States of carbon nanotube supported Mo-based HDS catalysts

Abstract

As HDS catalysts, the supported catalysts including oxide state Mo, Co–Mo and sulfide state Mo on carbon nanotube (CNT) were prepared, while the corresponding supported catalysts on γ-Al 2O 3 were prepared as comparison. Firstly, the dispersion of the active phase and loading capacity of Mo species on CNT was studied by XRD and the reducibility properties of Co–Mo catalysts in oxide state over CNTs were investigated by TPR while the sulfide Co–Mo/CNT catalysts were characterized by XRD and LRS techniques. Secondly, the activity and selectivity of hydrodesulfurization (HDS) of dibenzothiophene with Co–Mo/CNT and Co–Mo/γ-Al 2O 3 were studied. It has been found that the main active molybdenum species in the oxide state MoO 3/CNT catalysts were MoO 2, rather than MoO 3 as generally expected. The maximum loading before formation of the bulk phase was lower than 6%m (calculated in MoO 3). The TPR studies revealed that that active species in oxide state Co–Mo/CNT catalysts were more easily reduced at relatively lower temperatures in comparison to those in Co–Mo/γ-Al 2O 3, indicating that the CNT support promoted the reduction of active species. Among 0–1.0 Co/Mo atomic ratio on Co–Mo/CNT, 0.7 has the highest reducibility. It shows that the Co/Mo atomic ratio has a great effect on the reducibility of active species on CNT and their HDS activities and that the incorporation of cobalt improved the dispersion of molybdenum species on CNT and mobilization. It was also found that re-dispersion could occur during the sulfiding process, resulting in low valence state Mo 3S 4 and Co–MoS 2.17 active phases. The HDS of DBT showed that Co–Mo/CNT catalysts were more active than Co–Mo/γ-Al 2O 3 and the hydrogenolysis/hydrogenation selectivity of Co–Mo/CNT catalyst was also much higher than Co–Mo/γ-Al 2O 3. For the Co–Mo/CNT catalysis system, the catalyst with Co/Mo atomic ratio of 0.7 showed the highest activity, whereas, the catalyst with Co/Mo atomic ratio of 0.35 was of the highest selectivity.