Design and Synthesis of Hydroxide Ion–Conductive Metal–Organic Frameworks Based on Salt Inclusion

We
demonstrate a metal–organic framework (MOF) design for
the inclusion of hydroxide ions. Salt inclusion method was applied
to an alkaline-stable ZIF-8 (ZIF = zeolitic imidazolate framework)
to introduce alkylammonium hydroxides as ionic carriers. We found
that tetrabutylammonium salts are immobilized inside the pores by
a hydrophobic interaction between the alkyl groups of the salt and
the framework, which significantly increases the hydrophilicity of
ZIF-8. Furthermore, ZIF-8 including the salt exhibited a capacity
for OH– ion exchange, implying that freely exchangeable
OH– ions are present in the MOF. ZIF-8 containing
OH– ions showed an ionic conductivity of 2.3 ×
10–8 S cm–1 at 25 °C, which
is 4 orders of magnitude higher than that of the blank ZIF-8. This
is the first example of an MOF-based hydroxide ion conductor.