Model Fuel Deep Desulfurization Using Modified 3D Graphenic

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Model fuel deep-desulfurization using modified 3D graphenic adsorbents: Isotherm, kinetic and thermodynamic study Sotoudeh Sedaghat, Mohammad Mahdi Ahadian, Majid Jafarian, and Shadie Hatamie Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 29, 2019

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Industrial & Engineering Chemistry Research

Model fuel deep-desulfurization using modified 3D graphenic adsor bents: Isotherm, kinetic and thermodynamic study Sotoudeh Sedaghat†, M. Mahdi Ahadian†,*, Majid Jafarian‡, Shadie Hatamie†,1 †

Institute for Nanoscience and Nanotechnology (INST), Sharif University of Technology, Azadi Ave., 14588-89694, Tehran, Iran; Emails: [email protected] (S. Sedaghat) , [email protected] (M. M. Ahadian) [email protected] (S. Hatamie) ‡ Faculty of Science, K. N. Toosi University of Technology, Mirdamad Blvd., 15875-4416, Tehran, Iran; Email: [email protected] (M. Jafarian)

*

Corresponding Author:

Mohammad Mahdi Ahadian Institute for Nanoscience and Nanotechnology (INST), Sharif University of Technology, Azadi Ave., 14588-89694, Tehran, Iran. Tel: +98-21-66164123 Fax: +98-21-66164119 Email: [email protected]

1

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

1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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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. Nanoporous sponge-like structure of graphenic compounds was confirmed using various characterization techniques. Reduced graphene oxide (rGO), carbon black-graphene (CB-G) and nickelimpregnated graphene (Ni-G) showed adsorption capacities of 41.8, 46.9, and 43.3 mg DBT. g-1, respectively and the DBT concentration in model fuel has been diminished to less than 10 ppm. Thermodynamic parameters for adsorption process evidenced feasible and exothermic adsorption on rGO and CB-G with negative enthalpy changes. Adsorption isotherms for rGO and CBgraphene, best fitted with Langmuir isotherm indicating uniform adsorption sites. On the other hand, the isotherms for Ni-graphene, best fitted with Freundlich and Temkin models, showing especial active sites. Carbon black intercalation can effectively change the pore dimensions to meso-size while maintaining uniform graphenic morphology leading to high DBT adsorption capacities. Keywords: Deep-desulfurization, Adsorption isotherm study, Kinetic investigation, Nanoporous 3D graphene, DBT adsorption mechanism 1. Introduction Removal of sulfur contaminants from fuels has been receiving dramatic attention, not only due to the growing demand of cleaner air but also owing to the 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 ultra-low sulfur diesel (ULSD)

1, 2

.

These

environmental regulations are also implementing in many other countries including Japan, Brazil, and European Union 3. Several sulfur compounds are converted to sulfoxides (SO x) during combustion of fuels, 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 ultra-deep desulfurization of transportation fuels are more significant issues due to high demands of 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 2 ACS Paragon Plus Environment

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fuel cells need to have ultra-low amounts of sulfur pollutants, requiring to prepare a clean fuel source for hydrogen production 8. Hydro-desulfurization (HDS) is the most common strategy for removing organic sulfur contents from diesel fuels

9, 10

. HDS usually operates at hydrogen pressure

of 20 – 100 bar and 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, membrane, biodesulfurization (BDS), extractive desulfurization (EDS), ionic liquids, and adsorptive desulfurization are used for desulfurization from fuels

9, 13, 15-19

. Adsorptive desulfurization is an

outstanding and promising approach for sulfur removal in order to reach ultra-low concentrations, due to its mild process condition, selective removal possibility, various adsorbent availability, and relatively economically viable adsorption process

9, 20-22

. Carbon materials are well- known

adsorbents for various applications, for 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 resulting from their green and sustainable nature, high surface area and high resistance in 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, 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 graphene oxide34, 35

. Graphene derivatives have been used in reverse osmosis (RO) desalination process to control

biofouling which is a common problem in RO plants36,

37

. Hummers’ method is a common

approach to make GO which can be reduced to form graphene or partially-reduced graphene oxide. There are various methods for reduction of GO that differs in preparation conditions and final graphene structures and properties

38

. Hydrothermal reduction processes can produce 3-

dimensional graphene having 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 also showed improved properties, especially by providing active sites for adsorption, electrochemical and catalytic applications

22, 39, 42

.

