Silica Membrane Application for Pervaporation Process Hiroki Nagasawa, Toshinori Tsuru Hiroshima University, Higashi-Hiroshima, Japan
Acronyms AcOH BTESE BTESM CHA DDR EtOH FAU IPA LTA MER MFI MMM MOF MOR MWCNTs NaAlg PAA PEK-C PDMS PMMA PVA SPPO TEOS
acetic acid 1,2-bis(triethoxysilyl)ethane 1,2-bis(triethoxysilyl)methane chabazite DDR-type zeolite ethanol faujasite isopropyl alcohol zeolite A merlinoite MFI zeolite mixed matrix membrane metal organic framework moredenite multiwall carbon nanotubes sodium alginate polyacrylic acid cardo polyetherketone polydimethylsiloxane polymethyl methacrylate polyvinyl alcohol sulfonated poly (phenylene oxide) tetraethylorthosilicate
Current Trends and Future Developments on (Bio-) Membranes. http://dx.doi.org/10.1016/B978-0-444-63866-3.00009-1 # 2017 Elsevier B.V. All rights reserved.
218 Chapter 9 VOCs ZIF-8 ZSM-5
volatile organic compounds zeolitic imidazolate framework-8 ZSM-5-type zeolite
Symbols Pi p1,i p2,i p2,total Xi Yi α
permeance of component i partial pressure of component i in the feed partial pressure of component i in the permeate total pressure of permeate molar fraction of component i in the feed molar fraction of component i in the permeate separation factor
1 Introduction Pervaporation is a process that can achieve separations of liquid mixtures by selective permeation and vaporization through a membrane. A liquid mixture is fed to one side of the membrane and permeate comes out from the opposite side in a vapor phase. Pervaporation is known to have great advantages in the separation of azeotropes, close-boiling-point mixtures, isomers, thermally sensitive compounds, and in removing species present in low concentrations that are difficult to separate via conventional liquid separation processes such as distillation, extraction, and adsorption (Basile, Figoli, & Khayet, 2015). Although pervaporation requires heat of evaporation that is similar to distillation, pervaporation is nonetheless an energy-saving process that only requires the latent heat of evaporation for a small part of the feed mixture that selectively permeates through the membrane to be vaporized ( Jonquieres, Arnal-Herault, & Babin, 2013). Pervaporation can be operated continuously without regenerating extractants and adsorbates. Owing to these advantages, pervaporation is an attractive technology in many industries to yield a high-quality product. Pervaporation has been employed in many types of separations such as the dehydration of organics, the removal of organics from an aqueous phase, and for organic-organic separation. Polymeric, inorganic, and hybrid membranes have been applied in these processes. Polymeric membranes are the most commonly used because they are inexpensive and easy to scale up. Zeolite membranes have also been used in industries due to unique characteristics such as a porous crystalline structure, adsorption properties, mechanical strength, and chemical stability (Bowen, Noble, & Falconer, 2004). In particular, zeolite NaA membranes have been successfully commercialized for the dehydration of alcohol mixtures (Kita, Horii, Ohtoshi, & Okamoto, 1995). In recent years, hybrid and composite membranes such as metal-organic framework membranes (Liu et al., 2013) and mixed matrix membranes (MMMs; Vane, Namboodiri, & Bowen, 2008), as well as carbon molecular sieve membranes (Tanaka, Yasuda,
Silica Membrane Application for Pervaporation Process 219 Table 1: Typical membranes for pervaporation Membrane Type
SiO2, SiO2-TiO2, SiO2-ZrO2, hybrid silica
Zeolite Polymer Carbon
LTA, FAU, zeolite T, MOR, MFI, CHA PVA, polyamide, PDMS Carbonized polyimides, carbonized phenolic resins Zeolite-filled polymers, graphene oxidefilled polymers MOF-5, ZIF-8
Stable at high temperature and in acid Easy to control pore sizes Uniform intracrystal pores Difficult to apply to hydrocarbons High selectivity Low permeability Combining the benefits of both polymeric and inorganic materials Some MOF are instable in aqueous feed
Katayama, & Miyake, 2011) have also been developed and tested on a lab scale and on a pilot scale. Typical membranes that are used for pervaporation are summarized in Table 1. Microporous silica and silica-based membranes are also excellent candidates for pervaporation. The membranes prepared by sol-gel processing are highly permeable and selective and offer great advantages in the control of pore sizes across a wide range. Recent advances in the preparation of organic-inorganic hybrid silica membranes have improved performance with respect to selectivity, flux, and stability (Castricum et al., 2008a; Kanezashi, Yada, Yoshioka, & Tsuru, 2009; Wang, Kanezashi, Yoshioka, & Tsuru, 2012). Precise tuning of silica and silicabased microporous structures makes it possible to tailor membranes to meet the demand of individual separation applications. This chapter is an overview of the fundamentals and applications of silica and silica-based membranes for pervaporation processes. The synthesis and characterization of silica membranes are briefly presented. Current and potential applications of silica membranes in pervaporation are described.
