- Email: [email protected]
Seeded growth of beta zeolite membranes using zeolite structure-directing agent
Materials Letters 61 (2007) 1443 – 1445 www.elsevier.com/locate/matlet
Seeded growth of beta zeolite membranes using zeolite structure-directing agent Guolin Shao a , Jianhua Yang a , Xiongfu Zhang a , Gang Zhu a , Jinqu Wang a,⁎, Chang Liu b a
State Key Laboratory of Fine Chemicals, Institute of Adsorption and Inorganic Membrane, Dalian University of Technology, Dalian, 116012, PR China b Fushun Research Institute of Petroleum and Petrochemicals, Fushun, 113001, PR China Received 26 April 2006; accepted 16 July 2006 Available online 1 August 2006
Abstract In this work, beta zeolite membranes were prepared on seeded α-Al2O3 substrate tubes using zeolite structure-directing agent (ZSDA), instead of directly using organic agent as templates. This ZSDA method consists of two steps: preparation of beta ZSDA and growth of beta zeolite membranes with ZSDA by the secondary hydrothermal synthesis. The molar compositions of the synthesis solution of ZSDA and zeolite membrane are of 1SiO2/0.012Al2O3/0.167(TEA)2O/0.031Na2O/9.4H2O and 1SiO2/0.03Al2O3/0.027(TEA)2O/0.27Na2O/25H2O, respectively. Preparing the membranes only needed a little ZSDA, which was used to substitute for conventional templates in the secondary growth process of the membrane. The membranes were characterized by SEM, XRD and nitrogen permeance. It was found that the prepared membranes are continuous and defect-free. The mechanism of membrane growth with ZSDA was discussed simply. © 2006 Elsevier B.V. All rights reserved. Keywords: Beta zeolite membrane; Seeded growth; Preparation method; Zeolite structure-directing agent
1. Introduction Beta zeolite is a high-silica zeolite possessing a threedimensional interconnected channel system of 12-O ring large pore with pore diameters of 0.71 × 0.73 nm [1]. It is an intergrowth of two (or three) polymorphs, one of them being the only known real zeolite structure showing chirality [2,3]. Beta zeolite is a candidate for transformation into a valuable solid acid catalyst. It has potential application for separations and catalytic membrane reactors due to its unique pore structure and catalytic properties. In contrast to the small-pore [4] (0.42 nm, A-type) and medium-pore [5] (0.5–0.6 nm, MFI- and MELtype) zeolite membranes, beta zeolite membranes have the advantages for application of larger molecules. It can be used as a host to immobilize large complexes.
⁎ Corresponding author. Fax: +86 411 83653220. E-mail addresses: [email protected] (J. Yang), [email protected] (J. Wang). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.07.050
The key for such potential applications is whether a continuous and defect-free beta zeolite membrane can be prepared. The literatures report the need for a large amount of TEAOH as templates for the preparation of beta zeolite membranes [6–8]. These organic templates have to be removed by calcinations to open up the zeolite channels. Several studies have shown that cracks tend to form in the zeolite layer by changes in lattice parameters of zeolite crystals in the process of removing organic templates [9,10]. Using too many organic templates can therefore badly affect the quality of the membrane. If the amount of organic agent can be reduced, it should diminish the occurrence of cracking and formation of intercrystalline pores. The ZSDA method is expected to offer a solution to the above problem. Yin et al. [11] have utilized NaA ZSDA to synthesize high crystalline NaA zeolite particles. After preparing beta ZSDA, we used it to prepare the beta zeolite membranes successfully. The present work reports the synthesis of beta zeolite membranes on support surface by secondary synthesis with ZSDA instead of directly using organic templates. To our knowledge, this is the first report on the
1444
G. Shao et al. / Materials Letters 61 (2007) 1443–1445
Fig. 3. XRD patterns of the seeded α-Al2O3 support (a) and beta zeolite membranes (b).
synthesis of zeolite membrane with ZSDA. The á-Al2O3 tubes with a mean pore size of 2 μm supplied by the Nanjing University of Technology are used for supports. 2. Experimental section
Fig. 1. SEM images of α-Al2O3 supports surface before (a) and after (b) seeding.
Fig. 2. SEM images of the top and side view of the membranes.