Various studies introduced graphene-based materials as an efficient catalyst and adsorbent 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 of graphite and resulting in selective DBT adsorption in the presence of toluene. Graphene oxide/mixed metal oxide hybrid has been used for desulfurization of liquid fuels. The modified structure showed up to 170% increase in sulfur uptake, and the adsorption isotherm fitted by Freundlich model 46. Adsorption of DBT using bamboo charcoal was investigated by Zhao et al. 26

. They reported the pseudo-second-order equation as kinetic model and Freundlich model for

adsorption isotherm. A few-layered graphene-like boron nitride was synthesized by Xiong et al. and has been used in adsorptive desulfurization 47. This adsorbent showed 28.17 mg S. g-1 capacity and suggested pseudo-second-order kinetic with a Langmuir adsorption isotherm. However, there are numerous studies on adsorptive- desulfurization operation and macromolecular parameters, but there are few works discussing the surface effects and micromolecular mechanisms for sulfur removal. Focusing on surface of adsorbent can give us clues for further modification to enhance their adsorption efficiency. In this study, we synthesized hydrothermally reduced graphene-based materials possessing modified porous structure and investigated their performance in DBT adsorption. We have introduced carbon-black into graphene structure as a spacer to make it more appropriate for adsorption process. We used nickel due to its ability to bind to the sulfur atom 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, Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, and N2 adsorption-desorption measurement. These materials have been used for desulfurization of a model fuel comprising DBT in n-octane. Feasibility of the process was investigated by thermodynamic study. The adsorption isotherms and kinetics were calculated using Langmuir, Freundlich and Temkin equations, pseudo-first, and pseudo-second-order kinetic models and finally, best-fitted models were suggested. The effect of carbon black and Ni particles in the 4 ACS Paragon Plus Environment

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adsorption parameters is discussed. Our main purpose is to study the effect of micro and nanostructure properties of 3D graphenic materials on adsorption of aromatic sulfur from model fuel. Finally, we have suggested the probable mechanism for DBT adsorption over prepared compounds. 2. Experimental 2.1. Materials Graphite powder (mesh ≤200 m), sodium nitrate (NaNO3) and potassium permanganate (KMnO4) of 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 from Degussa. All 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 sulfuric acid, 2 g sodium nitrate was added to the mixture at 0 °C in an ice bath. Then, 12 g of potassium permanganate was added slowly under stirring to the mixture, followed by raising the temperature to 35-40 °C. The mixture was stirred at mentioned temperature for 4 h. Afterwards 160 mL water was added to the sluggish solution drop-wise. Subsequently, the temperature was evaluated to 85-90 °C, the color of the solution turned into yellow, which indicated graphite oxide formation. Then 400 mL deionized water and 12 mL hydrogen peroxide were added to the solution. The resulting product was dispersed in HCl and was washed using deionized water to reach the pH of 5-6. Finally, the obtained graphite oxide was exfoliated using ultrasonic treatment for 30 min to get graphene oxide sheets. 2.3. Synthesis of graphenic adsorbents Reduced graphene oxide was prepared using hydrothermal method at temperature of 180 °C and pH = 10-11 adjusted by ammonia, for 12 h. Synthesis of carbon black-graphene composite and nickel-graphene composite with a 1:10 weight ratio of GO to carbon black or Ni, was carried 5 ACS Paragon Plus Environment


