Quantitative investigations of acidity, and transient acidity, in zeolites and molecular sieves

To demonstrate that transient acidity exists in certain solid acid catalysts, a convenient and robust (1)H solid-state NMR spin-counting method is introduced for the accurate quantification of Bronsted acid sites in zeolites and molecular sieves. Poly (dimethylsiloxane) is used as an inert and easily handled spin-counting standard, allowing internal calibration of MAS NMR peaks arising from both acidic and nonacidic hydrogens in the catalyst. Results from example systems including H-ZSM5, H-ferrierite (H-FER), and SAPO-34 are presented, and the relevance of these measurements to zeolite synthesis conditions, postsynthetic treatments, and reaction mechanisms are discussed. Using this technique, we show the first direct spectroscopic proof that framework Bronsted acidity in SAPO-34 molecular sieves decreases with time following removal of the synthesis template molecules. This effect is similar to hydrothermally catalyzed dealumination in traditional zeolites, except that it occurs at ambient temperature and moisture levels. Our data indicates that these catalysts must be stored in a moisture-free atmosphere to preserve activity following template removal. These results are key to understanding hydrocarbon conversions in SAPO catalysts, particularly in the methanol-to-hydrocarbon chemistry area, where SAPO-34 is a leading candidate for commercialization.