Régis Mastretta

Rémi Poirot

Damien Bourgeois
email: damien.bourgeois@cea.fr

Daniel Meyer

Palladium isolation and purification from nitrate media: efficient process based on malonamides

Keywords: palladium, iron, solvent extraction, malonamide, waste reprocessing

Abstract: Solvents based on malonamides have been described to be very efficient and selective for liquid-liquid extraction of Pd(II) from nitrate media. The present study details the possibility to selectively extract Pd(II) over common metallic cations, Cu(II), Ni(II), Co(II), Zn(II), Nd(III), Fe(III) and Al(III) using N,N'-dimethyl,N,N'dibutyltetradecylmalonamide (DMDBTDMA) in toluene. The lowest selectivity was obtained considering Fe(III) and Nd(III), the extraction of all other cations being negligible. Pd(II) distribution and selectivity regarding Fe(III) have been fine-tuned using the aqueous HNO 3 concentration as the sole parameter, so that extraction, scrubbing and stripping steps can be simply designed without the need for other specific metal chelating reagents and/or aqueous media, and without the extensive generation of aqueous effluents. DMDBTDMA was also benchmarked with other classical Pd(II) extractants such as tributylphosphate (TBP), dihexylsulfide (DHS), tetraoctyl diglycolamide (TODGA) and bis(2-ethylhexyl)sulfoxide (BESO) in toluene and in n-heptane. Overall, this study reveals that malonamides are very well positioned and should be considered for further processing of aqueous nitrate wastes containing Pd contaminated with common base metals.

INTRODUCTION

Palladium (Pd) is a precious metal which has been the focus of extensive efforts for an efficient recovery from various wastes [START_REF] Işıldar | Electronic waste as a secondary source of critical metals: management and recovery technologies[END_REF][START_REF] Wang | Recovery of rare and precious metals from urban mines-A review[END_REF][START_REF] Jin | Review of critical material studies[END_REF][START_REF] Zhang | Selective recovery of palladium from waste printed circuit boards by a novel non-acid process[END_REF]. Generally, leaching of precious metals from wastes such as end of use automotive catalysts or shredded electronic scraps is performed in aqua regia, a mixture of nitric and chlorhydric acid, but there is also a peculiar interest on leaching with sole nitric acid (HNO 3 ) as Pd is highly soluble in this medium, along with silver (Ag), on the contrary to gold (Au) and platinum (Pt) [START_REF] Kaya | Recovery of metals and nonmetals from electronic waste by physical and chemical recycling processes[END_REF][START_REF] Sobral | Palladium: Extraction and Refining[END_REF]. As a consequence, various industrial effluents containing Pd in HNO 3 with variable amount of chloride ions have been described [START_REF] Nogueira | Oxidative leaching process with cupric ion in hydrochloric acid media for recovery of Pd and Rh from spent catalytic converters[END_REF][START_REF] Fernandes | Recovery and reuse of palladium from spent glucometer electrochemical test strips[END_REF]. Furthermore, the reprocessing of electronic scrap often involves a first leaching step in concentrated HNO 3 alone, to remove most of base metals and enable a more efficient subsequent Au leaching in aqua regia [START_REF] Kinoshita | Metal recovery from non-mounted printed wiring boards via hydrometallurgical processing[END_REF][START_REF] Sheng | Recovery of gold from computer circuit board scrap using aqua regia[END_REF]. Most Ag is lost during this first HNO 3 leaching, and the outcome of Pd is not clear: although it is likely that Pd is also contained in these solutions, no Pd content has been reported to our knowledge. Altogether, there is a need for robust solutions to process HNO 3 streams containing Pd more or less contaminated with other base metals such as Fe, Al, Cu, Ni, Zn… These streams may contain Pd at relatively high concentrations (up to several g/L), or at low concentrations (10-250 mg/L) according to their origin [START_REF] Cayumil | Concentration of precious metals during their recovery from electronic waste[END_REF][START_REF] Nguyen | Separation and Recovery of Precious Metals from Leach Liquors of Spent Electronic Wastes by Solvent Extraction[END_REF][START_REF] Prabaharan | A hydrometallurgical process for recovering total metal values from waste monolithic ceramic capacitors[END_REF][START_REF] Karwowska | Bioleaching of metals from printed circuit boards supported with surfactant-producing bacteria[END_REF][START_REF] Jung | Processing of high purity gold from scraps using diethylene glycol di-N-butyl ether[END_REF][START_REF] Yi | Selective Recovery of Au(III) and Pd(II) from Waste PCBs Using Ethylenediamine Modified Persimmon Tannin Adsorbent[END_REF]. Recovery of the Pd from these solutions has been envisioned according to different techniques, such as precipitation [START_REF] Kolarik | Recovery of value fission platinoids from spent nuclear fuel -Part I: General Considerations and Basic[END_REF], solvent extraction [START_REF] Charlesworth | Separating the Platinum Group Metals by Liquid-Liquid Extraction[END_REF], solvent impregnated [START_REF] Moon | Recovery of Platinum and Palladium from Spent Automobile Catalyst by Solvent-Impregnated Resins[END_REF] or chelating resins [START_REF] Gurung | Persimmon tannin-based new sorption material for resource recycling and recovery of precious metals[END_REF][START_REF] Venkatesan | Extraction of palladium (II) from nitric acid medium by imidazolium nitrate immobilized resin[END_REF]. Recently, we reported that N,N'-dimethyl,N,N'-dioctylhexylethoxymalonamide (DMDOHEMA) is an unexpectedly good candidate to extract Pd from nitrate media, as the 1,3-propanediamide (malonamide) moiety presents the adequate geometry to efficiently chelate the Pd(II) cation [START_REF] Poirot | Palladium Extraction by a Malonamide Derivative (DMDOHEMA) from Nitrate Media: Extraction Behavior and Third Phase Characterization[END_REF]. Also, we demonstrated that although this molecule was initially designed to purify lanthanides form minor actinides with solvent extraction, its selectivity for Pd(II) regarding Nd(III) can be tuned according to the diluent and/or the HNO 3 content of the aqueous phase [START_REF] Poirot | Metal Recognition Driven by Weak Interactions: A Case Study in Solvent Extraction[END_REF]. Selectivity factors (S Pd/Nd = DPd/ DNd) exceeding 20 with solvents based on DMDOHEMA at concentrations ranging from 0.3 to 0.6M have been obtained, either at low acidity in aliphatic diluents (nheptane), or at high acidity in aromatic diluents (toluene). This selectivity is the result of the control of the aggregation of the organic phase, and does not rely on modification of the aqueous speciation of the cations (with addition of specific chelating agents for instance). Furthermore, Pd distribution coefficient (DPd) lies around 10, without optimization, suggesting that both efficient extraction and easy stripping of Pd can be envisioned with this system. Thus, we decided to explore whether it is possible to exploit these results to develop a straightforward process for selective Pd recovery from nitrate aqueous solutions based on a malonamide solvent.

