Determination of the experimental minimal formula of metal-organic frameworks

Metal-organic frameworks have emerged as one of the most promising classes of materials in the last decade with potential applications in many domains of science. However, the determination of their precise chemical composition has often been overlooked, although it is crucial for many advanced applications such as catalysis or drug delivery. Here, we propose a rigorous yet simple method for the accurate determination of a MOF’s minimal formula. By integrating quantitative NMR and UV-Vis spectroscopy data alongside TGA analysis, we construct the complete minimal formula of several MOFs: MOF-808(Zr), UiO-66(Zr), MOF-5(Zn), and MIL-125(Ti). We show the critical influence of the MOF digestion method and the NMR measurement parameters on the accuracy of the minimal formula. Furthermore, we provide a quantitative method for determining the amount of residual chloride that originates from metal precursors used in MOF synthesis, which has been often ignored in minimal formula determination. In order to help improve the reproducibility and accuracy of MOF applications, we introduce the concept of the experimental molar mass that 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 can be achieved with very simple equations and procedures. It is easily generalized to new MOFs and even amorphous coordination networks such as Al-BDC.