Selective Desulfurization of Model Diesel Fuel by Carbon

Article pubs.acs.org/IECR

Selective Desulfurization of Model Diesel Fuel by Carbon Nanoparticles as Adsorbent Rahimeh Naviri Fallah,†,‡ Saeid Azizian,*,† Guy Reggers,‡ Sonja Schreurs,‡,§ Robert Carleer,‡ and Jan Yperman‡ †

Department of Physical Chemistry, Faculty of Chemistry, Bu-Ali Sina University, Hamedan 65167, Iran Laboratory of Applied and Analytical Chemistry, CMK, Hasselt University, Agoralaan Gebouw D, 3590 Diepenbeek, Belgium § NuTeC, XIOS, Agoralaan Gebouw H, 3590 Diepenbeek, Belgium ‡

S Supporting Information *

ABSTRACT: This work examines the effect of aromatic compounds (naphthalene and 1-methylnaphthalene) on the adsorption of sulfur thiophenic compounds (including benzothiophene, dibenzothiophene, and dimethyldibenzothiophene) from simulated diesel fuel by dispersed carbon nanoparticles (CNPs) in aqueous solution. To evaluate the equilibrium and kinetics of adsorptive desulfurization by synthesized CNPs, two model diesel fuels with 300 ppmw total concentration of sulfur compounds were used in a batch reactor at ambient conditions. The solid CNPs were characterized using FTIR, thermal analysis, elemental analysis, TEM, and surface pH. The equilibrium experimental data were fitted to Langmuir, Freundlich, and Langmuir−Freundlich models to estimate the adsorption parameters. Different equations were applied to fit the kinetics of adsorption and to determine its mechanism. The selectivity for benzothiophene (BT), dibenzothiophene (DBT), and dimethyl dibenzothiophene (DMDBT) adsorption was calculated with naphthalene as reference (NP). Both the adsorption capacity and the selectivity were in the order of BT > DBT > methyl NP ≈ DMDBT > NP. It was found that the rate of adsorption is high and aromatic compounds have no effect on the adsorption kinetics of thiophenic compounds. The results showed that the CNPs can act as a selective adsorbent for the removal of thiophenic carbons in competition with aromatics. zeolite,14 and mesoporous materials15 are the most well-known adsorbents. Adsorbents could be improved via incorporating adequate active sites on their surfaces. It has been shown that the oxidation of carbon surfaces increased their ability to perform the desulfurization. Oxidation is not only increasing the capacity but also the selectivity of the adsorption process via acid−base interaction between slightly basic thiophenic compounds and acidic oxygen-containing groups of adsorbent surfaces.11,12,16 Besides these, previous studies indicated that deposition of metallic species such as nickel, iron oxide, copper, and silver to activated carbon, zeolite, and mesoporous material can improve the efficiency of adsorptive desulfurization via πelectron interaction.8,10,15,17 Also, utilizing loaded nanoparticle adsorbents for the separation of organosulfur compounds exhibits high specificity.18 It seems that the use of nanotechnology in this field is therefore promising. Karvan and coworkers reported on the use of a nano-CuO/mesoporous SiO2 material as hot gas desulfurization sorbent.19 They found that mesoporous composite of nanosize CuO and amorphous SiO2 were effective in H2S adsorption. Vu et al. used composite of multiwalled carbon nanotubes (MWNTS) and titanium(IV)oxide (TiO2), which were prepared by heterogeneous gelatin method for the photocatalytic oxidative desulfurization of

1. INTRODUCTION Most countries have implemented stringent legislation to regulate the sulfur content of transportation fuels such as diesel fuel because it has an impact on the environmental pollution and it spoils the low-temperature activity of automotive catalytic converters.1 In the new regulation, the maximum allowable sulfur content for diesel fuel is 15 ppmw by 2006 in the U.S. and less than 10 ppmw by 2010 in the EU.2,3 So far, various approaches for desulfurization of liquid hydrocarbon fuel have been used. Hydrodesulfurization (HDS), the conventional removal of sulfur compounds, requires severe conditions to produce ultralow sulfur compounds fuel. Besides the fact that HDS is not an economical technique, HDS is also not successful in removing thiophenic compounds such as dibenzothiophenes and their derivatives, due to their low reactivity in the HDS process.4 Also, saturation of olefins reduces the octane number.5 Therefore, more efforts are being made in advanced desulfurization technologies such as selective oxidation,6 selective extraction,7 and selective adsorption.8−11 Recently, there is a lot of interest in developing sorbents for selective adsorption of organosulfur compounds from liquid fuel. In diesel fuel, a large number of aromatic compounds are present with structures similar to that of the sulfur compounds that need to be removed. Introduction of new adsorbents with high removal capacity, good selectivity, and high adsorption rate over other aromatic and olefin compounds is a key to an efficient adsorptive desulfurization process. In separation media, various adsorbents have been used. Activated carbon,2,3,8−13 © 2012 American Chemical Society

