Supported Gold Nanoparticles as Efficient Catalysts in the Solventless

Article pubs.acs.org/JPCC

Supported Gold Nanoparticles as Efficient Catalysts in the Solventless Plasmon Mediated Oxidation of sec-Phenethyl and Benzyl Alcohol Geniece L. Hallett-Tapley,† M. Jazmín Silvero,† Carlos J. Bueno-Alejo,† María González-Béjar,†,§ Christopher D. McTiernan,† Michel Grenier,† José Carlos Netto-Ferreira,*,‡ and Juan C. Scaiano*,† †

Department of Chemistry and Centre for Catalysis Research and Innovation, University of Ottawa, 10 Marie Curie, Ottawa K1N 6N5, Canada ‡ Departamento de Química, Universidade Federal Rural do Rio de Janeiro, Seropédica, 23851-970, Rio de Janeiro, Brazil S Supporting Information *

ABSTRACT: Surface plasmon excitation of supported gold nanoparticles in the presence of H2O2 leads to selective oxidation of secphenethyl and benzyl alcohols to the carbonyl products acetophenone and benzaldehyde, respectively, in the absence of additional solvents. Light-emitting diodes are compared with microwave irradiation as excitation sources. Hydrotalcite, ZnO, and Al2O3 have been chosen as the solid supports. The overall efficiency of the alcohol oxidation was found to be largely dependent on the nature of the support, with hydrotalcite-derived nanocomposites giving the highest conversions to product, yielding 90% acetophenone after 40 min of LED irradiation. The mechanism for plasmon-mediated alcohol oxidation is believed to involve a significant contribution from the support itself, with adsorption of the alcohol substrate and progression of the oxidation reaction being largely facilitated by the basicity of the support used.

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INTRODUCTION The development and implementation of reusable catalysts in organic chemistry is becoming ever more important with increasing environmental awareness and the current focus on green chemistry initiatives.1 Of particular importance is the use of noble metal catalysts in the oxidation of alcohols to their carbonyl products due to the significance of this fundamental reaction in both the cosmetic and pharmaceutical industries.2−6 Typical procedures require the use of both toxic and environmentally unfriendly chemical oxidants, such as chromates or permanganates, creating large quantities of toxic byproducts.5−7 Current interest is focused on the use of metal nanoparticles,8 specifically gold nanoparticles (AuNP), as a means of carrying out alcohol oxidation reactions under ambient conditions and in the absence of harsh additives. In particular, the development of heterogeneous-based catalysts, specifically those where AuNP can be successfully supported on an array of oxides, offers the advantage of simple catalyst removal and recovery, as well as catalyst recyclability, both highly desirable in the field of heterogeneous catalysis.9,10 Current interest in the scientific community has shifted to focus toward the development of green chemistry initiatives,1,11 particularly in the area of photocatalysis.12,13 Visible light energy, easily obtained from solar illumination,1,14,15 in conjunction with metal nanoparticles,13,15−18 is actively being pursued as a sustainable and renewable method of catalysis. © 2013 American Chemical Society

Reactions using photoinduced methods and supported AuNP are promising due to the strong absorption of the AuNP surface plasmon band (SPB) within the visible region of the electromagnetic spectrum (λmax ∼ 530 nm),18,19 in addition to the robustness and recyclability associated with heterogeneous nanocomposites. Excitation of AuNP by visible light induces an oscillating dipole of the nanomaterial surface electrons.18,20−22 This transient polarization of the nanoparticle surface electrons can be exploited in various ways, including photothermally20,22−26 and photochemically,27 as summarized in Figure 1. A recent publication from our group has estimated that organic molecules within nanometer distances from the NP surface can be subjected to temperatures of ∼500 °C over a submicrosecond time scale following AuNP plasmon excitation.24 In addition, work carried out by Branda and co-workers has exploited the thermal energy released via AuNP visible light illumination to examine tethered DNA denaturation26 as well as the photothermal release of biologically relevant probe dyes.23 Commonly, AuNP plasmon excitation is believed to have the catalytic benefit of performing high temperature reactions at Received: November 8, 2012 Revised: May 20, 2013 Published: May 29, 2013 12279

dx.doi.org/10.1021/jp311069v | J. Phys. Chem. C 2013, 117, 12279−12288

The Journal of Physical Chemistry C

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proton coupled electron transfer (PCeT).32 Recent work in our group has also suggested that laser-ablated AuNP can be generated and also successfully adsorbed, in situ, onto TiO2 P25.33 In this contribution, a series of supported AuNP composites synthesized using two methods already published will be tested as potential heterogeneous photocatalysts toward the plasmon-mediated solventless oxidation of sec-phenethyl and benzyl alcohols in the presence of H2O2.

