Introduction

Organically bridged polysilsesquioxanes, whose general formula is [O1.5SiRSiO1.5]n, have been extensively studied as precursors for organic–inorganic hybrid materials that are used in the forms of fillers, bulk gels, powders and films.1, 2, 3, 4 These polymers can be easily synthesized via hydrolysis/condensation reactions of bridged trialkoxysilane monomers [(R′O)3SiRSi(OR′)3], with water using an acid or base catalyst (sol–gel process). One of the characteristics of the organic bridging structure is its porosity, which arises from the organic bridges that separate the silicon centers to expand the silsesquioxane network; thus, organically bridged polysilsesquioxanes are expected to serve as various types of porous materials, such as adsorbents, catalyst supports and separation membranes.5, 6, 7

Reverse osmosis (RO) is an important water purification technology in which water molecules are selectively allowed to pass though a membrane to enable ions and large molecules to be removed from aqueous solutions by applying higher pressure than the osmosis pressure.8, 9 This method is applicable for water desalination, even starting from seawater. Currently, polyamide (PA) membranes are commercially used for water desalination. However, PA membranes usually exhibit a low tolerance to chlorine, which is generally used to clean the surfaces of membranes coated with biofilms.10, 11 Therefore, alternative membranes composed of polymer or ceramic materials that show chlorine resistance have also been studied as RO membranes.12, 13, 14

We recently reported the application of organically bridged polysilsesquioxane-based membranes as RO membranes. These bridged RO membranes showed thermal and chemical stability derived from their robust siloxane matrix. However, the bridged membranes showed lower water permeability (10−13 m3/m2·Pa·s) than that of PA membranes (>10−12 m3/m2·Pa·s), although it was higher than that of ceramic membranes (10−14 m3/m2·Pa·s) such as silicates or zeolites.15

To improve the water permeability of bridged silica RO membranes, we have studied the water separation properties of RO membranes prepared from a variety of bridged alkoxysilanes, as shown in Figure 1. For example, bis(triethoxysilyl)ethene (BTES-E2)- and bis(triethoxysilyl)ethyne (BTES-E3)-derived membranes were found to exhibit increased water permeance compared with bis(triethoxysilyl)ethane (BTES-E1)-derived membranes. This is because the rigid and polar ethylene and ethynylene units efficiently increased the membranes’ pore sizes and hydrophilicity, although a slight decrease in NaCl rejection was observed for the BTES-E3-derived membranes.16, 17 The incorporation of highly rigid norbornane units led to NaCl rejection as high as 98% but caused the water permeance to decrease (10−14 m3/m2·Pa·s) because of the hydrophobicity of the norbornane unit.18 Monomers with polar oxalyl urea and acetoxy groups were also investigated, and the results suggested that the presence of hydrophilic and rigid organic bridges improved the RO performance of the membranes.19, 20

Figure 1
figure 1

Precursors of the bridged silica membranes, reported previously.

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In this study, we synthesized a new organically bridged trialkoxysilane containing a 1,2,3-triazole unit linked by methylene units, as presented in Scheme 1. The bridge was anticipated to have sufficient rigidity and hydrophilicity to increase the water permeability of the resulting RO membrane. The properties of 1,2,3-triazole-containing membranes were investigated and compared with those of BTES-E1-derived membranes. An attempt to improve the hydrophilicity via the iodomethylation of BTES-MAz is also described.

Experimental procedures

General

All reactions were conducted in dry argon. The starting materials (chloromethyl)triethoxysilane, trichlorosilane, ethyldi(isopropyl)amine, triethylamine and propargyl chloride were obtained from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). Sodium azide, copper (I) chloride, copper (I) iodide and iodomethane were obtained from Wako Pure Chemical Industries, Ltd (Osaka, Japan). N,N-dimethylformamide, acetonitrile, diethyl ether, ethanol and 2-propanol were obtained from Kanto Chemical Co., Ltd (Tokyo, Japan). d-(+)-Glucose and d-(+)-maltose monohydrate were obtained from Sigma-Aldrich Co. LLC (St Louis, MO, USA). Ethanol, and diethyl ether, N,N-diisopropylethylamine, triethylamine and N,N-dimethylformamide were purified via distillation from Mg and CaH2, respectively, under a dry argon atmosphere and stored over activated molecular sieves. Other reagents were used as obtained.

