Aerobic Oxidation of Glucose to Glucaric Acid under Alkaline-Free

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Aerobic oxidation of glucose to glucaric acid under alkaline-free conditions: Au-based bimetallic catalysts and effect of residues in a hemicellulose hydrolysate. Elie Derrien, Modibo Mounguengui-Diallo, Noemie Perret, Philippe Marion, Catherine Pinel, and Michele Besson Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 06 Jun 2017 Downloaded from http://pubs.acs.org on June 7, 2017

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Industrial & Engineering Chemistry Research

Aerobic oxidation of glucose to glucaric acid under alkaline-free conditions: Au-based bimetallic catalysts and effect of residues in a hemicellulose hydrolysate. Elie Derrien1, 2, Modibo Mounguengui-Diallo1, Noémie Perret1, Philippe Marion2, Catherine Pinel1, Michèle Besson1* 1 – Univ Lyon, Univ Claude Bernard, CNRS, IRCELYON, UMR5256, 2 Avenue Albert Einstein, F-69626 Villeurbanne, France 2 – SOLVAY Research and Innovation Center of Lyon, 69192 Saint Fons, France

Dedicated to Professor Tapio Salmi, Åbo Akademi University, Turku (Finland)

ABSTRACT Au-Pt and Au-Pd bimetallic catalysts were prepared over various supports using different preparation methods and were compared in the base-free selective aerobic catalytic oxidation of glucose to glucaric acid. The method of preparation of the bimetallic catalysts, the support material for the Au-Pt bimetallic nanoparticles, and the metal molar ratios have a strong influence on the activity and the maximum yield of glucaric acid. The Au-Pt/ZrO2 catalyst with molar ratio Au/Pt = 1 provides a 50 % yield of glucaric acid at complete conversion of glucose and gluconic acid at 100°C, under 40 bar air, using a glucose/metal ratio of 80. The catalyst was stable upon sequential recycling in batch reactor and in long-term use in a continuous reactor. The influence of possible residual impurities has been studied. Furan derivatives or lignin residues might be problematic for catalytic oxidation of glucose in hemicellulose hydrolysates.

KEYWORDS: oxidation of glucose, glucaric acid, base-free, Au-Pt catalysts, hydrolysates

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Table of contents


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1. INTRODUCTION Sustainable biorenewable resources present a way to move away from our dependence on fossil hydrocarbons and move towards a sustainable bio-based chemicals industry. Monosaccharides are generally very useful compounds and carbohydrate biomass resources are abundant, widely distributed, and cheap starting materials.1,2 The cellulose and hemicellulose fraction of lignocellulosic feedstocks may serve for the production of sugars which could be converted to other potential platform chemicals; lignocellulose has crucial advantages over starch and sugar crops, because it is not edible and do not interfere with food supplies.3,4 However, even though lignocellulosic biomass is abundant, one of the main challenges is the efficient depolymerization of cellulose and hemicelluloses into their composing monomers.5 The major component cellulose is a homopolymer of glucose that has high crystallinity and is difficult to depolymerize. Hemicelluloses differ in structure and composition and, unlike cellulose, can easily yield hydrolysates containing a mixture of C5 and C6 sugars, using, for instance, autohydrolysis and dilute concentrations of sulfuric acid in water.6,7 After separation, the streams of soluble sugars offer a versatile platform that can provide a range of useful molecules.8 The oxidation of glucose provides a route for the production of glucaric acid, the corresponding polyhydroxy dicarboxylic acid with both terminal groups oxidized. It is an emerging platform chemical with an extremely diverse range of applications as a building block chemical and in direct end uses. It has been ranked as a “top value-added chemical” from biomass by the US Department of Energy,1 however, the full potential of glucaric acid has not yet been exploited. It is used in the formulation of phosphate-free detergents and metal complexation agents,9 as a monomer in the preparation of a variety of biopolymers,4,10-13 and it is an intermediate in the production of biobased adipic acid.14,15 It has been recently identified as a corrosion inhibitor that can be applied to road de-icers and cooling towers,16 and it has been qualified as a concrete admixture for set retardancy and water reduction.17 Its derivatives have also been studied for medical and cosmetic purpose.18-20 However the chemical has not yet been produced in sufficient volumes or cost-competitively enough, because of the difficulty of commercial scale production and restricts market uptake. The market demand is expected to grow over the next few years if a scalable cost-effective technology is developed.21 The standard industrial production route of glucaric acid involves the nitric acid oxidation of glucose.22-24 Despite the moderate selectivity and the non-sustainable approach, the nitric acid 3 ACS Paragon Plus Environment


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method still represents the most economical procedure due to the low cost of the oxidant. Kiely et al and the Rivertop Renewables Company have improved the process by capturing, recycling, and re-using unconsumed inputs.25-27 At present, this technology remains superior to more recent and more selective processes involving the use of TEMPO or related nitroxyl radical oxidants,2830

electrocatalytic oxidation,31 or fermentation route.32-34 Direct oxidation of glucose using air and

supported metal catalysts is also an attractive option for glucaric acid production. The

oxidation

of

several

classes

of

alcohols

including

monoalcohols,

5-

hydroxymethylfurfural, sugars, and polyols is an important research topic in the transformation of biomass.35 Selective oxidation of glucose in water solution to gluconic acid (where only the aldehyde function is oxidized) has been extensively studied. Usually, it is necessary to maintain the pH alkaline, thus high activity, by the continuous addition of a base. Gold catalysts have shown very high activity and selectivity towards gluconate compared to catalysts based on palladium.36-39 The development of catalysts for the base-free oxidation of glucose to gluconic acid was also recently reported.40-43 On the other hand, the oxidation of glucose to glucaric acid has been studied only at a limited extent. This reaction has been performed using platinum catalysts in alkaline conditions with maximum glucarate yield of 54% over Pt/C.44-50 Bimetallic PtCu and PtPd catalysts in moderate conditions (45°C, 1 bar O2) showed enhanced activity compared to monometallic catalysts with selectivity to glucarate of 25% and 44%, respectively, while gluconate was not completely converted (selectivity 10% and 21%, respectively).51,52 A severe limitation arises because of the necessary addition of homogeneous base, sand subsequently the salts of the product need to be neutralized to release free acids. A major topic of current research on alcohol oxidation is the application of bimetallic catalysts in the absence of base.53-55 Recently, Rennovia Inc. employed a high-throughput screening method which allowed them to identify selective heterogeneous catalysts for the production of glucaric acid under base-free conditions.53 Promising results were obtained over Pt and Pt-Au supported catalysts. At T > 100°C, under oxygen pressure > 5 bar, and using a molar ratio glucose/metal < 80 in multi-parallel millireactors, significant yields were reported over Pt/SiO2 (66%) and 4%Au-4%Pt/TiO2 (70%). A 74% yield was given in the presence of Pt/C at 80°C under 14 bar O2 in an initially neutralized solution.55 Bimetallic nanoparticles have been found to be more advantageous in terms of activity and selectivity, compared to their monometallic counterparts for oxidation.35 The improved catalytic 4 ACS Paragon Plus Environment

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performances were ascribed to synergistic effects, derived from electronic or geometrical interactions between the two metals.56 In this work, we report on a screening of a set of Au-based bimetallic catalysts for the oxidation of glucose to glucaric acid under non-neutralized conditions. Besides, in view of a set of study dealing with the selective oxidation of sugars, generated from hydrolysis of hemicelluloses, to aldaric acids, we investigated the possible inhibition effect of impurities contained in the hydrolysates. Indeed, during hydrolysis of hemicelluloses in an acidic medium, hexoses and pentoses degrade to 5-hydroxymethylfurfural and furfural, respectively; acetic acid is liberated from the side groups of hemicelluloses; incomplete hydrolysis releases sugar oligomers; lignin residues might be present. The authors previously studied the oxidation of glucose over Pt/C under alkaline conditions.49 On contrary, in this present work, the catalytic performance of Au-based catalysts will be examined at native pH.

