Article pubs.acs.org/IECR
Effects of Si/Al Ratio on Adsorptive Removal of Thiophene and Benzothiophene over Ion-Exchanged AgCeY Zeolites Hua Song,*,†,‡ Huijie Gao,† Hualin Song,*,§ Gang Yang,† and Xiaojuan Li† †
College of Chemistry & Chemical Engineering and ‡Provincial Key Laboratory of Oil & Gas Chemical Technology, Northeast Petroleum University, Daqing 163318, Heilongjing, China § Key Laboratory of Cancer Prevention and Treatment of Heilongjiang Province, Mudanjiang Medical University, Mudanjiang 157011, Heilongjing, China ABSTRACT: AgCeY-n zeolites with different Si/Al ratios n (n = 3.0, 4.8, and 5.3) were successfully prepared and characterized by XRD, BET analysis, SEM, IR spectroscopy, XPS, FTIR spectroscopy, and ICP analysis. The effects of the Si/Al ratio on the adsorptive desulfurization properties of the adsorbents were evaluated. The results showed that AgCeY-5.3 exhibited the best sulfur removal capacity and selectivity. The capacity for sulfur removal onto AgCeY-n decreased in the order AgCeY-5.3 > AgCeY-4.8 > AgCeY-3.0 for a model oil comprising thiophene (TP) and benzothiophene (BT), along with certain amounts of toluene or cyclohexene. The adsorption affinity of the sulfur compounds was in the order BT > TP. In addition, the inhibition effects on the AgCeY-n zeolites for sulfur removal were in the order cyclohexene > toluene. The removal of organic sulfur compounds onto AgCeY-n was mainly by π-electronic and direct coordination (S−M) interaction.
1. INTRODUCTION With growing awareness of the need to protect the environment and the great need to make ultralow-sulfur fuels for use in fuel cells and other applications, improving desulfurization technology has become increasingly important.1 Conventional hydrodesulfurization (HDS) is highly effective for the removal of thiols, sulfides, and disulfides; however, it is difficult to remove thiophene and its derivatives. Furthermore, HDS, which causes a loss of octane, is performed under harsh conditions.2 To avoid these problems, researchers have explored many new desulfurization technologies in recent years.3−5 Adsorptive desulfurization (ADS) shows some advantages, such as being able to obtain zero-sulfur fuels, operate under ambient conditions, and retain a high octane value. Therefore, ADS has received much attention for deep desulfurization.6,7 Y zeolites, such as AgY, CuY, and NiY, exhibit high sulfur adsorption capacities for thiophenic (TP) compounds, including 4,6-dimethyldibenzothiophene (4,6-DMDBT). Among all ion-exchanged Y zeolites, AgY shows the highest sulfur adsorption capacity under the same test conditions.8 Oliveira et al.9 prepared transition-metal- (5 wt % Ni-, Zn-, and Ag-) exchanged NaY zeolites and found that the adsorption capacities for TP at 30 °C were in the order AgY> NiY > ZnY > NaY. However, the sulfur adsorption capacities decreased rapidly when aromatic compounds were present in the fuel. It is © 2016 American Chemical Society
well-known that Ce-exchanged zeolites retain high selectivities to sulfur compounds in model oils containing toluene, olefins, and so on.8 Shi et al.10 prepared hierarchically structured CeY and found a gradual decrease in sulfur removal capacity on Ce(IV)Y in a model oil with toluene. Sato et al.11 studied the effects of the Si/Al ratio (2.4, 2.8, and 4.1) of the original NaY zeolites on the structure of NaY. Among the three zeolites tested, the zeolite with a Si/Al ratio of 2.8 showed the highest degrees of ion exchange, dealumination, and mesopore formation. These results confirmed that the Si/Al ratio of zeolites has a significant impact on structure. In a previous work,8 we prepared AgY, CeY, and AgCeY zeolites by liquid-phase ion exchange. The results showed that the AgCeY zeolite exhibited not only a high desulfurization capacity similar to that of AgY but also a high selectivity similar to that of CeY. The adsorption capacities for TP and benzothiophene (BT) in model oils containing toluene, cyclohexene, and pyridine were found to follow the order AgCeY > CeY > AgY. In the present work, bimetal ionexchanged AgCeY zeolites with different Si/Al ratios were successfully prepared, and the effects of the Si/Al ratio on the Received: Revised: Accepted: Published: 3813
December March 12, March 17, March 17,
2, 2015 2016 2016 2016 DOI: 10.1021/acs.iecr.5b04609 Ind. Eng. Chem. Res. 2016, 55, 3813−3822
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Industrial & Engineering Chemistry Research
analyzer. All samples were outgassed at 473 K until the vacuum pressure was 0.8 kPa. The adsorption isotherms for nitrogen were measured at 77 K. Pyrolysis infrared (Py-IR) spectra were recorded on a PE Spectrum GX FTIR spectrometer with a resolution of 8 cm−1. The samples (15 mg) were pressed into self-supporting wafers (diameter = 15 mm) and placed into an IR cell with CaF2 windows. The cell was then purged with flowing N2 at 573 K for 1 h. After the sample had cooled to room temperature, it was exposed to pyridine vapor. The IR spectra of the adsorbed pyridine were recorded after the sample had been degassed at 423 and 623 K. X-ray photoelectron spectroscopy (XPS) spectra were obtained using an ESCALAB MKII spectrometer equipped with a hemispherical analyzer. XPS measurements were performed using monochromatic Mg Kα radiation (E = 1253.6 eV) and a fixed analyzer pass energy of 40 eV. The recorded photoelectron binding energies were referenced to the C 1s contamination line at 284.8 eV. Fourier transform infrared (FTIR) spectoscopy of the catalysts was performed using a Brook Tensor Fu Liye 27 infrared spectrometer over a scanning range of 400−4000 cm−1 on KBr tablet. The surface morphological details of adsorbents were studied by scanning electron microscopy (SEM, JSM-5600LV). The chemical compositions of the ion-exchanged zeolites were determined by inductively coupled plasma (ICP) elemental analysis using a high-resolution magnetic sector ICP−MS spectrometer (Iris Advantage 1000) after the samples had been dissolved in HF acid. 2.5. Adsorption Desulfurization Experiments. 2.5.1. Static Adsorption Desulfurization Experiments. For each experimental run, 0.2 g of AgCeY was added to a conical flask containing 40 mL of model oil, which was then placed in a thermostatted water bath at 323 K and held for 1 h under magnetic stirring. The reactant was centrifuged, and the oil was analyzed for its residual sulfur concentration with a gas chromatograph equipped with a flame photometric detector (Shimadzu FPD-GC-14C) and a capillary column (PH-1, 60 m × 0.25 mm). 2.5.2. Dynamic Adsorption Desulfurization Experiments. A custom-made vertical stainless steel tube with an internal diameter of 10 mm and a length of 200 mm was used to perform dynamic adsorption desulfurization experiments. The apparatus consisted of a low-flow liquid pump, feed tanks, and a heating element. In all experiments, the adsorbent quantity was 1 g. Adsorptive desulfurization experiments were carried out at a feed flow rate of 20 mL·h−1 and a temperature of 323 K. Effluents were collected every 20 min until saturation was achieved and then analyzed for residual sulfur concentration. The breakthrough concentration was defined as a S concentration in the effluent of 20 mg·L−1.
