Dimethylpyridine-temperature programmed desorption (DMP-TPD) for measurement of strength of Bronsted and Lewis acid sites on metal oxide catalysts

Temperature-progammed desorption of dimethylpyridine (DMP-TPD) was performe d for the measurement of strength of Bronsted acid-and Lewis acid sites on metal oxide catalysts. Alumina, alumina-boria, silica-magnesia and silica-a lumina were employed as examples of solid acid catalysts. Infrared (IR) spe ctra of adsorbed pyridine (Py) and ammonia showed that Lewis acid sites wer e abundant on alumina, Bronsted acid sites were abundant on alumina-boria, and both were present on silica-magnesia and silica-alumina surfaces. 2,6-d imethylpyridine-TPD (2,6-DMP-TPD) spectra of alumina depended strongly on p urging temperatures. The amount of desorbed 2,6-DMP decreased with the incr ease in the purging temperature, and only a negligible amount of 2,6-DMP wa s detected in the TPD run from 523 K. On the other hand, 3,5-dimethylpyridi ne (3,5-DMP) adsorbed on Lewis acid sites on alumina. Selective adsorption of 2,6-DMP on Lewis acid sites was also confirmed from in-situ IR spectra. In the case of alumina-boria on which Bronsted acid sites are abundant, bot h 2,6-DMP-TPD and 3,5-DMP-TPD spectra gave similar desorption profiles. Pro files of acid strength determined from DMP-TPD agreed well with those deter mined from the IR spectra of adsorbed 3,5-DMP. These results demonstrated t he selective adsorption of 2,6-DMP on Bronsted acid sites because of steric hindrance of methyl groups, and the possibility of measurement of respecti ve strength of Bronsted acid and Lewis acid sites. Through a comparison of 2,6-DMP-TPD and 3,5-DMP-TPD spectra of silica-magnesia and:silica-alumina, the scope of DMP-TPD for the measurement of strength of Bronsted and Lewis acid sites on metal oxide catalysts was discussed. (C) 2000 Elsevier Scienc e B.V. All rights reserved.