Elevated pervaporation performance of polysiloxane membrane using channels and active sites of metal organic framework CuBTC

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Elevated pervaporation performance of polysiloxane membrane using channels and active sites of metal organic framework CuBTC Shengnan Yu, Zhongyi Jiang, He Ding, Fusheng Pan, Baoyi Wang, Jing Yang, Xingzhong Cao

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S0376-7388(15)00072-1 http://dx.doi.org/10.1016/j.memsci.2015.01.045 MEMSCI13447

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Journal of Membrane Science

Received date: 3 November 2014 Revised date: 21 January 2015 Accepted date: 22 January 2015 Cite this article as: Shengnan Yu, Zhongyi Jiang, He Ding, Fusheng Pan, Baoyi Wang, Jing Yang, Xingzhong Cao, Elevated pervaporation performance of polysiloxane membrane using channels and active sites of metal organic framework CuBTC, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2015.01.045 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Elevated pervaporation performance of polysiloxane membrane using channels and active sites of metal organic framework CuBTC Shengnan Yua,b, Zhongyi Jianga,b, He Dinga,b, Fusheng Pana,b,*, Baoyi Wangc, Jing Yangc and Xingzhong Caoc a

Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical

Engineering and Technology, Tianjin University, Tianjin 300072, China b

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072,

China c

Key Laboratory of Nuclear Analysis Techniques, Institute of High Energy Physics, Chinese

Academy of Sciences, Beijing 100049, China *

To whom correspondence should be addressed. Tel: +86-22-2350 0086; Fax: +86-22-2350 0086;

Email: [email protected].

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ABSTRACT: Hybrid membranes with polydimethyl siloxane (PDMS) as the polymer matrix and metal organic framework (MOF) CuBTC ([Cu3(BTC)2]) as the filler were fabricated and applied in pervaporative separation of thiophene from model gasoline. CuBTC submicroparticles with size of 0.2 to 0.3 ȝm were synthesized through a facile method at room temperature. The morphology, crystal structure and pore parameters of CuBTC, together with the morphology, functional groups, thermal and mechanical property, free volume characteristics, sorption and separation performance of PDMS-CuBTC hybrid membranes were studied. Primary functions of CuBTC particles are summarized as follows: (i) the channels of CuBTC provide expressways with low resistance for the transport of permeates; (ii) the submicroparticles interfere with the packing of PDMS chains, generating extra free volume of the membrane; (iii) the active metal sites of CuBTC facilitate the transport of thiophene; (iv) the channels of CuBTC sieve thiophene from the bulk of model gasoline. As a result, the normalized flux and selectivity of PDMS membranes are elevated simultaneously. The optimal separation performance is achieved when the weight ratio of normal CuBTC to PDMS reaches 8 %, with a permeation flux of 194.2 kg·ȝm/(m2·h) and an enrichment factor of 5.2 (increase by 100 % and 75 % compared with PDMS control membrane, respectively). KEYWORDS:

polydimethyl

siloxane;

metal

pervaporation; gasoline desulfurization

2

organic

framework;

hybrid

membrane;

1. INTRODUCTION Polymeric separation membranes, bearing advantages such as good processability, low cost and easy scaleup, are widely used in water, energy and environment related applications [1]. Despite these advantages, polymeric membranes often suffer from the compromise between permeability and selectivity [2]: increase of membrane permeability often results in loss of selectivity and vice versa. Fabricating hybrid membranes (or mixed matrix membranes) has been proved effective to overcome this tradeoff relation [3]. Research on fabricating hybrid membranes with inorganic fillers is often bothered by the incompatibility of the fillers with the polymer matrix, and further modification is required to reduce or eliminate the non-selective voids at the interface between fillers and polymer [4,5]. Organic fillers are preferred alternatives in terms of intrinsic compatibility with polymers [6,7]. Metal organic frameworks (MOFs) [8-11] are a series of 3-D coordinate polymers composed of metal ions and organic ligands. Due to the regular channel structure, large surface area and tunable adsorption capacity, MOFs have become popular organic fillers for hybrid membranes [12-14]. Several kinds of MOFs such as CuBTC [15,16], CuBDC [17], CuTPA [18], MIL-53 [19-21], MIL-101 [12], Mn(HCOO)2 [22], MOF-5 [23], Cu-MOF [24], ZIF-7 [25,26], ZIF-8 [27-31], ZIF-71 [32-34] and ZIF-108 [35] have been used to fabricate hybrid membranes for pervaporation, gas separation and nanofiltration. The channels of MOFs are essential to elevate membrane permeability, as they provide pathways with low resistance for permeate molecules [27,28,32], increasing membrane permeability effectively. The metal ions of MOFs also play an important role in stabilizing the materials structure. In some cases, the ions are not coordinated saturatedly, such as CuBTC [36], Mg-MOF-74 [37],