performance of the AgCeY zeolites in terms of adsorption desulfurization were studied with model oils containing toluene and cyclohexene.
2. EXPERIMENTAL SECTION 2.1. Materials. Thiophene (TP, 99%) and benzothiophene (BT, 98%) were purchased from J&K Chemical Ltd. (Shanghai, China). NaY zeolites (Si/Al = 3.0, 4.8, and 5.3) in powder form were purchased from the catalyst factory of Nankai University. AgNO3·3H2O, Ce(NO3)3·6H2O, toluene, and cyclohexene were obtained as commercial analytical-grade reagents without further purification. 2.2. Model Oil Sample. A model oil with a total sulfur concentration of 200 mg·L−1 was prepared by adding TP and BT to 1-octane solvent and is denoted as model oil M1. To investigate the effects of aromatics and olefins on selective adsorptive desulfurization over AgCeY zeolites, two additional model oils were prepared by adding certain amounts of toluene and cyclohexene together with TP and BT to 1-octane solvent. The compositions of the model oils are summarized in Table 1. Table 1. Compositions of Model Oils Used sulfur concentration (mg·L−1) model oil
TP
BT
toluene (mg·L−1)
cyclohexene (mg·L−1)
M1 M2 M3
100 100 100
100 100 100
− 500 −
− − 500
2.3. Adsorbent Preparation and Regeneration. In this work, the sorbents were prepared using the method of liquidphase ion exchange at room temperature. NaY zeolites with different Si/Al ratios (denoted as NaY-n, where n is the Si/Al molar ratio) were used as starting materials. First, ion exchange of NaY-n was performed with 0.1 mol·L−1 AgNO3 aqueous solution for 24 h at room temperature. After ion exchange, the zeolites were filtered, washed thoroughly with deionized water, and then dried at 383 K for 12 h. Subsequently, the samples were calcined at 773 K for 4 h in an air atmosphere, and AgY-n zeolites adsorbents were obtained. Second, ion exchange of AgY-n was performed with 0.25 mol·L−1 CeNO3 aqueous solution for 24 h at room temperature. After ion exchange, the zeolites were filtered, washed thoroughly with deionized water, and then dried at 383 K for 12 h. These samples were also calcined at 773 K for 4 h in an air atmosphere, and the obtained zeolites with different Si/Al ratios are denoted as AgCeY-n (n = 3.0, 4.8, and 5.3). The regeneration of the spent adsorbents was performed as follows: The spent adsorbents were washed thoroughly with anhydrous ethanol three times and then dried at 383 K for 12 h. These samples were calcined at 823 K for 4 h in an air atmosphere. 2.4. Adsorbent Characterization. X-ray diffraction (XRD) analysis of the samples was carried out on a D/max2200PC-X-ray diffractometer using Cu Kα radiation under the conditions of 40 kV, 30 mA, and a scan range from 10° to 80° at a rate of 10°·min−1. The typical physicochemical properties of the supports and catalysts were analyzed by N2-adsorption specific surface area measurements using the Brunauer−Emmett−Teller (BET) method using a Micromeritics NOVA 2000e physisorption
3. CHARACTERIZATION OF ADSORBENT 3.1. X-ray Diffraction (XRD) Analysis. The X-ray diffraction (XRD) patterns of NaY-n and AgCeY-n are shown in Figure 1. The similarity of the AgCeY-n zeolites to the original NaY zeolites in the XRD patterns, especially in the 2θ range of 20−35°, indicates that the original zeolite structure was retained.12 Also, the XRD patterns of AgCeY-n with different Si/Al ratios clearly show that all of the samples had similar structures. No new diffraction peaks belonging to Ag- or Ce-related compounds or to the pure metals were observed in 3814
DOI: 10.1021/acs.iecr.5b04609 Ind. Eng. Chem. Res. 2016, 55, 3813−3822
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3.3. Pyrolysis Infrared (Py-IR) Analysis. The nature and relative amounts of Brønsted and Lewis acid sites of the sorbents in this study were characterized by Py-IR spectra of pyridine adsorption at 423 and 623 K. The Lewis and Brønsted acidity values were calculated according to the method of Emeis.14 Figure 2 shows the Py-IR spectra of adsorbed pyridine
Figure 1. XRD patterns of metal-ion-exchanged Y zeolites.
