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An investigation of the interaction between polyvinylpyrrolidone and metal cations
Reactive & Functional Polymers 44 (2000) 55–64 www.elsevier.com / locate / react
An investigation of the interaction between polyvinylpyrrolidone and metal cations Manhong Liu, Xiaoping Yan, Hanfan Liu*, Weiyong Yu Polymer Chemistry Laboratory, Chinese Academy of Sciences and China Petro-Chemical Corporation ( PCLCC), Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’ s Republic of China Received 23 February 1999; received in revised form 4 August 1999; accepted 11 August 1999
Abstract The interaction between polyvinylpyrrolidone (PVP) and metal cations has been studied in absolute ethanol and water at room temperature and atmospheric pressure. The UV-VIS and IR absorption spectra of metal cations, PVP and PVP-M n 1 (M n 1 5 Fe 31 , Co 21 , Ni 21 ) were given. It was shown that PVP and metal cations formed unstable complexes, and the coordination stability constants were determined according to the Miller–Dorough method. The stability constants of PVP-Fe 31 and PVP-Co 21 were lower than 10 and 0.6, respectively, and that of PVP-Ni 21 was too low to be estimated. The solid complexes of PVP-Fe 31 and PVP-Co 21 were also synthesized under proper conditions. 2000 Elsevier Science B.V. All rights reserved. Keywords: Interaction; Coordination; Polyvinylpyrrolidone; Metal cations
1. Introduction Nanoscopic metal clusters or colloids, because of their unique chemical and physical properties as compared to either bulk metal or single metal atoms, have been attracting great attention in academia and industries, especially in the field of catalysis [1–8]. Liu et al. [9,10] described that in the selective hydrogenation of cinnamaldehyde to cinnamyl alcohol catalyzed by the polyvinylpyrrolidone (PVP)-stabilized platinum colloid, the activity was enhanced to 120% and the selectivity for cinnamyl alcohol *Corresponding author. Tel.: 1 86-10-6255-4487; fax: 1 8610-6255-9373. E-mail address: [email protected] (H. Liu)
was increased from 12.0% to 98.5% when employing Fe 31 or Co 21 as a modifier. It is generally accepted that an increase in the selectivity of a catalyst by incorporating additives may cause a decrease in the activity as compensation or vice versa. However, in the catalytic system described above, considerable increases in both the selectivity and the activity were observed when some metal cations were introduced. This unusual modification effect was also found in the selective hydrogenation of crotronaldehyde [10], citronellal [11] and chloronitrobenzene [12] over polymer-stabilized platinum colloid. Moreover, it was also observed that some metal cations added to the PVP-stabilized ruthenium monometallic and the ruthenium-containing bimetallic colloid systems
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increased the activity while the high selectivity (selectivity to chloroaniline . 99.9%) remained constant [13,14]. Therefore, further research should be done to explain the nature of the modification effect of some metal cations in the selective hydrogenation over the polymer-stabilized metal colloids. We suspected that the charge distribution and electronic properties of the catalysts were changed upon the adsorption of metal cations on the metal colloids. Hence, the change in electronic and conformational properties as well as the coordination ability of the surface metal atoms, which are main factors influencing the activity and the selectivity of the catalyst, should be determined by the adsorption of different amounts of the metal cations. However, in the course of examining the adsorption of nickel(II) chloride on PVP-stabilized platinum colloid, it was found that the concentration of nickel(II) had hardly changed after adsorption. This was probably due to the amount of the adsorption being too small to give a precise result. On the other hand, the evidence of interaction between metal cations and the reactants [10] or the products [12] have been reported. Interaction between PVP and palladium colloidal particles [15] has also been reported. When PVP is used as a stabilizer for metal colloids, it is possible that the metal cations interact with the oxygen or nitrogen atoms on the five-membered nitrogen-containing heterocycles of PVP. This interaction may affect the properties of the metal colloids, and consequently influence the catalytic performance. Under this consideration, we investigated the interaction between PVP and metal cations.
supplied by Beijing Chemicals. Absolute ethanol [16], anhydrous cobalt(II) chloride, and anhydrous nickel(II) chloride were prepared according to the literature [17], respectively.
2.2. Equipment All ultraviolet visible spectra were obtained on a Hitachi 340 type spectrophotometer. Infrared spectra were recorded on a Bruker IFS-IS spectrophotometer. Chemical analysis was carried out by means of inductively coupled plasma optical emission spectroscopy (ICP). The spectrometer was a Spex 1702 scanning ICP spectrometer.
2.3. Preparation of PVP-M n 1 complexes Metal cation solutions of Co 21 and Fe 31 were prepared from chloride salts in ethanol solution. Polymer complex formation of a metal cation with PVP was performed by addition of PVP ethanol solution (1.0 M, 10.0 ml) to a metal cation ethanol solution (0.15 M, 7.0 ml) to give amorphous precipitate. The resulting precipitate was centrifuged, washed with several portions of acetone until the color of washing liquor turned into colorless, and then dried in vacuo at room temperature. The blue precipitate of PVP-Co 21 complex and the light yellow precipitate of PVP-Fe 31 complex were designated as PVP-Co 21 (2) and PVP-Fe 31 (4), respectively. Another sample of the PVP-Fe 31 complex designated as PVP-Fe 31 (3) was obtained by adding PVP ethanol solution (0.2 M, 10.0 ml) to ferric ethanol solution (0.02 M, 10.0 ml). However, we failed to obtain the solid PVP-Ni 21 complex by various tests even by adding 1.0 M concentrated PVP ethanol solution to the saturated Ni 21 ethanol solution.