2. EXPERIMENTAL SECTION a. Materials Most of the powder supports were commercial: C (L3S, Arkema, 1089 m2 g-1), TiO2 (DT51, Cristal, 84 m2 g-1), ZrO2 (XZO 632-18, Mel Chemicals, 132 m2 g-1), Al2O3 (SCP 350, Rhône Poulenc, 265 m2 g-1), CeO2 (HAS 5, Solvay, 200 m2 g-1), zeolite β (H-Beta, IFPen, 564 m2 g-1), MgO (Merck Millipore, 64 m2 g-1). The hydrotalcite HT was prepared by co-precipitation.57 An aqueous solution (200 mL) of Mg(NO3)3.6H2O (84.6 g) and Al(NO3)3.9H2O (41.3 g) was added dropwise to 200 mL of an aqueous solution of NaOH (28.8 g) and Na2CO3 (22.3 g) at room temperature under vigorous stirring. The suspension was heated at 70°C for 14 h and the precipitate was recovered by filtration and washed with hot deionized water until neutrality of the filtrate. The solid HT was obtained after drying under N2 atmosphere at 80°C. A hydrothermal treatment was applied to MgO to increase its specific surface area.58 The solid was dispersed in 10 wt. equiv. of water and refluxed for 6 h. After filtration and drying in an oven at 110°C for 16 h, the crushed solid was calcined at 550°C for 5 h (heating rate 5°C min-1). The β-zeolite (Si/Al = 10.5) was activated by calcination at 450°C for 5 h under air (heating rate 1°C min-1). ZrO2 extrudates were provided by Saint Gobain-Norpro (SZ 31163, diameter 1/16”, SBET 37 m2 g-1). Tetrachloroauric acid (HAuCl4.3H2O, 99.99%), hexachloroplatinic acid (H2PtCl6.6H2O, 99.95%, and potassium tetrachloropalladate K2PdCl4 (99.99%), were purchased from Alfa-Aesar, platinum nitrate Pt(NO3)2 (99.9%) from Heraeus. The metallic precursors were either used as 5 ACS Paragon Plus Environment


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provided or solubilized in aqueous solution, which was analyzed by ICP-OES. NaBH4 (purity 98%) and poly(vinylalcohol) (PVA, purity > 99%) were purchased from Aldrich. Water was demineralized by resins and purified with an Elga water system. b. Catalyst preparation A range of bimetallic supported Au-based catalysts were prepared using various supports and using different methods of deposition of the two metals, either sequentially or simultaneously. Detailed catalyst preparation procedures are described below for catalysts containing a nominal Au/Pt or Au/Pd molar ratio of 1. 1. The (Au-Pt)DP-WI/TiO2 catalyst was prepared as described in the patent in a two-step process.54 First, Au was loaded onto the support by deposition-precipitation, then Pt by impregnation, before a final treatment under H2 to activate the catalyst. The TiO2 support (4 g) was introduced with water (500 mL) in a three-neck round-bottom flask of 1 L with magnetic stirring and a vertical water condenser. An aqueous solution containing 21 g L-1 Au as HAuCl4 (3.3 mL) was poured through a dropping funnel, the suspension was allowed to equilibrate for 30 min. Afterwards, an aqueous solution of 20 wt% urea (25 mL) was added, the suspension was heated to 80°C under reflux overnight. After cooling, the solid material was filtrated and washed with water until a negative test to the presence of chloride ions to a drop of silver nitrate solution. The solid was dried in an oven at 60°C under N2 and calcined at 300°C (ramp rate 2°C min-1) for 3 h in an air flow (50 mL min-1). It was recovered as a violet Au/TiO2 solid. The Au/TiO2 catalyst (3.86 g) was re-dispersed in water (50 mL) in a two-neck round-bottom flask. The pH was adjusted to 2.0 by addition of HNO3 1 M to be below the point of zero charge (PZC) of the support. Then, an aqueous solution of Pt at 48 g L-1 as Pt(NO3)2 (4.5 mL) was added within 1 h. The solid was filtrated, washed, dried under N2 at 60°C overnight, and reduced under H2 flow (50 mL min-1). After cooling, purging with Ar, and passivation in 1% O2/N2 the catalyst was recovered as a 3.8wt%Au-3wt.%Pt/TiO2 purple-red powder. 2. For the preparation of (Au-Pt)IWI/TiO2, incipient-wetness impregnation was used. In an agate mortar, an aqueous solution (10 mL, the amount of water that corresponds to the wetting volume of the support) containing 0.59 g HAuCl4. 3H2O and 0.76 g H2PtCl6. xH2O was poured dropwise over TiO2 (7.4 g) under vigorous mixing. The yellow solid was dried overnight under N2 at 60°C. To obtain the catalytically active form, the precursor was reduced under H2 flow at 300°C (ramp rate 2°C min-1) for 3 h, and recovered after passivation as a grey solid. 6 ACS Paragon Plus Environment

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3. (Au-Pt)DP/TiO2 was prepared by deposition-precipitation with urea of Au and Pt precursors. TiO2 (8 g) and 500 mL water were introduced in a three-neck round-bottom flask with water condenser, oil bath, and magnetic stirrer. An aqueous solution (6 mL) containing 53.4 g L-1 Au (as HAuCl4) and an aqueous solution (13.8 mL) containing 23.5 g L-1 Pt (as K2PtCl4) were mixed and added dropwise under N2. After stirring for 30 min, urea (60 mL, 20% solution) was added and the suspension was heated to 80°C and kept at this temperature overnight. After cooling, the solid was filtrated and washed with water until a negative test to the presence of chloride ions to a drop of silver nitrate solution. After drying at 110°C under N2, the beige powder was reduced as above and the grey solid was recovered. 4. (Au-Pt)NaBH4/TiO2 and Au-Pd/TiO2 were prepared using NaBH4 as the reducing agent.59 The support (7.4 g) and water (500 mL) were introduced in a 1 L three-neck round-bottom flask. Aqueous solutions containing 53 g L-1 Au (as HAuCl4) and 39 g L-1 Pt (or Pd) as H2PtCl6 (or K2PdCl4), respectively, were added and the impregnation went on for 1 h under moderate stirring (400 rpm). After cooling to 2°C with an ice bath, a freshly prepared NaBH4 solution (1.25 g in 100 mL) was slowly added with vigorous stirring (800 rpm). The suspension, which immediately turned black, was stirred for 3 h at 400 rpm. The solid was filtrated, washed until a negative test with silver nitrate, and dried under N2 at 60°C yielding a grey powder. The last procedure was also applied to prepare the Au-Pt catalysts on the other powder supports (ZrO2, Al2O3, CeO2, C, HT, MgO, H-β-zeolite) and on ZrO2 pellets. c.

