Isaline Bonnin

Raphaël Mereau

Karine De Oliveira Vigier

Thierry Tassaing
email: thierry.tassaing@u-bordeaux.fr

In operando simultaneous ATR-IR/Raman spectroscopic investigation of the catalytic activity of supported Pt, Ru, and Pd catalysts in H/D exchange reactions between H2 and D2O

Keywords: Isotopic Exchange, Deuterated water, Dihydrogen, Hydrogenation, Glucose, Ruthenium, Platinum, Palladium, In operando spectroscopy

In this study, catalytic H/D exchange experiments have been performed between H2 and D2O catalyzed by supported metal catalysts, namely, Ru/Al2O3, Pt/Al2O3 and Pd/Al2O3 in order to unveil the reaction mechanism at work in the glucose hydrogenation reaction on these supported metal catalysts. The conversion of H2 and D2O towards the formation of the exchange species namely HOD, HD and D2 was investigated using in operando simultaneous ATR-IR/Raman spectroscopy allowing the monitoring of the liquid/gas phase respectively. After 24 h of reaction at 22 °C under 30 bar of H2, it is found that Pt/Al2O3 has the highest efficiency for the H/D exchange between H2 and D2O, followed by Ru/Al2O3 and then Pd/Al2O3. On the other hand, the enhanced activity of Ru/Al2O3 for the hydrogenation of glucose is tentatively assigned to H atoms adsorbed and activated on the Ru surface leading to an Ru-H infrared band located around 1940-1970 cm -1 , that is observed only with Ru/Al2O3. This activated H* could be directly added to the carbon atom of the carbonyl of glucose to give sorbitol in a more efficient way compared to the other metals.

Introduction

Due to the use of lignocellulosic biomass as raw material, more and more catalytic reaction are performed in water. Hence, water allows the dissolution of carbohydrates contained in lignocellulosic biomass. An important reaction starting from sugars is the hydrogenation of sugars to sugar alcohol. For instance, sorbitol is a commodity chemical produced with high tonnage for paper industry, food and cosmetic applications. As sorbitol can be obtained from glucose hydrogenation (Fig. 1), contained in the carbohydrate part of lignocellulose, it was identified in 2004 by the US Department of Energy as one of the building blocks materials to high-value bio-based chemicals [START_REF] Bozell | Technology development for the production of biobased products from biorefinery carbohydrates-the US Department of Energy's "Top 10" revisited[END_REF]. During this reaction, it is of prime interest to understand the exact role of water on the catalyst. The synthesis of sorbitol from glucose is mostly performed with Raney-type nickel catalysts or nickel supported catalysts due to its high hydrogenation activity [START_REF] Gallezot | Glucose hydrogenation on promoted raney-nickel catalysts[END_REF][START_REF] Déchamp | Kinetics of glucose hydrogenation in a trickle-bed reactor[END_REF][START_REF] Li | Glucose Hydrogenation to Sorbitol over a Skeletal Ni-P Amorphous Alloy Catalyst (Raney Ni-P)[END_REF][START_REF] Li | Glucose hydrogenation over Ni-B/SiO2 amorphous alloy catalyst and the promoting effect of metal dopants[END_REF][START_REF] Wen | Preparation of sorbitol from D-glucose hydrogenation in gasliquid-solid three-phase flow airlift loop reactor[END_REF][START_REF] Schimpf | Ni/SiO2 catalysts prepared with ethylenediamine nickel precursors: Influence of the pretreatment on the catalytic properties in glucose hydrogenation[END_REF][START_REF] Geyer | New Catalysts for the Hydrogenation of Glucose to Sorbitol[END_REF]. However, due to the leaching of nickel, the study of more stable, selective and active catalysts was carried out. Ru catalyst was found as the best alternative for the selective hydrogenation of glucose to sorbitol in comparison to Pt or Pd noble metals [START_REF] Corma | Chemical Routes for the Transformation of Biomass into Chemicals[END_REF][START_REF] Besson | Conversion of Biomass into Chemicals over Metal Catalysts[END_REF][START_REF] Bonnin | Hydrogenation of Sugars to Sugar Alcohols in the Presence of a Recyclable Ru/Al2O3 Catalyst Commercially Available[END_REF]. Indeed, in a recent work, we have demonstrated that a commercial catalyst Ru/Al2O3 was very active and selective for the hydrogenation of glucose to sorbitol in comparison to Pt/Al2O3 and Pd/Al2O3 that were found to be inactive for this reaction (Table 1) [START_REF] Bonnin | Hydrogenation of Sugars to Sugar Alcohols in the Presence of a Recyclable Ru/Al2O3 Catalyst Commercially Available[END_REF]. The conversion and yield without catalyst that is found higher than that with Pd or Pt as catalyst could be related to thermal degradation of glucose into humins while the addition of catalyst Pd/Al2O3 and Pt/Al2O3 probably limits this side reaction.

Table 1: Screening of commercial metal catalysts (5 wt.% of metal, average particle size of 2 nm) for the hydrogenation of glucose in aqueous liquid phase under 30 bar of H2 at 120 °C for 1 h [START_REF] Bonnin | Hydrogenation of Sugars to Sugar Alcohols in the Presence of a Recyclable Ru/Al2O3 Catalyst Commercially Available[END_REF].

Glucose conversion (%)

Sorbitol yield (%) No catalyst [START_REF] Wu | Catalytic pathways and mechanistic consequences of water during vapor phase hydrogenation of butanal on Ru/SiO2[END_REF] Beyond the hydrogenation of glucose, the detailed review of Besson et al. emphasizes that Ru catalysts are also very efficient for the hydrogenation of various aldoses and carboxylic derivatives such as arabinose, galactose, rhamnose, levulinic acid or succinic acid [START_REF] Besson | Conversion of Biomass into Chemicals over Metal Catalysts[END_REF]. According to the literature, such activity is related to the presence of water that, on the one hand, stabilizes intermediates adsorbed on Ru metal surface by solvation effect [START_REF] Michel | Role of water in metal catalyst performance for ketone hydrogenation: a joint experimental and theoretical study on levulinic acid conversion into gamma-valerolactone[END_REF][START_REF] Michel | Why Is Ruthenium an Efficient Catalyst for the Aqueous-Phase Hydrogenation of Biosourced Carbonyl Compounds?[END_REF] and, on the other hand, directly plays a catalytic role in the hydrogenation mechanism [START_REF] Zhao | Solvent-mediated charge separation drives alternative hydrogenation path of furanics in liquid water[END_REF].

Investigated computationally [START_REF] Zhao | Solvent-mediated charge separation drives alternative hydrogenation path of furanics in liquid water[END_REF][START_REF] Demir | Effects of water on the kinetics of acetone hydrogenation over Pt and Ru catalysts[END_REF][START_REF] Wu | Catalytic pathways and mechanistic consequences of water during vapor phase hydrogenation of butanal on Ru/SiO2[END_REF][START_REF] Dai | On the role of water in selective hydrogenation of cinnamaldehyde to cinnamyl alcohol on PtFe catalysts[END_REF][START_REF] Hensley | Mechanistic Effects of Water on the Fe-Catalyzed Hydrodeoxygenation of Phenol. The Role of Brønsted Acid Sites[END_REF][START_REF] Shangguan | Kinetic Significance of Proton-Electron Transfer during Condensed Phase Reduction of Carbonyls on Transition Metal Clusters[END_REF][START_REF] Shangguan | The Role of Protons and Hydrides in the Catalytic Hydrogenolysis of Guaiacol at the Ruthenium Nanoparticle-Water Interface[END_REF][START_REF] Song | Selective hydrodeoxygenation of lignin phenols to alcohols in the aqueous phase over a hierarchical Nb2O5-supported Ni catalyst[END_REF] and experimentally [START_REF] Kim | Acidic Water Monolayer on Ruthenium[END_REF][START_REF] Merte | Water-Mediated Proton Hopping on an Iron Oxide Surface[END_REF][START_REF] Yuan | Microbehavior mechanism of water mediator on palladium in catalytic hydrogenation of aromatic carbonyl: Enhancement of hydrogen shuttling and modification of electronic structure[END_REF], the hydrogenation mechanism in the liquid aqueous phase is likely to proceed through a catalytic exchange between an adsorbed H (H*) from H2 and water. As the H* from H2 is transferred to water molecule, it suggests that the replacement of H2O by D2O, or gaseous H2 with D2, allows to follow the exchange of protons that occurs at the solid-liquid interface. Thus, to investigate the catalytic exchange of H * to water at the metal-liquid interface, few research works have reported the influence of metal for the production of exchange species HOD when water is replaced by deuterated water [START_REF] Rahkamaa-Tolonen | Investigation of NO Reduction by H2 on Pd Monolith with Transient and Isotopic Exchange Techniques: I. H2/D2 Exchange with H2O and NH3[END_REF][START_REF] Sajiki | Complete replacement of H2 by D2 via Pd/C-catalyzed H/D exchange reaction[END_REF][START_REF] Kurita | Facile and convenient method of deuterium gas generation using a Pd/C-catalyzed H2-D2 exchange reaction and its application to synthesis of deuterium-labeled compounds[END_REF]. In particular, exchange species like HOD, HD and D2 were detected for independent gas and aqueous phase investigations for the hydrodeoxygenation of guaiacol over Ni/Nb2O5 putting in evidence the D2O-H2 exchange involvement in the mechanism [START_REF] Song | Selective hydrodeoxygenation of lignin phenols to alcohols in the aqueous phase over a hierarchical Nb2O5-supported Ni catalyst[END_REF]. Analysis of the aqueous phase by infrared spectroscopy have proven the formation of HOD and a production of HD and D2 was detected in the gas phase by mass spectrometry. Others have studied the conversion of cinnamaldehyde into cinnamyl alcohol over Pt3Fe and suggest that water participated in the catalytic hydrogenation reaction by serving as a hydrogen-exchange bridge [START_REF] Dai | On the role of water in selective hydrogenation of cinnamaldehyde to cinnamyl alcohol on PtFe catalysts[END_REF].

In this context, we perform similar experiments in order to get insights into the mechanistic aspects of the hydrogenation of glucose to sorbitol in water over the supported metal catalyst Ru/Al2O3, Pt/Al2O3 and Pd/Al2O3 that were used in our previous study [START_REF] Bonnin | Hydrogenation of Sugars to Sugar Alcohols in the Presence of a Recyclable Ru/Al2O3 Catalyst Commercially Available[END_REF]. The main aim was to explain the high activity of Ru/Al2O3 in comparison to Pt/Al2O3 and Pd/Al2O3 (Table 1). To this aim, we have performed some in operando experiments on H2/D2O mixtures under high pressure at different temperatures in the presence of the supported metal catalysts cited above in order to compare their ability to perform the catalytic isotopic exchange between H2 and D2O. To monitor the exchange species as HOD, HD and D2, we have designed an innovative in operando spectroscopic method using a single high-pressure cell for the simultaneous coupling of ATR-IR and Raman spectroscopy allowing the monitoring of the liquid and gas phase respectively. Moreover, the H/D exchange was also followed using the same methodology during the hydrogenation of glucose to sorbitol over Ru/Al2O3 catalyst.

