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Letters to the Editor

Selective dibenzothiophene adsorption on modified activated carbons

Yongxing Yang a,b, Hongying Lu a,b, Pingliang Ying a, Zongxuan Jiang a,*, Can Li a,*

a State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, Chinab Graduate Schools of the Chinese Academy of Sciences, Beijing 100039, China

Received 2 September 2007; accepted 17 October 2007Available online 25 October 2007

World refining industry is facing new and more strin-

gently environmental regulations on the sulfur content indiesel fuel. Hydrodesulfurization (HDS) has been widelyused in current industry to remove the sulfur from the die-sel fuel. However, HDS has difficulty in reducing therefractory sulfur-containing compounds such as dibenzo-thiophene (DBT) and its derivatives to an ultra low level.Therefore, new strategies [14] for desulfurization havebeen explored to meet the urgent needs to produce cleanerdiesel fuel in current years. Among them, selective adsorp-tion is regarded as one of the most promising strategies toremove the refractory sulfur-containing compounds fromdiesel fuels. Carbon materials, particularly activated car-bons (ACs) are potential adsorbents for the removal of

the sulfur-containing compounds in the fuels [57]. How-ever, ACs usually have poor adsorption capacity and selec-tivity for the refractory sulfur-containing compounds.Therefore, it is still a challenging subject to increase theadsorption capacity and selectivity for the sulfur-contain-ing compounds on ACs to meet the industrial demands.In the present work, a series of modification approachessuch as steam and concentrated H2SO4 have been em-ployed to tune the textural structure and surface chemicalproperties of AC. It is found that the adsorption perfor-mance of the modified ACs for DBT from the model dieselis greatly improved.

The commercial granulated AC (Beijing GuanghuaWoods Plant, China) was used in this work. The carbonwas dashed with concentrated hydrochloric and hydroflu-oric acids and then dried in air at 120 C overnight. Thissample was denoted as AC. The AC sample treated bysteam at 900 C for 25 min was denoted as ACW900. The

AC sample treated by concentrated H2SO4 (96%) at

250

C for 4 h was denoted as ACS250. For a combinationtreatment by steam and H2SO4, the AC sample was firsttreated by steam at 900 C for 25 min and subsequentlytreated by concentrated H2SO4at 250 C for 4 h. The sam-ple obtained was denoted as ACWS. For further heat treat-ment, ACWS was heated at a rate of 10C/min underflowing nitrogen from room temperature to 900C andwas kept at 900 C for 12 h. The sample obtained was de-noted as ACWSN. All samples were characterized by N2sorption and base titration (Boehms method). The dy-namic adsorption tests of DBT on the modified ACs werecarried out using a fixed-bed flow reactor at ambient tem-perature and pressure. A model diesel fuel containing DBT

in heptane (sulfur concentration: 220 mg/dm3) was used.Adsorption operating conditions were as follows: flow rate:10 cm3/h; amount of adsorbents: 0.3 g. Sulfur content ofthe effluent was analyzed by microcoulometry every30 min until saturation was achieved (i.e. the effluent sulfurconcentration was equal to the feed sulfur concentration).

Table 1 lists the physicochemical properties of differentsamples. The steam treatment at 900 C could greatly en-hance the porosity of AC. The microporous volumes (Vmic)and total pore volumes (Vtotal) of ACW900are increased by50% and 83% as compared with the unmodified AC,respectively. The steam treatment at 900C hardly pro-

duces the surface acidic-oxygenated groups, because thesegroups are unstable at high temperature. The decrease inVmic of ACS250 is attributed to the opening of microporesthrough the reaction between the AC and H2SO4 at250 C. However, the H2SO4treatment can greatly increasethe surface acidic-oxygenated groups. The two-step modifi-cation by steam at 900 C and subsequently by H2SO4 at250 C can produce the maximum amount of theseacidic-oxygenated groups on AC surface. ACWS is post-treated by N2 at 900 C to remove the acidic-oxygenatedgroups and keep the textural structure unchanged. By

0008-6223/$ - see front matter 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.carbon.2007.10.016

* Corresponding authors. Fax: +86 411 84694447 (Z. Jiang), fax: +86411 84694447 (C. Li).

