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Y-codoped microporous organosilica membranes for high-performance pervaporation desalination
Journal of Membrane Science 584 (2019) 353–363
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Fabrication of La/Y-codoped microporous organosilica membranes for highperformance pervaporation desalination
T
Hua-Yu Zhang, Jiu-Li Wen, Qi Shao, Ai Yuan, Hai-Ting Ren, Fang-Ying Luo, Xiao-Liang Zhang∗ Institute of Advanced Materials, College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Organosilica membrane Desalination Rare earth doping Stability
La/Y-codoped microporous organosilica membranes were successfully fabricated by sol-gel procedure with 1,2bis(triethoxysilyl)ethane (BTESE) as silica precursor for efficient desalination. The rare earth elements were confirmed to be doped into silica frameworks during the sol-gel procedure by various characterizations, which would decrease particles size and size distribution of BTESE-derived sols and form hydrothermally stable Si–O–RE (RE for La and Y) bonds. It could significantly influence membrane surface morphology, microstructure properties and hydrophilcity of La/Y-codoped membranes, and then improve their desalination performance. Under the optimized preparation conditions, La25Y75-SiO2 (La:Y = 25:75, mol%) membranes exhibited the highest water flux of 10.3 kg m−2 h−1 and NaCl rejection of nearly 100% towards 3.5 wt% NaCl feed solution even at room temperature of 25 °C. It was attributed to strongest hydrophilcity, finest microstructure, and lowest mass resistance of water permeation for La25Y75-SiO2 membranes. Moreover, such these organosilica membranes demonstrated good stability for 200 h simulated seawater desalination tests without any significant changes of separation performance and promising hydrothermal durability under various salts solutions with different concentrations at 25–60 °C.
1. Introduction The global increasing demand of freshwater resources has gained considerable attentions over the past decades due to population growth and economic development [1–5]. The freshwater that humans can actually utilize is only about 0.2% of the total amount of water on the earth, which are from some rivers, lakes and groundwater. Thus, it is imperative to increase freshwater production from other water resources such as seawater (97.5% of the earth's total amount), wastewater and reuse-water, which may alleviate the current global water crisis [1–9]. Membrane-based technology for seawater desalination including reverse osmosis (RO), nanofiltration, electrodialysis, membrane distillation and pervaporation (PV), has been rapidly developed to dominate the market mainly due to its reduction in the energy consumption and low production cost [1–9]. Compared with highpressure driven processes like RO, PV has attracted increasing interests in recent years as a feasible potential desalination technique. It can efficiently reject salts to get freshwater from various containing salinity feed solution such as brackish water (0.1–3.5 wt% salinity), seawater (3.5–4.2 wt% salinity) and brine water (> 5 wt% salinity). It demonstrated main advantages of excellent salt reject even as high as 99.99% and high capability to cope with high-salinity solutions without
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increasing cost [2,4,6,7,9,10]. Polymers, inorganic materials and their hybrids were applied as PV membranes by many investigators, which exhibited adequate permeation flux and rejection [4,10]. Particularly, amorphous silica-based membranes have attracted significant attentions since late 2000s, which showed high salt rejections, reasonable flux and long-term running hydrostability towards all feed concentrations even for high-salinity brine water [4,8–10]. Microporous amorphous silica membranes have a pore size of about 0.3–0.5 nm and have been applied for gas separation, pervaporation and desalination processes [4,8–11]. Smaller molecules like water (0.26 nm) can easily permeate throughout the three-dimensional silica matrices channels, while larger hydrated ions such as Na+ and Cl− are firmly rejected with molecular sieving mechanism (hydrated ion size of Cl−, 0.66 nm, and Na+, 0.72 nm, respectively) [4,9,10]. However, pure silica membranes are lack of steady structural stability against water, particularly at high temperatures. The inherent nature of pure silica matrices produced from silane precursors (e.g. tetraethylorthosilicate, TEOS, Si(OEt)4) comprising siloxanes (Si–O–Si) and silanols (Si–OH) limits their application in water separation process [5,8–11]. As a consequence, it is of great urgency to improve the hydrothermal stability of amorphous silica networks for practical applications.
Corresponding author. E-mail addresses: [email protected], [email protected] (X.-L. Zhang).
https://doi.org/10.1016/j.memsci.2019.05.023 Received 3 November 2018; Received in revised form 6 May 2019; Accepted 8 May 2019 Available online 10 May 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.
Journal of Membrane Science 584 (2019) 353–363
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2. Experimental
Recently, two strategies have been developed to enhance hydrothermal stability of silica-based membranes for practical separation applications. One method is fabrication of hybrid silica membranes instead of above-mentioned pure silica membranes, which control the silica network by different organosilicon precursors with the “organic template” and “spacer” methods [8–10,12–19]. As reported by Duke et al. [12], carbonised templates were embedded into the silica microporous microstructure during the sol-gel process, which could block the movement of silica group and thus significantly enhance silica hydrostability in water steam atmosphere. Meanwhile, Castricum et al. [15] firstly developed organosilica hybrid membranes using bridged bis (silyl) precursors such as 1,2-bis(triethoxysilyl)ethane (BTESE, (EtO)3Si (CH2)2Si(EtO)3), i.e., partly replacing of Si–O–Si linkages with Si–R–Si (R for organic groups) linkages in the silica matrix, which might expand silica network resulting in relative looser structure (pore size distribution of 0.5–0.7 nm) and exhibited much stronger hydrothermal stability [8–10,15–19]. BTESE-derived silica network can be considered as being consist of Si–C–C–Si and Si–O–Si segments, whereas pure silica matrix has Si–O–Si segments [15]. Many meaningful works were performed on the BTESE-derived silica membranes in Tsuru's group, which were widely applied in gas separation, RO, pervaporation and catalytic membrane reactors [8,9,16,17]. Metal-doping offers another approach to fabricate silica membranes with enhanced hydrothermal stability [20–30]. Metal elements incorporate into the silica network, which result in the formation of Si–O–M (M for metal) bond bridges or the stable membrane architecture with metal oxides (MO) or metal particles dispersed into silica separation layers. Consequently, metal-doped silica membranes show good resistance to humid atmosphere or water steam. Elma et al. [20] prepared Co-doped TEOS-derived membrane with superior stability and desalination performance. The tricobalt tetroxide phase was well dispersed in the silica matrix, which formed a percolative channel for separating water from hydrated salt ions. The high flux of 11.3 kg m−2 h−1 and NaCl rejection over 99.7% were achieved, respectively, for the 3.5 wt% seawater feed solutions at 60 °C. For BTESEderived organosilica membranes, the transition metal doping of Pd [21–23], Nb [24–27], Ta [27], Zr [28–30] into silica networks had been claimed to date to develop hydrothermally stable membranes with improved gas permeability only for gas separations. Nevertheless, there are few works on the improvement of metal-doped BTESE-derived organosilica membranes for desalination. Recently the addition of rare earth (RE) element of Y into TEOSderived silica membranes was clearly demonstrated significantly hydrothermal stability and higher gas selectivity even exposure in steam [31]. As comparison with transition metals, Y element could stabilize network structure and tune the pore size with Si due to its distinct outstanding properties and the formation of hydrothermally stable Si–O–Y bonds, which could resistance against hydrolysis and nanoparticles aggregation [31,32]. Similarly, Si–O–La linkages were also formed in the TEOS-derived silica matrix, which was expected to exhibit good hydrothermal properties [33,34]. However, it is still much less explored for the RE-doping into organosilica matrix (e.g. BTESEderived SiO2 network) to better understand the influence of RE elements on the physical chemistry properties of organosilica sols and gels, membrane surface morphology, microstructure properties, and even desalination performance of RE-doped organosilica membranes. In this work, we develop a lanthanum/yttrium-codoped organosilica membrane prepared by sol-gel procedure with BTESE as silica precursor for efficient desalination. The effects of La and Y incorporating into silica separation layers are systematically investigated with various characterizations and correlated to their desalination performance. The membrane with optimized La/Y ratio shows good durability and promising hydrothermal stability during the desalination tests with different NaCl concentrations (0.3–7.5 wt%) feed solutions within the temperature ranges of 25–60 °C.
2.1. Materials The 1,2-bis(triethoxysilyl)ethane precursor (96.0%), La (NO3)3·6H2O (99.0%), Y(NO3)3·6H2O (99.5%), ethanol (≥99.7%), and NaCl (99.5%) were purchased from Aladdin, China. Nitric acid (65.0–68.0%) was obtained from Xilong Huagong Company, China. All chemicals were of analytical grade without further purification. Deionized water was home-made from a Millipore system. The commercial α-Al2O3 tubular ceramic substrates (100-mm length, 12 mm of outside diameter, 30% of porosity, average pore size of 100 nm) were obtained from Jiexi Lishun Technology Co. Ltd., China. 2.2. Preparation of La/Y-codoped organosilica sols and membranes La/Y-codoped organosilica sols were synthesized by sol-gel method with the molar ratio of BTESE/La(NO3)3·6H2O/Y(NO3)3·6H2O/HNO3/ H2O = 1/x/(0.075–x)/0.66/60 (x = 0–0.075). During the sol-gel process of hydrolysis and polymerization reaction, the equivalent weight of BTESE was kept at 5 wt% by controlling the amount of ethanol added into BTESE-derived sols. Firstly, a specific amount of BTESE precursor was mixed with ethanol, La(NO3)3·6H2O and Y(NO3)3·6H2O, following stirred for 12 h at room temperature. Then, the mixtures of deionized water and nitric acid diluted with ethanol were added into the above solutions by slowly adding drop-wise and vigorously stirred for 12 h. Thus, the stable La/Y-codoped BTESE-derived organosilica sols noted LamYn-SiO2 sols (m + n = 100) could be obtained. For comparison, pure BTESE-SiO2 sol was also prepared with the same process without adding lanthanum nitrate and yttrium nitrate. For the following characterization measurements, the organosilica gel powders, i.e., pure BTESE-SiO2 and LamYn-SiO2 derived gels, were also prepared with all the aforementioned organosilica sols by drying at 60 °C and then calcinated at 350 °C for 1 h in air atmosphere. Before membrane fabrication, the α-Al2O3 tubular ceramic substrates were modified to form a γ-Al2O3 interlayer with a boehmite sol via sol-gel method as described details in our previous works [35,36]. Then, La/Y-codoped organosilica membranes were prepared on the modified γ-Al2O3/α-Al2O3 substrates with dip-coating procedure, which was described elsewhere details [15]. After dip-coating with as-synthesized La/Y-codoped BTESE-derived organosilica sols for 15–20 s, the tubular membrane was dried at room temperature for 3 h and then calcined at 350 °C, to maintain Si-C-C-Si structure [15–17], for 1 h with heating and cooling rate of 0.5 °C min−1. The procedure was repeated several times and thus microporous silica membrane with uniform surface layers were formed, which marked as LamYn-SiO2 membrane (m + n = 100). Similarly, pure BTESE-SiO2 membrane was also prepared with the same dip-coating procedure using pure BTESE-SiO2 sol. 2.3. Desalination performance tests The pervaporation tests of membrane desalination performance towards NaCl/water solutions with different concentrations were carried out with a home-made thermostatic batch system. The inside of tubular membrane was evacuated by a vacuum pump and the permeated water vapor was collected by a cold trap cooled with liquid nitrogen. The downstream pressure was kept under 60 Pa. The effective area of organosilica membrane was about 24 cm2. The permeated flux was measured by collecting the condensed permeate. Based on the desalination experimental data, the membrane performance could be characterized in terms of permeation flux (J, kg m−2 h−1) and rejection ratio (Rej%) as the following equations:
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J = m / At
(1)
Rej% = (1 − Cp/ Cf ) × 100%
(2)
Journal of Membrane Science 584 (2019) 353–363
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where m is the mass (kg) of permeate collected over a certain time (t, h), A is the effective membrane area (m2), and Cp and Cf represent the mass concentrations of rejection salt in the permeate side and in the feed solution, respectively. The NaCl concentration was measured using a DDS-370A digital conductivity meter (Shanghai INESA Scientific Instrument Co., Ltd, China). 2.4. Characterization Colloid size distributions of the as-synthesized organosilica sols were characterized by dynamic light scattering (DLS) using a Malvern Zetasizer Nano-ZS instrument at 25 °C. The chemical composition and structure of organosilica gel powders were measured by Fourier transform infrared spectroscopy (FTIR, Spectrum One, Perkin Elmer) with KBr tablet method, X-ray diffraction (XRD, Rigaku Ultima IV, Cu Kα source, 40 kV, 20 mA) and X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi, mono-chromatic Al Kα source, 1486.8 eV). Nitrogen adsorption/desorption isotherm measurements of these powders were examined at 77 K using a Micromeritics ASAP2020HD88 system with the Brunauer-Emmett-Teller gas optometry method. All the samples were degassed under vacuum at 120 °C for 12 h before adsorption experiments. Pore size distribution was obtained from Horvath-Kawazoe model. The surface wettability of the organosilica membranes were measured on a contact angle system (OCA 15EC, Dataphysics Instruments GmbH, Germany) with captive bubble method [37]. These membranes were soaked in deionized water for 3 h before measurement. The morphology and elemental analysis of the organosilica gel powders and membranes calcinated at 350 °C were observed by field-emission scanning electron microscope (SEM, SU8020, Hitachi) with energy dispersive X-ray spectroscopy (EDX) for elemental mapping, and high-resolution transmission electron microscopy (HRTEM, JEM-2100, JEOL).