Materials and methods

Materials

H2 and D2O were used as received with the following characteristics: H2 (Air Liquid, 99.9999 %), D2O (CEA saclay, 99.9 % purity). The catalysts Pd/Al2O3, Pd/Al2O3 and Ru/Al2O3 were purchased from Alfa Aeser. The metal content and the average particles sizes of catalysts were measured by ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometry) and TEM (Transmission Electronic Microscopy). Catalysts exhibit a similar metal content (4.3 wt. % for Pd, 5.3 wt. % for Pt and 4.7 wt. %for Ru) and a similar average particle size (2 nm) allowing a proper comparison of these materials for the H/D exchange experiments. Especially, XRD analysis of Ru/Al2O3 have shown that the support is composed of a mixture of alumina phases identified as aluminium oxide (Al2O3) with a monoclinic crystalline structure (JCPDS 98-006-6560) and gamma alumina (γ-Al2O3) with tetragonal (JCPDS 98-009-9836) and cubic crystalline phases (JCPDS 10-1425).

High pressure simultaneous operando ATR-IR/Raman spectroscopy 2.1. High pressure cell

The monitoring of the catalytic H/D exchange by simultaneous operando ATR-IR/Raman spectroscopy is performed in a single high-pressure cell (Fig. 2 and Fig. S1 in ESI 1). The cell has two parts, an ATR-FTIR part located at the bottom for liquid phase measurements and the other located at the top is the Raman cell for gas phase analysis. The ATR part is composed of a silicon crystal to measure the wavelength range between 1000 cm -1 and 4000 cm -1 . The crystal has a visible surface of 50 mm 2 and it resists to high pressure thanks to its T-shape that allows blocking and sealing the crystal between two layers of the ATR accessory by the unsupported area principle. The ATR part of the cell is connected to the upper part of the cell composed of a sapphire window for Raman scattering. For a homogeneous temperature, the two parts of the cell can be heated independently using two cartridges' heaters each side of the ATR part and two cartridges' heaters disposed behind the Raman cell with thermocouples for each couple of cartridges to regulate the temperature with the accuracy of about 𝛥T = ± 0.5 °C. At the top of the Raman cell, a stainless capillary is connected to gas H2 and to a pressure sensor with an accuracy of about 𝛥P = ± 0.1 bar. To ensure a good mixing of the liquid phase, a stirrer is deposited on the ATR crystal and a magnetic plate is put next to the cell. The total volume of the cell is 5 cm 3 and to ensure the sealing between the two parts, flat Kapton® rings are used between plugs and the high-pressure cell body.

Infrared/Raman set-up

For the ATR-IR measurements, a Nicolet 6700 FTIR spectrometer equipped with a Globar source, a KBr/Ge beamsplitter and a DLaTGS/KBr thermal detector was used to investigate the spectral range from 400 to 4000 cm -1 . Single beam spectra with 4 cm -1 resolution are obtained by Fourier transformation of 100 accumulated interferograms in order to improve the signal-to-noise ratio.

For IR absorption measurements, a Nicolet 6700 FTIR spectrometer equipped with a Globar source, a KBr/Ge beamsplitter and a DLaTGS/KBr thermal detector was used to investigate the spectral range from 400 to 6500 cm -1 . Single beam spectra with 4 cm -1 resolution are obtained by Fourier transformation of 100 accumulated interferograms in order to improve the signal-to-noise ratio.

Experimental procedure

For a typical catalytic H/D exchange experiment, 10 mg of catalyst M/Al2O3 is deposited on the ATR crystal. Then, the Raman part is connected to the ATR part and.an excess of D2O (300 µL, 1.66 10 -2 mol) is injected with a syringe from the upper part to the ATR crystal where a magnetic stirrer constantly homogenizes the liquid during the experiment. Prior to the introduction of 30 bar of H2 at ambient temperature or 37 bar of H2 at 90 °C (that corresponds to 5.95 10 -3 mol of H2 according to the NIST database [START_REF] Informatics | Bienvenue sur le WebBook de Chimie NIST[END_REF]), acquisitions of both ATR-FTIR and Raman spectra are started at the same time to get a concomitant monitoring of D2O, HOD, H2, HD and D2 during 24 h. During the experiment, infrared acquisitions of 210 s are recorded continuously for 24 h for the monitoring of D2O and HOD species. Stretch and bending bands of D2O and HOD at 1205 cm -1 (D-O-D bending), 1456 cm -1 (H-O-D bending) and 3401 cm -1 (O-H stretch) are chosen for further kinetics analysis by using baseline integration from 1800 to 1330 cm -1 for D-O-D and H-O-D vibrations and from 3780 to 3050 cm -1 for O-H vibration. Regarding the Raman spectra, one background of the empty high-pressure cell is recorded after 1 scan of 15 seconds of time integration and no other background is performed during the experiment. During the experiment, an acquisition of 30 s of the gas phase for the monitoring of H2, HD and D2 is executed and re-executed after 270 s and this loop of acquisitions lasts during the whole experiment of 24 h. The rotational band of H2 at 580 cm -1 is integrated from 720 to 490 cm -1 for kinetic profiles. Vibrons of H2 at 4160 cm -1 is integrated from 4256 to 4070 cm -1 . The HD and D2 species at 3630 cm -1 and 2989 cm -1 respectively are integrated from 3695 to 3550 cm -1 for HD and from 3050 to 2915 cm -1 for D2. Finally, the methodology to calculate the conversion of D2O and H2 as well as the yield of HOD, HD and D2 from the ATR-FTIR and Raman spectra is presented in details in the ESI 2.

Results and discussion

Description of infrared and Raman spectra

In a first set of experiments, the catalytic H/D exchange was studied in the presence of Ru/Al2O3. The Fig. 3 displays ATR spectra of the liquid phase during this exchange. Initially, a spectrum is recorded before starting the kinetic experiments and vibrational bands of D2O are observed with the two characteristic bands at 1205 cm -1 and 2480 cm -1 which correspond to the bending mode 𝜈2 (D-O-D) and the asymmetric stretching contribution 𝜈3 (O-D) of D2O. After the insertion of H2, a weak peak at around 1700 cm -1 is observed probably due to traces of acetone residue (after the cleaning of the cell) that disappear after 10 min of reaction. ATR spectra are recorded during 24 h and show that two bands at 1456 cm -1 and 3401 cm -1 are increasing according to time. These bands are linked to the formation of HOD species and they correspond to the bending mode 𝜈2 (H-O-D) at 1456 cm -1 and the stretching contribution (O-H) at 3401 cm -1 . The stretching (O-D) of HOD is also visible around 2480 cm - 1 but it coincides with the 𝜈3 (O-D) of D2O explaining the shoulder observed around 2400 cm - 1 . It is also noticed that two bands at 1947 cm -1 (A) and at 1100 cm -1 (B) are observed only in the case of Ru/Al2O3 catalyst and the identification of these bands will be discussed later.

Simultaneously to the analysis of the liquid phase by ATR, Raman scattering is used for the monitoring of the gas phase during the catalytic H/D exchange of H2 and D2O with Ru/Al2O3. Spectra are displayed in the Fig. 4 in the low and high frequency range from 100 to 1100 cm -1 and from 2900 to 4250 cm -1 . Broad bands due to the fluorescence of the sapphire windows are present in these regions as it is shown in the black spectrum before the insertion of H2 in the cell. When H2 is added, rotational bands of H2 are observed at 343, 580, 806 and 1031 cm -1 and are related to the S branch (𝛥J=+2) from 0 to 3 rd rotational level respectively [START_REF] Veirs | Raman line positions in molecular hydrogen: H2, HD, HT, D2, DT, and T2[END_REF][START_REF] Li | Raman vibrational spectral characteristics and quantitative analysis of H 2 up to 400°C and 40 MPa[END_REF][START_REF] Petrov | Raman Spectra of Nitrogen, Carbon Dioxide, and Hydrogen in a Methane Environment[END_REF]. The stretching vibration of H2 (also called the vibrons) is Raman active with a main contribution at 4152 cm -1 that corresponds to Q1 (J=1) and a weak peak at about 4125 cm -1 that corresponds to Q1 (J=3) [START_REF] Veirs | Raman line positions in molecular hydrogen: H2, HD, HT, D2, DT, and T2[END_REF][START_REF] Li | Raman vibrational spectral characteristics and quantitative analysis of H 2 up to 400°C and 40 MPa[END_REF][START_REF] Howie | Phonon Localization by Mass Disorder in Dense Hydrogen-Deuterium Binary Alloy[END_REF] is also observed but in less extent after 24 h. Because the pressure of H2 takes time to equilibrate, the signal at 10 min is lowered compared to the signal at 1 h and, after this time, bands of H2 are decreasing. Concomitantly to this decrease, peaks at 253, 434, 610, 774 and 980 cm -1 are increasing during the experiment of 24 h and they correspond to rotational contributions of HD with the S branch from 0 to 4 th . In the high frequency range, vibrons of HD at 3630 cm -1 (Q1 (J=0)) is observed and it coincides with one band of fluorescence. More, a weak contribution at 2989 cm -1 appears and corresponds to the Q1 (J=0) stretch vibrons of D2 species.

Acquisitions of infrared and Raman spectra during the catalytic H/D exchange demonstrate that HOD, HD and D2 species are simultaneously produced in liquid and gas phase in the presence of Ru/Al2O3 and the attribution of spectral bands are summarized in the ESI 3.

To verify that the H/D exchange comes from the catalyst, same experiment is performed but without catalyst and no D2O nor H2 are consumed, as expected [START_REF] Sajiki | Complete replacement of H2 by D2 via Pd/C-catalyzed H/D exchange reaction[END_REF][START_REF] Kurita | Facile and convenient method of deuterium gas generation using a Pd/C-catalyzed H2-D2 exchange reaction and its application to synthesis of deuterium-labeled compounds[END_REF]. These first results already confirm that Ru catalyst is able to perform the H/D exchange.

Kinetic profiles of D2O, H2, HOD, HD and D2 species

Raman scattering spectra and infrared spectra are recorded simultaneously which allow to correlate without ambiguity the kinetic profiles of D2O and H2 that are converted into HOD, HD and D2 species. ) and HOD (1450 cm -1 ) obtained by ATR-FT-IR analysis (in red) as well as kinetic profiles of H2 (579 cm -1 ), HD (3625 cm -1 ) and D2 (2988 cm -1 ) obtained by Raman measurements (in green) during the catalytic H/D exchange between D2O and H2 over Ru/Al2O3 at 22 °C for 24 h under 30 bar of H2.

The Fig. 5 displays kinetic profiles obtained by infrared (D2O and HOD in red) and by Raman analysis (H2, HD and D2 in green) during the catalytic H/D exchange with Ru/Al2O3. Kinetics of D2O and H2 have similar profiles with a quick decrease at the first 200 min of the experiment following by a less marked decrease for the last 1440 min. The signal of H2 shows however, a rapid increase to higher intensities and then a fast decrease during the first 100 min of the reaction that is considered as an "induction" time for H2 to reach equilibrium between the liquid and the gas phase.

In accordance with the decrease of H2 and D2O signals, profiles of HOD, HD and D2 are increasing according to time justifying that these species are produced from the catalytic exchange between H2 and D2O. The formation of HOD and HD displays similar kinetic profiles with a strong increase at the first 200 min following by a less pronounced increase. Concerning D2, its formation is slower than HD and the intensity of the D2 signal is about half that of the HD signal.

Kinetic profiles in Fig. 5 demonstrate that D2O and H2 are both firstly converted into HOD and HD as described in Eq. 1. The HD previously produced can then react with D2O to produce D2 species like in Eq. 2. The Eq. 3 is the overall reaction starting from D2O and H2 to HOD and D2 without HD as intermediate as suggested by Kurita and Sajiki's group [START_REF] Sajiki | Complete replacement of H2 by D2 via Pd/C-catalyzed H/D exchange reaction[END_REF][START_REF] Kurita | Facile and convenient method of deuterium gas generation using a Pd/C-catalyzed H2-D2 exchange reaction and its application to synthesis of deuterium-labeled compounds[END_REF].

