Continuous Catalytic Oxidation of Glycerol to Carboxylic Acids Using

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Article Cite This: ACS Omega 2018, 3, 13862−13868

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Continuous Catalytic Oxidation of Glycerol to Carboxylic Acids Using Nanosized Gold/Alumina Catalysts and a Liquid-Phase Flow Reactor Naoki Mimura,* Natsumi Muramatsu, Norihito Hiyoshi, Osamu Sato, Yoshio Masuda, and Aritomo Yamaguchi

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AIST Tohoku Center, Research Institute for Chemical Process Technology, National Institute of Advanced Industrial Science and Technology, 4-2-1 Nigatake, Miyagino-ku, Sendai, Miyagi 983-8551, Japan S Supporting Information *

ABSTRACT: Here, we report the development of catalysts comprising highly dispersed Au on an alumina (Al2O3) support for the oxidation of glycerol to high-value carboxylic acids in a liquid-phase flow reactor. The catalysts were prepared by means of a deposition−precipitation method. To ensure that the catalysts could be used for long-term catalytic conversions in a liquid-phase flow reactor, we chose an alumina support with high temperature stability and a particle size (50−200 μm) large enough to prevent leakage of the catalyst from the reactor. One of the five catalysts had a high catalytic activity for the conversion of glycerol to the highvalue carboxylic acids, glyceric acid and tartronic acid (conversion of glycerol >70%), and the catalyst retained its catalytic activity over long-term use (up to 1770 min). Pretreatment of the catalyst with fructose, a mild reductant, increased the activity of the catalyst. Scanning transmission electron microscopy revealed three Au species highly dispersed on the surface of the alumina supportAu nanoparticles (mode = 7.5−10 nm), Au clusters (1−2 nm), and atomic Au.

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report8 is one of the few examples of liquid phase of flow oxidation of glycerol. Many other groups have also examined the catalytic activities of Pt-based catalysts for the oxidation of glycerol.9 In addition to Pt and Pd, nanosized Au has also been shown to have high catalytic activity for the oxidation of glycerol when molecular oxygen is used as an oxidant. For example, Hutchings’ group reported supported Au-based monometallic and bimetallic catalysts prepared by means of both an impregnation method and a sol immobilization method,10 and Villa and Prati’s group (or the research groups related to them) reported detailed characterizations and the catalytic activities of Au-based bimetallic and monometallic catalysts.11 It is not an overstatement that this research frame has been developed through the investigations under a conventional batch reaction. Flow-type reaction technique instead of batch type has been expected for conversion of large amount of raw materials such as biomass-derived reactants. Dumeignil’s group, including the main author of this letter, has also reported Au−Pd bimetallic catalysts prepared by means of ion-exchange12 and deposition−precipitation (DP) methods.13 Especially, the investigation of Au−Pd in an ion-

INTRODUCTION Since the discovery of the catalytic properties of Au,1 many Aubased catalysts for the conversion of many types of molecules have been developed. Also, catalytic conversions of lignocellulose2 and vegetable oil3 using both heterogeneous and homogeneous catalysts have been widely studied for the suppression of increasing of CO2 in the atmosphere, recently. Glycerol is a major byproduct (10% of the raw material) of the conversion of vegetable oil to biodiesel. To find uses for this major byproduct, Pagliaro et al. summarized the conversion of glycerol into high-value molecules4 and Dumeignil et al. examined the selective oxidation of glycerol.5 The oxidation of glycerol using molecular oxygen as an oxidant is an effective means of converting glycerol into high-value carboxylic acids such as glyceric acid, which is used as an additive in ink and as a cell activator, and tartronic acid, which is used as an auxiliary agent in detergents and moisturizers. Many research groups have examined the use of noble-metal catalysts for the production of carboxylic acids from glycerol, and conventional batch-type reactors were used in most of previous studies. Kimura’s group pioneered the research into glycerol oxidation, reporting the use of modified noble-metal catalysts (mainly Pt- or Pd-modified Bi catalysts) and either batch or fixed-bed flow reactors to oxidize glycerol to tartronic acid6 and dihydroxyacetone (DHA).7,8 The Kimura’s research © 2018 American Chemical Society

