Aqueous-phase Reforming Of Methanol For Hydrogen Production Catalyzed By Ru Complexes Bearing The Schiff Base Ligand

AbstractAqueous-phase reforming of methanol to produce hydrogen can link methanol with hydrogen energy,which is expected to establish a new sustainable and renewable energy cycle system.The design and synthesis of efficient catalysts is the most important part of methanol reforming for hydrogen production.In recent years,homogeneous catalysts with high activity and mild reaction conditions have gradually attracted attention,where the selection of organic ligands is central to the design of efficient catalysts.In this thesis,a series of Ru,Ru/Ln-Schiff base(Ln = La,Ce)compounds were synthesized with pincer-type Schiff base as the main ligands.In addition,the effects of the ratio of methanol to water,reaction temperature,base type and concentration on the catalytic performance were investigated.Moreover,disctinct catalytic mechanism were also explored and proposed based on kinetic isotope effect(KIE)experiments,in situ NMR,high-resolution mass spectrometry and intermediates catalytic activity.Details are described as follows:1.Two mononuclear Ru-PNNP Schiff base complexes 1 and 2 were obtained by the reaction of PNNP bis-Schiff base ligands synthesized by cyclohexanediamine or ethylenediamine with diphenylphosphine benzaldehyde as the main ligands with Ru precursors,and their structures were characterized in detail.Methanol reforming experiments showed that catalyst 2 was more active than 1,indicating that the type of substituent group plays a very important role in the reaction.Subsequently,the effects of methanol-water ratio,reaction temperature,and type and concentration of base on the methanol reforming catalytic reaction were investigated.Under the optimized reaction conditions,the number of conversions(TON)of catalyst 2 was 12000 at 180 h,which reached a moderate level compared to the literature,and the catalytically produced hydrogen was of high purity with no detectable CO formation.To explain the gradual decrease in the activity of catalyst 2 with time and the intermediate information observed by high-resolution mass spectrometry,compound 3 containing only one C=N bond and compound 4 completely without Schiff base double bond were designed and synthesized.Under the same reaction condition with 2,the results show that the catalytic effect 2 > 3 > 4,which indicates that the hydrogen-rich intermediate generated by catalyst 2 has the side reaction of reducing the C=N double bond while generating hydrogen,and the species without the C=N double bond becomes less active.Combined with the data through KIE experiments,in situ NMR,and high-resolution mass spectrometry,the catalytic mechanism of PNNP Schiff base Ru complexes in the catalytic reaction of methanol reforming was proposed.The catalytic performances of complexes 1-4 for hydrogen generation at 120 o C in the methanol/water(8:2 ml/ml)and 8M KOH mixture are shown as follows:Ru L1Cl2(1),V(H2)= 155 ml,TON = 1592Ru L2Cl2(2),V(H2)= 225 ml,TON = 2319Ru L3Cl2(3),V(H2)= 157 ml,TON = 1603Ru L4Cl2(4),V(H2)= 113 ml,TON = 1154L1 = n,n’-bis [O-(diphenylphosphine)phenylmethylene] Cyclohexanediamine),L2= n,n’-bis [O-(diphenylphosphine)phenylmethylene] ethylenediamine,L3 = [n-(diphenylphosphine)phenylmethylene-n’-(diphenylphosphine)benzyl] ethylenediamine,L4 = n,n’-bis [O-(diphenylphosphine)benzyl] ethylenediamine.2.In the second chapter of this thesisr,rare earth metal elements Ce and La were introduced into the Ru-salen type Schiff ligand system,and two heteronuclear metal salen type Schiff-base complexes [Ru Ce(L5)(NO3)2(CH3OH)3CH3O](5)and[Ru La(L5)(NO3)2(CH3OH)3CH3O](6)were successfully synthesized.We used two heteronuclear metal complexes containing rare earths for the catalytic reaction of methanol reforming and found that both complexes 5 and 6 showed significant catalytic effects on hydrogen production.Under the same conditions,complex 5 produced more hydrogen gas,which indicates that different rare earth metal ions have a significant effect on the catalytic reaction and that Ce has better effect than La.In addition,the catalytic reaction of methanol reforming was also carried out by complex 5 under the condition without alkali addition,and the gas composition was detected by gas chromatography as H2 and CO2,but the catalytic effect still needs to be improved at present.Complexe5 V(H2)= 66.5 ml,TON = 234Complexe 6 V(H2)= 34.5 ml,TON = 122...