Porous Carbon Microspheres: An Excellent Support To Prepare

Article pubs.acs.org/IECR

Porous Carbon Microspheres: An Excellent Support To Prepare Surface Molecularly Imprinted Polymers for Selective Removal of Dibenzothiophene in Fuel Oil Lei Qin,†,‡ Xiaorui Jia,†,‡ Yongzhen Yang,*,†,§ and Xuguang Liu*,†,‡ †

Key Laboratory of Interface Science and Engineering in Advanced Materials (Taiyuan University of Technology), Ministry of Education, Taiyuan 030024, China ‡ College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China § Research Center of Advanced Materials Science and Technology, Taiyuan University of Technology, Taiyuan 030024, China S Supporting Information *

ABSTRACT: Surface molecular imprinting is an effective way to prepare materials for selective removal of thiophene and its derivatives in transportation fuels and the support material is a crucial factor in determining the adsorption performance of resultant surface molecularly imprinted polymers (SMIP). In this work, a series of SMIP were prepared with dibenzothoiphene (DBT) as a template and various carbon materials as the support for desulfurization of fuel oil. The results demonstrated the porous carbon microspheres (PCMSs) as the support are far superior in preparing SMIP to nonporous carbon microspheres and conventional active carbon. Owing to its porous structure, regular spherical shape, high active surface area and thermal stability, SMIP/PCMSs obtained in this work exhibit excellent performance in the selective removal of DBT, with an outstanding adsorption capacity of 118 mg g−1 and favorable regenerability. In addition, the adsorption kinetic and isotherm outcomes suggest that physical interactions are mainly involved in the whole adsorption processes and the adsorption may be an integrated process of heterogeneous monolayer and multilayer adsorption. The thermodynamic analysis indicates that the adsorption takes place spontaneously and the adsorption process is endothermic, the pore diffusion and mass transfer as well as physical interaction between DBT and SMIP/PCMSs predominate in the whole adsorption process.

1. INTRODUCTION Severe environmental harassments like haze and acid rain that are caused by sulfur oxides (SOx) force public control of SOx emission. Because combustion of sulfur-containing organic compounds in fuel oils is the major source of SOx discharge, deep desulfurization of fuels has become an urgent task to relieve this issue.1,2 Sulfur-bearing compounds present in transportation fuels consist mainly of mercaptans, thioethers, disulfides, thiophene and their derivatives, in which thiophene and its derivatives account for more than 60% of the total sulfur. However, conventional hydrodesulfurization is less effective in removing such polyaromatic sulfur-containing compounds as benzothiophene (BT), dibenzothiophene (DBT) and their alkyl derivatives.3 Therefore, the removal of thiophene and its derivatives with high efficiency from fuels remains a great challenge in oil processing technology. In recent years, various alternative deep desulfurization approaches have been proposed, including oxidation,4,5 hydrodesulfurization,6,7 extraction,8,9 biodesulfurization,10,11 and adsorption desulfurization (ADS).12 Among these choices, ADS is considered as a promising approach for deep desulfurization because of its mild operation conditions and environmental benignity. Various adsorbents have been adopted or developed for the removal of thiophenic compounds, such as molecularly imprinted polymers (MIP),13 active carbon (AC),14,15 nanofibers16 and metal organic framework.17,18 Among them, MIP possesses unique advantages in selective removal of specific aromatic sulfur© 2015 American Chemical Society

containing compounds from fuel in the presence of other interferents. Thanks to the molecular recognition characteristic based on the memory effect of the space structure and binding sites of the template/target molecules, MIP shows great potential in detection,19 adsorption, extraction,20,21 separation22 and enrichment23 of particular molecules. However, traditional MIP prepared through bulk polymerization, dispersion polymerization or suspension polymerization embeds template molecules under thick polymer layer, which may hinder the subsequent elution of template molecules and the access of target molecules to the recognition sites. Moreover, it is also difficult to get MIP in uniform particles and tedious grinding and sieving are generally required. To overcome those problems, surface MIP (SMIP) has been developed by grafting a thin polymer film onto proper supports, such as TiO2,24,25 K2Ti6O13,26,27 SiO2, silica gel,28,29 carbon micropheres,30,31 carbon nanotubes32 and graphene.33 SMIP allows the imprinted sites to be distributed on the surface of the support, which shows the advantages of abundant accessible sites, high selectivity, easy elution, fast mass transfer and association kinetics. Moreover, the structures of template molecules such as thiophene and its alkylated derivatives are not destroyed by SMIP separation, which makes it possible to Received: Revised: Accepted: Published: 1710

August 3, 2015 October 23, 2015 November 13, 2015 November 13, 2015 DOI: 10.1021/acs.iecr.5b02837 Ind. Eng. Chem. Res. 2016, 55, 1710−1719

Article

Industrial & Engineering Chemistry Research Scheme 1. Schematic Procedure of SMIP Preparation

