Adsorption and Desorption of a Model Hydrocarbon Mixture Over HY

Article pubs.acs.org/JPCC

Adsorption and Desorption of a Model Hydrocarbon Mixture Over HY Zeolite Under Dry and Wet Conditions B. Azambre,*,† A. Westermann,*,† G. Finqueneisel,† F. Can,‡ and J. D. Comparot‡ †

Université de Lorraine - Laboratoire de Chimie et Physique Approche Multi-échelles des Milieux Complexes (LCPA2MC - EA 4632), Institut Jean-Barriol FR2843 CNRS, rue Victor Demange, 57500 Saint Avold, France ‡ Institut de Chimie des Milieux et Matériaux de Poitiers IC2MP − UMR 7285 CNRS-Université de Poitiers, 4 rue Michel Brunet, 86022 Poitiers, France ABSTRACT: Adsorption behavior of a hydrocarbon mixture (propene, toluene, decane) mimicking Diesel cold-start was investigated under dry and wet conditions for commercial HY zeolites with a Si/Al ratio ranging from 2.5 to 100. Textural and structural characterizations were carried out using N2 adsorption at 77 K and X-ray diffraction. In situ FTIR spectroscopy of adsorbed pyridine was exploited to probe acidic sites. The methodology used in this study consisted of adsorption phase at 35 °C with several kinds of mixtures followed by a Temperature-Programmed Desorption (TPD) at 10 °C/min. At high Si/Al ratio, a competitive thermodynamic adsorption between toluene and decane was demonstrated. To the opposite, propene is substantially not adsorbed whatever the Si/Al ratio of the zeolite. By decreasing the Si/Al ratio, the presence of large amounts of acidic sites enhances adsorption of unsaturated hydrocarbons. Water adsorption was found to be detrimental for HC storage due to hydrophilic nature.

1. INTRODUCTION Zeolites are used in many industrial adsorption and catalytic processes due to the wide availability of ordered microporous structures, their excellent heat resistance and the possibility to tailor the chemical and acid−base properties by ion exchange or tuning the Si/Al ratio. From a general viewpoint, HC adsorption in zeolites has been widely studied1−18 for more than 40 years because it is a common step in many petrochemical, fine organic synthesis and depollution processes. In the automotive depollution field, zeolites are now essential components of on-board Cu- and Fe-based NOx-SCR catalysts and also considered as very promising materials for the development of hydrocarbons (HC) traps in order to meet the more and more stringent regulations.8 Considering the latter application, HC in exhaust gases include branched and linear alkanes,2,3,13,16 alkenes,1,6−8,12,17 and aromatics1,2,4−7,9−12,14,16,18 of various molecular weights, ranging from C1 to C12.3,8 Propene,1,7,8,17 propane,2,16 or ethene6,12 are frequently used as model molecules to represent the lightest HC, whereas toluene1,2,4−7,9−12,14,16,18 is often used as representative of aromatics. Decane or higher n-alkanes have to be added to this mixture in order to mimic realistic Diesel exhausts. For Diesel cars, the HC trap has to assist the Diesel Oxidation Catalyst (DOC) during the cold-start period, that is, when the exhaust temperature is below the light-off temperature of the DOC (typically 200−300 °C). Once this light-off temperature is reached, the trapped HC may eventually desorb and can be effectively oxidized by the DOC so that the trap can be regenerated. © 2014 American Chemical Society

It is very challenging to meet all these requirements simultaneously and the basic processes governing the trapping of HC mixtures in absence/presence of inhibitors in relation with the diffusive and chemical properties of zeolitic materials are far from being fully understood. One common observed case is that the adsorption of aromatics and/or heavy (longchained) hydrocarbons is promoted at the expense of the light hydrocarbons.1,6 In most of literature studies, adsorption of no more than one or two types of HC is often considered due to some limitations induced by experimental setup, namely, those related to HC detection/quantification and the generation of accurate concentrations for the most condensable HC. Nevertheless, Burk et al.1 reported that *BEA (Beta) zeolite is a promising material to trap both propene and toluene, while Elangovan et al.5,14 found SSZ-33 is better than *BEA to trap toluene. On the other hand, Czaplewski et al.2 reported that one-dimensional zeolitic structures, such as mordenite, have some advantages above 3D zeolites for the trapping of HC mixtures owing to a “single-file diffusion mechanism”. In this concept, the bulkiest molecules, which have low diffusion kinetics and are usually more strongly adsorbed than the lighter ones, act as molecular “plugs” and constrain the small molecules within the 1D channels until they are themselves desorbed. Only a few works deal with the adsorption of HC binary mixtures relevant to cold-start applications, such as those Received: September 7, 2014 Revised: December 12, 2014 Published: December 15, 2014 315

DOI: 10.1021/jp509046n J. Phys. Chem. C 2015, 119, 315−331

Article

The Journal of Physical Chemistry C

ambient temperature under N2 and finally evacuated under dynamic vacuum for 6 h. Prior to pyridine adsorption, structural information on hydroxyl groups were obtained by collecting FTIR spectra in the hydroxyl region on outgassed zeolites. Pyridine was then adsorbed (200 Pa at equilibrium) at 25 °C and further desorbed until 450 °C (by steps of 50 °C from 50 to 450 °C). The total amount of Brønsted (BAS) and Lewis acid sites (LAS) were determined from the area of the ν19b band, using their molar coefficient: εPyH+ = 1.8 cm·μmol−1 and εPyL = 1.5 cm·μmol−1, respectively.20 Finally, the spectra obtained on the different HY samples were normalized to a disc of 10 mg·cm−2 in order to obtain quantitative results. 2.3. HC Adsorption and TPD Experiments. The adsorption properties of the different HY zeolites toward a multicomponent HC mixture (propene (1300 ppm, C3), toluene (600 ppm, C7), decane (300 ppm, C10) were studied under dry and wet conditions (H2O, 0 or 1%). When necessary, simple mixtures (mono or binary) were used but similar HC concentration ratios (close to those existing in Diesel exhausts) were maintained in order to ensure direct comparison of experimental data with those obtained with ternary (3 HC) or quaternary (3 HC + H2O) mixtures. The experimental setup can be divided in three parts: a device to generate constant and reproducible (±5%) HC (and water) concentrations, a fixed-bed reactor, and the detection/ quantification system composed by an IR spectrometer and a heating gas cell, all the parts being linked by transfer lines thermostated at 100 °C in order to prevent condensation. Pretreatment, adsorption and TPD experiments were performed in a single run for each sample. For the outgassing step, the temperature program consisted of thermally treating under He (Air Liquide, 99.99%, 60 mL· min−1) of the 200 mg zeolite samples placed into a quartz tube (itself loaded into a tubular Carbolite MTF furnace with Eurotherm controller) from 25 to 200 °C (30 min dwell) with a heating rate of 5 °C·min−1, followed by another heating from 200 to 500 °C (at 10 °C·min−1 for 30 min dwell at 500 °C). Following this in situ pretreatment, the temperature was cooled down under He to 35 °C and HC adsorption was started. The “so-called” breakthrough curve experiments were conducted at 35 °C ± 1 °C using more or less complex HC mixtures, as stated above. All the adsorption experiments were performed with a total flow rate of 150 mL·.min−1, which represents approximately a GHSV of 20000 h−1. Fixed concentrations of permanent gases and vapors were obtained thanks to Brooks 5850S mass-flow controllers and appropriate dilution with He (balance). The propene concentration (1300 ppm) was adjusted from a 5% propene/He bottle, (alphagaz 2 supplied from Air Liquide). Toluene (Carlo Arba-SDS, 99.9%), decane (Aldrich, 99%), and water (wet conditions only) vapors were generated from bubble towers placed into thermostated baths set, respectively, to 2, 15, and 30 °C in order to generate the appropriate partial pressures (600 ppm, 300 ppm, and 1%, respectively) after dilution with a He stream. When one or several components were removed from the mixture, the corresponding flows were replaced by pure He, in order to maintain a constant total flow rate of 150 mL·min−1. After saturation of the zeolite bed by the incoming HC, a degassing step of 1 h was performed under He (60 mL·min−1) at 35 °C to purge the system and eliminate most of the weakly held (physisorbed) species. Then, the TPD experiments were performed by ramping the temperature from 35 to 500 °C at 10 °C·min−1 under He.

of Sakuth et al.11 (toluene and propanol over Y with different Si/Al ratios), Wesson and Snurr16 (toluene and propane over CsMOR), and Kaliaguine and co-workers6,12 (toluene and ethene over ZSM-12 and SAPO). Hence, a more detailed study is needed in order to better understand the structural and chemical parameters affecting the adsorption of a multicomponent HC mixture in dry and wet conditions. In this study, the adsorption of a model ternary mixture of propene (C3), toluene (C7), and decane (C10) in the absence/ presence of water is investigated for a series of HY zeolites with increasing Si/Al ratio. Faujasite-type zeolites are examined here because of their open three-dimensional structures and large pores, which lead to lower mass transfer resistances and higher adsorption capacities compared to most of the other zeolites. First, the structural and chemical properties of the studied zeolites are characterized using XRD and FTIR of adsorbed pyridine. Then, the adsorption/desorption properties of each HC alone or in binary/ternary mixtures are investigated using breakthrough curves and subsequent TPD experiments. Besides, some correlations are attempted between the acid− base and hydrophilic/hydrophobic properties of the different HY on the one hand and the HC adsorption capacities, breakthrough parameters, and TPD profiles on the other hand.