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EXPERIMENTAL SECTION Reagents. Gold(III) tetrachloroauric acid hydrate (HAuCl4·3H2O), benzyl alcohol, hydrotalcite (HT), ZnO, 50% H2O2, and HPLC grade CH3CN were purchased from Sigma-Aldrich and used as received. sec-Phenethyl alcohol was purchased from Fisher Chemicals and required further purification before use. γ-Al2O3 was purchased from BDH Chemicals. Nano-Al2O3 (n-Al2O3) was purchased from Strem Chemicals and used as received. Irgacure 2929 (I-2959) was a gift from BASF Chemicals and was recrystallized twice from ethyl acetate prior to use. AuNP Composite Preparation: Method A. AuNP supported on HT, γ-Al2O3, and ZnO were prepared using the previously published dry, photochemical method.9 All nanocomposites consisted of a 1% nominal Au loading. Irradiation times were 8 h for Au on HT and 12 h for Au on γ-Al2O3 and ZnO. Following irradiation, the Au composites were washed for 48−72 h with CH3CN using Soxhlet extraction to remove any unreacted gold precursor and organic photoproducts from the surface of the catalyst prior to use. AuNP Composite Preparation: Method B. The method used for the preparation AuNP on HT and nanometer size Al2O3 (n-Al2O3) at 1% nominal loading has been previously discussed.33 20 mg of the solid support and 1 mL of Milli-Q water were placed in a 20 mL glass vial and sonicated for 1 min prior to use. Next, 3 mL of polymorphic AuNP was subjected to 532 nm laser drop ablation and collected in the support/ water slurry with continuous stirring. The suspension was left to separate for 2−3 h, and the supernatant was discarded. The resulting solid was washed with 4 mL of water and centrifuged at 3600 rpm for 30 min. This process was repeated three times, and the resultant solid was submitted to lyophilization for 14 h in a Labconco Freezone 4.5 lyophilizer. TEM and spectroscopic characterization of these materials can be found in the Supporting Information. Sample Preparation for 530 nm LED Plasmon Irradiation. The samples used for plasmon-mediated catalysis experiments containing Au supported on hydrotalcite and metal oxides were prepared using 25 mg of the desired AuNP composites (1% Au on HT, γ-Al2O3, n-Al2O3, or ZnO), 40 μL of sec-phenethyl alcohol, or 45 μL of benzyl alcohol and 100 μL of 50% H2O2. sec-Phenethyl alcohol was purified before use as previously described.27 Following excitation, the samples were diluted with 1 mL of HPLC grade CH3CN, placed in a 15 mL centrifuge tube, and centrifuged at 3500 rpm for 20 min to recover the supported Au catalyst. The remaining supernatant was filtered through a 45 μm PTFE syringe filter and analyzed by HPLC-PDA by diluting 100 μL of the solution with 100 μL of acetonitrile. HPLC results were extracted at λmax = 245 nm for sec-phenethyl alcohol and λmax = 250 nm for benzyl alcohol. Catalyst Recyclability Experiments. The supported AuNP composites were tested for potential reuse in the oxidation of sec-phenethyl alcohol. The reactions were scaled up to facilitate catalysis recovery and reuse. In brief, 160 μL of

Figure 1. Plausible reaction pathways to be considered following plasmon excitation of supported metallic nanoparticles. Commonly, plasmon irradiation and relaxation can result in high temperatures on the nanoparticle surface, which can induce chemical changes to surrounding organic molecules (Tc) or changes to the nanoparticle itself (Tn). Similarly, nanoparticle excitation may also result in various photocatalytic pathways, where the nanoparticle can act both as an electron donor (Et) or acceptor (Ht) via electron and hole transfer, respectively.