Characterization of supports and catalysts

The actual metal loading of the catalyst samples was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) using a Horiba Jobin Yvon Activa instrument. The point of zero charge (PZC) value of the supports was determined as follows: two identical 100 mL solutions containing 0.01 M NaOH and 0.1 M NaCl in deionized water were titrated with HCl 0.1 M, in the absence and in the presence of 1 g of the support under N2 atmosphere, using a M240 Meterlab Radiometer Analytical pH-meter and a XC 161 electrode. The PZC was estimated at the point where both titration curves crossed. Powder X-ray diffraction (XRD) patterns were recorded on a Bruker Advance D8A25 diffractometer using Ni-filtered Cu Kα radiation (λ = 0.1541 nm). The crystallite size was calculated from the XRD pattern by the Debye-Scherrer equation. The



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N2 sorption at -196 °C was conducted using a Micromeritics ASAP 2020 analyzer. Prior to the measurements, the samples were degassed under vacuum at 150°C for 4 h. The specific surface areas were determined by applying the BET (Brunauer-Emmett-Teller) method. Transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) measurements were performed using a JEOL 2010 instrument operated at an acceleration voltage of 200 kV. Samples were suspended in ethanol and dispersed ultrasonically. Drops of suspension were deposited onto carbon-coated copper grids followed by solvent evaporation in air at room temperature. d. Oxidation reaction tests The oxidation reaction tests were conducted batchwise in a C22 Hastelloy 300 mL autoclave. The reactor was equipped with a magnetically driven stirrer, and the temperature was maintained by an electric heating mantle with a temperature controller. Typically, 100 mL of a 0.25 M glucose aqueous solution and the desired amount of catalyst were introduced in the reactor. The reactor was purged with Ar three times and the reaction mixture was heated under Ar to 100°C with gentle stirring (400 rpm). Then, the pressure was adjusted with air to 40 bar and stirring was set at 1200 rpm. This time was taken as zero time of the reaction. Experiments were also performed in a high pressure fixed-bed reactor in the trickle-bed mode with co-current downflow of liquid and air, using catalysts prepared by the NaBH4 reduction method over ZrO2 pellets. The system consisted of a tube made of Hastelloy C (1 cm inner diameter and 15 cm length) and two sintered stainless-steel filters at the inlet and exit of the reactor.60 The aqueous glucose solution was introduced from a feed glass tank via an HPLC pump, the pressure and the flow rate of air were controlled with a back-pressure controller. The system was equipped with a heat exchanger, a high pressure gas-liquid separator, and the liquid was collected in a flask using a liquid level control device. Samples of the reaction medium were periodically withdrawn for analysis to plot concentration-time profiles. The products of glucose oxidation are various (Scheme 1) and effective methods were developed for the analysis of the oxidation mixtures using ion chromatography (IC), a Metrohm Professional IC850 system equipped with the 872 extension module and 889 autosampler.49 Separation and quantification of glucose, but also of guluronic, glucaric, and 2-keto-gluconic acids was performed on a CarboPac PA1 column (4x250 mm) at 17°C coupled to pulsed amperometric detection (PAD). The isocratic mobile phase consisted of 8 ACS Paragon Plus Environment

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2.5 mM sodium hydroxide and 200 mM sodium acetate. An IonPac AS11-HC (4x250 mm) column at 35°C and a suppressed conductivity detector were employed for separation and detection of the anionic salt forms of glucose oxidation products including guluronic acid, gluconic acid, 2-keto and 5-ketogluconic acids, glucaric acid, as well as the C-C broken bond byproducts such as tartaric, tartronic, (glyceric + glycolic) and oxalic acids. After 8 min, the gradient elution (1 mL min-1) of sodium hydroxide concentration was ramped from 2 to 35 mM over 25 min, followed by 100mM over 5 min to ensure that all potential contaminants were flushed off the column before the next sample. O

H O

H

OH OH

HO HO

OH

OH

OH

OH

CH2OH

COOH

D-glucose O

D-glucuronic acid

OH OH

O

side-products

OH HO

HO OH

OH

OH

OH

CH2OH

CH2OH

D-gluconic acid

OH

O

HO

O

O

OH

OH O CH2OH

5-ketoD-gluconic acid

2-ketoD-gluconic acid

OH O

OH

O

OH

O

O

OH

O

OH HO OH OH H

OH

HO OH

OH

HO

OH

OH

arabinonic acid

OH

OH

arabinaric acid

O

O

OH

OH

OH

OH

HO

HO OH

OH

O

O

OH OH

glyceric acid

tartronic acid

O

O

O

HO OH

OH

O

tartaric acid

OH

HO OH

H

OH

erythronic acid

OH HO

O

O

D-glucaric acid

OH HO

HO

HO

OH

breakdown products

OH

xylaric acid

O

D-guluronic acid O

OH

HO

O

glyoxylic acid glyoxylic acid

O

oxalic acid

O H

H

formic acid

Scheme 1. Catalytic oxidation of glucose to glucaric acid showing the by-products of the reaction.

Conversion is defined as the moles of glucose converted to the moles of glucose initially charged into the reactor. The yield is defined as the amount of product formed (e.g., glucaric acid) to the initial amount of glucose. The total organic carbon (TOC) in solution was measured using a TOC analyzer (Shimadzu TOC-VCSH) equipped with ASI-automatic sampler. The TOC analysis and the carbon balance 9 ACS Paragon Plus Environment


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determined from chromatography analysis of substrate and all formed products were compared to the initial loading concentration of glucose. The possible presence of metals by leaching from the catalysts during the reaction was detected by ICP-OES of the liquid samples.

3. RESULTS AND DISCUSSION Table 1 lists the catalysts prepared via the different methods of preparation, their metallic composition, the crystallite size of the metals (either as two components or as an alloy) by XRD, and the specific surface area.

Table 1. Metal content, crystallite size, and surface area of the prepared catalysts.

Catalyst

PZC

Metal loading

Au/(Pt or

Cristallite

SBET

support

(wt.%)a

Pd)

size

(m2 g-1)e

Au-Pt or Pd

molar ratio

(nm)

3.8-3.7

1.03

9-ndc

85

(Au-Pt)IWI/TiO2

3.2-3.7

0.86

30-18

81

(Au-Pt)DP/TiO2

3.2-3.2

1.00

33-12

86

(Au-Pt)NaBH4/TiO2f

3.4-3.5

0.97

7d

88

3.6-4.35

0.82

3.8-2.1

0.98

8d

90

(Au-Pt)DP-WI/TiO2

3.3

(Au-Pd)NaBH4/TiO2 (Au-Pt)NaBH4/ZrO2

4.9

3.5-3.45

1.03

6d

99

(Au-Pt)NaBH4/Al2O3

7.4

3.4-3.2

1.06

5d

253

(Au-Pt)NaBH4/CeO2

3.4

3.8-3.7

1.03

6d

214

(Au-Pt)NaBH4/C

7.2

3.7-3.7

1.00

4d

873

(Au-Pt)NaBH4/HT

10.5

4.1-3.5

1.17

6d

81

d

92

b

a

(Au-Pt)NaBH4/MgO

11.6

2.5-2.3

1.09

6

(Au-Pt)NaBH4/H-β zeol

2.6

2.9-2.8

1.04

6d

by ICP-OES.

b

469

the support was partially re-hydrated during the process of preparation. c not

detected. d alloy. e approximate relative error of ± 5%. f two batches.


