posted on 2020-11-10, 20:45authored byYunlei Shi, Dazhen Xiong, Zhiyong Li, Huiyong Wang, Jikuan Qiu, Hucheng Zhang, Jianji Wang
In
recent years, metal–organic frameworks (MOFs) have been
explored as emulsifiers for the fabrication of Pickering emulsions
and then used for hybrid material synthesis and interface catalysis.
Nevertheless, stimuli-responsive Pickering emulsions stabilized by
MOFs have been rarely reported so far, although they are of great
importance for fundamental research studies and practical applications.
Herein, for the first time, triethylenetetramine (TETA)-functionalized
MOFs (ZIF-90/TETA) have been designed, synthesized, and used for fabricating
CO2-/N2-response Pickering emulsions. It is
shown that even at the ZIF-90/TETA content of 0.25 wt %, the functional
MOF can still efficiently emulsify n-hexane and water
to form a high internal phase Pickering emulsion. Importantly, the
Pickering emulsion can be easily and reversibly switched between emulsification
and demulsification by bubbling of CO2 and N2 alternatively at atmospheric pressure. The possible mechanism of
the CO2/N2 switchable emulsion is investigated
by zeta potential, water contact angle, interfacial tension, 13C NMR spectroscopy, and an optical microscope. It is found
that the acid–base reaction of CO2 with TETA anchored
on the surface of ZIF-90 leads to the production of hydrophilic ammonium
bicarbonate and carbamate, which results in the emulsification of
the Pickering emulsion. However, when N2 is bubbled to
remove CO2, the reverse reaction takes place to cause the
demulsification of the Pickering emulsion. Moreover, the CO2/N2 switchable Pickering emulsion has been successfully
used as a microreactor for Knoevenagel reactions to demonstrate a
highly efficient integration of chemical reaction, product separation,
and ZIF-90/TETA recycling for a sustainable chemical process.