Adsorptive Denitrogenation and Desulfurization of Diesel Fractions by

Article pubs.acs.org/IECR

Adsorptive Denitrogenation and Desulfurization of Diesel Fractions by Mesoporous SBA15-Supported Nickel(II) Phosphide Synthesized through a Novel Approach of Urea Matrix Combustion Syed A. Shahriar, Hongfei Lin, and Ying Zheng* Department of Chemical Engineering, University of New Brunswick, 15 Dineen Drive, P.O. Box 4400, Fredericton, NB, Canada E3B 5A3 ABSTRACT: Stringent environmental regulations on the sulfur content in transportation fuels have made ultradeep desulfurization of diesel an important research topic. One of the approaches to promote the effectiveness of the conventional hydrodesulfurization (HDS) process is to remove the organic nitrogen-containing compounds from the feed before HDS. Adsorptive removal of nitrogen compounds at room temperature and pressure without the presence of hydrogen was studied using a high-capacity adsorbent, Ni2P/SBA15, which was prepared by the urea matrix combustion (UMxC) method. A metal loading of 7 wt % Ni was observed to be optimum among the loadings tested. The nitrogen adsorption capacity reached 9.1 mg/ g of adsorbent, which is higher than the capacities of most of the reported adsorbents. Ni2P/SBA15 was characterized by Brunauer−Emmett−Teller analysis, X-ray diffraction, Fourier transform IR spectroscopy, and transmission electron microscopy (TEM). The mesoporous nature of the adsorbent was confirmed by nitrogen adsorption/desorption analysis as well as TEM analysis. Uniform dispersion of Ni2P was observed in TEM images. Solvent-washing regeneration was studied, and four adsorption-and-regeneration cycles were carried out. Approximately 95% of the adsorptive capacity of the sorbent was recovered after four cycles.

1. INTRODUCTION Diesel vehicles are extensively operated both in highway and nonhighway transportation systems, which leads to an increasing demand for diesel fuels.1,2 The refining industries need to deal with heavier feedstock because worldwide petroleum reserves are becoming heavier and there is a substantial increase in sulfur and nitrogen contents.3 Low-grade crude contains a higher concentration of sulfur- and nitrogencontaining compounds. For heavy crude oil, the sulfur content is generally in the range 0.2−7.0 wt %. Nitrogen concentrations vary from trace amounts to more than 1.0 wt %, and the concentrations of metals vary from a few parts per million to over 1000 ppm.4 The maximum sulfur content in diesel fuel has been restricted to 15 ppmw since 2006 in many countries, including Canada. As a result of the increasingly stringent environmental regulations concerning the sulfur content in transportation fuels, refiners in most parts of the world are facing the inevitable reality that they need to produce clean automotive fuels with ultralow sulfur levels.5 Nitrogen-containing compounds (NCCs), which are naturally present in atmospheric gas oil (AGO) and light cycle oil (LCO) used as feedstocks for diesel fuel production, are traditionally responsible for color and gum formation. They have been identified as strong inhibitors of hydrodesulfurization (HDS) reactions during the hydrotreatment process, even when they are present in very low concentrations.6 Basic NCCs are considered as stronger inhibitors of HDS reactions than neutral ones.7,8 As a result, refractory sulfur-containing compounds (SCCs) are deprived of chances to occupy the active sites of catalysts. Therefore, the removal of NCCs before the hydrotreatment process can be an effective way to promote the efficiency of ultradeep desulfurization processes. An © 2012 American Chemical Society

