Characteristic and Adsorption Desulfurization Performance of Ag–Ce

Research Note pubs.acs.org/IECR

Characteristic and Adsorption Desulfurization Performance of Ag− Ce Bimetal Ion-Exchanged Y Zeolite Hua Song,† Xue-Han Cui,†,‡ Hua-Lin Song,*,§ Hui-Jie Gao,† and Feng Li† †

Provincial Key Laboratory of Oil & Gas Chemical Technology, College of Chemistry & Chemical Engineering, Northeast Petroleum University, Daqing 163318, Heilongjiang, China ‡ Oil Refineries, Daqing Petrochemical Company of PetroChina, Daqing 163711, Heilongjiang, China § Key Laboratory of Cancer Prevention and Treatment of Heilongjiang Province, Basic Medical College, Mudanjiang Medical University, Mudanjiang 157011, China ABSTRACT: AgY, CeY, and AgCeY zeolites were successfully prepared and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetry−differential thermogravimetry (TG-DTG), and Fourier transform infrared (FT-IR) spectroscopy. The adsorptive desulfurization properties of the adsorbents were evaluated in a fixed-bed unit through different types of model gasoline. The results showed that the AgCeY zeolite has a high desulfurization capacity, similar to that of AgY, and the adsorption affinity for sulfur compounds from model gasoline has the following order: benzothiophene > thiophene. In addition, AgCeY zeolite also has a high selectivity similar to CeY, and the effect on the zeolites for sulfur removal ranks in the following order: pyridine > cyclohexene > toluene. The AgCeY zeolite removes organic sulfur compounds by two types of adsorption modes: π-complexation and direct coordination (S-M) interaction.

1. INTRODUCTION Sulfur compounds in transportation fuels are the main group of air pollution sources.1 The major sulfur compounds remaining in the transportation fuels, after the conventional hydrodesulfurization (HDS) process, are refractory sulfur compounds, such as thiophene (TP), benzothiophene (BT), dibenzothiophene (DBT), and their alkylated derivatives.2−4 To remove TP and its derivatives selectively and meet increasingly stringent environmental regulations on sulfur content, adsorption desulfurization at ambient temperature and pressure was considered to be an efficient and economic technology.5,6 Recently, much attention has been focused on developing zeolites, mesoporous materials, mixed-metal oxides, activated alumina, and carbon as adsorbents. Among these porous material reported in the literature, zeolites are efficient and attractive adsorbents for desulfurization. Yang et al.7−9 determined that Ag-, Cu-, or Ni-exchanged Y zeolites adsorb thiophene from benzene selectively by π-complexation between sulfur compounds and the transition-metal cations from the zeolites, and Ag,Cu-exchanged zeolites both exhibit high capacities of desulfurization. However, when aromatics and many other impurities exist in transportation fuels, the capacities of the zeolites for adsorption desulfurization drop sharply. Thus, particular attention has been attached to selective adsorption desulfurization. Velu10 and co-workers studied Cu(II)Y, Zn(II)Y, Ni(II)Y, Pd(II)Y, and Ce(IV)Y for the removal of sulfur compounds from commercial aviation fuels and the result showed that Ce(IV)Y has higher selectivity than the other metal-exchanged zeolites via direct sulfur adsorbent (S-M) interaction. There is also an ongoing effort on bimetal ion-exchanged zeolites. Our group11 reported that the Cu−Ce bimetal ion-exchanged zeolite Cu(I)Ce(IV)Y not only has a high sulfur adsorption capacity, similar to that of © 2014 American Chemical Society

Cu(I)Y, but also has a high selectivity for sulfur compounds similar to Ce(IV)Y. The Cu(I)Ce(IV)Y zeolite binds sulfur compounds through two types of adsorption modes: specific πcomplexation and S-M interaction. Wang12 and co-workers used NaY, NiY, CeY, and Ni/Ce-loaded Y zeolite (NiCeY) zeolites as adsorbents for the removal of DBT from model fuel, and the experiment shows NiCeY has higher adsorptive selectivity for DBT than the other three zeolites. To our knowledge, a few articles have focused on the study of adsorption of sulfur compounds onto AgCeY zeolite, from the viewpoint of competitive adsorption. In this paper, AgY, CeY, and Ag−Ce bimetal ion-exchanged zeolites were synthesized, and the effects of the aromatics, nitrogen, and olefins on the selective adsorptive desulfurization of model gasoline over AgY, CeY, and AgCeY were studied, respectively.

