Metal-organic frameworks (MOFs) have been one of the most advancing classes of materials in the past few years, with immense potential in the field of catalysis,¹ gas storage,² drug delivery,³ water purification,⁴ etc. However, the fundamental aspect of determining the precise chemical composition of the synthesised MOFs is lacking sufficient attention, although it is crucial for many advanced applications, such as catalysis.

In this work, we present a simple yet robust methodology to derive the minimal formula of the synthesised MOF material.⁵ To achieve this, we combine Nuclear Magnetic Resonance (NMR) spectroscopy, Thermogravimetric analysis (TGA), and UV-Vis spectroscopy. We investigated the previously used methodologies that solely rely on TGA and demonstrated why the assumptions that were made in this technique are not justified. We further dive deep into the use of NMR to quantify the different organic molecules present in the framework. We have also shown the crucial influence of digestion methods and relaxation time on the accurate determination of the minimal formula. We also highlight the often-overlooked presence of residual inorganic species, particularly chloride and nitrate ions originating from the precursors such as ZrCl₄ or ZrOCl₂ (for Zr-based MOFs), or cerium ammonium nitrate (for Ce-based MOFs). These residues can remain embedded within the framework in significant quantities, impacting properties such as catalytic activity and framework stability. Our methodology provides a quantitative means of identifying and accounting for these species. 

To align MOF characterisation with practical applications, we introduce the concept of “experimental molar mass”, reflecting the true chemical identity of the MOF as synthesised and handled under laboratory conditions. We used our methodology to derive the experimental minimal formula of MOF-808(Zr), UiO-66(Zr), UiO-66(Ce), MOF-5(Zn), MIL-125(Ti), and MIL-100(Fe), to show the generality of our technique. We further extended our work to amorphous coordination networks, such as an Al-BDC MOF with no information on its crystal structure. Our work focuses on obtaining a fully charge-balanced and chemically feasible minimal formula. This work thus lays the foundation for a more rigorous, transparent, and application-relevant reporting of MOF compositions and serves as a critical step towards the rational design and development of MOFs in both fundamental research and industrial contexts.

[1] J. Pulparayil Mathew, C. Seno, M. Jaiswal, C. Simms, N. Reichholf, D. Van den Eynden, T. N. Parac-Vogt, J. De Roo, Small Science, 2024, 5, 2400369.

[2] N. Rosi, J. Eckert, M. Eddaudi, D. T. Vodak, J. Kim, M. O’Keeffe, O. M. Yaghi, Science, 2003, 300, 11271129.

[3] H. D. Lawson, S. P. Walton, C. Chan, ACS Applied Materials & Interfaces, 2021, 13, 7004–7020.

[4] D. T. Sun, N. Gasilova, S. Yang, E. Oveisi, W. L. Queen, Journal of the American Chemical Society, 2018, 140, 16697–16703.

[5] J. Pulparayil Mathew, C. Simms, D. E. Salazar Marcano, E. Dhaene, T. N. Parac-Vogt, J. De Roo, Advanced Science, 2025, e04713.