Fig. 2. FTIR spectra of the organosilica gel powders.
codoped-SiO2 sols. It also indicated that rare earth nitrate would promote organosilane hydrolysis-polymerization reaction and regulate the sol particles size during the BTESE-derived sols preparation processes. Adding one mole of rare earth nitrate (i.e., La(NO3)3·6H2O or Y (NO3)3·6H2O) as the sol precursors would enter 6 mol extra water into the as-synthesized sols and then the sols become diluted. Therefore, the diluted solution would prevent sols agglomeration and particle size growth, which was similar to the observation from Khanmohammadi et al. [32]. Moreover, the pH in the diluted solution would inevitably decrease by nitrate addition due to the nitrate solution acidity. Consequently, low branched silica clusters with microporous structure were inevitable formed at lower pH [32]. In this work, RE elements incorporate into the silica framework of BTESE-derived sols to decrease particles size and size distribution even to probably influence the microstructure properties of La/Y-codoped gels and their corresponding membranes. The characterization analysis of FTIR, XRD and XPS were further carried out to verify the structure properties of these organosilica gel powders calcined at 350 °C. As shown in Fig. 2, all samples represented the typical characteristic peaks of BTESE-SiO2 gels. Peaks centered at 1040 and 940 cm−1 could be ascribed to the stretching vibrations of siloxane (Si–O–Si) and silanol (Si–OH) bonds, respectively, while those obviously broad bands at around 3450 and 1640 cm−1 were could be assigned to the stretching vibrations of hydroxyl (O–H) groups and adsorbed water [25,31,32]. In addition, the characteristic peaks of –CH2– bonds from the bridging ethane groups of BTESE precursors emerged in the regions of 2855–2920 cm−1 (C–H stretching vibrations) and 1280–1420 cm−1 (C–H bending vibrations), and at approximately 702 cm−1 (Si–C stretching vibrations), which demonstrated the persistence of –CH2–CH2– groups in the organosilica network membranes after the calcination treatment at 350 °C [17,38,39]. Compared with those of pure BTESE-SiO2 gels, however, the peaks at 793 cm−1 assigned to Si–O stretching vibrations were red-shifted to ∼781 cm−1 and became stronger for the La/Y codoped-SiO2 gels, probably because of overlapping with the bands of Si–O–La (780 cm−1) [34] and Si–O–Y (751 cm−1) [32] vibrations formed in the La/Y codoped organosilica network by the condensation-polymerization reaction. To get more insights into the condensation degree of silanol group, the absorption peaks in the wavenumber region of 1200−850 cm−1 from Fig. 2 were deconvoluted six Gaussian-type peaks to identify the overlapping bands of siloxane and silanol bonds. As shown in Fig. S1 and Fig. 3a, the absorption band less than 940 cm−1 for peaks 5 and 6 (centered wavenumber around 935 and 895 cm−1, respectively)
3. Results and discussion 3.1. Characterization of organosilica sols and gels Fig. 1 shows the particle size distribution of the as-synthesized organosilica sols. Pure BTESE-SiO2 sol displayed a wide particle size distribution in the range of 3–21.0 nm, which was consistent with the literature values of about 1–30.0 nm [15]. Meanwhile, as seen in Fig. 1, the La/Y codoped-SiO2 sols had a narrower and more uniform particle size distribution around 2–10.0 nm. Moreover, the mean colloid sizes decreased from about 9.0 nm for BTESE-SiO2 sol to 4.2 nm of La100Y0, 3.1 nm of La25Y75, even to the smallest 2.7 nm of La0Y100 for La/Y
Fig. 1. Particle size distribution of the organosilica sols. 355
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Fig. 3. (a) FTIR spectrum and peak deconvolution of pure BTESE-SiO2 and La25Y75SiO2 gels. The bold black curves are raw data, and the red curves are the summation of the corresponding deconvolved peaks (peak #1–6 presented in the figures); (b) FTIR frequency shift of peak 3 and the FTIR peak area ratios of Si–OH/Si–O–Si for these organosilica gels. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
represented the Si–OH bending vibration. Meanwhile, peaks centered at around 1150, 1090, 1040 and 1000 cm−1, corresponding to the peaks 1∼4, respectively, were ascribed to Si–O–Si stretching modes, which was consistent with earlier reports [40,41]. Usually, the FTIR peak area ratios of Si–OH (uncondensed silicon species) to Si–O–Si (condensed silica species) groups was used as an indicator for analyzing the condensation degree of silanol group for the organosilica membranes as addressed in the literature [7,40–42]. Compared with that of pure BTESE-SiO2 gel, the peaks intensity for silanol groups (peaks 5 and 6) of La/Y codoped-SiO2 gels was reduced while the absorbance of Si–O–Si groups (peaks 1–4) was apparently increased (Fig. S1 and Fig. 3a). Consequently, the ratios of Si–OH/Si–O–Si decreased from 0.17 of pure BTESE-SiO2 gel to approximately 0.13 of La/Y codoped-SiO2 gels (Fig. 3b). It also indicated that incorporating La/Y rare-earth elements enabled an increase for the condensation degree of Si–OH group to form siloxane (Si–O–Si) bonds and thus a raise of the concentration of Si–O–Si groups of these membranes. Moreover, it was very interestingly observed that the peak wavenumbers of Si–OH vibration decreased (red-shift) while those of Si–O–Si vibrational mode increased (blueshift) with La/Y-codoped into BTESE-derived silica network. For example, La100Y0-SiO2 sample showed a Si–O–Si vibrational mode of peak 3 centered at about 1051 cm−1, as shown in Fig. 3b, which was obviously higher than that of pure BTESE-SiO2 gel (1039 cm−1). The Si–O–Si vibration is very sensitive to chemical environment, thus it can be used as dactylogram of dopant entering the silica framework. So, we also confirmed that the rare earth intercalated into silica matrices from the blue-shifted bending vibration of Si–O–Si and the red-shifted vibration of Si–OH, which would obviously influence the density and microstructure of organosilica network even to separation performance of these BTESE-derived membranes [40]. Fig. 4 shows the XRD patterns of these organosilica gel powders. All samples exhibited only a dispersive broad peak (25°–27°) without sharp diffraction peaks in the 2θ = 5°–50° ranges, indicating the presence of fully amorphous features of SiO2 network [32]. And no characteristic peaks indicative of crystalline phases of La2O3, Y2O3 and their related hydroxides (2θ = 27.2°, 29.2°, 30.0°, 39.5°, 42.0°, 46.0°, etc., from JCPDS cards 05–0602, 22–0369, 36–1481, 65–3178, 21–1422) [33,43] were observed in these samples. It could be ascribed to the formation of
Fig. 4. XRD patterns of the organosilica gel powders.
Si–O–RE (RE for La and Y) bonds, and very lower doped loading of rare earth elements ((La + Y)/Si = 3.75%, mol%, in the sols) with good dispersion in the organosilica matrices (also see Fig. 9 and Fig. 10) calcinated at relative lower temperature of 350 °C. XPS analysis was conducted to further quantitatively understand surface chemical structure (i.e. membrane surface composition along with the interactions RE elements with organosilica frameworks) of these samples. As shown in Fig. 5a of XPS survey spectra, the characteristic magnification peaks of lanthanum (La3d) and yttrium (Y3d) were presented in the rare earth-doped powders even to their lower contents. The La 3d5/2 and Y 3d5/2 peaks were observed at binding energy (BE) of 836.0 and 158.7 eV (Fig. S2), respectively, which was obviously higher than those of pure metallic lanthanum (BE 835.9 eV), La2O3 (BE 835.1 eV), pure metallic yttrium (BE 156.0 eV) and Y2O3 (BE 156.8 eV), respectively [44]. The chemical shifts of La and Y core levels also demonstrated the evidence of an interaction among rare earth, oxygen and silicon species, which probably produced hydrothermally 356
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Fig. 5. XPS survey (a) and deconvolutions of O1s (b) and Si2p (c) spectra for the organosilica powders. Raw data (black line), background (black dash dot line), the fitting envelope (red line), and deconvolved peaks (blue lines) are presented in the figures (b) and (c). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
stable Si–O–RE bonds. As illustrated in Fig. 5b and Table S1, each O1s spectrum was fitted with three peaks representing the main bonding environments with soft of XPSPEAK 4.1: Si–O–Si (532.3–532.8 eV), Si–O–Y (531.8–531.9 eV), and Si–O–La (531.1–531.4 eV) [33,34,44]. These peaks of Si–O–Y were located between the SiO2 (532.8 eV) and Y2O3 peaks (529.5 eV), respectively. Similarly, those of Si–O–La were located between the SiO2 and La2O3 (529.8 eV), respectively [33,34,44]. It was consistent with the formation of Si–O–RE bonding architecture in the silica matrix as the aforementioned results of FTIR and XRD [33,39]. Moreover, compared with that of pure BTESE-SiO2 sample, the binding energy of Si–O–Si peaks of La/Y codoped-SiO2
powders decreased from 532.8 eV to 532.3 eV with increasing La/Y percentage concentration, which agreed with results described elsewhere [33]. Similarly, the binding energies of Si–O–Y and Si–O–La peaks were also shifted depending on La/Y concentration (Table S1). Fig. 5c shows the deconvolutions of Si2p XPS spectra for the organosilica samples. It can be seen that the Si2p peaks of La/Y codopedSiO2 samples are slightly shifted by around 0.3 eV to lower binding energy values compared with that of pure BTESE-SiO2 sample. These shifts were associated with a change for three different silicon chemical environments of BTESE-derived silica framework: SiO4 at 103.5 eV, XSiO3 at 102.6 eV, and (X)2SiO2 at 101.8 eV (X = C, H, etc.), 357
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Fig. 6. Water permeated flux (a), rejection (b) and Arrhenius plot of temperature dependent water permeance (c) for the organosilica membranes towards 3.5 wt% NaCl feed solution at 25–60 °C, and water contact angles (d) of the corresponding membranes.
Fig. 7. N2 adsorption-desorption isotherms (a, close symbols for adsorption, open symbols for desorption) and pore size distribution (b) of the organosilica powders.
3.2. Influence of La/Y ratio on membrane desalination performance
respectively [38,39,45]. For pure BTESE-SiO2 sample, as displayed in Fig. 5c and Table S2, the contents of inorganic moieties (SiO4 and XSiO3) and organic moiety ((X)2SiO2) were 70.5% and 29.5%, respectively. However, the proportion of inorganic silicon environments increased to 80.9% (e.g. La100Y0-SiO2 sample) with prejudice of organic ones, when La/Y was incorporated into the organosilica networks. Such deconvolution within the Si2p region matched well with deconvolution within the O1s region confirming that there were the interactions between RE elements and silica architecture, which was consistent with the previous reports [31,43]. It would influence their microstructure even to separation properties for La/Y-codoped membranes.
To investigate the influence of La/Y ratio on the membrane surface microstructure and separation properties, six series of organosilica membranes with different La/Y ratio were prepared under the same conditions and then were tested to evaluate desalination performance towards 3.5 wt% NaCl solution (simulated seawater) at 25–60 °C. As shown in Fig. 6a and b, these membranes exhibited different desalination performance as a function of feed temperature. Water permeated flux of all these membranes increased with increasing feed temperatures, while their NaCl rejection slightly decreased from nearly 100% to the lowest of 99.1%, which might be related to their membrane surface 358
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Fig. 8. SEM images of La25Y75-SiO2 membrane: top surface (a, b) and cross sections (c, d).
