Adsorptive Removal of Thiophenic Compounds from Oils by Activated

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Adsorptive Removal of Thiophenic Compounds from Oils by Activated Carbon Modified with Concentrated Nitric Acid Chang Yu,† Xiaoming Fan,† Limei Yu,† Teresa J. Bandosz,‡ Zongbin Zhao,† and Jieshan Qiu*,† †

Carbon Research Laboratory, Liaoning Key Lab for Energy Materials and Chemical Engineering, State Key Lab of Fine Chemicals, Dalian University of Technology, Dalian 116024, People’s Republic of China ‡ Department of Chemistry, The City College of New York, 138th Street and Convent Avenue, New York, New York 10031, United States S Supporting Information *

ABSTRACT: Adsorptive removal of thiophenic compounds from oils by commercial coconut-based activated carbon (AC) and modified AC samples was studied systematically in a batch-type adsorption system. The modified AC samples were obtained by treating the commercial AC sample using 65 wt % concentrated nitric acid (HNO3) at different temperatures (30−120 °C). The effects of the modification temperature on morphology, pore structure, and surface chemistry of the AC samples were analyzed and compared. It has been found that oxidation with concentrated HNO3 at ambient conditions removes inorganic components or ashes of ca. 50% in the AC sample, produces carboxyl functional groups on the AC surface, and introduces high volume micropores with sizes around 0.54 nm. The effects of pore structure and surface features on adsorptive capability for the thiophenic compounds were investigated in detail. The results show that the as-received AC sample is able to adsorb the bigger size sulfur compounds. The adsorptive removal efficiency for the sulfur compounds decreases in the order of 4,6dimethyldibenzothiophene > dibenzothiophene > benzothiophene > thiophene. The modified AC samples can adsorb more thiophene and benzothiophene molecules, but this is not the case for dibenzothiophene and 4,6-dimethyldibenzothiophene molecules. On the basis of the results obtained, it was proposed that the pore structure and surface chemistry of the AC as well as frontier orbital energies of the thiophenic compounds and the AC samples govern the adsorption of these species on the surface of AC.

1. INTRODUCTION Consumption of fuel oils results in severe environment impact all over the world. It was found that the sulfur-containing organic compounds in fuel oils cause the severe corrosion of reactors and equipment in the oil-processing step. Moreover, SO x , emitted during the combustion of these sulfur compounds, is one of the main sources for acid rain. Therefore, how to remove the sulfur compounds from headstream remains a challenge and has become a necessary step in the oil processing technology. 1−4 Various methods have been established for the highly efficient removal of the organic sulfur compounds in fuel oils, such as hydrodesulfurization,5−7 oxidative desulfurization,8−13 adsorptive desulfurization,14−32 extractive desulfurization,33,34 and biodesulfurization.35,36 Of these approaches available now, the adsorptive desulfurization (ADS), one of the promising desulfurization approaches, has received much attention due to the fact that it can be operated at room temperature and under atmospheric pressure. In the case of ADS, various porous adsorbents have been adopted or developed for removal of thiophenic sulfur compounds, such as activated carbon (AC), carbon aerogels, aluminum oxide, and zeolite, etc.2,19−23,32,37−45 Of these adsorbents used now, it is believed that AC is one of the highly efficient candidates for removal of sulfur compounds from oil due to its very high surface areas, large pore volumes, and flexible surface chemistry, which can be tuned to a great degree via some methods, for example, metal modification,17,31 liquid-phase modification method using oxidant, etc.14,46 Also, it has been indicated in © 2013 American Chemical Society

the literature that the porous structure, the abundant surface oxygen-containing functional groups, and metal on AC surface can improve or promote the adsorptive capability of the AC adsorbents for the sulfur compounds.14,17,18,27,32 Nevertheless, the systematic data addressing the effects of modification conditions on the adsorptive capability of AC need to be widely analyzed. The mechanism involved in the adsorption of thiophenic compounds from oils is not clear. In our previous study, it had also been found that HNO3 was a good surface modification agent and oxidation with HNO3 at room temperature and 120 °C improved the adsorptive capability of AC for thiophene molecule.16 In the present work, the as-received AC sample is modified by treatment with concentrated HNO3 at various temperatures. The samples are extensively characterized from the point of view of surface chemistry and porosity, and the effects of these surface features on the removal efficiency of the thiophenic compounds from oil are analyzed. The frontier orbital energies of the thiophenic compounds and the AC adsorbents before and after modification are calculated using DMol3 method based on the density functional theory. The possible mechanism involved in adsorption of thiophenic compounds on AC is discussed and proposed in terms of the pore structure Received: November 1, 2012 Revised: February 25, 2013 Published: February 26, 2013 1499

