▪ Abstract This review covers the synthesis, characterization, and physico-chemical properties of microporous and mesoporous aluminophosphates and silicoaluminophosphates molecular sieves. Particular emphasis is given to the materials that have found applications as acid catalysts. We consider the evolution of the synthesis procedures from the first discoveries to the current methodologies and give perspectives for new possible synthesis strategies. Emphasis is given to the use of specially prepared precursors/reactants designed for the use as molecular sieves. Experimental (especially MAS-NMR and FTIR spectroscopy) and theoretical approaches to the description of the Si insertion into the ALPO framework and to the acidic properties of SAPOs and MeAPSOs materials are discussed.
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 clearcut 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.
This article reports the use of combined techniques (thermal desorption and spectroscopic analysis) for the characterization of Brønsted sites in microporous solid catalysts. The specific aim was to provide a quantitative determination of sites with slightly different acidities in chabazite-related silicoaluminophosphate (SAPO) materials, which display three bridged hydroxyls, OH A , OH B , and OH C , absorbing at 3630, 3614, and 3600 cm -1 , respectively. To this aim, different approaches were employed. First, the total concentration of Brønsted sites was calculated by classical NH 3 -TPD experiments. Ammonia was preferred to pyridine due to the small kinetic diameter, allowing easy access and diffusion inside the small cavities of chabazite. Second, FTIR spectroscopy was employed to study the adsorption of NH 3 and CO. The Lambert-Beer equation, using extinction coefficients from the literature, was employed to estimate the total amount of Brønsted sites involved in the formation of NH 4 + ammonium ions or in CO/H + adducts. The agreement among the data obtained by the three methods, which is excellent, is discussed critically. The fraction of each acid group in samples with different Brønsted site densities and strength distributions was determined with great accuracy. This finally allowed calculation of the extinction coefficients of the three hydroxyls of the H-SAPO-34 catalysts, which were A ) B ) 3.9 cm µmol -1 and C ) 6.0 cm µmol -1 . It is proposed that these values are of general use for determining the distribution of acid sites of SAPOs and zeolites whose hydroxyls absorb in the same range of wavenumbers.
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