Nitroxide supported on nanometric metal oxides as new hybrid catalysts for selective sugar oxidation

Journal of Colloid and Interface Science 536 (2019) 526–535

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Regular Article

Nitroxide supported on nanometric metal oxides as new hybrid catalysts for selective sugar oxidation Mehdi Omri a,c, Matthieu Becuwe b,c,⇑,1, Carine Davoisne b,c, Gwladys Pourceau a,c, Anne Wadouachi a,c,⇑,1 a

Laboratoire de Glycochimie, des Antimicrobiens et des Agroressources (LG2A), UMR CNRS 7378, Université de Picardie Jules Verne (UPJV), 33 rue Saint-Leu, 800039 Amiens, France Laboratoire de Réactivité et Chimie des Solide (LRCS), UMR CNRS 7314, Université de Picardie Jules Verne, 33 rue Saint-Leu, 800039 Amiens, France c Institut de Chimie de Picardie (ICP), FR CNRS 3085, Amiens, France b

g r a p h i c a l a b s t r a c t

. Bleach +

MOx

×2

-

a r t i c l e

i n f o

Article history: Received 18 July 2018 Revised 24 September 2018 Accepted 22 October 2018 Available online 28 October 2018 Keywords: Hybrid material Supported nitroxide Organocatalysis Sugar oxidation

a b s t r a c t A new series of supported organocatalysts, prepared by a simple method, were used for selective sugar oxidation. This approach is based on the immobilization of a nitroxide derivative through a carboxylic function on nanometric metal oxides (TiO2, Al2O3 and CeO2), allowing the recovery of the catalyst. These hybrid materials were carefully characterized by Diffuse Reflectance FT-IR spectroscopy (DRIFT), ThermoGravimetric Analysis (TGA), X-Ray Diffraction (XRD), Brunauer-Emmet-Teller surface area measurements (B.E.T.), elemental and electrochemical analyses, showing different characteristics and behaviors depending on the nature of the metal oxide used. The activity of the supported nitroxide catalyst was evaluated on methyl a-D-glucoside oxidation, used as model reaction. In all cases, high catalytic activity was highlighted, with up to 25 times less nitroxyl radical required for complete conversion than under homogeneous conditions. The influence of several experimental conditions such as the use of phosphate buffer and recyclability of the catalyst were also investigated. Ó 2018 Elsevier Inc. All rights reserved.

1. Introduction More and more valuable chemicals used in various fields such as cosmetics, medical, agrochemicals and food processing are ⇑ Corresponding authors at: Institut de Chimie de Picardie (ICP), FR CNRS 3085, Amiens, France. E-mail addresses: [email protected] (M. Becuwe), anne.wadouachi@ u-picardie.fr (A. Wadouachi). 1 Anne Wadouachi and Matthieu Becuwe contributed equally to this manuscript. https://doi.org/10.1016/j.jcis.2018.10.065 0021-9797/Ó 2018 Elsevier Inc. All rights reserved.

prepared from biomass feedstock [1]. The use of carbohydrates as renewable raw materials is becoming increasingly attractive to obtain eco-friendly and high-value products able to replace petroleum-based chemicals [2]. As example, glucaric acid, obtained from glucose oxidation, is one of the top 12 platform chemicals from biomass and can be a precursor to provide adipic acid, widely used in plastics and textile industries [1,3,4]. Nevertheless, sugar chemical modifications often require protection/deprotection steps which lead to multistep synthesis, and the use of hazardous reagents and/or toxic organic solvents.