adsorber may be coupled with a hydrotreater. The feed is introduced to the adsorber and hydrotreater in sequence. NCCs can then be selectively removed from liquid hydrocarbons in the adsorber by adsorption. This is a promising approach because the adsorption can be conducted at ambient temperature and pressure without using hydrogen.9 Developing an efficient adsorbent is a key to the selective removal of NCCs from diesel feedstock by the adsorption approach. Various materials, including activated carbon, zeolites, silica, etc., have been studied as sorbent materials.5 Transition-metal phosphides have attracted considerable attention as hydrotreatment catalysts.10 The combination of the iron-group metals Co and Ni with Mo and W in commercial hydroprocessing catalysts and the use of phosphorus as a promoter are well-known.11,12 The hydrodenitrogenation (HDN) activities of various binary and ternary transition-metal phosphides (Co2P, Ni2P, WP, MoP, NiMoP, CoMoP, and MoP) toward o-propylaniline were evaluated, and all of these compounds showed promising results.13 However,the use of transition-metal phosphides as sorbent materials for adsorptive removal of NCCs from diesel fuels has not been documented. Transition-metal phosphides can be loaded on various support materials to obtain better dispersion.14 Different types of single oxides as well as binary oxides have been used as support materials. During the last 16 years, significant progress has been achieved in the field of nanoporous Received: Revised: Accepted: Published: 14503

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concentration was kept within 2%. X-ray diffraction (XRD) data were acquired using a Bruker D8 Advance diffractometer equipped with a two-circle (θ and 2θ) goniometer housed in a radiation safety enclosure. The X-ray source was a sealed 2.2 kW Cu X-ray tube maintained at an operating current of 30 mA at 40 kV. The surface morphology of Ni2P/SBA15 was studied by transmission electron microscopy (TEM) on a JEOL 2010 scanning transmission electron microscope operated at 200 keV 2.4. Batch Adsorption. Batch adsorption experiments were performed at ambient temperature and pressure. LCO and a model oil containing model NCCs and SCCs were used as feeds. The model oil was blended to simulate diesel, and its composition is listed in Table 1. The composition of LCO is also listed in Table 1.

molecular sieves after the discovery of ordered mesoporous silicas (OMSs).15 Materials such as MCM-41 and FSM-16 have gained a lot of attention because of their range of pore sizes beyond those achievable in zeolites.16,17 However, these materials are hydrothermally unstable because of their limited pore sizes and thin walls. This drawback has been overcome by using amphiphilic block copolymers as structure-directing agents.18,19 The mesoporous material SBA-15 has gained considerable attention for several reasons: large mesopores, thicker pore walls, the presence of irregular interconnecting micropores, and high thermal and hydrothermal stabilities due to its thicker mesoporous walls.20 In this work, a novel sorbent material, nickel phosphide, was synthesized using the urea matrix combustion (UMxC) approach. Various loading ratios of nickel phosphides on SBA-15 were examined. Both LCO and a model diesel oil were studied as feeds.

Table 1. Composition of LCO and the Model Oil

2. EXPERIMENTAL SECTION 2.1. Synthesis of Mesoporous SBA15. Mesoporous SBA15 was synthesized by a sol−gel method.21 Tetraethyl orthosilicate (TEOS), the triblock copolymer Pluronic P123, hydrochloric acid, and water were purchased form SigmaAldrich and used as received as the silica precursor, template, acidulate, and solvent, respectively. A TEOS/P123/HCl/H2O molar ratio of 1.0:0.017:5.7:193 was used to synthesize mesoporous SBA-15. Initially, the neutral, nonionic template P123, which was used as the structure-directing agent in the SBA-15 synthesis, was added to the aqueous solution of HCl at 35 °C under continuous stirring for good dispersion. TEOS was added after the complete dissolution of the copolymer. The mixture was aged for 24 h when a white precipitate formed in order to obtain a homogeneous gel. After the aging period, the temperature was raised to 100 °C for 24 h for the hydrothermal reaction. The sample was then filtered, washed with distilled water to remove Cl−, and then dried at room temperature. The dried sample was placed in a furnace, which was heated from room temperature to 550 °C at a heating rate of 10 °C/min. The sample was calcined at 550 °C for 5 h in air. 2.2. Uploading of Metal Phosphides by the UMxC Method. In the UMxC method, a urea/metal molar ratio of 1:1 was used. The required amounts of urea and the metal and phosphorus precursors to give the desired Ni loading and a Ni/ P ratio of 1:2 were dissolved in water. The mass of water was twice the total mass of urea and the metal and phosphorus precursors. A few drops of concentrated nitric acid were added to dissolve the nickel nitrate to form a clear green solution. Next, the metal phosphate solution was poured onto the SBA15, and the combination was mixed together for 10 minutes. After the precursor was uploaded, the mixture was dried at 50 °C for 3 h and subsequently combusted at 500 °C for 10 min in air.22 The calcined sample was then reduced at 577 °C at a heating rate of 3 °C/min for 5 h under a 10 mL/ min H2 flow. The resulting sorbent material is denoted as Ni2P/SBA15. 2.3. Characterization. Nitrogen adsorption/desorption measurements were performed using a Quantachrome Autosorb-1 instrument. Specific surface areas were calculated using the multipoint Brunauer−Emmett−Teller (BET) equation within the linear region of the relative pressure (0.05−0.65 for SBA15 and 0.05−0.44 for Ni2P/SBA15). Surface functional groups of the sorbent were characterized using a Nicolet 6700 FTIR spectrometer at a resolution of 4 cm−1 over the range 4000−400 cm−1 by the KBr pellet method. The sample

