Recent advances in molecular imprinting technology for the deep desulfurization of fuel oils

NEW CARBON MATERIALS Volume 29, Issue 1, Feb 2014 Online English edition of the Chinese language journal Cite this article as: New Carbon Materials, 2014, 29(1):1–14.

REVIEW

Recent advances in molecular imprinting technology for the deep desulfurization of fuel oils Yong-zhen Yang1,2, Xu-guang Liu1,3,*, Bing-she Xu1, 2 1

Key Laboratory of Interface Science and Engineering in Advanced Materials (Taiyuan University of Technology), Ministry of Education,

Taiyuan 030024, China; 2

Research Center of Advanced Materials Science and Technology, Taiyuan University of Technology, Taiyuan 030024, China;

3

College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China

Abstract: As a novel adsorptive desulfurization method for the preparation of adsorbents, molecular imprinting technology is used to create specific molecular recognition sites in polymers to identify sulfur-bearing template molecules. It is a green process with potential applications because of its characteristics of mild conditions, simple operation, low investment, low pollution, high selectivity, no effect on octane value, and the possible reuse of the as-obtained benzothiophene-like compounds as fine chemicals. Recently, inorganic materials including silica gel, TiO2, K2Ti4O9, and carbon microspheres have been used as supports to prepare surface molecularly imprinted polymers for adsorbing dibenzothiophene and benzothiophene. Recent advances in molecular imprinting technology for deep desulfurization are summarized with carbon microsphere surface molecular imprinting technology highlighted. The review provides experimental references and theoretical guidance for designing and preparing green desulfurization materials. Key Words:

Molecular imprinting technology; Carbon microspheres; Surface molecular imprinting technology; Adsorption; Deep

desulfurization

1

Introduction

With growing environmental awareness, the deep desulfurization of transportation fuels has become an urgent task presently because of the emission of SO2 from burning fuel oils [1]. In order to protect the environment, more and more stringent exhaust discharge regulations and fuel standard specifications have been issued in many countries in an attempt to decrease world-wide sulfur emission. In China, the exposure draft of CHN V, equivalent to EUR V, has been issued and enacted in Beijing from June 1, 2012. The draft requires a no more than 0.01mg/mL of sulfur content in fuel oil. Sulfur-bearing compounds present in transportation fuels are mainly composed of mercaptans, thioethers, disulphides, thiophene and its derivatives, in which thiophene and its derivatives account for more than 60% of the total sulfur. The conventional hydrodesulfurization is effective in removing mercaptans, thioethers and disulphides, but is less effective in removing dibenzothiophene (DBT) and its alkylated derivatives such as 4-methyldibenzothiophene (4-MDBT) and [2-5] 4,6-dimethyldibenzothiophene (4,6-DMDBT) . Consequently, various alternative desulfurization technologies have been widely investigated in recent years, including

bio-desulfurization, extraction, oxidation, and adsorption desulfurization [5-10]. Adsorption desulfurization is a green process with extensive development space and application prospects, which has the characteristics of easy operation, low cost, no pollution, and accessibility to deep desulfurization. Furthermore, S-Zorb desulfurization as a kind of reactive adsorption desulfurization has been applied extensively in industry. Sulfur is typically reduced to about 0.005 mg/mL from feedstock of 0.5 mg/mL sulfur and even to very low level from feedstock containing greater than 2 mg/mL sulfur by this process [10]. However, the reactive nature of S-Zorb reaction desulfurization makes it occur in the presence of hydrogen at temperatures in the range of 300-400 oC and pressures in the range of 1.90-3.45 MPa, and at expense of destroying the structures of DBT or its alkylated derivatives which can otherwise be reused for other applications. Therefore, a major challenge in the use of adsorptive desulfurization is the design of the adsorbent materials which can selectively adsorb the organosulfur from fuel in the presence of a large portion of aromatic compounds. Fortunately, molecular imprinting technology has been developed as biosensors, separation media and affinity supports for the recognition of target molecules [11-15], which is expected to find applications in the deep desulfurization of

Received date: 23 August 2013; Revised date: 15 January 2014 *Corresponding author. E-mail: [email protected] Copyright©2014, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-5805(14)60121-9

Yong-zhen Yang et al. / New Carbon Materials, 2014, 29(1): 1–14

which can provide the advantages of high selectivity, easy elution, more accessible sites, fast mass transfer and association kinetics by designing the molecular recognition sites on the surface of imprinted materials, has been explored by grafting a very thin polymer film onto some supports, such as SiO2 [30], TiO2 [31], α-Al2O3 [32], silica gel [33], CdS [34], carbon nanotubes [28, 29], and ZnS Quantum dots [35], for different application requirements. Because the supports have good dispersion, uniform size, and good chemical, mechanical and thermal stabilities, surface MIPs (SMIPs) not only have good recognition ability and high adsorption selectivity, but also have good mono-dispersion, homogeneous size-distribution and good chemical, mechanical and thermal stabilities, showing potential applications in the fields of molecular separation and detection.

