Selective Oxidation Catalyzed By Molybdovanadophosphoric Acids And 8-quinolinolato Metal Complexes

Adipic acid industry is under the pressure of energy conservation and emission reduction.It's very important to improve the process of cyclohexane oxidation to KA oil(cyclohexanone and cyclohexanol)and to deal with the N2O emissionin when using nitric acid to oxidize KA oil.The former involves the activation of the C-H bond of inert alkanes and the oxygenation of the single oxygen atom.It is often necessary to control the cyclohexane conversion(<8%)to keep the good KA oil selectivity in harsh reaction conditions,which is one of the most challenging subjects in catalytic oxidation.The latter involves the activation of N-O bond in inert N2O with weak coordination capacity,and its application in organic synthesis is also the most challenging branch of oxidation chemistry.It's also a great challenge to use triplet O2 in the above selective catalytic oxidation under mild conditions.In addition,it is also a matter of concern that H2O2,as an active single oxygen donor,participates in the selective oxidation of olefins and cyclohexane.This thesis performed the following researches on the above aspects.1.Given the unique advantages of photocatalysis in the difficult chemical reactions,this thesis introduced this technology to the HCl-promoted cyclohexane oxidation to KA oil by N2O upon Keggin type molybdovanadophosphoric acids(HPAs),which can achieve about 26.2%cyclohexane conversion and 90.2%KA oil selectivity under normal temperature and pressure with visible light irradiation for 12 h.A mechanism was revealed by UV-Vis,cyclic voltammetry(CV)and XPS spectra,combined with condition experiments.After V(V)atoms were incorporated into HPAs,the oxidative capacity of HPAs was improved.Meanwhile,they can combine with HCl to form the donor-receptor(D-A)adducts,which can be stimulated by visible light to realize an electron transfer(ET)from Cl-to V5+,leading to the generation of Cl free radicals and the reduced state of heteropoly blues(HPBs),the former can complete the C-H activation of cyclohexane and the generated cyclohexyl radicals can capture the oxygen atom of N2O pre-activated by HPBs(D-A interactions),and thus the subsequent self-oxidation mainly generated KA oil.Then the excited state of HPBs as the active electron donor can reduce N2O in the protic medium to complete the catalytic cycle.This process realizes the selective oxidation of inert hydrocarbons by N2O,and has vital significance for the production of cyclohexane-derived KA oil and adipic acid and for the recycling of N2O.2.To further study the D-A interaction of HCl with V complexes and explore a new path to enhance the liquid phase catalytic oxidation performance of O2,we carried out the bis(8-quinolinolato)oxovanadium(IV)complexes(VIVOQ2)photo-catalyzed oxidation of cyclohexane under HCl promotion,and found that the catalytic efficiency of these complexes is similar to that of conventional V? or V? complexes and can also yield about 18%KA oil yield,but they can provide a higher KA oil selectivity.Particularly,dichloro-substituted complex achieved a target selectivity of 95%.Condition experiments and UV-Vis spectroscopic kinetics experiments showed that when the Cl substitution number on the ligand was below 2,the V?OQ2 complex was more sensitive to HCl and the characteristic band at 475 nm was positively correlated with this sensitivity.The formation rate of this characteristic band was much higher than that of the peroxovanadate species at 450 nm produced by the treatment of H2O2.However,there was no such a band in the UV-Vis spectrum in H2SO4 treatment.Therefore,such band should be attributed to an ET process from Cl' to V5+.Based on the FT-IR results,a possible mechanism was proposed as follows:a V-Ob bond of the V?OQ2 complex was dissociated to generate a species with V-Cl bond(CI axial coordination)via HCl protonation,followed by O2 addition to form superoxide radical species,generating a photoactive(PA)species with ligand semi-dissociated phenoxy radicals after multiple conversions.3.To further study the above photocatalytic systems,we have conducted the DFT studies of the activation effect of HCl on the V?OQ2 complexes and on the HPAs.Theoretical calculations found that a)the V?OQ2 complexes have central symmetric structures in acetonitrile with similar coordination stability.Their bridging oxygen atoms are more negative and are preferentially protonated by HCl.Their protonation capacity is weakened gradually with increasing Cl substituents on ligands.Moreover,protonated effect reduce their structural symmetry,thereby resulting in structural distortion effects.At least two interacted HCl are required to form a V?-species with Cl axially and non-axially coordinated structures.The proportion of axially-coordinated structure gradually increase with decreasing the Cl substituent on ligands.After combinating with O2 and undergoing a series of transformations,the photoactive(PA)species are generated in above two types of structures.The PA species with Cl-axial structure are more active,and their ET process could be significantly enhanced under the activation of multiple HCl as well as their ratios.This enhancing effect gradually is decreased with increasing of the Cl substituent on ligands.Furthermore,the interactions of multiple HCl play positive roles in the regeneration of V? species produced by photoexcitation,thereby completing the catalytic cycle.b)HPAs can easily interact with HCl to form the D-A adduct in the cooperation of their protonated bridging oxygens adjacent to V atoms,hence their forbidden band width are reduced,and most of their HOMO orbital are located in Cl atom,supporting the transfer process of electrons from Cl to Vv.Additionally,it's necessary for N2O activation to complete its oxygen transfer under the interactions of the reduced HPAs,HCl and solvent.These calculations well support the characterization and reaction results,and lay the foundation for the more detailed mechanism analysis.4.Based on the respective characteristics of 8-quinolinolato manganese(Q3Mn?),iron(Q3Fe?)and oxovanadium(V?OQ2)metal complexes,we performed catalytic oxidation studies of these complexes.The results showed that a)the Q3Mn? complexes with distortion structures can directly catalyze 50%H2O2 to selectively oxidize some olefins to their corresponding epoxides and a certain amount of diols at room temperature without any additives,their catalytic efficiency(highest TON 455)and the utilization rate of H2O2(90%)were obviously higher than those obtained by some reported systems containing additives.A reasonable mechanism was speculated based on the results of catalytic reactions and UV-Vis spectra.b)Ultrasonic field showed the acceleration effect on the oxidation of cyclohexane with H2O2 catalyzed by Q3Fe?,this effect became more outstanding upon relatively inert complex(5-chloro-7-iodine-substituted)Qe3Fe? and was gradually strengthened with increasing the temperature.For example,cyclohexane conversion over Qe3Fe? was ca.19.0%under an ultrasonic for 12 h at 45?,being three times higher than that of the conventional heating.This acceleration effect is likely related to the structural distortion of octahedral symmetric Q3Fe? complexes induced by ultrasonic,thus accelerating the activation of H2O2.c)Based on the above speculation that V?OQ2 complexes could be converted to a Vv-superoxide species under the co-activation of HCl and O2,we studied the benzyl alcohol oxidation in this kind of catalysis system.This system could realize the oxidation of benzyl alcohol,providing benzaldehyde as a main oxygenated product,along with a certain amount of chlorinated product benzyl chloride.Moreover,adding NaHCO3 could significantly inhibit chlorinated product and improve the selectivity of benzaldehyde.Under optimized conditions,the benzyl alcohol conversion was close to 80%and the benzaldehyde selectivity was 78.7%.