molecular imprinting for the selective adsorption of organosulphur compounds present in fuels

Analytica Chimica Acta 435 (2001) 83–90

Molecular imprinting for the selective adsorption oforganosulphur compounds present in fuels

Beatriz Castro a,d, Michael J. Whitcombe b, Evgeny N. Vulfson b,Rafael Vazquez-Duhalt c, Eduardo Bárzana d,∗

a Instituto Mexicano del Petróleo, Programa de Biotecnologıa del Petróleo, Eje Central L. Cárdenas 152, México DF 07730, Mexicob Institute of Food Research, Norwich Research Park, Colney, Norwich NR4 7UA, UK

c Instituto de Biotecnologıa, Universidad Nacional Autónoma de México, Apdo. Postal 510-3, Cuernavaca, Mor. 62250, Mexicod Facultad de Quımica, Universidad Nacional Autónoma de México, Cd. Universitaria, México DF 04510, Mexico

Received 4 July 2000; received in revised form 19 December 2000; accepted 3 January 2001

Abstract

A novel approach to the potential desulphurisation of fuels such as diesel is proposed. It relies on the creation of recognitionsites complementary to sulphur-containing compounds in highly cross-linked polymeric matrices using the molecular imprint-ing technique. Dibenzothiophene sulphone (DBTS) was used as template for the preparation of molecularly imprinted poly-mers (MIPs). Four different polymers were synthesised using 5-octyloxy-1,3-bis(4-ethenylphenyl)-benzenedicarboxamide ormethacrylic acid and divinylbenzenes or ethylene glycol dimethacrylate as functional monomers and cross-linkers, respec-tively. Three polymers showed better binding of DBTS than non-imprinted controls, and were also superior in adsorption oforganosulphur compounds such as dibenzothiophene (DBT) and benzothiophene (BT) present in a model mixture. A maximumadsorption of 66 mg DBT per gram of polymer was observed at a polymer load of 10 g l−1 and an initial DBT concentrationof 3.69 g l−1. The polymers also showed selectivity for fluorene. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: Desulphurisation; Dibenzothiophene; Organosulphur compounds; Molecular imprinting; Molecularly imprinted polymers;Solid-phase extraction

1. Introduction

The sulphur content of crude oil and its fractionsis one of the critical parameters associated with thequality of fuels since the combustion of organosulphurcompounds results in the emission of sulphur oxides.These oxides contribute significantly to atmosphericpollution (e.g. acid rain) and are hazardous to humanhealth [1]. Sulphur content of petroleum can be ashigh as 7%, depending on the source [2], posing a ma-jor problem to refiners and the petrochemical industry.

∗ Corresponding author.E-mail address: [email protected] (E. Barzana).

At present, organic sulphur is removed from fuel bythe well known hydrodesulphurisation process (HDS)which is essentially a catalytic reduction with hydro-gen, at high temperature (350–500◦C) and pressure(∼1000 psi). Although HDS is a well-established pro-cess that has been performed by oil refineries for sev-eral decades, the operational costs are rather high [3],especially when the reduction of sulphur content downto 50 ppm is sought. Consequently, alternatives to con-ventional HDS, particularly those based on new andemerging technologies are being actively researched.Among these, the use of microbiological methods [4]is probably the most promising approach but the possi-bility of employing enzymes with high specificity for

0003-2670/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.PII: S0 0 0 3 -2 6 70 (01 )00799 -1

84 B. Castro et al. / Analytica Chimica Acta 435 (2001) 83–90

organic sulphur [5,6] and some non-enzymatic hemo-proteins capable of oxidising dibenzothiophene (DBT)[7,8] have also been explored with some success. Theaim of this work was to assess the feasibility of usingmolecularly imprinted polymers (MIPs) for the selec-tive adsorption of organosulphur compounds.