Fig. 1 The preparation of traditional MIP.

fuel oil. As an emerging adsorptive desulfurization technique, molecular imprinting technology, which is the creation of specific molecular recognition sites in polymers to identify template molecules, has the unique pre-determinative characteristic, specificity and practicability. As-prepared polymer from this technology is named as molecularly imprinted polymer (MIP) because it is complementary to the template in space structure and binding sites. The conventional process to prepare MIP is generally illustrated in Fig. 1 [14,15]. MIP is prepared by copolymerizing a functional monomer with a cross-linker in the presence of a template molecule, and the functional monomers are initially arranged around the template molecules through either noncovalent or covalent interactions before copolymerization; after polymerization, the template is removed from the porous network by washing and cavities in the polymeric matrix are left to be complementary to the template in size, shape, and chemical functionality. Thus, MIP can rebind selectively with the template under certain experimental conditions. Traditional MIP preparations include bulk polymerization, dispersion polymerization and suspension polymerization [16-21]. However, the molecularly imprinted polymers (MIPs) prepared by these techniques have some disadvantages: the imprinted polymer matrices are usually thick and the number of recognition sites per unit volume of the polymer is relatively low; the template molecules are embedded in the matrices too deeply so that not only the subsequent elution is difficult, but also the diffusion barrier for the template molecules is introduced, leading to a low rate of mass transfer and a difficulty in binding template molecules with recognition sites. Other limitations facing conventional bulk MIPs include grinding difficulty, tedious sieving, and non-uniform particles. In order to overcome these limitations, the surface molecular imprinting technology has been developed in recent years, and the obtained surface MIP (SMIP) films allow the imprinted cavities to be distributed effectively for recognizing the template molecules. The surface molecular imprinting technology

[22-29]

,

Based on the recognition and selectivity mechanism of molecularly imprinted materials towards sulfur components in oil, the imprinted adsorption materials, which were synthesized using benzothiophene (BT), DBT and dibenzothiophene sulfone (DBTS) as the templates, could remove thiophene-like organosulfur components from fuel oils. The selectivity and removal rate of MIPs were beyond other physical adsorptions. After subsequent treatments, thiophene-like by-products could be obtained for other applications. As a result, molecular imprinting technology is growing into a new and promising desulfurization method. The surface molecular imprinting technology can enhance the synthesis controllability and binding rate between template molecules and polymers as a result of its excellent specific recognition, thus it will be very optimistic in deep desulfurization of fuel oils. In this review, the traditional and surface molecular imprinting technologies used in deep desulfurization of fuel oils at home and abroad are reviewed and prospected.

2 Traditional molecular imprinting technology for deep desulfurization In polymer network structure, specific recognition sites for thiophene-like organosulfur components were formed by the molecular imprinting technology. MIP was applied to selectively remove thiophene-like sulfur components in fuel oils. In 2001, three kinds of MIPs were synthesized by Castro [16] el al. using DBTs as the template, 5-octyloxy-1,3-bis(4-ethenylphenyl)-benzenedicarboxamide (receptor 4) or methacrylic acid (MAA) as the functional monomer and divinylbenzene (DVB) or ethylene glycol dimethacrylate (EDMA) as the cross-linker. Polymerization was conducted by adding the initiator into the mixture solution of the monomer and the template. After that, DBTs was eluted by methanol to prepare MIPs with recognition sites for DBTs. Besides, non-imprinted polymers (NIPs) were also synthesized in the same way except the addition of the template in order to compare adsorption performance with MIPs. The

Yong-zhen Yang et al. / New Carbon Materials, 2014, 29(1): 1–14

adsorption tests in acetonitrile solution containing organosulphur compounds indicate that three MIPs showed better binding for DBTs than non-imprinted controls, and were also superior in adsorption of organosulphur compounds such as DBT and BT present in a model mixture. The MIPs also showed a selectivity for fluorene. At room temperature, the adsorption capacity of MIP with MAA as the monomer and DVB as the cross-linker towards DBTs was 14.8 mg/g and the imprinting factor was about 1.12. Generally the specific adsorption of MIP is signified by the imprinting factor indicated by α, if α ≤ 1, MIP does not show selectivity for adsorbate; if 1≤α ≤ 1.5, MIP exhibits relatively weak selectivity; if α ≥ 1.5, selectivity is excellent. In their experiments, the most efficient adsorption towards DBTs was performed by MIP prepared using receptor 4 as the monomer and EDMA as the cross-linker, in which the imprinting factor was around 4.5 whereas the maximum adsorption capacity was merely 12.21 mg/g. The adsorption capacities of three MIPs towards DBT were larger than those of NIPs, and the maximum amount was 66.0 mg/g. These studies demonstrate the potential of MIPs as a practical tool for selective adsorption of organosulphur pollutants for the first time. In 2003, Chang et al. [17] reported the preparation of MIP by bulk polymerization using DBT as the template. The effects of functional monomer and solvent on the adsorption of DBT were evaluated. The results show that the MIP prepared using 4-vinyl pyridine (4-VP) as functional monomer, EDMA as cross-linker and toluene as porogen exhibited more remarkable adsorption capacity and selectivity. At 20 oC, the maximum binding capacity was 48.3 mg/g, as measured from adsorption isotherms, whereas the imprinting factor was only 1.21, indicating weak selectivity. Not only did the final MIP show excellent adsorption capacity for DBT, but it also performed good removing effect on other benzothiophenic sulfur compounds in petroleum such as BT and DBTs. The ultimate product could meet requirements of deep desulfurization. Chitosan is an aminopolysaccharide capable of forming gels through the Schiff base reaction between its amine groups and the aldehyde ends with glutaraldehyde used as a cross-linker. As a common natural polymer containing a large number of amino, hydroxyl and other active groups, chitosan can be applied to molecular recognition, waste water treatment, and drug release owning to its excellent biocompatibility, degradability and non-toxicity. The presence of these functional groups allows chitosan′s self-assembling with DBTs through the formation of hydrogen bonds, leading to a specific conformation, which may be retained by cross-linking. In 2004, Aburto et al. [18,19] reported the synthesis of imprinted chitosan hydrogels by bulk polymerization using glutaric dialdehyde as the cross-linker, DBTs as the template and acetonitrile/water as the solvent. Adsorption measurements in a simulated oil (acetonitrile solution) indicate that the imprinted chitosan hydrogel exhibited better selectivity and adsorption capacity toward DBTs. At 25 oC, the adsorption

