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Selectivity of Thiophene/Toluene Competitive Adsorptions onto NaY and NaX Zeolites C. Laborde-Boutet,*,† G. Joly,† A. Nicolaos,‡ M. Thomas,‡ and P. Magnoux† Laboratoire de Catalyse en Chimie OrganiquesChimie 7a, UMR 6503sUniVersite´ de Poitiers, 40 AVenue du Recteur Pineau, 86022 Poitiers Cedex, France, and Institut Franc¸ ais du Pe´ trole - site de Solaize, BP nο. 3, 69390 Vernaison, France
In the framework of research on desulfurization by adsorption, an experimental study of the adsorption selectivity between thiophene and toluene molecules in liquid n-heptane solutions onto NaY (Si/Al ) 2.4) and NaX (Si/Al ) 1.2) zeolites has been carried out. It investigates the possibility for these commercially available adsorbents to be used in a desulfurization process operated under ambient conditions without using H2. Dynamic adsorption experiments were performed at room temperature mainly at two concentration ranges: 25 mmol‚L-1 for competitive adsorptions and 1.25 mmol‚L-1 for single-solute experiments. Adsorption isotherms of thiophene and toluene on NaY and NaX sorbents have been obtained. On the NaY zeolite, toluene is favorably adsorbed (Rthio/tol ) 0.60) while an inverse behavior is observed with NaX sorbent (Rthio/tol ) 1.63). This exhibits many differences in sorbate/sorbent interactions with slight changes in the adsorbent nature. Besides, the selectivity on NaY sorbents has been found to depend significantly on the concentration range. Such a phenomenon agrees with the fact that the sorbates’ adsorption over NaY zeolites cannot be described by a simple Langmuir model. Thiophene adsorption onto NaY undergoes a peculiar phenomenon, since its isotherm exhibits an inflection at a coverage of 1 molecule per R-cage. 1. Introduction Recently, European directives have been set in order to reduce the release of sulfur pollutants by decreasing the sulfur compound content in fuels.1-3 The deep-desulfurization of gasoline can prevent the emission of sulfur oxides in the exhaust gases from motor vehicles. Moreover, the poisoning of catalytic converters and fuel cells catalysts can also be avoided.4 Thus, the specifications of sulfur concentration in gasoline have been reduced from 350 ppmw S by year 2000 to 50 ppm S by 20051,3 and eventually 10 ppm S by 2009.2 The need for desulfurization is an indirect consequence of the production of gasoline from the Fluid Catalytic Cracking unit, which is necessary to provide for the large demand in C6-C8 type of fuel. Since the feeding cut of the FCC unit has a high sulfur content (between 0.5 and 1.5 wt %), the outflow of the FCC unit has about 85-95% of the overall amount of sulfur derivatives in the resulting gasoline.5 Conventional hydrodesulfurization (HDS) is used to remove sulfur from FCC gasoline. By means of a reaction with H2 over Co/Mo-Ni/Mo catalysts, the sulfur compounds are converted into H2S. However, the required operating conditions of HDS for meeting the new specifications in sulfur content in gasoline can lead to the saturation of olefinic and aromatic molecules, resulting in a drop of octane index.6 Thus, research is carried out so as to obtain sulfur removal with a lesser loss of octane index. Since adsorption processes are efficient in the selective removal of traces from liquids, they have been considered as an interesting alternative to achieve gasoline deep-desulfurization. Moreover, the use of such a process could allow one to * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: +33 (0)549453404. Fax: +33 (0)549453779. † Universite´ de Poitiers. ‡ Institut Franc¸ ais du Pe´trole - site de Solaize.
