Article pubs.acs.org/IECR
Reduced Graphene Oxide Supported Silicotungstic Acid for Efficient Conversion of Thiols to Disulfides by Hydrogen Peroxide Sandip V. Nipane, Mukund G. Mali, and Gavisiddappa S. Gokavi* Kinetics and Catalysis Laboratory, Department of Chemistry, Shivaji University, Kolhapur 416004, India S Supporting Information *
ABSTRACT: Reduced graphene oxide (RGO) supported silicotungstic acid (STA) as a redox catalyst (STA-RGO) for efficient conversion of thiols to disulfide was prepared with an impregnation method. The catalyst was characterized by XRD, FTIR, UV− vis, TEM techniques, and zeta potential measurements. The uniform distribution of the STA on RGO was indicated by broadening of the XRD peaks of RGO. The Keggin unit of STA on the prepared catalyst was found to remain intact as noticed by FTIR. The increase in negative zeta potential and shifts in the characteristic UV−vis absorption bands of STA-RGO as compared with pure STA and RGO indicate the interaction between the two. The prepared catalyst was found to facilitate the oxidation of various thiols to their corresponding disulfides in the presence of hydrogen peroxide as an oxidant at room temperature. A probable mechanism involving sufenic acid as an intermediate is proposed.
1. INTRODUCTION Apart from its very good mechanical and thermal stability as well as very high conductivity, reduced graphene oxide (RGO) also possesses a large surface area which makes it a facile support for various catalysts. The inorganic catalysts supported on RGO include nanoparticles of metals,1 metal oxides,2 and polyoxometalates.3 The large surface area generates electrical double layer capacitance which is reduced considerably in the pure state of reduced graphene oxide due to aggregations as a result of van der Waals interaction between individual sheets.3 Attempts to increase this capacitance are made4−6 by the use of polyanailine, a material with pseudocapacitance, along with reduced graphene oxide. The difficulty in uniform deposition of polyanailine on the surface of reduced graphene oxide limits the increase in capacitance3 value of the composite material. Therefore, the polyoxometalates (POMs) are also used along with polyaniline3 to increase the capacitance value. The POMs are known to undergo irreversible adsorption7−9 on the carbon surfaces, which in turn prevent the aggregation of individual sheets of reduced graphene oxide. The POMs, the transition metal oxide clusters,10 have a unique structure incorporating both the heteroatom and the surrounding metal oxide moiety, generally a tungsten or molybdenum oxide. The redox activity of the POM depends on the heteroatom, while its conductivity both in solid and in solution depends on the structure of the surrounding metal oxide. Polyoxometalates are used in the fields of material science, chemistry, and biological applications10 due to their variable redox and acidic properties. Variation in the heteroatom and oxometalate moiety leads to a change in almost all physical as well as chemical properties of POMs, thus making them tailor-made reagents and catalysts for various inorganic and organic transformations. Their catalytic ability is utilized for both acid11−14 as well as redox15−17 catalyzed reactions. Due to their solubility in aqueous as well as nonaqueous solvents, POMs can be conveniently used in a variety of solvents. They are also used as analytical reagents, © 2014 American Chemical Society
staining agents, precipitating reagents, electron acceptors, and antiviral and antitumoral reagents10 in biochemical applications. Thiol to disulfide is an important organic transformation, and numerous studies are reported in the literature using hydrogen peroxide as an oxidant for this conversion.18−23 Polyoxometalates have been found to catalyze this transformation homogeneously,24 and it has been found that among the Keggin and Wells-Dawson types, POMs containing molybdenum are more effective25 than those containing tungsten due to the difference in their redox potentials. The redox behavior of molybdenum POMs is also not well-defined11 in aqueous solutions due to hydrolysis12 of these salts. Because of the lesser stability of phosphomolybdic acid in aqueous solution, vanadium substituted25 POMs are used as catalysts. Application of POMs as homogeneous catalysts suffers from disadvantages like interaction with solvent, leading to the formation of solvated ions and thus influencing their catalytic efficiencies. Therefore, several efforts have been made to disperse these POMs onto the surface of a support. Metal surfaces,26 silica,27 and reduced graphene oxide2 have been used as supports, and it has been observed that there is a decrease in the in the reduction potential phosphomolybdic acid2 when supported on reduced graphene oxide. Such variation in the physical property certainly is evidence of the interaction between reduced graphene oxide and the POMs. In the present study, we have supported Keggin type silicotungstic acid on reduced graphene oxide using an impegration method for its application as a redox catalyst to convert thiols to disulfides.