capacity for DBTs was 16.54 mg/g [19]. The adsorption equilibrium time was 420 min at 30 oC, as measured from adsorption kinetics [18]. The imprinted hydrogels were found to be stimuli-responsive with temperature. The steady-state fluorescence spectrofluorometry results from 4 to 76 oC show that the best specific adsorption toward DBTs (ligand) was at 50 oC because of the powerful interaction between ligand and gel, the adsorption capacity was 27.5 mg/g, and the imprinting factor was 1.22. In 2006, Ying et al. [20] used chitosan as the functional monomer and DBT as the template to prepare MIP by inverse suspension polymerization. The imprinted desulfurization adsorbents with homogeneous particle size were prepared using formaldehyde as the pre-cross-linker, epichlorohydrin as the cross-linker and ethyl ether as the solvent. The adsorption capacity towards DBT was 17.53 mg/g at room temperature. In 2010, Chang et al. [21] prepared chitosan MIPs by the dispersion polymerization using DBT as the template, paraffin as the dispersed phase, Span 80 as the surfactant, and epichlorohydrin or glutaric dialdehyde as the cross-linker. Adsorption measurements in simulated oil (50 mg of MIP dissolved in 50 mL of n-heptane) indicate that MIP using glutaric dialdehyde as the cross-linker had an excellent adsorption selectivity and capacity toward DBT. From adsorption kinetics, the adsorption equilibrium time was determined as 300 min. Adsorption capacity decreased with increasing temperature. At 15 oC, the imprinting factor was 2.45 and an adsorption capacity was 22.69 mg/g. At 25 oC, the adsorption capacity was 21.06 mg/g. From adsorption isotherms and kinetics, the adsorption was found to follow Freundlich model and pseudo-first-order kinetics, suggesting the adsorption of DBT onto the MIP was a multiple layer adsorption and the rate-controlling step may be the mass transfer rather than the chemical adsorption. Investigation on adsorption thermodynamics confirmed the spontaneous and endothermic nature of the adsorption. At 25 oC, adsorptive desulfurization in gasoline showed a saturated desulfurization capacity of 3.52 mg/g. The adsorption capacity remained unchanged after ten adsorption–regeneration cycles.

3 Surface molecular imprinting technology for deep desulfurization Compared with traditional molecular imprinting technology, surface molecular imprinting technology has been irrepressible as a novel deep desulfurization technology of fuel oil because SMIP has high selectivity, fast adsorption, good mechanical and thermal stability [36-46]. SMIP was obtained by grafting a thin imprinted polymer film with a vast majority of binding sites onto a support. Because the template molecule is more easily eluted and does not need to overcome the internal mass transfer resistance during recognition, this technology improves the adsorption rate between recognition sites and template molecules, and correspondingly the adsorption selectivity of MIPs. To date, some inorganic

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materials have been used as supports (i.e., silica gel, TiO2, K2Ti4O9, carbon microspheres (CMSs)) to prepare SMIPs for adsorbing DBT, as summarized in Table 1. The resultant materials have not only the imprinting function towards DBT molecule, but also good mechanical and thermal stability. 3.1

Silica gel as support

Because of its high porosity, large surface area, good compatibility, mechanical property and stability, silica gel is usually used as support for SMIP. In 2010, Hu et al and Zhang [36, 37] synthesized SMIP on silica gel surface modified by γ-aminopropyltriethoxy silane (APTS) with MAA as the functional monomer, EDMA as the crosslinking agent, azoisobutyronitrile (AIBN) as initiator and BT as template.

Table 1 Comparison of SMIPs for adsorbing DBT or BT SupportSMIP

Preparation and Raw materials

Q/mg·g-1

te/min

α

Regeneration

Ref.

Silica gel-SMIP

Grafting polymerization Support: silica gel modified by KH-550 Template: BT; Monomer: MAA; Cross-linking agent: EDMA; Initiator: AIBN; Solvent: toluene

57.4

140

1.55

No decrease after 30 cycles

[36]

TiO2-SMIP

Two step self-assembly polymerization Support: nano-TiO2 Template: DBT; Monomer: 4-VP; Cross-linking agent: EDMA; Initiator: AIBN; Solvent: toluene

18.2

300

2.50***

-

[38]

TiO2-SMIP

Two step self-assembly polymerization Support: nano-TiO2 Monomer: MAA; Others as Ref. [38]

12.1

400

1.39**

-

[39]

Hollow-SMIP using TiO2 as sacrificial support

As Ref. [38]

20.5

240

1.76***

Maintaining initial Q after 5 cycles

[40]

K2Ti4O9 -SMIP

Grafting polymerization Support: K2Ti4O9 modified by MPS Others as Ref. [38]

27.5

360

2.16

80% of the initial Q after 10 cycles

[41]

K2Ti6O13 -SMIP

In situ polymerization Support: K2Ti6O13 modified by MPS Others as Ref. [38]

23.2

300

2.11**

84% of the initial Q after 6 cycles

[42]

Carbon sphere-SMIP

In situ polymerization Support: carbon spheres modified by KH-570 Template: DBT; Monomer: MAA; Cross-linking agent: EDMA; Initiator: AIBN; Solvent: chloroform

109.5

300

1.99

-

[44]

Carbon sphere-SMIP

Iniferter-grafting polymerization Support: carbon spheres modified by CMTMS and NaDEDTC Others as Ref. [44]

88.71

180

4.60

80% of the initial Q after 6 cycles

[45]

Carbon sphere-SMIP

Grafting polymerization Monomer: AMPS; Cross-linking agent: APS; Others as Ref. [44]

