Thermochemical Investigation of Oxyanion Coordination in a Zirconium-Based Metal-Organic Framework

Porous materials possess high internal surface areas and void fractions that make them valuable in several applications, including gas storage, heterogeneous catalysis, and water purification. Despite the plentiful effort allocated to porous materials research annually, few methods exist to directly monitor and characterize chemical events occurring within a pore's confines. The crystalline nature of zeolites, covalent organic frameworks (COFs), and metal-organic frameworks (MOFs) permit structural characterization by X-ray diffraction; yet, quantifying the thermodynamics of chemical processes and transformations remains tedious and error ridden. Herein, we employ isothermal titration calorimetry (ITC) to determine the full thermodynamic profile of oxyanion adsorption in a zirconium-based MOF, NU-1000. To further validate this method, which we recently introduced to the field, we replicated ITC experiments as bulk adsorption measurements to demonstrate the correlation between the extracted stoichiometric parameter from ITC thermograms and the MOF uptake capacity. Moreover, based on the calculated association constants, we accurately predicted which analytes might be able to displace others. For example, dihydrogen phosphate can displace selenate and sulfate because of its higher association constant (ΔG_phosphate = -5.41 kcal/mol; ΔG_selenate = -4.98 kcal/mol; ΔG_sulfate = -4.77 kcal/mol). We monitored the exchange processes by titrating oxyanion-functionalized MOF samples with a more strongly binding analyte.