𝐷 2 𝑂 + 𝐻 2 → 𝐻𝑂𝐷 + 𝐻𝐷 Eq. 1 𝐷 2 𝑂 + 𝐻𝐷 → 𝐻𝑂𝐷 + 𝐷 2 Eq. 2 2𝐷 2 𝑂 + 𝐻 2 → 2𝐻𝑂𝐷 + 𝐷 2
Eq. 3

Then, in order to compare the efficiency of Ru/Al2O3 with Pt/Al2O3 and Pd/Al2O3, we have reported in Fig. 6 the kinetics profiles of D2O and HOD during the H/D exchange between D2O and H2 for 24 h at ambient temperature for the three catalysts. As for Ru/Al2O3, the H/D exchange occurs when Pt or Pd are used because D2O band decreases and HOD bands increase meaning that D2O is converted to HOD. However, we observe qualitatively that activities of H/D exchange depend on the metal used. These kinetics demonstrate that for all catalysts tested, the conversion of D2O to HOD is possible and the catalytic efficiency of the metal to perform the H/D exchange is as follows: Pt/Al2O3 > Ru/Al2O3 >> Pd/Al2O3. From these kinetic profiles, the conversion and yields of the H/D exchange were determined after 24 h of reaction at ambient temperature. Thus, Fig. 7 displays the number of mol of H2 and D2O that are consumed during the H/D exchange as well as the number of moles of produced HOD, HD and D2 for the different catalysts used. For the number of moles of H2 and of HOD, they were calculated using two different characteristic peaks of each species. Thus, we obtain an overall agreement between the two sets of data with an uncertainty of about +/-10% that is due to all the sources of errors associated with our methodology (baseline correction, molar extinction coefficient, signal-to-noise ratio, spectrometer and laser stability). On the other hand, we emphasize that the number of initial moles of D2O and H2 differs (1.66 10 -2 mol for D2O and 5.95 10 -3 mol for H2) and therefore conversion of D2O and H2 cannot be directly compared. However, the conversion of D2O with HOD yields and H2 conversion with HD and D2 yields can be quantitatively compared (conversions and yields are noted at the top of bars). In the case of the H/D exchange investigated with Pt/Al2O3 (red bars), the integration of the 𝜈2 infrared band of D2O at 1205 cm -1 gives a conversion of around 33% with 5.4 10 -3 mol of D2O consumed. From the two contributions of HOD (𝜈2 at 1456 cm -1 and 𝜈3 at 3400 cm -1 ), we calculated a yield for HOD production of 26% (4.4 10 -3 mol) and 31% (5.2 10 -3 mol), respectively, that is in the error bars discussed above. Thus, the conversion of D2O of 33% is roughly the same as the yield of HOD of 26-31% and demonstrate that D2O is selectively converted into HOD.

In the same way, integration of Raman contribution S1 at 579 cm -1 and Q1 at 4151 cm -1 are used for H2 measurements while stretch vibrons at 3625 cm -1 and at 2988 cm -1 are used to calculate the amount of HD and D2 respectively. During the H/D exchange with Pt/Al2O3, the H2 conversion is of 47-51% with 2.8-3.0 10 -3 mol consumed. This is higher than the production of total products HD + D2 given 36% of yields together and that corresponds to the formation of 2.2 10 -3 mol. Although we expect to have same values for the conversion and the total yields, the reported difference of more than 10% is probably due to analytical approximation considered in our methodology to determine the concentration of HD and D2. However, it is clear that more HD is produced than D2. An HD yield of 25% (1.5 10 -3 mol) is obtained compared to 11% (6.7 10 -4 mol) for D2, suggesting that some HD intermediate remained unreacted according to the Eq. 2. The number of moles comparison between D2O (5.4 10 -3 mol) and H2 (2.8-3.0 10 -3 mol) as well as HOD (4.4-5.2 10 -3 mol) and HD + D2 (2.2 10 -3 mol) agree with the Eq. 3 where twice more D2O and HOD are needed to perform the reaction to lead to D2 species.

When Ru/Al2O3 and Pd/Al2O3 are used for the H/D exchange of H2 with D2O, same interpretations as Pt/Al2O3 can be made except that conversion and yields are lower. In the case of Ru/Al2O3, 4.6 10 -3 mol of D2O gives 3.3-3.7 10 -3 mol of HOD and twice less of H2 with 2.1-2.4 10 -3 mol that is transformed into 1.3 10 -3 mol of HD and 6.7 10 -4 mol of D2. For Pd/Al2O3 the lowest conversions of D2O and H2 are recorded with 2.1 10 -3 mol and 2.8-5.3 10 -4 mol respectively giving 1.2-1.3 10 -3 mol of HOD, 2.8 10 -4 mol of HD and traces of D2 with 4.2 10 - 5 mol.

Altogether, kinetic and quantification studies demonstrate that the H/D exchange between D2O and H2 is effective whatever the catalyst used but performance is dependent on the metal used. Catalyst Pt/Al2O3 shows the best performance, followed closely by Ru/Al2O3 while Pd/Al2O3 has the worst performance. These results do not explain why Ru/Al2O3 was the most efficient catalyst in the hydrogenation of glucose to sorbitol (Table 1). Finally, the effect of pressure and temperature on the catalytic isotopic H/D exchange with Ru/Al2O3 has been investigated and the conversion of D2O and H2 as well as the yield of HOD, HD and D2 after 24 h of reaction are reported in the ESI 4. In particular, the increase of temperature enhances the conversion and yield of the reaction.

As proposed in recent articles, the catalytic isotopic H/D exchange between D2O and H2 could process through proton-shuttle and proton-electron transfer mechanism [START_REF] Zhao | Solvent-mediated charge separation drives alternative hydrogenation path of furanics in liquid water[END_REF][START_REF] Shangguan | Kinetic Significance of Proton-Electron Transfer during Condensed Phase Reduction of Carbonyls on Transition Metal Clusters[END_REF]. The gas H2 dissociates homolytically and two H* adsorb on the metal surface. Nearby water molecules interact with H* and form ionized H2O-H + accompanied by an electron that transfers to the metal's d-band. Proton of H2O-H + is located at the solid-liquid interface and can easily delocalize through water molecules by proton-shuttling to reach the carbonyl function when the substrate is added in the media. Following this mechanism, the H/D exchange with D2O and H2 involves a D2O + -H intermediate that can be schematized as in the Fig. 8. As explained by Hensley et al. [START_REF] Hensley | Catalytic consequences of hydrogen addition events and solvent-adsorbate interactions during guaiacol-H2 reactions at the H2O-Ru(0 0 0 1) interface[END_REF], the ability of the metal to convert an adsorbed H* into H + is two-fold. It implies to break the bond between H* and the metal but also to transfer an electron into the metal's d-band. These two characteristics were calculated by DFT for Pt(111), Ru(0001) and Pd(111) [START_REF] Ferrin | Hydrogen adsorption, absorption and diffusion on and in transition metal surfaces: A DFT study[END_REF] where the binding energy (BE) defines the strength of the M-H* bond while the work function (φ) is related to the minimum energy required to move electrons into the metal surface. BE and φ for each metal are reported in the Table 2. We emphasize that (0001) surface rather than (111) surface of Ru is chosen as Ru(0001) has become a benchmark system for investigation of Ru catalysed hydrogenation reactions in water [START_REF] Shangguan | Kinetic Significance of Proton-Electron Transfer during Condensed Phase Reduction of Carbonyls on Transition Metal Clusters[END_REF][START_REF] Michaelides | Different Surface Chemistries of Water on Ru{0001}: From Monomer Adsorption to Partially Dissociated Bilayers[END_REF][START_REF] Ramzan | Toward a Realistic Surface State of Ru in Aqueous and Gaseous Environments[END_REF]. The BE of Pt( 111) is the lowest one with -2.72 eV and its φ is the lowest one with -0.06 eV. In comparison to Ru(0001) and Pd(111), these results means that for Pt(111) the bond Pt-H* can disrupt more easily and its ability to transfer electrons is also higher, both properties that would favor the H/D exchange. The BE of Pd( 111) is lower by 0.09 eV compared to Ru(0001) whereas the φ of Ru( 0001) is lower by 0.05 eV to Pd(111). Therefore, although the disrupt of the Pd-H* bond is easier than for Ru, the lower energy needed to transfer electrons in Ru d-band compared to Pd d-band seems to be the main factor that drives the H/D exchange explaining the higher activity of Ru/Al2O3 in comparison with Pd/Al2O3. Therefore, from the BE and φ values reported in Table 2, we can qualitatively explain why the catalytic efficiency of the metal to perform the H/D exchange is in the order: Pt/Al2O3 > Ru/Al2O3 >> Pd/Al2O3.

Clearly, the mechanism involved in the hydrogenation of glucose cannot be fully interpreted from the kinetics investigation on the model reaction of H/D exchange between D2O and H2. However, when using Ru/Al2O3 as the catalyst, we have observed on the ATR-FTIR spectra of the water phase for both the catalytic H/D exchange or glucose hydrogenation an absorption band at about 1950 cm -1 that we defined as the Ru-band (see Fig. 9 and Fig. S4 in ESI 5). The next part is dedicated to the attribution of this band to a transient species that could explain the higher efficiency of Ru/Al2O3 for the hydrogenation of glucose.

Assignment of the Ru-band to Ru-H species

The Fig. 9 displays the spectra of the Ru-band (A) (1947 cm -1 ), of the Ru-band (B) (1100 cm -1 ) and of HOD (1450 cm -1 ) as well as their kinetic profiles obtained during the H/D exchange over Ru/Al2O3. The identity of the Ru-Band (A) and (B) band is unknown but it is supposed to be related to the metal Ru because they do not appear when Pt and Pd are used as catalysts for the H/D exchange. Moreover, the increase of the Ru-band (A) and (B) is not directly linked to the formation of HOD species obtained during the catalytic H/D exchange as their kinetic profiles differs. However, the kinetic of the Ru-band (A) and (B) are very similar showing that they might be related to the same species. Interestingly, the Ru-band (A) is also observed during the hydrogenation of glucose to sorbitol (see ATR-FTIR spectra in ESI 5-Fig. S4) suggesting that the same species is involved during both the H/D exchange and the hydrogenation reaction. However, the contribution of the Ru-band (B) is more difficult to analyze in the presence of glucose as some contributions in this frequency range are also related to glucose. In addition, this spectral range is also characteristic of Al2O3 that has a strong absorption around 1100 cm -1 that could eventually lead to fluctuations of the intensity of the ATR-IR signal in this region. Therefore, we will focus the discussion below on the Ru-band (A) that was clearly put in evidence in all the catalytic experiments performed in water using Ru/Al2O3.

