Microwave-Assisted Synthesis of HKUST-1 and Functionalized HKUST-1-@H3PW12O40: Selective Adsorption of Heavy Metal Ions in Water Analyzed with Synchrotron Radiation

A simple, rapid and efficient synthesis of the metal-organic framework (MOF) HKUST-1 [Cu-3(1,3,5-benzene-tri-carboxilic-acid)(2)] by microwave irradiation is described, which afforded a homogeneous and highly selective material. The unusually short time to complete the synthesis by microwave irradiation is mainly attributable to rapid nucleation rather than to crystal growth rate. Using this method, HKUST-1-MW (MW=microwave) could be prepared within 20 min, whereas by hydrothermal synthesis, involving conventional heating, the preparation time is 8 h. Work efficiency was improved by the good performance of the obtained HKUST-1-MW which exhibited good selective adsorption of heavy metal ions, as well as a remarkably high adsorption affinity and adsorption capacity, but no adsorption of Hg2+ under the same experimental conditions. Of particular importance is the preservation of the structure after metal-ion adsorption, which remained virtually intact, with only a few changes in X-ray diffraction intensity and a moderate decline in surface area. Synthesis of the polyoxometalate-containing HKUST-1-MW@H3PW12O40 afforded a MOF with enhanced stability in water, due to the introduced Keggin-type phosphotungstate, which systematically occluded in the cavities constituting the walls between the mesopores. Different Cu/W ratios were investigated according to the extrusion rate of cooper ions concentration, without significant structural changes after adsorption. The MOFs obtained feature particle sizes between 10-20 m and their structures were determined using synchrotron-based X-ray diffraction. The results of this study can be considered important for potentially wider future applications of MOFs, especially to attend environmental issues.