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Recovery of rhodium-containing catalysts by silica-based chelating ion exchangers containing N and S donor atoms
www.elsevier.nl/locate/ica Inorganica Chimica Acta 315 (2001) 183– 190
Recovery of rhodium-containing catalysts by silica-based chelating ion exchangers containing N and S donor atoms Jurjen Kramer, Arjen Scholten, Willem L. Driessen *, Jan Reedijk Leiden Institute of Chemistry, Leiden Uni6ersity, P.O. Box 9502, 2300 RA Leiden, The Netherlands Received 4 October 2000; accepted 22 January 2001
Abstract Two silica-based chelating ion exchangers have been synthesised for the recovery of the model catalysts RhCl3·3H2O and RhCl(PPh3)3. The N,S-donor ligands 2-aminocyclopentene-1-dithiocarboxylic acid and 4-amino-3-methyl-1,2,4-triazole-5-thione have been attached to the bifunctional spacer (3-glycidoxypropyl)trimethoxysilane prior to immobilisation on silica. Both ion exchangers showed a high Rh(III)-uptake capacity and selectivity over Cu(II), even in the presence of an excess of the latter. The Rh(I) complex was adsorbed to a much smaller extent, probably due to the bulky triphenylphosphane groups. Aquation negatively influenced the Rh(III)-uptake kinetics, whereas experiments performed in ethanol significantly increased the kinetic-uptake behaviour. Stripping of the loaded polymers proved to be very difficult, probably due to the strong Rhsulphur coordination. Only with thiourea a reasonable amount of Rh(III) could be stripped from the ion exchangers. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Rhodium complexes; Chelating ion exchange; Azathia ligand complexes; Kinetics
1. Introduction Homogeneous catalysts play an important role in the improvement of many organic reactions. These catalysts generally contain heavy metals, such as Co, Zn, Pd, Ru and Rh [1]. Both from an environmental point of view and because of the often high costs of the metal, the recovery of these metals is very important. Furthermore, contamination of the product with the catalyst and subsequent loss of the catalyst should be reduced to the lowest possible level. Hence, separation of the metal-containing homogeneous catalyst from product streams is very important. However, the complete recovery of the catalyst is a major difficulty in homogeneous catalysis, especially since the catalyst is usually present in very low concentrations [1,2]. Rhodium is a Group 9 metal and can be classified as a metal with a soft character, depending on the oxidation state, according to the Pearson classification [3,4]. * Corresponding author. Tel.: +31-71-5274345; fax: + 31-715274451. E-mail address: [email protected] (W.L. Driessen).
Hence, ligands that contain soft donor atoms, such as sulphur and phosphorus, will form relatively stable complexes with rhodium ions. Since both adsorption and subsequent desorption are the target, an optimal binding is required, as ligands that coordinate too strongly may prevent sufficient desorption. The immobilised ligands described in this paper contain both N and S donor atoms (see Scheme 1). The ligand 2-aminocyclopentene-1-dithiocarboxylic acid (Hacda, 1) is known to coordinate with Rh(I)-species via a six-membered chelate ring [5,6]. The ligand 4-amino-3-methyl-1,2,4-triazole-5-thione (Hamtt, 2) is a thiosemicarbazone in which the structural unit SCNN can bind to metal ions through S or through N, or through both atoms in a stable five-membered fashion, as deduced from the reported [7] crystal structures of several transition metals coordinated to 2. In this paper, the synthesis of silica-based chelating ion exchangers, modified with Hacda and Hamtt, is described. The ligands have been attached [8] via the bifunctional spacer (3-glycidoxypropyl)trimethoxysilane (Glymo) prior to immobilisation, see Scheme 1. Uptake and regeneration characteristics have been studied us-
0020-1693/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 0 1 ) 0 0 3 5 6 - 5
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J. Kramer et al. / Inorganica Chimica Acta 315 (2001) 183–190
ing the compounds RhCl3·3H2O and also RhCl(PPh3)3, i.e. Wilkinson’s catalyst, as a Rh-containing model catalyst. As ligand exchange of rhodium compounds is known to proceed much slower than that of compounds of other elements such as copper [9,10], the kinetic-uptake behaviour has also been studied in detail.
2. Experimental
2.1. Reagents and analytical methods All reactions were performed under an argon atmosphere with standard Schlenk techniques. All reagents and solvents were purchased from commercial sources and were used without further purification unless stated otherwise. Toluene was distilled from Na under argon and stored over molecular sieves. Methanol was distilled from Mg under an argon atmosphere and stored over molecular sieves. Silica (Aldrich Silica gel Davisil™, particle size 4 – 20 mm, surface area 480 m2 g − 1, pore volume 0.75 cm3 g − 1) was activated before use by heating in vacuo (200 mbar) at 50°C for at least 48 h. The ligand concentrations of the functionalised silica were calculated from the weight percentage of nitrogen. The compound RhCl(PPh3)3 was synthesised according to a literature procedure [11]. RhCl3·3H2O was purchased from Aldrich. Hamtt was synthesised according to a literature procedure [7] and was further purified by recrystallisation from water. Hacda was prepared following the method described by Nag et al. [12].
1
H and 13C NMR spectra were recorded on a 200 MHz JEOL JNM FT NMR spectrometer operating at 199.50 and 50.10 MHz for 1H and 13C NMR, respectively. Solid-state 13C NMR CP MAS NMR spectra were recorded on a Bruker MSL 400 spectrometer operating at 100.6 MHz with a rotational spin speed of 7 kHz. Metal analyses were performed on a Perkin – Elmer 3100 atomic absorption (AAS) and flame emission spectrometer using a linear calibration method. Elemental analyses (C, H, N, S) were carried out by the Analytical Service Centre of the Gorlaeus Laboratories or by the Microanalytical Laboratory of the University College in Dublin. Sulphur percentages were found to vary significantly whereas the other percentages were reproducible. Since carbon and hydrogen are also found in the Glymo spacer whereas nitrogen is only present in the ligands, the choice was made to base all ligand concentrations on nitrogen percentages, as is generally done.
