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Keywords:

  • Metal–organic frameworks;
  • Hydrothermal synthesis;
  • Chromium benzenedicarboxylates;
  • Acidity;
  • Water

Abstract

In the present work, two metal–organic frameworks (MOFs), chromium benzenedicarboxylates MIL-53 and MIL-101, have been synthesized. A wide range of reaction conditions were explored in order to understand the effects of water concentration and acidity. It was found that MIL-101 is preferentially obtained when the water content is high and the acidity is low. On the contrary, concentrated reactants and high acidity lead to the formation of MIL-53. The steady conversion of MIL-101 into MIL-53 during the reaction, due to the difference in relative stability, was also confirmed. The effect of water may therefore be explained by the increase in reaction rate with increasing concentration of reactants. The MIL-53 is selectively obtained at low water content, because the MIL-101, a transient phase, even if formed initially, may be converted into MIL-53 because of the high reaction rate. Relatively low acidity (a pH of about 3) is beneficial to the synthesis of MIL-101 even though increased MIL-53 yield was expected at low acidity due to the accelerated deprotonation of terephthalic acid. The selective formation of MIL-101 at low acidity is probably because the concentration of a chromium trimer (the chromium species that is essential for the MIL-101 structure) increases with increasing pH. The concentration of the Cr trimer is more important than the concentration of benzenedicarboxylate for the synthesis of MIL-101.