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Applied Catalysis A: General 456 (2013) 67– 74

Contents lists available at SciVerse ScienceDirect

Applied Catalysis A: General

j ourna l ho me page: www.elsev ier .com/ locate /apcata

atalytic oxidation-extractive desulfurization for model oil usingnorganic oxysalts as oxidant and Lewis acid-organic acid

ixture as catalyst and extractant

ongyan Songa,b, Jiajun Gaoa,c, Xingyu Chena,c, Jing Hea,b,∗, Chunxi Lia,c,∗

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, ChinaFaculty of Science, Beijing University of Chemical Technology, Beijing 100029, ChinaCollege of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China

r t i c l e i n f o

rticle history:eceived 3 December 2012eceived in revised form 15 February 2013ccepted 16 February 2013vailable online xxx

a b s t r a c t

The catalytic oxidation-extractive desulfurization (COEDS) performance for benzothiophene (BT-) anddibenzothiophene (DBT-) containing oils is studied at room temperature with respect to three inorganicoxysalt oxidants (KMnO4, K2Cr2O7, and NaClO4) under the catalysis of an acid mixture being composedof different kinds and amounts of Lewis acid (FeCl3, ZnCl2 or CuCl2) and organic acid (formic, acetic orpropionic acid). The results show that both KMnO4 and K2Cr2O7 are efficient oxidants, and their oxidiz-

eywords:atalytic oxidation extractiveesulfurizationenzothiopheneibenzothiopheneodel oil

ability can be adjusted effectively by the types and tuned slightly by the amounts of the Lewis acid used,and acetic acid is an appropriate organic acid. DBT is oxidized only to DBTO2, while BT is converted to twoto over six oxidized S-species depending on the Lewis acid used. 1-hexene presented in oil can slightlylower the S-conversion, but a promising desulfurization rate of over 92% for both BT and DBT can be alsoachieved under appropriate conditions.

© 2013 Elsevier B.V. All rights reserved.

ewis acid

. Introduction

Desulfurization is an essential process for removing organic sul-ur (S-) compounds from the fluid catalytic cracking (FCC) oils ton increasingly lower content, so as to meet the ever stricter cleanir regulations [1]. At present, catalytic hydrodesulfurization (HDS)s the mainstream industrial process, which is effective for thiolsnd sulfides, but less effective for some polycyclic thiophenic S-ompounds, such as benzothiophene (BT), dibenzothiophene (DBT)nd their alkyl-substituted derivatives. Such deficiency is intrin-ic due to the aromaticity of the thiophenic ring as well as theteric hindrance of the alkyl substitutes that make the cleavagef the C S bond and the accessibility of sulfur to the active sitesf the catalyst become more difficult. Therefore, the HDS processften requires severe conditions, e.g. high temperature (>573 K)nd pressure (>3 MPa) [2,3]. Till now, little progress has been made

n easing the HDS operation condition through development ofew catalysts [4], although extensive studies have been done. Iniew of the inherent difficulty of the HDS process for polycyclic

∗ Corresponding authors at: State Key Laboratory of Chemical Resource Engineer-ng, Beijing University of Chemical Technology, Beijing 100029, China.el.: +86 10 64410308; fax: +86 10 64410308.

E-mail addresses: hejing@mail.buct.edu.cn (J. He), licx@mail.buct.edu.cn (C. Li).

926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apcata.2013.02.017

thiophenic S-compounds, many alternative processes have beenattempted based on the principles of adsorption, extraction, com-plexation, oxidation, and their appropriate combinations, e.g. theS-Zorb process and the extractive catalytic oxidation desulfuriza-tion process. Among them, the oxidative desulfurization (ODS) ismore feasible as a succeeding process of HDS [5], since hydrogena-tion and oxidation are two complementary processes whereby theS-components that are difficult to be removed by hydrogenationcan just be removed easily by an oxidation process. This assumptionhas been justified by the facts, i.e., the hydrogenation activity of thearomatic S-compounds follows the order thiophenes > BTs > DBTs,whilst their oxidation activity is just the opposite [6]. Therefore,deep desulfurization of fuel oil may be achieved easily via a com-bined process of HDS and ODS by fully taking the advantages ofboth processes.

In comparison with HDS, all thiophenic S-components canbe oxidized under much milder conditions due to their strongreducibility and none necessity of the C S bond cleavage. Their oxi-dized products are corresponding sulfones or suloxides dependingon the nature of the S-compounds and the oxidation conditionsinvolved. Further, the oxidized products are of high polarity and

can be extracted efficiently by some polar solvents. Till now, manydifferent kinds of oxidants have been studied, e.g. ozone (O3) [7],oxygen (O2) [8], hydroperoxide (H2O2) [9–12], organic peroxides[13,14], and inorganic oxidants, etc. [15]. Among them, H2O2 is

