Tio2-Based Hybrid Nanocomposites Modified by Phosphonate

Article TiO2-Based Hybrid Nanocomposites Modiﬁed by Phosphonate Molecules as Selective PAH Adsorbents

Nadine Bou Orm 1 , Quoc An Trieu 2 and Stephane Daniele 2,*

1 College of Natural and Health Sciences, Zayed University, 144534 Abu Dhabi, UAE; [email protected] 2 Institut de Recherches sur la Catalyse et l’Environnement de Lyon (IRCELYON), CNRS—UMR 5256, Université de Lyon, 2 Avenue Albert Einstein, F-69626 Villeurbanne CEDEX, France; [email protected] * Correspondence: [email protected]; Tel.: +33-(0)472445360

Academic Editors: Ahmad Mehdi and Sébastien Clément Received: 25 October 2018; Accepted: 17 November 2018; Published: 21 November 2018

Abstract: A robust sol-gel process was developed for the synthesis of surface-functionalized titania nanocrystallites bearing unsaturated groups starting from molecular heteroleptic single-source precursors. Molecules and nanomaterials were thoroughly characterized by multinuclear liquid and solid-state nuclear magnetic resonance (NMR), infra-red (FT-IR, DRIFT) spectroscopies. Nitrogen adsorption-desorption (BET), thermogravimetric (TG) and elemental analyses demonstrated the reliability and the fine tuning of the surface functionalization in terms of ratio TiO2:ligand. The as-prepared materials were used as nano-adsorbents to remove mixture of 16 polycyclic aromatic hydrocarbon (PAHs) from aqueous solutions. Adsorption kinetic experiments were carried out for 24 h in solutions of one PAH [benzo(a)pyrene, 220 ppb] and of a mixture of sixteen ones [220 ppb for each PAH]. Most kinetic data best fitted the pseudo-second order model. However, in PAHs mixture, a competition process took place during the first hours leading to a remarkable high selectivity between light and heavy PAHs. This selectivity could be fine-tuned depending on the nature of the unsaturated group of the phosphonate framework and on the nanomaterial textures.

Keywords: alkoxides; phosphonates; titania; nanohybrid; sol-gel; PAH adsorption; selectivity

1. Introduction Polycyclic aromatic hydrocarbons (PAHs) are a family of nonpolar neutral chemical compounds resulting from the condensation of several benzene rings ranging from two to three (light molecular weight PAHs, LMW PAHs), to four and five (medium molecular weight PAHs, MMW PAHs) and up to seven (heavy molecular weight PAHs, HMW PAHs). The presence of PAHs in surface water comes mainly from the deposition of airborne particles produced via incomplete oil combustion, from the leaching releases from coal storage areas, from effluents of wood treatment plants and other industries and from the use of composts and fertilizers [1] The number of PAHs identified is in the order of 130, including PAHs containing sulfur, nitrogen or oxygen atoms. The U.S. Environmental Protection Agency (US-EPA) has compiled a list with 16 PAHs (Table S1) posing major environmental problems due to their persistence, their toxic effects on human health and are associated with a wide range of effects: degradation of the immune system, effects on the reproduction and development as endocrine disrupter and carcinogenic properties (carcinogenicity increasing for HMW PAH) [2]. PAHs are persistent pollutants in the environment since they are often resistant to biodegradation. Their removal using conventional physical and/or chemical technologies available for remediation of wastewater pollutants such as oxidation with ozone/hydrogen peroxide, photocatalytic degradation,

Molecules 2018, 23, 3046; doi:10.3390/molecules23113046 www.mdpi.com/journal/molecules Molecules 2018, 23, 3046 2 of 15 ion exchange, membrane (ultra)filtration, coagulation/flocculation, solvent extraction and reverse osmosis can also be not efficient. However, solid-phase extraction is generally considered to be superior for water re-use in terms of energy costs, flexibility and simplicity of design, ease of operation and insensitivity to toxic pollutants and because it does not result in the formation of harmful by-product or toxic concentrated sludge [3–5]. Hybrid (organic-inorganic) nanostructured materials of class II combining large surface-to-volume ratio and controllable surface functionalization can provide a powerful potential to update traditional adsorption treatment process. One of the goals of nanotechnology for the pollutant removal from wastewater is to develop efficient and selective nanosorbents that can remove low concentration contaminants in the presence of competing ones. A recent study on the removal selectivity of PAHs from a wastewater treatment plant (WWTP) located in an urban area in China shows that LMW PAHs such as naphthalene can be removed at a level of 80%, while HMW ones became even more concentrated with treatment in both the dissolved phase and the dewatered sludge [6]. Given the nature of PAHs, use of supported-aromatic derivatives to form "π-stacking" charge transfer complexes has already been reported since these interactions are effective in an aqueous medium, non-degrading and reversible [7–9]. Previous studies have reported the removal of PAHs from aqueous solutions using a large variety of supports such as clay, resins, activated carbon, periodic modified mesoporous organosilane (POMs), metal-organic frameworks (MOFs) [4,5,10]. However, studies dealing with the influence of the nature of the trapping organic molecule on adsorption properties towards PAH selectivity or affinity in mixture of LMW and HMW ones are scarce. In this study, we report the synthesis, characterization and properties of new hybrid (in this case organic-inorganic) nanotitania supports functionalized with a series of organophosphorus molecules (R–OPO2H2) holding different unsaturated groups (R = vinyl, phenyl, naphtyl) and spacers. The performances of such π-stacking complexation sites will be compared in terms of selectivity, capacity and kinetic of adsorption in an aqueous mixture of 16 PAH molecules. There is an abundant literature reporting the use of organophosphorus molecules for surface modification of metal oxides (MOx) because they allow the control of a mono-layer functionalization of metal oxide surfaces resulting from the non-competition between hetero-and homo-condensation but also but also because this ionocovalent anchoring is highly stable through multiple M-O-P bonds [11,12]. In particular, the Titanium oxide will be our nano-support of choice, because of the simple “soft chemistry” preparation routes of crystallized nanoparticles of titanium oxide widely studied in the literature and in our laboratory [13].

2. Results and Discussion

2.1. Heteroleptic Precursors Synthesis

A series of heteroleptic phosphonate derivatives of general formulae [Ti2(OiPr)6(O3P-R)]m [R = C2H3 (1), C6H5 (2), CH2CH2NHCH2(C10H7)(3), CH2CH2(NC9H10)(4)] was synthetized by Brönsted acid-base reactions between Ti(OiPr)4 and 0.5 equivalents of the corresponding phosphonic acid (Table1), at ambient temperature in isopropanol and under an argon atmosphere (Equation (1)).

iPrOH 2m Ti(OiPr)4 + m (HO)2 OP − R −→ [Ti2(OiPr)6(O3P − R)] + 2m HOiPr (1) 20◦C m

R = C2H3 (1); R = C6H5 (2); R = CH2CH2NHCH2(C10H7) (3); R = CH2CH2(NC9H10) (4) Higher equivalents of phosphonic acid led to insoluble precipitates. After a drying process, all products were orange-brown liquids. Compounds 1 and 2 were soluble in THF while 3 and 4 were only soluble in hot isopropanol. MoleculesMoleculesMolecules 20182018 2018,, 2323, 23,, xx, xFORFOR FOR PEERPEER PEER REVIEWREVIEW REVIEW 22 2ofof of 1616 16