dx.doi.org/10.1021/ef400029b | Energy Fuels 2013, 27, 1499−1505

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of the AC adsorbents and the thiophenic compounds and to calculate their frontier orbital energies.50 The molecular structure of the asreceived and modified AC samples (AC-COOH and AC-(COOH)2, referring to the oxidized AC adsorbents to different degrees) is shown in Figure 1. Within the framework of density functional theory, the

and surface chemistry of the AC as well as the frontier orbital energies of the AC and the thiophenic compounds.

2. EXPERIMENTAL SECTION 2.1. Materials. Commercial coconut-based activated carbon was purchased from Beijing Broad Activated Carbon Co., Ltd. (Beijing, China) and treated further using 65 wt % concentrated HNO3 at different temperatures (30−120 °C). For a typical run, 10 g of asreceived AC sample was mixed with a 100 mL solution of concentrated HNO3 and then heated at 30, 60, 90, or 120 °C for 3 h under stirring and refluxing conditions to obtain the modified AC samples, designated as AC-T, where T refers to the modification temperature. Thiophene (T) and n-octane were purchased from Dalian Chemical Co. (Dalian, China). Benzothiophene (BT), dibenzothiophene (DBT), and 4,6-dimethyldibenzothiophene (4,6DMDBT) were purchased from ACROS Organics. 2.2. Characterization Methods. The changes in morphology of the AC samples before and after modification were analyzed by scanning electron microscopy (SEM, A JEOL JSM-5600LV). The asreceived and modified AC samples were directly mounted to the sample holder with a piece of electrically conductive glue for SEM analysis. Ash content of the as-received and modified AC samples was determined by combusting about 1 g of adsorbent in a muffle furnace at 650 °C for 2 h. N2 adsorption isotherms of the as-received and modified AC samples were measured at −196 °C using an ASAP 2020 (Micromeritics). Before adsorption experiments, all of the AC samples were outgassed at 150 °C overnight under a vacuum of 10−6 Torr. The surface areas (SBET) were calculated using the BET equation; the micropore volumes (Vmic) and micropore areas (Smic) were calculated using the t-plot method; the total pore volumes (Vt) were obtained from the last point of the isotherm at a relative pressure of 0.99; and the pore size distributions were determined using the density functional theory method.47,48 The changes in the surface chemistry of the AC samples before and after modification with concentrated HNO3 were evaluated by FT-IR spectroscopy, thermogravimetric analysis (TGA), and potentiometric titration (PT). FT-IR spectra of the AC samples were recorded using a JASCO FT/IR-430 spectrometer with a resolution of 4 cm−1 (KBr pellet, carbon loading of ca. 0.5 wt %). The differential thermogravimetric (DTG) curves of the AC samples were monitored at 10 °C/min in N2 atmosphere using Mettler-Toledo TGA/ SDTA851e system. PT measurements were performed with a 716 automatic titrator (Metrohm). The instrument was set in the mode for the collection of equilibrium pH values. About 0.1 g of the AC adsorbent was added to a 50 mL solution of 0.01 N of NaNO3, and then the mixture was placed in a container thermostatted at 20 °C and equilibrated overnight with the electrolyte solution. To eliminate the influence of atmospheric CO2, the suspension was continuously saturated with N2. The carbon suspension was stirred constantly until the measurements were finished. Each sample was titrated by both acid (0.1 N of HNO3) and base (0.1 N of NaOH) starting from the initial pH of the suspension, respectively.49 2.3. Adsorption Experiments. The desulfurization performance of the as-received and modified AC samples was evaluated using model oils containing the n-octane and thiophenic compounds in a batchtype adsorption system, of which the details can be found elsewhere.16 For a typical run, the experiment conditions are as follows: room temperature; adsorptive time, 3 h; ratio of adsorbent to oil, 0.09 g/g. The sulfur contents in the model oils before and after adsorptive treatment were analyzed using a gas chromatography equipped with flame ionization detector (Agilent 6890 GC/FID with HP-5 capillary column, 30 m × 320 μm × 0.25 μm), of which inject and detector temperatures were 280 °C, column temperature was kept at 60 °C for 4 min, then increased to 150 °C at a rate of 10 °C/min and kept for 10 min, continually to 200 °C at a rate of 10 °C/min and kept for 8 min, and finally to 250 °C at a rate of 10 °C/min and kept for 10 min. The DMol3 module in the Materials Studio (MS) software package provided by Accelrys Inc. was used to optimize the geometric structure

