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Article Cite This: Energy Fuels 2017, 31, 11731-11742

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Removal of Sulfur and Nitrogen Compounds from Diesel Oil by Adsorption Using Clays as Adsorbents Luana V. Baia,† Wallace C. Souza,† Ricardo J. F. de Souza,‡ Cláudia O. Veloso,*,† Sandra S. X. Chiaro,‡ and Marco Antonio G. Figueiredo† †

Institute of Chemistry, Rio de Janeiro State University, Campus Maracanã, PHLC, Rua São Francisco Xavier, 524, Rio de Janeiro RJ, 20550-900, Brazil ‡ PETROBRAS-CENPES-Research and Development Center, Av. Horácio Macedo, 950, Rio de Janeiro RJ, 21941-915, Brazil S Supporting Information *

ABSTRACT: Stringent specifications for sulfur compounds content and the need to use oils with large amount of nitrogen compounds are challenges for fuel producers. Adsorption is an alternative process to remove sulfur and nitrogen compounds and clays are promising adsorbents for this removal. In this work, the adsorption performance of different commercial claysClay A (attapulgite), Clay B (bentonite), and Clay C (bentonite)for the removal of sulfur and nitrogen compounds from a real diesel stream was studied through kinetic and isothermal experiments. The bentonite clays showed the best adsorptive capacity for the removal of sulfur and nitrogen compounds, probably due to the presence of Brønsted acid sites. The highest adsorption capacity was observed for Clay B: 0.174 mol kg−1 for sulfur compounds and 0.127 mol kg−1 for nitrogen compounds. Clay A was more selective to the removal of nitrogen compounds. Equilibrium data showed that adsorbate−absorbate and adsorbate−surface interactions predominate for sulfur and nitrogen compounds, respectively, for Clay A and Clay B.

1. INTRODUCTION The producers of fuels for transport are now facing challenges such as more stringent specifications for sulfur compounds content to reduce the emission of sulfur oxides and, therefore, atmospheric pollution and the need to use heavier oils with large amount of nitrogen compounds.1 The removal of sulfur and nitrogen compounds from refinery streams is performed via hydrotreatment (HDT) processes that include hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) processes. The sulfur compounds are converted to H2S and hydrocarbons in the presence of a heterogeneous catalyst at high temperature and pressure, and under hydrogen atmosphere. The presence of refractory sulfur compounds and nitrogen compounds present in heavy crude oils hampers the HDS process. Nitrogen compounds are much less reactive than sulfur compounds and strongly adsorb on the active sites of the catalyst, hindering the adsorption of sulfur compounds.2−4 Adsorption is an alternative process to remove sulfur and nitrogen compounds in order to minimize the abovementioned HDT technical difficulties. An association of adsorption and HDT units can be a possible solution for the production of cleaner fuels under less-severe conditions and reducing process cost.5 Several adsorbents have been used for desulfurization and denitrogenation of liquid fuels, such as modified oxides,6,7 activated carbon,8,9 mesoporous,10−12 and microporous13,14 molecular sieves, and clays.15−22 The latter group of adsorbents is distinguished by their absorptive, adsorptive, and catalytic capacities,15 and it is greatly used in the oil industry for various separation and adsorption processes.18 Clays are composed by minerals that are hydrous silicates of aluminum and/or magnesium. Iron, nickel, chromium, and other cations can be also present in the crystalline structure as © 2017 American Chemical Society

an isomorphic substitution. The physicochemical properties of clays are dependent on its structure and composition. These properties also influence the adsorption and/or catalytic behavior of clays. The most used clays are bentonite and attapulgite. Bentonite is a clay mainly comprised of constituents from the montmorillonite group (Na-montmorillonite or Camontmorillonite), that is, aluminosilicate clay minerals designated as a 2:1 layer mineral composed of two silica tetrahedral sheets with a central aluminum octahedral sheet. Palygorskite and attapulgite are synonymous terms for the same hydrated magnesium aluminosilicate material, the chemical composition of which is (MgAl)5Si8O20(OH)2(H2O)4·4H2O. Similar to other clay minerals, attapulgite has tetrahedral (silica) and octahedral (Alumina) sheets as its basic building block. These units are connected to each other by shared O atoms.23 There are just a few papers about adsorption of sulfur and/or nitrogen compounds from model and real oil fractions using clays as an adsorbent. Mambrini et al. used hydrophobic bentonite clay for treating dibenzothiophene and quinoline solutions. Adsorption capacities of 38.7 mg g−1 and 54.5 mg g−1 were obtained for sulfur and nitrogen compounds, respectively.15 In another study, Zn-containing montmorillonite clay was used as adsorbents achieving 76% (kerosene) and 77% (diesel) of desulfurization. The authors suggested that these results were due to the increase in area, volume, and diameter of pores caused by the presence of Zn.18 Montmorillonite, kaolinite, vermiculite, and palygorskite were employed for the removal of sulfur compounds from petroleum fractions.16 The kaolinite showed the highest removal level, reaching desulfurReceived: July 5, 2017 Revised: October 5, 2017 Published: October 5, 2017 11731