The purpose of this article is to present in detail the design of a malonamide based solvent, that enables selective Pd(II) extraction, in the presence of base metals, such as Fe(III), Al(III), Zn(II), Cu(II), Ni(II), Co(II) and lanthanides, especially Nd(III). As previous reports on closely related N,N'-tetrasubstituted malonamides, bearing or not an alkyl chain on the central carbon atom, reported that Fe(III) can be very efficiently extracted [START_REF] Malik | A Novel Solvent Extraction Route for the Mutual Separation of Platinum, Palladium, and Rhodium in Hydrochloric Acid Media[END_REF][START_REF] Costa | The solvent extraction of iron(III) from chloride solutions by N,N′-tetrasubstituted malonamides: structureactivity relationships[END_REF][START_REF] Costa | Solvent extraction of iron(III) from hydrochloric acid solutions using N,N′-dimethyl-N,N′-diphenylmalonamide and N,N′-dimethyl-N,N′-diphenyltetradecylmalonamide[END_REF], we focused part of our study on the Pd(II)/Fe(III) separation. We also explored the scope and limitations of this solvent, with regard to some other potentially usable solvents based on commercially available extractants, tributylphosphate (TBP), dihexylsulfide (DHS), and bis(2-ethylhexyl)sulfoxide (BESO). Finally, we modelled a possible process with Fe(III) scrubbing and Pd(II) stripping sections employing only diluted HNO 3 aqueous solutions, ie without the need for changing aqueous media nor specific metal chelating anions.

EXPERIMENTAL Chemical Reagents

DMDOHEMA and N,N'-dimethyl,N,N'-dibutyltetradecylmalonamide (DMDBTDMA) (Figure 1

Solvent Extraction

Extractions were performed in vials with an organic phase containing extracting molecule and diluent and an aqueous HNO 3 solution containing a mixture of metallic cations in known concentrations. Aqueous phases were prepared by dilution of stock solutions of each metallic cation (5 g/L) into a 6M HNO 3 aqueous solution, prepared by dissolution of the corresponding salt into a 6M HNO 3 aqueous solution, optional addition of LiNO 3 , and the final HNO 3 concentration was adjusted with deionized water. Organic phases were prepared by dilution of DMDOHEMA, DMDBTDMA, TBP, DHS, BESO, or TODGA in the appropriate diluent and were pre-equilibrated with the adequate aqueous phase containing all reagents except the metallic cations to study. Unless otherwise noticed in the text, the volume of the organic phase was 1 mL, and the volume of the aqueous phase was chosen according to the A/O ratio detailed in the text. The two phases were vigorously shaken at 1000 rpm with an IKA-Vibrax VXR basic shaker in a 20-22°C environment. The tubes were removed and centrifuged for 5 min or until complete phase separation and the phases were separated, and 500 µL of each phase were taken for analysis.

The aliquot of the aqueous phase was directly diluted into a 2% HNO 3 /HCl aqueous solution (90/10). Metals contained in the aliquot of the organic phase were back-extracted with 800 µL of an aqueous 0.1 M thiourea solution at 20-22°C for 1 h. Both phases were separated, and 500 µL of the resulting aqueous solution were taken and diluted into a 2% HNO 3 /HCl aqueous solution (90/10). Preliminary studies showed that Pd(II) as well as other metals were totally stripped with this process, and that no metal precipitation occurred until analysis. Dilution factors were chosen so that metal concentrations are not above 20 mg/L, with a minimum 10 fold dilution to avoid matrix effects. Concentrations of each metal in both aqueous phases (extraction and stripping phases) were determined by inductively coupled plasma atomic emission spectroscopy (ICP/AES, SPECTRO ARCOS ICP Spectrometer, AMETEK Materials Analysis). Standards at 0 mg/L; 0,5 mg/L; 1 mg/L; 5 mg/L; 10 mg/L and 15 mg/L were prepared by dilution of 1,00 g/L solutions (SCP Science) into a 2% HNO 3 /HCl aqueous solution (90/10). Given concentrations are calculated as the means of three replicates on three different wavelengths for each metal; relative standard deviations were determined and lie between 1 and 4%. Quantification limits were determined for each metal from the background equivalent concentration (BEC) calculated by the spectrometer for each optical line and are summarized below (Table 1).

