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Redox switching of interlayer spacing in montmorillonite clay
JOURN&L OF
ELSEVIER
Journal of Electroanalytical Chemistry 406 (1996) 227-230
Preliminary note
Redox switching of interlayer spacing in montmorillonite clay S. Ravichandran, K.L.N. Phani *, S. Pitchumani, A. Mani * Emerging Concepts & Advanced Materials Group, EEB Division, Cenlral Electrochemical Research Institute, Karaikudi 630 006, India Received 7 November 1995; in revised form 18 December 1995
Keywords: Clay; Interlayer spacing; Montmorillonite clay; Pillared clay
1. Introduction Clay modified electrodes have attracted the attention of many electrochemists due to their ion-exchange properties and unique layered structure. Montmorillonite (MMT) belongs to the smectite group of clays. Intercalation of a variety of metal ions, metal complexes, organic monomers and polymers and their electroactivity in clays, particularly in MMT has been demonstrated [1-5] for various electrochemical applications. Most reports hitherto are devoted to the study of the effect of clay intercalation/encapsulation on the electrochemical behaviour of the "guest" species. However, none have attempted to study the effect of the "guest" (intercalate) species on the clay layers. Thus, it is intriguing to ask: Can we design an "elastic" host (clay)-guest system in which the host undergoes reversible volume changes in response to the changes in the guest species upon redox/electrochemical stimulus? Ideally, such changes demand the following criteria be fulfilled, in the first place: (1) the host should have a layered structure whose dimensions change upon intercalation; (2) the layered structure should resist chemical treatments without collapse; (3) the guest species exhibit reversible redox transformation with appreciably high cycle life; (4) the changes in the interlayer spacing should be in the molecular dimension; and (5) the porous nature of the host-guest system should be retained. This ideal system being "elastic" in nature is expected to show reversible shrinking and expansion upon redox/electrochemical stimulii. In the present communication, we present our first results on the effects of structural reorganization (upon redox stimulus) in the nickel(II) hydroxide/nickei(III)oxyhydroxide cou-
* Corresponding authors. 0022-0728/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved PH S 0 0 2 2 - 0 7 2 8 ( 9 6 ) 0 4 5 27-5
pie, on the shrinkage and expansion behaviour of the clay layers.
2. Experimental 10 mg of the Na-MMT clay (obtained from the Clay Mineral Repository, University of Missouri, USA) was suspended in 2 ml of water, sonicated for 15 min, followed by centrifugation for 30 rain at 2000 rev min -~. 2 ml of 0.01 M H3PO 4 (98%) was added to the clay suspension to avoid swelling of the clay film on indium tin oxide (ITO) glass substrates [6]. Aliquots of 0.1 ml of this suspension were employed for each coating on ITO (1 cm 2) and air dried for 2 days. The clay, coated on ITO, was kept immersed in 0.1 M Ni(NO3) 2 • 6H20 solution for 2 days for ion exchange to take place. These substrates were transferred to 25 ml of 0.01 M Ni(NO3)- 6H20 solution and slowly titrated [7,8] against 25 ml of 0.01 M NaOH (at a rate of 10 ml h - l ) with constant stirring. Stirring was continued for 3 days to pillar the clay layer with nickel hydroxide. The MMT film intercalated with nickel(II) hydroxide was then kept in 25 ml of 0.01 M NaOH solution. To this, 25 ml of 0.01 M K2S208 was added slowly at a rate of 10 ml h- l to oxidize nickel(II) hydroxide to the nickel(Ill) oxyhydroxide species within the clay layer. To the MMT-NiOOH film, which was kept in 0.01 M NaOH (25 ml), 1 ml of H 202 was added dropwise with stirring to reduce the film back to MMT-Ni(OH) 2. After the H202 treatment, the film was cycled in 0.1 M NaOH (0-600 mV vs SCE) to observe its electrochemical behaviour. At every stage of the experimental oxidation/reduction of the "guest" species in the clay layers, X-ray diffraction (XRD) patterns were recorded. The XRD patterns of the clay films on ITO glass were recorded using a computer-controlled JEOL-JDX8030 XRD system with Cu K a radiation, in the 2 0 range of
228
S. Ravichandran et aL / Journal of Electroanalytical Chemistry 406 (1996) 227-230
3-65 ° at a step-scan of 0.1 ° 219 per step. No pattern corresponding either Ni(OH) 2 or NiOOH bulk phases was observed; No XRD features of ITO glass were noticed. This serves to confirm that the recorded XRD patterns were corresponding to the clay layers exclusively. The UV-vis spectra were recorded on a Hitachi U-3400 spectrophotometer in the wavelength range 300-800 nm. The cyclic voltammetry was carried using a Wenking potentiostat (LB752), scan generator (VSG-72) and RikaDenki X-Y/t recorder.