DOI: 10.1021/acs.energyfuels.7b01928 Energy Fuels 2017, 31, 11731−11742

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Kinetic tests were performed in a Dubnoff reciprocal shaking bath, using a shaking frequency of 150 rpm at 40 °C. Seven flasks containing 10 mL of diesel and 2 g of adsorbent were used and samples were taken at 10, 30, 60, 120, 180, 240, and 300 min. All adsorbents were pretreated at 150 °C for 1 h to remove adsorbed compounds. 2.3. Isotherm Experiments. Adsorption experiments were performed at 40 and 70 °C, using a Dubnoff reciprocal shaking bath. Ten milliliters (10 mL) of diesel and different amounts of adsorbent (6.0, 4.0, 2.0, 1.0, 0.5, 0.33, and 0.2 g) were used for each equilibrium point after 480 min. All adsorbents were pretreated at 150 °C for 1 h before the experiments. The concentration of total sulfur and nitrogen compounds in the liquid phase was analyzed using an Elemental Analyzer 9000 system (Antek). 2.4. Experimental Data Treatment. First-order, pseudo-firstorder, and pseudo-second-order models were used to adjust adsorption kinetic data. An intraparticle diffusion model was also tested to study the adsorbate transport from solution to adsorbent surface. The first model is composed of two first-order differential equations that are usually used for a reaction kinetic model, where adsorbate sorption from the liquid phase to the solid phase can be considered as a reversible reaction with an equilibrium being established between two phases.26,27 This model neglects diffusion parameter and is composed of two first-order differential equations. The Euler−Cauchy numerical solution with a spreadsheet was used to solve the equations.28 The differential equations are shown in eqs 1 and 2.

ization of 60%, 76%, and 64% in crude oil, kerosene, and diesel samples, respectively. Ha evaluated the removal of sulfur compounds (2,4-dimethyldibenzothiophene and dibenzothiophene) from model fuels by modified clays.24 The most attractive results are those obtained by clays modified with benzyltrimethylammonium salts, because of its similarity with pollutants (aromatic compounds). The maximum adsorption capacity for 2,4-dimethyldibenzothiophene was 11.3 mg g−1 in model gasoline and 31.3 mg g−1 in model diesel. It is important to note that no information about the adsorption of nitrogen compounds using clays was found. Therefore, aiming at understanding the removal process of sulfur and nitrogen compounds using three commercial clays Clay A (attapulgite), Clay B (bentonite), and Clay C (bentonite)as adsorbents, an adsorption study through kinetic and isotherm experiments was carried out. For comparative purposes, an activated Alumina was also used, because of their promising properties as an adsorbent of various substances.25

2. EXPERIMENTAL SECTION 2.1. Adsorbent Characterization. Four commercial adsorbents were used, that is, an amorphous Alumina, one attapulgite clay, and two bentonite clays, named Alumina, Clay A, Clay B, and Clay C, respectively. The elemental composition of the adsorbents was determined by Xray fluorescence (XRF), using a Philips PW2400 WXRF spectrometer with a rhodium X-ray tube. The SuperQ/Quantitative PW2450 software, version 2.1d (Philips), was used to analyze the results. The crystalline phases of the adsorbents were identified by X-ray diffraction (XRD) using a Philips PW1710 spectrometer equipped with a copper anode and single-crystal graphite. The diffractometer operated at 40 kV and 20 mA using Cu Kα radiation over the range of 5−70° with a scan rate of 4° min−1. The samples were dried at 120 °C for 12 h before use. The textural analysis of the studied adsorbents involves the determination of specific area (Brunauer−Emmett−Teller (BET) method), microporous volume (t-plot method), mesoporous volume (Barret-Joyner-Halenda method), and pore diameter (Barrett−Joyner−Halenda (BJH) method). Textural properties were determined by N2 adsorption−desorption at −196 °C, using a Micromeretics ASAP 2400 system. Samples were degassed under vacuum (50 mTorr) at 300 °C for 1 h. The density and strength distribution of acid sites were determined by temperature-programmed desorption (TPD) of ammonia (2.91 mol % NH3 in He) using an AutoChem 2900 instrument (Micromeritics). The adsorbents were pretreated at 500 °C for 1 h using a 10 °C min−1 heating rate under a helium flow (30 mL min−1). The sample then was cooled to 30 °C and saturated with ammonia. The temperature was then increased to 100 °C for 2 h. Afterward, the sample was heated to 500 °C at 10 °C min−1. Desorption products were monitored by an OmniSTAR mass spectrometer (Pfeiffer). The nature of acid sites was determined by pyridine adsorption on adsorbents using Fourier transform infrared (FTIR) spectroscopy. The samples were dried at 500 °C using a heating rate of 5 °C min−1. After thermal treatment, the sample was cooled to 150 °C and exposed to a stream of pyridine/He (30 mL min−1) for 1 min. The sample then was outgassed under vacuum for 1 h at 150 °C. The spectra were recorded before pyridine adsorption and after desorption step. The spectra were acquired with a PerkinElmer FTIR Spectrum 100 spectrometer that was equipped with an MCT-A detector and a high-temperature chamber (Harrick) with ZnSe windows and scanned using 60 scans with a resolution of 4 cm−1. 2.2. Kinetic Experiments. Straight-run diesel oil from a Brazilian refinery, whose composition was 3906 mg kg−1 of sulfur and 522 mg kg−1 of nitrogen, representing Arabian crudes, was used to study the adsorption properties of the chosen adsorbents.

