Solid-State NMR Characterization of Acidity of Solid Catalysts

Abstract

Solid acid catalysts such as zeolites, metal oxides, and heteropolyacids are environmentally benign, and are widely applied in advanced chemical and petrochemical processes due to their inherent acid-base catalyzed activity, and advantageous for products separation and reuse. Solid-state NMR (SSNMR) spectroscopy is known to be a powerful technique for exploring the acidic properties of solid acid catalysts. In this section, we aim to provide a comprehensive overview and in-depth discussion on advanced SSNMR techniques for qualitative and quantitative characterization of the acidic properties of solid acid catalysts by utilizing assorted probe molecules. The acid type (Brønsted vs Lewis acid), strength (strong vs weak), concentration (more vs less), distribution/location (intra- vs extra-crystalline), and spatial proximity/interaction of acid sites can be identified by SSNMR experiments. Especially, correlations between the 1H, 13C, and 31P NMR chemical shifts of probe molecules (including deuterated pyridine, 2-13C-acetone, trimethylphosphine, and trialkylphosphine oxide) and the intrinsic Brønsted/Lewis acid strengths of solid acid catalysts have been established, which can be used to quantitatively determine the acid strength. Furthermore, spatial proximity and synergetic effect of various acid sites on solid acid catalysts (such as zeolites) can be ascertained by two-dimensional (2D) double-quantum magic-angle spinning (DQ MAS) NMR spectroscopy. Therefore, SSNMR is a unique approach to provide reliable information on the acidic properties, which should be beneficial for the design and/or modification of solid acid catalysts for practical applications.