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Separation of methanolrmmethyl-tert-butyl ether mixture by pervaporation using silicalite membrane
j o u r n a l of MEMBRANE SCIENCE E LS E V I E R
Journal of Membrane Science 107 (1995) 193-196
Short communication
Separation of methanol/methyl-tert-butyl ether mixture by pervaporation using silicalite membrane Tsuneji Sano a,., Masaru Hasegawa a, Yusuke Kawakami a, Hiroshi Yanagishita b a Japan Advanced Institute of Science and Technology, Tatsunokuchi, lshikawa 923-12, Japan b National Institute of Materials and Chemical Research, Tsukuba, Ibaraki 305, Japan Received 2 November 1994; accepted 21 April 1995
Abstract The polycrystalline silicalite membrane was prepared on a porous sintered stainless steel support and its pervaporation performance was investigated using a MeOH/MTBE mixture as a feed. It was found that the MeOH selectivity of pervaporation is considerably higher than that of distillation. This indicates that the silicalite membrane permeates methanol preferentially. From the results of the competitive adsorption experiments and the temperature dependence of permeation rates of MeOH and MTBE, it was suggested that the selective sorption of MeOH into the membrane takes place and that the transportation of MTBE within the membrane is strongly suppressed as compared with that of MeOH. Keywords: Zeolite; Silicalite membrane; Pervaporation; Methanol; Methyl-tert-butyl ether
1. Introduction Zeolite has recently been focused on as one of the materials for inorganic membranes because of its high chemical and thermal stability as well as its molecular sieving potential. There are many literature, reports and patents related to zeolite membranes, especially pure zeolite membranes such as silicalite, ZSM-5, NaA and so on. Although the zeolite membranes are polycrystalline films and are very fragile, the membrane performances for pervaporation and gas separation have been investigated [ 1-6]. More recently, we have also studied the liquid separation potential of the zeolite membrane and reported a high pervaporation performance of the silicalite membrane for the separation of alcohol/water mixtures [ 7 - 9 ] . The high alcohol permselectivity is attributable * Corresponding author. 0376-7388/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD10376-7388(95)00113-1
to the selective sorption of alcohol into the silicalite membrane. From the permeation experiments of various organic molecules with different kinetic diameters through the membrane, it was also suggested that pores with ca. 1 nm diameter originated from silicalite crystals other than intrinsic zeolitic pores (ca. 0.6 nm) exist within the membrane. Therefore, the separation of alcohol/water mixtures seems to take place mainly through the pores originated from silicalite crystals. In this short communication, the potential of the silicalite membrane for the separation of a mixture of organic compounds was investigated using a mixture of methanol (MeOH) and methyl-tert-butyl ether (MTBE). It is well known that methyl-tert-butyl ether manufactured by the liquid phase reaction of methanol and isobutene is used as an additive for reformulated fuels, and its demand is now increasing.
194
T. Sano et aL / Journal of Membrane Science 107 (1995) 193-196
2. Experimental
The hydrothermal synthesis of a silicalite membrane was performed as follows. Colloidal silica (Cataloid SI-30 from Shokubai Kasei Co.; 30.4 wt% SIO2, 0.38 wt% Na20, 69.22 wt% water) was added to a stirred mixture of tetrapropylammonium bromide (TPABr) and sodium hydroxide in solution, to give a hydrogel with a composition of 0.1TPABr-0.05Na20-SiO280H20. Then the hydrogel was transferred to a 300 ml stainless steel autoclave. A porous support of sintered stainless steel disc (5 cm diameter) with an average pore diameter of ca. 2 tzm was placed on the bottom of the autoclave. The autoclave was placed in an airheated oven at 170°C for 48 h. After the completion of crystallization under autogenous pressure without stirring, the autoclave was cooled down, and the support was recovered. The silicalite membrane on the support was washed with deionized water and dried at 100°C. The silicalite membrane was then calcined at 400°C for 20 h in order to decompose the organic amine occluded in the zeolite framework. The identification of a polycrystalline silicalite membrane was achieved by X-ray diffraction. A membrane thickness of about 460 /zm was measured by scanning electron microscopy. The pervaporation measurements using MeOH/ MTBE mixtures as a feed were performed on a standard pervaporation apparatus. Liquid nitrogen was used as a cooling agent for the cold trap. The compositions of the feed and the permeate were determined by gas chromatography. The pervaporation performance was evaluated by the following flux and separation factor a(MeOH/MTBE).