out by adding carbon black suspension or nickel solution to GO mixture under vigorous stirring and with the aid of ultrasound to reach a homogeneous mixture and well distribution of carbon black and Ni. Finally, the pH of each mixture was evaluated to 10-11 using ammonia, followed by hydrothermal process at the mentioned condition. 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 DBT adsorption properties of graphen-based synthetic materials have been investigated through batch adsorption tests. DBT was dissolved in n-octane to supply model fuel. For adsorption study, typically, 50 mg of adsorbent was dispersed in 20 mL model fuel with the DBT concentrations of 100-1000 mg. L-1. All the adsorption experiments were carried out under ambient atmosphere and at the temperatures of 288, 298, 313, and 333 K. Model fuel was magnetically stirred in presence of adsorbent for 24 h to make sure that it has reached the adsorption/desorption equilibrium. The concentration of each sample was measured by NanoDrop spectrophotometer (2000, Thermo Scientific, USA) at various time intervals. We calculated quantitative adsorption capacity of adsorbents using the equation below;

𝑞𝑒 =

(𝐶0 −𝐶𝑒 )𝑉

(1)

𝑤

where qe is sulfur equilibrium adsorption capacity (mg DBT.g-1), C0 and Ce represent to the initial and equilibrium concentrations (mg.L-1), V (L) is the model fuel volume, and w (g) is the weight of adsorbent 49. 2.5. Analytical methods The synthesized adsorbents were characterized using FT-IR performed by Spectrum RX I model of PerkinElmer instrument. XRD patterns collected by X’Pert PRO MPD diffractometer of PANalytical. FE-SEM and EDS data were obtained by MIRA3 microscope of the Tescan Company. 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, carried 6 ACS Paragon Plus Environment

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out by NanoDrop and Perkin-Elmer spectrophotometers. The N2 adsorption-desorption isotherms were measured using Belsorp mini II instrument. The specific surface areas obtained by volumetric nitrogen sorption at 77 K. Inductively coupled plasma optical emission spectroscopy (ICP-OES) was performed using Spectro Arcos instrument.

C-H

CO2

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



3. Result and Discussion 3.1. Characterization of adsorbents Figure 1a demonstrates FT-IR spectra for synthesized graphene oxide and hydrothermally reduced graphene oxide. The FT-IR spectra have confirmed the presence of the functional groups containing oxygen in graphene oxide at different wavenumbers included: the peak at about 3400 cm-1 for O-H stretching vibrations, at 1720 cm-1 and 1220 cm-1 for stretching vibrations of C=O and C-OH, and 1030 cm-1 peak for C-O stretching vibration. FT-IR spectra of reduced graphene oxide represent an obvious decrease of O-H peak at about 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 the Figure 1b exhibiting three obvious peaks. Dbonds and G-bond appeared at about 1340, 2700 and 1574 cm-1, cause by disordered structure of graphene and C-C bond stretching, respectively. The intensity ratio of D and G bonds (I D/IG) is greater than unity which is much higher than the reported values for common 2D graphene (~ 0.4)

Figure 2. UV-Vis Spectra of synthesized materials. The inset graphs show UV-Vis spectrum of reduced graphene oxide (rGO) and nickel loaded graphene (Ni-G).


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51

. The higher ID/IG expresses high defects in the compounds may arise from the sponge-like porous

structure. This value increased by adding carbon black as a spacer to the graphene. Highly disordered graphene structure can caused to higher surface area which is desirable for adsorption purposes 52. UV-Vis spectroscopy of the synthesized materials were carried out using a suspension of GO and hydrothermally reduced materials in distilled water. Using the UV-Vis, we estimated the ground state to the excited states transitions of the chromophores. Figure 2 demonstrates that GO spectra are in agreement with the special absorption peak at about 235 nm and a broad shoulder at 320– 370 nm that was expressed in the previous reports. The adsorption at about 235 nm, assigned to the electron transition from the bonding π orbital to the anti-bonding π*. This absorption peak has corresponded to the transition of the C=C bonds in the previous reports 53. The hydrothermally reduced products presented a red shift in the absorption peak of CB-G, due to the distance decrease between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). The broad shoulder at about 350 nm assigned to the transition from non-bonding n orbital to the anti-bonding π*. This case confirmed the existence of epoxide (C-O-C) and peroxide (R-O-O-R) linkages in the material. As it is comprehensible from the spectrums, this peak was GO (002)

