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

The acidity of crystalline silicoaluminophosphates with chabazite-related structure (CHA), was studied by using CO and C 2 H 4 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 C 2 H 4 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 clear-cut experimental evidence that such strong acid sites are present in SAPO materials. A combined FTIR and 29 Si MAS NMR study permitted explaining the strong acidity of OH B sites in terms of protons either at the borders of silica patches/islands or inside aluminosilicate domains.