Article pubs.acs.org/jced
Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Synergetic Effect of Cobalt-Incorporated Acid-Activated GAC for Adsorptive Desulfurization of DBT under Mild Conditions Prerana Sikarwar, U. K. Arun Kumar, Vijayalakshmi Gosu, and Verraboina Subbaramaiah* Department of Chemical Engineering, Malaviya National Institute of Technology Jaipur, Jaipur 302017, India
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S Supporting Information *
ABSTRACT: The present study explores the usage of acetic-acid-treated cobalt-modified activated carbon as an adsorbent material for the adsorptive desulfurization of DBT-containing model oil. Modification of granular activated carbon (GAC) with acetic acid treatment and incorporation of cobalt species in GAC framework significantly improved its adsorption capacity by improving oxygen-containing functional groups and acidic sites. The surface area of GAC improved from 257 to 393.8 m2/g when it was treated with cobalt species and acid. The average size of the Co3O4 crystallites in the Co-loaded activated carbon was found to be 13.8 nm. The effect of various operating parameters such as cobalt loading, adsorbent dose, temperature, and time was studied. Furthermore, the reusability of spent adsorbent and the effect of the presence of other aromatics compounds were also investigated. Under optimized operating conditions, ∼92% of DBT removal was achieved. The equilibrium data of DBT adsorption were analyzed by Langmuir, Freundlich, Temkin, and Redlich and Peterson (R−P) isotherm models by using nonlinear regression analysis. Among these, the Redlich−Peterson model is the best fit to represent the experimental data. The adsorption of DBT was found to be endothermic in nature. Values of changes in entropy and heat of adsorption were estimated to be 0.177 kJ/(mol K) and 35.67 kJ/mol, respectively. Moreover, a possible adsorption mechanism of DBT was also proposed. Finally, it can be concluded that cobalt-incorporated acetic-acid-activated GAC proves to be a potential adsorbent for the removal DBT from fuel oil in an economic and ecofriendly way.
1. INTRODUCTION Sulfur compounds present in liquid fuels are detrimental; they emit toxic SOx gases during their combustion, which is a serious threat to both the environment and mankind.1 Furthermore, the presence of these compounds also affects the efficiency of catalytic converter present in internal combustion engines of vehicle.2 Consequently, stringent environment rules/regulations have been enforced around the globe to limit the amount of sulfur in fuel. The United States Environmental Protection Agency and European Union have reduced the sulfur content to 15 and 10 ppm since 2006 and 2003, respectively.3 Immersive pressure is mounting on the petroleum refinery to achieve cleaner fuels. Currently, catalytic hydrodesulfurization (HDS) is widely used technology to reduce the sulfur content in transportation fuels. The HDS process demands severe operating conditions such as high temperature, high pressure, high hydrogen consumption, and a large amount of catalyst, which makes the overall process very expensive. In addition, the HDS process proves to be potential for the removal of thiols, sulfides, and disulfide, but it is ineffective for the removal of refractory sulfur compounds such as thiophene, benzothiophene, dibenzothiophene (DBT), and its alkylated derivatives.4 To obtain ultraclean fuels, several alternative techniques have been developed, including adsorptive desulfurization (ADS), oxidative desulfurization, biodesulfurization, and extractive desulfurization, by researchers in recent years.5−8 Among all of © XXXX American Chemical Society
