resolver

12 Chapter 2
the material is retained. Materials undergoing such kinds of phase transitions are often
called soft porous crystals. [37] The effect of large framework flexibility induced by specific
host-guest interaction is also known as “breathing”. [43] One of the best-studied flexible
systems is the MIL-53 system ([M(bdc)(OH)]n, M 3+ =Al 3+ , [44] Fe 3+ , [45] Cr 3+ , [46] Ga 3+ , [47]
In 3+ , [48] Sc 3+ , [49] ). These frameworks demonstrate different crystal phases according to the
activation state and the incorporation of guest molecules. Thus, after activation MIL-53
reveals a high-temperature phase with large pores (lp), which can shrinks to a narrowpore
phase (np) upon the adsorption of low amounts of polar molecules (Figure 2.8).
Moreover, a closed-pore phase was observed after activation of MIL-53(Fe) [45] and MIL-
53(Sc). [49] In addition to these famous MILs-53, some M2-PW based pillared-layered MOFs
also exhibit certain degree of flexibility upon guest adsorption. [50-52]
Figure 2.8.The different forms of MIL-53: (a) as synthesized (as); disordered terephthalic acid
molecules lie within the tunnels; (b) high temperature (open), lp; (c) room temperature hydrated
form (hydr.), np. Note the changes in the cell parameters during the thermal treatments
Reproduced from Chem. Soc. Rev., 2008, 37, 191-214 with permission of The Royal Society of
Chemistry. [53]
Features highlight 3-Good thermal stability even good chemical stability
Thermal stability of MOFs ranges from 250 °C to 500 °C. [25, 31, 54-55] . Chemically stable
MOFs, however, are challenging to be made due to their susceptibility to linkdisplacement
reactions under the treatment with solvents over extended periods of time.
Zeolitic imidazolate framework ZIFs, in particular ZIF-8 (Zn(MIm)2, MIm = 2-
methylimidazolate) is one of the prominent examples which reveals exceptional chemical
stability. [31] Remarkably, the structure integrity is retained after immersion ZIF-8 into
boiling methanol, benzene, or water for up to 7 days. Besides, its treatment in