The Catalyst Review December 2019 - 9

SPECIAL FEATURE
Figure 2. Comparison of performances between ZIF-8 and state of
the art catalysts (ZnAl2O3) in the transalkylation of rapeseed oil with
methanol at 200°C (left). Activation of methanol by a Zn-OH species
on a linker deficient cornerstone, the nucleophile oxygen of the MeOH
links to the Zn(II) Lewis site accompanied with a protonation of the
terminal hydroxide.

Source: Chizallet et al. 2010.

A similar story for MOF-5 another star of the early 2000. In the
framework of MOF-5, Zn4O cornerstones are linked in octahedral arrays
by benzendicarboxylate groups which complete the coordination
sphere of the Zn atoms. Hence, there are neither apparent nor latent
possible catalytic sites and little could be expected from MOF-5 in
catalytic applications. Unexpectedly, MOF-5 exhibits outstanding shape
selectivity for the alkylation of aromatics which overcomes reference
zeolites. It was obviously so puzzling that the same study has been
repeated by different teams to come to same conclusion! Based on
detailed structural studies it was reasoned that the possible presence
of Zn-OH species could be at the origin of the catalytic activities. In
another study, authors came to the same conclusions for Knoevenagel
condensation. In 2002, BASF researchers discovered that nanoporous
zinc carboxylates prepared from trimesic acid, very likely MOF-5, can
produce polypropylene carbonate with a molecular weight of ca. Mw =
60.000-75.000g/mol. Although characterization data was not provided,
we can suspect the presence of Zn-OH defects as discussed above.

We can see that structural defects are needed to understand the underneath catalytic activity of MOFs which should not be a priori
active when looking at their ideal crystalline structure Isomorphous metal substitution - a rational approach for site isolation. The
design of isolated catalytic sites in zeolites is definitively a source of inspiration. The isomorphous substitution of a few percent of
SiO4 tetrahedra by TiO4 tetrahedra in the silicalite-1 zeolite yields the famous TS-1, a Titanium "doped" Silicalite-1 which is currently
used for epoxidation conversion. Isomorphous metal substitution can be achieved in MOFs by a so-called "post-synthetic metal
cation exchange," at the inorganic bricks of various MOFs. For MIL-53 and UiO-66 frameworks, mixed Al/Fe-MIL-53 and Zr/TiUiO-66 have been obtained for example. Recently, the controlled incorporation of isolated Fe units into an Al-MIL-53 yield methane
oxidation catalyst that display a high activity and selectivity to methanol was reported.
Catalyst Stability: Friend or Enemy?

MOFs shall be regarded as intrinsically weaker materials than purely inorganic solids such as zeolites owing their composition
and higher porosity. Yet, stability criteria are process specific and one shall carefully consider in which conditions of temperature,
pressure and solvent, the materials are formulated and processed and then applied in a specific catalytic application. Regarding firing
temperature, MOFs which have been cited above are stable well above 300°C. Nevertheless, for heat-treatment of large amounts
of materials, it is recommended in practice to heat below 250°C because criteria for self-combustion are met in carboxylate-based
MOF (O-rich, carbon species and metal center which can act as catalyst!). While temperature of 150°C is enough for drying purpose,
higher temperature may be required to activate metal cluster by dehydroxylation. The stability of MOFs in a humid atmosphere has
been studied extensively. The exception of HKUST-1 which is known to slowly degrade in ambient atmosphere, others considered
MOFs here, namely Fe-MIL-100, Al-MIL-53, UiO-66, ZIF-8 are very robust even in presence of steam at 100°C for weeks. In this sense,
they are more stable than very large pore zeolites. In contrast, the stability of MOFs in aqueous solution is a controversy topic.
Whereas most of studies claim that MOF structures remain intact even in boiling aqueous solution (which is generally true), few
studies have analyzed the filtrate solution in order to determine whether part of the solid would have dissolved. A recent study on
UiO-66 demonstrates that washing at pH 7 is accompanied by a leaching of the linker (terephthalic acid) and therefore the creation
of defects. The study also showed that the hydrolysis rate of DMNP depends on the pH of the washing process for UiO-66 again
pointing out the key role played by the nature and nature of defects.
Two main mechanisms were reported which drive the MOF framework decomposition upon water exposure, namely hydrolysis and
ligand displacement. In the case of hydrolysis, the metal-ligand bond is broken, and water dissociates to form a hydroxylated cation
and a free protonated ligand (Equation 1). The ligand
displacement reaction involves the insertion of a
water molecule into the M-O metal-ligand bond of the
framework. This leads to the formation of a hydrated
cation and to the release of a free ligand (Equation 2).
For the said stable UiO-66, Walton and coworkers proposed a ligand displacement mechanism for the structural breakdown of UiO66 in the presence of either sodium hydroxide or water.

The Catalyst Review 										

December 2019

9



The Catalyst Review December 2019

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The Catalyst Review December 2019 - cover
The Catalyst Review December 2019 - contents
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