Study of Adsorptive Denitrogenation of Diesel Fuel over

Article pubs.acs.org/IECR

Study of Adsorptive Denitrogenation of Diesel Fuel over Mesoporous Molecular Sieves Based on Breakthrough Curves Han Zhang† and Haiyan Song*,‡ ‡

Department of Chemistry, College of Science, Northeast Forestry University, Harbin 150040, China China Tianchen Engineering Corporation, Harbin 150076, China

†

ABSTRACT: Adsorptive denitrogenation of model fuel and commercial diesel containing organic nitrogen compounds was studied over three typical mesoporous molecular sieves (Ti-HMS, HMS, and MCM-41) in a fixed-bed adsorption system at ambient pressure. The adsorbents were characterized by X-ray diffraction, UV−visible spectroscopy, transmission electron microscopy, N2 physical adsorption−desorption, and temperature-programmed NH3 desorption techniques. The adsorption capacities for various compounds (pyridine, quinoline, pyrrole, and indole) were examined and compared on the basis of the breakthrough curves, which provided insight into the adsorption behaviors over the different adsorbents. It is shown that the breakthrough time is shortened at increased temperature and liquid hourly space velocity (LHSV). The adsorption performance of HMS and Ti-HMS depends dominantly on the acid−base interaction. In addition, the polarity of the adsorbate is another factor that influences the adsorption capacity. The introduction of Ti improves the acidity of Ti-HMS, which thus exhibits a higher adsorption capacity than HMS, as confirmed by UV−visible spectroscopy and temperature-programmed NH3 desorption. The adsorption performance of nonacidic MCM-41 depends principally on the molecular weight of the adsorbate. For commercial diesel, the adsorption competition between sulfur and nitrogen compounds influences the adsorption of total nitrogen.

1. INTRODUCTION Recently, the U.S. Environmental Protection Agency issued regulations that will require refineries to reduce the sulfur and nitrogen contents of fuel oil.1 Traditional desulfurization has received considerable attention all over the world for many years. Currently, the denitrogenation of transportation fuels is receiving increasing attention in the worldwide research community because of the increase in stringent regulations and fuel specifications in many countries for environmental protection purposes. However, denitrogenation is usually more difficult than desulfurization because most organonitrogen compounds are much less reactive than organosulfur compounds.2−5 Two types of organic nitrogen-containing compounds (NCCs) are found in fuel oil. One consists of neutral compounds with pyrrole rings, such as pyrroles, indoles, carbazoles, and benzocarbazoles. The other consists of basic compounds with pyridine rings, such as pyridines, tetrahydroquinolines, quinolines, and acridines.2−5 It is necessary for refiners to monitor both neutral and basic nitrogen species during the refining process. On one hand, neutral NCCs tend to build gums through polymerization, which leads to the plugging of burners and injectors of petroleum processing units. On the other hand, basic NCCs have an inhibiting effect on currently used catalysts in catalytic converters for reducing CO and SOx.6 Moreover, the combustion of NCCs generates NOx which are responsible for acid rain. For these reasons, there is a strong need for nitrogen removal from fuels. Hydrodenitrogenation (HDN) requires high temperature and pressure, large reactor volume, and active catalysts. As is well-known, increasing the volume of a high-temperature and high-pressure reactor is very expensive, although further improvements in © 2012 American Chemical Society

catalyst activity over existing hydrotreating catalysts are still possible through continued catalytic research and development.7 Therefore, new approaches, such as adsorptive denitrogenation (ADN), oxidative denitrogenation (ODN), and acid extractive denitrogenation have been proposed. Among such methods, ADN using a solid material as an absorbent arouses economic interest and has been intensively studied. Reported adsorbents include reduced metals,8−11 metal oxides,12,13 zeolite-based materials,14−18 and carbon materials.19−22 Adsorption capacity and selectivity have been examined on the basis of breakthrough curves, and the electronic properties of adsorbents influence the adsorptive performance. However, commercially available adsorbents cannot effectively adsorb nitrogen compounds.23 The new challenge is to use suitable adsorbents to efficiently remove these nitrogen compounds from transportation fuels. In the present study, denitrogenation of model fuel and commercial diesel containing NCCs was studied using the mesoporous materials Ti-HMS, HMS, and MCM-41 as the adsorbents in a fixed-bed adsorption system at ambient pressure. The adsorption behaviors and mechanisms of pyridine, quinoline, pyrrole, and indole have been examined according to their breakthrough curves and a series of adsorbent characterizations. In addition, a tentative study of the adsorptive denitrogenation of commercial diesel was carried out, and a study of the regeneration of the adsorbents is in progress. Received: Revised: Accepted: Published: 16059

