Article pubs.acs.org/EF
Adsorptivity of a Hyper Cross-Linked Ionic Polymer Poly(vinyl imidazole)-1,4-bis(chloromethyl)benzene for Thiophenic Sulfurs in Model Oil Jing Zhang,†,‡ Hui-hui Xu,†,‡ Ying-zhou Lu,‡ Hong Meng,‡ Chun-xi Li,*,†,‡ Biao-hua Chen,†,‡ and Zhi-gang Lei†,‡ †
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China
Energy Fuels 2016.30:5035-5041. Downloaded from pubs.acs.org by TULANE UNIV on 01/20/19. For personal use only.
‡
ABSTRACT: A hyper cross-linked ionic polymer (HCIP) PVimBCmBn was synthesized through free radical polymerization of 1-vinylimidazole (Vim) and quaternized cross-linking with 1,4-bis(chloromethyl) benzene (BCmBn). Its composition, structure, morphology, specific area, porous structure and thermal stability were characterized, and the desulfurization performance was investigated. The results show that the HCIP prepared with stoichiometric ratio of PVim and BCmBn are ultrafine powders with specific area of 99.6 m2·g−1 and average pore size of 16.1 nm, being micro/mesoporous materials. The S-removability of HCIP is superior to its precursor (ethyl imidazole) and the imidazolium-based ionic liquids (ILs), which manifests the importance of the available micropores for the accommodation of sulfur molecules with less energy barrier. The adsorption isotherms of PVimBCmBn follow the Langmuir equation with its saturated adsorbance being 7.0, 5.2, and 4.1 mgS·g−1, respectively, for dibenzothiophene, benzothiophene, and thiophene in model oil at 293 K. However, its adsorptive removal ability for DBT in diesel oil is quite low due to its limited S-selectivity with respect to the abundant confused ring aromatics wherein.
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INTRODUCTION Ionic liquids (ILs) have attracted increasing interest in the development of alternative deep desulfurization technologies of fuel oils, such as extraction desulfurization1−5 and catalytic oxidation extraction desulfurization,6 because of their insolubility in oil and good extraction ability for thiophenic sulfurs. Their desulfurization mechanism is assumed to arise from specific interactions between ionic species and thiophenic ring, e.g., π−π complexation,7,8 dispersion, and hydrogen bonding9,10 interactions depending on the ILs involved. However, their industrial application in oil desulfurization is barely available and encounters many challenges, e.g., high viscosity and low mass transfer efficiency, inevitable contamination to the oil stream by entrainment and/or partial dissolution, as well as the restricted designability by the “liquid” attribute at ambient temperature, that is why the real number of ILs is far less than their estimated ones, being over 106.11 In contrast, the polymerized ILs (PILs)12,13 with porous structure might be a promising sorbent since, on one hand, they can remain the “extractive” desulfurization ability of the IL monomers, and on the other hand, provide additional desulfurization ability by the available micropores and large specific area. It is well-known that the microporous structure is essential for the adsorptivity due to its better accommodation for sulfurs with least energy barrier and the high specific area for the surface adsorption, and the excellent adsorptive desulfurization capability of metal− organic-frameworks (MOFs) can be attributed to these attributes.14,15 In comparison with ILs, PILs may get rid of cross contamination to the oil and have a better designability without the confinement of “liquidity” of ILs. Based on the above considerations, a series of HCIPs were prepared in this paper by polymerization of 1-vinyl imidazole and quaterniza© 2016 American Chemical Society
tion of the imidazole ring with 1,4-bis(chloromethyl) benzene. The structures of the polymers were characterized by various instrumental analysis, their adsorption performance for thiophenic sulfurs were studied and discussed in detail to elucidate the function of the imidazolium ring and the porous structure for adsorptive desulfurization.