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a. Effect of the method of preparation of Au-Pt/TiO2 catalysts The total metal loading of the Au-Pt/TiO2 catalysts prepared according to the four different procedures was in the range 6-8wt% and the Au/Pt molar ratio was close to 1. The specific surface areas of 84 ± 5% m2 g-1 were hardly affected by the preparation procedures for loading the metals. Figure 1 displays the XRD patterns of the four different Au-Pt/TiO2 catalysts.

a)

Figure 1. XRD patterns of the Au-Pt/TiO2 catalysts: (a) (3.8%Au-3.7%Pt)DP-WI/TiO2, (b) (3.0%Au-3.3%Pt)IWI/TiO2, (c) (3.2%Au-3.2%Pt)DP/TiO2, (d) (3.4%Au-3.5%Pt)NaBH4/TiO2, and TiO2. Powder Diffraction File of Au: PDF-04-001-2616, Pt: PDF-00-004-0802, Au-Pt alloy: PDF-00-046-1043.

The XRD patterns (Fig. 1) depended on the procedure of preparation. In the patterns associated with the samples prepared by DP-WI (a), IWI (b), or DP (c), reflections corresponding to Au and Pt were observed in addition to the typical features of TiO2 anatase. Moreover, the crystallite sizes also depended on the method. Indeed, for the DP-WI two-step synthesis (a), Au crystallites were relatively large (9 nm) and no clear reflections associated with Pt could be detected, which implies that Pt might exist as an amorphous phase or very small particles. In the



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catalysts prepared in one step by IWI (b) or DP (c), the crystallites were very large, above 10 nm for Pt and above 30 nm for Au. Conversely, the XRD pattern of (3.4%Au-3.5%Pt)NaBH4/TiO2 (d) prepared by wet impregnation and NaBH4 reduction clearly revealed the only presence of broad peaks between the peaks of the Au and Pt single phases, which suggests the formation of alloyed crystallites. The Vegard’s law states that the lattice constant results from the linear interpolation between the lattice constants of the pure constituent elements,61-63 its application gave the composition with an Au/Pt ratio of 1, identical to the nominal composition of the material. The crystallite size calculated from the [111] diffraction peak was of 7 nm, which is fairly low, taking into account the high loading of 8 wt.% of the material. Based on the XRD results it is apparent that only the bimetallic catalyst prepared by reduction with NaBH4 contained alloyed nanoparticles, and that the catalysts prepared by IWI or DP exhibited large monometallic particles. The size and distribution of the bimetallic nanoparticles were assessed by TEM micrography analysis. Figure S1 shows representative sets of TEM images for catalysts (AuPt)IWI/TiO2, (AuPt)DP/TiO2, and (Au-Pt)NaBH4/TiO2. It can clearly be observed that for the three catalysts analyzed, the particles were heterogeneously distributed, they showed a mixture of morphologies, and they were poorly dispersed. Quite small particles (< 10 nm) and very large particles were present, and there is visible evidence for multiple particle agglomeration. In conclusion of the TEM analysis (see Supporting Information), there are not evident differences among the catalysts prepared, except the NaBH4 catalyst exhibited nanoparticles containing consistently both metals. However, size distribution histograms could not be performed due to the extremely diverse morphology of the particles. b. Screening of TiO2-supported Au-Pt catalysts The selective catalytic oxidation of glucose to glucaric acid proceeds via gluconic and guluronic acids. Besides, oxidation of intermediate secondary alcohols to 2-keto and 5-keto gluconic acids and C-C bond scission to a range of shorter carboxylic acids may occur, which results in poor selectivity to glucaric acid (Scheme 1). Preliminary tests for glucose oxidation were performed at 100°C under 80 bar air. Figure 2 compares the temporal evolution of glucaric acid yield generated over the differently prepared Au-Pt/TiO2 catalysts.


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Figure 2. Yield of glucaric acid as a function of time during oxidation of glucose over the various AuPt/TiO2 catalysts. Reaction conditions: [glucose]0 = 0.25 M, glucose/metal molar ratio 40, T = 100°C, 80 bar air. () (3.8%Au-3.7%Pt)DP-WI/TiO2, () (3.6%Au-4.3%Pt)NaBH4/TiO2, () (3.2%Au-3.2%Pt)DP/TiO2, () (3.2%Au-3.7%Pt)IWI/TiO2. Before commenting on these results, it is worth mentioning the equilibration of D-glucaric acid in aqueous solution under neutral or acidic conditions (Scheme 2). In water, glucaric acid exists in an equilibrium of the acyclic compound, the two monolactones (D-glucaro-1,4- and Dglucaro-6,3- lactone) and the dilactone (D-glucaro-1,4;6,3-dilactone).64,65 At room temperature, the acyclic form is the major compound compared with the monolactones, and the dilactone is only detectable from 70°C. Given the high temperature and the low pH the reaction medium during oxidation of glucose, one might think that the formation of the lactones would be facilitated during reaction. However, prior to the ionic chromatography analysis, the samples were diluted and injected in an alkaline eluent (see experimental part). Under these conditions hydrolysis of the lactones should be high, and only the acyclic glucarate should be present. This assumption was ascertained by analysis of pure 1,4-glucarolactone by IC; only the peak corresponding to glucarate was detected.



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Scheme 2. Equilibrium of aqueous D-glucaric acid.

The Au-Pt/TiO2 catalysts prepared by IWI orange triangle) and DP (blue square) showed poor activity (Fig. 2), which may be attributed to the large crystallite sizes of Pt and Au observed by XRD (Fig. 1). On the other hand, the activity was enhanced remarkably when either (Au-Pt)DPWI/TiO2

or (Au-Pt)NaBH4/TiO2 was used. Both materials revealed a 20-fold initial reaction rate of

formation of glucaric acid in comparison with that of (Au-Pt)DP/TiO2 catalyst. The maximum yield of glucaric acid after 4 h attained 38% and 31%, over (Au-Pt)DP-WI/TiO2 and (AuPt)NaBH4/TiO2, respectively. In the case of (Au-Pt)DP-WI/TiO2, although their monometallic counterparts (Au, Pt) are poorly active in that reaction,49 a close contact created between Au and Pt as demonstrated by TEM-EDX analysis (Figure S1(d) was able to promote the formation of glucaric acid under base-free conditions. Also, the formation of alloyed nanoparticles for the catalyst prepared by NaBH4 reduction lead to an efficient catalyst for oxidation to glucaric acid. After attaining a maximum, the yield of glucaric acid decreased due to over-oxidation reactions, this will be discussed in more details later on. One must note that the Au/Pt ratio of the catalysts was not optimized. Moreover, this optimum ratio may depend on the preparation method. c. Effect of the nature of the metal associated to Au (Pt or Pd)


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Among the bimetallic catalysts studied for oxidation reactions of oxygenated compounds under base-free conditions, Au-Pd catalysts have also demonstrated interesting performances in oxidation of glucose,66 or HMF.67 A (3.8%Au-2.1%Pd)NaBH4/TiO2 catalyst (Au/Pd molar ratio of 0.98) was prepared using NaBH4 as the reducing agent, similarly as for the Pt bimetallic analog. XRD analysis of the material showed a diffraction peak at 2θ = 38.9° which was ascribed to a Au-Pd alloy (the XRD peak corresponding to metallic Pd and Au would be at 2θ of 40.0° and 38.4°, respectively). Figure 3 shows in more details the temporal evolution of the main products and pH over a) (3.6%Au-4.3%Pt)NaBH4/TiO2 (Au/Pt = 0.8) and b) (3.8%Au-2.1%Pd)NaBH4/TiO2 (Au/Pd = 0.98). It also gives the evolution of pH in the presence of both catalysts.

Figure 3. Oxidation of glucose to glucaric acid in the presence of (a) 3.6%Au-4.3%Pt/TiO2 (Au/Pt = 0.8) and (b) 3.8%Au-2.1%Pd/TiO2 (Au/Pd = 0.98). Reaction conditions: [glucose]0 =



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0.25 M, glucose/metal molar ratio 40, T = 100°C, 80 bar air. () glucose, () gluconic acid, () guluronic acid, () glucaric acid, () pH.