M. Omri et al. / Journal of Colloid and Interface Science 536 (2019) 526–535

Conventional oxidation conditions such as chromium oxidants exhibit several drawbacks such as extremely high toxicity and limited selectivity, needing partial protection of carbohydrates. In this context, scientific community was particularly active in the development of methodologies that allow selective oxidation of primary and anomeric positions of free carbohydrates to obtain aldonic, uronic or aldaric acids. Among others, nitroxide derivatives, mainly TEMPO ([2,2,6,6-tetramethylpiperidin-1-yl]oxy), are especially considered as the most interesting organocatalyst to achieve selective oxidation of carbohydrates [5–7]. Such selectivity comes from the oxidation mechanism which implies the reaction of the carbohydrate with the oxoammonium form of the nitroxide leading firstly to the formation of a carbonyl function during the first oxidation cycle and then to a carboxylic acid group after the second oxidation cycle (following the same mechanism) [7,8]. Indeed, numerous studies highlight the regioselectivity of the oxidation, the easiness of handling such catalyst and the possibility of shortening the reaction time when electrochemical activity of nitroxide is involved [9–14]. Although homogeneous procedure allows to achieve excellent catalytic activity, this approach is hampered by the use of organic solvent (often chlorinated) to solubilize reactants and to separate efficiently the final product from the active catalytic specie as well. Consequently, this procedure is impacting in terms of environmental footprint especially for development at the industrial scale. Moreover, TEMPO-mediated oxidation of free sugars requires excessively long reaction times and more amount of catalytic species. To circumvent these disadvantages and to increase the efficiency of the reaction, TEMPO was immobilized on various solid supports to first facilitate the recovery of the catalyst thus simplifying purification; and secondly to induce a higher local concentration of the catalyst allowing a better efficiency [15]. Different supports such as fullerene [16], graphene [17], clays [18], polymers [19,20], surface-coated iron [21] or graphene/cobalt metal [22] have been successfully used as support of TEMPO to oxidize selectively primary alcohols (such as benzylic alcohol) into carboxylic acids. However, silica-based materials still remain the most reported supports [23–28] owing to their easiness of synthesis associated to a perfect tuning of morphological and textural parameters [29,30]. This aspect is reinforced by the well-known and covalent grafting method using versatile organosilane chemistry [31–33]. However, the use of silica-based materials and organosilane spacer requires multi-step synthesis and drastic anhydrous conditions to obtain monomers grafting and a reproducible grafting rate. Additionally, precise characterization of the material and its organic surface layer is time-consuming and requires efficient tools like solid-state silicon NMR to clearly prove covalent grafting and to define precisely the degree of oligomerization which can hamper catalytic activity [34]. With this in mind, there is a great interest to design well-defined catalysts in order to rationalize catalytic activity and selectivity. In a continuation of our research concerning the selective oxidation of sugars [35], we prepared a series of new organic/ inorganic hybrid catalysts based on nanometric metal oxides (TiO2, Al2O3 and CeO2) decorated with nitroxide derivative and

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evaluated them as catalysts for the selective oxidation of methyl a-D-glucoside. Three common metal oxides (MOx: Al2O3, TiO2 and CeO2) were selected as support for preparing the hybrid catalysts. This choice was firstly motivated by their large employment in catalysis chemistry to support metal-based compounds as examples [36–39]. Another benefit of using MOx is the easiness of surface modification through coordinative anchoring [40] which is a more cost-effective immobilization strategy than silica-based material requiring multi-steps and specific grafting conditions (anhydrous conditions, heating,. . .) [41,42]. In addition, this approach facilitates the characterization of the anchoring and avoid formation of multi-layer (grafting of oligomers) which is a crucial point in the perspective to determine structure/catalytic activity relationship. The new hybrid catalysts were easily prepared in only one step by grafting nitroxide derivative using a carboxylic acid function through a coordinative anchoring (Fig. 1). Materials were characterized by different methods (FT-IR spectroscopy, TGA, XRD, BET, elemental and galvanostatic analyses) to identify/quantify organic catalyst and to ensure safeguarding of the support structure and morphology. Catalytic tests were carried out using methyl glucoside as model molecule to evaluate the efficiency of the as-prepared catalysts toward selective oxidation of sugars. In order to ensure dispersibility of material in water and to maximize interaction between material surface and carbohydrate, nanometric materials were preferred. 2. Materials and methods 2.1. Reagents and chemicals Chemical reagents such as methyl a-D-glucopyranoside, 4carboxy-TEMPO, sodium phosphate dibasic (Na2HPO4), potassium phosphate monobasic (KH2PO4), sodium chlorite (NaClO2), 11.5 wt% sodium hypochlorite solution (NaClO) were purchased from Sigma-Aldrich or TCI (France) and used as received. All the solvents (dichloromethane, acetonitrile) were purchased from Fisher Scientific and Acros and used without any purification. Metal oxide supports Al2O3 (c-Al2O3 nanopowder, particle size <50 nm, SSA > 40 m2/g), TiO2 (anatase nanopowder, primary particle size 21 nm, SSA = 32–65 m2/g) and CeO2 (nanopowder, particle size < 50 nm, SSA = 30 m2/g mesopore only) were purchased from Sigma-Aldrich and used as received. Milli-Q water was used for synthesis and analysis. 2.2. Instrumentation FTIR spectra in diffuse reflectance mode (DRIFT) were performed using a Nicolet AVATAR370 DTGS spectrometer from Thermo Electron Corporation. The spectra were acquired at room temperature over a range of 4000–400 cm 1. Thermogravimetric analyzes (TGA) were performed on a Netzsch thermal analyzer STA 449C Jupiter equipped with a Differential analysis microbalance coupled with a mass spectrometer QMS 403Aëolos equipped with SEV detector (Channeltron). The samples (10–15 mg) were