August 12, 2012 November 23, 2012 November 23, 2012 December 3, 2012 dx.doi.org/10.1021/ie302169r | Ind. Eng. Chem. Res. 2012, 51, 16059−16065

Industrial & Engineering Chemistry Research

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2. EXPERIMENTAL SECTION 2.1. Adsorbent Preparation. Three typical mesoporous molecular sieves (Ti-HMS, HMS, and MCM-41) were used as adsorbents for the nitrogen-containing compounds in fuel. TiHMS with a theoretical SiO2/TiO2 molar ratio of 50 was synthesized according to the literature method.24 HMS was synthesized using the same procedure as Ti-HMS, without the addition of titanium species. MCM-41 was synthesized according to the literature method.25 2.2. Adsorbent Characterization. The Ti content of TiHMS presented as the molar ratio of SiO2 to TiO2 was obtained on a Bruker SRS-3400 sequential X-ray fluorescence (XRF) spectrometer. Powder X-ray diffraction (XRD) patterns were recorded using a Rigaku D/Max 2400 diffractometer employing Cu Kα radiation. UV−visible (UV−vis) spectra were obtained on a JASCO UV550 spectrometer using BaSO4 as the internal standard. N2 adsorption isotherms were obtained using a Quantachrome Autosorb-1 physical adsorption apparatus. The pore size distribution was calculated using the Barrett−Joyner−Halenda (BJH) model from the adsorption branches of the isotherms. Transmission electron microscopy (TEM) images were recorded on a Tecnai G2 20 S-twin instrument (FEI Company) with an acceleration voltage of 200 kV. Temperature-programmed ammonia desorption (NH3 TPD) was employed to probe the acidities of the adsorbents. The samples were placed in homemade equipment that was connected to a gas chromatograph (HP GC-7890T) with a thermal conductivity detector. A 0.1-g sample was dried in a N2 flow (40 mL/min) at 200 °C for 1 h and then cooled to ambient temperature to absorb NH3. Desorption of NH3 was performed in N2 flow (40 mL/min) by increasing the temperature to 900 °C at a rate of 10 °C/min. 2.3. Model Fuel Preparation. Basic nitrogen-containing compounds pyridine (analytical reagent) and quinoline (analytical reagent) were purchased from the Guangxi Chemical Co. Neutral nitrogen-containing compounds pyrrole (analytical reagent) and indole (chemically pure) were purchased from Sinopharm Chemical Reagent Co. n-Octane was purchased from the Tianjin Bodi Chemical Co. Commercial diesel was purchased from China National Petroleum Corporation (CNPC). All chemicals were used without any pretreatment. Pyridine, quinoline, pyrrole, and indole were dissolved in n-octane as the model fuel. Detailed concentrations are listed in Table 1.

effluent from the top of the tube was stored in an airtight sample bottle for further analysis. The nitrogen concentrations of samples were analyzed on a KY-3000N instrument (Keyuan Inc. Co.) equipped with a nitrogen-chemiluminescence detector with a flameless burner.

3. RESULTS AND DISCUSSION 3.1. Adsorbent Characterization. Figure 1a shows the XRD patterns of the three adsorbents. HMS and Ti-HMS

Figure 1. (a) XRD patterns and (b) UV−visible spectra of Ti-HMS, HMS, and MCM-41.

