- Email: [email protected]
Fabrication of highly selective organosilica membrane for gas separation by mixing bis(triethoxysilyl)ethane with methyltriethoxysilane
Separation and Purification Technology 222 (2019) 162–167
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Fabrication of highly selective organosilica membrane for gas separation by mixing bis(triethoxysilyl)ethane with methyltriethoxysilane ⁎
Shaohua Chaia,b, Hongbin Dub, Yayun Zhaoa,b, Yichao Linb, Chunlong Kongb, , Liang Chenb, a b
T
⁎
University of Chinese Academy Sciences, 10049 Beijing, PR China Institute of New Energy Technology, Ningbo Institute of Material Technology and Engineering, CAS, 1219 Zhongguan West Road, 315201 Ningbo, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Organosilica BTESE-MTES Hybrid silica membrane Selectivity Gas separation
In the present study, the mixtures of bis(triethoxysilyl)ethane (BTESE) and methyltriethoxysilane (MTES) were proposed to design highly selective hybrid silica membranes for gas separation by the sol-gel method. The role of MTES in the hybrid membranes was investigated by changing the ratio of the two precursors. The membrane derived from mixture of 1 BTESE and 3 MTES exhibited the highest H2 and CO2 permeance with 7.99 × 10−7 mol m−2 s−1 Pa−1 and 3.53 × 10−7 mol m−2 s−1 Pa−1, respectively. In contrast, the superior selectivity was achieved on the membrane derived from mixture of 1 BTESE and 2 MTES, in which the H2/N2, H2/CH4, CO2/N2, and CO2/CH4 were found to be 64.4, 73.6, 28.5, and 32.6, respectively, which indicated that the structures of BTESE-MTES hybrid membranes can be adjusted by tuning the ratios of MTES to BTESE. Moreover, the membranes also performed well in the binary-component mixture gases and high-temperature test.
1. Introduction In the process of ammonia synthesis, the exhaust gas should be discharged to ensure a smooth reaction, but H2, CH4 and some rare gases contained in it can be recycled and reused [1]. On the other hand, carbon dioxide in natural gas, is believed to reduce the conversion rate and energy content of natural gas, thus it should be separated as much as possible [2]. However, the separation and purification of gas mixtures that are similar to the one previously described in the chemical industry always consume considerable energy. Among a series of methods solving these problems, membrane separation technology attracts more attention than the others due to its advantages of high efficiency, simplicity and low energy cost [3]. Silica membranes have been widely studied in gas- and liquid- separation due to their chemical stability, thermal stability, and improved mechanical strength. They can be utilized under aggressive conditions where organic polymer membranes are prone to failures [4]. Two methods for preparing the separation membrane by silica have been proposed: chemical vapor deposition (CVD) and sol-gel method [5]. The pore structure of the silica membranes can be controlled to a large extent by the sol-gel method, and the pore size of the membrane can be accurately controlled at approximately 0.3 nm, which is effective in the separation of small molecular gases [6]. Moreover, among the numerous precursors, tetraethoxysilane (TEOS) is commonly used to
⁎
prepare sol-gel-derived silica membranes, but its practical application is restricted due to poor hydrothermal stability which is related to the large number of hydroxyl groups that occupy the surface and are responsible for the strong H-bonding with water. In addition, SieOeSi bonds will dissociate in humid environments, especially at high temperatures [7–9]. Accordingly, some research groups have proposed to take advantage of organic-inorganic hybrid alkoxide that contains organic groups, which cannot be hydrolyzed, such as methyltriethoxysilane (MTES) [10–15]. De Vos et al. [13] first successfully fabricated hydrophobic silica membranes derived by copolymerization of TEOS and MTES. These membranes were 10 times more hydrophobic than TEOS–derived membranes because methyl groups can effectively eliminate the surface silanol groups which are hydrophilic. Kanezashi et al. [16] prepared hybrid silica membranes by using bis(triethoxysilyl)ethane(BTESE) as precursor. The presence of SieCeCeSi bonds which cannot be hydrolyzed in the silica networks, led to a loose silica network and enhanced hydrophobicity [17,18]. The BTESE-derived membranes showed extremely high H2 permeance (∼10−5 mol m−2 s−1 Pa−1), but a moderate selectivity for H2/N2, compared with inorganic silica membranes. The membrane pore sizes can be controlled by the size of the linking units which comprise the organic-inorganic hybrid alkoxides [15]. Commonly, the membrane prepared by the larger linking unit (Sie (CH2)2eSi > SieCH2eSi), the gas permeance is higher [16].
Corresponding authors. E-mail addresses: [email protected] (C. Kong), [email protected] (L. Chen).
https://doi.org/10.1016/j.seppur.2019.04.039 Received 31 October 2018; Received in revised form 13 April 2019; Accepted 13 April 2019 Available online 13 April 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
Separation and Purification Technology 222 (2019) 162–167
S. Chai, et al.
membranes was acquired by SEM (the Quanta FEG 250, FEI Company).
Therefore, the H2 permeance of BTESE-derived membranes was one order of magnitude higher than that of traditional pure silica membranes. On the basis of this result, tuning the pore size by adding a different alkoxide to the original precursors has been proposed [13,14,19]. Castricum et al. [20] observed that a series of unsupported silica materials prepared with different ratios of MTES and TEOS, with increasing MTES concentration, smaller water uptakes for materials, and the gels prepared from a combination of both precursors showed smaller micropore sizes. This result corroborates that changing the ratio of precursors used to prepare hybrid silica gels, can also effectively control the pore structure of the material. Inspired by this, we propose to design hybrid silica membranes by mixing MTES with BTESE. To the best of our knowledge, no report is currently available about BTESEMTES hybrid membranes used for gas separation, albeit Castricum et al. [14,15,21] fabricated a hybrid silica membrane by mixing BTESE with MTES for dehydration of n-butanol (5 wt% water). In the present work, a series of precursors prepared with different ratios of BTESE and MTES were designed to fabricate organic-inorganic hybrid silica membranes. Single gas permeances of H2, CO2, N2, and CH4 were first measured at 30 °C to acquire information on the performance of as-prepared membranes for separating small molecules. The effect of MTES concentration on the single gas permeances for silica membranes was evaluated. After the comparison, binary-component gas separations and hightemperature test were conducted on the membrane prepared by the optimum ratio of BTESE and MTES.