2.2. Immobilisation 2.2.1. The acda–Glymo ion exchanger (1B) The ligand was first coupled to Glymo in the following way: 4.3 g (17.1 mmol) of Glymo was added to 3.0 g (17.1 mmol) of the ammonium salt of Hacda in methanol (115 ml) and heated at 50°C for 17 h to give compound 1A. 1 H NMR (CDCl3, l ppm): 0.68 (t, 2H, SiCH2); 1.69, 1.85 (2× p, 4H, CH2CH2CH2-Hacda, SiCH2CH2); 2.5 (m, 2H, CH2S); 2.66, 2.82 (2× t, 4H, CH2CH2CH2Hacda); 3.3 –3.7 (m, 14H, Si(OCH3)3, CH2OCH2, CHOH); 7.0, 11.1 (2×bs, 2H, NH2). 13C NMR (CDCl3, l ppm): 4.6 (SiCH2); 19.9, 22.0 (CH2CH2CH2-
Scheme 1. Immobilisation of the ligands Hacda (1) and Hamtt (2) onto silica.
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Hacda, SiCH2CH2); 32.2, 35.1, 35.6 (CH2S, CH2CH2CH2-Hacda); 49.7 (Si(OCH3)3); 69.1 (CHOH); 72.5, 72.9 (CH2OCH2); 117.5 (CC(R)(NH2)); 168.5 (CC(R)(NH2); 198.8 (CS). After evaporation of the solvent, the product was dissolved in 115 ml toluene. 10.0 g of silica was added and the mixture was stirred at 90°C for 24 h. After filtration, the residual reagents were removed by washing with MeOH (3× ). The ion exchanger was then washed with Et2O (3 ×) and further purified by refluxing MeOH in a Soxhlet apparatus for 24 h and subsequently dried at 50°C in vacuo (200 mbar) for 24 h. Anal. Found for 1B: C, 12.66; H, 1.94; N, 0.63; S, 4.39%. The ligand concentration was calculated to be 0.44 mmol g − 1, based on the nitrogen content.
2.2.2. The amtt –Glymo ion exchanger (2B) Immobilisation of Hamtt was performed in a similar way: 3.0 g (25.4 mmol) of Hamtt was heated in 170 ml of refluxing methanol in the presence of 6.0 g (25.4 mmol) of Glymo during 17 h, yielding the coupled product 2A. 1H NMR (DMSO-d6, l ppm): 0.57 (t, 2H, SiCH2); 1.50 (p, 2H, SiCH2CH2); 2.26 (s, 3H, CH3 Hamtt); 3.0 –3.2 (m, 2H, Hamtt-S-CH2); 3.2 – 3.7, 3.8 – 3.9 (m, 14H, Si(OCH3)3, CH2OCH2, CHOH); 5.80 (s, 2H, NH2). 13C NMR (DMSO-d6, l ppm): 5.0 (SiCH2); 9.9 (CH3 Hamtt); 22.7 (SiCH2CH2); 35.7 (CH2S); 50.1 (Si(OCH3)3); 68.6 (CHOH); 72.8, 73.5 (CH2OCH2); 151.7, 153.4 (quat. C’s). The reaction with silica (10.9 g) was performed in 225 ml of toluene at 100°C for 72 h and purified as stated above. Anal. Found for 2B: C, 9.69; H, 1.84; N, 3.46; S, 2.50%. The ligand concentration was calculated to be 0.62 mmol g − 1, based on the nitrogen content.
2.3. Metal-uptake experiments The batch metal-uptake experiments using the Rh(III) model catalyst were performed using a freshly prepared solution of RhCl3·3H2O (Rh:ligand =2.5:1) in 25 ml of demineralised H2O. The uptake behaviour was studied as a function of pH, employing 0.6 M NaCl/ HCl buffer solutions (pH 1–2) and 0.6 M NaOAc/ HOAc buffer solutions (pH 3 – 6). A solution of 25 ml of RhCl3·3H2O and 25 ml of buffer solution was added to a batch of 100 mg ion exchanger. Blank uptake experiments with RhCl3 using pure silica and Glymo –Si have been performed under various acidic conditions (up to 5 M HCl), under neutral conditions, and using the above mentioned buffer system. No significant uptake was detected. Hence, all uptake capacity can be contributed to the supported ligand system.
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2.3.1. General procedure All experiments involving RhCl3·3H2O were performed in polyethylene bottles mounted on a shaker for 48 h at room temperature (r.t.). After shaking, the samples were filtered and washed with H2O, EtOH and Et2O, and dried in vacuo at 50°C for 24 h. Samples for metal analysis were prepared by heating approximately 100 mg of loaded samples overnight in 5 ml of concentrated H2SO4. Subsequently, concentrated HNO3 was added dropwise until the solutions became clear. After dilution in volumetric flasks the solution was filtered over a glass filter (P4) to remove residual, undigested silica. The filtrate was collected and analysed by AAS. Uptake experiments using Wilkinson’s catalyst were performed by using a fresh solution of RhCl(PPh3)3 (Rh:ligand=2.5:1) in 50 ml of CH2Cl2. The samples were shaken in glass bottles for 48 h, filtered and washed with CH2Cl2 (3×) and Et2O (3× ) and were further treated as described above. Distribution coefficients [D= (mmol Mn + g − 1 of dry ion exchanger)/(mmol Mn + ml − 1 of solution)] were determined as a function of pH using 100 mg samples of ion exchanger and an amount of RhCl3·3H2O corresponding to a ligand-to-metal ratio of 5:1. A solution of 25 ml of RhCl3·3H2O/water and 25 ml of the appropriate buffer was added to the samples. After shaking in the usual way (see above), the samples were filtered and 25 ml of the filtrate was transferred into a 50 ml volumetric flask and the metal content subsequently analysed by AAS. The loaded ion exchanger was analysed following the procedure described above. 2.4. Kinetic-uptake beha6iour For the determination of the rhodium-uptake kinetics, 1.0 g of ion exchanger was shaken in a 50 ml RhCl3·3H2O containing solution (2.5-fold excess with respect to the amount of ligand) in water or absolute ethanol. After 10 s, 30 s, 1 min, 5 min, 15 min, 1 h, 2 h, 4 h, 24 h and 48 h a sample of 100 mg of loaded ion exchanger was taken. The samples were then analysed according to the procedure described above. Uptake experiments using Wilkinson’s catalyst were done in CH2Cl2 in a similar way.
2.5. Competition experiments Complex 1B (100 mg) was shaken with 5 ml of freshly prepared solution of RhCl3·3H2O (2.5-fold excess with respect to the amount of ligand) together with 5 ml of standard metal chloride solutions (0.16 M) of copper, cadmium, nickel and zinc according to the general procedure. The solutions were buffered at pH 3 and 6. The Rh:M2 + ratio was 1:1 and in a different experiment the ratio was set to 1:7.3.