Received: May 31, 2018 Accepted: October 1, 2018 Published: October 23, 2018 13862

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exchange resin support focused on the development of a suitable catalyst for not only a batch reactor but also a flow reactor considering the characteristics of a flow reactor, including mechanical devices of a reaction system. This research report is also one of the few examples of flow oxidation of glycerol. It has been reported that ion-exchange resin is particularly suitable as a catalyst support for reactions conducted in liquid-phase flow reactors, which have recently attracted attention because of their greater efficiency compared with batch reactors.14 A continuous catalytic reaction (over 4000 min) in a liquid-phase flow reactor was successfully conducted using a Au−Pd/anion-exchange resin catalyst (glycerol conversion, 50−65%). However, since ion-exchange resin is unstable at high temperatures (>333 K), the reaction rate could not be increased by increasing the reaction temperature. Titanium dioxide is another suitable support material for Au catalysts; however, it cannot be used in liquidphase flow reactors because its small particle size means that particles can leak from the reaction tube and damage parts of the mechanical devices of the reactor. In this letter, we present a study in which we developed high-performance catalysts for the liquid-phase flow oxidation of glycerol to the high-value carboxylic acids such as glyceric acid and tartronic acid. The typical merits of using a flow reactor are high production efficiency and safer operation. As we described above, there are only a few reports11d,15 about the catalytic performance of glyceric acid and tartronic acid production using a flow reactor. Thus, we examined to prepare suitable catalysts for liquid-phase flow operation. The catalysts were prepared using a deposition−precipitation method to disperse nanosized Au on an alumina support. To ensure that the oxidation reaction could be conducted in a liquid-phase flow reactor instead of a conventional batch-type reactor, we selected a commercially available activated alumina powder that is commonly used for column chromatography (material: alumina N; activity: I; specific surface area: 150 m2/g; particle size: 50−200 μm; MP Biomedicals, Inc., California). This alumina has high heat-resistance, so higher reaction temperatures can be used, which provides a higher reaction rate and allows the use of a compact reaction system. In addition, the particle size ensures that there is no leakage of catalyst particles from the reactor through the filter and there is sufficient contact among the catalyst, reactant, and oxidant.

Table 1. Catalyst Au Content (ICP Results) and Changes in pH during Preparationa,b,c,d catalyst

Au content, wt %

pH before Au deposition

pH after catalyst precipitation (after stirring for 2 h)

1 2 3 4 5

0.27 0.38 0.69 0.88 0.89

7.0 5.6 4.6 3.6 3.2

7.6 7.4 6.7 4.5 3.7

a Initial amount of Au: 1.0 wt %. bSupport: MP alumina N; activity: I; specific surface area: 150 m2/g; particle size: 50−200 μm (as determined by the manufacturer, MP Biomedicals, Inc.). cDeposition−precipitation was performed at 343 K. dAu content was determined by means of inductively coupled plasma optical emission spectroscopy.

