Ultra-Deep Adsorptive Desulfurization of Light-Irradiated Diesel Fuel

Oct 10, 2013 - sorb) or 1.143 mg of sulfur per gram of adsorbent (mg-S/g-sorb) was achieved ... Ultra-deep desulfurization of diesel fuel has attracte...
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Ultra-Deep Adsorptive Desulfurization of Light-Irradiated Diesel Fuel over Supported TiO2−CeO2 Adsorbents Jing Xiao,†,‡ Xiaoxing Wang,† Yongsheng Chen,† Mamoru Fujii,† and Chunshan Song*,† †

Clean Fuels and Catalysis Program, EMS Energy Institute, and Department of Energy and Mineral Engineering, Pennsylvania State University, 209 Academic Projects Building, University Park, Pennsylvania 16802, United States ‡ School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China S Supporting Information *

ABSTRACT: This study investigates ultra-deep adsorptive desulfurization (ADS) from light-irradiated diesel fuel over supported TiO2−CeO2 adsorbents. A 30-fold higher desulfurization capacity of 95 mL of fuel per gram of adsorbent (mL-F/gsorb) or 1.143 mg of sulfur per gram of adsorbent (mg-S/g-sorb) was achieved from light-irradiated fuel over the original lowsulfur fuel containing about 15 ppm by weight (ppmw) of sulfur. The sulfur species on spent TiO2−CeO2/MCM-48 adsorbent was identified by sulfur K-edge XANES as sulfones and the adsorption selectivity to different compounds tested in a model fuel decreases in the order of indole > dibenzothiophenesulfone ≫ dibenzothiophene > 4-methyldibenzothiophene > benzothiophene > 4,6-dimethyldibenzothiophene > phenanthrene > 2-methylnaphthalene ∼ fluorene > naphthalene. The results suggest that during ADS of light-irradiated fuel, the original sulfur species were chemically transformed to sulfones, resulting in the significant increase in desulfurization capacity. For different supports for TiO2−CeO2 oxides, the ADS capacity increases with a decrease in the point of zero charge (PZC) value; for silica-supported TiO2−CeO2 oxides (the lowest PZC value of 2−4) with different surface areas, the ADS capacity increases monotonically with increasing surface area. The supported TiO2−CeO2/MCM-48 adsorbent can be regenerated using oxidative air treatment. The present study provides an attractive new path to achieve ultraclean fuel more effectively.

1. INTRODUCTION Ultra-deep desulfurization of diesel fuel has attracted great attention because of the stringent fuel specifications for diesel ( DMDBT > Phe > MNap ∼ Flu > Nap, which is consistent with the order of adsorption selectivity. The selectivity of refractory sulfur compounds follows the order of DBT > MDBT > DMDBT, suggesting that the presence of −CH3 groups neighboring sulfur atom in nonoxidized sulfur compounds sterically hinder ADS over the TiO2−CeO2/MCM-48 adsorbent. The result also indicates that the adsorption of nonoxidized sulfur compounds over the TiO2−CeO2/MCM-48 sorbent is likely through the sulfur atom, similar as the case of supported nickel adsorbent7 but different from the case of activated carbon.13,14 In contrast, the adsorption of DBT sulfone can go through the oxygen atom instead of sulfur atom, because of the higher electronegativity of oxygen than sulfur and much greater dipole moment of sulfones than nonoxidized sulfur compounds, resulting in a significantly higher adsorption selectivity and capacity, as indicated in Table 2. In this case, chemical transformation of initial refractory sulfur compounds in diesel fuel to sulfones over the TiO2−CeO2/MCM-48 adsorbent is likely to strengthen ADS by shifting the adsorbate from the sterically hindered sulfur atom on alkylated refractory sulfur molecules (e.g., 4,6-DMDBT) to a more attractive oxygen atom on the corresponding sulfone molecules. In other words, the adsorption between sulfone and adsorbent may go through a highly electronegative oxygen atom rather than a less electronegative sulfur atom in sulfone, resulting in higher ADS selectivity and capacity. Without light irradiation to generate peroxides, the refractory sulfur compound 4,6-DMDBT is not oxidized and, thus, is less strongly adsorbed on the surface of TiO2−CeO2/MCM-48 than DBT or BT, whereas the oxidized DBT compounds (e.g., DBTO2) is much more strongly adsorbed on TiO2−CeO2/ MCM-48, showing that the oxidation of refractory sulfur compounds to sulfones, with the help of light irradiation, is necessary and highly beneficial for achieving high adsorption capacity for ultra-deep adsorptive desulfurization. Moreover, in terms of sulfone formation, 4,6-DMDBT may show higher reactivity than DBT because of the higher electron density of the sulfur atom.36,37 Therefore, with the combination of light irradiation and adsorptive desulfurization, the present system is particularly promising for ultra-deep desulfurization of lowsulfur diesel fuel. Additionally, a strong adsorption capacity of indole was observed over the TiO2−CeO2/MCM-48 adsorbent as shown in Figure 4, which is similar to DBTO2. However, indole shows a lower dipole moment than DBTO2 as listed in Table 2, suggesting the interaction mode for the adsorption of indole may be different from DBTO2 over the TiO2−CeO2/MCM-48 adsorbent. As indole shows a bifunctional structure38 (i.e., electron acceptor from the hydrogen bonded to N and electron donor from conjugated π system), it behaves both as a weak acid due to the presence of the H−N bond and as a weak base due to the basicity of the N atom.39 With available silanol group sites and TiO2−CeO2 oxide sites on the surface of the TiO2−