A possible attribution of this Ru-band (A) band could be to formation of carbonyl species at the surface of Ru. Indeed, Ferri et al. have determined by ATR-FTIR studies that the addition of solvent dichloromethane can clean the metal surface at ambient temperature and that the addition of H2 in solvent allowed to desorb contaminants on the surface of Pt by the assignment of carbonyl species [START_REF] Ferri | Pt and Pt/Al 2 O 3 Thin Films for Investigation of Catalytic Solid-Liquid Interfaces by ATR-IR Spectroscopy: CO Adsorption, H 2 -Induced Reconstruction and Surface-Enhanced Absorption[END_REF]. Such behavior can be transpose to our experiment where solvent water and dissolved H2 enhance desorption of carbonyl to give the characteristic Ruband.The adsorption of carbonyl in linear or bridge configuration on metal surface was largely studied in the literature especially on Pt and Pd as well as Ru but to a lesser extent [START_REF] Mizushima | An EXAFS and IR study of the CO adsorption-induced morphology change in ruthenium catalysts[END_REF][START_REF] Marwood | In-situ surface and gas phase analysis for kinetic studies under transient conditions The catalytic hydrogenation of CO2[END_REF][START_REF] Zh | Infrared spectroscopy study of adsorption and coadsorption of carbon monoxide and hydrogen on Ru/Al203[END_REF][START_REF] Burgener | Supercritical Carbon Dioxide: An Inert Solvent for Catalytic Hydrogenation?[END_REF][START_REF] Chin | FTIR Studies of CO Adsorption on Al2O3-and SiO2-Supported Ru Catalysts[END_REF][START_REF] Baird | Attenuated Total Reflection Fourier Transform Infrared Spectroscopic Investigation of CO Adsorption on Hydrated Ru/Al2O3[END_REF][START_REF] Yoshida | In situ FTIR study on the formation and adsorption of CO on alumina-supported noble metal catalysts from H2 and CO2 in the presence of water vapor at high pressures[END_REF][START_REF] Bobadilla | Does shaping catalysts modify active phase sites? A comprehensive in situ FTIR spectroscopic study on the performance of a model Ru/Al2O3 catalyst for the CO methanation[END_REF][START_REF] Dongapure | Variations in activity of Ru/TiO2 and Ru/Al2O3 catalysts for CO2 hydrogenation: An investigation by in-situ infrared spectroscopy studies[END_REF]. Baiker's group especially assigned carbonyl adsorbed linearly on Ru/Al2O3 at 2006 cm -1 by ATR-FTIR after the insertion of 5 bar of CO at 50 °C and interestingly the appearance of the Ru-CO(L) band is like our study with a characteristic broad shape [START_REF] Burgener | Supercritical Carbon Dioxide: An Inert Solvent for Catalytic Hydrogenation?[END_REF].

In order to clarify the possible role of CO in the observation of the Ru-peak, gaseous CO is directly added on "deposited" Ru/Al2O3 in pure gas conditions or in aqueous environment. Ru/Al2O3 was deposited on the silicon crystal for ATR-FTIR measurements and on a metallic screw for IR microscopy measurements in reflection mode (see ESI 6-Fig. S5). The spectra are displayed in ESI 7-Fig. S6. The formation of carbonyl group adsorbed on the surface of Ru/Al2O3 in a water environment leads to two vibrational bands at 2060 cm -1 and 2010 cm -1 that could be assigned to linear and bridge carbonyls. As two bands are observed and that the shape of the band does not correspond to the Ru-band observed at 1947 cm -1 during the H/D exchange and the glucose hydrogenation, it confirms that the Ru-band cannot be carbonyl species.

Another possible assignment of the Ru-band (A) could be the stretching mode of Ru-H intermediate that are formed when H2 gas dissolves in D2O and then adsorbs on the surface of the metal before the exchange reaction. As it was previously mentioned in the Table 2, the BE of Ru( 0001) is the highest one compared to Pt(111) and Pd(111). It means that the Ru-H specie might be more stable than the Pt-H or Pd-H intermediate explaining the presence of the band at 1947 cm -1 only in the case of Ru/Al2O3.Therefore, the apparition of the Ru-band for both the H/D exchange and the hydrogenation can be an indication to explain why Ru/Al2O3 is active for the hydrogenation of glucose while Pt/Al2O3 and Pd/Al2O3 are inactive.

To demonstrate that the Ru-band at around 1950 cm -1 is actually a Ru-H intermediate, the Ru/Al2O3 is firstly studied by ATR-FTIR through the deposition of a thin film of the solid at the surface of the silicon crystal to probe intermediate species [START_REF] Andanson | Exploring catalytic solid/liquid interfaces by in situ attenuated total reflection infrared spectroscopy[END_REF]. But after 24 h under pressure of 30 bar of H2, the Ru-band is not observed. A deposit of Ru/Al2O3 under pressure of H2 is also studied by infrared reflection (see ESI 6-Fig. S5 for the description of the cell) where the sensitivity to detect adsorbed species should be enhanced (higher thickness of Ru/Al2O3 deposit) but no band at around 1950 cm -1 is observed. Consequently, when only pressure of gaseous H2 is in contact with Ru/Al2O3, no change is observed in infrared spectra whereas the Ru-band constantly appears in aqueous environment. On the other hand, the replacement of D2O by H2O does not result to a significant shift of the Ru-band when H2 pressure is added (see ATR-FTIR spectra in ESI 8-Fig. S7). Then, water plays a crucial role to observe the Ru-band but does not seem directly involved in the vibrating species. Therefore, the Ru-band could be an intermediate Ru-H as previously mentioned, however, on the contrary of adsorbed carbonyls, only few studies have been focused on infrared studies of the formation of Ru-H surface species in the gas phase and, to our knowledge none in the aqueous phase. Ishikawa et al. have studied the adsorption of H2 on Ru/ZrO2 by FTIR spectroscopy, they assume that the peak of Ru-H species should appear between 1800 and 2000 cm -1 , which is in accordance with our experimental data, but they do not have observed Ru-H species experimentally [START_REF] Ishikawa | Hydrogen Adsorption on Ru/ZrO2 Studied by FT-IR[END_REF]. Others have performed studies of H2 adsorption on Ru/Al2O3 and no band of H2 adsorbed on bare Ru surface appeared. However, when CO is pre-adsorbed, they assume the formation of a hydrocarbonyl species H-Ru-CO at around 2030 cm -1 [START_REF] Zh | Infrared spectroscopy study of adsorption and coadsorption of carbon monoxide and hydrogen on Ru/Al203[END_REF] which could be possibly formed in our study.

Recent articles have explored various adsorption states of H2 on Ru nanoparticles by infrared spectroscopy and performed DFT calculations using a nanoscale Ru particle model (Ru153) to assign the vibrational frequencies of the infrared spectra observed for Ru/MgO exposed to H2 gas [START_REF] Rivera Rocabado | Adsorption States of N2/H2 Activated on Ru Nanoparticles Uncovered by Modulation-Excitation Infrared Spectroscopy and Density Functional Theory Calculations[END_REF][START_REF] Rivera Rocabado | Uncovering the Mechanism of the Hydrogen Poisoning on Ru Nanoparticles via Density Functional Theory Calculations[END_REF]. It turns out that in the experiment involving switching between H2 and Ar at 250 °C, the introduction of H2 on Ru/MgO surface increased infrared absorbance and induced the appearance of two peaks at 1998 cm -1 and at 1863 cm -1 . DFT calculations assigned the former peak to H2 horizontally adsorbed on top of a bridge Ru atom. In contrast, the peak at 1863 cm -1 did not correspond to the adsorption of H2 on the Ru surface, as this activated H2 observed at 1998 cm -1 is found to have a short lifetime, but rather corresponds to the binding of a hydrogen atom on Ru153. Indeed, DFT calculations show that an H atom binding "on top site" of Ru153 (Ru-H) leads to a Ru-H stretching band at 1852 cm -1 in good agreement with the experimental peak at 1863 cm -1 . Therefore, these experimental and calculated frequencies of Ru-H species on MgO surfaces are in the same spectral range than the Ru-band (A) observed for our Ru/Al2O3 surface in water located between 1940 cm -1 and 1970 cm -1 hence supporting our hypothesis that the Ru band (A) can be assigned to a Ru-H stretching band that results from an activated form of H* binding "on top site" on the Ru/Al2O3 surface. In addition, these DFT calculation reports other possible Ru-H stretching bands at about 1000 cm -1 associated with H atoms in "bridge" configurations with 4 Ru atoms that might be associated with the experimental Ru-Band (B) peak at 1100 cm -1 (see fig. S9 of reference [START_REF] Rivera Rocabado | Uncovering the Mechanism of the Hydrogen Poisoning on Ru Nanoparticles via Density Functional Theory Calculations[END_REF]). Thus, the observation of the Ru-H band by ATR-FTIR could explain that these H atoms "on top" and "bridge" configurations are only detected on the surface of Ru but not for other metals. Such configuration observed only for Ru could be explained by the fact that the valence electronic structure of Ru (4d7 5s1) differs from that of Pt (5d9 6s1) and Pd (4d10). Indeed, a H binding to a unique Pd atom is very unlikely as its valence shell is full. Also, even if a H binding to a unique Pt atom is possible [START_REF] Ogasawara | Hydrogen adsorption on Pt(100), Pt(110), Pt(111) and[END_REF], it is much less probable than for Ru. On the other hand, H atoms that can interact with 2, 3 or 4 atoms in "bridge" configurations might exist for Pd-Pt metal surface but seems to be favored for Ru [START_REF] Rivera Rocabado | Uncovering the Mechanism of the Hydrogen Poisoning on Ru Nanoparticles via Density Functional Theory Calculations[END_REF]. Thus, the observation of the Ru-band could explain that adsorbed H* are more activated on the surface of Ru than on other metals. In particular, the presence of H atom binding "on top site" of Ru that are more sterically accessible and less bounded to the surface than in bridge configurations (see reference [START_REF] Rivera Rocabado | Uncovering the Mechanism of the Hydrogen Poisoning on Ru Nanoparticles via Density Functional Theory Calculations[END_REF]) could explain the easier H transfer to the carbon atom of the carbonyl for the hydrogenation of glucose to sorbitol. Indeed, in the glucose hydrogenation mechanism, we might infer that the kinetically relevant step concerns the addition of one adsorbed H* on the carbon atom of the carbonyl [START_REF] Shangguan | Kinetic Significance of Proton-Electron Transfer during Condensed Phase Reduction of Carbonyls on Transition Metal Clusters[END_REF] and the "better" activation of adsorbed H* on Ru could enhance this step (see Fig. 10). We emphasize that the prevalence of proton-electron transfer during the catalytic hydrogenation of the first H atom to the oxygen of the carbonyl in water on Ru surfaces is supported experimentally by kinetic measurements with H-D isotopic studies. [START_REF] Shangguan | Kinetic Significance of Proton-Electron Transfer during Condensed Phase Reduction of Carbonyls on Transition Metal Clusters[END_REF] In particular they used D2 instead of H2 in few experiments under a pressure of 1.3 bar. Such kind of experiments could have been performed in this work but this was beyond the scope of our study.

Conclusion

This work emphasizes a possible reason why the metal Ru is efficient for the hydrogenation of glucose, and to a larger extent, the hydrogenation of various aldoses and carboxylic derivatives in aqueous media. Indeed, we have demonstrated in a previous work that Ru/Al2O3 is very effective to convert glucose into sorbitol whereas the noble metal Pt and Pd are inactive. Herein, it was demonstrated here that the proton transfer of H ad-atoms to water plays a key role in the hydrogenation mechanism. This mechanism is followed by the catalytic H/D exchange between H2 and D2O to give HOD, HD and D2 that are measured in operando with the simultaneous coupling of ATR-IR and Raman spectroscopy. After 24 h at 22 °C under 30 bar of H2, the efficiency of catalysts for the H/D exchange is Pt/Al2O3 > Ru/Al2O3 >> Pd/Al2O3 showing that the H/D exchange between D2O and H2 is effective whatever the catalyst used but performance is dependent on the nature of the metal. Distinctively, an infrared band located around 1950 cm -1 observed only with Ru/Al2O3, is tentatively assigned to H atoms adsorbed and activated on the Ru surface. This H* could be directly added to the carbon atom of the carbonyl of glucose to give sorbitol in a more efficient way compared to the other metals where this band is not observed. Thus, this peculiarity observed only in the case of Ru/Al2O3 could explain its enhanced activity in the hydrogenation of glucose in aqueous media, and to a larger extent, its high efficiency for the hydrogenation of various aldoses and carboxylic derivatives in water.

