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

Temperature-programmed desorption of dimethylpyridine (DMP-TPD) was performed for the measurement of strength of Brønsted acid and Lewis acid sites on metal oxide catalysts. Alumina, alumina–boria, silica–magnesia and silica–alumina were employed as examples of solid acid catalysts. Infrared (IR) spectra of adsorbed pyridine (Py) and ammonia showed that Lewis acid sites were abundant on alumina, Brønsted acid sites were abundant on alumina–boria, and both were present on silica–magnesia and silica–alumina surfaces. 2,6-dimethylpyridine-TPD (2,6-DMP-TPD) spectra of alumina depended strongly on purging temperatures. The amount of desorbed 2,6-DMP decreased with the increase in the purging temperature, and only a negligible amount of 2,6-DMP was detected in the TPD run from 523 K. On the other hand, 3,5-dimethylpyridine (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 Brønsted acid sites are abundant, both 2,6-DMP-TPD and 3,5-DMP-TPD spectra gave similar desorption profiles. Profiles of acid strength determined from DMP-TPD agreed well with those determined from the IR spectra of adsorbed 3,5-DMP. These results demonstrated the selective adsorption of 2,6-DMP on Brønsted acid sites because of steric hindrance of methyl groups, and the possibility of measurement of respective strength of Brønsted 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 Brønsted and Lewis acid sites on metal oxide catalysts was discussed.