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UMCM-1 [38], Mn4Cl-MOF [39], CPO-27-Fe [40] and {[Zn(INAIP)(DMF)]·0.5DMF·4H2O}n [41]. The unoccupied metal sites have the function to interact with certain kinds of target molecules, such as thiophenic organosulfur [42] and CO2 [37]. This function can be used to fabricate hybrid membranes with the function of facilitated transport [43]. CuBTC (also known as HKUST-1, MOF-199 or BasoliteTM C300) is a suitable choice to fabricate hybrid membranes with the function of facilitated transport. It is a kind of MOF with open metal sites of Cu2+. CuBTC particles have been incorporated with several kinds of polymer such as polysulfone (PSf) [6], polyimide (PI) [15,16], polydimethyl siloxane (PDMS) [22], poly (L-lactic acid) (PLLA) [44] and ODPA-TMPDA [16] to fabricate hybrid membranes. The CuBTC particles used in the studies above were synthesized through hydrothermal or solvothermal method with sizes larger than 10 ȝm. In this case, the “sieve in a cage” structure can be observed, with noticeable voids at the interface between polymer and CuBTC particles [15]. Reducing the size of CuBTC particles will help reduce the voids, preventing severe decrease of selectivity. Several research works have been conducted on synthesizing CuBTC particles at room temperature [45-49]. In this study, submicro-sized CuBTC particles with open channels (designated as “unblocked CuBTC”) and blocked channels (designated as “blocked CuBTC”) were synthesized at room temperature. These particles were then incorporated into PDMS matrix to fabricate hybrid membranes with polyvinylidene fluoride (PVDF) ultrafiltration membranes as substrates. The morphology, crystal structure and pore parameter of CuBTC particles, as well as the morphology, functional groups, thermal and mechanical properties, free volume characteristics and swelling property were studied systematically. The hybrid membranes were employed for pervaporative desulfurization of model gasoline containing n-octane as the bulk and thiophene as a

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representative organosulfur impurity. 2. EXPERIMENTAL SECTION 2.1 Materials Copper hydroxide (Cu(OH)2), copper nitrate trihydrate (Cu(NO3)2·3H2O), diethylamine ((C2H5)2NH) and thiophene (C4H4S) were purchased from Jiangtian Technology Co., Ltd., Tianjin, China; trimesic acid (1,3,5-benzene tricarbonic acid, 1,3,5-C6H3(COOH)3, BTC for short), Ȗ-aminopropyl trimethoxysilane (APTMS, NH2(CH2)3Si(OCH3)3) and potassium bromide (KBr) were purchased from Aladdin Industrial Corporation, Shanghai, China; ethanol absolute, n-heptane (CH3(CH2)5CH3) and n-octane (CH3(CH2)6CH3) were purchased from Guangfu Fine Chemical Research Institute, Tianjin, China; hydroxyl-terminated PDMS (HO-[-Si(CH3)2O-]n-OH) oligomer with viscosity of 5000 cp (the molecular weight was about 50000) was purchased from Silicon Mountain Macromolecular Materials Co., Ltd., Shanghai, China; dibutyltin dilaurate ((CH3(CH2)3)2Sn[OCO(CH2)10CH3]2) was purchased from Sihuan’antong Commerce and Trade Co., Ltd., Beijing, China; polyvinylidene fluoride (PVDF) flat-sheet ultrafiltration membrane with molecular weight cutoff (MWCO) of 30000 was purchased from Synder Membrane Technology Co., Ltd., Vacaville, USA. KBr was of spectrum grade, all other reagents were of analytical grade and all the above were used without further purification. Deionized water was used throughout the experiment. 2.2 Preparation of CuBTC particles 2.2.1 Unblocked CuBTC particles Unblocked CuBTC submicro-sized particles were synthesized through a green and scalable acid-base reaction method [45]. In a typical synthesis, Cu(OH)2 powder was dispersed in water by