photocatalyticphotocatalyticphotocatalytic degradation,degradation,degradation, ionionion exchange,exchange,exchange, membranemembranemembrane (ultra)filtration,(ultra)filtration,(ultra)filtration, coagulation/flocculation,coagulation/flocculation,coagulation/flocculation, solventsolventsolvent extractionextraction extraction andand and reversereverse reverse osmosisosmosis osmosis cancan can alsoalso also bebe be notnot not efficient.efficient. efficient. However,However, However, solid-phasesolid-phase solid-phase extractionextraction extraction isis is generallygenerallygenerally consideredconsidered considered toto to bebe be superiorsuperior superior forfor for waterwater water re-usere-use re-use inin in termsterms terms ofof of energyenergy energy costs,costs, costs, flexibilityflexibility flexibility andand and simplicitysimplicity simplicity ofofof design,design, design, easeease ease ofof of operationoperation operation andand and insensitivityinsensitivity insensitivity toto to toxictoxic toxic pollutantspollutants pollutants andand and becausebecause because itit it doesdoes does notnot not resultresult result inin in thethe the formationformationformation ofof of harmfulharmful harmful by-productby-product by-product oror or toxictoxic toxic concentratedconcentrated concentrated sludgesludge sludge [3–5].[3–5]. [3–5]. HybridHybridHybrid (organic-inorganic)(organic-inorganic) (organic-inorganic) nanostructured nanostructurednanostructured materials materialsmaterials of ofof class classclass II IIII combining combiningcombining largelarge large surface-to- surface-to-surface-to- volumevolumevolume ratioratio ratio andand and controllablecontrollable controllable surfacesurface surface functionalizationfunctionalization functionalization cancan can provideprovide provide aa a powerfulpowerful powerful potentialpotential potential toto to updateupdate update traditionaltraditionaltraditional adsorption adsorptionadsorption treatment treatmenttreatment process. process.process. One OneOne of ofof the thethe goals goalsgoals of ofof nanotechnology nanotechnologynanotechnology for forfor the thethe pollutant pollutantpollutant removalremovalremoval fromfrom from wastewaterwastewater wastewater isis is toto to developdevelop develop efficientefficient efficient andand and selectiveselective selective nanosorbentsnanosorbents nanosorbents thatthat that cancan can removeremove remove lowlow low concentrationconcentrationconcentration contaminants contaminantscontaminants in inin the thethe presence presencepresence of ofof competing competingcompeting ones. ones.ones. AA A recent recentrecent study studystudy on onon the thethe removal removalremoval selectivityselectivityselectivity ofof of PAHsPAHs PAHs fromfrom from aa a wastewwastew wastewateraterater treatmenttreatment treatment plantplant plant (WWTP)(WWTP) (WWTP) locatedlocated located inin in anan an urbanurban urban areaarea area inin in ChinaChina China showsshowsshows thatthat that LMWLMW LMW PAHsPAHs PAHs suchsuch such asas as naphthalenenaphthalene naphthalene cancan can bebe be removedremoved removed atat at aa a levellevel level ofof of 80%,80%, 80%, whilewhile while HMWHMW HMW onesones ones becamebecamebecame eveneven even moremore more concentratedconcentrated concentrated withwith with treatmenttreatment treatment inin in bothboth both thethe the dissolveddissolved dissolved phasephase phase andand and thethe the dewatereddewatered dewatered sludgesludgesludge [6].[6]. [6]. GivenGiven Given thethe the naturenature nature ofof of PAHs,PAHs, PAHs, useuse use ofof of supported-aromaticsupported-aromatic supported-aromatic derivativesderivatives derivatives toto to formform form "" ππ"π-stacking"-stacking"-stacking" chargechargecharge transfertransfer transfer complexescomplexes complexes hashas has alreadyalready already beenbeen been reportreport reportededed sincesince since thesethese these interactiointeractio interactionsnsns areare are effectiveeffective effective inin in anan an aqueousaqueousaqueous medium,medium, medium, non-degradingnon-degrading non-degrading andand and reversiblereversible reversible [7–9].[7–9]. [7–9]. PreviousPrevious Previous studiesstudies studies havehave have reportedreported reported thethe the removalremoval removal ofofof PAHsPAHs PAHs fromfrom from aqueousaqueous aqueous solutionssolutions solutions usingusing using aa a largelarge large varietyvariety variety ofof of supportssupports supports suchsuch such asas as clay,clay, clay, resins,resins, resins, activatedactivated activated carbon,carbon,carbon, periodicperiodic periodic modifiedmodified modified mesoporousmesoporous mesoporous organosorganos organosilaneilaneilane (POMs),(POMs), (POMs), metal-organicmetal-organic metal-organic frameworksframeworks frameworks (MOFs)(MOFs) (MOFs) [4,5,10].[4,5,10].[4,5,10]. However,However, However, studiesstudies studies dealingdealing dealing withwith with thethe the influencinfluenc influencee e ofof of thethe the naturenature nature ofof of thethe the trappingtrapping trapping organicorganic organic moleculemolecule molecule ononon adsorptionadsorption adsorption propertiesproperties properties towardstowards towards PAHPAH PAH selectivityselectivity selectivity oror or affinityaffinity affinity inin in mixturemixture mixture ofof of LMWLMW LMW andand and HMWHMW HMW onesones ones areare are scarce.scarce.scarce. InInIn thisthis this study,study, study, wewe we reportreport report thethe the synthesis,synthesis, synthesis, charactericharacteri characterizationzationzation andand and propertiesproperties properties ofof of newnew new hybridhybrid hybrid (in(in (in thisthis this casecase case organic-inorganic)organic-inorganic)organic-inorganic) nanotinanoti nanotitaniataniatania supportssupports supports functionalizedfunctionalized functionalized withwith with aa a sese seriesriesries ofof of organophosorganophos organophosphorusphorusphorus moleculesmolecules molecules 2 2 (R–OPO(R–OPO(R–OPO2HH2H2))2 )holdingholding holding differentdifferent different unsaturatedunsaturated unsaturated groupsgroups groups (R(R (R == = vinyl,vinyl, vinyl, phenyl,phenyl, phenyl, naphtyl)naphtyl) naphtyl) andand and spacers.spacers. spacers. TheThe The performancesperformancesperformances ofof of suchsuch such ππ π-stacking-stacking-stacking complexationcomplexation complexation sitessites sites willwill will bebe be comparedcompared compared inin in termsterms terms ofof of selectivity,selectivity, selectivity, capacitycapacity capacity andandand kinetickinetic kinetic ofof of adsorptionadsorption adsorption inin in anan an aqueousaqueous aqueous mixturemixture mixture ofof of 1616 16 PAHPAH PAH molecules.molecules. molecules. ThereThere There isis is anan an abundantabundant abundant literatureliterature literature x reportingreportingreporting thethe the useuse use ofof of organophosphorusorganophosphorus organophosphorus moleculesmolecules molecules forfor for surfacesurface surface modificationmodification modification ofof of metalmetal metal oxidesoxides oxides (MO(MO (MOx))x ) becausebecausebecause theythey they allowallow allow thethe the controlcontrol control ofof of aa a mono-layermono-layer mono-layer functionalizationfunctionalization functionalization ofof of metalmetal metal oxideoxide oxide surfacessurfaces surfaces resultingresulting resulting fromfromfrom thethe the non-competitionnon-competition non-competition betweenbetween between hetero-andhetero-and hetero-and homohomo homo-condensation-condensation-condensation butbut but alsoalso also butbut but alsoalso also becausebecause because thisthis this ionocovalentionocovalentionocovalent anchoringanchoring anchoring isis is highlyhighly highly stablestable stable throughthrough through multiplemultiple multiple M-O-PM-O-P M-O-P bondsbonds bonds [11,12].[11,12]. [11,12]. InIn In particular,particular, particular, thethe the TitaniumTitaniumTitanium oxideoxide oxide willwill will bebe be ouou ourr rnano-supportnano-support nano-support ofof of choice,choice, choice, becausebecause because ofof of thethe the simplesimple simple "soft"soft "soft chemistry"chemistry" chemistry" preparationpreparation preparation routesroutesroutes ofof of crystallizedcrystallized crystallized nanoparticlesnanoparticles nanoparticles ofof of titaniumtitanium titanium oxideoxide oxide widelywidely widely studiedstudied studied inin in thethe the literatureliterature literature andand and inin in ourour our laboratorylaboratorylaboratory [13].[13]. [13].

2.2.2. ResultsResults Results andand and DiscussionDiscussion Discussion

2.1.2.1.2.1. HeterolepticHeteroleptic Heteroleptic PrecursorsPrecursors Precursors SynthesisSynthesis Synthesis

2 6 3 m AAA seriesseries series ofof of heterolepticheteroleptic heteroleptic phosphonatephosphonate phosphonate derivativesderivatives derivatives ofof of generalgeneral general formulaeformulae formulae [Ti[Ti [Ti2(OiPr)(OiPr)2(OiPr)6(O(O6(O3P-R)]P-R)]3P-R)]m m [R[R [R == = 2 3 6 5 2 2 2 10 7 2 2 9 10 CCC2HH2H3 3(( 11(),1),), CC C6HH6H5 5(( 22(),2),), CHCH CH2CHCH2CH2NHCHNHCH2NHCH2(C(C2(C1010HHH7))7 ()(3 3(),),3 ),CHCH CH2CHCH2CH2(NC(NC2(NC9HH9H10)10) (()4)4) (4)]] was]was was synthetizedsynthetized synthetized byby by BrönstedBrönsted Brönsted acid-baseacid-base acid-base Molecules 2018, 23, 3046 4 3 of 15 reactionsreactionsreactions betweenbetween between Ti(OiPr)Ti(OiPr) Ti(OiPr)4 4andand and 0.50.5 0.5 equivalentsequivalents equivalents ofof of thethe the correspondingcorresponding corresponding phosphonicphosphonic phosphonic acidacid acid (Table(Table (Table 1),1), 1), atat at ambientambientambient temperaturetemperature temperature inin in isopropanolisopropanol isopropanol andand and underunder under anan an argonargon argon atmosphereatmosphere atmosphere (Equation(Equation (Equation 1).1). 1). Table 1. Structures of the different phosphonic acids used in this study. TableTableTable 1.1. 1. StructuresStructures Structures ofof of thethe the differentdifferent different phosphphosph phosphoniconiconic acidsacids acids usedused used inin in thisthis this study.study. study. Different Phosphonic Acids Structures DifferentDifferentDifferent PhosphonicPhosphonic Phosphonic AcidsAcids Acids Structures Structures Structures OO O OHOH P OH VinylVinylVinyl phosphonicphosphonic phosphonicVinyl phosphonic acidacid acid (VPA)(VPA) (VPA) acid (VPA) PP OH OHOH OO O OHOH P OH P P Phenyl phosphonic acid (PPA) OH PhenylPhenyl phosphonPhenyl phosphon phosphonicicic acid acid (PPA) (PPA) acid (PPA) OHOH

