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Silylation of silicalite membrane and its pervaporation performance
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
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Silylation of silicalite membrane and its pervaporation performance T.Sano ~, K.Yamada ~, S.Ejiri a, M.Hasegawa a, Y.Kawakami a and H.Yanagishita b aSchooI of Materials Science, Japan Advanced Institute of Science and Technology, Tatsunokuchi, Ishikawa 923-12, Japan bNational Institute of Materials and Chemical Research, Tsukuba, Ibaraki 305, Japan To investigate the silylation condition of the silicalite membrane, the powdery silicalite crystals was silylated in gas or liquid phase using various silylating agents. Based on the results obtained, the silicalite membrane was silylated and its pervaporation performance was measured using an aqueous ethanol solution. The ethanol concentration of the permeate through the silylated silicalite membrane was markedly increased. It was concluded that the silylation is very effective for improvement of the separation performance of the silicalite membrane. 1. INTRODUCTION
Zeolite has been focused on as one of the materials for the inorganic membranes because of its molecular sieving property, thermal resistance and chemical stability [1]. We have recently reported that the polycrystalline silicalite membrane shows high pervaporation performance for the separation of alcohol/water mixtures [2-4] and acetic acid/water mixtures [5]. However, it was indicated that the separation of the liquid mixtures mainly takes place through not intrinsic zeolitic pores (0.53 x 0.56 nm) but the pores (ca.1 nm) which originate from silicalite crystals. The high alcohol or acetic acid permselectivity seems to be attributed to the high hydrophobic property of silicalite crystals. Therefore, in order to improve the separation performance of the silicalite membrane, the enhancement of the hydrophobicity of the membrane and the control of the pore sizes of small pores which originated from silicalite crystals must be conducted. In general, it is acknowledged that the silylation is very useful for modification of the surfaces of organic and inorganic materials. The modification is widely used to control adsorption and catalytic properties of the porous materials of zeolites by inactivation of the external surfaces and reduction of pore size [6-9]. Niwa et al. succeeded in controlling the pore size of mordenite, ZSM-5 and A zeolites by CVD (chemical vapor deposition) of silicon alkoxide [10-12]. There are a few reports concerning to the improvement of the membrane performance of porous glass and silica by using silylating agents [13,14]. From these standpoints, to clarify the silylation condition of the silicalite membrane, the powdery silicalite crystals were silylated using various silylating agents and the change in the hydrophobic property was monitored by water vapor adsorption. And
2180
then, effects of the silylation on the separation performance of the silicalite membrane was investigated. 2. E X P E R I M E N T A L
0.5 Stainless steel support
ssS
/s
sS sI Powdery silicalite crystals and ~ 0.4 s s iI SS the silicalite membranes on o iI iI I I porous supports were prepared ~- 0.3 -SI // l according to a procedure ~ I S , / previously described [1-4]. ~ 0.2 -Sintered stainless steel and alumina disc (5 cm diameter) with ~ 0.1 an average pore diameter of 0.5 ~/ I ~ 2 l~m were used as a porous "~ 300 \ 500 700 100 ure (~ support. As shown in Figure 1 the ~silicalite membrane experiences ,=:-'2-0.1 the irregular stress that arise from ~ o O -0.2 a difference in the thermal expansion between the stainless steel support and silicalite crystals. -0.3 Therefore, the membrane Figure 1 Thermal expansion curves of silicalite obtained was heated at 400~ for membrane and stainless steel support. 20 h in order to decompose the Heating 95~ Cooling 9be allowed to cool. organic amine occluded in the zeolite framework. The membrane was not disintegrated by this process. The silylation of powdery silicalite crystals in gas phase was conducted using a vacuum system. The powdery silicalite was evacuated at 400~ for 12h and then treated with an organic silane vapor (13.3 kPa) such as methyltrichlorosilane (CH3SiCI3), dimethyldichlorosilane ((CH3)2SICI2) and trimethylchlorosilane ((CH3)3SiCI) at 300~ for 3 h. Gaseous products and the silylating agent remained were removed by evacuation. The silylated silicalite was treated with water vapor (2.7 kPa) at 300~ for 1 h to hydrolyze Si-CI bonds on the deposited silylating agent. The amount of the silylating agent deposited was controlled by repeating the silylation after the treatment with water vapor. On the other hand, the silylation of powdery silicalite crystals and the silicalite membrane in liquid phase was conducted at 30~ using n-hexane containing a silylating agent (ca.3 wt%). The powdery silicalite and the membrane were washed several times with n-hexane to remove the free silylating agent. The amount of the silylating agent on the silicalite crystals or the membrane was evaluated using a Perkin-Elmer TGA 7 thermal analyzer. FT-IR spectra of silicalite crystals before and after the silylation was measured at room temperature using a JEOL JIR 7000 spectrometer equipped with an evacuable heatable chamber. The spectra were taken at 4 cm 1 resolution for 500 scans. The powdered zeolite was placed in a thin-walled ampule and then evacuated to about 10.5 torr at 400~ for 2 h. Concentration profiling for Si and C of the silylated membrane was measured by EDX attached to a Hitachi S-4100 scanning electron microscope. The water vapor (25~ and nitrogen (-196~ adsorption properties of the silicalite crystals before m
