Correlating NH3-TPD and 1H MAS NMR measurements of zeolite acidity: proposal of an acidity scale

The acidities (number and strength of the Bronsted acidic sites) of two zeolite samples, H-MFI and H-BEA, were measured by ammonia temperature-programmed desorption (ATPD) and proton nuclear magnetic resonance (1H MAS NMR). Deconvolutions of the ATPD and 1H MAS NMR spectra lead to acid sites distributions (number and strength) that are nearly similar. Acid sites of increasing acid strength are characterized by higher desorption temperatures and higher 1H NMR chemical shifts. A linear relationship was found between the activation energy for ammonia desorption and the 1H NMR chemical shift. The acid strength distributions determined by ATPD and 1H MAS NMR have been quantitatively correlated.