Irene Lara-Ibeas

Cristina Megías-Sayago

Alberto Rodriguez-Cuevas

Rubén Ocampo-Torres

Benoît Louis

Stéphane Colin

Stéphane Le Calvé
email: slecalve@unistra.fr

ADSORBENT SCREENING FOR AIRBORNE BTEX ANALYSIS AND REMOVAL

Keywords: Adsorption, BTEX, air pollution, desorption, air treatment

Adsorption is a commonly used technique for removal and analysis of gaseous pollutants due to its cost efficiency at low concentrations. In this work, single and competitive BTEX adsorptions were studied on three non-porous, mesoporous and microporous commercial adsorbents, namely Carbopack® B, SBA-16 and HKUST-1, respectively. For all these materials, C8 aromatics were preferentially adsorbed, preventing in some cases the adsorption of the most volatile species, i.e. benzene and toluene. This behavior indicates that the competition phenomenon is closely related to the strength of adsorbateadsorbent interactions. Activation energies for the desorption process were determined to be 33.8 and 33.7 and 35.9 kJ/mol for Carbopack® B, SBA-16 and HKUST-1, respectively, demonstrating that stronger interactions are present in microporous materials. Among the investigated adsorbents, SBA-16 seems to be the best candidate for air treatment and analysis since it exhibited high adsorption capacity, moderate hydrophobicity, minimal roll-up and low activation energy for the desorption. The experimental results obtained illustrate the complexity of multicomponent adsorption process on materials with different porosity and surface chemistry.

INTRODUCTION

Benzene, Toluene, Ethylbenzene and Xylenes isomers (BTEX) are a group of highly volatile gaseous pollutants frequently found in indoor [START_REF] Lara-Ibeas | BTEX near real-time monitoring in two primary schools in La Rochelle, France[END_REF]- [START_REF] Śmiełowska | Indoor air quality in public utility environments-a review[END_REF] and outdoor air [START_REF] Montero-Montoya | Volatile Organic Compounds in Air: Sources, Distribution, Exposure and Associated Illnesses in Children[END_REF], [START_REF] Verriele | The MERMAID study: indoor and outdoor average pollutant concentrations in 10 low-energy school buildings in France[END_REF]. It is known from the literature that these compounds have a negative impact on the environment since they contribute to the formation of ozone and other photochemical oxidants [START_REF] Kumar | Volatile Organic Compounds: Sampling Methods and Their Worldwide Profile in Ambient Air[END_REF]. Moreover, BTEX are either known for being, or suspected to be, irritants, neurotoxins, allergens or carcinogens [START_REF] Tsakas | Chemistry, Emission Control, Radioactive Pollution and Indoor Air Quality[END_REF] and their exposure on a long term basis presents a serious threat to the human health [START_REF] Ahmed | Impact of household air pollution on human health: source identification and systematic management approach[END_REF], [START_REF] Hoek | Long-term air pollution exposure and cardio-respiratory mortality: a review[END_REF]. Therefore, implementing effective strategies for pollution control is of paramount importance to limit human exposure and prevent the environment degradation.

Nowadays, numerous techniques based on physicochemical or biological processes have been developed for gaseous pollutant's removal such as thermal, plasma, catalytic or photocatalytic oxidation, condensation, membrane separation, biological degradation, absorption and adsorption [START_REF] Guieysse | Biological treatment of indoor air for VOC removal: Potential and challenges[END_REF]- [START_REF] Słomińska | Removal of BTEX Compounds From Waste Gases; Destruction and Recovery Techniques[END_REF].

However, the pollutant concentration in indoor air or industrial environments is relatively low, ranging from sub ppb level to a hundred of ppm. It is worthy to mention that not all removal techniques can be effective at such low concentration ranges [START_REF] Baur | Development of Adsorbents for Selective Volatile Organic Compounds Removal from Diluted Gas Streams[END_REF]. Furthermore, some of these methods are expensive or require regular maintenance limiting their use at domestic scale. Among them, adsorption has been demonstrated to be a technique that exhibits a good compromise between cost and efficiency for BTEX removal at low concentrations.

At the same time, analytical methods such as gas chromatography are required to monitor air quality and/or control the efficiency of the aforementioned depollution techniques. Since BTEX concentrations are usually very low, the integration of preconcentration devices is generally needed to increase the sensitivity of these methods. Thus, in the mentioned preconcentration unit, an adsorbent is used to trap pollutant molecules and concentrate the sample that will be, subsequently, analysed by conventional gas chromatography. The adsorbent requirements in pollutant removal as well as gas analysis include a minimal breakthrough, large adsorption capacity, thermal stability and selectivity to the targeted pollutants. Additionally, the desorption temperature should be moderate to enable an effective, inexpensive and rapid adsorbent regeneration.

In this regard, several materials differing in structure, porosity and surface chemistry have been lately investigated. Among these materials, carbon-based materials have been addressed as VOC/BTEX adsorbents including activated carbons [START_REF] Zhou | Activated carbons modified by magnesium oxide as highly efficient sorbents for acetone[END_REF], [START_REF] Khan | Comparative Analysis of Pressure Regions of Gaseous Benzene Using Ground Activated Carbon Sorbents Under Ambient Conditions[END_REF], ordered mesoporous carbons [START_REF] Rubahamya | Porous carbon screening for benzene sorption[END_REF] or carbon nanotubes [START_REF] Le | Investigation of BTEX Adsorption on Carbon Nanotubes Cartridges from Air Samples[END_REF]. However, graphitised carbon blacks, typically employed for sampling [START_REF] Senthilnathan | Indoor Air Pollution, Sorbent Selection, and Analytical Techniques for Volatile Organic Compounds[END_REF] and gas analysis [START_REF] Wang | Compact prototype microfabricated gas chromatographic analyzer for autonomous determinations of VOC mixtures at typical workplace concentrations[END_REF] applications, have been scarcely evaluated in terms of adsorption capacity. In contrast, BTEX adsorption capacity of a wide variety of silicas have been extensively studied as shown in Table 1. Multiple silica structures such as SBA-15, MCM-41, MCM-48, KIT-6 or SiO2 exhibited significant adsorption abilities ranging from 15.78 to 415.39 mg/g towards benzene and toluene. Several MOF have been also employed for VOC adsorption studies [START_REF] Bahri | A comparative study on metal organic frameworks for indoor environment application: Adsorption evaluation[END_REF]- [START_REF] Liu | Recent advances in metal-organic frameworks for adsorption of common aromatic pollutants[END_REF]. More precisely, HKUST-1 have been widely investigated for the adsorption of various aromatic compounds such as benzene, toluene, p-xylene and o-xylene (see Table 2).

Despite the huge number of BTEX adsorption related studies to date, most of them have been conducted at very high concentrations (5300 -800000 ppm) as recently reported by Szulejko et al. [START_REF] Szulejko | Seeking the most powerful and practical real-world sorbents for gaseous benzene as a representative volatile organic compound based on performance metrics[END_REF].

Nevertheless, there is a lack of studies employing more realistic BTEX concentrations, ranging from 0.5 to 99 ppm [START_REF] Chevalier | Performance of HKUST-1Metal-Organic Framework for a VOCs mixture adsorption at realistic concentrations ranging from 0.5 to 2.5 ppmv under different humidity conditions[END_REF]- [START_REF] Zhang | Comparison of dynamic adsorption/desorption characteristics of toluene on different porous materials[END_REF]. On the other hand, a considerable number of compounds (pollutants) with different polarities and molecular sizes are commonly coexisting in real environments [START_REF] Sarigiannis | Exposure to major volatile organic compounds and carbonyls in European indoor environments and associated health risk[END_REF], which may lead to competitive adsorption on available adsorption sites, thus resulting in preferential adsorption. Therefore, the study of multicomponent adsorption is a crucial issue to take into account when a material is assessed for pollutant removal or gas analysis. In this sense, only few studies have simulated the adsorption of gas mixtures including BTEX [START_REF] Chevalier | Performance of HKUST-1Metal-Organic Framework for a VOCs mixture adsorption at realistic concentrations ranging from 0.5 to 2.5 ppmv under different humidity conditions[END_REF], [START_REF] Peralta | Adsorption and Separation of Xylene Isomers: CPO-27-Ni vs HKUST-1 vs NaY[END_REF], [START_REF] Vellingiri | Metal organic frameworks as sorption media for volatile and semi-volatile organic compounds at ambient conditions[END_REF].

In this work, three commercially available materials differing in their composition, porosity and surface chemistry have been selected to perform single and multicomponent adsorption of low concentrated BTEX mixtures (10 -100 ppm). Carbopack® B is a non-porous graphitised carbon black traditionally used as a reference material in sorbent tubes for environmental monitoring [START_REF] Kim | Simulation of the breakthrough behavior of volatile organic compounds against sorbent tube sampler as a function of concentration level and sampling volume[END_REF], [START_REF] Tolnai | Evaluation of Carbopack B adsorbent for the tubetype diffusive sampling of volatile organic compounds at ambient concentration[END_REF] and preconcentration devices [START_REF] Ciccioli | Gas chromatographic evaluation of the organic components present in the atmosphere at trace levels with the aid of carbopack b for preconcentration of the sample[END_REF]- [START_REF] Wang | Belt-Mounted Micro-Gas-Chromatograph Prototype for Determining Personal Exposures to Volatile-Organic-Compound Mixture Components[END_REF]. SBA-16 is a mesoporous silica with relatively high specific surface area employed especially in catalytic applications [43, p. 16], [START_REF] Jourshabani | Fe-Supported SBA-16 Type Cagelike Mesoporous Silica with Enhanced Catalytic Activity for Direct Hydroxylation of Benzene to Phenol[END_REF], [START_REF] Kong | Oxidative dehydrogenation of ethane to ethylene over Mo-incorporated mesoporous SBA-16 catalysts: The effect of MoOx dispersion[END_REF] and, to a lesser extent, in gas analysis [START_REF] Hamdi | Mesoporous silica nanoparticle film as sorbent for in situ and real-time monitoring of volatile BTX (benzene, toluene and xylenes)[END_REF]- [START_REF] Luo | Improved separation of micro gas chromatographic column using mesoporous silica as a stationary phase support[END_REF]. Finally, HKUST-1 (Cu3(BTC)2) is a microporous MOF formed by copper nodes with 1,3,5-benzenetricarboxylic acid linkers between them. This material has been widely employed in catalysis [START_REF] Granato | Catalytic activity of HKUST-1 coated on ceramic foam[END_REF], [START_REF] Lee | Metal-organic framework materials as catalysts[END_REF], gas analysis [START_REF] Lara-Ibeas | Sub-ppb Level Detection of BTEX Gaseous Mixtures with a Compact Prototype GC Equipped with a Preconcentration Unit[END_REF] as well as in VOC adsorption [START_REF] Moellmer | High pressure adsorption of hydrogen, nitrogen, carbon dioxide and methane on the metal-organic framework HKUST-1[END_REF], [START_REF] Raganati | CO2 capture performance of HKUST-1 in a sound assisted fluidized bed[END_REF]. These three adsorbents were characterized and evaluated in terms of adsorption capacity in single and multicomponent dynamic adsorption experiments. Furthermore, toluene desorption activation energy was determined and compared between the three materials. The present study provides, therefore, new experimental data concerning the multicomponent adsorption of low concentrated BTEX mixtures, highlighting the importance of adsorbate-adsorbent interactions on the competitive adsorption of aromatics compounds.

