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SO2 gas separation
Journal of Membrane Science 496 (2015) 211–218
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Permeation properties of BTESE–TEOS organosilica membranes and application to O2/SO2 gas separation Lie Meng, Masakoto Kanezashi, Jinhui Wang, Toshinori Tsuru n Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 15 June 2015 Received in revised form 5 August 2015 Accepted 30 August 2015 Available online 1 September 2015
In the present study, a BTESE–TEOS mixed precursor was proposed for control of the pore sizes of organosilica networks in organic–inorganic hybrid silica membranes. FT-IR spectrometry confirmed the formation of a partially cross-linked polysiloxane structure with hydrocarbon units in a BTESE–TEOSderived silica network produced by the co-hydrolysis and condensation of BTESE with TEOS, which also showed an improved thermal stability. Single gas permeation measurements and normalized Knudsenbased permeance (NKP) established the order of the average membrane pore sizes as follows: BTESE 4BTESE–TEOS 4TEOS. The organosilica membranes derived from BTESE–TEOS exhibited a superior O2 permeance that was higher than 10 8 mol m 2 s 1 Pa 1 with an O2/SO2 selectivity of 7.3, which indicated that the pore size control in organosilica networks using BTESE–TEOS as a precursor was effective for selective O2/SO2 separation. Moreover, the effective molecular size of SO2 permeates through organosilica membranes was discussed. & 2015 Elsevier B.V. All rights reserved.
Keywords: Microporous membrane Organosilica Gas separation Sulfur dioxide Pore size control
1. Introduction One vital issue in recent years has been the development of effective approaches for the capture and separation of SO2, which is an origin of air pollution and global warming [1,2]. As an alternative to solvent absorption, membrane separation has been considered an attractive method in systems including sour gas purification and concentration of SO2 [3]. Thus far, polymer membranes have been extensively investigated to remove SO2 and purify exhaust gases, and these have shown remarkable SO2 solubility and selectivity, which favors SO2 permeation [3]. A novel application is the O2/SO2 separation in the Iodine–Sulfur thermochemical water-splitting cycle (IS process) for hydrogen production [4]. The separation of O2 and SO2 could significantly increase the O2 yield in SO3 decomposition, as confirmed by theoretical studies [5]. Since the SO3 decomposition reaction takes place at high temperatures under an oxygen atmosphere, polymeric membranes are not suitable for this application. Forsberg et al. [6] reported that ZrO2 membranes with pore sizes on the order of 1 nm showed an O2 permeance of 2.0 10 10 mol m 2 s 1 Pa 1 with an O2/SO2 permeation ratio of 1.05 at 130 °C. For dense inorganic membranes, He et al. [7] prepared oxygen ion conducting dense membranes and applied them to the separation of O2 and SO2 at high temperatures. Unfortunately, it was obvious that the n
Corresponding author. E-mail address: [email protected] (T. Tsuru).
http://dx.doi.org/10.1016/j.memsci.2015.08.066 0376-7388/& 2015 Elsevier B.V. All rights reserved.
membrane performance was degraded once SO2 was introduced into the feed stream. These results suggest that the main drawbacks of state-of-the-art inorganic membranes for O2/SO2 separation are low O2 selectivity and low chemical stability to SO2. Given the components involved in this system, O2 ( dk, O2 ¼0.346 nm) and SO2 ( dk, SO2 ¼0.360 nm) [8], membranes with a narrow pore size distribution are expected to enhance the separation efficiency. Moreover, for O2/SO2 separation, the thermal stability of membrane materials under an oxidizing atmosphere is required. Organosilica membranes [9,10] composed of covalent bonds between both hydrocarbons and oxygen to silicon, have recently attracted considerable attention because of their tunable membrane pore sizes, and improved hydrothermal stability due to a decrease in silanols on the surface, compared with pure inorganic silica membranes. To date, organosilica membranes have performed well in a variety of applications, including gas separation [11], pervaporation [12], reverse osmosis [13] and membrane reactors [14]. The pore size of organosilica membranes is dependent on the size of the organic–inorganic hybrid silica network, which can be controlled either by the organic-template method [15] or by spacer technology using Si–C bridges [16,17]. In the organic-template method, organic ligands were used as micropore templates that were embedded in the inorganic matrix and were removed during the pyrolysis process, to create a larger average pore size for membranes which resulted in an increased CO2 permeance [15]. The average pore size of a separation functional layer can be
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adjusted by varying the size of the minimum unit in organosilica networks [16]. A spacer technology [17] was proposed and confirmed using various types of bridged alkoxysilanes comprised of bridging units (Si–CH2–Si, Si–(CH2)2–Si). As opposed to silica membranes prepared from tetraethoxysilane (TEOS), organosilica membranes derived from bridged bis-silyl precursors such as bis (triethoxysilyl)methane (BTESM) and bis(triethoxysilyl)ethane (BTESE) show a hybrid silica network with looser micro-structures, which facilitates gas permeation. In addition, membrane pore sizes can also be controlled by the size of the linking units, the larger the linking unit (Si–(CH2)2–Si 4Si–CH2–Si), the higher the gas permeance. For BTESE-derived silica membranes [17], the H2 permeance was one order of magnitude higher (∼10 5 mol m 2 s 1 Pa 1) than that of pure silica membranes, and a high H2/SF6 selectivity of 25,500 with a low H2 to N2 permeance ratio (∼20) was recorded. In our recent work [18], the H2O/BTESE molar ratio for preparation of BTESE-derived silica sols was a key factor in controlling the membrane pore size. Less densification in organosilica networks was achieved by decreasing the H2O/BTESE molar ratio from 240 to 6, which resulted in an increase in the pore size of BTESE-derived membranes. Herein, we propose a novel strategy to control pore size by using a mixed precursor derived from different alkoxides. TEOS was mixed with BTESE for the possible tuning of pore sizes between TEOS and BTESE-derived silica membranes. In the present work, a BTESE–TEOS mixed sol was proposed as the Si precursor for the fabrication of organic–inorganic hybrid silica membranes with tunable pore sizes. BTESE–TEOS-derived silica sols were prepared via co-hydrolysis and condensation of BTESE with TEOS, and were characterized by the Fourier transform infrared (FT-IR) spectrometry and thermogravimetric–mass spectrometric (TG–MS) analysis. Single gas permeation measurements were conducted to investigate the effect of a hybrid precursor on the average pore size of organosilica membranes, which was also quantitatively evaluated via normalized Knudsen-based permeance (NKP). As a preliminary study, a binary-component gas separation of O2 and SO2 was carried out to investigate the gas transport properties, thermal stability and chemical resistance of organosilica membranes.
2. Experimental
initially mixed with ethanol. Subsequently, H2O and HCl were added and the mixture was stirred for 5 h at 50 °C to achieve a final sol. In the case of a composite organosilica sol (BTESE–TEOS), BTESE, TEOS and ethanol were mixed and vigorously stirred, followed by the addition of H2O and HCl with continuous stirring for 5 h at 50 °C to produce a hybrid sol. The molar ratios of the added reagents were (BTESE or BTESE–TEOS or TEOS)/H2O/HCl at 1/60/ 0.1, respectively, and the precursors were maintained at 5 wt%. For a BTESE–TEOS sol, the molar ratio of BTESE to TEOS was 1/1. Porous α-Al2O3 tubular substrates with a pore diameter of 1 μm, an outside diameter of 10 mm and a length of 100 mm were supplied by Mitsui Grinding Wheel Co., Ltd. and were employed as supports. To develop a smooth mesoporous particle layer for an organosilica membrane, α-Al2O3 powders (Sumitomo Chemical Co., Ltd.) with particle diameters of 1.9 and 0.2 μm were dispersed in a SiO2–ZrO2 colloidal sol. A fresh support was coated with the large-sized particle sols to eliminate the possibility of defects and relatively large pores on outer surface of support, followed by 0.5 h calcination at 550 °C, and then the coating procedure was repeated using the small-sized particle sols. A SiO2–ZrO2 intermediate layer was deposited by coating a 0.5 wt% concentration of the SiO2–ZrO2 (Si/Zr ¼1) sol that was obtained from the hydrolysis of zirconium tetrabutoxide (ZrTB) and TEOS onto the particle layer [19]. After coating and drying, the α-Al2O3 supported SiO2–ZrO2 membranes were calcined by heating to 550 °C for 0.5 h. For membrane fabrication, organosilica sols (BTESE, BTESE–TEOS) were applied onto the SiO2–ZrO2 intermediate layer by coating, followed by subsequent drying at room temperature and calcination at 300 °C for 0.5 h. 2.2. Characterization of organosilica gels The Fourier transform infrared (FT-IR) spectra for BTESE-derived and BTESE–TEOS-derived silica films coated on KBr pellets were recorded with a FT-IR spectrometer (FT/IR-4100, Jasco Co., Japan). Powdered samples of the BTESE-derived and BTESE–TEOSderived silica gels were obtained by drying organosilica sols at 40 °C under air. Thermogravimetric–mass spectrometric analysis (TG–MS, TG-DTA-410S, Rigaku Co., Japan) was conducted on the BTESE-derived and BTESE–TEOS-derived silica powders to characterize the thermal decomposition behavior of the membranes under an air flow of 300 cm3 min 1. The rate of temperature increase was 10 °C min 1 to 550 °C.