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2.6. Regeneration experiments Prior to the regeneration, the ion exchangers were loaded in a solution of 50 ml of RhCl3·3H2O/H2O (1B), RhCl3·3H2O/EtOH (2B) or RhCl(PPh3)3/CH2Cl2 using a Rh:ligand ratio of 2.5:1. After drying in the usual way, the samples were added to the following stripping solutions: 1 M PPh3/CH2Cl2, 2 M H2SO4, 2 M HCl, 2 M HNO3, 1 M NH4SCN and 1 M thiourea. For Rh(III), 0.5 M Na2H2edta was also tested. After 48 h, the samples were filtered, washed, dried and prepared for analysis following the usual procedure (see above).
3. Results and discussion
3.1. Immobilisation Since Hacda contains both an amine and a thiocarboxylic acid group, nucleophilic ring opening of Glymo is possible in two ways. Subsequent selective protection of each of these groups prior to reaction with the epoxide moieties clearly showed that reaction of unprotected Hacda took place solely via the thiocarboxylic acid group. Furthermore, the 13CCH2 signal of the epoxide group of Glymo is shifted from 44.7 to approximately 36 ppm, which is characteristic for reaction with thiol groups [13]. Hence, reaction of the ligand with the spacer has yielded the thiocarboxylic acid-coupled product 1A. The triazole ligand Hamtt can tautomerise from the thione into the thiol form in polar media. Coupling of Hamtt to Glymo in MeOH was found to proceed via the thiol to give product 2A. The solid-state 13C NMR spectra of the ion exchangers 1B and 2B are shown in Fig. 1. The spectrum of the grafted spacer is drawn for comparison purposes. The epoxide signal at 42 ppm, present in the Glymo –Si spectrum, is shifted towards 36 ppm and corresponds well to the CH2S signal in solution (2B) and the CH2CH2CH2 and CH2SCS signal (1B). The epoxide CH signal at 50.7 ppm is shifted upon reaction to 69 ppm in solution (CHOH) and shifted underneath the strong signal at 73 ppm in solid-state NMR, corresponding to the carbons next to the ether moiety (CH2OCH2). The residual peak at 51 ppm originates from unreacted Si(OCH3) groups that remain after immobilisation. The ligand peaks of both 1B and 2B in the solid-state spectrum correspond well with the spectrum obtained in solution.
3.2. Batch metal-uptake experiments Experiments were performed using an excess of Rhto-ligand ratio of 2.5, since uptake experiments with different excesses of Rh showed that a larger excess does not result in a higher uptake. The pH dependent
Rh(III) uptake pattern is depicted in Fig. 2. The uptake pattern of both ion exchangers is almost identical. A relatively low uptake capacity is observed at pH 1–2 and up to 0.16 (1B) and 0.43 (2B) mmol Rh g − 1 is adsorbed at about pH 3. Because both ion exchangers have different ligand concentrations, it is more correct to compare the ligand occupation (L (%)) as well. This value is defined in the following way: L (%)=[uptake capacity for Mn + (in mmol g − 1)]100/[ligand concentration (in mmol g − 1 silica)]. The values at pH 3 correspond to about 70% loading for both 1B and 2B. To investigate whether the low uptake at pH 1–2 could be ascribed to hydronation of the coordinating amine groups, another uptake experiment in a nonbuffered 1 M NaCl solution was performed. However, a similar uptake capacity was observed. Since Na+ ions do not compete with the ion exchangers, the difficult displacement of the Cl− ions by the ion exchanger apparently causes the drop in uptake capacity. This conclusion was confirmed by a different pH-dependent uptake experiment, using HNO3/NH4OH for the pHadjustment, since now a drop in the loading capacity was not observed at low pH. Clearly, NO3 − ions are more readily displaced by the ion exchanger than the Cl− ions present in the NaCl/HCl buffer system. The small uptake decrease in the range of pH 3–6 is probably due to increased competition of acetate ions with the ion exchangers, or, alternatively, could be due to the formation of hydrolysed and/or dinuclear rhodium-species at pH 4 and higher, thereby limiting the extraction of the metal ion [14 –17]. To gain insight into the stability of the binding of the N,S-containing ligands with the rhodium ions, distribution coefficients have been measured at different pH values. Both ion exchangers follow the same trend (see Fig. 3). The uptake- and distribution-coefficient results follow the same pattern (see Figs. 2 and 3), although the influence of the pH is less pronounced with 2B than with 1B. The log D values of up to 3.5 at pH 3–4 have been found. The distribution coefficients of nonbuffered solutions of RhCl3·3H2O/H2O (measured pH 3.8) fitted well with the results shown in Fig. 3.
3.3. Selecti6ity Because of the high stability of the complexes of 1B and 2B with Rh, the Rh-uptake behaviour in the presence of other metal ions is of interest. An uptake experiment under competitive conditions was performed with 1B at pH 3 and 6 using RhCl3·3H2O and various divalent metal ions, i.e. Cu(II), Zn(II), Ni(II) and Cd(II). Fig. 4 clearly shows a high selectivity of 1B for Rh over the other metal ions at pH 3. Even in the presence of a large excess of the other metal ions, 1B still remains over 10 times as selective for Rh as for Cu. At
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Fig. 1. CP MAS 13C NMR spectra of: (a) Glymo–Si; (b) 1B; and (c) 2B. Peaks originating from the Glymo epoxide group are marked O. Peaks originating from the ligands are marked L and peaks originating from the Glymo spacer are marked G. In spectrum (a), all peaks originate from Glymo.
pH 6, the selectivity for Rh is much lower, but in the presence of a large excess of Cu there is still some preference for Rh (Fig. 4B, pH 6). The lower selectivity can be attributed to a lower stability of the Rh –1B complex at this pH. This is consistent with the observations depicted in Fig. 3, with a lower log D value at pH 6 (log D =2.7) than at pH 3 (log D= 3.5). The other metal ions were hardly adsorbed at all. Apparently, the presence of the soft sulphur donor atom induces selectivity for the softer Rh ions.
3.4. Kinetic experiments Extensive aquation, with the probable formation of Rh(H2O)6 3 + , has been observed for the RhCl3·3H2O
Fig. 2. pH dependent Rh3 + -uptake behaviour of 1B and 2B.
