Pore size tuning of sol-gel-derived triethoxysilane (TRIES) membranes for gas separation

Journal of Membrane Science 524 (2017) 64–72

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Pore size tuning of sol-gel-derived triethoxysilane (TRIES) membranes for gas separation

MARK

⁎

Masakoto Kanezashia, , Rui Matsugasakoa, Hiromasa Tawarayamab, Hiroki Nagasawaa, Toshinori Tsurua a b

Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan Optical Communications Laboratory, Sumitomo Electric Industries Ltd, Japan 1, Taya-cho, Sakae-ku, Yokohama 244-8588 Japan

A R T I C L E I N F O

A BS T RAC T

Keywords: Sol-gel method Amorphous silica Pore size tuning Molecular sieving Gas separation

Triethoxysilane (TRIES), which consists of three ethoxy groups and a Si-H bond as a pendant-type alkoxysilane, was utilized as a Si precursor for the fabrication of a gas separation membrane. The effect of membrane fabrication parameters such as sol preparation conditions and calcination temperatures on Si-H groups and network structures was evaluated. The degree of dehydrogenation of Si-H groups in aqueous solution was independent of the H2O/Si molar ratio in the sol, but the degree of hydrolysis and polymerization of ethoxy groups (-OEt) depended on the H2O molar ratio. TRIES membranes calcined at 550 °C under N2 showed a decrease in network size with an increase in the H2O/Si molar ratio in the sol. The TRIES-derived network pore size also depended on the calcination temperature, and the network size was decreased under lower calcination temperatures. For example, a TRIES membrane calcined at 550 °C showed high selectivity for He/N2 and H2/ N2 at approximately 1000 and 600, respectively, however, in the case of calcination at 300 °C, Knudsen diffusion dominated for small molecules (H2/N2 selectivity: 4.3) and molecular sieving favored large molecules (H2/CF4: > 100, H2/SF6: > 400). When a TRIES membrane was fabricated by calcination at 300 °C, most Si-H groups were still present in the networks, the estimated network pore size for the TRIES membrane was 0.577 nm, which was larger than that of a tetraethoxysilane (TEOS) membrane calcined at 350 °C (0.426 nm). On the other hand, when TEOS and TRIES membranes were fabricated at 550 °C, both membranes showed approximately the same network pore size (TEOS: 0.385 nm, TRIES: 0.382 nm), due to the dehydrogenation of Si-H groups in the formation of Si-OH groups as well as for the forming of a Si–O–Si bond.

1. Introduction Sol-gel derived SiO2 membranes with pore sizes in the sub-nano meter range are quite attractive for applications to gas and liquid separation in a wide range of temperatures [1–3]. Aspects of membrane performance such as permeability and selectivity strongly depend on the thickness and pore sizes of the active separation layer. An γ-alumina with a pore size of 4 nm [3–7] and a SiO2-ZrO2 (pore size: < 2 nm) [2] layer, which can prevent the penetration of coating sol into substrate, is commonly used as the intermediate layer in the forming of a porous substrate for thin microporous silica layers. The pore size of a SiO2 structure can be controlled via the selection of Si precursors [3,6–13]. In previous studies, when the network structure was created by utilizing tetraethoxysilane (TEOS), the network size was suitable only for the separation of either helium or hydrogen from nitrogen and methane. On the contrary, when silsesquioxane (SQ,

⁎

RSiO3/2) categorized as bridged- [3,6–10] and pendant-type [11–15] was utilized for the network formation, the non-hydrolyzable groups created much looser networks compared with those of TEOS networks, as determined by positron annihilation lifetime spectroscopy (PALS) and gas permeation properties. The formation of loose networks could have been caused by bridged-type precursors, the spacer (Si-R-Si unit), that serve as a minimum unit within a network that is larger than ≡SiO (TEOS) during the construction of the network structure. On the other hand, pendant-type Si precursors such as methyltriethoxysilane (MTES) and phenyltriethoxysilane (PhTES) only have three ethoxy groups, which may contribute to a looser structure because of the smaller degree of cross linking. The incorporation of organic groups can also control the physicochemical properties such as hydrophobic/ hydrophilic and adsorptive characteristics. The incorporation of aminopropyl (-CH2CH2CH2NH2) groups in silica can enhance the CO2 adsorptive properties [14,15].

Corresponding author. E-mail address: [email protected] (M. Kanezashi).

http://dx.doi.org/10.1016/j.memsci.2016.11.006 Received 1 September 2016; Received in revised form 4 November 2016; Accepted 5 November 2016 Available online 13 November 2016 0376-7388/ © 2016 Elsevier B.V. All rights reserved.

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layer was formed on a SiO2-ZrO2 intermediate layer, followed by drying and calcination at 300–550 °C (calcination time: 0.5 h, N2 atmosphere).