Beta ZSDA was synthesized from the clear solution with a molar composition of 1SiO 2/0.012Al2O3/0.167(TEA)2O/ 0.031Na2O/9.4H2O at 407 K for 4 h. This clear synthesis solution was prepared by mixing with the desired amounts of silica gel, sodium aluminate, tetraethylammonium hydroxide, sodium hydroxide, and deionized water and stirring for 5 h at room temperature. Beta zeolite seeds were prepared using the procedure as described in the work of Xie et al. [12]. Prior to use, α-Al2O3 tubes with 12 mm OD and 9 mm ID were cut into smaller pieces with the length of 10 cm, and then the outer surface of the tubes was coated with beta zeolite seeds by dip coating in 1 wt.% seedcrystal suspension containing polyvinyl alcohol of 1 wt.% for 30 s. The seeded supports were dried at 333 K for 2 h and calcined in air at 573 K for 5 h. The precursor solution for the secondary grown membrane was prepared as follows. 0.816 g sodium aluminate (50 wt.% Al2O3, 38 wt.% Na2O) and 2.75 g sodium hydroxide (96 wt.%)
Fig. 4. Permeance of N2 vs. pressure difference through the prepared membrane.
G. Shao et al. / Materials Letters 61 (2007) 1443–1445 Table 1 Comparison of molar composition of the precursor solution among different methods Membrane
(TEA)2O/SiO2
Na2O/SiO2
Maloncy et al. [6] Tuan et al. [8] This study
0.25 0.32 0.027
0.04 0.05 0.27
were dissolved in 38.4 ml deionized water. Then 24.1 ml silicate sol (25 wt.%) was dropwise added into the above solution under stirring. Finally 5.4 ml ZSDA was dropwise added into the silicate–aluminate gel, and the precursor solution was further mixed under stirring for several hours. The total molar composition of the precursor solution was of 1SiO 2 / 0.03Al2O3/0.027(TEA)2O/0.27Na2O/25H2O. Subsequently, the precursor solution was transferred into a stainless-steel autoclave, and then the seeded tubes were immersed in the precursor solution. After crystallization at 423 K for 2–3 days the membranes were washed with distilled water and dried overnight. After four synthesis times the membranes were impermeable to N2 at room temperature and calcined to open the membrane pores at 723 K for 5 h in air with heating and cooling rates of 0.25 K/min. 3. Results and discussion Fig. 1 is the SEM images of α-Al2O3 tubes before and after seeding. Xomeritakis et al. [13] have reported that the film quality depends on the quality of the seed layer. Therefore we used the dip coating method to coat seeds on the surface of the supports. Compared with the extremely rough surfaces of the supports in Fig. 1(a), it is obviously seen in Fig. 1(b) that a uniform seeds layer can form on the surface of the supports by using dip coating in the seeds suspension containing polyvinyl alcohol. Fig. 2 shows the SEM images of the membranes obtained by the secondary growth with ZSDA. A continuous and dense zeolite membrane with around 9 ìm in thickness is formed on the seeded supports by the conducting growth function of ZSDA. The surface of the membrane is uniform and intergrown very well. But the surface morphology is different from that of the other reports [6–8]. This difference is probably attributed to the difference of the membrane growth mechanism which resulted from using ZSDA. The XRD spectra of the samples in Fig. 3 show that there exist no zeolite peaks on the seeded á-Al2O3 supports because the seeds layer is not thick enough. However, the synthesized membrane by the ZSDA method exhibits strong zeolite peaks at 7.5°, 21.3° and 22.3° 2θ with (h k l) values equal to (101), (205) and (116), which are characteristic of the beta zeolite structure [14]. This confirms the growth of beta zeolite membrane on the supports by the secondary growth with ZSDA. Results from the nitrogen permeation experiments through the prepared membrane are depicted in Fig. 4. The permeance decreases with increasing pressure difference across the membrane. This indicates that the permeation of N2 is mainly governed by surface diffusion. Surface diffusion occurs in zeolitic pores [15], and this confirms that the beta zeolite membrane prepared with ZSDA is defect-free. Table 1 shows the part feed compositions of beta zeolite membrane of this study and other reports. It is indicated that by using the ZSDA method the consumed amount of TEAOH was reduced by about 90% compared with the conventional method of directly using TEAOH as templates, and on the other hand, the ratio of Na2O/SiO2 increased about 6 times [6,7]. Bourgeat-Lami et al. have reported that a large amount of TEA+ cations
1445
were used to balance the negative charge of beta zeolite framework in the conventional method [16]. Further considering that sodium cation can balance the negative charge in the zeolite framework, it can be inferred that a lot of sodium cations substituting for TEA+ cations function to balance the negative charge of beta zeolite framework by this ZSDA method. Using this method to prepare beta zeolite membrane the consumed amount of organic agent was largely reduced. Several studies have shown that cracks tend to form in the zeolite layer by changes in the lattice parameters of zeolite crystals in the process of removing organic templates [9,10], accordingly such a great reduction of the consumed amount of organic agent made for diminishing the occurrence of cracking and formation of intercrystalline pores during the process of membrane calcinations largely improves the quality of the membrane.
