Zhuan Zhang

Pierre H Dixneuf
email: pierre.dixneuf@univ-rennes1.fr

Jean-François Soulé

Late Stage Modific ations of P-Containing Ligands using Transition-Metal-Catalysed C-H Bond Functionalisation

Phosphorus containing molecules have seen widespread applications for the development of innovative metal ligand catalysts and materials. In recent years, the implentation of late-stage modifications of phosphorus derivatives via ortho-regioselective C-H bond functionnalisations have led to invent faster and better ways for the synthesis of complex ligands by creating new reactions with low impact on the environement. This Feature article highlights transition-metal catalysed regioselective C-H bond functionalisations of phosphorus-containing molecules, in which the phosphorus containing unit play the role of the directing group for the C-H bond activation and the syntheses of new functional phosphorus derivatives.

Introduction

During the last decade efficient metal-catalysed activations of C-H bonds, followed by their selective functionalisations, have tremendously contributed to improve the synthetic methods of complex molecules and molecular materials with respect to classical catalytic cross coupling reactions, as they offer cleaner processes and much better atom economy.

The efficiency of catalytic C-H bond activation/functionalisation is now allowing late-stage diversification of functional molecules 2 and direct further functionalisation of metal catalysts, photocatalysts or molecular materials for optical properties.

Organophosphorus compounds, of which their oxidation state can be easily modified, constitute an important class of molecules for their numerous applications in medicinal, 4 synthesis, 5 materials, 6 organocatalysts, [START_REF] Methot | [END_REF] and especially as crucial ligand in metal-phosphine catalysts, 8 including the chiral ones. 9 Consequently, the application of the late stage functionalisations to phosphorous derivative C-H bonds constitutes a great challenge. The use of phosphines as directing groups for direct functionalisation of their aryl ortho C-H bonds has been rarely achieved as the P(III)-metal bonds in catalysts are usually too strong to be dissociated after functionalisation. In addition, the ortho C-H bond activation of aryl-phosphine-metal complexes is expected to lead to a difficult to reach 4-membered metallocycle intermediate. By contrast, phosphine oxides or sulphides, resulting from easy oxidation of phosphines, have been used as weakly coordinating directing groups for efficient C-H bond activation and functionalisation, via 5-or 6-membered metallacycle intermediates. Thus, they constitute excellent starting substrates for the late-stage functionalisation of their C-H bonds to afford after reduction new functional phosphines.

Illustrating recent studies in the field, including some of our own work, this feature article highlights the potential of C-H bond activations/functionalisations, in which a phosphorus containing moiety acts as a directing group, for the preparation of new functional phosphines. It is organized depending on the nature of the involved reactions: successively hydroarylation, alkenylation, alkylation, arylation, amination, C-O bonds formation, cyanation and borylation will be presented. Intramolecular C-H bond functionalisations leading to polycyclic phosphorous containing derivatives will not be retained in this review. [START_REF] Baba | For examples of intramolecular C-H bond functionalisations of phophorus derivatives see: a)[END_REF] In addition, annulation reactions of phosphine oxides and sulphides, [START_REF] Park | For examples of annulation reaction with phophorus derivatives see:a)[END_REF] which usually involve the formal insertion of alkyne or alkene into both ortho-aryl C-H and heteroatom-hydrogen bonds, will not be discussed here.

Hydroarylation of alkynes using phosphine oxides derivatives

In 2012, during their investigations on iridium-catalysed C-H bond alkenylation and alkylation of ferrocenes bearing different directing groups (e.g., 2-pyridine or imines), Shibata and co-workers reported the first example of the use of phosphine oxide as directing group for C-H bond activation/functionalisation (Scheme 1).

12

The C-H bond alkenylation of ferrocene derivatives was performed using 10 mol% [Ir(cod) 2 ]BARF [BARF = tetrakis(3,5trifluoromethylphenyl)borate] with 2 equivalents of diphenylacetylene in chlorobenzene at 135 ºC. Diphenylphosphine oxide is used as a directing group to form planar chiral ferrocenyl phosphine oxide, albeit in a low yield. But it is shown that C-H bonds on the ferrocene unit are more reactive that those of the phenyl group.

Scheme 1. Ir-catalysed ortho-alkenylation of ferrocenylphosphine oxide

In 2013, Glorius and co-workers reported the orthoalkenylation of aryl phosphonates and phosphonamides (Scheme 2). 10c [Cp*RhCl 2 ] 2 with AgSbF 6 in combination with 10 mol % Cu(OAc) 2 and 1.0 equiv PivOH as an additive at 110 ºC exhibited good catalytic activity with high stereoselectivity for the Ealkenylation of aryl phosphonamides; whereas, the aryl phosphonates are alkenylated in a poor E/Z ratio.

Scheme 2. Rh(III)-catalysed ortho-alkenylation of aryl phosphonates and phosphonamides with alkynes

In 2014, Satoh, Miura and co-workers demonstrated that phenylphosphine sulphides can also undergo proximal catalytic alkenylation with alkynes using Cp*Rh(III) under redox-neutral conditions but with acetic acid as proton source. 13 The reaction involves C-H bond cleavage directed by the P=S group to produce ortho-alkenylated phenylphosphine sulphides in moderate yields (Scheme 3). It is now also possible to perform the alkyne insertion into C-H bonds using phosphine oxide as formal directing group with ruthenium(II) catalysis (Scheme 4). 14 Tri-, di-, and monoarylphosphine oxides are alkenylated using [RuCl 2 (pcymene)] 2 catalyst in concert with AgSbF 6 as chloro abstractor in the presence of acetic acid. The reaction tolerates a wide range of substituents on both arylphosphine oxides and internal alkynes.

It is important to note that when unsymmetrical alkyne was used the reaction is completely stereo-and regioselective. The authors proposed a reaction pathway involving ortho-metallation -in which P=O acts as directing-followed by alkyne insertion into the 5 membered metallacycle and then in the presence of AcOH the protonolysis of Ru-C bond occurred with the regeneration of Ru(II) catalyst (see Figure 1).

Alkenylation of arylphosphine oxides with alkenes

In 2013, Satoh, Miura and co-workers described the orthoalkenylation of arylphosphine oxides with activated alkenes (Scheme 5). 15 In the presence of large excess of acrylates, mono arylphosphine oxides are ortho-alkenylated in moderate yields using [Cp*Rh(MeCN) 3 ][SbF 6 ] 2 /4AgOAc as catalytic system at 120 ºC. Interestingly, they succeeded in the latestage modification of Cy-JohnPhos oxide (dicyclohexyl(biphenyl-2-yl)-phosphine oxide) allowing the formation of ortho-alkylated product in 57% yield. In the case of sterically less hindered dimethyl(phenyl)phosphine oxide, the authors observed the dialkenylation. It is important to note that in this reaction the silver salt seem to act as an oxidant.

Scheme 5. Rh-catalysed ortho-alkenylation of arylphosphine oxides with alkenes

As demonstrated by Lee and co-workers, it is now possible to achieve the mono-alkenylation at ortho-position of P=O of dimethyl(phenyl)phosphine oxide with acrylate in 63% yield using [Cp*RhCl 2 ] 2 / 4 AgSbF 6 in combination with 1 equivalent of Cu(OAc) 2 and 1.0 equivalent Ag 2 CO 3 as an oxidant and base (Scheme 6). 16 However, again 19% of dialkenylation product was produced. They generalized the reaction to substituted dimethyl(phenyl)phosphine oxides in better yields. The reaction is also operative with styrene derivatives, provided that the reaction is conducted in absence of Cu(OAc) 2 and in THF. Phenylphosphine sulphides were also employed by Satoh, Miura and co-workers in Cp*Rh(III)-catalysed orthoalkenylation with activated alkenes (Scheme 7). The optimized reaction conditions are found using [Cp*Rh(MeCN) 3 ][SbF 6 ] 2 as catalyst in the presence of Cu(OAc) 2 as oxidant and base in diglyme at 120 ºC. The authors found that the addition of 1,2,3,4,5-pentaphenyl-1,3-cyclopentadiene (C 5 HPh 5 ) slightly improved the yield into the desired functionalized phenylphosphine sulphides, although its exact role is not clear. The ortho-alkenylated phenylphosphine sulphides are obtained in moderate yields with contamination by trace amounts of the Z-isomers. The authors did not observe a major effect of the substituent on the phenyl group. However, they did not demonstrate if the reaction with di-or triarylphosphine sulphides as substrates is also operative. Scheme 7. Rh-catalysed ortho-alkenylation of phenylphosphine sulphides with alkenes. E/Z ratio determined by [START_REF]C-H Bond Activation and Catalytic Functionalization[END_REF] H NMR.

In 2013, Glorious and co-workers also reported the C-H bond alkenylation of aryl phosphonates with acrylates in good yields using the pregenerated cationic rhodium species [Cp*Rh(MeCN) 3 ][SbF 6 ] 2 in the presence of stoichiometric amount of Cu(OAc) 2 as oxidant in DCE under air (Scheme 8). 10c

The reaction is not sensitive to the electronic and steric factors on the aryl group of phosphonates.

This orthofunctionalisation can also be performed with aryl phosphonamides. Scheme 8. Rh(III)-catalysed ortho-alkenylation of aryl phosphonates and phosphonamides with acrylate At the same time, Kim and co-workers reported the similar Rh(III)-catalysed ortho-alkenylation of aryl phosphonates and phosphonamides (Scheme 9). 17 The reaction conditions are analogous, albeit they employed only catalytic amount of Cu(OAc) 2 under air. While it is not mentioned by the authors, we believe that the trace of oxygen plays the role of the terminal oxidant for such oxidative couplings. The reaction is not limited to acrylates as vinyl sulfone and styrenes have been also successfully coupled with aryl phosphonates. During their investigation on annulation reaction between phosphonamide and activated alkenes or alkynes, Lee and coworkers also demonstrated that phosphonamide can act as a directing group in Rh(III)-catalysed ortho-alkenylation with styrenes even in the presence of TEMPO (Scheme 10). 18 The reaction was found to be sluggish with non-activated alkenes and only mono-alkenylation occurred even in the presence of access amount of styrene derivatives. In 2017, our group reported the first example of orthoalkenylation with alkenes of triarylphosphine oxides using Ru(II) catalyst (Scheme 11).

19

We employed [RuCl 2 (pcymene)] 2 in the presence of NaHCO 3 as base and Cu(OAc) 2 as oxidant in dichloroethane at 120 ºC. Diversely substituted triarylphosphine oxides are mono-alkylated at ortho-position in moderate yields with acrylates. An excess amount of triarylphosphine oxides is required to prevent di or trialkenylation side-reaction. Completed alkenylation or alkylation can now be controlled. 19 As shown in the Figure 1, we explained the key role of the reaction pH and oxidant on the selectivity of the reaction, namely, alkenylation under basic media versus alkylation in acidic conditions (Scheme 18). Activated alkenes such as acrylate or vinylsulfone displayed high reactivities, while with styrene double amount of catalyst loading is required. A wide variety of substituents on the biphenylphosphine oxides are tolerated but when there is no substituent at the ortho' position dialkenylation reaction occurred. Under these conditions, triphenylphosphine oxide is not reactive.

It is noteworthy that the authors also showed that using commercial chiral binaphthyl-based diphenylphosphine oxide as the substrate, they are able to synthesize different chiral alkene-phosphine hybrid oxide compounds without epimerization. The authors reasonably postulate that thus reaction involved 7-membered cyclopalladium intermediate.

Pd(OAc)2 (5 mol%) Ac-Gly-OH (10 mol%) O P R 2 R 1 AgOAc (3 equiv.) CF 3 CH 2OH, 100 ºC (3 equiv.) R 4 + R 3 O P R 2 R 1 R 3 R 4 O P Ph Ph R 87% 86% 75% 32% [a] 58% [b] R = CO 2 Et CO 2 Bn P(O)(OEt)2 SO 2 Ph Ph O P Ph Ph EtO 2 C 73% (m:d = 1.67) CO 2 Et O P Ph Ph EtO 2 C R R = Cl OMe F 62% 54% 67%
67% [b] (>99% ee)

O P Ph Ph CO 2 Et O P R 2 R 1
Pd OAc The same group has synthetized chiral biaryl phosphates using Pd(II)-catalysed asymmetric ortho'-direct alkenylation through dynamic kinetic resolution (Scheme 13).

