Gold Catalysis and Photoactivation: A Fast and Selective Procedure

Letter Cite This: ACS Catal. 2018, 8, 1635−1639

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Gold Catalysis and Photoactivation: A Fast and Selective Procedure for the Oxidation of Free Sugars Mehdi Omri,† Frédéric Sauvage,‡ Yan Busby,§ Matthieu Becuwe,‡ Gwladys Pourceau,*,† and Anne Wadouachi†

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†

Laboratoire de Glycochimie, des Antimicrobiens et des Agroressources (LG2A) UMR CNRS 7378−Institut de Chimie de Picardie FR 3085, Université de Picardie Jules Verne, 33 rue Saint Leu, FR-80039 Amiens Cedex, France ‡ Laboratoire de Réactivité et Chimie des Solides (LRCS), UMR CNRS 7314−Institut de Chimie de Picardie FR 3085, Université de Picardie Jules Verne, 33 rue Saint Leu, FR-80039 Amiens Cedex, France § Laboratoire Interdisciplinaire de Spectroscopie Electronique (LISE), Namur Institute of Structured Matter (NISM), University of Namur, 61 Rue de Bruxelles, 5000 Namur, Belgium S Supporting Information *

ABSTRACT: A fast and efficient methodology for the selective oxidation of sugars into corresponding sodium aldonates is herein reported. Hydrogen peroxide was used as a cheap oxidant and electron scavenger, in the presence of only 0.003−0.006 mol % of gold in basic conditions. Three photocatalysts were studied, namely Au/Al2O3, Au/ TiO2, and Au/CeO2, the latter being the most efficient (TOF > 750 000 h−1) and perfectly selective. Only a 10 min exposition under standard incident sunlight irradiation (A.M.1.5G conditions, 100 mW/cm2) affords total conversion of glucose into the corresponding sodium gluconate. Demonstrating its versatility, this methodology was successfully applied to a variety of oligosaccharides leading to the corresponding aldonates in quantitative yield and high purity (>95%) without any purification step. The photocatalyst was recovered by simple filtration and reused 5 times leading to the same conversion and selectivity after 10 min of illumination. KEYWORDS: carbohydrates, photocatalytic oxidation, gold catalysis, aldonates, oligosaccharides modification

T

oxidative selectivity and restricted spectral absorption into the UV owing to a too large bandgap value.4−6 This is for instance the case of the benchmark anatase TiO2 semiconductor for which its 3.20 eV bandgap value restricts the light absorption/ conversion below 410 nm, thus representing at best only 5% of the total solar spectrum. Among other approaches such as aliovalent doping, the use of SC-supported metal nanoparticles, especially gold or silver, was rapidly highlighted as an efficient approach to address this issue by extending the spectral absorption to the visible region thanks to localized surface plasmon resonance (LSPR) effect. In addition, it also affords to prolong electron/hole charge separation in the metal owing to the interfacial metal/SC Schottky barrier assisting the electron injection into the conduction band of the SC.7−12 Some studies on the selective photo-oxidation of organic compounds such as thiol into disulfide,13 aliphatic alcohol into carbonyl derivative14 or aminobenzyl alcohol into aminobenzaldehyde15 were recently reported in the presence of catalyst Au/MOx (MOx: metal oxide).13−17 On the other hand, only a few studies aimed

he use of sugars as renewable and abundant feedstock and their eco-friendly conversion into chemicals are becoming increasingly attractive to synthesize high-value-added products.1 Sugars are polyfunctional and serve as a chemical platform to synthesize a wide variety of derivative compounds after chemical modifications.2 However, the multistep protocols and hazardous chemicals used for these transformations penalize this chemistry to be transposed to the industrial scale. Among the different “glycotransformations”, C1oxidation is of particular interest since it leads to aldonic acids or their salts, widely used in cosmetics, pharmaceutical, food, and paper industries.3 Conventional procedures gather several drawbacks such as limited selectivity to aldonic acids, production of degradation byproducts owing to the weakness of the interglycosidic bond, separation issues of the products, or environmental toxicity. In this context, heterogeneous photocatalytic transformation is considered as one of the most promising sustainable and eco-friendly procedure. Although many efforts were devoted to design semiconducting (SC) materials exhibiting a high photocatalytic yield and selective charge transfer processes under visible light exposure rather than the lower UV flux of photons, the most promising photocatalyst materials developed so far are suffering from low © 2018 American Chemical Society

