Role of a Modulator in the Synthesis of an MIL-53 Metal–Organic Framework Revealed by <i>In Situ</i> Time-Resolved MAS NMR Spectroscopy

Metal-organic frameworks (MOFs) are attracting increasing attention due to their exceptional structural versatility and potential applications in catalysis and gas separation. Traditional synthesis methods involve solvothermal crystallization in the presence of modulators, whose involvement in the chemical processes occurring during nucleation and growth has not been well understood. Using time-resolved <i>in situ</i> magic-angle spinning ¹H, ²⁷Al, and ¹³C nuclear magnetic resonance (MAS NMR) spectroscopy, we demonstrate that the crystallite size of the final MOF can be controlled by adding malonic acid as a coordination modulator, influencing the nucleation and crystal growth at the molecular level. The variation of the amount of coordination (malonic acid) and pH modulator (NaOH) leads to significant changes in the fraction of deprotonated nitroterephthalic acid observed in the course of reaction: small amounts of malonic acid result in fast heterogeneous nucleation and the formation of small crystals. In contrast, a large amount of coordination modulator leads to protonation of the linker, homogeneous nucleation, and formation of large crystals. The study highlights the complex chemical processes that take place during the modulated synthesis of MOFs, which require investigation using time-resolved <i>in situ</i> methods capable of following the evolution of both the liquid and solid phases, making MAS NMR spectroscopy one of the most informative techniques.