⎛m ⎞ dC = − k1C + k −1q⎜ a ⎟ ⎝ v ⎠ dt

(1)

⎛ v ⎞ dq = k1C ⎜ ⎟ − k −1q dt ⎝ ma ⎠

(2)

They were rearranged and numerically integrated, giving eqs 3 and 4,

⎛m ⎞ Ct +Δt = Ct − kCt Δt + k −1qt Δt ⎜ a ⎟ ⎝ v ⎠

(3)

⎛ v ⎞ qt +Δt = qt − k −1qt Δt + kCt Δt ⎜ ⎟ ⎝ ma ⎠ −1

(4) −1

where Ct (mol L ) and qt (mol kg ) are sulfur or nitrogen compounds concentrations in the liquid and solid phase at time t, respectively; k1 is the adsorption coefficient (min−1); k−1 is the desorption coefficient (min−1); ma is the adsorbent weight (kg); v is the solution volume (L); t is time (min); Δt is the variation in time (min). The second model is a pseudo-first-order rate Lagergren model.29,30 It describes solid−liquid adsorption based on solid capacity. It considers that the adsorbate removal rate over time is directly proportional to the difference in saturation concentration and the number of active sites in the solid.31 It suggests that film diffusion is the rate-limiting step.32 This model has been chosen because it is one of the most applied models in the literature for kinetics data.25,29 Equation 5 describes this model and eq 6 is the former equation rearranged and numerically integrated using the Euler−Cauchy numerical solution, where qt is the sulfur or nitrogen compounds concentration in the solid phase at time t (mol kg−1); qe is the concentration of sulfur or nitrogen compounds in the solid phase at equilibrium (mol kg−1); and k2 is the adsorption coefficient (min−1).

k 2(qe − q) =

dq dt

(5)

qt +Δt = qt + k 2(qe − qt )Δt

(6) 29,30

It is used The third model is a pseudo-second-order rate model. to describe chemisorption involving the valence force by sharing or exchanging of electrons between the adsorbent and the adsorbate. The reaction velocity is dependent on the solute quantity adsorbed on the adsorbent surface and the quantity adsorbed at equilibrium.27 It 11732

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3. RESULTS AND DISCUSSION 3.1. Adsorbent Characterization. Four adsorbents were studied for the desulfurization and denitrogenation of a commercial diesel, that is, Clay A (attapulgite), Clay B (bentonite), Clay C (bentonite), and an Alumina. Bentonite is a clay mainly comprised of elements from the montmorillonite group, that is, aluminosilicate clay minerals, which are designated as a 2:1 layer mineral. Bentonite particles present cation exchange capacity and high surface area.23 Attapulgite is a hydrated magnesium aluminosilicate material that has tetrahedral (silica) and octahedral (Alumina) sheets as its basic building blocks. These units are connected to each other by shared O atoms.23 The chemical composition of the adsorbents (Table 1) is given in terms of the weight percentage

suggests that chemisorption is the rate-limiting step and it takes account of all of the adsorption steps, including external film diffusion, intraparticle diffusion, and adsorption.32 This model has been chosen because it is also largely used in the literature for kinetics data.30 Equation 7 represents this model and it was rearranged and numerically integrated using the Euler−Cauchy numerical solution (eq 8),

k 3(qe − qt )2 =

dq dt

(7)

qt +Δt = qt + k 3(qe − qt )2 Δt

(8)

where qt is the concentration of sulfur or nitrogen compounds in the solid phase at time t (mol kg−1); qe is the concentration of sulfur or nitrogen compounds in the solid phase at equilibrium (mol kg−1); and k3 is the adsorption coefficient (kg mol−1 min−1). Besides the three kinetic models presented above, the adsorbate diffusion within the adsorbent pores applying intraparticle diffusion model was investigated (eq 9).25 In this model, qt is the concentration of sulfur or nitrogen compounds in the solid phase at time t (mol kg−1); kid is the intraparticle diffusion rate constant (mol kg−1 min−1/2); and values of I (mol kg−1) give an idea about the thickness of the boundary layer. qt = k idt 1/2 + I

Table 1. Chemical Composition of Adsorbents Chemical Composition (wt %) sample Alumina Clay A Clay B Clay C

(9)

The adsorption isotherm parameters were estimated using Langmuir−Freundlich and BET isothermal models, according to experimental data. The BET isotherm is a theoretical equation that is widely applied in gas−solid equilibrium systems. It was developed considering physical adsorption with overlapping layers33 (eq 10), qe =

−1

(10)

−1

where Ce (mol L ) and qe (mol kg ) are the concentrations of sulfur and nitrogen compounds in the liquid and solid phases at equilibrium time; qs is the adsorption capacity of the adsorbent for the first layer (mol kg−1); K1 is the adsorption constant during contaminant− adsorbent interaction (formation of monolayer) (L mol−1); and Ka is the adsorption constant during contaminant−contaminant interaction (formation of the next layers) (L mol−1). The Langmuir−Freundlich isotherm (eq 11) is a combined form of Langmuir and Freundlich expressions. This isotherm can circumvent the limitation of the rising adsorbate concentration associated with the Freundlich isotherm model and predict the heterogeneous adsorption systems that are not included in the Langmuir equation.