2 Fundamentals of Pervaporation In pervaporation, the liquid mixture to be separated (feed) is placed in contact with one side of a membrane, and the permeated product (permeate) is removed as a low-pressure vapor from the other side, as shown in Fig. 1. The permeate vapor is condensed and collected. The chemical potential gradient across the membrane is the driving force for the mass transport. The driving force can be created by applying either a vacuum pump or an inert purge on the permeate side to maintain the permeate vapor pressure at a level that is lower than the partial pressure of the feed liquid. Pervaporation has potential applications in many categories, as shown in Table 2: (i) dehydration applications such as alcohol-water, organic-water, and chlorinated-hydrocarbon water systems; (ii) removal of trace organics such as volatile organic compounds (VOCs) from water; and (iii) separation of organic-organic mixtures. Pervaporation offers several
220 Chapter 9 Pump
Retentate (liquid) Membrane Permeate (vapor) Condenser
Vent Vac. pump
Fig. 1 Schematic diagram of a pervaporation system.
Table 2: Classifications of pervaporation separation Application
Dehydration of aqueous solution
Alcohols/water Organic acids/water Amines and diamines/water VOCs Polar/nonpolar mixtures Aromatic/aliphatic mixtures Aromatic/alicyclic mixtures Isomers
Removal of trace organics from water Organic-organic separation
advantages: very low capital and operating costs, environmentally friendly, and simple scale-up. An ideal membrane for pervaporation would be a defect-free top layer coated on a porous low-resistance support layer. The top layer is responsible for the selectivity of the membrane and should be adjusted to suit different applications. Pervaporation performance is evaluated using an experimental apparatus equipped with a screw for agitation, a heater, and a vacuum pump, as shown in Fig. 2. The feed solution is circulated vigorously to reduce the effects of concentration and temperature polarization. The feed-side total pressure is either at atmosphere or above (pressurized). The permeate is kept under a vacuum at a pressure lower than the vapor pressure in the feed, and it is then collected using a cold trap that is cooled using either liquid nitrogen or chilled water. Selectivity (or separation factor, α) can be used to express the separation capability of a pervaporation membrane for a binary mixture of components i and j and is obtained using the following equation: (1) α ¼ Yi =Yj = Xi =Xj
Silica Membrane Application for Pervaporation Process 221
1. PV module
5. Feed vessel
6. Tubing pump
3. VP module
7. Cold trap
4. Oven heater 8. Vacuum pump
(B) Fig. 2 Schematic image of a typical (A) pervaporation and (B) vapor permeation apparatus for a tubular membrane.
where Yi and Yj are mole fractions of the preferential and secondary permeants, respectively, in the permeate, and Xi and Xj are the corresponding mole fractions in the feed. Selectivity can vary from unity (no selective permeation) to infinity, and it is affected by membrane/ component solubility, feed hydrodynamic conditions, permeate resistance resulting from elevated partial pressures, and changes in the diffusion rate resulting from membrane swelling. Therefore choosing a membrane with the appropriate affinity is a crucial factor in pervaporation. Another index that is used to evaluate pervaporation performance is the permeate flux, which can be expressed in the dimensions, mol m2 s1 or kg m2 s1, that indicate how much permeate stream can be obtained per unit time and membrane area. Because flux is dependent on separation conditions such as concentration and temperature, which are inconsistent in the literature, permeance, P, is widely used for comparisons between different sets of data. Permeance is obtained by using the following equation: Pi ¼ Ji =ðp1, i p2, i Þ
where Ji is the permeate mole flux of the component i and p1,i and p2,i are the partial pressures for the feed and permeate stream, respectively. The partial pressure of the feed side is obtained by using saturated vapor pressure and an activity coefficient, which are calculated from the Antoine equation and the Wilson equation, respectively. The vapor pressure in the permeate is given by the following equation: p2, i ¼ p2, total Yi
222 Chapter 9 where p2,total is the total pressure of the permeate, but the pressure of the permeate is often assumed to be zero. Selectivity and productivity are critically important in pervaporation, but stability should also be considered. Membranes should be stable in hydrothermal environment as well as in concentrated acids and organic solvents. The commercialization of the pervaporation technique, to a large extent, can be attributed to the engineering approach of fabricating thin membranes in asymmetric and composite forms. The simplicity of membrane preparation is also important for large-scale production. Therefore a novel material for pervaporation membranes should be selected based on the development of the preceding points.
3 Membrane Synthesis 3.1 Sol-Gel Processing for Silica Membrane Preparation In general, silica membranes are prepared onto porous supports because the layer effective for separation should be thin enough to realize high permeance without sacrificing mechanical strength in order to withstand the pressure difference applied across the membrane. Porous alumina tubes or discs with approximately 1-μm pores are usually used as the support. An intermediate layer with pore sizes of several nanometers is formed on the surface of the support, so as to minimize the thickness of the top separation layer. Therefore a typical silica membrane has an asymmetric composite structure, as shown in Fig. 3. A typical procedure for silica membrane preparation via sol-gel processing is illustrated in Fig. 4. Sol-gel processing for the preparation of silica membranes starts with synthesizing a silica sol via acid- or base-catalyzed hydrolysis with a subsequent condensation of alkoxysilane precursors. The sol is coated onto a support to form a thin silica gel layer, and it is then calcined at high temperatures in order to fix the silica structure. In general, silica membranes have a highly porous structure with sub-nano-sized pores formed within silica networks and single
Fig. 3 A typical SEM image of the cross-section of a silica membrane.