(2-{[(Naphthalen-2-yl) methyl] amino} ethyl) phosphonic acid (NMAPA) (2-{[(Naphthalen-2-yl)Molecules(2-{[(Naphthalen-2-yl)(2-{[(Naphthalen-2-yl) 2018, 23, x FOR methyl] PEER methyl] REVIEW methyl]amino} amino} amino}ethyl) ethyl) phosphonic phosphonic ethyl) phosphonic acid acid (NMAPA) (NMAPA) acid (NMAPA) 3 of 16

[2-(3,4-Dihydroisoquinolin-2(1[2-(3,4-Dihydroisoquinolin-2(1H)-yl) ethyl]phosphonicH)-yl) ethyl]phosphonic acid (HQPA) acid (HQPA)

2m TiOiPr()+− m HO () OP R⎯⎯⎯iPrOH→[ Ti ()() OiPrOPr P −+ R] 2m HOi 4220 C° 23 6m (1) All the derivatives were obtained in quantitative yields without purification step and were fully characterized by FT-IR, 1H- and 31 P-NMR and P, Ti elemental analyses. All FT-IR spectra exhibited R = C2H3 (1); R = C6H5 (2); R = CH2CH2NHCH2(C10H7) (3); R = CH2CH2(NC9H10) (4) absorption bands at 1260 cm−1, between 1200 and 950 cm−1 and below 800 cm−1 corresponding toHigher P=O, P–O equivalents and Ti–O of bonds, phosphonic respectively. acid led Theto insoluble1H-NMR precipitates. spectrum of After1, at a roomdrying temperature process, all in productsCDCl3, were displayed orange-brown a 36:6:3 relative liquids. integration Compounds between 1 and CH 23 were, CH andsoluble vinylic, in THF respectively, while 3 and in agreement 4 were 31 onlywith soluble the proposed in hot isopropanol. general formulae [Ti2(OiPr)6(O3PC2H3)]m. The P-NMR spectrum showed the presenceAll the of derivatives only a single were signal obtained at 7.46 in ppm,quantitative high fielded yields in without comparison purification to the vinyl step and phosphonic were fully acid characterizedat 18.88 ppm. by Same FT-IR, characteristics 1H- and 31 P-NMR were and observed P, Ti elem for allental the derivativesanalyses. All2 –FT-IR4 and spectra were in exhibited agreement absorptionwith the literaturebands at 1260 data cm [11−].1, between 1200 and 950 cm−1 and below 800 cm−1 corresponding to P=O, P–O and Ti–O bonds, respectively. The 1H-NMR spectrum of 1, at room temperature in CDCl3, displayed2.2. Hybrid a 36:6:3 Nanomaterials relative integration Synthesis between CH3, CH and vinylic, respectively, in agreement with 31 the proposedDerivatives general1– 4formulaehave been [Ti2 used(OiPr) as6(O hydrolysable3PC2H3)]m. The precursors P-NMR inspectrum a further showed sol-gel the step presence to afford of only a single signal at 7.46 ppm, high fielded in comparison to the vinyl phosphonic acid at 18.88 hybrid (TiO2)x(O3P-R) nanomaterials, following a procedure we have already described for titania ppm.carboxylic Same characteristics surface modification were observed [14] (Equation for all (2)).the derivatives By varying 2 the–4 and co-hydrolysis were in agreement step parameters, with the we literaturehave checked data [11]. the influence of the amount and/or the nature of R moiety towards the performance of the resulting nanomaterials as nano-absorbents for a mixture of 16 PAHs. 2.2. Hybrid Nanomaterials Synthesis ) Derivatives 1–4 have been used as hydrolysable precursors1 iPrOH in a further sol-gel step to afford [Ti2(OiPr)6(O3P − R)]m + y Ti(OiPr)4 −−−−−−−−−−−−→ (TiO2)X(O3P − R) (2) 2)H2O,100◦C NR 4Br hybrid (TiO2)x(O3P-R) nanomaterials, following a procedure we have already described for titania carboxylic surface modification [14] (Equation 2). By varying the co-hydrolysis step parameters, we 2.2.1. Syntheses and Characterization of (TiO )x(VPA) Samples (Ti/P ratio (x) = 2–200) have checked the influence of the amount and /or2 the nature of R moiety towards the performance of the resultingThe first nanomaterials parameter to as modify nano-a wasbsorbents the amount for a ofmixture vinyl phosphonicof 16 PAHs. acid (VPA) by co-hydrolyzing the derivative 1 with different amounts of Ti(OiPr)4 in order to produce white powders with controlled [Ti()() OiPr O P−+ R] y Ti() OiPr⎯⎯⎯⎯⎯→1)iPrOH ()() TiO O P − R (2) Ti/P ratio (x) from23 2 to64 200. All X-raym powder diffractograms21)H2 O, 00°C?NR4 correspondedBr 23X to the anatase phase of TiO2 (file ICDD PDF-4+, 00-21-1272) with crystallite sizes estimated by the Debye-Scherrer equation at 2.2.1.around Syntheses 4–6 nm and in sizeCharacterization (Figure S1). This of (TiO result2)x(VPA) suggested Samples that (Ti/P the crystallite ratio (x) = size 2–200) of TiO 2, the nature of theThe phase first and parameter the crystallinity to modify were notwas affected the amount by the presenceof vinyl ofphosphonic the phosphonic acid acid(VPA) graft by up co- to a certain limit, since the highest concentrated phosphonic acid sample (x = 2) was amorphous. Unlike hydrolyzing the derivative 1 with different amounts of Ti(OiPr)4 in order to produce white powders withpure controlled TiO2, no traceTi/P ratio of the (x) brookite from 2 phase to 200. was Al detected.l X-ray powder diffractograms corresponded to the Figure1 shows the nitrogen adsorption-desorption isotherms for the bare TiO and some samples anatase phase of TiO2 (file ICDD PDF-4+, 00-21-1272) with crystallite sizes estimated2 by the Debye- ◦ Scherrer(TiO2)x equation(VPA) (x at = around 50, 100, 4–6 200) nm desorbed in size (Figure at 100 S1).C This for 6result h. All suggested displayed that type the crystallite IV(a) isotherms size with an H2(b)-type hysteresis loop (according to IUPAC terminology) [15], typical for mesoporous of TiO2, the nature of the phase and the crystallinity were not affected by the presence of the phosphonicmaterials withacid complexgraft up to pore-networks. a certain limit, Data since showed the highest a slight concentrated increase of thephosphonic specific surface acid sample area up (x to 269–333 m2/g for hybrid samples and that these latter exhibited larger final saturation plateau than = 2) was amorphous. Unlike pure TiO2, no trace of the brookite phase was detected. bare TiO , suggesting higher inter-particle interactions and extra-mesoporosity. Figure2 1 shows the nitrogen adsorption-desorption isotherms for the bare TiO2 and some samples (TiO2)x(VPA) (x = 50, 100, 200) desorbed at 100 °C for 6 h. All displayed type IV(a) isotherms with an H2(b)-type hysteresis loop (according to IUPAC terminology) [15], typical for mesoporous materials with complex pore-networks. Data showed a slight increase of the specific surface area up to 269–333 m2/g for hybrid samples and that these latter exhibited larger final saturation plateau than bare TiO2, suggesting higher inter-particle interactions and extra-mesoporosity.