O
t-
m
I
I
2181
and after the silylation were evaluated using conventional volumetric apparatus (Bell Japan BELSORP 18 and BELSORP 28SA). Prior to adsorption measurements, the sample (ca.0.1 g) was evacuated at 400~ for 12 h. The pervaporation measurements using an aqueous ethanol (5 vol%) or acetic acid (15 vol%) solution as a feed were performed using a standard pervaporation cell. 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 flux and separation factor. 3. RESULTS AND DISCUSSION 3.1. Silylation of powdery silicalite crystals To determine the suitable silylation condition, the silylation of powdery silicalite crystals instead of the silicalite membrane was conducted using alkyltrichlorosilane as a silylating agent. Figure 2-(A) shows FT-IR spectra of silicalite crystals before and after the silylation at 300~ with CH3SiCI3, (CH3)2SiCI2 and (CH3)3SiCI. In a spectrum of silicalite before the silylation, two peaks were observed near 3740 and 3500 cm 1. The high frequency peak has been attributable to terminal silanol groups (SiOH), while the low frequency broad peak to hydrogen bonding adjacent hydroxyl groups. On the other hand, in the spectrum of the silicalite after the silylation, organic C-H peaks as well as the two peaks were observed in the region of 2800 to 3000 cm 1. The loss of the 3740 cm ~ peak by the silylation was very small. To clarify a difference in the reactivity of the silylating agent, the difference spectra were calculated
"'a
8
8
o
o
-e
I
4000
3500 3000 Wavenumbe rs(cm- 1)
2500 4000
,
I
I
I
3500 3000 Waven umbe rs(cm- 1)
I
,
I
2500
Figure 2 FT-IR spectra of silicalite crystals before and after silylation. Silylation condition : Temperature=300~ Time=3h, P(silylating agent)=13.3 kPa Silylating agent : (a) CH3SiCI3, (b) (CH3)2SiCI2, (c) (CH3)zSiCI, (d) before sUylation (e) a - d, (f) b - d, (g) c - d
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on the basis of the spectrum before the silylation. As shown in Figure 2-(B), the clear loss of the 3740 and 3500 cm 1 peak intensities was observed for silicalite silylated with CH3SiCI3 or (CH3)2SiCI2, while only a very small loss for silicalite silylated with (CH3)3SiCI. The difference in the loss of the 3740 and 3500 cm 1 peak intensities was not observed between silicalite silylated with CH3SiCI 3 and (CH3)2SiCI2. This indicates that the silylating agent is reacted with the silanol groups on the silicalite crystals under the present reaction condition and that CH3SiCI3 and (CH3)2SiCI2 are more effective as the silylating agent. The reason why the loss of the 3740 cm ~ peak intensity by the silylation is very small is considered as follows. It is known that the external surface area of zeolite crystal is below 10% of the total surface area (determined by nitrogen adsorption) [15,16]. Therefore, most of silanol groups in the silicalite crystals exist in the zeolitic pores. As the kinetic diameter of the silylating agent used seems to be slightly larger than the size of the silicalite intrinsic pore, almost no silylating agent molecules can incorporate into the pores, indicating a very small loss of the peak intensity at 3740 cm 1. From the standpoint of the control of pore size which originate from silicalite crystals, the further deposition of the silylating agent is needed. For the purpose, the formation of Si-OH by the hydrolysis of the Si-CI bonds of the silylating agent deposited on the silicalite crystals must be conducted at first. Figure 3 shows FT-IR spectra of the silylated silicalite, in which the amount of the silylating agent (CH3SiCI3) deposited was controlled by repeating the silylation after the water vapor treatment. The peak B
A
f
..Q I.=.
o
o ..Q <
,
4000
I
,
I
3500 3000 Wavenumbers(cm -1 )
,
'
2500 4000
I
i
I
3500 3000 Wavenumbe rs(cm-1 )
,
2500
Figure 3 Change in FT-IR spectra of silicalite crystals by repeating silylation. Silylation condition : Temperature=300~ Time=3h, P(CH3SiCI3)=13.3 kPa Water vapor treatment condition : Temperature=300~ Time=l h, P(H20)=2.7 kPa Number of silylation : (a) 10, (b) 5, (c) 3, (d) 1, (e) before silyiation, (f) a - e, (g) b - e, (h) c- e, (i) d- e
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intensity at 2800 - 3000 cm 1 increased with the number of the silylation, indicating formation of the mutilayers of the silylating agent. To clarify the changes in the hydrophobicity and intrinsic pore volume of silicalite crystals by the silylation with CH3SiCI3 was investigated by adsorption of water vapor and nitrogen. As shown in Figure 4, the amount of water adsorbed on the silylated silicalite decreased with the number of silylation, while the amount of nitrogen adsorbed hardly changed by the silylation. This strongly suggests that the silylation mainly takes place on the external surface of the silicalite crystals and that the external surface become more hydrophobic. 200
35
A
B
3O _
0
25
A
150
9
100
-
0 0 A
O
~.. 20 o
15 10
o -r
5
.
