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Desulfurization of model diesel fuel on activated carbon modified with iron oxyhydroxide nanoparticles: Effect of tert-butylbenzene and naphthalene concentrations. Javier Antonio Arcibar-Orozco, Jose Rene Rangel Mendez, and Teresa J Bandosz Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef400999g • Publication Date (Web): 06 Aug 2013 Downloaded from http://pubs.acs.org on August 9, 2013
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Desulfurization of model diesel fuel on activated
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carbon modified with iron oxyhydroxide
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nanoparticles: Effect of tert-butylbenzene and
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naphthalene concentrations.
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Javier A. Arcibar-Orozco,[a] J. Rene Rangel-Mendez*[a] and Teresa J. Bandosz*[b]
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[a]
Instituto Potosino de Investigación científica y tecnológica A.C., División de Ciencias
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Ambientales. Camino a la presa de San José 2055, Col. Lomas 4a sección, San Luis Potosí, SLP
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(MEX)
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[b]
The City College of New York, Department of Chemistry and CUNY Energy Institute.
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160 Convent Avenue, New York, NY 10031 (USA).
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ABSTRACT. Commercially activated carbon Filtrasorb 400 (F400) was modified with iron
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oxyhydroxide nanoparticles and tested, in a continuous process, as a diesel fuel desulfurization
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medium. Model diesel fuel (MDF) was prepared as a mixture of decane-hexadecane, with 20
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ppm
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4,6dimethyldibenzothiophene-DMDBT), and different concentrations of tert-butylbenzene and
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naphthalene. The calculated selectivities and adsorption capacities for DBT and DMDBT were
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analyzed at different concentration of aromatics. The results indicate that while the concentration
of
sulfurfrom
dibenzothiophenes
(dibenzothiophene-DBT
and
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of naphthalene greatly affects the adsorption capacity of both DBT and DMDBT, the
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concentration of tert-butylbenzene affects only DMDBT uptake. At low concentration of
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aromatics, iron-containing carbon performs better than unmodified carbon and further oxidized
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species were identified on the surface of the iron modified activated carbon. This indicates that
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iron is acting as an acidic center attracting basic DBT and DMDBT and as an oxidant, for the
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adsorbed species. With an increase in the content of aromatics in MDF, the difference in the
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desulfurization performance between the studied materials diminishes. After thermal
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regeneration of the iron containing sample, 95 % of the pore volume is recovered, and only a 10
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% loss in the DBT and DMDBT adsorption capacity is found.
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KEYWORDS. carbon • iron oxyhydroxide
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dibenzothiophene.
nanoparticles • desulfurization • liquid fuel •
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1.
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Sulfur dioxide (SO2) in air creates a major environmental problem which has increased in its
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importance in the last years. The main source of SO2 is the burning of fossil fuels, especially
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diesel oils.1,2 For this reason, since the year 2010 strict environmental regulations require sulfur
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content in diesel oil to be less than 50 ppm. Since such levels are difficult to achieve by
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hydrodesulfurization (HDS), a traditional desulfurization method, there is a need for developing
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new highly efficient processes that can reduce operational costs for desulfurization.3 In recent
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years, great interest has emerged in the removal of sulfur compounds from diesel oil by
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adsorption at ambient conditions.4
Introduction
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Several materials have been tested for diesel desulfurization, including zeolites,5-7 alumina,8
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silica based adsorbents9, and activated carbon.10-12 Activated carbon has been reported as a
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material having a high adsorption capacity for dibenzothiophenes, the predominant sulfur
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compounds in diesel.13 However, the main constraint of carbon is its lack of selectivity for sulfur
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compounds. Therefore, physical and chemical modifications of carbon surfaces are used to
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improve their adsorption capacities and selectivities.12,
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oxyhydroxide nanoparticles addition improves the adsorption capacity of sulfur species and
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participates in dibenzothiophene (DBT) and 4,6 dimethyldibenzothiophene (DMDBT) catalytic
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degradation.17 Important surface features identified as governing desulfurization are: high
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specific surface area, heterogeneous structure with micropores mainly less than 10 Å in
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diameter,17 and surface chemistry enhancing the specific interactions of S-rings compounds.
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However, for an efficient desulfurization, it is also important to consider the presence of
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secondary compounds naturally present in diesel oil.
14-20
It has been demonstrated that iron
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According to Xiao and co-workers (2012),21 typical diesel fuel contains paraffins (65-75 %),
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mono-aromatic hydrocarbons (15-25 %), di-aromatic hydrocarbons (5-10%), and other
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compounds at ppm level (nitrogenated, sulfur-containing, additives). All of these affect the
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adsorption performance of activated carbons depending on the nature of various interactions. The
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addition of metal species can change these interactions and thus alter the adsorption capacity and
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selectivity for DBT and DMDBT.17 The extent of these effects may depend on the composition
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and concentration of secondary components of diesel oil. Since mono- and di-aromatics have a
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major presence in the components an understanding of how these compounds interact with the
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adsorbents’ performance is the first step in developing highly efficient materials for diesel fuel
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desulfurization.
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Based on the above, the objective of this paper is to evaluate the performance of activated
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carbon modified with iron oxyhydroxide nanoparticles in the desulfurization of a model diesel.
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The nature of interactions between the surface of activated carbon and mono- and di-aromatics
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present in model diesel is analyzed based on a study of surface chemistry and porosity. These
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interactions are expected to have a major effect on the adsorption capacity and selectivity of
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desulfurization process.
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2.