catalyst 4. The ICP analysis indicated the suitable pH for a DP process for the support material. Table 2 shows the results of continuous catalytic oxidation of glycerol using the prepared catalysts and a flow reactor. The behaviors in the time on stream are discussed in the next paragraph. The major products obtained were glyceric acid and tartronic acid. Of the five catalysts examined, at both 343 and 358 K, catalysts 3 and 4 indicated good performances for glycerol conversions. Comparing these two catalysts, catalyst 4 converted less glycerol but produced higher amounts of glyceric acid and tartronic acid than did catalyst 3. We therefore chose catalyst 4 for use in the subsequent experiments. No leakage of catalyst from the reaction chamber was observed during any of the reaction periods. Here, catalyst 1 also indicated good conversion, similar to that of the catalyst 4. The phenomena suggested formation of high-performance active sites, whose details are not clear now, on the support. However, according to ICP analysis, the amount of gold (as a rare noble metal) precursor lost in DP process was mostly among the tested catalysts. Thus, we considered that the preparation parameter (pH) was inappropriate. Figure 1 shows the catalytic performance of catalyst 4 with respect to the amount of glycerol converted and the amount of each carboxylic acid produced. This experiment was conducted continuously for a total of 690 min at four temperatures (343, 358, 353, and 343 K), with the initial temperature being repeated to examine whether the catalyst had an induction period. A pause for measurement was included at 360 min. We found that the catalytic activity of the second reaction at 343 K (570−690 min) was superior to that of the first reaction at 343 K (30−150 min), indicating that the catalyst had an induction period. The products detected by means of high-performance liquid chromatography were assigned to reasonable ingredients. Considering the product distribution, we concluded that the reaction pathway for this catalyst was comparable with that reported previously for glycerol oxidation using Au-based catalysts. Scheme 1 shows the reaction pathway for the conversion of glycerol into carboxylic acids using Au catalysts and batch reactors, as compiled on the basis of previously reported results from several research groups.13,16 The scheme suggests that the present catalysts pushed the equilibrium between glyceraldehyde and dihydroxyacetone to favor glyceraldehyde rather than dihydroxyacetone, which resulted in the formation of glyceric acid rather than lactic acid. In our previous study using a Au(−Pd)/TiO2 catalyst and a batch reactor, dihydroxyacetone was not detected. In contrast, dihydroxyacetone was detected

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RESULTS AND DISCUSSION Table 1 shows the Au content of the catalysts, as determined by inductively coupled plasma (ICP) optical emission spectroscopy, and the pH of the Au solution before and after deposition−precipitation (DP). Not all of the gold precursor was completely deposited on the surface of the alumina support, and we found that when a high pH was used, less Au was deposited compared to when a low pH was used; therefore, preparing the catalysts at a lower pH prevented the loss of Au. In the case of DP process, controlling the pHs is a very important factor in the preparation of better catalysts. The gold species are dispersed over the surface of the support by the electric affinity between the Au complexes and the surface of the support materials. After the pH adjustment, the electric charge of Au complexes, such as Au(OH)n, is negative in the aqueous solution. In our case, the electric affinities (interactions) were not so strong for fixing enough Au precursor in the higher pH conditions (4.6−7.0). Thus, the amounts of gold (catalysts 1−3) were lower than that of 13863

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Table 2. Results of Continuous Catalytic Oxidation of Glycerol Using the Prepared Catalysts and a Flow Reactora,b,c temperature = 343 K (0−180 min) catalyst 1 2 3 4 5 control

temperature = 358 K (180−360 min)

conversion of glycerol (%)

selectivity for glyceric acid (%)

selectivity for tartronic acid (%)

conversion of glycerol (%)

selectivity for glyceric acid (%)

selectivity for tartronic acid (%)

23.7 25.2 30.4 26.3 14.8

36.1 37.2 37.5 37.7 39.5

39.1 34.3 30.3 42.7 41.8

38.6 33.7 40.4 38.8 21.5 1.4

36.8 37.6 37.6 41.0 39.2 22.0

44.5 38.9 35.5 44.3 43.3 0

a

Conversions and selectivities are presented as the average of three measurements under identical conditions. bReactant solutions: 0.6 mol/L glycerol, 2.4 mol/L NaOH (glycerol/NaOH = 1:4 (mol/mol)); flow rate of the reactant: 0.25 mL/min; flow rate of oxygen: 6.0 mL/min; catalyst weight: 0.5 g. cControl: an Al2O3 support without Au as the active species.

Figure 1. Performance of catalyst 4 at various temperatures. (a) Conversion of glycerol, (b) selectivities for products. Reactant solutions: 0.6 mol/L glycerol, 2.4 mol/L NaOH (glycerol/NaOH = 1:4 (mol/mol)); flow rate of the reactant: 0.25 mL/min; flow rate of oxygen: 6.0 mL/min; catalyst weight: 0.5 g.