which give adsorption capacities of refractory sulfur compounds > aromatic compounds. To make a quantitative discussion of the adsorptive selectivity, a relative selectivity factor is defined in this study as αi − n = Q i/Q n

(1)

where Q i is the adsorptive capacity of compound i corresponding to the breakthrough point and Qn is the adsorptive capacity of the reference compound, naphthalene (Nap), corresponding to its breakthrough point. It should be mentioned that in using the kinetic breakthrough capacities instead of the equilibrium capacity in eq 1, the defined selectivity factor is not for the equilibrium selectivity.25 The adsorption selectivity for the ten compounds over the TiO2−CeO2/MCM-48 adsorbent decreases in the order of indole > DBTO2 ≫ DBT > MDBT > BT > DMDBT > Phe > MNap ∼ Flu > Nap with the relative selectivity factor (αi−n) of 27.6, 26.9, 3.7, 2.7, 2.4, 2.0, 1.5, 1.3, 1.3, and 1, respectively, as listed in Table 2. Both the breakthrough and saturation capacities of the ten compounds listed in Table 2 follow the 15750

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Table 3. BET Surface Area of Various Supports and Supported TiO2−CeO2 Adsorbents, and Desulfurization Capacity of the Supported TiO2−CeO2 Adsorbents support

SBET‑support (m2/g)

SBET‑supportedTiO2−CeO2 (m2/g)

PZC value

Q (mg-S/g-sorb)

Q (μg-S/m2-sorb)

pack. density (mg/cm3)

Q (mg-S/mL-sorb)

Al2O3 TiO2 AC EH-5

155 170 1087 315

104 90 930 196

8.9 5.5 7.4 2.4

0 0.010 0.048 0.354

0 0.10 0.05 1.80

0.80 0.80 0.20 0.40

0 0.008 0.010 0.142

CeO2/MCM-48 adsorbent, both hydrogen bonding interaction as well as acid−base interaction might play important roles in the adsorption of indole. The strong adsorption of indole over the TiO2−CeO2/MCM-48 adsorbent also suggests that the presence of nitrogen compounds in the fuel might suppress ADS over the TiO2−CeO2/MCM-48 adsorbent through competitive adsorption. An integration of two or more adsorbent beds may further improve the ultra-deep desulfurization process. 3.2. Effect of Support of TiO2−CeO2 Oxides on ADS. By comparison of the desulfurization performance of MCM-48supported TiO2−CeO2 and bulk TiO2−CeO2 in this study, the desulfurization capacity of the former is over 30 times higher than the latter, even though only 13.3 wt % of TiO2−CeO2 oxides were loaded on MCM-48. This suggests a strong effect of the support on ADS performance of the TiO2−CeO2 oxidebased adsorbents. In order to clarify support effect, various types of supports, including fumed silica (EH-5), activated carbon (AC-WPH), anatase TiO2, and gamma-Al2O3, were examined. Table 3 lists some physical properties and desulfurization capacities of bulk and supported TiO2−CeO2 adsorbents. Both the gravimetric and volumetric desulfurization capacities of various supported TiO2−CeO2 oxides follow the order of EH-5 > AC > TiO2 > Al2O3. The calculated desulfurization capacity on the basis of BET surface area is also listed in Table 3, which follows the order of EH-5 > TiO2 > AC > Al2O3. Supporting Information Figure S2 shows desulfurization capacity versus SBET of various supported TiO2−CeO2. No correlation is found between desulfurization capacity and SBET, suggesting desulfurization capacity is not mainly determined by BET surface area of supported TiO2− CeO2 adsorbents. More active sites (TiO2−CeO2) may be exposed on silica support (EH-5) followed by those on TiO2, AC, and Al2O3, which could be attributed to the electronic or chemical properties of the supported materials. The PZC value of the support affects the adsorption of metal-containing ions in metal precursors via electrostatic interaction, which could further influence the dispersion of formed metal oxides,40 and thus alter its adsorption performance. Figure 5 shows desulfurization capacity of various supported TiO2−CeO2 adsorbents versus the PZC value of the support. It can be seen that desulfurization capacity of supported TiO2−CeO2 adsorbents increases with decreasing PZC value of the support. It should be mentioned that the pH of the precursor solutions of mixed Ti−Ce salts to prepare supported TiO2−CeO2 adsorbent was as low as 0.185. Therefore, the surfaces of the studied supports (PZC > 2) were all positively charged during impregnation. It is likely that silica support had a stronger electrostatic adsorption to Ti−Ce precursors, yielding better dispersion of the supported oxides,41 resulting in a higher desulfurization capacity. High dispersion of TiO2−CeO2 on silica supports are also reflected by the absence of TiO2−CeO2 peak on EH-5 and MCM-48 supports, unlike the unsupported TiCeO in XRD patterns (Supporting