Fig. 9. SEM-EDX mapping images for La25Y75-SiO2 gel powder: SEM image (a), La/Y distribution (b), the La-LA (c) and Y-LA (d) X-ray maps taken in the same areas. The scale bar is 1 μm.
that of BTESE-SiO2 membrane, while the membranes (La25Y75, La50Y50 and La75Y25-SiO2) co-doped with two rare earth elements displayed significantly higher flux than that of BTESE-SiO2 membrane. At 60 °C, the membranes of La25Y75, La50Y50 and La75Y25-SiO2 showed water flux of 15.6, 6.2 and 10.7 kg m−2 h−1, respectively. Especially, La25Y75-SiO2 membrane exhibited the highest desalination performance at the same test conditions. Even at room temperature of 25 °C, the flux over 10.3 kg m−2 h−1 and rejection of nearly 100% were achieved for La25Y75-SiO2 membrane.
microstructure and hydrothermal stability [14]. For pure BTESE-SiO2 membrane, it showed relatively lower flux (∼3.0 kg m−2 h−1) even at higher temperature of 60 °C exhibiting high rejection over 99.9%. However, when rare earth doped into organosilica network, water flux was improved to 3.5–15.6 kg m−2 h−1 maintaining good rejection higher than 99.1%. Thus, the doping of rare earth into organosilica network had a highly positive effect on its desalination performance. It was very interesting that the membranes (La100Y0 and La0Y100-SiO2) doped with one rare earth element had slightly higher water flux than 359
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Fig. 10. TEM (a∼d) and HRTEM images (e, f) of La25Y75-SiO2 gel powder.
membrane pore size and surface hydrophilcity. Water contact angle of the membrane is smaller and then the membrane is more hydrophilic. As a result, this membrane with lower contact angle will preferentially adsorb water molecules to improve water permeated flux. So, water contact angles of these organosilica membranes were measured to evaluate surface hydrophilicity. As shown in Fig. 6d, as compared with pure BTESE-SiO2 membrane, water contact angles decreased from about 36° to approximately 20° for rare earth-dopedSiO2 membranes. La/Y-codoped membranes such as La25Y75-SiO2 membrane (water contact angle of 20.3 ± 0.9°) exhibited superior hydrophilicity than BTESE-SiO2 membrane. Fig. 7 shows the nitrogen adsorption-desorption isotherms and pore size distribution of the organosilica membranes. It can be seen that all the samples display high N2 sorption capacity following type I isotherm, demonstrating their typical microporous characteristics. The values of the BET specific surface area, micropore volume, and average pore size are listed in Table S3. These samples exhibited surface area of 138.6–192.1 m2 g−1 and produced more micropore volume of 0.05–0.07 cm3 g−1. Moreover, the calculated pore size distribution indicated that these samples contained average pore size around 0.58–0.62 nm (Fig. 7b and Table S3), which was in agreement with BTESE-derived membranes previously reported [9,41]. Thus, these pure BTESE and La/Y-codoped-SiO2 membranes can effectively reject Na+ (0.72 nm) and Cl− (0.66 nm) ions during the PV desalination process. It should be noted that La25Y75-SiO2 membrane exhibited the lowest contact angle (strongest hydrophilcity), smallest pore size (finest microstructure) and lowest mass resistance (Ep value) of water transport, which was contributed to the highest flux and rejection among the six organosilica membranes in this work. SEM and TEM analysis were used to further characterize the surface microstructure of La25Y75-SiO2 membrane. As shown in Fig. 8, a smooth and crack-free amorphous silica layer was uniformly deposited on the substrate surface, which was roughly 300 ± 30 nm in thickness. The thinner membrane exhibited higher permeation flux, which was consistent with our previous work [35]. Moreover, EDX mapping images of La and Y elements was measured to investigate the elementary
As shown in Fig. 6c, except for the differences of desalination performance, such these six membranes also showed Arrhenius-type relationships between water permeated flux normalized by the driving force of Δp (i.e. water permeance, P, mol m−2 h−1 Pa−1) and feed temperature (T, K) [46–48]:
J / Δp = P = P0 exp (−Ep /RT )
(3)
Δp = pasat − p p ≈ p sat − p p
(4)
where P0 is the pre-exponential factor, Ep is the activation energy, and R is the gas constant (=8.314 J mol−1 K−1), pasat is the actual water vapor pressure at membrane surface in the H2O-salts feed side, pp is the permeate pressure. Commonly, psat (the saturated vapor pressure of water) is used instead of pasat in the low concentration salt solutions to expediently calculated the driving force [48]. As seen in Fig. 6c, water permeance declined with increasing temperature and the resulting Ep values of these membranes were negative within the range of −24.1∼−34.0 kJ mol−1. Since during the pervaporation process Ep is determined by activation energy of diffusion (Ed) and enthalpy of exothermic sorption (ΔH), i.e. Ep = Ed+ΔH, so the negative ΔH usually dominates over the positive Ed resulting in the negative value of Ep [46,47]. Therefore, the increase of water permeated flux with increasing temperature is mainly owing to the significant influence of temperature on the psat or the increased driving force for water permeation [46,47]. In addition, for a given H2O-salts feed solution, Ep is more negative, Ed is the lower. So, activation energy is a remarkable indicator for mass transfer, revealing energy barrier for water to permeate the membrane layers. For example, La25Y75-SiO2 membrane displayed the most negative Ep value of −34.0 kJ mol−1 (Fig. 6c), which was consistent with reports elsewhere [46], and accordingly Ed value was the lowest for activation diffusion. It also indicates that adsorbed water molecules permeate more rapidly throughout the La25Y75SiO2 network channels than those of other membranes during pervaporation desalination process. Water permeation is dominated by molecular sieving and surface effect for PV desalination membranes [4], which depends on the 360
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distribution in the separation layer for La25Y75-SiO2 membrane (Fig. 9). The distribution of silica micron-size secondary particles was well consistent with those of La and Y elements (Fig. 9 b), which indicated that La (Fig. 9c) and Y (Fig. 9d) elements distributed uniformly in the organosilica matrices or separation layers even with their too low doped contents ((La + Y)/Si = 3.2%, mol%, see Fig. S3). Furthermore, TEM images (Fig. 10a~d) revealed that unordered La/Y particles without crystalline were continuously doped into the amorphous silica powders. Combined with the EDX mapping analysis, it indicated that the La/Y elements distributed uniformly into the micro-size silica matrices and further established Si–O–RE network through the separation layers. Meanwhile, as observed by the HRTEM images of Fig. 10e~f, the La/Y particles had a uniform nano-size of about 2–4 nm and covalently linked with siloxane to form a compact Si–O–RE network structure typical of silica matrix [21,31], which was further confirmed with the results as revealed by SEM and TEM images. Based on the results and discussion above, it demonstrated that nano-sized La/Y particles were distributed uniformly into the amorphous silica matrices and hydrothermally stable Si–O–RE bonds were formed in the silica network architectures instead of randomly dispersed in the homogenous organosilica layer, which was contributed to construct La/Y-codoped organosilica membranes with high desalination performance.
Fig. 12. The desalination performance of La25Y75-SiO2 membrane towards NaCl solutions with different concentrations at 25 °C.
NaCl), seawater (4.2 wt% NaCl) and brine water (7.5 wt% NaCl), respectively, whereas holding excellent salt rejections nearly 100% at 25 °C. It demonstrated that such this membrane with high desalination performance can be used to efficiently treat various saline waters within broad ranges of different salt concentrations even at room temperature of 25 °C. The long-term durability of silica membranes is one of the most important aspects as well as higher water permeation flux and rejection for practical desalination application. Fig. 13 shows the desalination performance of La25Y75-SiO2 membrane versus operating time towards simulated seawater of 3.5 wt% NaCl solution at room temperature of 25 °C. As shown in Fig. 13, the membrane illustrated high separation properties with virtually no change in flux and rejection performance after 200 h durability operation. The average values of permeation flux and rejection were 10.1 ± 0.6 kg m−2 h−1 and 99.97 ± 0.05% for prolonged periods of 200 h, respectively, which was approximately the same as the highest desalination performance of the La25Y75-SiO2 membrane described in section 3.2. It also indicated that the La25Y75SiO2 membrane was not destroyed after long-term exposure of seawater atmosphere in this work. Moreover, it could undergo tens of temperatures cycles and salts-exchanging (including NaCl, MgCl2, Na2SO4, MgSO4, etc.) cycles with different saline waters as feed solutions at 25–60 °C during the PV desalination tests (data not shown in the figure), as well as good hydrothermal stability for this membrane. It had such good durability for this La25Y75-SiO2 membrane, which might be attributed to the finer and more uniform amorphous silica network architectures constructed by hydrothermally stable Si–O–RE bonds
3.3. Desalination performance and stability of La25Y75-SiO2 membranes To further investigate the desalination performance of La/Y-codoped SiO2 membranes, La25Y75-SiO2 membrane prepared with the same optimized parameters as above-mentioned was used to separate NaCl solution with different concentrations at different feed temperatures. As displayed in Fig. 11, the permeation flux decreased as the NaCl concentration increased in the feed solutions, meanwhile keeping high rejection over 99.9% at the same test temperatures (also see Fig. 12). For example, the flux decreased from 13.4 kg m−2 h−1 for pure water feed to 10.3 kg m−2 h−1 for 3.5 wt% NaCl solutions at 25 °C, while the rejection ratio as high as nearly 100% was obtained. Especially, the membrane still exhibited high permeation flux of 7.9 kg m−2 h−1 and NaCl rejection as nearly 100% even towards 7.5 wt% NaCl feed solutions at 25 °C (Fig. 12), which was obviously superior to the desalination performance of other silica membranes under the same test conditions [13,20]. It also demonstrated such La25Y75-SiO2 membrane can be employed for potential applications in highsalinity wastewater treatment. Moreover, it can be seen from Fig. 11 that the permeated flux increased over ca. 1.5 times without reducing rejection performance (> 99.9%) with the increasing pervaporation temperature by 35 °C. In Fig. 12, La25Y75-SiO2 membrane also exhibited considerable high fluxes of 13.1, 9.7 and 7.9 kg m−2 h−1 for brackish (0.3 wt%
Fig. 11. The desalination performance of La25Y75-SiO2 membrane towards pure water and NaCl solutions at different feed temperatures.
Fig. 13. Stability of La25Y75-SiO2 membrane for 3.5 wt% NaCl solution at 25 °C. 361
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Acknowledgment
Table 1 Comparison of PV desalination performance for silica-based membranes. Membranes
C (NaCl wt %)
T (oC)
J (kg m−2 h−1)
Rej%
Ref
TEOS-SiO2 MTES-SiO2 Ni-doped TEOSSiO2 Co-doped TEOSSiO2 Carbonised-SiO2 ES40-SiO2 ES40-SiO2 La25Y75-doped BTESE-SiO2
0.3 0.3–3.5 0.3–3.5
22 25 25
9.5 4.7–2.5 7.0–2.5
99.6 93.0–83.0 99.9–97.0
[6] [12] [49]
0.3–7.5
60
20–7.7
> 99.7
[20]
3.5 3.5 3.5 3.5
25 25 60 25
9.5 2.8 17.8 10.3
99.5 99.0 99.0 > 99.9
[13] [14] [7] this work
This work was supported by the National Natural Science Foundation of China (grant Nos. 21566012, 21766011), Jiangxi Provincial Department of Science and Technology, China (grant Nos. 20171BAB203020, 20162BCB23025, 20151BDH80012), and in part by the National Undergraduate Training Programs for Innovation and Entrepreneurship (grant No. 201810414017). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.memsci.2019.05.023. References
Note: TEOS, tetraethylorthosilicate; MTES, methyl-triethoxy-silane; ES40, ethyl silicate 40; BTESE, 1,2-bis(triethoxysilyl)ethane.
