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Selectivity of Thiophene/Toluene Competitive Adsorptions onto Zeolites. Influence of the Alkali Metal Cation in FAU(Y) C. Laborde-Boutet,*,† G. Joly,† A. Nicolaos,‡ M. Thomas,‡ and P. Magnoux† Laboratoire de Catalyse en Chimie Organique, Chimie 7a, UMR 6503, UniVersite´ de Poitiers, 40 AVenue du Recteur Pineau, 86022 Poitiers Cedex, France, and Institut Franc¸ ais du Pe´ trolessite de Solaize, BP No. 3, 69390 Vernaison, France
In the framework of research considering adsorption for gasoline desulfurization purposes, dynamic adsorption experiments have been performed to study a comparison between thiophene and toluene affinities toward Y-faujasite adsorbents (Si/Al ) 2.4) exchanged with various alkali metal cations (Li+, Na+, K+, Rb+, Cs+). These experiments have been carried out at two concentration ranges in liquid n-heptane solutions: around 25 mmol‚L-1 for competitive adsorptions and around 1.25 mmol‚L-1 for single-solute adsorptions. Complementary microcalorimetry studies have been carried out in order to investigate the interaction of nucleophilic compounds toward alkaline Y zeolites, by measuring the adsorption enthalpy of CCl4 onto NaY and CsY sorbents. The results clearly exhibit two opposite trends for the adsorption of thiophene and toluene molecules onto alkaline Y faujasites. The thiophene affinity increases with cation electropositivity, while the toluene affinity decreases as the cations are more electropositive. Thus, from a favorable adsorption for toluene onto the LiY zeolite (Rthio/tol ) 0.38 at ca. 26 mmol‚L-1), the selectivities shift in favor of thiophene so that the Rthio/tol value reaches 3.00 with the CsY zeolite. The calorimetry measurements of CCl4 adsorption have exhibited an increase of the adsorption enthalpy with more electropositive cations, which seems to indicate that thiophene molecules are adsorbed similarly to CCl4, i.e., by strong interaction between the nucleophilic S atom of thiophene and the cationic charge. However, toluene should be mainly stabilized on the adsorption sites through other interactions, as combined attractions between the H atoms of toluene and the O atoms of the zeolite framework surrounding the SII cation can be suggested. 1. Introduction The first paper concerning our current studies1 states the context of our investigations about the selectivity of thiophene/ toluene competitive adsorptions onto zeolites, where the growing importance of the research on gasoline desulfurization is explained. The removal of sulfur traces from fluid catalytic cracking (FCC) gasoline is mainly achieved through conventional hydrodesulfurization (HDS), but meeting the new specifications in sulfur content in gasoline would require an increase in the operating temperature and pressure of this process. Under such conditions, olefinic and aromatic molecules are likely to undergo hydrogenation, leading to a substantial decrease in octane index. Therefore, alternative processes that could achieve sulfur removal without significant loss of octane index are investigated.2 Our research is being undertaken according to the possibility of removing sulfur derivatives from other hydrocarbon compounds by adsorption onto zeolites. Since a possible industrial application is considered, our investigations aim to find a sorbent exhibiting (i) a high selectivity between sulfur compounds and unsaturated hydrocarbons, (ii) a high sorbate capacity, and (iii) properties allowing its regeneration with a liquid solvent. Among zeolites, faujasite sorbents seem the most promising for fulfilling these criteria. On one hand, thiophenes constitute most of the sulfur derivatives contained in gasoline.3 On the other hand, in FCC * To whom correspondence should be addressed. Tel.: +33 (0)549453404. Fax: +33 (0)549453779. E-mail:
[email protected]. † Universite´ de Poitiers. ‡ Institut Franc¸ ais du Pe´trolessite de Solaize.