Figure 1. Molecular structure of the as-received and the modified AC samples. generalized gradient approximation with nonlocal exchange and correlation functions (GGA-PW91) was used.51,52 All-electron calculations were performed with double numerical polarization (DNP) basis set. The frontier orbital energies of the thiophenic compounds and the AC adsorbents, which refer to highest occupied molecular orbital energy (EHOMO) and lowest unoccupied molecular orbital energy (ELUMO), are obtained from the DMol3 module’s output file.

3. RESULTS AND DISCUSSION 3.1. Morphology and Component of the AC Samples. Figure 2 shows the SEM images for the AC samples before and

Figure 2. SEM images of the as-received and modified AC samples: (a) AC, (b) AC-30, (c) AC-60, (d) AC-90, (e) AC-120.

after oxidation with the concentrated HNO3 at different temperatures. It is clearly seen that no big differences are found in the modified AC samples; nevertheless, after the treatment, a number of fine particles with white color, which we assume are inorganic components or ashes, that can be observed on the surface of the as-received AC sample (Figure 2a), disappear partly (Figure 2b−e), as a result of their conversion into soluble nitrates. Further EDX element mapping of these fine particles by electron microprobe for the asreceived AC sample reveals that the fine particles are mainly made up of K, Na, and Si inorganic components; for the detailed information, please refer to Figure S1 in the Supporting Information. The ash content in the AC samples before and after modification was analyzed quantitatively, and the results are shown in Table 1. As a result of the treatment 1500


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significantly destroyed in comparison to that of other AC samples, indicating the strong oxidation force of the concentrated HNO3 at 120 °C. In the case of the AC samples treated between 30 and 90 °C, the pores in a range of 0.65− 1.58 nm are also affected, and the pore volume becomes smaller and the peaks in these regions shift toward the bigger pore size. It is important to notice that the volume of the micropores below 0.65 nm increases with an increase in the modification temperature and stabilizes at 60 °C. An interesting difference is that in the case of the AC-30 sample, the micropores centering around 0.59 nm that are observed in the as-received AC sample increase in volume; while for the AC-60 and AC-90 samples, the micropores with sizes around 0.59 nm are transformed into smaller sizes of about 0.54 nm gradually. This size of micropores is found to be beneficial for the adsorption of T molecule with a critical diameter of 0.53 nm from the oils.53 3.3. Surface Chemistry Properties of the AC Samples. The surface chemistry of the as-received and modified AC samples that is also an important factor affecting the adsorptive capability of the adsorbents was analyzed by FT-IR spectroscopy (Figure 4). In the region of 3100−3600 cm−1, for all of

Table 1. Ash Content and Pore Structure Parameters of the As-Received and Modified AC Samples sample AC AC-30 AC-60 AC-90 AC120

ash content (%)

SBET (m2/g)

Smic (m2/g)

Smeso (m2/g)

Vtotal (cm3/g)

Vmic (cm3/g)

1.47 0.65 0.59 0.62 0.58

1009 903 919 796 207

784 711 701 588 160

225 192 218 208 47

0.478 0.424 0.439 0.379 0.098

0.363 0.328 0.324 0.271 0.074

applied, about 50% of the inorganic components or ashes is removed. The modification temperature has no noticeable influence on the amount of inorganic components removed from AC, which is in agreement with the SEM results. 3.2. Pore Structure Properties of the AC Samples. It is well-known that the adsorptive capability of any solid adsorbents benefits from its porous structure and specific surface chemistry. Thus, it is a paramount importance to study the pore structure and surface properties of materials in details before and after modification with the concentrated HNO3. The pore structure parameters of the as-received and modified AC samples calculated from nitrogen adsorption isotherms are also summarized in Table 1. After the treatment, the structural parameters decrease for all of the AC samples. While in comparison to the as-received AC sample, there are relatively slight changes in the surface area and total pore volume of the modified AC samples treated between 30 and 90 °C, for the AC-120 sample, a dramatic change occurs. For example, the surface area and total pore volume of the AC-30, AC-60, and AC-90 samples decrease ca. 20% in comparison to that of the as-received AC sample. On the other hand, in the case of the AC-120 sample, the surface area and total pore volume are 207 m2/g and 0.098 cm3/g, respectively, which represent a decrease of ca. 80%. This indicates that the moderate modification temperature has no significant influence on the pore structure of the AC samples, while the higher temperature destroys the pore structure to a great degree. The pore size distributions of the AC samples were calculated using density functional theory method (Figure 3). Figure 3 shows that the pore structure of AC-120 sample is

