SESSION 1: CATALYSIS AND ADSORPTION IN MOFS
Catalytic and Adsorptive Applications of MOFs Based on Tetravalent Cations
c-MACS - Centre for Membranes, Adsorption, Catalysis and Spectroscopy, KULeuven
Keywords: Ti-MOF, Ce-MOF, MOF-808, bio-oil separation, catalytic C-H activation
In a first part of the lecture, we will explore the structure and reactivity of novel MOFs based on tetravalent cations, such as Ti4+, Ce4+ and Zr4+. We will elucidate the structure of a Ti-MOF material consisting of Ti4+-O2--Ti4+ chains, linked by 4,4’-biphenyldicarboxylate (bpdc) linkers. The eventual structural elucidation was only possible using a combination of advanced electron diffraction and solid state NMR. The redox photochemistry of the material will be discussed, and due to its high number of open coordination sites on Ti4+, the material is also an excellent catalyst for the oxidation of aromatic thiophenes. Next we will address new insights in the chemistry of Ce MOFs. While the Ce-UiO-66 material was originally reported to catalyze the TEMPO-mediated alcohol oxidation, we will now show that similar materials can also mediate redox processes of nitrogen oxides.
The chemistry of the Zr centres in UiO-66 and MOF-808 continues to be a rich source of inspiration in catalysis and adsorption. For instance, MOF-808 is one of the most active catalysts for the oxidation of aromatic thiophenes, and provided its polarity properties are well adjusted, it can also be used to catalyze epoxidation reactions. MOF-808 is a particularly useful material in complex separations of biobased compounds, like encountered in bio-oil: due to its large number of open coordination sites onthe Zr clusters, MOF-808 selectively adsorbs guaiacols (2-methoxyphenols) and dihydroxybenzenes from a large variety of other compounds, including furanics, alcohols, sugars, acids etc. Besides these applications that exploit the intrinsic reactivity of MOF framework atoms, MOFs can also be considered as one of the most powerful platforms to design immobilized homogeneous catalysts. We will illustrate this with the currently highly topical class of C-H activation reactions. The overall aim here is to find shortcuts to the currently established reactions like the Suzuki, Heck or Buchwald amination reactions. While the latter use pre-oxidized arene reactants (e.g., bromoarenes, iodoarenes) in combination with Pd catalysts, there is an interest to catalytically activate C-H bonds, rather than C-Br or C-I bonds. Such reactions are possible with Pd as well, but the reaction cycle then starts with Pd2+, which might act as an electrophile, but can also activate an arene C-H via a concerted-metallation deprotonation.
A first catalyst that will be discussed is a material allowing the dehydrogenative coupling of 2 arenes to form biaryl compounds. For instance, coupling of ortho-xylene results in bis-xylyl, which is a direct precursor in the production of the high performance Upilex polyamide material. We will show that Pd2+ can be directly grafted on uncovered Zr6 clusters of MOF-808, resulting in a three-fold increase of the lifetime of the Pd catalyst. Alternatively, we will demonstrate that the building brick of MOF-808 can as well be used to dock carboxylic organic ligands –as an example we will present a tetrahydrothiophene-2,5-dicarboxylic acid, which as a ligand can support the catalytic activity of Pd2+ in a variety of C-H activating reactions.
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 Van Velthoven, De Vos et al., Chem. Sci. 10, 3616, 2019