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Water-compatible gold and silver nanoparticles as catalysts for the oxidation of alkenes
Polyhedron xxx (2016) xxx–xxx
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Polyhedron journal homepage: www.elsevier.com/locate/poly
Water-compatible gold and silver nanoparticles as catalysts for the oxidation of alkenes Eugenia Fisher a,1, Lea Kenisgberg a,1, Monica Carreira a, Jacob Fernández-Gallardo a, Richard Baldwin b,⇑, María Contel a,c,⇑ a b c
Department of Chemistry, Brooklyn College, The City University of New York, Brooklyn, NY 11210, United States NanoComposix INC, 4878 Ronson Court Suite K, San Diego, CA 92111, United States Chemistry PhD Program, The Graduate Center, The City University of New York, 365 Fifth Avenue, New York, NY 10016, United States
a r t i c l e
i n f o
Article history: Received 30 March 2016 Accepted 9 June 2016 Available online xxxx This paper is dedicated to Emeritus Professor Martin A. Bennett on the occasion of his 80th birthday. Keywords: Oxidative cleavage Alkenes Gold Silver Nanoparticles
a b s t r a c t Water-compatible Ag/Au nanoparticles (NP) catalyze the liquid-phase oxidation of 1,1-diphenylethylene to afford benzophenone, the product of oxidative cleavage, and 1,1-diphenylepoxide. The metal nanoparticles are surface functionalized with tannic acid and/or citrate and have diameters of 10–12 nm. By proper selection of the reaction conditions, percent conversions/distributions of 85–100% of 1,1-diphenylethylene can be achieved at 90 °C using water as a solvent and TBHP (tert-butylhydroperoxide) as the co-oxidant. In addition, these nanoparticles have been further supported on functionalized silica to improve their stability and recyclability in this oxidation process. Conversions of up to 100% to benzophenone could be obtained with 1.5 mol% Au or Ag nanoparticles (10 nm) supported on functionalized silica. The supported NP could be recycled and re-used without significant catalytic activity loss for three runs. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction During the last two decades there has been much interest in finding new and less toxic metal-based oxidation catalysts, with reports that have been compiled in a number of recent reviews [1–10]. Research has been focused on developing highly selective and/or recoverable catalysts which employ oxygen or air as oxidants. While not all new oxidation catalysts have been able to fulfill all the above features, gold compounds and nanoparticles have been proven as an excellent alternative to other classical metal-based oxidation catalysts, and have been successfully used in heterogeneous [11–16] and to a lesser extent in homogeneous oxidations [13–18]. In contrast, oxidations of alkenes with silver compounds in homogeneous conditions have been less explored [12,19]. One relevant oxidation reaction is the oxidative cleavage of C@C double bonds to carbonyl compounds (usually achieved by stoichiometric oxidation or ozonation). In 2006, Shi and
⇑ Corresponding authors at: Department of Chemistry, Brooklyn College, The City University of New York, Brooklyn, NY 11210, United States (M. Contel). E-mail addresses: [email protected] (R. Baldwin), [email protected] (M. Contel). 1 These authors contributed equally to the work.
co-workers reported the oxidative cleavage of alkenes to form ketones and aldehydes with AuCl/neocuproine in water at 90 °C with TBHP (tert-butylhydroperoxide) as the oxidant (Scheme 1) [20]. Our group reported that gold(I) and especially silver(I) complexes containing a tripodal bis(imidazole) thioether ligand, could perform this oxidative cleavage in toluene [21]. Liu and co-workers synthesized fully substituted butenolides via a cascade reaction for the cleavage of a carbon–carbon triple bond involving: (a) gold-catalyzed addition of an oxygen nucleophile to the triple bonds in (Z)-enynols and, (b) subsequent oxygen-assisted oxidative cleavage of the exo-enolic double bond [22]. In the cases of the oxidative cleavage of alkenes with the Au(I) and Ag(I) compounds [20,21], we wondered if the active catalyst could be nanoparticles generated under reaction conditions and we set to explore the catalytic activity of Au and Ag NP in H2O. We report here on the use of commercially available water-compatible Ag/Au nanoparticles for the liquid-phase oxidation of 1,1-diphenylethylene in H2O at 90 °C with TBHP as oxidant. The oxidation products are benzophenone, product of the oxidative cleavage, and 1,1-diphenylepoxide (Scheme 2). We also describe catalytic studies with the nanoparticles further supported on functionalized silica in order to improve the recyclability of the systems.
http://dx.doi.org/10.1016/j.poly.2016.06.012 0277-5387/Ó 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: E. Fisher et al., Polyhedron (2016), http://dx.doi.org/10.1016/j.poly.2016.06.012
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E. Fisher et al. / Polyhedron xxx (2016) xxx–xxx
N
R1 R2
H R3
AV400 (1H NMR at 400 MHz, 13C{1H} NMR at 100.6 MHz). Chemical shifts (d) are given in ppm using CDCl3 as the solvent, and spectra was recorded at 25 °C. 1H and 13C NMR resonances were measured relative to solvent peaks with tetramethylsilane = 0 ppm. Coupling constants J are given in hertz. Nanoparticles were characterized by TEM and UV–Vis spectroscopy as well as ICP-MS for metal concentration by nanoComposix.
N
(5 mol %) AuCl (5 mol %)
R1
o
TBHP, H 2O, 90 C
O
R2
+
H O R3
R 1 = aryl, alkyl R 2 = aryl, alkyl, H R 3 = H or CH 3
Scheme 1. Oxidative cleavage of alkenes with a gold(I) catalyst [20].