2. Experimental
2.1. Materials
3. Results and discussion
Poly(N-vinyl-2-pyrrolidone) (av. Mw 10 000) was purchased from BASF, dried under vacuum below 1008C for 6 h, and stored in a drybox before use. Other analytical grade reagents were
3.1. Spectrophotometric measurements IR spectroscopy is useful for detecting the interaction between two species. Wuepper and
M. Liu et al. / Reactive & Functional Polymers 44 (2000) 55 – 64
Popov [18] reported that a shift of carbonyl band was observed in the IR spectrum of 2pyrrolidone in the presence of alkali metal ions. They concluded that the band shift was due to the interaction between the carbonyl oxygen of 2-pyrrolidone and the metal ions. The IR spectra of PVP-M n 1 complexes in methanol solution and in solid state are shown in Fig. 1a and b, respectively. The spectra of the PVP-M n 1 complexes in solid state are somewhat different from that in methanol solution. As shown in Fig. 1a, the peak of C=O double bond in PVP
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becomes somewhat asymmetric on adding Fe 31 , Co 21 and Ni 21 , and the C=O stretching frequencies of the complexes are almost the same as that of the free ligand, indicating that the interaction between PVP and M n1 in methanol solution is very weak. Fig. 1b gives the IR spectra of PVP, PVP-Fe 31 complex and PVPCo 21 complex in the solid state, respectively. The carbonyl band of PVP appears at 1661.5 cm 21 characteristic of uncomplexed pyrrolidone, whereas it shifts to 1655.7 cm 21 in PVPCo 21 , and 1656.2 cm 21 in PVP-Fe 31 complex,
Fig. 1. The IR spectra of PVP and PVP-M n 1 (a) in MeOH (298.0 K, 0.25 M): PVP (1), PVP-Fe 31 (2), PVP-Co 21 (3) and PVP-Ni 21 (4); (b) of powder samples: PVP (1), PVP-Co 21 (2), PVP-Fe 31 (3), and PVP-Fe 31 (4).
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respectively. It should be noted that the band shift is considerably greater than the one observed in methanol solution showing that in solution there is a competition of solvent molecules with PVP. In addition, the shift of carbonyl band in the PVP-Co 21 complex is a little larger than that in the PVP-Fe 31 complex. It is consistent with the Irving–Williams rule [19– 21], i.e. the magnitudes of the band shifts upon the metal complex formation follow the order: Mn(II) , Fe(II) , Co(II) , Ni(II) , Cu(II) . Zn(II). As shown in Fig. 1b, the peak of C=O double 31 bond in PVP-Fe becomes more asymmetric 21 than that in PVP-Co . This suggested that the 31 31 interaction between PVP and Fe in PVP-Fe 21 complex is stronger than that in the PVP-Co complex. UV-VIS spectroscopy is another convenient method to investigate the coordination compounds. The UV-VIS spectra of metal cations 31 21 21 (Fe , Co and Ni ) and PVP in water and
absolute ethanol are given, respectively (Figs. 2–5). It can be seen from Figs. 2 and 3 that the interaction between PVP and metal cations in absolute ethanol was stronger than that in water. This is attributed to the two reasons: (1) the solvated metal cations were formed in water and in absolute ethanol, and the aquo cations were more stable than the alcohol-solvated metal cations; (2) the hydrogen bonding of PVP with water was stronger than that of PVP with alcohol. The absorbances of Co 21 and PVP-Co 21 in absolute ethanol were 0.450 and 0.567 at 659 nm, and were 0.024 and 0.152 at 304 nm, respectively (Fig. 3). This indicated the presence of interaction between Co 21 and PVP. Fig. 4 illustrates that there was an interaction between Ni 21 and PVP for the absorbances of Ni 21 and PVP-Ni 21 in absolute ethanol were 0.075 and 0.084 at 412 nm, respectively. The interaction between Ni 21 and PVP is weaker than that in the PVP-Co 21 system. An obvious
Fig. 2. The UV-VIS spectra of PVP (1), Co 21 (2) and PVP-Co 21 (3) in water at 286.5 K. PVP: 0.225 M, Co 21 : 0.12 M.
M. Liu et al. / Reactive & Functional Polymers 44 (2000) 55 – 64
Fig. 3. The UV-VIS spectra of Co 21 (1) and PVP-Co 21 (2) in absolute ethanol at 286.5 K. PVP: 4 3 10 22 M, Co 21 : 4 3 10 23 M.
Fig. 4. The UV-VIS spectra of Ni 21 (1) and PVP-Ni 21 (2) in absolute ethanol at 286.5 K. PVP: 0.2 M, Ni 21 : 1 3 10 22 M.