o
~
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0
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0.2
0.4 0.6 P/Ps (-)
0.8
1
0
0.2
0.4 0.6 P/Ps (-)
0.8
1
Figure 4 Adsorption isotherms of (A) water vapor (25~ and (B) nitrogen (-196~ silylated powdery silicalite. Number of silylation : IZ] 10, O 5, Zx 1, <> before silylation
on
To evaluate the amount of the silylating agent on the silicalite crystals, the thermal gravimetric curves were measured. The weight loss, which was attributable to decomposition of the silylating agent, was observed near 450~ and increased with the number of the silylation. The weight loss of about 0.3 wt% was observed for the powdery silicalite crystals with the number of silylation of 10. The temperature was considerably higher than that of the boiling temperature of the silylaUng agent (CH3SiCI3, 66~ This suggests that the silylating agent was strongly bonded to the surface of silicalite crystals. Next, the silylation of silicalite crystals was conducted in liquid phase. The silylating agents used were n-octyltrichlorosilane (C,H~iCI3), n-octadecyltrichlorosilane (C1,H37SICI3) and phenyltrichlorosilane (C6HsSiCI3). It was confirmed by FT-IR spectra of the silicalite after the silylation that these silylating agents were also reacted with the silanol groups on silicalite crystals. The thermal gravimetric curves of silicalite after the modification with C8H,7SiCi3 shows the weight loss of about 1 wt% near
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600~ The temperature was considerably higher than that of the boiling temperature (233~ of CsHI~SiCI3.
3.2. Effect of silylation of silicalite membrane on its pervaporation performance Based on the above results, the sUicalite membrane prepared on the porous stainless steel support was Si treated with CsH~SiCI 3 or C~8H3~SiCI3 as the silylating agent. Figure 5 shows the chemical analysis by EDX for the surface and ~ the cross section of the silylated silicalite membrane. The peak ~, intensity of carbon on the surface ~ C O of the membrane was relatively stronger than that on the cross < A section, indicating that the silylation mainly takes place on the surface of membrane. From the thermal gravimetric analysis, the amounts of the C81-1~7SiCI3 and C~8H37SiCI3 I I I I reacted with the silicalite 0 0.5 1 1.5 2 2.5 membranes were 0.91 and 1.21 E (keV) wt%, respectively. Next, the pervaporation Figure 5 Chemical analysis by EDX for (A) performance of the silylated silicalite surface and (B) cross section of silylated membrane was measured using an silcalite membrane. Table 1 Pervaporation performances of silicalite membranes before and after silylation a Membrane Silylating No. agent 1b 1b 2b 2b 3b 3b 4c 4c 4c 4c
Conc. of permeate (vol%) EtOH Acetic acid
CsHI~SiCI 3 CsH17SiCI 3 C18H3~SiCI3 CsHI~SiCI3 C8H17SICI3
22.1 56.1 43.5 67.2 41.7 70.6 19.8 31.2 14.6 14.9
Flux Separation factor ( k g / m 2 h ) o~(R/H20) 0.843 0.206 0.759 0.251 0.138 0.0534 0.195 0.025 0.939 0.235
5 24 15 40 13 44 4 9 1 1
a Feed ; EtOH : 5 vol%, Acetic acid: 15 vol%, Feed temperature :30~ b Stainless steel support c Alumina support
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aqueous ethanol (5 vol%) or acetic acid (15 vol%) solution as a feed. In Table 1 the results of pervaporation experiments are listed. It is clearly indicated that in the case of the silicalite membrane on the stainless steel support, the ethanol concentration of the permeate is markedly increased by the silylation with 08H17SiCI3 or 018H37SICI3. These results indicate that the modification with the silylating agent is effective for improvement of the separation performance of polycrystalline silicalite membrane. On the other hand, in the case of the membrane on the alumina support, only a slight improvement of the pervaporation performance was observed. As the flux of the silylated membrane is considerably smaller than that without silylation, the silylation of the membrane seems to take pace. Taking into account the fact that the surface SiOJAI203 ratio of the membrane before the silylation was ca.400 due to a partial dissolution of the alumina support, it is considered that the small