carry out the desired separation at room temperature and atmospheric pressure. A first step consists of determining appropriate adsorbents that should satisfy three criteria: (i) a selective adsorption of the thiophenic compounds in comparison to the unsaturated hydrocarbons of the gasoline cut, (ii) a high sorbate capacity, (iii) a reversible adsorption allowing regeneration of the adsorbent at room temperature with a liquid solvent. The adsorption of sulfur compounds over various sorbents such as mixed metal oxides,7 activated carbon,8 clays, and zeolites has already been studied. Among these, zeolites have been reported as a promising type of adsorbent for removing selectively the sulfur derivatives from gasoline. Adsorption over zeolites for gasoline desulfurization purposes has been investigated since the 1970s, as Wardencki et al.9 have noted an effective adsorption of various sulfur compounds from n-heptane solutions over 13 X zeolite, but this efficiency dropped in the presence of benzene. These authors have carried out further studies10 on thiophene/benzene selectivity over various X-zeolites (NaX, HNaX, NiNaX, AgNaX), with the AgX zeolite leading to the best adsorption selectivity in thiophene. Various authors have studied the adsorption of thiophenic compounds onto intermediate pore size zeolites such as ZSM5. Weitkamp et al.,11 Garcia et al.,12 and King et al.13 considered gasoline desulfurization by adsorption onto alkaline ZSM-5 zeolites, with a high Si/Al ratio. In most of these cases, it was concluded that selectivity between sorbates can be deduced from their enthalpy of vaporization. Therefore, such materials do not seem suitable for removing sulfur derivatives from hydrocarbons of a same gasoline cut. The works of Garcia et al.,12 Koranyi et al.,14 and King et 13 al. showed the oligomerization of thiophenic compounds on protonated ZSM-5. Adsorption of thiophenic compounds onto HY zeolites also leads to oligomerization phenomena, as reported in the works of Geobaldo et al.,15 Richardeau et al.,16 and Chica et al.17,18 Given that oligomerization should be
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avoided for easy regeneration purposes, the removal of thiophenes by adsorption over acid zeolites does not seem appropriate in this context. Recently, several interesting studies of adsorption onto faujasite zeolites with the prospect of gasoline desulfurization have been carried out. Many studies have been performed by Yang et al.19-24 on thiophene adsorption by π-complexation on metal-ion exchanged zeolites. The higher selectivity in thiophene with regard to aromatic compounds has been obtained with a CuY zeolite. Velu et al.25 have investigated adsorption of thiophene onto CeY zeolites with promising results and studied reactive adsorption by chemisorption at elevated temperatures, finding the highest breakthrough capacities in thiophene for Ni-based zeolites at 200 °C.26 Last, Xue et al.27 have studied thiophene adsorption onto NaY, AgY, CuY, and CeY, with the consideration of the competition with aromatic molecules. In this work, the CeY zeolite has been reported to be the most promising sorbent for gasoline desulfurization purposes. Our research dedicated to the desulfurization by adsorption is focused on alkaline Y and X sorbents. These materials are currently considered for such purposes28 since they could satisfy the aforementioned criteria. First, the adsorption phenomena in such systems are mostly due to strong electrostatic interactions (≈50-100 kJ‚mol-1) between a given sorbate molecule with the cationic charges and the relatively basic framework oxygen atoms. It can be supposed that changes in the cationic sites, cation electropositivity, and framework basicity can induce many differences in the resulting thiophenic/aromatic selectivity. Then, faujasite-type adsorbents have a high porous volume, giving a high sorbate capacity. Last, those interactions are reversible despite their strength, which would allow an easy regeneration of the zeolite with a liquid solvent at room temperature. This paper shows a first stage in this global study, presenting our works on the selectivity of the thiophenic/aromatic competitive adsorption over NaY and NaX faujasites adsorbents. Those are the first sorbents to be investigated among the alkaline Y and X series, because they are commercially available and thus easy to include in the development of an industrial scale process. These results of dynamic competitive adsorption experiments are a first step in this ongoing research which aims to determine which parameters drive the adsorption of thiophenic and aromatic compounds in this kind of adsorbents. Once the various phenomena are defined, the adsorbent which exhibits the best properties for deep desulfurization purposes can be evidenced. 