1H, 13C and 29Si NMR spectra were obtained using a Varian System 500 spectrometer (Agilent Technologies, Santa Clara, CA, USA). FI GC/MS measurements were performed using a JEOL JMS-T100GCV mass spectrometer (Tokyo, Japan) and ESI MS measurements were performed using a Thermo Fisher Scientific (Waltham, MA, USA) LTQ Orbitrap XL mass spectrometer, both at the Natural Science Center for Basic Research and Development (N-BARD) of Hiroshima University. Fourier transform infrared (FT-IR) spectra were obtained using a Shimadzu (Kyoto, Japan) IR Affinity-1 spectrometer via the ATR method, applied to either the neat liquid or a film coated on a silicon wafer. The sol sizes were measured for 5 wt% ethanol solutions by means of dynamic light scattering (DLS) using a Malvern Zetasizer Nano analyzer (ZEN3600, Malvern, Worcestershire, UK). Surface contact angle measurements of water drops were performed using a Kyowa DM300 contact angle meter (Saitama, Japan) for films coated on glass plates. Solution conductivities were measured using a HORIBA ES-51 conductivity meter (Kyoto, Japan). Total organic carbon (TOC) was measured using a total organic carbon analyzer (Shimadzu, TOC-VE).

Synthesis

Synthesis of (azidomethyl)triethoxysilane (1)

In a flask, sodium azide (3.90 g, 60.0 mmol) and (chloromethyl)triethoxysilane (8.51 g, 40.0 mmol) were dissolved in DMF (100 ml), and the mixture was stirred for 12 h at 70 °C.21, 22 The mixture was filtered to remove the resulting precipitates. The solvent was evaporated from the filtrate, and the residue was distilled under reduced pressure (6 Torr, bp 78 °C) to produce 2.96 g (34% yield) of (azidomethyl)triehoxysilane (1) as a colorless liquid. 1H NMR (500 MHz, CDCl3) δ 3.99 (q, 6H, J=7.0 Hz, OCH2), 2.82 (s, 2H, SiCH2), 1.26 (t, 9H, J=7.0 Hz, CH3). 13C NMR (125 MHz, CDCl3) δ 59.09 (OCH2), 35.86 (SiCH2), 18.25 (CH3). 29Si NMR (100 MHz, CDCl3) δ −56.92. IR (neat, ATR) 2976 (C-H), 2891 (C-H), 2015 (-N3), 1099 (Si-O-C) cm−1. GC/MS (FI) m/z 219.10443 [M]+· (Calcd for C7H17N3O3Si: 219.10392).

Synthesis of (3-propynyl)triethoxysilane (2)

In a flask were placed copper (I) chloride (0.237 g, 3.2 mmol), ethyldi(isopropyl)amine (13.6 g, 106 mmol) and diethyl ether (120 ml) and a mixture of propargyl chloride (7.16 g, 24 mmol) and trichlorosilane (1.8 g, 24 mmol) in diethyl ether (80 ml) was slowly added in an ice bath.23 The mixture was warmed to room temperature and stirred for 2 h. The resulting precipitates were filtered, and the solvent and any volatile substances were evaporated from the filtrate. Distillation of the residue under reduced pressure (79 °C, 1.4 Torr) resulted in 7.13 g (35% yield) of a mixture of 85% (3-propynyl)triethoxysilane (2) and 15% (propa-1,2-dienyl)triethoxysilane (2′), as a colorless liquid. Compound 2′ could not be separated, and the mixed product was used for the subsequent reaction. 1H NMR (500 MHz, CDCl3) δ 4.84 (t, 0.15H, J=7.0 Hz, SiHC=of 2′), 4.50 (d, 0.30H. J=7.0 Hz, =CCH2 of 2′), 3.90 (q, 5.1H, J=7.0 Hz, OCH2 of 2), 3.88 (q, 0.9H, J=7.0 Hz, OCH2 of 2′), 1.84 (t, 0.85H, J=3.0 Hz, HC≡ of 2) 1.67 (d, 1.7H, J=3.0 Hz, SiCH2 of 2) 1.26 (t, 9H, J=7.0 Hz, CH3). 13C NMR (125 MHz, CDCl3) δ 216.23 (CHCCH2 of 2′), 79.91 (CCH of 2), 73.50 (SiCH of 2′), 68.43 (CCH2 of 2′), 67.55 (SiCH2 of 2), 59.36 (OCH2 of 2), 59.10 (OCH2 of 2′), 18.39 (CH3 of 2), 18.35 (CH3 of 2’), 1.60 (SiCH2 of 2). IR (neat, ATR) 3291 (C-H), 2974 (C-H), 2927 (C-H), 2891 (C-H), 2109 (C≡C), 1934 (C=C=C), 1072 (Si-O-C) cm−1. 29Si NMR (100 MHz, CDCl3) δ −54.09. GC/MS (FI) m/z 202.10173 [M]+· (Calcd for C9H18O3Si: 202.10252).

