Mixed-linker zeolitic
imidazolate frameworks (ZIFs) with the sodalite
(sod) topology type and based on ZIF-7 have been prepared
by direct synthesis from the mixtures of benzimidazole (BzIm) and
4,5-dichloroimidazole (dcIm). Incorporation of dcIm into the ZIF-7
structure gives ZIF-7/COK-17 hybrids with rhombohedral symmetry that
do not show the “open-to-closed form” structural transition
upon solvent removal exhibited by ZIF-7. They show Type I isotherms
for low molecular weight gases and high affinity for CO2 even at low partial pressures. Synthesis under mild conditions gives
ZIF nanoparticles (250–400 nm) suitable for incorporation into
mixed matrix membranes (MMMs): these were prepared with both glassy
(Matrimid) and rubbery (PEBAX 1657) polymers. Permeation tests at
298 K and 1.2 bar reveal that the incorporation of Zn(BzIm0.55dcIm0.45)2 nanoparticles at up to ca. 12 wt
% gives defect-free membranes with enhanced CO2 permeability
in both polymer matrices, with retention of selectivity (Matrimid)
or with an enhancement in selectivity that is most pronounced for
the smaller nanoparticles (PEBAX). The membrane with the best performance
exhibits a selectivity of ca. 200 for CO2/N2 (a 4-fold increase compared to the pure polymer) and a CO2 permeability of 64 Barrer. At the relatively low loadings investigated,
the MMMs’ performance obeys the Maxwell model, and the intrinsic
property of diffusivity of the ZIFs can be extracted as a result.