triisopropylbenzene, whose dimension was too large to enter into the intrinsic zeolitic pores of silicalite, as a solvent. Samples were left overnight to ensure equilibrium.
3. Results and discussion
We have already reported that silicalite membranes prepared in the same preparation method show a slight difference in the pervaporation performance [9]. Therefore, the pervaporation performance of the silicalite membrane used in this study was at first checked using an aqueous ethanol solution of 5 vol% at 30°C. The silicalite membrane exhibited a high separation factor a(EtOH/HzO) of 63 combined with a flux of 0.60 kg/m2h. Next, in order to investigate the potential of the silicalite membrane for the separation of the MeOH/ MTBE mixture, the pervaporation selectivity was compared to vapor-liquid equilibrium at 760 mmHg [ 10]. The pervaporation experiments were carried out at 30°C using MeOH/MTBE mixtures with MeOH concentrations of 5-50 vol%. As shown in Fig. 1, the methanol selectivity of pervaporation was considerably higher than that of distillation, indicating that the membrane permeates methanol preferentially. There appear to be few reports concerning inorganic membranes which show the high separation potential for the MeOH/MTBE mixture. The permeation rate, flux, increased slightly with an increase in the methanol con1
Flux(kg/m2h) ~
(weight of permeate, kg) (membrane area, m 2) X (permeation time, h)
Pervaporat~, .7 .
0.8
/
.__
(]) Separation factor et (MeOH / MTBE) -
[ C M e O H / C M T B E ] P. . . .
._~ 0.4 te
(2)
[ CMeO./CMTBE] Feed where CMeo. and CMTBEare the volume fractions of MeOH and MTBE, respectively. To evaluate the adsorption performance of the silicalite membrane, the competitive adsorption of MeOH and MTBE was also carried out at 30-50°C using 1,3,5-
--~ o 0.2 0
I
0
I
I
I
0.2 0.4 0.6 0.8 Molefraction of MeOHin liquid
Fig. 1. Comparison of selectivity between pervaporation and distillation. Pervaporation experiments were carried out at 30°C.
195
T. Sano et al. / Journal of Membrane Science 107 (1995) 193-196
A
10 ca I---
" A ~
Table 1 Competitive adsorption of MeOH and MTBE on silicalite membrane"
A~
"1-
o¢D
Adsorption temperature
(°C)
1 .{= cq
0.1
~O--
"O
30 40 50
O~
c
~ 0.01 20
...... 0 ~ J 30
MeOH
MTBE
176 155 118
37 36 38
"Silicalite membrane: 1.5 g. Amount of adsorbate solution: 10 ml. Initial concentrations of MeOH and MTBE: 6.0 wt%.
0~ ~ 40
Amounts ofMeOH and MTBE adsorbed (mg/g)
J 50
60
Feed temperature (°C)
Fig. 2. Effect of feed temperature on pervaporation performance. Feed composition: MeOH/MTBE = 50/50 vol%; O, flux (MeOH); O, flux (MTBE); A, separation factor a ( M e O H / M T B E ) .