to
1000
g('9
g
u:)
(c)
Ilao O *'¢
~r
500
o
I
~ 3-0 7.0
I 17-0
I
I 27-0
37.0
2000
3. Results and discussion The nickel(II) hydroxide, chosen here as the guest species possesses a layered structure and can be included inbetween the silicate sheets of the MMT clay [7,8], which in response to a redox stimulus undergoes noticeable changes. It is also because the molecular transformation of Ni(II) to other oxidation states is highly facile. Further, the chemical treatment conditions to effect these changes do not lead to the collapse of the clay layers, as seen on XRD examination. The intercalation of Ni(II) hydroxide in MMT is monitored using basal spacing (d001) of the clay layers in the XRD p~tems, and is shown in Fig. l(a). The spacing 14.359 A corresponds to the /3-Ni(OH) 2 intercalated with the clay layer; that is, the initial value for the dry MMT clay is 9.71 ,~ and the lattice expansion of 4.649 corresponds to the intercalated Ni(II) hydroxide, which matches well with the values reported earlier [9]. With the MMT-/3-Ni(OH) 2 layer, the redox transformation, particularly after the oxidation of the "guest" species from /3-Ni(OH) 2 to /3-NiOOH, the clay layer d-spacing is accompanied by the lattice expansion of 0.237 A~to yield the basal planar spacing of 14.596 ~,, as shown in Fig. l(b). On repeated oxidative treatments, interestingly, it changes from 14.596 to 19.840 ,~, which indicates the oxidation of /3-NiOOH to hydrated 7-NiOOH. The corresponding XRD pattern is seen in Fig. l(c). (The dimensional change from 14.359 to 19.840 A includes the value corresponding to the one water molecule associated with the y-NiOOH species (Fig. 2).) However, now the reduction of 7-NiOOH leads to /3-Ni(OH) 2, without going through the /3-NiOOH state. The change in lattice spacing of 0.237 A between /3-Ni(OH) 2 and /3-NiOOH within MMT layers is significant and cannot be mistaken for the reason that the accuracy of measurements of d-spacings at low 2t9 (or high d-values) is of the order of __+0.05 ,~, although normal accuracy of XRD analysis is +0.02 A. Hence, it is confirmed from standard basal spacings that the XRD patterns corresponding to the MMT-/3-Ni(OH) 2 (14.359 ~,) and MMT-/3-NiOOH (14.596 ~,) are distinctly different and are confirmed beyond doubt. The reduction of Ni(III) to Ni(II) has been observed when treated with H202. Thus the redox changes, /3o
(b)
) Z
g
~ooo;
U
Z Z N
0
[ 3"0 7.0
I
I
17:0
27.0
I 37.0
90 ; ¢.¢.o
(a)
2000
~~ ~,1 i.n -4"
0
I 3.0 7.0
¢,D
I 17.0
I 27.0
J 37-0
20 (degrees)
Fig. 1. XRD patterns corresponding to (a) MMT-/3-Ni(OH)2, (b) MMT/3-NiOOH and (c) MMT-hydrated 7-NiOOH.
Ni(OH) 2 ~/3-NiOOH are highly reversible. This is also observed by the colour change from light green (/3Ni(OH) 2) to black (/3-NiOOH) and vice versa. This conversion, as characterized by XRD, has been confirmed
14.T-51~I Cloy ,dyer I L._ ~- Ni(OH)~
[ Cloy,dyer 111!569~ ~. NiOOH~-.1
Oxidotion.
I Cloy,oyer I Reduction~
",
F-I
I C'oyloyer I Clay Ioyer
]
¢Ph hQSe or~e
~,=- Ni OOH~
I c,oy,oy
I
Fig. 2. Schematic of the basal plane changes of the MMT clay due to /3-Ni(OH) 2 ~/3-NiOOH, /3-NiOOH ~ y-NiOOH and y-NiOOH ~ / 3 Ni(OH) 2 .