2.54*

-

8.19*

-

[46]

Carbon sphere-SMIP

RAFT-grafting polymerization Support: carbon spheres modified by γ-chloropropyltrimethoxysilane and RAFT agent Others as Ref. [46]

2.27*

-

1.23*

-

-

Porous carbon sphere-SMIP

In situ polymerization Support: porous carbon spheres modified by KH-570 Others as Ref. [44]

376.0

360

1.57

-

-

Note: KH-550: γ-Aminopropyltriethoxysilane; MPS: 3-methacryloxypropyl trimethoxysilane; KH-570: γ-methacryloxypropyl trimethoxysilane; CMTMS: p-(chloromethyl) phenyl-trimethoxysilane; NaDEDTC: sodium N,N-diethyldithiocarbamate trihydrate; BT: benzothiophene; DBT: dibenzothiophene; MAA: methacrylic acid; 4-VP: 4-vinylpyridine; AMPS: 2-acrylamido-2-methylpropanesulfonic acid; EDMA: ethylene glycol dimethacrylate; AIBN: azoisobutyronitrile; APS: ammonium persulfate; RAFT: Reversible addition-fragmentation chain transfer. Q: Adsorption capacity; α: imprinting factor. (α=KMIP/KNIP, K=Cp/Cs. K: Distribution coefficient; Cp (mg/g): Equilibrium adsorption; Cs (mg/ml): Equilibrium concentration). Q and α were obtained by adsorption isotherms of static adsorption at 25 ℃ except for ref [46]. te: Equilibrium time: obtained by kinetic adsorption. # : Adsorbing BT. *: Dynamic adsorption test. **: Adsorption selectivity test at 25 oC. ***: Adsorption selectivity test at 45 oC. -: not available.

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The static adsorption in BT-containing simulated gasoline at 25 oC shows an adsorption capacity of 57.4 mg/g, an imprinting factor of 1.55 and a equilibrium time of 140 min for BT. MIP exhibited specific recognition performance and good spatial selectivity for BT. After 30 times of regeneration, the specific recognition ability of the SMIP for BT kept almost unchanged, illustrating that the SMIP could be reused because of its good mechanical strength and stable renewable performance. In addition, SMIP with composite templates was also synthesized on silica gel surface in the coexistence of thiophene, 3-methyl-thiophene, BT and DBT as template molecules. The results of dynamic adsorption in both real gasoline and simulated gasoline ( heptane, hexane, cyclohexane mixture, 1:1:1, v/v/v) show that SMIP dynamic saturated adsorption capacities for thiophene, 3-methyl-thiophene, BT and DBT in simulated gasoline were 24.56, 22.2, 33.82 and 34.85 mg/g, respectively, and the sulfur capacity was 12.56 mg/g in real gasoline, giving an estimated adsorption capacity for sulfur compounds of approximately 43.2 mg/g. Materials Studio 4.4 software was employed to compute the binding energy between template molecules with functional monomer and MIPs. The simulation results were basically consistent with the measured ones from above experiments, suggesting that molecular simulation technology is feasible for selecting polymer and determining polymer dosage rapidly and economically. 3.2

Nano-TiO2 as support

Nano-TiO2 is an ideal support material because of its nontoxic, low-cost, readily available, photostable and chemically stable nature. In 2010, Xu et al [38] prepared an MIP on TiO2 (T-MIP) for selective recognition of DBT using 4-VP as functional monomer by a two-step self-assembly polymerization, as shown in Fig. 2. In the first stage, the monomer 4-VP and template DBT were dissolved in toluene for pre-polymerization for 10 min. After adding TiO2 nanoparticles for 2.5 h, cross-linking agent EDMA and initiator AIBN were added successively into the pre-polymerization mixture. The final mixture was purged with N2 and placed in a water bath. In the second stage, the mixture solution was stirred at 60 oC for 24 h through changing the temperature twice, and then T-MIP was collected by cooling, filtration and washing. The adsorption behavior of the T-MIP in simulated oil (octane solution) was evaluated from 25 to 45 oC to determine the kinetic and thermodynamic parameters. The results indicate that the adsorption capacity became larger as temperature increased, and the adsorption behavior followed the pseudo second order kinetic model and Freundlich isotherm model, indicating the adsorption was a multi-molecular layer type. Values of the Gibbs’ free energy are in the range from –7.11 to –9.36 kJ/mol over the temperature interval 25-45 oC, indicating that the adsorption was spontaneous and endothermic. In addition, the adsorption capacity and imprinting factor of T-MIP toward DBT were larger than those toward other similar compounds, including