otherwise ambient temperatures and mild reaction conditions in the nanoenvironment surrounding the particle surface. The ability of supported metallic nanoparticles to also participate as an active redox catalyst is also an active area of research, particularly in the fields of photocatalysis. AuNP supported on semiconductor materials,12−14,28,29 such as TiO2, holds considerable promise. These composites tend to improve the flow of electrons within the material between the Au and TiO2, resulting in improved efficiency in photocatalytic reactions. Methyl viologen (MV) is a favorite probe for examining photoinduced electron transfer reactions, where donation of an electron from the NP surface results in the formation of the strongly absorbing MV radical cation, with characteristic blue color. This methodology has been used not only to probe the chemical dynamics at the semiconductor/ AuNP interface30 but also to examine the electron ejection abilities of AuNP stabilized in other organic media, such as cucurbiturils.31 Work from our group has shown that photochemically generated AuNP can be used to effectively photocatalyze the oxidation of benzylic alcohols in the presence of H2O2 using a variety of visible light excitation sources, such as lasers and light-emitting diodes (LEDs).27 This technique was found to give high conversion to the corresponding carbonyl products when light-emitting diodes were used as the photoexcitation source, providing a facile and economic way of initiating chemical reactions. Though this work proved effective at implementing a “greener” method of alcohol oxidation, recyclability of the catalyst (aqueous, colloidal AuNP) was unsuccessful. Supported AuNP may be a potential solution to this disadvantage. The photochemical approach to noble metal nanoparticle synthesis has recently been expanded in our research to include the dry, photochemical generation and adsorption of AuNP onto the surface of a variety of metal oxide supports.9 This result in itself was fascinating as it indicated that the ketyl radicals generated throughout by irradiation and required for the reduction of the gold precursor (HAuCl4) were sufficiently mobile in the solid state to successfully generate AuNP composites in the absence of coadded nanoparticle stabilizers. Further, finding a “home” for the proton produced in the reaction is essential, as this process is proposed to involve a 12280



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sec-phenethyl alcohol and 600 μL of 50% H2O2 were added to 100 mg of catalyst in a 2 mL Pyrex test tube and exposed to LED irradiation for 40 min. Following irradiation, the reaction mixture was diluted with 4 mL of CH3CN, transferred to a 15 mL centrifuge tube, and centrifuged at 3000 rpm for 20 min, and the supernatant was decanted. This procedure was repeated a total of four times to remove any adsorbed materials from the surface of the catalyst. After the final wash, the supernatant was decanted and the solid left to dry overnight. The support was then weighed and the reaction scaled down to contain the same ratio of sec-phenethyl alcohol/H2O2/support as described above, added to the same Pyrex test tube, and excited by LED irradiation for 40 min. HPLC analysis was as described for the LED sample preparation. Instrumentation. Irradiated samples were analyzed using a Waters Integrity HPLC in tandem with a reverse phase C18 Zorbax column using a 60:40 CH3CN/Milli-Q H2O eluent mixture and a 0.5 mL/min flow rate. In all experiments carried out for this work, the starting material, alcohols, H2O2, and the desired carbonyl products (acetophenone or benzaldehyde) were the only components detected in the reaction mixture. The light-emitting diode system (LED) consisted of four, 10 W 530 nm LedEngin LZ4-40G110 emitters and has been described in detail elsewhere.24,27 A solution of the desired alcohol, 50% H2O2, and supported AuNP were combined in a glass NMR tube, capped, and irradiated for varying amounts of time (0−40 min). The reaction work-up is explained above. The temperature within the LED reactor was monitored and did not exceed 30−35 °C. Conventional heating experiments were performed in a capped glass NMR tube for 40 min using sand bath heated to 40 °C on a conventional laboratory hot plate to mimic temperature conditions within the fan-cooled LED apparatus. Microwave irradiation was performed using a CEM Discover Laboratory Microwave. A solution of 200 μL of sec-phenethyl alcohol or 225 μL of benzyl alcohol, 500 μL of 50% H2O2, and 125 mg of supported AuNP were placed into a 10 mL thickwalled Pyrex microwave tube containing a magnetic stir bar and capped with a Teflon septa. The reaction was then microwaved for 1, 2.5, 5, and 10 min intervals at 60 °C for sec-phenethyl alcohol and 40 °C for benzyl alcohol, using a ramp time of 1 min for all trials. Following each time interval, an aliquot of the reaction mixture (140 μL for sec-phenethyl alcohol and 145 μL for benzyl alcohol) was taken, diluted with 1 mL of CH3CN, and centrifuged/analyzed as described above. Supported AuNP sizes were characterized using a JSM7500F field emission scanning electron microscope from Jeol Ltd. or a field emission JEM-2100F FTEM (average 50−150 measurements). In all cases, the supported nanoparticles were predominantly spherical, but more polydisperse than is characteristically observed in colloidal Au solutions. Diffuse reflectance spectra of the Au nanocomposites were recorded using a Cary-100 spectrometer using a diffuse reflectance accessory.

Scheme 1. Reaction Pathway for the Photoinitiated AuNPMediated Oxidation of Benzylic Alcohols

products is due to direct excitation and photocatalysis mediated by the AuNP composites. Five different AuNP supported materials were synthesized and tested in the oxidation of alcohols and are summarized in Table 1. Two separate preparation methods (see Experimental Table 1. Supported AuNP Nanocomposites Used in This Work support c

HT HT γ-Al2O3c n-Al2O3 ZnO

av NP sizea (nm)

PDIb

method

± ± ± ± ±

1.24 1.12 1.14 1.14 1.17

A: dry B: ablation A: dry B: ablation A: dry

21 9 30 11 29

10 3 11 4 12

a Standard deviation values were calculated from an average of 125− 150 particle measurements. bPDI = polydispersity index. For an explanation of calculation see Supporting Information. cValues taken from ref 9.