Molecules 2018, 23, 3046 4 of 15 Molecules 2018, 23, x FOR PEER REVIEW 4 of 16 Molecules 2018, 23, x FOR PEER REVIEW 4 of 16

FigureFigure 1. N2 1.adsorption-desorption N2 adsorption-desorption isotherms isotherms for bareTiO 2,2 and, and hybrid hybrid samples samples (TiO (TiO2)x(VPA)2)x (VPA) with x with x =Figure 50,= 10050, 1.100 and N 2and adsorption-desorption 200. 200. isotherms for bareTiO2, and hybrid samples (TiO2)x(VPA) with x = 50, 100 and 200. The mostThe most important important study study remained remained the surface the surface characterizations characterizations in order in order to compare to compare the differentthe sampledifferentThe natures most sample andimportant natures to correlate studyand to remained themcorrelate to them thethe synthesissurfaceto the synthesis characterizations procedure procedure and in and toorder to their their to ﬁnalcomparefinal properties properties the as PAHdifferent nano-absorbents.as PAH sample nano-absorbents natures and to correlate them to the synthesis procedure and to their final properties asAll PAH FT-IRAll nano-absorbents FT-IR spectra spectra exhibited exhibited strong strong absorption bands bands ascribed ascribed to Ti-O to Ti-Oand O-H and bonds O-H below bonds 800 below −1 −1 −1 cm−All1, aroundFT-IR spectra 1620 cm exhibited −and1 above strong 2800 absorption cm , (Figure −bands1 S2), ascribed however, to Ti-O the andconcentrations O-H bonds ofbelow VPA 800 were 800 cm−1 , around 1620− cm1 and above 2800−1 cm , (Figure S2), however, the concentrations of−1 VPA cmtoo, around small to 1620 display cm clearand aboveabsorption 2800 cmbands, (Figure assigned S2), to however, O–P–O groupthe concentrations between 1200 of and VPA 950 were cm . −1 weretoo tooOnly small small a toweak todisplay display band clear at clear 1260absorption absorption cm−1 wasbands bandsrelated assigned assigned to P=O to O–P–O bond. to O–P–O groupPhosphonate group between between and 1200 C=C and 1200 bonds950 and cm 950were−1. cm . −1 OnlyOnly aevidenced weak a weak band by band atDRIFT 1260 at 1260experiments cm cmwas−1 was related under related toargon P=O to atP=O bond. 150 bond.°C Phosphonate since Phosphonate spectra anddisplayed and C=C C=C bonds strong bonds were adsorption were evidenced ◦ by DRIFTevidencedbands experimentsat by around DRIFT 1000 experiments under cm−1 (symmetrical argon under at 150argon andC at unsymmetrical since 150 °C spectra since spectra stretching displayed displayed band strong of strong O–P–O) adsorption adsorption and weak bands at − aroundbandsones 1000 at at around 1260 cm cm1 1000(symmetrical−1 (ν P=O),cm−1 (symmetrical and 1619 and unsymmetricalcm −and1 (νC=C). unsymmetrical (Figure stretching 2) stretchingAbsorption band band bands ofO–P–O) of inO–P–O) the andrange and weak ofweak 3000– ones at 1260ones cm2800 −at1 cm1260(νP=O),−1 (C-Hcm−1and bending)(νP=O), 1619 and cmand −1619 1500–13501 (ν C=C).cm−1 (ν cmC=C). (Figure−1 (C-H (Figure2) stretching) Absorption 2) Absorption could bands be bands inexplained the in range the by range ofthe 3000–2800 presenceof 3000– of cm −1 (C-H2800residual bending) cm−1 (C-H alcohol and bending) 1500–1350 molecules and at1500–1350 cm the−1 surface.(C-H cm stretching) −1 (C-H stretching) could could be explained be explained by the by the presence presence of of residual alcoholresidual molecules alcohol molecules at the surface. at the surface.

◦ Figure 2. DRIFT spectra of samples (TiO2)100(VPA) at 150 C. In an attempt to quantify the ﬁnal VPA content or at least to estimate its control, TGA-TDA ◦ experiments of the (TiO2)x(VPA) samples were performed under air from 20 to 600 C. All the patterns Molecules 2018, 23, x FOR PEER REVIEW 5 of 16 Molecules 2018, 23, x FOR PEER REVIEW 5 of 16 Figure 2. DRIFT spectra of samples (TiO2)100(VPA) at 150 °C.

Figure 2. DRIFT spectra of samples (TiO2)100(VPA) at 150 °C. In an attempt to quantify the final VPA content or at least to estimate its control, TGA-TDA experiments of the (TiO2)x(VPA) samples were performed under air from 20 to 600 °C. All the patterns MoleculesIn 2018an ,attempt23, 3046 to quantify the final VPA content or at least to estimate its control, TGA-TDA5 of 15 wereexperiments similar ofand the displayed (TiO2)x(VPA) two samplesmajor weight were performedlosses ranging under from air from20 to 20 170 to °C600 (endothermic °C. All the patterns step) andwere from similar 170 and to 500displayed °C (exothermic two major step), weight corre lossessponding ranging to fromthe removal20 to 170 of °C water (endothermic and solvents step) ◦ were(alcohols)and similarfrom adsorbed 170 and to displayed 500 and °C to (exothermicthe two pyrolysis major weight step), the hydrox lossescorrespondingyl ranging groups from (–OH)to the 20 to andremoval 170 surfaceC (endothermicof vinylwater phosphonates, and step) solvents and ◦ fromrespectively(alcohols) 170 to adsorbed 500 (FigureC (exothermic andS3).The to the relationship pyrolysis step), corresponding the between hydrox theyl to groupsorganic the removal (–OH) content ofand and water surface the and weight vinyl solvents phosphonates,loss (alcohols)observed adsorbedinrespectively TGA was and difficult(Figure to the pyrolysis S3).Theto establish therelationship hydroxylsince phosphate between groups (–OH)ligands the organic and (PO surface4 )content still vinylremained and phosphonates, the ontoweight the loss TiO respectively observed2 surface (Figureuntilin TGA 900 S3).The was °C. difficult relationship to establish between since the phosphate organic content ligands and (PO the4) still weight remained loss observed onto the inTiO TGA2 surface was ◦ difﬁcultuntilHowever, 900 to °C. establish in comparison since phosphate with the ligands TDA patterns (PO4) still of remainedthe richest onto VPA the sample TiO2 surface (from the until hydrolysis 900 C. of 1)However, However,and the bare in in comparison comparison TiO2, one with withcould the the estimate TDA TDA patterns patterns that the of of the remova the richest richestl of VPA VPAvinyl sample sample phosphonic (from (from the theorganic hydrolysis hydrolysis units of 1occurredof) and 1) and the mainly barethe TiObare between2 ,TiO one2, could 300one and could estimate 450 estimate °C. that The the increasethat removal the inremova of the vinyl organicl phosphonicof vinyl content phosphonic organicat the surface units organic occurredwas unitsalso ◦ mainlyaccompaniedoccurred between mainly by 300 abetween reduction and 450 300 C.in and Thethe 450 increaseintensity °C. The in of the increasethe organic TDA in peak content the organicbetween at the content surface200 and at was 275 the also °C, surface accompaniedassociated was also to ◦ bytheaccompanied a loss reduction of –OH by in groups thea reduction intensity (Figure in of 3). the the intensity TDA peak of between the TDA 200 peak and between 275 C, associated200 and 275 to °C, the associated loss of –OH to groupsthe loss (Figure of –OH3). groups (Figure 3).

Figure 3. TDA of bare and some hybrid TiO2 samples (samples Ti/P= 25; 75, 125 and 200 are omitted Figure 3. TDA of bare and some hybrid TiO2 samples (samples Ti/P = 25; 75, 125 and 200 are omitted forFigure clarity). 3. TDA of bare and some hybrid TiO2 samples (samples Ti/P= 25; 75, 125 and 200 are omitted for clarity). for clarity). Then, by plotting the weight loss between 300 and 500 °C versus the starting Ti /P ratios (Figure Then, by plotting the weight loss between 300 and 500 ◦C versus the starting Ti /P ratios (Figure4), 4), one could demonstrate that weight losses followed a linear trend (y = −0.0189x + 4.5638) with a one couldThen, demonstrate by plotting the that weight weight loss losses between followed 300 aand linear 500 trend °C versus (y = − the0.0189x starting + 4.5638) Ti /P ratios with (Figure a high high regression coefficient R2 = 0.94. regression4), one could coefﬁcient demonstrate R2 = 0.94. that weight losses followed a linear trend (y = −0.0189x + 4.5638) with a high regression coefficient R2 = 0.94.

Figure 4. Variation of organic weight loss in the range of 300–500 ◦C in thermogravimetric analysis as a function of the starting Ti/P ratio. The most loaded sample (Ti/P = 25) having the highest weight loss of 4.25%, while for higher Ti/P ratios, a phenomenon of dilution of the phosphonate ligands at the TiO2 surface logically decreased Molecules 2018, 23, 3046 6 of 15 the percentages down to 1.05%. Such thermal analysis has thus demonstrated that the phosphonate loading in the hybrid nanomaterial has varied constantly with the initial stoichiometry of the solutions of hydrolysis (Ti/P ratio) and therefore, the degree of surface functionalization of TiO2 nanocrystallites could be controlled, to some extent, by the initial concentration of the heteroleptic precursor holding the phosphonate moiety. Ti and P elemental analyses on the samples (TiO2)x(VPA) were carried out and exhibited close values to the expected ones (Table2).

