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Aerobic oxidation of benzyl alcohols through biosynthesized palladium nanoparticles mediated by Oak fruit bark extract as an efficient heterogeneous nanocatalyst
Tetrahedron Letters 58 (2017) 4191–4196
Contents lists available at ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
Aerobic oxidation of benzyl alcohols through biosynthesized palladium nanoparticles mediated by Oak fruit bark extract as an efficient heterogeneous nanocatalyst Hojat Veisi a,⇑, Saba Hemmati a, Mahnaz Qomi b a b
Department of Chemistry, Payame Noor University, Tehran, Iran Active Pharmaceutical Ingredients Research Center (APIRC), Pharmaceutical Sciences Branch, Islamic Azad University, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 17 August 2017 Revised 15 September 2017 Accepted 19 September 2017 Available online 22 September 2017 Keywords: Green chemistry Biosynthesis Pd nanoparticles Oak Oxidation Alcohols
a b s t r a c t In this study, green synthesis of Pd nanoparticles (NPs) is outlined through application of Oak fruit bark extract as a reducing, capping and stabilizing agent. The characteristics and properties of the biosynthesized Pd NPs were revealed by FESEM, EDX, XRD, TEM, UV–Vis, and FT-IR spectroscopies. So that, UV–Vis spectroscopy of the Pd colloidal solution confirmed reduction of Pd ions, and XRD and TEM analysis identified fcc unit cell structure forming 5–7 nm spherical Pd NPs. Furthermore, catalytic activity of the prepared catalyst was investigated through aerobic oxidation of alcohols, as model reactions. Catalytic evaluations demonstrated achievement of good yields from primary and secondary benzyl alcohols. In general, the devised synthesis method is advantageous from several perspectives. For example, the synthesized catalysts give high product yields and are efficient, they eliminate the need for surfactant, chemical reductants, ligand and organic solvents, the approach is economically inexpensive, it results in cleaner reaction profiles, application of the simply prepared heterogeneous catalyst is convenient, and the catalyst is recoverable and reusable for at least six times without any significant loss of its catalytic activity. Ó 2017 Elsevier Ltd. All rights reserved.
Introduction Production of carbonyls through oxidation of alcohols is one of the essential organic transformations1 since carbonyls can be employed as intermediates in manufacturing dyes and pharmaceuticals.2 Such organic transformations can be facilitated by catalysts. In this respect, transition metal based catalysts can conduct selective oxidation of organic compounds as interesting alternatives to conventional waste-producing oxidation procedures, which demand for stoichiometric amounts of toxic inorganic salts.3,4 Recently, aerobic oxidation processes have gained more attention as utilizing oxygen has considerable benefits, from both economic and green chemistry points of view. In fact, molecular oxygen is inexpensive and produces just water as its byproduct. Consequently, extensive catalytic studies have concentrated on finding suitable active metals for homo- and heterogeneous catalytic oxidation with molecular oxygen, e.g. Fe,5 Ru,6 Co,7 Cu,8 Mn,9 Os10 and Pd.11 Also, solid supported transition metal nanoparticles (NPs) have attracted immense interest in development of ⇑ Corresponding author. E-mail address: [email protected] (H. Veisi). https://doi.org/10.1016/j.tetlet.2017.09.057 0040-4039/Ó 2017 Elsevier Ltd. All rights reserved.
new catalysts due to their high catalytic activity and reusability properties.12 So that, few recent reports have shown that heterogeneous palladium NPs can undertake aerobic oxidation of aromatic alcohols.13,14 To date, different chemical, physical and biological methods have been employed for production of Pd NPs. However, physical methods require expensive equipment and involve high vacuum technology. Also, chemical methods are not completely favorable since they need capping agents to avoid agglomeration of the produced particles, due to their high surface reactivity. Meantime, using the necessary toxic chemicals is a major concern and presence of non-polar solvents and toxic chemicals limit application of chemical synthesis of Pd NPs, in clinical fields. Therefore, researchers have shifted to ecofriendly NP synthesis through microorganisms and plant extracts. Green synthesis of Pd NPs is consisted of three main steps, which should be evaluated according to green chemistry perspectives and include (i) solvent medium selection, (ii) choosing an ecofriendly reducing agent and (iii) finding nontoxic NP stabilizers. Therefore, biosynthesis approach of Pd NPs through green chemistry reduction has been explored, in this study. It is noteworthy that bio-inspired, ecofriendly and greener synthesis methods of metal NPs are among the most interesting
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aspects of nanoscience and nanotechnology, nowadays.15,16 There are limited number of available works on biosynthesis of Pd NPs and Diospyros kaki. leaf,17 Cinnamomum zeylanicum bark,18 C. Camphora leaf,19 Curcuma longa tuber,20 banana peel,21 Hippophae rhamnoides Linn,22 Pistacia atlantica kurdica gum,23 Rosa canina fruit,24 pectin,25 Stachys lavandulifolia26 and Oak gum27 have been employed, to date. Oak tree is widely spread throughout Zagros Mountains, West of Iran, East and North of Iraq, Southern Turkey, Northern Syria and, also, in many other parts of the world. Tough acorns have had an important role in human diet for thousands of years, they are not widely used today as food or food ingredients, despite their noticeable availability. Moreover, many cultures have used acorns as a main staple and food, historically, and even many modern Asian countries are still using them. Acorns are rich in calorie, because of their high fat levels, protein and phenolic compounds.28 These facts motivated us to investigate possible bioreduction of Pd ions to Pd NPs. Therefore, this study continues our previous work29 and reports a facile and green method for biosynthesis of Pd NPs by Oak fruit bark extract (Fig. 1). In this regard, the bio-reduction process is monitored by UV–Visible, FT-IR, XRD, TEM, FESEM and EDX spectroscopies. Also, catalytic activity of Pd NPs in oxidation of benzyl alcohols to the associated aldehydes and ketones by molecular oxygen is studied. Primary and secondary benzyl alcohols are found out to give the corresponding products in good yields. Moreover, recyclability of the catalyst system is explored to show that the NPs can be recycled up to six times without significant loss of activity.