centration and the values were between 0.08 and 0.12 kg/m2h. A few introductory experiments were carried out to investigate the influence of the feed temperature on the pervaporation performance (Fig. 2). The feed methanol concentration was 50 vol%. The separation factor a ( M e O H / M T B E ) decreased with an increase in the feed temperature. The component flux of MTBE increased with the feed temperature, while the flux of MeOH was hardly dependent on the temperature. Taking into account that pores with ca. 1 nm diameter originated from silicalite crystals other than intrinsic zeolitic pores exist within the silicalite membrane [9] and that the molecular sizes (kinetic diameters) of MeOH and MTBE are 0.39 and 0.62 nm, respectively, these results suggest that the separation of MeOH/ MTBE mixtures takes also place through the voids among silicalite crystals and that the transportation of MTBE within the membrane is strongly suppressed as compared with that of MeOH. It is well known that the membrane performance in pervaporation depends upon the sorption of permeates in the feed side. Therefore, the liquid-phase adsorption experiments on the silicalite membrane were conducted. In Table 1 the amounts of MeOH and MTBE adsorbed on the membrane are presented. The amount of MeOH adsorbed was larger than that of MTBE. The difference in the adsorbed amount between MeOH and MTBE was, however, reduced with the adsorption tern-
perature. Taking into account that the silicalite shows the high hydrophobic property [ 11 ] and that MTBE is more organophilic than MeOH, this result indicates the selective sorption of MeOH into the membrane (the voids among silicalite crystals). From above results, it was concluded that although the silicalite membrane prepared here is polycrystalline, the membrane has the high methanol permselectivity. The high permselectivity is attributable to the selective sorption of methanol into the membrane. It is suggested that the silicalite membrane is effective in the separation process of MTBE synthesis and that the membrane has the high potential for the separation of organic mixtures.
References [1] J.G. Tsikoyiannis and W.O. Haag, Synthesis and characterization of a pure zeolitic membrane, Zeolites, 12 (1992) 126, [ 2 ] J. Dong, T. Dou, X. Zhao and L. Gao, Synthesis of membranes of zeolites ZSM-5 and ZSM-35 by the vapor phase method, J. Chem. Soc., Chem. Commun., (1992) 1056. [3] E.R. Geus, M.J. den Exter and H. van Bekkum, Synthesis and characterization of zeolite (MFI) membranes on porous ceramic supports, J. Chem. Soc,, Faraday Trans., 88 (1992) 3101. [4] E.R. Geus, H. van Bekkum, W.J.W. Bakker and A. Moulijn, High-temperature stainless steel supported zeolite (MFI) membranes: preparation, module construction and permeation experiments, Microporous Mater., I (1993) 131. [5] M.D. Jia, K.V. Peinemann and R.D. Behling, Ceramic zeolite composite membranes. Preparation, characterization and gas permeation, J. Membrane Sci., 82 (1993) 15. [6l M. Matsukata, N. Nishiyama and K. Ueyama, Preparation of a thin zeolitic membrane, Stud. Surf. Sci. Catal., 84 (1994) 1183.
196
T. Sano et al. /Journal of Membrane Science 107 (1995) 193-196
[7] T. Sano, H. Yanagishita, Y. Kiyozumi, D. Kitamoto and F. Mizukami, Separation of ethanol/water mixture by silicalite membrane, Chem. Lett., (1992) 2413. [8] T. Sano, H. Yanagishita, Y. Kiyozumi, f. Mizukami andK. Haraya, Separation of ethanol/water mixture by silicalite membrane on pervaporation, J. Membrane Sci., 95 (1994) 221. [9] T. Sano, H. Hasegawa, Y. Kawakami, Y. Kiyozumi, H. Yanagishita, D. Kitamoto and F. Mizukami, Potentials of silicalite membrane for the separation of alcohol/water mixture, Stud. Surf. Sci. Catal., 84 (1994) 1175.
[ 10] A. Vetere, I. Miracca and F. Cianci, Correlation and prediction of the vapor-liquid equilibria of the binary and ternary systems involved in MTBE synthesis, Fluid Phase Equilibria, 90 (1993) 189. [ 11 ] E.M. Flanigen, J.M. Bennett, R.W. Grose, J.P. Cohen, R.L. Patton, R.M. Kirchner and J.V. Smith, Silicalite, a new hydrophobic crystalline silica molecular sieve, Nature, 271 (1978) 512.