these techniques, ADS is often considered as a potential alternative technique with several advantages, such as process operation under ambient conditions, it does not require hydrogen, and selective removal of refractory sulfur compounds. However, the major challenge associated with ADS is to develop adsorbent that can specifically adsorb sulfur compounds from the fuel, is easily available, and possessed significant adsorption capacity. Various adsorbents have been tested in the ADS process, including mesoporous silica, ion-exchanged zeolites, alumina, metal−organic frameworks, activated carbon, and so on.9−12 Among these, activated carbon-based adsorbents have gained much attention owing to their high specific area and their controllable functional groups on their surface. The adsorption of sulfur compounds by activated carbon is governed by its porous nature and the functional group present on the activated carbon surface.13 However, mere activated carbon performance is not that satisfactory in ADS process due to limiting adsorption capacity, which may hinder the industrial application.14 Therefore, modification of activated carbon was carried out with active species, which plays a crucial role for selective adsorption of sulfur compounds from liquid fuel. Shah15 investigated tin (Sn)-modified activated charcoal for the Received: March 28, 2018 Accepted: June 13, 2018
A
DOI: 10.1021/acs.jced.8b00249 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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2.3. Preparation of Model Oil. Model oil was prepared by dissolving an appropriate amount of DBT in iso-octane solvent. The stock solution of 5000 mg/L was prepared by dissolving 5 g of DBT in 1 L of of iso-octane. Different concentrations of DBT ranging from 250 to 1000 mg/L were prepared from stock solution by appropriate dilution with iso-octane. The concentration of DBT in model oil was analyzed by using a UV−vis double-beam spectrophotometer (UV-1800, Shimadzu, Japan). Maximum absorbance was observed at a wavelength of 320 nm, similar to the previously reported literature.15,18 2.4. Adsorption Experiments. To understand the adsorption capacity of the synthesized adsorbent (Co/ ATGAC), batch ADS was performed with DBT-containing model oil. To perform desulfurization of DBT, a known amount of DBT solution was taken in a conical flask with a required amount of Co/ATGAC and stirred (140 rpm) in a constant temperature chamber (293−323 K) for all experimental runs. After the adsorptive desulfurization process, model oil was filtered with the help of a polytetrafluoroethylene syringe filter (hydrophobic, 0.5 μm). The concentration of DBT was analyzed with a UV spectrophotometer with the aid of the absorbance at 320 nm. The percentage removals of DBT and equilibrium adsorption capacity of the adsorbent were obtained using the following relationships (eqs 1 and 2). The percentage removal of DBT was calculated using following formula
removal of DBT from model oil and demonstrated that modified activated carbon shows better performance than virgin-activated carbon. It was proposed that enhanced adsorption of DBT is due to the acid−base interaction between Sn2+ (soft acid) and DBT (soft base). Manganese- and lanthanum-modified activated carbon were also found to exhibit more adsorption capacity as compared with mere activated carbon, and increase in adsorption capacity was attributed to the π complexation between active metal species and DBT.13,16 Furthermore, virgin activated carbon was activated by either physical or chemical process to increase the pore volume and surface area of activated carbon, which leads to the high adsorption capacity. Notably, chemically activated carbon contains defined micropore size distribution and high micropore volume.17 Inspired by these encouraging results, we decided to explore the potential of chemically modified activated carbon (acidic treatment along with cobalt species incorporation) for the ADS of model oil. Cobalt-loaded activated carbon has been used for several applications that are listed in Table S1 because cobalt proves to be an active metal when supported on GAC. The interaction mechanism between refractory sulfur compounds and cobaltbased GAC has not yet been reported to the best of our knowledge. The objective of the present study is to improve the adsorptive desulfurization performance of cobalt-based sorbents by immobilizing cobalt species on chemically activated GAC (ATGAC) and to investigate the interaction of refractory sulfur compounds with Co/ATGAC. Furthermore, kinetics and thermodynamic properties were investigated to find the feasibility of the adsorption process. In addition, reusability tests were also conducted using solvent regeneration technique.