as one of products in the present study, albeit in low amounts. Because of the short residence time in the catalyst bed, dihydroxyacetone (DHA) and glyceraldehyde did not reach equilibrium state. (Given sufficient time, DHA is converted into glyceric acid via glyceraldehyde, as reported previously, which is a competition between lactic acid production and glyceric acid production.) DHA is also a high-value molecule for sunless-tanning cosmetics. If DHA is also included into target molecules of the oxidation, the total selectivities (glyceric acid, tartronic acid, and DHA, which are threecarbon molecules without cleavage of C−C bond in glycerol) reach about 80% in our case (see Figure 2). Figure 2 shows the long-term (total time, 1650 min) performance of catalyst 4 at 343 K. In this experiment, we used

5 times the weight of the catalyst that was used in the previous experiment so that higher product yields would be obtained. Again, an induction period was observed, and no deactivation of the catalyst was observed at any point during the reaction period. The selectivity (44.6%) of tartronic acid, which is also a high-value chemical, at 1650 min is higher than that of typical results using a batch-type reactor.17 This is one of the superior points of this research. The time course of carbon balance is shown in the Supporting Information (Figure S2a). The average of the carbon balance from the first analysis point to the end is 95.0%. The amount reduced without reaching 100% is presumed to be CO2 formation (gaseous CO2 or sodium carbonate in aqueous solution) by complete oxidation, except for the error in quantitative analysis. 13864

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Scheme 1. Considerable Reaction Pathway for the Conversion of Glycerol into Carboxylic Acids Using Au Catalysts and Batch Reactorsa

a

This scheme was prepared from results reported by several research groups. Bold and underlined molecules are the products detected in the present study by means of high-performance liquid chromatography.

Figure 2. Long-term performance of catalyst 4 at 343 K. Reactant solutions: 0.6 mol/L glycerol, 2.4 mol/L NaOH (glycerol/NaOH = 1:4 (mol/ mol)); flow rate of the reactant: 0.25 mL/min; flow rate of oxygen: 6.0 mL/min; catalyst weight: 2.5 g.

We considered the induction period to reflect the reduction of Au to a metallic state by glycerol and the oxidation products. Therefore, we next examined what effect pretreatment with fructose, an organic reductant, would have on the performance of catalyst 4 at 343 K (Figure 3; see the Supporting Information for experimental details). With fructose pretreatment, the initial activity of the catalyst (conversion = 60%) was higher than that without pretreatment (conversion = 50%). In addition, the conversion of glycerol reached over 70% in less time (200 min) than without pretreatment (600 min). The highest conversion obtained with pretreatment was more than 80%. These results confirmed that reduction of Au was one of the reasons for the induction period observed in the earlier experiment (Figures 1 and 2). Next, we examined the effect of NaOH, an alkaline additive, on the reactant by not including NaOH in the reactant between 1110 and 1410 min (Figure 3). Removal of NaOH from the reactant produced a marked deactivation of the catalyst, which was recovered after the reintroduction of NaOH into the reactant. This confirmed that NaOH in the

reactant solution was essential for the conversion of glycerol to carboxylic acids, as has been reported by several groups.11d,18 The average turn-over frequency (TOF) between 1470 and 1770 min was 67.5 h−1. (The equation for calculation of TOF is shown in the Supporting Information.) The improvement in the TOF compared with that reported from our previous study12 (34.1 h−1) was likely a result of the higher operation temperature used in the present study, which was achievable because of the high heat-resistance of the alumina we selected as the support material. The carbon balance of this experiment is also shown in the Supporting Information (Figure S2b). The average carbon balance of the steady state (except the data from 1110 to 1410 min) is 94.7% in this case. Finally, we used scanning transmission electron microscopy to examine the surface morphology of catalyst 4 immediately after calcination (Figure 4). In the lower-magnification image (Figure 4a), several Au nanoparticles (mode = 7.5−10 nm (45.8% of counted nanoparticles)) can be seen. Size range of 96.6% of nanoparticles is 2.5−2.5 nm. The histogram is shown in the Supporting Information (Figure S3a). In the higher13865

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Figure 3. Effects of fructose pretreatment and the removal of NaOH from the reactant on the performance of catalyst 4. (a) Conversion of glycerol, (b) selectivities for products. Reactant solutions: 0.6 mol/L glycerol, 2.4 mol/L NaOH (glycerol/NaOH = 1:4 (mol/mol)); flow rate of the reactant: 0.25 mL/min; flow rate of oxygen: 6.0 mL/min; catalyst weight: 2.5 g; temperature: 343 K.

