Mechanism Study on Properties of Active Sites and Substituent Effects on the Hydrogenation Performance of Sulfur and Nitrogen Compounds

Author:Ding Si Jia

Supervisor:zhou ya song


Degree Year:2018





Hydrotreating is the mandatory technology for the production of the clean fuel and the superior industrial chemicals.The key of hydrotreating technology is the research and development(R&D)of the high active hydrotreating catalysts.One of the core indicators of the hydrotreating catalyst evaluation is the effective elimination of the refractory sulfur and nitrogen compounds which contain substituent groups.The foundation of the directed catalyst design is the deep understanding of the hydrotreating process of these refractory reactants.In this thesis,the quantum chemical calculations and the hydrogenation experiments are combined together and put the two essentials of hydrotreating,namely,the reactant system and the catalyst into one issue to study.In this way,the properties of the catalysts and the substituent effects on the hydrogenation performance of the sulfur and nitrogen compounds can be deeply investigated.The introduction of the Co or Ni promoters into the MoS2 active sites could lower the S coverage rate of Mo-edge and S-edge under the thermodynamically stable state,causing the easy generation of the active center.The active sites promoted by Co atoms need H2S desorption to generate active center whereas the ones promoted by Ni atoms could directly exposed Ni atoms and their nearby Mo atoms to the reaction atmosphere,which could directly adsorb the reactants.The Co and Ni promoters could expand the unoccupied molecular orbital on the active center,which brings about the suitable adsorption of the sulfur compounds.The Co promoter could evidently enhance the adsorption and hydrogenolysis of sulfur compounds,whereas the Ni promoter could evidently enhance the generation of the active center and the hydrogen activation.The corner sites of bimetal CoMoS active phase possess the catalytic advantages of both two edge sites.The corner sites could catalyze all the elementary reactions in the thiophene hydrodesulfurization independently and effectively.The sulfur compounds avoid transferring among the intermediates,and the catalytic efficiency enhances on the corner sites.When the S atoms on one side of MoS2 active sites are isoquant substituted by O atoms,the active sites will bend to the oxygen containing sites,and the stack number of active sites will reduce at the same time.Oxygen could enhance the interaction between Mo and H atoms,whereas the hydrogen saturation activity is hindered.However,the presents of oxygen could enhance the adsorptive selectivity of the radical sulfur compounds and promoted the hydrogenolysis activities.The substituent groups make the adsorption of the 4,6-dimethyldibenzothiophene(4,6-DMDBT)more flat on the active sites,resulting to more difficult to the hydrogen activation and transfer.The substituent groups also elevate the activation energy of the C-S bond cleavage elementary reaction on the NiMoS active sites.There are evident competitive adsorptions between 4,6-DMDBT and dibenzothiophene(DBT),which indirectly indicates that the adsorption difference between 4,6-DMDBT and DBT lies on the morphology rather than the abilities.Indole could rapidly convert to indoline,and then directly conducts the C-N bond cleavage,forming o-ethylaniline.When quinoline converts to tetrahydro-quinoline,the C-N bond could hardly cleave,until the tetrahydro-quinoline continuatively convert to decahydro-quinoline.The reason is that on the five-membered cycle of non-basic nitrogen compounds,there are only α-C and β-C on the nitrogen heterocycle.These two C atoms and N atom lies on the same plane when the C-N bond cleaves,which makes the molecular configuration more stable.There is an additional γ-C atom on the quinoline.Because of the rigidity of aromatic ring and the non-coplanar of sp3 C atoms,the benzene ring of quinoline has to be fully hydrogenated into six-membered naphthenic ring before the-NH group and the alkene group could be on the same plane,following with the cleavage of C-N bond.As to the non-basic nitrogen compound indole,whenever the substituent group is on the α-C or β-C atoms,the hydrogenation saturation activity of the nitrogen heterocycle will be reduced,consequently increasing the difficulty of the hydrodenitrogenation of indole.As to the basic nitrogen compound quinoline,when the substituent group is on the a-C atom,the influence of nitrogen compounds is slight,whereas the competitive adsorption of 2-methyl-quinoline to other basic nitrogen compounds is weakened.When the substituent group is on the β-C atom,the difficulty of the C-N bond cleavage of 3-methyl-quino line will increase,whereas the competitive adsorption of 3-methyl-quinoline is not affected virtually.