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 o n 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. Severa l types of interactions are shown by infrared and NMR, depending on th e pretreatment temperatures. The Al-27 and Si-29 MAS NMR spectra are m odified in the presence of bases. The protonic acidity, studied with t he two probes, decreases when the temperature rises, as usual in zeoli tes for the bridging Si-OH-Al hydroxyls, but very strong acid sites ar e created after the 1173 K heating. At least three kinds of Lewis site s 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 obs erved. The 1430-cm(-1) band decreases with the pretreatment temperatur e while the new 1455-cm(-1) one increases. Both could be assigned to p entacoordinated Al (Al-V) and possibly to some octahedral Al (Al-VI) r esulting from addition of pyridine on framework AlO4. Except for the c ase of pyridine adsorbed on the 1173 K pretreated sample where a parti al loss of crystallinity is observed, the adsorption of pyridine on th e two other materials (873 and 1073 K) narrows and shifts the Si-29 MA S NMR peaks, indicating a readjustment of the lattice (bond angles, bo nd lengths) in the presence of adsorbates. Nonacidic OH groups (possib ly 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 afte r the 1173 K pretreatment.