Received: Revised: Accepted: Published: 14419

August 30, 2012 October 11, 2012 October 12, 2012 October 12, 2012 dx.doi.org/10.1021/ie3023324 | Ind. Eng. Chem. Res. 2012, 51, 14419−14427

Industrial & Engineering Chemistry Research

Article

dibenzothiophene (DBT) and 4,6-dimethyl dibenzothiophene (DMDBT) in commercial diesel fuel.20 They found that more than 98% of the sulfur compounds in commercial diesel were oxidized and removed by the use of MWNTS/SiO2 composite as a photocatalyst. In our recent study,21 we investigated the efficiency of dispersed carbon nanoparticles in aqueous solution (CNPs) as adsorbent for the removal of benzothiophene (BT), DBT, and DMDBT from an n-heptane model fuel solution. The CNPs were prepared in a fast and simple way. We demonstrated that 70% removal efficiency for BT and DBT in less than 20 min was obtained. It was also found that CNPs are still active as desulfurization agent in four consequent runs, and this is without any regeneration. The objectives of the present study are: (1) CNPs characterization, (2) the effect of aromatic compounds, such as naphthalene (NP) and 1-methylnaphthalene (MNP), on the adsorption capacity of CNPs toward thiophenic compounds and in its selectivity, and (3) the study of aromatic compounds impacts on the kinetics of the sulfur compounds adsorption process by CNPs.

(C0 − Ce, i)w

qe, i =

m

(1)

where qe,i is the equilibrium amount of BT, DBT, DMDBT, NP, or MNP, respectively, adsorbed by the CNPs (mmol per gram of sorbent), and C0 and Ce,i are the initial and equilibrium concentrations of the sulfur compound (ppmw S) or aromatics (ppmw) in the solution, respectively. w is the mass of the liquid fuel phase, and m is the weight of the solid adsorbent (g). It should be mentioned that each 10 g of aqueous solution contains 0.0715 g of solid carbon nanoparticle. The adsorbed amount of compound i at any time, qt,i, was calculated by

qt , i =

(C0 − Ct , i)w m

(2)

where Ct,i is the bulk concentration of any compound i at any time. The relative selectivity factor was calculated by the equation defined as: αi − n =

2. EXPERIMENTAL SECTION 2.1. Materials. The preparation of dispersed CNPs in aqueous phase was performed according to Yang et al.’s method22 but with some changes in the amounts of reactants. One gram of +D-glucose and 15 mL of poly (ethylene glycol) (PEG-200) were dissolved in 5 mL of distilled water to produce a clear solution. Next, the solution is heated in a microwave oven for 9 min at 540 W and 2450 Hz; after microwave irradiation, the solution color turned dark brown. Without any further processing, the produced aqueous solution of CNPs was used as adsorbent for desulfurization experiments. For separation of suspended CNPs from aqueous solution, a saturated solution of NaCl was added, and after precipitation of the CNPs, the solid phase was separated from the liquid phase by centrifugation. To remove an excess of NaCl, the solid residue was washed three times with Milli-Q water. The sample was dried at 110 °C overnight. +D-Glucose and PEG-200 were supplied by Merck Co. Extra pure reagents of BT, DBT, and DMDBT (purity >98%) were purchased from Sigma-Aldrich Co., and n-heptane (>99%) was obtained from Merck Co. NP (99%) and MNP (99%) were obtained from Sigma-Aldrich Co. 2.2. Method. 2.2.1. Adsorption Equilibrium and Kinetics. Two model diesel fuels of n-heptane (MDF1and MDF2) were prepared to evaluate the selective adsorption of sulfur compounds from it. MDF1 contained equal concentrations of BT, DBT, and DMDBT. In the MDF2, in addition to the three thiophenic sulfur compounds, two aromatic compounds NP and MNP with the same concentration were added. The concentration of each component was about 2 mmol/kg (100 ppmw). The total sulfur compound concentration in the MDFs was thus 300 ppmw. The adsorption experiments were conducted in a stirred batch system at ambient temperature. Five grams of prepared MDF solution was added to various bottles containing different amounts (from 0.7 to 20 g) of CNPs aqueous solution, and the two-phase system was magnetically stirred at room temperature for 20 min. The samples were taken from the organic phase to measure the residual content of sulfur compounds and aromatics in the MDF solution. The equilibrium amount of compound i adsorbed per unit mass of adsorbent was calculated by:

