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Aug 26, 2013 - Poly[bis(p‑methyl phenyl) phosphazene] Pervaporative Membranes for Separating Organosulfur Compounds from n‑Heptane and Its. Surfac...
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Poly[bis(p‑methyl phenyl) phosphazene] Pervaporative Membranes for Separating Organosulfur Compounds from n‑Heptane and Its Surface Functionalization Zhengjin Yang,† Tao Wang,† Xia Zhan,‡ Jiding Li,*,† and Jinxun Chen† †

The State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing, 100084, P. R. China ‡ Beijing Technology and Business University, Fucheng Road, Haidian District, Beijing, 100048, P. R. China ABSTRACT: This work investigates the application of poly[bis(p-methyl phenyl) phosphazene] (PMePP) for separating organosulfur compounds from n-heptane. PMePP was synthesized and characterized by NMR, X-ray photo spectroscopy, thermogravimetric analysis, scanning electron microscopy, and Fourier transformed infrared spectroscopy. Membranes based on PMePP were fabricated and investigated for desulfurization of FCC (fluid catalytic cracking) model gasoline by pervaporation. The effects of operating temperature, feed composition, and feed sulfur content on the performance of PMePP membranes were investigated. The experimental results showed that the PMePP-based membranes are effective for FCC gasoline pervaporative desulfurization. To provide active spots for future grafting modification, the surface functionalization of PMePP was carried out to convert the methyl units into carboxylic units. The surface-functionalized membrane was also investigated for its pervaporation performance, and results showed the increased in enrichment factor and less temperature dependence.

1. INTRODUCTION Considered as a clean technology,1,2 pervaporation (PV) is a promising alternative to such conventional separation processes as distillation.3,4 It is particularly advantageous for the separation of close boiling liquids5 and azeotropic mixtures.6 Presently, pervaporation has been widely investigated for the production of biofuel7,8 and the desulfurization of FCC gasoline,9−12 which is an important aspect of environment protection. Gasoline desulfurization has become a top priority of environmental protection in both developed and developing countries, and stringent regulations have been established.13 The European Union has set a limit of sulfur content in gasoline to 10 ppm since 2009.10 In Beijing, China, the sulfur content in gasoline was limited to be less than 50 ppm in 2008,14 and this limit has been further reduced to 10 ppm since May 2012.15 Various pervaporation membranes are being developed for deep desulfurization of gasoline, including poly(ethylene glycol),16 poly(dimethyl siloxane),11,17 polyimides,18 hydroxyethyl cellulose,19 poly(ether sulfone),20 and polyphosphazene.21,22 To enhance the desulfurization performance and membrane stability, membrane modifications are often introduced by using filling additives,23 blending,24 copolymerization,25 and cross-linking.26 However, the above-mentioned polymeric membranes are relatively inert and chemical modifications are thus difficult. In addition, these membranes usually suffer from a severe decrease in the enrichment factor when the operating temperature is increased.10,11,24 Consequently, a high flux along with a high enrichment factor can hardly be obtained. On the other hand, an important aspect in developing desulfurization membranes that is rarely addressed is surface functionalization of the membrane. The surface properties of a membrane can significantly influence its pervaporation performance.27 It appears to be an effective approach to develop better desulfurization membranes by developing new membranes that © 2013 American Chemical Society

can be functionalized easily and/or modifying existing membranes through surface functionalization. The enrichment factor can be increased without sacrificing the pervaporative flux if the surface functionalization can be properly controlled so that the functionalization occurs only on the membrane surface without affecting the membrane interior. In this work, poly[bis(p-methyl phenyl) phosphazene](PMePP) was synthesized and characterized by NMR, X-ray photospectroscopy (XPS), and infrared with attenuated total reflectance (IR-ATR). Subsequently, PMePP-based composite membranes were fabricated and investigated for the removal of organosulfur compounds from n-heptane, which is a representative compound of gasoline. By oxidizing part of the methyl units on the PMePP membrane surface into carboxylic units, surface functionalized PMePP (SF-MePP) membranes were obtained. Such surface functionalization was shown to increase the sulfur removal efficiency and to enhance the membrane stability at different operating temperatures. Additionally, the surface modification provides potential linking spots for future grafting modifications.