2.1. Preparation of organosilica sols and membranes 2.3. Single and binary-component gas permeation measurement BTESE, BTESE–TEOS and TEOS were used as silica precursors. For the preparation of a BTESE or TEOS sol, BTESE or TEOS was
Single gas permeation testing was performed using an
Fig. 1. Schematic diagram of the experimental setup for single and binary-component gas permeation measurement.
L. Meng et al. / Journal of Membrane Science 496 (2015) 211–218
experimental setup, as schematically shown in Fig. 1. The feed gas was introduced to the upstream of the membrane with a furnace temperature that varied between 50 and 200 °C. The pressure in the retentate stream was regulated with a pressure gauge while the permeate stream was maintained at atmospheric pressure. The gas flow rates were measured via film flow meters (SF-2U, Horiba, Japan). O2/SO2 gas separation was performed at temperatures that ranged from 50 to 300 °C. A gas mixture of O2 and SO2 was fed upstream of the membrane module, while atmospheric pressure was maintained in the downstream. The SO2 concentration in the mixed gas was measured by trapping the SO2 with hydrogen peroxide (0.09 mol/L), which was prepared by diluting 0.3 g H2O2 (30%) to 100 mL. The conductivity increase corresponded to the amount of H2SO4 formed from the oxidation of the dissolved SO2 [20,21]. The O2 that was not dissolved in the H2O2 solution passed through the traps and was measured by a film flow meter. In binary-component gas permeation, the selective permeation causes a partial pressure distribution of the component along the membrane surface. Here, the logarithmic mean pressure difference for the component, i (Δpi, lm ) was employed to calculate the driving force for gas permeation [18,22]:
Δpi, lm =
Δpi, in − Δpi, out ln
(
Δpi, in Δpi, out
)
(1)
where Δpi, in and Δpi, out refer to the difference in the partial pressure of component i between the retentate and the permeate side at the inlet and at the outlet, respectively. Multiple (more than triplicate) measurements were performed to achieve reproducible results.
3. Results and discussion 3.1. Characterization of organosilica gels The presence of organic moieties in the silica network can be characterized using FT-IR analysis. Fig. 2 shows the FT-IR spectra of silica films, which were obtained by coating KBr pellets with sols derived from BTESE, BTESE–TEOS and TEOS with firing at 300 °C under air. Typically, the peaks of Si–O bonding can be characterized by attributing the most intensive peaks, which occurred at frequencies near 1070 cm 1, to the asymmetric stretching of oxygen atoms, while the peaks at around 800 cm 1 were due to the symmetric stretching of the oxygen atoms (vibrational mode of ring stretching). These results suggest that the sol–gel reaction was successful [23]. For BTESE and BTESE–TEOS-derived silica gels,
Fig. 2. FT-IR spectra of silica films derived from BTESE, BTESE–TEOS and TEOS on KBr pellets (firing at 300 °C in air).
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the bands at 1070 cm 1 and 800 cm 1 moved to lower wave numbers, which resulted from an increase in the carbon content of the organosilica networks [24]. Absorption at about 950 and 3750 cm 1 was ascribed to the silanol bonds (Si–OH), while that at 3000–4000 cm 1 was attributed to the OH groups and adsorbed water. BTESE-derived silica gels can be identified by the characteristic peaks at 2974, 2929 and 1340 cm 1 [25], which were attributed to the stretching vibrations of –CH3 bonding in the terminal-ethoxyl groups (–OCH2CH3), –CH2– bonding in the Si–(CH2)2–Si groups and symmetric C–H stretching vibration in the –CH2– fragments, respectively. Another strong absorption band at 1278 cm 1 was assigned to the Si–C bond [26]. Both symmetric and asymmetric stretching of the C–H bonds of the organic groups was also found in mixed BTESE–TEOSS-derived silica gels that were used for the preparation of organosilica membranes. The C–H bond, however, was completely featureless in the spectrum of purely inorganic silica films prepared from TEOS. Thus, the presence of methyl and ethyl groups, together with the molecular structure of the precursor (BTESE and TEOS) indicated that a partially cross-linked polysiloxane structure with hydrocarbon units was formed in the BTESE–TEOS-derived silica gels. The thermal stability of the silica gels derived from BTESE, BTESE–TEOS and TEOS was confirmed by TG–MS, and the measured curves are shown in Fig. 3. At temperatures below 200 °C, a minor weight loss was observed that was accompanied by the release of adsorbed water. The order of weight loss was TEOS4 BTESE–TEOS4 BTESE, which might be due to the hydrophilic property of TEOS. Above 200 °C, the residual weight decreased in a rapid manner, which corresponded to the water loss from condensation of the Si–OH groups when forming Si–O–Si bonds, as reported previously [27]. Additionally, there was a sharper decrease in weight loss for the BTESE-derived silica gel at 300–550 °C, compared with the silica gels derived from BTESE– TEOS and TEOS. This could be attributed to the degradation and/or combustion of ethoxide groups (–OCH2CH3) and ethylene bridges (–CH2–CH2–), according to the peak intensity at given mass-tocharge ratios (m/z) as a function of temperature that reported in our previous studies [18]. Thus, an increase in the thermal stability at 300–550 °C could be observed in the silica gels produced by the co-hydrolysis and condensation of BTESE with TEOS, compared with BTESE-derived silica gels. Moreover, the total weight loss of a BTESE or BTESE–TEOS gel was lower than 7% at 300 °C during the decomposition processes, which agreed with the previous report on bis(triethoxysilyl)methane (BTESM)-derived silica gels [28] and strongly demonstrated that organosilica membranes are highly stable in an oxidizing atmosphere at 300 °C.
Fig. 3. TG curve of silica gels derived from BTESE, BTESE–TEOS and TEOS (in air).
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organosilica networks is close to the kinetic diameter of He and H2, these molecules permeate via an activated permeation mechanism, whereby the permeance of He is higher than that of H2 because of the smaller kinetic diameter of He. Since a larger permeance of He than that of H2 was observed for the BTESE–TEOSderived silica membrane, and for which the permeance ratio of He/H2 was not as high as that for TEOS-derived silica membranes, the organosilica membranes derived from BTESE–TEOS possibly formed a separation layer with pores larger than those prepared from TEOS but smaller than those prepared from BTESE. The average pore size of a microporous membrane, dp , can be obtained by the normalized Knudsen-based permeance (NKP) method [28,32]. NKP is the ratio of the permeance of component i to the permeance predicted using He, which is the smallest molecule, under the Knudsen diffusion mechanism, as expressed in Eq. (2).
NKP = Fig. 4. Molecular diameter dependency of gas permeances for organosilica membranes derived from BTESE, BTESE–TEOS, and previously reported TEOS-derived silica membranes [29] at 200 °C.