gasoline, aromatics are the most severe competitors of thiophene adsorption onto faujasites,4 while also being valuable molecules for their high octane index. Thus, our study focuses on the thiophene/toluene adsorption selectivity onto faujasites. Our first paper1 gives the current state of art of thiophene adsorption for gasoline desulfurization purposes. In previous works, the thiophene/toluene adsorption selectivities onto NaFAU sorbents were investigated. This second article now presents an experimental study of the competitive adsorption between thiophene and toluene molecules onto FAU(Y) sorbents exchanged with various alkali metal cations. In such systems, sorbates mostly interact by strong electrostatic interactions (≈50-100 kJ‚mol-1) between a given 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 thiophene/toluene selectivity. These experimental results provide a basis for ongoing research where the parameters that drive the adsorption of thiophenic and aromatic compounds are sought. Once the various phenomena are defined, the adsorbent that 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 Na56FAU zeolite (Si/Al ) 2.4) provided by Zeolyst. This material is in powder form, without binding agent. Alkaline-exchanged FAU(Y) zeolites have undergone typical ion-exchange protocols. For a given ion exchange, sodic zeolite powder was dispersed in an alkaline salt solution. The salts used in the various ion exchanges and the corresponding concentra-
10.1021/ie060430j CCC: $33.50 © 2006 American Chemical Society Published on Web 10/24/2006
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Table 1. Characteristics of the Ion Exchanges and Properties of the Resulting Alkaline Y Zeolites Used in the Adsorption Experiments zeolite LiY NaY KY RbY CsY a
solution 1
mol‚L-1
LiNO3
0.4 mol‚L-1 NaCl + 0.7 mol‚L-1 KCl 1 mol‚L-1 RbCl 2 mol‚L-1 CsCl
no. exchange stages
hydration ratio, (mhyd - manhyd)/mhyd (%)
RTa
1
RT RT 80 °C
1 1 3
25.0 19.0 15.2 18.7 17.6
temp
ion exchange 55% Li+ 100% Na+ 58% K+ 65% Rb+ 90% Cs+
RT, room temperature.
Figure 1. Structure and cationic sites in Y faujasites.
tions are reported in Table 1, as well as the number of exchange stages and the temperature at which they were carried out. A 10 mL volume of solution was used per gram of hydrated NaY to be ion-exchanged. These suspensions were stirred for about 8 h in each exchange stage. Afterward, the zeolite was washed with ultrapure water, recovered by Bu¨chner filtration, and dried in an oven (80 °C, air). The hydration ratio and the ion-exchange percentage of the alkaline Y zeolites are also featured in Table 1. The ionexchange percentages were determined by means of the elemental analyses of the sorbents with the ICP (inductively coupled plasma) technique. One should note that these ionexchange ratios correspond to a substitution of most of the cations located in SII/SII′ sites of the zeolite structure (Figure 1), which are more easily exchanged than SI/SI′ cations. It is enough for having significant influences upon thiophene and toluene adsorption, since these sorbates can interact with SIItype surroundings but remain too large to enter the areas where the SI/SI′ cations are located. Complementary information concerning the alkali metal cations is gathered in Table 2. Prior to the experiments, the various zeolite sorbents were compacted into pellets by means of a hydraulic press (5 tons‚cm-2) before being ground and sieved in order to obtain particles with diameters 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. The main physicochemical characteristics of these compounds are featured in Table 3. 2.3. Dynamic Adsorption Procedures. Prior to the dynamic adsorption experiments, zeolite particles were activated at 350 °C under primary vacuum for 4 h. In dynamic adsorption experiments, thiophene and/or toluene solutions in n-heptane solvent were processed through an adsorption column (stainless steel, 150 mm length, 0.2 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 grain 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 flow rate of 1 mL‚min-1 (for competitive adsorption experiments) or 2 mL‚min-1 (singlesolute adsorption at low-concentration experiments). These flow rates correspond to space velocities of 42 and 84 g‚gads-1‚h-1, respectively. These experiments were carried out at room temperature. Solution samples were collected at the column outlet and analyzed by HPLC (high performance liquid chromatography) at room temperature. The HPLC chromatograph is constituted of a Varian Prostar injection pump, a Lichrospher RP 18 type column, and a UV-visible Varian Prostar 340 detector. In monocomponent adsorption experiments, methanol is used as HPLC solvent, while a 85-15 vol % methanol-water mixture is used to separate the solutes in competitive adsorption experiments. The estimated errors in the measurements are within (5% of the sample concentrations. 2.4. Breakthrough Curve 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) )