Metal doping such as Ni, Co, and Nb into a silica matrix is a wellestablished technique that is used to control the physicochemical properties of a network structure [2,16–22]. Tsuru et al. [17] reported that amorphous silica is stabilized in a hydrothermal atmosphere when Ni and Co is embedded in the silica matrix when network size and hydrophobic properties are controlled. A mixed-matrix structure with dispersed Pd nanoparticles in SiO2 can enhance hydrogen permeance by solution-diffusion throughout the Pd phase [20]. Recently, unique gas permeation properties were reported for cobalt-doped ethoxy polysiloxane (ES40) [21,22]. Control of the reducing and oxidizing states for cobalt particles incorporated into an ES40 matrix is used to tailor the molecular gap for gas separation, and has demonstrated enhanced levels of H2 and CO2/N2 selectivity. Thus, control of the dispersibility and the size of metal particles in silica is a key technology in the fabrication of high-performance membranes. Triethoxysilane (TRIES, HSi(OEt)3), which is a pendant-type due to the Si–H bond, can be used to fabricate a new class of amorphous silica membranes, because the reactivity of the Si-H bond can be used to tailor the molecular sieving and adsorptive properties. Several research groups have reported the activity of Si-H for the reduction of noble salts such as AgNO3, Pd (NO3)2, and H2PtCl6 to create nanoparticles dispersed in a SiO2 structure [23–26]. Morita et al. [26] demonstrated the onsite reduction of novel metal salts under ambient aqueous conditions by Si-H groups in porous hydrogen silsesquioxane (HSQ) monoliths, wherein highly dispersed nanoparticles of less than 40 nm in diameter were embedded in the HSQ monoliths. The reactivity of Si– H bonds with NH3 at high temperatures [27,28], which can form NH2 groups in SiO2 to tune the pore size and affinity by CO2 molecules, would also be an innovative way to control the network structure. However, only a few papers have reported on membrane fabrication utilizing TRIES as a Si precursor with modification of the network structure according to the reactivity of the Si-H groups. We reported only that the in-situ reaction between NH3 and Si-H groups at high temperatures effectively increased the hydrogen selectivity, which was caused by the formation of Si-NH2 and/or Si-NH groups [28]. To fabricate TRIES membranes by modifying the network structure via NH3 and nano particles that utilize the reactivity of Si-H groups, it is necessary to evaluate the effect that membrane fabrication parameters such as sol preparation conditions (H2O/Si molar ratio) and calcination conditions (temperature, atmosphere) will exert on Si-H groups and network size, which would affect the gas permeation properties for TRIES-derived membranes. This paper reports the fabrication of TRIES derived membranes for gas separation, and the evaluation into the effect of membrane fabrication parameters on Si-H groups as well as on the network structure. A modified gas translation (GT) model was applied to evaluate the network sizes of TRIES membranes.

2.2. Characterization of TRIES-derived sol and gels The TRIES sol size was measured by Dynamic Light Scattering (DLS, Malvern Zetasizer Nano ZS) at 25 °C. The FT-IR spectra for films coated onto the silicon wafer plate were measured utilizing a FT-IR spectrometer (FT/IR-4100, Jasco, Japan). TRIES-derived gel powder, which was prepared by drying at 40 °C, was used for the measurement of thermogravimetric analysis (TG) and 29Si MAS NMR. For TG measurement, each gel was kept at 200 °C for 2 h under He flow (100 cc min−1) to remove adsorbed water, and the temperature was increased to 1000 °C at a ramping rate of 20 °C min−1. The 29Si MAS NMR spectra were recorded at 119.17 MHz on a Varian 600PS solid NMR spectrometer using a 6 mm diameter zirconia rotor. The rotor was spun at 7 kHz. The spectra were acquired using 6.2 µs pulses, a 100 s recycle delay, and 1,000 scans. 3-(Trimethylsilyl) propionic2,2,3,3-d4 acid sodium salt was used as a chemical shift reference. Gaussian distribution fitting with the assumption that the baseline is linear and is tangent to the sides of each peak was applied to calculate the peak area ratio of FTIR and 29Si MAS NMR spectra, respectively. 2.3. Gas permeation measurement Fig. 1 shows a schematic diagram of the single-gas permeation measurement. Before the gas permeation measurement, the permeation cell was heated to 300 °C under a N2 flow (100 ml min−1), and the time course for N2 permeance was measured periodically to confirm the thermal stability of the TRIES-derived membranes. After confirming a steady state of gas permeance, single gasses (He, H2, CO2, N2, CH4, CF4, SF6) at 100–300 °C were fed onto the membrane surface at 200 kPa, while the permeation side was kept at atmospheric pressure. The permeation flow rate of each gas was measured using a bubble film meter. In the present study, the permeation measurement was conducted after reaching a steady-state (no concentration gradient in soap film), and the observed deviation in the permeation data was less than 5%, irrespective of permeating molecules. 3. Results and discussion 3.1. Characterization of TRIES sol and gel Fig. 2 shows the TRIES sol size distribution prepared with different H2O/Si molar ratios measured by dynamic light scattering at 25 °C. The sol size was slightly increased with increases in the H2O/Si molar ratio, but each sol had a sharp size distribution ranging from 0.7 to 1 nm, which would be suitable for the fabrication of molecular-sieving gas-separation membranes. Fig. 3 shows the FTIR spectra in the range of 500–4000 cm−1 for non-calcined TRIES-derived films (H2O/Si=6, 60, 240). Highly magnified FTIR spectra ranging from 2500 to 3500 cm−1 are also shown in Fig. 3(b). All samples showed a peak at 1100 cm−1, which can be assigned to Si-O-Si asymmetric stretching vibrations [29], suggesting the formation of silica networks caused by the hydrolysis and condensation of Si-OEt and Si-OH groups. The Si-H was assigned to 800 and 2250 cm−1, respectively [30,31]. Table 1 summarizes the peak area ratios of -CH3 (2974 cm−1)/Si-H (2250 cm−1) and Si-H (2250 cm−1)/ Si-O-Si (1100 cm−1) for non-calcined TRIES-derived film (H2O/Si=6, 60, 240). The peak area ratios of Si-H [30,31] and Si-O-Si were similar, irrespective of different H2O molar ratios. The CH3 peak in ethoxy groups (2974 cm−1) [32] clearly decreased with increases in the H2O molar ratio. An elevated H2O molar ratio can accelerate the reactivity of ethoxy (-OEt) groups and create Si–OH and Si–O–Si bonds. Thus, it can be concluded that the degree of hydrolysis and polymerization of