2. EXPERIMENTAL SECTION 2.1. Materials. Commercial HY zeolites with increasing global Si/Al ratio (2.5, 15, 40, and 100) were provided by Degussa (for Si/Al = 100, unit cell formula H2Al1.9Si190.1O384 0 EFAL) and by Zeolyst for the other ratio: CBV 300 (unit cell formula Na17H32Al49Si143O384 13 EFAL), CBV 720 (unit cell formula H10Al10Si182O384 2.1 EFAL), and CBV 780 (unit cell formula H3.29Al3.29Si188.1O384 1.41 EFAL), either in ammonium and protonated forms. Prior to adsorption and characterization studies, they were calcined under air (4 L/h/g) with a heating rate of 5 °C/min from room temperature to 200 °C (plateau of 1 h) and then to 500 °C (plateau of 4 h). 2.2. Characterizations. Porosimetric properties were obtained from N2 adsorption isotherms recorded at −196 °C on an automated Autosorb IQ sorptiometer supplied by Quantachrome. Prior to each adsorption measurement, samples were outgassed in situ in vacuum at 80 °C for 3 h and then at 150 °C for 12 h to remove any adsorbed impurities. Specific surface areas (SBET) were determined using the BET equation between 0.05 < P/P0 < 0.3. The micropore volume and the external surface area Sext were determined using the t-plot method.19 Powder X-ray diffraction (PXRD) measurements were carried out using a Brüker-AXS diffractometer and the Cu Kα radiation (1.5405 Å). Powdered diffraction patterns were recorded between 5 and 50° (2θ) using increments of 0.01° and a counting time of 2 s. Acidic properties were characterized by pyridine (Py) adsorption monitored by infrared spectroscopy. IR spectra were recorded on a Nicolet Nexus spectrometer equipped with a DTGS detector and a KBr beamsplitter using a resolution of 4 cm−1 and 64 scans. Samples were pressed as self-supported wafers and treated directly in the cell. A similar pretreatment procedure than the one applied for HC-adsorption tests (following section 2.3), was performed to characterize the various HY zeolites. Samples were progressively activated under nitrogen flow (30 mL·min−1) up to 200 °C (5 °C·min−1) for 30 min and then the temperature was further ramped to 450 °C (5 °C·min−1, 30 min.). Samples were thereafter cooled down to 316


Article


Table 1. Spectral Procedures Used for the Detection and Quantitation by FTIR of Propene, Toluene, and Decane and Their Reaction Products No.

compound

1

decane

2 3 4 5 6 7

propene toluene ethene CO2 CO H2O

wavenumber (cm−1)

quant. mode

2932 1550−1458 912 728 950 2400−2275 2142−2020 3780−3200

abs. area abs. abs. abs. area area area

subtraction 4 none none none none none

Figure 1. Example of a multicomponent breakthrough curve obtained for a HY-100 zeolite during the adsorption of a ternary (C3 + C7 + C10) HC mixture in absence of water. Qa (dotted area in green) is the adsorbed amount, and Qd (hatched area in red) is the desorbed amount of toluene. Qsat (adsorbed capacity at saturation) corresponds to the difference Qa − Qd.

The breakthrough curve then passes above C0. The corresponding desorbed capacity Qd was therefore subtracted from the adsorbed capacity Qa to obtain Qsat. Figure 1 illustrates the aforementioned notions. Φ represents the ratio Qa/Qd and the parameter t95% to t5% is used in order to better discuss the differences existing in the diffusion behavior of each HC under different conditions. The adsorption selectivity for each hydrocarbon in the mixture was calculated all along the adsorption phase (transient selectivity) as well as at saturation of the zeolitic bed. Thus, net adsorbed amounts of each HC at a given time were first transformed into “C1 equivalents” and each HC selectivity at time t was then calculated as the ratio of the transient adsorbed amount of the hydrocarbon (nads(HC); HC = C3, C7 and C10) by the transient total adsorbed amount (nads(CT); eq 2). The use of C1 equivalents was implemented to better compare the adsorption selectivities of each hydrocarbon.

t

D·(C0 ·tf − ∫ C(t ) ·dt ) 0 m

interferences subtractions 2−4

cases (as shown in Figure 1), a part of the amount adsorbed was desorbed before saturation owing to a displacement of the adsorption equilibrium due to competitive adsorption.

The composition of the reactor outflow was continuously monitored using a heated FTIR gas cell (C2 Cyclone Series − Specac, optical path length = 2 m, V = 0.19 L) coupled to a Varian Excalibur 4100 FTIR spectrometer and a DTGS detector. The temperature of the gas cell was maintained at 120 °C to avoid any condensation of the hydrocarbons during the tests. FTIR spectra, referenced to a He background, were recorded using a 2 cm−1 resolution and coaddition of 50 scans. These parameters allowed to obtain satisfying signal/noise ratio as well as a sufficient time resolution (the sampling rate was 1 spectrum per 20 °C intervals during the TPD) but also to detect the fine gas structure in most cases for identification purposes. The methodology used for the exploitation of time- or temperature-resolved IR spectra allowed to obtained quantitative data (with less than 10% error) for all the HC species, H2O (wet conditions) and the byproducts generated from secondary reactions in the course of the TPD (such as ethene and COx). First, a multipoints calibration curve (R2 > 0.99) was obtained for all individual species, which were quantitated either from absorbance or band area measurements (Table 1). For complex mixtures, the sometimes strong overlapping between absorptions pertaining to different species present simultaneously in the cell precluded their direct quantification. Hence, reprocessing of IR spectra was necessary by subtracting step by step the nondesired species and avoiding any spectral interference. Table 1 summarizes the bands chosen for the quantification and the interferences existing between the different hydrocarbons. 2.4. Description and Quantitative Exploitation of Breakthrough Curves. The adsorption capacity at equilibrium (Qsat) for each HC was calculated as the difference between the area under the breakthrough curve and the area under the curve in a blank experiment (eq 1). Q sat =

vibration mode νasymC−H (CH2) δC−H (CH2) δC−H δC−H δC−H νasymC−O νasymC−O νO−H

(1)

where D is the total flow rate (mL·min−1), m is the weight of material introduced into the reactor (g), C0 is the gas inlet concentration (ppmv), C(t) is the gas outlet concentration (ppmv), and tf is the time corresponding to bed saturation (min), that is, when C/C0 = 1. Because of the different diffusion behaviors of each HC through the experimental setup, blank experiments were performed using similar conditions but without catalyst. Then, the “blank breakthrough curves” corresponding to each HC were subtracted from those obtained with zeolites in order to have more accurate adsorption capacities. Equation 1 is only valid when no desorption was observed in the course of the experiments performed at 35 °C. In some

selectivity(t ) = [ndas(HC)/nads(C T)]× 100

(2)

3. RESULTS AND DISCUSSION 3.1. Properties of the Adsorbent Materials. 3.1.1. Structural and Textural Properties. First, the phase purity of the different HY zeolites was examined by X-ray powder diffraction (XRD). The XRD patterns of the different samples (not shown 317


Article


different lots. The HY-100 material coming from a different supplier (Degussa), this has more to do with being related to the synthesis conditions employed (not known). Regarding the external surface (Sext), an increase can be noticed, passing from 9 m2·g−1 for the HY-2.5 sample to a maximum of 102 m2·g−1 for the HY-15 one. This increase can be mainly explained, as mentioned before, by the fact that postsynthesis treatments applied to dealuminate zeolites create some mesoporosity in the sample. 3.1.2. Characterizations of Acid Sites. Surface properties of zeolites are a key parameter involved in hydrocarbon adsorption and processes. In this way, acidic properties, as well as residual OH groups of various samples, were investigated using pyridine adsorption monitored by IR spectroscopy. In Figure 3, IR spectra in the OH stretching region of HY zeolites from various Si/Al ratios are compared after activation

here) are consistent with those reported in the literature for the Faujasite structure (ICDD #01−074−1192).21 N2 adsorption isotherms recorded at −196 °C for the several samples are shown in Figure 2, whereas the specific surface areas (SBET), external surfaces (Sext), and micropore volumes determined from adsorption/desorption isotherms are gathered in Table 2.

