and plastic deformation, lowers $_6$0, suppresses elastic-driven radial crack propagation during unloading, and promotes crack deflection along low-energy grain boundaries when cracks form from shear faults at higher loads. For MOF-808 specifically, the combination of robust Zr-oxo connectivity, large pores that allow permanent deformation, and high grain-boundary density yields a mechanically resilient monolith despite low hardness and modest reduced modulus (Tricarico et al., 2022).

The processing literature provides the corresponding functional context. Monolithic Zr-MOFs obtained from gels are presented as promising for catalytic and adsorptive applications because hierarchical pore systems can mitigate mass-transfer limitations and improve handling relative to powders. In MOF-808, the same monolithic state that enables hierarchical porosity also underpins the consolidated nanostructure associated with crack suppression. This suggests that, for MOF-808, shaping and mechanics are not peripheral engineering considerations but part of the material definition relevant to packed beds, monolithic reactors, or adsorbers (Bueken et al., 2016, Tricarico et al., 2022).

Several limitations qualify the current evidence base. Mechanical testing was performed under laboratory ambient conditions, with humidity and atmosphere not explicitly controlled or reported. The monoliths were polycrystalline and therefore treated as effectively quasi-isotropic at the indentation scale because of random grain orientations. Standard deviations were reported from 32 indents, but the substantial viscoelastic recovery of residual imprints, approximately 43% for MOF-808, may influence post-test topography and apparent crack closure. On the processing side, MOF-808-specific N$_6$1 physisorption metrics, BET areas, pore volumes, and pore size distributions were not reported in the gel-paper dataset; aerogels were not explicitly reported for MOF-808 there; and phase purity in that study rested on PXRD consistency with the desired phase rather than MOF-808-specific microscopy. Accordingly, the best-supported description is that MOF-808 is a Zr$_6$2-BTC framework whose gel-derived or sol–gel-derived monolithic forms combine hierarchical porosity with a consolidated nanograined architecture that strongly resists indentation- and scratch-induced cracking through shear-dominated deformation and grain-boundary-mediated crack control (Bueken et al., 2016, Tricarico et al., 2022).