4. Conclusions A seeded growth method using beta ZSDA was developed for the preparation of beta zeolite membrane. The characterization of the membrane confirmed that a continuous and defect-free beta zeolite membrane was obtained by using ZSDA. The consumed amount of organic agent was reduced by about 90% compared with the other reported methods in literature. Such a great reduction diminished the occurrence of crack during membrane calcinations, which accordingly generated a membrane with high quality. This ZSDA method is expected to be extended to the preparation of other zeolite membranes needing organic agent as templates. Acknowledgement We are grateful for the financial support from the National Natural Science Foundation of China (NNSFC: No. 20176004). References [1] C. Baerlocher, W.M. Meier, D.H. Olson, Atlas of Zeolite Framework Types, Elsevier, Amsterdam, 2001, p. 77. [2] J.M. Newsam, M.M.J. Treacy, W.T. Koetsier, C.B. de Gruyter, Proc. R. Soc. Lond., Ser. A. 420 (1988) 375. [3] J.B. Higgins, R.B. Lapierre, J.L. Schlenker, A.C. Rohrman, J.D. Wood, G.T. Kerr, W.J. Rohrbaugh, Zeolites 8 (1988) 446. [4] X.F. Zhang, W.Q. Zhu, H.O. Liu, T.H. Wang, Mater. Lett. 58 (2004) 2223. [5] G.E. Romanos, V. Kasselouri, N.K. Kanellopoulos, Mater. Lett. 57 (2003) 2840. [6] M.L. Maloncy, A.W.C. van den Berg, L. Gora, J.C. Jansen, Microporous Mesoporous Mater. 85 (2005) 96. [7] Vu A. Tuan, S.G. Li, John L. Falconer, Richard D. Noble, Chem. Mater. 14 (2002) 489. [8] Vu A. Tuan, Laura L. Webber, John L. Falconer, Richard D. Noble, Ind. Eng. Chem. Res. 42 (2003) 3019. [9] J. Hedlund, J. Sterte, M. Anthonis, A.J. Bons, B. Carstensen, N. Corcoran, D. Cox, H. Deckman, W.D. Gijnst, P.P. de Moor, F. Lai, J. McHenry, W. Moriter, J. Reinoso, J. Perters, Microporous Mesoporous Mater. 52 (2002) 179. [10] M.J. Xxter, H. Bekkum, C.J.M. Jijin, F. Kapteijin, J.A. Moulijn, H. Schellevis, C.I.N. Beenakker, Zeolites 19 (1997) 13. [11] X.J. Yin, G.S. Zhu, W.S. Yang, Y.S. Li, G.Q. Zhu, R. Xu, J.Y. Sun, S.L. Qiu, R.R. Xu, Adv. Mater. 17 (2005) 2006. [12] Z.K. Xie, Q.L. Chen, B. Chen, C.F. Zhang, Cryst. Eng. 4 (2001) 359. [13] G. Xomeritakis, A. Gouzinis, S. Nair, T. Okubo, M. He, R.U. Overney, M. Tsapatsis, Chem. Eng. Sci. 54 (1999) 3521. [14] M.M.J. Treacy, J.B. Higgins (Eds.), Elsevier, Amsterdam, 2001, p. 78. [15] N. Nishiyama, L. Gora, V. Teplyakov, F. Kapteijn, J.A. Moulijn, Sep. Purif. Technol. 22–23 (2001) 295. [16] E. Bourgeat-Lami, F.D. Renzo, F. Fajula, J. Phy. Chem. 96 (1992) 3807.