LCO Chemical Composition (wt %) paraffins 11.40 cycloparaffins 5.50 alkylbenzenes 8.30 indans and tetralines 3.60 indenes 2.80 napthalene 0.10 C11 and napthalenes 35.40 acenaphthenes 13.00 acenaphthylenes 10.70 tricyclic aromatics 9.10 Nitrogen and Sulfur Contents (ppmw) nitrogen 494 sulfur 5034

model oil 47.30 − 40.92 − − 0.31 − − − − 999.90a 761.60b

a

Indole, quinoline, and carbazole were used the NCCs and had equal proportions by mass. Each had a concentration of 333.30 ppmw N (23.8 μmol N/g). bDBT or 4,6-DMDBT was used as the sulfurcontaining compound and had a concentration of 761.60 ppmw S (23.8 μmol S/g).

Adsorption was run for 24 h to reach the equilibrium. Equation 1 was used to calculate the nitrogen and sulfur adsorption capacities (in milligrams of N or S per gram of adsorbent, denoted as mgN,S/gads): (C0 − Ce)R (1) 1000 where C0 and Ce are the initial and equilibrium concentrations of nitrogen or sulfur (in ppmw), respectively, and R is the oilto-adsorbent ratio (in goil/gads). 2.5. Fixed-Bed Adsorption. Adsorption of nitrogen and sulfur from LCO was performed in a fixed bed at ambient temperature and pressure at a weight hourly space velocity (WHSV) of 0.29 goil gads−1 h−1. A low WHSV was used to provide more residence time. The nitrogen and sulfur adsorption capacities (in gN,S/gads) were calculated using eq 2:23 capacity =

capacity = 1000 × WHSV

∫0

t

ΔCt(t ) dt

(2)

where t is the run time (in h) and ΔCt(t) is the difference between the initial and final concentrations (in wt %). Figure 1 shows the experimental setup for the fixed-bed adsorption measurements. A high pressure protected pump (Gentech series III) was used to flow oil through the adsorbent bed. 14504

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Figure 2. Nitrogen adsorption/desorption isotherms of SBA15 and Ni2P/SBA15 with various Ni loadings (Po = atmospheric pressure).

Figure 1. Experimental setup for fixed bed adsorption.

2.6. Analysis of Oil Samples. The total nitrogen and sulfur contents of LCO and model oil were measured using Antek NS 9000 nitrogen/sulfur analyzer with detection ranges of 50 ppbw to 17 wt % and 40 ppbw to 40 wt % for nitrogen and sulfur, respectively. The analyzer was first calibrated with known concentrations of indole and dibenzothiophene (DBT) in dodecane solution. Nitrogen was detected by the emission of chemiluminescence, which is specific for chemically bound nitrogen only, and the sulfur was detected by fluorescence emission that is completely specific for sulfur in the original sample. The specialized data acquisition software (Antek model 9000) works in conjunction with the Antek 9000 series analyzer to simplify the instrument operation.