2. Experimental 2.1. Membrane fabrication Silsesquioxane (SQ) sols were prepared by the hydrolysis and polymerization of triethoxysilane (TRIES) in isopropyl alcohol (IPA) with water and HCl (TRIES/HCl=1/0.1 in a molar ratio), and the Si precursor was maintained at 2.0 wt%. In the present study, hydrolysis and polymerization was conducted at 25 °C under vigorous stirring, and the H2O molar ratio (H2O/Si molar ratio) was controlled within a range of from 6 to 240. Porous glass tubes (Sumitomo Electric Industries Ltd, Japan) with an average pore size of 500 nm and a porosity of 64% were used as the membrane substrates for TRIES membranes [28]. Silica glass particles (particle diameter: 300 nm) mixed with SiO2-ZrO2 sol were deposited onto the substrate to form the intermediate layer (calcination temperature: 550 °C, air atmosphere), which had an average pore size of 1–2 nm, as measured using nanopermporometry [2]. Then, a TRIES 65

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Fig. 1. Schematic diagram of the single-gas permeation measurement.

30 H2O/Si=240

H2O/Si=6

Number [-]

Table 1 Peak area ratio of -CH3 (2974 cm−1)/Si-H (2250 cm−1) and Si-H (2250 cm−1)/Si-O-Si (1100 cm−1) for non-calcined TRIES derived films (H2O/Si=6, 60, 240).

20

H2O/Si=60

10

0 -1 10

100

101

Fig. 2. TRIES sol size distribution prepared with different H2O/Si molar ratios at 25 °C.

ethoxy groups (-OEt) depends on the H2O molar ratio. Fig. 4 shows the 29Si NMR spectra of non-calcined TRIES-derived gel prepared with different H2O/Si molar ratios (6, 240). Both samples showed a peak at −85 ppm, corresponding to T3 (HSiO3) and a small peak for Q4 (SiO4) at −110 ppm [33]. The peak area ratios of the T unit for the samples prepared with H2O molar ratios of 6 and 240 were 0.93

Si-H

CH3/Si-H [-]

Si-H/Si-O-Si [-]

6 60 240

0.0144 0.0039 −

0.167 0.17 0.14

and 0.88, respectively. These results were consistent with that of the FTIR, and indicate a partial dehydrogenation of Si-H groups, as described in Eq. (1), during the hydrolysis and polymerization stages, but the degree of dehydrogenation in the Si-H groups was almost independent of the H2O/Si ratio in the sol. Since the reaction proceeds via the protonation of oxygen atoms and followed the electrophilic addition reaction of Si by H2O molecules under the acid condition [34], the reactivity of Si-H groups was prohibited and was independent of the H2O/Si molar ratio in the sol.

Size [nm]

-CH3

H2O/Si [-]

-Si-H+H2O →-Si-OH+H2

Fig. 5 shows the Si NMR spectra of TRIES-derived gel (H2O/ Si=6) calcined at different temperatures under an N2 (a) and an air (b)

Si-O-Si Si-H

-CH3

(a)

(b)

Absorbance [a. u.]

Absorbance [a. u.]

(1)

29

H2O/Si=6 H2O/Si=60

H2O/Si=6 H2O/Si=60 H2O/Si=240

H2O/Si=240

4000

3000

1000 500

2000

3500

3000 Wave number [cm ]

Wave number [cm ] −1

Fig. 3. FTIR spectra in the range of 500–4000 cm

2500 -1

-1

−1

(a), and a highly magnified range from 2500 to 3500 cm

66

(b) for non-calcined TRIES derived films (H2O/Si=6, 60, 240).

