Study On Biomimetic Catalytic Oxidation Of Cyclohexane With Trans-A₂B₂Type Metalloporphyrins Catalyst

Functionalization of inert alkanes to more valuable products (e.g., carboxylic acids,alcohols, ketones) has attracted much attention. Generally speaking, the oxidation ofalkane is problematic because of the inactivity of C–H bonds of alkane chemically.Catalytic oxidation of C–H bond in saturated hydrocarbons under mild conditions is akey step in the oxyfunctionalization of organic compounds. The cytochrome P-450monooxygenase enzyme, which can catalyze the oxidation of the inert C–H bonds undermild conditions in organisms, is able to effectively and stereospecially catalyze thehydroxylation and epoxidation of hydrocarbon in the metabolic system. There has beensignificant interest in modeling of the P-450enzyme active sites and developing thebiomimetic oxidation catalysts. The application of metalloporphyrins in syntheticchemistry and chemical industry is not popular because of its economic problem.Therefore, the selective oxidation of hydrocarbons and other organic compounds usingmetalloporphyrin is still a challenging and promising subject. Cyclohexane oxidationwith air or dioxygen in the absence of additives and solvents is one of the mostimportant industrial processes to produce KA oil (cyclohexanol/cyclohexanone) oradipic acid, which are keys intermediates used principally for the production ofcaprolactam, nylon-6,6and nylon-6.In our research, we synthesize a series of trans-A2B2-porphyrins through aTFA-catalyzed condensation reaction between various aldehydes and dipyrromethanesusing MacDonald “2+2” method, the range of the yeild is23-35%. Severaltrans-A2B2-metalloporphyrins were synthesized and used to catalyze cyclohexaneoxidation C–H bonds with dioxygen in the absence of additives and solvents, the yeildof A2B2-metalloporphyrins is83-98%. We found that the reaction conditions oftemperature, pressure and time have a strong impact on the cyclohexane conversion andselectivity to KA oil. The results show that the catalytic activities were relative to thenature of the substituent group and the central metal ion (ionization potential anddioxygen-binding ability) of trans-A2B2-metalloporphyrin. Research has shown that theadsorption of dioxygen is the first step for oxygen reduction; then the interactionbetween the catalyst and oxygen molecule and its effect of dissociation of the O O bondplays an important role in understanding the reaction mechanism. These processes aredeemed to proceed by a mechanism similar to the enzymatic systems. Hence, higher ionization potential and larger dioxygen-binding energy are associated with bettercatalytic activity. Cobalt metalloporphyrin presents better catalytic performance in theconversion of cyclohexane than the nickel metalloporphyrin under the same reactionconditions. At the1MPa and60min condition, the cyclohexane conversion andselectivity to KA oil and TON are7.93%and76%and0.6×106by Co-D(p-Cl)PPcatalyzed.Homogeneous systems based on metalloporphyrins have achieved good activitiesand high selectivities in catalytic oxidation of hydrocarbons. However, themetalloporphyrins are inclined to aggregation through interactions or decompose duringthe catalytic oxidation process. Furthermore, it is difficult to recover and recycle fromthe homogeneous media at the end of the first reaction. Immobilization of themetalloporphyrins on inert and stable inorganic supports is one of the most effectiveways to overcome the above drawbacks. In our research, trans-A2B2typemetalloporphyrins catalyst supported on boehmite, ZnO,(BM+ZnO), Zr(OH)2, ZrO2,MCM-41and kaolin were prepared and used to catalyze cyclohexane oxidation C–Hbonds with dioxygen in the absence of additives and solvents. The pore diameter of thecarriers has a significant impact on the catalytic performance in the conversion ofcyclohexane. Under the same reaction condition, immobilization of metalloporphyrin onboehmite and ZnO show better catalytic performance than others; this kind of catalystcan be facilely recovered and was recycled without significant decrease in catalyticperformance. At1MPa and120min conditions, the average cyclohexane conversion andselectivity to KA oil and TON are7.93%and83.30%and1.05×107byCo-D(p-Cl)PPCl/BM catalyzed. At1.5MPa and120min conditions, the averagecyclohexane conversion and selectivity to KA oil and TON are10.98%and84.34%and2.10×107by Co-D(p-Cl)PPCl/ZnO catalyzed.In order to study how oxygen activation is influenced by different substituents aswell by the central metal ion in metalloporphyrin complexes. We attempted a systematictheoretical study on metalloporpyrin, at the Dmol3(GGA/BLYP) model of theory. Bycomparison of the calculated HOMO and LUMO energies and geometry structure ofmetalloporphyrin, we have found that the HOMO and LUMO of metalloporphyrin areinsignificantly influenced by the presence of substituents in the metalloorphyrinmolecules. Electron-withdrawing substituents were beneficial to reduce LUMO energyand promote to activate the molecular oxygen. In metalloporphyrin/O2system, weproposed oxygen go through SN2mechanism (nucleophilic substitution) and form two kinds of transition state: O2-Co-D(p-Cl)PPCl and O2-Co-D(p-Cl)PPOH. Calculationsand theoretical simulations show that the proposed mechaism are better than the other.Meanwhile, the existence of high valent CoD(p-Cl)PP=O is very important and alsoplays the role of the radical chain initiator in the cyclohexane oxidation process. Thiswork provides new ideas for understanding and recognizing biomimetic catalyticoxidation of cyclohexane from metalloporphyrin; it might have outstanding theorysignificance.