J. Phys. Chem. B 2001, 105, 91-96
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An in Situ Energy-Dispersive X-ray Diffraction Study of the Hydrothermal Crystallization of Zeolite A. 2. Effect of Deuteration on Crystallization Kinetics Richard I. Walton† and Dermot O’Hare* Inorganic Chemistry Laboratory, UniVersity of Oxford, South Parks Road, Oxford, OX1 3QR, U.K. ReceiVed: July 30, 2000
Energy-dispersive X-ray diffraction has been used to follow the crystallization of zeolite A from amorphous aluminosilicate gels at 100 °C. Determination of changing Bragg peak intensity with time allows accurate quantitative kinetic information to be obtained. For the first time the effect of deuteration of reagents on the kinetics of a zeolite crystallization has been studied in detail. The kinetics of crystallization in NaOH/H2O and NaOD/D2O have been quantified using the nucleation growth model of Avrami and Erofe’ev. We find that the induction time of crystallization is lengthened when NaOH is replaced by NaOD, but no retardation of crystallization rate is observed. We suggest that this is due to the dissolution of starting materials being the rate-controlling step of the overall reaction. The new kinetic results are discussed in relation to previously postulated zeolite crystallization mechanisms.
Introduction In the preceding paper,1 we described the use of in situ energy-dispersive X-ray diffraction (EDXRD) to monitor the formation of zeolite A (LTA structure type) and its conversion into the dense aluminosilicate hydroxosodalite (SOD). This work was driven by the need to understand the formation mechanisms of the industrially important microporous solids. We illustrated the high quality of kinetic data that may be obtained by continual monitoring of Bragg peak intensity under real reaction conditions, and determined new information about the form of the crystallization curves. In this paper we describe a study of the effect of deuteration on the kinetics of crystallization of zeolite A using the EDXRD technique and relate our results to reaction mechanisms previously discussed in the literature for zeolite formation. Kinetic isotope effects on replacement of protons for deuterons are widely used in many areas of chemistry to probe the reaction mechanism of gas and solution phase reactions2 but have been much less applied to the study of heterogeneous reactions involving solids and liquids. In the case of zeolite crystallization, only two studies of the effect of deuteration on rate of reaction have been reported. The first of these, by Dutta et al., indicated that the growth of zeolite A was retarded in the presence of NaOD/D2O,3 and the second by Burkett and Davis showed that certain zeolites would crystallize more slowly when deuterated reagents were used, but that the effect was dependent on the sodium concentration and on the silica source used.4 Given the high-quality kinetic data we can now obtain using the EDXRD method, we decided to use the method to investigate the reported kinetic isotope effect more thoroughly.
crystallizations using gels of nominal composition Al2O3:2SiO2: xNaOL:20L2O (L+ ) lyonium, H+ or D+; x ) 2.5, 2.75, 3, 3.25, 3.5, 3.75, and 4) and Al2O3:2SiO2:xNaOL:80L2O (x ) 3, 3.25, 3.5, 3.75, and 4) were studied. The NaOD solution was purchased from Aldrich (∼40% NaOD in D2O), and the NaOH solution was prepared by dissolving NaOH pellets (BDH) in deionized water to produce a ca. 12 M solution. The concentrations of the NaOL/L2O solutions were determined accurately by titration against dilute HCl before the EDXRD experiments. For the preparation of a given gel, the required amount of solution was accurately weighed to allow the actual amount of NaOL used in a given experiment to be calculated. The solution was stirred with the premixed alumina and silica to produce a homogeneous opaque gel which was transferred into the Teflon liner of the cell. A constant percentage fill of the hydrothermal cell was used in all experiments. Data Analysis. Changing areas of characteristic Bragg reflections of LTA and SOD were determined using an automated Gaussian fitting routine5 to yield crystallization curves, as described in the preceding paper.1 Quantitative kinetic analysis of the crystallization curves was performed using the nucleation-growth model described by Avrami and Erofe’ev (eq 1).6-9
R ) 1 - exp{-(k(t - t0))n}
R is the extent of reaction scaled from zero at the beginning of reaction and unity at the end, t the time coordinate, k the rate constant, and n the Avrami exponent. The method of Sharp and Hancock10 was applied to produced linear plots from which the value of n and k may be easily determined (eq 2).