Figure 3. Pore size distributions of SBA15 and Ni2P/SBA15 with various Ni loadings.

Table 2. Physical Properties of the Support and the Adsorbents Prepared by the UMxC Method adsorbent SBA15 Ni2P/ SBA15

3. RESULTS AND DISCUSSION 3.1. Characterization. Nitrogen adsorption/desorption isotherms and pore size distributions for SBA15 and Ni2P/ SBA15 with various Ni loadings are shown in Figures 2 and 3 respectively. All four isotherms show typical type-IV behavior in the Brunauer−Deming−Deming−Teller (BDDT) classification characteristics of mesoporous materials.24 The BET surface area, average pore size, and total pore volume of SBA15 and the different Ni2P/SBA15 sorbents are listed in Table 2. The nitrogen adsorption/desorption isotherm of SBA15 shows a narrow hysteresis loop at P/Po = 0.7−0.8 due to capillary condensation.25 Compared with SBA15, the nitrogen adsorption−desorption isotherms of Ni2P/SBA15 showed a significant decrease in pore volume. Figure 3 shows that the average pore size became smaller when nickel phosphide was loaded. The specific surface area decreased with increasing metal loading. When the Ni loading reached 20 wt %, the surface area dropped to about one-third of that for the original support material. This is probably due to the fact that a high loading of the metal

metal loading (%)

specific surface area (m2/g)

total pore volume (cc/g)

pore size (nm)

0 7 15 20

945.70 354.50 299.30 279.30

1.32 0.55 0.52 0.62

7.70 6.20 7.30 4.40

blocks some inner pores and therefore decreases the surface area. Figure 4 shows the ammonia temperature-programmed desorption (NH3-TPD) profile of Ni2P/SBA15 with 7% Ni loading. The peak observed around 200 °C is due to physically adsorbed NH3.26 No peaks were observed at higher temperature, which indicates that no strong-acid sites were present in the adsorbent. Desorption of NH3 observed between 300 and 400 °C can be assigned to weak-acid sites, which might come from the weakly acidic solid SBA15. Figure 5 shows FTIR spectra of SBA15 and fresh, spent, and regenerated Ni2P/SBA15 containing 7 wt % Ni. The FTIR spectrum of SBA15 shows that the Si−O−Si asymmetric stretch at 1085 cm−1 dominates in all of the samples. The Si− O−Si symmetric stretch occurs at 804 cm−1, and the band at 466 cm−1 is assigned to the Si−O−Si bending mode. The band centered at 3430 cm−1 is assigned to the vibrations of the silanol groups.21 The band at 957 cm−1 can be assigned to Si− 14505

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Figure 4. NH3-TPD profile of Ni2P/SBA15 with 7 wt % Ni. Figure 6. XRD patterns of Ni2P/SBA15 with various Ni weight loadings. Asterisks denote Ni2P peaks.

L = Snf̅

(4) 2

in which S is the effective surface area (in m /g), n̅ is the average metal atom surface density (1.01 × 1015 atoms/cm2 for Ni2P22), and f is the fractional weight loading of the active phase in the sample (e.g., grams of metal phosphide per gram of adsorbent).30 The calculation results are summarized in Table 3. It should be noted that the crystallite size and metal site density of the Ni2P active phase increase linearly with the metal loading. Figure 5. FTIR spectra of SBA15 and fresh, spent, and regenerated Ni2P/SBA15.