Experimental
times with deionized water and then air dried under sunlight to remove the moisture completely. The barks were chopped and powdered in a ball mill. The final sieved powder was used for all the further studies. 2 g of powder were weighed, boiled for 20 min in 20 ml deionized water and the extracts were filtered through Whatman filter paper No. 1. The filtered extract was stored in refrigerator at 4 °C. This extracts were used as reducing as well as stabilizing agent. Bioinspired synthesis of palladium nanoparticles 20 ml of 1 10 3 M aqueous solution of PdCl2 were taken in Erlenmeyer flask and 10 ml of Oak fruit bark extract was added to it and refluxed at 100 °C. After 90 min the solution turns yellow to black indicating the formation of Pd nanoparticles. Then the solvent was evaporated. The dark gray solid achieved during this process was dried by the flow of air over a night and then under vacuum for 48 h. Aerobic oxidation of alcohols A mixture of K2CO3 (1 mmol) and the catalyst (10 mg) in toluene (5 mL) was prepared in a two necked flask. The flask was evacuated and refilled with pure oxygen. To this solution, the alcohol (1 mmol, in 1 mL toluene) was injected and the resulting mixture was stirred at 80 °C under an oxygen atmosphere. After completion of reaction, the reaction mixture was filtered off and the catalyst rinsed twice with CH2Cl2 (5 mL). The excess of solvent was removed under reduced pressure to give the corresponding carbonyl compounds.
Preparation of plant extract Procedure for reusing the catalyst The Oak fruit bark was obtained from Zagros Mountains, Kermanshah, Iran. The isolated barks were washed thoroughly several
After the reaction time, 5 mL of CH2Cl2 was added to the reaction mixture and stirred for 5 min. After this time, the catalyst was separated by centrifugation. In the next step, the recovered solid was washed using EtOH and dried under vacuum. Then, the recovered catalyst was used for another run. Results and discussion
Fig. 1. Schematic synthesis procedure of green Pd nanoparticles (a) PdCl2 solution, (b) collected Oak fruit bark, (c) solution of Oak fruit bark extract and (d) biosynthesized Pd.
Reduction of Pd ions to Pd NPs in the presence of the plant extract could be followed by color change and spectroscopic techniques, such as UV–Vis spectroscopy. As the Oak fruit bark extract was poured in the aqueous solution of PdCl2, the solution color started changing from yellow to black (Fig. 1). Surface Plasmon resonance phenomenon confirmed formation of Pd NPs. As NP formation continued, intensity of the color increased. In addition to optical tracking of the changes, UV–Vis spectra of the reaction solution was recorded and characterized a significant change as the peak about 400 nm disappeared (Fig. 2). This observation is a hallmark of Pd(II) conversion to Pd(0). This also verified noticeable reduction potency of the extract. Involvement of the available functional groups of Oak fruit bark extract in reduction and capping of the Pd NPs was well demonstrated by Fourier transform infrared spectroscopy (FTIR). Using FT-IR spectroscopy, the major factors responsible for biological reduction of palladium ions (Pd2+) to Pd NPs (Pd0) in oak fruit extract were defined. Fig. 3 displays the FT-IR spectra of aqueous Oak fruit bark extract with strong bands at 3423 cm 1 (OAH stretch), 2924 and 2853 cm 1 (CAH stretch), 1649 cm 1 (AC@O stretch), 1460 and 1386 cm 1 (CAC stretch) and 1024 cm 1 (CAN stretch). Therefore, based on the FT-IR data, it can be inferred that phenolic hydroxyl groups of flavones, terpenoids and tannins that
H. Veisi et al. / Tetrahedron Letters 58 (2017) 4191–4196
Fig. 2. UV–Vis spectra of PdCl2, Extract, Extract/Pd(II) and Extract/Pd(0).
Fig. 4. FESEM images of biosynthesized Pd NPs.
belong to the phyto-molecules of Oak fruit bark play a major role in reduction of Pd2+ ions. Also, these groups can strongly bind to Pd NPs. Morphology of the synthesized Pd NPs was scanned through recording FESEM images of the biosynthesized Pd NPs, which revealed formation of homogeneous and relatively capped Pd NPs by the biomolecules present in Oak fruit bark extract (Fig. 4). Energy dispersion X-ray spectroscopy (EDX) was conducted and the result is shown in Fig. 5. Observation of the characteristic peak of palladium metal in EDX spectrum provided another evidence for successful formation of Pd NPs. Also, presence of some other elements was indicated by EDX analysis, which include carbon, oxygen and nitrogen. So that, the EDX spectrum confirmed that existence of organic metabolites in the extract is the key to reducing, capping and stabilization of the Pd NPs. The Au signal in the EDX result corresponds to the gold grid in SEM analysis. Transmission electron microscopy (TEM) provided deeper insight about morphology, shape and size of the biosynthesized
Fig. 5. EDX spectrum of biosynthesized Pd NPs.
Fig. 3. FTIR spectra of Oak fruit bark extract.
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Fig. 6. (a) TEM image; and (b) particle size distribution of the Pd NPs.
Fig. 7. XRD spectra of Pd NPs using Oak fruit bark extract.
NPs. The TEM image illustrated nearly spherical Pd NPs, which were relatively well dispersed on the surface of the extract. The particle sizes were identified to be about 5 nm based on particle size distribution (Fig. 6).
