92
another in the linker, 1,4-benzene dicarboxylic acid, allowing for linear linkages to
an octahedral node (basic zinc acetate clusters). This results in the vertices of cubes
that are linked together in 3-dimensions, leaving relatively large pores that are stable even upon evacuation [16].
MOFs have found increasing utility in the last 10–20 years. Their permanent and
tunable porosity has been exploited in a number of ways. The first major application
was in hydrogen storage, as their low density leaves significant open space for guest
molecules. Other MOFs have found usefulness for controlled luminescence [18],
sensing, filtration, and CO 2 storage, to name a few [19]. The first class of MOFs
consisted of carboxylated organic molecules bound to Lewis acid metals or metal
clusters as nodes, but many of these prototypical MOFs exhibited water sensitivity
[16]. A newer subclass of MOFs, so-called “ZIFs,” consist of imidazole ligands
cross-linked to metals. Because of the angle of the bond (145° for the N-Zn-N linkage in ZIFs and the O-Si-O in zeolites), the geometry of these particular MOFs is
comparable to zeolites, and hence termed “Zeolitic imidazolate frameworks” or
ZIFs. These are often more stable to aqueous conditions than other MOFs, often
withstanding extreme pH conditions. Currently, one of the most stable MOFs is
ZIF-8, which is composed of 2-methylimidazole bound to Zn, and can withstand
boiling in high pH, aqueous solvents [17]. There are three particularly stable MOFs
of note that appear most often in the related literature: including MIL-101 (M = Cr
or Fe) (chromium terephthalate or iron terephthalate, named after the Materials
Institut Lavoisier ([M 3 F(H 2 O) 2 O(BDC) 3 ·n H 2 O] (n ≈ 0.25, BDC = 1,4- benzenedica
rboxylate)) [20], HKUST-1 (Copper benzene-1,3,5-tricarboxylate, named after
Hong Kong University of Science and Technology) [21], and the aforementioned
ZIF-8. (See Fig. 3).
MOFs can be treated as modular materials, wherein a particular functionality can
be added, and hence studied, separately. Not only do MOFs allow for a means to
increase the efficiency of ion conductivity, but present adjustable platforms for performing fundamental studies of mechanisms that are not well-understood. The
number of publications of MOFs utilized for ion conductivity has exponentially
grown since the first reports in 2009. Significant advances have been made with a
number of PCPs, particularly in terms of proton and hydroxide conductivity. Due to
their high porosity, which leads to organic moiety accessibility, MOFs are amenable
to internal functionalization of groups that have freedom of movement to allow for
ionic conductivity. This can also allow for incorporation of linkers that are unstable
or not selective during the initial synthesis conditions, or partial exchange of linkers
with new ones, for example. The alterations can be accomplished via post-synthetic
modification (PSM) [22, 23] or via incorporation of guests into the open spaces.
Such alterations to the framework can be correlated to the idea of “doping” in inorganic solids. MOFs have been shown to be flexible and will allow for movement to
encapsulate ions, either via moieties on the framework, or incorporation within the
pores, while maintaining a degree of regularity and directionality [24]. Thus, anisotropic ionic transport has been observed, allowing for a deeper understanding of the
mechanisms involved. A recent review of porous coordination polymers and MOFs
for ionic conductivity is very relevant here: Protic guests were used to allow for
C. A. Bauer
another in the linker, 1,4-benzene dicarboxylic acid, allowing for linear linkages to
an octahedral node (basic zinc acetate clusters). This results in the vertices of cubes
that are linked together in 3-dimensions, leaving relatively large pores that are stable even upon evacuation [16].
MOFs have found increasing utility in the last 10–20 years. Their permanent and
tunable porosity has been exploited in a number of ways. The first major application
was in hydrogen storage, as their low density leaves significant open space for guest
molecules. Other MOFs have found usefulness for controlled luminescence [18],
sensing, filtration, and CO 2 storage, to name a few [19]. The first class of MOFs
consisted of carboxylated organic molecules bound to Lewis acid metals or metal
clusters as nodes, but many of these prototypical MOFs exhibited water sensitivity
[16]. A newer subclass of MOFs, so-called “ZIFs,” consist of imidazole ligands
cross-linked to metals. Because of the angle of the bond (145° for the N-Zn-N linkage in ZIFs and the O-Si-O in zeolites), the geometry of these particular MOFs is
comparable to zeolites, and hence termed “Zeolitic imidazolate frameworks” or
ZIFs. These are often more stable to aqueous conditions than other MOFs, often
withstanding extreme pH conditions. Currently, one of the most stable MOFs is
ZIF-8, which is composed of 2-methylimidazole bound to Zn, and can withstand
boiling in high pH, aqueous solvents [17]. There are three particularly stable MOFs
of note that appear most often in the related literature: including MIL-101 (M = Cr
or Fe) (chromium terephthalate or iron terephthalate, named after the Materials
Institut Lavoisier ([M 3 F(H 2 O) 2 O(BDC) 3 ·n H 2 O] (n ≈ 0.25, BDC = 1,4- benzenedica
rboxylate)) [20], HKUST-1 (Copper benzene-1,3,5-tricarboxylate, named after
Hong Kong University of Science and Technology) [21], and the aforementioned
ZIF-8. (See Fig. 3).
MOFs can be treated as modular materials, wherein a particular functionality can
be added, and hence studied, separately. Not only do MOFs allow for a means to
increase the efficiency of ion conductivity, but present adjustable platforms for performing fundamental studies of mechanisms that are not well-understood. The
number of publications of MOFs utilized for ion conductivity has exponentially
grown since the first reports in 2009. Significant advances have been made with a
number of PCPs, particularly in terms of proton and hydroxide conductivity. Due to
their high porosity, which leads to organic moiety accessibility, MOFs are amenable
to internal functionalization of groups that have freedom of movement to allow for
ionic conductivity. This can also allow for incorporation of linkers that are unstable
or not selective during the initial synthesis conditions, or partial exchange of linkers
with new ones, for example. The alterations can be accomplished via post-synthetic
modification (PSM) [22, 23] or via incorporation of guests into the open spaces.
Such alterations to the framework can be correlated to the idea of “doping” in inorganic solids. MOFs have been shown to be flexible and will allow for movement to
encapsulate ions, either via moieties on the framework, or incorporation within the
pores, while maintaining a degree of regularity and directionality [24]. Thus, anisotropic ionic transport has been observed, allowing for a deeper understanding of the
mechanisms involved. A recent review of porous coordination polymers and MOFs
for ionic conductivity is very relevant here: Protic guests were used to allow for
C. A. Bauer