Table 3. XRD Data for Ni2P/SBA15

OH vibrations generated by the presence of defect sites. These defect sites disappear after loading of Ni2P onto SBA15 because Ni replaces the H+ of the Si−OH vibrations.27 Thus, it can be said that the metal phosphide reacts with the defect sites of SBA15, and as a result, the activity of SBA15 is increased after nickel phosphide loading. From the spectra of spent Ni2P/ SBA15, one may note that peaks at 1377−1558 cm−1 associated with the H−C−H bending vibration are created by the adsorption of alkanes. Peaks at 2854−3100 cm−1 due to the CC−H asymmetric stretching vibration can be also observed. Spent Ni2P/SBA15 was regenerated by washing in toluene followed by drying in a N2 flow at 450 °C. Regenerated Ni2P/ SBA15 presents nearly the same spectrum as the fresh adsorbent, indicating that Ni2P/SBA15 can be easily regenerated by this method. XRD patterns of fresh Ni2P/SBA15 with an initial Ni/P ratio of 1/2 and Ni loadings of 7, 15, and 20 wt % are presented in Figure 6. The active Ni2P forms after reduction under H2. The peaks centered at 2θ = 40.7, 44.6, 47.3, 54.1 can be assigned to the active phase, Ni2P.28 It was also observed the peak intensities at higher metal loadings are larger than for the 7 wt % Ni loading. The crystallite sizes (Dc) of the adsorbents were calculated using the Scherrer equation (eq 3),29 Dc =

Kλ β cos θ

metal loading (%)

metal site density (μmol/g)

crystallite size Dc (nm)

7 15 20

500.00 946.15 1139.50

1.72 3.11 4.19

Distinct differences in the morphologies and particle sizes of the samples were seen in the TEM images. The highly ordered mesostructures of synthesized SBA15 samples after calcination were further confirmed by TEM analysis. The TEM image of SBA15 is shown in Figure 7. The SBA15 sample displays a tubelike bundle with a typical pattern of hexagonal pore structure31 having an estimated diameter of 7.92 nm (as measured by Microsoft Office Picture Manager), which is almost the same as the value obtained by BET analysis with a very insignificant error; also, a center-to-center pore distance of around 7.88 nm was observed. The TEM image of Ni2P/SBA15 (Figure 8) shows that Ni2P was distributed uniformly all over the surface of SBA15. The crystal size of Ni2P was measured to be 1.43 nm. The image also shows that Ni2P diffused inside the pores of SBA15. However, the typical pore structure of SBA15 cannot be clearly distinguished in the Ni2P/SBA15 sample, presumably because of the presence of phosphide. In contrast to the typical stacked morphologies of molybdenum and tungsten sulfides, nickel phosphides are not layered but instead form spherical particles. The specific surface area of active Ni2P (ANi2P, in m2/g) was calculated using eq 5:24

(3)

where K is a constant (usually 0.9 for crystallite material), λ is the wavelength of the X-ray (in nm), β is the full width at halfmaximum (in nm), and θ is the angle of diffraction. From the crystallite size it is possible to calculate the theoretical metal site surface concentration, L (in μmol/g), which is given by eq 4:29

A Ni 2P =

N ⎡ ⎤ 1 ⎢ 6000 ⎛ 1 ⎞⎥ ⎜⎜∑ ⎟⎟ N ⎢⎣ ρ ⎝ i = 1 Dc ,i ⎠⎦⎥

(5)

where Dc,i is the crystallite sizes, ρ is the density of the active metal phosphide in the adsorbent and N is the number of 14506

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Figure 9. Effect of metal loading on the nitrogen and sulfur adsorption capacities.

and SCCs. The high capacity for sulfur adsorption is an added advantage of this method, as the treated LCO can then be used as a feed for HDS. At higher metal loadings there is a probability of blocking the pores, which might affect the total surface area of the adsorbents. BET analysis revealed that the surface area decreased with increasing metal loading. Therefore, the adsorption capacity at higher metal loadings was smaller. The Ni2P/SBA15 adsorbent with 7 wt % Ni was selected for further studies. 3.3. Effect of Oil-to-Adsorbent Ratio. The ratio of oil to adsorbent is an important factor for adsorption. Different oil-toadsorbent ratios were used to observe the effect of this ratio on the nitrogen adsorption and selectivity. The oil-to-adsorbent ratio ranged from 15 to 80 in this study. The observed nitrogen and sulfur adsorption capacities and the percent removal are

Figure 7. TEM image of SBA15.

