Model Fuel Deep Desulfurization Using Modified 3D Graphenic

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Cite This: Ind. Eng. Chem. Res. 2019, 58, 10341−10351

Model Fuel Deep Desulfurization Using Modified 3D Graphenic Adsorbents: Isotherm, Kinetic, and Thermodynamic Study Sotoudeh Sedaghat,† M. Mahdi Ahadian,*,† Majid Jafarian,‡ and Shadie Hatamie†,§ †

Institute for Nanoscience and Nanotechnology (INST), Sharif University of Technology, Azadi Avenue, 14588-89694 Tehran, Iran Faculty of Science, K. N. Toosi University of Technology, Mirdamad Boulevard, 15875-4416 Tehran, Iran

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‡

ABSTRACT: Three-dimensional graphenic adsorbents have been successfully synthesized by hydrothermal reduction and applied for deep removal of dibenzothiophene (DBT) from model fuel. The nanoporous spongelike structure of the graphenic compounds was confirmed using various characterization techniques. Reduced graphene oxide (rGO), carbon black−graphene composite (CB-G), and nickelimpregnated graphene (Ni-G) showed adsorption capacities of 41.8, 46.9, and 43.3 mg of DBT g−1, respectively, and the DBT concentration in the model fuel was diminished to less than 10 ppm. Thermodynamic parameters for the adsorption process evidenced feasible and exothermic adsorption on rGO and CB-G with negative enthalpy changes. Adsorption isotherms for rGO and CB-G were best fitted with the Langmuir isotherm, indicating uniform adsorption sites. On the other hand, the isotherms for Ni-G were best fitted with the Freundlich and Temkin models, showing special active sites. Carbon black intercalation can effectively change the pore dimensions to meso size while maintaining a uniform graphenic morphology, leading to high DBT adsorption capacities.

1. INTRODUCTION Removal of sulfur contaminants from fuels has been receiving dramatic attention, not only because of the growing demand for cleaner air but also because of strict environmental regulations. The established regulations by 2006 in the United States specified the sulfur content of diesel fuels to be less than 15 ppm, in the range of ultralow-sulfur diesel (ULSD).1,2 These environmental regulations are also being implemented in many other countries, including Japan, Brazil, and the European Union.3 During combustion of fuels, several sulfur compounds are converted to sulfoxides (SOx), which cause harmful effects on human organisms, animals, and also plants caused by acid rain.4 In addition, sulfur in the fuel can damage the industrial equipment and cause economic losses.5 Deep and ultradeep desulfurization of transportation fuels are more significant issues because of high demands for clean fuels in different fuel cells.6 Oil derivatives as desirable sources of hydrogen have been widely used in fuel cells.7 As the fuels used in different types of fuel cells need to have ultralow amounts of sulfur pollutants, preparation of a clean fuel source for hydrogen production is required.8 Hydrodesulfurization (HDS) is the most common strategy for removing organic sulfur contents from diesel fuels.9,10 HDS usually operates at a hydrogen pressure of 20−100 bar and a temperature range of 300−400 °C.11,12 On the other hand, HDS is not efficient in removing aromatic sulfur compounds like benzothiophene (BT) and its derivatives.4,13,14 Different alternatives such as oxidative desulfurization (ODS), electro-oxidation, use of membranes, biodesulfurization (BDS), extractive desulfuriza© 2019 American Chemical Society

tion (EDS), use of ionic liquids, and adsorptive desulfurization are used for desulfurization of fuels.9,13,15−19 Adsorptive desulfurization is an outstanding and promising approach for sulfur removal in order to reach ultralow concentrations because of its mild process conditions, selective removal possibility, various adsorbent availability, and relatively economically viable adsorption process.9,20−22 Carbon materials are well- known adsorbents for various applications because of their appropriate porous structures and compatibility.23,24 Activated carbon, mesoporous carbon materials (CMK), carbon nanotubes, carbon aerogels, graphenic materials, and their modified forms are mostly used for energy applications.25−32 Nanostructured carbons are known as promising materials because of their green and sustainable nature, high surface area, high resistance under different conditions, and tunable surface properties. Graphene is a significant material because of its two-dimensional (2D) structure formed by a honeycomb lattice. Graphene possesses π orbitals and is able to catch aromatic sulfur compounds through π−π interactions.33 Graphene, graphene oxide (GO), and their derivatives have been used as adsorbents, catalysts, and membranes for environmental purposes. Some cancerous heavy metals such as arsenic can be removed from water by means of modified forms of GO.34,35 Graphene derivatives Received: Revised: Accepted: Published: 10341