Silica Membrane Application for Pervaporation Process 223 Alkoxysilane (e.g. TEOS)
Hydrolysis Si-OC2H5 + H2O
H2O, HCl or NaOH
Si-OH + C2H5OH
Hydrolysis and condensation
Si-OC2H5 + HO – Si
Si O Si
Coating to a substrate (gel formation) Coated layer
Drying and calcination
Pore: network pore
Fig. 4 Schematic diagram of a typical procedure of silica membrane preparation via sol-gel processing.
nano-sized pores formed as either interparticle pores and/or grain boundaries. The final structures such as pore size, porosity, and morphology of the resultant membranes can be controlled by varying the processing conditions and the composition of the starting solution. During sol synthesis, the alkoxysilane precursors are hydrolyzed and condensed to form three-dimensional silica networks. The precursors first react with water to replace one or more of the alkoxy groups with hydroxyl groups. Subsequently, condensation reaction between two hydroxyl groups and/or between a hydroxyl group and an unhydrolyzed alkoxy group occurs to form a siloxane bond leading to polymerization of the precursors. Since the size and structure of silica networks and sols strongly affect the structure of the resultant membranes, as well as their permeation properties, the synthetic conditions, including the type and concentration of precursors and catalysis; the solvent; and the ratio of water to precursor should be carefully controlled. For example, acid and base catalysts affect both the reaction rates of hydrolysis and condensation. Therefore acid-catalyzed sol-gel-derived silica typically has a polymeric structure, whereas base-catalyzed silica has a colloidal structure (Brinker & Scherer, 1990). Higher water-to-precursor ratios tend to enhance the hydrolysis of alkoxy groups, resulting in a high degree of condensation and the formation of a dense silica network. Moreover, the structure can also be tuned by mixing different metal alkoxides to form a composite oxide structure (Tsuru, Wada, Izumi, & Asaeda, 1998), by doping cations (Kanezashi & Asaeda, 2006) and/or anions (Kanezashi et al., 2016), and by introducing organo-functional groups into the main and/or side chains of siloxane networks (Castricum et al., 2008a; Kanezashi et al., 2009).
224 Chapter 9
3.2 Amorphous Silica and Silica-Based Membranes Sol-gel-derived amorphous silica membranes were first reported by Asaeda and coworkers in 1990 (Kitao & Asaeda, 1990). These membranes were prepared by coating a thin silica layer onto the surface of α-alumina porous substrates. The silica layer was derived via acid-catalyzed hydrolysis and condensation of tetraethylorthosilicate (TEOS). Amorphous silica membranes have shown high permselectivity in pervaporation applications such as dehydration of alcohols and organic acids, and for the separation of nonaqueous organic-organic solvent mixtures (Ishida, Tasaka, & Asaeda, 2005; van Veenm, van Delft, Engelen, & Pex, 2001; Verkerk, van Male, Vorstman, & Keurentjes, 2001). Furthermore, the acid stability of amorphous silica membranes has proved to be much better than that of NaA zeolite membranes (Ishida et al., 2005). These membranes, however, are unstable under hydrothermal conditions. A densification of the silica matrix by water vapor at high temperature results in a reduction in the permeation flux during gas-phase separation (Kim & Gavalas, 1995; Tsapatsis & Gavalas, 1994). A dissolution of the silica layer in water can cause an increased flux and a simultaneous decrease in selectivity during pervaporation (Asaeda, Yang, & Sakou, 2002). Efforts to improve the hydrothermal stability of silica-based membranes have been ongoing since the late 1990s, and composites of silica with inorganic oxides such as zirconia (Tsuru et al., 1998) and titania (Asaeda, Ishida, & Waki, 2005) have been reported as alternatives to amorphous silica. Silica-zirconia composite membranes with well-controlled pore sizes below 1 nm were developed and evaluated for pervaporation of IPA/water mixtures. These membranes exhibited a stable flux of approximately 5.4 kg m2 h1 with a separation factor of 2500 at 73 mol% (90 wt%) of IPA in the feed mixture at 75°C and was proven to be highly stable even with high water concentrations (up to 60 mol% of water) at 75°C (Asaeda et al., 2002). For the pervaporation of aqueous organic acid solutions, silica-titania composite membranes were reported to have high stability in a 73 mol% acetic acid/water mixture at 100° C, although the water flux was not as high as that for silica membranes (Asaeda et al., 2005). Hydrothermal stability of silica membranes was also improved via the incorporation of hydrophobic groups. Hydrophobic methylated silica membranes have been successfully operated in the dehydration of butanol/water mixture at 95°C over a period that lasted for more than 18 months (Campaniello, Engelen, Haije, Pex, & Vente, 2004).