⎛ K sCens ⎞ qe = qmax,s⎜ n ⎟ ⎝ 1 + K sCe s ⎠ −1

(11) −1

where Ce (mol L ) and qe (mol kg ) are the concentrations of sulfur and nitrogen compounds in liquid and solid phases at equilibrium time, and Ks (L mol−1), qmax,s (mol kg−1) and ns (dimensionless) are the Langmuir−Freundlich characteristic parameters. Microsoft Office Excel 2007 was used to solve both kinetic and equilibrium models through data adjustment by minimizing the objective function OF (eq 12) for resolution of eq 1 and the objective function OF (eq 13) for resolution of eq 2, it was possible to determine the parameters using the SOLVER tool from the cited software. n

OF =

n

∑ (Cexp,i − Ccalc,i)2 + ∑ (qexp,i − qcalc,i)2 i=1

i=1

(12)

n

OF =

∑ (qexp,i − qcalc,i)2 i=1

Al2O3

68.7 74.9 72.1

99.6 1.1 16.3 16.1

CaO

MgO

Fe2O3

Si/Al molar ratio

10.0

13.6 5.6 6.1

3.9 1.5 2.3

53 3.9 3.8

of oxides. The Alumina adsorbent contains 0.3 wt % Na2O. Clay samples present alkaline-earth metals, mainly MgO. Clay A contained 23.6 wt % alkaline-earth oxides (MgO and CaO). Clay B and Clay C showed the same Si/Al molar ratio, while Clay A presented a very high ratio. The crystalline phases of clay adsorbents were determined via X-ray diffraction (XRD) (Figure 1). Clay B and Clay C are mainly composed of montmorillonite (ICCD File Card No. 130135) (see Figure S1 in the Supporting Information).34,35 However, the X-ray diffractogram of Clay C indicates the presence of minor amounts of quartz, K-feldspar, and plagioclase.35 The XRD pattern of Clay A shows that the major mineral is attapulgite, with characteristic diffraction peaks at 8.4°, 13.9°, 19.8°, 20.8°, 28.0°, and 28.8°. Besides attapulgite (ICCD File Card No. 31-0783) (see Figure S2 in the Supporting Information), the presence of quartz and dolomite was identified by the peaks at ∼26.6°36−38 and ∼30.8°,39 respectively. According to the supplier, the Alumina sample is an amorphous adsorbent. As can be seen in Table 2, Alumina is the adsorbent with the highest specific area, while Clay A (attapulgite type) showed the smallest area. Clay B and Clay C (bentonite type) present similar specific areas that are higher than that of Clay A, indicating that these clays were chemically treated by the suppliers.40 The acid treatment leads the leaching of cations from octahedral and tetrahedral sheets, dissolves impurities, and replaces exchangeable cations with hydrogen ions. It also opens the edges of the platelets. All these changes result in increased specific area and pore diameter. The pore volume and diameter of Alumina, Clay B, and Clay C are similar, while a higher value of these properties was observed for Clay A. The N2 adsorption/desorption isotherms of the adsorbents are illustrated in Figure 2 and they were identified according to IUPAC or BDDT (Brunauer, Deming, Deming, and Teller) classifications. All isotherms showed hysteresis loops that are usually associated with the filling and emptying of mesopores by capillary condensation. The bentonite clays, Clay B and Clay C, presented similar isotherms classified as Type IV that is characteristic of mesoporous solids,41 with H4 type hysteresis

qsK1Ce (1 − K aCe)(1 − K aCe + K1Ce)

SiO2

(13) 11733

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Figure 2. Nitrogen adsorption/desorption isotherms of (■,□) Clay A, (◆,◇) Clay B, (▲,△) Clay C, and (●,○) Alumina.

The determination of density, strength, and type of acid sites exposed on the surface of a solid is a useful tool to understand the adsorption capacity of adsorbents. Temperature-programmed desorption (TPD) of ammonia was used to measure total density and strength of acid sites on the surface of the studied samples, while acid site type was identified using pyridine adsorption. The acid site densities were measured by TPD of ammonia, and the results are shown in Table 2. Alumina is the adsorbent with the highest density, while Clay A is the adsorbent with the lowest density. The following sequence was observed: Alumina > Clay B > Clay C > Clay A. For all adsorbents, a broad peak with temperature maximum around 222−237 °C was observed, indicating that these are mainly weak acid sites. Infrared (IR) spectra of the samples in OH stretching region are shown in Figure 3. The hydroxyl stretching region of Clay A is complex with bands at 3739, 3718, 3673, 3638, 3619, and 3587 cm−1. According to the literature,50−52 bands below 3600 cm−1 are assigned to hydroxyl stretching vibrations of water, while bands above 3600 cm−1 are associated with coordinated hydroxyl groups. Frost et al. attributed bands at 3731 and 3710 cm−1 to hydroxyls attached to tetrahedral silicon in the attapulgite structure.50 They suggested that the first band is due to non-hydrogen-bonded hydroxyl group and the second band is associated with hydrogen-bonded hydroxyl on the tetrahedral silicon. These bands are frequently observed in layered silicates and zeolites and are assigned to terminal Si− OH groups. The remaining four bands are attributed to hydroxyl stretching vibrations of octahedral magnesium or equivalent cation.50 A band at ∼3616 cm−1 is always observed for palygorskite,50,53,54 and it is related to the contribution of stretching vibrations of OH in (Fe,Mg)−OH and (Al,Mg)− OH. The band at 3580 cm−1 was assigned to Al−Fe3+−OH or Al−Mg−OH bonds,55 while Chahi et al. attributed it to Al− Fe−OH stretching.53

Figure 1. X-ray diffractograms of Clay A, Clay B, and Clay C. [Symbol legend: (●) attapulgite, (▲) quartz, (■) dolomite, (○) montmorillonite, (◇) plagioclase.]