Seeded growth of beta zeolite membranes using zeolite structure-directing agent Guolin Shao a , Jianhua Yang a , Xiongfu Zhang a , Gang Zhu a , Jinqu Wang a,⁎, Chang Liu b a
State Key Laboratory of Fine Chemicals, Institute of Adsorption and Inorganic Membrane, Dalian University of Technology, Dalian, 116012, PR China b Fushun Research Institute of Petroleum and Petrochemicals, Fushun, 113001, PR China Received 26 April 2006; accepted 16 July 2006 Available online 1 August 2006
Abstract In this work, beta zeolite membranes were prepared on seeded α-Al2O3 substrate tubes using zeolite structure-directing agent (ZSDA), instead of directly using organic agent as templates. This ZSDA method consists of two steps: preparation of beta ZSDA and growth of beta zeolite membranes with ZSDA by the secondary hydrothermal synthesis. The molar compositions of the synthesis solution of ZSDA and zeolite membrane are of 1SiO2/0.012Al2O3/0.167(TEA)2O/0.031Na2O/9.4H2O and 1SiO2/0.03Al2O3/0.027(TEA)2O/0.27Na2O/25H2O, respectively. Preparing the membranes only needed a little ZSDA, which was used to substitute for conventional templates in the secondary growth process of the membrane. The membranes were characterized by SEM, XRD and nitrogen permeance. It was found that the prepared membranes are continuous and defect-free. The mechanism of membrane growth with ZSDA was discussed simply. © 2006 Elsevier B.V. All rights reserved. Keywords: Beta zeolite membrane; Seeded growth; Preparation method; Zeolite structure-directing agent
1. Introduction Beta zeolite is a high-silica zeolite possessing a threedimensional interconnected channel system of 12-O ring large pore with pore diameters of 0.71 × 0.73 nm [1]. It is an intergrowth of two (or three) polymorphs, one of them being the only known real zeolite structure showing chirality [2,3]. Beta zeolite is a candidate for transformation into a valuable solid acid catalyst. It has potential application for separations and catalytic membrane reactors due to its unique pore structure and catalytic properties. In contrast to the small-pore [4] (0.42 nm, A-type) and medium-pore [5] (0.5–0.6 nm, MFI- and MELtype) zeolite membranes, beta zeolite membranes have the advantages for application of larger molecules. It can be used as a host to immobilize large complexes.
⁎ Corresponding author. Fax: +86 411 83653220. E-mail addresses: [email protected] (J. Yang), [email protected] (J. Wang). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.07.050
The key for such potential applications is whether a continuous and defect-free beta zeolite membrane can be prepared. The literatures report the need for a large amount of TEAOH as templates for the preparation of beta zeolite membranes [6–8]. These organic templates have to be removed by calcinations to open up the zeolite channels. Several studies have shown that cracks tend to form in the zeolite layer by changes in lattice parameters of zeolite crystals in the process of removing organic templates [9,10]. Using too many organic templates can therefore badly affect the quality of the membrane. If the amount of organic agent can be reduced, it should diminish the occurrence of cracking and formation of intercrystalline pores. The ZSDA method is expected to offer a solution to the above problem. Yin et al. [11] have utilized NaA ZSDA to synthesize high crystalline NaA zeolite particles. After preparing beta ZSDA, we used it to prepare the beta zeolite membranes successfully. The present work reports the synthesis of beta zeolite membranes on support surface by secondary synthesis with ZSDA instead of directly using organic templates. To our knowledge, this is the first report on the
1444
G. Shao et al. / Materials Letters 61 (2007) 1443–1445
Fig. 3. XRD patterns of the seeded α-Al2O3 support (a) and beta zeolite membranes (b).
synthesis of zeolite membrane with ZSDA. The á-Al2O3 tubes with a mean pore size of 2 μm supplied by the Nanjing University of Technology are used for supports. 2. Experimental section
Fig. 1. SEM images of α-Al2O3 supports surface before (a) and after (b) seeding.
Fig. 2. SEM images of the top and side view of the membranes.