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Fig. 5. The UV-VIS spectra of PVP (1), Fe 31 (2) and PVP-Fe 31 (3) in absolute ethanol at 286.5 K. PVP: 2.4 3 10 22 M, Fe 31 : 3 3 10 23 M.
difference is also observed on comparing the UV-VIS spectra of ethanol solutions of Fe 31 and PVP-Fe 31 (Fig. 5), verifying that there was an interaction between Fe 31 and PVP. Bright et al. [22] have prepared a [Ni(NMPY) 6 ](ClO 4 ) 2 compound (NMPY 5 Nmethyl-pyrrolidone), consisting of a poorly defined light yellow crystal which could only be handled in a drybox. The molar absorption coefficient (´ max ) of the compound at lmax 5 414.1 nm in N-methyl-pyrrolidone was 14.5 l /(mol cm), which is of the same order of magnitude to that of PVP-Ni 21 (see Table 1). Thus, it suggested that the polymer-PVP and Table 1 The ´ max of M n 1 and M n 1 -PVP in absolute ethanol at 286.5 K System 31
Fe Fe 31 -PVP Co 21 Co 21 -PVP Ni 21 Ni 21 -PVP
lmax (nm)
´ max (l / mol cm)
386.5 386.5 659.0 659.0 412.0 412.0
3023.3 2336.7 112.5 141.7 7.5 8.4
metal cations formed complexes, and the structures of which were more complicated than those formed by the small molecule (N-alkylpyrrolidone) and metal cations. The interaction between PVP and metal cations may either be attributed to the donation of a pair of electrons from the carbonyl oxygen to the metal cations; or to the complex formation of the nitrogen on five-membered nitrogen-containing heterocycles with the metal cations, and the electron transfer from nitrogen to the metal cations may affect the polarizability of the neighbouring oxygen atom [15,18,22].
3.2. Determination of the coordination stability constants of PVP with metal cations In principle, stability constants can be determined by studying the concentrations of the various species present in a wide range of equilibrium mixtures containing the metal cations and the ligands in different proportions. Miller and Dorough [23] reported that the
M. Liu et al. / Reactive & Functional Polymers 44 (2000) 55 – 64
equilibrium constants of pyridinate complexes of some metallo-derivatives of tetraphenylporphine and tetraphenylchlorin were determined by spectrophotometric technique. We tried to use this method to obtain the coordination stability constants of PVP with metal cations. The coordination stability constants were determined according to the Miller–Dorough method as shown in Fig. 6. As for PVP-Fe 31 and PVP-Co 21 systems, the coordination constants are lower than 10 and 0.6, respectively, and that of PVP-Ni 21 is too low to be estimated. However, some factors obstructing us to obtain precise coordination stability constants based on the absorption spectra of the unstable complexes in ethanol solution, are summarized as follows. 1. The absorbance of PVP-Co 21 in different PVP concentration at 298.0 K is given. It is shown in Fig. 7 that the absorbance due to PVP-Co 21 complex increased with the successive addition of the PVP to a 10 23 M ethanol solution of cobalt(II) until the PVP concentration reached above 0.03 M. Increasing the PVP concentration over the range from 0.03 M to 0.05 M results in no appreciable change in the absorption spectrum, but increasing the PVP concentration above 0.05 M results in a reversal of the previous changes. This suggested that the possible complexes with multiple coordinating numbers were involved in the solution other than the formation of a single complex of 1:1 molar ratio of ligand (PVP in monomeric unit) to metal cations, which is also in accordance with the formation of the precipitates of PVP-Fe 31 and PVP-Co 21 under certain conditions (see above). 2. According to the Miller–Dorough method, a plot of 1 /(A PVP1M 2 A M ) against 1 /MPVP , will give a straight line from which the coordination stability constant may be calculated by evaluating the slope and the intercept. (A PVP1M and A M are the absorbance of n1 n1 PVP-M and M , respectively, MPVP is
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the uncomplexed PVP concentration.) When the complexed PVP concentration is much smaller than the total PVP concentration, we can substitute the total PVP concentration for the uncomplexed PVP concentration to get such a plot. The molar absorption coefficients (´ max ) of metal cations and PVP-metal cations were calculated according to the UVVIS spectra (Table 1). It is obvious from Table 1 that the values of ´ max are small, and 31 21 21 decrease in the order Fe . Co . Ni . These involve adjusting conditions so that the concentration of metal cations is large enough for a precise measurement, and the concentration of PVP needed to be much larger than that of metal cations. However, the PVP concentration is limited because the precipitates will occur when mixing the ethanol solution of metal cations with that of PVP in high concentration (as mentioned in Section 3.1). Therefore, if the complex concentration is not quite negligible, a plot of 1 /(A PVP1M 2 A M ) vs. 1 /MPVP without consideration of the complexed PVP concentration will give an error. 3. Although the ´ max value of FeCl 3 is relatively large, the stability constant still cannot be accurately determined. A special experimental difficulty is the possible influence of the presence of the ferrous ions and acetaldehyde, as in absolute ethanol the ferric ions will be partly reduced with the formation of ferrous ions meanwhile the oxidation of ethanol to acetaldehyde [17]. In order to obtain some idea on the coordination behavior of the complexes, we carried out elemental analysis of the solid complexes. The 21 atomic ratios of N:C are 1:6.13 for PVP-Co 31 (2), 1:6.02 for PVP-Fe (3), and 1:5.96 for 31 PVP-Fe (4), respectively, which agree with the number of atoms in the monomeric unit of PVP. Analysis for all the elements present in the complexes never reached 100%. Agnew [24] has reported that analysis of transition metal complexes of polyvinylpyridines never totals
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Fig. 6. Determination of coordination stability constant PVP-Co 21 (a) and PVP-Fe 31 (b) at 298.0 K.