[1] J.R. Werber, C.O. Osuji, M. Elimelech, Materials for next-generation desalination and water purification membranes, Nat. Rev. Mater. 1 (2016) 16018 https://doi. org/10.1038/natrevmats.2016.18. [2] G.Z. Liu, J. Shen, Q. Liu, G.P. Liu, J. Xiong, J. Yang, W.Q. Jin, Ultrathin two-dimensional MXene membrane for pervaporation desalination, J. Membr. Sci. 548 (2018) 548–558. [3] Z. Yang, X.H. Ma, C.Y. Tang, Recent development of novel membranes for desalination, Desalination 434 (2018) 37–59. [4] Q.Z. Wang, N. Li, B. Bolto, M. Hoang, Z.L. Xie, Desalination by pervaporation: a review, Desalination 387 (2016) 46–60. [5] A. Farsi, C. Malvache, O.D. Bartolis, G. Magnacca, P.K. Kristensen, M.L. Christensen, V. Boffa, Design and fabrication of silica-based nanofiltration membranes for water desalination and detoxification, Microporous Mesoporous Mater. 237 (2017) 117–126. [6] M. Elma, C. Yacou, J.C. Diniz da Costa, D.K. Wang, Performance and long term stability of mesoporous silica membranes for desalination, Membranes 3 (2013) 136–150. [7] S. Wang, D.K. Wang, J. Motuzas, S. Smart, J.C. Diniz da Costa, Rapid thermal treatment of interlayer-free ethyl silicate 40 derived membranes for desalination, J. Membr. Sci. 516 (2016) 94–103. [8] R. Xu, J. Wang, M. Kanezashi, T. Yoshioka, T. Tsuru, Development of robust organosilica membranes for reverse osmosis, Langmuir 27 (2011) 13996–13999. [9] R. Xu, P. Lin, Q. Zhang, J. Zhong, T. Tsuru, Development of ethenylene-bridged organosilica membranes for desalination applications, Ind. Eng. Chem. Res. 55 (2016) 2183–2190. [10] I. Agirre, P.L. Arias, H.L. Castricum, M. Creatore, J.E. ten Elshof, G.G. Paradis, P.H.T. Ngamou, H.M. van Veen, J.F. Vente, Hybrid organosilica membranes and processes: status and outlook, Sep. Purif. Technol. 121 (2014) 2–12. [11] Q. Wei, Y.L. Wang, Z.R. Nie, C.X. Yu, Q.Y. Li, J.X. Zou, C.J. Li, Facile synthesis of hydrophobic microporous silica membranes and their resistance to humid atmosphere, Microporous Mesoporous Mater. 111 (2008) 97–103. [12] M.C. Duke, S. Mee, J.C. Diniz da Costa, Performance of porous inorganic membranes in non-osmotic desalination, Water Res. 41 (2007) 3998–4004. [13] H. Yang, M. Elma, D.K. Wang, J. Motuzas, J.C. Diniz da Costa, Interlayer-free hybrid carbon-silica membranes for processing brackish to brine salt solutions by pervaporation, J. Membr. Sci. 523 (2017) 197–204. [14] S. Wang, D.K. Wang, S. Smart, J.C. Diniz da Costa, Improved stability of ethyl silicate interlayer-free membranes by the rapid thermal processing (RTP) for desalination, Desalination 402 (2017) 25–32. [15] H.L. Castricum, R. Kreiter, H.M. van Veen, D.H.A. Blank, J.F. Vente, J.E. ten Elshof, High-performance hybrid pervaporation membranes with superior hydrothermal and acid stability, J. Membr. Sci. 324 (2008) 111–118. [16] X. Yu, L. Meng, T. Niimi, H. Nagasawa, M. Kanezashi, T. Yoshioka, T. Tsuru, Network engineering of a BTESE membrane for improved gas performance via a novel pH-swing method, J. Membr. Sci. 511 (2016) 219–227. [17] N. Moriyama, H. Nagasawa, M. Kanezashi, K. Ito, T. Tsuru, Bis(triethoxysilyl) ethane (BTESE)-derived silica membranes: pore formation mechanism and gas permeation properties, J. Sol. Gel Sci. Technol. 86 (2018) 63–72. [18] X.C. Gao, G.Z. Ji, J.C. Wang, L. Peng, X.H. Gu, L. Chen, Critical pore dimensions for gases in a BTESE-derived organic-inorganic hybrid silica: a theoretical analysis, Sep. Purif. Technol. 191 (2018) 27–37. [19] A.P. Dral, J.E. ten Elshof, Organic groups influencing microporosity in organosilicas, Microporous Mesoporous Mater. 267 (2018) 267–273. [20] M. Elma, D.K. Wang, C. Yacou, J. Motuzas, J.C. Diniz da Costa, High performance interlayer-free mesoporous cobalt oxide silica membranes for desalination applications, Desalination 365 (2015) 308–315. [21] M. Kanezashi, M. Sano, T. Yoshioka, T. Tsuru, Extremely thin Pd-silica mixed-matrix membranes with nano-dispersion for improved hydrogen permeability, Chem. Commun. 46 (2010) 6171–6173. [22] H. Song, S. Zhao, J. Lei, C. Wang, H. Qi, Pd-doped organosilica membrane with enhanced gas permeability and hydrothermal stability for gas separation, J. Mater. Sci. 51 (2016) 6275–6286. [23] J.J. Lei, H.T. Song, Y.B. Wei, S.F. Zhao, H. Qi, A novel strategy to enhance hydrothermal stability of Pd-doped organosilica membrane for hydrogen separation, Microporous Mesoporous Mater. 253 (2017) 55–63.
during the sol-gel procedures in this work. Table 1 summarizes desalination performance for silica-based membranes reported in the literature and this work [6,7,12–14,2049]. As can be seen in Table 1, La25Y75-SiO2 membrane in this work has exhibited relatively higher permeation flux and rejection as compared to other silica membranes even under different test conditions. It may be caused by the much thinner membrane thickness of about 300 nm in this work compared to the micrometer-grade thickness of other membranes reported in the references. However, it is worth noting that it is difficult to compare directly the desalination performance such as flux and rejection even though these membranes have similar thickness in Table 1. It is a complex process of water transport though silica membranes for PV desalination, which is not only dependent on the membrane thickness, but also on the preparation technique and membrane microstructure properties. In addition, the substrate resistance and experimental conditions such as feed temperatures and seawater concentrations also take some effects. Furthermore, although the La25Y75SiO2 membrane in this work has exhibited relatively higher desalination performance and good stability towards 3.5 wt% NaCl solutions at 25 °C, further work is still required to verify high desalination efficiency for the microporous organosilica membranes with enhanced stability, which are potentially used for real seawater desalination applications.
4. Conclusions La/Y-codoped microporous organosilica membranes with enhanced performance and good stability for efficiently desalination were fabricated by the sol-gel method. The rare earth elements doped into silica frameworks significantly improved the water permeation flux and rejection properties for desalination. It might be attributed to more uniform silica network architectures instead of randomly dispersed in the homogenous organosilica separation layer, which were constructed by hydrothermally stable Si–O–RE bonds during the sol-gel procedures. The La25Y75-SiO2 membrane exhibited the highest permeation flux and rejection nearly 100%, which was due to its strongest hydrophilcity, finest microstructure and lowest mass resistance for water transport. Water molecules permeated rapidly throughout such the ordered organosilica-network channels. Moreover, this membrane kept stability for relatively long-term running of 200 h without significant changes for NaCl rejection ratio towards 3.5 wt% NaCl feed solutions, and sustained good hydrothermal durability under salts solutions with different concentrations over the temperature range of 25–60 °C. The development of such La/Y-codoped organosilica membrane with high desalination performance and good stability could open up the promising potential applications in seawater desalination and high-salinity wastewater treatment.
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[24] H. Qi, J. Han, N.P. Xu, H.J.M. Bouwmeester, Hybrid organic–inorganic microporous membranes with high hydrothermal stability for the separation of carbon dioxide, ChemSusChem 3 (2010) 1375–1378. [25] H. Qi, J. Han, N.P. Xu, Effect of calcination temperature on carbon dioxide separation properties of a novel microporous hybrid silica membrane, J. Membr. Sci. 382 (2011) 231–237. [26] H. Qi, H.R. Chen, L. Li, G.Z. Zhu, N.P. Xu, Effect of Nb content on hydrothermal stability of a novel ethylene-bridged silsesquioxane molecular sieving membrane for H2/CO2 separation, J. Membr. Sci. 421–422 (2012) 190–200. [27] H.F. Qureshi, R. Besselink, J.E. ten Elshof, A. Nijmeijer, L. Winnubst, Doped microporous hybrid silica membranes for gas separation, J. Sol. Gel Sci. Technol. 75 (2015) 180–188. [28] M. ten Hove, A. Nijmeijer, L. Winnubst, Facile synthesis of zirconia doped hybrid organic inorganic silica membranes, Sep. Purif. Technol. 147 (2015) 372–378. [29] H.T. Song, S.F. Zhao, J.W. Chen, H. Qi, Hydrothermally stable Zr-doped organosilica membranes for H2/CO2 separation, Microporous Mesoporous Mater. 224 (2016) 277–284. [30] M. ten Hove, M.W.J. Luiten-Olieman, C. Huiskes, A. Nijmeijer, L. Winnubst, Hydrothermal stability of silica, hybrid silica and Zr-doped hybrid silica membranes, Sep. Purif. Technol. 189 (2017) 48–53. [31] M.H. Zahir, T. Nagano, High-selectivity Y2O3-doped SiO2 nanocomposite membranes for gas separation in steam at high temperatures, J. Am. Ceram. Soc. 99 (2016) 452–460. [32] S. Khanmohammadi, E. Taheri-Nassaj, Micro-porous silica-yttria membrane by solgel method: preparation and characterization, Ceram. Int. 40 (2014) 9403–9411. [33] B. Ballinger, J. Motuzas, C.R. Miller, S. Smart, J.C. Diniz da Costa, Nanoscale assembly of lanthanum silica with dense and porous interfacial structures, Sci. Rep. 5 (2015) 8210, https://doi.org/10.1038/srep08210. [34] T. Gougousi, M.J. Kelly, D.B. Terry, G.N. Parsons, Properties of La-silicate high-K dielectric films formed by oxidation of La on silicon, J. Appl. Phys. 93 (2003) 1691–1696. [35] X.L. Zhang, H. Yamada, T. Saito, T. Kai, K. Murakami, M. Nakashima, J. Ohshita, K. Akamatsu, S.-I. Nakao, Development of hydrogen-selective triphenyl methoxysilane-derived silica membranes with tailored pore size by chemical vapor deposition, J. Membr. Sci. 499 (2016) 28–35. [36] X.L. Zhang, K. Akamatsu, S.-I. Nakao, Hydrogen separation in hydrogen-methyl cyclohexane-toluene gaseous mixtures through triphenylmethoxysilane-derived silica membranes prepared by chemical vapor deposition, Ind. Eng. Chem. Res. 55 (2016) 5395–5402. [37] Y. Baek, J. Kang, P. Theato, J. Yoon, Measuring hydrophilicity of RO membranes by
[38]
[39]
[40]
[41]
[42] [43]
[44] [45]
[46] [47]
[48]
[49]
363
contact angles via sessile drop and captive bubble method: a comparative study, Desalination 303 (2012) 23–28. P.H.T. Ngamou, J.P. Overbeek, R. Kreiter, H.M. van Veen, J.F. Vente, I.M. Wienk, P.F. Cuperus, M. Creatore, Plasma-deposited hybrid silica membranes with a controlled retention of organic bridges, J. Mater. Chem. A 1 (2013) 5567–5576. H. Song, Y. Wei, H. Qi, Tailoring pore structures to improve the permselectivity of organosilica membranes by tuning calcination parameters, J. Mater. Chem. A 5 (2017) 24657–24666. P.H.T. Ngamou, M.E. Ivanova, C. Herwartz, N. Lühmann, A. Besmehn, W.A. Meulenberg, J. Mayer, O. Guillon, Tailoring the structure and gas permeation properties of silica membranes via binary metal oxides doping, RSC Adv. 5 (2015) 82717–82725. G.H. Gong, H. Nagasawa, M. Kanezashi, T. Tsuru, Tailoring the separation behavior of polymer-supported organosilica layered-hybrid membranes via facile posttreatment using HCl and HN3 vapors, ACS Appl. Mater. Interfaces 8 (2016) 11060–11069. E. Paparazzo, M. Fanfoni, E. Severini, S. Priori, Evidence of Si-OH species at the surface of aged silica, J. Vac. Sci. Technol. A 10 (1992) 2892–2896. B. Ballinger, J. Motuzas, S. Smart, J.C. Diniz da Costa, Gas permeation redox effect on binary lanthanum cobalt silica membranes with enhanced silicate formation, J. Membr. Sci. 489 (2015) 220–226. J.J. Chambers, G.N. Parsons, Physical and electrical characterization of ultrathin yttrium silicate insulators on silicon, J. Appl. Phys. 90 (2001) 918–933. S. Roualdes, R. Berjoan, J. Durand, 29Si NMR and Si2p XPS correlation in polysiloxane membranes prepared by plasma enhanced chemical vapor deposition, Sep. Purif. Technol. 25 (2001) 391–397. D.H. Wu, A.R. Gao, H.T. Zhao, X.S. Feng, Pervaporative desalination of high-salinity water, Chem. Eng. Res. Des. 136 (2018) 154–164. K.C. Guan, Q. Liu, G.Y. Zhou, G.Z. Liu, Y.F. Ji, G.P. Liu, W.Q. Jin, Cation-diffusion controlled formation of thin graphene oxide composite membranes for efficient ethanol dehydration, Sci. China Mater. 62 (2019) 925–935, https://doi.org/10. 1007/s40843-018-9401-1. Z.S. Cao, S.X. Zeng, Z. Xu, A. Arvanitis, S.W. Yang, X.H. Gu, J.H. Dong, Ultrathin ZSM-5 zeolite nanosheet laminated membrane for high-flux desalination of concentrated brines, Sci. Adv. 4 (2018) eaau8634,https://doi.org/10.1126/sciadv. aau8634. A. Darmawan, L. Karlina, Y. Astuti, Sriatun, J. Motuzas, D.K. Wang, J.C. Diniz da Costa, Structural evolution of nickel oxide silica sol-gel for the preparation of interlayer-free membranes, J. Non-Cryst. Solids 447 (2016) 9–15.