MATERIALS AND METHODS

Materials

Commercially available Carbopack® B (60-80 mesh, SUPELCO), HKUST-1 (Basolite® C300, Sigma-Aldrich), and SBA-16 (ACS Materials) were used as received for adsorption tests. Toluene diluted in synthetic air (100 ppm, Air Products, France) and BTEX mixture diluted in nitrogen (10 ppm, Air Products, France) were employed in temperature programmed desorption (TPD) and dynamic adsorption experiments.

Characterization of adsorbents

X-ray diffraction patterns were recorded on a Bruker D8 Advance diffractometer, with a Ni detector side filtered Cu Kα radiation (1.5406 Å) over a 2θ range of 5-60°.

Scanning electron microscopy (SEM) images were acquired in a ZEISS GEMINI SEM 500 microscope using an electron high tension (EHT) voltage ranging from 2 to 6 kV.

The textural properties were analysed by means of nitrogen physisorption using a Micromeritics ASAP 2420 equipment. Prior to analysis, the samples were outgassed at 180 °C for 5 h under vacuum.

The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method. The total pore volume was estimated by the single point method from the amount of adsorbed N2 at a relative pressure (P/P0) of 0.99. Pore size distributions (PSD) were determined using Barrett, Joyner, and Halenda (BJH) method from the adsorption branch of the isotherm. The micropore volume and micropore surface area were calculated by a t-plot method. Micropore size distribution was calculated by Saito and Foley method [START_REF] Saito | Argon porosimetry of selected molecular sieves: experiments and examination of the adapted Horvath-Kawazoe model[END_REF].

Thermogravimetric analyses (ATG) were performed in a thermobalance coupled with a differential scanning calorimetry apparatus (SDT 650, TA Instruments). The samples were analysed in inert (N2) atmosphere from room temperature to 500°C by using a 10 K/min heating rate.

Temperature Programmed Desorption (TPD) experiments were used to calculate the toluene desorption energy from the adsorbent surfaces. TPD analyses were carried out in a Micromeritics

AutoChem II 2920 equipped with a TCD detector. Samples were outgassed in-situ at 280 °C during 1 h. Afterwards, they were cooled down to room temperature and a gas stream of 50 mL/min of toluene diluted in synthetic air (100 ppm, Air Products, USA) was passed through the adsorbent bed for 15 min.

Subsequently, samples were analysed using a helium flow rate of 30 mL/min at several temperature ramps between 10 and 30 K/min for Carbopack® B and SBA-16. Lower heating rates from 4 to 12 K/min were used on HKUST-1 samples to prevent adsorbent decomposition before toluene desorption.

Dynamic adsorption experiments

Carbopack® B, SBA-16 and HKUST-1 (15 mg) were packed separately in thermal desorption tubes (Sigma Aldrich, USA) using two glass wool plugs. Prior to analysis, the samples were conditioned for 2 h at 280 °C under a helium flow of 50 mL/min using thermal desorber (TurboMatrix 350, Perkin Elmer, USA) in order to ensure a clean surface. Then, the tubes were mounted on a self-made experimental device showed in Figure S1 (Supporting Information). For a single component breakthrough test, toluene diluted in synthetic air (100 ppm, Air Products, France) was flowed through each tube under ambient conditions (room temperature, T=23 °C ± 2 °C). As for the multicomponent adsorption, BTEX mixture diluted in nitrogen (10 ppm, Air Products, France) was employed and flowed at the same temperature, with a 5 mL/min flow rate in both experiments (Mass flow controller 1). The effluent gas stream was continuously analysed by a gas chromatograph (µBTEX-1 In'Air Solutions, France). Before the analysis and in order to avoid the detector saturation, the effluent was diluted using a second mass flow controller operating at 995 mL/min and 95 mL/min nitrogen flow (99.999% purity, Messer, Gemany) for single and multicomponent experiments, respectively, to obtain a concentration of 500 ppb at the analytical instrument inlet. Prior to each adsorption experiment, the peak area corresponding to the initial concentration (C0) was measured. To this purpose, the gas stream was flowed through a bypass, diluted and analysed. For a given compound, the obtained bypass peak area was used as reference to determine the time at which the adsorbent saturation is reached (C = C0).

The dynamic adsorption capacity (q) for given concentration and flow rate can be calculated as the area above the breakthrough curve using the following equation:

𝑞 = 𝐹 𝐶 0 𝑚 ∫ (1 - 𝐶 𝐶 0 ) 𝑑𝑡 𝑡 𝑠 𝑡 0 (1)
where q is the dynamic adsorption capacity per gram of adsorbent (mg/g ads), F is the volume gas flow rate (m 3 min -1 ), m is the mass of adsorbent (g), t0 is the initial time (min), ts is the saturation time (min), C0 is the initial concentration (mg m -3 ), C is the outlet concentration (mg m -3 ) at a given time.

RESULTS AND DISCUSSION

Characterization of the adsorbents

XRD analysis of commercial adsorbents was carried out to assess the crystal structure (Figure 1). Briefly, Carbopack® B exhibits the characteristic diffractions of graphitic carbon (ICDD #00-023-0064), HKUST-1 corresponds to those of C10H10N8Zn composite (ICDD #00-023-1971) while a broad peak is observed in the case of SBA-16, which is ascribed to amorphous silica, usually appearing in the XRD patterns of ordered mesoporous silicas. Related to the latter, the highest diffractions are observed at small angles, not detectable with the present method. Nevertheless, XRD profiles allow confirming the existing phases. (ranging from 10 to 180 µm) with highly grainy surface. In contrast, HKUST-1 (Fig. 2c) exhibits a homogeneous morphology constituted by smooth surface octahedral crystals having uniform sizes between 10 and 25 µm. The latter is congruent with previous literature reports [START_REF] Chevalier | Performance of HKUST-1Metal-Organic Framework for a VOCs mixture adsorption at realistic concentrations ranging from 0.5 to 2.5 ppmv under different humidity conditions[END_REF], [START_REF] Zhao | Competitive adsorption and selectivity of benzene and water vapor on the microporous metal organic frameworks (HKUST-1)[END_REF], [START_REF] Al-Janabi | Mapping the Cu-BTC metal-organic framework (HKUST-1) stability envelope in the presence of water vapour for CO2 adsorption from flue gases[END_REF], [START_REF] Lin | Synthesis and characterization of porous HKUST-1 metal organic frameworks for hydrogen storage[END_REF] or was purchased as commercial material [START_REF] Lamia | Adsorption of propane, propylene and isobutane on a metal-organic framework: Molecular simulation and experiment[END_REF]. The corresponding N2 adsorption-desorption isotherms are presented in Figure 3. Carbopack® B (Fig. 3a) exhibits a type III isotherm according to the IUPAC classification, which is characteristic of non-porous materials. The significant increase in adsorbed N2 observed at P/P0 = 0.8-0.95 suggests that, at this point, all the layers were saturated, and the adsorbate molecules are filling the interparticle space.

Thus, the estimated total pore volume may not correspond only to the pores present on the adsorbent but also to the space between particles. As displayed in Fig. 3b, SBA-16 exhibits a type IV isotherm, characteristic of mesoporous materials. A hysteresis loop can be noticed at intermediate P/P0 associated to capillary condensation occurring in mesopores [START_REF] Zhai | Organosilane-assisted synthesis of ordered mesoporous poly(furfuryl alcohol) composites[END_REF]. The form of the hysteresis curve, with two different widths, reveals the presence of two different types of pores. This evidence was confirmed by the pore size distribution (see Fig. S2) with pore diameters of nearly 3.4 and 6.0 nm. The high quantity absorbed by HKUST-1 at low pressures (Fig. 3c) followed by a long horizontal plateau at higher pressures is typical of type I adsorption isotherms and can be associated to a very strong adsorption within the micropores [START_REF] Vellingiri | Metal organic frameworks as sorption media for volatile and semi-volatile organic compounds at ambient conditions[END_REF], [START_REF] Lin | Synthesis and characterization of porous HKUST-1 metal organic frameworks for hydrogen storage[END_REF], [START_REF] Autie-Castro | Cu-BTC and Fe-BTC metal-organic frameworks: Role of the materials structural features on their performance for volatile hydrocarbons separation[END_REF]. Most probable pore diameter were determined by Saito and Foley method (see Figure S3) resulting to be 5.4 and 6.9 Å, similar to the diameters of the small (4.5 Å) and large cages (9 Å) theoretically presented in the MOF structure [START_REF] Raganati | CO2 capture performance of HKUST-1 in a sound assisted fluidized bed[END_REF]. These diameters imply that, although the dynamic diameters of BTEX molecules are larger (from 0.59 to 0.68 nm; see Table S1) than the size of the small cages, they may enter inside the larger pores thus interacting by means of π-π stacking with the organic linkers. The textural properties of all studied samples are summarized in Table 3. As expected, the specific surface area increases with the porosity (112, 572, 1734 m 2 g -1 for Carbopack® B, SBA-16 and HKUST-1, respectively) with the highest value for the microporous adsorbent. Concerning the pore volume, SBA-16 presents a total volume of 0.72 cm 3 g -1 , barely lower than that observed for HKUST-1, 0.89 cm 3 g -1 . These adsorbents exhibit therefore similar pore volumes but different pore diameters.

According to the pore size distribution, 95% of the total pore volume in SBA-16 corresponds to pores having 3.4 or 6.0 nm diameter (Vmeso). In contrast, most of the pores in HKUST-1 have diameters around 0.54 and 0.69 nm. The micropore volume of HKUST-1 sample was calculated to be 0.68 cm 3 g -1 which corresponds to 77% of the total volume, thus assessing a highly microporous nature. In order to evaluate both the thermal stability and water affinity of the adsorbents, a thermogravimetric study was carried out over the samples after 2 h of exposure to ambient air (Figure S4). All samples have in common a first weight loss situated around 100 °C, usually ascribed to physisorbed water. As deduced from the thermal profiles, Carbopack B is the most hydrophobic adsorbent (showing less than 2% weight loss at 100°C) followed by SBA-16 (6%) and HKUST-1

(exhibiting 32% weight loss between 80 and 150°C). It is well known from the literature that HKUST-1 exhibits a high affinity towards water [START_REF] Chevalier | Performance of HKUST-1Metal-Organic Framework for a VOCs mixture adsorption at realistic concentrations ranging from 0.5 to 2.5 ppmv under different humidity conditions[END_REF], [START_REF] Zhao | Competitive adsorption and selectivity of benzene and water vapor on the microporous metal organic frameworks (HKUST-1)[END_REF], [START_REF] Al-Janabi | Mapping the Cu-BTC metal-organic framework (HKUST-1) stability envelope in the presence of water vapour for CO2 adsorption from flue gases[END_REF] since water molecules can coordinate to the free copper sites [START_REF] Küsgens | Characterization of metal-organic frameworks by water adsorption[END_REF]. These results are in agreement with those observed by Seo et al. [START_REF] Seo | Microwave synthesis of hybrid inorganic-organic materials including porous Cu3(BTC)2 from Cu(II)-trimesate mixture[END_REF] reporting a weight loss of 30% at ~110°C associated to a presumed loss of 15 water molecules (29.7%) per Cu3 unit present in HKUST-1 structure. This great affinity to water prevents the use of HKUST-1 as an effective adsorbent in moist environments since water molecules may occupy adsorption sites resulting in a considerable decrease in the adsorption capacity. Nevertheless, this adsorbent can be employed for VOC removal in dry environments such as aircraft cabins where the relative humidity ranges between 2 and 23% [START_REF] Nagda | Low relative humidity and aircraft cabin air quality[END_REF].