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1

SiO4 Si-H peak area ratio [-]

HSiO3

H2O/Si=240

H2O/Si=6

0.8

Calcination in N2

0.6 0.4

Calcination in air

0.2 0 0

200

400

600

Temperature [°C]

-60

-80

-100

-120

-140

Fig. 6. T unit (Si-H) peak area ratio to Q unit (SiO4) of TRIES gel (H2O/Si=6) as a function of calcination temperature.

Chemical shift [ppm]

1

29

Relative weight [-]

Fig. 4. Si NMR spectra of non-calcined TRIES-derived gel prepared with different H2O/Si molar ratios (6, 240).

atmosphere. The peak area of the T3 unit was decreased with the calcination temperature, irrespective of the calcination atmosphere. Fig. 6 shows the T unit (Si-H) peak area ratio to Q unit (SiO4) as a function of the calcination temperature. The calculated peak area ratios of the T3 unit were 0.93, 0.72 and 0.13 for the sample before calcination and during calcinations at 300 °C and 500 °C under N2, respectively. The drastic decrease in the T unit between 300 and 550 °C under N2 can be ascribed to the dehydrogenation of the Si-H and SiOH groups to form Si–O–Si bonds, as described in Eq. (2) [35]. When TRIES gel was calcined under an air atmosphere, the peak area ratio of Si-H was lower than that calcined under N2, since oxygenation of Si-H to form Si-OH groups started at around 300 °C [36]. Thus, low temperature calcination under a N2 atmosphere, where oxygenation and dehydrogenation of Si-H groups can be prevented, is suitable for the fabrication of TRIES membranes with a high density of Si-H groups. In the present study, TRIES membranes were calcined at different temperatures under N2 to evaluate the effect of Si-H groups on network size and gas permeation properties. -Si-H+OH-Si~ → -Si-O-Si-+H2

H2O/Si=240

0.9 H2O/Si=60

0.85 H2O/Si=6

0.8 0.75 200

400 600 800 Temperature [ ഒ]

200 and 400 °C was 5–13% for H2O/Si molar ratio of 240-6, which can be ascribed to desorption of chemisorbed water and condensation of SiOH groups. The smallest weight loss between 200 and 400 °C for gel with a H2O/Si ratio of 240 indicates that the number of Si-OH groups was smaller than that prepared from H2O/Si ratios of 6 and 60. The total weight loss increased as the H2O/Si molar ratio decreased due to the different number of unreacted ethoxy and Si-OH groups in the

(2)

SiO4

HSiO3

SiO4

500oC, N2 500oC, air

300oC, N2

300oC, air

Gelation at 40oC

-60

-80 -100 -120 Chemical shift [ppm]

Fig. 5.

29

1000

Fig. 7. TG curve under a He atmosphere of TRIES gel prepared with different H2O/Si molar ratios.

Fig. 7 shows the TG curve under a He atmosphere of TRIES gel prepared with different H2O/Si molar ratios. The weight loss between

HSiO3

0.95

-140

-60

-80 -100 -120 Chemical shift [ppm]

Si NMR spectra of TRIES-derived gel (H2O/Si=6) calcined at different temperatures under N2 (a) and air (b) atmospheres.

67

-140

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Fig. 8. SEM image of the surface (a) and cross-section (b) of a TRIES-derived membrane.

noted that an organosilica sol has approximately the same size as a TRIES sol and was coated on an intermediate layer with an average pore size of 1–2 nm. The depth profile suggests that an organosilica layer with a thickness of less than 40 nm was formed inside the intermediate layer due to the penetration of the organosilica sol, which made it very difficult to distinguish each layer. It would be ideal to fabricate layer-by-layer structures to minimize the resistance of gas permeation, and further study is required to optimize the relationship between the properties of the coating sols (concentration, size, distribution) and the intermediate layers (pore size, distribution). Fig. 9(a) shows the kinetic diameter dependence of gas permeance at 300 °C of TRIES-derived membranes (H2O/Si=240) calcined at 300 and 550 °C under N2. Each gas permeance was decreased largely with increases in calcination temperature. Fig. S1 shows the time course of N2 permeance at 300 °C for a TRIES membrane (H2O/Si=240) calcined at 300 °C. The N2 permeance slightly decreased about 5% in the initial period of about 3 h and approached a steady state. In the present study, the molecular size and temperature dependence of gas permeances for TRIES membranes calcined at 300 °C was measured after confirming the steady-state permeance of N2 at 300 °C. A TRIES membrane calcined at 300 °C showed H2 permeance of 1.0×10−6 mol m−2 s−1 Pa−1 with H2/N2 of 6 and a H2/CF4 permeance ratio of 210. Knudsen diffusion dominated the permeation properties of small molecules; the permeance of H2 was higher than that of He,