2. EXPERIMENTAL SECTION 2.1. SMIP Preparation. 2.1.1. Supports Preparation. Three types of carbon materials were used. First, PCMSs were synthesized through a hydrothermal method with glucose as the carbon source. The reaction kettles filled with glucose solution (0.4 mol L−1) were heated at 180 °C for 24 h in an oven. The hydrothermal products were washed by acetone, ethanol and deionized water separately. Then, PCMSs were acquired after annealing at 800 °C for 2 h.40 Second, NCMSs were prepared by CVD method with acetylene as carbon source, and then acidified by dispersing and soaking into mixed acids of H2SO4/HNO3 (3:1, v/v) for 20 min. Third, AC was obtained by dispersing and soaking the active carbon (obtained from Institute of Coal Chemistry, Chinese Academy of Sciences) into mixed acids of H2SO4/HNO3 (3:1, v/v) for 20 min. 2.1.2. SMIP Preparation. The overall processes of SMIP preparation on three carbon supports are depicted in Scheme1. Molecular imprinting is a technique for the creation of tailormade binding sites that with memory of the shape, size and functional groups of template molecules. The special identifiable adsorption property of SMIPs comes from their particular structures. SMIPs can be obtained by copolymerization of the functional monomers and cross-linking agents in the presence of template molecules. After template molecules removal, the recognition cavities complementary to the template were formed on the polymer layer, which can selectively rebind with the template molecules from a mixture of similar structured compounds.41,42 Several steps are necessary during the preparation of SMIPs on a certain support. Typically, the PCMSs should be modified with KH570 at first. KH570 is a commonly used amphiphilic agent in the preparation of an inorganic/organic complex. It was used to modify the surface chemical conditions of PCMSs by introducing a −CC− bridge for subsequent MAA grafting. PCMSs (0.3 g), silanization agent 3-methacryloxypropyltrimethoxysilane (KH570) (1 mL) and a mixed solvent

reuse these valuable adsorbates such as BT and DBT for other applications. Meanwhile, the quality of oil product is not degraded through the desulfurization with SMIP. The support material is a crucial factor determining the performance of SMIP; the merits of a good SMIP such as good monodispersion, homogeneous size-distribution and good mechanical, chemical and thermal stabilities are greatly related to the support material used for imprinting. Porous carbon materials, especially the porous carbon microspheres (PCMSs), synthesized by hydrothermal carbonization method34,35 are supposed to be ideal support materials for preparing SMIP. This synthesis route gives PCMSs many attractive properties36 such as low density, high mechanical stability, high porosity, high surface area, large pore volumes, mechanical and thermal stablilities, easy functionalization and good biocompatibility. The microporous structure makes PCMSs suitable for gas or liquid molecular adsorption and separation applications owing to their ability to induce strong van der Waals interactions with molecules. Although PCMSs have been widely used in separation,37 electrochemical energy storage38 and catalysis39 in recent years, there are few reports on the application of PCMSs as supports for preparing SMIP. In this work, size-controllable PCMSs were obtained as support materials by a hydrothermal method with glucose as the carbon source. On the basis of PCMSs, SMIP/PCMSs were then prepared with DBT as a template. The performance of SMIP/PCMSs in the selective removal of DBT from gasoline was investigated by means of static adsorption; the adsorption capacity, kinetics, isotherms, selectivity as well as the regeneration ability of SMIP/PCMSs were considered. For comparison, another two carbon materials were also used as supports to prepare SMIPs, viz., nonporous carbon microspheres (NCMSs) with similar particle size obtained by chemical vapor deposition (CVD) and active carbon (AC) with similar surface area. The superiorities of PCMSs over NCMSs and AC in preparing SMIP were then illustrated. 1711

DOI: 10.1021/acs.iecr.5b02837 Ind. Eng. Chem. Res. 2016, 55, 1710−1719

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Industrial & Engineering Chemistry Research

mL of model oil with an initial DBT concentration C0 of 0.8 mmol L−1 in n-hexane. The beaker was placed in a water bath with magnetic stirring at a fixed temperature (288, 298 or 308 K). The mixture was sampled at specific intervals. To do this, 1 mL of mixture was drawn out with a syringe and filtered by a syringe filter. Then, each filtrate was analyzed by GC to determine the remaining concentration (Ct, mmol L−1). The adsorption capacity (Q, mg g−1) can be calculated according to eq 1.

