Spectroscopic Characterization of Acidity in Chabazite

Acid sites in potassium-containing chabazite catalysts, under evaluation for methylamines synthesis, were characterized by thermogravimetric analysis-infrared spectroscopy (TGA-IR), solid state magic angle spinning nuclear magnetic resonance (MAS NMR), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Spectroscopic results of samples activated between 350 and 650 °C indicate the presence of three distinguishable acid sites: (i) eight-ring Bronsted sites (high-frequency hydroxyls or HF OH), (ii) six-ring Bronsted sites (low-frequency hydroxyls or LF OH), and (iii) nonframework (pentacoordinate and octahedral) AlOH sites. The six-ring/LF OH sites desorb ammonia first with increasing temperature and appear to dealuminate before the eight-ring/HF OH sites. Potassium cation content was found to affect the stability of Bronsted acid sites toward dealumination. Improved catalytic activity and selectivity toward reduced trimethylamine yield, in favor of mono- and dimethylamine, is shown to be a result of optimizing potassium cation content and activation temperature.