Nanoscale Synthesis of Two Porphyrin-Based MOFs with Gallium and Indium

Two porphyrin-based
metal–organic frameworks (MOFs) containing gallium or indium,
[Ga2(OH)2(H2TCPP)]·3DMF·3H2O (Ga-PMOF) and [In2(OH)2(H2TCPP)]·3DMF·4H2O (In-PMOF) (H6TCPP
= 4-tetracarboxyphenylporphyrin), were discovered using high-throughput
methods. The structure was refined by the Rietveld-method starting
from the structure model of Al-PMOF, [Al2(OH)2(H2TCPP)]. The new PMOFs exhibit BET surface areas between
1150 and 1400 m2 g–1 and are also porous
toward CO2 (Ga-PMOF, 15.2 wt %; In-PMOF, 12.9 wt %). They
are thermally stable in air up to 330 °C, but show limited chemical
stabilities toward acids and bases. In order to achieve size control,
different synthesis routes were investigated, i.e., batch synthesis
at different temperatures (yield: In-PMOF-bs-th 96%, Ga-PMOF-bs-th
87%), ultrasound-assisted synthesis (yield: In-PMOF-bs-us 85%), and
continuous-flow synthesis (yield: Ga-PMOF-cf 71%). By using these
different methods we could control the nucleation rate and the crystal
size. The crystal sizes were found to vary about 60 to 160 nm and
70 to 130 nm for Ga- and In-PMOF, respectively, which was proven by
dynamic light scattering (DLS), powder X-ray diffraction (PXRD), scanning
electron microscopy (SEM), and transmission electron microscopy (TEM)
measurements.