2. Experimental 2.1. General procedure The unsupported water suspended metal nanoparticles employed are surface functionalized with tannic acid and/or citrate in the case of gold and citrate surface groups in the case of silver and have dimensions of 10–12 nm. The nanoparticles used are available commercially from nanoComposix (products AuB10, AuB12 and AgCB10 for gold at 10 and 12 nm and silver respectively). 1,1-diphenylethylene was purchased from Sigma Aldrich. All purchased reactants were used without further purification. Reaction and work-up solvents were purchased from Fisher Scientific (ACS Grade). Deuterated solvents were purchased from Cambridge Isotope Laboratories, Inc., were kept over molecular sieves (3 Å, beads, 4–8 mesh). NMR spectra were recorded using a Bruker
2.2. Catalytic oxidation procedure for unsupported nanoparticles To 1.5 ml of deionized H2O, TBHP (2.1 equiv.) and the alkene (1,1-diphenylethylene) were added at room temperature. Solutions of metal nanoparticles in water were subsequently added (different concentrations, see Tables 1–4 for mol per cent values). The reaction mixture was stirred under reflux at 90 °C (different times were studied, see Tables 1–4). After the reaction finished and the flask was cooled to room temperature, the aqueous layer was extracted with diethyl ether (3 20 mL). The combined organic layers were washed with a saturated sodium bisulfite solution (3 10 mL) followed by deionized H2O (3 20 mL), and dried over MgSO4 overnight. The solution was then filtered and concentrated under vacuum. The product obtained was analyzed by 1H NMR spectroscopy. Ketone and epoxide (Scheme 2) were observed in different percent conversions depending on the reaction conditions (Tables 1–4). Diagnostic signals in 1H NMR spectra (CDCl3): 1,1-diphenylethylene 5.5 ppm (2H, s, CH2), 7.6–7.3 ppm (10H, m, 6CH); benzophenone 7.8 ppm (4H, m, 4CH); 1,1-diphenylepoxide 4.84 ppm (2H, s, CH2).
Scheme 2. Oxidative cleavage of 1,1-diphenylethylene catalyzed by Au or Ag NP.
Table 1 Oxidations with 1 mol% of gold nanoparticles of size 10 nm (entries 1–4) and 12 nm (entries 5–8).a Entry
1 2 3 4 5 6 7 8
Type of NP
10 nm 10 nm 10 nm 10 nm 12 nm 12 nm 12 nm 12 nm
Au Au Au Au Au Au Au Au
[cat] mol%
1 1 1 1 1 1 1 1
Time (h)
18 5 2 1 18 5 2 1
Conversion (%)
Std dev
To ketone
To epoxide
Total
85 85 67 65 79 78 66 62
15 15 33 35 21 22 34 38
86 69 71 39 90 87 63 54
4 0 4 12 3 0 2 2
a Reaction conditions: 0.5 mmol diphenylethylene, 1.05 mmol TBHP, H2O, 90 °C. Conversion determined by 1H NMR spectroscopy. Values given are an average of two reactions. The standard deviation is given for the conversion of ketone.
Table 2 Oxidations with gold nanoparticles of size 12 nm (18 h).a Entry
1 1 2
Type of NP
12 nm Au 12 nm Au 12 nm Au
[cat] mol%
1 2 5
Time (h)
18 18 18
Conversion (%)
Std dev
To ketone
To epoxide
Total
79 83 88
21 17 12
90 70 84
3 3 0
a Reaction conditions: 0.5 mmol diphenylethylene, 1.05 mmol TBHP, H2O, 90 °C. Conversion determined by 1H NMR spectroscopy. Values given are an average of two reactions. The standard deviation is given for the conversion of ketone.
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E. Fisher et al. / Polyhedron xxx (2016) xxx–xxx Table 3 Oxidations with silver nanoparticles of size 10 nm.a Entry
Type of NP
[cat] (mol%)
Time (h)
Conversion (%)
Std dev
To ketone
To epoxide
Total
1 2 3
10 nm Ag 10 nm Ag 10 nm Ag
1 1 5
5 18 18
78 89 86
22 11 14
47 45 92
0 3 0
a Reaction conditions: 0.5 mmol diphenylethylene, 1.05 mmol TBHP, H2O, 90 C°. Conversion determined by 1H NMR spectroscopy. Values given are an average of two reactions. The standard deviation is given for the conversion of ketone.
Table 4 Reactions of gold nanoparticles (size 10 nm, entries 1–3 and size 12 nm entry 4) supported on APTMS functionalized silicaa. Entry
1 2 3 4
Type of NP
10 nm 10 nm 10 nm 12 nm
Au Au Au Au
Functionalized silica (g)
[cat] mol%
0.25 0.25 0.375 0.25 g
Time (h)
1 1.5 1.5 1.5
Conversion (%)
18 18 18 18
Std dev
To ketone
To epoxide
Total
93 100 90 97
7 0 10 3
98 100 95 97
7 0 1 3
a Reaction conditions: 0.5 mmol diphenylethylene, 1.05 mmol TBHP, H2O, 90 °C. Conversion determined by 1H NMR spectroscopy. Values given are an average of two reactions. The standard deviation is given for the conversion of ketone.
modified. Nanoparticles were provided at a concentration of 1 mg/mL. After impregnation, the silica supported nanoparticles were collected by filtration. The supported Au nanoparticles formed a light purple powder while the supported Ag nanoparticles formed a light brown powder. The filtrate was a clear solution in both cases.
2.3. Functionalization of silica Chromatographic silica was mixed with silane (Aminopropyltrimethoxysilane APTMS or Mercaptopropyltrimethoxysilane MPTMS) in ethanol. 19 mmol of silane per 1 g of silica were used. The solution was stirred for 10 min in the case of APTMS and for 2 h in the case of MPTMS, and then washed 3 times with 10 mL of ethanol. The functionalized-silica was filtered and dried under vacuum. The ninhydrin test was performed to check if the silica was functionalized with the amine group (APTMS). A solution of 0.2 g of ninhydrin and 50 mL of H2O was prepared. One mL of the ninhydrin solution in water was added to 2 mg of the functionalized silica. The mixture was boiled for 15–20 s. A blue/violet color indicated that surface was successfully functionalized.
2.5. Catalytic oxidation procedure for supported nanoparticles and study of the recyclability To the supported metal NP, 1.5 ml of deionized H2O, the starting material (1,1-diphenylethylene), and TBHP (2.1 equiv.) were added at room temperature. The reaction mixture was stirred under reflux at 90 °C for the times specified in Tables 4–6. After the reaction finished and the flask was cooled to room temperature, 4 mL of ether was added to the crude reaction. The mixture was stirred for a few minutes, and the liquid layer was decanted by pipette and run through a fine filter. This procedure was repeated 4 times to separate the products of the reaction. Deionized H2O (3 5 mL) was subsequently added to the reaction mixture and the solid
2.4. Procedure for supporting the nanoparticles Ag and Au nanoparticles in water were stirred at room temperature with functionalized silica overnight. The ratio of grams of functionalized silica to milliliters of nanoparticle solution was
Table 5 Oxidations using recycled supported gold nanoparticles (10 nm) supported on APTMS functionalized silicaa. Entry
Recycled NP?