2. EXPERIMENT The methods of preparing samples (AgY, CeY, and AgCeY) and model gasoline are the same as those given in a previously reported article.11 The compositions of different types of model gasoline are summarized in Table 1. Adsorptive desulfurization experiments are carried out at a feed flow rate of 20 mL/h and a temperature of 50 °C in a fixed-bed packed with 1 g of adsorbent. Effluent samples were collected every 20 min until saturation was achieved. The breakthrough concentration is defined as the point of 20 mg L−1 S in effluent for TP and BT. Received: Revised: Accepted: Published: 14552

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Table 1. Composition of Model Gasoline Used Concentration (mg L−1) Sulfur No.

TP

BT

M1 M2 M3 M4

100 100 100 100

100 100 100 100

toluene

pyridine

cyclohexene

500 500 500

3. CHARACTERIZATION OF ADSORBENT 3.1. XRD. Figure 1 shows the XRD patterns of the NaY, AgY, CeY, and AgCeY zeolites. The similarity in the XRD

Figure 2. XPS spectra of AgCeY and the Ag 3d, Ce 3d regions on the surface of AgCeY: (a) Ag 3d and (b) Ce 3d. Figure 1. XRD patterns of metal ion-exchanged Y zeolite adsorbents.

patterns of AgY, CeY, AgCeY to the original NaY indicates that the original zeolite structure is retained without any significance change upon Ag and Ce ion exchange followed by heat treatment, which is consistent with the results reported in the literature.10 However, slight decrease of the peak intensity after exchange process is found, particularly the Ce3+ ion-exchanged Y zeolite. These results indicate that a certain loss of crystallinity occurred after ion-exchanged and heat treatment. In the case of Ce-exchanged adsorbent (CeY), the Ce3+ has been oxidized to Ce4+ after calcination at 500 °C in air. 3.2. XPS. The XPS patterns of the Ag 3d spectra for Ag and AgCeY (Figure 2a) show peaks at binding energy of 368.8 and 367.9 eV, which originates from the Ag+, indicating that silver is present as Ag+ on the surface of the adsorbent AgY and AgCeY.13 The XPS patterns of the Ce 3d spectra for CeY and AgCeY (Figure 2b) show two broad regions, one located at ∼883.1 eV, which can be assigned to 3d5/2 emission; the other, at ∼901.9 eV, which can be assigned to 3d3/2 emission. This indicates that, in Ce-exchanged zeolite, the Ce species are present as Ce4+.14 Analysis of XPS spectra of the Ce 3d also demonstrates that the ion-exchanged Ce3+ ions over the zeolite by impregnation were oxidized to Ce4+ through calcination. 3.3. TG-DTG. TG-DTG curves for the AgCeY zeolite preloaded (saturated) with TP and BT are shown as Figures 3a and 3b. Generally, TG curves show an initial weight loss in the temperature range of 30−120 °C, which is attributed to adsorbed impurities (mostly water). Both TG curves show a huge peak in the temperature range of 120−310 °C, which is accompanied by a substantial weight loss. No distinct peak in the range of 310−570 °C is observed for each sample. The first observation is that both TP and BT can be desorbed at temperatures lower than 310 °C, which indicates that no very strong chemically interactions exist between the adsorbate and adsorbent. It is reminded that such strong interactions have

Figure 3. TG and DTG curves of TP and BT from AgCeY adsorbents: (a) TP and (b) BT.

been reported in the case of BT adsorption on Faujasite zeolites and are attributed to S-M bonds (σ-bonds).15 The second observation is based on the corresponding temperatures of the peaks maxima. The desorption temperature of TP (∼232 °C) is higher than that of BT (∼209 °C). It may be deduced that the adsorption affinity of TP and BT onto the AgCeY adsorbent is BT > TP. 14553

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of the band at 1454 cm−1 was larger than that of band at 1545 cm−1 for AgY and AgCeY; however, CeY exhibits lower Lewis acidity, compared to the AgY, and it shows higher Bronsted acidity than Lewis acidity. This indicates that AgY and AgCeY possess more Lewis acid sites than Bronsted acid sites. Hence, it can be concluded that the Ag ions mainly provide the Lewis acid centers. According to Lewis acid−base theory, thiophene and its derivatives belong to Lewis bases and are easier to be absorbed on the Lewis acid sites. 3.5. Fourier Transform Infrared (FT-IR) Spectroscopy. Figure 5 shows the FT-IR spectra of the NaY, AgY, CeY, and AgCeY zeolites before adsorbing the model gasoline (Figure 5a) and after saturation adsorption of the model gasolines M1 (Figure 5b), M2 (Figure 5c), M3 (Figure 5d), and M4 (Figure 5e). As can be seen from Figure 5b, it is observed that TP and BT loaded on the AgY, CeY, and AgCeY absorbents show peaks at 3547 cm−1 (the acid hydroxyl vibration peak on the surface of zeolites). In contrast with Figure 5a, the band at 2965 and 2921 cm−1 can be observed in Figure 5b, which is assigned to the C− H stretching vibration of saturated −CH3 and −CH2, indicating some of the adsorbed TP and BT can undergo the opening of their thiophenic ring in adsorption processes, the change on the molecular structure have taken place.16 There also exists two