21

From axially racemic biphenylphosphine oxides, bearing enantiopure menthyl phenylphosphate group as directing group, similar reaction conditions, excepted the presence of Cu(OAc) 2 as oxidant, allowed the formation of alkenylated products containing both axially chirality and the previous P stereogenic center. A wide substrate scope is demonstrated and the reaction occurred in all cases with a good diastereomeric ratio (dr>95:5). (2'-methyl-[1,1'-biphenyl]-2yl)diphenylphosphine oxide and ethyl acrylate in the presence of 5 mol% Pd(OAc) 2 and 3 equivalents of AgOAc, (protected)amino acids are evaluated toward enantioselective C-H alkenylation (Scheme 14). They demonstrated that the unprotected amino acids such as L-Val-OH inhibited the reaction, whereas monoprotected amino acids such as Ac-L-Val-OH, Boc-L-Pro-OH, Boc-L-Phe-OH, and others allowed the formation of phosphine oxide-olefin ligand in good yield with moderate ee. The best result is obtained using Boc-L-Val-OH as the ligand and the corresponding chiral biaryl phosphine-olefin hybrid ligands is isolated in 90% yield with 87% ee. An electronwithdrawing substituent at ortho'-position of the biphenylphosphine oxide gave lower yield without affecting the ee value, whereas electron-donating group improved the yield (Scheme 15). 22 The authors have confirmed the absolute configuration of the (S) products by single-crystal X-ray crystallography, which also explains why the transition state of the (S)-Pd complex is more favourable than the (R)-Pd complex in the C-H activation step. They have also used the racemic binaphthyl diphenyl oxide as a substrate to examine this enantioselective C-H alkenylation reaction and they obtained (S) phosphine-olefin ligand in 62% yield with 88% ee, which could be improved to 99% ee by simple recrystallisation (Scheme 16).

Pd(OAc)2 (5 mol%) Boc-L-Vol-OH (10 mol%) O PPh 2 AgOAc (12 equiv.) CCF 3 CH 2 OH/DME (1;1) 60 ºC (3 equiv.) R 3 + R 1 O PPh 2 R 1 R 3 R 2 R 2 O PPh 2 Me R 3 R 3 = CO 2 Me CO 2 t-

Alkylation of arylphosphine oxides with alkenes

During their investigation on ortho-alkenylation of arylphosphine oxides with alkynes using Ru(II) catalysis, Satoh, Miura et al. described only one example of direct alkylation by replacing the alkyne partners with n-butyl acrylate (Scheme 18). 14 In the presence of 1-adamantanecarboxylic acid, 33% of alkylated product is obtained in dioxane and 50% in diglyme. However, the scope of this transformation was not investigated. After production of ortho-alkylated of triarylphosphine oxides with acrylate, the hydrolysis of ester followed by the reduction of phosphine oxide using phenylsilane afforded the synthesis of novel bifunctional phosphine containing a pendent carboxylic group as hybrid ligands (Scheme 20). In acidic media -and due to the coordination ability of the P=O bond, avoiding classical β-H elimination-proto-demetallation of D affords the linear alkylation product F and regenerates the salt Ru 2+ . Thus, for alkylation the use of oxidant [Cu(OAc)2] is no longer required. Similarly, the complex E leads to the hydroarylation product G, via the protonation of the M-C bond in the presence of proton sources. On the other hand, under basic conditions the intermediate D undergoes a base-assisted β-H deprotonation to form the alkenylation product F. This deprotonation in basic medium generates M 0 species and an oxidant such as Cu(OAc)2 is required to regenerated the active species Ru 2+ .

Arylation of arylphosphine oxides

The first arylation of a biphenyl moiety directed by a R 2 P=O group was reported in 2014 by Yang's group using diaryliodonium trifluoromethyl sulphonate with the catalytic system PdCl 2 /Ag 2 CO 3 /PivOH in DMF under air atmosphere (Scheme 21). 24 A variety of dialkylP=O and Ph 2 P=O directing groups allowed the C-H bond phenylation at the ortho'-position, even with several substituents at meta' and para' positions. Actually, when diisopropylphenyl-phosphine oxide (thus without biphenyl group) is used, the phenylation of C-H bond did not occur, suggesting that the formation of the 5membered cyclometallated intermediate does not take place with Pd(II) catalyst and that the regioselective formation of the 7-membered cyclometallate is rather favoured. The monoarylations of a variety of biphenyl dialkyl phosphine oxides with (mesityl)(aryl)I + TfO -are easily performed. The prepared polyaromatic phosphine oxides could be transformed into phosphorous(III) compounds by classical reduction with trichlorosilane in the presence of NEt 3 . Thus, the phenyl phosphine oxides and 1,4-epoxydihydro naphtalene with the catalyst [Cp*Rh(MeCN) 3 (SbF 6 ) 2 ] and AgOAc in DCE at 120 °C form ortho-naphtyl phenylphosphine oxides. With the less bulky Me 2 P=O group and with an excess of bicyclic alkene, the bis ortho-naphtylation could be achieved.

(2-Thiophenyl) 3 P=O derivative leads to the naphtylation of one thiophenyl group at the C2 position. The naphtylation of 5-phenyldibenzophosphole oxide occurred on the phenyl moiety, not on the dibenzophosphole ring, probably because of the easier formation of the 5-membered metallacycle intermediate. The sulphide (Cy) 2 (Ph)P=S could not be arylated under standard conditions, but by using 3 equiv of AcOH instead of AgOAc and the reaction led to 39% of arylated product. The reaction of phenylphosphonamides with the same bicyclic alkene gave the hydroarylation without the formation of biaryl product as well as with N-Boc bridged similar bicyclic alkene.

The biarylphosphine oxides are converted into phosphine derivatives by reduction with trichlorosilane. The reaction operates with Cp*Rh(III) / 5 AgSbF 6 catalyst in the presence of stoichiometric amount Cu(OAc) 2 as oxidant. Only one example has been demonstrated with 1-bromo-3-chlorobenzene. The reaction occurred regioselectively at the meta-position of both halo substituents in a moderate yield.

Scheme 23. Rh(III)-catalysed dehydrogenative C-H bond arylation of aryl phosphonamide

In 2017 Shi et al. reported the mono and bis ortho'-arylation of biphenyl C-H bonds directed by a phosphorous(III) group instead of phosphine oxide, on reaction with aryl bromides or chlorides but catalysed by Rh(I) catalyst. The discovery came from the observation of the stoichiometric biphenyl C-H bond cleavage from the Rh(Cl)(CyJohnPhos)(COD) complex to form a 6-membered complex as intermediate, which led to arylation with PhBr in the presence of an excess of LiOtBu (Scheme 24).

Scheme 24. Stoichiometric C-H bond arylation of biphenyl phosphine

The catalytic biphenyl C-H bond arylation of phosphine was thus achieved by Shi's group using 2.5 mol% of [RhCl(COD)] 2 catalyst with PhBr or PhCl in the presence of 3 equiv. of LiOtBu at 110°C for 12h.

A variety of commercially available phosphines such as Davephos or JohnPhos are arylated in high yields (Scheme 25). 26 The ortho'-diarylation of commercial phosphines, directed by PCy 2 , PPh 2 but not P(tBu) 2 group are achieved using 2.4 equiv. of phenylbromide but at 140°C for 36 h in excellent yields (Scheme 25). The use of phosphine containing Rh(I) catalyst such as RhCl(PPh 3 ) 3 displayed a lower reactivity. A few examples of successive ortho'-arylations have been performed allowing the formation of mixed diarylated of the biphenyl phosphines. The direct transformation, via this arylation reaction, of chiral phosphines without racemization is achieved such as for the transformation of (R)-H-MOP and of the Tang's P-stereogenic phophine (Scheme 25). These new built chiral phosphines are evaluated in the enantioselective Pd-catalysed arylation of Me 2 CHCO 2 R and of the Rh-catalysed arylation of imines with arylstannate for which the Ar-MOP ligand A allowed to reach 76% yield of product with 98% enantioselectivity. Whereas Pd(II) catalysts favoured biphenyl C-H bond activation of biphenyl phosphine oxides 24 and Rh(I) catalysed the biphenyl C-H functionalisation of phosphines, 26 it was shown in 2017 by Yang et al. 27 that Ir(III) catalysts allow the ortho phenyl C-H bond functionalisation of phosphine oxides as it was also shown with Rh(III) catalysts by Miura. 25 Yang reported that ortho C-H bond arylation of phosphine oxides could be achieved with the catalytic system [Cp*IrCl 2 ] 2 / 4 AgSbF 6 with 2,4,6-trimethylbenzoic acid (TMBzOH) additive, which allows the ortho C-H bond deprotonation to form a 5membered metallacycle. The later in the presence of a diazo compound allows the carbene insertion into the C-Ir bond and tautomerization to form 2'-phosphorylbiphenyl-2-ols (Figure 3). The temporary protection of the indole nitrogen with a phosphinoyl group t-Bu 2 P=O has been used by Shi for the regioselective arylation of indole at C7 position.

28

The Pd(OAc) 2 catalyst in the presence of 2-chloro pyridine promotes the selective C7-H bond activation, via the preferred 6-membered cyclometallate formation, followed by its reaction with a variety of aryl boronic acids. However, the reaction requires 2 equiv of Ag 2 O plus CuO/Cu(OTf) 2 (Scheme 27). 28 The phosphinoyl group can be removed using LiAlH 4 reduction to give 7-arylindole. The N-phosphinoyl indole can also be alkenylated at C7 position with acrylate and Pd(OPiv) 2 /functional pyridine.

Scheme 27. Palladium(II)-catalysed C7-H arylation of indoles directed by R2P=O group

The same group took profit of the same N-P(=O)tBu 2 directing group to arylate the indole at C6 position using copper catalysis.

The arylation reagents are Ph 2 I(OTf) or Aryl(Mesityl)I(OTf) but the C-6 regioselectivity was obtained using CuO as catalyst at 80°C (Scheme 28). For the arylations described in Scheme 29-31 a similar plausible mechanism can be proposed based on initial coordination of the P=O oxygen atom to the Pd(II) species which promotes the ortho C-H bond deprotonation to give the 6-membered metallacycle A. The oxidative addition of Ar+ generates a Pd(IV) intermediate B which on reductive elimination gives the ortho arylated product (Scheme 32).

30-32

Although these reactions take place under rather mild conditions due to the easy formation of a 6-membered cyclometallate intermediate, they do not offer the direct access to arylated phosphine derivatives such as benzyl and biphenyl phosphines or by reduction of functionalized phosphine oxides.

Ortho-amination of arylphosphine oxides

The amidation of arylphosphoryl C-H bonds has first been performed in 2014 by Chang et al. to create Pstereogenic centers. 33

O P R 1 R 2 R 2 NHSO 2 R 3 O S O 2 H N P Ph O Bn

Scheme 33. Asymmetric C-H amidation of phosphine oxide aryl C-H bonds

Better amidation diastereoselectivity is obtained when the chiral group linked to the phosphorous atom has a C 2symmetric pyrrolidine moiety (Scheme 34). 33 Thus, at 25°C or 60°C diastereoselectivities above 80% could be obtained. The diastereoselectivity is also dependent on the nature of the azide used. When one enantiomer was subjected to the standard conditions an enantiomeric aminated product was obtained with a similar de (-) 81% (Scheme 34) when compared to the starting enantiomer.

[Cp*IrCl2]2 (2.0 mol%) AgNTf2 (8.5 mol%) PivOH (12 mol%) + R 2 SO 2 N 3 P DCE (a) 60 ºC, 12 h (b) 25 ºC, 24 h To improve the enantio or diastereoselectivity in aryl C-H bond amidation the mechanism of the amidation of phosphine oxides with azide was studied by Chang. 34 They demonstrated that the addition of 12 mol % of pivalic acid tremendously improved the catalytic efficiency of the [IrCl 2 Cp*] 2 / 4 AgNTf 2 system. They also showed, by KIE experiments, that the C-H bond cleavage is involved in the turnover-limiting step. Then, they were able to isolate and characterize the 5-membered cyclometallate from stoichiometric reaction of triphenylphosphine oxide with Cp*Ir(III) and K 2 CO 3 as base. In the presence of TsN 3, such intermediate led to the insertion of the nitrene into the Ir-C bond to give the 6-membered cationic cyclometallate (Scheme 36). They also showed that the coordination of the carboxylate to the Ir(III) center favoured first the cyclometallation and that the formed carboxylic acid leads to the protodemetallation of the nitrogen containing metallacycle.

