A caveat on the effect of modulators in the synthesis of the aluminum furandicarboxylate metal‐organic framework MIL‐160

Abstract

Modulators are widely used in the synthesis of metal-organic frameworks (MOFs) for improving the porosity and morphology. For aluminum MOFs modulation has been seldom reported and as is shown here for the example of the aluminum furandicarboxylate MOF MIL-160 the positive effects of modulators are small and disadvantageous effects will be more likely. Formic acid as modulator can slightly increase the BET surface area and pore volume of MIL-160 up to a modulator:linker ratio of 1.25 : 1. Acetic acid only shows some increase in both surface area and pore volume at the smallest tested ratio of 0.125 : 1. The stronger acids oxalic acid and hydrochloric acid with the also more aluminum-coordinating anions have no positive porosity effect and decrease surface area and pore volume already at small amounts. At a 1 : 1 modulator:linker ratio for oxalic acid and at 0.75 : 1 for hydrochloric acid no porous MOF is formed anymore from the analysis by powder X-ray diffraction and nitrogen sorption. Further, thermogravimetric analysis and scanning electron microscopy suggests that none of the tested modulators has any noticeable positive effect on the introduction of linker defects or the improvement of crystallinity or crystal size.

Expanding the horizons of porphyrin metal–organic frameworks via catecholate coordination: exploring structural diversity, material stability and redox properties

“…Steinert et al recently showed that the addition of various monocarboxylic acids has no significant positive effect on the growth of Al-carboxylate MOFs (at least MIL-160) and attributed this to the relative inertness of such a cation. 96 Given that catecholate presents a stronger affinity for Al( iii ) than carboxylate, 97 this suggests that in addition to using modulators and ligands with similar binding strength as generally accepted, 50 this binding strength must be high enough to achieve a proper control of the crystal growth.…”