exhibit similar (1 0 0) diffraction peaks at 2θ = 2.4° accompanied by broader unresolved higher-order reflections, indicating that the adsorbents have mesostructures with lack of long-range order.26 For MCM-41, an intense (1 0 0) peak at 2θ = 2.0° with three well-resolved (1 1 0), (2 0 0), and (2 1 0) peaks at 2θ = 3.0−6.0° can be observed, which is assigned to the hexagonal symmetry of mesoporous MCM-41, indicating the long-range order and good textural uniformity of the mesoporous structure.27 The UV−visible spectra of the adsorbents are shown in Figure 1b. There is no absorption for HMS and MCM-41 without the introduction of framework Ti. Ti-HMS presents an absorption band near 210−225 nm, which originates from the charge transfer of oxygen 2p electron to the empty 3d orbit of Ti4+, indicating the existence of framework Ti in tetrahedral coordination.28 The absorption band at 260−280 nm is attributable to the partially polymerized hexacoordinated Ti species in the materials.29 The lack of absorption at 330 nm indicates the absence of the anatase phase. According to XRF analysis, the actual molar ratio of SiO2 to TiO2 is 42. Nitrogen adsorption−desorption isotherms and pore size distribution of the adsorbents are shown in Figure 2. Both TiHMS and HMS exhibit type IV isotherms according to the IUPAC classification, with a typical H1 hysteresis loop of mesoporous solids. This type of hysteresis loop indicates that the materials have uniform pore sizes and shapes.30 Each slope with a sharp step at a relative pressure (P/P0) of 0.2−0.4 indicates the capillary condensation in mesopores.31 The specific surface area of Ti-HMS and HMS is 830.0 and 804.7 m2/g, respectively. For MCM-41, the type IV isotherm with a H2 hysteresis loop shows a distinct step over a narrow range of relative pressures P/P0 = 0.4−0.55, indicating the existence of ordered mesopores in the silica material.32 MCM-41 also shows a large specific surface area of 938.6 m2/g. The mean pore sizes of Ti-HMS, HMS, and MCM-41 were found to be 2.3, 2.3, and

Table 1. Concentrations of NCCs in Model Fuel and Commercial Diesel NCC pyridine quinoline pyrrole indole diesel

concentration of NCC (ppm)

concentration of N (ppm)

molar concentration of N (μmol/g)

840.79 1409.78 717.86 1304.21

149.00 153.00 150.00 153.00 55.00

10.64 10.93 10.71 10.93 3.92

2.4. Adsorption Tests. The adsorbents were pelletized, sieved by 20−40-mesh screen, and pretreated at 170 °C for 1 h to remove moisture and impurities. The adsorptive denitrogenation of model fuel was performed over an adsorbent weight of 0.3 g at temperatures of 15, 40, and 60 °C, with the fuel being sent into the adsorbent bed by a pulsation-free pump and flowing at different liquid hourly space velocities (LHSVs). The 16060

dx.doi.org/10.1021/ie302169r | Ind. Eng. Chem. Res. 2012, 51, 16059−16065


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attributed to the desorption of ammonia from the weak acidic sites.33 Obviously, the acidity of each Ti-HMS sample is higher than that of HMS, and the acidity increases with increasing Ti content, indicating that acid sites can be introduced into HMS by loading a transition metal such as Ti.34 In particular, the acidity of such materials is related to the presence of isolated silanol groups (Si−OH) and acid groups (Ti−OH and Ti−O− Si−OH).33 According to a related study based on pyridine adsorption FTIR spectroscopy, it can be inferred that the acidity in Ti-HMS originates from the symmetrical vibrations of both Lewis and Brønsted acid sites and that the introduction of framework Ti improves the acidity to a certain extent,35 which agrees with the NH3 TPD results. In addition, the MCM-41 sample shows no desorption band because of a lack of surface acidity.33 3.2. Effects of Temperature and LHSV on Adsorption. A breakthrough curve reflects the adsorption performance of an adsorbent. A longer breakthrough time indicates that the adsorbent has a better adsorption capacity for an adsorbate. The breakthrough curves of pyridine in the model fuel over TiHMS bed are shown in Figures 5 and 6 to investigate the effects

Figure 2. Nitrogen adsorption−desorption isotherms and pore size distributions of Ti-HMS, HMS, and MCM-41.