M. Liu et al. / Reactive & Functional Polymers 44 (2000) 55 – 64
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Fig. 7. The absorbance of PVP-Co 21 in different PVP concentration at 298.0 K.
100%. For the complexes of polyvinylpyrrolidone, the low total content of the elements may be due to water and solvent molecules, either occluded in the precipitate, or coordinated to the metal or forming hydrogen bonds either to the nitrogen or to the oxygen of the monomeric unit. The number of vinylpyrrolidone units per metal cation was calculated from the nitrogen and the metal contents obtained, and the molar ratios of monomeric unit of PVP to Fe 31 and Co 21 in the solid complexes were 6:1 and 5:1, respectively. It is known that PVP plays an important role as a reactive polymer in protecting and stabilizing the colloidal dispersions of noble metals. Hirai et al. [15] investigated the IR spectra of PVP-stabilized palladium colloid, the carbonyl bond was at 1655 cm 21 for PVP, and shifted to 1630 cm 21 for PVP-stabilized palladium colloid. This suggested that parts of carbonyl groups of PVP coordinated to palladium atoms on the surface of the palladium particles. Thus,
PVP works not only as a protective polymer to stabilize the colloidal dispersions, but also as a reactive polymer to obscure the efficient adsorption of reactants and desorption of products on the catalyst. It was observed that the extra addition of PVP in selective hydrogenation of o-chloronitrobenzene over PVP-stabilized ruthenium colloid resulted in a decrease in activity [13,14]. A similar effect was also found in the asymmetric hydrogenation of a-ketoesters over PVPstabilized platinum colloid [25,26]. If metal cations (Fe 31 , Co 21 or Ni 21 ) were added into the reaction system, PVP and metal cations would form unstable complexes. This would diminish the interaction of PVP with metal colloids and decrease the inhibiting effect of PVP. Thus, the activity increased by enhancing the adsorption rate of reactants or the desorption rate of the product on the metal surface of the catalyst. In the meantime, the metal colloids will not be destroyed because this interaction is
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so weak. As a consequence, the modification mechanism of metal cations could be explained to some extent. 4. Conclusion IR and UV-VIS characterization demonstrated that there is an interaction of PVP and metal cations and the coordination of PVP to the metal cations is weak. The stability constants of PVP-Fe 31 and PVP-Co 21 were lower than 10 and 0.6, respectively, and that of PVP-Ni 21 was too low to be estimated. The solid complexes of PVP with Fe 31 and Co 21 were also prepared under proper conditions. According to the weak interaction, the modification mechanism of metal cations to PVP-stabilized metal colloids catalytic systems could be partially explained. Acknowledgements Financial support for this work by the National Science Foundation of China (contracts no. 29774037, 29873058) and the Fund of the Chinese Academy of Sciences (contract no. KJ952-J1-508) is gratefully acknowledged. References [1] J.M. Thomas, Pure Appl. Chem. 60 (1988) 1517–1528. [2] A. Henglein, Chem. Rev. 89 (1989) 1861–1873. [3] G. Schmid, Chem. Rev. 92 (1992) 1709–1727.