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Fabrication of La/Y-codoped microporous organosilica membranes for highperformance pervaporation desalination
T
Hua-Yu Zhang, Jiu-Li Wen, Qi Shao, Ai Yuan, Hai-Ting Ren, Fang-Ying Luo, Xiao-Liang Zhang∗ Institute of Advanced Materials, College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Organosilica membrane Desalination Rare earth doping Stability
La/Y-codoped microporous organosilica membranes were successfully fabricated by sol-gel procedure with 1,2bis(triethoxysilyl)ethane (BTESE) as silica precursor for efficient desalination. The rare earth elements were confirmed to be doped into silica frameworks during the sol-gel procedure by various characterizations, which would decrease particles size and size distribution of BTESE-derived sols and form hydrothermally stable Si–O–RE (RE for La and Y) bonds. It could significantly influence membrane surface morphology, microstructure properties and hydrophilcity of La/Y-codoped membranes, and then improve their desalination performance. Under the optimized preparation conditions, La25Y75-SiO2 (La:Y = 25:75, mol%) membranes exhibited the highest water flux of 10.3 kg m−2 h−1 and NaCl rejection of nearly 100% towards 3.5 wt% NaCl feed solution even at room temperature of 25 °C. It was attributed to strongest hydrophilcity, finest microstructure, and lowest mass resistance of water permeation for La25Y75-SiO2 membranes. Moreover, such these organosilica membranes demonstrated good stability for 200 h simulated seawater desalination tests without any significant changes of separation performance and promising hydrothermal durability under various salts solutions with different concentrations at 25–60 °C.
1. Introduction The global increasing demand of freshwater resources has gained considerable attentions over the past decades due to population growth and economic development [1–5]. The freshwater that humans can actually utilize is only about 0.2% of the total amount of water on the earth, which are from some rivers, lakes and groundwater. Thus, it is imperative to increase freshwater production from other water resources such as seawater (97.5% of the earth's total amount), wastewater and reuse-water, which may alleviate the current global water crisis [1–9]. Membrane-based technology for seawater desalination including reverse osmosis (RO), nanofiltration, electrodialysis, membrane distillation and pervaporation (PV), has been rapidly developed to dominate the market mainly due to its reduction in the energy consumption and low production cost [1–9]. Compared with highpressure driven processes like RO, PV has attracted increasing interests in recent years as a feasible potential desalination technique. It can efficiently reject salts to get freshwater from various containing salinity feed solution such as brackish water (0.1–3.5 wt% salinity), seawater (3.5–4.2 wt% salinity) and brine water (> 5 wt% salinity). It demonstrated main advantages of excellent salt reject even as high as 99.99% and high capability to cope with high-salinity solutions without
∗
increasing cost [2,4,6,7,9,10]. Polymers, inorganic materials and their hybrids were applied as PV membranes by many investigators, which exhibited adequate permeation flux and rejection [4,10]. Particularly, amorphous silica-based membranes have attracted significant attentions since late 2000s, which showed high salt rejections, reasonable flux and long-term running hydrostability towards all feed concentrations even for high-salinity brine water [4,8–10]. Microporous amorphous silica membranes have a pore size of about 0.3–0.5 nm and have been applied for gas separation, pervaporation and desalination processes [4,8–11]. Smaller molecules like water (0.26 nm) can easily permeate throughout the three-dimensional silica matrices channels, while larger hydrated ions such as Na+ and Cl− are firmly rejected with molecular sieving mechanism (hydrated ion size of Cl−, 0.66 nm, and Na+, 0.72 nm, respectively) [4,9,10]. However, pure silica membranes are lack of steady structural stability against water, particularly at high temperatures. The inherent nature of pure silica matrices produced from silane precursors (e.g. tetraethylorthosilicate, TEOS, Si(OEt)4) comprising siloxanes (Si–O–Si) and silanols (Si–OH) limits their application in water separation process [5,8–11]. As a consequence, it is of great urgency to improve the hydrothermal stability of amorphous silica networks for practical applications.
Corresponding author. E-mail addresses: [email protected], [email protected] (X.-L. Zhang).
https://doi.org/10.1016/j.memsci.2019.05.023 Received 3 November 2018; Received in revised form 6 May 2019; Accepted 8 May 2019 Available online 10 May 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.
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2. Experimental
Recently, two strategies have been developed to enhance hydrothermal stability of silica-based membranes for practical separation applications. One method is fabrication of hybrid silica membranes instead of above-mentioned pure silica membranes, which control the silica network by different organosilicon precursors with the “organic template” and “spacer” methods [8–10,12–19]. As reported by Duke et al. [12], carbonised templates were embedded into the silica microporous microstructure during the sol-gel process, which could block the movement of silica group and thus significantly enhance silica hydrostability in water steam atmosphere. Meanwhile, Castricum et al. [15] firstly developed organosilica hybrid membranes using bridged bis (silyl) precursors such as 1,2-bis(triethoxysilyl)ethane (BTESE, (EtO)3Si (CH2)2Si(EtO)3), i.e., partly replacing of Si–O–Si linkages with Si–R–Si (R for organic groups) linkages in the silica matrix, which might expand silica network resulting in relative looser structure (pore size distribution of 0.5–0.7 nm) and exhibited much stronger hydrothermal stability [8–10,15–19]. BTESE-derived silica network can be considered as being consist of Si–C–C–Si and Si–O–Si segments, whereas pure silica matrix has Si–O–Si segments [15]. Many meaningful works were performed on the BTESE-derived silica membranes in Tsuru's group, which were widely applied in gas separation, RO, pervaporation and catalytic membrane reactors [8,9,16,17]. Metal-doping offers another approach to fabricate silica membranes with enhanced hydrothermal stability [20–30]. Metal elements incorporate into the silica network, which result in the formation of Si–O–M (M for metal) bond bridges or the stable membrane architecture with metal oxides (MO) or metal particles dispersed into silica separation layers. Consequently, metal-doped silica membranes show good resistance to humid atmosphere or water steam. Elma et al. [20] prepared Co-doped TEOS-derived membrane with superior stability and desalination performance. The tricobalt tetroxide phase was well dispersed in the silica matrix, which formed a percolative channel for separating water from hydrated salt ions. The high flux of 11.3 kg m−2 h−1 and NaCl rejection over 99.7% were achieved, respectively, for the 3.5 wt% seawater feed solutions at 60 °C. For BTESEderived organosilica membranes, the transition metal doping of Pd [21–23], Nb [24–27], Ta [27], Zr [28–30] into silica networks had been claimed to date to develop hydrothermally stable membranes with improved gas permeability only for gas separations. Nevertheless, there are few works on the improvement of metal-doped BTESE-derived organosilica membranes for desalination. Recently the addition of rare earth (RE) element of Y into TEOSderived silica membranes was clearly demonstrated significantly hydrothermal stability and higher gas selectivity even exposure in steam [31]. As comparison with transition metals, Y element could stabilize network structure and tune the pore size with Si due to its distinct outstanding properties and the formation of hydrothermally stable Si–O–Y bonds, which could resistance against hydrolysis and nanoparticles aggregation [31,32]. Similarly, Si–O–La linkages were also formed in the TEOS-derived silica matrix, which was expected to exhibit good hydrothermal properties [33,34]. However, it is still much less explored for the RE-doping into organosilica matrix (e.g. BTESEderived SiO2 network) to better understand the influence of RE elements on the physical chemistry properties of organosilica sols and gels, membrane surface morphology, microstructure properties, and even desalination performance of RE-doped organosilica membranes. In this work, we develop a lanthanum/yttrium-codoped organosilica membrane prepared by sol-gel procedure with BTESE as silica precursor for efficient desalination. The effects of La and Y incorporating into silica separation layers are systematically investigated with various characterizations and correlated to their desalination performance. The membrane with optimized La/Y ratio shows good durability and promising hydrothermal stability during the desalination tests with different NaCl concentrations (0.3–7.5 wt%) feed solutions within the temperature ranges of 25–60 °C.
2.1. Materials The 1,2-bis(triethoxysilyl)ethane precursor (96.0%), La (NO3)3·6H2O (99.0%), Y(NO3)3·6H2O (99.5%), ethanol (≥99.7%), and NaCl (99.5%) were purchased from Aladdin, China. Nitric acid (65.0–68.0%) was obtained from Xilong Huagong Company, China. All chemicals were of analytical grade without further purification. Deionized water was home-made from a Millipore system. The commercial α-Al2O3 tubular ceramic substrates (100-mm length, 12 mm of outside diameter, 30% of porosity, average pore size of 100 nm) were obtained from Jiexi Lishun Technology Co. Ltd., China. 2.2. Preparation of La/Y-codoped organosilica sols and membranes La/Y-codoped organosilica sols were synthesized by sol-gel method with the molar ratio of BTESE/La(NO3)3·6H2O/Y(NO3)3·6H2O/HNO3/ H2O = 1/x/(0.075–x)/0.66/60 (x = 0–0.075). During the sol-gel process of hydrolysis and polymerization reaction, the equivalent weight of BTESE was kept at 5 wt% by controlling the amount of ethanol added into BTESE-derived sols. Firstly, a specific amount of BTESE precursor was mixed with ethanol, La(NO3)3·6H2O and Y(NO3)3·6H2O, following stirred for 12 h at room temperature. Then, the mixtures of deionized water and nitric acid diluted with ethanol were added into the above solutions by slowly adding drop-wise and vigorously stirred for 12 h. Thus, the stable La/Y-codoped BTESE-derived organosilica sols noted LamYn-SiO2 sols (m + n = 100) could be obtained. For comparison, pure BTESE-SiO2 sol was also prepared with the same process without adding lanthanum nitrate and yttrium nitrate. For the following characterization measurements, the organosilica gel powders, i.e., pure BTESE-SiO2 and LamYn-SiO2 derived gels, were also prepared with all the aforementioned organosilica sols by drying at 60 °C and then calcinated at 350 °C for 1 h in air atmosphere. Before membrane fabrication, the α-Al2O3 tubular ceramic substrates were modified to form a γ-Al2O3 interlayer with a boehmite sol via sol-gel method as described details in our previous works [35,36]. Then, La/Y-codoped organosilica membranes were prepared on the modified γ-Al2O3/α-Al2O3 substrates with dip-coating procedure, which was described elsewhere details [15]. After dip-coating with as-synthesized La/Y-codoped BTESE-derived organosilica sols for 15–20 s, the tubular membrane was dried at room temperature for 3 h and then calcined at 350 °C, to maintain Si-C-C-Si structure [15–17], for 1 h with heating and cooling rate of 0.5 °C min−1. The procedure was repeated several times and thus microporous silica membrane with uniform surface layers were formed, which marked as LamYn-SiO2 membrane (m + n = 100). Similarly, pure BTESE-SiO2 membrane was also prepared with the same dip-coating procedure using pure BTESE-SiO2 sol. 2.3. Desalination performance tests The pervaporation tests of membrane desalination performance towards NaCl/water solutions with different concentrations were carried out with a home-made thermostatic batch system. The inside of tubular membrane was evacuated by a vacuum pump and the permeated water vapor was collected by a cold trap cooled with liquid nitrogen. The downstream pressure was kept under 60 Pa. The effective area of organosilica membrane was about 24 cm2. The permeated flux was measured by collecting the condensed permeate. Based on the desalination experimental data, the membrane performance could be characterized in terms of permeation flux (J, kg m−2 h−1) and rejection ratio (Rej%) as the following equations:
354
J = m / At
(1)
Rej% = (1 − Cp/ Cf ) × 100%
(2)
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where m is the mass (kg) of permeate collected over a certain time (t, h), A is the effective membrane area (m2), and Cp and Cf represent the mass concentrations of rejection salt in the permeate side and in the feed solution, respectively. The NaCl concentration was measured using a DDS-370A digital conductivity meter (Shanghai INESA Scientific Instrument Co., Ltd, China). 2.4. Characterization Colloid size distributions of the as-synthesized organosilica sols were characterized by dynamic light scattering (DLS) using a Malvern Zetasizer Nano-ZS instrument at 25 °C. The chemical composition and structure of organosilica gel powders were measured by Fourier transform infrared spectroscopy (FTIR, Spectrum One, Perkin Elmer) with KBr tablet method, X-ray diffraction (XRD, Rigaku Ultima IV, Cu Kα source, 40 kV, 20 mA) and X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi, mono-chromatic Al Kα source, 1486.8 eV). Nitrogen adsorption/desorption isotherm measurements of these powders were examined at 77 K using a Micromeritics ASAP2020HD88 system with the Brunauer-Emmett-Teller gas optometry method. All the samples were degassed under vacuum at 120 °C for 12 h before adsorption experiments. Pore size distribution was obtained from Horvath-Kawazoe model. The surface wettability of the organosilica membranes were measured on a contact angle system (OCA 15EC, Dataphysics Instruments GmbH, Germany) with captive bubble method [37]. These membranes were soaked in deionized water for 3 h before measurement. The morphology and elemental analysis of the organosilica gel powders and membranes calcinated at 350 °C were observed by field-emission scanning electron microscope (SEM, SU8020, Hitachi) with energy dispersive X-ray spectroscopy (EDX) for elemental mapping, and high-resolution transmission electron microscopy (HRTEM, JEM-2100, JEOL).