Figure 2. N2 adsorption isotherms for the several HY zeolites.

Basically, two kinds of FAU zeolites can be distinguished: one zeolite corresponding to a sample essentially microporous (isotherm of type I with a horizontal plateau at high relative P/ P0), with no or very weak mesopores contribution, and another group corresponding to microporous materials with a more important contribution of mesopores (isotherms I with a hysteresis loop between 0.5 ≤ P/P0 ≤ 1). Logically, the first group, consisting of zeolite HY-2.5, presents a low external surface (Sext = 9 m2·g−1, Table 2). On the other hand, the second group containing the remaining samples (with HY-15, HY-40 and HY-100) shows a higher external surface (>43 m2· g−1). The presence of substantial mesoporosity in the later samples could be attributed to the fact that those materials had all been subjected to a dealumination process (in order to increase the framework Si/AlIV ratio), as they present a rather high value (>5) when comparing with the less dealuminated samples (Si/ AlIV < 5). Surprisingly, the HY-100 sample, which is a highly dealuminated sample, does not present any significant increase of the mesoporosity (Sext). Hence, the micropore volumes of HY zeolites spread between 0.299 and 0.324 cm3·g−1. The specific surface area and micropore volumes do not follow a specific order in function of the Si/Al ratio, probably because they come from

Figure 3. FTIR spectra in the ν(OH) region obtained for the different HY zeolites activated under a N2 flow up to 450 °C and evacuated under dynamic vacuum at room temperature.

at 450 °C. IR spectra of catalysts are consistent with that reported in the literature, displaying the characteristic OH groups of HY zeolites at 3629 and 3566 cm−1. In addition, the IR spectrum of HY-2.5 zeolite displays a significantly different IR spectrum with (i) a single broad band at 3600 cm−1 instead of the well-resolved HF and LF bands and (ii) the presence of two new bands at 3693 and 3671 cm−1. The latter species are assigned to AlOH groups from aluminic extraframework (EFAL) debris. In fact, it is well-known that Brønsted acid sites carried by the framework (bridged OH groups) are present in high amounts at the lower Si/Al atomic ratio (i.e HY-2.5 for instance). Due to their high aluminum content, these zeolites are not very stable, and thermal treatment in the presence of steam of desorbing water can partially remove aluminum from the crystal framework. It is

Table 2. Textural, Structural, and Chemical Characteristics of the Used HY Zeolites aciditya (μmol·g−1)

a

−1

sample

Si/Al ratio

SBET (m ·g )

HY-2.5 HY-15 HY-40 HY-100

2.5 15 40 100

783 843 857 838

2

S

ext

−1

(m ·g ) 2

9 102 93 43

−1

micropore volume (cm ·g )

Lewis

Brönsted

total

0.299 0.300 0.324 0.302

137 43 7 8

172 71 49 16

309 114 56 24

3

Determined by pyridine adsorption, followed by IR spectroscopy (evacuation at 150 °C). See text for more details. 318


Article


Figure 4. FTIR spectra of adsorbed pyridine. (A) Pyridine evacuated on HY-2.5 at 150 (a), 200 (b), 250 (c), 300 (d), 350 (e), 400 (f), and 450 °C (g); (B) Pyridine evacuated at 150 °C after adsorption onto the different HY zeolites.

Figure 5. Distribution of acid sites determined by FTIR of adsorbed pyridine. (A) Strong acidity (pyridine evacuated at 450 °C) evolution vs Si/Al ratio; (B) Total acidic sites evolution vs pyridine temperature evacuation.

used in this study. Hence, it is more appropriate to investigate the acidic properties of our HY zeolites. Due to the nitrogen electron lone pair, pyridine should interact with acidic centers in a specific way to form (i) coordinated species on Lewis acid sites (PyL) and (ii) the pyridinium ion on protonic sites (PyH+), both giving rise to ν8a, ν8b, ν19a, and ν19b ring (CCN) vibration modes.27 In zeolites, Brønsted acid sites correspond to the bridged Si−OH−Al groups, and their amount and strength depend on the Si/Al atomic ratio. Pyridine was adsorbed (200 Pa at equilibrium) at room temperature and further desorbed up to 450 °C. An example of IR spectra of evacuated pyridine for HY-2.5 sample is shown in Figure 4A. Adsorption of pyridine gives rise to coordinated PyL species, characterized by bands at 1454, 1620, and 1632 cm−1. The appearance of the 1543 cm−1 band indicates the presence of Brønsted acid sites. In addition, a weak band is observed at 1443 cm−1, which disappears at 200−250 °C (Figure 4A). This band has been assigned by Khabtou et al.20 to pyridine Hbonded to LF hydroxyl groups. This feature is common to all the samples studied (Figure 4B). With the increase of the temperature, the balance between the intensities of the two bands at 1545 cm−1 (Brønsted) and 1455 cm−1 (Lewis) changes in favor of the latter. It is to note that these bands are still present at 450 °C (spectrum g, Figure 4A), which indicates the presence of residual strong Brønsted and Lewis acid sites. To compare the acidity of various zeolites, IR spectra of pyridine evacuated at 150 °C are presented in Figure 4B. At this

assumed that the aluminum removed from the framework remains in the cavities (EFAL) as small clusters whose surface induces high concentration of Lewis acid sites. In the present study, this assumption can be easily verified using the chemical formula computed for the different zeolites (see section 2.1). Indeed, it is found that the HY-2.5 zeolite possesses a much higher amount of EFAL (13 per unit cell) than the other investigated samples. On the other hand, the broadening and blue-shift of the HF and LF bands, resulting in the appearance of a single band at about 3600 cm−1, is explained by the development of H-bonding interactions between Si(OH)Al and AlOH species in close vicinity. Another possibility that could not be easily differentiated, lays in the existence of H-bonds between acidic groups and residual water molecules that were not evacuated at 450 °C. This point will be discussed with more details below, in a forthcoming paragraph dedicated to the characterization of the acidic surface properties by pyridine adsorption. Finally, all spectra show a sharp peak near 3745−3737 cm−1 characteristic of isolated noninteracting surface silanols.22−25 The acid sites of HY zeolites were thereafter examined by IR spectroscopy of pyridine adsorption, which is, with ammonia, one of the most widely used basic probe molecules for surface acidity characterization.22,23,26 In this study, pyridine was preferred to ammonia because the latter can access the small sodalite cages, while pyridine can only diffuses through the larger windows delimiting the supercages, similarly to the HC 319


Article

The Journal of Physical Chemistry C temperature, physisorbed and H-bonded pyridine are removed from the surface. Results are reported in Table 2 for pyridine evacuated at 150 °C. It clearly appears that, classically, the lower the Si/Al ratio, the higher the total amount of acid sites (both Brønsted and Lewis sites; Figure 5B). In addition, some Lewis acidity was observed for all samples, which indicates the presence of extra-framework aluminum species whatever the catalyst. The LAS concentration is much higher for the HY-2.5 zeolite, as expected from the presence of AlOH species described previously (Figure 3). In fact, this result confirms that HY zeolite with the lower Si/Al ratio has an important amount of aluminic extraframework debris. The strength of acid sites is discussed from Figure 5A, which displays the distribution of both strong Brønsted and Lewis versus the Si/Al ratio. Strong acid sites were determined from IR spectra of pyridine evacuated at 450 °C. It appears that the concentration of strong Brønsted acid sites is relatively constant in respect with the Si/Al atomic ratio. This indicates that HY zeolites with high Si/Al ratio display the lower amount of acidic OH group (Table 2), but the distribution of Brønsted acid sites seems to be more homogeneous than over HY-2.5 sample for instance. To the opposite, the concentration of strong LAS decreases with the Si/Al ratio. The evolution of the total amount of acidic sites in function of the temperature of pyridine evacuation is reported in Figure 5B. A drop in the amount of acid sites can be noticed for the lower Si/Al ratio. Zeolites with higher Si/Al ratio (Si/Al ≥ 15) present a more homogeneous distribution of acid sites. 3.2. Impact of the Presence of Single or Multiple Adsorbates on the Adsorption/Desorption Behavior; Case of HY-100. 3.2.1. Interpretation of Multicomponent Breakthrough Curves. In this section, we first provide insights onto the competitive nature of adsorption between the different HC (propene, toluene and decane) and their diffusion by comparing the shapes of single and multicomponents breakthrough curves. Hence, breakthrough curves obtained for a selected zeolite, HY-100, are displayed in Figure 6 for selected binary (C3 + C7 (A), C3 + C10 (B)) and ternary (C3 + C7 + C10 (C)) mixtures. Similar concentrations and total flow rates were used in each case in order to ensure a direct comparison between the different sets of experimental data. For the sake of brevity, single-component breakthrough curves (for a specific HC) are not displayed here because they were found to be identical to those of the corresponding HC in the binary mixture (for toluene and decane). With the same purpose, the adsorption behavior of HY in the presence of the C7 + C10 mixture was found to be virtually identical to the one obtained with the ternary mixture (Figure 6C) and was also omitted here. The reproducibility of experimental data was checked by repeating three times some experiments and the relative discrepancies were always found to be below 10%. Quantitative data were calculated from breakthrough curves and are compared for the different adsorption conditions in Table 3. Whatever the mixture composition, propene presents an extremely abrupt slope with a saturation time tf close to zero, which means that this hydrocarbon easily penetrates and diffuses within the zeolite pore system without being adsorbed. As previously stated, the breakthrough curves for the C7 (toluene) or C10 (decane) components are the same with or without C3 (propene, Figure 6A,B). Thus, this means that the