BT, 4-methyldibenzothiophene (4-MDBT), and 4,6-dimethyldibenzothiophene (4,6-DMDBT). The adsorption equilibrium was reached in 300 min and equilibrium adsorption amount was 18.2 mg/g at 25 oC. According to the non-competitive binding assays at 45 oC, the adsorption capacity and imprinting factor towards DBT were 14.07 mg/g and 2.50, respectively, indicating that SMIP had good selective recognition and adsorption properties towards DBT. In addition, T-MIP was also synthesized using MAA as functional monomer [39]. The adsorption at 25 oC shows the equilibrium time and capacity were about 420 min and 12.1 mg/g, respectively, and adsorption capacity by selectivity test was about 7.988 mg/g, the imprinting factor reached 1.39. Hollow micro/nanomaterials with high surface area and large pore volume were widely applied in absorptive separation owing to their unique structures and properties. In 2011, based on the synthesis of T-MIP using 4-VP as the functional monomer [38], hollow MIP (H-MIP) was obtained by Xu et al. [40], in which TiO2 acted as the sacrificial support and was dissolved with HF (Fig. 2). H-MIP possessed a larger surface area (365.80 m2/g) and a larger pore volume (0.53 mL/g) than T-MIP, hollow non-imprinting material (H-NIP) and TiO2. Results show that H-MIP could selectively recognize DBT in octane solutions and the absorption abilities towards DBT in octane solutions followed the order H-MIP > T-MIP > H-NIP > TiO2, as shown in Fig. 3. At 25 oC, the adsorption of H-MIP towards DBT reached equilibrium within 240 min and adsorption capacity was 20.5 mg/g in adsorption isotherm measurement. At 45 oC, the adsorption capacity towards DBT attained 23.75 mg/g and the imprinting factor was around 1.76. The adsorption capacity towards DBT was calculated to be 11.25 mg/g at 25 oC according to adsorption selectivity test. By adding 200 mg of H-MIP into 6 mL of gasoline containing 398.89 mg/L sulfur and shaking for 240 min, the total sulfur concentration decreased to 323.77 mg/L. The ultimate sulfur content could be reduced to 250.99 mg/L by further adsorption. The adsorption capacity of H-MIP did not decline after five adsorption-regeneration cycles. The adsorption behavior was best described by the pseudo-second-order kinetics and Freundlich isotherm model. The adsorption was confirmed to be spontaneous and endothermic in nature by thermodynamic analysis. 3.3

K2Ti4O9 and K2Ti6O13 whiskers as supports

K2Ti4O9 and K2Ti6O13, with multilayer structure, large surface area, and good mechanical and thermal stability, were chosen as new supports of SMIP. In 2011, Xu et al. [41] prepared SMIP by graft polymerization using DBT as the template and K2Ti4O9 as the carrier. In this method, K2Ti4O9 activated with HCl was first modified by 3-(methylacryloxyl) propyltrimethoxy silane (MPS), obtaining MPS-K2Ti4O9. Then, 4-VP and AIBN were added into the solution containing MPS-K2Ti4O9 and toluene to obtain grafted K2Ti4O9 (4-VP-K2Ti4O9). Thereafter, 4-VP-K2Ti4O9 and DBT were dissolved in toluene followed by

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regeneration of K2Ti4O9-MIP.

Fig. 2 Synthesis of T-MIP and H-MIP using nano TiO2 as support [40].

Fig. 3 Effect of binding capacity for DBT, BT, 4-MDBT and 4,6-DMDBT on adsorption by H-MIP, H-NIP, T-MIP and TiO2 [40]. (Initial concentration: 2.71 mmol/L (DBT: 500 mg/L; BT: 365 mg/L; 4-MDBT: 540 mg/L; 4,6-DMDBT: 580 mg/L); temperature: 45 oC; time: 240 min)

adding AIBN and EDMA. Finally, the final product K2Ti4O9-MIP was achieved after filtering, washing, eluting and drying. Both K2Ti4O9-MIP and K2Ti4O9-NIP needed 360 min to attain the adsorption equilibrium, and the adsorption capacity towards DBT increased with increasing temperature and initial DBT concentration. The kinetic adsorption of K2Ti4O9-MIP followed the pseudo-second-order model while that of K2Ti4O9-NIP obeyed the pseudo-first order model. As for the isotherm data of K2Ti4O9-MIP and K2Ti4O9-NIP, they were well fitted by Freundlich model. The selective adsorption measurements proved the better recognition capacity and binding affinity of K2Ti4O9-MIP towards DBT at 45 oC than K2Ti4O9-NIP. The adsorption capacity towards DBT reached 26.5 mg/g, and the imprinting factor was about 1.96. The adsorption capacity was estimated to be 14.5 mg/g at 25 oC. The adsorption capacity at 25 oC was 27.5 mg/g, the calculated imprinting factor was around 2.16, and the adsorption equilibrium was reached in 360 min, as measured from isothermal adsorption. Approximately 80% of the initial adsorption capacity was preserved in the tenth cycle in the

Furthermore, Xu et al. [42] synthesized SMIP for adsorbing DBT by in-situ polymerization using K2Ti6O13 as the support, 4-VP as the functional monomer, EDMA as the cross-linker, AIBN as the initiator and toluene as the solvent. Similar experimental results were obtained when K2Ti4O9 was utilized as the support. The adsorption reached equilibrium in 300 min, which was endothermic and spontaneous in nature. At 25 oC, the adsorption equilibrium time was 300 min, and the adsorption capacity reached 23.2 mg/g in adsorption isotherm test. In selectivity tests, the adsorption capacity was 24.15 mg/g and the imprinting factor was 2.11 (the temperature was not defined by the reference). After sixth adsorption-desorption cycles, 84% of the initial adsorption capacity was maintained. Based on the above-mentioned experiments, H-MIP, which could selectively recognize DBT, was prepared by Yang et al. [43] using K2Ti4O9 as the sacrificial support. K2Ti4O9-MIP was synthesized by bulk polymerization using K2Ti4O9 as the carrier. At first, K2Ti4O9 activated by HCl was modified by MPS, resulting in MPS-K2Ti4O9. Meanwhile, 4-VP and DBT were dissolved in toluene and purged with nitrogen for a few hours, then MPS-K2Ti4O9, AIBN and EDMA were added into the solution obtaining K2Ti4O9-MIP. After that, HF was utilized to dissolve K2Ti4O9 to gain H-MIP with a specific area of 245.15 m2/g and an average pore diameter of 3.44 nm. At 25 oC, the absorption reached equilibrium in 180 min, adsorption capacity was 17.8 mg/g, and the imprinting factor was around 2.12, as calculated from adsorption isotherms. An appropriate increase in temperature enhanced the adsorption. The selectivity performance of H-MIP was favorable. The adsorption was spontaneous and followed the pseudo-second-order model and Freundlich equation. Nevertheless, adsorption capacity of H-MIP was slightly decreased compared with that of K2Ti4O9-MIP, which was due to the loss of recognition sites when dissolving the carrier. 3.4