Molecular imprinting is a methodology forthe introduction of selective recognition sitesinto highly cross-linked polymeric matrices, viathe template-directed assembly of functionalisedmonomers into a polymer network [9–12]. Generally,polymerisation is carried out in the presence of a printmolecule or template, which forms a complex withthe constituent monomers. Subsequent removal of thetemplate leads to the formation of cavities within thepolymeric structure that function as specific recogni-tion sites. Most recent work in this area has focused onthe preparation of imprinted polymers for chiral reso-lutions [13–15] but numerous other applications havealso been described. Some examples of these include:the design of catalysts [16,17] and sensors [18–20],enzyme-linked immunoassays [21,22], solid-phaseextraction [23–26], removal of undesired componentsfrom complex mixtures [27,28], selective recoveryof metabolites from fermentation broths [29], controlof crystallisation [30], and separation or concentra-tion of proteins [31–33] and whole cells [34,35].This paper reports our initial results on the designand synthesis of MIPs for the selective adsorption oforganosulphur compounds normally present in fuels.DBT was chosen as the initial model compound sinceit can account for up to 70% of total organic sulphurin diesel.

2. Experimental section

2.1. Molecular modelling

Molecular modelling and preliminary energy min-imisation were performed by molecular mechan-ics techniques (PCModel for Windows). The inputfiles (z matrices) for MOPAC were generated andsemi-empirical methods of computational quantumchemistry as Hamiltonian AM1 [36] were applied toestimate enthalpies of formation, bond orders, inter-molecular distances, ionisation potentials and otherparameters.

Enthalpies of formation were calculated for eachfunctional monomer and complex, as well as the en-thalpies of formation of the complexes (��Hcomplex)according to the following equation [37]

��Hcomplex = �Hcomplex

−(m�Hreceptor + t�HDBTS) (1)

where m and t are the stoichiometric amounts offunctional monomers and template molecules in-volved in complex formation. Energy (kcal mol−1)for each bond was calculated according to Eq. (2) asfollows

bond energy =[

��Hcomplex∑ni=1bond order

](bond order)i (2)

2.2. Monomer synthesis

All reagents used in this work were purchased fromAldrich and were of the highest purity available.

2.2.1. Synthesis of5-octyloxybenzene-1,3-dicarboxylic acid (222)

Ten grams (47 mmol) of dimethyl 5-hydroxyben-zene-1,3-dicarboxylate (1) was added to anhydrouspotassium carbonate (8 g) and 1-bromooctane (10 g,52 mmol) in dry dimethylformamide (DMF, 100 ml).The mixture was heated to 80◦C for 2 h with stir-ring. After cooling, the reaction mixture was mixedwith distilled water (approximately 100 ml) and ex-tracted with diethyl ether (4 ml × 100 ml). The etherextracts were washed once with water, dried (MgSO4)and evaporated to yield an oily product, which crys-tallised after evaporation of residual DMF at high vac-uum. The intermediate ester was used without furtherpurification. This ester was then hydrolysed to (2) byreflux with an aqueous methanolic solution of sodiumhydroxide. The reaction mixture was diluted with wa-ter and washed with diethyl ether followed by acidi-fication with HCl. The product (2) was collected byfiltration and recrystallised from ethanol as colourlesscrystals.

2.2.2. Synthesis of 5-octyloxy-1,3-bis(4-ethenylphenyl)-benzenedicarboxamide (444)

An amount of 2 g (6.8 mmol) of the acid (2) wastreated with an excess of oxalyl chloride (4 ml) at