Table 2. Textural and Acid Properties of the Adsorbents Pore Volume (cm3 g−1)

sample

specific area (m2 g−1)

mesoporous

Alumina Clay A Clay B Clay C

357 95 198 273

0.34 0.45 0.21 0.30

a

microporous 0.02 0.01 0.01 0.02

b

pore diameter (nm)

acid site densityc (μmol NH3 g−1)

5 22 6 9

297 74 127 88

a c

Determined via the BJH method. bDetermined via the t-plot method. As measured by TPD of NH3.

loops representing slit-shaped pores and platelike particles with spaces between the parallel plates.42 Caglar et al. found similar isotherm profiles.43 On the other hand, Clay A, an attapulgite clay, presented a Type V isotherm, corresponding to a weak adsorbent−adsorbate interaction and saturation of the adsorbed amount at high pressures.44 A steep increase of adsorbed amount is observed for Clay A at p/p0 = 0.9 that is probably related to the filling of voids among particles. Similar N2 isotherms for attapulgite samples were observed in the literature.45−47 The N2 adsorption/desorption isotherm of the Alumina sample is Type IV with an H3 hysteresis loop, which is usually observed with aggregates of platelike particles giving rise to slit-shaped pores.48,49 11734

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Figure 4 shows the spectra of pyridine chemisorption on all samples at 150 °C. The spectra for Alumina (1616, 1599, 1576,

Figure 3. Infrared spectra in the OH stretching region for Alumina, Clay A, Clay B, and Clay C.

Figure 4. IR spectra of pyridine adsorbed on Alumina, Clay A, Clay B, and Clay C at 150 °C.

1492, 1451 cm−1) and Clay A (1621, 1610, 1577, 1494, 1448 cm−1) showed the presence of only Lewis acid sites. The Alumina sample showed a band at 1599 cm−1 that is related to labile species ascribed to hydrogen-bounded pyridine,56 and the band at 1616 cm−1 was assigned to Al3+ sites in octahedral symmetry.62,63 On the other hand, the spectra of Clay B and Clay C indicated the presence of Brønsted acid sites (1638, 1545, 1491 cm−1, and 1637, 1545, 1491 cm−1, respectively) and Lewis acid sites (1622, 1576, 1491, 1455 cm−1, and 1622, 1612, 1577, 1491, 1454 cm−1, respectively). Similar IR spectra were observed in the literature.43,64 Bodoardo et al.56 ascribed the formation of pyridinium ions on (AlOH) hydroxyl species, while Lewis acid sites were related to Al centers. At least two types of Lewis sites can be observed: one at 1622 cm−1, which was associated with Al sites on pillars, and the other one at 1614 cm−1, which was associated with Al sites in the tetrahedral layer. 3.2. Adsorption Kinetics. The adsorption kinetics describes the rate of adsorbate retention on an adsorbent, indicating adsorption efficiency. The adjusting of batch adsorption kinetics is useful for designing adsorption columns. The kinetic profiles for the adsorption of sulfur compounds on studied adsorbents are depicted in Figure 5. Clay A and Alumina achieved equilibrium after Clay C (0.128 mol kg −1) > Alumina (0.059 mol kg −1) > Clay A (0.018 mol kg −1)

Clay B and Clay C are commercial bentonites that exhibit similar chemical composition (Si and Al are the major elements) and specific area. These samples exhibit Brønsted and Lewis acid sites. The Alumina sample presents only aluminum, as well as high specific area and acid site density; however, only Lewis acid sites were noticed. Clay A, which is an attapulgite clay, shows low specific surface area and acid site density, and, for the Alumina, only Lewis acid sites were observed. The main element in its chemical composition is silicon, and the presence of Ca and Mg was also observed. It is important to highlight that the presence of Brønsted acid sites seems to be crucial to a better performance in the adsorption of sulfur compounds as Clay B and Clay C showed the higher values of adsorption capacity. However, the interaction of Lewis acid sites with sulfur and nitrogen compounds cannot be denied, because Alumina and Clay A, which have only this type

Table 3. Kinetic Parameters for Each Adsorbent for the Adsorption of Sulfur Compounds: Model 1, First-Order; Model 2, Pseudo-First-Order; and Model 3, Pseudo-Second-Order Model 1

Model 2

adsorbent

qe,exp (mol kg−1)

k1 (min−1)

k−1 (min−1 L−1)

qe,cal (mol kg−1)

Alumina Clay A Clay B Clay C

0.056 0.018 0.179 0.128

0.081 0.359 0.152 0.064

0.645 10.0 0.310 0.205

0.059 0.018 0.174 0.126

R2

k2 (min−1)

qe,cal (mol kg−1)