Figure 6. Effect of the gas mixture composition on breakthrough curves for HY zeolite with Si/Al ratio = 100: (A) binary mixture C3 + C7; (B) binary mixture C3 + C10; (C) ternary mixture C3 + C7 + C10. Conditions: T = 35 °C, total flow rate = 150 mL/min, and sample mass = 200 mg.

adsorptions of decane and toluene globally do not interfere with the one of propene for the HY-100 zeolite. Under our conditions, toluene begins to be detected in the gas phase at t = 65 min in the presence (or absence) of propene and in absence of decane (Figure 6A). Then, the toluene concentration gradually increases to reach a plateau, indicating the adsorbent saturation, at tf = 160 min. On the other hand, the adsorption of decane in absence of toluene follows a rather similar behavior. Decane breaks through later (at t = 120 min) than toluene (Figure 6B) but saturates the zeolite at almost the same time at tf = 170 min. Concerning the ternary C3 + C7 + C10 mixture (Figure 6C), it can be observed that (i) toluene (t0% = 40 min) and decane (t0% = 95 min) break through earlier than isolated or in binary mixtures with propene (Figure 6A,B); (ii) toluene adsorption is 320


Article


found to prevent diffusion into the micropores.17 In the present study, the quantitative data obtained from breakthrough curves indicate that the amount of adsorbed propene on HY-100 at equilibrium in binary (C3 + C7, C3 + C10) or ternary (C3 + C7 + C10) mixtures is very small (19, 38, and 12 μmol/g, respectively, as calculated from the data shown in Table 3). These values are rather consistent with the small amount of strong Brönsted acid sites (about 15 μmol/g) that was measured from FTIR of pyridine adsorption. This can be explained by the fact that for HY zeolite, propene diffuses very easily through the large supercage windows and does not undergo steric hindrances that prevent him from being adsorbed on BAS sites. Though, it can be noticed that the amount of adsorbed propene is slightly decreased when toluene is present in the feed, this being possibly due to competition effects (Table 3). By taking a density of 1.272 g/cm3 and a unit cell volume of 15100 Å3 for the dehydrated HY zeolite,28 the calculated amount of supercages (SC) is 4.168.1020 SC/g (since there are 8 SC per unit cell). Assuming that all the acid sites are located in the internal porosity induces that there is only 1 strong BAS site for 46 supercages in HY-100. Hence, this explains why propene does not really adsorb in HY zeolites with a high Si/Al ratio. 3.2.1.2. Toluene Adsorption. Coming now to toluene adsorption, it has been reported that for the Y-zeolite, the diffusivities were found to have almost the same value in both the adsorption and the desorption processes. It was concluded that the pore diameter of the Y-type zeolite is sufficiently large, compared with the molecular size, to result in a negligibly small resistance to mass transfer at the pore mouth.29 The preferred adsorption mode consists in a π interaction between the aromatic ring and a Lewis acid site on the one hand and the stabilization of the methyl group with negatively charged framework oxygens on the other hand.18 For our HY samples, FTIR of pyridine adsorption has revealed the formation of LAS sites due to the dealumination procedures used for their syntheses. Interactions with Al3+ sites of HY are probably stronger than other kinds of adsorption modes (with BAS sites or through dispersive interactions with pore walls), but the amounts of these sites are by several orders of magnitude smaller than the amount of adsorbed toluene. Hence, weak interactions with the framework predominate for HY-100, which has a homogeneous internal surface. In the binary C3 + C7 mixture, the adsorbed quantity of 0.290 g/g at equilibrium (Table 3) corresponds to 4.5 toluene molecules per SC. This value agrees remarkably well with adsorption data obtained in the study of Daems et al.30 from toluene adsorption in the liquid phase on NaY at room temperature (4.2 toluene/SC). From their study, they deduced that toluene has an adsorption behavior rather similar to benzene and p-xylene and almost completely fills the micropores space. In HY zeolite, the accessible framework pore volume that can be calculated theoretically (with a void fraction of 0.522) is, respectively, 0.410 and 0.348 mL/g with and without the sodalite cages. Taking the toluene density in liquid phase of 0.867 g/mL at 20 °C, the maximum toluene capacity that could be inferred is 0.301 g/g. By neglecting the adsorption at the external surface, a “packing ratio” of about 96% can be calculated for toluene in the binary mixture with propene (and also in single adsorption conditions). This means that toluene molecules efficiently occupy the confined spaces in the supercages when adsorbed alone or in mixture with propene.

Table 3. Adsorption Data Obtained for the HY-100 Zeolite in the Presence of Binary (C3 + C7, C3 + C10) or Ternary (C3 + C7 + C10) HC Mixtures in the Absence of Watera mixture

HC

Qsat (mg/gads)

Φ = Qd/Qa (%)

t5% (min)

tf (min)

tf − t5% (min)

binary

C3 C7 C3 C10 C3 C7 C10

0.8 290 1.6 171 0.5 26 172

0 0 0 0 0 75 0

0 60 0 120 0 40 95

2 160 2 170 2 70 200

2 100 2 50 2 30 105

binary ternary

Qsat represents the adsorption capacities at saturation, Φ is the percentage of HC desorbed during the adsorption phase (by competitive adsorption), tf − t5% is a parameter representative of diffusional constraints. a

displaced by the incoming decane molecules, as revealed both from the excess toluene observed in the gas phase between t = 70 min and t = 200 min and the unusual shape of the decane profile observed during the breakthrough (Figure 6C). For a detailed interpretation of these competitive adsorption effects, it is necessary to take into account both the possible sittings of each HC in the HY zeolite and the aspects related to their diffusion within the zeolite framework. The Faujasite Y framework is composed by sodalite (β) cages of diameter 66 Å, which are connected by hexagonal prisms through windows of 2.3 Å. Similar windows link the sodalite cages to the bigger α cages, which are also called supercages due to their size, approximately 12.4 Å in diameter. Supercages are linked together through windows of 7.4 Å in diameter, and this forms the accessible pore network. Molecules larger than water or ammonia can access only the supercages and cannot pass into the empty space inside sodalite cages. Thus, most aspects related to the HC adsorption in this study are confined to the three-dimensional internal network constituted by the interconnected supercages. Nevertheless, a small proportion of molecules should also be adsorbed at the external surface of the crystallites or in the empty spaces existing between them, as deduced also from the porous characteristics presented in Table 2 and the hysteresis in N2 adsorption isotherms (not shown here), which have revealed the existence of a small amount of mesopores and significant external surface areas for some samples. 3.2.1.1. Propene Adsorption. Considering the siting of each HC first, the most favorable configurations for the adsorption of alkanes, alkenes, and aromatics have already been described in the literature.17 Small alkene molecules (such as propene) entering the micropores of zeolites are reported to adsorb onto the OH groups (SiAlOH) of Brönsted acid sites by forming a hydrogen bond via the π-electron of the CC bond (πadsorption) as the most stable adsorption structure.17 However, the rate-determining step for the formation of the most stable adsorption structure was found to differ depending on the type of zeolite topology and molecular size, and the elementary adsorption process could be interpreted on this basis. While only π-adsorbed complexes were observed for 1-butene adsorption on mordenite, which has large micropores, alkyladsorbed complexes were observed for 1-butene adsorption on ZSM-5, which has slightly smaller pores.17 Species adsorbed on silanol groups at the external surface were present on ferrierite, with the smallest pores, and significant energy barriers were 321


