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Fig. 5.2
TGA records
of CO
2
adsorption for
the titanium phosphonate
samples, tested at 40 °C.
Reprinted with permission
from Ref. [
15
]. Copyright
2011, Royal Society of
Chemistry
It is noteworthy that the adsorption capacity of porous metal phosphonates
remained stable even after multiple cycles, making them promising in practical
applications [
10
]. Increasing the CO
2
-framework interactions and improving the
porosity of the phosphonate hybrid frameworks present the major challenge and
bottleneck in capturing CO
2
. Creating open metal sites and introducing basic sites
have been shown to effectively increase the interactions. Moreover, in compari-
son with the carboxylate-based MOFs, the storage of fuel gases such as hydrogen
and methane on porous metal phosphonates is scarcely reported. Rigorous inves-
tigation toward highly stable and affordable metal phosphonates for theoretical
research and practical applications is urgently needed.
5.1.2 Liquid Adsorption and Separation
Adsorption from liquid phase is much more complicated as compared with gas-
phase adsorption, due to the competitive behaviors between solute and solvent for
the solid surface. The adsorption of a solute is mainly dependent on the molecu-
lar sizes and physicochemical properties and on the textual properties and surface
chemistry of an adsorbent. It can be envisioned that metal phosphonates can per-
form as good adsorbents for the removal of heavy metal ions from waste waters,
due to that the organic functionalities serve the formation of complexes with metal
ions through acid-base interactions, and the easy separation of the loaded solid
adsorbent from liquid phase is preferable.
Besides the majority of efforts have been concentrated on the gas adsorption
or storage, mesoporous hybrid materials have recently been developed as adsor-
bents for the removal of heavy metal ions, in which the organic functionali-
ties of these adsorbents serve the formation of complexes with heavy metal ions
through acid-base interactions. For instance, the coupling organophosphonic
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