CMSs as support

A series of SMIP for adsorbing DBT were synthesized using CMSs as support by the authors of the present contribution. CMSs as the support material have the following advantages: rich binding sites after modification, which are favorable for grafting or anchoring molecules; good chemical, thermal and mechanical stabilities; possibility in introducing external magnetic field effect during the molecular recognition by Fe-containing carbon particles to help solid-liquid phase separation. On the basis of these advantages, CMSs coated by molecularly imprinted film have a very broad use in deep desulfurization of fuel oil, bio-medicine and other fields. Several SMIPs, which could specifically identify and adsorb DBT in fuel oils for deep desulfurization, were

Yong-zhen Yang et al. / New Carbon Materials, 2014, 29(1): 1–14

Fig. 4 Preparation of MIP-CMSs.

obtained using CMSs as the support by surface chemical modification, including oxidation, silanization, functional monomer grafting and cross-linking. The process for preparation is shown in Fig. 4. First, CMSs were oxidized by mixed acids to obtain oxidized-CMSs with oxygen-containing groups for further reaction. Then, the activated CMSs were modified by different coupling agents, including 3-methacryloxypropyl trimethoxysilane (KH-570) and p-(chloromethyl) phenyl-trimethoxy silane (CMTMS). Afterwards, different monomers, including MAA and 2-arylamide-2-methyl propane sulfonic acid (AMPS), were grafted onto the surface of modified CMSs upon thermal-initiating or photo-initiating. MIPs on the surface of CMSs (MIP-CMSs) were obtained after the polymers were crosslinked in presence of the template DBT molecule. Finally, the adsorptive properties of as-obtained MIP-CMSs were evaluated by dynamic and static methods. In addition, other effective preparation routes of SMIPs were also introduced, such as reversible addition fragmentation chain transfer polymerization, and imprinting on porous CMSs as support. 3.4.1 MIP prepared on the surface CMSs(MIP-PMAA/CMSs) by in-situ polymerization

of

In-situ polymerization is in fact a kind of one-pot polymerization by adding all of functional monomer, template, cross-linking agent and initiator into disperse phase. It is a simple, easy and low-cost method. Therefore, on the basis of surface modification of CMSs, MIP on CMS surface (MIP-PMAA/CMSs) for adsorbing DBT molecule was obtained by in-situ polymerization with DBT as the template, EDMA as cross-linking agent, AIBN as initiator, MAA as the functional monomer, and chloroform as the solvent. The detailed process is shown in Fig. 5 [44]. Firstly, the silanized CMSs were formed from oxidized CMSs, which had been treated by mixed acid of HNO3 and H2SO4 in 1:3 (v/v), by reaction with a silane coupling agent KH-570. Then 0.3 g of silanized CMSs and 1 mL of MAA were added into a flask filled with 20 mL of DBT (0.369 g) solution in chloroform. The mixture solution was continuously stirred for 30 min, followed by the addition of 4 mL of EDMA and 0.065 g of AIBN. The polymerization reaction lasted for 24 h with magnetic stirring at 60 oC in water bath. Afterwards, the resultant materials were washed with ethanol to remove the

polymer physically adhered on the surface of CMSs and then with the mixed solvent of acetic acid and methanol (v/v = 1:9) to remove template DBT. Finally, MIP-PMAA/CMSs were obtained after drying. Non-imprinted CMSs (NIP-PMAA/CMSs) were prepared in the same way in absence of the template. The static adsorption experiments were performed at room temperature by adding 0.1 g of MIP or NIP-PMAA/CMSs into 25 mL of DBT solution in n-hexane with initial concentrations of 8 mmol/L. The concentration of DBT in mixed solution was determined using ultraviolet visible (UV–vis) spectrophotometer at 25 oC. The results indicate that the adsorption equilibrium time was 5 h and MIP-PMAA/CMSs had a higher capacity for DBT than NIP-PMAA/CMSs. Fig. 6 shows the adsorption isotherms of MIP-PMAA/CMSs and NIP-PMAA/CMSs towards DBT, which were obtained by investigating the final adsorption amount after 0.1 g of MIP-PMAA/CMSs or NIP-PMAA/CMSs was added into 20 mL of n-hexane solution with different DBT initial concentrations (C0) for 6 h at 25 oC. It can be seen that the adsorption amount of MIP-PMAA/CMSs towards DBT increased with the increasing DBT concentration and achieved a saturation value. However, NIP-PMAA/CMSs were saturated faster than MIP-PMAA/CMSs. Maximum adsorption capacity of MIP-PMAA/CMSs (109.5 mg/g) towards DBT was higher than that of NIP-PMAA/CMSs (78.2 mg/g). The imprinting factor of 1.99 suggested the difference in structure between MIP-PMAA/CMSs and NIP-PMAA/CMSs. The specific recognition cavities, containing some functional groups matching to DBT, were formed in polymer with the same shape and size as template DBT. Thus, MIP-PMAA/CMSs had recognition ability and molecular memory towards DBT, resulting in high binding affinity and selective ability for DBT. Although NIP-PMAA/CMSs had the same functional groups for recognizing DBT, no cavities were complementary to DBT in shape. The adsorption of NIP-PMAA/CMSs towards DBT seemed to be a non-specific adsorption. As a result,

Yong-zhen Yang et al. / New Carbon Materials, 2014, 29(1): 1–13

Fig. 5 Preparation of MIP-PMAA/CMSs by in-situ polymerization with DBT as a template, MAA as a monomer, EDMA as a cross-linking agent and AIBN as an initiator[44].