the patterns of the AgCeY-n zeolites, which suggests that very little or no crystalline AgOx, CeOx, or pure metal formed in the AgCeY-n zeolites. As compared to those of the parent NaY zeolite, the intensities of the diffraction peaks of the AgCeY-n zeolites decreased significantly, indicating a decrease in crystallinity after ion exchange and calcination. The adsorbents’ color changed from white to yellow after calcination at 823 K in air, indicating that Ce3+ had been oxidized to Ce4+.13 3.2. Brunauer−Emmett−Teller (BET) Analysis. The textural parameters of NaY-n and AgCeY-n adsorbents obtained from Brunauer−Emmett−Teller (BET) analysis are reported in Table 2. All three NaY zeolites showed similar micropore surface areas of 572−594 m2/g and micropore volumes of approximately 0.31 cm3/g. The mesopore surface area and volume of the NaY-5.3 zeolite were slightly higher than those of the NaY-3.0 and NaY-4.8 zeolites. It is clear that the mesopore size was centered at 10 nm, so the presence of more mesopores in NaY-5.3 increased its pore size. NaY-3.0 had a slightly higher BET surface area than the other zeolites, but its pore volume and pore radius were lower than those of the NaY-4.8 and NaY5.3 zeolites. Sato et al.11 studied the textural properties of NaY zeolites with different Si/Al ratios (2.4, 2.8, and 4.1) and found that the NaY zeolites had similar micropore surface areas and micropore volumes. The surface areas and pore radii of the NaY-n zeolites clearly decreased after ion exchange, showing that the decrease in crystallinity occurred during ion exchange and calcination, which is consistent with the XRD analysis. Meanwhile, a higher mesoporous surface area and volume (especially for AgCeY-5.3, with a mesoporous surface area of 64 m2·g−1 and a mesoporous volume of 0.06 cm3·g−1) can be observed in the modified Y adsorbents. The pore radii of AgCeY-n showed trends similar to those of the starting parent NaY, which increased with increasing Si/Al ratio.
Figure 2. Py-IR pyridine thermodesorption spectra of AgCeY-n zeolites (423 K).
on AgCeY-n at 423 K. The band at 1490 cm−1 can be assigned to the contributions of both Lewis and Brønsted acid sites. The bands at 1454 and 1607 cm−1 are characteristic of Lewis acid sites, whereas those at 1543 and 1630 cm−1 are characteristic of Brønsted acid sites.15 Table 3 summarizes the Lewis and Brønsted acidities of each adsorbent at desorption temperatures of 423 and 623 K. It is generally believed that NaY is a nonacidic zeolite, but acidity could be generated by the liquid-phase ion exchange of NaY with metal ions.10 It is known that the Ag ions mainly provide Lewis acid sites,8 whereas Ce ions provide more Brønsted acid sites than Lewis acid sites.8,10,16 Obviously, Lewis acid sites were formed because Ag+ and Ce4+ were present as electron acceptors and could form OAg and OCe structures on NaY zeolite.17 Brønsted acid sites in rare-earth Ce-exchanged zeolites are generated by the hydrolysis of Ce cations in site I′ in the sodalite and supercages.18 Therefore, the amounts of Brønsted acid sites in AgCeY-n associated with hydroxyl bridges point to supercages.19 For all of the samples, the total number of acid sites in AgCeY-n at a desorption temperature of 423 K was much higher than that at a desorption temperature of 623 K, indicating that weak acidic sites were predominant.19 The Lewis acid sites for the samples decreased in the order AgCeY-5.3 > AgCeY-4.8 > AgCeY-3.0, showing that the number of Lewis acid sites increased with
Table 2. Textural Properties of NaY-n and AgCeY-n Zeolites surface area (m2·g−1) sample NaY-n
AgCeY-n
a
pore volume (cm3·g−1)
metal contenta(wt %)
Si/Al ratio
SBET
Smicro
Smeso
Vtotal
Vmicro
Vmeso
pore size (nm)
Ag
Ce
3.0 4.8 5.3 3.0 4.8 5.3
631 612 620 549 553 541
594 572 579 507 513 477
37 40 41 42 40 64
0.35 0.34 0.36 0.30 0.31 0.31
0.32 0.30 0.31 0.26 0.27 0.25
0.03 0.04 0.05 0.04 0.04 0.06
1.11 1.13 1.17 1.11 1.12 1.16
− − − 0.72 0.85 0.86
− − − 1.13 1.43 1.46
Data obtained by ICP analysis. 3815
DOI: 10.1021/acs.iecr.5b04609 Ind. Eng. Chem. Res. 2016, 55, 3813−3822
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Industrial & Engineering Chemistry Research Table 3. Distribution of Brønsted and Lewis Acidities in AgCeY-n Zeolites sample
desorption temperature (K)
Brønsted acidity (μmol·g−1)
Lewis acidity (μmol·g−1)
total acidity (μmol·g−1)
AgCeY-3.0
423 623 423 623 423 623
129.86 92.22 144.23 87.07 114.86 81.30
198.67 5.97 203.92 19.16 294.41 45.88
328.53 98.19 348.16 106.23 409.27 127.19
AgCeY-4.8 AgCeY-5.3
maximum (fwhm) was 1.9 eV (>1.8 eV), which originated from the Ag+, indicating that silver was present as Ag+ on the surface of the adsorbent AgCeY-n.15 The Ce 3d5/2 and Ce 3d3/2 spectra of AgCeY-n are shown in Figure 3b. Shyu et al.21 reported that the binding energies of 916.6, 906.3, 901.2, 897.8, 888.5, and 882.8 eV can be attributed to Ce4+ species of CeO2 and the binding energies of 903.7, 899.2, 885.8, and 880.8 eV can be attributed to Ce3+ species of Ce2O3. The pattern of the XPS spectra for AgCeY-n showed two broad regions of overlapping contributions: The one located at 882.9 eV can be associated with 3d5/2 emission, whereas that located at 905.7 eV can be associated with 3d3/2 emission. This implies that the Ce species were present as Ce4+ in AgCeY-n. Analysis of the Ce 3d XPS spectra also indicated that the ion-exchanged Ce3+ ions impregnated into the zeolite were oxidized to Ce4+ through calcination.22 In addition, the peaks area for Ag+ and Ce4+ of AgCeY-5.3 in the Ag 3d and Ce 3d spectra were both the highest among the AgCeY-n zeolites investigated. This further confirms that the surface contents of Ag+ and Ce4+ were higher for AgCeY-5.3 than for AgCeY-3.0 and AgCeY-4.8, in agreement with the ICP results (Table 2). 