Figure 4. FT-IR spectra of the as-received and modified AC samples: (a) AC, (b) AC-30, (c) AC-60, (d) AC-90, (e) AC-120.

the AC samples, there is a strong peak centering at 3400 cm−1 that can be mainly assigned to O−H stretching vibrations due to the chemisorbed water and oxygen-containing functional groups on the carbon surface. The shoulder peak at 3200 cm−1 observed only for the AC-120 sample is assigned to the O−H stretching vibrations of carboxylic groups, which appear as a main peak at about 1719 cm−1 (CO vibrations).14,54 In the wavenumber region below 2000 cm−1, a broad peak at 950− 1300 cm−1 due to the symmetrical stretching vibration of etheric epi-oxide and O−H bending mode can be found for all of the AC samples,55 and its relative intensity varies slightly with the modification temperature. For the modified AC samples, it is also noted that the peak at 1635 cm−1 that can be observed in the as-received AC sample disappears and is transformed into the new peaks at 1567 and 1719 cm−1, whose intensities vary slightly with the modification temperature. The peak at 1567 cm−1 is due to the presence of the ionoradical structures CO, while the peak at 1719 cm−1 is indicative of stretching vibrations of CO in carboxylic groups.54 These results imply that oxidation with HNO3 increases the amount of oxygen-containing functional groups on the AC surface, and

Figure 3. Pore size distributions of the as-received and modified AC samples. 1501


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high temperature oxidation is beneficial for the formation of more carboxylic groups. The AC samples were further studied using TGA technique to get more information about the functional groups on the surface. The DTG curves for the as-received and modified AC samples are shown in Figure 5. On all curves, three peaks can

Figure 6. pKa distributions of the as-received and modified AC samples.

previous studies, oxygen-containing functional groups can be classified as strong acids (carboxylic groups) at pKa < 8 and weak acids (phenolic groups) at pKa over 8.49,61,62 It is clearly seen that the carboxylic groups at pKa < 8 on the surface of AC oxidized at 90 and 120 °C increase obviously. Once again, these results support our FT-IR and TG finding that the high modification temperature favors formation of strongly acidic/ carboxylic groups on the AC surface.61 It is noteworthy that at the pKa < 5, the number of the carboxylic groups increases with an increase in the modification temperature from 30 to 120 °C. 3.4. Adsorption Desulfurization Capabilities of the AC Samples. The adsorption capabilities of the as-received and the modified AC samples for the thiophenic compounds were measured and compared, and the results are shown in Table 3. The mechanism involved in the adsorptive process for removing the thiophenic compounds will be further elucidated in terms of the pore structure and surface chemistry properties of the AC samples. As seen, the removal efficiencies of the asreceived AC adsorbent toward the thiophenic compounds follow in the order of 4,6-DMDBT > DBT > BT > T. This is the case for the adsorption amount of AC adsorbents for the thiophenic compounds. This indicates that the as-received AC sample has higher affinity to adsorb the bigger size sulfur compounds. These compounds, such as DBT and 4,6DMDBT, are considered as refractory and difficult to remove using HDS method; 7 that is to say, the adsorption desulfurization will hopefully make up for the deficiency using the HDS method in the future. The good adsorption behavior of the as-received AC toward these species can be linked to the strong interaction forces between the surface and the bigger size sulfur compounds due to the electron-donor effect of the benzyl or methyl groups on the pentagonal ring of the sulfur compounds.32 It is also noted from Table 3 that all modified AC samples, including the AC-120 sample for which the pore structure is seriously destroyed, have higher adsorption capability and removal efficiency for T molecule. One has to remember that the striking features of the AC-120 sample are the abundance of carboxylic groups on the surface. This implies that the carboxylic groups can favor the removal of T molecule. This is also why the AC-90 sample with the micropores being the predominant sizes of 0.54 nm that are similar to those of the AC-60 sample has higher removal efficiency for T molecule

