Determination of the Experimental Minimal Formula of Metal-Organic Frameworks

The determination of the precise chemical composition of metal organic frameworks (MOFs) is often overlooked, although it is crucial for many advanced applications such as catalysis or drug delivery. Here, a rigorous yet simple method is proposed for the accurate determination of a MOF's minimal formula. By combining quantitative nuclear magnetic resonance (NMR) and ultraviolet-visible (UV-vis) spectroscopy data with thermogravimetric analysis (TGA), the minimal formula of several MOFs - MOF-808(Zr), UiO-66(Zr), UiO-66(Ce), MOF-5(Zn), MIL-125(Ti), and MIL-100(Fe) - are constructed. The influence of the MOF digestion method and the NMR measurement parameters on the accuracy of the minimal formula is shown. A quantitative method is provided for determining the amount of residual chloride or nitrate from metal precursors used in MOF synthesis, which has been ignored so far in minimal formulae determination. To improve the reproducibility and accuracy of MOF applications, the concept of the experimental molar mass is introduced, which can deviate significantly from the idealized molar mass. Although the determination of the MOF experimental minimal formula is often perceived as a complex and tedious task, the general methodology presented here is straightforward and involves very simple equations and procedures. It is easily generalized to new MOFs and even coordination networks of unknown structure, as demonstrated here for Al-BDC.