2.8 nm, respectively. In addition, the total pore volumes of TiHMS, HMS, and MCM-41 were 0.69, 0.65, and 0.93 cm3/g, respectively. The TEM images in Figure 3 provide insight into the porosity structures of the adsorbents. Both Ti-HMS and HMS

Figure 3. TEM images of Ti-HMS, HMS, and MCM-41.

exhibit typical wormlike mesostructures with uniform dimension. For MCM-41, ordered channel arrays of parallel lines are clearly observed along the direction of the pore arrangement, indicating that the sample has a highly ordered long-range structure. Figure 4 shows the NH3 TPD results of the adsorbents, including a sample of Ti-HMS with a theoretical SiO2/TiO2 molar ratio of 200 for comparison. Both Ti-HMS and HMS show an ammonia desorption band between 250 and 380 °C, indicating that the adsorbent exhibits a heterogeneous distribution of acid strengths. The desorption band can be

Figure 5. Breakthrough curves of pyridine over Ti-HMS at 7.2 h−1 LHSV.

of temperature and LHSV on the adsorption. The breakthrough time at 15 °C is longer than those at 40 and 60 °C (Figure 5), which indicates that the adsorption capacities decrease with increasing temperature. This can be explained from a thermodynamic point of view. The adsorption occurring in a solid is classified as an exothermic process. Therefore, an

Figure 4. NH3 TPD profiles of (a) Ti-HMS (SiO2/TiO2 = 50), (b) Ti-HMS (SiO2/TiO2 = 200), (c) HMS, and (d) MCM-41.

Figure 6. Breakthrough curves of pyridine over Ti-HMS at 15 °C. 16061



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NCCs, inferred from its much longer saturation time. According to the breakthrough order, the adsorption capacities for the four adsorbates increase in the order indole < pyrrole < quinoline < pyridine, which can be attributed to the acid−base interaction. It is well-known that pyridine and quinoline are basic nitrogen compounds, whereas the other two are neutral. Based on the acid test of NH3 TPD (Figure 4), HMS contains a few acid sites although they are weak, resulting in a higher adsorption capacity of HMS for basic compounds than that for neutral ones. For basic NCCs, the adsorption capacity increasing from quinoline to pyridine is possibly related to the adsorption sites density,38 as well as the dipole magnitude of the adsorbate. The breakthrough and saturation capacities for the NCCs were calculated from the molar concentrations of N and the amounts of treated fuel and are listed in Table 2. 3.3.2. Adsorptive Denitrogenation over Ti-HMS. Figure 8 shows the breakthrough curves of the NCCs over Ti-HMS at

increase in temperature is unfavorable to the adsorption of nitrogen compounds on mesoporous molecular sieves, as confirmed in our previous study.36 At 15 °C, the breakthrough curve at 7.2 h −1 LHSV reached steady state slowly, compared with those at 9.6 and 12 h −1 LHSV (Figure 6). As the contact time between the liquid and solid phases increases, more NCCs are adsorbed by the active sites in the adsorbent bed. Generally, higher temperature and LHSV are both unfavorable for the adsorption process. Similar results were obtained for the adsorptions of the other NCCs. 3.3. Adsorptive Denitrogenation over Different Mesoporous Materials. The adsorption performance of an adsorbent usually depends on its chemical characteristics and physical properties such as acid sites, surface area and pore structure. Meanwhile, the characteristics of the adsorbate also influence the adsorption results. The molecular weight and polarity of the adsorbate might provide close insight into the adsorption behavior. The molecular weights of the NCCs studied in this work increase in the order pyrrole < pyridine < indole < quinoline. The polarities increase in the order pyridine < pyrrole < quinoline < indole, which can be inferred from the order of dipole magnitudes according to a relevant report.37 3.3.1. Adsorptive Denitrogenation over HMS. The breakthrough curves of pyridine, quinoline, pyrrole, and indole over HMS at 15 °C and 7.2 h−1 LHSV are shown in Figure 7. Indole

Figure 8. Breakthrough curves of NCCs over Ti-HMS at 15 °C and 7.2 h−1 LHSV.