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Activated carbon Filtrasorb 400 (F400), manufactured by Calgon, (ACF, particle size 250-500
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µm) was modified in order to deposit iron oxyhydroxide nanoparticles on its surface. The
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modification methods used has been described by Arcibar-Orozco and co-workers.22 500 mg of
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ACF were immersed with 10 mL of 3 M aqueous FeCl3 and 3M H3PO4 in a flask that was tightly
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closed and heated at 80 °C for 60 h. Then, the carbon was rinsed several times with deionized
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water until no iron, chloride or phosphorous was detected in the remaining solution. The
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modified carbon material has an iron content of 1-1.2%, as previously reported.22 This material
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was identified as ACM 1P.
Experimental Section
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2.1. Methods. Model diesel fuel (MDF) was prepared by adding tert-butylbenzene (TBB),
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and naphthalene –two chemicals that are often used to represent mono- and di-aromatics
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compounds-21,23-24 in a mixture of 1:1w/w decane-hexadecane that simulates paraffin
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compositions. Dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (DMDBT) were
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added in a concentration of 10 ppm-sulfur each. In order to evaluate the desulfurization effect of
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the model diesel composition, four different model diesel fuels were prepared with different
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aromatics concentrations. The details of the concentration of these MDFs are listed in Table 1.
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The selected concentrations correspond to Model Diesel Fuel with High Concentration
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(MDFHC), Low concentration (MDFLC) and two medium High and Low (MDFMH and
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MDFML) aromatics concentrations.
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Dynamic adsorption experiments were performed in a polyethylene column with a 4 mm
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inside diameter and a 60 mm length; the packed volume was 0.7 mL. The MDF was fed with a
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peristaltic pump at a constant flow of 5 mL/h. A contact time of 10 min was maintained. The
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concentrations of sulfur- and aromatics-compounds were analyzed in a Waters 2690 HPLC with
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a photodiode array detector. Separation of compounds was performed in a Lichrosphere RP-18
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column (100 Å, 5 µm, 4.0 mm×125 mm, EM Separations, Gibbstown, NJ). The gradient method
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utilized was: 10 µL of sample was injected at a constant flow of 1 mL/min; to separate
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naphthalene and tert-butylbenzene the starting composition was 80% methanol-20% H2O, after 7
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min the composition was changed to 100% methanol to separate DBT and DMDBT over 8 min,
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maintained for 5 min and then changed back to 80% methanol for 5 min to remove decane and
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hexadecane from the column. All adsorption experiments were carried out at 25 °C. After
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adsorption, materials were dried at 100°C for 24 h before analysis was begun.
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A thermal regeneration of exhausted sample was carried out at 500°C for 2 h under a constant flow of nitrogen of 100 mL/min. The regeneration samples are referred to with the suffix: R.
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2.2. Material characterization. Differential thermogravimetric curves (DTG) were obtained
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on a Thermogravimetric analyzer, SDT Q 600 from TA instruments. The heating rate was
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10°C/min to a final temperature of 1000°C. The analysis was performed under a constant flow of
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nitrogen (100 mL/min).
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The porosity and specific surface area of the activated carbons was determined from the
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nitrogen adsorption isotherm measured on Accelerated Surface Area and Porosimetry System
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ASAP 2020 (Micromeritics). Degassing of samples was carried out at 120°C until 10-5 Torr was
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reached. The specific surface area was determined using the BET method. Pore size distribution
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and volume of pores of different sizes (micropores Vmicro, mesopores Vmeso and total pore
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volumes Vt) were calculated using the density functional theory (DFT).27
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X-Ray Fluorescence analyses were performed in a SPECTRO Model 300T Benchtop Multi-
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Channel Analyzer from ASOMA Instruments, Inc. The acquisition conditions were: voltage 9.0
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kV, current 280 mA, and count time 100 s.
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The pKa distributions and proton binding curves were obtained from potentiometric titration
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measurements in a automatic titrator DMS titrino 881 (Metrohom). A 0.1 g of powdered sample
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was placed in the reaction vessel with 50 mL of 0.01 M NaNO3 .The solution was maintained at
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a constant stirring overnight. Next, the pH of solution was recorded and adjusted to pH 3 by
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adding 0.1 M HCl. Finally, the solution was titrated with 0.1 M NaOH. The solution was
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continuously purged with N2 to eliminate interference of atmospheric CO2. After titration, the
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proton binding curves Q(pH)26 and pKa distributions were calculated using the SAIEUS
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method27.
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In order to qualitatively determine the compounds present on the surface of carbons substrates
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after adsorption, an extraction with furan was carried out. 0.05 g of material were equilibrated
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with 5 mL of furan for 5 days. Then, the solution was filtered and injected into a mass
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spectrometer (MS) Q-TRAP 400 (Applied Biosystems). The MS parameters used are: ion spray
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voltage of 5500 V (highest sensibility); Collision energy and collision energy spread of 10 and
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40 V, respectively; the declustering potential was 80 V, Nitrogen gas was used as a curtain gas
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and a collision gas.
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3.
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The breakthrough curves for DBT and DMDBT for each model diesel fuel (MDF) are
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presented in Figures 1 and 2. For the sake of comparison, the adsorption capacities and
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selectivities were calculated from the breakthrough curves at an arbitrarily chosen equivalent bed
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volume (EBV), volume of treated effluent/volume of adsorbent packed of 65. At This value
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some materials start to be exhausted. For all MDFs used, the breakthrough points (the volume in
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which the concentration is first detected) of naphthalene and TBB could not be detected (only
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those for MDFLC are reported in supplementary information, Figure S1A), indicating a low
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affinity for adsorption of mono- and di-aromatics by these carbon surfaces and/or fast adsorption
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Results and Discussion
and saturation of the surface active centers.
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In all cases, DBT appears in the column outlet sooner than does DMDBT. Its smaller size than
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that of DMDBT (diameters of 5.5 Å28 and 6 Å for DMDBT11) probably result in faster diffusion.
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For
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MDFMH
