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20 Chapter 2<br />
Cr III CUSs of MIL-101, displaying enhanced activities in the Knoevenagel condensation of<br />
benzaldehyde and ethyl cyanoacetate.<br />
Beyond the above mentioned PSM approaches, numerous conceptually different postsynthesis<br />
routes have been emerging. Intriguing building block replacement (BBR), which<br />
involves replacement of key structural components of the MOF, opens up a very broad<br />
strategy for the synthesis of functionalized isostructural MOFs featuring gradient<br />
compositions and chemical properties. Solvent-assisted linker exchange (SALE), [109-113]<br />
which is also termed as “stepwise synthesis”, [114] , “bridging linker replacement”, [114] “postsynthetic<br />
ligand exchange”, [115] isomorphous ligand replacement, [116] , is one of the<br />
important method to achieve this kind of modification on MOFs. Conceptually, by the<br />
reaction of a parent MOF and a solution of a second linker, a daughter MOF retaining the<br />
parent MOF topology can be obtained. Besides, partial substitution of the metal ions in a<br />
given framework with other metal ions of similar size and coordination chemistry could<br />
be accomplished by the so-called “post-synthetic metal ion exchange”, [117] (also known as<br />
“transmetalation”, [118] “metal metathesis” [119] or “metal-ion exchange” [120-121] ). Kahr et al.<br />
obtained a series of mixed-metal material Mg/Ni-MOF-74 with various degrees of Ni 2+<br />
incorporation by the post-treatment of Mg-MOF-74 and Ni 2+ solution with weak acid. [122]<br />
These materials showed increased stability and improved accessible porosity compared<br />
with the parent Mg-MOF-74 and Ni-MOF-74.<br />
2.3.3.4 Mixed-component co-assembly MOFs<br />
In addition of the post-synthetic modification on MOFs, co-assembly mixed-component<br />
(MC) MOFs which is related to the co-crystallizing of linker or metal-nodes mixtures for<br />
the introduction of functionalization should not be overlooked. Several research groups<br />
reported simultaneous utilizing two or more functionalized linkers during synthesis<br />
yielding to the formation of mixed-linker MOFs (MIXMOFs) or multivariate MOFs (MTV-<br />
MOFs). [123-128] In fact, Yaghi et al. revealed that a number of linkers with distinct functional<br />
groups (up to eight) can be incorporated into MOF-5 within one phase (Figure 2.13). [123]<br />
Curiously, the properties of MTV-MOFs are not just simply a linear combination of those<br />
of the pure components. For instance, MTV-MOF-5-EHI, as a member of this series,<br />
demonstrates strikingly better selectivity for CO2/CO in comparison with its best samelink<br />
counterparts. Moreover, partial metal substitution can be achieved by directly<br />
copolymerization via mixed-metal solid solutions, [126, 129-135] where more than one type of