| Abstract
| - The acidity of crystalline silicoaluminophosphates with chabazite-related structure (CHA), was studied byusing CO and C2H4 probe molecules. SAPO-34 samples with similar silicon concentrations prepared usingdifferent structure-directing agents have been studied, and the results have been compared with asilicoaluminophosphate, CAL-1, having a similar structure and much higher silicon concentration. A detailedanalysis of the FTIR spectra in the OH stretching region, and of the downward shift of the OH bands uponCO 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 tocompute the fraction of the three sites. While the two main components OHA and OHC were already reportedand explained in terms of different crystallographic positions, the existence and nature of the OHB site arehere discussed for the first time. Protons at the OHB sites are shown to have an acidity (downward shift uponCO adsorption of ca. 330 cm-1) comparable to that usually measured in zeolites, aluminosilicate crystallinematerials, 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 and29Si MAS NMR study permitted explaining the strong acidity of OHB sites in terms of protons either at theborders of silica patches/islands or inside aluminosilicate domains.
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