Adsorption, Co-adsorption, and Reactions of Sulfur Compounds

Nov 13, 2012 - sulfur compounds, olefins, and aromatics over Ce-exchanged. Y zeolite .... Yang12 and Song13−15 have studied the interaction mechanis...
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Adsorption, Co-adsorption, and Reactions of Sulfur Compounds, Aromatics, Olefins over Ce-Exchanged Y Zeolite Linhai Duan,† Xionghou Gao,‡ Xiuhong Meng,† Haitao Zhang,‡ Qiang Wang,§ Yucai Qin,† Xiaotong Zhang,† and Lijuan Song*,† †

Key Laboratory of Petrochemical Catalytic Science and Technology, Liaoning Shihua University, Fushun 113001, Liaoning, PR China ‡ Petrochemical Research Institute, PetroChina Company Limited, 9 Dongzhimen North Street, Dongcheng District, Beijing 100007, PR China § College of Environmental Science and Engineering, Beijing Forestry University, Beijing 100083, PR China ABSTRACT: Adsorption, coadsorption, and reactions of sulfur compounds, olefins, and aromatics over Ce-exchanged Y zeolite (CeY) have been studied by N2 adsorption, intelligent gravimetric analyzers (IGA), X-ray diffraction (XRD), inductively coupled plasma mass spectrometry (ICPMS), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), Fourier-transform infrared spectroscopy (FT-IR), frequency response (FR) techniques, and batch and fixed bed methods. The coadsorption of olefins can effectively contribute to the alkylation and oligomerization reactions of thiophene and further decrease the desulfurization performance of CeY. The influence of aromatics on the desulfurization performance of CeY can be related to the competitive adsorption/diffusion processes between sulfur compounds and aromatics. The diffusion process is the rate-controlling step for benzene, and the sorption process is the rate-controlling step for thiophene. Thiophene molecules can be preferentially adsorbed onto the Lewis acid sites by direct interaction between sulfur on sulfur compounds and the Ce ion (S−M bond formation) or π-complexation, but the S−M bond interactions play a more dominant role than π-complexation in the CeY/sulfur compounds system. treating technology.3,4 Consequently, development of new deep desulfurization processes for liquid hydrocarbon fuels has become one of the major challenges to the refining industry and to the production of the ultralow-sulfur fuels for fuel cell application. Recently, many new desulfurization technologies have been developed including adsorption,5 extraction,6,7 oxidation,8,9 bioprocesses,3 etc. Among these, the selective adsorption desulfurization method is a rather promising alternative because it does not need to consume H2 and can be operated at ambient or rather low temperature and pressure conditions. The challenge for the selective adsorption desulfurization is to explore an adsorbent that can attract and selectively adsorb sulfur compounds but leave the aromatic and olefinic hydrocarbons untouched. Zeolites have been found to be promising materials for the adsorptive desulfurization. The refractory sulfur adsorption performance of the transition metal ion-exchanged Y zeolite prepared by using vapor-phase and solid-state ion-exchange techniques has been widely investigated. Due to its well-defined three-dimensional channels with a larger pore opening of 7.4 Å × 7.4 Å, a supercage cavity of

1. INTRODUCTION The production of ultralow-sulfur fuels has been receiving increasing attention worldwide due to the increasingly stringent regulations and fuels specifications for environmental protection purposes.1,2 In terms of the technology available, conventional hydrodesulfurization (HDS) is highly effective for removing organic sulfur compounds in gasoline, and the sulfur content can be reduced to less than 30 ppmw. However, the major problem is that the HDS technology results in a significant reduction of the octane number due to the saturation of olefins in naphtha from catalytic cracking. Furthermore, the reactor needs to be 5−15 times larger than those currently being used for producing ultralow-sulfur gasoline to meet the new regulations. On the other hand, ultralow-sulfur fuel is also needed for on-site or on-board use with fuel cell systems. For the automotive fuel cells and military fuel cells, gasoline is the ideal fuel because of its higher energy density, ready availability, and proven safety for transportation and storage. However, liquid hydrocarbon fuels usually contain certain sulfur compounds that are poisonous to both the catalysts used in fuel processors and the electrode catalysts in fuel cell stacks. Thus, the sulfur compounds in the liquid hydrocarbons need to be reduced to less than 0.1 ppmw. It is very difficult and uneconomical to meet such an extremely demanding fuel sulfur requirement with the current hydro© 2012 American Chemical Society