DBT removal % =
(Co − Ce) *100 Co
(1)
The amount of DBT adsorbed per gram of solid adsorbent q=
2. EXPERIMENTAL SECTION 2.1. Materials. All reagents used in this study were of analytic grade and procured from various firms. Granular activated charcoal (GAC) and DBT were purchased from Loba Chemie, Mumbai, India and Himedia, Mumbai, India, respectively. Iso-octane and cobalt nitrate hexahydrate were procured from Fisher Mumbai, India, and Merck, Mumbai, India. Glacial acetic acid was obtained from Rankem, Mumbai, India. 2.2. Preparation of Adsorbent. GAC pores were activated by physical method, followed by a chemical method, as mentioned below. Initially, GAC pores were activated by a physical method where GAC was soaked with hot deionized (DI) water (383 K) for 1 h to remove the impurities present in the pores of GAC. The washed GAC was dried in an oven at 383 K to remove the moisture present inside the GAC. In the second step, GAC was activated through a chemical process, and the dried GAC was pretreated at 323 K with acetic acid for 3 h. Chemically activated GAC was filtered and washed with plenty of DI water until the pH of the water became neutral. Washed GAC was kept in an oven for drying at 383 K for 10 h. The prepared sample was designated as ATGAC (acid-treated granular-activated carbon) Different weight ratios (1, 2, 3, and 4%) of cobalt active metal were loaded on ATGAC by using an incipient impregnation process where cobalt nitrate hexahydrate was used as a precursor. After the impregnation of active metal on ATGAC, the samples were kept for drying in an oven at 383 K for complete drying and then followed by calcination at 573 K for 3 h. The resulting adsorbent was designated as Co/ATGAC.
C0 − Ce *V m
(2)
Here Co (mg/L) and Ce (mg/L) represent the initial and equilibrium concentrations of DBT in the model oil, respectively, m (g) is the amount of adsorbent, and V (mL) is the volume of model oil. 2.5. Characterization of Adsorbents. The Fourier transform infrared (FTIR) spectra of blank and modified GACs were recorded on an FTIR spectrophotometer (FT-IR Spectrum 2, PerkinElmer) in the frequency range of 400−4000 cm−1. Prior to the analysis, samples were combined with KBr powder in a specific weight ratio to obtain a translucent circular shape. To explore the texture and morphology of the adsorbents, scanning electron microscope (SEM) analysis was carried out using Nova Nano FE-SEM 450 (FEI). Initially, the samples were coated with gold to provide conductivity to the sample using a spin coater (spin coating system, APEX) and further preceded. Energy-dispersive X-ray spectroscopy was employed to examine the distribution of active species on support material. Tecnai G2 20 S-Twin, FEI Netherlands was used to obtain high-resolution transmission electron microscopy (HR-TEM) images. X-ray diffraction (XRD) data of the blank and cobalt impregnated ATGAC were collected on a Panalytical X Pert Pro apparatus. The XRD was operated at 40 mA, 40 kV, Cu Kα radiation (k = 1.54060 Å) as a target over a 2θ range of 10−90° with a 0.02° step size and 4 s step time. The physical characteristics of the adsorbents were studied by using ASAP 2020 (Micromeritics). Prior to the analysis, samples were degassed at 473 K for 12 h to eliminate contaminations. The BET method was adopted for the calculation of surface area, whereas the BJH method was used for the determination of average pore diameter and pore volume. B
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The XPS analysis was carried out using an XPS spectrometer (Scienta Omicron) with a monochromatic Al Kα source.
3. RESULTS AND DISCUSSION 3.1. Adsorbent Characterization. FTIR spectra of mere GAC, ATGAC, and 1 wt % Co/ATGAC were performed to confirm the surface functionalities of the sample. Figure 1
Figure 2. X-ray diffraction of blank GAC, 1 wt % Co/ATGAC, and spent 1 wt % Co/ATGAC.
