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 Brønsted sites (high-frequency hydroxyls or HF OH), (ii) six-ring
Brønsted 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
Brønsted 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.
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