∫0t x 1
2πσ
exp{-[(t - µ1)/x2σ]2} dt
(1)
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
(2)
i,exp
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 flow rate Q, the mass of dried processed sample mads, and the mass of 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:
2K′ tm 2 σ2 + ) 2 Pe 1 + K′ tR tR
(4)
Ind. Eng. Chem. Res., Vol. 45, No. 24, 2006 8113 Table 2. Characteristics of Alkali Metal Cations
ionic radius (Å) element electronegativity
Li+
Na+
K+
Rb+
Cs+
0.76 0.98
1.02 0.93
1.38 0.82
1.52 0.82
1.67 0.79
Table 3. Physicochemical Properties of the Solvent and of the Sorbates compound
mol wt (g‚mol-1)
liquid density at 25 °C
molar vol (mL‚mol-1)
dipole moment (exptl)
dipole moment (mol modeling)
n-heptane thiophene toluene carbon tetrachloride
100 84 92 154
0.682 1.051 0.867 1.586
146.6 79.9 106.1 97.1
0.55 0.37
0.51 0.42
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. Microcalorimetry Measurements. A Setaram DSC111 microcalorimeter was used to determine the heats of adsorption of carbon tetrachloride onto NaY and CsY zeolites. The calorimeter was connected to a volumetry device allowing small additions of gaseous CCl4. Pressure sensors have a measurement range between 0 and 12 mbar. Calorimetry measurements were preceded by an activation stage dedicated to the removal of water molecules and impurities from the zeolite sample. Samples of approximately 50 mg in weight were heated at 350 °C for 8 h under primary vacuum. Experiments were carried out at 25 °C. They consisted in adding successive CCl4 increments, each CCl4 increment being sent onto the zeolite sample once the thermal equilibrium had been reached, which took between 2 and 10 min. The adsorption experiments were assumed as completed when the addition of CCl4 did not produce any measurable heat of adsorption. 3. Results and Discussion 3.1. Dynamic Adsorption Experiments Related to Henry’s Law. To study the evolution of the interaction strengths between thiophene or toluene molecules and the various alkaline Y zeolites, dynamic monocomponent adsorption experiments have been performed at low concentration (around 1.25 mmol‚L-1) to approach Henry’s law. In that case, the adsorption capacities should be proportional to the sorbate/sorbent affinities. Besides, the theoretical outcome of a competitive adsorption experiment in such a concentration range would be the superposition of the monocomponent breakthrough curves. Figure 2 gathers the breakthrough curves of thiophene and toluene onto LiY, NaY, and CsY adsorbents, while the corresponding adsorption capacities are listed in Table 4. These experimental results clearly exhibit two opposite trends for the adsorption of thiophene and toluene molecules onto the Y zeolites exchanged with the various alkali metal cations, whose characteristics are described in Table 2. Thiophene adsorption is very weak on LiY (0.16 molecule‚cage-1) and NaY zeolites (0.26 molecule‚cage-1), but the adsorption strength increases with cation electropositivity, with the strongest adsorption onto the CsY adsorbent (1.13 molecules‚cage-1). On the contrary, toluene is strongly adsorbed onto LiY (1.55 molecules‚cage-1) and NaY (1.46 molecules‚cage-1), but the adsorption strength decreases as cation electropositivity increases, with the weakest adsorption onto the CsY (0.49 molecule‚cage-1) zeolite. Consequently, these experiments show that the affinity of thiophene toward the sorbent strengthens while that of toluene weakens with higher cation electropositivity. These results may seem surprising, given that thiophene and toluene are both
Figure 2. Breakthrough curves of (a) thiophene and (b) toluene in dynamic monocomponent adsorption experiments in n-heptane solvent onto NaY zeolite. Thiophene and toluene concentrations approach 1.26 mmol‚L-1.