mapping of the synthesized adsorbent (1 wt % Co/ATGAC) is also performed to understand the dispersion of cobalt on Co/ ATGAC. The spatial distributions of each component of adsorbent, that is, carbon, oxygen, and cobalt, are shown in Figure 4. TEM image of 1 wt % Co/ATGC is presented in Figure 5a,b. Figure 5a illustrates the dark-colored particle representing the cobalt oxide species, whereas the light-colored matrix represents the support material (activated carbon).30 It can be seen from the Figure 5b that Co/ATGAC exhibits the orderliness and structural regularity. In addition, it reveals the fine dispersion of cobalt species into the pore channels of GAC. The average particle size was found to be ∼18 nm. The textural properties of mere GAC and 1 wt % Co/ATGAC are presented in Table 1. The surface area of Co/ATGAC increased significantly as compared with mere GAC. This increase in surface area can be attributed to the acidic treatment of GAC. Because of acid treatment, the inert materials present in the pores of GAC were removed and increased the pore volume.31,32 Nitrogen adsorption−desorption isotherms of GAC and 1 wt % Co/ATGAC are depicted in Figure 6; it exhibits the type-I isotherms, which confirm the microporous nature of adsorbent. Results concluded that modified GAC retained its original structure even after acid treatment and metal incorporation. 3.2. Effect of Various Adsorbents. To investigate the performance of modified activated carbon for the ADS of DBT, few preliminary experiments were conducted. Figure 8 shows that adsorption capacity of ATGAC exhibits more adsorption capacity (51% DBT removal) as compared with the mere GAC (40% DBT removal). The observed improvement on ATGAC was due to the activation of pores and the incorporation of oxygen-containing functional groups, which lead to the high sorption capacity.14 Previous literature confirmed that the oxygen-containing functional groups present on the activated carbon surface can remarkably increase the adsorption capacity.19,31,33 Figure 8 depicted the adsorption performance of different weight percentage of active metal on ATGAC (1 wt % Co/ATGAC, 2 wt % Co/ATGAC, 3 wt % Co/ATGAC, and 4 wt % Co/ATGAC). The improvement of DBT adsorption onto Co/ATGAC as compared with mere GAC and ATGAC may be due to the formation of the π complex between DBT and cobalt species. The maximum adsorption of DBT was observed for 1 wt % Co/ATGAC; DBT removal was decreased with the increase
Figure 1. FTIR spectra of GAC, ATGAC, 1 wt % Co/ATGAC, and spent 1 wt % Co/ATGAC.
depicted the FTIR spectrum of samples; a broad absorption band centered at 3435 cm−1 is assigned to O−H stretching vibrations of hydroxyl groups due to the presence of water molecules inside the pores of GAC framework.19 The bands at 2921 and 2845 cm−1 might be due to the stretching vibrations of C−H. The peaks at 1737 and 1628 cm−1 were associated with the carboxyl group and carbonyl group.20 The absorption band at 1035 cm−1 was ascribed to the C−O stretching vibration.21 Although these bands were observed in mere GAC, ATGAC, and Co/ATGAC, in acid-treated GAC, the increase in intensity of oxygen-containing bands (1722, 1630, and 1088 cm−1) may be due to the increase in oxygen-containing sites when treated with acetic acid. A similar trend was observed by Pradhan and Sandle22 for the oxidation of GAC with nitric acid. FTIR spectra of 1 wt % Co/ATGAC exhibited two additional peaks at 568 and 665 cm−1 as compared with GAC and ATGAC. These bands represent the Co−O bond in spinel-phase Co3O4, with the former peak aroused owing to the Co3+−O stretching vibration, and the latter peak contributed to the Co2+−O vibrations.23,24 XRD patterns of mere GAC and 1 wt % Co/ATGAC are depicted in Figure 2. A broad hump was observed at 24 and 44°, respectively, and these were attributed to the amorphous carbon originating from the reflection of (002) and (010) planes, respectively.25 The diffraction peak at 36.75° originating from (311) plane corresponds to Co3O4.26,27 The Scherrer equation was used to calculate the average Co3O4 particle size of the adsorbent with the help of the peak obtained in the XRD spectrum at 36.75°.28 The average size of the Co3O4 crystallites in the Co-loaded activated carbon was found to be 13.8 nm. The SEM micrographs of mere GAC and 1 wt % Co/ATGAC are presented in Figure 3a,b, respectively. As depicted in Figure 3a, GAC shows porous nature and possesses rough surface. Cobalt-loaded GAC exhibits lesser porosity and more smooth surface as compared with blank GAC. However, agglomeration has not appeared on the surface of GAC, which signifies that metal was dispersed uniformly on the GAC matrix.29 Elemental C
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Figure 3. SEM micrograph of (a) blank GAC, (b) 1 wt % Co/ATGAC, and (c) spent 1 wt % Co/ATGAC.