(60 mL) of C2H5OH/H2O (3:1, v/v) were added into a threeneck flask. Acetic acid was used to modify the pH value to 5. The mixture solution was stirred at 65 °C for 2 h. The products were filtered and washed with ethanol, then dried at 50 °C overnight to obtain PCMSs grafted with KH570 (KH570/ PCMSs). After that, KH570/PCMSs (0.2 g), H2O (20 mL), functional monomer methacrylic acid (MAA, 99%) (1 mL) and (NH4)2S2O8 (0.105 g) were sufficiently mixed in another three-neck flask. The mixture solution was stirred under N2 atmosphere at 80 °C for 24 h. The products were filtered and washed with ethanol, and then dried at 50 °C overnight to get polymethacrylic acid (PMAA) grafted KH570/PCMSs (PMAA/PCMSs). The grafting of a functional monomer is a vital step because certain intermolecular interactions are needed between functional monomer and template molecule. For the template molecule DBT, MAA is a favorable monomer because the complex DBT−MAA possesses a certain degree of binding energy to guarantee corresponding SMIPs the identification and adsorption of DBT. Finally, PMAA/PCMSs (0.1 g), DBT (0.1843 g) and chloroform (10 mL) were sufficiently mixed and stirred for 0.5 h at room temperature to preassemble DBT with PMAA/ PCMSs. The cross-linking agent ethylene glycol dimethacrylate (EDMA, 98%) (4 mL) was then added under continuous stirring and the temperature was raised to 50 °C for 10 h. The cross-linking agent EDMA is needed to form the reticularly structured polymer layer on the surface of MAA grafted support to fix the DBT−MAA complex sites. The products were washed successively by methanol/acetic acid (9:1, v/v) solution to remove DBT. The eluent was detected by GC to make sure there was no DBT remaining on SMIP/PCMSs. The washing away of the template molecule DBT left the imprinted sites on the polymer layer, which gives the selective adsorption ability to SMIPs. Finally, SMIP/PCMSs were collected after drying under vacuum overnight. Nonimprinted polymers based on PCMSs (NIP/PCMSs) were prepared in the same way only without adding DBT as the template. For comparison, SMIP supported on NCMSs and AC (SMIP/NCMSs and SMIP/AC) as well as NIP (NIP/NCMSs and NIP/AC) were also prepared. 2.2. Characterization. The morphologies and structures of the products were characterized by field emission scanning electron microscopy (FESEM; JSM-6700F, operated at 10 kV; Japan). Porous structures were characterized by automated surface area and pore size analyzer (Quadrasorb SI, USA). The changes in size distribution and appearance of agglomeration were characterized by dynamic light scattering (DLS, Malvern Zetasizer Nano-ZS90, UK). The modified products were characterized by Fourier transformation infrared spectroscopy (FT-IR; BRUKER TENSOR 27, Germany) and thermogravimetry (TG; Netzsch TG 209 F3, operating from 40 to 900 °C in N2 atmosphere at a heating rate of 10 °C min−1, Germany). The adsorption capacities of SMIP were determined from gas chromatography (GC; Shimadzu GC-2014C, detector temperature 300 °C, gasification temperature 300 °C, column temperature 220 °C; Japan). 2.3. Static Adsorption. Static adsorption tests were performed to investigate adsorption performance of SMIP and NIP. 2.3.1. Adsorption Kinetics. 50 mg of SMIP, NIP or support materials was introduced into a conical beaker containing 50

Q = MV (C0 − C t)/m

(1)

where M (g mol−1) is the molecular mass of adsorbate; V (L) is the volume of simulated oil solution; m (g) is the weight of adsorbent SMIP. 2.3.2. Adsorption Isotherms. SMIP/PCMSs samples (10 mg each) were introduced into different conical beakers. Then, 10 mL of the simulated oil with an initial DBT concentration (C0: 0.2, 0.4, 0.6, 0.8 and 1.0 mmol L−1) was added into each conical beaker. The conical beakers which contained those mixtures were placed in a water bath and stirred at a fixed temperature (288, 298 and 308 K separately) for 3 h. The mixtures were drawn out with a syringe and filtered by a syring filter. Then the filtrates were analyzed to determine the remaining concentration (Ce, mmol L−1) using GC. The equilibrium adsorption capacity (Qe, mg g−1) was calculated according to the following equation: Q e = MV (C0 − Ce)/m

(2)

where M (g mol−1) is the molecular mass of adsorbate; V (L) is the volume of simulated oil solution; m (g) is the weight of adsorbent SMIP. 2.4. Adsorption Selectivity. To investigate further the selectivity of SMIP/PCMSs toward DBT, the adsorption of DBT with BT, biphenyl and fluorene as interferents having similar molecular structure, on the SMIP/PCMSs was measured. The mixed solution of DBT/BT/biphenyl/fluorene was prepared and the concentration of each substance in the mixed solution was all kept as 0.8 mmol L−1. SMIP/PCMSs or NIP/PCMSs (40 mg) were added and dispersed into 40 mL of the mixed solution. Then the static adsorption experiments were carried out at 298 K for 3 h to make sure adsorption equilibria were reached. The concentrations of four substances in the supernatant were tested by GC. 2.5. Regeneration of Adsorbent. The reusability of MIPs was investigated through ten times of desorption−adsorption cycles. After static adsorption experiments, MIPs saturated with DBT upon equilibrium were collected by centrifugation. These saturated MIPs were washed with mixed solvent of methanol/ acetic acid (9:1, v/v) to elute DBT. The recovered MIPs were obtained by filtering and washing thoroughly with ethanol after completely removing residual DBT. Then the recovered MIPs were reused for the subsequent static adsorption experiments. The whole process of desorption−adsorption cycle was repeated for ten times in the same manner.

