Oxidation of Benzyl Alcohol using in Situ Generated Hydrogen Peroxide

Aug 29, 2014 - Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff, CF10 3AT, United Kingdom. ABS...
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Oxidation of Benzyl Alcohol using in Situ Generated Hydrogen Peroxide Marco Santonastaso, Simon J. Freakley, Peter J. Miedziak, Gemma L. Brett, Jennifer K. Edwards, and Graham J. Hutchings* Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff, CF10 3AT, United Kingdom ABSTRACT: Catalysts containing bimetallic gold−palladium nanoparticles are extremely active and selective for the oxidation of alcohols to aldehydes and the direct synthesis of hydrogen peroxide from molecular hydrogen and oxygen. We show that the oxidation of benzyl alcohol can be carried out at 50 °C and below by generating hydrogen peroxide in situ. The oxidation of benzyl alcohol to benzaldehyde has been achieved with high selectivity (>85%) at temperatures where no reaction is observed with only molecular oxygen in an autoclave. The effect of temperature, catalyst support, and solvent are studied in an autoclave system and reactions were carried out in a fixed bed reactor at a range of gas flow rates where the catalysts demonstrated stable conversion and selectivity.

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INTRODUCTION Heterogeneous catalysts comprising supported bimetallic nanoparticles can be prepared by a relatively simple impregnation method. Catalysts containing Au and Pd nanoparticles supported on metal oxides such as TiO2 are exceptionally active for a number of catalytic reactions.1−5 These include the oxidation of aromatic alcohols such as benzyl alcohol to products such as benzaldehyde, benzoic acid, and benzyl benzoate6,7 (Scheme 1) and the direct synthesis of H2O2 from molecular hydrogen and oxygen1 (Scheme 2). Many catalysts have been reported in the literature that are active for both of these reactions, and they typically contain bimetallic nanoparticles between 2 and 30 nm consisting of Au and Pd, which, after high-temperature heat treatments, have been shown to be gold-core−palladium-shell particles on supports such as TiO21 and SiO23 or homogeneous alloys on activated carbon supports.8 The addition of Au to Pd in these systems has a synergistic effect on reaction yields. An increase in conversion of benzyl alcohol with bimetallic catalysts occurs when compared to that of monometallic Au or Pd catalysts, whilst achieving high selectivity to benzaldehyde with limited overoxidation to benzoic acid and benzyl benzoate. A similar synergy occurs for the direct synthesis of H2O2: increased H2O2 yields are observed when using AuPd bimetallic catalysts because Au increases the selectivity of the reaction and suppresses the sequential over-hydrogenation of H2O2. Monometallic Pd catalysts show high hydrogenation activity toward H2O2, resulting in poor yields. Acid and halide additives can be added to minimize these sequential hydrogenation/ decomposition reactions; however, these are not desirable industrially. In an industrial context, benzladehyde is formed from the oxidation of toluene at high temperatures (170−220 °C),9 whereas in the lab, the oxidation is typically carried out with molecular oxygen at 100−120 °C10,11 and the direct synthesis of H2O2 is carried out at 2 °C to stabilize the synthesized H2O2. Both reaction schemes are thought to proceed through hydroperoxy (−OOH) intermediates, which are highly © XXXX American Chemical Society

Scheme 1. Benzyl Alcohol Oxidation

Scheme 2. Direct Synthesis of Hydrogen Peroxide

oxidizing intermediate species.12 Whilst high temperatures are needed to generate oxidative species with molecular oxygen and to perform the oxidation of benzyl alcohol, Au−Pd catalysts are capable of activating oxygen in the presence of hydrogen when synthesizing H2O2. This creates the opportunity to carry out the liquid phase oxidation of alcohols at low temperatures by utilizing the hydroperoxy intermediates generated during H2O2 synthesis reactions. This would allow for the possibility of Special Issue: Continuous Processes 14 Received: June 18, 2014

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oxidizing alcohols at much lower temperatures using sacrificial hydrogen to activate oxygen, and using the lower reaction temperature could facilitate access to higher reaction selectivities. In this article, we aim to demonstrate the feasibility of oxidizing benzyl alcohol to benzaldehyde in the presence of hydrogen and oxygen at ≤50 °C, an intermediate temperature between reported reaction temperatures for benzyl alcohol oxidation and H2O2 synthesis. We aim to demonstrate that this reaction is feasible at considerably lower temperatures than those used as part of the industrial process,9 which would represent a considerable savings in terms of energy. At this temperature, only limited activity occurs with molecular oxygen using catalysts known to be active for both benzyl alcohol oxidation and H2O2 synthesis, demonstrating the advantage of using the in situ method. The reaction was studied using previously reported Au−Pd catalysts in both autoclave and flow reactor systems.4