in cobalt loading. The observed results suggested that higher cobalt loading resulted in the decline of surface adsorption sites due to the multilayer deposition of active metal species and agglomeration of cobalt particles on the GAC channel. Subhan and Liu34 observed a similar trend for ADS of DBT using nickelsupported MCM-41. The obtained results indicate that the introduction of cobalt and acid treatment of GAC is the key factor that contributes to increase in acidic sites and cobalt interacting directly with the sulfur compounds. Therefore, 1 wt % Co/ATGAC was taken as the potential adsorbent for further evaluation of ADS process. 3.3. Effect of Adsorbent Dose. To understand the effect of adsorbent dose, various quantities of 1 wt % Co/ATGAC were utilized, that is, 5, 10, 15, 20, 25, and 30 g/L at 500 mg/L of DBT concentration. The effect of adsorbent quantity on desulfurization of DBT is shown in Figure 9. With an increase in 1 wt % Co/ ATGAC dose the percentage removal of DBT increased from 45 to 96%. This observation may be attributed to the availability of large vacant sites and hence more π complexation sites for the sulfur species adsorption.35 At a lower dose (5 g/L) of Co/ ATGAC, the active/vacant sites were saturated with the DBT molecules, and no further enhancement in removal was observed; from 5 to 20 g/L, the DBT removal was significant. However, at higher dose (above 20 g/L) only marginal increase in percentage removal (∼4% increases from 20 to 30 g/L) was
observed. It may be ascribed to the insufficient number of DBT molecules at a higher dose, and it was conducted at a fixed concentration of DBT molecules (500 mg/L). Finally, 20 g/L of 1 wt % Co/ATGAC adsorbent dose was selected as an optimum dose to perform further experiments. 3.4. Adsorption Equilibrium Study. The equilibrium adsorption of DBT with various concentrations ranging from 250 to 1000 mg/L at different temperature (283, 293, 303, and 313 K) was investigated using 1 wt % Co/ATGAC. Figure 10 depicts the adsorption capacity of 1 wt % Co/ATGAC increasing abruptly with an increase in operating temperature and adsorbate concentration. The rise in adsorption capacity with an increase in temperature may be due to the faster diffusion of DBT species into the pores of the catalyst. In addition, at higher temperature, less retarding forces are imposing on the molecules, which leads to the high adsorption.36 To better understand the interactive behavior between adsorbate and adsorbent, four different types of adsorption isotherms were studied, namely, Langmuir, Freundlich, Temkin, and Redlich and Peterson (R−P). In general, equilibrium expressions of Langmuir (eq 3), Freundlich (eq 4), Temkin (eq 5), and R−P isotherms (eqs 6) are expressed below. qe = D
qmCeKT (1 + KLCe)
(3) DOI: 10.1021/acs.jced.8b00249 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Figure 4. Spatial distribution of adsorbent’s component in mapping elemental analysis of 1 wt % Co/ATGAC.
Figure 5. HR-TEM images for 1 wt % Co/ATGAC. Scale bars (a) 50 and (b) 2 nm.
qe = Br ln (KL) + BT ln(Ce)
Table 1. Surface Characteristics of Raw GAC and CobaltLoaded GAC sample
surface area (m2/g)
pore volume (cm3/g)
pore diameter (Å)
GAC 1 wt % Co/ATGAC
257.403 393.868
0.1163 0.2030
24.380 28.353
qe = (KFCe1/ n)
pore width (Å) 6.747 7.248
qe =
(5)
KR Ce (1 + αR Ceβ)
(6)
where Ce (mg/L) and qe (mg/g) represent the equilibrium concentration and adsorption capacity, respectively. qm and KL are Langmuir equilibrium constant indicating the theoretical maximum adsorption capacity and the affinity of the adsorbate molecules to bind with adsorbent, respectively. Values obtained for qm and KL at various temperatures are depicted in Table S2.