Fig. 2. FTIR spectra of the organosilica gel powders.
codoped-SiO2 sols. It also indicated that rare earth nitrate would promote organosilane hydrolysis-polymerization reaction and regulate the sol particles size during the BTESE-derived sols preparation processes. Adding one mole of rare earth nitrate (i.e., La(NO3)3·6H2O or Y (NO3)3·6H2O) as the sol precursors would enter 6 mol extra water into the as-synthesized sols and then the sols become diluted. Therefore, the diluted solution would prevent sols agglomeration and particle size growth, which was similar to the observation from Khanmohammadi et al. [32]. Moreover, the pH in the diluted solution would inevitably decrease by nitrate addition due to the nitrate solution acidity. Consequently, low branched silica clusters with microporous structure were inevitable formed at lower pH [32]. In this work, RE elements incorporate into the silica framework of BTESE-derived sols to decrease particles size and size distribution even to probably influence the microstructure properties of La/Y-codoped gels and their corresponding membranes. The characterization analysis of FTIR, XRD and XPS were further carried out to verify the structure properties of these organosilica gel powders calcined at 350 °C. As shown in Fig. 2, all samples represented the typical characteristic peaks of BTESE-SiO2 gels. Peaks centered at 1040 and 940 cm−1 could be ascribed to the stretching vibrations of siloxane (Si–O–Si) and silanol (Si–OH) bonds, respectively, while those obviously broad bands at around 3450 and 1640 cm−1 were could be assigned to the stretching vibrations of hydroxyl (O–H) groups and adsorbed water [25,31,32]. In addition, the characteristic peaks of –CH2– bonds from the bridging ethane groups of BTESE precursors emerged in the regions of 2855–2920 cm−1 (C–H stretching vibrations) and 1280–1420 cm−1 (C–H bending vibrations), and at approximately 702 cm−1 (Si–C stretching vibrations), which demonstrated the persistence of –CH2–CH2– groups in the organosilica network membranes after the calcination treatment at 350 °C [17,38,39]. Compared with those of pure BTESE-SiO2 gels, however, the peaks at 793 cm−1 assigned to Si–O stretching vibrations were red-shifted to ∼781 cm−1 and became stronger for the La/Y codoped-SiO2 gels, probably because of overlapping with the bands of Si–O–La (780 cm−1) [34] and Si–O–Y (751 cm−1) [32] vibrations formed in the La/Y codoped organosilica network by the condensation-polymerization reaction. To get more insights into the condensation degree of silanol group, the absorption peaks in the wavenumber region of 1200−850 cm−1 from Fig. 2 were deconvoluted six Gaussian-type peaks to identify the overlapping bands of siloxane and silanol bonds. As shown in Fig. S1 and Fig. 3a, the absorption band less than 940 cm−1 for peaks 5 and 6 (centered wavenumber around 935 and 895 cm−1, respectively)
3. Results and discussion 3.1. Characterization of organosilica sols and gels Fig. 1 shows the particle size distribution of the as-synthesized organosilica sols. Pure BTESE-SiO2 sol displayed a wide particle size distribution in the range of 3–21.0 nm, which was consistent with the literature values of about 1–30.0 nm [15]. Meanwhile, as seen in Fig. 1, the La/Y codoped-SiO2 sols had a narrower and more uniform particle size distribution around 2–10.0 nm. Moreover, the mean colloid sizes decreased from about 9.0 nm for BTESE-SiO2 sol to 4.2 nm of La100Y0, 3.1 nm of La25Y75, even to the smallest 2.7 nm of La0Y100 for La/Y
Fig. 1. Particle size distribution of the organosilica sols. 355
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Fig. 3. (a) FTIR spectrum and peak deconvolution of pure BTESE-SiO2 and La25Y75SiO2 gels. The bold black curves are raw data, and the red curves are the summation of the corresponding deconvolved peaks (peak #1–6 presented in the figures); (b) FTIR frequency shift of peak 3 and the FTIR peak area ratios of Si–OH/Si–O–Si for these organosilica gels. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
represented the Si–OH bending vibration. Meanwhile, peaks centered at around 1150, 1090, 1040 and 1000 cm−1, corresponding to the peaks 1∼4, respectively, were ascribed to Si–O–Si stretching modes, which was consistent with earlier reports [40,41]. Usually, the FTIR peak area ratios of Si–OH (uncondensed silicon species) to Si–O–Si (condensed silica species) groups was used as an indicator for analyzing the condensation degree of silanol group for the organosilica membranes as addressed in the literature [7,40–42]. Compared with that of pure BTESE-SiO2 gel, the peaks intensity for silanol groups (peaks 5 and 6) of La/Y codoped-SiO2 gels was reduced while the absorbance of Si–O–Si groups (peaks 1–4) was apparently increased (Fig. S1 and Fig. 3a). Consequently, the ratios of Si–OH/Si–O–Si decreased from 0.17 of pure BTESE-SiO2 gel to approximately 0.13 of La/Y codoped-SiO2 gels (Fig. 3b). It also indicated that incorporating La/Y rare-earth elements enabled an increase for the condensation degree of Si–OH group to form siloxane (Si–O–Si) bonds and thus a raise of the concentration of Si–O–Si groups of these membranes. Moreover, it was very interestingly observed that the peak wavenumbers of Si–OH vibration decreased (red-shift) while those of Si–O–Si vibrational mode increased (blueshift) with La/Y-codoped into BTESE-derived silica network. For example, La100Y0-SiO2 sample showed a Si–O–Si vibrational mode of peak 3 centered at about 1051 cm−1, as shown in Fig. 3b, which was obviously higher than that of pure BTESE-SiO2 gel (1039 cm−1). The Si–O–Si vibration is very sensitive to chemical environment, thus it can be used as dactylogram of dopant entering the silica framework. So, we also confirmed that the rare earth intercalated into silica matrices from the blue-shifted bending vibration of Si–O–Si and the red-shifted vibration of Si–OH, which would obviously influence the density and microstructure of organosilica network even to separation performance of these BTESE-derived membranes [40]. Fig. 4 shows the XRD patterns of these organosilica gel powders. All samples exhibited only a dispersive broad peak (25°–27°) without sharp diffraction peaks in the 2θ = 5°–50° ranges, indicating the presence of fully amorphous features of SiO2 network [32]. And no characteristic peaks indicative of crystalline phases of La2O3, Y2O3 and their related hydroxides (2θ = 27.2°, 29.2°, 30.0°, 39.5°, 42.0°, 46.0°, etc., from JCPDS cards 05–0602, 22–0369, 36–1481, 65–3178, 21–1422) [33,43] were observed in these samples. It could be ascribed to the formation of
Fig. 4. XRD patterns of the organosilica gel powders.
Si–O–RE (RE for La and Y) bonds, and very lower doped loading of rare earth elements ((La + Y)/Si = 3.75%, mol%, in the sols) with good dispersion in the organosilica matrices (also see Fig. 9 and Fig. 10) calcinated at relative lower temperature of 350 °C. XPS analysis was conducted to further quantitatively understand surface chemical structure (i.e. membrane surface composition along with the interactions RE elements with organosilica frameworks) of these samples. As shown in Fig. 5a of XPS survey spectra, the characteristic magnification peaks of lanthanum (La3d) and yttrium (Y3d) were presented in the rare earth-doped powders even to their lower contents. The La 3d5/2 and Y 3d5/2 peaks were observed at binding energy (BE) of 836.0 and 158.7 eV (Fig. S2), respectively, which was obviously higher than those of pure metallic lanthanum (BE 835.9 eV), La2O3 (BE 835.1 eV), pure metallic yttrium (BE 156.0 eV) and Y2O3 (BE 156.8 eV), respectively [44]. The chemical shifts of La and Y core levels also demonstrated the evidence of an interaction among rare earth, oxygen and silicon species, which probably produced hydrothermally 356
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Fig. 5. XPS survey (a) and deconvolutions of O1s (b) and Si2p (c) spectra for the organosilica powders. Raw data (black line), background (black dash dot line), the fitting envelope (red line), and deconvolved peaks (blue lines) are presented in the figures (b) and (c). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
stable Si–O–RE bonds. As illustrated in Fig. 5b and Table S1, each O1s spectrum was fitted with three peaks representing the main bonding environments with soft of XPSPEAK 4.1: Si–O–Si (532.3–532.8 eV), Si–O–Y (531.8–531.9 eV), and Si–O–La (531.1–531.4 eV) [33,34,44]. These peaks of Si–O–Y were located between the SiO2 (532.8 eV) and Y2O3 peaks (529.5 eV), respectively. Similarly, those of Si–O–La were located between the SiO2 and La2O3 (529.8 eV), respectively [33,34,44]. It was consistent with the formation of Si–O–RE bonding architecture in the silica matrix as the aforementioned results of FTIR and XRD [33,39]. Moreover, compared with that of pure BTESE-SiO2 sample, the binding energy of Si–O–Si peaks of La/Y codoped-SiO2
powders decreased from 532.8 eV to 532.3 eV with increasing La/Y percentage concentration, which agreed with results described elsewhere [33]. Similarly, the binding energies of Si–O–Y and Si–O–La peaks were also shifted depending on La/Y concentration (Table S1). Fig. 5c shows the deconvolutions of Si2p XPS spectra for the organosilica samples. It can be seen that the Si2p peaks of La/Y codopedSiO2 samples are slightly shifted by around 0.3 eV to lower binding energy values compared with that of pure BTESE-SiO2 sample. These shifts were associated with a change for three different silicon chemical environments of BTESE-derived silica framework: SiO4 at 103.5 eV, XSiO3 at 102.6 eV, and (X)2SiO2 at 101.8 eV (X = C, H, etc.), 357
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Fig. 6. Water permeated flux (a), rejection (b) and Arrhenius plot of temperature dependent water permeance (c) for the organosilica membranes towards 3.5 wt% NaCl feed solution at 25–60 °C, and water contact angles (d) of the corresponding membranes.
Fig. 7. N2 adsorption-desorption isotherms (a, close symbols for adsorption, open symbols for desorption) and pore size distribution (b) of the organosilica powders.
3.2. Influence of La/Y ratio on membrane desalination performance
respectively [38,39,45]. For pure BTESE-SiO2 sample, as displayed in Fig. 5c and Table S2, the contents of inorganic moieties (SiO4 and XSiO3) and organic moiety ((X)2SiO2) were 70.5% and 29.5%, respectively. However, the proportion of inorganic silicon environments increased to 80.9% (e.g. La100Y0-SiO2 sample) with prejudice of organic ones, when La/Y was incorporated into the organosilica networks. Such deconvolution within the Si2p region matched well with deconvolution within the O1s region confirming that there were the interactions between RE elements and silica architecture, which was consistent with the previous reports [31,43]. It would influence their microstructure even to separation properties for La/Y-codoped membranes.
To investigate the influence of La/Y ratio on the membrane surface microstructure and separation properties, six series of organosilica membranes with different La/Y ratio were prepared under the same conditions and then were tested to evaluate desalination performance towards 3.5 wt% NaCl solution (simulated seawater) at 25–60 °C. As shown in Fig. 6a and b, these membranes exhibited different desalination performance as a function of feed temperature. Water permeated flux of all these membranes increased with increasing feed temperatures, while their NaCl rejection slightly decreased from nearly 100% to the lowest of 99.1%, which might be related to their membrane surface 358
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Fig. 8. SEM images of La25Y75-SiO2 membrane: top surface (a, b) and cross sections (c, d).