3.2. Gas permeation properties of organosilica membranes The as-prepared organosilica membrane includes an α-Al2O3 support layer, α-Al2O3 and SiO2-ZrO2 intermediate layers, and an organosilica layer which could be clearly identifiable, as shown in our recent work [12,14,28]. The gas permeation properties of the organosilica membranes were characterized by the values for single-gas permeance, which were given as a function of the molecular diameter of gas molecules. It’s worth mentioning that the separation performances of silica-based membranes reported in our previous studies have been widely demonstrated to be reproducible, which indicates the preparation process of silica-based membranes is controllable under optimized conditions. Fig. 4 shows the molecular diameter dependency of gas permeance for organosilica membranes derived from BTESE and BTESE–TEOS, and for a previously reported TEOS-derived silica membrane [29], at 200 °C. The TEOS-derived silica membrane showed a permeance to H2 of 2.0 10 7 mol m 2 s 1 Pa 1 with a H2/N2 selectivity as high as 600. The SF6 permeance was below the detection limitations of the gas permeation system, suggesting a pore size that was smaller than 0.4 nm. For a BTESE-derived silica membrane, a one order of magnitude higher H2 permeance of 1.4 10 6 mol m 2 s 1 Pa 1 was obtained together with selectivities of H2/C3H8 and H2/SF6 of 2000 and 130,000, respectively. A BTESE–TEOS-derived silica membrane showed a H2/C3H8 permeance ratio of up to 15,000, which was larger than the H2/C3H8 selectivity for a BTESE-derived silica membrane. These gas separation performances strongly demonstrated that a smaller pore size was achieved in the BTESE–TEOS-derived silica membrane. Moreover, the organosilica membranes derived from BTESE and BTESE–TEOS exhibited a relatively lower H2/N2 selectivity (o20), which could be attributed to the looser structure of organosilica networks [17]. According to the gas-translation model, the permeation of nonadsorbable or weakly adsorbable gases depends on the ratio of the kinetic diameter of the diffusing molecule, dk , to the average pore size of the membrane, dp [30,31]. For He and H2, when the pore size of the organosilica network is much larger than their kinetic diameters, the Knudsen permeation mechanism dominates the permeation behavior, and the H2 permeance will be higher than the He permeance due to the smaller molecular weight of H2. Fig. 4 shows the Knudsen selectivity of H2/He for a BTESE-derived silica membrane, and confirms the larger pore size of its organosilica network. On the other hand, when the pore size of
Pi PHe
Mi MHe
3
=
( dp − dk, i ) exp ⎛⎜ 3 ⎝ ( dp − dk, He )
−
EP, i − EP, He ⎞ ⎟ ⎠ RT
(2)
where Pi , Mi , EP , i , dk, i represents the gas permeance, molecular weight, activation energy and molecular size for the permeating components, respectively. Herein, the EP , i for different gases is assumed to be similar to EP , He due to the large pore sizes of organosilica membranes [28]. Thus, the membrane pore size, dp , can be calculated by the following equation (Eq. (3)).
NKP =
( dp − dk, i )3 ( dp − dk, He )3
(3)
Fig. 5 shows the NKP plot at 200 °C for organosilica membranes derived from BTESE and BTESE–TEOS membranes, as well as for previously reported TEOS-derived silica membranes [29], as a function of molecular size. Judging from the fitting curves based on Eq. (3), the average pore size of a BTESE–TEOS-derived silica membrane ( dp ¼ 0.43 nm) was larger than that for a TEOS-derived silica membrane ( dp ¼0.34 nm) but smaller than that for a BTESEderived silica membrane ( dp ¼0.51 nm). The temperature dependence on gas permeation is widely utilized to investigate the gas transport properties of microporous membranes [33,34]. Fig. 6 shows the temperature dependence on gas permeance for a BTESE–TEOS-derived silica membrane at temperatures ranging from 50 to 200 °C. In microporous silica membranes, the Knudsen permeation of N2 and CH4 molecules is often reported with the existence of pinholes which were created by the siloxane ring with large sizes and/or spaces between colloidal particles [35]. However, an organosilica membrane derived from BTESE–TEOS showed an activated permeation behavior for
Fig. 5. NKP plot at 200 °C as a function of molecular size for organosilica membranes derived from BTESE and BTESE–TEOS, and for previously reported TEOSderived silica membranes [29]; symbols are experimental data and curves are calculated based on Eq. (3).
L. Meng et al. / Journal of Membrane Science 496 (2015) 211–218
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differences in the minimum units of membrane structures. Fig. 7 schematically shows the amorphous silica networks derived from TEOS, BTESE–TEOS and BTESE. The Si–C–C–Si–O–Si, Si–C–C–Si and Si–O–Si bonds coexist in organosilica membranes derived from BTESE–TEOS, because the organic groups between the Si atoms in the BTESE precursor are not hydrolyzed during the hydrolysis and polymerization. Thus, a BTESE–TEOS precursor forms a looser structure for the minimum unit in the organic–inorganic hybrid silica membrane, and the average pore size of an organosilica network should be larger than that derived from TEOS, but smaller than that derived from BTESE. Therefore we concluded that the membrane pore size was successfully tuned by using the composite organosilica (BTESE–TEOS) precursor, as quantitatively presented using NKP in Fig. 5. Fig. 6. Temperature dependence on gas permeance for a BTESE–TEOS-derived silica membrane at temperatures ranging from 50 to 200 °C (curves were fitted with Eq. (4)).
N2 and CH4 molecules (permeance increased with an increase in temperature), indicating that a sharp pore size distribution with fewer pinholes was achieved in the membrane structure [36]. The permeance of both CO2 and C3H8 increased with decreasing temperature, suggesting their permeation behavior was governed by the surface diffusion mechanism, in which the gas molecules adsorbed onto the membrane surface and subsequently diffused along a concentration gradient. The activation energy, EP , i , and membrane structural factor, k 0 , were obtained using Eq. (4) with experimental data from the temperature dependence on gas permeance.
Pi =
⎛ EP, i ⎞ k0 exp ⎜ − ⎟ ⎝ RT ⎠ Mi RT
(4)
where i refers to the gas component i , P is the permeance of the gas component, M represents the molecular weight of the gas component, R is the gas constant, and T is the temperature. Table 1 summarizes the activation energies of He permeation and the gas permeation ratios for membranes derived from BTESE [9,31], TEOS [37,38] and BTESE–TEOS. The activation energy of gas permeation is described as the interaction between the pore wall and gas molecules, which can be used for the evaluation of membrane pore size, i.e., diffusion in tighter pores requires a higher activation energy of permeation [39]. The activation energy for the permeation of He, a non-adsorptive gas molecule, through BTESE– TEOS-derived silica membranes was higher than that through BTESE-derived silica membranes. Therefore, it is reasonable to conclude that the pore sizes of organosilica membranes derived from BTESE–TEOS are larger than that derived from TEOS but smaller than that derived from BTESE. This trend can also be confirmed via He/H2 and H2/N2 selectivities. The variations in the pore size distribution of membranes fabricated by different precursors can be illustrated by the
3.3. Binary-component gas separation of O2/SO2 for organosilica membranes Separation for a binary gas mixture consisting of O2 and SO2 was conducted to evaluate the separation performance of organosilica membranes derived from BTESE and BTESE–TEOS. Generally, the membrane separation performance depends on several operating conditions such as the temperature, pressure and molar ratio of the feed gas. In the present study, binary gas separation was performed with a molar ratio of SO2/O2 ¼1/10, and the feed side was pressurized while the permeate side was kept at ambient pressure. Fig. 8 illustrates the O2/SO2 selectivity for BTESE and BTESE–TEOS-derived silica membranes as a function of O2 permeance, and compares the results with those of ZrO2 membranes [6], poly(ether–b–amide) (PEBAX3533)/polyetherimide (PEI) composite membranes [40], and poly(amide-6–b–ethylene oxide) (PEBA1657)/PEI composite membranes [41]. For ZrO2 membranes, the O2/SO2 separation was conducted at 130 °C, while for the PEBAX3533/PEI and PEBA1657/PEI membranes, the values for the permeance of SO2 and N2 were measured at 30 and 20 °C, respectively. These PEBA membranes exhibited a high permeance for SO2 due to a strong affinity (high solubility) between polarizable molecules and the ether block in PEBA [40], particularly at low temperatures. The BTESE–TEOS and BTESE-derived silica membranes showed an impressive combination of permeance and selectivity for O2 at 200 °C, which was beyond the Knudsen selectivity. BTESE-derived silica membranes showed an O2 permeance of 5.0 10 8 mol m 2 s 1 Pa 1 with an O2/SO2 permeation ratio of 3.6. The organosilica membranes derived from BTESE–TEOS exhibited an O2/SO2 selectivity of up to 5.9 while maintaining an O2 permeance higher than 1.7 10 8 mol m 2 s 1 Pa 1, which indicated that the pore size control in amorphous silica networks using BTESE–TEOS as a precursor was effective in improving the selective separation of O2/SO2. Fig. 9 shows the time-course of gas permeance for a BTESE– TEOS-derived silica membrane. The permeance of single gases (He,
Table 1 Activation energies of He permeation and gas permeation ratios for various silica-based membranes. Membrane materials
EP [kJ mol 1]
He/H2 [dimensionless]
H2/N2 [dimensionless]
Temperature [°C]
Reference
BTESE
1.8 3.4 5.9
– 0.7 0.8
16.2 7.0 15.6
250 200 200
[9] [31] [31]
TEOS
12.4 14.6
6.6 5.5
– 235.3
200 300
[37] [38]
BTESE–TEOS
10.0
2.3
16.3
200
this work
216
L. Meng et al. / Journal of Membrane Science 496 (2015) 211–218
Fig. 7. Schematic images of amorphous silica networks derived from (a) TEOS, (b) BTESE–TEOS and (c) BTESE.