229
S. Ravichandran et al. / Journal of Electroanalytical Chemistry 406 (1996) 227-230
(a)
.=
<
(b) CO
m
/~00 WAVELENGTH
E/V Fig. 3. Cyclic voltammogramsof MMT-Ni(II) hydroxide and MMTNi(lil) oxyhydroxide in 0.1 M NaOH, sweep rate= 100 mV s -I (vs SEE).
through its cyclic voltammetric behaviour with corresponding redox peak potentials Epa = 0.5 V and Epc = 0.415 V vs SCE (Fig. 3). The U V - v i s absorbance spectra also show the presence of Ni(II) hydroxide (Fig. 4(a)) within the clay interlayers vide two maxima at 360 and 500 nm, and the Ni(III) oxyhydroxide formation vide a broad peak with a maximum at 500-550 nm (Fig. 4(b)). Finally, the reduction from M M T - ? - N i O O H to M M T /3-Ni(OH) 2 is also achieved by the same peroxide treatment. The distinction of the reduction from 7-NiOOH and /3-NiOOH to /3-Ni(OH) 2 could be identified by XRD. Thus, the oxidation from MMT-fl-Ni(OH) 2 to MMT-/3NiOOH and its reversal, and the phase change from MMT-fl-NiOOH to MMT-),-NiOOH due to over-oxidation and its reduction to MMT-/3-Ni(OH) 2 have been clearly brought out in the schematic diagram shown in Fig. 2. In summary, we have demonstrated in this work, that it is possible to change the interlayer spacing of MMT clay reversibly using the redox chemistry of Ni(OH)2/NiOOH system. The change in d-spacing is observed from the transformation of fl-Ni(OH) 2 to /3-NiOOH and vice versa. The conversion of /3-NiOOH to ?-NiOOH, though associo ated with a very large d-spacing change of 5.481 A, is not found to be reversible. Other cleverer ways of stabilizing this conversion equilibrium without unwanted phase changes, would be an interesting proposition to fulfil the
800
GO0 (
nrn
)
Fig. 4. Absorbancespectra of(a) MMT-Ni(OH)2 and (b) MMT-NiOOH.
conditions spelt out in Section 1. In addition, other chemical systems as guest species, which could undergo facile redox reactions associated with reproducible volume changes are the topic of our current research. Using different combinations of clay host-guest complexes, it may be possible to design functionalized pillared clays that undergo size changes [10]. More generally, to design a system that can be "elastic" may be achieved by embedding an active pillar element into the clay that interferes with clay (host) structural equilibrium in response to the redox/electrochemical changes in the guest species.
Acknowledgement S.R. thanks CSIR, New Delhi for the award of a Senior Research Fellowship.
References [1] A. Fitch, Clays Clay Minerals, 38 (1990) 391; and references therein. [2] D. Ege, P.K. Ghosh, J.R. White, J.-F. Equey and AJ. Bar, J. Am. Chem. Soc., 107 (1985) 5644. [3] P. De S. Kaviramaand T.J. Pinnavaia, J. Electroanal. Chem., 332 (1992) 135. [4] P. Labbe, B. Brahimi, G. Reverdy, C. Mousty, R. Blankespoor, A. Gautier and C. Degrand, J. Electroanal. Chem., 379 (1994) 103.
230
S. Ravichandran et al. / Journal of Electroanalytical Chemistry 406 (1996) 227-230
[5] A. Fitch and P. Subramanian, J. Electroanal. Chem., 362 (1993) 177. [6] M. Isayama and T. Kunitake, Adv. Mater., 6 (1994) 77. [7] S. Yamanaka and G.W. Brindley, Clays Clay Minerals, 26 (1978) 21. [8] K. Ohtsuka, M. Suda, M. Ono, M. Takahashi, M. Sato and S. Ishio, Bull. Chem. Soc. Jpn., 60 (1987) 871.
[9] J. McBreen, in R.E. White, J.O'M. Bockris and B.E. Conway (Eds.), Modern Aspects of Electrochemistry, Chap. 2, Vol. 21, Plenum, New York, 1990, pp. 29-63. [10] Meeting Briefs, Chem. Eng. News, September 5, 1994, p. 35.