Received: March 30, 2012 Revised: November 10, 2012 Published: November 13, 2012 25748

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°C overnight, and then calcined at 500 °C for 6 h in air atmosphere. The above processes were repeated twice to achieve a high cerium loading. 2.2. Model Fuels. In order to investigate the selective adsorption mechanism of different sulfur-containing complexes, 10 kinds of model fuels containing certain amounts of sulfur compounds in n-nonane were prepared: S-1: thiophene (TP) 300 μg(S)/g S-2: 2-methylthiophene (2-MTP) 300 μg(S)/g S-3: 3-methylthiophene (3-MTP) 300 μg(S)/g S-4: tetrahydrothiophene (THT) 300 μg(S)/g S-5: benzothiophene (BT) 300 μg(S)/g S-6: dibenzothiophene (DBT) 300 μg(S)/g S-7: 4-methyldibenzothiophene (4-MDBT) 300 μg(S)/g S-8: 4,6-dimethyldibenzothiophene (4,6-DMDTP) 300 μg(S)/g S-9: TP 60 μg(S)/g, 2-MTP 60 μg(S)/g, 3-MTP 60 μg(S)/g, THT 60 μg(S)/g, and BT 60 μg(S)/g) S-10: BT 75 μg(S)/g, DBT 75 μg(S)/g, 4-MDBT 75 μg(S)/g, and 4,6-DMDBT 75 μg(S)/g) All the sulfur-containing compounds used for the preparation of standard samples were purchased from Aldrich and used as such without further purification. 2.3. Adsorbent Characterizations. The XRD patterns were acquired on a D/Max-RD-type diffractometer system (Rigaku, Japan) equipped with monochromatic Cu Kα radiation. The IR was performed on a spectra GX IR spectrometer (Perkin-Elmer, U.S.A.) using the KBr method. The sulfur content of model fuels was measured by the WK-2D coulomb meter (Jiangfen Electroanalytical Instrument Co., Ltd., China). The metal ion content in the zeolites was analyzed using an ICP-MS X7 (Thermo Elemental, U.S.A.). N2 adsorption−desorption isotherms were determined using an intelligent gravimetric analyzer (IGA). A more detailed description of the experimental procedures was given previously.21 The specific surface areas and total pore volume of the CeY were calculated using the BET equation and t-plot method respectively. 2.4. Adsorption Experiment. The adsorption isotherm measurements were made gravimetrically by a fully automated and computer-controlled gravimetric system (IGA, Hiden Analytical Ltd., Warrington, UK). A sensitive microbalance with resolution of 0.1 μg was mounted in a thermostatted enclosure to remove the thermal coefficients of the weighing system and thus provide higher stability and accuracy.22 The selective adsorption/desulfurization performances of the sorbents were carried out using a static stirring method or a fixed-bed flow reactor. The static adsorption studies were conducted with a mass ratio of 14:1 of the model fuels and adsorbents in a flask with a magnetic stirrer at ambient temperature and pressure. After 4 h, the adsorbents were separated by filtration. A WK-2D coulomb meter with a detection limit of 0.5 μg/g was used for the quantitative analysis and determination of the total sulfur content in the treated model fuels. The breakthrough curve measurements were carried out in a fixed-bed flow reactor. Prior to each experiment, the adsorbent was heated at 400 °C under N2 for 1 h in order to remove the physically adsorbed water and then cooled to room temperature. Briefly, the adsorbent was packed in a quartz tube with an internal diameter of 6 mm and a length of 350 mm. The volume of the adsorbent bed was 0.23 mL. The model fuel was fed through the adsorbent column using a

