The targeted incorporation of defects into crystalline matter allows for the manipulation of many properties and has led to relevant discoveries for optimized and even novel technological applications of materials. It is therefore exciting to see that defects are now recognized to be similarly useful in tailoring properties of metal-organic frameworks (MOFs). For instance, heterogeneous catalysis crucially depends on the number of active catalytic sites as well as on diffusion limitations. By the incorporation of missing linker and missing node defects into MOFs, both parameters can be accessed, improving the catalytic properties. Furthermore, the creation of defects allows for adding properties such as electronic conductivity, which are inherently absent in the parent MOFs. Herein, progress of the rapidly evolving field of the past two years is overviewed, putting a focus on properties that are altered by the incorporation and even tailoring of defects in MOFs. A brief account is also given on the emerging quantitative understanding of defects and heterogeneity in MOFs based on scale-bridging computational modeling and simulations.
Mixed precious-group metal–organic frameworks [RuxRh3−x(BTC)2] of the HKUST-1-type were synthesized and characterized (PXRD, BET, IR, Raman, XPS, TGA, SS-UV/VIS, EA, and HR-TEM).
A methodology
is introduced for controlled postsynthetic thermal defect engineering
(TDE) of precious group metal–organic frameworks (PGM-MOFs).
The case study is based on the Ru/Rh analogues of the archetypical
structure [Cu3(BTC)2] (HKUST-1; BTC = 1,3,5-benzenetricarboxylate).
Quantitative monitoring of the TDE process and extensive characterization
of the samples employing a complementary set of analytical and spectroscopic
techniques reveal that the compositionally very complex TDE-MOF materials
result from the elimination and/or fragmentation of ancillary ligands
and/or linkers. TDE involves the preferential secession of acetate
ligands, intrinsically introduced via coordination modulation during
synthesis, and the gradual decarboxylation of ligator sites of the
framework linker BTC. Both processes lead to modified Ru/Rh paddlewheel
nodes. These nodes exhibit a lowered average oxidation state and more
accessible open metal centers, as deduced from surface-ligand IR spectroscopy
using CO as a probe and supported by density functional theory (DFT)-based
computations. The monometallic and the mixed-metal PGM-MOFs systematically
differ in their TDE properties and, in particular in the hydride generation
ability (HGA). This latter property is an important indicator for
the catalytic activity of PGM-MOFs, as demonstrated by the ethylene
dimerization reaction to 1-butene.
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