0.937 0.595 0.987 0.998

0.860 2.000 0.415 0.260

0.057 0.018 0.184 0.128

11736

Model 3 R2

k3 (kg g−1 min−1)

qe,cal (mol kg−1)

R2

0.928 0.595 0.966 0.997

2.426 10.00 0.121 0.128

0.057 0.018 0.183 0.127

0.927 0.573 0.987 0.999

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combination of different steps. The influence of intraparticle diffusion was evaluated using the Weber−Morris diffusion model,25,65−67 expressed by a plot of qt vs t1/2. When only intraparticle diffusion controlled the adsorption process, a linear relation of experimental data that passes through the origin is observed. However, multilinear plots indicate that two or more steps influence the adsorption process. Figure 7 shows the Weber−Morris plot for the adsorption of sulfur compounds. All adsorbents showed multilinear relations. Clay B and Clay C presented three linear portions. The first sharp line represented the diffusion of adsorbate from the solution to the external surface of the adsorbent or boundary layer diffusion. The second linear portion is attributed to the gradual adsorption stage controlled by intraparticle diffusion. The third stage is the final equilibrium stage, where intraparticle diffusion starts to slow down, because of the extremely low adsorbate concentration that remains in the solution.25 The desulfurization of a model solution containing dibenzothiophene (DBT) in n-heptane was carried out using untreated, acid-activated, and magnetite-nanoparticle-loaded bentonites as adsorbents. The plot of the intraparticle diffusion model of all adsorbents showed three different linear steps.19 For Alumina and Clay A, only the first and third stages were identified. Srivastav and Srivastava used a commercial activated Alumina as an adsorbent to remove DBT dissolved in n-hexane and observed that more than one process is controlling adsorption, because three stages were identified in the Weber−Morris plots.25 In a similar way, Neubauer et al. studied the desulfurization of Jet A1 fuel enriched with benzothiophene and DBT, using an Ag−Al2O3 adsorbent, and they identified three-stage behavior for these sulfur compounds.65 Weber−Morris plots for the adsorption of nitrogen compounds are depicted in Figure 8. Only one stage was observed for Alumina, which indicated that the adsorption process is controlled by intraparticle diffusion. Clay A, Clay B, and Clay C showed two steps. For Clay A, the first and third steps were noted; this observaton is similar to the result obtained for sulfur compound adsorption. On the other hand, stages one and two described the adsorption process for Clay B and Clay C in the tested time range. 3.3. Adsorbent Dose. Two adsorbents were chosen to study equilibrium adsorption: Clay A and Clay B. According to adsorption kinetics, Clay A has shown a high selectivity for nitrogen compounds, while Clay B has presented the highest adsorption capacity of sulfur and nitrogen compounds. The influence of adsorbent amount on the removal of adsorbates is associated with the availability and accessibility of adsorption sites. Figure 9 depicts the effect of adsorbent dose in the adsorption of sulfur and nitrogen compounds present in a diesel sample. For sulfur compounds (Figure 9A), Clay A and Clay B showed different behaviors. In the presence of Clay A,

Figure 6. Adsorption kinetics of nitrogen compounds on (●) Alumina, (■) Clay A, (◆) Clay B, and (▲) Clay C. Experimental conditions: 10 mL of diesel and 2 g of adsorbent under stirring of 150 rpm at 40 °C. Alumina was adjusted using the first-order model; Clay A and Clay B were adjusted using the pseudo-first-order model; Clay C was adjusted using the pseudo-second-order model.

Clay B (0.127 mol kg −1) > Clay C (0.109 mol kg −1) > Clay A (0.078 mol kg −1) > Alumina (0.059 mol kg −1)

With regard to sulfur compounds, Clay B and Clay C exhibit better adsorption capacity, indicating that Brønsted acid sites are crucial. It is important to highlight that Clay A is more selective to the adsorption of nitrogen compounds (0.078 mol kg−1), compared to sulfur ones (0.018 mol kg−1). The equilibrium was first reached by Clay A after 20 min; on the other hand, the other three adsorbents attained saturation after 120 min. To the best of our knowledge, no information on the adsorption of nitrogen compounds on clay adsorbents is available in the literature. The parameters of first-order, pseudo-first-order, and pseudo-second-order kinetic models for the adsorption of nitrogen compounds are shown in Table 4. For all adsorbents and kinetic models, qe,exp and qe,cal values are very similar. For Alumina, the best fit was observed with the first-order model, with higher values of R2. In the case of Clay A, the pseudo-firstorder model has shown the best results; this model suggests that film diffusion is the rate-limiting step.32 For Clay B, firstorder and pseudo-first-order models have shown the same fit, whereas for Clay C, the pseudo-second-order model presented the highest R2 values; this latter model suggests that chemisorption is the rate-limiting step, and it takes account of all of the adsorption steps, including external film diffusion, intraparticle diffusion, and adsorption.32 The complete adsorption process is composed of one or more steps, that is, film or external diffusion, pore diffusion, surface diffusion and adsorption on the pore surface, or a

Table 4. Kinetic Parameters for Each Adsorbent for the Adsorption of Nitrogen Compounds: Model 1, First-Order; Model 2, Pseudo-First-Order; and Model 3, Pseudo-Second-Order Model 1