January 15, 2019 April 17, 2019 May 22, 2019 May 22, 2019 DOI: 10.1021/acs.iecr.9b00250 Ind. Eng. Chem. Res. 2019, 58, 10341−10351

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Industrial & Engineering Chemistry Research

suggest a probable mechanism for adsorption of DBT on the prepared compounds.

have been used in the reverse osmosis (RO) desalination process to control biofouling, which is a common problem in RO plants.36,37 Hummers’ method is a common approach to make GO, which can be reduced to form graphene or partially reduced graphene oxide (rGO). There are various methods for reduction of GO that differ in preparation conditions and final graphene structures and properties.38 Hydrothermal reduction processes can produce three-dimensional (3D) graphene having a porous structure, high surface area, and suitable sites for adsorption processes.39 Moreover, structural modification of graphene for adjustable adsorption and transport of molecules has become an important research area in recent years.40,41 Metal-loaded graphene and carbon materials have also shown improved properties, especially by providing active sites for adsorption, electrochemical, and catalytic applications.22,39,42 Various studies have introduced graphene-based materials as efficient catalysts and adsorbents for sulfur removal.43−45 Song et al. prepared graphene by different methods for desulfurization of dibenzothiophene (DBT).33 They suggested phosphoric acid as an appropriate oxidation agent for graphite, resulting in selective DBT adsorption in the presence of toluene. A GO/mixed metal oxide hybrid has been used for desulfurization of liquid fuels. The modified structure showed an increase of up to 170% in sulfur uptake, and the adsorption isotherm was fitted by the Freundlich model.46 Adsorption of DBT using bamboo charcoal was investigated by Zhao et al.26 They reported the pseudo-second-order equation as a kinetic model and the Freundlich model for the adsorption isotherm. A few-layer graphene-like boron nitride was synthesized by Xiong et al. and used in adsorptive desulfurization.47 This adsorbent showed a capacity of 28.17 mg of S g−1 and suggested pseudo-second-order kinetics with a Langmuir adsorption isotherm. Although there have been numerous studies on adsorptive desulfurization operation and macromolecular parameters, there are few works discussing the surface effects and micromolecular mechanisms for sulfur removal. Focusing on the surface of the adsorbent can give us clues for further modification to enhance their adsorption efficiency. In this study, we synthesized hydrothermally reduced graphene-based materials possessing a modified porous structure and investigated their performance in DBT adsorption. We introduced carbon black into the graphene structure as a spacer to make it more appropriate for the adsorption process. We used nickel because of its ability to bind to sulfur atoms by π−π complexation. The synthesized materials were characterized using X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDS), UV−vis spectroscopy, Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, and N2 adsorption−desorption measurements. These materials were used for desulfurization of a model fuel comprising DBT in n-octane. The feasibility of the process was investigated by a thermodynamic study. The adsorption isotherms and kinetics were calculated using the Langmuir, Freundlich, and Temkin equations and pseudo-firstorder and pseudo-second-order kinetic models, respectively, and finally the best-fitted models were suggested. The effect of carbon black and Ni particles in the adsorption parameters is discussed. Our main purpose is to study the effect of microand nanostructure properties of 3D graphenic materials on the adsorption of aromatic sulfur from model fuel. Finally, we