Fig. 1. Anchoring of TEMPO on metal oxide (MOx) using carboxylic acid function.

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heated, in an alumina crucible, until 800 °C with a 10 °C min 1 heating rate under static air. The powder X-ray diffraction (XRD) experiments were performed at room temperature on a diffractometer (Bruker D8 Advance XRD) equipped with a Cu X-ray tube (40 kV, 40 mA). The step size was 0.02°. Specific surface area was obtained from nitrogen adsorption measurements. The samples were pretreated under vacuum at 120 °C for 3 h. Then, N2 adsorption isotherms were measured at liquid nitrogen temperature with a Micrometrics ASAP 2020 porosimeter. Prior to analysis, the sample surface was degassed at 110 °C for 3 h under vacuum (ca. 1 mm Hg) without degradation (FTIR performed before and after). B.E.T. surface area was calculated from the adsorption part of the isotherm between 0.03 < P/P0 < 0.3 using multi-points method. Elemental analyses (C, H and N) were recorded on a Thermo Finnigan EA 1112 with a Sartorius MC balance with a precision of ±0.2%. All the analyses were reproduced three times to confirm the organic content of materials. Electrochemical signals were obtained using 2 electrodes Swagelok-type cell assembled in an argon-filled glovebox. For this, a typical working electrode was composed of 10 mg of hybrid material (50%) manually mixed with 50% of Super P carbon. It was separated from a lithium foil playing both the role of counter and reference electrode by two Whatman fiberglass sheets soaked with LiPF6 (1 M) in ethylene carbonate-dimethyl carbonate (ECDMC) 1/1 (v/v) mixture (LP30 electrolyte, certified battery purity grade, Merck). The different galvanostatic measurements were recorded between 3 V and 3.9–4.1 V (vs. Li+/Li) using a Biologic VMP potentiostat/galvanostat. Transmission electron microscopy (TEM) observation was performed using a Philips TECNAI 200F20

a)

b)

Al2O3

2.3. Grafting procedure Hybrid catalysts were prepared at room temperature and under aerobic conditions by surface coordination of metal oxide (Al2O3, TiO2 or CeO2) using the carboxylic function of the nitroxide reagent (4-carboxy-TEMPO, commercially available), which has a natural affinity for these materials [43]. 4-carboxy TEMPO (50.2 mg, 0.25 mmol) was dissolved in 100 mL of dichloromethane at room temperature. To this solution were added 500 mg of metal oxide support and the suspension was stirred for 3 h. Hybrid catalyst was then recovered by centrifugation, washed several times with dichloromethane and finally dried under vacuum overnight at 40 °C. 3. Results and discussion 3.1. Characterization of hybrid catalysts The synthesized hybrid catalysts, labelled C-TEMPO/MOx, were first characterized using FT-IR spectroscopy in diffuse reflectance mode (Fig. 2a–c) in order to assess fulfilment of the anchoring. Comparison of obtained materials (red lines) with starting metal oxide support (blue lines) and 4-carboxy-TEMPO (green lines)

c)

TiO2

CeO2 2920

2920

2920

1250

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1690

1690

3650

1690

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Wavenumber (cm-1)

1420 and 1250 cm

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f)

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Wavenumber (cm-1) 100

1542

1547 1420

Intensity (a. u.)