2. EXPERIMENTAL SECTION 2.1. Materials. Hexachlorocyclotriphosphazene (HCCP) was purchased from XinYi Chemical. Co., Ltd. (Jiangsu, China). It was recrystallized twice in heptane and sublimed at 50 °C before use. 4-Methyl phenol and benzophenone ketyl were purchased from Alfa Aesar (Johnson Matthey, UK). Tetrahydrofuran and toluene were purchased from FuChen Chemicals Received: Revised: Accepted: Published: 13801

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Figure 1. Synthesis of poly[bis(p-methyl phenyl) phosphazene].

Figure 2. Schematic representation of the pervaporation apparatus utilized in this work.

solution of PMePP (1 g) in THF (20 mL) was cast onto the porous PVDF support (180 μm). After the THF was evaporated at 25 °C overnight, the as prepared membrane was heated at 80 °C for 4 h to further remove the residual solvent. The PMePP membranes were then stored in a dry and clean place for characterization and evaluation. By controlling the gap between the casting knife and the PVDF porous support, we found the thickness of the dense PMePP layer which was measured in the cross-section SEM image to be around 15 μm with a standard deviation of 1.7. 2.2.3. Surface Functionalization of PMePP Membrane. Both the homogeneous films and the composite membranes were immersed in an aqueous solution of NaOH (0.02 mol/L) and KMnO4 (0.04 mol/L) at 50 °C for 5 h. They were then removed from the reaction solution to an aqueous NaHSO3 (0.4 mol/L) solution for 5 h to remove the MnO2. After rinsing in deionized water for 12 h, the surface-functionalized PMePP membranes (SF-MePP) were dried in an oven at 50 °C for 6 h. 2.2.4. Characterization. The PMePP polymer was characterized by 1H NMR and 31P NMR (600 MHz, JNM-ECA600, JEOL Co., Japan) using CDCl3 as the solvent. Surface elemental compositions of the PMePP membrane before and after surface functionalization were analyzed by X-ray photospectroscopy (XPS) (PHI Quantera SXM, ULVAC-PHI, USA) under an Al Kα radiation source. The takeoff angle of the photoelectron was set to be 45°. The spectra over the range of 0−1200 eV were examined with a resolution of 0.5 eV. The high resolution spectra of C1s and O1s peaks were also examined. FTIR-ATR were

(Tianjin, China) and were purified by distillation over sodium benzophenone ketyl. Sodium bisulfite (NaHSO3, AR) was obtained from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). Potassium permanganate (KMnO4, AR) was purchased from Beijing Chemical Works (Beijing, China). All other chemicals were purchased from FuChen Chemicals (Tianjin, China) and used as received. PVDF (PVDF-1015) porous support membrane was prepared in our laboratory using the nonsolvent-induced phase separation (NIPS) method.28 2.2. Methods. 2.2.1. Synthesis of PMePP. Poly(dichloro phosphazene) (PDCP) was produced via thermal ring-opening polymerization of HCCP at 260 °C for 25 h, and the reaction was carried out in a sealed pre-evacuated glass tube. A 20 g sample of PDCP was dissolved in 300 mL of toluene, followed by reacting with an excess amount of sodium 4-methylphenoxide, which was prepared by reacting 4-methylphenol (37.29 g) with sodium (9.52 g) in THF (200 mL). Tetrabutylammonium bromide (TABr, 4g) was added as catalyst. This mixture was then heated at 95 °C for 48 h under vigorous stirring. After that, the obtained reaction mixture was precipitated into methanol. The crude product of PMePP was further purified by repeated precipitation from THF into hexane. It was then thoroughly washed with ethanol and deionized water before it was dried under vacuum at 60 °C. The PMePP so prepared has a molecular weight of 2.3 × 105 g/mol, as determined by gel permeation chromatography (GPC). Figure 1 illustrates the procedure for PMePP synthesis. 2.2.2. Membrane Fabrication. PMePP membranes were fabricated by the solution casting method. A homogeneous 13802

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Figure 3. 1H and 31P NMR spectra of PMePP.