Graphite (002)

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



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dramatically diminished for the reduced products, confirming deoxygenation of materials during the reduction process

54

. However the intensity of n to π* transition in the Ni-G spectrum is

significantly decreased, this peak is still visible (Figure 2-inside curve). The Ni particles can lead to the presence of more oxygen groups on the structure, followed by more epoxide and peroxide

(a)

(b)

5 µm

2 µm

(d)

(c)

5 µm

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


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linkages. Figure 3 shows XRD patterns for pristine graphite, graphene oxide and graphenic reduced products. Graphite and GO demonstrate intense diffraction at (2θ) 26.0° and 11.0°, indicating the intercalation of graphite sheets in GO. However, the peak at about 2θ = 27.0° relates to unreacted graphite sheets. The calculated d-spacing value 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 of graphite (∼3.36 Å) 55. The interlayer distance of CB-G and Ni-G was obtained 3.66 Å and 3.58 Å, respectively. Obviously, no new peak appeared by adding carbon black, but the peak position and d-spacing showed a little increase. As we expected, the presence of carbon black in the graphene structure increased the interlayer spaces of graphene. XRD pattern of Ni-G shows two main diffraction peaks at 2θ = 44.5°, 51.9° relating to (111), (200) crystalline plans 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. So, there appeared no characteristic peak for nickel oxide, representing the hydrothermal process reduced not only graphene oxide but also nickel cations. The crystallite size of Ni is estimated to be about 46.7 nm using Scherrer equation 57. In addition, Ni-G spectra shows rGO peaks too, that confirms the addition of Ni particles has not disassembled graphene structure. Intrinsically, most of the grafted Ni particles are attached to the edge of the graphene sheets containing more dangling bonds. FE-SEM micrograph of hydrothermally reduced graphene oxide (rGO) shows threedimensional sponge-like nanostructure (Figure 4a). This structure is more porous than 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 back-scattered micrograph of nickel impregnated graphene, shown by arrows, the nickel particles are well-dispersed all over the structure. Figure 4d as an EDS-map of Ni-contained compound, confirms the presence and welldispersion of Ni particles on the graphene surface. EDS analysis represents ∼5.2 wt% nickel, and ICP-OES showed 7.9 wt% nickel in the Ni-G nanocomposite. ICP-OES result is near to the 11 ACS Paragon Plus Environment


expected value of nickel (10 wt%). The inset Figures 4a-4c show the micrographs with other magnifications. The N2 adsorption results and Brunauer-Emmett-Teller (BET) equation were used to estimate the specific surface area. Micropore and mesopore size distributions and pore volume of materials were estimated using desorption data, MP plot, and Barrett-Joyner-Halenda (BJH) plot.

Figure 5. (a) N2 adsorption-desorption isotherms of rGO, CB-G and Ni-G, (b) BJH-plot for mesopore size distribution, the inset curve is MP-plot showing micropore size distribution of synthesize graphenic structures.


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Table 1. N2 adsorption-desorption pore size parameters of adsorbents. Total Surface Area (m2.g-1)

Pore Diameter (nm) (Micro- Meso)

Total Pore Volume (cm3.g-1)