Experimental Section
ln[-ln(1 - R)] ) n ln(t) + n ln(k)
The EDXRD Experiment. Energy-dispersive X-ray diffraction experiments were performed on Station 16.4 of the Daresbury SRS as described in the previous paper.1 A series of * Corresponding author. E-mail:
[email protected]. † Present address: School of Chemistry, Stocker Road, University of Exeter, Exeter, EX4 4QD, UK.
(1)
(2)
Results The time for onset of crystallization of each zeolite was determined by inspection of the growth curves. These parameters are shown graphically in Figure 1 for each lyonium isotope for reactions performed using the more concentrated gels of
10.1021/jp002712h CCC: $20.00 © 2001 American Chemical Society Published on Web 12/12/2000
92 J. Phys. Chem. B, Vol. 105, No. 1, 2001
Walton and O’Hare TABLE 1: Kinetic for the Growth of Zeolite A from Gels of Composition Al2O3:2SiO2:xNaOH:20D2O at 100 °Ca NaOD concn/mol dm-3
t0/s
nb
k/10-4 s-1 b
10.221 93 9.597 838 8.957 967 8.349 925 8.200 462 7.647 393 7.586 229 6.873 431
1110 1290 1380 1260 1590 1740 1920 3780
2.46 3.82 2.51 2.15 1.56 1.86 2.11
32 12.7 15.5 12.3 6.59 8.05 3.41
a The induction time (t ) was determined by inspection. n is the 0 Avrami coefficient and k the rate constant for crystallization. b The reaction performed at the highest NaOD concentration was not modeled due to the small number of data points.
Figure 2. Rate constants for zeolite A crystallization from gel of composition Al2O3:2SiO2:xNaOL:20L2O at 100 °C. For discussion of error bars see previous paper.1
plotted in Figure 1c (for the lowest concentrations, the conversion of zeolite A to hydroxosodalite was not allowed to go to completion because of the long time-scale of the process). As already discussed in the previous paper, the total time that LTA is present increases as the sodium hydroxide concentration is increased; however, over the concentrations studied here there is little difference between reactions performed using NaOH and NaOD.
Figure 1. Plot of induction times for crystallization from gels of composition Al2O3:2SiO2:xNaOL:20L2O at 100 °C as a function of concentration of NaOL for (a) zeolite A and (b) hydroxosodalite, and (c) the total time zeolite A is observed. L ) H, squares; L ) D, diamonds. For discussion of error bars see text. Lines are for guidance only and have no physical significance.
composition, Al2O3:2SiO2:xNaOL:20L2O. The estimated errors on induction time were derived by considering the results of repeated runs at each concentration and that the automated Gaussian-fitting routine produces spurious results when attempting to fit a peak to the smooth amorphous background before crystallization begins; bearing in mind both of these factors, the induction times proved to be reproducible to within ( 5 min, and these are the error bars shown in Figure 1. At the higher end of the concentrations used, the induction times are essentially the same whether reactions are performed in NaOH/ H2O or NaOD/D2O, but as the concentration is lowered the induction times for crystallization become slightly larger when deuterated reagents are used. Another measure of the kinetics of reaction is the total time that zeolite A is present, and this is
For the initial growth of zeolite A from the gels Al2O3:2SiO2: xNaOL:20L2O the growth curves were normalized to the maximum intensity of the zeolite A Bragg reflection to produce extent of reaction data. The results of the Sharp-Hancock treatment of these data are given in Table 1. Figure 2 shows the rate constants with associated errors as a function of NaOL concentration of both isotopes used. For the highest concentrations used, zeolite A crystallizes very rapidly, and the growth curves have few data points; these were not included in the Sharp-Hancock analysis. Crystallization from gels of composition Al2O3:2SiO2:xNaOL: 80L2O at 100 °C was also investigated. Figure 3 shows normalized growth curves determined as above for the different NaOD concentrations studied. Under these more dilute conditions, the transformation of zeolite A into hydroxosodalite is considerably slower than the reactions studied above; experiments in the laboratory showed that heating for longer than 24 h was necessary for appreciable amounts of hydroxosodalite to be produced. As before, the most striking effect observed is the retarding effect of lowering the NaOL concentration, in this case only over a narrow range. The crystallization curve of zeolite A for the lowest NaOD concentration studied shows a
Hydrothermal Crystallization of Zeolite A. 2
J. Phys. Chem. B, Vol. 105, No. 1, 2001 93
Figure 5. Sharp-Hancock plots for zeolite A crystallization from gels of composition Al2O3:2SiO2:xNaOD:80D2O at 100 °C.