Fig. 9. SEM-EDX mapping images for La25Y75-SiO2 gel powder: SEM image (a), La/Y distribution (b), the La-LA (c) and Y-LA (d) X-ray maps taken in the same areas. The scale bar is 1 μm.
that of BTESE-SiO2 membrane, while the membranes (La25Y75, La50Y50 and La75Y25-SiO2) co-doped with two rare earth elements displayed significantly higher flux than that of BTESE-SiO2 membrane. At 60 °C, the membranes of La25Y75, La50Y50 and La75Y25-SiO2 showed water flux of 15.6, 6.2 and 10.7 kg m−2 h−1, respectively. Especially, La25Y75-SiO2 membrane exhibited the highest desalination performance at the same test conditions. Even at room temperature of 25 °C, the flux over 10.3 kg m−2 h−1 and rejection of nearly 100% were achieved for La25Y75-SiO2 membrane.
microstructure and hydrothermal stability [14]. For pure BTESE-SiO2 membrane, it showed relatively lower flux (∼3.0 kg m−2 h−1) even at higher temperature of 60 °C exhibiting high rejection over 99.9%. However, when rare earth doped into organosilica network, water flux was improved to 3.5–15.6 kg m−2 h−1 maintaining good rejection higher than 99.1%. Thus, the doping of rare earth into organosilica network had a highly positive effect on its desalination performance. It was very interesting that the membranes (La100Y0 and La0Y100-SiO2) doped with one rare earth element had slightly higher water flux than 359
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Fig. 10. TEM (a∼d) and HRTEM images (e, f) of La25Y75-SiO2 gel powder.
membrane pore size and surface hydrophilcity. Water contact angle of the membrane is smaller and then the membrane is more hydrophilic. As a result, this membrane with lower contact angle will preferentially adsorb water molecules to improve water permeated flux. So, water contact angles of these organosilica membranes were measured to evaluate surface hydrophilicity. As shown in Fig. 6d, as compared with pure BTESE-SiO2 membrane, water contact angles decreased from about 36° to approximately 20° for rare earth-dopedSiO2 membranes. La/Y-codoped membranes such as La25Y75-SiO2 membrane (water contact angle of 20.3 ± 0.9°) exhibited superior hydrophilicity than BTESE-SiO2 membrane. Fig. 7 shows the nitrogen adsorption-desorption isotherms and pore size distribution of the organosilica membranes. It can be seen that all the samples display high N2 sorption capacity following type I isotherm, demonstrating their typical microporous characteristics. The values of the BET specific surface area, micropore volume, and average pore size are listed in Table S3. These samples exhibited surface area of 138.6–192.1 m2 g−1 and produced more micropore volume of 0.05–0.07 cm3 g−1. Moreover, the calculated pore size distribution indicated that these samples contained average pore size around 0.58–0.62 nm (Fig. 7b and Table S3), which was in agreement with BTESE-derived membranes previously reported [9,41]. Thus, these pure BTESE and La/Y-codoped-SiO2 membranes can effectively reject Na+ (0.72 nm) and Cl− (0.66 nm) ions during the PV desalination process. It should be noted that La25Y75-SiO2 membrane exhibited the lowest contact angle (strongest hydrophilcity), smallest pore size (finest microstructure) and lowest mass resistance (Ep value) of water transport, which was contributed to the highest flux and rejection among the six organosilica membranes in this work. SEM and TEM analysis were used to further characterize the surface microstructure of La25Y75-SiO2 membrane. As shown in Fig. 8, a smooth and crack-free amorphous silica layer was uniformly deposited on the substrate surface, which was roughly 300 ± 30 nm in thickness. The thinner membrane exhibited higher permeation flux, which was consistent with our previous work [35]. Moreover, EDX mapping images of La and Y elements was measured to investigate the elementary
As shown in Fig. 6c, except for the differences of desalination performance, such these six membranes also showed Arrhenius-type relationships between water permeated flux normalized by the driving force of Δp (i.e. water permeance, P, mol m−2 h−1 Pa−1) and feed temperature (T, K) [46–48]:
J / Δp = P = P0 exp (−Ep /RT )
(3)
Δp = pasat − p p ≈ p sat − p p
(4)
where P0 is the pre-exponential factor, Ep is the activation energy, and R is the gas constant (=8.314 J mol−1 K−1), pasat is the actual water vapor pressure at membrane surface in the H2O-salts feed side, pp is the permeate pressure. Commonly, psat (the saturated vapor pressure of water) is used instead of pasat in the low concentration salt solutions to expediently calculated the driving force [48]. As seen in Fig. 6c, water permeance declined with increasing temperature and the resulting Ep values of these membranes were negative within the range of −24.1∼−34.0 kJ mol−1. Since during the pervaporation process Ep is determined by activation energy of diffusion (Ed) and enthalpy of exothermic sorption (ΔH), i.e. Ep = Ed+ΔH, so the negative ΔH usually dominates over the positive Ed resulting in the negative value of Ep [46,47]. Therefore, the increase of water permeated flux with increasing temperature is mainly owing to the significant influence of temperature on the psat or the increased driving force for water permeation [46,47]. In addition, for a given H2O-salts feed solution, Ep is more negative, Ed is the lower. So, activation energy is a remarkable indicator for mass transfer, revealing energy barrier for water to permeate the membrane layers. For example, La25Y75-SiO2 membrane displayed the most negative Ep value of −34.0 kJ mol−1 (Fig. 6c), which was consistent with reports elsewhere [46], and accordingly Ed value was the lowest for activation diffusion. It also indicates that adsorbed water molecules permeate more rapidly throughout the La25Y75SiO2 network channels than those of other membranes during pervaporation desalination process. Water permeation is dominated by molecular sieving and surface effect for PV desalination membranes [4], which depends on the 360
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distribution in the separation layer for La25Y75-SiO2 membrane (Fig. 9). The distribution of silica micron-size secondary particles was well consistent with those of La and Y elements (Fig. 9 b), which indicated that La (Fig. 9c) and Y (Fig. 9d) elements distributed uniformly in the organosilica matrices or separation layers even with their too low doped contents ((La + Y)/Si = 3.2%, mol%, see Fig. S3). Furthermore, TEM images (Fig. 10a~d) revealed that unordered La/Y particles without crystalline were continuously doped into the amorphous silica powders. Combined with the EDX mapping analysis, it indicated that the La/Y elements distributed uniformly into the micro-size silica matrices and further established Si–O–RE network through the separation layers. Meanwhile, as observed by the HRTEM images of Fig. 10e~f, the La/Y particles had a uniform nano-size of about 2–4 nm and covalently linked with siloxane to form a compact Si–O–RE network structure typical of silica matrix [21,31], which was further confirmed with the results as revealed by SEM and TEM images. Based on the results and discussion above, it demonstrated that nano-sized La/Y particles were distributed uniformly into the amorphous silica matrices and hydrothermally stable Si–O–RE bonds were formed in the silica network architectures instead of randomly dispersed in the homogenous organosilica layer, which was contributed to construct La/Y-codoped organosilica membranes with high desalination performance.
Fig. 12. The desalination performance of La25Y75-SiO2 membrane towards NaCl solutions with different concentrations at 25 °C.
NaCl), seawater (4.2 wt% NaCl) and brine water (7.5 wt% NaCl), respectively, whereas holding excellent salt rejections nearly 100% at 25 °C. It demonstrated that such this membrane with high desalination performance can be used to efficiently treat various saline waters within broad ranges of different salt concentrations even at room temperature of 25 °C. The long-term durability of silica membranes is one of the most important aspects as well as higher water permeation flux and rejection for practical desalination application. Fig. 13 shows the desalination performance of La25Y75-SiO2 membrane versus operating time towards simulated seawater of 3.5 wt% NaCl solution at room temperature of 25 °C. As shown in Fig. 13, the membrane illustrated high separation properties with virtually no change in flux and rejection performance after 200 h durability operation. The average values of permeation flux and rejection were 10.1 ± 0.6 kg m−2 h−1 and 99.97 ± 0.05% for prolonged periods of 200 h, respectively, which was approximately the same as the highest desalination performance of the La25Y75-SiO2 membrane described in section 3.2. It also indicated that the La25Y75SiO2 membrane was not destroyed after long-term exposure of seawater atmosphere in this work. Moreover, it could undergo tens of temperatures cycles and salts-exchanging (including NaCl, MgCl2, Na2SO4, MgSO4, etc.) cycles with different saline waters as feed solutions at 25–60 °C during the PV desalination tests (data not shown in the figure), as well as good hydrothermal stability for this membrane. It had such good durability for this La25Y75-SiO2 membrane, which might be attributed to the finer and more uniform amorphous silica network architectures constructed by hydrothermally stable Si–O–RE bonds
3.3. Desalination performance and stability of La25Y75-SiO2 membranes To further investigate the desalination performance of La/Y-codoped SiO2 membranes, La25Y75-SiO2 membrane prepared with the same optimized parameters as above-mentioned was used to separate NaCl solution with different concentrations at different feed temperatures. As displayed in Fig. 11, the permeation flux decreased as the NaCl concentration increased in the feed solutions, meanwhile keeping high rejection over 99.9% at the same test temperatures (also see Fig. 12). For example, the flux decreased from 13.4 kg m−2 h−1 for pure water feed to 10.3 kg m−2 h−1 for 3.5 wt% NaCl solutions at 25 °C, while the rejection ratio as high as nearly 100% was obtained. Especially, the membrane still exhibited high permeation flux of 7.9 kg m−2 h−1 and NaCl rejection as nearly 100% even towards 7.5 wt% NaCl feed solutions at 25 °C (Fig. 12), which was obviously superior to the desalination performance of other silica membranes under the same test conditions [13,20]. It also demonstrated such La25Y75-SiO2 membrane can be employed for potential applications in highsalinity wastewater treatment. Moreover, it can be seen from Fig. 11 that the permeated flux increased over ca. 1.5 times without reducing rejection performance (> 99.9%) with the increasing pervaporation temperature by 35 °C. In Fig. 12, La25Y75-SiO2 membrane also exhibited considerable high fluxes of 13.1, 9.7 and 7.9 kg m−2 h−1 for brackish (0.3 wt%
Fig. 11. The desalination performance of La25Y75-SiO2 membrane towards pure water and NaCl solutions at different feed temperatures.
Fig. 13. Stability of La25Y75-SiO2 membrane for 3.5 wt% NaCl solution at 25 °C. 361
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Acknowledgment
Table 1 Comparison of PV desalination performance for silica-based membranes. Membranes
C (NaCl wt %)
T (oC)
J (kg m−2 h−1)
Rej%
Ref
TEOS-SiO2 MTES-SiO2 Ni-doped TEOSSiO2 Co-doped TEOSSiO2 Carbonised-SiO2 ES40-SiO2 ES40-SiO2 La25Y75-doped BTESE-SiO2
0.3 0.3–3.5 0.3–3.5
22 25 25
9.5 4.7–2.5 7.0–2.5
99.6 93.0–83.0 99.9–97.0
[6] [12] [49]
0.3–7.5
60
20–7.7
> 99.7
[20]
3.5 3.5 3.5 3.5
25 25 60 25
9.5 2.8 17.8 10.3
99.5 99.0 99.0 > 99.9
[13] [14] [7] this work
This work was supported by the National Natural Science Foundation of China (grant Nos. 21566012, 21766011), Jiangxi Provincial Department of Science and Technology, China (grant Nos. 20171BAB203020, 20162BCB23025, 20151BDH80012), and in part by the National Undergraduate Training Programs for Innovation and Entrepreneurship (grant No. 201810414017). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.memsci.2019.05.023. References
Note: TEOS, tetraethylorthosilicate; MTES, methyl-triethoxy-silane; ES40, ethyl silicate 40; BTESE, 1,2-bis(triethoxysilyl)ethane.
