109Ag NMR chemical shift as a descriptor for Brønsted acidity from molecules to materials

Abstract

Molecular-level understanding of the acid/base properties of heterogeneous catalysts requires the development of selective spectroscopic probes to establish structure–activity relationships. In this work we show that substituting the surface protons in oxide supports by isolobal N-heterocyclic carbene (NHC) Ag cations and measuring their ¹⁰⁹Ag nuclear magnetic resonance (NMR) signatures enables to probe the speciation and to evaluate the corresponding Brønsted acidity of the substituted OH surface sites. Specifically, a series of silver N-heterocyclic carbene (NHC) Ag(I) complexes of general formula [(NHC)AgX] are synthesized and characterized, showing that the ¹⁰⁹Ag NMR chemical shift of the series correlates with the Brønsted acidity of the conjugate acid of X⁻ (i.e., HX), thus establishing an acidity scale based on ¹⁰⁹Ag NMR chemical shift. The methodology is then used to evaluate the Brønsted acidity of the OH sites of representative oxide materials using Dynamic Nuclear Polarization (DNP-)enhanced solid-state NMR spectroscopy.