11.0 Å × 13.0 Å, and a large amount of cation-exchangeable sites, Y zeolite generally possesses higher adsorption capacity than other zeolites.10,11 Rates of adsorption and desorption in porous adsorbents are generally controlled by the mass transfer of the sorbates within the pore network, rather than by the intrinsic kinetics of sorption at the surface. In fine micropores, such as the intracrystalline pores of zeolites, the diffusing molecule never escapes from the force field of the adsorbent surface, and mass transfer occurs by an activated process involving jumps between adsorption sites. One of the bottlenecks to the industrial application of the Y zeolite is the smaller selective adsorption capacity of the adsorbent for sulfur compounds in spite of a high specific surface area. It was widely agreed that the low selective adsorption capacity of Y zeolites is caused by the competitive adsorption of aromatics and olefins to the sulfur compounds due to the similarity in molecular structures. Yang12 and Song13−15 have studied the interaction mechanisms of sulfur compounds and aromatics with ion-exchangeable Y zeolites and proposed π-complexation and S−M bond interaction, respectively, with the use of a molecular simulation method. The competitive adsorption, diffusion, and sorbate− sorbent interaction modes of hydrocarbons, olefins, aromatics, and sulfur compounds are very important to the desulfurization performance of Y zeolites. Another fact that cannot be ignored, however, is that the oligomerization and alkylation of sulfur compounds possibly form bulky compounds such as dimers, trimers, and tetramers, consequently decreasing the selective adsorption performance of sorbents to sulfur compounds.16,17 However, until now, the oligomerization and alkylation mechanisms are still not very clear. Some theoretical studies demonstrated that one of the oxygen atoms in the framework of the zeolite was the catalytic center for the reaction, while other studies showed that the Brønsted acid sites may play a key role.18−20 In this contribution, the adsorption of sulfur compounds, coadsorption of sulfur compounds/aromatics/olefins, sorbate− sorbent interaction modes, oligomerization and alkylation of sulfur compounds, intracrystalline diffusion, and the desulfurization performance of CeY have been systematically investigated, and the importance of Brønsted acid sites to the desulfurization performance of CeY has been elaborated. The desulfurization performance was evaluated using a batch method and a fixed-bed absorber under ambient conditions. CeY was characterized in detail using various techniques including ICP-MS, XRD, XPS, TEM, and HRTEM. The competitive adsorption, intracrystalline diffusion behaviors of the aromatics, olefins, and sulfur compounds were determined by intelligent gravimetric analyzers (IGA) and frequency response (FR) technologies. FTIR technologies have been used to study the oligomerization and alkylation mechanisms and the sorbate−sorbent interaction modes of aromatics, olefins, and sulfur compounds. The objective of this work is to shed some light on the bottleneck to the desulfurization performance of CeY zeolite.

2. EXPERIMENTAL SECTION 2.1. Adsorbent. The starting material used in this study was a sodium-type Y zeolite (NaY, from Nankai University, China, Si/Al = 2.55). The adsorbents were prepared by ion exchange with NaY using a 0.1 M aqueous solution of Ce(NO3)3 at 100 °C for 4 h. After ion exchange, the zeolite suspension was filtered, washed thoroughly using deionized water, dried at 100 25749