TABLE 2: Kinetic Parameters for the Growth of Zeolite A from Gels of Composition Al2O3:2SiO2:xNaOL:80L2O (L ) H or D) at 100 °Ca NaOD concn
k/10-4 s-1
n
NaOH concn
k/10-4 s-1
n
2.754 2.574 2.406 2.233 2.06
10.2 8.0 5.2 3.7 2.0
2.74 2.5 1.60 1.52 3.00
2.77 2.61 2.42 2.254 2.083
6.59 6.93 4.93 4.04 2.67
3.31 2.30 2.49 2.40 1.93
a
Legend as for Table 1.
Figure 3. (a) Normalized crystallization curves for zeolite A formation from gels of composition Al2O3:2SiO2:xNaOD:80D2O at 100 °C (b) expanded region of the crystallization curves illustrating the step in crystallization observed for the lower concentrations.
Figure 6. Avrami-Erofe’ev rate constants for zeolite A crystallization from gels of composition Al2O3:2SiO2:xNaOL:80L2O at 100 °C
Figure 4. Plot of induction times for crystallization of zeolite A as a function of concentration of NaOL from gels of composition Al2O3: 2SiO2:xNaOL:80L2O at 100 °C
very distinctive step at R ≈ 0.65. Repeating identical reactions indicated that this was a reproducible effect and this feature in the growth curves was also observed when NaOH is used, as we showed in the previous paper.1 Expanding the growth curves for the higher concentrations shows that a similar step is observed in all cases, albeit less pronounced for the higher concentrations (Figure 3b). For gels of composition Al2O3:2SiO2:xNaOL:80L2O at 100 °C the time for onset of crystallizations are plotted in Figure 4 for both L ) H and L ) D. As mentioned above, the induction times proved to be highly reproducible. This graph shows that there is an effect on changing NaOH for NaOD on the induction
time for crystallization. At the lower end of the concentration range studied, the effect of deuteration is most apparent, and time for Bragg reflections to be first detected doubles from ∼30 min to ∼1 h. Sharp-Hancock analysis of the crystallization data from the more dilute gels does not produce a linear graph, but rather two regions of linearity are produced when the step is most pronounced (Figure 5). Linear regression was performed on the first region of crystallization (0.1 < R < 0.5) to produce a rate constant to describe crystal growth, and the results are tabulated in Table 2. The rate constants are also shown graphically in Figure 6 for both L ) H and L ) D. Over this narrow concentration range there is clearly a significant effect on rate constant with changing concentration, but within experimental error the curves for both isotope are identical. Discussion Description of Zeolite Crystallization Mechanism. Zeolite crystallizations have been the focus of much attention for many years,11,12 but our study demonstrates that with development in experimental methods, new information can be obtained to further our understanding of the formation mechanism of zeolites. In order to relate our results to the previously postulated
94 J. Phys. Chem. B, Vol. 105, No. 1, 2001
Figure 7. Schematic of the possible processes involved in the hydrothermal crystallization of an aluminosilicate zeolite from alumina, silica, and sodium hydroxide solution.
