Study On Selective Oxidation Of Toluene Over Bismuth Molybdate Photocatalyst

Benzaldehydes are important in chemical industry,they are needed for the synthesis of commercial products such as pharmaceuticals,photosensitizers,flavors,and fragrances.The generation of them by selective oxidation of toluene has been regarded as viable and element economical.Nonetheless,it is hard to achieve high activity and selectivity simultaneously using traditional thermal-catalytic methods.It is because under reaction conditions that are relatively severe,a high reactivity would suggest low product selectivity.Photocatalytic selective oxidation of toluene into benzaldehyde has been regarded as an alternative because of its high selectivity towards targent products as well as the reactions were conducted at mild conditions.However,there exist some main drawbacks of researches in this area,such as low catalytic activity,low selectivity,and using of ultraviolet irradiation or noxious solvent.Bismuth molybdate?Bi₂MoO₆?as a low cost and easily prepared photocatalyst,has attracted a lot attention due to its good visible light response,suitable band location.It is known that photocatalytic activity of a semiconductor is greatly related to its physical properties such as the morphology,size,thickness as well as surface nature.So,we prepared Pd/Bi₂MoO₆nanosheet by a simple hydrothermal method using MoO₃ nanorods as precursor on one hand.The effect of the Pd amount on the photocatalytic activity was investigated.On the other hand,to avoid the use of novel metal,Bi₂MoO₆ nansheets of different thickness?6?120 nm?were prepared by hydrothermal or solvethermal methods,respectively.Furthermore,the structure-activity relationship between the properties and selective oxidation activity of the as-prepared Bi₂MoO₆ was analyzed.The main research contents of the paper are summarized as follows:1.The Pd/Bi₂MoO₆ nanosheet was prepared using a photodeposition method after achieving Bi₂MoO₆ by a simple hydrothermal method.The conversion of toluene increased with the increasing of Pd amount before 1%-Pd/Bi₂MoO₆,while decreased with further increasing the amount of Pd.At the optimized reaction conditions,highest activity was achieved with 11.5%conversion of toluene and 98%selectivity to benzaldehyde.It is worth pointing out that the production rate of benzaldehyde is 766.7?mol/g/h.The high activity and selectivity could be related to the formation of Schottky junction,which is benefit for the separation of charge carriers.Hole was found the main active species in this reacti on process and a possible reaction mechanism was proposed.2.Bi₂MoO₆ nanosheets different in thickness were facilely prepared and tested as photocatalysts for selective oxidation of toluene to benzaldehyde in the presence of O₂ and under visible light irradiation.It was found that the photocatalytic activity is closely related to the thickness of Bi₂MoO₆ nanosheets.Apparently,thinner nanosheets could induce higher photocatalytic activity.Among them,the ultrathin Bi ₂MoO₆ nanosheets?about 6 nm in thickness?which were prepared by a simple solvothermal method using MoO₃ rods as precursor and ethylene glycol as solvent showed the highest activity.The excellent activity is ascribed to the ultrathin sheet-like structure that facilitates efficient separation and transfer of charge carriers.Holes was found to be the main active species.The active species were investigated by addding trapping agents and holes were main active species,which interact with toluene molecules on Bi₂MoO₆ surface,and there is immediate formation of benzyl radicals.Then the benzyl radicals react with O₂ both on the surface of catalyst or in the reaction solution to produce benzaldehyde.