2. EXPERIMENTAL SECTION 2.1. Materials. Graphite powder (mesh ≤200 m), sodium nitrate (NaNO3) and potassium permanganate (KMnO4) from Merck were used for fabrication of graphene oxide. Sulfuric acid (H2SO4), hydrochloric acid (HCl), hydrogen peroxide (H2O2), ammonia (NH3), and nickel acetate were obtained from ChemLab, and carbon black was procured from Degussa. All of the materials were used without further purification. 2.2. Preparation of Graphene Oxide. GO was prepared by the modified Hummers’ method using graphite powder.48 Briefly, 2 g of graphite powder was dissolved in 92 mL of H2SO4, and 2 g of NaNO3 was added to the mixture at 0 °C in an ice bath. Next, 12 g of KMnO4 was slowly added to the mixture under stirring, after which the temperature was raised to 35−40 °C. The mixture was stirred at the mentioned temperature for 4 h. Afterward, 160 mL of water was added to the sluggish solution dropwise. Subsequently, the temperature was elevated to 85−90 °C, and the color of the solution turned into yellow, which indicated graphite oxide formation. Then 400 mL of deionized water and 12 mL of H2O2 were added to the solution. The resulting product was dispersed in HCl and washed using deionized water to reach a pH of 5−6. Finally, the obtained graphite oxide was exfoliated using ultrasonic treatment for 30 min to get GO sheets. 2.3. Synthesis of Graphenic Adsorbents. Reduced graphene oxide was prepared using the hydrothermal method at a temperature of 180 °C and pH 10−11 (adjusted using ammonia) for 12 h. The synthesis of the carbon black− graphene composite and nickel−graphene composite with a 1:10 weight ratio of GO to carbon black or Ni was carried out by adding carbon black suspension or nickel solution to the GO mixture under vigorous stirring with the aid of ultrasound to reach a homogeneous mixture and good distribution of carbon black or Ni. Finally, the pH of each mixture was elevated to 10−11 using ammonia, followed by the hydrothermal process under the mentioned conditions. The products were labeled as rGO (reduced graphene oxide), CB-G (carbon black−graphene composite), and Ni-G (nickel−graphene composite). 2.4. DBT Adsorption Tests. The DBT adsorption properties of the graphene-based synthetic materials were investigated through batch adsorption tests. DBT was dissolved in n-octane to supply the model fuel. For adsorption studies, typically, 50 mg of adsorbent was dispersed in 20 mL of model fuel with a DBT concentration of 100−1000 mg L−1. All of the adsorption experiments were carried out under ambient atmosphere and performed at temperatures of 288, 298, 313, and 333 K. The model fuel was magnetically stirred in the presence of the adsorbent for 24 h to make sure that the adsorption/desorption equilibrium was reached. The concentration of each sample was measured using a NanoDrop spectrophotometer (2000, Thermo Scientific, USA) at various time intervals. We calculated the quantitative adsorption capacity of each adsorbent using the equation below: qe =

(C0 − Ce)V w

(1)

where qe is the sulfur equilibrium adsorption capacity (mg of DBT g−1), C0 and Ce are the initial and equilibrium 10342

DOI: 10.1021/acs.iecr.9b00250 Ind. Eng. Chem. Res. 2019, 58, 10341−10351

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Industrial & Engineering Chemistry Research concentrations, respectively (mg L−1), V is the model fuel volume (L), and w is the weight of adsorbent (g).49 2.5. Analytical Methods. The synthesized adsorbents were characterized using FT-IR performed on a PerkinElmer Spectrum RX I instrument. XRD patterns were collected on a PANalytical X’Pert PRO MPD diffractometer. FE-SEM and EDS data were obtained using a TESCAN MIRA3 microscope. Raman analysis was performed using an Apus Raman microscope system from Teksan Company. The concentration of DBT in model fuel samples was measured using UV−vis spectroscopy carried out using NanoDrop and PerkinElmer spectrophotometers. The N2 adsorption−desorption isotherms were measured using Belsorp mini II instrument. The specific surface areas were obtained by volumetric nitrogen sorption at 77 K. Inductively coupled plasma optical emission spectroscopy (ICP-OES) was performed using a Spectro Arcos instrument.

3400 cm−1 due to deoxygenation.50 The CO peak at 1735 cm−1 was diminished, and the peak at 1560 cm−1 confirmed restoration of the sp2 carbon networks after hydrothermal reduction. Raman shifts for the materials are shown in Figure 1b, which exhibits three obvious peaks. The D bands appeared at about 1340 and 2700 cm−1 and the G band at 1574 cm−1, caused by the disordered structure of graphene and C−C bond stretching, respectively. The intensity ratio of the D and G bands (ID/IG) is greater than unity, which is much higher than the reported values for common 2D graphene (∼0.4).51 The higher ID/IG expresses a high number of defects in the compounds, which may arise from the spongelike porous structure. This value was increased by adding carbon black as a spacer to the graphene. A highly disordered graphene structure can have a higher surface area, which is desirable for adsorption purposes.52 UV−vis spectra of the synthesized materials were measured using suspensions of GO and the hydrothermally reduced materials in distilled water. Using the UV−vis data, we estimated the ground to excited state transitions of the chromophores. Figure 2 demonstrates that the GO spectrum is

3. RESULTS AND DISCUSSION 3.1. Characterization of Adsorbents. Figure 1a shows FT-IR spectra of the synthesized graphene oxide and