Synthesis of BTES-MAz

In a flask were placed compounds 1 (3.09 g, 14.1 mmol) and 2 (3.13 g, 15.5 mmol), acetonitrile (140 ml), copper (I) iodide (2.95 g, 15.5 mmol), and ethyldi(isopropyl)amine (2.00 g, 15.5 mmol), and the mixture was stirred for 1 h at room temperature.24 The resulting precipitate was filtered, and the solvent and any volatile compounds were evaporated from the filtrate. Distillation of the residue under reduced pressure (bp 132 °C, 5 × 10−3 Torr) resulted in 3.88 g (65% yield) of 1,4-bis(triethoxysilylmethyl)-1,2,3-triazole (BTES-MAz) as a pale yellow liquid. 1H NMR (500 MHz, CDCl3) δ 7.42 (s, 1H, ArH), 3.95 (s, 2H, NCH2), 3.84 (q, 6H, J=7.0 Hz, OCH2), 3.82 (q, 6H, J=7.0 Hz, OCH2), 2.26 (s, 2H, CCH2), 2.00 (t, 9H, J=7.0 Hz, CH3), 2.00 (t, 9H, J=7.0 Hz, CH3). 13C NMR (125 MHz, CDCl3) δ 143.23 (Ar), 122.69 (Ar), 59.76 (OCH2), 59.24 (OCH2), 36.61 (SiCH2N), 18.72 (CH3), 18.65 (CH3), 9.61 (SiCH2C). 29Si NMR (100 MHz, CDCl3) δ −50.60, −58.71. IR (neat, ATR) 2972 (C-H), 2926 (C-H), 2891 (C-H), 1069 (Si-O-C) cm−1. GC/MS (FI) m/z 421.20646 [M]+· (Calcd for C16H35N3O6Si2: 421.20644). Anal. Calcd for C16H35N3O6Si2 (%): C, 45.58; H, 8.37; N, 9.97. Found (%): C, 45.26; H, 8.86; N, 10.03.

Synthesis of BTES-MAz-MeI

In a flask were placed BTES-MAz (83 mg, 0.2 mmol), acetonitrile (8 ml), and iodomethane (246 μl, 1 mmol), and the mixture was stirred for 7 h at 70 °C.25 Volatiles were evaporated from the mixture under reduced pressure to produce 165 mg (90% yield) of BTES-MAz-MeI. 1H NMR (500 MHz, CDCl3) δ 8.54 (s, 1H, ArH), 4.30 (s, 2H, NCH2), 4.27 (s, 3H, NCH3), 3.91 (q, 6H, J=7.0 Hz, OCH2), 3.89 (q, 6H, J=7.0 Hz, OCH2), 2.58 (s, 2H, CCH2), 1.24 (t, 9H, J=7.0 Hz, CH3), 1.22 (t, 9H, J=7.0 Hz, CH3). 13C NMR (125 MHz, CDCl3) δ 141.85 (Ar), 128.82 (Ar), 59.86 (OCH2), 59.58 (OCH2), 40.76 (NCH3), 38.17(SiCH2N), 18.23 (CH3), 18.17 (CH3), 9.90 (SiCH2C). 29Si NMR (100 MHz, CDCl3) δ −56.92, −62.39. MS (ESI) m/z 436.22992 [M-I]+ (Calcd for C17H38O6N3Si2: 436.22937).

Preparation of sols and membranes

To an ethanol solution of the monomer BTES-MAz or BTES-MAz-MeI, in which the ethanol content was adjusted to be 5 wt% with respect to the BTES-MAz or BTES-MAz–MeI, was slowly added 60 eq of water. The mixture was stirred for 24 h at room temperature. The sol was diluted with ethanol to a sol content of 2 wt% and then coated on an NTR-7450 (Nitto Denko Corporation, Osaka, Japan) support membrane, and the membrane was heated at 120 °C for 10 min to produce an RO membrane. Coating films were prepared via drop casting onto substrates in air and were then subjected to IR spectrometry (on silicon wafers) and water contact angle measurements (on glass plates) after calcination at 120 °C.