(4)
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expressions for pseudo-first-order and pseudo-second-order models are given in eqs 7 and 8, respectively. qt = qe(1 − e−k1t ) qt =
(7)
k 2tqe2 (1 + k 2qet )
(8)
where qt (mg/g) represents the amount of DBT adsorbed per gram of adsorbent at time t (min) and k1(min−1) and k2 (g·mg−1· min−1) are the rate constant of pseudo-first-order and pseudosecond-order reactions, respectively. These two adsorption kinetic models have been utilized to fit the kinetics of adsorption of DBT onto 1 wt % Co/ATGAC by employing nonlinear regression. SSE was utilized to find out the most appropriate kinetic model to fit the experimental data. The SSE is represented by following mathematical equation.
Figure 6. Nitrogen adsorption−desorption isotherm plots for the adsorbents.
n
SSE =
The qm value rose with an increase in temperature, which confirms the endothermic nature of overall sorption process.37 Moreover, an increase in maximum adsorption capacity may also be attributed to the chemisorption.38 KF and 1/n are the Freundlich constants indicating adsorption capacity and intensity, respectively. As shown in Table S2, values of KF increase with the increase in temperature, similar to the trend obtained for values of qm.. Furthermore, the values of 1/n < 1 for different temperatures indicate that adsorption capacity is diminished marginally for lower Ce. This isotherm represents that cobalt-loaded ATGAC exhibits highly heterogeneous surface for the adsorption of DBT. BT and KT are the heat of sorption and Temkin isotherm constant, respectively. The R−P isotherm model finds its applicability in homogeneous as well as heterogeneous systems. Because this isotherm contains elements from Langmuir as well as Freundlich isotherm, the adsorption of DBT onto 1 wt % Co/ATGAC is a mixture of both and does not follow monolayer adsorption.39 When the β value approaches 1, the R−P isotherm represents the Langmuir equation. The adsorption of DBT molecules seems promising because the value of β lies between 0 and 1. The Langmuir, Freundlich, Temkin, and R−P isotherm parameters along with correlation coefficient (R2) and sum of square of error (SSE) are utilized to estimate the goodness of fit for the selection of optimum isotherms onto 1 wt % Co/ATGAC at various temperatures, as shown in Table S2. Recently, nonlinear regression analysis received more attention to minimize the quadratic error among the experimental data and model outputs. It can be clearly seen from the Table S2 that the R−P isotherm obtained the highest value of R2 and the lowest value for SSE. Hence, the R−P model is the best fit for the equilibrium data obtained at the different temperatures. Figure 10 indicates the fit of the R−P isotherm model for the obtained experimental results. 3.5. Adsorption Kinetic Study. To gain the better understanding about the reaction pathway and adsorption mechanism of DBT molecules onto 1 wt % Co/ATGAC adsorption, pseudo-first-order and pseudo-second-order kinetic models are employed to analyze the experimental data in the studied adsorption systems. The effect of contact time on the removal of the DBT from model fuel oil by 1 wt % Co/ATGAC is depicted in Figure 11. In the first 60 min, rapid adsorption was observed; thereafter, the adsorption rate slowly increased. After 3 h, the adsorption approaches equilibrium. The mathematical
∑ (qe,exp − qe,cal)i2 i=1
(9)