3.4.2 Preparation of MIP-PMAA/CMSs grafting-polymerization with iniferter

by

For preparing SMIP, in-situ polymerization is a simple,easy and low-cost route. But the process has some disadvantages: polymerization rate is low and molecular weight is low; polymerization is hardly controllable because reactant content and ratio have large influence on the product and reaction rate, causing product with a low density. Therefore, the authors [45] also prepared MIP-PMAA/CMSs by grafting-polymerization. Fig. 6 Adsorption isotherms of MIP-PMAA/CMSs and NIP-PMAA/CMSs by in-situ polymerization towards DBT at 25 oC for 6 h [44].

MIP-PMAA/CMSs had a higher binding capacity for DBT than NIP-PMAA/CMSs. Moreover, the adsorption kinetic followed the pseudo second order kinetics and the adsorption isotherm obeyed the Langmuir model.

Polymerization with iniferter (initiator-transfer agent-terminator) is a new technique for the living radical polymerization. Iniferter acts as an initiator, retarder, transfer agent and terminator at the same time [28]. Iniferter was chosen to modify CMSs for grafting functional monomer, because it can graft various monomers, control the polymerization degree and avoid gelation under mild reaction conditions. Polymerization with iniferter (initiator-transfer agent-terminator) is a new technique for the living radical

Yong-zhen Yang et al. / New Carbon Materials, 2014, 29(1): 1–13

Fig. 7 Preparation of MIP-PMAA/CMSs by iniferter-grafting polymerization [45].

show that the adsorption equilibrium was reached in 3 hand the maximal adsorption amounts of MIP-PMAA/CMSs and NIP-PMAA/CMSs towards DBT were 0.4821 mmol/g (88.71 mg/g) and 0.241 6 mmol/g (44.45 mg/g), respectively. The imprinting factor reached 4.60. The saturated adsorption capacities of MIP-PMAA/CMSs and NIP-PMAA/CMSs for diphenyl (DIP) were 0.099 4 mmol/g and 0.040 2 mmol/g, showing MIP-PMAA/CMSs possessed a better recognition and selectivity towards DBT. The adsorption followed the pseudo second order kinetics and Freundlich isotherm model. After sixth adsorption-desorption cycles, 80% of the initial adsorption capacity was maintained. Fig. 8 Adsorption kinetic curves of MIP-PMAA/CMSs and NIP-PMAA/CMSs by iniferter-grafting polymerization in 0.8 mmol/L of DBT at 25 oC [45].

polymerization. Iniferter acts as an initiator, retarder, transfer agent and terminator at the same time [28]. Iniferter was chosen to modify CMSs for grafting functional monomer, because it can graft various monomers, control the polymerization degree and avoid gelation under mild reaction conditions. The adsorption kinetics (Fig. 8) and competitive adsorption (Fig. 9) in DBT n-hexane solution at 25 oC were investigated with the aid of gas chromatography. The results

3.4.3 Preparation polymerization

of

MIP-PAMPS/CMSs

by

grafting

A strong intermolecular force between functional monomer and template is needed in the preparation of MIP with stable template-monomer composites and excellent properties. AMPS, in which –SO3H group has strong polarity to potentially combine DBT molecule through electrostatic interaction and hydrogen bond, was selected as the monomer to prepare stable composites of template and functional monomer. AMPS can be well soluble in water for aqueous polymerization to obtain a thermally stable polymer.

Yong-zhen Yang et al. / New Carbon Materials, 2014, 29(1): 1–14

Fig. 9 Adsorption kinetic curves of MIP-PMAA/CMSs by iniferter-grafting polymerization in 0.8 mmol/L of DBT and DIP at 25oC [45].

MIP-PAMPS/CMSs were syhthesized by grafting polymerization on the surface of CMSs [46], which were modified in prior by acidification and silanization, using DBT as a template, cholofrom as a solvent and AMPS as a functional monomer. The results of dynamic adsorption at 25 o C indicate that when 3 mL of DBT solution was injected into the packed column, the adsorption of 0.1 g of MIP-PAMPS/CMSs toward DBT reached saturation with the maximum adsorption amount of 2.54 mg/g, while the samecontent of NIP-PAMPS/CMSs adsorbed only 0.31 mg/g DBT. The imprinting factor reached 8.19. It is suggested that the MIP-PAMPS/CMSs had a much better recognition towards DBT than NIP-PAMPS/CMSs. 3.4.4 Reversible addition-fragmentation chain transfer method (RAFT) Living/controlled radical polymerization (CRP) has been well known for synthesis of polymers, because it can be used to synthesize a polymer with a well-defined structure and functional terminal group, controllable molecular weight with a narrow molecular weight distribution, and a copolymer with different structures. Compared with other CPR methods, RAFT radical polymerization now has been a hot topic with its advantages, including wide usable functional monomer range, mild polymerization condition, and being capable of adopting many polymerization methods. More reports about the surface modification of carbon materials containing carbon nanotubes by RAFT [47] provide a way for the modification of CMSs by RAFT. MIP-PAMPS/CMSs for adsorbing DBT were synthesized by RAFT radical polymerization with ultrasound method on the surface of silanized CMSs using AMPS as the monomer in our preliminary experiment. The dynamic adsorption experiments show that the adsorption capacity of MIP-PAMPS/CMSs was 2.27 mg/g better than that of NIP-PAMPS /CMSs. The selectivity factor of MIP-PAMPS/CMSs is 5.56 with BT as a competitive molecule in selectivity adsorption. 3.4.5