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piston pump at 2 h−1 liquid hourly space velocity (LHSV), and the effluent of the column was collected periodically for analysis. The sulfur compounds in fuel oils were analyzed by gas chromatography equipped with a capillary plus a sulfur chemiluminescence detector (SCD). 2.5. FT-IR Measurements. The adsorbent (after reaching an adsorption equilibrium of the sulfur compounds or the mixtures of sulfur compounds and aromatics and olefins in model fuels [S-1 to S-10]) was degassed and dried in a vacuum oven and kept for 8 h at an elevated temperature of 80 °C. After cooling down, typically only a small amount of the sample was dispersed in KBr by a hand mortar and then pressed to form a pellet. IR spectra were commonly recorded by the KBr pellet technique on a FT-IR spectrometer (TM GX PerkinElmer) with a resolution of 4 cm−1, and 16 scans. The spectra of the original adsorbents before adsorption were used as the reference spectra. 2.6. FR Measurements. The frequency response (FR) technique developed by Rees and co-workers has been proved to be a very powerful method for determining the intracrystalline mass transfer of molecules through zeolite crystals.23−28 An outstanding advantage of the FR method is its ability to distinguish multikinetic processes in a FR spectrum with a broad range of time constants; i.e. various “independent” rate processes that occur simultaneously can be investigated. An accurately known amount of sorbent sample was scattered into a plug of glass wool and then outgassed at a pressure of BT > DBT > 4-MDBT > 4, 6-DMDBT > THT, indicating that BT and its alkyl-substituted derivatives are more stable than TP. The two bands at 1452 cm−1 and 1379 cm−1, which are assigned to the deformation vibration of saturated CH3 and CH2, further confirm that sulfur compound oligomerization and alkylation reactions can occur induced by proton acids over CeY. According to our molecular simulation results, the thiophene ring can interact with CeY by two modes, namely π-complexation and S−M bond formation. The bonds of perturbed symmetric stretching vibration of CC in the fundamental ring caused by π-complexation, which should be at about 1396 cm−1, is shifted 17 cm−1 to lower wavenumbers 1379 cm−1 in coincidence with the bands of deformation vibration of saturated CH2. The shift to lower wavenumber is caused by a decrease in the electron density of the entire thiophene ring, implying that the ring of the adsorbed thiophene molecule is parallel to the surface of the adsorbent. In other words, thiophene is adsorbed onto Ce3+ in zeolite Y by π-electronic interaction. Ce3+ is a hard acid and prefers to combine with sulfur compounds by the direct S−M σ bond rather than by the π bond.38 In the study of organometallic complexes with thiophene ligands, the IR band of S−M bond interactions should be at about 1438 cm−1. While in this CeY/ sulfur compounds system, the IR band of S−M bond interactions is shifted to 1452 cm−1 which is close to the band of deformation vibration of saturated CH3. 3.3. Adsorption/Desulfurization Performance of CeY. Model fuel S-3 with sulfur concentration (300 μg/g) was used to study the desulfurization performance as a function of the number of the cerium ion exchanges (cf. Figure 8), the sulfur adsorption capacity, and the ion-exchange degree increase with the number of the ion exchanges. After two ion exchanges, the exchange degree was increased from 66.7% to 94.4%, and the

Table 3. Saturation Capacities, Adsorbed Volumes of the Sorbates, and POPVa in NaY, CeY at 29 °C saturation capacity (molecule/ unit cell)

a

adsorbed volume (mL/g)

POPV (%)

sorbate

NaY

CeY

NaY

CeY

NaY

CeY

thiophene benzene n-octane 1-octene

47 43 24 23

52 40 23 44

0.29 0.30 0.31 0.28

0.29 0.25 0.26 0.49

82 85 86 80

104 89 94 175

The percentage of occupation of pore volume in zeolites.

respectively. It can be noted that the POPV of all the sorbates on CeY has been increased compared with that on NaY. For benzene and octane, the POPV got close to the theoretical value, whereas, it is unbelievable that the POPV of thiophene and octene is 104 and 175, respectively, for CeY. Such abnormal phenomenon cannot simply be explained by pore filling. For the adsorption of thiophene, benzene, and octene on CeY, Yang32,33 and Song15,34,35 have proposed π-complexation and an S−M bond interaction mechanism according to molecular simulation results, but the π-complexation and S− M bond interaction mechanism cannot explain the high POPV of thiophene and octene. In order to make clear the adsorption and interaction mechanism of sulfur compounds over CeY, the sulfur compounds adsorbed over CeY were investigated by the FTIR method. The FT-IR spectra of different sulfur compounds adsorbed over CeY followed by degassing at 80 °C are shown in Figure 7. It is outstanding that the bands of 3180 cm−1, 3076

Figure 7. IR spectra of sulfur compound adsorption over CeY(−CeY, −TP, −2-MTP, −3-MTP, −THT, −BT, −DBT, −4-MDBT, and −4,6-DMDBT.

cm−1, and 1407 cm−1 are missing in all the spectra.36 The former two bands are ascribed to the stretching vibration of C− H in the fundamental thiophene ring, and the band of 1407 cm−1 is assigned to the stretching vibration of CC in the thiophene ring. The band at 3550 cm−1 is caused by the stretching vibration of OH in CeY, and this band becomes more intensified after the sulfur compounds have been adsorbed. Compared with the IR spectra of the parent CeY, three kinds of new bands at 2963, 2928, and 2859 cm−1, which can be ascribed to the stretching vibrations of saturated CH3 and CH2 radicals for all the sulfur compounds adsorbed over CeY, can be found. These findings indicate that some reactions

Figure 8. Variation of sulfur removal with the times (number) of ion exchanges in model fuel S-3. 25752