3. RESULTS AND DISCUSSION 3.1. Morphology and Structure of the SMIP Supports. The morphology of three kinds of support materials (PCMSs, NCMSs and AC) were assessed by FESEM. To graft effectively KH570 onto the support, NCMSs and AC were acidified in advance because the inert surfaces of NCMSs and AC cannot be effectively grafted with KH570.30 As illustrated in Figure 1712


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layer, which makes a contribution to the improvement of adsorption performance. For each support, SMIP has higher surface area and larger pore volume owing to the addition and elution of DBT during the SMIP preparation process. The FT-IR spectra of three SMIP and their corresponding supports and intermediates during molecular imprinting are shown in Figure 2. There are lots of oxygen-containing

1a,b,c, both NCMSs and PCMSs exhibit nice dispersibility and uniformity with an average diameter of 400 nm, whereas, AC is in the form of irregularly shaped blocks.

Figure 1. FESEM images of the support materials: (a) PCMSs, (b) NCMSs, (c) AC, (d) SMIP/PCMSs, (e) SMIP/NCMSs and (f) SMIP/AC.

As given in Table S1, the specific surface area of PCMSs is 607.5 m2 g−1, much larger than that of NCMSs (9.0 m2 g−1). Meanwhile, PCMSs have abundant micropores with an average pore size of 1.25 nm, whereas few pores can be determined on NCMSs. The specific surface area of AC is 1141.5 m2 g−1, with a lot of mesopores (average pore size of 2.10 nm). 3.2. Morphology and Structure of SMIP. The SEM images of SMIP grafted on three different supports (SMIP/ PCMSs, SMIP/NCMSs and SMIP/AC) are shown as Figure 1 (d, e, f). Compared with supports materials, SMIP displays rougher surfaces and little larger particles than the corresponding supports; the SMIP particles are agglomerated owing to the coating of an imprinted polymer layer on the supports. The changes in size distribution and appearance of agglomeration can be illustrated with DLS measurement, as shown in Figure S1. The kinetic diameter (dK, nm) was determined by the average of triplicate test valves at 25 °C. The dK value of PCMSs, NCMSs and AC is 533.1, 619.3 and 1076.0 nm, respectively. The particle size values are narrowly distributed. Almost no large particles can be detected. However, after imprinting, the dK value of SMIP/PCMSs, SMIP/NCMSs and SMIP/AC has increased to 580.1, 678.8 and 1312.3 nm, respectively, and the size distribution becomes wider. Large particles (5000−6000 nm) can be detected in the samples of SMIP/PCMSs and SMIP/NCMSs, which indicate the agglomeration during the imprinting. No larger particles were detected in the sample of SMIP/AC, because AC and SMIP/AC are irregularly shaped blocks, which prevents agglomerating. It should be noticed that the dK values tested by DLS are nearly 200 nm larger than particle sizes obtained from FESEM because the principles of these two methods are different. Thus, dK can only qualitatively describe the changes in size distribution and appearance of agglomeration, but cannot be used to determine the real size of particles in dry state. The surface area and porous structures of all SMIPs and SNIPs are listed in Table S1 to illustrate the influence of imprinting. Because the thermal stabilities of suppots and corresponding SMIPs are different, the samples have been preprocessed at diverse temperatures. Supports were preprocessed at 300 °C whereas SMIPs and NIPs were preprocessed at 100 °C. According to Table S1, for whichever support, the surface area and porous structures are increased after imprinting because of the covering of the loosen polymer

Figure 2. FT-IR spectra of different samples during the preparation of (a) SMIP/PCMSs, (b) SMIP/NCMSs and (c) SMIP/AC.

functional groups on the surface; the bands at 3435, 1631 and 1116 cm−1 are ascribed to the presence of hydroxyl groups − OH, −CO stretching vibration, and adsorption of −CO, respectively. From PCMSs to SMIP/PCMSs and from AC to SMIP/AC, no new absorption bands are found at other wave numbers, because no new functional groups are introduced onto the surface of PCMSs or AC. However, NCMSs shows a different situation. No obvious characteristic absorption bands appear for original NCMSs (Figure 2b), suggesting the inactivity feature of NCMSs, which hinders the further modification steps. Various oxygen-containing functional groups were introduced onto the surface of NCMSs by acidification and afterward modification processes.43 The results indicate that imprinted layer is formed to a certain degree. The TG curves in N2 from 100 to 900 °C of the SMIP products during various preparation stages are displayed in Figure 3 Compared with acidified NCMSs and AC, PCMSs prepared through annealing exhibit excellent thermal stability; the weight loss upon heating at 900 °C is only 3%. Correspondingly, the SMIP/PCMSs is also rather stable under high temperature with a total weight loss of 13%. The thermal stabilities of as-prepared NCMSs and as-obtained AC were excellent with almost no weight loss. However, they cannot be used directly for afterward modification for the lack of active sites in spite of their high thermal stability. After acidification, the stabilities of NCMSs and AC fell sharply with 1713