Figure 4. Scanning transmission electron microscopyhigh-angle annular dark-field images of catalyst 4. (a) Low-magnification image and (b) high-magnification image. Arrows in (b) indicate highly dispersed atomic Au.

magnification image (Figure 4b), highly dispersed atomic Au and small Au clusters (1−2 nm) can be seen. In the measured square field of Figure 4b (19.8 nm × 19.8 nm), there are about 35 atomic gold and 1 clusterlike gold species. Similar atomic and clusterlike Au species on the surface of an alumina support (or an Fe-modified alumina support) in catalysts developed for the hydrogenation of 5-hydroxymethylfurfural to afford 2,5bis(hydroxymethyl)furan were previously reported by Satsuma et al.;19 in addition, atomic Au species have been reported to be inactive or less catalytically active than the clusterlike species with respect to their hydrogenation activity. Details of the catalytic activities of the three Au species observed in the

present study remain to be elucidated. We are now advancing analysis to elucidate the gold nanospecies.

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CONCLUSIONS We prepared five high-performance Au/alumina catalysts by means of a deposition−precipitation method for the conversion of glycerol to high-value carboxylic acids. The reaction was conducted in a liquid-phase flow reactor, which was made possible by careful selection of a suitable alumina support material. The catalysts were prepared under different pH conditions, which resulted in the deposition of varying amounts of Au on the alumina support. After the activities of 13866

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Japan Society for the Promotion of Science (Grant IDs: JP16KT0038, JP16H03003, and JP25340130).

the catalysts were examined, the most active catalyst was selected and further characterized. We found that this catalyst could be used for long-term (up to 1770 min) conversion of glycerol into glyceric acid and tartronic acid. The TOFs for this catalyst were higher than those we reported previously. We also found that the catalyst had an induction period, which we considered to be due to the reduction of the Au in the catalyst to a suitable state for oxidation. No deactivation of the catalyst was observed during the long-term test. Finally, scanning transmission electron microscopy revealed three highly dispersed Au species on the surface of the alumina supportatomic Au, Au nanoparticles, and Au clusters.

