Different efficiency toward the biomimetic aerobic oxidation of benzyl alcohol in microchannel and bubble column reactors: Hydrodynamic characteristics and gas–liquid mass transfer

The selective aerobic oxidation of benzyl alcohol to benzaldehyde has attracted considerable attention because benzaldehyde is a high value-added product. The rate of this typical gas–liquid reaction is significantly affected by mass transfer. In this study, CoTPP-mediated(CoTPP: cobalt(II) mesotetraphenylporphyrin) selective benzyl alcohol oxidation with oxygen was conducted in a membrane microchannel(MMC) reactor and a bubble column(BC) reactor, respectively. We observed that 83% benzyl alcohol was converted within 6.5 min in the MMC reactor, but only less than 10% benzyl alcohol was converted in the BC reactor. Hydrodynamic characteristics and gas–liquid mass transfer performances were compared for the MMC and BC reactors. The MMC reactor was assumed to be a plug flow reactor,and the dimensionless variance was 0.29. Compared to the BC reactor, the gas–liquid mass transfer was intensified significantly in MMC reactor. It could be ascribed to the high gas holdup(2.9 times higher than that of BC reactor), liquid film mass transfer coefficient(8.2 times higher than that of BC reactor), and mass transfer coefficient per unit interfacial area(3.8 times higher than that of BC reactor). Moreover,the Hatta number for the MMC reactor reached up to 0.61, which was about 15 times higher than that of the BC reactor. The computational fluid dynamics calculations for mass fractions in both liquid and gas phases were consistent with the experimental data.

Liquid-phase epoxidation of propylene with molecular oxygen by chloride manganese meso-tetraphenylporphyrins

Propylene molecule owns two active sites,the direct epoxidation of propylene by dioxygen is still a challenge due to the limitation of selectivity.In this work,the direct liquid-phase propylene aerobic epoxidation protocol by chloride manganese meso-tetraphenylporphyrin(MnTPPCl)was developed.The conversion of propylene was 12.7%,and the selectivity towards PO(propylene oxide)reached up to 80.5%.The formation of PO was attributed to the mechanism via high-valent Mn species,which was confirmed by means of in situ UV–vis spectrum.

Manganese porphyrin-mediated aerobic epoxidation of propylene with isoprene: A new strategy for simultaneously preparing propylene epoxide and isoprene monoxide

The direct epoxidation of propylene by O_(2) is a significant and challenging topic. The key factor for this homogeneous aerobic epoxidation is the activation of molecular oxygen under mild conditions. In this work,the aerobic epoxidation of propylene catalyzed by manganese porphyrins was achieved in the presence of isoprene. Isoprene contains an allyl methyl group, and the α-H can be easily removed to achieve the activation of molecular oxygen. The conversion of propylene was 38% and the selectivity toward propylene oxide(PO) was up to 87%. The role of isoprene was demonstrated, and a plausible mechanism was proposed. The protocol reported herein is expected to provide a strategy for the simultaneous preparation of propylene oxide and isoprene monoxide.

Cyclohexene Promoted Efficient Biomimetic Oxidation of Alcohols to Carbonyl Compounds Catalyzed by Manganese Porphyrin under Mild Conditions

Selective oxidation of alcohols to corresponding carbonyl compounds is one of the most important processes both in academic and application research.As a kind of biomimetic catalyst,metalloporphyrins-catalyzed aerobic oxidation of alcohols with aldehyde as hydrogen donator is gathering much attention.However,using olefins as another kind hydrogen donator for aerobic oxidation of alcohols has not been reported.In this study,a system comprising managenese porphyrin and cyclohexene for biomimetic aerobic oxidation of alcohols to carbonyl compounds was developed.The catalytic system exhibited excellent catalytic performance and selectivity towards the corresponding products for most primary and secondary alcohols under mild conditions.Based on the results obtained from experiments as well as in situ EPR(electron paramagnetic resonance)and UV-vis spectroscopy,the role of cvclohexene was demonstrated.

Oxygen Atom Transfer Mechanism for Vanadium-Oxo Porphyrin Complexes Mediated Aerobic Olefin Epoxidation

The development of catalytic aerobic epoxidation by numerous metal complexes in the presence of aldehyde as a sacrificial reductant(Mukaiyama epoxidation)has been reported,however,comprehensive examination of oxygen atom transfer mechanism involving free radical and highly reactive intermediates has yet to be presented.Herein,meso-tetrakis(pentafluorophenyl)porphyrinatooxidovanadium(Ⅳ)(VOTPFPP)was prepared and proved to be efficient toward aerobic olefin epoxidation in the presence of isobutyraldehyde.In situ electron paramagnetic resonance spectroscopy(in situ EPR)showed the generation,transfer pathways and ascription of free radicals in the epoxidation.According to the spectral and computational studies,the side-on vanadium-peroxo complexes are considered as the active intermediate species in the reaction process.In the cyclohexene epoxidation catalyzed by VOTPFPP,the kinetic isotope effect value of 1.0 was obtained,indicating that epoxidation occurred via oxygen atom transfer mechanism.The mechanism was further elucidated using isotopically labeled dioxygen experiments and density functional theory(DFT)calculations.

Substrate specificity in the biomimetic catalytic aerobic oxidation of styrene and cyclohexanone by metalloporphyrins: kinetics and mechanistic study

Substrate specificity is a hallmark of enzymatic catalysis.In this work,the biomimetic catalytic oxidation of styrene and cyclohexanone by iron(III)porphyrins and molecular oxygen was carried out,and remarkable differences in efficiency were observed.The specificity of the substrates for biomimetic catalytic oxidation was investigated by kinetics and mechanistic studies.Kinetics studies revealed that the oxidation of styrene followed Michaelis-Menten kinetics with KM=8.99 mol L^(-1),but the oxidation of cyclohexanone followed first-order kinetics with kobs=1.46×10^(-4) s^(-1),indicating that the styrene epoxidation by metalloporphyrins exhibited characteristics of enzyme-like catalysis,while the oxidation of cyclohexanone was in agreement with the general rules of chemical catalysis.Different catalytic mechanisms for the two substrates were discussed by operando electron paramagnetic resonance spectroscopy,operando UV-vis spectroscopy,and KI/starch experiments.Substrate specificity was concluded to be attributed to the stability of high-valence species and oxygen transfer rate.

Propylene epoxidation coupled with furfural oxidation over Pt(Ⅱ) TPP porphyrin with molecular oxygen

The development of green route for preparing propylene oxide(PO)with molecular oxygen is of significance both in academic and industrial.In this work,propylene epoxidation coupled with furfural oxidation catalyzed by platinum meso-tetraphenylporphyrin(Pt(Ⅱ)TPP)has been developed.Propylene conversion and PO selectivity reached up to 56%and 83%,respectively.Meanwhile,furfural was almost completely converted to furoic acid.Based on operando characterizations and electron paramagnetic resonance(EPR)tests,a mechanism involved high-valent Pt species was proposed.This work is expected to provide a potential application prospects for producing PO and furoic acid simultaneously in chemical industry.