Model 2

adsorbent

qe,exp (mol kg−1)

k1 (min−1)

k−1 (min−1 L−1)

qe,cal (mol kg−1)

Alumina Clay A Clay B Clay C

0.063 0.079 0.125 0.114

0.005 0.070 0.013 0.013

0.011 0.081 0.033 0.005

0.058 0.074 0.125 0.116

R2

k2 (min−1)

qe,cal (mol kg−1)

0.9571 0.992 0.989 0.965

0.016 0.118 0.016 0.020

0.058 0.078 0.126 0.114

11737

Model 3 R2

k3 (kg g−1 min−1)

qe,cal (mol kg−1)

R2

0.9570 0.996 0.989 0.965

0.041 0.192 0.017 0.031

0.054 0.077 0.114 0.108

0.9537 0.987 0.973 0.968

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Figure 7. Weber−Morris intraparticle diffusion plot for sulfur compounds adsorption on (●) Alumina, (■) Clay A, (◆) Clay B, and (▲) Clay C. Experimental conditions: 10 mL of diesel and 2 g of adsorbent under stirring (150 rpm) at 40 °C.

Figure 8. Weber-Morris intraparticle diffusion plot for nitrogen compounds adsorption on Alumina (●), Clay A (■), Clay B (◆), Clay C (▲). Experimental conditions: 10 mL of diesel and 2 g of adsorbent under stirring (150 rpm) at 40 °C.

nitrogen compounds (Figure 9B), Clay A and Clay B showed similar behavior; that is, the removal increased constantly when adsorbent dose increased. 3.4. Adsorption Isotherm. The adsorption process in the liquid phase involves a competition between the solvent and the solute that must be considered for an accurate discussion of the process. The adsorption of a solute on an adsorbent at a liquid/solid interface is commonly followed by measuring the

the removal of sulfur compounds increased until the adsorbent dose was equal to 0.4 g mL−1; thereafter, this removal remained constant. This trend can be due to aggregation of adsorbent particles hindering the access to adsorbent active sites. On the other hand, for Clay B, the removal of sulfur compounds increased constantly when the adsorbent dose increased. This behavior can be attributed to the availability of larger specific area and adsorbent sites for sulfur compounds adsorption. For 11738

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according to Giles et al.69 Two interactions are important to the adsorption process, that is, adsorbate−adsorbent and adsorbate−adsorbate interactions. The Type S isotherm is observed when adsorbate−adsorbate interactions significantly contribute to increase adsorption, indicating a continuous progression with increasing loading from monolayer adsorption to multilayer adsorption. An increase of adsorption capacity in high range of concentration confirms the difficulty in sulfur adsorption. Both adsorbents presented isotherms with unfavorable profiles, indicating that, at low concentrations, there is a small amount of contaminant removal. At 40 °C, Clay B achieved lower equilibrium concentration and higher adsorption capacity for sulfur adsorption in the concentration range used, indicating that this is a better adsorbent for the removal of sulfur compounds. As expected, the increase of adsorption temperature led to higher equilibrium concentration and lower adsorption capacity, suggesting that the interaction of sulfur compounds and adsorption sites on Clay B is weak and also that the adsorptive forces between adjacent molecules of the adsorbed phase are also weak. The results for the adsorption isotherms of nitrogen compounds with the selected adsorbents are presented in Figure 11. The isothermal constants for the Langmuir−

Figure 9. Effect of adsorbent dose on the adsorption of (A) sulfur compounds and (B) nitrogen compounds on (■) Clay A and (◆) Clay B. Experimental conditions: 10 mL of diesel and 6.0, 4.0, 2.0, 1.0, 0.5, 0.33, 0.2 g of adsorbent under stirring (150 rpm) at 40 °C.

solute concentration at an equilibrium point.68 The adsorption isotherm is a correlation of the amount of adsorbed solute by unit mass of solid and the remaining equilibrium concentration of the solute at a specific temperature. This is important to describe the adsorption mechanism pathways and the design of adsorption systems. The adsorption performance of a solid is influenced by the accessibility of adsorption sites and the surface properties of the adsorbent. The equilibrium results are expressed as the number of moles of sulfur or nitrogen per unit mass of adsorbent, in order to allow the comparison between the removals of sulfur and nitrogen. Figure 10 illustrates the results for the sulfur

Figure 11. Adsorption isotherm of nitrogen compounds in diesel using Clay A and Clay B as adsorbents. Experimental conditions: 10 mL of diesel, 40 °C ((■) Clay A, (◆) Clay B) and 70 °C ((□) Clay A, (◇) Clay B), 8 h, and atmospheric pressure.