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Fig. 3. The distribution coefficient for Rh3 + as a function of the pH using 1B and 2B.
Fig. 4. Competitive metal-ion uptake experiment with 1B at pH 3 and pH 6 at an Rh3 + :M2 + ratio of 1:1 (A) and 1:7.3 (B).
solutions in water, as reported by Benguerel et al. [15,16]. This aquation negatively influences the Rh(III) uptake behaviour, as illustrated in Fig. 5. To investigate other possible solvent effects on the rate of the Rh uptake, freshly prepared solutions of RhCl3·3H2O in water and EtOH abs. were compared. The kinetic-uptake behaviour of 2B is depicted in Fig. 6. From Fig. 6(a) it can be deduced that EtOH drastically increases the uptake rate (up to twofold in the first hour), probably due to the much lower aquation. The t1/2 (the time needed for loading half of the total amount of Rh) is much shorter in EtOH ( 15 min) than in water (\ 60 min). These values are much lower than those found for Cu using a similar backbonespacer system [8], demonstrating the relative inertness of the ligand exchange on rhodium. The maximum ligand occupation of just over 60% (Fig. 6(b)) is about the same for both solvents (0.38 mmol g − 1 in water, 0.40 mmol g − 1 in EtOH). The uptake kinetics of the Rh(I)-species, RhCl(PPh3)3, were determined with 1B. A different batch was used, with a ligand concentration of 0.23 mmol g − 1. The t1/2 was found to be somewhat higher than the value obtained for RhCl3·3H2O in EtOH. The ligand occupation was over 80%, corresponding to a total amount of 0.20 mmol g − 1.
3.5. Regeneration experiments
Fig. 5. Effect of aquation on the uptake kinetics of RhCl3·3H2O in water. The aged solutions were prepared two days prior to the experiment and the fresh solutions were used several minutes after preparation.
Both 1B and 2B were loaded with the two model catalysts and subsequently treated with different stripping agents, such as mineral acids and competing (chelating) ligands. The results are shown below in Fig. 7. Both ion exchangers show higher RhCl3·3H2O uptake than RhCl(PPh3)3 uptake (1 in Fig. 7(a) and (b)). Probably the bulky PPh3 groups have a negative influence on the uptake. In addition, the presence of the hard nitrogen atoms in the ligands is likely to favour coordination to Rh(III)-species over the soft Rh(I) catalyst. It is evident that none of the used stripping agents were very effective, neither for the RhCl3·3H2O loaded
Fig. 6. Kinetic Rh3 + uptake behaviour of 2B in H2O and EtOH abs. during (a) the first 60 min and (b) five days.
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Fig. 7. Regeneration experiments of RhCl3·3H2O- and RhCl(PPh3)3-loaded 1B (a) and 2B (b) (entry 1). Entries 2 – 10 indicate the loading after stripping with 1 M PPh3/DCM (2), 2 M H2SO4 (3), 2 M HCl (4), 2 M HNO3 (5), 1 M NH4SCN (6), 1 M (NH2)2CS (7) and 0.5 M Na2EDTA (8). Stripping with Na2EDTA was not tested for RhCl(PPh3)3 loaded 1B and PPh3 was not tested for RhCl3·3H2O loaded 2B.
ion exchangers nor for the RhCl(PPh3)3 loaded ion exchangers. The best results for the RhCl3·3H2O loaded ion exchangers were obtained with thiourea, stripping the metal ions on 1B for 18% and on 2B up to 30% relative to the initial loading. The lower stripping percentage for 1B is probably caused by the formation of a somewhat more stable complex during the uptake (pH 3.8) since log D values using 1B are somewhat higher than using 2B at this pH (3.4 and 3.1, respectively, Fig. 3). A possible explanation for the best stripping potential of thiourea is its ability to coordinate strongly via its soft sulphur atom to the relatively soft platinum group metals [18]. Hydronation of the amine groups upon the addition of mineral acids did not render effective stripping with any of the loaded ion exchangers, except for RhCl(PPh3)3 loaded 1B. 25% could be stripped, corresponding, however, to only 0.04 mmol g − 1 reduction of the metal ion content. The best results for Wilkinson’s catalyst loaded ion exchangers were obtained with PPh3 (23%, 1B) and thiourea (31%, 2B). Multiple cycles of stripping without reloading appeared not to be successful; only 5% extra could be attained after four cycles of stripping with thiourea. Increasing the acid concentration by addition of up to 5 M of the acids mentioned above, did not improve the regeneration. The RhCl3·3H2O-loaded 1B was also stripped with 5 M HCl together with the oxidising agent 0.4 M NaClO3, as described by Alam et al. [19], which at room temperature did not result in improved removal of the metal ions, but resulted in complete removal of rhodium at 70°C. However, it effectively destroyed the system as well, since reloading proved to be impossible. None of the Rh(I)-loaded ion exchangers could be stripped, apparently due to their very strong coordination to the sulphur donor atoms of the ion exchangers. This is in agreement with the high values of the distribution coefficients for the Rh(I)-1B and
Rh(III)-2B systems, indicative for a high stability of these loaded ion exchangers. 4. Concluding remarks The silica-based chelating ion exchangers containing N and S donor atoms described in this paper have been shown to bind Rh ions very strongly. A high selectivity for Rh over other metal ions, such as Cu and Cd, has been observed. Ligand occupations as high as 65%, corresponding to 0.40 mmol Rh g − 1 ion exchanger, have been obtained. The Rh(III)-uptake kinetics were found to be negatively influenced by aquation, but positively influenced by using ethanol as a solvent. The RhCl3·3H2O-uptake appeared to be significantly higher than the RhCl(PPh3)3-uptake, probably due to steric hindrance of the PPh3 ligands. Stripping of the loaded polymers appeared to be very difficult and only up to 30% of the initially adsorbed Rh(III) ions could be removed using 1 M thiourea. Apparently the stability of the formed complexes is too high as a result of the very strong RhS coordination. Future work will focus on the development of chelating ion exchangers bearing ligands without sulphur donor atoms. Acknowledgements Dr. W. Buijs (DSM-Research) is acknowledged for many suggestions and fruitful discussions. Mr. J.J.M. van Brussel, Mr. F. Lefe`ber and Mr. J.G. Hollander are gratefully acknowledged for the collection of the solidstate CP MAS 13C NMR spectra and the elemental analyses of the ion exchangers presented in this paper. This research has been sponsored by IOP (Innovation Oriented research Programme, The Netherlands) Environmental Technology/Heavy Metals Cluster Separation, project number IZW97411.