Capi Capr

(3)

where Capi is the maximum adsorption capacity of compound i, and Capr is the maximum adsorption capacity of the reference compound NP in the model fuel, which contains both thiophenic (BT, DBT, and DMDBT) and aromatic (NP and MNP) compounds. 2.2.2. Analysis of the Treated MDF Samples. All samples were analyzed using a Thermo Trace GC Ultra, equipped with a flame ionization detector (FID). The compounds were separated by a DB-5 MS capillary column with 30 m of length, 0.25 mm of internal diameter, and 0.25 mm of film thickness. The oven temperature was initially set at 40 °C and ramped immediately at 6 °C/min to 120 °C, followed by a ramp at 15 °C/min from 120 to 290 °C, and held at 290 °C for 2 min. In these analyses, toluene was used as an internal standard for the quantitative analysis of each compound. 2.2.3. TEM. TEM images of carbon nanoparticles were obtained by a transmission electron microscope Philips EM208. 2.2.4. FTIR Analysis. Fourier transform infrared spectroscopy (FTIR) analyses were carried out on a Bruker Vertex 70 FTIR spectrometer using the attenuated total reflectance method (ATR) at a resolution of 4 cm−1. 2.2.5. TGA Analysis. Thermogravimetric curves were obtained using a TA Hi-Res 2960 Thermogravimetric Analyzer. About 20 mg of solid CNPs is pyrolyzed under approximately 35 mL/min N2 gas flow at a heating rate of 20 °C/min from room temperature to 600 °C. The gas flow was switched at 600 °C to oxygen with further heating to 800 °C at a heating rate of 20 °C/min. 2.2.6. TG−MS Analysis. A TGA Q5000 Thermogravimeric analyzer of TA Instruments coupled with a Pfeiffer Vacuum ThermoStar mass spectrometer was applied for analysis of separated solid CNPs. About 2 mg of sample was heated to 600 °C. The heating rate was 20 °C/min in a helium atmosphere with 100 mL/min flow rate. The mass spectrometer was set at the standard ionizing voltage of 70 eV with a mass range m/z of 10−210 and a scan rate of 5 scans/min. 2.2.7. CHN Analysis. The elemental composition (C, H, and N) of CNPs is determined with a FlashEA 1112 Elemental Analyzer of Thermo Electron Corp. The oxygen content was determined by the difference between 100% and the combined 14420

dx.doi.org/10.1021/ie3023324 | Ind. Eng. Chem. Res. 2012, 51, 14419−14427


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features of which dominate the IR pattern. In contrast, the IR spectrum of the solid CNPs reveals increased intensities of the oxidized moieties, carbonyl functionalities, and broadening of the absorption bands caused by condensation reactions and increase of MW. The absorption band at 1080 cm−1 can be related to the presence of ether vibration bond (C−O−C) in the CPNs and is in accordance with the enhanced oxygen content. All peaks that identified dispersed CNPs in aqueous solution are similar to the obtained CNPs after separation. Only the intensity of the absorption bands is different. In the 3600− 2500 cm−1 range, less intense signals for solid CNPs are observed as compared to dispersed CNPs, due to the removal of polyethylene glycol and water molecules. Figure 3 shows a typical thermal gravimetric graph and DTG curve of solid CNPs in two different atmospheres, initially nitrogen and then switching to an oxygen atmosphere. Initially, mass loss, dedicated to moisture, is observed (24%). Subsequently, a stepwise thermal degradation occurs, resulting in an additional mass loss of 48.5% with formation of a carbonized residue at 600 °C. Switching to an oxidative atmosphere induces combustion of the carbonaceous matter for finding the ash content of sample. Negligible amounts of ash residue are obtained ( DBT > DMDBT ≈ MNP > NP. The lower adsorption capacity for aromatics demonstrated the selectivity of CNPs toward sulfur