Figure 8. TEM image of fresh Ni2P/SBA15 with 7 wt % Ni.

Figure 10. Effect of oil-to-adsorbent mass ratio on the adsorption capacities and percent removals of nitrogen and sulfur.

nanocrystals used in the calculation. The specific surface area of active Ni2P was found to be 1.43 m2/g, which is very small compared with the overall surface area of Ni2P/SBA15. However, this small surface area plays an important role in the adsorption capacity. 3.2. Effect of Nickel Loading. Different weight loadings of nickel with respect to SBA15 at a Ni/P ratio of 1:2 were uploaded onto SBA15 using the UMxC method to study the effect of metal loading on adsorption. LCO was used as the feed. The results are shown in Figure 9. In comparison with pure SBA15, most of the Ni2P/SBA15 sorbents exhibited improved capacity in the adsorption of NCCs and SCCs regardless of the Ni loading. In particular, the capacity for SCCs was also doubled when Ni2P was incorporated into SBA15. This result confirms that metal phosphides provide active sites for nitrogen and sulfur adsorption. It was also observed that 7 wt % Ni gave the maximum adsorption capacity for both NCCs

plotted as a function of oil-to-adsorbent ratio in Figure 10. The percent removal is defined by eq 6: % removal =

C0 − Ce × 100% C0

(6)

Figure 10 shows that the nitrogen and sulfur adsorption capacities increased with increasing oil-to-adsorbent ratio. A similar trend was also observed in adsorptive denitrogenation by Ti−HMS.32 On the other hand, the removal percentage initially increased with increasing oil-to-adsorbent ratio but then decreased for oil-to-adsorbent ratios greater than 30. In an adsorption process, the concentration gradient is a dominating force to sustain the adsorption, while repulsive forces between the adsorbed compounds and the compounds in the oil phase retard the adsorption of more species from oil phase.33 The two forces become balanced at the adsorption equilibrium. At high 14507

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oil-to-adsorbent ratios, there are more NCCs and SCCs available in the LCO, leading to a slower decrease in contaminant concentration during adsorption. On the other hand, there are a limited number of active sites on the adsorbent, which soon get saturated before enough contaminant is removed. As a result, the equilibrium concentration Ce increases with increasing oil-to-adsorbent ratio, which leads a lower nitrogen and sulfur removal percentage. 3.4. Effect of Temperature on Adsorption. Temperature has a pronounced effect on the adsorption capacity of the adsorbents. The effect of temperature on the adsorption of nitrogen and sulfur from LCO was studied to evaluate the optimum temperature. Adsorption measurements were performed at room temperature (22 °C) and at 50, 75, 100, and 150 °C at atmospheric pressure. Adsorption was carried out for 24 h, after which the system was quickly cooled to room temperature and the sample was taken immediately. Figure 11

Figure 12. Nitrogen selectivity of Ni2P/SBA15.

replaced by the model oil, the nitrogen selectivity increased from less than 40% to more than 80%. 3.6. Fixed-Bed Adsorption. The ratio of the effluent concentration of nitrogen or sulfur to the concentration of nitrogen or sulfur in feed (C/C0) is plotted against time in Figure 13. The total nitrogen and sulfur adsorption capacities of

Figure 11. Effect of temperature on the equilibrium concentrations of nitrogen and sulfur.