Figure 4. XPS spectrum of PMePP.

recorded for PMePP membranes before and after oxidation using a Nicolet IR 560 spectrometer with horizontal ATR accessory equipped with ZnSe crystal. The spectra were recorded with a resolution of 4 cm−1 and each spectrum was collected 32 times. Thermal stability tests of PMePP were conducted on a thermal gravimetric analyzer (TGA) under nitrogen at a heating rate of 10 °C/min from 20 to 950 °C. The SEM image of PMePP before and after surface functionalization was obtained in a JEOL JSM7401F scanning electron microscope. 2.2.5. Pervaporation Setup. The pervaporation experiments were carried out using an apparatus described in Figure 2. Detailed information about the setup and experimental procedure was described previously.22 All the pervaporation runs were carried out with binary n-heptane/sulfur compound mixtures. When a steady state permeation flux was reached, the permeated mixture was collected in a nitrogen cold trap, and its sulfur content was analyzed using a microcoulometric analysis instrument (RPA-200, China). Each concentration measurement was repeated at least twice. Two major parameters, that is, sulfur enrichment factor (β) and permeation flux (J), were evaluated. J was determined from the weight of the permeated component over a given period of time, and β was determined from the permeate and feed concentration, as shown in eq 1:

β=

cp cf

(1)

where, cp and cf are the sulfur content of the permeate sample and the feed mixture, respectively.

3. RESULTS AND DISCUSSION 3.1. NMR of PMePP. NMR was used to investigate the structure of PMePP including the degree of chlorine substitution and the bonding of hydrogen atoms. The as prepared PMePP was characterized by 1H NMR and 31P NMR (Figure 3). A strong single peak was observed in 31P NMR spectrum, which was attributed to the phosphorus in the backbone of the polymer. A chemical shift of −18.8 ppm was observed, which was in good agreement with the results reported in the literature.29,30 The degree of chlorine substitution was shown to be 100%. Besides the peaks for tetramethylsilane (TMS) and CDCl3, three peaks can be observed in the 1H NMR spectrum: the methyl hydrogen atoms at 2.07 ppm and the 2- and 3- hydrogen atoms of phenyl rings at 6.73 and 6.56 ppm, respectively. The above results confirm the successful synthesis of PMePP. 3.2. XPS of PMePP. The synthesized polymer was also analyzed by XPS spectroscopy (Figure 4). XPS analysis is carried out to identify the elemental composition of the synthesized 13803

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Figure 5. FTIR-ATR spectrum of PMePP (a) before and (b) after surface modification.

polymer. A full elemental analysis was conducted from a binding energy of 0 to 1200 eV. Peaks for C, N, O, and P atoms were shown, but no peak for chlorine atoms was found, which suggests a complete substitution of chlorine. The C/O ratio of PMePP was obtained from the XPS at a high resolution of C 1s and O 1s. The C/O ratio of PMePP was calculated to be 7, which is close to the observed value of 7.3 from the XPS high resolution spectroscopy. These results also suggest the successful synthesis of PMePP. 3.3. FTIR-ATR of PMePP Pervaporative Membrane. The FTIR-ATR spectrum of the PMePP composite membrane is shown in Figure 5a. There was no evidence of the presence of P− Cl units. The vibrations of phenyl rings and methyl groups are represented by the peak at 1502.68 and 1403.56 cm−1, respectively. The peak at 857.27 cm−1 is a representation of the p-disubstituted phenyl ring. Other major infrared bands (in cm−1) are as follows: 1207.24 (PN), 1181.69 (Ph-O), 1069.46 and 974.58(P−O−C), and 761.71 (P−N). The infrared spectrum confirms the structure of PMePP, and it is consistent with that reported in the literature.31 3.4. Thermal Analysis. Thermal stability is a very important factor in pervaporation desulfurization since in most cases the membrane will be in contact with feeds of high temperature. The membrane should not degrade in order to obtain long durability. To investigate the thermal behavior of the synthesized PMePP membrane, thermal stability tests of PMePP were conducted, and results are shown in Figure 6. It can be clearly observed that the sample did not lose any weight before 380 °C in thermal tests, indicating high thermal stability. Concerning the experimental condition, PMePP will remain stable in pervaporation experiments since the operating temperature would not even exceed 100 °C. 3.5. Surface Functionalization of PMePP Membrane. The methyl units on the surface of the PMePP membrane are partially oxidized into carboxylic units by immersing the membrane into a KMnO4 aqueous solution. The oxidation process was investigated using FTIR-ATR (Figure 5b) and XPS spectroscopy.