3.4. Fourier Transform Infrared (FT-IR)-Pyridine Desorption. Fourier transform infrared (FT-IR)-pyridine desorption analyses of AgY, CeY, and AgCeY at 300 °C are shown in Figure 4. Different from NaY, which is a type of

Figure 4. FT-IR-pyridine thermo-desorption spectra of AgY, CeY, and AgCeY zeolites (300 °C).

nonacidic zeolite, AgY, CeY, and AgCeY showed both Lewis acid sites, corresponding to absorbance bands at 1454, 1608 cm−1, and Bronsted acid sites, corresponding to absorbance bands at ∼1545, 1630 cm−1. It is evident that the integral area

Figure 5. FT-IR spectra of AgY, CeY, and AgCeY adsorbing different types of model gasoline at the breakthrough point: (a) before absorption of model gasoline, (b) model gasoline M1, (c) model gasoline M2, (d) model gasoline M3, and (e) model gasoline M4. 14554

dx.doi.org/10.1021/ie404362f | Ind. Eng. Chem. Res. 2014, 53, 14552−14557

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Figure 6. Breakthrough curves for the adsorptive removal of TP and BT from model gasoline: (a) model gasoline M1, (b) model gasoline M2, (c) model gasoline M3, and (d) model gasoline M4.

adsorption peaks, at ∼1377 and 1450 cm−1. The band at 1377 cm−1 observed in AgY and AgCeY, which is lower than that for the symmetric stretching vibration of CC in the fundamental ring of gaseous TP, can been assigned to the perturbed symmetric stretching vibration of CC in the fundamental ring.17 The interaction between a metal cation and the πelectron from the ring of TP caused a decrease in the electron density of the entire TP ring, thus inducing the band shifts to lower wavenumbers. This indicates that some TP and BT is adsorbed onto AgY and AgCeY by π-electronic interaction, and the TP ring molecule is parallel adsorbed to the surface of the adsorbent.17,18 This conclusion is the same as that observed in Gil’s report.19,20 The band at 1450 cm−1 appears at CeY and AgCeY, which can be ascribed to a shift of ν(CC)sym to higher frequencies, is caused by a direct interaction between the S atoms from TP and BT and the Ce4+ ions from Ce ionexchanged zeolite,16,21 which is consistent with previous conclusion.22 The direct interaction increased the electron density of the CC−CC fragment of TP, giving rise to the partial recovery of double bond performance. Although AgCeY is a zeolite, for which Lewis acids represent the main acid on it, the FT-IR analysis demonstrate that TP and BT are absorbed on the AgCeY mainly via π-complexation and direct coordination (S-M) interaction. It can be observed that a band at 1496 cm−1 in Figure 5c can be assigned to aqueous toluene, which is the vibration band of the ring skeleton of toluene.18 This can be interpreted as an indication that there is an effect between the π-electron on the ring skeleton and the cation of zeolite, leading to a decrease in the electron cloud density on the ring skeleton, thus implying that toluene was adsorbed onto the AgY, CeY, and AgCeY zeolites by π-electronic interaction. Compared with CeY and AgCeY, AgY shows the strongest band at 1496 cm−1, indicating that toluene is easier to be adsorbed onto AgY. This phenomenon indicates that the organic sulfur compounds from model gasoline containing aromatics can be selectively

adsorbed over Ce-exchanged zeolite. Moreover, the disappearance of the bands at 2921 and 2965 cm−1 implies that the existence of toluene can control the opening of the thiophenic ring in adsorption processes. There exists a new band at ∼1490 cm−1 in Figure 5d, which can be ascribed to physically adsorbed pyridine.23 It also can be observed that the characteristic bands of TP and BT became weak after adsorption of the model gasoline M3, which shows the significant effect of pyridine on the adsorptive desulfurization performance. Therefore, the metal ion-exchanged zeolites give priority to adsorb nitrogen compounds than organic sulfur compounds. It can be observed that three new bands appeared: the bands at 2935 and 2864 cm−1 in Figure 5e can be ascribed to the stretching vibration of the strong absorption peak which originates from the methyl saturated bond; and the band at 1460 cm−1 should be ascribed to plane rocking vibrations of the strong absorption peak, which originate from the methyl saturated bond.16 The three characteristic peaks imply that cyclohexene was adsorbed on the AgY, CeY, and AgCeY zeolites via π-electronic interaction. Compared with CeY and AgCeY, the band intensity of AgY is the strongest, which indicates the existence of strong interactions between AgY and cyclohexene. Moreover, the characteristic bands of TP and BT become weak, thus implying that a certain amount of polymers may be formed on the surface of the AgY, CeY, and AgCeY zeolites with the existence of alkenes, which may occupy the active sites and clog the pores in metal ion-exchanged zeolites;20 this will reduce the adsorptive selectivity for removing organic sulfur compounds from the model fuelcontaining olefins.