The results obtained from fitted curves are displayed in Figure 8, and the calculated parameters are presented in Table S3. As can be seen from Table S3, the highest value of regression coefficient (R2) and the lowest value of SSE were obtained for the pseudosecond-order model as compared with the pseudo-first-order model. Therefore, the kinetics of DBT adsorption is best expressed by the pseudo-second-order model. The obtained results from the kinetic study indicated that chemical reaction is the rate-controlling step.40 Shi14 observed a similar trend for adsorptive desulfurization of DBT using activated carbons derived from hydrothermally carbonized sucrose. 3.6. Adsorption Thermodynamic Parameter. The study of thermodynamic parameters includes assessment of Gibbs free-energy change, enthalpy change, and entropy change to gain comprehensive knowledge about the adsorption of DBT onto 1 wt % Co/ATGAC process, such as spontaneity of the process and structural characteristics of the adsorbent after adsorption of DBT. Change in Gibbs free energy (ΔG°) in an ADS process is determined based on eq 10. ΔGo = −RT ln KD
(10)
where R is the universal gas constant, T represents the adsorption temperature in Kelvin, and KD indicates thermodynamic equilibrium constant and can be expressed as qe/Ce. The values of ΔG° are calculated at different temperature listed in Table S4. The negative value of ΔG° indicates the spontaneous adsorption at the studied temperatures on 1 wt % Co/ATGAC surface and the strong affinity of 1 wt % Co/ATGAC toward the DBT species. Values of ΔG° are decreasing with an increase in temperature, which clearly shows that the adsorption of DBT is more spontaneous at higher temperature. The van’t Hoff equation was employed to calculate entropy change (ΔS°) and enthalpy change (ΔH°). ln K =
ΔS° ΔH ° − R RT
(11)
The linear plot of ΔG° vs T was drawn by using eqs 10 and 11 (as shown in Figure 12) to find out the values of ΔS° (kJ/mol K) and ΔH° (kJ/mol). The calculated thermodynamic parameters are summarized in Table S4. The positive value of enthalpy change indicates that the adsorption of DBT is endothermic in F
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Figure 7. XPS deconvoluted (a) C 1s spectra of 1 wt % Co/ATGAC, (b) C 1s spectra of spent 1 wt % Co/ATGAC, (c) O 1s spectra of 1 wt % Co/ ATGAC, and (d) O 1s spectra of spent 1 wt % Co/ATGAC.
contribute to the adsorption of DBT.43 The XRD patterns of fresh and spent adsorbent are shown in Figure 2. The amorphous nature of adsorbent remains unchanged after adsorption of DBT.37 No additional peak was observed in the XRD of spent adsorbent, which indicates that the adsorption phenomenon involves the electrostatic interaction between the adsorbent and adsorbate.44 SEM images of fresh and spent adsorbents are depicted in Figure 3a,c. It can be clearly indicated from the images that DBT loaded onto the fissures and channels present on the surface of raw adsorbent signifies the successful adsorption of DBT onto 1 wt % Co/ATGAC. The deconvoluted XPS C 1s and O 1s spectra before and after adsorption are shown in Figure 7. The C 1s spectra of fresh and spent adsorbent are deconvoluted into four characteristic peaks, namely, graphitic carbon (C−C, 284.6 eV), carbon present in phenolic, alcohol, and ether aromatic (C−O, 286 eV), carbonyl and quinone
nature. The change of enthalpy due to physisorption is usually 2.8, and Co2+ belongs to borderline acids.15 Therefore, the loading of Co2+ ions on activated carbon could weaken the local hard acids present on the surface and lead to the increase in adsorption of DBT onto 1 wt % Co/ATGAC to a certain extent.51 Overall, the π complexation seemed to be the main mechanism that governed the adsorption of DBT onto Co/ATGAC.