The XRD pattern of the synthesized palladium NPs is presented in Fig. 7. The observed intense peaks at 2h of 40, 46, 68, 82 and 87 deg. represent (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) Bragg reflections, respectively. Furthermore, the XRD pattern was compared with JCPDS standard (#05-0681), which concluded in approval of palladium NPs generation with cubic (fcc) crystal structure. Such unit cell structure is consistent with the earlier reports. In order to examine catalytic activity of the prepared nanoparticles as a novel heterogeneous catalyst option, aerobic oxidation of alcohols was probed in presence of Pd NPs@Extract. In this way, oxidation of para-methylbenzyl alcohol by different amounts of the biosynthesized catalyst was carried out, under a specific pressure of O2 gas. Under optimum conditions and catalytic performance of Pd NPs@Extract, para-methylbenzaldehyde was achieved in an excellent yield. It should be noted that oxidation reaction did not occur when the palladium catalyst was not added to the reactor. Another point is that, the desired product was obtained in low yield (only 30%), when without using an appropri-
Table 1 Optimization of reaction conditions in oxidation of para-methylbenzyl alcohol.
O OH
a
H
Pd NPs@Extract, K2CO3, O2 o
toluene, 80 C, 12h
H3 C
H3 C
Entry
Catalyst amount (% mol)
T (°C)
Base
time (h)
run
Yield (%)a
1 2 3 4 5 6 7 8 9 10 11 12
1 1 2 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5
25 80 80 80 80 80 80 80 80 80 80 80
– – – – K2CO3 K2CO3 Et3N K2CO3 K2CO3 K2CO3 K2CO3 K2CO3
12 12 12 12 1 5 12 12 12 12 12 12
1 1 1 1 1 1 1 1 2 3 4 5
NR trace 15 30 54 80 85 95 90 88 85 80
Isolated yield.
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OH R1
Table 3 Comparison efficiency of Pd NPs@Extract with some reported catalysts for the oxidation reaction of benzyl alcohol.
O
Pd NPs@Extract, K2CO3, O2 o
toluene, 80 C, 12h
R2
R1
Entry
Reaction conditions
Time (h)
Yield %
Refs.
1
Pd NPs@Extract, toluene, K2CO3, 80 °C
12
95
2 3 4 5 6 7 8 9
Pd/MagSBA, solvent-free, 80 °C Au-Pd/SBA, solvent-free, 160 °C Pd/NMC, solvent-free, 160 °C Co-NG, DMF, 130 °C RMC-Au, toluene, K2CO3, 130 °C Fe3O4/Cys-Pd, solvent-free, 50 °C Pd/CeO2, solvent-free, 160 °C Au-Pd/ TiO2, solvent-free, 160 °C
9 4 1 5 3 1.5 1 1.4
71 67.2 59.8 92.4 85 85 20.8 34.2
This work 30 31 32 33 34 35 36 37
R2
Entry
R1
R2
Yield (%)
1 2 3 4 5 6 7
PhA 4-OMeAPhA 4-MeAPhA 4-NO2APhA PhA PhA PhACH@CHA
H H H H CH3 PhA H
95 90 90 88 90 90 85
ate base and 2.5% mol of the catalyst were used for 12 h, at 80 °C (Table 1, entries 1–4). However, application of K2CO3, as a proper base, ended up with 95% yield of the desired product, in the same conditions (Table 1, entry 8). Further evaluations were performed on oxidation reaction of different substituted primary and secondary benzyl alcohols. Both primary and secondary benzyl alcohols could be successfully oxidized to give good yields of the associated aldehydes and ketones. It was found out that secondary benzyl alcohols are reactive towards oxidation, under similar reaction conditions. Upon their oxidation reactions, the target products were obtained in excellent
Fig. 8. The recycling of the Pd NPs@Extract for the aerobic oxidation of paramethylbenzyl alcohol.
yields (Table 2, entries 5,6). Also, primary benzyl alcohols possessing electron donating and withdrawing groups gave products in noticeable yields (Table 2, entries 2 and 3). Furthermore, complete conversion of allylic alcohols, e.g. cinnamyl alcohol (Table 2, Entry 7), to the desired aldehydes were obtained within 12 h. The C@C double bonds of the substrates remained intact without any intra-molecular hydrogen transfer over Pd NPs@Extract, under the tested conditions. Fig. 7 investigates reusability of Pd NPs@Extract for aerobic oxidation of para-methylbenzyl alcohol after 24 h of catalytic reaction (Table 2, Entry 3). In the end of the catalytic reaction, the catalyst was separated from the liquid phase by centrifugation. Then, it was thoroughly washed with ethanol and the isolated catalyst was reused as catalyst in subsequent runs, under the same reaction conditions. The obtained results (Fig. 8) indicated no efficiency reduction. Moreover, no palladium leaching was inspected in aerobic oxidation of the alcohols for up to five runs. Once more, TEM and EDX images (Fig. 9) of the catalyst were recorded after six catalytic runs. It was confirmed that the nanostructure of the catalyst has been preserved. Moreover, TEM image of the catalyst verified that the Pd NPs have remained well dispersed and same, after being reused for six times. The oxidation reaction of benzyl alcohol was performed to compare the synthesized Pd NPs@Extract efficiency with previously reported catalyst (Table 3). From Table 3, it is visible that the current catalyst exhibited more conversions and yields in comparison to the other listed system.
Fig. 9. TEM and EDX image of reused catalyst after the 6th run.