aromatic hydrocarbons of similar size and dipole moment (Table 3). The equivalent selectivity Rthio/tol, defined as (qthio/Cthio)/(qtol/ Ctol), increases with cation electropositivity, as a result of both contributions. It is the lowest on the LiY (Rthio/tol ) 0.10) and the highest on the CsY (Rthio/tol ) 2.31) adsorbents. The effect of the cation type upon the interaction of the zeolite toward thiophene and toluene molecules is thus clearly underlined. In Y zeolites, it can be assumed that the main adsorption sites for thiophene and toluene molecules are cationic SII sites (Figure 1). In such surroundings, there is an interaction between the cationic charge and the nucleophilic part of the sorbate5-7 while the framework oxygen atoms of the D6R window surrounding the SII site can interact with the hydrogen atoms of the sorbates (as is the case for the adsorption of ammonia onto Y sorbents8). These two subinteractions taking place in the adsorption of thiophenic or aromatic molecules onto SII sites are combined in the adsorption phenomenon on this type of site, as represented in Figure 3. With alkali metal cations, those result in a strong electrostatic interaction whose magnitude can be about 110-125 kJ‚mol-1 in the case of xylene molecules.9 When shifting the cation from lithium to cesium, several parameters can change, inducing modifications in the overall adsorption strength of the molecules onto SII sites. First, as the size of the cation changes while bearing the same charge, it changes the cation polarizability and thus its electrophilic nature.
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Table 4. Summary of Thiophene and Toluene Adsorption in Dynamic Adsorption Experiments (≈1.25 mmol‚L-1, n-Heptane Solvent) onto LiY, NaY, and CsY Zeolites, with Equivalent Thiophene/Toluene Selectivities zeolite LiY NaY CsY
thiophene concn, Cthio (mol‚L-1)
thiophene ads, qthio (molecules‚cage-1)
toluene concn, Ctol (mol‚L-1)
toluene ads, qtol (molecules‚cage-1)
equiv selectivity, Rthio/tol
0.001 26 0.001 26 0.001 26
0.16 0.26 1.13
0.001 25 0.001 30 0.001 26
1.55 1.46 0.49
0.10 0.18 2.31
Figure 4. Enthalpy of CCl4 adsorption onto NaY and CsY sorbents versus zeolite loading. Figure 3. Means of interaction for a thiophenic or an aromatic molecule with SII site surroundings.
Second, the location of the cation in the D6R surroundings can vary according to its size, making differences in the distances between the cationic charge and the nucleophilic part of the sorbate. Moreover, this should also affect the distances between the hydrogen atoms of the sorbate and the oxygen atoms of the zeolite framework surrounding the SII cation. Last, changing the cations also shifts the charge distribution borne by the oxygen atoms of the zeolite framework, which can modify the interaction between the D6R oxygen atoms with the hydrogen atoms of the sorbate. Generally, it is assumed that bigger alkali metal cations are less acidic, leading to a decrease of interaction with aromatic molecules.7 It is also considered that zeolites with bigger alkali metal cations are more basic sorbents, meaning a higher charge density borne by the oxygen atoms of the framework. However, thiophene and toluene molecules are very similar in nature, since the two sorbates have both nucleophilic and electrophilic parts. Therefore, further investigations are required to obtain an explanation about the reasons for the sorbates to be differently influenced by a change in the cation nature. 