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ultrasound and stirring to form a suspension, while BTC was dissolved in ethanol to form a homogeneous solution. The molar ratio of Cu(OH)2, BTC, water and ethanol was 1:1:100:80. The suspension of Cu(OH)2 was added to the solution of BTC, and it was observed that the color of the suspension changed from dark green to light blue gradually, within about 3 h. The reaction was kept at ambient temperature for 48 h. Then, the particles in the suspension were separated with centrifugation and washed with ethanol. Then, the particles were dried in a vacuum oven at 40 oC for 24 h. Finally, the particles were activated in N2 atmosphere at 100 oC for 6 h, then 150 oC for 12 h, and it was observed that the color of the particles changed from light blue to dark blue. The product particles were stored in N2 atmosphere. 2.2.2 Blocked CuBTC particles Blocked CuBTC submicro-sized particles were synthesized through a complexation method [46]. In a typical synthesis, Cu(NO3)2·3H2O was dissolved in water, while BTC was dissolved in a mixture of ethanol and diethylamine. The molar ratio of Cu(NO3)2, BTC, water, ethanol and diethylamine was 1:1:220:55:3.5. The solution of Cu(NO3)2 was added to the solution of BTC, and it could be observed that blue particles were produced instantly. The reaction was kept at ambient temperature for 48 h. Then, the particles in the suspension were separated with centrifugation and washed with ethanol. Then, the particles were dried in a vacuum oven at 40 oC for 24 h. The particles were not activated. 2.3 Fabrication of hybrid membranes A certain amount of CuBTC particles were dispersed in n-heptane with ultrasound to avoid excess aggregation. Then PDMS oligomer and the crosslinker, APTMS, were dissolved in the suspension under gentle stirring. The catalyst, dibutyltin dilaurate, was added to accelerate the

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crosslinking of PDMS. The mass ratio of heptane, PDMS, APTMS and dibutyltin dilaurate was 1:0.3:0.023:0.003. The casting solution was kept stay for 5 to 10 seconds so as the bubbles could disappear, then cast onto pre-dried PVDF support layers. The membranes were dried overnight at room temperature to evaporate the solvent, then put in an oven at 75 oC for 4 h to complete crosslinking and evaporate residual solvent. All the membranes were stored in dust-free and dry environment. The membranes are designated according to the mass ratio percentage of CuBTC particles to PDMS oligomer as 0 %, 2 %, 4 %, 6 %, 8 %, 10 % and 12 % for simplicity. 2.4 Characterization of particles and membranes The morphology of CuBTC particles was observed with a transmission electron microscope (TEM, JEOL JEM-100CXII, Japan). The crystal structure of CuBTC powder was characterized with an x-ray diffractometer (XRD, Rigaku D/MAX-2500, Japan) at 40 kV, 200 mA, the scan speed was 2o/min, the step size was 0.02o and the 2ș range was 5~30o, using CuKĮ radiation. The pore volume, size distribution and Brunauer-Emmett-Teller (BET) surface area of CuBTC particles were measured with an automatic sorption analyzer (Micromeritics ASAP 2020 V4.01H, USA). The cross-section images of membranes were observed with a field emission scanning electron microscope (FESEM, FEI Nanosem 430, USA and Hitachi S-4800, Japan). The Fourier transform infrared spectra (FT-IR, Bruker Vertex 70, Germany) were obtained with a range of 4000~400 cm-1 and a resolution of 4 cm-1. The thermal gravimetric analysis (TGA, Netzsch TG 209, Germany) was carried out with a temperature range from room temperature to 800 oC and a heating rate of 10 oC/min in nitrogen atmosphere. The static tensile test at ambient temperature was conducted with an electronic universal material testing machine (Zhongke Measuring Instruments WDW-02, China), the stretching rate was 5 mm/min.