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Recovery of rhodium-containing catalysts by silica-based chelating ion exchangers containing N and S donor atoms Jurjen Kramer, Arjen Scholten, Willem L. Driessen *, Jan Reedijk Leiden Institute of Chemistry, Leiden Uni6ersity, P.O. Box 9502, 2300 RA Leiden, The Netherlands Received 4 October 2000; accepted 22 January 2001
Abstract Two silica-based chelating ion exchangers have been synthesised for the recovery of the model catalysts RhCl3·3H2O and RhCl(PPh3)3. The N,S-donor ligands 2-aminocyclopentene-1-dithiocarboxylic acid and 4-amino-3-methyl-1,2,4-triazole-5-thione have been attached to the bifunctional spacer (3-glycidoxypropyl)trimethoxysilane prior to immobilisation on silica. Both ion exchangers showed a high Rh(III)-uptake capacity and selectivity over Cu(II), even in the presence of an excess of the latter. The Rh(I) complex was adsorbed to a much smaller extent, probably due to the bulky triphenylphosphane groups. Aquation negatively influenced the Rh(III)-uptake kinetics, whereas experiments performed in ethanol significantly increased the kinetic-uptake behaviour. Stripping of the loaded polymers proved to be very difficult, probably due to the strong Rhsulphur coordination. Only with thiourea a reasonable amount of Rh(III) could be stripped from the ion exchangers. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Rhodium complexes; Chelating ion exchange; Azathia ligand complexes; Kinetics
1. Introduction Homogeneous catalysts play an important role in the improvement of many organic reactions. These catalysts generally contain heavy metals, such as Co, Zn, Pd, Ru and Rh [1]. Both from an environmental point of view and because of the often high costs of the metal, the recovery of these metals is very important. Furthermore, contamination of the product with the catalyst and subsequent loss of the catalyst should be reduced to the lowest possible level. Hence, separation of the metal-containing homogeneous catalyst from product streams is very important. However, the complete recovery of the catalyst is a major difficulty in homogeneous catalysis, especially since the catalyst is usually present in very low concentrations [1,2]. Rhodium is a Group 9 metal and can be classified as a metal with a soft character, depending on the oxidation state, according to the Pearson classification [3,4]. * Corresponding author. Tel.: +31-71-5274345; fax: + 31-715274451. E-mail address: [email protected] (W.L. Driessen).
Hence, ligands that contain soft donor atoms, such as sulphur and phosphorus, will form relatively stable complexes with rhodium ions. Since both adsorption and subsequent desorption are the target, an optimal binding is required, as ligands that coordinate too strongly may prevent sufficient desorption. The immobilised ligands described in this paper contain both N and S donor atoms (see Scheme 1). The ligand 2-aminocyclopentene-1-dithiocarboxylic acid (Hacda, 1) is known to coordinate with Rh(I)-species via a six-membered chelate ring [5,6]. The ligand 4-amino-3-methyl-1,2,4-triazole-5-thione (Hamtt, 2) is a thiosemicarbazone in which the structural unit SCNN can bind to metal ions through S or through N, or through both atoms in a stable five-membered fashion, as deduced from the reported [7] crystal structures of several transition metals coordinated to 2. In this paper, the synthesis of silica-based chelating ion exchangers, modified with Hacda and Hamtt, is described. The ligands have been attached [8] via the bifunctional spacer (3-glycidoxypropyl)trimethoxysilane (Glymo) prior to immobilisation, see Scheme 1. Uptake and regeneration characteristics have been studied us-
0020-1693/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 0 1 ) 0 0 3 5 6 - 5
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ing the compounds RhCl3·3H2O and also RhCl(PPh3)3, i.e. Wilkinson’s catalyst, as a Rh-containing model catalyst. As ligand exchange of rhodium compounds is known to proceed much slower than that of compounds of other elements such as copper [9,10], the kinetic-uptake behaviour has also been studied in detail.
2. Experimental
2.1. Reagents and analytical methods All reactions were performed under an argon atmosphere with standard Schlenk techniques. All reagents and solvents were purchased from commercial sources and were used without further purification unless stated otherwise. Toluene was distilled from Na under argon and stored over molecular sieves. Methanol was distilled from Mg under an argon atmosphere and stored over molecular sieves. Silica (Aldrich Silica gel Davisil™, particle size 4 – 20 mm, surface area 480 m2 g − 1, pore volume 0.75 cm3 g − 1) was activated before use by heating in vacuo (200 mbar) at 50°C for at least 48 h. The ligand concentrations of the functionalised silica were calculated from the weight percentage of nitrogen. The compound RhCl(PPh3)3 was synthesised according to a literature procedure [11]. RhCl3·3H2O was purchased from Aldrich. Hamtt was synthesised according to a literature procedure [7] and was further purified by recrystallisation from water. Hacda was prepared following the method described by Nag et al. [12].
1
H and 13C NMR spectra were recorded on a 200 MHz JEOL JNM FT NMR spectrometer operating at 199.50 and 50.10 MHz for 1H and 13C NMR, respectively. Solid-state 13C NMR CP MAS NMR spectra were recorded on a Bruker MSL 400 spectrometer operating at 100.6 MHz with a rotational spin speed of 7 kHz. Metal analyses were performed on a Perkin – Elmer 3100 atomic absorption (AAS) and flame emission spectrometer using a linear calibration method. Elemental analyses (C, H, N, S) were carried out by the Analytical Service Centre of the Gorlaeus Laboratories or by the Microanalytical Laboratory of the University College in Dublin. Sulphur percentages were found to vary significantly whereas the other percentages were reproducible. Since carbon and hydrogen are also found in the Glymo spacer whereas nitrogen is only present in the ligands, the choice was made to base all ligand concentrations on nitrogen percentages, as is generally done.