Beta ZSDA was synthesized from the clear solution with a molar composition of 1SiO 2/0.012Al2O3/0.167(TEA)2O/ 0.031Na2O/9.4H2O at 407 K for 4 h. This clear synthesis solution was prepared by mixing with the desired amounts of silica gel, sodium aluminate, tetraethylammonium hydroxide, sodium hydroxide, and deionized water and stirring for 5 h at room temperature. Beta zeolite seeds were prepared using the procedure as described in the work of Xie et al. [12]. Prior to use, α-Al2O3 tubes with 12 mm OD and 9 mm ID were cut into smaller pieces with the length of 10 cm, and then the outer surface of the tubes was coated with beta zeolite seeds by dip coating in 1 wt.% seedcrystal suspension containing polyvinyl alcohol of 1 wt.% for 30 s. The seeded supports were dried at 333 K for 2 h and calcined in air at 573 K for 5 h. The precursor solution for the secondary grown membrane was prepared as follows. 0.816 g sodium aluminate (50 wt.% Al2O3, 38 wt.% Na2O) and 2.75 g sodium hydroxide (96 wt.%)
Fig. 4. Permeance of N2 vs. pressure difference through the prepared membrane.
G. Shao et al. / Materials Letters 61 (2007) 1443–1445 Table 1 Comparison of molar composition of the precursor solution among different methods Membrane
(TEA)2O/SiO2
Na2O/SiO2
Maloncy et al. [6] Tuan et al. [8] This study
0.25 0.32 0.027
0.04 0.05 0.27
were dissolved in 38.4 ml deionized water. Then 24.1 ml silicate sol (25 wt.%) was dropwise added into the above solution under stirring. Finally 5.4 ml ZSDA was dropwise added into the silicate–aluminate gel, and the precursor solution was further mixed under stirring for several hours. The total molar composition of the precursor solution was of 1SiO 2 / 0.03Al2O3/0.027(TEA)2O/0.27Na2O/25H2O. Subsequently, the precursor solution was transferred into a stainless-steel autoclave, and then the seeded tubes were immersed in the precursor solution. After crystallization at 423 K for 2–3 days the membranes were washed with distilled water and dried overnight. After four synthesis times the membranes were impermeable to N2 at room temperature and calcined to open the membrane pores at 723 K for 5 h in air with heating and cooling rates of 0.25 K/min. 3. Results and discussion Fig. 1 is the SEM images of α-Al2O3 tubes before and after seeding. Xomeritakis et al. [13] have reported that the film quality depends on the quality of the seed layer. Therefore we used the dip coating method to coat seeds on the surface of the supports. Compared with the extremely rough surfaces of the supports in Fig. 1(a), it is obviously seen in Fig. 1(b) that a uniform seeds layer can form on the surface of the supports by using dip coating in the seeds suspension containing polyvinyl alcohol. Fig. 2 shows the SEM images of the membranes obtained by the secondary growth with ZSDA. A continuous and dense zeolite membrane with around 9 ìm in thickness is formed on the seeded supports by the conducting growth function of ZSDA. The surface of the membrane is uniform and intergrown very well. But the surface morphology is different from that of the other reports [6–8]. This difference is probably attributed to the difference of the membrane growth mechanism which resulted from using ZSDA. The XRD spectra of the samples in Fig. 3 show that there exist no zeolite peaks on the seeded á-Al2O3 supports because the seeds layer is not thick enough. However, the synthesized membrane by the ZSDA method exhibits strong zeolite peaks at 7.5°, 21.3° and 22.3° 2θ with (h k l) values equal to (101), (205) and (116), which are characteristic of the beta zeolite structure [14]. This confirms the growth of beta zeolite membrane on the supports by the secondary growth with ZSDA. Results from the nitrogen permeation experiments through the prepared membrane are depicted in Fig. 4. The permeance decreases with increasing pressure difference across the membrane. This indicates that the permeation of N2 is mainly governed by surface diffusion. Surface diffusion occurs in zeolitic pores [15], and this confirms that the beta zeolite membrane prepared with ZSDA is defect-free. Table 1 shows the part feed compositions of beta zeolite membrane of this study and other reports. It is indicated that by using the ZSDA method the consumed amount of TEAOH was reduced by about 90% compared with the conventional method of directly using TEAOH as templates, and on the other hand, the ratio of Na2O/SiO2 increased about 6 times [6,7]. Bourgeat-Lami et al. have reported that a large amount of TEA+ cations
1445
were used to balance the negative charge of beta zeolite framework in the conventional method [16]. Further considering that sodium cation can balance the negative charge in the zeolite framework, it can be inferred that a lot of sodium cations substituting for TEA+ cations function to balance the negative charge of beta zeolite framework by this ZSDA method. Using this method to prepare beta zeolite membrane the consumed amount of organic agent was largely reduced. Several studies have shown that cracks tend to form in the zeolite layer by changes in the lattice parameters of zeolite crystals in the process of removing organic templates [9,10], accordingly such a great reduction of the consumed amount of organic agent made for diminishing the occurrence of cracking and formation of intercrystalline pores during the process of membrane calcinations largely improves the quality of the membrane.