Received: October 5, 2017 Revised: December 22, 2017 Published: January 19, 2018 1635

DOI: 10.1021/acscatal.7b03394 ACS Catal. 2018, 8, 1635−1639

Letter

ACS Catalysis at selectively photo-oxidizing carbohydrates using TiO2, Ag/ TiO2, or Cr/TiO2 with limited success.18−21 Indeed, the results systematically report a low conversion yield and a lack of selectivity to the corresponding gluconic acid, leading to smaller chain monosaccharides and partial- and overoxidized byproducts. Interestingly, Zhou et al. reported very recently the aerobic oxidation of two monosaccharides (glucose and xylose) at micromolar scale in 4 h at 30 °C under high light power (300 mW/cm2) into the corresponding acids with good selectivity by using Au/TiO2 photocatalyst (3.8 mol %).22 In this work, we report on a simple and an efficient procedure affording a fast and selective photoelectrochemical reaction leading to pure sodium mono- and oligo-aldonates in less than 10 min of incident light exposure under standard photovoltaic A.M.1.5G conditions (100 mW/cm2) at room temperature (Scheme 1). Scheme 1. Photocatalyzed Oxidation of Saccharides into Corresponding Aldonates

Figure 1. Top: Conversion of glucose vs time using Au/Al2O3 (green line), Au/TiO2 (red line), or Au/CeO2 (blue line) catalysts; bottom: Selectivity toward sodium gluconate using Au/Al2O3 (green bar), Au/ TiO2 (red bar), or Au/CeO2 (blue bar) catalysts.

In this study, three different photocatalysts are investigated: Au/TiO2 and Au/CeO2 for which hot electrons from gold excited state can be injected into the related conduction band of the SC, and Au/Al2O3 for which the excessive value of the bandgap (≈ 4.4 eV) prevents any interfacial charge transfer to proceed.23 The photocatalysts were prepared by a deposition− precipitation method and characterized by transmission electron microscopy (TEM), flame spectrometry (AAS), and X-ray diffraction (XRD).24 The gold nanoparticles have a size of around 2−3 nm with good monodispersity (Figure S1). Gold loading of each catalyst is between 0.4 and 0.7 wt % (Table S1) and no change of crystallinity of the SC support was noticed subsequently to gold deposition (Figure S2). Gold nanoparticles show a broad plasmonic absorption band at ca. 550 nm as measured in UV−visible absorption spectroscopy in reflectance mode (Figure S3). The XPS analysis on SC supported gold nanoparticles shows that gold is mainly found in the metallic state (Au0 represent 85% of the peak area in Au/ CeO2 while Au+I oxidation state 15%, Figure S4). The photocatalysts performance was evaluated in water for the oxidation of glucose using a catalytic charge of 0.003−0.006 mol % of gold (i.e., a substrate/gold ratio between 15 000 and 30 000), 1.1 equiv of hydrogen peroxide as an electron scavenger and 1 equiv of sodium hydroxide. After filtration of the catalyst, the resulting reaction media was freeze-dried and analyzed by NMR and HPLC to evaluate both conversion and selectivity into sodium gluconate (Figure S5−S6). Otherwise, EDX analysis of lyophilized media did not evidence any dissolution of the photocatalyst (Figure S7). As shown in Figure 1, whatever the photocatalyst, moderate to high conversion yields (between 30 and 79%) into gluconate were already achieved in only 5 min of illumination (Figure S8). This is significantly higher than the values reported in the literature for oxidation using atmospheric air (7−16% in most cases).18−21 Surprisingly, Au/TiO2 exhibited the lower photocatalytic activity in terms of both conversion and selectivity, albeit still superior than values reported in the literature: 78% of