Figure 5. DTG curves of the as-received and modified AC samples.

be seen, and their relative intensities vary with the modification temperature. The first peak centered around 80 °C represents desorption of adsorbed water. After modification with the concentrated HNO3, the first peak of the AC samples shifts slightly toward high temperature, which is due to the increasing hydrophilicity of the modified AC samples.56 The relative intensity of the CO2 desorption peak in a range of 200−400 °C, representing decomposition of carboxyl, lactone, and lactol groups,57−59 increases slightly as the modification temperature increases, while the CO desorption peak at 600−800 °C, corresponding to decomposition of carbonyl and phenol groups on carbon surface,57−59 becomes weaker as compared to that of the as-received AC sample. This implies that oxidation with HNO3 is able to increase the amount of the carboxyl, lactone, and lactol groups on the AC surface, although it decreases the amount of the carbonyl or phenol groups. The high temperature treatment as shown from the above-mentioned FT-IR analysis favors incorporation of the carboxyl, lactone, and lactol functional groups to the carbon matrix. The distribution of oxygen-containing functional groups on the surface of the AC adsorbents obtained from PT experiments using SAIEUS approach is presented in Figure 6.60 It can be clearly seen that in comparison to the as-received AC sample, the intensities of the peaks for the modified AC adsorbents are increased significantly, and they vary with the modification temperature especially in the case of species having pKa < 5, whose acidity seems to increase. The strength of surface groups expressed as pKa values of the peak maximum along with the number of the oxygencontaining functional groups obtained from integration of the peaks on the pKa distributions is presented in Table 2. Obviously, after modification with the concentrated HNO3, the whole number of functional groups on the AC surface increases regardless of the number of functional groups of the AC samples expressed in the form of mmol/cm2 or mmol/g. To discuss conveniently, the number of functional groups on the AC surface was expressed in mmol/g. On the basis of the 1502


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Table 2. Peak Positions and Numbers of Groups (in Parentheses; [mmol/g]) for the As-Received and Modified AC Samples sample AC AC-30 AC-60 AC-90 AC120

pKa 3−4

pKa 4−5 4.53 4.63 4.34 4.30

pKa 5−6

(0.067) (0.071) (0.097) (0.190)

3.99 (0.591)

5.26 5.34 5.71 5.20 5.06

(0.002) (0.017) (0.052) (0.136) (0.559)

pKa 6−7 5.92 (0.049) 6.21 (0.090) 6.05 (0.127) 6.04 (0.456)

pKa 7−8 7.21 7.22 7.27 6.87 6.97

pKa 8−9

(0.057) (0.052) (0.034) (0.152) (0.403)

8.20 8.45 8.69 7.92 8.25

(0.007) (0.076) (0.116) (0.155) (0.365)

pKa 9−10 8.76 9.29 9.95 8.84 9.38

(0.030) (0.132) (0.195) (0.250) (0.530)

pKa 10−11

total (mmol/g)

total (mmol/m2)

0.301 0.653 0.494 1.44 2.904

303.71 589.66 453.98 1146.24 601.13

9.70 (0.089) 10.45 (0.215) 10.09 (0.430)

Table 3. Adsorption Capabilities of the AC Samples Before and After Modification for the Thiophenic Sulfur Compoundsa T

BT

DBT

4,6-DMDBT

sample

adsorption amount q (mmol/g)

removal efficiency x (%)







AC AC-30 AC-60 AC-90 AC120

0.0311 0.0508 0.0586 0.0693 0.0436

21.9 36.0 41.6 49.1 30.8

0.2435 0.2777 0.2853 0.2755 0.1551

66.6 75.9 78.1 75.5 42.3

0.2748 0.2675 0.2855 0.2887 0.1913

89.4 86.8 92.5 93.5 60.7

0.2875 0.2877 0.2883 0.2683 0.0063

97.0 97.1 97.1 90.3 2.1

a

Iinitial concentrations of T, BT, DBT, and 4,6-DMDBT are 400, 1000, 900, and 900 ppmwS, respectively.