Journal of Membrane Science 107 (1995) 193-196
Short communication
Separation of methanol/methyl-tert-butyl ether mixture by pervaporation using silicalite membrane Tsuneji Sano a,., Masaru Hasegawa a, Yusuke Kawakami a, Hiroshi Yanagishita b a Japan Advanced Institute of Science and Technology, Tatsunokuchi, lshikawa 923-12, Japan b National Institute of Materials and Chemical Research, Tsukuba, Ibaraki 305, Japan Received 2 November 1994; accepted 21 April 1995
Abstract The polycrystalline silicalite membrane was prepared on a porous sintered stainless steel support and its pervaporation performance was investigated using a MeOH/MTBE mixture as a feed. It was found that the MeOH selectivity of pervaporation is considerably higher than that of distillation. This indicates that the silicalite membrane permeates methanol preferentially. From the results of the competitive adsorption experiments and the temperature dependence of permeation rates of MeOH and MTBE, it was suggested that the selective sorption of MeOH into the membrane takes place and that the transportation of MTBE within the membrane is strongly suppressed as compared with that of MeOH. Keywords: Zeolite; Silicalite membrane; Pervaporation; Methanol; Methyl-tert-butyl ether
1. Introduction Zeolite has recently been focused on as one of the materials for inorganic membranes because of its high chemical and thermal stability as well as its molecular sieving potential. There are many literature, reports and patents related to zeolite membranes, especially pure zeolite membranes such as silicalite, ZSM-5, NaA and so on. Although the zeolite membranes are polycrystalline films and are very fragile, the membrane performances for pervaporation and gas separation have been investigated [ 1-6]. More recently, we have also studied the liquid separation potential of the zeolite membrane and reported a high pervaporation performance of the silicalite membrane for the separation of alcohol/water mixtures [ 7 - 9 ] . The high alcohol permselectivity is attributable * Corresponding author. 0376-7388/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD10376-7388(95)00113-1
to the selective sorption of alcohol into the silicalite membrane. From the permeation experiments of various organic molecules with different kinetic diameters through the membrane, it was also suggested that pores with ca. 1 nm diameter originated from silicalite crystals other than intrinsic zeolitic pores (ca. 0.6 nm) exist within the membrane. Therefore, the separation of alcohol/water mixtures seems to take place mainly through the pores originated from silicalite crystals. In this short communication, the potential of the silicalite membrane for the separation of a mixture of organic compounds was investigated using a mixture of methanol (MeOH) and methyl-tert-butyl ether (MTBE). It is well known that methyl-tert-butyl ether manufactured by the liquid phase reaction of methanol and isobutene is used as an additive for reformulated fuels, and its demand is now increasing.
194
T. Sano et aL / Journal of Membrane Science 107 (1995) 193-196
2. Experimental
The hydrothermal synthesis of a silicalite membrane was performed as follows. Colloidal silica (Cataloid SI-30 from Shokubai Kasei Co.; 30.4 wt% SIO2, 0.38 wt% Na20, 69.22 wt% water) was added to a stirred mixture of tetrapropylammonium bromide (TPABr) and sodium hydroxide in solution, to give a hydrogel with a composition of 0.1TPABr-0.05Na20-SiO280H20. Then the hydrogel was transferred to a 300 ml stainless steel autoclave. A porous support of sintered stainless steel disc (5 cm diameter) with an average pore diameter of ca. 2 tzm was placed on the bottom of the autoclave. The autoclave was placed in an airheated oven at 170°C for 48 h. After the completion of crystallization under autogenous pressure without stirring, the autoclave was cooled down, and the support was recovered. The silicalite membrane on the support was washed with deionized water and dried at 100°C. The silicalite membrane was then calcined at 400°C for 20 h in order to decompose the organic amine occluded in the zeolite framework. The identification of a polycrystalline silicalite membrane was achieved by X-ray diffraction. A membrane thickness of about 460 /zm was measured by scanning electron microscopy. The pervaporation measurements using MeOH/ MTBE mixtures as a feed were performed on a standard pervaporation apparatus. Liquid nitrogen was used as a cooling agent for the cold trap. The compositions of the feed and the permeate were determined by gas chromatography. The pervaporation performance was evaluated by the following flux and separation factor a(MeOH/MTBE).