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EXPERIMENTAL DETAILS
Chemical Materials. Thiophene (T, > 99%), benzothiophene (BT, > 97%) and dibenzothiophene (DBT, > 99%) were purchased from J&K Scientific Ltd. 2,2-azobis(isobutyronitrile) (AIBN, AR) was from Tianjin Guang-fu Fine Chemical Industry Institute. DMF, ethyl acetate, toluene, and octane with AR grade were from Beijing Yili Fine Chemical Ltd. 1,4-Bis(chloromethyl) benzene (BCmBn, AR) and 1vinyl imidazole(Vim, AR) were from Aladdin chemical reagent. All reagents were used as received. The diesel oil used was bought from a gas station and was pretreated three times at room temperature for 2 h each with 10 wt% anhydrous FeCl3 to remove the oxygen containing additives. Synthesis of PVim and PVimBCmBn. Synthesis of PVim. Twenty-five grams (0.266 mol) Vim, 150 g toluene, and 0.15 g AIBN were added into a 300 mL oxygen bomb reactor at room temperature and stirred magnetically. The gas phase of the reactor was purged five times with high pressure N2 to remove oxygen completely, and then the reactor was heated for 12 h at 70 °C in a thermostatic bath. Finally, the polymer PVim as light yellow particles was obtained by filtration, and dried by rotatory evaporator at 70 °C under reduced pressure with yield of 97.8%. Synthesis of PVimBCmBn. Three grams (0.032 mol) PVim and 140 g DMF were added to a 250 mL conical flask, stirred magnetically for 1 Received: January 25, 2016 Revised: May 19, 2016 Published: May 20, 2016 5035
DOI: 10.1021/acs.energyfuels.6b00179 Energy Fuels 2016, 30, 5035−5041
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Energy & Fuels
the random quaternization between −CH2Cl group and imidazole ring, which is helpful to form a highly cross-linked structure with good porosity and high specific area, leading to a higher adsorptivity for the thiophenic sulfurs. In contrast, as PVim solution is slowly dropped into excessive amount of BCmBn, most of the imidazole rings tend to react with one BCmBn molecule, forming a branched ionic polymer with structure B (see Scheme 1), and the residue −CH2Cl groups can only react partially with the subsequent PVim due to their steric hindrance effect. Therefore, PVimBCmBn(B) shows a much lower quaternization, cross-linkage, and porosity. Similarly, as BCmBn is slowly added into excessive PVim, its two −CH2Cl groups tend to react with the abundant imidazole rings, forming cross-linked structure (A) and noncross-linked structure (C), and the latter further reduces the possibility of cross-linking of PVim chains and pore-forming, leading to a poor porosity of PVimBCmBn(C). Influence of the Cross-Linking Agent Usage on the Adsorptivity of PVimBCmBn. BCmBn here is not only a cross-linking agent but also a quaternizing reagent, and the stoichiometric mole ratio of Vim to BCmBn is 2:1. In order to study its influence on the adsorptivity of PVimBCmBn, six cross-linked polymers were prepared with varying stoichiometric amount of BCmBn, i.e., 0.4, 0.6, 0.8, 1, 1.2, and 1.4, according to the procedure described in Synthesis of PVim and PVimBCmBn, and their adsorbance for DBT was compared, as shown in Figure 2. It is found that the sulfur adsorptivity of PVimBCmBn increases first and then decreases with the usage of BCmBn, and the best performance is achieved at the stoichiometric ratio of PVim to BCmBn. The results indicate that neither excessive nor insufficient usage of BCmBn is good for the formation of porous structure and cross-linking, since the former case is similar to (B) and the latter is similar to (C) in Scheme 1. On this basis, the appropriate synthesis condition of PVimBCmBn is determined, viz. DMF as solvent, PVim and BCmBn premixed directly at stoichiometric ratio and reacted at 70 °C for 30 h. Characterization of the Polymers. Molecular Weight of PVim. The molecular weight (Mw) of PVim was determined by gel permeation chromatography (GPC515−2410, American Waters) using DMF solvent. As shown in Table 1, the PVim polymer has a molecular weight of about 1 070 500, and its D value is close to 1, implying a narrow distribution of polymerization degree. The average polymerization degree of the PVim is estimated as 11 390. Elemental Analysis of the Polymers. Elemental analysis