15 °C and 7.2 h−1 LHSV. The breakthrough amounts for indole, pyrrole, quinoline, and pyridine were 12.02, 12.58, 13.10, and 15.72 gF/gA, respectively. The breakthrough order of the NCCs over Ti-HMS was the same as that over HMS. However, the adsorption capacity of Ti-HMS was much higher than that of HMS. It is reasonable to infer that a direct interaction of framework Ti plays an important role in the adsorption of nitrogen compounds. Upon the addition of Ti, which replaces some of the Si in the framework, the chemical character of HMS changes, and the acidity increases. Fu et al. demonstrated that the number of acid sites in HMS increases to some degree upon the introduction of Ti species.34 Meanwhile, the amount of lattice defects in a mesoporous material increases upon the addition of transition-metal ions such as Ti, which possibly improves the adsorption affinity.39 In addition, the added Ti contains an unoccupied orbital that easily adsorbs NCCs with lone-pair electrons. Therefore, more NCCs can be

Figure 7. Breakthrough curves of NCCs over HMS at 15 °C and 7.2 h−1 LHSV.

first breaks through at 11.76 g of treated fuel per gram of adsorbent (gF/gA), and the ratio of the outlet concentration to the initial concentration in the model fuel (C/C0) for indole increases sharply. Pyridine breaks through at an amount of 14.67 gF/gA, which is about 1.25 times higher than that of indole. The amount of treated fuel corresponding to the saturation point is 36.47, 57.64, and 59.76 gF/gA for indole, pyrrole, and quinoline, respectively. Meanwhile, HMS exhibits a higher adsorption capacity for pyridine than for the other

Table 2. Adsorption Capacities (mmol/g) of the Adsorbents for NCCs

HMS Ti-HMS MCM-41

breakthrough saturation breakthrough saturation breakthrough saturation

pyridine

quinoline

pyrrole

indole

total N of diesel

0.156 − 0.167 − 0.143 0.641

0.154 0.651 0.158 0.680 0.126 0.251

0.143 0.622 0.145 0.644 0.14 0.561

0.128 0.398 0.132 0.403 0.137 0.328

0.061 0.194 0.086 0.214 0.055 0.184

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anchored on the Ti active sites. For compounds with the same acid−base properties, HMS and Ti-HMS adsorb pyrrole over indole and pyridine over quinoline, indicating that the adsorbate with higher polarity is more easily adsorbed, which agrees with the previous report.37 The breakthrough amounts for the NCCs increased significantly, as reported in Table 2. 3.3.3. Adsorptive Denitrogenation over MCM-41. The breakthrough curves of the NCCs over MCM-41 at 15 °C and 7.2 h−1 LHSV are shown in Figure 9. Quinoline and indole

Figure 10. Breakthrough curves of commercial diesel at 15 °C and 7.2 h−1 LHSV.

Ti-HMS, HMS, and MCM-41 were found to be 21.87, 15.46, and 13.89 gF/gA, respectively. The adsorption capacities of total nitrogen are also listed in Table 2 for comparison. Notably, TiHMS exhibited the highest adsorption capacity for NCCs among the three adsorbents. The removal rate of total nitrogen compounds in the diesel was over 90% using Ti-HMS as the adsorbent, in which the basic N-containing compounds are removed completely and the neutral ones can achieve 80%, determined by the titration of perchloric acid-acetic acid solution using methyl violet as the indicator. The acidity value and the typical wormhole pseudo-ordered structure make the adsorbates more difficult to desorb from Ti-HMS. Therefore, the breakthrough capacity of Ti-HMS is about 1.10 times higher than that of HMS, and about 1.17 times higher than that of MCM-41 with well-ordered regular arrangement of hexagonal array. Meanwhile, the three adsorbents also exhibited good decoloration performance for diesel. 3.4. Adsorbent Regeneration. The employed adsorbents are deactivated by the adsorbates, which shortens the lifetimes of the adsorbents. Figure 11 shows the breakthrough curves of

Figure 9. Breakthrough curves of NCCs over MCM-41 at 15 °C and 7.2 h−1 LHSV.