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Conclusion This study proposes a successful green chemistry approach for synthesis of Palladium nanoparticles (Pd NPs) using Oak fruit bark extract. The outlined synthesis method is simple, reliable and clean while it can enhance green industrial production of Pd NPs and provide an ecofriendly route for synthesis of benign NPs. The biosynthesized Pd NPs were characterized by FESEM, EDX, XRD, TEM, UV–Vis and FT-IR spectroscopy techniques. According to the spectroscopy results, presence of flavonoids and polyols is the key of the extract to act as a reductant, stabilizer and capping agent in synthesis and catalytic activity of the Pd NPs. Moreover, the proposed nano-catalyst was found to be successful in aerobic oxidation of alcohols under convenient conditions. Furthermore, recyclability and reusability of the catalyst without significant loss of its activity are advantageous features of the biosynthesized catalyst. Acknowledgement We are thankful to Payame Noor University (PNU) and Pharmaceutical Sciences Branch, Islamic Azad university, Tehran-Iran for partial support of this work. References 1. (a) Sheldon RA, Arends IWCE, Dijksman A. Catal Today. 2000;57:157; (b) Beller M, Bolm C. Transition metals for organic synthesis. 2nd ed. Wiley-VCH; 2004; (c) Brink G, Arends IWCE, Sheldon RA. Science. 2000;287:1636. 2. (a) Sheldon RA, Kochi JK. Metal-catalyzed oxidations of organic compounds. New York: Academic Press; 1981; (b) Pillai UR, Sahle-Demessie E. Appl Catal A Gen. 2003;245:103; (c) Hudlicky M. Oxidations in organic chemistry. Washington, DC: ACS; 1990. 3. (a) Backvall JE. Modern oxidation methods. Wiley-VCH; 2004; (b) Mallat T, Baiker A. Chem Rev. 2004;104:3037. 4. (a) Marko IE, Giles PR, Tsukazaki M, et al. Adv Inorg Chem. 2004;56:211; (b) Sheldon RA, Arends IWCE, Ten Brink GJ, Dijksman A. Acc Chem Res. 2002;35:774; (c) Zhan BZ, Thompson A. Tetrahedron. 2004;60:2917. 5. Martin SE, Suarez DF. Tetrahedron Lett. 2002;43:4475. 6. (a) Yamaguchi K, Mizuno N. Angew Chem Int Ed. 2002;41:4538; (b) Mark I, Paul Giles R, Tsukazaki M, Chelle-Regnaut I, Urch CJ, Brown SM. J Am Chem Soc. 1997;119:12661; (c) Choi E, Lee C, Na Y, Chang S. Org Lett. 2002;4:2369. 7. (a) Sharma VB, Jain SL, Sain B. Tetrahedron Lett. 2003;44:383; (b) Gilhespy M, Lok M, Baucherel X. Chem Commun. 2005;1085. 8. (a) Naik R, Joshi P, Deshpande RK. Catal Commun. 2004;5:195; (b) Gamez P, Arends IWCE, Reedijk J, Sheldon RA. Chem Commun. 2003;2414; (c) Ragagnin G, Betzemeier B, Quici S, Knochel P. Tetrahedron. 2002;58:3985; (d) Tsai W, Liu YH, Peng SM, Liu STJ. Organomet Chem. 2005;6:415; (e) Marko IE, Gautier A, Dumeunier R, et al. Angew Chem. 2004;116:1614.
9. Son YC, Makwana VD, Howell AR, Suib SL. Angew Chem. 2001;113:4410. 10. (a) Shapley PA, Zhang N, Allen JL, Pool DH, Liang HC. J Am Chem Soc. 2000;122:1079; (b) Coleman KS, Coppe M, Thomas C, Osborn JA. Tetrahedron Lett. 1999;40:3723; (c) Dobler C, Mehltretter GM, Sundermeier U, Eckert M, Militzer HC, Beller M. Tetrahedron Lett. 2001;42:8447. 11. (a) Buffin BP, Clarkson JP, Belitz NL, Kundu A. J Mol Catal A Chem. 2005;225:111; (b) Pillai UR, Sahle-Demessie E. Green Chem. 2004;6:161; (c) Caravati M, Grunwaldt JD, Baiker A. Phys Chem Chem Phys. 2005;7:278; (d) Muzart J. Tetrahedron. 2003;59:5789; (e) Caravati M, Grunwaldt JD, Baiker A. Catal Today. 2004;91–92:1. 12. (a) Roucoux A, Schulz J, Patin H. Chem Rev. 2002;102:3757; (b) Ikegami S, Harmamoto H. Chem Rev. 2009;109:583; (c) Veisi H, Mohammadi P, Gholami J. Appl Organomet Chem. 2014;28:868. 13. (a) Karimi B, Abedi S, Clark JH, Budarin V. Angew Chem Int Ed. 2006;45:4776; (b) Mifsud M, Parkhomenko KV, Arends IWCE, Sheldon RA. Tetrahedron. 2010;66:1040; (c) Feng B, Hou Z, Yang H, et al. Langmuir. 2010;26:2505. 14. Veisi H, Kordestani D, Sajjadifar S, Hamelian M. Iran Chem Commun. 2014;2:27. 15. Mohanpuria P, Rana NK, Yadav SK. J Nanopart Res. 2008;10:507. 16. Gericke M, Pinches A. Hydrometallurgy. 2006;83:132. 17. Song JY, Kwon EY, Kim BS. Bioproc Biosyst Eng. 2010;33:159. 18. Sathishkumar M, Sneha K, Kwak IS, Mao J, Tripathy S, Yun YS. J Hazard Mater. 2009;171:400. 19. Yang X, Li Q, Wang H, et al. J Nanopart Res. 2010;12:1589. 20. Sathishkumar M, Sneha K, Yun Y. Int J Mater Sci. 2009;4:11. 21. Bankar A, Joshi B, Kumar AR, Zinjarde S. Mater Lett. 2010;64:1951. 22. Nasrollahzadeh M, Sajadi SM, Maham M. J Mol Catal A Chem. 2015;395:297. 23. Veisi H, Faraji AR, Hemmati S, Gil A. Appl Organomet Chem. 2015;29:517. 24. Veisi H, Rashtiani A, Barjasteh V. Appl Organomet Chem. 2016;30:231. 25. Khazaei A, Rahmati S, Hekmatian Z, Saeednia S. J Mol Catal A Chem. 2013;372:160. 26. Veisi H, Ghorbani-Vaghei R, Hemmati S, Haji Aliani M, Ozturk T. Appl Organomet Chem. 2015;29:26. 