Synthesis and Post-Synthetic Transmetalations of Metal-Organic Frameworks Containing Bimetallic Centers

The heterobimetallic metal-organic frameworks (MOFs) of the type HKUST-1 were obtained through post synthetic cation exchange processes, where Cu 3(BTC)2 (BTC = 1,3,5-benzenetricarboxylate) was exposed to solutions of Mn2+, Fe2+, Co2+, or Ni2+ at time intervals of 6, 12, 24, 36, 48, and 72 hours. The elemental composition of the products was analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES), showing partial substitution of copper ions in the frameworks. Transmetalations done with Mn2+ and Ni2+ show a relatively low cation exchanged through the different time intervals, with maximum transmetalation percentages of 2.4 % for Mn2+ and 2 % for Ni2+ after a 72 h and 24 h solvothermal reactions, respectively. An increasing linear trend in the degree of transmetalation is observed for Fe2+ and CO 2+ over time, having a maximum transmetalation percentage of 39 % for Fe and 18% for Co, after 72 h solvothermal reaction. Characterization methods such as powder x-ray diffraction (p-XRD) and infrared (IR) spectroscopy show a retention in the overall crystallinity and porosity of the transmetalated samples. The Brunauer-Emmett-Teller (BET) surface areas and gas adsorption isotherms for N2 and CO2 were obtained for each species. The CO2 adsorption capacities of the transmetalated samples show an overall increase, having Mn2+ transmetalations the highest adsorption capacity of 5.47 mmol g–1 , versus 3.08 mmol g–1 for HKUST-1