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8 H. Song et al. / Applied Cata

he most widely used one for its strong oxidizability and greenerttributes, however, its demerits are also noteworthy. First, it is apto be self-decomposed at higher temperature or in the presencef some metallic catalysts, leading to a lower using efficiency. Sec-nd, the H2O2 solution is immiscible to the oil phase, the oxidationccurs only on the interface, and thus some surfactants are needed.inally, efficient oxidation of S-compounds is achievable only underhe help of some polyoxometalates catalysts and ionic liquids,esulting in a higher material cost [16–24]. In comparison with2O2, O2 shows much lower oxidative ability for the S-compoundsven under the help of catalysts or UV irradiation [25], and isess feasible let alone the risk of explosive limit. In contrast, somenorganic oxidants like KMnO4 and K2Cr2O7 are commonly usedxidants with good stability and lower cost. They show strong oxi-izability for the S-compounds in the presence of strong Brønstedcids like hydrochloric acid [26], while their oxidizability in neatarboxylic acid is negligible. This implies that their oxidizabilityepends on the Brønsted acidity involved, which can be adjusted bydding different kinds and/or amounts of Lewis acid to the organiccid. Moreover, the (organic acid+ Lewis acid) mixture is also an effi-ient extractant for the oxidized S-compounds. As a matter of fact,any Lewis acid type metal chlorides (FeCl3, ZnCl2, SnCl2, AlCl3

nd CuCl) are used for desulfurization, e.g. as solid particulates fordsorptive desulfurization [27], as additives of adsorptive materi-ls [28–30], as anions of ionic liquids for extractive desulfurization31,32], or as catalysts for oxidative desulfurization [33].

Based on the above analysis, we proposed a catalytic oxidation-xtractive desulfurization (COEDS) process by using inorganicxysalts as oxidant and Lewis acid-organic acid mixture as bothatalyst for sulfur oxidation and extractant for the oxidized S-omponents. The feasibility and influencing factors of the presentrocess are studied by using DBT or BT as the model thiophenic-compounds, KMnO4, K2Cr2O7, or NaClO4 as oxidant, FeCl3, ZnCl2r CuCl2 as the Lewis acid for adjusting the acidity of the organiccids (formic, acetic or propionic acid). Besides, the interference of-hexene, as a representative of the coexistent olefins in real oil, onhe COEDS performance is studied also.

. Experimental

.1. Chemical materials

The following chemicals are purchased from different compa-ies and used as received, specifically, DBT (>98%) and BT (>98%)

rom J&K Scientific Ltd.; Formic acid (>98%), propionic acid (> 99%)nd n-octane (AR grade) from Tianjin Guangfu Fine Chemical Indus-ry Institute; Acetic acid (> 99%) and KMnO4 (>99%) from Beijinghemical Works; K2Cr2O7 (>99%) from Tianjin Hongyan Chemicaleagent; Anhydrous FeCl3 (>99%) and ZnCl2 (>98%) from Shantouilong Chemical Ltd.; CuCl2 (>98%) and NaClO4 (>98%) from Aladdinhemistry Ltd.; and 1-hexene (>98%) from Alfa Aesar.

The oxidized products of BT and DBT, i.e. BTOx and DBTOx, arerepared by oxidizing BT and DBT model oils at 298 K using K2Cr2O7s oxidant, acetic acid-ZnCl2 mixture as the catalyst system underhe following conditions, S/Oxidant = 2:1 and S/ZnCl2 = 5:1 in moleatio, acetic acid/oil = 3 ml/10 g, reaction time 60 min. Their separa-ion and purification process is referred to Section 2.5.

.2. UV and IR spectrum of acetic acid in presence of Lewis acids

To explore the interaction between Lewis acid and acetic

cid, definite amount of anhydrous FeCl3 or ZnCl2 is added intolacial acetic acid with magnetic stirring for 60 min. And thenhe supernate of acetic acid-FeCl3 and acetic acid-ZnCl2 mixtures obtained respectively via centrifugation, and characterized by

: General 456 (2013) 67– 74

UV–vis spectrophotometer (Labtech, Beijing) and FT-IR (NicoletNexus, 8700 IR spectrometer on the 400–4000 cm−1 range). Theirresults are compared with that of anhydrous acetic acid.

2.3. Oxidative desulfurization experiments in model oil

The model oils used here are binary solutions of n-octane and BT(or DBT) with their initial S-content all being 1000 ppmw (�g/g). Towhich certain amounts of oxidant (K2Cr2O7, KMnO4 or NaClO4) andLewis acid (FeCl3, ZnCl2 or CuCl2) are added directly, and then startstiming as soon as specific amount (3 ml/10 g oil) of organic acid(formic acid, acetic acid, or propionic acid) is added and mixed withmagnetic stirrer. All the experiments are done at room temperature(298 K). 10 g of model oil is used for most of the experiments, and30 g of model oil is used only for the experiments that need aboutfive samples (about 0.2 mL for each) for studying the S-conversionagainst time. The oil samples to be analyzed are washed by equalvolume of water to eliminate the remaining organic acids. To inves-tigate the oxidation selectivity, model oils containing 12.5 wt% of1-hexene are prepared with their S-content all being 1000 ppmw.After experiments, the used inorganic salts are reclaimed by evap-orating the acetic solution.