Table 2. Ti and P elemental analyses of the samples (TiO2)x(VPA).

Experimental Theoretical Ti/P Ratio % Ti % P Ti/P Ratio 25 50.69 1.03 32 50 51.29 0.72 46 75 82.00 0.82 65 100 50.03 0.35 92 125 68.43 0.36 123 150 79.84 0.35 148 200 106.31 0.36 190

31P{1H} MAS NMR experiments exhibited similar patterns show two distinct resonances upfield and downfield shifts (lower and higher ppm) at about 19 and 13 ppm and broadened compared to the resonance of the original ligand VPA at about 18 ppm (Figure S4). These data, in agreement with the data given in the literature, did again confirm the phosphonate grafting onto the TiO2 surface nanoparticles [16,17]. The width of the peaks and the presence of P=O absorption band in DRIFT spectra (Figure2) suggested a mixture of bidentate and tridentate surface grafting modes [ 11]. The data also demonstrated the absence of molecular titanium phosphonate phases formed by dissolution-precipitation process which are expected to give peak at higher field such as layered titanium phenyl-phosphonates at −4.0 ppm [18]. In an attempt to choose an appropriate Ti/P ratio for the following adsorption study, we estimated the density of VPA molecules onto 5 nm TiO2 nanocrystallite surface. The intermediate Ti/P ratio of 100 was selected for comparison because (i) sample contained enough phosphonic acid ligand to show an effect and (ii) only 12% of Ti atoms were coordinated or 5 % of the surface were covered, assuming only tridentate bonding mode or an area of 24 Å2 per molecule of phosphonic acid, respectively. This would achieve high dispersion of the un-saturated receptor, avoiding any side π−π stacking effect between aromatic rings onto the surface. In the same way, Stellaci et al. have demonstrated the use of mixed-ligand gold-coated nanoparticles (short and long thioethers) as efficient receptor for PAH because of supramolecular effect [19]. So for the following syntheses, the theoretical ratio Ti/P was kept constant to 100.

2.2.2. Syntheses and Characterizations of (TiO2)100(O3P-R) Samples Following the same procedure as for the VPA ligand, a series of three new hybrid nanomaterials of general formulae (TiO2)100(O3P-R) was synthetized as white powders. The elemental analyses data conﬁrmed a Ti/P ratio close to 100 (Table3).

Table 3. Ti and P elemental analysis of hybrid samples (TiO2)100(O3P-R).

R Group PPA NMAPA HQPA % Ti 54.52 59.62 59.06 % P 0.36 0.36 0.36 Ti/P 98 106 105 Molecules 2018, 23, x FOR PEER REVIEW 7 of 16 Molecules 2018, 23, 3046 7 of 15 Ti/P 98 106 105

31 1 PowderPowder XRD XRD (Figure (Figure S5), S5), FT-IR FT-IR (Figure S6), S6), TGA-TDA TGA-TDA (Figure (Figure S7) and S7) solid and state solid 31P state{1H} MASP{ H} MASNMR (Figure (Figure S8) S8) data data were were consistent consistent with with the theresults results already already obtained obtained with the with VPA the ligand. VPA ligand.In In addition,addition, nanomaterials nanomaterials belonging aromatic aromatic rings rings such such as (TiO as (TiO2)100(PPA),2)100(PPA), (TiO2)100 (TiO(NMAPA),2)100(NMAPA), and and (TiO(TiO22))100100(HQPA),(HQPA), displayed displayed a characteristic a characteristic shift of shift the ofcarbons the carbons linked to linked the aromatic to the ring aromatic at δ = 128 ring at δ = 128ppm ppm in the in the solid solid state state 13C {113H}C{ MAS1H} NMR MAS spectra. NMR spectra (Figure (FigureS9) S9). SimilarSimilar to VPA–based to VPA–based hybrid hybrid nanomaterials,nanomaterials, the the three three new newhybrid hybrid nanoma nanomaterialsterials exhibited exhibited close specific surface area (225–322 m2/g) and volume pore (0.39–0.51 cm3/g) and similar type IV(a) close speciﬁc surface area (225–322 m2/g) and volume pore (0.39–0.51 cm3/g) and similar type IV(a) isotherms than bare TiO2 (Figure 5 and Table S2). However, (TiO2)100(PPA) and (TiO2)100(NMAPA) isothermsmaterials than displayed bare TiO 2different(Figure 5adso andrbent Table textures S2). However, with type (TiO H3 2hy)100steresis(PPA) loops and (TiOsuggesting2)100(NMAPA) the materialspresence displayed of macroporosity different adsorbent [20]. textures with type H3 hysteresis loops suggesting the presence of macroporosity [20].

FigureFigure 5. Nitrogen5. Nitrogen adsorption-desorption adsorption-desorption isothermsisotherms of (TiO2))100100(PPA),(PPA), (TiO (TiO2)1002)(NMAPA),100(NMAPA), (TiO2)(TiO100(HQPA).2)100(HQPA).

2.3. Kinetic2.3. Kinetic PAH PAH Adsorption Adsorption Studies Studies of (TiOof (TiO2)1002)100(O(O33P-R) Samples Samples

BatchBatch absorption absorption tests tests using using 10 mg 10 of mg (TiO of 2(TiO)100(VPA)2)100(VPA) and and 50 mL 50 ofmL 200 of ppb200 PAHppb PAH aqueous aqueous solution for 24solution h at 20 ◦forC and24 h atat pH20 °C 6.5, and showed at pH that6.5, theshowed 16 PAH that moleculesthe 16 PAH were molecules extracted were with extracted efﬁciencies with of 93–100%efficiencies leading of to 93–100% a high leading total equilibrium to a high total adsorption equilibrium capacity adsorption of aroundcapacity 16–17of around mg ·16–17g−1 (or mg·g around−1 −1 1 mg·g(or−1 aroundof each 1 PAH).mg·g Thisof each was PAH). conﬁrmed This was by confirmed doubling theby doubling volume ofthe PAH volume aqueous of PAH solution, aqueous using solution, using 100 mL batches of 200 ppb PAH solution with 10 mg of the materials, whereupon the 100 mL batches of 200 ppb PAH solution with 10 mg of the materials, whereupon the adsorption adsorption percentage was half that of the previous tests (Figure 6). percentageMolecules was 2018,half 23, x FOR that PEER of the REVIEW previous tests (Figure6). 8 of 16

FigureFigure 6. Batch 6. Batch adsorption adsorption tests tests ofof (TiO2))100100(VPA)(VPA) after after 24 24h. h.

Close values were reported for nano-hybrids NH2-SBA-15 towards three PAHs (naphthalene, acenaphthylene and phenanthrene) [22] and periodic mesoporous organosilica functionalized with phenyl groups towards a mixture of five PAHs (naphthalene, acenaphthene, fluorine, fluoranthene, pyrene) [23] with adsorption capacity ranges of 0.76–1.92 mg g−1 and 0.72–1.69 mg g−1, respectively. Nano-absorbents combining both –NH2 and phenyl groups such as hybrid mesoporous p- aminobenzoic acid-MCM-41 (PABA-MCM-41) nano-materials were used to remove benzo[k]fluoranthene and benzo[b]fluoranthene from aqueous media [200 ppb] and have exhibited lower adsorption capacity in the range of 23–27 μg g−1 [24]. Adsorption kinetics studies were then carried out to determine the time required to reach adsorption equilibrium, the extraction efficiencies and the selectivities towards the PAHs of the different hybrid sorption systems. We carried out the experiment with (TiO2)100(VPA) in an aqueous solution containing only one PAH [i.e. benzo(a)pyrene] at a concentration of 220 ppb. The pattern showed that the equilibrium was reached after 90 min leading to an equilibrium capacity of 0.97 mg·g−1. Figure 7 showed that experimental data fitted very well with the pseudo-second order model (R2 value of 1.000) (Table S3) as largely reported for other absorbents such as zeolite, clays, sepiolite, activated carbon, biosorbents and hybrid mesoporous material [5]. Thanks to high surface area of (TiO2)100(VPA) nano-materials and stirring during the adsorption, mass transfer process of adsorbates to nano-TiO2’s surface was negligible that left interaction between adsorbates and active site on surface of adsorbent and adsorbate as a rate-limiting step in the adsorption process.