3.3 Organic-Inorganic Hybrid Silica Membranes In recent years, there has been a growing interest in developing organic-inorganic hybrid silica membranes because they are expected to bring about a breakthrough in improving permselective properties and stabilities as well as adding a new functionality to membranes. Castricum and coworkers used 1,2-bis(triethoxysilyl)ethane (BTESE) to develop new organic-inorganic hybrid silica membranes with organic linking groups in the silica networks (Castricum et al., 2008a, 2008b). These membranes showed superior hydrothermal stability in the dehydration of butanol containing 5 wt% of water at 150°C, with a separation factor of more
Silica Membrane Application for Pervaporation Process 225 OEt EtO
Si OEt OEt
Fig. 5 Schematic image of network size tuning via the “spacer” technique. The network structure derived from (A) TEOS, (B) BTESM, and (C) BTESE.
than 400 and a water flux of more than 20 kg m2 h1 (Castricum, Kreiter, et al., 2008). Kanezashi and coworkers proposed a “spacer” technique to control the size of silica networks, as shown is Fig. 5 (Kanezashi et al., 2009). In this technique, bis-silyl-type precursors such as bis(triethoxysilyl)methane (BTESM) or BTESE were used as the precursor instead of TEOS. The CH2 and CH2-CH2 linking groups between the Si atoms act as a spacer, which enlarges the size of the minimum unit in silica networks. As a result, the average pore size of the derived membranes can be successfully tuned on the order of TEOS < BTESM < BTESE (Kanezashi, Kawano, Yoshioka, & Tsuru, 2012). In a recent study, we proved that the network sizes of BTESE membranes could be precisely controlled by controlling the water-to-BTESE ratio during sol preparation (Nagasawa, Niimi, Kanezashi, Yoshioka, & Tsuru, 2014; Niimi, Nagasawa, Kanezashi, Yoshioka, & Tsuru, 2014). The spacer technique is a promising strategy to fabricate microporous silica membranes because controlling the porous structure of the silica layer is critical in achieving satisfactory separation. As was mentioned previously, a porous ceramic such as α-alumina is the most common material used for the fabrication of silica and silica-based membranes. In contrast, little work has been done on the fabrication of such membranes using polymeric supports, although the use of polymeric supports instead of the ceramic supports offers great potential for cost-saving and large-scale manufacturing. Development of polymer-supported silsesquioxane membranes has long been a challenging task, because the calcination step, which is essential in a sol-gel process, is usually performed at 200–600°C. Recently, we developed a new type of “layered-hybrid membrane,” as shown in Fig. 6, consisting of a BTESE-derived layer on a polymeric support (Gong et al., 2014a). This was accomplished by lowering the heat-treatment temperature in order
226 Chapter 9
Fig. 6 Schematic image of the concept of the layered-hybrid membrane.
to avoid thermal degradation of the polymeric supports. This BTESE-derived layered-hybrid membrane was prepared onto a polymeric nanofiltration membrane via heat-treatment temperature at 120°C and showed a stable water flux of 2.3 kg m2 h1 and a separation factor of 2500 for vapor permeation dehydration of 90 wt% isopropanol-water mixtures at 105°C (Gong et al., 2014b). We also reported that the pore sizes and structures of the BTESE-derived hybrid silica top layers on the membrane surface could be tuned at a mild temperature (60°C) by introducing either hydrochloric acid or ammonia vapor treatment. More recently, we demonstrated a photo-induced sol-gel synthesis of layered-hybrid membranes (Nagasawa, Nishibayashi, Kanezashi, Yoshioka, & Tsuru, 2017). The hybrid silica membranes coated onto polymeric supports were flexible and had good stability. Therefore the concept of the layeredhybrid membrane could represent substantial progress in the implementation of silica and silicabased membranes via novel module designs such as spiral or hollow fiber modules.
4 Pervaporation Applications 4.1 Dehydration of Alcohols The first and the most important application of pervaporation is the removal of water from azeotropic alcohol-water mixtures. An azeotrope is a mixture that exhibits the same concentration in both the vapor and the liquid phases. Therefore the components of an azeotrope cannot be purified using only one distillation column. For example, a mixture of 96.4 wt% ethanol and
Silica Membrane Application for Pervaporation Process 227 4.4 wt% water is a well-known azeotrope with a boiling point of 78.2°C. Ethanol is a widely used industrial solvent and has a considerable variety of other applications. Moreover, bioethanol has emerged as the most important renewable fuel to replace fossil fuels. IPA also forms an azeotropic mixture with water at an IPA composition of 87.8 wt%. IPA is widely used as a solvent in chemical industries and as a cleaning agent in electric and electronic industries, and, therefore, the recycling of used IPA is essential from both environmental and economic points of view. Alcohol dehydration membranes were first commercialized using polymeric membranes from polyvinyl alcohol (PVA) family (Ohya, Matsumoto, Negishi, Hino, & Choi, 1992; Wesslein, Heintzm, & Lichtenthaler, 1990). NaA zeolite membranes were developed by Kita and coworkers and were successfully implemented for the dehydration of aqueous alcohol mixtures (Kita et al., 1995). NaA zeolite membranes exhibit excellent