2. Experimental Section 2.1. Adsorbents. Zeolite adsorbents were prepared on the basis of Na56Y (Si/Al ) 2.4) and Na88X (Si/Al ) 1.2) powders, without binding agent, provided by Zeolyst (CBV100) and Ceca, respectively. The hydrated weight/anhydrous weight ratio for NaY and NaX are equal to 1.255 and 1.350, respectively. The BET surface areas of the NaY and NaX zeolites were equal to 1429 and 876 m2‚g-1, respectively. The microporous volume of the NaY sorbent was equal to 0.34 cm3‚g-1, representing 93.5% of its total porous volume. The microporous volume of the NaX sorbent was equal to 0.29 cm3‚g-1, representing 90.3% of its total porous volume. Prior to the experiments, zeolite samples were ground and sieved between 200 and 400 µm. 2.2. Sorbates and Solvent. Experiments have been carried out by processing thiophene and toluene in liquid n-heptane
solutions. Thiophene (99+%) was obtained from Aldrich, while toluene (>99%) and n-heptane (>99%) were provided by Fluka. 2.3. Dynamic Adsorption Procedures. Prior to the experiments, zeolite particles were activated at 350 °C under primary vacuum for 4 h. In the dynamic adsorption experiments, thiophene and/or toluene solutions in n-heptane solvent were processed through an adsorption column (stainless steel, 150 mm in length, 0.2 in. in diameter) filled with dried adsorbent particles. The equivalent of 1 g of hydrated zeolite was used in each experiment, which gave an adsorbent bed length of approximately 100 mm. The remaining space inside the column was filled with an inert carborundum grains bed. Thiophene and/or toluene solutions were pumped into the adsorbent column by means of a Gilson 307 pump, allowing the release of a constant flowrate of 1 mL‚min-1 (for competitive adsorption experiments) or 2 mL‚min-1 (single-solute adsorption at low-concentration experiments). These experiments were carried out at room temperature. Solution samples were collected at the column outlet and analyzed by HPLC at room temperature. The high-performance liquid chromatography (HPLC) chromatograph is constituted of a Varian Prostar injection pump, a Lichrospher RP 18 type column and of a UV-visible Varian Prostar 340 detector. In the monocomponent adsorption experiments, methanol is used as HPLC solvent, while a 85% - 15% vol. methanol-water mixture is used in order to separate the solutes in competitive adsorption experiments. 2.4. Breakthrough Curves Modeling. The dynamic adsorption data are fitted according to frontal chromatography principles. In fact, the response of a chromatography system to an inlet concentration step can be described by eq 1, when fronting or tailing phenomena are negligible:
F(t) )
∫0tx 1
e-[(t-µ1)/
x2σ]2
dt
(1)
2πσ
This signal is a Gaussian curve integral which can be characterized by its first- and second-order moments, µ1 and σ2. Considering a set of experimental data (ti,exp, Fexp(ti,exp)), a breakthrough curve F(t) is adjusted using µ1 and σ2 so that the following term is minimized:
(F(ti,exp) - Fexp(ti,exp))2 ∑ i,exp
(2)
The moments of the breakthrough curves are representative of the phenomena occurring in the dynamic adsorption process. The first-order moment µ1 corresponds to the retention time tR. Given the concentration C0, the flowrate Q, the mass of dried processed sample mads, and the mass of the zeolite unit cell Mads, the retention time tR allows the calculation of the adsorption capacity q (in molecules per R-cage) by means of eq 3:
Mads C0Q q) 8
∫0∞(1 - F(t)) dt mads
)
Mads C0QtR 8 mads
(3)
The second-order moment is representative of the hydrodynamic phenomena in the adsorption column, as stated by eq 4:
σ2 2K′ tm 2 + ) tR2 Pe 1 + K′ tR
(4)