RO experiments

RO experiments were performed at 25 °C under a feed pressure of 1.5 MPa, which was supplied by an N2 gas cylinder, using a 2000 p.p.m. NaCl aqueous solution. The water permeance (Lp) and NaCl rejection (R) were determined using equations (1) and (2), respectively, where Vp, S, ΔP, Δπ, and Cf and Cp are the permeate volume, membrane area, applied pressure, osmotic pressure, respectively, and the NaCl concentrations of the feed and permeate solutions as determined from their conductivity measurements.

Neutral organic solutes of EtOH, IPA, glucose and maltose were used in 500 p.p.m. aqueous solutions, and the contents in the permeates were detected as TOC. The experimental details of water RO measurements have been previously described in the literature.26

Results and discussion

Monomer synthesis and sol preparation

The synthetic route for the monomers is shown in Scheme 1. (Azidomethyl)triethoxysilane and (propargyl)triethoxysilane (1 and 2) were synthesized using the procedures reported in the literature.21, 22, 23 Product 2 contained 15% of the allene isomer (2′), which could not be separated from 2, and the mixed product was consequently used for the subsequent reaction. The Huisgen cycloaddition reaction, also known as the ‘click reaction’, of 1 and 2 was performed to obtain a 1,2,3-triazole-containing bridged alkoxysilane (BTES-MAz). Compound 2′ did not react under the reaction conditions and was readily separated from the BTES-MAz. The iodomethylated salt BTES-MAz-MeI was synthesized by treating BTES-MAz with iodomethane.

The monomers BTES-MAz and BTES-MAz-MeI were polymerized via hydrolysis/condensation reactions (sol–gel reactions) with water in ethanol solution. Generally, a sol-gel reaction requires a catalytic amount of acid or base to proceed in water.27 However, insoluble gels immediately formed from the BTES-MAz and BTES-MAz-MeI in the presence of an acid or base catalyst. Therefore, the reaction was performed without catalysts. The basic triazole unit seemed to function as a self-catalyst in these sol–gel reactions. The increase in size of the sol particles was traced by means of DLS measurements, as shown in Figure 2. For BTES-MAz, the sol size gradually increased as the reaction mixture was stirred at room temperature. For the production of a coating on a porous support membrane with a pore size of ~1 nm, the desired stirring time was determined to be 24 h, at which the sol size reached ~2.3 nm. The sol was then immediately coated onto a polysulfone support membrane (NTR-7450). In this study, NTR-7450 support membranes were used in place of the inorganic support membranes that were used in the previous studies by virtue of their ready commercial availability and their flexibility, which was expected to improve the processability of the membranes. For BTES-MAz-MeI, however, the sol size could not be precisely controlled, likely because of its aggregation behavior due to its ionic structure, and a sol with a particle size of 5 nm was used for coating. These coated membranes were calcined at 120 °C to produce the gel. The calcination temperature was chosen to be slightly below the thermal decomposition temperature of NTR-7450.

Figure 2
figure 2

Particle size distributions of (a) BTES-MAz and (b) BTES-MAz-MeI sols measured via DLS after various reaction times.

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Membrane preparation

To investigate the properties of the gel films, BTES-MAz and BTES-MAz-MeI films were coated on silicon wafers via the drop casting of the sols, followed by calcination at 120 °C for 10 min, and the ATR-IR spectra of the resulting films were obtained, as shown in Figure 3. Absorption bands are clearly seen near 1050–1200 cm−1 and near 3140 cm−1, which are ascribed to Si-O and aromatic C-H stretching of the 1,2,3-triazole moieties, respectively. These findings imply that the triazole components remained in the siloxane matrix after calcination.28, 29 For the BTES-MAz film, bands near 2800–3000 cm−1, ascribed to alkyl C-H stretching, are also seen. By contrast, the alkyl C-H bands of the BTES-MAz-MeI film are weak, and the O-H stretching band is broad and strong. This seems to indicate that the BTES-MAz film contained unreacted EtO- groups, whereas the BTES-MAz-MeI film contained adsorbed water because of its highly hydrophilic properties. It may be possible to assign the O-H band of the BTES-MAz-MeI film to SiOH units arising from the hydrolysis of the SiOEt groups. However, the O-H band is too strong to be wholly ascribed to such SiOH units.

Figure 3
figure 3

ATR-FT-IR spectra of BTES-MAz and BTES-MAz films on silicon wafers after heating at 120 °C for 10 min and of the BTES-MAz monomer.

