The Size Control Of MoO₃/MCM-41 Active Phase And Its Effect On The Catalytic Oxidative Desulfurization Performance

The size of active center is a critical factor for supported catalysts.The ratio of low coordination metal sites is increasing with the decreasing of activity compound size?bulk?nanocluster?subnanocluster?single atom?.And this increasing will lead to the superior activity of catalyst in lots of reactions.Because of the high surface free energy,the metal content is limited to a low level for preventing the aggregation of activity compounds.We need to consider both the activity of per metal site and the productivity of unit mass of catalyst when evaluating the catalyst.MCM-41 was used as the support for MoO₃ in the present investigation,and the MoO₃/MCM-41 was applied as the catalyst of DBT oxidative desulfurization.Catalyst with2.00 wt%Mo content kept the size of subnanocluster because of the great support ability of MCM-41.Catalyst with 2.00 wt%Mo content showed superior catalytic activity at 20^oC with TBHP,which means the achieving of room temperature oxidative desulfurization.Catalyst with 0.02 wt%Mo content was also prepared.The catalysts with 0.02 wt%and 2.00 wt%Mo content exist as the forms of subnanocluster MoO3 and atomically dispersed MoO3respectively which were confirmed by HAADF-STEM.The similar activities mean the similar catalytic process on subnanocluster and atomically dispersed MoO₃.Because of the higher content of subnanocluster MoO₃/MCM-41,subnanocluster MoO₃ possesses higher productivity.O₂ is a kind of green,economical,and safe oxidant.We investigated the application of MoO3/MCM-41 in O2 oxidative desulfurization.Different with TBHP oxidative desulfurization,catalyst with low content showed higher activity than catalyst with high content.So subnanocluster MoO3 is good at catalyzing the oxidative desulfurization with TBHP and atomically dispersed MoO₃ shows higer activity during the oxidative desulfurization with O2.We believe the activity site and mechanism are different in O2 and TBHP oxidative desulfurization.Combining with the results of HADDF-STEM,XPS,and Raman,we believe Mo=O group is the active site of TBHP oxidative desulfurization,while the vacancy of Mo=O group is the activite site of O₂ oxidative desulfurization.Both mechanisms were proposed according to the literature.