(630b) The Nature of Acid Sites in MIL-100 Catalyzed Acetalization of Benzaldehyde with Methanol

Metal-organic framework materials (MOFs) have emerged as promising materials for a range of heterogeneous catalysis applications, especially those involving acid catalysis due to their tendency for providing open-metal sites in uniform well-defined coordination environments. MIL-100 is a prototypical MOF in this regard, with prior literature evidencing its utility in a range of acid catalyzed reactions such as ethene oligomerization and (most commonly) the acetalization of carbonylic compounds by alcohols [1]. Although several reports have emerged evaluating MIL-type frameworks for acetalization reactions, the true source of activity during liquid phase catalysis still remains a point of major contention [1-2]. In this work, a combination of liquid-phase kinetic measurements, in-situ titrations, and in-situ infrared spectroscopy data were used to decipher the identity of active sites mediating acetalization of benzaldehyde with methanol. Greater than 90% reduction in initial rates following dehydration at 250 °C and the in-situ titration of sites with 2,6-di-tert-butylpyridine (DTBP) reveal the Brønsted acidic nature of measured catalytic activity. Quantitative analysis of infrared spectroscopic measurements point to polarized water molecules ligated to metal centers as the source of Brønsted acidity. Sequential titrations with DTBP and pyridine, combined with titration data on homologous nodes with varying degrees of diffusion constraints, reveal the presence of a fraction of sites accessible only to pyridine but not DTBP - a result of the large difference in kinetic diameters of the two titrants. The methodology and concepts explored in this study point to the importance of thorough investigations into MOF site speciation under reaction conditions, especially those in the liquid phase, to more fully exploit this emerging class of materials for catalytic applications.

References: [1] Zhang, F., et al., Chem. Eng. J. 259 (2015) 183. [2] Herbst, A., et al., Inorg. Chem. 53 (2014) 7319.