2.3. Single and binary-component gas permeation test Before we evaluated the permeation properties, all the membranes were dried at 120 °C in a vacuum oven for 24 h to remove the water adsorbed in the membranes. Subsequently, the membranes were mounted in a stainless steel module, and sealed at each end with silicone O-rings. The gas separation performance was measured on a homemade permeance apparatus. The feed and permeate pressures were kept at 0.3 MPa and 0.1 MPa, respectively. The permeation flow rate was measured by a soap film bubble flow meter. The single gas permeations of H2, CO2, N2, and CH4 were evaluated at 30 °C. After a comparison of single gas permeance and ideal selectivity of all membranes, the binary gas separation of H2/N2, CO2/CH4 and hightemperature test were further conducted on the most comprehensive membrane. The temperature dependence of permeance for the membrane was studied at temperatures ranging from 30 °C to 150 °C. For binary gas test, equimolar mixture gases (both 50 mL/min) were fed on the feed side of the permeation module. Sweep gas (100 mL/min) was fed on the permeate side to keep the concentration of permeating gas as low as possible thus providing a driving force for gas permeation. The pressure drop through the membrane was kept at 0.1 MPa. The fluxes of feed and sweep gases were determined with mass flow controllers, and a calibrated gas chromotograph (Agilent 7890 B) was used to measure the concentration of mixed gases on the permeate side. The gas mixture selectivity of the membrane was calculated by the following relationship:
2. Experimental 2.1. Preparation of hybrid sols and membranes
αi / j = In the present work, six types of sols were prepared. The molar ratios of BTESE to MTES were 1/0 (pure BTESE), 2/1, 1/1, 1/2, 1/3, and 0/1 (pure MTES), respectively. In the preparation procedure of BTESE-MTES hybrid sols, BTESE (95%, Aladdin) and MTES (98%, Aladdin) were mixed in established ratios and added to dry ethanol, with a molar ratio of silane/ethanol = 1/35–40. A mixture of water, nitric acid, and ethanol was added dropwise to the solutions under vigorous stirring in an ice bath. All the final solutions with molar ratios of silane/ethanol/H2O/H+ = 1:46:60:0.1. After 2 h, the solutions were kept in closed systems under continuous stirring at 60 °C for 1 h to develop hybrid sols. As for pure MTES-derived sols, nitric acid was replaced by acetic acid, and ice bath was not needed, the reaction time of the final solutions was extended to 8 h. The preparation of pure BTESE-derived sols and specific membranes fabrication has been presented in the previous report [22]. Porous αalumina tubes (average pore size: 1.5 μm, outside diameter: 12 mm) were employed as supports. The fresh supports were coated with colloidal silica (Aldrich) to eliminate the large pores and defects, followed by 20 min calcination at 550 °C. Then the large-sized particle pure BTESE-derived sols were coated on them by 20 min calcination at 300 °C, as the intermediate layer. After that, the prepared hybrid sols were coated onto the intermediate layers, followed by calcining at 300 °C for 20 min. The coating and calcination process was repeated three times in order to obtain separation layers. The corresponding hybrid membranes were named M-1/0, M-2/1, M-1/1, M-1/2, M-1/3, and M-0/1, respectively, the powders prepared by the sols were named as P-1/0, P-2/1, P-1/1, P-1/2, P-1/3, and P-0/1.
yi, perm / yj, perm yi, feed / yj, feed
where yi, perm and yj, perm are the mole fractions of the components in the permeate, yi, feed and yj, feed denote their corresponding mole fractions in the feed. And the gas permeance was defined as follows:
Pi =
Fi 1 × A ΔP
Fi (mol/s) is the molar flow rate of the permeate gas, A (m2) is the area of membrane and ΔP is the driving force through the membrane. The selectivity and permeance reported in the present work are an average of at least three membranes. 3. Results and discussion 3.1. Characterization of organic-inorganic hybrid gels Fig. 1 shows the FT-IR spectra of silica films derived from six ratios of BTESE and MTES, which were obtained by mixing KBr with powders prepared by calcining the precursors at 300 °C under air. All the films show strong peaks near 1070 cm−1, which are indicative for the asymmetric stretching of oxygen atoms, whereas the peaks at around 800 cm−1 were due to the symmetric stretching of the oxygen atoms (vibrational mode of ring stretching). This result proves that all the solgel reactions were successful [23]. Pure BTESE-derived powders can be identified by the characteristic peak at 2929 cm−1, which was attributed to the stretching vibrations of eCH2e bonding in the Sie (CH2)2eSi groups. The absorption at around 1340 cm−1 was attributed to the symmetric CeH stretching vibration in the eCH2e fragments. Pure MTES-derived powders were characterized by a strong peak around 1280 cm−1, which was ascribed to the presence of methyl groups [24], the peak that appeared on pure BTESE-derived powders in the same position can be attributed to the Si-C bond [25]. Another absorption peak of 2974 cm−1 was assigned to the eCH3 bonding in the eOCH2CH3 [26]. Both methyl and eCH2e groups could be found in all the BTESE-MTES mixture, which indicated that the cross-linked polysiloxane structure with hydrocarbon units was formed in the BTESE/
2.2. Characterization The powders of sols derived from different ratios of BTESE and MTES prepared by calcination at 300 °C were characterized by Fourier transform infrared spectroscopy (FT-IR, Nicolet 6700, Thermo Company) and N2 adsorption/desorption isotherms at 77 K on Accelerated Surface Area and Porosimetry System (ASAP 2020M, Micromeritics Company). The morphology of each as prepared 163