Over the TiO2-supported Au-Pt catalyst, glucose was totally converted within 6 h. In contrast to the reaction performed in the presence of Pt/C under alkaline conditions where the oxidation reaction of glucose to glucarate was sequential via gluconate,49 guluronic and glucaric acids were formed in addition to gluconic acid very rapidly after the beginning of the reaction under basefree conditions. The maximum yield of gluconic acid was around 20%, it was then progressively converted and disappeared completely after 10 h. A maximum yield of glucaric acid of 31% was measured using this catalyst. The pH, which was initially neutral, decreased sharply within the first minutes down to a plateau at pH 2.1; this is due to the formation of carboxylic acids by oxidation reactions and consistent with the pKa of glucaric acid of 3.08.68 Under the same reaction conditions, the Au-Pd/TiO2 catalyst was more active for glucose oxidation and led to complete glucose conversion within only 2 h. However, the Au-Pd catalyst was less efficient for the subsequent conversion of gluconic acid. The maximum yield of gluconic acid formed was 45%, subsequent oxidation was slow, and total conversion of gluconic was not yet attained after 24 h. The yield of glucaric acid reached a maximum of 22%, degradation occurred upon prolonged reaction times. Therefore, the Au-Pt catalysts were chosen for further study. d. Effect of the nature of the support for Au-Pt The support may also greatly affect the activity and selectivity of the oxidation reaction. For instance, over Au-Pt catalysts prepared by sol immobilization, the activity and selectivity to C3 products in glycerol oxidation was enhanced by using acidic H-mordenite compared to activated carbon or TiO2.69,70 A series of Au-Pt catalysts with a Au/Pt ratio around 1 were prepared on various supports revealing different chemical and textural characteristics according to the NaBH4 procedure (Table 1). The catalysts prepared were compared in glucose oxidation under the same conditions as previously. The yield of glucaric acid as a function of reaction time over the different catalysts is shown in Figure 4.


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Figure 4. Influence of the nature of the support of Au-Pt in the oxidation of glucose to glucaric acid. Reaction conditions: [glucose]0 = 0.25 M, glucose/metal = 40, T = 100°C, 80 bar air. () 3.5%Au-3.4%Pt/ZrO2, () 3.6%Au-4.3%Pt/TiO2, () 3.7%Au-3.7%Pt/C, () 3.8%Au3.7%Pt/CeO2, () 3,4%Au-3.2%Pt/Al2O3, () 2.9%Au-2.8%Pt/H-β, ().4.2%Au-3.5%Pt/HT. All materials showed the presence of Au-Pt alloy in the XRD patterns, whose composition was close to the nominal one. The crystallite size was not significantly modified by the support, it was fairly low below 7 nm. Nevertheless, it was found that the nature of the support affected to a great extent the maximum yield of glucaric acid produced. The lowest yield of 10% was observed over the CeO2 supported catalyst. The maximum yield over the β-zeolite supported catalyst was only 20% after 10 h, then it decreased to 15% after 24 h. The catalysts prepared on C, TiO2 and Al2O3 demonstrated a much higher initial reaction rate and they produced glucaric acid with a maximum yield of around 30% after 4-6 h. The Au-Pt catalyst supported on basic hydrotalcite (PZC = 10.5) showed a peculiar behavior. The reaction of formation of glucaric acid started rapidly and a maximum yield of 20% was attained after 1 h; thereafter, glucaric acid was very rapidly consumed and lower molecular weight dicarboxylic acids were formed (especially tartaric acid and arabinaric acid). At the same time, the TOC value measured in the aqueous phase decreased to attain less than 60% of the initial value after 8 h, suggesting the large formation of CO2. These results may be compared to those observed during glycerol oxidation at non-neutralized pH in the presence of Au-Pt catalysts prepared by colloid immobilization on various acidic, basic, and amphoteric supports; the selectivity to tartronic acid and to C-C cleavage products was demonstrated to be proportional to 17 ACS Paragon Plus Environment


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the strength of basic sites of the support.71 Moreover, by using the HT-supported catalyst for glucose oxidation, a different evolution of the pH as a function of time was also observed. Contrarily to the findings in the previous reaction tests (Fig. 3a for Au-Pt/TiO2), the initial pH value decreased rapidly from 6.8 to 4.2 within the first moments of the reaction, and then it gradually increased and was back again to 6.5 after 24 h. We suggest that the pH of the solution was significantly lowered due to high conversion of glucose to acids and diacids that caused Mg2+ dissolution from the unstable HT under these conditions. The selective leaching of Mg from the HT catalyst was mentioned in the literature in the presence of acids.72,73 In the presence of Au-Pt/MgO, similar phenomenon was observed. After 24 h, the TOC was no more than 12% of the initial loading. The support showed limited stability and brown deposits on the reactor walls evidenced the degradation of the organic compounds. Finally, by switching to the ZrO2-supported Au-Pt catalyst, the best activity was obtained and the maximum yield of glucaric acid of 45% was observed after 3 h reaction. This catalyst (3.5%Au3.45%Pt)NaBH4/ZrO2 which demonstrated interesting catalytic performance was used for further optimization. e. Further insight in the determination of the main products and side-products formed. Figure 5 displays the concentration of the different acids formed as a function of time in the presence of the 3.5%Au-3.45%Pt/ZrO2 catalyst. It also compares the carbon balance calculated from chromatography analysis with the measured TOC.


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Figure 5. Major acids (left) and minor acids (right) analysed during glucose oxidation in the presence of 3.5%Au-3.45%Pt/ZrO2. () gluconic acid, () guluronic acid, () glucaric acid, () tartaric acid, (–) TOC, (●) carbon balance, () 5-ketogluconic acid, () 2-ketogluconic acid, () glucuronic acid, () arabinaric acid, () tartronic acid, () glyceric acid + glycolic acid, () formic acid, ( ) oxalic acid. Reaction conditions: [glucose]0 = 0.25 M, glucose/metal = 40, T = 100°C, 80 bar air.

The same compounds as under alkaline conditions were analyzed (see Scheme 1), although in different relative concentrations.49 Besides glucaric acid, typical products mixtures include gluconic acid, guluronic acid (oxidation of the primary alcohol function of gluconic acid to an aldehyde), and at a lower extent glucuronic acid (oxidation of the alcohol function of glucose to a carboxylic acid), they were formed as intermediate compounds (Scheme 1). 5-Keto and 2-keto gluconic acids by oxidation of a secondary alcohol oxidation, as well as short-chain carboxylic acid and diacids by over-oxidation were also analyzed. The most important side-product was tartaric acid, the yield of which was around 10% and stable after 8 h. Tartronic and oxalic acids were formed in very low concentrations (yields < 2%). The yield of arabinaric acid, a C5 diacid, was significant and was 5% at the end of the reaction. Glyceric acid and glycolic acids, which could not be differentiated, were analyzed together and were present up to the end of the reaction. Formic acid was detected all along the reaction. The TOC value decreased drastically during the reaction to attain a value of less than 60% after 24 h. In parallel, the carbon balance fell down during the first hour. There was a large difference between the carbon balance and the TOC measurement during the first hours; thereafter the balance stabilized at 80% of the TOC value. The presence of formic acid and arabinaric acid support the loss of carbon from the aqueous phase as CO2. Analysis by GC of the gas phase collected in a gas bag after cooling off the reactor confirmed the qualitative presence of CO2. f. Influence of reaction conditions The effect of reaction conditions on the yield of glucaric acid were investigated over 3.5%Au3.45%Pt/ZrO2 catalyst. Substrate/metal molar ratio



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The selectivity to glucaric acid which was initially high decreased drastically as a function of time as the oxidation of glucose proceeded. One may then expect that acceleration of the main reactions of oxidation of glucose to glucaric acid would minimize the over-oxidation reactions. Figure 6 shows the effect of the glucose/metal molar ratio on the yield of glucaric acid and the TOC evolution as a function of time. Note that compared to the reaction tests described previously, pressure was 40 bar instead 80 bar, the temperature was identical at 100°C.

