Selective oxidation of methane to
methanol is one of the most difficult
chemical processes to perform. A potential group of catalysts to achieve
CH4 partial oxidation are Cu-exchanged zeolites mimicking
the active structure of the enzyme methane monooxygenase. However,
the details of this conversion, including the structure of the active
site, are still under debate. In this contribution, periodic density
functional theory (DFT) methods were employed to explore the molecular
features of the selective oxidation of methane to methanol catalyzed
by Cu-exchanged mordenite (Cu-MOR). We focused on two types of previously
suggested active species, CuOCu and CuOOCu. Our calculations indicate
that the formation of CuOCu is more feasible than that of CuOOCu.
In addition, a much lower C–H dissociation barrier is located
on the former active site, indicating that C–H bond activation
is easily achieved with CuOCu. We calculated the energy barriers of
all elementary steps for the entire process, including catalyst activation,
CH4 activation, and CH3OH desorption. Our calculations
are in agreement with experimental observations and present the first
theoretical study examining the entire process of selective oxidation
of methane to methanol.
The dehydrogenation of n-hexane and cycloalkanes giving n-hexene and cycloalkenes has been observed in the reaction of such hydrocarbons with hydrogen peroxide, in the presence of copper complexes bearing trispyrazolylborate ligands. This catalytic transformation provides the typical oxidation products (alcohol and ketones) with small amounts of the alkenes, a novel feature in this kind of oxidative processes. Experimental data exclude the participation of hydroxyl radicals derived from Fenton-like reaction mechanisms. DFT studies support a copper-oxo active species, which initiates the reaction by H abstraction. Spin crossover from the triplet to the singlet state, which is required to recover the catalyst, yields the major hydroxylation and minor dehydrogenation products. Further calculations suggested that the superoxo and hydroperoxo species are less reactive than the oxo. A complete mechanistic proposal in agreement with all experimental and computational data is proposed.
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