3.5. Fourier Transform Infrared (FTIR) Analysis. Panels a−d of Figure 4 show Fourier transform infrared (FTIR) spectra of the AgCeY-n zeolites before adsorption of model oil and after saturation adsorption of model oils M1, M2, and M3, respectively. The FTIR spectra after the adsorption of M1 are shown in Figure 4b. As compared to Figure 4a, four new bands are observed for AgCeY-n in Figure 4b. Shi et al.3,23 studied the effects of toluene on thiophene adsorption over NaY and Ce(IV)Y zeolites. They reported that the two minor bands at 2944 and 2859 cm−1 can be ascribed to αCH of saturated CH2 groups close to either the double bond (*CH2CHCH) or the sulfur atom (*CH2S). In Figure 4b, the bands at 2941 and 2860 cm−1 can also be assigned to the CH stretching vibration of saturated CH2 groups. The bands at 1402 and 1461 cm−1 are correlated to the interactions between TP and AgCeY-n and between BT and AgCeY-n, respectively. The band at 1402 cm−1, which is approximately 7 cm−1 lower than that for the symmetric stretching vibration of CC in the fundamental rings of TP and BT, can be assigned to the perturbed symmetric stretching vibration of the CC bond.12 The shift of the band of the symmetric stretching vibration of CC to lower wavenumbers was caused by a decrease in the electron density of the entire thiophene ring, indicating that the rings of the adsorbed TP and BT molecules were parallel to the surface of the adsorbent.10 This phenomenon can be explained by the adsorption of some TP and BT molecules onto AgCeY-n through π-electronic interactions. The band at 1461 cm−1 can be assigned to ν(CC)sym, which shifted to higher frequencies as a result of the increased electron density within the CC CC fragment when TP and BT were coordinated with AgCeY-n through the S atom. This confirmed the existence of a
increasing Si/Al ratio. The weak Brønsted acid sites at a desorption temperature of 423 K increased initially and then decreased with increasing Si/Al ratio. However, the strong Brønsted acid sites at a desorption temperature of 623 K decreased with increasing Si/Al ratio. It has been reported that hydroxyl ions associated with aluminum decrease significantly with increasing Si/Al ratio. Therefore, the Lewis acid sites increased with increasing Si/Al ratio. Another reason might have been the presence of nonframework aluminum. The Lewis acidities of zeolites with higher Si/Al ratios increased slightly for these two reasons.20 AgCeY-5.3 exhibited the highest Lewis acidity but the lowest Brønsted acidity as compared to AgCeY4.8 and AgCeY-3.0. 3.4. X-ray Photoelectron Spectroscopy (XPS). The AgCeY-n samples were studied by X-ray photoelectron spectroscopy (XPS), and the Ag 3d spectra are shown in Figure 3a. The XPS patterns of the Ag 3d spectra for AgCeY-n showed that the binding energy (BE) of Ag 3d5/2 was approximately 368.9 eV and its width at full width at half-
Figure 3. XPS spectra of the (a) Ag 3d and (b) Ce 3d regions on the surfaces of AgCeY-3.0, AgCeY-4.8, and AgCeY-5.3 zeolites. 3816
DOI: 10.1021/acs.iecr.5b04609 Ind. Eng. Chem. Res. 2016, 55, 3813−3822
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Figure 4. FTIR spectra of AgCeY-n adsorbing different types of model oil at the breakthrough point: (a) before absorption of model oil and (b−d) after absorption of model oils (b) M1, (c) M2, and (d) M3.
characteristic peaks indicate that cyclohexene was adsorbed onto the AgCeY-n zeolites through π-electronic interactions. 3.6. Scanning Electron Microscopy (SEM). Panels a and b of Figure 5 show scanning electron microscopy (SEM)
direct interaction between the S atoms of TP and BT and AgCeY-n through sulfur−metal (S−M) interactions. Therefore, there were two adsorption modes between TP and BT and AgCeY-n: TP and BT interacting with AgCeY-n through direct sulfur−metal (S−M) interactions and TP and BT adsorbing on the AgCeY-n zeolites through π-electronic interactions. The bands at 1640 cm−1 can be assigned to the hydroxide radical vibrations of water adsorbed in the zeolites. The FTIR spectra after the adsorption of M2, which contained toluene, are shown in Figure 4c. As compared to the FTIR spectra after absorption of M1 in Figure 4b, a new band can be seen at 1492 cm−1. This band, which shifted to lower frequencies compared to that of gaseous toluene at 1498 cm−1, can be attributed to the vibration band of the ring skeleton of toluene adsorbed on AgCeY-n adsorbents.10 This can be interpreted as an effect between the π electrons on the ring skeleton of toluene and the cations of the zeolite, which confirms that toluene was adsorbed onto the AgCeY-n zeolites by π-electronic interactions. Moreover, the disappearance of the bands at 2860 and 2934 cm−1 showed that the presence of toluene can significantly affect the opening of the thiophenic ring during adsorption processes. The FTIR spectra after the adsorption of M3, which contained cyclohexene, are shown in Figure 4d. The two bands at 2930 and 2856 cm−1 can be ascribed to ν(CH3)sym and ν(CH2)asym, respectively. The band at 1473 cm−1 should be ascribed to plane-rocking vibrations of the strong absorption peak originating from the methyl-saturated bond.12 The three
Figure 5. SEM images of (a) NaY-5.3 and (b) AgCeY-5.3 zeolites.
