Research Article pubs.acs.org/journal/ascecg
Magnetic Fe@g‑C3N4: A Photoactive Catalyst for the Hydrogenation of Alkenes and Alkynes R. B. Nasir Baig,‡ Sanny Verma,† Rajender S. Varma,† and Mallikarjuna N. Nadagouda*,‡ †
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Sustainable Technology Division, National Risk Management Research Laboratory, U.S. Environmental Protection Agency, MS 443, Cincinnati, Ohio 45268, United States ‡ WQMB, WSWRD, National Risk Management Research Laboratory, U.S. Environmental Protection Agency, Cincinnati, Ohio 45268, United States S Supporting Information *
ABSTRACT: A photoactive catalyst, Fe@g-C3N4, has been developed for the hydrogenation of alkenes and alkynes using hydrazine hydrate as a source of hydrogen. The magnetically separable Fe@g-C3N 4 eliminates the use of high pressure hydrogenation, and the reaction can be accomplished using visible light without the need for external sources of energy.
KEYWORDS: Photocatalysis, Hydrogenation, Nanoferrite, Graphitic carbon nitride, Heterogeneous catalysis
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INTRODUCTION The hydrogenation of double and triple bonds using metals such as Pd, Rh, Pt, and Ru and their complexes have been customarily used in pharmaceutical and petrochemical industries although they increase the cost of the production. These reactions often involve the use of highly flammable and explosive hydrogen gas under high pressure conditions.1,2 Consequently, cost-effective and safer protocols are being explored for hydrogenation chemistry. Progress has been made using first row transition metals for the catalytic hydrogenation of alkenes and alkynes. Recent discoveries involve the use of metals and metal oxide nanoparticles due to their homogeneous equivalence in terms of surface area. Commonly, these nanoparticles have been immobilized over solid supports for increased activity and ease of recyclability. Iron nanoparticles have been immobilized over polystyrene and used for the hydrogenation of alkene, but the reaction requires the use of hydrogen at high pressure.3 To circumvent this drawback of high pressure hydrogenation, iron and cobalt complexes were specially designed. However, these catalysts are poisoned under the reaction conditions, resulting in the accumulation of metal waste.4,5 Although iron nanoparticles immobilized over graphene surfaces have been employed for the hydrogenation reaction with better activity as compared to earlier reports, they still require 20 bar pressure and extended reaction time to hydrogenate olefins.6 Replacing high pressure hydrogen with hydrazine could also perform this reaction at elevated temperature. However, the use of graphene as a support is © 2016 American Chemical Society
strongly discouraged due to complexity in synthesis and risk of exposure to graphene dust.7 Thus, the discovery of a heterogeneous catalyst that could accomplish the aforementioned transformation at ambient conditions is desirable as it could eliminate the use of high pressure hydrogen.8,9 In this continuation of our research on the development of a sustainable protocol in organic synthesis10−13 and magnetically separable heterogeneous catalysts,14−17 herein we report the first example that demonstrates the use of graphitic carbon nitride surface (g-C3N4) for the immobilization of iron ferrite (Scheme 1) and its application in photocatalytic hydrogenation using hydrazine at room temperature.