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The surface properties of the films were also investigated, as summarized in Table 1. The water contact angle of the BTES-MAz film was 69.1°, which indicated that this film had less hydrophilic properties than the BTES-E1 film reported previously, contrary to our expectations.30 This increase in the contact angle of the BTES-MAz can be attributed to the presence of a larger number of hydrophobic EtO- groups compared with that in the BTES-E1 film. A pencil hardness test of the BTES-MAz film surface indicated an index of 2B, revealing that the surface of this film was softer than that of the BTES-E1 film.

Table 1 Surface properties of bridged silica films calcined at 120 °C.
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A cross-sectional FE-SEM image of the BTES-MAz membrane on the sulfonated polyethersulfone NTR-7450 support is shown in Figure 4. The membrane surface was flat and smooth without any cracks or pin-holes, indicating that the support was sufficiently well coated. The membrane thickness of the BTES-MAz separation layer was estimated to be ~200 nm.

Figure 4
figure 4

SEM image of the BTES-MAz membrane.

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Water separation properties

RO measurements were performed using a 2000 p.p.m. NaCl aqueous solution at a pressure of 1.5 MPa. Water separation performance can be evaluated in terms of water permeance and NaCl rejection. These measures usually show a trade-off relationship because the separation properties of a membrane depend primarily on its pore size and a smaller pore size usually leads to both lower permeance and higher rejection. This trade-off relationship for the investigated membranes is shown in Figure 5, and the RO parameters are summarized in Table 2. For BTES-MAz, the membrane exhibited desalination properties characterized by a water permeance of 3.7–5.4 × 10−13m3 /m2 Pa s and 95–96% NaCl rejection; these data were found to be reproducible in three experimental runs. In the trade-off plot, the BTES-MAz membrane lies approximately on the BTES-E1 trade-off line and shows better separation performance than the BTES-Nor and BTES-MOU membranes do. It is likely that the improved performance of the BTES-MAz membrane can be ascribed to the hydrophilic, rigid, and sterically bulky 1,2,3-triazole ring. Hydrophilicity and rigidity should lead to increased water permeability, whereas steric bulkiness should enhance salt rejection. Consequently, the appropriate combination of these characteristics exhibited by the triazole ring seems to be responsible for the overall improvement in membrane performance. The methylene bridges linking the triazole core and silyl units also seem to play an important role in providing moderate flexibility of the organic bridges. Compared with the data for a BTES-E1 membrane calcined at 120 °C, the water permeance of the BTES-MAz membrane was lower, whereas its NaCl rejection was slightly higher. By contrast, the BTES-MAz-MeI membrane showed no salt rejection properties. This was likely because the large sol particle size resulted in an excessively large pore size of the membrane, preventing it from exhibiting a molecular sieving effect.

Figure 5
figure 5

Trade-off plot of the RO performance of the bridged polysilsesquioxane-based membranes.

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Table 2 Summary of the RO performance of bridged silica membranes.
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Similar RO measurements using 500 p.p.m. of neutral solutes in aqueous solutions were also performed to investigate the pore sizes of the membranes. The molecular weight cutoff curves are shown in Figure 6. Compared with the bare NTR-7450 support membrane, the BTES-MAz membrane clearly blocked the permeation of neutral molecules. The cutoff molecular weight (solute molecular weight at 90% rejection) was estimated to be 165. Although the molecular sieving ability was slightly lower than that of the BTES-E1 membrane,31 the observed values are sufficiently high for practical use.

Figure 6
figure 6

Molecular weight cutoff (MWCO) curves.

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Conclusions

In this study, we synthesized two new bridged trialkoxysilanes, BTES-MAz and BTES-MAz-MeI, containing 1,2,3-triazole moieties as hydrophilic and rigid units and prepared films and RO membranes using the synthesized materials. SEM imaging of the BTES-MAz membrane revealed the successful coating of a separation layer derived from BTES-MAz on the support, with a thickness of ~200 nm. The BTES-MAz membrane exhibited RO separation properties characterized by a water permeance of 3.7–5.4 × 10−13 m3/m2 Pa s and 95–96% NaCl rejection for water desalination. The RO separation properties of the BTES-MAz membrane were comparable to those of a previously reported BTES-E1 membrane, whereas the hardness and contact angle of the film were softer and higher, respectively. The higher softness of the BTES-MAz membrane implies increased membrane flexibility, which is advantageous for membrane processability. RO measurements using neutral solutes also revealed that the membrane had a cut-off molecular weight of 165, slightly larger than that of the BTES-E1 membrane. These results indicate the potential of BTES-MAz as a precursor for high-performance separation membranes, and further studies to improve the performance of such membranes are underway.

scheme 1

Synthesis of the bridged alkoxysilane monomers containing 1,2,3-triazole.