2.4. S-conversion and total S-content analysis

The S-conversion with respect to BT or DBT in the model oilsis determined in terms of the concentration difference of BT (orDBT) before and after experiment. The concentration of BT (orDBT) is analyzed by high performance liquid chromatography(HPLC, Shimadzu 10A-VP, equipped with UV-vis detector and aC-18 column; wavelength = 251 nm for BT and 310 nm for DBT;methanol:water 8:2 for BT and 9:1 for DBT; flow rate= 1 mL/min).The relative errors of the analysis are within 2% (100–1000 ppmw),4% (10–100 ppmw), 10% (1–10 ppmw) and 30% (<1 ppmw), respec-tively. The total S-content in the oxidized oil phase includingall kinds of S-species is determined by sulfur and nitrogen ana-lyzer (KY-3000SN, Jiangyan Keyuan Electronic Instrument Ltd,China).

2.5. Analysis of the oxidized S-products

All the S-species in the oil phase is analyzed directly by HPLC.In fact, most of the oxidized S-species are dissolved in the acidphase due to their high polarity. To obtain the oxidized S-speciesin the acid layer, the oil phase in the flask is firstly poured outby decantation, and then specific volumes of CH2Cl2 and dilutehydrochloric acid are added in turn and shake for a while, formingtwo new layers, viz. the CH2Cl2 layer containing all kinds of organicS-species, and the aqueous layer containing all water soluble saltsand acid. The CH2Cl2 layer is washed four times by equal volumeof dilute hydrochloric acid to remove all inorganic salts and aceticacid. The oxidized sulfur derivatives are obtained as a solid pow-der by evaporating the CH2Cl2 layer completely, and then analyzedby FT-IR.

2.6. Solubility of acetic acid in oil

For measuring the solubility of sole HAc in oil, 10 g octane and6 ml glacial HAc is added to a glass vial, stirred magnetically for30 min at 298.15 K, and then put aside 60 min for a clear phase sep-aration. The saturated content of HAc in the oil layer is determined

via acid-base titration. For measuring the solubility of HAc in thepresence of coexistent inorganic components, viz. Lewis acid andoxidant, 10 g octane and 3 ml glacial HAc is used, and the amountof Lewis acid and oxidant is determined according to mole ratio of

H. Song et al. / Applied Catalysis A: General 456 (2013) 67– 74 69

Table 1Extractive desulfurization rate (%) of oil and S-distribution factors (D) between model oil and different extractants (or Lewis acid adsorbents).

Oil Extractant or adsorbent S-removal rate (%)a Db

10 g DBT oil, 1000 ppmSc FeCl3 solidd 33.1% 19.610 g BT oil, 1000 ppmS FeCl3 solid 47.0% 35.010 g DBT oil, 1000 ppmS ZnCl2 solidd 0% 010 g BT oil, 1000 ppmS ZnCl2 solid 0% 010 g DBT oil, 1000 ppmS 6 ml HAc 8.7% 0.6410 g BT oil, 1000 ppmS 6 ml HAc 9.4% 0.6910 g DBT oil, 1000 ppmS 3 ml (HAc + ZnCl2) 5.0% 0.5810 g BT oil, 1000 ppmS 3 ml (HAc + ZnCl2) 4.4% 0.5010 g DBT oil, 1000 ppmS 3 ml (HAc + FeCl3) 7.7% 0.7210 g BT oil, 1000 ppmS 3 ml (HAc + FeCl3) 5.8% 0.5410 g DBTOx model oil, 42.6 ppmS FeCl3 solid 100% /10 g BTOx model oil, 39.3 ppmS ZnCl2 solid 100% /10 g oil 6 ml HAc solution of DBTOx, 1000 ppmS 97.3% 243.010 g oil 3 ml (HAc + ZnCl2) solution of DBTOx, 1000 ppmS 96.5% 307.110 g oil 3 ml (HAc + FeCl3) solution of DBTOx, 1000 ppmS 96.9% 271.010 g oil 6 ml HAc solution of BTOx, 1000 ppmS 95.8% 153.610 g oil 3 ml (HAc + ZnCl2) solution of BTOx, 1000 ppmS 95.8% 252.410 g oil 3 ml (HAc + FeCl3) solution of BTOx, 1000 ppmS 96.2% 220.6

a S-removal rate is the relative sulfur amount removed from oil against the total sulfur in the system assuming all sulfurs are dissolved in oil initially, in %.b S-distribution factor refers to the ratio of S-content in acetic acid (or Lewis acid adsorbent) against S-content in oil for the extraction (or adsorption) system, in (g-S g−1-

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Ac)/(g-S g−1-oil) or (g-S g−1-adsorbent)/(g-S g−1-oil).c ppmS is the sulfur content in parts per million, 10−6g g−1.d The amount of Lewis acid used is 5-fold moles of sulfur in 10 g 1000 ppmS mod

/Lewis acid = 1:5 and S/oxidant = 1:2 as used in the COEDS processor 10 g of 1000 ppmS model oil.