Figure 1 :

 1 Figure 1: Hydrogenation of glucose to sorbitol catalyzed by a metal catalyst in aqueous liquid phase.

Figure 2 :

 2 Figure 2: High-pressure experimental device with a single cell coupled with ATR-FTIR and Raman spectrometers for in operando simultaneous measurements of the catalytic H/D exchange of D2O and H2 producing HOD, HD and D2 (1: gas/liquid entrance, 2: cartridges heaters, 3: thermocouple, 4: stirrer, 5: silicon crystal, 6: sapphire window).

Figure 3 :

 3 Figure 3: Spectra of the liquid phase obtained by ATR-FT-R spectroscopy during the catalytic H/D exchange between D2O and H2 in the presence of Ru/Al2O3 at 22 °C for 24 h under 30 bar of H2.

Figure 4 :

 4 Figure 4: Spectra of the gas phase obtained by Raman spectroscopy during the catalytic H/D exchange between D2O and H2 in the presence of Ru/Al2O3 at 22 °C for 24 h under 30 bar of H2.

Figure 5 :

 5 Figure 5: Kinetic profiles of D2O (1205 cm -1) and HOD (1450 cm -1 ) obtained by ATR-FT-IR analysis (in red) as well as kinetic profiles of H2 (579 cm -1 ), HD (3625 cm -1 ) and D2 (2988 cm -1 ) obtained by Raman measurements (in green) during the catalytic H/D exchange between D2O and H2 over Ru/Al2O3 at 22 °C for 24 h under 30 bar of H2.

Figure 6 :

 6 Figure 6: Kinetic profiles of D2O at 1205 cm -1 and of HOD at 1450 cm -1 (noisy) and at 3400 cm -1 during the H/D exchange between D2O and H2 over Ru, Pt and Pd/Al2O3 at 22 °C for 24 h under 30 bar of H2.

Figure 7 :

 7 Figure 7: Number of mol consumed for D2O (n0 = 1.66 10 -2 mol) and H2 (n0 = 5.95 10 -3 mol) and number of mol produced of HOD, HD and D2 after 24 h of H/D exchange at 22 °C with Pt, Ru or Pd/Al2O3 used as catalysts. On top of bars are indicated average conversions (for D2O and H2) and yields (for HOD, HD, D2) in percentages.

Figure 8 :

 8 Figure 8: Formation of the intermediate D2O-H + during the isotopic H/D exchange of D2O and H2 on metal surfaces inspired by reference [33].

Figure 9 :

 9 Figure 9: Spectra and kinetic profiles of HOD (1450 cm -1 ), the Ru-band (A) at 1947 cm -1 and Ru-band (B) at 1100 cm -1 obtained by ATR-FT-IR during the H/D exchange between D2O and H2 in the presence of Ru/Al2O3 at 22 °C.

Figure 10 :

 10 Figure 10: Proposed sequence of elementary steps for the hydrogenation of carbonyl over Pt, Ru and Pd metal surfaces.

Table 2 :

 2 Binding energies (BE) and work function (φ) on hydrogen adsorption for Pt(111), Ru(0001) and Pd(111) calculated at the vapor interface by Ferrin et al.[START_REF] Ferrin | Hydrogen adsorption, absorption and diffusion on and in transition metal surfaces: A DFT study[END_REF].

		BE (eV)	φ (eV)
	Pt	-2.72	-0.06
	Ru	-2.97	0.03
	Pd	-2.88	0.08

ACKNOWLEDGMENTS

The authors acknowledge the Region Nouvelle Aquitaine and the University of Poitiers for their financial support for the funding of the PhD of I. Bonnin. They are also grateful to the program "Défi in situ en conditions extrêmes" of the MITI of CNRS for the financial support to the infrared and Raman equipment, to INCREASE Federation and to the GDR 2035 SolvATE.


Joseph Grondin

Christian Aupetit

Jean-Marc Vincent

Thierry Tassaing
email: thierry.tassaing@u-bordeaux.fr

Cyclic carbonates through the photo-induced carboxylative cyclization of allylic alcohol with CO2: a comprehensive kinetic study of the reaction mechanism by in-situ ATR-IR spectroscopy

Keywords: CO2 organocatalysis, photo-promoted ATRA, cyclic carbonates

A one-pot multicomponent green process is investigated for the synthesis of perfluoroalkylated cyclic carbonate which merges the photo-promoted Atom Transfer Radical Addition (ATRA) of a perfluoroalkyl iodide (Rf-I) onto allyl alcohols with the Lewis base promoted carboxylative cyclization. The evolution of the complex mixture during the reaction was monitored by insitu ATR-IR and Raman spectroscopies that provided insights into the reaction mechanism. The effect on the kinetics and the carbonate yields of key parameters such as the stoichiometry of reagents, the nature of the Lewis base and the solvent, the temperature and the pressure of the solvent were evaluated. It was found that high yields were obtained using strong Lewis bases that played both the role of activating the allyl alcohol for the generation of the allyl carbonate in the presence of CO2 and promoting the ATRA reaction through the activation of C4F9I by halogen bonding. This protocol was also extended to various unsaturated alcohols.

Introduction

During the last decade, many academic and industrial researches have been devoted to the development of sustainable synthetic pathways to produce five membered cyclic carbonates by the catalytic coupling of CO2 with epoxides [1-5] using appropriate organic or transition metal catalysts. This 100% atom economic reaction is very attractive as it represents a greener and safer alternative to the conventional synthesis of cyclic carbonates from diols and toxic phosgene. Today, these precursors find applications as electrolytes in Li-ion batteries, intermediates for fine chemical synthesis and polar aprotic solvents replacing DMF, DMSO, NMP and acetonitrile. [START_REF] Shaikh | Organic carbonates[END_REF] They also serve as raw materials for the synthesis of polycarbonates [START_REF] Tempelaar | Synthesis and post-polymerisation modifications of aliphatic poly(carbonate)s prepared by ring-opening polymerisation[END_REF][START_REF] Grignard | Advances in the use of CO2 as a renewable feedstock for the synthesis of polymers[END_REF] and isocyanate-free polyurethanes [START_REF] Maisonneuve | Isocyanate-Free Routes to Polyurethanes and Poly(hydroxy Urethane)s[END_REF][START_REF] Grignard | CO2-blown microcellular non-isocyanate polyurethane (NIPU) foams: from bio-and CO2-sourced monomers to potentially thermal insulating materials[END_REF][START_REF] Gennen | CO2-Sourced α-Alkylidene Cyclic Carbonates: A Step Forward in the Quest for Functional Regioregular Poly(urethane)s and Poly(carbonate)s[END_REF][START_REF] Monie | Chemo-and Regioselective Additions of Nucleophiles to Cyclic Carbonates for the Preparation of Self-Blowing Non-Isocyanate Polyurethane Foams[END_REF] (NIPUs) that become more and more attractive for industry as an alternative to classical polyurethanes (PUs). In view of the broad scope of applications of cyclic carbonates and their strong economic potential, there is still a need to improve their synthesis and decrease their production costs. In particular, whereas the synthesis of terminal cyclic carbonates from the coupling of CO2 with epoxides is well established, the conversion of internal triand tetra-substituted epoxides into their corresponding five membered carbonates is still very challenging even though of great interest. [START_REF] Martín | Recent Advances in the Catalytic Preparation of Cyclic Organic Carbonates[END_REF] On the other hand, the synthesis of six membered cyclic carbonates by oxetane/CO2 coupling [START_REF] Martín | Recent Advances in the Catalytic Preparation of Cyclic Organic Carbonates[END_REF][START_REF] Rintjema | Highly Chemoselective Catalytic Coupling of Substituted Oxetanes and Carbon Dioxide[END_REF][START_REF] Alves | Organocatalytic Coupling of CO2 with Oxetane[END_REF] remains challenging because of i) the lower reactivity of four membered ether rings compared to epoxides even under harsh experimental conditions, and ii) a poor reaction selectivity. Interestingly, αalkylidene cyclic carbonates that are synthesized by coupling CO2 with propargylic alcohols using organocatalysts or metal based catalysts are intermediates of great interest in organic and polymer synthesis. [START_REF] Besse | Synthesis and applications of unsaturated cyclocarbonates[END_REF][START_REF] Zhao | Task-specific ionic liquid and CO2-cocatalysed efficient hydration of propargylic alcohols to [small alpha]-hydroxy ketones[END_REF][START_REF] Song | Advances in CO2 Capture, Sequestration, and Conversion[END_REF][START_REF] Rintjema | Substrate-Assisted Carbon Dioxide Activation as a Versatile Approach for Heterocyclic Synthesis[END_REF][START_REF] Boyaval | Organocatalytic Coupling of CO2 with a Propargylic Alcohol: A Comprehensive Mechanistic Study[END_REF][START_REF] Li | Carboxylative cyclization of propargylic alcohols with carbon dioxide: A facile and Green route to α-methylene cyclic carbonates[END_REF][START_REF] Huang | Selective Organocatalytic Preparation of Trimethylene Carbonate from Oxetane and Carbon Dioxide[END_REF][START_REF] Méreau | Tetrabutylammonium Salts: Cheap Catalysts for the Facile and Selective Synthesis of α-Alkylidene Cyclic Carbonates from Carbon Dioxide and Alkynols[END_REF][START_REF] Grignard | Boosting the Catalytic Performance of Organic Salts for the Fast and Selective Synthesis of α-Alkylidene Cyclic Carbonates from Carbon Dioxide and Propargylic Alcohols[END_REF] However, at present, the substrate scope is mainly restricted to tertiary alcohols as only few examples are reported using primary and secondary alcohols. [START_REF] Dabral | Silver-Catalyzed Carboxylative Cyclization of Primary Propargyl Alcohols with CO2[END_REF][START_REF] Cervantes-Reyes | Expanded Ring NHC Silver Carboxylate Complexes as Efficient and Reusable Catalysts for the Carboxylative Cyclization of Unsubstituted Propargylic Derivatives[END_REF][START_REF] Cervantes-Reyes | Copper-catalysed synthesis of α-alkylidene cyclic carbonates from propargylic alcohols and CO2[END_REF] Finally, 1,2 and 1,3-diols [START_REF] Kindermann | Synthesis of Carbonates from Alcohols and CO2[END_REF][START_REF] Tamura | Direct conversion of CO2 with diols, aminoalcohols and diamines to cyclic carbonates, cyclic carbamates and cyclic ureas using heterogeneous catalysts[END_REF][START_REF] Hosseinian | Advancements in six-membered cyclic carbonate (1,3-dioxan-2-one) synthesis utilizing carbon dioxide as a C1 source[END_REF][START_REF] Bobbink | Synthesis of cyclic carbonates from diols and CO2 catalyzed by carbenes[END_REF][START_REF] Brege | En Route to CO2-Based (a)Cyclic Carbonates and Polycarbonates from Alcohols Substrates by Direct and Indirect Approaches[END_REF] were subjected to carbonation in order to afford five-and six-membered cyclic carbonates, respectively, but their reaction with CO2 is limited due to the formation of water as a by-product. To circumvent this drawback, both homogeneous and heterogeneous catalysts (Bu2SnO, K2CO3, CeO2…) used in combination with dehydrating systems were developed. [START_REF] Honda | Catalytic CO2 conversion to organic carbonates with alcohols in combination with dehydration system[END_REF][START_REF] Honda | Direct Cyclic Carbonate Synthesis from CO2 and Diol over Carboxylation/Hydration Cascade Catalyst of CeO2 with 2-Cyanopyridine[END_REF] Although one efficient system involving CeO2 as the catalyst and 2-Cyanopyridine as dehydrating agent has been developed [START_REF] Honda | Direct Cyclic Carbonate Synthesis from CO2 and Diol over Carboxylation/Hydration Cascade Catalyst of CeO2 with 2-Cyanopyridine[END_REF], this process requires harsh reaction conditions (150 °C, 5 MPa) and a large excess of an expensive dehydrating agent. Alternative protocols have been recently suggested to overcome the limitation induced by the formation of water by combining the use of an organic base and an alkylating agent under mild conditions. For example, Zhang et al. [START_REF] Reithofer | Synthesis of cyclic carbonates with carbon dioxide and cesium carbonate[END_REF] and Buchard et al. [START_REF] Mcguire | Synthesis of 5-to 8-membered cyclic carbonates from diols and CO2: A one-step, atmospheric pressure and ambient temperature procedure[END_REF][START_REF] Gregory | Synthesis of 6-membered cyclic carbonates from 1,3-diols and low CO2 pressure: a novel mild strategy to replace phosgene reagents[END_REF] have combined strong Lewis bases and Tosyl chloride (TsCl) as the alkylating agent to promote the synthesis of 6-membered cyclic carbonates. A similar strategy has been proposed by Kitamura et al. [START_REF] Kitamura | Convenient synthesis of ethylene carbonates from carbon dioxide and 1,2-diols at atmospheric pressure of carbon dioxide[END_REF] and Dyson et al. [START_REF] Bobbink | Synthesis of cyclic carbonates from diols and CO2 catalyzed by carbenes[END_REF] using alkyl halides as the alkylating agents for the synthesis of cyclic carbonates. The effect of the addition of ionic liquids to these systems to obtain cyclic carbonates from diols was also investigated. [START_REF] Lim | Metal-Free Synthesis of Cyclic and Acyclic Carbonates from CO2and Alcohols[END_REF] A critical assessment of the reaction mechanism at work in such metal-free dual activating systems for the coupling of CO2 with 1,xdiols to afford (a)cyclic carbonates has been recently proposed by Brege et al. [START_REF] Brege | The coupling of CO2 with diols promoted by organic dual systems: Towards products divergence via benchmarking of the performance metrics[END_REF]. In particular, it was shown that choosing either DBU/EtBr or TEA/TsCl as the organic dual activating system, it was possible to control the product selectivity to substituted cyclic and/or acyclic carbonates depending on the nature of the substrate. Therefore, there is still a need to develop innovative routes to broaden the structural diversity of cyclic carbonates that could be exploited for the design of novel functionalized chemicals and polymers.