Fig. 8. O2/SO2 or N2/SO2 selectivity for various membranes as a function of O2 or N2 permeance.
significant change in the structure of the BTESE–TEOS-derived silica membrane. When the ethoxy groups (–OCH2CH3) that were unreacted during the hydrolysis and polymerization, and/or the Si–C–C–Si linking units, were oxidized, gas permeance would have become higher. Therefore, the absence of an obvious increase in O2 permeance at 300 °C confirmed the stable membrane structure. In addition, no absorption and/or reaction between SO2 and the organosilica network was observed [42]. Fig. 10 shows the values for gas permeance at 200, 250 and 300 °C for BTESE–TEOS-derived silica membranes before and after the separation of O2/SO2 as a function of molecular diameter. After O2/SO2 separation at a given temperature from 200 to 300 °C, no significant change could be observed in gas permeance, confirming the thermal and chemical stability of BTESE–TEOS-derived silica membranes. Moreover, the CF4 permeance remained virtually unchanged during exposure to O2 and SO2, indicating that the BTESE–TEOS-derived silica membranes were free of pinholes and voids. It should be noted that the gas permeance was increased with increasing temperature via the activated permeation mechanism, which is discussed in Fig. 6. Fig. 11 shows the temperature dependence of gas permeance
Fig. 9. Time-course of gas permeance at temperatures ranging from 200 to 300 °C for a BTESE–TEOS-derived silica membrane.
H2, N2, CF4) was measured before and after the binary gas separation of O2/SO2, and approximately 60 and 150 min was conducted for each single and binary gas mixture, respectively. The temperature was gradually increased in a stepwise fashion from 200 to 300 °C to confirm the thermal stability of the organosilica membrane under the oxidizing atmosphere of a SO2 flow (partial pressure of SO2: 14 kPa), which also established its chemical stability. All the measured values for single gas permeance were similar before and after exposure to O2 and SO2, indicating no
Fig. 10. Molecular diameter dependency of gas permeance at temperatures ranging from 200 to 300 °C for BTESE–TEOS-derived silica membranes before and after the separation of O2/SO2.
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Table 3 Kinetic diameters and L-J length constants for gas molecules. Kinetic diameter, [8] dk [nm]
He H2 O2 SO2 N2 CH4 C3H8 CF4 SF6 a
Fig. 11. Temperature dependence on the permeance of O2 and SO2 for a BTESE– TEOS-derived silica membrane at temperatures ranging from 150 to 300 °C.
(O2 and SO2) at temperatures ranging from 150 to 300 °C for a BTESE–TEOS-derived silica membrane. The O2 permeance became higher with increasing temperature, suggesting that it was governed by an activated permeation mechanism. The values for SO2 permeance were on the order of 10 9 mol m 2 s 1 Pa 1 and increased with decreasing temperature. This is a typical trend whereby a surface diffusion mechanism at low temperatures creates a larger degree of SO2 adsorption on the membrane surface, which allows SO2 to diffuse through a membrane via the concentration dependency [43]. The O2/SO2 permeance ratio reached 7.3 in this temperature range, and was increased with increasing temperature because the temperature dependence of O2 permeance was larger than that of SO2. Thus, the BTESE–TEOS-derived silica membrane is an attractive alternative material for the separation of O2/SO2 mixed gases at temperatures up to 300 °C. The effective molecular size of the diffusing component is an important parameter for the development of membranes and membrane processes. To date, only a limited number of papers have reported the permeation properties of SO2, although this information is critical for a determination of the effective molecular size of SO2 above 250 °C where the molecular sieving effect dominates the permeation behavior. Table 2 summarizes the permeance values of O2, N2 and SO2 in BTESE–TEOS-derived silica membranes. In this work, the BTESE–TEOS-derived silica membrane showed a lower permeance for SO2 than that for N2 at 250 and 300 °C, although the kinetic diameter of SO2 ( dk, SO2 ¼ 0.360 nm) was smaller than that of N2 ( dk, N2 ¼ 0.364 nm). However, in addition to the kinetic diameter, the Lennard-Jones (LJ) length constant [44] was also well accepted for the molecular size of gas components. Table 3 lists the kinetic diameter, dk , and L-J length constant, σLJ , for the gas molecules examined in the present study. The kinetic diameter, dk , was the minimum crosssectional diameter of gas molecules, while the L-J length constant (collision diameter), σLJ , was calculated either from the transport properties (viscosity, thermal conductivity) or from the second virial coefficients [44]. The kinetic diameters of the inorganic gas
Table 2 Gas permeance of various gases in a BTESE–TEOS-derived silica membrane. Kinetic diameter, [8] dk [nm]
O2 SO2 N2
0.346 0.36 0.364
Permeance [10 9 mol m 2 s 1 Pa 1] 200 °C
250 °C
300 °C
17.2 4.0 2.8
21.7 3.6 7.0
28.7 3.9 10.9
b
0.26 0.289 0.346 0.36 0.364 0.38 0.43 0.47 0.55
L-J length constant, [44] σLJ [nm]
σLJ a
σLJ b
0.258 0.292 0.343 0.429 0.368 0.382 0.506
0.263 0.287 0.346 0.369 0.382 0.563 0.47 0.551
σLJ / dk
1.00 1.00 1.00 1.19 1.01 1.01 1.24 1.00 1.00
L-J length constant calculated from the viscosity. L-J length constant determined derived from the second virial coefficient.
molecules approximated the L-J length constant, while for SO2 and hydrocarbons such as C3H8 the difference between dk and σLJ became larger, which suggested that a change in the shape of molecules can be predicted by the ratio of σLJ / dk [30]. When σLJ / dk equaled to 1, the molecules were considered spherical while SO2 and long branched molecules (hydrocarbons) showed a σLJ / dk that was higher than 1. Moreover, the σLJ of SO2 was larger than that of N2, which was consistent with the above-mentioned values for gas permeance. These results suggest that the L-J length constant should be used as the effective molecular size for SO2 permeating through a silica network. We can therefore conclude that the diffusion of a gas molecule depends not only on its minimum crosssectional diameter, but also on its effective length, which results in a molecular sieving effect for O2/SO2 separation.
4. Conclusions A novel strategy to control the pore size of organosilica membranes was developed by the introduction of a mixed BTESE–TEOS precursor. Organic–inorganic hybrid silica membranes with tunable pore sizes were prepared via a sol–gel processing, and were proposed for O2/SO2 gas separation. FT-IR confirmed the formation of a partially cross-linked polysiloxane structure with hydrocarbon units in a BTESE–TEOS-derived silica network produced by the cohydrolysis and condensation of BTESE with TEOS, which also showed an improved thermal stability compared with a BTESEderived silica network. Pore size distribution, as determined by the single gas permeation measurement, suggested that the order of membrane pore sizes was BTESE 4BTESE–TEOS 4TEOS, because of the differences in the minimum units when these were applied as precursors, which was consistent with the pore sizes predicted by the NKP plot. The organosilica membranes derived from BTESE– TEOS exhibited superior O2 permeance with an O2/SO2 selectivity as high as 7.3 in temperatures ranging from 150 to 300 °C, which demonstrated the potential of BTESE–TEOS-derived silica membranes for the selective separation of O2/SO2.
Acknowledgments This work was supported by the Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), of the Energy Carrier Project of the Japan Science and Technology Agency (JST).