4. Conclusions A seeded growth method using beta ZSDA was developed for the preparation of beta zeolite membrane. The characterization of the membrane confirmed that a continuous and defect-free beta zeolite membrane was obtained by using ZSDA. The consumed amount of organic agent was reduced by about 90% compared with the other reported methods in literature. Such a great reduction diminished the occurrence of crack during membrane calcinations, which accordingly generated a membrane with high quality. This ZSDA method is expected to be extended to the preparation of other zeolite membranes needing organic agent as templates. Acknowledgement We are grateful for the financial support from the National Natural Science Foundation of China (NNSFC: No. 20176004). References [1] C. Baerlocher, W.M. Meier, D.H. Olson, Atlas of Zeolite Framework Types, Elsevier, Amsterdam, 2001, p. 77. [2] J.M. Newsam, M.M.J. Treacy, W.T. Koetsier, C.B. de Gruyter, Proc. R. Soc. Lond., Ser. A. 420 (1988) 375. [3] J.B. Higgins, R.B. Lapierre, J.L. Schlenker, A.C. Rohrman, J.D. Wood, G.T. Kerr, W.J. Rohrbaugh, Zeolites 8 (1988) 446. [4] X.F. Zhang, W.Q. Zhu, H.O. Liu, T.H. Wang, Mater. Lett. 58 (2004) 2223. [5] G.E. Romanos, V. Kasselouri, N.K. Kanellopoulos, Mater. Lett. 57 (2003) 2840. [6] M.L. Maloncy, A.W.C. van den Berg, L. Gora, J.C. Jansen, Microporous Mesoporous Mater. 85 (2005) 96. [7] Vu A. Tuan, S.G. Li, John L. Falconer, Richard D. Noble, Chem. Mater. 14 (2002) 489. [8] Vu A. Tuan, Laura L. Webber, John L. Falconer, Richard D. Noble, Ind. Eng. Chem. Res. 42 (2003) 3019. [9] J. Hedlund, J. Sterte, M. Anthonis, A.J. Bons, B. Carstensen, N. Corcoran, D. Cox, H. Deckman, W.D. Gijnst, P.P. de Moor, F. Lai, J. McHenry, W. Moriter, J. Reinoso, J. Perters, Microporous Mesoporous Mater. 52 (2002) 179. [10] M.J. Xxter, H. Bekkum, C.J.M. Jijin, F. Kapteijin, J.A. Moulijn, H. Schellevis, C.I.N. Beenakker, Zeolites 19 (1997) 13. [11] X.J. Yin, G.S. Zhu, W.S. Yang, Y.S. Li, G.Q. Zhu, R. Xu, J.Y. Sun, S.L. Qiu, R.R. Xu, Adv. Mater. 17 (2005) 2006. [12] Z.K. Xie, Q.L. Chen, B. Chen, C.F. Zhang, Cryst. Eng. 4 (2001) 359. [13] G. Xomeritakis, A. Gouzinis, S. Nair, T. Okubo, M. He, R.U. Overney, M. Tsapatsis, Chem. Eng. Sci. 54 (1999) 3521. [14] M.M.J. Treacy, J.B. Higgins (Eds.), Elsevier, Amsterdam, 2001, p. 78. [15] N. Nishiyama, L. Gora, V. Teplyakov, F. Kapteijn, J.A. Moulijn, Sep. Purif. Technol. 22–23 (2001) 295. [16] E. Bourgeat-Lami, F.D. Renzo, F. Fajula, J. Phy. Chem. 96 (1992) 3807.