27. Lebaschi S, Hekmati M, Veisi H. J Colloid Interface Sci. 2017;485:223. 28. Michael HC. Encyclopedia of Earth. Washington DC: National Council for Science and the Environment; 2012. 29. (a) Veisi H, Taheri S, Hemmati S. Green Chem. 2016;18:6337; (b) Pirhayati M, Veisi H, Kakanejadifard A. RSC Adv. 2016;6:27252; (c) Bonyasi F, Hekmati M, Veisi H, Colloid J. Interface Sci. 2017;496:177; (d) Ghorbani-Vaghei R, Chegini M, Veisi H, Karimi-Tabar M. Tetrahedron Lett. 1861;2009:50; (e) Veisi H, Maleki B, hamelian M, Sedigh Ashrafi S. RSC Adv. 2015;5:6365; (f) Heidari F, Hekmati M, Veisi H. J Colloid Interface Sci. 2017;501:175; (g) Veisi H, Ghadermazi M, Naderi A. Appl Organomet Chem. 2016;30:341; (h) Veisi H, Masti R, Kordestani D, Safaei M, Sahin O. J Mol Catal A Chem. 2014;385:61; (i) Ghorbani-Vaghei R, Veisi H. Synthesis. 2009;945; (j) Veisi H, Hemmati S, Javaheri H. Tetrahedron Lett. 2017;58:3155. 30. Li Y, Huang J, Hu X, Lam FLY, Wang W, Luque R. J Mol Catal A Chem. 2016;425:61. 31. Chen Y, Wang H, Liu CJ, et al. J Catal. 2012;289:105. 32. Wang B, Lin M, Ang TP, Chang J, Yang Y, Borgna A. Catal Commun. 2012;25:96. 33. Li M, Wu S, Yang X, et al. Appl Catal A Gen. 2017;543:61. 34. Oliveira AAS, Costa DS, Teixeira IF, et al. Catal Today. 2017;289:89. 35. Zamani F, Hosseini SM. Catal Commun. 2014;43:164. 36. Chen Y, Zheng H, Guo Z, et al. Catalysis. 2011;283:34. 37. Cao E, Sankar M, Nowicka E, et al. Catal Today. 2013;203:146.
Contents lists available at ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
Aerobic oxidation of benzyl alcohols through biosynthesized palladium nanoparticles mediated by Oak fruit bark extract as an efficient heterogeneous nanocatalyst Hojat Veisi a,⇑, Saba Hemmati a, Mahnaz Qomi b a b
Department of Chemistry, Payame Noor University, Tehran, Iran Active Pharmaceutical Ingredients Research Center (APIRC), Pharmaceutical Sciences Branch, Islamic Azad University, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 17 August 2017 Revised 15 September 2017 Accepted 19 September 2017 Available online 22 September 2017 Keywords: Green chemistry Biosynthesis Pd nanoparticles Oak Oxidation Alcohols
a b s t r a c t In this study, green synthesis of Pd nanoparticles (NPs) is outlined through application of Oak fruit bark extract as a reducing, capping and stabilizing agent. The characteristics and properties of the biosynthesized Pd NPs were revealed by FESEM, EDX, XRD, TEM, UV–Vis, and FT-IR spectroscopies. So that, UV–Vis spectroscopy of the Pd colloidal solution confirmed reduction of Pd ions, and XRD and TEM analysis identified fcc unit cell structure forming 5–7 nm spherical Pd NPs. Furthermore, catalytic activity of the prepared catalyst was investigated through aerobic oxidation of alcohols, as model reactions. Catalytic evaluations demonstrated achievement of good yields from primary and secondary benzyl alcohols. In general, the devised synthesis method is advantageous from several perspectives. For example, the synthesized catalysts give high product yields and are efficient, they eliminate the need for surfactant, chemical reductants, ligand and organic solvents, the approach is economically inexpensive, it results in cleaner reaction profiles, application of the simply prepared heterogeneous catalyst is convenient, and the catalyst is recoverable and reusable for at least six times without any significant loss of its catalytic activity. Ó 2017 Elsevier Ltd. All rights reserved.
Introduction Production of carbonyls through oxidation of alcohols is one of the essential organic transformations1 since carbonyls can be employed as intermediates in manufacturing dyes and pharmaceuticals.2 Such organic transformations can be facilitated by catalysts. In this respect, transition metal based catalysts can conduct selective oxidation of organic compounds as interesting alternatives to conventional waste-producing oxidation procedures, which demand for stoichiometric amounts of toxic inorganic salts.3,4 Recently, aerobic oxidation processes have gained more attention as utilizing oxygen has considerable benefits, from both economic and green chemistry points of view. In fact, molecular oxygen is inexpensive and produces just water as its byproduct. Consequently, extensive catalytic studies have concentrated on finding suitable active metals for homo- and heterogeneous catalytic oxidation with molecular oxygen, e.g. Fe,5 Ru,6 Co,7 Cu,8 Mn,9 Os10 and Pd.11 Also, solid supported transition metal nanoparticles (NPs) have attracted immense interest in development of ⇑ Corresponding author. E-mail address: [email protected] (H. Veisi). https://doi.org/10.1016/j.tetlet.2017.09.057 0040-4039/Ó 2017 Elsevier Ltd. All rights reserved.