images of the morphologies of zeolite particles of NaY-5.3 and AgCeY-5.3, respectively, which were obtained at a magnification of 50000×. A comparison of the NaY-5.3 and AgCeY-5.3 images shows no significant difference between the two samples. This indicates that the morphology of zeolite particles of NaY-5.3 remained unchanged after ion exchange. The NaY5.3 and AgCeY-5.3 adsorbents showed a homogeneous polygonal structure approximately 0.2 μm in thickness and with an average particle diameter of 1.0 μm. 3.7. Inductively Coupled Plasma (ICP) Analysis. The chemical compositions of NaY and ion-exchanged zeolites are also summarized in Table 2. The metal content of Ag+ or Ce4+ on the AgCeY-n decreased in the order AgCeY-5.3 > AgCeY3817
DOI: 10.1021/acs.iecr.5b04609 Ind. Eng. Chem. Res. 2016, 55, 3813−3822
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TP sulfur removals by the AgCeY-n adsorbents with model oil M1 decreased in the order AgCeY-5.3 > AgCeY-4.8 > AgCeY3.0. AgCeY-n adsorbents with different Si/Al ratios provided a better desulfurization ratio for model oil M1 without any other competitive component. Figure 6b shows the static adsorption desulfurization results of AgCeY-n adsorbents with model oil M2. The BT removal of AgCeY-5.3 was 90.8%, and that of AgCeY-3.0 was a minimum of 83.4%, indicating that the influence of toluene on BT desulfurization on AgCeY-n was not serious. However, there was a noticeable decrease of TB sulfur removal on AgCeY-n as compared to model oil M1 without toluene. Specifically, the TP selective adsorption desulfurization on the AgCeY-n adsorbents obviously decreased because of the presence of toluene. The TP removal of AgCeY-5.3 was 63.8%, and that of AgCeY-3.0 was a minimum of 53.4%. The TP and BT removals by AgCeYn adsorbents with model oil M2 decreased in the order AgCeY5.3 > AgCeY-4.8 > AgCeY-3.0. This indicates that the selectivity to TP for model oil M2, which contained toluene, decreased with decreasing Si/Al ratio. The presence of olefins in fuels would strongly decrease the adsorption of sulfur compounds. The competitive adsorption of cyclohexene with TP and BT on AgCeY-n was investigated with model oil M3, which contained cyclohexene (Figure 6c). The BT removal by AgCeY-5.3 was 87.6%, and that by AgCeY-3.0 was a minimum of 79.5%, so the influence of cyclohexene was higher than the influence of toluene on AgCeY-n. Because of the influence of cyclohexene, the removal of TP by the AgCeYn adsorbents decreased more than the removal of BT. The TP removal by AgCeY-5.3 was 56.7%, whereas the TP removal by AgCeY-3.0 was the lowest, 48.6%. The decrease in TP removal by AgCeY-n adsorbents with model oil M3 was in the order AgCeY-5.3 > AgCeY-4.8 > AgCeY-3.0. This indicates that the selectivity for TP in model oil M3, which contained cyclohexene, decreased with decreasing Si/Al ratio. 4.2. Dynamic Adsorption Desulfurization Experiments. 4.2.1. Effects of Si/Al Ratio on Breakthrough and Saturation Loadings with M1. Figure 7 shows the breakthrough results for the AgCeY-n adsorbents with model oil M1 (100 mg·L−1 sulfur in the form of TP and BT). The breakthrough capacity for BT removal was higher than that for TP removal, which is consistent with the above results. Zhou et al.26 studied the effects of 4,6-DMDBT, dibenzothiophene (DBT), TP, BT, and benzene on desulfurization over AgY, and they found that the selectivity of adsorption followed the order BT > TP > DBT > 4,6-DMDBT > benzene, which is consistent with the bond order obtained using the method of natural bond orbital (NBO) analysis. This implies that AgY zeolite is promising for the removal of TP derivatives because of their π-complexation interactions with the adsorbent and their geometric structures. Moreover, it was reported that sulfur compounds were adsorbed over CeY zeolites through a direct sulfur−adsorbent (S−M) interaction and that the adsorption selectivity decreased in the order 5-MBT > BT > 2-MBT, which is consistent with decreasing electron density on the sulfur atoms.27 Thus, based on these two points, one can safely conclude that both AgY and CeY have higher selectivities to BT than to TP. Consequently, AgCeY-n zeolites will inevitably show higher selectivities to BT than TP. The breakthrough loadings for AgCeY-n are summarized in Table 4. For AgCeY5.3, the breakthrough loadings were 0.45 wt % for TP and 1.07 wt % for BT, values that are higher than those of AgCeY-3.0 and AgCeY-4.8. The saturation loadings were 0.48 wt % for TP
4.8 > AgCeY-3.0, showing that increasing the Si/Al ratio beneficial for increasing the metal content. It is known that Ag+ mainly interacts with the S atoms of TP and BT through πcomplexation,24 whereas Ce4+ mainly interacts with the S atoms of TP and BT through direct sulfur−metal (S−M) coordination.25The higher the contents of Ag+ and Ce4+ on the adsorbent would lead to a better adsorptive sulfur removal.
4. ADSORPTIVE DESULFURIZATION RESULTS AND DISCUSSION 4.1. Static Adsorptive Desulfurization Experiments. Figure 6a shows the static adsorption desulfurization results of AgCeY-n adsorbents with model oil M1. The BT removals were above 93% for all samples. The TP removal by AgCeY-5.3 was 78.8%, and that by AgCeY-3.0 was a minimum of 71.1%. The
Figure 6. Sulfur removals of AgCeY-n zeolites with model oils (a) M1, (b) M2, and (c) M3. 3818
DOI: 10.1021/acs.iecr.5b04609 Ind. Eng. Chem. Res. 2016, 55, 3813−3822
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Industrial & Engineering Chemistry Research
and 1.09 wt % for BT, which are also higher than those of AgCeY-3.0 and AgCeY-4.8. Obviously, the adsorption capacities of AgCeY-n exhibit trends similar to those of the Lewis acidity, which increased with increasing Si/Al ratio. This might be because, according to Lewis acid−base theory, thiophene and its derivatives are Lewis bases and are more easily absorbed onto Lewis acid sites.20 4.2.2. Effects of Si/Al Ratio on Breakthrough and Saturation Loadings with M2. In addition to the organic sulfur compounds present in commercial fuels, certain quantities of aromatics are also present, and the presence of aromatics also significantly affects the sulfur-removal performance of adsorbents. Therefore, it was necessary to test the effects of aromatics on adsorptive desulfurization using the AgCeY-n zeolites. In the present work, toluene was selected as the aromatic competitor. Competitive adsorption among toluene, TP, and BT was investigated with model oil M2, and the results are shown in Figure 8 and Table 4. It can be seen in Table 4 that, for all of the AgCeY-n zeolites, the breakthrough and saturation loadings of TP and BT decreased because of the influence of toluene. Toluene has an aromatic skeleton structure similar to thos of TP and BT, leading to competitive adsorption among toluene, TP, and BT on the AgCeY-n zeolites. As mentioned in section 3.5 (Figure 4b), TP and BT were absorbed onto the AgCeY-n zeolites through both π-complexation and direct coordination (S−M) interactions. Toluene can be adsorbed onto the AgCeY-n zeolites through πelectronic interactions, and the presence of toluene could significantly affect the opening of the thiophenic ring in the adsorption processes (see the FTIR analysis of Figure 4c). Therefore, the adsorption of toluene on the active sites of AgCeY-n would impact the breakthrough and saturation loadings of TP and BT.28 For AgCeY-5.3 (Table 4), the decreases in breakthrough loading were approximately 4.44% for TP and 4.67% for BT, whereas the decreases in saturation loading were approximately 4.17% for TP and 3.67% for BT. The decreases in the breakthrough and saturation loadings for TP and BT on AgCeY-5.3 were both the lowest as compared to those on AgCeY-3.0 and AgCeY-4.8. The sulfur loadings increased in the order AgCeY-5.3 < AgCeY-4.8 < AgCeY-3.0 (see Table 4). This indicates that the effects of toluene on the adsorptions of TP and BT on AgCeY-n zeolites were less severe with increasing Si/Al ratio. It was reported that Ce-exchanged zeolites mainly remove TP and BT through direct S−M interactions; therefore, Ce4+ played an important role in
Figure 7. Breakthrough curves for (a) AgCeY-3.0, (b) AgCeY-4.8, and (c) AgCeY-5.3 with model oil M1.
