Benzoic acid as a selector-modulator in the synthesis of MIL-88B(Cr) and nano-MIL-101(Cr).

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 similar to 100 nm average size and excellent S-BET = 3467 m(2) 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 (V-cell similar to 2000 angstrom(3)) after thermal activation. The degassing of MIL-88B(Cr) in a vacuum at 250 degrees C yields a porous material with a S-BET area of 1136 m(2) g(-1), which is around the theoretical maximum. The transition to the denser 'closed' form (V-cell similar to 1500 angstrom(3)) occurs only at 350 degrees C, when all of the benzoate/benzoic acid, hindering the process, is removed.