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In eq 4, Pe is the Pe´clet number, K′ is the capacity factor of the adsorption column, and tm is the mass transfer time. This relation shows that the second-order moment comprises the contribution of axial dispersion and the contribution due to the mass transfer with the adsorbent particles. 2.5. Thermodesorption analysis. A thermodesorption experiment has been performed to analyze thiophene desorption from a NaY zeolite, using a microbalance (Symetric thermogravimetric analyzer Setaram B24) coupled with a mass spectrometer (Balzer Thermostar). Temperature and sample weight are continuously recorded. It was used with a carrier gas controlled by a mass flowmeter, at a flowrate of 2 Normoliters per hour. The heating rate is fixed at 10 °C‚min-1, from an ambient temperature up to 450 °C. The initial sample weight was about 67 mg in this experiment. 3. Results and Discussion 3.1. Dynamic Adsorption Experiments Related to Henry’s Law. To study the strength of the interactions between the sorbates (thiophene, toluene) and the zeolite sorbents (NaY, NaX), dynamic adsorption experiments have been carried out with the aim of approaching Henry’s law. Here, monocomponent adsorptions were performed at a concentration around 1.261.30 mmol‚L-1, knowing that the curves from competitive adsorptions in such cases would be the superposition of monocomponent curves (low-loading). In a low concentration range where Henry’s law can be applied, the equilibrium between the adsorbed phase q (molec‚cage-1) and the concentration in the liquid phase C (mol‚L-1) is linear. The ratio q/C should be equal to Henry’s constant (noted as KH), which is proportional to the sorbent/sorbate interaction strength. Breakthrough curves corresponding to the monocomponent adsorptions of thiophene and toluene over NaY and NaX are presented in Figures 1 and 2, respectively, while Table 1 sums up the results. A competitive adsorption experiment processing thiophene and toluene onto NaY zeolite in the same concentration range (1.50 mmol‚L-1 for thiophene and 1.30 mmol‚L-1 for toluene) gave similar results to the superposition of the results from monocomponent adsorptions (the equivalent selectivity or separation factor29 Rthio/tol, defined as (qthio/Cthio)/(qtol/Ctol), was equal to 0.20). This confirms that this concentration range allows one to approach Henry’s law behavior fairly. On a NaY zeolite, a high capacity for toluene is obtained while thiophene is weakly adsorbed. The values for the respective Henry constants show a large difference of affinity between toluene and thiophene toward the NaY adsorbent. On the contrary, from the adsorption capacity results onto NaX, it can be deduced that thiophene has a slightly greater affinity than toluene on this zeolite. Moreover, when shifting from NaY to NaX sorbent, there is an increase in the affinity for thiophene and a decrease in affinity for toluene, as described by the evolution of the Henry constants. Though NaY and NaX could appear as close to one another (same structure and cation type), the change in the Si/Al ratio brings many differences to the potential adsorption sites in their interactions with thiophene and toluene molecules. In NaY zeolites, it is assumed that the main interaction between the sorbates and the sorbent occurs onto SII surroundings (Figures 3 and 4): the nucleophilic part of the molecule (aromatic ring, S atom) can interact with the cationic charge30-32 while the hydrogen atoms of the sorbates can also interact with the framework oxygen atoms of the D6R window surrounding the SII site (as it is the case for the adsorption of ammonia onto
Figure 1. Breakthrough curves of (a) thiophene and (b) toluene in dynamic monocomponent adsorption experiments in n-heptane solvent onto a NaY zeolite. Thiophene and toluene concentrations are equal to 1.26 and 1.30 mmol‚L-1, respectively.
Figure 2. Breakthrough curves of (a) thiophene and (b) toluene in dynamic monocomponent adsorption experiments in n-heptane solvent onto NaX a zeolite. Thiophene and toluene concentrations are equal to 1.26 and 1.28 mmol‚L-1, respectively.
Y sorbents33). With alkali cations, those result in a strong electrostatic interaction whose magnitude can be about 110-125 kJ‚mol-1 in the case of xylene molecules.34 With an increase of the Si/Al ratio, the additional cations for NaX zeolites are located in SIII sites (Figure 3). This surrounding is of a different type from the SII sites: the cationic charge is located inside the R-cage, enabling the nucleophilic parts of the
Ind. Eng. Chem. Res., Vol. 45, No. 20, 2006 6761 Table 1. Summary of Thiophene and Toluene Adsorption in Dynamic Adsorption Experiments (1.26-1.30 mol‚L-1, n-Heptane Solvent) onto NaY and NaX Zeolites, with Related Henry Constants and Equivalent Thiophene/Toluene Selectivities zeolite
sorbate
sorbate conc C (mol‚L-1)
sorbate adsorbed q (mol‚molcage-1)
Henry constant KH (L‚molcage-1)
equivalent selectivity Rthio/tol
NaY
thiophene toluene thiophene toluene
0.00126 0.00130 0.00126 0.00128
0.26 1.46 1.11 1.04
206.3 1123.1 881.0 812.5
0.18
NaX
1.08
Figure 5. Possible interaction of thiophene with both SIII and SII sites in the NaX zeolite.