Molecules 2018, 23, 3046 8 of 15

Close values were reported for nano-hybrids NH2-SBA-15 towards three PAHs (naphthalene, acenaphthylene and phenanthrene) [22] and periodic mesoporous organosilica functionalized with phenyl groups towards a mixture of five PAHs (naphthalene, acenaphthene, fluorine, fluoranthene, pyrene) [23] with adsorption capacity ranges of 0.76–1.92 mg g−1 and 0.72–1.69 mg −1 g , respectively. Nano-absorbents combining both –NH2 and phenyl groups such as hybrid mesoporous p-aminobenzoic acid-MCM-41 (PABA-MCM-41) nano-materials were used to remove benzo[k]fluoranthene and benzo[b]fluoranthene from aqueous media [200 ppb] and have exhibited lower adsorption capacity in the range of 23–27 µg g−1 [24]. Adsorption kinetics studies were then carried out to determine the time required to reach adsorption equilibrium, the extraction efficiencies and the selectivities towards the PAHs of the different hybrid sorption systems. We carried out the experiment with (TiO2)100(VPA) in an aqueous solution containing only one PAH [i.e. benzo(a)pyrene] at a concentration of 220 ppb. The pattern showed that the equilibrium was reached after 90 min leading to an equilibrium capacity of 0.97 mg·g−1. Figure7 showed that experimental data fitted very well with the pseudo-second order model (R2 value of 1.000) (Table S3) as largely reported for other absorbents such as zeolite, clays, sepiolite, activated carbon, biosorbents and hybrid mesoporous material [5]. Thanks to high surface area of (TiO2)100(VPA) nano-materials and stirring during the adsorption, mass transfer process of adsorbates to nano-TiO2’s surface was negligible that left interaction between adsorbates and active site on surface of adsorbent and adsorbate asMolecules a rate-limiting 2018, 23, x FOR step PEER in the REVIEW adsorption process. 9 of 16

Figure 7. Adsorption capacity of (TiO(TiO22)100(VPA)(VPA) toward toward benzo(a)pyrene benzo(a)pyrene as as a a function of time. Insert: Pseudo-second model for benzo(a)pyrenebenzo(a)pyrene adsorptionadsorption kinetics.kinetics.

As shown inin FiguresFigures8 –8–11,11, all all hybrid hybrid (TiO (TiO2)2100)100(O3P-R) nanomaterials demonstrated remarkable 100% of removal efficiencyefficiency ofof all 16 PAHs afterafter 2424 h. Such results confirmed confirmed that an aromatic function was notnot necessarynecessary toto havehave efficientefficient chargecharge transfer,transfer, asas reportedreported by by Bloom Bloom et et al al [ 9[9].]. Adsorption kinetics of VPA-, PPA- and NMAPA-based material followed similar and two-stage patterns with first one ascribed as a competition between light, medium and heavy PAHs followed by an equilibrium one to reach 100% of adsorption of all PAHs. Interestingly, while VPA-based (vinyl) nanomaterial had more affinity with light PAH molecules in the first seven hours, going through PPA (phenyl) to NMAPA (naphtyl) gave totally opposite adsorption kinetic behaviors. In particular the NMAPA-based ligand displayed the highest affinity for the heavy PAHs during the first hours of experiment. We could ascribe this result to strongest quadrupole interactions between naphtyl groups (two cycles) and PAHs with several aromatic cycles than vinyl ones. It was also well- known that the most stable conformations of benzene dimers are parallel displaced or T-shaped compared to sandwich one [24]. One could speculate that these conformations led to different steric hindrance at the surface of the nano-materials and time was mandatory to reach different supramolecular assembly organizations. Other parameter that could be taken into account was the change of the adsorbent texture since PPA-and NMAPA-based mesoporous materials exhibited type H3 hysteresis loops consisting in the presence of macroporosity (more convenient for the diffusion of heavy PAHs) while VPA one exhibited type H2(b) hysteresis loop revealing small pore neck diameters (cavitation and pore blocking effects) [20]. We have already reported how supramolecular spatial arrangement of hybrid titania nanocrystals through the use of bi-functional ligands (DTPA) enabled to reach efficient lanthanide ionic separation with very high selectivity (i.e., SLa/Gd = 160) [25].

Molecules 2018, 23, x FOR PEER REVIEW 10 of 16

Molecules 2018, 23, x FOR PEER REVIEW 10 of 16 Molecules 2018, 23, 3046 9 of 15 Molecules 2018, 23, x FOR PEER REVIEW 10 of 16

Light PAH ...... , Medium PAH -----, Heavy PAH Figure 8. Adsorption kinetics of 16 PAHs on (TiO2)100(VPA). Light PAH ...... , Medium PAH -----, Heavy PAH Light PAH ...... , Medium PAH —–, Heavy PAH — Light PAH ...... , Medium PAH -----, Heavy PAH Figure 8. Adsorption kinetics of 16 PAHs on (TiO2)100(VPA). Figure 8. Adsorption kinetics of 16 PAHs on (TiO2)100(VPA). Figure 8. Adsorption kinetics of 16 PAHs on (TiO2)100(VPA).

Light PAH ...... , Medium PAH -----, Heavy PAH Light PAH ...... , Medium PAH —–, Heavy PAH —

Figure 9. Adsorption kinetics of 16 PAHs on (TiO2)100(PPA). FigureLight 9.PAHAdsorption ...... , Medi kineticsum PAH of 16 -----, PAHs Heavy on (TiO PAH2)100 (PPA). Light PAH ...... , Medium PAH -----, Heavy PAH Figure 9. Adsorption kinetics of 16 PAHs on (TiO2)100(PPA).

Figure 9. Adsorption kinetics of 16 PAHs on (TiO2)100(PPA).

Light PAH ...... , Medium PAH —–, Heavy PAH —

Figure 10. Adsorption kinetics of 16 PAHs on (TiO2)100(NMAPA).

Molecules 2018, 23, x FOR PEER REVIEW 11 of 16

Light PAH ...... , Medium PAH -----, Heavy PAH Molecules 2018, 23, 3046 10 of 15 Figure 10. Adsorption kinetics of 16 PAHs on (TiO2)100(NMAPA).

Light PAH ...... , Medium PAH —–, Heavy PAH — Light PAH ...... , Medium PAH -----, Heavy PAH Figure 11. Adsorption kinetics of 16 PAHs on (TiO2)100(HQPA). Figure 11. Adsorption kinetics of 16 PAHs on (TiO2)100(HQPA). Adsorption kinetics of VPA-, PPA- and NMAPA-based material followed similar and two-stage patternsFinally, with (TiO first2)100 one(HQPA) ascribed sample as a competition has shown more between affinity light, to medium heavy PAH and heavybut did PAHs not exhibit followed any by competitionan equilibrium process one during to reach the 100% first ofhours, adsorption adsorpti ofon all percentages PAHs. Interestingly, of all PAH while being VPA-based constant all (vinyl) over thenanomaterial experiment had(Figure more 11). affinity In comparison with light with PAH the molecules PPA-based in the sample first sevencontaining hours, also going one phenyl through ring,PPA this (phenyl) last result to NMAPA demonstrated (naphtyl) that gave subtle totally chan oppositege of the adsorption phenyl 3D kinetic spatial behaviors. organization In particular and/or texturethe NMAPA-based [from H3 to H2(b) ligand hysteresis displayed loop] the might highest drastically affinity forimpact the heavythe selectivity PAHs during of our hybrid the first nano- hours absorbentsof experiment. towards We mixture could ascribe of PAHs. this result to strongest quadrupole interactions between naphtyl groupsThermodynamic (two cycles) andstudies PAHs and with computational several aromatic calculations cycles than are vinylnow ones.under It progress was also in well-known order to elucidatethat the mostwhether stable mono conformations or multilayer of or benzene T-shaped dimers or parallel are parallel displaced displaced conformations or T-shaped could compared be at theto origin sandwich of such one selectivity [24]. One couldperformances. speculate that these conformations led to different steric hindrance at the surface of the nano-materials and time was mandatory to reach different supramolecular 3. Experimental Section assembly organizations. Other parameter that could be taken into account was the change of the adsorbent texture since PPA-and NMAPA-based mesoporous materials exhibited type H3 hysteresis 3.1. General Information loops consisting in the presence of macroporosity (more convenient for the diffusion of heavy PAHs) whileAll VPA manipulations one exhibited have type been H2(b) carried hysteresis out under loop revealingan argon atmosphere small pore neck using diameters standard (cavitation Schlenk andand glovebox pore blocking techniques. effects) THF [20]. Weand have HOiPr already were reported dried using how either supramolecular an MBRAUN spatial SPS-800 arrangement system of (Garching,hybrid titania Germany) nanocrystals (THF) through or distillation the use ofover bi-functional [Al(OiPr) ligands3]4, respectively. (DTPA) enabled Deuterated to reach solvents efficient (CDCllanthanide3, MeOD) ionic were separation stored over with molecular very high sieves. selectivity Vinyl (i.e., phosphonic SLa/Gd = 160)acid [(VPA),25]. phenyl phosphonic acid (PPA)Finally, and (TiO ammonium2)100(HQPA) salt sample(NnBu4Br) has were shown purchased more affinity from toSigma heavy Aldrich PAH but(Saint-Louis, did not exhibit MO, USA)any competitionand were used process as duringreceived. the firstPhosphonic hours, adsorptionacids such percentages as (C10H7)CH of all2NHCH PAH2 beingCH2PO(OH) constant2 (NMAPA),all over the (C experiment9H10N)CH2 (FigureCH2PO(OH) 11). In2 (HQPA) comparison were with synthesized the PPA-based according sample containingto the published also one procedurephenyl ring, and this stored last resultunder demonstratedargon [26]. Ti(OiPr) that subtle4 (TTIP) change was ofpurchased the phenyl from 3D Sigma spatial Aldrich organization and distilledand/or under texture vacuum [from H3 prior to H2(b) used. hysteresis The standard loop] stock might solution drastically containing impact the sixteen selectivity US-EPA of ourPAHs hybrid at a nano-absorbentsconcentration of 500 towards ppm mixtureeach was of purchased PAHs. from Sigma Aldrich. It was diluted to a concentration of 200 Thermodynamicppb (μg L−1) each studiesin deionized and computational water: ethanol calculationsmixture prior are used now in underextraction progress tests. in order to elucidate whether mono or multilayer or T-shaped or parallel displaced conformations could be at the 3.2.origin Product of such Characterizations selectivity performances. 1H liquid and 13C solid state MAS NMR spectra were recorded on an AM-250 spectrometer 3. Experimental Section Bruker (Billerica, MA, USA). 31P solid state MAS NMR were recorded on a Bruker DSX400 spectrometer3.1. General Informationspectra with a 4 mm probe and rotation speeds of 5 to 10 Kz. Analytical data were obtained from the Institut des Sciences Analytique (UMR 5280, Villeurbanne, France) by ICP. FT-IR All manipulations have been carried out under an argon atmosphere using standard Schlenk