B. Castro et al. / Analytica Chimica Acta 435 (2001) 83–90 85

room temperature in a flask equipped with a gas scrub-ber. The reaction was initiated by the addition of onedrop of DMF. When all signs of reaction had ceased,the remaining oxalyl chloride was removed in vacuoto yield the acyl chloride (3), which was used with-out further purification. This residue was dissolved indry tetrahydrofuran (THF, 40 ml) and added dropwiseto an ice-cold solution of 4-amino-ethenylbenzene(1.6 g, 13.6 mmol) and triethylamine (12 ml) in THF(100 ml). The stirred mixture was left at room tem-perature overnight. After solvent removal the residuewas diluted with dichloromethane (100 ml) and ex-tracted sequentially with water, aqueous NaHCO3,dilute HCl and water, followed by drying (Mg2SO4)and solvent evaporation under vacuum. The productwas purified by column chromatography on silicagel, eluting with chloroform/methanol, yield = 55%(4, see Fig. 1). FT-IR, CHCl3, 3429 (N–H), 1676(amide I), 1589 (aromatic), 1517 (amide II) cm−1;13C-NMR, 75.412 MHz, CDCl3 + d6-DMSO, δ

(ppm), 13.57 (–CH3, C1), 22.06, 25.41, 28.56, 28.64,28.73, 31.19 (6 × –CH2, C2, C3, C4, C5, C6, C7),67.95 (–CH2, C8), 112.36 (=CH2, C19), 116.67,117.44, 119.99, 126.07 (4×aromatic –CH, C10, C12,

Fig. 1. Structures of DBTS and the receptor, 5-octyloxy-1,3-bis(4-ethenylphenyl)-benzenedicarboxamide (4), and computer output for themost stable configuration for the 1:1 complex of DBT sulphone (transversal to the plane) and 5-methacryloxy-1,3-bis(4-ethenylphenyl)-benzenedicarboxamide (parallel to the plane). The dashed lines represent hydrogen bonds. Obtained by molecular modelling usingsemi-empirical method (AM1).

C15, C16), 133.02 (Q, C14), 135.67 (=CH, C18),135.79, 159.01 (2 × Q, C17, C9), 164.59 (carbonyl,C13); 1H-NMR, 299.877 MHz, CDCl3 + d6-DMSO,δ (ppm), 0.89 (t, 3H, –CH3, J = 6.9 Hz), 1.31(m, 8H, –CH2), 1.80 (m, 2H, –CH2), 4.05 (m, 2H,O–CH2– J = 6.9 Hz), 5.21 (dd, 1H, Jcis = 10.8 Hz,Jgem = 0.9 Hz, cis-CH=CH2), 5.70 (dd, 1H Jtrans =17.7 Hz, Jgem = 0.9 Hz, trans-CH=CH2), 6.69 (dd,1H Jtrans = 17.6 Hz, Jcis = 10.9 Hz, –CH=CH2),7.40, 7.77 (aromatic AB system, 4H, JA,B = 8.7 Hz),7.66 (s, 2H, aromatic –CH, H10), 8.10 (s, 1H, aro-matic –CH, H12), 9.66 (s, –CONH–).

2.3. Measurement of association constants

The interaction between the synthesised receptor 4and the template dibenzothiophene sulphone (DBTS)was assessed by NMR titration. Receptor 4 (66 mMin deuterated chloroform, 800 �l), was titrated with5–50 �l of DBTS (92.4 mM) in the same solvent. 1HFT-NMR spectra were recorded on a Jeol 90 MHzusing TMS as the internal reference standard. Fromthese data the values of association constants werecalculated using host–guest software kindly provided


Fig. 2. 1H-NMR titration of receptor 4 (host) with DBTS (guest). Chemical shift (relative to TMS) of the receptor N–H resonance againstconcentration of added DBTS.

by Prof. Chris Hunter (University of Sheffield, UK).Fig. 2 shows the chemical shift (relative to TMS)of receptor 4 N–H resonance as a function of addedDBTS.