Type of NP
[cat] mol%
Time (h)
Conversion (%)
std dev
To ketone
To epoxide
Total
1 2 3
No From entry 1 From entry 2
10 nm Au 10 nm Au 10 nm Au
1.5 1.5 1.5
18 18 18
100 98 97
0 0 3
99 98 99
1 2 4
a Reaction conditions: 0.5 mmol diphenylethylene, 1.05 mmol TBHP, H2O, 90 °C. Conversion determined by 1H NMR spectroscopy. Values given are an average of two reactions. The standard deviation is given for the conversion of ketone.
Table 6 Reactions of silver nanoparticles (size 10 nm, 1.5 mol%) supported on silica functionalized with MPTMS and APTMSa. Entry
Recycled NP?
Type of NP
[cat] mol%
Time (h)
1 2 3
No From entry 1 From entry 2
10 nm Ag 10 nm Ag 10 nm Ag
1.5 1.5 1.5
18 18 18
Conversion (%)
Std dev
To ketone
To epoxide
Total
100 100 100
0 0 0
91 87 75
5 2 4
a Reaction conditions: 0.5 mmol diphenylethylene, 1.05 mmol TBHP, H2O, 90 °C. Conversion determined by 1H NMR spectroscopy. Values given are an average of two reactions. The standard deviation is given for the conversion of ketone.
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E. Fisher et al. / Polyhedron xxx (2016) xxx–xxx
supported nanoparticles were filtered out. The reaction flask was washed with diethyl ether (10 mL) to dissolve any remaining product. The solid supported nanoparticles were then washed with deionized H2O (1 10 mL) and diethyl ether (3 10 mL). Following the filtration process, the supported nanoparticles were dried under vacuum for further use. The organic layer was extracted from the combined organic and aqueous filtrate with diethyl ether (3 20 mL). Following extraction, the organic layer was washed with a saturated sodium bisulfite solution (3 10 mL), followed by deionized H2O (2 10 mL). The organic layer was then dried over MgSO4 overnight. The solution was filtered and poured into a pre-weighted round bottom flask and dried under high vacuum. The product obtained was weighed and analyzed by 1H NMR spectroscopy. Ketone and epoxide (Scheme 2) were observed in different percent conversions depending on the reaction conditions (Tables 4–6). 2.6. Transmission electron microscopy characterization TEM images were taken on a JEOL 1010 electron microscope. Samples of the catalysts were suspended in a 2:1 mixture of isopropanol and DI water. 5 lL of this solution was dropped onto the formvar side of a carbon coated formvar 300 mesh copper TEM grid. The sample was then dried under vacuum and imaged at 100 kV.
3. Results and discussion We studied the use of commercially available water-stable Ag/Au nanoparticles for the liquid-phase oxidation of 1,1-diphenylethylene in H2O at 90 °C with TBHP as oxidant (2.1 equivalents). The oxidation products are benzophenone, product of the oxidative cleavage, and 1,1-diphenylepoxide (Scheme 2). We started by investigating the effect of time in the catalytic activity of 1 mol% of unsupported 10 nm and 12 nm gold nanoparticles (Table 1). From the results depicted in Table 1 we infer that the total conversion and the percent conversion to ketone are directly proportional to the reaction time. Increasing the reaction time resulted in an increased total conversion and increased conversion to ketone. We next examined how the conversion percentage was related to the mol percent of catalyst used in the reactions for Au NP of size 12 nm (Table 2) with a constant reaction time of 18 h. We did not observe significant changes in terms of total conversion. However, we observed that increasing the amount of catalyst increased slightly the amount of conversion to ketone product. Our group had reported that silver(I) complexes containing a tripodal bis(imidazole) thioether ligand (10 mol%) could perform this oxidative cleavage at 90 °C in toluene. We therefore examined next the catalytic activity of silver nanoparticles of 10 nm in size and the dependence of the catalytic reaction in terms of time and mol percent of the catalyst (Table 3). The results in Table 3 show that as the reaction time increases (entry 1 to entry 2), the conversion to ketone and the total conversion increases as well. However, as the mol% of the catalyst increases the overall percent of conversion doubles but the ketone and epoxide distributions do not change significantly.
We therefore demonstrated that both Au and Ag water suspendable nanoparticles are able to catalyze the oxidation of 1,1-diphenylethylene affording the product of oxidative cleavage benzophenone as the major product. It should be noted that in the report of Shi and co-workers a mechanism involving the first step of epoxidation and a subsequent oxidation to produce carbonyl products was discarded since they could not detect epoxidation products or perform further oxidations on specific epoxides. [20] Gold NP of sizes 10 and 12 nm (1 mol%) are able to oxidize 1,1-diphenylethylene in 18 h affording conversions of 40–90% depending on the time. Silver nanoparticles catalyze this oxidative cleavage but the amount required for a conversion of 90% (18 h of reaction) is higher (5 mol%). While trying to recycle these nanoparticles and run subsequent reactions we observed a dramatic decrease in the percent total conversion. This is due to the decomposition and aggregation of the water-suspendable nanoparticles in solution under the reaction conditions. We decided to support these nanoparticles on a solid in order to improve their catalytic activity and their recyclability. Since the nanoparticles used in this study contain tannic acid/citrate as surface groups, a procedure was developed to displace these groups and coordinate the metal NP to the functional groups on the solid support. The surface of silica materials can be functionalized by various methods, e.g. by reaction of the silanol groups on the silica surface with alkoxysilanes [23]. Functionalizing the silica with aminopropyltrialkoxysilanes proved efficient for Au nanoparticles. The primary amine in the APTMS (Fig. 1) is known to possess a high affinity for Au nanoparticles and to prevent nanoparticles from aggregating. The amine groups give rise to a positive charge on the surface of the silica while the gold and silver nanoparticles with either a citrate or a tannic acid surface inherently have a negative charge, thus bringing the particles in close contact to the silica surface. Amines are better ligands for noble metal surfaces than the alcohol or acid groups in the citrate or tannic acid, thus the incorporation of amines was used to bind the metal nanoparticles to the silica surface. We tested supported Au NP in the oxidation reaction and observed that when the mol% of catalyst was increased from 1 to 1.5, the total conversion increased and the ratio to the product of oxidative cleavage increased (see entry 2). Also, when comparing entry 1 in Table 4 to entry 1 in Table 1, we can observe that the activity of 10 nm Au nanoparticles improves when the Au NP were supported on silica functionalized with APTMS. In the same manner, Au NP of size 12 mm afforded the same catalytic activity while supported on silica functionalized with APTMS affording high% of total conversion and a 97% conversion to benzophenone. In general the percentage of epoxide produced was considerably lower than with unsupported Au or Ag NP (never more than 10%). The 10 nm Au nanoparticles from entry 2, Table 4 (before and after catalysis) were sent for characterization by TEM imaging (Fig. 2) and we could observe that there were not considerable differences in the samples in terms of NP size. We examined next the recyclability of the supported Au nanoparticles (10 mm size). The results are collected in Table 5. In the recyclability tests we used 1.5 mol% of 10 nm Au nanoparticles that were supported on 0.25 g silica functionalized with APTMS (entry 1). After the reaction the nanoparticles were
O
O Si O
Si
NH2 O
3 aminopropyltrimethoxysilane (APTMS)
O
SH O
3 mercapto propyltrimethoxysilane (MPTMS)
Fig. 1. Two silanes (MPTMS and APTMS) used in this study to functionalize silica to make solid supports.