Table 4. Breakthrough and Saturation Loadings for TP and BT on AgCeY-n Zeolites decrease in loadings (%) breakthrough loading (wt %)
saturation loading (wt %)
breakthrougha
saturation
sample
feed
TP
BT
TP
BT
TP
BT
TP
BT
AgCeY-3.0
M1 M2 M3 M1 M2 M3 M1 M2 M3
0.41 0.39 0.38 0.44 0.42 0.41 0.45 0.43 0.42
1.03 0.97 0.93 1.06 1.00 0.96 1.07 1.02 0.97
0.45 0.41 0.40 0.46 0.44 0.42 0.48 0.46 0.44
1.06 0.99 0.95 1.08 1.02 0.98 1.09 1.05 0.99
− 4.88 7.32 − 4.55 6.82 − 4.44 6.67
− 5.83 9.71 − 5.66 9.43 − 4.67 9.35
− 8.89 11.11 − 4.35 8.70 − 4.17 8.33
− 6.61 10.37 − 5.56 9.26 − 3.67 9.17
AgCeY-4.8
AgCeY-5.3
a
a
Measured at a sulfur concentration in the effluent of 10% for TP and BT. 3819
DOI: 10.1021/acs.iecr.5b04609 Ind. Eng. Chem. Res. 2016, 55, 3813−3822
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Industrial & Engineering Chemistry Research
effects of alkenes on the sulfur removal efficiency of the AgCeYn zeolites. In the present work, cyclohexene was selected as an alkene competitor. Competitive adsorption among cyclohexene, TP, and BT was investigated with model oil M3, and the results are shown in Figure 9 and Table 4. Obvious
Figure 8. Breakthrough curves for (a) AgCeY-3.0, (b) AgCeY-4.8, and (c) AgCeY-5.3 with model oil M2.
retaining the selectivities of the ion-exchanged zeolites for removing sulfur from the model oil containing toluene.27 For all of the AgCeY-n zeolites, the decreases in the breakthrough and saturation loadings for TP and BT were no higher than 5.83% except for saturation loadings for TP and BT over AgCeY-3.0. This was because of the Ce4+ in AgCeY-n. As discussed in section 3.4, the surface metal contents of Ce4+ on the AgCeY-n zeolites decreased in the order AgCeY-5.3 > AgCeY-4.8 > AgCeY-3.0 (Table 2), which was contrary to the decreases in the breakthrough and saturation loadings. AgCeY5.3, which exhibited the highest surface metal content of Ce4+, showed the lowest decreases in breakthrough and saturation loadings for TP and BT. 4.2.3. Effects of Si/Al Ratio on Breakthrough and Saturation Loadings with M3. Usually, a certain amount of olefins is contained in gasoline to guarantee the octane number of the gasoline. It was therefore necessary to investigate the
Figure 9. Breakthrough curves for (a) AgCeY-3.0, (b) AgCeY-4.8, and (c) AgCeY-5.3 with model oil M3.
decreases in the breakthrough and saturation loadings were observed for all of the samples because of the influence of cyclohexene. These decreases can be explained in two ways: (i) Similarly to toluene, olefins can also be adsorbed on the AgCeY-n zeolites through π-electronic interactions; therefore, the adsorption of cyclohexene could compete with and decrease the adsorptions of TP and BT.29 (ii) Alkyl thiophene macromolecular compounds could form by the proton acid 3820
DOI: 10.1021/acs.iecr.5b04609 Ind. Eng. Chem. Res. 2016, 55, 3813−3822
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Industrial & Engineering Chemistry Research
with a high adsorption selectivity to organic sulfur compounds in model oils containing toluene or cyclohexene. The SEM and XRD results showed that the original zeolite structure was retained after ion exchange and calcination. XPS analysis indicated that silver was present as Ag+ on the surface of the AgCeY-n adsorbents and that Ce3+ was oxidized to Ce4+ after calcination. On the AgCeY-5.3 adsorbent, the surface contents of Ag+ and Ce4+ were both higher than those on the AgCeY-3.0 and AgCeY-4.8 zeolites. The reason for this difference could be that the pore volume and diameter of AgCeY-5.3 were higher than those of AgCeY-3.0 and AgCeY-4.8 (BET analysis). The Lewis acid sites of AgCeY-n decreased in the order AgCeY-5.3 > AgCeY-4.8 > AgCeY-3.0. The adsorption capacity of AgCeYn showed trends similar to those of Lewis acidity, which increased with increasing Si/Al ratio, showing that a higher content of Lewis acid sites was beneficial for increasing the adsorption capacity. From the reported results, we conclude that, as the Si/Al ratio of AgCeY zeolites increases, the structure of the zeolites changes, and both the quantity of metal carried and the number of acid sites increase. Static and dynamic adsorption desulfurization experiments showed that the capacity for sulfur removal on AgCeY-n decreased in the order AgCeY-5.3 > AgCeY-4.8 > AgCeY-3.0 and that the selectivity of adsorption followed the order BT > TP. The removal of organic sulfur compounds on AgCeY-n occurs mainly through two adsorption modes: π-electronic interactions and direct coordination (S−M) interactions. The inhibition effects on sulfur removal by the zeolites followed the order cyclohexene > toluene.