enhancement of the pervaporation performance is attributable to a difference in the hydrophobicity between ZSM-5 and silicalite crystals. A few introductory experiments were carried out to investigate the influence of the feed temperature on the separation factor c~(EtOH/H20) and the flux of ethanol and water. As shown in Figure 6, the flux increased with an increase in the feed temperature, while the separation factor hardly changed. Based on these data, the apparent activation energies of the pervaporation rate of each component (ethanol and water) were calculated. The activation energy of the pervaporation rate of water was 8~9 kcal/mol, while the apparent activation energy for ethanol was 9~ 10 kcal/mol. These values were the same as those obtained using the silicalite membrane without silylation. Although an exact reason why the separation performance is improved by the silylation at the present time due to a limited data, this strongly indicates that the improvement of the pervaporation performance of the silicalite membrane by the silylation is mainly 0.7
....... ! O
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~
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20 =
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I I I 3.1 3.2 3.3 1/T x 10 -3 (l/K)
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Figure 6 Influences of feed temperature on separation factor c~(EtOH/H20) and flux. Feed ethanol concentration 95 vol%
2186
attributable to the enhancement of the hydrophobicity of silicalite membrane. The control of the amount of the silylating agent deposited on the silicalite membrane was tried by repeating the silylation after the water vapor treatment. However, the further deposition could not be conducted due to the disintegration of the membrane. 4. CONCLUSIONS The silylation condition of powdery silicalite crystals in gas or liquid phase was investigated using various silylating agents. It was suggested that the silylation mainly takes place on the external surface of silicalite crystals and the silylated silicalite crystals are more hydrophobic. Based on the results obtained, the silicalite membrane was silylated and its pervaporation performance was measured using an aqueous ethanol solution. The ethanol concentration of the permeate through the silylated silicalite membrane was markedly increased, indicating that the silylation is very effective for improvement of the separation performance of the zeolite membrane. REFERENCES
1. for example ; (a) J.G.Tsikoyiannis and W.O.Haag, Zeolites, 12 (1992) 126, (b) E.R.Geus, H.van Bekkum, W.J.W.Bakker and A.Moulijn, Microporous Mater.,1 (1993) 131,(c)M.D.Jia, K.V.Peinemann,R.D.Behling, J.Membrane Sci., 82 (1 993) 15, (d) Y.Yan, M.Tsapatsis, G.R.Gavalas and M.E.Davis, J.Chem.Soc.Chem. Commun., (1995) 227, (e) N.Nishiyama, K.Ueyama and M.Matsukata, J.Chem. Soc.Chem.Commun., (1995) 1967. 2. T.Sano, H.Yanagishita, Y.Kiyozumi, D.Kitamoto and F.Mizukami, Chem.Lett., (1992) 2413. 3. T.Sano, H.Yanagishita, Y.Kiyozumi, F.Mizukami and K.Haraya, J. Membrane Sci., 95 (1994) 221. 4. T.Sano, H.Hasegawa, Y.Kawakami, Y.Kiyozumi, H.Yanagishita, D.Kitamoto and F.Mizukami, Stud.Surf.Sci.Catal., 84 (1994) 1175. 5. T.Sano, S.Ejiri, M.Hasegawa, Y.Kawakami, N.Enomoto, Y.Tamai and H.Yanagishita, Chem.Lett., (1995) 153. 6. Y.F.Chu, C.F.Keweshan and E.F.Vansant, Stud.Surf.Sci.Catal., 46 (1989) 749. 7. M.Huang, A.Adnot and S.Kaliaguine, J.Chem.Soc. Faraday Trans., 89 (1993) 4231. 8. M.Chamoumi, D.Brunel, F.Fajula, P.Geneste, P.Moreau and J.Solofo, Zeolites, 14 (1994)282. 9. L.V.Piryutko, O.O.Parenago, E.V.Lunina, A.S.Kharitonov, L.G.Okkel and G.l.Panov, React.Kinet.Catal.Lett., 52 (1994) 275. 10. M.Niwa, H.Itoh, S.Kato, T.Hattori and Y.Murakami, J.Chem.Soc., Chem.Commun., (1982) 819. 11. M.Niwa, K.Yamazaki and Y.Murakami, Ind.Eng.Chem.Research, 30 (1991) 38. 12. T.Hibino, M.Niwa and Y.Murakami, Zeolites, 13 (1993) 518. 13.r Y.T.Lee, K.Iwamoto and M.Seno, Maku, 13, (1988) 171. 14. M.Shibata, H.Ohya, M.Aihara and Y.Negishi, Maku, 19 (1994) 332. 15. I.Suzuki, S.Namba and T.Yashima, J.Catal., 81 (1983) 485. 16. M.Inaoka, M.Yamada,S.Okada, M.Niwa and Y.Murakami, J.Catal., 100 (1986)264.