.7. Extractive performance of HAc solutions for BT and DBT andheir oxidized products

Before extraction experiment, BT and DBT model oils as wells acetic solution of BTOx and DBTOx are prepared with their S-ontent all being 1000 ppm since the oxidized sulfurs are virtuallynsoluble in octane but soluble in HAc. In a typical extraction exper-ment, 10 g model oil (or octane) and definite amount of Lewis acidre added to a glass vial, followed by the addition of 3 ml aceticcid (or acetic solution of BTOx or DBTOx). The resulting mixtures stirred vigorously with a magnetic stirrer at 298 K for 60 min,nd then put aside 60 min for phase settling. The equilibrated oilamples to be analyzed are washed by equal volume of water toliminate the remaining HAc, and their S-content is analyzed by theulfur and nitrogen analyzer, while the S-content in the acid phases calculated via mass balance with the known solubility of HAc inctane at varying conditions. However, when Lewis acid is absentn the experiment, 6 ml HAc (or HAc solution of BTOx or DBTOx) is

ixed with 10 g model oil (or octane) so as to form a liquid-liquidquilibrium, which has a comparable acid phase volume (about

ml) with that in the presence of Lewis acid. On this basis, the-removal rate of model oils and the corresponding distributionactor (D) of sulfur are obtained.

. Results and discussion

.1. Extractive desulfurization performance of HAc andAc-Lewis acid mixtures

To discriminate the contribution of catalytic oxidation andxtraction for the sulfur removal in the present COEDS process,xtractive performance of different HAc solutions for BT andBT and their oxidized products are measured in terms of sul-

ur removal rate and distribution factor (D) between HAc and oil.

s seen from Table 1, both HAc and (HAc + Lewis acid) mixturehow weak extractive ability for BT and DBT oils with their D val-es around 0.6, but show excellent extractive ability for BTOx andBTOx with their D values ranging from 150 to 300.

i.e. 0.253 g FeCl3 and 0.213 g ZnCl2.

Regarding the desulfurization ability of sole Lewis acids, onlyFeCl3 shows a moderate adsorptive ability for BT and DBT. And thedesulfurization performance for BT is higher than for DBT, whichcan be explained by the hard soft acid and base (HSAB) theory [27].However, the sole Lewis acids show excellent adsorptive ability forthe oxidized sulfurs (BTOx and DBTOx) due to the much strongercomplexation between Lewis acid and oxygen atom of the oxidizedsulfurs. For example, the saturated content of BTOx and DBTOx inoil can be removed completely by both FeCl3 and ZnCl2 with smallamount of usage.

Despite the varying adsorptive ability of sole Lewis acids forBT and DBT as well as their oxidized derivatives, the extractivedesulfurization performance of HAc and (HAc + Lewis aid) mixturesshows only a little difference, because the dissolved Lewis acid haslost its adsorptive ability, and just results in a stronger polarityand acidity of HAc due to its complexing interaction with HAc.Further, as noted from the variation of the distribution factors ofsulfurs, adding Lewis acid can slightly decrease the extractive abil-ity of HAc for BT and DBT, but increase the extractive ability forBTOx and DBTx, since the increasing polarity of HAc is unfavorablefor the extraction of non-polar BT and DBT but favorable for highpolar BTOx and DBTOx. In short, HAc and its Lewis acid mixturesshow very low extractive performance for BT and DBT oils, but areexcellent extractants for BTOx and DBTOx. Therefore, the oxidizedsulfurs formed in the COEDS process can be removed instantly viaefficient extraction of HAc solution.

3.2. Catalytic performance of acetic acid-FeCl3 system

As a trial experiment, FeCl3-acetic acid solution is used for theCOEDS of model oils containing DBT (or BT) using K2Cr2O7 as oxi-dant at room temperature. It is found that 2-fold Cr(VI) againstsulfur (in mole ratio) has shown very good oxidative performancewith S-conversion above 99% for DBT and 95% for BT in 20 min,while the contrast S-conversion is only 9.6% for DBT and 0.5% forBT in the absence of Lewis acid, which highlights the catalysis ofthe HAc-Lewis acid mixture for the oxidation of thiophenic sulfurs.

The result is encouraging in terms of both desulfurization rate andability under such mild conditions, which along with the low cost ofraw materials make the present process attractive for practical use.Besides, the reactivity of the two S-compounds follows the order

70 H. Song et al. / Applied Catalysis A: General 456 (2013) 67– 74

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ig. 1. DBT conversion at varying amounts of Lewis acids. Time= 60 min; Cr:S = 2:1;cetic acid/oil = 3 ml/10 g.

BT > BT, as observed in many ODS processes [15,20,24,34], whichs in line with the higher electron density on S-atom of DBT [9].