) as a function of Figure 6. Evolution of TOC concentration (▬,) and glucaric acid yield (, the glucose/metal ratio. Reaction conditions: [glucose]0 = 0.25 M, T = 100°C, 40 bar air, 3.5%Au-3.45%Pt/ZrO2 catalyst. (, ▬) glucose/metal = 80, ( , ) glucose/metal = 20. As expected, by increasing the amount of catalyst by a factor 4 the reaction was drastically accelerated and the maximum yield of glucaric acid was attained after 2 h (glucose/metal = 20) compared with 8 h (glucose/metal = 80). A small positive effect was observed on the maximum yield of glucaric, which increased from 50% to 58%. In parallel, the TOC amount analyzed in the aqueous phase was also slightly higher when using a glucose/metal ratio of 20 (86% vs.79%for glucose/metal = 80). However, owing to the very large amount of catalyst and to the relatively small improvement involved, this strategy could only be valuable if the reaction can be conducted over a stable catalyst in a continuous reactor, which allows regulation of the contact time and limitation of side reactions. Further experiments were conducted using the glucose/metal molar ratio of 80. 20 ACS Paragon Plus Environment

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Temperature effect The same strategy was used by varying the temperature. In the previously mentioned patent by Rennovia, a positive effect of temperature on the yield of glucaric acid was noted over AuPt/TiO2 catalysts tested in millireactors.54 The effect of temperature in the range 60-120°C under 40 bar air on the TOC evolution and the yield of glucaric acid is displayed in Figure 7.

Figure 7. Temperature effect on the reaction of glucose to glucaric acid. Reaction conditions: [glucose]0 = 0.25M, glucose/metal = 80, 40 bar air, 3.5%Au-3.45%Pt/ZrO2 catalyst. (glucaric acid, TOC) = (,▬) 60°C, (,) 80°C, (, ) 100°C, (,) 120°C.

A minimum temperature of 100°C is required; indeed, at the temperatures of 60°C and 80°C, the maximum yield of glucaric acid was not yet attained after 24 h of reaction. Nevertheless, the analysis of a sample taken from the reaction medium at 80°C after 72 h afforded a yield of glucaric acid of 50%. Conversely, increasing the temperature from 100°C to 120°C led to a lowering of the maximum yield from 50% after 8 h to 45% after 2 h; in both cases, the TOC value at the maximum was 80% of the initial concentration. This observation suggests that the lowering of selectivity at 120°C is rather due to dehydration and/or condensation reactions, which form undetected unsaturated compounds as was further evidenced by the dark brown colour of the reaction medium at 120°C. The TOC value also decreased drastically, suggesting C-C cleavage recations. Effect of partial pressure of oxygen



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The influence of partial oxygen pressure was investigated. Figure S3 compares the results for the reaction performed at 40 bar (partial pressure of oxygen of 8 bar) with those of a reaction under 2 bar of pure oxygen (PO2 = 1 bar, if one takes into account the vapor pressure of water at 100°C), both were performed in the batch autoclave. Whereas gluconic acid formed intermediately could be converted to completion under 8 bar, the reaction was stopped after 4 h under 1 bar. This was attributed to a lack of sufficient oxygen to perform the reaction. Nevertheless, some features could be identified. The reaction started similarly at the beginning, suggesting a zero order relative to the oxygen pressure. The selectivity to gluconic acid and glucaric acid was slightly higher under 1 bar; this finding is corroborated with the relative TOC value after 3 h, which was 93% and 89% under 8 and 1 bar, respectively, though the yield of glucaric was identical at 43% after that reaction time. g. Influence of Au/Pt ratio in (Au-Pt)NaBH4/ZrO2 An important parameter which may deeply influence the catalytic performance is the composition of the nanoparticles in the Au-Pt catalysts. To examine this effect, a series of (AuPt)NaBH4/ZrO2 catalysts was prepared with various Au/Pt molar ratios, while keeping a total loading of approximately 7wt.% metal. The prepared catalysts with the actual metal loading and crystallite size determined by XRD are gathered in Table 2.

Table 2. Characterization of (Au-Pt)NaBH4/ZrO2 catalysts with variable compositions. Catalyst

Au wt%

Crystallite sizea

Metal loading

Molar ratio

(%)

Au/Pt

7%Pt/ZrO2

7.0

-

0

7

0.7%-4.9%Pt/ZrO2

5.6

0.14

12

6b

1.4%Pt-5.3%Au/ZrO2

6.7

0.26

21

6b

1.8%Pt-4.4%Au/ZrO2

6.2

0.41

29

6b

2.5%Pt-3.6%Au/ZrO2

6.1

0.69

41

7b

3.5%Pt-3.5%Au/ZrO2

7.0

1.00

50

7b

4.0%Pt-2.7%Au/ZrO2

6.7

1.48

60

7b

3.9%Pt-1.6%Au/ZrO2

5.5

2.43

71

7b

4.3%Pt-0.8%Au/ZrO2

5.1

5.37

84

9b

(nm)


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7%Au/ZrO2 a

7.0

-

100

18

XRD. b Bimetallic alloy

The total metal loading was within the range of 5.1wt.% to 7wt.% metal. Since no metal was detected in the filtration and washing solutions, the errors on the ICP-OES analysis after mineralization of the solids may be responsible for the difference to the nominal 7wt.% loading observed for some samples, especially since solubilization of the alloyed nanoparticles was more difficult than for the monometallic catalysts. The XRD patterns of the Au-Pt/TiO2 catalysts showed a smooth transition from a Pt like pattern to a Au like pattern with the increase of Au/Pt ratio. For example, the (111) reflection gradually shifted to higher angles from 2θ = 38.3° to 39.7° with increasing Pt loading. No additional monometallic Pt and/or Au diffraction peaks were observed, indicating that the AuxPt1x

nanoparticles bear the characteristics of an alloy-type compound involving Au and Pt. From the

linear dependence of the lattice parameters on the relative Au/Pt content, we estimated the relative bimetallic composition in the samples, which was very close to the actual composition determined by ICP-OES. Although the Au-Pt phase diagram exhibits a miscibility gap, and therefore Au-Pt nanoparticles are expected to phase segregate into Pt-rich and Au-rich phases, alloyed Au-Pt nanoparticles have been synthesized over almost the entire composition range.74 The crystallite size of the alloy nanoparticles was of 6-7 nm when the Au fraction was in the range 0-70%. It increased to 9 nm and 18 nm, respectively, when the Au fraction was over 80%, including the monometallic Au catalyst. The Au-Pt/ZrO2 catalysts with various compositions were tested for glucose oxidation. Figure 8a shows the initial reaction rate of glucose conversion. Figure 8b gathers the results of the maximum formation rate of glucaric acid and of the maximum yield in glucaric acid attained during the reaction as a function of the composition of the bimetallic Au-Pt/ZrO2 catalyst. The reaction rates were normalized with total metal content and were calculated from the tangent slope fitted with the concentration curves vs reaction time.