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Silylation of silicalite membrane and its pervaporation performance T.Sano ~, K.Yamada ~, S.Ejiri a, M.Hasegawa a, Y.Kawakami a and H.Yanagishita b aSchooI of Materials Science, Japan Advanced Institute of Science and Technology, Tatsunokuchi, Ishikawa 923-12, Japan bNational Institute of Materials and Chemical Research, Tsukuba, Ibaraki 305, Japan To investigate the silylation condition of the silicalite membrane, the powdery silicalite crystals was silylated in gas or liquid phase using various silylating agents. Based on the results obtained, the silicalite membrane was silylated and its pervaporation performance was measured using an aqueous ethanol solution. The ethanol concentration of the permeate through the silylated silicalite membrane was markedly increased. It was concluded that the silylation is very effective for improvement of the separation performance of the silicalite membrane. 1. INTRODUCTION
Zeolite has been focused on as one of the materials for the inorganic membranes because of its molecular sieving property, thermal resistance and chemical stability [1]. We have recently reported that the polycrystalline silicalite membrane shows high pervaporation performance for the separation of alcohol/water mixtures [2-4] and acetic acid/water mixtures [5]. However, it was indicated that the separation of the liquid mixtures mainly takes place through not intrinsic zeolitic pores (0.53 x 0.56 nm) but the pores (ca.1 nm) which originate from silicalite crystals. The high alcohol or acetic acid permselectivity seems to be attributed to the high hydrophobic property of silicalite crystals. Therefore, in order to improve the separation performance of the silicalite membrane, the enhancement of the hydrophobicity of the membrane and the control of the pore sizes of small pores which originated from silicalite crystals must be conducted. In general, it is acknowledged that the silylation is very useful for modification of the surfaces of organic and inorganic materials. The modification is widely used to control adsorption and catalytic properties of the porous materials of zeolites by inactivation of the external surfaces and reduction of pore size [6-9]. Niwa et al. succeeded in controlling the pore size of mordenite, ZSM-5 and A zeolites by CVD (chemical vapor deposition) of silicon alkoxide [10-12]. There are a few reports concerning to the improvement of the membrane performance of porous glass and silica by using silylating agents [13,14]. From these standpoints, to clarify the silylation condition of the silicalite membrane, the powdery silicalite crystals were silylated using various silylating agents and the change in the hydrophobic property was monitored by water vapor adsorption. And
2180
then, effects of the silylation on the separation performance of the silicalite membrane was investigated. 2. E X P E R I M E N T A L
0.5 Stainless steel support
ssS
/s
sS sI Powdery silicalite crystals and ~ 0.4 s s iI SS the silicalite membranes on o iI iI I I porous supports were prepared ~- 0.3 -SI // l according to a procedure ~ I S , / previously described [1-4]. ~ 0.2 -Sintered stainless steel and alumina disc (5 cm diameter) with ~ 0.1 an average pore diameter of 0.5 ~/ I ~ 2 l~m were used as a porous "~ 300 \ 500 700 100 ure (~ support. As shown in Figure 1 the ~silicalite membrane experiences ,=:-'2-0.1 the irregular stress that arise from ~ o O -0.2 a difference in the thermal expansion between the stainless steel support and silicalite crystals. -0.3 Therefore, the membrane Figure 1 Thermal expansion curves of silicalite obtained was heated at 400~ for membrane and stainless steel support. 20 h in order to decompose the Heating 95~ Cooling 9be allowed to cool. organic amine occluded in the zeolite framework. The membrane was not disintegrated by this process. The silylation of powdery silicalite crystals in gas phase was conducted using a vacuum system. The powdery silicalite was evacuated at 400~ for 12h and then treated with an organic silane vapor (13.3 kPa) such as methyltrichlorosilane (CH3SiCI3), dimethyldichlorosilane ((CH3)2SICI2) and trimethylchlorosilane ((CH3)3SiCI) at 300~ for 3 h. Gaseous products and the silylating agent remained were removed by evacuation. The silylated silicalite was treated with water vapor (2.7 kPa) at 300~ for 1 h to hydrolyze Si-CI bonds on the deposited silylating agent. The amount of the silylating agent deposited was controlled by repeating the silylation after the treatment with water vapor. On the other hand, the silylation of powdery silicalite crystals and the silicalite membrane in liquid phase was conducted at 30~ using n-hexane containing a silylating agent (ca.3 wt%). The powdery silicalite and the membrane were washed several times with n-hexane to remove the free silylating agent. The amount of the silylating agent on the silicalite crystals or the membrane was evaluated using a Perkin-Elmer TGA 7 thermal analyzer. FT-IR spectra of silicalite crystals before and after the silylation was measured at room temperature using a JEOL JIR 7000 spectrometer equipped with an evacuable heatable chamber. The spectra were taken at 4 cm 1 resolution for 500 scans. The powdered zeolite was placed in a thin-walled ampule and then evacuated to about 10.5 torr at 400~ for 2 h. Concentration profiling for Si and C of the silylated membrane was measured by EDX attached to a Hitachi S-4100 scanning electron microscope. The water vapor (25~ and nitrogen (-196~ adsorption properties of the silicalite crystals before m