Micropore Volume (cm3.g-1)

rGO

706.1

0.6 - 2.4

0.32

0.31

CB-G

620.5

0.6 - 2.4

0.32

Ni-G

552.9

1.1 - 2.4

0.26

0.28 0.25

Sample

Total and external specific surface area and pore volume obtained using t-plot (not shown). BET adsorption isotherms are depicted in Figure 5. The curves in Figure 5a demonstrates type I adsorption isotherms for all of the materials. These types of isotherms represent the presence of micropores with predominant pore size less than 2 nm. Table 1 shows total surface area, pore size and pore volume of 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 highest specific surface area, it seems the conjugation of carbon black or nickel particles reduces available surface area of graphene. Embedding carbon black particles between the graphene sheets introduces new mesoporous structure, confirmed by hysteresis in the BET isotherm. These pores provide new pathways for DBT molecules to conduct them reaching the micropores and adsorption sites. However, this two-step adsorption may diminish the total process rate. 3.2. Adsorption isotherms Adsorption isotherms for three adsorbents obtained by performing batch mode adsorption from DBT solutions in different concentrations for 24 hours. Adsorption experiments showed deep-desulfurization of model fuel decreasing the amount of DBT to less than 10 ppm. Figures 6a6c indicate the isotherms at 25 °C. Using adsorption equilibrium data, we can calculate adsorption performance of the adsorbents. We use Langmuir, Freundlich and Temkin equations to evaluate the adsorption constants and plot the isotherms. Langmuir, Freundlich and Temkin models are based on the following equations (2), (3) and (4), respectively: 𝐶𝑒 𝑞𝑒

=

𝐶𝑒 𝑞𝑚

+

1

(2)

𝑞𝑚 𝑘𝐿



1

𝐿𝑛 𝑞𝑒 = 𝐿𝑛 𝑘𝐹 + 𝑞𝑒 =

𝑅𝑇 𝑏𝑇

𝑛

𝐿𝑛 𝐴 𝑇 +

𝐿𝑛 𝐶𝑒

𝑅𝑇 𝑏𝑇

(3)

𝐿𝑛 𝐶𝑒

(4)

where qm is the maximum adsorption capacity, KL represents Langmuir adsorption constant (L.mg 1

), that depends on the adsorption energy, KF and n are the Freundlich adsorption constants 60. bT

is the Temkin constant depends on the heat of adsorption (J.mol-1), AT and R represent Temkin equilibrium binding constant (L.g−1) and gas constant (8.3145 J. mol−1. K−1), respectively. T refers to temperature (K) 61. Table 2 lists the calculated isotherm constants for adsorbents. As it stands from Figure 6 and Table 2, rGO and CB-G adsorption isotherms best match on Langmuir, indicating a uniform adsorption of DBT on the surface of adsorbents. This uniform adsorption may occur due to π-π stacking of aromatic groups of DBT molecules on graphene. For 50

(a) rGO

(b) CB.G

40

40

30

30

20

qe (experiment) qL (Langmuir) qF (Freundlich) qT (Temkin)

10

qₑ (mg.g-1)

qₑ (mg.g-1)

50

qe (experiment)

20

qL (Langmuir) 10

qF (Freundlich) qT (Temkin)

0

0 0

200

400

600

800

0

Cₑ (mg.L-1) 50

200

400

600

Cₑ (mg.L-1)

(c) Ni-G

40

qₑ (mg.g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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30 20

Figure 6. Adsorption isotherms of synthesized adsorbents (DBT adsorption in batch mode) at 25 °C, (a) rGO, (b) CB-G, and (c) Ni-G. Schematic adsorption mechanism (inset).

qe (experiment) qL (Langmuir) qF (Freundlich) qT (Temkin)

10 0 0

200

400

Cₑ

600

800

(mg.L-1) 14 ACS Paragon Plus Environment

800

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Ni-G (Figure 6c), there is a better match on Freundlich in lower DBT concentrations and match on Langmuir in higher DBT concentrations. This type of correlation suggests that the adsorption process starts at the specific sites over the adsorbents and then continues by adsorption on the uniform graphitic surface. In addition, Freundlich matching represents adsorption energy is exponentially diminished by filling the adsorbent sites. Also, Temkin isotherm indicates better correlation with adsorption isotherm on Ni-G compared to rGO and CB-G, demonstrating higher possibility of adsorbent-adsorbate interactions on Ni-G. This is likely based on π-complexation of Ni and DBT molecule.62 Table 2. Langmuir, Freundlich, and Temkin isotherm constants for adsorption of DBT by adsorbents at 298 K. Adsorbent type Isotherm