2.2. Immobilisation 2.2.1. The acda–Glymo ion exchanger (1B) The ligand was first coupled to Glymo in the following way: 4.3 g (17.1 mmol) of Glymo was added to 3.0 g (17.1 mmol) of the ammonium salt of Hacda in methanol (115 ml) and heated at 50°C for 17 h to give compound 1A. 1 H NMR (CDCl3, l ppm): 0.68 (t, 2H, SiCH2); 1.69, 1.85 (2× p, 4H, CH2CH2CH2-Hacda, SiCH2CH2); 2.5 (m, 2H, CH2S); 2.66, 2.82 (2× t, 4H, CH2CH2CH2Hacda); 3.3 –3.7 (m, 14H, Si(OCH3)3, CH2OCH2, CHOH); 7.0, 11.1 (2×bs, 2H, NH2). 13C NMR (CDCl3, l ppm): 4.6 (SiCH2); 19.9, 22.0 (CH2CH2CH2-
Scheme 1. Immobilisation of the ligands Hacda (1) and Hamtt (2) onto silica.
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Hacda, SiCH2CH2); 32.2, 35.1, 35.6 (CH2S, CH2CH2CH2-Hacda); 49.7 (Si(OCH3)3); 69.1 (CHOH); 72.5, 72.9 (CH2OCH2); 117.5 (CC(R)(NH2)); 168.5 (CC(R)(NH2); 198.8 (CS). After evaporation of the solvent, the product was dissolved in 115 ml toluene. 10.0 g of silica was added and the mixture was stirred at 90°C for 24 h. After filtration, the residual reagents were removed by washing with MeOH (3× ). The ion exchanger was then washed with Et2O (3 ×) and further purified by refluxing MeOH in a Soxhlet apparatus for 24 h and subsequently dried at 50°C in vacuo (200 mbar) for 24 h. Anal. Found for 1B: C, 12.66; H, 1.94; N, 0.63; S, 4.39%. The ligand concentration was calculated to be 0.44 mmol g − 1, based on the nitrogen content.
2.2.2. The amtt –Glymo ion exchanger (2B) Immobilisation of Hamtt was performed in a similar way: 3.0 g (25.4 mmol) of Hamtt was heated in 170 ml of refluxing methanol in the presence of 6.0 g (25.4 mmol) of Glymo during 17 h, yielding the coupled product 2A. 1H NMR (DMSO-d6, l ppm): 0.57 (t, 2H, SiCH2); 1.50 (p, 2H, SiCH2CH2); 2.26 (s, 3H, CH3 Hamtt); 3.0 –3.2 (m, 2H, Hamtt-S-CH2); 3.2 – 3.7, 3.8 – 3.9 (m, 14H, Si(OCH3)3, CH2OCH2, CHOH); 5.80 (s, 2H, NH2). 13C NMR (DMSO-d6, l ppm): 5.0 (SiCH2); 9.9 (CH3 Hamtt); 22.7 (SiCH2CH2); 35.7 (CH2S); 50.1 (Si(OCH3)3); 68.6 (CHOH); 72.8, 73.5 (CH2OCH2); 151.7, 153.4 (quat. C’s). The reaction with silica (10.9 g) was performed in 225 ml of toluene at 100°C for 72 h and purified as stated above. Anal. Found for 2B: C, 9.69; H, 1.84; N, 3.46; S, 2.50%. The ligand concentration was calculated to be 0.62 mmol g − 1, based on the nitrogen content.
2.3. Metal-uptake experiments The batch metal-uptake experiments using the Rh(III) model catalyst were performed using a freshly prepared solution of RhCl3·3H2O (Rh:ligand =2.5:1) in 25 ml of demineralised H2O. The uptake behaviour was studied as a function of pH, employing 0.6 M NaCl/ HCl buffer solutions (pH 1–2) and 0.6 M NaOAc/ HOAc buffer solutions (pH 3 – 6). A solution of 25 ml of RhCl3·3H2O and 25 ml of buffer solution was added to a batch of 100 mg ion exchanger. Blank uptake experiments with RhCl3 using pure silica and Glymo –Si have been performed under various acidic conditions (up to 5 M HCl), under neutral conditions, and using the above mentioned buffer system. No significant uptake was detected. Hence, all uptake capacity can be contributed to the supported ligand system.
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2.3.1. General procedure All experiments involving RhCl3·3H2O were performed in polyethylene bottles mounted on a shaker for 48 h at room temperature (r.t.). After shaking, the samples were filtered and washed with H2O, EtOH and Et2O, and dried in vacuo at 50°C for 24 h. Samples for metal analysis were prepared by heating approximately 100 mg of loaded samples overnight in 5 ml of concentrated H2SO4. Subsequently, concentrated HNO3 was added dropwise until the solutions became clear. After dilution in volumetric flasks the solution was filtered over a glass filter (P4) to remove residual, undigested silica. The filtrate was collected and analysed by AAS. Uptake experiments using Wilkinson’s catalyst were performed by using a fresh solution of RhCl(PPh3)3 (Rh:ligand=2.5:1) in 50 ml of CH2Cl2. The samples were shaken in glass bottles for 48 h, filtered and washed with CH2Cl2 (3×) and Et2O (3× ) and were further treated as described above. Distribution coefficients [D= (mmol Mn + g − 1 of dry ion exchanger)/(mmol Mn + ml − 1 of solution)] were determined as a function of pH using 100 mg samples of ion exchanger and an amount of RhCl3·3H2O corresponding to a ligand-to-metal ratio of 5:1. A solution of 25 ml of RhCl3·3H2O/water and 25 ml of the appropriate buffer was added to the samples. After shaking in the usual way (see above), the samples were filtered and 25 ml of the filtrate was transferred into a 50 ml volumetric flask and the metal content subsequently analysed by AAS. The loaded ion exchanger was analysed following the procedure described above. 2.4. Kinetic-uptake beha6iour For the determination of the rhodium-uptake kinetics, 1.0 g of ion exchanger was shaken in a 50 ml RhCl3·3H2O containing solution (2.5-fold excess with respect to the amount of ligand) in water or absolute ethanol. After 10 s, 30 s, 1 min, 5 min, 15 min, 1 h, 2 h, 4 h, 24 h and 48 h a sample of 100 mg of loaded ion exchanger was taken. The samples were then analysed according to the procedure described above. Uptake experiments using Wilkinson’s catalyst were done in CH2Cl2 in a similar way.
2.5. Competition experiments Complex 1B (100 mg) was shaken with 5 ml of freshly prepared solution of RhCl3·3H2O (2.5-fold excess with respect to the amount of ligand) together with 5 ml of standard metal chloride solutions (0.16 M) of copper, cadmium, nickel and zinc according to the general procedure. The solutions were buffered at pH 3 and 6. The Rh:M2 + ratio was 1:1 and in a different experiment the ratio was set to 1:7.3.