3.2. Adsorption of Nucleophilic Compounds onto SII Sites through Calorimetry Experiments. Adsorption of nucleophilic compounds onto SII sites of Y-faujasite sorbents has not been clearly defined through previous publications. Several works have reported that aromatics adsorption on SII sites diminished in strength as the cation elecropositivity increased: this phenomenon was attributed to a lowering interaction of the aromatic ring with the cation, supposed to be less acidic.7 However, others have obtained an increase in adsorption energy of nucleophilic compounds such as CHCl3 or CH2Cl2 onto Y-faujasites with higher cation electropositivities.5,10 The possible problem may arise from the consideration of the sorbate adsorption as a global phenomenon, without any distinction of the two “subinteractions” mentioned in Figure 3. The origin of the interaction increase or decrease could not clearly be stated, since all these sorbates have both a nucleophilic part (attracted by the SII cationic charge) and an electrophilic part (attracted by the 6R oxygen atoms of SII sites). To suppress this ambiguity, a study of CCl4 adsorption onto
NaY and CsY faujasites has been carried out. Actually, carbon tetrachloride is a sorbate that is only able to interact through a nucleophilic interaction with the cation. Thus, if the evolution of the adsorption strength of a given sorbate is similar to that of CCl4, it should mean that this sorbate is mainly stabilized by the interaction of its nucleophilic part with the cationic charge. Calorimetry measurements have been performed and are presented in Figure 4. The adsorption enthalpies of CCl4 onto NaY and CsY have been estimated at 49.1 and 68.5 kJ‚mol-1, respectively, at a loading of 1 molecule per R-cage. This result is a definite confirmation of the trends obtained with CH2Cl2 and CHCl3.5,10 Given the higher heat of adsorption of CCl4 onto the CsY zeolite, it can be concluded that the interaction between the nucleophilic part of a molecule and the cationic charge increases with higher cation electropositivity. Larger and more electropositive cations can have a greater acidbase affinity with nucleophilic compounds, with a higher polarization of the bond between the nucleophilic part and the cation. Therefore, it can be deduced that thiophene is adsorbed onto SII sites in a similar fashion, while toluene molecules are mainly stabilized through another interaction. Namely, we can suggest that such aromatic molecules are actually stabilized onto SII sites mostly by the combined attractions between their hydrogen atoms and the oxygen atoms of the D6R window surrounding the SII cation. The decrease in toluene affinity with larger alkali metal cations can be explained by the increasing distances between the toluene H atoms and the D6R oxygen atoms surrounding the SII cations. 3.3. Dynamic Competitive Adsorption Experiments at 0.025 mol‚L-1. Dynamic competitive adsorption experiments provide other means of comparing the affinities of thiophene and toluene molecules toward the alkaline Y adsorbents. In addition to the experiments performed with LiY, NaY, and CsY faujasites, the study was completed by competitive adsorptions onto KY and RbY adsorbents. Being carried out at a higher concentration range (around 0.025 mol‚L-1), these experiments lead to adsorption/desorption breakthrough curves. In these cases, both solutes are adsorbed prior to the first breakthrough. When all available adsorption sites are saturated, the sorbate that has the strongest interaction with the sorbent continues to adsorb by removing the other solute from the zeolite porosity.
Ind. Eng. Chem. Res., Vol. 45, No. 24, 2006 8115 Table 5. Summary of Thiophene and Toluene Adsorption in Dynamic Adsorption Experiments (≈25 mmol‚L-1, n-Heptane Solvent) onto LiY, NaY, KY, RbY, and CsY Zeolites, with Corresponding Thiophene/Toluene Selectivities zeolite LiY NaY KY RbY CsY
thiophene concn, Cthio (mol‚L-1)
thiophene ads, qthio (molecules‚cage-1)
toluene concn, Ctol (mol‚L-1)
toluene ads, qtol (molecules‚cage-1)
selectivity, Rthio/tol
0.0254 0.0238 0.0250 0.0250 0.0250
1.00 1.67 1.87 2.23 2.45
0.0275 0.0256 0.0250 0.0250 0.0250
2.81 3.00 1.54 1.10 0.84
0.38 0.60 1.21 2.02 3.00
Figure 6. Adsorption isotherms of thiophene in liquid n-heptane solution onto the NaY zeolite, at room temperature.