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RESULTS AND DISCUSSION Initial reactions were carried out with a 5 wt % Au−Pd/TiO2 prepared by a standard impregnation method that has been extensively studied in the literature for both H2O2 synthesis and benzyl alcohol oxidation.1,6 The catalyst has been shown to have metal particles in a core−shell configuration, with XPS and STEM-EDS showing that the outer shell is palladium-rich. The particles have a bimodal size distribution, with small particles in the range of 2−10 nm and with, >20 nm, particles also present.1 To establish if there was any conversion of the benzyl alcohol in the absence of a catalyst, a blank reaction was carried out under the standard reaction conditions (50 °C, 30 min, 10 vol % BnOH/MeOH) outlined in the Experimental Section. Without catalyst present, the conversion of benzyl alcohol was measured at 0.1%, which is within experimental error for this reaction system. A further blank test was carried out in the presence of catalyst (5 wt % Au−Pd/TiO2) but with H2O2 (the equivalent amount, as measured during the synthesis of H2O2 in the absence of benzyl alcohol) added to the benzyl alcohol and methanol. In this case, conversion of the benzyl alcohol was observed, but this was minimal (0.5%). Effect of Temperature. As we have previously reported, the reactions that we are combining in this process are ideally carried out at distinctly different reaction temperatures. Detailed studies into the direct synthesis of H2O2 have shown that the optimum temperature for the synthesis is 2 °C; this is owing to the stability of the peroxide at this temperature due to the suppression of sequential reactions. The oxidation of benzyl alcohol is typically carried out at >100 °C, temperatures at which H2O2 is very unstable. To investigate the optimum temperature at which the right balance of H2O2 stability after synthesis and reactivity of benzyl alcohol, we carried out the reaction using a standard 5 wt % Au−Pd/TiO2 catalyst: the temperatures studied were 2, 25, and 50 °C, and the results are shown in Figure 1. Although there is some activity at 2 °C (0.5%), this is not significantly greater than that of the blank reaction; it is possible at this temperature that any H2O2 formed is stable and therefore there is no radical formation to initiate the oxidation reaction. When the temperature is increased to 25 °C, the conversion significantly increases to 5.4%, and the major product at this temperature is benzaldehyde; however, there is significant toluene formation, a product we have observed

Figure 1. Conversion of benzyl alcohol and selectivity towards the major product during the in situ oxidation of benzyl alcohol at different temperatures. Conversion: black bars. Selectivity: benzaldehyde, spotted bars; toluene, striped bars; benzyl benzoate, gray bars; and benzoic acid, white bars.

previously in our benzyl alcohol oxidation studies.13 We have shown that the toluene is formed when there is a deficiency of oxidant in the reaction mixture, so it is possible that at this temperature there still is not enough H2O2 being formed and subsequently breaking down into radical species to carry out the oxidation reaction. Further raising the reaction temperature to 50 °C led to a further small increase in the benzyl alcohol conversion to 5.9%; we have previously shown that it is possible to form H2O2 in a water/methanol mix at higher temperatures,14 such as 50 °C, and we have also shown that toluene, a more difficult to oxidize molecule, can be activated at temperatures as low as 80 °C by a radical mechanism.15 It is clear in this work that we can activate benzyl alcohol at lower temperatures than this with reasonable conversion while carrying out the reaction at a considerably lower temperature than has previously been reported. Due to these results, we defined 50 °C as the temperature of our standard reaction conditions, and all further reactions discussed will be carried out at this temperature. Effect of Solvent Variation. In our previous reports, we have shown that the optimum solvent for the formation of H2O2 is a mixture of water and methanol; we have, however, also reported formation data in pure methanol.14,16 The productivity of the catalyst for H2O2 synthesis in pure methanol is higher than that in a water/methanol mix due to increased hydrogen solubility. However, the increased hydrogen solubility also leads to higher rates of over-hydrogenation. The oxidation of benzyl alcohol has, in this case, been carried out with solvent; however, the best results, in terms of turn over frequency (TOF), have been achieved using a solvent-free system.2 To determine which of these solvent systems is most appropriate for the combinations of the two reactions, we have carried out the reaction solvent free, in a water (34%)/methanol (66%) mixture, and in pure methanol, and the results are shown in Figure 2. From these results, it is observed that the optimum solvent for the formation of H2O2 is not the best solvent for the in situ B

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magnesium oxide and cerium oxide and compared them to the titania-supported catalyst; the results are shown in Figure 3, which are in agreement with the previous work.

