resolver

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