R 1 O R 1 N Ph Ph (R) (R) P R 1 O R 1 N Ph Ph (R) (R) NHSO 2 R 2 P O Ph N Ph Ph (R) (R) (R) 84%,

Scheme 36. Formation of Iradacycle intermediates from phosphine oxide

To show the influence of chiral acid on the enantioselective C-H amidation the prochiral alkydiphenylphospine oxides Ph 2 RP=O were amidated with TsN 3 in the presence of one diastereoisomer of O,O-dipivaloyl-L-tartaric acid. (4mol%). Very good yields were obtained >80% but with moderate enantioselectivities (ee : R= Me :11%, Benzyl: 19%, t-Bu: 27%, cyclohexyl: 32%). However this low asymmetric induction shows that the chiral acid additive is involved in the C-H bond cleavage and the formation of cyclometalate intermediate.

In 2017, Cramer et al. have reported an efficient enantioselective C-H bond amidation of phosphine oxides by taking profit of cooperative effects between a chiral carboxylic acid and a chiral Cpx-Iridium(III) catalyst. 35 They have operated with a chiral iridium(III) catalyst based on atropchiralcyclopentadienyl ligand of type [Cp x -IrI 2 ] 2 with AgNTf, with the cooperation of a chiral amino acid derivative such as (S)-phtaloyl tert-leucine, for the amidation of ortho phenyl C-H bonds of alkyl(Ph) 2 P=O with TaN 3 . They observed matched and mismatched effects of a variety of chiral carboxylic acids with the Ir(III) catalyst but they succeeded to obtain high yields of amidated phosphine oxides with high enantioselectivity (Scheme 37). Using several azides a large variety of amidated phosphine oxides are thus obtained with high yields > 95% and with high enantioselectivity up to 99:1 using (S)-phtaloyl tert-leucine as a chiral partner (Scheme 38). 35 The phosphine oxides were easily transformed into borane adducts of P-chiral phosphine.

Scheme 38. Examples of Ir(III) catalysed enantioselective amidation of phosphine oxides

Ortho-Cyanation of arylphosphine oxides

Ortho C-H bonds of arylphosphates have selectively been transformed into C-CN moieties using N-cyano-N-phenyl-p toluenesulfonamide as cyanation reagent with [RhCp*Cl 2 ] 2 (5 mol%), / AgSbF 6 (15 mol%) as catalytic system by Ding and coworkers as early as 2013 (Scheme 39). 36 Ortho cyanation took place on the aryl, naphtyl or 3-thienyl phosphonate in dichloroethane at 110°C with a variety of substituents. Dimethyl, diisopropyl arylphosphates and dimethylamino arylphosphate displayed similar reactivity allowing the formation of C-CN bond in good yields. From (S)-(-)-menthyl (2′methyl-[1,1′-biphenyl]-2-yl)(phenyl) phosphinate using PhI(OAc) 2 under 10 mol % Pd(OAc) 2 in CF 3 CH 2 OH at 100°C, ortho'-C-H acetoxylation occurred in 46% yield with excellent diastereomeric ratio (dr >95:5). Then, the authors have shown that the combination of the axially chiral substrates induced the kinetic resolution with C-H activation to construct synthetically useful axially chiral scaffolds. Indeed, when they used the axially chiral substrates, such as (R)-isopropyl (2′methyl-[1,1′-biphenyl]-2-yl)(phenyl)phosphine oxide and (R)-

Ortho-acylation of arylphosphine oxides

In addition to C-H bond alkenylation (Scheme 13) and hydroxylation (Scheme 43), Yang and co-workers also reported the kinetic resolution of (R)-alkyl (2′-methyl-[1,1′-biphenyl]-2yl)-(phenyl)phosphine oxides through C-H acylation (Scheme 44). 21 The reactions are carried out in the presence of 10 mol% palladium catalyst associated to 4 equivalents of peroxide such as TBHP (tert-Butyl hydroperoxide) and displayed the highest diastereoselectivity and good yields for both of product A and recovered starting materials B when the phosphine oxide bears a bulky substituent (t-Bu or i-Pr). In 2014 Clark et al. 39 reported the direct iridium(I)-catalyzed borylation of benzyl phosphines without initial formation of phosphine oxide as the ortho C-H bond activation in that case involves a 5-Membered cyclometallate intermediate. The reaction is catalyzed by [Ir(OMe)(cod)] 2 in the presence of B 2 pin 2 The resulting borylated phosphines are isolated as their borane complex (Scheme 45a)). 39 Mono ortho borylation is selectively performed.

As expected triphenylphosphine was unreactive due to the difficulty to generate a 4-membered intermediate. By contrast the biphenyl group of 2-dicyclohexylphosphino) biphenyl could be directly mono borylated (Scheme 45b)

.39

. At 150°C for 3 days this later phosphine led to 97% conversion with a 40/60 ratio of mono/diborylated biphenyl.

Conclusion

Recent advances in transition metal-catalysed C-H bond activation/functionalisation of organophosphorus compounds have greatly impacted synthetic strategies and design of new phosphine ligands. In most of the cases, phosphorus (V) derivatives (i.e., with P=O or P=S bond) have been employed to direct the ortho C-H bond cleavage via the formation of 5membered cyclometallated species with, Rh(III) and Ru(II) catalysis, allowing their functionalisations into of mostly C-C, but also C-N or C-O bonds. In contrast, with palladium catalysis, the formation of 5-membered cyclometallated species seems not be favoured. When biphenyl phosphine oxide is employed, the functionalisation occurred at the ortho'-position, suggesting the formation of 7-membered palladacycle. One of the most straightforward strategy to modify phosphine without protection/deprotection sequence is to employ P(III) as directing group. However, such strategy has been only succesful in the case of ortho' C-H bond arylation or borylation of biphenyl phosphines with rhodium(I) or iridium(I) catalysts, and with bulky phosphines only. These observations open new ways in the late-stage modification of ligands. Several optically active phosphines and diphosphines have been shown to be very useful when incorporated in metal-catalysts for enantioselective transformations. The door is now opened for late-stage C-H bond modification of their phosphine oxide or sulphide derivatives to quickly produce new active chiral functional phosphines.

13 Scheme 3 .

 133 Scheme 3. Rh-catalysed ortho-alkenylation of phenylphosphine sulphides with alkynes

Scheme 4 .

 4 Scheme 4. Ru-catalysed ortho-alkenylation of arylphosphine oxides with alkynes

Scheme 6 .

 6 Scheme 6. Rh-catalysed mono ortho-alkenylation of dimethyl(phenyl)phosphine oxide with alkenes [a] Reaction performed in absence of Cu(OAc)2 in THF insetad of 1,4dioxane.

Scheme 9 .

 9 Scheme 9. Rh(III)-catalysed ortho-alkenylation of aryl phosphonates and phosphonamides with acrylate.

Scheme 10 .

 10 Scheme 10. Rh(III)-catalysed ortho-alkenylation of aryl phosphonamide with alkenes

20

 20

via 7 -

 7 Scheme 12. Pd-Catalysed alkenylation of biphenylphosphine oxides. [a] Using 10 mol% of Pd(OAc)2. [b] Using 10 mol% of Pd(OAc)2, 20 mol% of Ac-Gly-OH and 5 equiv. of AgOAc.

5 )

 5 Scheme 13. Pd-Catalysed asymmetric alkenylation of biphenylphosphine oxides

22

 22

From

Scheme 20 . 1 .

 201 Scheme 20. Preparation of bifunctional phosphine-carboxylate ligands

Figure 1 .

 1 Figure 1. Unified mechanism for C-H bond functionalisation of phosphine oxides with alkynes, and alkenes.

Scheme 21 .

 21 Scheme 21. Regioselective monophenylation of various biphenyl C-H bonds directed by R2P=O groups. [a] PdCl2 (20 mol %), Ag2CO3 (0.4 equiv) were used.

25

 25

Scheme 22

 22 Scheme 22 Reaction of arylphosphine oxide derivatives with 1,4-epoxydihydro naphthalene. [a] 2 equivalents of 1,4-epoxydihydro naphtalene is used. [b] 3 Equiv. of AcOH was used instead of AgOAc.

Figure 2 .

 2 Figure 2. Mechanism for the catalysed naphtylation of arylphosphine oxides with 1,4epoxydihydro naphthalene

  10c

Scheme 25 .

 25 Scheme 25. Rodium(I)-catalysed arylation of commercially available phosphines. [a] Using 2.4 equiv. of ArBr; [b] Using 2.4 equiv. of ArCl, [c] using 10 equiv of ArBr.

Figure 3 .

 3 Figure 3. Proposed mechanism for the Ir(III) catalysed activation of arylphosphine oxides towards diazo compounds

Figure 4 .

 4 Figure 4. Proposed mechanism for Cu-catalysed C6-H arylation of indoles

Scheme 37 .

 37 Scheme 37. Examples of matched and mismatched effects of chiral carboxylic acids on enantioselective amidation of phosphine oxides.

Scheme 39 .

 39 Scheme 39. Rhodium(III) catalysed ortho-cyanation of arylphosphates

36 Figure 5 .

 365 Figure 5. Proposed mechanism for the ortho-cyanation of arylphosphates

  Scheme 44. Kinetic resolution (R)-alkyl (2′-methyl-[1,1′-biphenyl]-2-yl)-(phenyl)phosphine oxides through Pd-catalysed C-H acylation

  Scheme 45. Ortho borylation of benzylic and biphenyl phosphines The proposed mechanism involves the formation from [Ir(OMe)(cod)] 2 of an Ir(III) cyclometallated intermediate A giving B on B 2 pin 2 addition, followed by C-B bond formation leading to C. Decoordination of borylated phosphine and addition of benzylphosphine regenate the intermediate A. (Scheme 45c) .39.

  Scheme 18. Ru-catalysed ortho-alkylation of triphenylphosphine oxide with acrylates In 2017, as carboxylic acids are often a catalyst partner for C-H bond functionalisations, our group investigated this alkylation reaction for the straightforward synthesis of bifunctional phosphine bearing a carboxylate function (Scheme 19).

	19 We
	found that the [RuCl 2 (p-cymene)] 2 / 4 AgSbF 6 catalyst
	associated with simple AcOH in dichloroethane is able to
	promote the formation of ruthenacycle -via C-H bond
	activation at ortho-position of the P=O group-followed by
	alkene insertion and protonolysis of Ru-C bond to afford the
	ortho-alkylated arylphosphine oxide in good yields (Figure 1).
	To obtain a monoalkylation of one ortho C-H bond 2
	equivalents of phosphine oxide need to be used. It is
	demonstrated that the alkylation reaction takes place without
	oxidant such as Cu(OAc) 2 . Several substituted triarylphosphine
	oxides are ortho-alkylated in good yields with n-butylacrylate.
	In addition, desymmetrization of alkyl diarylphosphine oxides
	and methyl diphenylphosphinate was achieved using this late-
	stage alkylation leading to chiral phosphine oxides containing
	three different substituents. The reaction has been also
	extended to other activated alkenes.

