important enzyme-catalyzed oxidation reactions. ... Fenton reagents are used in the hydroxylation of aromatic substrates (e.g., ... (2a) together describe the so-called Haber-Weiss cycle, a chain reaction .... 1 with respect to the free reactants (see ... This step leads to the final product with the regeneration of the catalyst and ...
O2 evolution in the Fenton reaction.
F. Buda a y*, B. Ensing a , M. C. M. Gribnau b , and E. J. Baerends a * Department of Theoretical Chemistry, Vrije Universiteit, De Boelelaan 1083, NL-1081 HV Amsterdam, The Netherlands b Unilever Research Vlaardingen, The Netherlands ]
a]
]
]
]
]
Abstract We investigate the mechanism involved in the oxygen production in the Fenton chemistry by using Density Functional Theory calculations. This study extends a previous work in which we have supported the Fe(IV)O
2+
complex
as the key active intermediate in the Fenton reaction. Here we provide a consistent picture of the entire reaction cycle by analyzing how the active species Fe(IV)O can react with hydrogen peroxide to produce O and regenerate the 2+
Fe
2+
2
catalyst. These results are also relevant in view of the analogies with
important enzyme-catalyzed oxidation reactions.
Keywords: Oxidation catalysis, Fenton reaction, Ferryl-oxo complex, Density Functional Theory
I. INTRODUCTION The Fenton reagents, 1] a mixture of ferrous ions and hydrogen peroxide in water, has great relevance for the chemical industry in view of its powerful oxidizing properties. Indeed, y Present address: Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA, Leiden,
The Netherlands
1
Fenton reagents are used in the hydroxylation of aromatic substrates (e.g., production of phenol using benzene as a substrate), 2] or for the treatment of contaminated waters containing nonbiodegradable organic compounds. 3] Moreover, the oxidative reactions of the Fenton chemistry show analogies with fundamental processes in biology which are involved in the etiology of diseases. 4] Therefore an understanding of the microscopic mechanisms of the Fenton chemistry is of fundamental importance and may have an impact on very dierent elds and applications. Though the history of the Fenton chemistry extends over more than a century, still basic questions about its mechanism and the nature of the active intermediate remain controversial. 5{7] There are basically two alternative models which have been proposed in the literature: in the rst mechanism, introduced by Haber and Weiss 8] and subsequently modied by Barb et al., 9] the oxidative intermediate is identied with the free hydroxyl radical formed by the metal-catalyzed decomposition of the hydrogen peroxide the second mechanism involves the formation of a highly reactive, high valent iron complex, such as the ferryl-oxo complex rst proposed by Bray and Gorin. 10] Recently, we have used Density Functional Theory (DFT) calculations and ab-initio Molecular Dynamics simulations to investigate the initiation step in the Fenton reaction both in the vacuum 11] and in the presence of the water solvent. 12{14] The main conclusion of this work is that a ferryl-oxo complex can easily be produced in water starting from a primary intermediate with the hydrogen peroxide coordinated to the ferrous ion. The alternative model in which a free OH radical obtained from the hydrogen peroxide dissociation diuses into the solvent seems energetically unlikely. Thus, our results strongly support a mechanism in which the Fe(IV)-oxo complex acts as the key intermediate. Experimentally, in an excess of H O over Fe ions, catalytic decomposition of hydrogen peroxide to produce water and O accompanies the oxidation of Fe ions: 15] 2H O ! 2H O + O Evidently, in order for FeO to be a viable active intermediate, it should also be able to lead to O production in the presence of excess H O . It is the purpose of this paper to investigate possible reaction mechanisms for the O production with the FeO intermediate. 2
2+
2
2+
2
2
2
2
2
2+
2
2
2
2
2
2+
Within the widely accepted hydroxyl radical mechanism the O production has been explained, in the absence of other reagents, e.g. an organic substrate, according to the following reaction sequence: 2
hydroxyl radical HO mechanism: Initiation reaction (0) Fe + H O 2+
2
Fe + OH ; + HO 3+
!
2
O Production 2
(1) HO +H O 2
!
2
(2a) HOO +H O 2
2
(2b) HOO +Fe
3+
H O + HOO 2
!
!
O + H O + HO 2
2
Fe + H + O 2+
+
2
Degradation reaction (3) HO +Fe
2+
!
Fe + OH ; 3+
The step (0) is the initiation step producing the active intermediate HO and the Fe ion the steps (1) + (2) describe the H O consumption under O evolution the step (3) is a degradation step in which the catalyst is consumed by producing Fe + OH;. Steps (1) and (2a) together describe the so-called Haber-Weiss cycle, a chain reaction which consumes the H O reagens under O evolution. There is considerable evidence against this "Haber-Weiss cycle" (see the recent review by Koppenol 16]), and it is now considered much more likely that reaction (2b), which regenerates the Fe catalyst, is responsible for the O production, although the concentration ratio Fe ]