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RESULTS AND DISCUSSION The first step was to design and develop magnetically active photocatalysts that could facilitate hydrogenation of double and triple bonds. The motive was to use the most sustainable energy source: visible light, which would make the procedure environmentally efficient and adoptable for industries and academicians. Graphitic carbon nitride has been synthesized without the generation of any hazardous product in pure form which is not the case with some other carbon based support.18 Additionally, it has an affinity toward visible light19,20 which can overcome the energy barrier leading to the formation of the Received: December 1, 2015 Revised: January 12, 2016 Published: January 28, 2016 1661
DOI: 10.1021/acssuschemeng.5b01610 ACS Sustainable Chem. Eng. 2016, 4, 1661−1664
Research Article
ACS Sustainable Chemistry & Engineering Scheme 1. Synthesis of Fe@g-C3N4
the hydrogenated product in nearly quantitative yield (Table 1, entry 4; 98%). Further increase in the iron loading did not show any positive impact on the yield and the reaction time (Table 1, entry 5). Control experiment with bare FeO nanoparticles under similar conditions produced a meager 5% conversion over the period of 48 h (Table 1, entry 6). The hydrogenation using g-C3N4 as a catalyst under visible light did not generate any product after 48 h of stirring (Table 1, entry 7). Immobilization of iron oxide over the g-C3N4 surface not only increased the activity of the catalyst, but also provided the activation energy through visible light absorption. The higher reaction rate may be attributed to synergetic effect of nanoferrites and g-C3N4. After determination of the active catalyst and optimum reaction conditions, the general reactivity and substrate scope were explored further. Thus, a wide range of alkenes and alkynes were subjected to hydrogenation conditions. Treatment of different styrene derivatives with Fe@g-C 3 N 4 and NH2NH2.H2O under the optimized conditions led to the formation of corresponding hydrogenated products (Table 2, entries 1−10). The styrenes with electron-donating substituents underwent slow hydrogenation (Table 2, entries 2−3) compared to electron-withdrawing substituents on the similar substrates (Table 2, entries 4−5). The substituted alkenes, such as trans-stilbene and 1,1-diphenylethylene, were efficiently hydrogenated using Fe@g-C3N4 under photochemical conditions (Table 2, entries 6−7). Hydrogenation of 1, 1diphenylethylene (Table 2, entry 6) appeared to be faster than that of its isomer (Table 2, entry 7). The difference in reactivity of hydrogenation of stilbene and 1,1-diphenylethylene could be credited to their substituent patterns around double bonds. The aliphatic alkenes (Table 2, entries 8−9) hydrogenated smoothly. The reaction of norbornene (Table 2, entry 9) was found to be much faster than that of cyclooctene (Table 2, entry 8). The higher rate of hydrogenation in the norbornene derivative is presumably due to its confirmation and stereochemical arrangement at bridge head position. α,β-Unsaturated carbonyl compounds, such as chalcone, were selectively hydrogenated using Fe@g-C3N4 under photochemical conditions (Table 2, entry 10). A wider scope and applicability of the catalyst system is explained by the hydrogenation of alkynes to corresponding alkanes under similar conditions using hydrazine hydrate as a source of hydrogen (Table 3, entries 1−3). A plausible reaction mechanism based on the literature reports7 involves the coordination of hydrazine with nanoferrites and the simultaneous activation of alkene through π−π
desired products. Thus, we combined g-C3N4 with an iron oxide nanoparticle which, in turn, provided a synergic jump for hydrogenation. Ferrous sulfate was immobilized over a graphitic carbon nitride surface, reduced, and caged into gC3N4 cavities as iron oxide. The important aspect for using Fe@g-C3N4 as a photocatalyst for the hydrogenation was to find an effective stoichiometric ratio of iron oxide and g-C3N4 support.21 Accordingly, Fe@g-C3N4 was prepared with different weight percentagees of iron and screened toward the photocatalyzed hydrogenation of styrene using hydrazine as a source of hydrogen at room temperature (Table 1). The selection of hydrazine hydrate was important as it facilitates easy handling and generates inert nitrogen gas as a sole byproduct.22−24 Table 1. Catalyst Screening and Reaction Optimizationa
entry 1 2 3 4 5 6 7 8c 9 10d
catalyst Fe@g-C3N4 Fe@g-C3N4 Fe@g-C3N4 Fe@g-C3N4 Fe@g-C3N4 FeO g-C3N4 Fe@g-C3N4 Fe3O4 Fe@g-C3N4
(1% Fe) (2% Fe) (5% Fe) (10% Fe) (20% Fe)
(10% Fe) (10% Fe)
time
yieldb
48 h 24 h 24 h 8h 8h 48 h 48 h 24 h 48 h 24 h
34% 45% 85% 98% 95% 5% 13% 5%
Reaction conditions: styrene (1 mmol), catalyst (25 mg), NH2NH2· H2O (10 mmol), water (1.0 mL), 40 W domestic light bulb. bIsolated yield. cReaction was performed in dark. dReaction was performed in water without hydrazine. a
All reactions were performed using 1 mmol of styrene, 25 mg of Fe@g-C3N4 in water, and hydrazine hydrate under visible light irradiation. The percentage of iron played a crucial role in the development of photoactive magnetically separable Fe@gC3N4 catalysts. The catalyst with 1% Fe (Table 1, entry 1) required 48 h to produce 34% of the desired product. The increase of iron loading to 2% increased the yield to 45% and reduced the reaction time to 24 h (Table 1, entry 2). The Fe@ g-C3N4 catalyst with a 5% loading afforded 85% of yield (Table 1, entry 3), but 10% of iron loading was very effective and gave 1662
DOI: 10.1021/acssuschemeng.5b01610 ACS Sustainable Chem. Eng. 2016, 4, 1661−1664
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ACS Sustainable Chemistry & Engineering Table 2. Hydrogenation of Alkenes Catalyzed by Fe@gC3N4a
Figure 1. Plausible reaction mechanism for hydrogenation of alkene using Fe@g-C3N4.