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Nomenclature
dk dp Ep k0 M p P R T
molecular size, m pore diameter, m Kinetic energy, J mol 1 structural parameter, dimensionless molecular weight, g mol 1 pressure, Pa gas permeance, mol m 2 s 1 Pa 1 gas constant, J mol 1 K 1 absolute temperature, K
Greek symbols
σLJ
Lennard-Jones length constant, m
Subscripts
i lm in out
component i logarithmic mean inlet of the membrane module outlet of the membrane module
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Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Permeation properties of BTESE–TEOS organosilica membranes and application to O2/SO2 gas separation Lie Meng, Masakoto Kanezashi, Jinhui Wang, Toshinori Tsuru n Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 15 June 2015 Received in revised form 5 August 2015 Accepted 30 August 2015 Available online 1 September 2015
In the present study, a BTESE–TEOS mixed precursor was proposed for control of the pore sizes of organosilica networks in organic–inorganic hybrid silica membranes. FT-IR spectrometry confirmed the formation of a partially cross-linked polysiloxane structure with hydrocarbon units in a BTESE–TEOSderived silica network produced by the co-hydrolysis and condensation of BTESE with TEOS, which also showed an improved thermal stability. Single gas permeation measurements and normalized Knudsenbased permeance (NKP) established the order of the average membrane pore sizes as follows: BTESE 4BTESE–TEOS 4TEOS. The organosilica membranes derived from BTESE–TEOS exhibited a superior O2 permeance that was higher than 10 8 mol m 2 s 1 Pa 1 with an O2/SO2 selectivity of 7.3, which indicated that the pore size control in organosilica networks using BTESE–TEOS as a precursor was effective for selective O2/SO2 separation. Moreover, the effective molecular size of SO2 permeates through organosilica membranes was discussed. & 2015 Elsevier B.V. All rights reserved.
Keywords: Microporous membrane Organosilica Gas separation Sulfur dioxide Pore size control
1. Introduction One vital issue in recent years has been the development of effective approaches for the capture and separation of SO2, which is an origin of air pollution and global warming [1,2]. As an alternative to solvent absorption, membrane separation has been considered an attractive method in systems including sour gas purification and concentration of SO2 [3]. Thus far, polymer membranes have been extensively investigated to remove SO2 and purify exhaust gases, and these have shown remarkable SO2 solubility and selectivity, which favors SO2 permeation [3]. A novel application is the O2/SO2 separation in the Iodine–Sulfur thermochemical water-splitting cycle (IS process) for hydrogen production [4]. The separation of O2 and SO2 could significantly increase the O2 yield in SO3 decomposition, as confirmed by theoretical studies [5]. Since the SO3 decomposition reaction takes place at high temperatures under an oxygen atmosphere, polymeric membranes are not suitable for this application. Forsberg et al. [6] reported that ZrO2 membranes with pore sizes on the order of 1 nm showed an O2 permeance of 2.0 10 10 mol m 2 s 1 Pa 1 with an O2/SO2 permeation ratio of 1.05 at 130 °C. For dense inorganic membranes, He et al. [7] prepared oxygen ion conducting dense membranes and applied them to the separation of O2 and SO2 at high temperatures. Unfortunately, it was obvious that the n
Corresponding author. E-mail address: [email protected] (T. Tsuru).
http://dx.doi.org/10.1016/j.memsci.2015.08.066 0376-7388/& 2015 Elsevier B.V. All rights reserved.
membrane performance was degraded once SO2 was introduced into the feed stream. These results suggest that the main drawbacks of state-of-the-art inorganic membranes for O2/SO2 separation are low O2 selectivity and low chemical stability to SO2. Given the components involved in this system, O2 ( dk, O2 ¼0.346 nm) and SO2 ( dk, SO2 ¼0.360 nm) [8], membranes with a narrow pore size distribution are expected to enhance the separation efficiency. Moreover, for O2/SO2 separation, the thermal stability of membrane materials under an oxidizing atmosphere is required. Organosilica membranes [9,10] composed of covalent bonds between both hydrocarbons and oxygen to silicon, have recently attracted considerable attention because of their tunable membrane pore sizes, and improved hydrothermal stability due to a decrease in silanols on the surface, compared with pure inorganic silica membranes. To date, organosilica membranes have performed well in a variety of applications, including gas separation [11], pervaporation [12], reverse osmosis [13] and membrane reactors [14]. The pore size of organosilica membranes is dependent on the size of the organic–inorganic hybrid silica network, which can be controlled either by the organic-template method [15] or by spacer technology using Si–C bridges [16,17]. In the organic-template method, organic ligands were used as micropore templates that were embedded in the inorganic matrix and were removed during the pyrolysis process, to create a larger average pore size for membranes which resulted in an increased CO2 permeance [15]. The average pore size of a separation functional layer can be
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adjusted by varying the size of the minimum unit in organosilica networks [16]. A spacer technology [17] was proposed and confirmed using various types of bridged alkoxysilanes comprised of bridging units (Si–CH2–Si, Si–(CH2)2–Si). As opposed to silica membranes prepared from tetraethoxysilane (TEOS), organosilica membranes derived from bridged bis-silyl precursors such as bis (triethoxysilyl)methane (BTESM) and bis(triethoxysilyl)ethane (BTESE) show a hybrid silica network with looser micro-structures, which facilitates gas permeation. In addition, membrane pore sizes can also be controlled by the size of the linking units, the larger the linking unit (Si–(CH2)2–Si 4Si–CH2–Si), the higher the gas permeance. For BTESE-derived silica membranes [17], the H2 permeance was one order of magnitude higher (∼10 5 mol m 2 s 1 Pa 1) than that of pure silica membranes, and a high H2/SF6 selectivity of 25,500 with a low H2 to N2 permeance ratio (∼20) was recorded. In our recent work [18], the H2O/BTESE molar ratio for preparation of BTESE-derived silica sols was a key factor in controlling the membrane pore size. Less densification in organosilica networks was achieved by decreasing the H2O/BTESE molar ratio from 240 to 6, which resulted in an increase in the pore size of BTESE-derived membranes. Herein, we propose a novel strategy to control pore size by using a mixed precursor derived from different alkoxides. TEOS was mixed with BTESE for the possible tuning of pore sizes between TEOS and BTESE-derived silica membranes. In the present work, a BTESE–TEOS mixed sol was proposed as the Si precursor for the fabrication of organic–inorganic hybrid silica membranes with tunable pore sizes. BTESE–TEOS-derived silica sols were prepared via co-hydrolysis and condensation of BTESE with TEOS, and were characterized by the Fourier transform infrared (FT-IR) spectrometry and thermogravimetric–mass spectrometric (TG–MS) analysis. Single gas permeation measurements were conducted to investigate the effect of a hybrid precursor on the average pore size of organosilica membranes, which was also quantitatively evaluated via normalized Knudsen-based permeance (NKP). As a preliminary study, a binary-component gas separation of O2 and SO2 was carried out to investigate the gas transport properties, thermal stability and chemical resistance of organosilica membranes.
2. Experimental
initially mixed with ethanol. Subsequently, H2O and HCl were added and the mixture was stirred for 5 h at 50 °C to achieve a final sol. In the case of a composite organosilica sol (BTESE–TEOS), BTESE, TEOS and ethanol were mixed and vigorously stirred, followed by the addition of H2O and HCl with continuous stirring for 5 h at 50 °C to produce a hybrid sol. The molar ratios of the added reagents were (BTESE or BTESE–TEOS or TEOS)/H2O/HCl at 1/60/ 0.1, respectively, and the precursors were maintained at 5 wt%. For a BTESE–TEOS sol, the molar ratio of BTESE to TEOS was 1/1. Porous α-Al2O3 tubular substrates with a pore diameter of 1 μm, an outside diameter of 10 mm and a length of 100 mm were supplied by Mitsui Grinding Wheel Co., Ltd. and were employed as supports. To develop a smooth mesoporous particle layer for an organosilica membrane, α-Al2O3 powders (Sumitomo Chemical Co., Ltd.) with particle diameters of 1.9 and 0.2 μm were dispersed in a SiO2–ZrO2 colloidal sol. A fresh support was coated with the large-sized particle sols to eliminate the possibility of defects and relatively large pores on outer surface of support, followed by 0.5 h calcination at 550 °C, and then the coating procedure was repeated using the small-sized particle sols. A SiO2–ZrO2 intermediate layer was deposited by coating a 0.5 wt% concentration of the SiO2–ZrO2 (Si/Zr ¼1) sol that was obtained from the hydrolysis of zirconium tetrabutoxide (ZrTB) and TEOS onto the particle layer [19]. After coating and drying, the α-Al2O3 supported SiO2–ZrO2 membranes were calcined by heating to 550 °C for 0.5 h. For membrane fabrication, organosilica sols (BTESE, BTESE–TEOS) were applied onto the SiO2–ZrO2 intermediate layer by coating, followed by subsequent drying at room temperature and calcination at 300 °C for 0.5 h. 2.2. Characterization of organosilica gels The Fourier transform infrared (FT-IR) spectra for BTESE-derived and BTESE–TEOS-derived silica films coated on KBr pellets were recorded with a FT-IR spectrometer (FT/IR-4100, Jasco Co., Japan). Powdered samples of the BTESE-derived and BTESE–TEOSderived silica gels were obtained by drying organosilica sols at 40 °C under air. Thermogravimetric–mass spectrometric analysis (TG–MS, TG-DTA-410S, Rigaku Co., Japan) was conducted on the BTESE-derived and BTESE–TEOS-derived silica powders to characterize the thermal decomposition behavior of the membranes under an air flow of 300 cm3 min 1. The rate of temperature increase was 10 °C min 1 to 550 °C.