Intensity (a. u.)

1559 1420 1250

d)

microscope. Images were recorded in bright-field mode. The samples for TEM were prepared as follow: the catalyst was suspended in EtOH, a drop was deposited on a Cu grid with a holey carbon support membrane and the grid was let dried for 15 h.

1000

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Fig. 2. Comparison between C-TEMPO/MOx support (red curve) with starting material (blue curve) and commercial 4-carboxy-TEMPO (green curve) by DRIFT spectrum (a–c) and TGA under air (d–f). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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between 3 and 4 V regarding lithium metal (Fig. 3) and confirmed with EPR spectra realized as example for Al2O3-based hybrid material (see ESI section I). Characteristic electrochemical signal of nitroxide species was clearly observed for all hybrid materials displaying an oxidation plateau at 3.55 V vs Li (formation of oxoammonium ion) followed by a complete reduction of the oxoammonium part into the nitroxide radical. Additionally, it is interesting to note the good correlation between electron numbers exchanged during electrochemical test fitting with the organic content determined using elemental analyses. It can also be noticed that the electrochemical signal of ceria-based hybrid is not as flat as the two other materials (with more stable potential) which suggests that the material has an impact on the oxidation potential of the nitroxide making it more oxidative in the present case and probably more reactive. Further characterizations were also realized to verify the integrity of the metal oxide. First, materials were also studies by TEM (Fig. 4). Images realized with a magnification of 285,000 clearly show nanosize of the materials consisting of spheroidal nanoparticles with average size centred on 19 nm and 45 nm, for TiO2 and CeO2 respectively, while Al2O3 is mainly composed of rice-shaped particle with smaller particle size (5  20 nm). In all the case, grafting of the nitroxide do not induce increasing of the particles size or coalescence of the nanoparticles. We can also confirmed absence of excess of organic molecule and of grafting of multi-layers. Images with a lower magnification were also realized to attest the good homogeneity of the materials (see ESI). On the other side, powder X-ray diffractions confirmed safeguarding of material crystallographic structure (Fig. 5a–c). For the three materials, identical diffraction pattern was obtained after grafting without any phase transformation and widening of the peaks which suggest no increase of the particle size. To complete TEM images, nitrogen adsorption isotherms were carried out to evaluate the effect of molecule grafting on the textu-

clearly reveals the presence of nitroxides on the surface of the metal oxides. In all cases, slightly shifted characteristic bands attributed to the methylene fragment of the nitroxide molecule are clearly observed on the material surface at 2920, 1420 and 1250 cm 1. Additionally, the presence of absorption bands corresponding to the carboxylate function [44] at 1559, 1547 and 1542 cm 1 respectively for Al2O3, TiO2 and CeO2 prove the coordinative grafting of 4-carboxy-TEMPO. Surface grafting is also evidenced, in the case of ceria, by complete disappearance of free surface hydroxyl functions, observed initially at 3650 cm 1 (Fig. 2c). Absence of this peak for TiO2 and Al2O3 is probably due to water adsorption [45]. Concomitantly with these information, the absence of peaks at 1690 cm 1 corresponding to the asymmetric stretching of the carboxylic group and appearance of carboxylate signal observed at 1560 cm 1 also traduce that the signal observed from nitroxide derivative is only due to grafted species, probably as monomer, and not physically adsorbed ones. Comparison of the hybrid materials with the starting metal oxides using thermogravimetric analysis coupled to mass spectrometry (TGA/MS) allowed to estimate of nitroxide content, considering that the total degradation of organic matter occurred under air. Organic contents of 0.285 mmol/g, 0.147 mmol/g and 0.047 mmol/g were calculated for C-TEMPO/Al2O3, C-TEMPO/TiO2 and C-TEMPO/CeO2 hybrid materials respectively, which are in accordance with the variation of specific surface between the three starting metal oxides. Precise organic content was then determined by C, H, N elemental analyses and especially nitrogen content, since this element is only present in the nitroxide compound (Table 1). Consequently, grafting rates were refined giving 0.25 mmol/g, 0.158 mmol/g and 0.049 mmol/g for hybrid materials based on Al2O3, TiO2 and CeO2 respectively (Table 1). Entirety of nitroxide radical part, well studied in the energy storage field [46], was proved by electrochemical characterization using Galvanostatic Cycling with Potential Limitation (GCPL)

