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The results reveal that E (%) is 58.2%, 55.6%, 49.3
% for MIL-101, HKUST-1 and ZIF-8, respectively.
This trend could be attributed to the complex
formation reaction between TCNs and
coordinatively unsaturated (CUS) sites in MIL-101
and HKUST-1. The absence of open metal site may
have an impact in the case of ZIF-8 for its relatively
lowered adsorption performance.
3.3 Effect of Ionic Liquids
One of the alternatives considered to upgrade the
adsorption properties of those MOFs was to decorate
with ILs following a post-synthetic modification. It
was expected to lead to a higher performance for
adsorption than the pristine MOFs. Ten ionic liquids
were tested as potential guests, introduced into the
pores of virgin MOFs targeted for TCs adsorption.
Tetracyclines are complex hydrophilic drugs having
high solubility in water and can exist both in acidic
and basic form. The highest adsorption efficiency of
77.4% were obtained for the three TCs during the
combination of [HBth][PF6] with ZIF-8. The best
result was specifically obtained when [HBth][PF6]
to ZIF-8 ratio was increased to 25 % as depicted on
Table 1. The high adsorption efficiency of the
IL@MOFs could be due to the capability of ILs to
create complex interaction of columbic forces and π-
π interaction with TCs. Generally the ILs improves
the adsorption efficiency of all the studied MOFs to
a greater extent.
3.4 Doping by Capillary Action
Once [HBth][PF6] @ZIF-8 was selected as the best
adsorbent, improving the stability of the hybrid
composite, in order to keep the ILs inside the pores
of MOF was considered. The effect of capillary
action was studied as an alternative synthesis
strategy, since ZIF-8 has no open metal sites. The
[HBth][PF
6
] was mixed with activated ZIF-8 powder
using mortar and pestle in a molar ratio of 0.20:1,
0.25:1, 0.30:1 and 0.35:1. The mixture was heated
and stored overnight to enhance the diffusion of the
IL through pores of ZIF-8. As shown in Table 2, the
adsorbed quantities of TCNs increased to a
significant extent until the IL to MOF ratio became
0.30 to 1. The capillary action improves the
confinement of IL inside ZIF-8 pores and hence, the
stability of the resulting composite, which brought
great potential in the way of increasing the
adsorption efficiency of TCNs.
4 CONCLUSIONS
In summary, imidazolium, hydroxyquinolium and
benzothiazolium ionic liquids were impregnated on
water stable metal organic frameworks and
investigated for adsorption of TCNs. ZIF-8 when
combined with one type of benthothiazolium IL
showed highest adsorption efficiency, especially
after doping by capillary action. The result proves
that the adsorption properties of MOFs could be well
improved by using appropriate ILs.
ACKNOWLEDGEMENTS
The authors thank the National Natural Science
Foundation of China (No. 81373284, 81673316).
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