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E. Fisher et al. / Polyhedron xxx (2016) xxx–xxx
A
B
Fig. 2. TEM images (40) of Au NP (10 mm) supported on APTMS functionalized-silica before (A) and after (B) the catalytic oxidation.
recovered and used for the subsequent reactions (entries 2 and 3). We kept the ratio of catalyst/alkene/co-oxidant constant taking into consideration the amount of catalysts recovered. The catalytic activity of the Au NP was preserved for three consecutive reactions. The total conversion and the conversion to ketone only decreased very slightly. However after the third run, the yields decreased dramatically and we did not study subsequent runs. This deactivation of the catalyst may be due to the well-known effect of lixiviation of gold (and other metals) from stabilized nanoparticles. This deactivation mechanism has been previously described for gold nanoparticles in oxidation reactions [24]. We also ran reactions with 10 nm Au nanoparticles supported on silica functionalized with APTMS, using hydrogen peroxide as the co-oxidant (see table in the SI) but the total conversion yields were low (ranging from 35% to 42%). While supporting silver nanoparticles on silica functionalized with MPTMS (Fig. 1) we noticed that the filtrates always displayed color and that after impregnation the supported nanoparticles did not show a uniform color indicating that the impregnation procedure had not proceeded in a successful way. We then functionalized the silica with mixtures 50% MPTMS/50% APTMS so that the silver nanoparticles would have reduced degradation and aggregation. We reasoned that the positive charge of the amine would attract the negative silver colloid which will subsequently be held in place by the thiol. After impregnation, the filtrate was clear, and the supported nanoparticles formed a powder of uniform color. We carried the catalytic reaction and in a similar manner to the supported Au NP systems, we were able to recycle them two times (Table 6) since after the third run the yield decreased dramatically (again, most plausibly due to lixiviation of silver). In conclusion, we have demonstrated that the oxidative cleavage of 1,1-diphenylethylene can be performed catalytically by water-soluble Au (10 or 12 nm) or Ag (12 nm) NP in H2O at 90 °C and with TBHP as oxidant. The mol% of catalyst used is as low as 1 mol% for Au unsupported NP or 5 mol% for Ag unsupported NP (18 h reaction time). Supporting these NP in functionalized silica
(either with APTMS for Au or with a 50:50 ratio mixture of MPTMS and APTMS for Ag) allowed for the formation of more stable systems. The supported NP could be used in the oxidative cleavage under the same reaction conditions with almost 100% conversion to benzophenone (1.5 mol% of metal NP) and could be recycled and reused for another two runs without major loss of catalytic activity. These preliminary results suggest that some of the oxidation processes of organic compounds with gold and silver compounds may indeed be catalyzed by NP generated in situ under reaction conditions. We will further explore the reaction mechanisms and the scope of the oxidative cleavage with these and similar NP systems in the future. Acknowledgement The authors are grateful for financial support from the PSCCUNY Research Awards, grants PSCREG-39-68 and PSCOOC-38-78 (M.C.). M.C. is grateful to Mr. Leonard Tow and the Tow Foundation for a Tow Professorship (2015–2017). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.poly.2016.06.012. References [1] W.N. Oloo, Q. Lawrence Jr., Acc. Chem. Res. 48 (2015) 2612. [2] A. Villa, N. Dimitratos, C.E. Chan-Thaw, C. Hammond, L. Prati, G.J. Hutchings, Acc. Chem. Res. 2015 (48) (2015) 1403. [3] A. Parent, R. Alexander, K. Sakai, ChemSusChem 7 (2014) 2070. [4] H. Miyamura, S. Kobayashi, Acc. Chem. Res. 47 (2014) 1054. [5] Md.E. Ali, Md.M. Rahman, S.M. Sarkar, S.B. Abd Hamid, J. Nanomat. (2014). Article ID 192038, http://dx.doi.org/10.1155/2014/192038. [6] M.G. Clerici, O.A. Kholdeeva, Liquid Phase Oxidation via Heterogeneous Catalysis. Organic Synthesis and Industrial Applications, John Wiley & Sons, Hoboken, 2013. [7] L. Wu, Y. Zhang, Y.-G. Ji, Curr. Org. Chem. 17 (2013) 1288.