Acetic acid is a weak Brønsted acid with very low self disasso-iation constant. So the excellent catalysis of the acetic acid-FeCl3ixture is closely associated with its stronger acidity than the neat

cetic acid due to the complexing interaction between FeCl3 and-atom of acetic acid [35], which weakens the O-H bond and leads

o a higher dissociation and Brønsted acidity. Thereby, the oxida-ive activity of Cr(VI) can be exhibited under the catalysis of suchn acid mixture system.

.3. Influence of Lewis acid on sulfur oxidation

.3.1. Catalytic oxidation and extractive removal of DBTBesides FeCl3, other Lewis acids, e.g. AlCl3, ZnCl2 and CuCl2,

ay be also used as an additive to adjust the Brønsted acidity ofcetic acid. Here, AlCl3 is the strongest Lewis acid but not applica-le since its interaction with acetic acid is so strong that produceydrochloride smog as soon as they come into contact. In ordero investigate the influence of the Lewis acid and its amount onhe catalysis of acetic acid for the oxidation of DBT, three typesf acid mixtures, i.e. acetic acid-FeCl3 (or ZnCl2 or CuCl2) mixture,re used in the experiments under the following conditions, i.e. 2-old Cr(VI) against sulfur, fixed ratio of acetic acid to oil (3 ml/10 g),nd varying amounts of Lewis acid at 298 K. As presented in Fig. 1,nhydrous acetic acid shows very low catalysis for the conversionf DBT, however, its catalytic activity enhances greatly in the pres-nce of Lewis acid. Besides, DBT conversion increases drasticallyith the amount of Lewis acids first and then levels off. The S-

onversion in both FeCl3-acetic acid and ZnCl2-acetic acid exceeds9.5% with 5-fold Lewis acid usage. Moreover, the catalytic activityf the acid mixtures follows the order FeCl3-acetic acid > ZnCl2-

cetic acid » CuCl2-acetic acid, which is consistent with the acidityrder of the corresponding Lewis acids.

The interaction between Lewis acid (MCln) and Brønsted acidacetic acid) is described in Scheme 1. Obviously, the Brønsted

Scheme 1. Interaction between inorganic Lewis acid

Fig. 2. UV spectrum for the Lewis-Brønsted acid complexes.

acidity of the (Lewis acid + acetic acid) mixtures will increasewith the increasing acidity of Lewis acid as a result of a strongercomplexation between MCln and acetic acid. Thereby, the best cat-alytic activity of FeCl3-acetic acid may be ascribed to its strongestBrønsted acidity among three acid mixtures studied here. To iden-tify such interaction and its variation between different Lewis acidsand acetic acid, the UV–vis spectrums with respect to FeCl3-aceticacid, ZnCl2-acetic acid and neat acetic acid are recorded in Fig. 2.As shown from the figure, the absorption band of acetic acid broad-ens with the addition of Lewis acid, giving rise to a red shift of thespectrum. The absorption of FeCl3-acetic acid is wider than that ofZnCl2-acetic acid, indicating a stronger interaction between FeCl3and acetic acid. Moreover, their UV–vis spectrums are consistentwith their appearance, i.e. FeCl3-acetic acid is a red solution, whileZnCl2-acetic acid is colorless. Besides, the IR spectrum for the abovesystems is also recorded. It is noted that their spectrum appearanceis of little difference, but the adsorption strength for FeCl3- andZnCl2-acetic acid is stronger than that for neat acetic acid in therange of wavenumber 3600–2500 cm−1, which suggests a weakerO-H bond and a stronger bond vibration in carboxyl as a result ofthe complexation between O-atom and FeCl3 or ZnCl2.

To detect all the oxidized products of DBT and the total S-contentresidue in the oil phase, the oxidized oil samples are analyzed byHPLC and sulfur and nitrogen analyzer, respectively. The results arepresented in Fig. 3 and compared with the fresh model oil. It is seenthat the oxidized oil phase with FeCl3-acetic acid or ZnCl2-aceticacid shows just one peak at retention time 2.2 min, suggesting a soleoxidized S-specie. Moreover, the total S-contents in the oxidizedoil phase are very low, specifically, 22.8 ppmw for FeCl3-acetic acidsystem and 16.2 ppmw for ZnCl2-acetic acid system, indicating thatthe complex acetic acid solution is an efficient extractant for theoxidized DBT. In order to identify this oxidized S-species, solid pow-ders from the acid mixture solution are obtained via the procedure

as described in Section 2.5, and analyzed by IR spectrum. The resultsindicate that the sole oxidized product of DBT is DBTO2, since onlysulfone peaks are found at about 1290 and 1165 cm−1.

MCln and acetic acid, which is the organic acid.

H. Song et al. / Applied Catalysis A: General 456 (2013) 67– 74 71

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Fig. 5. S species and total S content of the oxidized BT model oil with different acid

ig. 3. S species and total S content of the oxidized DBT model oil with different acidixtures. (a) Fresh oil; (b) oxidized oil phase with FeCl3-acetic acid; (c) oxidized oil

hase with ZnCl2-acetic acid.