2.4. Preparation of polymers

Polymers were synthesised using a mixture ofcross-linker (96 mol%) and functional monomers(methacrylic acid (MAA) or receptor 4; 4 mol%).Either divinylbenzenes (DVB, tech grade 80%) orethylene glycol dimethacrylate (EGDMA) wereused as cross-linkers. All monomers were treatedto remove polymerisation inhibitors. The monomerand cross-linker mixture as well as the poro-genic solvent were transferred to a test tube andazo-bis(cyclohexane)carbonitrile (1 mol% with re-spect to polymerisable double bonds) was added. Forimprinted polymers, the template (DBTS, 16 mol%)was also added. The tube was closed with a glass jointand connected to a vacuum line. The polymerisationmixture was degassed by a series of freeze–thawcycles and sealed under reduced pressure. The poly-merisation reaction was carried out in a water bathat 65◦C for 24 h. The solid polymer product was re-moved from the tube, washed with methanol, dried

and ground to a particle size of approximately 50 �m(MM2000 mill, Retsch Co., Germany). It was thensubjected to an extensive washing process (Soxhletextraction) to remove the entrapped template withmethanol until no more template was detected byHPLC in the solvent (see later). DBTS recovery wasin all cases above 95% of the initial template loadin the polymerisation mixture. Non-imprinted (con-trol) polymers were treated in exactly the same way.The composition of the obtained polymers and theirpreparation are summarised in Table 1.

2.5. Binding studies

In a typical experiment 30 mg of polymer wereadded to an acetonitrile solution (3 ml) containing thespecified organosulphur compounds at different con-centrations as described in legends to Tables. Thescrew-capped vials with the former suspension wereleft under mechanical agitation overnight. The poly-mer was then removed by filtration and the resultingsolution was analysed by HPLC (Hewlett-Packard Se-ries 1100 with a diode array detector) at 225 nm usinga C18 ODS reverse phase column. Acetonitrile/water(4:1 (v/v)) was used as a mobile phase at a flow rateof 1.2 ml min−1.


Table 1Composition and binding properties of selected polymers

Polymer Cross-linker Monomer Porogena Template DBTS uptakeb

P1NI DVB MAA CHCl3 None 10.49P1I DVB MAA CHCl3 DBTS 11.78 (12%)P2NI DVB 4 CHCl3 None 8.41P2I DVB 4 CHCl3 DBTS 10.15 (21%)P3NI DVB 4 ACN/Tol None 8.02P3I DVB 4 ACN/Tol DBTS 8.14 (1%)P4NI EGDMA 4 ACN/Tol None 10.07P4I EGDMA 4 ACN/Tol DBTS 12.21 (21%)P4Ia EGDMA None ACN/Tol DBTS 5.07

a CHCl3 and acetonitrile/toluene (1:1 (v/v)) were used at 2 and 6 ml g−1 of monomers, respectively.b The uptake is expressed in milligram of DBTS bound per 1 g polymer. In these experiments an initial concentration of 86 mg of

DBTS per gram of polymer was used. The numbers in brackets show the enhancement of binding to imprinted polymers as compared tonon-imprinted controls. See Section 2 for further details.

3. Results and discussion

The objective of this research was to establish thefeasibility of using MIPs for the selective adsorptionof organosulphur compounds. DBTS (Fig. 1), the oxi-dised derivative of DBT, was selected as a template forthe initial investigation. Our reasoning for using thistemplate molecule was that the presence of at least onefunctional group capable of hydrogen bonding, shouldfacilitate the formation of well defined cavities in thepolymer structure. Polymers imprinted with DBTSwere also of interest in their own right as this com-pound is one of the products of diesel bio-oxidationwith whole cells [5] and enzymes [7]. With this ratio-nale in mind, polymers P1I and P1NI (non-imprintedcontrol) were prepared as described in the Section 2,using DVB as a cross-linker and a four-fold excess ofDBTS over MAA (functional monomer) to maximisethe interaction between the template and the polymerforming components. Once obtained, these polymerswere tested in a DBTS solution to assess whether thepresence of the template in the polymerisation mixtureled to significant enhancement of binding (“imprintingeffect”). The first two rows in Table 1 show that themethacrylic acid-based polymer showed a perceptibleimprinting effect (12%). However, further attempts toenhance the imprinting effect (i.e. varying the ratio oftemplate to MAA, using other solvents like tetrahy-drofuran, dichloromethane or acetonitrile, and replac-ing DVB with EGDMA) did not result in significantimprovements.