Separation and Purification Technology 222 (2019) 162–167
S. Chai, et al.
images of the membranes. No cracks or pinholes were observed on the surface from the surface image, indicating continuous and compact membrane was fabricated. As anticipated, three layers can be observed from the cross-sectional image, corresponding to the separation layer, BTESE intermediate layer and SiO2 layer, respectively. And the adhesion between the layers was quite good. The SiO2 layer was used to modify the substrate. We can observe that some SiO2 particles penetrated into the α-Al2O3 substrate. Large-sized BTESE-derived particles can further fill the defects on the SiO2 layer, but did not show any gas separation performance [22]. It is difficult to find penetration of the component of separation layer into the BTESE intermediate layer, which proves the superiority of BTESE as the intermediate layer and preparation method. The thickness of the separation layer was approximately 300 nm. 3.3. Gas permeation test of the hybrid membranes Figs. 4 and 5 summarize the single gas permeance and ideal selectivity of all the hybrid membranes which were prepared by six ratios, respectively. It can be seen that all the materials were effective for gas separation, H2 and CO2 permeances through the materials were obviously higher than that of N2 and CH4. Compared with the gas permeances for BTESE membrane, M-2/1 and M-1/1 were all lower, but M1/2 and M-1/3 were higher. Similarly, with 50% MTES as the dividing line, MTES tend to play a different role. MTES have lower functionality, which contribute to reduce the porosity due to the possibility of crosslinking during the condensation of BTESE and MTES was considerably reduced [28]. And owing to the space occupation by methyl groups, the effective pore space in the silica matrix for gas permeation was decreased [27]. These effects were thought to be mainly responsible for the lower gas permeance through BTESE-MTES hybrid membranes when the proportion of MTES was below 50%. In the case of hybrid membranes prepared from high concentration of MTES, several possible reasons could account for their high gas permeance. First, fast condensation rate of a great deal of MTES in the formation of silica network would make up for the limited possibility of cross-linking during the condensation of BTESE and MTES. Moreover, the pores in the silica matrix available for gas permeation may be formed since the aggregation of methyl groups in the molecular level between the silica networks. Therefore, the high permeances for rich-MTES hybrid membranes were due to the porosity in the silica matrix increased. In all of membranes we studied in this work, M-1/3 showed the highest permeance to H2 of 7.99 × 10−7 mol m−2 s−1 Pa−1 and CO2 permeance of 3.53 × 10−7 mol m−2 s−1 Pa−1. M-1/2 showed the best ideal separation factors of H2/N2, H2/CH4, CO2/N2, and CO2/CH4, which are determined as the ratio of the single component permeances, were found to be 64.4, 73.6, 28.5, and 32.6, respectively, which are much higher than the corresponding knudson coefficients and far exceeded some of the previous report about hybrid organosilica membrane [16,19,21,30]. This may related to the formation of relatively uniform and stable sesquisiloxane-type structure because of the good match between BTESE and MTES, resulting in the well-distributed pore size, which can sieve the gas more effectively according to their dynamic diameters. Given the good permeance and outstanding ideal selectivity of M-1/ 2, 1 BTESE and 2 MTES were considered to produce relatively perfect pore structure, which were identified as the optimum ratio in all comparisons. The binary gas separation of H2/N2, CO2/CH4 and hightemperature test were further conducted. The mixed gases permeances and separation factors at 30 °C and 0.1 MPa of three M-1/2 were summarized in Table 1, which proved that the excellent reproducibility of the membrane fabrication. And the temperature dependence of gas permeance for the M-1/2 was studied at temperatures ranging from 30 °C to 150 °C (Fig. 6). All the permeance of H2, N2 and CH4 became higher with increasing temperature, indicating that the pore size of silica networks is close to the tested gases in molecular size. Hence,
Fig. 1. FT-IR spectra of the powders prepared with different ratios of BTESE and MTES.
MTES-derived powders. And the peak intensity of methyl groups increased gradually with the increase of MTES in the sol, revealing that additional methyl groups are in the silica networks. As shown in Fig. 2, all the powders showed high N2 adsorption except pure MTES-derived powders, which was also consistent with the result of [27], because the space occupation of the methyl groups in the pure MTES-derived silica network lead to the inaccessible pores. N2 adsorption of BTESE-MTES powders were lower than that of BTESE when the proportion of MTES was below 50%, indicating fewer pores available in the BTESE-MTES hybrid silica for N2 adsorption, the methyl groups might still act as barrier in the BTESE-MTES silica network so that some pores were blocked. When the proportion of MTES was higher than 50%, the pores available in the silica for N2 absorption obviously increased, suggesting that high concentration of MTES may support the formation of pores.
3.2. Characterization of hybrid membranes Fig. 3 shows the representative surface and cross-sectional SEM
Fig. 2. N2 adsorption isotherms of the powders derived from different sols. 164
Separation and Purification Technology 222 (2019) 162–167
S. Chai, et al.
Fig. 3. Surface and cross-sectional SEM images of the membranes.
organic-inorganic silica materials. A larger amount of CO2 can be absorbed by silica at a lower temperature [29], therefore the mixture selectivity of CO2/CH4 decreased with increasing temperature. The H2/ N2 mixture selectivity also decreased with the increase of operation temperature, because the temperature dependence of N2 was larger than that of H2. Equimolar H2/N2 and CO2/CH4 mixture selectivities at 30 °C were found to be 19.0 and 14.8, respectively. The results were much lower than the ideal selectivity, which is in line with our previous work [22]. It could be attributed to the presence of a second gas in the membrane which has significant effects on the interaction between the gas molecules and the pores. Large molecules (N2 and CH4) could slow down the permeation of small molecules (H2 and CO2) [30]. Besides, in this work, the aggregate methyl groups in the silica networks may also produce a negative effect on the transportation of gases. The H2/CH4 and CO2/N2 mixed gases were also investigated on the membranes, and the corresponding selectivities at 30 °C were 31.5 and 14.4, respectively, which were also lower than the ideal values. Furthermore, M-1/2 has been tested for the separation of equimolar CO2/CH4 mixture at 150 °C for more than 24 h, both the CO2 permeance and CO2/CH4 selectivity kept unchanged (Fig. 7), indicating that the thermal stability of the membrane was relatively good.
Fig. 4. Molecular diameter dependency of gas permeances for six types of membranes.
4. Conclusion We successfully prepared BTESE-MTES hybrid silica membranes for gas separation by the sol-gel method. The role of MTES in the BTESEMTES hybrid membranes was investigated by changing the ratio of the precursors. It seemed that MTES served a different influence: the hybrid membranes showed higher gas permeance compared with the silica membranes prepared by BTESE when the proportion of MTES was higher than 50%, while with lower MTES content may reduce the porosity and effective pore size of the hybrid membranes, which lead to lower gas permeance. The formation of uniform pore size distribution may explain the high ideal selectivity of membranes derived from the copolymerization of 1 BTESE and 2 MTES, and the membranes were still effective for selective separation in mixture gases, though the mixture selectivities were lower than the ideal separation factors. Overall, the membrane structures were tuned successfully by changing the ratios of BTESE and MTES, which provides a valuable insight for further development in gas separation applications.
Fig. 5. Ideal selectivities of six types of membranes which determined as the ratio of the single component permeances.
these molecules can permeate the hybrid membrane via an activated mechanism. But the CO2 permeance decreased from 8.64 × 10−8 mol m−2 s−1 Pa−1 to 8.07 × 10−8 mol m−2 s−1 Pa−1 when the temperature increased from 30 °C to 150 °C, which proved again that compared with other gases, a strong interaction exits between CO2 and the hybrid
Acknowledgements We acknowledge the financial support from NSFC (No. 51672289 and 51502311), the aided program for science and technology innovative research team of Ningbo municipality (No. 2014B81004). 165
Separation and Purification Technology 222 (2019) 162–167
S. Chai, et al.
Table 1 Mixed gases permeances and separation factors at 30 °C and 0.1 MPa for M-1/2. Mixed gas separation performances of M-1/2 GasA/B
H2/N2
CO2/CH4
No.
