Strategies to Incorporate Mesoporosity - Mesoporous Organic-Inorganic Non-Siliceous Hybrid Materials - page 27

3.1.1 Ligand Extension
Crystalline hybrids are constructed from the regular linkages of metal centers and
organic groups, which can achieve well-defined pores, high surface area, and large
porosity. Nevertheless, their pore sizes are typically restricted to the microporous
range. Thus, the synthesis of mesoporous hybrids is envisioned to improve the trans-
mission capability of the pores while facilitating practical applications that require
the diffusion of bulky molecules [ 8 ]. Using extended ligands or bulky secondary
building blocks is an apparent strategy. Disappointedly, linker expansion tends to
result in the reduced surface areas and pore sizes due to the consequent interpen-
etrated structures and dramatically reduces the stability of the framework upon
solvent removal from the porous hosts [ 9 ]. An elaborately designed ligand with hier-
archical functional groups, 4,4′,4″- s -triazine-1,3,5-triyltri-p aminobenzoate, could
be devised to extend the linkers while inhibiting interpenetration and reinforcing the
framework against disintegration upon guest removal [ 10 ]. The mesoporous MOF
was prepared through a one-pot solvothermal method, followed by stabilization at
pH values around 3.0. The amino groups in the ligand were prearranged so that they
would not participate in the framework formation but could accept protons after the
network was generated, giving rise to a stable mesoporous MOF up to 300 °C. The
N 2 sorption isotherm exhibited a typical type IV behavior, and the X-ray diffraction
(XRD) data confirmed that the open channels were identical in size and as large as
22.5 26.1 Å. Similarly, Schröder et al. reported the synthesis of (3,24)-connected
mesoporous framework NOTT-119 by adopting a nanosized C 3-symmetric hexacar-
boxylate linker, presenting a high surface area of 4,118 m 2 g − 1 and pore sizes in the
range of 2.4-4.5 nm [ 11 ]. The hybrid framework is stable up to 315 °C. When the
dimension of the linker was enlarged beyond this point, the network could no longer
hold stability to thermal treatment and suffered disruption of the structure owing to
surface tension effects. A homologous series of palindromic oligophenylene deriva-
tives terminated with ʱ -hydroxy-carboxylic acid functions were targeted to afford
linear and robust building blocks for expanding the pore size to up to 9.8 nm [ 12 ],
which is the largest channel achieved via the ligand-expanding method to date. All
members had non-interpenetrating structures and exhibited robust architectures, as
evidenced by their permanent porosity and high thermal stability. This strategy for
making MOFs with large pore apertures and avoiding the problem of interpenetra-
tion is to start with a framework in which one can maintain a short axis with the
long organic links inclined to that axis. The short axis effectively eliminates the pos-
sibility of interpenetration because it is the distance between the links along that
axis joining the secondary building units.
A multidimensional ligand, tetrakis-1,3,5,7-(4-phosphonatophenyl)adaman-
tine (Fig. 3.2 ), which could impede the formation of a close-packed arrange-
ment of organic molecules in an organic-inorganic hybrid framework, was
used to fabricate mesoporous metal phosphonate materials [ 13 ]. The prepara-
tion was accomplished through a non-hydrolytic condensation process between
the tetraphosphonic acid and titanium(IV) isopropoxide in DMSO, permitting
the generation of mesopores. N 2 sorption experiments revealed the presence of