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(1) (a) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Novel Gold Catalysts for the Oxidation of Carbon Monoxide at a Temperature far Below 0 °C. Chem. Lett. 1987, 16, 405−408. (b) Hutchings, G. J. Vapor phase hydrochlorination of acetylene: Correlation of catalytic activity of supported metal chloride catalysts. J. Catal. 1985, 96, 292−295. (2) Zhou, C.-H.; Xia, X.; Lin, C.-X.; Tong, D.-S.; Beltramini, J. Catalytic conversion of lignocellulosic biomass to fine chemicals and fuels. Chem. Soc. Rev. 2011, 40, 5588−5617. (3) (a) Aransiola, E. F.; Ojumu, T. V.; Oyekola, O. O.; Madzimbamuto, T. F.; Ikhu-Omoregbe, D. I. O. A review of current technology for biodiesel production: State of the art. Biomass Bioenergy 2014, 61, 276−297. (b) Abbaszaadeh, A.; Ghobadian, B.; Omidkhah, M. R.; Najafi, G. Current biodiesel production technologies: A comparative review. Energy Convers. Manage. 2012, 63, 138−148. (4) Pagliaro, M.; Ciriminna, R.; Kimura, H.; Rossi, M.; Della Pina, C. From Glycerol to Value-Added Products. Angew. Chem., Int. Ed. 2007, 46, 4434−4440. (5) Katryniok, B.; Kimura, H.; Skrzynska, E.; Girardon, J.-S.; Fongarland, P.; Capron, M.; Ducoulombier, R.; Mimura, N.; Paul, S.; Dumeignil, F. Selective catalytic oxidation of glycerol: perspectives for high value chemicals. Green Chem. 2011, 13, 1960−1979. (6) Jpn. Kokai Tokkyo Koho JP08-151345H, 08-092156H, 06279352H. (7) Kimura, H.; Tsuto, K.; Wakisaka, T.; Kazumi, Y.; Inaya, Y. Selective oxidation of glycerol on a platinum-bismuth catalyst. Appl. Catal., A 1993, 96, 217−228. (8) Kimura, H. Selective oxidation of glycerol on a platinum-bismuth catalyst by using a fixed bed reactor. Appl. Catal., A 1993, 105, 147− 158. (9) (a) Ning, X.; Li, Y.; Yu, H.; Peng, F.; Wang, H.; Yang, Y. Promoting role of bismuth and antimony on Pt catalysts for the selective oxidation of glycerol to dihydroxyacetone. J. Catal. 2016, 335, 95−104. (b) Xiao, Y.; Greeley, J.; Varma, A.; Zhao, Z. J.; Xiao, G. An experimental and theoretical study of glycerol oxidation to 1,3dihydroxyacetone over bimetallic Pt-Bi catalysts. AIChE J. 2017, 63, 705−715. (c) Zhang, M.; Shi, J.; Sun, Y.; Ning, W.; Hou, Z. Selective oxidation of glycerol over nitrogen-doped carbon nanotubes supported platinum catalyst in base-free solution. Catal. Commun. 2015, 70, 72−76. (10) Dimitratos, N.; Lopez-Sanchez, J. A.; Anthonykutty, J. M.; Brett, G.; Carley, A. F.; Tiruvalam, R. C.; Herzing, A. A.; Kiely, C. J.; Knight, D. W.; Hutchings, G. J. Oxidation of glycerol using goldpalladium alloy-supported nanocrystals. Phys. Chem. Chem. Phys. 2009, 11, 4952−4961. (11) (a) Dimitratos, N.; Lopez-Sanchez, J. A.; Lennon, D.; Porta, F.; Prati, L.; Villa, A. Effect of Particle Size on Monometallic and Bimetallic (Au,Pd)/C on the Liquid Phase Oxidation of Glycerol. Catal. Lett. 2006, 108, 147−153. (b) Villa, A.; Wang, D.; Su, D.; Veith, G. M.; Prati, L. Using supported Au nanoparticles as starting material for preparing uniform Au/Pd bimetallic catalysts. Phys. Chem. Chem. Phys. 2010, 12, 2183−2189. (c) Bogdanchikova, N.; Tuzovskaya, I.; Prati, L.; Villa, A.; Pestryakov, A.; Farías, M. More Insights into Support and Preparation Method Effects in Gold Catalyzed Glycerol Oxidation. Curr. Org. Synth. 2017, 14, 377−382. (d) Villa, A.; Jouve, A.; Sanchez Trujillo, F.; Motta, D.; Prati, L.; Dimitratos, N. Exploring the Effect of Au/Pt Ratio on Glycerol Oxidation in Presence and Absence of a Base. Catalysts 2018, 8, 54. (12) Mimura, N.; Hiyoshi, N.; Fujitani, T.; Dumeignil, F. Liquid phase oxidation of glycerol in batch and flow-type reactors with oxygen over Au-Pd nanoparticles stabilized in anion-exchange resin. RSC Adv. 2014, 4, 33416−33423. (13) Mimura, N.; Hiyoshi, N.; Daté, M.; Fujitani, T.; Dumeignil, F. Microscope Analysis of Au−Pd/TiO2 Glycerol Oxidation Catalysts