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Figure 8. Dependence of (a) the initial reaction rate of oxidation of glucose (b) the initial formation rate of glucaric acid and the maximum yield in glucaric acid, on the fraction of gold in the bimetallic nanoparticles of Au-Pt/ZrO2 prepared according to the NaBH4 method. Reaction conditions: [glucose]0 = 0.25 M, glucose/metal = 80, 100°C, 40 bar air. Figure 8a demonstrates that there is a synergistic effect of both metals for the catalytic activity for oxidation of glucose. The monometallic Pt catalyst was poorly active (2 mol h-1 gmetal-1) and glucose conversion was not yet complete (96% conversion) after 24 h. The rate of oxidation of glucose increased with increasing percentages of Au in the alloy and went through a maximum at 10 mol h-1 gmetal-1 for the composition of 80% Au. Then, the rate dramatically decreased to 6.8 mol h-1 gmetal-1 for the monometallic gold catalyst; however, this value is pretty high if one considers the large crystallite size of Au (18 nm, Table 2). It is interesting to find that these results are in good agreement with those of Zhang et al, who evaluated PVP-stabilized Au-Pt colloidal nanoparticles with average diameter of 1.5 nm prepared by reduction of Au and Pt precursors.71 In the aerobic oxidation of glucose ([glucose]0 = 0.26 M, glucose/metal = 764), although under controlled pH of 9.5, at 60°C and under oxygen flow at atmospheric pressure, the average activity varied with the composition of the nanoparticles; the highest activity of the AuPt nanoparticles was achieved at the same atomic composition. The maximum reaction rate of formation of glucaric acid and the maximum yield of glucaric acid also varied with the composition as shown in Fig. 8b. In the presence of pure gold, formation of glucaric acid was very low and a yield of only 3% was achieved. As the relative proportion of Pt increased, this rate rose rapidly up to a molar ratio Au/Pt of ca. 1. The maximum yield of


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glucaric acid of 50% was observed at that composition. Then, with further decrease in the gold amount, the rate declined by 25%, however, it did not decrease so much below 30 wt.% Au; this observation is closely related with the reasonable activity of Pt for conversion of gluconic acid. On the other hand, the maximum yield of glucaric acid dropped down in the presence of monometallic Pt catalyst. We ascribed this phenomenon to the deactivation of the Pt catalyst which takes place in the absence of Au according to the literature, owing to irreversible oxidation of metallic Pt by air or irreversible adsorption of acid products on the catalyst surface.56 Thus, the role of Au was double: it prevented deactivation of the Pt catalyst and it induced a synergistic effect on the glucaric yield obtained. The findings also support the previous data related to the effect of the Au/Pt ratio on the yield of glucaric acid in the patent of Rennovia.54 They prepared 8%(Au-Pt) catalysts supported on TiO2 or ZrO2 using the two-step preparation via DP of Au and WI of Pt. In the oxidation of glucose ([glucose]0 = 0.54 M, glucose/metal = 40, 90°C, 5 bar O2), a maximum of yield of glucaric acid was achieved, although it was rather observed for 20-30% of Au in the catalyst. This result provides evidence of the importance of the support and the mode of preparation of bimetallic Au-Pt catalysts for aerobic oxidation. h. Stability of Au-Pt/ZrO2 catalysts In addition to catalytic activity and selectivity, the recyclability and the stability of the catalysts are important criteria. The 3.5%Au-3.45%Pt/ZrO2 catalyst was examined in recycling experiments. After 4 h of reaction in each test, the used catalyst was separated by filtration, washed with water, dried under N2 at 60°C overnight, weighted, and reutilized in a subsequent run under identical reaction conditions without special reactivation. The next reaction test was conducted by adapting the amount of glucose to the mass of catalyst which had been recovered, so that the glucose/metal molar ratio remained at 80. Figure 9 shows the results for a set of 5 experiments.



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Figure 9. Reuse of the 3.5%Au-3.45%Pt/ZrO2 catalyst. Reaction conditions: [glucose]0, glucose/metal molar ratio = 80, T= 100°C, 40 bar air, 4 h. Glucaric acid (solid bars) and gluconic acid (hatched bars) are indicated at each cycle.

The catalyst remained effective upon repeated use. In the three first successive experiments the reused catalysts performed as the fresh one; the yields of gluconic acid and glucaric acid after 4 h reaction were unchanged within experimental error (ca. 25% and 43%, respectively). The progress of the reaction was slightly lowered in the fourth to sixth cycle, yielding a slightly higher amount of gluconic acid remaining to be oxidized to glucaric acid after 4 h. The fresh and the used catalyst after a 24 h-reaction test under 40 bar air were compared by TEM. Representative images at different scales are presented in Figure S4.The comparison of the images for the fresh and used Au-Pt/ZrO2 catalyst did not evidence any change in the nanoparticles morphology or size. Moreover, EDX analysis on isolated particles demonstrated the concomitant presence of both Au and Pt as alloy. Furthermore, no traces of Au or Pt below the detection limit of the ICP-OES technique (Au, Pt < 0.2 mg L-1) were detected in the final reaction solution after the 24 h test. The decline in activity may be due to the multiple handling, washing, and drying of the catalytic material. Next, the long-term stability of such type of catalyst was further examined in a continuous trickle-bed reactor. Figure 10 compares conversion of glucose and yields of gluconic and glucaric acids as a function of time on stream over Au0.3wt%-Pt0.7wt%/ZrO2 and Au0.7wt%-Pt1.6wt%/ZrO2 (molar ratio Au/Pt ca. 0.45). Both catalysts showed stable conversion and yields of gluconic and glucaric acid over a period of 200 h on stream. Over the catalyst containing the higher metal loading, at total conversion of glucose and while 8% gluconic acid still remained to be converted, the yield of glucaric acid was stable at 55% with time on stream for at least one week. These 26 ACS Paragon Plus Environment

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catalysts were prepared using extrudates of zirconia, and the deposition of the metals was not as effective as it was on the zirconia powder. Optimization of the catalyst composition to approach the optimum Au/Pt ratio of 1 observed in batch reactor may still improve the yield in glucaric acid.

Figure 10. Glucose conversion (, ), yields of gluconic (, ), and glucaric (, ) acids over Au0.3wt%-Pt0.7wt%/ZrO2 (open symbols) and Au0.7wt%-Pt1.6wt%/ZrO2 (plain symbols). Reaction conditions: [glucose]0 = 0.25M (0.1 mL min-1), T = 100°C, 40 bar air (25 mL min-1), 5 g catalyst.

i.

Effect of residues and impurities in the oxidation of glucose over Au-Pt/ZrO2

In the scenario of the present project, pinewood was used as raw material for the hydrolysis of the hemicelluloses which were mainly constituted of galactoglucomannans and some arabinogluconoxylans. The sugars from the hemicelluloses were separated from the lignocellulosic biomass after hydrolysis using hot water s followed by sulfuric acid. However, the raw filtrate contained some impurities. The chemical analysis evidenced degradation products of hexoses and pentoses like 5-HMF and furfural, sulfuric acid, acetic acid liberated from the side groups of hemicelluloses during hydrolysis, some sugar oligomers, and some lignin degradation products.49 To examine the possible detrimental effect of the presence of these residues, we further examined the oxidation reaction of glucose over the 3.5%Au-3.45%Pt/ZrO2 catalyst in the presence of the mentioned impurities. The glucose/metal ratio was 80, and the reaction tests were 27 ACS Paragon Plus Environment


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conducted at 100°C under 40 bar air. Pure aqueous solutions of glucose under these conditions are converted to glucaric acid with a yield of 50% after 10 h of reaction. Figure 11 compares the oxidation of glucose in the absence and in the presence of the different impurities. For clarity, only the evolution of concentration of gluconic and glucaric acids are shown, as well as the evolution of TOC and pH values.