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The free volume characteristics of the hybrid membranes were measured with a positron annihilation lifetime spectroscope (PALS, EG&G ORTEC, USA) with a fast-fast coincidence system. The resolution was 194 ps, which was the full width at half maximum for Ȗ-rays under

22

60

Co prompt

Na window settings. The positron source (22Na, the radioactivity was 5×105 Bq)

was sandwiched between two pieces of membrane samples. Complete statistics for each spectrum were more than 2×106 coincidences. The spectrum was resolved with the LT-v9 workstation. The correlation between the radius of the free volume cavities (r3, r4), the ortho-positronium (o-Ps) pickoff lifetime (IJ3, IJ4) and the corresponding intensity (I3, I4) were analyzed according to literature [50,51]. It was assumed that the free volume was a spherical potential well surrounded by an electron layer with a thickness of ǻr, and the following equations were used to correlate IJ3, IJ4 and r3, r4.

1ª r 1 § 2ʌr · º τ = «1 − + sin ¨ ¸ 2 ¬ r + ∆r 2ʌ © r + ∆r ¹ »¼

−1

(1)

Vi =

4 3 ʌri 3 (2)

where ǻr is the thickness of the electron layer with an estimated value of 0.1656 nm. The fractional free volume F is calculated as F=F3+F4=V3I3 + V4I4. 2.5 Separation Experiment The separation performances of as-prepared composite membranes were measured with a self-made equipment, which was illustrated in previous study [52]. The feed liquid was model gasoline containing n-octane and thiophene with a concentration of 1300 ppm by weight, the flow rate was 40 L/h and the operation temperature was controlled at 40 oC. The effective area of the 8

membrane sample was 25.6 cm2. The downstream side of the membrane module was kept in vacuum with a pump, and the absolute downstream pressure was below 0.3 kPa. The vapor of permeate was collected with a cold trap, which was immersed in liquid nitrogen and replaced every 30 min till a steady state was achieved. Permeation flux (J, kg·m-2·h-1) is defined as:

J=

Q at (3)

where Q (kg) is the total amount of the permeate collected during the experiment time (t, h), and a (m2) is the effective membrane area. Normalized flux (JN, kg·ȝm·m-2·h-1) is used to exclude the influence of membrane thickness on flux, and is defined as:

J N = Jd (4) where d (ȝm) is the thickness of the active layer of the membrane. The concentration of sulfur in feed and permeate was measured with a microcoulomb sulfur detector (Taizhou East China Analytical Instruments DL-2B-EE, Jiangsu of China). The selectivity of the membrane, represented by the enrichment factor of thiophene (ȕ), was defined as:

β=

ωP ωF (5)

where ȦF and ȦP refer to the concentration of thiophene in the feed and the permeate, respectively. 2.6 Swelling and sorption Swelling and sorption experiments were carried out to determine the affinity of the membranes towards thiophene. Samples of homogenous membranes were dried in vacuum at 40 oC for 24 h, weighed and then immersed in the feed of separation experiment for 48 h. The swollen membranes 9

were weighed once the liquid drops on the surface of the membrane samples were wiped with filter paper. After weighing, the samples were put into a glass tube which was connected to a vacuum pump. The liquid adsorbed in the samples was desorbed and collected with a cold trap, and the concentration of sulfur was determined by the microcoulomb detector. Swelling degree (SD, %) is defined as:

SD =

WSW − WDR × 100 WDR (6)

where WSW and WDR represent the weight of swollen and pre-dried membrane samples, respectively. Adorption selectivity (ȕS) is defined as:

βS =

ωM ωF (7)

where ȦM and ȦF represent the sulfur content of liquid in the membrane and in the feed, respectively. According to the solution-diffusion theory, the diffusion selectivity (ȕD) is defined as:

βD =

β βS (8)

where ȕ represents the enrichment factor collected from the separation experiment. 3. RESULTS AND DISCUSSION 3.1 Characterization of CuBTC particles Fig. 1 10