O
t-
m
I
I
2181
and after the silylation were evaluated using conventional volumetric apparatus (Bell Japan BELSORP 18 and BELSORP 28SA). Prior to adsorption measurements, the sample (ca.0.1 g) was evacuated at 400~ for 12 h. The pervaporation measurements using an aqueous ethanol (5 vol%) or acetic acid (15 vol%) solution as a feed were performed using a standard pervaporation cell. 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 flux and separation factor. 3. RESULTS AND DISCUSSION 3.1. Silylation of powdery silicalite crystals To determine the suitable silylation condition, the silylation of powdery silicalite crystals instead of the silicalite membrane was conducted using alkyltrichlorosilane as a silylating agent. Figure 2-(A) shows FT-IR spectra of silicalite crystals before and after the silylation at 300~ with CH3SiCI3, (CH3)2SiCI2 and (CH3)3SiCI. In a spectrum of silicalite before the silylation, two peaks were observed near 3740 and 3500 cm 1. The high frequency peak has been attributable to terminal silanol groups (SiOH), while the low frequency broad peak to hydrogen bonding adjacent hydroxyl groups. On the other hand, in the spectrum of the silicalite after the silylation, organic C-H peaks as well as the two peaks were observed in the region of 2800 to 3000 cm 1. The loss of the 3740 cm ~ peak by the silylation was very small. To clarify a difference in the reactivity of the silylating agent, the difference spectra were calculated
"'a
8
8
o
o
-e
I
4000
3500 3000 Wavenumbe rs(cm- 1)
2500 4000
,
I
I
I
3500 3000 Waven umbe rs(cm- 1)
I
,
I
2500
Figure 2 FT-IR spectra of silicalite crystals before and after silylation. Silylation condition : Temperature=300~ Time=3h, P(silylating agent)=13.3 kPa Silylating agent : (a) CH3SiCI3, (b) (CH3)2SiCI2, (c) (CH3)zSiCI, (d) before sUylation (e) a - d, (f) b - d, (g) c - d
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on the basis of the spectrum before the silylation. As shown in Figure 2-(B), the clear loss of the 3740 and 3500 cm 1 peak intensities was observed for silicalite silylated with CH3SiCI3 or (CH3)2SiCI2, while only a very small loss for silicalite silylated with (CH3)3SiCI. The difference in the loss of the 3740 and 3500 cm 1 peak intensities was not observed between silicalite silylated with CH3SiCI 3 and (CH3)2SiCI2. This indicates that the silylating agent is reacted with the silanol groups on the silicalite crystals under the present reaction condition and that CH3SiCI3 and (CH3)2SiCI2 are more effective as the silylating agent. The reason why the loss of the 3740 cm ~ peak intensity by the silylation is very small is considered as follows. It is known that the external surface area of zeolite crystal is below 10% of the total surface area (determined by nitrogen adsorption) [15,16]. Therefore, most of silanol groups in the silicalite crystals exist in the zeolitic pores. As the kinetic diameter of the silylating agent used seems to be slightly larger than the size of the silicalite intrinsic pore, almost no silylating agent molecules can incorporate into the pores, indicating a very small loss of the peak intensity at 3740 cm 1. From the standpoint of the control of pore size which originate from silicalite crystals, the further deposition of the silylating agent is needed. For the purpose, the formation of Si-OH by the hydrolysis of the Si-CI bonds of the silylating agent deposited on the silicalite crystals must be conducted at first. Figure 3 shows FT-IR spectra of the silylated silicalite, in which the amount of the silylating agent (CH3SiCI3) deposited was controlled by repeating the silylation after the water vapor treatment. The peak B
A
f
..Q I.=.
o
o ..Q <
,
4000
I
,
I
3500 3000 Wavenumbers(cm -1 )
,
'
2500 4000
I
i
I
3500 3000 Wavenumbe rs(cm-1 )
,
2500
Figure 3 Change in FT-IR spectra of silicalite crystals by repeating silylation. Silylation condition : Temperature=300~ Time=3h, P(CH3SiCI3)=13.3 kPa Water vapor treatment condition : Temperature=300~ Time=l h, P(H20)=2.7 kPa Number of silylation : (a) 10, (b) 5, (c) 3, (d) 1, (e) before silyiation, (f) a - e, (g) b - e, (h) c- e, (i) d- e
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intensity at 2800 - 3000 cm 1 increased with the number of the silylation, indicating formation of the mutilayers of the silylating agent. To clarify the changes in the hydrophobicity and intrinsic pore volume of silicalite crystals by the silylation with CH3SiCI3 was investigated by adsorption of water vapor and nitrogen. As shown in Figure 4, the amount of water adsorbed on the silylated silicalite decreased with the number of silylation, while the amount of nitrogen adsorbed hardly changed by the silylation. This strongly suggests that the silylation mainly takes place on the external surface of the silicalite crystals and that the external surface become more hydrophobic. 200
35
A
B
3O _
0
25
A
150
9
100
-
0 0 A
O
~.. 20 o
15 10
o -r
5
.