2. EXPERIMENTAL SECTION 2.1. Synthesis of Reduced Graphene Oxide (RGO). Initially, graphene oxide (GO) was synthesized by graphite powder (SD. Fine) according to Hummers’ method,28 followed Received: Revised: Accepted: Published: 3924
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by its chemical reduction with sodium borohydride. Graphene oxide (0.3 g) was dispersed by sonicating in 50 mL of water. Sodium borohydride (0.5 g; SD. Fine) was then added to the GO dispersion. The mixture was kept at 90 °C for 4 h, and the solid reduced graphene oxide (RGO) was filtered off and repeatedly washed with water. The final product was dried in a vacuum oven at 80 °C overnight. 2.2. Preparation of Reduced Graphene Oxide Supported Silicotungstic Acid (STA-RGO). Reduced graphene oxide (RGO) supported silicotungstic acid (Loba Chemie) was prepared using an impregnation method. To an aqueous solution of silicotungstic acid (0.125 g in 20 mL) was added 0.250 g of reduced graphene oxide (RGO) with constant stirring. Water was slowly removed by evaporation by keeping the resulting solution at 40 °C for two days to obtain solid catalyst. The amount of STA supported on RGO is 0.5 g per 1 g of RGO. The other catalysts STA-GO and STA-graphite were prepared by replacing RGO with GO and graphite. 2.3. Procedure for Oxidation of Thiols. The oxidation of 4-chlorothiophenothiol (Alfa Aesar) by hydrogen peroxide at room temperature was carried out in a solution of 40% aqueous acetonitrile solution for 5 min. Pure graphite, graphene oxide, and RGO were used as heterogeneous catalysts. A sticky product was obtained in the presence of graphene oxide (GO), and about 61% disulfide is obtained for RGO, while with graphite the reaction did not occur. The STA supported on GO as a catalyst gave about 31% disulfide, and that on the RGO gave a yield of 90%. Therefore, STA-RGO was used for our further study. The typical experimental procedure involved the addition of 2 mmol (0.25 mL) of 30% H2O2 dropwise with constant stirring to a mixture of thiol (1 mmol) and STA-RGO catalyst (10 mg) in 10 mL of 40% aqueous acetonitrile (v/v). The progress of the reaction was monitored by TLC. After completion of the reaction, the excess H2O2 was destroyed by 2 mL of aqueous sodium sulphite. The mixture was treated with dichloromethane (2 × 25 mL), and the organic layer was dried on anhydrous Na2CO3. It was concentrated to get required solid product, which was filtered off and recrystallized. The product was analyzed by melting point and GC-MS. 2.4. Characterization. X-ray diffraction (XRD) powder patterns of the pure reduced graphene oxide (RGO), pure STA, and RGO supported STA (STA-RGO) were recorded on an X-ray diffractometer (Panalytical) using (Cu Kα) as the radiation source (k = 1.541841A°). Morphology has been studied by transmission electron microscope (TEM; Technai G2 F30). To obtain TEM pictures, the respective powders were dispersed in methanol, and a drop of this suspension was placed onto a carbon coated copper grid (Icon Analytical Equipment Pvt. Ltd., Prod. Code 01810) and dried. The FTIR spectra were recorded on a Perkin-Elmer 100 instrument in KBr. The zeta potentials of the pure reduced graphene oxide (RGO) and reduced graphene oxide (RGO) supported STA were measured with a Malvern Zetasizer instrument. 2.5. UV−Vis Spectra. The UV−vis spectra of the RGO, pure STA, and STA-RGO have been recorded by using a Shimadzu UV−vis NIR spectrophotometer (Model UV 3600) between 200 and 400 nm. The respective material was dispersed in water, and the spectra were recorded by using matched quartz cuvettes of path length 1 cm.