Table 1 Determination of nitroxide content in hybrid catalyst using thermogravimetric analysis (TGA) and confirmed with elemental analysis (EA). %C

%H

%N

NO content

C-TEMPO/Al2O3

ATG EA

– 3.587 ± 0.03%

– 0.501 ± 0.03%

– 0.419 ± 0.01%

0.285 mmol/g 0.25 mmol/g

C-TEMPO/TiO2

ATG EA

– 2.29 ± 0.03%

– 0.321 ± 0.03%

– 0.267 ± 0.01%

0.147 mmol/g 0.158 mmol/g

C-TEMPO/CeO2

ATG EA

– 0.582 ± 0.03%

– 0.135 ± 0.03%

– 0.069 ± 0.01%

0.047 mmol/g 0.049 mmol/g

b)

C-TEMPO-Al2O3

4

Potenal vs Li+/Li (V)

Potenal vs Li+/Li (V)

3.8

Ox 3.6

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3.4

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3

0

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e- exchanged/ NO.

0.8

1

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C-TEMPO-TiO2

4

3.8

Ox

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Red

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C-TEMPO-CeO2 4

Potenal vs Li+/Li (V)

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Ox

3.6

Red

3.4 3.2 3

0

0.2

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0.6

e- exchanged/ NO.

0.8

1

1

0.8

0.6

0.4

0 .2

0

e- exchanged/ NO.

Fig. 3. Electrochemical signature of (a) C-TEMPO/Al2O3, (b) C-TEMPO/TiO2 and (c) C-TEMPO/CeO2 realized using galvanostatic analysis operated with a current intensity corresponding to 1e exchanged in one hour under standard battery conditions.

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TEM

Fig. 4. TEM bright field images (magnification of 285,000) of pristine nanometric material (up) and corresponding hybrid including grafted-TEMPO (down) for Al2O3 (a), TiO2 (b) and CeO2 (c).

Al2O3

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96,71 m².g-1/0,825cm3.g-1

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m².g-1/0,826cm3.g-1

Volume (cm3.g-1)

115,08

Volume (cm3.g-1)

30

Scaering angle 2θ (°)-CuKα

500

48,56 m².g-1/0,217cm3.g-1

50

100 0

20

Scaering angle 2θ (°)-CuKα

Scaering angle 2θ (°)-CuKα

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CeO2

c)

Intensity (a. u.)

a)

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47.82 m².g-1/0.114cm3.g-1

50

17,10 m².g-1/0,063 cm3/g

40 30 20 10

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Fig. 5. Evolution of powder XRD patterns (a–c) and N2-adsorption/desorption isotherms (d–f) before (blue) and after grafting (red) of carboxy-TEMPO on Al2O3, (a, d) TiO2 (b, e) and CeO2 (c, f). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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appearance of characteristic peaks around 2900 cm 1, 1600 cm 1 and 1360 cm 1 and without ungrafted or physisorbed species (no bands corresponding to the carboxylic acid at 1720 cm 1) [51]. Organic content of 4.96% (1.07 mmol/g), 1.27% (0.276 mmol/g) and 0.77% (0.167 mmol/g) was estimated to 5.6 molecule/nm2, 3.14 molecule/nm2 and 2.14 molecule/nm2 for Al2O3, TiO2 and CeO2 respectively. These values were used as ‘‘references” to define a perfect monolayer and also to extract the grafting rate, which depends on the steric effect of the molecule [52]. From these data,