ELSEVIER
Journal of Electroanalytical Chemistry 406 (1996) 227-230
Preliminary note
Redox switching of interlayer spacing in montmorillonite clay S. Ravichandran, K.L.N. Phani *, S. Pitchumani, A. Mani * Emerging Concepts & Advanced Materials Group, EEB Division, Cenlral Electrochemical Research Institute, Karaikudi 630 006, India Received 7 November 1995; in revised form 18 December 1995
Keywords: Clay; Interlayer spacing; Montmorillonite clay; Pillared clay
1. Introduction Clay modified electrodes have attracted the attention of many electrochemists due to their ion-exchange properties and unique layered structure. Montmorillonite (MMT) belongs to the smectite group of clays. Intercalation of a variety of metal ions, metal complexes, organic monomers and polymers and their electroactivity in clays, particularly in MMT has been demonstrated [1-5] for various electrochemical applications. Most reports hitherto are devoted to the study of the effect of clay intercalation/encapsulation on the electrochemical behaviour of the "guest" species. However, none have attempted to study the effect of the "guest" (intercalate) species on the clay layers. Thus, it is intriguing to ask: Can we design an "elastic" host (clay)-guest system in which the host undergoes reversible volume changes in response to the changes in the guest species upon redox/electrochemical stimulus? Ideally, such changes demand the following criteria be fulfilled, in the first place: (1) the host should have a layered structure whose dimensions change upon intercalation; (2) the layered structure should resist chemical treatments without collapse; (3) the guest species exhibit reversible redox transformation with appreciably high cycle life; (4) the changes in the interlayer spacing should be in the molecular dimension; and (5) the porous nature of the host-guest system should be retained. This ideal system being "elastic" in nature is expected to show reversible shrinking and expansion upon redox/electrochemical stimulii. In the present communication, we present our first results on the effects of structural reorganization (upon redox stimulus) in the nickel(II) hydroxide/nickei(III)oxyhydroxide cou-
* Corresponding authors. 0022-0728/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved PH S 0 0 2 2 - 0 7 2 8 ( 9 6 ) 0 4 5 27-5
pie, on the shrinkage and expansion behaviour of the clay layers.
2. Experimental 10 mg of the Na-MMT clay (obtained from the Clay Mineral Repository, University of Missouri, USA) was suspended in 2 ml of water, sonicated for 15 min, followed by centrifugation for 30 rain at 2000 rev min -~. 2 ml of 0.01 M H3PO 4 (98%) was added to the clay suspension to avoid swelling of the clay film on indium tin oxide (ITO) glass substrates [6]. Aliquots of 0.1 ml of this suspension were employed for each coating on ITO (1 cm 2) and air dried for 2 days. The clay, coated on ITO, was kept immersed in 0.1 M Ni(NO3) 2 • 6H20 solution for 2 days for ion exchange to take place. These substrates were transferred to 25 ml of 0.01 M Ni(NO3)- 6H20 solution and slowly titrated [7,8] against 25 ml of 0.01 M NaOH (at a rate of 10 ml h - l ) with constant stirring. Stirring was continued for 3 days to pillar the clay layer with nickel hydroxide. The MMT film intercalated with nickel(II) hydroxide was then kept in 25 ml of 0.01 M NaOH solution. To this, 25 ml of 0.01 M K2S208 was added slowly at a rate of 10 ml h- l to oxidize nickel(II) hydroxide to the nickel(Ill) oxyhydroxide species within the clay layer. To the MMT-NiOOH film, which was kept in 0.01 M NaOH (25 ml), 1 ml of H 202 was added dropwise with stirring to reduce the film back to MMT-Ni(OH) 2. After the H202 treatment, the film was cycled in 0.1 M NaOH (0-600 mV vs SCE) to observe its electrochemical behaviour. At every stage of the experimental oxidation/reduction of the "guest" species in the clay layers, X-ray diffraction (XRD) patterns were recorded. The XRD patterns of the clay films on ITO glass were recorded using a computer-controlled JEOL-JDX8030 XRD system with Cu K a radiation, in the 2 0 range of
228
S. Ravichandran et aL / Journal of Electroanalytical Chemistry 406 (1996) 227-230
3-65 ° at a step-scan of 0.1 ° 219 per step. No pattern corresponding either Ni(OH) 2 or NiOOH bulk phases was observed; No XRD features of ITO glass were noticed. This serves to confirm that the recorded XRD patterns were corresponding to the clay layers exclusively. The UV-vis spectra were recorded on a Hitachi U-3400 spectrophotometer in the wavelength range 300-800 nm. The cyclic voltammetry was carried using a Wenking potentiostat (LB752), scan generator (VSG-72) and RikaDenki X-Y/t recorder.