Figure 1. XRD spectra of slicotungstic acid (STA), reduced graphene oxide (RGO), and reduced graphene oxide supported silicotungstic acid (STA-RGO).
characteristic peaks arising from the reflections29,30 of the bodycentered cubic planes 220, 222, 320, 332, 400, 411, 420, and 510. A broad peak at about 2θ ≈ 24° in the case of pure RGO is its characteristic31 pattern. These characteristic peaks of STA and RGO are broadened in the case of STA-RGO. Such broadening of XRD peaks of STA-RGO indicates that the material is amorphous in nature due to the uniform distribution of STA onto the surface of RGO. The reported XRD pattern31 of graphene oxide (GO) is at 2θ ≈ 11.6° and, in the case of the prepared RGO of the present study, shows no such peak, indicating that GO used initially to prepare RGO was totally reduced. The TEM of RGO and STA-RGO are shown in Figures 2 and 3. The TEM of pure RGO (Figure 2) indicates aggregation of its sheets, while no such aggregation is observed after impregnation of STA on the RGO (Figure 3). The interaction between STA and RGO sheets hinders their aggregation and also the TEM in the presence of STA shows uniform distribution of the STA on the RGO surface. The STA splits the RGO sheets along with the basal plane as shown in Scheme 1. The FTIR spectrum of pure RGO and that of pure STA are shown in Figure 4. The spectrum of RGO shows a peak at 1571 cm−1 due to the stretching of the benzene ring,32 while the peaks due to 1220 cm−1 and 1719 cm−1 due to the C−O and CO stretches are absent, indicating the absence of any oxygen moieties expected to be present in GO. The spectrum of STA shows a characteristic33 peak of the Keggin unit at 983 cm−1, 928 cm−1, 884 cm−1, and 800 cm−1 corresponding to W− Od, Si−O, W−Ob−W, and W−Oc −W vibrations. The characteristic peaks of the Keggin unit of STA are retained, while there is a shift of peak at 1571−1620 cm−1 of RGO in the FTIR spectrum of RGO supported STA. The presence of all the frequencies of STA in the RGO supported STA indicates that the Keggin unit is intact. The shift in the peak at 1571 cm−1 in
3. RESULTS AND DISCUSSION The XRD powder patterns of the RGO, STA, and STA- RGO are shown in Figure 1. The XRD pattern of pure STA shows 3925
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Figure 2. TEM image of reduced graphene oxide (RGO).
Scheme 1. Interaction between Silicotungstic Acid (STA) and Reduced Graphene Oxide (RGO)
the case of pure RGO to 1647 cm−1 in the STA supported RGO indicates interaction between them. The absorption spectra of polyoxometalates containing either W or Mo as addenda atoms are due to the charge-transfer transitions34 between oxygen and addenda atoms. The reported absorption maxima for such charge transfer transition34 for silicotungstic acid in aqueous acidic solutions is around 263− 266 nm, and in the present study the spectrum also shows a
peak at 264 nm (Figure 5). The spectrum of GO exhibits a maximum at 236 nm (Figure 5) corresponding to p−p transitions of aromatic C−C bonds,35 and the decrease in intensity in the case of the spectrum of RGO is an indication of its reduction. The spectrum of STA supported on RGO shows the maximum at 260 nm (Figure 5). The shift in the maximum of RGO at 236−260 nm in the case of the STA supported on RGO is due to the interaction between RGO and STA. 3926
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Figure 3. TEM image of reduced graphene oxide supported silicotungstic acid (STA-RGO).