HKUST-1 decomposition takes place at around 320°C as evidenced by a dramatic weight loss of ~30% accompanied by a heat release (exothermic peak), which is usually ascribed to the MOF decomposition into Cu2O and CuO [START_REF] Al-Janabi | Mapping the Cu-BTC metal-organic framework (HKUST-1) stability envelope in the presence of water vapour for CO2 adsorption from flue gases[END_REF], [START_REF] Seo | Microwave synthesis of hybrid inorganic-organic materials including porous Cu3(BTC)2 from Cu(II)-trimesate mixture[END_REF] and the subsequent benzene di-carboxylic group oxidation [START_REF] Banerjee | Synthesis of CuO nanostructures from Cu-based metal organic framework (MOF-199) for application as anode for Li-ion batteries[END_REF]. The material decomposition involves the loss of its chemical structure and, thus, its adsorption properties. Consequently, this temperature must not be exceeded during the activation/desorption process. No remarkable thermal events were observed at higher temperatures. As depicted in Figure S4a and S4b, Carbopack® B and SBA-16 suffered a total weight loss of 3 and 7%, respectively, up to 400°C, demonstrating high thermal stability.

Single component adsorption experiments

Single aromatic adsorption experiments were carried out using toluene as probe molecule (Figure 4). The breakthrough curves represent the evolution of the pollutant concentration in the effluent leaving the adsorbent bed as a function of time. They provide information about the bed adsorption capacity and the kinetics of the adsorption process. Three different zones can be distinguished in these consecutive curves: the unsaturated zone, mass transfer zone and saturated zone. The first zone ranges from the beginning of the experiment (t0) to the breakthrough time (tb). Usually, in air treatment, even a very low amount of pollutant in the effluent is not allowable; therefore, the breakthrough time is reached when 5% of the concentration of the feed (C0) is leaving the adsorbent bed. Prior to the breakthrough, adsorption can be considered complete. Industrially, the quantity of pollutant adsorbed during this stage represents the usable capacity of the reactor. From the breakthrough point, the concentration measured at the outlet progressively increases until the saturation time (ts) is reached (C/C0 = 1). The steepness of the concentration profile between tb and ts represents the mass transfer zone. The profile of this zone provides insights about the mass transfer from gas phase to the adsorption sites inside the sorbent particles. From an industrial point of view, a short mass transfer zone is preferred considering that from tb the adsorption is considered ineffective. From ts, all the adsorption sites are occupied, and no further adsorption takes place. Therefore, the measured concentration of the effluent at the outlet is equal to the concentration of the feed. At this point, the dynamic adsorption capacity of the adsorbent under the operating conditions can be determined. Table 4 shows the obtained breakthrough times and the calculated dynamic adsorption capacities.

Although the duration of the unsaturated zone is similar for the three adsorbents, HKUST-1 exhibits the shortest breakthrough time. It could be somehow related with the adsorbent density, which is different in each case. Certainly, HKUST-1 density (bulk density of 0.35 g/cm 3 ) was higher compared to Carbopack® B and SBA-16, which implies that the adsorbent bed length was slightly shorter and, thus, toluene molecules might pass through faster.

As evidenced by the breakthrough curves, the mass transfer zones have different shapes depending on the adsorbent, being the steepness of the concentration front related to the kinetics of the adsorption process. In this way, the breakthrough curve of HKUST-1 increases sharply from the breakthrough point to the saturation suggesting fast toluene adsorption kinetics. It might be related both to the particle size, as smaller sizes should enable higher intraparticle mass transfer rates, and with the presence of micropores. HKUST-1 consists of smaller particles (10 -25 µm, see Fig. 2), compared with the other two adsorbents (10 -180 µm), which may lead to shorten the mass transfer zone. The adsorbent adsorption capacities were calculated from the area below the respective breakthrough curves (Fig. 4). Surprisingly, the adsorbent with higher specific surface area (HKUST-1) led to the lowest adsorption capacity, suggesting that toluene adsorption may be influenced by different factors. Theoretically, in HKUST-1, the structure is composed by two large cages with a pore diameter of 9 Å and internal diameters of 13.2 and 11.1 Å connected to 6 Å sized smaller cages with 4.5 Å

diameter. These values are slightly different from those obtained experimentally (5.4 and 6.9 Å), however, in both cases, toluene molecules (kinetic diameter = 5.8 Å) are not expected to enter in the smaller cages, but they can be adsorbed in the larger ones. Nevertheless, if HKUST-1 is slightly hydrated, the water molecules may reduce the opening diameter of pores thus preventing the entrance of toluene molecules [START_REF] Chevalier | Performance of HKUST-1Metal-Organic Framework for a VOCs mixture adsorption at realistic concentrations ranging from 0.5 to 2.5 ppmv under different humidity conditions[END_REF], [START_REF] Farrusseng | Heats of Adsorption for Seven Gases in Three Metal-Organic Frameworks: Systematic Comparison of Experiment and Simulation[END_REF]. Despite the existence of adsorbed water molecules is highly unlikely after the adsorbent outgassing at 280 °C, it has been demonstrated that the powder pre-treatment at temperatures above 250 °C may cause a drastic decrease in porosity [START_REF] Raganati | CO2 capture performance of HKUST-1 in a sound assisted fluidized bed[END_REF], hence reducing the available surface area.

In contrast to HKUST-1, SBA-16 presents the highest adsorption capacity compared to other adsorbents. The results suggest that the presence of large pores (3.4 -6.0 nm) can lead to an increase in the adsorption capacity, as demonstrated by Zhang et al. for the toluene adsorption over UiO-66 MOF [START_REF] Zhang | Adsorption/desorption kinetics and breakthrough of gaseous toluene for modified microporous-mesoporous UiO-66 metal organic framework[END_REF]. They reported an enhancement of toluene dynamic adsorption capacity of the micro-mesoporous UiO-66 compared to the microporous counterpart, due to an increase in the molecular transfer rate that allows toluene molecules to diffuse in the material further thus increasing the residence time of the adsorbate molecules in the adsorbent bed. Moreover, it seems that the adsorption of toluene molecules in larger pores can promote the adsorption of new toluene molecules by π-π stacking within the mesopores, which may lead to higher adsorption capacity. Carbopack® B exhibited lower adsorption capacity than SBA-16 probably due to its lower specific surface area and, thus, less available adsorption sites.

Comparison with the literature

The adsorption of aromatics has been widely studied in the last years. However, the variability of the conditions used in the experiments render difficult the comparison of the results. Table 2 summarizes the results for aromatics adsorption previously reported over HKUST-1 [START_REF] Chevalier | Performance of HKUST-1Metal-Organic Framework for a VOCs mixture adsorption at realistic concentrations ranging from 0.5 to 2.5 ppmv under different humidity conditions[END_REF], [START_REF] Vellingiri | Metal-organic frameworks for the adsorption of gaseous toluene under ambient temperature and pressure[END_REF], [START_REF] Xu | Effect of Textural Properties on the Adsorption and Desorption of Toluene on the Metal-Organic Frameworks HKUST-1 and MIL-101, Effect of Textural Properties on the Adsorption and Desorption of Toluene on the Metal-Organic Frameworks HKUST-1 and MIL-101[END_REF]- [START_REF] Zhao | Competitive adsorption and selectivity of benzene and water vapor on the microporous metal organic frameworks (HKUST-1)[END_REF]. Since most of the studies were performed at very high concentrations (5925 -950000 ppm) compared to ours (10 ppm), the measured adsorption capacities in these studies were (in comparison) rather high. Only the results reported by Vellingiri et al. [START_REF] Vellingiri | Metal-organic frameworks for the adsorption of gaseous toluene under ambient temperature and pressure[END_REF] were conducted under similar conditions (initial concentration of 99 ppm and 25 °C) and can be compared to the adsorption capacity determined herein. In this regard, our measured toluene adsorption capacity over HKUST-1 sample (6.96 mg/g) was significantly lower than the value of 150 mg/g obtained by Vellingiri et al. The difference could be associated to the temperature employed during the outgassing process, 170 °C (Vellingiri et al.) instead of 280 °C (this study), which would further confirm that HKUST-1 is very sensitive to the temperature, starting its structural degradation at T < 280 °C.

Similarly, Table 1 summarizes the obtained results for both benzene and toluene adsorption over micro-and mesoporous silicas. As noted above, most of the experiments were conducted at very high initial concentrations, which do not provide representative data for realistic environments. Mesoporous silicas with diverse structures have been tested for aromatics adsorption, some of them combining mesoand micropores. The latter ones exhibited higher adsorption performance due to the stronger interactions of the molecules in the narrow pores that promotes their adsorption. Indeed, considering only the mesoporous silicas, the pore volume did not seem to be a key parameter influencing the adsorption, whilst a linear increase in adsorption capacity appears to be related to the decrease in the pore diameter, as shown in Figure S5 for toluene. This may explain the low toluene adsorption capacity of SBA-16 compared to other silicas with narrower mesopores.

As for graphitised carbon blacks solids, few data addressing BTEX total adsorption capacity have been found in literature. In contrast, these materials have been traditionally used for VOC sampling; the studies dealing mainly with its regeneration [START_REF] Ciccioli | Gas chromatographic evaluation of the organic components present in the atmosphere at trace levels with the aid of carbopack b for preconcentration of the sample[END_REF], uptake rates [START_REF] Tolnai | Evaluation of Carbopack B adsorbent for the tubetype diffusive sampling of volatile organic compounds at ambient concentration[END_REF] and samples stability [START_REF] Lee | Ozone Artifacts and Carbonyl Measurements Using Tenax GR, Tenax TA, Carbopack B, and Carbopack X Adsorbents[END_REF]. Furthermore, Carbopack® B has been lately replaced by Carbopack® X due to its higher specific surface area (240 m 2 /g to be compared with 112 m 2 /g for Carbopack® B). The adsorption capacity of this material was investigated by Khan et al. that reported a benzene adsorption capacity of 6.79 mg/g (50 ppm, 25 °C) [START_REF] Khan | A comparison of figure of merit (FOM) for various materials in adsorptive removal of benzene under ambient temperature and pressure[END_REF]. This value, relatively lower than the toluene adsorption capacity found for Carbopack® B in the present work, could be explained by the lower volatility of toluene, as it will be highlighted during the multicomponent experiments.