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Figure 11. Oxidation of glucose in the absence (open & grey symbols) and in the presence of (a) H2SO4 (0.15 mol eqv.), (b) CH3COOH (0.15 mol eqv.), (c) HMF (0.15 mol eqv.), (d) furfural (0.15 mol eqv.), (e) cellobiose (3.33 g L-1), and (f) guaiacol (3.3 g L-1). Reaction conditions: 3.5%Au-3.45%Pt/ZrO2 catalyst, [glucose]0 = 0.25 M, glucose/metal = 80, T= 100°C, 40 bar air. (, ) gluconic acid, ( , ) glucaric acid, (–, –) TOC, (, ) pH. The addition of sulfuric acid (Fig. 11a) or acetic acid (Fig. 11b) caused a drop in the pH value which was more important with the former one. In both cases, evolution of gluconic acid concentration was similar and a significant decrease of TOC was observed within the first hour. The major difference in the presence of one or the other added acid was actually found in the yield of glucaric acid at the end of the reaction, which was slightly lower. This finding suggests that in the presence of these acids some side reactions of glucaric acid are taking place. The possible leaching of the catalyst under acidic conditions in the presence of H2SO4 was checked by analysis of the final solution by ICP-OES. The filtrate contained 0.32 mg L-1 Pt corresponding to 0.1% leaching of Pt of the solid catalyst introduced, whereas no traces of gold were detected. The used catalyst was reintroduced in a reaction test in identical conditions in the presence of H2SO4. The results were similar to those obtained with the fresh catalyst, and no Pt anymore and no Au were detected in the final reaction medium. The traces of leaching of Pt could originate from Pt which was not allied to Au in the freshly prepared catalyst. It was evident that the presence of furanic derivatives, 5-HMF (Fig. 11c) and furfural (Fig. 11d), had a detrimental effect on the reaction rate of the reaction. The maximum of gluconic acid formed was delayed to 3 h and 8 h, respectively, instead of 0.5 h with the pure glucose solution. Glucaric acid was also formed at a much slower rate to attain around 40% yield after 24 h. At this stage of the reaction, gluconic was not yet completely converted. As it was observed in alkaline conditions,49 strong adsorption of the furanic ring of HMF or furfural prevents the access of glucose to the catalyst surface. High-performance liquid chromatography (HPLC) was used for the quantification of furanic compounds as described previously.76 Accordingly, HMF and furfural were totally converted within 2 h of reaction; 2,5-furan dicarboxylic acid (FDCA) and furan carboxylic acid (FCA) were formed respectively. However they were not stable under the reaction conditions and were further degraded. After 24 h, only a yield of 10% FDCA from HMF introduced was analyzed, and FCA was detected demonstrating that decarboxylation reactions 29 ACS Paragon Plus Environment


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occurred. FCA was formed from furfural with a maximum yield of 20% and was totally degraded after 4 h. The addition of 3.33 g L-1 cellobiose, chosen as a representative disaccharide residue, did not influence during the first hour of the reaction (Fig. 11e). Thereafter, oxidation of gluconic acid to glucaric acid was slowed down and was not complete after 24 h, whereas the maximum in glucaric acid yield attained beyond 8 h was a little lower. Analysis by ionic chromatography demonstrated that total conversion of cellobiose occurred after only 30 min. However, neither cellobionic nor cellobiaric acid were detected, contrary to the reaction under alkaline conditions over Pt/C.49 This suggests that cellobiose was mainly converted by hydrolysis to glucose. This is in accordance with results in the literature which observed the significant hydrolysis of cellobiose to glucose under O2 pressure at temperatures of 145°C.77,78 Nevertheless, the formation of glucose from cellobiose surprisingly involved no positive effect on the final yield of glucaric acid. Guaiacol, used as a model of the phenolic compounds released from lignin was problematic and it drastically decreased the oxidation reaction of glucose (Fig. 11f). The aromatic ring may be strongly adsorbed on the catalyst surface and thus hinders glucose oxidation. The pH and TOC concentration evolutions showed behaviors in accordance with the slow reaction which yielded less than 10% glucaric at the end of the reaction test. In conclusion, as was previously shown for the effect of residues and impurities during oxidation of glucose under alkaline conditions, the reaction rates dropped dramatically in the presence of unsaturated compounds such as furanic or phenolic compounds.

4. Conclusions In this work, supported bimetallic Au-Pt and Au-Pd catalysts were prepared and investigated in the aerobic oxidation of glucose to glucaric acid under base-free conditions. The catalyst prepared on different supports via the facile method of co-reduction of Au3+ and Pt2+ in aqueous solution with NaBH4 contained AuPt alloy nanoparticles. The ZrO2 support gave the most effective catalyst, yielding 50 % of glucaric acid at 100°C, under 40 bar of air. Tuning the composition of the catalyst by adjusting the molar ratios Au/Pt, the catalytic activity for glucose oxidation and the yield of glucaric acid go through a maximum as a function of the alloy composition. 30 ACS Paragon Plus Environment

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Moreover, no efficiency loss related to the yield of glucaric acid was observed upon repeated use in batch reactor or long-term use in a trickle-bed reactor of Au-Pt/ZrO2 catalysts. Finally, screening of the effect of residues possibly contained in hydrolysates of hemicelluloses was investigated. Introduction of acids (sulfuric or acetic acid) or disaccharides (cellobiose) was without detrimental consequence, whereas the presence of furan derivatives (5hydroxymethylfurfural or furfural) or phenolic fragments (guaiacol) was problematic, since these residues drastically lowered down the reaction rates, especially of gluconic acid intermediate to glucaric acid. Hence, these catalysts can be used for glucaric and aldaric acid production from sugar-rich liquors of acid hydrolysis of hemicelluloses after required cleaning procedures. This study will be described in a future report.

Acknowledgements. This work was funded by Polywood FUI (Fonds Unique Interministériel). We gratefully acknowledge Axelera for support. Special thanks to INP-Pagora and Novasep for characterization of the hydrolysate for the project, the Institut des Sciences Analytiques for help in developing analytical methods, Yoan Aizac and Laurence Burel (IRCELYON) for XRD and TEM analysis, Jialu Li for help in the oxidation experiments during her Master internship.

ASSOCIATED CONTENT Supporting Information: Representative TEM images of metallic nanoparticles loaded on TiO2 by various preparation methods (Figure S1); XRD pattern of the 3.8%Au-2.1%Pd/TiO2 catalyst (Figure S2); Effect of the partial pressure of O2 in glucose oxidation (Figure S3); Representative images of the fresh 3.5%Au-3.45%Pt/ZrO2 catalyst and after catalytic test (Figure S4).

AUTHOR INFORMATION Corresponding author •

Tel.: +33 472445358. Fax: +33 472445399. E-mail: michele.bessonrcelyon.univ-lyon1.fr

•

ORCID Michèle Besson : 0000-0003-0024-5676



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ORCID Noémie Perret : 0000-0003-4976-5189 Notes: The authors declare no competing financial interest.

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Aerobic Oxidation of Glucose to Glucaric Acid under Alkaline-Free

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