The Langmuir−Freundlich isotherm is: qe =

qm(KLFCe)1/ n 1 + (KLFCe)1/ n

(6)

where KLF and n are constants. The value of exponent n indicates the heterogeneity of the site energies. The isotherm parameters were calculated by fitting the experimental data (Table S2). On the basis of the correlation coefficients, the Langmuir−Freundlich model fits best the experimental data. The Langmuir−Freundlich isotherm is presented as full and 14422



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Figure 5. Ion-kinetograms of some dedicated mass ions of CNPs (TG/MS).

Figure 6. Evolution of CO2 as a function of temperature by TG/MS.

compounds in model diesel fuel and its possible application, if economical and practically reliable. The selectivity factor was obtained by dividing the total capacity of each sulfur compound in MDF2 by the total capacity of NP (eq 3). The calculated selectivity factors are presented in Figure 8. The order of critical diameter of the molecules or the area of the molecular plane increases in the order of NP < BT < MNP < DBT < DMDBT.16 For thiophenic compounds, the preferential adsorption can there-

fore be explained by their size. The interaction between adsorbate and adsorbent plays a significant role in adsorption. The surface pH value as well as TG/MS and FTIR results indicate that the CNPs surface is acidic and this acidity can affect the selectivity of adsorption. It can be expected that acid− base interaction will have a complementary effect on the adsorption process. Yet more research is needed to elucidate the correct reaction mechanism. Table S3 compares the capacity and calculated selectivity factor of different adsorbents 14423



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Figure 9. Variation of qt with time for BT, DBT, and DMDBT from (a) MDF1 and (b) MDF2 by the CNPs. Dashed and solid lines represent the predicted qt values by the modified pseudo n-order model.

Figure 7. Adsorption isotherms for (a) BT, (b) DBT, (c) DMDBT from MDF1 and MDF2, and (d) NP and MNP from MDF2 by the CNPs. Dashed and solid lines represent the predicted values by the Langmuir−Freundlich equation.

Figure 10. Removal percentage of different compounds from (a) MDF1 and (b) MDF2 during different runs.

of CNPs for DBT is similar to that of the modified activated carbon using metal nitrate solution.2 3.3. Adsorption Kinetics. The study of adsorption kinetics determines the adsorbate uptake rate. The kinetics adsorption of thiophenic compounds and aromatics by CNPs in model diesel fuels (MDF1 and MDF2) is shown in Figure 9. The results indicate that the uptake of adsorbate species is very fast, and approximately 70% of sulfur and aromatics were removed within 10 min, which is quite rapid in comparison with other adsorbents.21 To fit the experimental kinetic data, the pseudo-first-order (PFO),26 the pseudo-second-order (PSO),26,27 the modified

Figure 8. Adsorption selectivity of CNPs solution for different compounds.

for organosulfur compounds. In comparison with commercial activated carbon used by Song’s group,3 the adsorption capacity is higher but the selectivity for desulfurization is lower. The adsorption capacity of DBT for polystyrene polymer-derived carbon13 is larger than that for CNPs. Also, the selectivity factor 14424



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Figure 11. Variation of qt with t1/2 for different compounds from MDF2.

pseudo-n-order (MPnO),28 and the mixed-order (MOE)29 rate equations have been used. The PFO adsorption rate equation can be expressed as:

dqt dt

= k1(qe − qt )

qt = qe

(7)

F2 = (8)

dt

= k 2(qe − qt )2

(9)

with the corresponding integrated form: qt =

k 2qe2t 1 + k 2qet

(10)

where k2 is the PSO rate coefficient. A new empirical rate equation recently introduced by our research group is the modified pseudo-n-order (MPnO) model.27 The advantage of the MPnO model in comparison with previous kinetic models is the overall fitting performance in the whole time range.27 The MPnO has the following expression: dqt /dt = k(qen − 1/qtn − 1)(qen − qtn)