break through at 11.78 and 12.53 gF/gA, respectively. Unlike the other two adsorbents, MCM-41 without acid sites adsorbs indole over quinoline. The adsorption capacities for the four compounds increase in the order quinoline < indole < pyrrole < pyridine (Table 2), indicating that the acid−base interaction does not play an important role in adsorption over MCM-41. The adsorption order over MCM-41 depends principally on the molecular weights of the adsorbates. Adsorbates with lower molecular weights and higher polarity are more easily adsorbed on MCM-41. Therefore, MCM-41 shows higher adsorption properties for pyridine and pyrrole than for indole and quinoline. On the other hand, MCM-41 with a well-ordered regular hexagonal array has a different distribution of related functional groups on the surface and pores, which are the main locations of adsorption sites, benefitting adsorption.40 The adsorption of indole mainly depends on the high negative electrostatic potential and net atomic charge on the two sides of the indole molecular plane, inconsistent with one side of quinoline.37 3.3.4. Adsorptive Denitrogenation of Commercial Diesel. Adsorptive denitrogenation of commercial diesel has been studied tentatively. The initial total nitrogen and sulfur concentrations of the investigated commercial diesel were 55 and 640 ppm, respectively, which were determined by the nitrogen or sulfur chemiluminescence detector with a flameless burner. During the adsorptive denitrogenation of commercial diesel over the molecular sieves, the adsorption competition between the nitrogen and sulfur compounds, such as thiophene, benzothiophene, and their derivatives, results in a decrease of the nitrogen removal rate to a certain extent. This is a possible reason for the different adsorption capacities for model and real fuels. The breakthrough curves for total nitrogen of commercial diesel, which contains pyrrole, quinoline, indole, aniline, and so on, over the three different adsorbents at 15 °C and 7.2 h−1 LHSV are shown in Figure 10. The breakthrough amounts over

Figure 11. Breakthrough curves of pyridine over fresh and regenerated Ti-HMS at 15 °C and 7.2 h−1 LHSV.

model fuel containing pyridine over fresh and regenerated TiHMS at 15 °C. The adsorbent was regenerated by washing with ethanol at ambient temperature and drying at 100 °C. As expected, the two curves in Figure 11 are similar, and both break through at about 15.71 gF/gA, indicating that the regenerated adsorbent exhibits an adsorption capacity similar 16063