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Figure 3. TG curves of different samples during the preparation of (a) SMIP/NCMSs, (b) SMIP/PCMSs and (c) SMIP/AC. (TG conditions: 100−900 °C, 10 °C min−1, N2). Figure 4. Adsorption kinetic profiles of (a) SMIP/PCMSs, NIP/ PCMSs and PCMSs; (b) SMIP/NCMSs and NIP/NCMSs; (c) SMIP/AC, NIP/AC and AC. (Adsorption conditions: cDBT = 0.8 mmol L−1 (224 ppm), V = 10 mL, m = 10 mg, 25 °C.)

weight losses of 25% and 32%, respectively. Meanwhile, SMIP/ NCMSs and SMIP/AC performed worse at 900 °C with weight losses of 75% and 43%, respectively. Furthermore, PMAA grafting was the most pivotal step for the modification of the support surface, in which the functional groups for DBT recognizing and capturing were introduced. The amount of PMAA grafted on to the support surface determines adsorption ability toward DBT. The grafting degree (Dg) of PMAA on the support surface is defined as30 Dg = (Wlatter − Wformer)/Wformer·100%

conditions. The reason lies in the existence of imprinted cavities and suitable recognition sites on SMIP, which brings the specific binding of DBT molecule by electrostatic interaction and hydrogen bonds. Imprint factor ( f imp), defined in eq 4, can be used to illustrate intuitively the effectiveness of imprinting.28

(3)

fimp = Q e(SMIP)/Q e(NIP)

where Wformer and Wlatter are the weight losses of the SMIP products before and after the PMAA grafting, respectively. The Dg on the basis of TG results (Figure 3) of PMAA on PCMSs, NCMSs and AC is 83.3%, 32.5% and 7.7%, respectively. The poor grafting degree of PMAA on the surface of AC suggests that the irregular shape of support may hamper the covering of polymer layer. The lower grafting degree of PMAA on the surface of NCMSs than that of PCMSs may be astrided to the low specific area of NCMSs. The above results reveal that the SMIP was obtained as designed, with polymeric layers covering on the surfaces of three supports. Especially, owing to the porous structure, high surface area, uniform spherical shape, PCMSs are an ideal support to prepare SMIP; the resultant SMIP/PCMSs exhibit the best thermal stability and highest grafting degree. 3.3. Kinetic Adsorption of DBT on SMIP. The kinetic curves of DBT adsorption on various SMIP are shown in Figure 4. Because there was hardly any adsorption on NCMSs, the profile of DBT adsorption on NCMSs is not presented in Figure 4b. All the profiles are similar in adsorption tendency: the adsorption amount increases rapidly at the beginning 30 min, and then gradually levels off; saturated adsorption is achived after around 2.5 h. However, the adsorption capacities of SMIP are apparently higher than those of NIP at the same

(4) −1

The saturated adsorption amounts (Qe, mg g ), equilibrium times (t, min) and imprint factors of different SMIP are summarized in Table 1. The saturated adsorption amount (Qe) Table 1. Adsorption Performance of SMIP/PCMSs toward DBT Compared with Other Literature Outcomes support

te (min)

Qe (mg g−1)

f imp

K′ (beyond BT)

ref.

PCMSs NCMSs AC TiO2 K2Ti6O13 silica gel CMSs GO

150 150 150 240 240 140 180 35

118.03 95.67 120.21 26.74 22.03 57.4 88.83 181.9

1.81 2.37 1.16

2.83

this work this work this work 26 27 29 30 33

1.32 1.73 1.97 1.71

1.62

of DBT on SMIP/PCMSs and SMIP/AC are obviously larger than that of SMIP/NCMSs. Compared with other previous works (Table 1), the SMIP/PCMSs can be treated as a promising adsorbent for removal of DBT from gasoline, with outstanding adsorption capacity, though Table 1 can only give a simplified comparison of the adsorption ability of adsorbent 1714