networks, which was consistent with the results of FTIR. When 3 ethoxy groups were hydrolyzed and condensed, corresponding to the chemical formula of HSiO1.5, the theoretical calculation of residual weight was 0.93 with the assumption of the final structure of SiO2 at 1000 °C, which was approximately the same as TRIES prepared with an H2O/Si ratio of 240. 3.2. Effect of calcination temperature and H2O/Si molar ratio in sol on network size A thin defect-free separation layer was essential in fabricating porous membranes with molecular sieving properties that included high levels of gas permeability and selectivity. Fig. 8 shows the SEM image of the surface (a) and cross-section (b) of a TRIES-derived membrane fabricated at 550 °C. The fabricated membrane was an asymmetric structure with a thin, continuous TRIES layer on the top of the intermediate/glass particle layer. The thickness of the TRIES layer was clearly less than 100 nm. Some TRIES sols penetrated the pores of the intermediate layer, which made it difficult to distinguish each layer, since the size of a TRIES sol had a distribution ranging from 0.7 to 1 nm, which was somewhat smaller than that of the intermediate layer. In our previous work, X-ray photoelectron spectroscopy (XPS) analysis was carried out to show the depth profile of organosilica membranes [8]. It should be

10-6 10-7 10-8 10

101 He

300oC

H2

500oC CO2

Dimensionless permeance [-]

Permeance [mol m -2 s-1 Pa-1]

10-5

CH4 N2

CF4 SF6

-9

10-10 10-11 0.2

0.3 0.4 0.5 Kinetic diameter [nm]

100 10-1 10-2

CO2

CH4

N2

CF4

SF6

10-3 10-4 0.2

0.6

He

300oC 500oC

H2

0.3 0.4 0.5 Kinetic diameter [nm]

0.6

Fig. 9. Kinetic diameter dependence of gas permeance (a) and dimensionless permeance based on He permeance (b) at 300 °C for TRIES membranes (H2O/Si=240) calcined at 300 and 550 °C.

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dehydrogenation of most Si-H groups occurs at 550 °C to create Si-OH groups and further reacted with other –SiOH groups. Fig. 11(a) shows the kinetic diameter dependence of gas permeance at 300 °C of TRIES-derived membranes (H2O/Si=6, 60, 240) calcined at 550 °C under N2. Each of the values for gas permeance were decreased with increases in the water molar ratio, and the gas permeance was decreased with increases in the molecular size due to the molecular sieving property, irrespective of the H2O/Si molar ratio. A TRIES membrane with an H2O/Si ratio of 6 showed an H2 permeance of 1.0×10−6 mol m−2 s−1 Pa−1, which was 5 times higher than that with an H2O/Si ratio of 240. The permeance of H2 was higher than that of He for a membrane with an H2O/Si ratio of 6, but membranes with H2O/Si ratios of 60 and 240 showed a He permeance that was higher than that for H2. Fig. 11(b) shows dimensionless permeance based on a He permeance at 300 °C for TRIES-derived membranes (H2O/Si=6, 60, 240) calcined at 550 °C under N2 as a function of the kinetic diameter. The permeation cutoff of a membrane with an H2O/Si ratio of 6 appeared to roughly follow the range of the Knudsen mechanism for He (0.26 nm) and CO2 (0.33 nm). A membrane with an H2O/Si ratio of 240 showed the sharpest cutoff between H2 and gases with a molecular size larger than CO2. The permeation cutoff for a membrane with an H2O/Si ratio of 60 was between that for ratios of 6 and 240. These results indicate that the network pore size of TRIES depends on the H2O/Si molar ratio in the sol, and the network pore size decreases with an increase in the H2O/Si ratio. A similar trend has been reported in bis (triethoxysilyl) ethane (BTESE)-derived organosilica membranes. Niimi et al. [10] reported that the organosilica network pore size was successfully controlled by the H2O molar ratio in the sol, and that the network pore size became smaller when sol was hydrolyzed under a higher H2O molar ratio. As mentioned in Section 3.1, the degrees of hydrolysis and polymerization of the ethoxy groups (-OEt) largely depended on the H2O molar ratio. The enlarged network size in the case of a low H2O molar ratio can be explained as follows, and is schematically shown in Fig. 10(b). Since a pyrolysis of the ethoxy groups starts at above 400 °C under an inert atmosphere [10], this can serve as a “template” [38] in networks