zeolite crystallization mechanisms, we show in Figure 7 a simple scheme representing the possible steps involved in the crystallization of a sodium aluminosilicate zeolite. This includes the two extremes of reaction mechanism discussed in the literature: the homogeneous solution mediated formation of nucleation sites, and the heterogeneous formation of nucleation sites in the amorphous gel.11 Using X-ray diffraction, we can only observe the final step in this complex process: the growth of zeolite crystals beyond a critical size for coherent diffraction to be detected. None of the X-ray amorphous material (both solid and solution) before crystallization takes can be observed. NMR experiments have been performed by other workers to probe the amorphous precursors and solution phase prior to crystallization, and these studies have provided evidence that solution species produced by dissolution of the starting materials are the reactive species in the formation of zeolite nucleation sites;13-15 this is the basis for the scheme shown in Figure 7. Effect of Deuteration. The EDXRD technique has supplied crystallization data for zeolite formation of sufficient quality for the effect of deuteration on kinetics to be investigated systematically for the first time. The time of onset of crystallization observed by diffraction techniques is an important parameter that contains information about the initial steps of crystallization. It must be borne in mind that this induction time as measured is subject to some factors inherent in the experiment: in particular, the time taken for the reaction mixture to reach temperature, and the smallest particle size at which crystallites can be detected. These errors will be the same for all our experiments and so we believe the differences in the observed induction time are significant. We observe that as NaOL concentration is raised, the induction time is shortened, and this is a well-known phenomenon. More importantly, we observe an isotope effect; the onset time for the appearance of crystals is retarded and this becomes most obvious when the concentration is lowered (Figure 4). The rate of crystallization is not affected by deuteration. To the best of our knowledge, there are only two previous mentions of an isotope effect on replacing H with D in a zeolite crystallization. Dutta et al. studied the crystallization of zeolite A from a gel of composition 8.6Na2O:Al2O3:SiO2:556L2O at ∼90 °C over a period of ∼6 h, by quenching methods.3 These workers produced two growth curves showing the evolution of the zeolite by determining the percentage crystallinity of the product after various times of
Walton and O’Hare heating. An obvious difference was observed on deuteration and it was suggested that a kinetic isotope effect during the condensation polymerizations producing aluminosilicate anions and then nucleation sites which caused the retarded growth when NaOD/D2O was used. Burkett and Davis in their seminal NMR study of the synthesis of pure-silica zeolites, noted that the use of D2O instead of H2O could slow the kinetics of zeolite formation when certain combinations of sodium concentration and silica source were used.4 Again, quenching methods were used to produce crystallization curves. The authors suggested that nucleation processes (i.e., the steps of reaction happening prior to crystallization) are affected by deuteration. Our results show that only the induction time of crystallization is affected on deuteration and there is no evidence that the rate of crystallization is changed. The current, widely accepted view, of zeolite crystallizations is that they are solution-mediated processes which involve the condensation of anionic alumina and silica solution species formed by dissolution of the starting materials, rather than involving a direct reaction between the solid starting materials brought about by the presence of the concentrated sodium hydroxide solution (see Figure 7). The first step of reaction, taking place during the induction time is dissolution of the silica and alumina (or of an amorphous aluminosilicate formed when the chemicals are mixed13). The condensation processes which follow could potentially be affected by deuteration in a number of ways, the most obvious being if the formation of polyaluminosilicates is base-catalyzed, involving the cleavage of O-H/D bonds. Another possible effect of deuteration could be due to the fact that the hydrogen-bonding in D2O is stronger than in H2O, and potentially this could change the energetics of the polycondensation reactions since aluminate and silicate anionic species must interact with sodium cations involving restructuring of the solution. Other solvent isotope effects such as differing solubilites of reagents and change in vapor pressure must also be borne in mind, but these are likely to be relatively small, especially at elevated temperature.16 If the condensation reactions producing nucleation sites for crystallization are subject to a kinetic isotope effect then we would also expect the rate of crystallization of the zeolite to