Synthesis and characterization of conventional and mesoporous Ga-MFI for ethane dehydrogenation

Abstract

Gallium was for the first time incorporated into the crystal structure of a mesoporous MFI zeolite. As a reference material, a well-defined conventional MFI zeolite with gallium incorporated was also synthesized. The materials were characterized by XRPD, NH₃-TPD, elemental analysis, N₂ physisorption measurements, SEM, FT-IR as well as ²⁹Si and ⁷¹Ga MAS NMR spectroscopy. The Ga contents correspond to molar ratios of Si/Ga = 43 and 38 for the conventional and mesoporous Ga-MFI, respectively. The different types of analysis show that two almost identical zeolites were synthesized, the main difference being the morphology of the crystals. The catalytic properties of the samples were tested in ethane dehydrogenation at 823 K and at atmospheric pressure. The two Ga-MFI samples differ greatly in conversion of ethane, as a result of their difference in porosity, and it is found that the conversion is significantly improved by introducing mesopores into the structure. The major product is ethene for the conventional Ga-MFI, but there are significant amounts of benzene and toluene by-products for the mesoporous Ga-MFI. The ⁷¹Ga isotropic chemical shift and second-order quadrupolar effect parameter is reported for tetrahedral Ga incorporated in the MFI structure and it is shown that absorbed water molecules have an important impact on the ⁷¹Ga electric field gradients and thereby on the quantitative evaluation of the ⁷¹Ga MAS NMR intensities.