Concentrations of each metal after extraction experiments were compared to the concentrations of same metal in the initial aqueous phase before contacting the aqueous layer with the organic layer. The distribution ratio, D, was calculated for each metal as the ratio at equilibrium between the total concentration of said metal in the organic phase and the total concentration of said metal in the aqueous phase. Confidence intervals were determined taking into account the error of the spectrometer and the precision of the materials employed for dilution, and relative errors were found to lie between 5 and 10%.

Unless otherwise stated in this article, all results show a total mass balance (determined assuming that the volume change of each phase can be neglected) in each metal comprised between 95 and 105% of the initial metal load in the aqueous phase.

HNO 3 concentration determination

Concentration of HNO 3 in both aqueous and organic phases was obtained after potentiometric titration using a Mettler DL77 Titrator equipped with a combined glass electrode for pH measurements. The aqueous phase was directly analyzed, the organic phase was beforehand diluted with ethanol (5 mL for 100 µL of organic phase), then water (15 mL). Acid-base titration was carried out using 0,010 M aqueous NaOH. HNO 3 distribution ratio (DHNO 3 ) was then obtained after pre-equilibration of solvent in different conditions as the ratio at equilibrium between the HNO 3 concentration in the organic phase and the HNO 3 concentration in the aqueous phase.

RESULTS AND DISCUSSION

Composition of solvent

Following our fundamental studies related to the Pd(II)/Nd(III) separation [START_REF] Poirot | Palladium Extraction by a Malonamide Derivative (DMDOHEMA) from Nitrate Media: Extraction Behavior and Third Phase Characterization[END_REF][START_REF] Poirot | Metal Recognition Driven by Weak Interactions: A Case Study in Solvent Extraction[END_REF], we screened the extraction of several common metallic cations with DMDOHEMA in nheptane in order to determine the scope of the selectivity of Pd(II) extraction with this malonamide (Figure 2). We focused on +II and +III cations regularly found in metallic wastes such as waste of electric and electronic equipment (WEEE), varying the DMDOHEMA concentration in the solvent, and the aqueous HNO 3 concentration. Both parameters have a positive impact on metallic cations extraction, and a negative impact on Pd/Nd selectivity (S Pd/Nd ). This negative impact on selectivity is principally due to the increase of organic phase aggregation induced by the HNO 3 extracted from the aqueous phase. No noticeable extraction of Cu(II), Ni(II), Co(II), Zn(II) or Al(III) was found. The only other metallic cation extracted is Fe(III), only when the aqueous HNO 3 concentration is important ([HNO 3 ] aq ≥ 3M). Extraction yields of Nd(III) are always higher than those of Fe(III), except at 5 M aqueous HNO 3 , thus Nd(III) can be employed as a reference cation in order to propose a selective solvent. Pd(II) extraction becomes significant either when DMDOHEMA concentration in the solvent is high or when aqueous nitric acid concentration is high (as long as [HNO 3 ] aq remains below 6 M, otherwise competitive HNO 3 extraction leads to a drop in Pd(II) extraction [START_REF] Poirot | Palladium Extraction by a Malonamide Derivative (DMDOHEMA) from Nitrate Media: Extraction Behavior and Third Phase Characterization[END_REF]). As a consequence, there is a compromise to find between high selectivity and sufficient Pd distribution coefficient (DPd). In our previous studies, we demonstrated that a ca. ten fold increase in Pd/Nd selectivity can be obtained with toluene as a diluent [START_REF] Poirot | Metal Recognition Driven by Weak Interactions: A Case Study in Solvent Extraction[END_REF]. This change in diluent is important to take into consideration, as with DMDOHEMA in n-heptane, we previously reported that DPd above 10 are only obtained when Nd(III) is also significantly extracted (Table 2). Also, organic phase splitting can arise when Pd(II) load in the solvent reaches 2,5 -10 g/L [START_REF] Poirot | Palladium Extraction by a Malonamide Derivative (DMDOHEMA) from Nitrate Media: Extraction Behavior and Third Phase Characterization[END_REF]. Latter splitting is due to the precipitation of a Pd(II)-DMDOHEMA complex, fully soluble in toluene, so that no organic phase splitting was observed using this diluent. Using DMDOHEMA in toluene, it is thus possible to have both a DPd above 5 along with S Pd/Nd above 10, eg. using 0,3 M DMDOHEMA with a 4 M [HNO 3 ] aq aqueous solution, or 0,4 M DMDOHEMA with a 3 M [HNO 3 ] aq aqueous solution (Table 2).

