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

High field quantitative Al-27 single pulse (SP) MAS NMR combined with temperature programmed desorption (TPD) of ethanol is used to study the surface of gamma-Al2O3 during phase transformation processes induced by calcination in the temperature range of 500-1300 degrees C. Following ethanol adsorption, ethylene is generated during TPD with a desorption temperature above 200 degrees C. The amount of ethylene decreases monotonically with increasing calcination temperature prior to TPD. Significantly, Al-27 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 gamma-to-alpha Al2O3 phase transformations. (C) 2016 Published by Elsevier Inc