Figure 6. Thermal stability test of PMePP samples. The test was conducted under nitrogen at a heating rate of 10 °C/min from 20 to 950 °C.

Compared with the FTIR-ATR spectrum of PMePP, a new peak at 1605.58 cm−1 was observed on the functionalized surface. It was attributed to the vibration of carboxylic units in salt forms. This peak indicates the formation of carboxylic units during surface functionalization. The carboxylic units were also confirmed by the emergence of a new broad peak in C1s high resolution XPS spectrum (Figure 7). In the meantime, the peak at 1373.09 cm−1 representing a methyl unit was significantly decreased in strength, which was caused by the oxidation of the methyl units on the membrane surface during the surface functionalization. However, this peak did not vanish, indicating that not all methyl units were completely oxidized. As demonstrated in Figure 1b, both the carboxylic and methyl units are present on the functionalized surface. Since the relative quantity of carboxylic units to methyl units cannot be obtained in the FTIR-ATR spectrum due to the semiquantitative nature of infrared spectroscopy, XPS spectroscopy was utilized to determine the carboxylic/methyl unit ratio. The C/O ratio obtained from high resolution XPS spectroscopy can be used to determine the quantity of methyl units that were 13804

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Figure 7. C1s spectra of PMePP and SF-MePP pervaporative membrane.

Figure 9. Effect of temperature on total permeation flux. Feed sulfur content ≈400 ppm and permeate pressure ≈300 Pa (with n-heptane as the model gasoline component).

oxidized into carboxylic units. The C/O ratio of the SF-MePP membrane surface is 4.1 (Table 1), which means that 62.1% of

the meantime, when operating temperature increases, the vapor pressure of both organosulfur compounds and n-heptane will increase leading to the increase in driving force for mass transportation which in turn contributes to the increase of pervaporation flux. However, since the organosulfur compound contents in the feed and the permeate are relatively low (no more than 0.2 wt %), the total pervaporative fluxes are primarily determined by the permeation flux of n-heptane. In this study, a permeation flux of up to 3.0 kg/(m2 h) was achieved, which should be a good result for industrial application. As expected, the partial fluxes of the organosulfur species is also influenced by the operating temperature, as shown in Figure 10. An increase in the operating temperature increased all the partial permeation fluxes of thiophene, 2-methyl thiophene, ethyl thioether, and n-butyl mercaptan. On the other hand, the partial fluxes of the sulfur components are in the following order: thiophene > 2-methyl thiophene > ethyl thioether ≈ butyl mercaptan. This can be explained from the solubility parameter point of view. On the basis of the solubility parameter theory, the membrane tends to have a stronger affinity to the organosulfur permeant if there is a smaller difference in the solubility parameter between the membrane and the permeant. The solubility parameters of PMePP, thiophene, 2-methyl thiophene, ethyl thioether, and butyl mercaptan are 20.7, 20.1, 19.6, 17.3, and 23.6 MPa0.5, respectively21 (Table 2). Therefore, the PMePP membrane favors the permeation of thiophene as far as the

Table 1. C/O ratio for PMePP and SF-PMePP Investigated by XPS High Resolution Spectroscopy membrane