Permeances (A)(mol·m−2·s−1·Pa−1)
Permeances (B)(mol·m−2·s−1·Pa−1)
SF
Permeances (A)(mol·m−2·s−1·Pa−1)
Permeances (B)(mol·m−2·s−1·Pa−1)
SF
1 2 3
1.37 × 10−7 1.31 × 10−7 1.17 × 10−7
7.05 × 10−9 6.71 × 10−9 6.42 × 10−9
19.4 19.5 18.2
8.96 × 10−8 8.80 × 10−8 8.17 × 10−8
6.06 × 10−9 5.88 × 10−9 5.83 × 10−9
14.8 15.0 14.0
Average
1.28 × 10−7
6.72 × 10−9
19.0
8.64 × 10−8
5.92 × 10−9
14.6
Fig. 6. H2/N2, CO2/CH4 selectivities and gas permeances of M-1/2 as a function of temperature at 0.1 MPa. Colloids Surf. A. 173 (2000) 1–38. [10] N.K. Raman, C.J. Brinker, Organic “template” approach to molecular sieving silica membranes, J. Membr. Sci. 105 (1995) 273–279. [11] G. Cao, Y. Lu, L. Delattre, C.J. Brinker, G.P. López, Amorphous silica molecular sieving membranes by sol-gel processing, Adv. Mater. 8 (1996) 588–591. [12] K. Kusakabe, S. Sakamoto, T. Saie, S. Morooka, Pore structure of silica membranes formed by a sol-gel technique using tetraethoxysilane and alkyltriethoxysilanes, Sep. Purif. Technol. 16 (1999) 139–146. [13] R.M. de Vos, W.F. Maier, H. Verweij, Hydrophobic silica membranes for gas separation, J. Membr. Sci. 158 (1999) 277–288. [14] H.L. Castricum, A. Sah, R. Kreiter, D.H.A. Blank, J.F. Vente, J.E. ten Elshof, Hybrid ceramic nanosieves: stabilizing nanopores with organic links, Chem. Commun. 9 (2008) 1103–1105. [15] H.L. Castricum, A. Sah, R. Kreiter, D.H.A. Blank, J.F. Vente, J.E. ten Elshof, Hydrothermally stable molecular separation membranes from organically linked silica, J. Mater. Chem. 18 (2008) 2150–2158. [16] M. Kanezashi, K. Yada, T. Yoshioka, T. Tsuru, Design of silica networks for development of highly permeable hydrogen separation membranes with hydrothermal stability, J. Am. Chem. Soc. 131 (2009) 414–415. [17] M.C. Duke, J.C. Diniz da Costa, G. Lu, M. Petch, P. Gray, Carbonised template molecular sieve silica membranes in fuel processing systems: permeation, hydrostability and regeneration, J. Membr. Sci. 241 (2004) 325–333. [18] 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. [19] L. Meng, M. Kanezashi, J. Wang, T. Tsuru, Permeation properties of BTESE-TEOS organosilica membranes and application to O2/SO2 gas separation, J. Membr. Sci. 496 (2015) 211–218. [20] H.L. Castricum, A. Sah, M.C. Mittelmeijer-Hazeleger, C. Huiskes, J.E. ten Elshof, Microporous structure and enhanced hydrophobicity in methylated SiO2 for molecular separation, J. Mater. Chem. 17 (2007) 1509–1517. [21] H.L. Castricum, R. Kreiter, H.M. van Veen, D.H.A. Blank, J.F. Vente, J.E. ten Elshof, High-performance hybrid pervaporation membranes with superior hydrothermal and acid stability, J. Membr. Sci. 324 (2008) 111–118. [22] X. Yang, H. Du, Y. Lin, L. Song, Y. Zhang, X. Gao, C. Kong, L. Chen, Hybrid organosilica membrane with high CO2 permselectivity fabricated by a two-step hot coating method, J. Membr. Sci. 506 (2016) 31–37. [23] C. Lo, M. Lin, K. Liao, M.D. Guzman, H. Tsai, V. Rouessac, T. Wei, K. Lee, J. Lai, Control of pore structure and characterization of plasma-polymerized SiOCH films deposited from octamethylcyclotetrasiloxane (OMCTS), J. Membr. Sci. 365 (2010) 418–425. [24] V. Raman, O.P. Bahl, N.K. Jha, Synthesis of silicon oxycarbide through the sol-gel process, J. Mater. Sci. Lett. 12 (1993) 1188–1190. [25] M.A. Wahab, I.I. Kim, C. Ha, Hybrid periodic mesoporous organosilica materials prepared from 1,2-bis(triethoxysilyl)ethane and (3-cyanopropyl)triethoxysilane,
Fig. 7. Thermal stability evaluation of M-1/2 for the separation of an equimolar CO2/CH4 mixture at 150 °C and 0.1 MPa.
References [1] Y. Zhang, Y. Li, Recovery of hydrogen, methane and rare gases from nitrogen release gas, Chem. Eng. Des. Comm. 14 (1988) 14–22. [2] R.F. Service, The carbon conundrum, Science. 305 (2004) 962–963. [3] M. Pera-Titus, Porous inorganic membranes for CO2 capture: present and prospects, Chem. Rev. 114 (2014) 1413–1492. [4] G. Zhu, Q. Jiang, H. Qi, Effect of sol size on nanofiltration performance of a sol-gel derived microporous zirconia membrane, Chin. J. Chem. Eng. 23 (2015) 31–41. [5] N.W. Ockwig, T.M. Nenoff, Membranes for hydrogen separation, Chem. Rev. 107 (2007) 4078–4110. [6] R.M. de Vos, H. Verweij, High-selectivity, high-flux silica membranes for gas separation, Science 279 (1998) 1710–1711. [7] T. Yoshioka, M. Asaeda, T. Tsuru, A molecular dynamics simulation of pressuredriven gas permeation in a micropore potential field on silica membranes, J. Membr. Sci. 293 (2007) 81–93. [8] R.K. Iler, The Chemistry of Silica, Wiley & Sons Inc., New York, USA, 1979. [9] L.T. Zhuravlev, The surface chemistry of amorphous silica. Zhuravlev model,
166
Separation and Purification Technology 222 (2019) 162–167
S. Chai, et al.
Non-Cryst. Solids. 128 (1991) 231–242. [29] 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. [30] C. Zhou, C. Yuan, Y. Zhu, J. Caro, A. Huang, Facile synthesis of zeolite FAU molecular sieve membranes on bio-adhesive polydopamine modified Al2O3 tubes, J. Membr. Sci. 494 (2015) 174–181.
Microporous Mesoporous Mater. 69 (2004) 19–27. [26] J.H. Kim, Y.M. Lee, Gas permeation properties of poly(amide-6-b-ethylene oxide)–silica hybrid membranes, J. Membr. Sci. 193 (2001) 209–225. [27] 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. [28] M.J. van Bommel, T.N.M. Bernards, A.H. Boonstra, The influence of the addition of alkyl-substituted ethoxysilane on the hydrolysis-condensation process of TEOS, J.