and the permeance of CH4 was higher than that of N2. On the other hand, a TRIES membrane calcined at 550 °C showed high selectivity for He/N2 and H2/N2 at approximately 1000 and 600, respectively. Small network pore size dominated molecular sieving for He and H2 molecules, and showed a permeance of He (dm: 0.26 nm) that was higher than that of H2 (dm: 0.289 nm). Fig. 9(b) shows dimensionless permeance based on He permeance at 300 °C for TRIES-derived membranes (H2O/Si=240) calcined at 300 and 550 °C under N2 as a function of kinetic diameter. The broken line in this figure represents the calculated dimensionless permeance based on He under a Knudsen mechanism. Since the deviation between experimentally obtained He selectivity and Knudsen-based selectivity is caused by the molecular sieving effect, the order of the pore sizes can be estimated from permeation cut-off [37], which is the dimensionless permeance based on He permeance (He selectivity). The permeation cutoff of a membrane calcined at 300 °C appeared to roughly follow the range of a Knudsen mechanism for He (0.26 nm) and CH4 (0.38 nm). A membrane calcined at 550 °C showed the permeation cutoff between H2 and gases with a molecular size larger than CO2, which was much higher than the value expected for a Knudsen diffusion (H2/N2 Knudsen selectivity: 3.64). These results indicate that network pore size by TRIES depends on calcination temperature, and that network pore size was decreased with increases in calcination temperature. Fig. 10 shows a schematic image for the formation of a TRIES network structure calcined at high temperatures; effect of high H2O/Si molar ratio (a) and low H2O/Si molar ratio (b) on TRIES networks. It should be noted that the ethoxy groups was completely hydrolyzed when the TRIES sol was prepared with H2O in a molar ratio of 240. Thus, the formation of loose networks when calcined at 300 °C can be ascribed to the presence of Si-H groups, which is the same concept for loose network formation by pendant-type alkoxides such as MTES and PhTES [12]. The Si-H groups were non-hydrolyzable under acidcatalyzed conditions, as described in Section 3.1, where the network size was enlarged due to the smaller degree of cross-linking, which corresponds to fewer ethoxy groups. The formation of small network pore sizes could possibly have been caused by the condensation of SiOH groups when the TRIES membrane was calcined at 550 °C, since

Fig. 10. Schematic image of the formation of a TRIES network structure calcined at high temperatures; effect of high H2O/Si molar ratio (a) and low H2O/Si molar ratio (b) on TRIES networks.

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101

10

-6

He

H2O/Si=6 H2O/Si=60 H2O/Si=240

H2 CO2 N2

10-7

CH4

10-8 10

Dimensionless permeance [-]

Permeance [mol m -2 s-1 Pa-1]

10-5

CF4

-9

SF6

10-10 10-11 0.2

0.3 0.4 0.5 Kinetic diameter [nm]

100

He

CO2 N2 CH4

10-1 10-2

CF4

10-3 SF6

10-4 0.2

0.6

H2O/Si=6 H2O/Si=60 H2O/Si=240

H2

0.3 0.4 0.5 Kinetic diameter [nm]

0.6

Fig. 11. Kinetic diameter dependence of gas permeance (a) and dimensionless permeance based on He permeance (b) at 300 °C for TRIES-derived membranes (H2O/Si=6, 60, 240) calcined at 550 °C under N2.

from activation energy, Ed, resulting in the activation energy of permeation (Ep=Ed−ΔH). The heat of adsorption of CO2 by silica was reported to approximately 25 kJ mol−1 [39], which is much higher than that of H2 and N2 molecules, since these gases tend to adsorb on SiO2 by following Henry's law. Thus, a negative activation energy of CO2 permeation, corresponding to the surface diffusion mechanism, is reasonable. Most papers have reported Knudsen permeation of N2 and CH4 molecules for highly permeable H2 separation membranes [1,2]. This is because small molecules such as He and H2 can permeate through network pores by activated diffusion, but molecules larger than H2 can permeate only through a fewer number of grain boundaries in a membrane, which are created by spaces between gels and/or large size siloxane rings, by Knudsen diffusion. A TRIES membrane (H2O/

calcined at 550 °C under N2, and this can enlarge a network structure with unreacted ethoxy groups prepared at a low H2O molar ratio. 3.3. Temperature dependence of gas permeance for TRIES membranes Fig. 12(a) shows the temperature dependence of gas permeance for a TRIES membrane (H2O/Si=240) calcined at 550 °C under N2. He and H2 permeance was largely increased with an increase in the temperature (activated diffusion). N2, CH4 and CF4 permeance also showed activated diffusion properties. CO2 molecules showed surface diffusion, since they are adsorptive molecules with a strong affinity for SiO2. In the adsorption-diffusion model, molecular permeation occurs through adsorption with the heat of adsorption, ΔH, and is followed by diffusion

Temperature [oC]

10-5

200

300

150

Temperature [oC] 100

10-5

10-6

-1

10-7

10-8 CO2

10-9

N2

10

-10

10

-11

He

CO2 N2

10-7

CH4

-2

H2

100

H2

-1

Permeance [mol m s Pa ]

He

-2

-1

-1

Permeance [mol m s Pa ]

10-6

200

300

CF4

10-8

SF6

10-9

10-10

CH4 CF4

1.5

2

2.5 -1 1000/T [K ]

10-11 1.5

3

2

2.5 -1 1000/T [K ]

3

Fig. 12. Temperature dependence of gas permeance for a TRIES membrane (H2O/Si=240) calcined at 550 °C (a) and a membrane (H2O/Si=240) calcined at 300 °C (b).

