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 degreesC indicate the presence of three distinguishable acid sites: (i) eight-ring Brphinsted sites (high-frequency hydroxyls or HF OH), (ii) six-ring Brphinsted 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 Brphinsted 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.