was performed for PVim and PVimBCmBn by elemental analyzer (varioEL cube, Elementar, Germany) with respect to N, C, and H. As shown in Table 2, the total content of these elements in PVim is 91.3%, being less than 100%, while the C/N ratio is close to the theoretical value, which may be caused by the water impurity of PVim, as evidenced by the IR spectra of the polymers. The total content of N, C, and H in PVimBCmBn is 78.6%, and the rest is Cl and O arising from the hygroscopicity of polymer. Besides, the C/N ratio of PVimBCmBn is about 8% lower than its theoretical value, this may arise from the incomplete cross-linking of PVim, and thus some dissolution loss of PVim in the final HCIP product. FTIR Analysis of the Polymer. Infrared spectra of PVim and PVimBCmBn were analyzed by FTIR (Nicolet 8700, American Nicolet). As shown in Figure 3, the following characteristic peaks were found for the imidazole ring of PVim, namely,
h at room temperature to completely dissolve the polymer, and then 2.79 g (0.016 mol) BCmBn was added. The reaction was conducted in a water bath at 70 °C for 30 h with magnetic stirring, and light yellow particles were precipitated gradually as the cross-linking proceeded. The precipitate was filtered, washed three times with ethyl acetate, and dried overnight at 100 °C in an oven, with product yield of 89.5%. Characterization of the Polymer Sorbent. The composition, structure, morphology, specific area, pore structure, and thermal stability of the polymers were characterized by elemental analysis, gel permeation chromatography, infrared spectrum, scanning electron microscopy, physical adsorption analyzer, and thermogravimetric analysis methods, respectively. Adsorption Measurement of the Sorbent. The adsorption experiment was conducted in a 50 mL conical flask, to which mc grams of sorbent and m0 grams of model oil was added and stirred magnetically at room temperature for 2 h to reach adsorption equilibrium. The sulfur concentration of the oils before and after adsorption, viz. C0 and Ce, was analyzed by sulfur nitrogen analyzer, whereby the equilibrium adsorbance of the sorbent qe can be calculated as follows
qe =
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(C0 − Ce) × m0 mc
(1)
RESULTS AND DISCUSSION Influence of Synthetic Process on the Adsorptivity of PVimBCmBn. The molecular structure and architecture of PVimBCmBn depends on the stoichiometric ratio of PVim to the cross-linking agent BCmBn and the feeding mode. Here three cross-linked products, noted as PVimBCmBn(A), PVimBCmBn(B), and PVimBCmBn(C) were prepared at the following feeding modes under the same other conditions as described in Synthesis of PVim and PVimBCmBn. (A) All PVim and BCmBn were premixed and reacted. (B) PVim dilute solution of DMF was slowly added and reacted with BCmBn dilute solution. (C) With an opposite feeding process of (B). As shown in Figure 1, the adsorption capacity of PVimBCmBn(A) for DBT is much higher than that of (B)
Figure 1. Adsorption capacity of PVimBCmBn (A, B, C) for DBT model oil. Conditions: initial sulfur concentration750 ppm, mass ratio of sorbent and oil 1:20, 298 K, magnetic stirring 2 h.
and (C), indicating that the direct mixing approach is superior to the successive dropping one of the reactants. The performance difference of these sorbents may arise from their structure difference in cross-linkage, quaternization degree and microscopic assembly. The possible structure of PVimBCmBn prepared under the above feeding modes is presented in Scheme 1. As a matter of fact, mode (A) is more favorable for 5036
DOI: 10.1021/acs.energyfuels.6b00179 Energy Fuels 2016, 30, 5035−5041
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Energy & Fuels Scheme 1. Major Structure of PVimBCmBn(A, B, C) Prepared with Different Feeding Modes
Table 2. Element Analysis Result of PVim and PVimBCmBn
polymer
N [%]
C [%]
H [%]
total [%]
PVim PVimBCmBn
26.879 15.838
58.08 56.21
6.361 6.526
91.3 78.6
test value C/N (mol)
theoretical value C/N (mol)
2.52 4.14
2.50 4.5
Figure 2. Sulfur adsorptivity of PVimBCmBn prepared with different amount of BCmBn. Conditions: DBT-octane model oil with initial Scontent 1000 ppm, mass ratio of absorbent and oil 1:20, 298 K, magnetic stirring 2 h.