model fuel with aromatic molecules (benzene or toluene) under optimized reaction conditions. It was found that the addition of benzene (10 and 20 vol %) or toluene (10 and 20 vol %) decreases the adsorption of DBT onto adsorbent, as shown in Figure 13. This decrease in adsorption capacity could be due to
4. CONCLUSIONS In the current study, cobalt-incorporated ATGAC was synthesized by incipient wet impregnation technique and tested for adsorption of DBT from model oil. Impregnation of cobalt on acetic-acid-treated GAC significantly enhances the removal of DBT in comparison with raw GAC and acetic-acid-treated GAC. The kinetic data were found to best fit in the pseudosecond-order model as compared with pseudo-first-order model and clearly indicate that chemical reaction along with transfer and sharing of electron species plays a major role in the ratedetermining step. The thermodynamic study revealed that the adsorption of DBT onto 1 wt % Co/ATGAC was spontaneous and endothermic in nature; positive value of ΔS° represents the increase in disorder of DBT adsorption. Adsorption equilibrium data were well represented by the R−P isotherm under tested temperatures. On the basis of the static adsorption experiments, the adsorption process was most likely to be governed by the π complexation mechanism. In comparison with other commercially available adsorbent, Co-modified GAC is economical and easy to regenerate. Overall, the present adsorbent is efficient enough to remove most refractory sulfur compound from model oil; therefore, it can be used as a promising adsorbent for ADS of liquid fuels.
Figure 13. Effect of aromatic compounds on DBT adsorption (volume of model oil, 10 mL; temperature, 303 K; adsorbent dose, 20 g/L; adsorption time, 4 h; benzene, 10 and 20 vol %; toluene, 10 and 20 vol %).
the competition between aromatic molecules and DBT molecules to get adsorbed on active sites. With the addition of toluene (10 and 20 vol %) in the model oil, the adsorption performances were decreased, with the DBT removal of 80 and 75%, respectively. Similarly, by adding benzene (10 and 20 vol %) to the model oil, DBT removal reduced to 77 and 72%, respectively. The obtained results match well with the previously reported literature.49 1 wt % Co/ATGAC showed approximately the same affinity for the adsorption of both aromatics because they both might get adsorbed through the π complexation mechanism. The same preference may be given for the adsorption of both toluene and benzene because they both contained the same number of π electrons. Moreover, DBT contained a greater number of π electrons as compared with toluene and benzene; therefore, higher DBT removal therefore, higher DBT removal was observed as compared with benzene and toluene. Weaker bonding of aromatics with the adsorbent as compared with DBT molecules confirms the selectivity of Co/ ATGAC. Regeneration of spent adsorbent was performed using the solvent extraction technique. To regenerate, 1 g of spent adsorbent was mixed with 10 mL of toluene and stirred at room temperature for 15 min. Subsequently, adsorbents were filtered and dried overnight at 90 °C. Then, the regenerated adsorbent was reutilized for the ADS process. After the first cycle, the adsorption capacity was decreased from 23 (mg/g) to 21.67 (mg/g), and in the next cycle, it reduced to 21.14 (mg/g). 3.9. Adsorption Mechanism. The adsorption of DBT onto 1 wt % Co/ATGAC can be governed by following mechanisms such as π complexation, acid−base interaction, and other weak interactions such as van der Waals interactions. The generation of the π complex takes place through dual bonding of Co2+ atoms to π electrons of DBT. In the π complexation, both Co2+ species and DBT act as electron acceptor and donor. Molecular orbitals of DBT overlap with the vacant s orbital of Co to form the σ component of the bond. Simultaneously, the d orbitals of Co2+ back-donate electrons from it to the antibonding orbitals of
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b00249. Applications of cobalt-oxide-supported GAC, equilibrium parameters for the adsorption of DBT on 1 wt % Co/ ATGAC, kinetic parameters for the adsorption of DBT onto 1 wt % Co/ATGAC, and thermodynamics parameters for the adsorption of DBT onto 1 wt % Co/ ATGAC. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel: +91 1412713317. ORCID
Verraboina Subbaramaiah: 0000-0003-4457-6725 Funding
P.S. acknowledges the financial support of MHRD, Govt of India to carry out this research. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Sikarwar, P.; Kumar, U. A.; Gosu, V.; Subbaramaiah, V. Catalytic Oxidative Desulfurization of DBT Using Green Catalyst (Mo/MCM-
I
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DOI: 10.1021/acs.jced.8b00249 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
