Yakimov Alexander V, Mance Deni, Searles Keith, Copéret Christophe
ETH Zurich, Vladimir-Prelog Weg 1-5/10, 8093 Zurich, Switzerland.
J Phys Chem Lett. 2020 May 7;11(9):3401-3407. doi: 10.1021/acs.jpclett.0c00716. Epub 2020 Apr 17.
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 2 orders of magnitude, the NMR signals associated with surface sites. DNP-SENS mostly relies on using exogenous polarizing agents (PAs), typically dinitroxyl radicals, to boost the NMR signals. However, the PAs may interact with the surface or even react with surface sites, thus leading to loss or quenching of DNP enhancements. Herein, we describe the development of a DNP-SENS formulation that allows broadening 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 DNP formulation (TEKPol/TCE). The addition of pyridine not only preserves the PAs and thereby restores the DNP enhancement but also allows probing Lewis/Brønsted acid surface sites that are often present on these catalysts.
动态核极化表面增强核磁共振光谱法（DNP-SENS）已成为材料化学和多相催化领域一种强大的表征工具，它能将与表面位点相关的核磁共振信号显著增强多达2个数量级。DNP-SENS主要依靠使用外源性极化剂（PA），通常是二硝基自由基，来增强核磁共振信号。然而，这些极化剂可能会与表面发生相互作用，甚至与表面位点发生反应，从而导致DNP增强效果的丧失或淬灭。在此，我们描述了一种DNP-SENS配方的开发，该配方能够将DNP-SENS的应用扩展到含有高活性表面位点的样品，即齐格勒-纳塔丙烯聚合催化剂、硫酸化氧化锆负载的茂金属以及二氧化硅负载的阳离子钼亚烷基。该方案包括在进行DNP配方（TEKPol/TCE）之前吸附吡啶。吡啶的加入不仅能保护极化剂，从而恢复DNP增强效果，还能探测这些催化剂上常见的路易斯/布朗斯特酸性表面位点。