DMDOHEMA has been chosen for the partitioning of trans-uranium elements from high level liquid waste in the reprocessing of spent nuclear fuel according to its affinity for both lanthanides(III) and actinides(III) [START_REF] Modolo | DIAMEX Counter-Current Extraction Process for Recovery of Trivalent Actinides from Simulated High Active Concentrate[END_REF]. The former malonamide reference [START_REF] Facchini | Transientand Steady-State Concentration Profiles in a DIAMEX-like Countercurrent Process for An(III) + Ln(III)[END_REF],

N,N'-dimethyl,N,N'-dibutyltetradecylmalonamide (DMDBTDMA, Figure 1), has been discarded in these studies, although it is easier to prepare (the central carbon atom chain does not need to be synthetized separately). This extracting molecule is also very stable in nitrate media [START_REF] Lumetta | Review: Solvent Systems Combining Neutral and Acidic Extractants for Separating Trivalent Lanthanides from the Transuranic Elements[END_REF]. As a consequence, we compared both malonamides with toluene as a diluent (Figure 3). As expected, Nd distribution ratios are lower with DMDBTDMA than with DMDOHEMA: at 5 M [HNO 3 ] aq , DNd is 0,3 with DMDBTDMA and 0,7 with DMDOHEMA. More surprising is the increase in DPd: these were found about twice bigger with DMDBTDMA than with DMDOHEMA. Altogether, S Pd/Nd are 4 times bigger with DMDBTDMA than with DMDOHEMA, especially in the conditions where Pd is well extracted, ie above 3 M [HNO 3 ] aq . With DMDBTDMA in toluene, it is therefore possible to find adequate conditions for efficient Pd(II) extraction and separation from Nd(III), and as a consequence for other multivalent cations evaluated previously.

Modelling of counter-current process

With the so gathered data, it is possible to model the behavior of each metal during The Pd extraction step is easy to model as the aqueous HNO 3 concentration remains constant all along the different stages. In the chosen conditions, with an aqueous to organic flow phase ration (A/O) of 5, and previously determined DPd = 15,6 and DFe = 0,32, after the 6 stages, more than 99,9% of the Pd and 6,4% of the Fe are extracted (Figure 9). The extract contains then 500 mg/L Pd and 640 mg/L Fe. The A/O ratio can be chosen according to the distribution coefficients in order to maximize Pd extraction yield and minimize Fe extraction yield; both can be exactly calculated after each stage according to equation [START_REF] Işıldar | Electronic waste as a secondary source of critical metals: management and recovery technologies[END_REF], where N the stage number:

(1)

The loss in Pd can be deduced and its expression simplified into equation (2) taking into account that :

(

And the amount of Fe extracted can also well be estimated with equation (3) as in this case :

(

The Fe scrubbing step was then modelled following a two-stage operation. First of all, the HNO 3 concentrations in organic and aqueous phases were determined for each stage according to the procedure presented beforehand, ie using the linear model determined for DHNO3. Then, from the aqueous HNO 3 concentration at equilibrium in each stage, both DPd and DFe were deduced, and the concentrations of each metallic cation at equilibrium in each stage calculated consequently, taking into account mass balance. Overall Pd loss and Fe scrubbing yield are finally obtained through the comparison on input and output quantities. Different aqueous HNO 3 solutions and various O/A ratios were evaluated, and a good compromise consisted in the use of a 1,2 mol/L aqueous HNO 3 solution with an O/A flow ratio of 5 (Figure 10). With these parameters, the loss in Pd is 1,5%, and the Pd purity in the organic phase is over 99,5% (99,7% Fe scrubbing). The HNO 3 concentration in the organic phase decreases regularly along the stages, so that at final stage, the organic phase is equilibrated with a 1,8 M aqueous HNO 3 solution. At this point, DFe = 0,008, and S Pd/Fe = 465: the slight decrease in HNO 3 content in both phases enables to keep Pd in the solvent with minimal losses, along with a drastic increase in the selectivity regarding Fe, which enables complete scrubbing of Fe from the solvent.

A similar strategy was adopted in order to strip Pd from the organic phase, this time with water. The decrease in HNO 3 content in the organic phase is much more pronounced as the distribution coefficient of HNO 3 also decreases when the amount HNO 3 in the aqueous layer decreases ([HNO 3 ] aq is below 2, see Figure 8). Practically, more than 90% of the HNO 3 is stripped from the organic layer during the first stage. As a consequence, DPd drops and Pd can be easily stripped from the organic layer within a few stages. The only concern is the stability of the Pd(II) solution along the different stages: to avoid Pd(II) hydrolysis, we preferred to model the process employing a 5.10 -2 mol/L aqueous HNO 3 solution (Figure 11). The results are very similar to stripping with H 2 O only, except that aqueous and organic layers equilibrate so that [HNO 3 ] aq = 5.10 -2 mol/L. Pd is quantitatively (more than 99,9%) stripped from the organic phase after 6 stages, and the final aqueous Pd solution contains ca. 1 g/L Pd. At such a concentration aqueous Pd(II) can be easily processed in order to isolate a salt such as PdCl 2 (NH 3 ) 2 after precipitation with ammonium chloride (or ammonia then chlorohydric acid) [START_REF] Bernardis | A review of methods of separation of the platinum-group metals through their chloro-complexes[END_REF]. Overall, with such a process, starting from an aqueous HNO 3 solution containing rather diluted Pd(II) (100 mg/L) and concentrated Fe(III) (2 g/L), a 1 g/L Pd(II) aqueous solution can be easily obtained, with minimal Pd loss (1,5 %), excellent purity (more than 99,5 %), and generation of only 1/5 th of the initial volume of an aqueous effluent (resulting from Fe scrubbing), fully compatible with the raffinate for further elimination or possible valuation. Furthermore, the final organic phase does not contain any undesired reagent (specific Pd(II) chelating agents for instance) that could disturb re-introduction of same solvent in a next extraction step, so that solvent regeneration may be straightforward, even not necessary. The excellent stability of DMDBTDMA regarding aqueous HNO 3 may indeed protect from a systematic solvent regeneration.