ral properties (Fig. 5d–f). As generally observed after grafting, a decrease of the specific surface area was obtained in all cases passing from 115.1 m2 g 1, 52.8 m2 g 1 and 47 m2 g 1 to 96.7 m2 g 1, 48.6 m2 g 1 and 17.1 m2 g 1 for Al2O3, TiO2 and CeO2 respectively. This consistent decrease following grafting was accompanied by a reduction of the pore volume for Al2O3 and CeO2, which tends to indicate that the nitroxide compound is grafted not only on the material surface but also inside the material pore channels. (Fig. 5f). In order to compare the three hybrid catalysts, we estimated the surface density of nitroxide grafting using organic content obtained from elemental analysis and specific surface area. Thus, values of 1.3 molecule/nm2, 1.8 molecule/nm2 and 0.62 molecule/ nm2 for hybrid materials based on alumina, titania and ceria respectively were obtained and are quite consistent with the girth of the nitroxide derivative and number of active sites available for the grafting generally observed for these materials (between 4 and 6/nm2) [47,48]. To go beyond, we carried out additional experiment to determine ‘‘active grafting site” of the materials used in this study, by using a small organic molecule [49,50]. In order to mimic as close as possible our organ catalyst, formic acid was used to do probe the surface and grafted using the same procedure used for nitroxide grafting. Materials thus obtained were characterized by TGA and FTIR (Fig. 6) in order to estimate organic content and to prove the absence of ungrafted molecule respectively. As shown just after on Fig. 6 (a–c), formic acid is clearly chemisorbed (following dissociation of the carboxylic acid) on all the three metal oxides with

a)

Al2O3

Table 2 Catalytic performances of hybrid materials depending on the metal oxide used as support. Yields were determined by 1H NMR (available in ESI).

Catalyst

Run

Neq NO

Conversion (%)

Al2O3 TiO2 CeO2 Carboxy-TEMPO C-TEMPO/Al2O3

1 1 1 1 1 2 1 2 1 2

– – – 0.1 0.021 NA 0.013 NA 0.004 0.004

0 0 0 >99 94 11 >99 0 >99 >99

C-TEMPO/TiO2 C-TEMPO/CeO2

TiO2

b)

2923 2853

1662

CeO2

c) 2930 1632

1336

2830 2720

1570 1370

1358

Intensity (a. u.)

2915 2865 2816

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Wavenumber (cm-1)

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Mass Formic acid= 4,96% 0

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Mass Formic acid= 1,27% 0

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Mass Formic acid= 0,77% 0

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Fig. 6. Comparison between Formic acid grafted on MOx support (red curve) with starting material (blue curve) and by DRIFT spectrum (a–c) and TGA under air (d–f). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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approximately 23%, 57% and 29% of the active sites were exploited for the grafting on Al2O3, TiO2 and CeO2, respectively. 3.2. Evaluation of methylglucoside oxidation catalytic activity To evaluate the catalytic activity of these new hybrid materials for sugar oxidation, methyl glucoside was selected as carbohydrate model. For this purpose, tests were carried out using a classical procedure [9] in phosphate buffer at pH = 6.7 (see ESI for full procedure). Prior to any tests with the as-synthesized hybrid materials, each starting material, both metal oxide supports and 4carboxy-TEMPO were tested (Table 2). As intuited, metal oxide supports did not show any catalytic activity. On the other hand, Table 3 Catalytic performances without phosphate buffer of hybrid materials depending on the metal oxide used as support. Yields were determined by 1H NMR (available in ESI). Catalyst

Run

Neq NO

Conversion (%)

C-TEMPO/Al2O3

1 2 1 2 1 2

0.021 – 0.013 – 0.004 –

>99 66 >99 0 >99 >99

C-TEMPO/TiO2 C-TEMPO/CeO2

C-TEMPO/Al2O3

C-TEMPO/TiO2

b)

Intensity (a. u.)