Constants

rGO

CB-G

Ni-G

0.999

0.999

0.988

KL (L. mg )

0.057

0.060

0.045

qm (mg.g-1)

41.8

46.9

43.3

R2

0.966

0.853

0.991

KF (mmol1−(1/n). L1/n/g)

13.864

19.004

14.621

n

6.079

6.983

5.992

R2

0.937

0.946

0.987

AT (L.g-1)

1.908

1.381

1.806

bT (kJ. mol-1)

0.415

0.341

0.404

R2 Langmuir

Freundlich

Temkin

-1

Every isotherm constant listed in Table 2 relates to a special phenomena. Low values for the Langmuir constant (KL) indicates the formation of a monolayer on the uniform graphitic layer. In the Freundlich equation, n is greater than unity (here ≈ 6), which represents the favorable adsorption of DBT over adsorbents. Values of bT for the adsorption on prepared adsorbents is 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) is higher than that for rGO and Ni-G (41.8 and 43.3 mg/g, respectively)

47

. Carbon black particles as spacer between graphene sheets seem to 15 ACS Paragon Plus Environment


provide more spaces for DBT adsorption. Graphene sheets provided π- π stacking sites for DBT adsorption. In addition, Ni particles in Ni-G type create new adsorption sites that can grab sulfur atoms by π- π complexation

62

. This observation confirms the expected presence of dual active

sites. So the adsorption capacity is slightly increased by direct interaction between nickel and sulfur atoms. 3.3. Kinetic investigations The pseudo-first order and pseudo-second-order kinetic models were studied to investigate adsorption process on adsorbents. The pseudo-first and pseudo-second-order are given by these two equations, respectively; log(qe − qt ) = log qt − k1 t t qt

=

1 k2 q2e

+

(5)

t

(6)

qe

where t represents time (min), qe and qt are adsorbed DBT quantity at the equivalent point and at time t (mmol. g-1), respectively. k1 and k2 (min-1) are the pseudo-first and pseudo-second-order rate constants. The initial adsorption rate, h can be calculated using following equation 32; h = k 2 q2e

(7)

As the first and second-order kinetic investigation, log (Qe− Qt) and t/qt versus t were plotted for each adsorbent; Table 3 displays correlation factors of these plots and calculated k1, k2 and h for rGO, CB-G and Ni-G. From correlation factors, pseudo-second-order kinetic fits more to rGO and CB-G, indicating the adsorption over uniform graphene surface is controlled by liquid-solid interaction and depends on the DBT concentration near the graphene sites. Furthermore, pseudosecond-order kinetic model proves that an interaction between DBT molecules and n-octane is happening on adsorption sites. Moreover, the pseudo-second-order rate constant determines higher rate of adsorption on rGO (k2 = 0.0627 g.mg -1.min-1). The correlation factor of pseudo-secondorder kinetic diagram for Ni-G is slightly lower than this value for pseudo-first-order plot. So, it can be concluded that DBT adsorption over 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, 16 ACS Paragon Plus Environment

Page 16 of 27

Page 17 of 27

there are two mechanisms of adsorption that were 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 kinetic plots for each adsorbent. Kinetic parameters are provided in table 3. These 2

(a)

rGO

CB-G

Ni-G

log (qe-qt)

1.5 1 0.5 0 -0.5

-1 0

5

10

15

20

25

30

35

40

45

50

t (min) 7

(b)

rGO

CB-G

Ni-G

6

t/qt (min.g/mg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5 4 3 2 1 0 0

10

20

30

40

50

60

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



Page 18 of 27

data were obtained from batch-wised DBT adsorption for 60 min at 25 ᵒC and 100 mg. L-1 of DBT concentration. rGO shows higher initial adsorption rate and rate constant, indicates faster adsorption of DBT, different reports in the literature showed same results