E-mail addresses: zxjiang@dicp.ac.cn (Z. Jiang), canli@dicp.ac.cn (C.Li).

www.elsevier.com/locate/carbon

Available online at www.sciencedirect.com

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comparing ACWSN with ACWS, we observe the dramaticdecrease of all types of acidic-oxygenated groups and slightchange in pore volumes (Vmic and Vtotal).

Fig. 1 shows the variance of sulfur removal with effluentvolume and the total adsorption capacity of DBT on themodified ACs calculated from the breakthrough curves.

Both ACW900and ACS250 show enhanced adsorption per-formance as compared with the unmodified AC. As shownin Table 1, ACW900 has dramatic increase in the total porevolumes (Vtotal) as compared with AC. Thus higher DBTadsorption performance of ACW900 is directly related toits improvedVtotal. Adsorption of DBT on AC is governedby the dispersion interactions (pp interactions) in themicropores [6]. The enhanced DBT adsorption perfor-mance of ACW900is attributed to its largestVmic(as shownin Table 1). The acidic-oxygenated groups on the surfacemight act as adsorption sites for selective adsorption ofDBT [5,6]. The significant increase in adsorption perfor-mance of ACS250 may be due to the increase in surface

acidic-oxygenated groups as a result of H2SO4 treatment.ACWShas the highest adsorption capacity in all the inves-tigated samples because it has the maximum amount ofsurface acidic-oxygenated groups. ACWSN with similarpore volumes (Vtotal and Vmic) to ACWS shows higheradsorption performance as compared with AC, becauseits pore volumes are lager than AC. On the other hand,ACWSN shows rather lower adsorption performance ascompared with ACWS. This result is ascribed to the lossof the surface acidic-oxygenated groups due to N2 treat-ment at 900 C. This suggests that the surface acidic-oxy-genated groups play an important role in enhancing DBT

adsorption performance on the modified ACs.The arenes present in diesel fuel strongly compete with

the sulfur-containing compound in the adsorption process[2]. Therefore, it is important to investigate the adsorptionselectivity for sulfur-containing compound on ACs in thepresence of arenes. The tetrahydronaphthalene is addedto the heptane to mimic the arenes in the model dieseland its content is set to 5 and 30 wt%. Fig. 2 shows the ef-fect of arene on the DBT adsorption performance of ACWSand ACWSN. The DBT adsorption performance of ACWSN

is declined dramatically by the addition of arene. However,the effect of arene on the DBT adsorption performance ofACWS is very little. After 5 wt% of arene is added, theadsorption capacity of ACWSNis decreased by 60%. In con-trast, the adsorption capacity of ACWSis decreased by only10%, and decreased by only 30% even after 30 wt% of arene

Table 1

Physicochemical properties of activated carbon samples

Samplea SBETb (m2/g) Vtotal

b (cm3/g) Smicc (m2/g) Vmic

c (cm3/g) Surface acidity (mmol/g)d

Phenol Lactone Carboxyl Total

AC 1106 0.617 716 0.369 0.018 0.010 0.030 0.058ACW900 1979 1.138 1091 0.553 0.040 0.018 0.036 0.094ACS250 1271 0.740 543 0.308 0.301 0.381 1.271 1.953ACWS 1570 0.922 990 0.428 0.455 0.522 1.433 2.410ACWSN 1516 0.912 985 0.422 0.042 0.047 0.052 0.141

a AC: Untreated activated carbon, ACW900: AC treated by steam at 900 C, ACS250: AC treated by H2SO4 at 250 C, ACWS: AC treated by steam at900 C and subsequently treated by H2SO4at 250 C, ACWSN: ACWStreated by N2at 900 C.b The specific surface areas calculated using the BET equation by assuming the section area of nitrogen molecule to be 0.162 nm. The total pore volumes

estimated to be the liquid N2 volume at relative pressure of 0.98.c The micropore volumes and micropore areas calculated using the t-plot method.d

NaHCO3 (carboxyl), Na2CO3(carboxyl and lactonic), NaOH (carboxyl, lactonic and phenolic).

