Revisiting the nature of the acidity in chabazite-related silicoaluminophosphates: Combined FTIR and²⁹Si MAS NMR study

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 OH_A, OH_B, and OH_C) 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 OH_A and OH_C were already reported and explained in terms of different crystallographic positions, the existence and nature of the OH_B site are here discussed for the first time. Protons at the OH_B sites are shown to have an acidity (downward shift upon CO adsorption of ca. 330 cm ^-1) comparable to that usually measured in zeolites, aluminosilicate crystalline materials, which are known to possess stronger acidity than SAPOs. To our knowledge, this is the first clearcut experimental evidence that such strong acid sites are present in SAPO.materials. A combined FTIR and ²⁹Si MAS NMR study permitted explaining the strong acidity of OHB sites in terms of protons either at the borders of silica patches/islands or inside aluminosilicate domains.