Sol–gel synthesis, characterization by solid state NMR and HDS activity of Mo–alumina and Mo–P–alumina based catalysts

Abstract

The aim of the present study was to prepare different Mo–alumina and Mo–P–alumina catalysts by a sol–gel method. The wishes were to underline the influence of the way to introduce the different Mo and/or P elements in the architecture of the dried and calcined solids and the consequences on their catalytic properties. During the sol–gel alumina synthesis, molybdenum [precursor=(NH 4) 6Mo 7O 24·4H 2O] and phosphorus (precursor=H 3PO 4) can be introduced separately or simultaneously in 2-butanol (step 1) or in water (step 3) to obtain the final mixed oxide. For Mo–alumina, the influence of the amount of molybdenum (Mo/Al ratio=0.1, 0.2 or 0.3) has been studied, whereas for the Mo–P–Al solids, four combinations have been prepared with a fixed Mo/Al ratio at 0.2 and P/Al ratio at 0.03 and 0.2. The dried and calcined samples have been extensively characterized by 27Al, 31P MAS NMR and 27Al Multiple Quantum Magic Angle Spinning NMR (MQMAS NMR). After sulfidation, the solids have been tested in hydrodesulfurization (HDS) of thiophene. It appears that the activity of Mo–Al catalysts increases when the molybdenum amount increases and when molybdenum is dissolved in 2-butanol during the synthesis. A promotor effect of phosphorus has been observed in some cases, particularly, when the molybdenum and the phosphorus precursors are both dissolved in water. Moreover, this study has shown the importance of the step of introduction of phosphorus for the catalytic performances of the Mo–P–Al catalysts. These catalytic results are correlated with some nuclear magnetic resonance (NMR) characterizations, which evidence the formation of different aluminium phosphates.