Although they rapidly revealed possible issues from the extraction step, the use of sulfur based reagents such as BESO or DHS could also be envisioned. However, backextraction of Pd(II) from solvents based on these reagents requires the use of a specific aqueous layer containing a Pd(II) stabilizing agent (ammonia, thiourea, thiocyanate…).

The switch from nitrate media to other media generally leads to the generation of more aqueous effluents, as solvent needs to be washed and re-equilibrated with HNO 3 afterwards prior to re-employment in the extraction step. Although we did not investigate in detail these steps with the same parameters employed in the proposed process modelling, it remains certain that the malonamide based process is the more straightforward and simplest which can be envisioned.

CONCLUSION

  extraction and back-extraction steps. Therefore, we devised a counter-current process based on a classical three steps sequence, extraction, scrubbing and stripping, to demonstrate the potential interest of the DMDBTDMA based solvent. Modelling was performed with the assumption that equilibrium is reached in each stage, and that both phases are always far from saturation, which was checked at the end of the process. The model is base on an initial 3 M HNO 3 aqueous solution containing 100 mg/L Pd(II) and 2000 mg/L Fe(III), and 0,6 M DMDBTDMA in toluene. Each step was decomposed in six stages.

FIGURESFigure 1 .

 1 FIGURES

Figure 2 .Figure 3 .Figure 4 .Figure 5 .

 2345 Figure 2. Extraction yields of various metallic cations with DMDOHEMA in n-heptane according to extractant concentration in solvent and aqueous HNO 3 concentration. Initial metal concentration 100 mg/L (for each cation), aqueous/organic phase volume ratio 1, 1 h extraction.

Figure 6 .

 6 Figure 6. Distribution ratio of Pd(II), Nd(III) and Fe(III) after extraction with 0,6 mol/L DMDBTDMA in toluene according to 1/T. Initial [Pd] aq 100 mg/L, initial [Nd] aq 200 mg/L, initial [Fe] aq 200 mg/L, [HNO 3 ] aq 3 mol/L, aqueous/organic phase volume ratio 1, 1 h extraction.

5 yFigure 7 .Figure 8 .

 578 Figure 7. Distribution ratio of Pd(II), Nd(III) and Fe(III) after extraction with 0,6 mol/L DMDBTDMA in toluene according to aqueous HNO 3 concentration. Initial [Pd] aq 200 mg/L, initial [Nd] aq 500 mg/L, aqueous/organic phase volume ratio 1, initial [Fe] aq 2000 mg/L,

5 yFigure 9 .Figure 10 .Figure 11 .

 591011 Figure 9. Modelling of a six-stage counter-current extraction of an aqueous phase (initial [Pd] aq 100 mg/L, initial [Fe] aq 2000 mg/L, [HNO 3 ] 3 mol/L) with 0,6 mol/L DMDBTDMA in toluene. Aqueous/organic phase flow ratio 5, distribution ratio at equilibrium DPd = 15,6 and DFe = 0,32.

Table 1 .

 1 Quantification limits (LoQ) for ICP-AES analysis.

	3	4	5	6

Table 2 .

 2 Pd(II) distribution coefficient (DPd) and Pd(II)/Nd(III) selectivity (S Pd/Nd ) in selected conditions with DMDOHEMA based solvents.

Table 3 .

 3 Results from solvent scrubbing with H 2 O or 1 M HNO 3 : Pd loss, Fe elimination, and final aqueous HNO 3 concentration. Initial organic phase: DMDBTMA 0,6 M in toluene equilibrated with 3 M aqueous HNO 3 and containing 95 mg/L Pd(II) and 80 mg/L Fe(III).

	6,4	1,8	12	3,4	4,9	13,8	9,3	21


Prince Nana Amaniampong

François Jérôme
email: francois.jerome@univ-poitiers.fr

Microjet Radicals

Catalysis under ultrasonic irradiation: a sound synergy

Keywords: Ultrasound, Catalysis, Synergistic effect, Cavitation bubbles, Radical reactions, Shear forces Graphical Energy transfer Abrasion

Through selected examples, we report here that ultrasound is capable of assisting a solid catalyst in various reactions. Beside improvement of heat and mass transfer induced by the implosion of cavitation bubbles, we show that a synergistic effect can occur between catalysis and ultrasound. In particular, the physical and chemical effects locally exerted on a catalytic surface upon implosion of cavitation bubbles can oxidize metal ions, provide energy to the catalyst/reaction, in situ activate or create catalytic sites and even prevent the catalyst from deactivation. The impact of ultrasound on solid catalyst structures, which may positively or negatively impact the performances of catalysts, is also discussed.