Figure 2. Conversion of benzyl alcohol and selectivity towards the major products during the in situ oxidation of benzyl alcohol using different solvent systems (100% MeOH, 66% MeOH/34% H2O, and solvent free). Conversion: black bars. Selectivity: benzaldehyde, spotted bars; toluene, striped bars; benzyl benzoate, gray bars; and benzoic acid, white bars.

Figure 3. Conversion of benzyl alcohol and selectivity towards the major product during the in situ oxidation of benzyl alcohol with different supports. Conversion: black bars. Selectivity: benzaldehyde, spotted bars; toluene, striped bars; benzyl benzoate, gray bars; and benzoic acid, white bars.

oxidation; again, this could be because the H2O2 is too stable in this water/methanol solvent system. As discussed previously, the addition of purchased H2O2 to benzyl alcohol at this temperature gave only minimal conversion of the benzyl alcohol (0.5%), suggesting that conditions where the H2O2 is relatively stable are not ideal for the secondary step of this reaction. Furthermore, the solvent-free system yielded the lowest conversion of all of the combinations tested with 0.7% conversion; it seems likely in this case that the hydrogen and oxygen did not have sufficient solubility in the benzyl alcohol for the peroxide formation reaction to occur. This is supported by the selectivity data, with the solvent-free system having the lowest selectivity towards benzaldehyde and the highest selectivity towards toluene. We have previously reported the reaction of benzyl alcohol under an atmosphere of helium, in which case toluene is the favored product, along with benzaldehyde due to a disproportionation reaction pathway.13 The highest activity reported is for the pure methanol system, which indicates that this is the best compromise solvent system with sufficient solubility of the gases to form peroxide but also allowing the formation of radicals from the peroxide, which are capable of oxidizing the benzyl alcohol. The selectivity toward benzaldehyde is higher for the water/methanol mixture than that for pure methanol; however, this may simply be a function of conversion because the reaction has proceeded further than when a water/methanol mix is used. Catalyst Support Effects. It has also been shown previously that catalyst support can play a key role in both the activity of a catalyst and the subsequent selectivity toward the products.11,17 It has been demonstrated that the discussed disproportionation mechanism, which leads to the formation of the toluene byproduct, can be suppressed by the use of magnesium oxide as a support.18 We have also observed that cerium oxide-supported catalysts are particularly active for alcohol oxidations and have also been used for benzyl alcohol oxidation.18 We prepared catalysts supported on both

The magnesium oxide-supported catalyst has very good selectivity toward benzaldehyde, indicating that the disproportionation reaction is not occurring with this catalyst; however, also in agreement with the previous reports, the conversion with this catalyst is much lower than when a titania catalyst is used. The cerium oxide-supported catalyst gave similar results to that of the titania-supported catalyst, with similar activity and selectivity toward benzaldehyde. The ceria catalyst formed slightly less toluene than that from the titania catalyst; however, we have previously observed that ceria-supported catalysts leach over 90% of their metal during the oxidation of benzyl alcohol.19 These results demonstrate that the acidic/basic nature of the catalyst has a significant effect on the selectivity of the products; however, as the further oxidation product of benzyl alcohol oxidation is an acid, it is important that the catalyst is stable under the acidic conditions that could form at high conversions. Time-on-Line Analysis. To investigate the stability of the titania catalyst, we carried out a time-on-line study of the reaction, and the results are shown in Figures 4 and 5. It is clear from the results in Figure 4 that the initial rate of reaction is much higher than the overall rate and that the reaction is plateauing at around 5.9% conversion. The selectivity seems consistent throughout the reaction, indicating that both the initial oxidation and subsequent oxidation steps are being suppressed. There is no notable increase in the toluene formation, and the conversion of benzyl alcohol is below the theoretical limit of the conversion if all of the gas is converted to H2O2, which suggests that there is catalyst deactivation rather than a limit to the amount of gas available. To test this, we carried out a reuse experiment on the catalyst, and it was found that on the second use the conversion of C