Please cite this article in press as: E. Fisher et al., Polyhedron (2016), http://dx.doi.org/10.1016/j.poly.2016.06.012
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[17] A.S.K. Hashmi, F.D. Toste, Modern Gold Catalyzed Synthesis, Wiley-VCH Verlag & Co. KGaA, Weinheim, 2012. Chapter 10. [18] F.D. Toste, V. Michelet, Gold Catalysis: An Homogeneous Approach, Catalytic Science Series 13, Imperial College Press, London, 2014. Chapter 2. [19] M. Liu, H. Wang, H. Zeng, C.-J. Li, Sci. Adv. 1 (2015) e1500020. [20] D. Xing, B. Guan, G. Cai, Z. Fang, L. Yang, Z. Shi, Org. Lett. 8 (2006) 69. [21] F. Liu, R. Anis, E. Hwang, R. Ovalle, A. Varela-Ramírez, R.J. Aguilera, M. Contel, Molecules 16 (2011) 6701. [22] Y. Liu, F. Song, S. Guo, J. Organomet. Chem. 694 (2009) 502. [23] S.L. Westcott, S.J. Oldenburg, T.R. Lee, N.J. Halas, Langmuir 14 (1998) 5396. [24] J. Restrepo, R. Pocar, P. Lozano, M.I. Burguete, E. Garcia-Verdugo, S.V. Luis, ACS Catal. 5 (2015) 4743.
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Water-compatible gold and silver nanoparticles as catalysts for the oxidation of alkenes Eugenia Fisher a,1, Lea Kenisgberg a,1, Monica Carreira a, Jacob Fernández-Gallardo a, Richard Baldwin b,⇑, María Contel a,c,⇑ a b c
Department of Chemistry, Brooklyn College, The City University of New York, Brooklyn, NY 11210, United States NanoComposix INC, 4878 Ronson Court Suite K, San Diego, CA 92111, United States Chemistry PhD Program, The Graduate Center, The City University of New York, 365 Fifth Avenue, New York, NY 10016, United States
a r t i c l e
i n f o
Article history: Received 30 March 2016 Accepted 9 June 2016 Available online xxxx This paper is dedicated to Emeritus Professor Martin A. Bennett on the occasion of his 80th birthday. Keywords: Oxidative cleavage Alkenes Gold Silver Nanoparticles
a b s t r a c t Water-compatible Ag/Au nanoparticles (NP) catalyze the liquid-phase oxidation of 1,1-diphenylethylene to afford benzophenone, the product of oxidative cleavage, and 1,1-diphenylepoxide. The metal nanoparticles are surface functionalized with tannic acid and/or citrate and have diameters of 10–12 nm. By proper selection of the reaction conditions, percent conversions/distributions of 85–100% of 1,1-diphenylethylene can be achieved at 90 °C using water as a solvent and TBHP (tert-butylhydroperoxide) as the co-oxidant. In addition, these nanoparticles have been further supported on functionalized silica to improve their stability and recyclability in this oxidation process. Conversions of up to 100% to benzophenone could be obtained with 1.5 mol% Au or Ag nanoparticles (10 nm) supported on functionalized silica. The supported NP could be recycled and re-used without significant catalytic activity loss for three runs. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction During the last two decades there has been much interest in finding new and less toxic metal-based oxidation catalysts, with reports that have been compiled in a number of recent reviews [1–10]. Research has been focused on developing highly selective and/or recoverable catalysts which employ oxygen or air as oxidants. While not all new oxidation catalysts have been able to fulfill all the above features, gold compounds and nanoparticles have been proven as an excellent alternative to other classical metal-based oxidation catalysts, and have been successfully used in heterogeneous [11–16] and to a lesser extent in homogeneous oxidations [13–18]. In contrast, oxidations of alkenes with silver compounds in homogeneous conditions have been less explored [12,19]. One relevant oxidation reaction is the oxidative cleavage of C@C double bonds to carbonyl compounds (usually achieved by stoichiometric oxidation or ozonation). In 2006, Shi and
⇑ Corresponding authors at: Department of Chemistry, Brooklyn College, The City University of New York, Brooklyn, NY 11210, United States (M. Contel). E-mail addresses: [email protected] (R. Baldwin), [email protected] (M. Contel). 1 These authors contributed equally to the work.
co-workers reported the oxidative cleavage of alkenes to form ketones and aldehydes with AuCl/neocuproine in water at 90 °C with TBHP (tert-butylhydroperoxide) as the oxidant (Scheme 1) [20]. Our group reported that gold(I) and especially silver(I) complexes containing a tripodal bis(imidazole) thioether ligand, could perform this oxidative cleavage in toluene [21]. Liu and co-workers synthesized fully substituted butenolides via a cascade reaction for the cleavage of a carbon–carbon triple bond involving: (a) gold-catalyzed addition of an oxygen nucleophile to the triple bonds in (Z)-enynols and, (b) subsequent oxygen-assisted oxidative cleavage of the exo-enolic double bond [22]. In the cases of the oxidative cleavage of alkenes with the Au(I) and Ag(I) compounds [20,21], we wondered if the active catalyst could be nanoparticles generated under reaction conditions and we set to explore the catalytic activity of Au and Ag NP in H2O. We report here on the use of commercially available water-compatible Ag/Au nanoparticles for the liquid-phase oxidation of 1,1-diphenylethylene in H2O at 90 °C with TBHP as oxidant. The oxidation products are benzophenone, product of the oxidative cleavage, and 1,1-diphenylepoxide (Scheme 2). We also describe catalytic studies with the nanoparticles further supported on functionalized silica in order to improve the recyclability of the systems.
http://dx.doi.org/10.1016/j.poly.2016.06.012 0277-5387/Ó 2016 Elsevier Ltd. All rights reserved.
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E. Fisher et al. / Polyhedron xxx (2016) xxx–xxx
N
R1 R2
H R3
AV400 (1H NMR at 400 MHz, 13C{1H} NMR at 100.6 MHz). Chemical shifts (d) are given in ppm using CDCl3 as the solvent, and spectra was recorded at 25 °C. 1H and 13C NMR resonances were measured relative to solvent peaks with tetramethylsilane = 0 ppm. Coupling constants J are given in hertz. Nanoparticles were characterized by TEM and UV–Vis spectroscopy as well as ICP-MS for metal concentration by nanoComposix.
N
(5 mol %) AuCl (5 mol %)
R1
o
TBHP, H 2O, 90 C
O
R2
+
H O R3
R 1 = aryl, alkyl R 2 = aryl, alkyl, H R 3 = H or CH 3
Scheme 1. Oxidative cleavage of alkenes with a gold(I) catalyst [20].