new catalysts due to their high catalytic activity and reusability properties.12 So that, few recent reports have shown that heterogeneous palladium NPs can undertake aerobic oxidation of aromatic alcohols.13,14 To date, different chemical, physical and biological methods have been employed for production of Pd NPs. However, physical methods require expensive equipment and involve high vacuum technology. Also, chemical methods are not completely favorable since they need capping agents to avoid agglomeration of the produced particles, due to their high surface reactivity. Meantime, using the necessary toxic chemicals is a major concern and presence of non-polar solvents and toxic chemicals limit application of chemical synthesis of Pd NPs, in clinical fields. Therefore, researchers have shifted to ecofriendly NP synthesis through microorganisms and plant extracts. Green synthesis of Pd NPs is consisted of three main steps, which should be evaluated according to green chemistry perspectives and include (i) solvent medium selection, (ii) choosing an ecofriendly reducing agent and (iii) finding nontoxic NP stabilizers. Therefore, biosynthesis approach of Pd NPs through green chemistry reduction has been explored, in this study. It is noteworthy that bio-inspired, ecofriendly and greener synthesis methods of metal NPs are among the most interesting
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aspects of nanoscience and nanotechnology, nowadays.15,16 There are limited number of available works on biosynthesis of Pd NPs and Diospyros kaki. leaf,17 Cinnamomum zeylanicum bark,18 C. Camphora leaf,19 Curcuma longa tuber,20 banana peel,21 Hippophae rhamnoides Linn,22 Pistacia atlantica kurdica gum,23 Rosa canina fruit,24 pectin,25 Stachys lavandulifolia26 and Oak gum27 have been employed, to date. Oak tree is widely spread throughout Zagros Mountains, West of Iran, East and North of Iraq, Southern Turkey, Northern Syria and, also, in many other parts of the world. Tough acorns have had an important role in human diet for thousands of years, they are not widely used today as food or food ingredients, despite their noticeable availability. Moreover, many cultures have used acorns as a main staple and food, historically, and even many modern Asian countries are still using them. Acorns are rich in calorie, because of their high fat levels, protein and phenolic compounds.28 These facts motivated us to investigate possible bioreduction of Pd ions to Pd NPs. Therefore, this study continues our previous work29 and reports a facile and green method for biosynthesis of Pd NPs by Oak fruit bark extract (Fig. 1). In this regard, the bio-reduction process is monitored by UV–Visible, FT-IR, XRD, TEM, FESEM and EDX spectroscopies. Also, catalytic activity of Pd NPs in oxidation of benzyl alcohols to the associated aldehydes and ketones by molecular oxygen is studied. Primary and secondary benzyl alcohols are found out to give the corresponding products in good yields. Moreover, recyclability of the catalyst system is explored to show that the NPs can be recycled up to six times without significant loss of activity.
Experimental
times with deionized water and then air dried under sunlight to remove the moisture completely. The barks were chopped and powdered in a ball mill. The final sieved powder was used for all the further studies. 2 g of powder were weighed, boiled for 20 min in 20 ml deionized water and the extracts were filtered through Whatman filter paper No. 1. The filtered extract was stored in refrigerator at 4 °C. This extracts were used as reducing as well as stabilizing agent. Bioinspired synthesis of palladium nanoparticles 20 ml of 1 10 3 M aqueous solution of PdCl2 were taken in Erlenmeyer flask and 10 ml of Oak fruit bark extract was added to it and refluxed at 100 °C. After 90 min the solution turns yellow to black indicating the formation of Pd nanoparticles. Then the solvent was evaporated. The dark gray solid achieved during this process was dried by the flow of air over a night and then under vacuum for 48 h. Aerobic oxidation of alcohols A mixture of K2CO3 (1 mmol) and the catalyst (10 mg) in toluene (5 mL) was prepared in a two necked flask. The flask was evacuated and refilled with pure oxygen. To this solution, the alcohol (1 mmol, in 1 mL toluene) was injected and the resulting mixture was stirred at 80 °C under an oxygen atmosphere. After completion of reaction, the reaction mixture was filtered off and the catalyst rinsed twice with CH2Cl2 (5 mL). The excess of solvent was removed under reduced pressure to give the corresponding carbonyl compounds.
Preparation of plant extract Procedure for reusing the catalyst The Oak fruit bark was obtained from Zagros Mountains, Kermanshah, Iran. The isolated barks were washed thoroughly several
After the reaction time, 5 mL of CH2Cl2 was added to the reaction mixture and stirred for 5 min. After this time, the catalyst was separated by centrifugation. In the next step, the recovered solid was washed using EtOH and dried under vacuum. Then, the recovered catalyst was used for another run. Results and discussion
Fig. 1. Schematic synthesis procedure of green Pd nanoparticles (a) PdCl2 solution, (b) collected Oak fruit bark, (c) solution of Oak fruit bark extract and (d) biosynthesized Pd.