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In Situ Oxidation in a Continuous Flow System. At this stage of the study, we have demonstrated that we can carry out the oxidation of benzyl alcohol at 50 °C (this is lower than temperatures that are needed with molecular oxygen) by generating H2O2 in situ. Studies of the direct synthesis of H2O2 in a small-scale fixed bed reactor, using a 1% AuPd/TiO2 catalyst prepared by a modified impregnation methodology, outlined in the experimental procedures have previously been reported.4 This catalyst had been shown to be able to produce H2O2 under continuous conditions, whereas the 5% AuPd/ TiO2 catalyst prepared by standard impregnation methods leached metal when formed into pellets to test in the fixed bed reactor. The catalyst prepared by modified impregnation contains metal nanoparticles with a mean particle size of 2.9 nm that are homogeneous alloys which comprise roughly 50 wt % Pd in each particle.5 To investigate if the in situ oxidation is possible in a continuous system using this catalyst, which has been shown to be stable to continuous H2O2 production, we conducted experiments using 10% benzyl alcohol/MeOH (0.2 mL min−1) as substrate and H2/O2 of 1:1 where the reaction atmosphere consisted of 4 vol % of each gas diluted in CO2 at a pressure of 10 bar at 50 °C. Gas flow rates between 5 and 100 mL min−1 were investigated, which correspond to residence times on the order of milliseconds. Results shown in Figure 6 using only

Figure 4. Conversion of benzyl alcohol with in situ H2O2 at various times.

Figure 5. Selectivity towards the major products during the conversion of benzyl alcohol with in situ H2O2 at various times. Diamonds, benzaldehyde; triangles, toluene; squares, benzyl benzoate; and circles, benzoic acid.

Figure 6. Effect of total gas flow on benzyl alcohol conversion in a fixed bed reactor. Diamonds, 4 vol % H2 + 4 vol % O2 balance CO2; triangles, 4 vol % O2 + balance CO2; and circles, CO2 only.

benzyl alcohol decreased to below half of the original conversion, thereby indicating that either catalyst deactivation is occurring or there is product inhibition during the reaction. To confirm that the process was occurring through a radical mechanism, radical quenchers were added into the reaction in an attempt to suppress conversion. As can be observed in Table 1, the conversion of benzalcohol was suppressed when radical quenchers for both hydroxyl and hydroperoxy radical species were present, indicating that the reactions was proceeding through radicals generated through the synthesis of H2O2.

CO2 and catalyst (120 mg) showed that there was minimal conversion of benzyl alcohol over the range of flows used, yielding conversions ranging between 0.3 and 0.5%. When only oxygen was used, a larger conversion was measured, increasing to a maximum of 4% at a gas flow of 40 mL min−1. This result shows that, in contrast to the standard impregnation catalyst used previously in the autoclave study activity is seen at 50 °C

Table 1. Effect of Addition of Radical Quenchers on Benzyl Alcohol Conversion and Selectivitya selectivity (%) quencher

conversion (%)

benzaldehyde

toluene

benzylbenzoate

benzoic acid

none Na2SO3 NaNO2

5.9 2.4 0.4

89.5 96.5 100

6.2 1.3 0

0.0 0.5 0

4.2 1.8 0

Reaction conditions: 10 mL reaction volume, 10 vol % benzyl alcohol in MeOH, 5% H2/CO2 (2.9 MPa) and 25% O2/CO2 (1.1 MPa), 50 °C, 10 mg catalyst (2.5 wt % Au/2.5 wt % Pd/TiO2, 400 °C, 3 h), 1200 rpm, 30 min reaction time, radical quencher concentration = 0.05 M.

a

D

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temperatures that are intermediate to those for the oxidation on benzyl alcohol and H2O2 synthesis. The reaction proceeds through a radical mechanism, as the addition of quenchers for hydroxyl and hydroperoxy radicals suppressed conversion. Optimum conversion was observed when carrying out the reaction with methanol as solvent, as this promotes high H2O2 synthesis rates. The reaction was also carried out in a continuous fixed bed reactor with a catalyst prepared by a modified impregnation procedure where enhanced conversion was seen when H2 and O2 were present simultaneously, and it was observed that the catalysts experienced minimal deactivation. Currently, the conversion of the benzyl alcohol is limited at these temperatures; however, the successful application of the continuous fixed bed reactor could also present a solution to this problem via recycling of the product stream, which will be the focus of our future research into this reaction.