Intensity (a. u.)

a)

a complete conversion of methyl glucoside into glucuronate was observed 48 h after treatment with free 4-carboxy-TEMPO (0.1 eq.). Catalytic activity of the new hybrid materials was then evaluated using this procedure adapted to our materials and realized totally in water without the use of an additional organic solvent (Table 2). After recovering of the catalyst, the filtrate was freeze-dried and the crude product was analyzed by NMR to determine the conversion ratio. For TiO2 and CeO2 based materials, quantitative conversion of methyl a-d-glucopyranoside into corresponding uronate compound was observed within 48 h using up to 25 times less nitroxyl radical than in the homogeneous conditions. A slightly difference was observed for the C-TEMPO/Al2O3 which did not give a total conversion of methyl glucoside (94%) in the same time despite a higher nitroxide content. Difference of activity may be ascribed to different parameters. Probably, the support nature could be at the origin of such difference since the electrochemical potential of the nitroxide is higher which suggests a more oxidative behavior. Another explanation would be that higher loading for Al2O3 and TiO2 (1.3 and 1.8 molecule/nm2) can hamper activity owing to the steric hindrance around nitroxide molecules, between them and with the carbohydrate anchored during the catalytic cycle, or with a limited accessibility of the catalyst inside the material (agglomerated particle or non-accessible porosity). Less surely, we can also envisaged that recombination or deactivation of oxoammonium part can occur

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Fig. 7. Comparison between hybrid catalysts before (blue curves) and after two catalytic runs (red curves) realized using DRIFT (a, b) and TGA (c, d) for Al2O3 and TiO2-based materials. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Surprisingly, in the case of ceria-based hybrid, catalytic activity is maintained over multiple run without any loss of activity (Fig. 8a). Indeed, the C-TEMPO/CeO2 catalyst was found to be effective and very stable, resulting in quantitative conversion of methyl glucoside even after 5 runs and without any specific treatment between each run (Fig. 8a). Absence of leaching was attested by FTIR (Fig. 8b) and TGA analysis (Fig. 8c) of the catalyst after 5 cycles which reveals a nitroxide content (0.92%) very close to that of the initial catalyst (1%). Such stability for ceria-based hybrid material may signify a better affinity of carboxylic acid for this material compared to TiO2 and Al2O3 in basic medium. Generally, metal carboxylate stability depends mostly of the basicity of the anion and ionic radius of metal cation [57]. Based on ionic radius of the metals used in this study, it is obvious to think that this parameter is the key parameter in the stability of metal carboxylate link. Precisely, Al3+, Ti3+ (or Ti4+) and Ce3+ possess increasing ionic radius respectively [58]. Considering that carboxylate anion is quite big (approximatively 0.1 nm) and bigger than cerium III, the better stability for ceria-based hybrid is consistent with stability rules of metal carboxylate and better charge complementarity (HSAB theory) between carboxylate with surfacic cerium ion compared with other metal oxides [59]. Furthermore, experiments of grafting of formic acid on the materials revealed a crucial information. Indeed, after careful study of the spectra, we can easily conclude that formic acid is chemisorbed (carboxylate form) on the surface of all the metal oxides but with different binding modes. According to the literature [60], the large bands between 1570 and 1670 cm 1 correspond to two different types of anchoring (monodentate and bidentate) associated normally to two different bands overlapping together. Depending of the asymmetry of this large band, bonding mode can be determined. In the case of Al2O3, the asymmetry of the band is displayed toward the higher wavenumber which means that the monodentate anchoring mode is the most predominant one. On the contrary, for ceria, the asymmetry of the band is