Multicomponent adsorption experiments

Single component adsorption experiments allowed determining the adsorption capacity of each material, illustrating the differences on toluene adsorption among them. In indoor air, however, a broad spectrum of gaseous pollutants usually coexists, resulting in competitive adsorption phenomena.

Therefore, the capacity of an adsorbent for pollutant removal or analysis cannot be based solely on the adsorption of a single compound. To determine this capacity in a more realistic environment, a mixture of aromatic compounds was employed for dynamic adsorption experiments.

Figure 5 illustrates the obtained breakthrough curves for BTEX under ambient conditions (T=296 K ± 2 K), using a feed of 5 mL/min containing 10 ppm of each pollutant. For each adsorbent, the plot is composed of five breakthrough curves corresponding to each single compound in the mixture, excepting coeluted m-and p-xylenes that are represented by the same curve. In general, the obtained plots are somewhat comparable, and higher boiling point compounds are preferentially adsorbed following the order benzene > toluene > ethylbenzene > m/p-xylenes > o-xylene. Nevertheless, the concentration profiles and the calculated adsorption capacities (see Table 5) for each compound vary from one adsorbent to another suggesting that slightly different adsorbate-adsorbent interactions/diffusional processes are co-existing.

Initially, all compounds were completely adsorbed, excepting benzene over Carbopack® B that passed through the adsorbent bed from the beginning. Afterwards, the other species start to break through in accordance with their increasing boiling point. In this way, the outlet adsorbate concentrations start to rise, reaching, in the case of benzene and toluene, higher values than the inlet concentration (C/C0 > 1). This phenomenon, called roll-up, appears when the thermodynamic effects govern the adsorption process [START_REF] Peralta | Comparison of the Behavior of Metal-Organic Frameworks and Zeolites for Hydrocarbon Separations[END_REF] i.e. the adsorbate with weaker interactions is replaced by other species with more favourable adsorbate-adsorbent interactions. Roll-up was observed over all materials in different degrees, resulting in the desorption of the most volatile compounds in favour of the adsorption of the less volatile compounds.

SBA-16 and Carbopack® B present minimum benzene/toluene roll-up whereas over HKUST-1 the effect is more pronounced. Consequently, the adsorption capacities of each compound were strongly related to adsorbate-adsorbent interactions as well as to the presence of micropores. In Carbopack® B, London interactions are the prevailing adsorbate-adsorbent forces that govern the adsorption process due to the non-polar surface of this material. Consequently, the adsorption capacity for each compound is considerably different due to the different strength of these interactions and, logically, follows the order of the boiling points (o-xylene > m/p-xylenes > ethylbenzene > toluene > benzene) provided in Table S1. It should be noted that the effect of the competitive sorption between the different species in this material is remarkable, resulting in the total desorption of the most volatile compound, i.e. benzene. Therefore, special attention should be paid when this material is used to remove/analyze benzene or similar boiling points pollutants in presence of less volatile compounds.

Even when interactions of different nature can be established, London forces are usually the driving force in the adsorption of high boiling point molecules, having also high molecular mass. It is known from the literature that in mesoporous silicas, the hydroxyl groups located at the surface may behave as weak acid sites interacting with the π-electrons of the aromatic rings [START_REF] Serna-Guerrero | Applications of Pore-Expanded Mesoporous Silica. 7. Adsorption of Volatile Organic Compounds[END_REF], [START_REF] Zhao | Organophilicity of MCM-41 adsorbents studied by adsorption and temperature-programmed desorption[END_REF]. Hence, in SBA-16, adsorption can take place via London forces and weak π-system-hydrogen bonding of the aromatic ring with silanols. For these dipole forces, the strength increases with the polarizability of the molecule, which explains the low adsorption capacity obtained for benzene and toluene compared to As a priori expected, the results regarding total adsorption capacity in the multicomponent adsorption study are consistent with those obtained in single component experiments. Total adsorption capacity followed the order SBA-16 > Carbopack® B > HKUST-1. It is clear from our results that multicomponent adsorption is a complex process in which polarizability, molecular size, surface chemistry and porosity play a crucial role. In this context, the multicomponent breakthrough curves can provide valuable information about the key aspects in competitive adsorption of pollutants according to their characteristics. SBA-16 has demonstrated the highest adsorption capacity for aromatics in both single and multicomponent experiments. Moreover, among the investigated materials, the mesoporous silica exhibited the minimum roll-up, thus avoiding the release of compounds previously adsorbed, which can be dangerous since benzene is the most hazardous one. Therefore, SBA-16 can be a promising candidate for applications to pollutant removal and analysis. Nevertheless, for its use in absorbers at industrial scale, enough length should be expected to prevent early breakthroughs.

Desorption activation energy of toluene

In gas analysis as well as in pollutant removal, the activation energy of the desorption process of contaminants is a crucial parameter for a sound selection of an adsorbent. This energy should be relatively high to enable the trapping of pollutants at room temperature but low enough to permit its desorption at moderate temperatures, typically in the range 150 -350 °C. In this context, TPD is powerful surface analysis technique providing valuable information regarding desorption kinetics, reaction order or desorption activation energy, as well as the variation of each of these factors with respect to the adsorbate coverage [START_REF] Parker | Determination of the reaction order and activation energy for desorption kinetics using TPD spectra: Application to D2 desorption from Ag(111)[END_REF].

In this work, toluene TPD experiments were carried out at different heating rates, between 10 to 30 K/min, over Carbopack® B and SBA-16 samples. Lower heating rates from 4 to 12 K/min were used over HKUST-1 to prevent adsorbent decomposition before toluene desorption. The obtained desorption curves are displayed in Fig. 6.

Whatever the adsorbent, only one desorption peak was detected, thus indicating that the adsorption of toluene took place from a single adsorption site. As illustrated in Fig. 6, an increment in the heating rate led to an increase in the peak desorption temperature (Tp). SBA-16 and Carbopack® B exhibited similar desorption temperatures at the same heating rates, whilst higher temperatures were required for HKUST-1. This implies that higher energies are required to desorb toluene from this material and, consequently, either the toluene-adsorbent interactions are stronger or the diffusional limitations higher compared to those of SBA-16 and Carbopack® B. Relating this to the obtained results during the competitive adsorption experiments, where toluene was released from HKUST-1 (Fig. 5c), it might be expected that the adsorbate-adsorbent interactions of C8 aromatics could be even stronger. The desorption activation energy (Ed) of toluene can be estimated using the data from TPD experiments [START_REF] Peralta | Comparison of the Behavior of Metal-Organic Frameworks and Zeolites for Hydrocarbon Separations[END_REF], [START_REF] Parker | Determination of the reaction order and activation energy for desorption kinetics using TPD spectra: Application to D2 desorption from Ag(111)[END_REF], [START_REF] Zhang | Improvement of CO2 adsorption on ZIF-8 crystals modified by enhancing basicity of surface[END_REF] according to the Polanyi-Wigner equation [START_REF] Pan | Catalytic combustion of styrene over copper-based catalyst: Inhibitory effect of water vapor[END_REF]:

- 𝑑𝜃 𝑑𝑇 = 𝐴 𝛽 𝜃 𝑛 exp (-𝐸 𝑑 𝑅𝑇 ⁄ ) (2)
where θ is the surface coverage, T (K) is the temperature, A (s - ) is the desorption rate coefficient, β (K s - ) is the heating rate, n is the order of the reaction, Ed (J mol - ) is the activation energy of the desorption and R (J mol

In this way, by plotting ln( 𝑅𝑇 𝑝 2 𝛽 ) versus 1 𝑇 𝑝 ⁄ , the toluene desorption activation energy over the different materials can be calculated (Fig. 7). The obtained desorption activation energies are 33.7, 33.8 and 35.9 kJ/mol for SBA-16, Carbopack® B and HKUST-1, respectively. These values are very close, indicating that the strength of the adsorbate-adsorbent interactions is similar in all investigated materials.

Among them, HKUST-1 exhibited a barely higher desorption activation energy probably due to both the stronger toluene electrostatic interactions within the micropores and the expected lower diffusional coefficient. In addition, the presence of benzyl groups in HKUST-1 structure allows forming π-π interactions with toluene, hence enhancing the energy required for the desorption. In the case of mesoporous and non-porous materials such as SBA-16 and Carbopack® B, the molecule diffusion should not be limited, thus leading to a lower desorption activation energy. Nevertheless, the activation energy was found to be almost identical on SBA-16 and Carbopack® B, suggesting that the presence of pores much larger than the size of the adsorbate molecule did not significantly influence the adsorbateadsorbent interactions.

The activation energy of the desorption process was 35.9 kJ/mol for HKUST-1, being barely lower than 43.8 kJ/mol reported by Xu et al. [START_REF] Xu | Effect of Textural Properties on the Adsorption and Desorption of Toluene on the Metal-Organic Frameworks HKUST-1 and MIL-101, Effect of Textural Properties on the Adsorption and Desorption of Toluene on the Metal-Organic Frameworks HKUST-1 and MIL-101[END_REF] for toluene and slightly higher than the value of 33.8 kJ/mol determined by Zhao et al. for the desorption of benzene [START_REF] Zhao | Competitive adsorption and selectivity of benzene and water vapor on the microporous metal organic frameworks (HKUST-1)[END_REF]. This difference might be mostly related to the lower benzene cross-sectional area, which allows a better diffusion within the micropores. From an industrial point of view, a lower desorption energy is preferred since less energy is required for the adsorbent regeneration, thus resulting in more cost-effective processes. Therefore, considering only the energy aspect, SBA-16 seems again to be the best candidate for pollutant removal and analysis among the studied adsorbents.

CONCLUSIONS

In this study, three commercial adsorbents with different porosities and surface chemistries were characterized and their BTEX adsorption was assessed in terms of breakthrough time and adsorption capacity. Firstly, single component dynamic adsorption studies were carried out using toluene as a probe molecule. Those results suggest that breakthrough time is closely related to both particle size, being shorter for smaller particles, and the micropores presence. From the breakthrough curves, adsorption capacity values of those materials were calculated to be 18.7, 24.9 and 7.0 mg/g for Carbopack® B, SBA-16 and HKUST-1, respectively. It was expected to find an increasing adsorption capacity as specific surface area increases. However, the material with larger specific surface area (HKUST-1) exhibited the lowest adsorption capacity, pointing out possible diffusional limitations related to the microporous structure. The same trend in adsorption capacity was found during multicomponent adsorption experiments. Furthermore, roll up effect was observed to some extent with all adsorbents.