4. FIXED-BED BREAKTHROUGH EXPERIMENTS Figure 6 shows the adsorption capacities with cumulative effluent volume by using model gasoline M1, M2, M3, and M4. As can be seen from Figure 6a, the breakthrough capacity of TP 14555

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Table 2. Breakthrough and Saturation Loadings for TP and BT from Model Gasoline over AgY, CeY, and AgCeY Adsorbents Decline in Loadings (%) Breakthrough Loadinga (wt %) feed

a

TP

BT

Saturation Loading (wt %) TP

M1 M2 M3 M4

0.822 0.243 0.090 0.192

1.550 0.525 0.098 0.417

0.873 0.254 0.096 0.205

M1 M2 M3 M4

0.424 0.406 0.165 0.395

1.044 0.989 0.217 0.943

0.447 0.411 0.176 0.404

M1 M2 M3 M4

0.149 0.148 0.103 0.142

0.267 0.257 0.154 0.251

0.159 0.154 0.105 0.151

Breakthrough

BT Sample AgY 1.577 0.543 0.099 0.444 Sample AgCeY 1.061 1.007 0.233 0.997 Sample CeY 0.275 0.265 0.158 0.255

Saturation

TP

BT

TP

BT

70.438 89.051 76.606

66.129 93.677 73.129

70.905 89.003 76.518

65.568 93.722 71.833

4.245 61.085 6.769

5.268 79.215 9.646

8.054 60.626 9.642

5.090 78.040 6.051

0.671 30.872 4.698

3.745 42.322 6.067

3.145 33.962 4.843

3.636 42.545 7.200

Measured at a sulfur concentration in effluent of 20 mg L−1 for TP and BT.

that pyridine may block the pores of the molecular sieve, leaving no space for sulfur compounds. The cyclohexene is selected as model material of alkenes in fuels. It is observed that, in comparison with adsorbing model gasoline without cyclohexene (Figure 6a), the capacity for sulfur removal of AgCeY and CeY adsorbents do not show a remarkable decline (Figure 6d); however, the decrease in the breakthrough loadings of AgY is most severe, relatively. When cyclohexene and TP both exist in model gasoline, alkylation reaction will occur on the surface of the adsorbent, so the alkylated thiophene will be generated. Cyclohexene, TP, and BT are all adsorbed on AgY in the same way (π-complexation),24 so it is obvious that the competitive adsorption exists. However, TP and BT are adsorbed on the Ce-exchanged Y zeolite mainly by forming S-M bonds between Ce4+ and the sulfur atoms; there is less competitive adsorption, so the effect of cyclohexene on CeY and AgCeY is less than that on AgY. As shown in Table 2, the decline in the loading (%) of the three adsorbents decreases in the following order: AgY > AgCeY > CeY. Thus, the AgCeY zeolite not only has a high capacity for sulfur compound adsorption, but it also shows high selectivity for sulfur removal when alkenes exist.

