A Formulation Protocol with Pyridine to Enable DNP-SENS on Reactive Surface Sites: Case Study with Olefin Polymerization and Metathesis Catalysts

Dynamic nuclear polarization surface-enhanced NMR spectroscopy (DNP-SENS) has emerged as a powerful characterization tool in material chemistry and heterogeneous catalysis by dramatically increasing, by up to two orders of magnitude, the NMR signals associated with surface sites. DNP-SENS mostly relies on using exogenous polarizing agents (PAs) – typically di-nitroxyl radicals, to boost the NMR signals, that may interact with the surface or even react with highly reactive surface sites, thus leading to loss/quenching of DNP enhancements. Herein, we describe the development of a DNP-SENS formulation that allows us to broaden the application of DNP-SENS to samples containing highly reactive surface sites, namely a Ziegler-Natta propylene polymerization catalyst, a sulfated zirconia-supported metallocene and a silica-supported cationic Mo alkylidene. The protocol consists of adsorbing pyridine prior to the impregnation of the DNP formulation (TEKPol/TCE). The addition of pyridine does not only preserve the PAs and thereby restore the DNP enhancement, but it also allows probing the presence of Lewis acid and Brønsted acid surface sites that are often present on these catalysts.