The morphology of as-prepared CuBTC particles is shown in Fig. 1. The size of the particles is about 0.2~0.3 ȝm. The crystals with reduced size (submicrometer) [53] are favorable for the fabrication of membranes with thickness around several tens of micrometers. Non-selective voids at the interface between polymer matrix and particles can be reduced, and high membrane selectivity can be endowed. Moreover, the sizes and morphologies of the two kinds of CuBTC particles are similar. The crystal structures of unblocked and blocked CuBTC were analyzed with XRD, as shown in Fig. 2. The XRD pattern of unblocked CuBTC ([Cu3(BTC)2]) is in consistent with literatures [46,54], indicating that the crystal structure is well-developed. The XRD pattern of blocked CuBTC indicates another kind of MOF built upon Cu2+ and BTC, [Cu2(BTC)(OH)(H2O)] [55]. Blocked CuBTC is used as a reference material without pores to reveal the contribution of the pores in unblocked CuBTC to membrane separation performance. Fig. 2 and Fig. 3 The pore parameters of CuBTC were characterized by N2 adsorption test at 77 K, as illustrated in Fig. 3. BET surface area and pore volume are 1562 m2/g and 0.801 cm3/g for unblocked CuBTC, respectively; 150 m2/g and 0.099 cm3/g for blocked CuBTC, respectively. Density functional theory (DFT) pore width distribution plots of CuBTC are shown in the insert image of Fig. 3. The pore widths of unblocked particles are 0.6 nm and 0.9 nm, which can be due to intrinsic micropores of CuBTC [36,56], while pores of blocked particles are not pronounced. The surface area and pore volume of blocked CuBTC are much lower than those of unblocked CuBTC, indicating that the channels of blocked CuBTC are not open. 3.2 Characterization of membranes Fig. 4

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The cross-section morphology of PDMS control and PDMS-CuBTC (unblocked) hybrid membrane with the filler content of 8 wt% is illustrated in Fig. 4. It is observed that the CuBTC particles disperse homogeneously in PDMS matrix, and that no voids can be observed at the interface between PDMS and CuBTC particles. The thickness of the active layer of the membrane is around 28 to 37 ȝm. Fig. 5 The infrared spectra of unblocked CuBTC powder, PDMS control and PDMS-CuBTC (unblocked) hybrid membranes are shown in Fig. 5. It can be observed that the strong characteristic bands of CuBTC, which locate at 1646, 1375 and 730 cm-1 (marked with rectangles in Fig. 5) appear in the spectrum of hybrid membrane. The band at 1646 cm-1 stands for the stretching vibration of C=O bond, the band at 1375 cm-1 indicates the stretching vibration of C=C bond on the aromatic ring, and the band at 730 cm-1 refers to the scissoring vibration of carboxylate ion. No new band or shifted band is found in the spectra, indicating that no chemical interaction occurs between PDMS and CuBTC. The intensity of the three characteristic bands shows no obvious change with the content of filler particles. Fig. 6 The thermogravimetric analysis of PDMS control and PDMS-CuBTC (unblocked) hybrid membranes are illustrated in Fig. 6. In the temperature range of 300 oC to 600 oC, the hybrid membranes lose weight at a much slower rate than the PDMS control membrane. The thermal stability of the hybrid membrane is enhanced effectively. This can be ascribed to the non-covalent interaction [57] between the carboxylate ions of CuBTC and the amino groups of APTMS, the crosslinker of PDMS membrane. The interaction increases the cohesive energy of PDMS matrix,

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and the decomposition of PDMS happens at a higher temperature consequently. Fig. 7 The mechanical property of PDMS control and PDMS-CuBTC (unblocked) hybrid membranes was studied with mechanical stretching at room temperature, as shown in Figs. 7a and 7b. When the filler content increases from 0 % to 8 %, the elongation at break increases from 43 % to 215 %, and the elasticity modulus (Young modulus) decreases from 0.44 MPa to 0.26 MPa. When the filler content continues to increase from 8 % to 12 %, the plasticity of the membranes begins to decrease. Table 1 The free volume characteristics were studied with PALS, and the results are listed in Table 1. When the filler content of CuBTC increases from 0 % to 8 %, the fractional free volume of the membranes shows an increase. Considering the results of mechanical stretching and PALS, it is deduced that the chain packing of PDMS is interrupted by CuBTC particles, thus extra space for thermal motion of PDMS chains is generated at the interface between PDMS and CuBTC particles. When the filler content continues to increase from 8 % to 12 %, the aggregation of CuBTC particles becomes dominant, and the effective external surface area reduces, decreasing the extra free volume cavities at the interface between PDMS and CuBTC particles. Table 1 shows that the diameters of free volume cavities are 0.34 nm (2×r3) and 0.78 nm (2×r4). The kinetic diameters of thiophene and n-octane used in pervaporative desulfurization of model gasoline are 0.53 nm [58] and 0.63 nm [59], respectively. Moreover, the intrinsic pore widths of unblocked CuBTC are 0.6 nm and 0.9 nm. Therefore, it is assumed that the free volume cavities and the channels with width of 0.9 nm in CuBTC do not have significant sieving function in this