Figure 3. Structure and cationic sites in NaY and NaX faujasites. Cations are only located in SI/SI′ and SII/SII′ in NaY zeolite, while the higher aluminum content in the NaX structure is compensated by additional ions in SIII sites.
Figure 6. String of breakthrough curves of toluene adsorption onto NaX zeolites at different concentrations: (a) 1.3, (b) 4.0, (c) 10.0, (d) 17.5, (e) 24.7 mmol‚L-1. The resulting amounts of adsorbed toluene are featured in Figure 7d.
Figure 4. Means of interaction for a thiophenic and/or an aromatic molecule with a SII site surrounding.
sorbates to interact closer, while interactions between H atoms of sorbates and O atoms of the zeolite framework change. Moreover, the change in the Si/Al ratio leads to a sensitive shift in the framework oxygen basicity and also in the distribution of the negative charge along the oxygen atoms of the zeolite structure. It should be noted that a more basic adsorbent does not necessarily have a higher charge density on all its O framework atoms. For instance, works from Uytterhoeven et al. have shown a decrease of charge density on the O atoms surrounding SII sites when shifting from NaX to CsX zeolites, notwithstanding the higher global basicity of CsX sorbent.35 Possibly, a similar difference between NaY and NaX charge distributions could lead to the weaker adsorption of toluene onto the NaX zeolite. Last, the favorable thiophene adsorption on the NaX zeolite is possibly a consequence of a combined interaction that would not be feasible on a NaY zeolite. A reasonable hypothesis consists of the interaction of the nucleophilic sulfur atom with a SIII cationic charge with a simultaneous
interaction of thiophene aromaticity with another cationic charge located in SII (Figure 5). 3.2. Adsorption Isotherms. To complete the study on thiophene and toluene behavior onto NaY and NaX sorbents, dynamic monocomponent adsorption experiments have been undertaken to obtain the respective isotherms. An example of a string of such dynamic adsorption experiments is presented in Figure 6, where the breakthrough curves of toluene in n-heptane solutions on the NaX sorbent at different concentration ranges are gathered. The result from each breakthrough curve states an equilibrium point as q (molec‚cage-1) is plotted versus C (mol‚L-1). The data concerning the resulting isotherms of each sorbate (thiophene, toluene) on both adsorbents (NaY, NaX) are presented in Figure 7. They are fitted by Langmuirtype isotherms q ) qmax[KC/(1 + KC)] using a least-squares method. For each isotherm, the calculated parameters qmax (molec‚cage-1) and K (L‚mol-1) are reported in Table 2, as well as the variance of residuals (reduced χ2) expressing the accuracy of the Langmuir isotherm fitting. An estimation of the Henry constants for each isotherm can be retrieved from these parameters, since the Langmuir equation can be simplified into the linear relation q/C ) qmax‚K in the low concentration range. The corresponding values for KH ) qmax‚K are also included in Table 2. It can be observed that the adsorption isotherms of toluene on NaY and NaX zeolites can be described properly by the simple Langmuir model. On NaY zeolites, the maximum amount of adsorbed toluene qmax is equal to 3.85 molec‚cage-1, which
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Figure 7. Adsorption isotherms in a liquid n-heptane solution of thiophene in NaY (a), toluene in NaY (b), thiophene in NaX (c), toluene in NaX (d) and their fitting with a Langmuir model equation. The corresponding correlation coefficients expressed in terms of reduced χ2 are listed in Table 2. Table 2. Langmuir Parameters of the Isotherms Presented in Figure 7 and Retrieved Values for the Respective Henry Constantsa
a
zeolite
sorbate
qmax (mol‚molcage-1) (Langmuir isotherm)
K (L‚mol-1) (Langmuir isotherm)
KH (L‚molcage-1) (Langmuir isotherm)
reduced χ2 (Langmuir fitting)
NaY NaY NaX NaX
thiophene toluene thiophene toluene
6.0 3.85 3.47 3.40
101.8 376.5 317.2 247.1
610.9 1447.6 1102.3 840.2
0.172 0.030 0.009 0.027
The reduced χ2 corresponding to the various Langmuir isotherm fittings are proportional to the deviation between experimental and modeled data.