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involved in the adsorption processes and the whole adsorption rate is controlled by the intermolecular forces including hydrogen bonds and electrostatic interactions between template molecules and recognition sites. It can be speculated that by increasing the specific surface area,14 the microporous structures promote adsorption capacity of SMIP/PCMSs toward DBT. 3.5. Adsorption Isotherm of DBT on SMIP/PCMSs. The saturation adsorption capacities of SMIP/PCMSs toward DBT were determined at different DBT concentrations (0.2, 0.4, 0.6, 0.8, 1.0 mmol L−1) and temperatures (288, 298 and 308 K). The saturation adsorption data were fitted with the Langmuir (eq 7), Freundlich (eq 8), Dubinin−Radushkevich (eq 9) and Scatchard (eq 10) isothermal equations to estimate the adsorption features of SMIP/PCMSs:27,33,45

materials because the adsorption conditions are diverse in different reports. On the other hand, with respect to imprint factors, the f imp values of SMIP/PCMSs and SMIP/NCMSs are both larger than that of SMIP/AC. The poor imprint factor of SMIP/AC may be caused by its irregular morphology, which obstructs the process of modification and its large particle size, which widely limits the overall accessible surface area for imprinting and consequently limits the quantity of imprinted cavities. All these further illustrate that the porous structures and regular spherical shape make PCMSs a superior support for preparing SMIP. 3.4. Adsorption Kinetics of SMIP/PCMSs. The adsorption performance of SMIP/PCMSs was further investigated in consideration of its outstanding adsorption capacity. Figure 5

illustrates the adsorption profiles of DBT on the SMIP/PCMSs and NIP/PCMSs at various temperatures. The adsorption capacity of DBT increases rapidly at the beginning and levels off at last. During the first 40 min, there are large amounts of unoccupied imprinted cavities on the surface of SMIP/PCMSs. Thus, template molecules can easily access and be adsorbed on the binding sites, exhibiting a rapid rise of the adsorption amount toward DBT. The adsorption rate decreases after 40 min because most of cavities have been occupied and the DBT internal diffusion has to overcome increasing mass transfer resistance. The kinetic curve of NIP/PCMSs is similar to that of SMIP/ PCMSs, but with lower adsorption capacity (65.08 mg g−1), which can be attributed to the lack of suitable imprinted cavities and recognition sites. Moreover, the adsorption capacity of SMIP/PCMSs reaches 118 mg g−1 at 298 K. To understand further the adsorption kinetics, the pseudofirst-order model (eq 5) and pseudo-second-order model (eq 6) are applied to fit the kinetic data.33,44

2

t /Q t = 1/(k 2Q e ) + t /Q e

(7)

ln Q e = ln Ce/n + ln kF

(8)

ln Q e = ln Q m − βε 2

(9)

Q e/Ce = k Sb − Q eb

(10)

ε = RT ln(1 + 1/Ce)

(11)

where Qe (mg g−1) and Ce (mmol L−1) are adsorption amount and concentration of DBT at adsorption equilibrium, respectively; Qm (mg g−1) represents maximum adsorption; kL (L mmol −1) is the Langmuir constant, which is related to the affinity of binding sites; kF and n are both Freundlich constants that represent the adsorption capacity and adsorption favorability, respectively; β (mol2 kJ−2) is the coefficient related to the mean free energy of adsorption; ε is the Polanyi potential that can be obtained according to eq 11; kS (mg g−1) and b (mmol L−1) are the Scatchard adsorption isotherm parameters; R is the ideal gas constant (R = 8.314 · 10−3 kJ mol−1 K−1); T (K) is the absolute temperature. The calculated parameters are listed in Table S3. And a comparison of Langmuir, Freundlich and Dubinin−Radushkevich models for DBT adsorption on SMIP/PCMSs is illustrated by using nonlinear regression, as shown in Figure 6. The Scatchard models are not included in Figure 6 because the R2 values of Scatchard fittings are very low. To simulate the adsorption conditions in real oil, the maximal initial DBT concentrations were chosen around 0.8 mmol L−1, whereas the large adsorption capacity of SMIP/PCMSs makes the isotherm

Figure 5. Kinetic curves for adsorption of DBT dissolved in n-hexane solution on SMIP/PCMSs or NIP/PCMSs at different temperatures.

lg(Q e − Q t ) = lg Q e − k1t /2.303

Ce/Q e = Ce/Q m + 1/(Q mkL)

(5) (6)

−1

where Qe and Qt (mg g ) are the amounts of DBT adsorbed on SMIP or NIP at equilibrium and time t (min), k1 (min−1) and k2 (g mg−1 min−1) are the pseudo-first-order and pseudosecond-order adsorption rate constant, respectively. As given in Table S2, the pseudo-first-order model fits some better for all the adsorption data than the pseudo-second-order model. It was assumed that physical interactions are mainly

Figure 6. Adsorption isotherm curves for adsorption of DBT dissolved in n-hexane solution on SMIP/PCMSs at different temperatures. 1715


