Study of active sites on the MFI zeolite catalysts for the transformation of ethanol into propylene

Industrial demand for propylene is increasing. Alternative sources of propylene other than petroleum are of wide interest. The catalytic transformation of bioethanol is a promising avenue for the production of propylene. However, the reaction mechanism and the nature of active sites on the catalyst remained unclear. In this work, MFI-type zeolite catalysts with various Si/Al₂ ratios (73, 97, 128, 176, 207) were prepared by hydrothermal synthesis under atmospheric pressure. We characterized these MFI catalysts by BET surface area measurements, X-ray fluorescence, X-ray diffraction, scanning electron microscopy, temperature programmed desorption of ammonia, and ²⁷Al MAS NMR to understand the effect of the Si/Al ₂ ratio. The ethanol conversion reaction was systematically investigated using these catalysts according to the contact time and reaction temperature. The conversion of ethanol yielded similar product distributions over these catalysts at different contact times, indicating that the product distribution has no relationship with the Si/Al₂ ratio. Also, the propylene production rates per acidic site of these catalysts were similar. These results strongly imply that the acidic sites of the different MFI-type zeolite catalysts have the same nature, as well as the same reactivity. The catalytic activity was proportional to the number of acidic sites.