glucose were transformed into corresponding sodium gluconate with a selectivity of ∼85% in 30 min. Au/Al2O3 enables evaluating the intrinsic photocatalytic properties of gold itself because the 4.4 eV bandgap value of Al2O3 prevents any electron injection from Au to Al2O3 and any direct bandgap excitation. Although one would have expected this catalyst lagging far behind, light absorption by gold ensures a quantitative conversion into pure sodium gluconate with a selectivity greater than 95% in 30 min. Supporting gold to nanocrystalline CeO2 accelerates significantly glucose oxidation without penalizing the selectivity (Figure 1). Quantitative conversion to the gluconate was obtained in only 10 min of illumination. A turnover frequency (TOF) of 752 380 h−1 was calculated for this photocatalyst after 5 min of irradiation (i.e., at 79% conversion). Although the bandgap value of TiO2 and CeO2 are relatively comparable, the differences between TiO2 and CeO2-supported gold photocatalysts can be explained by three factors:10,14,25 (i) LSPR effect is stronger for Au/CeO2 than for Au/TiO2; this enables a better light capture by the gold nanoparticles (ii) the conduction band position of CeO2 is more oxidizing than TiO2 counterpart and the Schottky barrier is lower for CeO2 (0.8 eV) than in TiO2 (1.09 eV), and thus one could anticipate that gold excited states should inject faster in CeO2 than in TiO2 which for this latter already takes place in less than 240 fs (more efficient charge separation); and (iii) a strong quenching of CeO2 luminescence has been observed under UV excitation (375 nm) when supporting Au NPs suggesting either a reduction of recombination rate or also electron injection into the metal gold, thus offering more separated carriers to onset the photo-oxidation process. In addition, the oxygen vacancies at the CeO2 surface help to achieve a better gold dispersion at the surface of the semiconductor as well as to enhance the stability of gold nanoparticles. The lower selectivity of TiO2based photocatalysts has been previously pointed out.22 It has been endowed to the formation of strong oxidizing radicals 1636

DOI: 10.1021/acscatal.7b03394 ACS Catal. 2018, 8, 1635−1639

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ACS Catalysis

literature reporting the use of atmospheric air as oxidant for the oxidation of carbohydrates.7,8 This discrepancy may stem from the shorter reaction time (99%) and selectivity (>95%) after 10 min of irradiation. Postaging characterization by means of TEM and AAS showed no structural, chemical, and textural transformation of the photocatalyst upon cycling (Table 2, Figure S1, Table S1). No further oxidation of gold

formed from the direct hole transfer from the O 2p orbitals of TiO2 under UV irradiation. A very recent work showed that glucose can be converted into formate using nano-TiO2 in basic conditions as a result from the formation of two types of strongly oxidative radicals ·OH (E° = +2.38 V vs NHE) and O2− (E° = +1.72 V vs NHE) when in the presence of hydroxyl ions.26 Several blank tests were carried out to better highlight the role of the photocatalyst (Table 1). First, a reaction without any Table 1. Influence of Some Parameters on the Conversion of Glucose and on the Selectivity toward Gluconic Acida catalyst 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

TiO2 Al2O3 CeO2 Au/TiO2

Au/Al2O3

Au/CeO2

neq H2O2 neq NaOH 1.1 1.1 1.1 1.1 1.1 0 1.1c 1.1 0 1.1c 1.1 0.5 0 1.1 1.1b 1.1c

1 1 1 1 1 1 1c 1 1 1c 1 1 1 0.5 0b 1c

conversion (%)

selectivity (%)

0 0 0 0 49 0 0c 69 0 0c >99 75 0 63 29b 0c

95 -c >95 >95 >95 >95b -c

a

Reaction conditions: glucose 250 mg, catalyst 2.5 mg, water 5 mL, light power 100 mW/cm2 (A.M.1.5G), 10 min. bReaction time extended to 1 h. cReaction performed in darkness.

