CHANGES IN THE ENVIRONMENT OF SI AND AL IN SAPO-37 ZEOLITE DURING ACIDITY MEASUREMENTS

The interaction of bases (pyridine, 2,6-dimethylpyridine) is studied on SAPO-37, pretreated at nonusual high temperatures from 873 to 1173 K. XRD, infrared, and P-31, Al-27, and Si-29 MAS NMR spectroscopies are used, the spectra being recorded in the presence of the bases. Several types of interactions are shown by infrared and NMR, depending on the pretreatment temperatures. The Al-27 and Si-29 MAS NMR spectra are modified in the presence of bases. The protonic acidity, studied with the two probes, decreases when the temperature rises, as usual in zeolites for the bridging Si-OH-Al hydroxyls, but very strong acid sites are created after the 1173 K heating. At least three kinds of Lewis sites are seen by infrared. Besides the 1455-cm(-1) Lewis band of pyridine in oxides, new species vibrating also at 1430 and 1455 cm(-1) are observed. The 1430-cm(-1) band decreases with the pretreatment temperature while the new 1455-cm(-1) one increases. Both could be assigned to pentacoordinated Al (Al-V) and possibly to some octahedral Al (Al-VI) resulting from addition of pyridine on framework AlO4. Except for the case of pyridine adsorbed on the 1173 K pretreated sample where a partial loss of crystallinity is observed, the adsorption of pyridine on the two other materials (873 and 1073 K) narrows and shifts the Si-29 MAS NMR peaks, indicating a readjustment of the lattice (bond angles, bond lengths) in the presence of adsorbates. Nonacidic OH groups (possibly vibrating at 3625-3630 cm(-1)) are shifted to 3535 cm(-1) upon the adsorption of pyridine or 2,6-dimethylpyridine. They do not exist after the 1173 K pretreatment.