2. Experimental 2.1. General procedure The unsupported water suspended metal nanoparticles employed are surface functionalized with tannic acid and/or citrate in the case of gold and citrate surface groups in the case of silver and have dimensions of 10–12 nm. The nanoparticles used are available commercially from nanoComposix (products AuB10, AuB12 and AgCB10 for gold at 10 and 12 nm and silver respectively). 1,1-diphenylethylene was purchased from Sigma Aldrich. All purchased reactants were used without further purification. Reaction and work-up solvents were purchased from Fisher Scientific (ACS Grade). Deuterated solvents were purchased from Cambridge Isotope Laboratories, Inc., were kept over molecular sieves (3 Å, beads, 4–8 mesh). NMR spectra were recorded using a Bruker
2.2. Catalytic oxidation procedure for unsupported nanoparticles To 1.5 ml of deionized H2O, TBHP (2.1 equiv.) and the alkene (1,1-diphenylethylene) were added at room temperature. Solutions of metal nanoparticles in water were subsequently added (different concentrations, see Tables 1–4 for mol per cent values). The reaction mixture was stirred under reflux at 90 °C (different times were studied, see Tables 1–4). After the reaction finished and the flask was cooled to room temperature, the aqueous layer was extracted with diethyl ether (3 20 mL). The combined organic layers were washed with a saturated sodium bisulfite solution (3 10 mL) followed by deionized H2O (3 20 mL), and dried over MgSO4 overnight. The solution was then filtered and concentrated under vacuum. The product obtained was analyzed by 1H NMR spectroscopy. Ketone and epoxide (Scheme 2) were observed in different percent conversions depending on the reaction conditions (Tables 1–4). Diagnostic signals in 1H NMR spectra (CDCl3): 1,1-diphenylethylene 5.5 ppm (2H, s, CH2), 7.6–7.3 ppm (10H, m, 6CH); benzophenone 7.8 ppm (4H, m, 4CH); 1,1-diphenylepoxide 4.84 ppm (2H, s, CH2).
Scheme 2. Oxidative cleavage of 1,1-diphenylethylene catalyzed by Au or Ag NP.
Table 1 Oxidations with 1 mol% of gold nanoparticles of size 10 nm (entries 1–4) and 12 nm (entries 5–8).a Entry
1 2 3 4 5 6 7 8
Type of NP
10 nm 10 nm 10 nm 10 nm 12 nm 12 nm 12 nm 12 nm
Au Au Au Au Au Au Au Au
[cat] mol%
1 1 1 1 1 1 1 1
Time (h)
18 5 2 1 18 5 2 1
Conversion (%)
Std dev
To ketone
To epoxide
Total
85 85 67 65 79 78 66 62
15 15 33 35 21 22 34 38
86 69 71 39 90 87 63 54
4 0 4 12 3 0 2 2
a Reaction conditions: 0.5 mmol diphenylethylene, 1.05 mmol TBHP, H2O, 90 °C. Conversion determined by 1H NMR spectroscopy. Values given are an average of two reactions. The standard deviation is given for the conversion of ketone.
Table 2 Oxidations with gold nanoparticles of size 12 nm (18 h).a Entry
1 1 2
Type of NP
12 nm Au 12 nm Au 12 nm Au
[cat] mol%
1 2 5
Time (h)
18 18 18
Conversion (%)
Std dev
To ketone
To epoxide
Total
79 83 88
21 17 12
90 70 84
3 3 0
a Reaction conditions: 0.5 mmol diphenylethylene, 1.05 mmol TBHP, H2O, 90 °C. Conversion determined by 1H NMR spectroscopy. Values given are an average of two reactions. The standard deviation is given for the conversion of ketone.
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E. Fisher et al. / Polyhedron xxx (2016) xxx–xxx Table 3 Oxidations with silver nanoparticles of size 10 nm.a Entry
Type of NP
[cat] (mol%)
Time (h)
Conversion (%)
Std dev
To ketone
To epoxide
Total
1 2 3
10 nm Ag 10 nm Ag 10 nm Ag
1 1 5
5 18 18
78 89 86
22 11 14
47 45 92
0 3 0
a Reaction conditions: 0.5 mmol diphenylethylene, 1.05 mmol TBHP, H2O, 90 C°. Conversion determined by 1H NMR spectroscopy. Values given are an average of two reactions. The standard deviation is given for the conversion of ketone.
Table 4 Reactions of gold nanoparticles (size 10 nm, entries 1–3 and size 12 nm entry 4) supported on APTMS functionalized silicaa. Entry
1 2 3 4
Type of NP
10 nm 10 nm 10 nm 12 nm
Au Au Au Au
Functionalized silica (g)
[cat] mol%
0.25 0.25 0.375 0.25 g
Time (h)
1 1.5 1.5 1.5
Conversion (%)
18 18 18 18
Std dev
To ketone
To epoxide
Total
93 100 90 97
7 0 10 3
98 100 95 97
7 0 1 3
a Reaction conditions: 0.5 mmol diphenylethylene, 1.05 mmol TBHP, H2O, 90 °C. Conversion determined by 1H NMR spectroscopy. Values given are an average of two reactions. The standard deviation is given for the conversion of ketone.
modified. Nanoparticles were provided at a concentration of 1 mg/mL. After impregnation, the silica supported nanoparticles were collected by filtration. The supported Au nanoparticles formed a light purple powder while the supported Ag nanoparticles formed a light brown powder. The filtrate was a clear solution in both cases.
2.3. Functionalization of silica Chromatographic silica was mixed with silane (Aminopropyltrimethoxysilane APTMS or Mercaptopropyltrimethoxysilane MPTMS) in ethanol. 19 mmol of silane per 1 g of silica were used. The solution was stirred for 10 min in the case of APTMS and for 2 h in the case of MPTMS, and then washed 3 times with 10 mL of ethanol. The functionalized-silica was filtered and dried under vacuum. The ninhydrin test was performed to check if the silica was functionalized with the amine group (APTMS). A solution of 0.2 g of ninhydrin and 50 mL of H2O was prepared. One mL of the ninhydrin solution in water was added to 2 mg of the functionalized silica. The mixture was boiled for 15–20 s. A blue/violet color indicated that surface was successfully functionalized.