Section) were used in the making of these materials. The photochemical methodology9 has proven useful for the preparation of a variety of supported nanoparticle composites and, in particular, on bulk (micrometer sized) supports. Though this process has been used to successfully prepare supported AuNP, it has been ineffective toward preparing supported nanoparticles on nanoscale solid materials. Recently, our group has exploited another technique for supporting AuNP via laser ablation.33 This process has shown to be very useful toward supporting nanoparticles on nanoscale supports, a result that was unattainable using the dry photochemical method. As such both techniques have been incorporated within this study to demonstrate the versatility of both methodologies in the preparation of heterogeneous catalysts and to compare the ability of the corresponding materials to catalyze alcohol oxidations. Table 1 presents the size and polydispersity of the supported nanoparticles prepared for this work and illustrates considerable dependence on not only the support but also the method of preparation used. Each of the supported nanocomposites presented above was tested as potential photocatalysts toward the oxidation of secphenethyl and benzyl alcohols over 40 min. Following exposure to LED light, an immediate bubbling of the sample was observed as a result of the light-induced decomposition of the H2O2 initiator. This can be seen in Figure 2, where, following LED photoillumination, the sample immediately “jumps” in the reaction vessel due to the rapid decomposition of the peroxide substrate and resultant gas evolution. The conversions to acetophenone and benzaldehyde respectively are shown in Table 2 following 5 and 40 min of irradiation. This selection of data along with the graphical illustrations presented in Figure 3 (for sec-phenethyl alcohol and benzyl alcohol) shows the conversion to carbonyl products over 40 min for all the tested nanocomposites.

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RESULTS Light-Emitting Diode (LED) Photoexcitation. The plasmon-mediated oxidation of sec-phenethyl and benzyl alcohols (Scheme 1) was initially examined using 530 nm LEDs as the photoexcitation source, ensuring direct and selective excitation of the AuNP surface plasmon absorption and indicating that any conversion to the desired carbonyl 12281



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oxidation of both sec-phenethyl and benzyl alcohol after 40 min in the dark, though not as large as the values obtained with plasmon irradiation (Figure 3, vide inf ra). These results suggest that for heterogeneous catalyzed plasmon-mediated alcohol oxidations larger AuNP may be more suited for the process. Overall, for the five nanocomposites studied in this work, dark reactions conducted in the absence of AuNP (support only) conversions to the carbonyl products were very low, showing that AuNP mediate and are essential for the oxidation. Microwave Irradiation. Microwave irradiation was used to compare with photoexcitation techniques to establish if successful alcohol oxidation is not simply due to mere heating of the solvent medium. Because of the small volumes being employed in this work, longer microwave irradiation times and temperatures were avoided to limit loss of volume in the reaction medium. Figure 5 and Figure S4, as well as Table 3, summarize the percent conversions to acetophenone and benzaldehyde, respectively, over 10 min of microwave excitation. As is illustrated in the plots presented below, the presence of AuNP is clearly required to promote effective oxidation of sec-phenethyl alcohol, with the samples yielding larger quantities of carbonyl oxidation products with consecutive irradiation time in the presence of gold. Moreover, in the absence of AuNP, minimal amounts of the carbonyl oxidation products were observed. AuNP have been long known to be efficient absorbers of microwave radiation,34 attributed to the passive heating mechanism required to convert alcohol to carbonyl products under these reaction conditions. The low yields of acetophenone and benzaldehyde in the absence of Au further corroborate that simple heating of the reaction medium is not efficient enough to promote alcohol oxidations in the presence of H2O2 but more so, that AuNP plasmon excitation can be used as a means to rapidly induce this reaction and obtain comparable yields to the more commonly used chemical methods. Conventional Heating Experiments. Catalytic trials were also carried out in the absence of support or catalyst using both LED irradiation and conventional bench heating (at 40 °C) to complement those run in the absence of AuNP (support only). Such experiments were carried out to illustrate that the plasmon-mediated oxidation of benzylic alcohols is not an efficient process within the operating parameters of irradiation source. HPLC analysis of both sec-phenethyl and benzyl alcohols in the presence of H2O2, in the aforementioned concentrations, over 40 min showed no considerable conversion to the carbonyl oxidation products (Figures S8 and S9), demonstrating that supported AuNP catalysts are required to promote efficient oxidation under the described experimental conditions. Previous reports have suggested that similar benzylic oxidations can occur around room temperature.35,36 Therefore, further experiments using conventional heating were also carried out to ensure that the yields being observed were not simply a thermal phenomenon due to the operating conditions of the LED reactor. The 1% AuNP@HT catalysts prepared using methods A and B were chosen to test this hypothesis, as these nanomaterials demonstrated the highest activity toward plasmon-mediated benzylic alcohol oxidation. In addition to the control experiments run in the absence of catalyst and support, Figure 6 presents the results obtained for the oxidation of secphenethyl or benzyl alcohol in the presence of H2O2 and supported AuNP in a 40 °C sand bath. The results obtained from the purely thermal reaction are compared to the