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Si=240) calcined at 550 °C showed activated permeation behavior of N2, CH4 and CF4 molecules, but the slope of temperature dependence of N2, CH4, and CF4 was smaller than that of He and H2. If these molecules permeate through the same pores, the slope of temperature dependence of gas permeances, corresponding to activation energy, should increase as molecular size increases [40]. Thus, it is reasonable that the Knudsen effect through large pores of a TRIES membrane will continue to affect the overall permeation property of large molecules, but a TRIES membrane (H2O/Si=240) calcined at 550 °C could have a sharper pore size distribution than that of TEOS-derived SiO2 membranes. Fig. 12(b) shows the temperature dependence of gas permeance for a TRIES membrane (H2O/Si=240) calcined at 300 °C under N2. The permeance values for He and H2 were independent of temperature, and N2, CH4, CF4 and SF6 molecules showed a Knudsen type of permeation behavior, whereby permeance was slightly increased with a decrease in temperature. Gas selectivity for small molecules (H2/N2 selectivity: 4.3) was controlled by Knudsen diffusion through network pores, and for large molecules (H2/CF4: > 100, H2/SF6: > 400) by molecular sieving. The calcination temperature did not affect the permeation property of CO2 molecules, i.e., both membranes showed surface diffusion mechanisms. A modified gas translation (GT) model [41], which is shown in Eq. (3), was used to obtain the pre-exponential parameter (k0) and the apparent activation energy (Ep) via regression with the temperature dependence of gas permeance for the evaluation of network size. The pre-exponential parameter (k0) is expressed only by membrane configuration factors such as membrane thickness, L, tortuosity, τ, porosity, ε, pore diameter, dp, and molecular size of the permeating molecules, di, as described in Eq. (4). The network pore size (dp) can be obtained by establishing the relationship between the k01/3 of each gas molecule and its molecular size, based on the assumption of a monomodal structure. It should be noted that a is a constant and only depends on the membrane structure, irrespective of the permeating molecules (molecular size, weight).

(d p − d i ) 2 ε (d p − d i ) 3τL dp2

⎛ E p, i ⎞ 8 exp ⎜ − ⎟= ⎝ RT ⎠ πMi RT

ε (d p − d i )3 3τL dp2

8 = a (d p − d i )3 π

(4)

Fig. 13 shows a k01/3 plot of the TRIES membranes calcined at 300 (a) and 550 °C (b). The relationships between the k01/3 of each gas molecule and the kinetic diameter for the TEOS membranes calcined at 350 and 550 °C [41,42] are also shown in the same figure to allow a comparison of the network pore size. The k0 value of CO2 molecules was not plotted because of the contribution of surface diffusion caused by the adsorption effect between CO2 and the silica structure below 300 °C [39]. In the case of calcination at 300 °C, where most Si-H groups still remain in the networks, the estimated network pore size for the TRIES membrane was 0.577 nm, which was larger than that of the TEOS membrane calcined at 350 °C (0.426 nm). This can be explained as follows. The formation of networks larger than those in the TEOS membrane can be ascribed to the smaller degree of cross-linking, corresponding to a smaller number of ethoxy groups than what is found in TEOS. When both membranes were fabricated at 550 °C, both showed approximately the same network pore size (TEOS: 0.385 nm, TRIES: 0.382 nm). This is reasonable since the TRIES membrane should have the same network structure as a TEOS membrane, because of the dehydrogenation of Si-H groups to form Si-OH groups and subsequent formation of Si–O–Si bonds. It should be noted that the estimated network pore size was decreased with higher calcination temperatures, irrespective of the Si precursor.

4. Conclusions Triethoxysilane (TRIES) is a pendant-type alkoxysilane with a Si–H bond, and was utilized as a Si precursor for the fabrication of a gas separation membrane. The effect of membrane fabrication parameters such as sol preparation conditions and calcination temperatures on SiH groups and network structure was evaluated. A modified gas translation (GT) model was applied to evaluate the network size of TRIES membranes. The fabricated membrane was an asymmetric structure with a thin, continuous TRIES layer (thickness: < 100 nm) on top of an intermediate/glass particle layer. The thickness of the TRIES layer was clearly less than 100 nm. The degree of dehydrogenation for the Si-H groups was independent of the H2O/Si molar ratio in the sol, but the degree of hydrolysis

⎛ E p, i ⎞ exp ⎜ − ⎟ ⎝ RT ⎠ Mi RT k 0, i

(3)

0.04

0.04

closed symbols: TEOS open symbols: TRIES

closed symbols: TEOS open symbols: TRIES

0.03

0.03 Ne He H2

k0,i1/3 [-]

k0,i1/3 [-]

Pi =

k 0, i =

0.02

He

0.02 H2

N2

0.01

Ne

CH4

0.01 CF4 SF6 N2

0 0.2

0.3

0.4

0.5

0 0.2

0.6

0.3

CH4

0.4

CF4

0.5

Kinetic diameter [nm]

Kinetic diameter [nm]

Fig. 13. k01/3 plot of TRIES and TEOS-derived membranes calcined at 300 °C (a) and at 550 °C (b).