167
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Fabrication of highly selective organosilica membrane for gas separation by mixing bis(triethoxysilyl)ethane with methyltriethoxysilane ⁎
Shaohua Chaia,b, Hongbin Dub, Yayun Zhaoa,b, Yichao Linb, Chunlong Kongb, , Liang Chenb, a b
T
⁎
University of Chinese Academy Sciences, 10049 Beijing, PR China Institute of New Energy Technology, Ningbo Institute of Material Technology and Engineering, CAS, 1219 Zhongguan West Road, 315201 Ningbo, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Organosilica BTESE-MTES Hybrid silica membrane Selectivity Gas separation
In the present study, the mixtures of bis(triethoxysilyl)ethane (BTESE) and methyltriethoxysilane (MTES) were proposed to design highly selective hybrid silica membranes for gas separation by the sol-gel method. The role of MTES in the hybrid membranes was investigated by changing the ratio of the two precursors. The membrane derived from mixture of 1 BTESE and 3 MTES exhibited the highest H2 and CO2 permeance with 7.99 × 10−7 mol m−2 s−1 Pa−1 and 3.53 × 10−7 mol m−2 s−1 Pa−1, respectively. In contrast, the superior selectivity was achieved on the membrane derived from mixture of 1 BTESE and 2 MTES, in which the H2/N2, H2/CH4, CO2/N2, and CO2/CH4 were found to be 64.4, 73.6, 28.5, and 32.6, respectively, which indicated that the structures of BTESE-MTES hybrid membranes can be adjusted by tuning the ratios of MTES to BTESE. Moreover, the membranes also performed well in the binary-component mixture gases and high-temperature test.
1. Introduction In the process of ammonia synthesis, the exhaust gas should be discharged to ensure a smooth reaction, but H2, CH4 and some rare gases contained in it can be recycled and reused [1]. On the other hand, carbon dioxide in natural gas, is believed to reduce the conversion rate and energy content of natural gas, thus it should be separated as much as possible [2]. However, the separation and purification of gas mixtures that are similar to the one previously described in the chemical industry always consume considerable energy. Among a series of methods solving these problems, membrane separation technology attracts more attention than the others due to its advantages of high efficiency, simplicity and low energy cost [3]. Silica membranes have been widely studied in gas- and liquid- separation due to their chemical stability, thermal stability, and improved mechanical strength. They can be utilized under aggressive conditions where organic polymer membranes are prone to failures [4]. Two methods for preparing the separation membrane by silica have been proposed: chemical vapor deposition (CVD) and sol-gel method [5]. The pore structure of the silica membranes can be controlled to a large extent by the sol-gel method, and the pore size of the membrane can be accurately controlled at approximately 0.3 nm, which is effective in the separation of small molecular gases [6]. Moreover, among the numerous precursors, tetraethoxysilane (TEOS) is commonly used to
⁎
prepare sol-gel-derived silica membranes, but its practical application is restricted due to poor hydrothermal stability which is related to the large number of hydroxyl groups that occupy the surface and are responsible for the strong H-bonding with water. In addition, SieOeSi bonds will dissociate in humid environments, especially at high temperatures [7–9]. Accordingly, some research groups have proposed to take advantage of organic-inorganic hybrid alkoxide that contains organic groups, which cannot be hydrolyzed, such as methyltriethoxysilane (MTES) [10–15]. De Vos et al. [13] first successfully fabricated hydrophobic silica membranes derived by copolymerization of TEOS and MTES. These membranes were 10 times more hydrophobic than TEOS–derived membranes because methyl groups can effectively eliminate the surface silanol groups which are hydrophilic. Kanezashi et al. [16] prepared hybrid silica membranes by using bis(triethoxysilyl)ethane(BTESE) as precursor. The presence of SieCeCeSi bonds which cannot be hydrolyzed in the silica networks, led to a loose silica network and enhanced hydrophobicity [17,18]. The BTESE-derived membranes showed extremely high H2 permeance (∼10−5 mol m−2 s−1 Pa−1), but a moderate selectivity for H2/N2, compared with inorganic silica membranes. The membrane pore sizes can be controlled by the size of the linking units which comprise the organic-inorganic hybrid alkoxides [15]. Commonly, the membrane prepared by the larger linking unit (Sie (CH2)2eSi > SieCH2eSi), the gas permeance is higher [16].
Corresponding authors. E-mail addresses: [email protected] (C. Kong), [email protected] (L. Chen).
https://doi.org/10.1016/j.seppur.2019.04.039 Received 31 October 2018; Received in revised form 13 April 2019; Accepted 13 April 2019 Available online 13 April 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
Separation and Purification Technology 222 (2019) 162–167
S. Chai, et al.
membranes was acquired by SEM (the Quanta FEG 250, FEI Company).
Therefore, the H2 permeance of BTESE-derived membranes was one order of magnitude higher than that of traditional pure silica membranes. On the basis of this result, tuning the pore size by adding a different alkoxide to the original precursors has been proposed [13,14,19]. Castricum et al. [20] observed that a series of unsupported silica materials prepared with different ratios of MTES and TEOS, with increasing MTES concentration, smaller water uptakes for materials, and the gels prepared from a combination of both precursors showed smaller micropore sizes. This result corroborates that changing the ratio of precursors used to prepare hybrid silica gels, can also effectively control the pore structure of the material. Inspired by this, we propose to design hybrid silica membranes by mixing MTES with BTESE. To the best of our knowledge, no report is currently available about BTESEMTES hybrid membranes used for gas separation, albeit Castricum et al. [14,15,21] fabricated a hybrid silica membrane by mixing BTESE with MTES for dehydration of n-butanol (5 wt% water). In the present work, a series of precursors prepared with different ratios of BTESE and MTES were designed to fabricate organic-inorganic hybrid silica membranes. Single gas permeances of H2, CO2, N2, and CH4 were first measured at 30 °C to acquire information on the performance of as-prepared membranes for separating small molecules. The effect of MTES concentration on the single gas permeances for silica membranes was evaluated. After the comparison, binary-component gas separations and hightemperature test were conducted on the membrane prepared by the optimum ratio of BTESE and MTES.