[1] J.R. Werber, C.O. Osuji, M. Elimelech, Materials for next-generation desalination and water purification membranes, Nat. Rev. Mater. 1 (2016) 16018 https://doi. org/10.1038/natrevmats.2016.18. [2] G.Z. Liu, J. Shen, Q. Liu, G.P. Liu, J. Xiong, J. Yang, W.Q. Jin, Ultrathin two-dimensional MXene membrane for pervaporation desalination, J. Membr. Sci. 548 (2018) 548–558. [3] Z. Yang, X.H. Ma, C.Y. Tang, Recent development of novel membranes for desalination, Desalination 434 (2018) 37–59. [4] Q.Z. Wang, N. Li, B. Bolto, M. Hoang, Z.L. Xie, Desalination by pervaporation: a review, Desalination 387 (2016) 46–60. [5] A. Farsi, C. Malvache, O.D. Bartolis, G. Magnacca, P.K. Kristensen, M.L. Christensen, V. Boffa, Design and fabrication of silica-based nanofiltration membranes for water desalination and detoxification, Microporous Mesoporous Mater. 237 (2017) 117–126. [6] M. Elma, C. Yacou, J.C. Diniz da Costa, D.K. Wang, Performance and long term stability of mesoporous silica membranes for desalination, Membranes 3 (2013) 136–150. [7] S. Wang, D.K. Wang, J. Motuzas, S. Smart, J.C. Diniz da Costa, Rapid thermal treatment of interlayer-free ethyl silicate 40 derived membranes for desalination, J. Membr. Sci. 516 (2016) 94–103. [8] R. Xu, J. Wang, M. Kanezashi, T. Yoshioka, T. Tsuru, Development of robust organosilica membranes for reverse osmosis, Langmuir 27 (2011) 13996–13999. [9] R. Xu, P. Lin, Q. Zhang, J. Zhong, T. Tsuru, Development of ethenylene-bridged organosilica membranes for desalination applications, Ind. Eng. Chem. Res. 55 (2016) 2183–2190. [10] I. Agirre, P.L. Arias, H.L. Castricum, M. Creatore, J.E. ten Elshof, G.G. Paradis, P.H.T. Ngamou, H.M. van Veen, J.F. Vente, Hybrid organosilica membranes and processes: status and outlook, Sep. Purif. Technol. 121 (2014) 2–12. [11] Q. Wei, Y.L. Wang, Z.R. Nie, C.X. Yu, Q.Y. Li, J.X. Zou, C.J. Li, Facile synthesis of hydrophobic microporous silica membranes and their resistance to humid atmosphere, Microporous Mesoporous Mater. 111 (2008) 97–103. [12] M.C. Duke, S. Mee, J.C. Diniz da Costa, Performance of porous inorganic membranes in non-osmotic desalination, Water Res. 41 (2007) 3998–4004. [13] H. Yang, M. Elma, D.K. Wang, J. Motuzas, J.C. Diniz da Costa, Interlayer-free hybrid carbon-silica membranes for processing brackish to brine salt solutions by pervaporation, J. Membr. Sci. 523 (2017) 197–204. [14] S. Wang, D.K. Wang, S. Smart, J.C. Diniz da Costa, Improved stability of ethyl silicate interlayer-free membranes by the rapid thermal processing (RTP) for desalination, Desalination 402 (2017) 25–32. [15] H.L. Castricum, R. Kreiter, H.M. van Veen, D.H.A. Blank, J.F. Vente, J.E. ten Elshof, High-performance hybrid pervaporation membranes with superior hydrothermal and acid stability, J. Membr. Sci. 324 (2008) 111–118. [16] X. Yu, L. Meng, T. Niimi, H. Nagasawa, M. Kanezashi, T. Yoshioka, T. Tsuru, Network engineering of a BTESE membrane for improved gas performance via a novel pH-swing method, J. Membr. Sci. 511 (2016) 219–227. [17] N. Moriyama, H. Nagasawa, M. Kanezashi, K. Ito, T. Tsuru, Bis(triethoxysilyl) ethane (BTESE)-derived silica membranes: pore formation mechanism and gas permeation properties, J. Sol. Gel Sci. Technol. 86 (2018) 63–72. [18] X.C. Gao, G.Z. Ji, J.C. Wang, L. Peng, X.H. Gu, L. Chen, Critical pore dimensions for gases in a BTESE-derived organic-inorganic hybrid silica: a theoretical analysis, Sep. Purif. Technol. 191 (2018) 27–37. [19] A.P. Dral, J.E. ten Elshof, Organic groups influencing microporosity in organosilicas, Microporous Mesoporous Mater. 267 (2018) 267–273. [20] M. Elma, D.K. Wang, C. Yacou, J. Motuzas, J.C. Diniz da Costa, High performance interlayer-free mesoporous cobalt oxide silica membranes for desalination applications, Desalination 365 (2015) 308–315. [21] M. Kanezashi, M. Sano, T. Yoshioka, T. Tsuru, Extremely thin Pd-silica mixed-matrix membranes with nano-dispersion for improved hydrogen permeability, Chem. Commun. 46 (2010) 6171–6173. [22] H. Song, S. Zhao, J. Lei, C. Wang, H. Qi, Pd-doped organosilica membrane with enhanced gas permeability and hydrothermal stability for gas separation, J. Mater. Sci. 51 (2016) 6275–6286. [23] J.J. Lei, H.T. Song, Y.B. Wei, S.F. Zhao, H. Qi, A novel strategy to enhance hydrothermal stability of Pd-doped organosilica membrane for hydrogen separation, Microporous Mesoporous Mater. 253 (2017) 55–63.
during the sol-gel procedures in this work. Table 1 summarizes desalination performance for silica-based membranes reported in the literature and this work [6,7,12–14,2049]. As can be seen in Table 1, La25Y75-SiO2 membrane in this work has exhibited relatively higher permeation flux and rejection as compared to other silica membranes even under different test conditions. It may be caused by the much thinner membrane thickness of about 300 nm in this work compared to the micrometer-grade thickness of other membranes reported in the references. However, it is worth noting that it is difficult to compare directly the desalination performance such as flux and rejection even though these membranes have similar thickness in Table 1. It is a complex process of water transport though silica membranes for PV desalination, which is not only dependent on the membrane thickness, but also on the preparation technique and membrane microstructure properties. In addition, the substrate resistance and experimental conditions such as feed temperatures and seawater concentrations also take some effects. Furthermore, although the La25Y75SiO2 membrane in this work has exhibited relatively higher desalination performance and good stability towards 3.5 wt% NaCl solutions at 25 °C, further work is still required to verify high desalination efficiency for the microporous organosilica membranes with enhanced stability, which are potentially used for real seawater desalination applications.
4. Conclusions La/Y-codoped microporous organosilica membranes with enhanced performance and good stability for efficiently desalination were fabricated by the sol-gel method. The rare earth elements doped into silica frameworks significantly improved the water permeation flux and rejection properties for desalination. It might be attributed to more uniform silica network architectures instead of randomly dispersed in the homogenous organosilica separation layer, which were constructed by hydrothermally stable Si–O–RE bonds during the sol-gel procedures. The La25Y75-SiO2 membrane exhibited the highest permeation flux and rejection nearly 100%, which was due to its strongest hydrophilcity, finest microstructure and lowest mass resistance for water transport. Water molecules permeated rapidly throughout such the ordered organosilica-network channels. Moreover, this membrane kept stability for relatively long-term running of 200 h without significant changes for NaCl rejection ratio towards 3.5 wt% NaCl feed solutions, and sustained good hydrothermal durability under salts solutions with different concentrations over the temperature range of 25–60 °C. The development of such La/Y-codoped organosilica membrane with high desalination performance and good stability could open up the promising potential applications in seawater desalination and high-salinity wastewater treatment.
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Journal of Membrane Science 584 (2019) 353–363
H.-Y. Zhang, et al.
[24] H. Qi, J. Han, N.P. Xu, H.J.M. Bouwmeester, Hybrid organic–inorganic microporous membranes with high hydrothermal stability for the separation of carbon dioxide, ChemSusChem 3 (2010) 1375–1378. [25] H. Qi, J. Han, N.P. Xu, Effect of calcination temperature on carbon dioxide separation properties of a novel microporous hybrid silica membrane, J. Membr. Sci. 382 (2011) 231–237. [26] H. Qi, H.R. Chen, L. Li, G.Z. Zhu, N.P. Xu, Effect of Nb content on hydrothermal stability of a novel ethylene-bridged silsesquioxane molecular sieving membrane for H2/CO2 separation, J. Membr. Sci. 421–422 (2012) 190–200. [27] H.F. Qureshi, R. Besselink, J.E. ten Elshof, A. Nijmeijer, L. Winnubst, Doped microporous hybrid silica membranes for gas separation, J. Sol. Gel Sci. Technol. 75 (2015) 180–188. [28] M. ten Hove, A. Nijmeijer, L. Winnubst, Facile synthesis of zirconia doped hybrid organic inorganic silica membranes, Sep. Purif. Technol. 147 (2015) 372–378. [29] H.T. Song, S.F. Zhao, J.W. Chen, H. Qi, Hydrothermally stable Zr-doped organosilica membranes for H2/CO2 separation, Microporous Mesoporous Mater. 224 (2016) 277–284. [30] M. ten Hove, M.W.J. Luiten-Olieman, C. Huiskes, A. Nijmeijer, L. Winnubst, Hydrothermal stability of silica, hybrid silica and Zr-doped hybrid silica membranes, Sep. Purif. Technol. 189 (2017) 48–53. [31] M.H. Zahir, T. Nagano, High-selectivity Y2O3-doped SiO2 nanocomposite membranes for gas separation in steam at high temperatures, J. Am. Ceram. Soc. 99 (2016) 452–460. [32] S. Khanmohammadi, E. Taheri-Nassaj, Micro-porous silica-yttria membrane by solgel method: preparation and characterization, Ceram. Int. 40 (2014) 9403–9411. [33] B. Ballinger, J. Motuzas, C.R. Miller, S. Smart, J.C. Diniz da Costa, Nanoscale assembly of lanthanum silica with dense and porous interfacial structures, Sci. Rep. 5 (2015) 8210, https://doi.org/10.1038/srep08210. [34] T. Gougousi, M.J. Kelly, D.B. Terry, G.N. Parsons, Properties of La-silicate high-K dielectric films formed by oxidation of La on silicon, J. Appl. Phys. 93 (2003) 1691–1696. [35] X.L. Zhang, H. Yamada, T. Saito, T. Kai, K. Murakami, M. Nakashima, J. Ohshita, K. Akamatsu, S.-I. Nakao, Development of hydrogen-selective triphenyl methoxysilane-derived silica membranes with tailored pore size by chemical vapor deposition, J. Membr. Sci. 499 (2016) 28–35. [36] X.L. Zhang, K. Akamatsu, S.-I. Nakao, Hydrogen separation in hydrogen-methyl cyclohexane-toluene gaseous mixtures through triphenylmethoxysilane-derived silica membranes prepared by chemical vapor deposition, Ind. Eng. Chem. Res. 55 (2016) 5395–5402. [37] Y. Baek, J. Kang, P. Theato, J. Yoon, Measuring hydrophilicity of RO membranes by
[38]
[39]
[40]
[41]
[42] [43]
[44] [45]
[46] [47]
[48]
[49]
363
contact angles via sessile drop and captive bubble method: a comparative study, Desalination 303 (2012) 23–28. P.H.T. Ngamou, J.P. Overbeek, R. Kreiter, H.M. van Veen, J.F. Vente, I.M. Wienk, P.F. Cuperus, M. Creatore, Plasma-deposited hybrid silica membranes with a controlled retention of organic bridges, J. Mater. Chem. A 1 (2013) 5567–5576. H. Song, Y. Wei, H. Qi, Tailoring pore structures to improve the permselectivity of organosilica membranes by tuning calcination parameters, J. Mater. Chem. A 5 (2017) 24657–24666. P.H.T. Ngamou, M.E. Ivanova, C. Herwartz, N. Lühmann, A. Besmehn, W.A. Meulenberg, J. Mayer, O. Guillon, Tailoring the structure and gas permeation properties of silica membranes via binary metal oxides doping, RSC Adv. 5 (2015) 82717–82725. G.H. Gong, H. Nagasawa, M. Kanezashi, T. Tsuru, Tailoring the separation behavior of polymer-supported organosilica layered-hybrid membranes via facile posttreatment using HCl and HN3 vapors, ACS Appl. Mater. Interfaces 8 (2016) 11060–11069. E. Paparazzo, M. Fanfoni, E. Severini, S. Priori, Evidence of Si-OH species at the surface of aged silica, J. Vac. Sci. Technol. A 10 (1992) 2892–2896. B. Ballinger, J. Motuzas, S. Smart, J.C. Diniz da Costa, Gas permeation redox effect on binary lanthanum cobalt silica membranes with enhanced silicate formation, J. Membr. Sci. 489 (2015) 220–226. J.J. Chambers, G.N. Parsons, Physical and electrical characterization of ultrathin yttrium silicate insulators on silicon, J. Appl. Phys. 90 (2001) 918–933. S. Roualdes, R. Berjoan, J. Durand, 29Si NMR and Si2p XPS correlation in polysiloxane membranes prepared by plasma enhanced chemical vapor deposition, Sep. Purif. Technol. 25 (2001) 391–397. D.H. Wu, A.R. Gao, H.T. Zhao, X.S. Feng, Pervaporative desalination of high-salinity water, Chem. Eng. Res. Des. 136 (2018) 154–164. K.C. Guan, Q. Liu, G.Y. Zhou, G.Z. Liu, Y.F. Ji, G.P. Liu, W.Q. Jin, Cation-diffusion controlled formation of thin graphene oxide composite membranes for efficient ethanol dehydration, Sci. China Mater. 62 (2019) 925–935, https://doi.org/10. 1007/s40843-018-9401-1. Z.S. Cao, S.X. Zeng, Z. Xu, A. Arvanitis, S.W. Yang, X.H. Gu, J.H. Dong, Ultrathin ZSM-5 zeolite nanosheet laminated membrane for high-flux desalination of concentrated brines, Sci. Adv. 4 (2018) eaau8634,https://doi.org/10.1126/sciadv. aau8634. A. Darmawan, L. Karlina, Y. Astuti, Sriatun, J. Motuzas, D.K. Wang, J.C. Diniz da Costa, Structural evolution of nickel oxide silica sol-gel for the preparation of interlayer-free membranes, J. Non-Cryst. Solids 447 (2016) 9–15.