In this context, light-driven CO2 fixation through C-C bond formation to give added value chemicals and monomers remains a relatively unexplored area although it represents a very promising field and a green attractive route. [START_REF] Baran | Photocatalytic Carboxylation of Organic Substrates with Carbon Dioxide at Zinc Sulfide with Deposited Ruthenium Nanoparticles[END_REF][START_REF] Masuda | Light-Driven Carboxylation of o-Alkylphenyl Ketones with CO2[END_REF][START_REF] Wang | Photoinduced radical-initiated carboxylative cyclization of allyl amines with carbon dioxide[END_REF][START_REF] Gui | Photochemical Carboxylation of Activated C(sp3)-H Bonds with CO2[END_REF][START_REF] Murata | Construction of a visible light-driven hydrocarboxylation cycle of alkenes by the combined use of Rh(i) and photoredox catalysts[END_REF] A recent review reports several typical examples on the photochemical fixation of CO2 into heterocyclic scaffolds with detailed mechanistic descriptions of the various proposed synthetic routes. [START_REF] Ngo | Recent advances in the synthesis of heterocycles and pharmaceuticals from the photo/electrochemical fixation of carbon dioxide[END_REF] For example, an original radical synthetic route was proposed for the synthesis of oxazolidinones through the photopromoted base-catalyzed coupling of CO2 with allylamines [START_REF] Wang | Photoinduced radical-initiated carboxylative cyclization of allyl amines with carbon dioxide[END_REF] or propargyl amines [START_REF] He | Metal-Free Photocatalytic Synthesis of exo-Iodomethylene 2-Oxazolidinones: An Alternative Strategy for CO2 Valorization with Solar Energy[END_REF] in the presence of perfluoroalkyl iodides and iodine, respectively. In the work by Wang et al. [START_REF] Wang | Photoinduced radical-initiated carboxylative cyclization of allyl amines with carbon dioxide[END_REF] allyl alcohols were investigated to a limited extent although the reaction could be very useful for the preparation of cyclic carbonates. Also, the proposed mechanism was supported by a few mechanistic experiments conducted mostly using 1 H NMR. Very recently, Jain et al. reported the first photochemical synthesis of linear carbonates from the reaction of CO2 with alcohols using a silver-doped ceria nanocomposite at room temperature under visible light irradiation. [START_REF] Malik | Visible-light driven reaction of CO2 with alcohols using a Ag/CeO2 nanocomposite: first photochemical synthesis of linear carbonates under mild conditions[END_REF] In this context, we decided to investigate the synthesis of cyclic carbonates through a one-pot multicomponent metal-free process which merges the photo-promoted Atom Transfer Radical Addition (ATRA) of a perfluoroalkyl iodide (Rf-I) onto unsaturated alcohols with the Lewis base promoted carboxylative cyclization (Scheme 1). A detailed in situ ATR-IR and Raman spectroscopic study led to the optimization of reaction conditions and provided insights into the reaction mechanism. In addition, the stoichiometry of reagents, the nature of the Lewis base, the effects of the temperature, the pressure and the solvent on the kinetics and the carbonate yields were evaluated. Finally, using optimized conditions, the carbonation scope was extended to a variety of unsaturated alcohol substrates. Scheme 1. Proposed synthesis of 5-6 membered cyclic carbonates through a photo-promoted ATRA reaction followed by a Lewis base-promoted carboxylative cyclization.

Results and discussion

Synthesis of perfluorobutyl cyclic carbonate 1 from the coupling of CO2 with allylic alcohol and perfluorobutyliodide: Mechanistic study

Model reaction using allyl alcohol, DBU and C4F9I

An in-situ ATR-IR/Raman kinetic study for the preparation of carbonate l was first performed under UVA irradiation at 365 nm (LED) in the presence of 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU), perfluorobutyl iodide (C4F9I) and allyl alcohol. The reaction was conducted in acetonitrile (CH3CN) at T = 25 °C under a low carbon dioxide pressure of 0.3 MPa (Scheme 2) using a slight excess of DBU (1.05 eq) and C4F9I (1.25 eq) relative to allyl alcohol. Scheme 2. Benchmark reaction for the synthesis of the perfluorobutyl carbonate 1. DBUH + -I - is obtained as a by-product.

Typical ATR-IR spectra of the mixture obtained at different reaction times are depicted in Fig. 1 where specific spectral signatures of reactants, intermediates and products could be identified. From the spectrum of the reaction mixture before the addition of CO2, we have identified specific peaks of interest. Before addition of CO2 to the ternary mixture of allyl alcohol, DBU and C4F9I, a narrow band is observed at 1606 cm -1 that is characteristic of the C=N stretching mode of DBU indicating that there is no specific interaction and/or reaction between DBU and another reactant of the mixture (black curve). In particular, there is no deprotonation of the alcohol moiety by DBU, as well as no alkylation of DBU by C4F9I. We have also identified a peak at 684 cm -1 that is assigned to a C-I stretching mode coupled with the C-F bending modes of C4F9I. Unfortunately, we do not detect any contribution of the allyl alcohol. After the addition of CO2 at a pressure of 0.3 MPa (red curve), the DBU absorption band shifts at 1642 cm -1 , which has been assigned to the C=NH + stretching mode of protonated DBU. [START_REF] Brege | The coupling of CO2 with diols promoted by organic dual systems: Towards products divergence via benchmarking of the performance metrics[END_REF][START_REF] Heldebrant | The Reaction of 1,8-Diazabicyclo[5.4.0]undec-7ene (DBU) with Carbon Dioxide[END_REF] Concomitantly, a shoulder of this broad peak appears around 1660 cm -1 , as well as a new peak at 1275 cm -1 , both being attributed to C-O stretching modes of the allyl carbonate anion CH2=CHCH2CO3 -. [START_REF] Onwukamike | Detailed Understanding of the DBU/CO2 Switchable Solvent System for Cellulose Solubilization and Derivatization[END_REF] Thus in the dark, a trimolecular reaction between CO2, allyl alcohol and DBU occurs which leads to the formation of the organic salt CH2=CHCH2CO3 -/DBUH + . We emphasize that the peak of DBU is barely detected as a shoulder at about 1606 cm -1 whose intensity gives an estimated concentration between 0.05 and 0.1eq of DBU still present in the mixture under these experimental conditions that is consistent with the thermodynamic data of Heldebrant et al. [START_REF] Heldebrant | Organic liquid CO2 capture agents with high gravimetric CO2 capacity[END_REF]. Therefore, the allylic alcohol is quantitatively transformed into its carbonated form during this first step of the reaction that occurs within 5 minutes as it is mainly controlled by CO2 solubilisation kinetic in the organic phase. Then, when the LED is switched ON and during the course of the kinetic (green and blue curves), the main absorption of DBUH + is slightly shifted toward higher wavenumbers from 1642 cm -1 to 1648 cm -1 and gets thinner as the shoulder due to the carbonate anion tends to disappear. The slight shift is ascribed to anion metathesis of DBUH + /RCO3 -to DBUH + /I -that should occur upon carbonate cyclization (scheme 1). Accordingly, the characteristic peak of RCO3 -at 1275 cm -1 gets weaker as a function of time. Concomitantly, the new absorption band at 1813 cm -1 , assigned to the C=O stretching mode of the perfluoroalkylated cyclic carbonate, smoothly increases with time. In the same time the band at 684 cm -1 associated with C4F9I has vanished at the end of the reaction. We emphasize that the intermediate ATRA product with the formation of a C-I bond was not detectable in our experimental conditions as the C-I stretching mode is observable by ATR-IR at wavenumbers lower than 600 cm -1 . Thus, the rate of the concentration evolution of C4F9I, RCO3 -, and the cyclic carbonate could be monitored by ATR-IR from the height of the peak at 684 cm -1 , 1275 cm -1 and 1813 cm -1 , respectively. To have complementary information from the allyl function of the alcohol that is not detectable by ATR-IR, the kinetic of the same model reaction was monitored by in-situ Raman spectroscopy in order to follow the evolution of the height of the peak at 1645 cm -1 characteristic of the C=C bond of the allyl alcohol (or allyl carbonate anion at the same wavenumber) and that of cyclic carbonate observed at 1813 cm -1 (see ESI Figure S1). Figure 2 displays the kinetic profiles of the disappearance of the C4F9I substrate and allyl carbonate intermediate generated quantitatively before irradiation, as well as of the appearance of the cyclic carbonate product 1. For comparison, the kinetic profiles of the cyclic carbonate bands measured in Raman and ATR-IR have been scaled to their maximum intensity at 600 minutes. By the same token, the maximum of the kinetic profile at t=0 of the C=C and RCO3 -band have been scaled at about the maximum peak height of the C4F9I band. Good agreement is observed between the IR and Raman follow-ups of the cyclic carbonates thus validating the repeatability of our experimental procedure. The profiles show that the reaction is completed in about 8 h, time after which the reagents have been fully consumed, while the cyclic carbonate 1 was formed with a yield > 95% as determined by ATR-IR using equation (1) in paragraph 3.3. The reaction thus occurs with an excellent selectivity. The ATRA process is supported by the smooth decrease of the intensity of the Raman band assigned to the C=C stretching mode of the allyl carbonate. Interestingly, the kinetic profiles reveal that the rates of formation of 1 and consumption of the allyl carbonate and C4F9I occurs with the same rates, thus showing that the rate of the intramolecular cyclization is much higher than that of the ATRA process.

