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3.1 Template-Free Self-assembly Synthesis Strategy
In recent years, researchers have paid much attention toward the synthesis of
nanostructured porous hybrid materials through template-free self-assembly strate-
gies, which do not require the use of preformed templates or structure-directing
agents. These routes usually initiate the assembly from the interactions between
the precursor molecules, and the ordered attachment allows the formation of
porous morphologies.
Microporous organic-inorganic hybrid materials are often known as crystal-
line MOFs, which involve the strong and regular coordination of metal ions and
organic linkage moieties, thus leading to porous framework structures. Figure
3.1
presents some typical organic acid ligands in obtaining mesoporous non-sili-
ceous hybrid materials. Compared with typical microporous metal carboxylates,
metal phosphonates have exhibited higher chemical and thermal stability due
to the strong affinity and chelation of organophosphonic linkers to metal ions.
Metal phosphonate hybrids often come up in the form of dense layered motifs,
which have evolved into the field of organic-inorganic hybrids by appending
organic pillars of the rigid inorganic layers [
1
]. It should be recognized that the
pillars are too crowded and insufficient free space remains in the interlayer region,
and no or poor porosity is expected to be present [
2
]. Several tactics have been
adopted to create porosity in the metal phosphonate frameworks. The first route
is the substitution of aryl biphosphonic acid by some non-pillaring groups, such
as phosphoric, phosphors, and methylphosphonic acids, leading to the presence
of interlayer pores and an increase in the surface area [
3
]. Although porous phos-
phonates can be obtained, the problem of this approach is that the replacement
is random and uncontrollable, and the accurate structural characterization and
a narrow pore size distribution are still challenges. Secondly, the geometry of a
large and multidimensional polyphosphonic bridging molecules would disfavor
the formation of the layered motif and thereby necessitate an open framework.
A third approach would be to attach a second functional group to the phosphonate
ligand to coordinate with the metal centers and disrupt the structure away from the
layers [
4
].
Metal-sulfonate networks have been studied considerably less than other
kinds of hybrid materials because of the relatively weak coordination interactions
between the sulfonate anions and metal cations, making the frameworks insuf-
ficiently robust to sustain permanent porosity [
5
]. Metal sulfonates have been
considered as potential analogues of layered metal phosphonates [
6
]. The rigid
inorganic layers provide scaffolds of regular anchor points for pendant organic
groups. In keeping with the theme of using larger cores with regard to the porous
phosphonates to disperse the crossing sulfonate groups, the pillaring group,
1,3,5-tris(sulfomethyl)benzene could be envisioned to open channels between the
layers of a metal sulfonate [
7
].
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