2.5. Catalytic oxidation procedure for supported nanoparticles and study of the recyclability To the supported metal NP, 1.5 ml of deionized H2O, the starting material (1,1-diphenylethylene), and TBHP (2.1 equiv.) were added at room temperature. The reaction mixture was stirred under reflux at 90 °C for the times specified in Tables 4–6. After the reaction finished and the flask was cooled to room temperature, 4 mL of ether was added to the crude reaction. The mixture was stirred for a few minutes, and the liquid layer was decanted by pipette and run through a fine filter. This procedure was repeated 4 times to separate the products of the reaction. Deionized H2O (3 5 mL) was subsequently added to the reaction mixture and the solid
2.4. Procedure for supporting the nanoparticles Ag and Au nanoparticles in water were stirred at room temperature with functionalized silica overnight. The ratio of grams of functionalized silica to milliliters of nanoparticle solution was
Table 5 Oxidations using recycled supported gold nanoparticles (10 nm) supported on APTMS functionalized silicaa. Entry
Recycled NP?
Type of NP
[cat] mol%
Time (h)
Conversion (%)
std dev
To ketone
To epoxide
Total
1 2 3
No From entry 1 From entry 2
10 nm Au 10 nm Au 10 nm Au
1.5 1.5 1.5
18 18 18
100 98 97
0 0 3
99 98 99
1 2 4
a Reaction conditions: 0.5 mmol diphenylethylene, 1.05 mmol TBHP, H2O, 90 °C. Conversion determined by 1H NMR spectroscopy. Values given are an average of two reactions. The standard deviation is given for the conversion of ketone.
Table 6 Reactions of silver nanoparticles (size 10 nm, 1.5 mol%) supported on silica functionalized with MPTMS and APTMSa. Entry
Recycled NP?
Type of NP
[cat] mol%
Time (h)
1 2 3
No From entry 1 From entry 2
10 nm Ag 10 nm Ag 10 nm Ag
1.5 1.5 1.5
18 18 18
Conversion (%)
Std dev
To ketone
To epoxide
Total
100 100 100
0 0 0
91 87 75
5 2 4
a Reaction conditions: 0.5 mmol diphenylethylene, 1.05 mmol TBHP, H2O, 90 °C. Conversion determined by 1H NMR spectroscopy. Values given are an average of two reactions. The standard deviation is given for the conversion of ketone.
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E. Fisher et al. / Polyhedron xxx (2016) xxx–xxx
supported nanoparticles were filtered out. The reaction flask was washed with diethyl ether (10 mL) to dissolve any remaining product. The solid supported nanoparticles were then washed with deionized H2O (1 10 mL) and diethyl ether (3 10 mL). Following the filtration process, the supported nanoparticles were dried under vacuum for further use. The organic layer was extracted from the combined organic and aqueous filtrate with diethyl ether (3 20 mL). Following extraction, the organic layer was washed with a saturated sodium bisulfite solution (3 10 mL), followed by deionized H2O (2 10 mL). The organic layer was then dried over MgSO4 overnight. The solution was filtered and poured into a pre-weighted round bottom flask and dried under high vacuum. The product obtained was weighed and analyzed by 1H NMR spectroscopy. Ketone and epoxide (Scheme 2) were observed in different percent conversions depending on the reaction conditions (Tables 4–6). 2.6. Transmission electron microscopy characterization TEM images were taken on a JEOL 1010 electron microscope. Samples of the catalysts were suspended in a 2:1 mixture of isopropanol and DI water. 5 lL of this solution was dropped onto the formvar side of a carbon coated formvar 300 mesh copper TEM grid. The sample was then dried under vacuum and imaged at 100 kV.
3. Results and discussion We studied the use of commercially available water-stable Ag/Au nanoparticles for the liquid-phase oxidation of 1,1-diphenylethylene in H2O at 90 °C with TBHP as oxidant (2.1 equivalents). The oxidation products are benzophenone, product of the oxidative cleavage, and 1,1-diphenylepoxide (Scheme 2). We started by investigating the effect of time in the catalytic activity of 1 mol% of unsupported 10 nm and 12 nm gold nanoparticles (Table 1). From the results depicted in Table 1 we infer that the total conversion and the percent conversion to ketone are directly proportional to the reaction time. Increasing the reaction time resulted in an increased total conversion and increased conversion to ketone. We next examined how the conversion percentage was related to the mol percent of catalyst used in the reactions for Au NP of size 12 nm (Table 2) with a constant reaction time of 18 h. We did not observe significant changes in terms of total conversion. However, we observed that increasing the amount of catalyst increased slightly the amount of conversion to ketone product. Our group had reported that silver(I) complexes containing a tripodal bis(imidazole) thioether ligand (10 mol%) could perform this oxidative cleavage at 90 °C in toluene. We therefore examined next the catalytic activity of silver nanoparticles of 10 nm in size and the dependence of the catalytic reaction in terms of time and mol percent of the catalyst (Table 3). The results in Table 3 show that as the reaction time increases (entry 1 to entry 2), the conversion to ketone and the total conversion increases as well. However, as the mol% of the catalyst increases the overall percent of conversion doubles but the ketone and epoxide distributions do not change significantly.
