Synthesis, characterization and activity of homogeneous and heterogeneous (SiO2, NaY, MCM-41) iron(III) catalysts on cyclohexane and cyclohexene oxidation
Graphical abstract
Introduction
Selective oxidation of unsaturated hydrocarbons is a challenging reaction due to the similarity in the electronegativity of the carbon and hydrogen atoms [1]. In this context, the development of catalysts that may operate at low temperature and pressure for the transformation of relatively cheap and available substrates into valuable functionalized products has attracted the attention from both academy and industry [2]. For example, the production of cyclohexanone/cyclohexanol, phenol and methanol from cyclohexane, benzene and methane, respectively, are among the top 10 challenges of the modern chemistry [3].
Concerning alkanes, cyclohexane oxidation is particularly interesting in nylon 6 and nylon 6,6 polymers manufacturing, since the oxidation products, cyclohexanol and cyclohexanone (KA oil), are the precursors of adipic acid and caprolactam, obtained by subsequent oxidation using nitric acid [4]. Furthermore, cyclohexanol and cyclohexanone are applied in the production of detergent emulsions, plasticizers, herbicides, insecticides and pharmaceuticals [5].
Currently, the industrial oxidation of cyclohexane employs homogeneous cobalt catalyst and molecular oxygen at a temperature between 423 and 433 K and pressure about 0.9 MPa [6]. Despite these drastic conditions, the process obtains about 4% yield and selectivity around 78% for cyclohexanol [1]. This reveals that there is room for the development of more efficient and low energy catalytic processes.
Concerning alkenes, their oxidation reaction is also of relevance because it yields a lot of applicable products like alcohols, ketones, aldehydes, acids and epoxides, which are largely used in food, chemical and pharmaceutical industries [7].
There are different approaches in the literature devoted to the development of catalysts aiming hydrocarbon oxidation, including both homogeneous and heterogeneous processes. Some of them are inspired in biological systems, like cytochrome P-450 and methanemonoxygenase enzymes [8], [9], [10], [11]. These iron metalloenzymes are oxygenases, able to catalyze the stereospecific oxidation of hydrocarbons under mild conditions, using electron transfer reactions and oxygen transport [12]. These oxygenases generally use molecular oxygen as oxidant to introduce one or two oxygen atoms into their substrates [13]. Concerning cytochrome P450, it is a family of heme protein that can catalyze oxidation reactions like hydroxylation and epoxidation of numerous alkane and alkene substrates [14]. On the other hand, iron methanemonoxygenase is a non heme enzyme which converts methane to methanol, into methanotrofic bacteria metabolism [15], [16]. Other oxygenases also can oxidize alkenes like toluene ortho-monooxygenase, that convert substrates like toluene, naphthalene and trichloroethylene [13], and Rieske dioxygenases, a class of enzymes able to cis-hydroxylation of aromatic hydrocarbons [17].
The biotransformation, under mild conditions, by these enzymes is selective and efficient. This has called attention to the development of bioinspired catalysts that try to reproduce the active center of these enzymes and solve the oxidation mechanisms, as well [18]. However these enzymes have a limited application in industries, since their isolation and purification are very expensive, there are difficulties in stabilizing the biocatalyst in the reaction medium, problems with product separation, recovery and recycling [19].
Trying to overcome this drawback, iron complexes containing N, O, S donor ligands have been used as bio-inspired catalysts for hydrocarbon oxidation, since similar donor groups are present in the active site of oxidases [20], [21], [22], [23], [24], [25], [26]. These systems are characterized by working at room temperature and in the presence of oxidant agents such as peroxides or iodosilbenzene.
Due to the satisfactory results obtained previously with the bioinspired homogeneous catalyst [FeIII(HBPCℓNOL)(Cl)2] (Fig. 1, compound (2)) in the cyclohexane oxidation [27], we decided to evaluate if the similar, but simpler, compound [Fe2(BPA)2(μ-OCH3)2(Cl)2] would be more effective in the oxidation of cyclohexane and cyclohexene than the former one. Furthermore, the ligand H2BPCℓNOL was immobilized in silica, NaY-zeolite and MCM-41 matrices, resulting, after reacting with iron(III), in the heterogeneized versions of the catalyst [FeIII(HBPCℓNOL)(Cl)2], allowing a direct comparison between the efficiency of homogeneous and heterogeneous systems.
Section snippets
Materials and methods
Reagents and solvents that were used in the syntheses were of the high purity grade and were obtained from Sigma–Aldrich and Synth. Cyclohexane and cyclohexene, used in the oxidation assays, were distilled and stored under argon atmosphere. Hydrogen peroxide 30% v/v was obtained from Vetec and its concentration was previously determined by titration. The solid matrices employed in this study were silicagel-60 (35–70 mesh ASTM), NaY zeolite (Fábrica Carioca de Catalisadores) and MCM-41 [28]. NaY
Results and discussion
The aim of the present work is to evaluate the catalytic activity of an iron complex analogous to [Fe(BPCℓNOL)Cℓ2], in the oxidation of hydrocarbons. Our group has been developing iron coordination compounds [41], [42], [43], such as [Fe(BPCℓNOL)Cℓ2] [27], and evaluating their catalytic activity toward hydrocarbons oxidation. Here, our goal was to evaluate whether a ligand structurally simpler than H2BPCℓNOL, the bipodal tridentate HBPA molecule, would result in a more efficient iron catalyst
Conclusions
In this work, we developed homogenous and heterogeneous iron(III) catalytic systems in which the iron(III) centers are coordinated by similar chelating group, allowing a direct comparison between the efficiency of both systems on cyclohexane and cyclohexene oxidation.
We have observed that the efficiency on the oxidative process (homogenous x heterogeneous) is dependent on the substrate and that the amount of oxidant (H2O2) shows influence on the results.
The homogenous systems were more
Conflict of interest
The authors state that there is no conflict of interest.
Acknowledgments
The authors are grateful for the financial support received from CNPq, CAPES and FAPERJ.
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