Reduction of Pd ions to Pd NPs in the presence of the plant extract could be followed by color change and spectroscopic techniques, such as UV–Vis spectroscopy. As the Oak fruit bark extract was poured in the aqueous solution of PdCl2, the solution color started changing from yellow to black (Fig. 1). Surface Plasmon resonance phenomenon confirmed formation of Pd NPs. As NP formation continued, intensity of the color increased. In addition to optical tracking of the changes, UV–Vis spectra of the reaction solution was recorded and characterized a significant change as the peak about 400 nm disappeared (Fig. 2). This observation is a hallmark of Pd(II) conversion to Pd(0). This also verified noticeable reduction potency of the extract. Involvement of the available functional groups of Oak fruit bark extract in reduction and capping of the Pd NPs was well demonstrated by Fourier transform infrared spectroscopy (FTIR). Using FT-IR spectroscopy, the major factors responsible for biological reduction of palladium ions (Pd2+) to Pd NPs (Pd0) in oak fruit extract were defined. Fig. 3 displays the FT-IR spectra of aqueous Oak fruit bark extract with strong bands at 3423 cm 1 (OAH stretch), 2924 and 2853 cm 1 (CAH stretch), 1649 cm 1 (AC@O stretch), 1460 and 1386 cm 1 (CAC stretch) and 1024 cm 1 (CAN stretch). Therefore, based on the FT-IR data, it can be inferred that phenolic hydroxyl groups of flavones, terpenoids and tannins that
H. Veisi et al. / Tetrahedron Letters 58 (2017) 4191–4196
Fig. 2. UV–Vis spectra of PdCl2, Extract, Extract/Pd(II) and Extract/Pd(0).
Fig. 4. FESEM images of biosynthesized Pd NPs.
belong to the phyto-molecules of Oak fruit bark play a major role in reduction of Pd2+ ions. Also, these groups can strongly bind to Pd NPs. Morphology of the synthesized Pd NPs was scanned through recording FESEM images of the biosynthesized Pd NPs, which revealed formation of homogeneous and relatively capped Pd NPs by the biomolecules present in Oak fruit bark extract (Fig. 4). Energy dispersion X-ray spectroscopy (EDX) was conducted and the result is shown in Fig. 5. Observation of the characteristic peak of palladium metal in EDX spectrum provided another evidence for successful formation of Pd NPs. Also, presence of some other elements was indicated by EDX analysis, which include carbon, oxygen and nitrogen. So that, the EDX spectrum confirmed that existence of organic metabolites in the extract is the key to reducing, capping and stabilization of the Pd NPs. The Au signal in the EDX result corresponds to the gold grid in SEM analysis. Transmission electron microscopy (TEM) provided deeper insight about morphology, shape and size of the biosynthesized
Fig. 5. EDX spectrum of biosynthesized Pd NPs.
Fig. 3. FTIR spectra of Oak fruit bark extract.
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Fig. 6. (a) TEM image; and (b) particle size distribution of the Pd NPs.
Fig. 7. XRD spectra of Pd NPs using Oak fruit bark extract.
NPs. The TEM image illustrated nearly spherical Pd NPs, which were relatively well dispersed on the surface of the extract. The particle sizes were identified to be about 5 nm based on particle size distribution (Fig. 6).
The XRD pattern of the synthesized palladium NPs is presented in Fig. 7. The observed intense peaks at 2h of 40, 46, 68, 82 and 87 deg. represent (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) Bragg reflections, respectively. Furthermore, the XRD pattern was compared with JCPDS standard (#05-0681), which concluded in approval of palladium NPs generation with cubic (fcc) crystal structure. Such unit cell structure is consistent with the earlier reports. In order to examine catalytic activity of the prepared nanoparticles as a novel heterogeneous catalyst option, aerobic oxidation of alcohols was probed in presence of Pd NPs@Extract. In this way, oxidation of para-methylbenzyl alcohol by different amounts of the biosynthesized catalyst was carried out, under a specific pressure of O2 gas. Under optimum conditions and catalytic performance of Pd NPs@Extract, para-methylbenzaldehyde was achieved in an excellent yield. It should be noted that oxidation reaction did not occur when the palladium catalyst was not added to the reactor. Another point is that, the desired product was obtained in low yield (only 30%), when without using an appropri-
Table 1 Optimization of reaction conditions in oxidation of para-methylbenzyl alcohol.
O OH
a
H
Pd NPs@Extract, K2CO3, O2 o
toluene, 80 C, 12h
H3 C
H3 C
Entry
Catalyst amount (% mol)
T (°C)
Base
time (h)
run
Yield (%)a
1 2 3 4 5 6 7 8 9 10 11 12
1 1 2 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5
25 80 80 80 80 80 80 80 80 80 80 80
– – – – K2CO3 K2CO3 Et3N K2CO3 K2CO3 K2CO3 K2CO3 K2CO3
12 12 12 12 1 5 12 12 12 12 12 12
1 1 1 1 1 1 1 1 2 3 4 5
NR trace 15 30 54 80 85 95 90 88 85 80
Isolated yield.
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OH R1
Table 3 Comparison efficiency of Pd NPs@Extract with some reported catalysts for the oxidation reaction of benzyl alcohol.
O
Pd NPs@Extract, K2CO3, O2 o
toluene, 80 C, 12h
R2
R1
Entry
Reaction conditions
Time (h)
Yield %
Refs.
1
Pd NPs@Extract, toluene, K2CO3, 80 °C
12
95
2 3 4 5 6 7 8 9
Pd/MagSBA, solvent-free, 80 °C Au-Pd/SBA, solvent-free, 160 °C Pd/NMC, solvent-free, 160 °C Co-NG, DMF, 130 °C RMC-Au, toluene, K2CO3, 130 °C Fe3O4/Cys-Pd, solvent-free, 50 °C Pd/CeO2, solvent-free, 160 °C Au-Pd/ TiO2, solvent-free, 160 °C
9 4 1 5 3 1.5 1 1.4
71 67.2 59.8 92.4 85 85 20.8 34.2
This work 30 31 32 33 34 35 36 37
R2
Entry
R1
R2
Yield (%)
1 2 3 4 5 6 7
PhA 4-OMeAPhA 4-MeAPhA 4-NO2APhA PhA PhA PhACH@CHA
H H H H CH3 PhA H
95 90 90 88 90 90 85
ate base and 2.5% mol of the catalyst were used for 12 h, at 80 °C (Table 1, entries 1–4). However, application of K2CO3, as a proper base, ended up with 95% yield of the desired product, in the same conditions (Table 1, entry 8). Further evaluations were performed on oxidation reaction of different substituted primary and secondary benzyl alcohols. Both primary and secondary benzyl alcohols could be successfully oxidized to give good yields of the associated aldehydes and ketones. It was found out that secondary benzyl alcohols are reactive towards oxidation, under similar reaction conditions. Upon their oxidation reactions, the target products were obtained in excellent
Fig. 8. The recycling of the Pd NPs@Extract for the aerobic oxidation of paramethylbenzyl alcohol.