C/O ratio

PMePP (calculated) PMePP (observed) SF-MePP

7:1 7.3:1 4.1:1

the methyl units was oxidized into carboxylic units. Moreover, the depth of surface oxidation was shown to be less than 1 μm.29 It can thus be expected that the surface functionalization process would not affect the mass transport resistance of the membrane. The surface functionalization is very mild and there was no breaking or fracture in the membrane observed (Figure 8). 3.6. Pervaporation Evaluation of PMePP Membrane. 3.6.1. Effect of Temperature. In general, operating temperature has a considerable effect on both permeation flux and enrichment factor for pervaporative desulfurization.32,33 With the current membrane, the permeation flux was shown to increase with an increase in the operating temperature (Figure 9). This can be explained by the solution-diffusion model. The diffusivities of both n-heptane and the organosulfur compounds (i.e., thiophene, 2-methyl thiophene, ethyl thioether, and n-butyl mercaptan) increase when the operating temperature increases, thereby contributing to an increase in their permeation fluxes. In

Figure 8. Scanning electron microscope (SEM) image of PMePP before (a) and after (b) surface functionalization. 13805

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brane12,34,36 for pervaporative removal of thioether. Among the temperature range tested, the highest sulfur enrichment was obtained at 45 °C, and the enrichment factors are 4.87 and 3.92 for 2-methyl thiophene and n-butyl mercaptan, respectively. These values are similar to the enrichment factors of PDMS membrane.36 3.6.2. Effect of Feed Sulfur Content. The sulfur content in the feed is an important parameter in the removal of organosulfur compounds from the model gasoline solutions. 11,37 As pervaporation proceeds, the sulfur concentration in the feed decreases, influencing the instantaneous desulfurization performance of the membrane. The impact of feed sulfur concentration on the partial permeation flux of sulfur and the sulfur enrichment factors of PMePP membrane for pervaporative removal of thiophene, 2-methyl thiophene, and 2, 5-dimethyl thiophene were determined at an operating temperature 80 °C, and the results are shown in Figures 12 and 13. As one may expect, the partial fluxes of thiophene, 2-methyl thiophene, and 2,5-dimethyl thiophene increase with an increase in the feed sulfur content. This may be attributed to the increased driving force for mass transfer through the membrane when the feed sulfur content increased. At a given feed sulfur concentration, the partial flux of thiophene is the highest among the sulfur permeants studied here due to its smallest molecular size and highest affinity to the PMePP membrane. Additionally, the pervaporation fluxes of thiophene, 2-methyl thiophene, and 2, 5-dimethyl thiophene were measured with the same membrane, at the same operating temperature (80 °C), at a constant feed composition, and the other operating conditions were also kept constant. Thiophene has the highest vapor pressure among these three thiophenes in this situation. It exhibits the highest mass transfer driving force during this process, leading to the result that thiophene presents the highest permeation flux. However, the feed sulfur concentration had little effect on the sulfur enrichment factors of the PMePP membrane to thiophene, 2-methyl thiophene, and 2,5-dimethyl thiophene. The average enrichment factors for removing thiophene, 2-methyl thiophene, and 2,5-dimethyl thiophene are 5.5, 3.6, and 2.9, respectively. Among the sulfur compounds tested, thiophene had the highest enrichment factor, which is consistent with partial sulfur fluxes discussed above. As the pervaporation continues, the sulfur content in the feed gradually decreases, but the enrichment factor remains almost unchanged. This feature is particularly useful from an application perspective. 3.7. Pervaporation Performance of SF-MePP Membrane. The partial fluxes and enrichment factors of surface modified membrane SF-MePP for removing thiophene and nbutyl mercaptan from the model gasoline at different temperatures are shown in Figures 14 and 15. It can be observed that the

Figure 10. Effect of temperature on partial fluxes of thiophene (□), 2methyl thiophene (△), ethyl thioether (▽), and n-butyl mercaptan (◊). Feed sulfur content ≈ 400 ppm and permeate pressure ≈ 300 Pa (with nheptane as the model gasoline component).

