Modification and Potential Applications of Organic–Inorganic Non-Siliceous Hybrid Materials - Mesoporous Organic-Inorganic Non-Siliceous Hybrid Materials - page 78

for the separation of only neutral compounds [ 23 , 24 ]. However, packed columns
usually result in low resolution as a result of peak broadening, which impairs the
separation efficiency of these hybrid materials. Recently, a series of hexagonal
periodic mesoporous metal phosphonates with semicrystalline pore walls syn-
thesized by microwave irradiation were first employed as the stationary phase in
the open-tubular capillary electrochromatography (OTCEC) separation technique
[ 25 ], which combines the efficiency of capillary electrophoresis and the selectivity
of HPLC. The presence of functional groups inside the framework and the excel-
lent properties of the obtained metal phosphonates, including well-ordered pores
with large surface area and pore volume and high thermal stability, improved
selectivity and so encouraged their use as the stationary phase in wall-coated
OTCEC separation for various substances including acidic, basic, and neutral
compounds. In the case of neutral compounds including benzene, nitrobenzene,
naphthalene, and anthracene (Fig. 5.4 ), the elution order was benzene < nitroben-
zene < naphthalene < anthracene on the mesoporous titanium phosphonate-coated
capillary, suggesting a hydrophobicity mechanism for the separation of the spe-
cies. In contrast, for the ordered mesoporous titanium phosphonate constructed
from the same phosphonate linkers (EDTMP) but with an amorphous framework,
benzene, naphthalene, and anthracene could be well separated, while benzene and
nitrobenzene could not be completely separated. This indicated that, in addition to
hydrophobic interactions, suitable polarization of the crystalline mesoporous phos-
phonates is responsible for the effective separation of benzene and nitrobenzene.
Since the separation efficiency mainly depends on the host-guest interactions
between stationary and mobile phases, such as hydrophobicity-hydrophobic-
ity interaction, polarity, and intermolecular forces, phosphonate hybrid materi-
als, constructed by long alkyl chains [ 21 ], multidimensional linkages [ 26 , 27 ],
or phosphonic linkers with polar pendant groups [ 28 , 29 ], can also be used in
chromatography techniques, and this is worth exploring. Three non-, micro- and
mesoporous Cd-MOF isomers were used to prepare mesoMOF, and the resultant
Fig. 5.4 OTCEC separation
of neutral compounds at
pH = 3.0. PMTP-1 and
EDTMP-Ti stand for
amorphous and crystalline-
ordered mesoporous titanium
phosphonates, respectively.
Reprinted with permission
from Ref. [ 25 ]. Copyright
2011, Wiley-VCH