Revisiting the Nature of the Acidity in Chabazite-Related Silicoaluminophosphates: Combined FTIR and 29Si MAS NMR Study

Abstract

The acidity of crystalline silicoaluminophosphates with chabazite-related structure (CHA), was studied by using CO and C2H4 probe molecules. SAPO-34 samples with similar silicon concentrations prepared using different structure-directing agents have been studied, and the results have been compared with a silicoaluminophosphate, CAL-1, having a similar structure and much higher silicon concentration. A detailed analysis of the FTIR spectra in the OH stretching region, and of the downward shift of the OH bands upon CO and C2H4 adsorption, evidenced the presence of three distinct acid sites (named OHA, OHB, and OHC) absorbing at 3631, 3617, and 3600 cm-1, respectively (average values). A multipeak curve-fitting approach, along with hydrogen bond theory, following Makarova et al. (J. Phys. Chem. 1994, 98, 3619), allowed us to compute the fraction of the three sites. While the two main components OHA and OHC were already reported and explained in terms of different crystallographic positions, the existence and...