solubility aspect is concerned. Another aspect we should bear in mind is the mass driving force difference in different sulfur species. The driving force during sulfur compound permeation is mainly based on the difference in vapor pressure. During this investigation, the pervaporation fluxes of different sulfur compounds were measured with the same piece of membrane, at the same operating temperature, at a constant feed composition (400 ppm) and the other operating conditions were also kept constant. Thiophene has the highest vapor pressure, which in turn leads to the highest mass transportation driving force and thus the highest partial flux. In addition, the permeation of the organosulfur species is also influenced by the diffusion aspect. According to our previous study,34 the infinite dilute diffusion coefficients of thiophene, 2-methyl thiophene, and n-butyl mercaptan are in the following order: thiophene > 2methyl thiophene > n-butyl mercaptan. Therefore, the diffusion aspect augments the permeation of sulfur components in the same order. Ethyl thioether and n-butyl mercaptan have similar molecular structures and molecular weights, and thus these two components exhibited similar fluxes at a given operating temperature. At an operating temperature of 55 °C, the enrichment factors for thiophene and ethyl thioether are 6 and 4.0, respectively (Figure 11). The selectivity of the PMePP membrane is shown to be higher than that of PDMS35 and polyimide18 membranes at the operating conditions. Especially, the PMePP showed an enrichment factor that is twice that of the PDMS mem-

Table 2. Molecular Information of Sulfur Compounds and n-Heptane

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Figure 11. Effect of temperature on sulfur enrichment factor of thiophene (□), methyl thiophene (△), ethyl thioether (▽), and butyl mercaptan (◊) in removing organosulfur from heptane. Feed sulfur content ≈ 400 ppm (with n-heptane as the model gasoline component). Permeate pressure ≈ 300 Pa.

Figure 12. Effect of feed sulfur content on partial flux of thiophene (□), methyl thiophene (△), and 2,5-dimethyl thiophene (▽) at 80 °C with n-heptane as model gasoline component. Permeate pressure ≈ 300 Pa.

Figure 13. Effect of feed sulfur content on sulfur enrichment factor of thiophene, 2-methyl thiophene, and 2,5-dimethyl thiophene at 80 °C with n-heptane as a model gasoline component. Permeate pressure ≈ 300 Pa.

fluxes of both thiophene and n-butyl mercaptan increase when the operating temperature increases. Compared to the unmodified PMePP membrane, the thiophene flux of the SFMePP membrane decreased by about 32%, whereas there was only a slight reduction in the partial flux of n-butyl mercaptan. The decreases in the sulfur fluxes are attributed to the changes in chemical structure of the membrane surface. When methyl units are oxidized into carboxylic units, the affinity of thiophene and nbutyl mercaptan to the membrane decreases, resulting in a decrease in the solubility of the sulfur components in on the membrane surface. Moreover, because of the surface charges introduced by the functionalization of the membrane, desorption of thiophene and n-butyl mercaptan from the membrane became more difficult. Nevertheless, the decrease in partial flux is not significant and may be remediated by neutralizing the surface charges or by grating alkyl groups. It is interesting to notice that both the enrichment factors of thiophene and n-buthyl mercaptan increased when the PMePP membrane was functionalized. Thiophene showed an enrichment factor of above 6 in the temperature range studied. Compared to the PMePP membrane, the increase in the

enrichment factor with an increase in the operating temperature is more significant with the modified membrane. It may be pointed out that there is no drastic change in the sulfur enrichment for both thiophene and n-butyl mercaptan when the operating temperature increases, in spite of the significant increase in the permeation flux, as discussed before. A higher operating temperature is thus favorable as far as the SF-MePP membrane is concerned. However, in view of the stability of the gasoline feed solution and the safety aspect of the process, an operating temperature higher than 90 °C is considered unfavorable. Therefore, an operating temperature of ∼85 °C appears to be suitable for the SF-MePP membrane because at this temperature, a high flux can be obtained without sacrificing the sulfur enrichment factor. The advantages of surface functionalization of PMePP membrane lies in the following aspects: (1) Both sulfur enrichment factor and membrane stability are improved by converting parts of the methyl units into carboxylic units. (2) The membrane can be operated at a relatively high temperature 13807

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removing thioether was much higher than the enrichment factors with other membranes reported. The enrichment factor was improved by surface functionalization. With the surface modified membrane, increasing the operating temperature had little effect on the sulfur enrichment, and thus a high flux could be obtained by operating at a relatively high temperature without sacrificing the sulfur enrichment.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 086-10-62782432. Fax: 086-10-62782432. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial supports from National High Technology Research and Development Program of China (No. 2012AA03A607), National Natural Science Foundation of China (21176135, 21206001), and Beijing Foundation (PXM2013_178203_000004, 2132010) are gratefully acknowledged.

