MIL-53(Al) under reflux in water: Formation of γ-AlO(OH) shell and H2BDC molecules intercalated into the pores

Abstract

It is shown that treatment of MIL-53(Al) (Al(OH)BDC·H2O, BDC = 1,4-benzene dicarboxylate) under reflux in water results in a progressive transformation of the solid into a new well crystallized phase. After reflux for 10h or more the new phase is obtained in a pure form and its XRD pattern was indexed in a monoclinic system with the following cell parameters: a=19.47Å, b=8.98Å, c=6.60Å, β=107.7°. Characterization of the obtained solid by TGA, FT-IR, NMR, TEM and XRD has revealed that its composition is [0.8Al(OH)BDC·0.2H2BDC]+0.2γ-AlO(OH). Formation of this material indicates that under reflux in water a partial hydrolysis of the MOF network occurs producing H2BDC molecules (occluded in the pores) and γ-AlO(OH) species. The latter is shown to form a thick shell (100–200nm) consisting of strongly crumpled sheets of 3nm of thickness. Formation of γ-AlO(OH) under reflux in water allows to use such treatment as an easy way to modify the surface properties of MIL-53(Al). We showed that treatment of MIL-53(Al) under reflux in water for a short time (<1h) yields a thin layer of γ-AlO(OH) on the surface of the particles while only slightly decreasing their pore volume (from 0.57 to 0.50cm3/g). Such surface modification of MIL-53(Al) might be useful from two points of view. First, it creates a strongly hydrophilic layer on the surface of material and may thus facilitate its shaping. Second, the OH groups of γ-AlO(OH) surface layer may be used as anchoring sites for functionalization of MIL-53(Al) particles.