ACIDITY OF ALUMINOPHOSPHATE STRUCTURES .1. INCORPORATION OF SILICON INTO CHABAZITE-LIKE STRUCTURE-44

SAPO-44 samples have been prepared with different silicon contents up to 34.3 mass% SiO2. Characterization of the samples by X-ray diffracti on (XRD), thermal analysis, adsorption measurements, chemical analysis , microprobe analysis and IR spectroscopy reveals that the samples are highly crystalline, even those with the highest SiO2 content. The num ber of Bronsted acid sites determined by NH3 sorption (calorimetric an d temperature-programmed desorption (TPD) measurements) is independent of the SiO2 content of the samples and amounts to about one acid cent re per double-six-ring of the chabazite-like structure. Concerning the silicon ordering within the framework, it is proposed that aluminopho sphate-based as well as aluminosilicate-based double-six-rings form th e structure.