to
1000
g('9
g
u:)
(c)
Ilao O *'¢
~r
500
o
I
~ 3-0 7.0
I 17-0
I
I 27-0
37.0
2000
3. Results and discussion The nickel(II) hydroxide, chosen here as the guest species possesses a layered structure and can be included inbetween the silicate sheets of the MMT clay [7,8], which in response to a redox stimulus undergoes noticeable changes. It is also because the molecular transformation of Ni(II) to other oxidation states is highly facile. Further, the chemical treatment conditions to effect these changes do not lead to the collapse of the clay layers, as seen on XRD examination. The intercalation of Ni(II) hydroxide in MMT is monitored using basal spacing (d001) of the clay layers in the XRD p~tems, and is shown in Fig. l(a). The spacing 14.359 A corresponds to the /3-Ni(OH) 2 intercalated with the clay layer; that is, the initial value for the dry MMT clay is 9.71 ,~ and the lattice expansion of 4.649 corresponds to the intercalated Ni(II) hydroxide, which matches well with the values reported earlier [9]. With the MMT-/3-Ni(OH) 2 layer, the redox transformation, particularly after the oxidation of the "guest" species from /3-Ni(OH) 2 to /3-NiOOH, the clay layer d-spacing is accompanied by the lattice expansion of 0.237 A~to yield the basal planar spacing of 14.596 ~,, as shown in Fig. l(b). On repeated oxidative treatments, interestingly, it changes from 14.596 to 19.840 ,~, which indicates the oxidation of /3-NiOOH to hydrated 7-NiOOH. The corresponding XRD pattern is seen in Fig. l(c). (The dimensional change from 14.359 to 19.840 A includes the value corresponding to the one water molecule associated with the y-NiOOH species (Fig. 2).) However, now the reduction of 7-NiOOH leads to /3-Ni(OH) 2, without going through the /3-NiOOH state. The change in lattice spacing of 0.237 A between /3-Ni(OH) 2 and /3-NiOOH within MMT layers is significant and cannot be mistaken for the reason that the accuracy of measurements of d-spacings at low 2t9 (or high d-values) is of the order of __+0.05 ,~, although normal accuracy of XRD analysis is +0.02 A. Hence, it is confirmed from standard basal spacings that the XRD patterns corresponding to the MMT-/3-Ni(OH) 2 (14.359 ~,) and MMT-/3-NiOOH (14.596 ~,) are distinctly different and are confirmed beyond doubt. The reduction of Ni(III) to Ni(II) has been observed when treated with H202. Thus the redox changes, /3o
(b)
) Z
g
~ooo;
U
Z Z N
0
[ 3"0 7.0
I
I
17:0
27.0
I 37.0
90 ; ¢.¢.o
(a)
2000
~~ ~,1 i.n -4"
0
I 3.0 7.0
¢,D
I 17.0
I 27.0
J 37-0
20 (degrees)
Fig. 1. XRD patterns corresponding to (a) MMT-/3-Ni(OH)2, (b) MMT/3-NiOOH and (c) MMT-hydrated 7-NiOOH.
Ni(OH) 2 ~/3-NiOOH are highly reversible. This is also observed by the colour change from light green (/3Ni(OH) 2) to black (/3-NiOOH) and vice versa. This conversion, as characterized by XRD, has been confirmed
14.T-51~I Cloy ,dyer I L._ ~- Ni(OH)~
[ Cloy,dyer 111!569~ ~. NiOOH~-.1
Oxidotion.
I Cloy,oyer I Reduction~
",
F-I
I C'oyloyer I Clay Ioyer
]
¢Ph hQSe or~e
~,=- Ni OOH~
I c,oy,oy
I
Fig. 2. Schematic of the basal plane changes of the MMT clay due to /3-Ni(OH) 2 ~/3-NiOOH, /3-NiOOH ~ y-NiOOH and y-NiOOH ~ / 3 Ni(OH) 2 .