shows that the equilibrium concentrations of nitrogen and sulfur increased with temperature. This suggests that physical adsorption dominates and that the adsorption process at room temperature and atmospheric pressure is optimal. An abnormality was observed in nitrogen adsorption at 100 °C, as the equilibrium concentration of nitrogen at 100 °C was found to be slightly lower than that at 75 °C. In LCO there are significant amounts of indole and its derivatives, which are very unstable in oxygen. Thus, there is a probability that indole was oxidized at high temperature, leading to the higher saturation concentration of nitrogen at 100 °C than at 75 °C. 3.5. Nitrogen Selectivity. The nitrogen selectivity over Ni2P/SBA15 was calculated using the total adsorption capacity according to eq 7: N selectivity =

moles of adsorbed nitrogen × 100% total moles adsorbed

Figure 13. Breakthrough curves for nitrogen and sulfur adsorbed on Ni2P/SBA15 (LCO).

Ni2P/SBA15 were calculated using eq 2. The observed nitrogen and sulfur adsorption capacities were 20.34 mgN/gads and 64.37 mgS/gads, respectively. Figure 13 also shows that the breakthrough point for nitrogen is very short and that sulfur has no breakthrough point. Competitive adsorption of model nitrogen and sulfur compounds was performed in batch mode to reveal the reason for the short breakthrough point of nitrogen. Indole, carbazole, and quinoline were chosen as model nitrogen compounds, and DBT was used as the model sulfur compound. An oil-to-adsorbent ratio of 30 was used, and the adsorption was performed at ambient temperature and pressure. Table 4 shows the adsorption capacities for the different NCCs adsorbed from the model oil containing nitrogen and sulfur compounds. Carbazole, which is a three-ring compound and neutral in nature, has the lowest adsorption capacity, and the

(7)

The adsorption selectivity can be affected by many factors, including the surface chemistry of the sorbent material, the fuel properties, etc.34 The effect of fuel properties was considered in this work. The nitrogen selectivities obtained using model oil and LCO are plotted in Figure 12, which shows that the nitrogen selectivity was less than 50% when LCO was used as the feed. The sulfur content in the LCO used in this study was approximately 10 times the nitrogen content. Furthermore, the LCO had much more complex compounds than the model oil and contained only neutral NCCs (about 80% of the NCCs were carbazole and its derivatives). Ni2P/SBA15 has a relatively low adsorption capacity for carbazole. As a result, a low nitrogen selectivity was obtained for LCO. When LCO was

Table 4. Nitrogen and Sulfur Adsorption Capacities of Ni2P/ SBA15 with the Model Oil

14508

model compound

capacity (mg/gads)

indole carbazole quinoline DBT

12.6 2.56 11.84 5.47

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adsorbents for different types of oil. The comparison was performed on the basis of specific adsorption capacity as well as adsorption capacity per unit surface area of the adsorbent. The nitrogen adsorption capacity per unit surface area for Ni2P/ SBA15 is higher than those for the other adsorbents shown in Table 5. 3.9. Regeneration of the Adsorbent. One of the most important factors in commercializing an adsorbent is its regenerability.14 Ni2P/SBA15 spent in LCO was regenerated by solvent washing for 24 h in batch mode at room temperature and subsequently activated under a nitrogen flow at 450 °C for 5 h. Toluene was used as the solvent. The regenerated adsorbent was used to adsorb nitrogen and sulfur from LCO under the same conditions as used for fresh adsorbent. The regenerability of the adsorbent is expressed in terms of the recovery ratio, which is defined as shown in eq 8:

LCO used as the feed contained more than 80% carabazole and its derivatives. Therefore, a short breakthrough point was observed. 3.7. Effect of Contact Time on Adsorption. Contact time is an important factor for adsorption. In this study, the effects of contact time with the nitrogen and sulfur model compounds were evaluated in batch mode. Samples were taken from the oil/adsorbent mixture at regular intervals and analyzed using the NS 9000 analyzer. Figure 14 shows the adsorption

recovery ratio =

readsorption capacity capacity at first use

(8)

Figure 15 shows the recovery ratio of regenerated Ni2P/SBA15 as a function of the number of regeneration cycles. Ni2P/