Introduction

Ultrasonic irradiation of liquids locally leads to rapid changes of pressure. When the liquid is locally subjected to depression, the pressure becomes lower than the vapor pressure of the sonicated liquid, thus generating a cavitation bubble composed of gas and vapors of liquid (Fig. 1). Once formed, these cavitation bubbles absorb energy from the sound waves, grow and then collapse, locally resulting in the formation of shock waves, high speed jets or radicals. [1] The effect of the cavitation bubble collapse on a liquid is depending on the applied frequency. For instance, the popular Low Frequency Ultrasound (LFUS) generate large cavitation bubbles (~170 µm at 20 kHz) [START_REF] Hasan | Size and growth of cavitation bubble nuclei[END_REF] and their implosions induce mostly physical effects such as shock waves, high speed jets, etc. As a consequence, LFUS are typically used, for example, for the erosion of particles, emulsification, crystallization, extraction, etc. [START_REF] Suslick | Ultrasound: its chemical, physical, and biological effects[END_REF][START_REF] De Castro | Ultrasound-assisted crystallization (sonocrystallization)[END_REF] In contrast, High Frequency Ultrasound (HFUS, 300-800 kHz) generate a large amount of small size cavitation bubbles (4.6 µm at 550kHz) which leads to a higher production of radicals that can further initiate chemical reactions (Fig. 1). [START_REF] Hasan | Size and growth of cavitation bubble nuclei[END_REF][START_REF] Navarro | Effect of ultrasonic frequency on the mechanism of formic acid sonolysis[END_REF] More information on ultrasound can be found in a recent excellent review from Ashokkumar and Qiao. [START_REF] Mckenzie | Ultrasound and Sonochemistry for Radical Polymerization: Sound Synthesis[END_REF] With the growing demand of our society for alternative activation technologies, ultrasound is now witnessing a sort of renaissance, in particular in the field of organic chemistry and catalysis. In this context, the terms sonochemistry and sonocatalysis have been coined. [START_REF] Mckenzie | Ultrasound and Sonochemistry for Radical Polymerization: Sound Synthesis[END_REF] and Johansson et al. [START_REF] Johansson | Sustainable and energy efficient leaching of tungsten (W) by ultrasound controlled cavitation[END_REF] (A) (B)

Chemical reactions induced by ultrasound

From a chemical point of view, a cavitation bubble can be viewed as a micro-reactor. [START_REF] Lohse | Inside a micro-reactor[END_REF] Due to the extreme conditions of pressure (up to 1,000 bar) and temperature (up to 5,000 K) at the interior of the cavitation bubbles, pyrolysis takes place inside the cavitation bubble. For instance, when water is subjected to an ultrasonic irradiation at a high frequency (100-800 kHz), small size cavitation bubbles are formed inside which water is dissociated to • H and

• OH radicals. Upon implosion of the cavitation bubble, these radicals are propelled into the bulk solution where they react with organic solutes.

Conducting organic reactions under ultrasonic irradiation requires a careful selection of reactants. Reactants exhibiting a high vapor pressure tend to diffuse inside the cavitation bubbles where they are pyrolyzed. [START_REF] Cataldo | Ultrasound-induced cracking and pyrolysis of some aromatic and naphthenic hydrocarbons[END_REF] In contrast, chemicals with a low vapor pressure remain into the bulk solution where they are subjected to shock waves or react with radicals released by the implosion of the cavitation bubbles. Hence, according to the reaction conditions, two reaction pathways take place under ultrasonic irradiation (i) pyrolysis-like reactions or (ii) radical-like reactions. Recently, P. Amaniampong investigated the reactivity of aqueous and alcoholic solutions of glucose under HFUS (550 kHz) (Fig. 2). Using a high concentration of glucose (> 40 wt. %), the probability of contact between glucose and the cavitation bubbles is high leading to a pyrolysis-like mechanism, with the in situ formation of levoglucosane. [START_REF] Amaniampong | Catalyst-Free Synthesis of Alkylpolyglycosides Induced by High-Frequency Ultrasound[END_REF] Conversely, at low concentration (< 10 wt. %), glucose reacts with H • and • OH radicals propelled into the solution, leading to its major oxidation either to gluconic or glucuronic acid, depending on the atmosphere used.

Radical reactions

Pyrrolysis

These examples showed that the ultrasound technology could be considered as a tunable technology for the conversion of strategic biobased building blocks to industrially relevant specialty chemicals. However, being able to control the rate and, even more importantly, to optimize the reaction selectivity under ultrasonic irradiation is much more difficult to achieve and, to date, it remains a challenging task. To tackle this issue, few groups have investigated the addition of a catalyst into the ultrasonic reactor.