2.1. Preparation of organosilica sols and membranes 2.3. Single and binary-component gas permeation measurement BTESE, BTESE–TEOS and TEOS were used as silica precursors. For the preparation of a BTESE or TEOS sol, BTESE or TEOS was
Single gas permeation testing was performed using an
Fig. 1. Schematic diagram of the experimental setup for single and binary-component gas permeation measurement.
L. Meng et al. / Journal of Membrane Science 496 (2015) 211–218
experimental setup, as schematically shown in Fig. 1. The feed gas was introduced to the upstream of the membrane with a furnace temperature that varied between 50 and 200 °C. The pressure in the retentate stream was regulated with a pressure gauge while the permeate stream was maintained at atmospheric pressure. The gas flow rates were measured via film flow meters (SF-2U, Horiba, Japan). O2/SO2 gas separation was performed at temperatures that ranged from 50 to 300 °C. A gas mixture of O2 and SO2 was fed upstream of the membrane module, while atmospheric pressure was maintained in the downstream. The SO2 concentration in the mixed gas was measured by trapping the SO2 with hydrogen peroxide (0.09 mol/L), which was prepared by diluting 0.3 g H2O2 (30%) to 100 mL. The conductivity increase corresponded to the amount of H2SO4 formed from the oxidation of the dissolved SO2 [20,21]. The O2 that was not dissolved in the H2O2 solution passed through the traps and was measured by a film flow meter. In binary-component gas permeation, the selective permeation causes a partial pressure distribution of the component along the membrane surface. Here, the logarithmic mean pressure difference for the component, i (Δpi, lm ) was employed to calculate the driving force for gas permeation [18,22]:
Δpi, lm =
Δpi, in − Δpi, out ln
(
Δpi, in Δpi, out
)
(1)
where Δpi, in and Δpi, out refer to the difference in the partial pressure of component i between the retentate and the permeate side at the inlet and at the outlet, respectively. Multiple (more than triplicate) measurements were performed to achieve reproducible results.
3. Results and discussion 3.1. Characterization of organosilica gels The presence of organic moieties in the silica network can be characterized using FT-IR analysis. Fig. 2 shows the FT-IR spectra of silica films, which were obtained by coating KBr pellets with sols derived from BTESE, BTESE–TEOS and TEOS with firing at 300 °C under air. Typically, the peaks of Si–O bonding can be characterized by attributing the most intensive peaks, which occurred at frequencies near 1070 cm 1, to the asymmetric stretching of oxygen atoms, while the peaks at around 800 cm 1 were due to the symmetric stretching of the oxygen atoms (vibrational mode of ring stretching). These results suggest that the sol–gel reaction was successful [23]. For BTESE and BTESE–TEOS-derived silica gels,
Fig. 2. FT-IR spectra of silica films derived from BTESE, BTESE–TEOS and TEOS on KBr pellets (firing at 300 °C in air).
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the bands at 1070 cm 1 and 800 cm 1 moved to lower wave numbers, which resulted from an increase in the carbon content of the organosilica networks [24]. Absorption at about 950 and 3750 cm 1 was ascribed to the silanol bonds (Si–OH), while that at 3000–4000 cm 1 was attributed to the OH groups and adsorbed water. BTESE-derived silica gels can be identified by the characteristic peaks at 2974, 2929 and 1340 cm 1 [25], which were attributed to the stretching vibrations of –CH3 bonding in the terminal-ethoxyl groups (–OCH2CH3), –CH2– bonding in the Si–(CH2)2–Si groups and symmetric C–H stretching vibration in the –CH2– fragments, respectively. Another strong absorption band at 1278 cm 1 was assigned to the Si–C bond [26]. Both symmetric and asymmetric stretching of the C–H bonds of the organic groups was also found in mixed BTESE–TEOSS-derived silica gels that were used for the preparation of organosilica membranes. The C–H bond, however, was completely featureless in the spectrum of purely inorganic silica films prepared from TEOS. Thus, the presence of methyl and ethyl groups, together with the molecular structure of the precursor (BTESE and TEOS) indicated that a partially cross-linked polysiloxane structure with hydrocarbon units was formed in the BTESE–TEOS-derived silica gels. The thermal stability of the silica gels derived from BTESE, BTESE–TEOS and TEOS was confirmed by TG–MS, and the measured curves are shown in Fig. 3. At temperatures below 200 °C, a minor weight loss was observed that was accompanied by the release of adsorbed water. The order of weight loss was TEOS4 BTESE–TEOS4 BTESE, which might be due to the hydrophilic property of TEOS. Above 200 °C, the residual weight decreased in a rapid manner, which corresponded to the water loss from condensation of the Si–OH groups when forming Si–O–Si bonds, as reported previously [27]. Additionally, there was a sharper decrease in weight loss for the BTESE-derived silica gel at 300–550 °C, compared with the silica gels derived from BTESE– TEOS and TEOS. This could be attributed to the degradation and/or combustion of ethoxide groups (–OCH2CH3) and ethylene bridges (–CH2–CH2–), according to the peak intensity at given mass-tocharge ratios (m/z) as a function of temperature that reported in our previous studies [18]. Thus, an increase in the thermal stability at 300–550 °C could be observed in the silica gels produced by the co-hydrolysis and condensation of BTESE with TEOS, compared with BTESE-derived silica gels. Moreover, the total weight loss of a BTESE or BTESE–TEOS gel was lower than 7% at 300 °C during the decomposition processes, which agreed with the previous report on bis(triethoxysilyl)methane (BTESM)-derived silica gels [28] and strongly demonstrated that organosilica membranes are highly stable in an oxidizing atmosphere at 300 °C.
Fig. 3. TG curve of silica gels derived from BTESE, BTESE–TEOS and TEOS (in air).
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organosilica networks is close to the kinetic diameter of He and H2, these molecules permeate via an activated permeation mechanism, whereby the permeance of He is higher than that of H2 because of the smaller kinetic diameter of He. Since a larger permeance of He than that of H2 was observed for the BTESE–TEOSderived silica membrane, and for which the permeance ratio of He/H2 was not as high as that for TEOS-derived silica membranes, the organosilica membranes derived from BTESE–TEOS possibly formed a separation layer with pores larger than those prepared from TEOS but smaller than those prepared from BTESE. The average pore size of a microporous membrane, dp , can be obtained by the normalized Knudsen-based permeance (NKP) method [28,32]. NKP is the ratio of the permeance of component i to the permeance predicted using He, which is the smallest molecule, under the Knudsen diffusion mechanism, as expressed in Eq. (2).
NKP = Fig. 4. Molecular diameter dependency of gas permeances for organosilica membranes derived from BTESE, BTESE–TEOS, and previously reported TEOS-derived silica membranes [29] at 200 °C.