Effect of reagent stoichiometry

In order to investigate the influence of the stoichiometry of DBU and C4F9I on the reaction outcome, in situ ATR-IR kinetic studies of the photoinduced carboxylative coupling of CO2 with allyl alcohol were performed. Fixing the amount of C4F9I (1.25 equiv.), the quantity of DBU was varied from 0.5 to 1.5 equiv. (see figure 3). While increasing the amount of DBU from 0.5 to 1.5 equiv. did not affect the initial reaction rates, lower yields of 1 were obtained when deviating from the optimal amount of about 1-1.05 equiv. Using 0.5 and 0.75 equiv. of DBU led to decreased yields of 1 by about ~ 55% and 20%, respectively. This is what is expected when considering the mechanism of scheme 1 in which the DBU combined with CO2, first reacts quickly and quantitatively in the dark with allyl alcohol to generate the allyl carbonate/DBUH + ion pair which then, under light irradiation undergoes the ATRA reaction followed by intramolecular cyclization to deliver 1 and DBUH + /I -. More surprising is the detrimental effect on the yield when using an excess of DBU, the yield of 1 decreasing of ~ 10% and 20% with 1.25 and 1.5 equiv. of DBU, respectively. In agreement with our previous study, [START_REF] Grondin | Visible-light induced photochemistry of Electron Donor-Acceptor Complexes in Perfluoroalkylation Reactions: Investigation of halogen bonding interactions through UV-Visible absorption and Raman spectroscopies combined with DFT calculations[END_REF] this could be ascribed to the side-reactivity of DBU with RfI, most probably to generate DBUC4F9H + -I -. This would ultimately lead to the consumption of a significant amount of RfI. Then, using a slight excess amount of DBU (1.05 equiv.), we have varied the quantity of C4F9I from 1 to 1.5 equiv. (see figure 4). Although a barely detected improvement of the yield is observed upon an increase of the amount of C4F9I from 1 to 1.25, a further increase of the amount of C4F9I up to 1.5 equiv. has a detrimental effect on the yield. Finally, using 1 equiv. of DBU and 1 equiv. of C4F9I led to a significantly reduced yield.

Therefore, to further investigate our model reaction, we have used an excess amount of DBU (1.05 equiv.) and C4F9I (1.25 equiv.) for which we obtained the maximum yield.

Halogen bonding effect

The presence of DBU is mandatory in order to deprotonate the allyl alcohol in the presence of CO2 to form the allyl carbonate anion. We have also shown recently that DBU could form a halogen bond with RfI [START_REF] Grondin | Visible-light induced photochemistry of Electron Donor-Acceptor Complexes in Perfluoroalkylation Reactions: Investigation of halogen bonding interactions through UV-Visible absorption and Raman spectroscopies combined with DFT calculations[END_REF] thus potentially weakening the Rf-I bond, but also that the UV-absorption of the [RfI-DBU] halogen bond complex was enhanced and red-shifted compared to that of RfI, thus facilitating the direct photolysis of the Rf-I bond to generate Rf radicals which initiate a radical chain process. [START_REF] Postigo | Electron Donor-Acceptor Complexes in Perfluoroalkylation Reactions[END_REF] In order to verify that halogen bonding is playing a key role for the ATRA reaction, we performed first a reaction without DBU. The follow-up of the reaction displayed in figure 5 revealed that the peak height of C4F9I at 684 cm -1 was constant over time hence showing that C4F9I was not consumed during the reaction, confirming that DBU was playing a crucial role in the ATRA process. A reaction was then performed using allyl benzene as a substrate in the presence of catalytic amount of DBU, i.e. 10 mol%. Allyl benzene was chosen in order to avoid the presence of the hydroxyl group on the substrate in order to avoid the carbonatation step promoted by DBU and leading to its protonation. As seen in figure 5, using a catalytic amount of DBU, a fast consumption of C4F9I is observed, i.e. ~ 50% in 40 min while full consumption is achieved in about 5h. Therefore, as reported before, [START_REF] Grondin | Visible-light induced photochemistry of Electron Donor-Acceptor Complexes in Perfluoroalkylation Reactions: Investigation of halogen bonding interactions through UV-Visible absorption and Raman spectroscopies combined with DFT calculations[END_REF] DBU catalyzes the light-promoted iodoperfluoroalkylation reaction, most likely thanks to an halogen bond interaction with C4F9I. Thus, in the photo-induced carboxylative cyclization reaction, the Bronsted and Lewis basicities of DBU are both playing a key role.

Lewis base effect

The influence of the nature of the base, i.e. tertiary amines (TEA, TMEDA, DABCO) and guanidines (TMG, MTBD), was then studied on the model reaction.

In comparison with the reference amidine (DBU), the acyclic guanidine (TMG) showed similar reactivity, although the rate of formation of the cyclic carbonate with TMG is significantly faster. A fast initial rate is also observed with the cyclic guanidine MTBD but the reaction stops at ~ 65 %. This could be ascribed to secondary reactions such as the nucleophilic attack of the allyl alcohol to 1 that that was shown to occur in the MTBDcatalysed coupling of 2-methyl-3-butyn-2-ol with CO2. [START_REF] Boyaval | Organocatalytic Coupling of CO2 with a Propargylic Alcohol: A Comprehensive Mechanistic Study[END_REF] Alternatively, the strong absorption of the MTBD-RfI halogen bonded complex in the UVA spectral range as reported in reference [START_REF] Grondin | Visible-light induced photochemistry of Electron Donor-Acceptor Complexes in Perfluoroalkylation Reactions: Investigation of halogen bonding interactions through UV-Visible absorption and Raman spectroscopies combined with DFT calculations[END_REF] could be responsible for secondary photochemical reactions that would ultimately lead to the consumption of a significant amount of RfI.

Using tertiary amines, the reaction proceeds at a very slow rate, the yields of cyclic carbonate after 15 h reaching only ~ 25% with TEA and ~ 15% with TMEDA and DABCO. Therefore, it appears that although strong (Brønsted/Lewis) bases are needed in order to reach high yields for such reaction, TMG and DBU displays the higher selectivity towards the synthesis of 1. Figure 6. Influence of the nature of the base on the kinetic of formation of perfluorobutyl carbonate followed by in situ ATR-IR spectroscopy. Conditions: allyl alcohol (48 L, 0.5 mmol), Base (1.05 equiv.), C4F9I (1.25 equiv.), CH3CN (1 mL), 25 °C, CO2 (0.3 MPa), h = 365 nm (LED).

Mechanism

Thus, from the experimental results obtained above, a plausible mechanism for the photoinduced carboxylative coupling of CO2 with allyl alcohol is depicted in Scheme (3).

The first step, which occurs in the dark during the preparation of the reaction mixture, is the carbonation of the alcohol group that leads to the carbonate A along with unreacted DBU (~ 10%). When the LED is switched on, the CF2-I bond activated by DBU through a halogen bond undergoes a homolytic cleavage to generate I• and C4F9• radicals, the latter adding onto carbonate A to give the radical intermediate B. B could then react with I• or, more likely, abstracts the iodine atom of C4F9I to afford the iodocarbonate C and C4F9• which propagates a radical chain process. Finally, the fast intramolecular cyclization affords the cyclic carbonate 1 along with DBUH + I -. Scheme 3. Mechanism proposal for the formation of the perfluorobutyl carbonate 1.

Influence of experimental parameters

Considering the model reaction using allyl alcohol, 1.05eq DBU and 1.25eq C4F9I, the influence of different reaction conditions on kinetics and yields for the formation of perfluorobutyl cyclic carbonate were investigated including the temperature, the pressure, the irradiation wavelength and the nature of the solvent.

Temperature and pressure effect.

The influence of the temperature on the kinetics and yields is illustrated in Fig. 7. At a fixed pressure of 0.3 MPa, the temperature increase from 25 °C to 50 °C did not affect the reaction significantly. In contrast, heating up to 70 °C led to a maximum yield of about 30%. This agrees with previous works who showed that the carbonation of alcohols promoted by a super base such as DBU is exothermic and highly effective at room temperature, and that heating at 90 °C shifted the equilibrium towards the starting reactants. [START_REF] Heldebrant | Organic liquid CO2 capture agents with high gravimetric CO2 capacity[END_REF][START_REF] Smith | Insights into the Organocatalyzed Synthesis of Urethanes in Supercritical Carbon Dioxide: An In Situ FTIR Spectroscopic Kinetic Study[END_REF] Similar rates and yields were obtained when conducting the reaction at 0.15, 0.35 and 2 MPa. Thus, using DBU, the reaction proceeds very effectively in the presence of slight excess of the base with respect to allyl alcohol and at low CO2 pressure. In agreement with Heldebrant's work and the results of this study, the alcohol group predominantly exists in the form of a carbonate in the reaction conditions which ensures a highly effective carboxylative cyclization compared to the possible competitive epoxidation from the iodoperfluoroalkyl alcohol.

Irradiation wavelength effect

The effect of the irradiation wavelength on the kinetic and yield of the reaction have been studied using a UVA LED (365 nm), a blue LED (405 nm) and a white light LED (425-700 nm). As shown in the figure 9, the UVA and blue light irradiation led to similar reaction rates and yields while white light irradiation (> 425 nm) led to a slower rate and moderate yield (~ 60%) in 10 h. Thus, due to the formation of the halogen bond complex [C4F9I-DBU] whose absorption tails up to the blue region of the optical spectrum, [START_REF] Grondin | Visible-light induced photochemistry of Electron Donor-Acceptor Complexes in Perfluoroalkylation Reactions: Investigation of halogen bonding interactions through UV-Visible absorption and Raman spectroscopies combined with DFT calculations[END_REF] rather low energy blue photons promote the homolytic cleavage of the C4F9-I bond to generate Rf• which initiates a radical chain process.

Solvent effect

The influence of the nature of the solvent on the kinetics and the yields of the reaction was then evaluated with various organic solvents such as THF, DMSO and anisole, the latter being the less toxic. [START_REF] Prat | CHEM21 selection guide of classical-and less classical-solvents[END_REF] Although CH3CN appears to be the best solvent for this reaction in terms of kinetic and yield, the perfluorobutyl carbonate 1 could be formed in a good yield (~ 85%) using DMSO (see figure 10). Lower yields of about 60 and 30% were obtained with THF and anisole, respectively.

Substrate scope

In an effort to extend the applicability of the protocol used for the synthesis of perfluorobutyl carbonate, several commercially available allylic and propargylic alcohols were tested for the formation of both 5 and 6 membered cyclic carbonates (Scheme 4). Scheme 4. Substrate scope using optimal conditions of the model reaction.