Scheme 19. Ru-catalysed ortho-alkylation of arylphosphine oxides with alkenes

  In 2013 were reported several examples of arylation of ortho C-H bonds of aryl groups directed by a phosphate -O-P=O(OR) 2 or a phosphoamidate -NR-P=O(OR) 2 directing group. Thus the presence of a heteroatom between the P=O and aryl groups allows the easy formation of a 6-membered cyclometallate. However the directing groups do not give a fast access to phosphine derivatives as phosphine oxides do. Thus Moon Finally Kim and Lee performed the ortho arylation of arenes directed by a phosphoramide group with Pd(OAc) 2 . catalyst with CuO and TfOH but at room temperature with Ph 2 I(OTf) or Ar 2 I(OTf (Scheme 31)

	FG	H N R	P(OEt)2 O	+	(1.2 equiv.) Ph 2 IOTf	Pd(OAc)2 (5-10 mol %) CuO (3.0 equiv.) TfOH (20 mol %) 1,4-dioxane, 25 ºC	FG	R N Ph	P(OEt)2 O	R	O H	+ OEt OEt P O	(2 equiv.) Ph 2 IOTf	Pd(OTf)2 .2H 2 O (10 mol %) Na 2 CO 3 (1.0 equiv) 1,2-dichloroethane (0.25 M) T ºC, 1 h, Ar	R	O Ph	P O OEt OEt
		Ph	O		MeO		Ph	O		Br	Ph	O		O	P(OEt)2 O	Ph	O	P(OEt)2 O	O	O	O	P(OEt)2 O
		N H			P(OEt)2		MeO		N H	P(OEt)2			H N		P(OEt)2		Ph			Ph	Ph
	84%, 3 h					82%, 3 h		Cl		75%, 7 h				80 ºC, 64%			80 ºC, 30%	80 ºC, 67%
		Ph	O							Ph	O
		N			P(OEt)2		Et	N H	P(OEt)2 O	Br	N	P(OEt)2	MeO	OMe	O	P(OEt)2 O	MeO	OMe	O	P(OEt)2 O	MeO	OMe	O	P(OEt)2 O
	75%, 5 h					53%, 16 h				71%, 6 h									t-Bu
	Scheme 31. . Arylation of N-aryl Phosphoramidates.
	Ar	O						Pd II X 2							O	80 ºC, 61%			60 ºC, 73%	60 ºC, 72%
	E	P(OR)2								E		P(OR)2	Scheme 29. . Pd(II)-catalyzed arylation of arylphosphates with diaryliodonium triflates
																a) Arylphosphates
	30 Kim profit of the Pd(II) catalysed ortho C-H bond activation to perform 31 and Kim and Lee HX 32 took C-H bond arylation using Ar 2 I(OTf) reagents. Moon et al O X Pd IV X Ar OTf Pd II O E P(OR)2 P(OR)2 E A B	FG	O	+ OPO(OEt)2 P OR O OR Ph	Ph 2 IOTf (2 equiv.)	Pd(TFA)2 (10 mol %) DCE, 110 ºC, 15 h O Ph P O O O	FG Cl	O Ph	O Ph O P O P O O	OR OR
							Ar-I			Ar 2 IOTf						55%				65%	29%
																b) Monophosphoric acid
	Scheme 32. . Plausible mechanism for E= O, NH, NR.						FG	O	P OH O	OMe	+	(2 equiv.) Ph 2 IOTf	i) Pd(TFA)2 (10 mol %) DCE, 80 ºC, 15 h MeOH, rt, 30 min ii) TMSCHN 2 (5 equiv)	FG	Ph OPO(OMe)2
																t Bu					OPO(OMe)2
																	OPO(OMe)2	Ph	Br	OPO(OMe)2
																	Ph				Ph
																83%					90%	56%
																Scheme 30. . Arylation of arylphosphates and monophosphoric acids

30

used the Pd(OTf) 2 catalyst for ortho arylation of arylphosphates with both Ph 2 I(OTf) and Ar 2 I(OTf) reagents at moderate temperature (Scheme 29)..

30

Kim et al. used Pd(O 2 CCF 3 ) 2 catalyst for ortho phenylation of arylphosphonates and of aryl hydrogen phosphates with Ph 2 I(OTf) (Scheme 30) 31 .

  Profit of the P=O directing group was taken, using an efficient Ir(III) catalyst [Cp*IrCl 2 ] 2 . Starting from Ph 2 (R*)P=O derivative containing one chiral R* group, they are able to differentiate two phenyl ortho C-H bonds. It occurred via the expected formation of a 5-membered metallocycle, arising from C-H bond deprotonation, and followed by the amidation of the C-Ir bond with ptoluenesulfonyl azide. The catalytic amidation of several phosphine oxides containing a chiral group arising from chiral alcohol, such as menthol, led to good yield in the formation of aminated products with good diastereoselectivity. The nature of substituents on the phenyl groups has a strong influence on the diastereoselectivity (with chiral -2-phenyl cyclohaxanol : de = 19% for o-MeO-, 44% for p-MeO, 36% for o-Me, of 49% for Cl) (Scheme 33).

					33
	R 2	R 1	P	O	+ N 3	SO 2 R 3		[Cp*IrCl2]2 (2 mol %) AgNTf2 (8.5 mol %) PivOH (12 mol %)
				R 2	(1.1 equiv)		DCE, 85 ºC, 4 h
									Ph O	P	O	Ph	O Ph	P	O
											NHTs	NHTs
		75% (de = 4%)				68% (de = 24%)	62% (de = 41 %, after 12h)
				MeO
				Ph	O	P	O	NHTs		Ph	O	P	O	NHTs
					84% (de = 44%) OMe			51% (de = 36%)

  Catalytic ortho C-H bond amidation of a variety of triaryl and alkyldiaryl phosphine oxides were also performed at 85°C for 4 h with TsN 3 (1.1 equiv.) and led to high yield of monoamidation product (Scheme 35).

									33
	R 2	R 1	P	O	R 2	+	TsN 3 (1.1 equiv)	[Cp*IrCl2]2 (2 mol%) AgNTf2 (8.5 mol%) PivOH (12 mol%) DCE, 85 ºC, 4 h	R 2	R 1	P	O	R 2 NHTs
	R
				P	O	NHTs	P	O	NHTs		P	O
	R							(R= Me; 75%) (R= H; 84%)						NHTs
								(R= OMe; 84%)
							R	(R= F; 86%) (R= Cl; 87%)	89%			83%
	Scheme 35. Non chiral amidation of triarylphosphine oxides.
														(1.1 equiv)
														NHTs	TsHN	Me	NHTs	Me
														(S) Ph	P O	P O
														Ph	N	Ph	Ph	N	Ph
														(S)	(S)
														80% de (a) 87%, 84% de (b)	enantiomer 84%, (-)81% de (a)	83%, 82% de (a)
														OMe
														O 2 S	Ph	O 2 S	Ph
														O 2 S	NH	MeO	NH	OMe
														NH	O	O
														Ph	O P N	Ph	Ph	Ph	P N	Ph	Ph	P N	Ph
														83%, 80% de (a)	92%, 89% de (a) 93%, 90% de (b)	88%, 86% de (a) 92%, 89% de (b)
														Scheme 34. Asymmetric amidation of phosphinic amides

7. Ortho-Acetoxylation of arylphosphine oxides

  In 2013, Kim et al. have shown that organophosphate group could direct the ortho C-H bond activation of the phenyl group in benzylic phosphonic acid for acetoxylation (Scheme 40). Scheme 42). The Renantiomer OAc group was transformed in 3 steps in the OMe group and thus the MeO-MOP ligand was obtained in 77% yield and 99%ee.

	Scheme 41. Ortho acetoxylation of aryl C-H bonds with monophosphoric acid directing
	group.
	In 2014 Yang et al. succeeded the acetoxylation with PhI(OAc) 2
	of the ortho sp 2 C-H bond of a biphenyl group directed by a
	R 2 P=O group in the presence of Pd(OAc) 2 catalyst in CF 3 CH 2 OH
	(Scheme 42). 38 The reaction proceeds via the formation of a 7-
	membered intermediate via C-H bond deprotonation.
	Diacetoxylation of ortho C-H bonds could be performed. More
	importantly the acetoxylation of each R-and S-optically pure
	2-diphenylphosphine oxide-1,1'-binaphtyl leads to high yield
	(75%) and ee (99%) (Pd(OAc)2 (10 mol %) Ar 1	Ar 1
	Ar 2	H P(O)Ph2	PhI(OAc)2 (3.0 equiv) CF 3 CH 2OH, 100 ºC, Air	Ar 2	OAc P(O)Ph2	37 catalyst in dichloroethane at 110°C led to the acetoxylation of Their reaction with PhI(OAc) 2 in the presence of Pd(OAc) 2
						ortho C-H bonds selectively. The crude product is methylated
			CF 3			using TMS-diazomethane to generate methyl phosphonate
						esters in good yields. The hydroxyl group on the phosphorous
	P(O)Ph2 OAc	OAc P(O)Ph2	R	P(O)Ph2 OAc	is thought to favour the ortho cyclometallation via deprotonation of the C-H bond with Pd(II)-OAc to lead the
				MeO		formation of a 6-membered intermediate. The resulting
				R=H, 53%	Pd(II)-C bond then reacts with PhI(OAc) 2 to give the C-OAc
	68%		65%	R=OAc, 43%	bond.
	P(O)Ph2	P(O)Ph2		P(O)Ph2
	OAc		OAc		OAc
	73%		(R) 75% (99% ee)	(S) 75% (99% ee)
	Scheme 42. Pd-catalyzed Acetoxylation of ortho biphenyl C-H bonds directed by R2P=O
	In 2015, Yang and co-worker reported the asymmetric C-H
	bond hydroxylation /kinetic dynamic resolution of chiral
	phosphine oxides (Scheme 43).			Scheme 40. Ortho acetoxylation of benzylic C-H bonds with monophosphoric acid
						directing group.
						Similarly, the ortho C-H bond of phenylphosphoric monoacids
						is easily acetoxylated in dioxane under mild conditions at 80°C
						and tolerated a variety of substituents on the phenyl group.

The products are obtained after OH methylation with TMSdiazomethane. When a large excess of PhI(OAc) 2 is used the ortho diacetoxylation could be achieved (Scheme 41). 21

Scheme 28. Copper-catalysed C6-H arylation of indoles directed by tBu2P=O groupThe proposed mechanism suggests a formal regioselective insertion of the C6=C7 double bond in the Ph-Cu(III) bond of intermediate (Figure4).29

Acknowledgements

We acknowledge the China Scholarship Council (CSC) for a grant to ZZ.

Conflicts of interest

There are no conflicts to declare.


Tahar Zadi

Aymen Amin Assadi
email: aymen.assadi@ensc-rennes.fr

Noureddine Nasrallah

Rachida Bouallouche

Phuong Nguyen Tri

Abdelkrim Bouzaza

Mohamed Miftah

Rachida Maachi

Dominique Wolbert

Treatment of hospital indoor air by a hybrid system of combined plasma with photocatalysis: case of trichloromethane

Keywords: plasma DBD/photocatalysis process, cylindrical reactor, Chloroform removal, synergetic effect

The purpose of this study is to evaluate the efficiency of non-thermal plasma and heterogeneous photocatalysis processes for indoor air treatment using cylindrical continuous reactor at pilot scale and high flow rates. Trichloromethane (CHCl 3 ) also called chloroform was chosen as a model pollutant representing hospital indoor air. This pollutant is considered as carcinogenic, mutagenic and reprotoxic agent. The effect of several parameters such as inlet pollutant concentrations (25-300 mg m -3 ), flow rates (2 to 8 m 3 h -1

), relative humidity of the effluent (5, 30, 50 and 90 %) as well as input of the plasma discharge (9 to 21 kV) on the photodegradation of trichloromethane is investigated.

Our findings show that the increase of flow rate leads to a reduction of degradation efficiency, while the humidity promotes the degradation in the case of photocatalysis process due to the formation of

Moreover, the addition of a photocatalyst under UV radiation in the discharge zone enhances the reduction of ozone and CO gases compared to plasma process alone.

The combination of plasma DBD and photocatalysis enhances the removal efficiency with a synergetic effect, leading to removal efficiency higher than 10% if we consider the sum of the contribution of each process separately.