2.3. Single and binary-component gas permeation test Before we evaluated the permeation properties, all the membranes were dried at 120 °C in a vacuum oven for 24 h to remove the water adsorbed in the membranes. Subsequently, the membranes were mounted in a stainless steel module, and sealed at each end with silicone O-rings. The gas separation performance was measured on a homemade permeance apparatus. The feed and permeate pressures were kept at 0.3 MPa and 0.1 MPa, respectively. The permeation flow rate was measured by a soap film bubble flow meter. The single gas permeations of H2, CO2, N2, and CH4 were evaluated at 30 °C. After a comparison of single gas permeance and ideal selectivity of all membranes, the binary gas separation of H2/N2, CO2/CH4 and hightemperature test were further conducted on the most comprehensive membrane. The temperature dependence of permeance for the membrane was studied at temperatures ranging from 30 °C to 150 °C. For binary gas test, equimolar mixture gases (both 50 mL/min) were fed on the feed side of the permeation module. Sweep gas (100 mL/min) was fed on the permeate side to keep the concentration of permeating gas as low as possible thus providing a driving force for gas permeation. The pressure drop through the membrane was kept at 0.1 MPa. The fluxes of feed and sweep gases were determined with mass flow controllers, and a calibrated gas chromotograph (Agilent 7890 B) was used to measure the concentration of mixed gases on the permeate side. The gas mixture selectivity of the membrane was calculated by the following relationship:
2. Experimental 2.1. Preparation of hybrid sols and membranes
αi / j = In the present work, six types of sols were prepared. The molar ratios of BTESE to MTES were 1/0 (pure BTESE), 2/1, 1/1, 1/2, 1/3, and 0/1 (pure MTES), respectively. In the preparation procedure of BTESE-MTES hybrid sols, BTESE (95%, Aladdin) and MTES (98%, Aladdin) were mixed in established ratios and added to dry ethanol, with a molar ratio of silane/ethanol = 1/35–40. A mixture of water, nitric acid, and ethanol was added dropwise to the solutions under vigorous stirring in an ice bath. All the final solutions with molar ratios of silane/ethanol/H2O/H+ = 1:46:60:0.1. After 2 h, the solutions were kept in closed systems under continuous stirring at 60 °C for 1 h to develop hybrid sols. As for pure MTES-derived sols, nitric acid was replaced by acetic acid, and ice bath was not needed, the reaction time of the final solutions was extended to 8 h. The preparation of pure BTESE-derived sols and specific membranes fabrication has been presented in the previous report [22]. Porous αalumina tubes (average pore size: 1.5 μm, outside diameter: 12 mm) were employed as supports. The fresh supports were coated with colloidal silica (Aldrich) to eliminate the large pores and defects, followed by 20 min calcination at 550 °C. Then the large-sized particle pure BTESE-derived sols were coated on them by 20 min calcination at 300 °C, as the intermediate layer. After that, the prepared hybrid sols were coated onto the intermediate layers, followed by calcining at 300 °C for 20 min. The coating and calcination process was repeated three times in order to obtain separation layers. The corresponding hybrid membranes were named M-1/0, M-2/1, M-1/1, M-1/2, M-1/3, and M-0/1, respectively, the powders prepared by the sols were named as P-1/0, P-2/1, P-1/1, P-1/2, P-1/3, and P-0/1.
yi, perm / yj, perm yi, feed / yj, feed
where yi, perm and yj, perm are the mole fractions of the components in the permeate, yi, feed and yj, feed denote their corresponding mole fractions in the feed. And the gas permeance was defined as follows:
Pi =
Fi 1 × A ΔP
Fi (mol/s) is the molar flow rate of the permeate gas, A (m2) is the area of membrane and ΔP is the driving force through the membrane. The selectivity and permeance reported in the present work are an average of at least three membranes. 3. Results and discussion 3.1. Characterization of organic-inorganic hybrid gels Fig. 1 shows the FT-IR spectra of silica films derived from six ratios of BTESE and MTES, which were obtained by mixing KBr with powders prepared by calcining the precursors at 300 °C under air. All the films show strong peaks near 1070 cm−1, which are indicative for the asymmetric stretching of oxygen atoms, whereas the peaks at around 800 cm−1 were due to the symmetric stretching of the oxygen atoms (vibrational mode of ring stretching). This result proves that all the solgel reactions were successful [23]. Pure BTESE-derived powders can be identified by the characteristic peak at 2929 cm−1, which was attributed to the stretching vibrations of eCH2e bonding in the Sie (CH2)2eSi groups. The absorption at around 1340 cm−1 was attributed to the symmetric CeH stretching vibration in the eCH2e fragments. Pure MTES-derived powders were characterized by a strong peak around 1280 cm−1, which was ascribed to the presence of methyl groups [24], the peak that appeared on pure BTESE-derived powders in the same position can be attributed to the Si-C bond [25]. Another absorption peak of 2974 cm−1 was assigned to the eCH3 bonding in the eOCH2CH3 [26]. Both methyl and eCH2e groups could be found in all the BTESE-MTES mixture, which indicated that the cross-linked polysiloxane structure with hydrocarbon units was formed in the BTESE/
2.2. Characterization The powders of sols derived from different ratios of BTESE and MTES prepared by calcination at 300 °C were characterized by Fourier transform infrared spectroscopy (FT-IR, Nicolet 6700, Thermo Company) and N2 adsorption/desorption isotherms at 77 K on Accelerated Surface Area and Porosimetry System (ASAP 2020M, Micromeritics Company). The morphology of each as prepared 163
