Abstract:

The B-P-O catalysts with different P/B ratios from 0.8 to 1.3 were prapared and characterized. X-ray diffraction showed that B and P were of tetrahedral structure and Raman spectroscopy indicated that the catalysts with P/B>1 are abundant in P2O5 on the surface while the catalyst with P/B<1 is abundant in B2O3 on the surface. Microcalorimetric adsorption of ammonia, water and dimethyl ether(DME) was performed respectively to evaluate the surface acid properties of these catalysts. It was revealed that the catalyst with P/B<1 exhibited similar initial heat of adsorption of ammonia as γ-A12O3. However, the catalysts with P/B>1 displayed unusual low heat for the adsorption of ammonia. In addition, the curve of differential heat versus coverage of the ammonia passed through a maximum for the catalysts with P/B>1. According to the results of microcalorimetric adsorption of water, the above behavior can be explained by the dissociative adsorption of ammonia. This demonstrated that ammonia may not be a proper probe molecule for titration of acidity on B-P-O catalysts. Further study by using DME for the microcalorimetric adsorption showed that the surface arid strength measured in terms of initial adsorption heat increased with P content in the B-P-O catalysts.

Key words: Borophosphate catalysts, Surface acidity, Microcalorimetric adsorption, Selection of probe molecules