INTRODUCTION

Emissions of volatile organic compounds (VOCs) into atmosphere are detrimental to both human health and the global environment [START_REF]Pollutions olfactives : origine, législation, analyse, traitement[END_REF]. In recent years, plasma technology has been applied to remove VOCs and chlorinated volatile organic compounds (CVOCs) [START_REF] Shahna | Decomposition of gas-phase chloroform using nanophotocatalyst downstream the novel non-thermal plasma reactor: by-products elimination[END_REF][START_REF] Subrahmanyam | Nonthermal plasma abatement of trichloroethylene enhanced by photocatalysis[END_REF][START_REF] Ramaraju | Catalytic non-thermal plasma reactor for stripping the VOCs from air[END_REF]. The dielectric barrier discharge reactor (DBD) is often used in non-thermal plasma (NTP) technology. Several investigations showed that DBD reactors appeared to be most interesting from an economic point of view and they are recognized to be relevant for VOCs removal [START_REF] Vandenbroucke | Non-thermal plasmas for noncatalytic and catalytic VOC abatement[END_REF][START_REF] Van Durme | Combining non-thermal plasma with heterogeneous catalysis in waste gas treatment: A review[END_REF]. Additionally, a combination of photocatalytic process with discharge plasma leads to an improvement in the treatment efficiency of VOCs [START_REF] Vandenbroucke | Non-thermal plasmas for noncatalytic and catalytic VOC abatement[END_REF]. Titanium dioxide (TiO 2 ), the most used semiconductor, is activated by photons provided by an ultra violet (UV) radiation [START_REF] Pichat | Some views about indoor air photocatalytic treatment using TiO 2 : conceptualization of humidity effects, active oxygen species, problem of C1-C3 carbonyl pollutants[END_REF][START_REF] Pichat | Purification/ deodorization of indoor air and gaseous effluents by TiO2 photocatalysis[END_REF][START_REF] Assadi | Photocatalytic oxidation of trimethylamine and isovaleraldehyde in an annular reactor: influence of the mass transfer and the relative humidity[END_REF]. Many studies have shown [START_REF] Pichat | Purification/ deodorization of indoor air and gaseous effluents by TiO2 photocatalysis[END_REF][START_REF] Assadi | Photocatalytic oxidation of trimethylamine and isovaleraldehyde in an annular reactor: influence of the mass transfer and the relative humidity[END_REF][START_REF] Bouzaza | Photocatalytic degradation of some VOCs in the gas phase using an annular flow reactor: determination of the contribution of mass transfer and chemical reaction steps in the photodegradation process[END_REF] that the photocatalytic process (TiO 2 +UV) is an promising way for the treatment of air polluted by VOCs. Indeed, photocatalysts are semiconductors activated by photons which create electron-hole pairs, inducing oxidation or reduction reactions at the surface of the material with adsorbed molecules [START_REF] Pichat | Purification/ deodorization of indoor air and gaseous effluents by TiO2 photocatalysis[END_REF][START_REF] Assadi | Photocatalytic oxidation of trimethylamine and isovaleraldehyde in an annular reactor: influence of the mass transfer and the relative humidity[END_REF]. TiO 2 activated by UV radiation is one of the most efficient photocatalysts for VOC treatment. No ozone and no CO were produced with process. Moreover, other researchers have studied the combination of NTP with TiO 2 +UV for the removal of VOC at scale laboratory [START_REF] Assadi | Abatement of 3-methylbutanal and trimethylamine with combined plasma and photocatalysis in a continuous planar reactor[END_REF][START_REF] Taranto | Combining cold plasma and TiO 2 photocatalysis to purify gaseous effluents: a preliminary study using methanol-contaminated air[END_REF]. It is now well established that combined process improves the removal of pollutants [START_REF] Shahna | Decomposition of gas-phase chloroform using nanophotocatalyst downstream the novel non-thermal plasma reactor: by-products elimination[END_REF]. Several studies have demonstrated a synergistic effect between NTP and TiO 2 +UV processes [START_REF] Ramaraju | Catalytic non-thermal plasma reactor for stripping the VOCs from air[END_REF][START_REF] Lu | Synergistic effect of nanophotocatalysis and non-thermal plasma on the removal of indoor HCHO[END_REF][START_REF] Maciuca | synergetic effect by coupling photocatalysis with plasma for low VOCs concentration removal from air[END_REF][START_REF] Thevenet | Oxidation of acetylene by photocatalysis coupled with dielectric barrier discharge[END_REF].

The major drawback of the NTP removal process is the formation of by-products like CO x , ozone, NO x , which can be more harmful than the target VOC itself. Various researches deal to overcome this limit by a combination of NTP with heterogeneous catalysis [START_REF] Vandenbroucke | TCE abatement with a plasma-catalytic combined system using MnO 2 as catalyst[END_REF][START_REF] Li | Post plasma-catalysis for VOCs degradation over different phase structure MnO2 catalysts[END_REF][START_REF] Abou Saoud | Study of synergetic effect, catalytic poisoning and regeneration using dielectric barrier discharge and photocatalysis in a continuous reactor: Abatement of pollutants in air mixture system[END_REF][START_REF] Huang | Combination of photocatalysis downstream the non-thermal plasma reactor for oxidation of gas-phase toluene[END_REF].

In this work, Chloroform has been chosen as a model pollutant of a CVOC. This pollutant is the main hospital indoor air pollutant and is considered as carcinogenic, mutagenic and reprotoxic agent [START_REF] Elfalleh | Innovative and stable TiO2 supported catalytic surfaces removing aldehydes under UV-light irradiation[END_REF][START_REF]Assessment of a Substance -Chloroform -Specified on the Priority Substances List[END_REF].

Moreover, the novelty of this study is to investigate the synergetic effect between non-thermal plasma and heterogeneous photocatalysis processes during the chlorinated compound in the indoor air with a 2 significant influence of the coupling impact on ozone and CO through various operating parameters.

Moreover, the effect of the new configuration of inner electrode has been investigated by changing from inner electrode to spiral electrode [START_REF] Assadi | Abatement of 3-methylbutanal and trimethylamine with combined plasma and photocatalysis in a continuous planar reactor[END_REF][START_REF] Maciuca | synergetic effect by coupling photocatalysis with plasma for low VOCs concentration removal from air[END_REF][START_REF] Abou Saoud | Study of synergetic effect, catalytic poisoning and regeneration using dielectric barrier discharge and photocatalysis in a continuous reactor: Abatement of pollutants in air mixture system[END_REF] to stainless steel grid electrode..

EXPERIMENTAL

Pilot system and setup details

The experimental works were carried out on a cylindrical reactor which has a coaxial structure and 1500 mm in length. It consists of two concentric tubes made of Pyrex glass (Fig. 1) with equal internal diameters of 58 mm for internal tube and 67 mm for external tube. The internal tube contains the UV lamps (Philips CLEO performance 80W/10 model). The UV intensity, which is measured by a UV radiometer (VLX-3W equipped with a sensor CX 365, ALYS Technologies, Switzerland) at the photocatalyst surface, is equal to 20 ± 3 W.m -2

. The external tube is divided into four sections, the length of the reactive zone is 800 mm, and the surface of the catalyst media is 0.20 m 2 . The catalyst used, Glass Fiber Tissue (GFT), was supplied by Ahlstrom Research and Services [24]. It is containing colloidal silica, percentage of titanium dioxide nanoparticles, percentage of manganese dioxide nanoparticles and inorganic fibers. In fact, titanium dioxide or manganese dioxide has been deposited on inorganic fibers by impregnation using an industrial-size press. A dry mixture of 50 wt% colloidal silica and different wt% of titanium dioxide nanoparticles (PC500 Millennium) and manganese dioxide is suspended in pure water. In order to ensure the deposition of 13 g/m 2 of dry TiO 2 on fiber support, the suspension is composed of 40% of dry powder and 60% of pure water. The press is employed to impregnate fibers with the suspension; then they are dried. According to BET method, specific surface area is measured and is equal to 20.6 m 2 .g -1 of TiO 2 . The repetitive use of this GFT-TiO2 photocatalyst up to the 5th cycle leading to butyraldehyde degradation was done in our previous works. No loss of activity was detected by the photocatalyst during its repetitive recycling. [START_REF] Abou Saoud | Study of synergetic effect, catalytic poisoning and regeneration using dielectric barrier discharge and photocatalysis in a continuous reactor: Abatement of pollutants in air mixture system[END_REF] The gaseous effluent flows between the inner wall of the outer tube and the external wall of the inner tube. To generate the plasma on catalytic surface (Fig. 2), a reactor on discharge of dielectric barrier (DBD) at room temperature and atmospheric pressure is used. The cylindrical reactor is covered by copper electrode (0.5 mm thickness) rolled up and stuck on the external wall of the tube, and is connected to the ground through a 2.5 nF capacity to collect the charges. The internal electrode (high voltage electrode) is established of a rolling-up of two Railings of metal spread in stainless steel square stitch of 2 mm of thickness and 800 mm length in narrow contact with the internal wall of the outside tube of the reactor. The latter plays the role of the dielectric barrier. This barrier is connected to an amplifier (TREK model PD06035), which allows to amplify the signal of the generator 3000 times and thus to increase the voltage up to 21 kV coupled with a signal generator (BFI Optilas reference DS 335/1) which allows to deliver a sinusoidal signal having a range of voltage of 50 mV to 7 V. Detailed description of the pilot system is shown in (Fig. 2). The DBD surfacic plasma is generated on the surface of the catalyst installed in the annular space of the reactor. The construction of this reactor and the stainless steel grid allowed having uniform plasma. Likewise the grid ensures the fixation of the catalyst against the internal wall of the reactor. Moreover, the cylindrical geometry ensures a uniform UV radiation of the catalyst.

Two high voltage probes (NORTH STAR), connected with oscilloscope (LeCroy Wave Surfer 24 digital

Xs-A) to visualize the variations of signal (tension and current) as a function of time, this apparatus allows to measure the voltages across the reactor and coils of condensers. The air flow is controlled by a flow meter (Bronkhorst In-Flow type E-700) coupled to a precision valve.

The flow range can vary from 2 to 8 m 3 h -1

.

The main air flow is generated by two ways. When the relative humidity (RH) is low (RH<5%), we use the internal compressed air network. At higher RH, a centrifugal pump is used with the ambient air. A fraction of the inlet air passes counter-currently through a column filled with glass Raschig rings with water is used to control the RH (25 to 90% ).

The pollutant, air-Chloroform (VWR BDH Prolabo-99.4 %), is directly and continuously injected into the inlet air with the aid of a syringe / syringe drive system through a septum perpendicularly to the gas stream. To improve the pollutant evaporation, a heating system is used for covering the injection zone.

Fig. 3. General schematic view of the pilot system: photocatalysis and plasma DBD.

Apparatus and analysis

Gas Chromatography with flame ionization detection (GC-FID, GC 9000 series, Thermo Focus, USA) is used to evaluate the inlet-outlet chloroform concentrations. The capillary column used is Chrompact FFAP-CB (25 m length and 0.32 mm external diameter), and Nitrogen (N 2 ) gas is the mobile phase.

The injections are made manually from the samples taken in the reactors with syringes of 500 μL. The concentration of carbon monoxide (CO) at the reactor outlet is measured by CO/NOx ZRE gas device commercialized by Fuji Electric S.A.S.

The ozone produced by the plasma treatment is analyzed by colorimetric titration method after bubbling [START_REF] Rodier | The Analysis of Water: Natural Water[END_REF]. Thus, at the outlet of the plasma reactor, a constant flow rate (130 L h -1

) of the gas stream containing O 3 bubbles in a solution of KI at 10 -2 M. The presence of O 3 leads to the oxidation of I -to I 2 , giving the solution a yellow color. I 2 is then titrated until discoloration with a solution of Na 2 S 2 O 3 at 10 -3 M. This titration is carried out in an acid medium, after adding a few drops of HCl solution.

The reactions which will take place during bubbling and dosing are:

O 3 + 2 I -→ I 2 + O 2 + O -+ e - (Eq.1.1) I 2 + 2 S 2 O 3 -2 →2 I -+ S 4 O 6 -2
(Eq. 1. 2)

RESULTS AND DISCUSSION

The continuous removal of the Chloroform by different processes (plasma, photocatalysis and combined system) is studied by examining in various operating parameters such as pollutant concentration, flow rate, relative humidity, and injected energy. For each test, the degradation kinetic of chloroform and the by-products elimination (as CO and ozone) are estimated.

Then irradiation and/or electric discharge was turned on, removal begins. The input and output concentrations are measured until the steady-state is reached. At the end of the experiment, the reactor was cleaned by a current of unpolluted air during at least two hours.

In order to monitor the performance of the process, some operational conditions/parameters are studied: Specific energy (SE; J.L ))/1000 (Eq.2)

where P is input power adjusted by changing the applied voltage (U a ) at frequency = 50Hz and Q is the flowrate.