is relevant according to the number of adsorption sites (4 SII sites per R-cage). The lower amount of adsorbed toluene qmax in saturated NaX zeolite (3.40 molec‚cage-1) is due to the lower available volume resulting from the presence of cationic SIII sites. On both adsorbents, higher capacities of thiophene can be obtained at saturation, since thiophene molecules are smaller than toluene. The values of KH retrieved with the Langmuir parameters are higher than the ones obtained from the experiments in the low concentration range. With exception of thiophene adsorption onto NaY, those values are in the same order of magnitude and the differences lie in the fact that the aforementioned experiments only approach the Henry’s law domain. Thus, the results from the “low concentration range” experiments can only lead to underestimations of the slopes at the isotherm’s origin. Nevertheless, the agreement between these two ways for obtaining the KH constants is fair, and similar conclusions can be drawn from these last estimations. However, the large difference between the two estimations of KH for thiophene adsorption onto NaY has other grounds. In that case, thiophene adsorption exhibits a peculiar phenomenon, since its isotherm undergoes an inflection (Figure 8). Consequently, the Langmuir model is inappropriate for modeling this adsorption isotherm. This fact is confirmed by the high value of the reduced χ2 (equal to 0.172) resulting from this Langmuir fitting. This specific case, which has already been noticed previously,36 occurs at a coverage of about 1 molec‚cage-1. Therefore, it can be concluded that it is not a consequence of additional pore-filling mechanisms that take place at highloading. Thus, it can be suggested that this inflection is the consequence of the activation of an additional type of adsorption sites for thiophene.
Figure 8. Adsorption isotherms in a liquid n-heptane solution of thiophene in NaY and evidence of its peculiar behavior through a comparison with Henry’s law, represented by the dashed straight line.
To evidence this phenomenon, a thermodesorption experiment has been performed with adsorbed thiophene onto a NaY zeolite. Prior to the thermodesorption, the faujasite adsorbent was processed with a thiophene solution (0.1 mol‚L-1 in n-heptane solvent). The thermodesorption apparatus was coupled with a mass spectrometer allowing a count of the thiophene molecules along the temperature increase, which is presented in Figure 9. It appears clearly that the mass spectrometer signal can be deconvoluated into two peaks with characteristic temperatures. This result highlights the possibility for thiophene to adsorb in two different types of sites in NaY zeolites, with two distinct levels of adsorption strength. Several studies on benzene adsorption onto Y zeolites carried out by Barthomeuf et al.32 have evidenced another adsorption site for this molecule: the window (W) sites. Here, benzene molecules interact by their H atoms with the O atoms of the 12R window. Given the thiophene molecule geometry, the
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Figure 9. Count of the mass spectrometer signal calibrated on the thiophene main peak as a function of temperature (°C) in the desorption of thiophene from NaY zeolite. The plain curves depict the deconvolution of the signal into two peaks, corresponding to distinct adsorption energies.
