Development of Ethenylene-Bridged Organosilica Membranes for

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Development of ethenylene-bridged organosilica membranes for desalination applications. Rong Xu, Peng Lin, Qi Zhang, Jing Zhong, and Toshinori Tsuru Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b04439 • Publication Date (Web): 04 Feb 2016 Downloaded from http://pubs.acs.org on February 5, 2016

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Industrial & Engineering Chemistry Research

Development of ethenylene-bridged organosilica membranes for desalination applications Rong Xu,† †

Peng Lin,†

Qi Zhang,†

Jing Zhong,*† and Toshinori Tsuru*‡

Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of

Petrochemical Engineering, Changzhou University, Changzhou, 213164, China ‡

Department of Chemical Engineering, Hiroshima University, Higashi-Hiroshima

739-8527, Japan

ABSTRACT: A promising new ethenylene-bridged organosilica membrane has been developed for pervaporative desalination of water. Due to the introduction of polarizable and rigid ethenylene bridges in the silica networks, the membrane exhibited an improved water affinity and superior hydrothermal stability. The ethenylene-bridged organosilica membrane delivered a high water flux of up to 14.2 kg m-2 h-1 with a high NaCl rejection of 99.6% at for pervaporative desalination 70 °C. Moreover, the membrane was highly applicable and stable for desalination of saline waters under a wide range of salt concentrations. The rapid permeation of water molecules through the membrane was attributed to the open pore structure and the water/pore wall interactions, while the transport of hydrated salt ions was explained by the relatively broad pore size distribution of the ethenylene-bridged silica networks.

KEYWORDS: organosilica, membrane, desalination, pervaporation

ACS Paragon Plus Environment


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1. INTRODUCTION The development of technologies for water desalination is critical for addressing the increasing global water shortage.1 Reverse osmosis (RO) has become the state-of-the-art technology for seawater desalination. However, to drive the transport of water across a RO membrane, a high pressure of typically 5-8 MPa is required mainly to overcome the osmotic pressure of seawater. Moreover, the commercial polyamide RO membranes are prone to biofouling, and regeneration of these membranes is undesirable due to their low chemical stability.2 To address these challenges of RO, extensive studies have developed alternative desalination technologies, such as forward osmosis3 and membrane distillation4. Pervaporation is a widely used membrane separation technology for organic solvent dehydration and organic mixture separation. In pervaporation, mass transfer is driven by vapor pressure differences across a membrane, which are generally generated by employing a vacuum pump or a carrier gas on the permeate side.5 Recently, many efforts have focused on pervaporation with the hope of applying this technology to desalination. As an emerging technology, pervaporation may offer advantages of nearly 100% salt rejection and energy consumption that is independent of the feed salinity. Additionally, compared with pressure-driven RO processes, pervaporation has a lower fouling propensity due to the absence of applied hydraulic pressure.6 The main problem with pervaporative desalination is the requirement of the heat of evaporation, namely increasing the energy consumption. This problem may be ameliorated by using waste heat or solar to heat up the feed water, resulting in a


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significant reduction in the external energy use of the process. Therefore, desalination using a pervaporation process could be particularly interesting for those areas with a lot of low-grade heat energy.1At present, various membranes have been developed for pervaporative desalination by using different types of materials such as crosslinked PVA, zeolites (NaA, ZSM-5 and Silicalite) and carbonized-template silica.7–10 Although these membranes are capable of removing > 95-99.5% of monovalent salt ions, the water flux for most of them requires further improvement. Organically bridged silica is a highly promising material for application in molecular separation membranes. This new class of materials is synthesized by hydrolysis and condensation reactions of bridged organosilanes (R′O)3Si-R-Si(OR′)3. Incorporation of organic bridges (R) into the silica networks offers new opportunities to finely tune bulk properties such as flexibility, chemical stability, and surface hydrophilicity/hydrophobicity.11 Castricum et al.12,13 first developed microporous organically bridged silica membranes by co-condensation of bis(triethoxysilyl)ethane (BTESE) and methyltriethoxysilane. The resultant membranes delivered stable performance for 2 years during the pervaporative dehydration of butanol at 150 °C. Kanezashi et al. proposed a “spacer” technique to control silica networks, using BTESE as a silica precursor for the development of a highly permeable hydrogen separation membrane.14 Recently, the applications of bridged organosilica membranes have expanded to RO. BTESE-derived membranes have exhibited excellent chlorine tolerance under a wide range of chlorine concentrations.15 The main problem of BTESE membranes, however, has been a relatively low water flux, mainly due to the