triisopropylbenzene, whose dimension was too large to enter into the intrinsic zeolitic pores of silicalite, as a solvent. Samples were left overnight to ensure equilibrium.
3. Results and discussion
We have already reported that silicalite membranes prepared in the same preparation method show a slight difference in the pervaporation performance [9]. Therefore, the pervaporation performance of the silicalite membrane used in this study was at first checked using an aqueous ethanol solution of 5 vol% at 30°C. The silicalite membrane exhibited a high separation factor a(EtOH/HzO) of 63 combined with a flux of 0.60 kg/m2h. Next, in order to investigate the potential of the silicalite membrane for the separation of the MeOH/ MTBE mixture, the pervaporation selectivity was compared to vapor-liquid equilibrium at 760 mmHg [ 10]. The pervaporation experiments were carried out at 30°C using MeOH/MTBE mixtures with MeOH concentrations of 5-50 vol%. As shown in Fig. 1, the methanol selectivity of pervaporation was considerably higher than that of distillation, indicating that the membrane permeates methanol preferentially. There appear to be few reports concerning inorganic membranes which show the high separation potential for the MeOH/MTBE mixture. The permeation rate, flux, increased slightly with an increase in the methanol con1
Flux(kg/m2h) ~
(weight of permeate, kg) (membrane area, m 2) X (permeation time, h)
Pervaporat~, .7 .
0.8
/
.__
(]) Separation factor et (MeOH / MTBE) -
[ C M e O H / C M T B E ] P. . . .
._~ 0.4 te
(2)
[ CMeO./CMTBE] Feed where CMeo. and CMTBEare the volume fractions of MeOH and MTBE, respectively. To evaluate the adsorption performance of the silicalite membrane, the competitive adsorption of MeOH and MTBE was also carried out at 30-50°C using 1,3,5-
--~ o 0.2 0
I
0
I
I
I
0.2 0.4 0.6 0.8 Molefraction of MeOHin liquid
Fig. 1. Comparison of selectivity between pervaporation and distillation. Pervaporation experiments were carried out at 30°C.
195
T. Sano et al. / Journal of Membrane Science 107 (1995) 193-196
A
10 ca I---
" A ~
Table 1 Competitive adsorption of MeOH and MTBE on silicalite membrane"
A~
"1-
o¢D
Adsorption temperature
(°C)
1 .{= cq
0.1
~O--
"O
30 40 50
O~
c
~ 0.01 20
...... 0 ~ J 30
MeOH
MTBE
176 155 118
37 36 38
"Silicalite membrane: 1.5 g. Amount of adsorbate solution: 10 ml. Initial concentrations of MeOH and MTBE: 6.0 wt%.
0~ ~ 40
Amounts ofMeOH and MTBE adsorbed (mg/g)
J 50
60
Feed temperature (°C)
Fig. 2. Effect of feed temperature on pervaporation performance. Feed composition: MeOH/MTBE = 50/50 vol%; O, flux (MeOH); O, flux (MTBE); A, separation factor a ( M e O H / M T B E ) .