Freundlich and BET models, along with R2 values (which show adjustment quality), are shown in Table S2 in the Supporting Information. The experimental data for the adsorption of nitrogen compounds on Clay A at 40 and 70 °C was fitted by the BET model, whereas on Clay B, at 40 and 70 °C, the best model was the Langmuir−Freundlich model. According to Giles et al.,69 the four isotherms were classified as Type L, which shows concavity to the concentration axis near the origin. This type of isotherm usually indicates that molecules are adsorbed on the surface, and, sometimes, they can show a strong intermolecular attraction, resulting in a vertical adsorption. A well-defined plateau can also be observed and it is associated with the formation of a monolayer of the adsorbate. The isotherms obtained for Clay B at 40 and 70 °C were assigned to subgroup 1, while Clay A isotherms at both adsorption temperatures were classified as subgroup 3. The subgroups of the isotherm classes are defined according to the shape of the curves away from the origin, plateaus, and changes

Figure 10. Adsorption isotherm of sulfur compounds in diesel using Clay A and Clay B as adsorbents. Experimental conditions: 10 mL of diesel, 40 °C ((■) Clay A, (◆) Clay B) and 70 °C ((□) Clay A, (◇) Clay B), 8 h, and atmospheric pressure.

compounds adsorption isotherm with the selected adsorbents at 40 and 70 °C. Because of the profile of the isothermal data, only the BET isothermal model was used to adjust the experimental results. The isotherm constants for the BET model, along with R2 values (which shows adjustment quality), are shown in Table S1 in the Supporting Information. The BET model is the suitable equation for representing the adsorption isotherm data for sulfur compounds on Clay A and Clay B at 40 and 70 °C. Regardless of the adsorption temperature and adsorbent, all isotherms for sulfur compounds were classified as Type S, 11739

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presented different adsorption steps for sulfur and nitrogen compounds. For sulfur compounds, the three steps (diffusion of adsorbate from solution to external surface of adsorbent or boundary layer diffusion; intraparticle diffusion; final equilibrium stage) were observed, whereas, for nitrogen compounds, only diffusion of the adsorbate from the solution to the external surface of adsorbent, and the equilibrium stage, were noted. From equilibrium data, it was noted that adsorbate−absorbate and adsorbate−surface interactions predominate for sulfur and nitrogen compounds, respectively, for Clay A and Clay B.

of slope are considered. The subgroup 1 curves do not show a plateau or an inflection, indicating a clearly incomplete isotherm; that is, the saturation of the surface has not been reached. For subgroup 3 isotherms, a plateau and an inflection point are observed. The plateau is formed when adsorbed molecules cover the surface and the possible sites are filled. A short plateau, as can be seen for Clay A isotherms, means that, after the adsorbate interacts with the adsorbent surface, forming an adsorbate layer on the surface, the adsorbate in the solution shows the similar affinity to the adsorbent surface and to the adsorbate layer built on the adsorbent surface. As the attraction for the adsorbate layer increases, the curve steadily rises and a second rise is noted. Nitrogen compounds present in the studied diesel showed good interaction with Clay B adsorption sites as the highest adsorption capacity for nitrogen compounds was observed for this adsorbent. As mentioned above, the plateau of the Type L isotherm was not achieved throughout the concentration range, indicating that adsorption sites are still available on the surface. The adsorption capacity of Clay B at 70 °C was higher than that at 40 °C; this increase is due to the increase of fluidity of liquid phase, allowing the diffusion of the adsorbate into adsorbent pores. Clay A presented a lower adsorption capacity for nitrogen compounds, compared to Clay B, indicating that the Clay A surface has less adsorption sites for nitrogen than Clay B. The isotherm shapeType L and subgroup 3suggested that nitrogen compounds had a better attraction to each other than to the adsorption sites on Clay A. The increase of adsorption temperature from 40 °C to 70 °C did not influence the adsorption capacity of Clay A until the sharp rise in curve steepening, because of multilayer adsorption. The increase of temperature did not influence the nitrogen compounds adsorption in the region of the initial rise and plateau, indicating that these compounds strongly interact with adsorption sites. On the other hand, the adsorbate−adsorbate attraction is a weak interaction, because, when adsorption temperature increases, the adsorption capacity decreases. Analyzing characterization results and adsorption isotherms, it is not simple to establish a correlation between higher contaminant removal capacity and adsorbent properties. Although the adsorption is a surface phenomenon, it was not possible to establish a direct correlation with specific area. In this case, Alumina showed the worst performance, although it had the highest specific area. On the other hand, acid site type (Brønsted sites) seems to be more important than acid site density for the adsorption of sulfur compounds.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b01928. Isotherm constants for BET and Langmuir−Freundlich models and R2 values for adsorption experimental data on Clay A and B at 40 and 70 °C; and ICCD File Card Nos. 13-0135 (montmorillonite-15A) and 31-0783 (palygorskite) (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Cláudia O. Veloso: 0000-0002-9807-5830 Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS The authors thank Petrobras S.A. for financial support. REFERENCES

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4. CONCLUSION The bentonite clays (Clay B and Clay C) showed promising adsorptive capacity for nitrogen and sulfur compounds probably due to the presence of Brønsted acid sites and also higher specific area, whereas the attapulgite clay (Clay A) was selective to nitrogen compounds, whose adsorption can be influenced by chemical composition, that is, a high Si/Al molar ratio. Clays were more effective than Alumina for sulfur and nitrogen compounds removal from a real diesel stream. For Clay A, the adsorption process for sulfur and nitrogen compounds can be described by the diffusion of adsorbate from solution to external surface of adsorbent, and the equilibrium stage, where low adsorbate concentration is present in the solution. It is important to highlight that Clay A showed the highest pore volume. On the other hand, Clay B and Clay C 11740

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