be affected. This is because the process of crystallization involves the same condensation reactions at the surface of the nucleation sites, incorporating the anionic solution species. The nucleationgrowth models that we have deduced indicate that after crystallization begins, there is continued formation of nucleation sites after crystal growth has begun. The value of the Avrami coefficient contains information about the mechanism of crystallization, and although it is difficult to draw specific conclusions about reaction mechanism, values of n greater than 1 indicate the continued formation of nucleation sites after crystallization has started. The values of the Avrami coefficient obtained in our study all are ca. 1-4 suggesting such a model and this is typical for a zeolite crystallization.17 These considerations imply that if deuteration were to affect the rate of formation of nucleation sites, it should also affect the rate of crystal growth. Origin of the Kinetic Isotope Effect. On the basis of the above discussion we conclude that within the errors of our experiment, the kinetic isotope effect observed does not arise from slowing of the polycondensations and hence rate of formation of nucleation sites. The most likely cause of the retarding of crystallization induction time on deuteration is in the dissolution of the starting materials. This is likely to be basecatalyzed and involve cleavage of O-H/D bonds. Surprisingly few studies have been made on kinetic isotope effects of the reactions of water with solids, given the great utility of isotope
Hydrothermal Crystallization of Zeolite A. 2 studies in probing the mechanisms of solution reactions in many areas of chemistry.2 The studies that have been performed that are pertinent to the current discussion are those of Casey et al. who showed that the acid-catalyzed dissolution of silica is retarded by use of D2O,18 and those by King et al.19 and Thomas and Jennings20 who both showed that the rate of hydration of tricalcium silicate is slowed in D2O compared to H2O. Thus our conclusion regarding the dissolution of silica and alumina by sodium hydroxide being subject to a kinetic isotope effect is quite reasonable. It is also consistent with the findings of Burkett and Davis who noted that crystallization of the siliceous zeolite ZSM-5 when using a liquid silica source (tetraethyl orthosilicate) was not affected by deuteration, whereas when solid silica is used crystallization is retarded;4 presumably the liquid silicon source is much more rapidly broken down than the solid and no kinetic isotope effect is measurable. Since ODis a stronger base than OH-, we might expect reactions performed in sodium deuterioxide to be more rapid than those in sodium hydroxide, given the marked dependence of reaction rates on OL- concentration, and therefore pL of the solution (pL ) -log[L+]). The fact that the reverse trend is observed suggests that the kinetic isotope effect must be large, and over the range of concentrations we have studied outweighs any effect of increased basicity of the solution. The isotopic substitution study has highlighted the important role that the dissolution of starting materials can play in the kinetics of zeolite formation. This point was also suggested by the work we described in the previous paper1 when two-step crystallization curves were produced. Such growth curves were also observed in the work described in this paper (Figure 3). As we stated in the previous paper,1 one possible reason for the pause in crystallization is that a point is reached when the solution is depleted of the species necessary for crystal growth and a critical concentration of the silica and alumina anions must be again reached for crystallization to continue. Alternatively, the unusual growth curves might be evidence for the autocatalytic mechanism, which involves the dissolution of amorphous gel to produce nucleation sites.21 The choice of silica source can greatly affect the form of the zeolite crystallization curve. The use of colloidal silica gives a smooth growth curve, whereas the use of solid fumed silica gives the distinctive twostep curve. The small particles of colloidal silica are likely to be more rapidly dissolved than the solid silica source, and this gives further evidence to support the view that the dissolution of the starting materials controls the crystallizations we have studied here. Conclusions Although much work has previously been directed toward determining the kinetics of zeolite crystallizations, our in situ EDXRD study demonstrates that with recent developments in instrumentation new information can be found and provide an increasingly detailed picture of zeolite formation when combined with all available information. This study has highlighted two new important features of zeolite crystallization; the kinetic isotope effect on deuteration, and the two-step growth curves. Both observations highlight how the dissolution of starting materials can have a large influence on the crystal growth of zeolites and possibly provides new evidence for the autocatalytic mechanism, widely postulated in the literature. Although we have only used very simple kinetic model to analyze our data, the accurate, high-quality crystallization curves we have obtained will be important to enable computer modeling of zeolite crystallization to be performed: an increasingly important tool