The overall degradation rate is one of the criteria used for evaluating the performance of VOC's elimination.

It is defined as Chloroform Removal Efficiency (CRE):

CRE (%) = ([CHCl 3 ] inlet -[CHCl 3 ] outlet ) ×100 / [CHCl 3 ] inlet (Eq.3)
Other expressions can also be used to evaluate the process performance:

The Rate of Synergy (RS) calculated by the following expression:

RS = CRE combined process /(CRE plasma +CRE photocatalysis ) (Eq.4)
The and [CO] is the concentration in ppm of ozone and CO respectively.

Effect of flow rates and inlet concentrations on chloroform degradation

Removal of chloroform was investigated in the annular reactor under various inlet concentrations and flow rates. The inlet chloroform concentration varies from 25 to 300 mg.m -3

, and the flow rates of effluent were in the interval of 2-8 m

3

. h -1 .

Photocatalysis alone

(Fig. 4) shows the influence of gas flow rates and inlet concentrations on the removal efficiency of chloroform. The corresponding residence times in the annular plug-flow reactor for gas flow rate of 2, 4, 6 and 8 m 3 .h -1 are 2.4, 1.2, 0.8 and 0.6 s respectively.

As expected, this behavior is in line with those reported in the literature [START_REF] Bouzaza | Photocatalytic degradation of some VOCs in the gas phase using an annular flow reactor: determination of the contribution of mass transfer and chemical reaction steps in the photodegradation process[END_REF][START_REF] Wang | The kinetics of photocatalytic degradation of trichloroethylene in gas phase over TiO 2 supported on glass bead[END_REF]. For a given flow rate, it shows that the removal capacity first increases proportionally with the inlet concentration (Fig. 4). At low inlet concentration, the degradation fits as a first-order kinetic reaction. This is due to the availability of the photocatalytic sites at this concentration range. At higher pollutant concentration, the degradation rate tend to a limit [START_REF] Bouzaza | Photocatalytic degradation of some VOCs in the gas phase using an annular flow reactor: determination of the contribution of mass transfer and chemical reaction steps in the photodegradation process[END_REF][START_REF] Wang | The kinetics of photocatalytic degradation of trichloroethylene in gas phase over TiO 2 supported on glass bead[END_REF].

On the other hand, at different flow rates, it can be seen that the flow rate has a positive influence on the rate of degradation, independent on the input charge. The overall kinetics reaction would thus be completely controlled by the mass transfer step. In fact, two phenomena may occur when the flux increases at a constant load: i) the improvement of the mass transfer and ii) the reduction of the residence time, as described recently [START_REF] Assadi | Photocatalytic oxidation of trimethylamine and isovaleraldehyde in an annular reactor: influence of the mass transfer and the relative humidity[END_REF].

The plasma DBD alone

For the plasma tests, the UV lamps were turned off. Four voltage values were chosen to follow this oxidation process.

(Fig. 5) presents the oxidation of chloroform at two different initial concentrations by non-thermal plasma at various injected voltages and at a constant flow rate of 2 m

3 .h -1
.

It is worth mentioning that whatever the value of the inlet concentration, the increase of injected voltage leads to an increase of the CRE. By electron impact, the plasma produces radicals O • , HO

• [START_REF] Maciuca | synergetic effect by coupling photocatalysis with plasma for low VOCs concentration removal from air[END_REF][START_REF] Thevenet | Oxidation of acetylene by photocatalysis coupled with dielectric barrier discharge[END_REF]. When the injected voltage is increased, more radicals are produced which react more or less directly with chloroform.

The CRE decreases with increasing inlet concentration; the CRE was reduced two times when the inlet concentration increases from 25 to 100 mg m -3

. This is due the lack of electrons and reactive species (like O • and OH • ) when the inlet pollutant concentration increases. The effect of the residence time with a plasma DBD reactor was also studied. For an inlet concentration of 100 mg m -3

, the effect of injected voltages at different flow rates on the CRE was followed (Fig. 6).

At the same time, the increase of flow rates leads to a reduction of the CRE. This effect is similar to that observed for photocatalytic process alone.

Express the molecular diffusivity (D m ) of a two-component system as follows [START_REF] Perry | Perry's Chemical Engineers's Handbook[END_REF]:

( ) ( ) ( ) ( ) -3 1.75 0.5 2 1 / 3 1 / 3 1.43 10 1 1 ( ) m A B A B T D M M P υ υ × × = +   +     ∑ ∑ (Eq.7)
This expression [START_REF] Pichat | Some views about indoor air photocatalytic treatment using TiO 2 : conceptualization of humidity effects, active oxygen species, problem of C1-C3 carbonyl pollutants[END_REF] makes it possible to calculate the coefficient of diffusivity in air of chloroform (D m = 0. 01977 cm 2 .s -1

). The internal mass-transfer step can be neglected because TiO 2 is deposited on the substrate surface where the reaction occurs.

The external mass-transfer coefficient, k m is estimated using a semi-empirical correlation (Eq.8)

developed in annular reactors for laminar flow [START_REF] Mobarak | Mass transfer in smooth and rough annular ducts under developing flow conditions[END_REF]:

Sh = 1.029 × Sc 0.33 × Re 0.55 × � Ltot d � -0.472 (Eq.8)
Then the mass transfer coefficients are calculated and shown in Table 1. The increase in flow rate decreases the residence time and increases the mass transfer coefficient due to negative effect of flow rate, leading to an unfavorable or favorable effect. It seems that the reaction between the radicals and the chloroform does not take place in this condition.

We note also that whatever the flow rate, the increase of the injected voltage improves the conversion rate.

Fig. 6. Influence of applied voltages at different flow rates on the CRE in plasma alone:

[CHCl3]= 100 mg m -3 , HR= 50 %, T= 20 °C

DBD/TiO 2 +UV plasma coupling

As for each process mentioned above, the influence of operating parameters on process performance will be studied for the combined system plasma and photocatalysis. Firstly, the DBD treatment and TiO 2 +UV photocatalysis process can be subdivided into six phases (i) transfer of gaseous reactants (Chloroform and oxygen) to the discharge zone (ii) oxidation reaction on the discharge zone (iii) transfer of gaseous reactants and byproducts to the solid phase (iv) oxidation reaction in solid phase (v) desorption of gaseous photocatalytic reaction products and byproducts (vi) diffusion of products to bulk phase.

Removal efficiency

The same values of injected voltage and UV intensity are maintained during these experiments in order to compare the results between the three processes: photocatalysis, DBD plasma, and DBD/photocatalysis. The evolution of the chloroform removal as a function of injected voltage is similar to that observed for the DBD plasma alone (Fig. 7).

Quite logically, the conversion rate increases with the injected voltage. The elimination is complete for [CHCl 3 ]= 100 mg m -3 with a flow rate of 2 m 3 h -1 from 15 kV. With the coupling process, we also find an improvement in the CRE, which is higher than the sum of those in plasma alone and in photocatalysis. At can be seen from this figure that, the RS of the combined system is usually higher compared to each process considered separately. Moreover, we note the existence of a synergy due to the process combination. RS is mainly higher (about 1.17-1.33) than the sum of the CRE for each separated process whatever the flow is.

An increase of the injected voltage leads to an increase in RS for the combined process. This trend is still observed with different input voltages.

The RS by coupling plasma DBD and photocatalysis was 1.16-1.18 higher than the sum of the removals recorded at the same conditions for plasma DBD alone and photo-catalysis alone.

Therefore, a synergistic effect occurs on the surface of the GFT when coupling plasma and photocatalytic processes.

This trend was the same for all flow rates and voltage values applied in the reactor. This is due to the role of neutrals (including molecules, radicals, metastable species and atoms), ions, electrons, photons and electric fields, on the improvement of plasma-catalyst surface interactions in terms of surface chemical reactions and the improvement of mass transfer [START_REF] Assadi | Abatement of 3-methylbutanal and trimethylamine with combined plasma and photocatalysis in a continuous planar reactor[END_REF]. Similar observations have been found in the literature for various VOCs [START_REF] Assadi | Abatement of 3-methylbutanal and trimethylamine with combined plasma and photocatalysis in a continuous planar reactor[END_REF][START_REF] Maciuca | synergetic effect by coupling photocatalysis with plasma for low VOCs concentration removal from air[END_REF][START_REF] Thevenet | Oxidation of acetylene by photocatalysis coupled with dielectric barrier discharge[END_REF][START_REF] Assadi | Use of DBD plasma, photocatalysis and combined DBD plasma/ photocatalysis in a continuous annular reactor for isovaleraldehyde elimination -Synergetic effect and byproducts identification[END_REF][START_REF] Gharib-Aboughaida | Association of surface dielectric barrier discharge and photocatalysis in continuous reactor at pilot scale: Butyraldehyde oxidation, by-products identification and ozone valorization[END_REF].

The synergistic effect observed on the photocatalytic surface can be attributed to:

 Non-thermal plasma involving different processes since plasma produces many species such as high-voltage electrons, excited molecules such oxygen atomic (O°) in dry air and radicals

(O • , HO • , O 2 •-
, O 3 , NO x , etc.) in wet air. These species can interact with chloroform molecules leading to its degradation [START_REF] Lu | Synergistic effect of nanophotocatalysis and non-thermal plasma on the removal of indoor HCHO[END_REF][START_REF] Maciuca | synergetic effect by coupling photocatalysis with plasma for low VOCs concentration removal from air[END_REF].

 Plasma promoting the desorption of byproducts attached at the TiO 2 surface leads to its mineralization. In fact, Abou Sadou et al. showed a significant regeneration that has been occurred by combining photocatalysis and nonthermal plasma. Indeed, the surface atomic percentage (XPS) on the catalyst surface at time zero, after the catalyst poisoning and after the plasma treatment were studied and results give the evidence for surface regeneration with plasma. Thus, it has also been demonstrated that DBD-plasma may effectively regenerate poisoned catalysts. This trend can explain the synergetic effect during the sulfur pollutant degradation time [START_REF] Abou Saoud | Study of synergetic effect, catalytic poisoning and regeneration using dielectric barrier discharge and photocatalysis in a continuous reactor: Abatement of pollutants in air mixture system[END_REF].

 Activation of the TiO 2 due to Plasma can induce by creating hole/electron pairs through ion bombardment [START_REF] Vandenbroucke | Non-thermal plasmas for noncatalytic and catalytic VOC abatement[END_REF][START_REF] Assadi | Abatement of 3-methylbutanal and trimethylamine with combined plasma and photocatalysis in a continuous planar reactor[END_REF][START_REF] Sivachandiran | Isopropanol saturated TiO 2 surface regeneration by non-thermal plasma: influence of air relative humidity[END_REF].

 Enhancing of mass transfer between plasma and catalysis zones. In fact, Assadi et al.

supposed that the presence of the turbulences generated by plasma on the photocatalytic medium can enhance the mass transfer. Moreover they found that an increase of four times the value of mass transfer coefficient and argued that the model is able to predict the synergetic effect [START_REF] Assadi | modeling and simulation of VOCs removal by nonthermal plasma discharge with photocatalysis in a continuous reactor: Synergetic effect and mass transfer[END_REF].

Intermediate by-products

Injected voltages effect

Firstly, we note that no ozone and CO was produced by photocatalysis and/or photolysis. Moreover, with our system, the amount of NOx is lower than 1.5 ppm in each operating parameter tested.

Figure 9 shows the effect of the initial concentration of chloroform on the formation of ozone at the outlet of the plasma reactor DBD at different voltages. It can be seen that, for an increase of the initial concentration, the ozone concentration formed at the outlet of the reactor decreases. Ozone is formed by the recombination reactions of atomic oxygen. Indeed, by electron impact, plasma produces radicals and active species. For example, atomic oxygen is formed by dissociation of O 2 and then is consumed over a few hundred microseconds by combining and producing ozone [START_REF] Assadi | Removal of trimethylamine and isovaleric acid from gas streams in a continuous flow surface discharge plasma reactor[END_REF][START_REF] Maxime | Removal of gas-phase ammonia and hydrogen sulfide using photocatalysis, nonthermal plasma, and combined plasma and photocatalysis at pilot scale[END_REF].