Figure 2. UV−vis spectra of the synthesized graphene oxide (GO) and carbon black−graphene composite (CB-G). The inset graphs show the UV−vis spectra of reduced graphene oxide (rGO) and nickel-loaded graphene (Ni-G).

in agreement with previous reports, exhibiting the special absorption peak at about 235 nm and a broad shoulder at 320−370 nm. The adsorption at about 235 nm is assigned to the electronic transition from the bonding (π) orbital to the antibonding (π*) orbital. This absorption peak has corresponded to the transition of the CC bonds in the previous reports.53 The hydrothermally reduced CB-G product presented a red shift in the absorption peak due to the decreased energy difference between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). The broad shoulder at about 350 nm assigned to the transition from the nonbonding (n) orbital to the π* orbital. This case confirmed the existence of epoxide (C−O−C) and peroxide (R−O−O−R) linkages in the material. As is comprehensible from the spectra, this peak was dramatically diminished for the reduced products, confirming deoxygenation of materials during the reduction process.54 However, the intensity of the n → π* transition in the Ni-G spectrum is significantly decreased, although this peak is still visible (Figure 2, left inset). The Ni particles can

Figure 1. (a) FT-IR spectra of graphene oxide (GO) and reduced graphene oxide (rGO). (b) Raman spectra of rGO, CB-G, and Ni-G.

hydrothermally reduced graphene oxide. The FT-IR spectrum of graphene oxide confirmed the presence of the functional groups containing oxygen at different wavenumbers, including the peak at about 3400 cm−1 for O−H stretching vibrations, the peaks at 1720 and 1220 cm−1 for stretching vibrations of CO and C−OH, and the peak at 1030 cm−1 for the C−O stretching vibration. The FT-IR spectrum of reduced graphene oxide showed an obvious decrease in the O−H peak at about 10343


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by arrows) are well-dispersed all over the structure. Figure 4d presents an EDS map of the Ni-containing compound, which confirms the presence and good dispersion of Ni particles on the graphene surface. EDS analysis showed ∼5.2 wt % nickel and ICP-OES showed 7.9 wt % nickel in the Ni-G nanocomposite. The ICP-OES result is nearer to the expected value for nickel (10 wt %). The insets in Figure 4a−c show micrographs at other magnifications. The N2 adsorption results and Brunauer−Emmett−Teller (BET) equation were used to estimate the specific surface areas. Micropore and mesopore size distributions and pore volumes of the materials were estimated using desorption data, MP plot, and Barrett−Joyner−Halenda (BJH) plot. The total and external specific surface areas and pore volumes were obtained using a t plot (not shown). The BET adsorption isotherms are depicted in Figure 5. The curves in Figure 5a demonstrate type I adsorption isotherms for all of the materials. These types of isotherms represent the presence of micropores with predominant pore sizes less than 2 nm. Table 1 shows total surface areas, pore sizes, and pore volumes of the adsorbents. From the data for pore volumes, most of the porosity measured by BET analysis is in the micropore range, which is favorable for DBT adsorption.59 Pristine graphene (rGO) shows the highest specific surface area, and it seems that the conjugation of carbon black or nickel particles reduces the available surface area of graphene. Embedding carbon black particles between the graphene sheets introduces new mesoporous structure, as confirmed by the hysteresis in the BET isotherm. These pores provide new pathways to conduct DBT molecules, allowing them to reach the micropores and adsorption sites. However, this two-step adsorption may diminish the total process rate. 3.2. Adsorption Isotherms. Adsorption isotherms for the three adsorbents were obtained by performing batch-mode adsorption from DBT solutions with different concentrations for 24 h. The adsorption experiments showed deep desulfurization of the model fuel, decreasing the amount of DBT to less than 10 ppm. Figure 6a−c indicates the isotherms at 25 °C. Using adsorption equilibrium data, we calculated the adsorption performance of the adsorbents. We used the Langmuir, Freundlich and Temkin equations (eqs 2−4, respectively) to evaluate the adsorption constants and plot the isotherms:

lead to the presence of more oxygen groups on the structure, followed by more epoxide and peroxide linkages. Figure 3 shows XRD patterns for pristine graphite, graphene oxide, and the graphenic reduced products. Graphite and GO

Figure 3. XRD patterns of graphene oxide (GO) and reduced graphene-based materials (reduced graphene oxide (rGO), carbon black−graphene composite (CB-G), and nickel−graphene composite(Ni-G)).