when only oxygen is present. This is possibly due to the smaller Au−Pd particle size or morphology differences between the two catalysts. When H2 + O2 were used together, an increased conversion was seen that increased with the gas flow in a similar manner to that of the increase in H2O2 productivity previously reported.4 This can be explained in two possible ways: First, the higher flow of H2 results in less of a concentration gradient through the catalyst bed, and second, that faster flows improve the hydrodynamics of the reaction by decreasing the thickness of the liquid layer around the particles, hence increasing mass transfer to the catalyst particles. Both explanations indicate higher H2O2 synthesis at higher gas flows and, in this case, higher conversion of benzyl alcohol at higher gas flows. Maximum conversions of 11.3% were achieved at very short contact times between the catalyst and the gases. When only oxygen was used as reactant, a limit of 4.5% conversion is reached; this indicates that it is the mass transfer of hydrogen that is limiting the conversion during in situ reactions and that the increased generation of H2O2 results in increased conversion. Under these conditions, the catalyst was stable and gave constant conversion within error over extended times (each reaction was carried out for 5 h). These results show that it is possible to increase the conversion of benzaldehyde over this catalyst by the addition of hydrogen to generate an in situ oxidative species. The selectivity of the reaction at similar conversion with the different atmospheres is shown in Table 2.

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EXPERIMENTAL SECTION Catalyst Preparation. A typical preparation for 1 g of 2.5% Pd/2.5% Au on various supports by the impregnation method was carried out according to the following procedure, which has been previously reported in the literature.1 PdCl2 (0.0416 g, Sigma-Aldrich) was added to HAuCl4 (2.04 mL, Johnson Matthey, 12.25 g Au/1000 mL) and heated at 80 °C with stirring and left until the PdCl2 had completely dissolved. Catalyst support (0.95 g TiO2 (Degussa Evonik P25), MgO (Sigma-Aldrich), or CeO2 (Sigma-Aldrich)) was then added to the solution, and the water was allowed to evaporate until the mixture formed a paste. The samples were dried (110 °C, 16 h) and then calcined in static air at various temperatures, typically 400 °C for 3 h with a ramp rate of 20 °C min−1. Au−Pd catalysts were also prepared by a modified impregnation method described previously in the literature.5 A brief outline of the preparation method is as follows: For the preparation of 1% Au−Pd-supported catalyst, the requisite amounts of HAuCl4·3H2O solution (Sigma-Aldrich, 8.9 mg/ mL) and PdCl2/HCl solution (Sigma-Aldrich, 6 mg/mL; HCl concentration: 0, 0.58, 1, 2 M) were charged into a clean 50 mL round-bottomed flask, and the volume of the solution was adjusted using deionized water to a total volume of 16 mL and immersed into an oil bath on a magnetic stirrer hot plate. The solution was stirred at 1000 rpm, and the temperature of the oil bath was raised from room temperature to 60 °C over a period of 10 min. At 60 °C, metal oxide support material [1.98 g TiO2 (Degussa Evonik P25)] was added slowly over a period of 8− 10 min with constant stirring. The subsequent slurry was stirred at 60 °C for an additional 15 min. Following this, the temperature of the oil bath was raised to 95 °C for 16 h, leaving a dry solid. The solid powder was ground thoroughly to form a uniform mixture. Four hundred milligrams of the sample was reduced at 10 °C min−1 under a steady flow of gas (5% H2/Ar) Catalyst Testing. Catalyst testing was performed using a stainless steel autoclave (Parr Instruments) with a nominal volume of 50 mL. The autoclave was charged with the catalyst (0.01 g), solvent (9 mL of CH3OH), and benzyl alcohol (1 mL), purged three times with 5% H2/CO2 (100 psi), and then filled with 5% H2/CO2 (420 psi) and 25% O2/CO2 (160 psi) to a total pressure of 580 psi. Stirring (1200 rpm) was commenced on reaching the desired temperature (50 °C), and experiments were carried out for 30 min. Residual H2O2 was determined by titration of aliquots of the final filtered solution with acidified Ce(SO4)2 using ferroin as indicator. A

Table 2. Effect of Gas Phase Composition on Benzyl Alcohol Conversion and Selectivity in a Fixed Bed Reactora selectivity (%) gas CO2 O2/CO2 H2/CO2 + O2/CO2

conversion (%)

benzaldehyde

toluene

benzyl benzoate

benzoic acid

0.6 4.2 5.8

59.4 94.6 83.7

35.8 4.3 13.4

0.0 0.0 1.9

0.0 1.1 0.9

a

Reaction conditions: 120 mg catalyst (1% AuPd/TiO2 prepared by modified impregnation), 50 °C, 10 vol % benzyl alcohol in MeOH at 0.2 mL min−1, total gas flow 24 mL min−1, H2 or O2 concentration 4 vol % H2/O2 = 1:1 balance CO2, total pressure = 10 bar.