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hydrophobic nature of the ethylene bridges (–CH2−CH2–), which hinders the rapid transport of water molecules through the membrane. In order to overcome this obstacle, the chemical structure of the organic bridges must be modified to increase the membrane affinity for water. Here, we report on the development of a highly water-permeable organosilica membrane

that

uses

an

ethenylene-bridged

(–CH=CH–)

precursor,

bis(triethoxysilyl)ethylene, which is different from BTESE. The introduction of more polarizable ethenylene bridges in the networks may make the membrane more attractive to polar water molecules. Moreover, the unsaturated ethenylene bridges are reactive and hence easily accessible for further functionalization, such as sulfonation, bromination and ozonolysis.16–18 In the present study, using a sol-gel technique, we fabricated ethenylene-bridged organosilica membranes and applied them to pervaporative desalination of water. Various characterizations were carried out to provide information about pore chemistry, pore structure, and the morphology of the membranes. Here, we present the results of a series of studies that include the desalination performances, transport properties and the potential desalination applications for this innovative membrane. 2. EXPERIMENTAL 2.1 Synthesis of BTESEthy-derived Sols. 1,2-Bis (triethoxysilyl) ethylene (BTESEthy, 95%, ∼80% trans isomer, Gelest, Inc.) sol was synthesized via the hydrolysis and polymerization reaction of a precursor (EtO)3SiCH=CHSi(OEt)3 with water and HCl, and ethanol was used as a solvent. A required amount of BTESEthy


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was mixed with ethanol. Subsequently, premixed water and HCl were added dropwise to the solution under continuous stirring. The molar composition of the reactants was BTESEthy: H2O: HCl = 1: 60: 0.2, with the equivalent weight percent of BTESEthy kept at 5 wt %. The solution was stirred for 2 h at 40 °C before coating. The BTESEthy-derived gel powder was prepared by drying at 60 °C in air, and was ground using a mortar and pestle. 2.2 Membrane Preparation. Porous α-alumina disks (diameter, 30 mm; porosity, ∼35%; and average pore size, 200 nm) were used as supports. First, α-alumina particles (average particle size, 200 nm) were coated onto the surface of the support using a SiO2-ZrO2 colloidal (2 wt %) sol as a binder, and the support was fired at 550 °C for 30 min. This coating and firing process was repeated several times to remove the large pores that might have resulted in pinholes in the final membrane. Then, SiO2-ZrO2 (molar ratio of Si: Zr = 1:1, 0.5 wt %) sols were coated onto the particle layer and fired at 550 °C to form an intermediate layer. Finally, the BTESEthy-derived separation layer was fabricated by coating the BTESEthy (0.5 wt %) sol onto the intermediate layer, followed by flash calcination19 at 250 °C in air for 20 min. 2.3 Characterization. The size distribution of freshly prepared BTESEthy sols was determined by dynamic light scattering (DLS) using a Malvern Zetasizer Nano-ZS (ZEN3600) at 25 °C. The non-volatile residue concentration of the BTESEthy polymer was measured by thermogravimetric (TG, NETZSCH 209 F3) analysis to evaluate the reaction behavior during sol synthesis. The temperature was increased at