3.2. Gas permeation properties of organosilica membranes The as-prepared organosilica membrane includes an α-Al2O3 support layer, α-Al2O3 and SiO2-ZrO2 intermediate layers, and an organosilica layer which could be clearly identifiable, as shown in our recent work [12,14,28]. The gas permeation properties of the organosilica membranes were characterized by the values for single-gas permeance, which were given as a function of the molecular diameter of gas molecules. It’s worth mentioning that the separation performances of silica-based membranes reported in our previous studies have been widely demonstrated to be reproducible, which indicates the preparation process of silica-based membranes is controllable under optimized conditions. Fig. 4 shows the molecular diameter dependency of gas permeance for organosilica membranes derived from BTESE and BTESE–TEOS, and for a previously reported TEOS-derived silica membrane [29], at 200 °C. The TEOS-derived silica membrane showed a permeance to H2 of 2.0 10 7 mol m 2 s 1 Pa 1 with a H2/N2 selectivity as high as 600. The SF6 permeance was below the detection limitations of the gas permeation system, suggesting a pore size that was smaller than 0.4 nm. For a BTESE-derived silica membrane, a one order of magnitude higher H2 permeance of 1.4 10 6 mol m 2 s 1 Pa 1 was obtained together with selectivities of H2/C3H8 and H2/SF6 of 2000 and 130,000, respectively. A BTESE–TEOS-derived silica membrane showed a H2/C3H8 permeance ratio of up to 15,000, which was larger than the H2/C3H8 selectivity for a BTESE-derived silica membrane. These gas separation performances strongly demonstrated that a smaller pore size was achieved in the BTESE–TEOS-derived silica membrane. Moreover, the organosilica membranes derived from BTESE and BTESE–TEOS exhibited a relatively lower H2/N2 selectivity (o20), which could be attributed to the looser structure of organosilica networks [17]. According to the gas-translation model, the permeation of nonadsorbable or weakly adsorbable gases depends on the ratio of the kinetic diameter of the diffusing molecule, dk , to the average pore size of the membrane, dp [30,31]. For He and H2, when the pore size of the organosilica network is much larger than their kinetic diameters, the Knudsen permeation mechanism dominates the permeation behavior, and the H2 permeance will be higher than the He permeance due to the smaller molecular weight of H2. Fig. 4 shows the Knudsen selectivity of H2/He for a BTESE-derived silica membrane, and confirms the larger pore size of its organosilica network. On the other hand, when the pore size of
Pi PHe
Mi MHe
3
=
( dp − dk, i ) exp ⎛⎜ 3 ⎝ ( dp − dk, He )
−
EP, i − EP, He ⎞ ⎟ ⎠ RT
(2)
where Pi , Mi , EP , i , dk, i represents the gas permeance, molecular weight, activation energy and molecular size for the permeating components, respectively. Herein, the EP , i for different gases is assumed to be similar to EP , He due to the large pore sizes of organosilica membranes [28]. Thus, the membrane pore size, dp , can be calculated by the following equation (Eq. (3)).
NKP =
( dp − dk, i )3 ( dp − dk, He )3
(3)
Fig. 5 shows the NKP plot at 200 °C for organosilica membranes derived from BTESE and BTESE–TEOS membranes, as well as for previously reported TEOS-derived silica membranes [29], as a function of molecular size. Judging from the fitting curves based on Eq. (3), the average pore size of a BTESE–TEOS-derived silica membrane ( dp ¼ 0.43 nm) was larger than that for a TEOS-derived silica membrane ( dp ¼0.34 nm) but smaller than that for a BTESEderived silica membrane ( dp ¼0.51 nm). The temperature dependence on gas permeation is widely utilized to investigate the gas transport properties of microporous membranes [33,34]. Fig. 6 shows the temperature dependence on gas permeance for a BTESE–TEOS-derived silica membrane at temperatures ranging from 50 to 200 °C. In microporous silica membranes, the Knudsen permeation of N2 and CH4 molecules is often reported with the existence of pinholes which were created by the siloxane ring with large sizes and/or spaces between colloidal particles [35]. However, an organosilica membrane derived from BTESE–TEOS showed an activated permeation behavior for
Fig. 5. NKP plot at 200 °C as a function of molecular size for organosilica membranes derived from BTESE and BTESE–TEOS, and for previously reported TEOSderived silica membranes [29]; symbols are experimental data and curves are calculated based on Eq. (3).
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differences in the minimum units of membrane structures. Fig. 7 schematically shows the amorphous silica networks derived from TEOS, BTESE–TEOS and BTESE. The Si–C–C–Si–O–Si, Si–C–C–Si and Si–O–Si bonds coexist in organosilica membranes derived from BTESE–TEOS, because the organic groups between the Si atoms in the BTESE precursor are not hydrolyzed during the hydrolysis and polymerization. Thus, a BTESE–TEOS precursor forms a looser structure for the minimum unit in the organic–inorganic hybrid silica membrane, and the average pore size of an organosilica network should be larger than that derived from TEOS, but smaller than that derived from BTESE. Therefore we concluded that the membrane pore size was successfully tuned by using the composite organosilica (BTESE–TEOS) precursor, as quantitatively presented using NKP in Fig. 5. Fig. 6. Temperature dependence on gas permeance for a BTESE–TEOS-derived silica membrane at temperatures ranging from 50 to 200 °C (curves were fitted with Eq. (4)).
N2 and CH4 molecules (permeance increased with an increase in temperature), indicating that a sharp pore size distribution with fewer pinholes was achieved in the membrane structure [36]. The permeance of both CO2 and C3H8 increased with decreasing temperature, suggesting their permeation behavior was governed by the surface diffusion mechanism, in which the gas molecules adsorbed onto the membrane surface and subsequently diffused along a concentration gradient. The activation energy, EP , i , and membrane structural factor, k 0 , were obtained using Eq. (4) with experimental data from the temperature dependence on gas permeance.
Pi =
⎛ EP, i ⎞ k0 exp ⎜ − ⎟ ⎝ RT ⎠ Mi RT
(4)
where i refers to the gas component i , P is the permeance of the gas component, M represents the molecular weight of the gas component, R is the gas constant, and T is the temperature. Table 1 summarizes the activation energies of He permeation and the gas permeation ratios for membranes derived from BTESE [9,31], TEOS [37,38] and BTESE–TEOS. The activation energy of gas permeation is described as the interaction between the pore wall and gas molecules, which can be used for the evaluation of membrane pore size, i.e., diffusion in tighter pores requires a higher activation energy of permeation [39]. The activation energy for the permeation of He, a non-adsorptive gas molecule, through BTESE– TEOS-derived silica membranes was higher than that through BTESE-derived silica membranes. Therefore, it is reasonable to conclude that the pore sizes of organosilica membranes derived from BTESE–TEOS are larger than that derived from TEOS but smaller than that derived from BTESE. This trend can also be confirmed via He/H2 and H2/N2 selectivities. The variations in the pore size distribution of membranes fabricated by different precursors can be illustrated by the
3.3. Binary-component gas separation of O2/SO2 for organosilica membranes Separation for a binary gas mixture consisting of O2 and SO2 was conducted to evaluate the separation performance of organosilica membranes derived from BTESE and BTESE–TEOS. Generally, the membrane separation performance depends on several operating conditions such as the temperature, pressure and molar ratio of the feed gas. In the present study, binary gas separation was performed with a molar ratio of SO2/O2 ¼1/10, and the feed side was pressurized while the permeate side was kept at ambient pressure. Fig. 8 illustrates the O2/SO2 selectivity for BTESE and BTESE–TEOS-derived silica membranes as a function of O2 permeance, and compares the results with those of ZrO2 membranes [6], poly(ether–b–amide) (PEBAX3533)/polyetherimide (PEI) composite membranes [40], and poly(amide-6–b–ethylene oxide) (PEBA1657)/PEI composite membranes [41]. For ZrO2 membranes, the O2/SO2 separation was conducted at 130 °C, while for the PEBAX3533/PEI and PEBA1657/PEI membranes, the values for the permeance of SO2 and N2 were measured at 30 and 20 °C, respectively. These PEBA membranes exhibited a high permeance for SO2 due to a strong affinity (high solubility) between polarizable molecules and the ether block in PEBA [40], particularly at low temperatures. The BTESE–TEOS and BTESE-derived silica membranes showed an impressive combination of permeance and selectivity for O2 at 200 °C, which was beyond the Knudsen selectivity. BTESE-derived silica membranes showed an O2 permeance of 5.0 10 8 mol m 2 s 1 Pa 1 with an O2/SO2 permeation ratio of 3.6. The organosilica membranes derived from BTESE–TEOS exhibited an O2/SO2 selectivity of up to 5.9 while maintaining an O2 permeance higher than 1.7 10 8 mol m 2 s 1 Pa 1, which indicated that the pore size control in amorphous silica networks using BTESE–TEOS as a precursor was effective in improving the selective separation of O2/SO2. Fig. 9 shows the time-course of gas permeance for a BTESE– TEOS-derived silica membrane. The permeance of single gases (He,
Table 1 Activation energies of He permeation and gas permeation ratios for various silica-based membranes. Membrane materials
EP [kJ mol 1]
He/H2 [dimensionless]
H2/N2 [dimensionless]
Temperature [°C]
Reference
BTESE
1.8 3.4 5.9
– 0.7 0.8
16.2 7.0 15.6
250 200 200
[9] [31] [31]
TEOS
12.4 14.6
6.6 5.5
– 235.3
200 300
[37] [38]
BTESE–TEOS
10.0
2.3
16.3
200
this work
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Fig. 7. Schematic images of amorphous silica networks derived from (a) TEOS, (b) BTESE–TEOS and (c) BTESE.