Figure 2. Photos presenting the reaction vessel containing supported AuNP, H2O2, and sec-phenethyl alcohol (a) before and (b) after LED photoexcitation.

Table 2. Conversion (%) of sec-Phenethyl and Benzyl Alcohols to Carbonyl Oxidation Products Using Supported AuNP (1% Nominal Loading) support HTc HTd γ-Al2O3c n-Al2O3d ZnOc

LED irradiation (min)

sec-phenethyl alcohola (%)

benzyl alcoholb (%)

5 40 5 40 5 40 5 40 5 40

64 90 50 86 10 41 1 34 48 80

28 46 28 50 6 14 7 42 14 37

a c

Conversion to acetophenone. bConversion to benzaldehyde. Prepared using method A. dPrepared using method B.

Figure 3 shows that the support used in the synthesis of the nanocomposites plays a very important role in efficient alcohol oxidation. Overall, 1% Au@HT prepared using both methods A and B were the most efficient heterogeneous catalysts for alcohol oxidations with near-complete conversion to acetophenone, 90% and 86%, respectively, for the two preparation techniques, over 40 min. On the other hand, the Al2O3 catalysts (both the bulk and nanoderived materials) demonstrated much lower conversion yields. Importantly, control experiments were run for all the support/alcohol experiments and showed that, in the absence of AuNP, minimal conversions (3−6%, see Figure 3) to the acetophenone and benzaldehyde products were observed. As was observed in our previous publication on alcohol oxidations,27 in all cases, sec-phenethyl alcohol oxidation was favored over oxidation of the primary benzyl alcohol, occurring more rapidly and giving higher conversions to carbonyl products over the time periods investigated in this work (Table 2). Dark Reactions. To ensure that plasmon excitation was required to initiate the AuNP-mediated oxidation of both secphenethyl and benzyl alcohol, each reaction mixture was also run under dark conditions for 40 min to compare the overall conversion to acetophenone and benzaldehyde, respectively, in the absence of light at room temperature. The histogram presented in Figure 4 illustrates that 530 nm excitation of the supported AuNP surface plasmon absorption is required to efficiently oxidize both alcohols over a 40 min time span when using the catalysts prepared by method A. However, the Au@ HT catalyst prepared by method B shows considerable 12282



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Figure 3. Conversion of sec-phenethyl alcohol to acetophenone using 1% AuNP supported on (a) HT, (b) γ-Al2O3, n-Al2O3, and ZnO and conversion of benzyl alcohol to benzaldehyde using 1% Au supported on (c) HT and (d) γ-Al2O3, n-Al2O3, and ZnO. Note that the conversion to acetophenone and benzaldehyde in the absence of AuNP (blank 40 min) in all trials is negligible.

removal, considerable effort is focused on the ability to recycle or reuse these materials. Thus, heterogeneous supported nanoparticle catalysts are an attractive alternative due to the ability to recover, wash, and reuse these samples multiple times to reduce chemical waste. The supported AuNP nanocomposites discussed within this work were recovered and reused in the oxidation of sec-phenethyl alcohol to test the potential recyclability of these solid catalysts. Figure 7 presents the comparison of percent conversion to acetophenone after 40 min of LED irradiation over four turnover cycles. These results illustrate that recyclability is largely dependent upon the nanocomposite used. Overall, similar or a slightly higher conversion to acetophenone was observed after several reuses for three of the five supported AuNP catalysts. Namely, 1% AuNP@ZnO and 1% AuNP@n-Al2O3 faired poorly in the recycle testing, showing a considerable decrease in catalytic activity after only one reuse. Table S1 presents the average nanoparticle size, as determined by TEM, before and after catalysis. The average AuNP size is statistically similar for all the nanomaterials before and after catalysis, with 1%AuNP@nAl2O3 being the exception. In this case, the AuNP size was found to increase considerably after reaction, also evidenced by the appearance of large NP agglomerates on the support surface (Figure S10). Such nanoparticle aggregation accounts for not only the increase in average AuNP size but is likely the dominating factor contributing to the catalyst deactivation subsequent experiments, as shown in Figure 7. However, nanoparticle size cannot be used to explain the reduced catalytic efficiency of 1% AuNP@ZnO in the recyclability trials, as AuNP sizes before and after catalysis are statistically similar

