The Effect Of Microwave Field On The Properties Of Different Crystalline Aluminas

The development of clean oil technology is an inevitable requirement for environment protection at the current historical stage.In the progress of improving the original hydrodesulfurization(HDS)process,the key issue is to develop the HDS catalyst.To achieve the goal,on one hand,researchers started with the catalyst structure and composition to design catalysts with higher activity.On the other hand,they introduced microwave radiation and other auxiliary means to stimulate the potential of the catalyst.The mechanism of catalitic HDS and the interaction between microwave and catalyst are still two open questions,especially the role of alumina as a support with different crystal forms,the interaction mechanism between metal and alumina,microwave field and alumina.In this paper,the surfaces of ?-Al₂O₃(110)and ?-Al₂O₃(010)were selected as the typical representatives of different crystalline carrier alumina.Ab initio molecular dynamics,density functional theory,and classical molecular dynamics were used to study the chemical properties of the alumina surface and the interaction between the active component Mo and the Al₂O₃ surface.At the same time,it is proved that microwave has a"non-thermal effect"on the properties of alumina.These may provide theoretical guidance for the design of single atom HDS catalyst under microwave irradiation.First,the nature of the hydroxyl group on the surface of Al₂O₃ was studied,and it was found that the hydroxyl frequency and the bond length have a linear relationship.In the hydroxyl group,the bond length represents the overlap degree between the p orbital of O and the s orbital of H,which reflects the ability of the hydroxyl group to provide protons.Therefore,the frequency of hydroxyl stretching vibration reflects the strength of Bronsted acidity on the Al₂O₃ surface,that is,the lower the frequency,the stronger the acidity.The calculation of the electrostatic potential shows that the low-potential oxygen atom on the hydroxyl group is nucleophilic,and it easily undergoes ion exchange reaction with the electrophilic reagent molybdate ion to form Mo–O–Al bond,so that Mo is loaded on the Al₂O₃ surface.In addition,the frequency of the hydroxyl group that becomes a hydrogen bond donor is significantly red-shifted,so it is not accurate to classify the type of surface hydroxyl groups according to the vibration frequency,it is necessary to combine the coordination number and the amount of charge.Secondly,CO molecule was used as a probe to investigate the Lewis acid on the clean surface of alumina,which is the coordinated unsaturated aluminum atom.CO is adsorbed on the surface of Al₂O₃ in the form of combination of C atoms and Al.The interaction between the 5? orbital of the CO molecule and the empty 3p of Al is the driving force for CO adsorption on the surface,which strengthens the C–O bond and induces a blue shift in the stretching frequency.However,on the surface of ?-Al₂O₃(010),some electrons enter the 2?^*orbit of the CO molecule from alumina,weakening C–O bond,and the blue shift phenomenon is not obvious.Later,based on the Lewis acid strength and charge density of the aluminum atom,Mo was"anchored"on Al₂O₃ surface.Through natural bond orbital(NBO)analysis,it is found that the five d orbitals and three porbitals of Mo on the ?-Al₂O₃(110)surface all participate in hybridization to form five pairs of bond orbitals,whilist,on the surface of ?-Al₂O₃(010),one d orbital and one p orbital are not involved in the hybridization,forming non-bonding orbits,so Mo interacts more strongly with ?-Al₂O₃(110)surface.In addition,Mo"anchored"on the surface of Al₂O₃ can adsorb thiophene and hydrogen sulfide stably,so Al₂O₃ supported monoatomic Mo is a potential single-atom catalyst with HDS activity.Finally,the optical properties of the clean surface,hydroxylated surface,and the Mo-loaded surface indicate that Al₂O₃ acting as a microwave transparent material cannot absorb microwaves.But the time-varying electric field affects the interaction between molecules and the movement of molecules.When the surface of the aluminum oxide is dehydrated at high temperature,water molecules can be bonded to the Al₂O₃ surface at low frequency electric field,however,the high frequency electric field makes the water molecules suspended in the air,which affects the hydroxyl coverage over the Al₂O₃surface.In terms of thermodynamics,the coverage of hydroxyl groups on the surface is closely related to the crystal transformation process during the high-temperature heating of alumina.The effect of microwave electric field on the diffusion of water molecules over alumina surface proves the existence of microwave"non-thermal effect".