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2.6. Regeneration experiments Prior to the regeneration, the ion exchangers were loaded in a solution of 50 ml of RhCl3·3H2O/H2O (1B), RhCl3·3H2O/EtOH (2B) or RhCl(PPh3)3/CH2Cl2 using a Rh:ligand ratio of 2.5:1. After drying in the usual way, the samples were added to the following stripping solutions: 1 M PPh3/CH2Cl2, 2 M H2SO4, 2 M HCl, 2 M HNO3, 1 M NH4SCN and 1 M thiourea. For Rh(III), 0.5 M Na2H2edta was also tested. After 48 h, the samples were filtered, washed, dried and prepared for analysis following the usual procedure (see above).
3. Results and discussion
3.1. Immobilisation Since Hacda contains both an amine and a thiocarboxylic acid group, nucleophilic ring opening of Glymo is possible in two ways. Subsequent selective protection of each of these groups prior to reaction with the epoxide moieties clearly showed that reaction of unprotected Hacda took place solely via the thiocarboxylic acid group. Furthermore, the 13CCH2 signal of the epoxide group of Glymo is shifted from 44.7 to approximately 36 ppm, which is characteristic for reaction with thiol groups [13]. Hence, reaction of the ligand with the spacer has yielded the thiocarboxylic acid-coupled product 1A. The triazole ligand Hamtt can tautomerise from the thione into the thiol form in polar media. Coupling of Hamtt to Glymo in MeOH was found to proceed via the thiol to give product 2A. The solid-state 13C NMR spectra of the ion exchangers 1B and 2B are shown in Fig. 1. The spectrum of the grafted spacer is drawn for comparison purposes. The epoxide signal at 42 ppm, present in the Glymo –Si spectrum, is shifted towards 36 ppm and corresponds well to the CH2S signal in solution (2B) and the CH2CH2CH2 and CH2SCS signal (1B). The epoxide CH signal at 50.7 ppm is shifted upon reaction to 69 ppm in solution (CHOH) and shifted underneath the strong signal at 73 ppm in solid-state NMR, corresponding to the carbons next to the ether moiety (CH2OCH2). The residual peak at 51 ppm originates from unreacted Si(OCH3) groups that remain after immobilisation. The ligand peaks of both 1B and 2B in the solid-state spectrum correspond well with the spectrum obtained in solution.
3.2. Batch metal-uptake experiments Experiments were performed using an excess of Rhto-ligand ratio of 2.5, since uptake experiments with different excesses of Rh showed that a larger excess does not result in a higher uptake. The pH dependent
Rh(III) uptake pattern is depicted in Fig. 2. The uptake pattern of both ion exchangers is almost identical. A relatively low uptake capacity is observed at pH 1–2 and up to 0.16 (1B) and 0.43 (2B) mmol Rh g − 1 is adsorbed at about pH 3. Because both ion exchangers have different ligand concentrations, it is more correct to compare the ligand occupation (L (%)) as well. This value is defined in the following way: L (%)=[uptake capacity for Mn + (in mmol g − 1)]100/[ligand concentration (in mmol g − 1 silica)]. The values at pH 3 correspond to about 70% loading for both 1B and 2B. To investigate whether the low uptake at pH 1–2 could be ascribed to hydronation of the coordinating amine groups, another uptake experiment in a nonbuffered 1 M NaCl solution was performed. However, a similar uptake capacity was observed. Since Na+ ions do not compete with the ion exchangers, the difficult displacement of the Cl− ions by the ion exchanger apparently causes the drop in uptake capacity. This conclusion was confirmed by a different pH-dependent uptake experiment, using HNO3/NH4OH for the pHadjustment, since now a drop in the loading capacity was not observed at low pH. Clearly, NO3 − ions are more readily displaced by the ion exchanger than the Cl− ions present in the NaCl/HCl buffer system. The small uptake decrease in the range of pH 3–6 is probably due to increased competition of acetate ions with the ion exchangers, or, alternatively, could be due to the formation of hydrolysed and/or dinuclear rhodium-species at pH 4 and higher, thereby limiting the extraction of the metal ion [14 –17]. To gain insight into the stability of the binding of the N,S-containing ligands with the rhodium ions, distribution coefficients have been measured at different pH values. Both ion exchangers follow the same trend (see Fig. 3). The uptake- and distribution-coefficient results follow the same pattern (see Figs. 2 and 3), although the influence of the pH is less pronounced with 2B than with 1B. The log D values of up to 3.5 at pH 3–4 have been found. The distribution coefficients of nonbuffered solutions of RhCl3·3H2O/H2O (measured pH 3.8) fitted well with the results shown in Fig. 3.
3.3. Selecti6ity Because of the high stability of the complexes of 1B and 2B with Rh, the Rh-uptake behaviour in the presence of other metal ions is of interest. An uptake experiment under competitive conditions was performed with 1B at pH 3 and 6 using RhCl3·3H2O and various divalent metal ions, i.e. Cu(II), Zn(II), Ni(II) and Cd(II). Fig. 4 clearly shows a high selectivity of 1B for Rh over the other metal ions at pH 3. Even in the presence of a large excess of the other metal ions, 1B still remains over 10 times as selective for Rh as for Cu. At
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Fig. 1. CP MAS 13C NMR spectra of: (a) Glymo–Si; (b) 1B; and (c) 2B. Peaks originating from the Glymo epoxide group are marked O. Peaks originating from the ligands are marked L and peaks originating from the Glymo spacer are marked G. In spectrum (a), all peaks originate from Glymo.
pH 6, the selectivity for Rh is much lower, but in the presence of a large excess of Cu there is still some preference for Rh (Fig. 4B, pH 6). The lower selectivity can be attributed to a lower stability of the Rh –1B complex at this pH. This is consistent with the observations depicted in Fig. 3, with a lower log D value at pH 6 (log D =2.7) than at pH 3 (log D= 3.5). The other metal ions were hardly adsorbed at all. Apparently, the presence of the soft sulphur donor atom induces selectivity for the softer Rh ions.
3.4. Kinetic experiments Extensive aquation, with the probable formation of Rh(H2O)6 3 + , has been observed for the RhCl3·3H2O
Fig. 2. pH dependent Rh3 + -uptake behaviour of 1B and 2B.
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Fig. 3. The distribution coefficient for Rh3 + as a function of the pH using 1B and 2B.