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Industrial & Engineering Chemistry Research curves increase rapidly without reaching the final plateaus. However, the values of Freundlich constant n are all larger than 1, representing the favorable adsorption conditions. The Dubinin−Radushkevich isotherm is used to estimate the porosity of adsorbent. The high R2 values for Dubinin− Radushkevich fittings can be ascribed to the porous structures of SMIP/PCMS.45 At the same time, the Langmuir and Freundlich models also fit well with the experimental data. Actually, both of the two models have some limitations to present different (covalent or noncovalent) adsorption processes on SMIP at different concentration ranges. The Langmuir isotherm model refers only to the system with one kind of binding site, whereas the Freundlich equation does not model the saturated portion in the adsorption isotherm curves. The Scatchard plot, known as the independent site-oriented adsorption model, helps evaluate the nature of adsorption.45 The low R2 values for Scatchard fitting indicate a heterogeneous surface of SMIP/PCMSs and various types of active sites are involved in the adsorption process, which fits to the Freundlich model. It can be concluded that the SMIP layer on the surface of the support material PCMSs presents some degree of heterogeneity and the adsorption on the surface of SMIP/ PCMSs may be an integrated process of monolayer and multilayer adsorption. Temperature is an important factor that influences adsorption of DBT from n-hexane solution onto SMIP/ PCMSs. From 288 to 308 K, the saturation adsorption amount increases with increasing temperature, suggesting the endothermic nature of adsorption. On the contrary, the expected effect of temperature on adsorption isotherms is a decrease in adsorption with increasing temperature because the previous theoretically calculated results suggest that the interaction between template molecule DBT and functional monomer MAA is exothermic.46 However, most of the adsorptions occurring on solid−solute interface have been reported to be promoted by raising temperature.24−27,47,48 This phenomenon can be explained as the following two aspects. On one hand, as suggested above, physical interactions between DBT and SMIP/PCMSs are mainly involved in the adsorption processes than the intermolecular forces between template molecules and recognition sites. The mass transfer and pore diffusion processes as well as adjustment of DBT onto the imprinted site are accelerated along with the rising of temperature.47 On the other hand, under the real adsorption conditions, the influence of solvent should be taken into consideration.49 The rise in adsorption temperature weakens the interactions between adsorbate and solvent. Therefore, the increase in temperature favors adsorption of SMIP/PCMSs toward DBT. Thus, the whole adsorption exhibits as an endothermic process. 3.6. Adsorption Thermodynamics of SMIP/PCMSs. The standard free energy change ΔGθ, standard enthalpy change ΔHθ, and standard entropy change ΔSθ are calculated according to eqs 9−11).50 ΔG θ = −RT ln Kc

T (K) is the absolute temperature; R is the ideal gas constant (R = 8.314 J mol−1 K−1). The thermodynamic parameters for the adsorption of DBT on SMIP/PCMSs at 288, 298 and 308 K are presented in Table 2. Negative ΔGθ and positive ΔSθ values confirm that the Table 2. Thermodynamic Parameters for Adsorption toward DBT in n-Hexane Solution onto SMIP/PCMSs ΔGθ (kJ mol−1)

ΔHθ (kJ mol−1)

ΔSθ (kJ mol−1 K−1)

288 298 308

448.00 712.70 677.45

−14.62 −16.28 −16.69

15.46

0.0508 0.0547 0.0542

Figure 7. Selective adsorptions of SMIP/PCMSs toward DBT beyond BT, biphenyl and fluorene.

It is obviously ascribbed to that the cavities and imprinted sites are tailor-made for template DBT. The reason for this is that the cavities imprinted by DBT do not match the other three analogs in size, shape and spatial arrangement of interaction sites. Only molecules that are in specific size and conformational structure can be well trapped into the cavity. On the other hand, BT owns a smaller molecular size, which makes it easier to enter the cavity imprinted by DBT. However, its mismatched spatial shape30,33 and higher energy gaps14 hinder its stay in the cavity and thus lead to lower adsorption. Biphenyl and fluorene are dimensionally similar to DBT, but their relatively larger steric hindrance besides the lack of specific interaction sites also results in their lower adsorption by SMIP/ PCMSs. These results suggest that SMIP/PCMSs possess the property of selective adsorption toward DBT. The specific

(10)

ΔS θ = (ΔH θ − ΔG θ )/T

Kc (L mol−1)

adsorption takes place spontaneously. The absolute values of ΔGθ at various temperatures are lower than 20, which indicates that the physical adsorption is in charge of the whole adsorption process. This result is accordant with the adsorption kinetic analysis. Moreover, the positive ΔHθ represents the adsorption process is endothermic, which is accordant with the adsorption isotherm analysis, also suggesting that the physical interaction as well as pore diffusion and mass transfer predominate in the whole adsorption process. 3.7. Adsorption Selectivity of SMIP/PCMSs toward DBT beyond Interferents. The adsorption selectivity toward DBT on SMIP/PCMSs in the presence of interferents with similar molecular structure such as BT, biphenyl and fluorene was further considered by static adsorption. In comparison with NIP/PCMSs and PCMSs, as shown in Figure 7, SMIP/PCMSs exhibits higher adsorption capacity and also excellent selectivity toward DBT.

