Simultaneous characterization of acidic and basic properties of solid catalysts by a new TPD method and their correlation to reaction rates

We propose a new TPD method for simultaneously characterizing the acidic and basic properties of solid catalysts by utilizing the co-adsorption of NH3 and CO2 on catalysts. First CO2 was adsorbed on the catalyst sample; then NH3 was adsorbed on it. Another adsorption sequence of NH3 and CO2, and CO2 and NH3 single adsorptions were also conducted. The TPD measurements were carried out by heating the catalyst sample from 373 to 773 K at a heating rate of 2.5 K min−1 in a helium stream under a total pressure of 1.3 kPa. In solid acid catalysts, there is little difference in the NH3-TPD spectra between single and co-adsorption systems. This results from the absence of any induction effect between the acid and base sites, because the number of base sites in the solid acid catalyst is very small. In contrast, in a solid acid-base catalyst of alumina, a remarkable difference in the NH3-TPD spectra was observed between single adsorption and co-adsorption systems. The difference in the TPD spectra between single and co-adsorption systems was ascribed to a strong induction effect appearing on the acid and base sites, which was proved by an in situ IR measurement. The validity of the TPD method was examined by correlating the number of the strong acid sites to catalytic activities of dehydrolysis of ethanol over solid acid and solid acid-base catalysts. In solid acid-base catalysts, the number of strong acid sites was calculated from the activation energy distribution for the desorption of NH3 in a co-adsorption system because of the strong induction effect. A proportional relationship between the intrinsic reaction rate constant, which is based on the concentration of ethanol within the catalyst, and the number of strong acid sites could be obtained, regardless of the catalysts or their types or pore structure.