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capacity, suggesting that the interactions of sulfur compounds with CeY play an important role for the adsorption/ desulfurization. Combined with the breakthrough volume of sulfur compounds in Figure 9, it can be concluded that the interactions between sulfur compounds and CeY are the key factor for desulfurization. The pore fillings are accompanied by chemical reactions which inhibit the adsorption of sulfur compounds over CeY. With a very rich sulfur compound solution, essentially acid-catalyzed condensation of sulfur compounds occurs with production of dimers, trimers, and tetramers that inhibit the further adsorption of sulfur compounds. The extent of the chemical reactions that play an important role in the desulfurization of sulfur compounds depends on the topological structure of sulfur compounds and/ or the Brønsted acid sites of the zeolite. 3.4. Influence of Coadsorption of Aromatics/Olefins on the Desulfurization Performance of CeY. A certain amount of aromatics and olefins are present in commercial gasoline in addition to the organic sulfur compounds at the several hundred ppmw level. Also, because of the similarities of the structure and properties of organic sulfur compounds and those of aromatic and olefinic compounds, it is necessary to investigate the influence of aromatics and olefins on sulfur removal. The effect of the copresence of aromatics, olefins, and thiophene on sulfur removal using model fuel 1 is shown in Figure 10. When 1-hexene, cyclohexene, 1,5-hexadiene,

sulfur adsorption capacity was increased from 3.6 mg(S)/g to 4.1 mg(S)/g at static adsorption conditions (SAC), indicating that the ion-exchange degree of the cerium ion plays an important role in the desulfurization performance of CeY. The static adsorption studies on the desulfurization of model fuels indicated that the CeY exhibits good adsorption properties. Hence, this adsorbent was used to evaluate its desulfurization performance in the model fuels in a flow apparatus at ambient temperature and pressure with 2 h−1 liquid hourly space velocities (LHSV), and the results of breakthrough curves were collected using model fuels S3−S10 and are shown in Figure 9. It can be seen that the breakthrough

Figure 9. Breakthrough curves of sulfur compounds for desulfurization over CeY.

volume of sulfur compounds is in the order: THT > 4,6DMDBT > 4-MDBT > DBT > BT > 3-MTP ≈ 2-MT ≈ TP. It can be noted that desulfurization performance of CeY is not dependent on the molecular size of sulfur compounds, implying that the key factor for the selective adsorption/desulfurization of CeY is the sorbate−sorbent interaction mode rather than the steric hindrance of sulfur compounds. The breakthrough adsorption capacities and the ideal sulfur adsorption capacity (determined from the pore volume of CeY and the density of the sulfur compound) of sulfur compounds over CeY are shown in Table 4. It can be seen that the pore occupancy percentage of sulfur compounds over CeY is in the following order: 4,6-DMDBT > THT > 4-MDBT > DBT > BT > 3-MTP > 2-MT > TP. The sulfur adsorption capacity of sulfur compounds is much less than the ideal sulfur adsorption

Figure 10. Influence of aromatics and olefins on the adsorption/ desulfurization for CeY.

benzene, and o-xylene are each added at an amount of 3% into the model fuel 1, the adsorption/desulfurization performance is reduced accordingly, and the influence on the desulfurization is in the following order: 1,5-hexadiene > cyclohexene > 1-hexene ≈ o-xylene > benzene. These results indicate that olefins have more pronounced effects on the desulfurization performance of CeY than do aromatics. It has been well accepted that the competitive adsorption (via similar π-complexation or S−M bond formation) between sulfur compounds and aromatics or olefins plays a key role in the desulfurization performance of ion-exchanged FAU zeolite, whereas our results suggest that the influence of oligomerization and alkylation of sulfur-like sulfides along with that of olefins or aromatics induced by Brønsted acid sites is another key factor that cannot be ignored. In order to further evaluate the influence of aromatics and olefins on the desulfurization performance of CeY, the FT-IR spectra of the copresence of aromatics and thiophene and the copresence of olefins and thiophene with CeY were obtained

Table 4. Breakthrough Capacity of Sulfur Compounds over CeYa adsorbate

sulfur adsorption capacity (mg(S)/g)

ideal sulfur adsorption capacity (mg(S)/g)

pore occupancy (%)