Article

The Journal of Physical Chemistry C 3.2.1.3. Decane Adsorption. By comparison with toluene, the intracrystalline diffusion of C6−C8 n-alkanes in Y-type zeolite has been reported to be more difficult. When entering the pre-mouth, paraffins adopt a chain-like configuration by contrast with a circle-like one in the gas phase, which accounts for an increased resistance to mass transfer.13 N-Alkanes interact with the zeolite framework through nonspecific van der Waals (dispersive) interactions and the incremental heat of adsorption per alkane carbon atom has been reported to vary between 8 and 13 kJ/mol, depending on the pore size.13 In the binary C3 + C10 mixture, the adsorbed decane quantity of 0.171 g/g at equilibrium (Table 3) corresponds to 1.74 decane molecules per SC, provided that all the decane is adsorbed in the internal surface, which is probably not true. Nevertheless, this value represents a packing ratio of only 67%, when put in relation with the liquid density of decane at 20 °C (0.73 g/mL) and the theoretical accessible pore volume. Such discrepancies between the packing ratio of toluene and decane could be explained by the specificity of alkane adsorption in zeolites. Besides the expected preferential adsorption of longer alkanes over shorter ones, a periodic rise and fall of the diffusion coefficients with alkane chain length in zeolites with cages connected via narrow windows.31 The diffusion coefficient could increase by orders of magnitude when the effective length of the alkane chain approaches the cage size due to a “commensurate freezing” effect. To fit within the cage, longchain alkanes tend to adopt a coiled configuration, which implies large entropic effects and a less optimal energetic interaction. Because of the uncomfortable adsorption of such molecules, a maximum in diffusion coefficient is obtained for the largest alkane chain still fitting inside a cage. Once the effective chain length exceeds the cage dimensions, the molecules no longer adsorb in a curled conformation inside a single cage but stretch through the window across two cages. From molecular dynamics simulations through a siliceous faujasite zeolite,32 it was shown that the average decane chain length is close to the 12 Å supercage diameter. Consequently, the end groups of an average length decane molecule can interact with opposite sides of a supercage. The result is stabilization of the configuration and an increase in the center of mass probability density at the supercage center. Moreover, probability plots have shown that the most likely supercage occupancy is 1 or 2 decane molecules, which is in good agreement with the present result. 3.2.1.4. Ternary Mixture. The interpretation of the breakthrough curves obtained for the ternary C3 + C7 + C10 mixture, as shown in Figure 6C, is discussed now. Due to the low amounts adsorbed, propene will not be considered in order to make the discussion easier, so that the ternary mixture could be simplified to a binary C7 + C10 one. For a low loading of adsorbates in HY-100 (corresponding to the first 30−40 min in Figure 6C), the toluene and decane molecules globally do not “feel” each other and almost adsorb independently. Due to the rather homogeneous character of HY-100, toluene interacts only weakly with the pore walls and diffuses rather rapidly through the windows connecting the supercages, reaching most of the adsorption sites first. By contrast, decane has a lower packing ratio in the faujasite structure and develop stronger interactions with the hydrophobic almost all-silica surface. Hence, its diffusion throughout the 3-D structure proceeds slowly compared to toluene. Diffusion being a sequence of activated jumps from one site to another, a jump can only be successful if the site to which the molecule jumps is empty.

When the amount of slowly moving molecules (decane) increases, they will essentially block the supercages windows, and the number of successful jumps of the fast component (toluene) will be determined by the rate at which an empty site is created by a jump of the slow component. As the loading of HY-100 with decane increases, toluene molecules undergo more difficulties in finding empty adsorption sites , that is, doing successful jumps, and toluene becomes detected in the gas phase (t = 40 min, Figure 6C). Although residual empty sites for toluene are still available, adsorbed toluene molecules begin to be squeezed out by decane due to a competition onto the same adsorption sites. The point when toluene concentrations above C0 are detected (t = 70 min) represents the maximal transient toluene adsorption capacity in the ternary mixture. As the decane loading further increases, both components in the mixture are more and more hindered by the presence of the other, which further limits their diffusion. This is clearly visible in Figure 6C, by looking at the symmetric profiles observed above t = 95 min until the end. Due to these competition effects, the adsorption equilibrium for decane is reached much more slowly in the ternary mixture (Figure 6C) than in the binary one (Figure 6B), though the adsorption capacities are similar (1.74 decane/SC). Interestingly, about 9% of the toluene molecules that were adsorbed in the binary C3 + C7 mixture remained adsorbed in the presence of decane (Table 3). More details on the adsorption modes of each HC will be discussed in the next section. Considering the latter aspects, it has to be recalled that diffusion in the micropores of a zeolite is usually described as “configurational” and has been assimilated as surface diffusion because of the small distance between the molecules and the pore wall. The diffusivity in this regime has been reported to be much more temperature-dependent than gas-phase diffusion. It depends strongly on the adsorbate concentration, the pore diameter, the structure of the pore wall, the interactions between the surface atoms and the diffusing molecules, the shape of the diffusing molecules, and the way the channels are connected. 3.2.2. Interpretation of TPD Profiles. When equilibrium was reached in the breakthrough experiments performed at 35 °C, pure He was substituted to each HC mixture in order to purge the system prior to temperature-programmed desorption (TPD). During this period, some weakly adsorbed (physisorbed) HC species were removed due to a shift in the adsorption equilibrium. In the course of the TPD, a part of the adsorbed toluene and decane molecules underwent some acidcatalyzed reactions, as deduced from instance from the darkening of the samples that denotes coke formation. Although rather limited for HY zeolites, coking or transformation to primary or secondary products precludes an accurate quantification of the adsorbed amounts for each HC from desorption profiles. For instance, the accurate quantification of the desorbing decane was not possible due to the superimposition of its characteristic absorptions to those of the other n-alkanes produced by acid-catalyzed processes. Hence, desorption profiles for decane should be better interpreted as representative of the overall evolution of alkanes. TPD results linked with the breakthrough curves presented in the previous section are displayed in Figure 7. Whatever the type of binary or ternary HC mixtures, broad signals corresponding to propene desorption were observed. For HY-100, the amounts of desorbed propene are far behind those of the other HC, as expected. However, the extended 322


Article


instance, if a large proportion of windows that link the supercages are blocked by the other adsorbed HC, propene can hardly escape even when desorbed. Switching now to toluene desorption, an intense peak of toluene is detected at 155 °C for the toluene/propene mixture (Figure 7A) and a much smaller one at 115 °C for the ternary mixture (Figure 7C). Related desorbed quantities are overall consistent with the fact that only 9% of the toluene adsorption capacity was only preserved in the ternary mixture. As previously mentioned, this occurs as a consequence of the adsorption competition, which strongly promotes decane adsorption at the expense of toluene. Desorption temperatures in the 115−155 °C range are in agreement with that was previously observed for other H-forms zeolites.13 By comparing the Na- and H-forms, Yoshimoto et al. concluded that peaks in the 100−150 °C range corresponded to toluene adsorption on H+ sites, whereas more firmly anchored toluene on Na+ sites was characterized by peaks in the 150−300 °C range. However, adsorption on H+ sites as the most likely siting does not seem to be valid for HY-100, since it was stated in the previous section that the quantity of adsorbed toluene is about 100 times higher than the total amount of acid sites. Hence, most of the desorbed toluene arises from molecules interacting through dispersive interactions with the internal or external surface of the zeolite host. Interestingly, it can be noted that ethene (detected by FTIR at 950 cm−1; C2 in Figure 7A,C) was only produced when toluene was present in the HC mixture. The temperature domain between 200 and 400 °C is consistent with the occurrence of cracking, dismutation or transalkylation phenomena, which have shown to involve OH groups with Brönsted acid character.34,35 Light alkenes may be produced via a transalkylation (by involving two toluene molecules) or via a monomolecular cracking mechanism.35 If Brönsted acid sites are available, the alkenes produced may further react in oligomerization reactions that may ultimately lead to coke formation. By contrast with toluene, the desorption profile of decane in the binary mixture with propene displayed at least two broad and unresolved peaks about 155 and 275 °C (Figure 7B). The first peak is ascribed to decane physisorbed at the external surface of the intercrystallites or in the mesoporous voids between them or even at the pore mouth. The second peak should arise from decane adsorbed in supercages whereas contributions above 300 °C may better be ascribed to oligomers and other alkanes produced from acid-catalyzed reactions. As expected, following the analysis breakthrough curves, the TPD profiles in the ternary mixture exhibit a different and complex behavior compared to the binary ones. The main difference is related to the shift toward higher temperatures of the TPD peaks corresponding to decane and the other alkanes, the gas evolution being still not ended at 500 °C. It is difficult to give an accurate view of the chemical processes involved in this phenomenon. However, it seems clear that the diffusion of the desorbing decane is severely hampered by the presence of propene, toluene and/or their acid-catalyzed byproducts. As a result, the constrained decane diffusion during the TPD likely increases the probability for coking and oligomerization reactions to take place. To sum up on this part, it can be outlined that our experimental approach combining breakthrough curves and TPD with IR detection is very useful for the understanding of the adsorption behavior of complex HC mixtures. This