The following reaction shows the formation of ozone [START_REF] Nakagawa | Investigation of humidity effect on atmospheric plasma decomposition of toxic gas with direct optical measurement of oh radicals[END_REF][START_REF] Assadi | modeling and simulation of VOCs removal by nonthermal plasma discharge with photocatalysis in a continuous reactor: Synergetic effect and mass transfer[END_REF][START_REF] Delagrange | Combination of a non-thermal plasma and a catalyst for toluene removal from air: Manganese based oxide catalysts[END_REF]:

O • + O 2 + M → O 3 + M (Eq. 9)
where M may be molecular oxygen.

Atomic oxygen contributes to the removal of chloroform, its concentration decreases, reducing the probability of recombination with O 2 to form ozone. That could be explained by the fact that more electrons and reactive species (such as O • ) were formed and then leads to ozone production (Eqs. 9

and 10) [START_REF] Assadi | A study of pollution removal in exhaust gases from animal quartering centers by combining photocatalysis with surface discharge plasma: From pilot to industrial scale[END_REF][START_REF] He | Catalytic behavior and synergistic effect of nonthermal plasma and CuO/AC catalyst for benzene destruction[END_REF].

e -+ O 2 → e -+ O • + O • (Eq.10)
Fig. 10 shows IBE variation for ozone and CO at different applied voltages for an input concentration of100 mg. m -3

.

From figures 9 and 10, the concentration of ozone increases slowly with the injected voltages into the discharge zone, that can be linked to the removal efficiency of by-product for ozone decreases from 46 to 32% because of the accumulation and/or excess of ozone concentration in the reactor. The results show that the IBE of the CO decreases with the combined process. The presence of the catalyst may play an important role on the reduction of the CO formation. For the decomposition of chloroform, these results imply that the IBE of the CO decreases as the amount of chloroform decomposition increases. O-mediated species are thought to contain CO more readily than those without O.

When TiO 2 is incorporated in a DBD reactor, the IBE of the CO for chloroform could reach to 50 % at an applied voltage of 9 kV. Furthermore, the TiO 2 catalyst could effectively prevent the formation of residues in the chloroform decomposition. The results shown in Fig. 10 imply that the TiO 2 in the plasma reactor has a catalytic activity to oxidize the intermediate hydrocarbon by-products such as CO and ozone. Reduction of the residence time has an inverse effect on the IBE of CO. This allows us to proven that the reactive species do not have sufficient time to oxidize the by-products generated during the oxidation of pollutant. The flow is inversely proportional to the conversion rate due to the reduction of the residence time in the reactor. By reducing the residence times, the pollutant molecules have less time to react. This is despite the fact that an increase in the flow rate results in a decrease of the resistance to the transfer of material. This result is similar to that observed in a continuous reactor [START_REF] Geng | Adsorption and Photocatalytic Degradation Kinetics of Gaseous cyclohexane in an Annular Fluidized Bed Photocatalytic Reactor[END_REF][START_REF] Boulinguiez | Photocatalytic degradation of ammonia and butyric acid in plug-flow reactor: Degradation kinetic modeling with contribution of mass transfer[END_REF][START_REF] Thevenet | Photocatalytic degradation of acetylene over various titanium dioxide-based photocatalysts[END_REF].

Effect of flow rates

Regarding our findings for the combined process, ozone formation is usually less than that for plasma treatment.

The plasma DBD/TiO 2 +UV coupling, ozone removal is between 46 and 33% at 6m

. This reduction of O 3 formation is due to the activation of TiO 2 by UV radiation, which leads to following reactions [START_REF] Maciuca | synergetic effect by coupling photocatalysis with plasma for low VOCs concentration removal from air[END_REF]:

O 3 + e -→ O •-+ O 2 (Eq.11)
Or

O 3 + hv(λ < 310nm) → O 2 + O • (Eq.12)
The DBD/TiO 2 +UV plasma process therefore reduces slightly the production of ozone compared to that obtained during manipulations with plasma alone. In fact, the interaction between ozone and catalyst should be investigated in details.

Effect of the relative humidity

The relative humidity (RH) of air can play an important role on the process performance and thus, this parameter has to take into account on CRE and by-products formation.

Photocatalytic process

The photocatalytic degradation is usually affected by water vapor. In most cases, the total absence of water is fatal to photocatalysis because this directly affects the quantity of hydroxyl radicals formed.

On the other hand, high RH value can negatively affect the process due to a competitive effect between water and pollutant [START_REF] Aa Assadi | Study of synergetic effect by surface discharge plasma/TiO2 combination for indoor air treatment: Sequential and continuous configurations at pilot scale[END_REF].

Four relative humidity values (RH) of the air are set ranging from 5, to 90 % Figure12 indicates that the water molecules have a significant effect on the conversion rate with a maximum regardless of the initial concentration of chloroform. In fact, at low RH, a slight increase of RH enhances the production of OH • [START_REF] Ochiai | Photoelectrochemical properties of TiO 2 photocatalyst and its applications for environmental purification[END_REF] and thus the conversion rate. For example, for an inlet concentration of 25 mg m -3

, there is a 15% improvement in the conversion rate for a relative humidity increase from 5 to 50%.

When the amount of water in the gaseous effluent is increased, the excess water molecules compete with the pollutants for the adsorption sites of the photocatalytic media. The number of active sites remains constant, which results in a decrease in photocatalytic activity.

This behavior is observed whatever the inlet pollutant concentration.

Plasma DBD process

Removal efficiency

Relative humidity can significantly alter NTP performance [START_REF] Assadi | Removal of trimethylamine and isovaleric acid from gas streams in a continuous flow surface discharge plasma reactor[END_REF][START_REF] Vandenbroucke | Non-thermal plasmas for noncatalytic and catalytic VOCabatement[END_REF].Fig. 13 shows that CRE increases first and then decreases with RH. In fact, when RH increases, the competitive effect on the active sites becomes predominant, which leads to a CRE decrease. The optimum RH value is about 50% whatever the injected voltage is. As we can previously see for photocatalytic process, the increase of the RH favors the formation of OH • radicals which lead to an improvement in the conversion rate.

During the electronic impact of the plasma on the water molecules, a reaction of decomposition of the water leads to the formation of radicals OH

• and H

• [START_REF] Assadi | Removal of trimethylamine and isovaleric acid from gas streams in a continuous flow surface discharge plasma reactor[END_REF][START_REF] Nakagawa | Investigation of humidity effect on atmospheric plasma decomposition of toxic gas with direct optical measurement of oh radicals[END_REF][START_REF] Guo | Detection of hydroxyl radical in plasma reaction on toluene removal[END_REF].

H 2 O + e -→ H • + OH • + e -(Eq.13)
On the other hand, at higher levels of RH, an inverted trend occurs and the CRE becomes slightly lower. This is because the increase of water vapor content limits the electron density and quenches the active species. This behavior is similar to that reported in the literature [START_REF] Huang | Combination of photocatalysis downstream the non-thermal plasma reactor for oxidation of gas-phase toluene[END_REF]. , respectively.

From a qualitative point of view, the obtained results are similar to those in the case of treatment with plasma and photocatalysis. Indeed, there is an optimum level of the chloroform conversion. Similarly, it is found that this optimum (50%) is not changed with the combination system.

Synergetic effect

Fig. 15 presents the variation of RS at various voltages and RH using three processes at a fixed chloroform content obviously, the RS varies from 1.38 to 1.26 for RH when the RH varies from 5 to 90% and RS varies from 1.15 to 1.19 when input voltage varies from 9 to 21 kV, respectively. This synergistic effect is observed under different parameters and are in good agreement with those obtained by several groups [START_REF] Assadi | Photocatalytic oxidation of trimethylamine and isovaleraldehyde in an annular reactor: influence of the mass transfer and the relative humidity[END_REF][START_REF] Assadi | Abatement of 3-methylbutanal and trimethylamine with combined plasma and photocatalysis in a continuous planar reactor[END_REF][START_REF] Lu | Synergistic effect of nanophotocatalysis and non-thermal plasma on the removal of indoor HCHO[END_REF][START_REF] Maciuca | synergetic effect by coupling photocatalysis with plasma for low VOCs concentration removal from air[END_REF][START_REF] He | Catalytic behavior and synergistic effect of nonthermal plasma and CuO/AC catalyst for benzene destruction[END_REF].

Fig. 15. Variation of RS with RH (at V= 18 kV) and applied voltage (at RH= 50%) using three processes:

[CHCl3]= 100 mg m -3 , Q= 2 m 3 h -1 , I= 20 W m -2 , T= 20 °C.

Intermediate by-products 3.2.3.3.1. Injected voltages effect

Figure16 shows that the increase of the relative humidity (90%) induces a decrease of the IBE of CO at each value of voltage injected. This decrease is more pronounced at 21kV: the IBE CO 64% at a RH of 90 %. The IBE CO does not exceed to 50 % for the 9 and 12 kV.

Similarly, Thevenet et al. [START_REF] Thevenet | Oxidation of acetylene by photocatalysis coupled with dielectric barrier discharge[END_REF] showed that, when removing acetylene by cold plasma, the increase of water vapor reduced the amount of CO. For the IBE of CO, the elimination of CO increases with the voltage (approximately 65 %). In the case of high humid air (90%), water vapor leads to the formation of radicals of active oxygen as described in Eq. 13. The decomposition of chloroform gives carbon which reacts with the latter to form CO 2

When non-thermal plasma is made in a stream of oxygen-rich gas and water, ozone formation is generally unavoidable. In an earlier research, it has been proposed that the decomposition of ozone by transition metal oxides (TiO 2 ) leads to the formation of atomic oxygen (

) on the surface of TiO 2 . On the other hand, the oxidation of CO and chloroform in the presence of ozone showed that this form was, most likely, the oxidizer. Furthermore, it has been shown that in terms of reactivity, an oxygen atom is a more chemically active species than Ozone [46].

Therefore, it would be favorable for the oxidation reaction if the ozone can be decomposed into one oxygen atom before reacting with VOC molecules or forming CO. The peroxyl radical OH 2

• acts as intermediate active species in the chain mechanism for the transformation of OH radical and reduces the formation of CO and ozone.

OH + CO → CO 2 + H (9)
The IBE of CO increased in voltages above an HR (90%) (Fig. 16). According to the results, plasma DBD/TiO 2 +UV.CO was substantially low in this process, which could be related to low overall CO production, and / or conversion of the CO formed into CO 2 during the oxidation state of the process.

Radicals, namely O

• and OH • [START_REF] Parvulescu | Plasma Chemistry and Catalysis in Gases and Liquids[END_REF], could influence the oxidation of CO in the reactor [START_REF] Ge | Removal of low-concentration benzene in indoor air with plasma-MnO2 catalysis system[END_REF][START_REF] Zhang | Atmosphericpressure O2 plasma treatment of Au/TiO2 catalysts for CO oxidation[END_REF][START_REF] Zhang | CO oxidation activity at room temperature over Au/CeO2 catalysts: disclosure of induction period and humidity effect[END_REF].

Kogoma et al., reported an exclusive oxidation of CO with OH [START_REF] Kogoma | Highly efficient VOC decomposition using a complex system (OH radical, ozone-UV, and TiO2)[END_REF].

According to (Fig. 16), the ozone removal efficiency is higher (between 80 and 87.5%) in the highly humid 90% air.

For coupling, as for plasma, ozone drops with increasing relative humidity. As previously observed, the presence of UV-activated TiO 2 in the discharge area slightly reduces ozone formation by a few hundred ppm at high concentrations.

The energy is supplied by UV radiation interacts with ozone. The global reactions were shown in (eq.10) [START_REF] Kogoma | Highly efficient VOC decomposition using a complex system (OH radical, ozone-UV, and TiO2)[END_REF].