Fig. 8. O2/SO2 or N2/SO2 selectivity for various membranes as a function of O2 or N2 permeance.
significant change in the structure of the BTESE–TEOS-derived silica membrane. When the ethoxy groups (–OCH2CH3) that were unreacted during the hydrolysis and polymerization, and/or the Si–C–C–Si linking units, were oxidized, gas permeance would have become higher. Therefore, the absence of an obvious increase in O2 permeance at 300 °C confirmed the stable membrane structure. In addition, no absorption and/or reaction between SO2 and the organosilica network was observed [42]. Fig. 10 shows the values for gas permeance at 200, 250 and 300 °C for BTESE–TEOS-derived silica membranes before and after the separation of O2/SO2 as a function of molecular diameter. After O2/SO2 separation at a given temperature from 200 to 300 °C, no significant change could be observed in gas permeance, confirming the thermal and chemical stability of BTESE–TEOS-derived silica membranes. Moreover, the CF4 permeance remained virtually unchanged during exposure to O2 and SO2, indicating that the BTESE–TEOS-derived silica membranes were free of pinholes and voids. It should be noted that the gas permeance was increased with increasing temperature via the activated permeation mechanism, which is discussed in Fig. 6. Fig. 11 shows the temperature dependence of gas permeance
Fig. 9. Time-course of gas permeance at temperatures ranging from 200 to 300 °C for a BTESE–TEOS-derived silica membrane.
H2, N2, CF4) was measured before and after the binary gas separation of O2/SO2, and approximately 60 and 150 min was conducted for each single and binary gas mixture, respectively. The temperature was gradually increased in a stepwise fashion from 200 to 300 °C to confirm the thermal stability of the organosilica membrane under the oxidizing atmosphere of a SO2 flow (partial pressure of SO2: 14 kPa), which also established its chemical stability. All the measured values for single gas permeance were similar before and after exposure to O2 and SO2, indicating no
Fig. 10. Molecular diameter dependency of gas permeance at temperatures ranging from 200 to 300 °C for BTESE–TEOS-derived silica membranes before and after the separation of O2/SO2.
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217
Table 3 Kinetic diameters and L-J length constants for gas molecules. Kinetic diameter, [8] dk [nm]
He H2 O2 SO2 N2 CH4 C3H8 CF4 SF6 a
Fig. 11. Temperature dependence on the permeance of O2 and SO2 for a BTESE– TEOS-derived silica membrane at temperatures ranging from 150 to 300 °C.
(O2 and SO2) at temperatures ranging from 150 to 300 °C for a BTESE–TEOS-derived silica membrane. The O2 permeance became higher with increasing temperature, suggesting that it was governed by an activated permeation mechanism. The values for SO2 permeance were on the order of 10 9 mol m 2 s 1 Pa 1 and increased with decreasing temperature. This is a typical trend whereby a surface diffusion mechanism at low temperatures creates a larger degree of SO2 adsorption on the membrane surface, which allows SO2 to diffuse through a membrane via the concentration dependency [43]. The O2/SO2 permeance ratio reached 7.3 in this temperature range, and was increased with increasing temperature because the temperature dependence of O2 permeance was larger than that of SO2. Thus, the BTESE–TEOS-derived silica membrane is an attractive alternative material for the separation of O2/SO2 mixed gases at temperatures up to 300 °C. The effective molecular size of the diffusing component is an important parameter for the development of membranes and membrane processes. To date, only a limited number of papers have reported the permeation properties of SO2, although this information is critical for a determination of the effective molecular size of SO2 above 250 °C where the molecular sieving effect dominates the permeation behavior. Table 2 summarizes the permeance values of O2, N2 and SO2 in BTESE–TEOS-derived silica membranes. In this work, the BTESE–TEOS-derived silica membrane showed a lower permeance for SO2 than that for N2 at 250 and 300 °C, although the kinetic diameter of SO2 ( dk, SO2 ¼ 0.360 nm) was smaller than that of N2 ( dk, N2 ¼ 0.364 nm). However, in addition to the kinetic diameter, the Lennard-Jones (LJ) length constant [44] was also well accepted for the molecular size of gas components. Table 3 lists the kinetic diameter, dk , and L-J length constant, σLJ , for the gas molecules examined in the present study. The kinetic diameter, dk , was the minimum crosssectional diameter of gas molecules, while the L-J length constant (collision diameter), σLJ , was calculated either from the transport properties (viscosity, thermal conductivity) or from the second virial coefficients [44]. The kinetic diameters of the inorganic gas
Table 2 Gas permeance of various gases in a BTESE–TEOS-derived silica membrane. Kinetic diameter, [8] dk [nm]
O2 SO2 N2
0.346 0.36 0.364
Permeance [10 9 mol m 2 s 1 Pa 1] 200 °C
250 °C
300 °C
17.2 4.0 2.8
21.7 3.6 7.0
28.7 3.9 10.9
b
0.26 0.289 0.346 0.36 0.364 0.38 0.43 0.47 0.55
L-J length constant, [44] σLJ [nm]
σLJ a
σLJ b
0.258 0.292 0.343 0.429 0.368 0.382 0.506
0.263 0.287 0.346 0.369 0.382 0.563 0.47 0.551
σLJ / dk
1.00 1.00 1.00 1.19 1.01 1.01 1.24 1.00 1.00
L-J length constant calculated from the viscosity. L-J length constant determined derived from the second virial coefficient.
molecules approximated the L-J length constant, while for SO2 and hydrocarbons such as C3H8 the difference between dk and σLJ became larger, which suggested that a change in the shape of molecules can be predicted by the ratio of σLJ / dk [30]. When σLJ / dk equaled to 1, the molecules were considered spherical while SO2 and long branched molecules (hydrocarbons) showed a σLJ / dk that was higher than 1. Moreover, the σLJ of SO2 was larger than that of N2, which was consistent with the above-mentioned values for gas permeance. These results suggest that the L-J length constant should be used as the effective molecular size for SO2 permeating through a silica network. We can therefore conclude that the diffusion of a gas molecule depends not only on its minimum crosssectional diameter, but also on its effective length, which results in a molecular sieving effect for O2/SO2 separation.
4. Conclusions A novel strategy to control the pore size of organosilica membranes was developed by the introduction of a mixed BTESE–TEOS precursor. Organic–inorganic hybrid silica membranes with tunable pore sizes were prepared via a sol–gel processing, and were proposed for O2/SO2 gas separation. FT-IR confirmed the formation of a partially cross-linked polysiloxane structure with hydrocarbon units in a BTESE–TEOS-derived silica network produced by the cohydrolysis and condensation of BTESE with TEOS, which also showed an improved thermal stability compared with a BTESEderived silica network. Pore size distribution, as determined by the single gas permeation measurement, suggested that the order of membrane pore sizes was BTESE 4BTESE–TEOS 4TEOS, because of the differences in the minimum units when these were applied as precursors, which was consistent with the pore sizes predicted by the NKP plot. The organosilica membranes derived from BTESE– TEOS exhibited superior O2 permeance with an O2/SO2 selectivity as high as 7.3 in temperatures ranging from 150 to 300 °C, which demonstrated the potential of BTESE–TEOS-derived silica membranes for the selective separation of O2/SO2.
Acknowledgments This work was supported by the Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), of the Energy Carrier Project of the Japan Science and Technology Agency (JST).
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Nomenclature
dk dp Ep k0 M p P R T
molecular size, m pore diameter, m Kinetic energy, J mol 1 structural parameter, dimensionless molecular weight, g mol 1 pressure, Pa gas permeance, mol m 2 s 1 Pa 1 gas constant, J mol 1 K 1 absolute temperature, K
Greek symbols
σLJ
Lennard-Jones length constant, m
Subscripts
i lm in out
component i logarithmic mean inlet of the membrane module outlet of the membrane module
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