The Catalyst Review December 2019 - 8

SPECIAL FEATURE
The famous MOF, named HKUST-1, of formula Cu3(btc)2 · 3 H2O (btc=benzenedicarboxylate) consists in a dimeric copper characteristic
paddle-wheel moiety. Soft thermal treatment removes water ligands to leave a vacant coordination site (cus) as a Lewis acid which
has been characterized by adsorption of molecular probed followed by IR spectroscopy. The application of Lewis acidity of HKUST-1
and other Cu-based MOFs has been demonstrated for the isomerization of α-pinene oxide and the cyclization of citronellal. Beyond
Lewis acido-basicity, MOFs made of open transition metal sites have be applied in redox type catalysis such as selective oxidation
of xanthenes and hydrochinone and the epoxidation of olefins. Among the striking examples, we can highlight the hydroxylation of
aromatics which has shown the highest turnover frequency reported so far.
Using benzenetricarbolxylic acid (btc) as linker, MIL-100 can be made from different M3+ metal (Fe3+, Cr3+, Sc3+). The original and
most famous version of MIL-100 is constructed with Cr3+ trimers. The Korean research center KRICT has synthesized a fluorine-free
version of MIL-100(Fe) which is commercially available at gram scale under the name of F100. It is also known as Fe-btc material
commercialized as Basolite® F300 (BASF). In the Scandium version the trimers can lose coordinated water to leave five-fold
coordinated Scandium cus. It was reasoned to be a strong Lewis acid catalyst as Scandium triflate is a well-known molecular Lewis
catalyst. The MIL-100(Sc) shows 100% conversion and selectivity for an intermolecular carbonyl ene reaction, whereas Zeolite beta
and Scandium exchanged zeolite beta are much less active and selective. In another study, performances of HKUST-1(Cu) and MIL100(Cr) were systematically compared with zeolites in a series of acid-catalyzed test reactions (e.g., Pechmann and Knoevenagel
condensations, acylation, Bekmann rearrangement and Prins and annulations reactions). Although it is not possible to infer a Lewis
mechanism for all the cases, one can still observe that in many cases, MOFs show higher activity and/or selectivity than zeolites.
The UiO-66 is another star among the MOF family owing its stability and catalytic activity. The structure of UiO-66 consists of
Zr6O4(OH)4 inorganic cornerstones bridged to another by terephathalate linkers. All 12 edges of the Zr6 octahedron are bridged so
that the Zr6O4(OH)4 unit result in a continuous faced-centered cubic framework structure. Upon heat-treatment at ca. 250-300°C,
dehydration of the Zr6O4(OH)4 cornerstone yields to a new composition of Zr6O6 accompanied with the creation of oxygen vacancy.
As a general note, we can see that the removal of chemisorbed water either as a single molecule (case of HKUST-1) or dissociated as
bridging hydroxyl (case of UiO-66) on the cornerstones yields metal cations with lower coordination numbers.
Although the Zr6O6 cornerstone exhibits Lewis sites Figure 1. Scheme of UiO-66 with a missing linker showing the enhanced accessibility (left). Two
which can dissociate water and thus in principle
capping water molecules replace the missing linker (center) and a cluster model of a dehydrated
UiO-66 cornerstone with one linker vacancy (right).
can activate other molecules, the congestion of
cornerstone, surrounded by the 12 ligands, prevents
the accessibility of the sites by large reactant.
Nevertheless, while an ideal crystalline UiO-66
material should have all Zr clusters surrounded by
the 12 linkers, up to four are usually missing for
the real solids, (Figure 1, left). The linker vacancies
are usually compensated by anions from the
metal precursor source (Cl- or SO42-). The resulting
Source: Author.
"defective" UiO-66 are therefore more porous than
excepted. Starting from tri- and tetratopic linkers, instead of benzenedicarboxylic acid, it is also possible by design to generate
Zr-MOFs, such as MOF 808 and NU-1000, exhibiting very large pores and additional terminal hydrogen-bonding-competent -OH
and -OH2 groups. The group of Professor Omar Fahra at Northwestern University has conducted a systematic study on the catalytic
hydrolysis of dimethyl 4-nitrophenyk phosphate (DMNP) which is a simulant of nerve agents (Liu et al. 2017). Authors concluded
that the degree of connectivity with linkers inversely correlates with catalytic activity as gauged by hydrolysis reaction rates under
fixed model conditions. In other words, nodes that present larger numbers of terminal -OH groups (as defect or not) or Zr with lower
coordination number (Lewis site) after heat-treatment exhibit the greater activity.
In principle, the MOF named ZIF-8 standing for Zeolitic Imidazolate Framework-8, synthesized from 2-methylimidazole and a zinc
salt, contains fully coordinated Zn2+ cations. It is very thermally stable as Zn accommodates a very favorable tetrahedral coordination.
Although the ideal structure does not exhibit a priori catalytic site, the material has surprisingly proven to be one of the most active
catalysts for the Knoevenagel reaction. More surprisingly, ZIF-8 outperforms state of the art catalysts currently applied at industrial
level for the transesterification of rapeseed oil triglyceride in the production of fatty acid methyl esters (FAME) (Figure 2). Even more
striking, there is a second and a priori obvious reason to think that ZIF-8 could not be active for this reaction. Triglycerides are too
bulky to penetrate inside the sodalite cavity of the ZIF-8 which exhibit apertures of 3Å at hexagonal window. The only possibility
for the catalytic reaction to occur is at the surface of crystallites. FTIR-monitored CO adsorption and DFT studies reveal that several
species can coexist at the surface especially low coordination Zn-OH species which can activate methanol molecule.

8

The Catalyst Review											

December 2019



The Catalyst Review December 2019

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