47

. Although the

adsorption capacity of CB-G was higher than rGO, adsorption rate constant of DBT for CB-G is relatively low (4.823 × 10-3 g.mg-1.min-1). It may relate to the structure of this adsorbent, which leads to lower π-π interaction between graphene sheets and DBT molecules and reduces availability of adsorption sites. This probability increases in Ni-G case due to the presence of Ni particles over graphene sheets. However, mesopore structure on CB-G makes higher adsorption capacity (3.1) but it leads to lower adsorption rate. It looks there is a pathway from mesopores to micropores for DBT molecules on CB-G. Although this way can navigate molecules toward adsorption sites, that makes longer way to pass and diminishes the total adsorption rate. Table 3 Kinetic parameters for DBT adsorption on synthesized adsorbents. Pseudo- first-order kinetic

Pseudo-second-order kinetic

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

3.4. Adsorption energy Thermodynamic parameters for adsorption of DBT on rGO were calculated using experimental data obtained at different temperatures. Gibbs free energy is expressed by the following equation; ∆𝐺 = −𝑅𝑇 ln 𝐾

(8)

where R is constant, T represents temperature and K represents to equilibrium constant which depends on the temperature by;

ln 𝐾 =

∆𝑆 𝑇

−

∆𝐻

(9)

𝑅𝑇


Page 19 of 27

∆S and ∆H represent to the entropy and enthalpy of adsorption, respectively. The apparent adsorption equilibrium constant is the ratio of the quantity of adsorbed DBT to the DBT in the solution 32, 63;

𝐾𝑒 =

𝑞𝑒

(10)

𝐶𝑒

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 temperature inverse. The Ln K versus 1/T plots (Figure 8) demonstrate linear behavior for the adsorption on rGO and CB-G adsorbents, indicating the adsorption enthalpy and entropy changes are independent of temperature. Enthalpy and entropy changes of DBT adsorption on the adsorbents were calculated using the slopes and intercepts of these plots. The thermodynamic parameters are shown in Table 4. Negative values for ∆G indicating the adsorption process is feasible and thermodynamically possible at room temperature 63

. However, temperature increasing changes the adsorption to be a less favorable process that

shows an exothermic process. In addition, the Gibbs free energy and equilibrium constant, K values for different temperatures show this process can be performed better at lower temperature. The negative values for ∆H also confirm the adsorption of DBT on rGO and CB-G is exothermic. The negative quantity for ∆S shows the reduced randomness of the system by collecting the DBT 5

rGO

CB-G

4

R² = 0.9715

3

Ln K

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2 1 0

R² = 0.9272

-1 -2 2.95

3.05

3.15

3.25

3.35

3.45

3.55

1/T * 10-3 (K-1) Figure 8. Linear relationship between Ln K and 1/T for DBT adsorption on rGO and CB-G (K: equilibrium constant, and T: temperature).



Page 20 of 27

molecules from solution on the adsorbent surface. All the thermodynamic parameters demonstrate the DBT adsorption is more favorable over rGO rather than on CB-G. The difference between ∆H and ∆S values for every adsorbent express the DBT-graphene interaction can consider to be enthalpy driven, so the change in temperature can affect the adsorption process. Moreover, herein the spontaneous adsorption process (negative ∆G) on the adsorbents is rising from enthalpy change. The |∆H| < 80 kJ.mol-1 confirms physical adsorption. However, the greater ∆H value for adsorption on rGO can represent more unfavorable subsequent desorption possibility. A subsequent desorption is required and important due to regeneration and recycling of adsorbents. From the thermodynamic parameters, it can result the 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 thermodynamic parameters for DBT adsorption on activated carbon was performed by Wen et al.