Figure 4. Conversion of sec-phenethyl alcohol to acetophenone (■) and benzyl alcohol to benzaldehyde (□) in the dark for 40 min using supported AuNP as photocatalysts.

conversion to oxidation products obtained via AuNP plasmon excitation. Though there is a thermal component to the reaction, as expected from previous reports on the nanoparticle surface temperature following exposure to visible light,20,24−26 the yields obtained using LED irradiation are considerably higher than those obtained from a purely thermal activation. Importantly, conventional heating of reaction mixtures containing HT only (in the absence of AuNP) showed no appreciable oxidation product formation, further confirming the participation of the AuNP surface in the oxidation mechanism. Catalyst Recyclability. Interest in heterogeneous catalysis has risen in the past several years due to the inherent advantages of using supported samples. In addition to ease of 12283



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Figure 5. Microwave-promoted oxidation of sec-phenethyl alcohol to acetophenone as a function of time in the presence of heterogeneous 1% AuNP photocatalysts. Conversion to acetophenone by the solid supports only (in the absence of AuNP) was also examined: method A (▲) and method B (▼) for HT and Al2O3 and method A (■) for ZnO. For the trials performed with γ-Al2O3 and n-Al2O3, the yield of acetophenone is the same for both the supports, resulting in overlap of the control data.

Table 3. Conversion (%) of sec-Phenethyl and Benzyl Alcohols to Carbonyl Oxidation Products Using Microwave Irradiation of Supported AuNP (1% Nominal Loading) support HTc HTd γ-Al2O3c n-Al2O3d ZnOc

microwave irradiation (min)

sec-phenethyl alcohola (%)

benzyl alcoholb (%)

1 10 1 10 1 10 1 10 1 10

84 89 55 95 20 48 26 65 49 93

58 75 86 84 12 21 91 89 37 74

Figure 7. Recyclability tests of the nanomaterials employed for AuNP mediated sec-phenethyl alcohol oxidation to acetophenone.

a c

Conversion to acetophenone. bConversion to benzaldehyde. Prepared using method A. dPrepared using method B.

Because of the inherent synergistic effects between nanoparticles and the support,37 the change in the physical composition of ZnO is likely the reasoning behind the observed catalytic deactivation of this nanomaterial in subsequent reuse trials. Overall, the ability of several of the nanomaterials discussed in this contribution to be reused in plasmon-mediated alcohol oxidations suggests that supported AuNP catalysts prepared using these methodologies are candidates for further investigation as not only a more effective, but greener, alternative for oxidation of simple organic alcohols.

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DISCUSSION In the results presented above, the photo-oxidation of secphenethyl and benzyl alcohols is facilitated and enhanced when carried out in the presence of supported AuNP. Furthermore, Au/HT nanocomposites are clearly the preferred catalyst, with the overall conversion yields being consistently higher than when AuNP supported on ZnO or Al2O3 are used. Importantly, in all cases, there is no visible change in the appearance of the supported nanocomposites following exposure to reaction conditions (see Figure 2). This observation contrasts with our previous work on benzylic alcohol oxidations using colloidal AuNP,27 where a clear change in color from pink to purple was observed following plasmon excitation, indicating some NP aggregates were formed. The absence of any discernible color change of the supported NP materials after LED excitation further corroborates the robustness of supported AuNP compared to their aqueous colloidal counter-

Figure 6. Oxidation of benzylic alcohols in the presence of 1% AuNP@HT prepared using methods A and B and H2O2 using conventional bench heating at 40 °C (■) and AuNP plasmon excitation using 530 nm LEDs (□) for 40 min.

(based on standard deviation calculations). Closer examination of TEM images before and after catalysis (Figure S11) depicts a very significant change in the composition of the ZnO solid itself. Notably, a crystalline material appears to dominate prior to catalysis, followed by transformation into an amorphous solid after exposure to the described reaction conditions. 12284