.3.2. Catalytic oxidation and extractive removal of BTSimilar to Section 3.3.1, we also studied the COEDS performance

f three Lewis acid-acetic acid solutions for the BT-containingodel oil at varying conditions. As shown in Fig. 4, the BT conver-

ion increases first with the growing amount of Lewis acids used,nd then levels off, and the catalytic activity of the acid mixtureolutions follows the same order as observed for DBT conversion.or example, the BT-conversion in FeCl3-, ZnCl2- and CuCl2-aceticcid systems is 97.8%, 57.3% and 22.1%, respectively under 5-foldsage of Lewis acid.

The S-species distribution and the total S-content in the oxidizedil phase are analyzed also, as shown in Fig. 5. It is noted that thexidized products of BT are much more complex than that of DBT,epending on the Lewis acid used, for example, over six oxidizederivatives are found in the FeCl3-acetic acid solution, and neitheruloxide (BTO) nor BTO2 is the predominant S-species. The resultsndicate that all the S-species detected remain an aromatic ring,therwise they are not observable at UV-wavelength of 251 nm in

he HPLC analysis. The complicated S-species may be originatedrom a serial deep oxidation of BT and even cleavage of the thio-henic ring [25], forming corresponding sulfoacid, carboxyl acid,

ig. 4. BT conversion at varying amount of Lewis acids. Time = 60 min; Cr:S = 2:1;cetic acid/oil = 3 ml/10 g, T = 298 K.

mixtures. (a) Fresh oil; (b) oxidized oil phase with FeCl3-acetic acid; (c) oxidized oilphase with ZnCl2-acetic acid.

alcohols or ketones under strong acidity conditions. In contrast, theoxidized products in ZnCl2-acetic acid are only sulfone and sulox-ide, as shown in Fig. 5c, which indicates that only the S-atom isoxidized due to the relatively weak acidity of the ZnCl2-acetic acidsolution that restricts the oxidative ability of the Cr(VI) oxidant.The results suggest that the oxidizability of an inorganic oxidantcan be adjusted effectively by using different kinds of Lewis acidsand tuned slightly by their using amounts for a specific carboxylicacid.

3.4. COEDS performance in different oxidation systems

3.4.1. S-conversion with different organic acidsBesides acetic acid, the applicability of its homologues, i.e.

formic acid and propionic acid, is also studied under the sameconditions as described previously and compared with aceticacid. Fig. 6 presents the experimental results with respect to theS-conversion of BT and DBT, respectively, using the same oxidantCr(VI) and Lewis acid FeCl3 but three different organic acids. Asshown from the figure, both acetic acid and propionic acid showexcellent COEDS performance with S-conversion above 92% in20 min, while formic acid does not. Besides, propionic acid shows

a little superior desulfurization performance to that of aceticacid, especially for BT, which contradicts to its weaker Brønstedacidity, but coincides with its higher extractive ability for BT andDBT due to its lower polarity and higher dispersive interaction.

72 H. Song et al. / Applied Catalysis A: General 456 (2013) 67– 74

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Fig. 6. Comparison of the S conversion rate with different organic acids w

his implied the influence of some secondary factors besides therønsted acidity of the acid mixture and the assumed interfacialxidation mechanism. In effect, bulk oxidation also occurs in thecidic phase for the extracted thiophenic sulfurs. And a higher BToncentration in propionic acid means a faster bulk oxidation ofT in the acid phase, which as a supplementary oxidation processay compensate the negative effect of its lower Brønsted acidity,

nd even reverse the overall trend under specific conditions. Thebove results may be a compromise effect of the following factors,iz. the intrinsic Brønsted acidity and the oxidability of the organiccids. As a matter of fact, the intrinsic Brønsted acidity followshe order formic > acetic > propionic acid, as indicated by theirissociation constants of aqueous solution at 298 K being 3.751,.756, and 4.874, respectively in pKa [36]. However, formic acid is atrong reducer, it can be oxidized also, leading to the consumptionf oxidant Cr(VI) and deterioration of the ODS performance. Inhort, formic acid is not feasible, and both acetic and propioniccid are appropriate organic acids for the present COEDS process.