Fig. 4. Competitive metal-ion uptake experiment with 1B at pH 3 and pH 6 at an Rh3 + :M2 + ratio of 1:1 (A) and 1:7.3 (B).
solutions in water, as reported by Benguerel et al. [15,16]. This aquation negatively influences the Rh(III) uptake behaviour, as illustrated in Fig. 5. To investigate other possible solvent effects on the rate of the Rh uptake, freshly prepared solutions of RhCl3·3H2O in water and EtOH abs. were compared. The kinetic-uptake behaviour of 2B is depicted in Fig. 6. From Fig. 6(a) it can be deduced that EtOH drastically increases the uptake rate (up to twofold in the first hour), probably due to the much lower aquation. The t1/2 (the time needed for loading half of the total amount of Rh) is much shorter in EtOH ( 15 min) than in water (\ 60 min). These values are much lower than those found for Cu using a similar backbonespacer system [8], demonstrating the relative inertness of the ligand exchange on rhodium. The maximum ligand occupation of just over 60% (Fig. 6(b)) is about the same for both solvents (0.38 mmol g − 1 in water, 0.40 mmol g − 1 in EtOH). The uptake kinetics of the Rh(I)-species, RhCl(PPh3)3, were determined with 1B. A different batch was used, with a ligand concentration of 0.23 mmol g − 1. The t1/2 was found to be somewhat higher than the value obtained for RhCl3·3H2O in EtOH. The ligand occupation was over 80%, corresponding to a total amount of 0.20 mmol g − 1.
3.5. Regeneration experiments
Fig. 5. Effect of aquation on the uptake kinetics of RhCl3·3H2O in water. The aged solutions were prepared two days prior to the experiment and the fresh solutions were used several minutes after preparation.
Both 1B and 2B were loaded with the two model catalysts and subsequently treated with different stripping agents, such as mineral acids and competing (chelating) ligands. The results are shown below in Fig. 7. Both ion exchangers show higher RhCl3·3H2O uptake than RhCl(PPh3)3 uptake (1 in Fig. 7(a) and (b)). Probably the bulky PPh3 groups have a negative influence on the uptake. In addition, the presence of the hard nitrogen atoms in the ligands is likely to favour coordination to Rh(III)-species over the soft Rh(I) catalyst. It is evident that none of the used stripping agents were very effective, neither for the RhCl3·3H2O loaded
Fig. 6. Kinetic Rh3 + uptake behaviour of 2B in H2O and EtOH abs. during (a) the first 60 min and (b) five days.
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Fig. 7. Regeneration experiments of RhCl3·3H2O- and RhCl(PPh3)3-loaded 1B (a) and 2B (b) (entry 1). Entries 2 – 10 indicate the loading after stripping with 1 M PPh3/DCM (2), 2 M H2SO4 (3), 2 M HCl (4), 2 M HNO3 (5), 1 M NH4SCN (6), 1 M (NH2)2CS (7) and 0.5 M Na2EDTA (8). Stripping with Na2EDTA was not tested for RhCl(PPh3)3 loaded 1B and PPh3 was not tested for RhCl3·3H2O loaded 2B.
ion exchangers nor for the RhCl(PPh3)3 loaded ion exchangers. The best results for the RhCl3·3H2O loaded ion exchangers were obtained with thiourea, stripping the metal ions on 1B for 18% and on 2B up to 30% relative to the initial loading. The lower stripping percentage for 1B is probably caused by the formation of a somewhat more stable complex during the uptake (pH 3.8) since log D values using 1B are somewhat higher than using 2B at this pH (3.4 and 3.1, respectively, Fig. 3). A possible explanation for the best stripping potential of thiourea is its ability to coordinate strongly via its soft sulphur atom to the relatively soft platinum group metals [18]. Hydronation of the amine groups upon the addition of mineral acids did not render effective stripping with any of the loaded ion exchangers, except for RhCl(PPh3)3 loaded 1B. 25% could be stripped, corresponding, however, to only 0.04 mmol g − 1 reduction of the metal ion content. The best results for Wilkinson’s catalyst loaded ion exchangers were obtained with PPh3 (23%, 1B) and thiourea (31%, 2B). Multiple cycles of stripping without reloading appeared not to be successful; only 5% extra could be attained after four cycles of stripping with thiourea. Increasing the acid concentration by addition of up to 5 M of the acids mentioned above, did not improve the regeneration. The RhCl3·3H2O-loaded 1B was also stripped with 5 M HCl together with the oxidising agent 0.4 M NaClO3, as described by Alam et al. [19], which at room temperature did not result in improved removal of the metal ions, but resulted in complete removal of rhodium at 70°C. However, it effectively destroyed the system as well, since reloading proved to be impossible. None of the Rh(I)-loaded ion exchangers could be stripped, apparently due to their very strong coordination to the sulphur donor atoms of the ion exchangers. This is in agreement with the high values of the distribution coefficients for the Rh(I)-1B and
Rh(III)-2B systems, indicative for a high stability of these loaded ion exchangers. 4. Concluding remarks The silica-based chelating ion exchangers containing N and S donor atoms described in this paper have been shown to bind Rh ions very strongly. A high selectivity for Rh over other metal ions, such as Cu and Cd, has been observed. Ligand occupations as high as 65%, corresponding to 0.40 mmol Rh g − 1 ion exchanger, have been obtained. The Rh(III)-uptake kinetics were found to be negatively influenced by aquation, but positively influenced by using ethanol as a solvent. The RhCl3·3H2O-uptake appeared to be significantly higher than the RhCl(PPh3)3-uptake, probably due to steric hindrance of the PPh3 ligands. Stripping of the loaded polymers appeared to be very difficult and only up to 30% of the initially adsorbed Rh(III) ions could be removed using 1 M thiourea. Apparently the stability of the formed complexes is too high as a result of the very strong RhS coordination. Future work will focus on the development of chelating ion exchangers bearing ligands without sulphur donor atoms. Acknowledgements Dr. W. Buijs (DSM-Research) is acknowledged for many suggestions and fruitful discussions. Mr. J.J.M. van Brussel, Mr. F. Lefe`ber and Mr. J.G. Hollander are gratefully acknowledged for the collection of the solidstate CP MAS 13C NMR spectra and the elemental analyses of the ion exchangers presented in this paper. This research has been sponsored by IOP (Innovation Oriented research Programme, The Netherlands) Environmental Technology/Heavy Metals Cluster Separation, project number IZW97411.
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