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0.6

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and polymerization of the ethoxy groups (-OEt) depended on the H2O molar ratio. The peak area ratio of the T unit (Si-H) to Q unit (SiO4) was decreased with increases in calcination temperature, irrespective of the calcination atmosphere. A drastic decrease in T units between 300 and 550 °C under N2 was ascribed to the dehydrogenation of Si-H to form Si-OH groups and Si–O–Si bonds. Each gas permeance was decreased with increases in the water molar ratio, and gas permeance was decreased with increases in molecular size due to the molecular sieving properties, irrespective of the H2O/Si molar ratio. TRIES membranes calcined at 550 °C under N2 showed a decreased network size with an increase in the H2O/Si molar ratio in the sol, due to the pyrolysis of unreacted ethoxy groups, which can serve as a “template” for networks. The TRIES derived network size also depended on calcination temperature, and the network size was decreased with increases in the calcination temperature. For example, a TRIES membrane calcined at 550 °C showed high selectivity of He/N2 and H2/N2 of approximately 1000 and 600, respectively. However, for a membrane calcined at 300 °C, Knudsen diffusion dominated for small molecules (H2/N2 selectivity: 4.3) and molecular sieving was dominant for large molecules (H2/CF4: > 100, H2/SF6: > 400). The estimated network pore size for a TRIES membrane calcined at 300 °C was 0.577 nm, which was larger than that of a tetraethoxysilane (TEOS) membrane calcined at 350 °C (0.426 nm), because of the smaller degree of cross-linking, corresponding to a smaller number of ethoxy groups compared with that of TEOS. On the other hand, when TEOS and TRIES membranes were fabricated at 550 °C, both showed approximately the same network pore size (TEOS: 0.385 nm, TRIES: 0.382 nm), which was a result of the dehydrogenation of Si-H groups in the formation of Si-OH groups and Si–O–Si bonds. Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at doi:10.1016/j.memsci.2016.11.006. References [1] Y.S. Lin, I. Kumakiri, B.N. Nair, H. Alsyouri, Microporous inorganic membranes, Sep. Purif. Methods 31 (2002) 229–379. [2] T. Tsuru, Nano/subnano-tuning of porous ceramic membranes for molecular separation, J. Sol-gel Sci. Technol. 46 (2008) 349–361. [3] 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. [4] R.M. de Vos, H. Verweij, High-selective, high-flux silica membranes for gas separation, Science 279 (1998) 1710–1711. [5] C.-Y. Tsai, S.-Y. Tam, Y. Lu, C.J. Brinker, Dual-layer asymmetric microporous silica membranes, J. Membr. Sci. 169 (2000) 255–268. [6] H.L. Castricum, G.G. Paradis, M.C. M.-Hazeleger, R. Kreiter, J.F. Vente, J.E. ten Elshof, Tailoring the separation behavior of hybrid organosilica membranes by adjusting the structure of the organic bridging group, Adv. Funct. Mater. 21 (2011) 2319–2329. [7] H.L. Castricum, H.F. Qureshi, A. Nijmeijer, L. Winnubst, Hybrid silica membranes with enhanced hydrogen and CO2 separation properties, J. Membr. Sci. 488 (2015) 121–128. [8] M. Kanezashi, K. Yada, T. Yoshioka, T. Tsuru, Organic-inorganic hybrid silica membranes with controlled silica network size: preparation and gas permeation characteristics, J. Membr. Sci. 348 (2010) 310–318. [9] X. Ren, K. Nishimoto, M. Kanezashi, H. Nagasawa, T. Yoshioka, T. Tsuru, CO2 permeation through hybrid organosilica membranes in the presence of water vapor, Ind. Eng. Chem. Res. 53 (2014) 6113–6120. [10] T. Niimi, H. Nagasawa, M. Kanezashi, T. Yoshioka, K. Ito, T. Tsuru, Preparation of BTESE-derived organosilica membranes for catalytic membrane reactors of methylcyclohexane dehydrogenation, J. Membr. Sci. 455 (2014) 375–383. [11] G. Cao, Y. Lu, L. Delattre, C.J. Brinker, G.P. Lopez, Amorphous silica molecular sieving membranes by sol-gel processing, Adv. Mater. 8 (1996) 588–591. [12] G. Li, M. Kanezashi, T. Tsuru, Preparation of organic-inorganic hybrid silica membranes using organoalkoxysilanes: the effect of pendant groups, J. Membr. Sci. 379 (2011) 287–295. [13] G.G. Paradis, D.P. Shanahan, R. Kreiter, H.M. van Veen, H.L. Castricum, A. Nijmeijer, J.F. Vente, From hydrophilic to hydrophobic HybSi® membranes: a change of affinity and applicability, J. Membr. Sci. 428 (2013) 157–162. [14] G. Xomeritakis, C.-Y. Tsai, C.J. Brinker, Microporous sol-gel derived aminosilicate membrane for enhanced carbon dioxide separation, Sep. Purif. Technol. 42 (2005)

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