Figure 7. HC emission profiles obtained for the HY-100 zeolite in the course of a TPD under inert atmosphere under adsorption conditions corresponding to the experiments presented in Figure 6. (A) Binary mixture C3 + C7. (B) Binary mixture C3 + C10. (C) Ternary mixture C3 + C7 + C10. Conditions: total flow rate = 60 mL/min, sample mass = 200 mg, heating rate = 10 °C/min.

temperature range (70−300 °C or even above) in which propene is produced is related to several issues: (i) the desorption of molecular propene chemisorbed on Brönsted acid sites having increasing strength; (ii) its formation as byproduct from the cracking or oligomerization reactions underwent by the other HC;33 (iii) the limitations induced by the presence of the other trapped HC in its diffusion from the internal surface toward the exterior of the crystallites. In Figure 7, some propene peaks could be observed at the same temperatures than the other bulkier HC, which confirms this assumption. By doing a parallel with the behavior of HC in 1-D zeolitic structures such as mordenite, it can be deduced that single-file diffusion mechanisms also probably prevail for the 3-D faujasite pore system at high adsorbates loading. For 323


Article


Figure 8. Breakthrough curves corresponding to the adsorption of the HC mixture (C3 + C7 + C10) for selected HY zeolites with different Si/Al ratios in the absence (dry atmosphere) or in the presence of water (wet atmosphere): (A) Si/Al = 2.5, dry; (A′) Si/Al = 2.5, wet; (B) Si/Al = 15, dry; (B′) Si/Al = 15, wet; (C) Si/Al = 100, dry; (C′) Si/Al = 100, wet. Conditions: T = 35 °C, total flow rate = 150 mL/min, sample mass = 200 mg.

Table 4. Quantitative data corresponding to the adsorption of a ternary (C3+C7+C10) HC mixture for the HY-2.5, HY-15 and HY-100 zeolites in absence (dry) or presence (wet) of water dry atmosphere

wet atmosphere

Si/Al

HC

Qsat

Φ = Qd/Qa

tf − t5%

mol./SC

Qsat

Φ = Qd/Qa

tf − t5%

2.5

C3 C7 C10 H2O C3 C7 C10 H2O C3 C7 C10 H2O

9.5 67 72

0 2 0

28 21 83

0.33 1.05 0.73

0.7 43 106

0 49 0

7 12 40

0.2 0.68 1.07

0.5 26 172

0 75 0

2 36 97

0.2 0.41 1.75

2.2 5.4 11 94 1.2 29 103 8.5 0 25 136 0

11 72 17 0 5 52 0 72 0 66 0 0

12 16 22 33 2 51 116 2 2 53 117 2

15

100

3.3. Effect of the Si/Al Ratio and the Presence of Water on HC Adsorption. The influence of the Si/Al ratio on

methodology is now applied to other HY zeolites in order to assess the effect of the Si/Al ratio. 324


Article


same in absence/presence of water at the high Si/Al ratio. It can be explained both by the very low acidity and by the hydrophobic nature of these zeolites. (ii) For zeolites with a medium (15) or low (2.5) Si/Al ratio, a competitive adsorption can be noticed between HC and water with, however, opposite results. At intermediate Si/Al ratio, the competition is at the advantage of HC (Figure 8B′). The displacement of a part of the adsorbed water, simultaneously to the filling of the porosity by HC indicates that the competitive adsorption is globally in favor of decane and in a lesser extent to toluene and propene. Nevertheless, the presence of water molecules in the porous network also impact the diffusion of HC, as deduced from the changes observed in the slopes of the breakthrough curves under dry and wet conditions (Figure 8B and B′). Water diffuses faster than decane and toluene due to its smaller size, but the diffusion of “heavy” HC (C7 and C10) is slowed down by the presence of this fourth compound in the gas mixture (Figure 8B′). For the lowest Si/Al ratios, the competitive adsorption is in this case in favor of water. Consequently, displacements of adsorbed toluene and decane are observed during the filling of pores by water. It can be explained both by the hydrophilic/ polar behavior of this type of surface and by the presence of a high concentration of acid sites, which is known to promote water adsorption. In order to provide a better comparison between “dry” and “wet” experiments, Figure 9A represents the evolution of the adsorbed quantity at saturation according to the Si/Al ratio

the adsorption of the C3 + C7 + C10 ternary mixture has been studied on the HY zeolite under dry and wet atmospheres. For sake of brevity, only the results for Si/Al = 2.5, 15, and 100 will be discussed in detail. 3.3.1. Dry Conditions. The breakthrough curves obtained for the various Si/Al ratios (Figure 8A−C) display the same overall profiles: propene breaks through first, then toluene (which presents a desorption peak) and finally decane. Whatever the Si/Al ratio, it can be seen that propene starts to desorb at 0 min (Figure 8). Breakthrough curves for Si/Al ratio = 15 and 100 are very similar, with tf between 5 and 8 min (Table 4). Contrary to the other samples, an inflection in the slope is observed at t = 5 min for the HY-2.5 one. It is interesting to notice that, in absence of other hydrocarbons, the propene breakthrough curve does not present this inflection.9 The adsorbed quantities at saturation increase from 0.5 mg/g for the HY-100 zeolite to 0.7 mg/g for the HY-15 one and 9.5 mg/g for the most acidic HY-2.5 zeolite (Table 4). The adsorbed quantity of 227 μmol/g of propene on the HY-2.5 zeolite corresponds globally to the amount of Brönsted acid sites (172 μmol/g), even if an adsorption onto the Lewis sites can be also considered. Finally, the propene adsorption capacity of a ternary mixture follow the trend expected by the amount of acid sites (cf. 3.1.2.): HY-2.5 ≫ HY-15 ≈ HY-100 (Table 4). On the other hand, it was shown earlier by FTIR that HY-2.5 zeolite displays a high amount of extra-framework Al species. The presence of these EFAL species is expected to increase the toluene adsorption strength via the interactions between the aromatic ring and the LAS. The breakthrough curves in Figure 8, left, are in good agreement with this assumption, because it can be clearly seen that the adsorption competition between toluene and decane only occurs to a minor extent for the HY2.5 sample, whereas it is important at higher Si/Al ratio. Hence, the ratio of the desorbed quantity over the adsorbed one (Φ in %) falls down from 75% for the HY-100 zeolite to 49% for the HY-15 and finally to 2% for the Si/Al ratio = 2.5. This ratio, Φ, has also to be linked with the adsorbed amounts of toluene at saturation, which gradually rises from 26 mg/g (HY-100) to 67 mg/g (HY-2.5). In that respect, Daems et al.31 have also observed a reversal in the selectivity of benzene compared with n-octane as a function of the Si/Al ratio, while the diffusion of the aromatic stayed similar. Logically, the adsorbed amounts at saturation (Qsat) for decane decrease with the Si/Al ratio (Table 4), the highest capacity being obtained for the HY-100 zeolite (Qsat = 172 mg/ g) and the lowest one for HY-2.5 (Qsat = 72 m/g). The toluene diffusion being easier than decane, it will be adsorbed first (especially on acid sites). The interactions between the acid sites and toluene being energetically more favorable, these acid sites will not be available for the adsorption of alkane. A complementary explanation can be given by the hydrophilic/ hydrophobic behavior of zeolites, which changes in function of the Si/Al ratio. At high Si/Al ratios, the zeolite surface is globally hydrophobic, which promotes the adsorption of compounds such as decane. 3.3.2. Wet Conditions. The same experiences were carried out in the presence of 1% H2O in the feed (quaternary mixture) in order to study the impact of this potential inhibitor on the adsorption behavior (Figure 8A′, B′, and C′). It can be summarized as follows: (i) There is almost no competitive adsorption effect between HC and water for the Si/Al ratio above 15. In other words, the HC adsorption behavior is the