The intensity of this phenomenon was related to adsorbate-adsorbent interactions in each case, indicating that the strength of these interactions plays a major role in competitive adsorption. Roll-up effect is of great importance when using adsorbents in air treatment, since hazardous compounds can be released even when the adsorbent is far from saturation. Finally, the activation energy for toluene desorption determined by TPD technique was 33.8, 33.7 and 35.9 kJ/mol for Carbopack® B, SBA-16

and HKUST-1, respectively. The microporous material (HKUST-1) exhibited the highest desorption activation energy due most probably to both stronger molecule interactions and diffusional limitations within the micropores. Non-porous (Carbopack® B) and mesoporous solids (SBA-16) exhibited almost identical energies suggesting comparable adsorbate-adsorbent interactions and diffusional coefficients.

Among the investigated adsorbents, SBA-16 seems to be the most appropriate for air treatment due to its superior adsorption capacity, minimal roll-up and low desorption temperature. Nevertheless, a detailed study of the adsorption capacity of this material after several regeneration cycles should be performed to fully evaluate its suitability for air treatment and gas analysis.

Figure 1 .

 1 Figure 1. X-Ray diffraction patterns of commercial adsorbents.

Figure 2 .

 2 Figure 2. SEM micrographs of (a) Carbopack® B, (b) SBA-16 and (c) HKUST-1

Figure 3 .

 3 Figure 3. Nitrogen adsorption (black) and desorption (red) isotherms at 77 K of (a) Carbopack® B, (b) SBA-16, (c) HKUST-1

Figure 4 .

 4 Figure 4. Breakthrough curves of toluene over SBA-16 (blue), Carbopack® B (red) and HKUST-1

Figure 5 .

 5 Figure 5. Breakthrough curves for BTEX over Carbopack® B (a), SBA-16 (b), and HKUST-1

  ethylbenzene and m/p-xylenes. As in the previous case, the adsorption capacity follows the order of the boiling points (o-xylene > m/p-xylenes > ethylbenzene > toluene > benzene). Obviously, o-xylene, with its highest boiling point and high polarizability, demonstrated the highest adsorption capacity.Similarly to SBA-16, BTEX adsorption in HKUST-1 occurs through London forces as well as π-interactions but, in this case, these interactions are between the aromatic ring of the adsorbate and those presented in the HKUST-1 structure[START_REF] Peralta | Comparison of the Behavior of Metal-Organic Frameworks and Zeolites for Hydrocarbon Separations[END_REF] formed with benzene 1,3,5-tricarboxylate linkers. Once again, the adsorption capacity follows the order: o-xylene > m/p-xylenes > ethylbenzene > toluene > benzene. However, the roll-up observed in the breakthrough curve for benzene and toluene is considerably more pronounced than in the mesoporous silica. This difference is probably due to the fact that in HKUST-1, BTEX molecules are adsorbed within the micropores instead of mesopores, and the BTEX molecules size is very close to that of micropores. Since London forces are distance sensitive, they are expected to play an important role in the adsorption within the micropores resulting in a huge difference between the adsorption capacities of smaller molecules, i.e. benzene and toluene, compared to the larger molecules. These results are in line with those reported in other studies. Chevalier et al. performed breakthrough experiments of 1.25 ppm binary mixtures of toluene and o-xylene on HKUST-1 [26]. They observed a pronounced roll-up effect of toluene indicating highly selective adsorption of o-xylene over toluene. Peralta et al. investigated the single and competitive adsorption of xylene isomers on HKUST-1 [33]. In single experiments, they found similar adsorption capacities for the three isomers, with a slightly higher value for o-xylene. Nevertheless, in multicomponent experiments, a preferential adsorption of o-xylene over m-and p-xylenes was reported.

Figure 6 .

 6 Figure 6. Toluene desorption TPD curves over Carbopack® B (a), SBA-16 (b) and HKUST-1 (c)

Figure 7 .

 7 Figure 7. Linear dependence between ln (RT 2 p/β) and 1/Tp for TPD of toluene on SBA-16 (blue),

Table 1 .

 1 Summary of former studies on benzene and toluene adsorption on various types of silicas

	Silica type	SBET (m 2 /g)	Vtotal (mL/g)	Vmicro (mL/g)	Dmeso (Å)	Compound	Concentration (ppm)	Temperature (°C)	Adsorption Capacity (mg/g ads)	Reference
	SBA-16		0.72	-	3.4 and 6.0	Toluene	100	23	24.88	This work
	SBA-15		1.2	0.07	6.3	Benzene	1000	--	71.08	[29]
	MCM-41	1088	0.92	-	2.9	Benzene	1000	--	79.67	[29]
	MCM-48	1210	1.02	-	2.4	Benzene	1000	--	76.55	[29]
	KIT-6		1.29	0.13	6.3	Benzene	1000	--	98.42	[29]
	SBA-15-1		1.00	0.05	5.6	Toluene	71.65	30	147.39*	[28]
	SBA-15-2		0.84	0.06	6.55	Toluene	71.65	30	190.74*	[28]
	MCM-41		0.64	-	2.38	Toluene	71.65	30	130.05*	[28]
	SiO2		0.8	-	4.33	Toluene	71.65	30	86.70*	[28]
	SBA-15		1.143	-	8.15	Benzene	1000	20	15.78	[30]
	SBA-15 A		0.54	0.089	5.58	Toluene	n. a.	25	167.42	[31]
	SBA-15 B		0.77	0.122	6.47	Toluene	n. a.	25	187.27	[31]
	SBA-15 C		0.45	0.083	3.82	Toluene	n. a.	25	415.39	[31]
	SBA-15 D		0.81	0.094	7.72	Toluene	n. a.	25	321.58	[31]
	MCM-41		0.64	-	2.43	Toluene	n. a.	25	151.95	[31]

n. a.: not available data; * equilibrium adsorption capacity extracted from isotherm data; Dmeso: mesopore diameter

Table 2 .

 2 Summary of BTEX adsorption studies on HKUST-1.

	Reference	SBET (m 2 /g)	Vtotal (mL/g)	Vmicro (mL/g)	Pore diameter (Å)	Compound	Concentration (ppm)	Temperature (°C)	Adsorption Capacity (mg/g ads)
	This work	1733	0.89	0.68	5.4 and 6.9	Toluene	100	23	7
	[26]	1122.4	0.47	n. a.	7.5	Toluene	2.5	23	238
	[26]	1122.4	0.47	n. a.	7.5	o-xylene	2.5	23	147
	[27]	1237	0.47	n. a.	10.51	Toluene	99	25	150
	[32]	907.2	0.46	n. a.	n.a.	Toluene	920000	25	516*
	[33]	1718	0.68	n. a.	n. a.	o-xylene	5925	125	297*
	[33]	1718	0.68	n. a.	n. a.	m-xylene	5925	125	255*
	[33]	1718	0.68	n. a.	n. a.	p-xylene	5925	125	297*
	[34]	1188.3	0.77	0.41	n. a.	Toluene	950000	25	571*
	[35]	1568.5	0.75	0.61	6.51 and 8.25	Benzene	64187	15	794*

n. a.: not available data; * equilibrium adsorption capacity extracted from isotherm data

Table 3 .

 3 Textural properties of investigated adsorbents

	Sample	SBET (m 2 g -1 )	Vtotal (cm³ g -1 )	Vmeso (cm³ g -1 )	Vmicro (cm³ g -1 )	Dpore (nm)
	Carbopack® B	112	-	-	-	-
	SBA-16	572	0.72	0.68	-	3.4/6.0
	HKUST-1	1733	0.89	0.14	0.68	0.54/0.69

SBET: BET surface area; Vtotal: total pore volume; Vmeso: mesoporous volume; Vmicro: microporous volume; Dpore: pore diameter.

Table 4 .

 4 Breakthrough times and adsorption capacities obtained in toluene adsorption experiments

	Adsorbent	Breakthrough time (min)	Adsorption Capacity (mg/ g ads)
	Carbopack® B	54	18.74
	SBA-16	32	24.87
	HKUST-1	20	6.96

Table 5 .

 5 Breakthrough times and adsorption capacities obtained in multicomponent adsorption experiments

		Breakthrough time (min)		Adsorption Capacity (mg/ g ads)
	Compound	CB	SB	HK	CB	SB	HK
	Benzene	1	11	28	0	0.6	0.16
	Toluene	23	40	44	1.47	2.81	0.82
	Ethylbenzene	51	99	70	3.31	8.16	3.25
	m/p -Xylenes	116	129	76	6.13	8.96	3.45
	o -Xylene	190	159	73	7.32	9.67	3.59
	Total	-	-	-	18.22	30.2	11.28
	CB: Carbopack® B; SB: SBA-16; HK: HKUST-1

  - K - ) is the ideal gas constant. During the desorption, the desorption peak temperature Tp is reached when

	𝑑 2 𝜃 𝑑𝑇 2 = 0. Taking this into account and assuming that desorption follows
	first order kinetics, Eq. ((2) yields:
	ln(	𝑅𝑇 𝑝 2 𝛽	) = ln (	𝐸 𝑑 𝐴	) +	𝐸 𝑑 𝑅𝑇 𝑝

Acknowledgements

This work has received funding from the Clean Sky 2 Joint Undertaking under the European Union's Horizon 2020 research and innovation program under grant agreement No. 687014 (MACAO project) and under the Marie Skłodowska-Curie Grant Agreement No. 643095 (MIGRATE project) [H2020-MSCA-ITN-2014]. This project has also been supported by the European Union's through the LIFE SMART IN'AIR -Smart indoor air monitoring network to reduce the impacts of pollutants on environment and healthunder grant number LIFE17 ENV/FR/000330.


Cristina Megías-Sayago
email: megiassayago@unistra.fr

Irene Lara-Ibeas

Qiang Wang

Stephane Le Calvé

Benoît Louis

Volatile Organic Compounds (VOCs) Removal Capacity of ZSM-5 Zeolite Adsorbents for Near Real-Time BTEX detection

Keywords: BTEX abatement, ZSM-5 zeolite, adsorbent design, real-time detection

The environmental impact of air pollution along with the registered detrimental effects on human health due to BTEX exposure have forced most countries to set limit values in public spaces and buildings, requiring the development of in situ devices able to quantify in (near)-real time the pollutants concentration, even at ppb levels. In this work, several ZSM-5 zeolites with 79 < Si / Al < 316 have been synthesized and used as part of a pre-concentration unit connected to a miniaturized GC prototype, needed to drop the detection limit according to current legislation. The most important parameters influencing the multicomponent gas-phase adsorption of BTEX over ZSM-5 zeolites have been found to be the number of available acid sites (Al content), the zeolite pore size and its hydrophobicity. The experimental results demonstrate that a compromise between the hydrophobic character and the number of Al atoms should be found in order to properly design an adsorbent for near-real time BTEX detection.