Figure 14. Effect of contact time on the equilibrium concentrations of nitrogen and sulfur.

capacities of Ni2P/SBA15 toward various model NCCs and SCCs as functions of time. The adsorption reached the saturation limit within 15 min, which signifies a very high rate of adsorption. It is interesting to note that the rate of adsorption was closely associated with the molecular size and chemical nature of the adsorbate. The chronological order of adsorption was found to be quinoline > indole > DBT > carbazole > 4,6-dimethyldibenzothiophene (4,6-DMDBT). Both quinoline and indole have two-ring structures, but indole is neutral in nature and quinoline is basic; thus, the adsorption rate of quinoline would be expected to be higher than that of indole. Both DBT and carbazole have three-ring structures, and thus, their adsorption rates were low. Additionally, DBT is slightly basic, leading to a higher rate of adsorption than for carbazole. 4,6-DMDBT is an alkyl-substituted derivative of DBT, and as expected, its rate was lowest among these five compounds. 3.8. Comparative Study. The adsorption capacity of Ni2P/SBA15 for nitrogen was compared with those of adsorbents reported in the literature. Table 5 shows the comparative study of nitrogen adsorption capacities of different

Figure 15. Recovery ratio of regenerated Ni2P/SBA15 vs number of regeneration cycles.

SBA15 was regenerated successfully. Four adsorption/regeneration cycles were carried out. After four cycles, approximately 95% of the adsorptive capacities for both NCCs and SCCs were retained. This result provides evidence that toluene is an effective solvent for regenerating the Ni2P/SBA15 adsorbent. The FTIR spectrum of regenerated Ni2P/SBA15 (Figure 5) also indicates that the sorbent can be regenerated well. The peaks at 1377−1558 and 2854−3100 cm−1 representing adsorbed species disappeared after regeneration. Similar spectra were observed for the fresh and regenerated sorbent materials.

4. CONCLUSIONS Ni2P/SBA15 has been successfully synthesized using a novel urea matrix combustion (UMxC) method. This adsorbent material has been applied for adsorptive denitrogenation and desulfurization of LCO and a model diesel oil. Three different weight percentages of nickel were loaded onto the supporting material SBA15 to identify the optimal metal loading. The results showed that a Ni loading of 7 wt % gave the maximum nitrogen and sulfur adsorption capacities among the loadings tested. The nitrogen adsorption capacity per unit surface area for Ni2P/SBA15 was 0.0293 mgN/m2ads, which is higher than those for most of the adsorbents reported in the literature. Ni2P/SBA15 was also used for fixed-bed adsorption, and a very short breakthrough time was observed as a result of the fact that carbazole and its derivatives constituted up to 80% of the

Table 5. Comparative Study of Nitrogen Adsorption Capacities of Different Adsorbents

adsorbent

oil

surface area (m2/g)

Ni2P/SBA15 (UMxC) MSC-30 (activated carbon)35 Si−Zr cogel4 YSP-1 (silica)5 YSP-2 (silicazirconia)5

LCO

310.3

specific capacity (mgN/gads)

capacity per unit surface area (mgN/m2)

9.1

0.0293

LCO

3140

4.90

0.00156

LGO LGO LGO

716 1331 1092

3.35 8.14 8.05

0.00467 0.00612 0.00737

14509

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NCCs in the LCO and Ni2P/SBA15 has a low adsorption capacity for carbazole. The regenerability of the developed adsorbent was also investigated, and it was observed that both the sulfur and nitrogen adsorption capacities were approximately 95% recovered. It was found that toluene was an effective solvent for the regeneration of Ni2P/SBA15.

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AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support provided by the Atlantic Innovation Fund, the Natural Sciences and Engineering Research Council of Canada, and the EssoImperial Oil University Research Award.

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REFERENCES

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dx.doi.org/10.1021/ie3015044 | Ind. Eng. Chem. Res. 2012, 51, 14503−14510