and glovebox techniques. THF and HOiPr were dried using either an MBRAUN SPS-800 system Molecules 2018, 23, 3046 11 of 15

(Garching, Germany) (THF) or distillation over [Al(OiPr)3]4, respectively. Deuterated solvents (CDCl3, MeOD) were stored over molecular sieves. Vinyl phosphonic acid (VPA), phenyl phosphonic acid (PPA) and ammonium salt (NnBu4Br) were purchased from Sigma Aldrich (Saint-Louis, MO, USA) and were used as received. Phosphonic acids such as (C10H7)CH2NHCH2CH2PO(OH)2 (NMAPA), (C9H10N)CH2CH2PO(OH)2 (HQPA) were synthesized according to the published procedure and stored under argon [26]. Ti(OiPr)4 (TTIP) was purchased from Sigma Aldrich and distilled under vacuum prior used. The standard stock solution containing sixteen US-EPA PAHs at a concentration of 500 ppm each was purchased from Sigma Aldrich. It was diluted to a concentration of 200 ppb (µg L−1) each in deionized water: ethanol mixture prior used in extraction tests.

3.2. Product Characterizations 1H liquid and 13C solid state MAS NMR spectra were recorded on an AM-250 spectrometer Bruker (Billerica, MA, USA). 31P solid state MAS NMR were recorded on a Bruker DSX400 spectrometer spectra with a 4 mm probe and rotation speeds of 5 to 10 Kz. Analytical data were obtained from the Institut des Sciences Analytique (UMR 5280, Villeurbanne, France) by ICP. FT-IR spectra were recorded as Nujol suspensions (for air sensitive derivatives) or as KBr pellets (for hydrolyzed powders) on a Bruker Vector 22 FT-IR spectrometer at room temperature and registered from 4000 to 400 cm−1. DRIFT experiments were carried out on a Nicolet 6700 FTIR spectrophotometer (Thermo Fischer, Waltham, MA, USA) equipped with a MCT detector (number of scan: 64, resolution: 4 cm−1). The recording mode was diffuse reﬂectance and the optical unit, Praying Mantis (Harrick). TGA/TDA data were collected with a model 92 system (Setaram, Lyon, France) in air with a thermal ramp of 10 ◦C min−1. Powder X-ray diffraction data were obtained with a D5005 diffractometer (Siemens, Munich, Germany) by using Cu-Kα radiation at the scan rate of 0.02◦2θ s−1 with step times of 1 and 16 s for routine or for expanded region spectra, respectively. BET experiments were carried out with an ASAP 2010 system after the samples were desorbed at 100 ◦C for 6 h.

3.3. Synthesis of Titanium Precursors

3.3.1. Synthesis of [Ti2(OiPr)6(O3PC2H3)]m (1)

Under an argon atmosphere Ti(OiPr)4 (1.48 mL, 4.9 mmol) was dissolved in anhydrous THF (10 mL). In another Schlenck ﬂask, CH2=CH-PO(OH)2 (272 mg, 2.5 mmol) was added to anhydrous THF (23 mL). Under an inert atmosphere, the vinyl phosphonic acid solution was added dropwise to the Ti(OiPr)4 one. The mixture was stirred at room temperature for 18 h. After evaporation of the solvent, 1.62 g (quantitative yield relative to titanium) of 1, soluble in THF, were obtained as a 1 ◦ brown oil. Anal. Calcd for C20H45O9PTi2: Ti: 17.20; P: 5.56; Found Ti: 17.24, P: 5.50. H-NMR (20 C, 31 CDCl3); δ ppm 6.25–5.70 (m, 3H, –CH=CH2); 5.05–4.30 (m, 6H, –OCH); 1.45–1.06 (m, 36H, –CH3). P 1 ◦ −1 { H}-NMR (20 C, CDCl3); δ ppm: 7.46. FT-IR (Nujol), cm : 510 (s), 616 (s), 710 (m), 748 (m), 797(s), 851 (m) (ν Ti–O); 955 (s), 1008 (vs), 1056 (s), 1082 (s) (ν P–O); 1126 (vs), 1151 (sh), 1179 (sh) (ν C–O); 1261 (m) (ν P=O); 1331 (w); 1615(w) (ν C=C).

3.3.2. Synthesis of [Ti2(OiPr)6(O3PC6H5)]m (2)

Using the same procedure as previously described but with Ti(OiPr)4 (1.51 mL, 5 mmol) dissolved in anhydrous THF (6 mL) and C6H5PO(OH)2 (395 mg, 2.5 mmol) in anhydrous THF (17 mL) gave 1.76 g (quantitative yield relative to titanium) of 2 as a light brown oil soluble in THF. Anal. Calcd for 1 ◦ C24H47O9PTi2 Ti: 15.79, P: 5.11; Found Ti: 15.82, P: 5.13. H-NMR (20 C, CDCl3); δ ppm: 7.90–6.80 (m, 31 1 ◦ 5H, –C6H5); 5.14–4.23 (m, 6H, –OCH); 1.45–1.06 (m, 36H, –CH3). P{ H}-NMR (20 C, CDCl3); δ ppm: 9.70. FT-IR (Nujol), cm−1: 612(m), 625 (m), 702 (m), 748 (m), 838 (m), 853 (m) (ν Ti–O); 957 (s), 976 (s), 1009 (vs), 1034 (m), 1089 (s) (ν P–O); 1135 (vs), 1162 (sh) (ν C–O); 1257 (m) (ν P=O); 1320 (w); 1630 (w) (ν C=C). Molecules 2018, 23, 3046 12 of 15

3.3.3. Synthesis of [Ti2(OiPr)6(O3PCH2CH2NHCH2(C10H7)]m (3)

Using the same procedure as previously described with (C10H7)CH2NHCH2CH2-PO(OH)2 (0.290 g, 1 mmol) dissolved in hot isopropanol (20 mL) and Ti(OiPr)4 (0.268 g, 2 mmol) in isopropanol (3 mL) under reﬂux for 8 h gave 0.83 g (quantitative yield relative to titanium) of 3 as an orange oil soluble in isopropanol under reﬂux and insoluble in THF. Anal. Calcd for C31H56NO9PTi2Ti: 12.94, 1 ◦ P: 4.18; Found Ti: 12.97, P: 4.14. H-NMR (20 C, MeOD); δ ppm: 8.12 (m, 1H, –C10H7); 7.97–7.12 (m, 6H, –C10H7); 4.67–4.34 (m, 2H, –NCH2); 4.16–3.70 (m, 6H, –OCH); 3.42 (d, J = 33.5 Hz, 2H, –CH2); 2.04 31 1 ◦ (dd, J = 35.3; 28.3 Hz, 2H, –PCH2); 1.38–0.64 (m, 36H, –CH3). P{ H}-NMR (20 C, MeOD); δ ppm: 7.57. FT-IR (Nujol), cm−1: 614(m), 624 (m), 708 (m), 748 (m), 843 (m), 853 (m) (ν Ti–O); 956 (s), 977 (s), 1008 (vs), 1045 (m), 1086 (s) (ν P–O); 1125 (vs), 1162 (sh) (ν C–O); 1256 (m) (ν P=O); 1325 (w); 1631 (w) (ν C=C).