catalytic reaction between cyclohexene and thiophene on the surface of the AgCeY-n zeolites, in which case the newly formed alkyl thiophene macromolecular compounds would cover the surface of AgCeY-n, leading to a decrease in the exposed active adsorption sites.30,31 It can also be seen from Table 4 that, for AgCeY-5.3, the decreases in breakthrough loading were approximately 6.67% for TP and 9.35% for BT, whereas the decreases in saturation loading were approximately 8.33% for TP and 9.17% for BT. These decreases in breakthrough and saturation loadings for TP and BT on AgCeY-5.3 were both lowest as compared to those of AgCeY-3.0 and AgCeY-4.8. The decrease of the sulfur loading increased in the order AgCeY-5.3 < AgCeY-4.8 < AgCeY-3.0 (see Table 4). This indicates that the effects of cyclohexene on the adsorptions of TP and BT on the AgCeY-n zeolites were less severe with increasing Si/Al ratio, which agrees with the trend seen for toluene. The interpretation of this behavior is similar to that for toluene, namely, the surface Ce4+ metal ions on AgCeY-n played a role in improving the adsorptive selectivities for TP and BT on AgCeY-n. Again, AgCeY-5.3, which exhibited the highest surface metal content of Ce4+, showed the lowest decrease in breakthrough and saturation loadings for TP and BT.
5. REGENERATION OF AGCEY-5.3 ZEOLITE Five cycles of regeneration−adsorption experiments were performed to show the regenerability of AgCeY-5.3 (Table 5). For model oil M1, the sulfur removals of TP and BT over Table 5. Regeneration after AgCeY-5.3 Absorption of Model Oils M1, M2 and M3
■
sulfur removal (%) M1
M2
AUTHOR INFORMATION
Corresponding Authors
M3
number of times regenerated
TP
BT
TP
BT
TP
BT
0 1 2 3 4 5
99.8 99.6 98.9 98.0 97.0 95.0
100 100 99.9 99.8 99.6 99.0
76.5 74.2 72.4 70.6 68.2 63.6
99.8 99.7 98.7 97.7 96.0 94.0
68.2 66.6 64.4 62.1 60.0 56.2
99.4 99.2 98.4 97.1 95.5 93.1
*E-mail:
[email protected]. Tel.: +86 0459 6503167. Fax: +86 0459 6506498. *E-mail:
[email protected]. Tel.: +86 0453 6984321. Fax: +86 0453 6984321. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors acknowledge the financial support from the National Natural Science Foundation of China (21276048) and the Natural Science Foundation of Heilongjiang Province (ZD201201).
AgCeY-5.3 after five regeneration cycles were 95.0% and 98.4%, respectively, comparable to those obtained on the fresh adsorbent. For model oil M2, the sulfur removals of TP and BT over AgCeY-5.3 after five regeneration cycles decreased from 76.5% and 99.8% to 63.6% and 94.0%, respectively. Decreases of 16.9% and 5.8%, respectively, were thus observed compared to the corresponding results for the fresh materials. Finally, for model oil M3, decreases of 17.6% and 6.3% for TP and BT, respectively, were observed compared to the results for the fresh adsorbent. In summary, AgCeY-5.3 exhibited better regenerability characteristics for model oils M1, M2, and M3.
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REFERENCES
(1) Lin, L. G.; Zhang, C.; Liu, C. Y.; Dong, M. M.; Zhang, L. H.; Deng, P. S.; Sun, H.; Huang, H.; Liu, H. B.; Zhang, Y. Z. Y type zeolites/PI membranes for sulfur-free hydrogen source and for fuel cell applications. Int. J. Hydrogen Energy 2014, 39, 4704−4709. (2) Stanislaus, A.; Marafi, A.; Rana, M. S. Recent advances in the science and technology of ultra-low sulfur diesel (ULSD) production. Catal. Today 2010, 153, 1−68. (3) Shi, Y. C.; Zhang, W.; Zhang, H. X.; Tian, F. P.; Jia, C. Y.; Chen, Y. Y. Effect of cyclohexene on thiophene adsorption over NaY and LaNaY zeolites. Fuel Process. Technol. 2013, 110, 24−32. (4) Dasgupta, S.; Gupta, P.; Aarti; Nanoti, A.; Goswami, A. N.; Garg, M. O.; Tangstad, E.; Vistad, Ø.B.; Karlsson, A.; Stöcker, M. Adsorptive desulfurization of diesel by regenerable nickel based adsorbents. Fuel 2013, 108, 184−189. (5) Dooley, K. M.; Liu, D. X.; Madrid, A. M.; Knopf, F. C. Oxidative desulfurization of diesel with oxygen: Reaction pathways on supported metal and metal oxide catalysts. Appl. Catal., A 2013, 468, 143−149.