o
~
OA 00 / o A oO o
~'~ O
5o
i--i r--1
n
0
0
0
0.2
0.4 0.6 P/Ps (-)
0.8
1
0
0.2
0.4 0.6 P/Ps (-)
0.8
1
Figure 4 Adsorption isotherms of (A) water vapor (25~ and (B) nitrogen (-196~ silylated powdery silicalite. Number of silylation : IZ] 10, O 5, Zx 1, <> before silylation
on
To evaluate the amount of the silylating agent on the silicalite crystals, the thermal gravimetric curves were measured. The weight loss, which was attributable to decomposition of the silylating agent, was observed near 450~ and increased with the number of the silylation. The weight loss of about 0.3 wt% was observed for the powdery silicalite crystals with the number of silylation of 10. The temperature was considerably higher than that of the boiling temperature of the silylaUng agent (CH3SiCI3, 66~ This suggests that the silylating agent was strongly bonded to the surface of silicalite crystals. Next, the silylation of silicalite crystals was conducted in liquid phase. The silylating agents used were n-octyltrichlorosilane (C,H~iCI3), n-octadecyltrichlorosilane (C1,H37SICI3) and phenyltrichlorosilane (C6HsSiCI3). It was confirmed by FT-IR spectra of the silicalite after the silylation that these silylating agents were also reacted with the silanol groups on silicalite crystals. The thermal gravimetric curves of silicalite after the modification with C8H,7SiCi3 shows the weight loss of about 1 wt% near
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600~ The temperature was considerably higher than that of the boiling temperature (233~ of CsHI~SiCI3.
3.2. Effect of silylation of silicalite membrane on its pervaporation performance Based on the above results, the sUicalite membrane prepared on the porous stainless steel support was Si treated with CsH~SiCI 3 or C~8H3~SiCI3 as the silylating agent. Figure 5 shows the chemical analysis by EDX for the surface and ~ the cross section of the silylated silicalite membrane. The peak ~, intensity of carbon on the surface ~ C O of the membrane was relatively stronger than that on the cross < A section, indicating that the silylation mainly takes place on the surface of membrane. From the thermal gravimetric analysis, the amounts of the C81-1~7SiCI3 and C~8H37SiCI3 I I I I reacted with the silicalite 0 0.5 1 1.5 2 2.5 membranes were 0.91 and 1.21 E (keV) wt%, respectively. Next, the pervaporation Figure 5 Chemical analysis by EDX for (A) performance of the silylated silicalite surface and (B) cross section of silylated membrane was measured using an silcalite membrane. Table 1 Pervaporation performances of silicalite membranes before and after silylation a Membrane Silylating No. agent 1b 1b 2b 2b 3b 3b 4c 4c 4c 4c
Conc. of permeate (vol%) EtOH Acetic acid
CsHI~SiCI 3 CsH17SiCI 3 C18H3~SiCI3 CsHI~SiCI3 C8H17SICI3
22.1 56.1 43.5 67.2 41.7 70.6 19.8 31.2 14.6 14.9
Flux Separation factor ( k g / m 2 h ) o~(R/H20) 0.843 0.206 0.759 0.251 0.138 0.0534 0.195 0.025 0.939 0.235
5 24 15 40 13 44 4 9 1 1
a Feed ; EtOH : 5 vol%, Acetic acid: 15 vol%, Feed temperature :30~ b Stainless steel support c Alumina support
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aqueous ethanol (5 vol%) or acetic acid (15 vol%) solution as a feed. In Table 1 the results of pervaporation experiments are listed. It is clearly indicated that in the case of the silicalite membrane on the stainless steel support, the ethanol concentration of the permeate is markedly increased by the silylation with 08H17SiCI3 or 018H37SICI3. These results indicate that the modification with the silylating agent is effective for improvement of the separation performance of polycrystalline silicalite membrane. On the other hand, in the case of the membrane on the alumina support, only a slight improvement of the pervaporation performance was observed. As the flux of the silylated membrane is considerably smaller than that without silylation, the silylation of the membrane seems to take pace. Taking into account the fact that the surface SiOJAI203 ratio of the membrane before the silylation was ca.400 due to a partial dissolution of the alumina support, it is considered that the small enhancement of the pervaporation performance is