yields (Table 2, entries 5,6). Also, primary benzyl alcohols possessing electron donating and withdrawing groups gave products in noticeable yields (Table 2, entries 2 and 3). Furthermore, complete conversion of allylic alcohols, e.g. cinnamyl alcohol (Table 2, Entry 7), to the desired aldehydes were obtained within 12 h. The C@C double bonds of the substrates remained intact without any intra-molecular hydrogen transfer over Pd NPs@Extract, under the tested conditions. Fig. 7 investigates reusability of Pd NPs@Extract for aerobic oxidation of para-methylbenzyl alcohol after 24 h of catalytic reaction (Table 2, Entry 3). In the end of the catalytic reaction, the catalyst was separated from the liquid phase by centrifugation. Then, it was thoroughly washed with ethanol and the isolated catalyst was reused as catalyst in subsequent runs, under the same reaction conditions. The obtained results (Fig. 8) indicated no efficiency reduction. Moreover, no palladium leaching was inspected in aerobic oxidation of the alcohols for up to five runs. Once more, TEM and EDX images (Fig. 9) of the catalyst were recorded after six catalytic runs. It was confirmed that the nanostructure of the catalyst has been preserved. Moreover, TEM image of the catalyst verified that the Pd NPs have remained well dispersed and same, after being reused for six times. The oxidation reaction of benzyl alcohol was performed to compare the synthesized Pd NPs@Extract efficiency with previously reported catalyst (Table 3). From Table 3, it is visible that the current catalyst exhibited more conversions and yields in comparison to the other listed system.
Fig. 9. TEM and EDX image of reused catalyst after the 6th run.
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Conclusion This study proposes a successful green chemistry approach for synthesis of Palladium nanoparticles (Pd NPs) using Oak fruit bark extract. The outlined synthesis method is simple, reliable and clean while it can enhance green industrial production of Pd NPs and provide an ecofriendly route for synthesis of benign NPs. The biosynthesized Pd NPs were characterized by FESEM, EDX, XRD, TEM, UV–Vis and FT-IR spectroscopy techniques. According to the spectroscopy results, presence of flavonoids and polyols is the key of the extract to act as a reductant, stabilizer and capping agent in synthesis and catalytic activity of the Pd NPs. Moreover, the proposed nano-catalyst was found to be successful in aerobic oxidation of alcohols under convenient conditions. Furthermore, recyclability and reusability of the catalyst without significant loss of its activity are advantageous features of the biosynthesized catalyst. Acknowledgement We are thankful to Payame Noor University (PNU) and Pharmaceutical Sciences Branch, Islamic Azad university, Tehran-Iran for partial support of this work. References 1. (a) Sheldon RA, Arends IWCE, Dijksman A. Catal Today. 2000;57:157; (b) Beller M, Bolm C. Transition metals for organic synthesis. 2nd ed. Wiley-VCH; 2004; (c) Brink G, Arends IWCE, Sheldon RA. Science. 2000;287:1636. 2. (a) Sheldon RA, Kochi JK. Metal-catalyzed oxidations of organic compounds. New York: Academic Press; 1981; (b) Pillai UR, Sahle-Demessie E. Appl Catal A Gen. 2003;245:103; (c) Hudlicky M. Oxidations in organic chemistry. Washington, DC: ACS; 1990. 3. (a) Backvall JE. Modern oxidation methods. Wiley-VCH; 2004; (b) Mallat T, Baiker A. Chem Rev. 2004;104:3037. 4. (a) Marko IE, Giles PR, Tsukazaki M, et al. Adv Inorg Chem. 2004;56:211; (b) Sheldon RA, Arends IWCE, Ten Brink GJ, Dijksman A. Acc Chem Res. 2002;35:774; (c) Zhan BZ, Thompson A. Tetrahedron. 2004;60:2917. 5. Martin SE, Suarez DF. Tetrahedron Lett. 2002;43:4475. 6. (a) Yamaguchi K, Mizuno N. Angew Chem Int Ed. 2002;41:4538; (b) Mark I, Paul Giles R, Tsukazaki M, Chelle-Regnaut I, Urch CJ, Brown SM. J Am Chem Soc. 1997;119:12661; (c) Choi E, Lee C, Na Y, Chang S. Org Lett. 2002;4:2369. 7. (a) Sharma VB, Jain SL, Sain B. Tetrahedron Lett. 2003;44:383; (b) Gilhespy M, Lok M, Baucherel X. Chem Commun. 2005;1085. 8. (a) Naik R, Joshi P, Deshpande RK. Catal Commun. 2004;5:195; (b) Gamez P, Arends IWCE, Reedijk J, Sheldon RA. Chem Commun. 2003;2414; (c) Ragagnin G, Betzemeier B, Quici S, Knochel P. Tetrahedron. 2002;58:3985; (d) Tsai W, Liu YH, Peng SM, Liu STJ. Organomet Chem. 2005;6:415; (e) Marko IE, Gautier A, Dumeunier R, et al. Angew Chem. 2004;116:1614.
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