removal is lower than that of BT, which implies that the selectivity for the removal of sulfur compounds decreases in the order of BT > TP. It implies that the breakthrough capacity of TP removal is lower than that of BT, so the selectivity for the removal of sulfur compounds decreases in the order of BT > TP. One possible reason is the difference of the molecular weights or polarizabilities. The second observation is both AgY and AgCeY adsorbents have a strong sulfur removal capacity, but the breakthrough capacity for sulfur of CeY adsorbent is relatively weak. It is reported that Ag+ in Ag-exchanged Y zeolite is highly effective and capable in the removal of TP and BT via π-complexation.8,10 The high sulfur adsorption capacity of AgCeY is slightly below that of AgY but is much higher than that of CeY (see Table 2), indicating that the introduction of the Ag ion to Ce-exchanged Y zeolite can improve its capacity for sulfur removal. Figure 6b shows that a huge reduction in capacity for sulfur adsorption is observed with AgY. However, the influence of toluene on AgCeY and CeY is not so obvious. The declines in loadings are listed in Table 2. The loss of sulfur adsorption shows that competitive adsorption exists among TP, BT, and toluene; these three have similar aromatic skeleton structures. As shown in the FT-IR analysis, AgY can adsorb TP and BT via π-complexation, but toluene can also be adsorbed via the same complexation, which leads to the competitive adsorption. The least loss of sulfur adsorption over CeY shows that the Ce ion can keep the selectivity of ion-exchanged zeolite for removing sulfur in the model gasoline with the toluene. It is known that CeY removes sulfur compounds via S-M interaction.10 Hence, the toluene has little effect on the capacity of sulfur adsorption over CeY and AgCeY, indicating that the introduction of Ce ion to Ag-exchanged Y zeolite can improve its capacity for sulfur removal when competitive adsorption exists. It is well-known that nitrogen compounds that normally exist in gasoline associate with the sulfur compounds, and pyridine is chosen as a model material to represent this group in this manuscript. For all of the adsorbents (Figure 6c), a sharp decrease in capacity of sulfur removal is observed, when compared with Figure 6a. This may mainly due to the strong interaction between pyridine and the acid sites of zeolites, such

5. CONCLUSION AgCeY, exchanged by both Ag and Ce ions on NaY zeolite, exhibits high adsorptive capacity and high adsorption selectivity to organic sulfur compounds in different model gasolines containing toluene or cyclohexene. The capacity of sulfur removal with model gasoline containing only TP and BT has an order ranking of AgY > AgCeY > CeY, whereas, for any of the aromatics, nitrogen, and olefins that exist (experiment with M2M4) in model gasoline, the order is changed: AgCeY > CeY > AgY. AgCeY zeolite has good desulfurization capacity, which is similar to AgY, and the adsorption affinity has the following order: BT > TP. AgCeY adsorbent also has high selectivity, which is similar to that of CeY; the effect on the metal ionexchanged Y zeolites for sulfur removal had an order ranking of pyridine > cyclohexene > toluene. The characteristics of AgCeY zeolite determined by XRD, XPS, TG-DTG, FT IR-pyridine desorption, and FT-IR suggest that Ce3+ was oxidized to Ce4+ 14556

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(17) Garcia, C. L.; Lercher, J. A. Adsorption and surface reactions of thiophene on ZSM-5zeolites. J. Phys. Chem. 1992, 96, 2669−2675. (18) Shi, Y.; Yang, X.; Tian, F.; Jia, C.; Chen, Y. Effects of toluene on thiophene adsorption over NaY and Ce(IV)Y zeolites. Journal of Natural Gas Chemistry 2012, 21, 421−425. (19) Gil, B.; Karolina, M.; Inska, Szczerb M. In situ IR and catalytic studies of the effect of coke on acid properties of steamed zeolite Y. Microporous Mesoporous Mater. 2007, 99, 328−333. (20) Zhang, X. T.; Xu, J.; Song, L. J.; Jin, L. L.; Sun, Z. L. FT-IR studies on the effect of olefins on desulfurization of a CeY zeolite. Journal of Fuel Chemistry and Technology 2010, 38, 91−95. (21) Choung, J. W.; Nam, I. S. Characteristics of copper ion exchanged mordenite catalyst deactivated by HCl for the reduction of NOx with NH3. Applied Catalysis B: Environmental 2006, 64, 42−50. (22) Mills, P.; Korlann, S.; Bussell, M. E. Vibrational study of organo metallic complexes with thiophene ligands: models for adsorbed thiophene on hydrodesulfurization catalysts. J. Phys. Chem. A 2001, 105, 4418−4429. (23) Li, W. L.; Xing, J. M.; Xiong, X. C.; Huang, J. X.; Liu, H. Z. Feasibility study on the integration of adsorption/bioregeneration of π-complexation adsorbent for desulfurization. Ind. Eng. Chem. Res. 2006, 45, 2845−2849. (24) Yang, S. W.; Kondo, J. N.; Domen, K. Formation of alkenyl carbenium ions by adsorption of cyclic precursors on zeolites. Catal. Today 2002, 73, 113−125.

after calcination, the loading of which on AgCeY zeolite and its framework structure was not damaged after Na+ was ionexchanged by Ag+ and Ce4+. The AgCeY zeolite removes organic sulfur compounds by two types of adsorption modes: π-complexation (through Ag+) and direct coordination (S-M) interaction (through Ce4+).

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-0453-6984321. Fax: +86-0453-6984321. E-mails: [email protected], [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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