.4.2. S-conversion and removal with different Lewiscid-organic acid combination

The Lewis acids (FeCl3 and ZnCl2) and the organic acids (aceticnd propionic acid) are shown to be good candidates for making thecid mixtures system. Here, we further compare the desulfurizationerformance in the resulting four acid mixtures in terms of S-onversion and S-removal, respectively, for BT- and DBT-containingodel oils. As shown in Fig. 7, with respect to the conversion and

emoval of DBT, three acid mixture systems, viz. FeCl3-acetic acid,

eCl3-propionic acid and ZnCl2-acetic acid, show excellent per-ormance with their S-conversion and S-removal all above 97%,hile ZnCl2-propionic acid as a combination of weak Lewis acid

nd weak organic acid shows the lowest desulfurization ability. In

Fig. 7. Comparison of S-conversion and S-removal with different acid mixture.

e other conditions fixed. Cr:S = 2:1; Fe:S = 5:1; oil 30 g; organic acid 9 ml.

comparison with DBT, their desulfurization performance for theBT-containing oil shows two noteworthy trends. First, the catalyticoxidation activity of the acid mixtures in terms of S-conversion (orS-removal) follows the order FeCl3-acetic acid ≈ FeCl3-propionicacid > ZnCl2-acetic acid > ZnCl2-propionic acid, which is consistentwith their overall Brønsted acidity and accordingly the oxidationability of the oxidant exhibited. Besides, the S-removal is alwayslower than the S-conversion, for example, the S-conversion forFeCl3-acetic acid system is 97.8% while the S-removal is only 85.8%.The results indicate that some oxidized S-species still remain in theoil phase due to their lower extractability by the organic acid used,which is reasonable considering that the oxidized S-species of BTare much complicated than that of DBT, as indicated in Fig. 3 andFig. 5, respectively.

3.4.3. S-conversion with different oxidantsIn view of the workability of K2Cr2O7 under the catalysis of

FeCl3-acetic acid, other strong oxidants like KMnO4 and NaClO4may be also applicable under the same acid mixture system.Thereby, experiments are conducted for the catalytic oxidation ofBT and DBT-containing oil using different inorganic oxidants underfixed other conditions. The results in terms of S-conversion arepresented in Fig. 8. It is shown that KMnO4 is as good as K2Cr2O7 forthe efficient oxidation of both DBT and BT with their S-conversionall above 99.6%. In contrast, NaClO4 shows much lower oxidativeability, and the S-conversion is only 27.0% for DBT and 19.4% forBT. The bad performance of NaClO4 in FeCl3-acetic acid solutionmay be attributed to its inherent weak oxidizability and lower

solubility in the acid mixture. In effect, K2Cr2O7 and KMnO4 arewell known strong oxidants and more soluble in the acid mixture,while NaClO4 is only a mild oxidant and sparsely soluble in the acidphase. In the present COEDS process, the oxidants are converted to

Time = 60 min; Cr:S = 2:1; Lewis acid:S = 5:1; organic acid/oil = 3 ml/10 g.

H. Song et al. / Applied Catalysis A: General 456 (2013) 67– 74 73

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However, the dissolved HAc can be reclaimed conveniently and effi-ciently by lowering the oil temperature to about 273 K, whereby99.5% of the HAc is precipitated from oil as crystalline particles, and

Table 2Solubility of HAc in octane at different temperature and varying amount of coexis-tent inorganic salts as used in the COEDS process.

HAc or HAc mixture (mass ratio)a T (K) HAc solubility,g/100 g-oil

100%HAc 298.15 45.0100%HAc + CuCl2 (3:0.212) 298.15 29.7100%HAc + CuCl2+ K2Cr2O7 (3:0.212:0.18) 298.15 27.0100%HAc + ZnCl2 (3:0.213) 298.15 20.8100%HAc + ZnCl2 + K2Cr2O7 (3:0.213:0.18) 298.15 19.2100%HAc + FeCl3 (3:0.253) 298.15 18.5100%HAc + FeCl3 + K2Cr2O7 (3:0.253:0.18) 298.15 16.8

ig. 8. Oxidative activity of different oxidants in FeCl3-acetic acid. Time = 60 min;r, Mn or Cl:S = 2:1; Fe:S = 5:1; acetic acid/oil = 3 ml/10 g.

heir corresponding reductive salts being stable in acidic medium,.e. K2Cr2O7 and KMnO4 are converted to CrAc3, MnAc2 and KAc.his is justified by the color variation of the acid mixture before andfter the reaction when oxidants are used insufficiently in com-arison with the DBT amounts, i.e. from carmine to incarnadineor KMnO4 and from brown to blue for K2Cr2O7.

.5. Oxidative selectivity of Cr(VI) in the presence of olefin

In practice, the FCC oil is firstly treated with catalytic hydro-enation for controlling the amount of olefins and S-compounds,owever, some amounts of olefins ranging from 10% to 30 wt%till remain in the treated oil for various purposes, which mighte oxidized competitively in the succeeding COEDS process, andhus deteriorate the oxidative selectivity of the oxidant for thehiophenic S-components. Here, we prepare two model oils con-aining 12.5 wt% of 1-hexene with their original S-content being000 ppmw for BT- or DBT oils. Such oils are used to study the

nfluence of the coexistence olefins on the oxidative selectivity of2Cr2O7 in an acid mixture system. The S-conversion with respect

o BT and DBT oils under the catalysis of FeCl3-acetic acid is pre-ented in Fig. 9. As shown from the figure, the 1-hexene presentn oil always lowers the S-conversion of BT (or DBT), indicating