To improve the polymer performance, an alternativefunctional monomer that could interact more stronglywith DBTS was evaluated. To this end, a polymeris-able receptor (5-methacryloxy-1,3-bis(4-ethenyl-phenyl)-benzene dicarboxamide) was designed andthe strength of its interaction with DBTS wasevaluated using conventional molecular modellingprocedures. This analysis yielded an enthalpy ofcomplexation between the receptor and DBTS of��H complex = −11.0644 kcal mol−1. Two mainhydrogen bonds were theoretically established withhydrogen bond energies of 4.18 kcal mol−1, in ad-dition to the two other secondary bonds that con-tributed 1.35 kcal mol−1 (Fig. 1). By comparison, thecomplexation of two molecules of methacrylic acidwith DBTS resulted in a weaker calculated enthalpy(��H complex = −8.62 kcal mol−1), and free energiesfor the two corresponding hydrogen bonds of 4.09and 4.52 kcal mol−1.

The polymerisable receptor 4 was then synthesisedin three steps, modifying the substitution in position5 by an octyl chain instead of the methacryloxy sub-stitution that was considered for the molecular mod-elling. This modification was performed in order toincrease the solubility of the polymerisable receptorin the reaction mixture. The interaction of the re-ceptor with the template was evaluated by 1H-NMRtitration in deuterated chloroform by following thechemical shift of the amide protons. The associationconstant obtained from these data was Ka = 15.7 M−1

at 27◦C (Fig. 2). It was not possible to determine the


corresponding value for MAA presumably because itwas significantly lower. The incorporation of the poly-merisable receptor into a polymer P2I, which was anal-ogous to P1I but contained 4 instead of MAA, led toan almost two-fold increase in the imprinting effect(compare rows 2 and 4, Table 1). However, accord-ing to the absolute values obtained, the DBTS adsorp-tion to imprinted (P2I) and non-imprinted (P2NI) wasslightly lower than the one achieved with the MAApolymers.

In a second attempt to further enhance the polymerperformance, the amount and type of porogen andcross-linker were varied. In particular, P4I preparedwith EDGMA in a 1:1 mixture of acetonitrile andtoluene showed the highest DBTS uptake (12.21 mgDBTS per gram of polymer), as well as a 21% ad-sorption increase compared to the correspondingnon-imprinted control polymer (P4NI). The imprint-ing effect observed was clearly due to the presenceof the template–monomer complex in the polymeri-sation mixture rather than to the print molecule on itsown, because another control polymer prepared in theabsence of the functional monomer (P4Ia) showedrelatively poor binding of DBTS (see Table 1). In-terestingly, polymers P3I and P3NI which were syn-thesised in the 1:1 (v/v) mixture of acetonitrile andtoluene but with DVB as cross-linker showed no im-printing effect at all. These results demonstrate thatthe specificity of imprinted polymers is dependent onnumerous interrelated factors, and that further detailedinvestigation is required in order to achieve a betterinsight into the structure-binding relationship in thesepolymers.