Introduction

The uncontrolled pollutant emissions mainly derived from the transport sector and several industrial processes have turned air pollution as one of the most notorious worldwide problem nowadays. Climate change evidence along with the numerous publications dealing with air pollution impact in human health [START_REF] Bruce | The health effects of indoor air pollution exposure in developing countries[END_REF][START_REF] Buka | The effects of air pollution on the health of children[END_REF][START_REF] Kampa | Human health effects of air pollution[END_REF] have forced most developing countries to apply severe laws aiming its depletion. Volatile organic compounds (VOCs) usually denote organic compounds mainly originated during engine combustion and industrial emissions but they can be also released, to a minor extent, from building materials or domestic products. Amid the wide variety of VOCs, special interest has been paid to benzene, toluene, ethylbenzene and xylenes (BTEX) due to their toxic, mutagenic and/or carcinogenic effects even at low concentrations [START_REF] Ueno | Microfluidic Device for Airborne BTEX Detection[END_REF]. Indeed, several effects in human health as skin and eyes irritation, headache, memory problems and loss of coordination have been linked to BTEX exposure [START_REF] Hazrati | Preliminary assessment of BTEX concentrations in indoor air of residential buildings and atmospheric ambient air in Ardabil, Iran[END_REF][START_REF] Kandyala | Xylene: An overview of its health hazards and preventive measures[END_REF] along with more detrimental effects as the development of leukemia, ascribed to the highest carcinogenic compound, i.e., benzene [7]. These severe problems drove the European Union in 2013 to set a limit value of 1.6 ppb for benzene in public buildings, value that was further decreased to 0.6 ppb in 2016 in France [START_REF] Lara-Lbeas | Sub-ppb Level Detection of BTEX Gaseous Mixtures with a Compact Prototype GC Equipped with a Preconcentration Unit[END_REF]. Therefore, the adopted legislation forces somehow the development of new, in-situ fast and highly sensitive analytical devices.

Gas chromatography (GC) has been the most powerful technique to detect BTEX in air, having detection limits down to ppt levels. The main problem associated to classical chromatographs, however, has been their huge size and weight as well as the high-energy consumption, factor that limit their application for in-situ measurements. Therefore, most of the time the air is sampled on site on adsorbents tubes which are then analyzed in the laboratory after thermal or chemical desorption.

As a consequence, the development of miniaturized systems for real-time BTEX analysis has attracted much attention in the last years. Lately, low-size GC prototypes have been successfully developed. However, the detection limit in the ppt range remains a challenge in most of these small devices. Recently, Lara-Ibeas [START_REF] Lara-Lbeas | Sub-ppb Level Detection of BTEX Gaseous Mixtures with a Compact Prototype GC Equipped with a Preconcentration Unit[END_REF] have designed a compact GC prototype equipped with a pre-concentration unit which lowered the detection limit down to sub-ppb level. This unit is filled with an adsorbent that allows the BTEX pre-concentration during in-situ measurements in a way that the gaseous samples are taken during a given time, adsorbed (concentrated) in the solid adsorbent and, finally, desorbed (by increasing the temperature in the unit) to be analyzed by the miniaturized GC. Hence, low-polluted environments can be studied; being possible to tune the sampling time in order to increase BTEX injected amount as much as needed.

Activated carbons are the most useful adsorbents for VOCs uptake [START_REF] Lee | Adsorption characteristics of binary vapors among acetone, MEK, benzene, and toluene[END_REF][START_REF] Son | Electrothermal adsorption and desorption of volatile organic compounds on activated carbon fiber cloth[END_REF][START_REF] Choma | Adsorption properties of activated carbons prepared from waste CDs and DVDs[END_REF],

nevertheless the practical problems linked to their use such as the difficulty for regeneration [START_REF] Dragoi | Adsorption of organic pollutants over microporous solids investigated by microcalorimetry techniques[END_REF] and flammability [START_REF] Zhang | Adsorptive and catalytic properties in the removal of volatile organic compounds over zeolitebased materials[END_REF] limit their implementation in the mentioned application. An adsorbent with high specific surface area, non-toxic character, high thermal stability and easy to regenerate (able to bear charge / discharge cycles) is highly warranted. In this sense, zeolite-type materials exhibit all these features as well as tailorable surface hydrophobicity, which turn them perfect candidates to be used as adsorbents in the aforementioned pre-concentration unit. Another important factor is the zeolite pore size which differs among the different zeolitic frameworks, becoming a tunable and interesting property. Indeed, properly tuning the size of the pores allows selective adsorption, thus diminishing the interferences due to the presence of other atmospheric organic pollutants during the analysis. The latter became necessary in high-polluted environments, as is the case of airports, where the concentration of VOCs can reach high concentrations [START_REF] Mokalled | VOC tracers from Aircraft Activities at Beirut Rafic Hariri International Airport[END_REF][START_REF] Mokalled | An Assessment of Indoor Air Quality in the Maintenance Room at Beirut-Rafic Hariri International Airport[END_REF]. Hence, to ensure a wider application of the GC prototype, a zeolite with pore size close to the BTEX cross-sectional areas [START_REF] Mc Clellan | Cross-Sectional Areas of Molecules Adsorbed on Solid Surfaces[END_REF] is highly desired. In this context, ZSM-5 zeolite having a pore size of roughly 0.55 nm appears to be a suitable candidate.

In this work, several ZSM-5 zeolites differing in their Si/Al molar ratio are synthetized via fluoride route and characterized by different techniques.

Multicomponent gas-phase BTEX breakthrough experiments have been carried out over all as-prepared zeolites, packed separately in thermal desorption tubes connected to the miniaturized GC prototype. The impact of the zeolite properties on the adsorption performance has been deeply studied and discussed, aiming to establish the key parameters affecting the process which will help us to accurately design an adsorbent for near real-time BTEX detection.

Experimental

H-ZSM-5 synthesis

ZSM-5 zeolites were synthetized via fluoride-mediated route adapted from our previous studies [START_REF] Losch | H-ZSM-5 zeolite model crystals : Structurediffusion-activity relationship in methanol-to-olefins catalysis[END_REF][START_REF] Ocampo | Synthesis of zeolite crystals with unusual morphology: Application in acid catalysis[END_REF]. Typically, the aluminum source (different amounts to achieve Si/Al = 300, 200, 150, 100 and 50 mole ratios, respectively) was placed in a 150 mL-Erlenmeyer flask where 50 mL of distilled water was added under vigorous stirring (700 rpm, r.t.). Once dissolved, TPABr (0.498 g) and NH 4 F (1.103 g) were sequentially added. Finally, 1.607 g SiO 2 (Aeroperl 300/30, Evonik) was gradually added during 5 min. The gel was then aged 2h under vigorous stirring and finally autoclaved at 443 K for 144h. H-zeolite form was obtained after a single calcination at 823 K during 15h in static air. The samples were named as ZSM-5_1 to ZSM-5_5, respectively, being the number increase related with the Si/Al molar ratio (abbreviated as SAR) decrease.

Characterization

X-ray diffraction (XRD) analysis was performed in a Bruker D8 (Cu Kα) diffractometer operated at 40 kV and 40 mA. XRD patterns were recorded in the 2θ = 5 -60° range with a step size of 0.05°. SEM micrographs were acquired in a JEOL FEG-6700F microscope working at a 9 kV accelerating voltage.

Zeolite chemical compositions were determined by X-ray fluorescence (XRF) using a XEPOS (AMETEK) device equipped with a Rh radiation tube.

Surface areas (BET method) and pore volumes were determined by nitrogen physisorption using a Micromeritics ASAP 2420 equipment. Samples were pre-treated in-situ at 250°C under vacuum prior to the analyses.

The acid site density was evaluated by NH 3 -TPD (temperature programmed ammonia desorption) by using a Micromeritics AutoChem II chemisorption analyzer equipped with a TCD detector. Samples were pre-treated in-situ at 550°C during 1 h. Afterwards, samples were cooled down to room temperature and a 5% NH 3 /He flow was passed through the reactor. Once physisorbed ammonia was removed, the temperature gradually increased to 550°C. 27 Al (I = 5/2) MAS NMR was carried out with a Bruker Avance II 400 spectrometer operating at B0 = 9.4 T (Larmor frequency = 104.2 MHz) equipped with a Bruker 2.5 mm double channel probe. Measurements were carried out with [Al(H 2 O) 6 ] 3+ as external standard reference.

The hydrophilicity of the zeolites has been determined by contact angle measurements. For this purpose, 20 mg of powder was placed under a hydraulic press and let under 12 bar for 30 s to obtain a compact pellet. Wetting properties of the samples were determined by static contact angle using an optical tensiometer (Attension theta, Biolin Scientific). A 2 µl drop of purified water (milli Q) was deposited on the pellet surface and the angle was recorded by the camera mounted on the device. Thermogravimetric analysis (TGA) was employed to monitor toluene uptake measurements. Prior to analysis, the samples were sieved in order to have a particle diameter between 0.074 and 0.100 mm. After that, they were placed in the sample holder and pre-treated in-situ at 550°C for 30 min with a heating ramp of 10°C/min under a 40 mL/min nitrogen flow. Once conditioned, the temperature was diminished to 40°C, and gas switched for 0.28 mg/min toluene concentration. The mass variation during all the experiment was monitored until saturation. In some cases, toluene desorption was also recorded.

Breakthrough experiments

Single dynamic adsorption experiments were carried out for each adsorbent in a homemade set-up schematized in Figure S1 (Supporting Information, SI). For these tests, 15.0 ± 0.2 mg (Mettler Toledo AG204, USA) of the as-prepared zeolites were packed separately in thermal desorption (TD) tubes (Sigma Aldrich, USA) using two quartz wool plugs to fix the adsorbent bed. These tubes were placed in the aforementioned setup through which 5.00 ± 0.05 mL/min (F-201CV-020, Bronkhorst, Netherlands) constant flow of an equimolar mixture of BTEX diluted in nitrogen (10 ± 1 ppm, Air Products, France) was passed. Prior to analysis, the tube outlet flow was diluted with pure nitrogen (5 mL/min diluted with 95 mL/min pure N 2 flow, being 500 ppb the final concentration) to end up in the linear calibration range of the instrument and to avoid possible detector saturation. The gaseous effluent was then continuously analyzed by a gas chromatograph (µBTEX-1 In'Air Solutions, France). This instrument operates according to three steps: sampling, separation and detection. A gaseous sample is collected and introduced in a 200 µL sampling loop by means of a SP 570 EC-BL micropump (Schwarzer Precision, Germany). It is subsequently injected into the GC column (i.d. 0.18 mm, RXi-624 stationary phase, 1 μm film thickness, Restek, Bellefonte, PA, USA) for compounds separation and the detection is conducted employing an eVx Blue mini photoionization detector (PID) (Baseline MOCON, Lyons, CO, USA) [START_REF] Nasreddine | Development of a novel portable miniaturized GC for near real-time low level detection of BTEX[END_REF][START_REF] Nasreddine | Portable novel micro-device for BTEX real-time monitoring: Assessment during a field campaign in a low consumption energy junior high school classroom[END_REF][START_REF] Lara-Ibeas | BTEX near real-time monitoring in two primary schools in La Rochelle, France[END_REF].

Prior to each adsorption experiment, the gas stream was flowed through a bypass, directly diluted as previously mentioned and then analyzed to determine the peak area corresponding to the initial concentration. The zeolite samples were conditioned at 350 °C for 2 h under a helium flow of 50 mL/min using thermal desorber (TurboMatrix 350, Perkin Elmer, USA) to remove the possible adsorbed molecules.