TP 2-MTP 3-MTP THT BT DBT 4-MDBT 4,6-DMDBT

2.08 2.00 2.64 6.65 2.41 2.96 3.33 4.56

145.68 119.00 119.00 130.66 98.59 75.02 69.73 68.35

1.43 1.68 2.21 5.09 2.44 3.94 4.77 6.67

a

The breakthrough adsorption capacity of sulfur compounds was calculated from the breakthrough curves, considering the sulfur content at the breakthrough point was below 1 ppmw. 25753

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suggesting that only slight alkylation reactions occur for olefins. It also reasonably illustrates the reason why the n-octene adsorption isotherm on CeY deviates from the Langmuir equation. It can be noted that the intensities of the absorption bands at 2979, 2932, 2860, 1452, and 1398 cm−1 are strengthened after adding 1-octene to model 1. The higher the concentration of olefins, the stronger the intensity of the absorption bands, illustrating that olefins can effectively contribute to the alkylation reaction of thiophene. The results of the alkylation reaction of thiophene and olefins are the production of bulky molecules such as the dimer and trimer, which decrease further adsorption of sulfur compounds. The mechanism of thiophene alkylation catalyzed by Brønsted acid sites has been studied, and several mechanisms have been proposed.37,39,40 However, the mechanism of olefin alkylation and the contribution of olefins to the alkylation of sulfur compounds are not well-known, and more work in this direction is necessary. The decrease in the removal of sulfur compounds with the addition of benzene and o-xylene is related to the competition of the adsorption and diffusion processes between benzene, o-xylene, and the molecules of sulfur compounds for entering the pores and adsorbing onto the acidic sites. The nucleophilicity of sulfur compounds is higher than those of o-xylene and benzene, but the amount of o-xylene and benzene in the model fuel is much higher (3% vs 300 ppm, i.e. 100 times the mass content of o-xylene and benzene compared to that of sulfur content. The adsorption and diffusion behavior of benzene and thiophene will be discussed in detail in the following section. 3.5. Intracrystalline Mass Transfer of Benzene and Thiophene within NaY and CeY. The FR method has been found to be a very powerful and unique technique for determining the mass transport rates of multikinetic processes occurring in various sorbate/sorbent systems. From the shape and the pattern of the spectra, information about the kinetic mechanisms taking place in the system can be obtained, which cannot be achieved from other techniques. If the out-of-phase peak meets in-phase curve asymptotically at higher frequencies, it indicates that diffusion processes govern the mass transport. While the intersection of the in-phase curve and the out-ofphase peak in the FR spectra at half the step height and at the maximum of the out-of-phase peak indicates that sorption processes domain the mass transport. The FR spectra of thiophene and benzene adsorption over NaY and CeY at 100 °C are presented in Figure 13. From the FR spectra of benzene uptake on NaY and CeY at 100 °C, it can be seen that only a simple, single response can be observed, suggesting that only one kind of diffusion process occurs in benzene/FAU zeolite system, and the diffusion process is the rate-controlling step. The curve is fitted by single intracrystalline diffusion process model, and the diffusion coefficients of benzene on NaY and CeY are ∼1.17 × 10−11 m2 s−1 and 7.11 × 10−12 m2 s−1, respectively. Jörg Kärger et al.41 have systematically investigated the diffusion of aromatic molecules in NaX by microscopic pulsed field gradient (PFG) NMR method, and they revealed that the diffusivity of benzene molecule is ∼3 × 10−11 m2 s−1. On comparing the obtained results with the literature, we found that diffusivity of benzene on FAU zeolite in this work is slightly smaller than the PFG NMR data,41 but still much larger than the results obtained by macroscopic techniques of measurement (e.g., ZLC and sorption methods). Generally, the transport diffusivity measured by macroscopic technology is often larger than the self-diffusivity determined by

using FT-IR spectrometry (TM GX Perkin-Elmer). In Figure 11 the FT-IR spectra of adsorption and the copresence of

Figure 11. FT-IR of fresh CeY (blue) and CeY after adsorption of 3% (wt) benzene (red), 300 μg/g thiophene (green), and 300 μg/g thiophene + 3.0% (wt) benzene (black).