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a ramping rate of 10 °C·min-1 and kept at 125 °C for 30 min under N2 at a flow rate of 30 mL·min-1. Fourier Transform Infrared Spectrometer (FT-IR, Thermo Nicolet-460) was applied to confirm the chemistry structure of the BTESEthy films. To evaluate the decomposition behavior of the organosilica networks, TG analysis was conducted on the BTESEthy gel powder under both air and N2 at a heating rate of 10 °C·min-1. Before TG measurement, the powder was pretreated at 100 °C for 1 h under N2 flow (10 mL·min-1) to remove adsorbed water. The thickness and morphology of the membrane was examined using field-emission scanning electron microscopy (FE-SEM, Zeiss SUPRA 55). The microstructures and surface properties of the membranes were examined via nitrogen sorption at 77 K and water sorption at 298 K, using a Belsorp-Max (Bel Japan, Inc.) instrument. Before analysis, the samples were outgassed at 200 °C under vacuum for 10 h. The Brunauer-Emmett-Teller (BET) method was employed in a relative pressure range of P/P0= 0.01-0.25 to calculate the specific surface area. The pore size distribution was obtained through the analysis of the adsorption branch of nitrogen isotherms using the MP method, and the micropore volume was estimated using t-plots. 2.4 Membrane performance. The single gas permeation measurements were performed at 200°C using high-purity He, H2, CO2, N2, C3H8, and SF6. Before the measurement, the membrane was first pretreated for 8 h at 200 °C under a He flow of 20 mL·min-1 to remove the adsorbed water from the membrane pores. The permeate side was kept at atmospheric pressure, and the pressure drop across the membrane was maintained at 100 kPa. The permeation rate was measured using a soap-film flow


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meter. Pervaporative desalination experiments were carried out using a NaCl aqueous solution in concentrations from 0.2 wt % to 3.5 wt % in order to simulate the typical salinity of brackish water (0.2-1 wt %) and seawater (approximately 3.5 wt %). Figure 1 shows a schematic diagram of the pervaporation experimental apparatus. The feed solution was continuously circulated using a variable speed peristaltic pump to minimize the effect of concentration polarization on the contact membrane side. The permeate was collected by turns in liquid nitrogen cold traps during a predetermined time interval. The pervaporative desalination performances of the membranes were evaluated by measuring water flux and salt rejection. Water flux, J, was calculated using the following expression: =

∙ ∆

where is the mass of the permeate collected in the cold trap at the experimental time interval,∆ , and is the effective membrane area. The observed salt rejection, Robs, can be expressed as follows:

= 1 −

× 100%

where and are the salt concentrations of the permeate and the feed, which were measured using a conductivity meter (INESA Instrument, DDB-303A). Each experiment was first run for at least 3 h to confirm a steady state, and then the permeate sample was collected and analyzed. At the end of each test, the downstream of the membrane cell was flushed with 100 mL deionized water and the conductivity of the stream was measured to check for salt leakage or crystallization. During each



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test, the membrane remained clean and no salt precipitation on the permeate side of the membrane was observed. The results during each measurement were reproducible, and the variations for water flux and salt rejection were generally within ± 0.3 kg·m-2·h-1 and ± 0.2%, respectively. 3. RESULTS AND DISCUSSION 3.1 Characterization of sols, gels and membranes. The size distribution of BTESEthy-derived sols with a precursor concentration of 5 wt% was acquired via dynamic light scattering (DLS) at 25°C, as shown in Figure 2a. Freshly synthesized BTESEthy sol showed a bimodal size distribution with a mean radius of 5.1 nm. This sol size was sufficiently small to prepare a thin, microporous separation layer. The bimodal size distribution can be explained by the polymerization reaction of the monomer species, rather than by the aggregation of nanoparticals, since even a very small fraction of aggregation would result in a steep increase in sol size and a broadening of its size distribution.20 It should be noted that all the sols needed for membrane fabrication were filtered (filter size, 0.1 µm) to minimize the effect of larger sols. Figure 2b presents the non-volatile residue concentration of BTESEthy sols as a function of reaction time. BTESEthy sols were synthesized at 40 °C with a H2O/Si molar ratio of 60. Since the polymer sol (non-volatile residue) is less volatile than the unreacted BTESEthy monomer, the TG weight of the nonvolatile residue can be used to evaluate the hydrolysis and condensation behaviors. As shown in Figure 2b, the nonvolatile residue under hydrolysis and polymerization at a pH of 2-3 was generated