0 50 100 150 200 250 3000

20

40

60

80

100

Sulfurre

moval(%)

Effluent volume (cm3- elution/g-adsorbent)

ACWSACS250ACWSNACW900

AC

0

10

20

30

40

50

ACS250ACW900AC

Totaladsorptionc

apacity

(mg/g)

47.1

23.6

ACWS ACWSN

10.9

20.6

33.3

a

b

Fig. 1. Adsorption of dibenzothiophene (DBT) on the different activatedcarbons. (a) The variance of sulfur removal with effluent volume for thedifferent samples, (b) comparison of the total adsorption capacity on thedifferent samples. AC: unmodified activated carbon, ACW900: AC treatedby steam at 900 C, ACS250: AC treated by H2SO4at 250 C, ACWS: ACtreated by steam at 900 C and subsequently treated by H2SO4at 250 C,ACWSN: ACWS treated by N2at 900 C.

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is added (as shown in Fig. 2b). These results indicate thatACWS exhibits higher selectivity for DBT in the presenceof arene than ACWSNdoes. Since ACWSand ACWSNhavesimilar pore volumes (Vtotal and Vmic), the adsorptioncontribution from the dispersion interactions (ppinterac-tions) in the pore channel is similar. The acidic-oxygenated

functional groups appear to play an important role inenhancing adsorption selectivity for DBT on the modifiedACs. The specific interaction between the adsorbed DBTmolecules and surface acidic-oxygenated groups on ACWSmay be responsible for the high selectivity for DBT onthe modified ACs.

Acknowledgements

We acknowledge the financial support from the Na-tional Science Foundation of China (NSFC Grant No.20673114) and the State Key Project of China (Nos.2006CB202506).

Appendix A. Supplementary material

Supplementary data associated with this article can befound, in the online version, at .

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[2] Ma XL, Sun L, Song CS. A new approach to deep desulfurization ofgasoline, diesel fuel and jet fuel by selective adsorption for ultra-cleanfuels and for fuel cell applications. Catal Today 2002;77(12):10716.

[3] Lu HY, Gao JB, Jiang ZX, Jing F, Yano YX, Wang G, et al. Ultra-deep desulfurization of diesel by selective oxidation with[C18H37N(CH3)3]4[H2NaPW10O36] catalyst assembled in emulsion

droplets. J Catal 2006;239(2):36975.[4] Yang RT, Hernandez-Maldonado AJ, Yang FH. Desulfurization oftransportation fuels with zeolites under ambient conditions. Science2003;301(5629):7981.

[5] Zhou AN, Ma XL, Song CS. Liquid-phase adsorption of multi-ringthiophenic sulfur compounds on carbon materials with differentsurface properties. J Phys Chem B 2006;110(10):4699707.

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0

20

40

60

80

100ACWS/No arene

ACWS/5% arene

ACWS/30% arene

ACWSN/No arene

ACWSN/5% arene

0

10

20

30

40

50

32.4

43.8

47.1

10.4

23.6

ACWS

No arene

5% arene

30% arene

ACWSN

Sulfurre

moval(%)

Effluent volume (cm3- elution/g-adsorbent)

0 50 100 150 200 250 300

Totaladsorption

capacity

(mg/g)

a

b

Fig. 2. Adsorption of dibenzothiophene (DBT) on the different activatedcarbons. (a) Effect of arene on the DBT adsorption performance onACWSN and ACWS, (b) comparison of the arene effect on the total

adsorption capacity of ACWSand ACWSN. ACWS: AC treated by steam at900 C and subsequently treated by H2SO4 at 250 C, ACWSN: ACWStreated by N2 at 900 C.

3044 Letters to the Editor / Carbon 45 (2007) 30423059