dehydration performance and have achieved a high water permeation flux of 8.5 kg m2 h1 and a separation factor of >10,000 for a 90 wt% ethanol solution at 75°C (Sato, Sugimoto, & Nakane, 2008). Subsequent to the development of NaA zeolite membranes, zeolite membranes, including zeolite T (Cui, Kita, & Okamoto, 2004), mordenite (Lin, Kikuchi, & Matsukata, 2000), chabazite (Hasegawa, Hotta, Sato, Nagase, & Mizukami, 2010), and DDR (Kuhn, Yajima, Tomita, Gross, & Kapteijn, 2008) were developed to improve stability in water and were used in dehydration applications. Molecular sieve carbon membranes derived from the carbonization of polymeric membranes have also been developed and are known to have excellent stability in dehydration systems with a high water content (Tanaka et al., 2011). Dehydration of alcohols has also been achieved using silica and silica-based membranes including organic-inorganic hybrid silica membranes. Tables 3 and 4 show summaries of the separation performances of different types of membranes for the dehydration of ethanol and IPA, respectively. In general, silica and silica-based membranes exhibit moderate separation factors and moderate permeance compared with zeolite membranes. Asaeda et al. (2002) removed water from a 90 wt% IPA/water solution with pervaporation at a normal boiling point through a silica-zirconia composite membrane and reported a stable water flux that reached as high as 9 kg m2 h1 with a separation factor of approximately 2000 (Kondo et al., 2003). Wang et al. evaluated the pervaporation performances of BTESE-derived hybrid silica membranes for dehydration of ethanol and IPA (Wang et al., 2012). In this study, BTESEderived membranes prepared at higher calcination temperature tended to show a higher separation factor. The membrane calcined at 300°C showed a water flux of 3.4 kg m2 h1 and a separation factor of 4370 in the dehydration of 90 wt% IPA at 75°C (Wang et al., 2012). Application of hybrid silica membranes in a water-rich system and at high temperature: More recently, we evaluated the stability of BTESE-derived hybrid silica membranes during longterm pervaporation with a water-rich stream (up to 50 wt% water content in IPA) and at high temperature in a vapor permeation mode (up to 110°C using a 90 wt% IPA mixture) (Nagasawa, Matsuda, Kanezashi, Yoshioka, & Tsuru, 2016). Fig. 7 shows the time course of
Table 3: Pervaporation dehydration performance of membranes for EtOH/water mixture Temperature (°C)
Feed EtOH Conc. (wt%)
Total Flux (kg m22 h21)
H2O Permeance (1027 mol m22 Separation s21 Pa21) Factor
Silica-based membranes Codoped silica ECN silica BTESE-derived
75 70 75
90 96 90
1.10 1.60 3.15
9.6 33 25 Zeolite membranes
NaA NaA NaA Zeolite T Zeolite T CHA CHA DDR MER
75 70 75 75 65 75 75 100 75
90 90 90 90 90 90 90 92 95
8.50 1.12 11.1 1.10 1.23 4.14 14.00 2.00 1.00
75 12 96 9.4 16 36 120 5.1 14
PVA Chitosan PDMS UV/O3modified PDMS Perfluorinated polymer
30 60 25 25
90 90 90 90
0.28 0.472 0.23 0.18
2.2 4.1 1.3 17
2530 350 100 10,000 18,000 >10,000 900 >10,000 39,500 >10,000 1500 2000
Wang and Tsuru (2011) van Veenm et al. (2001) Wang et al. (2012) Sato et al. (2008) Sommer and Melin (2005) Shao et al. (2014) Cui et al. (2004) Zhou, Lui, Zhu, Liu, and Yang, (2009) Hasegawa, Hotta, et al. (2010) Hasegawa, Abe, et al. (2010) Kuhn et al. (2008) Nagase et al. (2009)
Polymeric membranes 104 1791 16 450
Praptowidodo (2005) Ge, Cui, Yan, and Jiang (2000) Lai et al. (2012) Lai et al. (2012)
Smuleac, Wu, Nemser, Majumdar, & Bhattacharyya (2010)
Carbon membranes Carbonized polyimide Carbon molecular sieve
Sungpet and Prapruddivongs (2010)
Dong, Nakao, Nishiyama, Egashira, and Ueyama (2010)
Mixed matrix membranes Chitosan/ MWCNTs PAA/zeolite 4A
Qui et al. (2010)
Amnuaypanich, Naowanon, Wongthep, and Phinyocheep (2012)
228 Chapter 9
Table 4: Pervaporation dehydration performance of membranes for IPA/water mixture Membrane Type
Feed IPA Conc. (wt%)
H2O Permeance (1027 mol m22 Total Flux 22 21 (kg m h ) s21 Pa21)
500 1500 >10,000 1150 4370 2500
van Gemert and Cuperrus (1995) Asaeda, Sakou, Yang, and Shimasaki (2003) Yang, Yoshioka, Tsuru, and Asaeda (2006) van Veenm et al. (2001) Wang et al. (2012) Gong et al. (2014b)
Okamoto, Kita, Horii, Tanaka, and Kondo (2001) Kondo et al. (2003) Zhou et al. (2009) Zhou, Zhang, Hu, Chen, and Kita (2011) Zhu et al. (2012) Sato, Sugimoto, Kyotani, Shimotsuma, and Kurata (2012)
Silica-based membranes Silica SiO2-ZrO2 SiO2-ZrO2 ECN silica BTESE-derived BTESE-derived layered hybrid
70 at normal bp 75 80 75 105 (VP-mode)
95 90 90 96 90 90
0.25 9.0 2.16 1.90 3.40 2.3
3.0 24 5.9 17 35 13 Zeolite membranes
NaA Zeolite T Zeolite T ZSM-5 MOR/ZSM-5
100 65 75 75 75
90 90 90 90 90
7.10 1.52 4.43 6.25 4.80
19 19 29 17 13
8210 >10,000 8200 1000 2100
Qiao, Chung, and Pramoda (2005) Adoor, Manjeshwar, Naidu, Sairam, and Aminabhavi (2006) Adoor et al. (2006) Rao, Subha, Sairam, Mallikarjuna, and Aminabhavi (2007) Huang, Pal, and Moon (1999)
Tanaka et al. (2011)
Carbon Carbon molecular sieve Carbonized SPPO hollow fiber
Yoshimune, Mizoguchi, and Haraya (2013)
Mixed matrix membranes Chitosan/NaY
Littur, Kulkarni, Aralaguppi, and Kariduraganavar (2005)
Silica Membrane Application for Pervaporation Process 229
230 Chapter 9 50/50
H2O/IPA = 10/90
Water flux (kg m–2 h–1)
15 20 Time (h)
Fig. 7 Time course of the pervaporation performance at 75°C for a BTESE-derived organosilica membrane for an IPA/water mixture with various IPA concentrations (Nagasawa et al., 2016).
