The concentration of benzoic acid was found to exercise efficient control over the formation of either MIL-101(Cr) or MIL-88B(Cr) under otherwise similar hydrothermal synthetic conditions. Nanocrystals of MIL-101(Cr) with ∼100 nm average size and excellent SBET = 3467 m2 g−1 are obtained at lower concentrations of benzoic acid, while at higher concentrations the microparticulated MIL-88B(Cr) product is formed. Hereby a new efficient synthetic method towards the elusive MIL-88B(Cr), yet reported only once without synthetic details, is proposed. The obtained MIL-88B(Cr) has an interesting and potentially valuable property of retaining its high-volume form (Vcell ∼ 2000 Å3) after thermal activation. The degassing of MIL-88B(Cr) in a vacuum at 250 °C yields a porous material with a SBET area of 1136 m2 g−1, which is around the theoretical maximum. The transition to the denser ‘closed’ form (Vcell ∼ 1500 Å3) occurs only at 350 °C, when all of the benzoate/benzoic acid, hindering the process, is removed.
Propionic acid could efficiently affect the formation of Cr-benzenedicarboxylate metal-organic frameworks (MOFs), giving rise to yielding products with various structures. Generally, nano-sized crystals of MIL-101(Cr) can be prepared with comparatively lower concentration of propionic acid (0.6 M), which possessed high porosity (S BET = 2720 m 2 /g). The hierarchically porous (HP) MIL-101(Cr) was obtained with medium concentration of propionic acid (1.2 M), and micro-rod like crystals of MIL-88B(Cr) was formed in high concentration of propionic acid (1.8 M). Compare to bulk MIL-101(Cr) and nano-sized MIL-101(Cr), HP-MIL-101(Cr) showed much higher uptake of methylene blue (119.1 mg/L) in aqueous. This approach can offer a facile and effective pathway to synthesize stable hierarchically porous Cr-benzenedicarboxylate MOF for the applications in pollutant treatment fields.
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