3110.7 cm−1 for CH stretching vibration, 1648.9 cm−1 for CC stretching vibration, 1498.4 cm−1 for CN stretching vibration, 1415.5 cm−1 for C−H plane stretching vibration, 1284.4 cm−1 for a mix peak of CN and CN stretching vibration, 916 cm−1 for the stretching vibration of the ring, and 663.4 cm−1 for CN stretching vibration.16 Besides, a wide OH absorption is found around 3413.4 cm−1 due to the presence of trace amount of water in the sample. Compared with PVim, some new peaks were found for PVimBCmBn, viz.1550.5 cm−1 for stretching vibration of benzene skeleton, 831.2 cm−1 for CH deformation vibration of the benzene ring, as well as the strong peak at 1153.2 cm−1 for the in-plane CH bending vibration of the imidazolium ring.17 Besides, the mix peak of CN and CN around 1284.4 cm−1 disappears and a weak single peak appears at 1290 cm−1, because the two CN bonds become identical in the imidazolium ring in terms of bond symmetry and connection, and accordingly the two peaks converged into one.
Figure 3. Infrared spectrum of PVIm and PVimBCmBn.
SEM Images of the Polymers. The microscopic images of the PVim and PVimBCmBn were taken by scanning electron microscopy (JMS-7800-f, Japanese Electronics Co., Ltd.). As shown in Figure 4, the PVim precipitated in toluene is granular aggregates with primary particle size of about 100 nm, while its highly cross-linked ionic derivative, PVimBCmBn, precipitated in DMF has much smaller size and good uniformity. The PVimBCmBn is spheric fluffy aggregates with average particle size of about 20−30 nm. Further, PVimBCmBn is insoluble and even nonswelling in oil because of its high molecular weight, ionic characteristics, and highly cross-linked structure like an ionic exchange resin. BET Analysis of the Polymers. N2 pressure adsorption curves were measured for PVim and PVimBCmBn by the
Table 1. GPC Results of PVim Mn (g·mol−1)
Mw (g·mol−1)
Mp (g·mol−1)
Mz (g·mol−1)
Mv (g·mol−1)
D
942 720
1 070 500
897 110
1 224 700
0
1.14
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DOI: 10.1021/acs.energyfuels.6b00179 Energy Fuels 2016, 30, 5035−5041
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Thermal Analysis of the Polymers. Thermal stability of PVim and PVimBCmBn were analyzed by TGA/DSC synchronous thermal analyzer (TGA/DSC 1/1100 SF, Switzerland mettler Toledo). As shown in Figure 6, about 5% weight
Figure 6. Tg curve of PVim and PVimBCmBn.
loss occurred before 100 °C for the PVim sample due to the evaporation of the residual solvent and water wherein, and another 5% loss in the temperature range of 100−400 °C may be associated with the vaporization of the oligomers. The thermal decomposition temperature of PVim is about 400 °C, being consistent with that reported by Pekel, N. and Strat, M.,19,20 which is slightly lower than that of polystyrene of about 450 °C.21 Compared to PVim, PVimBCmBn shows a lower thermal stability and two decomposition stages. Specifically, the initial pyrolytic temperature advances to about 300 °C along with a 60% and a 13% weight loss in the temperature range of (300−375)°C and (375−500) °C arising from the decomposition of imidazolium salt and the PVim chain, respectively. Adsorptivity of PVimBCmBn for Thiophenic Sulfurs in Oil. Adsorption isotherms of PVimBCmBn for DBT, BT, and T in model oil at 293 K were measured, and presented in
Figure 4. SEM images of the polymers (a) PVim and (b) PVimBCmBn.
physical adsorption analyzer (2020 m ASAP, American Micromeritics). As shown in Figure 5, the nitrogen adsorption
Figure 5. N2-adsorption−desorption curve of PVimBCmBn.
of PVimBCmBn in the low pressure range (0−0.1 atm) is relatively low, and the adsorption curve follows the Langmuir pattern, which indicates its partial microporous structure. When the pressure is above 0.45 atm, the N2-adsorption quantity rises exponentially and appears as a hysteresis loop, which suggests that PVimBCmBn is predominantly of mesoporous and macroporous structure.18 The adsorption curve of PVim is not shown here for its negligible N2 -adsorbance and incompatibility with that of PVimBCmBn. Specific areas of PVim and PVimBCmBn are calculated as 0.92 m2·g−1 and 99.6 m2·g−1, respectively, indicating that PVim is virtually nonporous, while PVimBCmBn is of mesoporous structure with average pore size of 16.1 nm.