°C. A new reaction was then set-up using the recovered catalyst and fresh reactants. It was found that Fe@g-C3N4 could be recycled at least 10 times without losing its activity (Supporting Information). The metal leaching of the Fe@g-C3N4 was studied using ICP-AES analysis. The iron concentration was found to be 9.97% before the reaction and 9.95% after the 10th cycle of the reaction. The TEM and SEM images of the catalyst, recorded after the 10th cycle of the reaction, did not show a significant change in the morphology (Supporting Information Figures S5 and S6) and nature of nanoparticles. The ICP-AES analysis of the mother liquor does not show any traces of Fe, which confirms that g-C3N4 holds the iron ferrites tightly.
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a Reaction conditions: alkene (1.0 mmol), catalyst (25 mg), NH2NH2· H20 (10 mmol), H2O (1.0 mL), 40 W domestic light bulb. bIsolated yield.
CONCLUSION An efficient and sustainable protocol has been developed for the hydrogenation of alkenes and alkynes using a magnetically separable photoactive catalyst that occurs efficiently at room temperature using aqueous hydrazine as a source of hydrogen. The Fe@g-C3N4 catalyst under photochemical conditions has successfully eliminated the usage of high pressure hydrogen and enabled the reaction at room temperature. The high reactivity of the catalyst is attributed to noncovalent interactions between nanoferrites and the photoactive g-C3N4 surface.
Table 3. Hydrogenation of Alkynes Catalyzed by Fe@gC3N4a
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EXPERIMENTAL SECTION
Synthesis and Characterization of Catalyst. The synthesis of catalyst, Fe@g-C3N4, involves two steps.16 First, the graphitic carbon nitride (g-C3N4) was synthesized by calcination of urea at 500 °C for 2 h. The g-C3N4 was dispersed in a 10% solution of PEG in water using sonication. FeSO4·7H2O was added to this mixture, and the stirring was continued for 8 h. The excess of sodium borohydride was added thus reducing the FeSO4 to magnetic ferrites. Magnetic Fe@g-C3N4 was separated using an external magnet, washed with water and methanol, and then dried under vacuum at 50 °C. The catalyst Fe@gC3N4 was characterized by X-ray diffraction (Supporting Information Figure S1), transmission electron microscopy (Supporting Information Figure S2), scanning electron microscopy (Supporting Information Figure S3), EDX (Supporting Information Figure S4) and ICP-AES analysis. General Procedure for the Hydrogenation of Alkenes and Alkynes. The alkene/alkyne (1.0 mmol), NH2NH2·H2O (10 mmol/ 20 mmol for alkyne), and catalyst Fe@g-C3N4 (25 mg) were suspended in 1 mL of water. The reaction mixture was exposed to visible light using a 40 W domestic light bulb. After the completion of the reaction, the catalyst was separated and recovered using an external
Reaction conditions: alkyne (1.0 mmol), catalyst (25 mg), NH2NH2· H2O (20 mmol), H2O (1.0 mL), 40 W domestic light bulb. bIsolated yield. a
stacking. The activation of hydrazine hydrogen and hydrogenation of double bonds may occur in a concerted process with inert nitrogen being the sole byproduct formed during the reaction, which escapes into the atmosphere (Figure 1). After demonstrating the general reactivity of the catalyst, it was important to study the stability and recyclability aspects of the catalyst. Hence, a set of experiments was performed using 4-chlorostyrene as a substrate and the magnetic graphitic carbon nitride (Fe@g-C3N4) as a catalyst under photochemical conditions. As the reaction went to completion, the catalyst was separated magnetically, washed with acetone, and dried at 50 1663
DOI: 10.1021/acssuschemeng.5b01610 ACS Sustainable Chem. Eng. 2016, 4, 1661−1664
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ACS Sustainable Chemistry & Engineering magnet. The product was then extracted using CH2Cl2, dried over sodium sulfate, concentrated, and characterized.