Sonocatalysis

The concept of sonocatalysis has been investigated for decades but mostly with the popular low frequency ultrasound (<80 kHz), i.e. the in situ formation of radicals was very low and mainly physical effects occur in this case. Here, the turbulent flow and shock waves produced by the implosion of cavitation bubbles improve the dispersion of catalyst/reactant and mass transfer and increase the temperature at the catalyst surface, thus resulting in an improvement of the reaction rate. More importantly, cavitation on catalyst particle surfaces generates unpassivated and highly reactive surfaces. A chemical reaction that will normally not occur as a result of its low kinetic energy can be feasible under ultrasonication. The rate enhancement induced by ultrasound was observed in various reactions, for instance in Fenton-like reactions such as oxidation of carbohydrates, [START_REF] Rinsant | Efficient and Selective Oxidation of D-Glucose into Gluconic acid under Low-Frequency Ultrasonic Irradiation[END_REF] vanillin, [START_REF] Behling | Sonochemical oxidation of vanillyl alcohol to vanillin in the presence of a cobalt oxide catalyst under mild conditions[END_REF] oxidative depolymerization of lignin, [START_REF] Napoly | H2O2-mediated kraft lignin oxidation with readily available metal salts: what about the effect of ultrasound?[END_REF] among many other examples.

In more interesting cases, a synergistic effect between the catalyst and ultrasound occurs, paving the way to reactions which are usually not feasible under silent conditions. This synergistic effect between ultrasound and catalysis was mainly observed in the presence of solid catalysts. Indeed, the formation of cavitation bubbles preferentially occurs on a particle surface via heterogeneous nucleation. In contrast to homogeneous solution, close to a surface, the implosion of cavitation bubbles is very asymmetric and thus the physical and chemical effects arising from the implosion of cavitation bubbles are concentrated on the catalyst surface, inducing the activation or generation of catalytic sites. [START_REF] Philipp | Damage of solid surfaces by single laser-produced cavitation bubbles[END_REF] For example, with a combined use of low frequency ultrasound (24 kHz) and Fe3O4 as catalyst, Taghizadeh and co-workers reported the sonocatalytic depolymerization of 2hydroxyethyl cellulose (HEC). [START_REF] Taghizadeh | Sonocatalytic degradation of 2-hydroxyethyl cellulose in the presence of some nanoparticles[END_REF] At low frequency ultrasound, the amount of in situ produced radicals is very low and, under catalyst-free conditions, the depolymerization of HEC is a very slow reaction. Interestingly, in the presence of Fe3O4 as catalyst, the implosion of cavitation bubbles on the Fe3O4 surface provides locally enough energy to induce an electron transfer between Fe3O4 and water, generating in situ • OH radicals and H + , both species being involved in the depolymerization of 2-hydroxyethyl cellulose (Fig. 3). As a result, the catalytic depolymerization rate of HEC was greatly enhanced under ultrasonic irradiation. The solid catalyst acting as a nuclei for the formation of cavitation bubbles, this reaction is of course sensitive to the type and the amount of catalyst used (particle size, surface area, etc.).

As above mentioned, under ultrasonic irradiation of water at a high frequency (300-800 kHz), • OH radicals stemming from the sonolysis of water are formed in a much larger amount than at low frequency. Parizot et al. [START_REF] Parizot | Sonocatalytic oxidation of EDTA in aqueous solutions over noble metal-free Co3O4/TiO2 catalyst[END_REF] reported a very elegant work where in situ produced radicals activate the catalyst. In this concept, authors investigated the sonocatalytic degradation of ethylenediaminetetraacetic acid (EDTA) in water under ultrasonic irradiation at a high frequency (345 kHz), and in the presence of a Co3O4/TiO2 catalyst. Under optimized conditions, a conversion as high as 90% was obtained at 40 °C under an Ar/O2 atmosphere.

From a mechanistic point of view, radicals stemming from the sonolysis of water are propelled onto the surface of the Co3O4/TiO2 catalyst where they induce the oxidation of Co 2+ site to Co 3+ . Adsorbed EDTA is then oxidized by Co 3+ , this later being consequently reduced to Co 2+ (Fig. 3). In the absence of the Co3O4/TiO2 catalyst, no reaction occurred, confirming the synergistic effect between ultrasound and the Co3O4/TiO2 catalyst.

Figure 3. Different scenario of synergistic effects between solid catalysts and ultrasound

Energy, radicals

In another case in point, ultrasound can be beneficial for catalysis, thanks to the in situ "cleaning" of the catalyst surface (e.g. removal of oxide layer or passivating coatings from the catalyst surface) during the reaction, an important aspect as regards catalyst deactivation. This was reported for instance by Disselkamp et al. who investigated the effects of sonication on the hydrogenation vs isomerization of 3-buten-1-ol in the presence of palladium black in water. [START_REF] Disselkamp | A comparison between conventional and ultrasound-mediated heterogeneous catalysis: hydrogenation of 3-buten-1-ol aqueous solutions[END_REF] They showed that applying ultrasound during the hydrogenation/isomerization of

