Short communicationGraphene oxide-catalysed oxidation reaction of unsaturated compounds under microwave irradiation
Introduction
During the last years, graphene and chemically modified graphenes (CMGs) have attracted much attention due to their outstanding physico-chemical properties [1], [2]. Moreover, CMGs are expected to have a significant impact in different fields such as chemistry, physics, engineering and materials science [3], [4], [5], [6], [7], [8]. Although in the early stages of research the synthesis of these materials was complex, the advances on this matter have already allowed access to high quality materials [3], [4], [6], [7].
The use of these materials in catalysis is gaining increasing interest [8], [9], [10], being a relatively new area with a huge potential but low developments. The application of graphene and CMGs in catalysis has been mainly focused on their use as supports for transition metal with catalytic activity [9], [10], [11]. However, the diminishing supplies of metals used in industrial processes as catalyst and the progress achieved in the use of metal-free carbons in synthetic chemistry drove a more direct application of CMGs as catalyst instead of their use as supports [9], [10]. Graphene oxide (GO) accounts as most relevant CMG as it has been observed that functionalized carbons are comparatively more effective in catalysis as compared to unfunctionalized counterparts (e.g. graphene) [8], [9], [10], [12], [13].
GO is a readily available and inexpensive material that has been proven catalytically active for a number of organic transformations including oxidation of alcohols, alkenes and alkynes [10], [12], [13], [14]. The main advantages of the use of GO as catalyst include simplicity and inexpensive nature, metal-free reactivity and easy recovery via simple filtration from the reaction media. However, the interesting results published to date have been obtained under extreme conditions, especially in terms of catalyst loading (400 wt.%) and reaction times (24 h +) [12], [13]. These conditions are an important drawback which needs to be overcome for a future application of GO as catalyst in the chemical industry. For this reason, recent research efforts have been focused on the improvement of the catalytic activity of GO to improve these extreme conditions of catalyst loading and reaction time.
Microwave heating is gaining interest due to its several advantages as compared to conventional heating [15], [16], [17]. Among these advantages it can be highlighted the rapid and homogeneous heating rates, selective heating, non-contact heating, quick start/stop and heating from within the material (i.e., energy conversion instead of heat transfer) [15], [17], [18]. In addition, its use in combination with carbon materials has given rise to a new generation of processes and materials that cannot be obtained under conventional heating [18], [19], [20] including catalytic applications [18], [21]. The use of microwave heating has been reported to provide higher conversions and occasionally better selectivities in chemical reactions as compared to those obtained using conventional heating under otherwise similar reaction conditions. Moreover, it is possible to obtain higher reaction rates which can result in lower reaction times [21], [22], [23], [24]. Particularly, microwaves as non-conventional heating source had a significant impact in the field of organic chemistry in recent years. A countless number of studies have reported reduced reaction times and improved yields and selectivity obtained in chemical processes due to the previously mentioned rapid and homogeneous as well as selective heating achieved under microwave irradiation [22], [23], [24].
However, no studies can be found about the use of microwave heating to improve the performance of unmodified GO as catalyst in oxidation reactions of alkenes and alkynes.
For these reasons we envisaged the use of microwave heating in combination with GO as a novel possibility to achieve high conversions in oxidation reactions using lower catalyst loadings and milder operation conditions. In this work, we present a study of the catalytic activity of graphene oxide in different oxidation reactions of styrene and phenylacetylene under microwave heating, with the aim of reducing reaction times and catalyst loading as well as working under milder conditions to those previously reported.
Section snippets
Graphene oxide synthesis
A series of experiments under different conditions was performed in order to study the catalytic properties of graphene oxide (GO). GO was provided by NanoInnova Technologies (Madrid, Spain) and it was synthesized by using a modified Hummers' method [25]. Briefly, graphite powder (< 150 μm Sigma-Aldrich) was chemically oxidized in a solution containing NaNO3, H2SO4 and KMnO4. Reduced GO (rGO) was also kindly provided by NanoInnova. rGO has a surface area of around 100 m2 g− 1 and different
Results and discussion
Characterisation studies of GO were conducted in NanoInnova (http://www.nanoinnova.com/Product) using several techniques including XRD, TGA XPS and IR spectroscopy. Fig. 1 depicts XRD patterns of as-prepared GO with respect to graphite (used as starting material), evidencing a complete oxidation of graphite and the formation of the characteristic diffraction lines of GO [10], [11], [12], [13], [14]. The clear XRD pattern indicates a high purity of GO achieved after graphite powder treatment.
Conclusions
Graphene oxide can be a very efficient and selective catalyst for the oxidation of unsaturated compounds under microwave irradiation, particularly alkenes. Quantitative conversion of styrene could be achieved under the investigated conditions, producing a mixture of benzaldehyde, styrene glycol and phenylethanol (and other minor compounds). The selectivity of products was found to be highly dependent on the reaction conditions. Regardless of the conditions employed, benzaldehyde and styrene
Acknowledgements
Rafael Luque gratefully acknowledges Spanish MICINN for the financial support via the concession of a RyC contract (ref: RYC-2009-04199) and funding under project CTQ2011-28954-C02-02 (MEC). Consejeria de Ciencia e Innovacion, Junta de Andalucia is also gratefully acknowledged for funding project P10-FQM-6711.
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