O 3 + hv + H 2 O → O 2 + O • + 2H • (EQ. 14) H 2 O + O 3 → H 2 O 2 + O 2 (Eq. 15) OH -+ O 3 → HO 2 • + O 2 -•
(Eq. 16)

O 3 + e -→ O 3 -•
(Eq. 17)

Fig. 16 shows that, whatever the value of the RH, the increase in the voltage-injected favors the IBE of CO. This is explained by the fact that, at high energy values, electrical discharges produce more reactive species that allow for further degradation (mineralization) of the pollutant. Several studies on plasma photocatalytic [START_REF] Maxime | Removal of gas-phase ammonia and hydrogen sulfide using photocatalysis, nonthermal plasma, and combined plasma and photocatalysis at pilot scale[END_REF][START_REF] Allegraud | Spatio-temporal breakdown in surface DBDs: evidence of collective effect[END_REF] assumed that the reduction of the amount of carbon monoxide is related to the increase of the pathway favoring the formation of other intermediates reaction than CO.

Relative humidity effect:

Figure 17shows that the increasing the relative humidity of the air reduces the ozone production.

Indeed, the elimination of ozone with the coupling reaches to 85%.

First of all, it can be seen (Fig. 17) that when water vapor (from 50 to 90%) is added, the ozone depletion is increasingly important to the voltage value applied 18 kV. Ozone can be decomposed by the effect of the radicals H • and HO • according to the following reactions [START_REF] Thevenet | Oxidation of acetylene by photocatalysis coupled with dielectric barrier discharge[END_REF][START_REF] Assadi | A study of pollution removal in exhaust gases from animal quartering centers by combining photocatalysis with surface discharge plasma: From pilot to industrial scale[END_REF]:

O 3 + OH • → O 2 + OH 2 • (Eq.18) O 3 + H • → O 2 + HO • (Eq.19)
Radical recombination reactions can also occur according to the following reaction [START_REF] Baulch | Evaluated kinetic data for combustion modeling: Supplement II[END_REF]:

OH • + O • → H • + O 2 (Eq.20)
This phenomenon appears as an indication that ozone is not the main oxidizing species, but only a marker. Despite the reduction of ozone in the moisture range from 5 to 50%, the conversion rate increases with the RH in the same range.

As previously observed with photocatalysis alone, the increase of moisture favors the formation of OH • radicals while leads to an improvement of the IBE level in the plasma zone. When electron plasma is impacted on water molecules, a decomposition reaction of the latter leads to the formation of OH • and H

• radicals [START_REF] Ochiai | Photoelectrochemical properties of TiO 2 photocatalyst and its applications for environmental purification[END_REF][START_REF] Atkinson | Atmospheric degradation of volatile organic compounds[END_REF].

Regarding the OH • radicals and the efficiency of their formation reaction, the concentration downstream of the reactor and the rate of occurrence are proportional to the voltages applied in the device. Thus, their effect on the process performance appears for High voltage values [START_REF] Maxime | Removal of gas-phase ammonia and hydrogen sulfide using photocatalysis, nonthermal plasma, and combined plasma and photocatalysis at pilot scale[END_REF].

With the increase of relative humidity, OH • radicals are formed, their probability of recombination and the interaction with certain species generated from O 2 by the plasma discharge, such as atomic oxygen, are also increased [START_REF] Wang | Piezoelectric nano-generators based on zinc oxide nanowire arrays[END_REF]. In the presence of water, the reaction medium a modification of the potentially oxidizing species is taken. Under dry air, these are O • and O 3 , while in humid air they are replaced by OH • . Other studies [START_REF] Maxime | Removal of gas-phase ammonia and hydrogen sulfide using photocatalysis, nonthermal plasma, and combined plasma and photocatalysis at pilot scale[END_REF] show that the formation of hydrogen peroxide dominates the kinetics process when water is added. H 2 O 2 is formed by the recombination of two OH • radicals.

Ozone can be decomposed by the effect of the radicals H • and HO • according to the following reactions (Eq. 18-19) [START_REF] Guo | Detection of hydroxyl radical in plasma reaction on toluene removal[END_REF][START_REF] Capitelli | Collision integrals of high-temperature air species[END_REF]:

Others intermediates by-products:

We wanted to follow closely the degradation of Trichloromethane, so we decided to identify the degradation by-products by gas chromatography coupled to mass spectrometry (GC-MS). Thus, the by-products generated during the plasma, photocatalysis and combined process degradation of Trichloromethane were identified using a Gas chromatography-mass spectrometry (GC-MS) Perkin Elmer Clarus 500. The GC-MS column is a 25 m long Chrompac TM capillary column FFAP-CB, its diameter is 0.15 mm and the stationary phase's thickness is 0.25 m. Due to their low concentrations, by-products are concentrated on a Carbotrap (25 ml) then its content is removed using thermal desorption unit coupled with the GC-MS Three samples were taken for Trichloromethane after each treatment process. Analyses of outlet unpolluted air flow composition after its treatment with each of the three processes have also been made in order to suppress any doubt while identifying Trichloromethane intermediates. The results are shown in figure 18. Compounds present in the outlet air were detected by GC-MS. Regarding Trichloromethane as pure substance and its oxidation by plasma system; the detected degradation by-products were acetone, ethanol, methanol CCl4 and acetaldehyde. Trichloromethane is also detected at 5,75min. The same intermediates as in combined system were detected when plasma was used alone. We note that, after treatment by photocatalysis, the presence of acetone and methanol (Figure 18).

CONCLUSIONS

This work deals to compare the behavior of the effectiveness of chloroform removal in a cylindrical reactor by photocatalysis (TiO 2 + UV), DBD plasma and combined processes.

At high flow rate, the oxidation process, generated by three systems seems to be controlled by the mass transfer step. Moreover, this study showed that combination of the plasma with photocatalysis improves the performance of the pollutant decomposition process. In fact, chloroform removal efficiency by combined process is higher than the sum of both photocatalysis and plasma DBD alone.

Intermediate byproducts of chloroform with each process were identified.

Fig. 1 .

 1 Fig.1. Drawing of the cylindrical photo reactor (a) and the cross-section (b).

Fig. 2 .

 2 Fig.2. General schematic view of the circuit electric of the photocatalysis and plasma DBD in an annular reactor.

Fig. 4 .

 4 Fig.4. Influence of gas flow rates and inlet concentrations on the removal efficiency of chloroform by photocatalysis alone: RH= 50 %, T= 20 °C, I= 20 W.m -2 .

Fig 5 :

 5 Fig 5: Influence of gas flow rates and inlet concentrations on the removal capacity of chloroform by photocatalysis alone: RH=50%, T=20°C, I = 20 W m -2 .

Fig. 7 .

 7 Fig.7. Influence of injected voltages at different flow rates on the CRE with plasma DBD/TiO2+UV: [CHCl3]= 100 mg m -3 , RH= 50 %, I= 20 W m -2 , T= 20 °C.

Fig. 8 .

 8 Fig.8. Variation of RS with flow rate (at V= 15 kV) and applied voltage (at Q= 2 m 3 h -1 ) using three processes: [CHCl3]= 100 mg m -3 , RH= 50%, I= 20 W m 2 , T= 20 °C.

Fig. 9 .

 9 Fig.9. Influence of applied voltages at different inlet concentrations on ozone formation with plasma/TiO2+UV: HR= 50%, T= 20 °C

Fig. 11 shows

 11 Fig.11shows variation the IBE of ozone and CO with different flow rates. The reduction of the residence time leads to a decrease of CO and the IBE of CO remains low even at high voltages and flow rates.

Fig. 10 .

 10 Fig.10. Variation the IBE of ozone and CO with applied voltage: [CHCl3]= 100 mg m -3 , Q= 6 m 3 h -1 , RH= 50%, I= 20 W m -2 , T= 20 °C.

Fig. 11 .

 11 Fig.11. Variation the IBE of ozone and CO with flow rate: [CHCl3]= 100 mg m -3 , V= 18 kV, RH= 50%, I= 20 W m -2 , T= 20 °C.

Fig. 12 .

 12 Fig.12. Influence of the RH to different inlet concentrations of chloroform on the CRE with photocatalysis: Q= 2 m 3 h -1 , I= 20 W m -2 .

Fig. 13 .

 13 Fig.13. Influence of the RH at different voltages injected on the CRE with plasma DBD alone: [CHCl3]= 100 mg m -3 , Q= 2 m 3 h -1 , I= 20 W m -2 .

Fig. 14 .

 14 Fig.14. Influence of RH at different applied voltages on the CRE with plasma DBD/TiO2+UV: [CHCl3]= 100 mg m -3 , Q= 2 m 3 h -1 , I= 20 W m -2 , T= 20 °C.

Fig. 16 .

 16 Fig.16. Variation the IBE of ozone and CO with applied voltage: [CHCl3]= 100 mg m -3 , Q= 2 m 3 h -1 , RH= 90%, I= 20 W m -2 , T= 20 °C.

Fig. 17 .

 17 Fig.17. Variation the IBE of ozone and CO with RH: [CHCl3]= 100 mg m -3 , Q= 2 m 3 h -1 , V= 18 kV, I= 20 W m -2 , T= 20 °C.

Figure 18 :

 18 Figure 18: By-products identified by GC-MS following Trichloromethane oxidation by the Three tested systems

FiguresFig. 1 .

 1 Figures

Fig. 2 .

 2 Fig.2. General Schematic view the circuit electric of the photocatalysis and plasma DBD in an annular reactor.

Fig. 3 .Fig 4 :

 34 Fig.3. General Schematic view of the pilot system: photocatalysis and plasma DBD.

Fig. 5 . 1 Fig. 6 .Fig. 7 . 2 Fig. 8 .

 516728 Fig. 5. Influence of applied voltages at different flow rates on the CRE in plasma alone: [CHCl3] = 100 mg m-3, HR = 50%, T = 20 °C.

Fig. 9 .Fig. 10 .

 910 Fig.9. Influence of applied voltages at different inlet concentrations on ozone formation with plasma/TiO2+UV: HR= 50%, T= 20 °C

Fig. 11 . 3 [CHCl 3 ]= 50 mg m - 3 [CHCl 3 ]= 100 mg m - 3 [CHCl 3 ]= 150 mg m - 3 [CHCl 3 ]= 200 mg m - 3 [CHCl 3 ]= 300 mg m - 3 Fig. 12 .Fig. 13 .Fig. 14 .

 1133333333333121314 Fig.11. Variation the IBE of ozone and CO with flow rate: [CHCl3]= 100 mg m -3 , V= 18 kV, RH= 50%, I= 20 W m -2 , T= 20 °C.

Fig. 15 .

 15 Fig.[START_REF] Thevenet | Oxidation of acetylene by photocatalysis coupled with dielectric barrier discharge[END_REF]. Variation of RS with RH (at V= 18 kV) and applied voltage (at RH= 50%) using three processes:[CHCl3]= 100 mg m -3 , Q= 2 m 3 h -1 , I= 20 W m -2 , T= 20 °C.

Fig. 16 .Fig. 17 .

 1617 Fig.16. Variation the IBE of ozone and CO with applied voltage: [CHCl3]= 100 mg m -3 , Q= 2 m 3 h -1 , RH= 90%, I= 20 W m -2 , T= 20 °C.

Figure 18 .

 18 Figure 18. By-products identified by GC-MS following Trichloromethane oxidation by the different processes. Uapp= 15kV, [CHCl3]0= 50 mg m -3 , Q= 2 m 3 h -1

Table 1 :

 1 Values of the residence times, Reynolds number and mass transfer coefficients in the

	annular reactor:
	Flow rate (Q, m	3 h	-1 )	Residence times (s)	Reynolds number	Mass transfer coefficient of Chloroform (km x 10 5 , m s -1 )
	2			2.4	380	0.76
	4			1.2	760	1.12

03-May-2018 + 10:15:07

	03052018_inlet reactor_2min											1: Scan EI+
	100			Trichloromethane								3.99e9 TIC
	%
	0												Time
	0.50	2.50	4.50	6.50	8.50	10.50	12.50	14.50	16.50	18.50	20.50	22.50	24.50
	1.37 03052018_Combined _2min 0 100 % 0 100 % 0 100 % 03052018_Plasma_2min 03052018_Photocatalyse_2min , 0.87 1.87 2.37 2.87 3.37 3.87 Air Carbon tetrachloride Air Carbon tetrachloride Acetone Acetaldehyde Ethanol Methanol Air Acetone Methanol Acetone Methanol Ethanol Acetaldehyde	4.37	4.87 Time 1: Scan EI+ TIC 4.48e7 1: Scan EI+ TIC 5.07e7 1: Scan EI+ TIC 5.74e7