(The) bronsted acidity of SAPO and their catalytic performance

Abstract

The acidity and acid sites in silicoaluminophosphate-5 were investigated using temperature programmed desorption (TPD), FT-IR, magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy and nuclear Overhauser effect (NOE) spectroscopy. The different strengths of acid sites exist on silicoaluminophosphate-5. Although the amount of acid among the silicoaluminophosphate-5 samples varied significantly, their strength remained unchanged with the variation of Si contents. To distinguish the acidity of different environments of bridging hydroxyl groups, [$^2H_6$]benzene was used as a probe molecule. Using [$^2H_6$benzene as a probe molecule, variable temperature FT-IR and MAS NMR experiments have permitted the measurement of relative strength of different types (or different locations) of bridging hydroxyl groups. These results show that two types of bridging hydroxyl groups in silicoaluminophosphate-5 framework are fairly acidic, and they may be classified into one being more acidic and the other of lower acidity. The specificity of distance between the adsorbate (benzene as a probe molecule) and two types of bridging hydroxyl groups has been confirmed by 1H NOE techniques. It indicates that the benzene is closer than the proton at 12-ring O-atom with the distance, but the rate of isotope exchange is slower than that at 6-ring O-atom. Combination of measurements of through space distance with $^1H$ NOE technique and of relative isotopic exchange rates between bridging hydroxyl groups and the probe molecule provides a way for characterizing their locations and acidity in silicoaluminophosphate-5. The activity of benzene alkylation over a series of SAPO-5 samples depends on the relative extent and strength of acidity, and the geometry of two kinds of bridging hydroxyl groups. The results of these studies have led us to conclude that the bridging hydroxyl groups in silicoaluminophosphate-5 are not identical. The ability to measure through space distance wi...