attributable to a difference in the hydrophobicity between ZSM-5 and silicalite crystals. A few introductory experiments were carried out to investigate the influence of the feed temperature on the separation factor c~(EtOH/H20) and the flux of ethanol and water. As shown in Figure 6, the flux increased with an increase in the feed temperature, while the separation factor hardly changed. Based on these data, the apparent activation energies of the pervaporation rate of each component (ethanol and water) were calculated. The activation energy of the pervaporation rate of water was 8~9 kcal/mol, while the apparent activation energy for ethanol was 9~ 10 kcal/mol. These values were the same as those obtained using the silicalite membrane without silylation. Although an exact reason why the separation performance is improved by the silylation at the present time due to a limited data, this strongly indicates that the improvement of the pervaporation performance of the silicalite membrane by the silylation is mainly 0.7
....... ! O
~E 0.4
~
1
0 50 ~
0.6 ~" 0.5
60
"1-
.,~/0 [ -
9
40o
----O
EtOH
E
uJ
0
v
X
30 ~ 94,-,
• 0.3 IT" 0.2
~0
0
20 =
0 "-~
/
H20
=0.1 r"
o
~E
o
0.1 O3
0
I
20
I
I
30 40 50 60 Feed temperature (~
0
0.01 3
I I I 3.1 3.2 3.3 1/T x 10 -3 (l/K)
3.4
Figure 6 Influences of feed temperature on separation factor c~(EtOH/H20) and flux. Feed ethanol concentration 95 vol%
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attributable to the enhancement of the hydrophobicity of silicalite membrane. The control of the amount of the silylating agent deposited on the silicalite membrane was tried by repeating the silylation after the water vapor treatment. However, the further deposition could not be conducted due to the disintegration of the membrane. 4. CONCLUSIONS The silylation condition of powdery silicalite crystals in gas or liquid phase was investigated using various silylating agents. It was suggested that the silylation mainly takes place on the external surface of silicalite crystals and the silylated silicalite crystals are more hydrophobic. Based on the results obtained, the silicalite membrane was silylated and its pervaporation performance was measured using an aqueous ethanol solution. The ethanol concentration of the permeate through the silylated silicalite membrane was markedly increased, indicating that the silylation is very effective for improvement of the separation performance of the zeolite membrane. REFERENCES
1. for example ; (a) J.G.Tsikoyiannis and W.O.Haag, Zeolites, 12 (1992) 126, (b) E.R.Geus, H.van Bekkum, W.J.W.Bakker and A.Moulijn, Microporous Mater.,1 (1993) 131,(c)M.D.Jia, K.V.Peinemann,R.D.Behling, J.Membrane Sci., 82 (1 993) 15, (d) Y.Yan, M.Tsapatsis, G.R.Gavalas and M.E.Davis, J.Chem.Soc.Chem. Commun., (1995) 227, (e) N.Nishiyama, K.Ueyama and M.Matsukata, J.Chem. Soc.Chem.Commun., (1995) 1967. 2. T.Sano, H.Yanagishita, Y.Kiyozumi, D.Kitamoto and F.Mizukami, Chem.Lett., (1992) 2413. 3. T.Sano, H.Yanagishita, Y.Kiyozumi, F.Mizukami and K.Haraya, J. Membrane Sci., 95 (1994) 221. 4. T.Sano, H.Hasegawa, Y.Kawakami, Y.Kiyozumi, H.Yanagishita, D.Kitamoto and F.Mizukami, Stud.Surf.Sci.Catal., 84 (1994) 1175. 5. T.Sano, S.Ejiri, M.Hasegawa, Y.Kawakami, N.Enomoto, Y.Tamai and H.Yanagishita, Chem.Lett., (1995) 153. 6. Y.F.Chu, C.F.Keweshan and E.F.Vansant, Stud.Surf.Sci.Catal., 46 (1989) 749. 7. M.Huang, A.Adnot and S.Kaliaguine, J.Chem.Soc. Faraday Trans., 89 (1993) 4231. 8. M.Chamoumi, D.Brunel, F.Fajula, P.Geneste, P.Moreau and J.Solofo, Zeolites, 14 (1994)282. 9. L.V.Piryutko, O.O.Parenago, E.V.Lunina, A.S.Kharitonov, L.G.Okkel and G.l.Panov, React.Kinet.Catal.Lett., 52 (1994) 275. 10. M.Niwa, H.Itoh, S.Kato, T.Hattori and Y.Murakami, J.Chem.Soc., Chem.Commun., (1982) 819. 11. M.Niwa, K.Yamazaki and Y.Murakami, Ind.Eng.Chem.Research, 30 (1991) 38. 12. T.Hibino, M.Niwa and Y.Murakami, Zeolites, 13 (1993) 518. 13.r Y.T.Lee, K.Iwamoto and M.Seno, Maku, 13, (1988) 171. 14. M.Shibata, H.Ohya, M.Aihara and Y.Negishi, Maku, 19 (1994) 332. 15. I.Suzuki, S.Namba and T.Yashima, J.Catal., 81 (1983) 485. 16. M.Inaoka, M.Yamada,S.Okada, M.Niwa and Y.Murakami, J.Catal., 100 (1986)264.