photocatalyst was performed under A.M.1.5G illumination showing no reaction as one would have expected (entry 1). Second, the properties of the semiconducting materials alone (without gold nanoparticles) were evaluated according to the same reaction conditions (entries 2−4). In all cases, whatever the nature of the metal oxide, glucose remained unchanged, thus confirming that gold-free semiconductors are not photocatalysts under A.M.1.5G white light, proving the photochemical stability of these latter together with stressing the need to combine synergistically the SC with the Au NPs to achieve an efficient and selective photon-induced oxidation reaction. This gives evidence that OH· formed subsequently to hole transfer from the SC valence band cannot oxidize the glucose in our conditions, therefore is not an intermediate involved in the glucose to gluconate conversion, at least under A.M.1.5G conditions. Finally, similar reactions were performed in darkness. In this case, whatever the catalyst, glucose remained unchanged (entries 7, 10, 16) confirming that glucose is chemically stable in our conditions and underlying that the oxidation reaction involves photon-induced charge transfer processes. Indeed, no glucose decomposition in the presence of hydrogen peroxide is herein observed (entries 1−4, 7, 10, 16), by contrast to what has been experienced by Comotti et al. using various mixture between glucose and hydrogen peroxide at pH = 9.5 without catalyst and in darkness.27 The role of hydrogen peroxide is crucial: even in the presence of gold catalyst (entries 6, 9, 13), neither oxidation nor degradation of glucose was observed if hydrogen peroxide was not added. This is in contrast to several works in the

Table 2. Characteristics of Au/CeO2 after 5 Runs catalyst

mean sizea (nm)

gold contentb (%)

initial after 5 runs

2.9 ± 0.7 3.2 ± 0.6

0.50 ± 0.01 0.49 ± 0.01

a

Mean size was determined by TEM. bGold content was determined by AAS.

was also evidenced by XPS (Figure S4). This stresses its high robustness in the different media herein investigated. Nevertheless, a decrease in the TOF value was observed: a TOF of 466 000 h−1, still higher than those reported in literature, was calculated for the sixth run after 5 min of irradiation (Figure S8). Such excellent results can be obtained also at 3g-scale keeping same conversion yield and selectivity into sodium gluconate. Finally, the optimized conditions (Table 1, entry 11) were successfully applied to a variety of saccharides from mono- to 1637

DOI: 10.1021/acscatal.7b03394 ACS Catal. 2018, 8, 1635−1639

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ACS Catalysis Table 3. Photocatalyzed Oxidation of Various Substratesa

a

Reaction conditions: sugar 250 mg, Au catalyst 2.5 mg, NaOH 1 equiv, H2O2 1.1 equiv, water 5 mL, hν (standard A.M.1.5G conditions), 10 min. Conversion rates (conv.) and selectivity (selec.) were determined by NMR

residues (glucose or galactose) reacted quantitatively in 10 min with a selectivity greater than 95%.This methodology was also successfully applied to higher DP oligosaccharides such as maltotriose and maltohexose (entries 5−6) demonstrating its versatility. This appealing result underlines once again the advantage of photochemistry against, for instance, microwaveassisted procedure in terms of substrate-dependency. We thus demonstrate systematically fast, quantitative conversion yield and excellent selectivity regardless of the type of sugar, leading to pure aldonates without any means of purification step.

oligosaccharides (degree of polymerization (DP) from 1 to 6) leading to the corresponding mono to hexa-aldonates with excellent conversion and selectivity (Table 3, Figure S9−S10). Interestingly, by contrast to the microwave-assisted procedure recently described,24 NaOH can be used as a base even for oligosaccharides without degradation and/or hydrolysis of the interglycosidic bond, probably thanks to the mild conditions (room temperature and short reaction time). Moreover, this photoelectrochemical reaction is not substrate-dependent: lactose, maltose or cellobiose (entries 2−4), which exhibits different types of glycosidic linkage (α or β) and of glycosidic 1638