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with an increase in reaction time and approached 3 wt% within a few hours, indicating a fast reaction rate in the initial stage. The π-bond of the alkylene, as a nucleophile, could interact with H3+O in solution, thus contributing to the hydrolysis and condensation reaction.21 Figure 3a shows the FTIR spectrum of the BTESEthy films. The following absorptions were mainly related to the BTESEthy networks: C−H stretching vibrations in the vinyl unit (∼2980 cm-1); O-H stretch (∼3000-3600 cm-1); C=C stretch (∼1620 cm-1); Si−O−Si stretch (1020-1200 cm-1); Si–OH stretch (∼920 cm-1); and Si– C stretch (∼790 cm-1).22 The presence of these characteristic bands verified the formation of the BTESEthy-derived network structure. The thermal stability of the ethenylene-bridged organosilica network was evaluated by Thermogravimetric (TG) analyses under both an air and N2 atmosphere. Before TG measurement, a sample gel powder was maintained at 100 °C for 1 h to remove the physisorbed water. As shown in Figure 3b, the TG curve shows three weight loss steps under an air atmosphere. The first weight loss (ca. 1%) occurred below ∼200 °C was related to the evaporation of a small amount of chemisorbed water. The second weight loss (ca. 6%) was observed for temperatures ranging from ∼200 °C to ∼370 °C, which was probably due to the dehydration of silanol groups. Above approximately 370 °C, a continuous decrease in weight (ca.16%) suggested the decomposition of organic components in the networks. By contrast, the weight loss of the sample in N2 was less pronounced than that in air. The total weight loss under N2 atmosphere was about 17%, which was smaller than the 23% measured under an air



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atmosphere, which suggested that some organic compounds (carbon) remained in the gel powder under a N2 atmosphere. Figure 4 shows a cross-sectional SEM image of a BTESEthy-derived composite membrane. A three-layer structure (top layer, nanoparticle layer, and α-alumina layer) can be observed clearly, although it is difficult to distinguish the uppermost separation layer from that of the intermediate one in the top layer. A crack-free, continous separation layer, with a total thickness of approximately 100nm, formed on top of the SiO2-ZrO2 intermediate layer (Figure 4, inset). To probe the pore structure of the membrane, nitrogen gas sorption measurements were performed on the BTESEthy and BTESE xerogels. As Figure 5a shows, both materials were type I isotherms with a significant uptake of N2 at low relative pressure (P/P0 < 0.01) and a saturated adsorption at approximately P/P0 > 0.2, which is typical for microporous materials.23 Details of the textural properties are given in Table 1. The BTESEthy sample possessed a BET specific surface area of 492 m2·g-1 and a pore volume of 0.32 cm3·g-1, which were both larger than those of the BTESE material (Table 1). This can be ascribed to increased rigidity of the ethylene-bridged (– CH=CH–) networks, which would prevent pore collapse and the formation of dead-end pores during calcination, and thus afford more open and accessible pores.24,25 On the other hand, the pore size distributions (Figure 5a, inset) clearly reveal that both samples have bimodal size distribution in the microporous region with a pore diameter centered at approximately 0.6 nm. Water adsorption was applied to further characterize the BTESEthy and BTESE


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networks due to its high sensitivity to both surface chemistry and pore structure. Figure 5b shows the water isotherms of the BTESEthy and BTESE materials measured at 298K. First, the adsorbed amount of water increased slowly with an increase in the relative pressure of approximately 0.1. The limited water uptake at low pressure (P/P0 < 0.1) suggested a weak affinity of water for the organosilica networks. This was related to the hydrophobic nature of the organic bridges. Subsequently, water uptake capacity increased gradually for relative pressures ranging from 0.1 to 0.9, which was indicative of pore filling and/or condensation of water into both networks. This wide range of pressures for P/P0 during water adsorption implied a broad distribution of pore sizes for the two samples, which was consistent with the PSD results obtained by nitrogen adsorption isotherms (Figure 5a). Finally, a higher sorption capacity of water was observed for BTESEthy (380 cm3 g-1) than that of BTESE (304 cm3 g-1) at P/P0 = 0.9, partly due to the larger surface area of the BTESEthy material (Table 1). In contrast with the nitrogen isotherms, the water desorption branches for both BTESEthy and BTESE showed an obvious hysteresis. This was probably related to the surface chemistry of BTESEthy and BTESE networks. The presence of silanol groups (Si-OH) within the networks provides a number of adsorption sites for the chemisorption of water molecules, which results in an incomplete desorption of adsorbed water from the pore surface of the networks, and thus both water isotherms remain open. Table 1. Physicochemical properties of BTESEthy and BTESE samples. Samples

SBET [m2·g-1]

Vp [cm3·g-1]

dp [nm]


Water uptake [cm3·g-1]


