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22 Chapter 2<br />
Figure 2.14. Schematic presentation of the metal-based solid solution approach by mixing two<br />
different metal centers during the synthesis. Inspired by Burrows. [126]<br />
2.3.3.5 Defect-engineered MOFs<br />
On the basis of the concept of mixed-component MOFs, a very new approach to introduce<br />
functionalized MOFs has been reported very recently. [136] This is related to generation of<br />
so-called “defect-engineering MOFs” (DEMOFs), where in most cases the CUSs are<br />
modified or generated through the incorporation of additional “defect” linkers,<br />
modulators or through post-synthetic treatment. [137] The distinction of DEMOFs mainly<br />
related to the degree of deviation of the local structure as well as the long-ranging<br />
ordering of a variable fraction of the linkers and metal nodes in comparison with the<br />
parent MOF. [138-143] However, this kind of deviation barely can be observed from MC-<br />
MOFs. Only intrinsic defect may be present in certain conditions. [144-145] Besides, another<br />
important features of DEMOFs which should not be overlooked is the correlation and<br />
clustering of defect sites. Goodwin et. al used a combination of measurements and<br />
demonstrated the controlled introduction of nanoscale correlated defects within a<br />
hafnium terephthalate MOF UiO-66. [146] In particular, Fischer et al. reported the<br />
incorporation of pyridine-3,5-dicarboxylate (pydc) into the HKUST-1 and its Ru analog<br />
(Ru-MOF). [138-139] Remarkably, doping pydc resulted in the formation of mCUSs in the<br />
obtained Ru-DEMOFs, in which along with the Ru 2+ -/Ru 3+ -species typically found in the<br />
parent Ru-MOF framework, reduced Ru δ+ -species (0 < δ < 2) were generated. Moreover,<br />
such Ru-mCUSs appeared to be responsible for the activity of Ru-DEMOFs in the CO<br />
sorption as well as the catalytic hydrogenation of olefins. [138] Furthermore, doping pydc<br />
or other similar defect linker (DL) such as 5-hydroxyisophthalic acid (5-OH-ip) into<br />
HKUST-1(Cu-MOF), the creation of mesopores in the final Cu-DEMOFs was observed<br />
dependent on the utilized Cu-precursors and the incorporated level of DL. [139] Due to
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