Fluoride modification of Mo/Al 2O 3 catalysts : Characterization of the changes induced in support and Mo phases

Abstract

The characterization of fluoride-modified Mo/Al 2O 3 catalysts was performed in order to investigate on the effect that low levels of fluoridation of the alumina support (0–2.0 wt.%) cause on the support itself and on the supported Mo oxide and sulfide phases. Fluoride-modified Al 2O 3 supports and Mo/Al 2O 3 catalysts where characterized by nitrogen physisorption, scanning electronic microscopy (SEM-EDX), isoelectric point (IEP), Fourier transform infrared spectroscopy (FT-IR), infrared spectroscopy of adsorbed CO 2 (IR-CO 2), and temperature programmed reduction (TPR). The dispersion of the sulfided catalysts was estimated by dynamic NO chemisorption. The results indicate that the different hydroxyl types present on the alumina surface react to a different extent with fluoride and that it is the most basic hydroxyl groups that are titrated first. The consumption of the alumina OH by F −, inhibits, during the deposition of Mo, the formation of tetrahedral molybdenum oxide species in strong interaction with the support, leading to an increased number of polymeric octahedral Mo surface species. The NO adsorption results put in evidence a drop in the dispersion of the MoS 2 phase present in the sulfided samples.