Separation and Purification Technology 222 (2019) 162–167
S. Chai, et al.
images of the membranes. No cracks or pinholes were observed on the surface from the surface image, indicating continuous and compact membrane was fabricated. As anticipated, three layers can be observed from the cross-sectional image, corresponding to the separation layer, BTESE intermediate layer and SiO2 layer, respectively. And the adhesion between the layers was quite good. The SiO2 layer was used to modify the substrate. We can observe that some SiO2 particles penetrated into the α-Al2O3 substrate. Large-sized BTESE-derived particles can further fill the defects on the SiO2 layer, but did not show any gas separation performance [22]. It is difficult to find penetration of the component of separation layer into the BTESE intermediate layer, which proves the superiority of BTESE as the intermediate layer and preparation method. The thickness of the separation layer was approximately 300 nm. 3.3. Gas permeation test of the hybrid membranes Figs. 4 and 5 summarize the single gas permeance and ideal selectivity of all the hybrid membranes which were prepared by six ratios, respectively. It can be seen that all the materials were effective for gas separation, H2 and CO2 permeances through the materials were obviously higher than that of N2 and CH4. Compared with the gas permeances for BTESE membrane, M-2/1 and M-1/1 were all lower, but M1/2 and M-1/3 were higher. Similarly, with 50% MTES as the dividing line, MTES tend to play a different role. MTES have lower functionality, which contribute to reduce the porosity due to the possibility of crosslinking during the condensation of BTESE and MTES was considerably reduced [28]. And owing to the space occupation by methyl groups, the effective pore space in the silica matrix for gas permeation was decreased [27]. These effects were thought to be mainly responsible for the lower gas permeance through BTESE-MTES hybrid membranes when the proportion of MTES was below 50%. In the case of hybrid membranes prepared from high concentration of MTES, several possible reasons could account for their high gas permeance. First, fast condensation rate of a great deal of MTES in the formation of silica network would make up for the limited possibility of cross-linking during the condensation of BTESE and MTES. Moreover, the pores in the silica matrix available for gas permeation may be formed since the aggregation of methyl groups in the molecular level between the silica networks. Therefore, the high permeances for rich-MTES hybrid membranes were due to the porosity in the silica matrix increased. In all of membranes we studied in this work, M-1/3 showed the highest permeance to H2 of 7.99 × 10−7 mol m−2 s−1 Pa−1 and CO2 permeance of 3.53 × 10−7 mol m−2 s−1 Pa−1. M-1/2 showed the best ideal separation factors of H2/N2, H2/CH4, CO2/N2, and CO2/CH4, which are determined as the ratio of the single component permeances, were found to be 64.4, 73.6, 28.5, and 32.6, respectively, which are much higher than the corresponding knudson coefficients and far exceeded some of the previous report about hybrid organosilica membrane [16,19,21,30]. This may related to the formation of relatively uniform and stable sesquisiloxane-type structure because of the good match between BTESE and MTES, resulting in the well-distributed pore size, which can sieve the gas more effectively according to their dynamic diameters. Given the good permeance and outstanding ideal selectivity of M-1/ 2, 1 BTESE and 2 MTES were considered to produce relatively perfect pore structure, which were identified as the optimum ratio in all comparisons. The binary gas separation of H2/N2, CO2/CH4 and hightemperature test were further conducted. The mixed gases permeances and separation factors at 30 °C and 0.1 MPa of three M-1/2 were summarized in Table 1, which proved that the excellent reproducibility of the membrane fabrication. And the temperature dependence of gas permeance for the M-1/2 was studied at temperatures ranging from 30 °C to 150 °C (Fig. 6). All the permeance of H2, N2 and CH4 became higher with increasing temperature, indicating that the pore size of silica networks is close to the tested gases in molecular size. Hence,
Fig. 1. FT-IR spectra of the powders prepared with different ratios of BTESE and MTES.
MTES-derived powders. And the peak intensity of methyl groups increased gradually with the increase of MTES in the sol, revealing that additional methyl groups are in the silica networks. As shown in Fig. 2, all the powders showed high N2 adsorption except pure MTES-derived powders, which was also consistent with the result of [27], because the space occupation of the methyl groups in the pure MTES-derived silica network lead to the inaccessible pores. N2 adsorption of BTESE-MTES powders were lower than that of BTESE when the proportion of MTES was below 50%, indicating fewer pores available in the BTESE-MTES hybrid silica for N2 adsorption, the methyl groups might still act as barrier in the BTESE-MTES silica network so that some pores were blocked. When the proportion of MTES was higher than 50%, the pores available in the silica for N2 absorption obviously increased, suggesting that high concentration of MTES may support the formation of pores.
3.2. Characterization of hybrid membranes Fig. 3 shows the representative surface and cross-sectional SEM
Fig. 2. N2 adsorption isotherms of the powders derived from different sols. 164
Separation and Purification Technology 222 (2019) 162–167
S. Chai, et al.
Fig. 3. Surface and cross-sectional SEM images of the membranes.
organic-inorganic silica materials. A larger amount of CO2 can be absorbed by silica at a lower temperature [29], therefore the mixture selectivity of CO2/CH4 decreased with increasing temperature. The H2/ N2 mixture selectivity also decreased with the increase of operation temperature, because the temperature dependence of N2 was larger than that of H2. Equimolar H2/N2 and CO2/CH4 mixture selectivities at 30 °C were found to be 19.0 and 14.8, respectively. The results were much lower than the ideal selectivity, which is in line with our previous work [22]. It could be attributed to the presence of a second gas in the membrane which has significant effects on the interaction between the gas molecules and the pores. Large molecules (N2 and CH4) could slow down the permeation of small molecules (H2 and CO2) [30]. Besides, in this work, the aggregate methyl groups in the silica networks may also produce a negative effect on the transportation of gases. The H2/CH4 and CO2/N2 mixed gases were also investigated on the membranes, and the corresponding selectivities at 30 °C were 31.5 and 14.4, respectively, which were also lower than the ideal values. Furthermore, M-1/2 has been tested for the separation of equimolar CO2/CH4 mixture at 150 °C for more than 24 h, both the CO2 permeance and CO2/CH4 selectivity kept unchanged (Fig. 7), indicating that the thermal stability of the membrane was relatively good.
Fig. 4. Molecular diameter dependency of gas permeances for six types of membranes.
4. Conclusion We successfully prepared BTESE-MTES hybrid silica membranes for gas separation by the sol-gel method. The role of MTES in the BTESEMTES hybrid membranes was investigated by changing the ratio of the precursors. It seemed that MTES served a different influence: the hybrid membranes showed higher gas permeance compared with the silica membranes prepared by BTESE when the proportion of MTES was higher than 50%, while with lower MTES content may reduce the porosity and effective pore size of the hybrid membranes, which lead to lower gas permeance. The formation of uniform pore size distribution may explain the high ideal selectivity of membranes derived from the copolymerization of 1 BTESE and 2 MTES, and the membranes were still effective for selective separation in mixture gases, though the mixture selectivities were lower than the ideal separation factors. Overall, the membrane structures were tuned successfully by changing the ratios of BTESE and MTES, which provides a valuable insight for further development in gas separation applications.
Fig. 5. Ideal selectivities of six types of membranes which determined as the ratio of the single component permeances.
these molecules can permeate the hybrid membrane via an activated mechanism. But the CO2 permeance decreased from 8.64 × 10−8 mol m−2 s−1 Pa−1 to 8.07 × 10−8 mol m−2 s−1 Pa−1 when the temperature increased from 30 °C to 150 °C, which proved again that compared with other gases, a strong interaction exits between CO2 and the hybrid
Acknowledgements We acknowledge the financial support from NSFC (No. 51672289 and 51502311), the aided program for science and technology innovative research team of Ningbo municipality (No. 2014B81004). 165
Separation and Purification Technology 222 (2019) 162–167
S. Chai, et al.
Table 1 Mixed gases permeances and separation factors at 30 °C and 0.1 MPa for M-1/2. Mixed gas separation performances of M-1/2 GasA/B
H2/N2
CO2/CH4
No.