(11)

or

dqt /dt = k′

qen − qtn qtn − 1

(12)

kqn−1 e

where k′ = and n is the order of rate equation. By integration at boundary conditions, one gets: qt = qe(1 − e−nk ′ t )1/ n

k 2qe k1 + k 2qe

(15)

The results of fitting experimental data with these models for adsorption of sulfur compounds and aromatics by CNPs solution are presented in Table S4. As can be seen from the correlation coefficients, both the MPnO and the MOE models can describe the adsorption kinetics of the different compounds in MDF1 and MDF2 better. This may be attributed to the fact that both MPnO and MOE models are valid for heterogeneous surfaces. This defines CNPs as a heterogeneous adsorbent with different sites for adsorption or different kinds of functional groups. The results confirm also the findings with the adsorption data fitted with the Langmuir−Freundlich isotherm. A higher rate of adsorption is observed for BT and DBT by CNPs than for DMBT in both model fuels. The lowest kinetic adsorption is related to adsorption of NP. It should be noticed that despite the competitive adsorption between sulfur compounds and aromatics, the rate of sulfur compound uptake is rather the same. This means that the CNPs can remove sulfur compound selectively in very short time in the presence of aromatics. To verify this observation, Figure 10a and b shows the removal efficiency of CNPs for desulfurization in the model fuels with aromatics and without aromatics in four consecutive runs. These figures illustrate that the efficiency of CNPs for sulfur compound uptake in the presence of aromatics is only slightly affected. The adsorption kinetics is controlled by various mechanisms. The most important of these mechanisms are diffusion mechanisms, which include external diffusion, boundary layer diffusion, and intraparticle diffusion.30 To determine the ratelimiting step of the adsorption process, the intraparticle diffusion model was used, which is expressed on the basis of eq 16.31

where qe is the amount of qt at equilibrium and k1 is the PFO rate constant. The first theoretical basis of the pseudo-second-order (PSO) was presented by Azizian.26 The PSO adsorption rate equation is: dqt

(14)

where F2 (F2 < 1) is determined as the share of the secondorder term in the total rate equation.

The integrated form of eq 7 is:

qt = qe(1 − e−k1t )

1 − e−k1t 1 − F2 e−k1t

qt = k i t + C

(13)

(16)

where ki is the intra particle diffusion rate constant (mmol g−1 min−0.5) and C (mmol g−1) is a constant related to the energy of adsorption. If the curve of qt versus t0.5 is linear, the adsorption process will be controlled by intra particle diffusion. Yet if the data exhibit multilinear plots, more than one step

The next empirical model for heterogeneous surface adsorption kinetics was recently presented by Marczewski28 as the mixed-order rate equation (MOE), which is a combination of first- and second-order equations. The MOE model is expressed as: 14425



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affects the adsorption process. The plots of qt versus √t, for the obtained experimental data of different compounds in MDF2, are given in Figure 11. The plots are not linear, which indicates that more than one process is dominating the adsorption process.

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4. CONCLUSION The equilibrium and kinetics of adsorption of three thiophenic compounds in model fuel by the CNPs solution were studied. The results show that the synthesized CNPs remove BT, DBT, and DMDBT selectively from model diesel fuels in a short time. The presence of aromatics in the model diesel fuel only slightly reduced the desulfurization performance of CNPs. Higher adsorption selectivity for BT and DBT was due to their smaller size and the acidic surface of CNPs. The Langmuir− Freundlich model provides good correlation for adsorption data, and the adsorption kinetic data are best described by MPnO and MOE models. These equilibrium and kinetic results indicate that CNPs provide a heterogeneous surface for adsorption of thiophenic compounds. The results of the present study indicate that CNPs, which were prepared by a simple and green method and have very low ash content, are a good adsorbent for the desulfurization of fuel also in the presence of aromatics.

■

ASSOCIATED CONTENT

* Supporting Information S

CHN elemental analysis of the prepared CNPs and the obtained constants of adsorption isotherms and adsorption kinetic models. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +98 811 8282807. Fax: +98 811 8380709. E-mail: [email protected], [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank Martine Vanhamel and Ivo Feytongs for their technical assistance. R.N.F. acknowledges financial support from Hasselt University.

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REFERENCES

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Selective Desulfurization of Model Diesel Fuel by Carbon

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