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parts and also demonstrates the enhanced stability and versatility of supported AuNP as heterogeneous catalysts for organic reactions. When 1% Au on HT, γ-Al2O3, n-Al2O3, and ZnO were used as alcohol oxidation catalysts, vigorous bubbling was observed immediately upon exposure to 530 nm LED irradiation. Because of the large excess of H2O2 present in the reaction mixture, it seems reasonable to conclude that the visible gas evolution is likely due to plasmon-induced decomposition of peroxide into H2O and O2, similar to that which occurs with colloidal AuNP,27 or to thermal decomposition, as the surface temperature surrounding the AuNP following plasmon excitation can be upward of 500 °C.24 Note that samples are stable in the dark under air. The ability of supported AuNP to participate in electron transfer (eT) pathways and enhance nanoparticle photocatalytic activity has recently received considerable attention in the literature.31 Early publications on the topic have suggested that upon direct excitation electron ejection from AuNP is a viable pathway, with small metal nanoparticles being capable of acting as an electron reservoir.27,38 Garciá et al. have reported AuNP@TiO2 and nanodiamondassisted degradation of H2O2 into the corresponding hydroxyl radical (•OH) and hydroxide anion (−OH), through an initial electron transfer process;30,39 thus, a similar pathway for the AuNP-mediated decomposition of H2O2 can also be considered. Following 530 nm illumination, the formation of •OH may occur via a photocatalytic single electron transfer (1) or through a thermal (2) pathway (Scheme 2).

Figure 8. Plasmon-mediated supported AuNP decomposition of H2O2 in the absence of alcohol substrate.

when examining the potential pathway for supported AuNP benzylic alcohol oxidations examined here. A proposed mechanism for the plasmon-mediated supported AuNP oxidation of benzylic alcohols is presented in Figure 9.

Scheme 2

The participation of the thermal degradation cannot be discounted due to the high NP surface temperatures, although we note that the bond dissociation energy for H2O2 is higher than for organic peroxides, such as those employed in earlier work.24 The direct participation of •OH in the overall reaction mechanism, however, can be ruled out, given the absence of phenol-derived products in the reaction mixture following work-up. These products are almost always formed when •OH is present in aromatic mixtures as a result of the highly unstable and reactive nature of this radical species.40 Therefore, as was proposed for colloidal AuNP alcohol oxidation process,27 the large excess of H2O2 in the reaction mixtures makes Habstraction from an additional peroxide molecule quite favorable, forming the more selective hydroperoxyl radical (•OOH). The proposed supported AuNP plasmon-mediated H2O2 decomposition pathway, in the absence of alcohol substrates, is presented in Figure 8. Following visible light exposure, it has been proposed that promotion of the intraband Au 6sp electrons result in an overall positive charge on the NP surface.15 Oxidation would now be more favorable as AuNP capture electrons from oxidizable organic molecules within proximity of the surface to achieve an overall neutral state. This mechanism must also be considered

Figure 9. Proposed mechanism for the plasmon-mediated oxidation of sec-phenethyl (R = CH3) and benzyl (R = H) alcohols in the presence of supported AuNP.

The first step in this mechanism entails 530 nm photoexcitation of the AuNP followed by either by electron transfer to H2O2 or photothermal peroxide cleavage of high concentrations of H2O2 present in the reaction mixture. Here, as is shown in Figure 8 (vide inf ra), H2O2 can then proceed to cleave to generate •OH and −OH, i.e., illustrating the electron transfer mechanism. As mentioned above, the participation of photothermally generated •OH cannot be discounted and may contribute to initiation of the oxidation process. Essentially, the supported AuNP can be viewed as an integral participant in a nanoscale 12285