Permeances (A)(mol·m−2·s−1·Pa−1)
Permeances (B)(mol·m−2·s−1·Pa−1)
SF
Permeances (A)(mol·m−2·s−1·Pa−1)
Permeances (B)(mol·m−2·s−1·Pa−1)
SF
1 2 3
1.37 × 10−7 1.31 × 10−7 1.17 × 10−7
7.05 × 10−9 6.71 × 10−9 6.42 × 10−9
19.4 19.5 18.2
8.96 × 10−8 8.80 × 10−8 8.17 × 10−8
6.06 × 10−9 5.88 × 10−9 5.83 × 10−9
14.8 15.0 14.0
Average
1.28 × 10−7
6.72 × 10−9
19.0
8.64 × 10−8
5.92 × 10−9
14.6
Fig. 6. H2/N2, CO2/CH4 selectivities and gas permeances of M-1/2 as a function of temperature at 0.1 MPa. Colloids Surf. A. 173 (2000) 1–38. [10] N.K. Raman, C.J. Brinker, Organic “template” approach to molecular sieving silica membranes, J. Membr. Sci. 105 (1995) 273–279. [11] G. Cao, Y. Lu, L. Delattre, C.J. Brinker, G.P. López, Amorphous silica molecular sieving membranes by sol-gel processing, Adv. Mater. 8 (1996) 588–591. [12] K. Kusakabe, S. Sakamoto, T. Saie, S. Morooka, Pore structure of silica membranes formed by a sol-gel technique using tetraethoxysilane and alkyltriethoxysilanes, Sep. Purif. Technol. 16 (1999) 139–146. [13] R.M. de Vos, W.F. Maier, H. Verweij, Hydrophobic silica membranes for gas separation, J. Membr. Sci. 158 (1999) 277–288. [14] H.L. Castricum, A. Sah, R. Kreiter, D.H.A. Blank, J.F. Vente, J.E. ten Elshof, Hybrid ceramic nanosieves: stabilizing nanopores with organic links, Chem. Commun. 9 (2008) 1103–1105. [15] H.L. Castricum, A. Sah, R. Kreiter, D.H.A. Blank, J.F. Vente, J.E. ten Elshof, Hydrothermally stable molecular separation membranes from organically linked silica, J. Mater. Chem. 18 (2008) 2150–2158. [16] M. Kanezashi, K. Yada, T. Yoshioka, T. Tsuru, Design of silica networks for development of highly permeable hydrogen separation membranes with hydrothermal stability, J. Am. Chem. Soc. 131 (2009) 414–415. [17] M.C. Duke, J.C. Diniz da Costa, G. Lu, M. Petch, P. Gray, Carbonised template molecular sieve silica membranes in fuel processing systems: permeation, hydrostability and regeneration, J. Membr. Sci. 241 (2004) 325–333. [18] 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. [19] L. Meng, M. Kanezashi, J. Wang, T. Tsuru, Permeation properties of BTESE-TEOS organosilica membranes and application to O2/SO2 gas separation, J. Membr. Sci. 496 (2015) 211–218. [20] H.L. Castricum, A. Sah, M.C. Mittelmeijer-Hazeleger, C. Huiskes, J.E. ten Elshof, Microporous structure and enhanced hydrophobicity in methylated SiO2 for molecular separation, J. Mater. Chem. 17 (2007) 1509–1517. [21] H.L. Castricum, R. Kreiter, H.M. van Veen, D.H.A. Blank, J.F. Vente, J.E. ten Elshof, High-performance hybrid pervaporation membranes with superior hydrothermal and acid stability, J. Membr. Sci. 324 (2008) 111–118. [22] X. Yang, H. Du, Y. Lin, L. Song, Y. Zhang, X. Gao, C. Kong, L. Chen, Hybrid organosilica membrane with high CO2 permselectivity fabricated by a two-step hot coating method, J. Membr. Sci. 506 (2016) 31–37. [23] C. Lo, M. Lin, K. Liao, M.D. Guzman, H. Tsai, V. Rouessac, T. Wei, K. Lee, J. Lai, Control of pore structure and characterization of plasma-polymerized SiOCH films deposited from octamethylcyclotetrasiloxane (OMCTS), J. Membr. Sci. 365 (2010) 418–425. [24] V. Raman, O.P. Bahl, N.K. Jha, Synthesis of silicon oxycarbide through the sol-gel process, J. Mater. Sci. Lett. 12 (1993) 1188–1190. [25] M.A. Wahab, I.I. Kim, C. Ha, Hybrid periodic mesoporous organosilica materials prepared from 1,2-bis(triethoxysilyl)ethane and (3-cyanopropyl)triethoxysilane,
Fig. 7. Thermal stability evaluation of M-1/2 for the separation of an equimolar CO2/CH4 mixture at 150 °C and 0.1 MPa.
References [1] Y. Zhang, Y. Li, Recovery of hydrogen, methane and rare gases from nitrogen release gas, Chem. Eng. Des. Comm. 14 (1988) 14–22. [2] R.F. Service, The carbon conundrum, Science. 305 (2004) 962–963. [3] M. Pera-Titus, Porous inorganic membranes for CO2 capture: present and prospects, Chem. Rev. 114 (2014) 1413–1492. [4] G. Zhu, Q. Jiang, H. Qi, Effect of sol size on nanofiltration performance of a sol-gel derived microporous zirconia membrane, Chin. J. Chem. Eng. 23 (2015) 31–41. [5] N.W. Ockwig, T.M. Nenoff, Membranes for hydrogen separation, Chem. Rev. 107 (2007) 4078–4110. [6] R.M. de Vos, H. Verweij, High-selectivity, high-flux silica membranes for gas separation, Science 279 (1998) 1710–1711. [7] T. Yoshioka, M. Asaeda, T. Tsuru, A molecular dynamics simulation of pressuredriven gas permeation in a micropore potential field on silica membranes, J. Membr. Sci. 293 (2007) 81–93. [8] R.K. Iler, The Chemistry of Silica, Wiley & Sons Inc., New York, USA, 1979. [9] L.T. Zhuravlev, The surface chemistry of amorphous silica. Zhuravlev model,
166
Separation and Purification Technology 222 (2019) 162–167
S. Chai, et al.
Non-Cryst. Solids. 128 (1991) 231–242. [29] 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. [30] C. Zhou, C. Yuan, Y. Zhu, J. Caro, A. Huang, Facile synthesis of zeolite FAU molecular sieve membranes on bio-adhesive polydopamine modified Al2O3 tubes, J. Membr. Sci. 494 (2015) 174–181.
Microporous Mesoporous Mater. 69 (2004) 19–27. [26] J.H. Kim, Y.M. Lee, Gas permeation properties of poly(amide-6-b-ethylene oxide)–silica hybrid membranes, J. Membr. Sci. 193 (2001) 209–225. [27] 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. [28] M.J. van Bommel, T.N.M. Bernards, A.H. Boonstra, The influence of the addition of alkyl-substituted ethoxysilane on the hydrolysis-condensation process of TEOS, J.
167