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study, while the channels with width of 0.6 nm in CuBTC can sieve thiophene from n-octane in desulfurization of model gasoline. 3.3 Separation experiment Fig. 8 The separation performances of the as-prepared membranes are evaluated in desulfurization of model gasoline, as illustrated in Fig. 8. When the filler content of unblocked CuBTC increases from 0 % to 8 %, the normalized flux and selectivity increase by 100 % and 75 %, respectively. The intrinsic channels of unblocked CuBTC and the extra free volume cavities can reduce the resistance against the transmembrane transport of permeate molecules, increasing the flux; while the transport of thiophene is facilitated by the active metal sites of CuBTC, increasing the selectivity. The sieving function of unblocked CuBTC channels with width of 0.6 nm can also contribute to the increase of selectivity. The optimal separation performance is achieved when the filler content of unblocked CuBTC reaches 8 %, with a normalized flux of 194.2 kg·ȝm/(m2· h) and an enrichment factor of 5.2. When the filler content increases from 8 % to 10 %, the permeation rate of thiophene begins to decrease while the permeation rate of octane still increases due to increased non-selective voids at the interface between polymer matrix and fillers. When the filler content continues to increase from 10 % to 12 %, the permeation rate of octane begins to decrease because aggregation of particles (reducing extra free volume) becomes dominant. The separation performance of hybrid membranes incorporated with blocked CuBTC was evaluated. When the filler content of blocked CuBTC increases from 0 % to 8 %, the normalized flux increases by 47 %, but the selectivity shows no significant change. This result shows that both the increase of fractional free volume and the intrinsic channels of unblocked CuBTC can increase

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normalized flux. Fig. 9 The results of swelling and sorption are shown in Fig. 9. With the increase of filler content, the sorption selectivity increases due to the preferential affinity of unblocked CuBTC to thiophene. The values of diffusion selectivity are far greater than sorption selectivity, showing that diffusion is dominant in the contribution to membrane selectivity, rather than sorption in this study. The variation trend of diffusion selectivity with the increase of filler content is consistent with membrane selectivity in Fig. 8. The swelling degree shows continuous decrease with the increase of filler content. This is due to the increase of cohesive energy, which has been discussed through the analysis of TGA data. The stability of PDMS membranes in organic solvent is increased, which is beneficial for the separation process. Fig. 10 The pervaporative desulfurization performance of this study with recent relevant literature [60-73] is compared, as shown in Fig. 10 and explained below. i) PDMS membranes have generally higher normalized flux than such membranes as polyimide, polyphosphazene, PebaxTM and polysulfone membranes, and this phenomenon is attributed to the weak interaction between PDMS chains; ii) The regular channels of CuBTC provide pathways for permeate molecules with low resistance, endowing PDMS membrane with relatively high normalized flux; iii) Membranes functionalized with carriers such as Cu, Ag, etc. can increase membrane selectivity effectively, as the carriers can perform facilitated transport function to accelerate the transmembrane permeation of organosulfur;

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iv) CuBTC used in this study combines the advantages of regular channels and facilitated transport function, so the PDMS-CuBTC hybrid membranes are endowed with higher normalized flux than those hybrid membranes with dense fillers containing facilitated transport carriers. Fig. 11 The influence of operating temperature on separation performance of PDMS-8 % CuBTC (unblocked) hybrid membrane is demonstrated in Fig. 11 (a). With the increase of operating temperature, the normalized flux increases while the enrichment factor decreases. When the operating temperature increases, the thermal motion of PDMS chains becomes more intense, thus the fractional free volume increases, increasing the normalized flux and decreasing the selectivity. Moreover, the vapor pressure of the feed liquid increases, increasing the driving force of transmembrane mass transport, and this can also contribute to the increase of normalized flux. The Arrhenius plot in Fig. 11 (b) can correlate the partial flux of thiophene and octane with temperature:

§ E · J = A exp ¨ − ¸ © RT ¹ (9) where J refers to the permeation flux, A is a constant, E represents the apparent activation energy for permeation, R is the universal gas constant, and T is the thermodynamic temperature. It can be seen that the activation energy of octane is higher than that of thiophene, indicating that the mass transfer of octane is more sensitive to temperature than thiophene. This can be attributed to the difference of molecule size, as the kinetic radius of n-octane is larger than that of thiophene. The diffusion of larger molecules is more sensitive to the variation of diffusion resistance. When the temperature increases, the increase of partial flux of octane is faster than thiophene, decreasing the 16

selectivity. With the increase of sulfur content of feed, the normalized flux of PDMS-8 % CuBTC (unblocked) hybrid membrane increases while the selectivity decreases, as shown in Fig. 11 (c). When the activity of thiophene in the feed increases, the dissolution of thiophene into the membrane is enhanced, increasing the normalized flux. The increased dissolution of thiophene also raises the swelling degree of the membrane, increasing the normalized flux while decreasing the selectivity. Fig. 12 The operational stability of PDMS-8 % CuBTC (unblocked) hybrid membrane was investigated, as shown in Fig. 12. Owing to the good adhesion between PDMS active layer and PVDF substrate and the decrease of swelling degree, the membrane shows relatively high stability. 4. CONCLUSION A typical kind of metal organic framework with active metal sites, CuBTC, was incorporated in PDMS matrix to fabricate hybrid membranes and then applied in pervaporative desulfurization of model gasoline. Due to the submicrometer size of CuBTC particles, the non-selective voids at the interface between polymer matrix and fillers are reduced. The channels of CuBTC and the extra free volume cavities at the interface between PDMS matrix and CuBTC particles decrease the permeation resistance, increasing the normalized flux; while the active metal sites and the channels of CuBTC render the facilitated transport and molecular sieving function for thiophene component, respectively, increasing the selectivity. Normalized flux and selectivity are thus increased simultaneously. The optimal separation performance is achieved with a flux of 194.2 kg·ȝm/(m2·h) (100 % higher than PDMS control membrane), and an enrichment factor of 5.2

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(75 % higher than PDMS control membrane) when the filler content of unblocked CuBTC reaches 8 %. Increased feed temperature and sulfur content in feed both lead to increased normalized flux and decreased selectivity of the membrane with the optimal separation performance. This study provides a preliminary understanding on the contributions of the chemical structure and topological structure of porous fillers to membrane separation performance. NOMENCLATURE Symbols A

Constant of Arrhenius equation (kg· m-2·h-1)

a

Effective membrane area (m2)

d

Thickness of active layer (ȝm)

E

Apparent activation energy (kJ·mol-1)

F

Fractional free volume

I

Intensity of free volume cavity (%)

J

Permeation flux (kg·m-2·h-1)

JN

Normalized permeation flux (kg·ȝm·m-2·h-1)

Q

Permeate amount (kg)

R

Universal gas constant

r

Radius of free volume cavity (nm)

SD Swelling degree (%) T

Temperature (K)

t

Experiment time (h)

V

Free volume (nm3)

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W

Weight of membrane sample (g)

ȕ

Enrichment factor

ȕD

Diffusion selectivity

ȕS

Sorption selectivity

ǻr Thickness of electron layer (nm) ș

XRD glancing angle (o)

IJ

Ortho-positronium pickoff lifetime (ns)

Ȧ

Thiophene concentration (ppm)

Subscripts 3

Third pickoff lifetime

4

Fourth pickoff lifetime

D

Diffusion

DR Dried F

Feed

M

Membrane

N

Normalized

P

Permeate

S

Sorption

SW Swollen ACKNOWLEDGEMENT The authors gratefully thank the financial support from the National Science Fund for Distinguished Young Scholars (21125627), the Major Program of the National Natural Science