pervaporation performance at 75°C for an IPA/water mixture using various IPA concentrations. The operation was initiated using a 90 wt% IPA solution. The water content in the feed was increased in a stepwise manner and finally returned to a 90 wt% IPA solution. The water flux increased with a decrease in the feed IPA concentration. This can be ascribed mainly to an increase in the partial pressure of the water in the feed, but, interestingly, the degree of increase in water flux was greater than that in the water partial pressure. The water flux was increased from 2.3 to 6.4 kg m2 h1 (a 2.5-fold increase) with a decrease in the feed IPA concentration of from 90 to 50 wt%, while water partial pressure was increased only 1.5-fold. This suggests that IPA molecules inhibited the diffusion of water molecules in the micropores. The adverse impact of IPA on water permeation can be explained as follows. With an increase in the feed IPA concentration, the adsorption of IPA by hydrogen bonding occurs at the surfaces and on the pore walls of the membranes. Adsorbed IPA can block the flow of water molecules into the micropore by reducing the effective pore size for water permeation. More importantly, the membrane showed excellent stability for pervaporation under high water content mixtures. The water flux and separation factor for a 50 wt% IPA aqueous solution were stable over several hours. The pervaporation performance for a 90 wt% IPA aqueous solution was unchanged even after the operation under high water content mixtures. This indicates that the BTESE-derived hybrid silica membranes are applicable for pervaporation in high water content systems.
Silica Membrane Application for Pervaporation Process 231 110°C 75°C
Flux (kg m–2 h–1)
5 4 3 2 1 0
15 20 Time (h)
Fig. 8 Time course of the pervaporation and vapor permeation performances for a BTESE-derived organosilica membrane in an IPA/water mixture (90/10 wt%) at various permeation temperatures (Nagasawa et al., 2016).
Fig. 8 shows the time course of pervaporation and vapor permeation performances at various permeation temperatures. The measurements at permeation temperatures under 75°C were conducted in the pervaporation mode, while those at more than 90°C were conducted in the vapor permeation mode. The concentration of IPA in the feed was maintained at 90 wt%. In the pervaporation operation, the water flux decreased with a decrease in the permeation temperature due to the decreased partial pressure of the water in the feed. It was interesting that the separation factor was increased with a decrease in the permeation temperature. This can be explained by the fact that the adsorption equilibrium of IPA to a BTESE-derived hybrid silica matrix moves toward a higher load at lower temperatures, which results in a decrease in the effective pore size through which IPA molecules can permeate. It should also be noted that the pervaporation performance at 75°C after vapor permeation at 90°C for 8 h was almost the same as that before the vapor permeation operation. After vapor permeation at 110°C, the vapor permeation performance at 90°C was essentially unchanged. These results indicate that the BTESE-derived hybrid silica membrane is stable in the vapor permeation mode for temperatures as high as 110°C.
232 Chapter 9 BTESE-derived silica layer SiO2-ZrO2 intermediate layer α-Al2O3 particle layer α-Al2O3 support
Fig. 9 Schematic drawing of a layered structure and the typical internal pressure profile of a BTESE-derived organosilica membrane (Nagasawa et al., 2016).