Figure 5. UV−vis spectra of slicotungstic acid (STA), reduced graphene oxide (RGO), graphene oxide (GO), and reduced graphene oxide supported silicotungstic acid (STA-RGO).
Figure 4. FTIR spectra of slicotungstic acid (STA), reduced graphene oxide (RGO), and reduced graphene oxide supported silicotungstic acid (STA-RGO).
known to adsorb on carbon surfaces and also cleave the highly oriented pyrolytic graphite along its basal plane.37 The nature of adsorption of these POMs on carbon is strong chemisorption. Because both RGO and STA are negatively charged, the proton on the STA will be delocalized8 onto the RGO surface. Such delocalization stabilizes the electrostatic bond between RGO and STA. These POMs are protonated anions and bear high negative charges; therefore, in the present study, when adsorbed on the RGO, the silicotungstic acid’s negative zeta potential increases further. The value of the zeta
The zeta potential of a nanomaterial is the electric potential at the edge of the layer of bound ions, and in the case of graphene, the negative zeta potential is found to increase in the presence of anionic surfactants,36 while cationic surfactants make it positive due to interaction between them. Therefore, the zeta potential of graphene changes when it interacts with substrates capable of adsorbing on it and the overall zeta potential attains the resulting value. Polyoxometalates are 3927
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potential of pure RGO is −23.9 mV, while that of STA supported RGO is found to be −50.2 mV, which justifies the interaction between STA and RGO. The oxidation of thiols by hydrogen peroxide in the presence of GO, RGO, graphite, STA-GO, STA-RGO, and STA-graphite was studied individually at room temperature to know their catalytic efficiency using 4-chlorothiophenol as a substrate (Table 1). The reaction did not occur in pure water as the solvent, while yields of disulfide in pure acetonitrile and 40% aqueous acetonitrile were 80% and 90%, respectively (Table 1). The formation of disulfide did not occur in the presence of graphite and STA-graphite, whereas in the presence of GO, sticky product was obtained. There is a synergistic increase in the disulfide yield when STA is supported on RGO, while in the presence of STA-GO it is only one-third. Therefore, the thiols to disulfide conversion by hydrogen peroxide is more efficient in the presence of STA-RGO. The catalytic efficiency
Table 1. Catalytic Efficiency of Graphite, Graphene Oxide (GO), Reduced Graphene Oxide (RGO), Silicotungstic Acid (STA)-GO, STA-RGO, STA-Graphite, and Solvent Composition on the Oxidation of 4-Chlorothiophenol by Hydrogen Peroxide at Room Temperaturea catalyst(10 mg)
solvent
% yield
graphite GO RGO STA-graphite STA-GO STA-RGO STA- RGO STA-RGO
40% aqueous acetonitrile ″ ″ ″ ″ ″ water acetonitrile
0 sticky product 61 0 31 90 0 80
a
Reaction conditions: 10 mL 40% aq. solvent (v/v), 10 mg of catalyst, 1 mmol (0.289 g) 4-chlorothiophenol, 2 mmol (0.25 mL) H2O2 in 5 min.
Table 2. Thiol to Disulfide Conversion in the Presence of Reduced Graphene Oxide Supported Silicotungstic Acida
a
Reaction conditions: 10 mL 40% aq. solvent (v/v), 10 mg of catalyst, 1 mmol (0.289 g) thiol, 2 mmol (0.25 mL) H2O2 at room temperature. 3928
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of STA-RGO for the oxidation of thiols to disulfides was tested using hydrogen peroxide as an oxidant in 40% v/v aqueous acetonitrile. The amount of catalyst, hydrogen peroxide, and the solvent composition was carried out in order to optimize the conditions of the reactions. The conversion of thiols to disulfides proceeds smoothly in 40% v/v aqueous acetonitrile solution, and 2 moles of hydrogen peroxide are required per mole of thiol. The reaction was carried out at room temperature, 25 °C, and 10 mg of the catalyst STA-RGO is required. Both aliphatic and aromatic thiols are oxidized efficiently by the present system (Table 2). The aromatic thiols containing electron withdrawing as well as electron donating groups are also oxidized without much difference in the time of completion and the yield. The presence of other functional groups like −COOH and −NH2 on the thiol moiety did not undergo any oxidation, indicating the selectivity of the present catalyst toward thiol. The catalyst was successfully recycled four times with a slight decrease in the yield from 90 to 87% (Table 3).