than the AC-60 sample. Support for this is also the fact that the more functional groups representing CO desorption are present on the surface of the as-received AC sample, and thus weakly acidic groups have lower removal efficiency for T molecule. Considering this, it is concluded that the functional groups resulting from CO desorption are a negligible factor and do not affect the T molecule adsorption, while the carboxylic groups that are desorbed as CO2 in TA analysis are a positive factor for removal of T molecule. Moreover, the micropores that are similar in size to the critical size of T molecule are also an important factor affecting the removal efficiency of T molecule. Those pores can attract T more quickly/strongly due to the enhancement in the adsorption potential. This can also partly explain why the samples oxidized at 30, 60, and 90 °C have higher removal efficiency for T molecule than of the asreceived AC and AC-120 samples. The above discussion leads one to believe that the combination of the carboxylic groups and micropores with a size similar to the critical diameter of T molecule is responsible for the high efficiency removal of T from oils. For BT, DBT, and 4,6-DMDBT sulfur compounds, the various desulfurization results are obtained over the modified AC samples. In the case of BT, the AC-120 sample with the highest amount of carboxylic groups shows the lowest removal efficiency, while under the identical experiment conditions, other modified AC samples have higher removal efficiency for this molecule despite the relatively small surface area and pore volume as compared to the as-received AC sample. This result once again implies that the carboxylic groups and the porous structure of the AC samples simultaneously play important roles in adsorption of BT. Nevertheless, the direct correlations between the pore diameter of the AC adsorbents and their removal efficiency for BT molecule are not observed. For DBT and 4,6-DMDBT, the modified AC samples below 90 °C show the similar removal efficiency in comparison to that of asreceived AC sample; that is to say, the abundant carboxylic groups have no noticeable influence on the removal efficiency of DBT and 4,6-DMDBT over the oxidized AC. It should be also found that the porous structure, as a limiting factor or

indispensable premise, needs to be considered for the bigger size sulfur compound DBT and 4,6-DMDBT, which is confirmed by the relatively low adsorption capability of AC120 with less pore, although the more carboxylic groups are present on the surface. To better understand the mechanism of thiophenic compounds adsorption on AC, the frontier orbital energies of the sulfur compounds and the AC adsorbents were calculated (Table 4). For all of the thiophenic compounds, the orbital Table 4. Orbital Energy for the Thiophenic Compounds and the AC Adsorbents T BT DBT 4,6-DMDBT AC AC-COOH AC-(COOH)2

ELUMO (eV)

EHOMO (eV)

ΔE (eV)a

1.4437 0.5535 0.2216 0.0439 −1.5426 −2.2234 −2.8866

−8.8545 −7.8419 −7.5022 −7.2558 −6.0482 −6.3420 −6.9445

10.2982 8.3954 7.7238 7.2997 4.5056 4.1186 4.0579

ΔE refers to the frontier orbital energy gap between the LUMO and the HOMO of the sulfur compounds or the adsorbents.

a

energy gap becomes smaller as the size of sulfur compound molecules increases. This suggests that the bigger size sulfur compound would interact with the adsorbent surface more easily. This would be why the as-received AC sample has higher removal efficiency for the sulfur compounds that are relatively big in size, such as DBT and 4,6-DMDBT. In the case of AC, the orbital energy gap decreases gradually as the number of carboxylic groups on the carbon surface increases, indicating the increasing activity of the AC adsorbents. This is why the modified AC adsorbents can improve the removal efficiency of T or BT molecules. It should be noted that no systematic correlation between the modified AC samples and their removal efficiency for the bigger size sulfur compounds has been observed in the present work. In this regard, more work and more data are needed. 1503


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To further understand the interaction mode between the AC adsorbents and the thiophenic compounds as well as their intensity, the frontier orbital energy gap between the adsorbents and the adsorbates was also calculated (Table 5).

with an increase in the molecular size of the adsorbates and with an increase in the number of the carboxylic groups on the AC surface. Nevertheless, the interaction mode between the adsorbents and the adsorbates is not included in Figure 7.