[4] L.N. Lewis, Chem. Rev. 93 (1993) 2693–2730. [5] H.J. Watzke, J.H. Fendler, J. Phys. Chem. 91 (1987) 854– 861. [6] G. Schmid, Endeavour 14 (1990) 172–178. [7] M. Boudart, Nature 372 (1994) 320. [8] Z. Xu, F.-S. Xiao, S.K. Purnell, O. Alexeev, S. Kawi, S.E. Deutsch, B.C. Gates, Nature 372 (1994) 346–348. [9] W. Yu, H. Liu, Q. Tao, Chem. Commun. (1996) 1773–1774. [10] W. Yu, M. Liu, H. Liu, Q. Tao, J. Mol. Catal. A: Chem. 138 (1999) 273–286. [11] W. Yu, H. Liu, M. Liu, Z. Liu, React. Funct. Polym. 44 (2000) 21–29. [12] X. Yang, H. Liu, Appl. Catal. A. 64 (1997) 197–203. [13] M. Liu, W. Yu, H. Liu, J. Mol. Catal. A: Chem. 138 (1999) 295–303. [14] M. Liu, W. Yu, H. Liu, J. Zheng, J. Colloid Interface Sci. 214 (1999) 231–237. [15] H. Hirai, H. Chawanya, N. Toshima, React. Polymers 3 (1985) 127–141. [16] N. Cheng, S. Hu, in: 2nd ed, Handbook of Solvent, Chemical Industry Press, Beijing, 1994, p. 283. [17] >.B. Rapzrby, B.B. AyueLod, Pure Chemical Reagents, (Chinese Translation, S. Cao Trans.), Higher Education Press, Beijing, 1989, p. 288, p. 409. [18] J.L. Wuepper, A.I. Popov, J. Am. Chem. Soc. 91 (1969) 4352–4356. [19] K. Nakamoto, in: 5th ed, Infrared and Raman Spectra of Inorganic and Coordination Compounds, Wiley, New York, 1997, p. 59, Part B. [20] W.L. Driessen, W.L. Groeneveld, Recl. Trav. Chim. Pays-Bas 90 (1971) 87–96. [21] W.L. Driessen, W.L. Groeneveld, F.W. Van der Wey, Recl. Trav. Chim. Pays-Bas 89 (1970) 353–367. [22] J.H. Bright, R.S. Drago, D.M. Hart, S.K. Madan, Inorg. Chem. 4 (1965) 18–20. [23] J.R. Miller, G.D. Dorough, J. Am. Chem. Soc. 74 (1952) 3977–3981. [24] N.H. Agnew, J. Polym. Sci. 14 (1976) 2819–2830. [25] X. Zuo, H. Liu, M. Liu, Tetrahedron Lett. 39 (1998) 1941– 1944. [26] X. Zuo, H. Liu, D. Guo, X. Yang, Tetrahedron 55 (1999) 7787–7804.
An investigation of the interaction between polyvinylpyrrolidone and metal cations Manhong Liu, Xiaoping Yan, Hanfan Liu*, Weiyong Yu Polymer Chemistry Laboratory, Chinese Academy of Sciences and China Petro-Chemical Corporation ( PCLCC), Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’ s Republic of China Received 23 February 1999; received in revised form 4 August 1999; accepted 11 August 1999
Abstract The interaction between polyvinylpyrrolidone (PVP) and metal cations has been studied in absolute ethanol and water at room temperature and atmospheric pressure. The UV-VIS and IR absorption spectra of metal cations, PVP and PVP-M n 1 (M n 1 5 Fe 31 , Co 21 , Ni 21 ) were given. It was shown that PVP and metal cations formed unstable complexes, and the coordination stability constants were determined according to the Miller–Dorough method. The stability constants of PVP-Fe 31 and PVP-Co 21 were lower than 10 and 0.6, respectively, and that of PVP-Ni 21 was too low to be estimated. The solid complexes of PVP-Fe 31 and PVP-Co 21 were also synthesized under proper conditions. 2000 Elsevier Science B.V. All rights reserved. Keywords: Interaction; Coordination; Polyvinylpyrrolidone; Metal cations
1. Introduction Nanoscopic metal clusters or colloids, because of their unique chemical and physical properties as compared to either bulk metal or single metal atoms, have been attracting great attention in academia and industries, especially in the field of catalysis [1–8]. Liu et al. [9,10] described that in the selective hydrogenation of cinnamaldehyde to cinnamyl alcohol catalyzed by the polyvinylpyrrolidone (PVP)-stabilized platinum colloid, the activity was enhanced to 120% and the selectivity for cinnamyl alcohol *Corresponding author. Tel.: 1 86-10-6255-4487; fax: 1 8610-6255-9373. E-mail address: [email protected] (H. Liu)
was increased from 12.0% to 98.5% when employing Fe 31 or Co 21 as a modifier. It is generally accepted that an increase in the selectivity of a catalyst by incorporating additives may cause a decrease in the activity as compensation or vice versa. However, in the catalytic system described above, considerable increases in both the selectivity and the activity were observed when some metal cations were introduced. This unusual modification effect was also found in the selective hydrogenation of crotronaldehyde [10], citronellal [11] and chloronitrobenzene [12] over polymer-stabilized platinum colloid. Moreover, it was also observed that some metal cations added to the PVP-stabilized ruthenium monometallic and the ruthenium-containing bimetallic colloid systems
1381-5148 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S1381-5148( 99 )00077-2
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increased the activity while the high selectivity (selectivity to chloroaniline . 99.9%) remained constant [13,14]. Therefore, further research should be done to explain the nature of the modification effect of some metal cations in the selective hydrogenation over the polymer-stabilized metal colloids. We suspected that the charge distribution and electronic properties of the catalysts were changed upon the adsorption of metal cations on the metal colloids. Hence, the change in electronic and conformational properties as well as the coordination ability of the surface metal atoms, which are main factors influencing the activity and the selectivity of the catalyst, should be determined by the adsorption of different amounts of the metal cations. However, in the course of examining the adsorption of nickel(II) chloride on PVP-stabilized platinum colloid, it was found that the concentration of nickel(II) had hardly changed after adsorption. This was probably due to the amount of the adsorption being too small to give a precise result. On the other hand, the evidence of interaction between metal cations and the reactants [10] or the products [12] have been reported. Interaction between PVP and palladium colloidal particles [15] has also been reported. When PVP is used as a stabilizer for metal colloids, it is possible that the metal cations interact with the oxygen or nitrogen atoms on the five-membered nitrogen-containing heterocycles of PVP. This interaction may affect the properties of the metal colloids, and consequently influence the catalytic performance. Under this consideration, we investigated the interaction between PVP and metal cations.
supplied by Beijing Chemicals. Absolute ethanol [16], anhydrous cobalt(II) chloride, and anhydrous nickel(II) chloride were prepared according to the literature [17], respectively.