Permeation resistance analysis of BTESE-derived hybrid silica membranes under pervaporation and vapor permeation operations: In order to understand the rate-limiting factor for sol-gel-derived silica membranes during pervaporation, permeation resistance across the membrane was analyzed using the data shown previously (Nagasawa et al., 2016). Fig. 9 is a schematic drawing of the layered structure and shows the typical internal pressure profile of a BTESE-derived organosilica membrane. The membrane consists of an α-alumina porous support, an α-alumina particle layer, a SiO2-ZrO2 intermediate layer, and a BTESE-derived organosilica layer. The organosilica layer is in contact with the liquid or vapor feed stream, and water molecules preferentially permeate through the organosilica layer based on the molecular sieving effect. The permeating molecules desorb at the interface between the organosilica layer and the SiO2-ZrO2 intermediate layer and are then transported in the gaseous phase since the permeated side of the membrane is evacuated at low pressure. The driving force of the gas
Silica Membrane Application for Pervaporation Process 233 phase transport in the supporting layers, including the α-alumina porous support, the α-alumina particle layer, and the SiO2-ZrO2 intermediate layer, is given by the pressure drop across the three supporting layers. Considering the overall transport process across the membrane in the pervaporation and vapor permeation operations, this pressure drop across the supporting layers is of key importance since it determines the effective driving force for permeation through the organosilica layer. The total resistance of water permeation was determined using the water permeance obtained from the pervaporation and vapor permeation measurements. The resistance of water permeation through the supporting layers was estimated by measuring the nitrogen permeance followed by a correction that was based on the model equation, assuming that the mass transfer through the supporting layers is governed by Knudsen diffusion and viscous flow contribution. Finally, the water permeation resistance of the organosilica layer was calculated by taking the difference between the total resistance and the resistances of the supporting layers.
1.8 1.6 1.4
Permeation resistance (106 m2 s Pa mol–1)
Permeation resistance (106 m2 s Pa mol–1)
Fig. 10A shows the breakdown of the water permeation resistance of a BTESE-derived organosilica membrane during the pervaporation of a water/IPA mixture with various IPA concentrations at 75°C. The contribution of the supporting layers on the total permeation resistance was 37% when the feed IPA concentration was 90 wt%. By contrast, the contribution of the supporting layers was increased when the IPA concentration in the feed was decreased, and reached 61% when the feed IPA concentration was 50 wt%. The water permeation resistance profiles of BTESE-derived organosilica membranes during the pervaporation and vapor permeation of water/IPA mixtures (IPA concentration of 90 wt%) at different
Substrate Particle layer Intermediate layer Organosilica layer
1.2 1.0 0.8 0.6 0.4 0.2 0
60 70 80 IPA concentration (%)
1.8 1.6 1.4
Substrate Particle layer Intermediate layer Organosilica layer
1.2 1.0 0.8 0.6 0.4 0.2 0
(B) Operating temperature (°C) Fig. 10 Resistance analysis of pervaporation and vapor permeation for a BTESE-derived hybrid silica membrane. Breakdown of permeation resistance as a function of (A) IPA concentration at 75°C, and (B) permeation temperature for a 90 wt% IPA solution (Nagasawa et al., 2016).
234 Chapter 9 temperatures are described in Fig. 10B. In the case of vapor permeation (90–110°C), the permeation resistance of the organosilica layer contributed to more than 80% of the total permeation resistance. However, the permeation resistance of the supporting layers became dominant in the case of pervaporation (75°C). The contribution of the supporting layers was 73% when pervaporation was performed at 40°C. More specifically, the contribution of the permeation resistance of a SiO2-ZrO2 intermediate layer became almost half that in a high-water-content system and/or at low operation temperature. These results clearly show that a great opportunity remains for improvement in the overall permeation performance by minimizing the pressure drop at the supporting layer.
4.2 Dehydration of Acids Dehydration of acid solutions such as highly concentrated acetic acid is also an interesting industrial application for pervaporation. Acetic acid is an important chemical reagent that is used for the production of vinyl acetate monomers and various kinds of esters. A mixture of acetic acid and water is not an azeotrope, but it is a close-boiling-point mixture that requires a large amount of energy because of the high reflux ratio in distillation. Therefore pervaporationbased dehydration of an acetic acid solution provides an efficient and energy-saving separation. Moreover, since acetic acid is also used as a solvent for the production of terephthalic acid, the raw material for polyethylene terephthalate, dehydration of acetic acid is important not only in acetic acid production, but also in the recovery of acetic acid from reaction mixtures. Polymeric membranes are difficult to apply in acid dehydrations because of their low tolerance for concentrated acid. Hydrophilic zeolite membranes such as NaA zeolite, which are commercialized for alcohol dehydration, are not stable in acids because of the leaching of aluminum from the framework. Therefore zeolite membranes, including zeolite T (Cui et al., 2004), mordenite (Li, Kikuchi, & Matsukata, 2003a), ZSM-5 (Li, Kikuchi, & Matsukata, 2003b), and silicalite-1 (Matsuda, Otani, Tsuji, Kitamura, & Mukai, 2003), were developed, and