Table 5. Orbital Energy Gap between the Thiophenic Compounds and the AC Adsorbents

4. CONCLUSIONS The adsorptive capability of the as-received commercial AC sample for the thiophenic compounds follows the order 4,6DMDBT > DBT > BT > T. This can be attributed to the strong interaction forces between the as-received AC sample and the bigger size sulfur compounds due to the electrondonating property of the benzyl or methyl groups on the pentagonal rings. In comparison to the as-received AC sample, the AC samples oxidized between 30 and 90 °C adsorb more T or BT molecules due to the developed porous structure and the abundance of carboxylic groups. On the other hand, for removal of the bigger size sulfur compounds such as DBT and 4,6-DMDBT, it is proposed that the dominating parameter or leading factor determining the adsorptive capability of the adsorbents is the porous structure of the adsorbents. The preliminary theoretical study of the frontier energy gap of the sulfur compounds and the AC adsorbents shows that the AC samples mainly interact with the thiophenic compounds via the LUMO of the AC samples and the HOMO of the thiophenic compounds. The interaction intensity between the adsorbents and the adsorbates increases as the molecular size of the adsorbates and the number of the carboxylic groups on the AC surface increase.

AC

T BT DBT 4,6DMDBT

AC-COOH

AC-(COOH)2

ΔE1 (eV)

ΔE2 (eV)

ΔE1 (eV)

ΔE2 (eV)

ΔE1 (eV)

ΔE2 (eV)

7.4919 6.6017 6.2698 6.0921

7.3119 6.2993 5.9596 5.7132

7.7857 6.8955 6.5636 6.3859

6.6311 5.6185 5.2788 5.0324

8.3882 7.4980 7.1661 6.9884

5.9679 4.9553 4.6156 4.3692

It is clearly seen from Table 5 that for the same AC adsorbent, the energy gap between LUMO of the adsorbent and HOMO of the sulfur compound (ΔE2) is smaller than that between the LUMO of the same sulfur compound and the HOMO of the adsorbent (ΔE1). This leads one to believe that the AC adsorbents would mainly interact with the thiophenic compounds by the LUMO of the AC adsorbents and the HOMO of the thiophenic compounds. It is also noteworthy that ΔE2 becomes smaller as the size of sulfur compounds increases, implying that the bigger size sulfur compounds interact with the AC sample more easily and thus the strong interaction force exists between them and the AC surface. In the case of the same sulfur compound, ΔE2 decreases gradually as the number of carboxylic groups on the AC surface increases, implying that the oxidized AC samples interact with the sulfur compounds with greater affinity/stronger. Nevertheless, as described above, the porous structure of the adsorbents is the limiting factor in the ADS process for removing the sulfur compounds that are bigger in size, such as DBT and 4,6DMDBT. The adsorption mechanism mentioned above for removal of thiophenic compounds from fuel oils on the asreceived and modified AC samples is briefly summarized in Figure 7. As shown in Figure 7a, the pore sizes of the initial AC

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ASSOCIATED CONTENT

S Supporting Information *

SEM image and element mapping of the as-received AC sample. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-411-84986024. Fax: +86-411-84986080. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by the National Natural Science Foundation of China (nos. 50902016, 20923006, and U1203292), Dalian Municipal Science & Technology Projects of China (no. 2011A15GX023), and the Specialized Research Fund for the Doctoral Program of Higher Education of China (no. 20120041110020). We are grateful to Ms. Anna Kleyman for performing PT experiment and to Dr. Jacek Jagiello for providing the SAIEUS software.

Figure 7. Schematic chart of the mechanism of as-received and modified AC samples for adsorptive removal of thiophenic compounds.

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REFERENCES

(1) Song, C. S.; Ma, X. L. Appl. Catal., B 2003, 41, 207−238. (2) Hernandez-Maldonado, A. J.; Yang, R. T. Catal. Rev. 2004, 46, 111−150. (3) Srivastava, V. C. RSC Adv. 2012, 2, 759−783. (4) Shen, Y. S.; Li, P. W.; Xu, X. H.; Liu, H. RSC Adv. 2012, 2, 1700− 1711. (5) Solis, D.; Agudo, A. L.; Ramirez, J.; Klimova, T. Catal. Today 2006, 116, 469−477. (6) Pawelec, B.; Navarro, R. M.; Castano, P.; Alvarez-Galvan, M. C.; Fierro, J. L. G. Energy Fuels 2009, 23, 1364−1372.

are important for DBT and 4,6-DMDBT adsorption. On the other hand, after modifications, the small pore sizes and carboxylic groups enhance interactions of the carbon surface with T and BT molecules. Figure 7b presents the order of the activities of adsorbates and adsorbents calculated using DMol3 method based on the density functional theory. As seen, when ΔE becomes smaller, the activities and the interaction intensity between the adsorbents and the adsorbates increase. It happens 1504


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