2.2. Equipment All ultraviolet visible spectra were obtained on a Hitachi 340 type spectrophotometer. Infrared spectra were recorded on a Bruker IFS-IS spectrophotometer. Chemical analysis was carried out by means of inductively coupled plasma optical emission spectroscopy (ICP). The spectrometer was a Spex 1702 scanning ICP spectrometer.
2.3. Preparation of PVP-M n 1 complexes Metal cation solutions of Co 21 and Fe 31 were prepared from chloride salts in ethanol solution. Polymer complex formation of a metal cation with PVP was performed by addition of PVP ethanol solution (1.0 M, 10.0 ml) to a metal cation ethanol solution (0.15 M, 7.0 ml) to give amorphous precipitate. The resulting precipitate was centrifuged, washed with several portions of acetone until the color of washing liquor turned into colorless, and then dried in vacuo at room temperature. The blue precipitate of PVP-Co 21 complex and the light yellow precipitate of PVP-Fe 31 complex were designated as PVP-Co 21 (2) and PVP-Fe 31 (4), respectively. Another sample of the PVP-Fe 31 complex designated as PVP-Fe 31 (3) was obtained by adding PVP ethanol solution (0.2 M, 10.0 ml) to ferric ethanol solution (0.02 M, 10.0 ml). However, we failed to obtain the solid PVP-Ni 21 complex by various tests even by adding 1.0 M concentrated PVP ethanol solution to the saturated Ni 21 ethanol solution.
2. Experimental
2.1. Materials
3. Results and discussion
Poly(N-vinyl-2-pyrrolidone) (av. Mw 10 000) was purchased from BASF, dried under vacuum below 1008C for 6 h, and stored in a drybox before use. Other analytical grade reagents were
3.1. Spectrophotometric measurements IR spectroscopy is useful for detecting the interaction between two species. Wuepper and
M. Liu et al. / Reactive & Functional Polymers 44 (2000) 55 – 64
Popov [18] reported that a shift of carbonyl band was observed in the IR spectrum of 2pyrrolidone in the presence of alkali metal ions. They concluded that the band shift was due to the interaction between the carbonyl oxygen of 2-pyrrolidone and the metal ions. The IR spectra of PVP-M n 1 complexes in methanol solution and in solid state are shown in Fig. 1a and b, respectively. The spectra of the PVP-M n 1 complexes in solid state are somewhat different from that in methanol solution. As shown in Fig. 1a, the peak of C=O double bond in PVP
57
becomes somewhat asymmetric on adding Fe 31 , Co 21 and Ni 21 , and the C=O stretching frequencies of the complexes are almost the same as that of the free ligand, indicating that the interaction between PVP and M n1 in methanol solution is very weak. Fig. 1b gives the IR spectra of PVP, PVP-Fe 31 complex and PVPCo 21 complex in the solid state, respectively. The carbonyl band of PVP appears at 1661.5 cm 21 characteristic of uncomplexed pyrrolidone, whereas it shifts to 1655.7 cm 21 in PVPCo 21 , and 1656.2 cm 21 in PVP-Fe 31 complex,
Fig. 1. The IR spectra of PVP and PVP-M n 1 (a) in MeOH (298.0 K, 0.25 M): PVP (1), PVP-Fe 31 (2), PVP-Co 21 (3) and PVP-Ni 21 (4); (b) of powder samples: PVP (1), PVP-Co 21 (2), PVP-Fe 31 (3), and PVP-Fe 31 (4).
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respectively. It should be noted that the band shift is considerably greater than the one observed in methanol solution showing that in solution there is a competition of solvent molecules with PVP. In addition, the shift of carbonyl band in the PVP-Co 21 complex is a little larger than that in the PVP-Fe 31 complex. It is consistent with the Irving–Williams rule [19– 21], i.e. the magnitudes of the band shifts upon the metal complex formation follow the order: Mn(II) , Fe(II) , Co(II) , Ni(II) , Cu(II) . Zn(II). As shown in Fig. 1b, the peak of C=O double 31 bond in PVP-Fe becomes more asymmetric 21 than that in PVP-Co . This suggested that the 31 31 interaction between PVP and Fe in PVP-Fe 21 complex is stronger than that in the PVP-Co complex. UV-VIS spectroscopy is another convenient method to investigate the coordination compounds. The UV-VIS spectra of metal cations 31 21 21 (Fe , Co and Ni ) and PVP in water and
absolute ethanol are given, respectively (Figs. 2–5). It can be seen from Figs. 2 and 3 that the interaction between PVP and metal cations in absolute ethanol was stronger than that in water. This is attributed to the two reasons: (1) the solvated metal cations were formed in water and in absolute ethanol, and the aquo cations were more stable than the alcohol-solvated metal cations; (2) the hydrogen bonding of PVP with water was stronger than that of PVP with alcohol. The absorbances of Co 21 and PVP-Co 21 in absolute ethanol were 0.450 and 0.567 at 659 nm, and were 0.024 and 0.152 at 304 nm, respectively (Fig. 3). This indicated the presence of interaction between Co 21 and PVP. Fig. 4 illustrates that there was an interaction between Ni 21 and PVP for the absorbances of Ni 21 and PVP-Ni 21 in absolute ethanol were 0.075 and 0.084 at 412 nm, respectively. The interaction between Ni 21 and PVP is weaker than that in the PVP-Co 21 system. An obvious
Fig. 2. The UV-VIS spectra of PVP (1), Co 21 (2) and PVP-Co 21 (3) in water at 286.5 K. PVP: 0.225 M, Co 21 : 0.12 M.