benzene and thiophene show no new IR bands for the adsorption of benzene over CeY, suggesting that the adsorption interaction between benzene and CeY is weak and almost all the benzene molecules can be removed when degassing at 80 °C. For the adsorption of thiophene and the coadsorption of thiophene and benzene, however, three new bands appear at 2979, 1452, and 1398 cm−1, which are ascribed to the stretching vibration of the saturated CH3 and CH2 radicals, and the bands of deformation vibrations of saturated CH3 and of saturated CH2 respectively, suggesting that thiophene oligomerization or alkylation reaction occurs. The intensity of the IR band of the coadsorption of thiophene and benzene shows very little change compared with that of thiophene, implying that benzene cannot contribute to the thiophene oligomerization or the alkylation reaction. The FT-IR spectra of adsorption of olefins and copresence of olefins with thiophene are shown in Figure 12. It can be seen

Figure 12. FT-IR of fresh CeY (a) and CeY after adsorption of 3% 1hexene (b), 3% 1,5-hexadiene (c), 3% cyclohexene (d), 500 μg·g−1 thiophene (e), 500 μg·g−1 1-octene + 500 μg·g−1 thiophene (f), 5% 1octene + 500 μg·g−1 thiophene (g), and 15% 1-octene + 300 μg·g−1 thiophene (h).

that new bands at 2963, 2860, 1454, and 1376 cm−1 appeared for the adsorption of 3% 1-hexene, 3% 1,5-hexadiene, and 3% cyclohexene, which implies that olefins cannot be removed from CeY by degassing at 80 °C. It suggests that the interaction force between CeY and olefins is stronger and the adsorption mechanism of olefins is different from that of benzene. The intensities of the the FT-IR bands of olefins at 2963, 2860, 1454, 1376 cm−1 are relatively weaker than that of thiophene, 25754

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thiophene/CeY system, implying that two sorption processes exist in this system and also that the sorption process is the rate-controlling step. Thiophene molecules can act either as an n-type donor by donating the latter’s lone pairs of electrons to CeY by forming an S−M bond or as a π-type donor by sharing the delocalized π electrons of the thiophenic ring with metal ions to form a π-complexation. In the thiophene/CeY system, PI is denoted as an S−M interaction, and PII is denoted as πcomplexation. From Table 5 it can be seen that the time constant (k−1) of strong sorption PI (S−M bond) is about dozens of times shorter than k−2 of the weak sorption process PII (π complexation), and the response intensity of PI (K1) is larger than that of PII (K2), implying that thiophene is first adsorbed by forming π-complexation and then most of thiophene molecules interact with CeY by S−M bond formation.

Figure 13. FR spectra of benzene (a) and thiophene (b) adsorption in NaY and CeY at 373 K. Symbols □ and ○ represent respectively the experimentally determined in- and out-of-phase characteristic functions, and lines are the fits of theoretical models.

4. CONCLUSIONS In the present work, a systematic investigation of the influences of competitive adsorption, intracrystalline mass transfer, sorbate−sorbent interaction modes, alkylation and oligomerization of sulfur compounds on the desulfurization performance of Ce ion-exchanged Y zeolite was performed, through which the importance of Brønsted acid sites to the desulfurization performance of CeY was elaborated. CeY with a high degree of ion exchange was prepared by liquid-phase metal ion exchange. The TEM, ICP, and BET results confirmed that Ce ions are well dispersed in the channels of the zeolite, and no cerium aggregation and no dealumination occurred. The XPS analyses show that the exchanged cerium ion in CeY remains trivalent when the calcination temperature is lower than 800 °C. The equilibrium adsorptions of thiophene, benzene, n-octane, and 1-octene were performed on NaY and CeY at 29 °C. The deviation of the isotherms of 1-octene and thiophene on CeY from the Langmuir equation can be explained by the oligomerization and alkylation catalyzed by Brønsted acid sites. The adsorption/desulfurization performance of CeY on sulfur compounds is in the following order: THT > 4,6-DMDBT > 4-MDBT > DBT > BT > 3-MTP ≈ 2-MT ≈ TP, and the pore occupancy percent is in the order of 4,6-DMDBT > THT > 4MDBT > DBT > BT > 3-MTP > 2-MT > TP. We found that the acid-catalyzed condensation reaction, which produces dimers, trimers, and tetramers, inhibits further adsorption of sulfur compounds. The oligomerization and alkylation reactions, which can destroy the conjugated system of the thiophene ring, were monitored by FTIR analysis. The extent of oligomerization and alkylation is in the order: TP > 2-MTP ≈ 3-MTP > BT > DBT> 4-MDBT > 4,6-DMDBT > THT, indicating that THT, DBT, BT, and their alkyl-substituted derivatives are more stable than TP and its alkyl-substituted derivatives. Meanwhile, a slight alkylation reaction can also occur for the adsorption of olefins on CeY. Such oligomerization and alkylation intensified with the coadsorption of olefins and thiophene, illustrating that olefins can effectively contribute to the oligomerization and alkylation reaction of thiophene and can further decrease the desulfurization performance of CeY, while the adsorption mechanisms for benzene and o-xylene are only van der Waals interaction and π-complexation. The coadsorption of aromatics has almost no influence on the thiophene alkylation reaction. The decrease in the removal of sulfur compounds at the addition of benzene and o-xylene is related to the competition of the adsorption and diffusion