demonstrate intense diffraction peaks at 2θ = 26.0° and 11.0°, respectively, indicating the intercalation of graphite sheets in GO. However, the peak at 2θ ≈ 27.0° relates to unreacted graphite sheets. The calculated d-spacing of GO was 8.02 Å, indicating the presence of oxygen-containing groups between the graphitic layers. The XRD pattern of rGO shows the diffraction peak at about 24.0°, and the d-spacing changes from 8.02 Å for GO to 3.52 Å for rGO, which is still a little larger than the intersheet distance in graphite (∼3.36 Å).55 The interlayer distances for CB-G and Ni-G were obtained as 3.66 and 3.58 Å, respectively. Obviously, no new peak appeared upon addition of carbon black, but the peak position and dspacing showed a little increase. As we expected, the presence of carbon black in the graphene structure increased the interlayer spacing of graphene. The XRD pattern of Ni-G shows two main diffraction peaks at 2θ = 44.5° and 51.9°, related to the (111) and (200) crystalline planes of Ni, respectively (JCPDS no. 01-1258).56 These peaks are in compliance with the patterns for the face-centered-cubic (fcc) Ni structure from the reference patterns. Thus, there appeared no characteristic peak for nickel oxide, indicating that the hydrothermal process reduced not only graphene oxide but also nickel cations. The crystallite size of Ni was estimated to be about 46.7 nm using the Scherrer equation.57 In addition, the Ni-G spectrum shows rGO peaks too, confirming that the addition of Ni particles did not disassemble the graphene structure. Intrinsically, most of the grafted Ni particles are attached to the edges of the graphene sheets, which contain more dangling bonds. The FE-SEM micrograph of hydrothermally reduced graphene oxide (rGO) shows a 3D spongelike nanostructure (Figure 4a). This structure is more porous than that of normal graphene and has variable pore sizes. As shown in Figure 4b, adding carbon black provides a relatively dense structure, although the interlayer spaces between graphene sheets may increase as a result of the presence of carbon black spacers.58 Figure 4c shows a backscattered micrograph of nickelimpregnated graphene, in which the nickel particles (shown

Ce C 1 = e + qe qm qmKL ln qe = ln KF + qe =

1 ln Ce n

RT RT ln A T + ln Ce bT bT

(2) (3)

(4)

where qm is the maximum adsorption capacity; KL is the Langmuir adsorption constant (L mg−1), which depends on the adsorption energy; KF and n are the Freundlich adsorption constants;60 bT is the Temkin constant, which depends on the heat of adsorption (J mol−1); AT is the Temkin equilibrium binding constant (L g−1); R is the gas constant (8.3145 J mol−1 K−1); and T is the absolute temperature (K).61 Table 2 lists the calculated isotherm constants for the adsorbents. As can be seen from Figure 6 and Table 2, the rGO and CBG adsorption isotherms best match the Langmuir isotherm, indicating uniform adsorption of DBT on the surface of these 10344


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Figure 4. (a−c) FE-SEM images of (a) rGO, (b) CB-G, and (c) Ni-G (red arrows point to the Ni particles) (d) EDS map of Ni-G. Micrographs in further view and magnification are shown in the inset images in (a−c).

adsorbents. This uniform adsorption may occur due to π−π stacking of aromatic groups of DBT molecules on graphene. For Ni-G (Figure 6c), there is a better match with the Freundlich isotherm at lower DBT concentrations and the Langmuir isotherm at higher DBT concentrations. This type of correlation suggests that the adsorption process starts at specific sites on the adsorbents and then continues by adsorption on the uniform graphitic surface. In addition, Freundlich matching indicates that the adsorption energy is exponentially diminished by filling of the adsorbent sites. Also, the Temkin isotherm shows a better correlation with the adsorption isotherm of Ni-G compared with rGO and CB-G, demonstrating a higher possibility of adsorbent−adsorbate interactions on Ni-G. This is likely based on π complexation of Ni and DBT molecules.62

Every isotherm constant listed in Table 2 relates to a special phenomenon. Low values for the Langmuir constant (KL) indicate the formation of a monolayer on the uniform graphitic layer. In the Freundlich equation, n is greater than unity (here ∼6), which represents favorable adsorption of DBT on the adsorbents. The values of bT for the adsorption on the prepared adsorbents are in the range of 0.34−0.42, indicating that the main adsorption mechanism is physisorption. The maximum adsorption capacity of CB-G (46.9 mg g−1) is higher than those of rGO and Ni-G (41.8 and 43.3 mg g−1, respectively).47 Carbon black particles as spacers between the graphene sheets seem to provide more spaces for DBT adsorption. Graphene sheets provide π−π stacking sites for DBT adsorption. In addition, Ni particles in Ni-G create new adsorption sites that can grab sulfur atoms by π−π complexation.62 This observation confirms the expected presence of 10345