centration and the values were between 0.08 and 0.12 kg/m2h. A few introductory experiments were carried out to investigate the influence of the feed temperature on the pervaporation performance (Fig. 2). The feed methanol concentration was 50 vol%. The separation factor a ( M e O H / M T B E ) decreased with an increase in the feed temperature. The component flux of MTBE increased with the feed temperature, while the flux of MeOH was hardly dependent on the temperature. Taking into account that pores with ca. 1 nm diameter originated from silicalite crystals other than intrinsic zeolitic pores exist within the silicalite membrane [9] and that the molecular sizes (kinetic diameters) of MeOH and MTBE are 0.39 and 0.62 nm, respectively, these results suggest that the separation of MeOH/ MTBE mixtures takes also place through the voids among silicalite crystals and that the transportation of MTBE within the membrane is strongly suppressed as compared with that of MeOH. It is well known that the membrane performance in pervaporation depends upon the sorption of permeates in the feed side. Therefore, the liquid-phase adsorption experiments on the silicalite membrane were conducted. In Table 1 the amounts of MeOH and MTBE adsorbed on the membrane are presented. The amount of MeOH adsorbed was larger than that of MTBE. The difference in the adsorbed amount between MeOH and MTBE was, however, reduced with the adsorption tern-
perature. Taking into account that the silicalite shows the high hydrophobic property [ 11 ] and that MTBE is more organophilic than MeOH, this result indicates the selective sorption of MeOH into the membrane (the voids among silicalite crystals). From above results, it was concluded that although the silicalite membrane prepared here is polycrystalline, the membrane has the high methanol permselectivity. The high permselectivity is attributable to the selective sorption of methanol into the membrane. It is suggested that the silicalite membrane is effective in the separation process of MTBE synthesis and that the membrane has the high potential for the separation of organic mixtures.
References [1] J.G. Tsikoyiannis and W.O. Haag, Synthesis and characterization of a pure zeolitic membrane, Zeolites, 12 (1992) 126, [ 2 ] J. Dong, T. Dou, X. Zhao and L. Gao, Synthesis of membranes of zeolites ZSM-5 and ZSM-35 by the vapor phase method, J. Chem. Soc., Chem. Commun., (1992) 1056. [3] E.R. Geus, M.J. den Exter and H. van Bekkum, Synthesis and characterization of zeolite (MFI) membranes on porous ceramic supports, J. Chem. Soc,, Faraday Trans., 88 (1992) 3101. [4] E.R. Geus, H. van Bekkum, W.J.W. Bakker and A. Moulijn, High-temperature stainless steel supported zeolite (MFI) membranes: preparation, module construction and permeation experiments, Microporous Mater., I (1993) 131. [5] M.D. Jia, K.V. Peinemann and R.D. Behling, Ceramic zeolite composite membranes. Preparation, characterization and gas permeation, J. Membrane Sci., 82 (1993) 15. [6l M. Matsukata, N. Nishiyama and K. Ueyama, Preparation of a thin zeolitic membrane, Stud. Surf. Sci. Catal., 84 (1994) 1183.
196
T. Sano et al. /Journal of Membrane Science 107 (1995) 193-196
[7] T. Sano, H. Yanagishita, Y. Kiyozumi, D. Kitamoto and F. Mizukami, Separation of ethanol/water mixture by silicalite membrane, Chem. Lett., (1992) 2413. [8] T. Sano, H. Yanagishita, Y. Kiyozumi, f. Mizukami andK. Haraya, Separation of ethanol/water mixture by silicalite membrane on pervaporation, J. Membrane Sci., 95 (1994) 221. [9] T. Sano, H. Hasegawa, Y. Kawakami, Y. Kiyozumi, H. Yanagishita, D. Kitamoto and F. Mizukami, Potentials of silicalite membrane for the separation of alcohol/water mixture, Stud. Surf. Sci. Catal., 84 (1994) 1175.
[ 10] A. Vetere, I. Miracca and F. Cianci, Correlation and prediction of the vapor-liquid equilibria of the binary and ternary systems involved in MTBE synthesis, Fluid Phase Equilibria, 90 (1993) 189. [ 11 ] E.M. Flanigen, J.M. Bennett, R.W. Grose, J.P. Cohen, R.L. Patton, R.M. Kirchner and J.V. Smith, Silicalite, a new hydrophobic crystalline silica molecular sieve, Nature, 271 (1978) 512.