microscopic methods because it is difficult for the macroscopic technology to eliminate the effects of structural defects and surface resistance. In this contribution, the diffusivity of benzene on NaY thus measured is very close to the PFG; NMR data also suggest that FR technology is powerful for providing a clearer discrimination between different mass transfer resistances (internal diffusion/surface barrier). It also can be noted that the lower diffusivity of benzene molecules on CeY compared with that on NaY is due to the inhibitory effect of introduced cerium ion. The diffusion process of benzene is relatively very slow because the molecular size of benzene is close to the pore dimension of Y zeolite and the movement of benzene molecules is intracage motion and cage-to-cage migration. For the thiophene uptake over NaY and CeY, two different sorption processes are detected in the FR spectra, implying that the sorption process is the rate-controlling step in this system. The low-frequency peak and high-frequency peak are denoted by process I (PI) and process II (PII), respectively. The interactions between thiophene and NaY include π-complexation and van der Waals forces, which correspond to the PI and PII adsorption processes, respectively. The relaxation times of PI and PII can be derived from their time constants. The dynamics parameters of thiophene uptake by NaY and CeY are listed in Table 5. For the thiophene/NaY system, the time constant of PII (k−2) is about dozens of times larger than that of PI (k−1), and the response intensity of PII (K2) is larger than that of PI (K1), suggesting that van der Waals forces (PII) are the main sorption process. Since the FR spectra of the thiophene/NaY and the thiophene/CeY systems are similar, two uptake processes also appear in the FR spectra of the Table 5. Dynamics parameters of TP Uptake over NaY and CeY at 100 °C sample

Pe (torr)

k−1 (s−1)

k−2 (s−1)

K1

K2

NaY

0.2 0.4 0.6 0.8 0.1 0.2 0.3

0.71 0.56 1.06 0.93 0.863 0.441 0.372

19.05 30.75 50.36 101.94 13.451 18.722 27.205

0.58 0.18 0.09 0.06 1.77 0.99 0.76

0.90 0.52 0.35 0.27 1.17 0.70 0.62

CeY

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dx.doi.org/10.1021/jp303040m | J. Phys. Chem. C 2012, 116, 25748−25756

The Journal of Physical Chemistry C

Article

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processes for entering the pores and adsorbing on the acidic sites. According to the FR results, the diffusion process is the ratecontrolling step for benzene, and the sorption process is the rate-controlling step for thiophene. The above results indicate that thiophene molecules can be preferentially adsorbed on the Lewis acid site and act either as an n-type donor by donating the latter’s lone pairs of electrons to CeY by forming a S−M bond or as a π-type donor by sharing delocalized π electrons of the thiophenic ring with metal ions to form a π complex, but the S−M bond interactions play a more dominant role than πcomplexation in the CeY/sulfur compounds system.

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AUTHOR INFORMATION

Corresponding Author

*No.1, West Dandong Road, Wanghua District, Fushun 113001, Liaoning Province, China. Telephone:+86 024 56860658. Fax: +86 024 56860658. E-mail address: lsong56@ 263.net (L.Song). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support by the National Natural Science Foundation of China (Grant Nos. 21076100, 20776064, and 20976077) and PetroChina Company Limited (Grant No. 10-01A-01-01-01).

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