J. Phys. Chem. B, Vol. 105, No. 1, 2001 95 in understanding the formation and stability of zeolites, and an area in which much recent progress has been made.22,23 The energy-dispersive X-ray diffraction technique allows high-quality kinetic data to be determined. One disadvantage of the EDXRD technique is that low-resolution of the energydiscriminating detector limits the amount of structural information that can be obtained; we are not able to produce growth curves from many Bragg reflections during the transformation of one phase to another because of considerable peak overlap, and changes in Bragg peak width (and hence particle size) are difficult to monitor because of the large experimental broadening of reflections. Future developments in methodology could overcome these problems; for example, we have recently described a hydrothermal vessel for time-resolved neutron diffraction studies which offers the real possibility of extracting high-resolution diffraction data from a hydrothermal reaction. 24 In situ diffraction methods clearly have an important to play in the fuller understanding of zeolite crystallizations, but alone will not hold the key to formulating a complete mechanism. This is because the processes occurring in solution, before ordered crystalline material is formed, are of great importance in understanding crystallization and are not characterized by diffraction methods. Elegant in situ studies using techniques such as NMR spectroscopy15,25 and small-angle X-ray and neutron scattering26-28 and detailed studies on quenched samples using, for example, electron microscopy29 and atomic force microscopy,30 have recently been described and these do examine the amorphous material present prior to crystallization. The results from these methods must be combined with the results of diffraction studies to build up a complete picture of zeolite crystal growth. Clearly, a multitechnique approach is necessary to unravel fully the complexities of hydrothermal crystallization. Acknowledgment. This work was supported by the EPSRC who provided financial support and access to synchrotron radiation facilities at Daresbury laboratory. We thank the station scientist and technicians of Station 16.4 for their assistance with running the EDXRD experiments. References and Notes (1) Walton, R. I.; Millange, F.; O’Hare, D.; Davies, A. T.; Sankar, G.; Catlow, C. R. A. J. Phys. Chem. B, previous paper in this issue. (2) Bell, R. P. The Proton in Chemistry, 2nd ed.; Chapman and Hall: London, 1973. (3) Dutta, P. K.; Puri, M.; Bowers, C. ACS Symp. Ser. 1989, 398, 98. (4) Burkett, S. L.; Davis, M. E. Chem. Mater. 1995, 7, 1453. (5) Clark, S. M. J. Appl. Crystallogr. 1995, 28, 646. (6) Avrami, M. J. Chem. Phys. 1939, 7, 1103. (7) Avrami, M. J. Chem. Phys. 1940, 8, 212. (8) Avrami, M. J. Chem. Phys. 1941, 9, 177. (9) Erofe’ev, B. V. C. R. Dokl. Acad. Sci. URSS 1946, 52, 511. (10) Sharp, J. D.; Hancock, J. H. J. Am. Ceram. Soc. 1972, 55, 74. (11) Barrer, R. M. Hydrothermal Chemistry of Zeolites; Academic Press: London, 1982. (12) Thompson, R. W.; Dyer, A. Zeolites 1985, 5, 202. (13) Engelhardt, G. Zeolites 1983, 3, 292. (14) Dutta, P. K.; Shieh, D. C. J. Phys. Chem. 1986, 90, 2331. (15) Shi, J. M.; Anderson, M. W.; Carr, S. W. Chem. Mater. 1996, 8, 369. (16) Arnett, E. M.; McKelvey, D. R. Solute-solVent interactions; Marcel Decker: London, 1969. (17) Thompson, R. W. Zeolites 1992, 12, 681. (18) Casey, W. H.; Lasaga, A. C.; Gibbs, G. V. Geochim. Cosmochim. Acta 1990, 54, 3369. (19) King, T. C.; Dobson, C. M.; Rodger, S. A. J. Mater. Sci. Lett. 1988, 7, 861. (20) Thomas, J. J.; Jennings, H. M. Chem. Mater. 1999, 11, 1907. (21) Gonthier, S.; Gora, L.; Guray, I.; Thompson, R. W. Zeolites 1993, 13.
96 J. Phys. Chem. B, Vol. 105, No. 1, 2001 (22) Catlow, C. R. A.; Coombes, D. S.; Pereira, J. C. G. Chem. Mater. 1998, 11, 3249. (23) Thomson, K. T.; Wentzcotovitch, R. M.; McCormick, A.; Davis, H. T. Chem. Phys. Lett. 1998, 283, 39. (24) Walton, R. I.; Francis, R. J.; Halasyamani, P. S.; O’Hare, D.; Smith, R. I.; Done, R.; Humpreys, R. ReV. Sci. Instrum. 1999, 70, 3391. (25) Geradin, C.; In, M.; Allouche, L.; Haouas, M.; Taulelle, F. Chem. Mater. 1999, 11, 1285. (26) Watson, J. N.; Brown, A. S.; Iton, L. E.; White, J. W. J. Chem. Soc., Faraday Trans. 1998, 94, 2181.
Walton and O’Hare (27) Kirschhock, C. E. A.; Ravishankar, R.; Looveren, L. V.; Jacobs, P. A.; Martins, J. A. J. Phys. Chem. B 1999, 103, 4972. (28) Moor, P.-P. E. A. d.; Beelen, T. P. M.; Komanschek, B. U.; Beck, L. W.; Wagner, P.; Davis, M. E.; Santen, R. A. v. Chem. Eur. J. 1999, 5, 2083. (29) Mintova, S.; Olson, N. H.; Bein, T. Angew. Chem., Int. Ed. Engl. 1999, 38, 3201. (30) Agger, J. R.; Peraiz, N.; Cheetham, A. K.; Anderson, M. W. J. Am. Chem. Soc. 1998, 120, 10754.