competitive oxidation with thiophenic S-compounds. However,he oxidative selectivity is quite high especially for DBT oil, andhe desulfurization ability for BT oil increases with the increasingmount of Lewis acid. This, however, does not impose extra costince the Lewis acid herein is non-consumptive but an additive fordjusting the acidity of acetic acid. Besides, the overall oxidativeelectivity of DBT is better than that of BT at fixed other condi-ions due to its higher oxidability. The oxidized amount of alkenes expected to be within the range of tens to hundreds ppm levelccording to the limited lowering in desulfurization rate at opti-um conditions. Further, the oxidized products of alkene may be

easonably assumed as some oxygenates (ketone, acids, alcohols)ith higher polarity, and thus predominantly present in acetic acidhase rather than in oil phase. This assumption is justified by theon-observable impurity peaks in the desulfurized oil.

.6. Solubility of acetic acid in oil and its reclamation

As a promising extractant, HAc may be used in the COEDS pro-ess, however, its dissolution contamination to the treated oil andeclamation should be considered also. Therefore, solubility dataf HAc in octane model oil at different temperature and varying

Fig. 9. Oxidative desulfurization selectivity with the using amount of Lewis acid inthe acid mixture. Time = 60 min; Cr:S = 2:1; acetic acid/oil = 3 ml/10 g.

amount of coexistent inorganic salts as used in the COEDS pro-cess are measured using acid-base titration method. As shown fromTable 2, the solubility of neat acetic acid in gasoline is as high as 45 gat 298.15 K, however, it decreases greatly with the addition of inor-ganic salts, i.e. Lewis acid and oxidants, due to their insolubility inoil and strong affinity with HAc. For example, the HAc solubilityin octane decreased from 45 g for glacial HAc to 18.5 g for HAc-FeCl3 mixture and further to 16.8 g for HAc-FeCl3-K2Cr2O7 mixture.

100%HAc 273.15 0.20

a The mass ratio is determined based on the sulfur content in 10 g of 1000 ppmSmodel oil according to mole ratio of S/Lewis acid = 1:5 and S/oxidant = 1:2, and 10 goil for 3 ml HAc.

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he remaining content is only about 0.2% in mass percent, which cane reclaimed easily via water washing.

.7. Recycling of this oxidative desulfurization system

In practical process, definite amount of Lewis acid and oxidantolids are firstly dissolved in organic acid, forming the requiredxidative acid solution. The resulting acid complex reacts with oiln a stirred tank reactor (STR) at specific temperature for a periodf time and then pumped into a settling tank for phase separa-ion. The upper oil phase is transferred to a reactor and cooled tobout 273 K under stirring, whereby the dissolved acetic acid cane reclaimed efficiently via crystallization and filtration, followedy a water washing process to remove the low acid residue, ca. 0.2 ger 100 g oil and get the desulfurized oil product. The lower acidolution can be reused when the oxidized thiophenic sulfurs areemoved. For this purpose, the organic acid and water is first sepa-ated from the solution via distillation under reduced pressure, andhe oxidized sulfurs as heavy components coexisted with the solidalts can be separated through extraction with benzene. The finalnhydrous solid mixture containing Lewis acid, KAc, CrAc3, can besed repeatedly, since the presence of KAc and CrAc3 shows little

nfluence on the oxidizing ability of the system, on the contraryhey help to decrease the solubility of organic acid in oil.

. Conclusion

A new and efficient COEDS process is proposed, where somenorganic oxysalts like K2Cr2O7 and KMnO4 are used as oxidants,nd the acid mixtures of Lewis acid-organic acid are used both asatalyst for the oxidation of BT and DBT and as extracting solventsor the oxidized S-species. As strong oxidants, K2Cr2O7 and KMnO4how negligible oxidative ability for BT or DBT-containing oils ineat acetic acid, however, their oxidizability can be adjusted effec-ively by different kinds of Lewis acids and tuned slightly by theirsing amounts for a specific carboxylic acid. The catalytic activityf the Lewis acid-organic acid mixtures is closely related to theirverall Brønsted acidity arising from the intrinsic acidity of bothewis acid and organic acid and their complexing interaction. Theatalytic activity follows the order of FeCl3- > ZnCl2- > CuCl2-aceticcid, and ZnCl2-acetic acid > ZnCl2-propionic acid. In the presentOEDS process, DBT is oxidized to DBTO2, while BT is convertedo two (BTO and BTO2) to over six oxidized S-species dependingn the Lewis acid used, and the higher the acidity of the acid mix-ure, the more complicated the oxidized S-derivatives. The olefinsresent in oil can cause competitive oxidation with thiophenic sul-urs, and a higher Brønsted acidity of the acid mixture is helpful

or improving the desulfurization performance. The overall desul-urization performance of the present process can be adjusted byarying the Lewis acid, organic acid, and oxidant, respectively, andorthy of further study.

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: General 456 (2013) 67– 74

Acknowledgement

The authors are grateful for the support from the FundamentalResearch Foundation of Sinopec (Grant No. X505015).

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