High field 27Al MAS NMR and TPD studies of active sites in ethanol dehydration using thermally treated transitional aluminas as catalysts

• Aluminas with varying amount of 5-coordinated Al3+ were prepared by thermal treatment.
• Amount of different coordinated Al3+ was quantified by high field 27Al MAS NMR.
• Ethanol dissociation site was correlated with different coordinated surface Al3+.
• A positive correlation was found between 5-coordinated Al3+ and dissociated ethanol.

High field quantitative 27Al single pulse (SP) MAS NMR combined with temperature programmed desorption (TPD) of ethanol is used to study the surface of γ-Al2O3 during phase transformation processes induced by calcination in the temperature range of 500–1300 °C. Following ethanol adsorption, ethylene is generated during TPD with a desorption temperature above 200 °C. The amount of ethylene decreases monotonically with increasing calcination temperature prior to TPD. Significantly, 27Al SP MAS NMR reveals that the amount of penta-coordinated Al3+ ions also decreases with increasing calcination temperature. A quantitative (within experimental error) correlation between the amount of penta-coordinated Al3+ ions and the amount of strongly adsorbed ethanol molecules (i.e., the ones that convert to ethylene during TPD) is obtained. These results provide good evidence for a proposal that the penta-coordinated aluminum sites are the catalytic active sites on alumina surfaces during ethanol dehydration reaction across the entire course of γ-to-α Al2O3 phase transformations.

Figure optionsDownload high-quality image (84 K)Download as PowerPoint slide