Figure 5. Desulfurization capacity of supported TiO 2 −CeO 2 adsorbents versus PZC value of the supports, SiO2 (EH-5), TiO2, AC, and Al2O3.

Information Figure S3), as the XRD technique has limitation on the identification of smaller particles. The result suggests that the PZC value of the support plays a more important role than surface area. An exceptional case was AC, which had a higher PZC value than TiO2 but a slightly higher desulfurization capacity than TiO2. This may be attributed to the significantly higher surface area of AC (930 m2/g) than that of TiO2 (90 m2/g). Another possibility is that TiO2−CeO2 may have more suitable interaction with silica support than other supports, which makes the TiO2−CeO2 sites more active for sulfur oxidation and/or sulfones adsorption. To clarify the effect of support, further investigations are needed and in progress in our laboratory by using the DFT method. As suggested from the above results, silica is the most effective support to load TiO2−CeO2 oxides for desulfurization among the supports investigated. Mesoporous silica materials, such as SBA-15, have PZC values similar to EH-5 (between 2 and 4)42,43 but differ in the surface area. In order to examine the effect of surface area of silica supported TiO2−CeO2 adsorbents, various silica supports with different surface area, including EH-5, SBA-15, MCM-41, and MCM-48, were investigated for desulfurization. Mesoporous silica supports were chosen to ensure the pore size is large enough so that diffusion of the bulky sulfur compounds in the diesel fuel (i.e., 4,6-DMDBT (0.6 nm44)) would not be limited. Table 4 lists BET surface area of the supports and supported TiO2−CeO2 adsorbents and the desulfurization capacity of various silicasupported TiO2−CeO2 adsorbents. The adsorption capacity per unit volume of adsorbent follows the order of MCM-48 > MCM-41 > EH-5 > SBA-15. The higher volume-based capacity of TiO2−CeO2/EH-5 than TiO2−CeO2/SBA-15 is due to the higher packing density of the EH-5 supported TiO2−CeO2. The correlation between BET surface area and desulfurization capacity of various silica-supported TiO2−CeO2 adsorbents is shown in Figure 6. It is worth noting that desulfurization capacity increases monotonically with increasing surface area of the silica supports. This is probably due to a better dispersion of metal oxides with higher surface area of silica support, resulting in a greater amount of accessible TiO2−CeO2 active 15751

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Table 4. BET Surface Area of Supports and Supported TiO2−CeO2 Adsorbents and Desulfurization Capacity of Various Silica Supported TiO2−CeO2 Adsorbents support for TiO2−CeO2

MCM-41

MCM-48

SBET‑support (m2/g) SBET‑supported TiO2−CeO2 (m2/g)

315 196

EH-5

SBA-15 950 750

1229 850

1281 1089

unsupported 249

Q (mg-S/g-sorb) Pack. Density(mg/cm3) Q (mg-S/mL-sorb)

0.354 0.40 0.142

0.636 0.20 0.127

0.778 0.20 0.156

1.143 0.20 0.229

0.035 0.46 0.016

Figure 7. The breakthrough curves for the regenerated TiO2− CeO2/MCM-48 adsorbent coincide well with that for the fresh

Figure 6. Desulfurization capacity of various silica supported TiO2− CeO2 oxides, bulk TiO2−CeO2, and MCM-48 support versus their surface areas.