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radical chain process. •OH can then continue to abstract additional hydrogen from another molecule of H2O2 to form the less reactive •OOH. The peroxyl radical then abstracts the secondary H-atom from the alcohol substrate to form a ketyl radical. This radical is then believed to undergo single electron transfer back to the NP surface followed by subsequent loss of a proton,27 or a proton-coupled electron transfer (PCeT),32 to result in the formation of the desired carbonyl oxidation product. In a catalytic mechanism electron balance is required and for every chain step that takes electrons from AuNP, another step must return electrons to the particle. As supported AuNP are used as catalysts in this system, the adsorption of the organic alcohol molecule onto the support surface is likely and reasonably thermodynamically favorable. The proximity of the AuNP and the alcohol on the support surface would facilitate interaction with the two entities and enhance the overall yields of carbonyl oxidation products. The ability of a supported nanoparticle catalyst to actively and efficiently participate in photocatalytic reactions can be dictated by NP size and NP/support interactions,17,41 since the support itself cannot be viewed as an inert component of the catalyst composition. Though the catalytic activity for AuNP generally increases with smaller particle sizes, this appears not to be the case in this system. In fact, the larger particles are superior catalysts in the plasmon mediated oxidation of both benzylic alcohols. For example, Au@HT (method A) has the largest nanoparticles of the catalysts used in this work, but has superior activity toward the oxidation of both alcohols. Conversely, 1% Au@n-Al2O3 has the smallest nanoparticle size of the five remaining catalysts being examined; however, this particular material gave the lowest yields of acetophenone and benzaldehyde. In addition to NP size, the NP/support interactions clearly play an important role in the catalytic efficacy of AuNP-promoted alcohol oxidations. Comparing the nanocatalysts used, those using HT tend to give the best conversion to carbonyl products, while Al2O3 derived materials the lowest. The overall trend is HT > ZnO > Al2O; thus, the answer to catalyst efficacy may lie in the support effects, not only with the AuNP but also with the alcohol starting materials and corresponding intermediates formed throughout the course of the oxidation reaction. One striking resemblance between the supports employed in this work is the inherent basicity of the solid materials.42−44 HT is well-known to be a considerably basic support (isoelectric point, IEP = 10),42 followed by ZnO (IEP = 9).44 Al2O3 derived materials are the least basic of the three (IEP = 8)42 having the capability to behave more in an amphoteric nature. As HT is the most basic support used in this work, one would expect the surface to interact more strongly with the acidic protons of the alcohol substrates, following the trend HT > ZnO > Al2O3. A closer examination of the proposed mechanism shows two distinct steps that may be highly influenced by the basicity of the supported material. First, adsorption of the alcohol onto the support surface must be considered an integral part of the mechanism due to the proximity required between the substrate and AuNP to facilitate the PCeT step of the Au-mediated mechanism. Thus, the more favored oxidations detected when using basic supported materials may be explained by stronger support/ alcohol interactions at the onset of the reaction (i.e., preferential binding between the alcoholic H and the basic sites within the support). This stronger interaction could then directly translate into more facile adsorption of H+ and act as a

thermodynamic driving force for the reaction, facilitating carbonyl formation. In this case HT would be expected to be the best support for such reactions, followed by ZnO and, last, Al2O3, based solely on the basicity of the materials used. The results obtained agree well with this proposed trend.

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CONCLUSIONS AuNP supported on a variety of supported materials have shown the ability to rapidly oxidize sec-phenethyl and benzyl alcohols via economically viable 530 nm LED illumination as well as microwave excitation; the former is a much simpler technique, requiring a minor investment when compared with laboratory microwaves. The AuNP nanocomposites were synthesized using two different methods, a dry photochemical technique and laser ablation. Plasmon excitation of AuNP supported on HT resulted in the best conversions to acetophenone and benzaldehyde from their respective alcohol substrates, while Al2O3 supports (γ-Al2O3 and n-Al2O3) afforded the lowest yields. The proposed mechanism for 530 nm light-induced alcohol oxidations is similar to that which has been previously proposed, involving both photothermal and photochemical AuNP-mediated H2O2 cleavage. Here, single electron transfer from the AuNP to the peroxide, ketyl radical formation, and single electron transfer, followed by loss of a proton are all integral steps in the AuNP-catalyzed mechanism. Moreover, alcohol adsorption by basic sites on the support surface is believed to be a crucial participant toward favorable proton loss in the final stages of the reaction, resulting in more rapid and proficient generation of alcohol oxidation products. Finally, from the methodology presented within, several techniques for supported AuNP have proven effective toward plasmon driven alcohol oxidations and also exploit the use of “greener” light-induced photocatalytic processes, as compared to the more commonly used chemical alternatives.

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ASSOCIATED CONTENT

S Supporting Information *

TEM images and diffuse reflectance spectra of 1% AuNP@HT (method B), 1% AuNP@ZnO, 1% AuNP@n-Al2O3; examples of typical HPLC spectra obtained throughout this work; microwave excitation results for benzyl alcohol; photos of laser drop and LED setup; HPLC experiments run using conventional heating in the absence of catalyst and support; comparison of TEM images of 1% AuNP@ZnO and 1% AuNP@n-Al2O3 before and after catalysis; calculation of percent conversion and PDI. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.C.S.); josecarlos@ photo.chem.uottawa.ca (J.C.N.-F.). Present Address §

M.G.-B.: Instituto de Ciencia Molecular, Departamento de ́ Quimica Orgánica, Universidad de Valencia, C/Catedrático José Beltrán, 2, 46980, Paterna, Valencia, Spain. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the Natural Sciences and Engineering Research Council of Canada for support of this research. 12286



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M.J.S. is grateful to the CREATE programs for financial support. J.CN.-F. acknowledges the University of Ottawa for a Visiting Professor fellowship. M.G.B. thanks the Spanish Ministry of Economics and Competitiveness for her Juan de la Cierva contract and FP7-PEOPLE-2011-CIG (NANOPHOCAT) for financial support. We would also like to thank Dr. Yun Liu of the University of Ottawa for help with SEM and TEM imaging.

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Supported Gold Nanoparticles as Efficient Catalysts in the Solventless

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