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(9) Fukunaga, T.; Katsuno, H.; Matsumoto, H.; Takahashi, O.; Akai, Y. Development of Kerosene Fuel Processing System for PEFC. Catal. Today 2003, 84, 197. (10) Ma, X.; Velu, S.; Kim, J. H.; Song, C. Deep Desulfurization of Gasoline by Selective Adsorption over Solid Adsorbents and Impact of Analytical Methods on ppm-Level Sulfur Quantification for Fuel Cell Applications. Appl. Catal. B: Environ. 2005, 56, 137. (11) Ma, X.; Sprague, M.; Song, C. Deep Desulfurization of Gasoline by Selective Adsorption over Nickel-Based Adsorbent for Fuel Cell Applications. Ind. Eng. Chem. Res. 2005, 44, 5768. (12) Turk, B. S.; Gupta, R. P. RTI’s TReND Process for Deep Desulfurization of Naphtha. Prepr. Pap.-Am. Chem. Soc., Div. Pet. Chem. 2001, 46, 392. (13) Watanabe, S.; Ma, X.; Song, C. Selective Sulfur Removal from Liquid Hydrocarbons over Regenerable CeO2−TiO2 Adsorbents for Fuel Cell Applications. Prepr. Pap.-Am. Chem. Soc., Div. Pet. Chem. 2004, 49, 511. (14) Yang, R. T.; Hernández-Maldonado, A. J.; Yang, F. H. Desulfurization of Transportation Fuels with Zeolites under Ambient Conditions. Science 2003, 301, 79. (15) Hernández-Maldonado, A. J.; Yang, R. T. Desulfurization of Commercial Liquid Fuels by Selective Adsorption via π-Complexation with Cu (I)−Y Zeolite. Ind. Eng. Chem. Res. 2003, 42, 3103. (16) Hernández-Maldonado, A. J.; Yang, R. T. New Sorbents for Desulfurization of Diesel Fuels via π-Complexation. AIChE J. 2004, 50, 791. (17) Velu, S.; Ma, X.; Song, C. Selective Adsorption for Removing Sulfur from Jet Fuel over Zeolite-Based Adsorbents. Ind. Eng. Chem. Res. 2003, 42, 5293. (18) Hernández-Maldonado, A. J.; Stamatis, S. D.; Yang, R. T.; He, A. Z.; Cannella, W. New Sorbents for Desulfurization of Diesel Fuels via Complexation: Layered Beds and Regeneration. Ind. Eng. Chem. Res. 2004, 43, 769. (19) Haji, S.; Erkey, C. Removal of Dibenzothiophene from Model Diesel by Adsorption on Carbon Aerogels for Fuel Cell Applications. Ind. Eng. Chem. Res. 2003, 42, 6933. (20) Sano, Y.; Choi, K. H.; Korai, Y.; Mochida, I. Adsorptive Removal of Sulfur and Nitrogen Species from a Straight Run Gas Oil over Activated Carbons for Its Deep Hydrodesulfurization. Appl. Catal. B Environ. 2004, 49, 219. (21) Sano, Y.; Choi, K. H.; Korai, Y.; Mochida, I. Selection and Further Activation of Activated Carbons for Removal of Nitrogen Species in Gas oil as a Pretreatment for Its Deep Hydrodesulfurization. Energy Fuels 2004, 18, 644. (22) Zhou, A.; Ma, X.; Song, C. Deep Desulfurization of Diesel Fuels by Selective Adsorption with Activated Carbons. Prepr. Pap.-Am. Chem. Soc., Div. Pet. Chem. 2004, 49, 329. (23) Yang, R. T. Adsorbents: Fundamentals and Applications; Wiley: New York, 2003. (24) Tanev, P. T.; Chibwe, M.; Pinnavaia, T. J. Tianium-Containing Mesoporous Molecular Sieves for Catalytic Oxidation of Aromatic Compounds. Nature 1994, 368, 321. (25) Wang, A.; Kabe, T. Fine-Tuning of Pore Size of MCM-41 by Adjusting the Initial pH of the Synthesis Mixture. Chem. Commun. 1999, 20, 2067. (26) Tanev, P. T.; Pinnavaia, T. J. A Neutral Templating Route to Mesoporous Molecular Sieves. Science 1995, 267, 865. (27) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vatuli, J. C.; Beck, J. S. Ordered Mesonorous Molecular Sieves Synthesized by a LiquidCrystal Template Mechanism. Nature 1992, 359, 710. (28) Zhang, W.; Fröba, M.; Wang, J.; Tanev, P. T.; Wong, J.; Pinnavaia, T. J. Mesoporous Titanosilicate Molecular Sieves Prepared at Ambient Temperature by Electrostatic (S+I−, S+X−I+) and Neutral (S°I°) Assembly Pathways: A Comparison of Physical Properties and Catalytic Activity for Peroxide Oxidations. J. Am. Chem. Soc. 1996, 118, 9164. (29) Blasco, T.; Camblor, M. A.; Corma, A.; Pérez-Pariente, J. The State of Ti in Titanoaluminosilicates Isomorphous with Zeolite β. J. Am. Chem. Soc. 1993, 115, 11806.

to that of the fresh one and that the regeneration method is possibly economical and feasible. For commercial diesel, the same method was used to regenerate Ti-HMS. However, the breakthrough amount over regenerated Ti-HMS was only 12.56 gF/gA, which was lower than that over the fresh one (21.87 gF/ gA). A further study on improving the regeneration performance of adsorbents for commercial diesel is in progress.

4. CONCLUSIONS The adsorptive denitrogenation of pyridine, quinoline, pyrrole, and indole in model fuel and commercial diesel over Ti-HMS, HMS, and MCM-41 has been studied in a fixed-bed adsorption system. The breakthrough curves for the different nitrogencontaining compounds provide new insight into the adsorption behavior over various adsorbents. Both temperature and LHSV influence the adsorption performance. The adsorption capacity of the adsorbent is mainly determined by the acidity and pore structure of the adsorbent, the acid−base interaction between adsorbent and adsorbate, as well as the polarity and molecular weight of the adsorbate. The present study shows that the adsorption capacities increase in the order MCM-41 < HMS < Ti-HMS, for the four NCCs and total nitrogen in the diesel, indicating that Ti-HMS is a more effective adsorbent for nitrogen removal. Further investigations of the competitive adsorption of complex components in real fuel and the regeneration of adsorbents are in progress.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-451-8219-0679. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by the Fundamental Research Funds for the Central Universities (DL12BB22). REFERENCES

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Study of Adsorptive Denitrogenation of Diesel Fuel over

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