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(10) Nasir Baig, R. B.; Nadagouda, M. N.; Varma, R. S. Carboncoated magnetic palladium: applications in partial oxidation of alcohols and coupling reactions. Green Chem. 2014, 16, 4333−4338. (11) Saha, A.; Nasir Baig, R. B.; Leazer, J.; Varma, R. S. A modular synthesis of dithiocarbamate pendant unnatural α-amino acids. Chem. Commun. 2012, 48, 8889−8891. (12) Varma, R. S. Journey on greener pathways: from the use of alternate energy inputs and benign reaction media to sustainable applications of nano-catalysts in synthesis and environmental remediation. Green Chem. 2014, 16, 2027−2041. (13) Verma, S.; Mungse, H. P.; Kumar, N.; Choudhary, S.; Jain, S. L.; Sain, B.; Khatri, O. P. Graphene oxide: an efficient and reusable carbocatalyst for aza-Michael addition of amines to activated alkenes. Chem. Commun. 2011, 47, 12673−12675. (14) Nasir Baig, R. B.; Varma, R. S. Magnetically retrievable catalysts for organic synthesis. Chem. Commun. 2013, 49, 752−770. (15) Nasir Baig, R. B.; Varma, R. S. Magnetic silica-supported palladium catalyst: synthesis of allyl aryl ethers in water. Ind. Eng. Chem. Res. 2014, 53, 18625−18629. (16) Nasir Baig, R. B.; Nadagouda, M. N.; Varma, R. S. Magnetically retrievable catalysts for asymmetric synthesis. Coord. Chem. Rev. 2015, 287, 137−156. (17) Verma, S.; Verma, D.; Sinha, A. K.; Jain, S. L. Palladium complex immobilized on graphene oxide-magnetic nanoparticle composites for ester synthesis by aerobic oxidative esterification of alcohols. Appl. Catal., A 2015, 489, 17−23. (18) Su, C.; Loh, K. P. Carbocatalysts: graphene oxide and its derivatives. Acc. Chem. Res. 2013, 46, 2275−2285. (19) Dong, F.; Wu, L.; Sun, Y.; Fu, M.; Wu, Z.; Lee, S. C. Efficient synthesis of polymeric g-C3N4 layered materials as novel efficient visible light driven photocatalysts. J. Mater. Chem. 2011, 21, 15171− 15174. (20) Zhu, J.; Xiao, P.; Li, H.; Carabineiro, S. A. C. Graphitic carbon nitride: synthesis, properties, and applications in catalysis. ACS Appl. Mater. Interfaces 2014, 6, 16449−16465. (21) Verma, S.; Nasir Baig, R. B.; Han, C.; Nadagouda, M. N.; Varma, R. S. Magnetic graphitic carbon nitride: its application in the C-H activation of amines. Chem. Commun. 2015, 51, 15554−15557. (22) Ratnayake, W. M. N.; Grossert, J. S.; Ackman, R. Studies on the mechanism of the hydrazine reduction reaction: applications to selected monoethylenic, diethylenic and triethylenic fatty acids of cis configurations. J. Am. Oil Chem. Soc. 1990, 67, 940−946. (23) Schmidt, E. W. Hydrazine and Its Derivatives: Preparation, Properties, and Applications, 2nd ed.; Wiley & Sons: New York, 2001; Vol. 1, p 475. (24) Rai, R. K.; Mahata, A.; Mukhopadhyay, S.; Gupta, S.; Li, P. Z.; Nguyen, K. T.; Zhao, Y. L.; Pathak, B.; Singh, S. K. Room-temperature chemoselective reduction of nitro groups using non-noble metal nanocatalysts in water. Inorg. Chem. 2014, 53, 2904−2909.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01610. XRD, TEM, SEM, EDX data of the catalyst Fe@g-C3N4; XRD, EDX data of support g-C3N4; TEM, SEM images of recycled catalyst Fe@g-C3N4; and 1H NMR of the products (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
Equal contributions from R.B.N.B. and S.V. Notes
The views expressed in this article are those of the authors and do not necessarily represent the views or policies of the U.S. Environmental Protection Agency. Any mention of trade names or commercial products does not constitute endorsement or recommendation for use. The authors declare no competing financial interest.
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ACKNOWLEDGMENTS N.B.R.B. and S.V. were supported by the Postgraduate Research Program at the National Risk Management Research Laboratory administered by the Oak Ridge Institute for Science and Education through an interagency agreement between the U.S. Department of Energy and the U.S. Environmental Protection Agency.
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