Figure 9. (A) Adsorption capacities at saturation for HY zeolites for C3, C7, and C10 in function of the Si/Al ratio. Dry atmosphere: solid line; Wet atmosphere: dashed line. (B) HC adsorption selectivities at saturation (in C1 equiv) of the adsorbent bed in function of the total acidities of the different zeolites under a dry atmosphere. 325


Article

The Journal of Physical Chemistry C under dry (solid lines) and wet atmospheres (dashed lines). Figure 9B represents the selectivity (%) in equiv C1 of each HC according to the total acidity under a dry atmosphere. Propene is few or not adsorbed on all HY zeolites except for the lowest Si/Al ratios (Figure 9A). As a result, the selectivity at saturation is very low ( HY-2.5 ≫ HY-100). This possibly suggests that ethene is the main precursor for the formation of coke. These TPD profiles corresponding to “dry” adsorption conditions can now be compared with those obtained under “wet” adsorption conditions (Figure 13A′, B′, and C′). In good agreement with the corresponding breakthrough curves (Figure 8), most of the differences are observed for HY-2.5, while the HC desorption profiles for HY-100 are not affected by the presence of water. The effect of water is especially striking for HY-2.5. All the low-temperature HC emissions corresponding to the weakest interactions between the HC and the zeolite disappeared in the presence of water due to competitive adsorption effects in favor of the latter. On HY-2.5 and HY-15, water desorption is observed as a very broad peak centered at 155 °C. Desorbed quantities follow a logical order with the Si/ Al ratio, considering the relative hydrophilicity of zeolites: HY2.5 > HY-15 ≫ HY-100 (almost zero). Desorption profiles and quantities (Table 6) on HY-2.5 and HY-15 zeolites are

consistent with the adsorbed ones measured from breakthrough curves (Table 3). For HY-2.5, the residual HC emissions after adsorption under wet conditions (Figure 13A′) are observed at medium/ high temperature (around 320 °C). This phenomenon may be attributed in part to endothermic effects related to water evaporation (present in condensed form in the pores), which can delay the adsorption of HC.38 Furthermore, some desorbing hydrocarbons may have been readsorbed onto stronger adsorption sites, that is, acid sites, once water itself is desorbed from the same sites.

4. CONCLUSIONS In this study, a homemade methodology based on breakthrough curves and TPD experiments was especially implemented in order to describe the complex phenomena associated with the adsorption of a ternary mixture of hydrocarbons (propene, toluene, and decane, representative to the one found in diesel exhausts) on HY zeolites. A special emphasis has been put on the effects of the Si/Al ratio (ranging 328


Article


Figure 13. HC, ethene, and H2O emissions profiles monitored during the TPD under inert atmosphere corresponding to the adsorption of the HC ternary mixture (C3 + C7 + C10) in the presence of 1% water for the different zeolites: (A) Si/Al = 2.5, dry; (A′) Si/Al = 2.5, wet; (B) Si/Al = 15, dry; (B′) Si/Al = 15, wet; (C) Si/Al = 100, dry; (C′) Si/Al = 100, wet. Conditions: total flow rate = 60 mL/min, sample mass = 200 mg, heating rate = 10 °C/min.

Table 5. Quantitative Data Corresponding to the Emissions of Each HC During the TPD Performed under an Inert Atmosphere after Adsorption of the Ternary (C3 + C7 + C10) HC Mixture in the Absence of Watera C3 b

zeolite/HC

Qdes

HY-2.5 HY-15 HY-100

0.13 0.011 0.019

C7

C10

%

Qdes

b

b

%

Qdes

57 69 105

0.60 0.154 0.041

82 33 24

0.39 0.781 0.761

C2

CT

%

Qdes

b

c

b

coke

Qdes

78 104 64

0.079 0.170 0.019

15.8 18.3 3.0

8.65 9.09 7.99

HC + cokec

%

134.1 147.9 115.9

91 99 63

a

Qdes denotes the desorbed amount of each hydrocarbon in mmol/g or as % of the fraction adsorbed; Coke formation, as determined by TGA under air and total carbon (CT, mg/g), are also given. bQuantity in mmol/g. cQuantity in mg/g.

from 2.5 to 100) and the presence of water on some adsorption data, such as the adsorption capacities and selectivities or even the storage stability, which is important for cold-start applications. The selected HY zeolites were characterized according to their structural, textural and acidic properties. Data from FTIR of adsorbed pyridine confirmed an increase in the amount of Brönsted acid sites when decreasing the Si/Al ratio. On the other hand, the concentration of strong acid sites was

roughly the same for all samples. A sizable amount of Lewis acid sites (extra-framework Al species) was found at all the Si/ Al ratio, but the amount of LAS was far greater for the HY-2.5 zeolite. In the course of the adsorption of the HC mixture, a competitive thermodynamic adsorption between toluene and decane was observed, namely, for the medium-high Si/Al ratio due to high hydrophobicity and low acidity for this type of 329


Article


Table 6. Quantitative Data Corresponding to the Emissions of Each HC, Ethene (By-Product), and Water during the TPD Performed under Inert Atmosphere after Adsorption of the Ternary (C3 + C7 + C10) HC Mixture in the Presence of Watera C2

H2O

zeolite/HC

Qdes

C3 %

Qdes

C7 %

Qdes

C10 %

Qdes

Qdes

Qdes

CT %

HY-2.5 HY-15 HY-100

0.023 0.003 0

43 11

0.058 0.147 0.067

99 46 25

0.134 0.648 0.839

173 91 89

0.012 0 0

4.024 1.000 0

1.979 7.518 8.946

147 80 79

a Qdes denotes the desorbed amount of each hydrocarbon in mmol/g or as % of the fraction adsorbed; Desorbed quantities (Qdes) of each hydrocarbon and water are given in mmol/g and total carbon CT is expressed in mmol equiv C1/g.

(3) De Moor, B. A.; Reyniers, M. F.; Gobin, O. C.; Lercher, J. A.; Marin, G. B. Adsorption of C2-C8 n-alkanes in zeolites. J. Phys. Chem. C 2011, 115, 1204−1219. (4) Elangovan, S. P.; Ogura, M.; Davis, M. E.; Okubo, T. SSZ-33: A promising material for use as a hydrocarbon trap. J. Phys. Chem. B 2004, 108, 13059−13061. (5) Elangovan, S. P.; Ogura, M.; Stefan, E. C.; Hartmann, M.; Tontisirin, S.; Davis, M. E.; Okubo, T. A comparative study of zeolites SSZ-33 and MCM-68 for hydrocarbon trap applications. Microporous Mesoporous Mater. 2006, 96, 210−215. (6) Iliyas, A.; Zahedi-Niaki, M. H.; Eic, M.; Kaliaguine, S. Control of hydrocarbon cold-start emissions: A search for potential adsorbents. Microporous Mesoporous Mater. 2007, 102, 171−177. (7) Kanazawa, T. Development of hydrocarbon adsorbents, oxygen storage materials for three-way catalysts and NOx storage-reduction catalyst. Catal. Today 2004, 96, 171−177. (8) Li, H. X.; Donohue, J. M.; Cormier, W. E.; Chu, Y. F. Application of zeolites as hydrocarbon traps in automotive emission controls. Stud. Surf. Sci. Catal. 2005, 158, 1375−1382. (9) Lopez, J. M.; Navarro, M. V.; Garcia, T.; Murillo, R.; Mastral, A. M.; Varela-Gandia, F. J.; Lozano-Castello, D.; Bueno-Lopez, A.; Cazorla-Amoros, D. Screening of different zeolites and silicoaluminophosphates for the retention of propene under cold start conditions. Microporous Mesoporous Mater. 2010, 130, 239−247. (10) Park, J. H.; Park, S. J.; Ahn, H. A.; Nam, I. S.; Yeo, G. K.; Kil, J. K.; Youn, Y. K. Promising zeolite-type hydrocarbon trap catalyst by a knowledge-based combinatorial approach. Microporous Mesoporous Mater. 2009, 117, 178−184. (11) Sakuth, M.; Meyer, J.; Gmehling, J. Vapor-phase adsorption equilibria of toluene plus 1-propanol mixtures on Y-zeolites with different silicon to aluminum ratios. J. Chem. Eng. Data 1995, 40, 895− 899. (12) Sarshar, Z.; Zahedi-Niaki, M. H.; Huang, Q.; Eic, M.; Kaliaguine, S. MTW zeolites for reducing cold-start emissions of automotive exhaust. Appl. Catal. B 2009, 87, 37−45. (13) Savitz, S.; Siperstein, F.; Gorte, R. J.; Myers, A. L. Calorimetric study of adsorption of alkanes in high-silica zeolites. J. Phys. Chem. B 1998, 102, 6865−6872. (14) Serra, R. M.; Miro, E. E.; Bolcatto, P.; Boix, A. V. Experimental and theoretical studies about the adsorption of toluene on ZSM5 and mordenite zeolites modified with Cs. Microporous Mesoporous Mater. 2012, 147, 17−29. (15) Smit, B.; Maesen, T. L. M. Molecular simulations of zeolites: adsorption, diffusion, and shape selectivity. Chem. Rev. 2008, 108, 4125−4184. (16) Wesson, P. J.; Snurr, R. Q. Modified temperature programmed desorption evaluation of hydrocarbon trapping by CsMOR zeolite under cold start conditions. Microporous Mesoporous Mater. 2009, 125, 35−38. (17) Yoda, E.; Kondo, J. N.; Domen, K. Detailed process of adsorption of alkanes and alkenes on zeolites. J. Phys. Chem. B 2005, 109, 1464−1472. (18) Yoshimoto, R.; Hara, K.; Okumura, K.; Katada, N.; Niwa, M. Analysis of toluene adsorption on Na-form zeolite with a temperatureprogrammed desorption method. J. Phys. Chem. C 2007, 111, 1474− 1479.