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In the pseudo-first-order and pseudo-second-order kinetic investigations, log(qe − qt) and t/qt, respectively, were plotted versus t for each adsorbent; Table 3 displays the correlation coefficients (R2) of these plots and the calculated k1, k2, and h values for rGO, CB-G, and Ni-G. From the correlation coefficients, pseudo-second-order kinetics fits the rGO and CB-G data better, indicating that the adsorption over the uniform graphene surface is controlled by liquid−solid interactions and depends on the DBT concentration near the graphene sites. Furthermore, the pseudo-second-order kinetic model proves that an interaction between DBT molecules and n-octane occurs at the adsorption sites. Moreover, the pseudosecond-order rate constant determines a higher rate of adsorption on rGO (k2 = 0.0627 g mg −1 min−1). The correlation coefficient for the pseudo-second-order kinetic plot for Ni-G is slightly lower than that for the pseudo-first-order plot. Thus, it can be concluded that DBT adsorption over the Ni-G adsorbent is controlled by both diffusion and liquid− solid interface processes. As Ni-G has dual adsorption sites of graphene and nickel particles, there are two mechanisms of adsorption, as suggested by the isotherm investigation above. As the adsorption on graphene sheet depends on the DBT concentration, the initial π−π complexation on Ni particles can decrease the graphene−DBT interaction probability, so the total adsorption rate is diminished. Figure 7 shows the kinetic plots for each adsorbent. The kinetic parameters are provided in Table 3. These data were obtained from batchwise DBT adsorption for 60 min at 25 °C and a DBT concentration of 100 mg L−1. rGO shows a higher initial adsorption rate and rate constant, indicating faster adsorption of DBT; different reports in the literature showed the same results.47 Although the adsorption capacity of CB-G was higher than that of rGO, the DBT adsorption rate constant for CB-G is relatively low (4.823 × 10−3 g mg−1 min−1). This may be related to the structure of this adsorbent, which leads to weaker π−π interactions between graphene sheets and DBT molecules and reduces the availability of adsorption sites. This probability increases in the Ni-G case because of the presence of Ni particles over graphene sheets. However, the mesopore structure of CB-G results in a higher adsorption capacity (3.1), but it leads to lower adsorption rate. It looks like there is a pathway from mesopores to micropores for DBT molecules on CB-G. Although this pathway can navigate molecules toward adsorption sites, that makes it a longer way to pass and diminishes the total adsorption rate. 3.4. Adsorption Energy. The thermodynamic parameters for adsorption of DBT on rGO were calculated using experimental data obtained at different temperatures. The Gibbs free energy is expressed by the following equation:

Figure 5. (a) N2 adsorption−desorption isotherms of rGO, CB-G, and Ni-G. (b) BJH plots for mesopore size distributions. The inset curves are MP plots showing micropore size distribution of the synthesized graphenic structures.

Table 1. N2 Adsorption−Desorption Pore Size Parameters of the Adsorbents sample

total surface area (m2 g−1)

pore diameters (micro−meso) (nm)

total pore volume (cm3 g−1)

micropore volume (cm3 g−1)

rGO CB-G Ni-G

706.1 620.5 552.9

0.6−2.4 0.6−2.4 1.1−2.4

0.32 0.32 0.26

0.31 0.28 0.25

dual active sites. Thus, the adsorption capacity is slightly increased by direct interactions between nickel and sulfur atoms. 3.3. Kinetic Investigations. The pseudo-first-order and pseudo-second-order kinetic models were studied to investigate the adsorption process on these adsorbents. The pseudofirst-order and pseudo-second-order equations are shown in eqs 5 and 6, respectively: log(qe − qt ) = log qe − k1t t 1 t = + 2 qt qe k 2qe

ΔG = −RT ln K

(5)

where K is the equilibrium constant, which depends on the temperature as follows:

(6)

ln K =

where t is the time (min), qe and qt are the amounts of adsorbed DBT at the equilibrium and at time t, respectively (mmol g−1), and k1 and k2 are the pseudo-first-order and pseudo-second-order rate constants, respectively (min−1). The initial adsorption rate, h, for the adsorption with pseudosecond-order kinetic can be calculated using the following equation:32 h = k 2qe 2