Results and discussion

Samples characterization

The diffraction patterns of all synthetized ZSM-5 zeolites confirm the sole presence of the MFI structure (Figure S2) [START_REF] Ocampo | Synthesis of zeolite crystals with unusual morphology: Application in acid catalysis[END_REF][START_REF] Ocampo | Design of MFI Zeolite-Based Composites with Hierarchical Pore Structure: A New Generation of Structured Catalysts[END_REF] together with high crystallinity degree in all cases. The latter is usually related with the fluoride medium used during the zeolite synthesis which provides milder conditions in comparison with alkaline medium, thus facilitating the formation of few nuclei under diluted conditions as consequence of a slow release of SiO x F y -species [START_REF] Losch | H-ZSM-5 zeolite model crystals : Structurediffusion-activity relationship in methanol-to-olefins catalysis[END_REF][START_REF] Flanigen | New Developments in Zeolite Science and Technology[END_REF]. This, along with high Si/Al ratios, leads to the formation of zeolites with fewer defects and more open structures than in alkaline medium [START_REF] Xu | The synthesis of SAPO-34 and CoSAPO-34 from a triethylamine-hydrofluoric acid-water system[END_REF]. The formation of highly crystalline zeolites is further evidenced by SEM (Figure 1). In all cases, large coffin-shaped crystals having between 30-60 μm in size are formed.

The chemical composition, textural properties and the total number of BrØnsted acid sites of fresh samples are summarized in Table 1. The targeted variation of acid properties is successfully accomplished as confirmed by XRF results and further Figure 2 shows the results obtained after the breakthrough tests over all synthetized ZSM-5 zeolites. For each sample, five breakthrough curves can be distinguished which correspond to each single adsorbate interaction with the zeolite. It should be noted that, due to the long breakthrough times registered, the room temperature was subjected to variations that finally affected the breakthrough curve.

As an example the first sample, ZSM-5_1, between 1600 -2400 min corresponding to nighttime when room temperature diminished till its lowest value.

Apparently, the obtained results seem comparable, as the general behavior remains quite similar. Generally speaking, a clear exclusion of o-xylene during sorption experiments is observed as well as a C/C 0 constant value of around 0.4 -0.5 for m/p-xylene mixture suggesting the exclusion of one of the isomers. Related to this observation, it should be considered that adsorbate molecules within the zeolite framework are exposed to different possible interactions among which acid-base ones must be taken into account [START_REF] Alejandro | BTX abatement using Chilean natural zeolite: The role of Brønsted acid sites[END_REF]. When comparing the relative basicities of benzene, toluene and p-xylene (0.61, 0.92 and 1.26, respectively) one may expect that the three considered VOCs exhibit different level of interactions toward BrØnsted acid sites, being xylenes the ones interacting strongly due to their highest relative basicity.

However, the obtained results show that xylenes present the lowest abatement. This adsorption reduction with respect to xylenes isomers could be linked to the small micropores of ZSM-5 -0.55×0.51 nm and 0.56×0.53 nm for [100] and [010] planes (Figure 3), respectively, and the differences between the cross-sectional areas of adsorbates. Indeed, m/o-xylene molecules cross-sectional areas are 0.56 nm [START_REF] Mc Clellan | Cross-Sectional Areas of Molecules Adsorbed on Solid Surfaces[END_REF],

higher than those of ethylbenzene (0.55 nm), p-xylene (0.54 nm), toluene (0.47 nm)

and benzene (0.43 nm) [START_REF] Mc Clellan | Cross-Sectional Areas of Molecules Adsorbed on Solid Surfaces[END_REF]. The latter explains the steric exclusion of m/o-xylene molecules in zeolite micropores [START_REF] Kurganov | Effect of Size of Solvent Molecule on the Adsorption of p-and o-Xylene on ZSM-5 Type Zeolites and Mechanism of Adsorption[END_REF] thus supporting the observed experimental data.

It should be underlined that, despite this steric exclusion of m/o-xylene prevents their adsorption, it allows the separation of m/p-xylene isomers, usually a difficult task in gas chromatography which could be reached by coupling a gas chromatograph to a preconcentration unit filled with this adsorbent. Indeed, the chromatographic analysis method used during this work does not allow the separated quantification of m/pxylenes since they are coeluted. Therefore, the steric exclusion over ZSM-5 zeolites could be an effective strategy to separate both isomers without the need for a very specific chromatographic column.

Concerning benzene, toluene and ethylbenzene breakthrough curves, there are mainly two aspects that should be taken into account. Firstly, the phenomenon known as roll-up, consisting in desorption of a molecule by competitive adsorption of a more strongly adsorbed molecule (C/C 0 values over 1) and, secondly, the uptake curve slope. Attending to the first one, benzene trends always to desorb whatever the zeolite composition, toluene is largely desorbed after interaction with samples 1, 2 and 3, respectively, but the desorption occurs to a much lesser extent in samples 4 and 5 and ethylbenzene stays trapped into the zeolite framework in all the experiments. As mentioned before, adsorbate molecules are exposed to different possible interactions among which BrØnsted acid-base ones seem to be the key interactions controlling the process as demonstrated by Alejandro et al. [START_REF] Alejandro | BTX abatement using Chilean natural zeolite: The role of Brønsted acid sites[END_REF]. They confirmed by means of FTIR that benzene, toluene and xylene molecules are adsorbed within the ZSM-5 framework due to a direct interaction between acid sites and the delocalized π -system of benzene ring (see Figure 3). Logically the interaction strength may change between all molecules due to slight variations in the π-cloud as a consequence of the (m)ethyl substituents in the ring. Methyl group (toluene) is an electron donor which leads to an electron-rich π -system thus enabling stronger interaction with H + sites in the zeolite.

The π-donor character of methyl and ethyl groups may be assumed the same.

Considering now the two aforementioned aspects, i.e. the desorption process and the slope of uptake curve, not only the π-interaction should be kept in mind but also the diffusion process of each molecule within zeolite channels. Benzene is the smallest interacting molecule (0.43 nm), followed by toluene (0.47 nm) and ethylbenzene (0.55 nm). One may therefore a priori expect the same diffusion trend through the zeolite cavities. Indeed, the curve slopes verify this trend, the greatest is observed for benzene, followed by toluene and ethylbenzene.

As for the desorption process, benzene is the molecule diffusing faster which is also characterized by a "weaker" interaction with the acid sites. The latter may explain its behavior during the breakthrough tests together with a certain degree of adsorptive competition with toluene and ethylbenzene. Toluene, for its part, has both bigger cross-sectional area and "stronger" interaction with zeolite. It is desorbed in the samples with low aluminum content (1, 2 and 3) but remains adsorbed in those with higher Al content (4 and 5). The latter confirms somehow that the interaction is directly related to the number of available BrØnsted acid sites. It seems that the toluene molecules that diffuse along high SAR zeolite channels face problems to find available active sites, then being desorbed. The same effect could be expected for ethylbenzene, however, as its cross-sectional area is comparable to zeolite pore size, its diffusion along the channels is limited (see its uptake slope), thus hindering its possible desorption. This phenomenon is also observed for adsorbed p-xylene.

In order to properly evaluate the effect of ZSM-5 chemical composition in BTEX abatement, the breakthrough times and adsorption capacities over the five asprepared samples have been evaluated for each compound separately. Table S1 summarizes the obtained breakthrough times, presented as a function of the Si/Al molar ratio in Figure 4. As it can be seen, there is a correlation between the breakthrough time values and the Al content in the zeolite, different for each adsorbed compound. In all cases, the lowest SAR sample (ZSM-5_5, SAR 79) is excluded from the trend, which may give us some insights about the physical and/or chemical parameters involved during the adsorption process.

Regarding the rest of zeolite samples (ZSM-5_1 to ZSM-5_4) a clear maximum is situated around SAR 220±10 (ZSM-5_2, SAR 198) for Benzene, Toluene and Ethyl Benzene. Conversely, p-Xylene exhibits a minimum value at SAR value of 250 and reaches its maximum value over ZSM-5_4 (SAR 113). It appears that a proper explanation of the observed results is far for being simple and not directly related to simply the aluminum content in the zeolite framework. Certainly, the Al percentage is directly related with the total number of BrØnsted acid sites but also with the zeolite hydrophilic character [START_REF] Kawai | Evaluation of hydrophilic-hydrophobic character of zeolites by measurements of their immersional heats in water[END_REF], even higher when some Al species are in a octahedral environment. This is considered as a defect in the zeolite network, usually denoted as EFAl species (extra-framework Al species). In this case and due to the hydrophobic character of the adsorbates, the higher the zeolite hydrophobic character, the stronger the interaction between them. In addition, the Al content (number of active sites, possible presence of EFAl) also affects the molecules diffusion thus altering the obtained breakthrough times and sorption capacities. Therefore, a compromise between all factors should lead to an optimized system.

In order to evaluate the EFAl percentage and hydrophobic character of the samples with respect to their Al content, ZSM-5_2 (SAR 198) and ZSM-5_5 (SAR 79) samples were chosen to perform both 27 Al MAS NMR and contact angles measurements (Figure S3). As 'a priori' expected, EFAl species percentage increases with higher Al content (1 and 3% for ZSM-5_2 and ZSM-5_5, respectively). The latter suggests that the hydrophilic character also increases from SAR 198 to 79 due to an increment in both framework Al and EFAl species. Contact angle measurements confirm these results, sample 2 exhibited the highest angle meaning that the hydrophobic character of high SAR zeolites is superior to lower SAR zeolites.

From these results, the obtained breakthrough time dependence with Si/Al ratio can be understood as a compromise between the number of acid sites needed for allowing the adsorption process and the hydrophobic character of the adsorbent. For benzene, toluene and ethylbenzene that compromise (optimum) is attained when SAR is 198.

Nevertheless, the following two questions arise: why is systematically sample 5 excluded from all the correlations shown in Figure 4? Are those differences related to molecules diffusion paths?

In order to evaluate the effective diffusivity, toluene was chosen as representative probe molecule. Uptake measurements were carried out in a thermobalance under non-equilibrium isothermal conditions (40ºC), in a way that the mass variation was monitored during the experiment as shown in Figure S4. The toluene adsorption capacity is directly obtained from the measured variations while for the effective diffusivity calculation some specific initial and boundary conditions should be fixed as presented in our recent work [START_REF] Bingre | Assessment of the Improvement of Effective Diffusivity over Technical Zeolite Bodies by Different Techniques[END_REF]. Derived from this, the effective diffusivity can be calculated from the linear part of the uptake curves that can be expressed as follows (Eq. 1):

𝑞-𝑞 0 𝑞 ∞ -𝑞 0 = 6 √𝜋 ( 𝐷 𝑒𝑓𝑓 𝑟 2 ) 1/2 √ 𝑡 (1)
where q is the sample weight at a given time, q 0 the initial mass sample, q ∞ the saturation weight, D eff the effective diffusivity, r the particles diameter and t the time.