Figure 7. Breakthrough curves of total sulfur compounds over the TiO2−CeO2/MCM-48 adsorbent in the first three regeneration cycles via oxidative treatment using air at 340 °C. (Inset: Desulfurization capacity versus regeneration cycles.)

sites or exposed surface area of the TiO2−CeO2 particles45 for sulfur oxidation and/or sulfone adsorption and thus a higher desulfurization capacity. The MCM-48-supported TiO2−CeO2 adsorbent shows the highest desulfurization capacity, which may be attributed to a better dispersion of TiO2−CeO2 oxides over MCM-48 support.7 The control experiment shows that the desulfurization capacity of MCM-48 support itself is only 0.012 mg-S/g-sorb as shown in Figure 6, indicating that MCM-48 support alone does not contribute to the significant increase in the desulfurization capacity. It should be mentioned that the TiO2−CeO2/MCM-48 adsorbent plays at least two main roles in ADS; one is to provide active sites for sulfur oxidation, which can be determined by the supported TiO2−CeO2 oxides, and the other is to provide adsorption sites for oxidized sulfones, which can be influenced by not only the supported TiO2−CeO2 oxides but also the silica support itself. The adsorption of oxidized sulfones is likely through two types of interactions, (a) acid−base interaction between acidic sites on TiO2−CeO2 oxides and basic sulfones (electronegative oxygen in sulfones acts as electron donor and thus it can behave as a weak Lewis base46) and (b) hydrogen bonding interaction between sulfones and silanol groups47 on the surface of the silica support (the average concentration of silanol groups on silica materials is 5 −OH/nm−2).48 The contribution of the two modes to the sulfone adsorption will be further studied in the future work. 3.3. Adsorbent Regeneration. For practical applications, a potential adsorbent should not only possess a high adsorption capacity and selectivity to sulfur species but also have excellent regenerability and stability.49 The regeneration of spent TiO2− CeO2/MCM-48 adsorbent was performed via oxidative treatment at 340 °C under an air flow rate of 50 cm3/min for 2 h. The adsorption performance of the regenerated adsorbents was examined and compared with that of the fresh ones. The breakthrough curves of total sulfur compounds and the desulfurization capacity over the regenerated TiO2−CeO2/ MCM-48 adsorbents in the first three cycles are shown in

one, with the breakthrough desulfurization capacities of 1.143, 1.083, and 1.095 mg-S/g-sorb, or 95, 90, and 91 mL-F/g-sorb, respectively, for the first three regeneration cycles. The standard deviations were 1.6%, 2.2%, and 1.1%, respectively. The results show that the adsorption capacity can be recovered by air oxidation. In other words, oxidative air treatment can be an effective regeneration method for the TiO2−CeO2/MCM48 adsorbent. Figure 8 shows sulfur K-edge XANES spectra of the spent and regenerated TiO2−CeO2/MCM-48 adsorbent and three

Figure 8. Sulfur K-edge XANES spectra of the spent and regenerated TiO2−CeO2/MCM-48 adsorbent and three reference compounds, 4,6-DMDBT, DBTO2, and NiSO4.

reference compounds, 4,6-DMDBT, DBTO2, and NiSO4. The peak positions of sulfur compounds vary with the different oxidation states of sulfur in XANES spectra. It can be seen that no sulfur species were detected on the regenerated TiO2− CeO2/MCM-48 adsorbent, indicating that the adsorbed sulfone species can be oxidatively decomposed during air treatment. Additionally, the fact that the spent TiO2−CeO2/MCM-48 15752

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Figure 9. Proposed selective desulfurization process for ultra-deep desulfurization of diesel fuel.