Figure 10. Breakthrough curves of thiophene and toluene in dynamic competitive adsorption experiments (0.025 mol‚L-1, n-heptane solvent) onto NaY (a) and NaX (b) zeolites.
adsorption of thiophene molecules in such surroundings can be supposed, despite the repulsive force that can be induced by its nucleophilic S atom toward the O atoms of the W site. Moreover, Barthomeuf et al. have pointed out that, in the case of weakly basic Y zeolites (NaY with a Si/Al ratio ) 4), adsorption of benzene in W sites occurred only at non-nil coverage.32 A similar behavior for thiophene adsorption in the W sites of NaY zeolites can be suggested, which would explain the peculiar shape of its adsorption isotherm: adsorption onto SII sites only would occur at low coverage, before the W sites are activated for thiophene adsorption once 1 molec‚cage-1 is adsorbed onto SII sites, leading to the isotherm inflection. 3.3. Dynamic Competitive Adsorption Experiments at 0.025 mol‚L-1. The comparison of the respective affinities of thiophene and toluene toward a given adsorbent can also be achieved through a direct competitive adsorption experiment. The breakthrough curves presented in Figure 10a are the result of the dynamic competitive adsorption of thiophene and toluene in n-heptane solution at 0.025 mol‚L-1 onto the NaY zeolite. These curves are basic cases of competitive adsorptions: Prior to the first breakthrough, both solutes are adsorbed on the sites
of the NaY zeolite. Then, the available sites for the adsorption of the solutes are saturated. Thus, the compound with the highest affinity toward the zeolite is adsorbed by means of desorbing the other solute, which has weaker interactions. In this case, the toluene molecules which have the higher affinity tend to remove thiophene from the adsorption sites of NaY, so that the regular breakthrough of toluene also leads to the adsorption/ desorption curve of thiophene. This favorable selectivity for toluene agrees with the results previously observed on monocomponent adsorption. The capacities in thiophene and toluene, obtained by integration of the breakthrough curves, are 1.68 and 3.00 molec‚cage-1, respectively. This corresponds to a Rthio/tol selectivity of 0.60. It can be observed that it noticeably differs from the behavior in the low concentration range, where the difference between the toluene and thiophene affinities was 3 times greater (Rthio/tol ) 0.20 in a concentration range around 1.30-1.50 mmol‚L-1). Such a large difference can be explained through the peculiar shape of the thiophene adsorption isotherm: the inflection implies a lesser affinity gap at high concentration than at low concentration. A similar experiment has been carried out with NaX zeolite, leading to the breakthrough curves displayed in Figure 10b. In this competitive adsorption experiment, 2.06 and 1.27 molec‚ cage-1 of thiophene and toluene were adsorbed, respectively. Competitive adsorption experiments carried out at 0.025 mol‚L-1 approach high-loading conditions. Thus, the maximum volume available to the sorbates within the cages has an impact on the global quantity of the adsorbed molecules in our experiments. NaX zeolites, when compared to NaY, have additional cations located in the R-cages (SIII), reducing the cage volume. That is why the overall capacity is higher on NaY (4.68 molec‚cage-1) than on NaX (3.33 molec‚cage-1) zeolites. In agreement with the results previously observed at low concentration, an inverse behavior is observed on the NaX adsorbent when compared to the competitive adsorption experiment onto NaY. On NaX faujasite, thiophene is favorably adsorbed with a Rthio/tol selectivity of 1.63, implying a partial desorption of toluene. The differences between NaY and NaX zeolites induced by the contribution of cationic SIII sites and a wide modification of the charge born by the framework O atoms are the main features defining thiophene/toluene selectivity, as is the case in the low-concentration range. Only, it is altered by the specific behavior of thiophene in the NaY sorbent, where an additional type of site seems to be available at a coverage greater than 1 molec‚cage-1. Even though they approach high-loading conditions, pore-filling mechanisms should be of a minor influence in these competitive adsorptions. They could be responsible however for the slight difference in thiophene/toluene selectivities onto NaX sorbents between the high and low concentration ranges, where the ratio of the selectivities is about 1.5. 4. Conclusion The develoment of a process for the desulfurization of gasoline by adsorption requires the knowledge of basic thermodynamical data such as adsorption isotherms. On top of that, it is helpful to understand the sorbate-sorbent interactions in order to be able to explain the selectivity changes that have been observed in our study. As a matter of fact, the experiments carried out on NaY and NaX zeolites with thiophene and toluene sorbates show that slight changes in the nature of the adsorbent can lead to significant differences in sorbate-sorbent interactions. Shifting