3.3.4. Synthesis of [Ti2(OiPr)6(O3PCH2CH2(NC9H10)]m (4)

Using the same procedure as previously described with (C9H10N)CH2CH2PO(OH)2 (0.256 g, 1 mmol) n hot isopropanol (20 mL) and Ti(OiPr)4 (0.268 g, 2 mmol) in isopropanol (3 mL) under reﬂux for 8 h gave 0.80 g (quantitative yield relative to titanium) of 4 as an orange oil soluble in isopropanol under reﬂux and not soluble in THF. Anal. Calcd for C29H56NO9PTi2 Ti: 13.84, P: 4.48; Found Ti: 13.78, 1 ◦ P: 4.50. H-NMR (20 C, MeOD); δ ppm: 7.68–7.42 (m, 2H, C6H4); 7.08 (dt, J = 11.8, 9.0 Hz, 2H, C6H4); 4.45 (d, J = 64.9 Hz, 2H, –CH2); 4.24–3.75 (m, 6H, –OCH); 3.74–3.32 (m, 6H, –NCH2); 2.26–1.81 (m, 2H, 31 1 ◦ −1 –PCH2); 1.63–0.77 (m, 36H, –CH3). P{ H}-NMR (20 C, MeOD); δ ppm: 8.53. FT-IR (Nujol), cm : 616(m), 621 (m), 738 (m), 787 (m), 848 (m), 875 (m) (ν Ti–O); 955 (m), 986 (m), 1010 (vs), 1034 (m), 1083 (s) (ν P–O); 1125 (vs), 1160 (sh), 1186 (s) (ν C–O); 1260 (m) (ν P=O); 1330 (w); 1630 (w) (ν C=C).

3.4. Synthesis of Hybrid Nanomaterials (TiO2)x(O3P-R)

3.4.1. Preparation of (TiO2)x(VPA) nanoparticles

A solution based on [Ti2(OiPr)6(O3PC2H3)]m (1, 0.165 mmol) + y mmol of Ti(OiPr)4 in THF (20 mL) was stirred for 15 min. The resulting clear solution was then injected directly using a cannula into an aqueous solution (100 mL) containing (y + 2)/10 mmol of N(nBu)4Br and heated until reflux. A white precipitate was immediately formed. Reflux was maintained for 3 h. The medium was then cooled to room temperature, and the white solid formed was recovered by centrifugation (4000 rpm, 1 h) and then washed once with water (75 mL) and twice with ethanol (2 × 75 mL). The resulting white ◦ solids [denoted as (TiO2)x(VPA)] were dried at 70 C for 18 h before characterization. Values of x were been fixed in order to get a series of hybrid TiO2 nanomaterials with Ti/P molar ratios ranging from 2 to 200 (Table4). For comparison, the hydrolysis of pure [Ti 2(OiPr)6(O3PC2H3)]m (1) solution (y = 0) was performed to produce a (TiO2)2(VPA) sample.

Table 4. Ratios of Ti/P and amounts of 1, [Ti2(OiPr)6(O3PC2H3)]m and y, Ti(OiPr)4 aimed at their co-hydrolysis

Ti/P (Molar Ratio) 2 25 50 75 100 125 150 200 1 (mmol) 6.711 0.165 0.165 0.165 0.165 0.165 0.165 0.165 y (mmol) 0.00 3.81 7.95 6.05 8.11 10.21 12.26 16.4

3.4.2. Preparation of (TiO2)100(PPA) Nanoparticles

The same procedure described below using [Ti2(OiPr)6(O3PC6H5)]m (2, 0.207 mmol) and Ti(OiPr)4 31 (10.143 mmol) afforded a white powder of (TiO2)100(PPA). P-MAS NMR: 19.15, 13.95 ppm. BET (N2): 285 m2·g−1. Molecules 2018, 23, 3046 13 of 15

3.4.3. Preparation of (TiO2)100(O3P-R) Nanoparticles: R = CH2CH2NHCH2(C10H7) (NMAPA), CH2CH2(NC9H10) (HQPA) A modiﬁed procedure as described below (replacing THF by hot isopropanol) was used starting from precursors 3 and 4 and about 10 mmol of Ti(OiPr)4 to afford white powders of 31 (TiO2)100(O3P-R). The hybrid nanomaterials were characterized by solid state P-MAS NMR, powder XRD, BET, elemental analysis and TGA. Powder XRD: Anatase phase (average particle size = 5 nm). 31 2· −1 31 (TiO2)100(NMAPA): P-MAS NMR δ ppm = 18.09; BET (N2): 225 m g . (TiO2)100(HQPA): P-MAS 2 −1 NMR δ ppm = 18.24; BET (N2): 322 m ·g .

3.5. PAH Batch Adsorption Test Procedure For all tests, a 200 ppb PAH solution was prepared by diluting in a water:ethanol (24:1) mixture 4 mL of a 500 ppm standard solution containing the US-EPA PAHs (the weight concentration of the 16 PAHs being equal in the standard stock solution). The adsorption tests were made using 10 mg of each solid material under stirring at 20 ◦C in 50 or 100 mL of PAH aqueous solution. Samples of 12 mL were collected after 1, 2, 4, 7, 10 and 24 h, centrifuged and 10 mL were analyzed by SPE using commercial strata-X C18 cartridges to extract non-adsorbed PAHs. The elution of the adsorbed compounds was carried out by passing 3 mL of ethyl acetate. The extract was concentrated at 40 ◦C under a stream of nitrogen to a ﬁnal volume of 200 µL. The eluates were analyzed by GC-ToF in SIM mode on a DB-5MS UI column (30 m × 0.25 mm I.D, thickness 0.25 µm) with helium as carrier gas. The injection was performed in splitless mode (1 µL) at a temperature of 300 ◦C. The heating gradient was: 60 ◦C (1 min) at 320 ◦C (25 ◦C/min), 32 ◦C for 10 min.

4. Conclusions In conclusion, a robust sol-gel process, starting from novel heteroleptic titane phosphonate-alkoxides precursors, was developed in order to tune finely the surface functionalization of nanocrystallites of titania with phosphonate ligand (PA). Series of hybrid nanomaterials bearing different amount and nature of unsaturated groups (vinyl, phenyl, naphtyl) were elaborated and thoroughly characterized. Their performances as nano-adsorbents through π−π stacking demonstrated remarkable removal efficiencies of 100% over a mixture of 16 PAHs (listed by the U.S. Environmental Protection Agency). The adsorption kinetics results have shown that systems with a ratio TiO2/PA = 100 reached the equilibrium after 24 h and high adsorption capacities of around 1 mg·g−1 per PAH were found. The selectivity of PAH sorption process depended on the nature of the unsaturated-based phosphonate ligands, on the self-agglomeration of the surface-functionalized nano-particles and on the textures of the nanomaterials. Hence, mesoporous vinyl-based titania nanomaterials have demonstrated high affinity for light PAHs while macro-, mesoporous phenyl-based one had high affinity for heavy PAHs. Such findings would be very helpful in identifying factors affecting selective PAHs remediation from aqueous media in the presence of competing species (PCB, diclofenac) or PAH mixtures.

Supplementary Materials: The supplementary materials are available online. Author Contributions: Conceptualization, S.D. and N.B.O.; Methodology, S.D. and N.B.O; Validation, S.D., N.B.O. and Q.A.T; Formal Analysis, N.B.O. and Q.A.T; Investigation, S.D., N.B.O. and Q.A.T; Data Curation, S.D., N.B.O. and Q.A.T.; Writing-Original Draft Preparation, N.B.O.; Writing-Review & Editing, S.D. and N.B.O.; Supervision, S.D.; Project Administration, S.D.; Funding Acquisition, S.D. Funding: The authors acknowledge the Region Rhône-Alpes for financial support (ADR 2009-contract n◦ 1101538402). Acknowledgments: The authors thank Laure WIEST, Cédric FRATTA and Emmanuelle VUILLET from Institut des Sciences Analytiques (Villeurbanne, UMR 5280), Chantal LORENTZ from IRCELYON (Villeurbanne, UMR 5256) and Kai Chung SZETO from LCOMS/C2P2 laboratory (Villeurbanne, UMR 5265) for carrying out CG-ToF, multinuclear MAS NMR and DRIFT experiments, respectively. Conflicts of Interest: The authors declare no conflict of interest. Molecules 2018, 23, 3046 14 of 15

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Sample Availability: Samples of the compounds are not available from the authors.

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