The toluene uptake curves are presented in Figure 5 where the y-axis represents catalyst mass percentage as a function of time. In general, two stages concerning the sorption process can be differentiated depending on the sample. Initially, a faster uptake reaction that occurs at the initial stage of toluene absorption and, secondly, a later long-period stage where it is possible to distinguish a continuous (but considerably slower) increase in the sample weight (sample 5). Zeolites with SAR over 100 (samples 2, 3 and 4) present similar behavior being the saturation time t s (Q/Q ∞ = 1) directly related with Al content in a way that the higher the SAR, the lower the t s . These results demonstrate, once again, that the active sites for toluene adsorption are the acid sites and, therefore, the lower the Al amount the lower the adsorption time.

Again, sample 5 behaves differently, showing the above-mentioned second step, the slower but continuous toluene sorption. Nevertheless, it is noted that for ZSM-5_2 the linear part of the curve possesses slightly higher slope value compared to samples 3 and 4, which means that higher effective diffusivity could be envisaged in this sample. The calculated diffusivity coefficients are presented in Table 2 together with the registered toluene capacities (μmol/g). D eff values follow this trend ZSM-5_2

> ZSM-5_5 > ZSM-5_4 ≈ ZSM-5_3 which is similar to those appearing in Fig. 4B.

Actually, the highest diffusivity values coincide with the longest breakthrough times.

The latter highlights the importance and effect of diffusivity in the sorption process.

The differences regarding the uptake curves of both samples (2 and 5) can be associated also to the Al atoms in the zeolite framework. ZSM-5_2 having low Al atoms and defects exhibits higher diffusion coefficient and faster saturation in comparison with ZSM-5_5, which seems to suffer from diffusional limitations. In this sample, the probability of encountering the next active site considerably raises as Al content is higher which means that a larger amount of toluene molecules diffuse and absorb within the pores, thus hindering the next toluene molecules diffusion along the cavities. This can be somehow contrasted by the observed toluene capacities, higher when Al content increases. These results suggest that the breakthrough time dependence with Si/Al ratio is mainly governed by the number of acid sites which directly influences both the hydrophobic character of the adsorbent and the effective diffusivity of the molecules along the channels.

The effect of the ZSM-5 chemical composition in BTEX abatement (Figure 2)

has also been evaluated in term of adsorption capacities for each compound. The dynamic adsorption capacity can be calculated from the breakthrough curve using the following equation (Eq. 2):

𝑞 = 𝐹 𝐶 0 m ∫ (1 - 𝐶 𝐶 0 ) 𝑑𝑡 𝑡 𝑠 𝑡 0 (2)
where q is the dynamic adsorption capacity per gram of adsorbent (mg/g ads ), F is the gas flow rate (m 3 /min), m is the mass of adsorbent (g), t 0 is the initial time (min), t s is the saturation time (min), C 0 is the initial concentration (mg/m 3 ), C is the outlet concentration (mg/m 3 ) at a given time. The error on the calculation of the dynamic adsorption capacity has been estimated as follows:

∆𝑞 𝑞 = ∆𝐹 𝐹 + ∆𝐶 0 𝐶 0 + ∆𝑡 𝑡 + ∆𝑚 𝑚
where ∆𝑞 is the error on the adsorption capacity. ∆𝐹 is the error on the flow rate, ∆𝐶 0 is the error on the initial concentration and ∆𝑚 is the error on the adsorbent mass.

The capacity values are presented in Table S2 and plotted vs. Si/Al molar ratio in Figure 6.

In contrast to the breakthrough time dependence, the capacity variation does not follow a clear trend, except for toluene. Benzene remains adsorbed only in low SAR samples, thus suggesting that when the number of acid sites is limited, the most volatile compounds such as benzene are desorbed in favor of more strongly adsorbing species. Ethylbenzene and p-xylene adsorption capacities are comparable, not being affected by the Al content. The latter could be attributed to their hindered diffusion along the cavities. Finally, toluene exhibits the same capacity as observed during TGA experiments for ZSM-5_2 to ZSM-5_5 samples, i.e., toluene capacity increases with Al content. It is clear then that, in terms of capacity values, the optimal system is that with lower SAR.

In order to evaluate the possible regeneration of the adsorbent, needed to employ it in realistic conditions, the total desorption of BTEX was evaluated by means of thermogravimetric measurements. In all cases, the final zeolite mass corresponds to that placed at the beginning of the experiment pointing out a successful regeneration of the adsorbent.

In summary, a proper adsorbent design involves the control of many factors dealing with both zeolite and adsorbate properties. The most important parameters influencing BTEX adsorption over ZSM-5 zeolites have been found to be the number of available acid sites (Al content), the zeolite pore size and the hydrophobicity. An optimum value regarding Al atoms and framework hydrophobic character is desirable to favor any pollutant adsorption. As mentioned in the characterization part, the used fluoride route leads to obtain zeolites with fewer defects compared to the classical alkaline route which, as a consequence, diminishes the adsorbent hydrophilicity. In fact, only 2% extra-framework Al atoms (EFAl species) variation was observed when ZSM-5_2 (SAR 198) and ZSM-5_5 (SAR 79) were analyzed by 27 Al MAS NMR. The latter renders possible the utilization of lower SAR zeolites, which remarkably increment the adsorbate uptake due to the direct interaction adsorbate-BrØnsted sites.

Conclusions

Several ZSM-5 zeolites with 79 < Si / Al < 316 have been successfully synthesized and used in the multicomponent gas-phase adsorption of BTEX. The samples were packed separately in thermal desorption tubes connected to a miniaturized GC prototype, as a representative approach to the pre-concentration unit needed to drop the detection limit according to current legislation. The design of an adsorbent for near real-time BTEX detection involves the control of several factors including both zeolite and adsorbate properties. The most important parameters influencing BTEX adsorption over ZSM-5 zeolites have been demonstrated to be number of available acid sites (Al content), the hydrophobicity and the zeolite pore size. An optimum value regarding Al atoms and framework hydrophobic character is desirable to favor the pollutant adsorption. In this context, the fluoride-mediated synthesis of ZSM-5 zeolite appears as an excellent strategy. Indeed, zeolites with fewer defects and more open structures are obtained, compared to the classical alkaline route. In addition, this effect leads to higher hydrophobic materials even at low SAR, rendering possible the utilization of lower SAR zeolites, which remarkably raised the BTEX uptake due to the direct interaction adsorbate-BrØnsted sites.

These experimental results demonstrate that a compromise between the hydrophobic character and the number of Al atoms should be tailored in order to properly design an adsorbent for near-real time BTEX detection, compromise that can be easily reached when zeolite-mediated fluoride route is used. In addition, such zeolites can be selective to p-Xylene whereas m-, and o-Xylenes are not trapped. This promising result suggests that it will be possible to synthesize in the future, a zeolite specific for benzene, which does not trap the other BTEX, since benzene is the only regulated aromatic compound in indoor air because of its very high toxicity. This could permit to avoid the long separation chromatographic step by injecting directly the desorbed benzene molecule into a detector.

contrasted after NH 3 -

 3 TPD experiments. SAR (Si/Al molar ratio) values ranging from 316 to 79 decrease within the series, hence providing a suitable acid site density variation (mmol(H + )/g zeolite ) range. This approach is very useful to evaluate the Al content (indirectly the acidity) effect on the zeolites adsorptive properties. Likewise for textural properties, similar values regarding both specific surface area and pore volumes are obtained, thus facilitating the sole evaluation of acid site density during the multicomponent vapor-phase adsorption of BTEX.BTEX adsorptionBreakthrough curves represent the evolution of the adsorbate concentration in the gaseous effluent leaving the adsorbent bed as a function of time. In this type of curves, two important features are obtained: breakthrough time (t b ) and saturation time (t s ). The definition of breakthrough time may vary depending on the application, but the typical values are 1% or 5% of the initial concentration. In this work, t b is defined as the time at which 5% of the molecules are leaving the adsorbent bed so that when 5% of the injected concentration (C 0 ) is passing through the adsorbent and, thus, the measured concentration at the outlet (C) is equal to 0.05 C 0 . Up to this point, the adsorption can be considered complete. Once t b is reached (C/C 0 = 0.05), the analyte concentration at the outlet starts to rise until the saturation point. The steepness of the concentration profile provides useful information about the mass transfer from gas phase to the sorption sites inside the sorbent particles. Once saturation time is reached (C/C 0 = 1), all the adsorption sites are occupied. At this point, sorbent is completely saturated and sorption in the bed does not occur. Consequently, the outlet concentration is constant and equal to the initial concentration injected upstream of the adsorbent.

Figure captions Figure 1 .Figure 2 .

 captions12 Figure captions

Figure 3 .

 3 Figure 3. Schematic representation of BTEX adsorption process over ZSM-5 zeolites.

Figure 4 .Figure 5 .Figure 6 .Figure 1 .Figure 2 .Figure 3 .Figure 4 .Figure 6 .

 45612346 Figure 4. Breakthrough time dependence on zeolite Si/Al molar ratio. A) Benzene, B)

Table 1 .

 1 Chemical composition, acid site density and textural properties of as-

	prepared H-ZSM-5 zeolites.
	Sample	SAR (XRF)	Acid site density (mmol(H + )/g zeolite ) a	S BET [m 2 /g]	V pore (cm	3	/g)	Mean pore diameter (nm)
	ZSM-5_1	316	0.044	386	0.19			0.55 ± 0.01
	ZSM-5_2	198	0.132	367	0.17			0.55 ± 0.01
	ZSM-5_3	132	0.133	352	0.18			0.55 ± 0.01
	ZSM-5_4	113	0.172	375	0.18			0.55 ± 0.01
	ZSM-5_5	79	0.249	348	0.18			0.55 ± 0.01

Table 2 .

 2 Toluene adsorption capacities and effective diffusional coefficients obtained

	from TGA experiments.
	Sample	Q (μmol toluene/g zeolite)	Deff (m 2 /s)
	ZSM-5_2	581	1.6E-12
	ZSM-5_3	640	6.0E-13
	ZSM-5_4	667	6.5E-13
	ZSM-5_5	728	1.1E-12

Table 3 .

 3 Obtained adsorption capacities over prepared samples.

			Adsorption capacity (μmol/g)
	Sample
		Benzene	Toluene	Ethylbenzene	P-Xylene	m/o-Xylene
	ZSM-5_1	0	62.3 ± 2.1	232.9 ± 7.7	174.4 ± 5.8	0
	ZSM-5_2	0	53.7 ± 1.8	247 ± 8.2	175.5 ± 5.8	0
	ZSM-5_3 10.9 ± 0.4 76.1 ± 2.5	196 ± 6.5	155.1 ± 5.1	0
	ZSM-5_4	0	75.1 ± 2.5	235.3 ± 7.8	170.4 ± 5.6	0
	ZSM-5_5	20 ± 0.7	119.5 ± 3.9	249.7 ± 8.2	188.4 ± 6.2	0

Acknowledgments: This work has received funding from the Clean Sky 2 Joint Undertaking under the European Union's Horizon 2020 research and innovation program under grant agreement No 687014 (MACAO Project). This work was also funded by the European Commission through the LIFE program (SMART'IN AIR, LIFE17 ENV/FR/000330).