Figure 11 shows the time dependence of the formation of 5 and 6 membered cyclic carbonates from various alcohols. Allylic alcohols such has 1-pentene-3-ol, crotyl alcohol and geraniol were converted into 5 membered cyclic carbonates but in lower yield than that obtained for allyl alcohol. No cyclic carbonate was formed from the tertiary allylic alcohol 2-methyl 3 butene-2-ol. Contrasting with other alcohols, the alkyl carbonate anion RCO3 -did not form upon addition of CO2 at a pressure of 0.3 MPa. Interestingly, it was possible to convert 3-butene-1-ol into a 6 membered cyclic carbonate albeit in a lower IR yield compared to allyl alcohol. Finally, propargyl alcohol could not be converted into any 5 membered cyclic carbonate. This is probably due to the fact that the intramolecular nucleophilic substitution on an sp2 carbon is more difficult to achieve.

Materials and Methods

Material

Allyl alcohol (Sigma Aldrich), 1-pentene-3-ol (Sigma Aldrich), Crotyl alcohol (Sigma Aldrich), Geraniol (Sigma Aldrich), 3-butene-1-ol (Sigma Aldrich), 2-methyl 3 butene-2-ol (Sigma Aldrich), propargyl alcohol (Sigma Aldrich), acetonitrile (CH3CN, Sigma Aldrich), dimethyl sulfoxide (DMSO, Sigma Aldrich), tetrahydrofuran (THF, Sigma Aldrich), Anisole (Alfa Aesar), 1,8-diazabicyclo [5.4.0]undec-7-ene (DBU, Alfa Aesar), triethylamine (Et3N, Sigma Aldrich), 7-Methyl-1,5,7-Triazabicyclo[4.4.0]dec-5-ene (MTBD, Sigma Aldrich), TetramethylGuanidine (TMG, Alfa Aesar), Tetramethylethylenediamine, (TMEDA, Sigma Aldrich), 1,4-Diazabicyclo[2.2.2]octane (DABCO, Sigma Aldrich) and Perfluorobutyl iodide (C4F9I, Sigma Aldrich) were used without further purification.

In situ kinetics methods

The reactions were monitored in situ by ATR-IR spectroscopy using a home-made Ge ATR accessory suitable for high-pressure measurements (up to 5 MPa) and high temperature (up to 100 °C) (See ESI, Fig. S2) coupled with a ThermoOptek interferometer (type 6700) equipped with a globar source, a KBr/Ge beamsplitter and a DTGS (Deuterated Tri-Glycine Sulphate) detector. Single beam spectra recorded in the spectral range (600-4000 cm -1 ) with a 4 cm -1 resolution were obtained after the Fourier transformation of 20 accumulated interferograms for the first 20 spectra (one spectrum every 80 sec), and then Fourier transformation of 120 accumulated intereferograms until the end of reaction time (one spectrum every 8 min).

The in-situ Raman scattering investigations were performed using a Jobin-Yvon Horiba XploRA confocal Raman microscope equipped with a 10X objective and a laser diode at a wavelength λ = 785 nm and 100 % laser power of 45 mW. The spectral range from 140 to 2600 cm -1 was recorded with a grating of 600 l/mm and a resolution of about 5 cm -1 . The spectra result from two acquisitions of 20 seconds each to improve the signalto-noise ratio. A home-made high pressure cell (up to 10 MPa with a volume of 3 mL) equipped with two sapphire windows was used in order to simultaneously measure the Raman spectra and irradiate the mixture with the same LED than that used on the ATR-IR set-up.

The LEDS used in this study were purchased from Thorlabs. The LED at 365nm (M365FP1) used for most of the experiments delivers an irradiance of about 5mW/cm 2 on the sample. Two other LEDS (M405FP1) 405nm and MWWHF2 (425-700nm) were used for comparison at the same irradiance (5mW/cm 2 ).

The alcohol (0.5 mmol) was solubilized in a 2 mL vial with 1 mL of solvent. C4F9I (1.25 eq) was added to the vial and then the Lewis base (1.05 eq) was added to the vial. The whole mixture was then transferred to the reaction chamber (volume: 5 mL) which was directly fixed on the Ge crystal of the ATR-IR device and on which a sapphire window allows the irradiation of the mixture with a LED. The mixture was constantly homogenized during the experiment using a magnetically driven stirrer disposed in the reaction chamber. An optical lens was connected to the sapphire window's support and the LED was connected to the optical lens via an optical fibre in order to ensure a good reproducibility for the irradiation of all the mixtures investigated. The setup was sealed with the CO2 feed pipe. A first spectrum was recorded and then, once CO2 was introduced in the sample chamber at the desired pressure, the kinetic was started. Few minutes later, the LED was then switched on in order to start the photoinduced carboxylative coupling of CO2 with various alcohols. At the end of the reaction, in order to determine the yield, the ATR-IR spectrum of the reaction mixture was compared with the corresponding spectrum of the neat carbonate recorded under the same conditions. The absorbance (A (1810)) of the peak associated to the ν(C=O) stretching mode at 1810 cm -1 of the cyclic carbonate was used to determine the yield for the entire kinetic deduced by proportionality according to Equation 1. A number of experiments were conducted at least twice in order to check for reproducibility. We emphasize that ATR-IR spectroscopy can be considered as a quantitative method like 1 H NMR as demonstrated in our previous work [START_REF] Brege | The coupling of CO2 with diols promoted by organic dual systems: Towards products divergence via benchmarking of the performance metrics[END_REF] with an accuracy of about ± 3 % and can highlight the potential formation of by-products.

𝑌𝑖𝑒𝑙𝑑 (%) =

[𝐴(𝑡)(1810)](reaction mixture) [𝐴 (1810)](neat carbonate) (1)

Conclusion

In this paper, we have investigated an original protocol for the synthesis of perfluoroalkylated cyclic carbonate by the photoinduced carboxylative cyclization of allyl alcohols with CO2 promoted by visible/UVA light irradiation using perfluoroalkyl iodides. Under optimised conditions, quantitative yields up to 99 % in perfluorobutyl carbonate were reached in less than 8 h at 298 K and 0.15 MPa under blue irradiation at 405nm. In situ kinetic studies by ATR-IR and Raman spectroscopy revealed the evolution of all the components of the complex mixture during the reaction while varying key parameters such as the stoichiometry of the reactants, the nature of the Lewis base, the temperature, the pressure, the irradiation wavelength, the solvent and the substrate scope. Such fundamental kinetic studies enabled the determination of the reaction mechanism and provided insight into the underlying reason for the observed kinetics and selectivity. In particular, it was found that strong Lewis bases are needed that played both the role of activating the allyl alcohol for the generation of the allyl carbonate in the presence of CO2 and promoting the ATRA reaction through the activation of C4F9I by halogen bonding. Various allylic alcohols such has 1-pentene-3-ol, crotyl alcohol and geraniol (a biobased substrate) were converted into 5 membered cyclic carbonates. Interestingly, it was possible to convert 3butene-1-ol into a 6 membered cyclic carbonate although to a lower yield. Thus, this route offers a sustainable and promising approach for the valorisation of CO2 as a chemical feedstock through efficient usage of light energy.

Figure 1 .

 1 Figure 1. ATR-IR spectra of the reaction medium at several times during the reaction. Conditions: allyl alcohol (48 L, 0.5 mmol), DBU (1.05 equiv.), C4F9I (1.25 equiv.), CH3CN (1 mL), 25 °C, CO2 (0.3 MPa), irradiation at 365 nm (LED) under stirring.

Figure 2 .

 2 Figure 2. Kinetic profiles followed on the same model reaction by ATR-IR and Raman spectroscopies. The kinetics were recorded once the LED (365 nm) was switched on, i.e. 5 min after the solution containing the allyl alcohol (0.5 mmol), DBU (1.05 equiv.), C4F9I (1.25 equiv.), CO2 (0.3 MPa) in CH3CN (1 mL) has been prepared.

Figure 3 .

 3 Figure 3. Influence of the amount of DBU on the rate of formation and yield of carbonate 1 followed in situ by ATR-IR spectroscopy. Conditions: allyl alcohol (48 L, 0.5 mmol), DBU (0.5-1.5 equiv.), C4F9I (1.25 equiv.), CH3CN (1 mL), 25 °C, CO2 (0.3 MPa), h = 365 nm (LED).

Figure 4 .

 4 Figure 4. Influence of the amount of C4F9I on the kinetic of formation of perfluorobutyl carbonate followed by in situ ATR-IR spectroscopy. Conditions: allyl alcohol (48 L, 0.5 mmol), DBU (1.05-1 equiv.), C4F9I (1-1.5 equiv.), CH3CN (1 mL), 25 °C, CO2 (0.3 MPa), h = 365 nm (LED).

Figure 5 .

 5 Figure 5. Evolution of the height of the peak at 684 cm -1 of C4F9I in the presence of DBU (with allyl benzene) or without DBU (with ally alcohol) followed by in situ ATR-IR spectroscopy. Conditions: allyl alcohol or allyl benzene (0.5 mmol) without DBU or DBU (0.1 equiv.) respectively, C4F9I (1.25 equiv.), CH3CN (1 mL), 25 °C, CO2 (0.3 MPa), h = 365 nm (LED).

Figure 7 .

 7 Figure 7. Influence of temperature on the kinetic of formation of 1 followed by in situ ATR-IR spectroscopy. Conditions: allyl alcohol (48 L, 0.5 mmol), DBU (1.05 equiv.), C4F9I (1.25 equiv.), CH3CN (1 mL), T (25, 40, 50, 70 °C), CO2 (0.3 MPa), h = 365 nm (LED).

Figure 8 .

 8 Figure 8. Influence of pressure of CO2 on the kinetic of formation of 1 followed by in situ ATR-IR spectroscopy. Conditions: allyl alcohol (48 L, 0.5 mmol), DBU (1.05 equiv.), C4F9I (1.25 equiv.), CH3CN (1 mL), T = 25 °C, CO2 (0.15, 0.3, 2 MPa), h = 365 nm (LED).

Figure 9 .

 9 Figure 9. Influence of the irradiation wavelength on the kinetic of formation of 1 followed by in situ ATR-IR spectroscopy. Conditions: allyl alcohol (48 L, 0.5 mmol), DBU (1.05 equiv.), C4F9I (1.25 equiv.), CH3CN (1 mL), T = 25 °C, CO2 (0.3 MPa), h = 365 nm, 405 nm and 425-700 nm (LED-).

Figure 10 .

 10 Figure 10. Influence of the solvent on the kinetic of formation of 1 followed by in situ ATR-IR spectroscopy. Conditions: allyl alcohol (48 L, 0.5 mmol), DBU (1.05 equiv.), C4F9I (1.25 equiv.), solvent (1 mL), T = 25 °C, CO2 (0.3 MPa), h = 365 nm (LED).

Figure 11 .

 11 Figure 11. Kinetic investigation of the substrate scope to afford 5 and 6 membered perfluorobutyl cyclic carbonate. Conditions: substrate (0.5 mmol), Base (1.05 Eq), C4F9I (1.25 Eq), CH3CN (1 mL), 25°C, CO2 (0.3 MPa), h = 365 nm (LED).

Acknowledgments:

The authors acknowledge the "Conseil Régional Nouvelle Aquitaine" (CRNA) for financial support to the infrared and Raman equipment.

Data Availability Statement:

The raw/processed data required to reproduce these findings can be shared upon request sent to the corresponding author by e-mail.

Supplementary Materials:

The following supporting information can be downloaded at: www.mdpi.com/xxx/s1, Figure S1: title; Table S1: title; Video S1: title.

Author Contributions: Conceptualization J-M. V., T.T.; methodology, J.G., T.T.; investigation, C.A., J.G.; data curation, J.G., T.T.; writing-original draft preparation, J-M. V., T.T.; writing-review and editing, J.G., T.T.; supervision, T.T.;

Conflicts of Interest: "The authors declare no conflict of interest."