The binding of organosulphur compounds normallypresent in fuels in a mixture with representative quan-tities was then evaluated. In particular, it was of in-terest to establish whether the polymers would bindDBT and the smaller benzothiophene (BT), as well asother aromatics like fluorene (FLR). Three imprintedpolymers (P1I, P2I and P4I) were tested by exposingthem to a mixture of four compounds (DBTS, DBT,BT and FLR) in the same solvent. The results obtainedare summarised in Table 2. Polymer P3I was not testedwith this former mixture as no imprinting effect wasobtained for DBTS uptake (∼1% as shown in Table 1).It is evident from these data that the imprinting ef-fect was by far the greatest for the EGDMA/4 poly-mers (P4I/P4NI), as compared to the other two pairs

Table 2Adsorption of ligands from a mixture of DBTS, DBT, BT andFLR using three different molecularly imprinted polymersa

Ligand/polymer P1I P1NI P2I P2NI P4I P4NI

DBTS 14.8 13.2 13.4 10.8 14.8 7DBT 66 60.84 57.81 47.71 32.7 7.26BT 17.16 16.06 15.54 13.27 9.11 2.79FLR 29.97 27.63 26.49 21.63 15.18 3.6

a Uptake is expressed as milligram of ligand adsorbed per 1 gof polymer. Initial concentrations of each compound in the mixturewere 89 mg (DBTS), 97 mg (BT), 176 mg (FLR) and 369 mg(DBT) per gram of polymer. See Section 2 for further details.

(P1I/P1NI and P2I/P2NI). Interestingly, both in ab-solute and relative terms, the non-oxygen containingunfunctionalised ligands bind to imprinted polymersin larger amounts than DBTS (compare columns inTable 2). The increase in uptake for all four ligands,which is due to the imprinting effect, is presented inFig. 3. It is relevant to notice that the imprinting ef-fect for DBTS increases when this ligand is foundin a mixture, compared to the results obtained whenpresent as a single compound. This is more remark-able for P4 polymer (21.2% in Table 1 versus 111%in Fig. 2). Based on the absolute adsorption numbers,it seems that the ligands compete for adsorption sitesin the non-imprinted preparation; this translates into asubstantial reduction in the base level of adsorption.In contrast, the imprinted version shows a minor in-crease in non-specific adsorption since it occurs in thepresence of DBTS analogues. The net result of botheffects acting simultaneously is an apparent enhance-ment in adsorption by the imprinting process. How-ever, further experiments are needed to explain thisobservation. Finally the DVB polymers P1I and P2Iadsorb about twice as much material as the “better”imprinted P4I. The latter effect is probably due to thestronger interaction between the aromatic ligands andthe DVB matrix of P1 and P2 as compared to theEGDMA-based P4.

In conclusion, we have shown that the inclusionof the template–monomer complex (DBTS-4) intothe polymerisation mixture leads to the formationof recognition sites for binding DBT and analogouscompounds. This result in a significantly higher up-take of the target by imprinted polymers as comparedto the corresponding non-imprinted controls. The


Fig. 3. Increase in the uptake of ligands (%) attributable to the “imprinting effect”. These data were obtained from the binding assaysperformed with a mixture of DBTS, DBT, BT and FLR. See Section 2 for initial concentrations and further details. Removal enhancement(%) = [([DBT]ads,PXI − [DBT]ads,PXNI)/[DBT]ads,PXNI] × 100, where X (1, 2 or 4) represents a different polymer composition.

data reported here, although rather preliminary, pro-vide clear evidence that MIPs have the potential tobecome a practical tool for selective adsorption oforganosulphur pollutants.

The experiments and results herein presented aredirected towards the development of a technology forfuels desulphurisation as the ultimate and long-termgoal. However, MIPs could also be eventually used forthe separation and/or analysis of organosulphur com-pounds in fuels by solid-phase extraction, representinga potentially cheaper option compared to current ana-lytical methodologies that rely on the use of expensivestationary phases.

Acknowledgements

Financial support of National University of Mex-ico (DGAPA IN502763) and Mexican PetroleumInstitute (FIES 95137-II and D.00020) is gratefullyacknowledged. The collaboration of Dr. Manuel Ru-bio and Guillermo Ramırez-G., from Institute ofChemistry-UNAM, on the modelling procedures isalso appreciated. We thank Rosalba Garfias for tech-nical assistance.

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molecular imprinting for the selective adsorption of organosulphur compounds present in fuels

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