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P/P0=0.1

P/P0=0.9

BTESEthy

492

0.32

0.6

26

380

BTESE

405

0.28

0.6

22

304

Figure 6 shows the molecular size dependence of gas permeance for a BTESEthy membrane at 200°C. The membrane showed a H2 permeance as high as 1.7×10-6 mol·m-2·s-1·Pa with a moderate perm-selectivity of approximately 40 for H2/N2. As mentioned in the introduction, the incorporation of organic bridges into the silica networks led to a loose network structure compared with conventional TEOS-derived silica membranes, which reportedly had high permselectivity for H2 over N2.27 Moreover, H2 showed a higher permeance than He, despite the larger molecular size of H2. This can be explained by Knudsen diffusivity, where H2 with a smaller molecular weight than He shows a higher diffusivity. For large molecules such as N2, C3H8, and SF6, the separation mechanism from small molecules is governed by molecular sieving,28 since molecules with a larger molecular size result in a lower permeance with permeance ratios for H2/N2 at 40, for H2/C3H8 at 1470, and for H2/SF6 at 2787. In addition, a high permeance of CO2 was observed with a CO2/N2 perm-selectivity of 9.6 at 200 °C, likely owing to an additional adsorption of CO2 on the ethenylene-bridged networks with π-bond electrons. 3.2

Pervaporative

desalination

performances.

The

influence

of

the

temperature-induced desalination performance of BTESEthy membranes is shown in Figure 7a. With an increase in temperature from 30 to 70 °C, the water flux of the membrane increased approximately 2.5-fold. This can be explained by the


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conventional solution-diffusion model where the permeate flux in pervaporation is expressed by the following simplified equation.29 = ( − ) According to this model, flux () is proportional to the vapor pressure difference across a membrane, ( − ), and to the permeance, . The saturation vapor pressure in the feed, , increased with an increase in the temperature, whereas the vapor pressure in the permeate, , was maintained at approximately 300 Pa. Hence, an increase in the feed temperature resulted in a higher water flux due to the increased driving force ( − ) for water transport. Interestingly, the NaCl rejection was almost constant, or even increased slightly as the temperature increased, and reached 99.6% at 70 °C from an initial value of 99.4% at 30 °C. This contrasts with typical polymeric desalination membranes, for which increased water flux generally comes at the expense of salt rejection due to thermal expansion of the polymer at elevated temperature.30 However, thermal expansion effects were assumed to be less pronounced for these organic/inorganic hybrid membranes prepared by calcination at 250 °C and consequently, the large hydrated ions were still severely hindered by the micropores of the BTESEthy networks. A similar trend can be observed in our previous studies.29,31 In order to further demonstrate the hydrothermal stability, the BTESEthy membrane was evaluated at an elevated temperature of 70 °C under a continuous pervaporative desalination process. It is noteworthy that the salt rejection remained almost unchanged (> 99.5%) for as long as 100 h at 70 °C (Figure 7b). This result also



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supported the previously mentioned idea that no alteration of effective pore size and structure occurred in the present operating temperature range. However, a slight decrease in water flux was observed over time. We found an initial water flux of as high as 15.5 kg·m-2·h-1, and a final flux of 12 kg·m-2·h-1 was changed by 22% at the end of this test. This can most likely be attributed to the fouling of the membrane surface from metal deposition in the membrane module. Inspection of the membrane after a continuous operation of 100 h revealed brownish-yellow spots on the membrane surface. Subsequent EDX analysis of the deposited layer confirmed the presence of the elements Fe and Cr, which were probably dissolved in the water and then deposited onto the membrane surface. Since these Fe and Cr elements were present in the membrane module made of stainless steel, it is plausible to presume that clogging of the membrane surface due to metal deposition led to a decrease in water flux after long-term operation. Figure 8 shows the effect of feed salt concentration on the water flux and salt rejection of the membrane at 25 °C. In general, the water flux decreased as the salt concentration increased. The water flux decreased from 4.8 to 3.8 kg·m-2·h-1 as the NaCl concentration increased from 2000 to 35000 ppm in the feed. Since the average pore size of the BTESEthy membrane is estimated to be approximately 0.5 nm,35 water molecules should pass through the membrane while the larger hydrated salt ions would be perfectly rejected by size exclusion. However, the BTESEthy-derived silica network is amorphous in nature resulting in a relatively broad pore size distribution, which is evidenced by the N2 adsorption (Figure 5a, inset). The pore structure was