Figure 7. Experimental and fitted Langmuir isotherms of PVimBCmBn for DBT, BT, and T. Conditions: 0.3 g sorbent, 15 g model oil, magnetic stirring at 293 K for 2 h.
Figure 7. The experimental data were fitted by the Langmuir equation as follows Ce C 1 = e + qe qmax qmax × b
where Ce is the equilibrium concentration of sulfur, qe is the equilibrium adsorbance, qmax is the saturated adsorbance, and b 5038
DOI: 10.1021/acs.energyfuels.6b00179 Energy Fuels 2016, 30, 5035−5041
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Energy & Fuels is the adsorption equilibrium constant. The b value is associated with adsorption heat, and the greater its value, the bigger the starting slope of the isothermal adsorption curve.22 Model parameters b and qmax were determined by linear regression, as listed in Table 3.
the adsorption performance of PVimBCmBn is compared with the extractive capability of EIM for DBT, BT, and T-containing model oil in terms of their phase equilibrium isotherms, as presented in Figure 8.
Table 3. Langmuir Parameters Regressed from the Experimental Adsorption Isotherms thiophenic sulfurs
qmax
b
R2
DBT BT T toluene-DBT
7.025 5.148 4.080 3.386
0.001556 0.001778 0.000674 0.002402
0.97 0.99 0.92 0.99
As shown from Figure 7 and Table 3, all the adsorption isotherms can be well expressed by the Langmuir equation, implying that the monolayer adsorption plays a more predominant role than the multilayer physical adsorption arising from the multiscale porous structure and higher specific area. As seen from the saturated adsorbance qmax, the adsorption capacity of PVimBCmBn for the thiophenic sulfurs follows the order of DBT > BT > T, which is consistent with the extractive desulfurization performance of N-ethyl imidazole and conventional imidazolium-based ILs.5 Thus, the desulfurization mechanism of mesoporous HCIP may be ascribed to two factors, viz. the available imidazolium rings, which form πcomplexation with aromatic thiophenes via Lewis soft acid−soft base interaction,23 and the free volume available for the accommodation of sulfur molecules with least energy barrier.24 Besides, the b values are very small, indicating that the π−π interaction between PVimBCmBn and thiophenic sulfur is quite weak, which can only lead to a weak chemical adsorption and a low adsorptivity in the low sulfur concentration range. Further, toluene in model oil tends to lower the adsorptive desulfurization of the sorbent, and 5% toluene can result in a 52% decrease in the saturated adsorbance for DBT. Moreover, its adsorptive removability for DBT from real oil is much lower than that from model oil, although this is true for all adsorptive and extractive desulfurization processes reported heretofore. The lowering desulfurization capacity herein may be ascribed to the coexistent aromatics, being about 25−35%, especially the confused ring aromatics in the commercial diesel oil, which may be adsorbed competitively with thiophenic sulfurs due to their similar structure and π-complexation with PVimBCmBn. Therefore, its desulfurization selectivity for real oil needs further enhancement, which may be achieved by increasing the Lewis acidity of the anions and introducing transitional metallic (M) ions for additional M−S interaction. Regeneration and Reusability of PVimBCmBn. To regenerate the used sorbent, ethanol was used as solvent, considering its good solubility for the thiophenic sulfurs, insolubility for PVimBCmBn, and free of contamination to oil. The used sorbent of about 0.5 g was ultrasonically treated with 20 mL of ethanol at room temperature for 1 h, and then poured out the supernatant and dried the residue for 2 h at 353 K under reduced pressure. The desulfurization performance of the regenerated sorbent shows negligible decrease after six times of reusing, being above 97%, which may be ascribed to its rigid and intact mesoporous structure. Desulfurization Performance of PVimBCmBn Versus EIM and Imidazolium-Based ILs. To contrast the effect of porous structure on the desulfurization performance of PILs,
Figure 8. Adsorption isotherms of PVimBCmBn and extractive isotherms of EIM5 for different S-containing model oils. Conditions: 15 g model oil, 0.3 g sorbent, 7.5 g EIM, magnetic stirring 2h at 293 K.