(8)

ΔS ΔH − R RT

(9)

where ΔS and ΔH are the entropy and enthalpy of adsorption, respectively. The apparent adsorption equilibrium constant is the ratio of the quantity of adsorbed DBT to the concentration of DBT in the solution at equilibrium:32,63 q Ke = e Ce (10)

(7) 10346


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Figure 6. Adsorption isotherms of the synthesized adsorbents (DBT adsorption in batch mode) at 25 °C: (a) rGO; (b) CB-G; (c) Ni-G. The insets show schematic adsorption mechanisms.

adsorbents were calculated using the slopes and intercepts of these plots. The thermodynamic parameters are shown in Table 4. The negative values of ΔG indicate that the adsorption process is feasible and thermodynamically possible at room temperature.63 However, increasing the temperature changes the adsorption to be a less favorable process, which shows that it is an exothermic process. In addition, the ΔG and K values for different temperatures show that this process can be performed better at lower temperature. The negative values for ΔH also confirm that the adsorption of DBT on rGO and CB-G is exothermic. The negative values for ΔS show the reduced randomness of the system, resulting from collection of the DBT molecules from solution on the adsorbent surface. All of the thermodynamic parameters demonstrate that the DBT adsorption is more favorable on rGO than on CB-G. The difference between the ΔH and ΔS values for every adsorbent show that the DBT−graphene interaction can be considered to be enthalpy driven, so the change in temperature can affect the adsorption process. Moreover, herein the spontaneous adsorption process (negative ΔG) on the adsorbents arises from the enthalpy change. The value |ΔH| < 80 kJ mol−1 confirms physical adsorption. However, the greater ΔH value for adsorption on rGO can represent a more unfavorable subsequent desorption possibility. Subsequent desorption is

Table 2. Langmuir, Freundlich, and Temkin Isotherm Constants for Adsorption of DBT by the Three Adsorbents at 298 K adsorbent isotherm

constants

rGO

CB-G

Ni-G

Langmuir

R2 KL (L mg−1) qm (mg g−1) R2 KF (mmol1−(1/n) L1/n g−1) n R2 AT (L g−1) bT (kJ mol−1)

0.999 0.057 41.8 0.966 13.864 6.079 0.937 1.908 0.415

0.999 0.060 46.9 0.853 19.004 6.983 0.946 1.381 0.341

0.988 0.045 43.3 0.991 14.621 5.992 0.987 1.806 0.404

Freundlich

Temkin

The thermodynamic equilibrium constant, K, was estimated by computing Ke at different initial concentrations of DBT at a constant temperature and extrapolating to zero.63 Figure 7 shows the relationship between ln K and the inverse temperature. The ln K versus 1/T plots (Figure 8) demonstrate linear behavior for the adsorption on the rGO and CB-G adsorbents, indicating that the adsorption enthalpy and entropy changes are independent of temperature. The enthalpy and entropy changes for adsorption of DBT on the

Table 3. Kinetic Parameters for Adsorption of DBT on the Synthesized Adsorbents pseudo-first-order

pseudo-second-order

adsorbent

R2

k1 (min−1)

R2

k2 (g mg−1 min−1)

h (mg g−1 min−1)