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Foundation of China (21490583), the National Natural Science Fundation of China (21306131), the Specialized Research Fund for the Doctoral Program of Higher Education (20120032120009), the Program of Introducing Talents of Discipline to Universities (B06006), and the Open Project of the State Key Laboratory of Chemical Engineering (SKL-ChE-13B03). We also thank Prof. Xianhe Bu and Dr. Ze Chang from Department of Chemistry, Nankai University, China, who have helped us in nitrogen adsorption analysis. REFERENCES [1] P. Shao, R.Y.M. Huang, Polymeric membrane pervaporation, J. Membr. Sci. 287 (2007) 162-179. [2] L.M. Robeson, Correlation of separation factor versus permeability for polymeric membranes, J. Membr. Sci. 62 (1991) 165-185. [3] F.B. Peng, L.Y. Lu, H.L. Sun, Y.Q. Wang, J.Q. Liu, Z.Y. Jiang, Hybrid organic-inorganic membrane: Solving the tradeoff between permeability and selectivity, Chem. Mater. 17 (2005) 6790-6796. [4] S. Shu, S. Husain, W.J. Koros, A general strategy for adhesion enhancement in polymeric composites by formation of nanostructured particle surfaces, J. Phys. Chem. C 111 (2007) 652-657. [5] J. Ahn, W.J. Chung, I. Pinnau, M.D. Guiver, Polysulfone/silica nanoparticle mixed-matrix membranes for gas separation, J. Membr. Sci. 314 (2008) 123-133. [6] B. Zornoza, B. Seoane, J.M. Zamaro, C. Téllez, J. Coronas, Combination of MOFs and zeolites for mixed-matrix membranes, ChemPhysChem 12 (2011) 2781-2785. [7] S. Shahid, K. Nijmeijer, High pressure gas separation performance of mixed-matrix polymer

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FIGURE CAPTIONS Fig. 1. TEM images of CuBTC particles: (a) unblocked and (b) blocked. Fig. 2. XRD spectra of CuBTC particles. Fig. 3. Nitrogen adsorption isotherms at 77 K and (insert) DFT pore width distribution of CuBTC. Fig. 4. Cross-sectional SEM images of (a) PDMS control and (b) (c) PDMS-8 % CuBTC (unblocked) membranes with PVDF membranes as the substrates. Fig. 5. FT-IR spectra of unblocked CuBTC powders, PDMS control and 8 % hybrid membrane. Fig. 6. TGA spectra of CuBTC (unblocked), PDMS control and PDMS-CuBTC hybrid membranes. Fig. 7. Mechanical properties of PDMS control and PDMS-CuBTC (unblocked) hybrid membranes. Fig. 8. Effect of filler content on separation performance of PDMS membranes. Fig. 9. Swelling and sorption properties of PDMS control and PDMS-CuBTC (unblocked) hybrid membranes. Fig. 10. Comparison on desulfurization performance in this study with literature. Fig. 11. Effect of (a) operating temperature and (c) sulfur content of feed on separation performance; (b) Arrhenius plot of PDMS-8 % CuBTC (unblocked) membrane. Fig. 12. Operational stability of PDMS-8 % CuBTC (unblocked) membrane.

30

Table 1. Free volume characteristics of PDMS control and PDMS-CuBTC (unblocked) hybrid membranes Content of CuBTC (wt%)

r3 (nm)

I3 (%)

F3

r4 (nm)

I4 (%)

F4

F

0 4 8 12

0.1626 0.1601 0.1794 0.1654

11.85 11.53 11.32 10.96

0.213 0.198 0.274 0.208

0.3897 0.3885 0.3944 0.3906

25.60 26.69 26.40 26.30

6.346 6.556 6.784 6.565

6.559 6.754 7.058 6.773

31

z

PDMS hybrid membranes filled with metal-organic framework CuBTC were prepared.

z

Extra free volume cavity and mass transfer channel are generated in PDMS matrix.

z

Active metal sites of CuBTC render facilitated transport function for thiophene.

z

Channels of CuBTC render molecular sieving function for thiophene.

z

Permeation flux and selectivity are increased simultaneously.

32

Figure

(a)

(b) Fig. 1

Figure2

Fig. 2

Figure3

Fig. 3

Figure4

(a)

(b)

(c) Fig. 4

Figure5

Fig. 5

Figure6

Fig. 6

Figure

(a)

(b) Fig. 7

Figure

Fig. 8

Figure

Fig. 9

Figure10

Fig. 10

Figure

(a)

(b)

(c) Fig. 11

Figure

Fig. 12