It is found that the desulfurization behavior of extraction and adsorption is different in the following aspects. First, the extraction performance of EIM almost linearly increases with the sulfur concentration in oil, implying a constant Nernst partition coefficient (KN).This is also true for almost all ILs as justified by the thermodynamic relation, i.e.,KN =
CS in sol CS in oil
=
γS in oil γS in sol
≈
γS∞in oil
γS∞in sol
= const , considering that the
S-concentration in both phases is as low as in the ppm level, and the activity coefficient γS can be approximated by its infinite dilution value γ∞ S . Besides, in a lower S-concentration range, the sulfur removal capability of PVimBCmBn is always higher than that of EIM and the imidazolium-based ILs, as shown by the KN values of DBT, i.e., PVimBCmBn(4−7)> [BMIM][AlCl 4 (4.0) > EIM(3.28) > [BMIM][OcSO4](1.9) > [EMIM][DEP](1.45) > [BMIM][PF6](0.9) > [BMIM][CF3SO3](0.8) > [BMIM][BF4](0.7) > [MMIM][DMP](0.46).25 This may largely arise from its mesoporous structure and high specific area, because the available mesopores are helpful to accommodate the sulfurs and facilitate the π−π and dispersive attraction between thiophenic sulfur and imidazolium ring. In contrast, in the extractive process, additional Gibbs energy is required to create micropores for the coming sulfur molecules, which results in a lower sulfur partition coefficient. Moreover, the cohesive energy of ILs is much higher than that of alkyl-imidazole26 due to the strong ionic electrostatic interaction,27 which can explain why the extraction desulfurization ability of alkyl imidazoles is always higher than their corresponding ILs except those with strong Lewis acidic anion like [AlCl4]− and [FeCl4]−.28,29 Furthermore, the adsorptive behavior of PVimBCmBn accords well with the Langmuir equation with its saturated adsorbance being 7.02, 5.15, and 4.08 mgS·g−1, respectively, for DBT, BT, and T. In contrast, the desulfurization capability of EIM increases linearly in a wider S-concentration range, and even exceeds that of PVimBCmBn for high S-content oil, e.g., above 1200 ppm. This is because the interfacial imidazolium group shows a stronger attraction to the sulfur molecules, but the rigid PILs also lost some desulfurization ability permanently due to the presence of inaccessible imidazolium ring in the bulk 5039
DOI: 10.1021/acs.energyfuels.6b00179 Energy Fuels 2016, 30, 5035−5041
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Energy & Fuels Table 4. Desulfurization Performance of Different Kinds of Sorbents for DBT Model Oila sorbent PILs
active carbon
MOF
CMPS-Im(Cl), SSA not available CMPS-Im(Cl)-CuCl HCIP, SSA = 99.6 m2/g mesoporous ACs SSA 990−2320 m2/g Meso porous AC, SSA = 1312 m2/g UMCM-150 (C30H14Cu3O14) SSA = 3100 m2/g MOF-505(Cu2(bptc)
MOF-5 (Zn4O(BDC)3) SSA = 2500−3000 m2/g
zeolite
qmax (mgS·g−1) or KN
sorbent details
HKUST-1(Cu3BTC2),SSA = 1601 m2/g MIL-53(Cr(OH)-BDC, SSA = 1511 m2/g MIL-100(Cr3FOBTC2), SA = 2116 m2/g Cu(I)-Y zeolite Ag/MCM-41
ref
octane-DBT oil, 313 K, qmax = 1.9 (Ce = 1000 ppm) octane-DBT oil, 313 K, KN = 1.7−5.0 (Ce = 300−1000 ppm) octane-DBT oil, qmax = 7.05 model oil 398 ppmS, 298 K, qmax = 12−14 mgS/g, and 1.5−4.0 mgS/g with 1.4% phenanthrene in oil DBT-n-hexane model oil at 298 K qmax = 21.7 isooctane-DBT oil, qmax = 83 isooctane-DMDBT oil, qmax = 41 isooctane/toluene (85/15)-DBT, qmax = 25 isooctane-DBT oil, qmax = 39 isooctane-DMDBT oil, qmax = 27 isooctane/toluene (85/15)-DBT, qmax = 33 isooctane-DBT oil, qmax = 33 isooctane-DMDBT oil, qmax = 14 isooctane/toluene (85/15)-DBT, qmax = 10 n-octane − DBT oil, qmax = 55 n-octane − DBT oil, qmax = 40 n-octane − DBT oil, qmax = 29 octane-DBT model oil, qmax = 32.6 octane-DBT oil, qmax = 20.5
35 this work 33 34 31
31
31
14
32 32
a
