Methane hydroxylation

the studied mimic catalyzes two interrelated, chemically conjugated catalase and monooxygenase reactions. The conjugation mechanism is comprehensively described by the following diagrams [11]  

As follows from the above, at short contact times (below 2.9 s) the monooxygenase activity of the mimic remains low, whereas catalase activity is maximal (molecular oxygen yield exceeds 90 wt.%). Methanol yield and methane conversion increase with contact time up to r = 10 s and then stabilize at a level of 49-50 wt.% with —96% selectivity. Formaldehyde and formic acid are side products, giving total 2.7 wt.% no CO and C02 are detected in gaseous products. 

The curve of 02 accumulation shows that short contact times, at which the methane oxidation rate is low, are enough for complete H202 dissociation. However, as observed from shapes of 02 and CH3OH accumulation curves, methanol yield increases synchronously with 02 yield decrease, and from the moment r = 10.2 s both curves stabilize. Such stabilization and synchronization of catalase and monooxygenase reaction product yields is the experimental proof of their interaction, displayed by chemical conjugation. The existence of the stabilization zone of 02 and CH3OH yields is associated with full H202 dissociation. 

Generalization of the experimental data lead to the conclusion that gas-phase oxidative activation of methane with hydrogen peroxide is highly effective in the presence of biomimic PPFe3+OH/AlSiMg in a limited time interval (high selectivity up to 97%, conversion exceeding 60 wt.%, relatively low temperature -180°C, atmospheric pressure) [89], 

The kinetic regularities observed lead to consideration of the hydroxylation mechanism in the context of modem ideas about enzymatic catalysis mechanism. 

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Methane Methanotrophic Methane hydroxylation EX29EX2HX64EX33EX2H Hydroxo, aqua... 

A systematic study to identify solid oxide catalysts for the oxidation of methane to methanol resulted in the development of a Ga203—M0O3 mixed metal oxide catalyst showing an increased methanol yield compared with the homogeneous gas-phase reaction.1080,1081 Fe-ZSM-5 after proper activation (pretreatment under vacuum at 800-900°C and activation with N20 at 250°C) shows high activity in the formation of methanol at 20°C.1082 Density functional theory studies were conducted for the reaction pathway of the methane to methanol conversion by first-row transition-metal monoxide cations (MO+).1083 These are key to the mechanistic aspects in methane hydroxylation, and CuO+ was found to be a likely excellent mediator for the reaction. A mixture of vanadate ions and pyrazine-2-carboxylic acid efficiently catalyzes the oxidation of methane with 02 and H202 to give methyl hydroperoxide and, as consecutive products, methanol and formaldehyde.1084 1085... 

Whereas several transient species have been observed for dioxygen activation by MMOH, no intermediates were found by rapid-mixing spectroscopic methods for the actual methane hydroxylation step. Mechanistic probes, i.e. certain non-natural substrates that are transformed into rearranged products only if the reaction proceeds via a specific intermediate such as a radical or a cation, give ambivalent results Some studies show that products according to a pathway via cationic intermediates are obtained in sMMO hydroxylations and at least one study suggests the presence of a radical intermediate [40]. Computational analyses of the reaction of MMOHq with methane suggest a so-called radical recoil/rebound mechanism in which MMOHq... 

An additional pathway of similar energy was found to proceed via a concerted oxygen atom insertion [45]. In summary, it appears as if different substrates can be differently activated and the reaction pathway hence proceeds via different intermediates. Clearly, more studies are necessary before the mechanism of methane hydroxylation by MMOH can be fully understood. 

Figure 2.3 Dependencies of methane hydroxylation outputs on (a) temperature and (b) contact time at 180°C. 1 CH4 conversion 2 CH3OH output 3 CH20 and HCOOH outputs 4 selectivity 5 02 output Ratios CH4 H202 = 1 1.4 (a) and 1 1.8 (b) VCh/ha = 0.8ml/h, [H2OJ = 20wt.%.

Catalytic activity of heterogeneously applied iron protoporphyrin biomimics in methane hydroxylation with hydrogen peroxide was kinetically studied [87],... 

Figure 7.26 Temperature dependencies of methane hydroxylation product yields. CH HjOj = 1 1.4 VcH4=0.0861/h VHA = 0.8ml/h [H202] = 20 wt.% (1 CH4 conversion 2 CH3OH yield 3 CHzO and HCOOH yield 4 selectivity and 5 02 yield).

Kinetic curves displaying the influence of contact time (r) on methane hydroxylation with hydrogen peroxide are shown in Figure 7.27. 

The experimental data on acidic-basic sites of the mimic and ideas about the mechanism of biomimetic propylene hydroxylation with hydrogen peroxide have justified the probable mechanism of methane hydroxylation with hydrogen peroxide ... 

In microorganisms utilizing methane, a methane-hydroxylating system, metan monooxigenase, (MMO) has been detected, which catalyzed the reaction ... 

Merkx, M., Kopp, D. A., Sazinsky, M. H., Blazyk, J. L., Muller, J., and Lippard, S.J. (2001) Dioxygen activation and methane hydroxylation by soluble methane monooxygenase a tale of two irons and three proteins. Angew. Chem., Int. Ed. 40, 2782-2807. 

Yoshizawa, K. (2000) Two-step concerted mechanism for methane hydroxylation on the diiron active site of soluble methane monooxygenase, J. Inorg. Biochem. 78, 23-34. 

In their search for effective methane hydroxylation catalysts Periana et al. have reported the synthesis and use of a stable iridium NNC pincer complex. Transition state 102 has been shown by DFT calculation to be involved in C-H bond activation. 

Is the ruthenium analogue of compound I of cytochrome P450 an efficient oxidant A theoretical investigation of the methane hydroxylation reaction. J. Am. Chem. Soc. 125, 2291—2300. 

This problem was taken on by the Jerusalem group who used DFT (B3LYP) computations to model the mechanism of methane hydroxylation -101-103 allylic hydroxylation of propene " - and... 

Figure 2.20. Key geometric parameters for high-spin rebound transition states in methane hydroxylation and allylic hydroxylation . The quantities below the structures correspond to the geometric parameters of the corresponding iron-hydroxo/alkyl radical clusters ( Cj in Figure 2.17).

Studies by Chan and Floss et al. [32a] with another kind of MMO, the so-called particulate form pMMO from Methylococcus capsulatus (Bath) (which is membrane bound and contains copper), confirmed that the rate controlling step of methane hydroxylation has a very high KIE, while ethane produces only a moderate KIE. The data obtained in this work point to a mechanism in which C-H bond cleavage is preceded by bond formation at the alkyl carbon, i.e., one proceeding through a pentacoordinated carbon species [32a]. 

Figure 7 Schematic diagram for two-state rebound mechanism for methane hydroxylation by an oxoiron(IV) porphyrin rr-cation radical (after Ogliaro et...

Methane hydroxylation A biomimetic approach (heterocycles as ligands... 

Dioxygen activation and methane hydroxylation by soluble methane monooxygenase a tale of two irons and three proteins. Angew. Chem. Int Ed.,... 

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