Scheme 2. Mechanism of Thiol to Disulfide Conversion in the Presence of Reduced Graphene Oxide Supported Silicotungstic Acid by Hydrogen Peroxide
5. CONCLUSION The morphology of the prepared STA-RGO catalyst as studied by XRD, FTIR, and UV suggested the uniform distribution of STA on the surface of RGO. The TEM examination of the catalyst and the RGO indicated that the aggregated sheets of pure RGO are split along their basal plane. The negative surface charge on the RGO was also found to increase considerably after supporting STA on its surface, thus making it more nucleophilic as noticed by the increase in zeta potential of the catalyst. Such an increase in the nucleophilicity of the catalyst facilitates the deprotonation of thiols generating the main reactive thiolate anions. The thiols having lesser pKa values were converted into disulfide faster, thus supporting the deprotonation as the initial step in the proposed mechanism.
Table 3. Recycling of STA-RGO Catalyst for the Oxidation of 4-Chlorothiophenol by Hydrogen Peroxidea cycles
initial
first
swcond
third
yield (%)
90
89
89
87
a
Reaction conditions: 10 mL 40% aq. solvent (v/v), 10 mg of catalyst, 1 mmol (0.289 g) 4-chlorothiophenol, 2 mmol (0.25 mL) H2O2 in 5 min.
The oxidation of thiols to disulfides, except cysteine, by hydrogen peroxide occurs in the presence32 of catalysts. The mechanism of oxidation of thiols, in the presence of an acid, involves the formation of a thiolate anion. The thiolate anion then reacts with the hydrogen peroxide generating corresponding38 sulfenic acid. The combination of another thiolate anion with the sulfenic acid leads to the disulfide formation. In the recent past, POMs have received much attention as catalysts for the oxidation of organic compounds by hydrogen peroxide. These POMs act as both acidic and redox catalysts in most of the organic transformations. The activation of hydrogen peroxide by these POMs involves either the metal-peroxide formation or the ionic mechanism generating hydroxyl radicals. Silicotunstic acid, a Keggin type POM, used in this study was found to be inert toward hydrogen peroxide, and its catalytic activity in thiols to disulfide25 conversion was also not promising. Therefore, at room temperature, the interaction of pure silicotungstic acid with either hydrogen peroxide or thiol is unlikely in aqueous solutions. On the other hand, pure GO reacts39 with thiols at about 60 to 80 °C, generating RGO as the product. Thus, the synergetic effect of STA-RGO, in the present study, is the enhanced formation of the thiolate anion. The zeta potential of pure RGO increases when STA was supported on it, thus making it more nucleophilic. The formation of a thiolate anion is facilitated through deprotonation of thiol if there is an increase in the negative charge of the nucleophile. Further support for the facilitated deprotonation by STA-RGO is from the fact that the thiols having lesser pKa values react faster while those having higher values require more time. The aggregated sheets (Figure 2) of RGO are split along their basal planes (Figure 3) as a result of the addition of STA, which also supports the interaction between the two. The detailed mechanism of the oxidation of thiols in the presence of STA-RGO by hydrogen peroxide can be represented as in Scheme 2.
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ASSOCIATED CONTENT
S Supporting Information *
GCMS spectra of the products of Table 2. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the Department of Science and Technology for the financial support in the form of FIST and PURSE programmes to our department and our university.
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