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also probed by the positron annihilation lifetime (PAL) measurement as reported previously,35 which further indicated that the BTESEthy networks had both micropores and mesopores. Therefore, there may be a small fraction of large pores in the membrane, probably derived from the wide pore size distribution of amorphous BTESEthy networks and/or small defects in the membrane. A small fraction of hydrated salt ions could have penetrated the membrane as a liquid through these large pores, which led to an incomplete rejection (∼99.5%) of salt ions. Further optimization of membrane preparation procedures might obtain a sharp pore size distribution and prevent the occurrence of such pores. For instance, the values for for 5000 and 35000 ppm NaCl solutions at 25 °C were 3157 and 3104 Pa, respectively.33 Contrary to this trend, the salt rejection for the BTESEthy membrane was generally in excess of 99.5%, irrespective of the feed salt concentration, which suggested the superior structural stability. This contrasted with desalination performances of the zeolite membranes in pervaporation. According to Drobek et al.,8 a significant decrease in the salt rejection efficiency was observed for the ZSM-5 membranes at high salt concentration, which was explained by the structural degradation of the membranes in the harsh salty environment. The silicalite-1 membranes also underwent a structural alteration as salt concentration increased, though not as severe as the ZSM-5 membranes. Figure 9 is a schematic image of water transport through the organosilica networks of a BTESEthy membrane. The interaction between the permeating molecules and membranes plays an important role in water transport. Considering that the polar C=C



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and Si-OH groups are present in large amounts in the BTESEthy-derived silica networks, water molecules can initially adsorb onto these hydrophilic portions of the networks via hydrogen bonding and/or dipole-dipole interactions, as illustrated in Figure 9. The adsorbed molecules then act as a nucleus to attract more water molecules through the formation of new hydrogen bond. These water/pore wall interactions and water-water intermolecular interactions are expected to facilitate the rapid transport of water molecules, thus leading to a relatively high flux. In terms of pore chemistry, Cohen-Tanugi et al.34 reported that OH-groups hydrogen bond with water and offer a smoother entropic landscape for water molecules to transport, therefore allowing a faster overall water flow. In fact, in the previous study,35 we also calculated the electrostatic potentials (ESPs) that are related to the electron density distribution where negative potential represents electron-rich polar groups, and vice versa, for positive potentials. The ESPs of BTESEthy showed that the negative potentials were mainly localized near the C=C groups and oxygen atoms, leading to an enhanced H2O-affinity for BTESEthy channels. For the transport of salt ions, the permeation of Na+ and Cl− ions in the micro-channels of the membrane is generally in a hydrated state due to a large degree of hydration energy.36 The reported hydrated diameters are 0.72 and 0.66 nm for Na+ and Cl− ions, respectively, which is larger than that for water molecules (d = 0.27 nm).37,38 Since the average pore size of the BTESEthy membrane is estimated to be approximately 0.5 nm,35 water molecules should pass through the membrane while the larger hydrated salt ions would be rejected by size exclusion. However, the


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BTESEthy-derived silica network is amorphous in nature resulting in a relatively broad pore size distribution, which is evidenced by the N2 adsorption (Figure 5a, inset) and positron annihilation lifetime (PAL) measurements as reported previously,35 which further indicated that the BTESEthy networks had both micropores and mesopores. Therefore, there may be a small fraction of large pores in the membrane, probably derived from the wide pore size distribution of amorphous BTESEthy networks and/or small defects in the membrane. A small fraction of hydrated salt ions could have penetrated the membrane as a liquid through these large pores, which led to an incomplete rejection (∼99.5%) of salt ions. Further optimization of membrane preparation procedures might obtain a sharp pore size distribution and prevent the occurrence of such pores. Figure 10 shows the trade-off relationship between NaCl rejection and permeate flux for zeolite, carbonized silica, organosilica and crosslinked PVA membranes. Generally speaking, BTESEthy-derived organosilica membranes showed better desalination performances than those reported for inorganic zeolites and carbonized silica membranes in pervaporation. Compared with ZSM-5 and silicalite-1 membranes, the water flux produced by BTESEthy membranes was 48% and 68% higher, respectively, together with higher salt rejections in water desalination with 35000 ppm NaCl at 25 °C. Although higher water flux with competitive salt rejections was reported for the polymeric PVA membranes,9 the inherently high robustness of the BTESEthy membrane will benefit wider potential applications in water purification under harsh environments.