qmax: saturated adsorbance or the maximum adsorbance at specified equilibrium S-concentration; KN: the Nernst distribution coefficient of sulfur between sorbent and oil; CMPS-Im(Cl): N-methylimidazole grafted chloromethyl polystyrene (CMPS); bptc =1,1′-biphenyl-3,3′,5,5′tetracarboxylate
make the ionic polymer have a fully developed micro/ mesoporosity and high specific area through varying approaches, e.g., copolymerization,35 cross-linking,36 as well as hard30 and soft template method.37
phase, whereas the expandability of the liquid makes a full use of the desulfurization ability of each imidazole/imidazolium ring, leading to an “expandable” desulfurization capability. Therefore, ILs seem suitable for the extractive desulfurization of oil with high S-content, and PILs are more feasible for the deep desulfurization without contamination to the oil product. Desulfurization Performance of HCIP Versus Other Sorbents. Until now, PILs sorbents are mainly used for the gas adsorption of CO2,30 while their potential use in oil desulfurization has been seldom reported. In order to avoid the interference of the chemical composition and sulfur species in real oil, desulfurization performance of different sorbents was compared for DBT model oil. As shown in Table 4, many porous materials with large specific surface area (SSA) have been studied for oil desulfurization, and their adsorptivity roughly follows the order of MOFs31 > zeolite32 > active carbons33,34 > PILs.35 The excellent desulfurization ability of MOFs mainly arises from their microporous crystal structure, very high SSA (1500−3000 m2·g−1), and the complexing interaction between metal ion and sulfur. However, they are very hydrophilic, and the desulfurization performance is often lowered greatly by the presence of moisture. Zeolites with mesoporous structure and high SSA are promising sorbents, but their desulfurization capability is mainly from the loading metals due to the direct and indirect M−S interactions.32 Active carbon is a universal sorbent due to its high SSA and strong van der Waals force field arising from its rich confused aromatic rings, and thus its sulfur selectivity versus the coexistent aromatics is quite mean. In contrast, PIL is a newcomer to the various sorbents, and its desulfurization potential needs to be explored further by increasing its SSA, finely tuning its mesostructure, and incorporating transitional metals to the PIL texture and so on. In summary, the porous structure and specific area of the PILs is instrumental for their desulfurization performance. In order to prepare a promising PILs sorbent, it is necessary to
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CONCLUSION Linear polymer of PVim with molecular weight of 1 070 000 was synthesized via radical polymerization, and quaternized in DMF with stoichiometric amount of BCmBn, forming a highly cross-linked ionic polymer PVimBCmBn with specific area 99.6 m2·g−1, average pore size 16.1 nm, and pyrolysis temperature above 300 °C. The desulfurization ability of PVimBCmBn for thiophenic sulfurs is superior to that of EIM and imidazoliumbased ILs within 1000 ppm of S-concentration range, manifesting the importance of the mesoporous structure and high specific area. The adsorption isotherm of PVimBCmBn follows the Langmuir equation, and the adsorptivity follows the order of DBT > BT > T, with their saturated adsorbance being 7.02, 5.15, and 4.08 mgS·g−1, respectively, at 293 K.
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AUTHOR INFORMATION
Corresponding Author
*
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful for the financial support from the National Science Foundation of China (Grant No. 21376011).
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REFERENCES
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DOI: 10.1021/acs.energyfuels.6b00179 Energy Fuels 2016, 30, 5035−5041
Article
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DOI: 10.1021/acs.energyfuels.6b00179 Energy Fuels 2016, 30, 5035−5041