in highly concentrated system since electron hoping can occur if nitroxide radicals are too closed [53]. 3.3. Recyclability and stability of hybrid catalysts In order to evaluate the recovery and the reuse abilities of the hybrid catalysts, these latter were filtered then dried and reused for the subsequent run without any treatment. Only CeO2-based material ensured a quantitative conversion of methyl glucoside for the second run. For C-TEMPO/Al2O3 a significant loss of activity was observed (only 11% of methyl glucoside converted) and no reaction was observed with reused C-TEMPO/TiO2. This loss of activity is explained by the leaching of nitroxide, clearly observed in NMR analysis (see ESI), probably due to a low affinity of CTEMPO for MOx in basic media or due to the presence of phosphate (hypothesis 1) or of the glucuronate formed in the media (hypothesis 2). Regarding the first hypothesis, phosphate compounds are known to show better affinity for transition metal oxides than carboxylates [52], Therefore, phosphate buffer might be at the origin of such loss of activity. Nevertheless, oxidation reaction carried out without phosphate buffer (see ESI for full procedure) led to the same observations, a loss of activity was observed except for ceria-based material (Table 3). For Al2O3-based material, only absorption bands attributed to the starting aluminium oxide and water were observed and revealed a complete disappearance of nitroxide signals on the FTIR spectra (Fig. 7a). TGA/MS confirmed a leaching of organic catalyst after the first run as no carbon dioxide was detected during heating (Fig. 7c). In the case of TiO2, a different behavior was observed since even if nitroxide signals have been annihilated (Fig. 7b), the loss of weight obtained by TGA is largely more important than the original hybrid catalyst (Fig. 7d). This results is explained by a leaching of the nitroxide caused probably by the adsorption of glucuronate molecule, as proved by the signals observed at 1640, 1360 and 900 cm 1 on the FT-IR spectrum (Fig. 7b) [54–56].

a)

b)

c) 100

100

95

Intensity (a. u.)

Conversion rate

80

60

40

90

85

20

0 1

2

3

4

Cycles number

5

4000 3500 3000 2500 2000 1500 1000 500

80

200

400

600

800

1000

Temperature (°C)

Wavenumber (cm-1)

Fig. 8. Evolution of conversion rate of methyl glucoside into corresponding uronate as a function of run number for ceria-based catalyst (a). Comparison between hybrid catalysts before (blue curves) and after five catalytic runs (red curves) using DRIFT (b) and TGA (c). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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͠͠

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Fig. 9. Estimation of the relative proportion of monodentate and bidentate anchoring in synthesized hybrid catalysts.

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much displaced toward lower value of wavenumber suggesting the bi-dentate mode as the major bonding type in that case. Furthermore, formic acid grafted on TiO2 exhibits both anchoring, with a small predominance for monodentate mode [60] (Fig. 9). In addition to the charge complementarity (HSAB theory), this difference of anchoring type is probably at the origin of the higher stability of ceria-carboxylate couple and for the whole hybrid catalyst. In the future, the selection of the support would be essential to ensure the stability of the catalyst. 4. Conclusions To conclude, we present a series of new supported organic/inorganic hybrid catalysts on nanometric metal oxides for selective carbohydrate oxidation, including a nitroxide radical. Hybrid materials were obtained in one step synthesis at room temperature by grafting carboxy-TEMPO on the surface of metal oxides and were carefully characterized by a pool of techniques to confirm the grafting and to determine the nitroxide content. Selective catalytic oxidation of methyl a-d-glucopyranoside into corresponding methyl a-d-glucopyranuronate reveals the high potential and efficiency of this approach in the presence of 25 times less of nitroxide reagent compared with homogeneous catalysis reported in literature. The best performances were obtained for ceria-based hybrid catalyst which was found to be very stable under catalytic conditions, contrary to alumina and titania based materials presenting rapid leaching of nitroxide after the first run. Such difference of activity is explained by the metal-carboxylate anchoring, stronger in the case of ceria, which enhances both reactivity and stability of the catalyst. In the future, other studies will be carried out on other sugars (mono- and oligosaccharides) to develop an efficient green heterogeneous oxidation methodology to obtain high-value biosourced compounds.

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Acknowledgements The Conseil Régional de Picardie is gratefully acknowledged for financial support through the OXYOL project and PhD grant accorded for M. Omri. The authors wish to acknowledge Matthieu Courty for ThermoGravimetric Analysis.

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Appendix A. Supplementary material

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Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2018.10.065.

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