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4. CONCLUSIONS Ethenylene-bridged organosilica membranes were prepared via an acid-catalyzed sol-gel method using bis(triethoxysilyl)ethylene (BTESEthy), and were applied to pervaporative desalination of water. A thin and defect-free microporous separation layer was obtained by optimizing the sol synthesis. The BTESEthy membrane exhibited a large BET surface area and a high affinity for water, due to the introduction of rigid and polarizable ethenylene bridges in the networks. Gas permeation measurements demonstrated a superior molecular separation ability of the membrane. In pervaporation tests, increasing the operating temperature resulted in a continuous increase in water flux, whereas the salt rejection remained almost constant. Meanwhile, the ethenylene-bridged organosilica membrane was relatively stable at 70 °C during a continuous pervaporation operation of 100 h. The salt rejection in pervaporation was only slightly sensitive to the feed salinity. As the salt concentration of the feed increased from 2000 (brackish water) to 35000 ppm (seawater), the membrane always delivered very high salt rejections of > 99.5%, together with competitive water flux of 3.8-4.8 kg·m-2·h-1. The rapid permeation of water molecules through the membrane benefited from the open pore structure and the water/pore wall interactions, while the transport of hydrated salt ions was explained by the relatively broad pore size distribution of the BTESEthy networks. The promising results presented in this study show the potential of ethenylene-bridged organosilica membranes in desalination applications. AUTHOR INFORMATION


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Corresponding Author * E-mail: [email protected]

* E-mail: [email protected] ACKOWNLEDGEMENT This research is

supported

by National Natural Science Foundation of

China

(21406018 and 21276029), Natural Science Foundation of the Jiangsu Higher Education Institutions of China (15KJB530001), Natural Science Foundation of Jiangsu Province (BK20131142), Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology (BM2012110), A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, Changzhou University. REFERENCES (1)

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For Table of Contents Only



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Figure 1. Experimental setup for evaluation of membrane performance in water desalination. 76x50mm (600 x 600 DPI)


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Figure 2. (a) Size distribution of the BTESEthy sol determined by DLS, and (b) Non-volatile residue changes of BTESEthy-derived sol at 125 °C as a function of reaction time. 58x20mm (600 x 600 DPI)



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Figure 3. (a) FTIR spectra of BTESEthy films fired at 25 °C, and (b) TG curves of BTESEthy gel powder measured in air and N2 atmospheres with a heating rate of 10 °C/min. 60x21mm (600 x 600 DPI)


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Figure 4. Cross-sectional SEM image of a BTESEthy membrane. 83x48mm (300 x 300 DPI)



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Figure 5. (a) Nitrogen (77 K) isotherms and pore size distribution (PSD, inset) of BTESE and BTESEthy samples plotted as a function of relative pressure, and (b) Water (298K) isotherms of the BTESEthy and BTESE samples, plotted as a function of relative pressure. 126x194mm (600 x 600 DPI)


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Figure 6. Gas permeation properties of a BTESEthy membrane as a function of molecular size at a permeation temperature of 200 °C. 87x99mm (600 x 600 DPI)



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Figure 7. (a) Water flux and salt rejection of the BTESEthy membrane as a function of the operation temperature, and (b) the pervaporation performance of the membrane as a function of operating time at an elevated temperature of 70 °C, during desalination of a 2000 ppm NaCl solution. 119x171mm (600 x 600 DPI)


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Figure 8. Influence of NaCl concentration on separation performances of the BTESEthy membrane at 25 °C. 59x42mm (600 x 600 DPI)



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Figure 9. Schematic images of water transport through an ethenylene-bridged organosilica membrane. 79x74mm (600 x 600 DPI)


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Figure 10. Trade-off of pervaporation desalination performances for PVA, zeolite, carbonized silica and organosilica membranes (35000-ppm NaCl feed, and 25 °C unless otherwise specified). 70x59mm (600 x 600 DPI)


Development of Ethenylene-Bridged Organosilica Membranes for

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