(9)

ln Kc = −ΔH θ /(RT ) + const

T (K)

(11) −1

where Kc is equilibrium constant (Kc = Qe/Ce, L mol ); Qe (mg g−1) and Ce (mmol L−1) are adsorption amount and concentration of DBT at adsorption equilibrium, respectively; 1716


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R2 values for Dubinin−Radushkevich fittings can be ascribed to the porous structures of SMIP/PCMS. At the same time, the Langmuir and Freundlich isothermal models also fit well with the experimental results. Besides, the low R2 values for Scatchard fitting indicate a heterogeneous surface of SMIP/ PCMSs and various types of active sites are involved in the adsorption process. The adsorption occurring on the surface of SMIP/PCMSs can be considered as an integrated process of heterogeneous monolayer and multilayer adsorption. At the same time, the thermodynamic results are accordant with the adsorption kinetic and isothermal analysis. It is the physical adsorption that is in charge of the whole adsorption process and the adsorption process is endothermic because the pore diffusion and mass transfer as well as physical interactions between DBT and SMIP/PCMSs predominate in the whole adsorption process. Moreover, the adsorption ability of SMIP/ PCMSs maintains above 90% after 10 cycles, indicating that SMIP/PCMSs is regenerable with nice structural stability. These suggest that SMIP/PCMSs can be a potential adsorbent for selective removal of DBT from gasoline.

recognition ability of SMIP/PCMSs stems from the accordant molecular size and shape together with intermolecular affinity between matrix and substance. The distribution coefficient (Kd, L mol−1), selectivity coefficient (K) and the relative selectivity coefficient (K′) are calculated according to eqs 12−14):26,30,31 Kd = Q e/Ce

(12)

K = Kd(DBT)/Kd(interferent)

(13)

K ′ = K(SMIP)/K(NIP)

(14)

where Qe (mg g−1) is the equilibrium adsorption capacity; Ce (mmol L−1) is the equilibrium concentration. As given in Table S4, the distribution coefficient (Kd) of SMIP/PCMSs for DBT is much greater than that for interferents BT, biphenyl and fluorene. The selectivity coefficients (K) of SMIP/PCMSs are higher than that of NIP/PCMSs, which indicates the selectivity of SMIP/PCMSs for template molecules. The relative selectivity coefficient K′ for BT, biphenyl and fluorene is 2.83, 2.14 and 2.18, suggesting the enhanced extent of adsorption affinity and selectivity of SMIP/ PCMSs toward template molecule DBT with respect to NIP/ PCMSs. Owing to their matched binding sites and cavities size, SMIP/PCMSs exhibits higher adsorption capacity for template molecules than NIP/PCMSs. These results illustrate the high selective binding ability of SMIP/PCMSs toward template DBT. 3.8. Regeneration of SMIP/PCMSs. As a promising adsorption desulfurization adsorbent for future practical applications, the regeneration potential and stability are vitally important. As shown in Figure S2, the adsorption capacity of SMIP/PCMSs toward DBT maintains above 90% after 10 cycles, which means that the binding and release ability of SMIP/PCMSs remains nearly unchanged. The special memory effect and recognition ability toward DBT are maintained, indicating that SMIP/PCMSs can be regenerated with nice structural stability. SMIP/PCMSs can be considered as an effective desulfurization adsorbent for recycled use with no remarkable decrease in adsorption capacity.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b02837. Specific surface areas and porous structures of each supports and SMIPs (Table S1); kinetic and isothermal constants of SMIP/PCMSs adsorption (Table S2, Table S3); selective adsorption performance of SMIP/PCMSs (Table S4); DLS outcomes of each supports and SMIPs (Figure S1); regeneration performance of SMIP/PCMSs (Figure S2) (PDF).

■

AUTHOR INFORMATION

Corresponding Authors

*X. Liu. E-mail: [email protected]. *Y. Yang. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

4. CONCLUSIONS With hydrothermally synthesized PCMSs as the support, SMIP/PCMSs were prepared with DBT as a template. The performance of SMIP/PCMSs in removal of DBT from gasoline was investigated by means of static adsorption; the adsorption capacity, kinetics, isotherms, selectivity as well as the regeneration ability of SMIP/PCMSs were considered. For comparison, two other carbon materials were also used as supports to prepare SMIPs, viz., NCMSs with similar particle size and AC with similar surface area. The results demonstrated that, in preparing SMIP, PCMSs as a support are far superior to NCMSs and AC. Owing to their porous structure, regular spherical shape, high active surface area and thermal stability. SMIP/PCMSs obtained in this work exhibit excellent performance in the selective removal of DBT, with an outstanding adsorption capacity of 118 mg g−1 at 298 K and the relative selectivity coefficient K′ for BT, biphenyl and fluorene is 2.83, 2.14 and 2.18, respectively. In addition, the adsorption kinetic outcomes suggest that the pseudo-first-order model fits better with all the adsorption data. It is the physical interactions that are mainly involved in the adsorption processes. In terms of adsorption isotherm analysis, the highest

ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (No. 21176169, 51152001), International Science & Technology Cooperation Program of China (No. 2012DFR50460), Research Project Supported by Shanxi Scholarship Council of China (No. 2012-038) and Shanxi Provincial Key Innovative Research Team in Science and Technology (No. 2012041011).

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