The selectivity to benzaldehyde is slightly higher for the reaction with only oxygen gas fed at conversion, with benzaldehyde selectivity of 94.6% at 4.2% conversion and toluene selectivity of 4.3%. When hydrogen was also present, selectivity to benzaldehyde was lower, 83.7% at 5.6% conversion with toluene selectivity of 13.4%. This observation suggests that when hydrogen is present more toluene is formed either through disproportionation of benzylaldehyde or hydrogenolysis of benzyl alcohol; because Pd is able to activate hydrogen very easily, side reactions with adsorbed hydrogen could be occurring in competition to the reactions with reactive oxygen species.

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CONCLUSIONS We have demonstrated that it is possible to oxidize benzyl alcohol with high selectivity to benzaldehyde using in situ generated H2O2 at temperatures where limited activity is seen with oxygen only. Catalysts containing bimetallic nanoparticles, which are active for both reactions, carry out this reaction at E

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mixture of an aliquot of the final filtered solution together with a GC standard (mesitylene) in a 1:1 volume ratio was then analyzed by GC fitted with a CP-Wax column. The in situ selective oxidation of the benzyl alcohol was also carried out in a continuous fixed bed flow reactor as previously reported.4 A typical benzyl alcohol in situ oxidation reaction was carried out using 0.5 wt % Au−0.5 wt % Pd/TiO2 (120 mg, prepared by the modified impregnation method) that had been pressed into a disk and sieved to a particle size of 425−250 μm. The sample was supported at the bottom of the catalyst bed in the reactor tube by glass wool. The catalyst was contained within a 10 cm stainless steel tube with an internal diameter of 1 /8 in.; this resulted in a catalyst bed length of around 4 cm. The reactor system was then pressurized, typically to 10 bar, with 5% H2/CO2 and 25% O2/CO2 to give 1:1 H2/O2, with 4 vol % of each gas and balance CO2. The reactor was then heated by the water bath at 50 °C. When the reactor had reached pressure and the flow through the system had stabilized, the reaction mixture flow, typically 0.2 mL min−1, was introduced into the system. Both gas and liquid flowed concurrently through the catalyst bed from top to bottom. Liquid samples were taken from the gas liquid separator every 60 min and analyzed by GC using a CP-Wax column in a 1:1 volume mixture with mesitylene as GC standard; the residual concentration of H2O2 was determined by titration against an acidified dilute Ce(SO4)2 solution using ferroin as an indicator.

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(15) Peneau, V.; He, Q.; Shaw, G.; Kondrat, S. A.; Davies, T. E.; Miedziak, P.; Forde, M.; Dimitratos, N.; Kiely, C. J.; Hutchings, G. J. Phys. Chem. Chem. Phys. 2013, 15, 10636. (16) Piccinini, M.; Edwards, J. K.; Moulijn, J. A.; Hutchings, G. J. Catal. Sci. Technol. 2012, 2, 1908. (17) Su, F. Z.; Chen, M.; Wang, L. C.; Huang, X. S.; Liu, Y. M.; Cao, Y.; He, H. Y.; Fan, K. N. Catal. Commun. 2008, 9, 1027. (18) Meenakshisundaram, S.; Nowicka, E.; Miedziak, P. J.; Brett, G. L.; Jenkins, R. L.; Dimitratos, N.; Taylor, S. H.; Knight, D. W.; Bethell, D.; Hutchings, G. J. Faraday Discuss. 2010, 145, 341. (19) Miedziak, P. J.; Tang, Z.; Davies, T. E.; Enache, D. I.; Bartley, J. K.; Carley, A. F.; Herzing, A. A.; Kiely, C. J.; Taylor, S. H.; Hutchings, G. J. J. Mater. Chem. 2009, 19, 8619.

AUTHOR INFORMATION

Corresponding Author

*E-mail: hutch@cardiff.ac.uk. Notes

The authors declare no competing financial interest.

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REFERENCES

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