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EXPERIMENTAL SECTION We prepared five Au/Al2O3 catalysts by means of the deposition−precipitation method established by Haruta et al.20 Briefly, an acidic Au solution was prepared and adjusted to the required pH by addition of NaOH. Powdered support material was then added to the solution, the mixture was stirred for 2 h at 343 K, and the catalyst precursor was washed several times with distilled water and filtered. The obtained catalyst powder was dried at room temperature overnight and then calcined at 673 K for 4 h. Catalytic reaction tests were conducted using a stainless-steel liquid-phase flow reactor (inside diameter = 9.4 mm, length = 50 mm; Eyela, Tokyo, Japan) with Teflon filters at the inlet and outlet of the reaction chamber. The reaction chamber was filled with the catalyst and then heated using an electric aluminum-block heater. Products were identified by means of high-performance liquid chromatography (Shimadzu, Kyoto, Japan). Calibration curves for quantitative measurements were prepared using commercial reagents of high purity. The Au content of the catalysts was determined by means of inductively coupled plasma optical emission spectroscopy. A scanning transmission electron microscope (ARM-200F; JEOL, Tokyo, Japan) was used to examine the surface morphology of the catalysts. Details of the experiments are described in the Supporting Information.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b01191. Experimental details (materials, preparation of the catalyst, catalytic reaction tests, analysis of the catalysts, and fructose pretreatment), calculation of TOF (turnover frequency), and carbon balance of the long-term catalytic test and histogram of STEM analysis (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81-29-861-8460. Fax: +81-22-237-5226. ORCID

Naoki Mimura: 0000-0001-7809-8894 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support received in the form of Grants-in-Aid for Scientific Research (B) and (C) from the 13867

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Prepared by Deposition−Precipitation Method. Catal. Lett. 2014, 144, 2167−2175. (14) (a) Wiles, C.; Watts, P. Continuous flow reactors: a perspective. Green Chem. 2012, 14, 38−54. (b) Ricciardi, R.; Huskens, J.; Verboom, W. Nanocatalysis in Flow. ChemSusChem 2015, 8, 2586−2605. (15) (a) Len, C.; Delbecq, F.; Cara Corpas, C.; Ruiz Ramos, E. Continuous Flow Conversion of Glycerol into Chemicals: An Overview. Synthesis 2018, 50, 723−741. (b) Motta, D.; Trujillo, F. J. S.; Dimitratos, N.; Villa, A.; Prati, L. An investigation on AuPt and AuPt-Bi on granular carbon as catalysts for the oxidation of glycerol under continuous flow conditions. Catal. Today 2018, 308, 50−57. (16) Bianchi, C. L.; Canton, P.; Dimitratos, N.; Porta, F.; Prati, L. Selective oxidation of glycerol with oxygen using mono and bimetallic catalysts based on Au, Pd and Pt metals. Catal. Today 2005, 102−103, 203−212. (17) Villa, A.; Dimitratos, N.; Chan-Thaw, C. E.; Hammond, C.; Prati, L.; Hutchings, G. J. Glycerol Oxidation Using Gold-Containing Catalysts. Acc. Chem. Res. 2015, 48, 1403−1412. (18) (a) Ketchie, W. C.; Murayama, M.; Davis, R. J. Promotional effect of hydroxyl on the aqueous phase oxidation of carbon monoxide and glycerol over supported Au catalysts. Top. Catal. 2007, 44, 307− 317. (b) Skrzyńska, E.; Zaid, S.; Girardon, J.-S.; Capron, M.; Dumeignil, F. Catalytic behaviour of four different supported noble metals in the crude glycerol oxidation. Appl. Catal., A 2015, 499, 89− 100. (19) (a) Ohyama, J.; Esaki, A.; Yamamoto, Y.; Arai, S.; Satsuma, A. Selective hydrogenation of 2-hydroxymethyl-5-furfural to 2,5-bis(hydroxymethyl)furan over gold sub-nano clusters. RSC Adv. 2013, 3, 1033−1036. (b) Ohyama, J.; Hayashi, Y.; Ueda, K.; Yamamoto, Y.; Arai, S.; Satsuma, A. Effect of FeOx Modification of Al2O3 on Its Supported Au Catalyst for Hydrogenation of 5-Hydroxymethylfurfural. J. Phys. Chem. C 2016, 120, 15129−15136. (20) Haruta, M.; Tsubota, S.; Kobayashi, T.; Kageyama, H.; Genet, M. J.; Delmon, B. Low-Temperature Oxidation of CO over Gold Supported on TiO2, α-Fe2O3, and Co3O4. J. Catal. 1993, 144, 175− 192.

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