Studies On The Aerobic Oxidation Of O/p-cresol To O/p-hydroxy Benzaldehyde Catalyzed By Metalloporphyrins

Aromatic aldehydes are important chemical intermediates for the synthesis ofvarious pharmaceuticals, pesticides, dyes, flavors, perfumes and chiral intermediates.There are many preparation methods for aromatic aldehydes, and directoxyfunctionalization of cresols at their benzylic positions provides an effectivemethod for synthesizing hydroxybenzaldehyde. However, conventional methods forpreparing o/p-hydrocybenzaldehydes have several disadvantages such as involvinglarge investment and high energy. In addition,-CH₃substituent of cresol can beinfluenced by other substituent to carry out deep oxidation under radical oxidationconditions, which cause the reduction of main product selectivity. While under mildconditions, cresols oxidation reactions might not carry out which influent the cresolconversion. Therefore, no doubt there will be a huge market if the aromatic aldehydescould be prepared from the direct catalytic oxidation of alkyl aromatics at highconversions and selectivities under mild conditions with dioxygen as economical andgreen oxidant. However, dioxygen exists in triplet state and hydrocarbons exist insinglet state, so the development and application of the above reaction are restrained,because the oxidation of hydrocarbons with molecular oxygen is spin-forbidden.Fortunately, metalloporphyrins can catalytically activate molecular oxygen even atmild temperature, so the spin-restrained problem of hydrocarbon oxidation is solved.Accordingly, this dissertation was focused on the study of the new method andoxidation mechenismof alkyl aromatics to aldehyde catalyzed by metalloporphyrinswith oxygen.A series of metalloporphyrins with or without axial ligand chloride weredesigned and synthesized firstly. Then, the conversions and selectivities of the abovemetalloporphyrins were studied systematically. Meawhile, the effect of the componentcatalysts between metalloporphyrins and metal salts on p-cresol oxidation reactionwere also studied. The main results are summarized as follows:1. Catalytic performance and rule of metalloporphyrins were investigated.Through the study of the catalytic effect of metalloporphyrins with different structuresin the above reaction, it was found that the catalytic activity order ofmetalloporphyrins in the reaction was iron porphyrin> manganese porphyrin> cobaltporphyrin; the order of selectivity of p-hydroxybenaldehyde was iron porphyrin>manganese porphyrin> cobalt porphyrin; the sequence of activity and selectivity ofiron porphyrins with different substitutes was-CH₃O>-CH₃>-H>-Cl>-NO₂; theaxial chlorine free ligand of metalloporphyrins did not effect the conversion ofp-cresol much, but for the selectivity of p-hydroxybenzaldehyde was influenced muchby the metalloporphyrins with and without axial chlorine, and the former oneperformed better on p-cresol conversion. Among the catalyst studied, T（p-CH₃O）PPFeCl performed the best, and78.7%conversion of p-cresol,63.9%selectivity and50.3%yield of p-hydroxybenzaldehyde were reached.2. The metalloporphyrins and metal salts co-catalysts were studied in thecatalytic oxidation of p-cresol to prepare p-hydroxybenzaldehyde. Through the studyof different metalloporphyrins and metal salts, it was found that there were obvioussynergistic effects between metalloporphyrins and metal salts. The oxidation ofp-cresol could carry out under lower reaction pressure with lower composite catalystsconcentration, meanwhile the p-cresol conversion and p-hydroxybenzaldehydeselectivity could be significantly improved. Among the composite catalysts studied,T（p-CH₃O）PPFeCl-Co（OAc）.4H2O performed best. The p-cresol conversion could beimproved from8.0%to99.9%, and the p-hydroxybenzaldehyde selectivity could beimproved from17.4%to82.6%, comparing with the reaction without catalyst.Comparing with the reaction results obtained by single metalloporphyrins, those usingcomposite catalysts could increase the conversion by21.2%andp-hydroxybenzaldehyde selectivity by18.7%. And also, the catalyst dosage wasreduced from1.110mmol/L to0.279mmol/L.3. Through the effect of different metalloporphyrins on the catalytic oxidationreaction of o-cresol, it was found that the catalytic activity order of metalloporphyrinsin the reaction was iron porphyrin> manganese porphyrin> cobalt porphyrin; the orderof selectivity of o-hydroxybenaldehyde was iron porphyrin> cobalt porphyrin>manganese porphyrin; the sequence of activity and selectivity of iron porphyrins withdifferent substitutes was-CH₃O>-CH₃>-H>-Cl>-NO₂. Among the catalyst studied,T（p-CH₃O）PPFeCl performed the best, and50.4%conversion of o-cresol,26.6%selectivity and13.4%yield of o-hydroxybenzaldehyde were reached.4. Through the studies on the catalytic activities of metalloporphyrins withdifferent symmetry on the o-cresol oxidation reaction, it was found that A₃B typemetalloporphyrins had better performances than A₄type metalloporphyrins, especiallyA₃B type zinc porphyrin which could increase the o-cresol conversion by21.9%. TheEHOMO, ELUMOand ΔEH-Lof A₃B type and A₄type metalloporphyrins were observedby the quantum chemistry calculation simulation, and the ΔEH-Lvalue of A₃B typezinc porphyrin was lower than that of A₄type zinc porphyrin, which explained thereason for the higher activity of A₃B type metalloporphyrins.5. The thermodynamics of preparation reaction of o/p-hydroxybenzaldehydefrom o/p-cresol were studied. It was found that, ΔH of all the steps of o/p-cresoloxidation reactions were below zero, indicating that the reactions were exothermicreaction; the ΔG of each step of o/p-cresol oxidation reactions was negative,indicating the reactions could be carried out spontaneously in theory; in addition,except for the ether formation reaction steps, ΔG of other steps of o/p-cresol oxidation reaction were increased with temperature rising, which indicated that lowertemperature was favorable to the steps above.6. The reaction mechanism study showed that the p-cresol oxidation reactioncatalyzed by metalloporphyrin was a free radical reaction. Firstly, the free radical ofalkyl aromatics was produced by the metalloporphyrin catalytic cycle. Secondly, thefree radicals reacted with molecular oxygen and formed peroxides, then furtherdecomposed into aldehydes; at the same time, the free radicals could also react withtrivalent metal hydroxy complexes to form quinoid structure. Thirdly, the quinoidstructure reacted with methanol or water to form p-hydroxylbenzylether orp-hydroxybenzylalcohol. Fourthly, p-hydroxybenzylalcohol further oxidized top-hydroxybenzaldehyde, and the aldehyde could be further oxidized top-hydroxybenzoic acid.