Figure 5. Breakthrough curves of thiophene and toluene in dynamic competitive adsorption experiment (0.025 mol‚L-1, n-heptane solvent) onto LiY (a) and CsY (b) zeolites.
That is why the less strongly adsorbed solute follows a desorption curve, as its outlet concentration becomes greater than its inlet concentration. The adsorption equilibrium is reached when the outlet concentration of both sorbates remains equal to their inlet concentration. Qualitatively, these results are similar to the trends that have been observed in the experiments at lower concentration. The greatest selectivity for toluene is obtained with the LiY adsorbent, while thiophene is the most selectively adsorbed onto the CsY zeolite. Figure 5 depicts the breakthrough curves of these two extreme cases and exhibits the reversed selectivities. Moreover, the experiments performed with NaY, KY, and RbY faujasites have intermediate behaviors that confirm the main trends. The resulting capacities and selectivities, retrieved from the integration of the breakthrough curves in each experiment, are summed up in Table 5. The resulting Rthio/tol selectivity increases gradually with cation size, from 0.38 on the LiY faujasite to 3.00 on the CsY faujasite. Toluene capacity is generally smaller on FAU exchanged with larger cations, while thiophene capacity follows an opposite trend. This fact matches the previous results at low concentration. From a quantitative point of view, there are some differences in the results between what has been obtained at high and low concentration ranges. Namely, Rthio/tol selectivities were 0.38 on LiY and 0.60 on NaY at the high concentration range while they were 0.12 and 0.18 at low concentration. Apparently, the difference in affinity between toluene and thiophene adsorption was at least 3 times greater at low concentration than at high concentration. These significant shifts can be related to the peculiar shape of thiophene adsorption isotherms in the case of
Figure 7. Adsorption isotherms of thiophene in liquid n-heptane solution onto the CsY zeolite, at room temperature.
Y zeolites with weakly electropositive cations,1,11 which is described in Figure 6 in the case of the NaY faujasite. The inflection of the thiophene adsorption isotherm can be attributed to the activation of an additional type of adsorption site that would occur at a coverage of 1 molecule per R-cage. The most probable location of this kind of site is the 12R window frames (W) of the faujasite, given the thiophene molecule geometry. Previous works from Barthomeuf et al.7 have noticed a similar behavior for the adsorption of benzene in the NaY zeolite. Thus, it can also be assumed that the phenomenon occurring in the adsorption of thiophene onto the NaY also takes place on the LiY adsorbent. The same study on benzene adsorption has noted that adsorption occurred in W sites and SII sites simultaneously from nil coverage onto alkaline Y zeolites exchanged with more electropositive cations, such as Rb+ or Cs+, because of the higher basicity of their 12R window oxygen atoms.7 Given the shape of the thiophene adsorption isotherm (Figure 7), which matches a Langmuir or a dual-Langmuir behavior, it can be considered that thiophene adsorption also takes place in W sites and SII sites of the CsY sorbent from nil coverage, as benzene does. The Rthio/tol selectivities at low and high concentration ranges obtained with the CsY zeolites have the same order of magnitude, being 2.31 around 1.25 mmol‚L-1 and 3.00 around 0.025 mol‚L-1. One could note a selectivity shift, but this increase can be considered as relatively slight (30%) when
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compared to the ones occurring in LiY and NaY zeolites (over 300%). The assumption of W site adsorption for thiophene since nil coverage on the CsY is still relevant, because it would not agree with the order of magnitude of the selectivity shift otherwise. The relatively minor change in selectivity on the CsY can be explained by additional pore-filling mechanisms. When considering the size of thiophene molecules and toluene ones based on their liquid molar volumes (80 and 106 mL‚mol-1, respectively), one could estimate their saturation capacities as inversely proportional. With such assumptions, the thiophene saturation capacity is about 1.33 times the toluene one in the CsY sorbent. The experimental results are in very close agreement, with a thiophene saturation capacity of 4.25 molecules‚cage-1 while the toluene saturation capacity is 3.23 molecules‚cage-1, leading to a saturation capacity ratio of 1.32. These data are very similar to the Rthio/tol change between low and high concentration ranges observed on the CsY faujasite. 4. Conclusion These further experiments, following the previous works comparing the thiophene/toluene selectivities onto NaY and NaX adsorbents, clearly show that the type of cationic charge can largely influence the sorbate/sorbent interactions so that it can lead to significantly different selectivity results. Thiophene and toluene molecules are not adsorbed in the same way onto the SII sites of alkaline Y zeolites, even though the sorbates are similar to one another in nature. Thiophene molecules, mainly stabilized through an interaction between the nucleophilic sulfur atom and the cationic charge, have higher affinities toward Y zeolites with more electropositive cations, while toluene affinity follows an opposite trend. Moreover, the role of window sites also plays in favor of thiophene molecules, specially since the strongest W sites are to be found on the faujasite with more electropositive cations, i.e., the CsY zeolite. Acknowledgment Financial support for this project was provided by IFP, which is gratefully acknowledged.