Stability of catalysts

Solid catalysts (particles) act as nuclei for the formation of cavitation bubbles. The collapse of cavitation bubbles on the catalyst surface induce high-speed jets of liquid towards the surface which generate strong shear forces on the catalyst surface that may fragment it into smaller particles or modifies the chemical composition of the catalyst surface. [START_REF] Philipp | Damage of solid surfaces by single laser-produced cavitation bubbles[END_REF][START_REF] Haosheng | Cavitation damages on solid surfaces in suspensions containing spherical and irregular microparticles[END_REF] On one hand, this could be viewed as a positive effect for catalysis since it results in (i) an erosion of the catalyst particles to smaller particle sizes, thus increasing the catalytic surface area and/or (ii) the in situ "cleaning" of the catalyst surface, both aspects dramatically enhancing the catalytic reaction rate. However, implosion of cavitation bubbles on the catalyst surface may also lead to severe damages on the catalytic surface. [START_REF] Shchukin | Ultrasonic cavitation at solid surfaces[END_REF] The effect of ultrasound on catalyst stability is closely depending on the catalyst particle sizes and applied frequencies. Solid catalysts with particle sizes much larger than the cavitation bubbles tend to be severely subjected to abrasion, which is often accompanied by the ejection of matter from the catalyst surface to the solution (i.e. leaching). Conversely, due to turbulent flow, small size catalyst particles tends to collide each other, which may also induce a leaching of active species or agglomeration of particles through mechanical friction (Fig; 4). Tripathi and co-workers reported the effect of ultrasound frequencies on the stability of a Lindlar catalyst during the hydrogenation of 2-methyl-3-butyn-2-ol. [START_REF] Tripathi | Ultrasound-assisted selective hydrogenation of C-5 acetylene alcohols with Lindlar catalysts[END_REF] They noticed a significant leaching of Pd and Pb (40%) for all catalysts treated by ultrasound. In addition, the particle sizes were also drastically reduced from 20 m to 5 m, further illustrating the fracturing of the catalyst induced by ultrasound. Interestingly, the fracturing effect of ultrasound was dependent on the applied ultrasound frequency, with the major effect being observed at low frequency. At high frequency (380, 850 kHz), cavitation bubbles are much smaller (< 10 m) than at low frequency (> 150 m), leading to lower cavitation energy, thus lowering the physical effects (shear forces, inter particle collisions, etc.) on the catalyst surface. As a taken home message, prior to coupling solid catalysts with ultrasound, a particular attention should be given to the applied frequency to keep a positive effect of ultrasound on the catalytic reaction rates, while preventing the solid catalyst from damage.

Challenges and Future Perspectives

The combination of catalysis and ultrasound, in most instances, leads to interesting effects

Ejection of matter

Interparticle collision

Fracturing effect Fracturing effect

Interparticle collision

Ejection of matter

heterogeneous conditions, mostly because solid particles act as nuclei for the growth of cavitation bubbles, thus locally concentrating the effect of ultrasound on the catalyst surface.

Although the term sonocatalysis has been coined few years ago, this is a fast moving field which is currently knowing a sort of renaissance, boosted by the demand of our society for alternative activation technology. However, few scientific questions still need to be addressed in the future. For instance, most of catalytic reactions under ultrasound have been done in water. Clearly, much efforts should be paid in the future to understand all phenomenon of sonocatalysis in organic solvents. In addition, most of previous works focused on the combination of LFUS with catalysis. The synergistic effect of catalysis with HFUS is however more scarcely investigated. The selectivity of radical reactions induced by HFUS is quite difficult to control and, for this reason, the combination of HFUS with catalysis was mainly used for the total free oxidation of aqueous pollutants. The ability to control or tune the selectivity of these radical oxidative reactions with the help of a catalyst is highly desirable to the implementation of this technology to the synthesis of specialty chemicals but, to date, it remains an elusive task. Finally, the design of microfluidic reactors is also an important perspective to this work, in particular to reduce the overall energy consumption of this technology.

Comments

Authors have no competing interests to declare

Figure 1 .

 1 Figure 1. (A) formation, growth and implosion of cavitation bubbles; (B) Effect of the ultrasound frequency on shear forces and radical formation. Adapted from Ashokkumar et al.,[START_REF] Mckenzie | Ultrasound and Sonochemistry for Radical Polymerization: Sound Synthesis[END_REF] and Johansson et al.[START_REF] Johansson | Sustainable and energy efficient leaching of tungsten (W) by ultrasound controlled cavitation[END_REF]

[ 11 ]Figure 2 .

 112 Figure 2. Difference of reaction selectivity according to the reaction zone

Figure 4 .

 4 Figure 4. Effect of cavitation bubbles on the catalyst particles

Funding:

  Authors are grateful to the CNRS, the University of Poitiers and the region Nouvelle Aquitaine for the funding of (i) the PNA's postdoc grant, (ii) the FR CNRS INCREASE 3707, (iii) the Chaire TECHNOGREEN and (iv) the CPER/FEDER program.

  3-buten-1-ol allowed a fast in situ reduction of Pd black, caused by the implosion of cavitation bubbles on the Pd surfaces, leading to a 5 fold increase of the reaction rate. Very interestingly, under conventional heating, the isomerization/hydrogenation rate ratio decreased as a function of the reaction time, suggesting a deactivation of catalytic sites responsible for the isomerization of 3-buten-1-ol during the course of the reaction. Under

ultrasound, no change of the isomerization/hydrogenation rate ratio was observed over time indicating that applying ultrasound protect Pd black against deactivation (Fig.

3

). A similar effect was observed by Carcenac et al. who reported an in situ surface cleaning of PtO2 in the catalytic hydrogenation of fluorinated alkenes.

[START_REF] Carcenac | Convenient synthesis of fluorinated alkanes and cycloalkanes by hydrogenation of perfluoroalkylalkenes under ultrasound irradiation[END_REF]