zeolite. By contrast, propene was only little or not adsorbed on this kind of zeolite. Adsorption phenomena are also accompanied by kinetic limitations due to mutual steric hindrance during the HC diffusion within the internal porosity. Furthermore, it was shown that the adsorption selectivities evolve throughout the filling of pores, due to different diffusional HC characteristics and the possible sittings offered by the zeolites. For the latter, the study of the adsorption mechanisms highlighted the existence of specific interactions between unsaturated hydrocarbons (toluene and propene) with the acid sites when they are present in sufficient concentration (Si/Al < 15) in supercages. On the other hand, decane is mainly adsorbed via (nonspecific) dispersive interactions with the pore walls. Finally, the presence of water vapor in the gas mixture has generally a detrimental effect on the HC adsorption capacities and also affects the storage stability. Whereas these effects are very limited or even do not exist for the hydrophobic (high Si/ Al ratio) zeolites due to the absence of significant water adsorption, it can be very strong for the more acidic zeolites (with low Si/Al ratio). In the latter case, a competitive adsorption between the HC and water is observed, at the advantage of water. According to TPD data, this inhibitor substitutes most of the hydrocarbons having the lowest interaction forces with the zeolitic framework (corresponding to the peaks observed in TPD signals at T < 200−250 °C), while only the most stable fraction of HC remained.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors wish to thank the International Research Group (GDRI CNRS-PAN) “Catalysis for polluting emissions aftertreatment and production of renewable energies” for support of this work and, also, Prof. M. F. Ribeiro and Dr. R. Bartolomeu from IST Lisbon (Portugal) for having supplied some of the zeolites.

■

REFERENCES

(1) Burke, N. R.; Trimm, D. L.; Howe, R. F. The effect of silica: alumina ratio and hydrothermal ageing on the adsorption characteristics of BEA zeolites for cold start emission control. Appl. Catal. B 2003, 46, 97−104. (2) Czaplewski, K. F.; Reitz, T. L.; Kim, Y. J.; Snurr, R. Q. Onedimensional zeolites as hydrocarbon traps. Microporous Mesoporous Mater. 2002, 56, 55−64. 330


Article

The Journal of Physical Chemistry C (19) Lippens, B. C.; Linsen, B. G.; Deboer, J. H. Studies on pore systems in catalysts 0.1. The adsorption of nitrogen - apparatus and calculation. J. Catal. 1964, 3, 32−37. (20) Khabtou, S.; Chevreau, T.; Lavalley, J. C. Quantitative infrared study of the distinct acidic hydroxyl-groups contained in modified Yzeolites. Microporous Mater. 1994, 3, 133−148. (21) Treacy, M. M. J., Higgins, J. B., Eds. In Collection of Simulated XRD Powder Patterns for Zeolites; Elsevier: Amsterdam, London, 2001; pp 137−138. (22) Knozinger, H.; Huber, S. IR spectroscopy of small and weakly interacting molecular probes for acidic and basic zeolites. J. Chem. Soc., Faraday Trans. 1998, 94, 2047−2059. (23) Manoilova, O. V.; Dakka, J.; Sheldon, R. A.; Tsyganenko, A. A. Infrared study of Ti-containing zeolites using CO as a probe molecule. Catal. Microporous Mater. 1995, 94, 163−170. (24) Morrow, B. A.; Mcfarlan, A. J. Infrared and gravimetric study of an aerosil and a precipitated silica using chemical and H/D exchange probes. Langmuir 1991, 7, 1695−1701. (25) Trukhan, N. N.; Panchenko, A. A.; Roduner, E.; Mel’gunov, M. S.; Kholdeeva, O. A.; Mrowiec-Bialon, J.; Jarzebski, A. B. FTIR spectroscopic study of titanium-containing mesoporous silicate materials. Langmuir 2005, 21, 10545−10554. (26) Parry, E. P. An infrared study of pyridine adsorbed on acidic solids characterization of surface acidity. J. Catal. 1963, 2, 371−379. (27) Nishino, T., Scanning probe microscopy and spectroscopy. In Characterization of Solid Materials and Heterogeneous Catalysts: From Structure to Surface Reactivity; Michel Che, J. C. V., Ed.; Wiley-VCH Verlag GmBH & Co. KGaA: New York, 2012; Vol. 2, pp 703−745. (28) L. Abrams, D. R. C., Probing intrazeolite space. In Inclusion Chemistry with Zeolites: Nanoscale Materials by Design; Corbin, N. H. a. D. R., Ed.; Kluwer Academic Publishers: Netherlands, 1995; Vol. 6, pp 4−42. (29) Fujikata, Y.; Masuda, T.; Ikeda, H.; Hashimoto, K. Measurement of the diffusivities within MFI- and Y-type zeolite catalysts in adsorption and desorption processes. Microporous Mesoporous Mater. 1998, 21, 679−686. (30) Daems, I.; Leflaive, P.; Methivier, A.; Denayer, J. F. M.; Baron, G. V. Size and packing related adsorption effects in the liquid phase adsorption of aromatics and alkenes on FAU type zeolites. Stud. Surf. Sci. Catal. 2005, 158, 1177−1184. (31) Daems, I.; Baron, G. V.; Punnathanam, S.; Snurr, R. Q.; Denayer, J. F. M. Molecular cage nestling in the liquid-phase adsorption of n-alkanes in 5 Å zeolite. J. Phys. Chem. C 2007, 111, 2191−2197. (32) Clark, L. A.; Ye, G. T.; Gupta, A.; Hall, L. L.; Snurr, R. Q. Diffusion mechanisms of normal alkanes in faujasite zeolites. J. Chem. Phys. 1999, 111, 1209−1222. (33) Williams, B. A.; Babitz, S. M.; Miller, J. T.; Snurr, R. Q.; Kung, H. H. The roles of acid strength and pore diffusion in the enhanced cracking activity of steamed Y zeolites. Appl. Catal., A 1999, 177, 161− 175. (34) Dupuy, B.; Laforge, S.; Morais, C.; Bachmann, C.; Magnoux, P.; Richard, F. Alkylation of 3-methylthiophene by 2-methyl-1-pentene over HY, HBEA and HMCM-22 acidic zeolites. Appl. Catal., A 2012, 413, 192−204. (35) Trudell, J. R. Contributions of dipole moments, quadrupole moments, and molecular polarizabilities to the anesthetic potency of fluorobenzenes. Biophys. Chem. 1998, 73, 7−11. (36) Calleja, G.; Pan, J.; Calles, J. A. Pure and multicomponent adsorption equilibrium of carbon dioxide, ethylene, and propane on ZSM-5 zeolites with different Si/Al ratios. J. Chem. Eng. Data 1998, 43, 994−1003. (37) Lee, H. Y.; Yoon, S. D.; Byun, H. S. Cloud-point and vaporliquid behavior of binary and ternary systems for the poly(dodecyl acrylate) plus cosolvent and dodecyl acrylate in supercritical solvents. J. Chem. Eng. Data 2010, 55, 3684−3689. (38) Yeon, T. H.; Han, H. S.; Park, E. D.; Yie, J. E. Adsorption and desorption characteristics of hydrocarbons in multi-layered hydrocarbon traps. Microporous Mesoporous Mater. 2009, 119, 349−355. 331


Adsorption and Desorption of a Model Hydrocarbon Mixture Over HY

Recommend Documents