229
S. Ravichandran et al. / Journal of Electroanalytical Chemistry 406 (1996) 227-230
(a)
.=
<
(b) CO
m
/~00 WAVELENGTH
E/V Fig. 3. Cyclic voltammogramsof MMT-Ni(II) hydroxide and MMTNi(lil) oxyhydroxide in 0.1 M NaOH, sweep rate= 100 mV s -I (vs SEE).
through its cyclic voltammetric behaviour with corresponding redox peak potentials Epa = 0.5 V and Epc = 0.415 V vs SCE (Fig. 3). The U V - v i s absorbance spectra also show the presence of Ni(II) hydroxide (Fig. 4(a)) within the clay interlayers vide two maxima at 360 and 500 nm, and the Ni(III) oxyhydroxide formation vide a broad peak with a maximum at 500-550 nm (Fig. 4(b)). Finally, the reduction from M M T - ? - N i O O H to M M T /3-Ni(OH) 2 is also achieved by the same peroxide treatment. The distinction of the reduction from 7-NiOOH and /3-NiOOH to /3-Ni(OH) 2 could be identified by XRD. Thus, the oxidation from MMT-fl-Ni(OH) 2 to MMT-/3NiOOH and its reversal, and the phase change from MMT-fl-NiOOH to MMT-),-NiOOH due to over-oxidation and its reduction to MMT-/3-Ni(OH) 2 have been clearly brought out in the schematic diagram shown in Fig. 2. In summary, we have demonstrated in this work, that it is possible to change the interlayer spacing of MMT clay reversibly using the redox chemistry of Ni(OH)2/NiOOH system. The change in d-spacing is observed from the transformation of fl-Ni(OH) 2 to /3-NiOOH and vice versa. The conversion of /3-NiOOH to ?-NiOOH, though associo ated with a very large d-spacing change of 5.481 A, is not found to be reversible. Other cleverer ways of stabilizing this conversion equilibrium without unwanted phase changes, would be an interesting proposition to fulfil the
800
GO0 (
nrn
)
Fig. 4. Absorbancespectra of(a) MMT-Ni(OH)2 and (b) MMT-NiOOH.
conditions spelt out in Section 1. In addition, other chemical systems as guest species, which could undergo facile redox reactions associated with reproducible volume changes are the topic of our current research. Using different combinations of clay host-guest complexes, it may be possible to design functionalized pillared clays that undergo size changes [10]. More generally, to design a system that can be "elastic" may be achieved by embedding an active pillar element into the clay that interferes with clay (host) structural equilibrium in response to the redox/electrochemical changes in the guest species.
Acknowledgement S.R. thanks CSIR, New Delhi for the award of a Senior Research Fellowship.
References [1] A. Fitch, Clays Clay Minerals, 38 (1990) 391; and references therein. [2] D. Ege, P.K. Ghosh, J.R. White, J.-F. Equey and AJ. Bar, J. Am. Chem. Soc., 107 (1985) 5644. [3] P. De S. Kaviramaand T.J. Pinnavaia, J. Electroanal. Chem., 332 (1992) 135. [4] P. Labbe, B. Brahimi, G. Reverdy, C. Mousty, R. Blankespoor, A. Gautier and C. Degrand, J. Electroanal. Chem., 379 (1994) 103.
230
S. Ravichandran et al. / Journal of Electroanalytical Chemistry 406 (1996) 227-230
[5] A. Fitch and P. Subramanian, J. Electroanal. Chem., 362 (1993) 177. [6] M. Isayama and T. Kunitake, Adv. Mater., 6 (1994) 77. [7] S. Yamanaka and G.W. Brindley, Clays Clay Minerals, 26 (1978) 21. [8] K. Ohtsuka, M. Suda, M. Ono, M. Takahashi, M. Sato and S. Ishio, Bull. Chem. Soc. Jpn., 60 (1987) 871.
[9] J. McBreen, in R.E. White, J.O'M. Bockris and B.E. Conway (Eds.), Modern Aspects of Electrochemistry, Chap. 2, Vol. 21, Plenum, New York, 1990, pp. 29-63. [10] Meeting Briefs, Chem. Eng. News, September 5, 1994, p. 35.