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ACS Catalysis

(15) Kowalska, E.; Prieto Mahaney, O. O.; Abe, R.; Ohtani, B. Phys. Chem. Chem. Phys. 2010, 12, 2344−2355. (16) Kowalska, E.; Abe, R.; Ohtani, B. Chem. Commun. 2009, 241− 243. (17) Tanaka, A.; Hashimoto, K.; Kominami, H. J. Am. Chem. Soc. 2012, 134, 14526−14533. (18) Colmenares, J. C.; Magdziarz, A. J. Mol. Catal. A: Chem. 2013, 366, 156−162. (19) Colmenares, J. C.; Magdziarz, A.; Kurzydlowski, K.; Grzonka, J.; Chernyayeva, O.; Lisovytskiy, D. Appl. Catal., B 2013, 134−135, 136− 144. (20) Da Vià, L.; Recchi, C.; Davies, T. E.; Greeves, N.; LopezSanchez, J. A. ChemCatChem 2016, 8, 3475−3483. (21) Da Vià, L.; Recchi, C.; Gonzalez-Yañez, E. O.; Davies, T. E.; Lopez-Sanchez, J. A. Appl. Catal., B 2017, 202, 281−288. (22) Zhou, B.; Song, J.; Zhang, Z.; Jiang, Z.; Zhang, P.; Han, B. Green Chem. 2017, 19, 1075−1081. (23) Gaeeni, M. R.; Ghamsari, M. S.; Majd Abadi, A. M.; Majles Ara, M. H.; Han, W.; Park, H.-H. J. Am. Ceram. Soc. 2015, 98, 1818−1822. (24) Omri, M.; Pourceau, G.; Becuwe, M.; Wadouachi, A. ACS Sustainable Chem. Eng. 2016, 4, 2432−2438. (25) Jiang, D.; Wang, W.; Sun, S.; Zhang, L.; Zheng, Y. ACS Catal. 2015, 5, 613−621. (26) Jin, B.; Yao, G.; Wang, X.; Ding, K.; Jin, F. ACS Sustainable Chem. Eng. 2017, 5, 6377−6381. (27) Comotti, M.; Della Pina, C.; Falletta, E.; Rossi, M. Adv. Synth. Catal. 2006, 348, 313−316. (28) Castillo, N. C.; Ding, L.; Heel, A.; Graule, T.; Pulgarin, C. J. J. Photochem. Photobiol., A 2010, 216, 221−227. (29) Tseng, D.-H.; Juang, L.-C.; Huang, H.-H. Int. J. Photoenergy 2012, 2012, Article ID 328526. (30) Nosaka, Y.; Nosaka, A. Y. Chem. Rev. 2017, 117, 11302−11336. (31) Diesen, V.; Jonsson, M. J. Phys. Chem. C 2014, 118, 10083− 10087. (32) Zope, B. N.; Hibbitts, D. D.; Neurock, M.; Davis, R. J. Science 2010, 330, 74−78. (33) Davis, S. E.; Ide, M. S.; Davis, R. J. Green Chem. 2013, 15, 17− 45.

In conclusion, a mild, very fast and versatile method to selectively produce aldonates from mono and oligosaccharides derived from biomass was successfully developed. This reaction is performed in water, using a cheap and green oxidant and involving a very few quantities of a reusable catalyst. The photocatalytic results herein reported were gathered using a class A calibrated standard A.M.1.5G light source (P = 100 mW/cm2) typically used for photovoltaic characterization, ie. a procedure reproducible and realistic with respect to the endapplication. These first results call for further development of novel oligo-aldonate derivatives in large and green scale for their utilization as detergents, pharmaceuticals or cosmetics to replace petroleum-based chemicals.

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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/acscatal.7b03394. Experimental details, materials, methods, comparison experiments, catalysts, and crude characterization (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yan Busby: 0000-0002-6826-6142 Matthieu Becuwe: 0000-0002-1949-3955 Gwladys Pourceau: 0000-0001-9991-5793 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.O., M.B., G.P., and A.W. wish to thank the Conseil Régional de Picardie for its financial support. F.S. acknowledges the French National Research Agency (ANR) for funding Photo2batt project.

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REFERENCES

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DOI: 10.1021/acscatal.7b03394 ACS Catal. 2018, 8, 1635−1639