RGO CB-G Ni-G

0.868 0.851 0.961

2.950 × 10−2 4.100 × 10−3 3.300 × 10−3

0.999 0.905 0.953

6.271 × 10−2 4.823 × 10−3 1.868 × 10−3

23.98 2.382 0.485

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required and important for regeneration and recycling of adsorbents. From the thermodynamic parameters, it can be concluded that CB-G can be regenerated easily by a slight increase in temperature. The regeneration and reuse of the adsorbents have been investigated and will be reported as our future research works. Investigation of the thermodynamic parameters for adsorption of DBT on activated carbon was performed by Wen et al.63 The reported Gibbs free energy and equilibrium constant at 298 K were 12.59 kJ mol−1 and 160.85, respectively. The adsorption process over our synthesized 3D graphene was implemented with ΔG = −7 kJ mol−1, which is acceptable in comparison with common adsorbents. The adsorption processes on the other adsorbents like alumina composites showed positive enthalpy and entropy changes, though the obtained Gibbs free energies were ultimately negative.64 From the combination of the adsorption isotherm (section 3.2), kinetic (section 3.3), and thermodynamic (section 3.4) data, along with the structural characterization of synthesized compounds (section 3.1), it can be inferred that graphene provides uniform adsorption sites for catching aromatic sulfur compounds by π−π stacking (Figure 6a,b insets). The 3D spongelike structure of graphene represents porosities composed of graphitic plates, making it an efficient adsorbent for DBT capture. The DBT adsorption capacity of these adsorbents was around 40 mg g−1, which is higher than the amounts for well-known adsorbents such as some kinds of modified activated carbon.65 Further adsorption sites were obtained by embedding nickel nanoparticles on graphene. Ni particles provide the possibility of π−π complexation of sulfur atoms, which leads to DBT adsorption (Figure 6c inset). The π−π stacking mechanism may operate faster than π−π complexation, so the adsorption rate on rGO is higher than that on Ni-G. In addition, the CB particles cover the graphitic π sites, leading to a lower adsorption rate. CB-G showed a higher adsorption capacity (Figure 6) due to its enhanced mesoporous structure (Figure 5) created by CB particles. The newly established mesopores on CB-G lead the solvent to reach the surface easily, and afterward the DBT molecules can meet micropores for adsorption. The new mesopores on CB-G may act as navigator channels to conduct DBT molecules toward the adsorption sites, so the adsorption capacity rises.

Figure 7. (a) Pseudo-first-order and (b) pseudo-second-order kinetic plots for adsorption of DBT onto rGO, CB-G, and Ni-G.

4. CONCLUSIONS Three types of graphenic adsorbents were hydrothermally synthesized and used for adsorption of DBT from a model fuel. The synthesized adsorbents were characterized using different nanostructure analysis methods. Besides analysis and characterization methods, investigation of macroscopic phenomena of

Figure 8. Linear relationship between ln K and 1/T for adsorption of DBT on rGO and CB-G. K is the equilibrium constant, and T is the temperature.

Table 4. Thermodynamic Parameters for Adsorption of DBT on rGO and CB-G adsorbent

adsorption temperature

K

ΔG (kJ mol−1)

rGO

288 298 313 333 288 298 313 333

72.270 7.912 3.735 0.733 6.269 1.430 0.692 0.306

−7.009 −5.125 −3.429 0.858 −4.548 −0.886 0.958 3.280

CB-G

10348

ΔH (kJ mol−1)

ΔS (J mol−1 K−1)

−69.01

−210.51

−50.42

−162.85


Article


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adsorption were conducted in this study. CB-G showed higher d-spacing and mesoporous structure, which may be caused by carbon black spacer particles. Investigation of DBT adsorption on the prepared compounds indicated deep desulfurization to DBT contents of less than 10 ppm. Uniform adsorption over rGO and CB-G was concluded from the conformity of their isotherm plots with the Langmuir isotherm, whereas the presence of new adsorption sites and possible stronger adsorption−desorption interactions were confirmed for Ni-G by matching its isotherm to the Freundlich and Temkin models. Kinetic investigations showed pseudo-second-order kinetics for all of the adsorbents. Furthermore, a significant correlation coefficient of the pseudo-first-order model for Ni-G confirmed the dual adsorption sites with both adsorption mechanisms of π−π stacking and π−π complexation. Although carbon black particles as spacers in the CB-G structure increase the adsorption capacity, they lead to a lower adsorption rate. The adsorption capacity of CB-G reached 46.9 mg g−1, and the adsorption rate of rGO was about 24 mg g−1 min−1. The higher adsorption capacity for CB-G depends on the mesoporous navigator channels. Energy evaluation and thermodynamic parameters of DBT adsorption over rGO and CB-G showed a negative Gibbs free energy and enthalpy values, indicating an exothermic feasible process. The possibility of regeneration was concluded to be easier for CB-G because of the relatively lower value of the adsorption enthalpy change (−50.4 kJ mol−1). We will provide our results on regeneration and reuse of the adsorbents in future reports.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +98-21-66164123. Fax: +98-21-66164119. E-mail: [email protected]. ORCID

M. Mahdi Ahadian: 0000-0002-5693-9848 Present Address §

S.H.: Stem Cell Technology Research Center, No. 9, East 2nd St., Farhang Blvd., Saadat Abad St., 19977-75555 Tehran, Iran. Notes

The authors declare no competing financial interest.

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Model Fuel Deep Desulfurization Using Modified 3D Graphenic

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