Literature Cited (1) Laborde-Boutet, C.; Joly, G.; Nicolaos, A.; Thomas, M.; Magnoux, P. Selectivity of thiophene/toluene competitive adsorptions onto NaY and NaX zeolites. Ind. Eng. Chem. Res. 2006, 45, 6758-6764. (2) Song, C. An overview of new approaches to deep desulfurization for ultra-clean gasoline, diesel fuel and jet fuel. Catal. Today 2003, 86, 211. (3) Tian, F.; Wu, W.; Jiang, Z.; Liang, C.; Yang, Y.; Ying, P.; Sun, X.; Cai, T.; Li, C. The study of thiophene adsorption onto La(III)-exchanged zeolite NaY by FT-IR spectroscopy. J. Colloid Interface Sci. 2006, 301, 395. (4) Wardencki, W.; Straszewski, R. Dynamic adsorption of thiophenes, thiols and sulfides from n-heptane solutions on molecular sieve 13X. J. Chromatogr. 1974, 91, 715. (5) Timonen, J.; Pakkanen, T. T. A qualitative 1H MNR study of CHCl3 adsorption on conjugated acid-base pair in cation exchanged Y-zeolites. Microporous Mesoporous Mater. 1999, 30, 327. (6) Sauer, J.; Deininger, D. Interaction of ethene, 2-methylpropene and benzene with Na+ ion. IIsQuantum chemical study of sorption complexes in faujasites. Zeolites 1982, 2, 114. (7) Barthomeuf, D.; de Mallmann, A. Molecular recognition in the interaction of aromatics with zeolites. Chem., Ecol. Health 1995, 279. (8) Gilles, F.; Blin, J. L.; Toufar, H.; Briend, M.; Su, B. L. Double interactions between ammonia and a series of alkali-exchanged faujasites evidenced by FT-IR and TPD-MS techniques. Colloids Surf., A: Physicochem. Eng. Aspects 2004, 241, 245. (9) Kitagawa, T.; Tsunekawa, T.; Iwayama, K. Monte Carlo simulations on adsorptions of benzene and xylenes in sodium-Y zeolites. Microporous Mater. 1996, 7, 227. (10) Pinard, L.; Mijoin, J.; Magnoux, P.; Guisnet, M. Dichloromethane transformation over bifunctional PtFAU catalysts. Influence of acidobasicity of the zeolite. C. R. Chim. 2005, 8, 457. (11) Ng, F. T. T.; Rahman, A.; Ohasi, T.; Jiang, M. A study of the adsorption of thiophenic sulfur compounds using flow calorimetry. Appl. Catal., B: EnViron. 2005, 56, 127.
ReceiVed for reView April 6, 2006 ReVised manuscript receiVed September 8, 2006 Accepted September 16, 2006 IE060430J