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from NaY to NaX zeolite implies the addition of cationic charges which bring another type of adsorption sites, if not a possibility for the sorbates to interact with different surroundings. Such a change also results in an increase in the framework basicity. The consequence of these changes is an inversion of selectivity in competitive thiophene/toluene adsorption experiments. It has been shown that the Langmuir model is not always satisfactory for the description of liquid-phase adsorption equilibria in zeolites. A peculiar adsorption phenomenon of thiophene onto the NaY zeolite has been evidenced, with an inflection occurring on the isotherm at ca. 1 molec‚cage-1 in coverage. Given that the NaX sorbent exhibits the best selectivity for thiophene in comparison to the aromatic molecule, it seems that relatively basic zeolites are more appropriate to the removal of sulfur derivatives from gasoline. Further studies are being carried out using alkali cations exchanged Y zeolites to deepen our understanding of thiophenic and aromatic adsorption mechanisms onto Y and X zeolites. This should evidence the phenomena that are responsible for the selectivity of such molecules with these kinds of sorbents and possibly confirm the performance of basic zeolites for selective desulfurization purposes. Acknowledgment Financial support for this project was provided by IFP, which is greatly acknowledged. Literature Cited (1) Directive 98/70/EC. Directive of the European parliament and of the council of 13th October 1998 relating to the quality of petrol and diesel fuels and amending council directive 93/12/EEC. Official J. Eur. Communities 1998, L350, 58. (2) Directive 2003/17/EC. Directive of the European parliament and of the council of 3rd March 2003 amending Directive 97/70/EC relating to the quality of petrol and diesel fuels - Text with EEA relevance. Official J. Eur. Communities 2003, L76, 10. (3) Babich, I. V.; Moulijn, J. A. Science and technology of novel processes for deep desulfurization of oil refinery streams: a review. Fuel 2003, 82, 607. (4) Ma, X.; Sun, L.; Song C. A new approach to deep desulfurization of gasoline, diesel fuel and jet fuel by selective adsorption for ultra-clean fuels and for fuel cell applications. Catal. Today 2002, 77, 107. (5) Brunet, S.; Mey, D.; Pe´rot, G.; Bouchy, C.; Diehl, F. On the hydrodesulfurization of FCC gasoline: a review. Appl. Catal., A: Gen. 2005, 278, 143. (6) Fredrick, C. Sulfur reduction: what are the options? Hydrocarbon Process. 2002, 26. (7) Padley, M. B.; Rochester, C. H.; Hutchings, G. J.; King, F. FTIR spectroscopic study of thiophene, SO2, and CO adsorption on Cu/Al2O3 catalysts. J. Catal. 1994, 148, 438. (8) Loccarino, E. P.; Lieberman, M.; Taylor, W. F. Pre-methanation purification study: remoVal of low concentration gaseous sulfur compounds (catalyst poison); Final report for the period July 1975-July 1977, Exxon Research and Engineering Company: 1978. http://www.fischer-tropsch.org/ DOE/DOE_reports/0059t1/0059_t1_toc.htm. (9) Wardencki, W.; Straszewski, R. Dynamic adsorption of thiophenes, thiols and sulfides from n-heptane solutions on molecular sieve 13X. J. Chromatogr. 1974, 91, 715. (10) Wardencki, W.; Straszewski, R. Sorption properties of some modified sieves 13X towards thiophene and benzene. J. Chromatogr. 1985, 329, 128. (11) Weitkamp, J.; Schwark, M.; Ernst, S. Removal of thiophene impurities from benzene by selective adsorption in zeolite ZSM-5. J. Chem. Soc., Chem. Commun. 1991, 1133. (12) Garcia, C. L.; Lercher, J. A. Adsorption and surface reactions of thiophene on ZSM-5 zeolites. J. Phys. Chem. 1992, 96, 2669. (13) King, D. L.; Faz, C.; Flynn, T. Desulfurization of gasoline feedstocks for application in fuel reforming. Soc. Automot. Eng. 2000, 200001-0002, 9.
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ReceiVed for reView February 10, 2006 ReVised manuscript receiVed June 26, 2006 Accepted July 20, 2006 IE060168E