Porous CMSs-based SMIP

Improving the surface activity and compatibility of support with solvent is very important to enhance the imprinting efficiency in the preparation of MIP-CMSs, including the surface oxidation, silanization, and polymerization. Usually, CMSs do not have enough oxygen-containing functional groups on their surface and large surface area until they are modified by oxidization. Owing to the introduction of porous structures, porous carbon spheres [48,49] having a higher specific surface area, a higher reaction activity and a better developed pores with adjustable pore size than solid CMSs, have been widely applied in the fields of adsorption, catalysis or electrochemical energy storage and so on. Therefore, porous CMSs (PCMSs) can be the candidate for the support to simplify the process and reduce the cost. The PCMSs could be obtained after the CMSs were annealed , which were prepared by hydrothermal method using the biomasses (glucose, starch, cellulose, etc) as raw materials. The PCMSs prepared under optimum conditions would have uniform size, smooth surface, regular morphology and good dispersability in different solvents, and several kinds of oxygen-containing functional groups including hydroxyl, carbonyl and carboxyl group on their surface. As-obtained PCMSs would eliminate the necessity of surface activation and improve the reactivity for grafting, overcoming the difficulty in modification in the preparation of SMIPs. At present, the molecularly imprinted materials on the surface of PCMSs (MIP-PCMSs) using methacrylic acid (MAA) as monomer have been prepared. At first, PCMSs, with a uniform diameter of 160 nm and a specific surface area of 468.3 m2·g-1, were synthesized by hydrothermal method at 180 oC for 28 h combined with an annealing using aqueous glucose solution as raw materials. The as-obtained PCMSs were abundant in oxygen-containing functional groups on the surfaces, which promoted the surface activation, improved the grafting reactivity and overcame the difficulty in modification in the preparation of MIP-PCMSs. And then, based on the optimized parameters, MIP-PCMSs were prepared using DBT as a template, chloroform as a solvent, AIBN as in initiator and EDMA as a crosslinking agent. The adsorption results show that the adsorption capacity of DBT on MIP-PCMSs was 376.0 mg/g, 3.4 times greater than our previous results (109.5 mg/g), higher than that of non-imprinted polymers (NIP-PCMSs, 240 mg/g); the recognition factor was 1.57 and the adsorption equilibrium time was 3 h at 25 ℃. The pseudo second order kinetic model provided a better correlation for the adsorption kinetic of DBT onto MIP-PCMSs. Selective adsorption suggests that MIP-PCMSs had a better selective recognition towards DBT and the selectivity factor was 1.55.

4

Conclusions

In this paper, the frontier research development of bulk and surface MIPs for removing organic sulfur compounds in fuel oils is summarized with MIP on the surface of CMSs for

Yong-zhen Yang et al. / New Carbon Materials, 2014, 29(1): 1–14

adsorbing DBT highlighted. Compared with traditional molecular imprinting technologies for deep desulfurization, the surface molecular imprinting technology is able to create special molecular recognition sites on the surface of imprinted materials for increasing the adsorption efficiency, which is favorable for removal and rebinding of sulfur-bearing compounds. The molecular recognition was implemented at normal temperatures and pressures. There are numerous advantages such as simple process, low investment, low pollution, high selectivity, good reproducibility, and satisfactory octane preservability. Meanwhile, fine chemical products can be obtained by the recovery of adsorbed benzothiophenes. So the surface molecular imprinting technology for deep desulfurization is economical and environmentally friendly and has a potential application. To date, some types of inorganic material have been used as supports, including silica gel, TiO2, K2Ti4O9, CMSs, to prepare SMIPs. The research results provide the experimental basis and theoretical guidance for designing and preparing the green desulfurization materials. On the other hand, it should be pointed out that surface imprinting technology is still in its infancy of development for desulfurization, with the desulfurization ability lower than that of industrially applied S-Zorb process, which, as a kind of reactive desulfurization process, operates in presence of pressurized H2 and elevated temperature (300 oC). There are a lot of works to be done in order to improve adsorption capacity, selectivity and regeneration performance of imprinted materials.

among template, functional monomer, cross-linking agent and auxiliary solvent, optimizing SMIP process for scale-up purpose, fully considering the reaction thermodynamics and kinetics and the physical properties of materials. Correspondingly, the improvement in utilization of raw materials and polymers can be realized by studying affinity, selectivity and recognition mechanism of SMIP. Meanwhile, by-products will be reduced and atom economy will be more easily realized in accordance with green and ecological requirements.

In the preparation of MIPs, either the selection and utilization of the raw materials or the influence of poisonous remainders on the ecological environment is still an issue from the perspective of green chemistry. Some factors, such as the types and amounts of solvents, functional monomers, and cross-linking agents, seriously affect the ability of combination and selective adsorption. Because most of researches were in laboratory scale, the accurate prediction is still difficult for the choice and utilization of monomers and solvents, and the yield and adsorption efficiency of MIPs, resulting in a distance from actual industrial production. Furthermore, the commonly used reagents during preparation, such as MAA, ABIN, and chloroform, are poisonous or hazardous to the environment and human being. So the key to develop SMIPs with a higher adsorption capacity and selectivity should be the extensive application, high efficiency and green synthesis. The type and proportion of reactants should be optimized in order to prevent the formation of poisonous and hazardous product, and develop a low-cost, high-efficiency and very safe preparation of SMIPs.

[5] LIU Xu-guang, ZHAO Hui-jun. Progress in deep desulfurization

Further elucidation is needed for recognizing mechanism, mass transfer and interaction of binding sites between SMIPs and template molecules. Aided design technology can provide a low-cost, convenient and efficient tool for a deep study of molecular imprinting technology, such as scientifically designing and synthesizing SMIPs in theory, utilizing molecular stimulation technique to study pre-assembly system

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