M. Liu et al. / Reactive & Functional Polymers 44 (2000) 55 – 64
Fig. 3. The UV-VIS spectra of Co 21 (1) and PVP-Co 21 (2) in absolute ethanol at 286.5 K. PVP: 4 3 10 22 M, Co 21 : 4 3 10 23 M.
Fig. 4. The UV-VIS spectra of Ni 21 (1) and PVP-Ni 21 (2) in absolute ethanol at 286.5 K. PVP: 0.2 M, Ni 21 : 1 3 10 22 M.
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Fig. 5. The UV-VIS spectra of PVP (1), Fe 31 (2) and PVP-Fe 31 (3) in absolute ethanol at 286.5 K. PVP: 2.4 3 10 22 M, Fe 31 : 3 3 10 23 M.
difference is also observed on comparing the UV-VIS spectra of ethanol solutions of Fe 31 and PVP-Fe 31 (Fig. 5), verifying that there was an interaction between Fe 31 and PVP. Bright et al. [22] have prepared a [Ni(NMPY) 6 ](ClO 4 ) 2 compound (NMPY 5 Nmethyl-pyrrolidone), consisting of a poorly defined light yellow crystal which could only be handled in a drybox. The molar absorption coefficient (´ max ) of the compound at lmax 5 414.1 nm in N-methyl-pyrrolidone was 14.5 l /(mol cm), which is of the same order of magnitude to that of PVP-Ni 21 (see Table 1). Thus, it suggested that the polymer-PVP and Table 1 The ´ max of M n 1 and M n 1 -PVP in absolute ethanol at 286.5 K System 31
Fe Fe 31 -PVP Co 21 Co 21 -PVP Ni 21 Ni 21 -PVP
lmax (nm)
´ max (l / mol cm)
386.5 386.5 659.0 659.0 412.0 412.0
3023.3 2336.7 112.5 141.7 7.5 8.4
metal cations formed complexes, and the structures of which were more complicated than those formed by the small molecule (N-alkylpyrrolidone) and metal cations. The interaction between PVP and metal cations may either be attributed to the donation of a pair of electrons from the carbonyl oxygen to the metal cations; or to the complex formation of the nitrogen on five-membered nitrogen-containing heterocycles with the metal cations, and the electron transfer from nitrogen to the metal cations may affect the polarizability of the neighbouring oxygen atom [15,18,22].
3.2. Determination of the coordination stability constants of PVP with metal cations In principle, stability constants can be determined by studying the concentrations of the various species present in a wide range of equilibrium mixtures containing the metal cations and the ligands in different proportions. Miller and Dorough [23] reported that the
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equilibrium constants of pyridinate complexes of some metallo-derivatives of tetraphenylporphine and tetraphenylchlorin were determined by spectrophotometric technique. We tried to use this method to obtain the coordination stability constants of PVP with metal cations. The coordination stability constants were determined according to the Miller–Dorough method as shown in Fig. 6. As for PVP-Fe 31 and PVP-Co 21 systems, the coordination constants are lower than 10 and 0.6, respectively, and that of PVP-Ni 21 is too low to be estimated. However, some factors obstructing us to obtain precise coordination stability constants based on the absorption spectra of the unstable complexes in ethanol solution, are summarized as follows. 1. The absorbance of PVP-Co 21 in different PVP concentration at 298.0 K is given. It is shown in Fig. 7 that the absorbance due to PVP-Co 21 complex increased with the successive addition of the PVP to a 10 23 M ethanol solution of cobalt(II) until the PVP concentration reached above 0.03 M. Increasing the PVP concentration over the range from 0.03 M to 0.05 M results in no appreciable change in the absorption spectrum, but increasing the PVP concentration above 0.05 M results in a reversal of the previous changes. This suggested that the possible complexes with multiple coordinating numbers were involved in the solution other than the formation of a single complex of 1:1 molar ratio of ligand (PVP in monomeric unit) to metal cations, which is also in accordance with the formation of the precipitates of PVP-Fe 31 and PVP-Co 21 under certain conditions (see above). 2. According to the Miller–Dorough method, a plot of 1 /(A PVP1M 2 A M ) against 1 /MPVP , will give a straight line from which the coordination stability constant may be calculated by evaluating the slope and the intercept. (A PVP1M and A M are the absorbance of n1 n1 PVP-M and M , respectively, MPVP is