6. CONCLUSIONS In this work, AgCeY-n (n = 3.0, 4.8, and 5.3) zeolites were prepared and their catalytic performance with model oils comprising thiophene and benzothiophene were investigated. In the present work, the effects of the Si/Al ratio on the performance of the AgCeY-n (n = 3.0, 4.8, and 5.3) zeolites for adsorptive desulfurization were studied. The results indicated that AgCeY-5.3 exhibited the highest sulfur adsorption capacity, 3821
DOI: 10.1021/acs.iecr.5b04609 Ind. Eng. Chem. Res. 2016, 55, 3813−3822
Article
Industrial & Engineering Chemistry Research (6) Shu, C. H.; Sun, T. H.; Zhang, H. B.; Jia, J. P.; Lou, Z. Y. A novel process for gasoline desulfurization based on extraction with ionic liquids and reduction by sodium borohydride. Fuel 2014, 121, 72−78. (7) Gao, X. H.; Geng, W.; Zhang, H. T.; Zhao, X. F.; Yao, X. J. Thiophenic compounds adsorption on Na(I)Y and rare earth exchanged Y zeolites: A density functional theory study. J. Mol. Model. 2013, 19, 4789−4795. (8) Song, H.; Cui, X. H.; Song, H. L.; Gao, H. J.; Li, F. Characteristic and Adsorption Desulfurization Performance of Ag−Ce Bimetal IonExchanged Y Zeolite. Ind. Eng. Chem. Res. 2014, 53, 14552−14557. (9) Oliveira, M. L. M.; Miranda, A. A. L.; Barbosa, C. M. B. M.; Cavalcante, C. L., Jr.; Azevedo, D. C. S.; Rodriguez-Castellon, E. Adsorption of thiophene and toluene on NaY zeolites exchanged with Ag(I), Ni(II) and Zn(II). Fuel 2009, 88, 1885−1892. (10) Shi, Y.; Yang, X.; Tian, F.; Jia, C.; Chen, Y. Effects of toluene on thiophene adsorption over NaY and Ce(IV)Y zeolites. J. Nat. Gas Chem. 2012, 21, 421−425. (11) Sato, K.; Nishimura, Y.; Matsubayashi, N.; Imamura, M.; Shimada, H. Structural changes of Y zeolites during ion exchange treatment: Effects of Si/Al ratio of the starting NaY. Microporous Mesoporous Mater. 2003, 59, 133−146. (12) Wang, H. G.; Song, L. J.; Jiang, H.; Xu, J.; Jin, L. L.; Zhang, X. T.; Sun, Z. L. Effects of olefinon adsorptive desulfurization of gasoline over Ce(IV)Y zeolites. Fuel Process. Technol. 2009, 90, 835−838. (13) Wang, J.; Xu, F.; Xie, W. J.; Mei, Z. J.; Zhang, Q. Z.; Cai, J.; Cai, W. M. The enhanced adsorption of dibenzothiophene onto cerium/ nickel-exchanged zeolite Y. J. Hazard. Mater. 2009, 163, 538−543. (14) Emeis, C. A. Determination of integrated molar extinction coefficients for infrared absorption bands of pyridine adsorbed on solid acid catalysts. J. Catal. 1993, 141, 347−354. (15) Tarafdar, A.; Panda, A. B.; Pramanik, P. Synthesis of ZrO2−SiO2 mesocomposite with high ZrO2 content via a novel sol−gel method. Microporous Mesoporous Mater. 2005, 84, 223−228. (16) Moulder J. F.; Stickle W. F.; Sobol P. E.; Bomben K. D. Handbook of X-ray Photoelectron Spectroscopy: A Reference Book of Standard Spectra for Identification and Interpretation of XPS Data; Perkin-Elmer: Eden Prairie, MN, 1992. (17) Gupta, P.; Paul, S. Solid acids: Green alternatives for acid catalysis. Catal. Today 2014, 236, 153−170. (18) Hunger, M.; Horvath, T.; Weitkamp, J. Conversion of propan-2ol on zeolite LaNaY investigated by in situ MAS NMR spectroscopy under continuous-flow conditions. Stud. Surf. Sci. Catal. 1997, 105, 853−860. (19) Yi, D. Z.; Huang, H.; Meng, X.; Shi, L. Adsorption−desorption behavior and mechanism of dimethyl disulfide in liquid hydrocarbon streams on modified Y zeolites. Appl. Catal., B 2014, 148−149, 377− 386. (20) Li, Q. Z.; Huang, Y. F.; Wei, D.; Hu, J. F.; Chen, X. D.; Tong, G. M. Acidity and catalytic performance of Y zeolite with different Si/Al ratios. Pet. Process. Petrochem. 1991, 2, 27−31. (21) Shyu, J. Z.; Weber, W. H.; Gandhi, H. S. Surface characterization of alumina-supported ceria. J. Phys. Chem. 1988, 92, 4964−4970. (22) Li, J. C.; Zeng, P. H.; Zhao, L.; Ren, S. Y.; Guo, Q. X.; Zhao, H. J.; Wang, B. J.; Liu, H. H.; Pang, X. M.; Gao, X. H.; Shen, B. J. Tuning of acidity in CeY catalytic cracking catalysts by controlling the migration of Ce in the ion exchange step through valence changes. J. Catal. 2015, 329, 441−448. (23) Shi, Y. C.; Yang, X. J.; Tian, F. P.; Jia, C. Y.; Chen, Y. Y. Effects of toluene on thiophene adsorption over NaY and Ce(IV)Y zeolites. J. Nat. Gas Chem. 2012, 21, 421−425. (24) Yang, R. T.; Hernández-Maldonado, A. J.; Yang, F. H. Desulfurization of transportation fuels with zeolites at ambient temperature and pressure. Science 2003, 301, 79−81. (25) Velu, S.; Ma, X. L.; Song, C. S. Selective adsorption for removing sulfur from jetfuel over zeolite-based adsorbents. Ind. Eng. Chem. Res. 2003, 42, 5293−5304. (26) Zhou, D.-H.; Wang, Y.-Q.; He, N.; Yang, G. The πcomplexation mechanisms of Cu(I), Ag(I)/zeolites for desulfurization. Acta Phys.-Chim. Sin. 2006, 22, 542−547.
(27) Song, H.; Wan, X.; Dai, M.; Zhang, J. J.; Li, F.; Song, H. L. Deep desulfurization of model gasoline by selective adsorption over Cu−Ce bimetal ion-exchanged Y zeolite. Fuel Process. Technol. 2013, 116, 52− 62. (28) Zhang, Z. Y.; Shi, T. B.; Jia, C. Z.; Ji, W. J.; Chen, Y.; He, M. Y. Adsorptive removal of aromatic organosulfur compounds over the modified Na-Y zeolites. Appl. Catal., B 2008, 82, 1−10. (29) Takahashi, A.; Yang, R. T.; Munson, C. L.; Chinn, D. Cu(I)Yzeolite as a superior adsorbent for diene/olefin separation. Langmuir 2001, 17, 8405−8413. (30) Richardeau, D.; Joly, G.; Canaff, C.; Magnoux, P.; Guisnet, M.; Thomas, M.; Nicolaos, A. Adsorption and reaction over HFAU zeolites of thiophene in liquid hydrocarbon solutions. Appl. Catal., A 2004, 263, 49−61. (31) Qin, Y. C.; Mo, Z. S.; Yu, W. G.; Dong, S. W.; Duan, L. H.; Gao, X. H.; Song, L. J. Adsorption behaviors of thiophene, benzene, and cyclohexene on FAU zeolites: Comparison of CeY obtained by liquidand solid-state ion exchange. Appl. Surf. Sci. 2014, 292, 5−15.
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