We therefore demonstrated that both Au and Ag water suspendable nanoparticles are able to catalyze the oxidation of 1,1-diphenylethylene affording the product of oxidative cleavage benzophenone as the major product. It should be noted that in the report of Shi and co-workers a mechanism involving the first step of epoxidation and a subsequent oxidation to produce carbonyl products was discarded since they could not detect epoxidation products or perform further oxidations on specific epoxides. [20] Gold NP of sizes 10 and 12 nm (1 mol%) are able to oxidize 1,1-diphenylethylene in 18 h affording conversions of 40–90% depending on the time. Silver nanoparticles catalyze this oxidative cleavage but the amount required for a conversion of 90% (18 h of reaction) is higher (5 mol%). While trying to recycle these nanoparticles and run subsequent reactions we observed a dramatic decrease in the percent total conversion. This is due to the decomposition and aggregation of the water-suspendable nanoparticles in solution under the reaction conditions. We decided to support these nanoparticles on a solid in order to improve their catalytic activity and their recyclability. Since the nanoparticles used in this study contain tannic acid/citrate as surface groups, a procedure was developed to displace these groups and coordinate the metal NP to the functional groups on the solid support. The surface of silica materials can be functionalized by various methods, e.g. by reaction of the silanol groups on the silica surface with alkoxysilanes [23]. Functionalizing the silica with aminopropyltrialkoxysilanes proved efficient for Au nanoparticles. The primary amine in the APTMS (Fig. 1) is known to possess a high affinity for Au nanoparticles and to prevent nanoparticles from aggregating. The amine groups give rise to a positive charge on the surface of the silica while the gold and silver nanoparticles with either a citrate or a tannic acid surface inherently have a negative charge, thus bringing the particles in close contact to the silica surface. Amines are better ligands for noble metal surfaces than the alcohol or acid groups in the citrate or tannic acid, thus the incorporation of amines was used to bind the metal nanoparticles to the silica surface. We tested supported Au NP in the oxidation reaction and observed that when the mol% of catalyst was increased from 1 to 1.5, the total conversion increased and the ratio to the product of oxidative cleavage increased (see entry 2). Also, when comparing entry 1 in Table 4 to entry 1 in Table 1, we can observe that the activity of 10 nm Au nanoparticles improves when the Au NP were supported on silica functionalized with APTMS. In the same manner, Au NP of size 12 mm afforded the same catalytic activity while supported on silica functionalized with APTMS affording high% of total conversion and a 97% conversion to benzophenone. In general the percentage of epoxide produced was considerably lower than with unsupported Au or Ag NP (never more than 10%). The 10 nm Au nanoparticles from entry 2, Table 4 (before and after catalysis) were sent for characterization by TEM imaging (Fig. 2) and we could observe that there were not considerable differences in the samples in terms of NP size. We examined next the recyclability of the supported Au nanoparticles (10 mm size). The results are collected in Table 5. In the recyclability tests we used 1.5 mol% of 10 nm Au nanoparticles that were supported on 0.25 g silica functionalized with APTMS (entry 1). After the reaction the nanoparticles were
O
O Si O
Si
NH2 O
3 aminopropyltrimethoxysilane (APTMS)
O
SH O
3 mercapto propyltrimethoxysilane (MPTMS)
Fig. 1. Two silanes (MPTMS and APTMS) used in this study to functionalize silica to make solid supports.
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E. Fisher et al. / Polyhedron xxx (2016) xxx–xxx
A
B
Fig. 2. TEM images (40) of Au NP (10 mm) supported on APTMS functionalized-silica before (A) and after (B) the catalytic oxidation.
recovered and used for the subsequent reactions (entries 2 and 3). We kept the ratio of catalyst/alkene/co-oxidant constant taking into consideration the amount of catalysts recovered. The catalytic activity of the Au NP was preserved for three consecutive reactions. The total conversion and the conversion to ketone only decreased very slightly. However after the third run, the yields decreased dramatically and we did not study subsequent runs. This deactivation of the catalyst may be due to the well-known effect of lixiviation of gold (and other metals) from stabilized nanoparticles. This deactivation mechanism has been previously described for gold nanoparticles in oxidation reactions [24]. We also ran reactions with 10 nm Au nanoparticles supported on silica functionalized with APTMS, using hydrogen peroxide as the co-oxidant (see table in the SI) but the total conversion yields were low (ranging from 35% to 42%). While supporting silver nanoparticles on silica functionalized with MPTMS (Fig. 1) we noticed that the filtrates always displayed color and that after impregnation the supported nanoparticles did not show a uniform color indicating that the impregnation procedure had not proceeded in a successful way. We then functionalized the silica with mixtures 50% MPTMS/50% APTMS so that the silver nanoparticles would have reduced degradation and aggregation. We reasoned that the positive charge of the amine would attract the negative silver colloid which will subsequently be held in place by the thiol. After impregnation, the filtrate was clear, and the supported nanoparticles formed a powder of uniform color. We carried the catalytic reaction and in a similar manner to the supported Au NP systems, we were able to recycle them two times (Table 6) since after the third run the yield decreased dramatically (again, most plausibly due to lixiviation of silver). In conclusion, we have demonstrated that the oxidative cleavage of 1,1-diphenylethylene can be performed catalytically by water-soluble Au (10 or 12 nm) or Ag (12 nm) NP in H2O at 90 °C and with TBHP as oxidant. The mol% of catalyst used is as low as 1 mol% for Au unsupported NP or 5 mol% for Ag unsupported NP (18 h reaction time). Supporting these NP in functionalized silica
(either with APTMS for Au or with a 50:50 ratio mixture of MPTMS and APTMS for Ag) allowed for the formation of more stable systems. The supported NP could be used in the oxidative cleavage under the same reaction conditions with almost 100% conversion to benzophenone (1.5 mol% of metal NP) and could be recycled and reused for another two runs without major loss of catalytic activity. These preliminary results suggest that some of the oxidation processes of organic compounds with gold and silver compounds may indeed be catalyzed by NP generated in situ under reaction conditions. We will further explore the reaction mechanisms and the scope of the oxidative cleavage with these and similar NP systems in the future. Acknowledgement The authors are grateful for financial support from the PSCCUNY Research Awards, grants PSCREG-39-68 and PSCOOC-38-78 (M.C.). M.C. is grateful to Mr. Leonard Tow and the Tow Foundation for a Tow Professorship (2015–2017). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.poly.2016.06.012. References [1] W.N. Oloo, Q. Lawrence Jr., Acc. Chem. Res. 48 (2015) 2612. [2] A. Villa, N. Dimitratos, C.E. Chan-Thaw, C. Hammond, L. Prati, G.J. Hutchings, Acc. Chem. Res. 2015 (48) (2015) 1403. [3] A. Parent, R. Alexander, K. Sakai, ChemSusChem 7 (2014) 2070. [4] H. Miyamura, S. Kobayashi, Acc. Chem. Res. 47 (2014) 1054. [5] Md.E. Ali, Md.M. Rahman, S.M. Sarkar, S.B. Abd Hamid, J. Nanomat. (2014). Article ID 192038, http://dx.doi.org/10.1155/2014/192038. [6] M.G. Clerici, O.A. Kholdeeva, Liquid Phase Oxidation via Heterogeneous Catalysis. Organic Synthesis and Industrial Applications, John Wiley & Sons, Hoboken, 2013. [7] L. Wu, Y. Zhang, Y.-G. Ji, Curr. Org. Chem. 17 (2013) 1288.
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