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the uncomplexed PVP concentration.) When the complexed PVP concentration is much smaller than the total PVP concentration, we can substitute the total PVP concentration for the uncomplexed PVP concentration to get such a plot. The molar absorption coefficients (´ max ) of metal cations and PVP-metal cations were calculated according to the UVVIS spectra (Table 1). It is obvious from Table 1 that the values of ´ max are small, and 31 21 21 decrease in the order Fe . Co . Ni . These involve adjusting conditions so that the concentration of metal cations is large enough for a precise measurement, and the concentration of PVP needed to be much larger than that of metal cations. However, the PVP concentration is limited because the precipitates will occur when mixing the ethanol solution of metal cations with that of PVP in high concentration (as mentioned in Section 3.1). Therefore, if the complex concentration is not quite negligible, a plot of 1 /(A PVP1M 2 A M ) vs. 1 /MPVP without consideration of the complexed PVP concentration will give an error. 3. Although the ´ max value of FeCl 3 is relatively large, the stability constant still cannot be accurately determined. A special experimental difficulty is the possible influence of the presence of the ferrous ions and acetaldehyde, as in absolute ethanol the ferric ions will be partly reduced with the formation of ferrous ions meanwhile the oxidation of ethanol to acetaldehyde [17]. In order to obtain some idea on the coordination behavior of the complexes, we carried out elemental analysis of the solid complexes. The 21 atomic ratios of N:C are 1:6.13 for PVP-Co 31 (2), 1:6.02 for PVP-Fe (3), and 1:5.96 for 31 PVP-Fe (4), respectively, which agree with the number of atoms in the monomeric unit of PVP. Analysis for all the elements present in the complexes never reached 100%. Agnew [24] has reported that analysis of transition metal complexes of polyvinylpyridines never totals
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Fig. 6. Determination of coordination stability constant PVP-Co 21 (a) and PVP-Fe 31 (b) at 298.0 K.
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Fig. 7. The absorbance of PVP-Co 21 in different PVP concentration at 298.0 K.
100%. For the complexes of polyvinylpyrrolidone, the low total content of the elements may be due to water and solvent molecules, either occluded in the precipitate, or coordinated to the metal or forming hydrogen bonds either to the nitrogen or to the oxygen of the monomeric unit. The number of vinylpyrrolidone units per metal cation was calculated from the nitrogen and the metal contents obtained, and the molar ratios of monomeric unit of PVP to Fe 31 and Co 21 in the solid complexes were 6:1 and 5:1, respectively. It is known that PVP plays an important role as a reactive polymer in protecting and stabilizing the colloidal dispersions of noble metals. Hirai et al. [15] investigated the IR spectra of PVP-stabilized palladium colloid, the carbonyl bond was at 1655 cm 21 for PVP, and shifted to 1630 cm 21 for PVP-stabilized palladium colloid. This suggested that parts of carbonyl groups of PVP coordinated to palladium atoms on the surface of the palladium particles. Thus,
PVP works not only as a protective polymer to stabilize the colloidal dispersions, but also as a reactive polymer to obscure the efficient adsorption of reactants and desorption of products on the catalyst. It was observed that the extra addition of PVP in selective hydrogenation of o-chloronitrobenzene over PVP-stabilized ruthenium colloid resulted in a decrease in activity [13,14]. A similar effect was also found in the asymmetric hydrogenation of a-ketoesters over PVPstabilized platinum colloid [25,26]. If metal cations (Fe 31 , Co 21 or Ni 21 ) were added into the reaction system, PVP and metal cations would form unstable complexes. This would diminish the interaction of PVP with metal colloids and decrease the inhibiting effect of PVP. Thus, the activity increased by enhancing the adsorption rate of reactants or the desorption rate of the product on the metal surface of the catalyst. In the meantime, the metal colloids will not be destroyed because this interaction is
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so weak. As a consequence, the modification mechanism of metal cations could be explained to some extent. 4. Conclusion IR and UV-VIS characterization demonstrated that there is an interaction of PVP and metal cations and the coordination of PVP to the metal cations is weak. The stability constants of PVP-Fe 31 and PVP-Co 21 were lower than 10 and 0.6, respectively, and that of PVP-Ni 21 was too low to be estimated. The solid complexes of PVP with Fe 31 and Co 21 were also prepared under proper conditions. According to the weak interaction, the modification mechanism of metal cations to PVP-stabilized metal colloids catalytic systems could be partially explained. Acknowledgements Financial support for this work by the National Science Foundation of China (contracts no. 29774037, 29873058) and the Fund of the Chinese Academy of Sciences (contract no. KJ952-J1-508) is gratefully acknowledged. References [1] J.M. Thomas, Pure Appl. Chem. 60 (1988) 1517–1528. [2] A. Henglein, Chem. Rev. 89 (1989) 1861–1873. [3] G. Schmid, Chem. Rev. 92 (1992) 1709–1727.
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