adsorbent was effectively regenerated at 340 °C also implies a good thermal stability of the adsorbent. The negligible loss in the adsorption capacity of the regenerated TiO2−CeO2/MCM48 adsorbent suggests that the TiO 2 −CeO 2 /MCM-48 adsorbent can be a promising air-regenerable adsorbent for ultra-deep desulfurization of diesel fuel. 3.4. Proposed Two-Stage Process for Ultra-deep Desulfurization. On the basis of the above study, a novel two-stage process for selective and ultra-deep desulfurization of diesel fuel is proposed for the production of ultraclean fuel, as shown in Figure 9. In the first stage, peroxides are generated in fuel through sunlight or visible light irradiation. In the second stage, light-irradiated fuel is fed into an adsorption bed packed with dual-functional adsorbent/catalyst material (e.g., TiO2− CeO2/MCM-48) which serves as the oxidation catalyst for refractory sulfur compounds to sulfone species by the peroxides generated in the first step, as well as the adsorbent for simultaneous sulfone adsorption. After the adsorption bed reaches its working capacity, it can be switched to the regeneration side using oxidative air treatment. With the proposed new process, ultra-deep adsorptive desulfurization of diesel fuels can be achieved effectively under ambient conditions without using costly hydrogen. Moreover, this process may incorporate a good utilization of natural sunlight, which may lead to a more sustainable and clean process.

For different silica supported TiO2−CeO2 oxides, the desulfurization capacity increases monotonically with the increasing of surface area in the order of MCM-48 > MCM41 > SBA-15 > EH-5, which may be attributed to a higher dispersion of TiO2−CeO2 oxides on the higher surface area support. The TiO2−CeO2/MCM-48 adsorbent can be regenerated by oxidative air treatment; no sulfur of any types (sulfones, or sulfates, or DMDBT) was detected on the regenerated TiO2− CeO2/MCM-48 adsorbent by sulfur K-edge XANES spectroscopy. On the basis of the present study, a new two-stage process for ultra-deep adsorptive desulfurization of diesel fuel is proposed that may provide an attractive path to achieve ultraclean fuel more effectively and efficiently under ambient conditions without using costly hydrogen.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Effect of TiO2−CeO2 loading on the desulfurization capacity over the TiO2−CeO2/MCM-48 adsorbent with TiO2−CeO2 loading of 10.0, 13.3, 15.0, and 20.0 wt % from light-irradiated diesel at a breakthrough point of 1 ppmw-S at 25 °C and LHSV of 9.6 h−1 (Figure S1), effect of BET surface area of adsorbents, desulfurization capacity (mg-S/m2-sorb) versus BET surface area (m2/g) of various supported TiO2−CeO2 adsorbents (Figure S2), and XRD patterns of various supported TiO2− CeO2 adsorbents (Figure S3). This information is available free of charge via the Internet at http://pubs.acs.org.

4. CONCLUSION A systematic study was carried out on ultra-deep adsorptive desulfurization from light-irradiated diesel fuel over supported TiO2−CeO2 adsorbents. Sulfur K-edge XANES identified sulfones as were the major sulfur species on the spent TiO2− CeO2/MCM-48 adsorbent. Adsorption selectivity of different compounds from a model fuel follows the order of DBTO2 ≫ DBT > MDBT > BT > DMDBT > Phe > MNap ∼ Flu > Nap, indicating a higher adsorption selectivity of sulfones over the original sulfur compounds and aromatics in fuel. We conclude that the original sulfur species are chemically transformed to sulfones that are more selectively adsorbed during ADS of lightirradiated diesel fuel over the supported TiO 2 −CeO 2 adsorbent, resulting in over 30-fold higher desulfurization capacity (95 mL-F/g-sorb or 1.143 mg-S/g-sorb) as compared to ADS of the original diesel fuel. Additionally, the adsorption of sulfones over supported TiO2−CeO2 adsorbent is likely through the highly electronegative oxygen atom rather than sulfur atom. Desulfurization capacity of light-irradiated fuel over supported TiO2−CeO2 adsorbents is found to increase with decreasing PZC value of the supports. Among the different supports examined in this work, silica material with the lowest PZC value (2−4) is the most effective support to load TiO2−CeO2 oxides for desulfurization, possibly through a strong electrostatic adsorption of Ti−Ce precursors over the silica support.

Corresponding Author

*C. Song. Phone: (814) 863-4466. Fax: (814) 865-3573. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the U.S. Department of Energy, National Energy Technology Laboratory, the U.S. Office of Naval Research, and the U.S. National Science Foundation−U.S. Environmental Protection Agency Joint TSE program. J. Xiao gratefully acknowledges the support by the National Natural Science Foundation of China (21306054), the Guangdong Natural Science Foundation (S2013040014747), and the Fundamental Research Funds for the Central Universities (2013ZM0047). Sulfur K-edge XANES work at the CMC Beamline is supported in part by the Office of Basic Energy Sciences of the U.S. Department of Energy and by the National Science Foundation Division of Materials Research. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC0215753

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06CH11357. We would like to thank Prof. Jim Adair at Penn State for the help on Zeta potential measurements.

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