great improvements in stability have been recently reported for pervaporation of acid solutions. The pervaporation performances of acetic acid/water mixtures for different types of membranes are summarized in Table 5. Silica is stable in aqueous acidic solutions, and therefore silica membranes are often applied for the dehydration of acetic acid solutions. Asaeda and coworkers reported the first dehydration of an aqueous acetic acid solution using silica membranes in 1990 (Kitao & Asaeda, 1990). They reported an improved performance in water flux of 3.06 kg m2 h1 with a high separation factor of 800 for a 90 wt% acetic acid/water mixture at 75°C (Asaeda & Kawasaki, 1998). Silica and silica-titania composite membranes were stable in highly concentrated acetic acid solutions, but degradation was observed for a water-rich system (Asaeda et al., 2005). Recently, Tsuru et al. reported that organic-inorganic hybrid silica membranes derived from BTESE showed excellent stability in the dehydration of acetic acid solutions with a water
Silica Membrane Application for Pervaporation Process 235 Table 5: Pervaporation dehydration performance of membranes for AcOH/water mixture Membrane Type
Feed AcOH H2O Permeance Conc. Total Flux (1027 mol m22 Separation 22 21 (wt%) (kg m h ) s21 Pa21) Factor
Silica-based membranes Silica
Silica SiO2-TiO2 ECN silica
100 100 80
90 90 90
5.90 2.20 1.90
28 10 17
525 2100 60
Kitao and Asaeda (1990) Asaeda and Kawasaki (1998) Asaeda et al. (2005) Asaeda et al. (2005) Sommer and Melin (2005) Tsuru et al. (2012)
Zeolite membranes Zeolite T MOR MOR
75 75 130
50 90 50
0.90 0.658 10.9
4.9 8.3 7.7
520 4832 500
Cui et al. (2004) Li et al. (2003a) Sato, Sugimoto, Kyotani, Shimotsuma, and Kurata (2011) Li et al. (2003b) Nagase, Kiyozumi, Hasegawa, and Inoue (2007)
Tanaka et al. (2011) Teli, Gokavi, Sairam, and Aminabhavi (2007) Adoor, Manjeshwar, Bhat, and Aminabhavi (2008)
Polymeric membranes Chen, Liu, Xiong, Zhanggm, and Zhu (2008) Asman and Sanli (2003)
Carbon Carbon molecular sieve
Mixed matrix membranes NaAlg/ silicotungstic acid NaAlg/zeolite beta
236 Chapter 9 flux of 2.0–4.0 kg m2 h1 and a separation factor of 200–500 at 90 wt% of acetic acid at 75°C (Tsuru et al., 2012). Stable pervaporation performance was confirmed after long-term exposure (70 days) to a 90 wt% acetic acid solution at 75°C and for acetic acid concentrations that ranged from 30 to 90 mol% (Tsuru et al., 2012).
4.3 Separation of Organic-Organic Mixtures Separation of organic-organic mixtures is important in the chemical and petrochemical industries. Pervaporation can provide a viable alternative to conventional separations such as distillation because of its efficient separation of azeotropic and close-boiling mixtures. Zeolite membranes such as silicalite, NaX, NaY, and ZSM-5 are used in a wide range of applications, which can be classified into several categories: polar/nonpolar solvent mixtures, aromatic/ aliphatic, aromatic/alicyclic, and isomers. Polymeric membranes can also be utilized after improving their stability by, for example, crosslinking, grafting, blending, or copolymerization (Smitha, Suhanya, Sridhar, & Ramakrishna, 2004). Table 6 summarizes the separation performance of different types of membranes for organic-organic mixture separation. Regarding the application of silica membranes for organic-organic mixtures, successful separation was reported for benzene/cyclohexane and methanol/dimethyl carbonate mixtures. Matsuyama et al. evaluated silica membranes prepared by a counter-diffusion chemical vapor deposition for the pervaporation of 50:50 wt% benzene/cyclohexane at room temperature Table 6: Pervaporation performance of membranes for organic-organic mixtures Membrane Type
Conc. of A (wt%)
Benzene/ cyclohexane MeOH/DMC
Flux Separation (mol m22 h21) Factor
2.2 104 kg m2 h1 178
Matsuyama et al. (2013) Tsuru, Sasaki, Kanezashi, and Yoshioka (2011)
Zeolite membranes MOR
p-Xylene/ o-xylene Benzene/ cyclohexane MeOH/MTBE MeOH/ Benzene MeOH/DMC
10 10 50
50 50 50
53 32 48
5300 7000 480
Nishiyama, Ueyama, and Matsukata (1996) Yuan, Lin, and Yang (2004) Nikolakis et al. (2001) Kita, Fuchida, Horita, Asamura, and Okamoto (2001)
Silica Membrane Application for Pervaporation Process 237 and reported a separation factor for benzene over cyclohexane of 113 (Matsuyama et al., 2013). Tsuru et al. demonstrated the pervaporation of methanol/dimethyl carbonate mixtures at 50°C using SiO2, SiO2-ZrO2, and SiO2-TiO2 membranes (Tsuru et al., 2011). Although SiO2-ZrO2 membranes showed a separation factor of <10, porous SiO2 membranes had an increased separation factor of from 10 to 160. Silica membranes with an average pore size of 0.3 nm showed the highest permselectivity of methanol with a separation factor of 140 and a total flux of 5.8 kg m2 h1 for a 50:50 mol% methanol/dimethyl carbonate mixture.
5 Conclusions Pervaporation has advantages for the separation of azeotropes and close-boiling mixtures. Microporous silica and silica-based membranes are promising candidates for pervaporation owing to their permselectivity and stability under harsh operating conditions such as at high temperatures or in highly concentrated acidic solutions. Various types of membranes consisting of amorphous silica, composites of silica and other oxides, and organic-inorganic hybrid silica have been developed and used in a wide range of applications. In recent years, organicinorganic hybrid silica membranes have been studied extensively because of their excellent stability under hydrothermal conditions where amorphous silica membranes cannot be applied. For the development of new pervaporation membranes, the flux must be increased while the selectivity either remains unchanged or is increased.
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