63

. The reported Gibbs free energy and equilibrium constant were 12.59

kJ.mol-1 and 160.85 at 298 K, respectively. Adsorption process over our synthesized 3D graphene was implemented with a ∆G = -7 kJ.mol-1, which is acceptable in comparison with common adsorbents. Adsorption process on the other adsorbents like alumina composites showed positive enthalpy and entropy changes, though the obtained Gibbs free energy was ultimately negative Table 4. Thermodynamic parameters for DBT adsorption on rGO and CB-G. Adsorbent rGO

CB-G

Adsorption Temperature

K

∆G (kJ.mol-1)

288

72.270

-7.009

298

7.912

-5.125

313

3.735

-3.429

333

0.733

0.858

288

6.269

-4.548

298

1.430

-0.886

313

0.692

0.958

333

0.306

3.280

∆H (kJ.mol-1)

∆S (J.mol-1.K-1)

-69.01

-210.51

-50.42

-162.85


64

.

Page 21 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


From combination of adsorption isotherm (Section 3.2), kinetic (Section 3.3) and thermodynamic (Section 3.4) data, along with structural characterization of synthesized compounds (Section 3.1), it can be inferred graphene provides uniform adsorption sites for catching aromatic sulfurs by π-π stacking (Figure 6a and 6b, inset schematics). 3D sponge-like structure of graphene represents porosities composed of graphitic plates which makes it efficient adsorbent for DBT capturing. The DBT adsorption capacity of adsorbents was around 40 mg.g

-1

which is higher than that amount for well-known adsorbents such as some kinds of modified activated carbon

65

. Further adsorption sites obtained by embedding nickel nanoparticles on

graphene. Ni particles provide π-π complexation possibility of sulfur atoms which leads to DBT adsorption (Figure 6c, inset schematic). π-π stacking mechanism may accomplish faster than π-π complexation, so the adsorption rate on rGO is higher than Ni-G. In addition, the CB particles cover the graphitic π sites and lead to lower adsorption rate. CB-G showed higher adsorption capacity (Figure 6) due to its enhanced mesoporous structure (Figure 5) created by CB particles. The new established meso-size pores on CB-G lead solvent to reach the surface easily and afterward the DBT molecules can meet micropores for adsorption. New mesopores on CB-G may act as navigator channels to conduct DBT molecules toward the adsorption sites, so the adsorption capacity rises. 4. Conclusions Three types of graphenic adsorbents were hydrothermally synthesized and used for DBT adsorption from model fuel. The synthesized adsorbents were characterized using different nanostructure analysis methods. Besides analysis and characterization methods, investigation of macroscopic phenomena of adsorption could conduct this study. CB-G showed higher d-spacing and mesoporous structure may cause by carbon black spacer particles. DBT adsorption investigation over prepared compounds indicated deep-desulfurization to less than 10 ppm of DBT contents. Uniform adsorption over rGO and CB-G was concluded from their isotherm plots conformity with Langmuir isotherm. Whereas, the presence of new adsorption sites and possible stronger adsorption-desorption interaction was confirmed for Ni-G by matching its isotherm to Freundlich and Temkin models. Kinetic investigation showed pseudo-second-order rate for all the adsorbents. Furthermore, a significant correlation factor of pseudo-first-order model for Ni-G 21 ACS Paragon Plus Environment


confirmed the dual adsorption sites with both adsorption mechanisms of π-π stacking and π-π complexation. Although, carbon black particles as spacer in CB-G structure increase the adsorption capacity but leads to lower adsorption rate. The adsorption capacity of CB-G reached to 46.9 mg/g, and adsorption rate of rGO obtained about 24 mg.g-1.min-1. 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 quantity of Gibbs free energy and enthalpy, indicating an exothermic feasible process. The possibility for regeneration is concluded to be easier for CB-G due to the relatively lower value of adsorption enthalpy change (-50.4 kJ. mol-1). We will provide our results around regeneration and reusing of the adsorbents in future reports.

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TOC Graphic

Langmuir Isotherm Pseudo-second-order kinetic model

Freundlich & Temkin Isotherm Pseudo-first & pseudo-second order kinetic models


Model Fuel Deep Desulfurization Using Modified 3D Graphenic

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