Quantifying the Kinetics of UiO-66 Metal−Organic Framework Crystallization from X-ray Attenuation in the Zr K-Edge Region

X-ray absorption spectroscopy (XAS) provides element-specific information on local structures but is rarely used to follow macroscopic phase transformations. Here, we show that non-normalized transmission XAS spectra can be used to track phase-change kinetics in systems undergoing a single density-changing transformation. Using crystallization of the UiO-66 metal−organic framework (MOF) consisting of [Zr 6 O 4 (OH) 4 ] 12+ oxo-clusters as nodes and terephthalate linkers as a representative example, we demonstrate that changes in X-ray attenuation in Zr K-edge XAS spectra quantitatively mirror the evolution of crystalline material measured independently by in situ X-ray diffraction (XRD), enabling determination of the extent of solid formation without diffraction measurements. Kinetic analysis based solely on non-normalized X-ray attenuation yields a nucleation-growth mechanism identical to that obtained from multivariate curve resolution-alternating least squares (MCR-ALS) analysis of the full XAS dataset. Both approaches are consistent with the Gualtieri model of crystal formation and give identical activation energies for nucleation (80 ± 7 kJ mol −1 ) and crystal growth (64 ± 5 kJ mol −1 ). This approach links atomic-scale changes with macroscopic phase evolution and extends XAS to real-time monitoring of density-changing processes, including crystallization.