Figure 14. Partial fluxes of thiophene and n-butyl mercaptan at varied temperatures and feed sulfur content ≈ 400 ppm and permeate pressure ≈ 300 Pa (with n-heptane as the model gasoline component).

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Figure 15. Enrichment factor of PMePP and SF-MePP in removing thiophene and n-butyl mercaptan at varied temperatures. Feed sulfur content ≈ 400 ppm and permeate pressure ≈ 300 Pa (with n-heptane as a model gasoline component).

to enhance the permeation flux without sacrificing the enrichment factor. (3) Further surface modifications may be done by chemical grafting and other manipulations. The surface functionalization of PMePP by converting methyl units into carboxylic groups results in a chemically more active surface because the carboxylic units can be reacted with various organic compounds, such as alcohols or amines. The surface properties of the membrane (e.g., surface energy, hydrophilicity/hydrophobicity) can thus be finely tuned. It is believed that PMePP can also be used for pervaporation dehydration of organic solvents, as well as other filtration membranes. (4) Polypeptide can be grafted onto SF-MePP surface via its bonding with carboxylic units29 to form a responsive membrane surface for niche applications.

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NOMENCLATURE PMePP = poly[bis (p-methyl phenyl) phosphazene] FCC = fluid catalytic cracking NMR = nuclear magnetic resonance XPS = X-ray photo spectroscopy TGA = thermal gravimetric analyzer FTIR = Fourier transform infrared spectroscopy SEM = scanning electron microscope PV = pervaporation SF-MePP = surface functionalized PMePP HCCP = hexachlorocyclotriphosphazene PDCP = poly(dichloro phosphazene) TABr = tetrabutylammonium bromide PVDF = polyvinylidene fluoride THF = tetrahydrofuran NIPS = nonsolvent induced phase separation GPC = gel permeation chromatography TMS = tetramethylsilane PDMS = polydimethylsiloxane β = enrichment factor J = permeation flux cp = sulfur content of the permeate cf = sulfur content of the feed REFERENCES

(1) Feng, X. S.; Huang, R. Y. M. Liquid separation by membrane pervaporation: A review. Ind. Eng. Chem. Res. 1997, 36 (4), 1048−1066. (2) Sae-Khow, O.; Mitra, S. Carbon nanotube immobilized composite hollow fiber membranes for pervaporative removal of volatile organics from water. J. Phys. Chem. C 2010, 114 (39), 16351−16356. (3) Shao, P.; Huang, R. Y. M. Polymeric membrane pervaporation. J. Membr. Sci. 2007, 287 (2), 162−179. (4) Smitha, B.; Suhanya, D.; Sridhar, S.; Ramakrishna, M. Separation of organic−organic mixtures by pervaporationA review. J. Membr. Sci. 2004, 241 (1), 1−21. (5) Zhang, Q. G.; Liu, Q. L.; Zhu, A. M.; Xiong, Y.; Ren, L. Pervaporation performance of quaternized poly(vinyl alcohol) and its crosslinked membranes for the dehydration of ethanol. J. Membr. Sci. 2009, 335 (1−2), 68−75. (6) Zhang, W.; Yu, Z.; Qian, Q.; Zhang, Z.; Wang, X. Improving the pervaporation performance of the glutaraldehyde crosslinked chitosan

4. CONCLUSIONS Poly[bis(p-methyl phenyl) phosphazene] (PMePP) was synthesized and studied as a membrane material for removing organosulfurs from model gasoline. An enrichment factor of 6 for thiophene was obtained, and the membrane also showed good performance for removing thioether, mercaptan, and substituted thiophene. The enrichment factor of the PMePP for 13808

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dx.doi.org/10.1021/ie402022a | Ind. Eng. Chem. Res. 2013, 52, 13801−13809